Characteristics Of Electrostatic Field Distribution In Polyethylene Fluidized Beds

Electrostatics can cause serious problems for fluidized beds, altering hydrodynamics, causing agglomeration, interfering with instrumentation, generating nuisance discharge and even creating the danger of explosion. The accumulated electrostatic charges on powder particles or plastic films inside large commercial fluidized bed reactors can interfere with their performance, leading to undesirable byproduct/waste. Despite these negative effect, the mechanism of charge generation and dissipation are still poorly understood. Although there have been some efforts to prevent electrostatic charges in fluidized beds, the mechanism of reducing electrostatic charge accumulation are still poorly understood.Aiming at the problems mentioned above, four aspects work were carried out: 1. The generation mechanism of electrostatics in fluidized beds was analysed; 2. Experimental study on electrostatic potential profile in the fluidized beds; 3. Study on a new method measuring the bed level basing on the characteristics of electrostatic potential profile; 4. Sheeting detection research basing on the mearsurement of triboelectrical current in the fluidized bed. The main results in this article can be summarized as following:1. The mechanism of static charge generation and electrostatic character inside gas-solid fluidized beds was analysed, and a theory on bipolar charging of fluidized particles was proposed, which consists of three parts: a physical model for charge generation and accumulation in fluidized beds, the principle on bipolar charging of different sizes particles and the relationship between particle size distribution and chage distribution in fluidized beds. The first one could interprete how to generate and accumulate static charges with band level model. By developing the physical model realating the effective work function of particles, particle diameter and relative permittivity, the principle on bipolar charging of different sizes particles showed that the effective work function of different sizes particles with similar materials would decrease with the increase of particle diameter, therefore, the polarity of the charge on the different sizes particles would be opposite after contacting each other and separating. If the transferable charge carriers were electrons or negative ions, the coarse particles would charge positively, and the small particles would charge negatively. While the transferable charge carriers were positive ions, the coarse particles would charge negatively, and the small particles would charge positively. Finally it was proposed that due to the axial particle size distribution in fluidized bed, there would be as well a static charge distribution in it and the electric field would be not uniform.2. An on-line measurement technique, which consists of the electrostatic probe, signal transmitter and data acquisition systems, was developed to study the electrostatic characters in gas-solid fluidized beds. By measuring the electrostatic potential at different bed height, it was found that the electric field inside the bed was non-uniform. The axial profiles of potential were Z-shaped, and the voltage polarity would reverse near the interface between dense and dilute phases. The axial height where voltage polarity reversal happened would increase with the increase of gas velocity and static bed level. Comparing the fluidization of particles with different sizes, it was discovered that the potential polarity would change significantly when there was a obvious interface between dense and dilute spaces, and the converse was not true. The experimental phenomena mentioned above could be explained plausibly combining the physical model for charge generation and accumulation with the principle on bipolar charging of different sizes particles.3. A new method to determine bed level basing on the characteristics of static potential in fluidized beds was proposed. The results showed that bed level could be successfully predicted with the maximum relative error 4.08% and the mean relative error 2.02% through the method in this article. The novel electrostatic approach measuring the bed level in gas-solid fluidized beds could replace theγ-ray in future, and monitor electrostatic potential in the bed and bed level simultaneously.4. A new method to detect sheeting basing on the triboelectric current was investigated preliminarily. The results showed that sheeting on the column wall and distributor would influence the mean values and standard deviationof triboelectric current. By detecting the triboelectric current at different radial and axial locations of the column, the position of sheeting could be found out approximately. Triboelectric current had sensible reflection on the sheetings both at wall and distributor and was promisingly to be a novel way to detect sheetings in the fluidized bed reactor.