UBC Theses and Dissertations
Effect of operating parameters and particle properties on electrostatics in gas-solid fluidized beds Moughrabiah, Wajeeh O.
The influences of operating pressure, temperature and gas velocity on electrostatics in a fluidized bed of glass beads and different grades of polyethylene resin were investigated in a fluidization column of 150 mm inner diameter and 2.0 m height. Eight collision probes at different levels and radial positions measured the electrostatics in the bed. The electrostatics increased as pressure increased, probably due to an increase in bubble rise velocity, frequency and volume fraction. As the pressure increased, particle-particle and particle-wall collisions near the distributor and wall contributed heavily to charge generation. Temperature also played a role. At higher temperatures (up to 90°C), the polarity of net cumulative charge in the bed reversed. As the superficial gas velocity increased, the electrostatics increased. However, at higher gas velocities, the polarity in the freeboard was opposite to that in the bed, indicating that fines entrained from the column carried charges, resulting in a net charge of opposite polarity to that inside the bed. For Geldart group B particles the degree of electrification in the bed slightly increased with decreasing particle size. Charging for group A particles was significantly greater than for group B particles. For binary mixtures of group A and B particles the electrostatics increased as the proportion of small particles increased. As the relative humidity (RH) of fluidizing air increased, the electrostatics decreased. For the RH range (5-30%) explored, the sensitivity of the charging to RH varied significantly depending on the location of the probes. As the proportion of fine glass beads (<30 µm) increased to 2.0 wt% in a fluidized bed of large glass beads (574 µm), the electrostatics in the bed decreased, likely because the fines acted as spacers between larger particles. The electrostatics decreased as the proportion of an antistatic agent (Larostat) increased from 0.0 to 0.5 wt%, because Larostat tends to adsorb moisture and attach to the surface of the glass beads, consequently enhancing their surface conductivity. However, the degree of electrification increased when the wt% of Larostat exceeded 1.0%, likely due to the tendency of Larostat particles to adsorb water and to agglomerate.
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