Hydrodynamics and oxygen transfer characteristics in airlift bioreactors

Abstract

Using a light transmission optical probe, the effect of superficial gas and liquid velocities on bubble properties (bubble size, bubble rising velocity, bubble frequency and local gas holdup) at axial and radial positions were determined in the riser and the downcomer of a concentric cylinder airlift reactor. Both the vertical bubble length, the bubble rising velocity and the bubble frequency at axial in the riser increased with increasing superficial gas velocity and the bed height, but not depended on the superficial liquid velocity in the given experimental range. The radial distribution of local gas holdup, vertical bubble length and bubble frequency in the riser and the downcomer were found to be non-uniform. The profiles of local gas holdup, vertical bubble length and bubble rising velocity in the riser were shown as parabolic shapes. The local gas holdup, the vertical bubble length and the bubble frequency in the downcomer changed with superficial gas velocity and the distance from the top of the draft tube. Recirculation and flow structures of the gas in the downcomer section of a concentric cylindrical airlift reactor of air-water system were studied using a light reflection probe with the cross-covariance technique. A semi-empirical model presented here for predicting the gas recirculation rates in airlift reactors was developed based on the concept of ideal bubbly flow. The entrained gas rate and the gas recirculation rate increased with increasing superficial gas velocity. There were non-uniform radial distributions of air flow in the downcomer. Air flow rates in the downcomer depended on superficial gas velocity and the distance from the top of the draft tube, but no tdepended on superficial liquid velocity. The effects of superficial gas velocity, the downcomer-to-riser crosssectional area ratio (0.11 $\leq\,A_d/A_r\,\leq\,$0.53), and the connecting section length (0.1 $\leq\,L_c\,\leq$ 0.6) on gas phase holdup, the circulation liquid velocity, th...