Process for the recirculation of nitrogen oxides

A process for reduction of the nitrogen oxide content of exhaust gases produced in the manufacture of nitric acid using at least two absorbers containing heat- and acid-resistant adsorbent consisting of a zeolite molecular sieve of the class consisting of mordenite and clinoptilolite in a heat alternating cycle where a first adsorber is in an adsorbing condition and a second adsorber is to be regenerated, wherein a stream of nitrogen monoxide is oxidized in an exothermic reaction with oxygen to form a hot nitrogen dioxide gas stream, consisting of the following sequential steps, (1) leading directly after the above oxidation said hot nitrogen dioxide gas through said second adsorber at a temperature of 300-350 degrees C. to regenerate it, (2) cooling the nitrogen dioxide gas exiting from the said regenerated adsorber, (3) converting some of the contained nitrogen dioxide with water to nitric acid to create an impure exhaust gas, (4) absorbing the nitrogen oxide content of said impure exhaust gas in said first adsorber to form purified exhaust gas at a temperature of 20-40 degrees C., (5) scavenging said second adsorber with a part of said purified exhaust gas, (6) recycling said part of purified exhaust gas into the aforesaid hot nitrogen dioxide gas stream before said cooling, (7) removing exhaust gas and replacing the exhausted first adsorber by the regenerated second adsorber.