Airlift bioreactors have the potential of being used in many fermentation processes, and understanding their hydrodynamics is key to their use. To this end, gas holdup (i.e., volumetric gas fraction) and superficial liquid velocity in the downcomer and riser are studied in an external loop airlift bioreactor with an area ratio of 1:16. Two downcomer configurations are investigated consisting of the downcomer open or closed to the atmosphere. Experiments for these two configurations are carried out, over a range of superficial gas velocities from UG = 0.5 to 20 cm/s, using three aeration plates with open area ratios of 0.62, 0.99 and 2.22%. These results are compared to bubble column bioreactors and external loop airlift bioreactors with larger area ratios for similar operating conditions.

Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Experimental results show that the gas holdup in the riser does not vary significantly with a change in the downcomer configuration or bubble column operation, while a considerable variation is observed in the downcomer gas holdup. Test results also show that the maximum gas holdup for the three aerator plates is similar, but that the gas holdup trends are different.

The superficial liquid velocity is found to vary for the two downcomer configurations. However, for both cases the superficial liquid velocity is a function of the superficial gas velocity and/or flow condition in the downcomer. These observed variations are independent of the aerator plate open area ratio. The superficial liquid velocity is also found to vary for both downcomer configurations when compared to external airlift loop reactors having larger area ratios.