Heat transfer in a bladed mixer: scale up and effect of material and process parameters
Creators: name: Hartmanshenn, Clara
Publisher: name: No Publisher Supplied
Year: 2020

description: Granular materials make up a significant portion of the products manufactured by a variety of industries, including the pharmaceutical, bulk chemical, food, and construction industries. Yet, despite the ubiquity of particulate systems, a strong fundamental understanding of their behaviors is lacking. In the pharmaceutical industry, agitated drying of active pharmaceutical ingredients (APIs) is often a complex manufacturing step because it requires a combined understanding of the flow, heat transfer, mass transfer, and physicochemical properties of granular materials. During the process, a wet bed of API is heated in a jacketed cylindrical vessel while being agitated by a rotating impeller until the moisture content is reduced to a desired level. 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 a drying protocol that optimizes performance 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 using a combination of computational and experimental techniques. More specifically, we studied the influence of material properties and operating conditions on both the rate of heat transfer and the heating uniformity for a bed of dry granular material in a bladed mixer. We conducted numerical simulations using the discrete element method (DEM) coupled with a conductive heat transfer model to assess the effect of the material thermal conductivity and the agitation rate on the heating performance. We also carried out experiments using a laboratory-scale agitated dryer and an infrared camera to assess the effect of the agitation rate and compare with the simulation results. Both the simulations and the experiments suggested that slowly agitating the bed considerably improved heat transfer, but that rapid agitation did not always enhance heat transfer. The results indicated that there is a critical rotation rate beyond which agitating the bed faster did not significantly improve heat transfer and that the critical rotation rate depends on the thermal conductivity of the material. Additionally, we developed a dimensionless scaling that enabled us to collapse the data together and obtain an equation relating the heating time of the bed to the thermal properties of the material and the agitation rate. We also quantified the heating uniformity and found that the temperature standard deviation depended on both the thermal conductivity and the agitation rate. For the parameters studied, we found that the scaling could be used to approximately predict both the mean temperature of the bed and the standard deviation over time. Finally, we demonstrated that heat transfer in a bladed mixer could also be studied using a more theoretical approach by calculating the conduction and granular convection fluxes in the bed. Overall, the findings from this work improve fundamental understanding of heat transfer in a bladed mixer and provide insights into how the performance of agitated filter dryers and scale up of these processes can be optimized.