Role of nitrous acid decomposition in absorber and bleacher in nitric acid plant

Abstract

The stringent statutory regulation for clean environment makes the absorber and the bleacher in the nitric acid plant two very important equipments. The NOx finally released to the atmosphere from the absorber, is based on the mathematical calculations of the nitric acid and nitrous acid concentration profiles along the length of the absorber. The present work focuses on the role of HNO₂ decomposition in the absorber and the bleacher in the nitric acid plant. The decomposition of HNO₂ is a heterogeneous reaction resulting into the formation of nitric acid and desorption of nitric oxide. At high nitric acid concentration (40-50 wt.%), the reversibility of HNO₂ decomposition due to NO oxidation with nitric acid is important. The model for bleacher column is presented, which takes into account the de-colorization of the product acid from the absorption column and makes it free from the nitrous acid. The model for the bleacher also takes into account the decomposition and oxidation of HNO₂ with dissolved oxygen. The models proposed for absorber and bleacher are validated from the data collected from a mono-pressure nitric acid plant as well as the data present in open literature.