First-Principle Study of Gas Dispersion and Oxygen Transfer Efficiency in Non-Newtonian Fluids Using Coaxial Mixers

<p>In 2021, the global market for bioreactors is expected to be around $4.3 billion. Improving the aeration efficiency is one of the important process objectives in bioreactors while providing sufficient oxygen for the cells with minimal energy input and shear rate. About 50-60% of the total operational cost in a wastewater treatment plant is associated with the aeration process. One of the major challenges in bioreactors for the applications of fermentation and wastewater treatment is maintaining a critical level of dissolved oxygen concentration in highly viscous fluids. In this perspective, we have devoted our efforts to better understand the challenges in the aeration process and develop the proof of principles to explain the oxygen transfer mechanism in the coaxial mixer setup. With both the central and anchor impellers attached to independent shafts, the coaxial mixer has more flexibility to operate at various speed ratios and rotation modes. Accordingly, for a coaxial mixer, we investigated how the aeration efficiency (gas holdup per unit power) can be impacted by the following conditions: 1) Operating the downward-pumping, high-solidity ratio, axial flow central impeller at a low critical speed with a slowly-rotating anchor impeller (10-30 rpm), and 2) Operating the radial flow impeller (Scaba) with slowly-rotating anchor impeller (10- 30 rpm). Electrical resistance tomography (ERT) and computational fluid dynamics (CFD) were extensively utilized to shed light on the hydrodynamics of coaxial mixer and its impact on the aeration efficiency in non-Newtonian fluids. The working fluid was carboxymethyl cellulose solution (CMC), which is a non-Newtonian fluid and obeys the power-law model. Our research findings challenge the paradigm that a high rotational speed of impellers can intensify the gas dispersion. Interestingly, we observed the deterioration in the gas holdup at the higher rotational speed with the Scaba-anchor coaxial mixer. The efficacy of impeller designs was evaluated using novel gas helicity analysis. The impeller design with fluctuating gas helicity sign led to poor gas holdup. Our research study also provided sufficient evidence to support that the oxygen transfer rate in non-Newtonian fluids is governed by shear rate and the coaxial mixer operating in co- rotation mode outperformed the counter-rotation mode. </p>