Experimental study on gas–liquid–liquid macro-mixing in a stirred tank (original) (raw)
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Experimental Study on Liquid–Liquid Macromixing in a Stirred Tank
Industrial & Engineering Chemistry Research, 2011
In this paper, the experimental data on the mixing time and power consumption of two immiscible liquids in a mechanically agitated baffled tank are presented. The electric conductivity method was taken for the measurement of the mixing time and the shaft-torque method for the power consumption measurement. Tap water was used as the continuous phase and kerosene the dispersed phase. The effects of the agitation speed, type of impeller, clearance of the impeller off the tank bottom, volume fraction of the dispersed phase, physical properties of the liquids, and probe position on the macromixing of the liquidÀliquid system were studied. The phenomena of macromixing are largely similar to those of single-liquid and gasÀliquid stirred tanks. The experiment indicates that the flow field and turbulence can be dampened at high volume fraction of the dispersed phase while enhanced at low percentage. The mixing time becomes longer with increasing viscosity of the dispersed phase. The results show that the pitched blade turbine downflow is more efficient for macromixing than the others tested in this work.
Liquid-Liquid Mixing in Stirred Vessels: A Review
Chemical Engineering Communications, 2013
Liquid-liquid mixing is a key process in industries that is commonly accomplished in mechanical agitation systems. Liquid-liquid mixing performance in a stirred tank can be evaluated by various parameters, namely minimum agitation speed, mixing time, circulation time, power consumption, drop size distribution, breakup and coalescence, interfacial area, and phase inversion. The importance of these liquid-liquid mixing parameters, the measurement method, and the results are discussed briefly. Input parameters such as impeller type, power number, flow pattern, number of impellers, and dispersed phase volume fraction, in addition to physical properties of phases such as viscosity and density, are reviewed. Scale-up aspects are also included. © 2013 Copyright Taylor and Francis Group, LLC. http://www.tandfonline.com/doi/pdf/10.1080/00986445.2012.717313
Numerical Simulation of Turbulent Flow and Mixing in Gas–Liquid–Liquid Stirred Tanks
Industrial & Engineering Chemistry Research, 2017
The turbulent flows and macro-mixing processes in gas-liquidliquid flat-bottomed cylindrical stirred vessels agitated by a Rushton turbine have been numerically simulated based on the Eulerian multi-fluid approach using the RANS technique. Both the isotropic k-ε model and anisotropic Reynolds stress model are used. The numerical models are validated by means of comparing simulated flow field of agitated immiscible liquid-liquid dispersions to the corresponding experimental data from literature. The predicted time traces of normalized concentration and values of mixing time in gas-liquid-liquid stirred tanks are compared to the experimentally measured ones as well. Both the k-ε model and the Reynolds stress model correspond reasonably well to the experimental data in both turbulent liquid-liquid and gas-liquid-liquid stirred tanks, and the anisotropic Reynolds stress model produces better results in terms of flow field, homogenization curve and mixing time than the k-ε model. While the better accuracy of the Reynolds stress model comes at the cost of more computational time.
2003
The study relates to the liquid phase mixing in dual-impeller agitated contactors of internal diameter 0.32 m in presence of floating solid particles. High-density polyethylene particles and tap water were used as the solid and liquid phase in all experiments. Mechanically agitated tank was provided with two down-pumping pitched blade turbines (PTD). The mixing time has been measured by conductivity measurements using a pulse technique, whereas the minimum impeller speed for the complete drawdown of floating solid particles, N JS , was determined using Joosten visual method. The effects of a number of variables such as floating solids concentrations, the impeller diameter, the off-bottom impeller clearance as well as the spacing between impellers on the critical impeller speed, mixing time and power consumption values were studied in detail. Previous investigations on this subject have been limited to single-impeller configuration only. The dual-impeller reactor behaviour has been found to be very complex and their design must be considered with a great care. Obtained results show that the presence of floating solids in the liquid significantly affects the mixing time of the liquid phase. The analysed parameters are a strong function of the flow patterns structure created by dual-impeller configuration applied. Using experimental data dimensionless correlation is proposed to predict mixing time beyond complete suspension regime.
Gas–liquid mixing in dual agitated vessels in the heterogeneous regime
Chemical Engineering Research and Design, 2018
Gas-liquid multi-phase processes are widely used for reactions such as oxidation and hydrogenation. There is a trend for such processes to increase the productivity of the reactions, one method of which is to increase the gas flow rate into the vessel. This means that it is important to understand how these reactors perform as high gas flow rates occurs well into the heterogeneous regime. This paper investigates the mixing performance for the dual axial radial agitated vessel of 0.61 m in diameter. 6 blade disk turbine (Rushton turbine) below a 6 Mixed flow Up-pumping and down-pumping have been studied at very high superficial gas velocities to understand the flow regimes operating at industrial conditions. Electrical Resistance Tomography have been used to produce the 3D images using Matlab, along with analysing the mixing parameters such as Power characteristics, gas holdup and dynamic gas disengagement. Minimal difference between the two configurations have been reported in terms of gas holdup , however with the choice of upward and downward pumping impeller power characteristics show significant difference at very high gas flow rates. Also at these high superficial gas velocities, this report introduces a 3rd bubble class, as seen in dynamic gas disengagement experiments, which corresponds to very large slugs of gas. Highlights At high gas flow rates, a maximum holdup is observed Power for 6MFD/6MFU problematic at high gas flow rates The gas liquid holdup results show minimal difference between the two configurations Large slugs observed as a third bubble classes at very high gas flow rates Keywords Gas-liquid mixing; Gas HoldUp ; Gassed Power; Heterogeneous regime; Axial-radial dual system; Electrical Resistance Tomography V Volume [m 3 ] Volume of dispersion [m 3 ] Volume of liquid [m 3 ] Volume of gas [m 3 ] Superficial gas velocity [m s-1 Greek symbols Gas Holdup [%] Specific total energy input per liquid mass [W kg-1 ] Σ Conductivity (normalised) [mS cm-1 ] Density of fluid [kg m 3 ] Dimensionless numbers
Effect of impeller design on the flow pattern and mixing in stirred tanks
2006
The flow pattern and power number in a vessel depend on the impeller blade angle, number of blades, blade width, blade twist, blade thickness, pumping direction and interaction of flow with the vessel wall. Measurements of the power consumption and flow pattern have been carried out in a stirred vessel of 0.5 m diameter for the range of impellers to study the effect of blade shape on the flow pattern. The comparison of the flow pattern (average velocity, turbulent kinetic energy, maximum energy dissipation rate, average shear rate and turbulent normal stress) has been presented on the basis of equal power consumption to characterize the flow generated by different impeller geometries. Comparisons of LDA measurements and CFD predictions have been presented. The good comparison indicates the validity of the CFD model.
Energies
The mixing process is a widespread phenomenon, which plays an essential role among a large number of industrial processes. The effectiveness of mixing depends on the state of mixed phases, temperature, viscosity and density of liquids, mutual solubility of mixed fluids, type of stirrer, and, what is the most critical property, the shape of the impeller. In the present research, the objective was to investigate the Newtonian fluids flow motion as well as all essential parameters for the mechanically agitated vessel with a new impeller type. The velocity field, the power number, and the pumping capacity values were determined using computer simulation and experimental measurements. The basis for the assessment of the intensity degree and the efficiency of mixing had to do with the analysis of the distribution of velocity vectors and the power number. An experimental and numerical study was carried out for various stirred process parameters and for fluids whose viscosity ranged from lo...
A Study of Mixing Structure in Stirred Tanks Equipped with Multiple Four-Blade Rushton Impellers
Archive of Mechanical Engineering, 2012
The effect of multiple Rushton impellers configurations on hydrodynamics and mixing performance in a stirred tank has been investigated. Three configurations defined by one, two and three Rushton impellers are compared. Results issued from our computational fluid dynamics (CFD) code are presented here concerning fields of velocity components and viscous dissipation rate. These results confirm that the multi-impellers systems are necessary to decrease the weaken zones in each stirred tanks. The experimental results developed in this work are compared with our numerical results. The good agreement validates the numerical method.
Indian Journal of Chemical Technology (IJCT), 2017
The optimum positions for the additive injection and probe location in a dual-Rushton stirred tank have been investigated using Response Surface Methodology. Therefore, by keeping these two parameters (probe location and additive injection point) fixed during consequent experiments, the single Rushton impeller has been compared with a dual Rushton impeller stirred tank. The results have shown that the mixing time for single impeller systems, is lower than that for double impeller systems, so to obtain a lower mixing time in multi impeller systems, it is necessary to obtain the relation between the height of the liquid and the number of impellers. In the next part, the obtained mixing time is compared against simulation results and published data based on experimental works for a Rushton impeller in different configurations of a vessel. Also in this work the effects of superficial gas velocity and impeller rotational speed for both systems have been investigated.