Process for obtaining nitric acid of a concentration higher than the azeotropic concentration by means of the absorption of nitrogen oxides in water or diluted nitric acid

#This invention is concerned with a process for the manufacture of nitric acid of a concentration higher than the azeotropic concentration (68% by weight) with or without the simultaneous production of nitric acid of a concentration lower than the azeotropic concentration, by means of absorption of nitrogen oxides having a high degree of oxidation in water or in aqueous solutions of nitric acid, for which the partial pressure of the nitrogen oxides referred to is increased partly by decomposition of nitric acid by means of nitrogen oxides having a low degree of oxidation, and partly by compression at very high pressures of the gases that contain them.

#Practically all the nitric acid produced industrially is obtained starting with ammonia by catalytic oxidation of the ammonia in accordance with the reaction:

#4 NH.sub.3 + 5 O.sub.2 .fwdarw. 4 NO + 6 H.sub.2 O + Heat (I)

#The FIGURE is a simplified flow diagram in a plant in which the oxidation of ammonia is effected at low pressure (which presents the advantage that on the condensation of the water formed in accordance with reaction (I), the quantity of nitric acid transported by the water is minimal and, of course, very much less than that obtained in condensation processes with higher pressures).

#In the diagram referred to, the tower in which the oxidation of NO to NO.sub.2 is produced also functions at low pressure which presents the advantage compared with operation at higher pressures that the larger part of the nitrogen oxides are in the form of NO and therefore, taking into account that 3 mols of NO.sub.2 are obtained for each mol of NO in accordance with reaction IV, the quantity of NO.sub.2 is greater than can be achieved by the processes that operate at higher pressure in which the nitrogen oxides are usually displaced towards NO.sub.2.

#In accordance with the schematic diagram, the mixture of ammonia and air is mixed and reacts in the reactor 1, in accordance with reaction (I). The reagent gases are cooled in various heat exchangers 2, 3, and 4, whose operation is irrelevant for the purposes of the patent. In the cooler 5, the dew point of the gases is achieved, and the water formed in the previous reaction condenses and is separated in the cooler 6.