Modeling of Oxygen Transfer Correlations for Stirred Tank Bioreactor Agitated with Atypical Helical Ribbon Impeller

Abstract

Problem Statement: Mixing filamentous fungi entails delicate balance between promoting high volumetric oxygen transfer coefficient (k_La) while keeping low hydrodynamic stresses in the microenvironment of cultures. This study examined the oxygen transfer capability of a prototype low shear helical impeller as a potential replacement for the standard Rushton turbine. Approach: The dependence of k_La upon specific power uptake, superficial gas velocity and apparent viscosity were examined under coalescent, non-coalescent and viscous pseudoplastic fluids scenario using dynamic gassing out technique. Subsequently, collected data were treated under historical data design of response surface methodology. Results: Three empirical power law k_La correlations were developed for each fluid. Correlation comparison with literature models for single turbine agitation suggest higher k_La augmentation within 1.5-3.6 folds for helical impeller in distilled water and as high as 78% improvement in electrolytes fluid. However impeller performance is comparatively 10% lower against theoretical k_La of triple turbines arrangement for power uptake between 0-1600 W m^-3. In case of carboxymethylcellulose, better oxygen transfer is expected for design with higher proportionality of impeller-to-tank internal diameter. Conclusions: Helical impeller performance as gas-liquid contactor is comparable to single and triple Rushtons turbines, with superficial gas velocity proved to be more influential than power uptake in non-viscous liquids and vice versa in viscous pseudoplastic fluids.