WO2013045921A1

WO2013045921A1 - Catalytic reactor and catalyst structure

Info

Publication number: WO2013045921A1
Application number: PCT/GB2012/052385
Authority: WO
Inventors: Bernard John Crewdson; Suzanne Rose Ellis
Original assignee: Compactgtl Limited
Priority date: 2011-09-28
Filing date: 2012-09-26
Publication date: 2013-04-04
Also published as: GB201116708D0

Abstract

A catalytic reactor (10) defines first and second flow channels (16, 17), with removable catalyst-carrying inserts (22, 24; 50) in each of those channels in which a reaction is to occur. The catalyst insert (22, 24; 50) comprises a plurality of foils (41, 42; 52) bonded together (40) and which subdivide the flow channel (16, 17) into a multiplicity of flow sub-channels. Along a major proportion (44) of the length of the catalyst insert (22, 24; 50), at least some of the foils (41, 42; 52) are not bonded together. This provides the insert with some resilience.

Description

Catalytic Reactor and Catalyst Structure

The present invention relates to a reactor for performing chemical reactions which involve heat

transfer, the reactor defining channels in which there is a catalyst structure, and to a catalyst structure for use in such a reactor.

The use of a catalytic reactor consisting of a stack of metal sheets that define first and second flow

channels, where catalyst is provided on removable inserts such as corrugated foils within the flow channels, is described for example in WO 03/033131, which describes use of such a reactor for performing various chemical reactions for example Fischer-Tropsch synthesis, steam methane reforming, or combustion. WO 2010/067097 also describes a catalytic reactor in which a catalyst insert may comprise one or more corrugated foils. The two sets of channels enable heat transfer to take place between the contents of those channels. For example steam methane reforming is an endothermic reaction that requires an elevated temperature, typically above 750 °C; and the requisite heat may be provided by a combustion reaction taking place in the other set of channels within the catalytic reactor. Fischer-Tropsch synthesis is an exothermic reaction, so in this case the channels

adjacent to those for the synthesis reaction may carry a coolant . The provision of removable catalyst-carrying inserts in a reactor of the type described above is advantageous in that it enables the lifespan of the reactor to exceed the lifespan of the catalyst, potentially by several times. However, in order to deploy fresh catalyst into the reactor, the inserts must be removed and fresh inserts inserted. For good heat transfer between the insert and the channel wall, the insert is desirably in contact with the wall; but for ease of removal or

insertion there is desirably a clearance between the inserts and the wall. According to one aspect of the present invention there is provided a reactor defining first and second flow channels within the reactor, with a removable catalyst insert provided in each of those channels in which a reaction is to occur, the catalyst insert

comprising a plurality of foils bonded together and which subdivide the flow channel into a multiplicity of flow sub-channels, and wherein, along at least one portion at least some of the foils are not bonded together, the non- bonded portion or portions constituting a major

proportion of the length of the catalyst insert, and the non-bonded portion or portions being able to bow apart to provide resilience to the catalyst insert.

Although mention has been made of there being first and second flow channels, for first and second fluids, it will be appreciated that the reactor might define flow channels for more than two different fluids.

In one embodiment at least some of the foils are bonded together only in the vicinity of their ends. In one specific embodiment, one pair of adjacent foils within the catalyst insert are bonded together only in the vicinity of their ends. A pair of foils within an insert, where the pair of foils can bow apart along a portion of their length, may be referred to as a "non- bonded pair", although they are bonded together, for example in the vicinity of their ends. To ensure

sufficient resilience of the insert, at least one non- bonded portion may be of length at least 100 mm,

preferably at least 200 mm, for example 400 mm, 500 mm or 600 mm . The remaining foils are bonded together at least at a multiplicity of positions along their length. This reduces the flexibility of the stack of foils that form the insert, which makes the insert easier to handle, and this also enhances heat transfer throughout the insert, so reducing temperature differences within the insert.

By way of example the foils that form the insert may be bonded by spot welding.

In one embodiment, all the foils are bonded together along their length apart from a pair of adjacent foils that are at or adjacent to the middle of the stack of foils. For example if there are five foils stacked up to form the insert, the "non-bonded pair" of foils may be the second and third foils, or the third and fourth foils. If there are four foils stacked up to form the insert, the "non-bonded pair" would preferably be the second and third foils.

The presence of at least one "non-bonded pair" of foils provides some resilience to said major proportion of the length of the catalyst insert. During insertion of the insert into a channel within the reactor, the insert can be squeezed to fit into the channel. After insertion the portions of the catalyst insert above and below the "non-bonded pair" of foils tend to bow apart, forming a narrow gap, so that the insert makes good contact with the channel walls. There is poor thermal contact across the narrow gap, but this is near the centre of the insert where heat fluxes are small.

After insertion, the ends of the inserts may be restrained, for example by an end grille, so that the ends cannot move relative to the flow channel, or their movement is restricted. During operation, as a result of thermal expansion of the insert relative to the channel, the insert bows to a greater extent, thereby enhancing contact with the channel wall.

A preferred material for the foils is a high- temperature corrosion-resistant steel alloy, for example an aluminium-containing 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 surface of alumina which protects the alloy against further

oxidation. This oxide layer also protects the alloy against corrosion under conditions that prevail within for example a methane oxidation reactor or a

steam/methane reforming reactor. 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 surface on the metal is believed to bind with the oxide coating, so ensuring the catalytic material adheres to the metal substrate.

Where the foils are corrugated, the corrugations may be square, rectangular, trapezoidal or hexagonal in cross-section; or arcuate or sinusoidal; or they may be of zigzag shape, defining triangular corrugations, or a sawtooth shape, for example with sloping portions

connected by flat peaks. The corrugations typically run parallel to the length of the foils. In some alternative configurations, the corrugations may be non-parallel or even perpendicular to the length of the foil.

If the corrugated foils have corrugations that would enable adjacent foils to intermesh then the corrugated foils may be spaced apart by foils that are flat or substantially flat, to ensure they do not intermesh.

Such flat foils are not necessary if the adjacent foils have corrugations that are not parallel, or are otherwise shaped to ensure the foils they do not intermesh. For example if the peaks and troughs are defined by flat portions of the profile, and the flat portions in

adjacent foils are aligned with each other, then the foils will not intermesh. Where flat foils are used, they may also be corrugated at a very small amplitude, for example to provide a total height of less than about 0.2 mm, for example 0.1 mm, as this makes them slightly less flexible and so easier to work with during assembly. The direction of the corrugation of the substantially flat foil may lengthwise along the foil or, alternatively, may be transverse. The shape of the corrugations of the flat foils may be sawtooth or rippled. The foils may be of thickness in the range 20-150 pm, for example 50 pm. A thicker foil, for example 100 pm thick, may provide benefits in enhanced heat transfer. Preventing

intermeshing of the corrugated foils, whether by the provision of flat foils or by the provision of adjacent foils with non-parallel corrugations, ensures that the height of the insert is more repeatable and controllable than a stack in which identical corrugated foils are deployed . To form the catalyst insert the foil structure would be provided with a catalyst on the surfaces of at least some of the foils. For example the foil structure may be coated with ceramic support material, for example based on alumina, and this would be impregnated with active catalytic material appropriate for the reaction that is to take place in the corresponding channel. The ceramic coating may be applied by techniques such as dip coating, or spraying, to achieve a ceramic thickness between 10 pm and 100 pm, depending on the reaction, and the coating may be applied to separate foils before they are

assembled into the catalyst insert, or after the foils have been bonded together. Within the reactor the first and second flow

channels may be defined by grooves in plates arranged as a stack, or by spacing strips and plates in a stack, the stack then being 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 stack of plates provides the requisite structure to ensure that the reactor can resist the differential pressures and thermal stresses that are applied during operation; the catalyst insert does not have to provide structural support. Consequently the catalyst inserts can be non-structural, as they do not have to hold the walls of the channels apart during operation . The channels may be square in cross-section, or may be of height either greater than or less than the width; the height refers to the dimension in the direction of the stack, that is to say in the direction for heat transfer. For example the plates might be 0.5 m wide and 1.0 m long, or 0.6 m wide and 0.8 m long; and they may define channels 7 mm high and 6 mm wide, or 3 mm high and 10 mm wide, or 10 mm high and 5 mm wide. These dimensions are merely exemplary, and the skilled person will

recognise that many different shapes and sizes are equally suitable. Arranging the first and second flow channels to alternate in the stack helps ensure good heat transfer between fluids in those channels. For example the first flow channels may be those for combustion (to generate heat) and the second flow channels may be for steam/methane reforming (which requires heat) . The catalyst inserts are inserted into the channels, and can be removed for replacement. Such reactors can be used for a variety of reactions including Fischer-Tropsch synthesis, and synthesis gas generation, for example by steam methane reforming. If the desired reaction is exothermic, adjacent channels may be provided with coolant to draw the heat of the reaction out of the reactor. Conversely, if the desired reaction is endothermic then heat must be provided to the flow channels. This may be achieved either by flowing hot fluids, preferably gases, through the channels or by undertaking an exothermic reaction in the adjacent channels. Catalytic combustion may be used to provide heat . In a further aspect, the present invention provides a catalyst insert for use in such a reactor, the catalyst insert comprising a plurality of foils bonded together and which define a multiplicity of flow sub-channels and wherein, along at least one portion of the length of the catalyst insert, at least some of the foils are not bonded together, the non-bonded portion or portions constituting a major proportion of the length of the catalyst insert, and the non-bonded portion or portions being able to bow apart to provide resilience to the catalyst insert.

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 shows a schematic perspective view, partly in section, of part of a reactor block suitable for

steam/methane reforming and including catalyst inserts (the section being on the line 1-1 of figure 2);

Figure 2 shows a side view of the assembled reactor block of figure 1 showing the flow paths;

Figures 3a and 3b show plan views of parts of the reactor block of figure 1 during assembly; Figure 4 shows a side view of a catalyst insert of Figure 1 ; and

Figure 5 shows an end view of an alternative catalyst insert .

The invention is applicable to a process for making synthesis gas, that is to say a mixture of carbon

monoxide and hydrogen, from natural gas by steam

reforming. The synthesis gas may, for example,

subsequently be used to make longer-chain hydrocarbons by a Fischer-Tropsch synthesis. The steam reforming reaction is brought about by mixing steam and methane, and

contacting the mixture with a suitable catalyst at an elevated temperature so the steam and methane react to form carbon monoxide and hydrogen. The steam reforming reaction is endothermic, and the heat may be provided by catalytic combustion, for example of hydrocarbons and/or hydrogen mixed with air, so combustion takes place over a combustion catalyst within adjacent flow channels within the reforming reactor.

Referring now to figure 1 there is shown a reactor block 10 suitable for use as a steam reforming reactor, or for use in a steam reforming reactor. The reactor block 10 defines channels for a catalytic combustion process and channels for steam methane reforming. The reactor 10 consists of a stack of plates that are

rectangular in plan view, each plate being of corrosion resistant high-temperature alloy such as Inconel 625, Incoloy 800HT or Haynes HR-120. Flat plates 12,

typically of thickness in the range 0.5 to 4 mm, in this case 2.0 mm thick, are arranged alternately with

castellated plates 14 or 15, so the castellations define channels 16 or 17. The castellated plates 14 and 15 are arranged in the stack alternately. The thickness of the castellated plates 14 and 15, typically in the range between 0.2 and 3.5 mm, is in each case 0.9 mm. The height of the castellations , typically in the range 2-10 mm, is 6 mm in each case, and solid bars 18 of the same thickness are provided along the sides. The wavelengths of the castellations in the castellated plates 14 and 15 may be different from each other, but in the embodiment shown in the figure the wavelengths are the same, so that in each case successive fins or ligaments are 7 mm apart. The castellated plates 14 and 15 may be referred to as fin structures. At each end of the stack is a flat end plate 19, which in this case is also of thickness 2.0 mm.

Although only five channels are shown as being defined by each castellated sheet 14 or 15 in figure 1, in a practical reactor there might be many more, for example over seventy channels in a reactor block 10 of overall width about 500 mm.

The stack of plates would be assembled and bonded together typically by diffusion bonding, brazing, or hot isostatic pressing. Into each of the channels 16 and 17 is then inserted a respective catalyst insert 22 or 24 (only one of each are shown in Figure 1), carrying a catalyst for the respective reaction. These inserts 22 and 24 comprise a metal substrate and a ceramic coating acting as a support for the active catalytic material. The metal substrate of each insert 22, 24 comprises a stack of corrugated foils and flat foils occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.2 mm, for example 100 microns; the stacks shown in figure 1 consist of three corrugated foils separated by two flat foils, bonded together. The channels 16 and 17 in this example are 6 mm high and 7 mm wide, while the catalyst inserts 22 and 24 in this case are 5.4 mm high and 6.6 mm wide, so providing a degree of clearance from the walls of the channels 16 and 17. This is necessary to allow for tolerances in manufacture of the reactor block 10. Referring now to figure 2 there is shown a side view of the assembled reactor block 10. The gas mixture undergoing combustion enters a header 30 at one end of the reactor block 10 (top, as shown) and after passing through a baffle plate flame arrestor 31 follows the flow channels 17 that extend straight along most of the length of the reactor 10. Towards the other end of the reactor block 10 the flow channels 17 change direction through 90° to connect to a header 32 at the side of the other end of the reactor 10 (bottom right as shown) , this flow path being shown as a broken line C. The gas mixture that is to undergo the steam methane reforming reaction enters a header 34 at the side of the one end of the reactor block 10 (top left, as shown) , passes through a baffle plate 35 and then changes direction through 90° to flow through the flow channels 16 that extend straight along most of the length of the reactor block 10, to emerge through a header 36 at the other end (bottom, as shown) , this flow path being shown as a chain dotted line S. The arrangement is therefore such that the flows are co- current; and is such that each of the flow channels 16 and 17 is straight along most of it length, and

communicates with a header 30 or 36 at an end of the reactor block 10, so that the catalyst inserts 22 and 24 can be readily inserted before the headers 30 or 36 are attached .

Each of the flat plates 12 shown in figure 1 is, in this example, of dimensions 500 mm wide and 1.0 m long, and that is consequently the cross-sectional area of the reactor block 10. Referring now to figure 3a there is shown a plan view of a portion of the reactor block 10 during assembly, showing the castellated plate 15 (this view being in a plane parallel to that of the view of figure 2) . The castellated plate 15 is of length 800 mm, and of width 460 mm, and the side bars 18 are of width 20 mm. The top end of the castellated plate 15 is aligned with the top edge of the flat plate 12, so it is open (to communicate with the header 30) . One of the side bars 18 (the left one as shown) is 1.0 m long, and is joined to an equivalent end bar 18a that extends across the end. There is consequently a 180 mm wide gap at the bottom right-hand corner (to communicate with the header 32) . The rectangular region between the bottom end of the castellated plate 15 and the end bar 18a is occupied by two triangular portions 26 and 27 of castellated plate: a first portion 26 has castellations parallel to the end bar 18a, and extends to the edge of the stack (so as to communicate with the header 32), whereas the second portion 27 has castellations parallel to those in the castellated plate 15.

Referring to figure 3b there is shown a view, equivalent to that of figure 3a, but showing a

castellated plate 14. In this case the castellated plate 14 is again of length 800 mm, and of width 460 mm, and the side bars 18 are of width 20 mm. The bottom end of the castellated plate 14 is aligned with the bottom edge of the flat plate 12, so it is open (to communicate with the header 36) . One of the side bars 18 (the right one as shown) is 1.0 m long, and is joined to an equivalent end bar 18a that extends across the end. There is consequently a 180 mm wide gap at the top left-hand corner (to communicate with the header 34) . In the rectangular region between the top end of the castellated plate 14 and the end bar 18a there are triangular

portions 26 and 27 of castellated plate: a first portion 26 has castellations parallel to the end bar 18a, and extends to the edge of the stack (so as to communicate with the header 34), while the other portion 27 has castellations parallel to those in the castellated plate 14. It will be appreciated that many other arrangements of portions of castellated plates may be used to achieve this change of gas flow direction. For example the castellated plate 15 and the portion of castellated plate 27 may be integral with each other, as they have

identical and parallel castellations ; and similarly the castellated plate 14 and the adjacent portion of

castellated plate 27 may be integral with each other. Preferably the castellations on the triangular portions 26 and 27 have the same shape as those on the channel- defining portions 14 or 15. In some cases the triangular portions 26 and 27 may be omitted, to leave a gas

distribution space between the flat plates 12 through which the gas flows between the end of the castellated plate 14 or 15 and the header 32, 34 at the side of the block 10.

As mentioned previously, after the stack of plates 12, 14, 15 has been assembled, catalyst inserts 22 and 24 are inserted into the reaction channels 16 and 17. The catalyst inserts 24 are of length 800 mm and incorporate active catalytic material along 600 mm of their length, corresponding to the bottom three-quarters of the

straight channels as shown in plan in figure 3a, this portion being indicated by the arrow P, and over the other 200 mm indicated by the arrow Q the inserts 24 are non-catalytic. Similarly in the channels 16 for the steam reforming gas mixture S the catalyst inserts 22 are of length 800 mm, and as indicated by the arrow R active catalytic material is provided along the portion

occupying the upper three-quarters of the straight channels as shown in plan in figure 3b; the other 200 mm of the length of the inserts 22 as indicated by the arrow Q are non-catalytic. After inserting the catalyst inserts 22 and 24, a wire mesh or grille (not shown) may be attached across the bottom end of the reactor block 10 so that the catalyst inserts 22 do not fall out of the flow channels 16 when the reactor block 10 is in its upright position (as shown in figure 2) . It will hence be appreciated that the active catalytic materials on the inserts 22 and 24 are only present in those portions P and R of the flow channels 16 and 17 which are

immediately adjacent to each other.

It will be appreciated that headers 30, 32, 34 and 36 might then be attached to the reactor block 10.

Alternatively it may be more convenient to provide a reactor of larger capacity, and this may be achieved by combining several such reactor blocks 10 together, before attaching headers. As indicated above the inserts 22 and 24 comprise a metal substrate and a ceramic coating acting as a catalyst or catalyst support. The metal substrate of each insert 22, 24 comprises a stacked assembly of corrugated foils and flat foils occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.2 mm, for example 50 pm or 100 pm; the assemblies shown in figure 1 consist of three corrugated foils separated by two flat foils, bonded together. The total length of each insert 22 and 24 is 800 mm, and all the foils are 800 mm long in this example.

Referring now to figure 4 there is shown a side view of an insert 22 on a larger scale, and with the foils separated for clarity; the insert 24 in this example has the same structure. The insert 22 consists of three corrugated foils 41 separated by two flat foils 42. Each foil is of the appropriate width to fit in the

corresponding channel, 6.6 mm wide in this example. The foils are spot welded together at multiple positions 40; at the positions 40a in the vicinity of the ends of the foils 41, 42, that is to say at the end portions 43, all five foils are spot welded together, whereas at the intermediate positions 40b the top two foils 41, 42 are spot welded together and the bottom three foils 41, 42 are spot welded together. The spot welds at the end portions 43 ensure that the insert 22 remains in one piece, for ease of handling during insertion into the reactor. The middle portion 44 of the insert 22 between the end portions 43 can

therefore bow apart slightly, the top two foils 41, 42 bowing upward and the bottom three foils 41, 42 bowing downward. This provides some resilience to the insert 22.

During insertion the insert 22 can be squeezed together, so that it can be fitted into the channel.

After insertion, along the middle portion 44, the

portions of the catalyst insert 22 above and below the "non-bonded pair" of foils 41, 42 tend to bow apart, forming a narrow gap, and this enhances contact of the insert 22 with the channel walls, which is beneficial for heat transfer. There is poor thermal transfer across the narrow gap, but this is near the centre of the insert 22 where heat fluxes are small.

It will be appreciated that this insert 22 is merely an example, and that it may be modified in various ways. In particular there may be a plurality of positions 40 near each end at which all the foils 41, 42 are bonded together. For example, with the insert 22 of length 800 mm, end portions 43 of length 50 mm or 100 mm may be spot welded together as described in relation to the positions 40a, leaving a middle portion 44 of length 700 mm or 600 mm respectively bonded as described in relation to the positions 40b. The end portions 43 may be of different lengths at the two ends. The end portions 43 may occupy no more than 25% of the total length, for example no more than 20%. Referring now to figure 5 there is shown an end view of an alternative insert 50, which might be used instead of the insert 22 or the insert 24 if provided with an appropriate catalyst. The insert 50 consists of four corrugated foils 52, the corrugations having flat

portions at the peaks and troughs. The foils 52 are stacked with the flat portions of the peaks and troughs aligned with each other, so the foils 52 do not

intermesh. The foils 52 are shown slightly spaced part for clarity.

As described above in relation to figure 4, the foils 52 are bonded together, for example by spot

welding, along the length of the insert 50. The top two foils 52 are bonded together along their entire length, and the bottom two foils 52 are bonded together along their entire length, but the second and third foils 52 are bonded together only at end portions. Hence there is a middle portion of the insert 50 along which the top pair and bottom pair of foils can slightly bow apart. In this example the resulting gap is at the centre of the insert 50, with the same number of foils 52 above the gap as below the gap. Hence during insertion into a flow channel of a reactor block 10 the insert 50 can be squeezed so that it fits into the channel, but after insertion the insert 50 will tend to bow apart, enhancing thermal contact with the wall of the channel and reducing the risk of gases bypassing the catalyst on the insert 50.

It will be appreciated that the insert 50 may be modified in various ways, for example if it is desired to increase the surface area available for catalyst, one or more flat foils might be placed between adjacent

corrugated foils 52. For example an alternative

embodiment has six foils stacked together: four corrugated foils 52 arranged as shown, with flat foils between the bottom two corrugated foils 52 and between the top two corrugated foils 52. A further modification would also include an additional flat foil between the second and third corrugated foils 52, spot welded to only one of the corrugated foils 52 along its entire length.

An alternative embodiment comprises a stack of foils in which most of the foils are bonded together throughout their length, but in which at least a pair of foils are bonded together only at one or more intermediate

positions. For example such a "non-bonded pair" of foils might be bonded together at a position near the midpoint, or at spaced-apart positions along their length. In the case of a foil assembly of length 800 mm, by way of example, a "non-bonded pair" of foils might be bonded together for example at positions 200 mm or 300 mm in from each end. In this case the end portions and the middle portion of the foil assembly will tend to bow apart.

The concept of the present invention is equally applicable in reactors for other chemical reactions. By way of example, a similar reactor may be used for a reaction such as Fischer-Tropsch synthesis, in which a chemical reaction takes place in one set of channels while a heat transfer medium flows in an adjacent set of channels. In this case no chemical reaction takes place in the channels carrying the heat transfer medium, so no catalyst insert is required in those channels.

Consequently the channels for the chemical reaction may extend straight through the reactor block from one end to the other, while the channels for the heat transfer medium may incorporate a central section that is parallel to the channels for the chemical reaction, and a

distributor section at each end to link to headers at the side or sides of the block. The catalyst inserts in the channels in which the chemical reaction takes place may be structured like the inserts 22, 24 and 50 described above . It will also be appreciated that the inserts 22, 24 and 50 would be suitable for use in any reactor which defines flow channels into which a catalyst structure is inserted, most particularly where the flow channels are of a square or rectangular cross-section. This may for example be a reactor in which the channels are defined by square or rectangular cross-section tubes, or in which the channels are machined out of a solid block of material, or are defined by a stack of plates.

Claims

1. A reactor defining first and second flow channels within the reactor, with a removable catalyst insert provided in each of those channels in which a reaction is to occur, the catalyst insert comprising a plurality of foils bonded together and which subdivide the flow channel into a multiplicity of flow sub-channels, and wherein, along at least one portion at least some of the foils are not bonded together, the non-bonded portion or portions constituting a major proportion of the length of the catalyst insert, and the non-bonded portion or portions being able to bow apart to provide resilience to the catalyst insert.

2. A reactor as claimed in claim 1 wherein at least some of the foils are bonded together only in the vicinity of their ends.

3. A reactor as claimed in claim 2 wherein one pair of adjacent foils within the catalyst insert are bonded together only in the vicinity of their ends.

4. A reactor as claimed in claim 1 wherein at least some of the foils are bonded together only at one or more intermediate positions along their length.

5. A reactor as claimed in any one of the preceding claims wherein the proportion of the length of the catalyst insert at which at least some of the foils are not bonded together is at least 75%.

6. A reactor as claimed in any one of the preceding claims wherein the foils that form the insert are bonded by spot welding.

7. A reactor as claimed in any one of the preceding claims also comprising means to restrain the ends of the inserts within the flow channels.

8. A reactor as claimed in any one of the preceding claims wherein the insert comprises both corrugated foils and flat foils.

9. A reactor as claimed in any one of claims 1 to 7 wherein the insert incorporates only corrugated foils, the corrugations being shaped and arranged such that the foils do not intermesh.

10. A catalyst insert for use in a reactor as claimed in any one of the preceding claims, the catalyst insert comprising a plurality of foils bonded together and which define a multiplicity of flow sub-channels and wherein, along a major proportion of the length of the catalyst insert, at least some of the foils are not bonded

together .

11. A catalyst insert as claimed in claim 10 wherein at least some of the foils are bonded together only in the vicinity of their ends.

12. A catalyst insert as claimed in claim 11 wherein one pair of adjacent foils within the catalyst insert are bonded together only in the vicinity of their ends.

13. A catalyst insert as claimed in claim 10 wherein at least some of the foils are bonded together only at one or more intermediate positions along their length.

14. A catalyst insert as claimed in any one of claims 10 to 13 wherein the proportion of the length of the

catalyst insert at which at least some of the foils are not bonded together is at least 75%.

15. A catalyst insert as claimed in any one of claims 10 to 14 wherein the foils that form the insert are bonded by spot welding.

16. A catalyst insert as claimed in any one of the preceding claims wherein the insert comprises both corrugated foils and flat foils.

17. A catalyst insert as claimed in any one of claims 10 to 15 wherein the insert incorporates only corrugated foils, the corrugations being shaped and arranged such that the foils do not intermesh.

18. A reactor substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings .

19. A catalyst insert substantially as hereinbefore described with reference to, and as shown in, the

accompanying drawings.