Characterization of mixing performance in bioreactors using flow-following sensor devices

A quantitative description of flow and mixing in bioreactors is necessary to assess the exposure of the microorganisms to suboptimal conditions, which may negatively impact key performance parameters of a process, such as yield and productivity. The traditional approach of quantifying mixing, by means of a terminal mixing time, is inadequate because it rests on the assumption that a homogeneous state is reached, and it provides little information on the mixing process itself. In this study, it has been demonstrated how flow-following sensor devices can be used to obtain detailed knowledge on macroscopic flow in stirred bioreactors under turbulent flow conditions. The flow generated by common impeller types; the Rushton disc turbine and the pitched blade turbine, has been examined in terms of the axial flow features and circulation times under various levels of agitation. It was demonstrated that useful features to characterize the flow in the vessel, such as the axial distribution and axial velocity profile of the sensor devices could be obtained for the examined conditions. Based on these features it was observed that the flow following behavior of the sensor devices were negatively affected at higher impeller speeds. This finding was corroborated by CFD simulations of the vessel. The mean circulation times determined by the sensor devices were found to be highly correlated to the mixing times determined from homogenization of a chemical tracer (tm = 2.2–2.6 t¯c), which demonstrates that flow-following sensor devices can be used to quantify macromixing in bioreactors. Furthermore, the underlying circulation time distributions could be derived, which were found to be well described by the lognormal probability distribution.