Bubbles in fluidised Beds

Abstract

The aim of the investigation was to measure accurately the growth rate of small bubbles in gas fluidised beds. This was achieved by recording individually injected known volumes of gas into a bed operating in its particulate regime. The systems investigated were ballotini, catalyst and diakon fluidised with both air and carbon dioxide under ambient conditions. The bubble volume, and hence growth rate, was measured by a capacitance technique and volumes from 0•1 to 8•Oom3 could be measured with an overall accuracy of better than ±10%. The accuracy of the measurement of capacitance was ±0•002pF and the range over which capacitance measurements were made was 0•30 to O•70pF. The instrument followed a square wave with no apparent time lag when using a galvanometer with a linear frequency response of up to 100Hz. It was found that a meta-stable bubble phenomenon exists for all the systems investigated. That is, under given particulate conditions, a bubble greater than the stable size will grow and one smaller will shrink and vanish. It was found that meta-stable bubble volumes less than 4com3 could be correlated with gas velocity by an equation of the form:(Meta-stable volume)(gas velocity in excess of the minimum fluidising velocity)n = a constant.(where n is a constant for a given system.) Growth rates measured covered the range -0•15 to +0•25 (cm3/cm travelled) and the growth rate constants were found to cover the range 10 to 70 (cm). It is shown that the meta-stable phenomenon can be explained in terms of bubble shape, assuming the shape is a function of a bubble Reynolds Number.