The effect of cycled‐pressure on drying conditions during freeze drying

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Abstract
Summary: Cyclic‐pressure freeze drying has been found to improve the drying rate compared to the constant pressure process. This is due principally to the increased thermal conductivity of the semi‐dried region which results from the cyclic operation. The pressure oscillation causes unsteady temperature and mass flux conditions within the material being dried. During the high pressure portion of the cycle, very little sublimation takes place and the temperatures within the material are raised. When the pressure is reduced the temperatures are reduced as sublimation occurs. The temperature oscillations were found to have the same period as the pressure cycles, but to lag behind because of the heat capacity of the material. Additionally, it was found that the cycling produced high intermittent loads on both the heating source and the vapour condenser.

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