Multi-bed fischer-tropsch catalyst process for co 2 conversion

Abstract

The present disclosure relates generally to a process for preparing hydrocarbons. The process includes providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising C5+ hydrocarbons, and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide from the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising C5+ hydrocarbons. The disclosure is especially concerned with implementing the above multi-stage process in a way which is adapted to the relatively low output of carbon monoxide in the first FT product stream, so as to require no (or minimal) use of a separate extraneous carbon monoxide source.

Description

[0001] MULTI-BED FISCHER-TROPSCH CATALYST PROCESS FOR CO2CONVERSION

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates generally to processes for the Fischer-Tropsch synthesis of hydrocarbons from carbon dioxide.

[0004] BACKGROUND

[0005] The conversion of synthesis gas (i.e. , a mixture of carbon monoxide and hydrogen, also known as syngas) into hydrocarbons by the Fischer-Tropsch (FT) process has been known for decades, but has historically lagged in performance compared to other hydrocarbon synthesis techniques. The growing importance of alternative energy sources has resulted in renewed interest in the FT process as it allows a direct route to high-quality fuels and feedstock chemicals.

[0006] FT processes are known for producing linear hydrocarbons, as well as oxygenates, that can be useful in fuels and can also serve as valuable feedstock chemicals. The hydrocarbon fuel derived from FT processes is typically better able to meet increasingly stringent environmental regulations compared to conventional refinery-produced fuels, as FT-derived fuels typically have lower contents of sulfur, nitrogen, and aromatic compounds, which contribute to the emission of potent pollutants such as SO2, NOX, and particulates. Olefins and alcohols and other oxygenates obtained may also be used as reagents in other processes, such as in the synthesis of lubricants.

[0007] Alongside development of conventional FT processes based on conversion of carbon monoxide and hydrogen, there is growing interest around FT processes in which a mixture of carbon dioxide and hydrogen are used as the feedstock - often referred to as “modified” FT processes. In such processes, a catalyst (typically an iron-based catalyst) is used to convert carbon dioxide to carbon monoxide through the so-called “reverse water-gas shift” (rWGS) reaction (CO2 + H2 ^=±CO + H2O), with the same catalyst then converting the carbon monoxide to hydrocarbons and oxygenates through a FT reaction. This modified FT approach holds great promise since carbon dioxide is far more readily accessible as a feedstock than carbon monoxide. However, it has been relatively challenging to achieve efficient conversion of CO2 towards C5+ hydrocarbons.

[0008] Thus, there remains a need to identify more efficient routes to hydrocarbons starting from CO2 as a feedstock.

[0009] SUMMARY OF THE INVENTION

[0010] The present inventors have identified that the product stream from modified FT processes can include reasonable amounts of unreacted carbon monoxide, produced by rWGS. Whilst it is possible to recycle this carbon monoxide back to the modified FT reactor for further production of hydrocarbons, the catalysts used in the modified FT process also have a propensity to convert the carbon monoxide back to carbon dioxide and hydrogen by a competing water-gas shift (WGS) reaction. In view of this, the present inventors have identified that it can be advantageous to take residual carbon monoxide from the product stream of modified FT processes and feed this to a different reaction zone, which favours conventional FT over WGS. Thus, at its broadest the present invention provides a process for preparing hydrocarbons, comprising providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising Cs+ hydrocarbons, and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide from the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising Cs+ hydrocarbons.

[0011] For the purposes of the following discussion the first FT reaction carried out in the first FT reaction zone may be referred to as the “first FT stage”, “first FT step”, “first FT reaction” or similar. Likewise, the second FT reaction carried out in the second FT reaction zone may be referred to as the “second FT stage”, “second FT step”, “second FT reaction” or similar.

[0012] Advantageously, by carrying out a multi-stage reaction with the output from the first FT reaction zone being used (at least in part) as the feedstock for the second FT reaction zone, conditions can be chosen to optimise the overall selectivity for Cs+ hydrocarbons compared to simply recycling output to a single reaction zone. In particular, conditions and catalysts in the different reaction zones can be independently tuned to optimise the selectivity / productivity of the overall reaction - e.g., the first reaction zone conditions and catalyst can be tailored to use of carbon dioxide as the feedstock and the second reaction zone conditions and catalyst can be tailored to use of carbon monoxide as the feedstock.

[0013] Whilst this approach has the advantage of making more efficient use of the carbon monoxide of the first FT product stream compared to simply recycling the carbon monoxide back to the first FT reaction zone, the inventors have discovered that a particular challenge with using the first FT product stream to create the second FT feed stream is that the molar amount of hydrogen relative to carbon monoxide in the first FT product stream is typically significantly higher than the ideal ratio of 2:1. Specifically, in modified FT processes the amount of residual carbon monoxide in the product stream is typically modest, resulting in the second FT feed stream being relatively “lean” in terms of carbon monoxide.

[0014] Whilst this can be compensated for by the addition of further extraneous carbon monoxide to the second FT feed stream, this requires the provision of an additional source of carbon monoxide, which obviates the advantages which comes from using carbon dioxide and hydrogen as the primary feedstocks for the process. Thus, the inventors have identified a number of ways to address this challenge, and the present invention is especially concerned with implementing the above multi-stage process in a way which is adapted to the relatively low output of carbon monoxide in the first FT product stream, so as to require no (or minimal) use of a separate extraneous carbon monoxide source. In particular, the inventors have identified that the low output of carbon monoxide in the first FT product stream can be compensated for by:

[0015] (a) adjusting the conditions of the first FT reaction zone to control the ratio of hydrogen to carbon monoxide output in the first FT product stream; and / or

[0016] (b) adjusting the ratio of hydrogen to carbon monoxide in the second FT reaction zone, e.g. through adapting the second FT feed stream; and / or

[0017] (c) adjusting the conditions of the second FT reaction zone to cope with a relatively high proportion of hydrogen relative to carbon monoxide.

[0018] The inventors have identified several implementations for carrying out (a)-(c), which can be used separately or in any combination to boost the overall selectivity and / or productivity of the multi-stage reaction.

[0019] In a particularly preferred first implementation, the present invention provides a process for preparing hydrocarbons, comprising: providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising Cs+ hydrocarbons, and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide from the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising Cs+ hydrocarbons; wherein the second FT feed stream has a molar ratio of hydrogen to carbon monoxide in the range of 1 :2 to 10:1 , preferably 1 :2 to 8:1 , more preferably 1 :1 to 5:1 .

[0020] Advantageously, keeping the hydrogen to carbon monoxide ratio in the second FT feed stream within these ranges ensures that the productivity and selectivity for production of Cs+ hydrocarbons in the second FT reaction zone is at a good level. The present inventors have identified a number of ways to help to achieve a molar ratio of hydrogen to carbon monoxide within the above ranges in the second FT feed stream, which can be implemented separately or together.

[0021] In a first proposal, the first FT reaction zone is operated under conditions to bring the ratio of hydrogen to carbon monoxide in the first FT product stream closer towards the ideal ratio of 2:1 . This can be achieved, for example, by flowing the first FT feed stream through the first FT reaction zone at a relatively high volumetric flow rate, having a relatively low hydrogen:carbon dioxide molar ratio, and operating the reaction at a temperature intermediate between those typically used for FT and those typically used for rWGS. For example, the first FT reaction zone may operate under at least one of the following conditions:

[0022] • a ratio of hydrogen to carbon dioxide of less than 4:1 , preferably less than 3:1 , more preferably less than 2:1 , e.g. between 0.5:1 to 4:1 , 0.5:1 to 3:1 , or 0.5:1 to 2:1 ;

[0023] • a temperature of greater than 250°C;

[0024] • a pressure of 10 to 40 barg; and / or

[0025] • a GHSV of greater than 2,000 h-1.

[0026] Whilst certain implementations may rely entirely on adjustment of the first FT feed stream and reaction conditions in the first FT reaction zone to generate a suitable ratio of hydrogen and carbon monoxide for providing to the second FT reaction zone, in generally preferred implementations, the multi-stage process is carried out such that the molar ratio of hydrogen to carbon monoxide in the second FT feed stream is adjusted to be lower than the molar ratio of hydrogen to carbon monoxide in the first FT product stream.

[0027] For example, the molar ratio of hydrogen to carbon monoxide in the second reaction zone may be adjusted by selectively reacting / removing hydrogen from the first FT product stream.

[0028] As an example of such an approach, in a second proposal, the molar ratio of hydrogen to carbon monoxide in the second feed stream is adjusted by selectively removing hydrogen from the first FT product stream so as to produce a modified product stream having a decreased molar ratio of hydrogen to carbon monoxide, and using this modified product stream to form the second FT feed stream.

[0029] Selective removal of hydrogen may be achieved by separation methods. In other words, the method may involve subjecting the first FT product stream to a separation method to selectively remove hydrogen.

[0030] This method can separate the first FT product stream into a hydrogen rich (or pure) stream and a hydrogen depleted stream, wherein the hydrogen depleted stream is used to form the second FT feed stream. Advantageously, the hydrogen rich (or pure) stream can be recycled to the first FT reaction zone.

[0031] Separation of hydrogen from the first FT product stream may be carried out by any suitable method. For example, separation may be achieved using a hydrogen selective membrane, e.g. a hydrogen permeable membrane. Alternatively, separation may be achieved using swing adsorption, e.g. pressure swing adsorption.

[0032] Additionally, or alternatively, the molar ratio of hydrogen to carbon monoxide is adjusted by selectively reacting hydrogen from the first FT product stream. This may be achieved without the need for separation.

[0033] For example, hydrogen from the first FT product stream can be consumed through a hydrogenation reaction with a suitable reactant, without affecting the level of carbon monoxide, to reduce the molar ratio of hydrogen to carbon monoxide in the second FT reaction zone. Examples of suitable reactants include, for example, light olefins, such as C2-C4 olefins.

[0034] Selective reaction of hydrogen from the first FT product stream may occur prior to introduction of the second FT feed stream to the second FT reaction zone - e.g., in a separate hydrogenation zone / unit / reactor. Preferably, however, reaction of hydrogen occurs within the second FT reaction zone, since this avoids the need for a separate zone / unit / reactor.

[0035] Typically, reaction of hydrogen from the first FT product stream will be a catalysed reaction, e.g. a catalysed hydrogenation reaction. In instances where the reaction of hydrogen occurs within the second FT reaction zone, the second FT catalyst may also serve as the hydrogenation catalyst.

[0036] The reactant for reacting with hydrogen may be added to the first FT product stream or obtained from the first FT product stream. Preferably, the reactant for reacting with hydrogen is obtained from (e.g. a component of) the first FT product stream.

[0037] Preferably, the reactant is a C2-C4 olefin from the first FT product stream. With this in mind, it is preferred that unsaturated C2-C4 hydrocarbons constitute a relatively high proportion of the total C1-C4 hydrocarbons in the first FT product stream. For example, unsaturated hydrocarbons may account for at least 10 mol%, preferably at least 20 mol%, more preferably at least 30 mol%, e.g. at least 40 mol% of the total C1-C4 hydrocarbons in the first FT product stream, and said unsaturated hydrocarbons are included in the second FT feed stream.

[0038] Additionally or alternatively, C2-C4 olefins may be added to the second FT feed stream from an alternative (e.g. extraneous) source, such as from a steam cracker, a fluid catalytic cracker (FCC), or from an alcohol dehydration stream.

[0039] The molar ratio of hydrogen to C2-C4 olefins used in the hydrogenation reaction may be in the range of 2:1 to 30:1 , for example 2:1 to 20:1 , such as 5:1 to 30:1 or 5:1 to 20:1 . For example, in instances where hydrogenation occurs in the second FT reaction zone, the molar ratio of hydrogen to C2-C4 olefins in the second FT feed stream may be in the range of 2:1 to 30:1 , for example 2:1 to 20:1 .

[0040] In a fourth proposal, the molar ratio of hydrogen to carbon monoxide in the second FT feed stream is adjusted by selectively reacting a component of the first FT product stream to form additional carbon monoxide. Preferably, the molar ratio of hydrogen to carbon monoxide in the second feed stream is adjusted by converting C1-C4 hydrocarbons of the first FT product stream to produce carbon monoxide, and including this carbon monoxide in the second FT feed stream.

[0041] Conversion of the C1-C4 hydrocarbons may be carried out, for example, by reforming. Reforming may be, for example, partial oxidation, steam reforming or dry CO2 reforming. The equations for these reactions are:

[0042] Partial Oxidation

[0043] Steam Reforming

[0044] Dry CO2 reforming

[0045] From these formulae it can be seen that the ratio of carbon monoxide to hydrogen produced by the conversion increases with the number of carbon atoms in the hydrocarbon, and that higher levels can be produced by partial oxidation or dry CO2 reforming. In one embodiment, the reforming is partial oxidation. In one embodiment, the reforming is steam reforming. In one embodiment, the reforming is CO2 reforming.

[0046] Additionally or alternatively, the molar ratio of hydrogen to carbon monoxide in the second FT feed stream is adjusted compared to the first FT product stream by adding carbon monoxide to the second FT feed stream from an external source.

[0047] As well as adjusting conditions and components upstream of the second FT reaction zone to optimise the molar ratio of hydrogen to carbon monoxide, the present invention also envisages operating the second FT reaction zone under conditions compatible with the second FT feed stream being relatively lean in carbon monoxide. In particular, a fifth proposal involves operating the second FT reaction zone at a temperature of between 150 to 230°C which is relatively lower than temperatures typically used for FT. The preferred temperature range is, for example, 180 to 220°C, optionally 150 to 190°C, such as 160- 185°C.

[0048] The skilled reader will understand that all of the proposals above can be combined in order to boost productivity and / or selectivity of the overall reaction.

[0049] The various proposals above also constitute separate aspects of the invention.

[0050] For example, in a second implementation the present invention provides a process for preparing hydrocarbons, comprising: providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising C5+ hydrocarbons and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide of the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst at a temperature of between 150 to 250°C to form a second FT product stream comprising C5+ hydrocarbons.

[0051] In a third implementation the present invention provides a process for preparing hydrocarbons, comprising: providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising C5+ hydrocarbons and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide of the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising Cs+ hydrocarbons; wherein the first FT reaction zone operates under at least one of the following conditions:

[0052] • a molar ratio of hydrogen to carbon dioxide of less than 4:1 , preferably less than 3:1 , more preferably less than 2:1 ;

[0053] • a temperature of greater than 250°C;

[0054] • a pressure of 10 to 40 barg; and

[0055] • a GHSV of greater than 2,000 h-1.

[0056] In a fourth implementation the present invention provides a process for preparing hydrocarbons, comprising: providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising CB+ hydrocarbons and carbon monoxide; selectively removing hydrogen from the first FT product stream so as to decrease the ratio of hydrogen to carbon monoxide to form a modified FT product stream; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the modified FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising CB+ hydrocarbons.

[0057] In a fifth implementation the present invention provides a process for preparing hydrocarbons, comprising: providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising CB+ hydrocarbons, C1-C4 hydrocarbons, and carbon monoxide; reforming at least a portion of the C1-C4 hydrocarbons of the first FT product stream to form carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide of the first FT product stream and carbon monoxide obtained through said reforming of C1-C4 hydrocarbons; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising C5+ hydrocarbons.

[0058] In a sixth implementation the present invention provides process for preparing hydrocarbons, comprising providing a first FT feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising CB+ hydrocarbons and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide of the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising CB+ hydrocarbons; wherein providing the second FT feed stream comprises adding C2-C4 olefins alongside at least a portion of the first FT product stream.

[0059] Given the aims of the present application set out above, the multi-stage FT reaction is preferably carried out without adding additional components subsequent to introduction of the first FT feed stream. In particular, whilst carbon monoxide may be fed into the reaction as part of the first FT feed stream, it is preferred that the multi-stage FT reaction is carried out without addition of carbon monoxide from an extraneous source subsequent to introduction of the first FT feed stream.

[0060] Optionally, at least 60%, preferably at least 70%, more preferably at least 80%, more preferably at least 90%, most preferably all of the second FT feed stream is derived solely from the first FT product stream (i.e. without the addition of further components) and / or components recycled from the second FT stage or subsequent stages.

[0061] Preferably, the first FT product stream is separated to provide a C5+ hydrocarbon rich stream and a C5+ hydrocarbon poor stream, wherein at least a portion (e.g. at least 20%, at least 30%, at least 40%, at least 50, at least 75%, at least 80%, at least 90%, optionally all) of the C5+ hydrocarbon poor stream is provided to (i.e. used to form) the second FT feed stream.

[0062] Preferably, the first FT product stream is separated to provide a water rich stream and a water poor stream, wherein at least a portion (e.g. at least 20%, at least 30%, at least 40%, at least 50, at least 75%, at least 80%, at least 90%, optionally all) of the water poor stream is provided to (i.e. used to form) the second FT feed stream.

[0063] Preferably, the first FT product stream is separated in a condensate separation zone to provide a product stream enriched in water and C5+ hydrocarbons, and a condensate-poor product stream lean in water and C5+ hydrocarbons, wherein at least a portion of (e.g., all of) the condensate-poor product stream is provided to the second FT feed stream.

[0064] Preferably, separation of C5+ hydrocarbons and / or water occurs upstream of any hydrogen separation of the first FT product stream. This approach can increase efficiency of the hydrogen separation process.

[0065] For the avoidance of doubt, the first FT reaction zone is separate to the second FT reaction zone. The second FT reaction zone is downstream of the first FT reaction zone. Suitably, the first FT reaction zone is provided in a first reactor and the second FT reaction zone is provided in a second reactor. Advantageously, this arrangement simplifies interventions between the first FT reaction zone and second FT reaction zone to adjust the ratio of hydrogen to carbon monoxide.

[0066] The first FT catalyst may comprise iron, and the second FT catalyst may comprise one or more of iron, cobalt, nickel, rhodium or ruthenium. In preferred implementations, the first FT catalyst is an iron-based catalyst, and the second FT catalyst is a cobalt-based catalyst (this preference applying to every reference to “first FT catalyst” and “second FT catalyst” throughout the present specification). Advantageously, iron-based catalysts are well-suited to use in the first FT reaction zone due to their activity for both rWGS and FT (required for the modified FT process), and cobalt-based catalysts are well-suited to use in the second FT reaction zone since they have strong FT activity and relatively low activity for rWGS / WGS.

[0067] Optionally, the first FT catalyst comprises iron, and is free or substantially free of cobalt.

[0068] The first FT catalyst and second FT catalyst may be the same type of catalyst - for example, both may be iron-based catalysts. In such instances, the conditions in the first FT reaction zone may be used to favour carbon dioxide and hydrogen as the feedstocks for FT, and the conditions in the second FT reaction zone are used to favour carbon monoxide and hydrogen as the feedstocks for FT.

[0069] Preferably, however, the first FT catalyst is different from the second FT catalyst. In particular, the first FT catalyst may be a FT catalyst for modified FT (starting from carbon dioxide and hydrogen), and the second FT catalyst may be a FT catalyst for conventional FT (starting from carbon monoxide and hydrogen). As noted above, it is particularly preferred that the first FT catalyst is an iron-based catalyst and the second FT catalyst is a cobalt-based catalyst.

[0070] Optionally, the first FT reaction zone includes only a single catalyst type. In other words, the first FT catalyst may be a single catalyst type. Differently stated, the first FT catalyst may not comprise a mixture of catalyst, for example may not be a co-mingled mixture of two or more catalysts.

[0071] Optionally, the second FT reaction zone includes only a single type of catalyst. In other words, the second FT catalyst may be a single catalyst type. Differently stated, the second FT catalyst may not comprise a mixture of catalyst, for example may not be a co-mingled mixtures of two or more catalysts.

[0072] For the avoidance of doubt, the first FT catalyst has both rWGS and FT activity. The FT activity means that C2+ hydrocarbons account for a substantial portion of the first FT product stream, and in particular that C5+ hydrocarbons account for a substantial portion of the first FT product stream. For example, the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone is conducted with a C5+ selectivity (i.e., for all C5+ species) of at least 40%, e.g., at least 50%, or at least 60%.

[0073] The invention includes the combination of the aspects, implementations, proposals and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0074] BRIEF DESCRIPTION OF THE FIGURES Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0075] FIG. 1 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, with the first FT reaction zone being provided in a first reactor, and the second FT reaction zone being provided in a downstream second reactor.

[0076] FIG. 2 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, similar to that depicted in FIG. 1 , showing catalyst activation streams and separation of products from the first FT reaction zone.

[0077] FIG. 3 is a schematic view of an alternative embodiment of a process for performing a Fischer-Tropsch process as described herein, where the first FT reaction zone and second FT reaction zone are carried out in the same reactor, with the second FT reaction zone being downstream from the first FT reaction zone, and additionally showing options for heat exchange between components of the system.

[0078] FIG. 4 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, similar to FIG.1 and 2, but showing further details of the heat exchange system.

[0079] FIG. 5 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, similar to FIG. 1 and 2, and showing further details of heat exchange and processing of the product streams.

[0080] FIG. 6 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, similar to FIG. 1 and 2, and showing the incorporation of a partial oxidation unit downstream of the second FT reactor.

[0081] FIG. 7 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, in which the first FT product stream is fed through a hydrogen separation membrane, thereby creating a second FT stream having a reduced molar ratio of hydrogen:carbon monoxide.

[0082] FIG. 8 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, showing combination of a light olefin stream into the second FT feed stream to adjust the hydrogen:carbon monoxide molar ratio within the second FT reactor.

[0083] FIG. 9 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, in which the first FT product stream is fed through a partial oxidation unit, thereby creating a second FT stream having a reduced molar ratio of hydrogen:carbon monoxide.

[0084] FIG. 10 is a schematic view of an embodiment of a process for performing a Fischer-Tropsch process as described herein, incorporating both a partial oxidation unit and hydrogen separation membrane.

[0085] FIGs. 11 and 12 are plots showing the effect of the ratio of hydrogen:carbon dioxide in the feed stream of an iron-catalysed modified FT process on the molar ratio of hydrogen:carbon monoxide in the product stream.

[0086] FIG. 13 is a plot showing the effect of reactor temperature on the molar ratio of hydrogen:carbon monoxide in the product stream of an iron-catalysed modified FT process. FIG. 14 is a plot showing the effect of reactor pressure on the molar ratio of hydrogen:carbon monoxide in the product stream of an iron-catalysed modified FT process.

[0087] FIG. 15 is a plot showing the effect of GHSV on the molar ratio of hydrogen:carbon monoxide in the product stream of an iron-catalysed modified FT process.

[0088] DETAILED DESCRIPTION OF THE INVENTION

[0089] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0090] The present disclosure is concerned with FT processes for converting CO2 to hydrocarbons.

[0091] The present inventors have noted that the overall conversion of CO2 to hydrocarbons in modified FT processes can be limited by the WGS equilibrium, i.e., in-situ formed CO and H2O can react back to a certain extent to CO2 and H2, limiting the CO2 conversion to a lower level. This is exacerbated by the fact that there is H2O produced by the Fischer-Tropsch reaction, shown below:

[0092] This water can push the water-gas shift equilibrium toward CO2, i.e., in the “forward” direction, thus limiting the amount of CO available for conversion to products. Accordingly, while the high water-gas shift activity of FT catalysts can be helpful to convert CO2 to CO for use in Fischer-Tropsch synthesis, the latter reaction makes water, which can cause CO to shift back to CO2. This can provide for an overall limit on the conversion of feedstock carbon to C5+ hydrocarbons, which are the generally-desired FT products.

[0093] Thus, the present inventors note that one of the challenges associated with using CO2 in the feed stream of modified-FT processes is to ameliorate the problem of lower-than-desired overall conversion of CO2 into C5+ products. Here, the present inventors have determined that the reverse water-gas shift / water-gas shift activity of FT catalysts can be used to provide better C5+ conversion in FT processes. To do so, the present inventors have determined that the combination of a first FT stage with a closely-coupled downstream second FT stage can provide increased overall CO2 conversion. This scheme allows the first FT stage to be operated in manner that provides a good compromise between CO2 conversion and C5+ selectivity without regarding co-produced CO as an undesired by-product. This is because the closely-coupled second FT stage can provide high conversion of CO from the first FT stage.

[0094] Moreover, the inventors have recognised that the molar amount of carbon monoxide exiting the first FT stage can be significantly lower than the molar amount of hydrogen, meaning that the second FT stage operates far away from the optimal ratio of ~2:1 hydrogen :carbon monoxide for conventional Fischer- Tropsch. Thus, the present disclosure is especially concerned with implementations of the multi-stage process which maximise productivity and selectivity for C5+ hydrocarbons of the process as a whole, in particular through adjusting conditions to bring the ratio of hydrogen:carbon monoxide in the second FT stage closer to the optimal value of 2:1 . As noted in the summary of the invention section above, and described in more detail below, the present invention encompasses various proposals for achieving this, based on:

[0095] (a) adjusting the conditions of the first FT reaction zone to control the ratio of hydrogen to carbon monoxide output in the first FT product stream; and / or

[0096] (b) adjusting the ratio of hydrogen to carbon monoxide in the second FT reaction zone, e.g. through adjusting the second FT feed stream; and / or

[0097] (c) adjusting the conditions of the second FT reaction zone to cope with a relatively high proportion of hydrogen relative to carbon monoxide.

[0098] An example of a multi-stage process according to the invention is shown schematically in FIG. 1. In FIG. 1 , the process 100 includes providing a first FT feed stream 111 comprising carbon dioxide and hydrogen, here, to a first FT reaction zone, e.g., a reactor 110. A first FT catalyst 113 is contacted with the first FT feed stream 111 under conditions sufficient to form a first FT product stream 112 comprising Cs+ hydrocarbons and carbon monoxide. The process of this aspect of the disclosure also provides a second FT feed stream comprising carbon monoxide (at least a portion of which is from the first FT product stream) and contacts the second FT feed stream with a second FT catalyst to form a second FT product stream. In the process 100 of FIG. 1 , at least a portion of carbon monoxide of the first FT product stream 112 is included in second FT feed stream 121 , which is contacted with the second FT catalyst 123, here, in a second FT reaction zone (e.g., a reactor 120). This provides a second FT product stream 122, which includes C5+ hydrocarbons.

[0099] As used herein, a “feed stream” is used to mean the total material input to a process step, regardless of whether provided in a single physical stream or multiple physical streams, and whether through a single inlet or multiple inlets.

[0100] Similarly, a “product stream” is used to mean the total material output from a process step, regardless of whether provided in a single physical stream or multiple physical streams, and whether through a single reactor outlet or multiple reactor outlets. For example, hydrogen and carbon dioxide of the first FT feed stream can be provided to the first FT catalyst in a single physical stream (e.g., in a single pipe to reactor 110), or in multiple physical streams (e.g., separate inlets for carbon dioxide and H2, or one inlet for fresh carbon dioxide and H2 and another for recycled carbon dioxide and / or H2).

[0101] As used herein, the term “hydrocarbons” is used as a term to describe FT synthesis products. It includes not only alkanes and olefins, but also the oxygenated hydrocarbons (e.g., alcohols) that are often present to some extent in FT product streams.

[0102] First FT feed stream

[0103] As described above, the first FT feed stream contains both H2 and CO2 (e.g., provided to a reaction zone in a single physical stream or multiple physical streams).

[0104] The molar ratio of H2 to CO2 in the first FT feed stream may be at least 0.1 :1 , e.g., at least 0.5:1 . For example, the molar ratio of H2 to CO2 in the first FT feed stream may be at least 0.9:1 , e.g., at least 1 :1 or at least 1 .5:1 . Preferably, the molar ratio of H2 to CO2 in the first FT feed stream is no more than 20:1 , e.g., no more than 15:1 or no more than 10:1 . For example, the molar ratio of H2 to CO2 in the first FT feed stream may be in the range of 1 :1 to 6:1 , e.g., in the range of 1 .5:1 to 3:1 . Preferably, the molar ratio of hydrogen to carbon dioxide is less than 4:1 , preferably less than 3:1 , more preferably less than 2:1 , e.g. between 0.5:1 to 4:1 , 0.5:1 to 3:1 , or 0.5:1 to 2:1 . The person of ordinary skill in the art will provide a desired ratio of H2:CO2 in the first FT feed stream, based on the disclosure herein, that provides a desirable conversion and selectivity; excess H2 can, if consistent with a desirable conversion and selectivity, be provided to flow through the system and provide a first FT product stream with a desirable ratio of H2 to CO for the second FT process.

[0105] Other gases may also be included in the first FT feed stream. For example, in some embodiments, the first FT feed stream further comprises CO. According to certain processes, the process used to generate hydrogen and carbon dioxide may result in the product of other gases which are included in the first FT feed stream - for example, syngas derived from biomass gasification directly produces a mixture of H2, CO and CO2 which can be used to form the first FT feed stream. Furthermore, in instances where the first FT feed stream includes a recyclate stream, for example a recyclate from the second FT stage, this can lead to the introduction of additional gas components, including residual CO from the second FT stage. The person of ordinary skill in the art will provide a desired ratio of hydrogen to carbon monoxide and carbon dioxide (i.e. , oxides of carbon), based on the disclosure herein, that provides a desirable conversion and selectivity. The present inventors hypothesize that including CO in the first FT feed stream can provide a compromise between the rWGS reaction and the FT reaction to provide the desired hydrocarbons.

[0106] The amounts of various feed components can be selected to provide a desirable balance of CO2 conversion and C5+ selectivity. The present inventors note that while hydrogen is necessary for the reverse water-gas shift reaction that forms CO for Fischer-Tropsch synthesis, higher ratios of hydrogen to oxides of carbon can provide for relatively shorter chain hydrocarbons, thus decreasing selectivity for the highly desirable C5+ hydrocarbons. Accordingly, the first FT feed stream may have a ratio of hydrogen to oxides of carbon in the range of 1 :1 to 6:1 , e.g., 1 .5:1 to 3:1 . For example, the first FT feed stream may have a ratio of hydrogen to oxides of carbon in the range of 1 :1 to 3:1 . Moreover, proportions in the feed stream of H2, CO and CO2 may be selected such that the molar ratio H2 / (2CO + 3CO2) is in excess of 0.5; the present inventors have found that this can help to ensure a positive conversion of CO2. The molar ratio CO2 / (CO+CO2) is desirably at least 0.33, e.g., at least 0.4, or at least 0.45, or at least 0.5; this, too, can help to ensure positive conversion of CO.

[0107] The first FT feed stream may further comprise one or more inert gases. For example, the first FT feed stream may further comprise nitrogen and / or methane. For example, it can be desirable to perform the first FT process step in the presence of a significant amount of inerts (i.e., components that are not H2 or CO2). For example, the first FT feed stream may include up to 80 mol% of one or more inerts, e.g., in the range of 3-80 mol%, or 5-80 mol%, or 10-80 mol%, or 15-80 mol%, or 30-80 mol% of one or more inerts. The first FT feed stream may include up to 70 mol% inerts, up to 60 mol% inerts, or up to 50 mol% inerts, e.g., 3-70 mol%, or 5-70 mol%, or 10-70 mol%, or 15-70 mol%, or 30-70 mol%, or 3-60 mol%, or 5-60 mol%, or 10-60 mol%, or 15-60 mol%, or 30-60 mol%, or 3-50 mol%, or 5-50 mol%, or 10-50 mol%, or 15-50 mol%, or 30-50 mol% inerts. Optionally, the first FT feed stream includes up to 80% of one or more inerts selected from methane and nitrogen, e.g., up to 70 mol%, up to 60 mol%, or up to 50 mol%, or 15-70 mol%, or 30-70 mol%, or 15-60 mol%, or 30-60 mol%, or 15-50 mol%, or 30-50 mol%.

[0108] It can be desirable to reduce the amount of water present in the first FT feed stream to control the WGS / rWGS activity of the first FT catalyst. Accordingly, the first FT feed stream may have a water content of no more than 10 mol%, e.g., or no more than 2 mol%, or no more than 0.5 mol%. Maintaining a low amount of water can have additional benefits, such as improving catalyst stability.

[0109] First FT product stream

[0110] The process described herein provides high CO2 conversion. As used herein, a “conversion” is a molar fraction of a relevant component feed that is converted to products (be it to desirable products or undesirable species).

[0111] Preferably, the first FT reaction zone has a CO2 conversion of at least 5%, e.g., at least 10%, or at least 20%. Optionally, the first FT reaction zone has a CO2 conversion of no more than 60%, e.g., no more than 55%, or no more than 50%. For example, the first FT reaction zone may have a CO2 conversion of no more than 45%, e.g., no more than 40%. The CO2 conversion may be in the range of 5-60%, e.g., 5- 55%, or 5-50%, or 5-45%, or 5-40%, or 10-60%, or 10-55%, or 10-50%, or 10-45%, or 10-40%, or 15- 60%, or 15-55%, or 15-50%, or 15-45%, or 15-40%, or 20-60%, or 20-55%, or 20-50%, or 20-45%, or 20- 40%. The person of ordinary skill in the art will, based on the disclosure herein, operate at a degree of conversion that provides a desirable product distribution. Notably, under conditions where there is significant selectivity for CO, e.g., under lower H2 / CO2 ratios, lower temperature ranges and higher GHSV values, CO2 conversion is generally modest. This is acceptable, as CO will be substantially reacted in the subsequent second FT stage, and CO2 can pass through the subsequent second FT stage and be recycled to be converted in a subsequent pass.

[0112] The process as described herein includes contacting a first FT catalyst with the first FT feed stream to perform an FT reaction. Notably, the present inventors have determined that the first FT catalyst, under the conditions described herein, can provide desirably high C5+ selectivity. As used herein, selectivity is the molar fraction of converted material that is converted to a particular product. In other words, unless stated otherwise “selectivity” corresponds to the molar amount of a particular component as a fraction of the total amount of product output by a particular reaction (i.e. as a proportion of the first FT product stream or second FT product stream). The skilled reader will be familiar with calculating these amounts, and how to subtract the contribution from any components which are not the product of a reaction taking place in the reaction zone (for example, if a component is added to a feed stream then the contribution from this is subtracted from the the amount detected in the product stream). The Fischer-Tropsch process is typically used to make C5+ hydrocarbons, for example, unsubstituted C5+ hydrocarbons (e.g., alkanes and alkenes) and oxygenated C5+ hydrocarbons (e.g., C5+ alcohols, aldehydes, ketones, carboxylic acids).

[0113] Preferably, the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone is conducted with a C5+ selectivity (i.e., for all C5+ species) of at least 40%, e.g., at least 50%, or at least 60%. For example, the selectivity for C5+ alkanes may be at least 40%, e.g., at least 50%, or at least 60%. Optionally, the contacting of the first FT catalyst with the first FT feed stream to provide the first FT product stream is performed with a C2-4 selectivity of no more than 30%, e.g., no more than 25%, or no more than 20%. Optionally, the contacting of the first FT catalyst with the first FT feed stream to provide the first FT product stream is performed with a methane selectivity of no more than 20%, e.g., no more than 15%, or no more than 10%, or no more than 5%. Optionally, the contacting of the first FT catalyst with the first FT feed stream to provide the first FT product stream is performed with a C2-8 oxygenate selectivity of no more than 30%, e.g., no more than 25%, or no more than 20%. Optionally, the contacting of the first FT catalyst with the first FT feed stream to provide the first FT product stream is performed with a C2-8 oxygenate selectivity of no more than 15%, e.g., no more than 10%, or no more than 5%.

[0114] In conventional modified FT processes, CO production due to rWGS activity of the catalyst is considered an unwanted by-product. Here, however, the present inventors have found that any CO produced can be provided to the second FT reaction zone for further processing. Thus, the first FT catalyst under the conditions described herein has selectivity for CO, i.e. , significant amounts of CO can be output in the first FT product stream. For example, the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone may be conducted with an overall CO selectivity of at least 10%. For example, the contacting in the first FT reaction zone is conducted with an overall CO selectivity of at least 15% or at least 20%. Optionally, the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone is conducted with an overall CO selectivity of no more than 80%, e.g., no more than 60%, or no more than 50%, or no more than 40%. For example, the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone may be conducted with an overall CO selectivity in the range of 10-80%, e.g., 15-80%, or 20-80%, or 10-60%, or 15-60%, or 20-60%, or 10-50%, or 15-50%, or 20-50%, or 10-40%, or 15-40%, or 20-40%.

[0115] Temperature of the first FT reaction zone

[0116] Contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone may be conducted at a temperature in the range of 200-500°C. For example, the contacting may be conducted at a temperature in the range of 200-450 °C, e.g., 200-400 °C, or 200-350 °C, or 200-300 °C, or 225-500 °C, or 225-450 °C, or 225-400 °C, of 225-350 °C, or 225-300 °C, or 250-500 °C, or 250-450 °C, or 250-400 °C, or 250-350 °C, or 250-300 °C, or 260-500 °C, or 260-450 °C, or 260-400 °C, or 260-350 °C. In particular, the contacting may be conducted at a temperature in the range of 180-280 °C, e.g., 180-260 °C, or 180-240 °C, or 180-220 °C, or 200-280 °C, or 200-260 °C, or 200-240 °C, or 220-280 °C, or 220- 260 °C.

[0117] One of the main challenges of using CO2 in the feed stream of the FT process is to convert the CO2 to CO at economical and energy-efficient conditions. Conventional rWGS reactions used to activate CO2 often require high temperatures, leading to a more energy-intensive and expensive process. For example, temperatures in excess of 500°C are typical for rWGS reactors to ensure efficient conversion of CO2 to CO. In contrast, typical temperatures for FT conversion of CO and hydrogen are in the range of 200-350°C. The FT processes described herein can be performed at temperatures that are lower than temperatures used in many conventional reverse water-gas shift processes. In particular, the present inventors have found that operating the first FT stage at temperatures intermediate between those conventionally used for rWGS and FT can help to ensure suitable productivity and selectivity of Cs+ hydrocarbons whilst also ensuring that the first FT product stream has a suitable ratio of hydrogen:carbon monoxide to permit efficient production of Cs+ hydrocarbons in the second FT stage.

[0118] More specifically, the present inventors have found that the multi-stage process of the present invention operates particularly efficiently when the temperature of the first FT reaction zone is greater than 250°C, preferably greater than 300°C, more preferably greater than 320°C, more preferably still greater than 330°C, such as greater than 350°C. The upper limit for the temperature may be, for example 400°C or 500°C. Particularly good results are observed in the range of 250°C to 500°C, in particular 300°C to 500°C, especially 300°C to 400°C. The present inventors have discovered, in particular, that the ratio of hydrogen:carbon monoxide output in the first FT product stream is particularly sensitive to the temperature of the first FT reaction zone, and that the temperature preferred temperature ranges are particularly effective at minimising the ratio of hydrogen to carbon monoxide, whilst also allowing operation with lower energy demand, as well as for facile integration with a subsequent second Fischer- Tropsch process step. At lower temperatures, relatively longer chain hydrocarbon products can be formed. While there may be lower conversion of CO2 at lower temperatures, unreacted CO2 can be recycled, and the lower conversion can be acceptable in view of the desirability of longer-chain hydrocarbon products.

[0119] Pressures in the first FT reaction zone

[0120] Additionally, the first FT stage described herein can be performed at a variety of pressures, as would be appreciated by the person of ordinary skill in the art.

[0121] For example, the contacting the first FT feed stream with a first FT catalyst is conducted at a pressure of at least 1 barg, e.g., at least 5 barg, or at least 10 barg. As is understood by the person skilled in the art, “barg” corresponds to the pressure of the system in bar relative to the atmospheric / ambient pressure outside of the system.

[0122] The contacting may be conducted at an first FT pressure in the range of, for example, 1 to 100 barg. For example, the contacting is conducted at an first FT pressure in the range of 1 to 80 barg, or 1 to 70 barg, or 1 to 60 barg, 5 to 100 barg, 5 to 80 barg, 5 to 70 barg, 5 to 60 bag, or 10 to 100 barg, 10 to 80 barg, or 10 to 70 barg, 10 to 60 barg.

[0123] The present inventors have identified that the use of a multi-stage process in the present invention means that the preferred pressure conditions for the first FT reaction zone differ somewhat compared to the conditions which would be used for the first FT reaction zone if it were used in a single stage process. In particular, the present inventors have discovered that using comparatively lower pressures leads to a ratio of hydrogen:carbon monoxide in the first FT product stream which is closer to the optimal ratio for FT in the second FT reaction zone, whilst ensuring suitable production of C5+ hydrocarbons in the first FT product stream . For this reason, the pressure is preferably 5 to 60 barg, more preferably 5 to 50 barg, more preferably still 10 to 40 barg, even more preferably still 10 to 30 barg.

[0124] GHSV in the first FT reaction zone

[0125] The first FT stage described herein can be performed at a variety of GHSV (gas hourly space velocity), as would be appreciated by the person of ordinary skill in the art.

[0126] As such, the GHSV for performing the first FT stage in the first FT reaction zone is not particularly limited. For example, the contacting of the first FT feed stream with the first FT catalyst may be conducted at a first catalyst FT GHSV up to 100,000 tr1, e.g., up to 75,000 h1, or up to 50,000 tr1, with suitable ranges being 1 ,000 to 100,000 h1, or 1 ,000 to 75,000 h1, or 1 ,000 to 50,000 h1, or 2,000 to 100,000 h1, or 2,000 to 75,000 h1, or 2,000 to 50,000 h1, or 5,000 to 100,000 h1, or 5,000 to 75,000 h1, or 5,000 to 50,000 h1. The contacting may be conducted at a first catalyst FT GHSV up to 4,000 tr1, e.g., up to 4,000 h1, or up to 20,000 h1, with suitable ranges being for example 1 ,000 to 40,000 tr1, or 1 ,000 to 30,000 h1, or 1 ,000 to 20,000 tr1, or 2,000 to 40,000 tr1, or 2,000 to 40,000 tr1, or 2,000 to 30,000 tr1, or 5,000 to 40,000 h1, or 5,000 to 30,000 tr1, or 5,000 to 30,000 tr1, or 10,000 to 40,000 tr1, or 10,000 to 30,000 h1, or 10,000 to 20,000 tr1.

[0127] The present inventors note that the CO selectivity and Cs+ selectivity of the first FT stage can depend in part on the GHSV at which the process is performed, with higher CO selectivities and lower Cs+ selectivities typically resulting from higher GHSV values. For this reason, the present invention generally prefers GHSV values for the first FT stage of greater than 2,000 tr1, such as greater than 5,000 tr1.

[0128] Combined conditions in the first FT reaction zone

[0129] As noted above, the present inventors have identified that the use of a multi-stage process in the present invention means that the preferred conditions for the first FT reaction zone differ somewhat compared to the conditions which would be used for the first FT reaction zone if it were used in a single stage process. They have identified that particularly good performance is achieved when one or more of the following conditions are satisfied: i. a temperature of greater than 250°C; ii. a pressure of 10 to 40 barg;

[0130] Hi. a GHSV of greater than 2,000 h1; and iv. a molar ratio of hydrogen to carbon dioxide of less than 4:1 , preferably less than 3:1 , more preferably less than 2:1 .

[0131] Preferably, both (i) and (ii) are satisfied, preferably all of (i), (ii) and (iii); (i), (ii) or (iv); and most preferably all of (i)-(iv) are satisfied.

[0132] Preferably, both (i) and (iv) are satisfied.

[0133] Preferably, both (ii) and (iv) are satisfied.

[0134] The skilled reader recognizes that the general options and preferred values for the temperature, pressure, GHSV and molar ratio of hydrogen to carbon dioxide apply in respect of the combined values above. First FT

[0135] Preferably, the first FT catalyst includes at least 10 wt% iron (e.g., at least 15 wt%, at least 20 wt%, or at least 25 wt%), on an elemental basis. Preferably, the first FT catalyst includes at least 30 wt% iron, e.g., at least 35 wt%, or at least 40 wt% iron, on an elemental basis. Advantageously, iron-based catalysts have both rWGS and FT activity.

[0136] Optionally, the first FT catalyst is an alkali-promoted first FT catalyst, preferably an alkali-promoted ironbased FT catalyst. For example, in some embodiments as described herein, the first FT catalyst includes at least 1 wt% (e.g., at least 2 wt%, or at least 3 wt%) alkali metal, on an elemental basis. In some embodiments as described herein, the alkali metal is one or more of sodium, potassium, rubidium, and cesium.

[0137] Optionally, the first FT catalyst is an iron-based catalyst further comprising copper, most preferably an alkali-promoted iron-based catalyst comprising copper.

[0138] The first FT catalysts suitable for use in the process as described herein can be a variety of forms and are not particularly limited. For example, the first FT catalyst may be a supported or unsupported catalyst.

[0139] Preferably, the first FT catalyst is a supported catalyst, wherein the support comprises at least one of titanium oxide, zirconium oxide, cerium oxide, aluminum oxide, silicon oxide and zinc oxide. For example, in various embodiments, the support comprises at least one or aluminum oxide and silicon oxide. Again, preferably the supported catalyst is an iron-based catalyst further comprising copper, most preferably an alkali-promoted iron-based catalyst comprising copper, with the support most preferably being aluminum oxide and / or silicon oxide

[0140] The person of ordinary skill in the art will appreciate that the first FT catalysts of the disclosure can be provided in many forms, depending especially on the particular form of the reactor system in which they are to be used, e.g., in a fixed bed or as a fluidized bed. The supports of the first FT catalysts can be provided themselves as discrete bodies of material, e.g., as porous particles, pellets or shaped extrudates, with the metals provided thereon to provide the first FT catalyst. However, the first FT catalyst can itself be formed as a layer on an underlying substrate. The underlying substrate is not particularly limited. It can be formed of, e.g., a metal or metal oxide, and can itself be provided in a number of forms, such as particles, pellets, shaped extrudates, or monoliths. The person of ordinary skill in the art will select an appropriate first FT catalyst for the particular reactor system.

[0141] Conventionally, catalyst materials such as iron-based catalyst are prepared for use as active catalysts by treating them in situ with a reducing gas such as hydrogen, under conditions sufficient to convert a substantial amount of the metal oxides of the calcined catalyst material to metal (e.g. iron oxide to metallic iron). Then, when exposed to Fischer-Tropsch reaction conditions, a substantial part of this metal is converted to carbide. It is thus not conventionally necessary to provide a separate carbiding treatment; rather, the carbiding is a natural result of reaction conditions. Accordingly, in various embodiments, the first FT catalyst is activated by contact with H2 and oxides of carbon (e.g., CO and CO2). The carbiding can be performed in any convenient manner. For example, in various embodiments, the carbiding includes a reduction step, in which the catalyst material is treated with a reducing gas stream (e.g., containing hydrogen) for a time and at a temperature sufficient to provide at least 50 atom% of the catalyst in metallic form. Without intending to be bound by theory, the inventors understand this step to reduce oxidic metal species to metallic species, so that they can be more easily carbided in a subsequent treatment with a carbiding gas. Upon treatment of the iron-based catalyst with the reducing gas stream, a portion of the iron components present in the FT catalyst as described herein react to form metallic iron (Fe°).

[0142] Optionally, the treatment with the reducing gas stream is performed in the substantial absence of carbon monoxide. For example, the reducing gas stream may comprise no more than 1 vol% carbon monoxide, e.g., no more than 0.5 vol%, or no more than 0.1 vol%, or no more than 0.05 vol%, or no more than 0.01 vol% carbon monoxide. The reducing gas stream may further comprise an inert gas, such as nitrogen. The hydrogen and inert gas may be present in the reducing gas stream in a ratio of at least 1 :1 .

[0143] Suitably, treating the first FT catalyst material with the reducing gas stream is conducted at a temperature in the range of 250-650 °C. For example, treating the first FT catalyst material with the reducing gas stream may be conducted at a temperature in the range of 250-600 °C, or 250-550 °C, or 250-500 °C, or at a temperature in the range of 300-650 °C, e.g., 300-600 °C, or 300-550 °C, or 300-500 °C, or 350-650 °C, e.g., 350-600 °C, or 350-550 °C, or 350-500 °C.

[0144] As described above, treating the catalyst material with the reducing gas stream is conducted for a time sufficient to provide at least 50 atom% of the metal of the catalyst material in metallic form (e.g. 50 atom% iron in metallic form). Treating the catalyst material with the reducing gas stream may be conducted for at least 12 hours, e.g., at least 14 hours. For example, treating the catalyst material with the reducing gas stream may be conducted for a time in the range of 12 to 30 hours, e.g., in the range of 12 to 24 hours, or 14 to 30 hours, or 14 to 24 hours.

[0145] The person of ordinary skill in the art will be able to determine appropriate reducing conditions to provide a catalyst material with at least 50 atom% iron in reduced form. The treatment with the reducing gas stream may be performed to provide a catalyst material in which at least 60 atom% of the metal (e.g. iron) is in reduced form, e.g., at least 70 atom, at least 80 atom%, or at least 85 atom%. The proportion of metal in reduced form is measured by XRD.

[0146] The carbiding can include treating the catalyst material with a carbiding gas stream comprising carbon monoxide, at a temperature of at least 180 °C for a time sufficient to provide at least 50 atom% of the metal of the catalyst material in carbided form. This can be performed, e.g., after a treatment with a reducing gas as described above.

[0147] Optionally, the reducing gas / carbiding gas comprises at least a portion of H2 and CO (if present) from the first FT feed stream. For example, the process may further comprise separating at least a portion of H2 and at least a portion of CO of the first FT feed stream and contacting it with the first FT catalyst to activate the first FT catalyst. In the process 200 shown schematically in FIG. 2, at least a portion of H2 and CO stream 225A is separated from the first FT feed stream 211 and contacted with the first FT catalyst 213 to activate it. However, separate carbiding processes are not necessary, as the first FT catalyst material can be carbided under the reaction conditions of the first FT reaction zone, especially when treated first with a reducing gas as described above.

[0148] It can be desirable to have a substantial fraction of the metal of the carbided first FT catalyst material in carbide form, as it is carbide forms that are of highest catalytic activity. For example, in the carbided first FT catalyst materials, at least 50 atom% of the metal (e.g. iron) is in a carbide form, e.g., at least 55 atom%, or at least 60 atom. Optionally, 50-95 atom% of the metal (e.g. iron) is in a carbide form, e.g., in the range of 50-90%, or 50-85%, or 50-80%. The carbided first FT catalyst materials have 55-95 atom% of the metal in a carbide form, e.g., in the range of 55-90%, or 55-85%, or 55-80%; for example, in the range of 60-95 atom% of the metal (e.g. iron) is in a carbide form, e.g., in the range of 60-90%, or 60- 85%, or 60-80%. The amount of metal that is in the form of carbide is determined by Mbssbauer spectroscopy, and as such is expressed as an atomic fraction of metal in the form of carbide of the total metal species visible to Mbssbauer spectroscopy.

[0149] The present inventors note that, while oxidic iron is not a highly active catalyst for Fischer-Tropsch synthesis, it can catalyze water-gas shift reactions. In cases where the feed to the FT synthesis includes a high proportion of CO2, the present inventors have determined that water-gas shift activity can be highly desirable to convert that CO2 to CO for use in the Fischer-Tropsch synthesis. Accordingly, the present inventors have determined that some oxidic iron in the carbided Fischer-Tropsch catalyst material can be beneficial. Accordingly, the iron-based carbided Fischer-Tropsch catalyst materials of the disclosure may have at least 5 atom% of the iron in an oxide phase, e.g., at least 10 atom%, or at least 15 atom%, or at least 20 atom%. However, the present inventors also note that oxidic iron forms are generally not active catalysts for Fischer-Tropsch synthesis. Accordingly, it can be desirable to limit the amount of oxidic iron in the carbided Fischer-Tropsch catalyst material. For example, in the carbided Fischer- T ropsch catalyst materials of the disclosure, in the range of 5-50 atom% of the iron may be in an oxide phase, e.g., 5-45 atom%, or 5-40 atom%; e.g. in the range of 10-50 atom% of the iron may be in an oxide phase, e.g., 10-45 atom%, or 10-40 atom%; e.g. in the range of 15-50 atom% of the iron is in an oxide phase, e.g., 15-45 atom%, or 15-40 atom%, e.g. in the range of 20-50 atom% of the iron is in an oxide phase, e.g., 20-45 atom%, or 20-40 atom%. The amount of iron that is in the form of oxide is determined by Mbssbauer spectroscopy, and as such is expressed as an atomic fraction of iron in the form of oxide of the total iron species visible to Mbssbauer spectroscopy. The person of ordinary skill in the art can, based on the disclosure herein, select carbiding conditions to provide a desired degree of oxidic iron in the carbided Fischer-Tropsch catalyst materials of the disclosure.

[0150] Optionally, at least 30 atom% of the oxidic iron of the carbided Fischer-Tropsch catalyst material is in the form of Fe3O4. The present inventors note that this partially-reduced oxide has especially good activity as a reverse water-gas shift catalyst. Preferably, at least 40 atom% of the oxidic iron of the carbided first FT catalyst material is in the form of FesO4, e.g., at least 50 atom%. Optionally, at least 60 atom% of the oxidic iron of the carbided first FT catalyst material is in the form of FesO4, e.g., at least 70 atom%. The person of ordinary skill in the art can select carbiding conditions, particularly with respect to conditions under which the material is reduced, to provide a desired amount of FeaCU. The amount of oxidic iron present as of Fea Ti is determined using Mossbauer spectroscopy.

[0151] Second FT feed stream

[0152] One of the advantages of using a first FT catalyst with rWGS activity is that CO produced can not only undergo Fischer-Tropsch synthesis in the first FT stage, it can also be passed to the downstream second FT reaction zone.

[0153] As described above, the second FT feed stream comprises at least a portion of the carbon monoxide of the first FT product stream. For example, the second FT feed stream may comprise at least 50%, at least 60%, at least 70%, or at least 80%, of the carbon monoxide of the first FT product stream. Preferably, the second FT feed stream comprises substantially all of the carbon monoxide of the first FT product stream.

[0154] However, CO can be provided to the second FT feed stream from other sources. For example, CO may be provided to the second feed stream from a CO source other than the first FT product stream. In FIG. 4, a stream of CO 426a from some other source is included in the second FT feed stream 421 . The person of ordinary skill in the art will appreciate that CO can be provided from a variety of sources, e.g., gasification, reforming, or electrochemical CO2 reduction. Preferably, any additional CO fed to the system is a biogenic source of CO. For example, the CO may be derived from biomass pyrolysis, which produces a hydrogen-deficient and CO-rich gas mixture which can be used to form the second FT feed stream, without needing to separate hydrogen from the mixture in advance. Moreover, as described in more detail below, CO can be recycled to the second FT feed stream from the second FT product stream. Optionally, CO fed to the second FT feed stream is derived only from the multi-stage reaction (e.g. from the first FT product stream or as a recyclate from the second FT product stream), instead of from an external source.

[0155] The first FT product stream comprises C5+ hydrocarbons and carbon monoxide. The first FT product stream may further comprise water. At least a portion of the first FT product stream (e.g., C5+ hydrocarbons, CO, and water) is provided to the second FT feed stream. For example, at least 50 mol%, e.g., at least 60 mol%, at least 70 mol%, or at least 80 mol%, or the first FT product stream may be provided to the second FT feed stream. Optionally, substantially all of the first FT product stream is provided to the second FT feed stream.

[0156] As described above, the first FT product stream further comprises water. Optionally, at least a portion of the water present in the first FT product stream is provided to the second FT feed stream. For example, at least 50%, at least 60%, at least 70%, or at least 80% or the water present in the first FT product stream may be provided to the second FT feed stream. Optionally, substantially all of the water present in the first FT product stream is provided to the second FT feed stream.

[0157] Alternatively, the process comprises separating the first FT product stream to provide a water-rich first FT product stream and a water-poor first FT product stream, wherein at least a portion (e.g., all of) the water-poor first FT product stream is provided to the second FT feed stream. An example of such a process is shown schematically in FIG. 2. In FIG. 2, the process 200 includes process for preparing hydrocarbons by providing a first FT feed stream 211 comprising carbon dioxide and hydrogen, here, to a first FT reaction zone, e.g., a reactor 210. A first FT catalyst 213, as described herein, is contacted with the first FT feed stream 211 under conditions sufficient to form a first FT product stream 212 comprising Cs+ hydrocarbons and carbon monoxide. The first FT product stream 212 is then separated in a water separation zone 216 to provide a water rich first FT product stream 217A and a water-poor first FT product stream 217B, wherein at least a portion of (e.g., all of) the water-poor first FT product stream 217B is provided to the second FT feed stream 221 . The person of ordinary skill in the art will appreciate that a variety of processes can be used to remove water from the first FT product stream. For example, the first FT product stream can be contacted with a water scavenger to remove water therefrom. For example, a molecular sieve guard bed can be used to remove water from the first FT product stream; water can be recovered from the molecular sieves of the guard bed, e.g., by heating and vacuum. Alternatively, a knockout vessel can be used. However, use of a knockout vessel can in some cases cool the first FT product stream enough so that it is desirably reheated for introduction to the second FT process step. In FIG. 2, the process 200, first FT reactor 210, first FT feed stream 211 , first FT product stream 212, first FT catalyst 213, second FT reactor 220, second FT feed stream 221 , second FT product stream 222 and second FT catalyst 223 are generally as described herein.

[0158] Optionally, the portion of the first FT product stream that is included in the second FT feed stream has a water content of no more than 10 mol%, e.g., of no more than 2 mol%, of no more than 0.5 mol%.

[0159] As described above, the first FT product stream also includes C5+ hydrocarbons. As would be understood by the person of ordinary skill in the art, the C5+ hydrocarbons and any water present in the first FT product stream can be conveniently condensed. These substances can be separated from the first FT product stream via condensation. For example, the process may further comprise separating the first FT product stream to provide a condensate-rich first FT product stream enriched in water and C5+ hydrocarbons and a condensate-poor first FT product stream lean in water and C5+ hydrocarbons, wherein at least a portion of (e.g., all of) the condensate-poor first FT product stream is provided to the second FT feed stream. An example of such a process is shown schematically in FIG. 4. In FIG. 4, the process 400 includes process for preparing hydrocarbons by providing a first FT feed stream 411 comprising carbon dioxide and hydrogen, here, to a first FT reaction zone, e.g., a reactor 410. A first FT catalyst 413, as described herein, is contacted with the first FT feed stream 411 under conditions sufficient to form a first FT product stream 412 comprising C5+ hydrocarbons and carbon monoxide. The first FT product stream 412 is then separated in a condensate separation zone 416 to provide a first FT product stream enriched in water and C5+ hydrocarbons 417A and a condensate-poor first FT product stream lean in water and C5+ hydrocarbons 417B, wherein at least a portion of (e.g., all of) the condensate-poor first FT product stream 417B is provided to the second FT feed stream 421 .

[0160] The person of ordinary skill in the art would appreciate that, based on the processes as described herein, the first FT product stream may include H2, CO, and CO2 and other components in various amounts. Components of the first FT product stream may be separated and used for various purposes in the integrated process.

[0161] For example, the process may further comprise separating the first FT product stream to recycle at least a portion (e.g., at least 5 mol%, at least 10 mol%, at least 25 mol%, at least 50 mol%, at least 75 mol%, or at least 90 mol%) of one or more components of the first FT product stream to the first FT feed stream. For example, when the first FT product stream includes CO2, the process can include recycling at least a portion (e.g., at least 5 mol%, at least 10 mol%, at least 25 mol%, at least 50 mol%, at least 75 mol%, or at least 90 mol%) of the CO2 of the first FT product stream to the first FT feed stream. The first FT product stream may also include H2; optionally, the process further includes recycling at least a portion of H2 of the first FT product stream (e.g., at least 5 mol%, at least 10 mol%, at least 25 mol%, at least 50 mol%, at least 75 mol%, or at least 90 mol%) to the first FT feed stream.

[0162] Such recycling is shown in the process 200 of FIG. 2. Here, the process 200 includes separating from the first FT product stream 212 at least a portion of CO2 (stream 215) to recycle to the first FT feed stream 211 . Similarly, the process 200 includes separating from the first FT product stream 212 at least a portion of H2 (stream 214) to recycle to the first FT feed stream 211 . While stream 215 is depicted as entering reactor 210 through a different inlet than the rest of the first FT feed stream 211 , it is considered to be part of the first FT feed stream, as it is part of the material input to the process step.

[0163] Ratio of hydrogen to carbon monoxide in the second FT feed stream

[0164] The second feed stream includes both hydrogen and carbon monoxide, with at least a portion of the carbon monoxide derived from the first FT product stream.

[0165] Preferably, the hydrogen in the second FT feed stream is derived from the first FT product stream. For example, at least 25% of the H2 of the first FT product stream, e.g., at least 50% of the H2, at least 75% of the H2, or at least 90% of the H2 of the first FT product stream may be included in the second FT feed stream. Of course, some of the H2 of the first FT product stream can be used for other purposes, e.g., catalyst activation as described herein. Optionally, substantially all of the H2 of the second FT feed stream comes from the first FT product stream.

[0166] Optionally, more H2 than necessary is provided for the first FT reaction in the first FT feed stream, to provide excess H2 in the first FT product stream that can then provide a desired amount of H2 to the second FT feed stream for the second Fischer-Tropsch process step. In other embodiments, H2 can be provided to the second FT feed stream from other sources. For example, H2 may be provided to the second FT feed stream from a H2 source other than the first FT product stream. In FIG. 4, a stream of H2 426b from some other source is included in the second FT feed stream 421 . The person of ordinary skill in the art will appreciate that H2 can be provided from a variety of sources, e.g., gasification, reforming, or H2O electrolysis. Moreover, as described in more detail below, H2 can be recycled to the downstream second FT feed stream from the second FT product stream.

[0167] Although it is desirable to carry forward a substantial portion, or all, of the hydrogen from the first FT product stream, the amount of hydrogen in the first FT product stream arising from unreacted hydrogen of the first FT stream is generally present in a significantly higher molar amount than carbon monoxide. The present inventors have recognized that it is particularly advantageous to carry out the multi-stage reaction such that the ratio of hydrogen to carbon monoxide in the second FT reaction zone is reduced compared to the hydrogen to carbon monoxide molar ratio in the first FT product stream. The present inventors have recognized that it is especially advantageous for the second FT feed stream to have molar ratio of hydrogen to carbon monoxide in the range of 0.5:1 to 20:1 , such as 0.5:1 to 10:1 , preferably 0.5:1 to 8:1 , more preferably 0.5:1 to 6:1 , more preferably 1 :1 to 5:1 , e.g. in the range of 1 :1 to 2.5:1 . For example, the second FT feed stream may have a H2:CO ratio in the range of 0.5:1 to 6:1 , 1 :1 to 3:1 , 1 .4:1 to 3:1 , or 1 .4:1 to 2:1 .

[0168] One option for arriving at a suitable ratio of hydrogen to carbon monoxide is to co-feed a Fh-deficient syngas stream (e.g., H2:CO is in the range of 0.5:1 to 1.5:1) to the second FT feed stream such that the second FT feed ends up with a desirable H2:CO ratio (e.g., in the range of 1 .5:1 to 3:1 , or 1 .5:1 to 2.5:1). This process is shown in process 400 of FIG. 4, wherein CO feed stream 426A and H2 feed stream 426B can supply a HF-deficient syngas stream to second FT feed stream 421 to adjust the ratio of H2:CO.

[0169] However, as discussed above, to realise the maximum potential of using carbon dioxide and hydrogen as feedstocks, it is preferable to adjust the ratio of hydrogen to carbon monoxide without requiring the supplementation of the first FT product stream with an extraneous source of carbon monoxide. As discussed above, one method for achieving these ratios without the addition of extraneous CO is to control the conditions of the first FT reaction. Additionally or alternatively, it is possible to achieve these ratios through adjustment of or addition to the first FT product stream when forming the second FT feed stream.

[0170] Selectively reacting / removing hydrogen

[0171] The molar ratio of hydrogen to carbon monoxide in the second feed stream may be adjusted by selectively removing hydrogen from the first FT product stream so as to produce a modified product stream having a decreased molar ratio of hydrogen to carbon monoxide, and using this modified product stream to form the second FT feed stream.

[0172] Selective removal of hydrogen may be achieved by separation methods. In other words, the method may involve subjecting the first FT product stream to a separation method to selectively remove hydrogen.

[0173] This method can separate the first FT product stream into a hydrogen rich (or pure) stream and a hydrogen depleted stream, wherein the hydrogen depleted stream is used to form the second FT feed stream. Advantageously, the hydrogen rich (or pure) stream can be recycled to the first FT reaction zone.

[0174] Separation of hydrogen from the first FT product stream may be carried out by any suitable method.

[0175] Optionally, separation of hydrogen is achieved using a hydrogen selective membrane, e.g. a hydrogen permeable membrane. This approach may be referred to as hydrogen skimming. Suitably, the hydrogen selective membrane is a hollow-fibre membrane, such as a PRISM® Hydrogen Membrane available from Air Products, Trexlertown, US; a HISELECT® membrane available from Linde GmbH, Pullack, Germany or an ALaS membrane availability from Air Liquide, Paris, France.

[0176] Optionally, separation is achieved by adsorption, for example through Pressure Swing Adsorption (“PSA”), Temperature Swing Adsorption (“TSA”) or the like. In a PSA unit, the vapours of the first FT product stream are fed at elevated temperature and pressure to a fixed bed of adsorbent, e.g. containing selective molecular sieves. Due to its low molecular weight and lack of polarity, hydrogen adsorbs more weakly to the adsorbent than the other components of the first FT product stream, creating a hydrogen- enriched stream. This hydrogen-enriched stream can then exit the PSA unit, and is preferably recycled to the first FT feed stream. The pressure in the PSA unit is then reduced and the remaining components of the first FT product stream are desorbed to form a hydrogen-depleted stream, which is used to form the second FT feed stream.

[0177] In implementing such approaches, the method preferably involves providing a hydrogen separation zone between the first FT reaction zone and second FT reaction zone. The hydrogen separation zone may be a separate unit to carry out such separation. For example, a unit (e.g. cartridge) containing a hydrogen separation membrane may be positioned between the first FT reaction zone and second FT reaction zone. Alternatively, an adsorption unit, such as a PSA, unit may be positioned between the first FT reaction zone and second FT reaction zone.

[0178] Figure 7 depicts a multi-stage reactor system 700 of the invention in which hydrogen levels are adjusted in the first FT product stream using a hydrogen separation unit 740. In this system, a first feed stream 711 containing carbon dioxide and hydrogen is fed to a first FT reactor 710 containing a catalyst bed 713, formed from an iron-based FT catalyst. A first product stream 712 exits the first FT reactor 710, the product stream including the reaction products of rWGS and FT, together with unreacted components such as hydrogen and carbon dioxide. The ratio of hydrogen to carbon monoxide is above 2:1 - for the purposes of this illustration we will hypothesise a molar ratio of 10:1 . The first product stream 712 is fed to a condenser 716A and subsequently to a gas-liquid separation unit 716B, which results in a water- and Cs+ hydrocarbon-enriched stream 717A which is collected in vessel 716C, and a water- and Cs+-depleted stream 717B. This stream 7171 B is fed to hydrogen separation unit 740, which is a hydrogen permeable membrane (but could equally be a pressure swing adsorption unit). The hydrogen separating unit 740 outputs a hydrogen-enriched stream 741 and a hydrogen-depleted stream 721 . The hydrogen-enriched stream 741 is recycled to first feed stream 711 . The hydrogen-depleted stream 721 has a ratio of hydrogen:carbon monoxide lower than first product stream 712 - for example, in the hypothetical situation above where the ratio is 10:1 for first product stream 712, the hydrogen-depleted stream 721 may have a molar ratio of 3:1 . This hydrogen-depleted stream 721 is used as the feed stream for second FT reactor 720, comprising a catalyst bed 723, in this case formed by a cobalt-based FT catalyst. The product stream 722 from the reactor is then subjected to separation in an analogous way to the first product stream, using a condenser 726A, and a gas-liquid separation unit 726B which delivers water and C5+ hydrocarbons to a collection vessel 726C, and delivers a recyclate stream 732 back to the first feed stream 711 via a recycle compressor / circulator 734. A purge line 733 can be operated in instances where it is desirable to pause or stop delivery of the recyclate stream 732.

[0179] The molar ratio of hydrogen to carbon monoxide may also be adjusted by selectively reacting hydrogen from the first FT product stream. This may be carried out in addition to, or instead of, the separation techniques above.

[0180] For example, hydrogen from the first FT product stream can be consumed through a hydrogenation reaction with a suitable reactant, without affecting the level of carbon monoxide, to reduce the molar ratio of hydrogen to carbon monoxide in the second FT reaction zone. Examples of suitable reactants include, for example, compounds containing an unsaturated moiety, in particular unsaturated hydrocarbons. The reactant may be, for example, an olefin (in particular light olefins such as C2-C4 olefins) or an aromatic compound. Preferably the reactant is an olefin, especially a C2-C4 olefin, or a mixture of such olefins.

[0181] Selective reaction of hydrogen from the first FT product stream may occur prior to introduction of the second FT feed stream to the second FT reaction zone - e.g., in a separate hydrogen reaction zone / unit / reactor. Preferably, however, reaction of hydrogen occurs within the second FT reaction zone, since this avoids the need for a separate zone / unit / reactor.

[0182] Typically, reaction of hydrogen from the first FT product stream will be a catalysed reaction, e.g. a catalysed hydrogenation reaction. In instances where the reaction of hydrogen occurs within the second FT reaction zone, the second FT catalyst may also serve as the hydrogenation catalyst.

[0183] The reactant for reacting with hydrogen may be added to the first FT product stream or obtained from the first FT product stream. Preferably, the reactant for reacting with hydrogen is part of or obtained from the first FT product stream.

[0184] Preferably, the reactant is a C2-C4 olefin from the first FT product stream. With this in mind, it is preferred that unsaturated C2-C4 hydrocarbons constitute a relatively high proportion of the total C1-C4 hydrocarbons in the first FT product stream. For example, unsaturated hydrocarbons may account for at least 10 mol%, preferably at least 20 mol%, more preferably at least 30 mol%, e.g. at least 40 mol% of the total C1-C4 hydrocarbons in the first FT product stream, and said unsaturated hydrocarbons are included in the second FT feed stream.

[0185] Additionally or alternatively, C2-C4 olefins may be added to the second FT feed stream from an alternative source, such as from a steam cracker, a fluid catalytic cracker (FCC), or from an alcohol dehydration stream.

[0186] The C2-C4 olefin may comprise or consist of ethylene.

[0187] The molar ratio of hydrogen to C2-C4 olefins used in the hydrogenation reaction may be in the range of 2:1 to 30:1 , for example 2:1 to 20:1 , such as 5:1 to 30:1 or 5:1 to 20:1 . For example, in instances where hydrogenation occurs in the second FT reaction zone, the molar ratio of hydrogen to C2-C4 olefins in the second FT feed stream may be in the range of 2:1 to 30:1 , for example 2:1 to 20:1 .

[0188] Figure 8 depicts a multi-stage reactor system 800 of the invention in which C2-4 olefins are added to the output of the first FT reactor in order to deplete hydrogen levels in the second FT reactor. In this system, a first feed stream 811 containing carbon dioxide and hydrogen is fed to a first FT reactor 810 containing a catalyst bed 813, formed from an iron-based FT catalyst. A first product stream 812 exits the first FT reactor 810, the product stream including the reaction products of rWGS and FT, together with unreacted components such as hydrogen and carbon dioxide. The ratio of hydrogen to carbon monoxide is above 2:1 - for the purposes of this illustration we will hypothesise a molar ratio of 10:1 . The first product stream 812 is fed to a condenser 816A and subsequently to a gas-liquid separation unit 816B, which results in a water- and C5+ hydrocarbon-enriched stream 817A which is collected in vessel 816C, and a water- and C5+-depleted stream 817B. This stream 8171 B is combined with olefin stream 818 (in this case a stream of ethylene provided from a steam cracker, not shown) to form a second feed stream 821 . The second feed stream 821 is fed to second reactor 820, comprising a catalyst bed 823, in this case formed by a cobalt-based FT catalyst. The cobalt-based FT catalyst also serves as a catalyst for hydrogenation of the ethylene, so as to deplete the level of hydrogen within the reactor, and thereby reduce the ratio of hydrogen:carbon monoxide within the reactor to 3:1 , away from the initial ratio of 10:1 for first product stream 812. The product stream 822 from the reactor is then subjected to separation in an analogous way to the first product stream, using a condenser 826A, and a gas-liquid separation unit 826B which delivers water and Cs+ hydrocarbons to a collection vessel 826C, and delivers a recyclate stream 832 back to the first feed stream 811 via a recycle compressor / circulator 834. A purge line 833 can be operated in instances where it is desirable to pause or stop delivery of the recyclate stream 832.

[0189] Although the implementation shown in Figure 8 depicts the second reactor 820 with a single catalyst bed 823, the design or conditions of the system can be adjusted so as to promote hydrogenation in a first (upstream) zone and to promote FT in a second (downstream zone). For example, the second reactor 820 may have a change / gradient of conditions from upstream to downstream so as to favour hydrogenation in an upstream zone and FT in a downstream zone. This may be, for example, a temperature gradient, a switch or gradient of catalyst composition (e.g. varying level of mixing of different catalysts through the reactor bed), a pressure change or flow rate change (e.g. by varying reactor internal diameter). Alternatively, the single second reactor 820 can be replaced with a multi-stage reactor, e.g. comprising a hydrogenation reactor and a downstream FT reactor. Although this discussion of the second reactor is described in relation to the embodiment shown in Figure 8, the skilled reader will appreciate that these features are general options which can be used across all of the embodiments described herein (e.g. regardless of catalyst type).

[0190] The system may incorporate a feedback loop, to dynamically adjust the system in response to a measured parameter, in particular in response to the hydrogen:carbon monoxide molar ratio detected in the first FT product stream. For example, the feedback loop may cause alteration of the amount of hydrogen removed from the first FT product stream, e.g. by adjusting conditions in a hydrogen membrane unit or PSA unit. In addition, or alternatively, the feedback loop may be used to adjust the flow rate of reactant which is to be subjected to hydrogenation, e.g. adjust the amount of C2-C4 olefin added to the second FT feed stream.

[0191] Producing additional carbon monoxide from components of the first FT product stream and / or second FT product stream

[0192] The molar ratio of hydrogen to carbon monoxide in the second FT feed stream can be reduced relative to the ratio of hydrogen to carbon monoxide in the first FT product stream through supplementing carbon monoxide from the first FT product stream with additional carbon monoxide.

[0193] Whilst such addition can be carried out using an extraneous source of carbon monoxide, it is preferred that a portion (e.g., at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90%), preferably a substantial portion, optionally all of any carbon monoxide used to supplement the carbon monoxide content of the first FT product stream is derived from the multi-stage process itself. In particular, any carbon monoxide used to supplement the carbon monoxide content of the first FT product stream may be derived from additional components of the first FT product stream and / or from the second FT product stream.

[0194] To achieve this, it is preferred that the molar ratio of hydrogen to carbon monoxide in the second FT feed stream be adjusted by selectively reacting a component of the first FT product stream and / or second FT product stream to form additional carbon monoxide.

[0195] Preferably, the molar ratio of hydrogen to carbon monoxide in the second FT feed stream is adjusted by converting C1-C4 hydrocarbons of the first FT product stream to produce carbon monoxide, and including this carbon monoxide in the second FT feed stream.

[0196] Conversion of the C1-C4 hydrocarbons may be carried out, for example, by reforming. Reforming may be, for example, partial oxidation, steam reforming or dry CO2 reforming. The equations for these reactions are:

[0197] From these formulae it can be seen that the ratio of carbon monoxide to hydrogen produced by the conversion increases with the number of carbon atoms in the hydrocarbon, and that higher levels can be produced by partial oxidation or dry CO2 reforming.

[0198] Figure 9 depicts a multi-stage reactor system 900 of the invention in which C1-C4 alkanes and C1-C4 alkenes output from the first FT reactor are subjected to oxidation in order to generate further carbon monoxide, which is used to adjust the molar ratio of hydrogen:carbon monoxide. In this system, a first feed stream 911 containing carbon dioxide and hydrogen is fed to a first FT reactor 910 containing a catalyst bed 913, formed from an iron-based FT catalyst. A first product stream 912 exits the first FT reactor 910, the product stream including the reaction products of rWGS and FT, together with unreacted components such as hydrogen and carbon dioxide. The ratio of hydrogen to carbon monoxide is above 2:1 - for the purposes of this illustration we will hypothesise a molar ratio of 10:1 . The first product stream 912 is fed to a condenser 916A and subsequently to a gas-liquid separation unit 916B, which results in a water- and C5+ hydrocarbon-enriched stream 917A which is collected in vessel 916C, and a water- and C5+-depleted stream 917B. This stream 9171 B (which contains C1-C4 alkanes, C1-C4 alkenes, as well as oxygenates thereof, and contains CO, CO2 and hydrogen) is combined with oxygen stream 951 and fed to a partial oxidation unit 950. The partial oxidation unit 950 converts hydrocarbons from stream 917B into CO, CO2, hydrogen and water, and outputs these as pOx stream 952. Water is removed from pOx stream 952 using condenser 956A and gas-liquid separator 956B, to create a water-lean stream 921 . The water-lean stream 921 has a molar ratio of hydrogen:carbon monoxide which is lower than that for product stream 912 - in this case, a molar ratio of 3:1 . This water-lean stream 921 is used as the feed stream for second FT reactor 920, comprising a catalyst bed 923, in this case formed by a cobalt-based FT catalyst. The product stream 922 from the reactor is then subjected to separation in an analogous way to the first product stream, using a condenser 926A, and a gas-liquid separation unit 926B which delivers water and C5+ hydrocarbons to a collection vessel 926C, and delivers a recyclate stream 932 back to the first feed stream 911 via a recycle compressor / circulator 934. A purge line 933 can be operated in instances where it is desirable to pause or stop delivery of the recyclate stream 932. Combination of approaches

[0199] Although the implementations shown in Figures 7-9 depict only a single intervention to adjust the hydrogen:carbon monoxide molar ratio of the first FT product stream, the skilled reader understands that each of these approaches can be used in combination. This may be required, for example, where the molar ratio of the first product stream is significantly above the molar ratio of 2:1 , and it is beneficial (e.g. for reasons of cost or space) to stepdown the ratio in multiple stages, through multiple methodologies. Alternatively or additionally, such an approach may be beneficial in terms of increasing lifetime or reducing energy consumption of the components of the reactor. Furthermore, certain steps can be applied multiple times - e.g. multiple separation steps using a hydrogen separation membrane may be utilised.

[0200] An example of such a combination is shown in Figure 10. In this case, first FT feed stream 1011 is fed to a first reactor 1010 including catalyst bed 1013 - an iron-based catalyst in this instance. First FT product stream 1012 exiting the reactor then passes through a separator 1016 to remove a water- and Cs+- enriched product stream 1017A, with the remainder of the product stream being fed to oxidation unit 1050, together with oxygen stream 1051. The output of oxidation unit 1050 is then fed to separator 1056, to remove a water-enriched stream 1057A, with the remainder of the stream then passing to hydrogen separation unit 1040 - in this case a hydrogen separation membrane unit. A hydrogen-enriched stream 1041 is returned to first FT feed stream 1011 , with a hydrogen-depleted stream 1021 passing to second FT reactor 1020 having a catalyst bed 1023 of a cobalt-based catalyst. The product stream 1022 is then separated to remove water and Cs+ hydrocarbons, with the remainder of the product stream recycled to first FT feed stream as recyclate stream 1032.

[0201] Further components of the second FT feed stream

[0202] It can be desirable to perform the second FT process step in the presence of a significant level of inerts. One such inert, CO2, can come from the first FT process, e.g., via the first FT product stream. Accordingly, the second FT feed stream includes at least a portion of CO2 of the first FT product stream. For example, at least 10% of the CO2 of the first FT product stream, e.g., at least 25% of the CO2, at least 50% of the CO2, at least 75% of the CO2, or at least 90% of the CO2 of the first FT product stream is included in the second FT feed stream. Accordingly, in various embodiments as otherwise described herein, the portion of the first FT product stream that is included in the second FT feed stream has a CO2 content in the range of 10-95 mol% CO2, e.g., 10-90 mol%, or 10-85 mol%, or 10-80 mol%, or 10-75 mol%, or 10-70 mol%, or 20-95 mol%, or 20-90 mol%, or 20-85 mol%, or 20-80 mol%, or 20-75 mol%, or 20-70 mol%, or 30-95 mol%, or 30-90 mol%, or 30-85 mol%, or 30-80 mol%, or 30-75 mol%, or 30-70 mol% CO2. Of course, alternatively, the second FT feed stream may not include any substantial amount of CO2 of the first FT product stream. While it can be desirable generally to recycle CO2 to the first FT feed stream for use in the first FT reaction, as described in more detail below, unreacted CO2 can be recycled from the second FT product stream to the first FT feed stream.

[0203] But it can additionally or alternatively be desirable to include additional inert content to the second FT feed stream, be it CO2 or other inerts such as nitrogen and methane. For example, one or more inerts (e.g., CO2, nitrogen and / or methane) may be provided to the second FT feed stream from a source other than the first FT product stream. For example, the second FT feed stream may include up to 80 mol% of one or more inerts, e.g., in the range of 3-80 mol%, or 5-80 mol%, or 10-80 mol%, or 15-80 mol%, or 30- 80 mol% of one or more inerts. Optionally, the second FT feed stream includes up to 70 mol% inerts, up to 60 mol% inerts, or up to 50 mol% inerts, e.g., 3-70 mol%, or 5-70 mol%, or 10-70 mol%, or 15-70 mol%, or 30-70 mol%, or 3-60 mol%, or 5-60 mol%, or 10-60 mol%, or 15-60 mol%, or 30-60 mol%, or 3- 50 mol%, or 5-50 mol%, or 10-50 mol%, or 15-50 mol%, or 30-50 mol% inerts. Optionally, the second FT feed stream includes up to 80% of one or more inerts selected from CO2, methane and nitrogen, e.g., up to 70 mol%, up to 60 mol%, or up to 50 mol%, or 15-70 mol%, or 30-70 mol%, or 15-60 mol%, or 30-60 mol%, or 15-50 mol%, or 30-50 mol%. Optionally, the second FT feed stream includes up to 80 mol% of CO2, e.g., up to 70 mol%, up to 60 mol%, or up to 50 mol%, or 15-70 mol%, or 30-70 mol%, or 15-60 mol%, or 30-60 mol%, or 15-50 mol%, or 30-50 mol%. In FIG. 4, a stream of inert(s) 426c from some other source is included in the second FT feed stream 421 . The person of ordinary skill in the art will appreciate that inerts can be provided from a variety of sources. Moreover, as described in more detail below, inerts can be recycled to the second FT feed stream from the second FT product stream.

[0204] It can be desirable to reduce the amount of water that is conducted to the second FT process step. Accordingly, the portion of the first FT product stream that is included in the second FT feed stream may have a water content of no more than 10 mol%, e.g., or no more than 2 mol%, or no more than 0.5 mol%. Maintaining a low amount of water can have additional benefits, such as improving catalyst stability.

[0205] The second Fischer-Tropsch process is typically used to make C5+ hydrocarbons, for example, unsubstituted C5+ hydrocarbons (e.g., alkanes and alkenes) and oxygenated C5+ hydrocarbons (e.g., C5+ alcohols, aldehydes, ketones, carboxylic acids). Preferably, the contacting of the second FT feed stream with the second FT catalyst in the second FT reaction zone is conducted such that the overall process has a C5+ selectivity (i.e., from CO2 input) of at least 65%, e.g., at least 70%, or at least 75% (taking into account the products from both the first FT product stream and second FT product stream). Optionally, the contacting the second FT feed stream with the second FT catalyst in the second FT reaction zone is conducted with a C5+ selectivity (i.e., for all C5+ species) of at least 75%, e.g., at least 80%, or at least 85%. For example, the selectivity for C5+ alkanes may be at least 75%, e.g., at least 80%, or at least 85%. The selectivity for C5+ alkanes and C5+ alcohols may be at least 75%, e.g., at least 80%, or at least 85%. Optionally, the contacting of the second FT feed stream with the second FT catalyst in the second FT reaction zone may be conducted such that the overall process has a C9+ selectivity (i.e., from CO2 input) of at least 50%, e.g., at least 55%, or at least 60% (taking into account the products from both the first FT product stream and second FT product stream).

[0206] Conditions of the second FT reaction zone

[0207] As described above, the process includes contacting a second FT feed stream with a second FT catalyst.

[0208] The person of ordinary skill in the art will select appropriate reaction conditions (e.g., temperature and pressure) in conjunction with the particular feed and catalyst used to provide desired Fischer-Tropsch processes. In some embodiments of the disclosure as described herein, the second FT temperature may be in the range of 150-280 °C. For example, the second FT temperature is in the range of 150-260 °C, or 150-240 °C, or 150-230°C, or 150-220°C, or 175-280 °C, or 175-260 °C, or 175-240°C, or 175-230°C, or 195-280 °C, or 195-260 °C, or 195-250 °C, or 195-240 °C, or 195-230 °C. Optionally, the second FT temperature is in the range of 150-250 °C, e.g., 195-230 °C. The present inventors note that the second FT feed stream may be lean in CO (i.e., H2:CO is greater than 2:1 ), especially if the preceding first FT process has low CO selectivity. In such situations, it can be advantageous to lower the operating temperature of the second FT process to maximize CB+ selectivity of the process. For example, the second FT temperature may be in the range of 150-250 °C, e.g., 195-230 °C. The present inventors have identified that a temperature of 150-230°C for the second FT reaction is particularly good at achieving a suitable CB+ selectivity at the hydrogen to carbon monoxide ratios arising in the first FT product stream.

[0209] Notably, the first FT temperature and the second FT temperature can be relatively close to one another. The present inventors have noted that the first FT catalysts and processes described herein can provide suitable activity of CO2 even at relatively low temperatures. Accordingly, the first FT product stream can be provided with a temperature that is suitable for, or at least close to suitable for, the second FT reaction step. This can desirably provide for increased process integration. For example, in various embodiments, the first FT temperature is within 100 °C of the second FT temperature, e.g., within 50 °C of the second FT temperature, or within 25 °C of the second FT temperature.

[0210] The contacting the second FT feed stream with the downstream second FT catalyst may be conducted at a pressure of at least 1 barg, e.g., at least 5 barg, or at least 10 barg. Optionally, the contacting is conducted at a downstream second FT pressure in the range of 10-60 barg. For example, the second FT pressure may be in the range of 10-50 barg, 20-50 barg, or 25-50 barg, or 10-40 barg, or 20-40 barg, or 25-40 barg or 10-35 barg, or 20-35 barg, or 25-35 barg. Optionally, the second FT pressure may be in the range of 20-50 barg.

[0211] The second FT processes described herein can be performed at a variety of GHSV (gas hourly space velocity) values, as would be appreciated by the person of ordinary skill in the art. As such, the GHSV for contacting the downstream cobalt FT feed stream with the downstream second FT catalyst is not particularly limited. For example, the contacting may be conducted at a second FT GHSV in the range of 1 ,000 to 2,000,000 h1, such as in the range of 1 ,000 to 1 ,200,000 tr1, or 1 ,000 to 500,000 h1, or 1 ,000 to 100,000 h1, or 5,000 to 1 ,200,000 h1, or 5,000 to 500,000 h1, or 5,000 to 100,000 h1, or 10,000 to 1 ,200,000 h1, or 10,000 to 500,000 tr1, or 10,000 to 100,000 h1. Optionally, the contacting is conducted at a second FT GHSV in the range of 1 ,000 to 50,000 tr1, or 2,000 to 50,000 h1, or 5,000 to 50,000 h1, or 10,000 to 50,000, or 1 ,000 to 40,000 h1, or 2,000 to 40,000 h1, or 5,000 to 40,000 h1, or 10,000 to 40,000 h1, or 1 ,000 to 30,000 h1, or 2,000 to 30,000 h1, or 5,000 to 30,000 h1, or 10,000 to 30,000 h1. The person of ordinary skill in the art will appreciate that the full range of space velocities described above may not be available for a given FT process. Second FT catalyst

[0212] The processes as described herein include contacting a second FT catalyst with the second FT feed stream as described herein. The second FT catalyst for use in the processes as described herein is not particularly limited.

[0213] Optionally, the first FT catalyst and second FT catalyst may be the same type of catalyst - i.e. have the same composition, and optionally the same form. Preferably, however, the first FT catalyst is different from the second FT catalyst. In particular, the first FT catalyst may be a FT catalyst for modified FT (starting from carbon dioxide and hydrogen), and the second FT catalyst may be a FT catalyst for conventional FT (starting from carbon monoxide and hydrogen).

[0214] The second FT catalyst may comprise one or more of iron, cobalt, nickel, rhodium or ruthenium. Optionally, the second FT catalyst includes at least (or only) two metals, such as a combination of cobalt and iron.

[0215] Preferably, the second FT catalyst comprises cobalt. For example, the second FT catalyst may comprise cobalt in an amount in the range of 5-25 wt%, calculated as Co(0). “Calculated as Co(0)” and analogous terms mean that the weight of cobalt atoms / ions themselves are used in the calculation, and not the total amount of any compound or polynuclear ion in which those cobalt atoms / ions might be bound. For example, the second FT catalyst may comprise cobalt in an amount in the range of 7-25 wt%, or 10-25 wt%, or 5-20 wt%, or 7-20 wt%, or 10-20 wt%, calculated as Co(0). As the person of ordinary skill in the art will appreciate, cobalt-based catalysts are often provided to the reaction zone in the form of cobalt oxide on a support; the cobalt can be reductively activated (e.g., with H2) in situ to provide an active catalyst species with a significant concentration of Co(0).

[0216] The second FT catalyst may further include manganese, as an additional component alongside the other metals listed above. In particular, second FT catalyst may comprise cobalt and further include manganese in an amount up to 15 wt%, e.g., up to 12 wt%, or up to 10 wt%, or up to 7 wt%, calculated as Mn(0). Optionally, a catalyst material includes manganese in an amount in the range of 0.1-15 wt%, e.g., 0.1-10 wt%, or 0.1-5 wt%, 0.5-15 wt%, or 0.5-10 wt%, or 0.5-5 wt%, or calculated as Mn(0). Of course, optionally substantially no manganese is present (e.g., less than 0.1 wt% or less than 0.5 wt% manganese is present).

[0217] The second FT catalysts suitable for use in the process as described herein can be a variety of forms and are not particularly limited. For example, the second FT catalyst may be a supported or unsupported catalyst. While the form of the catalyst is not particularly limited, the second FT catalyst may be a supported catalyst, wherein the support comprises at least one of titanium oxide, zirconium oxide, cerium oxide, aluminum oxide, silicon oxide and zinc oxide. For example, the support may comprise at least one or titanium oxide, aluminum oxide, and silicon oxide. Optionally, the support is a titanium dioxide support. In such embodiments, the cobalt-based catalysts discussed above may be used.

[0218] The person of ordinary skill in the art will appreciate that the second FT catalysts of the disclosure can be provided in many forms, depending especially on the particular form of the reactor system in which they are to be used, e.g., in a fixed bed or as a fluidized bed. The supports of the second FT catalysts can be provided themselves as discrete bodies of material, e.g., as porous particles, pellets or shaped extrudates, with the metals provided thereon to provide the second FT catalyst. Alternatively, the second FT catalyst of the disclosure can itself be formed as a layer on an underlying substrate. The underlying substrate is not particularly limited. It can be formed of, e.g., a metal or metal oxide, and can itself be provided in a number of forms, such as particles, pellets, shaped extrudates, or monoliths. The person of ordinary skill in the art will select a second FT catalyst for the particular reactor system.

[0219] Second FT catalysts are typically activated before use, to provide the catalyst in metal(O) form e.g., to provide cobalt(O) species on a cobalt-based catalyst. Such activation can be performed prior to contacting the second FT catalyst with the second FT feed stream.

[0220] For example, the second FT catalyst may be activated by contact with a reducing gas. For example, hydrogen can be an especially suitable gas for activating the second FT catalyst, e.g., when the activation is a reduction to metal(O) species, e.g., as for many cobalt-based catalysts. The reducing gas may comprise at least a portion of hydrogen from the first FT product stream. For example, the process may further comprise separating at least a portion of hydrogen of the first FT product stream and contacting it with the second FT catalyst to activate the catalyst. In the process 200 shown schematically in FIG. 2, at least a portion of hydrogen stream 225B is separated from the first FT product stream 212 and contacted with the second FT catalyst 223 to activate it. In other embodiments, H2 present in the second FT feed stream can be used to activate the catalyst. This process is shown schematically in FIG. 4, where at least a portion of hydrogen stream 425 is separated from the second FT feed stream 421 and contacted with the second FT catalyst 423 to activate it. As would be understood by the person of ordinary skill in the art, activation temperatures can vary depending on the catalyst used. As such, the person of ordinary skill in the art would be able to select an appropriate temperature for activating the catalyst, e.g., in the range of 200-400 °C.

[0221] Second FT product stream

[0222] Additional components may be in present in the second FT product stream. For example, the second FT product stream may include water, which is another product of the Fischer-Tropsch reaction. Also present can be one or more light hydrocarbons (i.e., C1-C4) as a side product. CO and / or H2 can be present, e.g., unreacted from the second FT feed stream. CO2 or other inerts as described herein can also be present. Such components of the second FT product stream can be separated and / or recycled in various manners. Optionally, the process further includes in a downstream separation zone, separating the second FT product stream to provide a light product stream rich in hydrogen, carbon monoxide, carbon dioxide and C1-C4 hydrocarbons; and a heavy product stream rich in C5+ hydrocarbons.

[0223] As described above, the light product stream also comprises light hydrocarbons, i.e., C1-C4 hydrocarbons and the light product stream may be further separated to provide a light hydrocarbon product stream. Light hydrocarbons, while often not a desired portion of a Fischer-Tropsch product to be used as a fuel or a lubricant, can themselves be useful for a number of purposes. Accordingly, the process may further include separating at least a portion of C1-C4 hydrocarbons from the light product stream to provide a light hydrocarbon stream. For example, in the process 300 of FIG. 3, at least a portion of the C1-C4 hydrocarbon from the light product stream 336 are separated to provide a light hydrocarbon stream 338. The light hydrocarbon stream can, for example, be recycled to the first FT feed stream or the second FT feed stream. In the process 300 of FIG. 3, light hydrocarbons can be provided as part of the recycle stream 336, which becomes part of the first FT feed stream 311. In the process 400 of FIG. 4, light hydrocarbons are recycled via recycle stream 436 to second FT feed stream 421 .

[0224] The other components of the light product stream, e.g., hydrogen, carbon monoxide, and carbon dioxide may be used in other feeds of the process as described herein. As such, at least a portion of hydrogen, carbon monoxide and carbon dioxide of the light product stream may be included in the first FT feed stream and / or the second FT feed stream. For example, it can be desirable to recycle hydrogen from the light product stream, for example, to the first FT feed stream. For example, in the process of FIG. 3, at least a portion of H2 of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the first FT feed stream 311 via recycle stream 336. Optionally, the process includes recycling at least a portion of H2 of the light product stream to the second FT feed stream. For example, in the process of FIG. 4, at least a portion of H2 of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the second FT feed stream 421 via recycle stream 436. As an example, at least 25%, e.g., at least 50% of H2 of the light product stream is recycled to the first FT feed stream or the second FT feed stream. Optionally, at least 75%, e.g., at least 90% of H2 of the light product stream is recycled to the first FT feed stream or the second FT feed stream.

[0225] In some cases, e.g., when H2 is provided to the second FT feed stream from an H2 source other than the first FT product stream, H2 from the light product stream can make up most of the H2 of the first FT feed stream, e.g., at least 90%, at least 95%, or at least 98% of the H2 of the first FT feed stream. This is shown, e.g., in FIG. 5. Here, the primary H2 input to the process is through stream 540, which becomes part of the second FT feed stream 521 . H2 of the light product stream is included in recycle stream 536, which becomes part of first FT feed stream 511 .

[0226] Similarly, it can be desirable to recycle CO of the light product stream, for example, to the first FT feed stream and / or the second FT feed stream. For example, it can be desirable to recycle carbon monoxide from the light product stream, for example, to the first FT feed stream. For example, in the process of FIG. 3, at least a portion of carbon monoxide of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the first FT feed stream 311 via recycle stream 336. Optionally, the process includes recycling at least a portion of carbon monoxide of the light product stream to the second FT feed stream. For example, in the process of FIG. 4, at least a portion of carbon monoxide of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the second FT feed stream 421 via recycle stream 436. As an example, at least 25%, e.g., at least 50% of carbon monoxide of the light product stream is recycled to the first FT feed stream or the second FT feed stream.

[0227] Optionally, at least 75%, e.g., at least 90% of carbon monoxide of the light product stream is recycled to the first FT feed stream or the second FT feed stream.

[0228] As with hydrogen and carbon monoxide, it can be desirable to recycle carbon dioxide of the light product stream, for example, to the first FT feed stream and / or the second FT feed stream. Since CO2 is the primary carbon source for the first FT step, it can be especially desirable to recycle CO2 to the first FT feed stream. Accordingly, the process may include recycling at least a portion (e.g., at least 50%, at least 75%, or at least 90%) of CO2 of the second FT product stream to the first FT feed stream. For example, in the process of FIG. 3, at least a portion of CO2 of the second FT product stream (e.g., at least 50%, at least 75%, or at least 90%) can be recycled to the first FT feed stream 311 via recycle stream 336. For example, it can be desirable to recycle carbon dioxide from the light product stream, for example, to the first FT feed stream. For example, in the process of FIG. 3, at least a portion of carbon dioxide of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the first FT feed stream 311 via recycle stream 336. Optionally, the process includes recycling at least a portion of carbon dioxide of the light product stream to the second FT feed stream. For example, in the process of FIG. 4, at least a portion of carbon dioxide of the light product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the second FT feed stream 421 via recycle stream 436. Optionally, at least 25%, e.g., at least 50% of carbon dioxide of the light product stream is recycled to the first FT feed stream or the second FT feed stream. Optionally, at least 75%, e.g., at least 90% of carbon dioxide of the light product stream is recycled to the first FT feed stream or the second FT feed stream. In some cases, e.g., when CO2 is provided to the second FT feed stream from a CO2 source other than the first FT product stream, CO2 from the second FT product stream can make up most of the CO2 of the first FT feed stream, e.g., at least 90%, at least 95%, or at least 98% of the CO2 of the first FT feed stream. This is shown, e.g., in FIG. 6. Here, the primary CO2 input to the process is through stream 640, which becomes part of the second FT feed stream 621. CO2 of the second FT product stream is included in recycle stream 636, which becomes part of first FT feed stream 611 .

[0229] In many cases, hydrogen, carbon monoxide, and carbon dioxide, of the light product stream will be recycled.

[0230] Moreover, when one or more inerts are used in the process steps, it can be desirable to recycle these. For example, the process may include recycling at least a portion of inerts of the second FT product stream to the first FT feed stream and / or the second FT feed stream. Optionally, the process includes recycling at least a portion of inerts of the second FT product stream to the first FT feed stream. For example, in the process of FIG. 3, at least a portion of inerts of the second FT product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the first FT feed stream 311 via recycle stream 336. Optionally, the process includes recycling at least a portion of inerts of the second FT product stream to the second FT feed stream. For example, in the process of FIG. 4, at least a portion of inerts of the second FT product stream (e.g., at least 25%, at least 50%, or at least 75%) can be recycled to the second FT feed stream 421 via recycle stream 436. Optionally, at least 25%, e.g., at least 50% of inerts of the second FT product stream is recycled to the first FT feed stream or the second FT feed stream. For example, at least 75%, e.g., at least 90% of inerts of the second FT product stream may be recycled to the first FT feed stream or the second FT feed stream. Optionally, a purge stream can be incorporated with the recycle stream to prevent uncontrolled accumulation of inerts in the recycle stream (not shown here).

[0231] As would be understood by the person of skill in the art, the second FT product stream will also include water. Optionally, the process further comprises separating at least a portion of water from the second FT process stream. This is shown schematically in FIG. 3. In the embodiment of FIG. 3, the first FT catalyst 313 and the second FT catalyst 323 are provided in separate beds in the same reactor. Thus, the first FT reaction zone 310 is a volume of the reactor 305 that includes the bed 314 containing the first FT catalyst 313, and the second FT reaction zone 320 is a volume of the reactor 305 that includes the bed 324 containing the second FT catalyst 323. First FT feed stream 311 is contacted with the first FT catalyst 313 to provide first FT product stream 312, which is passed directly as the second FT feed stream 321 to the second FT catalyst 323 to provide second FT product stream 322. Here, the process also optionally includes separating at least a portion of water (e.g., at least 50%, at least 75%, or at least 90%) from the second FT product stream 322 to provide water-containing stream 334.

[0232] The carbon dioxide of the light product stream can also be used as a source of carbon monoxide for the process described herein. For example, the process may further include reacting at least a portion of the carbon dioxide of the light product stream in a CO generating zone to convert carbon dioxide to carbon monoxide, and including a least a portion of the carbon monoxide from the CO generating zone in the first FT feed stream and / or the second FT feed stream. The conversion method of carbon dioxide to carbon monoxide is not particularly limited. For example, a reverse water-gas shift or a conversion by a solid oxide electrochemical cell can be used. For example, in the process 500 of FIG. 5, at least a portion of the carbon dioxide of the light product stream 536 is provided to a CO generating zone 562 to convert carbon dioxide to carbon monoxide, and at least a portion of the carbon monoxide from the CO generating zone 554 is provided to the first FT feed stream 511 and / or the second FT feed stream 521 . In FIG. 5, the process 500, first FT reactor 510, first FT feed stream 511 , second FT product stream 512, first FT catalyst 513, second FT reactor 520, second FT feed stream 521 , second FT product stream 522 and second FT catalyst 523 are generally as described above.

[0233] As described above, the light product stream may also include light hydrocarbons that may be recycled to the first and / or second FT feed stream. There are other uses for the light hydrocarbon stream. For example, the process may further comprise at least partially oxidizing at least a portion of the C1-C4 hydrocarbons of the light product stream in a partial oxidation reaction zone to provide a partial oxidation (pOX) stream comprising carbon monoxide e.g., a CO- and / or CO2-containing partial oxidation (pOX) stream, and including at least a portion of the pOX stream in the second FT feed stream. An example of such a process is shown schematically in FIG. 6, in which the process 600, the first FT feed stream 611 , the first FT product stream 612, the first FT catalyst 613, the second FT feed stream 621 , the second FT product stream 622 and the second FT catalyst 623 can be as otherwise described herein. Here, the process includes oxidizing at least a portion of the light hydrocarbon stream 638 in a partial oxidation reaction zone 692 to provide a CO- and / or CO2 containing pOX stream, and including at least a portion of the pOX stream 694a stream in the second FT feed stream 621 .

[0234] Similarly, the process may further include oxidizing at least a portion of C1-C4 hydrocarbons of the light product stream in an oxidation reaction zone to provide an oxidation (OX) product stream comprising carbon dioxide, and including at least a portion of the carbon dioxide of the oxidation product stream to the first FT feed stream. An example of such a process is shown in FIG. 6, where the process 600 includes oxidizing at least a portion of the light hydrocarbon stream 638 in a partial oxidation reaction zone 692 to provide a CO2 containing OX stream, and including at least a portion of the OX stream 694b stream in the first FT feed stream 611 .

[0235] Moreover, the light hydrocarbon stream can be burned to provide heat energy, which can be used to heat various process streams, or to generate electricity. Accordingly, in various embodiments, the process includes burning at least a portion of the light hydrocarbon stream to provide energy, e.g., heat energy or electrical energy. For example, in the process 500 of FIG. 5, a portion of light hydrocarbon stream 538 is burned in a power generation zone (here, in an electrical generator 570), to generate electricity stream 572. Optionally, the heat energy may be used to provide the needed heat duty for the first FT process. For example, in the process 500 of FIG. 5, a portion of the light hydrocarbon stream 538 is burned in a power generation zone (here, in a heat generator 580), to generate heat stream 582. The heat stream 582 is conducted to a heat exchange zone 590 to heat the first FT feed stream 511 .

[0236] The present inventors have noted that it can be desirable to provide for heat exchange with a relatively hot first FT feed stream to cool the first FT product stream to a temperature more appropriate for the second FT step and to provide heat elsewhere to the integrated process. For example, the process may further comprise exchanging heat between at least a portion of the first FT product stream and at least a portion of the first FT feed stream, thereby cooling at least a portion of the first FT product stream and heating at least a portion of the first FT feed stream. An example of such a process is shown schematically in FIG. 4. In FIG. 4, the process 400, first FT reactor 410, first FT feed stream 411 , first FT product stream 412, first FT catalyst 413, second FT reactor 420, second FT feed stream 421 , second FT product stream 422 and second FT catalyst 423 are generally as described above. Here, the process 400 includes exchanging heat between at least a portion of the first FT product stream 412 and a least a portion of the first feed stream 411 in a first heat exchange zone 430, thereby cooling at least a portion of the first product stream 412 and heating at least a portion of the first feed stream 411 . The person of ordinary skill in the art will appreciate that a wide variety of heat exchangers can be used for this purpose.

[0237] Of course, any excess heat in the first FT product stream can be additionally or alternatively used for other purposes. For example, the process may further comprise exchanging heat between at least a portion of the first FT product stream and a steam generation zone, thereby cooling at least a portion of the first FT product stream and providing heat to the steam generation zone. This is shown in FIG. 4. Here, after heat exchange with the first feed stream 411 , the first product stream 412 is conducted to steam generation zone 432, to cool the first FT product stream 412 and provide heat to the steam generation zone 432. Steam can be generated from the heat provided, and electricity can be generated from the steam. For example, in the embodiment of FIG. 4, electricity stream 464 is provided by the generation of electricity using steam generated in the steam generation zone 432. Of course, as would be understood to the person of ordinary skill in the art, the steam generated in the steam generation zone may be used in other processes. Optionally, the steam may be used to heat the first FT feed stream. For example, in the embodiment of FIG. 4, the steam stream 466 generated in the steam generation zone 432 is conducted to the heat exchange zone 490 to heat the first FT feed stream 411 .

[0238] As with the first FT product stream, heat can be exchanged from the second FT product stream to provide heat to, for example, a feed stream (e.g., the first FT or second FT feed streams as described herein) or a steam generation zone. For example, in various embodiments, the process further comprises exchanging heat between at least a portion of the second FT product stream and at least a portion of the first FT feed stream, thereby cooling at least a portion of the second FT product stream and heating at least a portion of the first FT feed stream. In process 300 of FIG. 3, heat is exchanged between at least a portion of the second FT product stream 322 and first FT feed stream 311 in a second FT heat exchange zone 330, thereby cooling the second FT product stream 322 and heating the first FT feed stream 311 . Of course, heat can also be exchanged from the second FT product stream to the second FT feed stream. For example, the process may further comprise exchanging heat between at least a portion of the second FT product stream and at least a portion of the second FT feed stream, thereby cooling at least a portion of the second FT product stream and heating at least a portion of the second FT feed stream. In process 500 of FIG. 5, heat is exchanged between at least a portion of the second FT product stream 522 and second FT feed stream 521 in a second FT heat exchange zone 530, thereby cooling the second FT product stream 522 and heating the second FT feed stream 521 . Similarly, in process 600 of FIG. 6, heat is exchanged between at least a portion of the second FT product stream 622 and second FT feed stream 621 in a second FT heat exchange zone 630, thereby cooling the second FT product stream 622 and heating the second FT feed stream 621 . The person of ordinary skill in the art will appreciate that a wide variety of heat exchangers can be used for this purpose.

[0239] Of course, any excess heat in the second FT product stream can be additionally or alternatively used for other purposes. For example, the process may further comprise exchanging heat between at least a portion of the second FT product stream and a steam generation zone, thereby cooling at least a portion of the second FT product stream and providing heat to the steam generation zone. This is shown in FIG. 3. Here, after heat exchange with the first FT feed stream 311 , the second FT product stream 322 is conducted to steam generation zone 332, to cool the second FT product stream 322 and provide heat to the steam generation zone 332. Steam can be generated from the heat provided, and electricity can be generated from the steam (not shown here).

[0240] As noted above, the second FT process step provides a second FT product stream that includes CB+ hydrocarbons (e.g., unsubstituted hydrocarbons like alkanes and alkenes, and / or oxygenated hydrocarbons such as alcohols). Accordingly, one or more products are provided from at least a portion of Cs+ hydrocarbons of the second FT product stream. The C5+ hydrocarbons can be used as the basis of a variety of fuels, e.g., gasoline, diesel, aviation fuel. Other products, like waxes and lubricants, can also be made. And alkenes and oxygenates can be used as feedstocks in a variety of other processes.

[0241] The person of ordinary skill in the art will use conventional post-processing techniques to convert the C5+ hydrocarbon-containing product to desirable products such as desirable fuels. For example, the process may further include hydroprocessing at least a portion of C5+ hydrocarbons of the second FT product stream. As the person of ordinary skill in the art will appreciate, hydroprocessing is a treatment of the hydrocarbon stream with hydrogen in the presence of a suitable catalyst. A wide variety of hydroprocessing techniques are known and the person of ordinary skill in the art will apply them here. For example, in the process 300, 500, and 600 of FIGs. 3, 5, and 6, respectively, second FT product streams 322, 522, and 622 are hydroprocessed in hydroprocessing reactors 350, 550, and 650, to provide hydro processed product streams 352, 552, and 652.

[0242] The processes described herein can be operated in a wide variety of reactor systems. Suitable, the first FT reaction zone (i.e. , in which the first FT process step is performed) comprises a first FT reactor in which a first FT catalyst is disposed, and the second FT reaction zone (i.e., in which the second FT process step is performed) comprises a second FT reactor in which the second FT catalyst is disposed. Examples of such processes are shown schematically in FIGS. 1 , 2, 4, 5, and 6. In these examples, the process (100, 200, 400, 500, 600) is performed in a reactor system that includes a first FT reactor (110, 210, 410, 510, 610) in which the first FT catalyst (113, 213, 413, 513, 613) is disposed, and a second FT reactor (120, 220, 420, 520, 620) in which the second FT catalyst (123, 223, 423, 523, 623) is disposed. The reactors used for the integrated process of the present disclosure as described herein are not particularly limited, and the person of ordinary skill in the art will be able to select an appropriate reactor.

[0243] But other embodiments are possible. For example, the process may be performed in a reactor system comprising first FT catalyst bed in which the first FT catalyst is disposed, and wherein the second FT reaction zone comprises a second FT catalyst bed in which the second FT catalyst is disposed. Optionally, the first FT reactor bed and the second FT reactor bed are disposed within the same reactor. Such a configuration is shown in FIG. 3, in which the first FT catalyst 313 is disposed in a first FT catalyst bed 314, and the second FT catalyst 323 is disposed in a second FT catalyst bed 324. Here, the catalyst beds 314 and 324 are in the same reactor, with process gases flowing between them. Such a configuration can be especially desirable when the first FT temperature and the second FT temperature are relatively close to one another. However, such a configuration is not particularly limited, as the person of ordinary skill in the art can implement more cooling or less heat input in the second FT section to accommodate Fischer-Tropsch processes where there is a relatively large difference in temperature between the first FT and second FT processes.

[0244] Optionally, the process is performed in a reactor system comprising one or more first FT catalyst containers in which the first FT catalyst is disposed, and wherein the second FT reaction zone comprises one or more second FT catalyst containers in which the second FT catalyst is disposed. These can be provided in the same reactor, such as described above with respect to catalyst beds.

[0245] As noted above, the first FT process step using the first FT catalysts described herein and the second FT process step can be performed under similar conditions.

[0246] In the embodiments particularly-described above, separate first FT and second FT catalysts are used, e.g., in separate reactors, or in separate regions of the same reactor.

[0247] As described above, CO2 and H2 are substantial inputs to the process as described herein. Advantageously, the present inventors have recognized that each of these can come from renewable sources.

[0248] CO2 can be captured from the environment generally or more directly from processes that form CO2 (especially in difficult-to-abate sectors). This can make the eventual hydrocarbon product of lower carbon intensity. Accordingly, at least a part of the CO2 of the first FT feed stream and / or the second FT feed stream is preferably from a renewable source. Optionally, at least part (e.g., at least 25%, at least 50%, or at least 75%) of the CO2 of the first FT feed stream and / or the second FT feed stream is from direct air capture. For example, at least part (e.g., at least 25%, at least 50%, or at least 75%) of the CO2 of the first FT feed stream and / or the second FT feed stream may be from a manufacturing plant such as a bioethanol plant (e.g., CO2 produced fermentation), a steel plant, or a cement plant. Accordingly, the rWGS-Fischer Tropsch integrated processes of the disclosure as described herein can, in some cases, be a net consumer of carbon dioxide. These benefits in particular make the integrated processes highly attractive for transportation fuels, for both automotive and aviation sectors, since the carbon monoxide produced by the rWGS activity of the first FT catalyst can be readily utilized by well-established technologies to synthesize liquid hydrocarbon fuels by, e.g. cobalt-based FT processes.

[0249] In some embodiments, at least a part of the H2 of the first FT feed stream and / or the second FT feed stream is from a renewable source. For example, at least part (e.g., at least 25%, at least 50%, or at least 75%) of the H2 of the first FT feed stream and / or the second FT feed stream can be so-called “green” hydrogen, e.g., produced from the electrolysis of water operated using renewable electricity (such as wind, solar, or hydro-electric power). Optionally, at least part (e.g., at least 25%, at least 50%, or at least 75%) of the H2 of the first FT feed stream and / or the second FT feed stream may be from a so- called “blue” source, e.g., from a natural gas reforming process with carbon capture. Of course, other sources of H2 can be used in part or in full. For example, optionally, at least part (e.g., at least 25%, at least 50%, or at least 75%) of the H2 of the first FT feed stream and / or the second FT feed stream is grey hydrogen, black hydrogen, brown hydrogen, pink hydrogen, turquoise hydrogen, yellow hydrogen, and / or white hydrogen.

[0250] The present inventors have noted that electrolysis of water is a desirable way to provide hydrogen to the claimed processes. Accordingly, optionally, the process includes providing at least a portion of H2 to the first FT feed stream and / or the second FT feed stream by electrolysis of water. Optionally, the electrolysis of water is performed using at least partially electricity from a renewable source, e.g., to provide so-called “green hydrogen.” However, the present inventors have noted that electricity can be generated as part of the claimed process, e.g., using heat exchange from the first FT or second FT product stream, or by burning light hydrocarbons as described above. In some embodiments, the electrolysis of water is performed using at least partially electricity generated according to the processes as described herein. For example, in the process 400 of FIG. 4, water 418 separated from the first FT product stream is electrolyzed in electrolyzer 460, using electricity 464 generated from steam made in the steam generation zone 432 by heat exchange from the first FT product stream. H2 generated in the electrolysis is provided via stream 465 to the first FT feed stream. Optionally, at least a portion of O2 generated in the electrolysis 463 is provided to a partial oxidation reaction zone as described herein.

[0251] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0252] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0253] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0254] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0255] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0256] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0257] EXAMPLES

[0258] Example 1 - effect of conditions on hydroqen:carbon monoxide ratio

[0259] A number of experiments were carried out to demonstrate the effect of reactor conditions on the ratio of hydrogen to carbon monoxide in the product stream of a modified FT process according to the first FT step of the present invention.

[0260] Catalyst testing was performed using a 4-fold parallel fixed bed reactor unit (Manufactured by ILS) with tube internal diameter of 6.5mm and independent temperature and gas feed flow control per reactor. Catalyst was diluted with SiC for better temperature distribution along the bed and to avoid hot spots. The unit had two product knockout pots, firstly a hot pot (at 180°C collecting high boiling waxes) and a cold pot (at 12°C collecting water and light organics). GC analysis was done by analysing the 4 feed composition inlets and 4 product exits after knockout pots using an Agilent Refinery Gas Analyser. The GC had two TCD channels, one for hydrogen and the other for permanent gases (CH4, CO2, argon, nitrogen and CO). The other channel was an FID which is for light organics (C1-C5).

[0261] At the start of each test, the catalyst was reduced with H2 (GHSV = 5000, 80% H2 / 20% Argon) at 400°C for ~24 Hrs. Then, the reactor was cooled to ~150°C, the gas was switched to the H2 / CO2 feed, the pressure was increased and subsequently the temperature of the reactor was increased to the desired reaction temperature. Experimental conditions and results are as summarised in Table 1 .

[0262] Table 1

[0263] The data show that the molar ratio of hydrogen:carbon monoxide is particular sensitive to the ratio of hydrogen:carbon dioxide in the feed stream. This is especially clear from Figure 11 , which is a scatter plot for all experiments showing that the lowest molar ratio of hydrogen :carbon monoxide in the product stream is achieved at lower molar ratios of hydrogen :carbon dioxide in the feed stream. Clear bands are observed as the feed ratio moves up from 1 to 2, 2 to 2.5 and 2.5 to 3. This same trend is also evident in Figure 12, which compares experiments which differ only in terms of feed ratio, specifically, Experiments 15 and 16 (triangular data markers), and Experiments 17 and 18 (circular data markers).

[0264] The data also show that the molar ratio of hydrogen:carbon monoxide is highly sensitive to the temperature of the reactor. This is shown clearly in Figure 13, which provides a comparison of the results of experiments in the table above which differ only in terms of temperature - that is, Experiments 1 and 2 (triangular data markers), Experiments 3 and 4 (circular data markers), Experiments 5 and 6 (circular data markers) and Experiments 7-10 (diamond data markers).

[0265] The data also show that increasing flow rate leads to a lower ratio of hydrogen:carbon monoxide when all other conditions are held equal, as shown in Figure 14, which compares the results of Experiments 5, 13 and 14. Likewise, increasing pressure led to a lower ratio of hydrogen:carbon monoxide when all other conditions were held equal, as shown in Figure 15, which compares the results of Experiments 2 and 15. However, the data suggest that the molar ratio of hydrogen:carbon monoxide is relatively less sensitive to pressure and flow rate changes as compared to alterations in feed ratio and temperature.

[0266] Example 2 - Partial oxidation of first FT product stream

[0267] The effect of partial oxidation on the first FT product stream was simulated, specifically the effect on the ratio of hydrogen :carbon monoxide. Partial oxidation was applied to the FT offgas and recycle purge of an FT reactor using a partial oxidation unit operating at a pressure of 40 bar and temperature of 1350°C, with the results as shown in Table 2.

[0268] Table 2

[0269] These data show that the output stream is enriched in both hydrogen and carbon monoxide, with the hydrogen to carbon monoxide ratio being reduced compared to the input stream.

Claims

CLAIMS:1 . A process for preparing hydrocarbons, comprising providing a first Fischer-Tropsch (FT) feed stream comprising carbon dioxide and hydrogen; in a first FT reaction zone, contacting the first FT feed stream with a first FT catalyst under conditions sufficient to form a first FT product stream comprising Cs+ hydrocarbons, and carbon monoxide; providing a second FT feed stream comprising carbon monoxide and hydrogen, the second FT feed stream comprising at least a portion of the carbon monoxide from the first FT product stream; and in a second FT reaction zone, contacting the second FT feed stream with a second FT catalyst under conditions sufficient to form a second FT product stream comprising Cs+ hydrocarbons.

2. The process of claim 1 , wherein the second FT feed stream has a molar ratio of hydrogen to carbon monoxide in the range of 1 :2 to 10:1 , preferably 1 :2 to 8:1 , more preferably 1 :1 to 5:1 .

3. The process of claim 1 or 2, wherein the first FT product stream has a molar ratio of hydrogen to carbon monoxide in the range of 1 :2 to 10:1 , preferably 1 :1 to 8:1 , more preferably 1 :1 to 5:1 , more preferably still 1 :1 to 3:

14. The process of any one of the preceding claims, wherein the first FT reaction zone operates under at least one of the following conditions: o a temperature of greater than 250°C; o a pressure of 10 to 40 barg.

5. The process of any one of the preceding claims, wherein the first FT feed stream is delivered to the first FT reaction zone under at least one of the following conditions: o a GHSV of greater than 2,000 h-1 ; o a ratio of hydrogen to carbon dioxide of less than 4:1 , preferably less than 3:1 , more preferably less than 2:1 .

6. The process of any one of the preceding claims, wherein contacting the second FT feed stream with a second FT catalyst occurs at a temperature of between 150 to 230°C.

7. The process of claim 6, wherein contacting the second FT feed stream with a second FT catalyst occurs at a temperature of between 180 to 220°C.

8. The process of claim 6, wherein contacting the second FT feed stream with a second FT catalyst occurs at a temperature of between 165 to 185°C.

9. The process of any one of the preceding claims, wherein the molar ratio of hydrogen to carbon monoxide in the second FT feed stream is adjusted to be lower than the molar ratio of hydrogen to carbon monoxide in the first FT product stream.

10. The process of claim 9, wherein the molar ratio of hydrogen to carbon monoxide is adjusted by selectively reacting hydrogen from the first FT product stream to create a modified product stream, and using this modified product stream to form the second FT feed stream.11 . The process of claim 10, wherein hydrogen from the first FT product stream is consumed through a hydrogenation reaction with a reactant.

12. The process of claim 11 , wherein the reactant is a C2-C4 olefin.

13. The process of claim 12, wherein the second FT feed stream comprises said C2-C4 olefins, and the molar ratio of hydrogen to C2-C4 olefins in the second FT feed stream is in the range of 2:1 to 30:1 , preferably 2:1 to 20.

14. The process of claim 12 or 13, wherein unsaturated hydrocarbons account for at least 10 mol%, preferably at least 30 mol%, of the total C1-C4 hydrocarbons in the first FT product stream, and said unsaturated hydrocarbons are included in the second FT feed stream.

15. The process of any one of claims 12 to 14, wherein providing the second FT feed stream comprises adding said C2-C4 olefins alongside at least a portion of the first FT product stream.

16. The process of claim 9, wherein the molar ratio of hydrogen to carbon monoxide is adjusted by selectively removing hydrogen from the first FT product stream to create a modified product stream, and using this modified product stream to form the second FT feed stream.

17. The process of claim 16, wherein selective removal of hydrogen from the first FT product stream is achieved using a hydrogen selective membrane.

18. The process of claim 16, wherein selective removal of hydrogen from the first FT product stream is achieved by pressure swing adsorption.

19. The process of any one of the preceding claims, wherein the molar ratio of hydrogen to carbon monoxide in the second feed stream is adjusted by converting C1-C4 hydrocarbons of the first FT product stream to produce carbon monoxide, and including this carbon monoxide in the second FT feed stream.

20. The process of claim 19, wherein conversion of the C1-C4 hydrocarbons is carried out by reforming, for example by partial oxidation, steam reforming or dry CO2 reforming.21 . The process of any one of the preceding claims, wherein the second FT feed stream is derived solely from the first FT product stream and / or components recycled from the second FT stage or subsequent stages.

22. The process of any one of the preceding claims, wherein the first FT product stream is separated to provide a water rich stream and a water poor stream, wherein at least a portion of the water poor stream is used to form the second FT feed stream.

23. The process of any one of the preceding claims, wherein the first FT catalyst comprises iron, and the second FT catalyst comprises one or more of iron, cobalt, nickel, rhodium or ruthenium.

24. The process of claim 23, wherein the first FT catalyst is an iron-based catalyst, and the second FT catalyst is a cobalt-based catalyst.

25. The process of any one of the preceding claims, wherein the contacting of the first FT feed stream with the first FT catalyst in the first FT reaction zone is conducted with a Cs+ selectivity of at least 40%.

26. The process of any one of the preceding claims, wherein additional carbon monoxide is added to the second FT feed stream from an external source.

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