RU2808516C2 - Catalytic system, and also method for catalytic combustion of ammonia to nitrogen oxides in medium pressure installation - Google Patents

Abstract

FIELD: catalytic systems.

SUBSTANCE: present invention relates to a catalytic system for the catalytic combustion of ammonia to form nitrogen oxides and a method for the catalytic combustion of ammonia to nitrogen oxides in a medium pressure unit. A catalytic system is described for the catalytic combustion of ammonia to nitrogen oxides in a medium-pressure unit with mesh layers of catalyst woven or knitted or crocheted from wire based on the precious metal platinum, which, located one after another in the direction of fresh gas flow, form a catalyst packing, wherein the catalyst packing consists of a front ensemble with three catalyst meshes having the first average surface density, and a downstream ensemble of catalyst mesh layers located behind the front ensemble, having a second average surface density, and the decrease in the average surface density of the front ensemble relative to the second average surface density is from 1.5% to 29%, and the fact that the first average surface density lies in the range from 410 to 530 g/m2, and the second average surface density in the range from 540 to 790 g/m2. Also described is a method for the catalytic combustion of ammonia to nitrogen oxides in a medium pressure unit by passing fresh gas containing ammonia and oxygen through the catalytic system, wherein the ammonia is burned, the fresh gas having an ammonia content of 9.5 to 12 vol.% under pressure in in the range from 3.5 to 7 bar and catalyst grid temperature in the range from 870°C to 920°C is carried out with a flow rate through the above-described catalytic system in the range from 6 to 16 tN/m²⋅days.

EFFECT: achievement of a yield of the main product NO comparable to the industry standard, with a reduced use of precious metals.

14 cl, 3 dwg, 6 tbl

Description

Field of technology

The present invention relates to a catalytic system for the catalytic combustion of ammonia to produce nitrogen oxides in a medium-pressure unit, with a plurality of catalyst meshes woven or knitted or crocheted from platinum-based metal wire, which, arranged one behind the other in the direction of fresh gas flow, form a catalyst packing.

In addition, the invention relates to a method for the catalytic combustion of ammonia to nitrogen oxides in a medium pressure plant, according to which fresh gas containing ammonia and oxygen is passed through the catalytic system and the ammonia is burned.

Nitric acid for the chemical industry and fertilizer production is produced on an industrial scale using the Ostwald method by oxidizing ammonia with oxygen by oxidizing ammonia with oxygen under conditions of heterogeneous catalysis with precious metals, producing nitrogen oxide.

State of the art

The catalytic systems used for this are installed in the reaction zone of a flow reactor in a plane perpendicular to the direction of fresh gas flow, and often several catalyst grids are placed one behind the other, combining them into what is called a catalyst pack. Catalyst meshes are woven or crocheted or knitted single or multi-layer structures made of fine precious metal wire. Precious metal wire consists primarily of platinum (Pt), palladium (Pd), rhodium (Rh) or alloys of these metals.

Ammonia oxidation can be carried out at low pressure (1-3 bar; PtRh10), medium pressure (3.5-7 bar; PtRh5) or high pressure (8-14 bar; PtRh3). A typical Pt-Rh alloy is given in parentheses for the corresponding pressure range, and the number indicates the rhodium content in weight percent).

When ammonia is burned with oxygen, it produces nitrous oxide ( _N2O ) as an unwanted byproduct, which is harmful to ozone and a greenhouse gas. It is therefore important to avoid, as far as possible, the formation of N ₂ O in the combustion reaction without reducing the yield of the desired nitrogen oxides.

Nitric acid production plants use, depending on the type of design, from 2 to 30 catalyst meshes with a diameter of up to 6 m. The use of precious metals represents a large potential investment and must therefore be kept to the minimum possible level. On the other hand, "catalytic efficiency", which is an important parameter and measure for long-term high feed conversions and good yields, depends on the precious metal content.

During the oxidation of ammonia, catalyst grids continuously lose precious metal due to oxidation and sublimation, so from time to time (resource, service life) they need to be replaced at a certain cost. A suitable compromise in terms of service life, catalytic efficiency and precious metal use has proven to be PtRh5 alloy, which has established itself as the industry standard for precious metal catalysts for use in medium pressure applications.

To reduce the use of precious metals while maintaining catalytic efficiency, DE 10105624 A1 proposes a flow reactor for the catalytic oxidation of ammonia in a medium-pressure process, which uses three-dimensional knitted catalyst networks in which the individual layers of loops are connected to each other by pile threads. Loop threads and pile threads are made of the same material, for example PtRh5 or PtRh8.

An example implementation uses a test reactor with a catalyst packing made of PtRh5 wire, the packing consists of an ensemble of two front single-layer knitted catalyst meshes with a surface density of 600 g/ ^m2 , a middle ensemble of three double-layer knitted catalyst meshes with a comparatively higher surface density of 900 g/ ^m2 onto the catalyst mesh layer and, finally, from a two-layer knitted and weft-stitched catalyst mesh with a surface density of 800 g/m ² per mesh layer. The total weight of the precious metal structure is 16.5 kg, which is 4 kg less than a reference reactor with comparable catalytic efficiency.

DE 60201502 T2 describes three-dimensional catalyst networks for carrying out heterogeneous catalytic gas reactions for the oxidation of ammonia with atmospheric oxygen to produce nitric acid and for the reaction of ammonia with methane in the presence of oxygen to produce hydrocyanic acid. Catalyst meshes consist of multiple mesh layers of crocheted precious metal wire, with weft threads of precious metal wire inserted between the mesh layers.

From WO 2018/065271 A1 there is known a wire for weaving or knitting a catalyst mesh, which contains from two to eight individual threads connected to each other and which contains at least 90 wt.% Pt and at least 5 wt.% Rh.

EP 2689841 A2 describes a catalyst consisting of several catalyst grids arranged one behind the other of a first precious metal-containing wire, each of the grids having a second precious metal-containing wire which gives the corresponding catalyst grid a preferred orientation. The catalyst meshes are placed one behind the other in such a way that the preferred orientations of the adjacent catalyst meshes form an angle of 45° to 135°, with an intermediate mesh made of a third wire containing precious metal being inserted between the adjacent catalyst meshes.

Statement of technical problem

The invention described in DE 10105624 A1 also requires a high use of precious metals.

The aim of the invention is to provide a catalyst system for use in a medium pressure ammonia oxidation plant with which, despite the reduced use of precious metals, it is possible to achieve a yield of the main NO product comparable to the industry standard.

In addition, the basis of the invention is the task of developing a method for the oxidation of ammonia in a medium-pressure installation, which makes it possible to achieve a relatively high yield of product with less use of precious metals.

The essence of the invention

With regard to the catalytic system, this problem is solved, according to the invention, on the basis of a known catalytic system of the type indicated in the introduction in that the catalyst packing consists of a front ensemble with three catalyst meshes having a first average surface density, and located behind the front ensemble, a downstream ensemble of mesh catalyst layers having a second average surface density, wherein the reduction in the average surface density of the front ensemble relative to the second average surface density ranges from 1.5% to 29%, while the first average surface density lies in the range from 410 to 530 g/m ² , and the second average surface density in the range from 540 to 790 g/ ^m2 .

The catalyst system comprises a catalyst pack that consists of mesh layers of upstream and downstream catalyst mesh layer assemblies, as well as possible lattice meshes, which are not discussed in more detail here. The catalyst mesh beds of the catalyst pack are formed from single or multi-layer platinum-based metal wire catalyst meshes. Unless otherwise indicated, for single-layer catalyst networks, the expression "catalyst network" is also used below as equivalent to "catalyst network layer".

The front ensemble combines the first three, when viewed in the direction of fresh gas flow, mesh catalyst layers. It is adjacent to the downstream ensemble, which also includes several mesh catalyst layers. Before their intended use, the catalyst mesh layers have an initial nominal surface density, which is established, for example, in a specification or other regulation. The network catalyst layers of the ensemble may have the same or different nominal surface densities. The average surface density of the ensemble considered here is defined as the quotient of the sum of the nominal (initial) surface densities divided by the number of catalyst networks in the corresponding ensemble. Single- or multilayer catalyst meshes consist of metal wire based on the precious metal platinum with a diameter of d. The surface density of the catalyst mesh layer depends, among other things, on the diameter of the corresponding precious metal wire. The larger the wire diameter, the higher the surface density for a given loop size. Typically the wire diameter is in the range from 40 to 120 µm; The diameter of the standard wire for catalyst meshes for medium pressure installation is 76 microns. The average surface density of the upstream ensemble lies in the range from 410 to 530 g/m ² , and the average surface density of the catalyst meshes of the downstream ensemble ranges from 540 to 790 g/m ² .

Due to the lower average surface density of the front ensemble, there is less space filling in the upstream length of the catalyst nozzle compared to the downstream ensemble, which is accompanied by significantly less precious metal use in the mesh catalyst layers of the front ensemble. Generally speaking, one would expect that the saving of precious metal would also be accompanied by a reduction in the catalytic efficiency of the catalyst system. However, it has surprisingly been found that the catalytic efficiency can be maintained substantially constant (within a measurement error of about 0.3 absolute percent) if the difference between the first average and second average surface densities is simultaneously kept small, namely in the range of 1.5%-29% from the second average surface density, preferably not more than 25%.

The desired maximum possible preservation of catalytic efficiency requires a relatively narrow range of average surface densities of the network catalyst layers of the first and second ensemble, as indicated above. A particularly well-proven embodiment of the catalytic system is one in which the first average areal density is in the range of 415 to 510 g/m ² and the second average areal density is in the range of 575 to 710 g/m ² . The difference in surface density may be caused by the thickness of the precious metal wire. However, it has been found to be particularly advantageous if the upstream and downstream catalyst mesh layers of the catalyst assemblies are made of precious metal wire of the same or approximately the same thickness, and if the difference in area density is provided primarily by the different weave or knitting of the catalyst meshes. Accordingly, the front ensemble catalyst mesh layers consist of a crocheted knit structure with a first knit weave and a first loop size, or a woven structure with a first weave and a first loop size, or a knitted knit structure with a first knit weave and a first loop size , and the net catalyst layers of the lower ensemble consist of a crocheted knit structure with a second knit weave and a second loop size, or a woven structure with a second weave and a second loop size, or a knitted knit structure with a second knit weave and a second loop size , wherein the first loop size is larger than the second loop size.

Thus, the difference in surface density is due not only or not exclusively to the difference in wire thickness, but mainly to the difference in the weaving or knitting of the catalyst meshes, namely the size of the loops. Thus, the loop size of the upstream assembly catalyst mesh layers results in a substantially average weight per unit area that is at most 29%, preferably at most 25% less than that of the downstream assembly mesh layers. The mesh layers of the downstream catalyst ensemble are made, for example, in the form of standard catalyst meshes for operation at medium pressure from wire with a diameter of 76 microns and a surface density of the mesh layers of 600 g/m ² .

The comparatively more open loop configuration in the front ensemble catalyst mesh beds may also contribute to better utilization of the flow established in the catalyst pack and thus higher selectivity of the catalyst system. This is because the reaction in a layered system typically occurs in the front (top) mesh layers, causing them to experience particularly high thermal and mechanical stress. Thanks to the more open mesh configuration in this area, the load on the upper mesh layers can be distributed to the mesh layers further down, so that the efficiency of the catalytic system is not significantly reduced, despite less precious metals being used.

The lower the surface density of an individual catalyst mesh layer, the greater the saving of precious metal. On the other hand, as the surface density decreases, the catalytic efficiency of the catalyst system may decrease. Therefore, in a preferred embodiment of the catalytic system, it is provided that the surface density of the mesh layers of the front ensemble catalyst is constant or increases successively in the direction of fresh gas flow.

The catalyst system may comprise catalyst grids of metal wire based on precious metals other than platinum and rhodium, such as palladium or iridium, or metal wire of an alloy that contains another precious metal or several other precious metals in addition to platinum and rhodium. In the simplest and preferred case, the upstream and downstream catalyst mesh layers consist of only platinum and rhodium.

In a further preferred embodiment of the catalyst system, the catalyst mesh layers comprise an upstream group of mesh layers with one mesh layer or more mesh layers of a first rhodium-rich precious metal wire, and a downstream group of mesh layers of a second, rhodium-poor precious metal wire. rhodium, metal wire, wherein the mesh layer or one of the mesh layers of rhodium-rich wire forms the most forward, fresh gas-facing mesh layer, and wherein the rhodium content of the rhodium-rich precious metal wire is greater than or equal to 7 wt.% and does not exceed 9 wt.%, and at least 1 percentage point higher than the proportion of rhodium in rhodium-poor precious metal wire.

In this embodiment, the plurality of mesh layers of single- or multi-layer platinum-based catalyst meshes are divided into at least two groups. The front group contains at least one mesh layer of catalyst. The downstream group typically constitutes the largest volume and weight portion of the catalyst system and contains a large number of mesh layers of catalyst.

The lighter front ensemble catalyst mesh layers with the first, lower, average areal density can simultaneously form the rhodium-rich alloy front ensemble mesh layers; however, typically and preferably, these catalyst packing structures are different. That is, “upstream group of catalyst network layers” and “upstream ensemble of catalyst network layers” do not have to be equivalent, similarly, “downstream ensemble of catalyst network layers” does not have to be the same as “downstream ensemble of catalyst network layers”.

Typically, the downstream group constitutes the largest portion of the volume and weight of the catalyst system and contains a large number of mesh catalyst beds. The upstream group and the downstream group can be separated from each other by catalyst grids with a different composition. However, in the preferred case they are directly adjacent to each other. In the simplest case, the front group contains the most forward, when viewed in the direction of fresh gas flow, mesh catalyst layer. The catalyst meshes of both groups are woven, crocheted or knitted from precious metal wires that differ in their chemical composition. In the precious metal wire of the front group catalyst mesh layers, the rhodium content is greater than or equal to 7 wt.% and less than or equal to 9 wt.%, and it is at least 1 percentage point higher than the weight fraction of rhodium in the precious metal wire of the mesh layers. catalyst of the downstream group, which thus has a rhodium weight fraction of no more than 6%.

The proportion of rhodium in the rhodium-rich front group precious metal wire of at least 7 wt.% is higher than the standard content optimized for efficiency, service life and prevention of N ₂ O formation in industrial nitric acid production plants for medium-temperature operation. pressure, which is 5 wt.%. It is therefore surprising that by using rhodium-rich precious metal wire, higher catalytic efficiency can be achieved without negatively affecting service life and N ₂ O formation. However, this result is not achieved when all the catalyst mesh layers downstream in the direction of fresh gas flow are also formed from rhodium-rich precious metal wire, but only when the rhodium content decreases in the direction of fresh gas flow, and the mesh layers of the downstream group catalyst are formed from rhodium-poor metal wire, and the weight fraction of rhodium does not exceed 6 wt.%, a is preferably in the industry standard range of 5 wt%.

When alloys with rhodium contents below 7 wt.% are used in rhodium-rich precious metal wire, the increase in catalytic efficiency will be less. When the rhodium content is more than 9 wt.%, under unfavorable conditions of pressure and temperature, rhodium oxides can form, which leads to a slow decrease in catalytic efficiency over the life of the catalytic system.

Typically, the largest proportion of the total weight of the catalyst grids of the catalyst system, for example, at least 70%, is accounted for by the rhodium-poor precious metal wire catalyst grid layers, and sufficient that the rhodium-rich precious metal wire catalyst grid layers of the front group the proportion by weight was less, for example less than 30%, preferably less than 25% and especially preferably less than 20%. It justifies itself, for example, if the front group contains no more than three mesh layers.

In a preferred embodiment, the weight reduction of the catalytic system is no more than 25%.

With regard to high catalytic efficiency with minimal or no negative impact on service life and N ₂ O formation, it has been found to be beneficial if the rhodium content of the rhodium-poor precious metal wire is between 4 and 6 wt.%, in particular if the rhodium content of the rhodium-rich precious metal wire The rhodium content of the precious metal wire is from 7.8 to 8.2 wt.%, and the rhodium content of the rhodium-poor precious metal wire is from 4.8 to 5.2 wt.%.

The front group of mesh layers has a fresh gas-facing frontmost mesh layer which is formed from rhodium-rich precious metal wire. This frontmost mesh layer may, for example, be part of a multi-layer catalyst mesh, or the frontmost mesh layer is formed as a single layer of a single-layer catalyst mesh. Surprisingly, it turned out that to increase the catalytic efficiency it is sufficient if the front group of mesh layers is formed exclusively by the frontmost mesh layer. This represents a particularly simple and therefore preferred embodiment of the catalytic system.

In this regard, it has also proven to be advantageous if the outermost mesh layer, for example in the form of a single layer or several layers of a multilayer catalyst mesh, is laid on the mesh layers of the downstream group. In this case, the flow reactor containing the catalyst packing is in a vertical position, and the flow direction is vertical and oriented from top to bottom.

Friction and gravity are sufficient to secure the frontmost mesh layer to the catalyst nozzle. This simplifies the production of the catalyst system and the upgrading of existing catalyst systems to obtain the catalyst system of the present invention.

As regards the method, the above technical problem is solved by means of a method of the type mentioned in the introduction in that fresh gas with an ammonia content of 9.5 to 12 vol.% is under pressure in the range of 3.5 to 7 bar and a catalyst grid temperature in the range of 870°C to 920°C is carried out at a flow rate in the range from 6 to 16 tN/m ² ⋅day through the catalytic system according to the present invention.

In this case, the abbreviation “tN/m ² ⋅day” means “tons of nitrogen (from ammonia) per day per effective cross-sectional area in square meters. The method is designed to operate in the range of average pressures from 3.5 to 7 bar. With the proportion of ammonia in fresh gas less than 9.5 vol.% and a flow rate of less than 6 tN/m ² ⋅day, the combustion process can spontaneously stop. The NH ₃ content in fresh gas is more than 12 vol.% close to the safety threshold for an explosive mixture. At a catalyst grid temperature below 870° Increased formation of rhodium oxide may occur at C, and evaporation of platinum oxide is enhanced at catalyst grid temperatures above 920° C. The temperature of the catalyst grid is affected by the fresh gas preheating temperature, which preferably lies in the range from 150° C to 220° C.

Definitions

Precious metal wire

By wire containing precious metal is meant wire consisting of precious metal or containing a significant proportion (>50 wt.%) of precious metal. The platinum-based alloy contains more than 50 wt.% platinum. Other alloy components include palladium, rhodium and ruthenium. Typical precious metal wire diameters are 40-120 microns.

Medium pressure units

In medium pressure units, ammonia oxidation occurs at pressures ranging from 3.5 to 7 bar. In this pressure range, standard precious metal catalysts are metal wires of the binary alloy PtRh5 with a diameter of 76 μm and a surface density of approximately 600 g/m ² .

Catalyst mesh

A single or multi-layer flat textile structure woven, crocheted or knitted from wire or multiple wires of precious metal. The formation of a textile flat structure is achieved by interweaving one or more thread systems or wire systems in the form of a mesh.

Catalytic system

The catalytic system includes a catalyst packing and typically a mesh screen or multiple mesh screens, which may also be woven or crocheted or knitted from precious metal wire.

Catalyst nozzle

A configuration of several catalyst grids located one behind the other in the direction of fresh gas flow.

Implementation example

The invention is further illustrated by examples of implementation and drawings. Shown:

figure 1: diagram of a flow reactor for heterogeneous catalytic combustion of ammonia, figure 2: a bar graph showing the catalytic efficiency results of the test reactors compared to the reference reactor, and figure 3: bar graph showing _N2O production results in test reactors compared to the reference reactor.

Figure 1 schematically shows a vertically installed flow reactor 1 for heterogeneous catalytic combustion of ammonia. The catalytic system 2 forms the actual reaction zone of the flow reactor 1. It contains a catalyst packing 3 and downstream mesh grids 4. The catalyst packing 3 contains several single-layer catalyst grids 6 located one behind the other in the direction 5 of the fresh gas flow, on which can be laid additional catalyst mesh 7 (or several mesh layers of catalyst), which may be a component of the catalyst packing. Embodiments are detailed in Tables 1-5. The effective diameter of the catalyst mesh is 100 mm.

In a preferred embodiment, the front group of the catalytic system contains a maximum of three mesh layers (7).

Fresh gas is a mixture of air and ammonia with a nominal ammonia content of 10.7 vol%. It is heated to a preheating temperature of 175°C and introduced into reactor 1 from above at an elevated pressure of 5 bar. Upon entering the catalyst nozzle 3, the gas mixture ignites, and then an exothermic combustion reaction occurs, which covers the entire catalyst nozzle 3. In this case, the following main reaction takes place:

$${\text{4NH}}_{\text{3}}+{\text{5O}}_{\text{2}}\stackrel{\text{Pt/Rh}}{\to }\text{4NO}+{\text{6H}}_{\text{2}}\text{O}$$

In this case, ammonia (NH ₃ ) is converted into nitrogen monoxide (NO) and water (H ₂ O). The resulting nitrogen monoxide (NO) reacts in the flowing reaction gas mixture (symbolically indicated by arrow 8, indicating the direction of flow of the flowing gas mixture) with excess oxygen to form nitrogen dioxide (NO ₂ ), which reacts with water in a downstream absorption unit to form nitric acid ( _HNO3 ).

Catalyst meshes 6, 7 are textile flat formations, which are machine-knitted from metal wire with a diameter of 76 microns from a binary platinum-rhodium alloy. In flow reactor 1, the catalytic systems defined in tables 1-5 were tested.

In most test reactors, the catalyst pack contains five single-layer catalyst grids 6; in one test reactor, the catalyst pack contains an additionally laid catalyst mesh 7. The catalyst meshes are connected from metal wire made of a binary alloy of precious metals PtRh. The designation sequence in Tables 1-5 indicates the location in the direction of fresh gas flow. In addition, all reactors are equipped with grid meshes 4, which consist of six active drainage mesh layers (“grid meshes”) made of Pd82.5Pt15Rh2.5 alloy. The test reactors differ from each other in the composition of the frontmost (upper) mesh catalyst layer 7 and/or the surface density of the mesh catalyst layers.

In this case, the reference reactor according to Table 1 is a reactor that meets the modern industry standard for medium pressure installations. Single-layer catalyst meshes are formed from precious metal wires with a wire diameter of 76 microns. The surface density of the PtRh5 catalyst networks used is, as indicated in the column "Area density per layer", 600 g/m ² . Thus, the sum of the surface densities of all layers L1-L5 of the catalyst packing is 3000 g/m ² .

Table 1: Reference reactor

mesh layer a precious metal surface density per layer [g/ ^m2 ] L1 PtRh5 600 L2 PtRh5 600 L3 PtRh5 600 L4 PtRh5 600 L5 PtRh5 600 Σ: 3000

The following Tables 2-5 provide data for test reactors R1-R4. In the "assembly membership" column, the number "1" refers to the assignment of the corresponding network catalyst layer to the upstream ensemble (hereinafter also referred to as "ensemble 1"), and the number "2" indicates the assignment of the corresponding network catalyst layer or layers to the downstream ensemble (hereinafter also referred to as "ensemble 2"). In all test reactors R1-R4, the catalyst mesh layers L1-L3 in the context of the invention belong to the "front ensemble"; this is additionally highlighted with gray shading.

The column "Average surface density per ensemble" (in g/m ² ) indicates the quotient of the sum of the individual surface densities of catalyst networks divided by the number of catalyst networks in the corresponding ensemble; it is also briefly called here "average surface density". The surface densities are the nominal initial surface densities that the standard can achieve with precious metal wires with a wire diameter of 76 µm.

The last column in the tables shows the difference between the average surface density of ensemble 1 and the average surface density of ensemble 2 in % (the percentage data is indicated in relation to the second average surface density). Thus, these data characterize the saving of precious metal in the corresponding test reactor compared to a reactor in which the catalyst pack consists entirely of mesh layers of second area density catalyst.

Table 2. Test reactor R1

In test reactor R1, the average areal density of the front ensemble is 481 g/m ² , which is approximately 20% less. than the average surface density of 600 g/m ² layers L4 and L5, which form the “downstream ensemble” of the catalyst packing.

Table 3. Test reactor R2

In test reactor R2, ensemble 1 is also formed from the upper mesh layers of catalyst L1-L3. Their nominal initial average areal weight is 500 g/m ² , which is approximately 7% less than the average areal weight of ensemble 2, which is 540 g/m ² .

Table 4. Test reactor R3

In test reactor R3, the front ensemble (1) is also formed by mesh catalyst layers L1-L3. Their areal weight is 421 g/ ^m2 each, which is approximately 30% less than the standard areal weight of 600 g/ ^m2 (with a precious metal wire diameter of 76 microns).

Table 5. Test reactor R4

In test reactor R4, the top mesh layer of catalyst L1 consists of a PtRh8 alloy and has a surface density of 600 g/m ² . The two immediately adjacent mesh catalyst layers are composed of PtRh5 alloy and have a knitted weave, which results in a comparatively lower basis weight of 421 g/m ² . These three layers form ensemble 1. At the last mesh catalyst layers L4-L6 of the catalyst packing form ensemble 2 and also consist of a PtRh5 alloy with a surface density of 600 g/m ² .

Here, too, the front ensemble is formed by mesh catalyst layers L1-L3. Their average surface density (481 g/ ^m2 ) is approximately 20% lower than the average surface density of ensemble 2, that is, layers L4-L6.

The frontmost layer L1 is laid on the remaining catalyst packing (position 2 in Fig. 1). It forms, in flow direction 5, a front catalyst mesh (position 7 in FIG. 1) of a metal wire rich in the precious metal rhodium and thus a “front group of catalyst mesh layers” in the context of a preferred embodiment of the invention. In this case, the remaining mesh catalyst layers L2-L6 of comparatively rhodium-poor metal wire form a “downstream group of mesh catalyst layers” in the context of this embodiment of the invention.

The test reactors were operated under the following identical test conditions:

pressure: 5 bar (abs) performance: 12 tons of nitrogen (from ammonia) per day per unit of effective cross-sectional area of the catalyst packing in square meters (abbreviated as 12 tN/m ² ⋅day) NH ₃ content: 10.7 vol.% in fresh gas heating temperature: 175°C (NH ₃ /air mixture temperature), resulting in a grid temperature in the test reactors of 890°C

The change in catalytic efficiency as NO yield and the proportion of N ₂ O by-product produced was measured at intervals of approximately 24 hours. Five measurements were obtained for each test reactor R1-R4.

Catalytic efficiency (i.e., NO product yield) was measured as follows:

1. It was first checked that the catalytic system was suitable for complete conversion of the ammonia used and that NH ₃ was no longer present in the product gas in any appreciable amount. This verification was carried out by mass spectrometric analysis of the product gas.

2. Simultaneous sampling of the NH ₃ /air mixture in front of the catalyst nozzle and a sample of the product gas after it into separate pumped out flasks. The gas mass was determined by weighing.

3. A mixture of NH ₃ with air was absorbed with distilled water and titrated with 0.1 N sulfuric acid and methyl red in accordance with the color change.

4. The resulting nitrous gases were absorbed with a 3% sodium peroxide solution and titrated with 0.1 N sodium hydroxide solution and methyl red in accordance with the color change.

5. The catalytic efficiency Eta was calculated from the expression: Eta = 100xCn/Ca, where Ca means the average NH ₃ concentration from 7 separate measurements in the fresh gas in weight percent, and Cn means the average NOx concentration from 7 separate measurements, expressed in weight percent NH ₃ , which is oxidized to NOx.

6. Separately, the volume fraction of N ₂ O in the product gas was determined using gas chromatography.

The measurement results are shown in Table 6. In the columns of Table 6 labeled "NO-NO _Ref ", the difference in nitrogen monoxide yield compared to the reference reactor is given in absolute percentage points (for example, measurement No. 1 for reactor R1 gives a NO yield of 95.2 % and thus for a measured value of 95.4% for the reference reactor a NO-NO _Ref difference of -0.2 percentage points is obtained). In the columns labeled "N ₂ O–N ₂ O _Ref ", the difference in nitrous oxide yield compared to the reference reactor is given in parts per million by volume.

Table 6

No. reference reactor reactor 1 reactor 2 reactor 3 reactor 4 output NO
about.% _N2O
v.h./million NO-NO _Ref [%_abs.] N ₂ ON ₂ O _Ref [ppm] NO-NO _Ref [%_abs] N ₂ ON ₂ O _Ref [ppm] NO-NO _Ref [%_abs] N ₂ ON ₂ O _Ref [ppm] NO-NO _Ref [%_abs] N ₂ ON ₂ O _Ref [ppm] 1 95.4 868 -0.2 7 2 95.3 835 0.0 24 3 95.2 745 -0.1 34 4 95.0 895 0.0 42 5 95.1 886 0.2 50 1 95.4 845 -0.1 -38 2 95.3 800 0 -29 3 95.2 730 0 -10 4 95.2 802 -0.2 44 5 95.1 807 0.1 47 1 95.4 845 -0.7 51 2 95.3 729 -0.6 68 3 95.2 730 -0.5 77 4 95.1 807 -0.6 151 5 95.2 843 -0.7 143 1 95.2 870 0.3 -18 2 95.3 834 0.2 -eleven 3 95.3 867 0.3 7 4 95.4 899 0.1 18 5 95.2 945 0.3 -12

Measurement results

The measurement results shown in Table 6 are presented graphically in the form of diagrams in Figures 2 and 3 and are explained further with reference to these figures.

Diagram in Fig. 2 shows for reactors R1-R4 a measure of catalytic efficiency for a nitrogen productivity of 12 N/m ² ⋅day. The y-axis shows the difference in nitrogen monoxide yield compared to the reference reactor "NO-NO _Ref " in absolute percentage points (%_abs.). On the x-axis, numbers from 1 to 5 indicate the serial number of the corresponding measurement.

Accordingly, the two reactors R1 and R2 have NO conversion efficiencies that are comparable, within measurement error, to the output of the industry standard (reference reactor). The measurement error is about ±0.3 percentage points, as shown by the dotted line.

However, in the case of reactor R3, the yield of the main NO product is not comparable to the industry standard. This is explained by the large difference (30%) between the mesh layers of the upstream ensemble with mesh catalyst layers L1-L3 and the downstream ensemble with mesh catalyst layers L4 and L5. Thus, reactor R3 represents a comparative example for the invention.

Reactor R4, despite using less precious metals, has a catalytic efficiency that is comparable within error to that of the reference reactor. The measurement error is approximately ±0.3 percentage points, as shown by the dotted line. However, since the first layer has a higher surface density than the lower layers of the first ensemble, there is no noticeable increase in efficiency, as in reactor 1.

Diagram in Fig. 3 shows the results of N ₂ O production measurements for test reactors R1-R4. The y-axis represents the difference, in ppm, in the amount of nitrous oxide (N ₂ ON ₂ O _Ref ) in the product gas compared to the reference reactor. On the x-axis, numbers from 1 to 5 again indicate the serial number of the corresponding dimension.

Accordingly, test reactors R1, R2 and R4 produce amounts of N ₂ O in the range of the reference reactor. The standard measurement error is approximately ±50 ppm and is also indicated by dotted lines.

However, in the case of reactor R3, the increase in N ₂ O production exceeds the measurement error. Therefore, from the point of view of reducing N ₂ O formation, reactor R3 is also unsuitable.

Claims (14)

1. Catalytic system for the catalytic combustion of ammonia to nitrogen oxides in a medium-pressure unit with mesh layers of catalyst (6; 7), woven or knitted or crocheted from wire based on the precious metal platinum, which, located one after another in the direction (5 ) flow of fresh gas, form a catalyst packing (4), characterized in that the catalyst packing consists of a front ensemble with three catalyst meshes having a first average surface density, and a downstream ensemble of catalyst mesh layers located behind the front ensemble, having a second average surface density density, and the decrease in the average surface density of the front ensemble relative to the second average surface density ranges from 1.5% to 29%, and the fact that the first average surface density lies in the range from 410 to 530 g/m ² , and the second average surface density in range from 540 to 790 g/ ^m2 . 2. Catalytic system according to claim 1, characterized in that the weight reduction is no more than 25%. 3. Catalytic system according to claim 1 or 2, characterized in that the first average surface density lies in the range from 415 to 510 g/ ^m2 , and the second average surface density in the range from 575 to 710 g/ ^m2 . 4. The catalytic system according to one of the previous claims, characterized in that the mesh layers of the upstream and downstream ensemble catalysts are made of precious metal wire having the same thickness, and in that the mesh layers of the upstream ensemble catalyst consist of a crocheted knitted structure with a first knitted weave and a first stitch size, or a woven structure with a first weave and a first stitch size, or a knitted knitted structure with a first knitted weave and a first stitch size, and the mesh catalyst layers of the lower ensemble consist of a crocheted knitted structure with a second knit weave and a second loop size, or a woven structure with a second weave and a second loop size, or a knitted knit structure with a second knit weave and a second loop size, wherein the first loop size is larger than the second loop size. 5. Catalytic system according to one of the previous paragraphs, characterized in that the surface density of the mesh layers of the front ensemble catalyst is constant or consistently increases in the direction of fresh gas flow. 6. Catalytic system according to one of the previous paragraphs, characterized in that the mesh layers of the catalyst of the upstream and downstream ensembles consist of platinum and rhodium. 7. Catalytic system according to one of the previous claims, characterized in that the mesh layers of the catalyst (6; 7) contain a front group (7) mesh layers with one mesh layer or several mesh layers of a first rhodium-rich precious metal wire, and located behind the front group, a downstream group (6) of mesh layers made of a second rhodium-poor metal wire, wherein the mesh layer (7) or one of the mesh layers of rhodium-rich metal wire forms the front, fresh gas-facing mesh layer (7), and wherein the rhodium content of the rhodium-rich precious metal wire is greater than or equal to 7 wt.% and does not exceed 9 wt.%, and is at least 1 percentage point higher than the rhodium content of rhodium-poor precious metal wire. 8. Catalytic system according to claim 7, characterized in that the proportion of rhodium in the rhodium-poor precious metal wire is from 4 to 6 wt.%. 9. Catalytic system according to claim 7 or 8, characterized in that the proportion of rhodium in the rhodium-rich precious metal wire is from 7.8 to 8.2 wt.%, and the proportion of rhodium in the rhodium-poor precious metal wire is from 4 .8 to 5.2 wt.%. 10. Catalytic system according to one of paragraphs. 7-9, characterized in that the weight fraction of the front group of mesh layers (7) is less than 30%, preferably less than 25% and especially preferably less than 20% of the total weight of the catalyst packing (4). 11. Catalytic system according to one of paragraphs. 7-10, characterized in that the front group contains a maximum of three mesh layers (7). 12. Catalytic system according to one of paragraphs. 7-11, characterized in that the front group of mesh layers (7) is formed from the most anterior mesh layer (7). 13. Catalytic system according to claim 12, characterized in that the frontmost mesh layer (7) is laid on the mesh layers (6) of the downstream group. 14. A method for the catalytic combustion of ammonia to nitrogen oxides in a medium pressure unit by passing fresh gas containing ammonia and oxygen through the catalytic system (2), wherein the ammonia is burned, the fresh gas having an ammonia content of 9.5 to 12 vol.% under pressure in the range from 3.5 to 7 bar and catalyst grid temperature in the range from 870°C to 920°C is carried out with a flow rate in the range from 6 to 16 tN/m ² ⋅day through the catalytic system (2) according to one of the paragraphs . 1-13.