The prediction of variability occurring in fluidized bed coating equipment. II. The role of nonuniform particle coverage as particles pass through the spray zone

The purpose of this work was to evaluate the relative importance of particle circulation and particle-to-particle mass coating distribution on the overall mass coating distribution obtained in a Wurster process. A series of batch coating experiments was carried out over a range of operating conditions, in order to evaluate the particle-to-particle variation in the mass distribution of coating material deposited during batch coating operations. Results showed that the major component of variance was due to the variation in the amount of coating received per particle per pass through the spray zone. The variation in the number of times a particle passed through the spray zone was considerably less important. Two models were developed to explain the results of the experimental program. The first model categorized particles moving through the spray zone as either receiving coating or not. Thus, the distribution of coating material per particle per pass is described by a Bernoulli probability distribution. Using this picture of the spray process, the number of particles receiving coating during any given pass through the spray zone was found to vary between 2 and 6%. A second model was developed to explain the major cause of variation. This model explains the variation in terms of the hindering or sheltering effect that particles close to the source of the spray have on particles farther away. Although the agreement of model predictions with experimental results is only fair, it is believed that this model captures the main cause of particle-to-particle variation occurring in batch coating operations and thus is the first model to explain this phenomenon.