Formation And Evolution Of Particle Agglomerates In Gas-Solid Fluidized Beds

Gas-solid fluidized beds are widely used in many areas such as energy,chemical industry,metallurgy,food processing,owing to its high heat and mass transfer efficiency,simplicity and low cost.Agglomeration is a very important phenomenon in the gas-solid fluidized beds.As to the olefin polymerization fluidized bed reactor,polyolefin particles may bond with each other by the adhesion force between them.If no effective actions are taken in time,agglomerates will grow bigger and bigger,thus forming large chunk or sheet which even lead to bed defluidization and shutdown of the reactor.Particle agglomeration has become a recognized technical problem.Therefore,revealing the mechanisms of particle agglomeration,predicting the process of its evolution and developing effective controlling method are critical to prevent particle agglomeration,maintain the safe operation of reactors and produce high-performance products.This work was aimed at the particle agglomeration in olefin polymerization fluidized bed reactors.Firstly,theoretical analyzes of the microscopic physical processes of particle collision and bubble shearing agglomerates were carried out.Models of particle coalescence collision,agglomerates break-up and bubble-particle action frequency were proposed to describe the formation and break-up of agglomerates.The Constant Number Monte Carlo method was used to solve these stochastic models and predict the evolution processes of agglomerates.Secondly,lab-scale fluidized bed based on electromagnetic induction heating was designed and set up,characterizations of pressure fluctuation,temperature distribution,sampling,microscopic visualization were conducted to obtain the evolution behavior of agglomeration under the control of liquid bridge force,solid bridge force and the cooperative action of them.Liquid bridge force induces-solid bridge force sequent acting mechanisms of agglomeration were revealed.Finally,investigation of the effect of particle agglomeration on the fluidization behaviors of particles were carried out,criterion based on ? distribution was proposed to judge the particle fluidization types which provide guidance for the operation of industrial fluidized beds.More specifically,this work can be divided into the following four parts:1.With regard to the particle agglomeration controlled by solid bridge force,taking the effects of solid bridge growing on its strength into consideration,criterions of particle coalescence and break-up of agglomerates were proposed based on the force balance analyzes of solid bridge force-repulsive force and solid bridge force-bubble expansion force,respectively.By calculating the bubble frequency,action border and concentration of particles surrounding,bubble-particle interaction frequency model was firstly established.Based on the above work,CNMC method was applied to simulate the evolution of agglomeration controlled by solid bridge force.Particle size distribution,mass fraction of agglomerates,coalescence efficiency?aand breakage efficiency ?b were predicted,which provides a way to obtain the agglomeration kernel.Results showed that the modeling mass fraction of agglomerates was consistent with the experiments with a relative error lower than 16.5%,which proved the accuracy of the proposed models.Besides,investigation of the effects of bed temperature and superficial gas velocity were both carried out in the simulation and experiments,and found that ?a increased and ?b decreased with increasing bed temperature,while?b increased and ?a decreased with increasing superficial gas velocity.Increasing superficial gas velocity and decreasing bed temperature are supposed to be efficient in the control of agglomeration dominated by solid bridge force.2.With regard to the particle agglomeration controlled by solid bridge force,lab-scale gas-solid fluidized bed with side-wall liquid injection based on electromagnetic induction heating was firstly designed and made.Induction heating of PE wax @ graphite particles with core-shell structure was used to mimic the exothermic process of polymerization occurred on particles and surface adhesion characteristics after heated.Correlation based on bed pressure drop was established to quantitatively calculate the mass fraction of wet agglomerates,thus solving the problem that wet agglomerates easily break and were hard to sample.Secondly,criterion of wet agglomerates formation was proposed based on the energy balance analysis of initial kinetic energy during particle separation and energy dissipation caused by liquid bridge force taking the effects of liquid bridge volume,configuration on its strength into account.Considering the effects of liquid evaporation on bubble size,breakage model of wet agglomerates based on the force balance analysis of liquid bridge force and bubble expansion force.On these bases,CNMC was used to simulate the evolution of wet agglomerates.Results demonstrated that the modeling mass fraction of wet agglomerates was consistent with the experiments with a relative error of 27.4%.Investigation of the effects of liquid injection flowrates,induction heating power,superficial gas velocity and liquid injection position on agglomeration were also carried out in the simulation and experiments.Results showed that the match of heat removal caused by liquid evaporation and heat releasing by particles determined the amount of liquid in the bed thereby the liquid bridge force.3.With regard to the agglomeration process controlled by liquid bridge force and solid bridge force,screening experimental conditions were firstly conducted to mimic the high heat releasing rate and liquid injection rate in industrial olefin polymerization fluidized beds,thus achieving the simultaneous existence of solid bridge force and liquid bridge force.Pressure fluctuation,sampling,bed temperature distribution,microscope visualization were used to study the variations of wet agglomerates and solid agglomerates with liquid injection flowrates.Results showed that with the increase of liquid injection flowrate,mass fraction of wet agglomerates increased while the mass fraction of solid agglomerates firstly decreased,then fluctuated and quickly increased.On the basis,three stages of particle agglomeration were proposed:solid bridge force dominated,liquid bridge force dominated,and the cooperative control of them.Secondly,through the characterization of bed pressure drop and temperature distribution,occurrences of local dead zone and hot spot were validated.Combined with the direct visualization of liquid bridge and solid bridge between particles during heating,liquid bridge force induces-solid bridge force sequent acting mechanisms were proposed.Finally,taking the number of particles within agglomerates and liquid evaporation into account,the radial temperature distribution of wet agglomerates inside the industrial olefin polymerization fluidized bed reactors was calculated.Results showed that hot spot appeared when the number of particles within the wet agglomerates was more than 400.4.Experimental investigation of the effects of agglomeration on particle fluidization behavior was carried out.It was found that particle fluidization behavior was controlled by liquid bridge force under low induction heating power.With increasing liquid injection flowrates,the particle fluidization behavior changed from stable fluidization to fast defluidization.Under higher induction heating power,the particle fluidization behavior was controlled by liquid bridge force and solid bridge force.With increasing liquid injection flowrates,the particle fluidization behavior experienced gradual defluidization,agglomerating fluidization controlled by solid bridge force,agglomerating fluidization controlled by liquid bridge force,gradual and fast defluidization controlled by liquid bridge force and solid bridge force.Based on the ? distribution under different liquid injection flowrates and heat releasing flowrates,the particle fluidization behavior in gas-solid fluidized bed with side wall liquid injection were classified into:agglomerating fluidization controlled by solid bridge force(AGFSB),agglomerating fluidization,gradual defluidization and fast defluidization controlled by liquid bridge force(AGFLB).Finally,a criterion of judging the particle fluidization behaviors was proposed.