Sublimation of a Cryogranule Layer with Conductive Heat Input

Abstract

A physical basis is given for the isothermal conditions in a frozen layer on drying by sublimation. The basis of the physical model is that the frozen layer of granulated material constitutes a system of heat tubes in series along the height.Sublimation is traditionally used to clean organic and inorganic materials, and it is now being introduced into cryochemical technologies for making solid-state products such as ferrites, ceramic electrolytes, optically transparent and porous piezoelectric ceramics, catalysts, adsorbents, and so on.A major advantage of cryochemical technology is the scope for exact dispensing and uniform mixing of initial components and doping additives.Drying by sublimation with contact (conductive) energy input can take various forms in accordance with the working parameters, the properties, structure, and sizes of the process material, and also the conditions for removing the vapor; the precise physical mechanisms and thus the mathematical descriptions may differ considerably.Corresponding models have been devised [1, 2] for various forms of the process, but inadequate research has been done on contact energy input for sublimation at present.The state of particles of subliming material is dependent on the preparation method and treatment during the process. The particles in an immobile layer of monodispersed material tend to freeze together into a monolith after a certain time, which is dependent on the process parameters and the thermophysical properties of the material [2, 3].If the particles move, no matter whether the stirring is mechanical or hydrodynamic, one can calculate the drying in relation to mixing time and the thermal resistance between the heat-transfer surface and the suspended bed.In calculations on a vapor-permeable layer formed from granules resulting from supplying a salt solution to vacuum, one makes the following assumptions [1][2][3][4][5]: the material is considered as a continuous medium with high vapor permeability, whose thermophysical characteristics remain unaltered in the process; the heat transfer in the frozen region can be neglected; and the advance of the sublimation front is in a plane-parallel fashion with a sharp boundary between the frozen material and the dried zone, i.e., the phase transition occurs only at the boundary between those two zones.Calculations on the sublimation kinetics for the vapor-permeable bed are dependent on whether the dried particles are transported by the vapor flux or not. The model used to calculate the process is dependent on the relation between the size of the vapor gap between the bed and the heat-supplying surface on the one hand and the characteristic size of the particles in the bed on the other. In the first case, the sublimation is described as for sublimation of ice particles. In the second case, as the drying pro-