NL1006477C2 - Reactor and method for carrying out a chemical reaction. - Google Patents

Description

Title: Reactor and method for carrying out a chemical reaction.

The invention relates to a reactor for carrying out chemical reactions, more in particular reactions with a high heat effect, and to a method for carrying out chemical reactions in such a reactor.

Many chemical reactions are characterized by a positive heat effect (exothermic reaction) or a negative heat effect (endothermic reaction). An efficient supply or removal of the heat of reaction is often indispensable for the chemical reactions to proceed in the desired manner. In some exothermic reactions, the thermodynamic balance shifts in an undesired direction as the temperature rises. Examples thereof are the synthesis of ammonia and methanol, the oxidation of sulfur dioxide to sulfur trioxide in the production of sulfuric acid, the reaction of sulfur dioxide with hydrogen sulfide in the Claus process, the selective oxidation of H2S to elemental sulfur and the reaction of carbon monoxide with hydrogen to methane . Since thermal energy is released during the course of these reactions, the temperature of the reaction mixture will increase and the thermodynamic equilibrium will adversely shift unless the released heat of reaction is removed from the reactor quickly and efficiently.

Also in endothermic reactions a shift in undesired direction of the thermodynamic equilibrium can occur, now due to the consumption of thermal energy. Examples are the methane steam reforming and the dehydrogenation of ethylbenzene to styrene. The problem may also arise that, as a result of the consumption of energy by the reaction, the temperature of the reaction mixture drops too much, as a result of which the desired reaction no longer proceeds.

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In addition to shifting the thermodynamic equilibrium in an unfavorable direction, a temperature change can also adversely affect the selectivity of catalytic reactions.

5 Examples of reactions where temperature affects selectivity are the production of ethylene oxide from ethylene (the undesired reaction is the formation of water and carbon dioxide), the selective oxidation of hydrogen sulfide to elemental sulfur (the undesired reaction is the formation of SO2) and the Fischer Tropsch synthesis. In all these cases, a temperature increase occurs due to the release of the reaction heat. If this temperature increase is not prevented by a rapid dissipation of the reaction heat, the selectivity decreases sharply.

Most conventional catalytic reactors use a fixed bed of catalyst particles. Porous bodies of catalyst particles are poured or stacked in such a catalyst bed.

In order to avoid an undesirably high pressure drop over such a catalyst bed, it is preferred to use bodies or particles with dimensions of at least 0.3 mm. These minimal dimensions of the catalyst bodies are necessary to maintain the pressure drop that occurs when a stream of reactants is passed through the catalyst bed within technically acceptable limits. In addition to the dimensions limited by the permissible pressure drop at the bottom, the necessary activity of the catalyst imposes an upper limit on the dimensions of the catalytically active particles. The high activity required for a number of types of technical catalysts can usually only be achieved with an active phase surface area of 25 to 500 m2 per ml of catalyst volume. Surfaces of such an order of magnitude are possible only with very small particles, for example with 35 particles of 0.05 µm. Since particles of such dimensions are no longer able to flow through a liquid or gas mixture, the primary extremely small particles must be formed into highly porous bodies with dimensions of at least about 0.3 mm, which may have a large catalytic surface. An important task in the production of technical catalysts is to combine the required high porosity with a sufficiently high mechanical strength. The catalyst bodies must not disintegrate when the reactor is filled and when it is exposed to sudden temperature differences ("thermal shock").

As can be seen from the examples given above, there is a great need for a rapid supply or removal of thermal energy in catalytic reactors in combination with a low pressure drop. According to the current state of the art, it is virtually impossible to efficiently supply or remove thermal energy to a conventional solid catalyst bed. This is evident from the manner in which chemical reactions, such as methane steam reforming and the selective oxidation of ethylene to ethylene oxide, are carried out in solid catalyst beds.

Selective oxidation of ethylene uses a very large heat-exchanging surface by using a reactor with no less than 20,000 long pipes. In the methane-steam reforming, attempts are made to optimize the heat supply and limit the pressure drop by adjusting the size and shape of the catalyst bodies. In the latter reaction too, a large number of expensive pipes have to be used in the reactor.

In a number of technically important cases, it is desired to work with a large to very large spatial throughput speed in catalytic reactions, whereby a large pressure drop over the reactor is regarded as a minor drawback. In the conventional fixed bed reactors, a high pressure drop with the associated large spatial throughput speed is not quite possible. If the pressure at the reactor inlet is increased, the catalyst can be blown out of the reactor (gaseous reactants) or purged (liquid reactants). It is also possible that "channeling" occurs at a certain critical value of the pressure at the reactor inlet. In that case, the catalyst particles start to move in a certain part of the reactor. In that case, the reactants appear to flow almost exclusively through that part of the catalyst bed that is in motion.

The fixed bed reactors currently clog the catalyst bed. It is therefore necessary to regularly open the reactor 10 and remove the accumulated dust layer. It would be beneficial if one could pass a pulse of high pressure gas through the reactor in a direction opposite to that of the flow of reactants. This pressure pulse would blow the dust from the catalyst bed; this could prevent clogging without opening the reactor, which is technically very attractive. However, this is not possible with the prior art fixed bed catalysts; the catalyst bodies are then blown out of the catalyst bed with the substance.

It has been proposed to apply the catalyst exclusively to the wall of the reactor. An example of such a system is described in the extract of JP-A 6/111838. According to this publication, a reforming catalyst has been placed in grooves of a plate, while a combustion catalyst has been placed in grooves of a second plate. These plates are arranged against each other, so that the heat generated with the combustion can be reformed.

Also in carrying out the Fischer Tropsch reaction, in which higher hydrocarbons are produced from a mixture of hydrogen and carbon monoxide, a system has been used in which a catalyst is applied to the wall of the reactor. This wall-mounted catalyst ensures good heat transfer from the catalyst to the outside of the reactor. The following method has been proposed for applying the catalyst to the wall, inter alia. The catalyst is applied to the wall as a Raney metal, an alloy of the active metal and aluminum. After application, the catalyst is activated by dissolving the aluminum with caustic. The major part of the reactor volume is empty, so that the contact of the reactants with the catalytically active surface is low and the conversion per passage through the reactor is strongly limited. The reactants therefore have to be recycled frequently through the reactor.

In a number of technically important cases, the pressure drop as the reactants pass through the catalyst bed must remain very small. This applies, for example, to reactors in which flue gas must be purified from large installations, such as in the catalytic removal of nitrogen oxides from flue gas. Because a flue gas flow is generally very large, a considerable pressure drop requires a great deal of mechanical energy. The same applies to the purification of automotive exhaust gases. In this case too, a high pressure drop is not permitted.

Currently, the use of honeycombed catalysts is one of the few options for achieving an acceptable pressure drop without inadmissibly reducing contact with the catalyst. 25 ceramic honeycombs ("honeycombs", "monoliths") are often used for this, in which the catalytically active material is applied.

A variant of the method in which the catalyst is only applied to the wall is formed by the use of monoliths built up from thin metal plates. Such a reactor is manufactured, for example, by rolling up a combination of corrugated and flat thin metal plates and then welding it on. The flat plates can also be stacked in a way that leads to a system with a large number of 35 channels. The catalyst is then applied to the wall of the channels thus obtained.

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As noted, thermal conductivity in a fixed catalyst bed is poor. This has been attributed to the low thermal conductivity of the highly porous supports to which the catalytically active material is applied. Therefore, Kovalanko, O.N. et al., Chemical Abstracts IL (18) 151409u proposed to improve thermal conductivity by increasing the conductivity of the catalyst bodies.

They did this by using porous metal bodies as a catalyst support. It has already been described by Satterfield that the thermal conductivity of a stack of porous bodies is not so much determined by the conductivity of the material of the bodies, but by the contacts between the bodies themselves (C.N.

Satterfield, "Mass Transfer in Heterogeneous Catalysis", MIT Press, Cambridge, MA., USA (1970), page 173). The inventors' own measurements have shown that the thermal conductivity of catalyst bodies does indeed not strongly influence the heat transport in a catalyst bed.

In WO-A 86/02016, a reactor is disclosed comprising a catalysed reaction bed consisting of sintered metal particles which are in good heat-conducting connection with the reactor wall, which wall is provided on the outside with sintered metal particles for the removal of reaction heat . A phase transition also takes place on the outside of the reactor.

US-A 4,101,287 describes a 'combined heat 30 exchanger reactor' consisting of a monolith, part of which channels are flowed through the reactants and part of it through a coolant. The same drawback occurs here as with the system of WO-A 8602016.

EP-A 416710 discloses a method based on the use of a catalytic reactor in which the reactor bed consists of elementary metal particles sintered together on one side of the reactor wall and on the other side of the reactor wall. sintered metal particles are present. When the diameter of the reactor bed is chosen in such a reactor in relation to the heat effects which are different from reaction to reaction, but known and, depending on the reaction conditions, to be calculated, reactions of the intended type can be optimally carried out.

WO-A 9632118 discloses a system in which two different reactions are carried out using at least two reactor beds which are in good heat-exchanging contact with each other. These reactor beds are preferably built up of sintered metal spheres.

Although the use of sintered metal provides a clear improvement of the heat balance in reactors for chemical reactions, a number of drawbacks still remain. Most notable is the problem that a bed of sintered metal particles produces a fairly large pressure drop. In some cases this can seriously hinder the application.

It is therefore an object of the invention to provide a reactor for carrying out chemical reactions, in particular reactions with a large heat effect, wherein these problems do not occur, or only to a lesser extent.

The invention therefore comprises a reactor for carrying out a chemical reaction, which reactor is provided with at least one bed of catalytically active material, which bed consists of sintered elemental metal bodies and which is in heat-exchanging contact with the wall of the reactor, the improvement of which is that the elemental metal bodies have an empty fraction between 0.25 and 0.95.

The invention is based on the surprising insight that through the use of formed metal particles 1006477 8 with an empty fraction of 0.25 to 0.95, a sintered bed can be obtained which meets the requirements of an optimized process operation.

The invention makes it possible for the reactor bed to have a large free volume on the one hand, which manifests itself in a small pressure drop, while on the other hand a large surface area is available for the catalyst. An advantage of the invention is also that the weight of the reactor per unit volume is also lower.

The metal particles used in the reactor according to the invention are characterized in that they have an empty fraction. This is understood to mean that the particle has a shape such that within the smallest enclosure of the particle there is always at least 25% free space. The term smallest enclosure can be visualized as a fleece with as small an area as possible, which is applied stress-free around the particle. An alternative formulation is the smallest volume around the particle that cannot be crossed twice by the same straight sections. The concept of empty fraction is then formed by the open volume within the smallest enclosure, divided by the volume of the smallest enclosure.

The metal bodies to be used according to the invention can consist of extruded metal, of sheet metal which has been pressed, punched or folded into a suitable shape, or of bodies formed of metal wire. This therefore involves bodies other than solid spheres or fibers.

The largest dimension of the metal bodies to be used according to the invention is preferably not less than 2.5 mm. This is the minimum value for the largest dimension of the smallest enclosure (as defined above) of the body.

The desired porosity of the bed can be adjusted by choosing the shape of the particles. It is also thereby possible to set the surface available for the catalyst 1006477 9, almost independently of the porosity.

The specific shape of the particles can be selected within the definition of the invention. Various groups of particles, which are not exhaustively enumerated, can be distinguished here, which are mainly characterized by the nature of the manufacturing method.

A first group of particles is formed by the extruded metal particles, such as open cylinders and profiles. A second group is formed by the bodies formed from metal wire, i.e. wire figures such as wire spring shaped bodies and rings. A third group consists of complex structures such as crow's feet. The fourth group are 15 die-cut and / or bent bodies. One can think of profiles but also of other shapes such as saucers and V½ caps.

In this context, extruded metal or extruded metal particles are metal which is formed by extrusion and which is in the form of tubes or other forms obtainable by extrusion. By bringing these extrusions to a suitable length, particles of the desired size are obtained.

The chemical reactions carried out according to the invention take place under the most suitable conditions for the selected reactions. Due to the good heat exchange, there is a better heat transfer, which is reflected in a flatter temperature profile in the reactor. This system also offers the possibility to process more heat production per volume unit.

The temperature maintained in the reactor depends on the nature of the reaction. Generally, an elevated temperature is maintained because the advantages of the system are the most pronounced.

Generally, the temperature will be above 100 ° C, with an upper limit being the maximum temperature at which the material is still stable, or the temperature at 1006477. chemical reaction can be achieved. However, temperatures above 1250 ° C are generally not preferred because of the difficulties in reaching them and the demands such temperatures place on the materials of the reactors and the supply and discharge systems.

The pressures at which the reaction is carried out can be varied within wide limits, it may be noted.

The contiguous porous structure used in the reactor according to the invention can be constructed in various ways, as will also become apparent from the further explanation and examples of suitable structures. In general, the contiguous porous structure must meet the requirement that there is a heat exchanging contact between the dividing wall and the structure, while the porous structure also extends through the entire reactor bed.

This means that the porous structure is preferably rigidly connected to at least one reactor wall, while the reactor bed consists of a structure that fills the entire reactor, or at least extends through the entire reactor, in the form of solidly connected elementary particles such as the sintered particles. It is especially important that the heat exchange is especially good in the direction transverse to the wall. The main heat flow takes place in that direction. This is less important in the length of the reactor.

It is noted that it is preferable to sinter the metal bodies to the wall. However, from the viewpoint of efficiency and economy of the construction of the reactor, it may sometimes be preferable that the bed is not sintered firmly to the wall, but does have good heat exchange contact therewith. The disadvantage of slightly less effectiveness is then more than offset by the simplicity of reactor construction and ease of reactor bed replacement.

The degree of porosity of the reactor bed in the invention can be varied within wide limits. This porosity, ie the part of the bed that can be flowed through by gas or liquid, is generally between 20 and 95% by volume. The most suitable value depends on the nature of the reactor, the desired surface area, the desired pressure drop, the type of reaction (ie the degree of heat production) and the desired degree of heat transport in the bed.

The degree of heat transfer is a relatively important factor in the reactor systems of the invention. Naturally, the heat conductivity of the entire system, ie from the partition wall to the beds, is partly determined by the heat conductivity of the material used of the catalyst support and of the construction material of the reactor.

Preferably, the heat conductivity is not less than 10% of the heat conductivity of the material used in the solid state; preferably this value is between 10 and 75%. In absolute terms, the heat conductivity is preferably between 0.2 and 300 W / m.K.

The thermal conductivity is strongly dependent on the thermal conductivity of the elementary materials used. A sintered body of 31SL has a value of 3-12 W / m.K. 316L powder, on the other hand, has a value of 0.55, while solid material has 20 W / m.K. 30 Solid copper has a heat conductivity of 398 W / m.K. All these values refer to the condition at room temperature. At other temperatures the absolute value of the numbers changes, but the mutual relationship remains approximately the same.

35 The thermal conductivity of the system as a whole is also important for its operation. As indicated in 1006477 12, there must be a heat exchanging contact between the reactor bed and the wall. More particularly, it is important that there is good contact, under reaction conditions, between the reactor wall and the reactor bed. This is preferably achieved by sintering the elemental metal particles to the wall.

The reactor system according to the invention is applicable to any heterogeneously catalyzed gas phase reaction, but is more particularly suitable for those reactions which have a strong thermal effect, ie strong endo or exothermic reactions, or reactions whose selectivity is strongly temperature -dependent.

It is possible to operate with a large to very large spatial throughput speed, without the catalyst being blown out of the reactor (gaseous reactants) or purged (liquid reactants). Nor does "channeling" occur. Since the catalyst particles are much better fixed in the reactor according to the invention, it is possible to work with such a reactor at a much higher speed of the reactants (and consequently a much higher pressure drop across the reactor). Another important advantage of fixing the catalyst bodies in the reactor according to this embodiment is evident in the deposition of dust on the catalyst bed. In reactors 25 of the present invention, a pulse of high pressure gas may be passed through the reactor in a direction opposite to that of the flow of reactants. This pressure pulse blows the dust off the catalyst bed; this can prevent clogging without opening the reactor, which is technically very attractive.

The invention is particularly suitable for carrying out strongly exothermic or endothermic catalytic reactions. In general, such a reaction can be selected from the group of reform reactions, shift reactions, oxidation reactions, and reduction reactions. Examples 1006477 13 thereof include hydrogenation and dehydrogenation reactions.

As an example of such a reaction, the oxidation of methane is described. As an example of a reaction, the selectivity of which is strongly determined by the temperature, the selective oxidation of sulfur hydrogen is taken. In this case, the removal of thermal energy is of great significance since above a temperature of about 300 ° C the oxidation of sulfur vapor to the undesired sulfur dioxide proceeds. The use of a reactor system according to the invention makes it possible to purify gas flows with a hydrogen sulphide content of, for example, 10% by volume, very efficiently. The hydrogen sulfur is selectively oxidized to elemental sulfur which is extremely easy to separate by condensation. Since such gas mixtures cannot be properly processed in a Claus process, the invention is of particular importance for this.

As already indicated, the reactor system 20 used according to the invention can be built up in a number of ways.

A possible embodiment is described in European patent application EP-A 416.710. Another embodiment is described in WO-A 9632118. The contents of these two publications are incorporated herein by reference.

The metal or metal alloy can then be catalytically active itself or be catalytically active by treatment, but it is also possible to apply a catalytically active material thereon. One of the advantages of a catalyst on such metal particles lies in the better heat distribution by using the metal. On a micro scale it is observed that the heat management in the catalyst is better, so that the catalyst is used more efficiently. This influences, inter alia, the activity, but may also be of importance for the selectivity, for instance in case the selectivity is strongly dependent on the temperature. This is because according to the invention a much more homogeneous temperature distribution is obtained in the catalyst.

The advantage of this is, among other things, that one does not get hot or cold spots in the bed. Hot spots lead to aging or degradation of the catalyst, while hot spots do not react. As a result of the more homogeneous temperature distribution, a more stable and more effective reaction is therefore obtained.

Suitable metals for use in elementary particles include nickel, iron, chromium, manganese, vanadium, cobalt, copper, titanium, zircon, hafnium, tin, antimony, silver, gold, platinum, palladium, tungsten, tantalum as well as the lanthanides and actinides. The elementary particles may consist of substantially pure metal or an alloy of two or more metals, which alloy may also contain non-metallic components, such as carbon, nitrogen, oxygen sulfur, silicon, and the like.

The reactor system according to the invention can, as already indicated, already be catalytically active by itself or be activated by treatment. However, it is also quite possible to apply a catalytically active material to the fixedly connected elementary bodies. More specifically, it is possible to first apply a (highly) porous support to the metal or alloy surface and then to apply the catalytically active component to the support. The latter may be significant if the catalytically active component is not allowed to come into direct contact with the material of the sintered bodies to avoid undesired interactions between the material of the bodies and the catalytically active component.

When applying a catalyst, a dispersion of a carrier and / or the catalytically active material (or a precursor therefor) is first prepared in a liquid and then this liquid is suitably applied to the tightly bonded elementary bodies. This can be done, for example, by vacuuming the bed on which the carrier and / or the catalytically active material is to be applied and then drawing the dispersion into the bed, so that the bed is impregnated. When a carrier is first applied, the operation can be repeated with the catalytically active component or precursor therefor.

The invention is further elucidated in the drawing by means of a few examples, which are not intended as a limitation of the invention. The figure shows four types of particles, with different open fractions.

A first group is formed by the extruded metal particles, such as open cylinders and profiles. A second group is formed by the bodies formed from metal wire, i.e. wire figures such as wire springs and rings. A third group consists of complex structures such as crow's feet. The fourth group are sheet metal cut and / or curved bodies.

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Claims (8)

1. Reactor for carrying out a chemical reaction, which reactor is provided with at least one bed of catalytically active material, which bed consists of sintered elemental metal bodies and which is in heat-exchanging contact with the wall of the reactor, the improvement consisting of the elemental metal bodies having an empty fraction between 0.25 and 0.95. Reactor according to claim 1, wherein the metal bodies have an empty fraction between 0.25 and 0.75. Reactor according to claim 1 or 2, wherein the bodies are selected from extruded metal bodies, filamentary bodies and shaped plate bodies. The reactor of claims 1-3, wherein the extruded metal bodies are tubular. Reactor according to claims 1-4, wherein the metal bodies are sintered to the wall. 6. Method for carrying out a chemical reaction, which is characterized in that a reactor according to claims 1-5 is used. The method of claim 6, wherein the reaction is selected from endothermic and exothermic reactions. The method of claim 6, wherein the method is selected from reform reactions, shift reactions, oxidation reactions, and reduction reactions. 1006477