CATALYST GAUZE

Abstract

A catalyst gauze for an ammonia oxidation process is described, containing a first layer of knitted first wire material, whereby said first wire material is made from a platinum-rhodium alloy, characterized in that said first layer contains an activator in the form of a second wire material which is knitted among the first wire material and which is made from un-alloyed platinum.

Claims

1-11. (canceled)

12. A catalyst gauze for an ammonia oxidation process, containing a first layer of knitted first wire material, wherein said first wire material is made from a platinum-rhodium alloy, characterized in that said first layer contains an activator in the form of a second wire material which is knitted among the first wire material and which is made from un-alloyed platinum.

13. The catalyst gauze according to claim 12, wherein the first wire material is made from a Pt—Rh alloy with 1 to 10% by weight Rh.

14. The catalyst according to claim 12, wherein the second wire material is made from ≥99.5% by weight Pt.

15. The catalyst gauze according to claim 12 wherein the weight percentage of the activator second wire material in the catalyst gauze is in the range of 1 to 45% by weight.

16. The catalyst gauze pack for the catalytic oxidation of ammonia to nitric oxide, comprising at least one activated catalyst gauze according to claim 12 as a top layer on a layer formed from one or more knitted gauzes made from a platinum-rich wire material comprising at least 85% by weight Pt.

17. The catalyst gauze pack according to claim 16 wherein the top layer consists of a single activated catalyst gauze.

18. A catalyst pack according to claim 16 wherein one or more knitted gauzes of a Pd—Pt alloy are placed below the one or more knitted gauzes made from the platinum-rich wire material to reduce nitrous oxide by-product formation.

19. The catalyst gauze pack according to claim 16, further comprising a catchment layer comprising one or more catchment gauzes of palladium or a palladium-rich alloy as a bottom layer.

20. A process for the catalytic oxidation of ammonia comprising the steps of: (i) heating a gauze pack according to claim 16 to a temperature ≥250° C., and (ii) passing a gas mixture comprising ammonia and air across the gauze pack to initiate ammonia oxidation.

21. A process according to claim 20 wherein the heating is performed by the combustion of hydrogen.

22. A process according to claim 20 used for the production of nitric acid.

23. A catalyst gauze according to claim 12, wherein the weight percentage of the activator second wire material in the catalyst gauze is in the range of 1 to 33% by weight.

24. A catalyst gauze according to claim 12, wherein the weight percentage of the activator second wire material in the catalyst gauze is in the range of 8 to 35% by weight.

25. A process for the catalytic oxidation of ammonia comprising the steps of: (i) heating a gauze pack according to claim 16 to a temperature ≥250° C., and (ii) passing a gas mixture comprising ammonia and air and methane across the gauze pack to initiate ammonia oxidation.

Description

[0027] The invention is further illustrated by reference to the drawings in which:

[0028] FIG. 1 is a depiction of one embodiment of an activated knitted catalyst gauze according to the invention; and

[0029] FIG. 2 is a depiction of a cross-section of a gauze pack comprising the activated knitted catalyst gauze.

[0030] In FIG. 1, the gauze is knitted in a single Jersey knit pattern using a circular knitting machine, where a first platinum-rhodium wire (1) was used in 10 of the wire feeds and a second platinum wire (2) was used in 2 of the wire feeds. The stitch pattern used was knit x1, miss x3, generating a series of float loops on the underside of the gauze. The diagram illustrates the technical face of the gauze that shows the face loops, which has been simplified for ease of understanding by hiding the float loops.

[0031] FIG. 2 shows one example a gauze pack comprising an activated ignition gauze according to the invention. The gauze pack comprises a plurality of gauzes. The activated ignition gauze (3) is uppermost, followed by a group of Pt-rich catalyst gauzes (4), high palladium gauzes (5) and finally Pd-catchment gauzes (6) at the bottom.

[0032] The invention is further illustrated by reference to the following Examples.

Example 1

[0033] Preparation of activated platinum/rhodium gauze. A circular knitting machine was set up to knit a gauze test piece using 10 feeds of 3RhPt gauze and 2 feeds of 490Pt wire. Both wire diameters were 0.076 mm, and the two spools of 490Pt wire were positioned at opposite sides of the machine cylinder. The stitch type was the single Jersey knit pattern in FIG. 1. While the machine settings were adjusted to accommodate the change in alloy composition, finishing cleaning steps were identical to existing 3RhPt gauzes. The resulting gauze was free of defects and missing wires and was similar in density to the standard 3RhPt gauze. The amount of 390Pt wire in the gauze was 17% by weight.

Example 2

[0034] Testing of the activated platinum/rhodium gauze. Three layers of the activated gauze, with a diameter of 25 mm, were placed inside a quartz reactor surrounded by an electric heater. A thermocouple was placed beneath the gauze to measure gas temperature. The reactor was pre-heated to 100° C. and a helium-oxygen gas was passed through the reactor at a rate of 35 L.Math.min.sup.−1. Ammonia was then introduced at a rate of 2 L.Math.min.sup.−1 and the reactor kept at a temperature of 100° C. for 30 minutes. The temperature was increased until light-off was achieved, which was defined as the point at which the increase in temperature change was greater than 0.6° C..Math.s.sup.−1. Results were repeated with a standard platinum/rhodium gauze and compared.

[0035] The results shown in Table 1 illustrate the beneficial effect of including the second wire in the catalyst gauze.

TABLE-US-00001 TABLE 1 Ignition temperatures for activated and standard platinum/rhodium gauzes Ignition Temperature (° C.) Activated platinum/rhodium gauze 255 Standard platinum/rhodium gauze 294