Simulation of pellet coating in Wurster coaters.

A combination of computational fluid dynamics (CFD), discrete element method (DEM), and discrete droplet method (DDM), i.e., a CFD-DEM-DDM model, was developed to simulate coating of pellets in a Wurster coater. The model equations were implemented in parallel using an approach that uses the computational resources of both CPU and GPU. Effects of the gas flow pattern, inlet gas temperature, partition gap, and spray characteristics were studied on the process. Decreasing the peripheral gas velocity, increasing the central jet velocity, and reducing the partition gap caused more uniform distributions of the circulation time and draft tube time, while the inlet gas temperature had negligible effect on them. Very high jet velocity caused a wider distribution of the circulation time. The dynamics of the spray and its interaction with pellets had significant effects on the coating mass distribution. Widening the spray angle while maintaining the droplet size constant caused the most uniform coating mass distribution and the highest deposition rate. Heat and mass transfer conditions as well as the deposition pattern changed the distributions of the solvent content and temperature of the pellets.