Electrostatic Charge Generation Of Polyolefin Particles In Gas Phase Fluidized Bed Reactor

In gas-solid fluidized bed reactors,emergencies such as sheeting and wall fouling caused by electrostatics occur from time to time.Electrostatics has always been a major hidden trouble plaguing the safe and stable operations of polyolefin fluidized bed reactors.However,the mechanism of electrostatic generation of polyolefin particles in gas-solid fluidized bed reactor remain unknown and the bipolar charging behavior of polyolefin particles conflicts with existing theories.In this dissertation,based on the process and polymerization reaction,the dominant mechanisms of electrostatic charging in Polypropylene(PP)Circulating Fluidized Bed(CFB)and Polyethylene(PE)Bubbling Fluidized Bed(BFB)are investigated by cold-mode experiments as well as CFD simulation and Monte Carlo modeling of bipolar charging of identical granular materials,respectively.The unipolar charging of particles dominated by particle-wall contacts and its interaction with gas-solid flow in a PP CFB riser were revealed and the special bipolar charging behavior dominated by particle-particle friction in a PE BFB reactor was clarified.In particular,by using electrostatic separation technology,the significant role of particle morphology in bipolar charging of PE particles was found for the first time and the effects of particle morphology evolution during particle growth on the bipolar charging behavior of PE particles was clarified.With a Monte Carlo method simulation with surface grid as collision unit developed and a bipolar charging model for identical granules considering both particle size and particle morphology established,a new mechanism of bipolar charging dominated by effective contact area was revealed,theoretically unifying the contradictory charging polarity of PE due to particle morphology differences and particle size differences,and completely revealing the bipolar charging mechanism of nascent PE particles.The results lay a new theoretical foundation for the study of electrostatics in fluidized bed reactors,and provide a new idea for the quantitative prediction and the precise control of electrostatic charges.The main research contents and conclusions are as follows:1.The dominant mechanism of particle charging in the CFB riser and its interaction with gas-solid flow were revealed by experiments and CFD simulations coupled with electrostatic model.In the CFB riser,the particle-wall contact charging is dominant,particles are unipolar charged,and the magnitude of particle charging is determined by the gas-solid flow pattern and the solids concentration.A CFD model coupled with electrostatics is established,and the quantitative experimental validation of the model is achieved for the first time in the open literature.Under fast fluidization,the electrostatic repulsion acting in the radial direction causes the particles to accumulate near the wall,which eventually leads to an increase of 25.5%in the average solids concentration in the riser.The solids concentration determines both the particle charge density and the electric field intensity generated by charged particles,thus revealing the dual enhancement mechanism of solids concentration on the electrostatic effects,which can provide guidance for the design of CFB reactor and the stable operation with high flux/density.2.Combined with electrostatic separation and particle charge measurement,the bipolar charging behaviors of multi-grade PE particles with dual particle size distribution and with narrow particle size distribution were investigated.The results show that particle-particle bipolar charging dominates electrostatic generation in PE BFBs,and eight grades of PE particles studied consistently show that small particles are more likely to be positively charged in the bed.Excluding the effects of size,the narrow size of PE particles still charging severely.It was found that the effective work function of PET is even between the positively charged and negatively charged particles of the same particle size.Through multi-property characterization,the bipolar charging of PE with narrow size distribution was found to be morphology-dependent for the first time,and particles with smooth surface and high sphericity are more likely to be positively charged while particles with rough surface and low sphericity are more likely to be negatively charged.3.Based on the results of 2,a new method of particle morphology sorting based on electrostatics was developed.The particle morphology evolution of PE during particle growth and its interaction with bipolar charging behaviors were investigated by separating bipolar charged particles with different narrow particle size distributions.The results show that as the PE particles grow,the particle morphology becomes worse and the sphericity decreases.The better particle morphology of small particles may be the reason for their positive charges.The Young's modulus differences in surface and internal of particles with different morphologies suggest that the excessive crystallization rate is one of the reasons for the morphology degradation of PE particles.The difference in particle morphology correlates with the degree of bipolar charging,and the larger the difference in particle morphology,the greater the bipolar charge effects.4.Based on the theory of non-equilibrium electron transfer,a Monte Carlo method simulation was established to simulate the bipolar charging of identical insulating particles with surface grid as collision unit.The experimental results of the size-dependent polarity of bipolar charging in literature were reproduced qualitatively and quantitatively by the simulation.For two standard spherical particles,the size-dependent polarity of bipolar charging is consistent with the literature.As the surface density of high-energy electrons of the smaller particles decreases faster than that of the larger particles,the high-energy electrons given by the smaller particles are less than those received from the larger particles.As a result,the smaller particles gain net electrons and are negatively charged.The larger the particle size difference,the higher the charge-to-mass ratio of bipolar charging.For the multi-dispersed particles,the simulated collision probability is consistent with the theoretical value.Based on this,the size-dependent charging polarity of the multi-dispersed particles is quantitatively reproduced for the first time.The smaller particles are mainly negatively charged,while the larger particles are mainly positively charged.The bipolar charging behavior is related to the ratio of large and small particles.5.Based on the MultiGrain Model,a two-dimensional model of particle morphology was established.A Monte Carlo method simulation was developed to simulate the bipolar charge of irregular particles with surface grid as the collision unit A new mechanism of bipolar charge dominated by effective contact area considering both the effects of particle size and particle morphology was revealed.The results show that electron transfer mainly occurs between PE particles,and the direction of electron transfer is determined by the effective contact area.The particles with smaller effective contact area are more likely to be negatively charged,and vice versa.The diverging bipolar charging phenomenon caused by morphology difference and size difference could be theoretically unified by effective contact area.The simulation results of two bipolar charging particles reproduced the "Mosaic" distribution of positive and negative charges on particle surface,which was experimentally measured by experiments in the literature at the micro scale.It was found that the local contact frequency on particle surface has an important effect on the micro net charges.The simulation results of multiple particles bipolar charging qualitatively explain and verify that the deterioration of PE particle morphology during particle growth is the key factor that causes the unique size-dependent polarity of PE particles on bipolar charging.