Oxygen transfer and mixing in mechanically agitated airlift bioreactors

Abstract

Gas holdup, mixing, liquid circulation and gas–liquid oxygen transfer were characterized in a large (∼1.5 m 3) draft-tube airlift bioreactor agitated with Prochem ® hydrofoil impellers placed in the draft-tube. Measurements were made in water and in cellulose fiber slurries that resembled broths of mycelial microfungi. Use of mechanical agitation generally enhanced mixing performance and the oxygen transfer capability relative to when mechanical agitation was not used; however, the oxygen transfer efficiency was reduced by mechanical agitation. The overall volumetric gas–liquid mass transfer coefficient declined with the increasing concentration of the cellulose fiber solids; however, the mixing time in these strongly shear thinning slurries was independent of the solids contents (0–4% w/v). Surface aeration never contributed more than 12% to the total mass transfer in air–water.