Exploring a coating design spacing using experimental and simulated coating techniques

This study aimed to enhance an understanding of a laboratory-scale tablet coating process using CFD-DEM simulations to model the momentum, heat and mass transfer between the tablets and the fluid phase. Eight process points were selected from an experimental design space study and simulated in silico. The simulations were analyzed regarding the tablet movement, the coating mass distribution, the tablet and air temperatures and the solvent evaporation. The results indicated that the drying airflow rate significantly affected the tablet movement and the coating mass distribution, with higher flow rates resulting in steeper tablet beds and higher velocities. Under all process conditions, a linear decay of the coefficient of variation over time occurred, aligning with the experimental findings. The predicted and experimental values of the tablet bed and outlet air temperatures matched well. However, higher temperatures led to over-predictions due to neglecting the heat loss to the environment. The spray evaporation and the loss on drying (LoD) primarily depend on the drying air temperature and the spray rate, which also influences the drying time. Higher drying air temperatures increased the spray evaporation and decreased the LoD. Suggested drying times to ensure that the tablets reenter the bed dry were below 1 s. The outlet air humidity is inversely related to the drying airflow rate. Overall, the simulations demonstrated the potential of using advanced CFD-DEM models to replicate experiments digitally, providing opportunities for optimizing tablet-coating processes through a digital exploration of process space.