Studies On The Distribution Of Electrostatic Charges In Gas-solid Fluidized Beds

The occurrence of electrostatic charges is almost unavoidable in particulates and multiphase systems such as gas-solid fluidization, due to repeated particle contacts and separation, supplemented by the friction of particles rubbing against each other and the vessel wall. These charges can be accumulated by dielectric materials. The built-up of high equilibrium charges in fluidized systems can interfere with the normal hydrodynamics of the bed, resulting in particle-wall adhesion, inter-particle cohesion, electrostatic discharges and even wall sheeting. The inevitability, unpredictability, and complexity of the generation of electrostatic charges make it one of the most puzzling problems in ethylene polymerization fluidized bed reactors. Therefore, it is not only very challenging but of profound theoretical and practical significance to systematically investigate the electrostatics phenomena in the gas-solid fluidized bed.On the scales of energy band, particle and fluidized bed, a micro-meso-macro multilevel generation mechanism of electrostatic charges in gas-solid fluidized beds was proposed. Subsequently, basing on an on-line technique to measure electrostatic voltage developed in this work, characteristics of the distribution of electrostatic field in the gas-solid fluidized bed was systematically investigated through a series of experiments, and it was found that the three-dimensional profile of electrostatic potential in the fluidized bed with linear low density polyethylene (LLDPE) fluidized was shaped a double-saddle with the bed level as the interface. Furthermore, the evolution law of electrostatic behavior with the variation of fine particles' size and content was discovered. An impact factor in electrostatics of fine particles was then proposed, and it was identified that a reasonable weight percent of fines should be controlled in the industrial production of polyethylene. Influences of several typical kinds of liquid and solid impurities on the electrostatic potential were investigated. On the basis of the electronegativity of chemical compositions, a rule to choose positive and negative electrostatic induction agents was developed to control access charge accumulation in the fluidized bed. Finally, the criterion of dividing the information to three characteristic scales was established by Hurst analysis of the wavelet decomposed signals. The main results in this work can be summarized as following:1. On the scales of energy band, particle and fluidized bed, the mechanism of charge generation and accumulation in the fluidized bed, the principle of bipolar charging of polymer particles, and the profiles of electrostatic charge in the bed were discovered. Besides, a micro-meso-macro multilevel mechanism of the generation of electrostatic charges in gas-solid fluidized beds was proposed.Firstly, basing on the theory of energy band, the generation mechanisms of electrostatic charges on the polymer particles without impurities, with liquid impurities, and with solid imourities in the gas-solid fluidized beds was analyzed. The results showed that the polarity and amount of charges on materials contacting each other were mainly dependent on their energy bands. The introduction of liquid impurity would provide a media and channel to help charges neutralization, redistribution, leakage and dissipation. With the introduction of solid powder impurity, the contact types in the bed would change, and then contact charging among impurity powders, polymer particles and vessel wall would bring the redistribution of charges in the bed.Secondly, by developing a mathematical model relating effective work function of dielectric particles, particle diameter and relative permittivity, the principle on bipolar charging of dielectric particles with the same material but different size was analyzed. The result showed that effective work function of dielectric particles with identical size would decrease with the increase of the relative dielectric constant. For particles with the same dielectric constant and without any impurities, their work function would decrease with the increase of particle diameter, which provides the theoretical support to the bipolar charging phenomenon. While for particles with the same dielectric constant and with impurities, the dielectric constants of fine particles with stronger ability of adsorbing impurities would rise more than large ones. Therefore, the influence of dielectric constant on the particle's effective work function is predominant in this case, i.e. the work function would increase with the increase of particle diameter.Finally, a series of experiments in the fluidized column and particle shaking experiments were carried out to investigate the charging mechanism of the dielectric powders in the bed. The results showed that charging due to the friction of gas-wall and gas-particles could be neglected, and only unipolar charging occurred after particles contact or friction with the column wall. Therefore, it was demonstrated that bipolar charging was due to interactions among particles. On the basis of the experimental results mentioned above, three main charging modes in the fluidized bed were then indentified, which includes contact/friction charging between particles and neutral column wall, contact/friction charging among coarse and small particles, and contact/friction charging between particles and charged column wall. Subsequently, characteristics of charge distribution in the fluidized bed were predicted according to the particle size distribution along the axis of the bed.2. On the basis of the measurement of electrostatic voltage, the axial, radial and overall profiles of the electrostatic potential in the gas-solid fluidized bed were systematically investigated through a series of experiments. It was found that the voltage at any axial cross section of the bed showed double saddles with bed level as the interface. Accordingly, three special zones to be emphasized were identified, including distributor zone, stagnant zone and bed level zone, of which the latter two have high voltages and are more readily disturbed by particle/wall adhesion and even wall sheets.By measuring the electrostatic potential along the axis of the fluidized bed, it was found that the electric field inside the bed was non-uniform. If there was a distinct interface between the dilute and dense phases, the voltage polarity would reverse near the bed level, resulting in a Z-shaped or S-shaped axial profile of potential. The heights where voltage polarity reversal happened rose with the increase of gas velocity and static bed height. For Z-shaped axial profile, the electrostatic potential was negative at the bottom of the bed and positive at the top of the bed. The reason was that large particles charged negatively but small particles were positively charged. Accordingly, polymer materials showing Z-shaped potential profiles have certain common characteristics, which are low crystallinity, high surface roughness and strong ability to adsorb impurityies. Therefore, influences on the effective work function of particles due to adsorbed impurities are predominant, and large particles have higher work function. For S-shaped axial profile, the electrostatic potential was positive at the bottom of the bed and negative at the top of the bed. It means that large particles charged positively but small particles were positively charged. Polymer powders showing S-shaped potential profiles have high crystallinity, smooth surfaces and weak ability to adsorb impurities. Therefore, the dielectric constants of small and large particles are the same, and the effective work function of particles depends only on the particle size. In this case, large particles have lower work function.It was also found that the electrostatic voltage rose with the increase of radial distances from the axis of the column. To sum up, the voltage at any axial cross section of the bed showed double saddles with bed level as the interface. Accordingly, three special zones to be emphasized were identified, including distributor zone, stagnant zone and bed level zone, of which the latter two have high voltages and are more readily disturbed by particle/wall adhesion and even wall sheets. Based on the characteristics of axial profile of electrostatic potential in the fluidized bed, a new technique which could successfully predict bed level was developed. A good agreement of bed level was observed between visual measurements and predictions by the electrostatic method, with the maximum relative error 4.08% and mean relative error 2.02%.3. On the basis of the evolution law of electrostatic behavior with the variation of fine particles' size and content, an impact factor of fine particles on electrostatics was proposed. Then it was identified that the reasonable weight percent of fines should be controlled in industrial productions, and static control strategy in fluidized bed was presented on particle scale.Experiments were performed in a gas-solid fluidized bed to determine the changes in the electrostatic charges with various fine polyethylene particles' addition, which were the same material as the coarse polyethylene particles, to better understand their role in influencing electrostatic charge generation/dissipation. The results showed that polymer particles having the same chemical makeup but different sizes would have their own special contributions to the generation of static charges, owing to the differences in catalyst residue and surface properties among them.By measuring the electrostatic potential at different bed axial heights, it was found that the electric field inside the bed was significantly influenced by the added fine polyethylene particles' sizes, weight fractions and catalyst residue. Consequently, an impact factor of fine particles (F_b) was proposed, which was the plus of particle diameter term F′_b and catalyst residue term F″_b.It was found that there was strong association between electrostatic potential in the bed and F_b of the fine particles added. Within the experimental limits in this thesis, after three kinds of fines which F_b were all less than 1 were added in the fluidized bed, the potential in the bed was just influenced slightly. However, the bed potential was changed significantly following the addition of the fines with smallest particle diameter and highest residual catalyst content. While F_b of this kind of fines was less than 1, the static potential didn't change much but even decreased slightly with its addition. Once its F_b was more than 1, the electrical potential in bed increased greatly. With F_b of the forth kind of fines increasing further, the potential fell a little, and meanwhile it was found that a large amount of fine particles adhered to the column wall, which would be likely to result in sheeting for industrial reactor. Therefore, during the production of polyethylene, the particle size distribution should be strictly controlled, and the weight percent of fines with diameter less than 185 urn should be less than 10%, better to be less than 5%.4. Electrostatic behavior in a polyethylene gas-solid fluidized bed with different liquid and solid impurities added was experimentally investigated. The impurities include aluminum alkyls, water, alcohol, and their reaction products. On the basis of relations between charging tendency and electronegativity of chemical compositions, a rule to choose inorganic oxides powders as positive or negative electrostatic induction agents was presented. The rule is that acid oxides with large electronegativity have a tendency to charge negatively, while metal ions with small electronegativity make base oxides more easily charging positively.The experimental results are as follows. Water and alcohol without aluminum alkyls presence would decrease the charge accumulation in the fluidized bed. While reaction product Al(OH)₃ of aluminum alkyls with water would initiate negative charges in the bed, and reaction product Al₂O₃ of aluminum alkyls with oxygen/alcohol would initiate positive charges in the bed. The results demonstrated that reaction products of aluminum alkyls with water and oxygen/alcohol play a key role in the induction of negative and positive charges when there are impurities such as water, oxygen/alcohol in the gas feeing into polymerization reactor.According to the rule choosing static induction agents, four inorganic and inert oxide powders involed in ethylene polymerization, with SiO₂ and TiO₂ serving as negative charge induction agents, while Al₂O₃ and MgO as positive charge induction agents. As expected, addition of inorganic powders changed both values and axial profile of the electrostatic potential inside bed. SiO₂ had little influence on the electrostatic potential. TiO₂ inducted negative charges in the bed. While Al₂O₃ and MgO have tendency of charging positively, so their addition decreased the initial negative charges in the bed. Because the positively charging tendency of MgO is stronger, its addition could make the axial profile of potential change from Z-shaped to S-shaped. Therfore, less amount of MgO would have the same effect as Al₂O₃ to eliminate the same amount of negative charges.5. The criterion of dividing the information to three characteristic scales was established by Hurst analysis of the wavelet decomposed signals: micro-scale signals with only one Hurst exponent less than 0.5 (D₁ and D₂ wavelet decomposing scales); meso-scale signals with one Hurst exponent more than 0.5 and the other less than 0.5 (D₃-D₆ wavelet decomposing scales); macro-scale with only one Hurst exponent more than 0.5 (A₆ wavelet decomposing scales), and the three scales reflected the action of particles, the action of bubbles and agglomeration, and average dynamics respectively.For bubbling fluidization, energy percent of meso-scale component is highest, confirming that the electrostatic fluctuation signals mainly reflect the meso-scale. It was also found that micro-scale and meso-scale components were sensitive to the flow regime transition. Furthermore, energy profiles of three components varied with the radial distance of the bed. The results showed that there were more reflections on the local dynamics in the electrostatic fluctuating signals.