Charge transfer and bipolar charging of particles in a bubbling fluidized bed

Abstract

Particle–particle and particle–wall collisions in gas–solid fluidized beds lead to charge accumulation on particles. This work evaluated the effect of fluidization time on charge transfer and bipolar charging (charge separation) and their influence on hydrodynamic structures in a fluidized bed. Experiments were performed with glass beads and polyethylene particles in a glass column. The pressure fluctuations and net electrostatic charge of particles were measured during fluidization. Wavelet and short-time Fourier transforms were used to analyze pressure fluctuations. The results revealed that bipolar charging is the dominant tribocharging mechanism in a bed of glass beads. Bipolar charging in a bed of particles with a narrow size distribution does not affect either hydrodynamic structures or the transition velocity to the turbulent regime. A large difference between the work functions of the wall and particle in the bed of polyethylene particles leads to high charge transfer. Formation of a stagnant particle layer on the wall eventually causes the energy of macro-structures to increase to its maximum. At longer fluidization times, the macro-structural energy decreases and bubbles shrink until the electrostatic charge reaches the equilibrium level. These results well describe the effect of fluidization time on hydrodynamic structures.