Understanding granular mixing to enhance coating performance in a pan coater: Experiments and simulations

Abstract The knowledge of the particle flow and mixing in a pan coater is critical to optimize the design and operation of coating equipment. Mixing is an important but poorly understood aspect of coating of pharmaceutical dosage forms (tablets). Our study focuses on the fundamental mechanisms of granular flow and mixing and their relationship to the coating performance. A quantitative method is developed and validated to characterize the mixing process throughout the mixing vessel. This method is used to establish a baseline determination of mixing homogeneity as a function of various mixing conditions. White and red non-pareils of 5–6 mesh size are loaded in the ellipsoid pan coater to check the effect of initial loading (side–side and front–back), fill level, orientation of the vessel and the vessel speed on granular mixing. Video-imaging and discrete-pocket samplers are used to quantify mixing and to finally estimate the optimal operating conditions. DEM (Discrete Element Method) based numerical model was also developed to study the effect of granular mixing in a pan coater. When the axis of rotation of the mixer is horizontal (no tilt), slower axial dispersion is observed in both the experiments and simulations, than the radial convection. However, tilt enhances axial mixing, and faster axial mixing is seen for higher tilt angles from the horizontal. The speed of the rotating vessel has a nominal effect on the rate of mixing in a coating pan, as observed from the experimental and simulation studies. Moreover, fill level has no significant effect on the rate of mixing. Coating experiments are performed in the pan coater where white non-pareils being coated by spraying Opadry II solution. DEM simulation of coating is performed with post processing particle dynamics data. The effects of various operational and spray parameters are determined on the coating performance. Optimal coating performance is attained at an optimal mixing condition.