GB456446A - Improvements in and relating to the production of concentrated nitric acid - Google Patents

Abstract

<PICT:0456446/III/1> Concentrated nitric acid is obtained from the gases produced in the catalytic oxidation of ammonia with air by rejecting a substantial proportion of the water formed in the oxidation, compressing the remaining gases to a pressure suitable for producing concentrated nitric acid by the oxidation of nitrogen tetroxide and intermediate strength nitric acid with air, separating at ordinary temperatures a substantial proportion of the nitrogen oxides in the liquid phase, introducing the residual gases into the upper section of an absorption apparatus, subjecting them therein to countercurrent treatment at ordinary temperature with an aqueous absorption medium in order to form intermediate strength nitric acid, subjecting the intermediate strength nitric acid in the lower section of the absorption apparatus to a counter current treatment with a stream of air at a raised temperature while introducing the condensed nitrogen oxides in the liquid phase into the absorption apparatus at a point intermediate in depth between the points of introduction of the residual gases and air, and withdrawing concentrated nitric acid from a position at or near the base of the absorption apparatus. Any oxygen containing gas containing 15--30 per cent. of oxygen by volume may be used. Air enters through a pipe 1 and preheater 2 and mixes with ammonia entering through a pipe 3. The mixture is catalytically oxidized in a converter 7. The oxidized gas is cooled in a heat-exchanger 8 by exhaust gases leaving the absorption system, is further cooled in the preheater 2 and then passes through a condenser 9 where sudden cooling separates water with little acid. The gases are then compressed in a compressor 12 to say 20 atmospheres and cooled in a final cooler 13 where the remaining water and a large proportion of the nitrogen oxides condense. The gases and condensate are separated in a chamber 14 and enter a tower 15 through the pipes 16 and 23 respectively. Weak condensate from the interstage coolers of the compressor together with some water are forced by a pump 17 to the top of the tower 15 while air preferably preheated is introduced through a pipe 24. The upper part of the column is cooled by cooling means 19 while a temperature of about 70 DEG C., say 50--85 DEG C., is maintained in the lower part of the column by heating means 22. Strong nitric acid containing some oxides of nitrogen is released through a pipe 25 to a bleaching column 26, under lower (say atmospheric) pressure, which is supplied with air through a pipe 27. The concentrated acid is withdrawn through pipe 29. Air containing oxides of nitrogen are returned to the compressors 12 through a pipe 28. The residual gases leaving the tower through a pipe 20 are reheated in the heat exchanger 8, and are expanded through the power recovery section 21 of the compressor 12. According to a modification, the oxidation of ammonia is also carried out at the same elevated pressure, say 20--100 atmospheres; and the length of column below the inlet 23 is made longer so that all the nitrogen oxides are removed from the acid by the air entering through the pipe 24, further bleaching being unnecessary. The column 15 may be built as two or more separate columns. Specifications 405,450 and 418,916, are referred to.