CATALYST SYSTEM AND METHOD FOR THE CATALYTIC COMBUSTION OF AMMONIA TO FORM NITROGEN OXIDES IN A MEDIUM-PRESSURE SYSTEM

Abstract

Known catalyst systems for the catalytic combustion of ammonia to form nitrogen oxides consist of a plurality of single- or multilayer catalyst gauzes warp-knitted, weft-knitted or woven from platinum-based noble metal wire, which, when arranged one behind the other in a fresh gas flow direction, form a front group of gauze layers and at least one downstream group of gauze layers arranged after the front group. To provide from this starting point a catalyst system for use in a medium-pressure plant for ammonia oxidation, with which a high service life and a high yield of the main product NO can be achieved, it is proposed that the front group comprises a gauze layer or a plurality of gauze layers made of a first, rhodium-rich noble metal wire, wherein the gauze layer or one of the gauze layers made of the rhodium-rich noble metal wire is a front gauze layer facing the fresh gas, and that the downstream group comprises gauze layers made of a second, rhodium-poor noble metal wire, wherein the rhodium content in the rhodium-rich noble metal wire is at least 7 wt. % and no more than 9 wt. % and is at least 1 percentage point higher than the rhodium content in the rhodium-poor noble metal wire

Claims

1. A catalyst system for the catalytic combustion of ammonia to form nitrogen oxides in a medium-pressure plant, having a plurality of single- or multilayer catalyst gauzes warp-knitted, weft-knitted or woven from platinum-based noble metal wire, which, when arranged one behind the other in a fresh gas flow direction, form a front group of gauze layers and at least one downstream group of gauze layers arranged after the front group, characterised in that the front group comprises at least one gauze layer made of a first, rhodium-rich noble metal wire, wherein the at least one gauze layer made of the rhodium-rich noble metal wire is a front gauze layer facing the fresh gas, and in that the downstream group comprises gauze layers made of a second, rhodium-poor noble metal wire, wherein the rhodium content in the rhodium-rich noble metal wire is at least 7 wt. % and no more than 9 wt. % and is at least 1 percentage point higher than the rhodium content in the rhodium-poor noble metal wire.

2. The catalyst system of claim 1, wherein the rhodium content in the rhodium-poor noble metal wire is in the range of 4 to 6 wt. %.

3. The catalyst system of claim 2, wherein the rhodium content in the rhodium-rich noble metal wire is in the range of 7.8 to 8.2 wt. % and the rhodium content in the rhodium-poor noble metal wire is in the range of 4.8 to 5.2 wt. %.

4. The catalyst system of claim 1, wherein the rhodium-rich noble metal wire and the rhodium-poor noble metal wire consist of platinum and rhodium.

5. The catalyst system of claim 1, wherein the front group of the gauze layers has a proportion by weight of less than 30%, of all the catalyst gauzes in a catalyst pack.

6. The catalyst system of claim 1, wherein the front group comprises no more than three gauze layers.

7. The catalyst system of claim 1, wherein the front group of gauze layers is formed by the frontmost gauze layer.

8. The catalyst system of claim 7, wherein the frontmost gauze layer lies on the gauze layers of the downstream group.

9. The catalyst system of claim 1, wherein the catalyst gauzes arranged one behind the other in the fresh gas flow direction form a catalyst pack composed of a front assembly with three catalyst gauzes having a first average grammage, and a downstream assembly of catalyst gauze layers arranged behind the front assembly having a second average grammage, wherein the average grammage of the front assembly has a weight reduction ranging from 1.5% to 29% relative to the second average grammage, and in that the first average grammage is in the range of 410 to 530 g/m.sup.2 and the second average grammage is in the range of 540 to 790 g/m.sup.2.

10. The catalyst system of claim 9, wherein the weight reduction is no more than 25% and the first average grammage is in the range of 415 to 510 g/m.sup.2, and wherein the second average grammage is in the range of 575 to 710 g/m.sup.2.

11. The catalyst system of claim 9, wherein the catalyst gauzes of the front assembly consist of a warp-knitted fabric with a first warp-knit pattern and a first mesh size, or of a woven fabric with a first weave pattern and a first mesh size, or of a weft-knitted fabric with a first weft-knit pattern and a first mesh size, and the catalyst gauzes of the downstream assembly consist of a warp-knitted fabric with a second warp-knit pattern and a second mesh size, or of a woven fabric with a second weave pattern and a second mesh size, or of a weft-knitted fabric with a second weft-knit pattern and a second mesh size, wherein the first mesh size is greater than the second mesh size.

12. The catalyst system of claim 9, wherein the individual grammage of the catalyst gauze layers of the front assembly is constant or increases in the order in the fresh gas flow direction.

13. A process for the catalytic combustion of ammonia to form nitrogen oxides in a medium-pressure plant by passing an ammonia- and oxygen-containing fresh gas through a catalyst system, whereby ammonia is burnt, wherein the fresh gas with an ammonia content of between 9.5 and 12 vol. % is passed through a catalyst system according to claim 1 under a pressure in the range of between 3.5 and 7 bar at a catalyst gauze temperature in the range of 870 to 920° C. and with a throughput in the range of 6 to 16 tN/m.sup.2 d.

14. The catalyst system of claim 1, wherein the front group of the gauze layers has a proportion by weight of less than 25% of all the catalyst gauzes in a catalyst pack.

15. The catalyst system of claim 1, wherein the front group of the gauze layers has a proportion by weight of less than 20% of all the catalyst gauzes in a catalyst pack.

Description

EXEMPLARY EMBODIMENT

[0055] The invention will be explained below with the aid of exemplary embodiments and a drawing. The figures show in:

[0056] FIG. 1 a flow reactor for the heterogeneous catalytic combustion of ammonia in a schematic diagram,

[0057] FIG. 2 a bar chart with results for the catalytic efficiency of test reactors compared with a reference reactor, and

[0058] FIG. 3 a bar chart with results for the N.sub.2O formation of the test reactors compared with the reference reactor

[0059] FIG. 1 is a schematic view of a vertically positioned flow reactor 1 for the heterogeneous catalytic combustion of ammonia. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. It comprises a catalyst pack 3 and downstream getter gauzes 4. The catalyst pack 3 comprises a plurality of single-layer catalyst gauzes 6, arranged one behind the other in the flow direction 5 of the fresh gas, composed of a first, comparatively “rhodium-poor”, PtRh noble metal wire, on which a further single-layer catalyst gauze 7 composed of a second, comparatively “rhodium-rich”, PtRh noble metal wire is laid. The front catalyst gauze 7 in the flow direction 5 forms the single layer of the “front group of catalyst gauze layers” and the remaining catalyst gauzes 6 form a “downstream group of catalyst gauze layers” within the meaning of the invention. Examples of rhodium-poor and rhodium-rich noble metal wire compositions and actions of these and similar catalyst gauze systems will be explained in more detail below. Embodiments are specified in more detail in Tables 1 to 4.

[0060] The fresh gas is an ammonia-air mixture with a nominal ammonia content of 10.7 vol. %. It is heated to a preheat temperature of 175° C. and fed into the reactor 1 from the top under an elevated pressure of 5 bar. When it enters the catalyst pack 3, an ignition of the gas mixture occurs followed by an exothermic combustion reaction, which covers the entire catalyst pack 3. The following primary reaction takes place here:

##STR00001##

[0061] Ammonia (NH.sub.3) is converted to nitrogen monoxide (NO) and water (H.sub.2O) in this reaction. The nitrogen monoxide (NO) that is formed reacts with excess oxygen in the downward-flowing reaction gas mixture (symbolised by the directional arrow 8 showing the flow direction of the reaction gas mixture) to form nitrogen dioxide (NO.sub.2), which is reacted with water to form nitric acid (HNO.sub.3) in a downstream absorption plant.

[0062] The catalyst gauzes 6, 7 are each textile fabrics produced by machine warp-knitting a noble metal wire with a diameter of 76 μm composed of various binary platinum-rhodium alloys. In the flow reactor 1, the catalyst systems specified in Tables 1 to 4 were tested.

[0063] In the test reactors, the catalyst pack comprises six single-layer catalyst gauzes 6, 7, which were produced by weft-knitting a noble metal wire composed of a binary PtRh alloy. The sequence in which items are named in the table reflects their order in the flow direction of the fresh gas. In addition, getter gauzes 4 are provided in all the reactors, consisting of six active catchment gauze layers (“getter gauzes”) composed of Pd82.5Pt15Rh2.5.

[0064] The reference reactor according to Table 1 represents a reactor according to the current industrial standard for medium-pressure plants. The single-layer catalyst gauzes are produced from a noble metal wire with a wire diameter of 76 μm. The grammage of each of the PtRh5 catalyst gauzes used is 600 g/m.sup.2 as stated in the column “grammage/layer” The sum of the grammages of all the layers L1 to L6 of the catalyst pack is therefore 3600 g/m.sup.2. The grammages are nominal, initial grammages, as can be achieved as standard with a noble metal wire having a wire diameter of 76 μm.

TABLE-US-00001 TABLE 1 Reference reactor Gauze Noble Grammage/layer layer metal [g/m.sup.2] L1 PtRh5 600 L2 PtRh5 600 L3 PtRh5 600 L4 PtRh5 600 L5 PtRh5 600 L6 PtRh5 600 Σ: 3600

[0065] In the following Tables 2 to 4, data relating to test reactors R1 to R3 are given. In the “Group allocation” column, the number “1” means that the respective catalyst gauze layer is allocated to the front group (also referred to below as “group 1”), and the number “2” shows that the respective catalyst gauze layers are allocated to the downstream group (also referred to below as “group 2”). In all the test reactors R1 to R3, the front catalyst gauze layer L1 alone forms the “front group” within the meaning of the invention; this group is additionally marked by grey shading.

TABLE-US-00002 TABLE 2 Test reactor Ref. 1 Gauze Noble Group Grammage/layer layer metal allocation [g/m.sup.2] L1 PtRh8 1 600 L2 PtRh5 2 600 L3 PtRh5 2 600 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Σ: 3600

[0066] In the test reactor R1, the top catalyst gauze layer consists of a PtRh8 alloy; the remaining catalyst gauzes consist of the conventional PtRh5 alloy as in the reference reactor.

TABLE-US-00003 TABLE 3 Test reactor R2 Gauze Noble Group Grammage/layer layer metal allocation [g/m.sup.2] L1 PtRh10 1 600 L2 PtRh5 2 600 L3 PtRh5 2 600 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Σ: 3600

[0067] In the test reactor R2, the top catalyst gauze layer consists of a PtRh10 alloy; the remaining catalyst gauzes again consist of the conventional PtRh5 alloy.

TABLE-US-00004 TABLE 4 Test reactor R3 Gauze Noble Group Grammage/layer layer metal allocation [g/m.sup.2] L1 PtRh8 1 600 L2 PtRh5 2 421 L3 PtRh5 2 421 L4 PtRh5 2 600 L5 PtRh5 2 600 L6 PtRh5 2 600 Σ: 3242

[0068] In the test reactor R3, the top catalyst gauze layer consists of a PtRh8 alloy and has a grammage of 600 g/m.sup.2. The two immediately following catalyst gauzes consist of a PtRh5 alloy and have a warp-knit pattern with a larger mesh size, leading to a comparatively low grammage of 421 g/m.sup.2. The last two catalyst gauzes of the catalyst pack again consist of the PtRh5 alloy and have a grammage of 600 g/m.sup.2. The use of noble metal in reactor R3 is 358 g/m.sup.2 lower than in the reference reactor and in test reactor R1.

[0069] The gauze layers L1 to L3 form a front assembly within the meaning of a preferred embodiment of the invention, in which a noble metal saving is obtained, compared with a standard reactor, by the fact that the catalyst gauzes of the front assembly have, within narrow limits, a lower noble metal content than the catalyst gauzes of the downstream assembly. In the exemplary embodiment, the average grammage of the front assembly is 481 g/m.sup.2, which is approximately 20% less than the average grammage of the downstream assembly with the catalyst gauze layers L4 to L6.

[0070] The test reactors were operated under the following test conditions, which were identical in each case. [0071] Pressure: 5 bar (absolute) [0072] Throughput: 12 tonnes nitrogen (from ammonia) per day and effective cross-sectional area of the catalyst pack in square metres (abbreviated as 12 tN/m.sup.2 d) [0073] NH.sub.3 content: 10.7 vol. % in the fresh gas [0074] Preheat temp: 175° C. (temperature of the NH.sub.3/air mixture), giving a gauze temperature of 890° C. in the test reactors.

[0075] At intervals of approximately 24 h, the NO yield and the proportion of N.sub.2O forming as a by-product were measured to determine changes in catalytic efficiency. Five test results were obtained for each of the test reactors R1 to R4.

[0076] The procedure for measuring the catalytic efficiency (i.e. the NO product yield) was as follows: [0077] 1. It was first ensured that the service life of the catalyst system is comparable with that of the reference reactor and that the catalyst system is suitable for the complete conversion of the ammonia being used. This means that NH.sub.3 is no longer present in the product gas in a significant quantity, as verified by mass spectrometry measurement of the product gas. [0078] 2. A sample of NH.sub.3/air was taken upstream of the catalyst pack at the same time as a sample of the product gas was taken downstream in separately evacuated flasks. The mass of the gas was determined by weighing. [0079] 3. The NH.sub.3/air mixture was absorbed in distilled water and titrated to colour change using 0.1 N sulfuric acid and methyl red. [0080] 4. The nitrous product gases were absorbed in 3% sodium peroxide solution and titrated to colour change using 0.1 N sodium hydroxide solution and methyl red. [0081] 5. The catalytic efficiency eta was obtained from: eta=100×Cn/Ca, wherein Ca is the average NH.sub.3 concentration from 7 individual measurements in the fresh gas as a percentage by weight, and Cn is the average NOx concentration from 7 individual measurements, expressed as a percentage by weight of NH.sub.3 that has been oxidised to form NOx. [0082] 6. Separately, the proportion by volume of N.sub.2O in the product gas was determined by gas chromatography.

[0083] The test results are compiled in Table 5. The sequential number of the measurement entered in column 1 corresponds approximately to the operating time of the catalyst system in days. In the columns labelled “NO—NO.sub.Ref” in Table 5, the yield difference of nitrogen monoxide is given in absolute percentage points compared with the reference reactor (e.g. measurement no. 1 in reactor R1 gives an NO yield of 96.2%, and therefore a difference NO—NO.sub.Ref of +0.9 percentage points compared with the measured value of 95.3% in the reference reactor). In the columns labelled “N.sub.2O—N.sub.2O.sub.Ref”, the difference in dinitrogen monoxide is given compared with the reference reactor in each case in ppm by volume (vol. ppm).

TABLE-US-00005 TABLE 5 Reference reactor Reactor 1 Reactor 2 Reactor 3 Yield NO − N.sub.2O − NO − N.sub.2O − NO − N.sub.2O − NO N.sub.2O NO.sub.Ref N.sub.2O.sub.Ref NO.sub.Ref N.sub.2O.sub.Ref NO.sub.Ref N.sub.2O.sub.Ref No. vol. % vol. ppm [%_abs] [ppm] [%_abs] [ppm] [%_abs] [ppm] 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 −11 3 95.3 867 0.3 7 4 95.4 899 0.1 18 5 95.2 945 0.3 −12

[0084] Test Results

[0085] The test results from Table 5 are illustrated graphically in the diagrams of FIGS. 2 and 3, and will be explained in more detail below with reference to these figures.

[0086] The diagram of FIG. 2 shows a measure of the catalytic efficiency for a nitrogen throughput of 12 tN/m.sup.2 d for each of the reactors R1 to R3. On the y-axis, the difference in the nitrogen monoxide yield compared with the reference reactor “NO—NO.sub.Ref” is entered in absolute percentage points (%_abs.). On the x-axis, the numerals 1 to 5 indicate the sequential number of each measurement.

[0087] According to the diagram, significantly higher efficiency in conversion to NO is obtained in reactor R1 compared with the reference reactor according to the industrial standard. The increase in efficiency varies around 0.6% which, for a typical quantity of ammonia used in an industrial reactor, approximately 12 tN/m.sup.2 d, means an additional mass of 154 kg NO/m.sup.2 d.

[0088] In reactor R3, despite the lower noble metal use, a catalytic efficiency is obtained which is comparable to the yield in the reference reactor within the limits of measurement error. The measurement error is approximately +/−0.3 percentage points, as marked by the broken line. However, since the first layer has a higher grammage than the lower layers of the first assembly, no significant gain in efficiency as in reactor 1 is visible.

[0089] Reactor R2 shows a yield of the main product NO which, taking account of measurement error, is no higher than that of the reference reactor. This effect can only be attributed to the particularly rhodium-rich front catalyst gauze layer L1 in R2. Reactor R2 therefore does not display improved catalytic efficiency and to this extent it represents a comparative example for the invention.

[0090] The diagram of FIG. 3 shows the test results for N.sub.2O formation in the test reactors R1 to R3. On the y-axis, the difference in the quantity of dinitrogen monoxide in the product gas (N.sub.2O—N.sub.2O.sub.Ref) by comparison with the reference reactor is entered in vol. ppm. On the x-axis, the numerals 1 to 5 again represent the sequential number of each measurement.

[0091] Accordingly, in all the test reactors R1 to R3 an N.sub.2O formation is obtained which is comparable to that of the reference reactor, taking account of measurement error.

[0092] The standard measurement error is approximately +/−50 vol. ppm and is again indicated by a broken line.