Modeling and simulation of a spray column for NO_x absorption

Abstract

Absorption of nitrogen oxides was studied in a spray column of 1.25 m i.d. and 11.5 m height. Mixed-acid solutions of nitric and sulfuric acid were used as absorbents. Specific rates of absorption were measured using a stirred cell with a flat interface. The mathematical model for the adiabatic absorption included: the gas phase reaction and equilibria, gas phase mass transfer, equilibria at interface, liquid phase reactions and associated heat effects. The following outstanding features were also incorporated: heterogeneous equilibria and its role in computing the volumetric absorption rates, formation of HNO₃, HNO₂ in the gas phase and their mass transfer to/from liquid phase, dependence of H(kD)^1/2 on the concentration of HNO₃ with/without other electrolytes and complete energy balance. The spray column has been modeled as a countercurrently operating mass transfer equipment with known initial conditions. Concentrations and flow of the gas stream at the inlet (bottom) of the column were known. Values of the concentration and the temperature of the liquid phase being withdrawn from the bottom of the column were also specified. To begin computing a temperature and concentration profile, a differential element along the height of the column is considered. Equilibrium in the bulk of the gas phase of this differential volume were computed by the Newton-Raphson iterative method. Interfacial partial pressures of water and nitric acid were interpreted from a database by a Newton-Gregory forward-backward difference method. Volumetric mass transfer rates in the gas and the liquid phases were computed by solving two film theory model equations. These rates are used in evaluating derivatives with respect to height and then integrated numerically by using a fourth-order Runge-Kutta method, establishing mass balance across the differential height. Heat changes due to equilibria in the gas phase, absorption and desorption of nitrogen oxides and evaporation of water were computed to estimate the temperature of the incoming liquid phase. Gas phase equilibria are computed for the next element. These computations were repeated for subsequent differential elements until they added up to the desired height. A favorable agreement has been shown between the model predictions and the experimental observations.