Numerical Study Of The Particle Flow And Mixing Characteristics In A Ribbon Mixer

In process industry,such as chemical,pharmaceutical,food,metallurgical and silicate industries,homogeneous products are always desired and homogen is the key indicator to evaluate products in these industries.It implies that mixing unit operation employed to ensure product homogeneity is very significant.In the past,experiment and theoretical modeling were two traditional approaches used by scientific researchers to investigate particle flow and mixing characteristics in the mixing equipment,while nowadays,with the upgraded improvement of computer calculation ability,Discrete Element Method(DEM)has been widely used and accepted to conduct numerical simulation research on particle systems.In this study,the particle system in a ribbon mixer which is widely used in industry is investigated.Based on DEM,particle flow and mixing characteristics in this device are numerically studied.The research mainly includes the following three aspects:(1)A discrete element model which can simulate the flow and mixing of cohesive particles is developed.As we all know,water droplets can be condensed among particles under certain temperature and humidity conditions,making it necessary to build the relevant cohesion(the liquid bridge force)model.Previous models assume that the contact angles between the two ends of the liquid bridge and the particles are equal.But the materials of particles needed to be mixed are not necessarily the same,and different materials have different contact angles.Therefore,a more universal liquid bridge model capable of involving different contact angles is developed.It has high accuracy from micro and macro verification and is added to the original DEM model of dry particles,which makes the existing DEM model can simulate both dry and wet particle flow and mixing.(2)An image analysis technique using for evaluating the mixing degree of particles with different properties is developed and the accuracy of the new DEM model is verified by this technique.To ensure the reliability of numerical simulation,the DEM model must be verified by experiment or theoretical derivation.For this reason,we developed an image analysis method to seek different characteristics of different particle components on the image,directly identifying different particles to directly calculate the mixing degree of different particle components.By obtaining the developed image analysis tool and experiments of particle segregation and mixing in a drum,the accuracy and reliability of the dry particle DEM model and the established wet particle DEM model are verified.(3)By employing DEM simulation,the effects of operational parameters and particle properties on particle flow,mixing,particle breakage and equipment wear in a ribbon mixer are studied.In the ribbon mixer,the operation parameters,such as rotation speed,particle loading mode,particle characteristics,such as sphericity,size,density,and whether liquid exists between the particles,all will affect the flow and mixing of particles in the mixing equipment.Seen from the numerical simulation results,it shows that the faster the rotational speed,the greater the particle mixing rate.Up and down,before and after loading have the same mixing speed and are significantly greater than the left and right loading.Particles with big aspect ratio behave gently and particles with sharper edges move more violently.And only when the liquid bridge force is dominant in the interaction between particles,wet particles tend to form a whole and move together with the ribbon,which will weak the mixing rate.Finally,based on the numerical simulation results,the optimal operating parameter of the ribbon mixer is given.The investigation work done in this article can help to provide reference for the practical operation and equipment design of the ribbon mixer which is widely used in industries.