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Designs and operating principles of vacuum-sublimation sprayers, results of investigations of process kinetics and the dispersitvity of the organic-salt powders produced, the formation mechanism of the solid-phase structure, and the process of evaporative freezing in a vacuum are discussed. The size distribution of KNO 3 crystals in porous granules is indicated as a function of cooling rate.Technologies of nanomaterials and ultradisperse powders (UDP) of various nature and purpose are of interest for the production of new materials with enhanced qualitative characteristics [1][2][3][4].Chemical methods (precipitation in liquid and gaseous media, heterogeneous synthesis, cryochemical method), electrochemical methods of synthesis using a plasma or laser beam, electroerosion, shock-wave, and electroexplosive methods, method of self-propagating high-temperature synthesis, and mechano-chemical synthesis are currently employed for the production of UDP [5].Of the existing methods of converting solid macroparticles to the ultradisperse state, cryochemical methods [6, 7], which have been successfully implemented in the production of such materials as chemical reagents, catalysts, pigments, composites, pharmaceutical preparations, etc., exhibit potential for process and ecological safety with a high productivity and low net processing cost.Theoretical and experimental investigations conducted at the MGUIÉ have indicated that the cryochemical method of producing UDP of inorganic salts (solid oxidizers) is implemented most effectively when vacuum-sublimation processes are carried out. The following sequence of basic production operations is employed for practical production of UDP by the vacuum-sublimation method: preparation of a homogeneous solution; dispersion of the solution; freezing of drops of the disperse medium; sublimation removal of solvent from the cryogranulate obtained in the previous step; passivation and preservation of the granules; utilization (desublimation) of the solvent; and, dispersion of the granules into ultradisperse fragments.Selection of the solvent in accordance with the following conditions plays a major role in the step where a solution of initial substance is obtained:• sufficiently high solubility of the initial substance;• a triple-point interval ranging from -5 to + 20°C; • a crystallization temperature (eutectic point) of the solution no lower than -(15-20)°C; and • fire resistance, nontoxic nature, chemical inertness, and economic effectiveness. In executing the freezing step, the salt solution or suspension is preliminarily dispersed into drops to increase the freezing rate and improve the engineering properties of the product.

Heat exchangers with longitudinal fins are studied. The freeze drying process is shown to develop with different process times in the space between fins as well as with time at different fin heights. The diagram and characteristics of the UFST-3.1 freeze-drier built by MGUIÉ and Vakuummash OAO are given.An effective method is available for speeding up the freeze drying of various materials and accelerating the heat and mass exchange process in a vacuum. That method consists in using finned trays of various designs with fins made of material with a low thermal resistance (copper, aluminum, etc.). The fins allow the trays to have a much higher specific load since in that case the layer thickness is half the fin spacing. Energy is delivered to both sides of the material being dried and the heat-supplying surface increases, thus shortening the drying time. That method, which is incorporated into various technical designs [1, 2], is used quite extensively today. Not one of the known devices, however, provides a sufficient drying rate when there is a continuous feed of cryogranules, since this takes place in vacuum-spraying freeze drying installations [3].We propose that the design of the energy delivery devices for freeze drying disperse products be based on a system of parallel tubes, arranged in one horizontal plane, with common transverse finning formed by thin vertical plates. The material to be dried is poured into the space between the tubes and heat is delivered through the tube walls (by electric heating or by a heat transfer agent). This system has been chosen because it can be used in vacuum spray dryers with continuous feed of material and a fairly uniform energy delivery and a well-developed technology for making the components of such a system is available.It is desirable to consider the two limiting cases when designing the equipment. First, the tubes are placed so closely together as to form a practically continuous surface (considered approximately to be a plane), which forms the heat source. Second, the tubes are so far apart that their interference can be ignored and the problem can be reduced to a simple case of a single axisymmetric heat source.The first case was considered by Kamovnikov et al. [4], who gave a relation for determining the drying time. The calculation, however, was based on the assumption that the heat flow is constant along the fin and, hence, the sublimation (or freeze-drying) front is parallel to the fin plane throughout the time it moves. That approximation is allowable only when the fin is sufficiently conductive. But none of these requirements is satisfied under the conditions that we are considering. The material being dried (e.g., a solution of a mixture of sulfate salts that are the starting materials for obtaining ferrite powders by the cryochemical method [5]); the solution is chemically active with respect to the metals used in heat exchangers (aluminum, steel, copper, etc.). Moreover, it is desirable to make the fins thicker because the metal content of the entire desig...

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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-