Characteristics of Vibro-Fluidized Bed Freeze Drying

Abstract

The freeze drying of granular or chopped materials was carried out in a laboratory size vibro-fluidized bed. The heater, made of stainless steel pipe, was set in the bed for minimizing heat losses. General characteristics of the vibro-fluidized bed freeze drying process were discussed in this study.

Under the appropriate conditions of vibration, the drying processes proceeded with almost uniform bed temperature and moisture content due to thorough mixing by vibro-fluidization. The whole-bed temperatures were kept constantly at near sublimation points balanced to pressures in the drying chamber until the drying stages reached near the end points. The drying rates obtained were up to 6 kg-H₂O/m²h, though these values were restricted by the abilities of equipment used such as the vacuum pump, and condenser. The thermal efficiencies of the heater were almost 100%. The overall heat transfer coefficient between heater and material depended on the bed moisture content. The values decreased from 300–500 W/m²K in the initial period of drying to 60–80 W/m²K in the latter half of the period in accordance with the progress of drying process, the heat generated in the bed by vibration contributed as significantly as the heat for drying. The values were measured when the intensities of vibration were higher than the acceleration due to gravity.