Permeability rate, freeze-drying

D Quast, M Karel. Dry layer permeability and freeze-drying rates in concentrated fluid systems. Journal of Food Science 33 170, 1968. 

Nakagawa et al. (2007a) interpreted the dependence of the water vapor mass transfer permeability of the dried layer on ice crystal morphologies by using this relationship for different nucleation temperatures. The increase in the nudeation temperature was expected to increase the water vapor mass transfer permeability of freeze-dried materials and to contribute to acceleration of the primary sublimation rates during freeze-drying. 

The term moisture, usually defined as wetness conferred by an unidentified liquid, is assumed here to be due to water. Thus, the scope of this article is the characterization of and consequences due to relatively small amounts of water associated with solids of pharmaceutical interest. Chemical stability, crystal structure, powder flow, compaction, lubricity, dissolution rate, and polymer film permeability are some properties of pharmaceutical interest that have been demonstrated to be influenced by the presence of moisture. Wet granulation, extrusion, spheronization, tray drying, freeze drying, spray drying, fluid-bed drying, tableting, and aqueous film coating are some unit operations that obviously depend on the amount and state of water present. 

Permeability rate, PA, as a function of sublimation temperature, T, during the freeze-drying of CA blend membranes (pure gases, 25 C, 40 bar to obtain... 

If the freeze-drying process is controlled by water vapor mass transfer through the freeze-dried layer - for example in the case of dried cake of very low permeability or for very favorable heat transfer conditions - the drying rate for one vial is proportional to the mean driving force P - Pc) - that is, to the water vapor pressure difference between the sublimation front interface and the drying chamber - and controlled by the freeze-dried cake mass transfer resistance, denoted J p, which is inversely proportional to the dried layer permeability defined by Darcy s law. Inthiscaseitholds ... 

The primary characteristic necessary for a liner, cover, or cutoff wall is low permeability, which essentially enables them to slow down the seepage or diffusion of chemicals. Clay is therefore the main material used to construct these containment systems. The thickness and chemical compatibility of containment systems are of concern in assessing the performance of a system. For example, clay liners are constructed as a simple liner that is 2 to 5 ft thick. In composite and double liners, the compacted clay layers are usually between 2 and 5 ft thick, depending on the characteristics of the underlying geology and the type of liner to be installed. Regulations specify that the clay used can only allow water to penetrate at a rate of less than 1.2 in./yr. However, the effectiveness of clay liners can be reduced by fractures induced by freeze-thaw cycles, drying out, and the presence of some chemicals. 

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