A theoretical model for droplet breakup in turbulent dispersions

Effect of aeration on oxygen transfer characteristics in integrated wastewater treatment systems utilizing mass transfer model and computation fluid dynamics methods

This paper investigates the aeration and oxygen transfer characteristics within the aeration tank of an integrated wastewater treatment system (IWTS) using Computational Fluid Dynamics coupled with Population Balance Model and oxygen transfer model. The findings suggest that increasing the air flow rate significantly enhances the oxygen transfer rate, albeit at a decreasing rate of growth. The oxygen overall mass transfer coefficient is primarily influenced by the interfacial area per unit volume and to a lesser by the oxygen mass transfer coefficient (kL). A strong positive correlation is found between turbulence intensity and kL, which, along with dissolved oxygen distribution, confirms the critical role of turbulence in the oxygen transfer process. For small-scale IWTS, an air flow rate of 30 L/min may be the optimal choice.

An in-silico analysis of hydrodynamics and gas mass transfer characteristics in scale-down models for mammalian cell cultures

Bioprocess scale-up and technology transfer can be challenging due to multiple variables that need to be optimized during process development from laboratory scale to commercial manufacturing. Cell cultures are highly sensitive to key factors during process transfer across scales, including geometric variability in bioreactors, shear stress from impeller and sparging activity, and nutrient gradients that occur due to increasing blend times. To improve the scale-up and scale-down of these processes, it is important to fully characterize bioreactors to better understand the differences that will occur within the culture environment, especially the hydrodynamic profiles that will vary in vessel designs across scales. In this study, a comprehensive hydrodynamic characterization of the Ambr® 250 mammalian single-use bioreactor was performed using time-accurate computational fluid dynamics simulations conducted with M-Star computational fluid dynamics software, which employs lattice-Boltzmann techniques to solve the Navier-Stokes transport equations at a mesoscopic scale. The single-phase and two-phase fluid properties within this small-scale vessel were analyzed in the context of agitation hydrodynamics and mass transfer (both within the bulk fluid and the free surface) to effectively characterize and understand the differences that scale-down models possess when compared to their large-scale counterparts. The model results validate the use of computational fluid dynamics as an in-silico tool to characterize bioreactor hydrodynamics and additionally identify important free-surface transfer mechanics that need to be considered during the qualification of a scale-down model in the development of mammalian bioprocesses.

Emulsion Formation by Homogenization: Current Understanding and Future Perspectives

Emulsion formation by homogenization is commonly used in food production and research to increase product stability and to design colloidal structures. High-energy methods such as high-pressure homogenizers and rotor–stator mixers are the two most common techniques. However, to what extent does the research community understand the emulsion formation taking place in these devices? This contribution attempts to answer this question through critically reviewing the scientific literature, starting with the hydrodynamics of homogenizers and continuing by reviewing drop breakup and coalescence. It is concluded that although research in this field has been ongoing for a century and has provided a substantial amount of empirical correlations and scaling laws, the fundamental understanding is still limited, especially in the case of emulsions with a high-volume fraction of the disperse phase, as seen in many food applications. These limitations in the current understanding are also used to provide future perspectives and suggest directions for further investigation.

CFD-PBM Coupled Simulation of Liquid-Liquid Dispersions in Spray Fluidized Bed Extractor: Comparison of Three Numerical Methods

A coupled numerical code of the Euler-Euler model and the population balance model (PBM) of the liquid-liquid dispersions in a spray fluidized bed extractor (SFBE) has been performed to investigate the hydrodynamic behavior. A classes method (CM) and two representatively numerical moment-based methods, namely, a quadrature method of moments (QMOM) and a direct quadrature method of moments (DQMOM), are used to solve the PBE for evaluating the effect of the numerical method. The purpose of this article is to compare the results achieved by three methods for solving population balance during liquid-liquid two-phase mixing in a SFBE. The predicted results reveal that the CM has the advantage of computing the droplet size distribution (DSD) directly, but it is computationally expensive if a large number of intervals are needed. The MOMs (QMOM and DQMOM) are preferable to coupling the PBE solution with CFD codes for liquid-liquid dispersions simulations due to their easy application, reasonable accuracy, and high reliability. Comparative results demonstrated the suitability of the DQMOM for modeling the spray fluidized bed extractor with simultaneous droplet breakage and aggregation. This work increases the understanding of the chemical engineering characteristics of multiphase systems and provides a theoretical basis for the quantitative design, scale-up, and optimization of multiphase devices.

Single Air Bubble Breakup Experiments in Stirred Water Tank

Single gas bubbles have been injected into an stirred liquid tank and the eventual breakup process of the bubbles was examined through high-speed imaging. Experimental observations of the breakup probability, breakup time, number of daughter bubbles and daughter bubble size distribution have been collected. The occurrence of non-equal-size breakup events dominated over equal-size breakup events. The frequency of binary and multiple breakup events was about equal. The largest uncertainty is associated with the determination of the breakup time because the bubbles take continuously altering deformed shapes already from the point of injection into the tank. The present experimental data do not support the standard model assumption regarding the number of daughter particles. The active breakup zone in the stirred tank was in the large velocity field close to the radial impeller. It is not evident whether the breakup events are due to time average shear or pressures and velocity fluctuations.