Method for obtaining carbon dioxide from gas streams using ph swing electrolysis

Abstract

The invention relates to the technical field of carbon dioxide deposition methods and provides a method and a system for obtaining, preferably processing and / or concentrating, gas streams containing CO2.

Description

[0001] Munich, July 3, 2025

[0002] Our reference: RM 5246-02WO HMS / dsr

[0003] Publisher / owner: RWTH Aachen University, a public corporation

[0004] Official file number: Subsequent registration for DE 10 2024 120 035.9

[0005] RWTH Aachen University, a public corporation, 52062 Aachen

[0006] Method for obtaining carbon dioxide from gas streams using pH-swing electrolysis

[0007] The present invention lies in the technical field of carbon dioxide capture processes and provides a method and a system for the recovery, preferably processing and / or concentration, of CO2-containing gas streams. One of the greatest challenges of the century is combating the climate crisis and limiting global warming. The main factor for global warming and the associated climatic changes is the increase in the CO2 concentration in the atmosphere. The Paris Agreement of December 2015 set the goal of limiting global warming to well below 2°C compared to the pre-industrial era. To achieve this, global CO2 emissions must be reduced by 43% by 2030 compared to 2019. One component in achieving this goal is the removal of CO2 from ambient air or exhaust gases, followed by storage in a solid, climate-neutral form.Alternatively, the extracted carbon dioxide can be used to produce usable materials, such as fuels or solvents.

[0008] CO2, also known as carbon dioxide, has a concentration of approximately 0.04% by volume in the air (400 ppm) and approximately 1-30% by volume in exhaust gases. The processes used are collectively referred to as carbon capture and storage (CCS) and / or carbon capture and utilization (CCU) and are the subject of numerous current development projects.

[0009] US patent application US 2022 / 0331740 A1 discloses a process in which the driving forces for absorption and desorption are generated by electrodialysis. The desorption process is based on an amine, which is present in stoichiometric excess in an aqueous solution. Gaseous CO₂ is introduced into an absorber, reacts with the amine to form a complex, and is released within the electrochemical cell during acidification. The amine then exists as a cation and is subsequently regenerated with an anion exchange resin. The regeneration capacity of the anion exchange resin is then restored using the parallel electrochemically generated alkaline solution. The energy required to release CO₂ from the bound amine form is expected to be very high.

[0010] European patent application EP 4 252 299 A1 discloses an electrochemical cell that can be used for carbon dioxide capture and also as a redox flow battery energy storage device. For carbon dioxide absorption and desorption, an alkaline or acidic solution is used. The solutions are generated via the electrochemical cell, and the desorption takes place within the cell.

[0011] Document DE 10 2023 109 225 B3 discloses an electrolytic process and a plant for the production of carbon dioxide (CO2), in particular from air or industrial sources. Solid precipitation is not disclosed, and the carbonate formed is present exclusively as part of an aqueous solution.

[0012] Document DE 10 2022 105 042 A1 discloses a method for separating carbon dioxide (CO2) from an air stream.

[0013] A problem with all the described systems and processes for carbon dioxide capture is the high energy demand, which must be supplied, preferably from sustainable energy sources. This hinders the implementation of such systems in industrial practice. The primary technical task was therefore to provide an energy-efficient process for carbon dioxide capture.

[0014] Brief description of the characters

[0015] Figure 1 shows a first process alternative or system for the recovery of CO2.

[0016] Figure 2 shows a second process alternative or system for CO2 recovery.

[0017] Figure 3 shows the schematic structure of an electrochemical cell stack.

[0018] Figure 4 shows the CO2 content in the CO2-depleted phase as a function of different ratios of added hydroxide ion equivalents to added CO2. A precipitating agent from the alkaline earth metal class was used.

[0019] Figure 5 shows the pH values ​​of the outflowing currents of the cathode and anode chambers for different current densities.

[0020] Figure 6 shows the percentage of CO2 equivalents of the added solid that were carried away in the CO2-rich gas, plotted against different concentrations of hydrochloric acid in the incoming liquid phase.

[0021] Figure 7 shows the molar fraction of CO2 in the exhausted gas (in mol-%) plotted against different concentrations of hydrochloric acid in the incoming liquid phase.

[0022] Definitions

[0023] Within the scope of the present invention, "solid" means the presence of a substance as a solid or predominantly as a solid. "Predominantly" as a solid means that the solid cake may still contain a water content of up to 50% by weight of the total amount, and that this aqueous content also contains the dissolved substance.

[0024] In the context of the present invention, a "precipitating agent" refers to a substance or mixture of substances that is capable of reacting with the carbon dioxide from the input stream to form a product that precipitates as a solid. The reaction with the precipitating agent is reversible.

[0025] Whenever the present invention refers to "carbonate" or "hydrogen carbonate," this term also includes the associated ionic states as well as the presence of the substance as a dissolved salt and as a solid salt. Preferably, the carbonate and / or hydrogen carbonate are present primarily as a solid salt after gas-liquid reactive precipitation.

[0026] An “electrolyte” within the meaning of the present invention refers to the aqueous solution present in the system containing at least one salt, preferably selected from the group consisting of sulfate salts or chloride salts. Preferably, the salts are present at a concentration close to their maximum solubility. With the exception of bicarbonate and / or carbonate salts, precipitation of any other salt preferably does not occur during the entire process. The “acidic solution” describes the acidified solution of the electrolyte, which is present as the anolyte in the anode chamber, and the “alkaline solution” describes the alkalized solution of the electrolyte, which is present as the catholyte in the cathode chamber.

[0027] A "cation exchange membrane" within the meaning of the present invention is any material that serves as a cation-permselective separator. A cation exchange membrane also includes, for example, cation-permselective diaphragms. Detailed description

[0028] The primary object of the present invention was achieved by a process for obtaining, preferably processing and / or concentrating, CO2 from CO2-containing gas streams, comprising the steps of: i. providing a CO2-containing input gas stream (101, 201) and an alkaline solution; ii. contacting the gas stream and the alkaline solution and carrying out a gas-liquid reactive precipitation (102, 202); iii. obtaining hydrogen carbonate and / or carbonate, preferably

[0029] Hydrogen carbonate, as a solid; iv. Providing an acidic solution and contacting the acidic solution with the solid hydrogen carbonate and / or carbonate obtained in step iii. for reactive desorption (107, 207) and release of CO2 as the initial gas stream (108, 208), wherein the alkaline solution in step i. and the acidic solution in step iv. are reacted by means of an electrochemical cell (125, 225) containing a

[0030] Cation exchange membrane (110, 210) are provided and the acidic solution is obtained in the anode chamber (124, 224) and the alkaline solution in the cathode chamber (123, 223), wherein the electrochemical cell (125, 225) is configured such that the hydrogen obtained at the cathode is directed to the anode (119, 219) and oxidized there by means of a gas diffusion electrode, and preferably wherein the acidic and alkaline solutions contain precipitating agents, and preferably wherein the electrochemical cell (125, 225) is an electrolysis cell in which the hydrogen oxidation reaction takes place at the anode and the hydrogen evolution reaction takes place at the cathode. The process according to the invention can be described by way of example with reference to Figure 1. A CO2-containing input stream (101) is directed into a reactor for gas-liquid reactive precipitation (102). An alkaline solution is also added to these via the further input stream (114).The alkaline solution has a pH value of preferably 7 to 15, preferably 8 to 14.

[0031] In gas-liquid reactive precipitation, the CO2 contained in the feed gas dissolves in the alkaline solution and subsequently reacts with water to form carbonic acid (hydration) or with hydroxide ions to form bicarbonate (hydroxylation). If a dissolved amine is present in the electrolyte, this can also occur via the formation of carbamic acid followed by deprotonation to carbamate and subsequent hydrolysis of the carbamic acid to carbonic acid or hydrolysis of the carbamate to bicarbonate. Depending on the pH, either bicarbonate or carbonate is formed predominantly. The electrolyte contains precipitating agents that react with bicarbonate or carbonate to form a salt with a low solubility product (solubility of the bicarbonate or carbonate salt preferably < 10 g / L), resulting in the precipitation of the bicarbonate or carbonate as a salt immediately after its formation.The majority of the hydrogen carbonate and carbonate formed is subsequently in the solid phase. The precipitating agents produce hydrogen carbonate and carbonate solubilities of preferably < 10 g / L. The present invention also includes conditions in which a residue of the precipitating agent or of the component to be precipitated remains unreacted in the solution.

[0032] Within the scope of the present invention, carbonate, hydrogen carbonate or a mixture thereof can be present during precipitation.

[0033] Precipitating agents may preferably be species from the groups of aliphatic and cyclic amines, aminosilanes, imines, guanidines, amino acids, aminosiloxanes, amides, and alkaline earth metals. Furthermore, species that form a low solubility product with hydrogen carbonate and carbonate may preferably be present.

[0034] In the process according to the invention, the formation of solid hydrogen carbonate is preferred, since only one hydroxide ion is consumed per separated CO2, thus making the overall process more energy-efficient.

[0035] The product obtained by gas-liquid reactive precipitation is a suspension consisting of a solid and a liquid phase. The suspension can be recycled back into the apparatus. The recycled stream can be in the form of a suspension. Alternatively, after separation or partial separation of the solid phase, preferably only the aqueous phase can be recycled.

[0036] The solid and liquid phases are subsequently separated in a solid-liquid separator (105). The liquid phase, which is fed as an input current (121) into the anode chamber (124) of the electrochemical cell (125), has a pH value of preferably 5 to 13. The solid phase is fed via the current (106) to the device for reactive desorption (107).

[0037] Furthermore, an acidic solution is obtained in the anode chamber (124) of the electrochemical cell (125) and introduced into the reactive desorption apparatus (107) via the current (120). The current (120) has a pH value preferably of -1 to 6. The acidic solution dissolves the hydrogen carbonate or carbonate, which is initially present as a solid. The driving force for the dissolution results from the protonation of dissolved hydrogen carbonate and carbonate ions, so that carbonic acid is preferably formed, which then preferably reacts to form CO2 by dehydration with the elimination of water. As soon as the CO2 exceeds its solubility in the aqueous phase, it begins to transition into the gaseous phase and can be discharged at the top of the apparatus. The CO2 content in the output gas stream (108) is higher than the CO2 content in the input stream (101). The input current (109) into the cathode chamber (123) has a pH value preferably of 0 to 7.

[0038] The pH driving forces for gas-liquid reactive precipitation (102) and reactive desorption (107) are generated by pH-switching electrolysis. This can alternatively be referred to as electrochemical pH shift or electrochemical pH swing. The electrochemical cell (125) contains an anode (124) and a cathode chamber (123), which are separated from each other by a cation exchange membrane (110).

[0039] Within the scope of the present invention, the presence of a cation exchange membrane is essential, as such a membrane exhibits high long-term stability and robustness, for example, compared to anion exchange membranes. Preferably, within the scope of the present invention, the material of the cation exchange membrane is selected from the group consisting of persulfonic acid-based membranes.

[0040] In the present invention, a gas diffusion electrode containing a noble metal catalyst (e.g., platinum-, iridium-, or ruthenium-based catalyst) is preferably used as the anode. A catalyst made of nickel or a nickel alloy is preferably used at the cathode. The anode and the cathode are preferably contacted by current collectors.

[0041] The alkaline solution, which is introduced into the gas-liquid reactive precipitation reactor (102), is provided in the cathode chamber (123). The output stream (112) from the cathode chamber (123) is separated into a liquid and a gas phase via a gas-liquid separator (113), and the liquid phase is introduced as the input stream (114) into the reactor for the gas-liquid reactive precipitation reactor (102). The previously separated gas phase, containing hydrogen, is fed via stream (115) to a gas purification unit (117), and excess water and impurities are removed via stream (116).

[0042] Preferably a gas purification step is included, since the electrochemical cell according to the present invention requires a high purity, preferably > 80 vol.%, preferably > 99 vol.%, particularly preferably > 99.9 vol.% of the hydrogen.

[0043] The purified hydrogen can be temporarily stored in a reservoir (118) and is then supplied to the electrochemical cell (125) on the anode side. At the anode (119), the hydrogen is converted into protons (H₂) by a gas diffusion electrode. +The protons accumulate in the aqueous phase of the anode chamber (anolyte) (124), leading to acidification of the anolyte. Optionally, the gas diffusion electrode can be equipped with an additional cation exchange membrane to protect components in the aqueous solution, with the exception of H₂, that are prone to electrochemical oxidation. Furthermore, the gas diffusion electrode can optionally be provided with a hydrophobic protective layer to prevent breakthrough of the aqueous phase to the gas side. At the cathode (111), water reacts to form hydrogen and hydroxide ions (OH⁻). The accumulation of hydroxide ions leads to alkalization of the aqueous phase in the cathode chamber (catholyte) (123). In order for an accumulation of protons in the anode chamber or of hydroxide ions in the cathode chamber, additional cations are present in the electrolyte (e.g. sodium ions (Na⁻). + ) or potassium ions (K +)), which are dissolved therein by means of a salt (e.g., Na₂SO₄). During operation, these cations migrate from the anode chamber (124) via the cation exchange membrane (110) to the cathode chamber (123) to maintain the charge balance in both cell chambers. It follows that, unlike in conventional water electrolysis, the charge balance in both electrochemical chambers is not primarily maintained by the migration of protons or hydroxide ions, but primarily by the migration of cations from a salt previously dissolved in the electrolyte. This leads to the accumulation of protons in the anode chamber and hydroxide ions in the cathode chamber. The current (120) leaving the anode chamber can be partially recycled and mixed with the aqueous phase coming from the solid-liquid separator or the anode input current (121).Likewise, the current (112) leaving the cathode chamber can be partially recycled and mixed with the aqueous phase from the reactive desorption or the cathode inlet current (109). Recycling of the current (112) leaving the cathode chamber can also occur after the gas-liquid separator (113) in the form of a partial recycling of the current (115) leaving the gas-liquid separator.

[0044] Another possible process configuration is shown by way of example in Figure 2. A CO2-containing input gas stream (201) is fed into a gas-liquid reactive precipitation apparatus (202), where hydrogen carbonate or carbonate, preferably hydrogen carbonate, is precipitated as described. The CO2-depleted stream (203) is then released. In contrast to the process schematically shown in Figure 1, the process configuration shown in Figure 2 does not include a solid-liquid separator, and the resulting suspension is fed directly as an input stream (204) into the reactive desorption apparatus (207). For reactive desorption, an acidic solution, which is obtained in the anode chamber (224) of an electrochemical cell (225), is supplied as an input stream (221), and the CO2-enriched gas stream is released as an output gas stream (208).The aqueous phase obtained after the release of CO2 is supplied as a current (209) to the electrochemical cell on both the anode side (209a) and the cathode side (209b). In the cathode chamber (223), the catholyte is alkalized and supplied as an alkaline solution to the gas-liquid reactive precipitation as an input current (214). A gas-liquid separator (213) is used to separate the hydrogen produced in the cathode chamber. The hydrogen is fed to a gas purification unit (217) to remove water and impurities (216). The hydrogen obtained is then temporarily stored (218) and subsequently supplied to the anode side of the electrochemical cell (225) and oxidized to protons. Parts of the outflowing anode (221) or cathode current (212, 215) can preferably be fed back to the electrochemical cell as partial currents at the anode (209a) and cathode side (209b) respectively, in order to efficiently acidify the anolyte orTo achieve alkalization of the catholyte.

[0045] The process according to the present invention is characterized by the combination of the described electrochemical cell with gas-liquid reactive precipitation and reactive desorption, resulting in a very small pH difference between the anode and cathode chambers. This leads to significantly reduced open-circuit voltages compared to prior art processes and thus allows for considerably more energy-efficient operation. If the oxygen evolution reaction (OER) is used for the electrochemical generation of protons and the hydrogen evolution reaction (HER) for the generation of hydroxide ions, a minimum thermodynamic cell voltage of approximately 1.23 V is obtained for electrolysis operation. To further reduce this minimum thermodynamic cell voltage, the hydrogen oxidation reaction (HÖR) is used instead of the OER according to the present invention.This reduces the minimum thermodynamic cell voltage to 0 V when the pH values ​​in the anode and cathode chambers are the same, since the same electrochemical reaction takes place at both electrodes, only in opposite directions. In both cases, the minimum cell voltage increases as the pH values ​​differ between the two chambers.

[0046] A small difference in pH values ​​is preferable within the scope of the present invention in order to achieve high energy efficiency. The hydrogen consumed during the HÖR (hydrogen hysteresis) is generated in parallel in the cathode chamber (123, 223) and, after separation from the electrolyte and subsequent purification, supplied to the anode chamber (124, 224).

[0047] The following reaction equations result for the described reactions at the anode and cathode: OER: 2 H₂O -> 4H + + O2+ 4e-

[0048] HER: 4H2O + 4e~^ 4OH“ + 2H2

[0049] LISTEN: 2H2->• 4H + + 4e'

[0050] The present invention leads to an improved decoupling of energy efficiency, kinetics and capacity of a carbon dioxide capture plant, so that these can be scaled significantly independently of each other.

[0051] A further advantage is that the inventive method does not rely on heat, but only requires electrical energy for the pH swing process. This makes it suitable for use in conjunction with sustainably generated wind or solar energy.

[0052] The advantage of the process according to the invention is that a high driving force and high kinetics of CO2 absorption are maintained, since only a small amount of bicarbonate or carbonate, preferably bicarbonate, accumulates in the aqueous phase and is therefore removed from the equilibrium of the CO2-equivalent species by means of precipitation.

[0053] The advantage of the process according to the invention is that in gas-liquid reactive precipitation only one hydroxide ion is required for the precipitation of the hydrogen carbonate, thus reducing the power requirement for the electrochemical reaction, which reduces the electrical energy requirement compared to processes known from the prior art.

[0054] Preferably, the gas-liquid reactive precipitation (102, 202) in step ii. of the process according to the invention takes place in at least one reactor. Carrying out the gas-liquid reactive precipitation in several reactors, preferably connected in series, optionally with intermediate solid-liquid separators, is also possible. Furthermore, it is possible for the precipitation reaction and the separation to take place in a single reactor.

[0055] Preferably, the process according to the invention proceeds continuously, and the liquid reaction mixture obtained in step iv. is fed as an input stream (109, 209b) into the cathode chamber (123, 223) of the electrochemical cell (125, 225) after the release of CO2 as a gas phase in the initial gas stream (108, 208). Following the process flow diagram in Figure 2, this liquid reaction mixture can also be fed to the anode chamber (209a). Preferably, the input stream (109, 209) has a pH value between 0 and 7.

[0056] Preferably, in the context of the process according to the invention, the gas-liquid reactive precipitation (102, 202) in step ii. is carried out via a spray tower, a bubble column and / or a packed column, preferably via a spray tower.

[0057] Here, the alkaline solution is fed into the top of the tower and sprayed. The CO2-containing inlet gas stream (101, 201) enters above the bottom of the tower and flows against the downward flow of the alkaline solution. In the gas-liquid contact zone, CO2 dissolves in the alkaline solution and reacts to form hydrogen carbonate and carbonate. A cation dissolved in the alkaline solution has a low solubility product with hydrogen carbonate or carbonate. As soon as the ion product exceeds the solubility product of the species, solid hydrogen carbonate or carbonate species precipitate. The suspension collects in the bottom and is then optionally fed to a solid-liquid separation unit (105). The CO2-depleted stream (103, 203) exits at the top of the tower. During absorption and precipitation, the pH of the alkaline solution decreases continuously downwards along the column or tower, as one hydroxide ion is formed for each hydrogen carbonate produced.Two hydroxide ions are consumed for each carbonate formed. Similarly, the CG2 concentration of the counterflowing gas stream decreases continuously upwards along the column or tower. Only species that exhibit low solubilities with hydrogen carbonate or carbonate, preferably below 10 g / L, are suitable as cations or precipitating agents for hydrogen carbonate or carbonate.

[0058] As already described, the proportion of CO2 in the output gas stream (108, 208) is higher than in the input gas stream (101, 201). Preferably, the CO2 proportion in the input gas stream (101, 201) is 0.02 vol% to 30 vol%, more preferably 0.04 vol% to 15 vol%, and in the gas stream obtained in step iv., more than 50 vol%, more preferably more than 95 vol%, and particularly preferably more than 99 vol%, based on the total volume of the gas stream.

[0059] The initial gas stream consists primarily of gaseous CO2 and gaseous H2O. In addition to H2O, it may also contain small amounts of other impurities. Preferably, the initial gas stream (108, 208) is subjected to purification to remove water and other impurities, thereby meeting the necessary purity requirements for subsequent processes or for discharge into a CO2 gas network. Furthermore, compression of the gas may also be necessary. Compared to purely absorptive processes, precipitation offers the advantage of an additional integrated processing step, which reduces impurities in the CO2-concentrated initial gas stream (108, 208).

[0060] Reactive desorption is preferably carried out in a continuously operated stirred tank reactor. Hydrogen carbonate or carbonate, preferably hydrogen carbonate, is fed into the reactor as a solid. An acidic solution is also added as described. The solids dissolve in the acidic solution, and the dissolved hydrogen carbonate or carbonate reacts with the protons in the acidic solution to form CO₂, which is released as the output gas stream. Since protons are consumed during the reaction, the reactive desorption process leads to an increase in the pH value of the aqueous phase. A backpressure valve at the gas outlet of the stirred tank reactor regulates the gas pressure. This allows CO₂ to be supplied at the desired gas pressure, depending on the optional subsequent process. The reactive desorption process can also be interpreted as a chemical compressor.

[0061] Preferably, the electrochemical cell (125, 225) is an electrolysis cell in which the hydrogen oxidation reaction takes place at the anode and the hydrogen evolution reaction takes place at the cathode. The electrochemical cell is particularly preferably configured as part of an electrolysis stack.

[0062] A schematic diagram of a cell stack is shown in Figure 3. A cell stack consists of a plurality of electrochemical cells (325), preferably connected in series. Each of these cells contains an anode (319), a cathode (311), an anode chamber (324), a cathode chamber (323), and a cation exchange membrane (310). During the execution of the process according to the invention, an aqueous solution (catholyte) (309), preferably uniformly distributed, is introduced into the cathode chambers (323) of the electrochemical cells and alkalized there. This is used as alkalized catholyte (312) or as an alkaline solution in the process according to the invention for gas-liquid reactive precipitation. An aqueous solution (anolyte) (321), preferably uniformly distributed, is introduced into the anode chambers (324) of the electrochemical cells and acidified there. The acidified anolyte (320) orThe acidic solution is then used for reactive desorption in the process according to the invention. A cell stack preferably comprises end plates with flow distributors (330) and preferably at least one bipolar plate with flow distributors (331).

[0063] The electrical contact between two cells is preferably achieved by two short-circuited monopolar plates or by a bipolar plate. A flow distributor is preferably used between the cells to supply hydrogen to the anode. This flow distributor is preferably integrated into the bipolar plate. Furthermore, the cells can be equipped with current collectors to ensure a low-resistance electrical supply to the electrodes. Anolyte and catholyte are distributed as evenly as possible across all cells.

[0064] Another possible configuration of a cell stack within the scope of the present invention is the "zero-gap configuration." In this configuration, the cation exchange membrane is connected to the cathode, and a porous electrode is used as the cathode. This is also referred to as a "membrane-electrode assembly." In such a configuration, an additional flow distributor can be provided between the cells for the supply of aqueous solution (catholyte) and the removal of generated hydrogen. In this configuration, both sides of the bipolar plate are equipped with flow distributors.

[0065] Another aspect of the present invention relates to a system (100, 200) for the recovery, preferably for the processing and / or concentration of CO2 from CO2-containing gas streams, comprising an electrochemical cell (125, 225) with a cation exchange membrane (110, 210), at least one device for gas-liquid reactive precipitation (102, 202) and at least one

[0066] Device for reactive desorption (107, 207), wherein the devices are arranged such that a CO2-containing

[0067] The incoming gas stream (101, 201) is first subjected to a gas-liquid reactive precipitation (102, 202) and then preferably in solid form as

[0068] Hydrogen carbonate and / or carbonate bound CO2 is released as an output gas stream (108, 208) in a reactive desorption device (107, 207).

[0069] Preferably, the system according to the invention comprises a gas-liquid separator (113, 213) and preferably a solid-liquid separator (105), preferably wherein the gas-liquid separator (113, 213) is selected from the group consisting of centrifuge, free-fall separator, impactor, deflection separator, centrifugal separator and / or the solid-liquid separator (105) is selected from the group consisting of filtration device, centrifuge, centrifugal separator and sedimentation device, particularly preferably from the group consisting of filter press, belt filter, candle filter, drum filter, disc filter, belt filter press and cross-flow filtration.

[0070] Furthermore, the electrochemical cell (125, 225) preferably has a gas diffusion electrode at the anode (119, 219).

[0071] The statements made herein regarding the method according to the invention also apply to the system according to the invention. The invention is subsequently characterized by selected, non-limiting examples.

[0072] Examples

[0073] Example 1: Gas-liquid reactive precipitation

[0074] Gas-liquid reactive precipitation was carried out in a spray tower approximately 1.2 m high and 32 mm in diameter. A gas mixture containing approximately 15 vol% carbon dioxide was continuously fed in just above the sump. The remainder of the gas consisted primarily of nitrogen. The gas flow rate ranged from 0.5 to 8.0 SLPM (standard liters per minute) and was pre-saturated with water using a gas washing bottle. The alkalized solution containing the precipitant was continuously and evenly distributed across the cross-sectional area of ​​the spray tower head, thus contacting the upward-flowing gas in a countercurrent flow. The volumetric flow rate was 144 mL per minute. For the liquid phase, a species from the alkaline earth metal class was used as the precipitant / cation (0.05 mol hydroxide ion equivalent per L).The CO2 content in the CO2-depleted phase is plotted in Figure 4 for this class of substances as different ratios of added hydroxide ion equivalents to added CO2. The values ​​shown in Figure 4 were recorded after reaching steady-state flow.

[0075] Example 2: Electrochemical acidification or alkalization

[0076] Electrochemical acidification and alkalization were tested using a custom-constructed electrochemical cell. According to the process flow diagram in Figure 2, the anode and cathode chambers were supplied with the same electrolyte (approximately 1 nmol sodium sulfate per liter). A gas diffusion electrode with a platinum-based catalyst was used as the anode. The cathode was an electrode with a nickel-based catalyst. The anode was additionally equipped with a titanium-based current collector, and the cathode with a nickel-based current collector. The membrane was a cation exchange membrane with a thickness of approximately 110–130 pm and a maximum surface resistance of 0.8 Ω·m. 2The cell frame and spacers were made of polypropylene or materials with good chemical stability against acids and bases. The cell had a width of 200 mm, a height of 410 mm, and a depth of 75 mm. The geometric electrode area was approximately 100 cm². 2The galvanostatic electrolysis process was implemented using a potentiostat. During operation, hydrogen was supplied to the gas diffusion electrode (anode). This hydrogen was then electrochemically oxidized to protons at the same electrode. The hydrogen supply was implemented via a cross-flow configuration. The anode and cathode chambers were each fed with a volumetric flow rate of approximately 6 mL per minute. The liquid phase had a residence time of approximately 10 minutes in each chamber. The pH of the incoming liquid phase was approximately 6 to 9. Figure 5 shows the pH values ​​of the outgoing flows for different current densities. It is evident that with increasing current densities, the number of protons in the anolyte and the number of hydroxide ions in the catholyte increase. Thus, the anolyte becomes more acidic and the catholyte more alkaline. The values ​​shown in Figure 5 were recorded after reaching steady state.

[0077] Example 3: Reactive Desorption

[0078] Reactive desorption was investigated in a continuously operated stirred tank reactor. The system was operated at 1 bar and 25 °C. The solid and the acidic solution were continuously fed into the reactor. The solid was a carbonate salt from the alkaline earth metal class. The liquid phase contained hydrochloric acid. The continuously discharged CO₂-rich gas was in equilibrium with the equally continuously discharged liquid stream. Figure 6 plots the percentage of CO₂ equivalents of the added solid that were discharged in the CO₂-rich gas against different concentrations of hydrochloric acid in the incoming liquid phase. It is evident that the amount of CO₂ recovered from the solid increases with increasing hydrochloric acid concentration.Furthermore, Figure 7 shows the molar fraction of CO2 in the discharged gas (in mol%) as a function of different concentrations of hydrochloric acid in the incoming liquid phase. This shows a behavior that is hardly dependent on the hydrochloric acid concentration.

Claims

Claims 1. A process for obtaining, preferably processing and / or concentrating, CO2 from CO2-containing gas streams, comprising the steps of: i. providing a CO2-containing input gas stream (101, 201) and an alkaline solution; ii. contacting the gas stream and the alkaline solution and carrying out a gas-liquid reactive precipitation (102, 202); iii. obtaining hydrogen carbonate and / or carbonate, preferably hydrogen carbonate, as a solid; iv. providing an acidic solution and contacting the acidic solution with the solid hydrogen carbonate and / or carbonate obtained in step iii. for reactive desorption (107, 207) and release of CO2 as an output gas stream (108, 208), wherein the alkaline solution in step i. and the acidic solution in step iv.by means of an electrochemical cell (125, 225) containing a cation exchange membrane (110, 210) and the acidic solution is obtained in the anode chamber (124, 224) and the alkaline solution in the cathode chamber (123, 223), wherein the electrochemical cell (125, 225) is configured such that the hydrogen obtained at the cathode is directed to the anode (119, 219) and is oxidized there by means of a gas diffusion electrode, wherein the acidic and alkaline solutions contain precipitating agents, and wherein the electrochemical cell (125, 225) is an electrolysis cell in which the hydrogen oxidation reaction takes place at the anode and the hydrogen evolution reaction takes place at the cathode.

2. Method according to claim 1, wherein the electrochemical cell is part of an electrolysis stack.

3. Method according to claim 1 or 2, wherein the gas-liquid reactive precipitation (102, 202) in step ii. takes place in at least one reactor.

4. Method according to one of the preceding claims, wherein the precipitating agent is selected from the group of substances which have a solubility of less than 10 g / L with hydrogen carbonate or carbonate.

5. Method according to any of the preceding claims, wherein the precipitating agent is selected from the group consisting of aliphatic and cyclic amines, aminosilanes, imines, guanidines, amino acids, aminosiloxanes, amides and alkaline earth metals.

6. Method according to one of the preceding claims, wherein the method is continuous and the liquid reaction mixture obtained in step iv. is fed into the cathode chamber (123, 223) of the electrochemical cell (125, 225) as input stream (109, 209b) after the release of CO2 as output gas stream (108, 208).

7. Method according to one of the preceding claims, wherein the gas-liquid reactive precipitation (102, 202) in step ii. is carried out via a spray tower, a bubble column and / or a packed column, preferably via a spray tower.

8. Method according to one of the preceding claims, wherein the CO2 content in the provided input gas stream (101, 201) is 0.02 vol% to 30 vol%, preferably 0.04 vol% to 15 vol%, and in the gas stream obtained in step iv. is more than 50 vol%, preferably more than 95 vol% and particularly preferably more than 98 vol%, particularly preferably more than 99 vol%, based on the total amount of the gas stream.

9. System (100, 200) for the recovery, preferably for the processing and / or concentration of CO2 from CO2-containing gas streams, comprising at least one electrochemical cell (125, 225) with a Cation exchange membrane (110, 210), at least one device for gas-liquid reactive precipitation (102, 202) and at least one device for reactive desorption (107, 207), wherein the devices are arranged such that a CO2-containing input gas stream (101, 201) is first subjected to gas-liquid reactive precipitation (102, 202) and subsequently the CO2 bound in solid form as hydrogen carbonate and / or carbonate is released as an output gas stream (108, 208) in a device for reactive desorption (107, 207), wherein the electrochemical cell (125, 225) has a gas diffusion electrode at the anode (119, 219).

10. System according to claim 9, wherein the device for gas-liquid reactive precipitation (102, 202) is selected from spray tower, bubble column and packed column, preferably wherein the device for gas-liquid reactive precipitation is a spray tower.

11. System according to claim 9 or 10 additionally comprising a gas-liquid separator (113, 213).

12. System according to claim 11, wherein the gas-liquid separator (113, 213) is selected from the group consisting of centrifuge, free-fall separator, impactor, deflection separator or centrifugal separator.

13. System according to any one of claims 9 to 12, further comprising a solid-liquid separator (105).

14. System according to claim 13, wherein the solid-liquid separator (105) is selected from the group consisting of filtration device, centrifuge, centrifugal separator and sedimentation device, preferably from the group consisting of filter press, belt filter, candle filter, drum filter, disc filter, belt filter press and cross-flow filtration.

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