This invention relates to a method of carrying out a catalytic reaction in which an endothermic chemical reaction such as steam/methane reforming is performed in channels in a reactor, in which the heat is provided by a combustion reaction in adjacent channels in the reactor.

A plant and process are described in WO 2005/102511 (GTL

Microsystems AG) 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 reforming reaction is typically carried out at a temperature of about 800°C, and the heat required may be provided by catalytic combustion in channels adjacent to those in which reforming is carried out, the combustion channels containing a catalyst which may comprise palladium or palladium/platinum on an alumina support in the form of a thin coating on a metallic substrate. An inflammable gas mixture such as a mixture of methane and air is supplied to the

combustion channels. Combustion occurs at the surface of the catalyst without a flame. However, it has been found that the combustion reaction tends to occur most vigorously near the start of the combustion channel, which can lead to an unsuitable temperature distribution along the channel. One solution of this problem is described in WO 2009/101434

(CompactGTL pic). In this case the reaction takes place in two successive reactor blocks, through which the reforming gas mixture flows in series. An inflammable gas mixture of methane and air, containing only 60% of the required fuel and 80% of the required air, is preheated to 550°C before being supplied to combustion channels of the first reactor block; between the blocks the remaining air and fuel are mixed with the outflowing gases from the combustion channel, so that the inflammable gas mixture is supplied at about 600°C to the second reactor block. These temperatures are said to be below the auto-ignition temperatures for the respective inflammable gas mixtures. Nevertheless it would be desirable to achieve temperature distributions that are more uniform along the channels for the reforming reaction, without risking auto-ignition of an inflammable gas mixture. According to the present invention there is provided a method for performing an endothermic reaction in a reactor defining a multiplicity of first and second flow channels arranged alternately within the reactor to ensure thermal contact between the first and second flow channels, the reactor comprising an inlet header communicating with a plurality of the first flow channels, the method comprising supplying a fluid mixture to undergo the endothermic reaction to the second flow channels, and supplying a combustible gas mixture through the header to the first flow channels, wherein the method comprises preheating the combustible gas mixture to above 620°C while ensuring that the flow rate is sufficiently high that the temperature of the combustible gas mixture in the header is below the auto-ignition temperature for the flowing gas mixture. This may correspond to a residence time of less than 2 s, after formation of the preheated combustible gas mixture, before the gas mixture reaches the first flow channels. The term residence time (in a header) means the time that the gas mixture actually remains within the header, and can be calculated from the volume of the header divided by the gas flow rate at its actual temperature and pressure.

In one option, the combustible gas mixture is preheated to above 635°C. In another option it is preheated to above 640°C. The method may involve ensuring that the residence time of the combustible gas mixture at the preheated temperature is less than 1200 ms. In a preferred option the residence time is less than 1000 ms. Indeed, in one example, with a residence time less than 400 ms the gas mixture may be preheated to above 650°C. Surprisingly it has been found that the flow of the combustible gas mixture has a significant impact on the auto-ignition temperature, and that the auto-ignition temperature increases as the flow rate increases or as the residence time decreases. The method is applicable either to a single stage reaction, or to a two-stage or multi-stage reaction in which each stage takes place in a respective reactor. According to another aspect of the present invention there is provided a method for performing an endothermic reaction in a reactor defining a multiplicity of first and second flow channels arranged

alternately within the reactor to ensure thermal contact between the first and second flow channels, the reactor comprising an inlet header communicating with a plurality of the first flow channels, the method comprising supplying a fluid mixture to undergo the endothermic reaction to the second flow channels, and supplying a combustible gas mixture through the header to the first flow channels, wherein the method comprises preheating the combustible gas mixture to a temperature which is above the auto-ignition temperature for the static combustible gas mixture, while ensuring that the flow rate is sufficiently high that the temperature to which the gas mixture is preheated does not exceed the auto-ignition temperature for the flowing gas mixture.

The auto-ignition temperature for the static combustible gas mixture means the auto-ignition temperature for that gas mixture when it is static, that is to say not flowing. The auto-ignition temperature depends upon the gas composition, as previously recognised, but is also significantly affected by the flow rate of the gas. The invention therefore enables the gas streams to be preheated to a temperature closer to the desired operating temperature of the reactor. Hence it is possible to pre-heat to above 600°C, and preferably above 620°C. In each case the combustible gas mixture is supplied at an elevated temperature below its auto-ignition temperature, and the temperature may be raised at least in part as a result of combustion of combustible gas mixture in one or more reaction stage. Indeed preferably the combustible gas mixture provided to each reaction stage of a two-stage or multi-stage reactor is at such an elevated temperature. For at least some of the stages the temperature may be raised by heat exchange with gases emerging from the second gas flow channels of one or more of the reaction stages. In one preferred option the combustible gas mixture is arranged to flow in series through the reaction stages in the same order as the endothermic gas mixture. In this case the combustible gas mixture provided to a second or subsequent reaction stage is at an elevated temperature as a result of having at least partly undergone combustion in the preceding reaction stage. The combustible gas mixture comprises a fuel (such as methane) and a source of oxygen (such as air). The fuel may be pure methane, or natural gas containing methane. Other suitable fuels would be methanol, ethanol, or other flammable gases and vapours. In the case of a two- stage or multi-stage reaction it is preferable that between successive reaction stages the outflowing gas mixture that has undergone

combustion is subjected to treatment, for example to change its

temperature, or to introduce and mix in additional fuel, and optionally also to introduce additional air into the mixture. By staging the provision of fuel between different reaction stages and by staging the introduction of air, greater control over the temperature distribution can be achieved. For example, if there are two reaction stages, the proportion of the fuel provided at the first stage is preferably between 50% and 70% of the total required fuel, the remainder being provided for the second stage.

The combustible gas mixture may comprise air and between 2 vol% and 6 vol% fuel. For methane as fuel that would correspond, in each case, to there being an excess of oxygen. Whatever the fuel, the process may be operated such that the oxygen :fuel ratio is equal to or greater than that required for complete combustion. For example, for methane as fuel, complete combustion to form water vapour and carbon dioxide requires a ratio of methane to oxygen of 1 :3 by volume. It has been found beneficial if within a reactor the first flow channels and the second flow channels extend in parallel directions, and the combustible gas mixture and the endothermic reaction mixture flow in the same direction (co-flow). Preferably the flow channels are of length at least 300 mm, more preferably at least 500 mm, but preferably no longer than 1500 mm. A preferred length is between 500 mm and 1000 mm, for example 600 mm. It has been found that co-flow operation gives better temperature control, and less risk of hot-spots.

In a preferred option each first flow channel (the channels for the endothermic reaction) and each second flow channel (the channels for the combustion reaction) contains a removable catalyst structure to catalyse the respective reaction, each catalyst structure preferably comprising a metal substrate, and incorporating an appropriate catalytic material. 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 (TM)). 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 less than 200 pm, which is corrugated to define the longitudinal sub-channels. The or each reactor may comprise a stack of plates. For example, the first and second flow channels may be defined by grooves in

respective plates, the plates being stacked and then bonded together. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. To ensure the required good thermal contact both the first and the second gas flow channels may be between 10 mm and 2 mm high (in cross-section); and each channel may be of width between about 3 mm and 25 mm. The stack of plates forming the reactor block is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing.

Preferably a flame arrestor is 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 invention is equally applicable to other reactor designs. For example the catalyst within the channels may be coated onto the channel walls, or may be in the form of pellets within the channel. The channels may be smaller than those described above, for example the channels may have a transverse dimension of less than 2 mm, or even less than 1 mm, which may be referred to as microchannels.

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 suitable for steam/methane reforming (the section being on the line 1-1 of figure 2);

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

Figures 3a and 3b show plan views of parts of the reactor of figure 1 during assembly; and

Figure 4 shows a schematic flow diagram of a plant that includes the reactor of figure 1.

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 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 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.

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 forty channels in a reactor block 10 of overall width about 500 mm. The castellations might be of different sizes, for example of height 6 mm and with fin separation of 7 mm. At each end of the stack is an end plate 19, which may be the same as the flat plates 12, or may be thicker for example between 3 mm and 10 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 Figure 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 or assembly 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 0.2 mm, for example 100 microns or 50 microns.

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 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 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. 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, in this example, are of length 600 mm so as to occupy the bottom three-quarters of the straight channels as shown in plan in figure 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 figure 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 figure 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.

The headers 30, 32, 34 and 36 may then be attached to the reactor block 10. It may in some cases be more convenient to provide a reactor of larger capacity, and this may be achieved by combining several such reactor blocks together, and the headers may in that case communicate with flow channels in different reactor blocks.

In any event the steam/methane mixture is preheated, for example to over 600°C, for example 630°C or 640°C, before being introduced through the header 34 into the reactor. The temperature in the reformer channels therefore typically increases from the inlet temperature to about 750-800°C at the outlet. Heating the combustion gases to an elevated temperature suppresses heat transfer from the steam/methane mixture, which would otherwise occur, and suppresses carbon deposition which may otherwise occur. The total quantity of fuel (e.g. methane) that is required is that needed to provide the heat for the endothermic reaction, and for the temperature increase of the gases (sensible heat), and for any heat loss to the environment; the quantity of air required is up to 10% more than that needed to react with that amount of fuel.

A mixture of air and natural gas (predominantly methane, but which may also contain ethane, propane and butane, and possibly higher hydrocarbon gases) is supplied to the header 30 to supply the combustion channels 17. If the reaction is taking place as a single stage reaction, then all the required fuel and air would be supplied to the header 30 of the reactor 10, whereas if the reaction is being carried out in stages, using two reactors 10, then only a proportion of the required fuel may be supplied to the reactor 10 for the first stage, with the remaining portion of fuel being supplied to the reactor 10 for the next stage. This mixture is preheated to a similar elevated temperature, for example 640°C or 650°C, before being supplied to the header 30. It will be appreciated that the risk of auto-ignition is considerably greater in the header 30 (and any associated ducts) than in the individual channels 17, because the channels 17 are of much smaller cross-sectional area. Although this temperature may be above the auto-ignition temperature for a static gas mixture, the temperature is below the auto-ignition temperature for a flowing gas mixture, as long as the residence time at that elevated temperature is sufficiently short.

This is illustrated in the experimental results shown in Table 1 below. Experiments were carried out using a test reactor channel, not containing a catalyst, whose temperature could be varied; and the temperature at various positions in the test reactor channel were monitored using thermocouples. The test reactor channel was a tubular vessel of internal diameter 46 mm and of length 160 mm, with hemispherical end portions leading to end fittings, giving a total overall length of 227 mm and a total volume of 300 ml. Onset of auto-ignition is apparent from a sudden increase in temperature of the thermocouples. In each case the fuel consisted of 68 wt% methane and 32 wt% of C2-C4 alkanes, and this was mixed with air (79 vol% nitrogen and 21 vol% oxygen), to obtain a desired fuel concentration, FC: 2 vol%, 4 vol% or 6 vol%. The flow rate was adjusted to provide a preset residence time, t(r); for example a residence time t(r) of 150 ms corresponds to a flow rate of 3710 g/h. And the temperature was then gradually ramped up from 450° to 650°C, or, in some of the tests, to a higher temperature or until auto-ignition was observed. Each test was repeated three times to check for consistency.

The auto-ignition temperature is referred to as AIT. In some cases no auto-ignition was observed up to the highest ramp temperature, so the AIT is indicated as being above that temperature (by the symbol >).

Where a range of different AITs were obtained for the same conditions, the range of variation is indicated after the ± symbol.

Table 1

From the experimental results it is clear that for flow rates that give a residence time t(r) less than 500 ms, the auto-ignition temperature AIT is clearly above 650°C. Hence the combustible gas mixture can be preheated to a temperature as high as 650°C before being supplied to the header 30 of the reactor 10, without risking auto-ignition within the header 30, as long as the flow rate is sufficiently high to ensure a residence time t(r) at the elevated temperature of less than 500 ms. This elevated temperature is that to which the combustible gas mixture has been preheated. This is true for fuel concentrations over the range 2 vol% up to 6 vol%. Preheating to 650°C is also acceptable even for a residence time t(r) of 1 s, as long as the fuel concentration does not exceed 4 vol%. However, if the fuel concentration is 6 vol% and the residence time t(r) is 1 s, then it would be advisable not to preheat to above 640°C.

Operating with both the gas streams supplied to the reactor 10 preheated to above 640°C is beneficial in achieving a more uniform temperature distribution, and in increasing the conversion and yield achieved in the steam/methane reforming reaction. As will be appreciated, steam methane reforming may form part of a process for converting methane to longer-chain hydrocarbons, the synthesis gas produced by reforming then being subjected to Fischer- Tropsch synthesis. Alternatively, the synthesis gas may be subjected to a catalytic process to form methanol. The steam methane reforming in any such plant may be carried out using one or more reactors 10 as described above. A plant may incorporate several such reactors arranged in parallel, so that the plant capacity can be adjusted by changing the number of reactors that are utilised.

Referring now to figure 4 there is shown a schematic flow diagram of a chemical plant for performing steam methane reforming, using the reactor 10 described above. Although only one such reactor 10 is shown, it will be appreciated that the plant might incorporate more than one such reactor, either arranged in parallel (to increase capacity) or in series (to increase conversion and yield). The reactor 10 is indicated schematically, showing a flow path 16 for reforming and a flow path 17 for combustion. The exhaust gases from the combustion channels 17 (which emerge from the header 36 shown in figure 2) pass through a first heat exchanger 40 and a second heat exchanger 42 in series before being discharged through an exhaust outlet 44. A third heat exchanger 46 is provided in a duct 45 carrying the gas mixture that emerges from the reforming channels 16, this gas mixture comprising hydrogen and carbon monoxide. The combustible gas mixture, which is indicated as comprising methane and air, is preheated by passage through the first heat

exchanger 40 before being supplied to the reactor 10 along duct 41.

Similarly a mixture of steam and methane is preheated firstly by passage through the heat exchanger 42 and then through the heat exchanger 46, before being supplied to the reforming channels 16 of the reactor 10 along a duct 48.