Catalyst systems for flow reactors and methods for the catalytic oxidation of ammonia

Abstract

This invention relates to a catalyst system for a flow reactor, characterized by the sequence of precious metal alloys used to form the catalyst mesh. By using palladium alloys in the second and third catalyst mesh groups, the overall platinum content of the catalyst system can be kept relatively low. Furthermore, this invention relates to a method for the catalytic combustion of ammonia, wherein fresh gas containing at least ammonia is guided through the catalyst system.

Description

[0001] This invention relates to a catalyst system for a flow reactor, characterized by the sequence in which a noble metal alloy is used to form the catalyst network of the system. Furthermore, this invention relates to a method for the catalytic combustion of ammonia, wherein a fresh gas containing at least ammonia is directed through the catalyst system.

[0002] Catalyst systems in the sense of this invention are particularly used for gaseous reactions. They are used, for example, for the preparation of hydrogen cyanide via the Androusso process or for the preparation of nitric acid via the Ostwald process. To provide a large catalytically active surface for these reactions, such catalysts typically include a permeable spatial structure. Collection systems for recovering evaporated catalytically active components are also often based on such a mesh structure. Typically, multiple meshes are advantageously arranged one after another and combined to form a catalyst system. Catalyst meshes are typically composed of single or multiple layers of woven, interwoven, or fabricated textiles. Individual meshes consist of fine precious metal wires primarily containing platinum (Pt), palladium (Pd), rhodium (Rh), or alloys of these metals. In particular, the trapping mesh may also include other components, such as nickel.

[0003] The design utilizes a system with 2 to 50 catalyst meshes, each up to 6 meters in diameter, in a flow reactor. The use of precious metals represents a high investment, which must be kept as low as possible. On the other hand, “catalytic efficiency,” a crucial parameter and metric for sustained high conversion and good yield of reactants, depends on the precious metal content. Due to the increasing price of precious metals and the resulting investment burden on the catalyst system, the objective is to minimize the precious metal content of the catalyst system while maintaining efficiency.

[0004] During operation, the catalyst mesh continuously loses precious metals due to oxidation and sublimation, and therefore must be replaced periodically at a cost (for durability and lifespan). PtRh5 alloy, already established as the industry standard for precious metal catalysts in medium-pressure systems, has proven to be a suitable compromise regarding lifespan, catalytic efficiency, and the use of precious metals.

[0005] EP 3680015 A1 discloses a catalyst system consisting of at least two mesh layers, wherein a binary PtRh alloy is preferably used, the rhodium content of which decreases in the flow direction. Due to the use of a platinum alloy, the overall platinum content of the catalyst system is high.

[0006] To reduce the use of precious metals while maintaining catalytic efficiency, EP 1284927 A1 proposes a catalyst system consisting of at least two mesh layers, wherein, viewed from the flow direction, the first mesh is formed of a platinum-rhodium alloy and the second mesh is formed of a palladium-rhodium alloy.

[0007] CN 101554585 A describes a catalyst system comprising at least three mesh layers made of alloys with different compositions. The platinum alloy of the middle mesh layer contains a high platinum content of 50% to 73% by weight.

[0008] The object of this invention is to provide a catalyst system for a flow reactor in which the precious metals used can be utilized with maximum efficiency.

[0009] Furthermore, the object of the present invention is to provide a method for using such a catalyst system.

[0010] This objective is achieved by a catalyst system for a flow reactor, comprising at least three catalyst mesh groups arranged sequentially in the flow direction, wherein in each case, each catalyst mesh group is formed by at least one catalyst mesh, the at least one catalyst mesh being composed of at least one noble metal wire, and

[0011] - The first catalyst mesh assembly includes at least one catalyst mesh, which is composed of at least one first noble metal wire made of a platinum alloy, the platinum alloy being composed of 80 wt% to 98 wt% platinum, 2 wt% to 20 wt% rhodium, and 0 wt% palladium, excluding impurities.

[0012] - The second catalyst mesh assembly includes at least one catalyst mesh composed of at least one second noble metal wire made of a palladium alloy. The palladium alloy of the second noble metal wire, excluding impurities, comprises 70% to 97% palladium, 0% to 10% rhodium, and 3% to 30% of at least one other metal selected from nickel, tungsten, platinum, and gold.

[0013] - The third catalyst mesh assembly includes at least one catalyst mesh, which is composed of at least one third noble metal wire made of a palladium alloy. The palladium alloy of the third noble metal wire, excluding impurities, consists of 72% to 97% palladium, 0% to 10% rhodium, and 3% to 28% at least one other metal selected from nickel, tungsten, platinum, and gold.

[0014] The method is characterized in that the rhodium content of the precious metal wire of the catalyst network decreases or remains constant in the flow direction, and the palladium content of the precious metal wire of the catalyst network increases in the flow direction.

[0015] By using palladium alloys in the second and third catalyst meshes, the overall platinum content of the catalyst system can be kept relatively low. Within the scope of this invention, it has been surprisingly found that the positioning of catalyst meshes with different compositions in the rear region of the catalyst system, viewed in the flow direction, has a significant impact on the system efficiency.

[0016] This invention relates to a catalyst system for a flow reactor. In a flow reactor, a catalyst, in the form of a breathable fabric, is typically incorporated into a reaction zone in a plane perpendicular to the flow direction of the fresh gas. This breathable fabric is typically used in the form of a catalyst mesh. The catalyst system should be understood to refer to an assembly of such catalyst meshes.

[0017] The catalyst system according to the invention comprises at least three catalyst mesh groups arranged one after another in the flow direction. In each case, each catalyst mesh group is formed by at least one catalyst mesh, which is composed of at least one noble metal wire. A catalyst mesh group is understood to mean an assembly of at least one catalyst mesh in which the composition of the noble metal wire is not different. Typically, a catalyst mesh group comprises more than one catalyst mesh. Thus, the catalyst system according to the invention comprises at least three catalyst meshes composed of noble metal wires having three different compositions.

[0018] In the flow direction, the reactant gas first passes through the first catalyst mesh group, then through the second catalyst mesh group, and finally through the third catalyst mesh group.

[0019] Catalyst mesh should be understood as referring to single-layer or multi-layer breathable fabric. The surface of the catalyst mesh can be formed by interlocking one or more precious metal threads to create a mesh. Catalyst meshes can be produced, for example, by weaving, interlacing, or braiding one or more precious metal threads. Therefore, the structure of the catalyst mesh can be configured in a targeted manner by using different weaving, interlacing, or braiding patterns and / or different mesh sizes.

[0020] One or more catalyst nets from the first catalyst net group, one or more catalyst nets from the second catalyst net group, and one or more catalyst nets from the third catalyst net group can be independently interwoven, woven, and / or braided. Therefore, the interwoven, woven, and braided catalyst nets can be combined with each other as needed. For example, one or more catalyst nets from the first catalyst net group can be interwoven, and one or more catalyst nets from the second and third catalyst net groups can be woven. Similarly, one or more catalyst nets from the first catalyst net group can be woven, one or more catalyst nets from the second catalyst net group can be woven, and one or more catalyst nets from the third catalyst net group can be braided.

[0021] One or more catalyst meshes in the first catalyst mesh group include a first woven, interlaced, or braided pattern and a first mesh size. One or more catalyst meshes in the second catalyst mesh group include a second woven, interlaced, or braided pattern and a second mesh size, and one or more catalyst meshes in the third catalyst mesh group include a third woven, interlaced, or braided pattern and a third mesh size.

[0022] It has proven advantageous that at least two of the first, interlaced or braided pattern, the second, and the third are identical; particularly advantageously, that the first, interlaced or braided pattern, the second, and the third are all identical.

[0023] Furthermore, it may be advantageous that at least two of the first, second, and third mesh sizes are the same; particularly advantageously, that the first, second, and third mesh sizes are all the same.

[0024] The mass per unit area of ​​the catalyst mesh is not further limited. The mass per unit area of ​​the catalyst mesh can be 100 g / m². 2 Up to 950g / m 2 Within the range, especially at 150g / m 2 Up to 800g / m 2 Within the range of [specific parameters], the mass per unit area of ​​the catalyst mesh in one of at least three catalyst mesh groups may be the same or different. It has proven advantageous that the mass per unit area of ​​the catalyst mesh in a catalyst mesh group is the same.

[0025] The mass per unit area of ​​the catalyst network in at least three catalyst network groups can remain equal, decrease, or increase in the flow direction.

[0026] In a preferred embodiment, at least one catalyst mesh may include a three-dimensional structure. In the context of this application, "mesh" is understood as a flat, two-dimensional object. A three-dimensional structure should be understood to mean that the catalyst mesh extends into a third spatial dimension in addition to its planar extension. A catalyst mesh with a three-dimensional structure has a larger surface area, which advantageously affects catalytic performance and can reduce pressure drop in a flow reactor. A three-dimensional structure can be obtained by using at least one noble metal wire with a two-dimensional or three-dimensional structure or by texturing the catalyst mesh. The three-dimensional structure of the catalyst mesh can be, for example, wave-shaped or coil-shaped. To produce such a structure, the initially planar catalyst mesh can be subjected to process steps in which a three-dimensional structure is imprinted or produced by folding.

[0027] Three-dimensional structures can be obtained by placing a planar catalyst mesh on a rigid, permeable, but non-planar surface (e.g., a pre-formed metal mesh). This textured, permeable surface structure does not necessarily need to be catalytically efficient before being transferred to the catalyst mesh. Catalyst meshes with this structure are also called corrugated.

[0028] It has been shown that it is advantageous if at least one catalyst mesh of the catalyst system according to the invention comprises a three-dimensional structure, particularly if the catalyst mesh is a corrugated catalyst mesh. It may also be advantageous if at least one catalyst mesh in each catalyst mesh group comprises a three-dimensional structure.

[0029] The amount of catalyst mesh used depends on the operating conditions of the flow reactor. Among other factors, the throughput of fresh gas (especially pressure-dependent) is a key factor. For example, in flow reactors operating at low pressures (e.g., up to about 5 bar abs.), a catalyst mesh typically less than 15, usually between 5 and 10, can be used, while at higher pressures (e.g., up to 15 bar), a larger number of catalyst meshes can be used, typically greater than 20, usually between 30 and 50.

[0030] In each case, the catalyst mesh is formed by at least one noble metal wire. The noble metal is preferably selected from platinum, gold, and silver. Platinum should be understood to refer to the so-called platinum group metals, namely platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os), and ruthenium (Ru). The noble metal wire is understood to be a wire composed of a noble metal or a noble metal alloy.

[0031] Preferredly, a precious metal wire is used, having a diameter of 40 μm to 250 μm, preferably 50 μm to 200 μm, and particularly preferably 60 μm to 150 μm.

[0032] The precious metal wires can be designed as round wires, i.e., having a circular cross-section. In another embodiment, at least one of the precious metal wires can be designed as a flat round wire or as a wire with a different cross-section.

[0033] Precious metal wires may consist of multiple wires, in which case they are also called filaments. The filaments may all be composed of the same material, i.e., all containing precious metals, or they may be composed of different materials, and the different materials do not necessarily contain precious metals.

[0034] Fine filaments can be twisted together; in these cases, precious metal wires include rope-like structures.

[0035] The precious metal wire may include one or more helical longitudinal segments, or may be formed as a helical curve over its entire length. If the precious metal wire includes helical longitudinal segments, the active catalytic surface of the catalyst mesh and the mass of the catalyst mesh relative to the surface unit can be adjusted, for example, via the wire thickness or via the number of turns of the helical longitudinal segments. When using such a precious metal wire, the catalyst mesh produced from the precious metal wire may include a three-dimensional structure.

[0036] In many cases, it may be advantageous if the catalyst mesh is formed from two or more precious metal wires. In these cases, the precious metal wires can be composed of the same or different materials. Multiple precious metal wires can have the same or different diameters.

[0037] The first catalyst mesh of the catalyst system according to the invention comprises at least one catalyst mesh consisting of at least one first noble metal wire. The first noble metal wire comprises a platinum alloy, which, in addition to impurities, consists of 80% to 98% platinum, 2% to 20% rhodium, and 0% to 20% palladium.

[0038] Platinum alloy is understood to mean an alloy composed of platinum in amounts exceeding 50% by weight. The fact that the alloy is composed of 80% to 98% by weight platinum means that the weight percentage of platinum accounts for 80% to 98% of the total weight of the alloy. The platinum alloy of the first precious metal line preferably contains 85% to 97% by weight, particularly 90% to 95% by weight platinum.

[0039] The precious metal alloys described herein may contain impurities. In the present context, impurities are understood to mean expected or unavoidable impurities resulting from the production of one or more alloys. Unless otherwise stated, for all the described precious metal alloys, the total proportion of impurities, based on the total weight of the particular precious metal alloy, shall not exceed 1% by weight, preferably not more than 0.5% by weight. The precious metal alloys of this application particularly include platinum alloys of the first precious metal wire and palladium alloys of the second and third precious metal wires.

[0040] In a preferred embodiment, the platinum alloy of the first precious metal wire contains 3% to 18% by weight, particularly 5% to 15% by weight, of rhodium.

[0041] The platinum alloy of the first precious metal wire preferably contains no more than 15% by weight, particularly no more than 10% by weight, palladium.

[0042] In addition to platinum, rhodium, and optionally palladium, the platinum alloy of the first precious metal wire may also contain impurities. The total proportion of impurities shall not exceed 1% by weight of the platinum alloy, preferably not more than 0.5% by weight.

[0043] Particularly preferably, the first noble metal wire comprises a binary platinum alloy composed of platinum and rhodium. The binary platinum alloy may also contain impurities as described above. In this case, the platinum alloy of the first noble metal wire is, for example, PtRh3, PtRh5, PtRh8, PtRh10, or PtRh15. PtRh(X) here means that the alloy contains X wt% rhodium and, in addition to impurities, (100-X) wt% platinum.

[0044] In a preferred embodiment, the first catalyst mesh group comprises 1 to 10 catalyst meshes, preferably 3 to 8 catalyst meshes. In particular, the first catalyst mesh group may include 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 catalyst meshes.

[0045] Surprisingly, it has been found that sufficient catalytic efficiency can be achieved if the first catalyst network consists of only one catalyst network. This represents a particularly simple and therefore preferred embodiment of the catalyst system.

[0046] The second catalyst mesh of the catalyst system according to the invention comprises at least one catalyst mesh made of at least one second noble metal wire made of palladium alloy. Except for impurities, the palladium alloy of the second noble metal wire consists of 70% to 97% palladium, 0% to 10% rhodium, and 3% to 30% at least one other metal selected from nickel, tungsten, platinum, and gold.

[0047] Palladium alloy is understood to mean an alloy consisting of more than 50% by weight of palladium. The palladium alloy of the second precious metal wire preferably contains 75% to 95% by weight, preferably 80% to 90% by weight of palladium.

[0048] In addition to palladium, nickel, tungsten, platinum or gold, and optionally rhodium, the palladium alloy of the second precious metal wire may also contain impurities. The total proportion of impurities shall not exceed 1% by weight of the palladium alloy of the second precious metal wire, preferably not exceeding 0.5% by weight.

[0049] The palladium alloy of the second precious metal wire preferably contains at least one other metal in the range of 5% to 28% by weight, particularly preferably in the range of 8% to 25% by weight.

[0050] The palladium alloy of the second precious metal line may contain any combination selected from nickel, tungsten, platinum, and gold.

[0051] In a preferred embodiment, the palladium alloy of the second precious metal wire contains a rhodium content ranging from 1% to 10% by weight, particularly from 3% to 8% by weight. Considering the high catalytic efficiency and the low or no negative impact on lifespan, it may be advantageous for the palladium alloy of the second precious metal wire to contain at least 1.5% by weight of rhodium. In this case, the second precious metal wire is particularly preferably composed of at least 1.5% by weight of platinum.

[0052] The second precious metal wire may contain a ternary palladium alloy composed of palladium, platinum, and rhodium, or it may contain a binary palladium alloy containing nickel, tungsten, platinum, or gold in addition to palladium. The binary or ternary palladium alloys may also contain impurities as described above.

[0053] For example, the palladium alloy of the second noble metal wire can be PdPt10Rh5, PdPt15Rh3, PdPt20Rh1, PdNi5, PdW5, PdPt5, PdAu5, PdNi3, PdW3, PdPt3, or PdAu3. In this text, PdPt(Z)Rh(X) means that the alloy contains Z wt% platinum, X wt% rhodium, and, excluding impurities, (100 - (Z + X)) wt% palladium.

[0054] In a preferred embodiment, the second catalyst group comprises 1 to 10 catalyst meshes, preferably 3 to 8 catalyst meshes. Specifically, the second catalyst mesh group may include 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 catalyst meshes. The second catalyst mesh group preferably comprises more catalyst meshes than the first catalyst mesh group.

[0055] The third catalyst mesh of the catalyst system according to the invention comprises at least one catalyst mesh consisting of at least one third noble metal wire made of palladium alloy.

[0056] Apart from impurities, the palladium alloy of the third precious metal wire consists of 72% to 97% palladium, 0% to 10% rhodium, and 3% to 28% at least one other metal selected from nickel, tungsten, platinum, and gold.

[0057] The palladium alloy of the third precious metal wire preferably contains a palladium content in the range of 75% to 95% by weight, and particularly preferably in the range of 78% to 90% by weight.

[0058] The palladium alloy of the third precious metal wire preferably contains at least one other metal in the range of 5% to 25% by weight, particularly preferably in the range of 8% to 20% by weight.

[0059] The palladium alloy of the third precious metal line can contain any combination of nickel, tungsten, platinum, and gold.

[0060] In addition to palladium, nickel, tungsten, platinum or gold, and optionally rhodium, the palladium alloy of the third precious metal wire may also contain impurities. The total proportion of impurities shall not exceed 1% by weight of the palladium alloy of the third precious metal wire, preferably not exceeding 0.5% by weight.

[0061] According to the present invention, the palladium content of the noble metal wires in the catalyst mesh is increased in the flow direction. This increase in palladium content has proven to be particularly advantageous for the catalytic efficiency of the catalyst system. The palladium content of the third noble metal wire is preferably at least 2 weight percentage points higher than that of the second noble metal wire, particularly 4 weight percentage points higher, and more preferably at least 8 weight percentage points higher.

[0062] According to the present invention, the rhodium content of the precious metal wires in the catalyst mesh is reduced or kept the same. It has been surprisingly found that catalyst systems with this rhodium gradient are more efficient than systems in which one or more catalyst meshes comprising precious metal wires made of an alloy with a higher rhodium content are arranged in the rear or final region of the catalyst system.

[0063] In a preferred embodiment, the rhodium content of the third precious metal wire is at least 2 weight percentage points lower than that of the second precious metal wire, and particularly preferably at least 3 weight percentage points lower.

[0064] Preferably, the third precious metal wire does not contain rhodium. In these cases, the palladium alloy of the third precious metal wire is preferably a binary alloy, that is, the binary alloy consists of palladium and only one other component in addition to impurities. The other component is preferably platinum or nickel.

[0065] It may be advantageous if the third precious metal wire is platinum-free. In such embodiments, the platinum content in the catalyst system can be further reduced. Particularly preferred is that, in this case, the third precious metal wire comprises a binary palladium alloy composed of palladium and nickel, tungsten, or gold.

[0066] Examples of preferred palladium alloys for the third precious metal wire include PdPt5Rh5, PdPt15Rh3, PdPt20Rh1, PdNi5, PdW5, PdPt5, PdAu5, PdNi3, PdW3, and PdAu3.

[0067] In a preferred embodiment, the third catalyst mesh group comprises 1 to 10 catalyst meshes, preferably 2 to 8 catalyst meshes. Specifically, the third catalyst mesh group may include 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 catalyst meshes. The third catalyst mesh group preferably comprises more catalyst meshes than the first or second catalyst mesh group.

[0068] Typically, the catalyst meshes of the second and third catalyst groups occupy a maximum proportion of, for example, at least 70% of the total volume of the catalyst system, and it is sufficient for the catalyst mesh of the first catalyst mesh group, composed of rhodium-rich noble metal wires, to occupy only a small proportion of the volume, for example, less than 30%, preferably less than 25%, and particularly preferably less than 20%. In each case, the volume of the catalyst system or catalyst mesh group is primarily determined by the number of catalyst meshes used.

[0069] It may be advantageous that neither the second nor the third precious metal wire contains platinum. In this case, the catalyst system contains a particularly low platinum content.

[0070] In a preferred embodiment of the catalyst system according to the invention,

[0071] - The first precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0072] - The second precious metal wire is made of a ternary palladium alloy composed of palladium, platinum, and rhodium, and

[0073] - The third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold.

[0074] Specifically, the first noble metal wire may be composed of a PtRh(2-20) alloy, the second noble metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, and the third noble metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy. PtRh(ab) here means, for example, that the alloy contains rhodium in a weight percentage ranging from a wt% to b wt%, and the remaining percentage (100–(a to b)) wt% consists of platinum, excluding impurities.

[0075] Alternatively, the preferred option is...

[0076] - The first precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0077] - The second precious metal wire is made of a ternary palladium alloy composed of palladium, platinum, and rhodium, and

[0078] - The third precious metal wire is made of a ternary palladium alloy composed of palladium, platinum and rhodium.

[0079] Specifically, the first precious metal wire may be composed of a PtRh(2-20) alloy, the second precious metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, and the third precious metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy.

[0080] In another preferred embodiment,

[0081] - The first precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0082] - The second precious metal wire is made of a binary palladium alloy composed of palladium and nickel, tungsten, platinum, or gold.

[0083] and

[0084] - The third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold.

[0085] Specifically, the first precious metal wire may be composed of a PtRh(2-20) alloy, the second precious metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy, and the third precious metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy.

[0086] In another preferred embodiment of the catalyst system according to the invention,

[0087] - The first precious metal wire is made of a ternary platinum alloy composed of platinum, palladium, and rhodium.

[0088] - The second precious metal wire is made of a ternary palladium alloy composed of palladium, platinum, and rhodium, and

[0089] - The third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum and gold.

[0090] Specifically, the first noble metal wire may be composed of a PtPd(1-20)Rh(2-10) alloy, the second noble metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, and the third noble metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy.

[0091] The catalyst system may also include additional components.

[0092] The catalyst system may include, for example, an ignition layer upstream of a first catalyst network. The ignition layer comprises wires of noble metal containing only platinum and impurities.

[0093] It may also be advantageous that, viewed from the flow direction, at least the foremost catalyst mesh contains noble metal wires made of platinum, which contain no other components besides impurities. This foremost catalyst mesh can be the catalyst mesh of the first catalyst mesh group.

[0094] The catalyst system according to the invention may further include at least one additional catalyst mesh assembly, which consists of at least one catalyst mesh, which is composed of at least one additional noble metal wire.

[0095] At least one additional precious metal wire may contain the same composition as the first, second, or third precious metal wire, but the composition may also be different. In this case, it is also preferable that the rhodium content of the precious metal wire decreases in the flow direction. In this case, the palladium content of the precious metal wire may remain constant, increase, or decrease in the flow direction.

[0096] It has been shown that it is advantageous to have at least one additional catalyst mesh arranged upstream of the first catalyst mesh. In this case, it is particularly advantageous if at least one additional noble metal wire contains a platinum alloy, especially a binary PtRh alloy.

[0097] The catalyst system according to the invention may include, for example, at least four catalyst mesh groups, each catalyst mesh group comprising at least one catalyst mesh composed of at least one noble metal wire. At least one additional catalyst mesh group is preferably arranged upstream of the first catalyst mesh group. The rhodium content of the noble metal wire decreases or remains constant in the flow direction.

[0098] The preferred option is

[0099] -At least one additional precious metal wire contains a binary platinum alloy composed of platinum and rhodium.

[0100] - The first precious metals line contains a binary platinum alloy composed of platinum and rhodium.

[0101] - The second precious metal wire contains a ternary palladium alloy composed of palladium, platinum, and rhodium, and

[0102] - The third precious metals line includes binary palladium alloys composed of palladium and nickel, tungsten, platinum, or gold.

[0103] Specifically, the first precious metal wire may be composed of a PtRh(2-20) alloy, the second precious metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, the third precious metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy, and at least one additional precious metal wire may be composed of a PtRh(2-20) alloy.

[0104] Alternatively, the preferred option is...

[0105] -At least one additional precious metal wire contains a binary platinum alloy composed of platinum and rhodium.

[0106] - The first precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0107] - The second precious metal wire is made of a ternary palladium alloy composed of palladium, platinum, and rhodium, and

[0108] - The third precious metal wire is made of a ternary palladium alloy composed of palladium, platinum and rhodium.

[0109] Specifically, the first precious metal wire may be composed of a PtRh(2-20) alloy, the second precious metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, the third precious metal wire may be composed of a PdPt(3-30)Rh(1-10) alloy, and at least one additional precious metal wire may be composed of a PtRh(2-20) alloy.

[0110] In another preferred embodiment,

[0111] -At least one additional precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0112] - The first precious metal wire is made of a binary platinum alloy composed of platinum and rhodium.

[0113] - The second precious metal wire is made of a binary palladium alloy composed of palladium and nickel, tungsten, platinum, or gold.

[0114] and

[0115] - The third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold.

[0116] Specifically, the first precious metal wire may be composed of a PtRh(2-20) alloy, the second precious metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy, the third precious metal wire may be composed of a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy, and at least one additional precious metal wire may be composed of a PtRh(2-20) alloy.

[0117] In a preferred embodiment, the catalyst system according to the invention may include at least one separating element between two catalyst mesh groups, for example, in the form of at least one intermediate mesh. This intermediate mesh can be used to counteract compression of adjacent catalyst mesh groups under pressure loads. One or more intermediate meshes preferably have limited flexibility compared to the catalyst meshes of the catalyst mesh groups.

[0118] Suitable separating elements are, for example, elements or meshes made of heat-resistant steel (typically FeCrAl alloys such as Megapyr or Kanthal), stainless steel, or heat-resistant alloys (such as nickel-chromium alloys). One or more separating elements may also include a catalytically active coating comprising at least one noble metal.

[0119] It has been shown that it is advantageous to arrange a separating element, particularly an intermediate mesh, between the first and second catalyst mesh groups. Intermediate meshes made of Megapyr or Kanthal have proven to be particularly advantageous.

[0120] Separating elements in the form of intermediate mesh can also be arranged within the catalyst mesh assembly.

[0121] The catalyst system according to the invention is suitable for preparing nitric acid via the Ostwald process. An ammonia-oxygen mixture flows through the catalyst system; in other words, this involves the catalytic combustion of ammonia.

[0122] The catalyst system according to the invention is also suitable for the preparation of hydrogen cyanide via the Andruse process. An ammonia-methane-oxygen mixture flows through the catalyst system.

[0123] The present invention also relates to a method for the catalytic oxidation of ammonia, wherein a fresh gas containing at least ammonia is directed through a catalyst system according to the invention. For a preferred embodiment of the catalyst system, refer to the foregoing description.

[0124] The ammonia content of the fresh gas is preferably 9.5% to 12% by volume.

[0125] The pressure of the fresh gas is preferably from 1 bar to 14 bar, particularly from 3 bar to 10 bar. The temperature of the catalyst network is preferably in the range of 500°C to 1300°C, and more preferably in the range of 800°C to 1100°C.

[0126] Preferably, the fresh gas is supplied at 6 tN / m 2 d to 60tN / m 2 Throughput in the range of d is directed through the catalyst system according to the invention. (abbreviation "tN / m") 2 "d" represents "tons of nitrogen (from ammonia) / day and 1 square meter of standardized effective cross-sectional area of ​​the catalyst system".

[0127] The invention is explained below with reference to the accompanying drawings, exemplary embodiments, and experimental results regarding catalytic activity.

[0128] Figure 1A vertically positioned flow reactor 1 for heterogeneous catalytic combustion of ammonia is schematically shown. A catalyst system 2 forms the actual reaction zone of the flow reactor 1. The catalyst system 2 includes multiple catalyst meshes (4, 5, 6) arranged one after another in the direction of fresh gas flow 3.

[0129] The fresh gas is an ammonia-air mixture containing 10.7% by volume of nominal ammonia. The fresh gas is heated to a preheated temperature of 175°C and introduced from above into reactor 1 under a high pressure of 5 bar. Upon entering catalyst system 2, the gas mixture is ignited, and subsequently, an exothermic combustion reaction occurs. The following main reaction occurs:

[0130] 4NH3 + 5O2 → 4NO + 6H2O

[0131] In this process, ammonia (NH3) is converted into nitric oxide (NO) and water (H2O). The nitric oxide (NO) formed reacts with excess oxygen in the outflowing reaction gas mixture (symbolized by directional arrow 7 indicating the flow direction of the outflowing reaction gas mixture) to form nitrogen dioxide (NO2), which is added to water in the downstream absorption system to form nitric acid (HNO3).

[0132] In each case, the catalyst mesh is a woven fabric produced from a relevant precious metal alloy by machine weaving together precious metal threads with a diameter of 76 μm. Exemplary embodiments (E1 to E11) of the catalyst system that can be used in the flow reactor 1 are specified in Tables 1 and 2.

[0133] Table 1 :

[0134] E1 E2 E3 E4 E5 E6 G1 PtRh5 PtRh5 PtRh5 PtRh5 PtRh5 PtRh5 G2 PdPt15Rh3 PdPt15Rh3 PdPt10Rh3 PdPt15Rh3 PdPt15 PdPt20Rh1 G3 PdPt5 PdNi5 PdNi5 PdNi5 PdNi5 PdNi5

[0135] Table 2 :

[0136] E7 E8 E9 E10 E11 G1 PtRh5 PtRh5Pd5 PtRh5Pd5 PtPd15Rh5 PtPd15Rh5 G2 PdPt10Rh1 PdPt20 PdPt10Rh5 PdPt10Rh5 PdPt5Rh5 G3 PdW5 PdNi5 PdPt5Rh5 PdPt5 PdW5

[0137] Each catalyst system comprises three catalyst mesh groups, totaling 30 catalyst meshes; the order of naming G1 to G3 reflects the arrangement of the fresh gas in the flow direction.

[0138] Table 3 indicates the composition of catalyst system E12 with four catalyst mesh groups, wherein the rhodium-rich group G0 is arranged upstream of the catalyst system with G1 to G3 according to the invention.

[0139] Table 3 :

[0140] E12 G0 PtRh8 G1 PtRh5 G2 PdPt15Rh3 G3 PdNi5

[0141] According to Figure 1In the test reactor, catalyst systems E1 to E12 were compared with catalyst systems in which the arrangement of catalyst mesh groups G2 and G3 was reversed. In the comparison reactor, the catalyst mesh group with rhodium-rich alloy was therefore located after the rhodium-poor alloy. Both reactors had the same amount of catalytically active noble metal.

[0142] In each case, the test reactor was operated under the same test conditions.

[0143] Pressure: 5 bar (absolute)

[0144] Throughput: 12 tons of nitrogen (from ammonia) per day and per effective cross-sectional area per square meter of catalyst packing (abbreviated as 12 tN / m²). 2 d)

[0145] NH3 percentage: 10.7% by volume in fresh gas

[0146] Preheating temperature: 175℃ (NH3 temperature / air mixture), resulting in a mesh temperature of 890℃.

[0147] At approximately 12-hour intervals, over a 4-day period, the catalytic efficiency of the catalyst (NO yield in %) and the development of the amount of nitrous oxide (N₂O) as an undesirable byproduct were measured.

[0148] The measurement of catalytic efficiency (i.e., the product yield of NO) follows this order:

[0149] 1. Ensure the catalyst system is suitable for the complete conversion of ammonia. This means that there should no longer be a significant amount of NH3 in the product gas, which can be checked by mass spectrometry analysis of the product gas.

[0150] 2. NH3 / air samples upstream of the catalyst packing and samples from downstream product gases are simultaneously removed in their respective independent evacuation pistons. The mass of the gases is determined by weighing.

[0151] 3. Absorb the NH3 / air mixture in distilled water and titrate with 0.1N sulfuric acid and methyl red after the color changes.

[0152] 4. Absorb the nitrous acid product gas in a 3% sodium peroxide solution and titrate with 0.1N sodium hydroxide solution and methyl red after the color changes.

[0153] 5. The catalytic efficiency Eta is calculated using the following formula: Eta = 100 × C n / C a C a It is the average NH3 concentration measured individually seven times in fresh gas, expressed as a weight percentage, and C n It is the average NO measured individually 7 times. xConcentration, expressed as the amount oxidized to form NO x The weight percentage of NH3.

[0154] 6. Determine the volume percentage of N2O in the product gas separately using gas chromatography.

[0155] In the reactor equipped according to the present invention, with a considerable proportion of N2O, an average increase in efficiency of 0.4% can be observed throughout the test period. In this technical field, an increase of 0.4% represents a significant and economically meaningful enhancement.

Claims

1. Catalyst system for a flow reactor, comprising at least three catalyst mesh groups arranged one after the other in a flow direction, in each case, each catalyst mesh group is formed by at least one catalyst mesh, which is composed of at least one noble metal wire, and - the first catalyst mesh group comprises at least one catalyst mesh, which is composed of at least one first noble metal wire made of a platinum alloy consisting, apart from impurities, of 80 to 98% by weight of platinum, 2 to 20% by weight of rhodium and 0 to 20% by weight of palladium, which platinum alloy contains the sum of the individual component contents to 100%, - the second catalyst mesh group comprises at least one catalyst mesh, which is composed of at least one second noble metal wire made of a palladium alloy consisting, apart from impurities, of 70 to 97% by weight of palladium, 0 to 10% by weight of rhodium and 3 to 30% by weight of at least one further metal selected from the group consisting of nickel, tungsten, platinum and gold, and - the third catalyst mesh group comprises at least one catalyst mesh, which is composed of at least one third noble metal wire made of a palladium alloy consisting, apart from impurities, of 72 to 97% by weight of palladium, 0 to 10% by weight of rhodium and 3 to 28% by weight of at least one further metal selected from the group consisting of nickel, tungsten, platinum and gold, characterized in that - the rhodium content of the noble metal wires of the catalyst mesh groups decreases or remains constant in the flow direction, and - the palladium content of the noble metal wires of the catalyst mesh groups increases in the flow direction.

2. Catalyst system according to claim 1, wherein the catalyst meshes are woven independently of one another.

3. Catalyst system according to claim 1, wherein the catalyst meshes are interwoven independently of one another.

4. Catalyst system according to claim 1, wherein the catalyst meshes are knitted independently of one another.

5. Catalyst system according to claim 1, wherein at least one of the catalyst meshes comprises a three-dimensional structure.

6. Catalyst system according to claim 2, wherein at least one of the catalyst meshes comprises a three-dimensional structure.

7. Catalyst system according to claim 3, wherein at least one of the catalyst meshes comprises a three-dimensional structure.

8. Catalyst system according to claim 4, wherein at least one of the catalyst meshes comprises a three-dimensional structure.

9. Catalyst system according to claim 5, wherein at least one of the catalyst meshes is corrugated.

10. Catalyst system according to claim 6, wherein at least one of the catalyst meshes is corrugated.

11. Catalyst system according to claim 7, wherein at least one of the catalyst meshes is corrugated.

12. The catalyst system of claim 8, wherein at least one of the catalyst gauzes is corrugated.

13. The catalyst system of any one of claims 1-12, wherein the noble metal wire comprises a diameter of 40 pm to 250 pm.

14. The catalyst system of any one of claims 1-12, wherein the first noble metal wire comprises a binary platinum alloy consisting of platinum and rhodium, in addition to impurities.

15. The catalyst system of claim 13, wherein the first noble metal wire comprises a binary platinum alloy consisting of platinum and rhodium, in addition to impurities.

16. The catalyst system of any one of claims 1-12, wherein the second noble metal wire comprises a ternary palladium alloy consisting of palladium, platinum, and rhodium, in addition to impurities, or the second noble metal wire comprises a binary palladium alloy consisting of palladium and nickel, tungsten, platinum, or gold, in addition to impurities.

17. The catalyst system of claim 15, wherein the second noble metal wire comprises a ternary palladium alloy consisting of palladium, platinum, and rhodium, in addition to impurities, or the second noble metal wire comprises a binary palladium alloy consisting of palladium and nickel, tungsten, platinum, or gold, in addition to impurities.

18. The catalyst system of any one of claims 1-12, wherein the third noble metal wire has a palladium content at least 2 percentage points higher than the palladium content of the second noble metal wire.

19. The catalyst system of claim 17, wherein the third noble metal wire has a palladium content at least 2 percentage points higher than the palladium content of the second noble metal wire.

20. The catalyst system of any one of claims 1-12, wherein the third noble metal wire has a rhodium content at least 2 percentage points lower than the rhodium content of the second noble metal wire.

21. The catalyst system of claim 19, wherein the third noble metal wire has a rhodium content at least 2 percentage points lower than the rhodium content of the second noble metal wire.

22. The catalyst system of any one of claims 1-12, wherein the third noble metal wire comprises a binary palladium alloy consisting of palladium and nickel, tungsten, platinum, or gold, in addition to impurities.

23. The catalyst system of claim 21, wherein the third noble metal wire comprises a binary palladium alloy consisting of palladium and nickel, tungsten, platinum, or gold, in addition to impurities.

24. The catalyst system of any one of claims 1-12, wherein the third noble metal wire is free of platinum.

25. The catalyst system of claim 21, wherein the third noble metal wire is free of platinum.

26. The catalyst system of any one of claims 1-12, wherein the third noble metal wire is free of rhodium.

27. The catalyst system of claim 25, wherein the third noble metal wire is free of rhodium.

28. Catalyst system according to any one of claims 1 to 12, wherein the catalyst system comprises at least one further catalyst gauze group, which is composed of at least one catalyst gauze, which is composed of at least one further noble metal wire.

29. Catalyst system according to claim 27, wherein the catalyst system comprises at least one further catalyst gauze group, which is composed of at least one catalyst gauze, which is composed of at least one further noble metal wire.

30. Catalyst system according to any one of claims 1 to 12, wherein the catalyst system comprises at least one separating element between two of the catalyst gauze groups.

31. Catalyst system according to claim 29, wherein the catalyst system comprises at least one separating element between two of the catalyst gauze groups.

32. A process for the catalytic oxidation of ammonia, wherein at least a fresh gas containing ammonia is guided through a catalyst system according to any one of claims 1 to 31.

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