Suspending Characteristics And Mechanisms Of Soild Particles In Stirred Vessels

Suspending solid particles in a stirred reactor is widely used in industrial process,such as reaction systems with solid catalyst,crystallization or dissolution of solid particles,fermentation process of biomass,and so on.The key point of the operation of the stirred reactor is that all the particles can be suspended after minimum power is input.As a result,many researchers have explored the characteristics of solid-liquid suspension based on the research of the critical suspension of particle.Previous studies emphasized the macroscopic suspension characteristics of abundant particles in stirred tanks,but few of them involved the suspension mechanisms of particle.In this research,high-speed imaging and image processing methods were used to accurately measure the critical impeller speeds and the trajectories of particle,and direct numerical simulations based on the lattice Boltzmann method(LBM)were implemented to study the suspension mechanisms of particle.The main contents and conclusions of this research are as follows:In a square tank with a rotating disk or a Rushton turbine,the initial lift-off stage of a spherical solid particle in a laminar flow was investigated.The conclusions are in the following terms:(1)The vertical velocity of the particle is relatively smaller,and the inertial force acting on the particle is two orders of magnitude smaller than the lubrication force.Therefore,the particle is in a "quasi-steady state".(2)As a result of the spacing between the particle and the bottom of the tank(h)as a logarithmic-linear function of time in a limited range,the lubrication force model can be used to obtain the axial hydrodynamic force acting on the particle and the initial spacing(h0).It is verified by atomic force microscope that the particle surface roughness is related to the h0,and the order of magnitude of h0 is about 10-6 m.(3)The axial hydrodynamic force increases with the h in a linear fashion so that the lubrication force model is improved.The nonlinear fitting for the curve of particle motion is performed.The prediction results used by the new model are in good agreement with the experimental data,and the tolerance is within 4%.We have studied the dynamic behaviors of eight spherical particles in critical conditions:a laminar fluid flow was generated by the Rushton turbine.The conclusions are in the following terms:(1)The critical impeller speed(NLO)increases with the increase of suspended particle number.To lift off all the particles,there are apparent increases of the NLO caused by the formation of two stable patterns of particle on the bottom wall.(2)Good agreement between simulated results and experimental data is obtained,such as the trajectories and the vertical velocities of particle.The Shields number used in simulations(?sim)is 10%higher than the Shields number obtained by experimental data(?exp=?N2D2/(g??dp)).The lift-off sequence and corresponding location of particle are random due to the local surface roughness of particle.(3)Two torque models are proposed to describe the interaction between particles.Although the axial hydrodynamic force acting on the particle is smaller than its net gravity,the particle can be suspended due to the effect of torque.The parameters of critical suspension of a single cylindrical particle in a laminar flow were obtained in the square tank with the rotating disk.By implementing quaternion,contact force model,and dry friction force model in the LBM simulations,the lift-off process of cylinders with diameter dp=12 mm and the aspect ratio lp/dp=2?6 was resolved.The conclusions are as follows:(1)There is a rising trend of the critical impeller speed with the increase of the lp/dp.It indicates that the hydrodynamic force acting on the cylinder is approximately positively correlated to the cylinder length.The viscous Shields number(?vis=(?vNSp)/[g(?p-?)Vp])can be used as a control parameter of the cylinder suspension,and the deviation of the cylinder position from the center of the tank bottom is a necessary condition for the cylinder being lifted off the bottom wall.(2)The simulated results of the cylinder moving on an inclined plane are compared with literature data,and the accuracy of the dry friction model is verified.The predicted trajectories,velocities,and orientation of the cylinder are in good agreement with experimental data.(3)The torque model is proposed when the cylinder is in contact with the bottom of the tank.The result shows that the torque of the fluid to the rotating shaft is the key factor for the cylinder suspension.The suspension characteristics of a single spherical particle for different particle densities,dimensionless off-bottom clearances of impeller(C/T),and impeller types were investigated in the range of the Reynolds number based on impeller from 300 to 1310.The particle image velocimetry(PIV)technique and the LBM simulations were used to analyze the flow characteristics of the fluid in the tank.The results are as follows:(1)As for the particle with dimensionless density(??/?)less than 1,with the decrease of the C/T,the impeller speed required for multiple suspension(NLO,m)decreases and is closed to the impeller speed required for once suspension(NLO,1).By contrast,there is a negative correlation between the impeller speed(NLO,R)and the C/T for the particle with ??/? greater than 1.(2)The particle suspension in the transitional stirred tank flow can be divided into four categories;that is,the spiral rise,the spiral fall,the reciprocating rotation,and the jumping.The bulk flow and the turbulent kinetic energy are responsible for the particle suspension,and the former plays a dominant role in the lift-off process of the particle.(3)The distribution of resultant velocity under the impeller,the change of the symmetry of the flow field,and the formation of vortex behind the impeller blade can be used as the reference for judging the laminar-to-transitional transition.In summary,this research has established a set of models and methods for studying critical suspension of particles in stirred tanks,obtained the critical suspension characteristics of particles under typical conditions,and explored the effect of forces and torques on particle suspension.These could provide theoretical basis and data reference for a relevant topic of fundamental research.