Study On Bipolar Charging Law Of Fluidized Bed By Controlling The Morphology Of Polyethylene Particles By Minor Component

In the operation of gas-solid fluidized bed,accidents such as sticky wall agglomeration caused by static electricity accumulation often occur,and in severe cases,it may cause emergency shutdown and explosion of the reactor.Static electricity has become a major hidden danger that has plagued the safe and stable operation of polyethylene fluidized bed reactors for a long time.The bipolar charge of the same particle is the main source of static electricity in the fluidized bed.It is of great significance to study the bipolar charging problem in the gassolid fluidized bed to ensure the safe and stable operation of the reactor.However,the mechanism of bipolar charging of homogeneous polyethylene particles in gas-solid fluidized bed reactors has not yet been clarified,and the influencing factors of bipolar charging of homogeneous particles have not been unified.Based on previous studies,the influencing factors of bipolar charging point to the properties of particle morphology,such as particle size,particle surface curvature,roughness,contact area,etc.There is a correlation between bipolar charging and polyethylene particle morphology.Therefore,this dissertation takes the bipolar charging problem of polyethylene particles in gas-solid fluidized bed reactor as the research object.Using the "complex effect" of particle growth during the polymerization process,the catalysts with physicochemical properties controlled by trace poisons are polymerized to produce polyethylene particles with different morphologies,so as to explore the relationship between the morphology difference of polyethylene particles and the bipolar charge.First,the effect of O2 on the physical and chemical structures of the Ziegler-Natta catalyst was explored.Further,using catalysts regulated by different O2 content to carry out slurry polymerization reaction,polyethylene particles with different morphology were obtained by polymerization,and the relationship between polyethylene particle morphology and bipolar charge was studied by means of electrostatic separation device.Finally,the production of polyethylene particles with different morphology was regulated by trace CO and H2O.With the help of electrostatic separation device,the bipolar charging mechanism affected by the morphology difference of the polyethylene particles in the fluidized bed was revealed.The main research contents and conclusions of this dissertation are as follows:1.SEM and BET studies show that when O2 reacts with the catalyst,the catalyst still maintains the initial spherical shape,but its specific surface area is significantly increased,and the catalyst will produce many fine pores with smaller pore sizes,which will make the catalyst matrix loose and expand and weaken the strength of the catalyst.In situ DR UV-Vis and XPS results show that the addition of O2 makes the active center of the catalyst oxidize,and Ti2+ and Ti3+are oxidized to Ti4+.CO adsorption in-situ DRIFTS results show that O2 destroys the coordination between Ti and Mg2+,resulting in a coordination vacancy for Mg2+.O2 destroys the coordination between MgCl2 and hydroxyl groups on the surface of SiO2 in the catalyst,so that the silica support exposes unsaturated coordination O2-.XRD results show that the content of MgCl2 crystals in the catalyst poisoned by O2 increases,which further proves the destruction of MgCl2 coordination.The TEA activation process and ethylene polymerization process of the initial catalyst and O2 poisoned catalyst were dynamically monitored by in situ DRIFTS.The results show that the characteristic peak intensity of polyethylene molecular chain increase with time,indicating that the polymer is constantly being formed.Taking the infrared peak at 2851 cm-1 as the characteristic peak of polyethylene to evaluate the kinetic properties of the catalyst online,the results show that O2 reduces the polymerization activity of the catalyst.2.With the enhancement of O2 poisoning,the polyethylene particles experienced the process of morphology deterioration of "overall regular large particles-mixing of large particles and small particles-small particles slag",and the morphological difference first increased and then decreased,which is consistent with the trend of bipolar charging of polyethylene particles.There is a correlation between the bipolar charging behavior of polyethylene particles and their morphology differences.The degree of bipolar charging of particles with small morphology differences is weak.The mechanism of O2 regulating the catalyst and then affecting the morphology of polyethylene was clarified.First,O2 changes the pore structure of catalysts,resulting in different growth mechanisms of polymers.The initial catalysts follow the growth mode of"layer-by-layer crushing mechanism",while O2 poisoned catalysts follow the growth mode of"local crushing mechanism".Secondly,O2 desorbs part of the active center Ti element from the support,resulting in uneven spatial distribution of the active center.Finally,O2 weakens the physical strength of the catalyst.In this experiment,the catalyst was broken under the action of stirring.According to the "replication effect",it can be seen that the polymer product grown on this template also showed the morphology of slag.These factors together affect the final morphology of polyethylene particle growth.3.The research results show that with the increase of CO poisoning,the morphology of polyethylene undergoes a deterioration process of "mainly large particles-mixed with large and small particles-small particles of slag".With the enhancement of H2O poisoning,the morphology of polyethylene undergoes a deterioration process of "mainly regular large particles-mixing of large and small particles-main bonding large particles".The morphology difference of polyethylene increases first and then decreases,and the electrostatic separation results show that the particles with large morphology difference have strong bipolar charge.