H.SUB.2 .recovery and CO.SUB.2 .separation using membrane

Abstract

A method of treating a gas stream that includes: performing a first pressure swing adsorption (PSA) process generating a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, the first PSA tail gas including a residual H.sub.2 and CO.sub.2; separating CO.sub.2 from the first PSA tail gas using a CO.sub.2 selective membrane, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; performing a second PSA process generating a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, the second PSA tail gas including another residual H.sub.2; mixing the CO.sub.2-lean rejected gas and the second PSA tail gas to generate a mixed tail gas; and separating the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

Claims

1. A method of treating a gas stream, the method comprising: performing a first pressure swing adsorption (PSA) process generating a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, the first PSA tail gas comprising a residual H.sub.2 and CO.sub.2; separating CO.sub.2 from the first PSA tail gas using a CO.sub.2 selective membrane, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; performing a second PSA process generating a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, the second PSA tail gas comprising another residual H.sub.2; mixing the CO.sub.2-lean rejected gas and the second PSA tail gas to generate a mixed tail gas; and separating the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

2. The method of claim 1, further comprising mixing the recovered H.sub.2 permeate gas with the first feed gas.

3. The method of claim 1, further comprising compressing the first PSA tail gas prior to separating the CO.sub.2 from the first PSA tail gas.

4. The method of claim 1, further comprising passing the CO.sub.2-rich permeate gas through another CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas.

5. The method of claim 4, further comprising compressing the CO.sub.2-rich permeate gas prior to passing the CO.sub.2-rich permeate gas through the another CO.sub.2 selective membrane.

6. The method of claim 4, further comprising adding the another CO.sub.2-lean rejected gas to the mixed tail gas prior to separating the H.sub.2 from the mixed tail gas.

7. The method of claim 4, further comprising adding the another CO.sub.2-lean rejected gas to the first PSA tail gas or the second PSA tail gas.

8. The method of claim 1, further comprising performing a steam reforming, generating the first feed gas.

9. The method of claim 1, wherein a CO.sub.2 concentration of the first feed gas is between 10% and 30%.

10. The method of claim 1, wherein a CO.sub.2 concentration of the second feed gas is 0.1% or less.

11. A gas treatment system comprising: a first pressure swing adsorption (PSA) column to generate a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, the first PSA tail gas comprising CO.sub.2 and a residual H.sub.2; a CO.sub.2 selective membrane, connected to the first PSA column and configured to separate CO.sub.2 from the first PSA tail gas, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; a second PSA column to generate a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, the second PSA tail gas comprising another residual H.sub.2; a tail gas mixer to mix the CO.sub.2-lean rejected gas and the second PSA tail gas, generating a mixed tail gas; and an H.sub.2 selective membrane to separate the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

12. The gas treatment system of claim 11, further comprising: a first compressor between the first PSA column and the CO.sub.2 selective membrane; and a second compressor between the second PSA column and the H.sub.2 selective membrane.

13. The gas treatment system of claim 11, further comprising: another CO.sub.2 selective membrane to receive the CO.sub.2-rich permeate gas from the CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas; and a third compressor between the CO.sub.2 selective membrane and the another CO.sub.2 selective membrane.

14. The gas treatment system of claim 13, wherein the tail gas mixer is configured to add the another CO.sub.2-lean rejected gas to the mixed tail gas.

15. The gas treatment system of claim 13, further comprising a CO.sub.2 mixer between the first PSA column and the CO.sub.2 selective membrane, the CO.sub.2 mixer configured to add the another CO.sub.2-lean rejected gas to the first PSA tail gas.

16. The gas treatment system of claim 11, further comprising a heater to heat the mixed tail gas, the heater being between the mixer and the H.sub.2 selective membrane.

17. The gas treatment system of claim 11, further comprising a feed mixer to add the recovered H.sub.2 permeate gas to the first feed gas.

18. An H.sub.2 recovery system comprising: a steam reformer to generate a first feed gas comprising H.sub.2 and CO.sub.2, wherein a CO.sub.2 concentration of the first feed gas is between 10% and 30%; a first pressure swing adsorption (PSA) column to generate a first H.sub.2-product gas and a first PSA tail gas from the first feed gas, the first PSA tail gas comprising the CO.sub.2 and a residual H.sub.2; a CO.sub.2 selective membrane, connected to the first PSA column and configured to separate CO.sub.2 from the first PSA tail gas, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; an H.sub.2 production facility to generate a second feed gas comprising H.sub.2, wherein a CO.sub.2 concentration of the second feed gas is less than 0.1%; a second PSA column to generate a second H.sub.2-product gas and a second PSA tail gas from the second feed gas, the second PSA tail gas comprising another residual H.sub.2; a tail gas mixer to mix the CO.sub.2-lean rejected gas and the second PSA tail gas, generating a mixed tail gas; an H.sub.2 selective membrane to separate the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas; and a feed gas mixer to add the recovered H.sub.2 permeate gas to the first feed gas.

19. The H.sub.2 recovery system of claim 18, further comprising another CO.sub.2 selective membrane to receive the CO.sub.2-rich permeate gas from the CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas.

20. The H.sub.2 recovery system of claim 19, further comprising a cryogenic distillation system to purify the another CO.sub.2-rich permeate gas from the another CO.sub.2 selective membrane.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a block diagram of a tail gas H.sub.2 recovery system.

(2) FIG. 2 is a block diagram of a tail gas H.sub.2 recovery system with single-stage CO.sub.2 separation.

(3) FIG. 3 is a block diagram of a tail gas H.sub.2 recovery system with two-stage CO.sub.2 separation.

(4) FIG. 4 is a block diagram of a tail gas H.sub.2 recovery system with two-stage CO.sub.2 separation.

(5) FIG. 5 is an example process flow diagram of a method of tail gas H.sub.2 recovery.

DETAILED DESCRIPTION

(6) Embodiments described herein provide methods and systems of H.sub.2 recovery and CO.sub.2 separation, particularly for residual H.sub.2 recovery from multiple pressure swing adsorption (PSA) tail gases. Generally, H.sub.2 in refinery is produced from reforming naphtha or hydrocarbon as an off-gas with H.sub.2 concentration about 70-90%. The rest of gases in the off-gas are mainly hydrocarbons, e.g., un-converted feedstock, with traces of hydrogen sulfide (H.sub.2S). H.sub.2S in the off-gas is typically removed using H.sub.2S selective absorption. The sweet off-gas after the H.sub.2S removal can be then sent to a PSA unit to produce pure H.sub.2. In the typical PSA unit, hydrocarbons are adsorbed using molecular sieves and other adsorbents while H.sub.2 are passing through the adsorbent beds and exiting the process at concertation above 99%. Beside producing the pure H.sub.2 stream, another stream as a PSA tail gas is produced during the desorption step. The PSA tail gas stream has H.sub.2 content about 40-50% and used mostly as a fuel gas. It is therefore desired to recover the residual H.sub.2 from the PSA tail gas to improve the overall H.sub.2 production efficiency and yield. In many refineries, additional H.sub.2 production is needed to meet its H.sub.2 demand. For example, a steam reforming (SR) can be used in addition to H.sub.2 recovery from the refinery off-gas. The SR product gas stream typically contains about 70% H.sub.2, about 20% CO.sub.2, and about 7% methane or other hydrocarbon, and about 3% carbon monoxide (CO). One common PSA column can be used to purify both the SR outlet and the refinery off-gas by mixing both the streams. Alternately, two PSA columns can be used to separately treat the two H.sub.2-containing streams. This two-PSA process typically achieves about 85% H.sub.2 recovery.

(7) FIG. 1 is an example of a conventional tail gas H.sub.2 recovery system 100. Solid arrows are used in FIG. 1 to indicate gas or liquid flows, e.g., inflow and outflow. Generally, to meet the pure H.sub.2 demand, most refineries use multiple H.sub.2 production routes, e.g., a refinery off-gas and steam reforming (SR). Pressure swing adsorption (PSA) processes may be employed to separate H.sub.2 but some residual H.sub.2 escapes in the tail gases. The residual H.sub.2 may be recovered from the tail gases using an H.sub.2 selective membrane. In FIG. 1, a steam reforming (SR) product gas 102 from a SR process, e.g., steam methane reforming (SMR) and a refinery off-gas 104 are combined and sent to a pressure swing adsorption (PSA) column 106 and separated into an H.sub.2 product gas 108 and a tail gas 110. The tail gas 110 is then sent to an H.sub.2 selective membrane 112 to form a recovered H.sub.2-rich gas 114 and a final tail gas 116. The recovered H.sub.2-rich gas 114 is sent to a recycle gas compressor 118 and then looped back to the PSA column 106 for improving the overall H.sub.2 recovery. The final tail gas 116, as a retentate stream can be used as a fuel gas.

(8) However, the enrichment rate of H.sub.2 in the permeate stream of a membrane process in this conventional method can be limited due to the high CO.sub.2 concentration in the tail gas 110. Various embodiments described in this disclosure can improve the overall H.sub.2 recovery by employing (1) two PSA columns to separately recover H.sub.2 from the two streams, and (2) a CO.sub.2 selective membrane to first recover CO.sub.2 from the first PSA tail gas, e.g., a CO.sub.2-rich gas, before mixing it with the second PSA tail gas, e.g., a CO.sub.2-free tail gas.

(9) In the following, various embodiments of the methods and systems of H.sub.2 recovery including separate tail gas treatment are described. Embodiments with one CO.sub.2 selective membrane is first described referring to FIG. 2, and embodiments with two CO.sub.2 selective membranes are then described referring to FIG. 3-4. FIG. 5 is an example process flow diagram. In this disclosure, unless otherwise noted, concentrations of gases components in a fluid are provided based on volumetric concentration in percentile, referred to as %.

(10) Single-Stage CO.sub.2 Separation

(11) FIG. 2 is an example of a tail gas H.sub.2 recovery system 200 with single-stage CO.sub.2 separation. In FIG. 2, unlike the conventional method described referring to FIG. 1, two H.sub.2-containing gas streams are separately treated with two PSA columns to recover H.sub.2. Each PSA yields a tail gas containing residual H.sub.2, which can be recovered by the tail gas H.sub.2 recovery system 200. The two tail gas compositions are different at least in the CO.sub.2 concentration.

(12) First, a steam reforming (SR) process, e.g., steam methane reforming (SMR), generates a SR product gas 102 containing H.sub.2 and CO.sub.2. Accordingly, in some embodiments, the tail gas H.sub.2 recovery system 200 includes a steam reformer to generate the SR product gas 102. The SR process catalytically convert hydrocarbons such as methane, propane, or butane, to a syn-gas, a mixture of H.sub.2 and CO. The SR process can include water-gas shift (WGS) reaction between CO and water (H.sub.2O) to produce more H.sub.2. CO.sub.2 is thus generated as a by-product, making H.sub.2 and CO.sub.2 the main gas components of the SR product gas 102. In various embodiments, the SR product gas 102 contains about 10% to 30% CO.sub.2, for example, about 20%. A first pressure swing adsorption (PSA) column 202 processes the SR product gas 102 and generates an H.sub.2 product gas 108 and a first PSA tail gas 204. In some embodiments, the first PSA tail gas 204 contains about 30% to about 70% CO.sub.2, for example, about 50%. The CO.sub.2 concentration of the first PSA tail gas 204 can be greater than that of the SR product gas 102 as a result of the PSA process. The first PSA tail gas 204 is first processed for CO.sub.2 recovery prior to mixing with another tail gas.

(13) In various embodiments, as illustrated in FIG. 2, the first PSA tail gas 204 is sent to a first tail gas compressor 206 and then a first CO.sub.2 selective membrane 208. In some embodiments, the first PSA tail gas 204 is compressed to have a pressure between about 2 bar (200 kPa) and about 20 bar (2,000 kPa), for example, at about 10 bar (1,000 kPa). The first tail gas compressor 206 can compress the first PSA tail gas 204 to optimize its pressure, e.g., about 10 bar, as a feed gas to the first CO.sub.2 selective membrane 208. The first CO.sub.2 selective membrane 208 can separate the first PSA tail gas 204 into a CO.sub.2-rich permeate gas 210 and a CO.sub.2-lean rejected gas 212. The first CO.sub.2 selective membrane 208 can include polymer-based rubbery materials such as polyether-polyamide copolymers, e.g., Pebax polymer, and polydimethylsiloxane (PDMS). In some embodiments, the tail gas H.sub.2 recovery system 200 can include a fluid temperature control system to maintain the temperature of the first PSA tail gas 204, e.g., above its dew point to prevent condensation of a water vapor present in the first PSA tail gas 204. In some embodiments, the process temperature for CO.sub.2 separation is between about 25 C. and 60 C. In one or more embodiments, the tail gas H.sub.2 recovery system 200 can further include, prior to the first CO.sub.2 selective membrane 208, an additional pretreatment unit such as heater and carbon bed and particle filter.

(14) In some embodiments, the CO.sub.2 concentration in the CO.sub.2-rich permeate gas 210 can be about 70-75%, achieving about 95% CO.sub.2 recovery from the first PSA tail gas 204. In some embodiments, the CO.sub.2-rich permeate gas 210 can be sent to another purification system 214, e.g., a cryogenic distillation system, for further purification to produce a high purity CO.sub.2 stream, e.g., >99% CO.sub.2.

(15) On the other hand, the residual H.sub.2 of the first PSA tail gas 204 is concentrated in the CO.sub.2-lean rejected gas 212. Further, other gas components, e.g., methane (CH.sub.4), N.sub.2, and the like, can also be concentrated in the CO.sub.2-lean rejected gas 212.

(16) Still referring to FIG. 2, a second PSA column 216 is used to separate a refinery off-gas 104 into the H.sub.2 product gas 108 and a second PSA tail gas 218. Although not specifically illustrated in FIG. 2, the tail gas H.sub.2 recovery system 200 includes an H.sub.2 production facility to generate the refinery off-gas 104. Further, the refinery off-gas 104 can be processed in one or more upstream facilities for acid gas removal, e.g., H.sub.2S and CO.sub.2. For example, an amine-based stripper, e.g., a diglycolamine absorber (DGA) unit, can be used. With the acid removal, the refinery off-gas 104 becomes a so-called sweetened product stream. Accordingly, in various embodiments, the second PSA tail gas 218 is a CO.sub.2-free gas. In this disclosure, a gas is considered CO.sub.2-free if the CO.sub.2 concentration is below 0.1%. The second PSA tail gas 218 is then compressed by a second tail gas compressor 220. In some embodiments, the second PSA tail gas 218 is compressed to have a pressure between about 2 bar (200 kPa) and about 20 bar (2,000 kPa), for example, at about 10 bar (1,000 kPa).

(17) The CO.sub.2-lean rejected gas 212 from the first CO.sub.2 selective membrane 208 and the second PSA tail gas 218 from the second PSA column 216 can be mixed using a tail gas mixer 222 to form a mixed tail gas 224. The mixed tail gas 224 is then sent to a heater 226 and an H.sub.2 selective membrane 112 to form a recovered H.sub.2-rich gas 124 and a final tail gas 116. In various embodiments, the mixed tail gas 224 is heated to between about 60 C. and 200 C., for example, about 150 C. The H.sub.2 selective membrane 112 can include a polymer-based membrane in a glassy form, or a dense protonic ceramic membrane based on a single-phase and mixed-phase perovskite-type oxidic protonic ceramic material.

(18) In various embodiments, the recovered H.sub.2-rich gas 114 is obtained as a permeate gas. The recovered H.sub.2-rich gas 114 can be sent to a recycle gas compressor 118 and then looped back to be mixed using a feed gas mixer 228 with the SR product gas 102 for improving the overall H.sub.2 recovery. In some embodiments, the recovered H.sub.2-rich gas 114 is compressed to have a pressure at a same level of the SR product gas 102 that is being fed to the first PSA column 202, e.g., between about 2 bar (200 kPa) and about 40 bar (4,000 kPa), for example, at about 20 bar (2,000 kPa).

(19) On the other hand, the final tail gas 116 is obtained as a rejected gas. The final tail gas 116 can contain about 30-50% of CH.sub.4 or other hydrocarbons and 10-15% H.sub.2. The final tail gas 116 can be used, for example, as a fuel gas, mixed with a sale gas stream, or recycled as an additive to a feed to the steam reforming (SR) process. In some embodiments, especially when the heating value is not sufficient for other applications, the final tail gas 116 can be sent to an amine-based CO.sub.2 removal facility. By integrating the CO.sub.2 separation as a part of H.sub.2 recovery, the final tail gas 116 contains less CO.sub.2 compared to the conventional methods, thereby improving its heating value.

(20) In various embodiments, the tail gas H.sub.2 recovery process with CO.sub.2 separation can reduce the membrane area required for the H.sub.2 selective membrane 112 compared to the conventional methods, thereby also reducing the overall capital expenditure for H.sub.2 recovery

(21) Two-Stage CO.sub.2 Separation

(22) FIG. 3 is an example of a tail gas H.sub.2 recovery system 300 with two-stage CO.sub.2 separation. Like numbered items are as described with respect to FIG. 2 and will not be repeated in detail. In FIG. 3, the tail gas H.sub.2 recovery system 300 uses two CO.sub.2 selective membranes in series for two-stage CO.sub.2 separation. The H.sub.2 production stage can be identical to prior embodiments described referring to FIG. 2. An H.sub.2 product gas 108 is obtained from each of a SR product gas 102 and a refinery off-gas 104 using a first PSA column 202 and a second PSA column 216, respectively. Further, the first PSA tail gas 204 is separated using a first CO.sub.2 selective membrane 208 into a CO.sub.2-rich permeate gas 210 and a CO.sub.2-lean rejected gas 212.

(23) Unlike the prior embodiments described referring to FIG. 2, in FIG. 3, the CO.sub.2-rich permeate gas 210 is processed for further H.sub.2 recovery. Using two CO.sub.2 selective membranes, it is possible to further concentrate the CO.sub.2 in the tail gas, e.g., the CO.sub.2-rich permeate gas 210 while recovering more H.sub.2 in the reject sides of both membranes. The CO.sub.2-rich permeate gas 210 is sent to a third tail gas compressor 302 for compression and then to a second CO.sub.2 selective membrane 304 that generates a second CO.sub.2-rich permeate gas 306 and a second CO.sub.2-lean rejected gas 308. In some embodiments, the second CO.sub.2-rich permeate gas 306 contains about 90% CO.sub.2. The second CO.sub.2-lean rejected gas 308 and the CO.sub.2-lean rejected gas 212 can be mixed using a tail gas mixer 222 with the second PSA tail gas 218 to form a mixed tail gas 224. The mixed tail gas 224 can be processed for H.sub.2 recovery in the same way as described above referring to FIG. 2.

(24) In various embodiments, the second CO.sub.2 selective membrane 304 can include polymer-based rubbery materials such as polyether-polyamide copolymers, e.g., Pebax polymer and polydimethylsiloxane (PDMS). In some embodiments, the second CO.sub.2 selective membrane 304 can use the same membrane material as the first CO.sub.2 selective membrane 208, but it is not necessary. The membrane area can be larger in the first CO.sub.2 selective membrane 208 than the second CO.sub.2 selective membrane 304. In some embodiments, the CO.sub.2-rich permeate gas 210 after the first CO.sub.2 separation has a pressure of about 1 bar (100 kPa), which can be compressed again using the third tail gas compressor 302 to have a pressure between about 2 bar (200 kPa) and about 20 bar (2,000 kPa), for example, at about 10 bar (1,000 kPa), prior to the second CO.sub.2 separation. In one or more embodiments, the tail gas H.sub.2 recovery system 300 can include one or more additional pretreatment units between the first CO.sub.2 selective membrane 208 and the second CO.sub.2 selective membrane 304.

(25) FIG. 4 is an example of a part of a tail gas H.sub.2 recovery system 400 with two-stage CO.sub.2 separation. Like numbered items are as described with respect to FIGS. 2-3 and will not be repeated in detail. In FIG. 4, some components of the tail gas H.sub.2 recovery system 400, e.g., PSA columns and H.sub.2 selective membrane, are omitted for illustration purpose.

(26) In FIG. 4, the second CO.sub.2-lean rejected gas 308 is sent back to be added to a feed gas to the first CO.sub.2 selective membrane 208. Accordingly, the second CO.sub.2-lean rejected gas 308 is mixed with the first PSA tail gas 204 in FIG. 4 instead of mixing with the CO.sub.2-lean rejected gas 212 as described in FIG. 3.

(27) In FIG. 2-4, the recovered H.sub.2-rich gas 114 is sent back to be mixed with the SR product gas 102 for improving the overall H.sub.2 recovery and CO.sub.2 capture. By this loop, any residual CO.sub.2 in the recovered H.sub.2-rich gas 114 can be subject to another cycle of CO.sub.2 separation after the first PSA column 202. In some embodiments, although not specifically illustrated in FIGS. 2-4, the recovered H.sub.2-rich gas 114 can instead be sent back and mixed with the refinery off-gas 104 to be treated with the second PSA column 216.

(28) FIG. 5 is a process flow diagram of a method 500 of tail gas H.sub.2 recovery. In FIG. 5, the method 500 starts with performing a first pressure swing adsorption (PSA) process 502 generating a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, where the first PSA tail gas includes a residual H.sub.2 and CO.sub.2. Subsequently, CO.sub.2 is separated 504 from the first PSA tail gas using a CO.sub.2 selective membrane, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas. A second PSA process is performed 506 generating a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, where the second PSA tail gas includes another residual H.sub.2. Next, the CO.sub.2-lean rejected gas and the second PSA tail gas are mixed 508 to generate a mixed tail gas, followed by separating 510 H.sub.2 in the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

EXAMPLES

(29) The H.sub.2 recovery and CO.sub.2 separation was simulated by Aspen HYSYS V12 to examine its efficiency in an example of two tail gas treatment in accordance with an embodiment. Membrane unit was designed using the Saudi Aramco proprietary Industrial Membrane Process Simulator (IMPS) (External extension within HYSYS). Two feed gasses, a steam reforming (SR) product gas and a refinery off-gas, are first simulated for PSA processes. Table 1 summarizes the two feed gas compositions and the tail gas compositions after the PSA processes. It was confirmed that both tail gases 1 and 2 still contain residual H.sub.2 at 26% and 42%, respectively, after the PSA processes. Further, the tail gas 1 contains 47% CO.sub.2 while the tail gas 2 does not contain any CO.sub.2.

(30) TABLE-US-00001 TABLE 1 Aspen HYSYS simulation results for PSA process. Tail gas 2 SR product Refinery Tail gas 1 (from gas off-gas (from SR) off-gas) Temp. ( F.) 105.0 105.0 101.4 102.9 Press. (bar) 20.0 20.0 1.5 1.5 Molar flow (lb-mole/h) 1.0 10.sup.4 1.0 10.sup.4 4341.5 2603.5 Mass flow (ton/day) 1326.0 708.1 1251.1 545.3 H.sub.2 (%) 70.00 85.00 26.34 42.4 CO (%) 3.00 0.00 7.05 0.00 CO.sub.2 (%) 20.00 0.00 47.32 0.00 CH.sub.4 (%) 6.80 5.00 18.65 19.19 C.sub.2H.sub.6 (%) 0.00 5.00 0.13 19.19 C.sub.3H.sub.8 (%) 0.00 3.00 0.04 11.51 C.sub.4H.sub.10 (%) 0.00 2.00 0.01 7.67 H.sub.2O (%) 0.20 0.00 0.46 0.00

(31) Further, the H.sub.2 recovery and CO.sub.2 separation from these tail gases using two-stage CO.sub.2 separation were simulated. Membrane permeance used in the simulation and the result of the simulation are summarized in Tables 2 and 3, respectively. The CO.sub.2 separation from the tail 1 using two CO.sub.2 selective membranes was simulated at following process conditions: temperature at 105 F. (40.6 C.) and pressure at 10.0 bar. The first permeated gas (permeate CO.sub.2-1) is sent to the second CO.sub.2 selective membrane. The rejected gases from the two CO.sub.2 selective membranes (rejected CO.sub.2-1 and rejected CO.sub.2-2) are combined for the subsequent H.sub.2 separation using an H.sub.2 selective membrane simulated at a temperature of 300 F. (148.9 C.) and a pressure of 10 bar. The residual H.sub.2 present in the two tail gases is recovered as the permeate gas. The unrecovered H.sub.2 is 6.8% H.sub.2 in the final CO.sub.2 permeate (permeate CO.sub.2-2 in Table 3) and 10.7% H.sub.2 in the rejected gas after the H.sub.2 separation. Based on the initial volume of H.sub.2 in the two feed gases, the PSA processes followed by the tail gas H.sub.2 recovery yielded about 97% H.sub.2 recovery. The use of this tail gas H.sub.2 recovery process improved the H.sub.2 recovery from about 85%. On the other hand, the CO.sub.2 in the tail gas 1 is recovered as the final CO.sub.2 permeate (permeate CO.sub.2-2 in Table 3). A high concentration of about 90% CO.sub.2 is obtained, and more than 85% of CO.sub.2 is recovered. Consequently, the final tail gas contains about 38% methane and only about 8% CO.sub.2, this tail gas has a heating value greater than a tail gas from a conventional process.

(32) TABLE-US-00002 TABLE 2 Membrane permeance used in Aspen HYSYS simulation. CO.sub.2 selective membrane H.sub.2 selective membrane (GPU) (GPU) H.sub.2 37.5 300 N.sub.2 10 10 CO 20 2 CO.sub.2 300 30 CH.sub.4 20 10 C.sub.2H.sub.6 50 1

(33) TABLE-US-00003 TABLE 3 Aspen HYSYS simulation results for tail gas H.sub.2 recovery using two-stage CO.sub.2 separation. Rejected Permeate Rejected Permeate Rejected Permeate CO.sub.2-1 CO.sub.2-1 CO.sub.2-2 CO.sub.2-2 H.sub.2 H.sub.2 Temp. ( F.) 105.0 105.0 105.0 105.0 300.0 300.0 Press. (bar) 10.0 1.0 10.0 1.0 10.0 1.0 Molar flow 1860.6 2480.9 521.7 1959.2 2991.3 1994.4 (lb-mole/h) Mass flow 299.5 951.5 92.0 859.5 842.9 93.3 (ton/day) H.sub.2 (%) 41.20 15.18 46.65 6.81 10.74 89.94 CO (%) 13.21 2.43 9.10 0.65 9.59 0.32 CO.sub.2 (%) 10.41 75.01 19.85 89.69 8.06 2.82 CH.sub.4 (%) 34.96 6.42 24.08 1.71 38.29 6.52 C.sub.2H.sub.6 (%) 0.18 0.09 0.24 0.05 16.66 0.28 C.sub.3H.sub.8 (%) 0.03 0.05 0.06 0.04 9.99 0.08 C.sub.4H.sub.10 (%) 0.00 0.02 0.00 0.02 6.66 0.03 H.sub.2O (%) 0.01 0.81 0.01 1.02 0.00 0.01

EMBODIMENTS

(34) An embodiment described herein provides a method of treating a gas stream, where the method includes: performing a first pressure swing adsorption (PSA) process generating a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, the first PSA tail gas including a residual H.sub.2 and CO.sub.2; separating CO.sub.2 from the first PSA tail gas using a CO.sub.2 selective membrane, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; performing a second PSA process generating a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, the second PSA tail gas including another residual H.sub.2; mixing the CO.sub.2-lean rejected gas and the second PSA tail gas to generate a mixed tail gas; and separating the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

(35) In an aspect, combinable with any other aspect, the method further includes mixing the recovered H.sub.2 permeate gas with the first feed gas.

(36) In an aspect, combinable with any other aspect, the method further includes compressing the first PSA tail gas prior to separating the CO.sub.2 from the first PSA tail gas.

(37) In an aspect, combinable with any other aspect, the method further includes passing the CO.sub.2-rich permeate gas through another CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas.

(38) In an aspect, combinable with any other aspect, the method further includes compressing the CO.sub.2-rich permeate gas prior to passing the CO.sub.2-rich permeate gas through the another CO.sub.2 selective membrane.

(39) In an aspect, combinable with any other aspect, the method further includes adding the another CO.sub.2-lean rejected gas to the mixed tail gas prior to separating the H.sub.2 from the mixed tail gas.

(40) In an aspect, combinable with any other aspect, the method further includes adding the another CO.sub.2-lean rejected gas to the first PSA tail gas or the second PSA tail gas.

(41) In an aspect, combinable with any other aspect, the method further includes performing a steam reforming, generating the first feed gas.

(42) In an aspect, combinable with any other aspect, a CO.sub.2 concentration of the first feed gas is between 10% and 30%.

(43) In an aspect, combinable with any other aspect, a CO.sub.2 concentration of the second feed gas is 0.1% or less.

(44) An embodiment described herein provides a gas treatment system that includes: a first pressure swing adsorption (PSA) column to generate a first H.sub.2-product gas and a first PSA tail gas from a first feed gas, the first PSA tail gas including CO.sub.2 and a residual H.sub.2; a CO.sub.2 selective membrane, connected to the first PSA column and configured to separate CO.sub.2 from the first PSA tail gas, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; a second PSA column to generate a second H.sub.2-product gas and a second PSA tail gas from a second feed gas, the second PSA tail gas including another residual H.sub.2; a tail gas mixer to mix the CO.sub.2-lean rejected gas and the second PSA tail gas, generating a mixed tail gas; and an H.sub.2 selective membrane to separate the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas.

(45) In an aspect, combinable with any other aspect, the gas treatment system further includes: a first compressor between the first PSA column and the CO.sub.2 selective membrane; and a second compressor between the second PSA column and the H.sub.2 selective membrane.

(46) In an aspect, combinable with any other aspect, the gas treatment system further includes: another CO.sub.2 selective membrane to receive the CO.sub.2-rich permeate gas from the CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas; and a third compressor between the CO.sub.2 selective membrane and the another CO.sub.2 selective membrane.

(47) In an aspect, combinable with any other aspect, the tail gas mixer is configured to add the another CO.sub.2-lean rejected gas to the mixed tail gas.

(48) In an aspect, combinable with any other aspect, the gas treatment system further includes a CO.sub.2 mixer between the first PSA column and the CO.sub.2 selective membrane, where the CO.sub.2 mixer is configured to add the another CO.sub.2-lean rejected gas to the first PSA tail gas.

(49) In an aspect, combinable with any other aspect, the gas treatment system further includes a heater to heat the mixed tail gas, where the heater is between the mixer and the H.sub.2 selective membrane.

(50) In an aspect, combinable with any other aspect, the gas treatment system further includes a feed mixer to add the recovered H.sub.2 permeate gas to the first feed gas.

(51) An embodiment described herein provides an H.sub.2 recovery system that includes: a steam reformer to generate a first feed gas including H.sub.2 and CO.sub.2, where a CO.sub.2 concentration of the first feed gas is between 10% and 30%; a first pressure swing adsorption (PSA) column to generate a first H.sub.2-product gas and a first PSA tail gas from the first feed gas, the first PSA tail gas including the CO.sub.2 and a residual H.sub.2; a CO.sub.2 selective membrane, connected to the first PSA column and configured to separate CO.sub.2 from the first PSA tail gas, generating a CO.sub.2-rich permeate gas and a CO.sub.2-lean rejected gas; an H.sub.2 production facility to generate a second feed gas including H.sub.2, where a CO.sub.2 concentration of the second feed gas is less than 0.1%; a second PSA column to generate a second H.sub.2-product gas and a second PSA tail gas from the second feed gas, the second PSA tail gas including another residual H.sub.2; a tail gas mixer to mix the CO.sub.2-lean rejected gas and the second PSA tail gas, generating a mixed tail gas; an H.sub.2 selective membrane to separate the residual H.sub.2 and the another residual H.sub.2 from the mixed tail gas using an H.sub.2 selective membrane, generating a recovered H.sub.2 permeate gas and a final rejected gas; and a feed gas mixer to add the recovered H.sub.2 permeate gas to the first feed gas.

(52) In an aspect, combinable with any other aspect, the H.sub.2 recovery system further includes another CO.sub.2 selective membrane to receive the CO.sub.2-rich permeate gas from the CO.sub.2 selective membrane, generating another CO.sub.2-rich permeate gas and another CO.sub.2-lean rejected gas.

(53) In an aspect, combinable with any other aspect, the H.sub.2 recovery system further includes a cryogenic distillation system to purify the another CO.sub.2-rich permeate gas from the another CO.sub.2 selective membrane.

(54) While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.