Graduation date: 2000

Thesis (M. Sc.)(Chemistry)--University of Pretoria, 2006. / Summaries in English and Afrikaans. Includes bibliographical references. Available on the Internet via the World Wide Web.

Catalytic supercritical water gasification (SCWG) is a promising technology for hydrogen and methane production from wet organic feedstocks at relatively low temperatures (e.g. <500 oC). However, in order to make this process technically and economically viable, solid catalyst with enhanced activity and improved hydrogen selectivity should be developed. In this study, different aspects of catalytic SCWG have been investigated. The performance of several supported-nickel catalysts were examined to identify catalysts that lead to high carbon conversion and high hydrogen yields under near-critical conditions (i.e. near 374 oC). Moreover, for the first time, the effects of several parameters which dominated the activity of the supported nickel catalysts have been systematically investigated. Among the several different catalyst supports evaluated at 5% nickel loading, α-Al2O3, carbon nanotube (CNT), and MgO supports resulted in highest carbon conversions, while SiO2, Y2O3, hydrotalcite, yttria-stabilized zirconia (YSZ), and TiO2 showed modest activities. Comparing the XRD patterns for the support materials before and after the exposure to supercritical water, α-Al2O3, YSZ, and TiO2 were found to be hydrothermally stable among the metal oxide supports. Using the same amount of nickel on α-Al2O3, the methane yield decreased by increasing the nickel to support ratio whereas the carbon conversion was only slightly affected. At a given nickel to support ratio, a threefold increase in methane yield was observed by increasing the temperature from 350 to 410 oC. The catalytic activity also increased by the addition small quantity of potassium. The activity of Ni/γ-Al2O3 catalyst varied based on the affinity of the catalyst to form nickel aluminate spinel. This is also the first report on the role of oxidative pretreatment of the carbon nanotubes by nitric acid on the performance of these catalysts for the supercritical water gasification process. Using different lignocellulosic feeds, it was found that the gasification of glucose, fructose, cellulose, xylan and pulp resulted in comparable gas yields (± 10%) after 60 min, whereas alkali lignin was substantially harder to gasify. Interestingly, gasification yield of bark, which had a high lignin content, was comparable to those of cellulose. In summary, the Ni/α-Al2O3 catalyst had a higher hydrogen selectivity and comparable catalytic activity to the best commercially available catalysts for SCWG of carbohydrates.

Supercritical fluid technology is a well documented and emergent technology used in many industries today for the formation of micro- and nano- particles. The use of supercritical fluids allows synthesis of various types of particles since their properties can be varied with temperature or pressure, which sequentially can control the physical and chemical properties of the particles produced. Several different processes designed to generate powders and composites using supercritical fluids have been proposed in the past 20 years which can be used to synthesize materials with high performance specifications and unique functionality. In this research work, an extrusion micronization process using supercritical fluid has been proposed. This powder production technique could be a promising alternative to conventional techniques in terms of improvement in product quality as it provides a better control over particle size, morphology and particle size distribution, without degradation or contamination of the product. In addition, as extrusion is globally used for polymer production and processing, particle production by extrusion will allow production and processing in a single process step, eliminating the need for secondary particle production methods. The micronization process designed and described in this thesis involves a twin screw extruder equipped with a converging die and a high resistance spraying nozzle for particle production. A special CO2 injection device and polymer collection chamber was designed for CO2 supply and powder collection. To ensure complete dissolution of CO2 into the polymer matrix, stable injection of CO2, pressure generation and constant spray of micronized polymer particles, a special screw configuration was carefully designed for the extrusion process. The feasibility and the performance of this process have been demonstrated by experimental studies performed with low molecular weight polyethylene wax. Carbon dioxide at supercritical conditions was used as a solvent for processing the polymer. The generated polyethylene particles from the polyethylene wax/carbon dioxide solution system were analyzed and studied using an optical microscope, scanning electron microscope, capillary rheometer and differential scanning calorimeter. A detailed study on the effects of the processing parameters, such as temperature, pressure, flow rate and supercritical fluid on properties of polyethylene particle produced was carried out. The particle size data collected using an optical microscope indicate a significant impact of temperature and CO2 content on particle size. The obtained size data were utilized to generate particle size distribution plots and studied to analyze the effect of the processing variables. It was found that particle size distribution is affected by processing temperature and CO2 content. Studies of the SEM images reveal that the morphology of particles can be controlled by varying processing variables like temperature, polymer feed rate and CO2 content. The particles generated during this study indicate that particle production in an extrusion process using supercritical carbon dioxide is achievable and appears to be a promising alternative to conventional polymer particle production methods such as grinding, milling and other supercritical fluid-based precipitation methods. To validate and generalize the applicability of this process, micronization of other polymeric material should be performed. Commercialization of this technology will further require predictability and consistency of the characteristics of the product, for which a detailed understanding of the influence of all relevant process variables is necessary. In addition, development of theoretical models will further assist in the scale-up and commercialization of this supercritical fluid assisted micronization technology in the near future.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (Ph. D.)--University of Missouri-Columbia, 2001. / Typescript. Vita. Includes bibliographical references. Also available on the Internet.

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M. Tech. Chemical Engineering. / The solubility of highly volatile non polar solutes (hexane, toluene and benzene) was determined in CO2, in the vicinity of supercritical conditions using a method employing dielectric constants. This method is based on the measurement of the dielectric constant of the saturated supercritical solution and that of the supercritical solvent as a function of pressure along different isotherms, by using the Clausius- Mossotti function. Results were in agreement with literature values, proving that this method can be used to determine the solubility of a solute in a pressurized gas. The results obtained demonstrate the capability of the dielectric constant method as a nonintrusive, simple and efficient means of determining accurately the solubility of liquids in supercritical solvents.