Experimental And Discrete Element Method Study On Particle Mixing Characteristics In A Cylindrical Mixer

Particle mixing is an important unit operation in chemical process.The mixing process of glass balls with a diameter of 5mm in a cylindrical mixer with a diameter of 210 mm was studied.The effects of impeller configuration(blade diameter,inclined angle,and blade number),rotational speed,and fill level on the flow and mixing of particles in a cylindrical mixer was analysed.The coefficient of rolling friction,the coefficient of static friction,and the coefficient of restitution were experimentally determined before the simulation,and the simulation results of torque and surface particle distribution were validated by experiments.By analyzing DEM results,the mixing performance and mixing efficiency of particles under different conditions can be obtained.The important factor which affecting particle mixing most can be obtained by comparing the circumferential velocity,axial diffusion coefficient and coordination number with mixing performance.The flow-circulating of particles in the mixer is obtained by analyzing the velocity distribution and velocity vector of particles.The DEM simulation results show that the mean deviation between the simulated and the experimental torque value with real Young's modulus is 2.019%,and the mean deviation by using Young's modulus E=108 Pa is 4.075%.When the diameter of the impeller increases,the Lacey mixing index,mixing efficiency,circumferential velocity,and axial diffusion coefficient increase,but the coordination number decreases.The mixing performance and efficiency of a three-bladed mixer is better than those of a two-and four-bladed mixer.For pitched blades,a three-flat-bladed mixer provides better mixing performance than a three-45°-bladed down pumping or a three-45°-bladed up pumping mixer,but the mixing efficiency of the three-450 bladed up pumping mixer is the best of these three mixers.When the fill level is H/T=0.4,The circumferential velocity has the greatest influence on mixing performance at side-side initial loading by comparing the circumferential velocity,axial diffusion coefficient and coordination number with the mixing performance.When the fill level is H/T=0.8 and the number of revolutions is lower than 7,circumferential velocity determines the mixing performance.When the number of revolutions is higher than 7,axial diffusion coefficient becomes the greatest influence on mixing performance.By analyzing particle velocity distribution and particle vector,it can be found that when the filling level is H/T1=0.4 the particle reaches its maximum velocity at about the position of 0.68-0.78.So the maximum pressure on the blade is at the position of 0.68-0.78 of the blade length,and this value is 0.61-0.78 when the filling level is H/T=0.8.There was a similar main cycle in the two-,three-and four-bladed mixer,but the particles of the two-bladed mixer has a significant pull-down process along the axis while the pull-down process of the three-and four-bladed mixers is less obvious than two-bladed mixer.The reason is that some particles fail to fall back down along the axis to the bottom of the blade in time and the next blade has come and pull the particles to the next cycle.This phenomenon results in the axial diffusion coefficient being lower than that of the two-bladed mixer.