Effect of sparger position on bubble size distribution and gas-liquid mass transfer in internal airlift contactors

Abstract

To investigate the influence of sparger position on the bubble size distribution in internal loop airlift contactors. To complete the examination, the effects of other main design/operating parameters, i.e. the ratio between cross-sectional areas of downcomer and riser (A[subscript d]/A[subscript r]) and the superficial gas velocity, were also considered. The aeration in the ALC was achieved via the PVC ring sparger. The number of holes in the sparger was fixed at 60. The air flow rate was controlled in a range of superficial gas velocity, u[subscript sg] , from 4 to 10 cm s[superscript -1] . The ratios between cross-sectional areas were varying from 1.4 to 0.9. The sparger position was varied by changing the position of annulus sparger (0, 6 and 12 cm above the bottom of the contactor). A photographic technique was used to measure the bubble size and its distribution. The results revealed that the Sauter mean diameter of bubble was the smallest when the sparger was located at base of the reactor, and became larger when the sparger was moved to higher positions. It should be noted that at the lowest position, the sparger position was lower than the draft tube, therefore the recirculating fluid from the downcomer pushed the new-born bubbles towards the wall of the reactor and bubble breakage was observed. In addition, bubble size was also controlled by other flow conditions in the system, especially the superficial gas velocity (u[subscript sg] ), where an increase in u[subscript sg] decreased the bubble size due to the induction of high turbulence which rendered the bubble breakup. The system operated with smaller riser area had a higher level of turbulence, thus, the average bubble size was found to be smaller than system with larger riser area. At the bottom section of the reactor, the Sauter mean bubble diameter appeared to be larger than those at the middle and top sections. In terms of gas-liquid mass transfer, the sparger at its lowest position provided the highest level of overall volumetric mass transfer coefficient in the system. The specific interfacial area at the lowest sparger position was also found to be higher that other positions which suggested that this specific interfacial area could have strong positive impact on the gas-liquid mass transfer.