Multi-scale modeling of a spray coating process in a paddle mixer/coater: the effect of particle size distribution on particle segregation and coating uniformity

• DEM is used to study size induced particle segregation in a paddle mixer/coater.
• A new post-processing approach is used to analyze the spray zone in detail.
• Small particles are preferentially coated due to better exposure in the spray zone.
• The particle size dependence of coating growth rate agrees well with experiments.

The purpose of this paper is to investigate the effect of particle size distribution on the flow dynamics, segregation, and coating uniformity in an industrial paddle mixer/coater. First principles modeling approaches with the discrete element method (DEM) and spray post-processing analysis were applied to model particle motion and coating growth. A monosize seed distribution with a particle size of 10.52 mm was used as a baseline DEM simulation. Important spray region variables such as the residence time in the spray region, number of spray region visits, and area exposure percentage were analyzed to characterize the mixing dynamics in the spray. The seed particles were then changed to a log normal size distribution with a relative standard deviation varying from 0.134 to 0.389. Small particles appear preferentially in the spray zone in all polydisperse particle systems, and further analysis on the size dependence of the spray variables shows that small particles have a relatively high frequency of spray zone visits and low shielding. The binary particle DEM simulations with 11.69 mm large particles and 9.01 mm small particles demonstrate segregation patterns in which large particles collect in the end wall region and small particles collect in the spray region, which explains the observed spray preference for small particles. The results of this study highlight the need to understand the effect of mixer geometry on segregation in the particle flow field in order to predict the coating preference with particle size.