TRANSIENT-BEHAVIOR OF PULSED PARTICULATE FLUIDIZED-BEDS

Transient phenomena in solid-liquid fluidized-bed systems are importan t in designing pulsed, countercurrent (multistage) fluidized-bed conta ctors of the Cloete-Streat type at high-solids flow rate. Of particula r interest are the residence times or corresponding velocities of poro sity gradients in the bed and the excess or overshoot height of the be d after refluidization. Theory assuming local equilibrium between hold up and velocity of the phases (local-equilibrium model) for stepwise p erturbations in the liquid flow is readily available. It is investigat ed whether the local-equilibrium theory can be used for more complex p erturbations and whether inertia effects, such as are encountered in c ountercurrent multistage fluidized-bed systems, can be ignored. Theref ore, the detailed particle-bed model of Foscolo and Gibilaro, which in corporates inertia effects, was applied to investigate the transient b ehavior of fluidized-bed systems. Transient fluidization experiments w ere performed with a broad range of water-fluidized particles in a lab oratory-scale multistage fluidized-bed contactor. The operating condit ions corresponded to those for countercurrent contact. Numerical simul ations with the particle-bed model predict satisfactory experimental r esults. The ''overshoot'' heights of the fluidized bed were estimated correctly by the particle-bed model, whereas the local-equilibrium mod el only provides a conservative estimate. However, the local-equilibri um model allows an analytical solution that is more interesting for de sign, as it avoids tedious calculations. The residence time of the las t perturbation before the fluidized bed relaxes to steady state was es timated with similar accuracy by both models.