Effects of geometrical design on hydrodynamic and mass transfer characteristics of a rectangular-column airlift bioreactor

Gas holdup and gas–liquid mass transfer coefficients were measured in a 21-L rectangular-column airlift bioreactor with aspect ratio of 10 and working volumes ranging from 10 to 16 L. The effect of the bottom and top clearances was investigated using water and mineralized CMC solutions and covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N m−1. The gas holdup and mass transfer results were successfully correlated using expressions derived via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift bioreactor. The downcomer gas holdup (ɛd) increased two-fold when the bottom clearance (hb) was increased from 0.014 to 0.094 m while the top clearance (ht) had no effect. Increasing hb decreased the mass transfer by 50% compared to 31% when the top clearance (ht/Dhr) was increased. It was found that the gas–liquid separator diameter ratio (Dhs/Dhc) exerted the maximal influence of over 65% on mass transfer as compared to both clearances.