Vacuum Sublimation of Ice in Bulk

Abstract

AN ice sublimation or freeze-drying process will have maximum rate of vaporization per unit exposed area of frozen product at a specified temperature if the following conditions are satisfied : (i) The vapour pressure of ice sublimed on the condenser is negligible compared with ice at the product temperature, (ii) The condensing and product surfaces have a uniform and minimum separation. (iii) The permanent gas (usually air) pressure in the product-condenser interspace is a minimum, (iv) Mechanical obstruction (in the form of radiation shields, sterile caps, etc.) to vapour transfer from product to condenser is a minimum, (v) The product is of uniform and minimum thickness. The first four conditions follow from kinetic considerations and ensure that reversed vapour transfer, length of vapour path, and obstruction to vapour transfer are a minimum. The last condition as a consequence of the increase of drying-time with product thickness.