Study of Local Gas Holdup and Specific Interfacial Area in a Split-Column Airlift Bioreactor Using Sophisticated Four-Point Optical Probe for Culturing Microalgae/Cyanobacteria

Local gas holdup (epsilon) and interfacial area (a) at different axial locations of the riser and downcomer of a split-column airlift bioreactor were investigated using a sophisticated four-point optical probe. Such a type of a reactor has been found to outperform both bubble-column and draft-tube airlift bioreactors for culturing microalgae. The effect of superficial gas velocity (0.3-2.8 cm/s) on both gas holdup and interfacial area was studied using air-water system. It was found that both gas holdup and interfacial area significantly decrease from the top to the bottom of the downcomer for all superficial gas velocities, while their variation from the bottom to the top for the riser was found to be much less than that of the downcomer at the same superficial gas velocities. It was found that the interfacial area of the riser tends to increase by 35% from the bottom to the upper middle point of the column (6.15 Z/D from the bottom), then declines by 10% at the top location (7.7 Z/D from the bottom). Empirical correlations were obtained relating the gas holdup and specific interfacial area to superficial gas velocity of the riser and the downcomer of the bioreactor. It was found that the riser has to be represented as upper and lower halves to be best correlated, while the only upper half of the downcomer was successfully correlated. Having obtained variable interfacial area (a) at different locations of both the riser and the downcomer of the bioreactor, the local K(L)a consequently changes as a function of the location of the bioreactor and hence needs to be investigated locally as opposed to the current studies that have only measured and correlated the overall K(L)a.