Catalyst gauze and installation for the catalytic oxidation of ammonia

Abstract

Catalyst gauze (1) for the reduction of the amount of N.sub.2O in an ammonia oxidation process, containing a first layer (2) of woven or knitted first wire material (4), whereby said first wire material (4) is made from Pd or a Pd-rich alloy, whereby said first layer (2) contains a reinforcement in the form of a second wire material (5) which is woven or knitted among the first wire material (4) and which has a different composition than the first wire material (5).

Claims

1. A catalyst gauze for the reduction of the amount of N.sub.2O in an ammonia oxidation process, the gauze comprising a first layer, the first layer comprising: a woven or knitted first wire material, wherein said first wire material comprises Pd or a Pd-rich alloy, and a second wire material, which is woven or knitted among the first wire material thereby reinforcing the first layer, wherein the second wire material has a different composition than the first wire material and wherein the second wire material is disposed in loops which extend outside the first layer, such that at least 50 wt % of the second wire material is disposed outside the first layer.

2. Catalyst gauze of claim 1, wherein the second wire material comprises Pt or a Pt-rich alloy.

3. Catalyst gauze of claim 2, wherein the second wire material comprises a PtRh alloy with 1-10% Rh.

4. Catalyst gauze of claim 1, wherein the first layer is a knitted layer.

5. Catalyst gauze of claim 1, wherein the first wire material comprises an alloy with at least 75.0 wt % Pd.

6. Catalyst gauze of claim 1, wherein the first wire material comprises a PdPtRh alloy with at least 80.0 wt % Pd and at least 10.0 wt % Pt.

7. Catalyst gauze of claim 1, wherein the first wire material comprises an alloy with more than 80.0 wt % and less than 82.0 wt % Pd.

8. An installation for the catalytic oxidation of ammonia to NO, comprising a first catalyst gauze comprising a Pt or Pt-rich wire material having at least 70 wt % Pt and, downstream from said first catalyst gauze, a second catalyst gauze, wherein the second catalyst gauze comprises the catalyst gauze of claim 1.

9. A catalyst gauze for the reduction of the amount of N.sub.2O in an ammonia oxidation process, the gauze comprising a first layer and a second layer, the first layer comprising: a woven or knitted first wire material, wherein said first wire material comprises Pd or a Pd-rich alloy, and a second wire material, which is woven or knitted among the first wire material thereby reinforcing the first layer, wherein the second wire material has a different composition than the first wire material, and the second layer comprising: a woven or knitted third wire material, wherein said third wire material comprises a Pd or a Pd-rich alloy, and the second wire material, which is woven or knitted among the third wire material.

10. Catalyst gauze of claim 9, wherein the second layer is a knitted layer and the first wire material and the third wire material have the same composition.

11. Catalyst gauze of claim 9, wherein the first layer and second layer are disposed parallel, and the second wire material is present as a pile thread, which runs back and forth between the first layer and the second layer.

12. Catalyst gauze of claim 9, wherein the second wire material, in the part of the catalyst gauze between the first layer and the second layer, is disposed at an angle or angles (?,?) to a plane defined by the first layer, wherein the angle or angles (?,?) are between 30? and 90?.

13. Catalyst gauze of claim 9, wherein the second layer is a knitted layer.

14. Catalyst gauze of claim 9, wherein the summed weight percentage of the first wire material and the third wire material is between 33 and 67 wt %.

15. An installation for the catalytic oxidation of ammonia to NO, comprising a first catalyst gauze comprising a Pt or Pt-rich wire material having at least 70 wt % Pt and, downstream from said first catalyst gauze, a second catalyst gauze, wherein the second catalyst gauze comprises the catalyst gauze of claim 9.

Description

(1) In order to explain the invention a non-limiting example of a specific embodiment of a catalyst gauze according to the invention and its use is given below, with reference to the following figures:

(2) FIG. 1, which shows a perspective view of the structure of a catalyst gauze, whereby for clarity only a few stitches are shown, and

(3) FIG. 2, which shows a side view of the structure of the catalyst gauze of FIG. 1.

(4) The catalyst gauze 1 of the figures mainly consists of two layers, more specifically a first layer 2 and a second layer 3 which are both knitted.

(5) The first layer 2 is made from a first wire 4 with, in this example but not necessarily, a diameter of 0.076 mm, made from an alloy having a composition 81.5 wt % Pd, 15 wt % Pt and 3.5 wt % Rh. The second layer 3 is, in this example but not necessarily, made from the same wire material as the first layer 2.

(6) The first layer 2 and second layer 3 have a thickness d of circa 0.55 mm and are placed at a distance D of circa 1.4 mm, giving an overall thickness D+2d of 2.5 mm.

(7) In between the stitches of the knitted wires 4 of the first layer 2 and second layer 3 a pile thread 5 is provided, that runs back and forth between the first layer 2 and the second layer 3. This pile thread 5 is made from a second wire with, in this example but not necessarily, a diameter of 0.07 mm and made from an alloy with a composition of 95 wt % Pt and 5 wt % Rh.

(8) In general the wire diameters of the first and second wire may range from 0.06 mm to 0.105 mm, but may exceptionally also be outside this range,

(9) In order to make the difference between both types of wire clear, the pile thread 5 is indicated in the figures with thin lines, and the first wire 4 is indicated with thicker lines.

(10) The pile thread 5, in the part of the catalyst gauze 1 between the first layer 2 and the second layer 3, makes two average directions with the geometrical plane 6 in which the first layer 2 extends, indicated by A and B. These directions A, B make a smallest angle ?, respectively ? of circa 60? respectively circa 45? with the geometrical plane 6 in which the first layer 2 extends.

(11) Note that FIG. 2 is a side view in the direction in which angles ? and ? are minimal. In all other side views these angles ? and ? will appear larger.

(12) The weight of the first wire 4 used for both the first layer 2 and the third layer 3 is 48% of the total weight of the catalyst gauze 1 and the weight of the pile thread/second wire 5 is 52% of the total weight of the catalyst gauze 1. The total weight of the catalyst gauze 1 is circa 1220 g/m.sup.2.

(13) A catalyst gauze 1 as described above may be manufactured on commercially available industrial flat-bed knitting machines. In accordance with EP0504723 the setting on the flat-bed knitting machine are preferably between about 3.63 mm and about 0.81 mm with respect to gauge and between 2 and 6 mm for the mesh length.

(14) The tensile strength of a strip of the catalyst gauze 1 was measured and compared with two mutually identical strips placed on top of each other of a knitted catalyst gauze not according to the invention and made from only the first wire 4, having a total weight of 770 g/m2

(15) All test pieces had a width of 50 mm and were heat treated before testing for 0.5 hrs at 1000? C. in air to simulate aging during use. The results were as follows:

(16) TABLE-US-00001 Tensile Tensile strength per mass unit Material strength (N) of first wire (N/(g/m.sup.2)) Gauze 1 according to the 199.4 0.341 invention Comparative test piece 187.5 0.244

(17) The results show that for the same amount of first wire 4, the catalyst gauze 1 according to the invention, is circa 40% stronger than a traditional knitted Pd rich catalyst gauze and will therefore have longer life and more stable operating conditions.

(18) The catalyst gauze 1 was tested for its catalytic properties as follows:

(19) A test reactor was used with an effective diameter of 22 cm. In this test reactor, the following catalyst gauzes were installed, in order: A standard knitted catalyst gauze made of wires of 0.076 mm diameter of Pt+5 wt % Rh alloy with total weight of 600 g/m2 A catalyst gauze 1 according to the invention as described above Two further catalyst gauzes which were the same as in the first position A catalyst gauze with the same geometry as the catalyst gauze in the first position, but made from Pd alloy with 15 wt % Pt and 3.5 wt % Rh. Two further catalyst gauzes which were the same as in the first position

(20) The bed of catalyst gauzes was heated to 890? C. The reactor was operated at 5.0 bar absolute pressure.

(21) A feed of 29.6 kg/h NH.sub.3 in the form of a 10.20% NH.sub.3 mixture in air was supplied to the bed of catalyst gauzes. The N.sub.2O content was determined after certain times on stream, and was as follows:

(22) TABLE-US-00002 Days 2 3 5 7 9 12 N.sub.2O (ppm) 280 265 330 335 355 360

(23) Note that the N.sub.2O content was determined after bringing the O.sub.2 content to 0%.

(24) The selectivity to NO of the oxidation reaction, determined after 9 days, was 93.4%.