RU2808515C2 - 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 in a medium pressure unit and to a method for the catalytic combustion of ammonia to nitrogen oxides in a medium pressure unit. A catalytic system (1) is described for the catalytic combustion of ammonia to nitrogen oxides in a medium-pressure unit, with single- or multi-layer catalyst meshes (6; 7), woven or knitted or crocheted from metal wire based on the precious metal platinum, which, located one after another in the fresh gas flow direction (5), form a front group of mesh layers (7) and at least one downstream group of mesh layers (6) located behind the front group, wherein the front group contains at least one mesh layer (7) from the first rhodium-rich precious metal wire, wherein said at least one mesh layer (7) of rhodium-rich precious metal wire is the most forward mesh layer (7) facing the fresh gas, wherein the the downstream group contains mesh layers (6) of a second rhodium-poor precious metal wire, wherein the rhodium-rich precious metal wire and the rhodium-poor precious metal wire are composed primarily of platinum and rhodium, and the rhodium content of the rhodium-rich precious metal wire metal is greater than or equal to 7 wt.% and less than or equal to 9 wt.%, and the rhodium content of the rhodium-poor precious metal wire is in the range of 4 to 6 wt.%. 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 (2), wherein the ammonia is burned, the fresh gas having an ammonia content of 9.5 to 12% by volume. 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/m²⋅ days through the above-described catalytic system (2).

EFFECT: provision of a catalytic system for use in a medium pressure installation, with which it is possible to achieve a high service life and a high yield of the main product NO.

12 cl, 6 tbl, 3 dwg

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 comprising a plurality of single or multi-layer catalyst meshes woven or knitted or crocheted from platinum-based precious metal wires which are arranged one after the other in the direction of fresh gas flow, form a forward group of mesh layers and at least one, located behind the front group, a downstream group of mesh layers.

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 under conditions of heterogeneous catalysis with precious metals, producing nitric 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 single or multi-layer structures woven or crocheted or knitted from 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).

From DE 10105624 A1 a flow reactor for the catalytic oxidation of ammonia is known, in which the catalytic system is formed as a packing of several catalyst networks arranged parallel to each other. The nozzle is placed in the reaction zone in such a way that the planes formed by the catalyst grids are perpendicular to the direction of flow of the medium containing the starting materials to be converted. To increase the catalytic activity and selectivity of the catalytic reaction, as well as to improve the mechanical strength and service life of the catalyst packing, it is proposed to use bulk knitted catalyst meshes, in which 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.

In EP 3056267 A1, to reduce the formation of N ₂ O during the oxidation of ammonia at medium pressure, a modified precious metal catalyst mesh is proposed, in which a second wire of precious metal (Pd or a Pd-rich alloy) is inserted into the mesh layer for reinforcement. precious metal with a different composition, which is obtained, for example, from a PtRh alloy with 1-10% Rh. To determine the catalytic properties of the modified catalyst mesh, it is introduced into a test reactor along with other industry standard catalyst meshes, and N ₂ O production is determined as a function of time at a pressure of 5 bar and a temperature of 890°C. In this case, a crocheted or knitted catalyst mesh made of metal wire made of PtRh5 alloy with a diameter of 76 microns and a surface density of 600 g/m ² is used as a standard.

A catalytic system known from WO 01/87771 A1 for reducing the formation of nitrous oxide, which is formed during the catalytic combustion of ammonia and oxygen to nitrogen oxides, comprises, when viewed in the direction of fresh gas flow, at least one first catalyst network and at least one second catalyst mesh. Catalyst meshes have been woven, crocheted or knitted, or produced by laying nonwoven fabric. The first catalyst grid consists of a PtRh alloy with 5-10 wt.% rhodium, and the second catalyst grid consists of a PdRh alloy with at least 92 wt.% palladium and 2-4 wt.% rhodium, the rest platinum. Alternatively, the second catalyst network consists of 82-83 wt.% palladium and 2.5-3.5 wt.% rhodium, the balance platinum. In another embodiment, the front catalyst grid is a PtRh alloy with 8 wt% rhodium and the rear catalyst grid is a PdRhPt alloy with 3 wt% rhodium, 15 wt% platinum, the balance palladium. The _N2O production in reactors equipped with this catalyst system is compared to a commercial reactor which operates at a reactor pressure of 4.8 bar and a temperature of 880°C and is configured with two standard catalyst systems, one of which is a mesh catalyst pack. PtRh5 and PdNi5, and the other catalyst packing made of PtRh5 and PdNi5 grids.

WO 2002/062466 A2 describes a multilayer catalyst system with interconnected network layers that differ in their surface density, but which are all composed of a PtRh8 or PtRh5 alloy.

DE 2145842 A1 mentions that after using the catalytic system, a loss of palladium was determined which was high compared to the loss of platinum. This is explained by the fact that the catalyst grids on the gas outlet side again absorb platinum, carried away with the gas flow.

Statement of technical problem

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.

In installations for the production of nitric acid, depending on the type of design, from 2 to 30 catalyst grids with a diameter of up to 6 m are used. Due to oxidation and sublimation, catalyst grids lose precious metal in the process of ammonia oxidation, so that from time to time they are lost (resource, service life ) must be replaced, which entails certain costs. The use of precious metals is expensive and should therefore be kept to the lowest possible level. On the other hand, "catalytic efficiency", which is an important parameter and measure for long-term high feed conversions and good yields, generally increases with increasing proportion of precious metals.

A suitable compromise in terms of service life, catalytic efficiency and use of precious metals has proven to be PtRh5 alloy, which, as mentioned in EP 3056267 A1 and WO 01/87771 A1, has established itself as the industry standard for precious metal catalysts for use in medium-volume applications. pressure.

The aim of the invention is to provide a catalyst system for use in a medium pressure ammonia oxidation plant, with which it is possible to achieve a higher yield of the main product NO compared to the industry standard and thus higher catalytic efficiency without reducing the service life (loss of precious metals ) and without increasing the formation of the unwanted by-product N2O.

In addition, the basis of the invention is to develop a method for the oxidation of ammonia in a medium pressure plant, which allows to achieve a relatively high yield of the product, NO, without negatively affecting the service life and the formation of N ₂ O.

The essence of the invention

As regards 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 front group contains at least one mesh layer of a first rhodium-rich precious metal wire, said at least one, mesh layer of rhodium-rich precious metal wire is the most upstream mesh layer facing the fresh gas, and that the downstream group contains mesh layers of a second, rhodium-poor precious metal wire, the rhodium content of the rhodium-rich wire precious metal wire is greater than or equal to 7 wt.% and less than or equal to 9 wt.% and at least 1 percentage point higher, the rhodium content of the rhodium-poor precious metal wire.

The catalyst system comprises a catalyst pack that consists of an upstream group of platinum-based catalyst mesh beds and a downstream group of mesh catalyst beds, and optionally mesh screens, 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 group contains at least one mesh layer of catalyst. The downstream group typically 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 of different compositions. However, in the preferred case they are directly adjacent to each other.

The front group contains the most forward mesh catalyst layer in the direction of fresh gas flow, which consists of rhodium-rich precious metal wire.

The catalyst meshes of both groups are woven, crocheted or knitted from precious metal wires that differ in their composition. In rhodium-rich precious metal wire, the front group has a rhodium content of at least 7 wt.% and no more than 9 wt.%, and this is at least 1 percentage point higher than the rhodium weight fraction in rhodium-poor precious metal mesh wire layers of catalyst in the downstream group (thus, the weight for rhodium in it is no more than 6%).

The rhodium content of 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 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 packing in the catalytic system, for example, at least 70%, is accounted for by the rhodium-poor precious metal wire catalyst grid layers, and enough that the front group grid layers of rhodium-rich precious metal wire the precious metal comprised a smaller proportion by weight, for example less than 30%, preferably less than 25% and particularly preferably less than 20%. It justifies itself, for example, if the front group contains no more than three mesh layers.

With regard to high catalytic efficiency while simultaneously having minimal or no negative impact on service life and _N2O formation, it has proven 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 precious metal wire is 4 to 6 wt.%. the rhodium-rich precious metal wire is 7.8 to 8.2 wt.%, and the rhodium content of the rhodium-poor precious metal wire is 4.8 to 5.2 wt.%.

The catalyst system may comprise catalyst grids of metal wire based on another precious metal, 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. However, it is preferred that both the rhodium-poor precious metal wire and the rhodium-rich precious metal wire are composed of a PtRh binary alloy.

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 variant 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.

In this case, the superimposed frontmost mesh layer forms part of the catalyst packing. Friction and gravity are sufficient to fix the anterior mesh layer. This simplifies the production of the catalyst system and the upgrading of existing catalyst systems to obtain the catalyst system of the present invention.

In addition, an embodiment has proven to be advantageous in which catalyst grids arranged one behind the other in the direction of fresh gas flow form a catalyst packing of an upstream ensemble with three catalyst grids having a first average surface density and a downstream ensemble located behind the front ensemble mesh catalyst layers having a second average surface density, and the average surface density of the front ensemble is 1.5-29% less than the second average surface density, and 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 front ensemble combines the first three, when looking in the direction of fresh gas flow, a layer of catalyst networks. It borders the downstream ensemble, which also contains several mesh layers of catalyst.

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.

The catalyst meshes combined in the upstream ensemble have a lower average surface density than the catalyst meshes of the downstream ensemble. This reduces the amount of precious metal used by between 1.5% and a maximum of 29%, preferably by a maximum of 25% (compared to the second average areal density) and, in addition, there is less space filling at the front of the length of the catalyst packing, which is accompanied by less use of precious metal and at the same time results in less wastage of precious metal.

It has been shown that reducing the space filling provides good flow through the catalyst packing while maintaining high mechanical stability, and that reducing the amount of precious metal used does not lead to a significant reduction in the yield of nitrogen oxides when the front ensemble is limited to a small number of lighter mesh catalyst layers.

Generally speaking, one would expect that the saving of precious metal would also be accompanied by a noticeable 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 areal 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 areal density or average areal density can be caused by the thickness of the precious metal wire. However, it has turned out that it is particularly advantageous if the mesh layers of the upstream and downstream catalyst ensembles are made of precious metal wire with the same or approximately the same thickness, and the difference in surface density is provided mainly by the different weaving 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 downstream catalyst mesh layers 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. loop size, the first loop size being 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 the average weight per unit area of the downstream assembly catalyst mesh layers. The mesh layers of the downstream catalyst 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 have a surface density 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.

Lighter "front ensemble" catalyst meshes with a first, lower average areal density can simultaneously form "front ensemble" mesh layers of rhodium-rich alloy; however, typically, and preferably, these catalyst packing elements are different from each other.

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

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 with 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 precious metal wire is meant a wire consisting of a precious metal or containing a significant proportion (> 50 wt.%) of a precious metal. The platinum-based alloy contains more than 50 wt.% platinum. Rhodium-rich precious metal wire and rhodium-poor precious metal wire are composed primarily of platinum and rhodium. 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, catalysts based on precious metals made of metal wire of the binary alloy PtRh5 with a diameter of 76 μm and a surface density of approximately 600 g/m ² are accepted as the standard.

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 of fresh gas flow 5 from the first, relatively “poor” rhodium" PtRh wire made of precious metal, on which the following single-layer catalyst grid 7 is laid from a second, relatively "rhodium-rich" PtRh wire made of precious metal. The most upstream catalyst mesh 7 in the flow direction 5 forms a single layer of the "upstream group of catalyst mesh layers" and the remaining catalyst meshes 6 form the "downstream group of mesh catalyst layers" in the context of the invention. Examples of rhodium-rich and rhodium-poor precious metal wire compositions and the effectiveness of these and similar catalyst mesh systems are explained in more detail below. Embodiments are further defined in Tables 1-4.

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:

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 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 various binary platinum-rhodium alloys. In flow reactor 1, the catalytic systems defined in tables 1-4 were tested.

In the test reactors, the catalyst pack contains six single-layer catalyst grids 6,7 which are connected by metal wire spokes made of a binary PtRh alloy. The designation sequence in the tables 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.

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-L6 of the catalyst packing is 3600 g/m ² . 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.

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 L6 PtRh5 600 Ʃ˸3600

The following Tables 2-4 provide data for test reactors R1-R3. In the “assembly membership” column, the number “1” refers to the assignment of the corresponding network catalyst layer to the upstream group (hereinafter also called “group 1”), and the number “2” indicates the assignment of the corresponding network catalyst layers to the downstream group (hereinafter called also "group 2"). In all test reactors R1-R3, only the forwardmost mesh layer of catalyst L1 forms the "front group" in the context of the invention; this group is additionally highlighted with gray shading.

Table 2 . Test reactor R1

mesh layer a precious metal belonging to the ensemble on top density per layer
[g/ ^m2 ] L1 PtRh8 1 600 L2 PtRh5 2 600 L3 PtRh5 2 600 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Ʃ˸3600

In test reactor R1, the top mesh catalyst layer consists of a PtRh8 alloy, and the remaining catalyst meshes consist of a conventional PtRh5 alloy, as in the reference reactor.

Table 3 . Test reactor R2

mesh layer a precious metal belonging to the ensemble on top density per layer
[g/ ^m2 ] L1 PtRh10 1 600 L2 PtRh5 2 600 L3 PtRh5 2 600 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Ʃ˸3600

In test reactor R2, the top mesh layer of the catalyst consists of a PtRh10 alloy, and the remaining catalyst meshes also consist of the usual PtRh5 alloy, as in the reference reactor.

Table 4 . Test reactor R3

mesh layer a precious metal belonging to the ensemble on top density per layer
[g/ ^m2 ] L1 PtRh8 1 600 L2 PtRh5 2 421 L3 PtRh5 2 421 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Ʃ˸3242

In test reactor R3, the uppermost mesh catalyst layer 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 with a larger loop size, resulting in a comparatively low basis weight of 421 g/m ² . The last two layers of the catalyst mesh of the catalyst packing also consist of a PtRh5 alloy with a surface density of 600 g/m ² . The amount of precious metals used in reactor R3 is 358 g/m ² less than in the reference reactor and test reactor R1.

The mesh layers L1-L3 form the front ensemble in the context of a preferred embodiment of the invention, wherein the precious metal savings compared to a standard reactor result from the fact that the front ensemble catalyst meshes have a precious metal content that is within narrow limits lower than that of the bottom catalyst meshes. along the flow of the ensemble. In an exemplary embodiment, the average weight of the upstream assembly is 481 g/m ² , which is approximately 20% lower than the average weight of the downstream assembly with catalyst mesh layers L4-L6.

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 measurement results were obtained for each test reactor R1-R4.

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

1. It was first verified that the service life of the catalyst system was comparable to that of the reference reactor and that the catalyst system was suitable for complete conversion of the ammonia used. This means that NH ₃ is no longer present in the product gas in any appreciable amount, as verified 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 concentration of NH ₃ from 7 separate measurements in fresh gas in weight percent, and Cn means the average NOx concentration from 7 separate measurements, expressed in weight percent of 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 summarized in Table 5. The serial number of the measurement indicated in column 1 approximately corresponds to the duration of operation of the catalytic system in days. In the columns of Table 5 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 96.2% and thus for a measured value of 95.3% for the reference reactor results in a NO-NO _Ref difference of +0.9 percentage points). In the columns labeled "N ₂ ON 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 output NO
about.% N ₂ O
v.h./million NO-NO _Ref [%_abs.] N ₂ O-N ₂ O _Ref [h/million] NO-NO _Ref [%_abs] N ₂ O-N ₂ O _Ref [h/million] NO-NO _Ref [%_abs] N ₂ O-N ₂ O _Ref [h/million] 1 95.3 868 0.9 -37 2 95.4 867 0.6 -23 3 95.3 889 0.7 -49 4 95.6 899 0.5 -16 5 95.4 936 0.8 -61 1 95.4 840 0.1 -33 2 95.3 830 0.0 -59 3 95.2 730 0.2 -10 4 95.2 866 0.2 -20 5 95.1 860 0.2 -6 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 5 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-R3 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, reactor R1 has a markedly higher conversion efficiency to NO compared to the industry standard reference reactor. The efficiency increase ranges from 0.6%, which for a typical amount of ammonia used in an industrial reactor with a capacity of about 12 tN/m ² day means an additional mass of 154 kg NO/m ² day.

Reactor R3, 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.

Reactor R2 demonstrates the yield of the main product NO, which, taking into account the measurement error, does not exceed the yield of the reference reactor. This effect for reactor R2 can only be explained by the very high rhodium content in the front layer of the catalyst grid L1. Thus, reactor R2 does not demonstrate improved catalytic efficiency and therefore represents a comparative example for the invention.

Diagram in Fig. 3 shows the results of measurements of N ₂ O production in test reactors R1-R3. The y-axis represents the difference (N ₂ ON ₂ O _Ref ), in ppm, in the amount of nitrous oxide 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, in all test reactors R1-R3, the amount of N ₂ O produced, taking into account the measurement error, is comparable to that in the reference reactor. The standard measurement error is approximately ±50 ppm and is also indicated by the dotted line.

Claims (12)

1. Catalytic system (1) for the catalytic combustion of ammonia to nitrogen oxides in a medium-pressure unit with single- or multi-layer catalyst meshes (6; 7), woven or knitted or crocheted from metal wire based on the precious metal platinum, which, located one after another in the direction (5) of fresh gas flow, form a front group of mesh layers (7) and at least one downstream group of mesh layers (6) located behind the front group, characterized in that the front group contains at least one mesh layer (7) of the first rhodium-rich precious metal wire, wherein said at least one mesh layer (7) of rhodium-rich precious metal wire is the most forward fresh gas-facing mesh layer (7) , and in that the downstream group comprises mesh layers (6) of a second rhodium-poor precious metal wire, the rhodium-rich precious metal wire and the rhodium-poor precious metal wire consisting primarily of platinum and rhodium, and the rhodium content of the rhodium-rich precious metal wire is greater than or equal to 7 wt% and less than or equal to 9 wt%, and the rhodium content of the rhodium-poor precious metal wire is in the range of 4 to 6 wt%. 2. The catalytic system according to claim 1, characterized in that the rhodium content in the rhodium-rich precious metal wire is from 7.8 to 8.2 wt.%, and the rhodium content in the rhodium-poor precious metal wire is from 4.8 up to 5.2 wt.%. 3. The catalytic system according to one of the previous paragraphs, characterized in that the rhodium-rich precious metal wire and the rhodium-poor precious metal wire consist of platinum and rhodium. 4. Catalytic system according to one of the previous claims, characterized in that the front group of mesh layers (6) constitutes less than 30%, preferably less than 25% and especially preferably less than 20% of the total weight of all catalyst mesh layers of the catalyst pack (3). 5. Catalytic system according to one of the previous paragraphs, characterized in that the front group contains a maximum of three mesh layers (7). 6. Catalytic system according to one of the previous paragraphs, characterized in that the front group of mesh layers (7) is formed from the frontmost mesh layer (7). 7. Catalytic system according to claim 6, characterized in that the frontmost mesh layer (7) is laid on the mesh layers (6) of the downstream group. 8. The catalytic system according to one of the previous paragraphs, characterized in that the catalyst grids (6; 7), located one after another in the direction of the fresh gas flow (5), form a catalyst packing (3) from a front ensemble with three catalyst grids having the first average surface density, and located behind the front ensemble of the downstream ensemble of mesh catalyst layers with a second average surface density, and the average surface density of the front ensemble is less than the second average surface density by 1.5-29 wt.%, and in that the first average the basis weight is from 410 to 530 g/m ² and the second average basis weight is from 540 to 790 g/m ² . 9. Catalytic system according to claim 8, characterized in that the weight reduction is a maximum of 25%, and the average surface density lies in the range from 415 to 510 g/m ² , and the second average surface density in the range from 575 to 710 g / ^m2 . 10. The catalyst system of claim 8 or 9, wherein 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 downstream assembly are comprised 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 from a knitted knit structure with a second knit weave and a second loop size, the first loop size being larger than the second loop size. 11. Catalytic system according to one of paragraphs. 8-10, characterized in that the surface density of the mesh layers of the front ensemble catalyst is constant or consistently increases in the direction (5) of fresh gas flow. 12. 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 . 1-11.