Mathematical Model for the Removal of Sulfur Dioxide in a Pilot Scale Spray Dryer

The purpose of this study was to develop a mathematical model for the absorption of SO₂ into droplets of a Ca(OH)² slurry in a pilot scale spray dryer. This model is an expansion of the two film theory and uses basic spray drying expressions to predict contact times.

The developed model uses a series of resistances and a series of drying periods. The resistances are: (a) gas film, (b) liquid film, (c) solid dissolution, and (d) ash film. The drying periods are: (a) constant rate and (b) falling rate. The model also includes droplet breakup into particles between the constant and falling rate drying periods.

The results from the model indicate that as the stoichiometric ratio between SO₂ and Ca(OH)₂ is increased, SO₂ removal efficiency increases because of the increasing number of particles being generated. The effect of reducing the temperature approach to saturation (difference between dry bulb and wet bulb temperatures) in the spray dryer increases the SO₂ removal efficiency because of the increased contact time during the falling rate drying period.

The model shows that a rigorous development for the dry scrubbing process is possible. Further research should consider product recycle effects and the competing reaction by CO₂.