Electrostatic Influence On Hvdrodvnamics And Charge Control Investigation In Gas-solid Fluidized Bed

Electrostatics is generally existed in gas-solid fluidized beds, especially in polyolefin fluidized bed reactors, where highly insulated particles, water-free and oxygen-free environment provide natural conditions for triboelectric charging and charge accumulation. The excess accumulation of electrostatic charges in industrial fluidized beds leads to wall sheeting and explosion occasionally. Current research has mostly focused on electrostatic charge detection, generation and dissipation mechanism, charge distribution and control in fluidized beds. However, there is a serious lack of study on electrostatic influence on hydrodynamics and heat/mass transfer between fluidizing gas and particles, which results in a narrow and limited understanding of electrostatic hazards. Rational regulation and control of charge levels will eliminate the potential electrostatic hazards and even intensify the hydrodynamics without altering fluidization conditions. In further, heat and mass transfer processes can be strengthened. Therefore, it is of vital importance to investigate the influence mechanism of electrostatic charges on hydrodynamics and heat/mass transfer processes, develop novel charge control methods and achieve electrostatic utilization. This work provides a first insight of electrostatic influence on hydrodynamics in fluidized bed through rational control of average charge levels by injecting traces of liquid anti-static agents (LAA). The influences of charge levels on particle motions and bubble behaviors are systematically investigated by using a combination of multiphase detection methods including acoustic emission (AE) technique, electrostatic detection and pressure fluctuations. Then a novel local charge control method based on in-situ electrode corona discharge has been developed and verified by experiment. The mass and heat transfer between fluidized gas and particles can be strengthened through electrostatic control based on adding fines or corona discharge method. Therefore, investigations of electrostatic influences on hydrodynamics and charge control in fluidized beds are beneficial to electrostatic control and utilization, stability improvement of industrial reactors, which possess great theoretical significance and industrial application value. Research work has been carried out on the following aspects:1. A general charge control method is established to alter the whole charge levels of fluidized particles through LAA injection. By injecting 0-50 ppm LAA near the gas distributor, the charge-to-mass ratios of particles can be controlled in the range of-2.75 nC/g to 0 nC/g without changing fluidization conditions. Results indicate that an increase in LAA injection amount results in a gradual decrease of charges on particles and wall sheeting is also reduced.2. By altering the whole charge levels through injecting different amounts of LAA, electrostatic influences on gas bubble sizes and dynamic bed levels are revealed. Electrostatic repulsion force between mono-charged particles leads to an increase of average spaces among particles and mean voidage in emulsion phase, thus bubble sizes are reduced and bed fluctuations are weakened. The electrostatic influence is found to be more significant under a larger charge level. Comparing to uncharged situation, the average gas bubble sizes and bed voidage are reduced by almost 21% and 6% respectively, when fluidized particles are fully charged.3. The influence mechanism and action law of Geldart group B/D particles charging degrees on particle-wall average impact angles, particle velocity and flow pattern have been established by a combination of AE technique and multi scale wavelet analysis method. The classic correlation of minimum fluidization velocity has been revised by taking into consideration of electrostatic force for the first time. Results indicate that electrostatic influence on flow pattern is not significant. Electrostatic charges on fluidized particles lead to an obvious increase of average impact angles and decrease of particle velocity under a lower excess gas velocity. While drag force becomes dominant under a larger gas velocity, the electrostatic influence on particle motions weakens. Experimental results indicate that when charges are totally eliminated, the mean impact angles between polypropylene (PP) particles and wall are reduced by 3 degrees and 0.8 degrees under a lower gas velocity and higher gas velocity, respectively. The velocities of PP and LLDPE particles estimated from AE signals are found to be improved by 240% and 143% due to charge elimination. Besides, the classical Wen-Yu correlation has been revised theoretically in consideration of electrostatic influence. Results indicate that wall sheeting leads to a decrease of pressure drop of the whole bed while the electrostatic attraction force between charged particles and wall increases the pressure drop and minimum fluidization velocity. The minimum fluidization velocity is found to be increased by 5.4% when particles are fully charged.4. A novel charge neutralization method has been developed in the gas-solid fluidized bed based on in-situ corona discharge of fluidized gas. The performance of corona discharging has been studied through voltage-current characteristic curve, minimum corona voltage and corona affecting ranges. A comparison of electrostatic voltage, charge-to-mass ratio and field strength of dilute phase before and after application of corona discharge has been made to verify the feasibility of this new method. Results show that the corona charging efficiency is determined by both tribo-charging and corona charging process parameters. A larger superficial gas velocity leads to a smaller minimum corona voltage, thus results in a better corona discharging efficiency. Comparing to ion wind discharging method, in-situ electrodes based corona discharging method achieve a higher charge elimination efficiency.5. Electrostatic charges control methods and relevant theoretical models have been developed for rational regulations of charges on fluidized particles or complete charge elimination based on adding fines and/or altering corona conditions. Firstly, corona charging model of fluidized particles is developed based on particles tribo-charging and corona charging models. Influences of tribo-charging and corona charging process parameters like equilibrium charge and charging time constants on final charges of fluidized particles have been theoretically analyzed and experimentally verified.. In addition, another electrostatics control method by fines injection has also been developed based on particles tribo-charging model. It is demonstrated that the variations of electrostatics in fluidized beds are mainly determined by the diameter ratio of large and fine particles, fines weight ratio and contact potential difference between large and fine particles. This model unifies the influence mechanisim of fines on electrostatics in fluidized beds for the first time and can also well explain the experimental results from literatures.6. The influence mechanism of electrostatics on mass and heat transfer has been theoretically analyzed for first the time. It is shown that the variation of Reynolds number is responsible for electrostatic influence on mass and heat transfer. The decrease of Reynolds number leads to an increase of mass and heat transfer coefficients in bubble phase and a decrease of those in emulsion phase, while the mass and heat transfer coefficients of the whole bed are reduced. According to theoretical analysis, the mass and heat transfer process can be strengthened through appropriate electrostatics control through corona charging, adding fines and injecting traces of LAA.