Process for producing a gas stream containing carbon monoxide - Patent Application 20070122997

Abstract

A process is described for producing a gas stream comprising carbon monoxide, the process comprising: (a) supplying a gas mixture comprising carbon dioxide and hydrogen to a burner disposed within a reverse water gas shift vessel and combusting the gas mixture with a sub-stoichiometric oxygen gas stream to form a combustion gas mixture comprising carbon monoxide, carbon dioxide, hydrogen, and water vapor; (b) passing the combustion gas mixture through a bed of a reverse water gas shift catalyst disposed within the reverse water gas shift vessel to form a crude product gas mixture comprising carbon monoxide, water vapor, hydrogen, and carbon dioxide; (c) cooling the crude product gas mixture below the dew point and recovering condensate to form a dehydrated product gas; (d) removing carbon dioxide from the dehydrated product gas in a carbon dioxide removal unit to form a gas stream comprising carbon monoxide; and (e) combining the carbon dioxide recovered by the carbon dioxide removal unit with the gas mixture comprising hydrogen and carbon dioxide that was supplied to the reverse water gas shift vessel.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a gas stream comprising carbon monoxide from a feed gas comprising carbon dioxide and hydrogen by the reverse water gas shift reaction. [Background technology]

[0002] Gas streams containing carbon monoxide can be used in processes for the synthesis of a variety of chemicals, including hydrocarbons and oxygenates, such as alcohols.

[0003] The reverse water gas shift reaction can be expressed as follows: CO2 + H2 ⇔ CO + H2O ΔH == + 9.8 kcal / mol

[0004] The reverse water gas shift process is favored at high temperatures.

[0005] WO2019175476A1 discloses a method for producing carbon monoxide by combining oxygen with a carbon dioxide stream to form a carbon dioxide-based mixture, combining the carbon dioxide-based mixture with a hydrogen-based stream to form a gaseous feed, feeding a hydrocarbon-containing stream to the hydrogen-based stream before feeding the carbon dioxide-based mixture, and feeding the gaseous feed to a reactor containing at least one catalyst. The gaseous feed is treated by partial oxidation in the reactor so that the carbon dioxide reacts with hydrogen in the presence of oxygen in the reactor to form heat.

[0006] WO2020114899A1 discloses a process for performing a reverse water gas shift reaction at high temperatures in a reaction vessel, in which no catalyst is present in the reaction vessel, and carbon dioxide, hydrogen, and oxygen are introduced into the reaction vessel separately, with the hydrogen and oxygen being introduced into the reaction vessel via a burner so that the temperature in the reaction vessel is maintained in the range of 1000-1500°C by changing the molar ratio of hydrogen to oxygen. However, this process increases the amount of excess hydrogen that needs to be burned in the process to close the heat balance.

[0007] The present inventors have discovered an improved method for more efficiently carrying out the reverse water gas shift reaction with sufficient conversion to produce a gas stream containing carbon monoxide. Summary of the Invention

[0008] Accordingly, the present invention provides a process for producing a gas stream comprising carbon monoxide, the process comprising: (a) supplying a gas mixture comprising carbon dioxide and hydrogen to a burner disposed in a reverse water gas shift vessel and combusting the gas mixture with a sub-stoichiometric oxygen gas stream to form a combustion gas mixture comprising carbon monoxide, carbon dioxide, hydrogen, and water vapor; (b) passing the combustion gas mixture through a bed of reverse water gas shift catalyst disposed in the reverse water gas shift vessel to form a crude product gas mixture comprising carbon monoxide, water vapor, hydrogen, and carbon dioxide; (c) cooling the crude product gas mixture below the dew point and recovering condensate to form a dehydrated product gas; (d) removing carbon dioxide from the dehydrated product gas in a carbon dioxide removal unit to form a gas stream comprising carbon monoxide; and (e) combining the carbon dioxide recovered by the carbon dioxide removal unit with the gas mixture comprising hydrogen and carbon dioxide supplied to the reverse water gas shift vessel.

[0009] The present invention further provides a system for producing a gas stream comprising carbon monoxide by the present process.

[0010] In this process, a stream of carbon dioxide and a stream of hydrogen are combined to form a feed gas mixture. Optionally, a portion of the hydrogen may be fed separately to the reverse water gas shift vessel. Optionally, a portion of the carbon dioxide may be fed separately to the reverse water gas shift vessel.

[0011] Hydrogen is burned in the reverse water gas shift vessel to generate heat for the reverse water gas shift reaction. Therefore, hydrogen should be provided in excess of carbon dioxide so that sufficient hydrogen remains after combustion to drive the reaction over the reverse water gas shift catalyst. Excess hydrogen is also desirable given the potential end use of carbon monoxide-containing gas in the Fischer-Tropsch synthesis of hydrocarbons, where the H:CO ratio is preferably approximately 2:1. The molar ratio of hydrogen to carbon dioxide in the gas mixture fed to the burner, including recycled carbon dioxide, may range from 1:1 to 5:1. This ratio may vary depending on the carbon dioxide conversion achieved in the reverse water gas shift unit and the desired hydrogen to carbon monoxide ratio for downstream processes.

[0012] The gas mixture containing carbon dioxide and hydrogen supplied to the burner, including the carbon dioxide recovered in step (d), may contain 15 to 50% by volume, preferably 25 to 40% by volume, of carbon dioxide. The gas mixture containing carbon dioxide and hydrogen supplied to the burner preferably contains less than 10% by volume in total of other gases such as water vapor, nitrogen, carbon monoxide, and methane.

[0013] Any suitable source of carbon dioxide may be used. Thus, the carbon dioxide stream may be a stream recovered from a conventional ammonia plant that uses a hydrocarbon or carbonaceous feed, or it may be recovered from the flue gas of a furnace or boiler that is heated by the combustion of a fossil fuel such as natural gas or coal, biomass, or carbonaceous waste such as plastic. Alternatively, the carbon dioxide may be a stream separated from air or seawater.

[0014] The gas mixture comprising hydrogen and carbon dioxide further comprises at least a portion of the recovered carbon dioxide obtained from the carbon dioxide removal unit.

[0015] Any suitable hydrogen source may be used. Two or more hydrogen sources may be used. The process preferably utilizes non-fossil fuel-based hydrogen. Thus, hydrogen may be produced by catalytic or non-catalytic partial oxidation of biomass or plastic, optionally followed by steam reforming of the partial oxidation product gas. Alternatively, hydrogen may be provided by decomposing water. Preferably, the hydrogen is electrolytic hydrogen, e.g., hydrogen formed by water electrolysis. Intermediate storage of hydrogen may be used to reduce any variability in the production of hydrogen from electrolysis.

[0016] Any suitable oxygen source may be used. The oxygen purity may be at least 94% by volume, preferably at least 98% or 99% by volume, to minimize inerts such as nitrogen in the carbon monoxide product stream. Unlike WO2019175476, the oxygen does not need to be combined with the carbon dioxide stream. The oxygen may be recovered from the air using an air separation unit (ASU), which may be powered by a renewable power source, or steam generated in a reformed gas boiler, or other sources, including from downstream processes. Preferably, the oxygen comprises electrolytic oxygen, e.g., oxygen formed by water electrolysis. If desired, steam may be included with the oxygen.

[0017] Therefore, both hydrogen and oxygen for the process are preferably produced using an electrolysis unit supplied with a water source. The water may include condensate recovered from the crude product gas mixture and / or may include water recovered from a downstream conversion unit, such as a Fischer-Tropsch hydrocarbon synthesis unit. Optionally, the water may be treated to remove contaminants, such as organic compounds or salts, that may adversely affect the electrolysis unit.

[0018] Desirably, the electricity for the electrolysis unit is not derived from the combustion of fossil fuels. Power for the electrolysis may be provided by nuclear power, or preferably by renewable power sources such as solar power, wind energy, tidal energy, water or hydroelectric power, marine energy sources, geothermal energy and / or biomass. Electricity for the electrolysis may also be provided using a turbine driven by steam generated using heat recovered from the product gas stream produced by the partial oxidation of biomass or plastic waste. Power may be stored in intermediate facilities such as pumped hydro storage or battery storage to provide a more constant supply of power to the electrolysis unit.

[0019] An electrolysis unit typically comprises one or more electrolytic cells operating according to the following general formula: Electricity + 2H2O → 2H2 + O2

[0020] Electrolysis is a process in which water is chemically decomposed to obtain oxygen and hydrogen under the action of an electric current. In one configuration, alkaline cell electrolysis may be used in the process. Alkaline cell electrolysis may be performed at temperatures below 200°C by combining water with potassium hydroxide, the concentration of which may vary as a function of temperature (typically from 25% by weight at 80°C to 40% by weight at 160°C). Potassium hydroxide is preferred over sodium hydroxide because of its superior conductivity at essentially equivalent temperature levels. Alternatively, a polymer electrode membrane electrolyzer may be used. Alternatively, high-temperature electrolysis may be used in the process. High-temperature electrolysis operates at high temperatures (700-900°C) and under reduced pressure. High-temperature electrolysis is more efficient than ambient-temperature processes because part of the energy required for the reaction is contributed via heat, which is often obtained more cheaply than electricity, and the electrolysis reaction has better yields at high temperatures. High-temperature electrolysis may also enable the conversion of carbon dioxide in water to carbon monoxide. The carbon monoxide can be advantageously used to supplement the synthesis gas supplied to the downstream FT unit.

[0021] The carbon dioxide and hydrogen stream, or gas mixture comprising carbon dioxide and hydrogen, may be compressed, if necessary, to a pressure in the range of 0.8 to 4 MPa, or optionally 5 MPa (gauge), preferably 1.2 to 3.2 MPag.

[0022] The oxygen stream is desirably provided at a pressure higher than that of the gas mixture fed to the burner, for example up to 8 bar higher than that of the gas mixture fed to the burner, to create a velocity differential and promote mixing in the burner flame. The oxygen stream may be preheated if desired to improve combustion.

[0023] The gas stream fed to the reverse water gas shift vessel may be preheated before compression, preferably after compression. The preheat temperature of the feed gas to the reverse water gas shift vessel may be in the range of 400-1000°C or 450-800°C to support combustion. The hydrogen and carbon dioxide streams may be premixed before preheating, or may be preheated and mixed. Preheating of the feed to the preheat temperature may be achieved by exchange with the crude product gas mixture and / or by steam heating, or by using a fired heater, or by electrical heating, or a combination of two or more of these. Preferably, the feed gas mixture comprising carbon dioxide and hydrogen is heated by exchange with the crude product gas mixture.

[0024] Although it is generally preferred to minimize the amount of water vapor supplied to the reverse water gas shift vessel, it may be advantageous to include water vapor in the oxygen gas stream for safe transitions between stages of operation, particularly during process start-up or shutdown. The amount of water vapor in the oxygen stream may range from 0 to 50% by volume.

[0025] The amount of oxygen supplied to the burner is substoichiometric, i.e., the amount of oxygen is insufficient to combust all of the hydrogen in the gas mixture. Combustion of hydrogen consumes two hydrogen molecules per oxygen molecule. Thus, the molar ratio of oxygen to hydrogen (O2:H2) is typically less than 0.5:1 and may range from 0.02 to 0.2:1 or from 0.05 to 0.15:1.

[0026] Oxygen and a gas mixture containing carbon dioxide and hydrogen are supplied to a burner disposed within the reverse water gas shift vessel. Any burner design may be used, such as a burner used in an autothermal reformer or secondary steam reformer. The flows may be supplied at a single point or multiple points. A preferred burner design is one in which the gas mixture is supplied to the neck region of the reverse water gas shift vessel, and oxygen is supplied through the neck region to a central conduit that opens into the combustion zone. Combustion generates a flame in a combustion zone upstream of the water gas shift catalyst within the reverse water gas shift vessel. Local conditions in the combustion section, particularly in the flame front region, can be controlled by managing the momentum of the oxidant flow and the gas flow. The water gas shift vessel may be oriented so that the combustion zone is above a bed of reverse water gas shift catalyst. Such a configuration, used in an autothermal reforming vessel or secondary steam reforming vessel, may be used in the present process and may be referred to as autothermal reverse water gas shift (ARWGS). However, other configurations of the burner and catalyst may be used.

[0027] A reverse water gas shift vessel contains two reaction zones. The first zone, the combustion zone, is defined by the area between the burner and the inlet to the catalyst bed. The burner in the reverse water gas shift vessel is located in the neck region and may discharge into a void space, e.g., a truncated cone or cylinder with a vertical axis. In this zone, the process gas and process oxidant mix, consuming oxygen present in less than stoichiometric proportions. The second reaction zone is defined by a bed of reverse water gas shift catalyst. This zone is typically cylindrical, with a vertical axis. The goal in reverse water gas shift vessel design is to reduce temperature and composition fluctuations in the process gas stream exiting the first reaction zone and entering the second reaction zone. Non-uniform conditions can result in catalyst damage and / or loss of catalytic activity. Intimate mixing of the process gas with the process oxidant is necessary to achieve a uniform gas mixture at the inlet to the catalyst. In oxygen-based reactors, the mass flow rate of the oxidant is much less than the mass flow rate of the process gas. Dispersing a relatively small flow of oxidant into a relatively large flow of process gas requires accelerating the oxidant to a higher velocity. A preferred approach is to use a burner mounted in the cylindrical neck region of the vessel above the combustion zone mentioned above. The dimensions of the burner and neck are selected to stabilize the flame on the burner and enhance mixing between the process oxidant and process gas streams. The gas mixture is heated by combustion, typically to a temperature in the range of 800-1300°C. Oxygen is consumed in the combustion zone. The heated gas mixture, containing carbon monoxide, carbon dioxide, water vapor, and unreacted hydrogen, then passes through a bed of reverse water gas shift catalyst disposed within the reverse water gas shift vessel downstream of the burner.

[0028] The reverse water gas shift catalyst may be any suitable transition metal oxide catalyst, such as a catalyst based on nickel oxide, iron oxide, or chromium oxide, although other catalysts commonly used as reverse water gas shift catalysts may also be used. Preferably, the catalyst is a nickel oxide-based catalyst. Such catalysts are active for reverse water gas shift catalysis, but are advantageously also active for steam reforming hydrocarbons that may be present in the feed gas mixture. Therefore, the catalyst preferably comprises nickel oxide on a suitable refractory metal oxide support. Refractory metal oxide supports may include zirconia, alumina, calcium aluminate, magnesium aluminate, titania-magnesia, or mixtures thereof. More preferably, the catalyst comprises nickel oxide on zirconia, nickel oxide on α-alumina, nickel oxide on calcium aluminate, or nickel oxide on magnesium aluminate. The nickel content, expressed as NiO, may range from 3 to 20 wt. %.

[0029] The reverse water gas shift catalyst may be in particulate form, e.g., in the form of shaped units such as pellets, rings, or extrudates, which may be leaf- or groove-shaped. The catalytically active metal, e.g., nickel, may be dispersed throughout the particulate catalyst or may be present only in an eggshell layer 200 to 1000 micrometers thick on the surface of the refractory support. Alternatively, the catalyst may comprise one or more monolithic supports, such as metal or ceramic foams or honeycombs, supporting the catalytically active metal. Preferably, the catalyst is a particulate catalyst, more preferably a four-hole cylinder, especially one that is leaf- or groove-shaped to provide a higher geometric surface area (GSA) than a solid cylinder of similar size without increasing pressure drop. 400 to 550 m per cubic meter. 2 Catalysts having a GSA in the range of

[0030] If desired, a layer of zirconia balls, pellets or tiles may be placed on top of the catalyst to protect the surface of the catalyst from irregularities in the combustion gas flow. The advantage of this layer is that it prevents disturbance of the surface of the catalyst bed.

[0031] By controlling the preheat temperature and the amount of oxygen supplied to the burner, it is possible to control the outlet temperature of the reverse water gas shift vessel, which may be in the range of 700°C to 1050°C, preferably 750°C to 950°C.

[0032] In addition to producing a carbon monoxide gas stream via the reverse water gas shift reaction, a reverse water gas shift vessel with an appropriate selection of catalyst can also be used to convert waste gas from downstream processes to carbon monoxide. Thus, the reverse water gas shift vessel can be supplied with a pre-reformed gas mixture derived from hydrocarbons or oxygenates, or preferably, hydrocarbons or oxygenates that do not contain hydrocarbons higher than methane. The use of a pre-reformed gas mixture is preferred because it reduces the risk of undesirable carbon formation in the reverse water gas shift vessel or on the reverse water gas shift catalyst.

[0033] Pre-reforming can be carried out by passing a feed gas containing a hydrocarbon-containing or oxygen-containing gas stream mixed with an appropriate amount of steam through a pre-reformer vessel containing a fixed bed of pre-reforming catalyst. Steam introduction can be achieved by direct injection of steam and / or by saturating the feed gas by contact with a heated water stream. The heated water can include condensed water from a downstream process containing soluble organic compounds. Alternatively, the steam used for direct injection can be used to strip organic compounds from condensed water from the downstream process. In this way, the organic compounds can be converted to hydrogen and carbon oxides in the pre-reformer, reducing the wastewater treatment burden for the downstream process. The amount of steam introduced can be such that the steam-to-carbon ratio is 1:1 to 5:1, preferably 1:1 to 3:1, i.e., 1 to 3 moles of steam per mole of carbon atoms contained in the hydrocarbons in the pre-reformer feed gas. The pre-reformer feed gas may be passed adiabatically through a bed of steam reforming catalyst, such as a nickel steam reforming catalyst having a high nickel content, for example greater than 40 wt%, at an inlet temperature typically in the range of 350-650° C., more suitably 350-500° C. During the adiabatic pre-reforming step, any hydrocarbons higher than methane react with steam to produce a mixture of methane, carbon oxides, and hydrogen.

[0034] The gas mixture comprising hydrogen and carbon monoxide may be combined with the hydrocarbon-containing or oxygen-containing stream or pre-reformed gas stream and preheated upstream of the burner, or the hydrocarbon-containing or oxygen-containing stream or pre-reformed gas stream may be preheated separately and fed to the burner.

[0035] In some embodiments, the reverse water gas shift vessel may be supplied with 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, such as a Fischer-Tropsch product upgrading unit, such as a hydrocracker.

[0036] The crude product gas mixture from the reverse water-gas shift vessel contains water vapor formed by the reverse water-gas shift reaction and, optionally, water vapor added with the feed gas. Water is recovered from the crude product gas mixture by cooling the product gas mixture below its dew point and separating the condensate, for example, using one or more conventional gas-liquid separators. Removal of the water condensate from the crude product gas mixture produces a dehydrated product gas. Cooling can be achieved by raising the water vapor and / or by preheating one or more of the hydrogen stream, the carbon dioxide stream, the mixed gas stream comprising hydrogen and carbon dioxide, and, optionally, the pre-reformer feed gas and the pre-reformer effluent (if present). Further cooling with chilled water and / or air may also be achieved. Process steam produced by cooling may be used in the pre-reforming step or in downstream processes and / or for power generation.

[0037] The condensate may be at least partially recycled to the process, if desired. The condensate may be used as boiler feedwater, after treatment, if desired. Additionally, or alternatively, the condensate, optionally after treatment for use of contaminants, may be fed to an electrolysis unit used to generate hydrogen for the process. Thus, in some embodiments, a water stream recovered from the raw product gas mixture may be fed to the electrolysis unit. The condensate may also be used as boiler feedwater, again, after treatment, if desired.

[0038] The crude product gas mixture contains carbon dioxide, which is removed from the dehydrated product gas using a carbon dioxide removal unit. The majority of the carbon dioxide can be separated using a membrane, a solid absorbent, or preferably a scrubbing system, such as a system that operates by countercurrently contacting the crude product gas mixture or the dehydrated product gas with an absorbing liquid over packing in a column. The absorbing liquid can be a physical solvent such as potassium carbonate (sold as the Benfield process), methanol (sold as the Rectisol process), or glycol (sold as the Selexol process), or a chemical solvent such as an amine. Thus, the carbon dioxide removal unit may include one or more vessels providing a physical or reactive scrubbing system, preferably a reactive scrubbing system, particularly an amine scrubbing system. Carbon dioxide may also be removed by a conventional acid gas recovery unit (AGRU). In a conventional AGRU, the dehydrated gas stream is contacted with a suitable absorbing liquid, such as an amine, for example, monoethanolamine (MEA), methyldiethanolamine (MDEA), or dimethylethanolamine (DMEA), particularly a stream of aqueous solution containing methyldiethanolamine (MDEA), so that the carbon dioxide is absorbed by the liquid to obtain a loaded absorbing liquid and a gas stream with reduced carbon dioxide content. The loaded absorbing liquid is then regenerated by heating and / or depressurization to desorb the carbon dioxide and obtain a regenerated absorbing liquid, which is then recycled to the carbon dioxide adsorption stage. Heat from the regeneration of the loaded absorbent may be recovered from within the process. For example, a portion of the crude product gas mixture or steam generated by cooling the crude product gas mixture may be used to heat the loaded absorbent.

[0039] Alternatively, instead of washing with an amine, cold methanol or glycol may be used in a similar process to the amine to remove carbon dioxide.

[0040] The recovered carbon dioxide obtained from the carbon dioxide removal unit is preferably recompressed, if necessary, and returned to the reverse water gas shift vessel to increase the overall conversion to carbon monoxide.

[0041] The recovered carbon dioxide may be combined with the carbon dioxide feed, hydrogen gas feed or gas mixture containing hydrogen and carbon monoxide prior to preheating, preferably with the carbon dioxide feed stream prior to its compression.

[0042] Removal of carbon dioxide from the dehydration product gas produces a gas stream containing carbon monoxide. Hydrogen is also present in the product gas, the amount of which depends on the excess amount of hydrogen fed to the reverse water-gas shift vessel. Small amounts of inert gases such as carbon dioxide, methane, and nitrogen may also be present, which is undesirable to prevent their accumulation in downstream processes, especially if the product gas is used to produce Fischer-Tropsch hydrocarbons. Additionally, small amounts of catalyst poisons such as ammonia, hydrogen cyanide, and sulfur compounds such as hydrogen sulfide may also be present. Therefore, one or more purification units may be provided downstream of the carbon dioxide removal unit.

[0043] The carbon monoxide-containing gas stream from the present process comprises carbon monoxide and hydrogen, and the hydrogen to carbon monoxide molar ratio may be in the range of 1.0 to 2.5:1, preferably 1.2 to 2.5:1, and more preferably 1.6 to 2.2, which is particularly suitable for hydrocarbon synthesis by the Fischer-Tropsch reaction.

[0044] In a preferred use, the product gas is fed to a Fischer-Tropsch hydrocarbon synthesis unit which synthesizes a mixture of hydrocarbon products.

[0045] The Fischer-Tropsch hydrocarbon synthesis unit may include one or more Fischer-Tropsch reactor vessels containing a Fischer-Tropsch catalyst. The Fischer-Tropsch conversion step may be carried out according to and using any one of the known catalysts, but is advantageously applied to processes using a cobalt catalyst.

[0046] The Fischer-Tropsch process is ideally carried out according to the formula (C n H 2n+2 The more useful reactions produce alkanes as follows: (2n+1)H2+nCO→C n H 2n+2 +nH2O In the formula, n is typically 5 to 100 or more, with preferred products having n in the range of 10 to 20.

[0047] Typically, a portion of the carbon monoxide is converted in one or more Fischer-Tropsch reactors to produce liquid hydrocarbon products and water, and a gaseous mixture containing unreacted hydrogen and carbon monoxide, as well as carbon dioxide and gaseous light hydrocarbons including methane, ethane, propane, and butane. The reaction product mixture may be cooled, and aqueous and liquid hydrocarbon streams may be separated from the gaseous mixture using one or more gas-liquid separators. Optionally, the cooling may be such that propane and butane are also condensed and removed as liquids at this stage. Byproduct water may be separated using known hydrocarbon-water separators. In some embodiments, byproduct water in a Fischer-Tropsch hydrocarbon synthesis unit may be treated to remove organic compounds and used in the process. For example, steam can be used to strip a portion of the byproduct water of organic compounds, and the stripped water, after optional additional purification, may be used as feed to the electrolysis unit. Alternatively, the byproduct water may be treated to remove organic compounds and fed to a boiler to generate steam for the process. The separated gas mixture, referred to as "tail gas," can be used in a number of ways. Preferably, a first portion of the tail gas is recycled to one or more Fischer-Tropsch reactors in the synthesis loop to increase the overall conversion of carbon monoxide to hydrocarbons. The fraction recycled to form the loop may be configured to control the accumulation of inert gases, such as methane, in the Fischer-Tropsch hydrocarbon synthesis unit to an acceptable level. The remaining portion still contains a valuable carbon source. Therefore, in some embodiments, a second portion of the tail gas may be recycled to the reverse water-gas shift unit. If desired, undesired hydrocarbons produced in the Fischer-Tropsch process may be recycled to the process by mixing them with the tail gas fed to the reverse water-gas shift unit. Preferably, recycling to the reverse water-gas shift unit is via a steam reformer, preferably an adiabatic steam reformer or "pre-reformer," which converts ethane and any higher hydrocarbons present in the second portion of the tail gas to methane.Steam may be added to the second portion to provide a suitable steam-to-carbon ratio for the steam reforming step. The portion not recycled to the reverse water gas shift unit may be referred to as the "purge gas" and is removed from the process to prevent the accumulation of inert gases. This may be before or after the steam reforming step, if present.

[0048] The purge gas may optionally be treated to separate a stream rich in inert components or a stream depleted in carbon-containing components, for example, by passing the purge gas through a membrane that is more permeable to the inert gas than to the carbon-containing components, or by cooling the purge stream to condense condensables, or by using a solid absorbent such as a zeolite.

[0049] The purge gas may be exported as fuel or used within the process in a calciner heater or thermal oxidizer to heat the feed to the reverse water gas shift vessel or to superheat steam. Preferably, the purge gas is combusted as fuel. If the purge gas is combusted, a portion of the carbon dioxide in the resulting combustion or flue gas may be separated to reduce carbon dioxide emissions from the process. The carbon dioxide may be separated using the same methods used to recover carbon dioxide from the reverse water gas shift reactor product gas, optionally sharing equipment such as a regenerator column. [Brief explanation of the drawings]

[0050] The present invention will now be described with reference to the accompanying drawings. [Figure 1] 1 is a schematic flow sheet of one embodiment of the present invention.

[0051] The drawings are schematic and those skilled in the art will understand that in a commercial plant additional items of equipment may be required, such as reflux drums, compressors, pumps, vacuum pumps, columns, heat exchangers, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, etc. Providing such equipment accessories does not form part of the present invention and is in accordance with conventional chemical engineering practice. DETAILED DESCRIPTION OF THE INVENTION

[0052] In Figure 1, a carbon dioxide stream, such as one recovered from flue gas, is supplied to the process via line 10 and combined with a hydrogen stream provided via line 12 to form a mixed gas stream in line 14. A carbon dioxide recycle stream provided via line 16 is combined with the mixed gas in line 14, and the resulting mixed gas is supplied via line 18 to a gas-gas exchanger 20 where it is heated. The heated mixed gas is supplied from exchanger 20 via line 22 and combined with a pre-reformed tail gas mixture containing hydrogen, carbon dioxide, methane, and steam provided via line 24. The resulting mixed gas is provided via line 26 to a heater 28 where it is heated to the inlet temperature of the reverse water gas shift reaction. Alternatively, the pre-reformed tail gas mixture 24 may be added downstream of the heater 28.

[0053] The heated feed gas mixture is passed from heater 28 via line 30 to the inlet of reverse water gas shift vessel 32. The heated gas mixture is passed to the top of vessel 32. A burner (not shown) located at the top of vessel 32 receives a compressed, heated oxygen gas stream 34. The mixed gas and oxygen combust at the inlet temperature, resulting in combustion of a portion of the hydrogen in a flame within a combustion zone 36 adjacent to the burner within vessel 32. Vessel 32 further includes a bed of refractory metal oxide-supported nickel oxide reverse water gas shift catalyst 38 disposed below 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.

[0054] The resulting crude product gas mixture is withdrawn from vessel 32 via line 40 and subjected to cooling in boiler 42 connected to steam drum 44, which is supplied with water via line 46. The partially cooled crude product is supplied from boiler 42 via line 48 to heat exchanger 50, which heats a mixture of Fischer-Tropsch tail gas and steam, provided via line 52. The heated mixture passes from heat exchanger 50 via line 54 to pre-reforming vessel 56, which contains a bed of nickel pre-reforming catalyst, to form 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 supplied to exchanger 20, which heats the feed gas mixture in line 18. From exchanger 20, the partially cooled product gas is supplied via line 58 to one or more further heat exchangers 60, which may be supplied with chilled water and / or air, where it is cooled below its dew point to condense water vapor present in the crude product gas. The 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.

[0055] A dehydrated product gas comprising hydrogen, carbon monoxide, and carbon dioxide is withdrawn via line 68 and supplied to a conventional carbon dioxide removal unit 70 operated with a reactive liquid absorbent which recovers carbon dioxide from the dehydrated product gas. A carbon dioxide gas stream is withdrawn from unit 70 via line 72 and compressed in compressor 74 to form carbon dioxide recycle stream 16. A product gas mixture comprising carbon monoxide and hydrogen is withdrawn from carbon dioxide removal unit 70 via line 76.

[0056] In this embodiment, the product gas containing carbon monoxide in line 76 is subjected to one or more further purification steps (not shown) and then supplied to a Fischer-Tropsch hydrocarbon synthesis unit 78, which includes 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 unit 78 via line 80. A by-product water stream is recovered from Fischer-Tropsch unit 78 via line 82. Within unit 78, a Fischer-Tropsch tail gas stream is separated from the aqueous and liquid hydrocarbon streams. A portion of the tail gas stream, which includes 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 withdrawn from unit 78 via line 84 and combined with steam provided by line 86 to form a Fischer-Tropsch tail gas and steam mixture in line 52 that is fed to pre-reformer 56. The remaining portion of the tail gas is removed via line 84 as purge gas 85.

[0057] In this embodiment, an electrolysis unit 90 is used to electrolyze water to form hydrogen stream 12 and provide oxygen stream 90, which is compressed in compressor 92 and heated in heater 94 to form oxygen stream 34 which is fed to reverse water gas shift vessel 32.

[0058] Water for 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 supplied to the electrolysis unit 84 via dotted line 98.

[0059] Additionally, the steam provided in line 86 may be derived at least in part from by-product water 82 recovered from the Fischer-Tropsch hydrocarbon synthesis unit 78 .

Claims

1. 1. A process for producing a gas stream comprising carbon monoxide, the process comprising: (a) supplying a gas mixture comprising carbon dioxide and hydrogen to a burner disposed within a reverse water gas shift vessel and combusting the gas mixture with a sub-stoichiometric flow of oxygen gas to form a combustion gas mixture comprising carbon monoxide, carbon dioxide, hydrogen, and water vapor; (b) passing the combustion gas mixture through a bed of a reverse water gas shift catalyst disposed within the reverse water gas shift vessel to form a crude product gas mixture comprising carbon monoxide, water vapor, hydrogen, and carbon dioxide; (c) cooling the crude product gas mixture below its dew point and recovering a condensate to form a dehydrated product gas; (d) removing carbon dioxide from the dehydrated product gas in a carbon dioxide removal unit to form the gas stream comprising carbon monoxide; and (e) combining the carbon dioxide recovered by the carbon dioxide removal unit with the gas mixture comprising hydrogen and carbon dioxide supplied to the reverse water gas shift vessel.

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

1.

3. 3. The process of claim 1 or claim 2, wherein the gas mixture comprising carbon dioxide and hydrogen supplied to the burner comprises 15 to 50% by volume of carbon dioxide.

4. 4. A process according to any one of claims 1 to 3, wherein the carbon dioxide is obtained from an ammonia plant using a hydrocarbon or carbonaceous feed, or from the flue gases of a furnace or boiler, the furnace or boiler being heated by the combustion of a fossil fuel or carbonaceous waste, or from air or seawater.

5. A process according to any one of claims 1 to 4, wherein hydrogen and oxygen for the process are produced using an electrolysis unit supplied with a source of water.

6. 6. The process of any one of claims 1 to 5, 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 of any one of claims 1 to 6, wherein the carbon dioxide removal unit comprises a physical scrubbing system or a reactive scrubbing system.

8. 6. The process of claim 5, wherein the condensate is fed to the electrolysis unit.

9. The process of any one of claims 1 to 8, wherein the gas stream comprising carbon monoxide is fed to a Fischer-Tropsch hydrocarbon synthesis unit.

10. 10. The process of claim 9, wherein a gas mixture comprising methane and carbon dioxide formed by pre-reforming a Fischer-Tropsch tail gas is fed to the reverse water gas shift vessel.

11. 11. The process of claim 10, wherein water by-produced in the Fischer-Tropsch hydrocarbon synthesis unit is treated to remove organic compounds and used as feed to an electrolysis unit or fed to a boiler to generate steam for the process.

12. 12. A system for producing a gas stream by the process of any one of claims 1 to 11, comprising: (a) a supply line for supplying a gas mixture comprising carbon dioxide and hydrogen; (b) a reverse water gas shift vessel operably connected to the supply line, the reverse water gas shift vessel including: (i) a burner operably connected to an oxygen source; (ii) a combustion zone adjacent to the burner; (iii) a bed of reverse water gas shift catalyst disposed downstream of the combustion zone; and (iv) an outlet for a raw product gas mixture comprising carbon monoxide, water vapor, and carbon dioxide; (c) means operably connected to the reverse water-gas shift vessel for cooling the raw product gas mixture below the dew point and recovering a condensate from the raw product gas mixture to produce a dehydrated product gas; (d) a carbon dioxide removal unit coupled to the means for cooling the raw product gas mixture and recovering the condensate, the carbon dioxide removal unit recovering carbon dioxide from the dehydrated product gas; and (e) a conduit for supplying at least a portion of the recovered carbon dioxide from the carbon dioxide removal unit to the supply line for supplying the gas mixture comprising carbon dioxide and hydrogen to the reverse water-gas shift vessel.

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