Mathematical Model for the Prediction of Cycle-Time Distributions for the Wurster Column-Coating Process

The application of functional and nonfunctional coatings in the agricultural, food, and pharmaceutical industries is commonly carried out in the Wurster process, which is essentially a bottom-sprayed coating process occurring in a fluidized bed containing a central draft tube or partition. To predict the variability in the distribution of coating mass in a batch operation, information on the cycle-time distribution (CTD) of particles in the bed is required. The current work presents a compartment model to describe the CTD for the Wurster process. The basic model uses a plug flow compartment with dispersion, a mixed tank, and a second mixed tank with bypass, all connected in series. The resulting CTD is asymmetric with a sharp leading edge and a broader trailing edge with a significant tail. The addition of other compartments is used to model some of the anomalous results seen in published results from this type of equipment.