WO2014114933A1

WO2014114933A1 - Removal of carbon from a catalytic reaction module

Info

Publication number: WO2014114933A1
Application number: PCT/GB2014/050172
Authority: WO
Inventors: Ockert Rudolph Minnie
Original assignee: CompactGTL Ltd
Current assignee: CompactGTL Ltd
Priority date: 2013-01-23
Filing date: 2014-01-22
Publication date: 2014-07-31
Anticipated expiration: 2015-07-23
Also published as: GB201301201D0

Abstract

Carbon deposits are removed from a catalytic reaction module (10) which in normal operation is intended to perform steam reforming, the module comprising at least one reactor (12) defining first flow channels (15) for the steam reforming reaction and second flow channels (16) to provide heat to the first flow channels, with a first inlet (14) communicating with the first flow channels 15, and a second inlet (21 ) communicating with second flow channels. The removal performs combustion of the carbon deposits at progressively higher temperatures. The process comprises: (a) supplying non-reactive gas streams to the first and second flow channels (15, 16), and arranging for the reactor module (10) to cool down to a first temperature below its normal operating temperature; and (b) then supplying to the first inlet an oxidising gas stream which is predominantly non-reactive but also comprises oxygen, the proportion of oxygen being gradually increased from 0% to a raised proportion above 1.6% (vol), while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has been achieved; these steps may be carried out at 350°C. The next stage is: (c) lowering the proportion of oxygen to not less than 0.25% (vol), and increasing the temperature of the reactor to a second temperature below its normal operating temperature, such as 500°C; and gradually increasing the proportion of oxygen to a raised proportion while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has achieved. The third stage is: (e) ensuring the proportion of oxygen is not less than 0.25% (vol), while increasing the temperature of the reactor to a third temperature equivalent to its normal operating temperature; and (f) increasing the proportion of oxygen gradually to at least 3% (vol), while monitoring the temperature of the reaction module.

Description

Removal of Carbon from a Catalytic Reaction Module

This invention relates to a process for removing carbon from a catalytic reaction module with channels for performing a steam reforming reaction.

A plant and process are described in WO 2005/10251 1 (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. WO 2009/101434 (CompactGTL pic) describes a module for performing this steam methane reaction in two stages.

However, when performing steam reforming of a hydrocarbon such as methane there is the possibility, under some reaction conditions, that carbon will be deposited. In the case of a two stage reforming process, this is most likely to occur at the inlet of the second stage, where the gas mixture contains both carbon monoxide and methane. Such carbon deposits can be detrimental to the operation of the reactor, so it would be beneficial to be able to remove them without disrupting operation of the reactor.

According to the present invention there is provided a process for removing carbon from a catalytic reaction module which in normal operation is intended to perform steam reforming, the module comprising at least one reactor defining first flow channels for the steam reforming reaction and second flow channels to provide heat to the first flow channels, the module defining a first inlet which in normal operation is supplied with the reactants for the steam reforming reaction, and defining at least one second inlet communicating with second flow channels of the or the first reactor, the process starting in a state in which the reactor is at a first temperature below the normal operation temperature, the first flow channels contain a gas that is non-reactive within the first flow channels, containing no oxygen, the process comprising performing a controlled combustion operation which comprises supplying to the first inlet an oxidising gas stream which is predominantly non-reactive but also comprises oxygen, the proportion of oxygen being gradually increased from its initial value to a raised proportion above 1 .0% (vol) (i.e. 1 .0% by volume), while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has been achieved, and then lowering the proportion of oxygen to not less than 0.10% (vol); and then repeating this controlled combustion operation at a plurality of progressively higher temperatures. Preferably in the starting state of the process, the second flow channels contain a gas that is non-reactive within the second flow channels.

The presence of oxygen leads to a very exothermic reaction, which risks generating very high temperatures. This can cause damage to a reactor. It is therefore important to ensure that the controlled combustion operations are carried out with careful monitoring of the temperature of the reaction module, and with a low proportion of oxygen, as indicated. By way of example the controlled combustion operation may be carried out at three, four or five successively higher temperatures. The first temperature must be sufficiently high for combustion to be initiated, and should therefore be at least 250°C.

The process of the present invention may also include the step of taking the reaction module from an operating state to the said starting state. Hence the present invention provides a process for removing carbon from a catalytic reaction module which in normal operation is intended to perform steam reforming, the module comprising at least one reactor defining first flow channels for the steam reforming reaction and second flow channels to provide heat to the first flow channels, the module defining a first inlet which in normal operation is supplied with the reactants for the steam reforming reaction, and defining at least one second inlet communicating with second flow channels of the or the first reactor, the process comprising:

(a) supplying to the first inlet a first gas stream which is non-reactive within the first flow channels, and supplying to the second inlet a second gas stream which is non-reactive within the second flow channels, and arranging for the reactor module to cool down to a first temperature below its normal operating temperature; (b) then supplying to the first inlet an oxidising gas stream which is predominantly non- reactive but also comprises oxygen, the proportion of oxygen being gradually increased from 0% to a raised proportion above 1 .0% (vol) (i.e. 1 .0% by volume), while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has been achieved;

(c) lowering the proportion of oxygen to not less than 0.1 % (vol), and increasing the temperature of the reactor to a second temperature below its normal operating temperature;

(d) gradually increasing the proportion of oxygen to a raised proportion while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has achieved;

(e) ensuring the proportion of oxygen is not less than 0.1 % (vol), while increasing the temperature of the reactor to a final temperature equivalent to its normal operating temperature;

(f) increasing the proportion of oxygen gradually to at least 3% (vol), while monitoring the temperature of the reaction module.

This multistage combustion process has been found satisfactory as a way of removing carbon deposits from the first flow channels, without causing detrimental temperature changes or a thermal runaway.

The first temperature may for example be between 250°C and 350°C, for example 290°C; the second temperature may be between 50°C and 150°C higher than the first temperature, for example 350°C; and the final temperature, which is equivalent to the normal operating temperature, may be between 700°C and 800°C, for example about 750°C. It will be appreciated, as indicated above, that the process may be performed at a larger number of temperatures, for example a third temperature might be selected between 400°C and 500°C, and a fourth temperature might be selected between 500°C and 700°C, before performing the step at the final temperature.

In each step in which oxidation occurs, the temperature of the reaction module is monitored. The temperature may for example be taken as the weighted average bed temperature of the catalyst. In each step in which oxidation occurs, if the monitored temperature increases rapidly or if the temperature difference between the first inlet and an outlet from the channels for the steam reforming reaction exceeds a pre-set threshold, the proportion of oxygen may be decreased, until the rapid temperature increase has ceased, or the temperature difference has dropped below the pre-set threshold; the proportion of oxygen can then be increased again. In step (b) the proportion of oxygen is gradually increased from 0% to a raised proportion above 1 .0% (vol) for example above 1 .6% (vol), but the most severe exotherm is expected in the range between 0.1 % and 0.8%, so the increase of oxygen concentration may be most gradual within that range. The raised proportion in step (b) and the raised proportion in step (d) may be different, or may be the same. In one example the raised proportion is 2.0% (vol) in each case. At the end of each controlled combustion operation the oxygen proportion is lowered, but it is not lowered to less than 0.10% (vol) to ensure that reduction of the catalyst does not occur. This lowered proportion may be not less than 0.25%, or not less than 0.30%. In each of the steps (a) to (d) the temperature of the reactant module may be controlled by controlling the temperature of the second gas stream supplied to the second inlet, for example by electrical heating. The second gas stream in each of these steps (a) to (d) may be air, without any fuel. In steps (e) and (f) the temperature of the reactor module may be raised to the third temperature by initiating flow of a combustible gas mixture to the second inlet, so that the temperature is raised by catalytic combustion occurring in the second flow channels; alternatively it may be provided by hot exhaust gases from an exothermic reaction process taking place outside the catalytic reaction module.

In step (f), the proportion of oxygen in the first gas stream may be raised to 5% (vol), even higher, for example 8% (vol).

The oxidising gas stream, as indicated above, predominantly comprises non- reactive gases. Suitable non-reactive gas components would be nitrogen or steam; steam is non-reactive at least for temperatures below about 700°C.

The steps (a) to (f) ensure removal of carbon deposits from the first flow channels. It may then be desirable to supply to the first inlet a purging gas stream which is non-reactive within the first flow channels, to purge oxygen from the first flow channels. For example this purging gas may be nitrogen. This purging step may also be combined with decreasing the temperature of the reactor module, for example to 500°C or less. After purging, normal operation of the reactor module can be initiated by providing methane and steam to the first flow channels.

The process is carried out for a sufficient period of time to remove carbon deposits. The duration of treatment depends on the quantity of deposited carbon, and on the process conditions, but since the reaction between the carbon deposits and the oxygen is exothermic, completion of the carbon removal can be detected by monitoring the temperature of the reactor module, or by monitoring the composition of gases leaving the first flow channels. The reactor module may comprise a compact catalytic reactor in the form of a reactor block defining a multiplicity of first and second flow channels arranged alternately within the block to ensure thermal contact between the first and second flow channels. Preferably the pressure in the first channels is at ambient pressure, that is to say about 100 kPa.

In the method of the present invention, where a combustible gas mixture is provided to the module to produce heat, the combustible gas mixture may be preheated to an elevated temperature below its auto-ignition temperature.

The combustible gas mixture typically comprises a fuel (such as methane) and a source of oxygen (such as air). Within a reactor of the reactor module the first flow channels and the second flow channels may extend in parallel directions. The flow channels may be of length at least 300 mm, for example at least 500 mm, but for example no longer than 1000 mm. One suitable length is between 500 mm and 700 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 embodiment each channel which is intended for a chemical reaction during normal operation 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. 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)). The substrate may be a foil, a wire mesh or a felt sheet, which may be corrugated, dimpled or pleated; a suitable substrate is a thin metal foil for example of thickness typically between 50 μηη and 200 μηη, for example 100 μηη, which is corrugated to define the longitudinal subchannels.

Each reactor block 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.

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 diagrammatic side view of a reaction module of the invention; and Figure 2 shows graphically the variation of temperature through the reactor module of figure 1 , and the corresponding variation of conversion in the steam methane reaction. Normal Operation

The steam reforming reaction of methane 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 (which may be referred to as synthesis gas or syngas). The steam reforming reaction is endothermic, and the heat is provided by catalytic combustion, for example of methane mixed with air. The combustion takes place over a combustion catalyst within adjacent flow channels within a reforming reactor. Preferably the steam/methane mixture is preheated, for example to over 600°C, before being introduced into the reactor. The temperature in the reformer reactor therefore typically increases from about 600°C at the inlet to about 750-800°C at the outlet.

In normal operation, if the reactant mixture is steam and methane, then the steam/carbon ratio is typically between 1.4 and 1 .5. If the reactant mixture also contains carbon dioxide, then the steam/carbon ratio under normal operating conditions may be somewhat lower, for example between 0.9 and 1 .1 , for example 1 .0.

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.

Referring now to figure 1 there is shown a reaction module 10 suitable for use as a steam reforming reactor. The reaction module 10 consists of two reactor blocks 12a and 12b each of which consists of a stack of plates that are rectangular in plan view, each plate being of corrosion resistant high-temperature alloy. Flat plates are arranged alternately with castellated plates so as to define straight-through channels between opposite ends of the stack, each channel having an active part of length 600 mm. By way of illustration, the height of the castellations (typically in the range 2-10 mm) might be 3 mm in a first example, or might be 10 mm in a second example, while the wavelength of the castellations might be such that successive ligaments are 20 mm apart in the first example or might be 3 mm apart in the second example. All the channels extend parallel to each other, there being headers so that a steam/methane mixture can be provided to a first set of channels 15 and an air/methane mixture provided to a second set of channels 16, the first and the second channels alternating in the stack (the channels 15 and 16 being represented diagrammatically), such that the top and bottom channels in the stack are both combustion channels 16.

Appropriate catalysts for the respective reactions are provided on corrugated foils (not shown) in the active parts of the channels 15 and 16, so that the void fraction is about 0.9. A flame arrestor 17 is provided at the inlet of each of the combustion channels 16.

By way of example there may be over fifty such castellated plates in each stack. The steam/methane mixture is supplied through an inlet 14, and flows through the reactor blocks 12a and 12b in series, there being a duct 20 connecting the outlet from the channels 15 of the first reactor block 12a to the inlet of the channels 15 of the second reactor block 12b. Similarly the combustion mixture is supplied through and inlet 21 , and also flows through the reactor blocks 12a and 12b in series, there being a duct 22 connecting the outlet from the channels 16 of the first reactor block 12a to the inlet of the channels 16 of the second reactor block 12b. The duct 22 includes an inlet 24 for additional air, followed by a static mixer 25, and then an inlet 26 for additional fuel, followed by another static mixer 27. Temperature sensors 30 are provided to monitor the temperature in each reactor block 12a and 12b. Although only one temperature sensor 30 is shown in each case, in practice there may be multiple temperature sensors at different positions within each reactor block 12a and 12b. In use of the reaction module 10, the steam/methane mixture is preheated to

620°C, and supplied to the reaction module 10 to flow through the reactor blocks 12a and 12b. A mixture of 80% of the required air and 60% of the required methane (as fuel) is preheated to 550°C, which is below the auto-ignition temperature for this composition, and is supplied to the first reactor block 12a. In both cases the preheating may be carried out by heat exchange with exhaust gases that have undergone combustion within the module 10. The temperature rises as a result of combustion at the catalyst, and the gases that result from this combustion emerge at a temperature of about 700°C. They are mixed with the remaining 20% of the required air (by the inlet 24 and the static mixer 25), and then with the remaining 40% of the required methane (by the inlet 26 and the static mixer 27), so that the gas mixture supplied to the combustion channels 16 of the second reactor block 12b is at about 600°C, which is again below the auto-ignition temperature for this mixture (which contains water vapour and carbon dioxide as a consequence of the first stage combustion). By adjusting the temperature of the additional air supplied at the inlet 24, the temperature of the resulting mixture can be controlled to be below the auto-ignition temperature.

By way of example the gas flow rates may be such that the space velocity is preferably between 14000 and 20000 /hr and possibly more particularly between 15000 and 18000 /hr for the steam methane reforming channels (considering the reaction module 10 as a whole), and is preferably between 19000 and 23000 /hr for the combustion channels (considering the reaction module 10 as a whole).

Referring now to figure 2, this shows graphically the variations in temperature T along the length L of the combustion channels 16 (marked A), and that along the reforming channels 15 (marked B). The portion of the graph between L = 0 and L = 0.6 m corresponds to the first reactor block 12a, while the portion of the graph between L = 0.6 m and L = 1 .2 m corresponds to the second reactor block 12b. It will be noted that the temperature T in a reforming channel 15, once combustion has commenced, is always lower than the temperature T in the adjacent combustion channel 16. The combustion gas temperature undergoes a downward step change as a result of the added air (from inlet 24) between the first reactor block 12a and the second reactor block 12b (at position L = 0.6 m). The variation of conversion of methane, C, in the steam reforming reaction with length L is shown by the graph marked P. The conversion increases continuously through the reaction module 10 and reaches a value of about 80%, which is close to the equilibrium conversion under the reaction conditions.

It will be appreciated that the variations in temperature and conversion shown in figure 2 are by way of example only, and that the temperature distribution and consequently the conversion will be slightly different for example if the combustion catalysts are altered or if the ratio of fuel to air is altered. It will also be appreciated that the module described above is by way of example only and that many changes may be made while remaining within the scope of the present invention. For example the dimensions of the channels 15 and 16 and of the reactor blocks 12 may differ from those indicated above. The proportions of air and methane supplied to the first reactor block 12a may differ from the proportions mentioned above.

Carbon Removal

As discussed earlier, there is a risk that carbon may be deposited in the channels 15 carrying the gas mixture that is to undergo steam/methane reforming. In the case of the two-stage module 10, this is most likely to occur in the vicinity of the inlet to the second reactor block 12b, where the gas mixture contains both CO and methane, and the temperature is locally reduced, but it may also occur in other portions of the second reactor block 12b or in the first reactor block 12a.

The presence of such carbon deposition can, for example, be detected by monitoring temperatures in combustion channels 16 adjacent to the channels 15 for steam/methane reforming (which may be referred to as SMR channels).

A process has now been developed that enables such carbon deposits to be removed while leaving the module 10 in situ. The carbon deposits are removed by performing a controlled combustion within the SMR channels 15. The controlled combustion must be performed carefully, as the combustion reaction is highly exothermic, and the reactor temperature can experience a thermal runaway if care is not taken.

First Combustion Step

If the presence of carbon in the SMR channels 15 is detected, the gases supplied to the SMR channels 15 and to the combustion channels 16 are changed. Methane is no longer supplied any of the channels 15, 16, nor to the inlet 26, but air is supplied through the inlet 21 to the combustion channels 16, and nitrogen is supplied through a secondary inlet 31 to the SMR channels 15. These gases are non-reactive under these circumstances. The temperature of the blocks 12a and 12b can be controlled by heating the air using electrical heaters. The temperature is gradually reduced, at a rate of for example 60°C/hr, down to 270°C. The nitrogen supplied to the SMR channels 15 may be at ambient pressure, about 100 kPa. Once a steady temperature of 270°C has been reached, oxygen is introduced into the nitrogen stream. This may be achieved by combining an oxygen/nitrogen mixture, for example an 8% oxygen/nitrogen mixture, with the pure nitrogen stream. The initial proportion of oxygen is about 0.1 % (vol), and this proportion is very gradually increased, while monitoring the temperature of the reactor blocks 12a and 12b. If the temperature difference between the inlet and outlet of the first reactor block 12a exceeds a threshold, which may for example be between 50°C and 100°C, for example 70°C, this suggests the exothermic combustion of carbon is taking place too rapidly, so the proportion of oxygen would be reduced, until the temperature stabilised. Similarly, if the rate of increase of temperature is too large (say greater than 3-10°C/min), that would also suggest the exothermic combustion is taking place too rapidly, leading to a thermal runaway, and in this case also the proportion of oxygen would be reduced, until the temperature stabilised. The proportion of oxygen is gradually increased, over a period of several hours, up to a proportion of 2.0% (vol). The most vigorous exothermic reaction is expected to take place at a level of oxygen between about 0.1 and 0.8% (vol), initially in the first reactor block 12a, and then in the second reactor block 12b, so this stage must be carried out particularly carefully.

The first step of carbon combustion may be assumed to be complete when a steady state temperature has been achieved for an hour.

Second Combustion Step

The proportion of oxygen supplied to the secondary inlet 31 is then reduced, though to no less than 0.3% (vol) to ensure that an oxidising atmosphere is maintained. The reactor block temperatures are increased to achieve a weighted average catalyst bed temperature of 330°C, by heating the air supplied through the inlet 21 . The temperature at the outlet of the reactor module 10 is monitored, until it stabilises. The proportion of oxygen is then gradually increased. The proportion of oxygen may be increased either to achieve an oxygen level between 0.3 and 0.8% (vol), or alternatively until the temperature difference across any one reactor block 12a or 12b is 70°C. The oxygen proportion is then held steady until the temperatures are steady. The oxygen proportion is then gradually increased again, either to achieve a level between 0.8 and 2.0% (vol) or to achieve a temperature difference across any one reactor block 12a or 12b of 70°C, whichever is observed first. The oxygen proportion is then held constant, until the temperature at the outlet from the second reactor block 12b has dropped to the stabilised value it had had before the increase of the oxygen proportion for this second combustion step.

The oxygen proportion may then be reduced, but not below 0.3% (vol). Third Combustion Step The temperature is then raised to 750°C. This is achieved by providing methane into the air stream supplied through the inlet 21 , and by introducing methane through the inlet 26, as is done during normal operation, so that the temperature is increased by catalytic combustion within the combustion channels 16. With a proportion of oxygen between 0.3% and 0.8% (vol) in the gas stream provided through the secondary inlet 31 , the gases emerging from the reforming channels 15 of the second reactor block 12b are monitored, using a gas analyser 32 which can monitor the level of carbon dioxide, to see if any combustion is occurring. The gas analyser 32 may also monitor the levels of oxygen and of carbon monoxide.

The concentration of oxygen is then gradually increased, in this example to a maximum of 5% (vol) over a period of at least 2 hours, while monitoring the

temperatures of the reactor blocks 12a and 12b, to ensure a thermal runaway does not occur.

Process Completion

Once a steady state has been achieved, as for example indicated by steady temperatures for a period of 30 min, it can be assumed that the combustion has been completed. The supply of oxygen to the secondary inlet 31 is then stopped, but nitrogen continues to be supplied. The flow of nitrogen purges oxygen from the reforming channels 15 and the duct 20. The temperature can be allowed to drop, by ceasing combustion in the combustion channels 16, so the temperature gradually drops to about 500°C. After purging for sufficiently long to ensure removal of oxygen, the flow of steam and methane through the inlet 14 can be restarted, and the flow of gases through the secondary inlet 31 stopped. Hence operation of the reaction module 10 can be restarted.

As is well-known, the performance of the reactor module 10, when performing steam methane reforming, depends on the temperature. Assuming optimum catalyst performance, a particular temperature corresponds to a particular production of carbon monoxide and hydrogen. The performance under other circumstances can be compared, by comparing the temperature to which the reactor would have to be raised in order to obtain the desired output at 750°C. This is the standard output.

In one experimental test, the reactor performance had deteriorated to such an extent that the standard output required a temperature increase of 24.7°C. After performing the carbon removal by combustion as described above, the reactor performance had significantly improved, as the standard output required a temperature increase of only 3.6°C.

It will be appreciated that the controlled combustion process described above may be modified while remaining within the scope of the invention. In particular, controlled combustion may be performed at a larger number of successively higher temperatures, that is to say in a larger number of combustion stages. Furthermore the gas mixture might comprise superheated steam in place of all or part of the nitrogen supplied to the secondary inlet 31 . In some situations it is more convenient to produce superheated steam, rather than requiring pure nitrogen, as water is readily available.

Decoke using steam

As an alternative approach, which also makes use of superheated steam, the reactor blocks 12a and 12b may be heated up to above 700°C by performing catalytic combustion in the combustion channels 16, as described above. Superheated steam at a temperature of above 500°C can then be supplied to the secondary inlet 31 (without any oxygen), and passed through the reforming channels 15 of the reactor blocks 12a and 12b. The steam reacts with the carbon via the following reaction: C(s) + H₂0(g) -> CO(g) + H₂ (g) ΔΗΓ = +131 .30 kJ mol^"1

The reaction is endothermic and therefore requires a high temperature (T > 700°C) for the reaction to occur. The reaction is also favoured by operating at a low pressure, for example at atmospheric pressure (about 100 kPa). This can be used as the sole process for removing carbon deposits from the reforming channels 15.

It will be appreciated that this use of steam at a temperature above 700°C may be combined with the controlled combustion process described above. For example after performing a plurality of controlled combustion operations at successively higher temperatures, a final treatment may use superheated steam without oxygen.

Claims

1 . A process for removing carbon from a catalytic reaction module which in normal operation is intended to perform steam reforming, the module comprising at least one reactor defining first flow channels for the steam reforming reaction and second flow channels to provide heat to the first flow channels, the module defining a first inlet which in normal operation is supplied with the reactants for the steam reforming reaction, and defining at least one second inlet communicating with second flow channels of the or the first reactor, the process starting in a state in which the reactor is at a first temperature below the normal operation temperature, the first flow channels contain a gas that is non-reactive within the first flow channels, containing no oxygen, the process comprising performing a controlled combustion operation which comprises supplying to the first inlet an oxidising gas stream which is predominantly non-reactive but also comprises oxygen, the proportion of oxygen being gradually increased from its initial value to a raised proportion above 1 .0% (vol), while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has been achieved, and then lowering the proportion of oxygen to not less than 0.10% (vol); and then repeating this controlled combustion operation at a plurality of progressively higher temperatures.

2. A process as claimed in claim 1 wherein the first temperature is between 250°C and 400°C.

3. A process as claimed in claim 1 or claim 2 wherein, in the starting state of the process, the second flow channels contain a gas that is non-reactive within the second flow channels.

4. A process as claimed in claim 3 with the catalytic reaction module initially in an operating state, wherein the process also comprises:

(a) supplying to the first inlet a first gas stream which is non-reactive within the first flow channels, and supplying to the second inlet a second gas stream which is non-reactive within the second flow channels, and arranging for the reactor module to cool down to the first temperature,

so as to achieve the starting state of the process.

5. A process as claimed in any one of the preceding claims, starting with the reactor module containing non-reactive gas streams and being at the first temperature, comprising:

(b) then supplying to the first inlet an oxidising gas stream which is predominantly non- reactive but also comprises oxygen, the proportion of oxygen being gradually increased from 0% to a raised proportion above 1 .6% (vol), while monitoring the temperature of the reaction module, and then maintaining the raised proportion of oxygen until a steady state has been achieved;

(c) lowering the proportion of oxygen to not less than 0.25% (vol), and increasing the temperature of the reactor to a second temperature below its normal operating temperature;

(e) ensuring the proportion of oxygen is not less than 0.25% (vol), while increasing the temperature of the reactor to a final temperature equivalent to its normal operating temperature;

6. A process as claimed in claim 5 wherein the second temperature is between 300°C and 550°C.

7. A process as claimed in claim 5 or claim 6 wherein the final temperature is between 700°C and 800°C.

8. A process as claimed in any one of the preceding claims wherein, in a step in which oxidation occurs, if the monitored temperature increases rapidly or if the temperature difference between the first inlet and an outlet from the channels for the steam reforming reaction exceeds a pre-set threshold, the process involves decreasing the proportion of oxygen until the rapid temperature increase has ceased, or the temperature difference has dropped below the pre-set threshold, and then increasing the proportion of oxygen again.

9. A process as claimed in any one of the preceding claims, wherein in at least one controlled combustion operation the raised proportion of oxygen is 2% (vol).

10. A process as claimed in any one of the preceding claims, wherein, in at least one controlled combustion operation, the proportion of oxygen is increased either to achieve a level between 0.8 and 2.0% (vol) or to achieve a temperature difference across a reactor of the reaction module above a threshold, whichever is observed first.

1 1 . A process as claimed in any one of the preceding claims wherein, in a final controlled combustion operation, the proportion of oxygen in the first gas stream is raised to 5% (vol).

12. A process as claimed in any one of the preceding claims wherein the reactor of the reactor module is a compact catalytic reactor in the form of a reactor block defining a multiplicity of first and second flow channels arranged alternately within the block to ensure thermal contact between the first and second flow channels.

13. A process as claimed in any one of the preceding claims wherein the pressure in the first channels is 100 kPa.

14 A process as claimed in any one of the preceding claims wherein, after performing a plurality of controlled combustion operations, the catalytic reaction module is heated to above 700°C, and superheated steam at above 700°C, without any oxygen, is supplied to the first inlet.

15. A process for removing carbon from a catalytic reaction module which in normal operation is intended to perform steam reforming, the module comprising at least one reactor defining first flow channels for the steam reforming reaction and second flow channels to provide heat to the first flow channels, the module defining a first inlet which in normal operation is supplied with the reactants for the steam reforming reaction, and defining at least one second inlet communicating with second flow channels of the or the first reactor, the process comprising ensuring the temperature of the reactor is above 700°C, and supplying superheated steam without oxygen and at a temperature above 700°C to the first inlet.

16. A process as claimed in claim 15 wherein the temperature of the reactor is achieved by performing catalytic combustion in the second flow channels.