WO2013164583A1

WO2013164583A1 - A process for purging a catalytic reactor used to convert synthesis gas

Info

Publication number: WO2013164583A1
Application number: PCT/GB2013/051068
Authority: WO
Inventors: Edwin NTAINJUA NDIFOR; André RITTERMEIER
Original assignee: Compactgtl Limited
Priority date: 2012-05-02
Filing date: 2013-04-26
Publication date: 2013-11-07
Also published as: TW201402205A; GB201207716D0

Abstract

The catalytic reactor comprises a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions. The catalyst is in the form of solid catalytic bodies without free liquid. Operation of the reactor is stopped by performing a shutting-in procedure. After the shutting-in procedure, operation of the reactor is restarted. The restarting comprises removing products such as hydrocarbons and water from the surface of the catalyst gradually so as to avoid damage to the pore structure of the catalyst by purging the catalyst with a flowing gas that contains a reducing agent e.g. hydrogen, for between 6 and 12 hours, at a temperature at or below the normal operating temperature of the catalyst.

Description

A PROCESS FOR PURGING A CATALYTIC REACTOR USED TO CONVERT SYNTHESIS GAS

This invention relates to a process for use with a catalytic reactor, particularly where the reactor contains a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions, for example Fischer-Tropsch synthesis or methanol synthesis.

The Fischer-Tropsch synthesis process is a well-known process in which synthesis gas, i.e. a combination of hydrogen and carbon monoxide, reacts in the presence of a suitable catalyst to produce hydrocarbons. This may form the second stage of a process for converting natural gas to a liquid or solid hydrocarbon, as natural gas can be reacted with either steam or small quantities of oxygen to produce the synthesis gas. A range of different types of reactor are known for performing the Fischer-Tropsch synthesis; and a range of different catalysts are suitable for Fischer- Tropsch synthesis. For example cobalt, iron and nickel are known catalysts, with different characteristics as to the resulting product.

During operation it may occasionally be necessary to cease operation of a catalytic reactor, and this may be referred to as a shut-in process. This may be a scheduled shutdown, or may be unscheduled. For example this may be necessary in a modular plant, where the number of reactors that are in use is changed in accordance with the flow rate of the gas to be treated. The shut-in process involves introducing gases into the reactor such that the catalytic reaction stops, without damaging the catalysts. By way of example hydrogen has been used as a shut-in gas, as have gases that are inert such as nitrogen and argon. It has been found that problems can arise when operation of the reactor is subsequently restarted, and that there can be a significant reduction in the performance of the catalyst and the productivity of the reactor. By way of example US 6 878 655 described a process for regenerating a hydrocarbon synthesis catalyst in the form of a slurry , in which a first stage involves contacting the slurry of the catalyst with a dry stripping gas, so as to remove water from the catalyst slurry. The stripping process is preferably carried out at a temperature above 200 °C, such as 230 °C, but at a pressure which is lower than that used for Fischer-Tropsch synthesis. Some examples also include a second stage in which the catalyst is reactivated, for example using hydrogen at a temperature significantly above the Fischer-Tropsch operating temperature. According to a first aspect of the present invention there is provided a process for use with a catalytic reactor, the catalytic reactor comprising a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions, the catalyst being in the form of solid catalytic bodies without free liquid, wherein operation of the reactor is stopped by performing a shutting-in procedure, and wherein after the shutting-in procedure operation of the reactor is restarted, wherein the restarting comprises removing products such as hydrocarbons and water from the surface of the catalyst gradually so as to avoid damage to the pore structure of the catalyst by purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours.

The flowing gas removes vapours of products such as hydrocarbons and water from the catalyst surface. This purging is carried out at a temperature at or below that at which, during normal operation of the reactor, the synthesis gas is converted to the product (hereinafter referred to as "the normal operating

temperature"). The purging may be carried out at a temperature well below the normal operating temperature, for example at ambient temperature.

The process may entail reducing the pressure to below 0.2 MPa (2 bar) (absolute pressure). The process may also comprise heating the catalyst up to an elevated temperature, unless the catalyst is already at or above the elevated temperature, the elevated temperature being below the normal operating

temperature of the catalyst by not more than 35 K, the heating being at less than 20 K/h; and may then also comprise purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours at the elevated temperature. The purging may also be carried out during the heating process.

After performing this process the catalyst can be returned to normal operation by contacting it with synthesis gas, raising the pressure to a normal operating pressure, and raising the temperature to the normal operating temperature.

The catalyst is in the form of solid catalytic bodies without free liquid, that is to say the catalytic bodies are not immersed or suspended in liquid. In operation a thin film of liquid may form on surfaces of the catalytic items, this liquid consisting of products of the synthesis reaction, but there is a continuous flow path for reactant gases past the catalytic items. The pressure reduction step may entail reducing the pressure to below 0.5 MPa (5 bar), or to below 0.2 MPa (2 bar), and may be to 0.1 MPa (1 bar). The low pressure enhances the rate of evaporation of any liquid or solids on the catalyst surface.

The purging steps may be for longer periods. For example each purging step may last for at least 4 hours, for example each may last for 8 hours. The flowing gas or, in other words, the purging gas may be synthesis gas, or a nitrogen/hydrogen mixture, and is preferably a dry gas, so not containing a significant concentration of water vapour.

The heating up to the elevated temperature should be performed slowly. It must be no more than 20 K/h, and may be significantly slower, for example 10 K/h, 5 K/h or 3 K/h. Such a slow temperature increase avoids rapid evaporation of any liquid, such as water, on the catalyst surface.

The elevated temperature may be somewhat higher, for example not more than 20 K below the normal operating temperature; but is typically at least 10 K below the normal operating temperature.

The purging gas may comprise less than 10% of a reducing agent, in combination with at least one inert gaseous component. The purging gas may comprise 5% hydrogen, in combination with at least one inert gaseous component. The restarting may also comprise changing, throughout a period of at least one hour, the composition of the purging gas from an initial composition to a final composition corresponding to synthesis gas. The temperature during this step may be the normal operating temperature of the catalyst and the pressure may be a normal operating pressure.

According to a second aspect of the present invention, there is provided a catalytic reactor comprising: a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions, the catalyst being in the form of solid catalytic bodies without free liquid; and at least one controller operable to stop operation of the reactor by performing a shutting-in procedure, and, after the shutting-in procedure, to restart operation of the reactor, wherein the restarting comprises removing products such as hydrocarbons and water from the surface of the catalyst gradually so as to avoid damage to the pore structure of the catalyst by purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours, at a temperature at or below the normal operating temperature of the catalyst.

The at least one controller may be operable to perform any one or more of the abovedescribed process steps.

According to a third aspect of the present invention, there is provided a catalytic reactor for converting a synthesis gas to a product, the product being in a liquid or a solid state at ambient conditions, the reactor comprising: a structure configured to hold solid catalytic bodies such that there is a continuous flow path for the synthesis gas past the catalytic bodies; and a controller operable to temporarily stop the conversation of the synthesis gas to the product and subsequently purge the solid catalytic bodies with a flowing gas that includes a reducing agent, for between 6 and 12 hours, at a temperature at or below a temperature at which the synthesis gas is converted to the product, to remove hydrocarbons and water from surfaces of the solid catalytic bodies without damaging pore structures of the solid catalytic bodies. Not only does the treatment process prevent the decrease in catalyst performance that would otherwise occur, surprisingly the treatment process has been found to enhance the catalyst performance. In one example the treatment process when applied to a Fischer-Tropsch catalyst was found to increase the productivity of C5+ hydrocarbons by at least 40%.

The shutting-in of the reactor, so as to suppress the catalytic reaction or, in other words, to temporarily stop the conversion of the synthesis gas to the product, may be either scheduled or unscheduled. The reactor would subsequently be brought back on stream by restarting the supply of the reactant gas stream. The process of the present invention has been found to improve catalyst performance when the catalytic reaction is restarted.

The purging gas may for example comprise a tail gas from a Fischer-Tropsch synthesis reaction that, if necessary, has been treated to remove at least some of the constituents. It will be appreciated that such a tail gas contains not only hydrogen and carbon monoxide, but other components, such as carbon dioxide, ethane and methane, which are inert under these conditions. The invention will now be further and more particularly described, by way of example only.

The present invention is particularly suitable for treatment of catalysts within compact catalytic reactors, wherein each reactor consists of a stack of plates that define synthesis flow channels and coolant flow channels arranged alternately within stack. Within each reactor the first and second 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 then being bonded together. Alternatively the flow channels may be defined by spacing strips and plates in a stack, or by grooves in plates arranged as a stack, the stack then being bonded together. The stack of plates forming the reactor is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. In such a compact catalytic reactor, good thermal contact between the synthesis reaction and the coolant stream can be achieved by having both the first and the second flow channels between 10 mm and 2 mm high (in cross-section); and each channel may be of width between about 3 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 1 mm and 20 mm. For example the plates might be 0.5 m wide and 0.8 m long; and they might define channels for example 7 mm high and 6 mm wide, or 3 mm high and 10 mm wide, or 10 mm high and 5 mm wide. Catalyst structures are inserted into the channels for the synthesis reaction, and can if necessary 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. There may, in some cases, be two or more catalyst structures within a channel, arranged end to end. 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 coating of ceramic support material on the metal substrate, which provides a support for the catalyst. The ceramic support is preferably in the form of a coating on the metal substrate, for example a coating of thickness 100 μηι on each surface of the metal. 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)), but other materials such as stainless-steel may also be suitable. 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 μηι, which is corrugated to define the longitudinal subchannels. The catalyst element may for example comprise a single shaped foil, for example a corrugated foil of thickness 50 μηι; this is particularly appropriate if the narrowest dimension of the channel is less than about 3 mm, but is also applicable with larger channels. Alternatively, and particularly where the channel depth or width is greater than about 2 mm, the catalyst structure may comprise a plurality of such shaped foils separated by substantially flat foils. The active catalytic material would be incorporated in the ceramic coating.

The invention is of relevance to a chemical plant for converting natural gas (primarily methane) to longer chain hydrocarbons. The first stage of this process is to produce synthesis gas, and preferably involves steam reforming, that is to say the reaction:

H₂0 + CH₄ → CO + 3 H₂

This reaction is endothermic, and may be catalysed by a rhodium or

platinum/rhodium catalyst in a first gas flow channel. The heat required to cause this reaction may be provided by combustion of a fuel gas such as methane, or another short-chain hydrocarbon (e.g. ethane, propane, butane), carbon monoxide, hydrogen, or a mixture of such gases, which is exothermic and may be catalysed by a palladium/platinum catalyst in an adjacent second gas flow channel. Alternatively the synthesis gas may be produced by a partial oxidation process or an autothermal process, which are well-known processes; these produce synthesis gases of slightly different compositions.

The synthesis gas mixture is then used to perform a Fischer-Tropsch synthesis to generate longer chain hydrocarbons, that is to say: n CO + 2n H₂→ (CH₂)_n + n H₂0 which is an exothermic reaction, occurring at an elevated temperature, typically between 190°C and 280 °C, and an elevated pressure typically between 1 .8 MPa and 2.8 MPa (absolute values), in the presence of a catalyst such as iron, cobalt or fused magnetite. The preferred catalyst for the Fischer-Tropsch synthesis comprises a coating of gamma-alumina of specific surface area 140-230 m²/g with about 10-40% cobalt (by weight compared to the alumina), and with a promoter such as ruthenium, platinum or gadolinium which is less than 10% the weight of the cobalt, and a basicity promoter such as lanthanum oxide. Other suitable ceramic support materials are titania, zirconia, or silica. The preferred reaction conditions are at a temperature of between 200 °C and 240 °C, and a pressure in the range from 1 .5 MPa up to 4.0 MPa, for example 2.1 MPa up to 2.7 MPa, for example 2.6 MPa.

Although described in relation to a Fischer-Tropsch synthesis process, the invention would also be applicable for example in a reactor for producing methanol from synthesis gas. As mentioned above the reaction channels contain catalyst in the form of catalytic bodies without free liquid. For example the body may include a coating of ceramic support material on a corrugated foil metal substrate, the coating providing a support for the catalyst material. The catalytic bodies locate within flow channels, and reactant gases flow through the flow channels. There may be a thin film of liquid on surfaces of the catalytic bodies as a result of the reaction, but the catalytic bodies are not immersed or suspended in liquid, and there is a continuous flow path for reactant gases past the catalytic bodies.

It is important that the characteristics of the catalyst are not detrimentally affected during a shut-in. The shut-in gas may comprise an inert gas such as argon or nitrogen, and may contain a reducing agent such as hydrogen or carbon monoxide. This reduces the risk of oxidation of the catalyst. Nevertheless it has been found that the reactor productivity decreases after a shut-in. A plant for performing Fischer-Tropsch synthesis may comprise a number of

Fischer-Tropsch synthesis reactors operated in parallel, each reactor being provided with cut-off valves so that it can be disconnected from the plant. A reactor that has been cut-off in this way would conventionally be flushed through with an inert gas to suppress further reactions, or with a shut-in gas.

The shutting-in gas may be tail gas from the Fischer-Tropsch synthesis reaction that has been treated, if necessary, to remove some constituents. In particular it may be beneficial to treat such a tail gas to lower the concentration of water vapour. The purging steps of the catalyst treatment process may use the same gas as the shutting-in gas. It will be appreciated that at least one controller may be provided to fully or partially control one or more aspects of the operation of the reactors, e.g. the feed rates of the various gases or constituents thereof.

Example 1

As an example, a compact catalytic Fischer-Tropsch reactor has a normal operating condition of 220 °C at a pressure of 2.6 MPa (26 bar) (absolute). It is shut in using nitrogen with 5% hydrogen gas, and the reactor is allowed to cool to ambient temperature. The subsequent treatment is as follows.

The pressure within the reactor is then lowered to atmospheric pressure (0.1 MPa), and the reactor is purged with dry nitrogen with 5% hydrogen gas at ambient temperature (between 15° and 25 °C) for 8 hours. The reactor is then gradually heated up to 190°C at only 3 K/h, that is 0.05 K/min, over 56 hours, while continuing to purge. The reactor is then purged with dry nitrogen with 5% hydrogen gas at this elevated temperature of 190°C for a further 8 hours.

After this purging process, the gas mixture is changed back to synthesis gas, the pressure is increased to the normal operating pressure, 2.6 MPa, and the temperature gradually increased back to the normal operating temperature. This may be at successively lower rates, for example at 6 K/h from 190°C up to 212°C, then at 1 K/h from 212 ^<Ό up to 217Ό, and then at 0.2 K/h from 217^<Ό up to 220 °C.

In this example the reactor productivity for C5+ hydrocarbons (i.e.

hydrocarbons with at least 5 carbon atoms) was 0.68 g/(g cat. h) After performing the shut-in and treatment described above, the C5+ productivity was found to have increased to 0.95 g/(g cat. h). After a second shut-in and treatment as described above the C5+ productivity was found to have increased to 1 .0 g/(g cat. h). A third such shut-in and treatment did not alter the productivity. Example 2

As a second example, a compact catalytic Fischer-Tropsch reactor has a normal operating condition of 225°C at a pressure of 2.6 MPa (26 bar) (absolute). It is shut in using nitrogen with 5% hydrogen gas, and allowing the reactor to cool to ambient temperature. The subsequent treatment is as follows.

The pressure within the reactor is then lowered to atmospheric pressure (0.1 MPa), and the reactor is purged with dry nitrogen gas with 5% hydrogen at ambient temperature (between 15° and 25 °C) for 8 hours. The reactor is then gradually heated up to 190°C at only 3 K/h, that is 0.05 K/min, over 56 hours, while continuing the flow of the purging gas mixture. The reactor is then purged with dry nitrogen with 5% hydrogen gas at this elevated temperature of 190 °C for a further 8 hours. After this purging process, the gas mixture is changed back to synthesis gas, the pressure is increased to the normal operating pressure, 2.6 MPa, and the temperature gradually increased back to the normal operating temperature. This was at progressively lower rates, at 6 K/h from 190°C up to 215°C, then at 1 K/h from 215°C up to 222^<Ό, and then at 0.2 K/h from 222 ^<Ό up to 225°C.

In this example the reactor productivity for C5+ hydrocarbons was 0.51 g/(g cat. h) After performing the shut-in and treatment described above, the C5+ productivity was found to have increased to 0.85 g/(g cat. h). Example 3

As a third example, a compact catalytic Fischer-Tropsch reactor has a normal operating condition of 225 °C at a pressure of 2.6 MPa (26 bar) (absolute). It is shut in using nitrogen with 5% hydrogen gas, and allowing the reactor to cool to Ι θδ'Ό. The subsequent treatment is as follows.

The pressure within the reactor is lowered to atmospheric pressure (0.1 MPa), and the reactor is purged with dry nitrogen with 5% hydrogen gas at ^~\ 95°C for 12 hours. This temperature is above the elevated temperature of the previous examples, and no gradual heating step is required.

After this purging process, the gas mixture is changed back to synthesis gas, the pressure is increased to the normal operating pressure, 2.6 MPa, and the temperature gradually increased back to the normal operating temperature. This may again be at progressively lower rates, at 4 K/h from 195°C up to 215^qC, then at 1 .5 K/h from 215°C up to 221 °C, and then at 0.3 K/h from 221 °C up to 225 °C.

It is believed that on shut-in of such a Fischer-Tropsch reactor, pores of the catalyst are at least partly filled with water and/or liquid or solid hydrocarbons, both of which are formed by the Fischer-Tropsch reaction. The prolonged purges at both the initial temperature and the elevated temperature encourage evaporation of such materials out of the pores, while the slow temperature rise to the elevated

temperature avoids any rapid phase change causing damage to the catalyst structure.

It will also be appreciated that the purging process of the invention can lead to an enhancement in the catalyst performance. It will be understood that no additional reactivation or regeneration of the catalyst is required.

Example 4 As a fourth example, a compact catalytic Fischer-Tropsch reactor has a normal operating condition of 220 °C at a pressure of 2.6 MPa (26 bar) (absolute). The normal gas flow is syngas (1 .9:1 H₂:CO) containing 12.7% nitrogen. The reactor is shut in at the normal operating temperature of 220 °C by stopping the flow of the gases. After 10 minutes of this syngas shut-in, the subsequent treatment is as follows:

The pressure and temperature within the reactor are maintained as during normal operation (2.6 MPa and 220°C), and the reactor is purged with substantially pure dry nitrogen at a normal nitrogen feed rate for 5 minutes.

The nitrogen feed rate is then increased, e.g. doubled, and syngas is then introduced in small steps, e.g. an additional 2% of the normal feed rate is added every 3 to 10 minutes, whilst slowly reducing the nitrogen feed rate to the normal nitrogen feed rate target. This process lasted 7 hours from when feed to the reactor was stopped to when the normal or target syngas feed rate to the reactor was achieved. In this example, the reactor productivity for C5+ hydrocarbons was

0.42 g/(g cat. h). After performing the shut-in and treatment described above, the C5+ productivity was found to have increased to 0.65 g/(g cat. h). It is believed that the reason for the increase in performance is as follows:

Initial syngas re-introduction at significantly lower flow rates, e.g. 2 to 25% of normal flow rate, leads to complete CO conversion which increases the hydrogen concentration on the catalyst or favours catalyst reduction in syngas. This is believed to favour reduction of partially oxidized cobalt, removal of liquid products and carbon- based impurities and other impurities from the catalyst, or selective formation of more active metallic Co with a hexagonal close-packed structure (hep) from Co carbide (which has been identified in the used catalyst, and is known to contribute to catalyst deactivation). During this mode, it is anticipated that the formation of methane and gaseous products over the catalyst is significantly higher than the formation of liquid products which is favoured during normal operation. This is believed to favour catalyst reduction.

Example 5 As a fifth example, a compact catalytic Fischer-Tropsch reactor containing a pellet catalyst has a normal operating condition of 235 °C at a pressure of 2.6 MPa (26 bar) (absolute) with CO conversion of around 56%. The CO feed to the reactor is stopped for around 10 to 12 hours whilst normal H₂ and N₂ flows continue feeding the reactor at the same temperature and pressure. An attempt to reintroduce a normal CO flow under the normal operating conditions (235°C, 26 bar) led to a runaway temperature rise which tripped the reactor. An alternative and more satisfactory way of reintroducing CO is as follows:

CO is re-introduced under the normal operating conditions (235°C, 26 bar) in a step-wise manner. By way of example, 50% of the normal CO flow is first added and held for 30 minutes to ensure that all internal temperatures are approximately constant. The CO flow is then increased to 75% and then 87.5% of the normal CO flow, followed by an increase of the CO flow to the normal CO flow target, with a 20 minutes hold in between each increase.

In this example the reactor productivity for C5+ hydrocarbons per channel volume was 231 g/l h with 56% conversion. After performing the treatment described above, the C5+ productivity and CO conversion were found to have increased to 279 g/l h and 69% respectively.

It is believed that this increase in performance is related to hydrogen- treatment of the catalyst during the 10 to 12 hours during which there was no CO flow to the reactor and/or due to the reasons given above in relation to Example 4.

Although the process of the invention has been described above in relation to Fischer-Tropsch reactors, it will be appreciated that it would be equally applicable to a range of different reactors, such as methanol-forming reactors. It has been described in some Examples in relation to reactors in which the catalyst is supported on a corrugated foil, but it is equally applicable to reactors where the catalyst is coated on to channel walls, and to fluidised pellet bed reactors.

Claims

1 . A process for use with a catalytic reactor, the catalytic reactor comprising a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions, the catalyst being in the form of solid catalytic bodies without free liquid, wherein operation of the reactor is stopped by performing a shutting-in procedure, and wherein after the shutting-in procedure, operation of the reactor is restarted, wherein the restarting comprises removing products such as hydrocarbons and water from the surface of the catalyst gradually so as to avoid damage to the pore structure of the catalyst by purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours, at a temperature at or below the normal operating temperature of the catalyst.

2. A process as claimed in claim 1 wherein the flowing gas is at a pressure below 0.2 MPa.

3. A process as claimed in claim 1 or claim 2 wherein the temperature is an elevated temperature, the elevated temperature being below the normal operating

temperature of the catalyst by not more than 35 K.

4. A process as claimed in claim 1 or claim 2 also comprising heating the catalyst up to an elevated temperature, the elevated temperature being below the normal operating temperature of the catalyst by not more than 35 K, the heating being at less than 20 K/h.

5. A process as claimed in claim 4 which comprises the subsequent step of purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours at the elevated temperature.

6. A process as claimed in claim 4 or claim 5 wherein the heating up to the elevated temperature is at no more than 3 K/h.

7. A process as claimed in claim 2 or any one of claims 4 to 6 wherein the elevated temperature is below the normal operating temperature by not more than 20 K.

8. A process as claimed in claim 7 wherein the elevated temperature is at least 10 K below the normal operating temperature.

9. A process as claimed in any one of the preceding claims wherein the flowing gas comprises less than 10% of a reducing agent, in combination with at least one inert gaseous component.

10. A process as claimed in claim 9 wherein the flowing gas comprises 5% hydrogen, in combination with at least one inert gaseous component.

1 1 . A process as claimed in claim 9 wherein the flowing gas is obtained from a tail gas produced during operation of the reactor.

12. A process as claimed in claim 1 wherein the restarting comprises a subsequent step of changing, throughout a period of at least one hour, the composition of the flowing gas from an initial composition to a final composition corresponding to synthesis gas.

13. A process as claimed in claim 12 wherein the temperature during the

subsequent step is the normal operating temperature of the catalyst and the pressure is a normal operating pressure.

14. A process as claimed in any one of claims 1 to 12 wherein the catalyst is then returned to normal operating conditions by exposing the catalyst to synthesis gas, raising the pressure to a normal operating pressure, and raising the temperature to the normal operating temperature.

15. A process substantially as hereinbefore described with reference to any one of the Examples.

16. A catalytic reactor comprising:

a catalyst for converting synthesis gas to a product which is liquid or solid at ambient conditions, the catalyst being in the form of solid catalytic bodies without free liquid; and

at least one controller operable to stop operation of the reactor by performing a shutting-in procedure, and, after the shutting-in procedure, to restart operation of the reactor, wherein the restarting comprises removing products such as

hydrocarbons and water from the surface of the catalyst gradually so as to avoid damage to the pore structure of the catalyst by purging the catalyst with a flowing gas that contains a reducing agent, for between 6 and 12 hours, at a temperature at or below the normal operating temperature of the catalyst.

17. A catalytic reactor according to claim 16, wherein the at least one controller is operable to perform a process according to any one of claims 2 to 14.