Method and system for removing carbon dioxide from a carbon dioxide-containing gas mixture

Abstract

The invention relates to a method (100) for separating carbon dioxide from a gas mixture (101) containing at least carbon dioxide, nitrogen, oxygen, argon and water, the method (100) comprising a first non-cryogenic separation step (21), a second non-cryogenic separation step (25), and a cryogenic separation step (24), the first non-cryogenic separation step (21) and the second non-cryogenic separation step (25) being pressure swing adsorption steps or the first non-cryogenic separation step (21) being a pressure swing adsorption step and the second non-cryogenic separation step (25) being a membrane separation step. The present invention also relates to a corresponding system.

Description

[0001] Description

[0002] Process and plant for removing carbon dioxide from a carbon dioxide-containing gas mixture

[0003] The invention relates to a method and a plant for removing carbon dioxide from a carbon dioxide-containing gas mixture, in particular a flue gas.

[0004] background

[0005] There are various approaches to reducing carbon dioxide emissions from industrial processes. In addition to increasing efficiency and thus avoiding carbon dioxide emissions, carbon capture and storage (CCS) can also be used to capture and store the carbon dioxide produced. The goal of reducing or avoiding carbon dioxide emissions is decarbonization.

[0006] Various CCS processes based on research studies are described primarily in the scientific literature, but these have so far found little to no commercial application. In particular, amine scrubbing and combined membrane and pressure swing adsorption (PSA) processes can be used to separate carbon dioxide from flue gases. Details on the individual processes are explained below.

[0007] Current CCS processes are extremely cost-intensive, both in terms of capital expenditures (CAPEX) and operating expenses (OPEX). Therefore, there is a need for improved CCS processes in terms of CAPEX and / or OPEX.

[0008] Disclosure of the invention

[0009] Against this background, a method and a plant for separating carbon dioxide from a carbon dioxide-containing gas mixture are achieved with the features of the respective independent patent claims. Further refinements are the subject of the dependent patent claims and the following description. Within the scope of the present invention, process variants are proposed to overcome the disadvantages of the prior art explained above, each of which comprises the use of two non-cryogenic separation steps and one cryogenic separation step. A "separation step," as understood here, can comprise the use of one or more separation devices or separation units, which can be arranged in any desired manner in series or in parallel, for example, different pressure swing adsorption units and / or membrane separation units.

[0010] The proposed process serves to separate carbon dioxide from a gas mixture containing at least carbon dioxide, nitrogen, oxygen, argon, and water, in particular from a flue gas. The process comprises a first non-cryogenic separation step, a second non-cryogenic separation step, and a cryogenic separation step. The first non-cryogenic separation step and the second non-cryogenic separation step are pressure swing adsorption steps, or the first non-cryogenic separation step is a pressure swing adsorption step and the second non-cryogenic separation step is a membrane separation step. Merely to simplify the explanations, and without any intended limitation, the second non-cryogenic separation step is described below predominantly as a pressure swing adsorption step.

[0011] Since pressure swing adsorption steps and, if applicable, membrane separation steps are used in the process proposed here, the technical principles are first explained below and corresponding definitions are given.

[0012] PSA is routinely used to separate gaseous mixtures. PSA utilizes the varying adsorption strengths of individual gas molecules or atoms towards the adsorbent used. PSA and its variants, as well as technical principles, are described in the specialist literature, for example in an article by H.-J. Bart and U. v. Gemmingen, "Adsorption," in Ullmann's Encyclopedia of Industrial Chemistry, online publication 2012, DOI: 10.1002 / 14356007.b03_09.pub2. The use of PSA for carbon dioxide separation from flue gases is well known. For example, US 3,838,553 A describes a combination of PSA with cryogenic separation. The removal of carbon dioxide from a carbon dioxide-rich stream is also described, for example, in US 766,251 B2. US 945286 A discloses a process with a similar objective, but in which membrane separation is used.

[0013] PSA is based on physical adsorption phenomena, whereby highly volatile, low-polarity compounds such as hydrogen or helium are practically impossible to adsorb compared to molecules such as carbon dioxide, carbon monoxide, nitrogen, and hydrocarbons. The latter components can be separated due to their different adsorption forces. PSA processes operate between two pressure stages. Adsorption takes place at a comparatively high adsorption pressure, during which a gas or gas mixture freed from the adsorbed components is discharged from the PSA. This is also referred to here as a "high-pressure stream." Desorption, in contrast, takes place at a comparatively low desorption pressure, providing a gas or gas mixture referred to here as a "low-pressure stream." A PSA in which desorption takes place at a subatmospheric pressure level is typically also referred to as a "vacuum" PSA (VPSA).

[0014] PSA typically involves several pressure vessels connected in parallel, each filled with an adsorbent. Using connecting lines and appropriate control valves, the feed gas, referred to here as the "separation insert," can flow through at least one of the pressure vessels at the adsorption pressure, thus forming a high-pressure stream. One or more additional pressure vessels are regenerated during this time while maintaining the low-pressure stream. PSA therefore appears externally as a continuous process, while internally it is a discontinuous process consisting of a series of parallel sequences.

[0015] Membrane separation processes use a membrane, i.e. an element that is more or less selectively permeable to components of a mixture. The membrane forms a selective barrier between two phases, the feed and the permeate. Components with high permeance can pass through the barrier and collect on the permeate side, while the other components with low permeance remain on the feed side of the membrane, which is also referred to as the retentate. In membrane separation processes, there is a pressure drop across the membrane, so that the permeate is obtained as a low-pressure stream (and is referred to accordingly here), whereas a high-pressure stream is obtained on the feed side (and is referred to accordingly here). The terms “high pressure” and “low pressure” are to be understood relative to one another.

[0016] Membranes for membrane separation processes can be made from various materials, such as polymers and ceramics. Their material, structure, and homogeneity determine the membrane's properties, which essentially consist of permeability and physical and chemical stability. The membranes used in membrane separation processes can be arranged in a closed unit called a module or cartridge. Such modules can vary greatly in shape, packing density, and flow configuration. The driving force in membrane separation is the partial pressure difference. Since temperature also affects membrane performance, operating pressure and temperature are key factors for membrane operation.

[0017] The use of the present invention results in lower CAPEX and OPEX compared to absorptive solutions. Furthermore, no chemicals are required, and there are no problems with amine emissions in the exhaust gas and in the carbon dioxide product. Because there is no need for scrubbing agent handling and the compensation of scrubbing agent losses, no corresponding running costs are generated. This results in simpler operation, especially with regard to changes in process conditions, e.g., fluctuating product demand. In embodiments of the invention, no external cooling medium is required for carbon dioxide purification, especially when a gaseous carbon dioxide product is obtained. Drying of the carbon dioxide product is not necessary. The pressure of the carbon dioxide product can be significantly higher than with amine scrubbing, although lower compressor power and compressor size are required for product compression.This is particularly relevant if the carbon dioxide is to be provided, for example, for sequestration at high pressure. In one embodiment of the present invention, the method comprises using the gas mixture or a portion thereof to provide a first separation insert containing at least a portion of the carbon dioxide, nitrogen, oxygen, argon, and water from the flue gas and subjecting it to the first non-cryogenic separation step.

[0018] In general, the "provision" of a specific material stream "using" another material stream is understood to mean any type of provision, for example, simply passing a corresponding material stream to another process step, i.e., provision without any material, physical, or quantitative change. Generally, a second material stream provided using a first material stream, as understood here, contains at least some components contained in or obtained from the first material stream.A second material stream provided in this sense can be obtained from the first material mixture, for example, by separating or branching off a part or one or more components, enriching or depleting one or more components, chemically or physically reacting one or more components, heating, cooling, pressurizing, decompressing and the like.

[0019] As used herein, material streams can also be enriched or depleted in one or more components. These terms refer to a content in another material stream used to form the material stream. A material stream under consideration is "enriched" if it has at least 2 times, 5 times, 10 times, 100 times, or 1,000 times the content of the designated component(s), and "depleted" if it has at most 0.5 times, 0.1 times, 0.01 times, or 0.001 times the content of the designated component(s), in each case with respect to the material stream used to form the material stream under consideration.

[0020] In a corresponding embodiment, the first non-cryogenic separation step provides a first low-pressure stream, which is enriched in carbon dioxide compared to the first separation insert and contains a portion of the nitrogen, oxygen, and argon from the first separation insert, as well as a first high-pressure stream. Using the first low-pressure stream or a portion thereof, a second separation insert, each containing at least a portion of the carbon dioxide, nitrogen, oxygen, and argon from the low-pressure stream, is further provided and subjected to the cryogenic separation step.

[0021] The cryogenic separation step provides a residual gas stream that is depleted in carbon dioxide compared to the second separation insert and contains carbon dioxide as well as a portion of the nitrogen, oxygen and argon from the second separation insert, as well as a carbon dioxide stream.

[0022] Using the residual gas stream or a portion thereof, a third separation insert containing at least a portion of the carbon dioxide, nitrogen, oxygen and argon from the residual gas stream is provided and subjected to the second non-cryogenic separation step, and by means of the second non-cryogenic separation step, a second low-pressure stream enriched in carbon dioxide compared to the third separation insert and containing a portion of the nitrogen, oxygen and argon from the third separation insert, and a second high-pressure stream are provided.

[0023] The second low-pressure stream or a part thereof is used in the provision of the second separation insert and can, for example, be fed to an intermediate stage compressor used in the course of this provision.

[0024] For the advantages of such a design, reference is made to the above explanations regarding the general features of the invention. Further features and advantages are explained below.

[0025] The gas mixture used, for example a flue gas, can be cooled to approximately 40°C within the scope of the present invention. If necessary, condensate (water) can then be separated off in a separator. The water-saturated gas is compressed and fed as the first separation feed to the first non-cryogenic separation step, in particular a VPSA. The high-pressure stream obtained in the first non-cryogenic separation step represents a carbon dioxide-depleted fraction. A low-pressure stream obtained here can be carbon dioxide-rich, in particular having a carbon dioxide content of more than 50% by volume. This can be compressed and subsequently dried, thereby providing the second separation feed. Drying can be carried out by adsorption, and, as mentioned again below, the anhydrous high-pressure stream from the first or second non-cryogenic separation step can be used as the regeneration gas.The second separation insert is fed to the cryogenic separation step.

[0026] In the cryogenic separation step, the gas is cooled to a temperature below the condensation temperature of carbon dioxide at the separation pressure used, and in a separator, condensed carbon dioxide can be separated from the (under the conditions used) non-condensable components (such as nitrogen, oxygen and argon). The fraction of non-condensed components is also referred to as high-pressure flash gas. The condensed liquid carbon dioxide can be subjected to further purification in order to increase the carbon dioxide content or to achieve the maximum permissible concentration of impurities, e.g. an oxygen concentration of less than 100 ppm by volume. This can be achieved by means of expansion, flashing, cooling, stripping, distillation or a combination of the aforementioned process steps.

[0027] The high-pressure flash gas, which contains carbon dioxide in addition to nitrogen, oxygen, and argon, is used to provide the third separation feed and is fed to the second non-cryogenic separation step, e.g., a second PSA, to increase the overall carbon dioxide yield of the process. In the second non-cryogenic separation step, as in the first non-cryogenic separation step, the corresponding third separation feed is separated to obtain a low-carbon dioxide high-pressure stream and a carbon dioxide-rich low-pressure stream. As mentioned, the low-pressure stream can be compressed with the low-pressure stream from the first non-cryogenic separation step (in particular, an intermediate feed into an installed compressor can be made) and can thus be fed to the drying and non-cryogenic separation step together with the first low-pressure stream.Compared to a fundamentally possible recirculation of the high-pressure flash gas before the first non-cryogenic separation, particular advantages can be achieved here. The second non-cryogenic separation step, in particular a PSA, can be carried out at a significantly higher adsorption pressure (approx. 15 to 40 bar instead of 2 to 15 bar absolute pressure) and desorption pressure (approx. 1.5 to 3.5 bar absolute pressure instead of a subatmospheric pressure level) than the first non-cryogenic separation step, in particular a VPSA. Without the second non-cryogenic separation step, i.e. when recirculating before the first non-cryogenic separation step, the high-pressure flash gas would have to be expanded to approx. 3 to 10 bar absolute pressure, or the gas mixture used, for example flue gas, which only contains approx. 20% carbon dioxide and 80% ballast (mainly nitrogen), would have to be compressed to 15 to 35 bar absolute pressure.Furthermore, the carbon dioxide-rich fraction of the second non-cryogenic separation step (or a corresponding portion of the low-pressure stream of the only existing first non-cryogenic separation step) would have to be compressed at a significantly lower intake pressure, which increases the energy requirement and the size of the machine. In addition, the higher adsorption pressure improves the selectivity between carbon dioxide and nitrogen separation. Due to the aforementioned advantages of the second non-cryogenic separation step, especially a second PSA, the energy requirement can be significantly reduced.

[0028] Particular advantages can be achieved in embodiments of the invention in which the second non-cryogenic separation step comprises, in particular, pressure swing adsorption, i.e., by using specific pressure levels.These embodiments can in particular comprise that the first separation insert is provided at a pressure in a first pressure range of 2 to 15 bar absolute pressure and is subjected to the first non-cryogenic separation step, that the first low-pressure stream is provided by means of the first non-cryogenic separation step at a pressure in a second pressure range below 1.5 bar absolute pressure, that the second separation insert is provided at a first pressure in a third pressure range of 15 to 40 bar absolute pressure and is subjected to the cryogenic separation step, that the residual gas stream is provided by means of the cryogenic separation step at the or a further pressure in the third pressure range and is subjected to the second non-cryogenic separation step, and that the second low-pressure stream is provided by means of the second non-cryogenic separation step at a pressure in a fourth pressure range above 1 bar absolute pressure.The advantages of such pressures have just been explained.

[0029] The same applies to process variants according to embodiments of the invention in which the provision of the second separation insert comprises compression and drying, and in which the first or second high-pressure stream or a part thereof is used as regeneration gas in the drying.

[0030] Further embodiments of the present invention may include subjecting the first and / or second high-pressure streams, or a portion thereof, to expansion in one or more expansion turbines. One or more such expansion machines may be coupled to a compressor (booster) or a generator, allowing compression of suitable process streams or energy generation. The high-pressure streams may be preheated to increase the pressure or adjust the temperature after appropriate expansion. For example, waste heat from flue gas treatment and compression may be utilized.

[0031] In embodiments of the present invention, the second high-pressure stream or a portion thereof can be fed to a nitrogen recovery process and / or used to generate cold. This results in particularly efficient process variants. The second low-pressure stream or a portion thereof can also be used as a heavy rinse stream in the first non-cryogenic separation step. This allows for increased nitrogen removal and thus reduced energy consumption during compression.

[0032] In embodiments of the present invention, the gas mixture can have a content of 50 to 70% nitrogen, a total content of 1 to 15% oxygen and argon, a content of 10 to 20% carbon dioxide and a content of 15 to 20% water, in each case by volume, and / or can be provided at a temperature in a temperature range of 100 to 200 °C and / or at a pressure in a pressure range of 0.1 to 1.5 bar absolute pressure and / or can be selected from a flue gas of a particularly fired manufacturing process, a power plant flue gas and an exhaust gas from cement production. Embodiments of the present invention can comprise a separation of 90 to 99% of the carbon dioxide contained in the gas mixture.Using the separated carbon dioxide or a portion thereof, a carbon dioxide product with more than 99% carbon dioxide content and less than 100 ppm oxygen by volume can be provided in this way.

[0033] A plant for separating carbon dioxide from a gas mixture containing at least carbon dioxide, nitrogen, oxygen, argon and water is also the subject of the present invention, wherein the method is designed to carry out a first non-cryogenic separation step, a second non-cryogenic separation step and a cryogenic separation step, wherein the first non-cryogenic separation step and the second non-cryogenic separation step are pressure swing adsorption steps or the first non-cryogenic separation step is a pressure swing adsorption step and the second non-cryogenic separation step is a membrane separation step.

[0034] With regard to the system provided according to the invention, express reference is made to the above explanations regarding the method according to the invention, as these also apply to a corresponding device. The same applies in particular to a configuration of a corresponding device, which is advantageously configured to carry out a corresponding method in any desired configuration.

[0035] The invention is further explained below with reference to the figures, which illustrate an embodiment of the present invention.

[0036] Short description of the characters

[0037] Figure 1 illustrates a method according to an embodiment of the invention.

[0038] Detailed description of the drawings

[0039] The following description explains methods according to embodiments of the invention. For the sake of simplicity and to avoid unnecessary repetition, the same reference numerals are used for method steps and system components, for example, a VPSA step and a VPSA unit used for this purpose. Explanations regarding such method steps generally apply to corresponding system components in the same way. Conversely, explanations regarding system components also apply to method steps.

[0040] As an example of a first non-cryogenic separation step, a VPSA is shown and as an example of a second non-cryogenic separation step, another PSA is shown, referred to for simplicity as “first pressure swing adsorption step” and “second pressure swing adsorption step”.

[0041] In Figure 1, a method according to an embodiment of the invention is schematically illustrated and designated overall by 100.

[0042] A gas mixture 101, for example a flue gas stream, is fed to the process 100, which is cooled in a pretreatment step 10 to a temperature of, for example, approximately 40 °C and is then at least partially freed of condensate, ie, in particular water, in a separator (not separately illustrated).

[0043] The still water-saturated gas is compressed in the pretreatment step 10, as also not separately illustrated, and fed in the form of a gas stream 102 as the first separation feed to the VPSA, i.e., the first non-cryogenic separation step 21, which is described here as part of a process group 20. A carbon dioxide-depleted fraction is withdrawn from the first non-cryogenic separation step 21, i.e., the VPSA, as the first high-pressure stream 103, and a carbon dioxide-rich fraction with a carbon dioxide content of more than 50% is withdrawn as the first low-pressure stream 104.

[0044] The carbon dioxide-rich fraction or the low-pressure stream 104 is compressed in a raw carbon dioxide compression step 22 to an absolute pressure of, for example, more than 15 bar, in particular more than 20 bar absolute pressure, and then, as a pressure stream 105, is fed to a drying step 23 and dried therein. The drying step 23 can be carried out in particular by adsorption, wherein the carbon dioxide-depleted fraction or the high-pressure stream 109 from the PSA (as not shown separately here) or 103 from the VPSA step (as shown in Figure 1) can be used as the regeneration gas, which can be discharged from the process 100 after this use and, for example, released to the atmosphere or, as required, fed to further flue gas treatment or expanded via an expander for energy recovery.

[0045] The dried, carbon dioxide-rich gas is fed to the cryogenic separation step 24 as a second separation insert 106. There, the gas is cooled. Subsequently, the condensed carbon dioxide can be separated from non-condensable components (nitrogen, oxygen, argon) in a separator.

[0046] The condensed carbon dioxide or a corresponding carbon dioxide stream can be provided by the cryogenic separation step with a desired carbon dioxide content or an oxygen concentration at a maximum permissible value, for example, less than 100 ppm by volume. This can be achieved by depressurization, flashing, cooling, stripping, distillation, or any combination of the aforementioned process steps.

[0047] The condensation of carbon dioxide in the cryogenic separation step 24 takes place at an elevated pressure level, for example, 5 to 40 bar absolute pressure. The non-condensable components can therefore be fed as high-pressure flash gas 108, which contains carbon dioxide in addition to nitrogen, oxygen, and argon, at this pressure level to a further PSA step 25, previously referred to as the "second" PSA step. The high-pressure flash gas 108 thus simultaneously forms a third separation insert 108.

[0048] The second non-cryogenic separation step 25, i.e., the further PSA, increases the carbon dioxide yield of the overall process. In the further PSA step, the high-pressure flash gas 108 is separated, as in the VPSA step, into a carbon dioxide-poor fraction or a second high-pressure stream 109 and a carbon dioxide-rich fraction or a second low-pressure stream 110. The carbon dioxide-rich fraction or the second low-pressure stream 110 can also be fed to the raw carbon dioxide compression step 22 (for example, via an intermediate feed) and, together with the low-pressure stream 104 from the first non-cryogenic separation step 21, i.e., the PSA, subjected to the drying step 23 and the cryogenic separation step 24.

Claims

A method (100) for separating carbon dioxide from a gas mixture (101) containing at least carbon dioxide, nitrogen, oxygen, argon, and water, wherein the method (100) comprises a first non-cryogenic separation step (21), a second non-cryogenic separation step (25), and a cryogenic separation step (24), wherein the first non-cryogenic separation step (21) and the second non-cryogenic separation step (25) are pressure swing adsorption steps, or the first non-cryogenic separation step (21) is a pressure swing adsorption step and the second non-cryogenic separation step (25) is a membrane separation step. The method (100) according to claim 1, comprising: using the gas mixture (101) or a portion thereof, a first separation insert (102) containing at least a portion of the carbon dioxide, the nitrogen, the oxygen, the argon, and the water from the gas mixture (101),provided and subjected to the first non-cryogenic separation step (21), by means of the first non-cryogenic separation step (21), a first low-pressure stream (104) which is enriched in carbon dioxide compared to the first separation insert (102) and contains a portion of the nitrogen, the oxygen, the argon and the water from the first separation insert (102), as well as a first high-pressure stream (103) are provided, using the first low-pressure stream (104) or a portion thereof, a second separation insert (106) which each contains at least a portion of the carbon dioxide, the nitrogen, the oxygen and the argon from the low-pressure stream (104), is provided and subjected to the cryogenic separation step (24), by means of the cryogenic separation step (24), a residual gas stream (108) which is depleted in carbon dioxide compared to the second separation insert (106) and contains carbon dioxide as well as a portion of the nitrogen, the oxygen and the argon from the second separation insert (106), as well as a carbon dioxide stream (107) are provided, using the residual gas stream (108) or a portion thereof, a third separation insert (108) which contains at least a portion of the carbon dioxide, the nitrogen, the oxygen and the argon from the residual gas stream (108) is provided and is subjected to the second non-cryogenic separation step (25), and by means of the second non-cryogenic separation step (25), a second low-pressure stream (110) which is enriched in carbon dioxide compared to the third separation insert (108) and contains a portion of the nitrogen, the oxygen and the argon from the third separation insert (108), as well as a second high-pressure stream (109) are provided,the second low-pressure stream (110) or a portion thereof is used in providing the second separation insert (106). The process according to claim 2, comprising providing the first separation insert (102) at a pressure in a first pressure range of 2 to 15 bar absolute pressure and subjecting it to the first non-cryogenic separation step (21), providing the first low-pressure stream (104) by means of the first non-cryogenic separation step (21) at a pressure in a second pressure range below 1.5 bar absolute pressure, providing the second separation insert (106) at a first pressure in a third pressure range of 15 to 40 bar absolute pressure and subjecting it to the cryogenic separation step (24), the residual gas stream (108) is provided by means of the cryogenic separation step (24) at the or a further pressure in the third pressure range and is subjected to the second non-cryogenic separation step (25), the second low-pressure stream (110) is provided by means of the second non-cryogenic separation step (25) at a pressure in a fourth pressure range above 1 bar absolute pressure.

4. The method according to claim 2 or 3, wherein providing the second separating insert (106) comprises compaction (22) and drying (23).

5. The method according to claim 4, wherein the second high-pressure stream (109) or the first high-pressure stream (103) or a part thereof is used as regeneration gas in the drying (23).

6. Method according to one of claims 2 to 5, in which the first and / or the second high-pressure stream (103, 109), or a part thereof, is subjected to expansion in one or more expansion turbines and / or the expansion turbines are combined with a compressor.

7. Method according to one of claims 2 to 6, in which the second high-pressure stream (109) or a part thereof is fed to a nitrogen recovery plant and / or is used to generate cold.

8. A method according to any one of claims 2 to 7, wherein the second low pressure stream (110) or a portion thereof is used as a purge stream in the first non-cryogenic separation step (21).

9. Method according to one of the preceding claims, in which the gas mixture (101) has a content of 50 to 70% nitrogen, a total content of 1 to 15% oxygen and argon, a content of 10 to 25% carbon dioxide and a content of 15 to 20% water, in each case in volume fraction, and / or at a temperature in a temperature range of 100 to 200 °C and / or at a pressure in a pressure range of 0.1 to 1.5 bar absolute pressure is provided and / or is selected from a flue gas, a power plant flue gas, and an exhaust gas from cement production. A process according to any one of the preceding claims, wherein 90 to 99% of the carbon dioxide contained in the gas mixture (101) is separated. A process according to any one of the preceding claims, wherein, using the separated carbon dioxide or a portion thereof, a carbon dioxide product having a carbon dioxide content of more than 99% and less than 100 ppm oxygen by volume is provided.Plant for separating carbon dioxide from a gas mixture (101) containing at least carbon dioxide, nitrogen, oxygen, argon, and water, wherein the method (100) is configured to carry out a first non-cryogenic separation step (21), a second non-cryogenic separation step (25), and a cryogenic separation step (24), wherein the first non-cryogenic separation step (21) and the second non-cryogenic separation step (25) are pressure swing adsorption steps, or the first non-cryogenic separation step (21) is a pressure swing adsorption step and the second non-cryogenic separation step (25) is a membrane separation step. Plant according to claim 12, which is configured to carry out a method according to one of claims 1 to 11.

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