(607e) Mapping the Droplet Drying Phenomena within a Fluidized Bed Coater as a Prediction Tool for Coating Qualities

For coating purpose, either agglomeration of pellets or excessive evaporation of the coating droplets should be avoided. This means that the coating process can often only be performed within a narrow operating window. Consequently, the right selection of the process conditions and a delicate process control are required in order to assure a good coating quality. A rapid and continuous process development can be established by understanding the process and describing it from the first principles.

In attempt to obtain a rational process development, in this work, the physical phenomena occurring during a single droplet drying within a fluidized bed coater has been modeled. Moreover, the droplet shape after impact was calculated. Using the model, the influence of various process conditions, e.g. inlet air temperatures, spraying rates and atomization pressures on the extent of the droplet drying could be studied comprehensively. Furthermore, the coatings obtained from the corresponding processes were characterized using a previously developed quantitative image analysis method [1,2], comprising the minimum coating thickness, the span of the thickness distribution, the porosity and the pore size distribution. In this way, the relationship between the extent of the droplet drying and the obtained coating qualities could be assessed, which makes the model also usable as a development tool.

Using the model, the changes of the solvent retained in the droplet and the changes of the droplet size upon drying were simulated, which were coupled with the droplet traveling history. In this way, the changes in the droplet dimensions before and after the collision with the particle could be modeled more accurately. From the simulation results, the influence of the impact kinetics between the droplet and the particle on the spreading extent of the droplet on the particle could be described, next to the influence of the drying conditions, the initial droplet size and contact angle.

It was found that the coating qualities are dependent on both the characteristic time of the droplet drying and the spreading extent of the drop impinging on the particle surface. Big spreading of the coating drop resulting in a thin coating layer leads to a more uniform coating thickness and lower coating porosity. From the given exercise, it has been shown that it is possible using the presented model to better predict the coating qualities, by determining the spreading extent of the coating droplet at any given process conditions.

References

1. F.L.Laksmana, A. Koning, P.J.A. Hartman Kok, H. Vromans, H.W. Frijlink, K. Van der Voort Maarschalk. Imaging and Quantifying the Effects of Different Process Parameters on the Quality of Particle Coating in Fluidized Bed Coater. Proceedings of the 6th World Meeting on Pharmaceutics, Biopharmaceutics and Pharmaceutical Technology. Barcelona, 7-10 April 2008.

2. Y.S.Wu, L. J. Van Vliet, H.W. Frijlink, K. Van der Voort Maarschalk. Pore size distribution in tablets measured with a morphological sieve. Int.J.Pharm.(2007)342(1-2):176-183.