Scaling Particle Dynamics in Rotating Drum Coaters

Scaling Particle Dynamics in Rotating Drum Coaters

J. Ban, R. Kumar, and C. Wassgren

School of Mechanical Engineering

Purdue University

Abstract

Scaling  particle coating operations is an important issue for a number of industries, including those that process pharmaceuticals, consumer products, and chemicals.  Scaling of particulate processes, and coating in particular, has been the subject of numerous experimental, computational, and analytical studies. Unfortunately, a dependable scaling method that fully translates data gathered at the lab scale to the industrial scale does not exist, especially since geometric similarity is often neglected.  This study investigates how imperfect particle size scaling affects the particle dynamics and inter-particle coating variability in geometrically similar pan coaters.  DEM computer simulations were used to model particle movement in five different-sized rotating cylindrical drums, resulting in five different drum to particle diameter ratios, with identical fill fractions and Froude numbers.  Measurements of particle surface velocities and time spent in the coating zone were made.  The spray coating zone residence time coefficients of variance (CoVs) were used to determine the degree of inter-particle coating variability.  Surface velocity profiles show good agreement with those found in the literature; however, the velocity scaling behavior is different.  The size of the coating zone was found to have no influence on the long term CoV behavior, a result that has not been described previously in the literature.  Decreasing the drum to particle diameter ratio results in reduced particle mixing and smaller CoV values after the same number of drum rotations.  This last finding demonstrates that the current approach to scaling without maintaining geometric similarity between the particles and the drum can result in significant errors.