CFD Simulation Of Immiscible Liquid-Liquid Dispersion In A Stirred Tank

The mean droplet size(d₃₂)and droplet size distribution（DSD）are two most important parameters for the description of immiscible liquid-liquid dispersion.They are closely related to the contact areas between the two phases and determine the rates of mass and heat transfer and chemical reactions.It is significant to define the factors of immiscible liquid-liquid and realize effective control of dispersion process for the theoretical research and practical application.In this paper,the CFD method was used to study the immiscible liquid-liquid dispersion in stirred tanks.The MRF technique was used to model the impeller rotation,and the k-ε model based on Eulerian-Eulerian approach was used to model the turbulent flow.The discrete method was implemented in population balance model to investigate the breakage and coalescence of droplets.For the water-oil system（heavy phase dispersed in light phase）,the effects of impeller speed（N）,dispersed phase volume fraction（a）and continuous phase viscosity（μc）on the d₃₂ 和 DSD were studied in detail.Results showed that increasing N and μc were both beneficial to the dispersion of two phase when a was constant.The droplet size distributions based on number were found to have bimodal distribution with the variation of N and μc,while the droplet size distributions based on volume were unimodal distribution all the time.With the increase of a,d₃₂ and S/V both increased.Besides,the plots of d₃₂ versus N and of d₃₂ versus a gave good log-linear correlations,and the relative coefficients were up to 0.999.For the oil-water system（light phase dispersed in heavy phase）,results showed the effects of N,a,μc on the oil-water dispersion were similar to the effects on the water-oil dispersion.The clearance（C）,deflection angle（q）and type of impeller could change the flow field and had great effect on the oil-water dispersion.The dispersion of suspending droplets was related to the intensity of flow field,especially near the free surface above the impeller.The effect of flow field below the impeller on the dispersion was poor.When clearance was 3T/4 and impeller was up-pumping at q = 60 o,the dispersion effect was best.The four blade flat blade impeller（FBI）was more effective than the Rushton turbine（RT）.