  Citation: Lins Barros, P.; Ein-Mozaffari, F.; Lohi, A. Gas Dispersion in Non-Newtonian Fluids with Mechanically Agitated Systems: A Review. Processes 2022, 10, 275. https://doi.org/10.3390/pr10020275 Academic Editor: Václav Uruba Received: 5 December 2021 Accepted: 27 January 2022 Published: 30 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Review Gas Dispersion in Non-Newtonian Fluids with Mechanically Agitated Systems: A Review Paloma Lins Barros , Farhad Ein-Mozaffari * and Ali Lohi Department of Chemical Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, Canada; paloma.barros@ryerson.ca (P.L.B.); alohi@ryerson.ca (A.L.) * Correspondence: fmozaffa@ryerson.ca 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 vol- umetric 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. Keywords: mixing; gas dispersion; gas holdup; non-Newtonian fluids; mass transfer coefficient 1. Introduction Multiphase flow operations have been extensively investigated due to their wide industrial applications [1–5]. These operations are often evaluated by considering the degree of contact between the phases. For instance, the phases interaction is crucial to determine the performance of multiphase mixing processes. In view of that, stirred vessels are commonly utilized for multiphase mixing operations due to their versatility in promot- ing adequate mixing characteristics for different types of fluids. For gas-liquid mixing, for example, high mass transfer coefficients can be achieved when employing sparged agitated vessels [6]. Hence, many studies have investigated these mixing systems, especially with regard to gas dispersion in Newtonian fluids [7–10]. Although 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]. More specifically, gas dispersion in non-Newtonian fluids with shear-thinning characteris- tics can be encountered in antibiotic and polysaccharide production, fungal fermentation, wastewater treatment, and cell culture [12–16]. The aeration and mixing of non-Newtonian fluids have many challenges especially because of their complex rheological characteristics. Since the viscosity varies depending on the local shear rate, poor mixing regions may exist, which can lead to process inefficien- cies [17]. Thus, impeller configurations in aerated non-Newtonian applications are often designed to target the reduction of stagnant regions created by the shear rate gradient in the vessel and to enhance the mixing performance [18]. Some of the mixing configurations Processes 2022, 10, 275. https://doi.org/10.3390/pr10020275 https://www.mdpi.com/journal/processes