Particle motion, coating and drying in Wurster fluidized beds - An experimental and discrete element modeling study
This thesis focuses on developing a mathematical model that is capable of predicting the fluidized bed coating process. First a basic understanding of pellet motion in a Wurster fluidized bed was established by conducting a series of experiments using the positron emission particle tracking (PEPT) technique. The PEPT results, such as the particle velocity, the cycle time distribution (CTD) and the residence time distribution (RTD) of particles in different regions of the fluidized bed, were selected to evaluate the model for pellet motion based on the discrete element method, computational fluid dynamics (DEM-CFD). In an effort to refine the sub-models involved in the DEM, the effects of the drag model were investigated. With the validated DEM-CFD model, the detailed pellet motion in the spray zone of the fluidized bed was studied. In order to identify the underpinning mechanisms by which coating thickness changes, pellets of different sizes were employed, and a simple model for predicting the growth of pellets was developed based on data available from the DEM-CFD results and the PEPT experiments. This predictive model was then evaluated experimentally. In addition, a model for drying of pellets was developed.
In the PEPT experiments, it was found that, for the parameters studied, particles spend approximately 12–29% of the cycle time in the Wurster tube. It was observed that particles tend to recirculate in the Wurster tube and sneak out from below the tube. In comparison with the PEPT experiments, the coupled DEM-CFD simulations showed close agreement with respect to the CTD and the RTD of particles in different regions. Using the validated DEM-CFD model, large particles were found to spend longer time in the spray zone and move closer to the spray nozzle. The latter effect provides evidence that large particles can shield small particles from spray droplets. Both of these effects suggest that large particles receive a greater amount of coating solution per particle cycle. A simple conceptual model was then developed to predict the effects of the residence time of particles in the spray zone, the particle cycle time, and the entrance distance on the relative rate of the increase in film thickness between large and small particles. By comparing predicted and measured relative rates of increase in coating thickness between large and small particles, it was confirmed that large particles grow faster than small particles.