Influence of dual-impeller type and configuration on oxygen transfer, power consumption, and shear rate in a stirred tank bioreactor

• Average shear rate and Kolmogorov's microscale were quantified in the systems.
• Correlations for γ˙av were obtained as function of N, ϕ, and rheological properties.
• Results showed that the EEDP-EEUP combination was the least harmful to the cells.
• The dual-impeller EEDP-RT had 87% higher mass transfer efficiency than RT-RT association.
• Associations with elephant ear impeller had shear conditions up to 60% lower than RT-RT.

In conventional stirred and aerated tank bioreactors, the choice of impeller type and configuration is critical to ensure suitable hydrodynamic conditions that maximize oxygen transfer while minimizing power consumption and shear rate. In this work, a systematic comparative evaluation of the performance of seven different dual-impeller configurations in a stirred tank bioreactor was carried out using Rushton turbines (RT) and Elephant Ear impellers in down-pumping (EEDP) and up-pumping (EEUP) modes. A 22 factorial design methodology was used to assess the effects of impeller speed (600–1000 rpm) and specific air flow rate (0.4–1.2 vvm) on the response variables. The EEDP-EEUP, RT-EEDP, and EEDP-RT combinations gave the best results in terms of oxygen transfer (kLa) and power consumption (P), with mass transfer efficiency (EMT = kLa/P) up to 5.09 J−1, which was 87% higher than for the RT-RT combination. For the selected impeller combinations (EEDP-EEUP, RT-EEDP, and RT-RT), the shear effects were quantified in terms of average shear rate (γ˙av) and using the Kolmogorov microscale (λ). The EEDP-EEUP dual-impeller combination proved to be the best system, with shear conditions (2174 < γ˙av (s−1) < 4287) up to 60% lower than for the conventional RT-RT configuration. The results indicated that the EEDP-EEUP system was most suitable for the growth of shear-sensitive aerobic microorganisms, ensuring adequate mass transfer and minimizing cell damage.

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