462777 Comparison and Validation of Coupled CFD-DEM and CFD-TFM Simulation Predictions for a Laboratory-Scale Wurster-Coating Process

Tuesday, November 15, 2016
Grand Ballroom B (Hilton San Francisco Union Square)
Peter Böhling1, Dalibor Jajcevic1, Matej Zadravec1, Conrad Davies2, Alan Carmody2, Pankaj Doshi3, Mary T. am Ende3, Johannes G. Khinast4,5 and Avik Sarkar3, (1)Research Center Pharmaceutical Engineering GmbH, Graz, Austria, Graz, Austria, (2)Worldwide Research and Development, Pfizer Inc., Sandwich, Kent, UK, (3)Worldwide Research and Development, Pfizer Inc., Groton, CT, (4)RCPE GmbH, Graz, Austria, (5)Institute of Process and Particle Engineering, Graz University of Technology, Graz, Austria

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 bed—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 Fire™ (AVL List GmbH Graz, Austria). The CFD-TFM simulations used Ansys Fluent™ (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 (180o) and quarter (90o) models, taking advantage of azimuthal symmetry, are also performed—such a strategy would be important to enable simulation of larger-scale Wurster coaters. The CFD-DEM method shows better agreement between the full- (360o), 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

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