Patent GB-691709-A (GB)
Title: Process for the manufacture of nitric acid
IPC: C01B21/26
Filed: 1951-05-21
Abstract: <PICT:0691709/III/1> <PICT:0691709/III/2> Nitric acid is manufactured from gases from the oxidation of ammonia by passing the gases, after cooling and passage through an oxidation zone, through a packed concentration tower and then through a series of absorption towers, the gases being countercurrently contacted in the absorption towers with an aqueous absorption liquid recirculated through the towers and being countercurrently contacted in the concentration tower with nitric acid withdrawn from the first or second absorption tower and passed through the tower without recirculation, the gases in the concentration tower being at as low a temperature as possible. The gases pass from cooler 1 through oxidation tower 2 to concentration tower 3 and thence to absorption towers 4 wherein absorption liquid is recirculated by pumps 10 through coolers 11 to distributers 9. Excess liquid is withdrawn through by-passes 12 from each tower to the preceding one and through by-pass 14 to the concentration tower 3.  Water is added to the last tower at 13.  Residual gases are withdrawn at 8 to an alkaline absorption section and washed with sodium carbonate solution.  The concentration tower, Fig. 2, has superposed beds of packing 16 each with cooling coils 17-20, the coils 17-19 being water cooled and the coil 20 being supplied with brine at 5 DEG C.  The lower part of concentration tower 3 may be used for treating NO2-containing gases, from the decomposition of sodium nitrite obtained in the alkaline absorption section, by passing the gases through packing 24 against a separate circulation of strong nitric acid entering at 28 from pump 26 and cooler 27; the remaining gases pass through tray 21 and packing 25 and are mixed with the oxidized gases entering at 6.  Concentrated nitric acid is withdrawn at 22.  The gases before entering the concentration tower 3 may be cooled by treatment in the upper section of the oxidation tower with an acid solution cooled to say 7.5 DEG C by 40 per cent of the cooling effect of the vaporization of the ammonia oxidized.  The remainder of the ammonia vaporization cooling effect may be used to cool the liquid circulating in later absorption towers, some of which may be used wholly or partly as oxidation towers by being partly filled with or devoid of packing. The alkaline absorption section may be improved in efficiency by adjusting the NO : NO2 ratio in the residual gases from the last absorption tower to 1 : 1 to 1 : 1.25 by increasing the NO2 content; the NO2 content may be increased by adding gases, from the concentration tower, or from the decomposition of the sodium nitrite from the alkaline absorption with nitric acid, to the residual gases, or by using the last absorption towers for oxidation of the residual gases, with continuous circulation of liquid therein without addition or subtraction.  In an alternative apparatus the concentration tower is mounted on the oxidation tower and is not provided with a cooling system; the first absorption tower has superposed beds of packing, and the acid for the concentration tower is supplied from the second absorption tower.