| As an important means to promote solid-liquid mixing and dispersion,stirring is widely used in process industry.Previous scholars’evaluation indexes for solid-liquid mixing research are mostly just suspension speed and power,but in this state,the solid distribution in the tank is always bad,and even there will be a clear liquid layer at the bottom of the tank,which will directly affect efficiency of the mass transfer and reaction of the system.Therefore,it is necessary to study the solid concentration distribution of solid-liquid mixing.In this paper,experimental and simulation methods are used to study the drawdown and dispersion characteristics of floating particles driven by a concentric coaxial mixer.In the experiment,the coaxial mixer was combined with the drawdown and dispersion of floating particles.The critical uniform suspension was adopted for the first time to investigate the effects of the operation mode of the inner and outer impellers,the impeller type,the impeller diameter,the immersion depth,the solid holdup,the solid particle size and the liquid viscosity on the drawdown and dispersion of floating particles,and the mechanism of drop-down dispersion of light particles was revealed.Firstly,the operation modes of the inner and outer impellers(co-rotation,counter-rotation,single inner impellers)and the pumping modes of the inner impellers(up-pumping and down-pumping)were compared,and the important role of the outer impellers in the process of achieving uniform solid dispersion was analyzed.Using the optimized operation mode,the performance of three kinds of agitators(axial flow,radial flow and mixed flow)to achieve uniform suspension of light particles under different impeller diameters and liquid viscosity was studied.The dominant impeller types with small diameter and large diameter,lower viscosity and higher viscosity were obtained respectively.The results show that:when the impeller diameter is small,the critical uniform suspension speed and power of the radial flow impeller are the lowest,and the performance is the best;when the impeller diameter increases,the power consumption of the radial flow impeller becomes the maximum of the three,while the mixed flow impeller shows the advantages.However,when the viscosity of the system increases,the radial flow impeller becomes the most efficient impeller again,indicating that it is suitable for the system with higher viscosity.The experimental results show that the outer impeller plays an important role in the process of achieving the uniform dispersion of floating particles.Therefore,the influence of the structure of the outer impeller on the dispersion of the floating particles is studied by numerical simulation.Firstly,the numerical simulation model and method are confirmed to be suitable for the dispersing process of viscous particles driven by coaxial mixers.The solid concentration field and liquid phase velocity field under different outer impeller diameter and width were simulated.The trend and reason of the distribution of solid phase in different areas in the tank were analyzed,and the optimal diameter of outer impeller was obtained(D_o/T=0.69).The outer impeller was modified by adding beams at different height positions of the outer impeller to enhance the liquid flow around the mixing shaft and liquid level,and improve the phenomenon of particle accumulation.The drawdown and dispersion performance of the modified outer impeller to floating particles is much better than that of the original outer impeller.Finally,the drawdown and dispersion processes of floating particles under single-axial and coaxial mixers are simulated,and the reasons why coaxial mixer is better than single-axial mixer are analyzed. |
