PROCESS

Abstract

A process for producing a methane-containing gas mixture includes the steps of: (i) passing a first feed gas mixture including hydrogen and carbon dioxide through a bed of methanation catalyst to react a portion of the hydrogen with at least a portion of the carbon dioxide and form a methane-containing gas mixture containing residual hydrogen, (ii) adding an oxygen-containing gas to the methane-containing gas mixture containing residual hydrogen to form a second feed gas mixture, and (iii) passing the second feed gas mixture through a bed of an oxidation catalyst to react the residual hydrogen and oxygen to form a hydrogen depleted methane-containing gas mixture.

Claims

1-12. (canceled)

13. A process for producing a methane-containing gas mixture comprising the steps of: (i) passing a first feed gas mixture comprising hydrogen and carbon dioxide through a bed of methanation catalyst to react a portion of the hydrogen with at least a portion of the carbon dioxide and form a methane-containing gas mixture containing residual hydrogen, (ii) adding an oxygen-containing gas to the methane-containing gas mixture containing residual hydrogen to form a second feed gas mixture, and (iii) passing the second feed gas mixture through a bed of a selective oxidation catalyst at an inlet temperature in the range 150 to 350 C. to selectively react the residual hydrogen and oxygen to form a hydrogen depleted methane-containing gas mixture.

14. The process according to claim 13, wherein the hydrogen concentration in the first feed gas mixture is 20% by volume.

15. The process according to claim 13, wherein the first feed gas mixture is a synthesis gas comprising hydrogen, carbon dioxide and carbon monoxide.

16. The process according to claim 13, wherein the first feed gas mixture is a prepared by mixing a hydrogen-containing gas mixture with a carbon dioxide-containing gas mixture.

17. The process according to claim 16, wherein the hydrogen-containing gas mixture is a methane-containing gas mixture.

18. The process according to claim 16, wherein the first feed gas mixture or hydrogen-containing gas mixture and/or the carbon dioxide-containing gas mixture are subjected to a desulphurisation step prior to the methanation step.

19. The process according to claim 13, wherein the methanation catalyst is a ruthenium- or nickel-containing methanation catalyst.

20. The process according to claim 13, wherein the methanation catalyst is operated at an inlet temperature in the range 200 to 350 C.

21. The process according to claim 13, wherein the temperature of the methane-containing gas mixture containing residual hydrogen is adjusted before mixing it with the oxygen containing gas.

22. The process according to claim 13, wherein the oxidation catalyst is a supported precious metal oxidation catalyst.

23. The process according to claim 13, wherein the process is performed at a pressure in the range 5 to 80 bar abs.

24. The process according to claim 13, wherein the methanation catalyst is operated at an inlet temperature in the range 200 to 300 C.

25. The process according to claim 13, wherein the methanation catalyst is operated at an inlet temperature in the range 230 to 280 C.

Description

[0030] The invention is further illustrated by reference to the accompanying drawing in which;

[0031] FIG. 1 is a depiction of a flowsheet of one embodiment according to the present invention.

[0032] In FIG. 1, a carbon dioxide-containing gas mixture 10 is pre-heated in heat exchanger 12 and passed through a desulphuriser vessel 14 containing fixed bed of a zinc oxide desulphurisation material 16 in to remove hydrogen sulphide and form a desulphurised carbon dioxide-containing gas mixture 18. A hydrogen-containing gas mixture 20 is pre-heated in heat exchanger 22 and the resulting heated gas in line 24 mixed with the desulphurised carbon dioxide-containing gas mixture 18 to form a first feed gas mixture in line 26. The first feed gas mixture 26 is passed at an inlet temperature of 200-350 C. and a pressure of 5-80 bar abs to the inlet of a methanation vessel 28 containing a fixed bed of a particulate nickel-containing methanation catalyst 30. Methanation reactions occur as the gas passes adiabatically through the catalyst bed to form a methane-containing gas mixture containing residual hydrogen 32. The methane-containing gas mixture containing residual hydrogen 32 is passed to a heat exchanger 34 where is it cooled in heat exchange with a coolant to a temperature below about 350 C. The resulting cooled gas mixture 36 is mixed with an oxygen stream fed via line 38 at a temperature of about 30 C. to form a second feed gas mixture 40. The second feed gas mixture 40 is passed at an inlet temperature of 150-350 C. to the inlet of an oxidation vessel 42 containing a fixed bed of a particulate alumina-supported platinum selective oxidation catalyst 44. Oxidation reactions occur as the gas passes adiabatically through the catalyst bed to form a hydrogen-depleted methane-containing gas mixture 46.

[0033] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0034] The invention is further illustrated by reference to the following calculated Example.

Example 1

[0035] 1000 kgmols/hr of a hydrogen-containing gas mixture containing 90% by volume methane and 10% by volume hydrogen is mixed with 27.5 kgmols/hr of a desulphurised carbon dioxide gas mixture consisting essentially of carbon dioxide to form a first feed gas mixture. The first feed gas mixture is fed to a methanator containing a bed of Katalco CRG-S2R at an inlet temperature of about 250 C. and a pressure of 30 bara. The methanation reaction proceeds to reduce the hydrogen concentration to approximately 1% by volume and heat the reacting gas to about 330 C.

[0036] The resultant gas stream is cooled down to 300 C. in a heat exchanger and mixed with 5.9 kgmols/hr of an oxygen-containing gas containing 99.5% oxygen with the resultant gas stream passed to an oxidation vessel containing a bed of Puravoc 73 selective oxidation catalyst, where oxygen reacts selectively with hydrogen. The resulting gas has a composition of approx. 93.8% vol CH.sub.4, 0.5% vol CO.sub.2, 5.6% vol H.sub.2O, 800 ppmv oxygen and <100 ppmv hydrogen and leaves the reactor at about 348 C. and 28 bara.

[0037] If, alternatively, the resultant gas stream from the methanator is fed to a second methanation stage at about 250 C., then the hydrogen level in the product gas from the oxidation vessel is about 0.36% vol. Hence, the invention allows a significant reduction in the hydrogen level of the product methane containing gas to be achieved in comparison to using a second methanation stage.