Reactor with Channels

The present invention relates to a reactor with channels for performing chemical reactions at elevated temperatures, for example Fischer-Tropsch synthesis, or steam methane reforming, and to a reactor block that may be used to form the 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 03/006149, which describes use of such a reactor for performing various chemical reactions including steam methane reforming. In such reactors the channels may be defined by flat plates spaced apart by castellated plates, or flat plates space apart by spacer bars, or by grooved plates. Another type of reactor utilises tubes. 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. Although this approach is effective, it would be desirable to reduce thermal gradients within the reactor, as these lead to stresses in the material forming a reactor. Similar reactors may also be used for Fischer-Tropsch synthesis. Fischer-Tropsch synthesis is an exothermic reaction, so in this case the channels adjacent to those for the synthesis reaction may carry a coolant.

Not only do thermal gradients within a reactor tend to lead to stresses within the material forming a reactor, but there is also a further risk of thermal runaway. With some exothermic catalytic reactions the rate of reaction may increase as the temperature increases; and in such a case there is a positive feedback between the reaction rate and the temperature within the reactor. This can lead to a rapid increase of temperature, referred to as a thermal runaway, and this can result in damage to the catalyst or to the reactor, or both, and would reduce the useful life of the reactor.

According to one aspect of the present invention there is provided a reactor defining first and second flow channels within the reactor, wherein the first flow channels are for fluids that undergo an exothermic reaction and the second flow channels are for a heat-removing fluid, wherein the channels at each end of the reactor are such that no heat is generated within them.

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.

Preferably the channels in which no heat is generated are not flow channels, that is to say no fluids flow through those channels, as they are blocked off at one or both of their ends (“non-flow channels”). Indeed there may be a plurality of such non-flow channels at the end of the reactor, for example two or three. Preferably the flow channel nearest to each end of the reactor is a second flow channel, and may be of smaller cross-sectional area than other second flow channels in the reactor.

Such a reactor may be made of blocks, each block defining a plurality of first and second flow channels, wherein the first flow channels are for fluids that undergo an exothermic reaction and the second flow channels are for a heat-removing fluid, wherein the channels at each end of the block are second flow channels. In this case these channels may be of smaller cross-sectional area than other second flow channels in the block, by being less high (in the direction of heat transfer). Since they are provided with heat on only one side they are preferably no more than 50% as high as other second flow channels within the block.

In an alternative, a reactor may be made of blocks, each block defining a plurality of first and second flow channels, wherein the first flow channels are for fluids that undergo an exothermic reaction and the second flow channels are for a heat-removing fluid, wherein the channels at each end of the block are first flow channels and are of smaller cross-sectional area than other first flow channels in the block, by being less high (in the direction of heat transfer). They are preferably no more than 50% as high as other first flow channels within the block.

The heat-removing fluid may be a fluid that undergoes an endothermic reaction. Alternatively the heat-removing fluid may be a coolant.

When the reactor is constructed by combining the reactor blocks end to end, there will be a small gap between successive reactor blocks, which inhibits heat transfer. This gap is preferably less than 5 mm wide.

Preferably each reactor block comprises a stack of metal sheets that are arranged to define the first and second flow channels, the first and second flow channels being arranged alternately within the stack, and there are removable catalyst-carrying gas-permeable non-structural elements within each flow channel in which a reaction is to be performed.

Within each reactor block 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.

To ensure the required good thermal contact both the first and the second flow channels may be between 20 mm and 1 mm high (in cross-section); and each channel may be of width between about 1.5 mm and 25 mm. By way of example the plates (in plan view) might be of width in the range 0.05 m up to 1 m, and of length in the range 0.2 m up to 2 m, and the flow channels are preferably of height between 2 mm and 10 mm (depending on the nature of the chemical reaction). 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 for example 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. 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 structures are inserted into the channels, and can be removed for replacement, and do not provide strength to the reactor, so the reactor itself must be sufficiently strong to resist any pressure forces or thermal stresses during operation.

Preferably each such catalyst structure is shaped so as to subdivide the flow channel into a multiplicity of parallel flow sub-channels. Preferably each catalyst structure includes a ceramic support material on the metal substrate, which provides a support for the catalyst. The metal substrate provides strength to the catalyst structure and enhances thermal transfer by conduction. Preferably the metal substrate is of a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example a ferritic steel alloy that incorporates aluminium (eg Fecralloy™), although the metal substrate may alternatively be of a different material such as stainless steel or aluminium, depending on the temperature and the chemical environment to which it is to be exposed. The substrate may be a foil, a wire mesh or a felt sheet, which may be corrugated, dimpled or pleated; the preferred substrate is a thin metal foil for example of thickness no more than 200 μm, which is corrugated to define the longitudinal sub-channels.

If the exothermic reaction is combustion, a flame arrestor is preferably provided at the inlet to each flow channel for combustion to ensure a flame cannot propagate back into the combustible gas mixture being fed to the combustion channel. This may be within an inlet part of each combustion channel, for example in the form of a non-catalytic insert that subdivides a portion of the combustion channel adjacent to the inlet into a multiplicity of narrow flow paths which are no wider than the maximum gap size for preventing flame propagation. For example such a non-catalytic insert may be a longitudinally-corrugated foil or a plurality of longitudinally-corrugated foils in a stack. Alternatively or additionally, where the combustible gas is supplied through a header, then such a flame arrestor may be provided within the header.

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 in the direction for heat transfer. The catalyst element may for example comprise a single shaped foil, for example a corrugated foil; this is particularly suitable where the channel's minimum cross-sectional dimension is no more than about 3 mm, although it is also applicable in wider channels. Alternatively, and particularly where the channel's minimum cross-sectional dimension is greater than about 2 mm, the catalyst structure may comprise a plurality of such shaped foils separated by substantially flat foils. To ensure the required good heat transfer, for example in a steam/methane reforming reactor, the combustion channels are preferably less than 10 mm high. But the channels are preferably at least 1 mm high, or it becomes difficult to insert the catalyst structures, and engineering tolerances become more critical. As one example, the channels might all be 7 mm high and 6 mm wide, and in each case the catalyst element may comprise a single shaped foil, or a plurality of shaped foils.

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

FIG. 1 shows a schematic perspective view, partly in section, of part of a reactor block suitable for steam/methane reforming (the section being on the line 1-1 of FIG. 2);

FIGS. 1
a and 1b show modifications to the reactor of FIG. 1;

FIG. 2 shows a side view of the assembled reactor block of FIG. 1 showing the flow paths;

FIGS. 3
a, 3b and 3c show plan views of parts of the reactor block of FIG. 1 during assembly; and

FIG. 4 shows a perspective view, partly exploded, of a reactor that incorporates reactor blocks similar to that of FIG. 1.

The invention would be 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 FIG. 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 3.9 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 as shown in the figure in a preferred embodiment the wavelengths are the same, so that in each case successive fins or ligaments are 10 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. As explained below in relation to FIG. 3c, the channels defined in the last two castellated plates 14a and 15a adjacent to the end plate 19 are non-flow channels 20. In a modification the end plate may be of different thickness, typically a greater thickness in the range 2.0 up to 10 mm. In this example the number of castellated plates 14, 14a, 15 and 15a in the reactor block 10 is thirteen, so that the overall height of the reactor block 10 is 78.7 mm.

Although only five channels are shown as being defined by each castellated sheet 14 or 15 in FIG. 1, in a practical reactor there might be many more, for example over forty 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 catalytic insert 22 or 24 (only one of each are shown in FIG. 1), carrying a catalyst for the respective reaction. These inserts 22 and 24 preferably have a metal substrate and a ceramic coating acting as a support for the active catalytic material, and the metal substrate may be a thin metal foil. For example the insert 22, 24 may comprise a stack of corrugated foils and flat foils, or a single corrugated foil, occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.1 mm, for example 50 microns. (There are no such catalytic inserts in the non-flow channels 20.)

Referring now to FIGS. 1a and 1b there are shown some modifications to the reactor block 10. Whereas the channels 16 and 17 of the reactor block 10 are wider than they are high, as illustrated in FIGS. 1a and 1b they may instead be higher than they are wide. The inserts 22 and 24 shown in FIG. 1 consist of a single corrugated foil within each channel; in figure la the insert 22a is again of a single corrugated foil, whereas in FIG. 1b the insert 22b comprises a stack of corrugated foils and flat foils.

Referring now to FIG. 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 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. It may be preferable to provide catalyst inserts 22 and 24 only along those parts of the straight portions of the flow channel 16 and 17 that are adjacent to each other.

Each of the flat plates 12 shown in FIG. 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 FIG. 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 FIG. 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 FIG. 3b there is shown a view, equivalent to that of FIG. 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.

Referring to FIG. 3c there is shown a view, equivalent to that of FIGS. 3a and 3b, but showing a castellated plate 14a defining one of the non flow channels 20. In this case the castellated plate 14a is of length 960 mm, and again of width 460 mm. In this case both the side bars 18 are 1.0 m long, and they connect to end bars 18a at each end. Consequently there is no fluid flow through the non-flow channels 20. However there are small bleed holes 28, at the top right-hand corner and bottom left-hand corner as shown, so that the non-flow channels 20 are at the pressure of the surroundings.

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.

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. Preferably in the channels 17 for the combustion gases C the catalyst inserts 24 are of length 600 mm so as to occupy the bottom three-quarters of the straight channels as shown in plan in FIG. 3a, this portion being indicated by the arrow P, and the other 200 mm indicated by the arrow Q are occupied by a non-catalytic spacer which may be in the form of a loosely-fitting corrugated foil. Similarly in the channels 16 for the steam reforming gas mixture S the catalyst inserts 22 are of length 600 mm, and as indicated by the arrow R the catalyst inserts 22 occupy the upper three-quarters of the straight channels as shown in plan in FIG. 3b; the other 200 mm as indicated by the arrow Q are occupied by a non-catalytic spacer. After inserting the catalyst inserts 22 and 24, a wire mesh (not shown) may be attached across the bottom end of the reactor block 10 so that the spacers and 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 FIG. 2). It will hence be appreciated that the catalytic inserts 22 and 24 are only present in those portions 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. However it may be more convenient to provide a reactor of larger capacity, and this may be achieved by combining several such reactor blocks together.

Referring now to FIG. 4 there is shown a reactor 40. This consists of several reactor blocks 10a and 10b that are similar to the reactor block 10 of FIG. 1. There are two reactor end blocks 10a that are at the ends of the reactor 40. These end blocks 10a differ from the reactor block 10 in that they have non-flow channels 20 at only one end of the stack, that being the end which forms the end of the reactor 40; at the other end of the reactor block 10a there are flow channels 16 for the steam methane reforming gas flow S. Between these end blocks 10a are several inner blocks 10b which differ from the end blocks 10a in having no non-flow channels 20; at both ends of each inner block 10b are flow channels 16 for the steam methane reforming gas flow S.

During assembly of the reactor 40, the reactor blocks 10a or 10b are welded to one another in such a way as to leave gaps 2.3 mm wide between successive blocks, the welding filling in the gaps around the edges in those positions where headers 30, 32, 34 and 36 (see also FIG. 2) are to be attached, but leaving an open gap 41 (only three shown) on those portions of the sides where no header is to be attached. This may be achieved either by holding the blocks at the desired spacing, and welding across the gap; or by placing spacer bars 2.3 mm thick between the blocks along those portions that are to be filled in, and welding the blocks and the spacer bars together.

Headers 30, 32, 34 and 36 are then attached to the reactor 40. In this example each header extends over the entire length of the reactor 40, which in this case is of total length 1.0 m, each header 30, 32, 34 and 36 having a single fluid inlet or outlet duct 42, 43, 44 and 45 for the respective fluids C, S.

Hence in operation the reactor block 10 or the reactor 40 may be used as part of a plant for producing synthesis gas from a mixture of methane and steam. A combustible gas mixture (see arrows C) would be supplied to the header 30, so as to flow along the flow paths 17 in which it undergoes catalytic combustion, the exhaust gases emerging into the header 32. A mixture of methane and steam (see arrows S) would be supplied to the header 34 so as to flow along the flow paths 16 in which are the catalyst inserts 22, typically being supplied at a temperature of about 600° C., and the mixture is raised to a temperature of about 770° C. as it passes through the reactor 40. The resulting synthesis gas emerges into the header 36, so as to emerge through the outlet duct 45.

The outermost flow channels in which gas flow occurs in the reactor 40 are reforming channels 16. Heat transfer from these outermost channels is restricted by the provision of the non-flow channels 20. This reduces the thermal gradients within the reactor 40, and so decreases the thermal stresses to which it is subjected.

In a modification, since the outermost reforming channels 16 experience heat in-flow on only one side, those outermost reforming channels 16 may be of smaller height than the other reforming channels 16 in the reactor block 10. For example they may be of height between 30 and 70% that of the other reforming channels, most preferably between 45 and 55% that of the other reforming channels 16. The corresponding inserts 22 would therefore have also to be of less height.

Since within each inner reactor block 10b the outermost flow channels are reforming channels, the reactor design described above ensures that combustion channels are not adjacent to combustion channels, which is advantageous for reducing thermal gradients. The air gap between successive blocks 10a, 10b may be open at the sides to allow air circulation, as indicated above, or alternatively the blocks may be welded together around their entire periphery so that the air is enclosed. Such an air gap inhibits heat transfer.

It will be appreciated that the reactor block 10 and the reactor 40 may be modified in various ways while remaining within the scope of the present invention. As indicated above the channel arrangements within the reactor block 10 is NNSCSCSCSCSNN (i.e. thirteen layers of channels alternating between steam reforming (S), and combustion (C), the outermost being steam reforming, but with two non-flow layers (N) at the ends). In a less preferred alternative the outermost layers are combustion, so that the layers would be NNCSCSCSCSCNN. Similarly within each inner reactor block 10b the channel arrangements is SCSCSCSCSCSCS.

In an alternative and less preferred arrangement the outermost layers are combustion: CSCSCSCSCSCSC; in this situation the outermost channels are of smaller height than the other combustion channels 17 in the block 10; they may for example be between 40% and 70% of the cross-sectional area of the other combustion channels, for example 50%. It will be appreciated that the number of layers within a reactor block may differ from that described. For example each inner reactor block might have only three layers, and these might be arranged either SCS or CSC.

It will also be appreciated that although the flow directions of the first flow channels and the second flow channels are shown as being parallel, in co-flow, in the reactor described above, the flow directions may be parallel in counter-flow, or alternatively the flow directions may be in transverse directions, or may be at an oblique angle.

Reactor with Channels

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