Hydrodynamics and mass transfer behavior in large scale multiple draft tube airlift contactors

Abstract

Developes of the basic knowledge regarding the behavior of the large scale multiple draft tube airlift contactor especially in the case of a large cross sectional area. Three main aspects were investigated including: (i) the influence of airlift configuration; (ii) the influence of the downcomer to riser cross sectional area ratio (Ad/Ar); and (iii) the effect of salinity of liquid phase on the airlift performance. The configuration of draft tube was found to significantly affect the performance of the airlift contactor. The multiple draft tube configuration demonstrated a better gasliquid mass transfer performance when compared with the conventional one draft tube system. The airlift with a larger number of draft tubes allowed a higher level of bubble entrainment which rendered a high gas holdup in downcomer. This resulted in a higher overall gas holdup in the contactor. Liquid velocity was also higher in the system with a larger number of draft tubes. This was believed to be due to the effect of internal liquid circulation which could take place more significantly in the airlift contactor with one large draft tube than in the system with multiple draft tubes. The ratio between downcomer and riser cross sectional areas, Ad/Ar, was also shown to have great effects on the system performance. The larger Ad/Ar exhibited the larger downcomer area which caused the lower downcomer liquid velocity and less quantity of gas bubbles being dragged into the downcomer. Therefore the overall, riser and downcomer gas holdups decreased with an increase in Ad/Ar. As a large fraction of gas bubbles left the system with large Ad/Ar, the interfacial area for mass transfer also decreased which led to a reduction in the overall volumetric mass transfer coefficient. This work also examined the influence of salinity on the airlift performance. Salinity raised the liquid phase surface tension which resulted in smaller bubble formation. This greatly enhanced the gas entrainment within in the system. This enhanced both the gas holdup and the gas-liquid interfacial area which resulted in a higher rate of gas-liquid mass transfer.