GB337847A - A process for working up oxides of nitrogen - Google Patents

Abstract

337,847. Caro, N., and Frank, A. R. Aug. 13, 1929. Drawings to Specification. Nitrogen tetroxide or nitric acid in concentrated form is obtained from mixtures containing nitrogen oxides, oxygen, and water vapour, such as ammonia combustion mixtures, by a three phase process in which part only of the water is first removed avoiding any considerable oxidation, the residual water is separated after or during the oxidation, which is promoted by lowering the temperature, as a tetroxide containing nitric acid of low water vapour pressure, and in the third phase the gas may be cooled or treated with substances which lower the vapour pressure of the tetroxide or have absorbing properties. Dispersed and dissolved substances, such as nitric or nitrous acids, solvents or solid adsorption agents such as silica gel are suitable. The gas may be liquefied or solid nitrogen oxides frozen out, and the treatments may take place in stages. The nitrogen tetroxide removed may be added to the liquefied gas or to the crude mixture from the second phase. This crude mixture can be worked up to give a 98 to 100 per cent tetroxide-free acid by treatment with oxygen at increased pressure and increased temperature. The amount of water passing to the second phase may be sufficient to allow the condensates from the second and third phases to be combined or employed for the mutual irrigation or scrubbing and adsorption. The residual gas may be compressed and washed with the condensates from the second or third phases, or the compressed gas from the second phase may be treated with condensate from the second phase. The second and third phases may be combined. Part or the whole of the plant may be run under increased pressure. The waste gas may be washed with water from the first phase to obtain aqueous nitric acid. With mixtures obtained by combustion of ammonia with pure oxygen the discharge temperature of the first phase may be above 100‹ C.; when air is used the temperature may be only 45‹ C., the conditions for the other phases varying similarly. The residual gas of the condensation can be worked up to nitrogen or oxygen. Specification 290,679 is referred to.