A mechanistic spray dryer mathematical model based on film theory to predict sulfur dioxide absorption and reaction by a calcium hydroxide slurry in the constant rate period

Removal of SO₂ from flue gases is an important air pollution abatement phenomena. In the spray dryer, flue gas from a coal-fired boiler is contacted with an atomized lime slurry; during this contact SO₂ absorbs and reacts with dissolved Ca(OH)₂. The mathematical model developed in this activity superimposes SO₂ absorption and reaction phenomena on existing mathematical descriptions of spray dryer operation. The SO₂ removal appears to occur primarily in the constant rate period where a continuous liquid phase exists in the atomized slurry droplet. The constant rate period proceeds until evaporation has reduced the liquid phase volume to the point where the Ca(OH)₂ sorbent particles touch and the diffusion paths for reactants are restricted. The SO₂ absorption flux involves liquid phase as well as gas phase resistances. The liquid phase resistance includes mass transfer and chemical reaction phenomena associated with the absorption and reaction of SO₂ and Ca(OH)₂ and the dissolution of Ca(OH)₂. Instantaneous reaction occurs between SO₂ and Ca(OH)₂ in the liquid phase. Solid dissolution in the liquid film is unimportant and solid dissolution and reaction occur in series. A comprehensive model was developed for the constant rate period. The model is based on film theory and treats the atomized slurry droplet as a sphere of discrete sorbent particles with the fluid phase uniformly distributed around the individual sorbent particles. This concept allows prediction of the mass transfer coefficients and the enhancement due to increasing solids concentration as evaporation proceeds. Efficiency predictions using the model were compared with pilot plant data taken at different inlet flue gas temperatures, stoichiometric ratios and slurry flow rates.