(406c) Detailed Analysis of a Large-Scale Wurster Coating Process

The Wurster-coating process is commonly used to layer drug, protective coatings, and other ingredients onto multiparticulate beads in the pharmaceutical industry. Beads can be coated with multiple functional films that control the release rate of drug during dissolution. The coating process takes place inside a fluidized-bed Wurster coater, which achieves a recirculation pattern of the bead flow via a combination of geometric features (Wurster tube insert, air-distributor plate) and operating conditions (air flow rate, spray properties, etc.).

Our past modeling efforts (AIChE 2016:736d, AIChE 2017:776a) have focused on the bead flow and coating in a cold-flow situation; thermodynamics of the process were not included. Similar simplifications were made in other numerical studies (Askarishahi et al. [1]) or, sometimes, the mass transfer effects were completely neglected (Pietsch et al. [2]). However, the thermal phenomena inside the fluid bed are quite complex and influence the final coating attributes. In our present work, we develop a fully integrated CFD-DEM environment including fully-coupled interphase mass and energy exchanges between the solid and fluid phases, in addition to momentum exchange (using the commercial code eXtended Particle System (XPS) coupled with AVL-FIRE^TM). The evaporation from the particle surfaces is modeled via constitutive drying models and species balance between the phases. The spray is idealized using a ray-tracing approach which incorporates the droplet size and droplet velocity distribution in the spray region.

We attempt to connect the bead-scale properties (temperature and drying rates) to the process-scale parameters such as spray rate, spray-liquid temperature, and fluidization-air-flow temperature. This understanding allows us to adjust the process parameters to achieve the desired particle-scale properties; e.g., attaining the correct film forming temperature and bead-drying time scales. After careful validation (against Liang et al. [3]) and verification (against Askarishahi et al. [1]) of the newly-implemented physics, we present simulations of the Wurster coating process. We compare our predictions with experimental data from a pilot-scale Glatt GPCG 1.1 Wurster coater; our comparisons are based on process-scale measurements such as the relative humidity, product temperature, and fluid temperature above the bed region. This coupled CFD-DEM model, once validated, will serve as the basis for subsequent production-scale studies and virtual scale-up efforts.

References

[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.

[3] L. Liang, J. Remmelgas, B. G. M. van Wachem. Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD. Powder Technology, 280, 124-134, 2015.