Study On CFD Simulation Of Agitated Extraction Column

Solvent extraction is an important separation technology,widely used in petrochemical industry,hydrometallurgy,atomic chemical industry and other important chemical fields.However,there is a serious amplification effect in industrial agitated extraction column,and the axial mixing is extremely serious.Therefore,in this paper,computational fluid dynamics(CFD)technology is used to study the complex flow of fluid in the column,to explore the causes of axial backmixing,to modify internal components to suppress axial backmixing,and to explore the behavior of droplets in the two-phase flow combined with population balance model(PBM),so as to provide a reference for the design of large-scale agitated extraction columns.Firstly,ANSYS Fluent 19.2 is used to simulate the single-phase flow residence time distribution(RTD)of agitated extraction column with different structural elements,such as channel area,annular gap position,opening mode,height of settling compartment,etc.Combined with RTD curves,backmixing model and steady-state flow field information,it is found that under the condition of a certain channel area,the agitated extraction column A6 with no pores in the stators and the inter-stage baffles has the smallest backmixing and simple structure.However,considering the liquid-liquid dispersion and the dead zone of the light phase,if pores are made in the inter-stage baffles and the stators,the agitated extraction column B4 with pores in the inter-stage baffles above the impellers and pores in the stators below the impellers not only has relatively light backmixing,but also improves the throughout,so its performance can be further studied.Secondly,based on the agitated extraction column B4,the results of Sauter average diameter d32 and droplet size distribution of dispersed phase simulated by different breakage and coalescence kernels and PBM solution methods were compared.It is found that the error of simulated d32 by Luo kernel and Luo kernel without coalescence is the largest,followed by the combination of Lehr breakage kernel and Luo coalescence kernel.The simulated d32 by CT kernel is in good agreement with the experimental data because of its adjustable parameters.For the discrete method(DM)and the quadrature method of moment(QMOM),the simulated d32 by the two methods are both consistent with the experimental data,but only DM could obtain reasonable droplet size distribution.Considering the accuracy and CPU time consuming,8 bins are suitable for DM and 2 nodes are suitable for QMOM.Finally,the d32 and hold-up of dispersed phase in the agitated extraction columnfor three different interfacial tension systems under different operating conditions were investigated by QMOM.It is found that the simulated values are in good agreement with the experimental results,indicating that CFD simulation could describe the behavior of dispersed phase accurately in the agitated extraction column.When the rotational speed is constant,the d32 and the hold-up of the dispersed phase increase with the increase of the feed flow rate of the dispersed phase.When the two phase feed flow rate is constant,the d32 decreases and the holdup of dispersed phase increases with the increase of the rotational speed.When the twophase feed flow rate and the rotational speed are both constant,from bottom to top,with the increase of axial height,the droplets of dispersed phase are broken by multi-stage shearing,the d32 decreases and the hold-up of dispersed phase increases.At the same flow rate,with the decrease of the interfacial tension of the system,the energy required for droplet breakage decreases,and the dispersed phase are more easily broken into small droplets.