Processing of particulates plays an important role in a wide variety of industries, including the pharmaceutical industry, the bulk chemical industry, and the food industry, to name a few . Yet, despite the ubiquity of particulate systems, a strong fundamental understanding of their behavior is lacking. We have focused on heat transfer in granular materials in order to better understand the drying of particulates. Drying is an important manufacturing step in the production of active pharmaceutical ingredients (APIs) . Several approaches exist to carry out the drying procedure, but a common method used by the pharmaceutical industry is agitated drying. During the process, a wet bed of API is heated in a jacketed cylindrical vessel while being mixed by a rotating impeller until the moisture content is reduced to a desired level. The complexity of the process stems from the fact that heat transfer, mass transfer, and changes in physicochemical properties can occur simultaneously throughout drying. Complications often plague the procedure, including issues such as lengthy drying times, over-drying, nonuniform drying, agglomeration, attrition, and form changes. These circumstances make agitated drying a complicated process to understand and control. When considering scale up, these challenges are coupled with the difficulties typically associated with transferring knowledge from lab scale to pilot or manufacturing scale. As a result, it can be difficult to design an appropriate drying protocol that optimizes heat transfer and can be translated from scale to scale while minimizing the risk for adverse conditions. In this work, we decouple the problem and focus on studying the heat transfer aspect of agitated drying. More specifically, we investigate the influence of process and material parameters on heat transfer in an agitated bed of particle. Our approach consists of employing a combination of experiments and discrete element method (DEM) modeling techniques to analyze how heat transfer scales with bed parameters. We have carried out a design of experiments and computed heating times as well as heat transfer coefficients for the different conditions. We find that scaling influences the flow and compressibility of the bed and therefore creates a balance between conduction and granular convection as the dominant mode of heat transfer.
1. Nedderman RM. Statics and Kinematics of Granular Materials. 2005.
2. Sahni EK, Chaudhuri B. Contact drying: A review of experimental and mechanistic modeling approaches. International Journal of Pharmaceutics. 2012;434, pp 334-348.