The Effects of Geometry and Operational Conditions on Gas Holdup, Liquid Circulation and Mass Transfer in an Airlift Reactor

In airlift reactors transport phenomena are achieved by pneumatic agitation and circulation occurs in a defined cyclic pattern through a loop. In the present work, the effect of geometrical relations on gas holdup and liquid velocity, and consequently on the gas-liquid mass transfer coefficient, was studied in a 6liter airlift bioreactor with AD/AR = 0.63; AD, downcomer cross-sectional area, and AR, riser cross-sectional area. Measurements of the volumetric oxygen transfer coefficient (kLa) were taken in a water-air system using a modified sulfite oxidation method. Different conditions were examined by varying parameters such as superficial air velocity in the riser (UGR), bottom clearance (d1) and top clearance (d2). It was observed from the experimental results that d1 and d2 have a remarkable effect on kLa values. The effect is due to their influence on gas holdup and liquid velocity, consequently affecting kLa. Superficial air velocity in the riser (UGR) ranged from 0.0126 to 0.0440 m.s and kLa varied between 40 to 250 h, whereas gas holdup (ε) reached values up to 0.2. The volumetric oxygen transfer coefficient (kLa), gas holdup in the riser (εR) and downcomer (εD) and superficial liquid velocity in the riser (ULR) for all the geometrical relations were successfully correlated with dimensionless numbers, namely, the Sherwood number (Sh) and the Froude number (Fr) as well as with geometrical relations such as the bottom space ratio (B = d1/DD) and top space ratio (T = (d2 + DD)/DD).