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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 μm 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.

Electrostatic precipitators are high efficiency gas cleaning devices widely used in industry for removing particulates from process gases. A major factor affecting their performance is particle migration, which is governed by the complex interaction of electrical and hydrodynamic phenomena. A fuller understanding of these fundamental mechanisms is therefore essential to the development of realistic mathematical models. The work described in this thesis concentrates on the fluid-particle interactions in a wire-plate-system. A pilot-scale rig was built using actual components from an industrial precipitator, allowing realistic conditions to be simulated in the laboratory. Hot-wire anemometry and laser-Doppler photon correlation techniques were employed to study the time-averaged velocity field. Several designs of wall strengthener were considered, and in each case the effect on the surrounding flow field was investigated using helium bubble visualisation. The turbulent nature of the fluid was characterised by local dispersion coefficient values and fluctuating velocity components. Alumina test dust in the size range 1-10 pm was used in the precipitator under a variety of operating conditions, and a technique was established for extracting representative dust samples. The samples allowed simultaneous measurement of concentration and size distribution, from which concentration profile development and collection efficiency information was obtained. Two alternative numerical models of the precipitator were developed, both incorporating the results from the fluid flow field experimentation. The first approach was based on the finite difference solution of the convective-diffusion equation, using appropriate boundary conditions. In the second approach, the transport of dust down the precipitator duct was simulated by the step-wise progression of a series of vertical line-sources, whose motion was governed by electrical migration and lateral diffusive spread. The validity of the models was tested by comparison of the predicted concentration profiles with corresponding experimental results.

Thesis (Ph.D.)--University of Texas at Dallas, 2008. / Includes vita. Includes bibliographical references (leaves 105-110)

Thesis (Ph. D.)--University of Wisconsin--Madison, 1967. / Vita. Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (p. 230-238).

Thesis (Ph. D.)--Drexel University, 2005. / Includes abstract and vita. Includes bibliographical references (leaves 144-156).

The study explored misconceptions in electrostatics and their origins amongst learners at entry point in a South African University. Available literature showed misconceptions in electrostatics amongst High School learners and confirmed textbooks as one of the sources of misconceptions. It was therefore important to look for misconceptions in electrostatics amongst first year Bachelor of Science (B.Sc 1) learners in physics courses and their origins at the start of the academic year. The study also explored educators' misconceptions in the topic to check whether they could also be a source of learners' misconceptions. The results were intended to give guidance on how to eliminate learners' misconceptions at school rather than carrying them to higher education institutions. The study used the ex-post facto research design and was a case-study. The ex-post facto research design enabled the researcher to investigate whether one or more pre-existing conditions have possibly caused the existing problem of misconceptions. The sample consisted of 198 learners from B.Sc 1 physics course and 28 educators from 15 High Schools in one education district in the Eastern Cape Province of South Africa. The data were collected through questionnaires, analysis of textbooks, and interviews. The Statistical Package for Social Sciences (SPSS) version 17 was used for quantitative analysis whereas categorization and coding were used for qualitative analysis. The study revealed that learners had misconceptions in electrostatics. The origins of misconceptions were traced to educators, textbooks, intuition, daily language and lack of hands-on activities. It emerged from the study that educators also had misconceptions and the cause of their misconceptions were textbooks, websites and gaps in content knowledge. The recommendations from the study were the following: identify preliminary knowledge of learners during introduction of the lesson; introduce the iii constructivist approach to teaching in the teacher training curriculum so that learners at school can be taught using the same approach; frequent upgrading of educators through inservice workshops to reduce educators' misconceptions which, in turn, will help to reduce the misconceptions among learners; introduction of conceptual change textbooks.