Rate Analysis of Freeze Drying of a Model System by a Uniformly Retreating Ice Front Model

Abstract

Prior to investigating the freeze drying of liquid foods, the freeze drying of a water-glass bead system was analyzed by a conventional uniformly retreating ice front (URIF) model. The drying rate gradually changed as drying proceeded (phase I) until a certain critical time, and then sharply decreased (phase II), probably due to instability in the sublimation plane. The apparent thermal conductivity of the frozen layer estimated by using the drying data from phase I agreed well with the thermal conductivity calculated by the Kunii-Smith model. This result shows that the heat flow through the dry layer, to which both heat conduction and heat transfer accompanied by vapor flow would contribute, was small enough compared with that from a heater in the present drying system. The drying rate and the temperature of the frozen layer in phase I were described well by the conventional URIF model.