Numerical Simulation Of Liquid-Liquid Two Phase Flow In Mixer-settlers

Heterogeneous dispersion and mixing is essential to unit operation such as liquid-liquid extraction.Multistage mixer-settlers are adopted for liquid-liquid extraction.In multistage mixer-settlers,the extraction efficiency is dependent on efficient dispersion and coalescence between continuous and dispersed liquid phase.In this paper,in order to compare the mixing efficiency of the large delta impeller and the shrouded disc turbine,computational fluid dynamics simulation coupled with population balance model(PBM)has been applied to simulate the flow field and the droplet size distribution of dispersed phase in the mixing chamber.Realizable ?-? turbulence model is adopted and multiple reference frame method is used for the modeling of interaction between static tank and rotating impeller.Eulerian model is used as multi-phase model.In order to verify the multiphase flow model and turbulence model,experiment was carried out to investigate the droplet size distribution of dispersed phase in liquid-liquid two phase flow at low power level.By comparing the Luo breakage model and the Lehr breakup model in the population balance model,it was found that the oil droplet size distribution(DSD)presents a lognormal distribution,and the Luo breakage model is more accurate for the prediction of oil droplet breakage.The flow field and dispersed phase distribution in mixing chamber with different types of impeller were investigated.It was found that although the shrouded disc turbine exhibits greater suction capacity than the large delta impeller,the flow field created by the large delta impeller is even more uniform.The circulation generated by the shrouded turbine is single-looped at the lower section of mixing chamber,and the radial disturbance is insufficient.The axial and radial flow field generated by the large delta impeller is strong and evenly distributed with smaller portion of dead zone.At low power level,accumulation of oil droplets can be found in the upper part of mixing chamber for both of the two types of impellers.However,with the increase of the power level,this situation can be obviously improved with the large delta impeller.Based on the model validated by experiment,the influence of impeller on the dispersion and the droplet size distribution of dispersed phase in the mixing chamber was investigated by simulation.At the same power level,the large delta impeller generates weaker shear force,which results in more uniform size distribution of the droplets.The shrouded disc turbine can form a larger portion of fine oil droplets.The influence of impeller diameter on droplet size distribution is not as obvious as that of power level and total volume fraction of oil phase in mixing chamber.However,larger impeller can form smaller droplets and narrower droplet size distribution,which is conducive to improving mass transfer efficiency and producing better overall mixing.The coalescence of oil droplets in the settling chamber was simulated on the basis of the above work.The impact of oil droplets size and volume of settler on coalescence and layering were investigated.The oil-water dispersion band in the settler gradually gets widened and the interface between the light phase and heavy phase becomes less obvious with the decrease of the oil droplets size in the feed.Furthermore,the entrainment rate of oil increases at water outlet.The residence time has great influence on clarification.When the minimum diameter of the oil droplet is 6?m,coalescence of very fine droplets is rather slow and the residence time in the settling chamber should be longer than 5.5min.