Invited review absorption of NOX gases

Abstract

Oxidation of nitric oxide is an important step in NO_x absorption. Recent advances namely catalytic oxidation and the use of slrong oxidizing agents have been reviewed. The refinements which have been made in the kinetics of NO oxidation have been discussed. Several liquid phase absorbents have been suggested for the NO removal, particularly for the purpose of pollution abatement. Different models for NO oxidation using nitric acid have been analysed and the limitations of published information have been brought out. Absorption of tetravalent nitrogen oxide (NO₂ and N₂O₄) has received considerable attention. The absorbents include water, nitric and sulfuric acids, sodium hydroxide, sodium sulfite and sodium chlorite. Absorption in water and nitric acid is important in the manufacture of nitric acid. The published information on the mechanism of NO₂ and N₂O₄ absorption in water has been critically analysed. The problem of nitrous acid decomposition has been analysed on the basis of film theory. The predictions agree favourably with the available experimental data. A new mathematical model has been developed for a packed column. Performance charts have been prepared which take into account the effects of concentration of nitric acid, superficial liquid velocity, temperature and the partial pressures of various components. A simplified model on the basis of NO oxidation has also been presented. Various process design aspects have been discussed in detail. Optimum values for various operating parameters exist and basis for their selection has been explained. Some engineering aspects such as the selection of equipment and material of construction have also been discussed. A comparison between semi-batch and continuous modes of operation has been presented. The problem of the manufacture of pure nitrites has been analysed. Specific recommendations have been made regarding the use of equations and procedures in design. The knowledge gaps have been clearly brought out and suggestions have been made for future work.