WO1999001210A1

WO1999001210A1 - Reactor and method for carrying out a chemical reaction

Info

Publication number: WO1999001210A1
Application number: PCT/NL1998/000381
Authority: WO
Inventors: Marco Robin Van Hessem
Original assignee: Gastec NV
Current assignee: Kiwa Gastec Holding NV
Priority date: 1997-07-04
Filing date: 1998-07-03
Publication date: 1999-01-14
Anticipated expiration: 2000-01-04
Also published as: NL1006477C2; AU8245698A

Abstract

The invention relates to a reactor for carrying out a chemical reaction, which reactor comprises at least one bed of catalytically active material, which bed consists of elementary metal bodies sintered to each other and which is in heat exchanging contact with the wall of the reactor, the improvement being that the elementary metal bodies possess a void fraction that lies between 0.25 and 0.95.

Description

Title : Reactor and method for carrying out a chemical reaction

This invention relates to a reactor for carrying out chemical reactions, more particularly reactions with a great thermal effect, and to a method for carrying out chemical reactions in such a reactor. A great many chemical reactions are characterized by a positive thermal effect (exothermic reaction) or a negative thermal effect (endothermic reaction) . To allow chemical reactions to proceed in the desired manner, an efficient supply or removal of the reaction heat is often indispensable. In some exothermic reactions, the thermodynamic equilibrium shifts in an undesired direction if the temperature rises. Examples 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 in the course of these reactions, the temperature of the reaction mixture will rise and the thermodynamic equilibrium will shift in an unfavorable direction, unless the reaction heat released is removed from the reactor fast and efficiently.

In endothermic reactions too, a shift of the thermodynamic equilibrium in an undesired direction can occur, now due to the consumption of thermal energy. Examples are methane-steam reforming and the dehydrogenation of ethylbenzene to styrene. A problem may also arise in that, as a result of the consumption of energy by the reaction, the temperature of the reaction mixture decreases too strongly, so that the desired reaction no longer proceeds.

Not only can a temperature change cause a shift of the thermodynamic equilibrium in an unfavorable direction, it can also adversely affect the selectivity of catalytic reactions. Examples of reactions where the 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 S0₂) and the Fischer Tropsch synthesis. In all these cases, the release of the reaction heat causes a temperature rise. If this temperature rise is not prevented by a prompt removal of the reaction heat, the selectivity decreases strongly.

In most conventional catalytic reactors, a fixed bed of catalyst particles is used. In such a catalyst bed, porous bodies of catalyst particles are provided in bulk or stacked. To .avoid an undesirably high pressure drop across such a catalyst bed, it is preferred to use bodies or particles of dimensions of at least 0.3 mm. These minimum dimensions of the catalyst bodies are necessary to keep the pressure drop occurring upon the passage of a stream of reactants through the catalyst bed within technically acceptable limits. While the dimensions are limited at the lower end of the range by the allowable pressure drop, 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 mostly be achieved only with a surface area of the active phase of 25 to 500 m² per ml of catalyst volume. Surface areas of such an order of magnitude can only be provided by very small particles, for instance particles of 0.05 μm. Since particles of such dimensions no longer allow a liquid or gas mixture to flow through them, the primary, extremely small particles have to be formed into high-porous bodies having dimensions of at least about 0.3 mm, which can possess a large catalytic surface area. 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 should not disintegrate upon being filled into the reactor and upon exposure to sudden temperature differences (thermal shock) .

As appears from the above examples, there is a very great need for a fast supply or removal of thermal energy in catalytic reactors, in combination with a low pressure drop. According to the present state of the art, it is virtually impossible to supply thermal energy to, or remove it from, a conventional fixed catalyst bed in an efficient manner. This is indeed 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 fixed catalyst beds.

In a selective oxidation of ethylene, a very large heat exchanging surface area is employed by the use of a reactor with as many as 20,000 long pipes. In methane-steam reforming, it is attempted to optimize the heat supply and to limit the pressure drop by adapting the dimensions and shape of catalyst bodies. In this last reaction, too, a large number of costly pipes must be used in the reactor. In a number of technically important cases, it is desired, in catalytic reactions, to work with a high to very high space velocity, while a great pressure drop across the reactor is considered a less serious drawback. In the conventional fixed bed reactors, a high pressure drop with the corresponding high space velocity is not properly possible. If the pressure at the reactor inlet is increased, the catalyst may be blown (gaseous reactants) or washed (liquid reactants) out of the reactor. It is also possible that at a particular critical value of the pressure at the reactor inlet, "channeling" occurs. In that case, the catalyst particles in a particular part of the reactor begin to move. In that case, the reactants are found to flow virtually exclusively through the part of the catalyst bed that is in motion. In the current fixed bed reactors, the catalyst bed clogs up. The reactor must therefore be regularly opened and the layer of cumulated dust removed. It would be favorable if a pulse of gas of high pressure could be sent through the reactor in a direction opposite to that of the stream of reactants. This pressure pulse would blow the dust off the catalyst bed; thus, clogging could be prevented without opening the reactor, which is technically very attractive. In the fixed bed catalysts according to the present state of the art, however, this is not possible: along with the dust, the catalyst bodies would be blown out of the catalyst bed. 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 abstract of JP-A 6/111838. According to this publication, a reform catalyst is provided in grooves of a plate, while in grooves of a second plate a combustion catalyst is provided. These plates are arranged against each other, so that reforming can take place with the heat generated by the combustion.

Also in carrying out the Fischer Tropsch reaction, where higher hydrocarbons are produced from a mixture of hydrogen and carbon monoxide, a system has been used with a catalyst provided on the wall of the reactor. This catalyst provided on the wall ensures a good heat transfer from the catalyst to the outside of the reactor. For providing the catalyst on the wall, inter alia the following method has been proposed. The catalyst is applied as a Raney metal, an alloy of the active metal and aluminum. After being applied, the catalyst is activated by dissolving the aluminum with lye. The greater part of the reactor volume is empty, as a result of which the contact between the reactants and the catalytically active surface is slight and the conversion per passage through the reactor is greatly limited. The reactants must therefore be frequently recirculated through the reactor.

In a number of technically important cases, the pressure drop upon passage of the reactants through the catalyst bed must remain very low. This applies, for instance, to reactors in which flue gas from large plants is to be purified, as in the catalytic removal of nitrogen oxides from flue gas. Because a flue gas stream is generally very large, a fair pressure drop requires a very great deal of mechanical energy. The same applies to the purification of exhaust gases of automobiles. In this case too, a high pressure drop is unallowable .

Currently, the use of catalysts provided on a honeycomb is one of the few possibilities of achieving an acceptable pressure drop without unallowably reducing contact with the catalyst. For this purpose, often ceramic honeycombs or monoliths are used, in which the catalytically active material is provided.

A variant of the method in which the catalyst is provided exclusively on the wall is the use of monoliths made up of thin metal sheets. Such a reactor is manufactured, for instance, by rolling up a combination of corrugated and planar thin metal sheets and then welding them together. It is also possible to stack the planar sheets in a manner yielding a system with a large number of channels. The catalyst is then provided on the wall of the thus-obtained channels .

As was noted, thermal conduction in a fixed catalyst bed is poor. This has been ascribed to the low thermal conductivity of the high-porous supports on which the catalytically active material is applied. Kovalanko, O.N. et al . , Chemical Abstracts 31_ (18) 151409u have accordingly proposed to improve thermal conduction by increasing the conductivity of the catalyst bodies. They did this by using porous metal bodies as catalyst support. Now, it has already been described by Satterfield that the thermal conductivity of a pile of porous bodies is determined not so much by the conductivity of the material of the bodies as by the mutual contacts between the bodies (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 indeed does not strongly influence thermal transport in a catalyst bed.

WO-A 86/02016 discloses a reactor comprising a reaction bed provided with a catalyst, which bed consists of sintered metal particles which are in good heat conducting communication with the reactor wall, which wall is externally provided with sintered metal particles for removing reaction heat. Further, on the outside of the reactor, a phase transition occurs. US-A 4,101,287 discloses a combined heat exchanger reactor consisting of a monolith, in which the reactants flow through a part of the channels and a cooling agent flows through a part of the channels. Here, the same disadvantage as in the system of WO-A 86/02016 presents itself. EP-A 416710 discloses a method which is based on the use of a catalytic reactor in which the reactor bed consists of elementary particles of metal sintered to each other and to one side of the reactor wall, while on the other side of the reactor wall no sintered metal particles are present. When in such a reactor the diameter of the reactor bed is selected in relation to the thermal effects, which vary from one reaction to another but are known and, depending on the reaction conditions, can be calculated, reactions of the type referred to can be carried out optimally. WO-A 9632118 discloses a system in which, using at least two reactor beds which are in good heat exchanging contact with each other, two different reactions are carried out. Preferably, these reactor beds are made up of metal spheres sintered together. Although the use of sintered metal yields a clear improvement of the heat economy in reactors for chemical reactions, a number of disadvantages still remain. Most prominent is the problem that a bed of sintered metal particles yields a rather high pressure drop. In a number of cases, this can seriously hamper their use. It is therefore an object of the invention to provide a reactor for carrying out chemical reactions, more particularly reactions with a great thermal effect, which does not entail these problems, or does so to a lesser extent .

The invention accordingly comprises a reactor for carrying out a chemical reaction, which reactor comprises at least one bed of catalytically active material, which bed consists of elementary metal bodies sintered to each other and which is in heat exchanging contact with the wall of the reactor, the improvement being that the elementary metal bodies have a void fraction which is between 0.25 and 0.95.

The invention is based on the surprising insight that by the use_, of shaped metal particles with a void fraction of 0.25 to 0.95, a sintered bed can be obtained which meets the requirements of optimized process control.

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

The metal particles that are used in the reactor according to the invention are characterized in that they have a void fraction. This is understood to mean that the particle has a shape such that within the smallest envelope of the particle, there is always at least 25% free space. The concept of smallest envelope can be visualized as a fleece with a smallest possible surface, which is provided around the particle without tension. An alternative formulation is the smallest volume around the particle that cannot be cut twice by the same straight line. The concept of void fraction is then formed by the open volume within the smallest envelope, divided by the volume of the smallest envelope. The metal bodies to be used according to the invention can consist of extruded metal, of sheetlike metal pressed, punched or folded into a suitable shape, or bodies formed from metal wire. Accordingly, the bodies involved here are different bodies from solid spheres or fibers.

The greatest 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 body's greatest dimension of the smallest envelope (as defined above) .

Through the choice of the shape of the particles, the desired porosity of the bed can be set. This also makes it possible to set the surface area available for the catalyst, substantially independently of the porosity.

The specific shape of. the particles can be chosen within the definition of the invention. Various groups of particles - the enumeration is not limiting - can be distinguished, which are characterized in particular 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 bodies in the shape of a wire spring and rings. A third group is formed by complex structures such as crow's-feet. The fourth group are bodies punched and/or bent from sheet. Sections can be considered here, but also other shapes, such as dishes and half-caps. Extruded metal or extruded metal particles in this connection are understood to mean metal which has been shaped through extrusion and which has the shape of tubes or other shapes obtainable by extrusion. By making these extrusions of the proper length, particles of the desired dimension are obtained.

The chemical reactions that are carried out according to the invention take place under the most suitable conditions for the reactions chosen. In connection with the good heat exchange, there is a better heat transfer, which is evidenced by a flatter temperature profile in the reactor. Further, this system provides the possibility of processing more heat production per unit of volume.

The temperature employed in the reactor depends on the nature of the reaction. In general, an elevated temperature is employed, because then the advantages of the system are most pronounced. In general, the temperature will be above 100 °C, an upper limit being the maximum temperature at which the material is still stable, or the temperature that can be achieved with a chemical reaction. Temperatures in excess of 1250°C, however, are generally not preferred because of the difficulties in achieving them and the requirements that such temperatures impose on the materials of the reactors and the supply and discharge systems.

The. pressure at which the reaction is carried out, can be varied within wide limits.

The continuous porous structure which is used in the reactor in accordance with the invention can be constructed in various ways, as will also appear from the further explanation and the examples of suitable structures. In general, the continuous porous structure should meet the requirement that there is a heat exchanging contact between the partition and the structure, while further the porous structure extends through the entire reactor bed.

This means that the porous structure is preferably fixedly connected to at least one reactor wall, while the reactor bed consists of a structure which fills the entire reactor, at least, extends through the entire reactor, in the form of fixedly interconnected elementary particles, such as the particles sintered together. It is then of particular importance that the heat exchange is good especially in the direction transverse to the wall. In that direction, the most important heat flow occurs. In the longitudinal direction of the reactor, that is less important.

It is noted that it is preferred to sinter the metal bodies fixedly onto the wall. From the point of view of efficiency and economy of the construction, however, it may sometimes be preferred that the bed, while having a good heat exchanging contact with the wall, is not fixedly sintered onto it. The disadvantage of a slightly lesser effectiveness is then more than compensated by the simplicity of constructing the reactor and the convenience in replacing the reactor bed.

In the invention, the degree of porosity of the reactor bed can be varied within wide limits. This porosity, that is, the portion of the bed that allows gas or liquid to flow through, 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 kind of reaction (i.e. the extent of heat production) and the desired extent of heat transport in the bed. The extent of heat transport is a relatively important factor in the reactor systems according to the invention. Obviously, the heat conductivity of the total system, that is, from the partition into the beds, is partly determined by the heat conductivity of the catalyst support material used 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 massive condition; 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 heat conductivity is highly dependent on the heat conductivity of the elementary materials used. A sintered body of 316L has a value of 3-12 W/m.K. Powder of 316L, by contrast, has a value of 0.55, while massive material possesses 20 W/m.K. Massive copper has a heat conductivity of 398 W/m.K. All of these values relate to the condition at room temperature. At other temperatures, the absolute value of the numbers changes, but the mutual ratio remains approximately the same . The heat conductivity of the system as a whole is also important for the operation thereof. As has been indicated, there should be a heat exchanging contact between the reactor bed and the wall. More particularly, it is of importance that there is a good contact, under reaction conditions, between the reactor wall and the reactor bed. This is preferably obtained by sintering the elementary metal particles fixedly onto the wall.

The reactor system according to the invention is applicable for each heterogeneously catalyzed gas phase reaction, but is more particularly suitable for those reactions that have a strong thermal effect, that is, highly endothermic or exothermic reactions, or reactions whose selectivity is highly temperature-dependent .

It is possible to work with a high to very high spatial velocity without the catalyst being blown (gaseous reactants) or washed (liquid reactants) from the reactor. Nor does

"channeling" occur. Because in the reactor according to the invention the catalyst particles are much better fixed, such a reactor allows working at a much higher velocity 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 when dust is deposited on the catalyst bed. In reactors according to the present invention, a pulse of gas of high pressure can be directed through the reactor in a direction opposite to that of the stream of reactants.

This pressure pulse blows the dust off the catalyst bed; as a result, clogging can be prevented without opening the reactor, which is technically very attractive.

The invention is particularly suitable for carrying out highly 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 include hydrogenation and dehydrogenation reactions. As an example of such a reaction, the oxidation of methane is described. As an example of a reaction whose selectivity is to a large extent determined by the temperature, the selective oxidation of hydrogen sulfide 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 starts to proceed. Use of a reactor system according to the invention makes it possible to purify highly efficiently gas streams of a hydrogen sulfide content of, for instance, 10% by volume. The hydrogen sulfide is selectively oxidized to elemental sulfur which is extremely easy to separate by condensation. Since such gas mixtures cannot properly be processed in a Claus process, the invention is of particularly great importance for this purpose.

As .has already been indicated, the reactor system that is used according to the invention can be made 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 the metal alloy may then be catalytically active itself or it can be rendered catalytically active by treatment, but it is also possible to provide a catalytically active material on it. One of the advantages of a catalyst on such metal particles resides in the better heat distribution by the use of the metal. On a microscale, it is observed that the heat economy in the catalyst is better, so that more efficient use is made of the catalyst. This has an influence, inter alia, on the activity, but may also be of importance for the selectivity, for instance in the case where the selectivity is highly dependent on the temperature. In fact, according to the invention, a much more homogeneous temperature distribution is obtained. One advantage of this is that no hot or cold spots in the bed are obtained. Too hot spots cause ageing or degradation of the catalyst, while at too cold spots no reaction occurs. Consequently, as a result of the more homogeneous temperature distribution, a stabler and more effective reaction is obtained.

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

The reactor system according to the invention, as has already been indicated, may already be catalytically active of itself or be activated by treatment. However, it is also very well possible to provide a catalytically active material on the fixedly connected elementary bodies. More particularly, it is possible first to provide a (high-) porous support on the metal surface or alloy surface and then to provide the catalytically active component on the support. This last can be of significance if the catalytically active component should not come into direct contact with the material of the bodies sintered together, so as to prevent undesired interactions between the material of the bodies and the catalytically active component. In applying the catalyst, first a dispersion of a support and/or the catalytically active material (or a precursor therefor) in a liquid is prepared and then this liquid is suitably applied to the fixedly connected elementary bodies. This can be done, for instance, by evacuating the bed on which the support and/or the catalytically active material are to be provided and then sucking the dispersion into the bed, so that the bed is impregnated. If first a support is provided, the operation can be repeated with the catalytically active component or precursor therefor. In the drawing, the invention is further elucidated on the basis of a few examples, which are not intended as a limitation of the invention. In the drawing, 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 groups is formed by the bodies formed from metal wire, i.e., wire figures such as wire springs and rings. A third group is formed by complex structures such as crow's-feet. The fourth group are bodies punched and/or bent from sheet.

Claims

1. A reactor for carrying out a chemical reaction, which reactor comprises at least one bed of catalytically active material, which bed consists of elementary metal bodies sintered to each other and which is in heat exchanging contact with the wall of the reactor, the improvement being that the elementary metal bodies possess a void fraction which is between 0.25 and 0.95.

2. A reactor according to claim 1, wherein the metal bodies possess a void fraction which is between 0.25 and 0.75.

3. A reactor according to claim 1 or 2 , wherein the bodies are selected from extruded metal bodies, wire bodies and shaped sheetlike bodies.

4. A reactor according to claims 1-3, wherein the bodies of extruded metal are tubular.

5. A reactor according to claims 1-4, wherein the metal bodies are sintered fixedly to the wall.

6. A method for carrying out a chemical reaction, which is characterized in that a reactor according to claims 1-5 is used.

7. A method according to claim 6, wherein the reaction is selected from endothermic and exothermic reactions.

8. A method according to claim 6, wherein the method is selected from reform reactions, shift reactions, oxidation reactions, and reduction reactions.