Catalyst system having a catalyst network comprising a novel metal wire for

Abstract

The present invention relates to a catalyst system comprising more than one catalyst net, wherein at least one of the catalyst nets contains at least one noble metal wire consisting of a binary PtRh alloy. The binary PtRh alloy consists of 3.4 wt% to 4.6 wt% of rhodium, impurities and the balance of platinum. The catalyst system does not include a catalyst mesh comprising a noble metal wire made of a further binary PtRh alloy comprising more than 7% by weight rhodium. The invention also relates to a method for the catalytic oxidation of ammonia, wherein the catalyst system according to the invention is used.

Description

[0001] The invention relates to a catalyst system comprising more than one catalyst mesh, wherein at least one of the catalyst meshes contains at least one noble metal wire consisting of a binary PtRh alloy. The binary PtRh alloy consists of 3.4 to 4.6% by weight of rhodium, impurities and the remainder of platinum. The catalyst system does not include a catalyst mesh containing a noble metal wire, which is made of another binary PtRh alloy containing more than 7% by weight of rhodium. The invention also relates to a method for the catalytic oxidation of ammonia, wherein the catalyst system according to the invention is used.

[0002] Catalyst systems with catalyst nets are particularly used in flow reactors 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. Suitable catalysts must provide a large catalytically active surface. Therefore, generally speaking, a catalyst net in the form of a three-dimensional, air-permeable structure of a noble metal wire is used. The collection system for recovering the catalytically active components of evaporation is also often based on such a grid structure. Typically, a plurality of nets are advantageously arranged one by one and combined to form a catalyst system. The catalyst system consists of 2 to 50 catalyst nets stacked up and down, wherein the number depends substantially on the conditions in the reactor, such as the operating pressure and mass flow rate of the gas. The diameter of the net reaches up to 6m in some burners.

[0003] Catalyst net is usually composed of a single layer or multilayer net in the form of a knitted fabric and / or a woven fabric. A separate net is made of a fine precious metal wire mainly containing platinum (Pt), palladium (Pd), rhodium (Rh) or an alloy of these metals. The choice of the material of the precious metal wire is determined in particular by the position and function of the catalyst net in the catalyst system. Specifically, in addition to the precious metal, the capture net may also contain other components, such as nickel. The use of precious metals is expensive, and precious metals are kept as little as possible. On the other hand, the "catalytic efficiency", which is an important parameter and measure of the conversion rate of the reactant and therefore the yield, increases as the precious metal content rises to a maximum value. The use of binary PtRh alloys in the catalytic oxidation of ammonia with a rhodium content of 2 wt % (weight percentage) to 50 wt % is known, such as known in US 1706055A.

[0004] For the planned activities, the operators of the ammonia oxidation system define the amount of ammonia they provide as a function of the minimum amount of nitrogen monoxide that will be produced. It has been established in the industry that the design of mesh catalysts for ammonia oxidation is based on a reference variable of catalytic efficiency, which is understood as the amount of ammonia used per day and per square meter of reactor cross-sectional area. Usually, only the initial efficiency of the catalyst is considered when designing a catalyst system. Due to oxidation and sublimation, the catalyst mesh loses precious metals and therefore loses efficiency during use, so that they must be replaced after a certain service life.

[0005] The PtRh5 alloy, which has been established as the industry standard for use in medium-pressure systems, has proven to be a suitable compromise in terms of service life, catalytic efficiency and precious metal usage, as described, for example, in EP 1284927 A1. Typical alloys in industry also include PtRh8 and PtRh10; in specific cases, especially at higher operating pressures, PtRh3 is also used.

[0006] In order to optimize the catalyst efficiency, different binary PtRh alloys are often combined with each other. For example, EP 3680015A1 discloses a catalyst system, which is composed of at least two mesh layers, in which different binary PtRh alloys are used, and the rhodium content of the binary PtRh alloy decreases in the flow direction. Due to the relatively high rhodium content of more than 7% by weight in at least one of the mesh layers, the rhodium content of the entire system is also high as a whole.

[0007] However, in particular in the case of relatively long reactors, it has surprisingly been found that the use of binary platinum alloys with rhodium contents between the industry standard 3 wt % and 5 wt % leads to an unexpected increase in the total conversion achievable by the catalyst system. The use of such alloys has also proven to be advantageous for longer total operating times and the average efficiency achievable during these operating times. In this technical field, a difference of 1 wt % in rhodium content also makes a significant difference in the cost of the mesh and catalyst system.

[0008] Therefore, the object of the present invention is to provide a catalyst system optimized for long active lengths in ammoxidation. In particular, it is part of the object to provide a catalyst system that allows a maximum overall product yield. Another object is to provide a catalyst system comprising a rhodium content optimized for this purpose.

[0009] This object is achieved by a catalyst system comprising more than one catalyst mesh, wherein at least one catalyst mesh contains at least one precious metal wire consisting of a binary PtRh alloy, characterized in that the binary PtRh alloy of the at least one precious metal wire consists of 3.4 wt. % to 4.6 wt. % rhodium, impurities and the remainder platinum, and the catalyst system does not include a catalyst mesh containing a precious metal wire, which is made of a further binary PtRh alloy containing more than 7 wt. % rhodium.

[0010] The present invention also provides a method for using such a catalyst mesh.

[0011] Surprisingly, it has been found that the average efficiency of the catalyst system according to the invention is unexpectedly higher than the average efficiency of a system containing the industry standard alloy PtRh3 and is comparable to a system with the more rhodium-rich PtRh5 of the same industry standard. In this case, the average efficiency is understood to mean the average efficiency of the catalyst system over the total possible operating time before replacement is necessary. Replacement becomes necessary when the catalyst system becomes unstable due to corrosion phenomena or the catalyst efficiency drops below a certain value, which is typically 94% in the pressure range of 4 to 6 bar.

[0012] 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 catalyst meshes.

[0013] The catalyst system includes more than one catalyst mesh. The catalyst mesh should be understood to mean a single layer or multiple layers of air permeable fabric. The surface formation of the catalyst mesh is preferably achieved by interweaving one or more noble metal wires to form a mesh. The catalyst mesh can be produced, for example, by weaving or knitting one or more noble metal wires. The structure of the catalyst mesh can be set in a targeted manner by using different weaving or knitting patterns and / or different mesh sizes. The catalyst meshes of the catalyst system can be woven or knitted independently of each other.

[0014] In a preferred embodiment, the catalyst net can include a three-dimensional structure. In the context of the present application, the net is understood to be a flat, two-dimensional object. The three-dimensional structure is understood to mean that the catalyst net also includes an extension to the third spatial dimension except that it is planar and two-dimensionally extended. The catalyst net with a three-dimensional structure includes a larger free surface area and better material transport conditions between gas and surface, which advantageously affects the catalytic effectiveness 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 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 stamped or produced by folding.

[0015] The mass per unit area of ​​the catalyst mesh is not further limited and can be, for example, between 100 g / m 2 Up to 950g / m 2 In the range of 150g / m 2 Up to 900g / m 2 The mass per unit area of ​​the catalyst mesh is influenced, inter alia, by the noble metal thread or threads used and by the knitting or weaving pattern used, in particular by the relevant mesh dimensions.

[0016] The catalyst meshes each contain at least one noble metal wire. A noble metal wire is understood to be a wire consisting of a noble metal or a noble metal alloy. A noble metal alloy is understood to mean an alloy consisting of a noble metal to an extent of more than 50% by weight. The fact that an alloy consists of more than 50% by weight of a noble metal means that the weight proportion of the noble metal is at least 50% by weight of the total alloy weight.

[0017] Preferably, noble metal wires are used which have a diameter of 40 μm to 150 μm, preferably 50 μm to 130 μm, in particular 60 μm to 120 μm.

[0018] The one or more noble metal wires can be designed as round wires, ie with a round cross section. In another embodiment, the one or more noble metal wires can be designed as flat round wires or as wires with different cross sections.

[0019] One or more precious metal wires may have a two-dimensional structure or a three-dimensional structure. One or more precious metal wires may include, for example, one or more wavy, stepped or spiral longitudinal sections over their entire length, or may be formed as wavy, stepped or spirally curved wires. If one or more precious metal wires include a spiral longitudinal section, the active catalyst surface of the catalyst mesh and the mass of the catalyst mesh relative to the surface unit can be adjusted via the number of windings of the spiral longitudinal section. When a precious metal wire having a two-dimensional structure or a three-dimensional structure is used, the catalyst mesh produced therefrom has a three-dimensional structure.

[0020] The one or more precious metal wires may comprise a plurality of wires, in which case also referred to as filaments. In such cases, the one or more precious metal wires may have an increased strength, which improves the long-term stability of the catalyst mesh. The filaments may be twisted together; in these cases, the one or more precious metal wires comprise a rope-like structure. It may also be advantageous if the one or more precious metal wires comprise at least one filament, which at least one filament is helically wound around at least one further filament.

[0021] At least one of the catalyst webs contains at least one noble metal wire consisting of a binary PtRh alloy.A binary alloy (also called two-component alloy) is understood to mean an alloy which contains only two alloying elements apart from impurities.

[0022] The desired optimization of the average efficiency for the longest possible maximum operating time requires a relatively narrow range of the composition of the binary PtRh alloy. The binary PtRh alloy consists of 3.4 to 4.6 wt. % rhodium, impurities and the remainder platinum. The binary PtRh alloy preferably consists of 3.6 to 4.4 wt. %, particularly preferably 3.8 to 4.2 wt. % rhodium. For example, the binary PtRh alloy can consist of 3.6 to 4.0 wt. % rhodium.

[0023] At this point it should be noted that the statements about the composition of one or more precious metal wires relate to the state before use in the reactor system. In other words, "fresh" precious metal wires are therefore referred to. During use, the composition of the precious metal wires changes as described above, for example due to evaporation of platinum fractions.

[0024] Impurities of the binary PtRh alloy are understood to mean conventional impurities which are intended to enter the binary PtRh alloy or which inevitably enter the starting material during the preparation process or which cannot be (completely) removed from the starting material with reasonable efforts. The impurity proportion in total preferably does not exceed 1% by weight, preferably does not exceed 0.5% by weight, of the binary PtRh alloy.

[0025] At least one catalyst mesh may consist entirely of at least one noble metal wire of a binary PtRh alloy, but it may also comprise additional components, such as one or more additional noble metal wires made of non-noble metals.

[0026] In many cases it may be advantageous if the catalyst mesh comprises at least one further noble metal wire, ie is formed from two or more noble metal wires. The at least two noble metal wires may have the same or different diameters and / or the same or different structures.

[0027] The noble metal of the further noble metal wire is preferably selected from the group consisting of platinum metal, gold and silver and combinations thereof. Platinum metal is understood to mean the so-called platinum group metals, namely platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os) and ruthenium (Ru).

[0028] The catalyst system comprises more than one catalyst mesh; in other words, the catalyst system comprises at least two catalyst meshes. The at least two catalyst meshes may be identical or different. Depending on the intended use of the catalyst system and the reaction conditions in the flow reactor, catalyst meshes having different or identical structures and different or identical noble metal wires may be combined with each other.

[0029] The catalyst system does not include a catalyst mesh containing noble metal wires made of a further binary PtRh alloy containing more than 7% by weight of rhodium. The binary PtRh alloy used consists of not more than 7% by weight of rhodium, impurities and the remainder of platinum. The proportion of impurities of the further binary PtRh alloy preferably does not exceed 1% by weight, in particular not more than 0.5% by weight.

[0030] The absence of a binary PtRh alloy containing a relatively high rhodium content allows a particularly cost-effective implementation of the catalyst system and has also been shown to be advantageous in terms of average efficiency. However, the catalyst system may comprise a further catalyst mesh with precious metal wires made of a binary PtRh alloy containing less than 7 wt. % rhodium, for example made of PtRh6.

[0031] Particularly advantageously, the catalyst system does not comprise a catalyst mesh containing noble metal wires made of a further binary PtRh alloy containing more than 6.5 wt.-% rhodium, in particular containing not more than 6 wt.-% rhodium. It may be particularly preferred that the catalyst system does not comprise any further catalyst mesh containing noble metal wires made of another binary PtRh alloy.

[0032] The catalyst system may comprise one or more catalyst mesh groups. A catalyst mesh group is understood to mean a collection of catalyst meshes formed by at least one noble metal wire having the same composition. Typically, a catalyst mesh group comprises more than one catalyst mesh.

[0033] The mass per unit area of ​​the catalyst meshes in the catalyst mesh group can be the same or different. It has been proven to be advantageous that the mass per unit area of ​​the catalyst meshes of the catalyst mesh group is the same. The mass per unit area of ​​the catalyst meshes of the catalyst mesh group can remain equal, decrease or increase in the flow direction.

[0034] For example, the catalyst system may include at least three catalyst meshes comprising precious metal wires made from a binary PtRh alloy comprising 3.4 wt % to 4.6 wt % rhodium, more preferably at least five catalyst meshes. Preferably, at least 50% of the catalyst meshes of the catalyst system are catalyst meshes comprising precious metal wires made from a binary PtRh alloy comprising 3.4 wt % to 4.6 wt % rhodium.

[0035] Typically, the catalyst system may contain at least one catalyst mesh or catalyst mesh group containing precious metal wires made of a palladium alloy. The catalyst mesh or the catalyst mesh group is preferably arranged downstream in the flow direction. Such catalyst meshes can be used as capture meshes, i.e., they can collect evaporated platinum from the catalyst mesh group arranged further upstream in the flow direction.

[0036] A palladium alloy is understood to mean an alloy consisting of palladium to an extent of more than 50% by weight. The palladium alloy may contain between 50% and 97% by weight of palladium, preferably more than 60% by weight of palladium, particularly preferably more than 70% by weight of palladium. The palladium alloy may be a ternary palladium alloy, which consists of palladium, platinum and rhodium, except for impurities, or a binary palladium alloy, which consists of palladium and nickel, tungsten, platinum or gold, except for impurities. The proportion of impurities does not exceed 1% by weight of the palladium alloy, preferably does not exceed 0.5% by weight. By way of example, a palladium alloy of a noble metal wire may consist of 70% to 97% by weight of palladium, 0% to 10% by weight of rhodium and 3% to 30% by weight of nickel, tungsten, platinum or gold, except for impurities. The noble metal wire may include, for example, a PdPt(3-30)Rh(1-10) alloy, a PdNi(3-30) alloy, a PdW(3-30) alloy, a PdPt(3-30) alloy, or a PdAu(3-30) alloy. In this case, PdM(ab) means, for example, that the alloy contains a metal M in a weight proportion in the range of a weight % to b weight %, and the remaining proportion (100-(a to b)) weight % is composed of palladium excluding impurities.

[0037] For example, the catalyst system may include at least three catalyst meshes containing precious metal wires made of a binary PtRh alloy containing 3.4 wt % to 4.6 wt % rhodium, and at least one catalyst mesh containing precious metal wires made of a palladium alloy. In particular, the catalyst system may include at least three catalyst meshes containing precious metal wires made of a binary PtRh alloy containing 3.4 wt % to 4.6 wt % rhodium, and at least two catalyst meshes containing precious metal wires made of a palladium alloy.

[0038] The catalyst system may also include additional components.

[0039] For example, the catalyst system may include an ignition layer as a first catalyst mesh or as a first catalyst mesh group. The ignition layer includes a noble metal wire containing only platinum and impurities.

[0040] It can be particularly advantageous if the catalyst mesh furthest upstream, as seen in the flow direction, consists of noble metal wires which, apart from impurities, contain only platinum and no further components.

[0041] In a preferred embodiment, the catalyst system may include a separator element, for example, in the form of an intermediate net. Such separator elements may be used to resist compression and / or melting or sintering of adjacent catalyst nets or catalyst net groups under pressure loads. Compared to the catalyst net, the separator element preferably has limited flexibility.

[0042] Suitable separation 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. The separation element may also comprise a catalytically active coating comprising at least one noble metal.

[0043] The catalyst system according to the invention is particularly suitable for the preparation of nitric acid by the Ostwald process. An ammonia-oxygen mixture flows through the catalyst system for catalytic combustion of ammonia.

[0044] The invention further relates to a method for the catalytic oxidation of ammonia, wherein a fresh gas containing ammonia is conducted via the catalyst system according to the invention. With regard to preferred embodiments of the catalyst system, reference is made to the preceding description.

[0045] The ammonia content of the fresh gas is preferably between 9% and 12% by volume.

[0046] 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 600°C to 1100°C, preferably in the range of 700°C to 1000°C.

[0047] Preferably, the fresh gas is heated at 3 tN / m 2d to 90tN / m 2 Throughput in the range d is conducted via the catalyst system according to the invention. Abbreviation "tN / m 2 "d" stands for "tonnes of nitrogen (from ammonia) per day and per square meter of normalized effective cross-sectional area of ​​the catalyst system".

[0048] The present invention is explained below with reference to the accompanying drawings and experiments on overall catalytic efficiency.

[0049] Figure 1 A vertically positioned flow reactor 1 for the heterogeneously catalytic oxidation of ammonia is schematically shown. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. The catalyst system 2 comprises a plurality of catalyst nets 4 arranged one behind the other in the flow direction 3 of the fresh gas, and behind which a plurality of capture nets 5 may be arranged. The effective catalyst net diameter may be up to 6 m. The nets used are in each case textile fabrics produced by weaving or knitting of precious metal threads.

[0050] The fresh gas is an ammonia-air mixture which is heated to the preheating temperature and introduced under high pressure from above into the reactor 1. Upon entering the catalyst system 2, the gas mixture is ignited and an exothermic combustion reaction subsequently occurs. The following main reactions occur:

[0051] 4NH3+5O2→4NO+6H2O

[0052] 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 6 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).

[0053] In accordance with Figure 1 In test reactors of 1000 nm, three catalyst systems were compared, each comprising five catalyst meshes made of a binary PtRh alloy and additionally six capture meshes made of a PdNi5 alloy. The test reactor according to the invention (IE) contained a PtRh4 mesh, and the comparison reactors contained a PtRh3 mesh (CE1) or a PtRh5 mesh (CE2). The catalyst meshes were produced from the relevant alloys by machine knitting of precious metal wires with a diameter of 76 μm. The fabric structure was identical for all meshes and was 600 g / m 2 is related to the mass per unit area of ​​PtRh5.

[0054] In each case, the test reactor was operated under the following identical test conditions.

[0055] Pressure: 5 bar (absolute)

[0056] Throughput: 12t / m 2 d N (12 tons of nitrogen (from ammonia) per day and per square meter of effective cross-sectional area of ​​the catalyst packing)

[0057] NH3 ratio: 10.7% by volume in fresh gas

[0058] Preheat temperature: 175°C (temperature of the NH3 / air mixture), resulting in a web temperature of 890°C.

[0059] The development of the average daily catalyst efficiency (NO production in %) of the relevant catalyst system is determined until the efficiency drops below a predetermined threshold of 94% or the catalyst package becomes too unstable for further operation of the reactor.

[0060] Table 1 compares the average efficiency of the catalyst systems on the first operating day (d1), which shows a linear dependence on the Rh ratio of the relevant catalyst system. In addition, the table shows the determined maximum operating time and the average efficiency of the test systems measured during this time.

[0061] Table 1 :

[0062] Efficiency of d1 [%] Maximum running time [d] Average efficiency [%] CE1(PtRh3) 94.5 330 95.3 IE(PtRh4) 95.1 335 95.7 CE2(PtRh5) 95.5 285 95.7

[0063] Figure 2 The total yields (maximum Y) of HNO3 obtained in the relevant time periods, normalized to the area of ​​the catalyst system, are compared. Surprisingly, the maximum is observed here in the system according to the invention, which has an average Rh content that is not usually used in industry (IE with PtRh4 as binary PtRh alloy).

[0064] With the reactor system according to the invention, an average increase in efficiency of about 0.4% over the maximum achievable operating time was observed compared to a system with a lower Rh content (CE1). Combined with a longer possible activity duration, this average increase represents a significant economic advantage. Compared to a system with a higher Rh content (CE2), the average efficiency is comparable. However, due to the longer maximum operating time, the increased overall yield and the lower proportion of price-determining rhodium, the system according to the invention also offers a significant economic improvement over the standard system.

Claims

1. A catalyst system for catalytic oxidation of ammonia, said system comprising more than one catalyst mesh, wherein at least one of said catalyst meshes contains at least one noble metal wire consisting of a binary PtRh alloy, It is characterized in that said binary PtRh alloy of said at least one precious metal wire consists of 3.4 wt. % to 4.6 wt. % rhodium, impurities and the remainder platinum, And the catalyst system does not include a catalyst mesh comprising precious metal wires made of another binary PtRh alloy comprising more than 7 wt % rhodium.

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

3. The catalyst system according to claim 1 or 2, wherein the catalyst system comprises at least three catalyst meshes containing precious metal wires made of the binary PtRh alloy comprising 3.4 wt. % to 4.6 wt. % rhodium.

4. The catalyst system according to any one of the preceding claims, wherein the catalyst system comprises a plurality of catalyst mesh groups.

5. The catalyst system according to any one of the preceding claims, wherein at least one of the catalyst webs comprises a three-dimensional structure.

6. The catalyst system according to any of the preceding claims, wherein the noble metal wires of the catalyst network have a diameter of 40 to 150 μm.

7. Catalyst system according to any of the preceding claims, wherein the proportion of the impurities in the binary PtRh alloy does not exceed 1 wt.-%.

8. The catalyst system of any one of the preceding claims, wherein the binary PtRh alloy of the at least one precious metal wire consists of 3.6 wt. % to 4.4 wt. % rhodium, impurities and the remainder platinum.

9. The catalyst system according to any of the preceding claims, wherein the catalyst system does not comprise a catalyst mesh containing precious metal wires made of a further binary PtRh alloy comprising more than 6.5 wt.-% rhodium, preferably not more than 6 wt.-% rhodium.

10. The catalyst system according to any one of the preceding claims, wherein the catalyst system does not comprise any additional catalyst mesh containing noble metal wires made of another binary PtRh alloy.

11. The catalyst system according to any one of the preceding claims, wherein the catalyst system comprises at least one catalyst mesh comprising noble metal wires made of a palladium alloy.

12. The catalyst system according to any one of the preceding claims, wherein the catalyst system comprises at least one separation element. 13 . A method for the catalytic oxidation of ammonia, wherein a fresh gas containing ammonia is passed via the catalyst system according to claim 1 .

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