Mapping the Droplet Drying Phenomena within a Fluidized Bed Coater as a Prediction Tool for Coating Qualities

For a high coating quality, agglomeration and excessive evaporation of the coating droplets during fluidized bed coating process should be avoided (1-3). This means that the coating process can often only be performed within a narrow operating window. Consequently, the selection of the process conditions and the process control are delicate steps to assure a good coating quality. In spite of being an established technology, the process development of a fluidized bed coating process has often been performed in an empirical way. This approach obviously results in a large experimental program and the interpretation of the results stays far from first principles. In this manner, a rapid and continuous process development is difficult to be achieved. Process modeling is one of the methods that can aid the process understanding, as it enables a comprehensive parametric study of the involving sub-processes and further can be used as a process prediction tool once it is validated. In this study, the physical phenomena occurring during a single droplet drying within a fluidized bed coater have been modeled, comprising of the changes in the droplet mass, water fraction, droplet size and droplet shape in time, before and after collision with the core particles. Using the models, the influence of various process conditions, i.e. inlet air temperatures, atomization pressures, and spraying rates could be studied comprehensively. The coatings obtained from the corresponding processes were characterized using a previously developed quantitative image analysis method (4, 5). In this way, the relationship between the extent of the droplet drying and the obtained coating quality could be assessed, which makes the model also usable as a development tool.