A PROCESS FOR DECOMPOSING NITROUS OXIDE FROM A GAS STREAM

Abstract

The invention relates to a process for decomposing nitrous oxide from a gas stream (1). comprising: (a) heating the gas stream (1) and splitting the gas stream (1) into at least two partial streams (3, 5) or splitting the gas stream (1) into at least two partial streams (3, 5) and heating the partial streams (3, 5): (b) feeding each of the partial streams (3, 5) into a separate decomposition reactor, wherein each reactor (31) comprises a catalyst: (c) decomposing the nitrous oxide into nitrogen and oxygen in the decomposition reactors to obtain purified streams (13, 15): (d) optionally feeding each purified stream (13, 15) into a unit (11) for decomposing nitrogen dioxide and/or nitrogen monoxide or combining at least two purified streams (13, 15) and feeding the combined purified streams into a unit (11) for decomposing nitrogen dioxide and/or nitrogen monoxide, wherein the catalysts of the decomposition reactors (31) are changed alternatingly and wherein one of the catalysts is changed when the arithmetic mean of the lifetime of the catalysts in the other reactors has reached 25 to 75% of the lifetime of one catalyst.

Claims

1-12. (canceled)

13. A process for decomposing nitrous oxide from a gas stream, comprising: (a) splitting the gas stream into at least two partial streams, and heating the partial streams; (b) feeding each of the partial streams into a separate decomposition reactor, wherein each reactor comprises a catalyst; (c) decomposing the nitrous oxide into nitrogen and oxygen in the decomposition reactors to obtain purified streams; (d) optionally feeding each purified stream into a unit for decomposing nitrogen dioxide and/or nitrogen monoxide or combining at least two purified streams and feeding the combined purified streams into a unit for decomposing nitrogen dioxide and/or nitrogen monoxide, wherein the catalysts of the decomposition reactors are changed alternatingly and wherein one of the catalysts is changed when the arithmetic mean of the lifetime of the catalysts in the other reactors has reached 25 to 75% of the lifetime of one catalyst.

14. The process according to claim 13, wherein the catalyst is a fixed bed catalyst.

15. The process according to claim 13, wherein the gas stream is split into a first and a second partial stream and the first partial stream is fed into a first decomposition reactor and the second partial stream into a second decomposition reactor.

16. The process according to claim 13 wherein the partial stream which is fed into the decomposition reactor containing an older catalyst is reduced and the partial stream which is fed into a decomposition reactor containing a fresher catalyst is increased.

17. The process according to claim 13, wherein during the change of the catalyst of one decomposition reactor all partial streams are fed into the other decomposition reactors.

18. The process according to claim 13, wherein the temperature of each partial stream fed into the decomposition reactors can be set individually.

19. The process according to claim 13, wherein before splitting the gas stream into partial streams and heating the partial streams, the whole gas stream is preheated.

20. The process according to claim 13, wherein the partial streams are preheated by heat transfer from the respective purified streams being withdrawn from the decomposition reactors.

21. The process according to claim 13, wherein the partial streams are additionally heated in a heater.

22. The process according to claim 13, wherein a part of the purified stream withdrawn from the decomposition reactor is recycled into the partial stream fed into the decomposition reactor before heating.

23. The process according to claim 22, wherein the part of the purified stream which is recycled into the partial stream can be set individually for each decomposition reactor.

24. The process according to claim 13, wherein the nitrous oxide comprising gas stream is obtained in a process for producing adipic acid.

Description

IN THE FIGURES

[0070] FIG. 1 shows a schematic flow diagram of the process for decomposing nitrous oxide,

[0071] FIG. 2 shows a decomposition unit for decomposing nitrous oxide.

[0072] FIG. 1 shows a schematic flow diagram of the process for decomposing nitrous oxide.

[0073] In a process for decomposing nitrous oxide according to the invention, a gas stream 1, which contains nitrous oxide and optionally nitrogen monoxide and nitrogen dioxide, is split into a first partial stream 3 and a second partial stream 5. The first partial stream 3 is fed into a first decomposition unit 7 and the second partial stream 5 into a second decomposition unit 9.

[0074] In the first and second decomposition units 7, 9, the nitrous oxide in the first and second partial streams 3, 5 is decomposed into nitrogen and oxygen and a first purified stream 13 and a second purified stream 15 are obtained. In the embodiment shown in FIG. 1, after passing the first and second decomposition units 7, 9, the obtained first and second purified streams 13, 15 are recombined and fed into a unit 11 for decomposing nitrogen monoxide and/or nitrogen dioxide. The unit 11 for decomposing nitrogen monoxide and/or nitrogen dioxide thereby can be any decomposition unit for nitric oxides known to a skilled person.

[0075] Besides splitting the gas stream 1 into two partial streams as shown in FIG. 1, the gas stream also can be split in more than two partial streams. In this case, each partial stream is fed into a separate decomposition unit. Further, besides recombining the purified streams before feeding into the unit 11 for decomposing nitrogen monoxide and/or nitrogen dioxide, it is also possible to feed each purified stream into a separate unit for decomposing nitrogen monoxide and/or nitrogen dioxide. After having passed the at least one unit 11 for decomposing nitrogen monoxide and/or nitrogen dioxide, the purified gas can be emitted into the atmosphere. If more than one unit for decomposition of nitrogen monoxide and/or nitrogen dioxide is used, each purified stream can be emitted into the atmosphere or at least two purified streams are combined and then emitted into the atmosphere.

[0076] Further, if the gas stream 1 is split into more than two partial streams, it is possible to recombine groups of at least two of the purified streams and feed each of combined streams into a separate unit for decomposition of nitrogen monoxide and/or nitrogen dioxide.

[0077] Using the first and the second decomposition units 7, 9 or optionally more than two decomposition units allows for splitting the gas stream 1 in such a way that each decomposition unit is fed with an amount of gas by which the nitrous oxide which did not decompose is minimized. Further, the temperature in each decomposition unit can be optimized in regard to the activity of the catalyst in the respective decomposition unit. Optimizing the temperature, for example, can be realized by setting the degree of the bypass 43 and/or by setting the power of the heater 29 and/or by setting a recycle stream from the outlet of the decomposition unit 7, 9 back into the inlet of the respective decomposition unit 7, 9. Further, using the at least first and second decomposition units 7, 9 allows for continuing operation even in case one decomposition unit is shut down, for example for maintenance like changing the catalyst. In this case, the gas streams 1 is fed into the decomposition unit 7, 9 which is not shut down if two decomposition units are comprised and, if more than two decomposition units are comprised, the gas stream is split into a number of partial streams which corresponds to the number of all decomposition units minus the number of the decomposition units which are shut down and the partial streams are fed into those decomposition units which still are in operation.

[0078] If the device comprises two decomposition units 7, 9, the first decomposition unit 7 and the second decomposition unit 9 preferably have the same design which corresponds to the design of a decomposition unit 21 as shown in FIG. 2. If more than two decomposition units are comprised, all decomposition units preferably have the same design which preferably corresponds to the design of the decomposition unit 21 shown in FIG. 2.

[0079] A partial stream 23 which may be the first partial stream 3 or the second partial stream 5 depending on whether the decomposition unit 21 corresponds to the first decomposition unit 7 or the second decomposition unit 9, is fed into a heat exchanger 25 in which the partial stream 23 is preheated by heat transfer from the purified stream 27 which is withdrawn from the decomposition unit 21. If the heat transferred from the purified stream 27 is not sufficient to heat the partial stream 23 to the temperature which is necessary for decomposition of nitrous oxide, the partial stream 23 after having left the heat exchanger 25 is fed into a heater 29 in which the partial stream 23 is heated to the temperature with which the partial stream 23 is fed into a decomposition reactor 31 in which the nitrous oxide is decomposed into nitrogen and oxygen. The heater 29 preferably is an electrical heater.

[0080] By further setting the heat which is transferred to the partial stream 23 in the heater 29, the temperature of the partial stream 23 can be set to a predefined temperature with which the partial stream 23 is fed into the decomposition reactor 31. The temperature with which the partial stream 23 is fed into the decomposition reactor 31 preferably is in the range from 430 to 650? C.

[0081] The decomposition reaction of the nitrous oxide usually is carried out in the presence of a catalyst. Therefore, the decomposition reactor 31 contains a catalyst, preferably a fixed bed catalyst 35. In the fixed bed catalyst 3, the nitrous oxide is decomposed forming oxygen and nitrogen. The gas stream obtained by the reaction then is fed into the heat exchanger 25 for heating the partial stream 23 which then is fed into the decomposition reactor 31. By this heat transfer, simultaneously the purified stream 27 withdrawn from the decomposition reactor 31 is cooled. The purified stream 27 may be further cooled in an additional heat exchanger 37. The additional heat exchanger 37 preferably is a heat exchanger for producing steam by evaporation of water or for superheating water. The purified stream 27 withdrawn from the additional heat exchanger 37 can be fed into the unit 11 for decomposing nitrogen dioxide and/or nitrogen monoxide or released into the atmosphere.

[0082] For a largely complete or particularly a complete decomposition of the nitrous oxide it is necessary to keep the amount of nitrous oxide in the gas stream fed into the decomposition reactor 31 below a predefined upper range, particularly below 11 wt-%. For reducing the amount of nitrous oxide in the gas stream fed into the decomposition reactor 31 if this gas stream contains too much nitrous oxide, a recycle line 39 with a recycle blower 41 is provided, the recycle line 39 connecting a feed line for the partial stream 23 and an exit line for the purified stream 27.

[0083] Further, for setting the temperature of the partial stream 23 fed into the decomposition reactor 31 a bypass 43 may be provided, bypassing the regenerative heat exchanger 25. The bypass 43 may be closed so that the whole gas stream flows through the regenerative heat exchanger 25 or the bypass 43 is open and the feed line into the regenerative heat exchanger 25 is closed so that the whole gas stream flows through the bypass 43 or in a third alternative, the gas stream is split and a part flows through the regenerative heat exchanger 25 and a part through the bypass 43.

[0084] Providing each of the decomposition units 21 with a heat exchanger 25 and a heater 29 allows for individually setting the temperature of the partial stream 23 which enters the decomposition reactor 31. However, besides such a design as shown in FIG. 2, it is also possible to at least partly heat the whole gas stream 1 before splitting into the partial streams 3, 5, 23. If the gas stream 1 is heated partly before splitting into the partial streams 3, 5, 23, a final heating of the partial streams takes place and this final heating allows for individually setting the temperature of each partial stream. If the gas stream is heated completely before splitting into the partial streams, it is not possible to further set the temperatures of the partial streams and therefore, an individual setting of the temperature for an optimized decomposition of the nitrous oxide in each decomposition unit 21 is not possible. For this reason, it is preferred that each partial stream 23 is heated at least partly in the individual decomposition unit 21.

[0085] Further, providing the recycle line 39 allows for setting the concentration of nitrous oxide in each partial stream 23 fed into the respective decomposition reactor 31 by setting the amount of gas being recycled from the purified gas 27 into the partial stream 23. Therefore, it is preferred that each decomposition reactor 23 is equipped with a recycle line 39 as shown here.

EXAMPLES

Example

[0086] An off-gas stream of 32000 Nm.sup.3/h comprising 8 vol-% nitrous oxide with a temperature of 40.5? C. and a pressure of 6.8 bar(abs) which is withdrawn from a production process of adipic acid is heated in a first heat exchanger to a temperature of 273?C. Then the off-gas stream is split into two partial streams.

[0087] The first partial stream has a volume flow of 15600 m.sup.3/h and enters a regenerative heat exchanger of a first nitrous oxide-decomposition unit. In this regenerative heat exchanger, the first partial stream is heated by electrical heating to a temperature of 490? C. and then the stream fed into a reactor with a fixed bed containing 8 t of a nitrous oxide-decomposition catalyst. As catalyst, the commercially available BASF-catalyst O3-81 was used. The first purified gas stream withdrawn from the fixed bed catalyst having a temperature of 680? C. and a N.sub.2O-concentration of 90 vol.-ppm is fed to the regenerative heat exchanger of the first nitrous oxide-decomposition unit. The first purified gas stream leaves the regenerative first heat exchanger of the first nitrous oxide-decomposition unit and passes a first steam generator.

[0088] The second partial steam having a volume flow of 16400 m.sup.3/h enters the regenerative heat exchanger of the second nitrous oxide-decomposition unit in which this stream is heated by electrical heating to a temperature of 490? C. and then is fed to a reactor with a fixed bed with 8 t of a nitrous oxide-decomposition catalyst. As catalyst the commercially available BASF-catalyst O3-81 was used. The second purified gas stream withdrawn from the fixed catalyst bed had a temperature of 675? C. and a N.sub.2O-concentration of 70 vol-ppm and is fed to the second regenerative heat exchanger of the second nitrous oxide-decomposition unit. The second purified gas stream leaves the regenerative heat exchanger and passes a second steam generator.

[0089] The first and the second purified gas streams were mixed. This mixed gas-stream is fed to a Denox-unit and is released after passing an expansion turbine to atmosphere. The concentration of nitrous oxide of the mixed off-gas flow is 80 vol.-ppm.

Comparative Example

[0090] An off-gas stream of 30400 Nm.sup.3/h comprising 8 vol-% nitrous oxide with a temperature of 34? C. and a pressure of 7.2 bar(abs) which is withdrawn from a production process of adipic acid is heated in a first step in a heat exchanger to a temperature of 254? C. and in a second step by regenerative heat exchange and by electrical heating to a temperature of 512? C. and then fed into a reactor with a fixed bed with 8 t of a nitrous oxide-decomposition catalyst. As catalyst the commercially available BASF-Catalyst O3-81 was used. The purified gas stream withdrawn from the fixed catalyst bed had a temperature of 710? C. and was fed to the regenerative heat exchanger and afterwards to a steam generator. The off-gas of the steam generator had a concentration of 660 vol.-ppm nitrous oxide and was fed to a Denox-unit and is released after passing an expansion turbine to atmosphere.