Microwave–convective drying characteristics of pharmaceutical powders

Common pharmaceutical excipients and active ingredients, wetted with specific solvents, were dried in a combined microwave–convective system (2.45 GHz, 90 W). The drying curves showed a constant drying rate period, followed by two falling rate periods. Cross-flow air velocity had little effect on solvent evaporation rate in the initial stages; however, an increase in drying rate was observed during the falling rate period. Increasing air temperature increased the drying rate throughout the entire process, with reductions in drying time of up to 78% being observed. Average and maximum sample temperatures were found to decrease on addition of air-flow, the extent of which was dependent on material and operating conditions.