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Electrostatic charging in gas-solid fluidized beds Park, Ah-Hyung Alissa

Abstract

Electrostatic charges in fluidized beds, generated through various mechanisms such as triboelectrification, ion collection, thermionic emission, and frictional charging, can cause particle agglomeration and hazardous electrical discharges. In this study, electrostatic charging in gas-solid fluidized beds was investigated by means of a simplified mechanistic model and bubble injection and free bubbling experiments in twodimensional and three-dimensional fluidized beds. An electrostatic ball probe measured static charges in fluidized beds, while an optical fiber probe was used to determine the simultaneous movement of the bubbles. Preliminary experiments indicated that a negatively charged object results in a negative voltage peak followed by a positive voltage peak. The opposite pattern occurred for a positively charged object. The current calculated from the voltage output was integrated to estimate the charge induced and transferred. It was shown that the voltage signal can be considered to consist of two components: induced voltage and that due to the direct charge transfer between charged particles and the probe. A simplified model was also developed by applying the method of images to distinguish induced and transferred charges. Spherical bubbles surrounded by a monolayer of charged particles and a medium of dielectric constant 1 were assumed in the models. Bubble injection experiments in a 2-D bed showed that both 321 pm glass beads and 378 pm polyethylene particles were charged positively, while 318 urn polyethylene particles were charged negatively. As bubble size increased the charge induced and transferred increased accordingly. The model gave reasonable predictions of the charge output. Increasing the relative humidity of the fluidizing air between 60% and 80% reduced the electrostatic charge accumulation by increasing the surface conductivity, enhancing the rate of charge dissipation. Adding group C fines and Larostat 519 reduced or eliminated particle buildup on the inner wall of the fluidization column, but the former probably also affected other interparticle forces such as Van der Waals forces and altered the fluidization behaviour. 1 wt% Larostat 519 clearly reduced electrostatic charge accumulation during free bubbling in a 3-D bed of 318 urn polyethylene particles; within 1.5 hr charge accumulation decreased to an insignificant level.

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