Reduction of the nox waste gas concentration in the production of nitric acid during a shutdown and/or start-up process of the production device

A process for reducing a concentration of NOx nitrogen oxides in residual gas obtained during a startup or a shutdown of an apparatus for preparing nitric acid from NOx nitrogen oxides, the apparatus for preparing nitric acid comprising a reactor configured to produce NOx nitrogen oxides, an absorption apparatus configured to absorb at least part of the NOx nitrogen oxides produced in an aqueous composition, a residual gas purification plant configured to at least one of decompose or reduce unabsorbed NOx nitrogen oxides, feed means configured to feed the NOx nitrogen oxides from the reactor to the absorption apparatus, discharge means configured to discharge the unabsorbed NOx nitrogen oxides from the absorption apparatus to the residual gas purification plant, and a bypass configured to transfer a gas mixture from the reactor to the residual gas purification plant while bypassing the absorption apparatus during the startup or the shutdown of the apparatus for preparing nitric acid, the process comprising:

• stopping feeding of the gas mixture from the reactor to the absorption apparatus;
• and
• transferring the gas mixture from the reactor to the residual gas purification plant, bypassing the absorption apparatus.