The present invention relates to a chemical reactor, particularly but not exclusively a chemical reactor in which an exothermic reaction occurs. The invention also relates to the control of a reaction taking place in such a chemical reactor. It would be relevant, for example, to a plant and a process for treating natural gas to produce a liquid product.

It is well known that most oil wells also produce natural gas, which may be referred to as associated gas. At many oil wells natural gas is produced in relatively small quantities along with the oil. When the quantities of associated gas are sufficiently large or the well is close to pre-existing gas transportation infrastructure, the gas can be transported to an off- site processing facility. When oil production takes place in more remote places it is difficult to introduce the associated gas into existing gas transportation infrastructure. In the absence of such infrastructure, the associated gas has typically been disposed of by flaring or re-injection. However, flaring the gas is no longer environmentally acceptable, while re- injection can have a negative impact on the subsequent quality of the oil from the well.

Gas-to-liquids technology can be used to convert the natural gas into liquid hydrocarbons and may follow a two-stage approach to hydrocarbon liquid production comprising syngas generation, followed by Fischer-Tropsch synthesis. In general, syngas (a mixture of hydrogen and carbon monoxide) may be generated by one or more of partial oxidation, auto-thermal reforming, or steam methane reforming. Where steam methane reforming is used, the reforming reaction is endothermic and so requires heat, and a catalyst such as platinum/rhodium; the heat may be provided by a combustion reaction. The syngas is then subjected to Fischer-Tropsch synthesis, for which a suitable catalyst is cobalt on a ceramic support, and which is an exothermic reaction.

Such a process is described for example in WO 01 / 51 194 (AEA Technology) and WO 03/048034 (Accentus pic). Reactors for performing the reactions, such as

steam/methane reforming and Fischer-Tropsch synthesis, may include a stack of plates shaped to define first and second sets of flow paths: an exothermic reaction may take place in one set of flow paths, and an endothermic reaction (or a coolant) be provided in the other set of flow paths. During operation it would be desirable to monitor the performance of the reactor, and one relevant parameter is the reactor temperature. However, measurement of temperatures within a reactor is not straightforward. According to an aspect of the present invention there is provided a reactor comprising a reactor block having a plurality of alternately arranged flow channels, wherein the plurality of flow channels are arranged such that fluids in at least two adjacent flow channels can exchange heat through an intervening wall, and wherein the intervening wall defines at least one channel extending within the reactor block and communicating with an outside surface of the block and dimensioned to accommodate a temperature sensor.

The reactor may comprise a reactor block defining multiple first and second flow channels, the first and second flow channels being arranged alternately within the block and separated by intervening walls, such that fluids in the first and second flow channels can exchange heat through the intervening walls, wherein at least some of the intervening walls define at least one channel extending within the thickness of the wall and communicating with an outside surface of the block, to accommodate a temperature sensor. The reactor block may comprise 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. In each flow-channel in which a chemical reaction is to be performed, a catalyst may be provided. The catalyst may be coated onto the walls of the flow channel, or be provided in discrete pellets, such as extrudates or spheres, or as particles or powders, packed within the channel, or may be provided on gas-permeable non-structural elements.

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 may be bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. The temperature sensor is preferably selected to be suitable for operation at the temperature at which the reactor block is expected to operate, and one such temperature sensor is a thermocouple.

Ideally it would be feasible to monitor the temperature of the fluids undergoing the chemical reaction, but this may not be practical. A temperature sensor within a flow channel may disrupt the flow; and replacing such a temperature sensor may be inconvenient or impractical. Since heat is transferred between the fluids in the first flow channels and the second flow channels, there is a temperature difference between the first flow channels and the second flow channels, and a temperature gradient through the intervening wall.

Nevertheless measuring the temperature within the intervening walls has been found to provide sufficiently accurate information to enable the reaction to be monitored and controlled. Since the channel communicates with the outer surface of the reactor block, the temperature sensor can be removed and replaced during operation of the reactor. 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. It is therefore advantageous to monitor the performance of the reactor on a continuous basis, and to adjust the flow rates of any reactants in such a way as to prevent thermal runaway.

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 block might define flow channels for more than two different fluids.

To ensure the required thermal contact between fluids flowing within the first and second flow channels, 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). As mentioned above, catalyst structures may be inserted into the channels, and may be removed for replacement. At least in some cases the catalyst structures are nonstructural, 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.

Where catalyst structures are provided, each such catalyst structure may be shaped so as to subdivide the flow channel into a multiplicity of parallel flow sub-channels. Each catalyst structure may include 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. The metal substrate may be 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)), 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 μιη, which is corrugated to define the longitudinal subchannels. 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 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:

Figure 1 shows a schematic perspective view, partly in section, of part of a reactor block suitable for steam/methane reforming;

Figure 2 shows a perspective view of one of the plates forming the reactor block of figure 1 ; Figure 3 shows a perspective view of a temperature sensor for use in the reactor block of figure 1 ; and

Figure 4 shows graphically a temperature variation during operation of a steam/methane reforming reactor.

The invention would be applicable in a reactor 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 portion of a reactor 5 suitable for use as a steam reforming reactor, or for use in a steam reforming reactor. The reactor 5 includes a reactor block 10 which defines channels 16 for a catalytic combustion process and channels 17 for steam methane reforming. The reactor block 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 or 13 are arranged alternately with castellated plates 14 or 15, so the castellations define the 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 is in each case 0.9 mm. The height of the castellations 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 the wavelengths may be 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.

Each of the flat plates 12 is a single 2 mm-thick plate, whereas each of the flat plates 13 is formed of two flat plates 13a of thickness 1 mm that are bonded together. In the reactor block 10 two such flat plates 13 are shown, but in practice the flat plates 13 would be provided at any height within the reactor block 10 at which the temperature is to be monitored. For example a reactor block might be made only using the flat plates 13, without any flat plates 12.

As shown in figure 2, each of the flat plates 13a defines a groove 13b extending from one edge of the plate 13a, and in this case each groove 13b is of semicircular shape and of radius 0.55 mm. The grooves 13b on adjacent surfaces of each pair of flat plates 13a are aligned with each other, so that the grooves 13b together define a straight channel 20 which, as shown in figure 1 , communicates with an outer surface of the reactor block 10. At each end of the stack is a flat end plate 19, which in this case is of thickness 4.0 mm. In one example the number of castellated plates 14 and 15 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 figure 1 , in a practical reactor 5 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.

If the reactor 5 is to be used for steam/methane reforming, providing the heat by catalytic combustion, then appropriate catalysts for those reactions would then be provided in each of the channels 16 and 17. For example a respective catalytic insert 22 or 24 (only one of each are shown in Figure 1 ), carrying a catalyst for the respective reaction, may be inserted into each channel 16 or 17. 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 μιη. A thermocouple 25, as shown in figure 3, of diameter 1 mm, can be installed into one or more of the channels 20. The length of each thermocouple 25 is selected so that the end of the thermocouple 25 is at a position within the reactor block 10 where the temperature is to be measured. It will be appreciated that the reactor block 10 is shown by way of example only. For example the thickness of the plates 12 and 13 may be in the range 0.5 to 4 mm, and may differ from each other; while the thickness of the castellated plates 14 and 15 may be in the range 0.2 to 3.5 mm. The heights of the castellations may be between 2 mm and 10 mm, and the separation between successive fins may be between 10 mm and 50 mm. So by way of example the channels 16 and 17 may instead be of height 6 mm and of width 7 mm. The reactor block 10 may also be modified, for example in that the outermost channels, those adjacent to the end plate 19, may for example be blocked off so that no gases pass through them, hence reducing heat loss. Other configurations for the reactor block 10 may be used according to the present invention. For example, the reactor block 10 may be formed by structures other than plates.

By way of example, in the context of a reactor block 10 whose components have been bonded by brazing, there may be an upper temperature limit for safe operation, for example that the metal of the plates 12, 13, 14 and 15 should not exceed 820 °C and that the braze which bonds them together should not exceed 815°C. During operation the temperature within the combustion channels 16 must be higher than that in the reforming channels 17, and there will be a temperature variation within each channel. For example, within the centre of a combustion channel 16 the temperature may be between 40° and 90 °C above that at the adjacent flat plate 12 or 13, while within the centre of a reforming channel 17 the temperature may be between 20° and 40 °C below that at the adjacent flat plate 12 or 13; this temperature difference will tend to be greater, the greater the height of the channel 16 or 17. The fins separating adjacent channels 16 (or adjacent channel 17) will be at a lower temperature than at the centre of the channel 16 (or 17), because of heat transfer through the fin.

Hence, during normal operation, the metal within the fins separating adjacent combustion channels 16 may be at about 780 °C, and the flat plates 12 or 13 may be at about 770°C; within the combustion channels 16 the maximum gas temperature may be about 830 °C, while within the reforming channel 17 the maximum gas temperature may be about 740 °C.

If there were to be a thermal runaway within a combustion channel 16, the temperature at the catalyst insert 22 will rapidly increase; there is a time delay before the temperature at the adjacent flat plate 12 or 13 starts to increase; and a further time delay before the temperature in the adjacent reforming channel 17 starts to increase. To control operation of the reactor 5 and to ensure that the safe operating temperatures are not exceeded, it would therefore be necessary to adjust the gas flows to the combustion channels 16 as soon as such a temperature increase is detected. Although the

thermocouples 25 do not enable the maximum temperature within the combustion channels 16 to be measured, they do enable the temperature to be monitored in the vicinity of the brazed bonds, and so they enable the reactor 5 to be controlled in such a way that the safe operating temperatures are not exceeded.

Referring now to figure 4, this shows graphically the variation in temperature, T, with time, t, in an experimental test carried out in such a steam/methane reforming reactor; and also shows the variation in fuel flow rate F. The line A shows the variation of temperature within a plate 13 separating a combustion channel 16 from a reforming channel 17, as measured by a thermocouple 25 as described above. The line B shows measurements of the temperature of the gas within the reforming channel 17, while the line C shows measurements of the temperature of the gas within the combustion channel 16. The line D shows the variations in the fuel flow rate, F. The time, t, merely indicates the start of the graph; the observations were actually started after 1223 hours of operation. The fuel flow rate, F, is adjusted with the aim of maintaining a constant temperature. Referring to line C, it is observed that at a time t = 0.62 hours the temperature in the combustion channel 16 starts to increase from its initial, steady value of about 720 °C, and at a time t = 0.70 hours it has reached a value of 733 °C, which is steady. At the time t = 0.78 hours the fuel flow rate is reduced from about 0.2268 kg/hr down to 0.2240 kg/hr, and the temperature gradually decreases over the following half an hour.

Referring to line A, it is observed that there is a similar increase in temperature, starting at substantially the same time: the temperature increases from an initial steady temperature of about 693 °C, and gradually rises to a peak temperature of about 703 °C. When the temperature in the combustion channel 16 drops, the temperature in the plate 13 drops but the change is less abrupt. Referring to line B, it is observed that there is also an increase in temperature in the reforming channel 17, starting at substantially the same time: the temperature increases from an initial steady temperature of about 646 °C, and reaches a maximum temperature of about 650 °C. When the temperature in the plate 13 drops, the temperature in the reforming channel 17 also drops, but the change is less abrupt.

Comparing the increases in temperature: that for the combustion gas was 13°C; that for the plate 13 was 10°C, and that for the reforming gas was 4°C. The abruptness of the temperature change varies in a similar way, being greatest for the combustion gas channel 16 and being least for the reforming channel 17. It is nevertheless clear that the

thermocouple 25 within the plate 13 responds closely to combustion gas temperatures, so that by monitoring the temperature within the plate 13, changes in the temperature in the combustion channel 16 can be detected. Hence the thermocouples 25 enable the operation of a steam/methane reforming reactor 5 to be monitored and therefore controlled. If the temperature in the plate 13 rises more rapidly than desired, or rises to a higher temperature than desired, then the operation of the reactor 5 would be modified either by decreasing the supply of fuel to the combustion channels 16 or by increasing the flow of the

steam/methane mixture supplied to the reforming channels 17.