Method of starting up a fischer-tropsch plant

Abstract

A method of starting up a Fischer-Tropsch plant for synthesizing hydrocarbon products, the method comprising: heating an activation gas; and passing the heated activation gas over a Fisher-Tropsch catalyst within a Fisher-Tropsch reactor to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor of the Fischer-Tropsch plant, wherein the heating of the activation gas comprises heating the activation gas to a first temperature in a range 200°C to 260°C using a steam heater and then boosting the temperature of the activation gas to a second temperature in a range 270°C to 350°C using an electric booster heater.

Description

Field The present specification relates to a method of starting up a Fischer-Tropsch plant. The specification is particularly concerned with providing a start-up method which activates Fischer-Tropsch catalyst insitu within the plant. Background Examples of Fischer-Tropsch (FT) plants and their operation are described in WO2021140227A1, WO2018146276A1, WO2017037175A1, WO2015140100A1, WO2015140099A1, WO2015010939A1 and WO2009128865A1. The Fischer-Tropsch process is a collection of chemical reactions that convert a mixture of carbon monoxide and hydrogen (also known as "synthesis gas" or "syngas") into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150-300 °C and pressures of one to several tens of atmospheres. The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (C„H2n+2). The more useful reactions produce alkanes as follows: (2n +1) H2 + n CO -> CnH2n+2 + n H2O where n is typically 1-100 or higher. The formation of methane (n = 1) is unwanted. Most of the alkanes produced tend to be straight-chain and are suitable to be upgraded to produce middle distillate fuels such as diesel and jet fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. The Fischer-Tropsch reaction is a highly exothermic reaction due to a standard reaction enthalpy (AH) of -165 kJ / mol CO combined. In a Fischer-Tropsch plant, the syngas fed to the Fischer-Tropsch unit can be prepared, at least in part, by subjecting a feed gas comprising hydrogen and carbon dioxide to a reverse-water-gas-shift reaction to convert some of the carbon dioxide to carbon monoxide. Other options for making syngas include gasification of biomass or municipal solid waste. WO2023 / 119236 (co-owned by BP PLC and Johnson Matthey Davy Technologies Ltd) describes that in a typical preparation of supported cobalt-containing FT synthesis catalysts, a solid support material is contacted with a solution of a soluble cobalt compound, such as cobalt nitrate. The impregnated support is subsequently calcined and / or oxidized to form a cobalt oxide. However, such oxides typically have poor FT catalytic activity and must be reduced to form the preferred catalytically active species of cobalt metal. WO2023 / 119236 further describes that typical activation (at lab-scale) involves the use of high temperatures greater than 300°C and pure hydrogen gas, but that such simple activation protocols are poorly suited to large-scale reactors that utilize large amounts of catalyst and reduction gas. As such, WO2023 / 119236 describes a lower temperature activation method for the activation of a Fischer-Tropsch synthesis catalyst in an industrial scale process. The method comprises: (i) contacting the catalyst with a first gaseous composition comprising at least 80% N2 at a temperature of no more than 250°C; (ii) introducing a second gas composition to the first gas composition below 175°C where the second gaseous composition comprises at least 80% H2 to form a H2 / N2 gaseous composition; and (iii) increasing the temperature of the activation gas mixture to a range of 220°C to 260°C. It is taught that this lower temperature activation process is better suited to large-scale reactors when compared to optimal higher temperature lab-based methods. WO2016 / 091693 also relates to Fischer-Tropsch (FT) processes and indicates that known FT processes typically utilise a stable catalyst composition comprising oxidic cobalt and employ a reduction step to activate the catalyst by reducing the cobalt oxide to elemental (metallic) cobalt. It is taught that during the activation process, reducing gas is passed over the catalyst bed at a temperature of from 200°C to 300°C, preferably from 220°C to 280°C, more preferably from 230°C to 250°C. US10029245B2 also describes a method for establishing catalyst activation and / or regeneration in a Fischer-Tropsch system. The method comprises: heating a heat transfer fluid ("HTF") to create an HTF vapor; providing a stream of the HTF to a shell side of an FT reactor, the FT reactor containing at least partially non-active FT catalyst in a plurality of FT catalyst-filled tubes; and while continuing to provide the stream of HTF vapor into the FT reactor on the shell side sufficient to maintain a predetermined reactor temperature, providing at least one FT catalyst activity-related gas into the FT reactor on a tube-side of the reactor to contact the at least partially non-active FT catalyst. The present specification seeks to provide an improved process for starting up a Fischer-Tropsch plant. In particular, the present specification seeks to provide an improved method for activating FT catalyst in a large-scale FT production plant. The methodology may be applied on initial start-up of the plant to activate the FT catalyst for the first time and / or may be applied to re-start a plant and / or re-activate an FT catalyst. Summary Certain industrial scale Fischer-Tropsch plant designs utilize a medium pressure steam (MPS) heater to heat activation gas to reduce and activate an FT catalyst at start-up of the plant (e.g., after fresh catalyst material has been loaded into the system or after a shut-down period). Such a steam heater system is limited to a maximum temperature of around 250°C for in-situ activation of the FT catalyst due to mechanical limitations of using steam for the heating source. The mechanical limitations are related to the mechanical design pressure of the FT reactor shell and associated steam drum. This can limit the maximum operating pressure which dictates the highest temperature (equivalent saturated steam temperature) that can be achieved with steam. It has been found that this can result in a sub-optimal reduction of the FT catalyst, with tests showing around 30-40% activity loss verses a more ideal reduction carried out at 300°C. The present specification provides an improved method of starting up a Fischer-Tropsch plant for synthesizing hydrocarbon products, the method comprising: heating an activation gas; and passing the heated activation gas over a Fisher-Tropsch catalyst within a Fisher-Tropsch reactor to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor of the Fischer-Tropsch plant, wherein the heating of the activation gas comprises heating the activation gas to a first temperature in a range 200°C to 260°C using a steam heater and then boosting the temperature of the activation gas to a second temperature in a range 270°C to 350°C using an electric booster heater. This start-up / catalyst activation method is advantageous in that: (i) it can be utilized in an industrial scale plant; (ii) it can be made energy efficient utilizing (internally) generated steam to perform the initial heating of the activation gas and only requiring imported (advantageously renewable) energy / electricity for boosting the temperature of the activation gas using the electrical heater; and (iii) it can simultaneously provide more optimal conditions for FT catalyst activation in an industrial scale plant. The Fisher-Tropsch reactor may comprise one or more tubes in which the Fisher-Tropsch catalyst is disposed, and the heated activation gas can be passed through the one or more tubes. Advantageously, the heated activation gas is also passed through the Fisher-Tropsch reactor outside of the one or more tubes (i.e., on a shell side of the reactor) such that the heated activation gas contacts both an inner and an outer side of the one or more tubes. This methodology of circulating hot activation gas through both a shell-side and a tube-side of an FT reactor avoids excessive thermal differentials across the reactor tube walls, shell and tube sheet and can reduce heat losses. The steam heater can be configured to heat the activation gas via heat exchange between a heated steam stream and the activation gas stream. The steam stream can be produced from a boiler. The activation gas can be warmed up via heat exchange with a process gas or waste stream of the Fischer-Tropsch plant prior to heating in the steam heater to the first temperature. Additionally, or alternatively, the input activation gas can be pre-heated via heat exchange with hot activation gas which has already been passed through the FT reactor. The first temperature of the activation gas after heating with the steam heater is in a range 200°C to 260°C. For example, the steam heater can heat the activation gas to a temperature of: at least 200°C, 210°C, 220°C, 230°C, 240°C, or 250°C; no more than 260°C, 255°C, or 250°C; and / or within a range defined by any combination of the aforementioned lower and upper limits. Such temperatures are achievable in an industrial scale FT plant using an MPS (medium pressure steam) heater. Subsequently (in series), the activation gas is boosted to the second temperature in the range 270°C to 350°C using the electric heater. For example, the electrical booster heater can further heat the activation gas to a temperature of: at least 270°C, 280°C, 290°C, or 300°C; no more than 350°C, 340°C, 330°C, 320°C, or 310°C; and / or within a range defined by any combination of the aforementioned lower and upper limits. Such temperatures are achievable using commercially available electrical heaters and the present specification isn't limited to a particular type of electrical heater. The electrical booster heater may be a resistive heater, an inductive heater, or another type of electrical heater such as a turbomachinery heater that imparts kinetic energy to a gas, thereby heating the gas, by means of a rotatable shaft assembly. The electrical heater may be configured to heat the activation gas directly or it may be configured to heat an intermediate fluid (liquid or gas) which then heats the activation gas. The system enables internally generated hot steam within the FT plant system to be utilized to heat the activation gas with a further boost using an electrical heater to reach a more optimal temperature for FT catalyst activation. The activation gas may comprise at least 50 mol%, 60 mol%, 70 mol%, 80 mol%, 90 mol%, 95 mol%, or 99 mol% hydrogen. The activation gas may comprise a mixture of a reducing gas, optionally hydrogen, and an inert gas, optionally nitrogen. The activation gas can be passed over the Fisher-Tropsch catalyst at a pressure of: at least 5, 10, or 15 barg; no more than 30, 28, or 25 barg; or within a range defined by any combination of the aforementioned lower and upper limits. For example, within a pressure range of 5 to 30 barg or within a range of 15 to 25 barg. The activation gas can be passed over the Fisher-Tropsch catalyst at a gas hour space velocity (GHSV) of: at least 2000, 3000, or 4000 Nm3 / h.m3 of catalyst; no more than 8000, 7000, or 6500 Nm3 / h.m3 of catalyst; or within a range defined by any combination of the aforementioned lower and upper limits. For example, within a range 2000 to 8000 Nm3 / h.m3 of catalyst, preferably within a range 4000 - 6500 Nm3 / h.m3 of catalyst. One or both of the steam heater and the electrical booster heater, or the combination of the steam heater and the electrical booster heater, may heat the activation gas at a heating rate of: at least 5, 8, or 10°C / hr; no more than 20, 15, or 12°C / hr; or within a range defined by any combination of the aforementioned lower and upper limits. Once the target temperature is reached by the combination of the steam heater and the electrical booster heater then the target temperature is maintained for a time period in a range 2 to 96 hours. The composition of the activation gas and the flow parameters of the activation gas can be adapted to achieve a more optimal activation of the FT catalyst at the boosted temperature provided by the combination of a steam heater and an electrical booster heater and / or reduce the activation time required to achieve an acceptable level of activation for the FT catalyst. The present specification also provides a system for performing the method as described herein. The system comprises: a steam heater configured to heat an activation gas to a first temperature in a range 200°C to 260°C; an electrical booster heater configured to boost the temperature of the activation gas to a second temperature in a range 270°C to 350°C; and a Fisher-Tropsch reactor comprising a Fisher-Tropsch catalyst, the Fisher-Tropsch reactor being configured to receive the heated activation gas and pass the heated activation gas over the Fisher-Tropsch catalyst to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor. In certain configurations, the Fisher-Tropsch reactor system is also configured to pass the heated activation gas through a shell-side of the reactor in addition to a tube-side of the reactor to avoid excessive thermal differentials across the reactor tube walls, shell and tube sheet and / or to reduce heat losses. Additional features of the system are as described in relation to the method and are not repeated here for reasons of conciseness. However, it will be understood that features described in relation to the method can be combined with features of the system. After activation of the Fisher-Tropsch catalyst within the Fisher-Tropsch reactor, synthesis gas comprising carbon monoxide and hydrogen is passed over the activated Fisher-Tropsch catalyst within the Fisher-Tropsch reactor to synthesize hydrocarbon products. Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a schematic of a system for performing the method of the present specification; Figure 2 shows a schematic of an FT reactor comprising a plurality of tubes in which FT catalyst is disposed; and Figure 3 shows a more detailed example of an FT system for performing the method of the present specification. A summary of the reference numerals used in the figures is set out in the table below. Reference Item 2 FT reactor 4 Synthesis gas stream / activation gas inlet to FT reactor tube side 6 Hydrocarbon product stream / activation gas exit FT reactor tube side 7 Activation gas stream 8 Steam heater 9 Heated activation gas stream 10 Electrical booster heater 12 Heated activation gas stream input to FT reactor 14 Activation gas stream exiting FT reactor shell side 16 FT reactor tubes 18 Shell-side of FT reactor 20 FT reactor 22 MPS heater 24 Electrical booster heater 26 Valve A 28 Valve B 30 Valve C 32 Filter / strainer 34 Activation drier 36 Cooler(s) 38 Steam drum 40 Fresh feed Detailed Description As described in the summary section, the present specification provides a Fischer-Tropsch (FT) plant system and method for activating an FT catalyst on initial plant start-up and / or when a new batch of catalyst is loaded into the system and / or when a used FT catalyst requires re-activation (each considered a plant start-up within the context of this specification). Figure 1 shows a simple schematic of an FT reactor system according to the present specification. In normal operation a synthesis gas 4 comprising or consisting of hydrogen and carbon monoxide is fed through an FT reactor 2 comprising FT catalyst to produce a hydrocarbon product stream 6. For starting up the system, an activation gas 7 is passed through a steam heater 8 configured to heat the activation gas to a first temperature in a range 200°C to 260°C. The heated activation gas 9 is then passed through an electrical booster heater 10 to boost the temperature of the activation gas to a second temperature in a range 270°C to 350°C. The heated activation gas 12 is then passed through the Fisher-Tropsch reactor 2, the Fisher-Tropsch reactor being configured to receive the heated activation gas 12 and pass the heated activation gas over the Fisher-Tropsch catalyst to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor. The activation gas then exits the reactor via stream 14. In more detail, for a Fisher-Tropsch reactor comprising a shell containing tubes, with catalyst within the tubes, the activation gas passes through the reactor shell and then through the tubes containing the catalyst material prior to exiting the reactor. The system as illustrated schematically in Figure 1 provides an improved method of starting up a Fischer-Tropsch plant for synthesizing hydrocarbon products, the method comprising: heating an activation gas; and passing the heated activation gas over a Fisher-Tropsch catalyst within a Fisher-Tropsch reactor to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor of the Fischer-Tropsch plant, wherein the heating of the activation gas comprises heating the activation gas to a first temperature in a range 200°C to 260°C using a steam heater and then boosting the temperature of the activation gas to a second temperature in a range 270°C to 350°C using an electric booster heater. The first temperature of the activation gas after heating with the steam heater is in a range 200°C to 260°C. For example, the steam heater can heat the activation gas to a temperature of: at least 200°C, 210°C, 220°C, 230°C, 240°C, or 250°C; no more than 260°C, 255°C, or 250°C; and / or within a range defined by any combination of the aforementioned lower and upper limits. Such temperatures are achievable in an industrial scale FT plant using an MPS (medium pressure steam) heater. Subsequently (in series), the activation gas is boosted to the second temperature in the range 270°C to 350°C using the electric heater. For example, the electrical booster heater can further heat the activation gas to a temperature of: at least 270°C, 280°C, 290°C, or 300°C; no more than 350°C, 340°C, 330°C, 320°C, or 310°C; and / or within a range defined by any combination of the aforementioned lower and upper limits. As described in the summary section, such temperatures are achievable using commercially available electrical heaters and the present specification isn't limited to a particular type of electrical heater. The electrical booster heater may be a resistive heater, an inductive heater, or another type of electrical heater such as an electrically powered turbo-machinery heater that imparts kinetic energy to a gas, thereby heating the gas, by means of a rotatable shaft assembly. The electrical heater may be configured to heat the activation gas directly or it may be configured to heat an intermediate fluid (liquid or gas) which then heats the activation gas. The system enables internally generated hot steam to be utilized to heat the activation gas with a further boost using an electrical heater to reach a more optimal temperature for FT catalyst activation. As described in the summary section, this start-up / catalyst activation method is advantageous in that it can be utilized in an industrial scale plant, can be made energy efficient utilizing internally generated steam to perform the initial heating of the activation gas and only requiring imported (advantageously renewable) energy / electricity for boosting the temperature of the activation gas using an electrical heater, and can simultaneously provide more optimal conditions for FT catalyst activation. As also described in the summary section and illustrated in Figure 2, the Fisher-Tropsch reactor 2 may comprise one or more tubes 16 in which the Fisher-Tropsch catalyst is disposed, and the heated activation gas is passed through the one or more tubes 16. Advantageously, the heated activation gas is also passed through the Fisher-Tropsch reactor outside of the one or more tubes (i.e., on a shell side of the reactor 18) such that the heated activation gas contacts both an inner and an outer side of the one or more tubes 16. This methodology of circulating hot activation gas through both the shell-side and the tube-side of the FT reactor avoids excessive thermal differentials across the reactor tube walls, shell and tube sheet and / or can reduce heat losses. The steam heater can be configured to heat the activation gas via heat exchange between a heated steam stream and the activation gas stream. The steam stream can be produced from a boiler. Optionally, the activation gas can also be warmed up via heat exchange with a process gas or waste stream of the Fischer-Tropsch plant prior to heating in the steam heater to the first temperature. Additionally, or alternatively, the input activation gas can be pre-heated heat via heat exchange with hot activation gas which has already been passed through the FT reactor. Additionally, or alternatively, the heating to produce the steam for the steam heater can be at least partially provided via heat exchange with a (hot) process gas or waste stream of the Fischer-Tropsch plant. The activation gas may comprise at least 50 mol%, 60 mol%, 70 mol%, 80 mol%, 90 mol%, 95 mol%, or 99 mol% hydrogen. For example, the activation gas may comprise a mixture of a reducing gas, optionally hydrogen, and an inert gas, optionally nitrogen. The activation gas can be passed over the Fisher-Tropsch catalyst at a pressure of: at least 5,10, or 15 barg; no more than 30, 28, or 25 barg; or within a range defined by any combination of the aforementioned lower and upper limits. For example, within a pressure range of 5 to 30 barg or within a range of 15 to 25 barg. The activation gas can be passed over the Fisher-Tropsch catalyst at a gas hour space velocity (GHSV) of: at least 2000, 3000, or 4000 Nm3 / h.m3 of catalyst; no more than 8000, 7000, or 6500 Nm3 / h.m3 of catalyst; or within a range defined by any combination of the aforementioned lower and upper limits. For example, within a range 2000 to 8000 Nm3 / h.m3 of catalyst, preferably within a range 4000 - 6500 Nm3 / h.m3 of catalyst. One or both or the steam heater and the electrical booster heater, or the combination of the steam heater and the electrical booster heater, may heat the activation gas at a heating rate of: at least 5, 8, or 10°C / hr; no more than 20, 15, or 12°C / hr; or within a range defined by any combination of the aforementioned lower and upper limits. Once the target temperature is reached by the combination of the steam heater and the electrical booster heater then the target temperature can be maintained for a time period in a range 2 to 96 hours. The composition of the activation gas and the flow parameters of the activation gas can be adapted to achieve a more optimal activation of the FT catalyst at the boosted temperature provided by the combination of a steam heater and an electrical booster heater and / or reduce the activation time required to achieve an acceptable level of activation for the FT catalyst. In relation to the above, it may be noted that FT reactor tubes can typically be made of a suitable grade of stainless steel which has a temperature limit which can accommodate the preferred activation temperature. That said, system components such as the shell side of the FT reactor, steam drum and associated pipework can be upgraded from the typically used carbon steel to chromium molybdenum steel (e.g., 1 % Cr 1 / 2 Mo steel) so that the system can operate with high partial pressures of hydrogen and at high temperature without suffering hydrogen embrittlement. As such, in accordance with certain configurations, the Fisher-Tropsch reactor system is at least partially constructed from a chromium molybdenum steel. Furthermore, the Fisher-Tropsch reactor can be subjected to an acid wash prior to activation of the Fisher-Tropsch catalyst within the Fisher-Tropsch reactor to remove any scale, rust or other foreign matter. For example, a shell-side of the FT reactor can be acid washed prior to activation of a catalyst batch. Additionally, a strainer or filter may be located in an exit line from the FT reactor shell to capture scale, rust and other foreign matter. Figure 3 shows a more detailed example of an FT system for performing the method of the present specification. In Figure 3, the normal process route and equipment for the FT synthesis loop is depicted in solid black and start-up lines as dashed. During catalyst activation, the medium pressure steam (MPS) heater 22 can heat the circulating activation gas (a mixture of hydrogen and nitrogen) to ~250°C and a "booster" electric heater 24 is deployed to heat the activation gas further to ~300°C. The hot activation gas (mixture of hydrogen and nitrogen) is circulated through the FT reactor 20 at or around 300°C. Activation gas can be directed through a shell side of the FT reactor 20 via start-up jump over lines (through Valve A and B with normal route Valve C closed - references 26, 28, and 30 respectively in Figure 3). The activation gas can be injected into the shell of the FT reactor 20 via a nozzle at the bottom of the reactor and flows through an exit nozzle at the top of the reactor. If there is a concern about distribution, ring mains with several inlet and outlet nozzles located round the circumference of the shell can be used. In order to avoid valving in the risers and downcomers from the steam drum 38, it is preferred that these are provided as a large dead leg in the system along with the steam drum 38 which can be isolated at the steam outlet and boiler feed water (BFW) inlet (noting that the BFW inlet to the steam drum is not shown in Figure 3). Alternatively, a small once-through flow of electrically heated LP nitrogen could be used and vented to atmosphere to simplify the system and again this may be sufficient to overcome the heat losses from the system. This once through nitrogen can flow through the shell side of the FT reactor instead of the activation gas which would only flow through the tube side. In Figure 3, fresh feed gas 40 is input to the system and an activation drier 34 can be used to dry the activation gas. A hot product stream / hot activation gas from the FT reactor 20 can be heat exchanged to warm input feed gas / activation gas and then the product stream / activation gas can be further cooled by cooler(s) 36. During activation the fresh feed gas can be isolated from the reactor loop and lines provided into the loop to enable the fresh activation gas to be added, i.e., the hydrogen and nitrogen. Once the FT catalyst has been activated using the methodology as described herein, the system can be switched to a normal operating mode in which synthesis gas is fed to the FT catalyst in the FT reactor / synthesis unit to synthesise a mixture of hydrocarbon products. The Fischer-Tropsch hydrocarbon synthesis unit may comprise one or more Fischer-Tropsch reaction vessels containing a Fischer-Tropsch catalyst. The Fischer-Tropsch conversion stage can be carried out according to any one of the known processes. The process may be operated at pressures in the range 0.1 to lOMPa and temperatures in the range 170 to 350°C. The gas-hourly-space velocity (GHSV) for continuous operation is in the range 1000 to 25000hr-1. Preferably the Fischer-Tropsch synthesis is carried out using one or more fixed bed reactors, i.e., a reaction vessel with a bed of catalyst fixed within the vessel through which the purified synthesis gas is passed. Any Fischer-Tropsch catalyst may be used, but cobalt-based Fischer-Tropsch catalysts are preferred over iron-based catalysts due to their lower carbon dioxide selectivity. Suitable cobalt Fischer-Tropsch catalysts are known, but preferred catalysts in the process comprise 9 to 20 wt% Co supported on a suitable support material. Suitable catalysts therefore include agglomerates, pellets or extrudates comprising metal oxides such as alumina, zinc oxide, titania or silica, or mixtures thereof, on which the catalytically active metal, preferably cobalt, is deposited. In a particularly preferred arrangement, the Fischer-Tropsch catalyst is used in combination with a catalyst carrier suitable for use in a tubular Fischer-Tropsch reactor where the catalyst carrier containing the catalyst is disposed within one or more tubes that are cooled by circulating coolant, such as water under pressure. By "catalyst carrier" we mean a catalyst container, for example in the form of a cup or can, configured to allow a gas and / or liquid to flow into and out of the carrier and through a bed of the catalyst or catalyst precursor disposed within the carrier. Any suitable catalyst carrier may be used. In one arrangement, the catalyst carrier is that described in WO2011 / 048361, the contents of which are incorporated herein by reference. In an alternative arrangement, the catalyst carrier may include a catalyst monolith as disclosed in WO2012 / 136971, the contents of which are also incorporated herein by reference. In yet another alternative arrangement, the catalyst carrier may be that disclosed in WO2016 / 050520, the contents of which are also incorporated herein by reference. In preferred embodiments, the Fischer-Tropsch hydrocarbon synthesis unit comprises a tubular reactor in which catalyst carriers containing a Fischer-Tropsch catalyst are disposed within one or more tubes cooled by a cooling medium. Typically, a portion of the carbon monoxide is converted in the one or more Fischer-Tropsch reactors to produce a mixture of liquid hydrocarbon products, co-produced water, and a gaseous mixture containing unreacted hydrogen and carbon monoxide, plus carbon dioxide and gaseous light hydrocarbons including methane, ethane, propane and butane. The reaction product mixture may be cooled, and the aqueous and liquid hydrocarbon streams separated from the gas mixture using one or more gas-liquid separators. The co-produced water may be separated using known hydrocarbonwater separators. The separated gas mixture, which may be termed "tail gas", may be used in a number of ways. Preferably a first portion of the tail gas is recycled to the one or more Fischer-Tropsch reactors in a synthesis loop to increase the overall conversion of carbon monoxide to hydrocarbons. The fraction that is recycled to form the loop may be set to control the build-up of inert gases, such as methane, in the Fischer-Tropsch hydrocarbon synthesis unit to an acceptable level. The remaining portion still contains a valuable source of carbon. Accordingly, a portion of the tail gas can be recycled to a reverse water-gas shift unit via a derichment vessel containing a derichment catalyst that converts any C2+ higher hydrocarbons present in the tail gas to methane. Steam can also be added to the tail gas to provide a suitable steam to carbon ratio for the derichment step. The portion of tail gas that is not recycled to the reverse water-gas shift unit, which may be termed "purge gas", may be removed from the process to prevent the build-up of inert gases. The purge gas may be exported as fuel or used within the process in a fired heater or thermal oxidiser to heat feed to the reverse water-gas shift vessel or superheat steam. According to certain configurations, in addition to recycling of a tail gas from the FT reactor to a reverse water-gas shift reactor via a derichment reactor as described above, a portion of the hydrocarbon product stream from the FT reactor (e.g., naphtha) can also be separated and recycled to a reverse water-gas shift reactor via a derichment reactor. In this way, unwanted product and / or waste streams from the FT process can be utilized to generate synthesis gas which is fed back into the FT system to synthesize desired hydrocarbon products. Liquid hydrocarbons recovered from the hydrocarbon synthesis unit are subjected to upgrading in an upgrading unit to provide more valuable hydrocarbon products. The upgrading unit may be fed with one or more liquid hydrocarbon streams produced by the hydrocarbon synthesis unit, including but not limited to a molten hydrocarbon wax and a light hydrocarbon condensate, which is liquid at ambient temperature. Desirably, the hydrocarbon synthesis unit is operated to produce a molten hydrocarbon wax liquid, which is subjected to upgrading treatments in a hydrotreating unit to generate liquid fuels. Accordingly, in some embodiments, at least a portion and preferably all of the liquid hydrocarbon mixture resulting from the hydrocarbon synthesis is fed as a feedstock, in the presence of hydrogen, to an upgrading unit comprising a hydrotreating unit. The hydrotreating unit may perform various conversions such as hydroisomerization, hydrogenation, hydrodeoxygenation, and / or hydrocracking using one or more vessels containing suitable catalysts. Hydrogen is required by the hydrotreating unit. This may be provided by various sources but is desirably provided by an electrolysis unit to minimise carbon dioxide emissions from the process. Accordingly, in some embodiments, a portion of the hydrogen stream from an electrolysis unit is fed to the hydrotreating unit. The hydrotreating unit may be operated at a temperature generally of between 200 and 450°C, preferably from 250 to 450°C, more preferably from 300 to 450°C and most preferably between 320 to 420°C; a pressure of between 0.2 and 15 MPag, preferably between 0.5 and 10 MPag and more preferably from 1 to 9 MPag; a liquid hourly space velocity of between 0.1 and 10 h'1, preferably between 0.2 and 7 h-1 and more preferably between 0.5 and 5.0 h'1, and the hydrogen content may be between 100 and 2000 litres H2 per litre of feedstock and preferably between 150 and 1500 litres H2 per litre of feedstock. The hydrotreating stage may suitably be carried out under conditions such that the conversion per pass of products with a boiling point of greater than or equal to 370° C into products having boiling points of less than 370° C is greater than 40% by weight and more preferably at least 50% by weight, so as to obtain middle distillates (gas oil and kerosene) having sufficiently good cold properties (pour point, freezing point) to satisfy the specifications in force for this type of fuel. The catalysts used in this stage are known. For example, hydroisomerization and hydrocracking can be carried out according to any one of the known processes, using any one of the known catalysts, and it is not limited to a specific process or catalyst. The majority of the catalysts suitable for hydroisomerization / hydro-cracking are of the bifunctional type combining an acid function with a hydrogenating function. The acid function is generally provided via supports of high specific surface area (150 to 800 m2 / g generally) exhibiting a surface acidity, such as halogenated (in particular chlorinated or fluorinated) aluminas, phosphorated aluminas, combinations of boron and aluminium oxides, or silicas / aluminas. The hydrogenating function is generally provided either by one or more metals from Group VIII of the Periodic Table of the Elements, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, or by a combination of at least one metal from Group VI, such as chromium, molybdenum and tungsten, and at least one metal from Group VIII. Most conventional hydrocracking catalysts are composed of weakly acidic supports, such as silicas / aluminas. These systems are typically used to produce middle distillates of very good quality. Many catalysts of the hydrocracking market are based on silica / alumina in combination with a metal from Group VIII. These systems have a very good selectivity for middle distillates and the products formed are of good quality. According to one preferred embodiment, the hydroisomerization / hydrocracking catalyst comprises at least one hydro-dehydrogenating element chosen from the noble metals of Group VIII, preferably platinum and / or palladium, and at least one amorphous refractory oxide support, preferably silica / alumina. The hydrocarbon products recovered from the hydrotreatment unit may be fed to separation apparatus to recover the valuable hydrocarbon products. The separation apparatus may comprise one or more atmospheric distillation columns and optionally one or more vacuum distillation columns that separate the upgrader hydrocarbon off-gas, the naphtha fraction, and preferably at least one kerosene and / or gas oil fraction and a heavy fraction. The heavy fraction generally exhibits an initial boiling point of at least 350°C, preferably of greater than 370°C. This fraction is advantageously recycled to hydrotreatment unit. It may also be advantageous to recycle a portion of the kerosene to the hydrotreatment unit. The gas oil and kerosene fractions may or may not be recovered separately and the cut points may be adjusted to produce the desired hydrocarbon product. While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of starting up a Fischer-Tropsch plant for synthesizing hydrocarbon products, the method comprising:heating an activation gas; andpassing the heated activation gas over a Fisher-Tropsch catalyst within a Fisher-Tropsch reactor to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor of the Fischer-Tropsch plant,wherein the heating of the activation gas comprises heating the activation gas to a first temperature in a range 200°C to 260°C using a steam heater and then boosting the temperature of the activation gas to a second temperature in a range 270°C to 350°C using an electric booster heater.

2. A method according to claim 1,wherein the Fisher-Tropsch reactor comprises one or more tubes in which the Fisher-Tropsch catalyst is disposed, and the heated activation gas is passed through the one or more tubes.

3. A method according to claim 2,wherein the heated activation gas is also passed through the Fisher-Tropsch reactor outside of the one or more tubes such that the heated activation gas contacts both an inner and an outer side of the one or more tubes.

4. A method according to any preceding claim,wherein the steam heater is configured to heat the activation gas via heat exchange between a steam stream and the activation gas.

5. A method according to any preceding claim,wherein the first temperature of the activation gas after heating with the steam heater is at least 210°C, 220°C, 230°C, 240°C, or 250°C, optionally no more than 255°C, or 250°C, or within a range defined by any combination of the aforementioned lower and upper limits.

6. A method according to any preceding claim,wherein the second temperature of the activation gas after heating with the electric booster heater is at least 280°C, 290°C, or 300°C, optionally no more than 340°C, 330°C, 320°C, or 310°C, or within a range defined by any combination of the aforementioned lower and upper limits.

7. A method according to any preceding claim,wherein the activation gas comprises at least 50 mol%, 60 mol%, 70 mol%, 80 mol%, 90 mol%, 95 mol%, or 99 mol% hydrogen.

8. A method according to any preceding claim, wherein the activation gas comprises a mixture of a reducing gas, optionally hydrogen, and an inert gas, optionally nitrogen.

9. A method according to any preceding claim,wherein the activation gas is passed over the Fisher-Tropsch catalyst at a pressure of: at least 5, 10, or 15 barg; no more than 30, 28, or 25 barg; or within a range defined by any combination of the aforementioned lower and upper limits.

10. A method according to any preceding claim,wherein the activation gas is passed over the Fisher-Tropsch catalyst at a gas hour space velocity (GHSV) of: at least 2000, 3000, or 4000 Nm3 / h.m3 of catalyst; no more than 8000, 7000, or 6500 Nm3 / h.m3 of catalyst; or within a range defined by any combination of the aforementioned lower and upper limits.

11. A method according to any preceding claim,wherein the activation gas is heated at a heating rate of: at least 5, 8, or 10°C / hr; no more than 20, 15, or 12°C / hr; or within a range defined by any combination of the aforementioned lower and upper limits.

12. A method according to any preceding claim,wherein the Fisher-Tropsch reactor is at least partially constructed from a chromium molybdenum steel.

13. A method according to any preceding claim,wherein the Fisher-Tropsch reactor is subjected to an acid wash prior to activation of the Fisher-Tropsch catalyst within the Fisher-Tropsch reactor.

14. A method according to any preceding claim,wherein, after activation of the Fisher-Tropsch catalyst within the Fisher-Tropsch reactor, synthesis gas comprising carbon monoxide and hydrogen is passed over the activated Fisher-Tropsch catalyst within the Fisher-Tropsch reactor to synthesize hydrocarbon products.

15. A system for performing the method according to any preceding claim, the system comprising:a steam heater configured to heat an activation gas to a first temperature in a range 200°C to 260°C;an electrical booster heater configured to boost the temperature of the activation gas to a second temperature in a range 270°C to 350°C; anda Fisher-Tropsch reactor comprising a Fisher-Tropsch catalyst, the Fisher-Tropsch reactor being configured to receive the heated activation gas and pass the heated activation gas over the Fisher-Tropsch catalyst to reduce and activate the Fisher-Tropsch catalyst in-situ within the Fisher-Tropsch reactor.

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