Modeling the absorption of SO2 in a spray scrubber using the penetration theory

A model based on the penetration theory has been developed to calculate the dynamic absorption rate of sulfur dioxide into a droplet of limestone slurry. The model includes both instantaneous equilibrium reactions and reactions with finite rates; limestone dissolution, sulfite oxidation, gypsum crystallization and the hydrolysis reaction of CO2. The model has been used to quantify the mass transfer within a spray scrubber and to estimate the impact of the reactions with finite rate of the SO2 mass transfer. The variations within the physical mass transfer conditions of a spray scrubber have been simulated by assuming high mass transfer coefficients close to the nozzles and low coefficients below the spray region. The developed concentration profiles of the diffusing species and the depth of penetration have been determined for different penetration times. The calculations show that the absorption of SO2 into a limestone spray scrubber to a large extent is liquid-side controlled. Only at the very top of the absorber, where the partial pressure of SO2 is low, is the gas film resistance above 50%. Limestone dissolution close to the gas-liquid interface has been shown to be of significance at low pH and in the parts of the absorber where the internal circulation inside the droplets is low. The impact of the hydrolysis reaction of CO2 on the absorption rate of SO2 has been studied by varying the reaction rate constant. Simulations show that the rate constant has a large impact on local absorption rates. Depending on the length of the contact time between the gas and the liquid, the assumption of an instantaneous hydrolysis reaction has an impact on the overall SO2 absorption rate. (C) 1997 Elsevier Science Ltd.