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Abstract
The fluidizing characteristic of a powder may be identified by studying the subsidence curve of a fully fluidized bed after abrupt termination of the fluid supply. The full bed collapsing process consists of three consecutive stages: a rapid initial stage for bubble escape, an intermediate stage of hindered sedimentation with constant velocity of solids descent, and a final decelerating stage of solids consolidation. For certain fluid-solid systems, either one or two of these stages may be missing. Mathematical models proposed for the individual stages and corroborated by high-speed cinematographic experimental measurements, led to the formulation of a dimensionless subsidence time Θ, correlating the physical properties of the solids and the fluid as well as the operating parameters to the fluidizing characteristic of the system. The larger the value of Θ, the smoother the fluidization.
For automatic determination of the subsidence behavior of a collapsing bed, an instrument using an optical-fiber projector-receiver set, for rapid tracking and recording of the subsiding bed surface, was developed. When the instrument output was fed into a microprocessor, the relevant parameters defining fluidizing characteristics were computed and displayed. By using this instrument, the fluidizing characteristics of binary mixtures of two closely sized powders were studied. For binary mixtures consisting of Geldart's Group A and Group B powders, fluidizing characteristics were noted to be intermediate between those of the two consistent components according to their relative proportions. When a Group C powder was added to either a Group A or Group B powder, however, the fluidizing characteristic of either component was found to be improved by the presence of the other, and there existed, in general, an intermediate composition with optimum fluidizing characteristic surpassing that of either of the constituents.
