Method of producing a liquid hydrocarbon

Abstract

A method of producing a liquid hydrocarbon, the method comprising: providing a first reactant stream comprising water and carbon dioxide; passing the first reactant stream to a first electrolysis unit to form a syngas comprising hydrogen and carbon monoxide; passing the syngas to a hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas; passing the effluent gas and steam to a derichment reactor to form a methane- enriched effluent gas; passing the methane-enriched effluent gas to the first electrolysis unit to form a gas mixture comprising hydrogen and one or both of carbon monoxide and carbon dioxide; and introducing the gas mixture into the syngas.

Description

[0001] METHOD OF PRODUCING A LIQUID HYDROCARBON

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of producing a liquid hydrocarbon.

[0004] BACKGROUND OF THE INVENTION

[0005] Electrolytic hydrogen is increasingly proposed for the preparation of liquid hydrocarbons, particularly liquid hydrocarbon fuels - so called e-fuels. The electrolytic hydrogen is used in liquid hydrocarbon synthesis either “directly” via Fischer-Tropsch units or indirectly via methanol, which is reacted further in a methanol-to-hydrocarbon unit, e.g. a methanol to gasoline, unit. Waste hydrocarbon by-products are Created alongside the synthesis of the liquid hydrocarbons. Examples of this are fusel oils in methanol synthesis and LPGs and methane in methanol-to-gasoline process. Methane is also a significant byproduct of the Fischer-Tropsch process. Combustion of these wastes e.g. in a fired steam reformer, or fired heater, is undesirable as this creates CO2 emissions.

[0006] A method of preparing hydrocarbon fuels using electrolytic hydrogen is proposed by Xu et al in “Low carbon fuel production from combined solid oxide CO2 co-electrolysis and Fischer-Tropsch synthesis system: A ijiodelling study”, Applied Energy, 242, 2019, 911-918. The method involves the use of a solid oxide electrolyser cell (SOEC) to convert carbon dioxide and water to carbon rhonoxide and hydrogen, followed by conversion of methane from a Fischer-Tropsch prbcess and water to carbon monoxide and hydrogen. Conversion of the methane increases the carbon efficiency of the method. However, C2+ hydrocarbons separated from the Fischer-Tropsch product hydrocarbons are disposed of, thereby rendering the method still relatively carbon inefficient.

[0007] WO2021214214 (Al) discloses a solid oxide cell system operating method in which hydrogen or syngas is produced from steam or a mixture comprising steam and carbon dioxide in an electrochemical reaction using an electric current. An additional quantity of at least one compound selected from the group consisting of natural gas, methane, or another hydrocarbon and is added to the steam or the mixture to carry out a conversion into syngas. An endothermic reformation of the added hydrocarbon is carried out by coupling in exhaust heat from the electrochemical reaction, and the additional quantity of the at least one compound is added in to provide hydrogen in order to compensate for the effects of a degradation of the solid oxide cells of the solid oxide cell system so that the total quantity of the hydrogen generated by the solid oxide cell system is kept constant over time.

[0008] The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto.

[0009] SUMMARY OF THE INVENTION

[0010] One aspect of the present disclosure is directed to a method of producing a liquid hydrocarbon, the method comprising: providing a first reactant stream comprising water and carbon dioxide; passing the first reactant stream to a first electrolysis unit to form a syngas comprising hydrogen and carbon monoxide; passing the syngas to a hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas; passing the effluent gas and steam to a derichment reactor to form a methane- enriched effluent gas; passing the methane-enriched effluent gas to the first electrolysis unit to form a gas mixture comprising hydrogen and one or both of carbon monoxide and carbon dioxide; and introducing the gas mixture info the syngas.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a diagram of a first embodiment of a method according to the present invention;

[0013] Figure 2 is a diagram of a second embodiment of a method according to the present invention.

[0014] DETAILED DESCRIPTION OF THE INVENTION

[0015] The present disclosure is directed to a method of producing a liquid hydrocarbon, the method comprising: providing a first reactant stream comprising water and carbon dioxide; passing the first reactant stream to a first electrolysis unit to form a syngas comprising hydrogen and carbon monoxide; passing the syngas to a hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas; passing the effluent gas and steam to a derichment reactor to form a methane- enriched effluent gas; passing the methane-enriched effluent gas to the first electrolysis unit to form a gas mixture comprising hydrogen and one or both of carbon monoxide and carbon dioxide; and introducing the gas mixture into the syngas.

[0016] Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.

[0017] Advantageously, in contrast to prior art methods, the method of the present invention may produce a liquid hydrocarbon in a more carbon-efficient manner. Recycling effluent gas back to the first electrolysis unit may ensure that a higher proportion of the carbon content of the first reactant stream is used in the method. In contrast to the method of the aforesaid WO2021214214 (Al), the inclusion of the derichment unit to convert higher hydrocarbons in the effluent gas recovered from the hydrocarbon synthesis unit into methane, operation of the electrolysis unit is enhanced without requiring full conversion of the hydrocarbons to hydrogen and carbon oxides. In addition, there is no need to dispose of the effluent gas, e.g. by burning, thereby potentially reducing the cost and environmental impact of the method.

[0018] In certain prior art methods, methane may be recovered from effluent gas and then recycled back to the electrolysis unit. While this may increase the carbon efficiency of the method, the carbon content of C2+ hydrocarbons contained in the effluent gas is still lost from the method, and such C2+ hydrocarbons need to be disposed of, thereby increasing the environmental impact of the method. In contrast, in the present invention, by passing the effluent gas to the derichment reactor before the passing the effluent gas to the first electrolysis unit, C2+ hydrocarbons contained in the effluent gas are converted to methane. Accordingly, substantially the entire carbon content of the effluent gas may be recycled meaning that the carbon efficiency of the method increases. Furthermore, the need to dispose of effluent gas is significantly reduced.

[0019] The term “liquid hydrocarbon” has its usual meaning in the art. As used herein the term may encompass species formed of carbon and hydrogen that are liquid at room temperature and pressure. The hydrocarbons typically comprise alkanes, and typically comprise from 10 to 20 carbon atoms per molecule. The liquid hydrocarbon product is preferably a liquid hydrocarbon fuel. Examples of liquid hydrocarbon fuels include diesel, gasoline and jet fuel or kerosine. Such fuels are commercially valuable.

[0020] The method comprises providing a first reactant stream comprising water and carbon dioxide. The water is typically provided in the form of steam. The carbon dioxide may be any source of carbon dioxide. For example, the carbon dioxide may be a by-product of combustion or a component of a product gas stream produced by partial oxidation, steam reforming or gasification of a carbonaceous material, or the carbon dioxide may be separated from air, sea-water, landfill-gas or biogas, or derived from a biogenic source. Carbon dioxide recovery from any of these sources may be achieved by known methods.

[0021] The method comprises passing the first reactant stream to a first electrolysis unit to form a syngas comprising hydrogen and carbon monoxide. Electrolysis units are known in the art. Typical electrolysis units comprise a Cathode and an anode. Typically, coelectrolysis of carbon dioxide and water (typically in the form of steam) takes place at the cathode according to the following reaction:

[0022] CO2+ H2O CO + H2+ 2O2'

[0023] The oxygen ions produced at the cathode may be transported to the anode and form molecular oxygen (i.e. () >) at the anode.

[0024] The term “syngas” or “synthesis gas” as used herein may encompass a fuel gas mixture. In the method of the present invention, the syngas comprises hydrogen (i.e. molecular hydrogen EE) and carbon monoxide (i.e. CO). The syngas may comprise other species such as, for example, carbon dioxide (i.e. CO2).

[0025] The method comprises passing the syngas to a hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas. Hydrocarbon synthesis units are known in the art. The hydrocarbon product may be recovered from the hydrocarbon synthesis unit. The effluent gas comprises hydrocarbons, typically lower hydrocarbons not forming the liquid hydrocarbon product. The effluent gas may comprise other species, for example unreacted hydrogen, carbon monoxide and / or carbon dioxide.

[0026] The method comprises passing the effluent gas and steam to a derichment reactor to form a methane-enriched effluent gas. The effluent gas is preferably heated upstream of the derichment reactor in heat exchange with the syngas. This may reduce the energy requirement of the method.

[0027] The derichment reactor typically comprises a bed of derichment catalyst. Suitable derichment reactors and derichment catalysts are known in the art. In the derichment reactor, at least a portion of C2+ hydrocarbons contained in the effluent gas may be converted to methane. Typically, the majority of C2+ hydrocarbons contained in the effluent gas are converted to methane, more typically substantially all of C2+ hydrocarbons contained in the effluent gas are converted to methane. The methane-enriched effluent gas may therefore also be considered to be a C2+ hydrocarbon-depleted effluent gas.

[0028] The derichment reactor may usefully operate by adiabatically steam reforming hydrocarbons contained in the effluent gas. Accordingly, a supply of steam to the derichment reactor is required. Furthermore, in order to satisfactorily steam reform the effluent gas without deactivation of the catalyst by carbon formation, a source of hydrogen may also be provided to the derichment reactor.

[0029] The amount of steam introduced to the derichment reactor may be such as to give a steam to carbon molar ratio in the feeds to the derichment reactor of 1 : 1 to 5 : 1. By steam to carbon molar ratio we mean the molar ratio of steam to the sum of the carbon-containing components in the feed, including hydrocarbons, CO and CO2.

[0030] The feed gases may be passed adiabatically through a bed of a derichment catalyst, such as a particulate nickel catalyst having a nickel content in the range of 30-60% by weight, for example above 40% by weight. Such catalysts are available commercially.

[0031] A stream of hydrogen may be fed with the effluent gas to the derichment reactor to reliably convert the C3-C4 to methane. The hydrogen may be a pure hydrogen stream or may comprise a suitably high hydrogen content to provide the hydrogen for the derichment.

[0032] The derichment reactor preferably operates at a temperature of from 300 °C to 650 °C and / or at a pressure of from 10 to 100 bara. In some arrangements the derichment reactor may be operated with an inlet temperature in the range of from 400 to 500°C, a steam to carbon molar ratio of from 1 : 1 to 5:1 and a H2 content of at least 0.001 kg H2 per kg of carbon-containing components in the feed. The minimum inlet steam content recommended at a 400°C inlet temperature is 1.8 kg / kg steam to carbonaceous feed ratio. Operating temperatures higher than 400°C may require more steam and hydrogen to avoid carbon deposition. An inlet temperature of at least 350 °C is preferable to provide a high reaction rate.

[0033] In some arrangements the derichment vessel may be operated at a pressure in the range of, for example, 1.0 to 7.0 MPag, preferably 1.5 to 6.6 MPag.

[0034] In the event that the effluent gas contains olefins that can react exothermically with hydrogen over the derichment catalyst, a hydrogenation catalyst may be provided upstream of the derichment catalyst. However, this is generally not necessary unless the olefin levels are above e.g. 10mol%.

[0035] The derichment reactor produces a methane-enriched effluent gas.

[0036] The method comprises passing the methane-enriched effluent gas to the first electrolysis unit to form a gas mixture comprising hydrogen and one or both of carbon monoxide and carbon dioxide. In the electrolysis unit, methane in the methane-enriched effluent gas is converted to hydrogen and one or both of carbon monoxide and carbon dioxide.

[0037] The method comprises introducing the gas rhixture into the syngas. The gas mixture may be recovered from the first electrolysis unit before being introduced into the syngas. Alternatively, the gas mixture may be introduced into the syngas within the electrolysis unit.

[0038] Preferably, the first electrolysis unit comprises a solid oxide electrolyser cell (SOEC), the solid oxide electfolyser cell comprising an anode, a cathode and a solid electrolyte membrane disposed between the anode and the cathode. Solid oxide electrolysis cells are known in the art. A solid oxide cell may be particularly suitable for the (co)- electrolysis of water and carbon dioxide at the cathode and / or the oxidation of methane at the anode. A solid oxide electrolyser cell is composed of a cathode where the reduction reaction is happening, a solid electrolyte membrane for the transport of ions and an anode where the oxidation reaction takes place. The solid electrolyte membrane may transport O2' or H+. A catalyst can be deposited together with the cathode or the anode to enable tandem reactions.

[0039] The first electrolysis unit preferably comprises a stack of solid oxide electrolysis cells. This may enable a higher volume of electrolysis to take place. Passing the reactant stream to the first electrolysis unit preferably comprises passing the first reactant stream to the cathode. Cathode materials in a solid oxide electrolysis cell may include Ni, or Ag based oxides mixed or supported on yttria-stabilised zirconia, or mixed oxides such as BaCeo.2Zro.7Yo.i03-5 or Lao / zsSro.isCro.sMno.sCh-s. In a solid oxide electrolysis cell, co-electrolysis of carbon dioxide and of steam may take place at the cathode according to the following reaction:

[0040] In the cathode: CO2 + H O — CO + H + 2O2'

[0041] Oxygen ions are produced at the cathode together with CO and H and are transported through the solid oxide electrolyte and react at the anode to produce O2.

[0042] Passing the methane-enriched effluent gas to the first electrolysis unit preferably comprises passing the methane-enriched effluent gas to the anode. In this case, the hydrocarbons contained in the methane-enriched effluent gas (mainly methane) may react with a suitable electrocatalyst at the anode to be oxidized to carbon dioxide and water or partially oxidized to produce carbon monoxide and hydrogen. In such case, the anode preferably comprises an electrocatalyst. Suitable electrocatalysts are perovskites, such as lanthanum strontium cobalt ferrite (LSF), gadolinia-doped ceria (GDC), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium manganate (LSM) e.g. of formula (Lao.8Sro.2)o.95Mn03-<5, lanthanum strontium cobalt manganate (LSCM) and nickel-iron or nickel-copper manganate materials such as Ni-Sr2Fei.5Moo.506-5-Ceo.8Sm0.20i.9;) and Ni- Cu / LSCM.

[0043] Methane-enriched synthesis gas and steam are provided to the anode. The anode therefore preferably comprised a steam reforming catalyst. The steam reforming catalyst may suitably comprise nickel and / or PGM based on a suitable catalyst support such as yttria-stabilised zirconia (YSZ). The steam reforming catalyst may enable formation of carbon monoxide and hydrogen. The oxygen ions transported through the solid oxide electrolyte may then react with hydrogen and carbon monoxide to make water and carbon dioxide at the anode that could be recirculated to the cathode to produce carbon monoxide and hydrogen. In this case, the reactions at the anode may be as follows:

[0044] At the anode; catalytic: CPU + H2O — H2 + CO (endothermic) In the anode: H2 + O2' — H2O + e. (exothermic)

[0045] CO + O2- CO2+ e-

[0046] By having hydrogen and carbon monoxide oxidation at the anode, electricity could be also generated. This may improve the overall energy efficiency of the system. In addition, hydrogen oxidation may produce heat that may balance the heat loss in the steam reforming, which is an endothermic reaction.

[0047] A proton-conducting solid oxide cell (H-SOC), where the solid electrolyte exhibits proton conductivity, can also be used. In this case, a catalyst may be employed with the H- SOC at the cathode for reforming both methane and carbon dioxide to carbon monoxide and hydrogen. In electrolysis mode, steam is oxidized at the anode to produce H+which are transported across a solid proton electrolyte membrane to react in the cathode to make H2. H2 can react then with CO2 that is fed at the cathode to make CO and H2O. CH4 is fed at the cathode in the same time with CO2 to make syngas over a reforming catalyst. The steam can also react with CH4 to further make syngas using a steam reforming catalyst at the cathode.

[0048] Anode: H2O 2H++ 0.5O2

[0049] Cathode: 2H+^ H2

[0050] At the cathode catalytic: CO2 + H2 — CO + H2O

[0051] CO2+ CH4^ 2CO + 2H2

[0052] CH4+ H2O CO + H2

[0053] The cell can also be used to produce power if operated in reverse mode, if power to operate the electrolysis unit is not available. When in fuel cell mode, reforming catalysts are provided at the anode, where CO2 and the methane enriched gas are fed. Part of the hydrogen produced may be oxidized at the anode and produce H+ ions that are transported via the electrolyte to produce water at the cathode. The benefit of such a system is that power and heat may be generated, which may be integrated into the system. The water can be recirculated at the anode, where it can make additional syngas by reacting with methane over the steam reforming catalyst. In this case, the reactions may be as follows:

[0054] At the anode catalytic: CPU + CO2 — 2CO + 2H2

[0055] CH4+ H2O co + H2

[0056] In the anode: H2 — 2H+

[0057] In the cathode: 2H++ O.5O2 — H2O

[0058] The first electrolysis unit preferably operates at a temperature of from 500 to 1000 °C, more preferably from 700 to 900 °C. Lower temperatures may results in an unfavourable low level of hydrolysis. Higher temperatures may decrease the energy efficiency of the method. The water or steam to carbon dioxide molar ratio in the first reactant stream may be 0.5: to 2: 1, preferably about 1 : 1. Additionally, steam will be present in the methane- enriched effluent gas recovered from the derichment reactor. The steam to methane molar ratio in the methane-enriched effluent gas fed to the first electrolysis unit is preferably in the range of about 1 : 1 to 3 : 1, more preferably about 2: 1 to 3 : 1.

[0059] In a preferred embodiment, the hydrocarbon synthesis unit comprises a Fischer- Tropsch reactor that converts syngas to a liquid hydrocarbon product, an aqueous stream and the effluent gas. Fischer-Tropsch reactors are known in the art.

[0060] The temperature of the Fischer-Tropsch reactor is preferably from 150 °C to 300 °C. Lower temperatures may result in unfavourably low levels of liquid hydrocarbon being generated. Higher temperatures may increase the energy cost of the method without a significant increase in the levels of liquid hydrocarbon being produced.

[0061] The Fischer-Tropsch reactor preferably comprises a catalyst comprising cobalt, iron and / or ruthenium. Such a catalyst may be particularly effective at catalysing Fischer- Tropsch reactions and / or enable the reaction to proceed at favourably low temperatures and / or with high yield.

[0062] The molar ratio of hydrogen to carbon monoxide in the syngas is preferably from 1.8 to 2.2 since this is close to the stoichiometric ratio of the Fischer-Tropsch reaction of about 2.

[0063] The effluent gas preferably comprises Cl to C4 hydrocarbons. Such hydrocarbons are preferably not contained in the liquid hydrocarbon, or are preferably contained in the liquid hydrocarbon in only very low levels. Cl, i.e. methane, may be readily converted in the first electrolysis unit. C2, C3 and C4 may be readily converted to methane in the derichment reactor. The effluent gas may comprise a tail gas containing unreacted hydrogen and carbon monoxide or may consist of hydrocarbons, for example, liquid petroleum gas, i.e. LPG.

[0064] Where the hydrocarbon synthesis unit includes a Fischer-Tropsh reactor, passing the syngas to the hydrocarbon synthesis unit (Fischer-Tropsch rector) preferably comprises passing the syngas to a carbon dioxide removal unit to form a carbon-dioxide depleted syngas and a carbon dioxide stream, and then passing the carbon-dioxide depleted syngas to the hydrocarbon synthesis unit. This may increase the efficiency of the method since carbon dioxide is not used in the Fischer-Tropsch reaction. The method preferably further comprises passing the carbon dioxide stream to the first electrolysis unit. For example, the carbon dioxide stream may be introduced into the first reactant stream prior to the first reactant stream being passed to the first electrolysis unit. This may increase the carbon efficiency of the method. The carbon dioxide stream is preferably passed to the cathode of the first electrolysis unit.

[0065] The method preferably further comprises passing at least a portion of the aqueous stream to the first electrolysis unit in the form of steam. This may reduce the water requirement of the method. For example, the aqueous stream may be introduced into the first reactant stream prior to the first reactant stream being passed to the first electrolysis unit. The aqueous stream is preferably passed to the cathode of the first electrolysis unit.

[0066] The aqueous stream is preferably at least partially purified prior to being passed to the first electrolysis unit. For example, hydrocarbons may be removed from the aqueous stream. Impurities, such a hydrocarbons, may clog the first electrolysis unit and / or poison catalysts contained in the cathode and / or anode of the first electrolysis unit.

[0067] Preferably, the method further comprises: passing the liquid hydrocarbon product to an upgrading unit to provide an upgraded liquid hydrocarbon product and a waste hydrocarbon; and passing the waste hydrocarbon arid steam, via the derichment reactor or a further derichment reactor, to the first electrolysis unit. The upgrading unit may include a hydroprocessing unit, a hydrotreatment unit, a hydrocracking unit and a fractionation unit. This may result in a higher value liquid hydrocarbon product without reducing the carbon efficiency of the method. The waste hydrocarbon may be passed to the derichment reactor before being passed to the first electrolysis unit. For example, the waste hydrocarbon may be introduced into the effluent gas prior to the effluent gas being passed to the derichment reactor. Alternatively, a separate further derichment reactor may be used in parallel to derich the waste hydrocarbon. Using the same or a parallel derichment unit to de-rich the waste hydrocarbon and produce a deriched waste hydrocarbon gas protects the first electrolysis unit from clogging by hydrocarbons higher than methane in the waste hydrocarbon. Using a parallel derichment reactor to derich the waste hydrocarbon in addition to the derichment reactor fed with the effluent gas stream, allows the derichment reactors to be operated at different conditions that better convert the different feeds to methane. The deriched waste hydrocarbon is preferably passed to the anode of the first electrolysis unit.

[0068] In another preferred embodiment: the hydrocarbon synthesis unit comprises a methanol synthesis unit upstream of a methanol-to-hydrocarbon unit; and the step of passing the syngas to the hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas comprises: passing the syngas to the methanol synthesis unit to convert syngas to a methanol stream and an off gas stream comprising hydrogen; passing the methanol stream to the methanol-to-hydrocarbon unit to form a hydrocarbon mixture and the effluent gas; and recovering from the hydrocarbon mixture a hydrocarbon fuel product selected from a diesel product, a gasoline product and a jet fuel product.

[0069] Methanol synthesis units and methanol-to-hydrocarbon units are known in the art. Methanol-to-hydrocarbon units may be particularly effective at producing liquid hydrocarbon fuels. The methanol-to-fuel unit may generate fuel using the Exxon process, where methanol is processes to olefins and then olefins are oligermerised to obtain the fuel range for gasoline or jet fuel. Recovering from the hydrocarbon mixture at least one of a diesel product, a gasoline product and a jet fuel or kerosine product may comprise distillation. Methanol may be formed in the methanol synthesis unit according to the following reactions:

[0070] CO231 b > CH3OH + I TO

[0071] The methanol synthesis unit may comprise a catalyst, preferably a copper-based catalyst. The off gas stream is preferably recycled into the syngas prior to the syngas being passed to the hydrocarboh synthesis unit. This may improve the carbon efficiency of the method.

[0072] Preferably, prior to the off gas being recycled into the syngas, the off gas is passed to a hydrogen recovery unit to increase the hydrogen concentration of the off gas and to form a carbon-containing off gas; and the method further comprises introducing the carbon- containing off gas to the effluent gas prior to the effluent gas being passed to the derichment reactor. The hydrogen recovery unit may comprise, for example, a membrane hydrogen recovery unit and / or a pressure swing adsorption (PSA) hydrogen recovery unit. Since methanol is formed from carbon monoxide with H2:CO ratio of 2: 1 and from carbon dioxide with a H2:CO2ratio of 3 : 1, increasing the hydrogen concentration of the off gas may improve the efficiency of the method. Introducing the carbon-containing off gas to the effluent gas prior to the effluent gas being passed to the derichment reactor may increase the carbon efficiency of the method.

[0073] The syngas is preferably passed to the methanol synthesis unit has a stoichiometry number R defined as R = ([H2] - [CO2]) / ([CO2] + [CO]) in the range 1.95 to 2.15. Such an R value may be particularly suitable for methanol synthesis.

[0074] Passing the methanol stream to the methanol-to-fuel reactor preferably comprises: passing the methanol stream to a methanol distillation unit to form a purified methanol stream and a water stream; and passing the purified methanol stream to the methanol-to-fuel unit.

[0075] This may increase the efficacy of the methanol-to-fuel reaction and / or reduce poisoning of catalyst used in the methanol-to-fuel reactor. The water stream is preferably introduced into the first reactant stream prior to the first reactant stream being passed to the first electrolysis unit. This may reduce the water requirement of the method.

[0076] The method preferably further comprises separating a fusel oil from the methanol distillation unit and introducing the fusel oil stream into the effluent gas prior to passing the effluent gas to the derichment reactor. This may increase the carbon efficiency of the method.

[0077] The method preferably further comprises introducing the water stream into the first reactant stream. This may reduce the water requirement of the method.

[0078] The method preferably further comprises introducing carbon dioxide into the syngas prior to passing the syngas to the hydrocarbon synthesis unit. As discussed above, additional methanol may be produced from reaction of the carbon dioxide and hydrogen.

[0079] The method preferably further comprises: providing a second reactant stream comprising water; passing the second reactant stream to a second electrolysis unit to form a hydrogen stream; and introducing the hydrogen stream into the syngas prior to passing the syngas to the hydrocarbon synthesis unit, wherein the second electrolysis unit operates at a lower temperature than the first electrolysis unit.

[0080] This may enable a portion of the hydrogen in the syngas to be produced at a lower temperature (i.e. in a more energy efficient manner) than the hydrogen produced in the first electrolysis unit. This may be particularly beneficial when the hydrocarbon synthesis unit requires a higher proportion of hydrogen to carbon monoxide and / or carbon dioxide on a molar basis. This may be the case, for example, when the hydrocarbon synthesis unit comprises a Fischer-Tropsch unit, or comprises a methanol synthesis unit and a methanol- to-fuel unit. The second electrolysis unit may be operated at temperatures below 100°C, typically in the range 20 °C or 30 °C to 90 °C.

[0081] The second electrolysis unit preferably comprises an alkaline electrolysis unit or a polymer electrolyte membrane (PEM) electrolysis unit. Such electrolysis units are known in the art. Such electrolysis units may be particularly effective at carrying out electrolysis at such a lower temperature.

[0082] The first electrolysis unit and / or the second electrolysis unit are preferably powered by renewable energy. This may render the method more environmentally friendly.

[0083] EXAMPLES

[0084] Figure 1 shows a diagram of a first embodiment of a method according to the present invention. A first electrolysis unit 1, powered by renewable energy, receives a first reactant stream 2 comprising steam and carbon dioxide. The first electrolysis unit 1 converts the first reactant stream 2 to a syngas 5 comprising hydrogen and carbon dioxide. A second electrolysis unit 3, powered by renewable energy, operating at a lower temperature than the first electrolysis unit 1, receives a second reactant stream 4 comprising water. The second electrolysis unit 3 converts the second reactant stream 4 to a hydrogen stream 6, which is then introduced into the syngas 5. Optionally, a carbon dioxide stream 7 may be introduced into the syngas 5. The syngas 5 is passed to a methanol synthesis unit 8 to convert syngas 5 to a methanol stream 9 arid an off gas stream 10 comprising hydrogen. The off gas stream 10 is passed to a hydrogen recovery unit 11 to increase the hydrogen concentration of the off gas stream 10 and to form a carbon-containing off gas 12. The hydrogen-enriched off gas stream from hydrogen recovery unit 11 is then recycled into the syngas 5 prior to the syngas 5 being passed to a methanol synthesis unit 8. The methanol stream 9 is passed to a methanol distillation unit 13 to form a purified methanol stream 14, a water stream 15 and a fusel oil stream 16. The water stream 15 may be disposed of or passed to the first electrolysis unit 1. The purified methanol stream 14 is passed to a methanol-to-fuel unit 17 to form a liquid hydrocarbon fuel product 18 and an effluent gas 19. The effluent gas 19 is passed to the first electrolysis unit 1. The fusel oil stream 16 is introduced into the effluent gas 19 prior to passing the effluent gas 19 to a derichment reactor 20. The derichment reactor 20 containing a derichment catalyst that converts the effluent gas 19 into a methane- enriched effluent gas 21, which is then passed to the first electrolysis unit 1.

[0085] Figure 2 shows a diagram of a second embodiment of a method according to the present invention. The second embodiment differs from the first embodiment in that rather than passing the syngas 5 to a methanol synthesis unit, carbon dioxide is removed from the syngas 5 in a carbon dioxide removal unit 22 to form a carbon dioxide stream 23 and a carbon dioxide depleted syngas 24. The carbon dioxide stream 23 is recycled to the first electrolysis unit 1. The carbon dioxide-depleted syngas 24 is then passed to a Fischer- Tropsch unit 25, which converts the carbon dioxide-depleted syngas 24 to a liquid hydrocarbon product 18, an aqueous stream 26 and the effluent gas 19. The aqueous stream 26 is passed, after optional purification (not shown) to the first electrolysis unit 1 in the form of steam. The liquid hydrocarbon product 18 is passed to a hydrocarbon upgrading unit 27 to provide an upgraded liquid hydrocarbon fuel product 28 and a waste hydrocarbon 29. The waste hydrocarbon 29 is then introduced into the effluent gas 19 prior to the effluent gas 19 being passed to the derichment reactor 20.

[0086] Alternatively, the waste hydrocarbon 29 may be mixed with steam and fed to a parallel derichment reactor to form a deriched waste hydrocarbon stream that may be fed to the first electrolysis unit 1.

[0087] An effluent gas from a methane-to-gasoline unit exhibited the following composition of light gas (methane / ethane), C3s and C4s: The effluent gas was passed to a derichment reactor containing a nickel CRG derichment catalyst (Johnson Matthey). The following performance was achieved:

[0088] This corresponds to:

[0089] Carbonaceous feed = 4557.0 kg / hr

[0090] Steam : Carbonaceous feed = 1.80 w / w H2 : Carbonaceous feed = 0.015 Nm3 / kg

[0091] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended clainjs. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

CLAIMS1. A method of producing a liquid hydrocarbon, the method comprising: providing a first reactant stream comprising water and carbon dioxide; passing the first reactant stream to a first electrolysis unit to form a syngas comprising hydrogen and carbon monoxide; passing the syngas to a hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas; passing the effluent gas and steam to a derichment reactor to form a methane-enriched effluent gas; passing the methane-enriched effluent gas to the first electrolysis unit to form a gas mixture comprising hydrogen and one or both of carbon monoxide and carbon dioxide;and introducing the gas mixture into the syngas.

2. The method of claim 1, wherein the liquid hydrocarbon comprises a liquid hydrocarbon fuel, preferably diesel, gasoline, jet fuel or kerosine.

3. The method of claim 1 or claim 2, wherein the first electrolysis unit comprises a solid oxide electrolyser cell (SOEC), the solid oxide electrolyser cell comprising an anode, a cathode and a solid electrolyte membrane disposed between the anode and the cathode.

4. The method of claim 3, wherein the first electrolysis unit comprises a stack of solid oxide electrolysis cells.

5. The method of claim 3 or claim 4, wherein passing the first reactant stream to the first electrolysis unit comprises passing the first reactant stream to the cathode.

6. The method of any of claims 3 to 5, wherein passing the methane-enriched effluent gas to the first electrolysis unit comprises passing the methane-enriched effluent gas to the anode.

7. The method of claim 6, wherein the anode comprises a steam reforming catalyst.

8. The method of any of claims 3 to 7, wherein the first electrolysis unit operates at a temperature of from 500 to 1000 °C, preferably from 700 to 900 °C.

9. The method of claim 1 or claim 2, wherein the first electrolysis unit comprises a protonconducting solid oxide cell (H-SOC) in which a catalyst is employed at the cathode for reforming both methane and carbon dioxide to form carbon monoxide and hydrogen.

10. The method of any preceding claim, wherein the hydrocarbon synthesis unit comprises a Fischer-Tropsch reactor that converts syngas to a liquid hydrocarbon product, an aqueous stream and the effluent gas.

11. The method of claim 10, wherein the effluent gas comprises Cl to C4 hydrocarbons.

12. The method of claim 10 or claim 11, wherein passing the syngas to the hydrocarbon synthesis unit comprises passing the syngas to a carbon dioxide removal unit to form a carbon-dioxide depleted syngas and a carbon dioxide stream, and then passing the carbon-dioxide depleted syngas to the hydrocarbon synthesis unit.

13. The method of claim 12, further comprising passing at least a portion of the carbon dioxide stream to the first electrolysis unit.

14. The method of any of claims 10 to 13, further comprising passing at least a portion of the aqueous stream to the first electrolysis unit in the form of steam, preferably wherein the aqueous stream is at least partially purified prior to being passed to the first electrolysis unit.

15. The method of any of claims 10 to 14, further comprising passing the liquid hydrocarbon product to an upgrading unit to provide an upgraded liquid hydrocarbon product and a waste hydrocarbon; and passing the waste hydrocarbon to the derichment reactor or a further derichment reactor and then the first electrolysis unit.

16. The method of any of claims 1 to 9, wherein the hydrocarbon synthesis unit comprises a methanol synthesis unit upstream of a methanol-to-hydrocarbon unit; and the step of passing the syngas to the hydrocarbon synthesis unit to form a liquid hydrocarbon product and an effluent gas comprises: passing the syngas to the methanol synthesis unit to convert syngas to a methanol stream and an off gas stream comprising hydrogen; passing the methanol stream to the methanol-to-hydrocarbon unit to form a hydrocarbon mixture and the effluent gas; and recovering from the hydrocarbon mixture at least one of a hydrocarbon fuel product selected from a diesel product, a gasoline product and a jet fuel product.

17. The method of claim 16, wherein the off gas stream is recycled into the syngas prior tothe syngas being passed to the hydrocarbon synthesis unit.

18. The method of claim 17, wherein prior to the off gas stream being recycled into the syngas, the off gas stream is passed to a hydrogen recovery unit to increase the hydrogen concentration of the off gas stream arid to form a carbon-containing off gas; and the method further comprises introducing the carbon-containing off gas to the effluent gas prior to the effluent gas being passed to the derichment reactor.

19. The method of any of claims 16 to 18, wherein the syngas passed to the methanol synthesis unit has a stoichiometry number R defined as R = ([H2] - [CO2]) / ([CO2] + [CO]) in the range 1 95 to 2.15.

20. The method of any of claims 16 to 19, wherein passing the methanol stream to the methanol-to-hydrocarbon unit comprises: passing the methanol stream to a methanol distillation unit to form a purified methanol stream and a water stream; and passing the purified methanol stream to the methanol- to-hydrocarbon unit.

21. The method of claim 20, further comprising separating a fusel oil from the methanol distillation unit and introducing the fusel oil stream into the effluent gas prior to passing the effluent gas to the derichment reactor.

22. The method of claim 20 or claim 21, further comprising introducing the water stream into the first reactant stream.

23. The method of any of claims 16 to 22, further comprising introducing carbon dioxide into the syngas prior to passing the syngas to the hydrocarbon synthesis unit.

24. The method of any preceding claim, wherein the derichment reactor comprises a nickel catalyst, preferably a nickel catalyst having a nickel content, expressed as NiO, in the range of from 30 to 60% by weight based on the total weight of the catalyst.

25. The method of any preceding claim, wherein the derichment reactor operates at a temperature of from 300 °C to 650 °C and / or at a pressure of from 10 to 100 bara.

26. The method of any preceding claim, wherein the effluent gas is heated upstream of the derichment reactor in heat exchange with the syngas.

27. The method of any preceding claim, further comprising: providing a second reactant stream comprising water; passing the second reactant stream to a second electrolysis unit to form a hydrogen stream; and introducing the hydrogen stream into the syngas prior to passing the syngas to the hydrocarbon synthesis unit, wherein the second electrolysis unit operates at a lower temperature than the first electrolysis unit.

28. The method of claim 27, wherein the second electrolysis unit comprises an alkaline electrolysis unit or a polymer electrolyte membrane (PEM) electrolysis unit.

29. The method of any preceding claim, wherein the first electrolysis unit and / or the second electrolysis unit are powered by renewable energy.

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