Catalyst system for flow reactor and method for catalytic oxidation of ammonia

Abstract

The present invention relates to a catalyst system for a flow reactor, said catalyst system being characterized by a sequence of noble metal-containing alloys used in the catalyst mesh forming the catalyst system. By using a ternary platinum alloy for the second catalyst net group and a palladium alloy for the third catalyst net group, the platinum and rhodium content of the catalyst system can be generally maintained relatively low while having high efficiency. The invention also relates to a method for the catalytic combustion of ammonia, wherein a fresh gas containing at least ammonia is conducted through the catalyst system.

Description

[0001] The invention relates to a catalyst system for a flow reactor, the catalyst system being characterized by the sequence of noble metal-containing alloys used in the catalyst mesh forming the catalyst system. In addition, the invention relates to a method for the catalytic combustion of ammonia, wherein a fresh gas containing at least ammonia is led through the catalyst system.

[0002] Catalyst systems in the sense of the present invention are particularly used for gas reactions. They are, for example, used to prepare hydrocyanic acid by the Andrussow process or to prepare nitric acid by the Ostwald process. In order to provide a large catalytically active surface for these reactions, such catalysts generally include a gas-permeable spatial structure. The collection system for recovering the catalytically active components of evaporation is also often based on such a grid structure. Generally, a plurality of nets are conveniently arranged one by one and combined to form a catalyst system. The catalyst net is generally composed of a single or multi-layer knitted fabric, a braided fabric or a woven fabric. A separate net is composed of a fine noble metal wire mainly containing platinum (Pt), palladium (Pd), rhodium (Rh) or an alloy of these metals. In particular, the collection net may also contain other components, such as nickel.

[0003] Depending on the design, a system comprising 2 to 50 catalyst screens with a diameter of up to 6 m is used in the flow reactor. The use of precious metals represents a high investment capital, so the use of precious metals is kept as little as possible. On the other hand, the "catalytic efficiency", which is an important parameter and measure of a sustained high conversion rate of reactants and a good yield, depends on the precious metal content. Due to the rising prices of precious metals and the resulting capital investment in the catalyst system, the goal 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 has to be replaced from time to time at a certain expense (durability, service life). The PtRh5 alloy, which has been established as the industrial standard for precious metal catalysts in medium-pressure systems, has proven to be a suitable compromise in terms of service life, catalytic efficiency and precious metal usage.

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

[0006] In order 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 the first mesh is formed of a platinum-rhodium alloy and the second mesh is formed of a palladium-rhodium alloy as viewed in the flow direction.

[0007] CN 101554585 A describes a catalyst system comprising at least three mesh layers made of alloys with different compositions. The first mesh layer and the second mesh layer are made of a platinum alloy, and the platinum alloy of the middle mesh layer contains a lower platinum content.

[0008] The object of the present invention is to provide a catalyst system for a flow reactor in which the use of platinum and rhodium can be reduced by using a higher proportion of palladium without having to accept a loss in catalytic efficiency.

[0009] Furthermore, it was an object of the present invention to provide a process for using such a catalyst system.

[0010] This object is achieved by a catalyst system for a flow reactor, which comprises at least three catalyst mesh groups arranged one behind the other in the flow direction, each catalyst mesh group being formed in each case by at least one catalyst mesh consisting of at least one noble metal wire, and

[0011] - the first catalyst mesh group comprises at least one catalyst mesh, the at least one catalyst mesh being composed of at least one first noble metal wire made of a platinum alloy, the platinum alloy of the first noble metal wire being composed of 80 wt % to 98 wt % platinum, 2 wt % to 20 wt %

[0012] rhodium and 0% to 15% by weight of palladium,

[0013] - the second catalyst mesh group comprises at least one catalyst mesh, the at least one catalyst mesh being composed of at least one second noble metal wire made of a platinum alloy, the platinum alloy of the second noble metal wire being composed of 75 wt % to 98 wt % platinum, 1 wt % to 16 wt % excluding impurities

[0014] rhodium and 1 to 20 weight percent palladium, and

[0015] - the third catalyst mesh group comprises at least one catalyst mesh, the at least one catalyst mesh being composed of at least one third noble metal wire made of a palladium alloy, the palladium alloy of the third noble metal wire being composed of 70 wt % to 97 wt % palladium, 0 wt % to 10 wt %

[0016] rhodium and 3 to 30% by weight of at least one additional metal selected from the group consisting of nickel, tungsten, platinum and gold,

[0017] The catalyst system is characterized in that the rhodium content of the noble metal wires of the catalyst gauze group decreases or remains constant in the flow direction and the palladium content of the noble metal wires of the catalyst gauze group increases in the flow direction.

[0018] In the context of the present invention, it has surprisingly been found that the positioning of catalyst screens with different compositions in the rear region of the catalyst system, viewed in the flow direction, has a significant effect on the efficiency of the system. By using a ternary platinum alloy for the second catalyst screen group and a palladium alloy for the third catalyst screen group, the platinum and rhodium content of the catalyst system can be kept relatively low overall while achieving high efficiency. Such a system offers economic advantages, particularly in view of the prospect of lower palladium prices.

[0019] The present invention relates to a catalyst system for a flow reactor. In a flow reactor, a catalyst in the form of a gas-permeable fabric is usually incorporated into a reaction zone in a plane perpendicular to the flow direction of the fresh gas. Such a gas-permeable fabric is usually adopted in the form of a catalyst mesh. The catalyst system should be understood to mean an assembly of such a catalyst mesh.

[0020] The catalyst system according to the present invention includes 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, and the at least one catalyst mesh is composed of at least one precious metal wire. The catalyst mesh group is understood to mean an assembly of at least one catalyst mesh, wherein the composition of the precious metal wire is not different. Typically, the catalyst mesh group includes more than one catalyst mesh. Therefore, the catalyst system according to the present invention includes at least three catalyst meshes composed of precious metal wires with three different compositions.

[0021] In the flow direction, the reaction gas first passes through the first catalyst mesh group, then passes through the second catalyst mesh group, and finally passes through the third catalyst mesh group.

[0022] Catalyst net should be understood to mean a single layer or multiple layers of breathable fabric. The surface formation of the catalyst net can be achieved by interlocking one or more precious metal wires to form a mesh. The catalyst net can be produced, for example, by weaving, braiding or knitting one or more precious metal wires. Therefore, the structure of the catalyst net can be set in a targeted manner by using different weaving, braiding or knitting patterns and / or different mesh sizes.

[0023] One catalyst net or multiple catalyst nets of the first catalyst net group, one catalyst net or multiple catalyst nets of the second catalyst net group, and one catalyst net or multiple catalyst nets of the third catalyst net group can be braided, woven and / or knitted independently of each other. Therefore, the catalyst nets braided, woven and knitted can be combined with each other as needed. For example, one catalyst net or multiple catalyst nets of the first catalyst net group can be braided, and one catalyst net or multiple catalyst nets of the second catalyst net group and the third catalyst net group can be knitted. Similarly, one catalyst net or multiple catalyst nets of the first catalyst net group can be knitted, one catalyst net or multiple catalyst nets of the second catalyst net group can be knitted, and one catalyst net or multiple catalyst nets of the third catalyst net group can be woven.

[0024] The catalyst mesh or meshes of the first catalyst mesh group include a first weave, braid or knit pattern and a first mesh size. The catalyst mesh or meshes of the second catalyst mesh group include a second weave, braid or knit pattern and a second mesh size, and the catalyst mesh or meshes of the third catalyst mesh group include a third weave, braid or knit pattern and a third mesh size.

[0025] It has proven to be advantageous if at least two of the first weaving, knitting or knitting pattern, the second weaving, knitting or knitting pattern and the third weaving, knitting or knitting pattern are identical; particularly advantageously, the first weaving, knitting or knitting pattern, the second weaving, knitting or knitting pattern and the third weaving, knitting or knitting pattern are all identical.

[0026] Furthermore, it may be advantageous if at least two of the first web size, the second web size and the third web size are of the same size; particularly advantageously, the first web size, the second web size and the third web size are all of the same size.

[0027] The mass per unit area of ​​the catalyst mesh is not further restricted. The mass per unit area of ​​the catalyst mesh may be between 100 g / m 2 Up to 950g / m 2 In the range of 150g / m 2 Up to 800g / m 2 The mass per unit area of ​​the catalyst meshes in one of the at least three catalyst mesh groups may be the same or different. It has proven to be advantageous if the mass per unit area of ​​the catalyst meshes of the catalyst mesh groups is the same.

[0028] The mass per unit area of ​​the catalyst meshes of the at least three catalyst mesh groups can remain equal, decrease or increase in the flow direction.

[0029] In a preferred embodiment, at least one catalyst net in the catalyst net may include a three-dimensional structure. In the context of the present application, net is understood to be a flat, two-dimensional object. The three-dimensional structure should be understood to mean that the catalyst net also includes an extension to the third spatial dimension except that it extends in the plane. The catalyst net with a three-dimensional structure includes a larger surface area, which advantageously affects the catalytic effect and can reduce the pressure drop in the flow reactor. The three-dimensional structure can be obtained by using at least one precious metal wire with a two-dimensional or three-dimensional structure or by texturing the catalyst net. The three-dimensional structure of the catalyst net can be, for example, a waveform or coil-shaped. In order to produce this structure, the catalyst net of the original plane can be subjected to a process step in which the three-dimensional structure is imprinted or produced by folding.

[0030] The three-dimensional structure can be obtained by placing a planar catalyst mesh on a rigid, permeable but non-planar surface (e.g., a preformed metal mesh). This textured permeable surface structure does not have to be catalytically active and is then transferred to the catalyst mesh. A catalyst mesh with such a structure is also referred to as corrugated.

[0031] It has proven to be advantageous if at least one catalyst mesh of the catalyst system according to the invention comprises a three-dimensional structure, in particular if the catalyst mesh is a corrugated catalyst mesh. It may be advantageous if at least one catalyst mesh of each catalyst mesh group comprises a three-dimensional structure.

[0032] The number of catalyst screens used depends on the conditions under which the flow reactor is operated. Among other things, the throughput of fresh gas (which depends in particular on pressure) is a key factor. For example, in a flow reactor operating at low pressure (e.g., up to about 5 bar (absolute)), typically less than 15, typically between 5 and 10 catalyst screens may be used, while at higher pressures (e.g., up to 15 bar), a greater number of catalyst screens may be used, typically greater than 20, typically between 30 and 50.

[0033] In each case, the catalyst mesh is formed by at least one noble metal wire. The noble metal is preferably selected from platinum metal, gold and silver. Platinum metal is to be understood as meaning the so-called platinum group metals, i.e. platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os) and ruthenium (Ru). A noble metal wire is to be understood as a wire consisting of a noble metal or a noble metal alloy.

[0034] Preferably, noble metal wires are used having a diameter of 40 μm to 250 μm, preferably 50 μm to 200 μm, particularly preferably 60 μm to 150 μm.

[0035] The noble metal wires can be designed as round wires, ie with a round cross section. In a further embodiment, at least one of the noble metal wires can be designed as a flat round wire or as a wire with a different cross section.

[0036] The precious metal wire may comprise a plurality of wires, in which case also referred to as filaments. The filaments may all consist of the same material, ie all contain the precious metal, or of different materials, which do not necessarily all contain the precious metal.

[0037] The filaments may be twisted together; in these cases, the precious metal wire comprises a rope-like structure.

[0038] The noble metal wire may comprise one or more helically formed longitudinal sections, or may be formed as a helical curve over the entire length. If the noble metal wire comprises a helically formed longitudinal section, both the active catalytic surface of the catalyst web and the mass of the catalyst web relative to the surface unit may be adjusted, for example via the wire thickness or via the number of turns of the helical longitudinal section. When such a noble metal wire is used, the catalyst web produced from the noble metal wire may comprise a three-dimensional structure.

[0039] In many cases, it may be advantageous if the catalyst mesh is formed from two or more noble metal wires. In these cases, the noble metal wires may consist of the same material or different materials. The multiple noble metal wires may have the same or different diameters.

[0040] The first catalyst mesh group of the catalyst system according to the invention comprises at least one catalyst mesh consisting of at least one first precious metal wire. The first precious metal wire comprises a platinum alloy which, apart from impurities, consists of 80 to 98% by weight of platinum, 2 to 20% by weight of rhodium and 0 to 15% by weight of palladium.

[0041] A platinum alloy is understood to mean an alloy consisting of platinum to an extent of more than 50% by weight. The fact that an alloy consists of 80% to 98% by weight of platinum means that the weight proportion of platinum is 80% to 98% by weight of the total alloy weight. The platinum alloy of the first noble metal wire preferably contains 85% to 97% by weight, in particular 90% to 95% by weight, of platinum.

[0042] The noble metal alloys described herein may contain impurities. In the present case, impurities are understood to mean expected or unavoidable impurities caused by the production of one or more alloys. Unless otherwise specified, for all the noble metal alloys described, the proportion of impurities does not exceed 1% by weight, preferably does not exceed 0.5% by weight, based on the total weight of the particular noble metal alloy. The noble metal alloys of the present application particularly include platinum alloys of the first noble metal wire and the second noble metal wire and palladium alloys of the third noble metal wire.

[0043] In a preferred embodiment, the platinum alloy of the first noble metal wire contains 3% to 18% by weight, in particular 5% to 15% by weight, of rhodium.

[0044] The platinum alloy of the first noble metal wire preferably contains not more than 15% by weight, in particular not more than 10% by weight, particularly preferably not more than 5% by weight, of palladium.

[0045] The first precious metal wire may include a ternary platinum alloy consisting of platinum, rhodium and palladium. The ternary platinum alloy may also contain impurities as described above. In this case, the platinum alloy of the first precious metal wire is, for example, PtRh5Pd5 or PtRh5Pd15. PtRh(X)Pd(Y) here means that the alloy contains X weight % rhodium and Y weight % palladium, and consists of (100-(X+Y)) weight % platinum excluding impurities.

[0046] Particularly preferably, the first precious metal wire comprises a binary platinum alloy consisting of platinum and rhodium. The binary platinum alloy may also contain impurities as described above. In this case, the platinum alloy of the first precious metal wire is, for example, PtRh3, PtRh5, PtRh8, PtRh10 or PtRh15. PtRh(X) here means that the alloy contains X weight % rhodium and, apart from impurities, consists of (100-X) weight % platinum.

[0047] In a preferred embodiment, the first catalyst mesh group includes 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.

[0048] Surprisingly, it has been found that it is sufficient to achieve a sufficient catalytic efficiency if the first catalyst mesh group comprises only one catalyst mesh. This represents a particularly simple and therefore preferred embodiment of the catalyst system.

[0049] The second catalyst mesh group of the catalyst system according to the invention comprises at least one catalyst mesh consisting of at least one second noble metal wire consisting of a platinum alloy. The platinum alloy of the second noble metal wire consists of 75% to 98% by weight of platinum, 1% to 16% by weight of rhodium and 1% to 20% by weight of palladium, excluding impurities.

[0050] The platinum alloy of the second noble metal wire preferably contains 80% by weight to 95% by weight, in particular 85% by weight to 90% by weight, of platinum.

[0051] In a preferred embodiment, the platinum alloy of the second noble metal wire contains 2% to 15% by weight, in particular 5% to 12% by weight, of rhodium.

[0052] The platinum alloy of the second noble metal wire preferably contains more than 1% by weight, in particular more than 5% by weight, particularly preferably more than 10% by weight, of palladium.

[0053] The platinum alloy of the second noble metal wire is a ternary platinum alloy and may contain impurities in addition to platinum, rhodium and palladium. The proportion of impurities does not exceed 1% by weight of the platinum alloy, preferably does not exceed 0.5% by weight.

[0054] The platinum alloy of the second precious metal wire may be, for example, PtRh5Pd5, PtRh1.5Pd13, PtRh2.5Pd17, PtRh4Pd14 or PtRh3.5Pd16. PtRh(X)Pd(Y) here means that the alloy contains X weight % rhodium and Y weight % palladium, and consists of (100-(X+Y)) weight % platinum excluding impurities.

[0055] In a preferred embodiment, the second catalyst mesh group includes 1 to 10 catalyst meshes, preferably 3 to 8 catalyst meshes. In particular, 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 includes more catalyst meshes than the first catalyst mesh group.

[0056] The third catalyst mesh group 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 a palladium alloy.

[0057] The palladium alloy of the third precious metal wire consists, excluding impurities, of 70 to 97 wt % palladium, 0 to 10 wt % rhodium, and 3 to 30 wt % of at least one additional metal selected from the group consisting of nickel, tungsten, platinum and gold.

[0058] Palladium alloy is understood to mean an alloy consisting of palladium to an extent of more than 50% by weight. The palladium alloy of the third noble metal wire preferably has a palladium content in the range from 75% to 96% by weight, preferably from 80% to 95% by weight.

[0059] The palladium alloy of the third noble metal wire preferably contains at least one further metal in the range from 4% by weight to 25% by weight, particularly preferably in the range from 5% by weight to 20% by weight.

[0060] The palladium alloy of the third noble metal wire may contain any combination selected from nickel, tungsten, platinum and gold.

[0061] The palladium alloy of the third noble metal wire may contain impurities in addition to palladium, nickel, tungsten, platinum or gold and optionally rhodium. The third noble metal wire may comprise a ternary palladium alloy, which in addition to impurities consists of palladium, platinum and rhodium, or a binary palladium alloy, which in addition to impurities contains palladium and nickel, tungsten, platinum or gold. The proportion of impurities does not exceed 1% by weight, preferably does not exceed 0.5% by weight, of the palladium alloy of the third noble metal wire.

[0062] In a preferred embodiment, the palladium alloy of the third noble metal wire has a rhodium content in the range of 1% to 10% by weight, in particular in the range of 3% to 8% by weight. With regard to high catalytic efficiency and at the same time low or no negative impact on the service life, it may be advantageous for the palladium alloy of the third noble metal wire to contain at least 1% by weight of rhodium. In this case, the third noble metal wire particularly preferably has a platinum content of at least 1.5% by weight.

[0063] The palladium alloy of the third noble metal wire may be, for example, PdPt10Rh5, PdPt5Rh5, PdPt15Rh3 or PdPt20Rh1. PdPt(Z)Rh(X) herein means that the alloy contains Z wt% platinum and X wt% rhodium, and is composed of (100-(Z+X)) wt% palladium excluding impurities.

[0064] According to the invention, the rhodium content of the noble metal wires of the catalyst mesh group is reduced or kept the same. It has surprisingly been found that a catalyst system having such a rhodium gradient is more effective than a system in which one or more catalyst meshes comprising noble metal wires made of an alloy having a higher rhodium content are arranged in the rear or last region of the catalyst system.

[0065] In a preferred embodiment, the rhodium content of the third noble metal wire is at least 2 weight percentage points lower, particularly preferably at least 3 weight percentage points lower, than the rhodium content of the second noble metal wire.

[0066] Preferably, the third noble metal wire contains no rhodium. In these cases, the palladium alloy of the third noble metal wire is preferably a binary palladium alloy, ie the binary palladium alloy consists of only one further component in addition to palladium and impurities. The further component is preferably platinum or nickel.

[0067] According to the invention, the palladium content of the noble metal wires of the catalyst mesh pack increases in the flow direction. Surprisingly, it has been found that catalyst systems with an increased palladium gradient in the configuration according to the invention have a high catalytic efficiency despite a concomitant reduction in platinum and rhodium.

[0068] It may be advantageous if the third noble metal wire is free of platinum. In such an embodiment, the platinum content in the catalyst system can be further reduced. Particularly preferably, in this case, the third noble metal wire comprises a binary palladium alloy consisting of palladium, impurities and nickel, tungsten or gold.

[0069] Examples of preferred binary palladium alloys for the third noble metal wire include PdNi5, PdW5, PdPt5, PdAu5, PdNi3, PdW3, and PdAu3. PdPt(Z) ​​means that the alloy contains Z wt% platinum and consists of (100-Z) wt% palladium excluding impurities.

[0070] In a preferred embodiment, the third catalyst mesh group includes 1 to 10 catalyst meshes, preferably 2 to 8 catalyst meshes. In particular, 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 includes more catalyst meshes than the first catalyst mesh group or the second catalyst mesh group.

[0071] Typically, the catalyst meshes of the second and third catalytic groups account for a maximum proportion of, for example, at least 70% of the total volume of the catalyst system, and it is sufficient that the catalyst meshes of the first catalyst mesh group consisting of rhodium-rich noble metal wires account for only a small volume proportion of, 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 of the catalyst mesh group is determined primarily by the number of catalyst meshes used.

[0072] In a preferred embodiment of the catalyst system according to the invention,

[0073] - the first precious metal wire is made of a binary platinum alloy consisting of platinum and rhodium,

[0074] - the second noble metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium, and

[0075] - The third noble metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold.

[0076] In particular, the first precious metal wire may consist of a PtRh (2-10) alloy, the second precious metal wire may consist of a PtRh (1-16) Pd (1-20) alloy, and the third precious metal wire may consist 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 proportion in the range of a weight % to b weight %, and the remaining proportion (100 - (a to b)) weight % consists of platinum excluding impurities.

[0077] It is also preferred that

[0078] - the first precious metal wire is made of a binary platinum alloy consisting of platinum and rhodium,

[0079] - the second noble metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium, and

[0080] - The third precious metal wire is made of a ternary palladium alloy consisting of palladium, platinum and rhodium.

[0081] In particular, the first noble metal wire may be composed of a PtRh (2-10) alloy, the second noble metal wire may be composed of a PtRh (1-16) Pd (1-20) alloy, and the third noble metal wire may be composed of a PdPt (3-30) Rh (1-5) alloy.

[0082] In a further preferred embodiment of the catalyst system according to the invention,

[0083] - the first precious metal wire is made of a ternary platinum alloy consisting of platinum, palladium and rhodium,

[0084] - the second noble metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium, and

[0085] - The third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum and gold.

[0086] In particular, the first precious metal wire may be composed of a PtPd(1-15)Rh(2-20) alloy, the second precious metal wire may be composed of a PtRh(1-16)Pd(1-20) 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.

[0087] In a further preferred embodiment of the catalyst system according to the invention,

[0088] - the first precious metal wire is made of a ternary platinum alloy consisting of platinum, palladium and rhodium,

[0089] - the second noble metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium, and

[0090] - The third precious metal wire is made of a ternary palladium alloy consisting of palladium, platinum and rhodium.

[0091] In particular, the first precious metal wire may be composed of a PtPd(1-15)Rh(2-20) alloy, the second precious metal wire may be composed of a PtRh(1-16)Pd(1-20) alloy, and the third precious metal wire may be composed of a PdPt(3-30)Rh(1-5) alloy.

[0092] The catalyst system may also include additional components.

[0093] The catalyst system may include, for example, an ignition layer upstream of the first catalyst mesh group. The ignition layer includes precious metal wires containing only platinum and impurities.

[0094] It may also be advantageous if, viewed in the flow direction, at least the foremost catalyst web contains noble metal wires made of platinum which contain no further components besides impurities. The foremost catalyst web may be a catalyst web of the first catalyst web group.

[0095] The catalyst system according to the invention may further comprise at least one further catalyst gauze group, the at least one further catalyst gauze group consisting of at least one catalyst gauze consisting of at least one further noble metal wire.

[0096] At least one further noble metal wire may comprise the same composition as the first, second or third noble metal wire, but the composition may also be different. In this case, it is also advantageous if the rhodium content of the noble metal wire decreases or remains constant in the flow direction. In this case, the palladium content of the noble metal wire may remain constant or increase in the flow direction.

[0097] The catalyst system according to the invention may contain, for example, at least four catalyst mesh groups, each of which comprises at least one catalyst mesh consisting of at least one noble metal wire. At least one further catalyst mesh group may be arranged upstream of the first catalyst mesh group or downstream of the third catalyst mesh group. The rhodium content of the noble metal wire decreases or remains unchanged in the flow direction.

[0098] It has proven to be advantageous if at least one further catalyst gauze group is arranged downstream of the third catalyst gauze group. In this case, it is particularly advantageous if at least one further noble metal wire contains a palladium alloy.

[0099] The palladium alloy of the at least one further noble metal wire particularly preferably consists, 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.

[0100] The palladium alloy of the at least one further noble metal wire preferably has a palladium content in the range from 75% by weight to 95% by weight, particularly preferably in the range from 78% by weight to 90% by weight.

[0101] The palladium alloy of the at least one further noble metal wire preferably contains the at least one further metal in the range from 5% by weight to 25% by weight, particularly preferably in the range from 10% by weight to 22% by weight.

[0102] The palladium alloy of the at least one additional noble metal wire may contain any combination selected from nickel, tungsten, platinum and gold.

[0103] Besides palladium, nickel, tungsten, platinum or gold and optionally rhodium, the palladium alloy of the at least one further noble metal wire may also contain impurities. The impurity proportions amount to not more than 1% by weight, preferably not more than 0.5% by weight, of the palladium alloy of the at least one further noble metal wire.

[0104] In a preferred embodiment, the palladium alloy of the at least one further noble metal wire is a binary alloy which, in addition to palladium and impurities, also contains nickel, tungsten, platinum or gold.

[0105] In a preferred embodiment of the catalyst system according to the invention,

[0106] - the first precious metal wire is made of a binary platinum alloy consisting of platinum and rhodium,

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

[0108] - the third noble metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold,

[0109] and

[0110] - at least one further noble metal wire comprises a binary palladium alloy consisting of palladium and an impurity, nickel, tungsten, platinum or gold.

[0111] In particular, the first precious metal wire may be composed of a PtRh (2-10) alloy, the second precious metal wire may be composed of a PtRh (1-16) Pd (1-20) 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 PdNi (3-28) alloy, a PdW (3-28) alloy, a PdPt (3-28) alloy or a PdAu (3-28) alloy.

[0112] It is also preferred that

[0113] - the first precious metal wire is made of a binary platinum alloy consisting of platinum and rhodium,

[0114] - the second precious metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium,

[0115] - the third noble metal wire is made of a ternary palladium alloy consisting of palladium, platinum and rhodium, and

[0116] - at least one further noble metal wire comprises a binary palladium alloy consisting of palladium and an impurity, nickel, tungsten, platinum or gold.

[0117] In particular, the first precious metal wire may be composed of a PtRh (2-10) alloy, the second precious metal wire may be composed of a PtRh (1-16) Pd (1-20) alloy, the third precious metal wire may be composed of a PdPt (3-30) Rh (1-5) alloy, and at least one additional precious metal wire may be composed of a PdNi (3-28) alloy, a PdW (3-28) alloy, a PdPt (3-28) alloy or a PdAu (3-28) alloy.

[0118] In a further preferred embodiment of the catalyst system according to the invention,

[0119] - the first precious metal wire is made of a ternary platinum alloy consisting of platinum, palladium and rhodium,

[0120] - the second precious metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium,

[0121] - the third precious metal wire is made of a binary palladium alloy consisting of palladium and nickel, tungsten, platinum and gold,

[0122] and

[0123] - at least one further noble metal wire comprises a binary palladium alloy consisting of palladium and an impurity, nickel, tungsten, platinum or gold.

[0124] In particular, the first precious metal wire may be composed of a PtPd(1-15)Rh(2-20) alloy, the second precious metal wire may be composed of a PtRh(1-16)Pd(1-20) 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 PdNi(3-28) alloy, a PdW(3-28) alloy, a PdPt(3-28) alloy or a PdAu(3-28) alloy.

[0125] In a further preferred embodiment of the catalyst system according to the invention,

[0126] - the first precious metal wire is made of a ternary platinum alloy consisting of platinum, palladium and rhodium,

[0127] - the second precious metal wire is made of a ternary platinum alloy consisting of platinum, rhodium and palladium,

[0128] - the third noble metal wire is made of a ternary palladium alloy consisting of palladium, platinum and rhodium, and

[0129] - at least one further noble metal wire comprises a binary palladium alloy consisting of palladium and an impurity, nickel, tungsten, platinum or gold.

[0130] In particular, the first precious metal wire may be composed of a PtPd(1-15)Rh(2-20) alloy, the second precious metal wire may be composed of a PtRh(1-16)Pd(1-20) alloy, the third precious metal wire may be composed of a PdPt(3-30)Rh(1-5) alloy, and at least one additional precious metal wire may be composed of a PdNi(3-28) alloy, a PdW(3-28) alloy, a PdPt(3-28) alloy or a PdAu(3-28) alloy.

[0131] In a preferred embodiment, the catalyst system according to the present invention may include at least one partitioning element between two catalyst mesh groups in the catalyst mesh group, for example in the form of at least one intermediate mesh. This intermediate mesh can be used to offset the compression of adjacent catalyst mesh groups under pressure load. One or more intermediate meshes preferably have limited flexibility compared to the catalyst mesh of the catalyst mesh group.

[0132] Suitable separator elements are, for example, elements or meshes made of heat-resistant steel, typically a FeCrAl alloy such as Megapyr or Kanthal, stainless steel or a heat-resistant alloy such as a nickel-chromium alloy. One or more separator elements may also comprise a catalytically active coating comprising at least one noble metal.

[0133] It has proven to be advantageous if a separation element, in particular an intermediate mesh, is arranged between the first catalyst mesh group and the second catalyst mesh group. An intermediate mesh made of Megapyr or Kanthal has proven to be particularly advantageous.

[0134] Separating elements in the form of intermediate meshes can also be arranged within the catalyst mesh pack.

[0135] The catalyst system according to the invention is suitable for the preparation of nitric acid by the Ostwald process. An ammonia-oxygen mixture flows through the catalyst system; in other words, this involves catalytic combustion of ammonia.

[0136] The catalyst system according to the invention is also suitable for preparing hydrocyanic acid by the Andrussow process. An ammonia-methane-oxygen mixture flows through the catalyst system.

[0137] The invention also relates to a process for the catalytic oxidation of ammonia, wherein a fresh gas containing at least ammonia is conducted through the catalyst system according to the invention. With regard to preferred embodiments of the catalyst system, reference is made to the preceding description.

[0138] The ammonia content of the fresh gas is preferably between 9.5% and 12% by volume.

[0139] The pressure of the fresh gas is preferably between 1 bar and 14 bar, in particular between 3 bar and 10 bar. The catalyst mesh temperature is preferably in the range of 500°C to 1300°C, preferably in the range of 800°C to 1100°C.

[0140] Preferably, the fresh gas is heated at 6 tN / m 2 d to 60tN / m 2 The throughput in the range d is conducted through the catalyst system according to the invention. The abbreviation "tN / m 2 "d" stands for "tons of nitrogen (from ammonia) per day and per square meter of normalized effective cross-sectional area of ​​the catalyst system".

[0141] The invention is explained below with reference to the drawings and exemplary embodiments and experiments on catalytic activity.

[0142] Figure 1 Schematic diagram of a vertically positioned flow reactor 1 for heterogeneous catalytic combustion of ammonia. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. The catalyst system 2 comprises a plurality of catalyst mesh groups (4, 5, 6) which are arranged one behind the other in the flow direction 3 of the fresh gas.

[0143] The fresh gas is an ammonia-air mixture containing a nominal ammonia content of 10.7% by volume. The fresh gas is heated to a preheating temperature of 175° C. and introduced into the reactor 1 from above at a high pressure of 5 bar. Upon entering the catalyst system 2, the gas mixture is ignited and, subsequently, an exothermic combustion reaction occurs. The following main reactions occur:

[0144] 4NH3+5O2→4NO+6H2O

[0145] In this case, ammonia (NH3) is converted into nitrogen monoxide (NO) and water (H2O). The nitrogen monoxide (NO) formed reacts with excess oxygen in the outflowing reaction gas mixture (symbolized by the directional arrow 7 indicating the flow direction of the outflowing reaction gas mixture) to form nitrogen dioxide (NO2), which reacts with water in the downstream absorption system to form nitric acid (HNO3).

[0146] Catalyst meshes are fabrics produced by machine knitting of noble metal threads, which typically have a diameter of 76 μm and consist of the relevant noble metal alloys. In Tables 1 and 2, exemplary embodiments (E1 to E11) of catalyst systems which can be used in the flow reactor 1 are specified.

[0147] Table 1 :

[0148] E1 E2 E3 E4 E5 E6 G1 PcqI PcqI PcqI PcqI PcqI PcqI G2 PtRh5Pd5 PtRh3.5Pd16 PtRh2.5Pd17 PtRh5Pd15 PtRh4Pd10 PtRh3Pd12 G3 PdNi5 PdW PdPt5 PdPt5Rh5 PdPt15 PdAu5

[0149] Table 2 :

[0150] E7 E8 E9 E10 E11 G1 PcqI PtRh5Pd5 PtRh5Pd5 PtPd15Rh5 P8 G2 PtRh3.5Pd16 PtRh4Pd10 PtRh3Pd12 PtRh2.5Pd17 PtRh5Pd5 G3 PdPt15Rh3 PdPt20Rh1 PdPt10Rh1 PdPt20 PdPt10Rh5

[0151] The catalyst systems each include 3 catalyst mesh groups, with a total of 30 catalyst meshes; the order of nomenclature G1 to G3 reflects the arrangement of the fresh gas in the flow direction.

[0152] In accordance with Figure 1In a test reactor of , the catalyst systems E1 to E11 were compared with catalyst systems in which only platinum alloys were used in each case. For this purpose, the choice of alloys was limited to those alloys of the catalyst gauze groups G2 and G3 or G1 and G3. In order to ensure a comparable catalytically active noble metal content in the reactor equipped according to the invention and in the comparative reactor, the number of catalyst gauze was adjusted accordingly.

[0153] In each case, the test reactor was operated under the following identical test conditions.

[0154] Pressure: 5 bar (absolute)

[0155] Throughput: 12 tons of nitrogen (from ammonia) per day and per square meter of effective cross-sectional area of ​​the catalyst packing (abbreviated as 12 tN / m 2 d)

[0156] NH3 ratio: 10.7% by volume in fresh gas

[0157] Preheating temperature: 175°C (NH3 temperature / air mixture), resulting in a web temperature of 890°C.

[0158] At intervals of approximately 12 hours, over a period of 4 days, the development of the catalytic efficiency of the catalyst (NO production in %) and of the amount of nitrous oxide N2O occurring as an undesirable by-product was measured.

[0159] The catalytic efficiency (i.e., product yield of NO) is measured in the following order:

[0160] 1. Ensure that the catalyst system is suitable for complete conversion of the ammonia used. This means that no significant amounts of NH 3 are present in the product gas anymore, which is checked by means of mass spectrometry of the product gas.

[0161] 2. The NH3 / air sample upstream of the catalyst packing and the sample from the downstream product gas are removed simultaneously in respective independent evacuation pistons. The mass of the gas is determined by weighing.

[0162] 3. The NH3 / air mixture is taken up in distilled water and titrated according to the color change with the aid of 0.1 N sulfuric acid and methyl red.

[0163] 4. Absorb the nitrous acid product gas in 3% sodium peroxide solution and use the color change to

[0164] Titrate with 0.1N sodium hydroxide solution and methyl red.

[0165] 5. Catalytic efficiency Eta is generated by the following formula: Eta = 100*C n / C a , where C ais the average NH3 concentration in the fresh gas from seven separate measurements, expressed as weight percent, and C n is the average NO of 7 individual measurements x concentration, which is expressed as the amount of NO that has been oxidized to form x The weight percentage of NH3.

[0166] 6. Separately, the volume fraction of N2O in the product gas was determined by means of gas chromatography.

[0167] Compared to a system consisting only of the alloy of the catalyst mesh groups G1 and G3, in a reactor equipped according to the invention, despite a reduction in platinum weight of up to 17% and rhodium weight of up to 27%, comparable efficiencies were observed over the entire test period at comparable N2O ratios. In this technical field, a saving of up to 17% platinum and up to 27% rhodium represents a significant and economically important reduction of these precious metals.

[0168] Compared to the catalyst system according to the invention, a reactor equipped with a system consisting only of the alloy of catalyst mesh groups G2 and G3 achieves an ammonia oxidation efficiency that is about 1% lower and is therefore classified as uneconomical, despite considerable savings in platinum and rhodium.

Claims

1. A catalyst system for a flow reactor, the catalyst system comprising 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 consisting of at least one noble metal wire, and - a first catalyst mesh group comprises at least one catalyst mesh, said at least one catalyst mesh being composed of at least one first noble metal wire made of a platinum alloy, said platinum alloy of said first noble metal wire consisting of 80% to 98% by weight of platinum, 2% to 20% by weight of rhodium and 0% to 15% by weight of palladium, excluding impurities, - the second catalyst mesh group comprises at least one catalyst mesh, said at least one catalyst mesh consisting of at least one second noble metal wire made of a platinum alloy, said platinum alloy of said second noble metal wire consisting of 75% to 98% by weight of platinum, 1% to 16% by weight of rhodium and 1% to 20% by weight of palladium, excluding impurities, and - a third catalyst mesh group comprises at least one catalyst mesh consisting of at least one third noble metal wire made of a palladium alloy, the palladium alloy of the third noble metal wire 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, It is characterized in that - the rhodium content of the noble metal wires of the catalyst mesh group decreases or remains constant in the flow direction, and The palladium content of the noble metal wires of the catalyst mesh pack increases in the flow direction.

2. The catalyst system according to claim 1, wherein the catalyst meshes are independently woven, knitted or braided from each other.

3. The catalyst system according to claim 1 or 2, wherein at least one of the catalyst meshes comprises a three-dimensional structure. The catalyst system of claim 3 , wherein at least one of the catalyst meshes is corrugated. 5 . The catalyst system according to claim 1 , wherein the noble metal wire has a diameter of 40 μm to 250 μm. 6 . The catalyst system according to claim 1 , wherein the proportion of the impurities in the alloy of the noble metal wire does not exceed in each case 1% by weight.

7. The catalyst system according to any one of the preceding claims, wherein the platinum alloy of the first precious metal wire contains not more than 10 wt. % palladium.

8. The catalyst system according to any one of the preceding claims, wherein the palladium content of the third noble metal wire is at least 2 weight percentage points higher than the palladium content of the second noble metal wire.

9. The catalyst system according to any one of the preceding claims, wherein the rhodium content of the third precious metal wire is at least 2 weight percentage points lower than the rhodium content of the second precious metal wire.

10. The catalyst system according to any one of the preceding claims, wherein the third precious metal wire comprises a ternary palladium alloy consisting of palladium, platinum and rhodium, or comprises a binary palladium alloy consisting of palladium and nickel, tungsten, platinum or gold, except for impurities.

11. The catalyst system according to any one of claims 1 to 9, wherein the third precious metal wire is free of rhodium.

12. The catalyst system according to any one of the preceding claims, wherein the catalyst system comprises at least one additional catalyst mesh group, the at least one additional catalyst mesh group consisting of at least one catalyst mesh, the at least one catalyst mesh consisting of at least one additional precious metal wire.

13. The catalyst system according to any one of the preceding claims, wherein the catalyst system comprises at least one separation element between two of the catalyst mesh packs.

14. A method for the catalytic oxidation of ammonia, wherein a fresh gas containing at least ammonia is passed through a catalyst system according to any of the preceding claims.

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