Abstract
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.
Highlights
Fewer studies refer to the aeration of non-Newtonian fluids, a broad range of industrial applicability can still be observed in food, chemical, biochemical, pulp and paper, and painting industries [11]
Jamshidzadeh et al [9] studied the transition between loading and complete dispersed regimes by determining the critical impeller speed for different impeller types used for gas dispersion in yield stress fluids and power-law fluids, respectively
Moilanen et al [97] investigated the gas dispersion in a yield-stress fluid using computational fluid dynamics (CFD), which enabled the prediction of the cavern formation within the vessel
Summary
Power consumption is a relevant variable to assess the mixing performance since it is directly related to the cost of operation. Gabelle et al [22] included the impeller type and vessel geometry effects on the estimation of the gassed power uptake for CMC and xanthan gum solutions described by Processes 2022, 10, 275 the power law model They considered different scales and multiple impeller configurations by proposing a novel expression in terms of the gas flow rate, tank diameter, impeller diameter, and power number of the stirrer closer to the sparger. Jamshidzadeh et al [32] verified different correlations for gas dispersion in a power-law fluid using coaxial mixers, which are comprised of a high-speed central impeller and a close clearance impeller rotating slowly They obtained different fitted parameters for each pumping direction including the effect of both co-rotation and counterrotation modes. Where N is the impeller speed, and M is the corrected torque, which is calculated by subtracting the friction torque (i.e., the torque measured in a vessel prior to filling it with the fluids to be evaluated) from the actual measured value (Equation (3)):
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