Gas-solid fluidized beds are widely applied in numerous chemical processes such as coal combustion and gasification, granulation and drying, olefin polymerization, etc. The vigorous particle motions and circulation in fluidized beds provide good mixing and contact of gas and solid phases, which thus results in efficient heat and mass transfer. Particle velocity is a key parameter to characterize particle motion and flow pattern, which further affects the heat and mass transfer, and the performance of the fluidized bed. Therefore, the measurement of particle velocity is significant for the fluidization quality monitoring, proper design and scale-up of fluidized bed reactors. Several experimental techniques have been employed to measure the particle velocity in fluidized beds, such as positron emission particle tracking technique (PEPT), particle image velocimetry (PIV), laser doppler velocity measurement (LDV), fiber optical probe, etc. However, due to the addition of tracking particles, limited applications in apparent apparatus, single point measurement and invasion of flow field, current techniques cannot be used directly in industrial reactors or large bench-scale fluidized beds to quantitatively characterize particle motions.
In olefin polymerization fluidized beds, insulated particle-particle and particle-wall contacts and frictions lead to the generation and accumulation of electrostatic charges. Electrostatic charges are found to be significantly influenced by hydrodynamics. Consequently, the variation of electrostatic signals contain rich dynamic information related to bubble behaviors and particle motions. Hence, combined with the cross correlation method, electrostatic signals have been utilized to measure particle velocities in dilute gas-solid systems such as pneumatic conveying pipelines, risers and downers of the circulating fluidized beds. However, particle velocity measurement in dense phase gas-solid fluidized beds based on electrostatic signals has not been implemented so far.
This work makes a first attempt to measure the particle velocity in the dense fluidized bed by electrostatic technique combined with the cross correlation method. Electrostatic sensors with a width of 6 mm and an arc of 60° were installed on the outside wall of the experimental fluidized bed at different axial positions. Induced electrostatic voltage was measured by the electrostatic sensor array and further analyzed by the cross correlation method. Since the interval between two sensors was fixed, particle correlation velocity could be calculated. Results demonstrated that there existed a remarkable correlation between induced electrostatic voltages measured by adjacent sensors, which is similar to that in dilute gas-solid systems. Below the dynamic bed level, particle correlation velocity increased with the axial height. With the superficial gas velocity increasing, particle correlation velocity increased and the probability density distribution of particle correlation velocity broadened. Both the upward and downward motions of particles could be identified near the dynamic bed level, resulting a particle correlation velocity distribution with two peaks. Moreover, the ratio of particle correlation velocity and bubble rising velocity at a certain height was in the range of 0.4-0.7 in the bubble growing zone of the fluidized bed, which is similar to the experimental results obtained through PEPT reported by Stein et al.. Therefore, the particle motions monitoring by electrostatic technique combined with the cross correlation method is very promising in the dense gas-solid fluidized bed and may be utilized to quantitatively characterize particle motions and fluidization quality.
Key words: particle motions, electrostatic signal, cross correlation method, online monitoring, gas-solid fluidized beds
 Stein, M., Y.L. Ding, J.P.K. Seville, and D.J. Parker, Solids motion in bubbling gas fluidised beds. Chemical Engineering Science, 2000. 55(22), 5291-5300.