Freeze-Drying Mechanisms and Drying Rates Under Various Drying Conditions
Freeze-drying rates of relatively homogeneous frozen materials and frozen particles in a shallow tray by radiative and conductive heating are studied both experimentally and theoretically. The drying mechanism of frozen materials of which initial ice cont
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ABSTRACT Freeze-drying rates of relatively homogeneous frozen materials and f~ozen particles in a shallow tray by radiative and conductive heating are studied both experimentally and theoretically. The drying mechanism of frozen materials of which initial ice content is less than 1.0 is quite different from the flat plane sublimation, which was found previously by one of the authors to become unstable under some conditions. The total drying times under various conditions are discussed for this kind of frozen materials and also for a packed bed of frozen particles using dimensionless heat and mass transfer equations to obtain some information for shorter drying time.
2. THEORETICAL CONSIDERATION
1. INTRODUCTION
Freeze-drying, which is probably the most sophisticated method of drying giving the highest quality of dried products, has been extended in application to various fields, especially to food processing. Many hours are usually required to dry materials by this method, ·however, because it has to be conducted below the freezing point of wet materials to be dried, which usually gives a quite small vapour pressure or a quite small driving force for mass transfer. In order to increase the mass transfer rate freeze-drying is usually conducted under vacuum, which gives, on the other hand, an inverse effect on drying rate due to a small thermal conductivity of dried-up zone of the frozen materials under drying. For the cost reduction of freeze-dried materials it is necessary to establish economical design standards of freeze-dryers and to determine their optimum operating condition. For these purposes it is essential to descriminate the freeze-drying mechanisms under various conditions taking into account the physico-chemical properties of frozen materials. There are many physico-chemical factors which control the drying mechanism and the drying rate, for example, thermal conductivities of frozen and dried up layers , permeability of vapour in dried-up layer, conductance of evacuation system, condition of ice contained, initial ice content and so on. All of these must be taken into account to discriminate the drying mechanism and to find its optimum drying conditions for shorter drying time. Because of the large variety of the properties
R. Toei et al. (eds.), Drying ’85 © Springer-Verlag Berlin Heidelberg 1985
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In order to estimate the drying rate of frozen materials it is indispensable to know the drying mechanism which depends not only on the physicochemical properties of frozen materials but also on the drying conditions such as the methods of heat supply and removal of the vapour generated. Freezedrying by radiative heat transfer is a rather simple process, that is, uniform retreating of frozen front in most cases [1~]. On the other hand the drying mechanism by conductive heat transfer is little complicated. In this paper freeze-drying mechanisms of three types of frozen materials are treated in a wide and shallow tray, heated simultaneously by radiation and conduction. Classification of frozen materials
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