Characteristics,Model And Mechanism Of Dense Solid Suspension Driven By Coaxial Mixers

The solid-liquid mixing is one of common unit operations in process industry,but the characteristics of viscous and high solids loading systems increase the difficulty of solid suspension,which bring challenges to traditional single-shaft mixers.For the first time,the coaxial mixers with wide adaptability were applied into the dense solid suspension in viscous systems,and the characteristics,model and mechanism were investigated.A new method based on the RGB brightness analysis was proposed to measure the cloud height,and the characteristics of dense solid suspension driven by coaxial mixers were studied.Additionally,the optimal parameters of structure and operation were obtained,and the effect degree of different factors on the suspension performance was analyzed with the help of grey correlation method.The results showed that at the higher outer impeller speed and co-rotation mode,the coaxial mixer had the superior performance.The PropellerU + Anchor was the most energy efficient,whose just-suspended power was 60% ? 83% lower than that of the single-shaft mixer,but it also showed the disadvantage of relatively low cloud height.Correspondingly,the use of higher inner impeller off-bottom clearance and combined inner impeller could effectively overcome this shortcoming.The higher power consumption and the worse suspension quality were distinctive features of dense solid-liquid mixing,and when the solid volume fraction was in the range of 0.18 to 0.2,the PropellerU + Anchor had the lowest just-suspended power per unit mass.Both the larger particle diameter and liquid viscosity also leaded to the greater difficulty of suspension,and the particle diameter had the most significant impact among physical parameters.On the basis of fully considering the collisional and frictional interactions between solids,the numerical model of dense solid suspension driven by coaxial mixer was established.Compared with the kinetic theory of granular flow and constant solid-phase viscosity models,the frictional-kinetic model could provide more accurate results.The solid-solid interactions played an important role at low speed,while the effect of frictional stress on solid distributions was limited.In addition,the RNG k-? model and Clift model were used to the calculation of turbulence and drag force,respectively,and the inclusion of turbulent dispersion force was also helpful to improve predictions.Finally,the turbulent dispersion coefficient was modified,and when the value of the coefficient was 0.6,the reasonable simulation results could obtained at various conditions.Correspondingly,relative deviations between predictions and measurements were mostly within 30%.The operation rule of coaxial mixer was clarified,and the mechanism of outer impeller on inner impeller's suspension ability was also revealed.By means of the co-rotation anchor,the PropellerU could more easily eliminate solid deposition,but its pumping action would also be weakened,and a larger inner impeller off-bottom clearance was help to comprehensively improve the suspension performance of PropellerU + Anchor.For the mechanism,taking the location of inner impeller as boundary,the axial flow in stirred tank showed enhanced and inhibited states with the rotation of outer impeller.Additionally,the higher outer impeller speed would make the effect more significant.The reason was that the tangential flow induced by outer impeller could change the location of high axial velocity region,and as the increase of outer impeller speed,the location of high axial velocity region moved in the opposite pumping direction.