PROCESS FOR PRODUCING A GAS STREAM COMPRISING CARBON MONOXIDE

Abstract

A process for producing a gas stream comprising carbon monoxide comprising the steps of (a) feeding a gas mixture comprising carbon dioxide and hydrogen to a burner and combusting it with a sub-stoichiometric amount of an oxygen gas stream to form a combusted gas mixture comprising carbon monoxide, carbon dioxide, hydrogen and steam, (b) passing the combusted gas mixture through a bed of reverse water-gas shift catalyst to form a crude product gas mixture containing carbon monoxide, steam, hydrogen and carbon dioxide, (c) cooling the crude product gas mixture to below the dew point and recovering a condensate to form a dewatered product gas, (d) removing carbon dioxide from the dewatered product gas in a carbon dioxide removal unit to form the gas stream comprising carbon monoxide, and (e) combining carbon dioxide recovered by the carbon dioxide removal unit with the gas mixture comprising hydrogen and carbon dioxide.

Claims

1. A process for producing a gas stream comprising carbon monoxide comprising the steps of (a) feeding a gas mixture comprising carbon dioxide and hydrogen to a burner disposed in a reverse water-gas shift vessel and combusting it with a sub-stoichiometric amount of an oxygen gas stream to form a combusted gas mixture comprising carbon monoxide, carbon dioxide, hydrogen and steam, (b) passing the combusted gas mixture though a bed of reverse water-gas shift catalyst disposed within the reverse water-gas shift vessel to form a crude product gas mixture containing carbon monoxide, steam, hydrogen and carbon dioxide, (c) cooling the crude product gas mixture to below the dew point and recovering a condensate to form a dewatered product gas, (d) removing carbon dioxide from the dewatered product gas in a carbon dioxide removal unit to form the gas stream comprising carbon monoxide, and (e) combining carbon dioxide recovered by the carbon dioxide removal unit with the gas mixture comprising hydrogen and carbon dioxide fed to the reverse water-gas shift vessel.

2. The process according to claim 1, wherein the gas mixture comprises carbon dioxide and hydrogen has a hydrogen to carbon dioxide molar ratio in the range of 2:1 to 10:1.

3. The process according to claim 1, wherein the gas mixture comprising carbon dioxide and hydrogen fed to the burner comprises 15 to 50% by volume, preferably 25 to 40% by volume, of carbon dioxide.

4. The process according to claim 1, wherein the carbon dioxide is obtained from an ammonia plant that uses a hydrocarbon or carbonaceous feed, or from a furnace or boiler flue gas, wherein the furnace or boiler is heated by combustion of a fossil fuel or carbonaceous wastes, or from air or seawater.

5. The process according to claim 1, wherein hydrogen and oxygen for the process are generated using an electrolysis unit to which a source of water is fed.

6. The process according to claim 1, wherein the reverse water-gas shift catalyst comprises 3 to 20% wt nickel, expressed as NiO, on a refractory metal oxide support.

7. The process according to claim 1, wherein the carbon dioxide removal unit comprises a physical wash system or a reactive wash system.

8. The process according to claim 5, wherein the condensate, optionally after treatment to remove contaminants, is fed to the electrolysis unit.

9. The process according to claim 1, wherein the product gas stream comprising carbon monoxide is fed to a Fischer-Tropsch hydrocarbon synthesis unit.

10. The process according to claim 9, wherein a gas mixture comprising methane and carbon dioxide formed by pre-reforming a Fischer-Tropsch tail gas and optionally non-condensable hydrocarbons recovered from a downstream Fischer-Tropsch process are fed to the reverse water-gas shift unit.

11. A process according to claim 10, wherein water co-produced in the Fischer-Tropsch hydrocarbon synthesis unit is treated to remove organic compounds and used in the process.

12. A system for producing a gas stream by the process of claim 1 comprising (b) a feed line for feeding a gas mixture comprising carbon dioxide and hydrogen, (b) a reverse water-gas shift vessel operatively connected to the feed line, said reverse water-gas shift vessel comprising (i) a burner operatively connected to a source of oxygen, (ii) a combustion zone adjacent the burner, (iii) a bed of reverse water-gas shift catalyst disposed downstream of the combustion zone, and (iv) and outlet for a crude product gas mixture containing carbon monoxide, steam and carbon dioxide, (c) means operatively connected to the reverse water-gas shift vessel for cooling the crude product gas mixture to below the dew point and recovering a condensate therefrom to form a dewatered product gas, (d) a carbon dioxide removal unit coupled to the means for cooling the crude product gas mixture and recovering the condensate to recover carbon dioxide from the dewatered product gas, and (e) a conduit that feeds at least a portion of the recovered carbon dioxide from the carbon dioxide removal unit to the feed line for feeding the gas mixture comprising carbon dioxide and hydrogen to the reverse water-gas shift vessel.

13. A process according to claim 1, wherein the gas mixture comprising carbon dioxide and hydrogen fed to the burner comprises 25 to 40% by volume, of carbon dioxide.

Description

[0051] The invention is illustrated by reference to the accompanying drawing in which:

[0052] FIG. 1 is a diagrammatic flowsheet of one embodiment of the invention.

[0053] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, compressors, pumps, vacuum pumps, towers, heat exchangers, 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. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0054] In FIG. 1, a carbon dioxide stream, such as a carbon dioxide stream recovered from a flue gas, is fed to the process via line 10 and combined with a hydrogen stream provided by line 12 to form a mixed gas stream in line 14. A carbon dioxide recycle stream provided by line 16 is combined with the mixed gas in line 14 and the resulting mixed gas fed via line 18 to a gas-gas interchanger 20 where it is heated. The heated mixed gas is fed from the interchanger 20 via line 22 and combined with a pre-reformed tail gas mixture containing hydrogen, carbon dioxide, carbon dioxide, methane and steam, provided by line 24. The resulting mixed gas is provided via line 26 to a heater 28 where it is heated to the inlet temperature for the reverse water-gas shift reaction. Alternatively, the pre-reformed tail gas mixture 24 may be added downstream of the heater 28.

[0055] A heated feed gas mixture is passed from the heater 28 via line 30 to the inlet of a reverse water-gas shift vessel 32. The heated gas mixture is passed to the top of the vessel 32. A burner (not shown) located at the top of the vessel 32 receives a compressed and heated oxygen gas stream 34. The mixed gas and the oxygen combust at the inlet temperature, resulting in combustion of a portion of the hydrogen in a flame within a combustion zone 36 adjacent the burner within the vessel 32. The vessel 32 further comprises a bed of refractory metal oxide-supported nickel oxide reverse water-gas shift catalyst 38 disposed beneath the combustion zone 36. The catalyst promotes the reverse water gas shift reaction thereby forming carbon monoxide. The catalyst also steam-reforms methane in the pre-reformed tail gas from line 24 to form hydrogen and carbon oxides.

[0056] The resulting crude product gas mixture is recovered from the vessel 32 via line 40 and subjected to cooling in a boiler 42, connected to steam drum 44, fed with water via line 46. The partially cooled crude product is fed from the boiler 42 via line 48 to a heat exchanger 50 where it heats a mixture of Fischer-Tropsch tail gas and steam provided by line 52. The heated mixture is passed from the heat exchanger 50 via a line 54 to a pre-reformer vessel 56 containing a bed of nickel pre-reforming catalyst, to form the pre-reformed tail gas mixture 24. The crude product gas mixture is further cooled in heat exchanger 50. From heat exchanger 50, the partially cooled crude product gas is fed to interchanger 20 where it heats the feed gas mixture in line 18. From the interchanger 20 the partially cooled product gas is fed via line 58 to one or more further heat exchangers 60, which may be fed with cold water and/or air, where it is cooled to below the dew point to condense steam present in the crude product gas. A mixture of gas and condensate is passed from the one or more heat exchangers 60 via line 62 to a gas-liquid separator 64, where the condensate is separated and recovered via line 66.

[0057] A dewatered product gas comprising hydrogen, carbon monoxide and carbon dioxide is recovered via line 68 and fed to a conventional carbon dioxide removal unit 70, operating by means of a reactive liquid absorbent, that recovers carbon dioxide from the dewatered product gas. A carbon dioxide gas stream is recovered from the unit 70 via line 72 and compressed in compressor 74 to form the carbon dioxide recycle stream 16. A product gas mixture comprising carbon monoxide and hydrogen is recovered from the carbon dioxide removal unit 70 via line 76.

[0058] In this embodiment, the product gas comprising carbon monoxide in line 76 is subjected to one or more further steps of purification (not shown) and fed to a Fischer-Tropsch hydrocarbon synthesis unit 78 containing one or more Fischer-Tropsch reactors containing a cobalt Fischer-Tropsch hydrocarbon synthesis catalyst. The Fischer-Tropsch hydrocarbon synthesis unit converts the product gas into hydrocarbon products, which are recovered from the unit 78 via line 80. A co-produced water stream is recovered from the Fischer-Tropsch unit 78 via line 82. Within the unit 78, a Fischer-Tropsch tail gas stream is separated from the aqueous and liquid hydrocarbon streams. A portion of the tail gas stream comprising hydrogen, carbon monoxide, carbon dioxide, methane and higher hydrocarbons is recycled to the one or more Fischer-Tropsch reactors. A further portion of the Fischer-Tropsch tail gas stream is recovered from the unit 78 via line 84 and combined with steam provided by line 86 to form the mixture of Fischer-Tropsch tail gas and steam in line 52 fed to the pre-reformer 56. A remaining portion of the tail gas is taken from line 84 as a purge gas 85.

[0059] In this embodiment, an electrolysis unit 90 is used to electrolyse water to form the hydrogen stream 12 and to provide an oxygen stream 90 that is compressed in compressor 92 and heated in heater 94 to form the oxygen stream 34 fed to the reverse water-gas shift vessel 32.

[0060] Water for the electrolysis is provided to the electrolysis unit 88 via line 96. This water may optionally be supplemented by at least a portion of the condensate 66 fed to the electrolysis unit 84 via the dotted line 98.

[0061] In addition, the steam provided in line 86 may be derived at least in part from the co-produced water 82 recovered from the Fischer-Tropsch hydrocarbon synthesis unit 78.