Method of treating a fischer-tropsch catalyst

Abstract

A method of treating a Fischer-Tropsch catalyst used in a Fisher-Tropsch process for manufacturing hydrocarbon products, the method comprising: disposing the Fischer-Tropsch catalyst within a treatment gas; and subjecting the Fischer-Tropsch catalyst to an electrically generated plasma treatment within the treatment gas to increase activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process.

Description

Field The present specification relates to a method of treating a Fischer-Tropsch catalyst. The specification is particularly concerned with providing a treatment method to activate and / or re-activate Fischer-Tropsch catalyst, optionally in-situ within a Fischer-Tropsch 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 (CnH2n+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 by subjecting a feed gas comprising hydrogen and carbon dioxide to a reverse water-gas shift reaction to convert some of the carbon dioxide and hydrogen to carbon monoxide and water. WO2023 / 119236 (co-owned by 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 describes an activation method which involves heating the catalyst in a reducing gas to activate the catalyst. WO2016 / 091693 also relates to Fischer-Tropsch 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. Summary Certain methods of activating a cobalt catalyst by heating in a reducing gas during start-up of a Fisher Tropsch reactor have been found to lead to sintering of the cobalt, loss of cobalt surface area, and hence some loss of activity. It has also been found that further deactivation of the cobalt catalyst occurs during extended operation of the Fischer-Tropsch process due to deposition of carbon containing materials on the catalyst. Accordingly, after an extended period of operation it is required to replace the catalyst with fresh material. The present specification seeks to alleviate problems associated with the deactivation of Fischer-Tropsch catalysts and the requirement for fresh catalyst material. The present specification provides a method of treating a Fischer-Tropsch catalyst used in a Fisher-Tropsch process for manufacturing hydrocarbon products, the method comprising: disposing the Fischer-Tropsch catalyst within a treatment gas; and subjecting the Fischer-Tropsch catalyst to an electrically generated plasma treatment within the treatment gas to increase activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process. It has been found that subjecting a spent Fischer-Tropsch catalyst to an electrically generated plasma treatment can recover substantially all of the activity of fresh catalyst material. Furthermore, it has been found that plasma-regenerated catalysts are more selective to C5+ hydrocarbons in a Fischer-Tropsch process, with lower methane, CO2 and C2-C4 hydrocarbons being produced when compared to fresh parent catalyst samples. This is advantageous as typically the longer chain C5+ hydrocarbons are the desired products of a Fischer-Tropsch process. As such, in this respect the plasma treated catalysts appear to be advantageous over fresh catalyst material which is a surprising result. The Fischer-Tropsch catalyst can be a spent catalyst which has been used in the Fischer-Tropsch process and comprises deposits of carbon containing material. In this case, the electrically generated plasma treatment removes at least some of the deposits of carbon containing material to increase the activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process thereby regenerating the spent catalyst for re-use. Alternatively, or additionally, the Fischer-Tropsch catalyst comprises at least some oxide material, and the electrically generated plasma treatment reduces at least some of the oxide material to metallic material to increase the activity of the Fischer-Tropsch catalyst prior to use in the Fischer-Tropsch process. It has been found that electrically generated plasma treatments are effective at re-generating or reactivating spent catalyst which has already been used in a Fischer-Tropsch process. As indicated above, such spent catalyst material comprises deposits of carbon containing material. An electrically generated plasma treatment can remove at least some of the deposits of carbon containing material to increase the activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process thereby regenerating the spent catalyst for re-use and reducing the requirement for fresh catalyst while avoiding undue sintering of the catalyst. As also indicated above, selectivity of the catalyst material for C5+ hydrocarbons is actually improved for the plasma treated catalyst material when compared to fresh catalyst material. It has been found to be advantageous to perform the electrically generated plasma treatment at relatively low power, temperature, and / or pressure. Experiments (described later in the detailed description) indicate that regeneration of the catalyst material may be performed at or near the lowest temperature required to remove deposits of carbon containing material in order to achieve the best performance for the regenerated catalyst material. High power, high temperature, and / or high pressure plasma processes can lead to the sintering of the catalyst material with an associated loss of cobalt surface area and activity. It has been found that there is a suitable operating parameter window for the plasma processing treatment which enables carbon containing material to be removed from the catalyst without causing undue sintering of the catalyst material, thereby enabling regeneration of the catalyst without an associated loss of surface area and activity. In light of the above, during the electrically generated plasma treatment the Fischer-Tropsch catalyst may be maintained at a temperature of: no more than 500, 400, 300, 250, 200, or 150°C; no less than 0, 50, or 100°C; and / or within a range defined by any combination of the aforementioned upper and lower temperature limits. Furthermore, the electrically generated plasma treatment may be performed at a power (expressed as watts per gram of catalyst) of: no more than 1000, 500, 200,100, 50, or 30 W / g; no less than 0.01, 0.1, 1.0, 10 or 15 W / g; and / or within a range defined by any combination of the aforementioned upper and lower power limits. Further still, the electrically generated plasma treatment may be performed at a pressure of: no more than 10, 5, or 2 bar; no less than 0.001, 0.01, or 0.1 bar; and / or within a range defined by any combination of the aforementioned upper and lower pressure limits. Finally, the electrically generated plasma treatment can be performed for a time period of: no more than 24, 10, 8, 2, 1, or 0.5 hours; no less than 10 seconds, 1 minute, 10 minutes, or 20 minutes; and / or within a range defined by any combination of the aforementioned upper and lower time limits. Certain processes may involve pulsed or repeated applications of the electrically generated plasma. It will be noted that the precise optimal conditions for the plasma treatment will depend on the specific nature of the Fischer-Tropsch catalyst, the extent of the carbon deposits on the catalyst, and the operational set up for performing the plasma treatment. However, plasma treatment conditions can readily be optimized applying the principle that the best results in terms of a highly active regenerated catalyst will be achieved using a process which is near the limits required to remove carbon containing material so as to avoid undue sintering of the catalyst material thereby enabling regeneration of the catalyst without an associated loss of surface area and activity. In order to "burn-off" the carbon containing material, the treatment gas used in the plasma processing may contain oxygen (or other oxidising gas such as ozone or NOx) and the process emits carbon dioxide. The treatment gas may have an oxygen (or other oxidising gas) content of: no more than 100, 50, or 21%; no less than 0.1, 1, or 5%; or and / or within a range defined by any combination of the aforementioned upper and lower limits. For example, the treatment gas can be a mixture of oxygen and an inert gas, optionally nitrogen. In the case that the plasma treatment causes partial oxidation of the catalyst material, or if the Fischer-Tropsch catalyst comprises at least some oxide material for any other reason (e.g., formed during synthesis or prior to use), then an electrically generated plasma treatment can be used to reduce at least some of the oxide material to metallic material to increase the activity of the Fischer-Tropsch catalyst. In this case, a reducing gas can be used as the treatment gas for the plasma treatment, optionally comprising or consisting of hydrogen. For example, the treatment gas can be a mixture of hydrogen and an inert gas, optionally nitrogen. Such a reducing plasma treatment can be utilized to reduce and activate the Fischer-Tropsch catalyst prior to use in a Fischer-Tropsch process. Similar principles apply for the reductive plasma treatment as described above for the oxidative plasma treatment. That is, the operating conditions (power, temperature, pressure) should be sufficiently high to achieve the desired change in the Fischer-Tropsch catalyst (in this case reduction of oxide material) while ensuring that the conditions are not unduly high as to result in sintering of the catalyst and an associated loss of surface area and activity. Accordingly, an electrically generated plasma treatment can be performed at two stages of a Fischer-Tropsch process: (i) an electrically generated plasma treatment in an oxidizing gas to remove carbon containing material from the Fischer-Tropsch catalyst; and (ii) an electrically generated plasma treatment in a reducing gas to reduce oxide material to metal in the Fischer-Tropsch catalyst. Stage (i) can be performed after running the Fischer-Tropsch process to re-generate the Fischer-Tropsch catalyst. Furthermore, stage (ii) can be performed prior to initiating (or restarting) the Fischer-Tropsch process to activate the Fischer-Tropsch catalyst. One or both of steps (i) and (ii) can be performed insitu within a Fischer-Tropsch reactor. Cold plasma treatments of the kind discussed herein can thus be used to remove deposits of carbon material, reduce oxide material to active metallic form, and can also function to re-disperse the active phase via the Kirkendall effect. 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(a) illustrates an experimental set-up used to perform plasma treatments on samples of Fischer-Tropsch catalyst material; Figure 1(b) illustrates another experimental set-up used to perform plasma treatments on samples of Fischer-Tropsch catalyst material; and Figure 2 shows CO2 generation as a function of time during plasma treatment of Fischer-Tropsch catalyst samples to remove carbon deposits at two different powers (20 W and 30 W) with subsequent testing of catalyst activity indicating that: (i) lower power plasma treatments lead to regenerated catalysts with a higher activity approaching that of freshly fabricated Fischer-Tropsch catalyst material; and (ii) plasma treated samples have a better selectivity for C5+ hydrocarbons in a Fischer-Tropsch process, with lower methane, CO2 and C2-C4 hydrocarbons being produced when compared to freshly fabricated parent catalyst samples. A summary of the reference numerals used in the figures is set out in the table below. Reference Item 2 Quartz tubing (outer) 4 Quartz tubing (inner) 6 Quartz wool 8 High voltage electrode 10 Metal mesh ground electrode 12 Catalyst 20 Quartz (dielectric) 22 Ground electrode 24 High voltage electrode 26 Catalyst pellets 30 CO2 emission - 30 Watt Plasma 32 CO2 emission - 20 Watt Plasma Detailed Description As described in the summary section, the present specification provides a method of treating a Fischer-Tropsch catalyst used in a Fisher-Tropsch process for manufacturing hydrocarbon products, the method comprising: disposing the Fischer-Tropsch catalyst within a treatment gas; and subjecting the Fischer-Tropsch catalyst to an electrically generated plasma treatment within the treatment gas to increase activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process. The Fischer-Tropsch catalyst can be a spent catalyst which has been used in the Fischer-Tropsch process and comprises deposits of carbon containing material. In this case, the electrically generated plasma treatment removes at least some of the deposits of carbon containing material to increase the activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process thereby regenerating the spent catalyst for re-use. Alternatively, or additionally, the Fischer-Tropsch catalyst comprises at least some oxide material, and the electrically generated plasma treatment reduces at least some of the oxide material to metallic material to increase the activity of the Fischer-Tropsch catalyst prior to use in the Fischer-Tropsch process. The method is particularly useful when applied to Fischer-Tropsch catalysts comprising a cobalt material on a support material (e.g., silica or a metal oxide material such as titania or alumina, SiC, or Zirconia). Preferably, the Fischer-Tropsch catalyst comprises a cobalt material on a titania support. As also described in the summary section, it has been found to be advantageous to perform the electrically generated plasma treatment at relatively low power, temperature, and / or pressure. Experiments indicate that regeneration of the catalyst material may be performed at or near the lowest temperature required to removal deposits of carbon containing material in order to achieve the best performance for the regenerated catalyst material. High power, high temperature, and / or high pressure plasma processes can lead to the sintering of the catalyst material with an associated loss of surface area and activity. As such, during the electrically generated plasma treatment the Fischer-Tropsch catalyst may be maintained at a temperature of: no more than 500, 400, 300, 250, 200, or 150°C; no less than 0, 50, or 100°C; and / or within a range defined by any combination of the aforementioned upper and lower temperature limits. Furthermore, the electrically generated plasma treatment may be performed at a power (expressed as watts per gram of catalyst) of: no more than 1000, 500, 200, 100, 50, or 30 W / g; no less than 0.01, 0.1, 1.0, 10 or 15 W / g; and / or within a range defined by any combination of the aforementioned upper and lower power limits. Further still, the electrically generated plasma treatment may be performed at a pressure of: no more than 10, 5, or 2 bar; no less than 0.001, 0.01, or 0.1 bar; and / or within a range defined by any combination of the aforementioned upper and lower pressure limits. Finally, the electrically generated plasma treatment is performed for a time period of: no more than 24, 10, 8, 2,1, or 0.5 hours; no less than 10 seconds, 1 minute, 10 minutes, or 20 minutes; and / or within a range defined by any combination of the aforementioned upper and lower time limits. The treatment gas can be a mixture of oxygen and an inert gas, optionally nitrogen. The treatment gas may have an oxygen (or other oxidising gas) content of: no more than 100, 50, or 21%; no less than 0.1, 1, or 5%; or and / or within a range defined by any combination of the aforementioned upper and lower limits. The plasma operating conditions in combination with the treatment gas composition can be selected to achieve removal of the carbon containing material without unduly oxidizing the catalyst material. Alternatively, the catalyst material can be subsequently subjected to a reducing treatment to convert oxidized material to active metal material. Such a reducing treatment may also be a plasma treatment method. In this case, a reducing gas can be used as the treatment gas for the plasma treatment, optionally comprising or consisting of hydrogen. For example, the treatment gas can be a mixture of hydrogen and an inert gas, optionally nitrogen. Optionally, the Fischer-Tropsch catalyst can be subjected to a solvent treatment to remove some of the deposits of carbon containing material prior to application of the electrically generated plasma treatment. It has been found that while solvent treatments can remove some of the carbon deposits, a subsequent plasma treatment achieves further significant reductions in the carbon deposits to regenerate a catalyst with better activity. For example, the Fischer-Tropsch catalyst may comprise no more than 1, 0.5, or 0.3 wt% of the carbon containing material after the electrically generated plasma treatment. While a plasma treatment may be utilized without a solvent treatment to achieve such reduced levels of carbon containing material, removing some of the carbon material with a solvent treatment process can reduce carbon dioxide emissions in a subsequent plasma treatment. In this regard, it is noted that carbon dioxide is evolved during the electrically generated plasma treatment and the treatment can be terminated after no more carbon dioxide is being evolved. As such, the treatment process can be monitored by monitoring carbon dioxide emissions from the catalyst material during treatment. In addition to achieving removal of carbon containing material from the catalyst, the electrically generated plasma treatment can also increase cobalt metal surface area of the Fisher-Tropsch catalyst as measured by a chemisorption technique (e.g., N2O chemisorption or H2 chemisorption). For example, the cobalt metal surface area of the Fisher-Tropsch catalyst after the electrically generated plasma treatment can be: at least 4.8 m2 / g, 5.0 m2 / g, 5.2 m2 / g, 5.4 m2 / g, or 5.6 m2 / g as measured by a chemisorption technique. It has been found that plasma treatments as described herein can regenerate catalyst with a surface area approaching that of freshly fabricated Fisher-Tropsch catalyst. In use, the Fischer-Tropsch catalyst is loaded into a Fischer-Tropsch reactor for performing the process of hydrocarbon production from synthesis gas. According to certain preferred methods, the Fischer-Tropsch catalyst is loaded into a catalyst carrier and these loaded carriers are then introduced into a Fischer-Tropsch reactor. When starting-up the Fischer-Tropsch reactor, the loaded catalyst can be subjected to an activation step comprising heating in a reducing gas to reduce and activate the Fischer-Tropsch catalyst. In accordance with an aspect of the present specification, this catalyst reduction step to reduce and activate the Fischer-Tropsch catalyst in-situ within the Fischer-Tropsch reactor can be replaced with an in-situ electrically generated plasma treatment to reduce and activate the Fischer-Tropsch catalyst in-situ within the Fischer-Tropsch reactor. In this case, the reactor configuration is modified to include electrodes to generate the plasma. In the configuration in which the catalyst is loaded into a catalyst carrier, the catalyst carrier can be modified to include electrodes to generate the plasma in situ. For the aforementioned activation process, the treatment gas is a reducing gas, optionally comprising or consisting of hydrogen. For example, the treatment gas is a mixture of hydrogen and an inert gas, optionally nitrogen. The plasma treatment method can be advantageous over prior activation processes in that it can reduce the catalyst without causing sintering of the catalyst, loss of cobalt surface area, and hence loss of activity. During the electrically generated plasma activation treatment the Fischer-Tropsch catalyst may be maintained at a temperature of: no more than 500,400, 300, 250, 200, or 150°C; no less than 0, 50, or 100°C; and / or within a range defined by any combination of the aforementioned upper and lower temperature limits. Furthermore, the electrically generated plasma treatment may be performed at a power (expressed as watts per gram of catalyst) of: no more than 1000, 500, 200, 100, 50, or 30 W / g; no less than 0.01, 0.1, 1.0, 10 or 15 W / g; and / or within a range defined by any combination of the aforementioned upper and lower power limits. Further still, the electrically generated plasma treatment is performed for a time period of: no more than 24,10, 8, 2, 1, or 0.5 hours; no less than 10 seconds, 1 minute, 10 minutes, or 20 minutes; and / or within a range defined by any combination of the aforementioned upper and lower time limits. In such an activation process, the Fischer-Tropsch catalyst comprises at least some oxide material and the electrically generated plasma treatment reduces at least some of the oxide material to metallic material to increase the activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process. After running the Fischer-Tropsch process for an extended period of time, the process can be stopped to regenerate the catalyst. This may be done in-situ within the Fischer-Tropsch carrier / reactor, or the catalyst can be removed / discharged from the catalyst carrier and / or the Fischer-Tropsch reactor to regenerate the catalyst. As previously described, the regeneration of the catalyst involves an electrically generated plasma treatment, advantageously in a treatment gas comprising some oxygen to aid in removing carbon material which have been deposited on the catalyst material in use. As such, two different electrically generated plasma treatments can be performed: (i) an electrically generated plasma treatment in an oxygen containing gas to remove carbon containing material from the Fischer-Tropsch catalyst; and (ii) an electrically generated plasma treatment in a reducing gas to reduce oxide material to metal in the Fischer-Tropsch catalyst. Stage (i) can be performed after running the Fischer-Tropsch process to re-generate the Fischer-Tropsch catalyst. Furthermore, stage (ii) can be performed prior to initiating (or restarting) the Fischer-Tropsch process to activate the Fischer-Tropsch catalyst. One or both of steps (i) and (ii) can be performed in-situ within a Fischer-Tropsch reactor. Cold plasma treatments of the kind discussed herein can thus be used to remove deposits of carbon material, reduce oxide material to active metallic form, and can also function to redisperse the active phase via the Kirkendall effect. Surprisingly, it has been found that regenerated spent catalysts can exhibit a higher selectivity for C5+ hydrocarbons in a Fischer-Tropsch process compared to fresh parent catalyst material. This is advantageous as the longer chain hydrocarbons are typically desired. In relation to the above, it has been noted that plasma processes have previous been used in methods for fabricate fresh catalyst materials. Several prior art references describe methods of preparing fresh catalyst materials in which a plasma calcination step is utilized in place of a more conventional thermal calcination process. Examples of prior art references are briefly discussed below. 1. "Plasma-Assisted Preparation of Highly Dispersed Cobalt Catalysts for Enhanced Fischer-Tropsch Synthesis Performance" (ACS Catal. 2018, 8, 7, 6177-6185): This paper proposes the substitution of the thermal calcination of the catalyst (Co-Pt / TiCh) with a plasma calcination in the preparation method. 2. "Plasma-assisted design of supported cobalt catalysts for Fischer-Tropsch synthesis - Studies in Surface Science and Catalysis" (volume 175, 2010, Pages 253-257): This paper describes the preparation of Co-lr / ALOs by replacing the calcination step with a plasma calcination. 3. "Products selectivity and reaction stability of cobalt-based Fischer-Tropsch catalysts affected by glow discharge plasma treatment and silica structure" (Catalysis Today - Volume 337, 15 October 2019, Pages 139-146): Similar to the papers above, this reference discloses that a plasma is used to decompose the catalyst precursors instead of using a thermal calcination method. 4. "Plasma Science and Technology" (book chapter "Cold Plasma Produced Catalytic Materials"; DOI: 10.5772 / 61832): This book chapter also discloses replacement of thermal calcination or reduction of catalysts with plasma treatments as part of their preparation. 5. CN103111298: This reference describes the use of a hydrogen plasma to decompose and reduce cobalt nitrate in a single step. The final catalyst is cobalt supported on carbon nanotubes. 6. CN113996301: This reference describes the use of a plasma as a pre-treatment method for a carbon-based support which is then used to make a cobalt catalyst for Fischer-Tropsch processes. 7. CN110918098: This reference describes that a glow discharge plasma is introduced into the preparation of a Co / CNT (carbon-nanotubes) catalyst. 8. CN110639583: This reference describes a preparation method in which the surface of an inorganic oxide support material is coated with a carbon-based coating, a cobalt salt is loaded onto the prepared support, and the cobalt salt is decomposed by a glow-discharge plasma. 9. CN106492863: This reference discloses a method for preparing a non-precious metal molybdenum carbide catalyst using a plasma process. 10. CN105833870: This reference discloses a preparation method of a cobalt-based carbon nanotube catalyst for a Fischer-Tropsch synthesis reaction. The aforementioned references are focussed on the use of plasma processes in the fabrication of fresh catalyst materials and generally utilize a plasma process as a replacement for a thermal calcination step or a thermal reduction step in the fabrication processes. None of the aforementioned references describe the present process for regenerating spent Fischer-Tropsch catalyst material by using an electrically generated plasma treatment to remove deposits of carbon containing material in order to increase the activity of the spent Fischer-Tropsch catalyst thereby regenerating the spent catalyst for re-use. Furthermore, none of the aforementioned references describe the present process for activating passivated Fischer-Tropsch catalyst material by using an electrically generated plasma treatment prior to use of the catalyst material in a Fischer-Tropsch process (i.e., using a plasma treatment at start-up of the Fischer-Tropsch process). Further still, none of the prior art documents disclose the use of a plasma treatment process to increase the selectivity of the catalyst material for longer chain hydrocarbons in a Fischer-Tropsch process. Following the above, it has also been noted that the use of plasma processes for catalyst regeneration has been discussed in the following paper: "Current state and perspectives of plasma applications for catalyst regeneration" (Catalysis Today 337 (2019) 15-27). The document talks generally about various types of catalysts and catalyst deactivation processes (in section 2) including a disclosure of how cobalt catalysts used in a Fischer-Tropsch process can be re-oxidized by steam (in section 2.2). The document then goes on to describe some examples of how plasma treatments can be used to regenerate certain types of catalyst for certain types of applications (in section 3). There is no further mention of cobalt catalysts used in Fischer-Tropsch processes in the section about plasma treatments, or indeed any type of cobalt catalyst. There is certainly no suggestion of the present process for regenerating spent Fischer-Tropsch catalyst material by using an electrically generated plasma treatment to remove deposits of carbon containing material in order to increase the activity of the spent Fischer-Tropsch catalyst thereby regenerating the spent catalyst for re-use. Furthermore, there is no disclosure that such a treatment method for spent Fischer-Tropsch catalysts can regenerate activity levels corresponding to fresh Fischer-Tropsch catalyst material or that the selectivity for longer chain length hydrocarbons in a Fischer-Tropsch process can actually be improved when compared to fresh Fischer-Tropsch catalyst material. Experimental A catalyst (cobalt on titania) was operated in a Fischer-Tropsch process for 5700 hours. The catalyst was then recovered and dewaxed using solvents (e.g., xylene, iso-octane, n-pentane). The spent catalyst as discharged from the reactor contained 18.2 wt% carbon. After the solvent de-waxing process, the amount of residual carbon left in the catalyst was 2.2 wt%. Samples of the catalyst material (each sample = 2 g) were then subjected to plasma treatments at either 30 Watts or 20 Watts for 1 hour in a 10% O2 in N2 gas flow. The apparatus set up is illustrated in Figure 1(a). An alternative version of the apparatus set up is illustrated in Figure 1(b). The CO2 generation as a function of time and the power used to generate the plasma is shown in Figure 2. The amount of residual carbon after the plasma treatments was 0.2 wt%. While not been bound by theory, it is believed that the carbon is in elemental (graphitic) form. Hydrocarbons will have been washed away by solvents. The following references confirm the existence of graphitic carbon on Fischer-Tropsch catalysts: "Bert M.Weckhuysen, Combined operando x-ray diffraction / Raman spectroscopy of catalytic solids in the laboratory: The Fischer-Tropsch synthesis catalyst showcase, Abstracts of papers, 253rd ACS National Meeting & Exposition, San Francisco, April 2-6, California, USA"; and "Paul Hazemann, Dominique Decottignies, Sylvie Maury, Severine Humbert, Frederic C.Meunier, Yves Schuurman, Selectivity loss in Fischer-Tropsch synthesis: The effect of carbon deposition, Journal of Catalysis, 401, 2021, 7-16". Cobalt surface area (Co SA) values for the fresh, the spent-dewaxed, and the spent-dewaxed-plasma regenerated samples were measured using N2O chemisorption and H2 chemisorption techniques. See, for example, "Robert C. Reuel, Calvin H. Bartholomew, The stoichiometries of H2 and CO adsorptions on Cobalt: Effects of support and preparation, Journal of Catalysis, 85, 1984, 63-77" for a description of chemisorption. Results are presented in the table below: Sample N2O Co SA (m2 / g) H2 Co SA (m2 / g) Fresh catalyst 5.7+ / - 0.2 5.70 + / - 0.06 Spent-dewaxed catalyst 3.9 + / - 0.3 2.20 + / - 0.02 Spent-dewaxed-plasma regenerated catalyst 5.6+ / - 0.2 - (Nb. H2 chemisorption has not been performed for the plasma-regenerated material) The fresh catalyst samples have the same cobalt surface area result (5.7 m2 / g) for both the N2O chemisorption and H2 chemisorption methods of analysis. There is a difference in the cobalt surface area measurements for the spent-dewaxed catalyst samples using the N2O chemisorption and H2 chemisorption methods (3.9 m2 / g versus 2.2 m2 / g). This may be due to residual carbon being burned by N2O (leading to an overestimation of the Co SA value by N2O chemisorption) and / or incomplete carbon removal during the pre-treatment prior to hydrogen chemisorption (leading to an underestimation of the Co SA value by H2 chemisorption). In any case, after plasma regeneration the cobalt surface area is almost equal to that of the fresh catalyst (5.6 + / - 0.2 m2 / g which is within the experimental error of the fresh material) indicating that the plasma treatment method is effective at regenerating the cobalt surface area of the spent catalyst material. Samples of fresh catalyst and samples of plasma regenerated spent catalyst were subjected to catalyst activity testing in a Fischer-Tropsch process. Results for three fresh catalyst samples and three plasma regenerated spent catalyst samples are shown in the table below. Catalyst Conversion Syngas Conversion CO Selectivity to Methane Selectivity to CO2 Selectivity to C2-C4 hydrocarbons Selectivity to C5+ hydrocarbons Productivity of C5+ hydrocarbons Relative Activity Fresh Catalyst 1 48.72 47.46 4.39 0.07 4.89 90.65 361 0.45 Plasma Regenerated Spent Catalyst 1 (30 W / 1 hr) 48.81 47.61 4.09 0.01 4.40 91.51 303 0.38 Fresh Catalyst 2 48.78 47.53 4.46 0.07 5.07 90.41 367 0.46 Plasma Regenerated Spent Catalyst 2 (30 W / 1 hr) 48.97 47.75 4.10 0.03 4.42 91.45 322 0.40 Fresh Catalyst 3 48.95 47.67 4.42 0.06 4.99 90.52 378 0.48 Plasma Regenerated Spent Catalyst 3 (20 W / 1 hr) 48.67 47.46 4.27 0.02 4.47 91.25 365 0.45 The productivity is expressed in g wax / kg catalyst per hour. These results were obtained at 210°C, 20 barg pressure, using a H2:CO ratio of 2:1. The flow rates of syngas were adjusted to achieve isoconversion conditions, so that the selectivity values can be directly compared. In the above table, it is informative to compare the relative values of the plasma regenerated spent catalysts compared to the values for the fresh "parent" catalyst materials as discussed below. For the three samples of fresh parent catalyst material (the first, third and fifth samples in the table), catalyst activity values (RA) were measured as 0.45, 0.46 and 0.48 respectively (average 0.46). For the two spent catalyst samples which were subjected to a plasma regeneration process at 30 Watts for 1 hour (the second and fourth samples in the table), catalyst activity values (RA) were measured as 0.38 and 0.40 respectively (average 0.39). As such, it can be noted that the spent catalyst materials regenerated using a plasma regeneration process at 30 Watts for 1 hour recovered 84% of the parent material activity. For the spent catalyst sample which was subjected to a plasma regeneration process at 20 Watts for 1 hour (the sixth sample in the table), a catalyst activity value (RA) of 0.45 was measured. As such, it can be noted that the spent catalyst materials regenerated using a plasma regeneration process at 20 Watts for 1 hour recovered 94% of the parent material activity. Within experimental error, the activity of the regenerated catalyst equates to that of the fresh parent catalyst samples. This suggests that the regeneration and burning of carbon deposits should be performed at low temperature and power to achieve regenerated catalyst material with the best performance. That is, the plasma power needs to be sufficient to remove carbon deposits and recover activity but not unduly high as to sinter and / or oxidize the catalyst material which can lead to a reduction in activity. It is also very interesting (and surprising) to note that the plasma-regenerated catalysts appear to be more selective to C5+ hydrocarbons with lower methane, CO2 and C2-C4 hydrocarbons when compared to the fresh parent catalyst samples (comparing selectivity values in the above table for fresh catalyst samples versus plasma regenerated spent samples). This is advantageous as typically the longer chain C5+ hydrocarbons are the desired products of a Fischer-Tropsch process. As such, in this respect the plasma regenerated catalysts appear to be advantageous over the fresh catalyst samples. 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 treating a Fischer-Tropsch catalyst used in a Fisher-Tropsch process for manufacturing hydrocarbon products, the method comprising:disposing the Fischer-Tropsch catalyst within a treatment gas; andsubjecting the Fischer-Tropsch catalyst to an electrically generated plasma treatment within the treatment gas to increase activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process.

2. A method according to claim 1,wherein the Fischer-Tropsch catalyst is a spent catalyst which has been used in the Fischer-Tropsch process and comprises deposits of carbon containing material, andwherein the electrically generated plasma treatment removes at least some of the deposits of carbon containing material to increase the activity of the Fischer-Tropsch catalyst in the Fischer-Tropsch process thereby regenerating the spent catalyst for re-use.

3. A method according to claim 1 or 2,wherein the Fischer-Tropsch catalyst comprises at least some oxide material, andwherein the electrically generated plasma treatment reduces at least some of the oxide material to metallic material to increase the activity of the Fischer-Tropsch catalyst prior to use in the Fischer-Tropsch process.

4. A method according to any preceding claim,wherein, during the electrically generated plasma treatment, the Fischer-Tropsch catalyst is maintained at a temperature of: no more than 500, 400, 300, 250, 200, or 150°C; no less than 0, 50, or 100°C; and / or within a range defined by any combination of the aforementioned upper and lower temperature limits.

5. A method according to any preceding claim,wherein the electrically generated plasma treatment is performed at a power, expressed as watts per gram of catalyst, of: no more than 1000, 500, 200,100, 50, or 30 W / g; no less than 0.01, 0.1, 1.0, 10 or 15 W / g; and / or within a range defined by any combination of the aforementioned upper and lower power limits.

6. A method according to any preceding claim,wherein the electrically generated plasma treatment is performed at a pressure of: no more than 10, 5, or 2 bar; no less than 0.001, 0.01, or 0.1 bar; and / or within a range defined by any combination of the aforementioned upper and lower pressure limits.

7. A method according to any preceding claim,wherein the electrically generated plasma treatment is performed for a time period of: no more than 24,10, 8, 2, 1, or 0.5 hours; no less than 10 seconds, 1 minute, 10 minutes, or 20 minutes; and / or within a range defined by any combination of the aforementioned upper and lower time limits.

8. A method according to any preceding claim,wherein the Fischer-Tropsch catalyst is subjected to a solvent treatment to remove some deposits of carbon containing material prior to the electrically generated plasma treatment.

9. A method according to any preceding claim,wherein the Fischer-Tropsch catalyst comprises no more than 1, 0.5, or 0.3 wt% of carbon containing material after the electrically generated plasma treatment.

10. A method according to any preceding claim,wherein the electrically generated plasma treatment increases surface area of the Fisher-Tropsch catalyst as measured by a chemisorption technique.wherein the surface area of the Fisher-Tropsch catalyst after the electrically generated plasma treatment is: at least 4.8 m2 / g, 5.0 m2 / g, 5.2 m2 / g, 5.4 m2 / g, or 5.6 m2 / g as measured by a chemisorption technique.

12. A method according to any preceding claim,wherein the treatment gas has an oxidizing gas content, optionally oxygen, of: no more than no more than 100, 50, or 21 vol%; no less than 0.1, 1, or 5 vol%; or and / or within a range defined by any combination of the aforementioned upper and lower limits.

13. A method according to claim 12,wherein the treatment gas is a mixture of oxygen and an inert gas, optionally nitrogen.

14. A method according to any preceding claim,wherein the Fischer-Tropsch catalyst is loaded into a catalyst carrier and the electrically generated plasma treatment is performed in-situ within the catalyst carrier.

15. A method according to claim 14,wherein the electrically generated plasma treatment is performed in-situ within the catalyst carrier before and / or after being used in the Fischer-Tropsch process.

16. A method according to any preceding claim,wherein the electrically generated plasma treatment is performed in-situ within a Fischer-Tropsch reactor.

17. A method according to any one of claims 1 to 13,wherein the Fischer-Tropsch catalyst is discharged from a catalyst carrier and / or a Fischer-Tropsch reactor prior to performing the electrically generated plasma treatment.

18. A method according to any preceding claim,wherein the Fischer-Tropsch catalyst comprises a cobalt material on a support material.

19. A method according to claim 18,wherein the support material is a metal oxide material, optionally titania or alumina, or silica, SiC, or Zirconia.

20. A method according to any preceding claim,wherein carbon dioxide is evolved during the electrically generated plasma treatment and the treatment is terminated after no more carbon dioxide is being evolved.

21. A method according to any preceding claim,wherein the treatment gas is a reducing gas, optionally comprising or consisting of hydrogen.

22. A method according to claim 21,wherein the treatment gas is a mixture of hydrogen and an inert gas, optionally nitrogen.

23. A method according to any preceding claim,wherein the electrically generated plasma treatment is performed in two steps: (i) an electrically generated plasma treatment in a reducing gas to reduce oxide material to metal in the Fischer-Tropsch catalyst; and (ii) an electrically generated plasma treatment in an oxidizing gas to remove carbon containing material from the Fischer-Tropsch catalyst.

24. A method according to claim 23,wherein step (i) is performed prior to initiating the Fischer-Tropsch process to activate the Fischer-Tropsch catalyst, andwherein step (ii) is performed after running the Fischer-Tropsch process to re-generate the Fischer-Tropsch catalyst.

25. A method according to claim 24,wherein one or both of steps (i) and (ii) are performed in-situ within a Fischer-Tropsch reactor.

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