Probing the electrostatics and hydrodynamics in gas-solid fluidized beds

Novel electrostatic dual-tip probes, combined with suitable signal analysis methods, were developed for in-situ measurement and monitoring of particle charge density levels and bubble properties inside gas-fluidized beds. The probes were calibrated in several particulate flow devices : ejector-funnel, motor-pulley, vertical tube and vibration tray setups, as well as a two-dimensional fluidized bed. The effects of particle charge density, solid flux, particle velocity and angle of impact on the transferred current received by the probe from charged particles were quantified. For dual-tip (two-material) probes, substantial differences were observed in the signals from the two tips made of different materials, arising mainly from charge transfer and depending on the hydrodynamics and charge density inside the bed. The probes were deployed with glass beads and polyethylene particles for both single bubble injection and freely bubbling experiments in two- and three-dimensional fluidization columns of different scales. Statistical and Fast Fourier Transform analysis showed that current signals were strongly affected by the local hydrodynamics in the fluidized bed. The amplitudes of current signal peaks, peak frequencies, as well as mean and standard deviations of the current increased with increasing superficial gas velocity. Local particle charge density and bubble behaviour were estimated by a signal processing procedure with decoupling methods. The probes were tested in steady state experiments, as well as in dynamic experiments by abruptly changing the superficial gas velocity or adding antistatic agent. Both particle charge density and bubble rise velocity obtained from the probes were of the same order of magnitude and followed similar trends as those directly measured by a Faraday cup and video images, respectively. The electrostatic probe signal was found to not always be consistent with the charge polarity and charge density on the particles. The probe signals and particles charge densities may have different polarity and relative magnitudes for different operating conditions and particle properties : density, mean size and size range, dielectric constant, sphericity, roughness and hydrophobicity. Particles with narrow size distribution and larger mean size generated higher charge densities. The novel probe has potential for in-situ monitoring electrostatic charges and hydrodynamic behaviour in gas-solid fluidized beds.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat