Evaporation effect on spreading time of a droplet in fluidized bed wet granulation

Abstract

The spreading rate of fine droplets on solid particles is an effective parameter in the probability of particles aggregation in fluidized bed wet granulation. In this paper, evaporation effect on spreading time of a binder droplet on a solid substrate is investigated. Employing force balance between the surface tension force and the binder viscous force and considering the droplet volume change due to evaporation, a mechanistic model is developed. Based on mass conservation equation, evaporation process is modeled by considering diffusion and convection. Also, the local dependence of physico-chemical properties of gas on its temperature is considered. The effect of evaporation rate on droplet spreading time is evaluated for air-ethanol and air-water systems at different conditions of temperature, vapor pressure, contact angle, and droplet diameter by the presented model. The results show that evaporation plays a significant role in spreading of ethanol droplet in comparison with water droplet because of more volatility and smaller contact angle. Moreover, the dimensional analysis is utilized to obtain effective dimensionless numbers (Ohnesorge number, tanθ, humidity, and Schmidt number), and finally, the range of dimensionless numbers in which the binder evaporation plays a significant role in the droplet spreading time is specified for two systems.