Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles

Abstract

Experiments were conducted in an 89 mm inner diameter Plexiglas fluidized bed with polyethylene resin particles to study the local instantaneous electrostatic charges using a miniature collision ball probe installed inside the column. Local differential pressure fluctuations were also measured to monitor the local fluidization quality. The power spectrum and probability density distributions from both the differential pressure fluctuations and the local instantaneous electric voltage signals were similar, confirming that the amplitude of voltage signals from a ball probe is mainly induced by passing bubbles. The standard deviation of voltages from the ball probe increased with increasing superficial gas velocity, probably due to an increase in the charge density on particles surrounding the rising bubble and increasing bubble rise velocity. The net charge transfer rate, however, showed no clear change with increasing superficial gas velocity because of the balance between the increase in particle velocity and the decrease in dense phase fraction. As the relative humidity increased at a given gas velocity, the standard deviation of the voltage signals and the differential pressure fluctuations both decreased, indicating that the charge buildup in the vicinity of the bubble decreased as the bubble size decreased. The net charge transfer rate, however, increased with increasing relative humidity at RH>50%, likely due to an increase in the charge dissipation rate due to increased electrical conductivity of the fluidizing media. At relative humidities below 50%, the net charge transfer rate was insensitive to the relative humidity because the increase in conductivity was offset by the decrease in charge buildup on particle surfaces. The addition of antistatic powders was effective in reducing the electrostatic charge buildup in the polyethylene resin bed. One hour after the addition of 1 wt% Larostat powders, both the standard deviation of voltage signals and the net charge transfer rate were reduced to insignificant levels.