NEW GEOMETRY FOR DEN2O CATALYST

Abstract

A process for the catalytic decomposition of nitrous oxide N2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger, over a catalyst for the decomposition of N.sub.2O which is installed between the noble metal gauze catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N.sub.2O before subsequent cooling, characterized in that the catalyst is in the form of a star-shaped body having six lobes.

Claims

1.-20. (canceled)

21. A process for the catalytic decomposition of nitrous oxide N.sub.2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, in a reactor which contains in this order in the flow direction a noble metal gauze catalyst and a heat exchanger, over a catalyst for the decomposition of N.sub.2O which is installed between the noble metal gauze catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N.sub.2O before subsequent cooling, characterized in that the catalyst is in the form of a star-shaped body having six lobes, wherein the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes is in the range from 1.9 to 3.61, the ratio of the area F1 inside this circle to the summed area F2 of the lobes outside this circle is in the range of from 0.54 to 0.90, the ratio of the distance x2 between the two intersections I of one lobe with neighboring lobes and the radius r1 of the circle is in the range of from 0.67 to 1.11.

22. The process according to claim 21, wherein the catalyst for the decomposition of N.sub.2O contains compound of the formula M.sub.xAl.sub.2O.sub.4, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, and x is from 0.6 to 1.5.

23. The process according to claim 21, wherein the ratio of radius r2 to radius r1 is from 1.9 to 3.0.

24. The process according to claim 21, wherein each lobe has straight outer walls with a rounded top, wherein the ratio of the length x1 from the intersection I of one lobe and neighboring lobes to the end of the straight outer wall to the distance x2 between two intersections I of one lobe and neighboring lobes is from 0.87 to 1.45.

25. The process according to claim 21, wherein each lobe has straight outer walls with a rounded top, wherein the angle ? between the straight outer wall and the straight line x2 between two intersections I of one lobe and neighboring lobes is from 70 to 140 degrees.

26. The process according to claim 21, wherein each lobe has straight outer walls with a rounded top, wherein the ratio of the length x2 between two intersections I of one lobe and neighboring lobes to the length x3 between the ends of the straight outer walls is from 0.9 to 1.8.

27. The process according to claim 21, wherein each lobe has straight outer walls with a rounded top and the ratio of the lobe area of the trapeze F3 confined by the straight outer walls of a lobe and the outer lobe area F4 outside this trapeze is from 2.5 to 14.35.

28. The process according to claim 21, wherein the cross-section area is from 0.19 to 13.9 mm.

29. The process according to claim 21, wherein the maximum radius r2 is from 0.4 to 6 mm.

30. The process according to claim 21, wherein the circle radius r1 is from 0.25 to 3.4 mm.

31. The process according to claim 21, wherein the elemental composition of the catalyst corresponds to an oxidic mixture containing from 50 to 80% by weight of Al.sub.2O.sub.3, 0.1 to 30% by weight of CuO and 0.1 to 40% by weight of oxides M(II)O of the one or more of the further metals.

32. The process according to claim 30, wherein the further metal is Zn.

33. The process according to claim 21, wherein at least 70% by weight of the catalyst for the decomposition of N.sub.2O is in the spinel phase.

34. The process according to claim 21, wherein the height of the catalyst bed of the catalyst for the decomposition of N.sub.2O is from 2 to 50 cm.

35. The process according to claim 21, wherein the decomposition of N.sub.2O is carried out at from 600 to 950? C. at a pressure of from 1 to 15 bar.

36. A star shaped catalyst body having six lobes for the decomposition of N.sub.2O containing compound of the formula M.sub.xAl.sub.2O.sub.4, where M is Cu or a mixture of Cu with one or more further metals M, selected from the group consisting of Sn, Pb, Zn, Mg, Ca, Sr and Ba, and x is from 0.6 to 1.5, wherein the ratio of the maximum radius r2 in the star to radius r1 of a circle connecting the intersections of the lobes being in the range from 1.9 to 3.61, the ratio of the area F1 inside this circle to the summed area F2 of the lobes outside this circle being in the range of from 0.54 to 0.90, the ratio of the distance x2 between the two intersections I of one lobe with neighboring lobes and the radius r1 of the circle being in the range of from 0.67 to 1.11.

37. The star shaped catalyst body for the decomposition of N.sub.2O according to claim 36, wherein the elemental composition of the catalyst corresponds to an oxidic mixture containing from 50 to 80% by weight of Al.sub.2O.sub.3, 0.1 to 30% by weight of CuO and 0.1 to 40% by weight of oxides M(II)O of the one or more of the further metals.

38. The star shaped catalyst body for the decomposition of N.sub.2O according to claim 36, wherein at least 70% by weight of the catalyst for the decomposition of N.sub.2O are in the spinel phase.

39. A reactor for the catalytic decomposition of nitrous oxide N.sub.2O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia which contains in this order in the flow direction a noble metal gauze catalyst, a heat exchanger and a catalyst bed for the decomposition of N.sub.2O which is installed between the noble metal catalyst and the heat exchanger so that the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N.sub.2O before subsequent cooling, characterized in that the catalyst bed contains the catalyst for the decomposition of N.sub.2O in the form of a star-shaped body having six lobes as defined in claim 21.

40. A method for decomposing N.sub.2O in N.sub.2O containing gas mixtures comprising utilizing a star shaped catalyst body as defined in claim 21.

Description

EXAMPLE

[0073] The table shows an overview of the calculated properties of the prior art geometries (FIG. 3a, 3b) and the geometry according to the invention (FIG. 3c) in two different sizes. The small tri-lobes have a high GSA/volume ratio, the large five-pointed stars have a low pressure drop. The new geometry combines these two beneficial features.

[0074] The values for delta p are calculated based on CFD (Computational Fluid Dynamics). GSA is the surface area of the modeled catalyst geometry in the simulations and the volume respectively. Both are the numerically calculated values from the catalyst model and based on the real geometries.

[0075] The geometric surface area (GSA) and pressure drop for a packed bed of the extrudates of different shapes and sizes were obtained from a detailed numerical simulation.

[0076] First, a random packing is generated with a simulation using a representative geometry of a reactor tube and the real geometry of the catalyst. The packing is generated by virtually dropping the catalyst particles into the reactor tube and calculating the movement and impacts between particle-particle and particle-wall contacts according to Newton's second law of motion. A discrete element soft-sphere algorithm is used as numerical method. The pressure drop is the result of a detailed simulation applying computational fluid dynamics. The fluid volume is extracted from the numerically generated random packed bed. The fluid dynamics around each pellet as well as all interstitial flow phenomena are fully resolved. The pressure drop is then calculated for an assumed bed height of 84 mm and an assumed inner tube diameter of 56 mm. Compressed air is used as fluid. The pressure at the end of the packed bed is ambient pressure. The assumed temperature is ambient temperature. The assumed flow rate is 1.5 Nm.sub.3/h. The six-star extrudates according to the invention are compared to a five-star extrudate and a modified trilobe extrudate as references. All extrudates had an assumed length of 10 mm.

TABLE-US-00001 die diameter delta p GSA/volume [mm] [mbar] at 1 m/s [10.sup.3 m.sup.2/m.sup.3] trilobe FIG. 3a 3 117 1.135 five-star FIG. 3b 6 50 0.585 six-star FIG. 3c 4.2 43 1.02 six-star FIG. 3c 4.8 30 0.799