Effect of operational and geometric parameters on the hydrodynamics of a Wurster coater: A CFD-DEM study

A coupled CFD-DEM method was used to study the hydrodynamics of a Wurster coater. Firstly, the CFD part of the model was validated by the accurate prediction of the pressure drops over a pseudo-2D fluidized bed under various gas velocities. The effect of gas velocity, gap height, tube length and batch volume of the particles on the cycle time and the residence time of the particles was thoroughly investigated. The central jet gas velocity u1 was found to speed up the particle cycle but undermine the coating efficiency. The gas velocity at the horizontal transport zone u2 was able to promote the horizontal transport of the particles but should not be too high, otherwise, it would obstruct the normal falling back of the particles in the downward zone. Big gap heights would decrease the coating efficiency but tube length had little impact on that. The increment of batch volume would commonly abase the cycle time and the working efficiency under a given u1. The de-fluidization problem arose when the batch volume increased to 550 mL. However, this problem could be swept out by the optimization of u1 and u2. In a mixture of different sizes, the coarse particles enjoyed higher coating efficiency and could travel closer to the nozzles. This may shield the fine particles from getting enough coating liquids, and thus coarse particles and fine particles were not recommended to get coated in the same batch.