Catalytic device for carrying out a reaction within a gaseous medium at high temperature

The present invention relates to catalytic reactions in a gaseous medium at high temperature, for example the oxidation of ammonia and the synthesis of HCN. A certain object of the invention is an improved catalytic device that can be used in this type of reaction and a reactor containing it.

Prior art

Oxidation of ammonia is widely used in the production of nitric acid. The process, known as the Ostwald process, involves an operation in which a preheated ammonia / air mixture, usually containing 5 to 15% by volume, is passed. , in particular 10 to 12% by volume. , air, with a high linear velocity (measured at standard temperature and pressure conditions), by a catalytic device arranged across the cross section of the reactor.

Synthesis of hydrocyanic acid (HCN) in a single operation from ammonia and hydrocarbon gas, in which the heat required for the endothermic reaction is generated by a simultaneous combustion reaction with oxygen or an air-containing gas in the presence of a catalyst, is an operation known for many years (U.S. Patent 1,934,838). It is known as the Andruss process.

These two types of reaction use platinum group catalysts, usually in the form of a flat woven mesh. The working cross-section of these catalysts is limited by the dimensions of the reactor.

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In order to increase the productivity of these reactors, the number of catalytic tissues can be increased. However, beyond a certain thickness, the resulting pressure drop counteracts the increase in reactant flow and eliminates the effects of better conversion yields. In addition, side reactions can occur as the thickness increases. Therefore, the difficulties in increasing production result from the current state of the art:

- pressure drop from the number of active sites of the catalyst (contact surface area),

- from the time of contact of the catalyst with the reactants.

In order to increase the effective surface area of ​​the catalyst, U.S. Pat. No. 5,160,722 and U.S. Pat. No. 5,356,603 describe the use of catalytic webs having transverse corrugations. Although the surface area is thus increased, these undulations have low amplitudes, and the preservation of the shape of this system is indeed only possible for temperatures below 800 ° C. In addition, the mechanical properties of the metal become insufficient and, due to the pressure drop, the folds or corrugations tend to collapse. The service life of such equipment is therefore very short, which is incompatible with industrial production.

Patent application EP 931 585, in part, describes the use of a catalytic mesh in the form of radially corrugated disks or cones, so that a rotating torch can follow the corrugation as it rotates about its axis. However, the above problems remain.

It is therefore an object of the present invention to provide a catalyst device comprising a catalyst which has a larger geometric surface area and which withstands the reaction conditions without substantially increasing the pressure drop or side reaction.

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The essence of the invention

The present invention relates to a catalytic device for carrying out a reaction in a gaseous medium at a high temperature, such as for the synthesis of HCN or the oxidation of ammonia, the essence of which comprises:

• at least one structured material which is effective as a catalyst for these reactions, • a support comprising at least one ceramic part, the structure of which allows the passage of gases, this part of this support has a corrugated face, so that the surface area (β) formed by the corrugation area is at least equal to the magnification (a) of the sawtooth corrugations and between about 1.1 and about 3, where the structured material is arranged such that to be held on the corrugated end face of a part of the support and follow its shape.

The means for holding the structured material on the corrugated face of the support part preferably consists of a second ceramic support part, the structure of which allows the passage of gases, a second part having a corrugated face which is substantially congruent and complementary to the corrugated face of the first support part. , and the second part is arranged such that the corrugated faces of the first and second parts lie opposite each other, and the structured material is inserted between the corrugated faces and follows their shape. Therefore, the pressure drop is preferably substantially homogeneous throughout the catalyst equipment thus formed.

Other conventional means for retaining structured material on the corrugated faces of the first portion of the support may be used by those skilled in the art.

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The term structured material means, in the sense of the present invention, any set of strips or wires which are linear and / or in the form of helical components through which gases can pass. This system is, for example, made of mesh, woven fabric, knitted fabric or as felt and can be obtained by various techniques such as weaving, knitting, sewing, embroidery and the like. Preferably it is a mesh.

The term two corrugated faces of substantially congruent and complementary shape means, in the sense of the present invention, any combination of two surfaces having undulations of similar size and shape, i.e. having the same increase in surface area β, which is designed so that when these surfaces lie opposite each other , the ripples are complementary (they are complementary).

Other objects of the invention and its advantages will be apparent to those skilled in the art from the detailed description and reference to the accompanying drawings.

Overview of pictures in the drawing

An exemplary embodiment of the present invention is illustrated in the accompanying drawings, in which Fig. 1 is a schematic representation of a catalytic device according to the invention;

Fig. 2 shows parameters which make it possible to calculate the increase in surface area (a) created by the corrugations in the shape of the saw teeth;

Example of an embodiment of the invention

The support 2 according to the present invention is made of a ceramic whose structure allows the passage of gases. Examples of such ceramics are, without limitation, ceramic foams or ceramic composites. The term ceramic preferably means, in the sense of the present invention, any refractory material capable of withstanding the temperatures to which the platinum catalytic mesh reaches in the reaction of a medium containing, inter alia, steam. When used in the synthesis of HCN according to the Andrussow process, this temperature can reach up to 120 ° C. Materials that are suitable are therefore based on alumina and may contain varying amounts of silica (10 to 60% by weight) and magnesium, zirconium, titanium and cerium oxide (1 to 20% by weight for each of these components). may contain, without limitation, one or more of the following compounds: silicon dioxide (SiO 2)₂), silicon carbide SiC, silicon nitride Si₃N₄, silicon boride, silicon boronitride, Alumina Al₂ O₃), aluminosilicate, carbon fibers, zirconia (ZrO₂), yttrium oxide (Y₂O₃), calcium oxide (CaO), magnesium oxide (MgO) and cordierite (MgO - Al₂O₃ - SiO₂) .

Preferably, Stetta® G29 ceramic from Stettner is used, the characteristics of which are as follows:

<td>Porous nost</td><td>Relati- in her density in kg / cm³</td><td>Flexural strength N / mm²</td><td>Linear coefficient. extensibility 1 / K.10⁶at 20</td><td>Linear coefficient. extensions 1 / K.10 '⁶</td><td>Thermal conductivity W / (mK)</td><td>Endurance against thermal shocks and ° C</td><td>Maximum. traffic. temperature in ° C</td><td>Obj em. resistance to 800 ° C</td>

<td> &gt; 3</td><td> 2</td><td> 4,5</td><td> 1,5-3</td><td> 2-4</td><td> 1,3 1,7</td><td> 380</td><td> 1000</td><td> 10=</td>

Prior to use, the materials used to form the support (2) are usually made by known techniques of forming, pressing, agglomeration and the like. They are then calcined at a high temperature (&gt; 1300 ° C) to obtain mechanical properties comparable to their future operating conditions. These combined operations must give them a structure through which gases can pass, which can be, for example, in the form of cells interconnected in 3 directions (foam) or honeycombs with a circular or polygonal (square, rectangular, hexagonal, etc.) cross-section.

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<img file="CZ20030174A3_D0001.tif" />

The structured material 1 which is effective as a catalyst is in particular a catalytic metal from the platinum group and can be prepared from platinum, rhodium, iridium, palladium, osmium, ruthenium or a mixture or alloy of two or more of these metals. Preferably it is a platinum or platinum / rhodium alloy. Alternatively, the catalyst may be from the platinum group as described above and at least one material including, but not limited to, cerium, cobalt, manganese, magnesium, and ceramics.

The corrugation of the end faces 6 and 7 of the support can be of any type, in particular in the shape of saw teeth.

The sawtooth wave will be defined by the height h of each wave and the distance d between the two waves. The increase in surface area and created by corrugation of this type can therefore be calculated from these three parameters (Fig. 2) as follows:

a =&gt; / (4h² + d²) / d

Enlargement of the surface area J3_OF generated by any type of wave according to the invention, will be at least equal to a and will be chosen in the range of about 1.1 to about 3. This is because a = 1.1 corresponds to an increase in surface area of ​​10%. Below this value, the advantages of this wave are not very noticeable. Above β = 3, the use of such a device becomes difficult. The sawtooth waves according to the invention preferably have the profile of an isosceles triangle sd = 2h, which results in a ratio a of about 1.40 and therefore an increase in surface area of ​​about 40%.

The present invention also relates to a reactor for high temperature exothermic reactions in a gaseous medium which has a substantially circular cross-section and comprises a catalytic device according to the invention passing through its cross-section.

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<img file="CZ20030174A3_D0002.tif" />

<img file="CZ20030174A3_D0003.tif" />

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It also relates to a reaction process in a gaseous medium at high temperature, such as the oxidation of ammonia or the synthesis of HCN, in which the catalytic device or reactor according to the invention is used.

In a certain embodiment of the invention, the process according to the present invention is the synthesis of HCN and comprises the operation of passing a gaseous mixture containing hydrocarbon, preferably methane, ammonia and oxygen through the catalyst device according to the invention at a temperature between 800 to 1400 ° C; obtained a gas stream containing at least 5% by volume of HCN.

The hydrocarbon used in the HCN synthesis process of the invention may be a substituted or unsubstituted and an aliphatic, cyclic or aromatic hydrocarbon or a mixture thereof. Examples of such hydrocarbons include, without limitation, methane, ethylene, ethane, propylene, propane, butane, methanol, and toluene. The hydrocarbon is preferably methane.

The present invention also relates to a method of manufacturing a catalytic device according to the invention, in which the structured material 1 is unrolled on the corrugated end face 6 of the part 2 of the support 2 so as to follow its shape and is held thereon by immobilizing means.

These immobilizing means are preferably mechanical and consist of a second part 4 of the support 2, the corrugated end face 7 of which covers the end face of the material 1, which lies on the opposite side to the corrugated end face 6 of the part 2 of the support 2.

Even more preferably, the combination thus formed produces a small pressure drop that is substantially homogeneous across the cross section of the reactor.

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A certain schematic exemplary embodiment of the device according to the invention (Fig. 1) consists of:

- combination of corrugated mesh 1,

corrugated ceramic supports 2 composed of two parts 2 and X, each of which has a corrugated end face £ and 7.

The meshes 1 are placed between the end faces £ and 7 of the two parts £ and X of the support 2.

Parts 2 and 4 of the support 2 are made of ceramic, having a honeycomb construction with a circular or polygonal (square, rectangular, hexagonal, etc.) cross section.

Example of preparation of a catalytic device according to the invention:

The support 2 according to the present invention can be given a corrugated shape, either before calcination (during the forming operation) or after calcination, folding and gluing of the triangular cross-sectioned prisms.

The combination of platinum meshes 1 forming the catalyst batch is then inserted between 2 layers 2 and X of the corrugated support 2. This positioning operation is performed by rolling a combination of elliptical webs 1, which will be deposited on the corrugated part 2 of the support, on the corrugated part 2 of the support 2. , the support is first placed on the hollow bricks XI, which form the basis of the reactor with a circular cross-section. The width of the elliptical meshes corresponds to the inner diameter of the reactor. The length is equal to the width multiplied by the previously determined coefficient β. The upper layer X of corrugated material makes it possible to immobilize the mesh X mechanically, while contributing to a homogeneous pressure drop on the combination over the entire exposed surface. A heat shield 10 is then placed on the catalytic device thus formed. This allows • · · «9 ···· *

to maintain the reaction and all the activation energy at the lowest point of the surface of the tissues 1. The hollow bricks 11 are themselves placed on the tubes 12 of the reactor boiler, which are composed of refractory cement 13.

Performance

The use of the catalyst device according to the present invention makes it possible, for the same reactor, to increase the surface area for contact between the catalyst and the reagents. This means, for all reagents, increased productivity and a minimal and substantially constant pressure drop, which allows for much longer production campaigns, when in this way the device withstands the reaction conditions, it is not substantially prone to mechanical deformations.

The table below makes it possible to compare the technical data of a representative system according to the prior art with the catalytic device according to the invention.

<td></td><td>State of the art: Flat system</td><td>Invention: Corrugated system</td>

<td>Coefficient a</td><td> 1,0</td><td> 1,4</td>

<td>Productivity: kg HCN per tonne of air and kg catalyst</td><td> 2,00</td><td> 2,16</td>

<td>Increase in pressure drop: % per month of the original pressure drop</td><td> 29</td><td> 5</td>

<td>Test duration: hours of production</td><td> 995</td><td> 3138</td>

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