Experimental And Numerial Studies On The Improvement Of Fluidization Of Nanoparticle Agglomerates

Nanoparticles have excellent physical and chemical properties and are widely used due to their small particle size and huge specific surface area.Fluidization,as is a promising technique that can enhance the gas-solid contact efficiency,heat/mass transfer efficiency,has advantages in processing of nanoparticles such as transport,mixing and surface modification massively.Due to the strong cohesive forces between nanoparticles,the nanoparticles are usually fluidized as the form of agglomerates which makes the fluidization process is different from that of common particles.In addition,the fluidization quality of nanoparticle agglomerates is poor and usually need to be improved with assisted methods.In this paper,the research of fluidization of nanoparticle agglomerates are is carried out by experimental study and numerical simulation,which can provide some theoretical guidance for application of fluidization of nanoparticle agglomerations.Firstly,the fluidization characteristics of SiO₂,Al₂O₃ and TiO₂ nanoparticles were studied in a fluidized bed with an inner diameter of 40mm.The effects of amplitude and frequency on the bed pressure drop,minimum fluidization velocity,bed expansion ratio and agglomeration at bottom are investigated by introducing mechanical vibration and the mechanism of vibration is discussed.The experimental results express that SiO₂ shows agglomerate particulate fluidization and Al₂O₃and TiO₂show agglomerate bubbling fluidization.Vibration can improve fluidization quality,reduce agglomerate size and pressure fluctuation,weaken hysteresis phenomenon,decrease the minimum fluidization velocity,increase the bed expansion ratio at low fluidization velocity.However,with the increase of frequency and amplitude,the improvement is gradually weakened.With the increase of amplitude and frequency,the bed expansion ratio of Al₂O₃ and TiO₂ nanoparticles agglomerates increases and the agglomerate size decreases,while SiO₂ nanoparticles agglomerates shows the opposite trend.Through the comparison,it is found that the influence degree of vibration is different for different materials.Vibration enhances the fluidization quality of nanoparticle agglomerates by promoting the fracture of“chains between particles”.But vibration has a dual effect on the agglomerates.It can not only promote the collision and breakage of the agglomerates,but also promote the densification of the agglomerates.Secondly,the fluidization experiments assisted by combining vibration and stirring of SiO₂,Al₂O₃ and TiO₂ nanoparticles were carried out in a fluidized bed with an inner diameter of 50 mm.The effects of vibration and stirring on bed pressure drop,minimum fluidization velocity,bed expansion ratio,agglomerates and agglomerate size distribution were investigated.The experimental results show that both vibration and stirring can improve the fluidization quality of nanoparticle agglomerates.However,the fluidization quality is no longer improved,or even worsen when the vibration or stirring parameter increases to a critical value.The combined action of vibration and stirring can further improve the fluidization quality.Compared with the fluidization without assistance,the bed expansion ratios are increased by 1.5-2 times,the minimum fluidization velocities are reduced by 4-7 times with the assistance of combined methods.The greater the cohesive forces between,nanoparticle,the more obvious the effect of combination of vibration and stirring.For different kinds of nanoparticles,the effects of vibration and stirring on fluidization are different.Under the optimal vibration and stirring parameters,stirring plays a leading role in improving the fluidization of SiO₂ nanoparticles agglomerates,while for the other two nanoparticles,the effect of vibration is slightly greater than that of stirring.The combination of vibration and stirring can reduce the agglomerates size and its distribution range of SiO₂ nanoparticles.There is a serious segregation phenomenon that the large agglomerates are at the bottom,and the small agglomerates are on the top when Al₂O₃ and TiO₂ nanoparticle agglomerates are fluidized.The combined methods can weaken the segregation phenomenon and make the agglomerates size distribution tends to be uniform along the height of the fluidized bed.The effect of breakage on the agglomerates at the bottom of the fluidized bed is more obvious.Then,a two-dimensional model was established to simulate the fluidization of nanoparticle agglomeration by coupling the two-fluid model and population balance model.This model considers the change of the diameter of agglomerates during fluidization and modifies the drag force equivalently by determining the change of agglomerate density with the diameter and uses agglomerate model and breakage model to determine the change of agglomerate number.The effects of fluidization velocity,cohesive force and types of material on fluidization are studied.The simulation results show that the simulation results are in good agreement with the experimental data.The agglomerate diameter increases firstly with fluidization time and then reaches a stable value.The agglomerate concentration and diameter distribution along the axial direction can be divided into three regions:the high concentration/large agglomerate region at the bottom,the transition region and the low concentration/small agglomerate region on the top.The agglomerate concentration and diameter gradually decrease along the bed height,but little change along the radial direction.In a fluidized bed,the total volume of agglomerates is mainly composed of large agglomerates,while the total number of agglomerates is the contribution of small agglomerates.With the increase of fluidization velocity,the decrease of cohesive force,it is found that the bed expansion ratio increases,the agglomerate size decreases and the segregation phenomenon is reduced Although the above factors have a certain influence on the fluidization of nanoparticle agglomerates,the type of materials and the cohesive force are the most important factors affecting the fluidization characteristics and agglomerates.Finally,the fluidization of nanoparticle agglomerates assisted by vibration and stirring respectively was simulated by using dynamic mesh and sliding mesh.The effects of difference assisted methods on the fluidization were studied.The results show that,the vibration can increase the bed height and decrease the agglomerate size.The diameter of agglomerates is further reduced by increasing the amplitude,but vibration would aggravate the segregation phenomenon when the amplitude is low.Stirring can make the bed expand smoothly and the concentration and diameter of agglomerates distributed uniformly along the height.Increasing the rotating speed can slightly increase the bed expansion height,but has no obvious effect on the agglomeration size.