Improvements in and relating to the production of nitric acid

摘要

Nitric acid is produced in a series of stages which involves contacting ammonia and an oxygen-containing gas with a catalyst, preferably a platinum-containing gauze, at an elevated temperature, and a pressure of between 2 and 10 atmospheres absolute. The resulting gaseous mixture is immediately cooled, mixed with further oxygen-containing gas, and passed through at least one oxidizing zone, in which the nitric oxide is oxidized to nitrogen dioxide, the temperature not being allowed to rise above 375 DEG C. This gas is then cooled by indirect heat exchange to between 70 DEG and 150 DEG C. and then passed through a cooler-condenser, where further cooling to between 40 DEG and 50 DEG C. occurs; water-vapour is condensed and a final oxidation occurs, a gaseous mixture having at least 90 per cent of its nitrous constituents in the form of nitrogen dioxide (or tetroxide) being obtained. This is then passed through an absorption tower to absorb the nitrogen oxides in water. In the Figure, air is drawn in through filter 1 by compressor 2 and enters the gas mixing zone 11 and some enters pipe 19. Gaseous ammonia from evaporator 6 passes through preheater 8 and filter 9 into the gas mixing zone 11. The mixed gases at 171 DEG C. pass through filter 12 into a combined converter/cooler 13, which contains catalyst 15, where the temperature rises to 825 DEG C. and a cooling section in which cooling to 250 DEG C. occurs by indirect heat exchange with water from feed-water heater 25. The gases leaving 13 enter the oxidation zone 23, mixed with air from line 19. The nitric oxide present is oxidized. The gases now at 340 DEG C. are cooled by passage through exhaust gas reheater 24 and feed-water heater 25, and at 120 DEG C. enters the cooler/condenser tower 26, in which sections 27 and 28 are packed with earthenware rings, and 29 and 30 are empty oxidation spaces. Weak nitric acid is introduced at the top of the tower and recycled via pump 40, cooler 41 and pipes 42a, 42 and 43. Some of the cycled nitric acid passes through heating coils 42b to evaporate liquid ammonia, and some is passed to the absorption tower 31. The gaseous mixture passes from tower 26 at 41 DEG C. into the absorption tower 31, which contains sieve-type plates 32 which are provided with cooling coils. Cooled condensate or water enters via pipe 33 and the nitric acid formed collects at the base of the tower and passes via pipe 35 into the top of the bleacher 36 where it passes downwards in counter-current to air from compressor 2 which enters via pipe 37. The gas issuing from the bleacher enters the bottom of tower 31. The 60 per cent strength nitric acid is removed via line 39. The exhaust gases from tower 31 pass through pipe 45, through reheater 24 where the temperature is raised to 105 DEG C., and thence to the gas turbine 4 where they assist the motor 3 to drive the compressor 2. The gases are then discharged to the atmosphere. Specifications 775,567, [Group XII], 801,806 and 803,212, [Group XII], are referred to.