Enrichment of carbon dioxide in gas streams by pressure swing adsorption

Abstract

Systems and methods (e.g., methods using the systems) for enriching carbon dioxide in a source gas. Systems can comprise pressure swing adsorption (PSA) units comprising adsorbent beds configured to adsorb carbon dioxide from a source gas comprising carbon dioxide; vacuum pumps configured to desorb carbon dioxide from the adsorbent bed and remove a carbon dioxide-rich stream; and diverters configured to divert percentages of the carbon dioxide-rich streams back to the PSA units, which can expel nitrogen from void spaces of the adsorbent beds.

Description

ENRICHMENT OF CARBON DIOXIDE IN GAS STREAMS BY PRESSURE SWING ADSORPTIONBACKGROUND

[0001] Earth’s atmosphere contains about 414 ppm carbon dioxide (US EPA, “Climate Change Indicators: Atmospheric Concentrations of Greenhouse Gases,” www.epa.gov / climate-indicators / climate-change-indicators-atmospheric-concentrations- greenhouse-gases, last updated November 1, 2023). The carbon dioxide concentration has increased about 50% from an estimated 280 ppm at the start of the Industrial Revolution circa 1750 (Ibid.).

[0002] A major source of carbon dioxide emissions is the combustion of carbon-containing compounds from fossil sources, such as methane, propane, coal, gasoline, and other petroleum products. In 2021, United States power plants emitted about 951 million metric tons of carbon dioxide equivalent (US EPA, “Sources of Greenhouse Gas Emissions,” www.epa.gov / ghgeniissions / sources-greenhouse-gas-emissions, last updated on November 16, 2023).

[0003] Natural gas- and coal-fired power plants provide about 60% of the United States’ electricity (US Energy Information Administration, “Total Energy” (interactive page), www.eia.gov / totalenergy / data / browser / inde .php?). There is an interest in capturing carbon dioxide before it is emitted from natural gas- and coal-fired power plants and sequestering it, e.g., in long-term subterranean storage.

[0004] Elowever, optimal geological sequestration of carbon dioxide may utilize carbon dioxide concentrations of 90 mol% or more. Typical flue gas emissions from power plants are much less, typically 8-12 mol% carbon dioxide. Even other industrial gas streams rarely comprise more than 40 mol% carbon dioxide. Accordingly, enrichment of carbon dioxide from power plant flue gases and similar sources is desirable.

[0005] Amine scrubbing is currently the leading technique for carbon dioxide enrichment. Typically, gas streams containing 8-12 mol% carbon dioxide are contacted with a monoethanolamine solution. Purified carbon dioxide can be removed from the solution by heat. However, amine scrubbing utilizes large amounts of solvent that are degraded during use, and utilizes complex operations to achieve levels of carbon dioxide enrichment suitable for sequestration.

[0006] There is a need in the art for techniques for carbon dioxide enrichment that do not utilize large volumes of solvent, do not generate degraded waste solvent, and are relatively straightforward to perform.SUMMARY

[0007] In various aspects, the present disclosure provides systems for enriching carbon dioxide in nitrogen-containing source gases, e.g., flue gases.

[0008] In one aspect, the disclosure provides a system comprising a pressure swing adsorption (PSA) unit comprising an adsorbent bed configured to adsorb carbon dioxide from a source gas comprising carbon dioxide and nitrogen; a vacuum pump configured to desorb the carbon dioxide from the adsorbent bed and remove a carbon dioxide-rich stream via the carbon dioxide outlet; and diverter configured to divert a percentage of the carbon dioxiderich stream to the PSA unit.

[0009] In another aspect, the disclosure provides a system comprising means for adsorbing carbon dioxide from a source gas comprising carbon dioxide; means for desorbing the carbon dioxide to yield a carbon dioxide-rich stream; and means for diverting a percentage of the carbon dioxide-rich stream to the means for adsorbing.

[0010] The systems make use of pressure swing adsorption (PSA) units comprising adsorbent beds which adsorb carbon dioxide. Desorption of carbon dioxide from the PSA unit yields a carbon dioxide-rich stream.

[0011] Some amount of nitrogen can remain in void spaces of the adsorbent beds. By using a portion of the carbon dioxide-rich stream as a rinse, nitrogen can be expelled from the void spaces and the carbon dioxide concentration of the carbon dioxide-rich stream can be increased.

[0012] Systems for enriching carbon dioxide from source gases according to the present disclosure are described herein.

[0013] In further aspects, the present disclosure provides methods for carbon dioxide enrichment.

[0014] Methods according to the present disclosure for enriching carbon dioxide, e.g., using the systems of the present disclosure, are described herein.BRIEF DESCRIPTION OF THE FIGURES

[0015] FIG. 1 schematically represents an exemplary system of the present disclosure suitable for enrichment of carbon dioxide from source gases predominantly comprising carbon dioxide and nitrogen. Solid lines represent gas flows; dashed-dot-dashed lines represent liquid flows; and dashed lines represent data flows.

[0016] FIG. 2 schematically represents an exemplary system of the present disclosure suitable for enrichment of carbon dioxide from source gases predominantly comprising carbon dioxide, nitrogen, and a third gas, in which nitrogen is separated from both carbon dioxide and the third gas, followed by separation of carbon dioxide from the third gas.

[0017] FIG. 3 schematically represents an exemplary system of the present disclosure suitable for enrichment of carbon dioxide from source gases predominantly comprising carbon dioxide, nitrogen, and a third gas, in which nitrogen and the third gas are separated from carbon dioxide.DETAILED DESCRIPTIONSystems for carbon dioxide enrichment

[0018] In some aspects, the present disclosure provides systems for enriching carbon dioxide in a source gas.

[0019] Source gases can include mixtures of nitrogen and carbon dioxide. Source gases can further comprise water vapor. Source gases can further comprise other molecules, such as oxygen, hydrogen, and / or methane. Source gases can be generated from combustion of coal, methane, or other organic material (e.g., other organic compounds such as hydrocarbons). For example, source gases can be flue gases from power plants. Flue gases typically have carbon dioxide concentrations of 8-12 mol%. However, flue gases are not limited to this range of carbon dioxide concentrations. Further, source gases are not limited to flue gases. Source gases can be any gas streams comprising nitrogen, and carbon dioxide at concentrations below 90 mol%. In some embodiments, source gases can comprise carbon dioxide at concentrations of 12-20 mol%. In some embodiments, source gases can comprise carbon dioxide at concentrations of 20-30 mol%. In some embodiments, source gases can comprise carbon dioxide at concentrations of 30-40 mol%. In some embodiments, source gases can comprise carbon dioxide at concentrations of 40 mol%-50 mol%. In some embodiments, source gases can have carbon dioxide concentrations of 20% to 40 mol%, or 30 mol% to 50 mol%.

[0020] Systems of the disclosure can comprise PSA units comprising adsorbent beds. Adsorbent beds can be in fluid communication with source gas inlets (e.g., inlets for gas streams containing carbon dioxide at relatively low concentrations, nitrogen, and in some embodiments, other gases), rinse gas inlets, reject gas outlets (e.g., outlets for reject gases containing nitrogen and, in some embodiments, other gases, with carbon dioxide being present at lower concentrations than in source gases), and carbon dioxide outlets (e.g. , outlets for carbon dioxide-rich streams containing carbon dioxide at relatively high concentrations). The various inlets, outlets, and gas streams are described in more detail below.

[0021] Adsorbent beds can be configured to adsorb carbon dioxide from source gases comprising carbon dioxide and nitrogen, and allow nitrogen-rich streams to exit via reject gas outlets. Known adsorbents, including but not limited to activated carbon adsorbents and silicalite adsorbents, have greater adsorption affinities for carbon dioxide than for nitrogen and O2. These greater adsorption affinities hold whether the adsorbents are equilibrium adsorbents (e.g., adsorbents which more strongly adsorb carbon dioxide than nitrogen or O2) or rate-based adsorbents.

[0022] Adsorbent beds can make use of various types of adsorbent materials, e.g., activated carbon (e.g., molecular sieved activated carbon (“MSC”), crushed activated carbon (“AC-1”), granular activated carbon (“AC-2”), pelleted activated carbon, or combinations of two or more thereof), silicalites, zeolites, such as crystalline aluminosilicate zeolite or a high aluminum X zeolite (e.g., zeolites having a silicon-to-aluminum ratio of about one (i.e., 1:1 ± 25% or 1: 1 ± 10%)), or amorphous adsorbents (e.g., silica gel or carbon), among others.

[0023] When hydrophilic adsorbents are used, water will tend to co-adsorb with carbon dioxide. To limit water co-adsorption, hydrophobic adsorbents can be used, and / or water vapor can be separated from source gases as described elsewhere herein.

[0024] Adsorbent beds can comprise multiple layers of adsorbents, such as from two to ten layers, such as of different materials described above. In some embodiments, adsorbent beds comprise two layers. In some embodiments, adsorbent beds comprise three layers. In some embodiments, adsorbent beds comprise 4-10 layers or subranges thereof.

[0025] Multiple layers of adsorbent beds can be disposed in any orientation, e.g., vertically (stacked together) or horizontally. Multiple layers can be disposed such that multiple layers are disposed between source gas inlets and / or rinse gas inlets on the one hand and reject gas outlets and / or carbon dioxide outlets on the other. For example, if an inlet is at the bottom ofa unit and an outlet is at the top of the unit, the layers can be vertically oriented. The layers can be discrete, having one adsorbent material per layer, or can be gradients from a region of one adsorbent material to another. A discrete layer may include some portion of an adsorbent material from an adjoining layer, arising from interactions between the layers during initial loading and / or during operation of the unit. The masses and / or volumes of each of multiple layers can be the same or can vary.

[0026] In some embodiments, the adsorbent beds comprise two layers, with the mass and / or volume ratio of the layers in a range from 1 :9 to 9: 1. For example, the mass and / or volume ratio of the two layers can be 1: 1 ± 20% or 1 : 1 ± 10%. In some embodiments, the adsorbent beds comprise three layers, with the mass and / or volume ratio of the layers in a range from 1:1:18 to 1 : 18: 1 to 18: 1: 1. For example, the mass and / or volume ratio of the three layers can be 1 : 1: 1 ± 20% or 1: 1 :1 ± 10%.

[0027] Independently of the number of layers of adsorbents, if adsorbent beds comprise two or more layers, the layers can be arranged so that each pair of adjacent layers comprises or consists of different materials, e.g., a first material in a first layer and a second material in a second, adjacent layer. If a third layer is included and is adjacent to the second layer but not the first, the third layer can comprise or consist of a third material or the first material, and the like holds for a fourth layer, fifth layer, etc., if such are included.

[0028] Independently of the number of layers in any adsorbent bed, in some embodiments, PSA units can comprise one, two, three, four, or more (e.g., 5 to 10) adsorbent beds. Thus, in some embodiments, PSA units comprise one adsorbent bed. In some embodiments, PSA units comprise two adsorbent beds. In some embodiments, PSA units comprise three adsorbent beds. In some embodiments, PSA units comprise from 4 to 10 adsorbent beds.

[0029] Multiple adsorbent beds in a single PSA unit can be the same or can differ from one another in number of adsorbent layers, adsorbent material(s) used, and / or other parameters. Multiple adsorbent beds within a single PSA unit can be used for a common subprocess, e.g., multiple adsorbent beds in a given PSA unit can be used for adsorbing carbon dioxide, passing nitrogen-enriched reject gas streams to reject gas outlets, and desorbing carbon dioxide to a carbon dioxide-enriched stream. Adsorbent beds can perform different aspects of a subprocess at different times. Continuing the example, one adsorbent bed of a PSA unit can adsorb carbon dioxide and pass nitrogen-enriched reject gas streams, while an additional adsorbent bed of the PSA unit can desorb carbon dioxide to carbon dioxide-rich streams.

[0030] Various properties of PSA units, such as dimensions, shape, and volume, number of adsorbent beds in each PSA unit, and number of adsorbent layers in each adsorbent bed, and bed and layer dimensions, shapes, and volumes can each independently be selected based on the source gases, the desired throughputs, and other parameters.

[0031] As illustrated in FIG. 1 , an exemplary carbon dioxide enrichment system 100 is operable to adsorb carbon dioxide from a source gas, such as a flue gas containing roughly 8- 12 mol% carbon dioxide with the balance being substantially nitrogen, the source entering from a gas source 102.

[0032] Prior to adsorption of carbon dioxide in a PSA unit 130, the source gas can be subjected to one or more treatments. If the source gas provided by gas source 102 is at too high a temperature to be adequately adsorbed / d esorbed in the PSA unit 130, the source gas can be cooled in a cooling unit 110, such as an air- fan cooling unit, in which a propeller drives ambient air over a heat exchanger, or a gas-to-liquid heat exchanger. Cooling in the cooling unit 110 can be desirable when the source gas is a flue gas, which typically has a temperature of about 1600°F. Regardless of the temperature of the source gas provided by the gas source 102, in some embodiments, the source gas is cooled to 150°F or less (e.g., 150°F, 140°F, 130°F, 120°F, 110°F, 100°F, 90°F, 80°F, 70°F, 60°F, 50°F, 40°F, or a temperature in a range bounded by any of the foregoing values such as 40°F to 150°F, 40°F to 140°F , 40°F to 130°F , 40°F to 120°F, 40°F to 110°F, 40°F to 100°F, 40°F to 90°F, 50°F to 150°F, 50°F to 140°F , 50°F to 130°F , 50°F to 120°F, 50°F to 110°F, 50°F to 100°F, 50°F to 90°F, 60°F to 150°F, 60°F to 140°F , 60°F to 130°F , 60°F to 120°F, 60°F to 110°F, 60°F to 100°F, 60°F to 90°F, 70°F to 150°F, 70°F to 140°F , 70°F to 130°F , 70°F to 120°F, 70°F to 110°F, 70°F to 100°F, 70°F to 90°F, 80°F to 150°F, 80°F to 140°F , 80°F to 130°F , 80°F to 120°F, 80°F to 110°F, 80°F to 100°F, 80°F to 90°F,90°F to 150°F, 90°F to 140°F , 90°F to 130°F , 90°F to 120°F, 90°F to 110°F, or 90°F to 100°F.

[0033] For source gases at relatively low temperatures, such as 150°F or less, cooling unit 110 (or its use if present) can be omitted. In the system 100, a source gas can be routed around or from cooling unit 110 to a separator 120 or a compressor 125, as described below.

[0034] Regardless of initial temperature, source gases may contain water vapor, up to near or at 100% relative humidity. For example, flue gas typically contains water vapor resulting from the combustion of hydrocarbons. As illustrated in FIG. 1, the source gas can pass through a separator 120, in which water vapor is condensed and passed to condensatehandling 122 (e.g., filtration or other water purification apparatus, evaporation ponds, discharge to municipal water systems, discharge to natural bodies of water, recycle to steam turbines in power plants, or recycle to cooling unit 110 as a heat exchange fluid).

[0035] Independently of whether source gases undergo cooling and / or separation of water vapor, source gases can be compressed prior to passing to adsorbent beds. Compressing source gases may enhance the adsorption of carbon dioxide and / or the rejection of nitrogen, and / or provide other benefits known to persons of ordinary skill in the art. FIG. 1 depicts a compressor 125 configured to receive source gas from a gas source 102, a cooling unit 110, and / or a separator 120.

[0036] The direction of flow of the source gas through the cooling unit 110, separator 120, and compressor 125 shown in FIG. 1 is exemplary. Flow can be organized between the units 110, 120, and / or 125 in any order. Typically, however, separation of condensate (e.g., by separator 120) can be performed after cooling (e.g., by cooling unit 110), because more condensate would be expected at lower temperatures.

[0037] As depicted in FIG. 1 , source gas, which may be pretreated as described above, can enter PSA unit 130 via source gas inlet 131. PSA unit 130 comprises an adsorbent bed 136 (with adsorbents described in more detail elsewhere herein). The adsorbent bed 136 can be pressurized, e.g., with nitrogen, prior to influx of source gas via source gas inlet 131. Pressurization may increase the efficiency of adsorption of carbon dioxide by the adsorbent bed 136.

[0038] During adsorption, carbon dioxide is adsorbed and nitrogen is rejected, passing to reject gas outlet 134 and thence to nitrogen handling 137. The rejected stream can comprise some carbon dioxide. For example, the rejected stream can comprise about 98 mol% nitrogen and 2 mol% carbon dioxide. The carbon dioxide concentration in the rejected stream can be determined using modalities and apparatus (e.g., an NDIR analyzer) described elsewhere herein.

[0039] During carbon dioxide adsorption in PSA unit 130, the temperature can be maintained in a range from 40°F to 150°F, such as 70°F to 120°F.

[0040] The adsorption pressure in the PSA units, such as PSA unit 130, can be maintained in the range of from about one psia (pounds per square inch absolute) to about 200 psia, such as from five to 80 psia. In some embodiments, the adsorption pressure in PSA units, such asPSA unit 130, can be from 20 psia to 200 psia. In some embodiments, the adsorption pressurein PSA units, such as PSA unit 130, can be greater than atmospheric pressure. In some embodiments, the adsorption pressure in PSA units, such as PSA unit 130, can be from atmospheric pressure to 200 psia, such as from atmospheric pressure to 80 psia, or from atmospheric pressure to 20 psia.

[0041] In some embodiments, the reject gas from PSA units, such as PSA unit 130, can be produced by co-current depressurization.

[0042] After adsorption, carbon dioxide is desorbed, recovered, and passed from PSA unit 130 via carbon dioxide outlet 135 as a carbon dioxide-rich stream. Desorption can be at any pressure less than the adsorption pressure, e.g., a pressure from the pressure of the feed gas down to vacuum or near-vacuum, e.g., less than 1.5 psia. In some embodiments, desorption pressures are less than 10 psia (e.g., 5 psia to less than 10 psia, 3 psia to less than 10 psia, 3 psia to 5 psia, or 1.5 psia to 3 psia). In some embodiments, the desorption pressures are less than 5 psia. In some embodiments, the desorption pressures are less than 3 psia. In some embodiments, the desorption pressures are less than 1.0 psia. In some embodiments, desorption pressures are less than 0.5 psia.

[0043] In some embodiments, such as that shown in FIG. 1, systems comprise vacuum pumps configured to desorb carbon dioxide from adsorbent beds and remove carbon dioxiderich streams via carbon dioxide outlets. The vacuum pump can be configured to draw a vacuum, e.g., to a pressure or pressure range described in the preceding paragraph.

[0044] Systems such as system 100 can be configured to desorb carbon dioxide in a countercurrent fashion or a non-countercurrent fashion.

[0045] In some embodiments, the carbon dioxide-rich stream comprises at least 90 mol% carbon dioxide, such as over 95 mol% carbon dioxide, and less than 10 mol%, such as less than 5 mol%, of nitrogen and other gases; and / or the carbon dioxide-rich stream comprises at least 90% by volume carbon dioxide, such as over 95% by volume carbon dioxide, and less than 10% by volume, such as less than 5%, by volume of nitrogen and other gases.

[0046] Achieving such levels of carbon dioxide enrichment can be complicated by the tendency of nitrogen to remain in void spaces of adsorbent beds, e.g., adsorbent bed 136. Desorption by application of vacuum, absent other operations, can yield carbon dioxide-rich streams comprising 10 mol%-40 mol% nitrogen and / or 10% by volume-40% by volume nitrogen. Although these streams are enriched in carbon dioxide relative to source gasstreams, carbon dioxide levels from 60 mol%-90 mol% and / or from 60% by volume-90% by volume can be undesirably low for carbon sequestration operations.

[0047] Accordingly, portions of carbon dioxide-rich streams can be diverted, such as by diverter 146, to a rinse gas inlet 133 of the PSA unit 130. The diverted portions can comprise carbon dioxide and nitrogen, e.g., 90 mol% carbon dioxide and 10 mol% nitrogen, at any pressure, such as 5 psi (pounds per square inch absolute). Rinse gases, comprising carbon dioxide, can displace nitrogen from void spaces in adsorbent beds, e.g., adsorbent bed 136. The displaced nitrogen can then be discharged from, e.g., PSA unit 130 via reject gas outlet 134 to nitrogen handling 137.

[0048] In some embodiments, nitrogen handling can provide nitrogen for pressurization of PSA units. For example, nitrogen handling 137 can provide nitrogen to a pressurization unit 139, which is in fluid communication with a pressurization gas inlet 138, thereby pressurizing the PSA unit 130 with a nitrogen-rich stream. However, pressurizing the PSA unit 130 with a nitrogen-rich stream is optional. In either scenario, excess nitrogen can be vented to atmosphere.

[0049] The rinse gas operation can increase the carbon dioxide concentration of carbon dioxide-rich streams to at least 90 mol% carbon dioxide, such as over 95 mol% carbon dioxide. The resultant carbon dioxide-rich streams can be suitable for carbon dioxide handling 160, which can include carbon sequestration operations, among others.

[0050] Diverters, such as diverter 146, can comprise flow control dampers with modulating positioners, or splitter valves. The diverted gas stream can be compressed, cooled, and / or have condensate separated therefrom prior to return to the rinse gas inlet 133. The cooling unit, separator, and / or compressor can be as described above regarding cooling unit 110, separator 120, and compressor 125.

[0051] Systems can comprise controllers, e.g., hardware, software, and / or firmware configured to determine the percentage of the carbon dioxide-rich stream to be diverted to the rinse gas inlet of the PSA unit to achieve and maintain the carbon dioxide concentration of the carbon dioxide-rich stream at 90 mol% or greater. The carbon dioxide concentration of the carbon dioxide-rich stream can be regulated by controlling operations of diverters by controllers, such as by controller 144. For example, if the carbon dioxide concentration of a carbon dioxide -rich stream is relatively low, a greater percentage of the carbon dioxide-rich stream can be diverted to the PSA inlet, and if the carbon dioxide concentration of a carbondioxide-rich stream is relatively high, a lesser percentage of the carbon dioxide-rich stream can be diverted to the PSA inlet.

[0052] Input(s) to controllers can comprise information pertaining to the carbon dioxide concentrations of one or more streams at one or more locations in the systems, such as source gas streams prior to entry into PSA units and / or carbon dioxide-rich streams after exit from PSA units. Other information that can be received by controllers includes, but is not limited to, information pertaining to relative humidity of a stream, types and / or concentrations of gases other than carbon dioxide, and / or flow rate in streams at one or more locations in the systems, among other information.

[0053] Information can be input to the controllers by manual entry, using human / computer interface devices such as keyboards, number pads, touchscreens, knobs, dials, or virtual and / or augmented reality input devices, among others. Alternatively or in addition, information can be input to the controllers by automated devices.

[0054] In some embodiments, such as those depicted in FIG. 1 , systems comprise outlet detectors, such as outlet detector 142. Outlet detector 142 can gather information relating to the carbon dioxide concentration of the carbon dioxide-rich stream exiting the PSA unit 130. This can be achieved by use of carbon dioxide detection modalities and apparatus known to persons of ordinary skill in the art, such as non-dispersive infrared (NDIR) analyzers. In some embodiments, outlet detector 142 is an NDIR analyzer.

[0055] In some embodiments, such as those depicted in FIG. 1 , systems comprise inlet detectors, such as inlet detector 126. Inlet detector 126 can gather information relating to the carbon dioxide concentration of the source gas prior to entry into the PSA unit 130, by use of detection modalities and apparatus known to persons of ordinary skill in the art. In some embodiments, inlet detector 126 is an NDIR analyzer.

[0056] FIG. 1 depicts a system in which the source gas inlet 131 and the rinse gas inlet 133 are discrete components of the PSA unit 130. In some embodiments (not shown), source gas and rinse gas are admitted to the PSA unit 130 via a single inlet, with appropriate flow lines and valves to permit entry of one or the other at any single time point.

[0057] In some embodiments, some or all excess rinse gas can be discharged from the PSA unit 130 via reject gas outlet 134 (not shown). Alternatively or additionally, and as shown in FIG. 1, some or all excess rinse gas can be recycled, via rinse recycle outlet 132, to join the source gas entering the PSA unit 130 via source gas inlet 131.

[0058] FIG. 1 depicts a system in which the reject gas outlet 134, the carbon dioxide outlet 135, and the rinse recycle outlet 132 are discrete components of the PSA unit 130. In some embodiments (not shown), any two or all three of reject gas, rinse gas to be recycled, and a carbon dioxide-rich stream are released by the PSA unit 130 via a unitary outlet, with appropriate flow lines and valves to permit release of any one gas stream at any single time point.

[0059] The adsorption capacities of PSA units, e.g., PSA unit 130, can vary depending on the nitrogen, carbon dioxide, and other gas content of the source gas. The adsorption capacities can be optimized by adjusting adsorption cycle times, pressure of source gas and / or rinse gas streams, or other parameters of the process.

[0060] PSA units, such as PSA unit 130, can be skid-mounted, thus providing easy mobility to and between processing locations.

[0061] System 100 depicted in FIG. 1 is shown as comprising one PSA unit. Systems can comprise one, two, three, four, or more PSAs.

[0062] In some embodiments, systems comprise multiple PSA units such that at least one PSA unit is in each one of several different stages of system operation at any given moment, such as is described in Section 0.

[0063] Systems such as system 100 depicted in FIG. 1 can be effective in enriching carbon dioxide in source gases comprising nitrogen, carbon dioxide, and water vapor. Traces of other compounds can be present without impinging on operation of the system 100.

[0064] If source gases further comprise greater than trace amounts of molecules, such as hydrogen and / or methane, other systems can be used to separate hydrogen and / or methane from nitrogen and carbon dioxide. The term “third gas” is used herein to refer to any mixture of one or more gases other than nitrogen and carbon dioxide.

[0065] FIG. 2 depicts a system 200. Reference numerals that are like between FIG. 1, FIG. 2, and / or FIG.3 refer to like components of systems and will not be described in detail. In FIG.2 and FIG. 3, inlet detector 126 is omitted for readability, but can be included in any system exemplified by either figure. Also, FIG. 2 and FIG. 3 have as a component upstream units 201. Upstream units 201 encompass any combination of a gas source 102 with zero, one, two, or three of a cooling unit 110, a separator 120, and a compressor 125.

[0066] FIG. 2 depicts a system 200 in which a source gas comprising carbon dioxide and third gas can be adsorbed by a PSA unit 130 while nitrogen is rejected. The rejected stream can comprise small amounts of carbon dioxide and the third gas, e.g., the rejected stream can comprise 95 mol% or more nitrogen, with carbon dioxide and the third gas in any proportion making up the balance. After adsorption and rejection of nitrogen to nitrogen handling 137, carbon dioxide and the third gas can then be co-desorbed from PSA unit 130 and passed to a carbon dioxide / third gas separator, e.g., carbon dioxide / third gas separator 235. The operating modality of the carbon dioxide / third gas separator 235 can be selected based on the type(s) and / or concentration(s) of third gas compounds, the desired purity of the separated third gas, and / or the temperature and / or other parameters of a stream comprising carbon dioxide and the third gas exiting the PSA unit 130, among other considerations that will be apparent to persons of ordinary skill in the art.

[0067] The third gas can comprise water vapor. The carbon dioxide / third gas separator 235 can comprise a water vapor separator, such as separator 120 depicted in FIG. 1 . In such embodiments, the adsorbent bed 136 of the PSA unit 130 can comprise one or more layers of a hydrophilic adsorbent material.

[0068] Additionally or alternatively, the third gas can comprise methane.

[0069] Further alternatively or additionally, the third gas can comprise methane.

[0070] In systems such as system 200, upon separation of carbon dioxide and third gas, the third gas is sent to third gas handling 237, and the resultant carbon dioxide-rich gas stream can be processed as in system 100, with a portion of the carbon dioxide-rich gas stream diverted as a rinse gas to PSA unit 130 and the remainder sent to carbon dioxide handling 160.

[0071] In FIG. 3, a system 300 for handling source gas comprising carbon dioxide, nitrogen, and third gas is depicted. The system 300 can be suitable for processing source gas streams wherein the third gas is not expected to co-adsorb with carbon dioxide in PSA unit 130, but instead is expected to be co-rejected with nitrogen. The co-rejected stream, predominantly comprising nitrogen and third gas (e.g., 95 mol% or more nitrogen and third gas, with carbon dioxide making up the balance) can pass to a nitrogen / third gas separator 335, upon which nitrogen and third gas can be separated, with nitrogen being handled by nitrogen handling 137 and third gas being handled by third gas handling 237. carbon dioxide can be desorbed from PSA unit 130 and further processed as in system 100.Methods for enriching carbon dioxide

[0072] In some aspects, the present disclosure provides methods for enriching carbon dioxide in source gases comprising carbon dioxide and nitrogen (e.g., using systems of the present disclosure). The methods comprise adsorbing carbon dioxide from source gases having first carbon dioxide concentrations in pressure swing absorption (PSA) beds (e.g., adsorbent beds of PSA units) to yield first nitrogen streams. The methods can be carried out using systems described in Section 0, but the methods are not limited thereto.

[0073] In some embodiments, PSA beds comprise activated carbon adsorbents. In some embodiments, PSA beds comprise silicalite adsorbents. The embodiments of this paragraph are not mutually exclusive.

[0074] Any source gas comprising carbon dioxide and nitrogen, and optionally water vapor, methane, hydrogen, and / or other third gases, can be provided to PSA beds. Source gases can comprise less than 15 mol% carbon dioxide. Source gases can comprise less than 12.5 mol% carbon dioxide. Source gases can comprise less than 10 mol% carbon dioxide.

[0075] In some embodiments, methods comprise cooling source gases to 150°F or less before adsorbing carbon dioxide. In some embodiments, methods comprise condensing water from source gases before adsorbing carbon dioxide.

[0076] First nitrogen streams can pass through PSA beds and be processed as desired.

[0077] The methods also comprise rinsing PSA beds with rinse streams having second carbon dioxide concentrations higher than first carbon dioxide concentrations, to yield second nitrogen streams. Second nitrogen streams can be expelled from PSA beds. Second nitrogen streams can but need not be combined with first nitrogen streams prior to handling. Second nitrogen streams can but need not be processed in the same way as first nitrogen streams.

[0078] The methods also comprise desorbing carbon dioxide from PSA beds by reducing pressure, to yield carbon dioxide streams having second carbon dioxide concentrations. Reducing pressure can comprise applying vacuum. The reduced pressures can be less than 1.5 psia. The reduced pressures can be less than 1.0 psia. The reduced pressures can be less than 0.5 psia.

[0079] Desorption can be performed in a countercurrent fashion or a non-countercurrent fashion. In some embodiments, desorption is performed in a non-countercurrent fashion.

[0080] The methods comprise directing at least portions of carbon dioxide streams to rinse streams to achieve and maintain second carbon dioxide concentrations at selected levels. In some embodiments, selected levels are 90 mol% or greater. In some embodiments, after rinsing, rinse streams can be combined with source gases for re-adsorption of carbon dioxide in the rinse streams.

[0081] In some embodiments, methods comprise determining first carbon dioxide concentrations. In some embodiments, methods comprise determining second carbon dioxide concentrations. The first and / or second carbon dioxide concentrations can be used to determine how much of carbon dioxide streams are to be directed to rinse streams to achieve and maintain second carbon dioxide concentrations at selected levels.

[0082] In some embodiments, after desorbing carbon dioxide, methods can comprise repressurizing PSA beds with first nitrogen streams and / or second nitrogen streams. Repressurizing with first nitrogen streams and / or second nitrogen streams is optional.

[0083] In some embodiments, PSA beds can be repressurized with source gases before adsorbing carbon dioxide.

[0084] Methods can be performed on multiple PSA beds simultaneously, such that one or more steps of the method are performed using one or more PSA beds at the same time. Table 1 represents a duty cycle table for a method performed using four PSA beds, in which the duty cycle comprises four phases of equal length.

[0085] For source streams comprising a third gas, methods can comprise adsorbing the third gas with carbon dioxide, desorbing the third gas with carbon dioxide, and separating carbon dioxide from the third gas after desorbing.100861 For source streams comprising a third gas, methods can comprise rejecting the third gas with first nitrogen streams, rejecting the third gas with second nitrogen streams, and separating the third gas from rejected nitrogen streams.

[0087] Carbon dioxide streams can be further processed as desired. In some embodiments, methods further comprise sequestering at least portions of fractions of enriched carbon dioxide streams that are not directed to rinse streams. Carbon sequestration can comprise geological sequestration, e.g., the injection of carbon dioxide, typically as a supercritical fluid into an underground rock formation of low porosity and low permeability. Worldwide, oil, gas, and coal reservoirs, and saline formations are expected to provide long-term (-millions of years) storage of between 8 Tt (trillion metric tons) to 55 Tt of CO2 (International Energy Association (2021) “The world has vast capacity to store CO2: Net zero means we’ll need it,” www.iea.org / commentaries / the-world-has-vast-capacity-to-store-co2-net-zero-means-we-ll- need-it).

[0088] As described, purifying carbon dioxide enriched streams to 90 mol% can be sufficient for carbon sequestration. If desired, such as for uses other than carbon sequestration, greater than 90 mol% purity can be achieved using the methods disclosed herein (e.g., by selecting suitable operating parameters of a particular deployment of a system of the disclosure) and / or by use of additional techniques and apparatus known in the art, e.g., amine scrubbing.

[0089] In some embodiments, at least a portion of the fraction of the enriched carbon dioxide stream that is not directed to the rinse stream comprises at least 99.5 mol% carbon dioxide. A stream comprising at least 99.5 mol% carbon dioxide can be suitable for medical and industrial applications. In some embodiments, at least a portion of the fraction of the enriched carbon dioxide stream that is not directed to the rinse stream comprises at least 99.9 mol% carbon dioxide. Such concentrations of carbon dioxide can be achieved by purifying (e.g., by amine scrubbing) at least the portion to at least 99.5 mol% carbon dioxide or at least 99.9 mol% carbon dioxide after desorbing.

[0090] In some embodiments, at least a portion comprising at least 99.9 mol% carbon dioxide can be added to a food or a beverage. Techniques and apparatus for adding carbon dioxide to a food or a beverage will be known to persons of ordinaiy skill in the art.CITATION OF REFERENCES

[0091] All publications, patents, patent applications and other documents cited in this application are hereby incorporated by reference in their entireties for all purposes to thesame extent as if each individual publication, patent, patent application or other document were individually indicated to be incorporated by reference for all purposes. In the event that there is an inconsistency between the teachings of one or more of the references incorporated herein and the present disclosure, the teachings of the present specification are intended.

Claims

CLAIMS1. A system for enriching carbon dioxide in a source gas, comprising: a pressure swing adsorption (PSA) unit comprising an adsorbent bed in fluid communication with a source gas inlet, a rinse gas inlet, a reject gas outlet, and a carbon dioxide outlet, the adsorbent bed configured to adsorb carbon dioxide from a source gas comprising carbon dioxide and nitrogen, and allow a nitrogen-rich stream to exit via the reject gas outlet; a vacuum pump configured to desorb the carbon dioxide from the adsorbent bed and remove a carbon dioxide-rich stream via the carbon dioxide outlet; and a diverter configured to divert a percentage of the carbon dioxide-rich stream to the rinse gas inlet of the PSA unit.

2. The system of claim 1, further comprising an outlet detector configured to determine a carbon dioxide concentration of the carbon dioxide-rich stream.

3. The system of claim 2, wherein the outlet detector is a non-dispersive infrared (NDIR) analyzer.

4. The system of claim 1, comprising a controller configured to determine the percentage of the carbon dioxide-rich stream to be diverted to the rinse gas inlet of the PSA unit to achieve and maintain a carbon dioxide concentration of the carbon dioxide-rich stream at 90 mol% or greater.

5. The system of claim 1, comprising an inlet detector configured to determine a carbon dioxide concentration of the source gas.

6. The system of claim 5, wherein the inlet detector is an NDIR analyzer.

7. The system of claim 1, comprising a cooling unit configured to cool the source gas to 150°F or less prior to entry of the source gas into the PSA unit.

8. The system of claim 7, wherein the cooling unit is an air-fan cooling unit.

9. The system of claim 1, comprising a separator configured to condense water from the source gas prior to entry of the source gas into the PSA unit.

10. The system of claim 1, comprising a compressor configured to pressurize the source gas prior to entry of the source gas into the PSA unit.

11. The system of claim 1, wherein the PSA unit comprises a pressurization gas inlet, and a pressurization unit configured to pressurize the PSA unit with the nitrogen-rich stream via the pressurization gas inlet.

12. The system of claim 1, wherein the vacuum pump is configured to draw a vacuum less than 3.0 psia.

13. A system for enriching carbon dioxide in a source gas, comprising: means for adsorbing carbon dioxide from a source gas comprising carbon dioxide and nitrogen and for rejecting a nitrogen-rich stream; means for desorbing the carbon dioxide to yield a carbon dioxide-rich stream; and means for diverting a percentage of the carbon dioxide-rich stream to the means for adsorbing.

14. The system of claim 13, comprising means for determining a carbon dioxide concentration of the carbon dioxide-rich stream.

15. The system of claim 13, comprising means for determining the percentage of the carbon dioxide-rich stream to be diverted to the means for adsorbing to achieve and maintain a carbon dioxide concentration of the carbon dioxide-rich stream at 90 mol% or greater.

16. The system of claim 13, comprising means for determining a carbon dioxide concentration of the source gas.

17. The system of claim 13, comprising means for cooling a temperature of the source gas to 150°F or less prior to entry of the source gas into the means for adsorbing.

18. The system of claim 13, comprising means for condensing water from the source gas prior to entry of the source gas into the means for adsorbing.

19. The system of claim 13, comprising means for pressurizing the source gas prior to entry of the source gas into the means for adsorbing.

20. The system of claim 13, comprising means for pressurizing the means for adsorbing with the nitrogen-rich stream.

21. A method for enriching carbon dioxide in a source gas comprising carbon dioxide and nitrogen, comprising: adsorbing carbon dioxide from a source gas having a first carbon dioxide concentration in a pressure swing absorption (PSA) bed to yield a first nitrogen stream; rinsing the bed with a rinse stream having a second carbon dioxide concentration higher than the first carbon dioxide concentration, to yield a second nitrogen stream; desorbing carbon dioxide from the PSA bed by reducing pressure, to yield a carbon dioxide stream having the second carbon dioxide concentration; and directing at least a portion of the carbon dioxide stream to the rinse stream to achieve and maintain the second carbon dioxide concentration at 90 mol% or greater.

22. The method of claim 21, wherein reducing the pressure, to yield the carbon dioxide stream having the second carbon dioxide concentration comprises applying vacuum.

23. The method of claim 21, comprising determining the first carbon dioxide concentration.

24. The method of claim 21, comprising determining the second carbon dioxide concentration.

25. The method of claim 21, comprising cooling the source gas to 150°F or less before adsorbing the carbon dioxide from the source gas having the first carbon dioxide concentration in the pressure swing absorption (PSA) bed to yield the first nitrogen stream.

26. The method of claim 21, further comprising condensing water from the source gas before adsorbing the carbon dioxide from the source gas having the first carbon dioxide concentration in the pressure swing absorption (PSA) bed to yield the first nitrogen stream.

27. The method of claim 21, wherein the source gas comprises a third gas, and the method comprises adsorbing the third gas from the source gas, desorbing the third gas from the PSA bed, and subsequently separating an amount of carbon dioxide from the third gas.

28. The method of claim 21, wherein the source gas comprises a third gas, and the method comprises rejecting the third gas with the first nitrogen stream, rejecting the third gas with the second nitrogen stream, and separating the third gas from the nitrogen streams after desorbing the carbon dioxide from the PSA bed.

29. The method of claim 28, wherein the third gas is methane.

30. The method of claim 21, comprising repressurizing the bed with the first nitrogen stream and / or the second nitrogen stream after desorbing the carbon dioxide from the PSA bed.

31. The method of claim 21, comprising repressurizing the source gas before adsorbing the carbon dioxide from the source gas.

32. The method of claim 21, wherein the rinse stream is combined with the source gas after rinsing the bed with the rinse stream.

33. The method of claim 21, comprising sequestering at least a portion of the enriched carbon dioxide stream that is not directed to the nnse stream.

34. The method of claim 21, wherein at least a portion of the enriched carbon dioxide stream that is not directed to the rinse stream comprises at least 99.5 mol% carbon.

35. The method of claim 34, comprising purifying at least the portion to at least 99.5 mol% carbon dioxide after desorbing the carbon dioxide from the PSA bed.

36. The method of claim 35, wherein the purifying comprises amine scrubbing.

37. The method of claim 34, further comprising adding at least the portion comprising at least 99.9 mol% carbon dioxide to a food or a beverage.

38. The method of claim 21, wherein the source gas comprises less than 15 mol% carbon dioxide.

39. The method of claim 21, wherein desorbing the carbon dioxide from the PSA bed is performed in a non-countercurrent fashion.

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