SELF-FREEZING IN AN INITIALLY WET POROUS-MEDIUM

Freeze-drying has received renewed interest in connection with biotechnology, food preservation and chemical industries. Freeze-drying under reduced pressure causes sublimation to occur. Simultaneous freezing of the moisture phase couples the two processes of freezing and sublimation and creates a self-driven process without need for any additional application of external energy other than that required to lower the surface temperature and pressure initially. A mathematical model consisting of coupled diffusion equations describing thermal and mass equilibrium in the solid-liquid and solid-vapour regions is solved analytically. The liquid-solid region is included with unknown freezing interfacial conditions. The effects of various parameters, such as conductivity ratio, latent heats, initial concentration, surface pressure and surface temperature, are examined with respect to the freezing and sublimation rates. The freezing rate is faster than the sublimation rate, though slower than the corresponding rate for constant freezing temperature. The freezing temperature is higher than the sublimation temperature for a constant composition ice pack.