Pneumatically agitated bioreactors take advantage of the injection of a gaseous stream (often air) to provide mixing and mediate transfer of gaseous substances (i.e., O2 and CO2) with the liquid phase. However, unlike in classical pneumatically agitated reactors where liquid mixing is random (i.e., bubble column), the specific design of airlift reactors (ALRs) causes the liquid to circulate between two interconnected zones known as the riser and the downcomer. The riser and the downcomer are connected by a specific reactor base allowing for liquid circulation and by a gas–liquid separator at the top. Under typical operation conditions, air is injected below the riser section and the removal of gas in the separator generates a mean density gradient between the riser and downcomer zones that causes the liquid broth to circulate. The function of the gas separator is to support efficient gas–liquid disengagement. The fraction of gas introduced in the downcomer section depends on design and operational variables. This fraction has a significant effect on fluid dynamics and, consequently, reactor performance.

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