A Method for Optical Quantification of Local Oxygen Limitations in Multiphase Bioreactors

Spatial oxygen limitation is a major source of physiological stress and metabolic, scale-dependent heterogeneity in aerobic bioprocesses. In gas-liquid stirred tank reactors, oxygen availability is governed by hydrodynamics and mass transfer and is therefore inherently non-uniform. Despite its importance, experimentally accessing spatial information on oxygen local supply remains challenging, as most established techniques rely on point-wise sensing or intrusive tracer-based methods. This work presents an optical method for visualizing local oxygen limitation based on the reversible redox chemistry of methylene blue in alkaline glucose solutions. After driving the reactor into a uniformly reduced, colorless state, the spatial reoxidation of the indicator during oxygen aeration is recorded and evaluated pixel-wise. The resulting two-dimensional fields of colorization time provide a spatially continuous indicator of local oxygen supply governed by hydrodynamic transport and gas-liquid mass transfer. Systematic experiments were conducted to assess the influence of glucose and sodium hydroxide concentration on the stability and reproducibility of the redox system, enabling the selection of a robust reaction medium. The method was then applied to a laboratory-scale stirred tank operated at different impeller speeds. The resulting colorization fields reveal characteristic spatial structures associated with oxygen-limiting regions, including insufficiently supplied zones under weak agitation, progressive homogenization with increasing turbulence, and residual transport limitations near reactor walls at high agitation rates. The presented approach requires no local tracer injection and operates under normal aeration conditions, providing qualitative yet physically meaningful insight into oxygen-supply and mixing related heterogeneities. It therefore offers a simple and cost-effective tool for identifying limiting operating points, supporting future scale-transfer studies, and complementing CFD-based analyses of oxygen transport in stirred tank bioreactors.