GB2441983A

GB2441983A - Catalytic structures for use in catalytic reactors

Info

Publication number: GB2441983A
Application number: GB0613935A
Authority: GB
Inventors: Anthony Henry Reading
Original assignee: AEA Technology PLC; CompactGTL PLC
Current assignee: CompactGTL PLC
Priority date: 2006-07-14
Filing date: 2006-07-14
Publication date: 2008-03-26
Also published as: GB0613935D0

Abstract

A catalytic structure incorporating a corrugated foil is made by a sequence of stamping steps, the number of contiguous corrugations being increased by each stamping step. The corrugated foil may be coated with a precursor for a ceramic coating using a piezoelectric ink jet printer to produce a uniform coating rapidly. This is dried and calcined; an active catalytic material is then deposited using a piezoelectric ink jet printer, and is activated by subsequent heat treatments.

Description

Catalytic Structures for Use in Catalytic Reactors This invention

relates to a process for producing catalytic structures, particularly but. not exclusively comprising a metal foil, for use in a catalytic reactor, and to the catalytic structures so made.

A process is described in WO 01/51194 and WO 03/033131 (Accentus plc) 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 longer chain 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 catalytic reactors are described for each stage. In each case the catalyst may comprise a corrugated foil coated with catalytic material. In each case the foils may be of an aluminium-containing steel, and the corrugations may be for example 2.5 mm high. A reactor for large productivity will have a multiplicity of channels containing such catalyst structures, and there is consequently a requirement to simplify the production of the necessary catalyst structures.

According to the present invention there is provided a process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, and forming the foil into corrugations by a plurality of stamping steps, the said corrugations extending parallel to each other, the number of contiguous corrugations along a line transverse to the orientation thereof being increased by each said stamping step.

A known technique for making corrugated foils entails passing the foil between rollers, but this rolling technique tends to put a curve into the resulting foil. This is not satisfactorily where the foils are to be inserted into a narrow channel between flat upper and lower surfaces, and the stamping process of the invention avoids this problem, and also can produce the corrugated foils more rapidly. Increasing the number of corrugations in successive stamping steps enables foils to be shaped which are of a material of comparatively low ductility.

In a second aspect the present invention provides a process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, cutting out foil blanks by chemical milling, and then subjecting resulting foil blanks to a corrugating process.

This chemical milling process can produce foil blanks whose dimensions are very accurate: accuracy to within 25 nm can be achieved. Although this accuracy is not essential, it has been found to be desirable to provide foil dimensions with an accuracy of better than 0.3 mm, more preferably better than 0.2 mm, and most preferably better than 0.1 mm to ensure that the resulting corrugated foil fits precisely within a reactor channel, to ensure that channelling (that is to say significant flow through a bypass channel between an edge of the foil and a channel wall, rather than along the channels defined by the corrugated foil) does not occur.

The catalyst structure preferably incorporates a ceramic coating to carry the active catalytic material.

Preferably the metal substrate for the catalyst structure is a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example an aluminium-bearing ferritic steel such as iron with 15% chromium, 4% aluminium, and 0.3% yttrium (eg Fecralloy (TM)). When this metal is heated in air it forms an adherent oxide coating, which protects the alloy against further oxidation and against corrosion. Where the ceramic coating is of alumina, this appears to bond to the oxide coating on the surface. The substrate may be a wire mesh or a felt sheet, but the preferred substrate is a thin metal foil for example of thickness less than 100 tm.

But in an alternative the metal substrate may be of a different type of stainless steel, for example one that forms a surface oxide of chromia when subjected to heat treatment in air, which again protects the alloy against corrosion, and enhances adhesion of the ceramic coating.

The corrugations of the foil may take a wide variety of different cross-sectional shapes, for example sinusoidal corrugations, circular arcs, circular arcs linked by straight portions, or square or rectangular castellations; in each case the corrugations define a multiplicity of parallel flow paths. Other shapes of corrugations are also possible.

Preferably all the surfaces forming the catalyst structure incorporate catalytic material.

In another aspect the present invention provides a process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, forming corrugations along the foil, and then coating the corrugated surface with a precursor for a ceramic coating using a piezoelectric ink jet printer. This is particularly suitable for providing ceramic coatings that are not required to have large pores, where the precursor for the ceramic coating does not contain particulate material larger than 2 zm, preferably no larger than 1 sum, and more preferably only dispersible material. It can enable uniform coatings to be deposited over the corrugated surface.

In another aspect the invention provides a process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, forming corrugations along the foil, coating the corrugated surface with a ceramic coating, and then depositing an active catalytic material or a precursor therefor using a piezoelectric ink jet printer.

It will also be appreciated that an aspect of the present invention is a catalyst structure made by such a process. Where the channel depth in a reactor is no more than about 3 mm, then the catalyst structure to be inserted into the channel may for example be a single shaped foil. Alternatively, and particularly where the channel depth is greater than about 2 mm, the catalyst structure may comprise a plurality of such shaped foils separated by substantially flat foils; the shaped foils and flat foils may be linked to each other, or alternatively may be inserted as separate items. To ensure the required good thermal contact with adjacent channels, for example with a Fischer-Tropsch reactor, the channels are preferably less than 20 mm deep, and more preferably less than 10 mm deep, and for a steam/methane reforming reactor the channels are preferably less than 5 mm deep. But the channels are preferably at least 1 mm deep, or it becomes difficult to insert the catalyst structures, and engineering tolerances become more critical. 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 invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: Figures la to ic show successive cross-sectional views of a foil during the formation of corrugations.

The invention is applicable to a wide range of different chemical reactions, particularly those involving gaseous reactants and requiring a catalyst. For example it would be applicable in a chemical process for converting natural gas (primarily methane) to longer chain hydrocarbons. This can be achieved by a two-stage process, and each stage might use a reactor incorporating a catalyst structure of the invention. The first stage is to make a synthesis gas for example by steam reforming, in which steam is mixed with natural gas and heated to an elevated temperature (so as to reach say 800 C) so that reforming occurs: H20+CH4 -* CO+3H2 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 gas mixture produced by the steam/methane reforming can then be used to perform a Fischer-Tropsch synthesis to generate a longer chain hydrocarbon, that is to say: nCO+2nH2 -(CH2)+nH2O which is an exothermic reaction, occurring at an elevated temperature, typically between 190 C and 280 C, and an elevated pressure typically above 1.5 MPa (absolute value), in the presence of a catalyst such as cobalt. It will be appreciated that all three of these reactions -reforming, combustion and synthesis -may be carried out using a compact catalytic reactor with flow channels in which catalyst structures of the invention are located, in each case the catalyst structure incorporating an appropriate catalytic material f or the respective reaction. In each case there may be a corrugated metal foil with an alumina coating providing the support for the catalytic material.

Suitable catalyst structures may be made from a steel alloy such as an aluminium-bearing ferritic steel, or a stainless steel, the alloy not containing any elements which act as a poison for the catalyst. This may be achieved either by using an alloy whose composition does not contain such an element, or that the element, although present, does not diffuse out of the metal to reach the active catalytic sites. Preferably the metal is in the form of a foil of thickness no more than 0.1 mm, for example of thickness 50 m. Such a foil may be produced by a rolling process, as is known. The foil is first formed into blanks of a suitable size to make a catalyst structure that fits the reactor channels into which it is to be inserted. The blanks are preferably made by a chemical milling or etching process, the desired area being coated with a suitable masking material, and this being a photoresist material so that the regions of the surface that are to become the edges of the blanks can be defined by a photographic technique.

For example the blanks might be rectangular in shape, typically of the same length as the reactor channel (eg 600 mm), and of width say 30 mm; this photoresist method enables the blanks to be cut to size to an accuracy better than 0.1 mm.

The blanks are then formed into corrugations running parallel to the longer side and of amplitude 2.0 mm. As shown in figure 1, which shows successive stages in the formation of such corrugations, the corrugations are formed by a stamping process between opposed dies. In this example a foil 10 of initial width 30 mm (Figure la) is first subjected to a stamping process to form a ridge 12 and two troughs 13 extending along the centreline by dies 20 and 21 (see Figure lb). In a second stamping process the initial ridge 12 and troughs 13 are clamped between the dies 20 and 21, and the outer parts of the foil 10 are each formed into two ridges 14 and an intervening trough 15 by dies 23 and 24 (see Figure ic) The resulting corrugated foil 10 is of width about 20 mm.

The corrugated foil 10 is preferably then thoroughly cleaned, for example using an etching solution, and rinsed and dried, and subjected to a heat treatment to ensure development of an oxide layer on the surface.

The subsequent steps are described in relation to a catalyst for reforming or for combustion, for example.

The corrugated foil 10 is then coated with a ceramic coating using an ink jet printer. A desired coating thickness of ceramic can be built up on the foil substrate by several successive spraying and drying steps, so that for example the final thickness of the ceramic layer may be in the range 30 to 200 jim on each side of the substrate. The droplets contain alumina as a sol, i.e. dispersible alumina, which has a primary particle size of about 15 rim and which forms a colloidal sol in water, so the resulting ceramic has a mainly mesoporous character (i.e. the pores are no larger than nm), subject to any sintering that occurs during calcination. For example the droplets may comprise Aerodisp W630 (trade mark) alumina suspension. Such a mesoporous ceramic layer is suitable for a catalyst support with reactions such as combustion or reforming.

After the ceramic precursor has been coated onto the foil 10, it is then dried and gradually heated up to a temperature of 950 C to ensure that the ceramic is firmly adhered to the foil and is thermally stable up to that temperature. This drying and initial calcining is carried out in a stream of dry air, at least initially, to remove any water vapour and so prevent hydrothermal damage to the ceramic.

The active catalytic material, platinum/rhodium for reforming or platinum/palladium for combustion, is then incorporated into the ceramic by impregnating the ceramic with a solution of a suitable salt, this then being dried and calciried, and finally reduced to the metallic form (if necessary) by contacting with a reducing atmosphere at elevated temperature. If necessary the impregnation, drying and calcining (to convert the catalytic material to an oxide form) may be repeated a number of times to achieve a desired loading of the active catalytic material, prior to the reduction step. The impregnation steps can also be performed using an ink jet printer to ensure uniformity over the surface of the corrugated foil 10, or to obtain a predetermined non-uniformity (for example a gradually varying concentration along the length of the foil) The above production process is given by way of example only, and may be modified in various aspects while remaining within the scope of the present invention. For example, rather than cutting out the foil blanks by chemical milling, they may be cut out by a different process, for example as part of the stamping mechanism. The preferred cutting process depends on the thickness of the foil and on the material of which it is made. For example a 50 m thick foil of Fecralloy (TM) steel that has been annealed can be cut with a guillotine. The size of the foils, in particular their width and hence the number of corrugations across the width of the foil, may affect the number of stamping steps required to corrugated the foil. For example the procedure may utilise additional pairs of dies 20 and 21 outside the dies 22 and 23 to corrugate a wider foil, producing an additional peak 12 on each side. After the foils have been corrugated and oxidised, a multiplicity of foils may be mounted in a support frame, and subjected to the subsequent process steps -the deposition of the ceramic support, and the incorporation of the active catalytic material -without being removed from the support frame. This avoids the need to handle the individual foils. The nature of the ceramic coating and of the active catalytic metal may be different from those described above, for example when making catalysts for different reactions.

Claims

-10 -Claims 1. A process for producing a catalytic structure

incorporating a corrugated foil, wherein the process comprises selecting a metal foil, and forming the foil into corrugations by a plurality of stamping steps, the said corrugations extending parallel to each other, the number of contiguous corrugations along a line transverse to the orientation thereof being increased by each said stamping step.
2. A process as claimed in claim 1 wherein the process also comprises cutting out foil blanks by chemical milling, prior to forming the corrugations.
3. A process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, cutting out foil blanks by chemical milling, and then subjecting resulting foil blanks to a corrugating process.
4. A process as claimed in claim 2 or claim 3 wherein the foil dimensions are accurate to better than 0.2 mm.
5. A process as claimed in any one of claims 1 to 4 comprising the subsequent step of coating the corrugated surface with a precursor for a ceramic coating using a piezoelectric ink jet printer.
6. A process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, forming corrugations along the foil, and then coating the corrugated surface with a precursor for a ceramic coating using a piezoelectric ink jet printer.

-11 -
7. A process for producing a catalytic structure incorporating a corrugated foil, wherein the process comprises selecting a metal foil, forming corrugations along the foil, coating the corrugated surface with a ceramic coating, and then depositing an active catalytic material or a precursor therefor using a piezoelectric ink jet printer.
8. A process as claimed in claim 5 or claim 6 comprising the subsequent step of depositing an active catalytic material or a precursor therefor into a ceramic coating using a piezoelectric ink jet printer.
9. A process for producing a catalytic structure incorporating a corrugated foil, the process being substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
10. A catalytic structure incorporating a corrugated foil, the structure being made by a process as claimed in any one of the preceding claims.
11. A catalytic structure as claimed in claim 10 wherein the metal is a stainless steel.

15998 Mdli P T Mansfield

Chartered Patent Attorney Agent for the Applicant