GB2640371A - A method for synthesising hydrocarbons - Google Patents

Abstract

A method for synthesising hydrocarbons, the method comprising: (a) feeding hydrogen and carbon dioxide to a reverse water-gas shift (rWGS) reactor to form synthesis gas comprising hydrogen and carbon monoxide; (b) passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream and one or more hydrocarbon containing recycle streams formed by the Fischer-Tropsch system itself and/or formed by an upgrading unit coupled to the Fischer-Tropsch system; (c) feeding at least a portion of the one or more hydrocarbon containing recycle streams to one or more derichment reactors containing derichment catalyst generating one or more methane containing recycle streams; and (d) feeding the one or more methane containing recycle streams to the reverse water-gas shift reactor where the methane undergoes a steam methane reforming (SMR) reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the method further comprises supplementing the one or more methane containing recycle streams with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.

Description

A METHOD FOR SYNTHESISING HYDROCARBONS

Field

This specification relates to a method for synthesising hydrocarbons from synthesis gas comprising hydrogen and carbon monoxide prepared using reverse water-gas shift and steam methane reforming.

Background

Gas streams comprising hydrogen and carbon monoxide (syngas or synthesis gas) are used in processes for the synthesis of chemicals including hydrocarbons and oxygenated hydrocarbons such as alcohols. Typically, these syngas streams are optimally low in inert content (CO2, CH4, N2, etc) and are produced at a target ratio of H2/CO, typically in the range 1.8-2.2.

Syngas generation using a reverse water-gas shift (RWGS) reaction can be beneficial since it makes use of carbon dioxide that may have been destined to be released to the atmosphere. Furthermore, hydrogen for the reverse water-gas shift reaction can be produced by water electrolysis using renewably produced electricity (so called, green hydrogen). Following this route to producing syngas, and using the syngas to produce hydrocarbons, can thus provide a fully integrated process to synthesise green hydrocarbons from carbon dioxide and hydrogen from the electrolysis of water.

The reverse water-gas shift reaction may be depicted as follows: H2+ CO2.# CO + H20 The reverse water-gas shift reaction is endothermic and thus a higher conversion of hydrogen and carbon dioxide to carbon monoxide and water is favoured by high temperatures. Various processes for carrying out RWGS are known in the art including: (i) Combusting part of the CO2/H2 feed with oxygen in a burner and passing the hot gases over a bed of RWGS catalyst so that the RWGS reaction proceeds towards equilibrium adiabatically.

(ii) Passing the feed CO2/H2 through RWGS catalyst in tubes in a furnace and combusting a fuel in the furnace to provide the heat for the RWGS reaction.

(iii) Preheating the CO2/H2 feed upstream of the RWGS catalyst using electrical energy. For instance, this could be by using a static electric resistance heater or by heating the gas directly, such as by using Turbo Machinery Heating (TMH).

(iv) Providing electrical resistance heating within the RWGS catalyst.

(v) Passing the feed CO2/H2 feed through tubes containing catalyst in a heat exchanger, where the tubes are heated by a fluid in a secondary heating circuit (the secondary heating utilising, for example, electrical energy).

Another known method for generating syngas is via a steam methane reforming reaction which his depicted below: CH4 + H2O '-CO + 3H2 This reaction is also endothermic and thus a higher conversion of methane and steam to carbon monoxide and hydrogen is also favoured by high temperatures.

Syngas produced using methodologies as described above can be converted to liquid hydrocarbons using a Fischer-Tropsch (FT) process. Fischer-Tropsch reactions occur in the presence of metal catalysts, typically at temperatures of 150-300°C and pressures of one to several tens of atmospheres. The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (C,,H2,,,,2). The more useful reactions produce alkanes as follows: (2n +1) H2 +n CO 4 C,F12,2 n H2O where n may be 1-100, or higher. The formation of methane (n = 1) is unwanted. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. The Fischer-Tropsch reaction is a highly exothermic reaction due to a standard reaction enthalpy (4H) of -165 ki/mol CO combined.

W02022079408 describes a reverse water-gas shift process for producing a gas stream comprising carbon monoxide by feeding a gas mixture comprising carbon dioxide and hydrogen to a burner disposed in a reverse water-gas shift reactor and combusting it with a sub-stoichiometric amount of an oxygen gas stream to form a combusted gas mixture containing carbon monoxide, carbon dioxide, hydrogen and steam. The mixture is then passed through a reverse water-gas shift catalyst to form a crude product gas comprising carbon monoxide, carbon dioxide, hydrogen and steam with increased CO content. The gas is then cooled so that the water content condenses and can be separated and removed. The gas stream then passes to a carbon dioxide removal unit to remove carbon dioxide and produce a syngas comprising carbon monoxide and hydrogen. The carbon dioxide which is removed from the syngas can be recycled to the feed gas mixture to the reverse water-gas shift reactor.

W02022079408 also describes a process for synthesising hydrocarbons where at least a portion of the H2/CO containing gas from the RWGS process as described above is fed to a Fischer-Tropsch (FT) unit to make hydrocarbons and FT water. At least a portion of this water can be recycled back to an electrolysis unit which makes the hydrogen feed to the RWGS reactor. It is also described that 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, can be recycled and fed to the reverse water-gas shift reactor. In this regarding a pre-reformer (also referred to herein as a derichment reactor) is a catalyst containing reactor which converts C2+ hydrocarbons to methane. Such reactors and catalysts are known in the art. Commercially available pre-reformer / derichment catalysts are typically nickel catalysts. Reactions which occur in the pre-reformer / derichment reactor include: CnHm + nH2O 4 nCO + (n+m/2)H2 CO + 3H2 <-> CH4 + H20 The net result is that a stream comprising C2+ hydrocarbons is converted into a methane rich stream. As such, a derichment reactor is a pre-reformer which converts higher hydrocarbons to methane and a deriched tail gas is one which has been treated to reduce the amount of higher hydrocarbons by converting them to methane. In this regard, the term "enrichment" in the context of a hydrocarbon stream is used in the art as describing a process for increasing the content of heavier hydrocarbons. Derichment is the opposite of this process, i.e. reducing the content of heavier hydrocarbons and increasing the methane content. The purpose of pre-reforming is to break down higher hydrocarbons such as propane, butane or naphtha into methane (CH4), which allows for more efficient reforming downstream.

W02022079408 thus discloses that hydrogen for the reverse water-gas reaction to produce syngas can be at least partially provided via electrolysis of water, optionally recycled from downstream of the reverse water-gas shift reactor. W02022079408 also discloses that carbon dioxide for the reverse water-gas shift reaction can be at least partially provided by recycling carbon dioxide from downstream of the reverse water gas shift reactor. W02022079408 also discloses that a hydrocarbon stream can be recycled from downstream via a pre-reformer / derichment reactor and introduced into the reverse water-gas shift reactor such that both the reverse water-gas shift reaction and the steam methane reforming reaction can occur in the same reverse water-gas shift reactor to produce syngas.

Summary

The present specification provides a method for synthesising hydrocarbons via a combination of reverse water-gas shift (syngas generation) and Fischer-Tropsch (hydrocarbon generation using syngas) which has several advantages over prior art arrangements. The aim is to improve the hydrogen efficiency of the process, reduce capital and operating costs, reduce renewable energy demands, and reduce overall carbon emissions and required import of carbon dioxide for the process. These improvements can be at least partially addresses by recycling hydrocarbon containing streams from the Fischer-Tropsch system (and/or an upgrading unit coupled to the Fischer-Tropsch system) back to the reverse water-gas shift reactor via one or more derichment reactors or pre-reformers (which convert C2+ hydrocarbons to methane). This recycling and derichment / pre-reforming process produces one or more methane containing recycle streams which are fed into the reverse water-gas shift reactor where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide. The methane also provides a fuel to generate heat to drive the reverse water-gas shift reaction, resulting in less hydrogen being consumed as a fuel in the process. In order to further improve hydrogen efficiency, in accordance with the present specification, the one or more methane containing recycle streams are supplemented with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide. The biomethane also provides a fuel to generate heat to drive the reverse water-gas shift reaction, resulting in less hydrogen being consumed as a fuel in the process. Biomethane or biogas is a renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, wastewater, and food waste. Biomethane/biogas is composed primarily of methane and carbon dioxide but also may comprise small amounts of hydrogen sulfide and siloxanes.

The present configuration thus comprises a reverse water-gas shift system which utilizes both: (i) methane generated via recycling of hydrocarbon waste streams from the downstream hydrocarbon synthesis process; and (ii) a biogas feed stream which supplements the recycled methane from the downstream hydrocarbon synthesis process. This combination of methane streams, which includes replacing a fraction of the H2/CO2 primary feed to the reverse water-gas shift reactor with a biomethane feed, supplements a loss of hydrogen in the reverse water-gas shift process when converting carbon dioxide to carbon monoxide and/or can offset a reduction in hydrogen supply from an electrolyser powered by renewable energy during periods of low renewable energy availability.

The use of biomethane in this manner is particularly useful in reverse water-gas shift reactors in which a proportion of the hydrogen is combusted to generate the heat to drive a higher conversion of carbon dioxide to carbon monoxide (i.e., autothermal reverse water-gas shift reactors). The biomethane can function as both a feedstock for syngas production via the steam methane reforming reaction and also as a fuel to generate heat to drive both the reverse water-gas shift and steam methane reforming reactions, resulting in less hydrogen being consumed as a fuel in the process. Several benefits are thus derived from this approach including: a reduction in hydrogen consumption and an associated increase in hydrogen efficiency; a reduction in capital costs by reducing the size and capital expenditure of the upstream electrolysis unit required for hydrogen production; a reduction in operating costs and renewable energy requirements for the upstream electrolysis unit required for hydrogen production; freely available biomethane can be used within the plant instead of being released to atmosphere, reducing overall carbon emissions and requiring less imported CO2.

In light of the above, according to the present specification there is provided a method for synthesising hydrocarbons, the method comprising: (a) feeding hydrogen and carbon dioxide to a reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide; (b) passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream and one or more hydrocarbon containing recycle streams formed by the Fischer-Tropsch system itself and/or formed by an upgrading unit coupled to the Fischer-Tropsch system; (c) feeding at least a portion of the one or more hydrocarbon containing recycle streams to one or more derichment reactors containing derichment catalyst generating one or more methane containing recycle streams; and (d) feeding the one or more methane containing recycle streams to the reverse water-gas shift reactor where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the method further comprises supplementing the one or more methane containing recycle streams with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.

The present specification also provides a system for synthesising hydrocarbons according to the aforementioned method, the system comprising: a reverse water-gas shift reactor configured to receive hydrogen and carbon dioxide and form synthesis gas comprising hydrogen and carbon monoxide; a Fischer-Tropsch system configured to receive the synthesis gas and form a hydrocarbon product stream comprising a mixture of hydrocarbons and one or more hydrocarbon containing recycle streams formed by the Fischer-Tropsch system itself and/or formed by a hydrocarbon product stream upgrading unit coupled to the Fischer-Tropsch system; one or more derichment reactors containing derichment catalyst and configured to receive the one or more hydrocarbon containing recycle streams and generate one or more methane containing recycle streams to the reverse water-gas shift reactor where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the system further comprises a biomethane feed configured to supplementing the one or more methane containing recycle streams with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.

Examples of methods and systems according to the present specification are provided in the detailed description.

Brief Description of the Drawings

Figure 1 shows illustrates an example of a method for synthesising hydrocarbons according to the present specification.

Figure 2 shows a more detailed flow sheet illustrating an example of a method for synthesising hydrocarbons according to the present specification.

A summary of the reference numerals used in the figures is set out in the table below.

Reference Item 2 Hydrogen Feed 4 Carbon Dioxide Feed 6 Feed Heater 8 RWGS Reactor / Reformer 9 Synthesis Gas Oxygen Feed 12 Steam Feed 14 Syngas Cooling (& Water Separation) 16 Carbon Dioxide Separation 17 Carbon Dioxide Recycle 18 FT Synthesis (Fischer-Tropsch Reactor) Hydrocarbon Products (e.g., Diesel, Jet Fuel) 22 Naphtha 24 FT Tails Gas Recycle 26 Steam Feed 28 Offgas Derichment Interchanger 32 Derichment Reactor 33 Methane Recycle Stream 34 Biomethane Feed 36 Biomethane Purification Unit

Detailed Description

As described in the summary section and illustrated in Figure 1, the present specification provides a method for synthesising hydrocarbons, the method comprising: feeding hydrogen 2 and carbon dioxide 4 to a reverse water-gas shift reactor 8 to form synthesis gas 9 comprising hydrogen and carbon monoxide; passing the synthesis gas 9 though a Fischer-Tropsch reactor/system 18 containing a FischerTropsch catalyst to form a hydrocarbon product stream 20 and one or more hydrocarbon containing recycle streams 24 which may be formed by the Fischer-Tropsch system itself and/or by an upgrading unit coupled to the Fischer-Tropsch system; feeding at least a portion of the one or more hydrocarbon containing recycle streams 24 to one or more derichment reactors 32 containing derichment catalyst generating one or more methane containing recycle streams 33; and feeding the one or more methane containing recycle streams 33 to the reverse water-gas shift reactor 8 where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the method further comprises supplementing the one or more methane containing recycle streams 33 with a biomethane containing feed gas 34 which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.

The biomethane containing feed gas 34 can be mixed with the one or more hydrocarbon containing recycle streams 24 prior to the one or more derichment reactors 32 as illustrated in Figure 1. Alternatively, the biomethane containing feed gas 34 can be mixed with the one or more methane containing recycle streams 33 after the one or more derichment reactors 32. In that case, the biomethane containing feed gas 34 can be deriched in a separate derichment reactor to the derichment reactor(s) 32 used for the one or more hydrocarbon containing recycle streams 24 prior to mixing the deriched biomethane containing feed gas 34 with the one or more methane containing recycle streams 33. The biomethane containing feed gas is advantageous passed through a purification unit (not shown in Figure 1 but shown in Figure 2) prior to supplementing the one or more methane containing recycle streams with the biomethane containing feed gas.

The one or more hydrocarbon containing recycle streams 24 may comprise a tail gas stream from the Fischer-Tropsch system, and at least a portion of the biomethane containing feed gas 34 is mixed with the tail gas stream prior to entering the reverse water-gas shift reactor. Additionally or alternatively, the one or more hydrocarbon containing recycle streams 24 may comprise a naphtha stream from an upgrading unit coupled to the Fischer-Tropsch system, and at least a portion of the biomethane containing feed gas 34 is mixed with the naphtha stream prior to entering the reverse water-gas shift reactor. The recycled tails gas and naphtha streams are passed through a derichment reactor to convert C2+ hydrocarbons to methane prior to entering the reverse water-gas shift reactor. Advantageous, when both tails gas and naphtha are recycled, these streams are deriched in separate derichment reactors operating with different conditions optimized for the two different recycle streams. The biogas feed stream can be mixed with one or both of the tails gas and naphtha streams either prior to or after the derichment reactors. If mixed after, the biogas can be separately deriched prior to mixing.

Additionally, steam is advantageously mixed with the biomethane containing feed gas prior to entering the reverse water-gas shift reactor, optionally prior to derichment of the biomethane. Furthermore, the hydrogen and/or carbon dioxide feed gas can be mixed with the biomethane containing feed gas prior to entering the reverse water-gas shift reactor, optionally following derichment of the biomethane. Further still, one or more of the methane containing recycle streams, the biomethane containing feed gas, the hydrogen feed gas, the carbon dioxide feed gas, or a mixture of two or more of the aforementioned feed gas streams are advantageously pre-heated prior to entering the reverse water-gas shift reactor. Pre-heating the feed gases improves the efficiency of the reverse water-gas shift reactor.

In order to provide an optimal ratio of methane to hydrogen in the reverse water-gas shift reactor, the feed gas streams (e.g., prior to derichment of the one or more hydrocarbon containing recycle streams) can provide a ratio of total flow rate of methane containing streams to flow rate of the hydrogen feed in a range 0.01 to 4. The feed gas streams, at an inlet of the reverse water-gas shift reactor, can provide a molar ratio of methane to H2 in a range 0.01 to 2.5.

In accordance with the present specification, the H2/CO2 primary feed to the reverse water-gas shift reactor is supplemented with both a methane recycle stream and a biomethane feed in order to reduce the loss of hydrogen in the reverse water-gas shift process when converting carbon dioxide to carbon monoxide and/or offset a reduction in hydrogen supply from an electrolyser powered by renewable energy during periods of low renewable energy availability. The biomethane can function both as a feedstock for syngas production via a steam methane reforming reaction and also as a fuel to generate heat to drive both the reverse water-gas shift and steam methane reforming reactions, resulting in less hydrogen being consumed as a fuel in the process. As indicated in the summary section, several benefits are thus derived from this approach including: a reduction in hydrogen consumption and an associated increase in hydrogen efficiency; a reduction in capital costs by reducing the size and capital expenditure of the upstream electrolysis unit required for hydrogen production; a reduction in operating costs and renewable energy requirements for the upstream electrolysis unit required for hydrogen production; and freely available biomethane can be used within the plant instead of being released to atmosphere, reducing overall carbon emissions and requiring less imported CO2.

Figure 2 shows an example of a suitable flow sheet. Hydrogen 2 and carbon dioxide 4 are pre-heated in a feed gas heater 6 and passed to a reverse water-gas shift reactor 8. Oxygen 10 and steam 12 can also be fed to the reverse water-gas shift reactor 8. The oxygen can be combusted with a portion of the hydrogen to provide heat and the process gas passed over a reverse water-gas shift catalyst within the reactor 8 to drive a reverse water-gas shift reaction and generate crude syngas. The crude syngas can be cooled in a syngas cooling unit 14 and water separated from the syngas. The syngas can then be passed to a carbon dioxide separation unit 16 to separate carbon dioxide from the syngas. The separated carbon dioxide 17 can be recycled to the feed gas for the reverse water-gas shift reactor 8. The syngas may be further processed to remove other contaminants if required prior to passing the syngas to a Fischer-Tropsch synthesis unit 18 where the syngas is passed over a Fischer-Tropsch catalyst to produce a hydrocarbon product stream 20. Unwanted hydrocarbons such as naphtha 22 can be separated and recycled via a derichment interchanger 30 and a derichment reactor 32 to produce methane which is mixed into the feed gas to the reverse water-gas shift reactor. This recycled methane can serve as both a feedstock for syngas generation and a fuel for combustion within the reverse water-gas sift reactor to produce heat to drive the reverse water-gas shift reaction.

The FT synthesis unit also produces a tails gas stream 24 which can also be recycled via the derichment interchanger 30 and the derichment reactor 32 to produce methane which is mixed into the feed gas to the reverse water-gas shift reactor. Again, this recycled methane can serve as both a feedstock for syngas generation and a fuel for combustion within the reverse water-gas sift reactor to produce heat to drive the reverse water-gas shift reaction. This tails gas recycle loop may use the same derichment reactor 32 as the naphtha recycle loop or may use a different derichment reactor and/or interchanger. Steam 26 and any other sources of offgas 28 can also be fed to one or more of the derichment reactors 32 via one or more derichment interchanges 30.

The above-described recycling of naphtha and tails gas to generate methane which is introduced into the H2/CO2 feed for the reverse water-gas shift reactor aids in increasing the hydrogen efficiency of the overall process by using the methane as both a feedstock for syngas generation via steam methane reforming in the reverse water-gas shift reactor 8 and also as a fuel for heating the process gas to drive the reforming and reverse water-gas shift reactions. In accordance with the present specification, a biomethane feed stock 34 is also used to supplement the H2/CO2 feed to increase the hydrogen efficiency of the overall process by using the biomethane as both a feedstock for syngas generation via steam methane reforming in the reverse water-gas shift reactor 8 and also as a fuel for heating the process gas to drive the reforming and reverse water-gas shift reactions.

In the illustrated example, the biomethane 34 is passed through a purification unit 36, mixed with offgas 28, steam 26, and recycled FT tails gas 24, heated in the interchanger 30 and passed through the derichment reactor 32 to generate a methane containing gas stream which is mixed with the H2/CO2 feed 2, 4 and recycled CO2 17 to provide a mixed feed gas for the reverse water-gas shift reactor.

The present process is primarily for use when excess biomethane is available on-site and would otherwise be wasted. A stream of biomethane available on-site is purified to remove contaminants (e.g., sulphur) that would poison the downstream derichment reactor and RWGS reactor catalyst before being combined with the FT tails gas recycle (plus any other off-gas streams) and passed through the derichment reactor to turn longer chain hydrocarbons into methane. This feed is then combined with CO2 to match production requirements and H2 to provide the correct Hz:CO ratio required for the FT loop. Since methane can then be burned in the RWGS burner to provide heat for the reactions, less H2 needs to be burned and more is available for RWGS reaction. The biomethane serves as both a feedstock and replacement fuel in the RWGS burner to save hydrogen consumption.

This approach can represent an improvement over prior flow sheets which use an autothermal reformer (ATR) to combust green hydrogen with oxygen to supply the enthalpy, at high temperature, for the endothermic RWGS (and reforming) reactions. Use of biomethane addition will help improve the hydrogen efficiency of the process as less hydrogen is burned in the RWGS burner and more can instead be used in the RWGS reaction. A further description of the process as illustrated in Figure 2 is provided below with some additional features covered.

A stream of pressurised hydrogen (e.g., renewable hydrogen from a pressurised electrolysis unit) is imported into the plant. A stream of carbon dioxide is also introduced, compressed if required, (and optionally purified to remove compounds that poison catalysts, e.g., sulphur compounds). One part of the CO2 stream is imported into the plant. Another part may be separated from the cooled, dewatered syngas and recycled back to the RWGS unit. The CO2 and hydrogen streams are mixed together then heated, typically to a temperature of 250-500°C, preferably 350-450°C, which is hot enough to be at a margin above the dew point if steam is subsequently added. Alternatively, the hydrogen and CO2 streams may be heated separately, then mixed.

A hydrocarbon stream comprising the imported biomethane alongside other optional recycle streams such as recycled FT tails gas and ISO-cracker/distillation offgas (from the downstream upgrading unit) is then combined with steam and heated before passing to a derichment (pre-reforming) reactor. The function of this reactor is to convert higher (C2+) hydrocarbons into methane so that they won't crack to form carbon in the RWGS downstream. It may also contain naphtha (typically C3-C7 hydrocarbons), which may not be of value as a final product. These streams may be processed in a single derichment reactor or in two or more separate derichment reactors operating at different conditions. The hydrocarbon stream(s) may also contain hydrocarbons from other sources.

Feed streams for the RWGS reactor thus include a hydrogen stream, a carbon dioxide stream, a biomethane feed, and one or more methane recycle streams (e.g., deriched tails gas from the FT unit and/or deriched naphtha from the upgrading unit). In the RWGS reactor, both steam methane reforming and reverse water-gas shift reactions produce a hot CO rich syngas.

The reacted CO rich gas from the RWGS reactor can be cooled in several steps. This cooling can be used to raise steam (as used in the process and optionally exported) and to preheat feed streams. Further cooling is needed to condense and separate water content from the gas. The gas is then fed to a CO2 removal unit, where unreacted CO2 is separated and can be recycled upstream of the RWGS unit so that the remaining syngas is substantially a H2/CO syngas.

The H2/CO syngas can be purified (to remove FT catalyst poisons) and optionally compressed before passing to a FT (or other liquid hydrocarbon) synthesis unit. As well as producing FT liquids, this also produces a hydrocarbon rich tailgas stream and FT water. The latter water stream may be (optionally purified and) recycled and provided as feed to an electrolysis unit if one is being used locally to provide renewable hydrogen feed. Furthermore, if the liquid hydrocarbon production produces a naphtha stream surplus to requirements, this can be recycled to the RWGS process as previously described.

Variations of the specific flow sheet illustrated in Figure 2 are envisaged as discussed below.

In the illustrated arrangement the biomethane containing feed gas is passed through a purification unit prior to being passed to the reverse water-gas shift reactor. However, if the biomethane is already of sufficient purity then a purification step integrated into the present process may not be required.

In the illustrated arrangement the biomethane containing feed gas is fed to a derichment reactor containing derichment catalyst prior to being passed to the reverse water-gas shift reactor. However, if the biomethane already has a low content of C2+ hydrocarbons then derichment of the biomethane to convert C2+ hydrocarbons to methane may not be required.

In the illustrated arrangement, the biomethane containing feed gas is mixed with at least one of the hydrocarbon containing recycle streams prior to being fed to the reverse water-gas shift reactor via the derichment heat interchanger and derichment reactor. However, the biomethane containing feed gas could be fed to the reverse water-gas shift reactor without being mixed with such recycle streams. For example, two or more derichment reactors can be provided for separately deriching various C2+ hydrocarbon containing feed streams. In that case, the biomethane could be purified and deriched in a dedicated derichment reactor prior to feeding to the reverse water-gas shift reactor rather than being mixed and deriched with another hydrocarbon stream. Alternatively, the biomethane could be mixed with one or both of the hydrogen and carbon dioxide feeds prior to feeding to the reverse water-gas shift reactor. As such, it will be appreciated that while the illustrated arrangement shows all hydrocarbon feed gases being mixed, deriched, and then mixed with Hz/CO2 to form a single feed stream to the reverse water-gas shift reactor, alternative feed configurations could be implemented for the Hz, CO2, biomethane, and recycle streams.

The process as described above involves recycling of both FT tails gas and also naphtha separated from the FT product stream along with any other hydrocarbon offgas streams. However, one or more of the hydrocarbon recycle streams could be provided individually or in combination. For example, the one or more hydrocarbon containing recycle streams may comprise a tail gas stream from the FischerTropsch reactor, and at least a portion of the biomethane containing feed gas is mixed with the tail gas stream prior to entering the reverse water-gas shift reactor. Alternatively, or additionally, the one or more hydrocarbon containing recycle streams comprises a naphtha stream from the FischerTropsch reactor, and at least a portion of the biomethane containing feed gas is mixed with the naphtha stream prior to entering the reverse water-gas shift reactor.

While not specifically illustrated in Figure 2, water produced in the process can be recycled and electrolysed or thermochemically split to produce hydrogen which is fed to the reverse water-gas shift reactor and/or the one or more derichment reactors. Oxygen produced by the electrolysis or thermochemical splitting of water can also be used as a feed for combustion in the reverse water-gas shift reactor. For example, the synthesis gas generated by the reverse water-gas shift reactor can be passed to a water removal unit to remove water prior to passing the synthesis gas to the FischerTropsch reactor and at least a portion of the water removed in the water removal unit can be electrolysed or thermochemically split to produce hydrogen (and oxygen) which is fed to the reverse water-gas shift reactor and/or the one or more derichment reactors. Alternatively or additionally, the Fischer-Tropsch reactor generates a water stream in addition to the product stream, and at least a portion of the water stream is electrolysed or thermochemically split to produce hydrogen, and optionally oxygen, which is fed to the reverse water-gas shift reactor and/or the one or more derichment reactors. The use of biomethane according to the present specification is particularly advantageous when the hydrogen is produced by an electrolyser, as the improvement in hydrogen efficiency can enable a reduction in capital costs by reducing the size and capital expenditure of the electrolysis unit required for hydrogen production and a reduction in operating costs and renewable energy requirements for the electrolysis unit required for hydrogen production.

It will be appreciated that various modifications to the configuration shown in Figure 2 can be made while still implementing the core features of the present invention. Accordingly, while this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims (19)

1. Claims 1. A method for synthesising hydrocarbons, the method comprising: (a) feeding hydrogen and carbon dioxide to a reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide; (b) passing the synthesis gas though a Fischer-Tropsch system to form a hydrocarbon product stream and one or more hydrocarbon containing recycle streams formed by the Fischer-Tropsch system itself and/or formed by an upgrading unit coupled to the Fischer-Tropsch system; (c) feeding at least a portion of the one or more hydrocarbon containing recycle streams to one or more derichment reactors containing derichment catalyst generating one or more methane containing recycle streams; and (d) feeding the one or more methane containing recycle streams to the reverse water-gas shift reactor where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the method further comprises supplementing the one or more methane containing recycle streams with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.
2. 2. A method according to claim 1, wherein the biomethane containing feed gas is mixed with the one or more hydrocarbon containing recycle streams prior to the one or more derichment reactors.
3. 3. A method according to claim 1, wherein the biomethane containing feed gas is mixed with the one or more methane containing recycle streams after the one or more derichment reactors.
4. 4. A method according to claim 3, wherein the biomethane containing feed gas is deriched in a separate derichment reactor to the one or more hydrocarbon containing recycle streams.
5. 5. A method according to any preceding claims, wherein the biomethane containing feed gas is passed through a purification unit prior to supplementing the one or more methane containing recycle streams with the biomethane containing feed gas.
6. 6. A method according to any preceding claims, wherein the one or more hydrocarbon containing recycle streams comprises a tail gas stream from the Fischer-Tropsch system, and at least a portion of the biomethane containing feed gas is mixed with the tail gas stream prior to entering the reverse water-gas shift reactor.
7. 7. A method according to any preceding claims, wherein the one or more hydrocarbon containing recycle streams comprises a naphtha stream from the hydrocarbon product stream upgrading unit, and at least a portion of the biomethane containing feed gas is mixed with the naphtha stream prior to entering the reverse water-gas shift reactor.
8. 8. A method according to any preceding claims, wherein steam is mixed with the biomethane containing feed gas prior to entering the reverse water-gas shift reactor, optionally prior to derichment.
9. 9. A method according to any preceding claims, wherein the hydrogen and/or carbon dioxide feed gas is mixed with the biomethane containing feed gas prior to entering the reverse water-gas shift reactor, optionally following derichment.
10. 10. A method according to any preceding claim, wherein one or more of the methane containing recycle streams, the biomethane containing feed gas, the hydrogen feed gas, the carbon dioxide feed gas, or a mixture of two or more of the aforementioned feed gas streams are pre-heated prior to entering the reverse water-gas shift reactor.
11. 11. A method according to any preceding claim, wherein the feed gas streams provide a ratio of total flow of methane containing streams to flow of the hydrogen feed in a range 0.01 to 4.
12. 12. A method according to any preceding claim, wherein the feed gas streams, at an inlet of the reverse water-gas shift reactor, provide a molar ratio of methane to H2 in a range 0.01 to 2.5.
13. 13. A method according to any preceding claim, wherein the synthesis gas generated by the reverse water-gas shift reactor is passed to a water removal unit to remove water prior to passing the synthesis gas to the Fischer-Tropsch system.
14. 14. A method according to claim 13, wherein at least a portion of the water removed in the water removal unit is electrolysed or thermochemically split to produce hydrogen which is fed to the reverse water-gas shift reactor and/or the one or more derichment reactors.
15. 15. A method according to claim 14, wherein oxygen is also produced by the electrolysis or thermochemical splitting of water and the oxygen is also fed to the reverse water-gas shift unit and/or the one or more derichment reactors.
16. 16. A method according to any preceding claim, wherein the synthesis gas generated by the reverse water-gas shift reactor is passed to a carbon dioxide removal unit to remove carbon dioxide prior to passing the synthesis gas to the FischerTropsch system.
17. 17. A method according to claim 16, wherein at least a portion of the carbon dioxide removed in the carbon dioxide removal unit is recycled to the reverse water-gas shift reactor.
18. 18. A method according to any preceding claim, wherein the Fischer-Tropsch system generates a water stream in addition to the hydrocarbon product stream, and at least a portion of the water stream is electrolysed or thermochemically split to produce hydrogen, and optionally oxygen, which is fed to the reverse water-gas shift reactor and/or the one or more derichment reactors.
19. 19. A system for synthesising hydrocarbons according to the method of any preceding claim, the system comprising: a reverse water-gas shift reactor configured to receive hydrogen and carbon dioxide and form synthesis gas comprising hydrogen and carbon monoxide; a Fischer-Tropsch system configured to receive the synthesis gas and form a hydrocarbon product stream comprising a mixture of hydrocarbons and one or more hydrocarbon containing recycle streams formed by the Fischer-Tropsch system itself and/or formed by a hydrocarbon product stream upgrading unit coupled to the Fischer-Tropsch system; one or more derichment reactors containing derichment catalyst and configured to receive the one or more hydrocarbon containing recycle streams and generate one or more methane containing recycle streams to the reverse water-gas shift reactor where the methane undergoes a steam methane reforming reaction to form synthesis gas comprising hydrogen and carbon monoxide, wherein the system further comprises a biomethane feed configured to supplementing the one or more methane containing recycle streams with a biomethane containing feed gas which also undergoes a steam methane reforming reaction in the reverse water-gas shift reactor to form synthesis gas comprising hydrogen and carbon monoxide.