(448n) Comparison and Validation of Coupled CFD-DEM and CFD-TFM Simulation Predictions for a Laboratory-Scale Wurster-Coating Process

Fluidized-bed Wurster coaters are commonly employed in the pharmaceutical industry to coat beads containing drug products. The Wurster-coating process is comprised of an internally-circulating fluidized bedâ??an internal cylindrical riser tube (Wurster insert) is used to channel the air and bead flow to establish the desired circulating bead-flow pattern. Two computational fluid dynamics (CFD) based approaches to simulate a lab-scale Wurster coater were compared: CFD-DEM (discrete-element method) and CFD-TFM (two-fluid model). The CFD-DEM model provides better particle trajectories but is computationally more expensive, whereas the CFD-TFM simulations are numerically faster but sacrifice the resolution of particle trajectories and collisions.

Predictions from both approaches are validated against the experimental measurements reported by Li et al. [AIChE J. (2015), 61(3), 756-768]. The CFD-DEM simulations were performed using the XPS code (eXtended Particle System, developed internally at RCPE) coupled with commercial CFD software AVL Fireâ?¢ (AVL List GmbH Graz, Austria). The CFD-TFM simulations used Ansys Fluentâ?¢ (ANSYS Inc. Canonsburg, USA). The objective of this study to determine which modeling methodology is best-suited to simulate the Wurster coating process. More specifically, the ability of CFD-TFM simulations to generate the desired residence-time-distribution data in various regions of the coater is contrasted to the more-expensive CFD-DEM method.

For both CFD-DEM and CFD-TFM approaches, the bead flow velocities, residence time in various zones, and the bead-cycle-time distributions were measured to allow direct comparisons to Li et al. The present results, using both CFD-DEM as well as CFD-TFM, agree quantitatively and qualitatively with the experimental results given by Li et al. However, a comparison of the clustering and bubbling dynamics reveal significant differences between the CFD-DEM and CFD-TFM models. Although it is difficult to precisely identify the source of these differences, it can be stated that the CFD-DEM predictions are more visually similar to the flow of actual beads.

Simulations using half (180^o) and quarter (90^o) models, taking advantage of azimuthal symmetry, are also performedâ??such a strategy would be important to enable simulation of larger-scale Wurster coaters. The CFD-DEM method shows better agreement between the full- (360^o), half-, and quarter-sized systems, whereas the CFD-TFM approach was not amenable to this type of geometric idealization. Therefore, CFD-DEM has been demonstrated to be better suited to simulate the current Wurster-coating