(336a) Cross-Scale Validation and Evaluation of Wurster Coating Processes

The Wurster process is of major interest throughout the pharmaceutical industry and is commonly used to modify the release rate or the taste of granulated beads. The coating process takes place in the Wurster tube, in which the particles are transported centrally upwards followed by a fountain region and subsequent downwards movement outside the tube. The beads are gathered in the downbed region before transported through the Wurster tube again. Thus, the particle flow resembles a torus-shaped recirculation pattern.

Due to various equipment scales and complex thermal effects, a process transfer, e.g., from lab to production scale, might be challenging. Furthermore, several combinations of operating conditions can lead to similar outcome in terms of thermodynamic behavior. Basic scale-up routines rely on the scaled mass flow through the bottom plate with correction factors for batch mass, distribution plate and process time. Nevertheless, they neglect differences with respect to the specific coating solvent, thermal operating conditions and particle trajectories (i.e., cycle and residence times).

Due to the complexity of the process, recent numerical investigations were limited to simplifications regarding geometry, batch size or spray modeling (see e.g. Askarishahi et al. [1] and Pietsch et al. [2]). To our knowledge, also the direct comparison between different real scale application devices has not been demonstrated before.

In our work a CFD-DEM framework is presented where a commercial code XPS (eXtended Particle System) is used for the DEM simulation and is coupled to AVL-FIRE^TM for the CFD simulation. Both implement state-of-the-art models for fully coupled momentum, heat and mass exchange. For the lab scale, a numerical model of the Glatt GPCG-2 is built, whereas for the production scale, the Glatt GPCG-30/60 Wurster coater is used. Most importantly, we aim for a novel view on the scale up between the two devices. This is based on (i) particle trajectories, (ii) particle temperature distribution and (iii) drying times. All numerical analysis are validated with large scale experiments and performed at both scales which enables a direct comparison.

[1] M. Askarishahi, S. Mohammadsadegh, S. Radl. Full-Physics Simulations of Spray-Particle Interaction in a Bubbling Fluidized Bed. AIChE Journal, 63(7), 2569-2587, 2017.

[2] S. Pietsch, S. Heinrich, K. Karpinski, M. Müller, M. Schönherr, F. Kleine Jäger. CFD-DEM modeling of a three-dimensional prismatic spouted bed. Powder Technology, 316, 245-255, 2017.