Title of Invention

A PROCESS AND DEVICE FOR REDUCING THE NITROUS OXIDE

Abstract

The present invention relates to a process and device for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least one second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where the minimum of one first catalyst mesh element consists of platinum- rhodium mesh, characterized in that at least one second, downstream catalyst mesh element consists of palladiumrhodium mesh with at least 92 wt.% of palladium, 2-4 wt.% of rhodium, and the remainder of platinum.

Full Text

The invention pertains to a process and to a device for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least a second catalyst mesh element is used for the catalytic combustion of am¬monia and oxygen to form nitrogen oxides, and where at least one first catalyst mesh element is a platinum-rhodium mesh element. A familiar problem in the production of nitric acid by the combustion of ammonia with oxygen to form suitable nitrogen oxides is the fact that nitrous oxide (laughing gas) is also formed, which is under suspicion of contributing to the destruction of the earth's ozone layer. In the production of nitric acid, therefore, it is extremely important to effec¬tively destroy the N2O which has formed or to prevent nitrous oxide from being formed as completely as possible in the first place. The attempt to achieve these ends should not impair the yield of the desired nitrogen oxides. DE 198 19882A1 discloses a process for the catalytic decomposition of the N20 present in a gas mixture obtained during the production of nitric acid by the catalytic oxi¬dation of ammonia. This process makes use of a catalyst for the decomposition of N2O and is characterized in that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the N20-decomposing catalyst before the mixture is cooled. DE-OS 22 39 514 discloses an arrangement of catalyst gauze strips for accelerat¬ing the reaction between two or more gases flowing through the gauze, which is charac¬terized in that, on the downward-directed flow side of a group of gauze strips of precious metal material, a second group of gauze strips consisting of non-precious metal material, relatively difficult-to-vaporize metal material, and metal material which can be carried away by the gases is provided, which second groups supports the acceleration of the re¬action during the operation of the system. DE-OS 19 59 137 discloses a catalyst with a reduced platinum and rhodium con¬tent of 12-20 wt.% for the oxidation of ammonia to nitrogen oxide. In Hollemann-Wiberg, Lehrbuch der anorganischen Chemie [Textbook of Organic Chemistry], 71-80* edition, Verlag Walter de Gruyter & Co., Berlin, 1971, p. 360, an ammonia combustion element for recovering nitrogen oxide with a platinum mesh cata¬lyst and larger systems with several stacked wire mesh elements per combustion ele¬ment are described. EP 0 359 286 B1 describes a process for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides. The nitrous oxide is cooled in a heat recovery unit and then absorbed in water and/or di¬lute nitric acid. The process is characterized in that the hot combustion gases are given a retention time of 0.1-3 seconds before they are cooled. In this way, it is possible to de¬compose up to 90% of the N20 which has formed. EP 0 611 041 B1 discloses a process for reducing the N20 emissions during the startup phase of an ammonia oxidation reaction. A catalyst mesh based on platinum is used together with a platinum collector mesh element, which comprises fibers of a palla¬dium alloy, said palladium alloy containing 0.1-5 wt.% of cobalt. DE 198 05 202 A1 describes a process for the production of nitric acid in which ammonia is burned on at least one catalyst mesh element, especially on a platinum mesh element, as oxygen is being supplied. The reaction gases are then cooled. Before the reaction gases are cooled downstream of the catalyst mesh, they are conducted over a heat-resistant catalyst to convert the N2O contained in the reaction gases. The processes indicated above either have unsatisfactory N2O decomposition rates or require a very complicated apparatus, which must be considered disadvantageous in an economic sense. The problem is therefore to find a novel method and a novel device for reducing ni¬trous oxide while avoiding at least some of the disadvantages listed above and especially to provide an efficient process and a corresponding device which are low in cost in terms of the required apparatus. This problem is solved according to the invention by a process according to Claim 1 or Claim 2, by a device according to Claim 6 or Claim 7, and by a use according to Claim 11. Both in the process according to the invention and in the correspondingdevice Re¬cording to the invention, ammonia is mixed with oxygen and burned at a temperature of approximately 860°C by passing it quickly over a first catalyst mesh element, which con¬sists of platinum-rhodium mesh typically containing 5.0-10.0 wt.% of rhodium, and then by passing it quickly over a second catalyst mesh element, this second catalyst consist¬ing of palladium-rhodium mesh with at least 92 wt.% (or more) of palladium, 2-4 wt.% of rhodium, and the remainder of platinum or alternatively of 82-83 wt.% of palladium, 2.5-3.5 wt.% of rhodium, and the remainder of platinum, as a result of which the proportion of nitrous oxide is reduced in a highly efficient manner. The mesh used here is a gas-permeable structure which is essentially metallic and which is produced by, for example, knotting, knitting, weaving, or the random laying of fibers. The surprising result was obtained that, through the simultaneous use of the two catalyst mesh elements according to the invention, the proportion of N20 that forms couid be decreased significantly right from the start, and simultaneously NO could be produced in a highly efficient manner. It is advantageous, first, for the palladium-rhodium mesh to have a rhodium content of 2.5-3 wt.% and a palladium content of 82.5%, because in this way an especially low L level of N20 formation can be reached. It is also advantageous for the palladium-rhodium mesh element to be separated from the platinum-rhodium mesh element by least one third mesh element, so that in this way it is possible to prevent the different types of mesh elements from becoming welded together. As material for the third mesh element, a heat-resisting steel (FeCrAI alloy, e.g., Megapyr or Kanthal) has been found to give good results. Finally, it can be advantageous for the palladium-rhodium mesh to contain an addi¬tional metal from the group consisting of iridium, ruthenium, rhenium, cobalt, nickel, cop¬per, and gold to increase the mechanical stability of the palladium-rhodium catalyst mesh elements without interfering with their catalytic function. The following examples are presented to explain the invention in greater detail. 1. Experiments in the Test Reactor System Data: — reactor: — reactor pressure: — load: — temperature: — running time: test reactor with an effective diameter of 100 mm; 5 bars; 10tN/m2day; 865°C; 20 days. Example 1_ (Standard System) Selectivity of conversion from ammonia to nitrogen oxide: N20 output: PtRh8 meshes + PdNi5 meshes 95.5-96% 1,000-1,200 ppm Example 2 (Comparison Example) Selectivity of conversion from ammonia to nitrogen oxide: N20 output: Example 3 (Comparison Example) Selectivity of conversion from ammonia to nitrogen oxide: N2O output: PtRh8 meshes + PdRhl .5 meshes 95.5-96% 1,000-1,200 ppm PtRhS meshes + PdRh5 meshes 94.5-95% 900-1,200 ppm Example 4 (Invention) PtRh8 meshes + PdRh3 meshes Selectivity of conversion from ammonia to nitrogen oxide: 95.5-96% N20 output: 300-500 ppm Remark: Pd meshes mechanically very weak after use. Cracks were present. Example 5 (Invention) PtRh8 meshes + PdRh3Pt5 meshes Selectivity of conversion from ammonia to nitrogen oxide: 95.5-96% N20 output: 300-500 ppm Remark: Pd meshes mechanically very weak after use, but no cracks present. PtRh8 meshes + PdRh3Pt15 meshes Example 6 (Invention) Selectivity of conversion 95.5-96% 300-500 ppm from ammonia to nitrogen oxide: N2O output: Remark: Pd meshes mechanically stable after use, and no cracks present. 2.1. Experiments in the Industrial Reactor System Data: — reactor: — reactor pressure: — load: — temperature: — running time: industrial system with two parallel reactors; 4.8 bars; 11.2tN/m2day; 880°C; 180 days Reactor 1 (Standard System) Conversion of ammonia To nitrogen oxide: N2O output: PtRh5 meshes + PdNi5 meshes 95% 1,500 ppm Reactor 2 (Invention) Conversion of ammonia To nitrogen oxide: N2O output: PtRh5 meshes + PdPt15Rh2.5 n 95% 1,000 ppm 2.2. Experiments in the Industrial Reactor System Data; — reactor: — reactor pressure: — load: — temperature: — run time: industrial reactor; 3.5 bars 5.85 tN/m2day; 860X; 300 days. Example 1_ (Standard System) PtRh5 meshes + PdNi5 meshes Conversion of ammonia To nitrogen oxide: 94-96% N20 output: 1,200-1,600 ppm Example 2 (Invention) PtRh5 meshes + PdPtl 5Rh3 meshes Conversion of ammonia To nitrogen oxide: 94-96% N20 output: 400-800 ppm WE CLAIM: 1. A process for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least one second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where the minimum of one first catalyst mesh element consists of platinum- rhodium mesh, characterized in that at least one second, downstream catalyst mesh element consists of palladium-rhodium mesh with at least 92 wt.% of palladium, 2-4 wt.% of rhodium, and the remainder of platinum. 2. A process for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least one second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where the minimum of one first catalyst mesh element consists of platinum- rhodium mesh, characterized in that the minimum of one second, downstream catalyst mesh element consists of palladium-rhodium mesh with 82-83 wt.% of palladium, 2.5-3.5 wt.% of rhodium, and the remainder of platinum. 3. The process as claimed in claim 2, wherein the palladium-rhodium mesh has a rhodium content of 2.5-3 wt.% and a palladium content of 82.5 wt.%. 4 The process as claimed in any one of claims 1 to 3, wherein the palladium rhodium mesh element is separated from the platinum-rhodium mesh element by at least one third mesh element. 5. The process as claimed in claim 4, wherein the third mesh element consists of a heat-resisting steel. 6. A device for reducing the nitrous oxide which is formed during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least one second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where the minimum of one first catalyst mesh element consists of platinum-rhodium mesh, characterized in that the second, downstream catalyst mesh element consists of palladium-rhodium mesh with at least 92 wt.% of palladium, 2-4 wt.% of rhodium, and the remainder of platinum. 7. A device for reducing the nitrous oxide which is obtained during the catalytic combustion of ammonia and oxygen to form nitrogen oxides, where a catalyst system consisting of at least one first catalyst mesh element and at least one second catalyst mesh element is used for the catalytic combustion of ammonia and oxygen to form nitrogen oxides, and where the minimum of one first catalyst mesh element consists of platinum- rhodium mesh, characterized in that the second, downstream catalyst mesh element consists of palladium-rhodium mesh with 82-83 wt.% palladium, 2.5-3.5 wt.% rhodium, and the remainder of platinum. 8. The device as claimed in claim 7, wherein the palladium-rhodium mesh has a rhodium content of 2.5-3 wt.% and a palladium content of 82.5 wt%. 9. The device as claimed in any one of claims 6 to 8, wherein the palladium rhodium mesh element is separated from the platinum-rhodium mesh element by at least one third mesh element. 10. The device as claimed in claim 6 to 9, wherein the third mesh element consists of a heat-resisting steel. Dated this 9 day of January 2002

Documents:

in-pct-2002-0051-che abstract duplicate.pdf

in-pct-2002-0051-che abstract.pdf

in-pct-2002-0051-che claims duplicate.pdf

in-pct-2002-0051-che claims.pdf

in-pct-2002-0051-che correspondence-others.pdf

in-pct-2002-0051-che correspondence-po.pdf

in-pct-2002-0051-che description (complete) duplicate.pdf

in-pct-2002-0051-che description (complete).pdf

in-pct-2002-0051-che form-1.pdf

in-pct-2002-0051-che form-18.pdf

in-pct-2002-0051-che form-26.pdf

in-pct-2002-0051-che form-3.pdf

in-pct-2002-0051-che form-5.pdf

in-pct-2002-0051-che pct search report.pdf

in-pct-2002-0051-che pct.pdf

in-pct-2002-0051-che petition.pdf