CATALYST SYSTEM FOR A FLOW REACTOR AND METHOD FOR CATALYTIC OXIDATION OF AMMONIA

Abstract

The present invention relates to a catalyst system for flow reactors which is characterized by the sequence of the noble metal-containing alloys used of the catalyst networks forming the catalyst system. By using palladium alloys for a second and third catalyst network group, the platinum content of the catalyst system can be kept relatively low overall. In addition, the invention relates to a method for catalytic combustion of ammonia, in which a fresh gas containing at least ammonia is conducted through a catalyst system.

Claims

1. A catalyst system for a flow reactor, comprising at least three catalyst network groups arranged one behind the other in the flow direction, each catalyst network group being formed from at least one catalyst network composed of at least one noble metal wire in each case, and the first catalyst network group comprising at least one catalyst network composed of at least one first noble metal wire made of a platinum alloy, the platinum alloy consisting of, in addition to impurities, 80-98 wt. % platinum, 2-20 wt. % rhodium and 0-20 wt. % palladium, the second catalyst network group comprising at least one catalyst network composed of at least one second noble metal wire made of a palladium alloy, the palladium alloy of the second noble metal wire consisting of, in addition to impurities, 70-97 wt. % palladium, 0-10 wt. % rhodium and 3-30 wt. % of at least one further metal, the at least one further metal being selected from the group consisting of nickel, tungsten, platinum and gold, and the third catalyst network group comprising at least one catalyst network composed of at least one third noble metal wire made of a palladium alloy, the palladium alloy of the third noble metal wire consisting of, in addition to impurities, 72-97 wt. % palladium, 0-wt. % rhodium and 3-28 wt. % of at least one further metal, the at least one further metal being selected from the group consisting of nickel, tungsten, platinum and gold wherein the rhodium content of the noble metal wires of the catalyst network groups decreases or remains constant in the flow direction, and the palladium content of the noble metal wires of the catalyst network groups increases in the flow direction.

2. The catalyst system according to claim 1, wherein the catalyst networks are woven, braided or knitted independently of one another.

3. The catalyst system according to claim 1, wherein at least one of the catalyst networks comprises a three-dimensional structure.

4. The catalyst system according to claim 3, wherein at least one of the catalyst networks is corrugated.

5. The catalyst system according to claim 1, wherein the noble metal wires comprise a diameter of 40-250 μm.

6. The catalyst system according to claim 1, wherein the first noble metal wire comprises a binary platinum alloy which consists of, in addition to impurities, platinum and rhodium.

7. The catalyst system according to claim 1, wherein the second noble metal wire comprises a ternary palladium alloy which consists of, in addition to impurities, palladium, platinum and rhodium, or comprises a binary palladium alloy, which consists of, in addition to impurities, palladium and nickel, tungsten, platinum or gold.

8. The catalyst system according to claim 1, wherein the palladium content of the third noble metal wire is above the palladium content of the second noble metal wire by at least 2 weight percentage points.

9. The catalyst system according to claim 1, wherein the rhodium content of the third noble metal wire is below the rhodium content of the second noble metal wire by at least 2 weight percentage points.

10. The catalyst system according to claim 1, wherein the third noble metal wire comprises a binary palladium alloy, which consists of, in addition to impurities, palladium and nickel, tungsten, platinum or gold.

11. The catalyst system according to claim 1, wherein the third noble metal wire is platinum-free.

12. The catalyst system according to claim 1, wherein the third noble metal wire is rhodium-free.

13. The catalyst system according to claim 1, wherein the catalyst system comprises at least one further catalyst network group composed of at least one catalyst network composed of at least one further noble metal wire.

14. The catalyst system according to claim 1, wherein the catalyst system comprises at least one separating element between two of the catalyst network groups.

15. A method for catalytic oxidation of ammonia, wherein a fresh gas containing at least ammonia is conducted through a catalyst system according to claim 1.

Description

DETAILED DESCRIPTION

[0127] FIG. 1 schematically shows a vertically positioned flow reactor 1 for heterogeneously catalytic combustion of ammonia. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. The catalyst system 2 comprises a plurality of catalyst network groups (4, 5, 6) arranged one behind the other in the flow direction 3 of the fresh gas.

[0128] The fresh gas is an ammonia-air mixture comprising a nominal ammonia content of 10.7% by volume. It is heated to a preheating temperature of 175° C. and introduced from above into the reactor 1 at an elevated pressure of 5 bar. Upon entry into the catalyst system 2, the gas mixture is ignited and a subsequent exothermic combustion reaction takes place. The following main reaction takes place:

4NH.sub.3+5O.sub.2.fwdarw.4NO+6H.sub.2O

[0129] In this case, ammonia (NH.sub.3) is converted to nitrogen monoxide (NO) and water (H.sub.2O). The nitrogen monoxide (NO) formed reacts in the outflowing reaction gas mixture (symbolized by the directional arrow 7 indicating the flow direction of the outflowing reaction gas mixture) with excess oxygen to form nitrogen dioxide (NO.sub.2), which is added to water in a downstream absorption system to form nitric acid (HNO.sub.3).

[0130] The catalyst networks are in each case textile fabrics which are produced from the relevant noble metal alloy by machine braiding a noble metal wire with a diameter of typically 76 μm. In Tables 1 and 2, exemplary embodiments (E1-E11) for catalyst systems are specified which can be used in a flow reactor 1.

TABLE-US-00001 TABLE 1 E1 E2 E3 E4 E5 E6 1 PtRh5 PtRh5 PtRh5 PtRh5 PtRh5 PtRh5 2 PdPt15Rh3 PdPt15Rh3 PdPt10Rh3 PdPt15Rh3 PdPt15 PdPt20Rh1 3 PdPt5 PdNi5 PdNi5 PdNi5 PdNi5 PdNi5 indicates data missing or illegible when filed

TABLE-US-00002 TABLE 2 E7 E8 E9 E10 E11 G1 PtRh5 PtRh5Pd5 PtRh5Pd5 PtPd15Rh5 PtPd15Rh5 G2 PdPt10Rh1 PdPt20 PdPt10Rh5 PdPt10Rh5 PdPt5Rh5 G3 PdW5 PdNi5 PdPt5Rh5 PdPt5 PdW5

[0131] The catalyst systems each comprise 3 catalyst network groups with a total of 30 catalyst networks; the sequence of the naming G1 to G3 reflects the arrangement in the flow direction of the fresh gas.

[0132] Table 3 indicates the composition of a catalyst system E12 with 4 catalyst network groups in which a rhodium-richer group G0 is arranged upstream of the catalyst system according to the invention with G1-G3.

TABLE-US-00003 TABLE 3 E12 G0 PtRh8 G1 PtRh5 G2 PdPt15Rh3 G3 PdNi5

[0133] In a test reactor according to FIG. 1, the catalyst systems E1-E12 were compared to catalyst systems in which the arrangement of the catalyst network groups G2 and G3 was reversed. In the comparative reactors, the catalyst network group with the rhodium-rich alloy was thus behind the rhodium-poorer alloy. Both reactors each have the same content of catalytically active noble metal.

[0134] The test reactors were operated under the following identical test conditions in each case. [0135] Pressure: 5 bar (absolute) [0136] Throughput: 12 tons of nitrogen (from ammonia) per day and effective cross-sectional area of the catalyst packing in square meters (abbreviated to 12 tN/m.sup.2d) [0137] NH.sub.3 proportion: 10.7% by volume in the fresh gas [0138] Preheating temp.: 175° C. (NH 3 temperature/air mixture), resulting in a network temperature of 890° C.

[0139] At an interval of about 12 h, over a period of 4 days, the development of the catalyst efficiency of the catalyst (yield of NO in %) and the amount of nitrous oxide N.sub.2O occurring as an undesired by-product was measured.

[0140] The measurement of the catalytic efficiency (i.e., the product yield of NO) has the following sequence: [0141] 1. It is ensured that the catalyst system is suitable for the complete conversion of the ammonia used. That means that NH.sub.3 in the product gas is no longer present in a significant amount, which is checked by means of mass spectrometric analysis of the product gas. [0142] 2. Simultaneous removal of a sample of NH.sub.3/air upstream of the catalyst packing and a sample from the product gas downstream in respectively independent evacuated pistons. The mass of the gas is determined by weighing. [0143] 3. The NH.sub.3/air mixture is absorbed in distilled water and titrated by means of 0.1 N sulfuric acid and methyl red after color change. [0144] 4. The nitrous product gases are absorbed in 3% sodium peroxide solution and titrated by means of 0.1 N sodium hydroxide solution and methyl red after color change. [0145] 5. The catalytic efficiency Eta results from: Eta=100×C.sub.n/C.sub.a, where C.sub.a is the average NH.sub.3 concentration of 7 individual measurements in the fresh gas in percent by weight and C.sub.n is the mean NO.sub.x concentration of 7 individual measurements expressed as percent by weight of the NH.sub.3 which has been oxidized to form NO.sub.x. [0146] 6. Separately, the volumetric proportion of N.sub.2O in the product gas is determined by means of gas chromatography.

[0147] In the reactors equipped according to the invention, it was possible to observe an efficiency increased over the entire test period by an average of 0.4% with a comparable proportion of N.sub.2O. In this field of technology, the increase by 0.4% represents a significant and economically significant enhancement.