Gas dispersion in non-Newtonian fluids is encountered in a broad range of chemical, biochemical, and food industries. Mechanically agitated vessels are commonly employed in these processes because they promote high degree of contact between the phases. However, mixing non-Newtonian fluids is a challenging task that requires comprehensive knowledge of the mixing flow to accurately design stirred vessels. Therefore, this review presents the developments accomplished by researchers in this field. The present work describes mixing and mass transfer variables, namely volumetric mass transfer coefficient, power consumption, gas holdup, bubble diameter, and cavern size. It presents empirical correlations for the mixing variables and discusses the effects of operating and design parameters on the mixing and mass transfer process. Furthermore, this paper demonstrates the advantages of employing computational fluid dynamics tools to shed light on the hydrodynamics of this complex flow. The literature review shows that knowledge gaps remain for gas dispersion in yield stress fluids and non-Newtonian fluids with viscoelastic effects. In addition, comprehensive studies accounting for the scale-up of these mixing processes still need to be accomplished. Hence, further investigation of the flow patterns under different process and design conditions are valuable to have an appropriate insight into this complex system.

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The operations of mixing systems utilized in a variety of industrial applications for dispersing gas in non-Newtonian fluids are complex. The main challenge is to maintain a homogeneous medium throughout the vessel. In fact, coaxial mixers have demonstrated an energy-efficient performance for systems containing rheologically complex fluids. However, the accurate characterization of the power demand for gas dispersion in yield-stress fluids using coaxial mixers remains unclear. Hence, the objective of the present work was to investigate the power characteristics of a coaxial mixing system comprising the xanthan gum solution, which is a shear-thinning fluid possessing yield stress. The power consumption of an Anchor-PBT mixing configuration was measured experimentally at different central and anchor impeller speeds, rotation modes, and aeration rates. The main effects of the operating variables and their interactions were quantified using a central composite design of experiments. Significant effects of the central impeller speed and the interaction between the anchor speed and the aeration rate were identified. Also, a generalized power curve for this mixing system was proposed based on a predefined equivalent impeller speed and impeller diameter expressions to develop the novel power number and Reynolds number. The findings of this work provide an asset in designing and scaling up of energy-efficient aerated coaxial mixers by rapidly estimating the power demand.

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