Process for preparing nitric acid with a concentration in the range from 50 to 77.8% by weight

A process for preparing nitric acid in a concentration of from 50 to 77.8% by weight, the process comprising:

• (I) catalytically oxidizing ammonia in the gas phase with a superstoichiometric proportion of process air, to obtain a gas mixture comprising NO and O2;
• (II) cooling and condensing the gas mixture, thereby oxidizing NO with O2 in the gas mixture, to obtain a gas stream comprising NO2/N2O4 and an aqueous solution comprising nitric acid;
• and
• (III) countercurrently absorbing the NO2/N2O4 from the gas stream in an absorber with process water (H2O), to obtain the nitric acid in a concentration of from 50 to 77.8% by weight and an NOx-comprising absorber offgas,
• wherein the water content of the process air is continuously measured and, as a function thereof, the amount of process water, (Q-H2O), fed to the absorber is adjusted, such that the concentration of the nitric acid is from 50 to 77.8% and the NOx content of the NOx-comprising absorber offgas is from 20 to 500 ppm.