Primary Drying The Sublimation of Ice

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. 

In practice, at the beginning of the primary drying stage, more than 90% of the water in the initial solution has frozen. The ice is then removed by sublimation. Unlike the freezing process, ice sublimation is amenable to some measure of control. The heat flow to the ice front must be adjusted to balance exactly the heat absorbed by the sublimation of ice at the operating temperature of sublimation. In this chapter, we discuss the contributing mechanisms by which heat is transferred from the shelves of the freeze-drier to the ice front and the mechanisms by which water vapour is transferred to the condenser (mass transfer), and their relative contributions to the overall sublimation process. 

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. 

The effects of pressure variations, as they occur when pressure is controlled independently via calibrated leaks, are more complicated [6-12]. In many cases, an increase in chamber pressure favors ice sublimation and is reflected in a shorter primary drying time because of the improved thermal exchanges and of the higher product temperature [6,10 12]. 

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