Influence of draft tube configuration on hydrodynamic properties and mass transfer in large scale airlift contactors

Abstract

Large scale internal loop airlift contactors were examined for their hydrodynamic and mass transfer behavior using tap water and saline solution. These behavior was manipulated by changing the number of draft tubes while keeping the ratio between downcomer and riser cross sectional area constant at 2.1. The results showed that the number of draft tube could have effects on: gas holdup, liquid velocity, and gas-liquid mass transfer. Increasing the number of draft tubes seemed to promote a better liquid movement with higher gas holdup where the overall gas holdup of 0.008 was obtained in the airlift with a conventional one draft tube airlift, whereas this was increased to 0.010 when there were five draft tubes. Liquid velocity seemed to also increase with an increase in the draft tube number and this velocity was 24 cm/s with one draft tube, and 28 cm/s with five draft tubes. Five draft tubes seemed to provide the best level of mass transfer where k[subscript L]a increased from 0.0024 to 0.0031 1/s with an increase in the number of draft tubes from 1 to 5, respectively. In the airlift with five and six draft tubes, two configurations were examined, one with draft tube installed around the peripheral of the outer column, and the other with one draft tube in the center and the rest surrounding the central draft tube. However, the pattern of draft tube configurations did not seem to have great effect on overall gas holdup, and riser liquid velocity and mass transfer. The salinity raised the liquid phase surface tension which resulted that smaller bubble formation. Overall gas holdup in tap water was always less than in saline water, but changes in salinity of the water medium from 15, 30 and 45 ppt did not show impacts on overall gas holdup and riser liquid velocity. However, smaller bubble enhanced the surface area, and hence, increased the rate of mass transfer within the system.