Thermal decomposition of N2O

Details Thermal decomposition of N2O Thermal decomposition of N2O

2000-01-11

2001-12-11

Langel, Wayne (Department: 1754)

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

Nitrogen or nitrogenous component

C423S239100, C423S351000, C423S579000

Reexamination Certificate

active

06328941

ABSTRACT:

The invention relates to a process for the thermal decomposition of N
2
O in N
2
O-containing gases and reactors suitable for this purpose.
N
2
O is formed, for example, as a by-product in processes in which HNO
3
is used as oxidizing agent in the liquid phase. Particularly in the oxidation of alcohols, aldehydes and ketones, e.g. cyclohexanol and cyclohexanone to adipic acid, acetaldehyde to glyoxal or glyoxal to glyoxylic acid, considerable amounts of N
2
O are liberated. Even in air fractionation plants, small amounts of N
2
O-containing gas streams are obtained. The N
2
O-containing gases can contain comparable amounts of NO
x
.
N
2
O has a certain damage potential in respect of the earth's atmosphere. In the stratosphere, it is a significant source of NO. This in turn contributes substantially to ozone depletion in the stratosphere. Moreover, N
2
O acts as a greenhouse gas whose global warming potential is said to be about 290 times that of CO
2
.
The N
9
O content of industrial waste gases should therefore be reduced as much as possible. It is known that in the absence of a catalyst N
2
O begins to decompose appreciable only at above 800° C. It is also known that as the temperature increases the decomposition no longer proceeds as desired to form N
2
and O
2
, but forms increasing proportions of NO above 1000° C. The exothermic decomposition of NO into N
2
and O
2
occurs at a high rate only at above 1500° C.
The catalytic decomposition of N
2
O has been known for a long time. WO 93/04774 describes silver-containing catalysts suitable for decomposing N
2
O.
The catalytic processes for decomposing N
2
O have the disadvantage that even small amounts of chemical and mechanical (dust) impurities in the ppm range and/or heating, above about 700 to 800° C. can lead to an activity loss through to complete destruction of the catalyst. A constant, well-defined gas composition is therefore a prerequisite for catalytic processes. Furthermore, wide fluctuations in the N
2
O content of the gases to be purified can, owing to the strongly exothermic nature of the decomposition reaction, lead to wandering temperature fronts and thus to intense hot spot temperatures which can result in local damage to the catalyst. There is therefore great interest in methods which make purely thermal decomposition of N
2
O possible.
Thermal processes for purifying waste gases contaminated by organic constituents are known. Known apparatuses are classical incineration muffles in which the waste gas is burnt together with fuel/air mixtures. Such an incineration process for the incineration of N
2
O-containing waste gases is described, for example, in DE-A-41 16 950. Owing, to the complexity of the combustion chamber and the associated high capital costs, the process is mainly suitable for small amounts of gas.
According to EP-A-0 555 110, an N
2
O-containing, gas is, with addition of fuel, burnt at very high temperatures directly in the flame in order to achieve formation of NO
x
. However, the reaction occurs only in low yields.
EP-A-0 359 286 describes a process for removing N
2
O from the product gas from ammonia incineration. Here, the product gas from ammonia incineration is not cooled immediately but only after a hold time of from 0.1 to 3 sec. As a result, a major part of the N
2
O is decomposed into nitrogen and oxygen. The waste gases from the incineration can also be contacted with a metal or metal oxide catalyst. The process is tailored to use for N
2
O removal after ammonia incineration and is only economical when the N
2
O-containing, gas is obtained at the necessary high temperature.
All known thermal incineration or decomposition processes for decomposing N
2
O have the disadvantage that additional fuels have to be burnt to produce the necessary high temperatures. Furthermore, it is technically difficult to ensure that the temperature in the combustion zone can be regulated to prevent decomposition of NO also present in the reaction gas or to prevent formation of NO
x
, from the N
2
O.
It is an object of the present invention to provide a process for the thermal decomposition of N
2
O into nitrogen and oxygen without the necessity of using a catalyst, which process can be carried out inexpensively with a minimal fuel consumption. In addition, in the case of waste gases which are free of NO
x
but contain N
2
O, the N
2
O should be decomposed while preferably minimizing the formation of NO
x
from the N
2
O. If the N
2
O-containing gas to be purified contains relatively large amounts of NO
x
which, after removal of N
2
O, which is all to be available for chemical utilization, for example for conversion into nitric acid, only the N
2
O should be selectively decomposed while leaving the NO completely undecomposed in the purified gas.
We have found that this object is achieved by the use of recuperative heat exchangers or regenerative heat exchangers for the thermal decomposition of N
2
O in N
2
O-containing gases.
In addition, the object of the invention is achieved by a process for the thermal decomposition of N
2
O in N
2
O-containing gases at from 800 to 1200° C., in which the N
2
O-containing gas is passed through one or more recuperative heat exchangers or regenerative heat exchangers in such a way that when the gas to be reacted is passed through a charge of heat transfer material it is heated to a temperature in the range from 800 to 1200° C. and the N
2
O present is decomposed thermally, and the reacted gas is cooled by heat exchange so as to heat the charge of heat transfer material and the gas to be reacted.
Here, the charge of heat transfer material can be, for example, a charge of inert particles in the heat exchanger.
According to the present invention, it has been found that the thermal decomposition of N
2
O described in EP-A-0 359 286 can be advantageously carried out in recuperative heat exchangers or regenerative heat exchangers which are filled with inert particles, with no further energy input being required after the start-up phase. It has been found that N
2
O can be completely decomposed thermally on inert surfaces at temperatures as low as about 800° C. and residence times of about 2 seconds. Here, N
2
O can be decomposed completely in the presence of NO
x
with no further NO
x
being formed.
Such gases can come from the sources described at the outset.
The decomposition is carried out at from 800 to 1200° C., preferably from 850 to 1100° C., particularly preferably from 900 to 1000° C. The residence time of the gas in the inert charge at the decomposition temperature is preferably from 0.5 to 60 s, particularly preferably from 2 to 20 s.
The residence time is matched to the respective temperature so that the desired degree of N
2
O decomposition is achieved and, correspondingly, NO
x
formation or NO
x
decomposition is restricted to the extent corresponding to the respective operating boundary conditions.
The decomposition pressure selected can be adapted within a wide range to the external boundary conditions (process pressure, etc.) so as to achieve the economically most advantageous solution. The process is, in principle, not restricted to a particular pressure range. On the basis of experience, the economically most advantageous working range is from 1 to 10 bar.
In a recuperative heat exchanger, heat exchange between the gases occurs via the tube walls. In the regenerative heat exchanger reactors, beds of generally inert material function as heat storages and heat exchange media. As heat storage media, it is possible to use particles of a wide variety of shapes such as randomly shaped particles or regular molded bodies. The use of honeycombs is also possible. For example, the &ggr;-Al
2
O
3
rings (Pural® from Condea) of the dimensions 5×5×3 mm have been found to be useful in laboratory tests. Ceramic materials such as SiC, SiO
2
and many more are generally useful. Metallic fittings are also conceivable. Thermal stability up to about 1200° C. and the presence of a large specific surface area are important, since the impact of the thermally

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