BED-HEIGHT DYNAMICS OF EXPANDED BEDS

In this work, we develop a distributed-parameter model which is capabl e of predicting the dynamic response of bed height as a function of th e salient properties of the solid and fluid phases. We show that a mod el which captures both the convective and dispersive nature of solid t ransport in a solid-liquid fluidized bed can adequately predict the be d-height evolution under both step increases and step decreases in the fluidization velocity. The convective term is described by an augment ed form of the Richardson-Zaki expression. We propose a novel Bingham- like model to describe the diffusive characteristics of the solid-liqu id system. It is shown that the dispersion scales as a function of the dimensionless column-average solid velocity, which is proportional to a yield stress. When this velocity is large, as in the initial period after a step change, the dispersion mechanism is not operable in the system. After the column-average solid velocity drops below a critical level, the dispersion mode is operable. A series of experiments were performed for both step increases and step decreases in the fluidizati on velocity. Model predictions for the bed-height transients are in go od agreement with experimental data. (C) 1997 Elsevier Science Ltd.