Temperature Variations in Powder Beds during Combined Microwave-Convective Drying

Abstract

The center temperature of wetted pharmaceutical powder samples dried under combined microwave-convective conditions in an experimental system (2.45 GHz, 90 W) was monitored. Introduction of convective air to the microwave system facilitated a reduction in the material temperature, with the extent of the decrease being dependent on the air temperature and velocity. In general, the variation in center temperature of a sample dried under microwave-convective conditions consisted of three distinct stages: a short “warming-up” period; a constant temperature stage, and a decreasing temperature phase. Towards the end of the constant-rate period, a local “peak” superimposed on the temperature curve was observed. The temperature history was found to be material dependant, with the dielectric, physical and thermal properties of the solvent and powder being contributory factors. The variation was also dependent on bed geometry (surface area and depth), with an increase in surface area and decrease in depth providing reduced product temperatures. The surface temperature remained lower than the center temperature throughout processing, with the gradient increasing on reducing the moisture content.

Keywords:

• Convection
• Drying
• Internal temperature
• Microwave
• Pharmaceuticals
• Powders

Acknowledgments

The authors gratefully acknowledge the support of Dr. Ian McConvey and Mr. Phillip Shering (AstraZeneca), Mr. Paul Richardson (GlaxoSmithKline) and Mr. Albert Evans (Rosenmund-Guedu).