The Determination of Minimum Bubbling Velocity, Minimum
Fluidization velocity and Fluidization Index of Fine Powders
using Gas-Solid Tapered Beds

Abstract

Tapered beds have a velocity gradient along the axis unlike conventional beds. Tapered beds can be used in place of conventional beds to deal with problems such as slugging. Previous studies using gas-solid tapered beds have reported that stagnant bed height has no effect on minimum fluidization velocity. They have thus come up with correlations to predict minimum fluidization velocity without incorporating the initial bed height. 3 different particle systems namely Hematite, Dolomite and Limestone in the size range 20 – 80 μm were used in the study. Two tapered beds having the tapered angles of 7.36º and 9.28º were used and air was the fluidizing medium. Minimum bubbling velocities at each of the bed heights have also been observed and the fluidization index was calculated. The fluidization index is the ratio of minimum bubbling velocity to the minimum fluidization velocity. The other models for determining umf , also define umf to be that superficial velocity through the entrance at which the maximum pressure drop is obtained , i.e when the bed becomes partially fluidized from a stagnant condition whereas correlations developed here are for when the bed becomes totally fluidized (minimum full fluidization velocity) and pressure drop becomes constant. As the minimum full fluidization velocity and minimum bubbling velocity were found to increase with an increase in bed height; new correlations have been developed for predicting these values for particles in the size range of 20– 80 μm.