Mathematical model based on film theory to predict the absorption of sulfur dioxide by a calcium hydroxide slurry in a spray dryer during the falling rate period

The purpose of this study was to expand on an existing mathematical model describing SO₂ absorption by a calcium hydroxide slurry in a spray dryer. The existing model provided a mechanistic approach to the mass and heat transfer occuring within the spray dryer. It also provided a rate equation based on film theory to predict SO₂ absorption during the constant rate period. Within the spray dryer, the constant rate period occurs when there is a continuous liquid phase in the atomized slurry droplet. The constant rate period continues until evaporation causes the diffusion paths to become restricted by the calcium hydroxide absorbent particles touching. At this point, the falling rate period begins and continues until evaporation causes an equilibrium moisture concentration to be reached. This study continues the modeling effort of the constant rate period by modeling the falling rate period. The model developed for the falling rate period is based on film theory and a series of resistances. The resistances are the gas film, liquid film, solid dissolution and the product ash film. The shrinking core model was used to describe the formation of dried product (ash film) during the falling rate period. An effective diffusivity parameter was regressed such that this model agreed with the experimental data. This model fit the data very well, however, the regressed parameter was unrealistically high. The modelling activity was terminated at this point.