Freeze drying of cryogranules of salts of ferrite-forming components on a finned surface

Abstract

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...

Cryocrystallization during preparation of ferrites by cryochemical technology

“…In particular, freezing of a suspension with alloying of ferrite powders [8], by sublimation dehydration [9], and thermal decomposition of ferrite powders [10], is performed on belts made of nickel, platinum or corrosion-resistant steels. With contact of two bodies (ferrite powders-substrate) there are different physical constants and initial temperatures T 1 and T 2 at the contact boundary between them and intermediate contact temperature T c is established that is determined from the relationship Physical constants of some materials, their initial temperatures T 1 and T 2 and the contact temperature T c , are provided in Tables 1 and 2.…”