Counter-current spray drying with stream separation: Computational modeling of a novel dryer design

Spray drying has been in use for producing fine powder of pharmaceuticals in order to make the drug respirable and thus accessible to the large absorptive area of the lungs. However, traditional designs for spray dryers have turned out to be inappropriate for processing very fine particles. Therefore, a novel design has been suggested in which the drying agent is separated from the moisture-containing material by using a vapor-permeable membrane (Gerde, 2009). A computational model of the new design has been developed for design and optimization purposes. The model has been shown to satisfactorily reproduce the experimental measurements. A linear correlation has been obtained between the swirl flow rate and the outflow concentrations, predicting a maximum possible flow rate of 1 L/min in order to meet the required quality of the product. The performance of the dryer has been shown to be very sensitive to the injection flow rate as well as the effective diffusivity of the membrane. Using ethanol as the solvent, the model predicts a maximum injection of 12 mg/min. Based on the simulation results, a fairly low probability of wall deposition is expected.