Basic trends in the improvement of domestic equipment for mixing particulate materials

Abstract

Area I: Establishing the Field of Use of Standard Mixers. In a particulate material there is a very complex relationship between the particles that are in contact. This relationship is governed by the properties of the particles (their material nature, shape, relative configuration, size) and the external conditions (humidity, temperature, the occurrence of electric charges on the particles, the degree of compression, etc.). As a result it is virtually impossible to predict the behavior of a particulate material when a mixing element acts on it without conducting experiments. At the present time it is only possible to identify the class of mixer according to the physicomechanical properties of the particulate material to be mixed and the volume of a single portion. A definite model can only be selected from several designs by experimental studies, which require a suitable stock of laboratory mixing equipment. In terms of their process mechanism, standard batch mixers can be divided into two classes: circulatory and bulk mixing. In circulatory mixers the material being mixed is displaced by the operating element along a given circuit, directly acting on the particles in a relatively small area of the internal volume of the mixing compartment. They are only effective for materials that flow well (according to the classification of the Severodonetsk Scientific-Research Institute of Chemical Engineering for noncohesive and cohesively flowing bulk materials [1]). Centrifugal paddle mixers of type TsL and planetary screw mixers of type PSh can be assigned to this class [2, 3]. In bulk mixers the particulate material is displaced by the working surfaces of the agitator in a random manner around the internal volume of the mixing chamber. In this case the blades of the mixing element cover virtually all the internal zones of the mixer's body. Mixers for cohesive particulate materials are also recommended, but cohesively flowing particulate materials can also be mixed in them. Bulk mixers also include LN and GN ribbon mixers, PZh plow mixers, ZL Z-shaped mixers, and ZSh Z-shaped mixers with discharge screw [2, 3]. The choice of the class and type of mixer can be made initially according to the required linear rate of rotation of the mixer operating element, which is calculated from an empirical equation taking into account the required homogeneity of the mixture, the weight fraction of the key component, and the tensile strength of the particulate material [4]. Area II: Choice of Optimum Operating and Design Parameters for Mixers with an Established Design, In most studies in this area the limiting value of the coefficient of heterogeneity of the mixture and the time for attaining this value, the optimum values of the coefficient for filling the mixing chamber with material, the rate of rotation of the operating element, and the energy consumption are determined experimentally on the selected mixer (with respect to a specific mixture of particulate material). While there is an overall benefit, the general disadvantage of these experiments should be noted. The recommendations or formulas obtained from the experiments can only be applied to a given model of mixer over the studied range of operating conditions and for a specific particulate material. Mixer optimization using mathematical models of the mixing process has unfortunately not yet become a sufficiently well established method amongst the developers of mixing equipment. In order to determine the parameters of the models