(663c) Detailed Numerical Analysis of Tablet Coating Processes

Spray coating is frequently used in the pharmaceutical industry to control, e.g., the release of the active pharmaceutical ingredient of a tablet or to mask its taste. Coating is typically performed in rotating drums where the coating liquid is sprayed onto the moving particle bed. Coating uniformity is of extreme importance in the pharmaceutical industry as the coating usually has critical functional properties.

Coater performance depends on different parameters, where the following parameters seem to be the most important, i.e., (i) the extension of the coating zone where particles are effectively coated and (ii) the transport between the coating zone and the zone where particles are not seen by the spray. Beside experimental work, numerical simulations of particle motion using the Discrete Elements Method (DEM) have become an extremely important tool in particle technology problems (i.e. Radl et al., 2009) and are frequently used for coater simulations (e.g., Kalbag et al., 2008). Typically, these simulations are concerned with particle flow only and focus on the residence time of the tablets in the spray zone. For example, the statistical approaches presented in many publications (e.g. Dubey et al., 2008) completely neglect the complex particle-gas-liquid system. However, the interaction between the coating spray and the tablets strongly impacts the uniformity of the film on the tablet itself. The effects leading to this ?on tablet? uniformity have not been analyzed in detail yet by means of simulations. Hence, the mechanisms controlling the uniformity of the film on a single tablet are currently not fully understood.

The major objective of this work is to locally analyze the coating process by using sophisticated CFD (Computational Fluid Dynamics) multiphase spray simulations. The impact and the deposition of droplets on tablets with different shape, as well as the production and evolution of the liquid film on the surface of the tablets are numerically modelled. Furthermore, the evaporation of droplets in the spray and from the wall film is taken into account in order to estimate the effects of the drying gas flow. Spray droplets are simulated with a sophisticated DDM (Discrete Droplets Method) Euler-Lagrange approach, also known as Lagrangian Monte Carlo method (Dukowitz, 1980). Models for multi-component evaporation and particle/wall interaction are also taken into account. The wall film is treated with a two-dimensional model incorporating submodels for interfacial shear force, film evaporation and heat transfer between film, solid wall and gas phase.

In our work, we used detailed DEM simulations of non-spherical particles in a coater as input for our sophisticated CFD simulations. The subsequent Euler-Lagrange simulations show how different physical parameters of the coating spray affect the coating process on a single tablet. Thus, the deposition behaviour of the droplets on the tablet surface is analysed for the first time in terms of film thickness and its homogeneity on the tablet. The outcome of this work provides a deeper understanding of the local interaction between the spray and the tablet bed, allowing a step ahead in the design, optimization and operation of industrial coating devices.

References

Aliseda, A., Hopfinger, E.J., Lasheras, J.C., Kremer, D.M., Berchielli, A., Connolly, E.K., 2008. Atomization of viscous and non-newtonian liquids by a coaxial, high-speed gas jet. Experiments and droplet size modeling. Int. J. Multiphase Flow 34, 161-175.

Dubey, A., Portillo, P.M., Muzzio, F.J., 2008. Study of Tablet Coating Thickness Uniformity Using LIBS, DEM and Compartment Modeling. 2008 AIChE Annual Meeting.

Dukowicz, J. K., 1980. A Particle Fluid Numerical Model for Liquid Sprays. J. Comp. Physics, 35, 229-253.

Kalbag, A., Wassgren, C., Penumetcha, S.S., Perez-Ramos, J.D., 2008. Inter-tablet coating variability: Residence times in a horizontal pan coater. Chemical Engineering Science 63, 2881-2894.

Radl, S., Kalvoda, E., Glasser, B.J., Khinast, J.G., 2009. Mixing Characteristics of Wet Granular Matter in a Bladed Mixer. Submitted to Powder Technology.

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