Evaluation of pellet cycle times in a Wurster chamber using a photoluminescence method

Particle cycle time variation in a fluid bed coater is an important parameter that affects inter-particle coating uniformity. For the purpose of analyzing the pellet circulation process, a tracer particle photoluminescence detection system was developed and used in a laboratory scale fluid bed Wurster chamber. In order to tackle the problem of optically dense flow within the Wurster tube, transitions of a photoluminescent pellet were recorded in the down-flow region. Tracer pellets were representative for evaluated populations of pellets in terms of size and density. Pellet cycle time distributions were assessed for processes with varying particle size, gap, fluidizing air flow rate and loading mass. A higher air flow rate and gap decreased the distribution mode whereas a decrease in gap and an increase in the air flow rate and loading mass decreased the magnitude of pellet cycle time variation. Additionally, the variation in the position of tracer pellet successive passes was assessed and the change in transition azimuth was connected to stability of gas-solids flow. We also estimated that for the homogeneous spray case the average circulation process variability contribution to the total coating mass variability is relatively small. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.