Method for reducing n2o formation in an ammonia oxidation process and catalyst unit for this purpose

Abstract

The present invention relates to a method for reducing the formation of N2O in an ammonia oxidation process, comprising the steps of: - providing a process gas mixture comprising ammonia and oxygen; - passing the process gas mixture under oxidizing conditions successively through a first catalyst group and at least one second catalyst group, thereby obtaining a gas containing nitrogen oxides; wherein the first catalyst group comprises a first type of catalytic wire, and wherein the second catalyst group comprises a second type of catalytic wire, wherein the cross-sectional area of the first type of catalytic wire is larger than the cross-sectional area of the second type of catalytic wire; and, wherein the cross-sectional area of the first type of catalytic wire is at least 9,503 µm 2 The present invention further relates to a catalyst unit suitable for use in the method.

Description

Technical Field

[0001] This invention relates to the optimization of catalyst arrangements. More specifically, it relates to a catalyst arrangement that can be used in the catalytic oxidation of ammonia (e.g., in a larger nitric acid production process). Preferably, the catalyst arrangement is optimized to reduce the amount of N2O that can be formed in the catalytic oxidation process. Preferably, the catalyst arrangement is optimized to have high NO selectivity in the catalytic oxidation process. Background Technology

[0002] The catalytic oxidation of ammonia is a common step in processes such as nitric acid production. The goal of catalytic oxidation is to achieve a high conversion of ammonia to nitric oxide (NO). The formation of nitrous oxide (N₂O, also known as dinitrogen oxide) is a common side reaction in this catalytic oxidation step. N₂O formation is problematic because it firstly reduces NO yield and pollutes the desired NO gas, which is then further converted to nitric acid. Perhaps even more problematic is that N₂O is a very potent greenhouse gas with a global warming potential (GWP) of approximately 265 (i.e., 1 ton of N₂O is equivalent to 265 tons of CO₂). Furthermore, N₂O can deplete the ozone layer and lead to hypertrophication of sensitive habitats.

[0003] Therefore, it is necessary to reduce the formation and / or emission of N2O in catalytic oxidation processes. It is also necessary to improve the yield and / or selectivity of NO in catalytic oxidation processes (preferably ammonia catalytic oxidation processes). Furthermore, it is necessary to extend the lifespan of the catalysts used in catalytic oxidation processes.

[0004] Therefore, one of the objectives of this invention is to overcome or improve one or more of the aforementioned disadvantages existing in the market, or to meet any needs existing in the market. Preferably, this invention provides a solution that can be easily implemented in existing production plants and manufacturing facilities and / or without requiring major modifications to existing equipment. Summary of the Invention

[0005] The inventors have now unexpectedly discovered that one or more of these objectives can be achieved by providing, in the catalyst unit of a catalytic oxidation process, a mesh made of relatively coarse catalytic metal wires as a first mesh (one or more), and a mesh made of relatively fine catalytic metal wires (one or more). Preferably, the mesh openings of these first meshes are wider than those of the meshes deeper within the catalyst unit.

[0006] In a first aspect, the present invention provides a method for reducing N2O formation in an ammonia oxidation process, comprising the following steps: - Provides process gas mixtures containing ammonia and oxygen; - Under oxidizing conditions, the process gas mixture is sequentially passed through a first catalyst group and at least one second catalyst group to obtain a gas containing nitrogen oxides; The first catalyst assembly comprises a first type of catalytic metal wire, and the second catalyst assembly comprises a second type of catalytic metal wire. The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, Preferably, the cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 Preferably at least 12,272 µm 2 More preferably at least 15,394 µm 2 .

[0007] In some embodiments, the first catalyst assembly comprises one or more woven meshes made of catalytic metal wires of the first type.

[0008] In some embodiments, the diameter or width of the first type of catalytic metal wire is at least 110 µm, preferably at least 125 µm, and more preferably at least 140 µm.

[0009] In some embodiments, the distance between the centers of two adjacent filaments in the first catalyst assembly is at least 400 µm, preferably at least 500 µm, and more preferably at least 600 µm.

[0010] In some embodiments, the first catalyst assembly comprises at least two woven meshes oriented parallel to each other and rotated relative to each other by an angle of 0.0° to at most 75.0°, preferably at least 25.0° to at most 60.0°, preferably at least 35.0° to at most 50.0°, and more preferably about 45.0°.

[0011] In some embodiments, the second catalyst assembly comprises one or more woven or knitted meshes made of the second type of catalytic metal wires; preferably, the second catalyst assembly comprises one or more woven meshes made of the second type of catalytic metal wires.

[0012] In some embodiments, the mesh size of the woven mesh of the first catalyst group is larger than that of the woven or knitted mesh of the second catalyst group, and / or the cross-sectional area of ​​the woven mesh of the first catalyst group is larger than that of the woven or knitted mesh of the second catalyst group.

[0013] In some embodiments, the cross-sectional area of ​​the second type of catalytic metal wire is at most 7,854 µm. 2 Preferably up to 6,362 µm2 Preferably up to 5,027 µm 2 More preferably up to 4,536.5 µm 2 .

[0014] In some embodiments, the ratio of the mesh size of the woven mesh of the first catalyst group to the mesh size of the woven or knitted mesh of the second catalyst group is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.9.

[0015] In some embodiments, the ratio of the cross-sectional area of ​​the first type of catalytic metal wire to the cross-sectional area of ​​the second type of catalytic metal wire is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.9.

[0016] In some implementations, the first catalyst group and the second catalyst group are housed in the same catalyst unit.

[0017] In some embodiments, the second catalyst assembly comprises at least six, preferably at least seven, more preferably at least eight woven and / or knitted meshes, which are oriented parallel to each other and optionally rotated relative to the preceding mesh.

[0018] In some embodiments, the first type of catalytic metal wire and / or the second type of catalytic metal wire comprises platinum, palladium, rhodium, or an alloy containing platinum, rhodium, and / or palladium.

[0019] The present invention also provides a catalyst unit comprising: - A first catalyst assembly, comprising a first type of catalytic metal wire; and, - A second catalyst assembly containing a second type of catalytic metal wire; The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, The cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 Preferably at least 12,272 µm 2 More preferably at least 15,394 µm 2 .

[0020] The present invention also provides the use of the catalyst unit according to claim 14 for reducing N2O formation in an ammonia oxidation process.

[0021] Preferred embodiments of the invention are disclosed in the detailed description and the appended claims. In the following paragraphs, different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other one or more aspects unless explicitly indicated otherwise. In particular, any feature indicated as preferred or advantageous may be combined with any other one or more features indicated as preferred or advantageous. A (preferred) embodiment of one aspect of the invention is also a (preferred) embodiment of all other aspects of the invention. Attached Figure Description

[0022] Figure 1a and Figure 1b Two views of a catalyst unit according to an embodiment of the present invention are depicted.

[0023] Figure 2 A cross-sectional view of a catalyst unit according to an embodiment of the present invention is depicted. Detailed Implementation

[0024] In describing this invention, the terminology used should be interpreted according to the following definitions, unless the context otherwise requires.

[0025] Unless otherwise defined, all terms used in disclosing this invention (including technical and scientific terms) have the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. For further guidance, terminology definitions are included herein to better understand the teachings of this invention.

[0026] The various aspects of the invention are defined in more detail in the following paragraphs. Each aspect so defined may be combined with any other one or more aspects unless explicitly indicated otherwise. In particular, any feature indicated as preferred or advantageous may be combined with any other one or more features indicated as preferred or advantageous.

[0027] Throughout this specification, the phrase "one embodiment" or "an embodiment" refers to a specific feature, structure, or characteristic described in that embodiment being included in at least one embodiment of the invention. Therefore, the phrases "in one embodiment" or "in an embodiment," appearing in various places throughout this specification, do not necessarily refer to the same embodiment, but may refer to the same embodiment. Furthermore, specific features, structures, or characteristics may be combined in one or more embodiments in any suitable manner (which will be apparent to those skilled in the art based on this disclosure). Moreover, as will be understood by those skilled in the art, while some embodiments described herein include some features included in other embodiments but not others, combinations of features from different embodiments are intended to fall within the scope of the invention and form different embodiments. For example, any embodiment may be used in any combination in the following claims and statements.

[0028] As used herein, the terms “comprising,” “comprises,” and “comprisedof” are synonymous with “including,” “includes,” or “containing,” and are inclusive or open-ended, and do not exclude additional, unlisted members, elements, or method steps. It should be understood that the terms “comprising,” “comprises,” and “comprisedof” as used herein include the terms “consisting of,” “consists,” and “consists of.”

[0029] As used in the specification and appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context clearly specifies otherwise. For example, “a step” refers to one step or more steps.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art.

[0031] A range of values ​​enumerated by endpoints includes all integers and (where appropriate) fractions contained within that range (e.g., when referring to, for example, the number of elements, 1 to 5 may include 1, 2, 3, 4, and when referring to, for example, a measure, 1 to 5 may also include 1.5, 2, 2.75, and 3.80). The enumeration of endpoints also includes the endpoint values ​​themselves (e.g., 1.0 to 5.0 includes both 1.0 and 5.0). Any range of values ​​enumerated herein is intended to include all subranges contained therein.

[0032] As used herein, when referring to measurable values ​​such as parameters, quantities, durations, etc., the term "about" is intended to include variations of + / -10% or less, preferably + / -5% or less, more preferably + / -1% or less, and deviations from the specified value of + / -10% or less, preferably + / -5% or less, more preferably + / -1% or less, provided such variations are suitable for implementation in the disclosed invention. It should be understood that the values ​​referred to by the modifier "about" are themselves specifically and preferably disclosed.

[0033] The terms “wt%”, “vol%”, or “mol%” refer to the weight percentage, volume percentage, or mole percentage of a component based on the total weight, volume, or number of moles of the material comprising that component.

[0034] In describing this invention, the terminology used should be interpreted according to the following definitions, unless the context otherwise requires.

[0035] In a first aspect, the present invention provides a method for reducing N2O formation in an ammonia oxidation process, comprising the following steps: - Provides process gas mixtures containing ammonia and oxygen; - Under oxidizing conditions, the process gas mixture is sequentially passed through a first catalyst group and at least one second catalyst group to obtain a gas containing nitrogen oxides; The first catalyst assembly comprises a first type of catalytic metal wire, and the second catalyst assembly comprises a second type of catalytic metal wire. The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, Preferably, the cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 (Corresponding to a diameter of 110 µm for a circular wire), preferably at least 12,272 µm. 2 (Corresponding to a diameter of 125 µm for a circular filament), more preferably at least 15,394 µm. 2 (Corresponding to a diameter of 140 µm for a circular wire).

[0036] It has been found that this first type of catalytic wire reduces N2O formation while still providing satisfactory selectivity and / or yield for NO formation in the ammonia oxidation process. The thicker first-type catalytic wire appears to reduce N2O formation, while the thinner second-type catalytic wire ensures high selectivity and / or yield for NO formation. Because the gas mixture first impacts the thicker first-type catalytic wire, these wires protect the thinner second-type catalytic wire from the high temperatures / harsh conditions of the ammonia oxidation process, thus extending the catalyst bed's lifespan. This allows for less downtime for catalyst bed replacements and enables more efficient use of typically expensive catalytic wires.

[0037] In some embodiments, the first catalyst assembly comprises one or more, preferably two or more, woven meshes made of catalytic metal wires of the first type. Preferably, the woven mesh has an open weave structure, and the mesh openings are larger than the cross-sectional area of ​​the wires used to make the weave structure. This reduces the back pressure when the gas mixture passes through the mesh.

[0038] In some embodiments, the first type of catalytic metal wire and / or the second type of catalytic metal wire have an elliptical, but preferably circular, cross-section. Because such wires have no corners, they are likely to experience less wear, as corners are typically particularly sensitive to wear and corrosion. Therefore, the lifespan of the catalyst can be extended with such wires.

[0039] In some embodiments, the diameter of the first type of catalytic metal wire ( d (or the width is at least 110 µm, preferably at least 125 µm, more preferably at least 140 µm.)

[0040] In some embodiments, the diameter of the first type of catalytic metal wire ( d The width is at least 110 µm to at most 300 µm, preferably at least 125 µm to at most 250 µm, and preferably at least 140 µm to at most 200 µm.

[0041] In some embodiments, the diameter of the second type of catalytic metal wire ( d The wires can be at least 20 µm wide, preferably at least 50 µm, preferably at least 70 µm, and most preferably at least 75 µm. Such wires can provide sufficient service life for the catalyst.

[0042] In some embodiments, the diameter of the second type of catalytic metal wire ( dThe width is at least 20 µm to at most 150 µm, preferably at least 50 µm to at most 125 µm, preferably at least 70 µm to at most 100 µm, and preferably at least 75 µm to at most 90 µm.

[0043] In some embodiments, the diameter of the second type of catalytic metal wire ( d The width may be up to 150 µm, preferably up to 125 µm, preferably up to 100 µm, or preferably up to 90 µm.

[0044] In some embodiments, the distance between the centers of two adjacent filaments in the first catalyst assembly ( l The diameter is at least 400 µm, preferably at least 500 µm, and more preferably at least 600 µm. This ensures an open-weave structure for the passage of gas mixtures.

[0045] In some embodiments, the distance between the centers of two adjacent filaments in the first catalyst assembly ( l The diameter of the filament is at least 400 µm to at most 1500 µm, preferably at least 500 µm to at most 1200 µm, preferably at least 600 µm to at most 1000 µm, and preferably at least 600 µm to at most 800 µm. This ensures an open braided structure for the passage of gas mixtures, but not so open that the gas mixture is sufficiently in contact with the filaments for catalytic reactions.

[0046] In some embodiments, the distance between the centers of two adjacent filaments in the second catalyst assembly ( l The diameter is at least 100 µm, preferably at least 200 µm, and more preferably at least 300 µm. This ensures an open-weave structure for the passage of gas mixtures.

[0047] In some embodiments, the distance between the centers of two adjacent filaments in the second catalyst assembly ( l The diameter of the filament is at least 100 µm to at most 750 µm, preferably at least 150 µm to at most 600 µm, preferably at least 200 µm to at most 500 µm, and preferably at least 250 µm to at most 400 µm. This ensures sufficient contact between the filament and the gas mixture for the catalytic reaction to achieve high NO production.

[0048] In some embodiments, the distance between the centers of two adjacent filaments in the first catalyst assembly is at least 2.5 times, at least 2.6 times, at least 2.7 times, at least 3.0 times, at least 3.3 times, at least 3.5 times, and preferably at least 4.0 times the diameter or width of the first type of catalytic metal filament.

[0049] In some embodiments, the distance between the centers of two adjacent filaments in the second catalyst assembly is at least 2.5 times, at least 2.6 times, at least 2.7 times, at least 3.0 times, at least 3.3 times, at least 3.5 times, and preferably at least 4.0 times the diameter or width of the second type of catalytic metal filament.

[0050] In some embodiments, the first catalyst assembly comprises at least two woven meshes oriented parallel to each other and rotated relative to each other by an angle of 0.0° to a maximum of 75.0°, preferably at least 25.0° to a maximum of 60.0°, preferably at least 35.0° to a maximum of 50.0°, and more preferably about 45.0°.

[0051] A wire mesh typically extends in two directions, which define the two-dimensional planes of each wire mesh. As used herein, the term "parallel-oriented wire mesh" refers to the orientation of these two-dimensional planes. However, within these parallel-oriented planes, the wire mesh can be rotated relative to wire meshes from different planes. Preferably, the different wire meshes are stacked, more preferably stacked one on top of the other.

[0052] In some embodiments, the second catalyst assembly comprises one or more woven or knitted meshes made of the second type of catalytic metal wires; preferably, the second catalyst assembly comprises one or more woven meshes made of the second type of catalytic metal wires.

[0053] In some embodiments, the second catalyst assembly comprises at least four, preferably at least five, preferably at least six, preferably at least seven, preferably at least eight woven or knitted meshes made of the second type of catalytic metal wire. Such catalyst assemblies ensure high NO production.

[0054] In some embodiments, the cross-sectional area of ​​the second type of catalytic metal wire is at most 7,854 µm. 2 (Corresponding to a diameter of 100 µm for a circular wire), preferably up to 6,362 µm. 2 (Corresponding to a diameter of 90 µm for a circular wire), preferably up to 5,027 µm. 2 (Corresponding to a diameter of 80 µm for a circular filament), more preferably up to 4,536.5 µm. 2 (Corresponding to a diameter of 76 µm for a circular wire).

[0055] In some embodiments, the mesh size of the woven mesh of the first catalyst group is larger than that of the woven or knitted mesh of the second catalyst group, and / or the cross-sectional area of ​​the woven mesh of the first catalyst group is larger than that of the woven or knitted mesh of the second catalyst group.

[0056] As used in this article, the term "cross-sectional area" may refer to the area of ​​the cross-section of a filament, which is perpendicular to the filament.

[0057] In some embodiments, the catalyst unit may include a third catalyst assembly comprising a third catalytic wire with a cross-sectional area smaller than that of the second catalytic wire. In some embodiments, the mesh size in the third catalyst assembly is smaller than that in the second catalyst assembly. Preferably, the third catalyst assembly comprises a woven or knitted mesh made of the third catalytic wire. Preferably, the third catalyst assembly is positioned downstream of the second catalyst assembly. It should be understood that in some embodiments, the catalyst unit may even include a fourth catalyst assembly, optionally a fifth catalyst assembly, etc., each comprising finer catalytic wires and / or smaller mesh sizes compared to the previous catalyst assembly.

[0058] In some embodiments, the ratio of the cross-sectional area of ​​the first type of catalytic metal wire to the cross-sectional area of ​​the second type of catalytic metal wire is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.9.

[0059] In some implementations, the first catalyst group and the second catalyst group are housed in the same catalyst unit.

[0060] In some embodiments, the ratio of the mesh size of the woven mesh of the first catalyst group to the mesh size of the woven or knitted mesh of the second catalyst group is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.9.

[0061] In some embodiments, the second catalyst assembly comprises at least six, preferably at least seven, more preferably at least eight woven and / or knitted meshes, which are oriented parallel to each other and optionally rotated relative to the preceding mesh.

[0062] In some embodiments, the first catalyst assembly comprises at least two woven meshes woven from first-type catalytic metal wires, wherein two consecutive woven meshes are preferably rotated relative to each other by an angle of at least 25° to at most 65°, preferably at least 30° to at most 60°, preferably at least 35° to at most 55°, preferably at least 40° to at most 45°; and / or, the second catalyst assembly comprises at least four, preferably at least six, preferably at least eight woven meshes woven from second-type catalytic metal wires, wherein two consecutive woven meshes are preferably rotated relative to each other by an angle of at least 3° to at most 45°, preferably at least 5° to at most 25°, preferably at least 7° to at most 15°, preferably at least 9° to at most 12°.

[0063] In some embodiments, the first catalyst group and the second catalyst group are rotated relative to each other by an angle of at least 25° to at most 65°, preferably at least 30° to at most 60°, preferably at least 35° to at most 55°, and preferably at least 40° to at most 45°.

[0064] In some embodiments, the first type of catalytic metal wire and / or the second type of catalytic metal wire comprises platinum, palladium, rhodium, or an alloy containing platinum, rhodium, and / or palladium.

[0065] In some embodiments, the obtained nitrogen oxide-containing gas contains at least 75.0 vol% NO, preferably at least 80.0 vol% NO, preferably at least 85.0 vol% NO, preferably at least 90.0 vol% NO, preferably at least 95.0 vol% NO, preferably at least 98.0 vol% NO, preferably at least 99.0 vol% NO, wherein vol% is expressed relative to the total volume of the obtained nitrogen oxide-containing gas.

[0066] In some implementations, the method includes the step of oxidizing NO to NO2.

[0067] In some embodiments, the method includes the step of contacting the obtained nitrogen oxide-containing gas with water, preferably by washing with water. This may result in the formation of nitric acid or a nitric acid solution.

[0068] In some embodiments, the method includes a step of venting the remaining gas after the step of contacting water.

[0069] In some embodiments, the gas mixture is at a temperature of at least 100°C, preferably at least 125°C, preferably at least 150°C, preferably at least 175°C, preferably at least 200°C, preferably at least 225°C, and preferably at least 250°C before flowing through the first catalyst group.

[0070] In some embodiments, the gas mixture is at a pressure of at least 1.0 atm, preferably at least 1.5 atm, preferably at least 2.0 atm, preferably at least 2.5 atm, preferably at least 3.0 atm, preferably at least 3.5 atm, preferably at least 4.0 atm, preferably at least 4.5 atm, preferably at least 5.0 atm before flowing through the first catalyst group.

[0071] In some embodiments, the gas mixture comprises about 4 parts by volume of ammonia to about 5 parts by volume of oxygen.

[0072] This invention provides a catalyst unit comprising: - A first catalyst assembly, comprising a first type of catalytic metal wire; and, - A second catalyst assembly containing a second type of catalytic metal wire; The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, Preferably, the cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 (Corresponding to a diameter of 110 µm for a circular wire), preferably at least 12,272 µm. 2 (Corresponding to a diameter of 125 µm for a circular filament), more preferably at least 15,394 µm. 2 (Corresponding to a diameter of 140 µm for a circular wire).

[0073] In some embodiments, the catalyst unit is a catalyst unit configured for use in the methods or uses disclosed herein.

[0074] The present invention provides the use of a catalyst unit according to the embodiments described herein for reducing the formation of N2O in an ammonia oxidation process.

[0075] It should be understood that the (preferred) implementation of the method is also the (preferred) implementation of the catalyst unit and the use, and vice versa.

[0076] The invention will be more readily understood by referring to the following embodiments, which are included herein for the purpose of illustrating certain aspects and embodiments of the invention only and are not intended to limit the invention.

[0077] Example In these embodiments, the meshes are numbered starting with the mesh intended to be struck first by the process gas flow. In other words, the process gas strikes mesh #1 first, then mesh #2, then mesh #3, and so on. Therefore, mesh #1 is located upstream of mesh #2, and mesh #8 is located downstream of mesh #7.

[0078] As used herein, the parameter “rotation” in the examples refers to the relative angle of rotation of one mesh in the catalyst unit relative to the first mesh.

[0079] In these embodiments, the composition of the filaments is the same.

[0080] Comparative example: The following catalyst unit was prepared as a comparative example, comprising eight woven meshes made of catalyst filaments with circular cross-sectional areas, and possessing the following characteristics:

[0081] Table 1: Catalysts used as comparative examples Example 1 The following catalyst unit according to an embodiment of the present invention was prepared, comprising eight woven meshes made of filaments having a circular cross-sectional area, and having the following characteristics:

[0082] Table 2: Catalyst Units According to Embodiments of the Present Invention Figure 1a and Figure 1b The catalyst unit of Example 1 is described, wherein Figure 1a This is a front view of catalyst unit (1). Figure 1b This is a perspective view of the catalyst unit (1). The catalyst unit (1) comprises a first catalyst assembly (10), which consists of a first woven mesh #1 (11) and a first woven mesh #2 (12), which are oriented in parallel (arranged vertically above each other) but rotated at a 45° angle, as shown. Figure 1a and Figure 1b As shown. The first woven wire mesh #1 (11) and the first woven wire mesh #2 (12) are woven from the first catalyst wire (100).

[0083] The catalyst unit (1) also includes a second catalyst assembly (20) stacked below the first catalyst assembly (10), wherein the second catalyst assembly (20) consists of second woven wire meshes #3 (23) to #8 (not distinguishable in the figure). The second woven wire meshes are arranged in parallel orientation (upper and lower than each other) and rotate relative to wire mesh #1, as shown in Table 2. The second woven wire meshes are woven from second catalyst filaments (200).

[0084] The diameter of the wire used d And the distance between the centers of the wires in different meshes l As defined in Table 2.

[0085] In similar oxidation experiments, the selected characteristics of the comparative catalyst unit and the catalyst unit of Example 1 are compared in the table below:

[0086] Table 3: Characteristics of the catalyst unit in: - A cat The total surface area of ​​all catalyst wires in the catalyst unit - V cat The volume of all catalyst wires in the catalyst unit These experiments show that, for the catalyst unit according to Example 1, a significant decrease in N2O selectivity was observed compared to experiments using the catalyst unit according to the comparative example. It can also be noted that NO selectivity and NO yield increased, while NH3 conversion remained acceptable.

[0087] Example 2: Figure 1 depicts a schematic cross-sectional view of a catalyst unit (1) according to an embodiment of the present invention placed in a pipe (2), through which a gas mixture (containing ammonia and oxygen) flows in the direction indicated by arrow (3). The catalyst unit (1) comprises a first catalyst assembly (10) placed upstream of a second catalyst assembly (20). Both the first catalyst assembly (10) and the second catalyst assembly (20) are housed in the same housing (4). The first catalyst assembly (10) consists of a stack of two first woven wire meshes (11, 12) woven from a first catalytic metal wire (100). In this specific embodiment, the diameter (d) of the first catalytic metal wire (100) is 150 µm. The rotation between the first wire meshes (11, 12) is 0°. However, to obtain a catalyst unit similar to the catalyst unit in Example 1, the woven wire mesh (12) may be rotated by an angle of 45° relative to the woven wire mesh (11).

[0088] The second catalyst assembly (20) consists of a stack of six second woven wire meshes (23, 24, 25, 26, 27, 28) arranged parallel to each other and woven from second catalytic metal wires (200). In this specific embodiment, the diameter (d) of the second catalytic metal wires (200) is 76 µm. The rotation between the second wire meshes (23, 24, 25, 26, 27, 28) is 0°. However, to obtain a catalyst unit similar to the catalyst unit in Example 1, the six consecutive second woven wire meshes (23, 24, 25, 26, 27, 28) can each be rotated by 20°.

[0089] It should be understood that the embodiments can be easily modified by changing the number of wire meshes or wire mesh groups in the catalyst unit. For example, a catalyst unit containing two first woven wire meshes and eight second woven wire meshes can be obtained by placing two additional second woven wire meshes in the second catalyst group.

[0090] It should be understood that although preferred embodiments and / or materials for providing embodiments according to the present invention have been discussed, various modifications or changes may be made without departing from the scope and concept of the invention.

Claims

1. A method for reducing N2O formation in an ammonia oxidation process, comprising the following steps: - Provides process gas mixtures containing ammonia and oxygen; - Under oxidizing conditions, the process gas mixture is sequentially passed through a first catalyst group and at least one second catalyst group to obtain a gas containing nitrogen oxides; The first catalyst assembly comprises a first type of catalytic metal wire, and the second catalyst assembly comprises a second type of catalytic metal wire. The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, The cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 Preferably at least 12,272 µm 2 More preferably at least 15,394 µm 2 .

2. The method of claim 1, wherein the first catalyst assembly comprises one or more woven meshes woven from the first type of catalytic metal wires.

3. The method according to claim 1 or 2, wherein the diameter or width of the first type of catalytic metal wire is at least 110 µm, preferably at least 125 µm, more preferably at least 140 µm.

4. The method according to any one of the preceding claims, wherein the distance between the centers of two adjacent filaments in the first catalyst assembly is at least 400 µm, preferably at least 500 µm, and more preferably at least 600 µm.

5. The method according to any one of the preceding claims, wherein the first catalyst assembly comprises at least two woven meshes oriented parallel to each other and rotated relative to each other by an angle of 0.0° to at most 75.0°, preferably at least 25.0° to at most 60.0°, preferably at least 35.0° to at most 50.0°, more preferably about 45.0°.

6. The method according to any one of the preceding claims, wherein the second catalyst assembly comprises one or more woven or knitted meshes made of the second type of catalytic metal wires; preferably wherein the second catalyst assembly comprises one or more woven meshes made of the second type of catalytic metal wires.

7. The method according to any one of the preceding claims, wherein the mesh size of the woven mesh of the first catalyst group is larger than the mesh size of the woven or knitted mesh of the second catalyst group, and / or wherein the cross-sectional area of ​​the woven mesh of the first catalyst group is larger than the cross-sectional area of ​​the woven or knitted mesh of the second catalyst group.

8. The method according to any one of the preceding claims, wherein the cross-sectional area of ​​the second type of catalytic metal wire is at most 7,854 µm. 2 Preferably up to 6,362 µm 2 Preferably up to 5,027 µm 2 More preferably up to 4,536.5 µm 2 .

9. The method according to any one of the preceding claims, wherein the ratio of the mesh size of the woven mesh of the first catalyst group to the mesh size of the woven or knitted mesh of the second catalyst group is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.

9.

10. The method according to any one of the preceding claims, wherein the ratio of the cross-sectional area of ​​the first type of catalytic metal wire to the cross-sectional area of ​​the second type of catalytic metal wire is at least 1.2, preferably at least 1.5, preferably at least 2.0, preferably at least 2.5, preferably at least 3.0, preferably at least 3.5, preferably at least 3.8, for example 3.

9.

11. The method according to any one of the preceding claims, wherein the first catalyst group and the second catalyst group are contained in the same catalyst unit.

12. The method according to any one of the preceding claims, wherein the second catalyst assembly comprises at least 6, preferably at least 7, more preferably at least 8 woven and / or knitted meshes, which are oriented parallel to each other and optionally rotated relative to the preceding mesh.

13. The method according to any one of the preceding claims, wherein the first type of catalytic metal wire and / or the second type of catalytic metal wire comprises platinum, palladium, rhodium, or an alloy containing platinum, rhodium and / or palladium.

14. A catalyst unit comprising: - A first catalyst assembly, comprising a first type of catalytic metal wire; and, - A second catalyst assembly containing a second type of catalytic metal wire; The cross-sectional area of ​​the first type of catalytic metal wire is larger than that of the second type of catalytic metal wire; and, The cross-sectional area of ​​the first type of catalytic metal wire is at least 9,503 µm. 2 Preferably at least 12,272 µm 2 More preferably at least 15,394 µm 2 .

15. Use of the catalyst unit according to claim 14 for reducing N2O formation in an ammonia oxidation process.

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