Intensification of spray drying granulation process by gas absorption accompanied by chemical dissociation reactions

Abstract

• Model for slurry droplet drying in gas mixture containing soluble gas is developed. • Filtration of a vapor–gas mixture inside a porous shell is described by Darcy's law. • Presence of active gas enhances evaporation rate during the entire drying process. • Suggested alternative drying method can be used as part of a novel technology. In the present study, we developed a transient model for the drying of a single slurry droplet moving in a multicomponent gaseous mixture containing a soluble gas. The comprehensive model accounts for the effects of the soluble gas absorption/desorption, filtration, and compressibility of the gas–vapor mixture inside the porous crust on the intensity of the drying of the slurry droplet. The model is based on an application of the theory of heat and mass transfer during slurry droplet evaporation, in conjunction with a model for gas absorption/desorption as accompanied by a chemical dissociation reaction. It is shown that the presence of the active gas increases the evaporation rate during the entire drying process. As shown by numerical calculations, in a gas mixture containing air and ammonia with an ammonia mass fraction of 0.2, at a temperature of 293 K and humidity of 50%, the drying time of silica-aqueous slurry droplets with a radius of 250 μ m is approximately 35% shorter than that in a gas mixture not containing an active gas. We also found that at the second stage of a slurry droplet drying, the desorption of the dissolved gas from wet core decreases the temperature of the porous shell and reduces the mechanical stresses inside it that prevent the destruction of porous granules. The computational results obtained using the developed model are validated based on a good agreement with the available experimental data. Accordingly, the suggested model can be considered as a basis for alternative drying technologies.