CFD simulation and PIV measurement of liquid–liquid two-phase flow in pump-mix mixer
Graphical abstract
Introduction
Pump-mix mixer-settler is one of the most widely used liquid–liquid extraction equipment in rare earth element separation with many advantages in efficiency, simplicity and flexibility. It is used for continuous operations, which involve continuous suction of phases from feed tanks or neighboring mixer-settlers into mixer. In a pump-mix mixer, liquid phases are sucked into mixer due to the suction developed by impeller rotation. After extraction of rare earth element in mixer, the mixture of two phase dispersion overflows into the settler for phase separation. Mixing of two phases is governed by drop size distribution produced by impeller. In general, it is necessary to produce drops having a narrow drop size distribution as well as to ensure the drops distribute uniformly throughout the mixer. The main ideas in the development of solvent extraction mixer-settler focused on achieving sufficient mixing without emulsion, clean phase separation, minimizing the loss of reagents or decreasing the surface area of settler, preventing swirling and vortexing of the liquid in mixer. Types of mixer geometry [1], [2] have been reported in literature but not standardized yet. Therefore, study of flow characteristics in the mixer is of crucial importance in design, optimization and scale-up of a mixer-settler.
As far as pumping effects are concerned, many researchers [3], [4], [5] studied the effects of impeller type, diameter, speed, clearance and flow rate on suction head in single phase system. For the static suction head, it is acceptable to simplify the complicated simulation by using a single fluid with simulated density and viscosity. However, the actual liquid–liquid mixing process includes drop breakup and coalescence. Holdup and drop size distribution are two major factors to judge the extent of mixing and the efficient utilization of a mixer, which cannot be obtained from single phase investigation. Therefore, studies on flow characteristics of liquid–liquid system are of great importance for analyzing two-phase flow in pump-mix mixer.
Studies on liquid–liquid two-phase flow are much more difficult due to unstable interface, small difference in density and considerable mass transfer resistance in separate phase compared to gas–liquid and solid–liquid system. Until now, studies have been concentrated on two main fields: drop size, which controls the rate of mass transfer on interface and time for dispersed and continuous phase to completely settled, and flow field, which affects the fluid flow and mixing time.
Population balanced model (PBM), developed by Ville et al. [6], is an important and effective method to study drop size distribution and its effect on mass transfer in a mixer [7], [8], [9]. Efforts have been also made to control the drop size. Zaheri et al. [10] and Khakpay et al. [11] studied the addition of surfactants, such as aniline and sodium dodecyl sulfate (SDS), to mean drop size distribution. Addition of surfactants controlled effectively the mean drop size but increase the difficulty of two-phase settling. Further studies are needed for mixing and mass transfer in a mixer and phase separation in a settler by combining the CFD simulation, PIV measurements together with PBM to get a deeper understanding of mixer-settler.
Laurenzi et al. [12] studied two-phase turbulent flow fields of a dilute liquid–liquid system by CFD simulation and PIV measurement. The mean velocity and turbulent characteristics of water and oil droplets flow were compared with those of the corresponding single-phase phase flow and the influence of dispersed phase on the continuous one was assessed. They found that the addition of dispersed phase increased the turbulence intensity of continuous phase in a mixer. Several models have been reported to simulate the flow field and holdup in liquid–liquid system [13], [14], [15] under the assumptions of steady flow. The standard k-ε model is the most common model for the turbulence model [16].
Tabib et al. [17] applied large eddy simulation (LES) model, namely, one-equation sub-grid scale turbulent kinetic energy LES model to study the design features, turbulence and flow characteristics in a multi-phase liquid–liquid pump-mixer. The LES model they used could capture the variations in instantaneous flow structures as well as turbulence parameters with variations in geometric configurations of the multi-phase pump-mixer.
The present work aims to study flow characteristics of liquid–liquid two-phase flow in pump-mix mixer via a RANS based CFD model and experimental PIV measurements for a pump-mixer. Single and dual-impeller configurations were studied. Effects of impeller speed, flow ratio and drop size on flow field and holdup were discussed. The investigations included simulations and experiments of liquid–liquid two-phase flow with water as aqueous phase and kerosene as organic phase.
Section snippets
Experiment
Flow field measurements via PIV in liquid–liquid system were carried out to investigate the flow structures and validate the CFD simulation. The experimental setup, shown in Fig. 1, consisted of two peristaltic pumps (Longer Precision Pump Co., WT600-2 J), two feed tanks, a motor (speed controllable) and a mixer-settler. Deionized water (density 998.3 kg/m3, viscosity 0.001 Pa s) and kerosene (density 780 kg/m3, viscosity 0.0024 Pa s) from feed tanks were pumped into a buffer chamber underneath
Simulation strategy
CFD simulations have been carried out to get deep understanding of flow features, analyze the detailed information and predict more results under conditions that were unable to achieve in experiments. The geometry of the mixer was generated by GAMBIT 2.4. The mixer was divided into two parts: inner volume and outside volume. The inner volume was a cylinder containing the rotating impeller, and the outside one is the remaining volume of the mixer including the inlets, outlet, shaft and the
Validation of simulation results
Comparison of velocity field between experiments and simulations is shown in Fig. 7, where the same reference vector, 0.282 m/s, is used. From the experimental results, fluid discharged by impeller flows to the bottom of mixer and then rises up. The whole flow forms an axial field, which was caused by low location of BSTRTB impeller and has been reported in our previous study [18]. It is obvious that the predicted flow field agrees qualitatively with that in experiments. Further analysis in
Conclusion
To generate high suction head, single impeller was always located near the bottom of pump-mix mixer and caused remarkable heterogeneity of flow field and holdup. Increasing impeller speed and flow ratio decreases the relative fluid velocity around impeller significantly. Less homogeneous holdup distribution occurs with higher impeller speed, larger flow ratio and greater drop size.
Dual-impeller configuration, lower one for suction and the upper for mixing, was then proposed for pump-mix mixer.
Acknowledgment
This research was carried out under the National Key Basic Research Program of China (No. 2012CBA01203) and the Specialized Research Fund for Doctoral Programme of Higher Education of MOE of China (No. 20130002110018) in the State Key Laboratory of Chemical Engineering of Tsinghua University, Beijing, China. The authors gratefully acknowledge these grants.
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