Catalyst system for a flow reactor and process for catalytic oxidation of ammonia

Abstract

The present invention relates to a catalyst system for a flow reactor, which catalyst system comprises at least two meshes, at least one of which is a precious metal mesh. At least one of the meshes has a cutout. The invention also relates to a reactor comprising such a catalyst system and to a process for catalytic oxidation of ammonia.

Description

[0001] CATALYST SYSTEM FOR A FLOW REACTOR AND METHOD FOR CATALYTIC OXIDATION OF AMMONIA

[0002] The present invention relates to a catalyst system for flow reactors comprising at least two meshes, at least one of which is a precious metal mesh. At least one of the meshes has a recess. Furthermore, the invention relates to a reactor having such a catalyst system and to a process for the catalytic combustion of ammonia, in which a fresh gas containing at least ammonia is passed through the catalyst system.

[0003] Catalyst systems within the meaning of the present invention are used in particular for gas reactions. They are employed, for example, in the production of hydrogen cyanide by the Andrussow process or in the production of nitric acid by the Ostwald process. In order to provide a large catalytically active surface for these reactions, such catalysts generally have a spatial, gas-permeable structure. Collection systems for the recovery of vaporized catalytically active components are also frequently based on such net-like structures. It is usually advantageous to arrange several nets one behind the other and combine them to form a catalyst system. The catalyst nets usually consist of single- or multi-layer knitted, warp-knitted, or woven fabrics. The individual nets consist of fine precious metal wires, predominantly containing platinum (Pt), palladium (Pd), rhodium (Rh), or alloys of these metals.The choice of precious metal wire material is determined, among other things, by the position and function of the catalyst mesh within the catalyst system. Collecting meshes, in particular, can contain precious metals as well as other components such as nickel. To increase the catalytically active available surface area, the catalyst meshes often have a three-dimensional structure in addition to their planar dimensions. Depending on the design, systems with 2 to 50 catalyst meshes with a diameter of up to 6 m are used in flow reactors. In addition to catalyst meshes containing precious metals, such catalyst systems can also include other mesh components that serve, for example, to stabilize the overall system. Such meshes are typically made of base materials, for example, Fe-Cr-Al alloys such as Kanthai or Megapyr, or comparable steel variants.State-of-the-art meshes extend across the entire flow cross-section of the reactor. The catalyst systems are used at high temperatures (> 800 °C) in the reactors. Due to the cyclical temperature fluctuations between room temperature and operating temperature during operation, including startup and shutdown processes, the reactor expands during operation compared to its cold base state and contracts again during cooling. This thermal cycle, with the resulting variation in the reactor diameter, can lead to both the catalytic meshes and the support meshes becoming loose from their anchorage around the reactor and / or to cracking in the meshes. Both phenomena cause gas slip in the reactor and negatively impact the overall efficiency of the process.

[0004] Furthermore, the gas volume flow may be inhomogeneous across the cross-sectional area of ​​the reactor, typically with an increased flow in the peripheral regions of the reactor and a reduced gas flow in the center.

[0005] It is therefore desirable to improve the mechanical stability of the catalyst systems and to homogenize the volume flow within the catalyst systems.

[0006] The object of the present invention was to provide a catalyst system for a flow reactor in which at least one of the problems identified in the prior art is solved.

[0007] A further object was to provide a reactor comprising the catalyst system according to the invention.

[0008] Furthermore, the invention was based on the object of specifying a method for the use of such a catalyst system.

[0009] According to the present invention, at least one of the objects is achieved by a catalyst system for a flow reactor, comprising at least two nets, wherein at least one of the nets is a noble metal net and at least one of the nets has a recess.

[0010] Within the scope of the invention, it was surprisingly discovered that it has an advantageous effect on the stability of the catalyst system if at least one of the meshes does not span the entire area of ​​the reactor. Presumably, the presence of such an opening allows for a reduction in the shear forces acting on the meshes during a thermal cycle. This prevents the meshes from tearing away from their attachment points on the reactor wall or from tearing in the cross-section of the meshes.

[0011] Furthermore, such catalyst systems can optimize the distribution of the gas flow across the reactor cross-sectional area, especially in the case of inhomogeneous gas flow rates. This is especially true for meshes that do not contain precious metal, such as the typically used support meshes.

[0012] If the mesh having the recess is at least a catalytically active noble metal mesh, the catalytic efficiency of the noble metal can also be adapted to an inhomogeneous volume flow in the reactor and controlled in a targeted manner.

[0013] The present invention relates to a catalyst system for a flow reactor. In flow reactors, catalysts in the form of gas-permeable sheet materials are typically installed in the reaction zone in a plane perpendicular to the flow direction of the fresh gas. Such gas-permeable sheet materials are usually used in the form of catalyst meshes. Catalyst systems are usually secured in the reactor using clamping devices to prevent slippage.

[0014] A catalyst system is understood to be an ensemble of meshes comprising at least one catalyst mesh. In other words, a catalyst system comprises a stack of meshes, of which at least one mesh has catalytic activity. In this case, the catalyst mesh is a precious metal mesh. A catalyst system can also comprise one or more meshes without a catalytic function, for example, catchment meshes, which serve to recover precious metal evaporated during operation, or support meshes, which serve to stabilize the catalyst system and typically do not contain any precious metal.

[0015] The catalyst system has a diameter designed for use in the intended reactor, for example in a range of 0.5 m to 6 m.

[0016] The networks of the catalyst system can be connected to each other, for example by welding at the edges, whereby the welding can be carried out over the entire edge area or only partially and / or selectively. In this case, all networks of the catalyst system can be connected to each other or, if the catalyst system has more than

[0017] 2 networks, only parts of the networks can be connected to each other.

[0018] The catalyst system according to the invention comprises at least two meshes. A mesh is understood to be a single- or multi-layer, gas-permeable sheet-like structure, wherein the structure comprises a plurality of interconnected thread elements. The thread elements are, for example, wires or yarns. The surface formation of such meshes is achieved by the mesh-like interlacing of one or more thread elements. Meshes can be produced, for example, by weaving, knitting, or crocheting. The structure of the meshes can be specifically adjusted by using different weaving, knitting, or crocheting patterns and / or different mesh sizes.

[0019] The materials of the at least two meshes of the catalyst system can be the same or different, i.e. both meshes can be precious metal meshes, but they can also be a precious metal mesh and a non-precious metal mesh.

[0020] The shape of the meshes is not further restricted, however, round meshes are typically used in flow reactors. Depending on the type and function of the meshes in the catalyst system, they have different characteristics with regard to the mesh size, structure, as well as the type and dimension of the thread element. The mesh size of the meshes is usually in the range of less than 3 cm, in particular less than 1 cm. In the context of this application, the mesh size refers to the distance between the individual meshes in a mesh; in other words, the mesh size is understood to be the internal distance between two adjacent mesh legs at the point of greatest extension of a mesh. It describes the size of the openings in the mesh. A smaller mesh size means smaller openings in the mesh.

[0021] At least one of the meshes of the catalyst system is a precious metal mesh. In other words, the catalyst system's catalyst mesh is a precious metal mesh.

[0022] A precious metal mesh is a mesh containing at least one precious metal wire. A precious metal wire is defined as a wire that consists of at least one precious metal or that contains a significant proportion (> 50 wt%) of precious metal. For the purposes of the present invention, precious metals are defined as a metal selected from the group consisting of platinum group metals, gold, and silver. Platinum group metals are the metals of the so-called platinum group, i.e., platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os), and ruthenium (Ru).

[0023] The precious metal wire preferably consists of platinum, a platinum alloy, palladium, or a palladium alloy. A platinum alloy contains more than 50 wt.% platinum; other alloying components include, in particular, palladium, rhodium, and ruthenium. A palladium alloy contains more than 50 wt.% palladium; other alloying components include, in particular, platinum, rhodium, ruthenium, nickel, tungsten, and gold. In the following, precious metal meshes comprising a precious metal wire made of platinum or a platinum alloy are also referred to as platinum-containing precious metal meshes or, for short, platinum meshes. Precious metal meshes comprising a precious metal wire made of palladium or a palladium alloy are also referred to as palladium-containing precious metal meshes or palladium meshes.

[0024] The precious metal wire preferably consists of an alloy selected from the group consisting of platinum with 1 - 15 wt.% rhodium, platinum with 1 - 15 wt.% rhodium and 0.1 - 20 wt.% palladium, platinum with 1 - 15 wt.% rhodium, 0.1 - 20 wt.% palladium and 0.1 - 5 wt.% ruthenium, platinum with 1 - 15 wt.% rhodium, 0.1 - 40 wt.% palladium and 0.001 - 5 wt.% iridium, platinum with 1 - 15 wt.% rhodium, 0.1 - 20 wt.% palladium and 0.001 - 5 wt.% tantalum, platinum with 1 - 15 wt.% rhodium, 0.001 - 5 wt.% iridium and 0.001 - 5 wt.% tantalum, palladium with 1 - 25 wt.% platinum, palladium with 1 - 25 wt% platinum and 1 - 15 wt% rhodium, palladium with 1 - 25 wt% tungsten, palladium with 1 - 15 wt% nickel, palladium with 0.001 - 5 wt% rhodium, palladium with 1 - 15 wt% copper, palladium with 1 - 15 wt% copper and 1 - 15 wt% nickel and palladium with 1 - 30 wt% cobalt, palladium with 0.001 - 10 wt% rhodium and 3 - 30 wt.-% of at least one further metal, wherein the at least one further metal is selected from the group consisting of nickel, tungsten, platinum and gold. The precious metal alloys described herein may contain impurities. In the present case, an impurity is understood to be an intentional contamination or an contamination caused and unavoidable contamination during the production of the alloy or alloys. Unless otherwise stated, the total proportion of impurities for all described precious metal alloys based on the total weight of the respective precious metal alloy, preferably a maximum of 0.5 wt.%. If ranges are mentioned in this application, the range limits are deemed to be included.

[0025] In preferred embodiments, the at least one precious metal mesh is a platinum-containing precious metal mesh or a palladium-containing precious metal mesh, particularly preferably a platinum-containing precious metal mesh.

[0026] Preferably, precious metal wires are used that have a diameter of 40-250 pm, preferably 50-200 pm, and particularly preferably 60-150 pm. The precious metal wire can be designed as a round wire, i.e., with a round cross-section. In another embodiment, the wire can be designed as a flattened round wire or as a wire with a different cross-section.

[0027] The precious metal wire can have a two- or three-dimensional structure. For example, the precious metal wire can comprise one or more wave-shaped, stepped, or helical sections, or it can be formed as a wave-shaped, stepped, or helical wire bent over its entire length. When using such precious metal wires, the catalyst mesh produced from them can have a three-dimensional structure.

[0028] The precious metal wire can comprise several wires, in this case also referred to as filaments, which can preferably be twisted together. The filaments can all be made of the same material, i.e., all contain precious metals, or they can be made of different materials, which in turn do not all have to contain precious metals. The filaments can be twisted together; in these cases, the precious metal wires have a rope-like structure.

[0029] In many cases, it may be advantageous for the precious metal mesh to be formed from two or more precious metal wires. In these cases, the at least two precious metal wires may be made of the same or different materials. The at least two precious metal wires may have the same or different diameters and / or the same or different structures.

[0030] The precious metal mesh can be made entirely of precious metal wire or wires, but it can also include other components, such as wires or yarn made of non-precious metals or non-metallic materials. Examples of non-metallic materials include organic and ceramic materials.

[0031] The weight per unit area of ​​the precious metal mesh is not further limited; it can, for example, be in the range of 100 to 950 g / m 2 , especially in the range of 150 to 800 g / m 2 .

[0032] The mesh size of the precious metal net is also not further limited; it can, for example, be in the range from 0.01 to 10 mm, in particular in the range from 0.1 to 5 mm, particularly preferably in the range from 0.5 to 3 mm.

[0033] The number of precious metal meshes used in the catalyst system, especially the catalyst meshes, depends on the conditions under which the flow reactor is operated. For example, the number of precious metal meshes can range from 5 to 50; depending on the pressure, 5 to 10 or even 20 to 50 precious metal meshes can be used.

[0034] The catalyst system can comprise one or more precious metal mesh groups. A precious metal mesh group is understood to be an ensemble of precious metal meshes formed from at least one precious metal wire of the same composition. Typically, a catalyst mesh group comprises more than one precious metal mesh, for example, 2 to 10 precious metal meshes.

[0035] Preferred catalyst systems have a noble metal network group with platinum-containing noble metal networks and a noble metal network group with palladium-containing noble metal networks.

[0036] In preferred embodiments, the catalyst system according to the invention can comprise at least one precious metal-free mesh. The precious metal-free mesh does not comprise any wire or wires containing precious metal; it therefore comprises only precious metal-free wires. Such precious metal-free meshes can be, for example, support meshes. Support meshes can be used to counteract the compression of adjacent precious metal mesh groups under compressive loads or to provide a support surface for precious metal meshes. In the former case, such meshes are also referred to as separator screens. Precious metal-free meshes, in particular separating or support meshes, preferably have limited flexibility compared to precious metal meshes. Typically, the precious metal mesh and another precious metal-free mesh have the same size and / or shape.

[0037] Suitable non-precious metal nets include nets made of heat-resistant steel, typically a FeCrAl alloy such as Megapyr or Kanthai, stainless steel, or heat-resistant alloys such as nickel-chromium alloys. Support nets made of Megapyr or Kanthai have proven particularly advantageous.

[0038] Preferably, precious metal-free wires are used, which have a diameter of 0.01–3 mm, more preferably 0.1–2 mm. Typically, the diameter of the precious metal-free wires is larger than the diameter of the precious metal wires. The precious metal-free wire can be designed as a round wire, i.e., with a round cross-section. In another embodiment, the wire can be designed as a flattened round wire or as a wire with a different cross-section.

[0039] The weight per unit area of ​​the precious metal-free mesh is not further limited; it can, for example, be in the range of 0.05 to 3 kg / m2 , especially in the range of 0.01 to 1 kg / m 2 Typically, the basis weight of the precious metal-free mesh is higher than the basis weight of the precious metal mesh or meshes.

[0040] The mesh size of the precious metal-free mesh is also not further limited; it can, for example, range from 0.01 to 10 cm, especially from 0.05 to 1 cm. Typically, the mesh size of the precious metal-free mesh is larger than the mesh size of the precious metal mesh(es).

[0041] The noble metal-free mesh(es) can be arranged above or below the at least one noble metal mesh (relative to the flow direction of a fresh gas in the flow reactor), within noble metal mesh groups and / or between different noble metal mesh groups. It has proven advantageous for the stability of catalyst systems if at least one noble metal-free mesh is arranged between different noble metal mesh groups.

[0042] At least one of the meshes of the catalyst system according to the invention has a recess. A recess is understood to be a defined and locally limited, free area within an otherwise homogeneous mesh. Accordingly, the recess is not a random hole in otherwise homogeneous meshes, but rather a deliberately introduced opening.

[0043] The recess is typically many times larger than the mesh size of the net having the recess, for example by a factor of at least 10, more preferably by a factor of at least 100, particularly preferably by a factor of at least 500.

[0044] Preferably, the cross-sectional area of ​​the recess is at least 5% of the cross-sectional area of ​​the catalyst system, in particular at least 10%, particularly preferably at least 25%.

[0045] The mesh with the recess can be at least one precious metal mesh, but the recess can also be located in a precious metal-free mesh. It has been shown that crack formation in the catalyst system is particularly reduced when at least one precious metal mesh has a recess. If a precious metal-free mesh of the catalyst system has the recess, this has a particularly beneficial effect on the stability of the systems during thermal cycling during reactor operation.

[0046] The shape of the recess is not further limited; it can have the same shape as the catalyst system or have a different shape, for example, the recess can be round, oval, polygonal, in particular rectangular, square, triangular or pentagonal.

[0047] Preferably, the recess is arranged concentrically in the catalyst system, in other words, the recess is preferably located in the center of the network having it.

[0048] Particularly preferably, the net having the recess is an annular net, i.e. a round net with a circular, concentrically arranged recess.

[0049] It may be advantageous for a mesh to have multiple recesses; the catalyst system may also comprise multiple meshes that have at least one recess. In such cases, the recesses can be arranged one above the other or offset from one another.

[0050] If the catalyst system has more than two nets, the net with the recess can be arranged outside or in the middle of the net package, i.e. between other nets.

[0051] The catalyst system can also comprise one or more additional meshes that have a smaller diameter than the other meshes of the catalyst system. Such meshes can, for example, be arranged above the at least one recess. Such a mesh can be a precious metal mesh or a precious metal-free mesh. Such meshes make it possible to design the catalyst system more variably, further optimize the flow behavior, and simultaneously utilize the advantages of meshes with a recess. A precious metal mesh is particularly suitable in this case for optimizing the catalytic activity within the catalyst system and utilizing the full reactor surface for the catalytic reaction. A precious metal-free mesh makes it possible to prevent the precious metal meshes from sagging in the center of the catalyst package.

[0052] Advantageous catalyst systems comprise, for example, at least one precious metal mesh with a recess and one precious metal mesh without a recess. Such systems are particularly advantageous when the precious metal mesh with a recess is arranged between two other precious metal meshes or precious metal mesh groups; in such cases, the catalyst system thus comprises at least three precious metal meshes. Such precious metal mesh groups can also comprise more than three meshes. Such precious metal meshes can be the meshes of one precious metal mesh group or meshes of different precious metal mesh groups.

[0053] Particularly advantageous are catalyst systems which have at least two different precious metal nets or precious metal net groups, of which one precious metal net or one precious metal net group has a platinum alloy and the other precious metal net or the other precious metal net group has a palladium alloy.

[0054] In a particularly preferred embodiment, the catalyst system comprises a platinum mesh group comprising a platinum mesh with a recess, followed by a noble metal-free support mesh without a recess, followed by a palladium mesh group without palladium meshes with a recess and a final noble metal-free mesh, which may again have a recess.

[0055] Further advantageous examples of catalyst systems according to the invention comprise at least one precious metal mesh without a recess and a precious metal-free mesh with a recess. The precious metal mesh(es) is / are in particular a platinum-containing precious metal mesh or a palladium-containing precious metal mesh, and the precious metal-free mesh is / are a support mesh made of heat-resistant steel. Particularly advantageous are catalyst systems that comprise at least two different precious metal meshes or precious metal mesh groups, of which one precious metal mesh or precious metal mesh group comprises a platinum alloy and the other precious metal mesh or precious metal mesh group comprises a palladium alloy. In such cases, the palladium mesh, the palladium mesh group, or part of the palladium mesh group is particularly advantageously arranged between the platinum mesh or the platinum mesh group and the precious metal-free mesh with a recess.Particularly suitable catalyst systems also comprise precious metal-free nets between the precious metal net groups and / or within a precious metal net group, which can be designed with or without a recess.

[0056] In a particularly preferred embodiment, the catalyst system comprises a platinum mesh group without platinum meshes with a recess, followed by a noble metal-free support mesh without a recess, followed by a palladium mesh group without palladium meshes with a recess and a final noble metal-free mesh having a recess.

[0057] Further embodiments of catalyst systems according to the invention comprise at least one precious metal mesh without a recess, a precious metal-free mesh with a recess, and a precious metal-free mesh arranged above the recess with a smaller diameter than the entire catalyst system. The precious metal mesh(es) is / are in particular a platinum-containing precious metal mesh or a palladium-containing precious metal mesh, and the support mesh and the mesh arranged above the recess are meshes made of heat-resistant steel. Particularly advantageous in this case are catalyst systems that have at least two different precious metal meshes or precious metal mesh groups, of which one precious metal mesh or precious metal mesh group comprises a platinum alloy and the other precious metal mesh or precious metal mesh group comprises a palladium alloy.In such cases, the palladium mesh, the palladium mesh group, or part of the palladium mesh group is particularly advantageously arranged between the platinum mesh or the platinum mesh group and the precious metal-free meshes. Particularly suitable catalyst systems also comprise precious metal-free meshes between the precious metal mesh groups and / or within a precious metal mesh group, which can be designed with or without a recess.

[0058] A catalyst system comprising a precious metal mesh with a recess and a precious metal-free mesh without a recess can also be advantageous. The precious metal mesh is, in particular, a platinum-containing precious metal mesh or a palladium-containing precious metal mesh, and the precious metal-free mesh is a support mesh made of heat-resistant steel. Particularly advantageous are catalyst systems comprising at least two different precious metal meshes or precious metal mesh groups, of which one precious metal mesh or precious metal mesh group comprises a platinum alloy and the other precious metal mesh or precious metal mesh group comprises a palladium alloy. In such cases, the palladium mesh, the palladium mesh group, or part of the palladium mesh group is particularly advantageously arranged between the platinum mesh or the platinum mesh group and the precious metal-free mesh.Particularly suitable catalyst systems also comprise precious metal-free nets between the precious metal net groups and / or within a precious metal net group, which can be designed with or without a recess.

[0059] Further embodiments of catalyst systems according to the invention comprise at least one precious metal mesh with a recess, a precious metal-free mesh without a recess, and a precious metal mesh arranged above the recess with a smaller diameter than the entire catalyst system. The precious metal mesh(es) is / are in particular a platinum-containing precious metal mesh or a palladium-containing precious metal mesh, and the support mesh and the mesh arranged above the recess are meshes made of heat-resistant steel. Particularly advantageous in this case are catalyst systems that have at least two different precious metal meshes or precious metal mesh groups, of which one precious metal mesh or precious metal mesh group comprises a platinum alloy and the other precious metal mesh or precious metal mesh group comprises a palladium alloy.In such cases, the palladium mesh, the palladium mesh group, or part of the palladium mesh group is particularly advantageously arranged between the platinum mesh or the platinum mesh group and the precious metal-free meshes. Particularly suitable catalyst systems also comprise precious metal-free meshes between the precious metal mesh groups and / or within a precious metal mesh group, which may be designed with or without a recess. It may also be advantageous if the precious metal mesh having the recess and the smaller precious metal mesh are arranged between further precious metal meshes that do not have a recess. Such precious metal meshes are preferably the meshes of a precious metal mesh group; in other words, such an advantageous precious metal mesh group comprises at least four meshes, with two meshes without a recess enclosing a precious metal mesh with a recess and a precious metal mesh with a smaller diameter.Such precious metal network groups can also include more than 4 networks.

[0060] In a particularly preferred embodiment, the catalyst system comprises a platinum mesh group comprising a platinum mesh with a recess and a platinum mesh with a smaller diameter, followed by a noble metal-free support mesh without a recess, followed by a palladium mesh group without meshes with a recess and a final noble metal-free mesh, which may again have a recess.

[0061] Further combinations of the embodiments described herein are also possible.

[0062] The present invention also relates to a reactor comprising a catalyst system according to the invention. For preferred embodiments of the catalyst system, reference is made to the preceding explanations.

[0063] During operation, gases enter the reactor through an inlet in the direction of flow, then flow through the catalyst system and finally out the bottom of the reactor. Typically, the catalyst system is installed in the reaction zone of a flow reactor in a plane perpendicular to the flow direction of the fresh gas. The catalyst system can be arranged so that the mesh with the cutout is positioned at the front of the flow direction, but it can also be positioned further back in the flow direction.

[0064] The reactor is preferably an ammonia oxidation reactor. The size of the reactor varies, but is typically a domed cylindrical vessel with an internal diameter of 0.5 to 6 m. The reactor may have additional components, such as support structures or fixing structures for the catalyst system, additional catalyst units, or cooling devices.

[0065] The present invention also relates to a process for the catalytic oxidation of ammonia, in which a fresh gas containing ammonia is passed over a catalyst system according to the invention. For preferred embodiments of the catalyst system, reference is made to the preceding explanations.

[0066] The catalyst system according to the invention is particularly suitable for the production of nitric acid by the Ostwald process. An ammonia-oxygen mixture flows through the catalyst system; in other words, this process involves catalytic ammonia combustion.

[0067] The catalyst system according to the invention is also suitable for the production of hydrogen cyanide by the Andrussow process. In this process, an ammonia-methane-oxygen mixture flows through the catalyst system.

[0068] The ammonia content of the fresh gas is preferably between 9.5 and 12 vol%. The pressure of the fresh gas is preferably between 1 and 14 bar, in particular between 3 and 10 bar. The catalyst system temperature is preferably in the range from 500 to 1300 °C, preferably in the range from 800 to 1100 °C.

[0069] Preferably, the fresh gas is supplied with a flow rate in the range of 6 to 60 tN / m 2 d through a catalyst system according to the present invention. The abbreviation "tN / m 2 d" stands for "tons of nitrogen (from ammonia) per day and a standardized effective cross-sectional area of ​​the catalyst system of one square meter.

[0070] The invention is explained below with reference to illustrations and exemplary embodiments. Like reference numerals designate like components in different embodiments; they are not explicitly mentioned in every embodiment. The illustrations are neither to scale nor do they correctly reflect the described proportions. Figure 1 shows a schematic of a vertically positioned flow reactor (1) for the heterogeneous catalytic combustion of ammonia. The catalyst system (2) forms the actual reaction zone of the flow reactor (1). The catalyst system (2) shown comprises several meshes (4, 5) arranged one behind the other in the flow direction (3) of the fresh gas. The catalyst system (2) comprises a catalyst mesh group (4) and a noble metal-free support mesh (5) arranged underneath, which has a centrally positioned recess (6).

[0071] The fresh gas, for example, is an ammonia-air mixture that is heated to a preheating temperature and introduced into the reactor (1) from above under increased pressure. Upon entering the catalyst system (2), the gas mixture is ignited, followed by an exothermic combustion reaction. The following main reaction takes place:

[0072] 4 NH3 + 5 O2 -> 4 NO + 6 H2O

[0073] In this process, ammonia (NH3) is converted to nitrogen monoxide (NO) and water (H2O). The resulting nitrogen monoxide (NO) reacts with excess oxygen in the outflowing reaction gas mixture (symbolized by arrow 7, which indicates the flow direction of the outflowing reaction gas mixture) to form nitrogen dioxide (NO2), which is then reacted with water in a downstream absorption system (not shown) to form nitric acid (HNO3).

[0074] Figure 2 shows a side view of a cross-section of embodiments of catalyst systems (2) according to the invention. In the present case, all meshes can have a round cross-section, with the mesh with the recess being annular. Other shapes of the meshes and recesses are also conceivable. Meshes shown individually can also represent mesh groups, i.e., a pack of several meshes of the same type. The precious metal meshes can be, for example, platinum- or palladium-containing meshes or mesh groups, while the precious metal-free meshes or mesh groups can be, for example, Kanthal meshes.

[0075] Fig. 2 A shows a catalyst system (2) comprising a precious metal-free mesh (5) with a recess (6) and a precious metal mesh (4). Fig. 2 B shows a catalyst system (2) in which a precious metal mesh (8) has a recess (6) and is arranged above a precious metal-free mesh without a recess (9). In the embodiment according to Fig. 2 C, another smaller precious metal-free mesh (10) is arranged above a precious metal-free mesh (5) with a recess (6). The embodiment shown in Fig. 2 D comprises two types of precious metal mesh (4, 11) above a precious metal-free mesh (5) with a recess (6). The catalyst system from Fig. 2 E comprises an additional precious metal-free mesh (9) which has no recess. A precious metal-free mesh (5) with a recess (6) is arranged between this mesh (9) and a precious metal mesh (4). The system shown in Fig. 2 F corresponds to the configuration shown in Fig.2 D, but comprises an additional precious metal-free net (9) arranged between the different precious metal nets (4, 11). The system according to Fig. 2 G corresponds to the configuration in Fig. 2 B, but comprises an additional precious metal net (11) arranged between the precious metal-free net (9) and the precious metal net (8). In the configuration shown in Fig. 2 H, this system is extended by a further precious metal net (4) which closes off the catalyst system in one direction and has no opening. The catalyst system in Fig. 2 I corresponds to the configuration shown in Fig. 2 B and comprises an additional precious metal net (12) which has a smaller diameter and is arranged above the opening (6) of the precious metal net (8). Such a system can also have further precious metal nets (11), as shown in Fig. 2 J.In an additional extension, the system can be completed with another precious metal mesh (4) (Fig. 2 K). A combination of several meshes with recesses (5, 8) is also possible, as shown in Fig. 2 L.

Claims

CLAIMS 1. A catalyst system for a flow reactor, comprising at least two meshes, at least one of the meshes being a noble metal mesh, characterized in that at least one of the meshes has a recess.

2. Catalyst system according to claim 1, wherein the noble metal mesh is a platinum-containing noble metal mesh or a palladium-containing noble metal mesh.

3. Catalyst system according to one of claims 1 or 2, wherein the recess is larger by a factor of at least 10 than the mesh size of the mesh with recess.

4. Catalyst system according to one of the preceding claims, wherein the cross-sectional area of ​​the recess is at least 5% of the cross-sectional area of ​​the catalyst system.

5. Catalyst system according to one of the preceding claims, wherein the Recess is arranged concentrically in the catalyst system.

6. Catalyst system according to one of the preceding claims, wherein the Catalyst system comprises another mesh which has a smaller diameter than the other meshes of the catalyst system.

7. Catalyst system according to one of the preceding claims, wherein the catalyst system comprises at least one noble metal-free mesh.

8. The catalyst system according to claim 7, wherein the noble metal-free mesh has the recess.

9. Catalyst system according to one of claims 7 or 8, wherein the mesh size of the noble metal-free mesh is larger than the mesh size of the at least one noble metal mesh.

10. Catalyst system according to one of claims 7 to 9, wherein the basis weight of the noble metal-free mesh is higher than the basis weight of the at least one noble metal mesh.

11. Catalyst system according to one of the preceding claims, wherein the Catalyst system comprises several precious metal network groups.

12. Reactor comprising a catalyst system according to one of claims 1 to 11.

13. Process for the catalytic oxidation of ammonia, in which at least Ammonia-containing fresh gas is passed through a catalyst system according to one of claims 1 to 11.

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