Temperature Characteristics of Pharmaceutical Powders During Microwave Drying

The internal temperature of wetted pharmaceutical powder samples dried in an experimental microwave system were monitored. In general, the temperature profile consisted of three distinct stages: short preheating stage, constant temperature period, and decreasing temperature phase. Approaching the end of the constant rate period, a local maximum in the temperature curve was observed. The development of the internal temperature profile was observed to be material dependent, with the dielectric, physical, and thermal properties of the powder and solvent being contributory factors. The characteristics were also dependent on bed dimensions (surface area and depth), with an increase in surface area and decrease in depth providing lower product temperatures. The surface temperature remained lower than the center temperature throughout the drying process, with the differential through the sample depth increasing on reducing the moisture content. A nonuniform temperature distribution developed across the sample cross section. Sample agitation was found to aid temperature leveling.