Experimental Study On Interphase Mass Transfer Process Of Tmmiscible Liquid-liquid System In A Stirred Tank

Stirred tanks are widely used in industrial processes,such as chemical,metallurgical,environmental and biomedical industries,because of its advantages of flexible operating conditions and efficient mass transfer/heat transfer.The interphase mass transfer of liquid-liquid systems in a stirred tank is one of the most common unit operations in industrial processes.In recent years,there have been many researches on the dispersion process in immiscible liquid-liquid systems in a stirred tank,but they mainly focused on the flow and mixing characteristics,and did not consider the interphase dynamic mass transfer process.In addition,from the perspective of visualized measurement,the tracer currently used is only dissolved in the dispersed phase or the continuous phase,and cannot comprehensively reflect the interaction between the dispersed and the continuous phasees on the interphase mass transfer process.Therefore,the visualization of multi-process coupling behaviors of the flow,macromixing and interphase mass transfer has not been found in the stirred tankIn view of this,NaI solution and 1-octanol are used to construct an immiscible liquid-liquid system.Rhodamine B is chosen as the fluorescence tracer.The planar laser-induced fluorescence(PLIF)method combined with the refractive index matching is used to visually measure the interphase mass transfer process in the immiscible liquid-liquid system in a stirred tank.By measuring the interphase mass transfer equilibrium time and calculating the mass transfer coefficient,it is convenient to analyze the influence of different operating conditions on the interphase mass transfer of the immiscible liquid-liquid systemFirstly,the effects of agitation speed,dispersed phase volume fraction,impeller types(RDT,PBTU,PBTD and VBT),impeller clearance and eccentric stirring on the mass transfer equilibrium time are investigated.The experimental results indicate that the interphase mass transfer efficiency increases and the mass transfer equilibrium time decreases with the increase of agitation speed.The mass transfer efficiency of RDT impeller is higher than that of other impeller types,and the dispersion and mass transfer efficiency of VBT impeller are the worst of them.For the impeller clearance,the double circulation loop of the RDT impeller is gradually recovered to enhance the shear intensity when the impeller clearance increases from T/10 to 2T/5,so the interphase mass transfer efficiency is enhanced.Furthermore,the interphase mass transfer process is dampened and the equilibrium time increases continuously when the volume fraction of NaI solution increases from 2%to 50%.When the impeller eccentricity increases,the mass transfer equilibrium time increases correspondingly and the trend of its change in the immiscible liquid-liquid system is contrary to that of the mixing time in the single phase.Then,the PLIF method is applied to measure the effect of the solute concentration and agitation speed on the interphase mass transfer coefficient in Lewis cell by using the same immiscible liquid-liquid system.It is found that the increase of agitation speed will enhance the shear rate and the amplitude of wave motion at the interphase,and the mass transfer coefficient will increase obviously.On the other hand,there is a positive correlation between the interphase mass transfer coefficient and the solute concentration.The mass transfer coefficient is more significantly affected by the concentration when the agitation speed is lower(50 rpm)and the solute concentration is in the higher range(60 μg/L-120μg/L).However,in the case of the doubled agitation speed(100 rpm)and the lower concentration(30 μg/L-60μg/L),the mass transfer coefficient changes more obviously with the increase of the solute concentration.Finally,an empirical correlation formula of mass transfer coefficient of this immiscible liquid-liquid system is obtained by fitting the experimental data,and the average relative deviation between the calculated and the experimental values of the mass transfer coefficient is as low as 2.70%.