SELF-FREEZING IN AN INITIALLY WET POROUS-MEDIUM

Freeze-drying has received renewed interest in connection with biotech nology, food preservation and chemical industries. Freeze-drying under reduced pressure causes sublimation to occur. Simultaneous freezing o f the moisture phase couples the two processes of freezing and sublima tion 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 consi sting of coupled diffusion equations describing thermal and mass equil ibrium in the solid-liquid and solid-vapour regions is solved analytic ally. The liquid-solid region is included with unknown freezing interf acial conditions. The effects of various parameters, such as conductiv ity ratio, latent heats, initial concentration, surface pressure and s urface temperature, are examined with respect to the freezing and subl imation rates. The freezing rate is faster than the sublimation rate, though slower than the corresponding rate for constant freezing temper ature. The freezing temperature is higher than the sublimation tempera ture for a constant composition ice pack.