Bubble characteristics and liquid circulation in internal loop airlift contactors

Abstract

This work aimed at the development of the basic knowledge regarding the behavior of the internal loop airlift contactor (ALC). Three main aspects were investigated including: (i) the bubble size distribution in the contactor and its effect on gas-liquid mass transfer; (ii) the internal liquid flow within riser of the contactor; and (iii) the development of the mathematical model for the prediction of gas-liquid mass transfer behavior in the system. The bubble size distribution in the riser of the ALC was found to follow a normal distribution when the supplied superficial gas velocity was lower than 1 cmํs[superscript -1]. The average bubble size in the riser of the system at this condition was found to be about 7-8 mm. As the superficial gas velocity increased to approximately 2-4 cmํs[superscript -1], the size distribution changed from normal to multimodal types with two dominant bubble sizes at 3-5 and 7-8 mm. At a high range of superficial gas velocity (5 cmํs[superscript -1] up to the upper limit of this work at 12 cmํs[superscript -1]), the bubble size followed a lognormal distribution with a small bubble at 3-5 mm dominating the system. The cross-sectional area ratio between downcomer and riser of the ALC was found to affect the bubble size only when the superficial gas velocity was greater than 2.9 cmํs[superscript -1] and the ratio of 0.4 was observed to give the lowest average bubble size. The analysis of the bubble size distribution revealed that the increase in the superficial gas velocity did not have significant effect on the gas-liquid mass transfer coefficient in the ALC, rather it caused the bubble to become smaller. This increased the mass transfer area between gas and liquid which increased the overall gas-liquid mass transfer rate. The comparison between liquid velocity from experiment and from mass/energy balances indicated that there must exist an internal liquid circulation within the riser. The downflow liquid flowrate in theriser was found to increase with superficial gas velocity. The ratio between downcomer and riser cross sectional areas (Ad/Ar) was found not to influence the fraction of downflow and upflow areas but the extent of internal circulation was less pronounced in the system with a large Ad/Ar. The mathematical model for the ALC divided the system into three sections, each of which had different mixing performance. The first two sections were riser and downcomer for which the mixing were described by a plug flow with dispersion. The gas separation section at the top of the ALC, on the other hand, was described by a completely mixed model. Not only did the simulation results on the oxygen concentration profiles agreed well with the experimental results obtained from this work, it also could satisfactorily explain the results of the experiments reported in literature.