GB456518A - Improvements in or relating to the production of highly concentrated nitric acid - Google Patents

Abstract

<PICT:0456518/III/1> Nitric acid of a concentration exceeding 90 per cent is obtained from the gases produced in the catalytic oxidation of ammonia at ordinary or increased pressure with air or oxygen-enriched air by stripping the gases of most of their water, oxidizing them so that they contain gaseous nitrogen peroxide, treating them at elevated pressure with an absorbent consisting of nitric acid of a concentration exceeding 90 per cent and oxidizing the resulting solution at a still higher pressure and at elevated temperature, the proportion of concentrated nitric acid used as an absorbent being sufficiently great and the proportion of water present as vapour in the oxidized gases or as liquid in the concentrated nitric acid being sufficiently small to ensure that no separation of nitrogen peroxide as a separate liquid phase occurs either during the absorption or subsequently. Water or dilute nitric acid may be added to the solution before oxidation provided the quantity employed is insufficient to cause phase separation. The absorption stage may operate at pressures of 3--5 atmospheres and a temperature of 0--10 DEG C. The solution of nitrogen peroxide in concentrated nitric acid may be oxidized with air at 50--200 atmospheres pressure and at 60--120 DEG C. An ammonia-air mixture is passed through a converter 1 at atmospheric pressure. The oxidized gases leaving at a temperature of 400 DEG C., pass through pipes (not shown), to a heat-exchanger 27, through a heat-exchanger 33 and to a watercooled condenser 2 in which they are rapidly cooled, a large fraction of their water content being condensed before appreciable oxidation to the peroxide stage can take place. The gases are pumped by a turbo-compressor 3 into a water cooled vessel 6 at a pressure of 3 atmospheres. Here oxidation takes place and residual water is condensed. The substantially dried and completely oxidized gases then pass to the foot of an absorption vessel 7 consisting of a bubbler cap tower irrigated with concentrated nitric acid. The gases leave the top of the tower through a spray catcher 8 and are finally scrubbed free from nitric acid or tetroxide in a scrubber 10 irrigated with a portion of the condensate from the condenser 2. The residual gases pass through the heat - exchanger 33 to power recovery in the recovery section 11 of the turbo-compressor. The tetroxide-enriched acid leaving the bottom of the tower 7 is admixed with dilute nitric acids from the condenser 6 and the scrubber 10 and the resulting solution is sent to one of a group of acid eggs 16 from which it is forced by means of compressed air into the top of the series of oxidation vessels 21, 22, 23. These vessels contain suitable packing, internal baffles and internal coils for heating or cooling. Air at about 95 atmospheres pressure is supplied to the lowermost oxidation vessel preferably in a finely divided state from a compressor 36 and reservoir 37. The bottom vessel may be maintained at a temperature of 100 DEG C., while the top vessel has a temperature of 60 DEG C. The air moves in counter-current to the acid and reaches the top nearly free from oxygen. The residual nitrogen passes through a condenser 31, a scrubber 32 and the heat-exchanger 27, is expanded to about 3 atmospheres with power recovery at 34 and subsequently joins the residual gases from the absorption vessel. The concentrated nitric acid from the bottom oxidation vessel is bleached in a steam heated bleaching column 28 by means of air at 4 atmospheres released from the acid eggs. The tetroxide containing gases from the bleaching column is fed into the main gas stream either in front or immediately after the compressor 3. The hot nitric acid is cooled in a cooler 25 a portion being sent through a valve 26 into the absorption vessel 7. The "make" acid is drawn off at valve 29.