Mixing efficiency studies in an airlift bioreactor with helical flow promoters for improved reactor performance

Abstract

Cultivation of aerobic microorganisms typically requires rapid, but gentle and simultaneously energy efficient mixing in bioreactors to prevent high gradients between local concentrations. The mixing efficiency in a pilot scale airlift bioreactor with helical flow promoters was investigated with varying air inputs. Helical flow promoters were helical tubes attached to riser or downcomer, or both and besides acting as flow promoter they were used as additional gas spargers. Mixing time, circulation time, dimensionless mixing time and Bodenstein number were used as assessment parameters to analyze the mixing efficiency of different airlift setups with helical flow promoters. Introducing air through the helical flow promoter in the downcomer zone prolonged the circulation time, but shortened the mixing time and decreased the Bodenstein number. Low Bodenstein number indicated an improved axial dispersion. Mixing was more related to dispersion in the airlift reactor with helical flow promoter in the downcomer zone whereas in a regular airlift reactor mixing was related to liquid circulation speed.