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Characterization of electrostatic charges in gas-solid fluidized beds Mehrani, Poupak


A novel on-line measurement technique was developed in this work based on the Faraday cup method by constructing a copper fluidization column of diameter 0.1 m as the inner cup and a second surrounding copper column as the outer cup to gain better understanding of charge generation inside gas-solid fluidized beds. Net charges generated inside fluidized beds were investigated for relatively large glass beads (566 p.m mean diameter) fluidized by extra dry air. It was concluded that particle-gas contacting had negligible effect on the particle charging mechanism for the conditions studied. Also, air ionization is expected to have played a negligible role with respect to dissipation of charges on the particles. Free bubbling fluidization of mono-sized and binary mixtures of particles consisting of relatively large glass beads (566 urn mean diameter) and fine glass beads (30 urn mean diameter) showed that the net charges generated inside the fluidized bed were caused by entrained charged fine particles from the fluidization column. The effect of adding different varieties of fine (<45 urn) particles (Larostat 519, glass beads, silver-coated glass beads, a catalyst and silica) on charge generation/dissipation inside beds of the relatively large glass beads and 558 um polyethylene particles was studied by investigating the change of the electrostatic behaviour of fines after their addition to the fluidized bed. It was found that fine Larostat 519, two types of glass beads and two types of silver-coated glass beads carried positive charges out of the fluidized bed of relatively coarse glass beads at different relative humidities of the fluidizing air (0, 15, 35 and 60%). Comparison of charge-to-mass ratios of different fines showed that the finer the particles, the higher the charges carried per unit mass. The Larostat fines helped to dissipate the initial bed charges by attaching themselves to the large glass beads. It was found that the higher the surface conductivity of the fines, the easier it was for them to lose their charges to the column walls, thereby dissipating the initial bed charges. As the relative humidity of the fluidizing gas increased, the charge-to-mass ratios decreased, as expected. Free bubbling fluidization of binary mixtures of fines (Larostat 519, catalyst, silica and silver-coated glass beads) with relatively large polyethylene particles showed that the polarity of the charges transported out of the fluidized bed depended on the relative humidity of the fluidizing gas. It was concluded that the relative humidity of the fluidizing gas can affect the bed material (polyethylene particles) and/or the electrical behaviour of added fines. Fine catalyst and silver-coated glass beads behaved similarly, probably due to having high surface electrical conductivities. Charge-to-mass ratios were higher for the catalyst and silica particles than for the other fines. Observations after fluidizing the binary particles mixtures confirmed that there were fewer polyethylene particles clinging to the column walls when Larostat 519 and silver-coated glass bead fines were present. Bi-polar charging was also investigated. For both the coarse glass beads and polyethylene particles tested, smaller particles were charged positively and larger particles negatively. Different fines charging mechanisms, charge transfer and charge separation between the fines and the coarse particles, as well as the column wall, and their significance were investigated. For different added fines, different leading charging mechanisms were determined. The fines charging mechanisms considered in this study included particleparticle, as well as particle-wall, interactions. The latter were important here because the fluidization column in this study was of laboratory scale, so that particle-wall contacts were significant. In industrial-scale units, particle-particle interactions are likely to be dominant. Such factors as the material, physical and chemical surface properties of the solid phases, as well as the moisture content of the fluidizing gas are also important. Overall fines added to an initially charged fluidized bed carry significant charges from the column. This is a significant finding since fines are always elutriated in fluidized bed processes. It also suggests that since electrostatic forces play a role in determining the flux of entrained fines from a fluidized bed, they should be incorporated into models developed to predict entrainment flux and, perhaps also, transport disengagement height.

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