WO2015198066A1

WO2015198066A1 - Catalytic reactors

Info

Publication number: WO2015198066A1
Application number: PCT/GB2015/051868
Authority: WO
Inventors: Josephus Johannes Helena Maria Font Freide; Lary Lane KOCHER; Johannes Gerhardus Koortzen; Ross Alexander Morgan
Original assignee: Compact Gtl Plc
Priority date: 2014-06-27
Filing date: 2015-06-26
Publication date: 2015-12-30
Also published as: GB201411477D0; GB2527592A

Abstract

A catalytic reactor defines a multiplicity of first and second flow channels (17, 15) for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively. One of more of the first flow channels (17) contains particles (22) including catalyst material and may contain a structure or insert (20) that is embedded within the particles and that fully or partly transects the flow channel to assist heat transfer between the catalyst material and one or more first flow channel walls.

Description

CATALYTIC REACTORS

This invention relates to catalytic reactors and processes that use such catalytic reactors. Such catalytic reactors may be suitable for use in a chemical process to convert natural gas to longer-chain hydrocarbons, particularly but not exclusively for performing Fischer-Tropsch synthesis. The invention also relates to a plant including such a catalytic reactor.

A process is described in WO 01/51194 and WO 03/048034 (Accentus pic) in which methane is reacted with steam, to generate carbon monoxide and hydrogen in a first catalytic reactor; the resulting gas mixture is then used to perform Fischer-Tropsch synthesis in a second catalytic reactor. The overall result is to convert methane to hydrocarbons of higher molecular weight, which are usually liquids or waxes under ambient conditions. The two stages of the process, steam/methane reforming and Fischer-Tropsch synthesis, require different catalysts, and heat to be transferred to or from the reacting gases, respectively, as the reactions are respectively endothermic and exothermic.

Reactors for these reactions may be formed as a stack of plates, with flow channels defined between the plates, the flow channels for the different fluids alternating in the stack. In those channels that require a catalyst, this is described as being in the form of a metal substrate carrying the catalyst in a ceramic coating, such structures being removable from the channels when catalyst is spent, such as a corrugated foil with a catalyst-containing ceramic coating.

Although such a catalyst-carrying metal substrate provides some advantages, in some situations it would be desirable to provide a higher loading of catalytic material per unit volume of reactor channel. According to an aspect of the present invention there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively; wherein at least one of the first flow channels contains particles including catalyst material and a structure or insert that is embedded within the particles and/or that fully or partly transects the flow channel to assist heat transfer between the catalyst material and one or more (e.g. two or more, three or more, or four) first flow channel walls.

A further aspect of the invention provides a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively; wherein at least one of the first flow channels contains particles including catalyst material and a heat transfer structure or insert that transects and subdivides the flow channel into two or more passageways.

A further aspect of the invention provides a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively; wherein at least one of the first flow channels contains particles including catalyst material and a structure within the flow channel in contact with the particles to assist heat transfer to or between the catalyst material and at least two of the flow channel walls, wherein the particles are in contact with at least two of the flow channel walls.

A further aspect of the invention provides a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively; wherein at least one of the first flow channels contains particles including catalyst material and a structure within the flow channel in contact with the particles to assist heat transfer to or between the catalyst material and at least two of the flow channel walls, wherein substantially all (e.g. greater than or equal to 80%, greater than or equal to 90% or around 100%) of the surface of the or each structure or insert is in contact with a volume occupied by the particles. In other words, substantially all of the surface of the or each structure or insert is in contact with the particles or could be in contact with the particles if the particles were differently packed.

The provision of a structure within the flow channel that assists heat transfer to or from the catalyst material enables an effective solution for minimising the formation of hot spots in the reactor. Furthermore, the structure aids catalyst removal, especially under circumstances where the catalyst has been fouled causing the particles to agglomerate. In these circumstances, the structure can dislodge catalyst particles and thus aid the removal of these particles from the flow channel. The term "particles" is used to mean, for example, pellets, spheres, extrudates, trilobes, powders, granules, fibres, particulates, particulate solids or any solid components comprising the catalyst material that are suitable for use in the catalytic reactor. The particles may be arranged in a bed of particles. The bed may be a fixed bed.

The structure or insert may comprise one or more walls, which may be continuous and/or which may subdivide the first flow channel into two or more (e.g. a multiplicity of) discrete passageways. The or each structure or insert or wall or walls thereof may be continuous, or substantially continuous and/or non-porous, but may include one or more holes through its thickness. The holes, or perforations with which the insert may be provided aid mixing of the reagents within the flow channel. Furthermore, if one or more of the subchannels created by the structure suffers from a blockage or other catalyst fouling, then the presence of perforations within the structure will allow the gases to flow around the blockage. This may aid the minimisation of the pressure drop across the reactor as a whole. The or each structure or insert may comprise a unitary structure, which is preferably formed integrally, such as from a single sheet or foil, or formed by joining or bonding or welding, e.g. one or more portions or parts or walls together. Such an arrangement may be in preference to, for example, a structure or insert formed of a plurality of portions or parts or walls simply held together by the channel walls.

Additionally or alternatively, the structure or insert may be formed of one or more sheets, foils or plates, which may be corrugated.

The or each structure or insert or wall or walls thereof may have a thickness of less than or equal to 1 mm, or 800 μηι or less, or 500 μηι or less or 200 μηι or less, or 100 μηι or less. Preferably, the cross-sectional area of the or each structure or insert is less than or equal to half of the cross-sectional area of the first flow channel within which it is received, more preferably less than or equal to one third, most preferably less than or equal to one quarter, for example less than or equal to one tenth. For example, the total cross-sectional area of the passageways may be greater than or equal to the total cross- sectional area of the structure or insert. In some embodiments, the total cross-sectional area of the passageways is ten or twenty or fifty or one hundred times greater than or equal to the total cross-sectional area of the structure or insert.

The or each structure or insert or wall or walls thereof may be in direct contact with, or at least heat transfer contact with, opposed portions of the or its respective first flow channel, e.g. thereby subdividing the or its respective first flow channel. The or each structure or insert may comprise two or more walls, each of which may extend across the or its respective first flow channel and/or may be in direct contact with, or at least heat transfer contact with, opposed portions of the or its respective first flow channel, e.g. thereby subdividing the or its respective first flow channel.

Preferably, two or more of the first flow channels, more preferably most of the first flow channels and most preferably all of first flow channels comprise a respective structure or insert.

The at least one first flow channel may be subdivided into a plurality of passageways, for example four or more five or more, eight or more, or ten or more passageways. In some embodiments, the structure or insert comprises a substantially constant cross-sectional shape along the channel, but the orientation of the cross-sectional shape may vary along the length of the structure or insert. For example, the structure or insert may comprise a cross-section with a plurality of radial extensions, which may twist along the length of the structure or insert. Alternatively, the structure or insert may comprise a shaped sheet or foil or plate, e.g. folded or corrugated sheet or foil or plate.

The structure may comprise at least one thin section extending in a direction along the first flow channel and in a direction perpendicular thereto. Thus, obstruction of flow of the gas mixture can be minimised.

The structure may comprise at least one section extending from a central part of the cross section of the first flow channel to an outer part thereof. Thus, the section can provide a heat transfer path from the central to outer parts, and thence heat can be transferred to one or more of the second flow channels. The structure may comprise a plurality of sections spaced from one another in a direction along the first flow channel. Thus, a more even radial distribution of particles can be achieved, e.g. during loading of the reactor. According to a further aspect of the present invention there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively, wherein each first flow channel contains particles comprising catalyst material, the average diameter of the particles being greater than or equal to 100 μηι and less than or equal to 1000 μι ι.

The average diameter of the particles may be greater than or equal to 300 or 400 or 500 or 600 μηι and less than or equal to 1000 μηι. The average diameter of the particles may be greater than or equal to 300 or 400 or 500 or 600 μηι and less than or equal to 900 μηι. The average diameter of the particles may be greater than or equal to 300 or 400 or 500 or 600 μηι and less than or equal to 800 μηι. The average diameter of the particles may be greater than or equal to 300 or 400 or 500 or 600 μηι and less than or equal to 700 μηι. Thus, particles with the above-described dimensions can form a network which transfers heat more effectively to or from the catalyst material. This can be attributed to the fewer boundaries between particles in the heat transfer paths.

The term "diameter" is used to mean, for example, an equivalent spherical diameter. The average diameter may be a mass median diameter, a volume equivalent diameter, a hydraulic diameter, or any other suitable average diameter. The diameter may be measured using standard sifting techniques or in any other suitable way.

Moreover, the smallest dimension of the cross section of the first flow channel may be greater than or equal to 1 mm and less than or equal to 50 mm. The smallest dimension of the cross section of the first flow channel may be greater than or equal to 2 or 4 or 6 or 8 or 10 mm and less than or equal to 50 mm. The smallest dimension of the cross section of the first flow channel may be greater than or equal to 2 or 4 or 6 or 8 or 10 mm and less than or equal to 20 mm. The smallest dimension of the cross section of the first flow channel may be greater than or equal to 2 or 4 or 6 or 8 mm and less than or equal to 10 mm. The smallest dimension of the cross section of the first flow channel may be greater than or equal to 2 or 4 or 6 mm and less than or equal to 8 mm.

The smallest dimension of the cross section of the first flow channel may be the same or similar to the dimension of the cross section of the first flow channel that extends orthogonally to the smallest dimension. A reactor provided with at least first flow channels that are substantially square in cross section is particularly advantageous where an insert structure is used, either a catalyst-carrying insert or a solely heat transfer insert. Conversely, a reactor provided with elongate channels, in which a first dimension considerably exceeds the extent of the channel in the orthogonal direction, provides advantages when a particulate catalyst alone is used, i.e. with no insert for either heat transfer or catalysis. The elongation of the channel in the first direction reduces the total number of layers provided within a reactor of a given size and therefore the amount of metal used is also reduced. This results in considerable cost savings. This elongate dimension is balanced by the smaller extent of the channel in the orthogonal direction. This provides a reduced heat transfer distance and aids the structural integrity of the reactor. According to a further aspect of the present invention there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively, wherein each first flow channel contains particles comprising catalyst material, the ratio of the average diameter of the particles to the smallest dimension of the cross section of the first flow channel being greater than or equal to 1 :4 and less than or equal to 1 : 100.

The ratio may be greater than or equal to 1 :4 or 1 :6 or 1 :8 and less than or equal to 1 : 100. The ratio may be greater than or equal to 1 :4 or 1 :6 or 1 :8 and less than or equal to 1 :50. The ratio may be greater than or equal to 1 :4 or 1 :6 or 1 :8 and less than or equal to 1 :25. The ratio may be greater than or equal to 1 :4 or 1 :6 or 1 :8 and less than or equal to 1 : 12.

Thus, with the above-described particle sizes and channel dimensions or the above-described ratios therebetween, particles are large enough for acceptable heat transfer while being small enough to enable acceptable catalytic activity in the channel. According to a further aspect of the present invention there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively, wherein each first flow channel contains particles comprising catalyst material, the standard deviation of the diameters of the particles being less 200 μηι.

The standard deviation may be less than or equal to 100 or 50 or 25 μηι. Thus, the amount of smaller particles can be reduced and, thus, so too can the formation of hot spots due, for example, to the increased local reactivity and decreased thermal conductivity related to these smaller particles.

Moreover, the standard deviation may be greater than or equal to 10 or 25 or 50 μηι. Thus, the particles may pack together to form a bed with desired properties, e.g. porosity.

The standard deviation may be a geometrical standard deviation or any other suitable standard deviation. The standard deviation may be measured using standard sifting techniques or in any other suitable way.

According to a further aspect of the present invention there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively, wherein each first flow channel contains particles comprising catalyst material, the proportion of particles with a diameter of less than or equal to 50 μηι being less than or equal to 10%.

The proportion may be less than or equal to 5%, less than or equal to 1 % or less than or equal to 0.5%.

Thus, the formation of hot spots due to smaller particles can be reduced.

According to a further aspect of the present invention, there is provided a catalytic reactor defining a multiplicity of first and second flow channels for carrying a gas mixture which undergoes a chemical reaction, and a heat transfer fluid, respectively, wherein each first flow channel contains particles comprising catalyst material. The particles may be particles as specified above in relation to the fifth, sixth, seventh and/or eighth aspects of the present invention. Each first flow channel may contain a structure or insert as specified above in relation to the first, second, third and/or fourth aspects of the present invention.

Where the particles are arranged in a bed, they may be graded, for example with a thermally conductive material that is preferably inert (e.g. alumina, aluminium, copper, diamond powder, graphite or silicon carbide) and may be interspersed with the active catalyst or incorporated at the centre of the catalyst or within the catalyst particles.

The particles or bed or fixed bed of particles may have a thermal conductivity of greater than or equal to 0.5 watts per metre-kelvin (W m^"1 K^"1) or greater than or equal to 2 W m^"1 K^"1 or greater than or equal to 4 W m^"1 K^"1.

The activity of the catalyst material and/or the shape, configuration and material of the structure or insert may be adapted such that, in use, the rate at which heat is released in a region of the particles is less than or equal to the maximum rate at which heat transfers from the region. For example, the rate may be between 75 and 95% of the maximum rate.

The catalyst material may comprise cobalt. The cobalt load may be greater than or equal to 5 or 10 or 15 or 20 or 25% and less than or equal to 40%. The cobalt load may be greater than or equal to 5 or 10 or 15 or 20% and less than or equal to 25%. The cobalt load may be greater than or equal to 5 or 10 or 15% and less than or equal to 20%.

There may be one or two or more regions of the particles. Each region of the particles may include particles with different properties, catalysts with different activities, and/or a heat-transfer structure with different properties and/or no heat-transfer structure. In some embodiments, two or more regions may be arranged longitudinally along the flow path. In some embodiments, the passageways may provide the regions, while in other embodiments the or each first flow channel may comprise two or more structures or inserts each providing one or more of the regions, and in yet further embodiments the regions are provided by one or more groups of first flow channels. Thus, the properties of the region can be matched to the reactivity of the gas mixture in that region.

The first and second flow channels may be arranged alternately in the reactor.

Where the chemical reaction is exothermic, then the heat transfer fluid in the second flow channels may be a coolant fluid. Where the chemical reaction is endothermic, then the heat transfer fluid in the second flow channels may be a hot fluid, or may be a fluid which undergoes an exothermic reaction such as combustion in the second flow channels. By way of example the catalytic reactor may be for performing Fischer-Tropsch synthesis.

It should be understood that the Fischer-Tropsch reaction is a comparatively slow reaction. The purpose of the Fischer-Tropsch synthesis is to generate hydrocarbons in which the carbon chain is longer than that of methane, and indeed preferably at least C5 and so are normally liquids or waxes. A practical reactor must therefore generate a significant quantity of such longer-chain hydrocarbons per unit time, and should be selective towards the formation of such longer-chain hydrocarbons rather than methane. It has been found that providing a greater volumetric loading of active catalyst material can enhance both the conversion of CO to hydrocarbons, and also the productivity of the desired hydrocarbons.

The Fischer-Tropsch reaction is typically carried out at a temperature less than or equal to 300°C and typically about 200°C, so a wide range of materials may be selected for the reactor. For example the reactor may be made of an aluminium alloy, stainless steel, high-nickel alloys, or other steel alloys.

The structure or insert may similarly be of a wide range of different materials, including those of which the reactor may be made. The structure or insert may be formed of a metal. One suitable type of metal structure is a thin metal foil for example of thickness no more than 100 μηι, which may be corrugated, pleated or otherwise shaped so as to subdivide the flow channel, so defining a multiplicity of flow paths and also enhancing heat transfer within the channel. The metal structure may comprise copper or aluminium. At the point of reactor assembly, the or each structure may be sized to slide easily into the first flow channel. This aids the loading of the or each structure into the first flow channel. In order to effect this, the material from which the or each structure or insert is formed may be selected such that it has a greater thermal expansion co-efficient than that of the material forming the first flow channel into which it is to be inserted. For example, the flow channels may be formed from stainless steel, such as

Stainless 316 which has a typical thermal expansion coefficient of 16-18x10^"6 K^"1. In this case, it may be preferable to provide an aluminium insert as aluminium has a typical thermal expansion coefficient of 22-23x10^"6 K^"1, which, being greater than that of the steel reactor channel, may result in improved thermal contact in use.

In a further example, if the flow channels are formed from Inconel 625, which is particularly appropriate for higher temperatures, such as those required for Steam Methane Reforming, this has a thermal expansion coefficient of 12.8x10^"6 K^"1. In this case, it may be preferable to provide a structure or insert formed from Stainless 316, with typical thermal expansion coefficient of 16-18x10^"6 K^"1.

In each case, the structure(s) or insert(s) may be sized to have at least 5 μητι, or 10 μηι or 50 μηι clearance at room temperature, but come into contact with the opposed portions of the first flow channels when the reactor is at its operating temperature. The operating temperature will vary depending on the reaction taking place but may be in the region of 200°C in the first example when used for Fischer Tropsch synthesis.

Such a metal structure may be inserted into a flow channel of a reactor in which flow channels for the Fischer-Tropsch reaction alternate with flow channels to remove heat. The particulate catalyst material is also inserted into the flow channel, into flow paths defined in the metal structure. The metal structures may be removable from the channels in the module, and removal of the metal structure makes it easier to remove the particulate catalyst material so it can be replaced if the catalyst becomes spent. The flow paths defined by the metal structure may have any suitable cross-sectional shape. At least some of the flow paths may communicate with each other along their length, or alternatively the flow paths may all be separated from each other by the metal structure.

The length of the heat transfer structure or insert may be selected so that, in use, the flow channel and the heat transfer structure are coterminous. Alternatively, the heat transfer structure may be selected to exceed the length of the first flow channel when the reactor is at room temperature. This configuration provides a heat transfer structure that is easily removable and in turn aids the removal of the particulate catalyst from the first flow channel. The smallest dimension of the cross section of the first flow channel, e.g. the channel depth, may be between 1 mm and 15 mm, so the channel is large enough that the metal structure and the particulate catalyst can easily be inserted, while being sufficiently narrow for there to be good heat transfer. A preferred depth of the channel is no more than 10 mm, for example 7 mm or 5 mm or 3 mm. Desirably the temperature within the channels is maintained uniformly across the channel width, within about 2-4°C, and this is more difficult to achieve the larger the channel becomes. The channel depth can be measured in a direction orthogonal to a wall separating a first flow channel from a second flow channel. The reactor may comprise a stack of plates. For example, first and second flow channels may be defined by grooves in respective plates, the plates being stacked and then bonded together. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. The stack of plates forming the reactor is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing.

The catalyst-containing particles may be of a diameter between 0.1 mm and 2 mm, although clearly they must be sufficiently small to fit into the flow paths within the metal structure, or otherwise to fit within the channel. The particles are preferably of size between 0.1 mm and 1 mm, and as a general rule larger particles are preferable because they provide better heat transfer. Although not essential, there is a benefit in having all the particles of substantially the same size, for example having a standard deviation as specified above or varying in diameter by no more than a factor of 2, or varying by no more than a factor of 1.5, or varying by no more than a factor of 1.25. Smaller particles, which may be referred to as fines, may cause blockage, preventing through flow of the reactants and products, and may also lead to localised overheating.

The catalyst-containing particles may comprise a ceramic core, and the material of the ceramic may be selected from a wide range of inert ceramic materials, for example being silicon carbide, aluminium oxide, titanium dioxide, beryllium oxide, etc. It is generally desirable if the material of the ceramic has a comparatively high thermal conductivity, and therefore that the ceramic material is of low porosity. For example, dense cordierite has a thermal conductivity of about 2.0 W m^"1 K^"1 ; alumina (85%) has a thermal conductivity of about 12 W m^"1 K^"1 ; alumina (about 100%) has a thermal conductivity of about 29 W m^"1 K^"1 ; and beryllium oxide (about 100%) has a thermal conductivity of about 220 W m^"1 K^"1 ; magnesia (30% porous) has a thermal conductivity of about 7 W m^"1 K^"1; and sintered titania has a thermal conductivity of about 3 W m^"1 K^"1; the beta-polymorph of silicon carbide has a thermal conductivity of about 3 W m^"1 K^"1; each of these values is at 100°C.

A reactor may include within each first flow channel not only the metal structure to enhance heat transfer, and the bed of catalyst-containing particles, but also a gas- permeable catalyst-bearing structure with a metal substrate. Such a catalyst-bearing structure may for example comprise one or more metal foils coated with a catalyst- containing ceramic coating. For example a reactor may include a metal structure to enhance heat transfer and a bed of catalyst-containing particles at one end of each first flow channel, where a first catalytic activity is required, and a catalyst-bearing structure with a metal substrate at the other end of each first flow channel, where a different catalytic activity is required. A suitable metal for the metal substrate of such a catalyst- bearing structure is the aluminium-bearing steel alloy discussed above; this forms a secure bond to a ceramic coating for example of alumina or zirconia, which may be used to support a catalytically active material.

A further aspect of the invention provides a kit of parts for assembly into a chemical reactor as described above, the kit comprising one or more of the catalytic reactor or components thereof and/or a structure or insert receivable within one of the flow channels that is adapted to transect the flow channel when so received and/or particles including catalyst material. The kit may include any one or more additional features described above in relation to the other aspects of the invention and/or any further features that the skilled person would understand to be advantageous.

Hence a plant for processing natural gas to obtain longer chain hydrocarbons may incorporate a steam/methane reforming reactor, to react methane with steam to form synthesis gas, and a Fischer-Tropsch reactor to generate longer-chain hydrocarbons, and at least one of these reactors may be a reactor as described above.

A yet further aspect of the invention provides a method of processing a gas mixture, the method comprising:

a) providing a catalytic reactor with a flow channel within which is contained particles including catalyst material that is embedded within the particles and that transects the flow channel;

b) passing a gas mixture through a flow channel so that the gas mixture undergoes a chemical reaction; and

c) transferring heat between the particles and the flow channel wall or walls through the structure or insert.

The method may include one or more additional steps or features described above in relation to the other aspects of the invention and/or any further steps that the skilled person would understand to be advantageous.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 a shows a sectional view of part of a reactor suitable for Fischer-Tropsch synthesis;

Figure 1 b shows a sectional view of a modification to the reactor of Figure 1a;

Figure 2 shows a transverse sectional view of a metal structure for use in a reactor as shown in Figure 1 a or 1 b;

Figure 3 shows a transverse sectional view of an alternative metal structure for use in a reactor as shown in Figure 1a or 1 b;

Figure 4 shows a sectional view of part of another reactor suitable for Fischer- Tropsch synthesis; and

Figure 5 shows productivity and temperature variations in a reactor using two different sizes of Fischer-Tropsch catalyst particles.

The invention is of relevance to a chemical process for converting natural gas (primarily methane) to longer chain hydrocarbons. The first stage of this process may use steam reforming to form synthesis gas, that is to say the reaction of the type:

This reaction is endothermic, and may be catalysed by a rhodium or platinum/rhodium catalyst in a flow channel. The heat required to cause this reaction may be provided by combustion of an inflammable gas such as methane or hydrogen, which is exothermic and may be catalysed by a platinum/palladium catalyst in an adjacent second gas flow channel. The synthesis gas is then used to perform Fischer-Tropsch synthesis to generate a longer chain hydrocarbon, that is to say: n CO + 2n H₂ → (CH₂)_n + n H₂0 which is an exothermic reaction, occurring at an elevated temperature, typically between 190°C and 280°C, and an elevated pressure typically between 1.5 MPa and 2.5 MPa (absolute values), in the presence of a catalyst such as iron, cobalt or fused magnetite. A suitable catalyst for the Fischer-Tropsch synthesis comprises a coating of gamma- alumina of specific surface area 140-230 m²/g with about 10-40% cobalt (by weight compared to the alumina), and with a promoter such as ruthenium, platinum or gadolinium which is less than or equal to 10% the weight of the cobalt, and a basicity promoter such as lanthanum oxide.

The synthesis gas stream is cooled and compressed to the elevated pressure, say 2.0 MPa, and is then fed to a catalytic Fischer-Tropsch reactor. The Fischer-Tropsch reactor may be a compact catalytic reactor formed from a stack of plates as described above; the reactant mixture flows through one set of channels, while a coolant flows through the other set.

Referring now to Figure 1a there is shown a reactor block 10 suitable for use in performing Fischer-Tropsch synthesis, the reactor block 10 being shown in section and only in part. The reactor block 10 consists of a stack of flat plates 12 of thickness 1 mm spaced apart so as to define channels 15 for a coolant fluid alternating with channels 17 for the Fischer-Tropsch synthesis. The coolant channels 15 are defined by sheets 14 of thickness 0.75 mm shaped into flat-topped sawtooth corrugations. The height of the corrugations (typically in the range 1 to 4 mm) is 2 mm in this example, and correspondingly thick solid edge strips 16 are provided along the sides, and the wavelength of the corrugations is 12 mm (the arrangement being described in more detail below). The channels 17 for the Fischer-Tropsch synthesis are of height 5 mm (typically within a range of 2 mm to 10 mm), being defined by bars 18 of square or rectangular cross-section, 5 mm high, spaced apart by 80 mm (the spacing typically being in a range of 20 - 100 mm) and so defining straight through channels.

Within each of the channels 17 for Fischer-Tropsch synthesis is a heat transfer structure or insert in the form of a corrugated, 50 μηι thick foil 20 (typically of thickness in the range from 20-150 μηι) to enhance heat transfer (only two such foils 20 are shown); this foil 20 is corrugated. Each corrugation transects and subdivides the relevant channel 17, so it defines multiple flow paths 21 , which in this example are each of width about 2 to 3 mm and have substantially the same cross-sectional area. All the flow paths 21 are filled with a bed of small ceramic beads 22, in this case each of width 0.6 mm, which are of alumina impregnated with 25% (by weight) cobalt (the beads 22 are shown only in a few of the flow paths 21). The foil 20 is non-porous in this embodiment and forms a unitary, integral corrugated structure that enhances heat transfer between the particles in the centre of the channel and the walls of the channel 17.

The reactor block 10 may be made by stacking the components that define the channels 15 and 17, and then bonding them together for example by brazing or by diffusion bonding. The reactor block 10 is then turned through 90° so that the channels 15 and 17 are upright, and the corrugated foils 20 along with the beds of ceramic beads 22 are inserted into the channels 17.

Referring now to Figure 1 b there is shown an alternative reactor block 1 10 suitable for use in performing Fischer-Tropsch synthesis, the reactor block 1 10 being shown in section and only in part. In many respects the reactor block 1 10 resembles the reactor block 10, identical components being referred to by the same reference numerals. The reactor block 1 10 consists of a stack of flat plates 12 of thickness 1 mm spaced apart so as to define channels 15 for a coolant fluid alternating with channels 1 17 for the Fischer- Tropsch synthesis. The coolant channels 15 are defined in addition by sheets 14 of thickness 0.75 mm shaped into flat-topped sawtooth corrugations as described above, with solid edge strips 16. The channels 1 17 for the Fischer-Tropsch synthesis are sealed by solid edge bars 18 and are defined in addition by sheets 1 19 of thickness 1.0 mm shaped into castellations of height in the range of 4 mm to 12 mm, preferably 5 mm. In the preferred example the resulting channels 1 17 are of width 7 mm and of height 6 mm and extend straight through the stack from one face to the opposite face. As with the channels 15, 17 within the reactor block 10, the channels 15 and 1 17 in reactor block 110 extend in parallel.

Within each of the channels 117 for Fischer-Tropsch synthesis is a heat transfer structure or insert in the form of a zigzag-shaped thin foil 120 of thickness in the range from 20-150 μηι, preferably 50 μηι, that transects and subdivides the relevant channel 1 17 to enhance heat transfer (only three such foils 120 are shown). This foil 120 defines four flow paths 121 , all of which flow paths 121 are filled with a bed of small ceramic beads 122, each of width 0.7 mm, which are of silicon carbide impregnated with 25% (by weight) cobalt (the beads 122 are shown only in a few of the flow paths 121). The foil 120 in this embodiment is also non-porous and forms a unitary, integral corrugated or zigzag- shaped structure that enhances heat transfer between the particles in the centre of the channel and the walls of the channel 1 17.

Each plate 12 may for example be 1.3 m by 1.3 m, or 1.2 m by 0.8 m, so the channels 17 or 1 17 would be 1.3 m long or 0.8 m long, respectively. Preferably the channels 17 or 1 17 are no more than 1.5 m long, and preferably at least 0.3 m long. Although only a few layers of the stack are shown, reactor block 10 or 1 10 might in practice have ten, twenty or thirty layers containing the reaction channels 17 or 1 17, or as many as 100 such layers. The flat plates 12, the bars 18, and the castellated sheets 14 and 1 19 may be of a high-temperature steel alloy, or of an aluminium alloy, for example 3003 grade (aluminium with about 1.2% manganese and 0.1 % copper).

It will be appreciated that the size of the reaction channel 17 or 117 may differ from that described above. However, the reaction channels 17 or 1 17 are preferably at least 1 mm deep, preferably at least 2 mm deep, to provide adequate space for catalyst; and are preferably no more than 20 mm deep, more preferably no more than 10 mm deep, as it is difficult to ensure substantially uniform temperature throughout such a deep channel. It will be appreciated that a range of differently shaped metal inserts may be used within each reaction channel 17 or 117. The role of this metal insert is to enhance the transfer of heat away from the region of the catalyst bed of catalyst particles 22 or 122 where heat is generated, and to transfer that heat to the adjacent coolant channels 15. Although shaped foils 20 or 120 have been described above for this purpose, an alternative metal insert would be a flat foil which extends diagonally across the reaction channel 17 or 1 17 between diagonally opposite corners, so dividing the channel 17 or 1 17 into two triangular flow paths. Another alternative, as shown in Figure 2, would be a flat foil 30 extending across the centre plane of each reaction channel 17 or 117, with punched-out projecting tabs 31 extending above and below the foil to contact the walls at the top and bottom of each reaction channel 17 or 117.

Yet another alternative heat transfer structure or insert, shown in Figure 3, would be a metal strip or wire 40 with strips 41 projecting radially in multiple different directions, so as to contact different walls of the reaction channel; a single such metal wire 40 with radial strips 41 could be inserted into the channel 1 17, and a number of such metal wires

40 with radial strips 41 could be inserted side-by-side into the wider channel 17. There could be or fewer or more radial strips 41 than the six shown in the Figure. For example, there could be four radial strips 41 arranged in a 'cross' shape. Each of the radial strips 41 could extend the complete length of the insert. Alternatively, each of the radial strips 41 could be subdivided into a number of, e.g. 100, smaller radial strips which are spaced from each other along the length of the insert. The spaces between the strips can allow a more even radial distribution of the particles to be achieved, e.g. during loading of the reactor. Alternatively, the insert could include a number of, e.g. 400, smaller radial strips

41 which are arranged around the central structure 40 in a twisted or helical or random manner. The strips 41 may be bonded to or formed integrally with the wire 40.

The structure or insert in all of the embodiments shown include a constant cross- section along the length of the channel. While this may be preferred as it minimises the pressure drop of the gas passing through the channel in use, it may be preferable in some applications to vary the cross section. This may be achieved in the case of the insert 40, 41 , for example, by arranging the radial strips 41 around the wire 40 in a twisted or helical manner. Several other configurations would be apparent to the skilled person.

Referring now to Figure 4, there is shown another reactor block 210 suitable for use in performing Fischer-Tropsch synthesis, the reactor block 210 being shown in section and only in part. In many respects the reactor block 210 resembles the reactor block 10, identical components being referred to by the same reference numerals. The reactor block 210 consists of a stack of flat plates 12 of thickness 1 mm spaced apart so as to define channels 15 for a coolant fluid alternating with channels 217 for the Fischer- Tropsch synthesis. The coolant channels 15 are defined in addition by sheets 14 of thickness 0.75 mm shaped into flat-topped sawtooth corrugations as described above, with solid edge strips 16. The channels 217 for the Fischer-Tropsch synthesis are sealed by solid edge bars 18 and are defined in addition by sheets 219 of thickness 1.0 mm shaped into castellations of height in the range of 4 mm to 12 mm, preferably 5 mm. In the preferred example the resulting channels 217 are of width 7 mm and of height 6 mm and extend straight through the stack from one face to the opposite face. As with the channels 15, 17 within the reactor block 10, the channels 15 and 217 in reactor block 1 10 extend in parallel. The channels 217 for Fischer-Tropsch synthesis do not contain any heat transfer structures like the foils 20 or 120 in the reactor blocks 10, 110. All the flow paths 221 are filled with a bed of small ceramic beads 222, each of width 0.7 mm, which are of silicon carbide impregnated with 25% (by weight) cobalt (the beads 222 are shown only in a few of the flow paths 221). In this case, the ratio of the width of the ceramic beads 222 to the height of the channels 217 is 1 : 10. As regards the ceramic beads 22 or 122 or 222 which act as the catalyst carriers, it has been found that the size of these beads is a significant consideration. Larger particles, e.g. those with sizes greater than or equal to 100 μηι, can lead to more effective heat transfer through the bed of particles. Moreover, smaller particles tend to become hotter and/or transfer the heat less effectively, and fines can cause obstruction to the flow. It is therefore desirable to have a fairly narrow range of particle sizes. For example, a maximum standard deviation of particle sizes, e.g. 50 μηι, can be specified. Alternatively or additionally, a maximum proportion, e.g. 10%, of small particles, e.g. with a diameter of less than or equal to 10%, can be specified. The particle size distributions can be controlled and measured using standard methods such as sieving. For example in the reaction channels 217, which are of width 7 mm and of height 6 mm, the optimum particle size for the ceramic beads 222 is in the range 0.5 to 0.7 mm. Smaller particles, for example between 0.2 mm and 0.4 mm, tend to produce excessive pressure drop, whereas larger particles, for example between 0.8 mm and 1.0 mm lead to a reduction in the total quantity of catalytic material within the reaction channel 217, because so many fewer ceramic beads 222 can fit into the space. Thus, there may be an optimum ratio of the size of the particles to the depth of the channels, e.g. between 1 :8 and 1 :12.

It will be appreciated that the metal insert 20 or 120 and/or the particles or bed of particulate or ceramic beads 22 or 122 or 222 may extend the entire length of the reaction channel 17 or 1 17 or 227. Alternatively, a part of the length of the reaction channel 17 or 1 17 or 227 may be occupied by the metal insert 20 or 120 and/or the bed of particles or particulate or ceramic beads 22 or 122 or 222, while an adjacent part of the reaction channel 17 or 117 or 227 may also receive a gas-permeable catalyst-bearing structure (not shown) with a metal substrate. Such a catalyst-bearing structure may for example comprise one or more metal foils coated with a catalyst-containing ceramic coating; for example the catalyst-bearing structure may resemble the corrugated foil 20, differing only in that it also includes a ceramic coating, for example of alumina, which acts as a support for an active catalytic material such as cobalt. A suitable metal for the metal substrate of such a catalyst-bearing structure is the aluminium-bearing steel alloy Fecralloy™.

In some embodiments, reaction channels 17 or 1 17 or 217 may be divided longitudinally into several regions, each region having different properties, e.g. different catalytic activity, different thermal conductivities, etc. The activity of the catalyst material in each region may be adapted such that the rate at which heat is absorbed or released in the region is less than or equal to the maximum rate at which heat transfers to or from the region. For instance, the amount of catalyst or the concentration of active catalyst may be lower in a region near the inlet to the channel 17 or 117 or 217 (where the gas mixture is more reactive) and higher in a region near the outlet of the channel 17 or 1 17 or 217 (where the gas mixture is less reactive). Instead of discrete regions, the properties may vary gradually.

Referring to Figure 5, the productivity and temperature variations during operation of a single-channel reactor will now be described. The reactor comprises Fischer-Tropsch catalyst particles comprising cobalt and an alumina core. The cobalt load is 30%. The Figure shows data for two different sizes of the Fischer-Tropsch catalyst particles. In the first case, the particles have a distribution of sizes from 200 to 500 μηι. In the second case, the particles have a distribution of sizes from 500 to 700 μηι. The productivities of the reactor, i.e. grams of product per litre of reactor channel volume per hour, are similar in both cases, being around 200 grams per litre per hour. In the first case, the difference between the average temperature in the reactor and the maximum temperature in the reactor is around 1 °C (1 degree Celsius). In the second case, the difference between the average temperature in the reactor and the maximum temperature in the reactor is significantly more, i.e. around 4°C (4 degrees Celsius). Thus, the temperature gradients in the reactor are significantly less in the case of the larger particles (500 to 700 μηι) particles than in the case of the smaller particles (200 to 500 μηι). This can be attributed to better heat transfer through the bed of particles in the case of the larger particles.

Methods of processing a gas mixture using one or more features of the chemical reactor of the invention, as described above, would be readily appreciated by the skilled person and may include any number of steps or features described herein and/or any further steps that the skilled person would understand to be advantageous.

It should be realized that the foregoing example embodiments should not be construed as limiting. Other variations and modifications will be apparent to persons skilled in the art upon reading the present application.

For example, instead of the ceramic beads 22 or 122 or 222, the reactor blocks 10 or 110 or 210 may use other types of particles such as pellets, spheres, extrudates, trilobes, powders, granules fibres or particulates. These need not include ceramic materials, and need not include non-catalytic materials.

Moreover, the disclosure of the present application should be understood to include any novel features or any novel combination of features either explicitly or implicitly disclosed herein or any generalization thereof and during the prosecution of the present application or of any application derived therefrom, new claims may be formulated to cover any such features and/or combination of such features.

Claims

1. A catalytic reactor defining:

a multiplicity of first flow channels for carrying a gas mixture which undergoes a chemical reaction, and

a multiplicity of second flow channels for carrying a heat transfer fluid,

wherein each first flow channel contains particles comprising catalyst material, the average diameter of the particles being greater than or equal to 100 μηι and less than or equal to 1000 μηι.

2. A reactor as claimed in claim 1 , wherein the average diameter of the particles is greater than or equal to 100 μηι and less than 500 μηι.

3. A reactor as claimed in claim 1 or claim 2, wherein the average diameter of the particles is greater than or equal to 100 μηι and less than or equal to 300 μηι.

4. A reactor as claims in claim 1 or claim 2, wherein the average diameter of the particles is greater than or equal to 300 μηι and less than 500 μηι.

5. A reactor as claimed in claim 1 , the proportion of particles with a diameter less than or equal to 50 μηι being less than or equal to 10%.

6. A reactor as claimed in claim 5, wherein the proportion is less than or equal to 5%.

7. A reactor as claimed in claim 5, wherein the proportion is less than or equal to 1 %.

8. A reactor as claimed in claim 5, wherein the proportion is less than or equal to 0.5%.

9. A reactor as claimed in any one of the preceding claims, wherein the standard deviation of the diameters of the particles is less than or equal to 200 μηι.

10. A reactor as claimed in claim 9, wherein the standard deviation is less than or equal to 100 μηι.

11. A reactor as claimed in claim 9, wherein the standard deviation is less than or equal to 50 μηι.

12. A reactor as claimed in claim 9, wherein the standard deviation is less than or equal to 25 μηι.

13. A reactor as claimed in any one of the preceding claims, wherein the ratio of the average diameter of the particles to the smallest dimension of the cross section of the first flow channel is less than or equal to 1 :4 and greater than or equal to 1 : 100.

14. A reactor as claimed in claim 13, wherein the smallest dimension of the cross section of the first flow channel is greater than or equal to 1 mm and less than 15 mm.

15. A reactor as claimed in claim 14, wherein the smallest dimension of the cross section of the first flow channel is less than 10 mm.

16. A reactor as claimed in any one of the preceding claims, further comprising a non- catalytic heat transfer structure or insert that is embedded within the particles and that fully or partly transects the flow channel to assist heat transfer between the catalyst material and the first flow channel wall or walls.

17. A reactor as claimed in claim 16, wherein the structure or insert comprises one of more continuous walls that subdivide the first flow channel into two or more discrete passageways.

18. A reactor as claimed in claim 17, wherein each of the two or more discrete passageways comprises substantially the same cross-sectional area.

19. A reactor as claimed in any one of claims 16 to 18, wherein the particles are in contact with at least two of the flow channel walls.

20. A reactor as claimed in any one of claims 16 to 19, wherein the or each wall has a thickness of less than or equal to 1 mm.

21. A reactor as claimed in any one of claims 16 to 20, wherein the or each structure or insert comprises a unitary structure.

22. A reactor as claimed in any one of claims 16 to 21 , wherein the structure or insert is formed of one or more sheets, foils or plates.

23. A reactor as claimed in claim 22, wherein at least one of the one or more sheets or foils or plates is corrugated.

24. A reactor as claimed in any one of claims 16 to 23, wherein the cross-sectional area of the or each structure or insert is less than or equal to half of the cross-sectional area of the first flow channel within which it is received.

25. A reactor as claimed in any one of claims 16 to 24 wherein the material from which the or each structure or insert is formed has a greater thermal expansion co-efficient than that of the material forming the first flow channel into which it is received.

26. A reactor as claimed in claim 25 wherein the or each structure or insert is sized to have at least 5 μηι clearance at room temperature, but to come into contact with the opposed portions of the first flow channels when the reactor is at its operating

temperature.

27. A reactor as claimed in any one of claims 16 to 26, wherein the structure is a metal structure or a metal structure comprising an aluminium alloy, stainless steel, high- nickel alloy, a steel alloy, copper or aluminium.

28. A reactor as claimed in any one of claims 16 to 27, wherein the structure comprises at least one section extending from a central part of the cross section of the first flow channel to an outer part thereof.

29. A reactor as claimed in any one of claims 16 to 28, wherein the structure comprises a plurality of sections spaced from one another in a direction along the first flow channel.

30. A reactor as claimed in any one of claims 17 to 29, wherein substantially all of the surface of the or each structure or insert is in contact with a volume occupied by the particles.

31. A reactor as claimed in any one of the preceding claims, wherein the catalyst material comprises cobalt and the cobalt load is between 20% and 40% by weight.

32. A reactor as claimed in any one of the preceding claims comprising two or more regions respectively comprising particles with different properties, catalyst materials with different activities and/or a different structure to assist heat transfer to or from the catalyst material in the region or no such structure.

33. A reactor as claimed in any one of the preceding claims, wherein the catalyst- containing particles comprise a ceramic core of an inert ceramic material.

34. A reactor as claimed in any one of the preceding claims, wherein each first flow channel further comprises a gas permeable catalyst-bearing structure with a metal substrate.

35. A reactor as claimed in any one of the preceding claims, wherein the first and second flow channels are arranged alternately.

36. A plant for processing natural gas to obtain longer chain hydrocarbons comprising a steam/methane reforming reactor, to react methane with steam to form synthesis gas, and a Fischer-Tropsch reactor to generate longer-chain hydrocarbons, wherein at least one of these reactors is a reactor as claimed in any one of the preceding claims.

37. A method of processing a gas mixture, the method comprising:

providing a catalytic reactor with a flow channel within which are contained particles including catalyst material and a non-catalytic heat transfer structure or insert that is embedded within the particles and that transects the flow channel;

passing a gas mixture through the flow channel so that the gas mixture undergoes a chemical reaction; and

transferring heat between the particles and the flow channel wall or walls through the structure or insert.