(166f) The Fluidization Pattern of Density-Segregating Binary Mixtures

Although widely exploited in chemical conversions (combustion, pyrolysis, polimerization, etc.) or physical treatments (solid mixing or separation, granulation, drying, etc.), fluidization of mixtures of dissimilar particles still constitutes a source of problems for processes whose efficiency is related to the state of mixing of bed components at given operating conditions. Devising effective design and conduction criteria for fluid bed operations employing multisolid charges requires a better understanding of the dynamics of segregating fluidization, an unsolved problem, to date, even for two-solid beds, which should represent the simplest case. Indeed, the main difficulty in the analysis of their behaviour is that segregation and fluidization get on simultaneously under the action of the gas flowing across the particulate mass, so that component distribution and fluidization conditions turn out to be functions of each other. Because the opposite tendencies to stratification and remixing are mainly influenced by density and size differences between the two solid species, a separate investigation on different types of binary beds can be of help in isolating the role played by each of the two factors. To this purpose, the paper analyses the process of fluidization of mixtures of two spherical solids differing only in density. Experimental results demonstrate that extending to these systems the concept of minimum fluidization velocity leads to neglecting some important aspects of the actual phenomenology of binary fluidization, as it fails to account for its dependence on the state of mixing of bed components. It is shown that achievement of the fluidized state covers a finite velocity range, whose boundaries can be defined in a general way. The location of these boundaries, as well as the amplitude of the velocity range of fluidization is related to the component density ratio, mixture composition and packed bed configuration.