The following steps are examined in the freezing of drops in a liquid coolant: initial, crystallization of the solution, and formation of granules. A working diagram and system of equations describing the crystallization of a liquid drop of solution are presented. Investigations conducted on the freezing of saline solutions made it possible to determine the following kinetic parameters: the time and average cooling rate of drops to the crystallization temperature, as well as the time and crystallization rate of different sizes of drops of the solution, and the geometric dimensions and process parameters of the freezing plant.The cryochemical method is widely used to produce nanomaterials from organic and inorganic salts. Freezing of solutions of material-forming components is a basic process of the cryochemical technology of solid-phase materials. Here, it is necessary to ensure the highest solidification rate of both the solvent, and also the dissolved substances, and also to retain the high chemical homogeneity of the solid phase, which is inherent to the initial solution.To produce ultra-disperse materials in accordance with cryochemical technology, the initial solution is atomized by some means or other in a container filled with a cryogenic liquid (for example, liquid nitrogen), where vigorous freezing of drops of the solution (DS) occurs.When DS of a certain radius strike the surface of the cryogenic liquid, a violent boiling process is initiated, and a vapor interlayer of the coolant liquid is formed along the surface wetted by the DS. Under the action of this vapor layer, granules are rotated about their own axis, and are chaotically displaced over the surface of the cryogenic liquid. In a number of cases, the DS become submerged in the cryogenic liquid on contact. Thus, the freezing of DS situated on the surface of a coolant with a boiling point appreciably lower than the crystallization temperature of the solution is rather complex.Owing to vigorous boiling of the liquid, which is caused by a high temperature differential ∆T = T d -T l (T d and T l are the temperatures, respectively, of the DS and cryogenic liquid), a vapor interlayer with a thickness h is formed between the surface of a sphere and the liquid (Fig. 1). The thermal flux from the warmer sphere is drawn away through the low-heatconducting layer of liquid vapor. According to experimental investigations, the following two regimes of coolant-vapor flow are possible: laminar and turbulent. Numerical assessment indicates [2] that for liquid nitrogen, a turbulent regime will set in when the radius of a drop R d > 8 mm.Since appreciably smaller sizes of DS are used for the cryochemical method of porous-granule synthesis, it is possible to assume the presence of a laminar regime for the movement of vapor in the boundary film.
Production rotor presses (RP) have come into widespread use in the chemical and allied branches of industry for the mass production of articles of relatively simple shape, which are formed from granular materials [1]. In these machines, the basic process of compacting the initial material is carried out simultaneously in several sets of pressing tools with their continuous movement around the vertical (more rarely the horizontal) axis together with the effective rotor. The pressing tool consists of a die and two plungers; the die is rigidly fixed to the rotor, while the plungers move along the axis parallel to the axis of the rotor's rotation. The complete production cycle, which includes the batching of material, pressing, ejection, and removal of the article, and other operations when necessary, is accomplished in one revolution of the rotor.The basic force effect on the granular material being compacted occurs during the pressing segment, which is determined by the central pressing angle ~% (Fig. 1). The bulk compression of the material in die 4 is accomplished as a result of the relative forward two-way motion of plungers 3, their transportation motion together with rotor 5, and the rolling of rollers 2 along stationary master cam 1 with a slope "r. Plungers with fiat ends are used most frequently for the fabrication of cylindrical articles, and a needle is installed in the die for articles with a central channel.The determination of the density and pressure throughout the volume of the article being formed as a function of pressing conditions is one of the most critical problems of the theory of compaction of granular materials. Solution of the direct problem of the calculation of force parameters (distributed and integral) in accordance with the given geometrical description of the article being pressed is of special interest in constructing RP.The fa'ictional forces that develop daring pressing between the material being compacted and the wall of the die and needle, and the forces of interparticle wedging and bridging result in nonuniform pressure (stress) and density distribution throughout the volume of the article being pressed. Maximum pressure and density are observed directly beneath the plungers, and decrease with increasing distance from the plungers. Nonuniformity of their distribution is especially high when articles are pressed with a high length-to-diameter ratio.The stress-strain state of the article being pressed is extremely complex. Not only longitudinal forces, which are a result of the action of the plungers, but also radial and circumferential forces caused by restriction of particle displacement by the walls of the die and needle, develop in the material being pressed. A pressure lower than that in the direction of pressing is transmitted to the wails of the die. Experimental data [2] on the density distribution throughout the volume of pressed articles suggest that the stresses are distributed uniformly over not only the height of the article, but also its cross-section.The complexity ...
In chemical and related industries, screw presses of various designs are widely used for compaction (densification) of powdered materials [1, 2]. The basic determining technical parameters for such machines are the throughput and the input power.Methods for calculating the throughput for low-speed and high-speed screw presses of various designs are given in [2]. Let us calculate the input power, which determines the correct choice of the drive and the strength characteristics of the structural elements of screw presses.The input power during compaction of powdered materials depends on the rate of energy consumption within the limits of the transport, prepressing, and pressing zones. As we know [3], the highest power consumption is required in the pressing zone, where the highest pressures arise in the material to be compacted. In the transport and prepressing zones, the input power required is not high (10-15% of the total power), and in engineering calculations it can be neglected.
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