Catalytic reactor

This invention relates to a catalytic reactor suitable for use in performing gas phase reactions at elevated pressures, and particularly but not exclusively for performing endothermic reactions, and also to a chemical process using the catalytic reactor.

The use of catalytic material supported on a metal substrate is well known. For example <patcit id="pcit0001" dnum="GB1490977A"><text>GB 1 490 977</text></patcit> describes a catalyst comprising an aluminium-bearing ferritic alloy substrate, coated with a layer of a refractory oxide such as alumina, titania or zirconia, and then with a catalytic platinum-group metal. As described in <patcit id="pcit0002" dnum="GB1531134A"><text>GB 1 531 134</text></patcit> and <patcit id="pcit0003" dnum="GB1546097A"><text>GB 1 546 097</text></patcit>, a catalyst body may comprise substantially flat sheets and corrugated sheets of such material arranged alternately so as to define channels through the body, either several such sheets arranged in a stack, or two such sheets wound together to form a coil. In these examples both the flat sheets and the corrugated sheets have small-scale corrugations superimposed upon them to help in the formation of the coating. Such catalyst bodies are described as being suitable for use in treating exhaust gas from vehicles.

According to the present invention there is provided a catalytic reactor comprising a plurality of metal sheets arranged as a stack and bonded together, the sheets being shaped so as to define a plurality of first gas flow channels between adjacent sheets and to define a plurality of second gas flow channels between adjacent sheets, first gas flow channels alternating with second gas flow channels in the stack, and portions of each channel-defining sheet between gas flow channels being in contact with the adjacent metal sheet and so providing thermal contact, such that there is good thermal contact between fluids in the first and the second gas flow channels, and providing structural support such that fluids in the first and the second gas flow channels may differ in pressure; the first gas flow channels extending the entire width of the stack between opposed faces of the stack, and the second gas flow channels extending the entire width of the stack between opposed faces of the stack, and the orientation of the first gas flow channels being transverse to the orientation of the second gas flow channels; headers to supply fluids to the flow channels, the headers enabling different fluids to be supplied to the first and the second flow channels; and catalyst-carrying corrugated metal foils in each of the gas flow channels.

The good thermal contact between gases in adjacent flow channels is enhanced by sandwiching corrugated metal foil within each gas flow channel. This foil may also act as a carrier for the catalytic material. The adjacent metal sheets may be bonded together for example by diffusion bonding. To ensure the required good thermal contact, both the first and the second gas flow channels are preferably less than 5 mm wide in at least one direction transverse to the gas flow direction. More preferably both the first and the second gas flow channels are less than 2 mm wide in at least one such direction.

For example the sheets might be flat, with grooves machined across their surfaces to define gas flow channels. The reactor might therefore comprise a stack of such flat plates, the grooves in adjacent plates following different paths. The grooves themselves might be for example 20 mm wide, each groove accommodating a corrugated sheet or foil of material coated with catalytic material. To ensure that the gas flow channels are gas tight the plates are bonded together.

In use of the catalytic reactor, the gas mixture supplied to each flow channel is different from the gas mixture supplied to the adjacent channels, and the corresponding chemical reactions are also different. Preferably one of the reactions is endothermic while the other reaction is exothermic. In that case heat is transferred through the wall of the sheet separating the adjacent channels, from the exothermic reaction to the endothermic reaction.

Preferably the sheets themselves are also coated with suitable catalytic material.

This reactor is particularly suitable for performing methane/steam reforming (which is an endothermic reaction, generating hydrogen and carbon monoxide), and the alternate channels might contain a methane/air mixture so that the exothermic oxidation reaction provides the necessary heat for the endothermic reforming reaction. For the oxidation reaction several different catalysts may be used, for example palladium or platinum on a ceramic support; for example platinum on a lanthanum-stabilised alumina support, or palladium on zirconia. The preferred catalyst for the oxidation reaction is platinum on stabilised alumina. For the reforming reaction also several different catalysts may be used, for example nickel, platinum, palladium, ruthenium or rhodium, which may be used on ceramic coatings; the preferred catalyst for the reforming reaction is rhodium or platinum/rhodium on alumina. The oxidation reaction may be carried out at substantially atmospheric pressure, while the reforming reaction is preferably carried out at elevated pressure, for example up to 2 MPa (20 atmospheres), more typically 300 kPa or 500 kPa.

It will be appreciated that the materials of which the reactors are made are subjected to a severely corrosive atmosphere in use, for example the temperature may be as high as 900°C, although more typically around 750°C. The reactor may be made of a metal such as an aluminium-bearing ferritic steel, in particular of the type known as Fecralloy (trade mark) which is iron with up to 20% chromium, 0.5 - 12% aluminium, and 0.1 - 3% yttrium. For example it might comprise iron with 15% chromium, 4% aluminium, and 0.3% yttrium. When this metal is heated in air it forms an adherent oxide coating of alumina which protects the alloy against further oxidation. Where this metal is used as a catalyst substrate, and is coated with a ceramic layer into which a catalyst material is incorporated, the alumina oxide layer on the metal is believed to bind with the oxide coating, so ensuring the catalytic material adheres to the metal substrate.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> shows a longitudinal sectional view of a catalytic reactor which does not fall within the present invention;</li><li><figref idref="f0001">Figure 2</figref> shows a cross sectional view of the reactor of <figref idref="f0001">Figure 1</figref>;</li><li><figref idref="f0002">Figure 3</figref> shows a flow diagram of a chemical process that may be performed with the reactor of <figref idref="f0001">figures 1 and 2</figref>; and</li><li><figref idref="f0003">Figure 4</figref> shows a sectional view of plates stacked to form a catalytic reactor of the invention.</li></ul>

Referring to <figref idref="f0001">figure 1</figref> a catalytic reactor 10 (which does not fall within the present invention) consists of several nested concentric pressure tubes 12 of Fecralloy steel, each of wall thickness 0.5 mm (only four are shown in the figure, but the number of tubes 12 might in practice be say fifteen or sixteen). The innermost tube 12 contains an electrical heating element 14. As shown in <figref idref="f0001">Figure 2</figref>, the annular channels 15 between the tubes 12 locate foils 16 of corrugated Fecralloy steel whose corrugations are typically 2.0 mm high (peak to peak) with a pitch of 2.0 mm.

When all the tubes 12 and corrugated foils 16 have been assembled, the surfaces of the first, third, fifth etc. annular channels 15a are coated with a zirconia sol, and the surfaces of the second, fourth, sixth etc. annular channels 15b are coated with an alumina sol. This may be performed by temporarily blocking the end of one set of annular channels, for example with wax, and immersing the assembly in the appropriate sol. The assembly is then dried slowly, and then sintered, for example in an air furnace, raising the temperature to for example 1100°C over a period of four hours and then holding it at that temperature for a further four hours. After cooling the coated assembly, catalyst materials are then introduced for example in the form of a salt of the appropriate metal: palladium is introduced onto the zirconia coating in the channels 15a, and rhodium is introduced onto the alumina coating in the channels 15b in this example. The catalyst metals are then formed by a heat treatment to decompose (or reduce) the salt.

Annular end caps 18 are then laser welded onto the ends of each annular channel 15, each end cap 18 communicating with an inlet or outlet duct 20. The external diameter of the resulting reactor 10 is 50 mm, and it is of length 500 mm.

The reactor 10 is particularly suitable for performing steam/methane reforming, that is to say the reaction:         H₂O + CH₄ → CO + 3H₂

This reaction is endothermic, and is catalysed by the rhodium catalyst in the channels 15b. The heat required to cause this reaction may be provided by combustion of methane, that is to say:         CH₄ + 2O₂ → CO₂ + 2H₂O which is an exothermic reaction, and is catalysed by the palladium catalyst in the channels 15a. The heat generated by this combustion reaction is conducted through the walls of the tubes 12 into the adjacent channels 15b. Thus in use the reactor 10 is initially heated using the electrical heating element 14. A mixture of methane and air is then supplied to all the channels 15a at approximately atmospheric pressure, where it undergoes catalytic combustion. A mixture of steam and methane is supplied to the alternate channels 15b, where the steam/methane reforming reaction occurs; the steam and methane mixture is preferably at an elevated pressure, as this raises the mass flow rate and so enables a larger quantity of methane gas to be treated. For example these channels 15b may be at a pressure of 1 MPa.

The gas mixture produced by the steam/methane reforming can then be used to perform a Fischer-Tropsch synthesis, that is to say:         carbon monoxide + hydrogen → paraffin or olefin (say C₁₀) + water which is an exothermic reaction, occurring at an elevated temperature, for example 320°C, and an elevated pressure (e.g. 1.8-2.2 MPa) in the presence of a catalyst such as iron, cobalt or fused magnetite, with a potassium promoter. The exact nature of the organic compounds formed by the reaction depends on the temperature, the pressure, and the catalyst, as well as the ratio of carbon monoxide to hydrogen. The heat given out by this synthesis reaction may be used to provide at least part of the heat required by the steam/methane reforming reaction, for example a heat transfer fluid such as helium may be used to transfer the heat from a reactor in which the Fischer-Tropsch synthesis is occurring, the heat being used to preheat at least one of the streams of gases supplied to the reactor 10.

Referring now to <figref idref="f0002">figure 3</figref>, the overall chemical process is shown as a flow diagram. Most of the fluids are at an elevated pressure of 10 bar (1 MPa). The feed gas 24 consists primarily of methane, with a small percentage (say 10%) of ethane and propane at 10 bar. It is passed through a heat exchanger 25 so it is at about 400°C and is then supplied via a fluidic vortex mixer 26 to a first catalytic reactor 28; in the mixer 26 the feed gas is mixed with a stream of steam that is also at about 400°C and 10 bar, these streams entering the mixer 26 through tangential inlets and following a spiral path to an axial outlet so they become thoroughly mixed. The first part of the reactor 28 is a pre-reformer 29 with a nickel methanation catalyst at 400°C, in which the higher alkanes react with the steam to form methane (and carbon monoxide). The second part of the reactor 28 is a reformer 30 with a platinum/rhodium catalyst, in which the methane and steam react to form carbon monoxide and hydrogen. This reaction may be performed at 800°C, the heat being provided by combustion of methane over a palladium (or platinum) catalyst. The hot gases from the reformer 30 are then quenched by passing through a heat exchanger 31 to provide the hot steam that is supplied to the vortex mixer 26, and then through the heat exchanger 25 in which they lose heat to the feed gas.

The stream of carbon monoxide and hydrogen is then supplied to a third reactor 32 in which the carbon monoxide and hydrogen react, undergoing Fischer-Tropsch synthesis to form a paraffin or similar compound. This reaction is exothermic, preferably taking place at about 350°C, and the heat is used to preheat the steam supplied to the heat exchanger 31, using a heat exchange fluid such as helium circulated between heat exchange channels in the reactor 32 and a steam generator 33. During this synthesis the volume of the gases decreases, so this process is also performed at the elevated pressure of 10 bar. The resulting gases are then passed into a condenser 34 in which they exchange heat with water initially at 25°C. The higher alkanes (say C5 and above) condense as a liquid, as does the water, this mixture of liquids being passed to a gravity separator 35; the separated higher alkanes can then be removed as the desired product, while the water is returned via the heat exchangers 33 and 31 to the mixer 26. Any lower alkanes or methane, and remaining hydrogen, pass through the condenser 34 and are then supplied to a refrigerated condenser 36 in which the gases and vapours are cooled to about 5°C. The remaining gases, consisting primarily of hydrogen, carbon dioxide, methane and ethane, are passed through a pressure-releasing vent valve 37 to a flare 38. The condensed vapours, consisting primarily of propane, butane and water, are passed to a gravity separator 39, from which the water is combined with the recycled water from the separator 35, while the alkanes are recycled to the inlet of the Fischer-Tropsch reactor 32.

The temperature to which the vapours are lowered in the first condenser 34 determines the molecular weights of the alkanes that are condensed, and so emerge as the product. Hence by changing the temperature of the water supplied to the condenser 34 the characteristics of the product can be modified. The above reaction scheme relies on the steam/methane ratio being close to the stoichiometric requirement for the reformer 30, the rhodium catalyst being particularly resistant to coking; this has the benefit that negligible quantities of carbon dioxide are formed in the reformer 30, so that it is unnecessary to further treat the gases (using the reverse water gas shift reaction) to convert carbon dioxide back to carbon monoxide. It will also be appreciated that if the feed gas consists solely of methane, then the pre-reformer 29 may be omitted.

When used in this fashion the overall result of the processes is that methane is converted to higher molecular weight hydrocarbons which are typically liquids at ambient temperatures and pressures. The processes may be used at an oil or gas well to convert natural gas into a liquid hydrocarbon which is easier to transport.

It will be appreciated that the reactor 10 of <figref idref="f0001">Figures 1 and 2</figref> may be used for performing a variety of chemical processes, and that the catalyst within each channel 15 must be appropriate to the corresponding process.

Referring now to <figref idref="f0003">figure 4</figref> a reactor 40 of the invention comprises a stack of plates 42 each of Fecralloy steel, in this case the plates being 200 mm square and 3 mm thick (only parts of two plates are shown, in section, in the figure). Grooves 44 of width 8 mm and depth 2.5 mm extend across the entire width of each plate 42 parallel to one side, separated by lands 45 of width 3 mm, the grooves 44 being machined. A carrier foil 46 of Fecralloy steel 50 µm thick coated with a ceramic coating containing a catalyst material, and with corrugations 2.5 mm high, locates in each such groove 44. A stack of such plates 42 with the catalyst foils 46 is assembled, the orientation of the grooves 44 differing by 90° in successive plates 42, and is covered with a flat top plate of Fecralloy steel; the stack is then diffusion bonded together by heating the stack to a temperature in the range 600°C to 1200°C in an inert atmosphere. The stack of plates may be provided with headers either at this stage, or subsequently. Thus the gas flow channels are defined by the grooves 44, one set of channels extending from say right to left in the stack, and the other set of channels (in the alternate plates 42) extending from front to back of the stack.

It will be understood that the type of ceramic deposited on the corrugated foils 46 in the gas flow channels may be different in successive plates 42 in the stack, and that the catalyst materials may differ also. For example (as with the reactor 10 of <figref idref="f0001">figures 1 and 2</figref>) the ceramic might comprise alumina in one of the gas flows channels, and zirconia in the other gas flow channels.

Preferably, after diffusion bonding, the stack of plates 42 is then held at about 900°C while passing an oxidising gas stream through all the grooves 44 defining the gas flow channels. This promotes the formation of an alumina-rich oxide layer on the surfaces of the channels. After this oxidation step, the stack is cooled to room temperature, and an aqueous suspension of either alumina or zirconia sol is pumped through the grooves 44 and then allowed to drain out (so leaving a coating of sol on the walls of the channels); the viscosity of the sol suspension can be adjusted either by changing its pH or concentration, and the removal of excess sol may rely upon draining under gravity, or may require pumping, depending on the viscosity. The stack is then sintered in an oxidising atmosphere at a temperature of, for example, approximately 800°C, such that the alumina sol particles sinter onto the oxide layer on the surface of the Fecralloy steel so forming a ceramic catalyst-carrier layer. This layer is desirably of thickness in the range 10-50 µm, and the steps of coating with the appropriate sol and then sintering may be repeated, if necessary, to achieve the desired thickness. Finally a solution of an appropriate catalytic metal salt is pumped through the channels 44, and the stack is then dried, and thermally treated in a reducing (or oxidising) atmosphere to produce the desired form of dispersed catalyst metal on the ceramic carrier layer within the gas flow channels 44.

As with the reactor 10, the reactor formed from the plates 42 would be suitable for performing steam/methane reforming, for example using a rhodium catalyst. The heat required to cause this reaction may be provided by combustion of methane, which may be catalysed by a palladium catalyst. Because the plates 42 forming the stack are bonded together the gas flow channels are gas tight (apart from communication with headers at each end), and the pressures in the alternate gas flow channels may also be different, as mentioned in relation to the reactor 10.

It will be appreciated that the benefits of such narrow gas flow passages are that the diffusion path lengths are short, and that heat and mass transfer rates are increased because there is less effect of the boundary layer. Hence the rate of chemical reaction, which requires diffusion of the reacting species into contact with the catalytic surfaces, is enhanced, and also the rate of transfer of heat between the exothermic reaction and the endothermic reaction is also enhanced. Consequently such catalytic reactors can provide a high power density.

As described above, the ceramic coatings may be deposited from a material in the form of a sol, that is to say a dispersion containing particles with a particle size between 1 nm and 1 µm. For a particular sol, such as alumina sol, the way in which the sol is prepared determines the particle size. Some alumina sols have individual particles as the primary sol particles (so-called unaggregated), whereas some alumina sols have sol particles that are aggregates of smaller particles. In general, the aggregated type of sol will give a more porous ceramic coating than an unaggregated sol. Thus by selecting the type of sol used, or by mixing various amounts of different types of sol, the porosity of the ceramic coating can be controlled. The catalytic activity of the ceramic coating can be controlled by adjusting the porosity of the ceramic and the loading of the catalytic material. When making a catalytic reactor for performing a very exothermic reaction it may be desirable to adjust the catalytic activity along the flow path, for example to provide low catalytic activity initially, and higher catalytic activity further along the flow path, so as to prevent formation of hot spots. This may, for example, be appropriate in the case of reactors for performing Fischer-Tropsch synthesis. When using a zirconia sol to form a zirconia ceramic coating similar considerations apply; and in addition it may be desirable to include cations such as yttrium so as to form stabilized zirconia, particularly where the ceramic coating may reach high temperatures during operation, as stabilised zirconia provides a stable surface area.

Referring again to <figref idref="f0003">figure 4</figref> it will be appreciated that the gas flow channels 44 may vary in width and depth along their length, so as to vary the fluid flow conditions, and the heat or mass transfer coefficients, so as to control the chemical reactions at different places within the reactor 40. This is particularly applicable in a reactor for Fischer-Tropsch synthesis, in which the gas volume decreases, as by appropriate tapering of the channels 44 the gas velocity may be maintained as the reaction proceeds. Furthermore the pitch or pattern of the corrugated foils 46 may vary along a reactor channel 44 to adjust catalytic activity, and hence provide for control over the temperatures or reaction rates at different points in the reactor 40. The corrugated foils 46 may also be shaped, for example with perforations, to promote mixing of the fluid within the channels 44.