A PRESSURE SWING ADSORPTION PROCESS FOR PRODUCING HYDROGEN AND CARBON DIOXIDE

Abstract

A pressure swing adsorption process (PSA) comprising the following steps: feeding an input gas containing H.sub.2, CO.sub.2 and impurities through a CO.sub.2 adsorbent material in a pressure vessel under a high pressure; withdrawing a first H.sub.2-rich product gas due to adsorption of CO.sub.2 in the adsorbent material; setting the pressure to an intermediate pressure causing the adsorbent material release a second gas stream; passing a CO.sub.2-rich purge stream through the adsorbent material, obtaining a purge gas; setting the pressure to a sub-atmospheric low pressure, so that a CO.sub.2-rich product gas is released under vacuum by the adsorbent material; re-pressurizing the vessel to said high pressure; said steps being performed cyclically in a pressure vessel or in a plurality of parallel pressure vessels of a multiple vessel setup.

Claims

1-24. (canceled)

25. A pressure swing adsorption process (PSA) for recovery of H.sub.2 and CO.sub.2 at high purity from an input feed gas which contains at least 40% vol of H.sub.2, the pressure swing adsorption process comprising: a) feeding the input feed gas, which contains H.sub.2, CO.sub.2 and other components or impurities, through an adsorbent material suitable to adsorb CO.sub.2, in a pressure vessel under a high pressure, and withdrawing from the pressure vessel a first gas stream which is a H.sub.2-rich product gas containing less CO.sub.2 than the input gas due to adsorption of CO.sub.2 in the adsorbent material; b) lowering the pressure in the pressure vessel to a target intermediate pressure, which is lower than said high pressure, causing the adsorbent material to release a second gas stream, which is a gas containing impurities, H.sub.2 and CO.sub.2, and withdrawing said second gas from the pressure vessel; c) purging the adsorbent material by passing a CO.sub.2-rich purge stream through the adsorbent material, obtaining a third gas which is a purge gas containing impurities desorbed from the material, and withdrawing said third gas from the pressure vessel; d) lowering the pressure in the pressure vessel to a low pressure, which is lower than said intermediate pressure, and is a sub-atmospheric pressure, causing the adsorbent material to release under vacuum a fourth gas stream, which is a CO.sub.2-rich product gas containing CO.sub.2 desorbed from the material, and withdrawing said fourth gas from the pressure vessel; e) re-pressurizing the vessel to said high pressure, wherein said steps a) to e) are performed cyclically in a pressure vessel or in a plurality of parallel pressure vessels of a multiple vessel setup; wherein the sequence of steps performed by each pressure vessel includes an additional purge step which is performed after the step d) and before the step e), and said additional purge step is performed using part of H.sub.2-rich product gas produced by the same or at least another vessel, or said additional purge step is performed using a stream obtained from the blowdown step b) of at least another vessel, and said additional purge step produces an additional CO.sub.2-rich stream; and wherein as a result of the above steps, CO.sub.2 and H.sub.2 contained in the input gas are recovered separately in the H.sub.2-rich stream and in the CO.sub.2-rich stream respectively, and impurities are removed in at least one further stream which is not recycled into the PSA process.

26. The pressure swing adsorption process according to claim 25 wherein at least 50% in volume of the input gas is represented by the sum of H.sub.2 and CO.sub.2.

27. The pressure swing adsorption process according to claim 25 wherein said H.sub.2-rich product gas has an H.sub.2 purity of at least 90% and/or said CO.sub.2-rich product gas has a CO.sub.2 purity of at least 90%.

28. The pressure swing adsorption process according to claim 25 wherein the removal of CO.sub.2 relative to the CO.sub.2 contained in the feed is at least 90% and the rate of recovery of H.sub.2 in the H.sub.2 stream is at least 90%.

29. The pressure swing adsorption process according to claim 25 wherein step a) is performed by passing the input gas from bottom to top of the pressure vessel and the first gas stream is withdrawn from top of the vessel.

30. The pressure swing adsorption process according to claim 29 wherein step a) is performed in such a way that at least a portion of the adsorbent material is not saturated with CO.sub.2 and the unsaturated adsorbent material is located in the upper part of the vessel.

31. The pressure swing adsorption process according to claim 25 wherein step b) includes lowering the pressure by blowing down the pressure vessel from the upper part of the vessel and the second stream is withdrawn from top of the vessel.

32. The pressure swing adsorption process according to claim 25 wherein step c) is performed by passing the CO.sub.2-rich purge stream from bottom to top of the adsorbent material.

33. The pressure swing adsorption process according to claim 25 wherein said fourth gas, that is the CO.sub.2-rich product gas, is withdrawn from the bottom of the vessel.

34. The pressure swing adsorption process according to claim 25, wherein the step e) includes a plurality of pressurization steps performed with the same or a different pressurizing medium.

35. The pressure swing adsorption process according to claim 25 wherein the CO.sub.2-rich purge stream used in step c) includes CO.sub.2-rich product gas originated from at least one vessel performing the step d).

36. The pressure swing adsorption process according to claim 25 wherein the step e) includes the feeding, as a pressurizing medium, of at least part of the second gas stream originated from at least one pressure vessel performing the step b).

37. The pressure swing adsorption process according to claim 25 wherein the step e) includes the feeding, as a pressurizing medium, of at least part of the H.sub.2-rich product gas originated from at least one pressure vessel performing the step a).

38. The pressure swing adsorption process according to claim 36 wherein step e) includes a first pressurization step e1) with the second gas stream originated from at least one vessel performing the step b), up to an intermediate pressure, followed by a final pressurization step e2) with the H.sub.2-rich product gas originated from at least one vessel performing the step a), to reach the feed pressure.

39. The pressure swing adsorption process according to claim 25 wherein the step b) includes: b1) lowering the pressure to a first intermediate pressure and withdrawing during said step b1) a first stream containing hydrogen, impurities and small amount of CO.sub.2, then b2) lowering the pressure to the target intermediate pressure and withdrawing during said step b2) a second stream containing impurities and small amount of CO.sub.2 and H.sub.2.

40. The pressure swing adsorption process according to claim 39 wherein said first stream obtained in the step b1) of at least one vessel is used as a pressurizing medium in the step e1) of at least one vessel.

41. The pressure swing adsorption process according to claim 25, wherein at least part said additional CO.sub.2-rich stream is used to perform the step c) of at least one vessel, and/or at least part of said additional CO.sub.2-rich stream form part of the CO.sub.2 product.

42. The pressure swing adsorption process according to claim 25, further comprising more pressure equalization steps, wherein an equalization step comprises sending an equalization stream withdrawn from at least one vessel during a de-pressurization stage to at least one vessel during a pressurization stage, so that said stream acts as a pressurising medium for the vessel under pressurization.

43. The pressure swing adsorption process according to claim 42, wherein step b) includes one or more sub-steps of de-pressurization and equalization streams originated from at least one vessel performing step b) are introduced into at least one vessel performing the re-pressurization step e).

44. The pressure swing adsorption process according to claim 25, further comprising: i) adsorption at a high pressure producing a H.sub.2-rich hydrogen product stream, ii) depressurization to a first intermediate pressure and production of a first output stream containing hydrogen, impurities and small amounts of CO.sub.2, iii) depressurization to a second intermediate pressure and production of a second stream containing impurities and small amounts of CO.sub.2 and H.sub.2, iv) purge with a CO.sub.2-rich purge stream and withdrawal of a stream rich of impurities and containing a small amount of CO.sub.2, v) depressurization to sub-atmospheric pressure and production of a first CO.sub.2-rich product stream, vi) purge under vacuum using part of the hydrogen product produced by step i), obtaining a second CO.sub.2-rich product stream, vii) first pressurization by feeding in the pressure vessel at least part of the output stream from step ii), viii) final pressurization by feeding in the pressure vessel a part of the hydrogen product produced by step i).

45. The pressure swing adsorption process according to claim 44, wherein the pressure swing adsorption process is performed in a multiple vessel setup, including a plurality of vessels running in parallel, and the process comprises at least one of the following: the step vi) comprising purge under vacuum using part of the hydrogen product produced by step i) in at least another vessel of the setup; the step vii) comprising feeding at least part of the stream originated from step ii) of at least another vessel of the setup; or the step viii) comprising feeding in the pressure vessel a part of the hydrogen product produced by at least another vessel of the setup at step i).

46. The pressure swing adsorption process according to claim 44, wherein the CO.sub.2-rich purge stream of step iv) includes at least a portion of the first CO.sub.2-rich product stream from step v), at least a portion of the second CO.sub.2-rich product stream from step vi), or a mixture thereof.

47. The pressure swing adsorption process according to claim 44 wherein the step vi) includes at least one of: a first substep vi-1), a second substep vi-2) and a third substep vi-3), wherein the first substep vi-1) produces part of the CO.sub.2-product, the second substep vi-2 produces a stream which is entirely recycled to step iv) of one or more vessels, and the third substep vi-3) produces a stream which is wasted.

Description

DESCRIPTION OF FIGURES

[0102] FIG. 1 is a scheme of a first embodiment of the invention.

[0103] FIG. 2 is a scheme of a second embodiment.

[0104] FIG. 3 is a scheme of a third embodiment featuring pressure equalization.

[0105] FIG. 4 is a plot of CO.sub.2 purity Vs. CO.sub.2 recovery in an embodiment of the invention.

[0106] FIG. 5 is a plot of H.sub.2 purity Vs. H.sub.2 recovery in an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0107] In FIG. 1, the blocks 101 to 105 denote different process steps performed cyclically by columns 110 of a multiple column setup. Each column contains one or more beds of an adsorbent material with a strong affinity to CO.sub.2.

[0108] In step 101, a feed gas 120 containing H.sub.2, CO.sub.2 and impurities is fed to the bottom of the column 110 and traverses the column from bottom to top. CO.sub.2 contained in the feed gas is adsorbed by the adsorbent material and a H.sub.2 product gas, which is substantially free of CO.sub.2, is withdrawn from top. Said step 101 is performed at a high pressure P.sub.high. The step 101 is conducted in such a way that not all the adsorbent material is saturated with CO.sub.2. Particularly, the adsorbent material located in an upper region 112 the column 110 is left unsaturated.

[0109] In step 102, the column 110 is de-pressurized to an intermediate pressure P.sub.hv-rc (pressure of heavy recycle) and a waste stream 122 is withdrawn from top of the column. This waste stream 122 contains impurities and low amounts of CO.sub.2 and medium amounts of H.sub.2 and can be used as a fuel.

[0110] Step 103 is performed at the pressure P.sub.hv-rc that the column reaches at the end of step 102. The column is purged with a CO.sub.2-rich stream 123 taken from step 104 performed in the same or another column of the setup and a waste stream 124 is withdrawn from top. Also this waste stream 124 contains impurities and low amounts of CO.sub.2 and H.sub.2, It generally contains more CO.sub.2 than the waste stream 122 and less H.sub.2.

[0111] The steps 102 and 103 provide a two-steps sequence of removing impurities from the adsorbent material. In step 102, the column 110 is blown down from top and most of the impurities are removed with the waste stream 122. The impurities are removed together with some residual hydrogen. Some CO.sub.2 may also desorb due to the reduction of pressure. However, the CO.sub.2 desorbed in the lower part 111 of the column is re-captured in the unsaturated upper region 112, resulting in a low content of CO.sub.2 in the waste stream 122. Said partial re-capture of desorbed CO.sub.2 is made possible by the combination of bottom feed in step 101 and blowdown from top in step 102.

[0112] The step 103 achieves a further reduction of impurities contained in the column 110, by purging the column with a high-purity CO.sub.2 stream 123. As the purge is carried out bottom to top, the column gas phase is replaced with high purity CO.sub.2 starting from the bottom and adsorbed impurities at the bottom of the column are replaced by CO.sub.2 due to the stronger affinity of the CO.sub.2 for the adsorbent material and to its higher partial pressure. At the end of the step 103 the column 110 contains very little impurities and can produce a substantially pure carbon dioxide stream upon a further depressurization.

[0113] Step 104 produces CO.sub.2 by de-pressurization of the column 110 from the pressure P.sub.hv-rc to a low pressure P.sub.low below atmospheric pressure (less than 1 atm abs as above defined). Evacuation is carried out from the bottom end of the column 110, obtaining a substantially pure CO.sub.2 stream 125. Part of this stream 125 forms the CO.sub.2 purge stream 123 used in step 103 and the remaining part forms a CO.sub.2 product 126 which is exported.

[0114] At the end of the step 104, the adsorbent material is regenerated and the column 110 is pressurized back to the high pressure P.sub.high of step 101 by admitting again the feed 120 to the bottom of the column. At the end of the pressurization step, i.e. when the column 110 reaches an internal pressure of P.sub.high, the step 101 is performed, starting the withdrawal of the H.sub.2 stream 121 from the column top.

[0115] The products of the process are therefore the high-purity H.sub.2 stream 121 and the high-purity CO.sub.2 stream 126.

[0116] It shall be noted that the CO.sub.2 stream 125 obtained in step 104 is partially used as a process stream 123 (namely a purge stream) to carry out the step 103. Particularly, the purge stream 123 is an aid to remove adsorbed and gas-phase impurities from the column at intermediate pressure.

[0117] In a multiple-column setup, this purge stream 123 can be transferred directly from a first source column performing the step 104 to a second target column performing the step 103. More preferably, the stream 123 is stored temporarily in a suitable tank.

[0118] In other embodiments, columns of a multiple setup may exchange other process streams, and any exchanged process stream may be transferred directly from one or more source columns to one or more target columns, or may be temporarily stored.

[0119] FIG. 2 discloses a second embodiment of the invention involving steps 201 to 208. Compared to FIG. 1, the embodiment of FIG. 2 provides a greater number of process streams exchanged between columns of the setup:

[0120] a part of the hydrogen product delivered at the high pressure P.sub.high is used as a purge aid to purge a column under vacuum, and another part is used as a pressurizing medium of a column after regeneration;

[0121] the depressurization of the column from the above mentioned pressure P.sub.high to P.sub.hv-rc is carried out in two steps, including a first step passing from P.sub.high to a pressure P.sub.BD1>P.sub.hv-rc and then a second step to reach the target pressure P.sub.hv-rc during the first step of depressurization from P.sub.high to P.sub.BD1, a H.sub.2-rich recycle stream is obtained which is used as a pressurizing medium;

[0122] part of the CO.sub.2 product is used as purging aid.

[0123] More in detail, the step 201 is an adsorption step similar to the above described step 101. However only a part 222 of the so obtained hydrogen product 221 is exported. A portion 223 of the hydrogen product 221 is used as pressurizing medium in a column performing the step 208 and another portion 224 of the hydrogen product is use to purge a column under vacuum performing the step 206.

[0124] In step 202, the column is depressurized from the top end to a first intermediate pressure P.sub.BD1. The H.sub.2-recycle stream 225 leaving the column contains predominantly H.sub.2 with impurities and small amounts of CO.sub.2. Said stream 225 is used to pressurize a column undergoing step 207 via a compressor 226.

[0125] In step 203, the column is further depressurized from the top end, from said first intermediate pressure P.sub.BD1 to the target heavy recycle pressure P.sub.hv-rc. The stream 227 leaving the column during the step 203 is rich in impurities and has a low content of CO.sub.2 and hydrogen. Depending on the impurities of the stream, it can be wasted or used as fuel, e.g. for a reformer.

[0126] In an option, the step 203 includes substeps 203a and 203b. Particularly, an embodiment provides that the first substep 203a is a blowdown to atmospheric pressure and the following substep 203b is an intermediate evacuation where the column reaches a subatmospheric pressure.

[0127] Step 204 is performed at the above mentioned heavy recycle pressure P.sub.hv-rc. The column is purged from the bottom end using a CO.sub.2 rich stream 228, either from a column under step 205 or step 206 or a mixture of the two. During this step 204 the impurities that are still adsorbed within the column are displaced by the more strongly adsorbing CO.sub.2. The outlet stream 229 is enriched in impurities and contains small amounts of CO.sub.2 and H.sub.2. Said step 204 can be named heavy recycle step.

[0128] Step 205 provides evacuation and generation of the CO.sub.2 product (substantially pure CO.sub.2). The column is depressurized from the bottom end to the target low pressure P.sub.low (less than 1 atm abs) to produce a high purity CO.sub.2 stream 230. Part of the CO.sub.2 stream 230 concurs to the purge stream 228. A remaining part of said stream 230 forms the CO.sub.2 product 231, which is exported.

[0129] The step 205 may be split into a first substep 205a and a second substep 205b. In the first substep 205a, the effluent withdrawn from bottom of the column is entirely sent as heavy recycle stream 228 to a column performing the step 204. In the second substep 205b, which is carried out before or after the first substep 205a, the effluent withdrawn from bottom of the column is exported as CO.sub.2 product 231.

[0130] Step 206 provides a further purge of the column at the low pressure P.sub.low. The column is purged from the top end under vacuum using the part 224 of the hydrogen product taken from another column performing the step 201. During this step, the top end of the column is cleaned from impurities and CO.sub.2.

[0131] During the beginning of the vacuum purge 206, the stream 232 has a relatively high content of CO.sub.2 and can concur to form the CO.sub.2 purge stream 228 for the heavy recycle step 204, as denoted by the line 233 or part of the CO.sub.2 product 231. For long purge durations, the remaining part 234 is a waste stream.

[0132] The step 206 may be split into a first substep 206a and a second substep 206b and a third substep 206c. In the first substep 206a, the effluent withdrawn from bottom of the column is entirely sent as heavy recycle stream 228 to a column performing the step 204. In the second substep 206b, which is carried out before or after the first substep 206a, the effluent withdrawn from bottom of the column is exported as CO.sub.2 product 231. In the third and final substep 206c, the effluent is wasted.

[0133] The step 206 helps achieving a full regeneration of the adsorbent material, removing the CO.sub.2 still adsorbed at the end of the step 205. In addition, said step 206 provides additional CO.sub.2 that can be used for purging another column, e.g. with stream 233. This increase the CO.sub.2 recovery because a smaller amount of the CO.sub.2 product gas 230 is required for the purge step 204, i.e. the exported CO.sub.2 product 231 can be increased. In case the CO.sub.2 concentration of stream 232 is sufficiently high, said stream 232 or a part thereof can also directly form part of the CO.sub.2 product thereby also increasing the recovery.

[0134] The CO.sub.2 product stream 231 and the vacuum purge outlet stream 232 are extracted from the column with a compressor 235.

[0135] Step 207 is termed recycle pressurization. The column is pressurized from P.sub.low to a medium pressure P.sub.mid using the hydrogen rich stream 225 delivered by the compressor 226 and withdrawn from a column performing the step 201. The final pressure at the end of this step 207 is usually below the highest pressure P.sub.high.

[0136] In some embodiments, the H.sub.2 recycle stream 225 may be mixed with the feed 120 and/or the H.sub.2 recycle stream 225 may be admitted into a column also or only during the adsorption step 201.

[0137] Step 208 is termed product pressurization. The column undergoes a final pressurization to reach the feed pressure P.sub.high using the hydrogen rich product 223 withdrawn from a column performing the step 201.

[0138] Said step 208 is preferably performed top to bottom using the hydrogen product 223, as shown. As an alternative, the step 208 may be performed bottom to top using the feed 120.

[0139] FIG. 3 discloses a third embodiment involving steps 301 to 312 which substantially operates according to the process of FIG. 2 and further includes some pressure equalization steps.

[0140] The hydrogen stream 321 obtained at step 301 is partly used as a pressurizing medium, as stream 323 directed to step 312, and as a vacuum purge aid as stream 324 directed to step 308. The remaining part 322 is exported.

[0141] The lowering of pressure from P.sub.high to P.sub.hv-rc includes intermediate de-pressurization steps bringing the column to intermediate pressure P.sub.PE1, P.sub.PE2, P.sub.PE3 in steps 302 to 304 wherein P.sub.PE1>P.sub.PE2>P.sub.PE3. Similarly, the raising of pressure from P.sub.low to P.sub.high (after evacuation and regeneration of the adsorbent material and withdrawal of the CO.sub.2 product) includes intermediate pressurization steps reaching the pressure P.sub.PE3, P.sub.PE2, P.sub.PE1 in steps 309 to 311.

[0142] A gaseous stream containing mainly hydrogen, some impurities and little CO.sub.2 withdrawn from each intermediate de-pressurization step is used as pressurizing medium in a corresponding intermediate pressurization step. During this step, two columns are directly connected, so that the final pressure at the end of the intermediate de-pressurization step is equal to the final pressure of the intermediate pressurization step. This is called pressure equalization (PE). Accordingly, the stream 325 is passed from step 302 to step 311; the stream 326 from step 303 to step 310, and the stream 327 from step 304 to step 309.

[0143] After the step 304, the column is further depressurized in step 305 from the column top to reach the pressure P.sub.hv-rc. The effluent of said step 305 (withdrawn from the top of the column) contains mainly impurities and little H.sub.2 and CO.sub.2.

[0144] After the step 305 is completed, the column is purged with a CO.sub.2-rich stream 328 taken from steps 307 and/or 308. In step 307 the pressure is lowered to P.sub.low releasing the CO.sub.2 product 329 and in step 308 the column is vacuum purged with some H.sub.2 product 324 from step 301 and a vacuum purge stream 330 is extracted.

[0145] The CO.sub.2-rich stream 328 used in the step 306 may include part of the stream 329 and/or of the stream 330 if the latter has a sufficient concentration of CO.sub.2.

[0146] After the vacuum purge step 308, the column undergoes the PE-pressurization steps 309 to 311, where pressure is raised with the aid of the above mentioned streams 325, 326 and 327 withdrawn from one or more columns performing the steps 302 to 304.

[0147] The number of pressure vessels (columns) may vary. For the implementation of the process of FIG. 1, the minimum number of columns is two, using also storage tanks for the recycle. For the implementation of the process of FIG. 3, including three pressure equalization steps, the minimum number of columns is four. If a continuous feed is required, a number of columns greater than the minimum is appropriate. Preferred embodiments may be implemented preferably with 8 to 12 columns.

EXAMPLES

[0148] The following examples relate to processing a feed gas at a temperature of 298 K, a pressure of 30 bar abs and the following composition: N.sub.2:H.sub.2:CO.sub.2=25:50:25 (vol %).

Example 1

[0149] The above described feed gas is processed with a cycle configuration similar to FIG. 2 but with feed pressurization in the step 208 (i.e. step 208 receives the feed 120 from bottom instead of product from top) and without the second purge (step 206 not used).

[0150] A hydrogen purity of 95% and a hydrogen recovery of 90%, together with a CO.sub.2 purity of 95% and a recovery of >89% are achieved.

[0151] The estimated energy consumption of the process is 2000 kJ/kgCO.sub.2 (energy per kg of CO.sub.2 separated) including about 370 kJ/kg for recompression of CO.sub.2 to 110 bar for storage (for CO.sub.2 storage, energy requirement for recompression approx. 370 kJ/kgCO.sub.2). This energy consumption was estimated for the production of H.sub.2>95% purity, >90% recovery and CO.sub.2>90% recovery and >95% purity.

[0152] When also the second purge 206 is performed, the above energy consumption drops to around 1500 kJ/kg.

[0153] FIG. 4 is a plot obtained when optimizing the cycle of the present example to maximize the CO.sub.2 purity and recovery, coproduction of H.sub.2 at 95% purity and 90% recovery, fixed evacuation pressure.

Example 2

[0154] The above described feed gas is processed with a cycle configuration according to FIG. 3:

[0155] A CO.sub.2 recovery of >90% and a CO.sub.2 purity of >95% is achieved, while coproducing hydrogen with a purity >99% and a recovery >86%.

[0156] The estimated energy consumption for the separation is approx. 800 kJ/kgCO.sub.2, including CO.sub.2 compression to 110 bar (approx. 370 kJ/kgCO.sub.2 for recompression). This energy consumption was estimated for the production of H.sub.2 at purity >95% and recovery >90% and production of CO.sub.2 with recovery >90% and purity >95%.

[0157] FIG. 5 is a plot obtained when optimizing the cycle depicted in FIG. 3 to maximize the purity and recovery of H.sub.2 with coproduction of CO.sub.2 at purity 95% and recovery 90% and fixed evacuation pressure.