Characteristics of Vibro-fluidized Bed Freeze Drying

Abstract

A new method for minimizing heat loss in a vibro-fluidized bed freeze dryer was examined. A heater was set in a bed of granular materials. They were mixed smoothly by suitable conditions of vibration, and dried with uniform moisture content throughout the bed. Temperature at various positions in bed were almost the same. They were kept nearly constant at below 0°C during most of the drying period. The heater had no appreciable heat loss until the bed temperature became higher than the temperature of the wall of the drying chamber. The apparent thermal efficiency of the heater was evaluated as almost or more than 100%, because of the contribution of the heat generated by the vibration of the bed which was measured when the intensities of vibration were higher than the acceleration due to gravity. The heat transfer coefficient between the heater and the bed was about 500W/mm²·K at the initial point of the drying of the high moisture content materials. The value decreased with the drying process. After a period of drying, however, it showed a constant value of nearly 70W/mm²·K until the end of the drying process. The drying rates of this method were obtained up to about 6 kg-H₂O/mm²·h. These drying rates will be improved by the use of an appropriate proportion of capacity of the cold trap and the vacuum pump to the heat input.