Distribution of Electrostatic Potential in a Gas−Solid Fluidized Bed and Measurement of Bed Level

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Abstract

A theory on the electrostatic charge distribution in gas−solid fluidized beds was proposed. It consists of an interpretation of the causes of bipolar charging, the charging mechanisms in the fluidized bed, and the relationship between charge distribution and particle-size distribution in fluidized beds. Experiments were performed in a three-dimensional column gas−solid fluidized bed to measure the electrostatic charge distribution. It was found that the electric field inside the bed was nonuniform. If there was a distinct interface between the dilute and dense phases, the voltage polarity would reverse near the bed level, resulting in a Z-shaped axial profile of potential. The heights where voltage polarity reversal happened rose with the increase of gas velocity and static bed height. It was also found that the electrostatic voltage rose with increasing radial distances from the axis of the column. To sum up, the voltage at any axial cross section of the bed showed double saddles, with bed level as the interface. Accordingly, three special zones to be emphasized were identified: distributor, stagnant, and bed level zones, of which the latter two have high voltages and are more readily disturbed by particle/wall adhesion and even wall sheets. Based on the characteristics of axial profile of electrostatic potential in the fluidized bed, a new technique that could successfully predict bed level was developed. Good agreement was observed between visual measurements of bed level and predictions by the electrostatic method, with maximum relative error of 4.08% and mean relative error of 2.02%.

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