Numerical Study on Spray Drying Process: Effect of Nonuniform Temperature Field and Interaction between Droplets on Evaporation Rates of Individual Droplets

Spray drying process is widely used to produce particulate materials in the pharmaceutical industries, such as porous materials for direct compression, solid dispersion for improvement of drug dissolution properties, micro encapsulation to stabilize active compounds, taste masking, preparation of dry powder for inhalation. However, as many factors affect the physical properties of dried particles and the spray drying processes have complex behaviors in which heat and mass transfer occur simultaneously, the detailed mechanisms of dry particle generation have yet to be sufficiently elucidated. In this study, computational fluid dynamics was used to simulate water droplet evaporation in a spray dryer, and the evaporation kinetics of "individual droplets" in the droplet aggregate (group) were analyzed. The numerical simulation revealed that each droplet had different evaporation rates owing to the following two reasons. First, the driving force of evaporation, Delta T, changed every moment as the droplets traveled through different temperature fields in the drying tower. Second, it was calculated the driving force for droplet evaporation differed from the ideal system because the evaporation of other droplets changed the fluid characteristics around the droplets. The obtained results are important findings that lead to the understanding the spray drying process to design and manufacture the pharmaceutical products.