Method of preparing C8 to C18 alkanes

Abstract

A method of preparing C8 to C18 alkanes comprising hydrogenation alkenes in a reactor. The reactor comprises a bulk catalyst bed and a minor catalyst bed, a means for supplying hydrogen along with fr

Description

The invention relates to a method of preparing C8 to Cl 8 alkanes. BACKGROUND OF THE INVENTION Jet fuel, also known as aviation turbine fuel (ATF), is a type of fuel designed for use in aircraft powered by gas-turbine engines. Jet fuel comprises a mixture of hydrocarbons, including a large proportion of C8 to Cl 8 alkanes. Sustainable aviation fuel (SAF) is a biofuel used as a more environmentally friendly alternative to jet fuel derived from fossil fuels. WO2022 / 063994A1 describes a method of manufacturing SAF from methanol. Many of the proposed or commercialised SAF flowsheets, including that set out in WO2022 / 063994A1, utilise the process steps of oligomerisation and then hydrotreatment. The hydrotreatment is typically operated under a high hydrogen partial pressure at temperatures of 300 °C or more and 30-40 bara across a catalyst comprising metal particles, such as platinum, palladium and / or nickel, on an alumina support. Commercial examples of such catalysts include A302099-5 (5% Pd on alumina), B301013-5 (5% Pt on alumina) and HTC nickel available from Johnson Matthey. The catalytic hydrogenation of the oligomerisation product, typically C8 to Cl 8 olefins, into the corresponding paraffins that are suitable for SAF or renewable diesel, is highly exothermic. The potentially high temperatures may result in losses to methane and / or thermal runaway. To control the temperature, large hydrogen recycles may need to be employed. This may involve recovering an unreacted hydrogen gas stream from the catalyst bed, cooling the unreacted hydrogen gas stream, for example using a heat exchanger, repressurising the unreacted hydrogen gas stream to a desirable operating pressure of the catalyst bed, for example using a costly compressor, and then recycling the cooled, repressurised stream to the catalyst bed. This may add cost and / or complexity to the method. US2014 / 0024863A1 describes a liquid / gas reactor for carrying out exothermic reactions, such as the hydrogenation of an aldehyde to an alcohol, the selective hydrogenation of a diene or an alkyne to an olefin, and the hydrogenation of the aromatic ring in an aromatic compound. Such reactions are typically less exothermic than the hydrogenation of C8 to Cl 8 olefins to paraffins. For example, the heat of hydrogenation of an aldehyde to an alcohol is typically in the range of -60 to -65 KJ / mol, whereas the heat of hydrogenation of an alkene to an alkane is typically in the region of -90 to -130 KJ / mol. The reactor of US2014 / 0024863A1 comprises an annular (bulk) catalyst bed and a core (minor) catalyst bed extending substantially vertically through the catalyst bed. The annular catalyst bed produces an at least partially converted liquid product stream. The annular catalyst bed receives hydrogen gas, fresh feed and at least partially converted liquid product stream, whereas the core catalyst bed receives hydrogen gas and at least partially converted liquid product stream only, i.e. no fresh feed. A product stream is recovered from the core catalyst bed. By recycling the at least partially converted liquid product stream, a temperature rise resulting from the exothermic nature of the reaction can be limited. This is because the concentration of reactive molecules is reduced. By having the core catalyst bed supplied with only feed that has already been subjected to the reaction, and will therefore be partially converted, the product stream recovered from the minor catalyst bed will be more fully converted than a product stream recovered from a reactor without the core catalyst bed. In the reactor of US2014 / 0024863A1, the hydrogen gas is fed to a shared vapour space above the core and annulus catalyst beds. The gas flow through each catalyst bed is determined by the pressure drop through each catalyst bed, and to achieve the same superficial gas flowrate in both annulus and core reactor these pressure drops need to be identical. This is in practice hard if not impossible to achieve as the pressure drop is a function of the particle size, bed voidage and liquid flow. The particle size of the catalyst may be different from the start as they may be different catalysts, both may come from a different batch or experienced slightly different history e.g. mechanical attrition due to movement and storage may result in a different particle size. The voidage of the two beds can be different because of differences in loading of the two catalysts in the two different zones or because the catalyst "settles" in use. It is quite conceivable that different catalyst loading techniques could be loaded for the two reaction zones, because they are a different size or shape, and one may be more difficult to access than the other. If the pressure drop in the core is higher than that in annulus, then the hydrogen flow to the core reaction zone may be significantly less. To address the pressure drop issue, WO2023161612A1 describes a similar reactor to that of US2014 / 0024863A1 but with separate hydrogen feeds for the core and annulus catalyst beds, respectively. This means that hydrogen feed to each catalyst volume can be independently controlled thereby allowing to feed the precise flow of hydrogen to both reactor volumes as desired to balance pressure drops / flows and reduce the size of a hydrogen recycle compressor if there is one. This gives better control on the reaction conditions and enables the operator to approach the maximum conversion. The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. In particular, the invention seeks to provide a method of preparing C8 to Cl8 alkanes by hydrogenation of corresponding alkenes that exhibits reduced risk of thermal runaway without the need to incur the costs associated with hydrogen recycle. SUMMARY OF THE INVENTION The present disclosure is directed to a method of preparing C8 to Cl 8 alkanes by hydrogenation of corresponding alkenes, the method comprising: providing a fresh feed comprising C8 to Cl 8 alkenes; supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream; supplying hydrogen gas to the bulk catalyst bed; allowing hydrogenation reactions to occur in the bulk catalyst bed; collecting an at least partially converted liquid product stream; recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed; supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor; supplying hydrogen gas to the minor catalyst bed; allowing hydrogenation reaction to occur in the minor catalyst bed; and collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C8 to Cl 8 alkanes, wherein the reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed; a separating wall between said bulk catalyst bed and said minor catalyst bed; a means for collecting a product stream from said minor catalyst bed, the product stream comprising C8 to Cl 8 alkanes; and a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. DETAILED DESCRIPTION OF THE INVENTION In a first aspect, the present disclosure is directed to a method of preparing C8 to Cl 8 alkanes by hydrogenation of corresponding alkenes, the method comprising: providing a fresh feed comprising C8 to Cl 8 alkenes; supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream; supplying hydrogen gas to the bulk catalyst bed; allowing hydrogenation reactions to occur in the bulk catalyst bed; collecting an at least partially converted liquid product stream; recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed; supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor; supplying hydrogen gas to the minor catalyst bed; allowing hydrogenation reaction to occur in the minor catalyst bed; and collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C8 to Cl 8 alkanes, wherein the reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed; a separating wall between said bulk catalyst bed and said minor catalyst bed; a means for collecting a product stream from said minor catalyst bed, the product stream comprising C8 to Cl 8 alkanes; and a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. Advantageously, in contrast to conventional methods of preparing C8 to Cl 8 alkanes by hydrogenation of corresponding alkenes, the method of the present disclosure may be carried out with a reduced risk of thermal runaway. Typically, the method may be carried out without the temperature increase across the reactor exceeding 40 °C, more typically without the temperature increase across the reactor exceeding 20 °C. Furthermore, such a reduced risk of thermal runaway may exist without the need to provide cooling by means of hydrogen recycling, or at least may enable lower levels of hydrogen recycling to be employed. As a result, the use of a compressor may be avoided, or the size of the compressor may be reduced, thereby reducing the cost and complexity of the method. The alkanes may be linear and / or branched. The alkanes may be cyclic and / or acyclic. The alkanes may comprise SAF, in particular SAF in compliance with ASTM D7566 and ASTM D4054. The alkanes may have a boiling point in the jet fuel range, i.e. in the range of about 130 to about 300 °C at atmospheric pressure. The method is to prepare C8 to Cl8 alkanes by hydrogenation of corresponding alkenes. As will be appreciated, different SAFs, having different requirements, may require different molecular masses. Accordingly, the invention may be, for example, a method of preparing C8 to Cl 6 alkanes by hydrogenation of corresponding alkenes (e.g. for kerosene-type jet fuel), or a method of preparing C5 to Cl 5 alkanes by hydrogenation of corresponding alkenes (e.g. for naphtha-type jet fuel), or a method of preparing alkanes having a boiling point in the jet fuel range. The alkenes may be linear or branched. The alkenes may be derived from oxygenates such as, for example, methanol, for example via a methanol-to-olefin (MTO) process followed by oligomerisation. The term “hydrogenation” as used herein may encompass the reaction between molecular hydrogen and another compound, in this case an alkene, to saturate the compound. The method is typically a continuous method, i.e. with the individual steps being carried out concurrently. The method comprises providing a fresh feed comprising C8 to Cl 8 alkenes. The term “fresh feed” as used herein may encompass a feed that has not yet passed over the bulk or minor catalyst beds. The fresh feed comprises C8 to Cl8 alkenes. The fresh feed preferably comprises at least 90 vol.% C8 to Cl 8 alkenes based on the total volume of the fresh feed, more preferably at least 95 vol.%, even more preferably at least 98 vol.%, still even more preferably at least 99 vol.%. Preferably, the fresh feed consists of or consists essentially of C8 to Cl 8 alkenes. The term “consists essentially of’ as used herein may encompass the situation where the feed contains species other than C8 to Cl8 alkenes, namely those not materially affecting the essential characteristics of the fresh feed. For example, in addition to C8 to Cl 8 alkenes, the fresh feed may comprise small amounts of one or more of water, oxygenates, aromatics, C2 to C7 alkenes and Cl 9+ alkenes. The feed is supplied to a bulk catalyst bed disposed within a reactor. The term “bulk” in relation to the bulk catalyst bed indicates that it has a greater volume than the “minor” catalyst bed. In other words, it occupies a greater volume and comprises a larger weight of catalyst than does the minor catalyst bed. The method comprises supplying hydrogen gas to the bulk catalyst bed. Accordingly, the hydrogen gas and the feed will be in contact with each other and with the catalyst of the bulk catalyst bed. The method comprises allowing hydrogenation reactions to occur in the bulk catalyst bed. In other words, allowing the hydrogen gas to saturate double bonds of the alkenes. The method comprises collecting an at least partially converted liquid product stream. By “at least partially converted” it is meant that the C8 to Cl 8 alkenes are either completely converted to C8 to Cl8 alkenes, or partially converted to alkenes. The term “partially converted” may encompass the situation in which only some of the alkenes are converted to alkanes, and / or in which only some of the double bonds of the alkenes are saturated, i.e. in which the alkenes comprise multiple double bonds and only some have been hydrogenated. The method comprises recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed. The at least a portion of said partially converted liquid product stream becomes part of the feed. In other words, recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed may comprise introducing the at least a portion of said partially converted liquid product stream to the feed. The method comprises supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor. Preferably, the ratio by volume of the partially converted liquid product stream recycled to the bulk catalyst bed to the partially converted liquid product stream supplied to the minor catalyst bed is from 1:1 to 10:1, more preferably from 3:1 to 6:1. The method comprises supplying hydrogen gas to the minor catalyst bed. Accordingly, the hydrogen gas and the feed will be in contact with each other and with the catalyst of the minor catalyst bed. The method comprises allowing hydrogenation reactions to occur in the minor catalyst bed. In other words, allowing the hydrogen gas to saturate double bonds of the alkenes. The method comprises collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C8 to Cl8 alkanes. The hydrogenated product stream preferably comprises at least 90 vol.% C8 to Cl8 alkanes based on the total volume of the hydrogenated product stream, more preferably at least 95 vol.%, even more preferably at least 98 vol.%, still even more preferably at least 99 vol.%. Preferably, the hydrogenated product stream consists of or consists essentially of C8 to Cl 8 alkanes. The term “consists essentially of’ as used herein may encompass the situation where the hydrogenated product stream contains species other than C8 to Cl 8 alkanes, namely those not materially affecting the essential characteristics of the hydrogenated product stream. For example, in addition to C8 to Cl8 alkanes, the fresh feed may comprise small amounts of one or more of water, oxygenates, C2 to Cl 8 alkenes, Cl9+ alkenes, C2 to C7 alkanes, Cl 9+ alkanes and aromatics. After being collected, the hydrogenated product stream may undergo one or more purification steps to remove certain species other than C8 to Cl 8 alkanes. The method is carried our using a reactor. The reactor is typically what is known as a liquid / gas reactor in that fluid passing through the reactor is in both the liquid phase and the gas phase. The fluid passes over a solid, that is, heterogeneous, catalyst in the reactor. The reactor and its components may be constructed of any suitable materials. Non-limiting examples of suitable reactors are those shown in Figure 3 and Figure 4 of WO2023 / 161612A1, the disclosure of which is hereby incorporated by reference. The reactor comprises a bulk catalyst bed. The catalyst of the bulk catalyst bed may comprise a single type of catalyst. In other words, the composition of the bulk catalyst bed may be substantially homogeneous. Alternatively, the bulk catalyst bed may comprise different types of catalyst along its length. For example, the bulk catalyst bed may comprise a relatively more reactive catalyst (e.g. a PGM catalyst) at the upstream end of the catalyst bed and a relatively less reactive catalyst (e.g. a Ni-containing catalyst) at the downstream end. The relatively more reactive catalyst may serve to initiate the reaction in the top (upstream) of the major catalyst bed meaning that the maximum temperature could be reached higher in the bed and thus reduce the overall size (volume of catalyst) of the bulk catalyst bed. Alternatively, if the feed has any minor components that could poison the general catalyst (leached from the oligomerisation catalyst or made in oligomerisation - Cl, S, metals etc.), then a sacrificial top layer (e.g. Ni-containing catalyst) could be used to capture these and hence protect the majority of the catalyst (e.g. PGM catalyst) below. The reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; and means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed. Suitable means for supplying, collecting and recycling are known in the art. The reactor comprises a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed. The minor catalyst bed is supplied only with feed which has already been subjected to reaction and is therefore at least partially converted. Consequently, the product stream exiting the minor catalyst bed will provide a more fully converted final product than a reactor without the minor catalyst bed. The minor catalyst bed may be disposed in any suitable location in the bulk catalyst bed, although it will be understood that the minor catalyst bed extends vertically through the bulk catalyst bed so that the inlet and outlet ends of the minor catalyst bed are not blocked by the bulk catalyst bed. In some embodiments, the minor catalyst bed is located in the centre of the bulk catalyst bed, such that the bulk catalyst bed forms an annulus around the minor catalyst bed. Alternatively, the secondary catalyst bed may be offset to the side of the bulk catalyst bed, or located against the wall of the reactor. The reactor may further comprise a means for cooling the recycled at least partially converted liquid product stream prior to supplying it to the bulk catalyst bed and to the minor catalyst bed. The means for cooling the recycled at least partially converted liquid product stream may comprise, for example, a heat exchanger. The reactor comprises a separating wall between said bulk catalyst bed and said minor catalyst bed. In some embodiments, the separating wall is formed from an insulating material. This may be particularly useful where the bulk and minor catalyst beds are operated at different temperatures. The separating wall may be of any suitable structure. For example, the separating wall may be formed by an internal pipe, in which the minor catalyst bed is located. The separating wall may be of any suitable cross-sectional shape, such as circular. Alternatively, the separating wall may be formed by a half pipe fastened to the wall of the reactor, for example. The reactor comprises a means for collecting a hydrogenated product stream from said minor catalyst bed, the product stream comprising C8 to Cl 8 alkanes. The reactor may comprise means for collecting the hydrogenated product stream from the outlet end of the minor catalyst bed. In some embodiments, the means for collecting the hydrogenated product stream from the outlet end of the minor catalyst bed includes a conduit for diverting the product stream from the minor catalyst bed to a receiving portion of the reactor, the receiving portion being isolated, at least in terms of liquid flow, from the outlet end of the bulk catalyst bed. The receiving portion may be a receptacle or a part of the reactor that is suitable for receiving the hydrogenated product stream exiting from the minor catalyst bed and keeping it separate from the product stream exiting from the bulk catalyst bed. For example, the receiving portion may comprise a baffle offset to one side of the bottom of the reactor, which acts as a weir flooded with product from the minor catalyst bed. Alternatively, the baffle may be combined with a roof which the conduit passes through, to create a closed receptacle for receiving the hydrogenated product from the minor catalyst bed; in such a case provision may be made for equalising vapour pressure between each side of the baffle and for overflow of product from the minor catalyst bed to the outlet end of the bulk catalyst bed. The reactor comprises a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. By providing means for supplying separate hydrogen gas streams to the bulk and minor catalyst beds, the flow of hydrogen gas to the bulk and minor catalyst beds are independent of each other and can optionally be individually controlled. This may allow the effect of non-identical pressure drops across the catalyst beds to be mitigated, resulting in a more consistent and improved overall conversion rate. Accordingly, in some embodiments the reactor comprises means for controlling the flowrate of the primary gas stream and the flowrate of the secondary gas stream individually. The flowrate of the gas streams may be controlled by any suitable means, such as by flow control valves. In one arrangement, the primary and secondary gas streams may be supplied to the reactor from separate sources with individually controlled flowrates. Alternatively, the primary and secondary gas streams may be supplied from a single source through a conduit which is branched to provide separate primary and secondary gas streams. In such embodiments, a flow control valve may be provided on each branch to control the flowrates of the primary and secondary gas streams individually. The gas flowrate should be sufficient to keep the liquid feed saturated with gas reactant across the entire area of both catalyst beds. Saturation can, for example, be determined by the formation of bubbles of excess gas reactant. Alternatively, or additionally, hydrogen levels in a vent flow can be analysed; if the hydrogen level is sufficiently high it can be inferred that the liquid feed was saturated with gas reactant. If either bed is determined not to be saturated with gas reactant, the flowrate of gas to that bed may be increased. The relative flow rates of hydrogen gas to the bulk catalyst bed and the minor catalyst bed may vary depending on, for example, the conversion target in the bulk catalyst bed, but may be, for example, from 15:1 to 25:1, preferably from 18:1 to 22:1. In some embodiments, the minor catalyst bed comprises a cover to isolate the inlet end of the minor catalyst bed from the inlet end of the bulk catalyst bed, with the secondary hydrogen gas and feed streams being supplied to the inlet end of the minor catalyst bed inside the cover. It will be understood that the cover defines a closed cavity above the inlet end of the minor catalyst bed, into which the secondary hydrogen gas and feed streams are supplied. As such, the cover may be in the form of a convexly curved plate or a dome, or may comprise either sidewalls and a roof or a single continuous sidewall and a roof. In some embodiments, the cover at least partly comprises an extension of the separating wall above the bulk and minor catalyst beds, such that the extension of the separating wall forms the sidewall(s) of the cover. In some embodiments, the cover comprises a removable cap. Providing a removable cap allows convenient access to the minor catalyst bed when required, for example to replace the catalyst. In some embodiments, the cover further comprises a gasket for creating a gastight seal with the removable cap. In some embodiments, the reactor comprises means for adjusting the temperature of the recycled product stream, e.g. a heater and / or a cooler. In some embodiments, the reactor comprises means for controlling the flowrate of the primary feed stream and the flowrate of the secondary feed stream individually. For example, the primary feed stream and the secondary feed stream may each be controlled by a flow control valve on their respective lines. The flowrate of the secondary feed stream supplied to the minor catalyst bed may be equal to the final product rate. However, for ease of control, an excess of up to 100% is preferably supplied. The excess may, for example, be combined with the recycled stream from the bulk catalyst bed. In a particularly preferred embodiment, the excess floods the weir and is thus combined with the output from the bulk reactor bed. Preferably the final product rate is then controlled to maintain a desired liquid level on the bulk catalyst bed side of the weir, by overflow of excess across the weir. Any suitable catalyst may be used in the bulk and minor catalyst beds, and may include, for example, active components selected from nickel, copper, cobalt, platinum, palladium, rhodium, ruthenium or any mixture thereof on a suitable catalyst support. The catalyst may also be of any suitable form, such as pellets, extrudates, resins, fibres or impregnated packing, for example. Suitable catalyst supports may, for example, include oxidic supports such as alumina, silica, vanadia, zirconia, or non-oxidic supports such as or carbon, polymer resin or silicon carbide. The catalyst used in the bulk and minor catalyst beds may be the same or different. The ratio of fresh feed to recycled at least partially converted liquid product stream by volume in the feed (i.e. the feed supplied to the bulk catalyst bed) is preferably from 1:7 to 1 :<10, more preferably from 8.5 to 9.5. Surprisingly, this may provide a favourable combination of particularly reduced risk of thermal runaway and a high conversion rate. Ratios lower than 1:7 (e.g. 1:66 in Example 4 of US2014024863 Al) may have an increased risk of thermal runaway. Ratios higher than 1:10 (e.g. 1:10 to 1:20 described on page 2, lines 10 to 12 of WO2023161612 Al in relation to the hydrogenation of alkene to alkane) may result in a slow conversion rate, thereby unfavourably extending the time of the method and decreasing its efficiency. Given the highly exothermic nature of the hydrogenation of alkenes to alkanes, it is surprising that ratios between 1:7 and 1 :<10 can be employed without an increased risk of thermal runaway. The bulk catalyst bed and / or the minor catalyst bed preferably operate at an inlet temperature from 90 to 150 °C. Lower temperatures may result in lower conversion rates. Higher temperatures may lead to unfavourable side reactions thereby reducing the purity of the final product. The bulk catalyst bed and / or the minor catalyst bed preferably operate at a pressure of from 30 to 80 bara. Lower pressures may result in lower conversion rates. Higher pressures may increase the cost of the method without any significant increase in conversion. Without being bound by theory, it is considered that increasing the pressure may increase the reactivity by forcing hydrogen and feed into pores of the catalyst. The partially converted liquid product stream is preferably cooled to 90 °C or lower before being recycled to the bulk catalyst bed and supplied to the minor catalyst bed. This may contribute to reducing the risk of thermal runaway. Such cooling may be carried out, for example, by the use of a heat exchanger. The catalyst of the bulk catalyst bed is preferably different to the catalyst of the minor catalyst bed. This may enable one to select combinations of different catalysts suitable for achieving a desired goal, for example reducing costs or increasing conversion rates. Since the reactor comprises a means for separately supplying hydrogen gas to the bulk catalyst bed and to the minor catalyst bed, different catalyst can be employed in the bulk and minor catalyst beds without issues relating to differences in back pressures between the two beds. The catalyst of the bulk catalyst bed preferably comprises nickel, more preferably nickel on an oxide support. The catalyst of the bulk catalyst bed preferably comprises a so-called “high-nickel” catalyst comprising from 10 to 35 wt.% Ni on an oxide support, for example from 13 to 17 wt.% Ni, or from 18 to 22 wt.% Ni, or from 28 to 32 wt.% Ni. An example if this catalyst is Ni HTC™ available from Johnson Matthey. Such a high-nickel catalysts may reduce aromatics present in the feed, i.e. by converting aromatics to cycloalkanes. Another suitable catalyst include nickel-impregnated silica fibres such as, for example, FlexCat™ available from Alkegen. Such nickel catalysts may be lower cost and may be less susceptible to poisoning than hydrogenation catalysts comprising platinum group metals. The catalyst of the minor catalyst bed preferably comprises: a platinum group metal, more preferably wherein the platinum group metal comprises platinum and / or palladium. The minor catalyst bed may may also comprise nickel, preferably nickel on an oxide support. Such a catalyst may be particularly suitable for catalysing the hydrogenation of C8 to Cl 8 alkenes to alkanes. A platinum group metal catalyst may exhibit higher activity than, for example, a nickel-containing hydrogenation catalyst. In a particularly preferred embodiment, the catalyst of the bulk catalyst bed comprises nickel, preferably nickel on an oxide support, and the catalyst of the minor catalyst bed comprises a platinum group metal, preferably wherein the platinum group metal comprises platinum and / or palladium. This may provide the benefit of the high activity platinum group metal catalyst together with reduced risk of poisoning of the platinum group metal catalyst and without incurring the high costs associated with employing a platinum group metal catalyst in both beds. The platinum group metal is preferably supported on alumina. This may increase the thermal resistance of the catalyst. The minor catalyst bed is preferably located such that it is central to the bulk catalyst bed which forms an annulus therearound. Preferably, all of the partially converted product stream is recycled with a portion being recycled to the bulk catalyst bed and a portion being passed to the minor catalyst bed. This may increase the yield of the method. Preferably, the ratio of the minor catalyst bed area to the bulk catalyst bed area will be from about half to about twice the ratio of the flowrates of recycle to the minor bed to the feed plus the recycle to the bulk bed. This may allow the minor catalyst bed to maintain a liquid velocity which gives good vapour / liquid mixing and good wetting, which will generally be of the same level as that achieved in the bulk catalyst bed. More preferably, the ratio is substantially 1:1. In a further aspect, the present disclosure is directed to a method of preparing alkanes having a boiling point in the jet fuel range by hydrogenation of corresponding alkenes, the method comprising: providing a fresh feed comprising alkenes (e.g. C8 to C18 alkenes, C8 to C16 alkenes, or C5 to Cl5 alkenes); supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream; supplying hydrogen gas to the bulk catalyst bed; allowing hydrogenation reactions to occur in the bulk catalyst bed; collecting an at least partially converted liquid product stream; recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed; supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor; supplying hydrogen gas to the minor catalyst bed; allowing hydrogenation reaction to occur in the minor catalyst bed; and collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising alkanes having a boiling point in the jet fuel range, wherein the reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed; a separating wall between said bulk catalyst bed and said minor catalyst bed; a means for collecting a product stream from said minor catalyst bed, the product stream comprising alkanes having a boiling point in the jet fuel range; and a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. In a further aspect, the present disclosure is directed to a method of preparing C8 to Cl 6 alkanes by hydrogenation of corresponding alkenes, the method comprising: providing a fresh feed comprising C8 to Cl6 alkenes; supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream; supplying hydrogen gas to the bulk catalyst bed; allowing hydrogenation reactions to occur in the bulk catalyst bed; collecting an at least partially converted liquid product stream; recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed; supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor; supplying hydrogen gas to the minor catalyst bed; allowing hydrogenation reaction to occur in the minor catalyst bed; and collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C8 to Cl 6 alkanes, wherein the reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed; a separating wall between said bulk catalyst bed and said minor catalyst bed; a means for collecting a product stream from said minor catalyst bed, the product stream comprising C8 to Cl 6 alkanes; and a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. In a further aspect, the present disclosure is directed to a method of preparing C5 to Cl 5 alkanes by hydrogenation of corresponding alkenes, the method comprising: providing a fresh feed comprising C5 to Cl 5 alkenes; supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream; supplying hydrogen gas to the bulk catalyst bed; allowing hydrogenation reactions to occur in the bulk catalyst bed; collecting an at least partially converted liquid product stream; recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed; supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor; supplying hydrogen gas to the minor catalyst bed; allowing hydrogenation reaction to occur in the minor catalyst bed; and collecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C5 to Cl 5 alkanes, wherein the reactor comprises means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed; a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed; a separating wall between said bulk catalyst bed and said minor catalyst bed; a means for collecting a product stream from said minor catalyst bed, the product stream comprising C5 to Cl 5 alkanes; and a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; and a means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

1. A method of preparing C8 to Cl 8 alkanes by hydrogenation of corresponding alkenes, the method comprising:providing a fresh feed comprising C8 to Cl 8 alkenes;supplying to a bulk catalyst bed disposed within a reactor a feed comprising the fresh feed and recycled at least partially converted liquid product stream;supplying hydrogen gas to the bulk catalyst bed;allowing hydrogenation reactions to occur in the bulk catalyst bed;collecting an at least partially converted liquid product stream;recycling at least a portion of said partially converted liquid product stream to the bulk catalyst bed;supplying at least a portion of said partially converted liquid product stream to a minor catalyst bed disposed within the reactor;supplying hydrogen gas to the minor catalyst bed;allowing hydrogenation reaction to occur in the minor catalyst bed; andcollecting a hydrogenated product stream from the minor catalyst bed, the hydrogenated product stream comprising C8 to Cl8 alkanes, wherein the reactor comprisesmeans for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed;means for collecting a partially converted liquid product stream from said bulk catalyst bed and recycling at least a portion thereof to said means for supplying fresh feed and recycled at least partially converted liquid product stream to said bulk catalyst bed;a minor catalyst bed extending substantially vertically through the bulk catalyst bed and means for supplying recycled at least partially converted liquid product stream only to said minor catalyst bed;a separating wall between said bulk catalyst bed and said minor catalyst bed;a means for collecting a product stream from said minor catalyst bed, the product stream comprising C8 to Cl 8 alkanes;a means for supplying a primary hydrogen gas stream only to the inlet end of the bulk catalyst bed; anda means for supplying a secondary hydrogen gas stream only to the inlet end of the minor catalyst bed.

2. The method of claim 1, wherein the ratio of fresh feed to recycled at least partially converted liquid product stream by volume in the feed is from 1:7 to 1 :<10, preferably from 8.5 to 9.5.

3. The method of claim 1 or claim 2, wherein the bulk catalyst bed and / or the minor catalyst bed operate at an inlet temperature from 90 to 150 °C.

4. The method of any preceding claim, herein the bulk catalyst bed and / or the minor catalyst bed operate at a pressure of from 30 to 80 bara.

5. The method of any preceding claim, wherein the partially converted liquid product stream is cooled to 90 °C or lower before being recycled to the bulk catalyst bed and supplied to the minor catalyst bed.

6. The method of any preceding claim, where the catalyst of the bulk catalyst bed is different to the catalyst of the minor catalyst bed.

7. The method of claim 6, wherein the catalyst of the bulk catalyst bed comprises nickel, preferably nickel on an oxide support.

8. The method of claim 6 or claim 7, wherein the catalyst of the minor catalyst bed comprises:a platinum group metal, preferably wherein the platinum group metal comprises platinum and / or palladium; and / ornickel, preferably nickel on an oxide support.

9. The method of claim 8, wherein the platinum group metal is supported on alumina.

10. The method of any preceding claim, wherein the minor catalyst bed is located suchthat it is central to the bulk catalyst bed which forms an annulus therearound.

11. The method of any preceding claim, wherein all of the partially converted product stream is recycled with a portion being recycled to the bulk catalyst bed and a portion being passed to the minor catalyst bed.

12. The method of any preceding claim, wherein the ratio of the minor catalyst bed area to the bulk catalyst bed area will be from about half to about twice the ratio of the flowrates of recycle to the minor bed to the feed plus the recycle to the bulk bed.

13. The method of claim 12, wherein the ratio is substantially 1:1.A