Heat and Mass Transfer Mechanisms in Sublimation Dehydration

Abstract

An analytical study is presented for sublimation dehydration which shows the effect of the important mechanisms involved in the process. The effects of the presence of a binary gas mixture are considered. The boundary conditions for this solution are all directly controllable external conditions; namely, the temperature, total pressure, water-vapor partial pressure at the heated surface, and the temperature of the face opposite to the heated surface. The analysis involves the simultaneous solution of the appropriate equations of continuity, momentum, and energy, as well as the equation of state for each of the gas components present. Typical results are given for freeze-drying of bovine muscle. The drying rate is seen to increase with decreasing total pressure and water-vapor concentration. The relative importance of diffusional transport compared with bulk transport is presented. It is shown that drying rates can be substantially increased by judiciously making use of heat transfer through the frozen region.