(418s) Investigating Contact Drying In a Filter Dryer: Experiments and Simulation

Pharmaceutical manufacturing processes often consist of a series of unit operations, each intended to alter certain properties of the materials being processed leading to the embodiment of certain desired properties of the final product. To ensure acceptable variations, significant importance should be given to the mechanistic and parametric attributes of each of the unit processes. Our study focuses on quantitative investigation of mixing and heat transfer in a filter dryer in the quest to determine the optimum drying conditions.  Fundamental process understanding and efficient process modeling schemes needed for the development of robust process are lacking. We use discrete element method (DEM) based numerical model, to improve the understanding of agitated vacuum drying process, and hence develop quantitative approach for optimization. DEM explicitly considers inter-particle and particle-boundary interactions, providing an effective tool to solve the transient heat transfer equations.

Once the dryer vessel is charged with the slurry (1L), nitrogen is purged through the drying chamber. The granular bed is then dried by applying heat to the inner wall of the vessel. During each batch run, at repeated intervals, the impeller is stopped and a thermocouple is placed through one of the ports in the dryer to measure the temperature of the bed as a function of time. At the same time, the samples of wet glass beads/Lactose are withdrawn using a cup sampler and stored in tared vials to measure the loss in weight. This leads to calculation of drying rate as a function of material properties and operational conditions of the filter dryer. Simulations are performed to check the effect of various system variables: impeller speed (5rpm, 15rpm, and 25rpm), fill ratio (25%, 45%, and 65%) and the wall temperature (313 K, 323 K, and 333 K) on the drying performance of the two granular systems (glassbeads or lactose with ethanol). Typical system with glass beads and lactose powder are numerically simulated using appropriate material properties.

Higher wall temperature showed an increase in the drying rate for both glass beads and lactose systems, thereby decreasing the total time for drying operation. Also at given wall temperature, the increase in fill volume (bed depth) resulted in a decline in the drying rate. Furthermore, as the rotational speed of the impeller decreases, the heat transfer in the granular bed increases. The heat transfer coefficient are also calculated for different parameters under study. Hence the optimal drying condition is determined. Current study can subsequently be used to direct future design of experiments based on a rationalized approach rather than the simple and usual trial and error method.