The freeze-drying process is studied under various operational policies through the use of a sorption-sublimation model. The operational policy that provides the shortest drying times keeps the pressure at its lowest value. The upper and lower heating plates are independently controlled so that the material constraints are encountered and held throughout the free water removal phase.Under certain conditions, and for the case of samples of small thickness, the sorbed water profiles may have segments whose bound water concentrations are higher than those at the start of the free water removal phase. It is shown that the criterion used in terminating the freeze-drying process is of extreme importance, since it may lead to an undesirable sorbed water profile which may deteriorate the quality of the dried product. M. J. MILLMAN, SCOPECertain biological materials, pharmaceuticals, biochemical products, and foodstuffs which may not be heated even to moderate temperatures in ordinary drying, are freeze-dried. Recent developments in biotechnology and biochemical engineering (Hill and Hirtenstein, 1983) have established the importance of lyophilization-freeze-drying-in biochemical processing.As a rule, freeze-drying produces the highest quality product obtainable by any drying method (Goldblith et al., 1975;Mellor, 1978). Prominent factors are the structural rigidity of the product, which facilitates rapid and almost complete rehydration at a later time, little loss of flavor and aroma, and minimization of degradative reactions which normally occur in ordinary drying processes, such as nonenzymatic browning, protein denaturation, and enzymatic reactions.However, freeze-drying is an expensive form of drying because of the slow drying rates and the use of vacuum. Thus, it is essential to develop operational policies which would reduce the drying times by improving the drying rates.The interactions in the variables of the freeze-drying process are complex, and an experimental approach of examining the various operational policies is tedious, expensive, and timeconsuming. But the experiment will finally confirm or disprove the validity of operational policies suggested by theoretical work, and which are supposed to improve or minimize the drying times. A mathematical model which can predict known freezedrying experimental results accurately, can be used in the analysis of the freezedrying process and in the examination of various operational policies which may maximize the drying rates. This work develops a sorption-sublimation model which is used to study operational policies that may significantly reduce the drying times. This model is also used to examine the effects on the bound water profiles of the criteria employed to end the terminal drying phase. Skim milk was used as a model material.The development of bound water profiles incorporates an important factor for optimization policies for quality retention in freeze-drying. CONCLUSIONS AND SIGNIFICANCEA sorption-sublimation model is presented and used to simulate the fr...

A sublimation model of the freeze drying process, which accounts for the removal of free water, is presented and is used to study the operation and design of freeze driers whose plates are heated by controlled means. The analysis is applied to driers wherein heat can be transferred either through the dried layer or through both the frozen and dried layers of the material being dried. In the latter case and for the material chosen for evaluation (non-fat milk), it is shown that most (up to 95%) of the heat used during drying is transported through the frozen layer.It was found that the shortest drying times are obtained when the condenser temperature and chamber pressure are kept at their lowest values, and the plate temperatures are controlled independently so that the scorch and melting constraints are both held throughout the drying period. The effect of sample thickness on the drying time is significant.

An economic analysis of various operational policies associated with a batch freeze dryer is presented. Skim milk is used as a model material. It is shown that the lowest cost of production corresponds to the highest production rate. This occurs when large sample thicknesses are used, the chamber pressure is at its lowest value and the upper and lower heating plates are independently controlled such that the melting and scorch constraints are both encountered and held during the free water removal phase. Therefore, the operational regime that provides the shortest drying times during the free water removal phase (Millman, Liapis & Marchello, 1984, 1985 will also provide the smallest production costs and highest capacities.