Analysis Of Electrostatic Induction Signals And Effects Of Applied Electric Field Onthe Hydrodynamic Of Fluidized Bed

The gas-solid fluidized bed has become an important reactor in the production of ethylene polymerization due to its simple structure, stable performance as well as the higher rate of heat and mass transfer. However, in the fluidized bed, the charge is easy to generate and accumulate through friction among particles owing to the high insulating property of polyethylene. This leads to two consequences. On the one hand, the hydrodynamics in the reactor is affected by the static, which may cause agglomeration and sheeting. On the other hand, the electrostatic induction performance also provides a new perspective for us to study the hydrodynamics in fluidized beds. Therefore, it is of great theoretical and practical value to study the hydrodynamics in the gas-solid ethylene polymerization fluidized bed by analyzing the electrostatic induction signals.The experiment is carried out by an on-line electrostatic detector with independent intellectual property rights. The research of this work is composed of electrostatic signal mechanism analysis, particle circulation time measurement, sheeting detection and influence of the applied electric field study. Firstly, an electrostatic induction detector which is sensitive to the induction charge is developed and used to measure the electrostatic induction potential in fluidized bed, obtaining the electrostatic induction potential distribution characteristics, by analyzing the relation between the spectral distribution of signals and the motion of particles, the fundamental basis for the hydrodynamic parameters detection is provided. Secondly, according to signal analysis, the functional relationship between the dominant frequency of electrostatic signals and particle circulation time is obtained, which can be used to guide the circulation time measurement in industrial production. Thirdly, the influences of sheeting size and position on the electrostatic induction signals are studied, discovering the fluctuant regularity of induction potential when sheeting falls along the wall, and the relationship between dominant frequency of electrostatic induction signals and the sheeting size and position, providing theoretical guidance for sheeting detection in ethylene polymerization fluidized bed reactors. Lastly, the influence of applied electric field on particles which adhere to the wall is explored.the The influences of electric field strength on the adhering particle quantity and size distribution are obtained, which can provide a new perspective for reducing the particle quantity adhering to the wall.The main results in this thesis can be summarized as follows:Firstly, an electrostatic induction detector which is sensitive to the induction charge is developed and used to measure the electrostatic induction potential in fluidized bed, obtaining the electrostatic induction potential distribution characteristics, by analyzing the relation between the spectral distribution of signals and the motion of particles, the fundamental basis for the hydrodynamic parameters detection is provided. By changing the length, diameter and thickness of insulation medium, the variation of triboelectric charge, translation charge and induction charge amount with the change of probe geometrical parameters is obtained. It is observed that the charge on probe is mostly induction charge when the thickness of insulating medium is three times more than the diameter of probe, and the probe can effectively reflect the motion tendency of the whole particles in the induction area at the same time. On this basis, an electrostatic induction detector which is sensitive to the induction charge is developed and used to measure the electrostatic induction potential in fluidized bed, obtaining the electrostatic induction potential distribution characteristics as follows. The maximum negative electrostatic potential appears nearby the distribution plate. As the elevation rises, the negative electrostatic potential decreases, and then the voltage polarity turns to positive and increasing. The positive electrostatic potential reaches the maximum1in the bed level. Upon the bed level, the positive electrostatic potential decreases.Secondly, according to signal analysis, the functional relationship between the dominant frequency of electrostatic signals and particle circulation time is obtained, which can be used to guide the circulation time measurement in industrial production. Based on the Faraday’s law of electromagnetic induction, the relation between the dominant frequency of electrostatic induction signal and the circulation time of particles is derived as Te=gr/fo (gr is the geometric characteristics constant, gr=5.668). It was found in the experiments that the circulation time was determined by gas velocity, and it decreases with the increase of gas velocity and elevation.Thirdly, the influences of sheeting size and position on the electrostatic induction signals are studied, discovering the fluctuant regularity of induction potential when sheeting falls along the wall, and the relationship between dominant frequency of electrostatic induction signals and the sheeting size and position, providing theoretical guidance for sheeting detection in ethylene polymerization fluidized bed reactors. In the experiment, when the sheeting moves at a constant speed, the electrostatic induction potential presents V shape. The relation between the potential and the sheeting is U=C·r-3(C is the constant determined by electricity, speed, magnetic conductance and probe sectional area.). The sheeting can cause dominant frequency changes. When the size of the sheeting is larger, and the electrostatic detector is closer, the dominant frequency of induction electrostatic signal is lower. However, compared with the distribution area and bed level area, the signal dominant frequency in stagnant area suffers less effect.Fourthly, the influence of applied electric field on particles which adhere to the wall is explored. The influences of electric field strength on the particle quantity and size distribution are obtained, which can provide a new perspective for reducing the particles amount adhering to the wall. It is verified that the applied alternating current electric field (AACEF) could reduce the particle quantity which adhere to the fluidized bed wall, and the particle quantity on insulation wall reduces as the electric field strength (EFS) increases. The particle quantity on carbon steel electrode greatly decreased when the EFS is low. Also, the AACEF broadens the particle size distribution, while the particle size distribution on carbon electrode and insulation wall trends to consistency at the same time.