Method for the catalytic decomposition of N2O

Details Method for the catalytic decomposition of N2O Method for the catalytic decomposition of N2O

2000-10-25

2004-06-01

Silverman, Stanley S. (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Nitrogen or nitrogenous component

C423S392000

Reexamination Certificate

active

06743404

ABSTRACT:

The present invention relates to a process for the catalytic decomposition of N
2
O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia. The invention further relates to a reactor which is suitable for carrying out the process.
In the industrial preparation of nitric acid by the Ostwald process, ammonia is reacted with oxygen over a noble metal catalyst to form oxides of nitrogen which are subsequently absorbed in water. In this process, ammonia and oxygen or air are reacted at from 800 to 955° C. over a catalyst gauze comprising noble metals in a reactor. The catalyst gauze generally comprises platinum and rhodium as active metals. In the catalytic reaction, ammonia is firstly oxidized to nitrogen monoxide which is subsequently further oxidized by oxygen to give nitrogen dioxide or dinitrogen tetroxide. The gas mixture obtained is cooled and then passed to an absorption tower in which nitrogen dioxide is absorbed in water and converted into nitric acid. The reactor for the catalytic combustion of ammonia also contains, downstream of the catalyst gauze, a recovery gauze for depositing and thus recovering catalyst metals which have been vaporized at the high reaction temperatures. A heat exchanger is located downstream of the recovery gauze to cool the gas mixture obtained. Absorption is carried out outside the actual reactor in a separate absorption column.
The combustion and the absorption can be carried out at the same pressure level. It is possible to employ an intermediate pressure of from about 230 to 600 kPa or a high pressure of from about 700 to 1100 kPa. In the case of a process with two pressure stages, the absorption is carried out at a higher pressure than the combustion. The pressure in the combustion is then from about 400 to 600 kPa and the pressure in the absorption is from about 900 to 1400 kPa.
An overview of the Ostwald process may be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Volume A 17, pages 293 to 339 (1991).
The combustion of ammonia forms not only nitrogen monoxide and nitrogen dioxide or dinitrogen tetroxide but generally also N
2
O (dinitrogen monoxide) as by-product. In contrast to the other oxides of nitrogen formed, N
2
O is not absorbed by the water during the absorption step. If no further step for removing N
2
O is provided, N
2
O can be emitted into the environment in a concentration of from about 500 to 3000 ppm in the waste gas.
Since N
2
O is a greenhouse gas and participates in the depletion of the ozone layer, very substantial removal from the waste gas is desirable. A number of methods of removing N
2
O from waste gas streams have been described.
DE-A-195 33 715 describes a process for removing nitrogen oxides from a gas stream, in which the nitrogen oxides apart from N
2
O are absorbed in an absorption medium and remaining N
2
O is subsequently decomposed catalytically at from 700 to 800° C. in a decomposition reactor. Since nitrogen oxides can be formed in this decomposition, a selective catalytic reduction (SCR) can follow.
U.S. Pat. No. 5,478,549 describes a process for preparing nitric acid by the Ostwald method, in which the N
2
O content is reduced by passing the gas stream after the oxidation over a catalyst bed of zirconium oxide at a temperature of at least 600° C. However, the precise position of the catalyst bed is not indicated. It is only stated that zirconium oxide in the form of cylindrical pellets is located below the recovery gauze. The space velocity in the reactor is 30,000 h
−
at 4 bar and 600° C.
EP-B 0 359 286 describes a process for the reduction of N
2
O. For this purpose, a reactor for carrying out the Ostwald process is modified in such a way that the gases obtained after the catalytic combustion are subjected to a retention time of from 0.1 to 3 seconds before cooling by means of the heat exchanger. If desired, a catalyst for the selective decomposition of N
2
O can be additionally provided.
It is an object of the present invention to provide a process and an apparatus for preparing nitric acid by the Ostwald method, by means of which the N
2
O content of the waste gases can be reduced very effectively and at low cost. The process should also be able to be integrated into existing plants and preferably increase the proportion of useful product. In addition, additional heating and cooling steps for the waste gas should be avoided, so that the removal of N
2
O can be carried out economically.
We have found that this object is achieved by a reactor for the catalytic oxidation of ammonia to nitrogen oxides, which contains a noble metal gauze catalyst and a heat exchanger in that order in the direction of flow and has a catalyst for the decomposition of N
2
O located between the noble metal gauze catalyst and the heat exchanger.
The object is also achieved by a process for the catalytic decomposition of N
2
O in a gas mixture obtained in the preparation of nitric acid by catalytic oxidation of ammonia, where the N
2
O is decomposed catalytically over a catalyst for the decomposition of N
2
O, wherein the hot gas mixture obtained from the catalytic oxidation of ammonia is brought into contact with the catalyst for the decomposition of N
2
O prior to subsequent cooling.
According to the present invention, it has been found that N
2
O can be reacted directly in the reactor for the catalytic oxidation of ammonia when a suitable catalyst is located between the noble metal gauze catalyst and the heat exchanger. In this way, N
2
O formed as by-product is decomposed immediately after it is formed. The decomposition occurs at the temperature prevailing in the catalytic oxidation of ammonia. Heating or cooling of the gaseous reaction mixture is thus unnecessary. The catalyst for the decomposition of N
2
O which is used according to the present invention is located directly in the reactor, preferably between the position of a noble metal recovery gauze located downstream of the noble metal catalyst and the position of the heat exchanger. Reactors for the Ostwald process are usually provided with inserts for accommodating the noble metal catalyst and the noble metal recovery gauze. These reactors can easily be modified by additionally providing a holder for the N
2
O decomposition catalyst.
The low catalyst bed height required according to the present invention allows installation in existing reactors without great rebuilding of the reactors. Thus, existing reactors can be modified to enable the process of the present invention to be carried out, without replacement of the reactor being necessary. The Ostwald process can be carried out at one pressure level or at two pressure levels, as described above. The height of the catalyst bed is preferably from 2 to 50 cm, particularly preferably from 5 to 10 cm. In production, the residence time over the catalyst is preferably less than 0.1 s. The pressure drop caused by installation of the catalyst is therefore very low, a small amount of catalyst can be employed, and the gas has to be held at a high temperature level for only a short time after the oxidation, so that secondary reactions can largely be suppressed.
According to the present invention, the decomposition of N
2
O is carried out in the reactor for the oxidation of ammonia at the oxidation temperature, generally at a temperature in the range from 600 to 950° C., preferably from 800 to 930° C., in particular from 850 to 920° C. The pressure is, depending on the pressure level at which the Ostwald process is carried out, generally from 1 to 15 bar.
As noble metal gauze catalyst, it is possible to use any noble metal gauze catalyst suitable for the catalytic oxidation of ammonia. The catalyst preferably comprises platinum and possibly rhodium and/or palladium as catalytically active metals.
The noble metal recovery gauze is preferably made of palladium. The catalyst used according to the present invention for the decomposition of N
2
O is preferably selected from among catalysts which still have sufficient activity at above 900° C

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