| Gas-solid fluidized beds are widely applied in numerous industrial processes, such as coal combustion and gasification, granulation and drying, olefin polymerization, etc. The bubble-induced solids motion and circulation within the bed lead to good contact and mixing of gas and solid phases, and high rates of heat and mass transfer. Electrostatic charge generation and accumulation on insulated particles are almost unavoidable due to repeated particle-particle and particle-wall frictions in the polyolefin gas-solid fluidized bed. An excess charge buildup can cause problems such as particles adhesion and wall sheeting, which could lead to loss of fluidization or emergency shutdown of industrial reactors. To reveal the generation mechanism of electrostatic charges, or to avoid the adverse effects of electrostatics outlined above, accurate and effective measurement techniques of electrostatic charges are indispensable. Two general methods have been mainly used to measure electrostatic charges in gas-solid fluidized beds, namely the electrostatic probe and the Faraday cup. The Faraday cup is an off-line measurement tool, and it is unable to monitor electrostatic charges online and continuously. The electrostatic probe is always intrusive and interferes with the flow field to some extent. Furthermore, it is quite difficult to interpret the complex electrostatic signals registered by the probe. Therefore, it is of vital importance to develop an online and non-intrusive measurement technique to monitor particles charge-to-mass ratios inside the fluidized bed. A suitable charge density measurement tool can identify potential risks earlier and prevent hazards due to the excess buildup of electrostatic charges.Electrostatic signals generated by charged particles motions contain useful information on charge levels in the fluidized bed coupled with local hydrodynamics, which are poorly understood and interpreted. Measurement of particle motions by cross-correlating electrostatic signals derived from a pair of axially spaced electrostatic sensors has been extensively investigated and successfully applied in pneumatic conveying pipes. However, this method has not been reported or studied in gas-solid fluidized beds. If monitoring of particle motions by electrostatic sensors combined with cross correlation method could be implemented in the fluidized bed, it is potential to provide a new way to characterize particle motions inside the fluidized bed and decouple hydrodynamics from electrostatic variations. To accomplish the online monitoring of key parameters related to particle motions in the fluidized bed reactors is significant and essential to enhance stable operation and optimal control of industrial processes.This work conducted a systematic comparison and interpretation of the similarity between electrostatic current and pressure drop, and established the quantitative relation between these two signals. Based on this relation, an online measurement technique of particles charge-to-mass ratio by ring-shape induced electrostatic sensors was proposed. By combining electrostatic sensors array with cross correlation method, the correlation velocity of particle clouds in the fluidized bed has been measured. Feasibility and repeatability of this method applied in the monitoring of particle motions in the fluidized bed have been analyzed and verified. The influence of charge levels on particle motions were systematically investigated by measuring the correlation velocities of Geldart B and D particles. A prediction model of particles charge-to-mass ratio was regressed based on the induced electrostatic voltage signals and average correlation velocities under various charge levels. The accuracy of this model to predict particles charge-to-mass ratios under different superficial gas velocities was validated. The online instrumentation of particles charge-to-mass ratios and monitoring of particle motions in the fluidized bed is significant and beneficial to assist in the prevention of charge buildup, measurement of flow parameters and stability enhancement of industrial reactors. Research work has been conducted in the following aspects:1. The similarity of electrostatic current and pressure drop was systematically compared and analyzed by spectral analysis in the self-designed multistage Faraday cup fluidized bed. The quantitative relation between these two signals was established. It was found that variations of both electrostatic current and pressure drop were simultaneously affected by the average voidage. When it was assumed that particles possessed the same volume and identical charges, the electrostatic current was proportional to the first-order derivative of pressure drop. The proportionality coefficients were related to the particles charge-to-mass ratio and dimensions of the fluidized bed.2. Based on the quantitative relation mentioned above, an online measurement technique of particles charge-to-mass ratio by ring-shape induced electrostatic sensors was proposed. Results showed that particles average charge-to-mass ratios measured by electrostatic sensors increased with the superficial gas velocity, and decreased with increasing content of liquid antistatic agent (LAA), which showed the same tendency with those measured by Faraday cup. The relative error of electrostatic sensor with a width of 20 mm was within 23%. The feasibility and accuracy of this online and non-intrusive measurement technique of particles charge-to-mass ratio was verified.3. The feasibility of monitoring particle motions by electrostatic sensors combined with cross correlation method in the fluidized bed was explored. The correlation velocities of Geldart B and D particles were measured, and the average and probability density distribution of correlation velocities were compared. In the dense-phase region of the fluidized bed, the average correlation velocity of particle clouds increased consistently with theoretical bubble rise velocity with increasing superficial gas velocity. Under the same excess gas velocity, the average correlation velocity of Geldart D particles was smaller than that of Geldart B particles. A measurement technique of dynamic bed level of fluidized bed was proposed based on the electrostatic sensors array and the distribution of correlation velocity distribution with a relative error no more than 10%.4. An online method to accomplish simultaneous measurement of particle motions and particles charge-to-mass ratio was established based on electrostatic sensors array. The charge levels in the fluidized bed were controlled by injection of LAA and the influence of charge levels on particle motions was systematically investigated. It was found that with the decrease of charge level, the average correlation velocity of particle clouds increased and the probability density distribution of correlation velocities broadened. When the excess gas velocity was 0.35 m/s, the average correlation velocities of Geldart D and B particles were found to be decreased by 26% and 50%, respectively, when particles were fully charged. The prediction model of particles average charge-to-mass ratio was regressed based on the induced electrostatic voltage signals and average correlation velocities under various charge levels. The predictive results of this model and the results measured by Faraday cup showed the same tendency under different superficial gas velocities. The relative error of this model was less than 40%. |
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