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Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xxvii, 334 p.; also includes graphics (some col). Includes bibliographical references (p. 322-334). Available online via OhioLINK's ETD Center

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Thesis (M.S.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

The triboelectrification of particles by contact or frictional charging is known to be an operational challenge in the polyolefin industry. Particularly in polyethylene production, gas-solid fluidized bed reactors are known to be susceptible to electrostatic charging due to the rigorous mixing of polyethylene and catalyst particles in a dry environment. The presence of charged particles coupled with a highly exothermic polymerization reaction results in sheet formation on the reactor walls. This behaviour can decrease reactor performance and obstruct the system, consequently forcing a shutdown for reactor maintenance. The generation of electrostatic charge in fluidized beds has been widely studied throughout the years; however, limited attention has been paid to the simulation and modeling of this phenomenon. Since it is difficult to accurately quantify the charge generation in industrial fluidized beds, developing an electrostatic model based on material properties would considerably aid in providing insight on this occurrence and its effects. A computational fluid dynamics (CFD) model that incorporates this electrostatic model can then be used as a predictive tool in research and development. Simulating electrostatic charging in gas-solid fluidized beds would be a cost-effective alternative to running experiments on them, especially for industrial-scale test runs. In this thesis, an electrostatic charging model was developed to be used in conjunction with an Euler-Euler Two-Fluid CFD model to simulate triboelectrification and its effects in gas-solid flows. The electrostatic model was first established for mono-dispersed gas-particle flows and was validated using past experimental findings of particle charging for gas-solid fluidization runs. With the goal of providing a realistic representation of gas-solid fluidization of polyethylene resins with a wide particle-size distribution, the electrostatic model was extended to consider bi-dispersed particulate flow systems. Simulation results using this model show the prediction of bipolar charging when the particles have different sizes, even though they are made of the same material. This phenomenon is analyzed and is shown to be driven by the electric field produced by the charge accumulated on the particles. Experimental studies of particle-wall and particle-particle contact charging were performed to investigate the electrostatic and mechanical parameters that are crucial for modeling the magnitude and direction of charge transfer in gas-solid flow systems. Particle-wall contact charging due to single and repeated collisions were tested with various particles, including commercial linear low-density polyethylene, to determine their rates of charging as well as their charge saturation limits when colliding with a metal surface. Plotting the charge saturation value of the particles against their respective surface areas revealed a linear trend which could be used to calculate the charge saturation of the particle for a given particle size. Additional particle-wall charging studies include the effect of initial charge, collision frequency, particle type, impact angle, impact velocity and the presence of impurities on particle charging. To study particle-particle contact charging, a novel apparatus was designed, built, and tested to determine the magnitude and direction of charge transfer due to the individual particle-particle collisions of insulator particles. This apparatus was the first of its kind, and it ensured that the measured charge transfer for each experimental trial was solely due to the binary collision between the particles. It was observed that the direction of charge transfer in identical particle collisions is not dictated by the net initial charges of the particles, but the localized charge difference at the particles’ contacting surface. Moreover, particle-particle collisions of nylon particles of varying sizes confirmed the bipolar charging phenomena, where the direction of charging was dictated by the relative size of the colliding particles. These findings, among others, contradict the charge transfer behavior predicted by electrostatic charging models currently proposed for particle-particle collisions. As such, it was concluded that an empirically accurate charge transfer model needs to be established to simulate the electrostatic charging of particles in poly-dispersed gas-solid flow systems.

This work explored the viability of two solid adsorbents, limestone and cement powder, for use in a flow-through packed-bed column for HCl and HF gas neutralization following refrigerant destruction. Neutralization tests performed at 408 K using 5% HCl in N2 and 5% HF in N2, showed that limestone had a significantly higher adsorption capacity for both HF & HCl, future tests therefore utilized limestone only. The results showed that ~49% of the fed HCl and between 7.8% - 16.2% of the fed HF gases were adsorbed by 0.007 kg of limestone for a 6.67×10-6 m3/s (STP) flow rate over 30 – 180 minutes. Applying the shrinking core model, effective diffusivities (De) of HCl & HF into the limestone particles were 1.5×10-9 & 2.2×10-9 m2/s respectively. Under these conditions, complete particle conversion times were 227 hours for HCl–limestone and 154 hours for HF–limestone. Estimating De values at plasma-reactor temperatures gave 5.61x10-9 m2/s & 8.24x10-9 m2/s for HCl–limestone and HF–limestone respectively. Correspondingly, particle consumption times were reduced to 61 and 41 hours for HCl–limestone and HF–limestone. Considering the conversion times for the 1 mm particle sizes, shorter conversion times would require micron-scale particle sizes, suitable for entrained flow but not for a packed-bed arrangement.

Fundamental studies on the mechanisms and kinetics of reduction of dense synthetic nickel oxide have been carried out in H2-N2 and H2-H2O mixtures. The influences of temperature, hydrogen partial pressure, and hydrogen-steam ratio on the reduction process were systematically investigated. The kinetics of the reduction process were followed metallographically by measuring the advance of the nickel product layer. The microstructures of the reduction products and their changes during heating were characterized using a high resolution scanning electron microscopy. In H2-N2 mixtures and H2-H2O mixtures with low steam content, it was found that the initial reduction rates were first order with respect to hydrogen partial pressure. In both sets of mixtures, it was found that the progress of NiO reduction was not a monotonic function of temperature. A minimum rate of advancement of NiO reduction was observed in the temperature range 700oC – 800oC depending on the hydrogen partial pressures and reduction time. A number of distinctly different nickel product microstructures, originating at the Ni-NiO interface were observed under various reduction conditions, namely coarse fibrous nickel with fissures, fine porous nickel-planar interface, large porous nickel-irregular interface and dense nickel product layer. The microstructures of reduction product were found to change with temperature and time. It was found that heating the coarse fibrous nickel structure resulted in a re-crystallization, grain growth and densification of nickel product. When the heat treatments were carried out on the porous nickel structures, the number of pores decreases with increasing temperature and time, which was accompanied by the increase in the pore sizes. The microstructures and kinetics of the reduction of nickel oxide were found to be a function of temperature, gas composition and reaction time. The study provides strong evidence for a link between the reduction kinetics and the changes in the reduction product microstructures. Mechanisms and kinetics of the reduction of nickel oxide have been discussed by considering reduction conditions, information on the structures and elementary processes involving in the reduction process.

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 24, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Jerry L. Atwood. Vita. Includes bibliographical references.

The overall objective of the work described in this Dissertation was to develop and verify a general reaction and diffusion model for non-catalytic reactions between gases and porous solids, particularly those relevant to the clean use of fossil fuels. Here, the internal pore structure of the solid was characterised by observing the kinetics in a regime limited only by intrinsic chemical reaction. It was hypothesised that a simple arbitrary function, f(X), determined from experimental measurements of rate vs. conversion in a kinetically-controlled regime, could be used in place of formal, mathematical pore models, to describe the evolution of pore structure during a reaction influenced by intraparticle mass transfer. The approach was used to study (i) the gasification of chars by CO2, where the only product was gaseous, (ii) the calcination of CaCO3 cycled between calcined and carbonated states, where the products were a gas and a solid, and (iii) the sulphation of virgin and sintered CaO by SO2, the only product being solid. Studies of calcination showed that, at least for limestones subjected to a history of cycling between the calcined and carbonated states, a correctly-determined f(X) could be applied to different sizes of particles at temperatures different to that at which f(X) was determined. Somewhat surprisingly, it was found that the f(X) determined from one, cycled, limestone was successful in predicting the conversion of other cycled limestones of different geological origin. It was concluded that the process of cycling between the calcined and carbonated states at the same process condition had significantly reduced the differences apparent in the pore structures of the different limestones when first calcined from the virgin materials. The experimentally-observed effects of pressure, concentration of CO2 and temperature described in the literature were explained successfully by the mathematical model. Finally, the study of sulphation explained satisfactorily (i) the reason for there being a maximum in the ultimate conversion of CaO to CaSO4 at a specific temperature, and (ii) the processes controlling the overall uptake of SO2 by sintered CaO, such as might be produced from a calcium-looping cycle for capturing CO2 from flue gases. For both the virgin and the cycled calcines, the ultimate conversion to CaSO4 seemed to be limited by the pore volume below 300 nm diameter. Two mechanisms were identified to explain why CaO cannot be fully sulphated to CaSO4. In summary, this work has demonstrated the applicability of the general reaction and diffusion model to gasification, calcination and sulphation reactions, and verified the f(X) approach for describing pore evolution during reaction.