Modeling Multiphase Heat and Mass Transfer in an Agitated Filter Dryer by Integrating Experiment, Computations, and Analytical Solutions.

The drying of a wet cake consisting of an active pharmaceutical ingredient and solvent in an agitated filter-dryer is a critical and challenging unit operation in the pharmaceutical industry. The complexity of this operation is attributed to the constraints on product quality in terms of its physical properties in addition to the residual solvent content. In this manuscript, a better understanding of the drying mechanism is gained by integrating insights from three-dimensional analytical solutions and computational fluid dynamics simulations into a zero-dimensional model to explain experimental data. The approach provides the time evolution of the mass flow rate of solvent from the wet cake and the center-point temperature of the cake with good accuracy. Further investigation of the zero-dimensional model reveals important parameters such as the mass transfer rate number that predicts whether the process is convection-controlled or diffusion-controlled, and the thermal load of vaporization that estimates the fraction of solvent vaporized per unit time. These parameters can be useful in devising a drying protocol for agitated-filter dryers.