Plenary Talk: Scale-up of Fluidized Bed Drying: Hydrodynamics, Mixing and Heat and Mass Transfer

Fluidized beds are used extensively in the pharmaceutical and chemical industries as a batch or continuous process for drying moist powders and granular solids. Fluidized bed dryers provide good mixing of solids and intensive heat and mass transfer between the solid and hot gas phases in the system. It has been reported that an average of 12% of all energy consumed is used for drying, and the cost of drying could reach up to 60%-70% of the total cost of investments. Therefore, optimal design and operation of the drying process is sought for cost-effective manufacturing and to ensure a robust process with good product quality. In the pharmaceutical industry, the drying process removes moisture or solvents to provide pharmaceutical materials (active pharmaceutical ingredients (APIs) and granulations) with stable properties. 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. Complications include lengthy drying times, nonuniform drying, agglomeration and attrition. In order to understand fluidized bed drying we have to consider flow of the gas and solids, mixing, heat transfer and mass transfer which makes the process difficult to scale-up.

In this talk, we will first present results for scale-up of batch fluidized bed drying for a pharmaceutical application. Parametric analysis of four fluidized bed dryer operating parameters– initial moisture content, material loading, heating air temperature and air flux – and their effect on drying was studied. Two different dryer scales were used to dry a pharmaceutical excipient powder. Based on these results we are able to introduce a practical scaling rule which can predict the drying behavior in the large scale unit based on results from a bench scale unit. We then discuss using knowledge gained from the batch fluidized bed dryers to predict the behavior of a continuous fluidized bed dryer, where solids continuously flow into and out of the drying chamber. Several pharmaceutical manufacturers are moving towards continuous manufacturing due to its potential to improve agility, flexibility, and robustness. The fluidized bed unit operation can be readily implemented as a continuous process for drying. A model for predicting the effluent moisture content for the continuous dryer was developed by combining the drying kinetics and residence time distribution (RTD) of powder in the dryer. The drying kinetics were obtained from the batch fluidized bed drying process. The RTD was characterized by carrying out a tracer response test in the continuous fluidized bed. A tank-in-series model was used to describe the RTD curves. A maximum mixedness model (MMM) was adopted for characterizing the micromixing in the continuous fluidized bed. The results of the model were compared to experimental results and we discuss how the operating conditions affect the resulting effluent moisture content. Finally, we will discuss the application of these results to improving the operation of pharmaceutical fluidized bed drying.