Hydrodynamics in external-loop airlift bioreactors with static mixers

Liquid circulation superficial velocity and gas holdup behaviours were investigated in an external-loop airlift bioreactor of 0.170?m³ liquid volume in gas-induced and forced-circulation-loop operation modes, in the presence of static mixers made of corrugated stainless steel pieces, resulting in packets with the height-to-diameter ratio equal to unity and using non-Newtonian starch solutions as liquid phase. The static mixers were disposed in the riser in three blocks, each with three mixing packets, successively turned 90° to the adjacent mixing element. It was found that in the presence of static mixers and forced-loop operation mode, liquid circulation superficial velocity in the riser section was significantly diminished, while gas holdup increased in a great measure. It was considered that static mixers split the fluid into individual streams and break up the bubbles, resulting in small bubble sizes with a relative homogeneous bubble distribution over riser cross section. They act as supplementary resistances in liquid flow, reducing riser cross sectional area, equivalent with A _D /A _R area ratio diminishing.