Electrostatic Charge Generation and Wall Fouling in a High-Pressure Gas-Solid Fluidized Bed: Implementation and Preliminary Testing of a Measurement Technique.

摘要

Due to the nature of gas-solid fluidized beds, providing continuous contacts between fluidizing particles and between particles and the reactor wall, the occurrence of electrostatic charges is unavoidable. In the polyethylene industry, electrostatics is a major problem. Large amounts of electrostatic charges are generated causing polyethylene and catalyst particles to adhere to the reactor wall, forming sheets. Particle sheets can break off and block the distributor plate, causing long shutdown periods for clean-up which result in economic loss due to decreased production and higher maintenance costs. The overall purpose of the project of which this thesis is part of is to help industry in minimizing this problem by examining the mechanisms underlying this phenomenon.;Towards this goal, an experimental technique for the measurement of the degree of wall fouling and its charge distribution was previously developed and implemented in an atmospheric system with a column of 0.102 m in diameter. This technique was extended in this thesis to a pilot-scale unit (0.154 m in diameter) designed to be capable of operating at pressures and temperatures up to 2 600 kPa and 100°C respectively and gas velocities up to 1 m/s, which are operating conditions of industrial polyethylene reactors. Preliminary experiments showed that increasing the operating pressure from 101 kPa to 401 kPa almost doubled the amount of polyethylene wall fouling due to the higher bubble rise velocity at this pressure, enhancing charge generation within the fluidized bed. Changing the particle size distribution by removing particles smaller than 250 microm had no significant effect on the extent of the wall fouling. Increasing the column diameter from 0.102 m to 0.154 m decreased wall fouling due to the lower column wall area per mass of particles. Overall, particle-particle contacts generated positively and negatively charged particles, but did not produce a net charge in the bed due to the negligible elutriation. However, particle-wall contacts produced a net charge. The formation of the wall layer was due to the image force created by the net charge and the layering effect created by the attraction between oppositely charged particles.