Characterization of coating shells in a Wurster fluidized bed under different drying conditions and solution viscosities

Abstract

Particle coating in fluidized beds is a widely used technique, while the particle growth mechanisms are complex. In this study, porous Al2O3 particles are coated with a sodium benzoate solution in a Wurster fluidized bed with different spray rates, fluidizing gas temperatures, and solution viscosities. The shell porosity, particle size, particle surface, and internal morphology of coated particles are characterized. The growth mechanisms of the coating shell including the effects of drying conditions and solution viscosities are summarized. The growth of the coating shell is distinguished into three modes: layer growth, island growth, and irregular growth. At a slow drying condition, the solute precipitates in the form of crystallization, resulting in the island growth. At a fast drying condition, many fine powders and vapor disturbance exist, leading to the irregular growth. Droplets with smaller viscosity are easier to spread and fill the existing pores, which promotes the layer growth.