Microwave drying of pharmaceutical powders

Although the application of microwave dryers has proven successful in many industries, the uptake rate of microwave technology by pharmaceutical companies has been slow, particularly in the primary manufacturing sector. Pharmaceutical powders are temperature sensitive thus, drying conditions are primarily dictated by stringent quality criteria. The heating effect of microwaves on a material is controlled by the dielectric loss factor. The loss factors of single and two-component systems of some common pharmaceutical excipients and active ingredients were determined within an experimental atmospheric microwave drying system. Formulations with a high percentage of ammonium acetate exhibited comparatively high loss factors, whilst the Aspirin-based samples presented low loss factors. However, both were significantly less than that of water; the solvent used in this work. The global loss factor of a multi-component system was found to be dependent on the loss factors of the individual components and their relative concentrations. The effect of moisture content on dielectric properties was also examined. Results showed that the loss factor of bound moisture is significantly less than that of free water, however, as moisture content increases, the loss factor of a wetted powder approached that of pure water. The effect of agitation on the drying behaviour of the powders was also examined, with the results revealing that the rate of drying increases with increasing frequency of agitation.