Abstract
Spray drying is a method for producing dry powder particles (e.g., instant coffee, milk, starch, cheese, bio-therapeutics, etc.) by atomizing a slurry and rapidly drying it using a hot gas in a drying chamber. 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. Such a model can be later incorporated into full-scale drying chamber simulations, where CFD modeling of the drying gas flow can be coupled to these single droplet models. The drying model is divided into two stages of drying with distinct equations accounting for heat and mass transfer during these stages. The first stage consists of droplet shrinkage with loss of moisture and diffusion of dissolved or suspended solid particles inside the droplet. At the end of the first stage, the volume fraction of the dissolved or suspended solid content on the droplet surface reaches a critical saturation level at which point a porous crust is formed indicating the start of the second stage. Heat and mass transfer from the droplet is assumed to occur through the porous crust. The model is validated using single droplet drying experiments from the literature showing reasonable agreement with the experimental data for temperature and mass evolution. The model is then used to conduct parametric studies to understand the role of the different factors that influence the final droplet morphology. The results show that the droplet mass and temperature evolution are affected greatly by the drying gas velocity and temperature, while the relative humidity of the drying gas has only a moderate effect. The model can be applied to ascertain the optimal external conditions needed to obtain particular particle characteristics.
