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Thesis (Ph. D.)--Drexel University, 2007. / Includes abstract and vita. Includes bibliographical references (leaves 178-183).

Thesis (B.S.)--University of Wisconsin--Madison, 1965. / eContent provider-neutral record in process. Description based on print version record. Bibliography: l. 34.

Thesis (Ph. D.)--Cornell University, May, 2007. / Vita. Includes bibliographical references.

In gas-solid fluidized bed reactors, such as those employed for polyethylene production, the generation of electrostatic charge is almost unavoidable. Electrostatic charges are generated due to the continuous contacts between particles and particles and the reactor wall. In such processes, accumulation of electrostatic charge causes a layer of particles to adhere to the reactor wall, a problem known as “sheeting” in polyolefin industry. Sheeting results in frequent reactor shutdowns for clean-up and in turn significant economic loss. The overall focus of this research is to better understand the underlying mechanisms of charge generation in gas-solid fluidized beds to ultimately be able to find means to reduce or eliminate this problem. The specific objective of this thesis is to determine the effect of fluidizing particle size on the degree of bed electrification and reactor wall coating. The experimental program involved the fluidization of polyethylene resins received directly from commercial reactors (i.e., having a wide size distribution of 20-1500 micron), as well as mono-sized large particles (600-710 micron) and binary mixture of small particles (200-300 micron and 300-425 micron with fractions up to 20 wt%) and large particles (600-710 micron). Experiments were carried out under atmospheric conditions in 3D fluidization columns housing two Faraday Cups for electrostatic charge measurement. For all conditions, the charge, mass and size distribution of particles fouled on the reactor wall as well as the layer thickness were measured and compared. Fluidization of the resins as received resulted in a certain size of particles (400 µm and smaller) to adhere to the column wall. For binary mixtures, the particles layer formed on the reactor wall mainly consisted of the smaller particles. Although the extent of wall coating declined as the amount of the smaller particles increased, but the smaller particles had a much higher net specific charge and thus replaced the large particles within the wall coating. Such high charge of small particles accumulated on the column wall in turn prevented the wall coating growth due to repelling the oppositely charged particles to the bulk of the bed. Regardless of the charge polarity of the bulk and wall particles, the wall fouling formation mechanism was found to be similar. Between the two sizes of small particles tested, the 212-300 micron particles gained a higher net specific charge than 300-425 micron particles. Bipolar charging due to small and large particles contacts was detected within the bulk of the bed and the wall coating.

Solar energy installations located in dry, arid regions chosen for their availability of sunlight often face the problem of dust accumulation, or "soiling", on their solar collector surfaces, requiring this dust to be cleaned regularly in order to maintain optimal power production. The electrodynamic screen (EDS) is a technology that can clean this dust off the surfaces of photovoltaic panels and concentrating solar power mirrors using no water and a minimum of power. The EDS is a series of conductive electrodes embedded between two thin dielectric layers, where voltages applied across the electrodes create a patterned electric field which directs the motion of charged dust particles off the EDS surface. As the dust in different desert regions across the world have different physical characteristics, a different set of design parameters is required for the optimal EDS for each region. This optimization work could be easily conducted using a computer model of the physics of an EDS and the dust it clears off its surface. In this thesis, a computer model of the EDS system is created using COMSOL Multiphysics. This model simulates the voltages applied across the electrodes and the resulting electric fields, and then use these to simulate the trajectories of the charged dust particles as they move across the EDS surface. This particle tracing work is validated using experimental data from high-speed camera trials and performance data for different EDS designs. This validation work shows both agreements and disagreements between the predicted and observed dust particle motion, and the beginnings of the investigation into this difference is presented. Finally, the ability to further develop this model for EDS design optimization is discussed.

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Electrostatic charging is a widely known natural phenomenon that has been observed sinceancient times. This phenomenon is also reported in solids handling and processing industrieswith varying extent of its interference in established processes and operations. Particlecollisions lead to electrostatic charge generation through triboelectrification. Sustainedcharging leads to particle agglomeration and adhesion, or even explosions. Hence, the presenceof charged particles are seen as a hindrance and a risk in certain processes. The focus of thisthesis is directed at gas-solid fluidization and pneumatic conveying where this holds true.Polyethylene is commonly manufactured in catalytic gas-solid fluidized bed reactors. Theinsulative nature of the polymer particles, the catalyst particles and the surrounding gas set upa conducive environment for electrostatic charge generation. Charged particles adhere on tonearby surfaces forming fused masses of polymer sheets. Presence of sheets in the reactorhinders the reactor productivity, thus warranting reactor shutdown and maintenance. On theother hand, catalyst introduction into the polyethylene reactor is performed through pneumaticconveying systems. In general, solids pneumatic conveying is known to cause the largestdegree of triboelectrification among many gas-solid systems. Therefore, the charging ofcatalyst particles may also contribute to operational challenges faced by this industry.Numerous studies have attributed particle characteristics, system variables, and operatingparameters as probable sources contributing to electrostatic charging in both fluidizationprocess and pneumatic conveying systems. However, a comprehensive consensus explainingparticle charging in real-world scenarios and suitable methods to mitigate or prevent chargingstill require further investigation. Thus, desirable control of charging in affected industrialsectors is still not present.Beyond the scope of fluidization and pneumatic conveying, certain studies have investigatedthe influence of dielectric strength of gases on the charging behaviour of solids. The worksclaim that gases with low dielectric strength perform better in minimizing electrostatic chargeof solids in controlled environments due to gas discharge and subsequent charge dissipation.Thus, applicability of such gases in dynamic processes like fluidization and pneumaticconveying must be investigated in hopes of reproducing similar observations. The principalaim of this thesis was to uncover a functional method to limit charging and particle adhesionin fluidization and other solids handling systems. As a means of accomplishing this, theobjective of this thesis was directed to study the efficacy of argon, which has a low dielectricstrength, against nitrogen in reducing triboelectrification of polyethylene particles influidization and pneumatic conveying operations.A stainless-steel fluidization apparatus was used to study the charging behaviour of acommercially produced polyethylene resin at atmospheric pressure. Results were drawn forpure argon and compared against pure nitrogen. Aiming to minimize the quantity of argonwhile simultaneously retaining as much efficacy as possible, binary mixtures of nitrogen andargon were also tested along with successive fluidization trials. Pure argon resulted in 90%reduction in fouling compared to pure nitrogen. Even binary mixture of 10 vol % argon showeda reduction of 50% in fouling values. Successive fluidization resulted in fouling valuescomparable to pure argon trials. Multiple pulse pneumatic conveying was carried out in astainless-steel tube with dehydrated amorphous silica that is a commonly used catalyst base inpolyethylene process. The net specific charge of the particles and the fouling inside the tubewere smaller under argon in the first injections. Subsequent injections were not as significant.Results from the above operations were validated through bench-scale shake tests performedunder controlled gaseous environment. Single large polyethylene particle charging was firsttested in nitrogen and argon atmosphere followed by multiple smaller particles. Bench-scaleshake tests showed argon influenced the saturation charges, reducing it and reaching it earliercompared to nitrogen. However, the degree of charge and fouling reduction was not assignificant as observed in fluidization trials.The thesis concludes that argon is indeed influential in reducing particle charge and particleadhesion in applicable systems. Influence of argon was observed in all operations withfluidization exhibiting the greatest degree of reduction in charging and fouling values.Furthermore, even small quantities of argon can make a non-linear impact on said parameters.The results also suggest that the majority of gas discharge and subsequent charge dissipationoccurs in areas of considerable electric fields. These are observed to entertain large number ofparticles contact and separation, providing plenty of opportunities for gas molecules to ionize.

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