Separation of H.sub.2, CO, and CH.sub.4 synthesis gas with methane wash

• US 4,102,659 A
• Filed: 10/06/1976
• Issued: 07/25/1978
• Est. Priority Date: 06/04/1976
• Status: Expired due to Term

• Current Assignee: UOP Des Plaines IL A NY General Partnership
• Original Assignee: Union Carbide Corporation (Dow, Inc.)
• Inventors: Martin, Jay Robert
• Primary Examiner(s): Yudkoff, Norman
• Application Number:
  US05/730,287
• Time in Patent Office: 657 Days
• Field of Search: 62/17, 62/40, 62/21, 62/28, 62/31, 62/26, 55/68
• US Class Current: 62/625
• CPC Class Codes: F25J 2200/70 Refluxing the column with a condensed part of the feed stream, i.e. fractionator top is stripped or self-rectified
• CPC Class Codes: F25J 2200/74 Refluxing the column with at least a part of the partially condensed overhead gas
• CPC Class Codes: F25J 2200/76 Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
• CPC Class Codes: F25J 2205/04 in the feed line, i.e. upstream of the fractionation step
• CPC Class Codes: F25J 2205/30 using a washing, e.g. "scrubbing" or bubble column for purification purposes
• CPC Class Codes: F25J 2210/04 Mixing or blending of fluids with the feed stream
• CPC Class Codes: F25J 2215/60 Methane
• CPC Class Codes: F25J 2235/60 the fluid being (a mixture of) hydrocarbons
• CPC Class Codes: F25J 2245/02 Recycle of a stream in general, e.g. a by-pass stream
• CPC Class Codes: F25J 2270/02 Internal refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/04 Internal refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 2270/08 Internal refrigeration by flash gas recovery loop
• CPC Class Codes: F25J 2270/12 External refrigeration with liquid vaporising loop
• CPC Class Codes: F25J 2270/14 External refrigeration with work-producing gas expansion loop
• CPC Class Codes: F25J 2270/20 Quasi-closed internal or closed external hydrogen refrigeration cycle
• CPC Class Codes: F25J 2270/24 Quasi-closed internal or closed external carbon monoxide refrigeration cycle
• CPC Class Codes: F25J 2270/30 Quasi-closed internal or closed external helium refrigeration cycle
• CPC Class Codes: F25J 2270/40 Quasi-closed internal or closed external air refrigeration cycle
• CPC Class Codes: F25J 2270/42 Quasi-closed internal or closed external nitrogen refrigeration cycle
• CPC Class Codes: F25J 2270/50 Quasi-closed internal or closed external oxygen refrigeration cycle
• CPC Class Codes: F25J 2270/58 : Quasi-closed internal or closed external argon refrigeration cycle
• CPC Class Codes: F25J 2270/60 : Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons
• CPC Class Codes: F25J 2280/02 : Control in general, load changes, different modes ("runs"), measurements
• CPC Class Codes: F25J 3/0223 : H2/CO mixtures, i.e. synthesis gas; Water gas or shifted synthesis gas production of carbon monoxide containing gas in general C01B32/40, C10J, C10K; production of hydrogen containing gas C01B3/00
• CPC Class Codes: F25J 3/0233 : separation of CnHm with 1 carbon atom or more
• CPC Class Codes: F25J 3/0252 : separation of hydrogen production of hydrogen containing gas in general C01B3/00, e.g. separation of hydrogen or hydrogen containing gases form gaseous mixtures at low temperatures C01B3/506
• CPC Class Codes: F25J 3/0261 : separation of carbon monoxide production of carbon monoxide containing gas in general C01B32/40, C10J, C10K
• CPC Class Codes: Y10S 62/92 : Carbon monoxide
• CPC Class Codes: Y10S 62/932 : From natural gas

Abstract

A process for separation of a synthesis feed gas mixture containing hydrogen, carbon monoxide and methane. The feed gas mixture is countercurrently contacted with methane wash liquid in a first adsorption zone and the recovered absorber bottoms liquid is contacted with hydrogen rich vapor in a second adsorption zone to recover residual hydrogen gas as overhead. Hydrogen rich vapor is generated for the second absorption zone by vaporization of second absorption zone bottoms liquid, with the bottoms liquid recovered from the second absorption zone being fractionated to recover carbon monoxide overhead gas. The process achieves the rejection of hydrogen impurity in the second absorption and associated vaporization steps with minimum degradation of refrigeration temperature levels in the process and high recovery of carbon monoxide product.

Claims

1. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) cooling the feed gas mixture;

(b) countercurrently contacting the cooled feed gas mixture with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(c) throttling the bottoms liquid from the first absorption zone to lower pressure;

(d) countercurrently contacting the throttled bottoms liquid with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the throttled bottoms liquid therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide, and warming second absorption zone bottoms liquid to vaporize a gaseous fraction therefrom containing hydrogen and carbon monoxide, as said hydrogen rich vapor for said second absorption zone;

(e) fractionating the bottoms liquid recovered from said second absorption zone in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane; and

(f) recirculating at least part of the bottoms liquid recovered from the fractionation zone to the first absorption zone as the methane wash liquid therefor.

2. A process according to claim 1 wherein the vaporization warming of said second absorption zone bottoms liquid is conducted internally in said second absorption zone by indirect heat exchange of said bottoms liquid with an in-process fluid stream.

3. A process according to claim 1 wherein bottoms liquid is withdrawn from the second absorption zone for said warming in step (d) to vaporize a gaseous fraction therefrom containing hydrogen and carbon monoxide, the vaporized gaseous fraction is separated from the warmed bottoms liquid and the vaporized gaseous fraction is passed to the second absorption zone as the hydrogen rich vapor therefor, with the warming being conducted so as to maintain a temperature gradient in the fluid being warmed, from the initial part of the warming to the final part thereof.

4. A process according to claim 3 wherein the bottoms liquid recovered from the fractionation zone is heat exchanged with the bottoms liquid withdrawn from the second absorption zone for the vaporization warming of the latter in step (d) and cooling of the fractionation zone bottoms liquid, the cooled fractionation zone bottoms liquid is pumped to higher pressure and a portion thereof is subcooled, with the sub-cooled portion being passed to the first absorption zone as the methane wash liquid therefor and the remaining portion of the cooled pumped fractionation zone bottoms liquid being warmed and vaporized by heat exchange with the feed gas mixture as at least part of the cooling of step (a) to form product methane gas.

5. A process according to claim 1 wherein the feed gas mixture contains 50 to 70 mol % hydrogen, 15 to 45 mol % carbon monoxide, and 2 to 6 mol % methane.

6. A process according to claim 1 wherein the second absorption zone comprises between two and five theoretical separation stages.

7. A process according to claim 1 wherein the residual hydrogen overhead gas recovered from the second absorption zone is warmed by heat exchange with the feed gas mixture as at least part of the cooling of step (a).

8. A process according to claim 1 wherein the overhead hydrogen gas recovered from the first absorption zone is warmed by heat exchange with the feed gas mixture as at least part of the cooling of step (a).

9. A process according to claim 8 wherein the overhead hydrogen gas warmed by heat exchange with the feed gas mixture is further warmed to about ambient temperature and withdrawn as product hydrogen gas.

10. A process according to claim 1 wherein a part of the carbon monoxide overhead gas recovered from the fractionation zone is warmed by heat exchange with the feed gas mixture as at least part of the cooling of step (a).

11. A process according to claim 1 wherein the feed gas mixture contains at least 30 mol % carbon monoxide and the first absorption zone is internally cooled for removal of the heat of solution of carbon monoxide in the methane wash liquid generated in the contacting of the cooled feed gas mixture with the methane liquid therein.

12. A process according to claim 1 comprising the further steps of:

dividing the overhead gas recovered from the fractionation zone into two portions comprising a first portion and a second portion of the overhead gas recovered from the fractionation zone for warming thereof with the feed gas mixture as at least part of the cooling of the latter in step (a);

compressing the heat exchanged overhead gas second portion to higher pressure and withdrawing a part thereof as carbon monoxide product gas;

cooling the remaining part of the compressed overhead gas portion and dividing same into a minor portion for isentropic expansion and a major refrigeration fluid portion;

isentropically expanding the minor portion and joining the isentropically expanded minor portion with the first portion of the overhead gas recovered from the fractionation zone to form a recycle minor part gas;

warming the recycle minor part gas and joining same with the heat exchanged overhead gas second portion for compression therewith;

dividing the major refrigeration fluid portion into two parts, cooling one part for condensation thereof to form cooled liquid by heat exchange with fractionation zone bottoms liquid for vaporization of the latter to provide reboil vapor for the fractionation zone, subcooling the cooled liquid one part by heat exchange with the recycle minor part gas for the warming of the latter, throttling the other part of the major refrigeration fluid portion to low pressure and joining the subcooled liquid one part with the throttled other part to form a refrigeration gas-liquid mixture;

throttling the refrigeration gas-liquid mixture to lower pressure and warming same to form a first gaseous fraction and a first liquid fraction therefrom by heat exchange with a portion of the bottoms liquid recovered from the fractionation zone being recirculated to the first absorption zone as the methane wash liquid therefor, for subcooling of the latter;

separating the first gaseous fraction of the refrigeration gas-liquid mixture from the first liquid fraction thereof;

throttling the separated first liquid fraction of the refrigeration gas-liquid mixture to still lower pressure and warming same to form a second gaseous fraction and a second liquid fraction therefrom by heat exchange with the heat gas mixture as at least part of the cooling in step (a);

separating the second gaseous fraction of the refrigeration gas-liquid mixture first liquid fraction from the second liquid fraction thereof;

heat exchanging the separated second liquid fraction with fractionation zone overhead to provide reflux cooling for the fractionation zone; and

joining the second gaseous fraction and the first gaseous fraction with the recycle minor part gas.

13. A process according to claim 12 wherein the feed gas mixture is heat exchanged with the bottoms liquid recovered from the fractionation zone for cooling of the latter prior to the subcooling of a portion thereof by heat exchange with the throttled lower pressure refrigeration gas-liquid mixture, as part of the cooling of the feed gas mixture in step (a).

14. A process according to claim 12 comprising the further steps of:

sensing the temperature of the fractionation zone at a point in the lower section thereof;

converting the fractionation zone temperature sensing into a transmittable signal; and

adjusting the fraction of the major refrigeration fluid portion which is divided as the other part thereof by means of the transmittable signal to maintain a predetermined temperature in the fractionation zone lower section.

15. A process according to claim 1 wherein a portion of the overhead gas recovered from the fractionation zone is circulated in a refrigeration circuit to provide cooling of the bottoms liquid recovered from the fractionation zone recirculated to the first absorption zone as the methane wash liquid therefor, at least part of the cooling of the feed gas mixture in step (a) and reflux cooling for the fractionation zone.

16. A process according to claim 15 wherein overhead gas in the refrigeration circuit is heat exchanged with fractionation zone bottoms liquid for vaporization of the latter to provide reboil vapor for the fractionation zone and cooling of the heat exchanged overhead gas.

17. A process according to claim 1 wherein a portion of the overhead gas recovered from the fractionation zone is warmed by heat exchange with the feed gas mixture as at least part of the cooling of step (a), compressed to higher pressure and withdrawn as carbon monoxide product.

18. A process according to claim 1 wherein an externally supplied refrigeration fluid is circulated in a closed loop refrigeration circuit to provide refrigeration for the process, comprising the further steps of:

compressing the refrigeration fluid, cooling the compressed refrigeration fluid and dividing same into a minor portion for isentropic expansion and a major refrigeration fluid portion;

isentropically expanding the refrigeration fluid minor portion, warming the isentropically expanded refrigeration fluid minor portion and joining same with the refrigeration fluid for compression therewith;

dividing the major refrigeration fluid portion of the compressed refrigeration fluid into two parts, cooling one part for condensation thereof to form cooled liquid by heat exchange with fractionation zone bottoms liquid for vaporization of the latter to provide reboil vapor for the fractionation zone, subcooling the cooled liquid one part by heat exchange with the isentropically expanded refrigeration fluid minor portion for the warming of the latter, throttling the other part of the major refrigeration fluid to low pressure and joining the subcooled liquid one part with the throttled other part to form a refrigeration gas-liquid mixture;

throttling the refrigeration gas-liquid mixture to lower pressure and warming same to form a first gaseous fraction and a first liquid fraction therefrom by heat exchange with a portion of the bottoms liquid recovered from the fractionation zone being recirculated to the first absorption zone as the methane wash liquid therefor, for subcooling of the latter;

separating the first gaseous fraction of the refrigeration gas-liquid mixture from the first liquid fraction thereof;

throttling the separated first liquid fraction of the refrigeration gas-liquid mixture to still lower pressure and warming same to form a second gaseous fraction and a second liquid fraction therefrom by heat exchange with the feed gas mixture as at least part of the cooling in step (a);

separating the second gaseous fraction of the refrigeration gas-liquid mixture first liquid fraction from the second liquid fraction thereof;

heat exchanging the separated second liquid fraction for vaporization thereof with fractionation zone overhead to provide reflux cooling for the fractionation zone; and

joining the vaporized second liquid fraction, the second gaseous fraction and the first gaseous fraction with the isentropically expanded refrigeration fluid minor portion.

19. A process according to claim 18 wherein the refrigeration fluid has a normal boiling point of less about -178°

C.

20. A process according to claim 18 wherein the externally supplied refrigeration fluid comprises a substantially pure component selected from the group consisting of helium, argon, carbon monoxide, nitrogen, deuterium, air, oxygen, and flourine.

21. A process according to claim 18 wherein the externally supplied refrigeration fluid is nitrogen.

22. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) cooling the feed gas mixture;

(b) countercurrently contacting the cooled feed gas mixture with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(c) throttling the bottoms liquid from the first absorption zone to lower pressure;

(d) countercurrently contacting the throttled bottoms liquid with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the throttled bottoms liquid therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide;

(e) withdrawing bottoms liquid from the second absorption zone and warming same to vaporize a gaseous fraction therefrom containing hydrogen and carbon monoxide, separating the vaporized gaseous fraction from the warmed bottoms liquid, and passing the vaporized gaseous fraction to the second absorption zone as the hydrogen rich vapor therefor, the warming being conducted so as to maintain a temperature gradient in the fluid being warmed from the initial part of the warming to the final part thereof;

(f) fractionating the warmed bottoms liquid in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane; and

(g) recirculating at least part of the bottoms liquid recovered from the fractionation zone to the first absorption zone as the methane wash liquid therefor.

23. A process according to claim 22 wherein the bottoms liquid recovered from the fractionation zone is passed in countercurrent flow heat exchange relationship with the bottoms liquid withdrawn from the second absorption zone in the vaporization warming of the latter in step (e).

24. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) cooling the feed gas mixture;

(b) countercurrently contacting the cooled feed gas mixture with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead product and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(c) throttling the bottoms liquid from the first absorption zone to lower pressure;

(d) countercurrently contacting the throttled bottoms liquid with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the throttled bottoms liquid therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide;

(e) withdrawing bottoms liquid from the second absorption zone and warming same to vaporize a gaseous fraction therefrom containing hydrogen and carbon monoxide, separating the vaporized gaseous fraction from the warmed bottoms liquid, and passing the vaporized gaseous fraction to the second absorption zone as the hydrogen rich vapor therefor, the warming being conducted so as to maintain a temperature gradient in the fluid being warmed from the initial part of the warming to the final part thereof;

(f) fractionating the warmed bottoms liquid in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane;

(g) heat exchanging the bottoms liquid recovered from the fractionation zone with the bottoms liquid recovered from the second absorption zone for the vaporization warming of the latter in step (e) and cooling of the fractionation zone bottoms liquid;

(h) pumping the cooled fractionation zone bottoms liquid to higher pressure and subcooling a portion of same, passing the subcooled portion to the first absorption zone as the methane wash liquid therefor and warming the remaining portion of the cooled pumped fractionation zone bottoms liquid for vaporization thereof by heat exchange with the feed gas mixture as at least part of the cooling of step (a);

(i) dividing the overhead gas recovered from the fractionation zone in step (f) into two portions comprising a first portion and a second portion;

(j) heat exchanging the second portion of the overhead gas recovered from the fractionation zone for warming thereof with the feed gas mixture as at least part of the cooling of the latter in step (a);

(k) comprsssing the heat exchanged overhead gas second portion to higher pressure and withdrawing a part thereof as carbon monoxide product gas;

(l) cooling the remaining part of the compressed overhead gas portion and dividing same into a minor portion for isentropic expansion and a major heat pumping fluid portion;

(m) isentropically expanding the minor portion and joining the isentropically expanded minor portion with the first portion of the overhead gas recovered from the fractionation zone in step (f) to form a recycle minor part gas;

(n) warming the recycle minor part gas and joining same with the heat exchanged overhead gas second portion from step (j) for compression therewith in step (k);

(o) dividing the major heat pumping fluid portion into two parts, cooling one part for condensation thereof to form cooled liquid by heat exchange with fractionation zone bottoms liquid for vaporization of the latter to provide reboil vapor for the fractionation zone, subcooling the cooled liquid one part by heat exchange with the recycle minor part gas for the warming of the latter in step (n), throttling the other part of the major heat pumping fluid portion to low pressure and joining the subcooled liquid one part with the throttled other part to form a heat pumping gas-liquid mixture;

(p) throttling the heat pumping gas-liquid mixture to lower pressure and warming same to form a first gaseous fraction and a first liquid fraction therefrom by heat exchange with a portion of the bottoms liquid recovered from the fractionation zone being recirculated to the first absorption zone as the methane wash liquid therefor, for the subcooling of the latter in step (h);

(q) separating the first gaseous fraction of the heat pumping gas-liquid mixture from the first liquid fraction thereof;

(r) throttling the separated first liquid fraction of the heat pumping gas-liquid mixture to still lower pressure and warming same to form a second gaseous fraction and a second liquid fraction therefrom by heat exchange with the feed gas mixture as at least part of the cooling in step (a);

(s) separating the second gaseous fraction of the heat pumping gas-liquid mixture first liquid fraction from the second liquid fraction thereof;

(t) heat exchanging the separated second liquid fraction with fractionation zone overhead to provide reflux cooling for the fractionation zone; and

(u) joining the second gaseous fraction and the first gaseous fraction with the recycle minor part gas.

25. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) cooling the feed gas mixture;

(b) countercurrently contacting the cooled feed gas mixture with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(c) throttling the bottoms liquid from the absorption zone to lower pressure;

(d) countercurrently contacting the throttled bottoms liquid with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the throttled bottoms liquid therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide, and warming second absorption zone bottoms liquid to vaporize a gaseous fraction therefrom containing hdrogen and carbon monoxide, as said hydrogen rich vapor for said second absorption zone;

(e) fractionating the bottoms liquid recovered from said second absorption zone in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane;

(f) recirculating at least part of the bottoms liquid recovered from the fractionation zone to the first absorption zone as the methane wash liquid therefor;

(g) sensing the flow rate of the feed gas mixture introduced to the first absorption zone;

(h) converting the feed gas flow rate sensing into a transmittable signal; and

(i) adjusting the flow rate of the methane wash liquid recirculated to the first absorption zone by means of the transmittable signal to maintain a predetermined ratio of the flow rate of the recirculated methane wash liquid to the flow rate of the feed gas mixture introduced to the first absorption zone.

26. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) first partial cooling the feed gas mixture to condense a first feed liquid fraction therefrom;

(b) separating the first feed liquid fraction from the uncondensed first partial cooled feed gas mixture;

(c) throttling the separated first feed liquid fraction to a lower pressure level;

(d) second partial cooling the uncondensed first partial cooled feed gas mixture to condense a second feed liquid fraction from the uncondensed second partial cooled feed gas mixture;

(e) throttling the separated second feed liquid fraction to said lower pressure level of step (c);

(f) joining the throttled second feed liquid fraction with the throttled first feed liquid fraction to form a combined feed liquid;

(g) warming the combined feed liquid for partial vaporization thereof by heat exchange with the uncondensed first partial cooled feed gas mixture for the second partial cooling of the latter;

(h) separating the vapor portion of the partially vaporized combined feed liquid from the liquid portion thereof;

(i) joining the separated vapor portion of step (h) with the feed gas mixture for said first partial cooling of the feed gas mixture in step (a);

(j) countercurrently contacting the uncondensed second partial cooled feed gas mixture of step (d) with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(k) throttling the bottoms liquid from the first absorption zone to lower pressure;

(l) countercurrently contacting the throttled bottoms liquid of step (k) with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the throttled bottoms liquid therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide, and warming second absorption zone bottoms liquid to vaporize a gaseous fraction thereof containing hydrogen and carbon monoxide, as said hydrogen rich vapor for said second absorption zone;

(m) fractionating the bottoms liquid recovered from said second adsorption zone in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane;

(n) passing the liquid portion separated from the partially vaporized combined feed liquid in step (h), comprising substantially pure carbon monoxide, to the fractionation zone in step (m) for enhancement of the purity of carbon monoxide in the overhead gas recovered therefrom; and

(o) recirculating at least part of the bottoms liquid recovered from the fractionation zone to the first absorption zone as the methane wash liquid therefor.

27. A process according to claim 26 wherein the uncondensed first partial cooled feed gas mixture contains at least 15 mol % carbon monoxide.

28. A process for the separation of a feed gas mixture containing hydrogen, carbon monoxide and methane comprising the steps of:

(a) providing the feed gas mixture at a pressure of at least 200 psia;

(b) partial cooling the feed gas mixture to condense a feed liquid fraction therefrom;

(c) separating the feed liquid fraction from the uncondensed partial cooled feed gas mixture;

(d) work expanding the separated uncondensed partial cooled feed gas mixture to lower pressure to form cooled feed gas mixture;

(e) countercurrently contacting the cooled feed gas mixture with a methane wash liquid in a first absorption zone to recover hydrogen gas as overhead and bottoms liquid comprising methane, carbon monoxide and residual hydrogen;

(f) throttling the bottoms liquid from the first absorption zone to lower pressure;

(g) joining the separated feed liquid fraction of step (c) with the throttled first absorption zone bottoms liquid to form a liquid mixture;

(h) countercurrently contacting the liquid mixture of step (g) with hydrogen rich vapor in a second absorption zone for absorption of carbon monoxide from the hydrogen rich vapor by the liquid mixture therein to recover residual hydrogen gas as overhead and bottoms liquid enriched in carbon monoxide, and warming second absorption zone bottoms liquid to vaporize a gaseous fraction therefrom containing hydrogen and carbon monoxide, as said hydrogen rich vapor for said second absorption zone;

(i) fractionating the bottoms liquid recovered from said second absorption zone in a fractionation zone to recover overhead gas comprising carbon monoxide and bottoms liquid comprising methane; and

(j) recirculating at least part of the bottoms liquid recovered from the fractionation zone to the first absorption zone as the methane wash liquid therefor.