Experimental And Numerical Simulation Of The Dispersion Of Floating And Sinking Particles In A Stirred Tank With Viscous Fluid

The dispersion of particles in viscous fluid is a typical solid-liquid multi-phase operation.This operation requires the particles to be uniformly dispersed and has certain requirement for the efficiency achieving uniform dispersion.In some processes,the floating particles and sinking particles coexist in the viscous fluid.Therefore,the status of the floating and sinking particles needs consideration simultaneously.In this work,an experimental study on the drawing down of floating particles under different operating conditions was carried out.Based on the experimental research,the numerical simulation method was used to study the solid dispersion of the floating particles in the liquid,and the particle dispersion in a three-phase system with both floating and sinking particles.The optimal impeller type and the correlation for predicting the just drawdown(just off-bottom)suspension speed are proposed.The experiment was carried out under the maltose solution with a liquid viscosity of 0.042 Pa·s with the SBS particle.The critical suspension characteristics of the impellers WH,CBY and PBT at different immersed depths and the relationship between rotation speed and cloud height were studied.The results show that the just drawdown power of WHd is the lowest when the immersed depth is relatively small,and that of CBY is the lowest when the immersed depth is relatively deep.The cloud height of particles decreases as the immersed depth increases.The closer the dimensionless cloud height is to 1,the greater the power required to increase the cloud height per unit height.The velocity distribution,turbulent kinetic energy distribution,solid volume fraction distribution,as well as the mixing time to reach a stable dispersion state,the power consumption and the uniformity of particle dispersion are analyzed by using numerical simulation.It was found that the drawdown of the floating particles mainly depended on the circulation effect near the liquid surface.The down-pumping impeller pulls the particles down through the main circulation and the up-pumping impeller draws down the particles by generating the secondary circulation on the liquid surface.If the main circulation velocity is higher,the particle drawdown is quicker.When using the up-pumping impeller,the blade can generate large turbulent kinetic energy on the liquid surface,which is beneficial for the dispersion of the particle cluster in the liquid.Floating and sinking particles have different dispersion characteristics in viscous fluid.The mixing time of the floating particles to achieve stable dispersion increases with the increase of the immersed depth of the impeller,and the mixing time of the sinking particles is to the opposite.The uniformity of the floating particles decreases with the increase of the immersed depth of the impeller,while the sinking particles are hardly affected by the immersed depth.By using CFD simulation,it is found that when using double CBY impeller in a viscous system,a short baffle at the liquid level/tank bottom is beneficial to suspension of both floating and sinking particles.