Primary Drying - Sublimation

T = temperature of shelves and product containers, TE = temperature of the ice core, 

TS = temperature of the sublimation zones, P = pressure and PS = pressure at the sublimation zones. 

the most important parameters for an economical sublimation process 

The data obtained in preliminary tests on the freezing behaviour of the product at certain temperatures, therefore, must be monitored during the entire process and the process itself must be controlled in such a way that the sublimation temperature is maintained throughout. Sublimation can be considered ended when the water frozen as ice crystals has been removed completely from the product to be dried. 

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Primary drying (sublimation) to remove bulk, free water... 

The large amounts of water vapour (1 g ice at the usual process pressure of approximately 0.1 mbar has a vapour volume of approximately 10 m ) which are produced during primary drying (sublimation) can only be pumped off economically by a cold condensation surface. 

Figure 8 An example of the decreasing heat requirement during primary drying at a chamber pressure of 0.15 torr. 5% mannitol maintained at -20°C during primary drying. Results obtained by computer simulation of freeze drying (see Ref. 3). Heavy curve Shelf Fluid. Light curve Shelf surface. Lightweight dashed curve Product Bottom. Heavy dashed curve Sublimation.

Sublimation (or Primary Drying). For the sublimation phase of the process, the frozen material usually is subjected to a vacuum of about 4.6 millimeters of mercury. The ice-crystal sublimation process can be regarded as comprised of two basic processes (l)Heal transfer, and (2) mass transfer. In essence, heat is furnished to the ice crystals to sublime them he generated waler vapor resulting is transferred out of the sublimation interface. Thus, it is evident thal sublimation will be rare-limited by both resistances to heat and mass transfer as they occur within the material. 

Before concentrating on process parameters (Section III), we will briefly outline the behavior of the product during the three separate but interdependent stages of freeze-drying freezing, sublimation (primary drying), and desorption (secondary drying). 

Once the product is adequately frozen, the next step is the removal of ice, i.e., primary drying. During primary drying, the rate of ice sublimation is dependent on the amount of heat supplied to the product. The temperature of the product equilibrates as a function of two opposite effects the transfer of heat from the shelf or from the gaseous atmosphere to the product, and the cooling due to ice sublimation. As the ice-vapor interface (moving front) moves toward the bottom of the containers, the rate of ice sublimation tends to diminish because the nascent porous matrix in the upper part of the pellet offers some resistance to vapor flow. 

Despite the practical importance of determining the point where primary drying terminates, there is no easy or universally recognized method to do this. Part of the difficulty arises from the fact that the boundary between primary and secondary drying is not clear-cut. A reason for this is that the upper part of the cake is subject to a limited water desorption whereas the bottom of the cake is still undergoing sublimation. Another reason is that all of the samples do not necessarily dry at the same rate [3] because freezing-drying is inherently a statistical process in many respects. 

The primary drying time of the product, which can be defined as the time elapsed from the moment when the vacuum is created in the chamber to the disappearance of the last ice crystals in the product, is obviously a dependent parameter as the rate of ice sublimation is controlled by the same factors as product temperature. This is also the case for the secondary drying time of the product, i.e., the time elapsed from the moment when ice sublimation is complete to the end of the cycle. During this period, water is desorbed from the product at a rate dictated both by technical factors and by product characteristics. 

During primary drying, condenser temperature results from the rate at which water vapor is being condensed. Condensation rate is dependent on the rate at which water is being sublimed from the product. Effective sublimation can only be achieved if a sufficient temperature difference, which corresponds to a driving force, is maintained between the product and the condenser. 

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