A Comparative CFD Study on Gas-Liquid Dispersion in A Stirred Tank with Low and High Gas Loadings

2018 ◽  
Vol 16 (8) ◽  
Author(s):  
Li Liangchao ◽  
Chen Ning ◽  
Xiang Kefeng ◽  
Xiang Beiping
Keyword(s):  
Flow Pattern ◽  
Bubble Size ◽  
Power Input ◽  
Gas Holdup ◽  
Stirred Tank ◽  
Balance Model ◽  
Wall Region ◽  
Gas Dispersion ◽  
Loop Region ◽  
Liquid Dispersion

Abstract The computational fluid dynamics (CFD) combined with a population balance model (PBM) was applied to simulate gas-liquid dispersion in a stirred tank with low and high gas loadings. The model predictions were validated by using the data in the literature. The simulation results show that the flow patterns and gas dispersion characteristics are very different in the stirred tank for low and high gas loadings. A typical two-loop flow pattern forms as that in single-phase stirred tank for low gas loadings, while a triple-loop flow pattern, with two recirculation loops above and one below the impeller is found in the tank for high gas loadings. Shaft power input of impeller agitation plays a major role for gas dispersion with low gas loadings. For high gas loadings, the potential energy due to gas sparging has significant effect on gas dispersion and can not be neglected. Compared to low gas loading, high gas loading causes average gas holdup increased in the stirred tank, while relative local gas holdup in the lower circulation-loop region and near-wall region reduced. The ability of impeller agitation for gas dispersion reduces with high gas loadings, and mean bubble size becomes larger and the volume-averaged bubble size distribution is wider.

CFD Simulation of Gas Dispersion in a Stirred Tank of Dual Rushton Turbines

2017 ◽  
Vol 15 (4) ◽  
Author(s):  
Xinju Li ◽  
Xiaoping Guan ◽  
Rongtao Zhou ◽  
Ning Yang ◽  
Mingyan Liu
Keyword(s):  
Flow Field ◽  
Size Distribution ◽  
Liquid Flow ◽  
Bubble Size ◽  
Great Influence ◽  
Gas Holdup ◽  
Bubble Size Distribution ◽  
Stirred Tank ◽  
Treatment Methods ◽  
Gas Dispersion

Abstract3D Eulerian-Eulerian model was applied to simulate the gas-liquid two-phase flow in a stirred tank of dual Rushton turbines using computational fluid dynamics (CFD). The effects of two different bubble treatment methods (constant bubble sizevs. population balance model, PBM) and two different coalescence models (Luo modelvs. Zaichik model) on the prediction of liquid flow field, local gas holdup or bubble size distribution were studied. The results indicate that there is less difference between the predictions of liquid flow field and gas holdup using the above models, and the use of PBM did not show any advantage over the constant bubble size model under lower gas holdup. However, bubble treatment methods have great influence on the local gas holdup under larger gas flow rate. All the models could reasonably predict the gas holdup distribution in the tank operated at a low aeration rate except the region far from the shaft. Different coalescence models have great influence on the prediction of bubble size distribution (BSD). Both the Luo model and Zaichik model could qualitatively estimate the BSD, showing the turning points near the impellers along the height, but the quantitative agreement with experiments is not achieved. The former over-predicts the BSD and the latter under-predicts, showing that the existing PBM models need to be further developed to incorporate more physics.

Imaging a Gas-Sparged Stirred-Tank Reactor With X-Ray CT

2007 ◽  
Author(s):  
Jason J. Ford ◽  
Theodore J. Heindel ◽  
Terrence C. Jensen
Keyword(s):  
Gas Flow ◽  
Gas Holdup ◽  
Operating Conditions ◽  
Stirred Tank ◽  
Gas Dispersion ◽  
Stirred Tank Reactor ◽  
X Ray ◽  
Tank Reactor ◽  
X Ray Computed ◽  
Ct Slices

X-ray computed tomography (CT) is used to explore the differences in gas dispersion in a gas-sparged stirred-tank reactor (STR) for different operating conditions. X-ray CT imaging is completed for various impeller speeds and gas flow rates in a 0.21 m ID STR made out of acrylic and equipped with a nylon Rushton-type impeller. From the CT slices, major differences in local time-averaged gas holdup can be seen, depending on the operating condition. Completely dispersed conditions have a relatively uniform holdup profile while flooded conditions have an increase in gas holdup towards the center of the tank. The high resolution of the X-ray system allows for visualizing time-averaged gas flow details such as low gas holdup regions directly above the impeller region under certain operating conditions and recirculation regions behind the baffles.

CFD modeling of gas dispersion and bubble size in a double turbine stirred tank

2006 ◽  
Vol 61 (10) ◽  
pp. 3313-3322 ◽  
Author(s):  
F. Kerdouss ◽  
A. Bannari ◽  
P. Proulx
Keyword(s):  
Bubble Size ◽  
Stirred Tank ◽  
Cfd Modeling ◽  
Gas Dispersion

The Influence of Impeller Combination on the Gas-Liquid Dispersion Performance of a Coaxial Mixer in Viscous Fluids

2017 ◽  
Vol 15 (6) ◽  
Author(s):  
Baoqing Liu ◽  
Fangyi Fan ◽  
Xiaoge Chen ◽  
Jinliang Liu ◽  
Zhijiang Jin
Keyword(s):  
Flow Field ◽  
Bubble Size ◽  
Gas Holdup ◽  
Experimental Results ◽  
Local Bubble ◽  
Rushton Turbine ◽  
Liquid Dispersion ◽  
Pitched Blade Turbine ◽  
Coaxial Mixers ◽  
Rushton Impeller

Abstract As a kind of mixer with wide adaptability, coaxial mixer has a wide-application in the process industry. With the help of experiment and numerical simulation, the gas-liquid dispersion performance of four impeller combinations in viscous system was studied in terms of the global gas holdup, local gas holdup, local bubble size and liquid phase flow field. Wall-scraping anchor is the common outer impeller of the four impeller combinations, and the four inner impellers are Rushton turbine, six-straight-blade turbine (SBT), 45° six-pitched-blade turbine pumping downwards (PBTD) and 45° six-pitched-blade turbine pumping upwards (PBTU). The experimental results indicate that the global gas holdup of the Rushton impeller combination is the highest among the four impeller combinations and that of PBTU is the lowest. The local gas holdup of Rushton and PBTU combinations are all high, but the gas distribution with the Rushton combinations is more uniform. Besides, the local bubble size of the four impeller combinations has the same tendency with the local gas holdup. Under the same conditions, the flow field and distribution of gas holdup of the four impeller combinations in stirred tank were simulated numerically. Compared with the experimental results, the simulation results show a good agreement on the value and distribution of local gas holdup. Among the four coaxial mixers, the Rushton impeller combination is more suitable for gas-liquid dispersion.

Numerical Simulation of Gas-Liquid Dispersion in A Stirred Tank Agitated by Punched Rigid-Flexible Impeller

2019 ◽  
Vol 17 (4) ◽  
Author(s):  
Deyin Gu ◽  
Zuohua Liu ◽  
Changyuan Tao ◽  
Jun Li ◽  
Yundong Wang
Keyword(s):  
Stirred Tank ◽  
Balance Model ◽  
Shear Strain Rate ◽  
Impeller Blade ◽  
Aperture Diameter ◽  
Dispersion Process ◽  
Aperture Ratio ◽  
Liquid Dispersion ◽  
Superficial Gas Velocity ◽  
Impeller Type

Abstract The hydrodynamics of gas-liquid dispersion process in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers were investigated using a combined computational fluid dynamics (CFD) and population balance model (PBM) approach. A classical Eulerian-Eulerian approach coupled with standard k-e turbulence model was employed to simulate gas-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, flexible connection piece length, aperture size/ratio, impeller speed and superficial gas velocity on the local gas holdup and bubble size distribution for the gas-liquid dispersion process were investigated. Results showed that a long flexible connection piece length was conductive to the gas-liquid dispersion process. The optimum aperture ratio and aperture diameter were 12 % and 8 mm, respectively, for the gas-liquid dispersion process. Punched rigid-flexible impeller could reduce the differential pressure ahead and behind the impeller blade, decrease the size of low-pressure zone, and improve the water shear strain rate compared with rigid impeller and rigid-flexible impeller at the same Pg,v. It was found that punched rigid-flexible impeller was more efficient in terms of gas dispersibility than rigid impeller and rigid-flexible impeller.

scholarly journals Three-Way Coupling Simulation of a Gas-Liquid Stirred Tank using a Multi-Compartment Population Balance Model

2016 ◽  
Vol 11 (3) ◽  
pp. 205-216 ◽  
Author(s):  
Jolius Gimbun ◽  
Shi Yan Liew ◽  
Zoltan K. Nagy ◽  
Chris D. Rielly
Keyword(s):  
Bubble Size ◽  
Population Balance ◽  
Compartment Model ◽  
Stirred Tank ◽  
Balance Model ◽  
Population Balance Model ◽  
Stirred Tanks ◽  
Two Phase ◽  
Bubble Liquid ◽  
Local Bubble

Abstract Modelling of gas-liquid stirred tanks is very challenging due to the presence of strong bubble-liquid interactions. Depending upon the needs and desired accuracy, the simulation may be performed by considering one-way, two-way, three-way or four-way coupling between the primary and secondary phase. Accuracy of the prediction on the two-phase flow generally increases as the details of phase interactions increase but at the expense of higher computational cost. This study deals with two-way and three-way coupling of gas-liquid flow in stirred tanks which were then compared with results via four-way coupling. Population balance model (PBM) based on quadrature method of moments (QMOM) was implemented in a multi-compartment model of an aerated stirred tank to predict local bubble size. The multi-compartment model is regarded as three-way coupling because the local turbulent dissipation rates and flow rates were obtained from a two-way computational fluid dynamics (CFD) simulation. The predicted two-phase flows and local bubble size showed good agreement with experimental data.

scholarly journals Gas phase hydrodynamics in a surface-aerated tank with a long-short blades agitator

2021 ◽  
Author(s):  
Yongjun Wu ◽  
Jian Wang ◽  
Pan You ◽  
Peicheng Luo
Keyword(s):  
Gas Phase ◽  
Bubble Size ◽  
Driving Force ◽  
Gas Holdup ◽  
Bubble Size Distribution ◽  
Balance Model ◽  
Large Bubble ◽  
Transfer Coefficients ◽  
Liquid Mass Transfer ◽  
Gas Suction

This work aims to study the gas phase hydrodynamics in a stirred tank with a surface-aerated long-short blades agitator by the Eulerian‒Eulerian approach coupled with population balance model. Predicted local gas holdup and bubble size distribution agree well with those measured by a conductivity probe technique. The predictions demonstrate that the pressure depression in the center is the main driving force for gas suction and the downward flow carries the bubbles down to redistribute in the whole tank. The gas phase has higher gas holdup with large bubble size in the upper part and lower gas holdup but with small bubble size in the lower part of the tank. The predicted gas-liquid mass transfer coefficients agree well with our previous experimental results and just depends on the power consumption per unit volume when the aspect ratio of the liquid height to the tank diameter varies from 1.1 to 2.0.

Design of Impeller Blades for Intensification of Gas-Liquid Dispersion Process in a Stirred Tank

2018 ◽  
Vol 16 (12) ◽  
Author(s):  
Deyin Gu ◽  
Zuohua Liu ◽  
Changyuan Tao ◽  
Jun Li ◽  
Yundong Wang
Keyword(s):  
Liquid System ◽  
Gas Holdup ◽  
Stirred Tank ◽  
Chaotic Mixing ◽  
Largest Lyapunov Exponent ◽  
Power Demand ◽  
Aperture Diameter ◽  
Dispersion Process ◽  
Liquid Dispersion ◽  
Free Area

Abstract Gas-liquid dispersion characteristics were experimentally investigated by measuring largest Lyapunov exponent (LLE), relative power demand (RPD), and local gas holdup in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers. Results showed that punched rigid-flexible impeller could enhance the value of LLE, namely, the chaotic mixing degree of gas-liquid system compared with rigid impeller and rigid-flexible impeller. RPD for punched rigid-flexible impeller was higher than that for rigid impeller and rigid-flexible impeller. The local gas holdup of punched rigid-flexible impeller system was higher than those of rigid impeller system and rigid-flexible impeller system at the same Pg,m. In addition, a long flexible connection piece length could improve the chaotic mixing degree, RPD, and local gas holdup. The aperture diameter of 8 mm and free area ratio of 12 % of punched rigid-flexible impeller were particularly suitable for the gas-liquid dispersion process in this work.

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