Coating of particles (tablets) is done to mask the unpleasant taste or odor of the drug, to control the bioavailability of the API, and to increase shelf-life. The coating solution is sprayed in specific locations of the granular bed and uniformity of coating is achieved by interparticle collisions and overall mixing behavior. Thus, the knowledge of particle flow and mixing in a pan coater is critical to optimize the design and operation of coating equipment. Our study focuses on the fundamental mechanisms of flow, mixing and their relationship to coating performance.

In a lab scale oblate spheroid coater, mixing studies have been performed with different colors of lactose nonpareils. Discrete pocket samplers are used to collect samples as a function of time and space. Discrete Element Method (DEM) based model is also developed to simulate the flow, mixing and coating of particles. Digitally recorded mixing states from experiments are used to fine tune the model. A parametric study is subsequently performed to quantify the effect of vessel speed, tilt of the vessel, fill level on mixing and coating behavior. Similar to the experiments, our model predicts that the axial mixing enhances with the increase in tilt of the coater and that there is slowest axial mixing at the vertical orientation of the coater. In general, the radial convection is estimated to be faster than axial dispersion transport. The speed of the rotating vessel has no prominent effect on the rate of mixing in both the experiments and simulations. Vessel fill level has no effect on the rate of mixing.

At the optimal mixing conditions, a black coating dye is sprayed at different levels of concentrations and flow rates to a granular bed of white nonpareils to study the coating behavior. Coating on the particles is measured by the change in diameter of the particles using vernier calipers. The coating performance is found to be directly proportional to the concentration of the dye and the spray rate. The higher tilt of the vessel, lower vessel speed and higher spray rate enhances coating performance. Moreover, numerical simulation provides the residence time distribution of all the particles passing through the spray zone.