WO2011055130A1

WO2011055130A1 - Catalytic reactor shutting-in process

Info

Publication number: WO2011055130A1
Application number: PCT/GB2010/051659
Authority: WO
Inventors: Neil Maxted; Robert Peat
Original assignee: Compactgtl Plc
Priority date: 2009-11-03
Filing date: 2010-10-04
Publication date: 2011-05-12
Also published as: GB0919182D0; TW201119735A

Abstract

A catalytic reactor that is used for treatment of a reactant gas stream comprising hydrogen can be shut-in by contacting the catalyst with a treatment gas that contains carbon monoxide as a higher proportion of the reactive components than in the reactant gas stream fed to the reactor during normal operation. A preferred treatment gas contains carbon monoxide as the only reactive component. This is applicable to a Fischer-Tropsch reactor with active catalytic material such as cobalt, incorporated into a ceramic support layer on a metal substrate. This process suppresses the risk of thermal runaway when the reactor is restarted.

Description

Catalytic Reactor Shutting-in Process

This invention relates to a process for the

treatment of a catalytic reactor prior to operation. It is applicable, particularly but not exclusively, to catalytic reactors for which the reactants comprise hydrogen and carbon monoxide (synthesis gas or syngas), for example Fischer-Tropsch synthesis and methanol synthesis. It relates, in particular, to a shut-in process for such a reactor.

The Fischer-Tropsch synthesis process is a well- known process in which synthesis gas 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.

According to the present invention there is provided a process for shutting-in a catalytic reactor for a catalytic reaction between reactants in a reactant gas stream, the reactants comprising hydrogen and carbon monoxide, the process comprising introducing shutting-in gas into the reactor to suppress the catalytic reaction, the shutting-in gas comprising at least one reducing agent, wherein the shutting-in gas comprises carbon monoxide, and the ratio of carbon monoxide to hydrogen in the treatment gas is greater than that in the reactant gas stream.

For example in the case of a Fischer-Tropsch catalytic reactor for operation with a reactant gas stream comprising synthesis gas with a ratio of hydrogen to carbon monoxide in the range 2.6 to 1.9 (corresponding to a carbon monoxide proportion between 27,8% and 34.5%) the shutting-in gas would preferably comprise at least 40% CO, more preferably at least 60%, still more

preferably at least 80% CO, as a proportion of the reactive gases . Indeed the treatment gas might consist entirely of carbon monoxide. Alternatively the shutting- in gas may comprise an inert gas such as argon or nitrogen in combination with reactive gas, for example a 10% CO/90% nitrogen mixture. The shutting-in of the reactor, so as to suppress the catalytic reaction, 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 reduce the risk of thermal runaway when the catalytic reaction is restarted. The shutting-in 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 hydrogen. It will be appreciated that such a tail gas also contains other components, such as carbon dioxide, ethane and methane, but these are inert under these conditions.

It will be appreciated that shutting-in of a reactor is intended to suppress the reaction occurring in the reactor, while also protecting the catalyst from any changes, so that its characteristics are not altered. This may be contrasted with a catalyst activation or regeneration process, which brings about the reduction of the catalyst material, for example converting cobalt oxide to cobalt metal.

The invention will now be further and more

particularly described, by way of example only.

The present invention is particularly suitable for treatment of Fischer-Tropsch 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 grooves in plates arranged as a stack, or by spacing strips and plates in a stack, the stack then being bonded together. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. The stack of plates forming the reactor is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing.

To ensure the required good thermal contact between the synthesis reaction and the coolant stream both the first and the second 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. 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 pm 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 μm, which is corrugated to define the longitudinal sub-channels. The catalyst element may for example comprise a single shaped foil, for example a corrugated foil of thickness 50 μm ; 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 enables the catalyst structures within the channels for the synthesis reaction to be protected during shut-in of the reactor.

The invention is of relevance to a chemical process 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:

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:

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.

The activity and selectivity of the catalyst depends upon the degree of dispersion of cobalt metal upon the support, the optimum level of cobalt dispersion being typically in the range 0.1 to 0.2, so that between 10% and 20% of the cobalt metal atoms present are at a surface. The larger the degree of dispersion, clearly the smaller must be the cobalt metal crystallite size, and this is typically in the range 5-15 nm. Cobalt particles of such a size provide a high level of

catalytic activity, but may be oxidised in the presence of water vapour, and this leads to a dramatic reduction in their catalytic activity. The extent of this

oxidation depends upon the proportions of hydrogen and water vapour adjacent to the catalyst particles, and also their temperature, higher temperatures and higher proportions of water vapour both increasing the extent of oxidation. It is understood that during a regeneration process, this oxidation of the small cobalt particles is reversed, and they are converted back to the metal.

It is important that the characteristics of the catalyst are not significantly altered during a shut-in. Although shut-in can be performed using a gas such as hydrogen, which ensures there is no risk of oxidation of the catalyst, it has been found that there is a potential for thermal runaway when the catalytic reaction is restarted. It is envisaged that this may occur because, if hydrogen is already present on the surface of the catalyst, then when the reaction restarts methane is produced in preference to longer-chain molecules. This methane production produces more heat than does the production of longer-chain molecules.

It has been found that by using a treatment gas for shut-in that is high in carbon monoxide, for example pure carbon monoxide or a mixture of nitrogen and carbon monoxide, these problems are avoided. By way of example such a mixture may contain between 10% and 90% CO, with the remainder nitrogen. When operation restarts, there is an increase in the production of longer-chain

molecules, and the risk of thermal runaways is

suppressed .

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. In accordance with the present invention, as described above, the reactor is instead flushed through with CO, or a gas mixture containing CO, and is shut-in in this state. It has been found that if the reactor is then brought back online there is a decrease in methane formation during the initial bedding-in stage before steady-state operation is achieved. This is a clear benefit from shutting in with CO.

The shutting-in gas may be tail gas from the

Fischer-Tropsch synthesis reaction that has been treated, if necessary, to remove hydrogen. The hydrogen removal may be achieved using a membrane, or by pressure swing absorption. Hence a gas composition may be obtained that comprises less than 20% hydrogen, and at least 80% CO, as proportions of the reactive components, and such a gas composition is suitable for use in the shutting-in process.

Most previously-known catalyst regeneration

processes have used hydrogen as the reducing agent.

Although this is effective at regenerating the catalyst, when the catalyst is subsequently brought back on line it is found that methane is produced in preference to longer chain molecules, and there is a significant time delay (typically several days of operation) before steady-state operation is achieved, with the formation of longer chain molecules. This problem may be avoided by using carbon monoxide as a reducing agent.

After regeneration of the Fischer-Tropsch catalyst, the reactor can then be brought back on line as desired. During the bed-in process the reactor is preferably provided with synthesis gas with a comparatively low proportion of hydrogen, for example with hydrogen: CO ratio of 1.5:1. This suppresses methane formation while hydrocarbon intermediates are gradually formed on the catalyst surface. After a bedding-in time of for example 200 hr operation, it can be assumed that the catalyst has reached its steady-state; and the synthesis gas

composition can then be returned to a higher value (with a hydrogen: CO ratio between 1.8 and 3.0:1, for example 1.9:1) while retaining selectivity to longer chain hydrocarbons, because hydrocarbon intermediates are now covering the catalyst surface, and/or because the catalyst at this stage is coated with a thin layer of waxy hydrocarbons through which the hydrogen and the CO of the synthesis gas must diffuse in order to react, and which therefore moderates the reaction. 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 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 shutting-in a catalytic reactor for a catalytic reaction between reactants in a reactant gas stream, the reactants comprising hydrogen and carbon monoxide, the process comprising introducing shutting-in gas into the reactor to suppress the catalytic reaction, the shutting-in gas comprising at least one reducing agent, wherein the shutting-in gas comprises carbon monoxide and the ratio of carbon monoxide to hydrogen in the shutting-in gas is greater than that in the reactant gas stream.

2. A process as claimed in claim 1 wherein at least 60% of the reducing agent in the shutting-in gas is CO.

3. A process as claimed in claim 2 wherein at least 80% of the reducing agent in the shutting-in gas is CO.

4. A process as claimed in claim 3 wherein the reducing agent in the shutting-in gas is CO.

5. A process as claimed in any one of the preceding claims wherein the shutting-in gas also comprises an inert gas,

6. A process as claimed in claim 5 wherein the inert gas comprises nitrogen.

7. A process as claimed in any one of the preceding claims wherein the catalytic reaction is Fischer-Tropsch synthesis .

8. A process as claimed in claim 7 wherein the shutting- in gas comprises a tail gas from the Fischer-Tropsch synthesis ,

9. A process as claimed in claim 8 wherein the tail gas is processed to remove at least some of the hydrogen, before use in the shutting-in process.