The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m3 NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, Ad/Ar, ranging from 0.11 ≤ Ad/Ar ≤ 0.69 and for concentric-tube types in the range 0.13 ≤ Ad/Ar ≤ 0.56.
The results are compared and contrasted for the two types of airlift contactors and also with corresponding results obtained in a bubble column (Ad/Ar = 0). Both the riser gas holdup, (εG)r, and the overall volumetric mass transfer coefficient, (KLaD)T, were found to increase with increasing gas superficial velocity and the corresponding specific gassing power input, (PG/VD)T. However, at a specified power input both (εG)r and (KLaD)T decreased with increasing Ad/Ar due to a resultant increase in circulating liquid velocity. Unifying generalized correlations for both types of airlift contactors are given for both (εG)r and (KLaD)T. These correlations are expected to be applicable as well to continuous-liquid flow bubble columns at comparable liquid velocities.
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