Systems and methods for capturing carbon dioxide and regenerating capture solution

Abstract

Techniques according to the present disclosure include capturing carbon dioxide from a dilute gas source with a CO2 capture solution to form a carbonate-rich capture solution; separating at least a portion of the carbonate from the carbonate-rich capture solution; forming an electrodialysis (ED) feed solution; passing a water stream and the ED feed solution through a bipolar membrane electrodialysis (BPMED) unit; applying an electrical potential to the BPMED unit to form at least two ED product streams, including a first ED product stream comprising hydroxide; and passing the first ED product stream for use in capturing carbon dioxide from the dilute gas source with the CO2 capture solution.

Description

[Technical field]

[0001] The present disclosure relates to systems and methods for capturing carbon dioxide (CO2) and regenerating a CO2 capture solution. [Background technology]

[0002] Capturing carbon dioxide (CO2) from the atmosphere is one approach to mitigate greenhouse gas emissions and slow climate change. However, many technologies designed for capturing CO2 from point sources, such as flue gases from industrial facilities, are generally ineffective at capturing CO2 from the atmosphere due to the extremely low CO2 concentrations and the large amounts of air required for the process. In recent years, progress has been made in finding technologies better suited to capture CO2 directly from the atmosphere.

[0003] Some direct air capture (DAC) systems use a liquid sorbent (sometimes called a solvent or capture solution) to capture CO2 from the air. An example of such a gas-liquid contacting system may be based on a cooling tower design, where a fan is used to draw air across a high surface area packed fill wetted with the capture solution containing the liquid sorbent. In some cases, the capture solution may be an aqueous alkaline solution that forms a carbonate-rich solution when reacted with CO2 in the air. The carbonate-rich capture solution is further processed downstream to regenerate a carbonate-lean solution and release a concentrated carbon stream, such as CO, CO2, or other carbon products.

[0004] A known thermochemical process for regenerating the capture solution and releasing CO2 is the pelletized calcium technology. This process involves precipitating carbonate to form calcium carbonate (CaCO3) solids, calcining the CaCO3 solids to recover the captured CO2, thereby producing calcium oxide (CaO), which is hydrated to produce calcium hydroxide (Ca(OH)2). The Ca(OH)2 is then reacted with an alkali carbonate (e.g., potassium carbonate K2CO3 or sodium carbonate Na2CO3) to regenerate the capture solution (e.g., potassium hydroxide KOH or sodium hydroxide NaOH). The integration of the thermochemical regeneration process with DAC synthesis has made commercial-scale CO2 capture and capture possible. Summary of the Invention [Means for solving the problem]

[0005] In an implementation of the embodiment, the method includes capturing carbon dioxide from a dilute gas source with a CO2 capture solution to form a carbonate-rich capture solution; separating at least a portion of the carbonate from the carbonate-rich capture solution; forming an electrodialysis (ED) feed solution; flowing a water stream and the ED feed solution through a bipolar membrane electrodialysis (BPMED) unit; applying an electrical potential to the BPMED unit to form at least two ED product streams, including a first ED product stream comprising hydroxide; and flowing the first ED product stream for use in capturing carbon dioxide from the dilute gas source with the CO2 capture solution.

[0006] In an aspect that can be combined with implementation of the embodiment, applying an electric potential to the BPMED unit includes applying at least a portion of the electric potential to the BPMED unit to form a first ED product stream and a second ED product stream.

[0007] In another embodiment that can be combined with any of the preceding embodiments, the second ED product stream comprises carbon dioxide.

[0008] Another embodiment that can be combined with any of the previous embodiments further includes recovering at least a portion of the carbon dioxide gas stream from the second ED product stream.

[0009] In another embodiment that can be combined with any of the previous embodiments, recovering a portion of the carbon dioxide gas stream from the second ED product stream comprises recovering a portion of the carbon dioxide gas stream from the second ED product stream to form a brine stream, and separating at least a portion of the carbonate from the carbonate-rich capture solution comprises crystallizing a portion of the carbonate to form a crystalline carbonate hydrate.

[0010] Another embodiment that can be combined with any of the previous embodiments further includes dissolving the crystalline carbonate hydrates and mixing the dissolved crystalline carbonate hydrates with a brine stream to form an ED feed solution, and then flowing the water and ED feed solution to the BPMED unit.

[0011] In another embodiment that can be combined with any of the previous embodiments, recovering a portion of the carbon dioxide gas stream from the second ED product stream to form the brine stream includes recovering a portion of the carbon dioxide gas stream in a flash tank.

[0012] Another embodiment that can be combined with any of the previous embodiments further includes flowing a brine stream from the flash tank for use in dissolving the crystalline carbonate hydrates, and mixing the dissolved crystalline carbonate hydrates with the brine stream to form the ED feed solution.

[0013] Another embodiment that can be combined with any of the previous embodiments includes flowing the carbon dioxide gas stream to a downstream process including at least one of a compression unit, a fuel synthesis system, a syngas generation reactor, or an electrolyzer cell; and obtaining one or more downstream products including at least one of syngas, CO, H, or water.

[0014] In another embodiment that can be combined with any of the preceding embodiments, the second ED product stream comprises a proton shuttle species.

[0015] Another embodiment that can be combined with any of the previous embodiments further includes reacting the second ED product stream with a portion of the carbonate salt to recover carbon dioxide gas and form an ED feed solution.

[0016] In another embodiment that can be combined with any of the previous embodiments, reacting the second ED product stream with a portion of the carbonate salt to recover carbon dioxide gas includes reacting a proton shuttle species of the second ED product stream with a portion of the carbonate salt to form carbonic acid and the ED feed solution.

[0017] Another embodiment that can be combined with any of the previous embodiments further includes flowing the carbon dioxide gas to a downstream process comprising at least one of: a compression unit, a fuel synthesis system, a syngas generation reactor, or an electrolyzer cell; and obtaining one or more downstream products comprising at least one of syngas, CO, H, or water.

[0018] In another embodiment that can be combined with any of the preceding embodiments, reacting the second ED product stream with a portion of the carbonate comprises reacting a proton shuttle species comprising bisulfate with a portion of the carbonate.

[0019] Another embodiment that can be combined with any of the previous embodiments further includes flowing the ED feed solution through an ion exchanger and then flowing the water and ED feed solution to a BPMED unit.

[0020] Another embodiment that can be combined with any of the previous embodiments further includes separating the BPMED recycle stream from the first ED product stream and returning the BPMED recycle stream to the BPMED unit.

[0021] In another embodiment that may be combined with any of the previous embodiments, the method includes separating a portion of the carbonate from the carbonate-rich capture solution, flowing the carbonate-rich solution through a nanofiltration unit to form a nanofiltration (NF) retentate stream comprising the carbonate-rich mixture, and forming a NF permeate stream comprising the hydroxide-rich mixture, and flowing the water and ED feed solution to the BPMED unit includes flowing an ED feed solution comprising at least a portion of the NF retentate stream to the BPMED unit.

[0022] Another embodiment that can be combined with any of the previous embodiments further includes recovering carbon dioxide gas from the second ED product stream to form a brine stream; flowing the brine stream to a reverse osmosis (RO) unit to form an RO retentate stream comprising a bicarbonate-rich solution and to form an RO permeate stream comprising water; and combining the RO retentate stream with the NF retentate stream to form the ED feed solution.

[0023] Another embodiment that can be combined with any of the previous embodiments further includes flowing at least a portion of the NF permeate stream for use in capturing carbon dioxide from a dilute gas source with a CO capture solution.

[0024] Another embodiment that can be combined with any of the previous embodiments further includes passing the NF retentate stream through an ion exchanger downstream of the nanofiltration unit to form an ion exchange regenerated waste stream and at least a portion of the ED feed solution.

[0025] Another embodiment that can be combined with any of the previous embodiments further includes separating a BPMED recycle stream comprising hydroxide from the first ED product stream and flowing the BPMED recycle stream to a BPMED unit.

[0026] Another embodiment that can be combined with any of the previous embodiments further includes combining at least a portion of the RO permeate stream comprising water with the first ED product stream to form a BPMED recycle stream.

[0027] Another embodiment that can be combined with any of the previous embodiments further includes operating the BPMED unit at a pH between 7 and 12.

[0028] Another embodiment that can be combined with any of the previous embodiments further includes operating the BPMED unit at a hydrogen ion concentration ranging between 0.001M and 2.5M.

[0029] Another embodiment that can be combined with any of the previous embodiments further includes increasing the concentration of hydroxide in the first ED product stream to form a CO2 capture solution, and then capturing carbon dioxide from the dilute gas source with the CO2 capture solution.

[0030] In another embodiment that can be combined with any of the previous embodiments, increasing the concentration of hydroxide in the first ED product stream includes evaporating water from the first ED product stream.

[0031] In another embodiment that can be combined with any of the previous embodiments, separating the portion of the carbonate from the carbonate-rich capture solution includes increasing the concentration of carbonate in the carbonate-rich capture solution by crystallizing the carbonate-rich capture solution to form mother liquor and crystalline carbonate hydrate.

[0032] Another embodiment that can be combined with any of the previous embodiments further includes dissolving at least a portion of the crystalline carbonate hydrate to form a carbonate portion; and mixing the carbonate portion with a brine stream to form the ED feed solution.

[0033] Another embodiment that can be combined with any of the previous embodiments further includes dissolving at least a portion of the crystalline carbonate hydrate to form a carbonate portion; and reacting the carbonate portion with a proton shuttle species in the second ED product stream to form an ED feed solution.

[0034] Another embodiment that can be combined with any of the previous embodiments further includes evaporating water from the carbonate-rich capture solution to increase the concentration of carbonate, and then crystallizing the carbonate-rich capture solution.

[0035] Another embodiment that can be combined with any of the previous embodiments further includes combining at least a portion of the mother liquor with at least a portion of the CO2 capture solution for use in capturing carbon dioxide from the dilute gas source together with the CO2 capture solution.

[0036] In another embodiment that can be combined with any of the preceding embodiments, the crystalline carbonate hydrate includes at least one of: potassium carbonate sesquihydrate (K2CO3·1.5H2O), sodium carbonate decahydrate (Na2CO3·10H2O), potassium sodium carbonate hexahydrate (KNaCO3·6H2O), or anhydrous carbonate.

[0037] In another aspect that may be combined with any of the previous aspects, separating at least a portion of the carbonate from the carbonate-rich capture solution includes crystallizing the carbonate-rich capture solution to form a low solids stream and a high solids stream comprising crystalline carbonate hydrates, the low solids stream having a higher liquid to solids ratio than the high solids stream.

[0038] Another embodiment that can be combined with any of the previous embodiments further includes dissolving the crystalline carbonate hydrates of the high solids stream in an aqueous solution and mixing with a brine stream to form an ED feed solution; and returning the low solids stream for use in the crystallization of the carbonate-rich capture solution.

[0039] In another embodiment that can be combined with any of the preceding embodiments, separating a portion of the carbonate in the carbonate-rich capture solution includes flowing the carbonate-rich capture solution through a nanofiltration unit.

[0040] In another embodiment that can be combined with any of the previous embodiments, flowing the carbonate-rich capture solution through the nanofiltration unit includes flowing the carbonate-rich capture solution through the nanofiltration unit to form a NF retentate stream comprising a carbonate-rich mixture, and to form a NF permeate stream comprising a hydroxide-rich mixture.

[0041] Another embodiment that can be combined with any of the previous embodiments further includes returning at least a portion of the NF permeate stream for use in capturing carbon dioxide with the CO capture solution; and crystallizing at least a portion of the NF retentate stream to form mother liquor and crystalline carbonate hydrate.

[0042] In another embodiment that can be combined with any of the previous embodiments, flowing the carbonate-rich capture solution through a nanofiltration unit includes rejecting at least 85% of the carbonate ions.

[0043] Another embodiment that can be combined with any of the previous embodiments further includes flowing the cell feed solution, which comprises a bicarbonate-rich solution, through a CO2 electro-reduction unit; applying an electrical potential to the CO2 electro-reduction unit to perform one or more reduction reactions on the cell feed solution; and performing the reduction reactions on the cell feed solution to obtain one or more reduction products.

[0044] In another embodiment that can be combined with any of the preceding embodiments, obtaining one or more reduction products includes obtaining at least one of: syngas, CO, H2, formate, methane, ethylene, or ethanol.

[0045] In another embodiment that can be combined with any of the previous embodiments, capturing carbon dioxide from a dilute gas source with a CO capture solution to form a carbonate-rich capture solution includes capturing carbon dioxide with the CO capture solution in at least one of: a gas-liquid contactor, an air contactor, a spray tower, a liquid-gas scrubber, a venturi scrubber, a packed tower, a single cell air contactor, a dual cell air contactor, or a multiple cell air contactor.

[0046] In another exemplary implementation, an electrochemical system for regenerating a CO2 capture solution for capturing carbon dioxide from a dilute gas source, the electrochemical system includes a carbonate separation subsystem configured to receive a carbonate-rich capture solution from the CO2 capture subsystem and separate at least a portion of the carbonate from the carbonate-rich capture solution; and a regeneration subsystem fluidly coupled to the carbonate separation subsystem, the regeneration subsystem including a bipolar membrane electrodialysis (BPMED) unit fluidly coupled to the carbonate separation subsystem. The BPMED unit includes at least one cation exchange membrane alternating with at least one bipolar membrane. The BPMED unit is configured: to receive an electrodialysis (ED) feed solution and a water stream; and to obtain at least two ED product streams including a first ED product stream including hydroxides.

[0047] In an embodiment compatible with the exemplary implementation, the at least one cation exchange membrane is configured to transport alkali metal ions and the at least one bipolar membrane is operable to provide hydroxyl ions.

[0048] In another embodiment that can be combined with any of the previous embodiments, the carbonate-rich capture solution includes at least one of: K2CO3, Na2CO3, or a combination thereof.

[0049] Another embodiment that can be combined with any of the previous embodiments further includes a CO2 capture subsystem fluidly connected to the carbonate separation subsystem and fluidly connected to the regeneration subsystem, the CO2 capture subsystem configured to receive a CO2 capture solution including at least one of: KOH, NaOH, an additive, or a combination thereof.

[0050] In another embodiment that can be combined with any of the previous embodiments, the carbonate separation subsystem includes a primary caustic evaporator fluidly connected to the CO2 capture subsystem and operable to concentrate the carbonate-rich capture solution.

[0051] In another aspect that may be combined with any of the previous aspects, the carbonate separation subsystem includes a crystallizer fluidly connected to the primary caustic evaporator, the crystallizer operable to concentrate the carbonate-rich capture solution received from the primary caustic evaporator.

[0052] In another embodiment that may be combined with any of the previous embodiments, the primary caustic evaporator includes at least one of: a mechanical vapor recompression (MVR) evaporator or a multiple effect evaporator.

[0053] In another embodiment that can be combined with any of the previous embodiments, the carbonate separation subsystem includes a nanofiltration unit operable to concentrate the carbonate-rich capture solution; and a crystallizer fluidly connected to the nanofiltration unit and operable to crystallize the carbonate-rich capture solution received from the nanofiltration unit to form crystalline carbonate hydrates.

[0054] In another embodiment that may be combined with any of the preceding embodiments, the nanofiltration unit is operable to reject at least 85% of carbonate ions.

[0055] In another embodiment that may be combined with any of the preceding embodiments, the nanofiltration unit is operable in a pH range of 2 to 14.

[0056] In another embodiment that can be combined with any of the previous embodiments, the crystallizer comprises at least one of: a chiller crystallizer, an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer, or a membrane distillation crystallizer.

[0057] In another embodiment that can be combined with any of the previous embodiments, the regeneration subsystem includes an auxiliary caustic evaporator fluidly connected to the CO2 capture subsystem and to the BPMED unit, the auxiliary caustic evaporator operable to concentrate the first ED product stream having hydroxides.

[0058] In another embodiment that may be combined with any of the previous embodiments, the auxiliary caustic evaporator includes at least one of: a mechanical vapor recompression (MVR) evaporator or a multiple effect evaporator.

[0059] In another aspect that may be combined with any of the previous aspects, the regeneration subsystem includes a crystallizer operable to crystallize the carbonate-rich capture solution to form crystalline carbonate hydrates; and a dissolution tank fluidly connected to the crystallizer and configured to dissolve the crystalline carbonate hydrates.

[0060] In another embodiment that can be combined with any of the previous embodiments, the regeneration subsystem includes a flash tank fluidly connected to the BPMED unit and operable to recover a carbon dioxide gas stream from a second product stream of the at least two product streams provided by the BPMED unit.

[0061] In another embodiment that can be combined with any of the previous embodiments, the BPMED unit of the regeneration subsystem is electrically coupled to a low carbon intensity power source that includes an intermittent power source.

[0062] Another embodiment that may be combined with any of the previous embodiments further includes at least one of: a compression unit, a fuel synthesis system, a syngas generation reactor, or an electrolyzer cell.

[0063] In another aspect that may be combined with any of the previous aspects, the carbonate separation subsystem includes a crystallizer operable to concentrate the carbonate-rich capture solution into crystalline carbonate hydrates; a solids separator fluidly connected to the crystallizer, the solids separator configured to form a high solids stream comprising the crystalline carbonate hydrates to form a low solids stream; and a dissolution tank fluidly connected to the solids separator, the dissolution tank configured to receive the high solids stream from the solids separator and for dissolving the crystalline carbonate hydrates in the high solids stream.

[0064] In another aspect that may be combined with any of the previous aspects, the regeneration subsystem includes an ion exchanger fluidly connected to the dissolving tank and the BPMED unit, the ion exchanger configured to remove a portion of the divalent and multivalent cations flowing to the BPMED unit.

[0065] In another embodiment that can be combined with any of the previous embodiments, the carbonate separation subsystem is configured to receive a CO2 capture solution that includes potassium hydroxide KOH, sodium hydroxide NaOH, an additive, or a combination thereof.

[0066] In another embodiment that can be combined with any of the previous embodiments, the carbonate-rich capture solution includes potassium carbonate, K2CO3, sodium carbonate, Na2CO3, or a combination thereof.

[0067] In another embodiment that can be combined with any of the previous embodiments, the CO2 capture subsystem includes at least one of: a gas-liquid contactor, an air contactor, a spray tower, a liquid-gas scrubber, a venturi scrubber, a packed tower, a single cell air contactor, a dual cell air contactor, or a multi-cell air contactor.

[0068] In another exemplary implementation, an electrochemical system for generating reduction products from carbon dioxide from a dilute gas source includes a CO2 capture subsystem configured to generate a carbonate-rich capture solution; a carbonate separation subsystem fluidly coupled to the CO2 capture subsystem and operable to receive the carbonate-rich capture solution, the carbonate separation subsystem including a crystallizer configured to at least partially form crystalline carbonate hydrates from the carbonate-rich capture solution; and a product generation subsystem fluidly coupled to the CO2 capture subsystem and fluidly coupled to the carbonate separation subsystem via the crystallizer. The product generation subsystem includes a dissolution tank fluidly coupled to the crystallizer and configured to dissolve the crystalline carbonate hydrates; and a CO2 electroreduction unit fluidly coupled to the dissolution tank, the CO2 electroreduction unit including one or more bipolar membranes and a catalyst configured to provide one or more reduction products.

[0069] In an embodiment compatible with the exemplary implementation, the CO2 electrolytic reduction unit provides one or more reduction products from a cell feed solution that includes a bicarbonate-rich solution.

[0070] In another embodiment that can be combined with any of the preceding embodiments, the one or more reduction products include at least one of: syngas, CO, H2, formate, methane, ethylene, or ethanol.

[0071] In another exemplary embodiment, an electrochemical system for regenerating a capture solution from a dilute gas source includes a carbonate separation subsystem configured to receive the CO capture solution and form a carbonate-rich capture solution having a concentration of carbonate greater than a concentration of carbonate in the CO capture solution; and a regeneration subsystem fluidly coupled to the carbonate separation subsystem, the regeneration subsystem including an electrodialysis (ED) unit including a gas diffusion electrode (GDE) and a cation exchange membrane.

[0072] In another exemplary implementation, a method for regenerating a capture solution for capturing carbon dioxide from a dilute gas source includes separating at least a portion of carbonate from a carbonate-rich capture solution; flowing an electrodialysis (ED) feed solution including a carbonate-rich bicarbonate mixture through an ED unit including a gas diffusion electrode (GDE); flowing a water stream and a hydrogen feed stream through the ED unit including the GDE; applying an electric potential to the ED unit to form multiple ED product streams, the multiple GDE product streams including a first ED product stream including hydroxide and hydrogen, a second ED product stream including bicarbonate and water, and a gaseous carbon dioxide stream; separating the first ED product stream into a CO2 capture solution and a hydrogen-rich stream; and diffusing the gaseous carbon dioxide stream through the GDE of the ED unit.

[0073] The details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. [Brief description of the drawings]

[0074] [Figure 1] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating capture solution and capturing CO2 by using direct protonation. [Diagram 2] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating capture solution and capturing CO2 by using indirect protonation. [Diagram 3] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating the capture solution and capturing CO2 by using a nanofiltration unit and direct protonation. [Figure 4] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating the capture solution and capturing CO2 by using a nanofiltration unit and indirect protonation. [Diagram 5] 1 is a flow chart illustrating an example method for regenerating a CO2 capture solution and capturing CO2 via an electrochemical system. [Figure 6] FIG. 1 is a block flow diagram showing an example electrochemical system for generating reduction products by using a CO electrolytic reduction unit. [Figure 7] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating capture solution and capturing CO2 using a chiller crystallizer and direct protonation. [Figure 8] FIG. 1 is a block flow diagram showing an example electrochemical system that regenerates the capture solution and recovers CO2 using a nanofiltration unit and a reverse osmosis unit. [Figure 9] FIG. 1 is a schematic diagram of an example bipolar membrane electrodialysis (BPMED) unit having a membrane stack including cation exchange membranes alternating with bipolar membranes. [Figure 10] FIG. 1 is a schematic diagram of an example electrodialysis (ED) unit including a gas diffusion electrode. [Figure 11] FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating the capture solution and capturing CO2 using a filtration unit and swinging the pH of the feed provided to the ED unit. [Figure 12]FIG. 1 is a block flow diagram showing an example electrochemical system for regenerating the capture solution and capturing CO2 using a filtration unit and an ED unit. [Figure 13] FIG. 1 is a schematic diagram of an example ED unit including a gas diffusion electrode. [Figure 14] FIG. 1 is a schematic diagram of an example control system that can be used to implement a computer-assisted process according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075] This disclosure describes systems and methods for capturing CO2 from a dilute source (such as atmosphere or ambient air) with a capture solution, regenerating the capture solution, and recovering the CO2 using an electrochemical process. The CO2 concentration in a dilute source such as atmosphere (approximately 400-420 ppm) is much lower than the CO2 concentration in a point source such as flue gas (approximately 5-15% v / v). Mass transfer kinetics favor CO2 capture from a point source. Thus, design concerns for the CO2 capture subsystem and capture solution regeneration subsystem are different for dilute sources when compared to point sources. The electrochemical systems and methods described herein include a CO2 capture subsystem coupled to a capture solution regeneration subsystem via a carbonate separation subsystem. The carbonate separation subsystem bridging the CO2 capture subsystem to the capture solution regeneration system allows the subsystems to be operatively decoupled from one another, which provides several advantages.

[0076] The CO2 capture subsystem can operate in a wider range of ambient conditions because it is decoupled from the downstream process by the carbonate separation subsystem. The carbonate separation subsystem forms crystalline carbonate hydrates, and the load required to form this product is determined by the capture solution composition and its position relative to the saturation curve of the DIC species (e.g., carbon saturation curve). For example, in conditions where a high ionic strength capture solution is used (e.g., high hydroxide for faster capture kinetics, or high carbonate for higher crystallizer recovery), the carbonate separation subsystem may require a lower load to reach saturation and form crystalline carbonate hydrates because the capture solution is closer to the saturation curve. In contrast, in conditions where a dilute capture solution is required (e.g., dry climates with high evaporative losses), the carbonate separation subsystem may require a higher load to reach saturation and form crystalline carbonate hydrates. Thus, the CO2 capture subsystem can equilibrate with the ambient temperature and relative humidity without significantly affecting the downstream capture solution regeneration subsystem. The evaporative load resulting from equilibrium is accommodated by the units of the carbonate separation subsystem that bridge the other two subsystems. The solubility of carbonate in the capture solution governs the load on the Carbonate Separation Subsystem, as the Carbonate Separation Subsystem is a buffer that provides pure or nearly pure carbonate to the capture solution regeneration subsystem. Thus, the capture solution used in the CO2 capture subsystem can be optimized for capture (e.g., high hydroxide, low carbonate) or to improve water balance and evaporation costs, taking into account the relative humidity balance.

[0077] The electrochemical system described herein allows for advantages such as adaptability to environmental conditions and cold weather operation. Because the carbonate separation subsystem can hold the load of the CO2 capture subsystem, a wide range of operating conditions are possible for the CO2 capture subsystem. The CO2 capture subsystem can operate anywhere under the carbonate saturation curve while maintaining a pure or nearly pure carbonate flow to the electrodialysis (ED) unit (which can be achieved, for example, by separating and dissolving crystalline carbonate hydrates). This allows the CO2 capture system to operate near the saturation line at high ionic strength, with the freezing point of the solution being significantly reduced as a result. For example, the CO2 capture subsystem can operate with a capture solution containing a high hydroxide concentration, which can enable higher capture rates.

[0078] Another advantage of the electrochemical system described herein is load flexibility (ramping). The main energy driver of the system is the ED unit. In some implementations, the ED unit can include a bipolar membrane electrodialysis unit (BPMED). In some implementations, the ED unit can include a gas diffusion electrode (GDE). Because the ED unit is an electrochemical cell, it can simply have its power supply cut, reduced, or ramped up as needed. This is an advantage over equipment sometimes used in calcium regeneration processes, such as fluidized bed reactors and high temperature calciners, which cannot be easily ramped. Furthermore, the electrochemical system allows for the elimination of insoluble precipitates. Sodium and potassium carbonates in particular are highly water soluble, and if these salts crystallize in undesirable locations or in dirty equipment, a simple water wash will remove the dirt.

[0079] Throughout this disclosure, the terms "air contactor" and "gas-liquid contactor" are used interchangeably to describe elements of a CO2 capture subsystem that uses a sorbent (e.g., a liquid capture solution) to absorb carbon dioxide from a dilute gas source such as ambient air or atmosphere.

[0080] The electrochemical systems and methods described herein can enable the capture solution regeneration subsystem to be used in conjunction with a variety of different styles of CO2 capture subsystems, including air contactors, e.g., cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a specific gas component from a larger gas stream using liquid sorbents. The electrochemical system has the benefit of flexibility of scale because the subsystems can be easily disconnected from one another and are modular. The subsystems can be sized to accommodate capacities ranging from laboratory scale to industrial or commercial scale. The electrochemical system is adaptable to a variety of environmental conditions and low carbon intensity power generation, including intermittent sources (e.g., wind, solar). The resulting CO2 product as a feedstock for downstream product production can result in other carbon products that have inherently low carbon emissions. Intermittent sources of power, such as wind and solar energy, tend to fluctuate and are difficult to control. Intermittent sources typically generate electricity only periodically. The subsequent electrochemical systems and methods have relatively fast ramp rates such that they can be coupled with low carbon intensity power sources, including intermittent and non-intermittent sources such as hydro, nuclear, and biomass.

[0081] The electrochemical system can include a CO2 capture subsystem coupled to a capture solution regeneration subsystem via a carbonate separation subsystem. CO2 from ambient air can be captured by contacting the air with a capture solution containing an alkali hydroxide (e.g., KOH, NaOH, or a combination thereof) in a gas-liquid contactor, such as an air contactor. In some cases, the air contactor or gas-liquid contactor can be designed based on cooling tower technology or from retrofitting existing cooling tower systems. Reacting the CO2 in the air with the alkali capture solution can form a carbonate-rich capture solution (e.g., K2CO3, Na2CO3, or a combination thereof), which must be treated to recover the captured CO2 for downstream use and to regenerate the alkali hydroxide in the capture solution.

[0082] Implementations of the present disclosure for treating carbonate-rich capture solutions can include the use of an ED unit. In some implementations, the ED unit can include a bipolar membrane electrodialysis unit (BPMED) consisting of a membrane stack including a bipolar membrane (BPM), an example of which is shown in FIG. 9. In some implementations, the ED unit can include a gas diffusion electrode (GDE) and a single cell membrane stack, an example of which is shown in FIG. 10. The BPMED unit can combine a BPM with either a cation exchange membrane (CEM) or an anion exchange membrane (AEM), or both. The ED unit can generate a pH swing in the solution. One membrane stack configuration that can be used in the ED unit is one in which the BPM is interleaved with the CEM. This membrane arrangement forms a BPMED unit, where the membranes define interleaved feed release (proton generation) and alkali regeneration (hydroxide generation) compartments. In some implementations, the ED unit can have three compartments (feed compartment, acid compartment, and base compartment) defined by AEMs interleaved with one or more CEMs or BPMs. The ED unit can be an element of a regeneration subsystem. To regenerate a capture solution containing alkali hydroxide, the BPM provides hydroxyl ions to the alkali regeneration compartment via dissociation of water and protons to the feed discharge compartment. The regeneration protons replace cations, such as alkali metal ions, that have been selectively transported across the CEM to the alkali regeneration compartment. For example, to regenerate potassium hydroxide (KOH) in the capture solution, the BPM provides hydroxyl ions to the alkali regeneration compartment via dissociation of water and protons to the feed discharge compartment. - ions, CEM provides K + The BPMED unit is designed to operate with low voltage drops (e.g., BPM voltage drop of less than 2 V and CEM voltage drop of less than 1 V) and high current densities (e.g., 50 mA / cm 2The ED unit may be selected or designed to have desired properties such as a BPM current density above 100 kcal / s. Both the BPM and CEM may be stable at a wide range of operating temperatures, especially at high operating temperatures, since they are capable of reduced voltages. Both the BPM and CEM may be stable at a wide range of pH and in highly concentrated alkaline solutions. This allows for a regenerative capture solution with a high hydroxide concentration, which can improve CO2 capture rates and reduce evaporator costs. In embodiments, the CEM has a low anion permeability that mitigates ion leakage that can reduce stack efficiency. An example BPM ED unit is illustrated and described with respect to FIG. 9 below. In some implementations, the ED unit may include a gas diffusion electrode (GED) and a single cell membrane stack, such as a CEM, an example of which is shown in FIG. 10 below. The ED unit is thus the electrochemical component of the regeneration subsystem in that the ED unit couples the input of electrical energy to facilitate chemical reactions (e.g., enabling salt splitting and acid-base recovery). Thus, in the configurations of FIGS. 1 through 10, the regeneration subsystem may be described as the "electrochemical" regeneration subsystem of the electrochemical system.

[0083] To achieve the desired process conditions for operating the ED unit, it may be advantageous to separate carbonate from other species in the carbonate-rich capture solution to provide a pure carbonate stream to the regeneration subsystem that includes the ED unit. One approach to this is to integrate a carbonate separation subsystem that bridges the gas-liquid contactor of the CO2 capture subsystem with the regeneration subsystem. The carbonate separation subsystem may include one or more units that selectively separate carbonate species from the hydroxide-containing process solution. For example, a caustic evaporator or a nanofiltration unit may be used to concentrate the carbonate-rich capture solution from the gas-liquid contactor. The caustic evaporator may increase the ion concentration of the capture solution, thereby shifting the composition of the solution against the carbonate saturation curve to reduce the solubility of carbonate in the solution and produce a concentrated carbonate-rich solution. Some examples of caustic evaporators include mechanical vapor recompression (MVR) evaporators and multiple effect evaporators. The concentrated carbonate-rich stream may then be sent to a crystallizer to form crystalline carbonate hydrates. This solid-phase intermediate product allows the carbonate to be easily separated from other components of the stream to form a pure or relatively pure carbonate stream for use in the ED unit. The crystalline carbonate hydrate dissolves in an aqueous solution that is used to feed the ED unit of the regeneration subsystem. The aqueous solution is primarily water, but may contain some non-aqueous components. In some cases, the nanofiltration retentate, which contains the concentrated carbonate solution from the nanofiltration system, is purified with an ion exchange system to remove undesirable ionic species (Ca +2 , Mg +2 , B.A. +2 , Sr +2At least a portion of the hydrates (e.g., silicates, borates) can be removed and then flowed to the ED unit. In some cases, a nanofiltration unit can be used downstream of the CO2 capture subsystem to produce a carbonate-rich or bicarbonate-rich mixture that is fed to the regeneration subsystem. The nanofiltration unit can include polyethersulfone as the membrane material and can have a molecular cutoff of 100-1000 Daltons. In some cases where the upstream CO2 capture subsystem ramps up or down (e.g., operates at a different capacity than the previous one), the resulting load can be accommodated by a caustic evaporator, nanofiltration unit, crystallizer, or combination thereof in the carbonate separation subsystem. This is particularly useful when the CO2 capture system operates in a high ionic strength solution or when the system is connected to an intermittent power source (or both). For example, when the ED unit operates at limited capacity or is out of service (e.g., due to maintenance), the carbonate separation subsystem can include one or more buffer tanks to store crystalline carbonate hydrates. In some cases, a buffer volume dissolution tank or overflow tank can help absorb the load of operational changes since crystalline carbonate hydrate is a highly soluble salt. For example, during non-peak periods, there is little electricity from intermittent sources (e.g., wind, solar) and the electrodialysis system, the most energy-consuming subsystem in the process, is ramped down, but the air contactor and crystallizer can continue to operate, capturing CO2 and producing crystalline carbonate hydrate. The crystalline carbonate hydrate solids can be stored and fed to the electrodialysis system to be regenerated when electricity is more readily available.

[0084] The ED unit consists of a brine loop that carries the absorbed CO2 in the form of dissolved inorganic carbon (DIC), and a caustic loop that regenerates the capture solution. When a significant current density is applied to the ED unit, water can split into protons and hydroxyl ions. With the generation of sufficient protons, the ED unit can shift the local pH and the local equilibrium of DIC species in the feed solution. The DIC species are carbonate CO3 2- , bicarbonate HCO3- , carbonate H2CO3, dissolved CO2, or a combination thereof. The ED unit directly protonates one or more of the DICs, resulting in the following reaction:

[0085] Reaction 1: CO3 2- +H + →HCO3 - (carbonate to bicarbonate)

[0086] Reaction 2: HCO3 - +H + →H2CO3 (bicarbonate to carbonic acid)

[0087] Reaction 3: Dissociation: H2CO3→H2O+CO2

[0088] At high H2CO3 concentrations, the equilibrium CO2 concentration becomes sufficient to outgas the CO2 from the capture solution. In some cases, the CO2 may be partially degassed within the ED unit cell. Since CO2 bubble formation due to CO2 degassing in the ED unit cell can increase electrical resistance and reduce the active area of ​​the cell, it may be beneficial to completely degas the external CO2. Complete CO2 degassing can be caused by an external flash tank, where the HCO3 - is returned to the ED unit as a brine stream. In other configurations, the formation of H2CO3 and the degassing of CO2 can both be carried out in separate tanks by using a proton-shuttle loop that allows the ED unit to indirectly protonate the DIC species.

[0089] In some embodiments, the ED unit can indirectly protonate the DIC species via intermediate dissolved ionic species to avoid CO2 degassing within the cells of the ED unit, which are first protonated within the ED unit and then donate these protons to the DIC species in the external unit (e.g., SO4 2- , HPO4 2- In one embodiment, the proton-shuttle species is sulfate, SO4. 2-, bisulfate HSO4 - or a combination thereof. The ED unit protonates the sulfate via the following reaction:

[0090] Reaction 4: SO4 2- +H + →HSO4 - (sulfate to bisulfate)

[0091] The external tank receives the sulfate-bisulfate mixture from the ED unit and the carbonate stream. The sulfate reacts with the carbonate to produce H2CO3 via the following reaction:

[0092] Reaction 5: 2KHSO4+K2CO3→2K2SO4+H2CO3

[0093] Therefore, bisulfate can then affect the same DIC pH swing in the external tank via reactions 1 through 3.

[0094] In some embodiments, it may be advantageous to use a process solution that includes a mixture of potassium-based and sodium-based species. In some cases, potassium-based capture solutions can achieve better capture kinetics than sodium-based capture solutions, but sodium-based capture solutions can have lower solubility that can reduce crystallizer loading. Thus, CO2 capture solutions can include mixtures of KOH, NaOH, K2CO3, and Na2CO3. These mixed sodium-potassium systems allow for adjustment of mass transfer kinetics, water balance, and operating temperature. The effective ratio of potassium-based components to sodium-based components can depend on the operating environment and is described herein.

[0095] FIG. 1 is a block flow diagram illustrating an example electrochemical system 100 for regenerating a capture solution and capturing CO2 by using direct protonation. The electrochemical system 100 includes a CO2 capture subsystem 102 fluidly connected to a carbonate separation subsystem 162 and a regeneration subsystem 164. The carbonate separation subsystem 162 includes a primary caustic evaporator 112 fluidly connected to a crystallizer 104. In some implementations, the crystallizer 104 can be fluidly connected to a solids separator, such as a centrifuge, pressure or vacuum filter, scraper, cyclone, and the like. The carbonate separation subsystem 162 exploits the solubility difference between carbonates and hydroxide salts in the capture solution to enable efficient separation of carbonates. The primary caustic evaporator 112 receives the carbonate-rich capture solution 120 from the CO2 capture subsystem 102. In some implementations, the CO2 capture subsystem 102 can include one or more air contactors 105. The air contactor 105 may include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactor 105 may include a single or multiple cell air contactor, a dual cell air contactor, a dual flow air contactor, or combinations thereof. The air contactor may operate in cross flow, counter flow, co-flow, or combinations thereof.

[0096] The carbonate-rich capture solution 120 can be an aqueous mixture that includes primarily carbonate ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or a combination thereof. The carbonate-rich capture solution 120 can also include minor amounts of other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 120 can include between 0.4M and 6M K2CO3 and between 1M and 10M KOH. In another implementation, the carbonate-rich capture solution 120 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0097] In some implementations, the primary caustic evaporator 112 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The primary caustic evaporator 112 removes water from the carbonate-rich capture solution 120 to form a concentrated carbonate-rich capture solution 118. The primary caustic evaporator 112 discharges a water stream 119. The concentrated carbonate-rich capture solution 118 can include a higher carbonate concentration and a higher hydroxide concentration than the carbonate-rich capture solution 120. For example, the concentrated carbonate-rich capture solution 118 can include between 0.4M and 6M K2CO3 and between 1M and 14M KOH. In another implementation, the concentrated carbonate-rich capture solution 118 can include a concentrated aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3. The primary caustic evaporator 112 thus increases the respective concentrations of carbonates and hydroxides such that the carbonates in the carbonate-rich capture solution 118 become less soluble, reducing the crystallizer load (evaporative heating or cooling refrigeration) on the crystallizer 104.

[0098] In some implementations, the crystallizer 104 includes an evaporative crystallizer, a freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooling), a film distillation crystallizer, or a combination thereof. The crystallizer 104 can be based on forced circulation, draft tube baffles, fluidized bed designs, or a combination thereof. The crystallizer 104 increases the hydroxide concentration, thereby decreasing the solubility of carbonate in the concentrated carbonate-rich capture solution 118. In some cases, the crystallizer 104 evaporates a portion of the concentrated carbonate-rich capture solution 118 to reach supersaturation. This concentration step forms crystalline carbonate hydrate 122, mother liquor 142, and water stream 124. The crystallizer 104 discharges the water stream 124 for downstream processing (e.g., a filtration system, a water treatment system, or a waste system) or for use in another application within or beyond the system 100. The crystalline carbonate hydrate 122 is at least partially separated from the mother liquor 142 to form pure or nearly pure carbonate that can be used in the feed solution for the ED unit. The mother liquor 142 can include the remaining components of the concentrated carbonate solution 118, such as water and hydroxides, after the crystalline carbonate hydrate 122 is separated. The crystalline carbonate hydrate 122 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 122 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). Potassium carbonate sesquihydrate crystals can be formed by dissolving potassium carbonate in water or by dissolving potassium carbonate in water. - In another example, the crystalline carbonate hydrate 122 can include sodium carbonate decahydrate (Na2CO3·10H2O), and the mother liquor 142 can include a mixture of NaOH, K2CO3, and / or HCl. -In another example, the crystalline carbonate hydrate 122 can include potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 122 can include different stoichiometric amounts of water molecules per unit of carbonate in the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value). After separation from the mother liquor 142, the crystalline carbonate hydrate 122 is sent to the regeneration subsystem 164 and the mother liquor 142 is returned to the CO2 capture subsystem 102.

[0099] The regeneration subsystem 164 includes a dissolution tank 106 fluidly connected to the BPMED 108 and the flash tank 110. The BPMED 108 is an example of an ED unit that uses a BPM and a CEM, but in some cases the regeneration subsystem 164 can include different ED units (e.g., ED units that include one or more CEMs, AEMs, BPMs, or combinations thereof). Taken together, the process streams flowing into and out of the dissolution tank 106, the BPMED 108, and the flash tank 110 form a brine loop in which the DIC is protonated and the CO2 is released. The dissolution tank 106 can receive the water stream 128 and the crystalline carbonate hydrate 122 from the crystallizer 104. In some cases, a purified aqueous solution can be used instead of or in addition to the water stream 128. The purified aqueous solution can be a solution with reduced or minimized particulates and dissolved contaminants. The crystalline carbonate hydrate 122 dissolves in water and forms bicarbonate HCO3 in the brine stream 138 received from the flash tank 110. - is combined with ED to form ED Feed Solution 126. ED Feed Solution 126 is composed of bicarbonate HCO3 - The rich solution may include a mixture of carbonates and other components such as water.

[0100] The BPMED 108 can include a stack of cells positioned between two electrodes. Each cell can be arranged in a configuration that includes alternating BPMs and CEMs. The electrodes are operable to apply a potential to enable salt decomposition and acid-base recovery. In some cases, the electrodes of the BPMED 108 can be coupled to an intermittent low carbon intensity power source (e.g., solar, wind, geothermal) or a non-intermittent low carbon intensity power source (e.g., hydro, nuclear, biomass, recycled natural gas). The BPMED 108 can include multiple feed discharge and alkali regeneration compartments defined by the BPMs and CEMs.

[0101] The BPMED 108 is configured to receive the ED feed solution 126 and a water stream 134 at one or more feed discharge compartments. The BPM of the BPMED 108 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 108 enables a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations to the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form a first ED product stream 132 having a hydroxide concentration between 0.5M and 12M. For example, the ED feed solution 126 can include a solution rich in potassium bicarbonate, KHCO3. The CEM of the BPMED 108 can transport potassium ions, KHCO3, and KHCO3. + are selected for K and transported to the alkaline regeneration compartment, where they + OH - to form KOH in the first ED product stream 132. In the feed discharge section, bicarbonate HCO3 - The ions are directly protonated in the BPMED 108 to form a second ED product stream 130 containing carbonate, HCO. In some cases, the carbonate dissociates into CO and water, and the CO may be partially degassed in the BPMED 108.

[0102] In some implementations, a reduction in pH can cause carbonic acid to dissociate and release CO2 within the cells of the BPMED 108. In some implementations, the BPMED 108 can include an inter-membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, a current of 50 mA / cm 2 to 2000mA / cm 2 A current density between can be applied to the BPMED 108.

[0103] The second ED product stream 130 is composed of carbonate HCO3 and bicarbonate HCO3 - For example, the second ED product stream 130 can include an aqueous mixture of carbonate, H2CO3, and potassium bicarbonate, KHCO3. The carbonate, H2CO3, dissociates into CO2 and water. The second ED product stream 130 can be sent to the flash tank 110, where the CO2 stream 136 is partially or completely released from the flash tank 110 and sent to one or more downstream processing units described in subsequent implementations (e.g., compression unit, purification unit, electrolytic reduction subsystem, carbon product production system, syngas generation reactor). Bicarbonate, HCO3 - A brine stream 138 containing the aqueous mixture of may be sent to the dissolving tank 106 completing the brine loop of the regeneration subsystem 164.

[0104] The first ED product stream 132 may include an aqueous mixture with hydroxide as the predominant chemical species. For example, the first ED product stream 132 may include an aqueous solution of potassium hydroxide, KOH. The first ED product stream 132 may be returned from the BPMED 108 to the CO2 capture subsystem 102 as a CO2 capture solution 144. In some implementations, the CO2 capture solution 144 may include a hydroxide concentration between 0.5M and 10M. In some implementations, the regeneration subsystem 164 may optionally include an auxiliary caustic evaporator 114. The auxiliary caustic evaporator 114 may include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The auxiliary caustic evaporator 114 concentrates the first ED product stream 132 by removing water to form the CO2 capture solution 144 and releasing the water stream 140. In such an implementation, the CO2 capture solution 144 comprises a carbonate-lean mixture and has a higher hydroxide concentration than the first ED product stream 132. Collectively, the auxiliary caustic evaporator 114, the CO2 capture subsystem 102, the carbonate separation subsystem 162, the dissolution tank 106, and the process streams flowing into and out of the BPMED 108 form a caustic loop in which the capture solution is regenerated.

[0105] The example electrochemical system 100, as well as other exemplary implementations according to the present disclosure, includes a process stream (also referred to as a "stream") within the electrochemical system that is used to capture CO2, regenerate the capture solution using a pH swing, and release the CO2. The capture of CO2 is also referred to herein as off-gassing of CO2. The process stream can be flowed using one or more flow control systems 999 implemented throughout the electrochemical system. The flow control system 999 can include one or more flow pumps to pump the process stream, and one or more flow tubes through which the process stream flows, and one or more valves to regulate the flow of the stream through the tubes. The control system 999 can include one or more pH monitoring devices and one or more conductivity monitoring devices. In some implementations, the control system 999 can include one or more chemical analysis devices (e.g., Fourier transform near infrared spectroscopy devices) that measure DIC species. In some implementations, the control system 999 may include one or more temperature sensors (e.g., thermocouples, thermistors, thermometers) and temperature controllers to monitor and control one or more aspects of the flow control system 999 in response to heat generated from one or more elements of the electrochemical system.

[0106] In some implementations, the control system 999 can be operated manually. For example, an operator can set the flow rate for each pump and set the open / closed positions of the valves to regulate the flow of the process stream through the piping of the control system 999. Once the operator has set the open / closed positions of the valves of all the control systems 999 distributed throughout the electrochemical system to capture CO2 and regenerate the capture solution, the control system 999 can run the stream under constant flow conditions, e.g., constant volumetric flow rate or other flow conditions. To change the flow conditions, the operator can manually operate the control system 999, e.g., by changing the pump flow rate or the open / closed positions of the valves.

[0107] In some implementations, the flow control system 999 can operate automatically. For example, the flow control system 999 can be connected to a computer or computer readable medium that stores instructions (such as flow control instructions and other instructions) executable by one or more processors to perform operations (such as flow control operations). An operator can set the flow rates and open / closed valve positions for all flow control systems 999 distributed throughout the electrochemical system to capture CO2 and regenerate the capture solution using the flow control systems 999. In such implementations, the operator can manually change the flow conditions by providing input through the flow control systems 999. Also, in such implementations, the flow control system 999 can automatically (i.e., without manual intervention) control one or more of the flow control systems using, for example, a feedback system connected to the flow control system 999. For example, sensors (such as pressure sensors, temperature sensors, or other sensors) can be connected to pipes through which the process stream flows. The sensors can monitor and provide flow conditions (such as pressure, temperature, or other flow conditions) of the process stream to the flow control system 999. In response to a flow condition exceeding a threshold value (such as a threshold pressure value, a threshold temperature value, or other threshold value), the control system 999 can automatically take action. For example, if the pressure or temperature in a line exceeds a threshold pressure value or a threshold temperature value, respectively, the flow control system 999 can provide a signal to a pump to reduce flow, a signal to open a valve to relieve pressure, a signal to shut off a process flow, or other signal.

[0108] FIG. 2 shows an example electrochemical system 200 for regenerating the capture solution and recovering CO2 by using indirect protonation and proton shuttle species. In some cases, the electrochemical system 200 can be advantageous over the electrochemical system 100 of FIG. 1 because the DIC species are protonated outside the ED unit, avoiding CO2 degassing in the cells of the ED unit. Degassing CO2 in the ED unit can be undesirable because it can increase electrical resistance and reduce the active area of ​​the cells. Thus, a brine loop that includes an indirect protonation system can be more efficient than a brine loop that includes a direct protonation system.

[0109] The electrochemical system 200 includes a CO2 capture subsystem 202 fluidly connected to a carbonate separation subsystem 262 and a regeneration subsystem 264. The carbonate separation subsystem 262 includes a primary caustic evaporator 212 fluidly connected to a crystallizer 204. In some implementations, the crystallizer 204 can be fluidly connected to a solids separator, such as a centrifuge, pressure or vacuum filter, scraper, cyclone, and the like. The carbonate separation subsystem 262 exploits the solubility difference between carbonates and hydroxide salts in the capture solution to enable efficient separation of the carbonates. The primary caustic evaporator 212 receives the carbonate-rich capture solution 220 from the CO2 capture subsystem 202. In some implementations, the CO2 capture subsystem 202 can include one or more air contactors 205. The air contactor 205 may include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactor 205 may include a single or multiple cell air contactor, a dual cell air contactor, a dual flow air contactor, or combinations thereof. The air contactor may operate in cross flow, counter flow, co-flow, or combinations thereof.

[0110] The carbonate-rich capture solution 220 can be an aqueous mixture containing primarily carbonate ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or a combination thereof. The carbonate-rich capture solution 220 can also contain other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and other dissolved species, including process additives (e.g., chloride, sulfate, acetate, phosphate, surfactants) and non-process elements (e.g., silicates, borates, calcium, magnesium, strontium, barium, iron, nickel), in small amounts. For example, the carbonate-rich capture solution 220 can include between 0.5M and 6M K2CO3 and between 1M and 10M KOH. In another implementation, the carbonate-rich capture solution 220 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0111] In some implementations, the primary caustic evaporator 212 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The primary caustic evaporator 212 removes water from the carbonate-rich capture solution 220 to form a concentrated carbonate-rich capture solution 218. The primary caustic evaporator 212 discharges a water stream 219. The concentrated carbonate-rich capture solution 218 can include a higher carbonate concentration and a higher hydroxide concentration than the carbonate-rich capture solution 220. For example, the concentrated carbonate-rich capture solution 218 can include between 0.5 M and 6 M K2CO3 and between 1 M and 14 M KOH. In another embodiment, the concentrated carbonate-rich capture solution 218 can include a concentrated aqueous Na2CO3-NaOH mixture. In some implementations, the concentrated carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3. The primary caustic evaporator 212 thus increases the respective concentrations of carbonates and hydroxides such that the carbonates in the carbonate-rich capture solution 218 become less soluble, reducing the crystallizer load (evaporative heating or cooling refrigeration) on the crystallizer 204.

[0112] In some implementations, the crystallizer 204 includes an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooled), a film distillation crystallizer, or a combination thereof. The crystallizer 204 can be based on forced circulation, draft tube baffles, fluidized bed designs, or a combination thereof. The crystallizer 204 increases the hydroxide concentration, thereby decreasing the solubility of carbonate in the concentrated carbonate-rich capture solution 218. In some cases, the crystallizer 204 evaporates a portion of the concentrated carbonate-rich capture solution 218 to reach supersaturation. This forms crystalline carbonate hydrate 222, mother liquor 242, and water stream 224. The crystallizer 204 discharges water stream 224 for downstream processing (e.g., in a filtration system, a water treatment system, or a waste system) or for use in another application within or beyond the system 200. The crystalline carbonate hydrate 222 is at least partially separated from the mother liquor 242 to form pure or nearly pure carbonate that can be used in the feed solution for the ED unit. The mother liquor 242 can include the remaining components of the concentrated carbonate solution 218, such as water and hydroxides, after the crystalline carbonate hydrate 222 is separated. The crystalline carbonate hydrate 222 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 222 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). The potassium carbonate sesquihydrate crystals can be at least partially isolated from the mother liquor 242, which can include a KOH-K2CO3 mixture. In another example, the crystalline carbonate hydrate 222 can include sodium carbonate decahydrate (Na2CO3·10H2O) and the mother liquor 242 can include NaOH. -In another embodiment, the crystalline carbonate hydrate 222 can include potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 222 can include different stoichiometric amounts of water molecules per carbonate in the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value). After separation from the mother liquor 242, the crystalline carbonate hydrate 222 is sent to the regeneration subsystem 264 and the mother liquor 242 is returned to the CO2 capture subsystem 202.

[0113] The regeneration subsystem 264 includes a dissolution tank 206 fluidly connected to a BPMED 208. The BPMED 208 is an example of an ED unit that uses a BPM and a CEM, although in some cases the regeneration subsystem 264 can include a different ED unit (e.g., an ED unit that includes one or more CEMs, AEMs, BPMs, or combinations thereof). In some cases, the dissolution tank 206 can operate at pressures of up to 40 bar. In summary, the process streams flowing into and out of the dissolution tank 206 and the BPMED 208 contain proton shuttle species (e.g., sulfate, SO4 2- , bisulfate HSO4 - ) forms a brine loop in which it is protonated in the BPMED, and the protons are shuttled to the dissolution tank 206 via a proton shuttle species. In some implementations, the proton shuttle species is Cl - , I - , Br - , HPO4 -2 , and H2PO4 -1 , acetate, and citrate. In the dissolution tank 206, the proton shuttle species protonates the DIC species to form carbonate, H2CO3. Thus, through the brine loop, the BPMED 208 indirectly protonates the DIC species.

[0114] The dissolving tank 206 can receive a water stream 228, crystalline carbonate hydrates 222 from the crystallizer 204, and a brine stream 238 from the BPMED 208. In some cases, a purified aqueous solution can be used in place of or in addition to the water stream 228. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. The crystalline carbonate hydrates 222 dissolve in water and react with proton shuttle species in the brine stream 238 received from the BPMED 208. For example, the crystalline carbonate hydrates 222 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O), which dissolves in the water of the dissolving tank 206 to provide potassium carbonate, K2CO3. The brine stream 238 can include a bisulfate-rich solution. For example, the brine stream 238 can include potassium bisulfate, KHSO4, as a proton shuttle species. The bisulfate-rich solution can react with carbonate in the dissolution tank 206 to produce a bisulfate-lean solution and carbonic acid. For example, potassium bisulfate KHSO4 can react with potassium carbonate K2CO3 to produce potassium sulfate K2SO4 and carbonic acid H2CO3. As a result, the pH will decrease. The carbonic acid will have sufficient equilibrium CO2 partial pressure (e.g., less than 1 bar) to dissociate into water and gaseous CO2. The dissolution tank 206 can partially or completely release the CO2 stream 236. The CO2 stream 236 can be sent to one or more downstream processing units described in subsequent implementations (e.g., compression unit, purification unit, electrolytic reduction subsystem, carbon product production system, syngas generation reactor). The reaction in the dissolution tank 206 also forms an ED feed solution 226. The ED feed solution 226 can include a bisulfate-lean solution. For example, the ED feed solution 226 can include potassium sulfate along with a mixture of other components such as potassium bisulfate and water. The dissolution tank 206 is configured to flow the ED feed solution 226 to the BPMED 208 .

[0115] The BPMED 208 can include a stack of cells positioned between two electrodes. In some cases, the electrodes of the BPMED 208 can be coupled to an intermittent low carbon intensity power source (e.g., solar, wind, geothermal) or a low carbon intensity power source (e.g., hydro, nuclear, renewable natural gas). The cells can be arranged in a configuration that includes alternating BPMs and CEMs. The electrodes are operable to apply an electrical potential to enable salt decomposition and acid-base recovery. The BPMED 208 can include multiple feed discharge and alkali regeneration compartments defined by the BPMs and CEMs.

[0116] The BPMED 208 is configured to receive the ED feed solution 226 and a water stream 234 at one or more feed discharge compartments. The BPM of the BPMED 208 is capable of a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 208 is capable of a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations to the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form an ED product stream 232 having a hydroxide concentration between 0.5M and 10M.

[0117] For example, in a potassium-based system, the ED feed solution 226 may include a solution rich in potassium sulfate. The CEM of the BPMED 208 may be a solution rich in potassium ion, K + Select and transport it to the alkaline regeneration compartment to produce K + OH - In the feed discharge section, the proton shuttle species can be protonated and combined with the cations to form the brine stream 238. For example, sulfate ion SO 2- is protonated to form potassium ion, K + In combination with the above, potassium bisulfate, KHSO4, can be formed in the brine stream 238.

[0118] The dissolution tank 206 is configured to receive the brine stream 238 from the BPMED 208 and completes the brine loop of the regeneration subsystem 264. In some implementations, the brine stream 238 can include between about 1 M and about 2.5 M sulfate and bisulfate.

[0119] For example, in a potassium-based system, the brine stream 238 may include a potassium sulfate, K2SO4, and potassium bisulfate, KHSO4, concentration of about 1 M. In another example, the brine stream 238 may include sodium sulfate, Na2SO4, and sodium bisulfate, NaHSO4, concentration of about 2.5 M. In some cases, the brine stream 238 may include a combination of K2SO4 / KHSO4 and NaSO4 / NaHSO4 at a total concentration of 2.5 M or less. In some cases, the brine stream 238 may include a KNaSO4 concentration of 1 M. In some cases, the brine stream 238 and the ED feed solution 226 may include other sulfate or bisulfate concentrations (or both) depending on the operating temperature of the BPMED 208 and the conversion of sulfate to bisulfate in the BPMED 208. The respective sulfate and bisulfate concentrations of the ED feed solution 226 and the brine stream 238 will depend on the least soluble species for any given operating temperature.

[0120] In some implementations, the BPMED 208 can include a membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, the BPMED 208 can include a membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. 2 to 2000mA / cm 2 A current density between can be applied to the BPMED 208.

[0121] The ED product stream 232 generated in the BPMED 208 can include an aqueous mixture having hydroxide as the predominant chemical species. For example, the ED product stream 232 can include an aqueous solution of potassium hydroxide, KOH. The ED product stream 232 can be returned from the BPMED 208 to the CO2 capture subsystem 202 as a CO2 capture solution 244. In some implementations, the CO2 capture solution 244 can include a hydroxide concentration between 0.5M and 10M. In some implementations, the regeneration subsystem 264 can optionally include an auxiliary caustic evaporator 214. The auxiliary caustic evaporator 214 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The auxiliary caustic evaporator 214 concentrates the ED product stream 232 by removing water to form the CO2 capture solution 244 and releases a water stream 240. For example, up to 20 m per t-CO2 delivered. 3 The water can be removed by the auxiliary caustic evaporator 214. In such an implementation, the CO2 capture solution 244 includes a carbonate-lean mixture and has a higher hydroxide concentration than the ED product stream 232. Collectively, the auxiliary caustic evaporator 214, the CO2 capture subsystem 202, the carbonate separation subsystem 262, the dissolution tank 206, and the process streams flowing into and out of the BPMED 208 form a caustic loop in which the capture solution is regenerated.

[0122] FIG. 3 is a block flow diagram illustrating an example electrochemical system 300 for regenerating a capture solution and recovering CO2 by using a nanofiltration unit 350 and direct protonation. In some cases, the electrochemical system 300 may be advantageous over the electrochemical system 100 of FIG. 1 and the electrochemical system 200 of FIG. 2 because, unlike a caustic evaporator, nanofiltration can selectively produce a specific concentration of carbonate without requiring water removal to achieve carbonate saturation. The electrochemical system 300 includes a CO2 capture system 302 fluidly coupled to a carbonate separation subsystem 362 and a regeneration subsystem 364. The carbonate separation subsystem 362 includes a nanofiltration unit 350 fluidly coupled to a crystallizer 304. In some implementations, the crystallizer 304 includes an evaporative crystallizer, a cooling crystallizer (e.g., vacuum or surface cooling), a membrane distillation crystallizer, or a combination thereof. The crystallizer can be based on forced circulation, draft tube baffle, or fluidized bed designs, or a combination thereof.

[0123] In some implementations, the crystallizer 304 can be fluidly connected to a solids separator, such as a centrifuge, pressure or vacuum filter, scraper, cyclone, and the like. The carbonate separation subsystem 362 exploits the solubility difference between carbonates and hydroxide salts in the capture solution to enable efficient separation of carbonates. The nanofiltration unit 350 can receive the carbonate-rich capture solution 320 from the CO2 capture subsystem 302. In some implementations, the CO2 capture subsystem 302 can include one or more air contactors 305. The air contactors 305 can include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactors 305 can include single or multi-cell air contactors, dual cell air contactors, dual stream air contactors, or combinations thereof. The air contactor may operate in cross-flow, counter-current, co-current, or combinations thereof. In some implementations, the primary caustic evaporator 312 may include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or combinations thereof.

[0124] The carbonate-rich capture solution 320 can be an aqueous mixture that includes primarily carbonate ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or a combination thereof. The carbonate-rich capture solution 320 can also include minor amounts of other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 320 can include between 0.4M and 6M K2CO3 and between 1M and 10M KOH. In another implementation, the carbonate-rich capture solution 320 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0125] The nanofiltration unit 350 can concentrate carbonate salts through selective rejection, which can reduce the evaporative load on the crystallizer 304 and can reduce feed flow rates and crystallizer sizing. The nanofiltration unit 350 can include one or more filtration membranes that are impermeable to or selective for large divalent ions such as carbonate ions. Nanofiltration membranes can have a unique surface charge, making them particularly suitable for separating ion mixtures. Rejection of chemical species can depend on size, ionic charge, and membrane affinity. The nanofiltration unit 350 can include membranes that have a wide pH tolerance and are durable enough to operate at pHs ranging from 0 to 14 or hydroxide concentrations up to 10 M. In some implementations, the nanofiltration unit 350 can include membranes that can operate at hydroxide concentrations up to 10 M. In some implementations, the nanofiltration unit 350 can include membranes that can stably handle hydroxide concentrations between about 10 M. In some implementations, the nanofiltration unit 350 can reject 85% to 100% of divalent ions (e.g., carbonate ions) to obtain a carbonate-rich retentate 352 and a hydroxide-rich or carbonate-lean permeate 354. In some cases, the nanofiltration unit 350 can reject between 50% and 100% of divalent ions. In some cases, the nanofiltration unit 350 can include a forward osmosis style filtration unit that uses a high ionic strength draw solution and a pressure gradient to obtain a carbonate-rich retentate 352. The high ionic strength draw solution is an electrolyte solution that can reduce the osmotic pressure difference across the membrane, allowing water to flow more easily from the feed solution to the draw solution. The nanofiltration unit 350 can include a plate and frame module that holds several nanofiltration membranes (e.g., flat membrane sheets) clamped together with spacers and supports.

[0126] The nanofiltration unit 350 can receive the carbonate-rich capture solution 320 as a feed solution and the mother liquor 342 as a draw-in solution. The filtration membrane of the nanofiltration unit 350 can select and reject carbonate ions, thereby producing a retentate 352 containing mainly concentrated carbonates and a permeate 354 (draw-out solution) containing mainly hydroxides. For example, the nanofiltration unit 350 can receive a K2CO3-rich solution as a feed and a mother liquor (which can include KOH, water, and small amounts of K2CO3) as a draw-in solution. The nanofiltration unit 350 can then produce a concentrated K2CO3-rich solution as the retentate 352 and a KOH-rich solution as the permeate 354. The retentate 352 can include a higher carbonate concentration than the carbonate-rich capture solution 320. In some implementations, the retentate 352 can include between about 0.5M and 6M K2CO3. In some implementations, the nanofiltration unit 350 can receive a Na2CO3-rich capture solution as a feed and can produce a concentrated Na2CO3-rich solution as retentate 352 and a NaOH-rich solution as permeate 354. In some implementations, the nanofiltration unit 350 can receive a mixed K2CO3 / Na2CO3-rich capture solution as a feed and can produce a concentrated and mixed K2CO3 / Na2CO3-rich solution as retentate 352 and a mixed KOH / NaOH-rich solution as permeate 354.

[0127] In some implementations, the nanofiltration unit 350 can include a feed tank configured to receive the carbonate-rich capture solution 320 and a reject collection tank configured to receive the retentate 352. The nanofiltration unit 350 thus increases the concentration of each of the carbonates such that the carbonates in the retentate 352 are less soluble, reducing the crystallizer load on the crystallizer 304 (evaporative heating or cooling refrigeration).

[0128] The crystallizer 304 receives the retentate 352 from the nanofiltration unit 350. In some implementations, the crystallizer 304 includes an evaporative crystallizer, a freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooling), a film distillation crystallizer, or a combination thereof. The crystallizer 304 can be based on forced circulation, draft tube baffles, fluidized bed designs, or a combination thereof. The crystallizer 304 increases the hydroxide concentration, thereby decreasing the solubility of carbonate in the retentate 352. The solubility of carbonate in the retentate 352 is determined by the composition of the retentate 352 and its position on the saturation curve. In some cases, the crystallizer 304 receives the retentate 352 from the nanofiltration unit 350 and evaporates a portion of the retentate 352 to reach supersaturation. This forms the crystalline carbonate hydrate 322, the mother liquor 342, and the water stream 324. The crystallizer 304 discharges a water stream 324 for downstream processing (e.g., a filtration system, a water treatment system, or a waste system) or for use in another application within or beyond the system 300. The crystalline carbonate hydrate 322 is at least partially separated from the mother liquor 342 to form a pure or nearly pure carbonate that can be used in the feed solution for the ED unit. The mother liquor 342 can include water, hydroxide, and small amounts of carbonate. The crystalline carbonate hydrate 322 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 322 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). The potassium carbonate sesquihydrate crystals can be formed by mixing KOH, HCl, or HCl. - In another example, the crystalline carbonate hydrate 322 can include sodium carbonate decahydrate (Na2CO3·10H2O) and the mother liquor 342 can include NaOH. -The crystalline carbonate hydrate 322 may include a mixture of potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In another example, the crystalline carbonate hydrate 322 may include potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 322 may include different stoichiometric amounts of water molecules per unit carbonate in the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value). After separation from the mother liquor 342, the crystalline carbonate hydrate 322 is sent to the regeneration subsystem 364 and the mother liquor 342 is returned to the nanofiltration unit 350 as a draw-in solution.

[0129] The regeneration subsystem 364 includes a dissolution tank 306 fluidly connected to a BPMED 308 and a flash tank 310. The BPMED 308 is an example of an ED unit that uses a BPM and a CEM, but in some cases the regeneration subsystem 364 can include different ED units (e.g., ED units that include one or more CEMs, AEMs, BPMs, or combinations thereof). Collectively, the process streams flowing into and out of the dissolution tank 306, the BPMED 308, and the flash tank 310 form a brine loop in which DIC is protonated and CO2 is released. The dissolution tank 306 can receive a water stream 328 and crystalline carbonate hydrates 322 from the crystallizer 304. In some cases, a purified aqueous solution can be used instead of or in addition to the water stream 328. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. The crystalline carbonate hydrate 322 dissolves in water and forms bicarbonate HCO3 in the brine stream 338. - is combined with ED to form ED Feed Solution 326. ED Feed Solution 326 is composed of bicarbonate HCO3 -The BPMED 308 may include a stack of cells positioned between two electrodes. In some cases, the electrodes of the BPMED 308 may be coupled to an intermittent low carbon intensity power source (e.g., solar, wind, geothermal) or a low carbon intensity power source (e.g., hydro, nuclear, renewable natural gas). Each cell may be arranged in a configuration that includes alternating BPMs and CEMs. The electrodes are operable to apply an electrical potential to enable salt decomposition and acid-base recovery. The BPMED 308 may include multiple feed-discharge and alkaline regeneration compartments defined by the BPMs and CEMs.

[0130] The BPMED 308 is configured to receive the ED feed solution 326 and a water stream 334 in one or more feed-discharge compartments. The BPM of the BPMED 308 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 308 enables a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations into the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form a first ED product stream 332 having a hydroxide concentration between 0.5M and 10M.

[0131] For example, the ED feed solution 326 may include a solution rich in potassium bicarbonate, KHCO3. The CEM of the BPMED 308 is rich in potassium ions, K + and transports them to the alkaline regeneration compartment, where K + OH - In the feed-discharge section, bicarbonate HCO3 is combined with KOH to form KOH in the first ED product stream 332. - The ions are directly protonated in the BPMED 308 to form a second ED product stream 330 containing carbonic acid, HCO. In some cases, the carbonic acid dissociates into CO and water, and the CO may be partially degassed in the BPMED 308.

[0132] In some implementations, the reduction in pH can dissociate carbonic acid to release CO2 within the cells of the BPMED 308. In some implementations, the BPMED 308 can include an inter-membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, the current can be 50 mA / cm 2 to 2000mA / cm 2 A current density between 0.01 and 0.1 can be applied to the BPMED 308.

[0133] The second ED product stream 330 generated by the BPMED 308 is composed of carbonate HCO and bicarbonate HCO - For example, the second ED product stream 330 can include an aqueous mixture of carbonate, H2CO3, and potassium bicarbonate, KHCO3. The carbonate, H2CO3, dissociates into CO2 and water. The second ED product stream 330 can be sent to a flash tank 310, where a CO2 stream 336 is partially or completely released from the flash tank 310 and sent to one or more downstream processing units described in subsequent implementations (e.g., a compression unit, a purification unit, an electrolytic reduction subsystem, a carbon product production system, a syngas generation reactor). The bicarbonate, HCO3 - A brine stream 338 comprising an aqueous mixture of may be sent to the dissolving tank 306 completing the brine loop of the regeneration subsystem 364.

[0134] The first ED product stream 332 may include an aqueous mixture with hydroxide as the predominant chemical species. For example, the first ED product stream 332 may include an aqueous solution of potassium hydroxide, KOH. The first ED product stream 332 may be returned from the BPMED 308 to the CO2 capture subsystem 302 as a CO2 capture solution 344. In some implementations, the CO2 capture solution 344 may include a hydroxide concentration between 0.5M and 10M. In some implementations, the regeneration subsystem 364 may optionally include an auxiliary caustic evaporator 314. The auxiliary caustic evaporator 314 may include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The auxiliary caustic evaporator 314 concentrates the first ED product stream 332 by removing water to form the CO2 capture solution 344 and releasing a water stream 340. In such an implementation, the CO2 capture solution 344 comprises a carbonate-lean mixture and has a higher hydroxide concentration than the first ED product stream 332. Collectively, the auxiliary caustic evaporator 314, the CO2 capture subsystem 302, the carbonate separation subsystem 362, the dissolution tank 306, and the process streams flowing into and out of the BPMED 308 form a caustic loop in which the capture solution is regenerated.

[0135] In each element of the electrochemical system 100 of FIG. 1 or the electrochemical system 300 of FIG. 3, one or more process streams include CO2, HCO3, - , and CO3 2- The process stream may contain a mixture of dissolved CO2, dissolved H2CO3, and HCO3 in various concentrations based on the partially and completely completed reactions and based on the process conditions. - , and CO3 2- The ratio of concentrations of HCO3 may depend on pH. For example, as the pH decreases from 10 to 7.5, - Concentration and CO2 concentration are CO3 2-The concentration can increase while decreasing. In some cases, significant amounts of bicarbonate are present in each process stream included in the brine loop. The BPMED 108 of the electrochemical system 100 of FIG. 1 or the BPMED 308 of the electrochemical system 300 of FIG. 3 can operate in a pH range of about 1 to 14. For example, the BPMED 108 and the BPMED 308 can operate in a pH range of about 7 to 12.

[0136] FIG. 4 is a block flow diagram showing an example electrochemical system 400 for regenerating the capture solution and recovering CO2 by using a nanofiltration unit 450 and indirect protonation by a proton shuttle species. In some cases, the electrochemical system 400 can be advantageous over the electrochemical system 100 of FIG. 1 and the electrochemical system 300 of FIG. 3 because the DIC species are protonated outside the ED unit, avoiding CO2 degassing in the cells of the ED unit. CO2 degassing in the ED unit can be undesirable because it can increase electrical resistance and reduce the active area of ​​the cells. Thus, a brine loop including an indirect protonation system can be more efficient than a brine loop including a direct protonation system. Unlike a caustic evaporator, the nanofiltration unit 450 can selectively produce a specific concentration of carbonate without requiring water removal to achieve carbonate saturation.

[0137] The electrochemical system 400 includes a CO2 capture subsystem 402 fluidly connected to a carbonate separation subsystem 462 and a regeneration subsystem 464. The carbonate separation subsystem 462 includes a nanofiltration unit 450 fluidly connected to a crystallizer 404. In some implementations, the crystallizer 404 can be fluidly connected to a solids separator, such as a centrifuge, pressure or vacuum filter, scraper, cyclone, and the like. The carbonate separation subsystem 462 exploits the solubility difference between carbonates and hydroxide salts in the capture solution to enable efficient separation of the carbonates. The nanofiltration unit 450 can receive the carbonate-rich capture solution 420 from the CO2 capture subsystem 402. In some implementations, the CO2 capture subsystem 402 can include one or more air contactors 405. The air contactor 405 may include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactor 405 may include a single cell air contactor, a dual cell air contactor, a multi-cell air contactor, a dual flow contactor, or combinations thereof. The air contactor may operate in cross-flow, counter-current, co-current, or combinations thereof.

[0138] The carbonate-rich capture solution 420 can be an aqueous mixture that includes primarily carbonate ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or a combination thereof. The carbonate-rich capture solution 420 can also include minor amounts of other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 420 can include between 0.4M and 6M K2CO3 and between 1M and 10M KOH. In another implementation, the carbonate-rich capture solution 420 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0139] Nanofiltration unit 450 can concentrate carbonates through selective rejection, reduce crystallizer load on crystallizer 404 (evaporative heating or cooling refrigeration), and reduce feed flow rate and crystallizer sizing. Nanofiltration unit 450 can include one or more filtration membranes that are impermeable to or selective for large divalent ions such as carbonate ions. Nanofiltration unit 450 can receive carbonate-rich capture solution 420 as a feed solution and mother liquor 442 as a draw-in solution. The filtration membrane of nanofiltration unit 450 can selectively reject carbonate ions, thereby producing retentate 452 containing mainly concentrated carbonates and permeate 454 (draw-out solution) containing mainly hydroxides. For example, nanofiltration unit 450 can receive K2CO3-rich solution as a feed and mother liquor 442 (which can include KOH, water, and small amounts of K2CO3) as a draw-in solution. The nanofiltration unit 450 can then produce a concentrated K2CO3-rich solution as retentate 452 and a KOH-rich solution as permeate 454. The retentate 452 can include a higher carbonate concentration than the carbonate-rich capture solution 420. In some implementations, the retentate 452 can include between about 0.5M and 6M K2CO3. In some implementations, the nanofiltration unit 450 can receive the Na2CO3-rich capture solution as a feed and produce a concentrated Na2CO3-rich solution as retentate 452 and a NaOH-rich solution as permeate 454. In some implementations, the nanofiltration unit 450 can receive the concentrated mixed K2CO3 / Na2CO3-rich capture solution as a feed and produce a mixed K2CO3 / Na2CO3-rich solution as retentate 452 and a mixed KOH / NaOH-rich solution as permeate 454.

[0140] The nanofiltration unit 450 can include a membrane that has a wide pH tolerance and is durable enough to operate at a pH ranging from 0 to 14. In some implementations, the nanofiltration unit 450 can include a membrane that can operate at a pH range of 3 to 14. In some implementations, the nanofiltration unit 450 can include a membrane that can stably handle hydroxide concentrations up to 10M. In some implementations, the nanofiltration unit 450 can reject 85% to 100% of divalent ions (e.g., carbonate ions) to result in a carbonate-rich retentate 452 and a hydroxide-rich or carbonate-lean permeate 454. In some cases, the nanofiltration unit 450 can reject between 50% and 100% of divalent ions. In some cases, the nanofiltration unit 450 can include a forward osmosis style filtration unit that uses a draw solution and a pressure gradient to result in a carbonate-rich retentate. The nanofiltration unit 450 can include a plate and frame module that holds several nanofiltration membranes (eg, flat membrane sheets) clamped together with spacers and supports.

[0141] In some implementations, the nanofiltration unit 450 can include a feed tank configured to receive the carbonate-rich capture solution 420 and a reject collection tank configured to receive the retentate 452. The nanofiltration unit 450 thus increases the concentration of each of the carbonates such that the carbonates in the retentate 452 are less soluble, thus reducing the crystallizer load (evaporative heating or cooling refrigeration) on the crystallizer 404.

[0142] The crystallizer 404 receives the retentate 452 from the nanofiltration unit 450. In some implementations, the crystallizer 404 includes an evaporative crystallizer, a freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooling), a film distillation crystallizer, or a combination thereof. The crystallizer 404 can be based on forced circulation, draft tube baffles, a fluidized bed design, or a combination thereof. The crystallizer 404 increases the hydroxide concentration, thereby decreasing the solubility of carbonate in the retentate 452. The solubility of carbonate in the retentate 452 is determined by the composition of the retentate 452 and its position on the saturation curve. In some cases, the crystallizer 404 evaporates a portion of the retentate 452 to reach supersaturation. This forms the crystalline carbonate hydrate 422, the mother liquor 442, and the water stream 424. The crystallizer 404 discharges a water stream 424 for downstream processing (e.g., in a filtration system, a water treatment system, or a waste system) or for use in another application within or beyond the system 400. The crystalline carbonate hydrate 422 is at least partially separated from the mother liquor 442 to form a pure or nearly pure carbonate that can be used in a feed solution for the ED unit. The mother liquor 442 can include the remaining components of the concentrated carbonate solution 418, such as water and hydroxides, after the crystalline carbonate hydrate 422 is separated. The crystalline carbonate hydrate 422 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 422 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). The potassium carbonate sesquihydrate crystals can be formed by dissolving the crystalline carbonate sesquihydrate in water at 1000° C. for 10 minutes at 20° C. for 10 minutes at 20° C. for 10 minutes at 20° C. - In another embodiment, the crystalline carbonate hydrate 422 can include sodium carbonate decahydrate (Na2CO3·10H2O), and the mother liquor 442 can include NaOH. -In another embodiment, the crystalline carbonate hydrate 422 can include potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 422 can include different stoichiometric amounts of water molecules per unit carbonate in the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value). After separation from the mother liquor 442, the crystalline carbonate hydrate 422 is sent to the regeneration subsystem 464 and the mother liquor 442 is returned to the nanofiltration unit 450 as a draw-in solution.

[0143] The regeneration subsystem 464 includes a dissolution tank 406 fluidly connected to a BPMED 408. The BPMED 408 is an example of an ED unit that uses a BPM and a CEM, although in some cases the regeneration subsystem 464 can include a different ED unit (e.g., an ED unit that includes one or more CEMs, AEMs, BPMs, or combinations thereof). In some cases, the dissolution tank 406 can operate at pressures up to 40 bar. In summary, the process streams flowing into and out of the dissolution tank 406 and the BPMED 408 contain proton shuttle species (e.g., sulfate, SO4 2- , bisulfate HSO4 - ) is protonated in the BPMED and the protons are shuttled to the dissolution tank 406 via the proton shuttle species, forming a brine loop. In some implementations, the proton shuttle species is Cl - , I - , Br - , HPO4 -2 , and H2PO4 -1 , acetate, and citrate. In the dissolution tank 406, the proton shuttle species protonates with the DIC species to form carbonate, H2CO3. Thus, in the brine loop, the BPMED 408 indirectly protonates the DIC species.

[0144] The dissolving tank 406 can receive the water stream 428, the crystalline carbonate hydrate 422 from the crystallizer 404, and the brine stream 438 from the BPMED 408. In some cases, a purified aqueous solution can be used instead of or in addition to the water stream 428. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. The crystalline carbonate hydrate 422 dissolves in water and reacts with the proton shuttle species in the brine stream 438. For example, the crystalline carbonate hydrate 422 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O) that dissolves in the water of the dissolving tank 406 to provide potassium carbonate, K2CO3. The brine stream 438 can include a bisulfate-rich solution. For example, the brine stream 438 can include potassium bisulfate, KHSO4, as a proton shuttle species. In some implementations, the brine stream 438 can include between about 1 M and about 2.5 M sulfate. For example, the brine stream 438 may include a potassium sulfate K2SO4 concentration of about 1 M. In another example, the brine stream 438 may include a sodium sulfate Na2SO4 concentration of about 2.5 M. In some cases, the brine stream 438 and the ED feed solution 426 may include other sulfate or bisulfate concentrations (or both) depending on the operating temperature of the BPMED 408. In the dissolution tank 406, the bisulfate-rich solution may react with carbonate to produce a bisulfate-lean solution and carbonic acid. For example, potassium bisulfate KHSO4 may react with potassium carbonate K2SO3 to produce potassium sulfate K2SO4 and carbonic acid H2CO3. As a result, the pH will decrease. The carbonic acid will have sufficient equilibrium CO2 partial pressure (e.g., less than 1 bar) to cause dissociation into water and gaseous CO2. The dissolution tank 406 may partially or completely release the gaseous CO2 stream 436. The gaseous CO2 stream 436 can be sent to one or more downstream processing units described in subsequent implementations (e.g., a compression unit, a purification unit, an electrolytic reduction subsystem, a carbon product production system, a syngas generation reactor).

[0145] The reaction in the dissolution tank 406 forms an ED feed solution 426. The ED feed solution 426 can include a dilute solution of bisulfate. For example, the ED feed solution 426 can include potassium sulfate along with a mixture of other components, such as potassium bisulfate and water. The dissolution tank 406 is configured to flow the ED feed solution 426 to the BPMED 408.

[0146] The BPMED 408 can include a stack of cells positioned between two electrodes. In some cases, the electrodes of the BPMED 408 can be coupled to an intermittent low carbon intensity power source (e.g., solar, wind, geothermal) or a low carbon intensity power source (e.g., hydro, nuclear, renewable natural gas). Each cell can be arranged in a configuration that includes alternating BPMs and CEMs. The electrodes are operable to apply an electrical potential to enable salt decomposition and acid-base recovery. The BPMED 408 can include multiple feed-discharge and alkaline regeneration compartments defined by the BPMs and CEMs.

[0147] The BPMED 408 is configured to receive the ED feed solution 426 and a water stream 434 in one or more feed-discharge compartments. The BPM of the BPMED 408 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 408 enables a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations into the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form an ED product stream 432 having a hydroxide concentration between 0.5M and 10M.

[0148] For example, in a potassium-based system, the ED feed solution 426 may include a solution rich in potassium sulfate. The CEM of the BPMED 408 is the potassium ion, K + and transport them into the alkaline regeneration compartment, where K + OH -In the feed-discharge section, the proton shuttle species can be protonated and combined with the cations to form the brine stream 438. For example, sulfate ion SO 2- is protonated to form potassium ion K + In combination with this, potassium bisulfate KHSO4 can be formed in the brine stream 438.

[0149] The dissolution tank 406 is configured to receive the brine stream 438 from the BPMED 408, completing the brine loop of the regeneration subsystem 464. In some implementations, the brine stream 438 can include between about 1 M and about 2.5 M sulfate and bisulfate.

[0150] For example, in a potassium-based system, the brine stream 438 can include potassium sulfate, K2SO4, and potassium bisulfate concentrations of about 1 M. In another example, the brine stream 438 can include sodium sulfate, Na2SO4, and sodium bisulfate concentrations of about 2.5 M. In some cases, the brine stream 438 can include a combination of K2SO4 / KHSO4 and NaSO4 / NaHSO4 at a total concentration of 2.5 M or less. In some cases, the brine stream 438 can include a KNaSO4 concentration of 1 M.

[0151] In some cases, the brine stream 438 and the ED feed solution 426 may contain other sulfate or bisulfate concentrations (or both) depending on the operating temperature of the BPMED 408 and the conversion of sulfate to bisulfate in the BPMED 408. The respective sulfate and bisulfate concentrations of the ED feed solution 426 and the brine stream 438 will depend on the least soluble species for any given operating temperature.

[0152] In some implementations, the BPMED 408 can include a membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, the BPMED 408 can include a membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. 2 to 2000mA / cm2 A current density between can be applied to the BPMED 408.

[0153] The ED product stream 432 can include an aqueous mixture with hydroxide as the predominant chemical species. For example, the ED product stream 432 can include an aqueous solution of potassium hydroxide, KOH. The ED product stream 432 can be returned from the BPMED 408 to the CO2 capture subsystem 402 as a CO2 capture solution 444. In some implementations, the CO2 capture solution 444 can include a hydroxide concentration between 0.5M and 10M. In some implementations, the regeneration subsystem 464 can optionally include an auxiliary caustic evaporator 414. The auxiliary caustic evaporator 414 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The auxiliary caustic evaporator 414 concentrates the ED product stream 432 by removing water to form the CO2 capture solution 444 and releasing a water stream 440. For example, up to 20 m per t-CO2 delivered. 3 The water can be removed by the auxiliary caustic evaporator 414. In such an implementation, the CO2 capture solution 444 contains a carbonate lean mixture and has a higher hydroxide concentration than the ED product stream 432. Collectively, the auxiliary caustic evaporator 414, the CO2 capture subsystem 402, the carbonate separation subsystem 462, the dissolution tank 406, and the process streams flowing into and out of the BPMED 408 form a caustic loop in which the capture solution is regenerated.

[0154] In each element of the electrochemical system 200 of FIG. 2 or the electrochemical system 400 of FIG. 4, one or more process streams may contain a mixture of DIC or proton shuttle species with varying concentrations based on the partially or fully completed reactions and process conditions. - , and CO3 2- Ratio of HSO4 - and SO4 2- The concentration of HCO3, as well as the total ionic strength in a process stream, can depend on the pH. For example, as the pH decreases from 10 to 7.5, -The CO2 concentration can increase, and so can the CO3 2- The concentration decreases. The BPMED 208 of the electrochemical system 200 of FIG. 2 or the BPMED 408 of the electrochemical system 400 of FIG. 4 can operate in a pH range of about 1 to 14. For example, the BPMED 208 and BPMED 408 can operate with hydrogen ion concentrations ranging between 0.001 M and 2.5 M. For example, as the cations in K2CO3 replace the protons to form KHCO3, the three ions in the K2CO3 molecule (two K + and 1 CO3 2- ) and two ions in the KHCO3 molecule (one K + and 1 HCO3 - ), the total ionic strength will decrease.

[0155] In some implementations, the electrochemical system 200 of FIG. 2 or the electrochemical system 400 of FIG. 4 can include an optional flash tank. The optional flash tank can be fluidly connected to the dissolving tank 206, 406 and to the BPMED 208, 408. The optional flash tank can receive an outlet stream from the dissolving tank 206, 406. The CO2 stream 236, 436 can be degassed from the optional flash tank in addition to or instead of the dissolving tank 206, 406. The ED feed solution 226, 426 can flow from the flash tank to the BPMED 208, 408 as needed.

[0156] Nanofiltration unit 350 of FIG. 3 and nanofiltration unit 450 of FIG. 4 can each be preceded by a primary filtration system (e.g., an ultrafiltration system) configured to remove solids such as silicates, hard water, surfactant additives, or salts that cause salinity problems. In this configuration, nanofiltration units 350 and 450 can enable the use of non-portable water sources such as brackish water bodies. This configuration can protect nanofiltration units 350 and 450 from potentially harmful contaminants and can prevent carryover of chemical species to downstream processes and units.

[0157] 5 is a flow chart illustrating an example method 500 for regenerating a CO2 capture solution and capturing CO2 via an electrochemical system in accordance with at least one exemplary embodiment of the present disclosure. Method 500 includes steps 502 through 516, although in other implementations, certain steps may be omitted and additional steps may be added. Steps 502 through 516 may be performed sequentially as shown, or may be performed in a different order than the method shown.

[0158] At 502, the carbonate-rich capture solution is passed to a carbonate separation subsystem including a crystallizer. The carbonate-rich capture solution including K2CO3, KOH, HO, NaOH, Na2CO3, or a combination thereof can be passed to the carbonate separation subsystem. In some implementations, the carbonate-rich capture solution can be passed to an evaporative crystallizer, a eutectic freeze crystallizer, a cooling crystallizer (e.g., vacuum or surface cooling), a membrane distillation crystallizer, or a combination thereof. In some implementations, the carbonate-rich capture solution can be passed to a primary caustic evaporator (e.g., a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof) prior to the crystallizer.

[0159] At 504, crystalline carbonate hydrate and a mother liquor are formed in the crystallizer. In some implementations, the crystalline carbonate hydrate can include potassium carbonate sesquihydrate (K2CO3·1.5H2O) or anhydrous potassium carbonate formed by increasing the hydroxide concentration of a carbonate-rich capture solution to decrease the solubility of the carbonate. In some implementations, the mother liquor can include potassium hydroxide KOH, water, and a small amount of potassium carbonate K2CO3. In some implementations, the crystalline carbonate hydrate can include sodium carbonate hydrate or anhydrous sodium carbonate. In some implementations, the mother liquor can include sodium hydroxide NaOH.

[0160] At 506, the crystalline carbonate hydrate is dissolved in a dissolving tank. The dissolved carbonate is mixed with bicarbonate HCO3 in the brine stream. -In some implementations, the dissolved carbonate is combined with the brine stream to form an ED feed solution that is a bicarbonate-rich solution.

[0161] At 508, the water stream and the ED feed solution are passed through an ED unit. In some implementations, the water stream is passed through one or more alkaline regeneration compartments of the ED unit. In some implementations, the ED feed solution is passed through one or more feed-discharge compartments of the ED unit.

[0162] At 510, an electrical potential is applied to the ED unit. In some implementations, the electrical potential is applied by an intermittent low carbon intensity power source (e.g., solar, wind, geothermal) or a low carbon intensity power source (e.g., hydro, nuclear, renewable natural gas). In some implementations, the electrical potential is applied by a power source up to 2000 mA / cm 2 can be applied to the ED unit. The BPM of the ED unit can separate water into hydroxyl ions and protons and provide the hydroxyl ions to one or more alkaline regeneration compartments. The CEM of the ED unit can transport cations (e.g., alkali metal ions) of the ED feed solution into one or more alkaline regeneration compartments.

[0163] At 512, an ED product stream is formed via a caustic loop that includes the ED unit. The cations are combined with hydroxyl ions from the alkaline regenerator section to form the ED product stream. In some implementations, the ED product stream may have a hydroxide concentration between 1M and 10M.

[0164] At 514, the CO2 gas stream is released through a brine loop including an ED unit. Protonation of the bicarbonate forms carbonic acid that can dissociate to release the CO2 gas stream. In some implementations, the brine loop includes a direct protonation system. In direct protonation, the DIC species are protonated in the feed-discharge section of the ED unit. In some implementations, the brine loop includes an indirect protonation system. In indirect protonation, the ED unit protonates proton shuttle species (e.g., sulfate, bisulfate) that are transferred to an external flash tank where the DIC species are protonated and release the CO2 gas stream. It can be beneficial to avoid CO2 degassing in the ED unit by pressurizing the ED unit to reduce bubble formation and electrical resistance. Operating the ED unit at pressure can also reduce the cost of downstream CO2 compression. In some implementations, the ED unit can operate at a pressure approximately equal to the CO2 off-gas pressure (e.g., about 40 bar). In some implementations, the ED unit may operate at a pressure approximately equal to the first stage of compression in the downstream compressor (eg, 1 bar or higher).

[0165] At 516, the CO2 capture solution comprising the ED product stream is returned to the CO2 capture subsystem. In some implementations, the ED product stream can be concentrated in an auxiliary caustic evaporator to form a CO2 capture solution, which is returned to the CO2 capture subsystem. In some implementations, the CO2 capture solution can include a hydroxide concentration between 0.5M and 10M.

[0166] Each of the above processes can be modified to accommodate CO2 capture and capture solution regeneration via different chemistries, for example, the NaOH-based reaction can be replaced with a KOH-based reaction as described in method 500.

[0167] In each of the electrochemical systems 100, 200, 300, and 400 of FIG. 1 through FIG. 4, and the method 500 of FIG. 5, it may be beneficial to off-gas the CO2 stream 136, 236, 336, 436 at high pressure to reduce the cost of downstream compression. For example, the CO2 may be released at a pressure equal to or higher than the first stage of compression in the downstream compressor (e.g., 1 bar or higher). In some cases, the CO2 may be released at about 40 bar. In some cases, the BPMEDs 108, 208, 308, 408 may be pressurized to reduce bubble formation and electrical resistance, thereby increasing stacking efficiency. For example, the BPMEDs 108, 208, 308, 408 may be operated at a pressure approximately equal to the CO2 off-gas pressure (e.g., about 40 bar). In some implementations, the BPMED 108, 208, 308, 408 may operate at an operating temperature ranging from 25° C. to 90° C. For example, the BPMED 108, 208, 308, 408 may operate at an operating temperature ranging from 40° C. to 60° C.

[0168] The implementations described herein relate to ED units that include an alternating CEM-BPM arrangement. In other possible configurations of ED units, each of the BPMEDs 108, 208, 308, 408 in Figures 1 to 4 and the ED units described in the method 500 in Figure 5 can include an anion exchange membrane (AEM). For example, the BPMEDs 108, 208, 308, 408 can include an iterative BPM-CEM-AEM arrangement interposed between electrodes. The BPMEDs 108, 208, 308, 408 can include separate alkaline regeneration, supply, and release compartments. In another example, the BPMEDs 108, 208, 308, 408 can include an alternating BPM-AEM arrangement interposed between electrodes. The BPMEDs can include separate supply and release compartments. In such cases, the electrochemical systems 100, 200, 300, 400 may include one or more additional process streams or may exclude one or more of the illustrated process streams.

[0169] In the electrochemical systems 100, 200, 300, 400, 700, 800 of Figures 1-4, 7, 8, 10, and the method 500 of Figure 5, respectively, the CO2 streams 136, 236, 336, 436, 736, 836, 1036, 1136, 1236, 1336 of Figures 1-4, 7, 8, 10-13 can be sent to a downstream processing system. In some implementations, the CO2 can be compressed in a downstream compression unit that can include a single-stage or multi-stage gas compressor (e.g., piston compressor, reciprocating compressor). In some cases, the CO2 can be sent to a cleaning unit (e.g., purification unit) that removes at least a portion of the residual water and / or other impurities. In some cases, the downstream compression unit can include a refrigeration system that liquefies the CO2 at low pressure, allowing it to be pumped by a liquid pump. In some cases, the downstream compression unit can include a CO2 stream 136, 236, 336, 436, 736, 836, 1036 at up to about 40 bar. The compressed CO2 can be sent downhole and sequestered in geological formations, subsurface reservoirs, carbon sinks, and the like. In certain downhole conditions, the CO2 can be mineralized into solid products such as calcium carbonate. In some cases, the compressed CO2 can be used for high oil recovery by injecting into one or more wells to enhance the production of hydrocarbons from the reservoir. In some implementations, the CO2 stream 136, 236, 336, 436, 736, 836, 1036 can be fed to a downstream fuel synthesis system, which can include a syngas generation reactor. The syngas generation reactor can generate a syngas product stream by a reverse water gas shift reaction, a steam methane reforming reaction, a direct methane reforming reaction, or a combination thereof. The downstream fuel synthesis system can also include a Fischer-Tropsch reactor, in which the syngas and hydrogen can react to produce hydrocarbon products, such as fuels. In some implementations, the downstream fuel synthesis system can include electrochemical alternatives to a Fischer-Tropsch reactor, such as an electrolytic reduction unit or a gas diffusion electrode.

[0170] In some implementations, the CO stream 136, 236, 336, 436, 736, 836, 1036 of FIGS. 1-4, 7, 8, 10 can be sent to an electrolyzer cell that performs one or more of the following reactions:

[0171] Reaction 6: CO2+2e - →CO+O 2-

[0172] Reaction 7: H2O+2e - →H2+O 2-

[0173] Reaction 8: O 2- →1 / 2O2+2e -

[0174] The electrolyzer cell can form downstream products such as syngas, pure carbon monoxide, or pure hydrogen from a feed such as CO2 stream 136, 236, 336, 436, 736, 836, 1036, 1136, 1236, 1336, or water (or both) in Figures 1 to 4, 7, 8, 10 to 13. The electrolyzer cell can include a nickel-based catalyst, a silver-based catalyst, or a precious metal-based catalyst. In some implementations, the electrolyzer cell is a solid oxide electrolyzer cell. In some implementations, the electrolyzer cell can provide a ratio of syngas suitable for downstream Fischer-Tropsch reactions to form value-added carbon products (e.g., short chain hydrocarbons, FT liquids, wax, etc.). For example, the electrolyzer cell can provide a syngas ratio of 2.5 or higher.

[0175] Carbon products derived from direct air capture technology are desirable because these products generally have low or zero net emissions on a lifecycle basis. Each of the electrochemical systems 100 to 400 in Figures 1 to 4 can be modified or integrated in a manner to generate value-added carbon products, such as syngas or short chain hydrocarbons, in-situ without the need for gaseous CO2 feedstock for the electrochemical cell. This is desirable because gaseous CO2 can lower the pH of the cell, creating conditions that favor hydrogen formation. Figure 6 shows an example electrochemical system 600 that produces reduction products, including carbon products, from a bicarbonate solution and includes one or more elements of the electrochemical systems 100, 200, 300, 400 according to the implementations described in Figures 1 to 4.

[0176] 6 is a block flow diagram illustrating an example electrochemical system 600 for generating reduction products by using a CO electroreduction unit 610. Electroreduction can also be referred to as electrochemical reduction. The electrochemical system 600 includes a CO capture subsystem 602 fluidly coupled to a product generation subsystem 606 via a carbonate separation subsystem 604.

[0177] The CO2 capture subsystem 602 may be substantially similar to one or more of the CO2 capture subsystems 102, 202, 302, 402 and may include one or more process streams or reactions described in Figures 1 to 4. In some implementations, the CO2 capture subsystem 602 may include one or more air contactors 605 similar to the air contactors 105, 205, 305, 405. The air contactors 605 may include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactors 605 may include single or multiple cell air contactors, dual cell air contactors, dual flow air contactors, or combinations thereof. The air contactors may operate in cross flow, counter flow, parallel flow, or combinations thereof. The CO2 capture subsystem 602 provides a carbonate-rich solution 612 to the carbonate separation subsystem 604.

[0178] The carbonate-rich solution 612 can be an aqueous mixture containing primarily carbonate ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or a combination thereof. The carbonate-rich capture solution 612 can also contain other components in small amounts, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 612 can include between 0.4M and 6M K2CO3 and between 1M and 10M KOH. In another implementation, the carbonate-rich capture solution 612 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0179] The carbonate separation subsystem 604 receives the carbonate-rich capture solution 612. The carbonate separation subsystem 604 can include one or more elements, process streams, and reactions from the carbonate separation subsystems 162, 262, 362, 462 according to the implementations described in Figures 1 to 4. In some implementations, the carbonate separation subsystem 604 can include a caustic evaporator or crystallizer (or both) and one or more process streams flowing into or out of these units according to the implementations described in Figures 1 and 2. In some implementations, the carbonate separation subsystem 604 can include a nanofiltration unit or crystallizer (or both) and one or more process streams flowing into or out of these units according to the implementations described in Figures 3 and 4. The carbonate separation subsystem 604 produces crystalline carbonate hydrate 614.

[0180] The crystalline carbonate hydrate 614 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 122 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). In another example, the crystalline carbonate hydrate 614 can include sodium carbonate decahydrate (Na2CO3·10H2O). In another example, the crystalline carbonate hydrate 614 can include potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 614 can include different stoichiometric amounts of water molecules per unit carbonate of the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value).

[0181] The product generation subsystem 606 receives the crystalline carbonate hydrate 614. The product generation subsystem 606 includes a dissolution tank 608 fluidly connected to a CO2 electrolytic reduction unit 610. The product generation subsystem 606 can include one or more elements, process streams, or reactions of the regeneration subsystems 164 and 364 according to the implementations described in Figures 1 and 3. In some implementations, the product generation subsystem 606 can include a caustic evaporator and one or more of the process streams flowing into or out of this unit according to the implementations described in Figures 1 and 3.

[0182] The dissolution tank 608 can receive the crystalline carbonate hydrates 614 from the carbonate separation subsystem 604, a water stream 620, and a brine stream 622. In some cases, a purified aqueous solution can be used in place of or in addition to the water stream 620. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. The crystalline carbonate hydrates 614 dissolve in the water and are converted into bicarbonate HCO3 in the brine stream 622. - to form cell feed solution 616. Cell feed solution 616 can include a bicarbonate HCO3 rich solution along with a mixture of other components such as carbonate and water.

[0183] The CO2 electro-reduction unit 610 receives the cell feed solution 616 and a water stream 620. The CO2 electro-reduction unit 610 includes one or more BPMs and one or more catalyst layers interposed between two electrodes. The electrodes can include nickel, silver, or non-metallic. In some implementations, the CO2 electro-reduction unit 610 can also include a CEM, an AEM, or a combination thereof. In some implementations, the CO2 electro-reduction unit 610 uses a catalyst including silver, mercury, tin, copper, or a combination thereof to perform any of the subsequent reduction reactions. In some implementations, the CO2 electro-reduction unit can provide a reduction product stream 624 from gaseous CO2, dissolved CO2, or a combination thereof. In some implementations, a current of 50 mA / cm 2 to 2000mA / cm 2A current density between can be applied to the CO2 electrolytic reduction unit 610.

[0184] In some implementations, the cell feed solution 616 can flow to a BPMED, such as BPMED 108, 208, 308, 408, 708, 808, 900 in Figures 1-4 and 7-9, or the ED unit 1000 in Figure 10, before flowing to the CO2 electroreduction unit 610. In such implementations, the BPMED can process or prepare the cell feed solution 616 by shifting the pH to a preferred range that improves the operation of the CO2 electroreduction unit 610 while avoiding complete degassing of CO2 in the BPMED. For example, the BPMED can shift the pH of the cell feed solution 616 to between 8 and 10 by performing one or more of reactions 1 to 3 before flowing the cell feed solution 616 to the CO2 electroreduction unit 610.

[0185] The BPM in the CO2 electroreduction unit 610 provides protons to the cathode, enabling a water splitting reaction. With sufficient flux, the protons can react with bicarbonate in the cell feed solution 616 (e.g., via reactions 1 through 3) to provide dissolved CO2 or gaseous CO2 locally (or both). Additionally, the CO2 electroreduction unit 610 can electrochemically reduce the locally provided CO2 (formed by protonation of DIC species) via one or more of the following reduction reactions:

[0186] Reaction 9: CO2+2H + +2e - →CO+H2O

[0187] Reaction 10: CO2+8H + +8e - →CH4+2H2O

[0188] Reaction 11: 2CO2+12H + +12e - →C2H4+4H2O

[0189] Electrochemical reduction typically occurs at a catalyst surface. In some cases, the CO2 electroreduction unit 610 can result in other short chain hydrocarbons or alcohols. The CO2 electroreduction unit 610 can be advantageous over an electrolyzer because it uses bicarbonate in the cell feed solution 616 as a carbon source for the electroreduction reaction, thus eliminating the need for a gaseous CO2 feed material. This allows the cell to operate at a pH that favors the production of reduced carbon products over the production of hydrogen. The reduction product stream 624 can include CO, H2, syngas, formate, methane, ethylene, ethanol, water, or combinations thereof.

[0190] The CO2 electroreduction unit 610 is configured to combine hydroxyl ions provided by the BPM with cations in the cell feed solution 616 to form a carbonate-lean solution 618. For example, the cell feed solution 616 can include a solution rich in potassium bicarbonate, KHCO3. The CO2 electroreduction unit 610 can combine potassium ions with hydroxyl ions provided by the BPM to form KOH in the carbonate-lean solution 618. The carbonate-lean solution 618 can be sent to the CO2 capture subsystem 602 as a regenerated capture solution.

[0191] FIG. 7 is a block flow diagram illustrating an example electrochemical system 700 for regenerating capture solution and capturing CO2 by using a chiller crystallizer 704 and direct protonation. In some cases, the electrochemical system 700 can be advantageous over the electrochemical systems 100, 200, 300, 400 of FIGS. 1-4 because the chiller crystallizer can be relatively economical and can eliminate the need for an upstream evaporator. The electrochemical system 700 includes a CO2 capture subsystem 702 fluidly coupled to a carbonate separation subsystem 762 and a regeneration subsystem 764. The carbonate separation subsystem 762 includes a chiller crystallizer 704 fluidly coupled to a solids separator 750. Examples of the solids separator 750 can include centrifuges, pressure or vacuum filters, purifiers, scrapers, cyclones, salt baskets, and the like. The chiller crystallizer 704 receives the carbonate-rich capture solution 720 from the CO2 capture subsystem 702. In some implementations, the CO2 capture subsystem 702 can include one or more air contactors 705. The air contactors 705 can include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a specific gas component from a larger gas stream using a liquid sorbent. The air contactors 705 can include single or multiple cell air contactors, dual cell air contactors, dual flow air contactors, or combinations thereof. The air contactors can operate in cross flow, counter flow, parallel flow, or combinations thereof.

[0192] The carbonate-rich capture solution 720 can be an aqueous mixture that includes primarily carbonate ions, alkali metal ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or combinations thereof. The carbonate-rich capture solution 720 can also include minor amounts of other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 720 can include between 0.1 M and 6 M K2CO3 and between 1 M and 10 M KOH. In another implementation, the carbonate-rich capture solution 720 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0193] The chiller crystallizer 704 increases the hydroxide concentration, thereby decreasing the solubility of carbonate in the carbonate-rich capture solution 720. In some cases, the crystallizer 704 concentrates a portion of the carbonate-rich capture solution 720 to reach supersaturation. This forms crystalline carbonate hydrate 722, mother liquor 742, and water stream 724. The chiller crystallizer 704 discharges water stream 724 for downstream processing (e.g., in a filtration system, water treatment system, or waste system) or for use in another application within or beyond the system 700. The crystalline carbonate hydrate 722 is at least partially separated from the mother liquor 742 to form pure or nearly pure carbonate that can be used in the feed solution for the ED unit. The mother liquor 742 can include the remaining components of the carbonate-rich capture solution 720, such as water and hydroxide, after the crystalline carbonate hydrate 722 is separated. The crystalline carbonate hydrate 722 can include carbonate sesquihydrate (M2CO3·1.5H2O) or anhydrous carbonate. For example, the crystalline carbonate hydrate 722 can include potassium carbonate sesquihydrate (K2CO3·1.5H2O). Potassium carbonate sesquihydrate crystals can be formed by the addition of KOH - In another example, the crystalline carbonate hydrate 722 can include sodium carbonate decahydrate (Na2CO3·10H2O), and the mother liquor 742 can include a mixture of NaOH, K2CO3, and / or HCl. -The crystalline carbonate hydrate 722 may include a mixture of potassium sodium carbonate hexahydrate (KNaCO3·6H2O). In some implementations, the crystalline carbonate hydrate 722 may include different stoichiometric amounts of water molecules per unit carbonate in the crystalline carbonate (e.g., M2CO3·nH2O, where M is an alkali metal and n is an integer or decimal value). After separation from the mother liquor 742, the crystalline carbonate hydrate 722 may be sent to a solids separator 750 or a regeneration subsystem 764, and the mother liquor 742 is returned to the CO2 capture subsystem 702. The solids separator 750 may further isolate the crystalline carbonate hydrate 722 from the remaining liquid and form a high solids stream 752 that may flow to the dissolution tank 706 and a low solids stream 760 that may be returned to the chiller crystallizer 704. High solids stream 752 contains primarily crystalline carbonate hydrates 722. Low solids stream 760 has a higher liquid to solids ratio than high solids stream 752. High solids stream 752 has a lower liquid to solids ratio than both low solids stream 760 and the inlet stream of solids separator 750.

[0194] In an embodiment, the chiller crystallizer 704 of the electrochemical system 700 includes mechanical refrigeration equipment. Such mechanical refrigeration equipment may include one or more pumps, one or more heat exchangers for circulating a refrigerant, piping, and / or other components. In an alternative embodiment, the chiller crystallizer 704 increases the hydroxide concentration, thereby decreasing the solubility of carbonates in the carbonate-rich capture solution 720 using other cooling means. For example, the chiller crystallizer 704 concentrates a portion of the carbonate-rich capture solution 720 to reach supersaturation using ambient or environmental cooling methods to form crystalline carbonate hydrates 722. In such a configuration, the chiller crystallizer 704 may saturate the carbonate-rich capture solution 720 to form crystalline carbonate hydrates 722 without the use of or with less reliance on mechanical refrigeration means, thus reducing the associated refrigeration energy requirements.

[0195] The regeneration subsystem 764 includes a dissolution tank 706 fluidly connected to a BPMED 708 and a flash tank 710. The BPMED 708 is an example of an ED unit that uses a BPM and a CEM, but in some cases the regeneration subsystem 764 can include different ED units (e.g., ED units that include one or more CEMs, AEMs, BPMs, or combinations thereof). Collectively, the process streams flowing into and out of the dissolution tank 706, the BPMED 708, and the flash tank 710 form a brine loop in which DIC is protonated and CO2 is released. In some implementations, the dissolution tank 706 can receive a water stream 728 and crystalline carbonate hydrates 722 from the crystallizer 104. In some cases, a purified aqueous solution can be used instead of or in addition to the water stream 728. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. In some implementations, the dissolution tank 706 can receive a high solids stream 752 containing crystalline carbonate hydrates 722 from the solids separator 750. The crystalline carbonate hydrates 722 in the high solids stream 752 can be dissolved in water to form bicarbonate HCO3 in the brine stream 735 received from the flash tank 710. - is combined with ED Feed Solution 726. ED Feed Solution 726 contains bicarbonate HCO3 - The ED feed solution 726 may contain a mixture of undesirable divalent and multivalent cations (e.g., Ca, Na, Cl ... 2+ , Mg 2+ The trapped ions can flow through an ion exchanger 754 to reduce or remove at least a portion of the ions (e.g., iron, zinc, etc.). The trapped ions can be removed from the ion exchanger 754 by using a new column and regenerating the used column by replacing the trapped ions with an acid or base. The columns can be configured for smooth operation such that some columns remain in operation while others are regenerated. Regeneration of the used columns can produce an ion exchange regenerated waste stream 770 that includes waste salts.

[0196] The BPMED 708 can include a stack of cells positioned between two electrodes. In some cases, the electrodes of the BPMED 708 can be coupled to an intermittent power source (e.g., solar, wind, geothermal) or a low carbon intensity power source (e.g., hydro, nuclear). Each cell can be arranged in a configuration that includes alternating BPMs and CEMs. The electrodes are operable to apply a potential that enables salt decomposition and acid-base recovery. The BPMED 708 can include multiple feed-discharge compartments and alkaline regeneration compartments defined by the BPMs and CEMs.

[0197] The BPMED 708 is configured to receive the ED feed solution 726 and the water stream 734 in one or more feed-discharge compartments. The BPM of the BPMED 708 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 708 enables a salt splitting reaction that splits salt into its cations and anions. The CEM is operable to transport the cations into the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form a first ED product stream 732 having a hydroxide concentration between 0.5M and 12M. For example, the ED feed solution 726 may include a solution rich in potassium bicarbonate, KHCO3. The CEM of the BPMED 708 may include a solution rich in potassium ions, KHCO3. + and transports them into the alkaline regeneration compartment, where K + OH - and form KOH in the first ED product stream 732. In the feed-discharge section, bicarbonate HCO - The ions are directly protonated in the BPMED 708 to form a second ED product stream 730 containing carbonate, H2CO3. In some cases, the CEM also protonates potassium ions, K + , sodium ion Na + , or a combination thereof.

[0198] In some implementations, the reduction in pH can dissociate carbonic acid and release CO2 into the cells of the BPMED 708. In some implementations, the BPMED 708 can include an inter-membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, the 50 mA / cm 2 to 2000mA / cm 2 A current density between 0.01 and 0.1 can be applied to the BPMED 708.

[0199] The second ED product stream 730 is composed of carbonate HCO and bicarbonate HCO - For example, the second ED product stream 730 can include an aqueous mixture of carbonate H2CO3 and potassium bicarbonate KHCO3. For example, the second ED product stream 730 can include an aqueous mixture of carbonate H2CO3, potassium bicarbonate KHCO3, and sodium bicarbonate NaHCO3. The carbonate H2CO3 dissociates into CO2 and water. The second ED product stream 730 can be sent to a flash tank 710, where a CO2 stream 736 is partially or completely released from the flash tank 710 and sent to one or more downstream processing units described in subsequent implementations (e.g., a compression unit, an electrolytic reduction subsystem, a carbon product production system, a synthesis gas generation reactor). Bicarbonate HCO3 - A brine stream 735 containing an aqueous mixture of the above may be sent to the dissolving tank 706 to complete the brine loop of the regeneration subsystem 764.

[0200] The first ED product stream 732 can include an aqueous mixture with hydroxide as the predominant chemical species. For example, the first ED product stream 732 can include an aqueous solution of potassium hydroxide KOH. The first ED product stream 732 can be returned from the BPMED 108 to the CO2 capture subsystem 702 as a CO2 capture solution 744. In some implementations, the CO2 capture solution 744 can include a hydroxide concentration between 0.5M and 10M. In implementations, a portion of the first ED product stream 732 can be returned to the BPMED 708 as an ED recycle stream 765, and treated water 766 can be added to the ED recycle stream 765. For example, the treated water 766 can include demineralized, upstreamed, filtered, purified, or treated water. The ED recycle stream 765 and treated water 766 can maintain the composition of the solutions in the feed-discharge and alkaline regeneration sections of the BPMED 708.

[0201] In some implementations, the regeneration subsystem 764 can optionally include an auxiliary caustic evaporator 714. The auxiliary caustic evaporator 714 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. The auxiliary caustic evaporator 714 concentrates the first ED product stream 732 by removing water and releasing a water stream 740 to form a CO2 capture solution 744. In such implementations, the CO2 capture solution 744 includes a carbonate-lean mixture and has a higher hydroxide concentration than the first ED product stream 732. Taken together, the process streams flowing into and out of the auxiliary caustic evaporator 714, the CO2 capture subsystem 702, the carbonate separation subsystem 762, the dissolution tank 706, and the BPMED 708 form a caustic loop in which the capture solution is regenerated.

[0202] Although the electrochemical system 700 employs a chiller crystallizer 704 and direct protonation, in some implementations it may be advantageous to employ the chiller crystallizer 704 and indirect protonation as described in the electrochemical systems 200, 400 of FIGS. 2 and 4, where the flash tank 710 is omitted from the electrochemical system 700. In some cases, indirect protonation may be advantageous since the DIC species are protonated outside of the ED unit, avoiding CO2 degassing within the cells of the ED unit. For example, the high solids stream 752 containing crystalline carbonate hydrates 722 may flow to the dissolution tank 706. In summary, the process streams flowing into and out of the dissolution tank 706 and the BPMED 708 are free of proton shuttle species (e.g., sulfate SO4 2- , bisulfate HSO4 - ) is protonated in the ED unit and the protons are shuttled to the dissolution tank 706 via the proton shuttle species, forming a brine loop. In some implementations, the proton shuttle species is Cl - , I - , Br - , HPO4 -2 , and H2PO4 -1 , acetate, and citrate. In the dissolution tank 706, the proton shuttle species protonates the DIC species to form carbonate, H2CO3. The brine loop thus allows the BPMED 708 to indirectly protonate the DIC species.

[0203] In an indirect protonation configuration in which the flash tank 710 is eliminated, the dissolution tank 706 can receive the water stream 728, the high solids stream 752 containing the crystalline carbonate hydrates 722 from the chiller crystallizer 704, and the brine stream 735 from the BPMED 708. In some cases, a purified aqueous solution can be used in place of or in addition to the water stream 728. The purified aqueous solution can be substantially free of particulates and dissolved contaminants. The crystalline carbonate hydrates 722 dissolve in the water and react with the proton shuttle species in the brine stream 735 received in the BPMED 708. The brine stream 735 can include a bisulfate-rich solution. For example, the brine stream 735 can include potassium bisulfate KHSO4 as a proton shuttle species. For example, sulfate ion SO4 2- is protonated to form potassium ion K + and may form potassium bisulfate KHSO4 in the brine stream 735. The bisulfate rich solution may react with carbonate in the dissolution tank 706 to provide a bisulfate lean solution and carbonic acid. For example, potassium bisulfate KHSO4 may react with potassium carbonate K2CO3 to provide potassium sulfate K2SO4 and carbonic acid H2CO3. As a result, the pH will decrease. The carbonic acid will have an equilibrium CO2 partial pressure (e.g., less than 1 bar) sufficient to dissociate into water and gaseous CO2. The dissolution tank 706 may partially or completely release the gaseous CO2 stream 736. The reaction in the dissolution tank 706 also forms an ED feed solution 726. The ED feed solution 726 may include a bisulfate lean solution. For example, the ED feed solution 726 may include potassium sulfate along with a mixture of other components such as potassium bisulfate and water. The dissolution tank 706 is configured to pass the ED feed solution 726 to the BPMED 708.

[0204] In some implementations, the brine stream 735 can include between about 1M and about 2.5M sulfate and bisulfate. For example, the brine stream 735 can include a potassium sulfate K2SO4 concentration and potassium bisulfate of about 1M. In another example, the brine stream 735 can include sodium sulfate Na2SO4 and sodium bisulfate of about 2.5M. In some cases, the brine stream 735 and the ED feed solution 726 can include other sulfate or bisulfate concentrations (or both) depending on the operating temperature of the BPMED 708 and the conversion of sulfate to bisulfate in the BPMED 708. In some cases, the brine stream 735 can include a 1M KNaSO4 concentration.

[0205] FIG. 8 is a block flow diagram illustrating an example electrochemical system 800 for regenerating capture solution and recovering CO2 using a nanofiltration unit 850 and a reverse osmosis unit 868. In some cases, the electrochemical system 800 can be advantageous because the nanofiltration unit 850 and reverse osmosis unit 868 can eliminate the need for solids in the process and reduce the amount of water required to form hydroxides in the BPMED 808. The BPMED 808 is an example of an ED unit that uses a BPM and a CEM, but in some cases the regeneration subsystem 864 can include different ED units (e.g., ED units that include one or more CEMs, AEMs, BPMs, or combinations thereof). The nanofiltration unit 850 can produce a carbonate concentration of up to about 2M. The reverse osmosis unit 868 can withstand operation at high pH (e.g., pH greater than 10) and can easily remove or incorporate excess water to maintain water balance. The electrochemical system 800 includes a CO2 capture subsystem 802 fluidly coupled to a carbonate separation subsystem 862 and a regeneration subsystem 864. The carbonate separation subsystem 862 includes a nanofiltration unit 850 fluidly coupled to a reverse osmosis unit 868 and a BPMED 808. The nanofiltration unit 850 can receive the carbonate-rich capture solution 820 from the CO2 capture subsystem 802. In some implementations, the CO2 capture subsystem 802 can include one or more air contactors 805. The air contactors 305 can include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a particular gas component from a larger gas stream using a liquid sorbent. The air contactors 805 can include single or multiple cell air contactors, dual cell air contactors, dual stream air contactors, or combinations thereof. The air contactor may be operated in cross-flow, counter-current, co-current, or combinations thereof.

[0206] The carbonate-rich capture solution 820 can be an aqueous mixture that includes primarily carbonate ions, alkali metal ions, alkali metal carbonates (e.g., K2CO3, Na2CO3), or combinations thereof. The carbonate-rich capture solution 820 can also include minor amounts of other components, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), water, and impurities. For example, the carbonate-rich capture solution 820 can include between 0.1 M and 6 M K2CO3 and between 1 M and 10 M KOH. In another implementation, the carbonate-rich capture solution 820 can include an aqueous Na2CO3-NaOH mixture. In some implementations, the carbonate-rich capture solution can include a mixture of K2CO3 and Na2CO3.

[0207] The nanofiltration unit 850 can concentrate carbonate salts through selective rejection. The nanofiltration unit 850 can include one or more filtration membranes that are impermeable to or selective for large divalent ions such as carbonate ions. Nanofiltration membranes can have a unique surface charge, which is particularly suitable for separating ion mixtures. The rejection of chemical species can depend on size, ionic charge, and membrane affinity. The nanofiltration unit 850 can include membranes that have a wide pH tolerance and are durable enough to operate at pHs ranging from 0 to 14. In some implementations, the nanofiltration unit 850 can include membranes that can operate in a pH range of 3 to 14. In some implementations, the nanofiltration 850 can include membranes that are stable to handle hydroxide concentrations between about 4% and about 20%. In some implementations, the nanofiltration unit 850 can reject 85% to 100% of divalent ions (e.g., carbonate ions) to obtain a nanofiltration (NF) retentate 852. In some cases, the nanofiltration unit 850 can reject between 50% and 100% of divalent ions. In some cases, the nanofiltration unit 850 can include a forward osmosis style filtration unit that uses a draw solution and a pressure gradient to result in a carbonate-rich nanofiltration retentate 852. The nanofiltration unit 850 can include a plate and frame module that holds several nanofiltration membranes (e.g., flat membrane sheets) clamped together with spacers and supports.

[0208] Nanofiltration unit 850 can receive carbonate-rich capture solution 820 as a feed. The filtration membrane of nanofiltration unit 850 can select and reject carbonate ions, thereby producing nanofiltration retentate 852 including a mixture that is primarily carbonate-rich, and nanofiltration permeate 854 including a mixture that is primarily hydroxide-rich. For example, nanofiltration unit 850 can receive a K2CO3-rich solution as a feed and then produce concentrated K2CO3 as nanofiltration retentate 852 and KOH as nanofiltration permeate 854. Nanofiltration retentate 852 can include a higher carbonate concentration than carbonate-rich capture solution 820. In some implementations, nanofiltration retentate 852 can include between about 0.5M and 6M K2CO3. In some implementations, nanofiltration unit 850 can receive a Na2CO3-rich capture solution as a feed and produce concentrated Na2CO3 as nanofiltration retentate 852 and NaOH as nanofiltration permeate 854. In some implementations, the nanofiltration unit 850 can produce a nanofiltration retentate 852 that includes a mixture of K2CO3 and Na2CO3.

[0209] In some implementations, the nanofiltration retentate 852 is sent to the BPMED 808 as the ED feed solution 826. In some implementations, the nanofiltration retentate 852 is passed through an ion exchanger 860 to remove undesirable divalent and multivalent cations (e.g., Ca) in the ED feed solution 826. 2+ , Mg 2+ , Sr 2+ , B.A. 2+ The trapped ions can be removed from the ion exchanger 860 by using a new column and regenerating the spent column by replacing the trapped ions with an acid or base. Regeneration of the spent column can produce an ion exchange regenerated waste stream 870 that includes waste salts.

[0210] The regeneration subsystem 864 includes the BPMED 308 fluidly connected to a flash tank 810 and a reverse osmosis (RO) unit 868. Collectively, the process streams flowing into and out of the BPMED 808, the flash tank 810, and the RO unit 868 form a brine loop in which DIC is protonated and CO2 is released. The ED feed solution 826 contains bicarbonate, HCO3, - The BPMED 808 may include a mixture of alkaline-rich solutions with a mixture of other components such as carbonate salts and water. Each cell of the BPMED 808 may be arranged in a configuration including alternating BPMs and CEMs. The electrodes are operable to apply a potential that enables salt decomposition and acid-base recovery. The BPMED 808 may include multiple feed-discharge and alkaline regeneration compartments defined by the BPMs and CEMs.

[0211] The BPMED 808 is configured to receive the ED feed solution 826 and the water stream 834 in one or more feed-discharge compartments. The BPM of the BPMED 808 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 808 enables a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations into the alkaline regeneration compartment. In the alkaline regeneration compartment, the cations combine with the hydroxyl ions to form a first ED product stream 832 having a hydroxide concentration between 0.5M and 10M. For example, the ED feed solution 826 can include a solution rich in potassium bicarbonate, KHCO3. The CEM of the BPMED 308 can transport potassium ions, KHCO3, and KHCO3. + and transports them into the alkaline regeneration compartment, where K + OH - and KOH are combined to form the first ED product stream 832. In the feed-discharge section, bicarbonate HCO3 - The ions are directly protonated in the BPMED 808 to form a second ED product stream 830 containing carbonate, HCO. In some cases, the CEM protonates potassium ions, K + , sodium ion Na + , or a combination thereof.

[0212] In some implementations, the reduction in pH can dissociate the carbonic acid and release CO2 into the cells of the BPMED 808. In some implementations, the BPMED 808 can include an inter-membrane distance of less than 1 mm between each BPM and the CEM. For example, the BPM can be positioned 0.7 mm from the CEM. In some implementations, the current can be 50 mA / cm 2 to 2000mA / cm 2 A current density between can be applied to the BPMED 808.

[0213] The second ED product stream 830 generated by the BPMED 808 is a mixture of carbonate HCO and bicarbonate HCO - For example, the second ED product stream 830 can include an aqueous mixture of carbonate, H2CO3, and potassium bicarbonate, KHCO3. The carbonate, H2CO3, dissociates into CO2 and water. The second ED product stream 830 can be sent to the flash tank 810, while the gaseous CO2 stream 836 is partially or completely released from the flash tank 810 and sent to one or more downstream processing units (e.g., compression unit, electrolytic reduction subsystem, carbon product production system, syngas generation reactor) described in subsequent implementations. Bicarbonate, HCO3 - The brine stream 835, which comprises an aqueous mixture of the above, may be sent to a reverse osmosis (RO) unit 868.

[0214] The reverse osmosis unit 868 has a primary function of removing water from the brine loop to maintain water balance. The membrane of the reverse osmosis unit 868 can select and reject carbonate ions, thereby producing an RO retentate 863 that includes a bicarbonate-rich solution and an RO permeate 866 that includes primarily water. The RO retentate 863 can include a higher bicarbonate concentration than the brine stream 835. In some implementations, the RO retentate 863 can include a potassium bicarbonate KHCO3 concentration of 2.4 M or less. In some implementations, the RO retentate 863 can include a sodium bicarbonate NaHCO3 concentration of 2.4 M or less. In some implementations, the RO retentate 863 can include a mixture of potassium bicarbonate KHCO3 and sodium bicarbonate NaHCO3 at a concentration of 2.4 M or less. The RO retentate 863 can then be combined with the NF retentate 852 to form the ED feed stream 826, thereby completing the brine loop of the regeneration subsystem 864. The water in the RO permeate 866 can be integrated into the caustic loop to produce hydroxide in the BPMED 808 .

[0215] The first ED product stream 832 can include an aqueous mixture with hydroxide as the predominant chemical species. For example, the first ED product stream 832 can include an aqueous solution of potassium hydroxide KOH, NaOH, or a combination thereof. The first ED product stream 832 can be returned from the BPMED 808 to the CO2 capture subsystem 802 as a CO2 capture solution 844. In some implementations, the CO2 capture solution 844 can include a hydroxide concentration between 0.5M and 10M. In some implementations, the regeneration subsystem 864 can optionally include an auxiliary caustic evaporator 814 to remove water 840 from the CO2 capture solution 844. The auxiliary caustic evaporator 314 can include a mechanical vapor recompression (MVR) evaporator, a multiple effect evaporator, or a combination thereof. In such implementations, the CO2 capture solution 844 includes a carbonate-lean mixture and has a higher hydroxide concentration than the first ED product stream 832. Collectively, the process streams flowing into and out of the auxiliary caustic evaporator 814, the CO2 capture subsystem 802, the carbonate separation subsystem 862, and the BPMED 808 form a caustic loop in which the capture solution is regenerated.

[0216] In some implementations, the water in the RO permeate 866 can be combined with the first ED product stream 832. A portion of the combined stream can be returned to the BPMED 808 as an ED recycle stream 865, and another portion of the combined stream can be returned to the CO2 capture subsystem 802 as the CO2 capture solution 844. In some implementations, process water 834 can be added to the ED recycle stream 865. For example, the process water 834 can include water that has been demineralized, distilled, filtered, purified, or otherwise treated.

[0217] Although the electrochemical system 800 employs a reverse osmosis unit 868 and direct protonation, in some implementations it may be advantageous to employ a reverse osmosis unit and indirect protonation as described in the electrochemical systems 200, 400 of FIGS. 2 and 4. In some cases, indirect protonation may be advantageous since the DIC species are protonated outside of the ED unit, avoiding CO2 degassing within the cells of the ED unit. For example, carbonate hydrates may flow to the off-gas tank. In summary, the process streams flowing into and out of the off-gas tank and ED unit may be free of proton shuttle species (e.g., sulfate SO4 2- , bisulfate HSO4 - ) is protonated in the ED unit to form a brine loop in which protons are shuttled to the off-gas tank via a proton shuttle species. In some implementations, the proton shuttle species is Cl - , I - , Br - , HPO4 -2 , and H2PO4 -1 , acetate, and citrate. In the off-gas tank, the proton shuttle species protonates the DIC species to form carbonate, H2CO3. Thus, in the brine loop, the ED unit indirectly protonates the DIC species.

[0218] In an indirect protonation configuration, the reverse osmosis unit can receive a brine stream from the ED unit and produce an RO retentate containing primarily bisulfate and an RO permeate containing primarily water. The off-gas tank can receive a carbonate-rich NF retentate from the nanofiltration unit and a bisulfate-rich RO retentate from the RO unit. In the off-gas tank, the carbonate reacts with the proton shuttle species. The RO retentate can include a bisulfate-rich solution. For example, the RO retentate can include potassium bisulfate KHSO4 as a proton shuttle species. For example, sulfate ion SO4 2- is protonated to form potassium ions, K +In combination with potassium bisulfate KHSO4, potassium bisulfate KHSO4 can be formed in the ED unit. Potassium bisulfate KHSO4 can then be selected, thereby producing an RO retentate comprising a bisulfate-rich solution. The bisulfate-rich solution can react with carbonate in the off-gas tank to produce a bisulfate-lean solution and carbonic acid. For example, potassium bisulfate KHSO4 can react with potassium carbonate K2CO3 to produce potassium sulfate K2SO4 and carbonic acid H2CO3. As a result, the pH will decrease. The carbonic acid will have sufficient equilibrium CO2 partial pressure (e.g., less than 1 bar) to dissociate into water and gaseous CO2. The off-gas tank can partially or completely release the gaseous CO2 stream. The reaction in the off-gas tank also forms an ED feed stream. The ED feed solution can include a bisulfate-lean solution. For example, the ED feed solution can include potassium sulfate along with a mixture of other components such as potassium bisulfate and water. The off-gas tank is configured to flow the ED feed solution to the ED unit.

[0219] FIG. 9 is a schematic diagram of an example BPMED 900 having a membrane stack including a CEM 906 alternating with a BPM 908. The BPMED 900 may include more or fewer CEMs 906 and BPMs 908 than shown in FIG. 9. The membrane stack is positioned between a cathode 902 and an anode 904. The membranes define alternating feed-discharge (proton generation) compartments 910 and alkaline regeneration (hydroxide generation) compartments 912. The BPMED 900 can be included in an electrochemical system as an element of a regeneration subsystem. For example, the BPMEDs 108, 208, 308, 408, 708, 808 in each of the regeneration subsystems can include at least some of the same elements as the BPMED 900.

[0220] To regenerate a capture solution, such as a first product stream 930 containing alkali hydroxide MOH, the BPM 908 converts hydroxyl ions OH via dissociation of water. - to the alkaline regeneration section 912, and protons H +to the supply-discharge compartment 910. The generated protons are selectively transported across the CEM 906 to the alkali regeneration compartment 912, where they react with alkali metal ions (e.g., K + , Na + etc.) Cation M + For example, to regenerate potassium hydroxide (KOH) in the capture solution as the first product stream 930, the BPM 908 is replaced by hydroxyl ions OH - CEM 906 provides potassium ion K + is passed inside the alkaline regeneration section 912. The alkali metal ions and hydroxyl ions thus form a first product stream 930 comprising a regenerated CO2 capture solution along with the alkali hydroxide. The first product stream 930 can flow from the BPMED 900 to a downstream CO2 capture subsystem or to a caustic evaporator.

[0221] The supply-discharge compartment 910 contains bicarbonate HCO3 - (for direct protonation) or sulfate SO4 2- The BPM 908 receives an ED feed solution 926, which may contain proton shuttle species such as carbonate, HCO (for indirect protonation), or bisulfate, HSO (for indirect protonation). The BPM 908 provides protons to the feed-discharge compartment 910, and species in the ED feed solution 926 are protonated to form a second product stream 932. The second product stream 932 is carbonate, HCO (for direct protonation), or bisulfate, HSO (for indirect protonation), and the species in the ED feed solution 926 are protonated to form a second product stream 932. - (for indirect protonation). The second product stream 932 can flow from the BPMED 900 downstream to a flash tank or dissolution tank.

[0222] The BPMED 900 offers low voltage drop (e.g., BPM voltage drop less than 2V and CEM voltage drop less than 1V) and high current density (e.g., 50mA / cm 29)。 In some implementations, the BPMED 900 can be selected or designed to have desired characteristics, such as BPM current density above 1000 nm. In some implementations, the BPMED 900 can be made of a membrane stack including BPM alternating with AEM. The BPM 908 can include 3D joints, planar joints, or a combination thereof. The BPM 908 with 3D joints has nanofibers that intertwine to improve catalytic surface area and mechanical strength. The 3D joints can be fabricated by electrospinning. The BPM 908 with planar joints has a catalyst layer sandwiched between a cation exchange layer and an anion exchange layer. In the BPM 908, the cation exchange layer is positioned on the cathode 902 side and the anion exchange layer is positioned on the anode 904 side. Water dissociation occurs at the intervening catalyst layer, and protons are transported through the cation exchange layer while hydroxyl ions are transported through the anion exchange layer. The membrane stack shown in FIG. 9 is for illustrative purposes and can vary.

[0223] FIG. 10 is a schematic diagram of an example ED unit 1000 including a gas diffusion electrode (GDE) 1004. This configuration can be advantageous because it can allow for high current density and hydroxide concentration up to 35% w / w. This can lower capital costs, reduce water usage, and reduce water treatment costs. The membrane stack includes a CEM 1006 positioned between a cathode 1002 and a GDE 1004. The ED unit 1000 of FIG. 10 uses the GDE 1004 as the anode, but in some implementations, the ED unit 1000 can use the GDE 1004 as the cathode. The GDE 1004 includes a gas diffusion layer 1010 that supports a catalyst layer 1012. The gas diffusion layer 1010 is porous and allows gas movement toward the catalyst layer 1012. In some implementations, the catalyst layer 1012 can include platinum or a non-precious metal catalyst (e.g., nickel, nickel iron, cobalt, metal alloys). Because the gas diffusion layer 1010 is hydrophobic, its pores are not significantly blocked by the aqueous electrolyte solution, thus maintaining gas transport to the catalyst layer 1012. In some cases, the ED unit 1000 with the GDE 1004 can operate at a pH ranging from 0 to 14. The cathode 1002 and the CEM 1006 define an alkaline regeneration (hydroxide generation) compartment and regenerate the capture solution. The CEM 1006 and the GDE 1004 define a feed-discharge (proton generation) compartment and form CO2. The ED unit 1000 can be used for direct or indirect protonation of DIC species. The ED unit 1000 can be included as an element of a regeneration subsystem of an electrochemical system, such as any of the electrochemical systems 100-400, 700, 800.

[0224] Direct protonation can be used to regenerate a CO2 capture solution and recover gaseous CO2. To regenerate a CO2 capture solution, such as a first ED product stream 1032 containing an alkali hydroxide MOH, the ED unit 1000 receives a gaseous hydrogen feed stream 1024 and an ED feed solution 1026 containing a carbonate-bicarbonate mixture in the feed-discharge compartment. The ED unit 1000 receives water in the alkaline regeneration compartment. An electrical potential is applied to the ED unit 1000. The hydrogen H2 oxidation reaction occurs in the GDE 1004. The generated protons are selectively transported across the CEM 1006 to the alkaline regeneration compartment by alkali metal ions (e.g., K + , Na + etc.) Cation M + is replaced by.

[0225] At the cathode 1002, a water dissociation reaction occurs to produce hydroxyl ions OH - and hydrogen H2. In the alkali regeneration section, alkali metal ions M + and hydroxyl ion OH - forms a first ED product stream 1032 that includes a regenerated capture solution along with alkali hydroxide MOH and hydrogen H2. For example, potassium ions K + may be transported across the CEM 1006 to form a regenerated capture solution with potassium hydroxide KOH. The first ED product stream 1032 may flow from the ED unit 1000 to a separation unit where hydrogen is separated from the regenerated CO2 capture solution. The CO2 capture solution 1044 may then flow to an air contactor of the CO2 capture subsystem. In some implementations, the separated hydrogen may be recycled to the GDE 1004 of the ED unit 1000 as the hydrogen feed stream 1024.

[0226] In the supply-discharge compartment, protons displacing alkali metal ions acidify the carbonate-bicarbonate mixture, producing gaseous carbon dioxide 1036, water, and bicarbonate MHCO3. -, carbon dioxide H2CO3, or a combination thereof. In some implementations, the gaseous carbon dioxide 1036 can be off-gassed through the GDE 1004. In some cases, the protonation in the feed-discharge section may not react to completion, forming a second ED product stream 1034 that includes bicarbonate MHCO3. - can leave the ED unit 1000 through the GDE 1004. In some cases, the ED unit 1000 can be fluidly connected to a downstream flash tank or off-gas tank for gaseous carbon dioxide to be degassed.

[0227] The ED unit 1000, including the GDE 1004, provides low voltage drop (e.g., CEM voltage drop of less than 1.6 V) and high current density (e.g., 50 mA / cm 2 to 1000mA / cm 2 The current density can be selected or designed to be between 0.01 and 0.1.

[0228] Indirect protonation via a proton shuttle species, such as sulfate, bisulfate, or a combination thereof, can be used to regenerate the CO2 capture solution. In some cases, the proton shuttle species is Cl. - , I - , Br - , HPO4 -2 , and H2PO4 -1 , acetate, citrate, or combinations thereof. To regenerate a CO2 capture solution, such as a first ED product stream comprising alkali hydroxide MOH, the ED unit receives a gaseous hydrogen feed stream and an ED feed solution comprising a sulfate-bisulfate mixture in the feed-discharge compartment. The ED unit receives water in the alkaline regeneration compartment. An electrical potential is applied to the ED unit. A hydrogen H2 oxidation reaction occurs in the GDE. Generated protons are selectively transported across the CEM to the alkaline regeneration compartment by alkali metal ions, such as K + , Na + etc.) Cation M + is replaced with.

[0229] In the indirect protonation configuration, the reactions occurring at the cathode and in the alkaline regeneration compartment are substantially the same as those occurring at the cathode and the alkaline regeneration compartment in the direct protonation configuration. The ED unit forms a first ED product stream comprising an alkali hydroxide MOH and a regenerated capture solution having hydrogen H2. The hydrogen is separated in a separation unit and the regenerated capture solution is sent to the CO2 capture subsystem.

[0230] In the indirect protonation configuration, in the feed-discharge section, protons displacing alkali metal ions acidify the sulfate-bisulfate mixture to form a second ED product stream containing water and bisulfate, MHSO4. In some cases, the second ED product stream can also contain sulfuric acid, H2SO4. The second ED product stream can then be sent to a dissolution tank to react the bisulfate with carbonate to produce carbonic acid, H2CO3. The carbonic acid will have sufficient equilibrium CO2 partial pressure to dissociate into water and gaseous CO2. The dissolution tank can partially or completely discharge the CO2 stream and recover it for use in downstream processes.

[0231] 11 is a block flow diagram illustrating an example electrochemical system 1100 for regenerating the capture solution and capturing CO2 using a filtration unit 1150 and a pH swing of the feed provided to the ED unit 1107. The electrochemical system 1100 includes a CO2 capture system 1102, a carbonate separation subsystem 1162, and an ED subsystem 1164.

[0232] The CO2 capture subsystem 1102 may include one or more air contactors 1105. The air contactors 1105 may include cooling tower style gas-liquid contactors, spray towers, liquid-gas scrubbers, venturi scrubbers, packed towers, and other systems designed to remove at least a portion of a specific gas component from a larger gas stream using liquid sorbents. The air contactors 1105 may include single cell air contactors, dual cell air contactors, dual flow air contactors, or combinations thereof. The air contactors 1105 may be operated in a cross-current, counter-current, co-current, or combinations thereof configuration. The CO2 capture solution 1144 is an aqueous mixture that includes small amounts of one or more of alkali metal carbonates (e.g., K2CO3, Na2CO3), water, facilitators, and other chemical species, such as hydroxide ions, alkali metal hydroxides (e.g., KOH, NaOH), and impurities. In the air contactor 1105, the carbonate and water of the CO2 capture solution 1144 react with CO2 from a dilute gas source (e.g., atmosphere or ambient air) to produce bicarbonate ions (HCO3 - ) can be formed.

[0233] Reaction 9: CO3 - +H2O+CO2→HCO3 -

[0234] The bisulfate ions may be neutralized by the alkali metal hydroxides of the CO2 capture solution 1144 to form metal carbonates / bicarbonates (e.g., K2CO3 / KHCO3, Na2CO3 / NaHCO3). In one possible configuration, the carbonates react with the CO2 from the dilute gas source and the water in the CO2 capture solution 1144 to form bicarbonate ions. In one possible configuration, the water in the CO2 capture solution 1144 can react with the CO2 from the dilute gas source in the air contactor 1105 to form carbonic acid (H2CO3), which reacts with the alkali metal hydroxides of the CO2 capture solution 1144 to form metal carbonates / bicarbonates (e.g., K2CO3 / KHCO3, Na2CO3 / NaHCO3).

[0235] The CO2 capture kinetics of carbonate may be improved by the introduction of additives such as promoter species in the CO2 capture solution 1144. Non-limiting examples of promoters for enhancing CO2 capture by carbonate include carbonic anhydrase, amines (primary, secondary, tertiary), zwitterionic amino acids, and boric acid. The resulting capture solution 1120 produced by the CO2 capture subsystem 1102 includes carbonate and bicarbonate as well as a promoter. An example composition of the capture solution 1120 may include K2CO3 / KHCO3 and a promoter. The capture solution 1120 may have a pH in the range of 11-13, and there may be little residual hydroxide from the CO2 capture solution 1144.

[0236] 11, the carbonate separation subsystem 1162 includes a filtration unit 1150 fluidly coupled to the CO2 capture subsystem 1102 to receive the capture solution 1120 from the air contactor 1105. The filtration unit 1150 is fluidly coupled to the ED subsystem 1164. The filtration unit 1150 may be any device used to separate the capture solution 1120 into a retentate 1152 and a permeate. In the configuration of FIG. 11, the permeate is shown flowing from the filtration unit 1150 to the air contactor 1105 as part of the CO2 capture solution 1144, and thus may be referred to herein as the "permeate 1144." In one possible configuration, the filtration unit 1150 is a nanofiltration unit 1150N. In another possible configuration, the filtration unit 1150 is an ultrafiltration unit 1150U. In yet another possible configuration, the filtration unit 1150 includes a combination of nanofiltration and ultrafiltration. For example, in one such configuration, the filtration unit 1150 may be a nanofiltration unit 1150N preceded or downstream of a primary filtration system (e.g., an ultrafiltration system) configured to remove solids including, but not limited to, silicates, hard water, accelerators, surfactant additives, or salts that pose salinity concerns. This configuration of the filtration unit 1150 may enable the use of non-portable water sources, such as brackish water bodies, with the electrochemical system 1100, such as a source of make-up water for the electrochemical system 1100. This configuration may protect the filtration unit 1150N from potentially harmful contaminants and may prevent carryover of chemical species to downstream processes and units.

[0237] The filtration unit 1150 may include one or more filtration membranes that are impermeable or selective to larger ions such as carbonate and bicarbonate ions. The filtration unit 1150 may receive the capture solution 1120 as a feed solution. The filtration membranes of the filtration unit 1150 may selectively reject salt species including carbonate and bicarbonate ions, thereby producing a retentate 1152 that includes primarily concentrated metal carbonates / bicarbonates (e.g., K2CO3 / KHCO3, Na2CO3 / NaHCO3). The filtration membranes of the filtration unit 1150 may reject larger molecules such as promoter species, thereby producing a permeate 1144 (drawout solution) of the filtration unit 1150. The permeate 1144 includes the promoter used to enhance CO2 capture with the carbonates in the CO2 capture solution 1144. The permeate 1144 including the promoter is returned to the CO2 capture subsystem 1102 for use with the CO2 capture solution 1144.

[0238] In one possible configuration, referring to FIG. 11, filtration unit 1150 receives a K2CO3 / KHCO3 / promoter feed solution. Filtration unit 1150 then produces a concentrated potassium carbonate / bicarbonate (e.g., K2CO3 / KHCO3) solution as retentate 1152 and a promoter-rich solution as permeate 1144. In another possible configuration, filtration unit 1150 receives a Na2CO3 / NaHCO3 / promoter feed solution. Filtration unit 1150 then produces a concentrated sodium carbonate / bicarbonate (e.g., Na2CO3 / NaHCO3) solution as retentate 1152 and a promoter-rich solution as permeate 1144. In another possible configuration, filtration unit 1150 receives a mixed Na2CO3 / NaHCO3 / K2CO3 / KHCO3 / promoter feed solution. The filtration unit 1150 then produces a sodium and potassium carbonate / bicarbonate (e.g., Na2CO3 / NaHCO3 / K2CO3 / KHCO3) solution as retentate 1152 and a promoter-rich solution as permeate 1144.

[0239] The filtration unit 1150 may selectively produce a specific concentration of carbonate / bicarbonate in the retentate 1152 without requiring removal of water by evaporation to achieve carbonate / bicarbonate saturation. The filtration unit 1150 may include a membrane with wide pH tolerance and durable enough to operate at a pH ranging from 0 to 14. In some implementations, the filtration unit 1150 may include a membrane capable of operating at a pH range of 10 to 14. In some implementations, the filtration unit 1150 may reject at least 85% of the larger ions (e.g., carbonate / bicarbonate ions) resulting in a carbonate / bicarbonate rich retentate 1152 and a carbonate / bicarbonate lean permeate 1144. In some cases, the filtration unit 1150 may reject between 50% and 100% of the larger ions. The permeate 1144 and / or the retentate 1152 may include small amounts of hydroxides. The filtration unit 1150 can include a plate and frame module that holds several filtration membranes (e.g., flat membrane sheets) clamped together with spacers and supports. In some implementations, the filtration unit 1150 can include a feed tank configured to receive the capture solution 1120 and a reject collection tank configured to receive the retentate 1152. The filtration unit 1150 can operate to increase the concentration of carbonates and bicarbonates such that the carbonates / bicarbonates in the retentate 1152 are less soluble. By helping to separate at least a portion of the enhancer from being carried to the ED unit 1107, the filtration unit 1150 can act to protect sensitive enhancers (e.g., carbonic anhydrase) so that they can be reused in the CO2 capture subsystem 1102 while suffering less degradation than if they continued through the ED unit 1107. By helping to separate the facilitator from being carried to the ED unit 1107, the filtration unit 1150 may help protect the membrane of the ED unit 1107 which may be sensitive to organic molecules and other ionic species.

[0240] In an alternative embodiment of the electrochemical system 1100, the filtration unit 1150 is not present. In such an embodiment, the promoter species used in the CO capture subsystem 1102 is capable of withstanding the large variations or swings in pH experienced in the ED unit 1107. In such an embodiment, the electrochemical system 1100 does not include the filtration unit 1150, and the electrochemical system 1100 routes the promoter in the capture solution 1120 through both the acid and base compartments of the ED unit 1107 and then routes the promoter-rich stream back to the air contactor 1105.

[0241] The concentrated carbonate / bicarbonate retentate 1152 forms the ED feed solution. The ED unit 1107 is configured to receive the ED feed solution. Referring to FIG. 11, the ED unit 1107 is or includes a BPMED 1108. The BPMED 1108 can include a stack of cells positioned between two electrodes. In some cases, the electrodes of the BPMED 1108 can be coupled to a low carbon intermittent power source (e.g., solar, wind, geothermal) in addition to or in addition to other low carbon intensity sources of baseload electricity (e.g., hydro, nuclear). Each cell can be arranged in a configuration that includes alternating membranes (BPM and CEM). The membranes define alternating feed-discharge (proton generating) and alkali generating (hydroxide generating) compartments. The electrodes are operable to apply an electrical potential to enable salt decomposition and acid-base recovery. The BPMED 1108 can include multiple feed-discharge compartments and alkaline compartments defined by BPMs and CEMs.

[0242] The BPMED 1108 is configured to receive the retentate 1152 and the water stream into one or more of the feed-discharge compartments. The BPM of the BPMED 1108 enables a water splitting reaction that splits water into hydroxyl ions and protons. The BPMED 1108 enables a salt splitting reaction that splits salts into their cations and anions. The CEM is operable to transport the cations into the alkali generation compartment.

[0243] To regenerate the capture solution, BPM converts the hydroxyl ions (OH) - in the alkali generation compartment, and protons H + The generated protons are selectively transported across the CEM to the alkali generation compartment, where they are converted to alkali metal ions (e.g., K + , Na + etc.) Cation M + For example, to provide potassium hydroxide (KOH) as the first product stream of BPMED 1108, BPM converts the hydroxyl ion OH - CEM provides potassium ions, K + to the alkali generation section. The alkali metal ions and hydroxyl ions thus form a first ED product stream 1132 that includes alkali hydroxide. The first ED product stream 1132 with hydroxide is returned to the CO2 capture subsystem 1102 for use with the CO2 capture solution 1144 as part of the regenerated CO2 capture solution loop.

[0244] To generate the second ED product stream 1138, the feed-discharge section of the BPMED 1108 is configured to feed carbonate CO3 2- and bicarbonate HCO3 - The BPM receives an ED feed solution containing dissolved inorganic carbon species such as ED CO, ED NH, ED NH OH, etc. The BPM provides protons to the feed-discharge compartment, and the species in the ED feed solution are protonated to form a second ED product stream 1138. The second ED product stream 1138 contains carbonate, HCO.

[0245] For example, in potassium-based systems, the CEM of BPMED 1108 is potassium ion, K + and transport them to the alkali-generating compartment to produce K + OH -and combine to form KOH in the first ED product stream 1132. Thus, the first ED product stream 1132 may include an aqueous mixture having hydroxide as the predominant species. For example, the first ED product stream 1132 may include an aqueous solution of potassium hydroxide, KOH. The first ED product stream 1132 may be returned from the BPMED 1108 to the CO2 capture subsystem 1102 to be used with or as part of the CO2 capture solution 1144. In the same example, in the feed-release section, species of the ED feed solution may be protonated and combined with cations to form the second ED product stream 1138. For example, carbonate CO3 of the ED feed solution may be protonated and combined with cations to form the second ED product stream 1138. 2- and bicarbonate HCO3 - The ions can be protonated to form primarily carbonate, H2CO3, in the second ED product stream 1138.

[0246] Referring to FIG. 11, the second ED product stream 1138 can flow from the ED unit 1107 to a degassing scrubber 1106 downstream of the ED unit 1107. The degassing scrubber 1106 is fluidly connected to the BPMED 1108. The carbonate H2CO3 in the second ED product stream 1138 will have an equilibrium CO2 partial pressure (e.g., about 1 bar) sufficient to dissociate into water and gaseous CO2 in the degassing scrubber 1106. The degassing scrubber 1106 can partially or completely release the gaseous CO2 stream 1136. The gaseous CO2 stream 1136 can be sent to one or more downstream processing units described herein (e.g., compression units, purification units, electrolytic reduction subsystems, carbon product production systems, syngas generation reactors).

[0247] The degassing scrubber 1106 provides a residual product stream 1112 that is primarily comprised of water, which can flow directly to the BPMED 1108. 2- and bicarbonate HCO3 -In configurations where it is desired to prevent ions from flowing into the alkaline compartment of the BPMED 1108, the electrochemical system 1100 may include a reverse osmosis (RO) unit 1128, an example of which is shown in FIG. 11. In such an embodiment, the residual product stream 1112 is fed indirectly to the BPMED 1108 via the RO unit 1128. The RO unit 1128 converts carbonate CO3 2- and bicarbonate HCO3 - The membrane of the RO unit 1128 can select and reject carbonate and bicarbonate ions from the residue product stream 1112, thereby producing an RO retentate 1168 containing a carbonate-bicarbonate mixture, and an RO permeate 1166 containing primarily water. The RO retentate 1168 may contain a higher concentration of carbonate and bicarbonate than the residue product stream 1112. The RO retentate 1168 is returned to the degassing scrubber 1106 to remove the carbonate CO3 2- and bicarbonate HCO3 - Ions are removed from the water in the residual product stream 1112. The water-containing RO permeate 1166 flows to the BPMED 1108 to produce hydroxides therein.

[0248] The electrochemical system 1100 of FIG. 11 allows for the electrochemical capture of CO2 from a capture solution using a pH swing. Protons generated from water splitting in the ED unit 1107 lower the pH of the retentate 1152 fed to the ED unit 1107 from the filtration unit 1150, thereby generating a more acidic output by increasing the concentration of carbonate in the second ED product stream 1138. The ED unit 1107 also produces a first ED product stream 1132 having a higher pH due to its aqueous mixture with hydroxide as the predominant species. For example, the retentate 1152 feed solution to the ED unit 1107 may have a pH between 11 and 13. The pH is lowered by protonation in the ED unit 1107 to generate a second ED product stream 1138 having a pH between 5 and 8. The pH is further raised by hydroxyl anions generated from water splitting in the ED unit 1107 to generate a first ED product stream 1132 having a pH of about 14.

[0249] In the electrochemical system 1100 of FIG. 11, the retentate 1152 fed to the ED unit 1107 can contain residual hydroxides, which can be neutralized by protons generated in the ED unit 1107 to form water. Thus, any residual hydroxides introduced into the ED unit 1107 via the retentate 1152 feed solution will act as a dead load for the electrochemical system 1100. In such a configuration, the electrochemical system 1100 is a two-step (capture + regeneration process) system, with the regeneration step operating at a low pH solution with little residual hydroxide. Although described with respect to FIG. 11 as including a BPMED 1108, in some implementations, the ED unit 1107 of the electrochemical system 1100 may take the place of, include, or use a gas diffusion electrode (GDE) as disclosed herein.

[0250] Figure 12 is a block flow diagram illustrating an example electrochemical system 1200 for regenerating capture solution and collecting CO2 by using an ED unit 1207. The electrochemical system 1200 of Figure 12 has similar components, features, processes, and / or functionality to the electrochemical system 1100 of Figure 11. Thus, the above descriptions of the components, features, processes, advantages, and / or functionality of the electrochemical system 1100 of Figure 11 apply mutatis mutandis to the electrochemical system 1200 of Figure 12. Reference numerals applied to features of Figure 11 apply mutatis mutandis to features of the electrochemical system 1200 of Figure 12.

[0251] 12, the CO2 capture solution 1244 is an aqueous mixture that includes a capture solvent, such as an amino acid or amine. In the air contactor 1205, the water and capture solvent of the CO2 capture solution 1244 react with CO2 from the dilute gas source to produce bicarbonate ions (HCO3 - For example, amine (AMP) scavenging solvents can react with CO2 and water to form bicarbonate salts according to the following reaction:

[0252] [ka]

[0253] In some cases, the reaction may not go to completion and the resulting bicarbonate-rich capture solution 1220 produced by the CO capture subsystem 1202 may contain unreacted capture solvent. An example composition of the bicarbonate-rich capture solution 1220 is primarily HCO - The filtration unit 1250 may receive the bicarbonate-rich capture solution 1220 as a feed solution. The filtration membrane of the filtration unit 1250 may select and reject salt species, including bicarbonate ions, thereby producing a retentate 1252 that contains primarily concentrated bicarbonate. The filtration membrane of the filtration unit 1250 produces a permeate that may contain the capture solvent used in the CO2 capture solution 1144. The permeate containing the capture solvent is fed back to the CO2 capture subsystem 1202 for use with the CO2 capture solution 1244. Water may be added to the electrochemical system 1200 at several locations, such as to the CO2 capture solution 1244 or to the collector of the air contactor 1205 to make up for losses in the air contactor 1205.

[0254] 12, the bicarbonate-rich retentate 1252 is provided as a feed solution to the ED unit 1207, where a proton-generating water splitting reaction allows the bicarbonate-rich feed solution to be protonated to form a primarily carbonic acid-containing ED product stream 1238. Hydroxyl ions and protons generated by water splitting in the ED unit 1207 may be provided as a water ED product stream 1232, which is returned to the CO2 capture subsystem 1202 to be used with the CO2 capture solution 1244 as part of the regenerated CO2 capture solution loop. The ED product stream 1238 may flow from the ED unit 1207 to a degassing scrubber 1206, which is downstream of the ED unit 1207. The degassing scrubber 1206 is fluidly connected to the ED unit 1207. The carbonated HCO in the ED product stream 1238 has sufficient equilibrium CO partial pressure (e.g., about 1 bar) to cause dissociation of water and gaseous CO in the degassing scrubber 1206. Gaseous CO 1236 is released from the degassing scrubber 1206. The electrochemical system 1200 may be provided with a reverse osmosis (RO) unit 1228, an example of which is shown in FIG. 12. The RO unit 1228 converts the carbonated CO into 2- and bicarbonate HCO3 - The membrane of the RO unit 1228 can select and reject carbonate and bicarbonate ions from the residual product stream 1212 of the degassing scrubber 1206, thereby producing an RO retentate 1268 containing a carbonate-bicarbonate mixture and an RO permeate 1266 containing primarily water. The RO retentate 1268 may contain a higher concentration of carbonate and bicarbonate than in the residual product stream 1212. The RO retentate 1268 is returned to the degassing scrubber 1206 to remove carbonate CO3. 2- and bicarbonate HCO3 - The ions are removed from the water in the residual product stream 1212. The RO permeate 1266 water flows to the ED unit 107 to produce hydroxyl ions and protons.

[0255] Thus, the use of amino acids and amines as a capture solution in electrochemical system 1200 of Figure 12 eliminates the need for initial carbonate formation for subsequent generation of bicarbonate. The capture solution can be reacted with CO2 and water to directly form bicarbonate ions, which may then be processed by electrochemical system 1200 as described above.

[0256] FIG. 13 is a schematic diagram of an example ED unit 1300 including a GDE 1304. This configuration allows some of the advantages similar to the ED unit 1000 including the GDE 1004 of FIG. 10, such as high current density and high hydroxide concentration. The membrane stack includes a CEM 1306 positioned between the anode 1302 and the GDE 11304. The GDE 1304 includes a gas diffusion layer 1310 that supports a catalyst layer 1312. The gas diffusion layer 1310 is porous and allows gas to move toward the catalyst layer 1312. In some implementations, the catalyst layer 1312 can include rhodium sulfide or various metals / metal alloys and their oxides, including platinum, palladium, iridium, silver, rhodium, or non-precious metal / metal oxide catalysts (e.g., nickel, iron, cobalt). In some cases, the ED unit 1300 with the GDE 1304 can operate at a pH ranging between 0 and 14. The GDE 1304 and CEM 1306 define an alkaline regeneration (hydroxide generation) compartment where the capture solution is regenerated. The CEM 1306 and the anode 1302 define a feed-discharge (proton generation) compartment where products including oxygen and proton shuttle species are formed. The ED unit 1300 can be used for indirect protonation of DIC species. The ED unit 1300 can be included as an element of a regeneration subsystem of an electrochemical system, such as any of the electrochemical systems 200, 400, 700, and 800.

[0257] Indirect protonation via a proton shuttle species, such as sulfate, bisulfate, or a combination thereof, can be used to regenerate the CO2 capture solution. In some cases, the proton shuttle species is Cl. - , I - , Br - , HPO4-2 , and H2PO4 -1 , acetate, citrate, or combinations thereof. The ED unit 1300 constitutes part of the brine loop of the indirect protonation configuration. The ED unit 1300 receives water and an ED feed solution 1326 comprising a sulfate-bisulfate mixture in the feed-discharge compartment. An electric potential is applied to the ED unit 1300. An oxygen evolution reaction occurs at the anode 1302. The oxygen evolution reaction is a reaction in which molecular oxygen is generated, for example, by electrochemical decomposition of water. In some implementations, the catalyst layer on the anode 1302 can include platinum group metals and their oxides, for example, iridium / iridium oxide or ruthenium / ruthenium oxide. In some implementations, these catalysts can be incorporated into titanium / titanium oxide electrodes to form dimensionally stable anodes (DSA) or mixed metal oxide (MMO) electrodes. The generated protons are selectively transported across the CEM 1306 to the alkaline regeneration compartment to generate alkali metal ions (e.g., K + , Na + etc.) Cation M + Replace with.

[0258] To regenerate the capture solution containing alkali hydroxide MOH, the ED unit 1300 receives water and oxygen feed 1324 in the alkali regeneration compartment. In some implementations, air or an oxygen-containing feed stream can be used in place of or in combination with the oxygen feed 1324. In the GDE 1304, an oxygen reduction reaction occurs to produce hydroxyl ions OH - In the alkaline regeneration section, alkali metal ions M + and hydroxyl ion OH - forms a first ED product stream 1332 that includes a regenerated capture solution together with an alkali hydroxide MOH. For example, potassium ions K + may be transported across the CEM 1306 to form a regenerated capture solution having potassium hydroxide, KOH. The first ED product stream 1332 may flow from the ED unit 1300 to an air contactor of the CO2 capture subsystem as a regenerated CO2 capture solution.

[0259] In the feed-discharge section, the protons replacing the alkali metal ions acidify the sulfate-bisulfate mixture to form a second ED product stream 1334 containing evolved oxygen and bisulfate salt MHSO4. In some cases, the second ED product stream 1334 can also contain sulfuric acid H2SO4. The second ED product stream 1334 can then be sent to a flash tank 135 or a degassing sparge where an oxygen stream 1342 can be degassed and removed. In some implementations, the oxygen stream 1342 can be recycled to the GDE 1304 and used instead of or in combination with the oxygen feed 1324. After the oxygen is degassed from the flash tank 1350, a brine stream 1338 containing a bisulfate-rich solution flows to a dissolution tank 1352. The dissolution tank 1352 receives carbonate-hydrate salts, for example, from an upstream carbonate separation subsystem.

[0260] In the dissolution tank 1352, the bisulfate-rich solution can react with carbonate to produce a sulfate-rich (bisulfate-lean) solution and carbonic acid according to reaction 5. For example, potassium bisulfate KHSO4 can react with potassium carbonate K2CO3 to produce potassium sulfate K2SO4 and carbonic acid H2CO3. As a result, the pH will decrease. The carbonic acid will have sufficient equilibrium CO2 partial pressure (e.g., 1 bar) to cause dissociation into water and gaseous CO2 stream 1336. The dissolution tank 1352 can partially or completely release the gaseous CO2 stream 1336. The gaseous CO2 stream 1336 can be sent to one or more downstream processing units described in subsequent implementations (e.g., compression unit, purification unit, electrolytic reduction subsystem, carbon product production system, syngas generation reactor). The reaction in the dissolution tank 1352 also forms the ED feed solution 1326. The ED Feed Solution 1326 can include a sulfate-rich (bisulfate-lean) solution. For example, the ED Feed Solution 1326 can include potassium sulfate along with a mixture of other components, such as potassium bisulfate and water.

[0261] Any one of the electrochemical systems 100, 200, 300, 400, 600, 700, 800, 1100, 1200 in Figures 1-4, 6-8, 11, 12, the BPMED 900 in Figure 9, the ED unit 1000 in Figure 10, the ED unit 1300 in Figure 13, and the method 500 in Figure 5 can include flowing a stream (e.g., carbonate-rich capture solution, crystalline carbonate hydrate, ED feed solution, product stream, etc.) to at least one auxiliary unit or device, such as one or more buffer tanks, filtration systems, water treatment systems, holding tanks, mixing tanks, settlers, clarifiers, conveyors, or other units that facilitate performance of the aforementioned electrochemical systems and methods.

[0262] In the electrochemical systems 100, 200, 300, 400, 600, 700, 800, 1100, 1200 of Figures 1-4, 6-8, 11, 12, the BPMED 900 of Figure 9, the ED unit 1000 of Figure 10, the ED unit 1300 of Figure 13, and the method 500 of Figure 5, additives such as simple salts (monovalent salts) can be included in one or more process streams. Simple salts such as sodium chloride can enhance performance by increasing conductivity and lowering the freezing point for the CO2 capture subsystem.

[0263] The primary caustic evaporator 112, 212, 312, 412, auxiliary caustic evaporator 114, 214, 314, 414, 714, 814 and crystallizer 104, 204, 304, 404, 704 of Figures 1 through 4 and 7 and 8 each discharge one or more water streams. In some cases, the water streams can be partially or completely recycled to other units that require water as an incoming stream, such as the BPMED unit 108, 208, 308, 408, 708, 808, 900, CO2 electrolytic reduction unit 610, ED unit 1000, 1107, 1207, 1300, or dissolution tank 106, 206, 306, 406, 608, 706, 1352. For example, in the electrochemical system 100, the primary caustic evaporator 112 discharges a water stream 119 that can replace or be combined with the water stream 134 flowing to the BPMED 108. For example, the auxiliary caustic evaporator 114 discharges a water stream 140 as condensate that can replace or be combined with the water stream 134 that is fed to the BPMED 108. In some cases, the discharged water stream may require treatment (e.g., in a filtration system or water treatment system) before flowing to downstream units. The primary caustic evaporators 112, 212, 312, 412 and the auxiliary caustic evaporators 114, 214, 314, 414, 714, 814 each have a maximum capacity of 20 m per t-CO2. 3 of water can be removed. If additional water removal from the process is advantageous, water can be removed from the air contactor, crystallizer, nanofiltration unit, reverse osmosis unit, auxiliary unit, or combinations thereof to maintain the water balance of the process. For example, water removal from the process can be advantageous to adjust for seasonal or diurnal weather conditions, fresh water is required to operate an ED unit, or combinations thereof. The water that is removed can be reused elsewhere, stored for future use, or transported off-site.

[0264] In each of the electrochemical systems 100, 200, 300, 400, 600, 700, 800, 1100, 1200 of Figures 1 to 4, 6 to 8, 11, 12, the BPMED unit 900 of Figure 9, the ED unit 1000 of Figure 10, the unit 1300 of Figure 13, and the method 500 of Figure 5, one or more elements that flow or receive a water stream can flow or receive a certain amount of suspended solids, dissolved solids, or impurities (or combinations thereof). For example, the BPMED units 108, 208, 308, 408, 708, 808, 900, the CO2 electrolytic reduction unit 610, and the ED units 1000, 1107, 1207, 1300 can receive respective water streams that each contain various amounts of suspended solids, dissolved solids, impurities, or combinations thereof. In some cases, the BPMED units 108, 208, 308, 408, 708, 808, 900, the CO2 electrolytic reduction unit 610, and the ED units 1000, 1107, 1207, 1300 can receive treated water streams. For example, the treated water can include water that has been demineralized, distilled, filtered, purified, or otherwise treated.

[0265] In each of the electrochemical systems 100, 200, 300, 400, 600, 700, 800, 1100, 1200 of Figures 1-4, 6-8, 11, 12, water is fed to the BPMED to generate a first ED product stream and a second ED product stream. Water can be evaporated from one or more streams to maintain water balance. For example, water can be evaporated from one or more of the following streams: carbonate-rich capture solution 120, 220, 320, 420, 720, 820, retentate 352, 452, 852, or permeate 354, 454, 854. Water can be evaporated using an MVR evaporator, a multiple effect evaporator, or a membrane filtration unit (e.g., ultrafiltration, nanofiltration, reverse osmosis, and the like). Water can also be evaporated through an air capture unit, where the evaporation is at least partially dependent on the ambient environmental conditions.

[0266] In each of the electrochemical systems 100, 200, 300, 400, 600, 700, 800, 1100, 1200 of Figures 1-4, 6-8, 11, 12, the BPMED 900 of Figure 9, the ED unit 1000 of Figure 10, the ED unit 1300 of Figure 13, and the method 500 of Figure 5, a wide range of hydroxide concentrations in the CO2 capture solution can absorb some amount of CO2 from the dilute gas source, but the most effective composition can be adjusted to improve capture efficiency and to accommodate a particular operating environment. For example, the CO2 capture solution 144, 244, 344, 444, 618, 744, 844, 930, 1044, 1144, 1244, 1332 can include potassium hydroxide KOH concentrations ranging from 1 M to 8 M and potassium carbonate K2CO3 concentrations ranging from 0.1 M to 1.8 M. For example, the CO2 capture solution 144, 244, 344, 444, 618, 744, 844, 930, 1044, 1144, 1244, 1332 can include a sodium hydroxide NaOH concentration ranging from 1M to 3M and a sodium carbonate Na2CO3 concentration ranging from 0.1M to 1M.

[0267] For example, the CO2 capture solution 144, 244, 344, 444, 618, 744, 844, 930, 1044, 1144, 1244, 1332 can include a mixture including a KOH concentration ranging from 2.5M to 5M, a NaOH concentration ranging from 2.5M to 1M, a potassium carbonate K2CO3 concentration ranging from 0.1M to 0.7M, and a sodium carbonate Na2CO3 concentration ranging from 0.05M to 0.2M.

[0268] The electrochemical systems and methods described herein are modularly scalable and therefore may include a different number of elements and subsystems than the implementations illustrated in Figures 1 through 13. For example, an electrochemical system may include multiple air contactors or ED units (or a combination of both) fluidly coupled to a carbonate separation subsystem. For example, an electrochemical system may include one or more caustic evaporators in the carbonate separation subsystem for each of the air contactors or ED units. The combination of multiple air contactors, multiple carbonate separation subsystems, and multiple regeneration subsystems and their respective elements may require a distribution system including one or more trains fluidly coupled to the systems and elements.

[0269] 14 is a schematic diagram of a control system (or controller) 1400 for an electrochemical system for regenerating capture solution and capturing CO. System 1400 may be used for the operations described in connection with any of the computer-assisted methods described above, for example as or as part of control system 999 or any other controller described herein.

[0270] System 1400 is intended to include various forms of digital computers, such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other suitable computers. System 1400 may also include mobile devices, such as personal digital assistants, cell phones, smart phones, and other similar computing devices. Additionally, the system may include a portable storage medium, such as a Universal Serial Bus (USB) flash drive. For example, the USB flash drive may store an operating system and other applications. The USB flash drive may include input / output components, such as a wireless transmitter or a USB connector that may be inserted into a USB port of another computing device.

[0271] The system 1400 includes a processor 1410, a memory 1420, a storage device 1430, and an input / output device 1440. Each of the components 1410, 1420, 1430, and 1440 are interconnected using a system bus 1450. The processor 1410 is capable of processing instructions for execution within the system 1400. The processor may be designed using any of a number of architectures. For example, the processor 1410 may be a CISC (Complex Instruction Set Computer) processor, a RISC (Reduced Instruction Set Computer) processor, or a MISC (Minimum Instruction Set Computer) processor.

[0272] In one implementation, the processor 1410 is a single-threaded processor. In some implementations, the processor 1410 is a multi-threaded processor. The processor 1410 is capable of processing instructions stored in the memory 1420 or the storage device 1430 to display graphical information for a user interface on the input / output device 1440.

[0273] The memory 1420 stores information within the system 1400. In one implementation, the memory 1420 is a computer-readable medium. In one implementation, the memory 1420 is a volatile memory unit. In some implementations, the memory 1420 is a non-volatile memory unit.

[0274] The storage device 1430 is capable of providing mass storage for the system 1400. In one implementation, the storage device 1430 is a computer-readable medium. In various implementations, the storage device 1430 may be a floppy disk device, a hard disk device, an optical disk device, or a tape device.

[0275] The input / output device(s) 1440 provide input / output operations to the system 1400. In one implementation, the input / output device(s) 1440 include a keyboard and / or a pointing device. In some implementations, the input / output device(s) 1440 include a display unit for displaying a graphical user interface.

[0276] Certain features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. An apparatus can be implemented in a computer program product tangibly embodied in an information carrier, for example in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by the programmable processor executing a program of instructions that performs the functions of the described implementations by operating on input data and generating output. The features described can be advantageously implemented in one or more computer programs executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, by a computer to perform a particular activity or bring about a particular result. Computer programs can be written in any form of programming language, including compiled or interpreted languages, and can be deployed as stand-alone programs or in any form including modules, components, subroutines, or other units suitable for use in a computing environment.

[0277] Suitable processors for executing a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer. Typically, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memories for storing instructions and data. Typically, a computer will also include, or be operatively linked to communicate with, one or more mass storage devices for storing data files; such devices include magnetic disks, e.g., internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and memory may be supplemented by, or incorporated in, ASICs (Application Specific Integrated Circuits).

[0278] To provide for interaction with a user, features may be implemented on a computer having a display device, such as a CRT (cathode ray tube) or LCD (liquid crystal display) monitor for displaying information to the user, and a keyboard and pointing device, such as a mouse or trackball, by which the user can provide input to the computer. Additionally, such operation may be accomplished via touch screen flat panel displays and other suitable mechanisms.

[0279] Features may be implemented in a control system that includes back-end components such as data servers, or that includes middleware components such as application servers or Internet servers, or that includes front-end components such as client computers having a graphical user interface or an Internet browser, or any combination thereof. The components of the system may be connected by any form or medium of digital data communication, such as a communications network. Examples of communications networks include a local area network ("LAN"), a wide area network ("WAN"), a peer-to-peer network (with ad-hoc or static members), a grid computing infrastructure, and the Internet.

[0280] As used in this description, the term "couple" and its variations, such as "coupled," "couples," and "coupling," are intended to include indirect and direct connections unless otherwise indicated. For example, when a first device is coupled to a second device, the coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, when a first device is fluidly coupled to a second device, the flow may be through a direct connection or through an indirect connection via other devices and connections. In particular, fluidly coupled means that a direct or indirect path for fluid flow is provided between the two fluidly coupled devices.

[0281] As used in this description, terms used to describe an action or result with respect to one or more of the process streams or elements of the aforementioned systems and methods (e.g., "flow," "form," "return," "receive," "produce," "release," "use," "apply," "provide," "dissolve," and their respective gerunds) are intended to include partial and complete actions or results. For example, flowing a solution through a subsystem can include flowing at least a portion or all of the solution through the subsystem. For example, a subsystem that receives a solution can include a subsystem that receives a portion of the solution or the entire solution.

[0282] Several embodiments of the present disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims. Further modifications and alternative embodiments of various aspects will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as merely illustrative. It will be understood that the forms shown and described herein are to be construed as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features may be utilized independently, all as will be apparent to those skilled in the art after having the benefit of this description. Changes may be made in the elements described herein without departing from the spirit and scope of the following claims. [Explanation of symbols]

[0283] 100 Electrochemical Systems 102 CO2 Capture Subsystem 104 Crystallizer 105 Air Contactor 106 Dissolving Tank 108 BPMED 110 Flash Tank 112 Primary caustic evaporator 114 Auxiliary caustic evaporator 118 Concentrated carbonate-rich capture solution 119 Water flow 120 Carbonate-rich capture solution 122 Crystalline Carbonate Hydrates 124 Water flow 126 ED supply solution 128 Water flow 130 Second ED Product Stream 132 First ED product stream 134 Water flow 136 CO2 flow 138 Brine Flow 140 Water flow 142 Mother liquor 144 CO2 capture solution 162 Carbonate Separation Subsystem 164 Playback Subsystem 999 Flow Control System

Claims

1. CO ２ A step of capturing carbon dioxide from a dilute gas source with a capture solution to form a capture solution rich in carbonate; A step of separating at least a part of the carbonate from the capture solution rich in carbonate; A step of forming an electrodialysis (ED) feed solution; A step of flowing a water stream and the ED feed solution through a bipolar membrane electrodialysis (BPED) unit; A step of applying a potential to the BPED unit to form at least two ED product streams including a first ED product stream containing a hydroxide, and the CO ２ A step of flowing the first ED product stream for use in capturing carbon dioxide from the dilute gas source together with the capture solution. A method comprising.

2. The method according to claim 1, wherein the step of applying the potential to the BPED unit includes a step of applying at least a part of the potential to the BPED unit to form the first ED product stream and the second ED product stream.

3. The method according to claim 2, wherein the second ED product stream contains carbonic acid, and the method further includes a step of recovering at least a part of a carbon dioxide gas stream from the second ED product stream.

4. The step of recovering the portion of the carbon dioxide gas stream from the second ED product stream includes a step of recovering the portion of the carbon dioxide gas stream from the second ED product stream to form a brine stream, and the step of separating at least a part of the carbonate from the capture solution rich in carbonate includes a step of crystallizing the portion of the carbonate to form a crystalline carbonate hydrate, and the method further includes, Before the step of flowing water and the ED feed solution through the BPED unit, dissolving the crystalline carbonate hydrate and mixing the dissolved crystalline carbonate hydrate with the brine stream to form an ED feed solution The method according to claim 3, comprising.

5. The step of recovering a portion of the carbon dioxide gas stream from the second ED product stream to form the brine stream includes the step of recovering the portion of the carbon dioxide gas stream in a flash tank, and the method further comprises flowing the brine stream from the flash tank for use in dissolving the crystalline carbonate hydrate, and mixing the dissolved crystalline carbonate hydrate with the brine stream to form the ED feed solution The method according to claim 4, comprising.

6. flowing the carbon dioxide gas stream to a downstream process comprising at least one of a compression unit, a fuel synthesis system, a synthesis gas generator reactor, or an electrolytic cell; and producing one or more downstream products comprising at least one of synthesis gas, CO, H ２ , or water. The method according to claim 4, further comprising.

7. The second ED product stream includes a proton shuttle species, and the method further comprises reacting the second ED product stream with the carbonate moiety to recover carbon dioxide gas and form the ED feed solution The method according to any one of claims 2 to 5, comprising.

8. The step of reacting the second ED product stream with the carbonate moiety to recover the carbon dioxide gas includes reacting the proton shuttle species of the second ED product stream with the carbonate moiety to form carbonic acid and the ED feed solution. The method according to claim 7, comprising.

9. flowing the carbon dioxide gas to a downstream process comprising at least one of a compression unit, a fuel synthesis system, a synthesis gas generator reactor, or an electrolytic cell; and producing one or more downstream products comprising at least one of synthesis gas, CO, H ２ , or water. The method according to claim 7, further comprising.

10. The method according to claim 7, wherein the step of reacting the second ED product stream with the carbonate moiety comprises reacting the proton shuttle species comprising bisulfate with the carbonate moiety.

11. The method according to any one of claims 1 to 6, further comprising flowing the ED feed solution through an ion exchanger before flowing water and the ED feed solution through the BP MED unit.

12. The method according to any one of claims 1 to 6, further comprising separating the BP MED recycle stream from the first ED product stream and returning the BP MED recycle stream to the BP MED unit.

13. The step of separating the carbonate moiety from the carbonate-rich capture solution comprises flowing the carbonate-rich capture solution through a nanofiltration unit to form a nanofiltration (NF) retentate stream comprising a carbonate-rich mixture and an NF permeate stream comprising a hydroxide-rich mixture, and the step of flowing water and the ED feed solution through the BP MED unit comprises flowing the ED feed solution comprising at least a portion of the NF retentate stream through the BP MED unit, and the method further comprises, recovering carbon dioxide gas from the second ED product stream to form a brine stream; flowing the brine stream through a reverse osmosis (RO) unit to form an RO retentate stream comprising a bicarbonate-rich solution and an RO permeate stream comprising water; combining the RO retentate stream with the NF retentate stream to form the ED feed solution; and the method according to claim 2.

14. the CO ２ The method according to claim 13, further comprising flowing at least a portion of the NF permeate stream for use in capturing the carbon dioxide from the dilute gas source with the capture solution.

15. The method according to claim 13 or 14, further comprising flowing the NF holding liquid stream to an ion exchanger downstream of the nanofiltration unit to form an ion exchange regeneration waste liquid stream and at least a portion of the ED supply solution.

16. The method according to claim 13 or 14, further comprising separating a BP MED recycle stream containing hydroxide from the first ED product stream and flowing the BP MED recycle stream to the BP MED unit.

17. The method according to claim 16, comprising combining at least a portion of the RO permeate stream containing water with the first ED product stream to form the BP MED recycle stream.

18. The method according to any one of claims 1 to 6, 13, and 14, further comprising operating the BP MED unit at a pH between 7 and 12.

19. The method according to any one of claims 1 to 6, 13, and 14, further comprising operating the BP MED unit at a hydrogen ion concentration ranging from 0.001 M to 2.5 M.

20. Before the step of capturing carbon dioxide from the dilute gas source with the CO ２ capture solution, increasing the concentration of hydroxide in the first ED product stream to form the CO ２ capture solution. The method according to any one of claims 1 to 6, 13, and 14, further comprising this step.

21. The method according to claim 20, wherein the step of increasing the concentration of hydroxide in the first ED product stream includes evaporating water from the first ED product stream.

22. The method according to claim 2, wherein the step of separating a carbonate portion from the carbonate-rich capture solution includes increasing the concentration of carbonate in the carbonate-rich capture solution by crystallizing the carbonate-rich capture solution to form a mother liquor and a crystalline carbonate hydrate.

23. dissolving at least a portion of the crystalline carbonate hydrate to form a carbonate portion; mixing the carbonate portion with a brine stream to form the ED feed solution; The method according to claim 22, further comprising: **Claim 24** dissolving at least a portion of the crystalline carbonate hydrate to form a carbonate portion; reacting the carbonate portion with a proton shuttle species in the second ED product stream to form the ED feed solution; The method according to claim 22, further comprising: **Claim 25** The method according to any one of claims 22 to 24, further comprising evaporating water from the carbonate-rich capture solution to increase the concentration of the carbonate prior to the step of crystallizing the carbonate-rich capture solution. **Claim 26** said CO ２ combining at least a portion of the mother liquor with at least a portion of the CO ２ capture solution for use in capturing the carbon dioxide from the lean gas source with the capture solution; The method according to any one of claims 22 to 24, further comprising: **Claim 27** The crystalline carbonate hydrate is potassium carbonate sesquihydrate (K ２ CO ３ ·1.5H ２ O), sodium carbonate decahydrate (Na ２ CO ３ ·10H ２ O), potassium sodium carbonate hexahydrate (KNaCO ３ ·6H ２ O), or at least one of the anhydrous carbonates; The method according to any one of claims 22 to 24, further comprising: **Claim 28** The step of separating the carbonate portion from the carbonate-rich capture solution includes the step of crystallizing the carbonate-rich capture solution to form a low solids stream and a high solids stream containing crystalline carbonate hydrate, the low solids stream having a higher liquid-to-solid ratio than the high solids stream, and the method further comprises, dissolving the crystalline carbonate hydrate of the high solids stream in an aqueous solution and mixing it with a brine stream to form the ED feed solution; returning the low solids stream for use in the crystallization of the carbonate-rich capture solution; The method according to any one of claims 1 to 6, 13, 14, and 22 to 24, comprising.

29. The method according to claim 1, wherein the step of separating the carbonate portion from the carbonate-rich capture solution includes flowing the carbonate-rich capture solution into a nanofiltration unit.

30. The step of flowing the carbonate-rich capture solution into the nanofiltration unit includes flowing the carbonate-rich capture solution into the nanofiltration unit to form an NF retentate stream containing a carbonate-rich mixture and an NF permeate stream containing a hydroxide-rich mixture, and the method further comprises, returning at least a portion of the NF permeate stream for use in capturing the carbon dioxide together with the CO ２ capture solution; crystallizing at least a portion of the NF retentate stream to form a mother liquor and crystalline carbonate hydrate; The method according to claim 29, comprising.

31. The method according to claim 29 or 30, wherein the step of flowing the carbonate-rich capture solution into the nanofiltration unit includes excluding at least 85% of the carbonate ions.

32. flowing a cell feed solution containing a bicarbonate-rich solution into a CO ２ electrolytic reduction unit; the CO ２Applying a potential to the electrolytic reduction unit to perform one or more reduction reactions in the cell feed solution; Performing the one or more reduction reactions in the cell feed solution to yield one or more reduction products; The method according to claim 1, further comprising:

33. The step of yielding one or more reduction products includes the step of yielding at least one of syngas, CO, H ２ , formate, methane, ethylene, or ethanol; the method according to claim 32.

34. The step of capturing carbon dioxide from the dilute gas source with the CO ２ capture solution to form the carbonate-rich capture solution includes the step of capturing carbon dioxide with the CO ２ capture solution in at least one of a gas-liquid contactor, an air contactor, a spray tower, a liquid-gas scrubber, a Venturi scrubber, a packed tower, a single cell air contactor, a double cell air contactor, or a multi-cell air contactor; the method according to claim 1.

35. An electrochemical system for regenerating a CO ２ capture solution for capturing carbon dioxide from a dilute gas source, comprising: A carbonate separation subsystem configured to receive a carbonate-rich capture solution from a CO ２ capture subsystem and separate at least a portion of the carbonate from the carbonate-rich capture solution; A regeneration subsystem fluidly connected to the carbonate separation subsystem, the regeneration subsystem including a bipolar membrane electrodialysis (BPED) unit fluidly connected to the carbonate separation subsystem, the BPED unit including at least one cation exchange membrane alternately arranged with at least one bipolar membrane; And the BPED unit Receives an electrodialysis (ED) feed solution and a water stream, Yielding at least two ED product streams including a first ED product stream containing a hydroxide; An electrochemical system configured as such.

36. The electrochemical system according to claim 35, wherein the at least one cation exchange membrane is configured to transport alkali metal ions, and the at least one bipolar membrane is operable to provide hydroxyl ions.

37. The carbonate-rich capture solution contains K ２ CO ３ 、Na ２ CO ３ 、or at least one of these combinations, the electrochemical system according to claim 35 or 36.

38. The electrochemical system further includes a CO ２ capture subsystem fluidly connected to the carbonate separation subsystem and fluidly connected to the regeneration subsystem, and the CO ２ capture subsystem is configured to receive the CO ２ capture solution containing at least one of KOH, NaOH, an additive, or a combination thereof, the electrochemical system according to claim 35 or 36.

39. The carbonate separation subsystem includes a primary caustic evaporator fluidly connected to the CO ２ capture subsystem and operable to concentrate the carbonate-rich capture solution, the electrochemical system according to claim 35 or 36.

40. The carbonate separation subsystem includes a crystallizer fluidly connected to the primary caustic evaporator, and the crystallizer is operable to concentrate the carbonate-rich capture solution received from the primary caustic evaporator, the electrochemical system according to claim 39.

41. The primary caustic evaporator includes at least one of a mechanical vapor recompression (MVR) evaporator or a multiple effect evaporator, the electrochemical system according to claim 39.

42. The carbonate separation subsystem is A nanofiltration unit operable to concentrate the carbonate-rich capture solution, A crystallizer fluidly connected to the nanofiltration unit and operable to crystallize the carbonate-rich capture solution received from the nanofiltration unit to form a crystalline carbonate hydrate The electrochemical system according to claim 35, comprising: **Claim 43** The electrochemical system according to claim 41, wherein the nanofiltration unit is operable to reject at least 85% of carbonate ions. **Claim 44** The electrochemical system according to claim 41, wherein the nanofiltration unit is operable in a pH range of 2 to 14. **Claim 45** The electrochemical system according to claim 40, wherein the crystallizer comprises at least one of a chiller crystallizer, an evaporative crystallizer, a eutectic freezing crystallizer, a cooling crystallizer, or a membrane distillation crystallizer. **Claim 46** The regeneration subsystem, wherein the CO ２ The electrochemical system according to claim 35, further comprising an auxiliary caustic evaporator fluidly connected to the capture subsystem and the BPME unit, the auxiliary caustic evaporator being operable to concentrate the first ED product stream having the hydroxide. **Claim 47** The electrochemical system according to claim 46, wherein the auxiliary caustic evaporator comprises at least one of a mechanical vapor recompression (MVR) evaporator or a multiple effect evaporator. **Claim 48** The regeneration subsystem, A crystallizer operable to crystallize the carbonate-rich capture solution to form a crystalline carbonate hydrate, A dissolution tank fluidly connected to the crystallizer and configured to dissolve the crystalline carbonate hydrate The electrochemical system according to claim 35, comprising: **Claim 49** The flash tank, wherein the regeneration subsystem is fluidly coupled to the BPME unit and is operable to recover a carbon dioxide gas stream from a second product stream of the at least two product streams provided by the BPME unit, according to any one of claims 35, 36, 42, and 46 to 48 of the electrochemical system.

50. The electrochemical system according to any one of claims 35, 36, 42, and 46 to 48, wherein the BPME unit of the regeneration subsystem is electrically coupled to a low-carbon intensity power source including an intermittent power source.

51. The electrochemical system according to any one of claims 35, 36, 42, and 46 to 48, further comprising at least one of a compression unit, a fuel synthesis system, a syngas generator reactor, or an electrolytic cell.

52. The carbonate separation subsystem is A crystallizer operable to concentrate the carbonate-rich capture solution into a crystalline carbonate hydrate; A solid-liquid separator fluidly coupled to the crystallizer, the solid-liquid separator configured to form a high solids stream including crystalline carbonate hydrate and a low solids stream; A dissolution tank fluidly coupled to the solid-liquid separator, the dissolution tank configured to dissolve the crystalline carbonate hydrate of the high solids stream received from the solid-liquid separator. The electrochemical system according to claim 35, comprising

53. The regeneration subsystem includes an ion exchanger fluidly coupled to the dissolution tank and the BPME unit, the ion exchanger configured to remove a portion of divalent and polyvalent cations flowing to the BPME unit, according to claim 52 of the electrochemical system.

54. The carbonate separation subsystem includes potassium hydroxide KOH, sodium hydroxide NaOH, an additive, or a combination thereof, CO ２ The electrochemical system according to claim 35, configured to receive a capture solution.

55. wherein the capture solution rich in carbonate is potassium carbonate K ２ CO ３ , sodium carbonate Na ２ CO ３ , or a combination thereof, of the electrochemical system according to any one of claims 35, 36, 42, 46 to 48, and 52 to 54.

56. wherein the CO ２ capture subsystem includes at least one of a gas-liquid contactor, an air contactor, a spray tower, a liquid-gas scrubber, a Venturi scrubber, a packed tower, a single cell air contactor, a double cell air contactor, or a multi-cell air contactor, of the electrochemical system according to any one of claims 35, 36, 42, 46 to 48, and 52 to 54.

57. An electrochemical system for generating a reduction product from carbon dioxide from a dilute gas source, a CO ２ capture subsystem configured to generate a capture solution rich in carbonate, and the CO ２ a carbonate separation subsystem fluidly connected to the capture subsystem and operable to receive the capture solution rich in carbonate, the carbonate separation subsystem including a crystallizer configured to form at least partially crystalline carbonate hydrate from the capture solution rich in carbonate, the CO ２ a product generation subsystem fluidly connected to the capture subsystem and fluidly connected to the carbonate separation subsystem via a crystallizer, a dissolution tank fluidly connected to the crystallizer and configured to dissolve the crystalline carbonate hydrate, a CO ２ electrolytic reduction unit fluidly connected to the dissolution tank and including one or more bipolar membranes and a catalyst configured to provide one or more reduction products A product generation subsystem including An electrochemical system including

58. Said CO ２ The electrochemical system according to claim 57, wherein the electrolytic reduction unit provides one or more reduction products from a cell feed solution containing a solution rich in bicarbonate.

59. Said one or more reduction products include at least one of synthesis gas, CO, H ２ The electrochemical system according to claim 57 or 58, including formate, methane, ethylene, or ethanol.

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