Practice question

Airlift bioreactors achieve efficient mixing and oxygen transfer without mechanical agitators, using gas-induced density differences to drive liquid circulation, thereby minimizing shear damage critical for fragile filamentous fungi like Aspergillus and mammalian CHO cells producing monoclonal antibodies. Design includes central draft tube dividing vessel into riser region where sparged air bubbles decrease apparent density to 800 kg per cubic meter driving upward flow at 0.5 to 2 meter per second, and downcomer region where degassed denser liquid returns by gravity creating continuous loop. Internal-loop version has draft tube inside vessel, external-loop variant has separate downcomer for better heat exchange and disengagement. Oxygen transfer coefficient kLa 100 to 300 per hour remains high due to extended bubble residence and low coalescence, while power input solely from isothermal expansion of gas reduces energy cost. Absence of shaft seals lowers contamination risk. Applications encompass single cell protein, wastewater treatment and animal cell cultivation where low shear preserves viability, though scale-up faces challenges of bubble coalescence at high cell density.