Effects of Bubble Interactions in Circulating Liquid Flow on Liquid Phase Mass Transfer Coefficient in an External Loop Airlift Bubble Column

Abstract

The liquid phase mass transfer coefficient kL for an external loop airlift has been investigated to elucidate a mechanism and degree for the gas–liquid concurrent, circulating liquid velocity uL to affect it. The approach is based on the Higbie model, i.e. kL∝uS0.5 at a fixed average bubble diameter dB, where the average bubble slip velocity uS is derived from the drift flux model. The uS and kL values have been deduced to decrease in the order of the gas–liquid concurrent flow, batch liquid and countercurrent flow. The result is elucidated by variation of the bubble wake development with the turbulent liquid flow direction and rate, in addition to variations of liquid inertia and drag on individual bubbles. The observed kL, surpassing kLS for the single bubble and kL,BF for the bubble flow (BF), increases from the enhancement effect of concurrent uL on kL through its preventing bubbles from clustering, coalescence and breakup. The kL values could reasonably be reproduced by the Higbie model in which the terminal rise velocity uBS is replaced by uS,BF=(gdB/2)0.5 for the BF. A factor F=kL/kL,BF derived from the above uS,BF-based equation for kL well reproduces the observed F.