Tablet coating in lab-scale drum coaters: Combining DEM simulations and spray experiments to predict tablet coating

In pharmaceutical industry the coating of granulates or tablets is an essential production step, which is often realised in a rotating drum moving the tablet bed and a number of spray nozzles installed above this bed. The nozzles atomise the coating liquid through a high-velocity annular air-stream and apply steering air nozzles for producing an oval-shaped spray. The operational conditions determine the produced droplet size spectrum. The collision of the droplets with the solid particles or tablets in the drum and the resulting coating and film thickness strongly influences the product properties of granulates or tablets. In order to be able to predict the coating result, without running time-consuming lab-scale experiments, it is intended to numerically simulate the entire tablet coating process by combining Computational Fluid Dynamics (CFD), the Discrete Element Method (DEM) and the Lagrangian Parcel Concept (LPC) for simulating the spray propagation originating at the nozzle. For doing so, knowledge on the spray droplet properties produced by the two-fluid spray nozzles is indispensable. Therefore, it is necessary to characterise as a first step the local droplet size and velocity distributions in different spray cross-sections, which is the main focus of the present contribution. Such measurements are efficiently done by applying Phase Doppler Anemometry (PDA). It was found that the velocity of the spray emitted from the two-fluid nozzle exceeds almost 200 m/s and the number mean droplet diameter was well below 20 μm. In the initial spray region droplet break-up was observed whereas downstream of 50 mm a slight growth of the mean droplet size indicated further coalescence. Due to the applied pattern air, the spray flux distribution at the normally located tablet bed (i.e., 100–150 mm downstream the nozzle) has an oval shape and the mean droplet velocity still exceeds almost 50 m/s with a mean droplet diameter of about 10 μm. The cross-sectional measurements are used to provide proper inlet conditions for the spray simulations, while the other cross-sections are used for validating the computed spray expansion and dispersion. For demonstrating the effect of a real measured spray flux distribution on the tablet coating quality, additionally DEM simulations were conducted. Here, the measured spray flux data were mapped onto the temporarily evolving and moving tablet surface simulated for a lab-scale test drum by DEM. The uniformity of tablet coating (inter-tablet variability) was judged by calculating the Coefficient Of Variation (COV) for the considered different tablet shapes and operational conditions.

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Lars Pasternak, Martin Sommerfeld, Pradeep Muramulla, Fei-Liang Yuan, Srikanth Gopireddy, Nora Urbanetz, Thomas Profitlich, Tablet coating in lab-scale drum coaters: Combining DEM simulations and spray experiments to predict tablet coating, Powder Technology, 2023, 118683, ISSN 0032-5910,
https://doi.org/10.1016/j.powtec.2023.118683.

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