Process intensification in manufacture of nitric acid: NOx absorption using enriched and pure oxygen

Abstract

Absorption column occupies a dominant place in the manufacturing of nitric acid. A mathematical model has been developed for the absorption of NOx gases using air and enriched oxygen in packed columns. For the first time, this paper considers the kinetics of nitrous acid decomposition and oxidation in the liquid phase. Till date, the published literature considers the instantaneous decomposition of nitrous acid in the liquid phase. The same case has also been considered in this paper as the base case. In addition, three more cases have been considered: (1) decomposition and oxidation kinetics of nitrous acid with a typical concentration of oxygen which is usually employed in nitric acid plants (2) complete decomposition and oxidation with enriched oxygen (3) complete conversion of nitrous to nitric acid by liquid phase nitrous acid oxidation. Enriched oxygen results in substantial reduction in column volume and/or the operating pressure. The enriched oxygen enhances the rate of nitrous acid oxidation (to nitric acid) and hence the nitrous acid available for decomposition reduces throughout the column. This implies that lesser quantity of nitric oxide gets desorbed to the gas phase. This feature has crucial implication because the need for gas phase oxidation of NO reduces and hence the absorption column becomes compact due to reduction in oxidation volume. Thus, the enriched oxygen plays a significant role in process intensification and equipment miniaturization. Additionally, the liquid phase oxidation of nitrous acid carried out in a suitable oxidizer was found to give substantial additional reduction in the column volume.For packed column absorber, the effects of design parameters viz. column diameter, heights of packed and empty sections, and operating parameters such as pressure and temperature have been examined.