Microwave drying of multi-component powder systems

The rate of uptake of microwave technology within the pharmaceutical sector has been slow due to stringent regulatory procedures and unsuccessful microwave trials. However, microwave systems offer significant energy and time savings compared with conventional dryers. Common pharmaceutical excipients, lactose, stearic acid, and active ingredients, Aspirin, Paracetamol, were wetted with water and dried within an experimental atmospheric microwave drying system (2.45 GHz, 90 W). The drying curves obtained showed a constant drying rate, followed by two falling rate periods. Two- and three-component mixtures were dried under the same conditions. The drying characteristics were found to lie between those of the pure components. Drying kinetics of samples wetted with a range of solvents (methanol, ethanol, acetone), and two-component solvent mixtures, were examined. The relative evaporation rates of the solvents showed some dependence on their dielectric properties; latent heat of vaporisation and boiling point having greater influence on drying characteristics. Clearly this research provides information vital to achieve increased acceptance of this technology within the pharmaceutical industry.