Fluidization Characteristics Of Coarse Cohesive Particles In Gas-Solid Fluidized Beds

In various industrial fluidization processes,such as olefin polymerization,solid combustion and so on,the fluidizing particles with a diameter of millimeter often show cohesiveness due to their physical properties and high local temperature of fluidized bed.Solid bridge force is easy to be produced by contact between particles,which leads to bonding,agglomeration as well as defluidization.To maintain stable fluidization of coarse cohesive particles system,the cohesive force model,agglomeration mechanism and fluidization characteristics of coarse cohesive particles are studied at the particle scale,meso-scale as well as reactor scale combined with experiment and simulation research methods.Around this goal,three research work has been carried out:experimental study on the bonding properties of single particle;study on the agglomeration mechanism of coarse cohesive particles in fluidized bed;study on the fluidization characteristics of coarse cohesive particles in fluidized bed.The following research results have been achieved:(1)On the particle scale,through real-time observation of the growth dynamics of solid bridge,it is found that solid bridge growth,contraction and compact happen during the single particle bonding process.According to the neck length index,it is found that the growth mechanism of solid bridge will transform from cohesive flow to surface diffusion during the bonding process.The non-dimensional tensile strength of solid bridge was calculated by the penetration parameters.The effect of the rheological properties of the material on the tensile strength of the solid bridge was considered.It was found that the tensile strength of the solid bridge increased first and then decreased with the increase of temperature in the temperature range studied.(2)On the meso-scale,based on the experimental value and simulation value of solid bridge force,the relationship between simulation cohesiveness parameter(square-well depth)and experimental cohesiveness condition(temperature)was established for the first time with the experimental research and computational fluid dynamis-discrete element method(CFD-DEM)simulation coupled with square-well model.Through the experimental study and simulation study of the bonding process,the bridging law of coarse cohesive particle flow is obtained.The whole bridging process is divided into four stages:in the first stage,coarse cohesive particles are bonded to nucleation in the high frequency zone.In the second stage,the nucleation particles are accumulated forming a single side bridge.And in the third stage,the single-side bridges are growing in each other,and connected to form a bilateral bridge.When unstable,the bilateral bridges are easily dispersed and restore the state of the single side.In the fourth stage,the process in third stage recurs repeatedly,while the overall cohesiveness of the bed is enhanced,resulting in a stable bilateral bridge and defluidization.(3)On the reactor scale,the influence of cohesiveness on fluidization behavior was obtained,and the monitoring of coarse cohesive particles fluidization was realized.With the increase of the cohesiveness parameters,the particles flow transforms from the normal bubbling flow to the agglomeration fluidization,the aggregative fluidization until defluidization.Through the bed pressure drop,pressure fluctuation deviation and wavelet analysis,it is found that coarse cohesive particles begin the nucleation process with cohesiveness increasing.The average diameter of the bed bubble increases while the bubbles as well as agglomeration gather and break up continuously.When the cohesiveness increases to a certain extent,the force balance is reached.Then the fluidized bed will undergo a short agglomeration fluidization which have similar bubbles behavior to the normal bubbling fluidization.Under higher cohesiveness conditions,the particles quickly form the single-side bridges and the bilateral bridge which cause defluidization.