Investigation of gas-solid heat and mass transfer in a Wurster coater using a scaled CFD-DEM model

Detailed knowledge about the gas-solid heat and mass transfer is essential for the design and optimization of the fluidized bed coating process. Coupled computational fluid dynamics and discrete element method (CFD-DEM) is a powerful tool to understand such a process. However, in previous CFD-DEM studies, the inter-phase heat and mass transfer appeared to be substantially simplified or even neglected. In this work, a complete and reliable CFD-DEM model is proposed to investigate the Wurster coating process. Considerable attention is paid to numerical methods on the particle-fluid heat convection, coating liquid spraying and the evaporation of sprayed liquid on the particle surface. To allow the simulation of coating processes with many particles in an acceptable runtime, a novel scaling approach accounting for the interphase heat and mass transfer is proposed. The proposed CFD-DEM model is then validated by the simulation results under different scaling degrees and the temperature/humidity data in experiments. The simulations reveal three modes of particle drying in the coating process, and multiple humidity probes mounted along the vertical direction of the annulus region would be helpful to predict the over-spraying in advance. The proposed methodology can be extended to the spouted and top-spray beds.