Numerical simulation of spray drying using droplet drying kinetics

Spray drying is the most widely used method for producing dry powder particles (e.g., instant coffee, milk, starch, cheese, bio-therapeutics, etc.) by atomizing a liquid or slurry and rapidly drying it using a hot gas as the drying medium. We are developing a numerical model to accurately predict spray droplet evaporation and evolution of particle formation in spray drying. Understanding the evolution of particle formation in a spray dryer is important as it directly influences the final product quality. Experiments inside a spray dryer are often not able to shed light on the physical processes governing the particle formation at the scale of an individual particle. Hence, we are developing a numerical method that uses complex mass and heat transfer equations to account for the change in properties of a single droplet based on advanced droplet drying kinetics. The drying kinetics model consists of two stages of drying. First stage consists of droplet shrinkage with loss of moisture and diffusion of dissolved or suspended solid particles inside the droplet. Second stage starts when the concentration of the dissolved or suspended solid content on droplet surface reaches a critical saturation level at which point a porous crust is formed. The drying stops when the moisture content within the droplet reaches a desired value. The key feature of the model is the inclusion of first order dominating physics at all stages of drying without compromising on speed and efficiency of the model. The model is validated using single droplet drying experiments from literature. Details of the methodology along with simulation results will be discussed.