HYDRODYNAMIC BEHAVIOR AND GAS-LIQUID MASS TRANSFER OF NOVEL NON-BAFFLED AIRLIFT CONTACTOR

Abstract

This work purposed to study hydrodynamic behavior and gas-liquid mass transfer of novel non-baffle airlift contactors in which the fluid cyclic flow pattern was induced without the need of physical partition between riser and downcomer. Two geometries were examined, i.e. flat panel and cone bottom cylindrical airlifts. The flat panel of various widths, i.e. at 20, 30, 40, and 50 cm and the cone of various angles of bottom cone, i.e. at 30o, 45o and 53o were employed as a model study. The parameters for analysis were aeration rate and unaerated liquid height (40, 50, 60 cm and at volume 100 L). Results showed that most hydrodynamic parameters, i.e. riser gas-holdup, downcomer gas-holdup, overall gas-holdup, bubble size, downcomer liquid velocity and mass transfer (kLa) increased with aeration rate. Riser and downcomer cross sectional area did not depend on air flow rate and unaerated liquid height, but did depend on the width of contactor for flat panel and angle of cone bottom for flat plate and cone geometries, respectively. Bubbles size distribution could only be observed in flat panel configuration as the employed photographic technique could not be used accurately with the cone bottom airlift. At low usg (<0.4 cm/s), bubble sizes were small (0.3-0.4 cm) and presented in bimodal distribution. The high air flow rate (usg > 1 cm/s) bubble sizes were large (0.5-0.6 cm) and with a broader distribution. Bubble Sauter mean diameter was within the range of 0.2-0.8 cm. Flat panel showed that riser gas-holdup, downcomer gas-holdup, overall gas-holdup and kLa increased with the width of contactor from 30 to 50 cm, whereas the contactor with the width of 20 cm exhibited similar behavior with those of 50 cm. The maximum values of riser gas-holdup, downcomer gas-holdup, overall gas-holdup and kLa were 0.10, 0.075, 0.08 and 0.014 1/s, respectively. On the other hand, it was demonstrated that riser gas-holdup, downcomer gas-holdup and overall gas-holdup from the cone geometry decreased when the angle of bottom increased. The maximum values of riser gas-holdup, downcomer gas-holdup and overall gas-holdup were 0.09, 0.06 and 0.07. The kLa of the cone geometry only depended on air flow rate and not on other design parameters. The unaerated liquid height had strong effect on downcomer liquid velocity. The maximum of downcomer liquid velocity in the flat panel and cone geometries were 27 and 24 cm/s, respectively.