(224g) Particle Dynamic Simulations of Heat Transfer in a Bladed Mixer: Effect of Material and Process Parameters

Drying of active pharmaceutical ingredients (APIs) is an important step in the production of pharmaceuticals that controls the moisture content of the drug crystals. The drying step is carried out after crystallization and filtration of the API. Insufficient drying can lead to microbial growth and polymorph transformations. Often the API is heat sensitive so the temperature during drying must be carefully controlled. While a variety of dryers are used in the pharmaceutical industry, one of the most popular types is an agitated filter dryer. Such dryers are fairly simple in form in that they can be approximated as a cylinder with a rotating impeller (bladed mixer) with heated walls. During the drying step, the API particles undergo simultaneous transfer of mass and heat such that moisture content is reduced to a desired level. In this work, we focus on understanding heat transfer in a bladed mixer. Due to the complex flow dynamics associated with particles, heat transfer through a particle bed in agitated systems remains difficult to understand and optimize. As a result, the process can prove to be challenging to adapt from the laboratory scale to the pilot plant or even manufacturing scale. In this work, we make use of the discrete element method (DEM) to examine both heat transfer and flow of granular material in a bladed mixer geometry. We capture two important modes of heat transfer: conduction and dry granular convection. The first mode involves heat conduction through physical contacts between particles, while the other one describes heat transfer due to movement of particles. We observe that these modes of heat transfer can be expressed via two relevant time scales. The first time scale is a conductive time scale that describes the time needed for a single particle in contact with a heated surface to attain the temperature of said surface. The second time scale is a mixing time scale and defines the rate at which thermal energy is distributed throughout the bulk material. By looking at the relative significance of these competing time scales, we witness and characterize different regimes for heat transfer. Furthermore, we evaluate the influence of material properties such as conductivity and operating conditions such as agitation rate on heating performance. Developing such fundamental understanding of heat transfer in a bladed mixer provides an insight into how the performance of agitated filter dryers and their scale-up can be optimized.