3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis of dimethyl ether using natural gas as a feed via the C-H-O ternary diagram(2017) Masindi, AndisaniIn this research, the C, H and O bond equivalent diagram was used to design processes for DME synthesis using natural gas as a feed. This research proposes alternative ways of producing DME using natural gas (a cleaner gas) compared to the traditional routes. The different feed combinations were assessed for the production of syngas. The crucial step is the H2:CO ratio in each feed which determines the DME synthesis process route and yield. The syngas process was developed under equilibrium and non-equilibrium conditions (assuming 100% methane conversion). The region of operation on the ternary bond diagram was limited by mass and energy balance and carbon deposition boundaries. The feed composition was as follows, (1) Feed 1: methane, steam and oxygen (2) Feed 2: methane, oxygen and carbon dioxide (3) Feed 3: methane, oxygen, carbon dioxide and water. Feed (2) had the highest DME yield. The most optimal reaction route produced DME via the JFE reaction route (H2:CO =1). The yield of DME was 0.67 moles of DME per mole methane processed under non-equilibrium conditions. The proposed route does not emit CO2, excess CO2 is recycled back to the reforming reactor. Under equilibrium, the yield of DME was 0.25 mole DME per mole methane processed. The results indicate that a combination of partial oxidation and dry reforming produces a syngas composition which results in a high DME yield compared to (1) and (3).Item Direct methane transformation into higher hydrocarbons and oxy-products(1996) Eskendirov, IgorIn present thesis the results of a study of the combined action of a solid catalyst and a gas-phase inintiator, hydrogen peroxide, in the methane partial oxidation and oxidative coupling reactions are presented. [Abbreviated Abstract. Open document to view full version]Item Methane decomposition : characterization of the carbon produced and possible use in direct carbon fuel cells(2011-12-15) Salipira, Ketulo LacksonInvestigations into methane conversion (both catalytic and non-catalytic) and characterization of the carbon produced for use in high efficiency DCFCs were performed. Under non-catalytic processes, a high methane conversion (> 80%) was achieved at 1200 oC at flow rates of between 10-60 ml/min. Analysis of the carbon using Raman spectroscopy showed that the carbon was highly disordered and the degree of disorder increased with increase in methane flow rate (from aD/aG = 1.54 at 10 ml to aD/aG = 2.24 at 60 ml/min). Further analysis of the carbon using thermogravimetric analysis (TGA) demonstrated that the carbon produced at higher flow rates e.g. 100 ml/min were easily oxidized (746 oC) compared with those produced at lower flow rates (10 ml/min, 846 oC). Therefore, a high temperature coupled with high flow rates (60-100 ml/min) produced carbon with desired qualities (high disorder, low crystallinity and more thermally reactive) for DCFC uses. In the catalytic decomposition of methane, Ni supported on TiO2 and a 1:1 mixture of TiO2/Al2O3 gave high and stable methane conversions of about 60% at only 600 oC compared to 1200 oC required for the non-catalytic conversion. These catalysts were found to be the best catalyst systems of the tested catalysts. Considering the thermal oxidation and crystallinity data which are some of the properties of the carbon required for direct carbon fuel cells (DCFCs), the carbon produced can potentially be used in DCFC systems.Item The nonoxidative conversion of light alkanes over metal-loaded H-ZSM-5 zeolite catalysts(2008-06-20T11:36:20Z) Ngobeni, Maropeng WalterThe study of the aromatisation of methane was conducted at 750oC over metalimpregnated H-ZSM-5 catalysts with a feed flow rate of 13 ml/min and the composition of the feed was 90% methane balance argon. Typical products that were detected from the outlet stream were ethene, ethane, benzene and toluene. The amount of coke produced was determined by using 10% argon as an internal standard. The effects of different parameters such as the type of the support material, the molybdenum content, the %XRD crystallinity and SiO2/Al2O3 ratio of H-ZSM-5, the reaction temperature, the feed flow rate, the type of the molybdenum precursor, the catalysts preparation method, the addition of dopants, silanation and the regenerability of the catalysts were investigated. The results obtained showed that H-ZSM-5 was a better support for the preparation of catalysts used for the aromatisation of methane. Mo/H-ZSM-5 catalysts were more active when the molybdenum loading was between 2 and 4 wt% and loadings higher than 4% led to lower activities. The lower activities observed at higher molybdenum loadings was related to the poor dispersion and decrease in the pore volumes and surface areas observed due to the formation of MoO3 crystallites. Furthermore, the zeolite structure collapsed under the reaction conditions when the molybdenum loading was more than 4 wt%. The study showed that the conversion of methane increased linearly with increasing reaction temperature and the apparent activation energy of the reaction was found to be 64.5 kJ/mol. The results of the effect of the %XRD crystallinity of H-ZSM-5 on the performance of H-ZSM-5 catalysts showed that 2%Mo/H-ZSM-5 catalysts were more active when the crystallinity of the zeolite was between 50 and 70%. The conversion of methane decreased with an increase in the SiO2/Al2O3 ratio of H-ZSM-5. Higher aromatisation activities were observed when the SiO2/Al2O3 ratio of H-ZSM-5 was iii 60. The type of the molybdenum precursor used in the preparation of 2%Mo/HZSM- 5 catalysts did not have a significant influence on the conversion of the catalysts, but higher selectivities for aromatics were observed when ammonium heptamolybdate was used as a source of molybdenum. The catalysts prepared by physical mixing of MoO3 and H-ZSM-5 catalysts were more active than those prepared by impregnation with solutions of ammonium heptamolybdate. The presence of dopants such as boron, silver and alkali metal ions (Li+, Na+ and K+) in 2%Mo/H-ZSM-5 catalysts was also investigated. Boron (0.05-0.2 wt%) did not affect the conversion level of the catalysts but changed their selectivity properties. The selectivity for C2 hydrocarbons increased with boron content, while the selectivity for aromatics decreased. The addition of silver ions (0.5 wt%) significantly improved the conversion of the catalysts. This was attributed to the enhancement of the acvidity of the catalysts upon addition of silver ions which was observed by temperature programmed desorption of ammonia and pyridine adsorption studies of the infrared spectra of the catalysts. The addition of alkali metal ions in the Mo:Metal ratio of 0.5 led to decreased catalytic activities, due to the lowered acidities of the catalysts. The silanation of H-ZSM-5 improved the conversion of methane but lowered the selectivity for aromatics. A comparative study of the W-based and Mo-based catalyst at equivalent molar contents showed that molybdenum-based catalysts were more active than tungsten based catalysts. The study also showed that the catalytic performance of 2%Mo/H-ZSM-5 catalysts could be regenerated to appreciable levels by treatment of the catalysts in air at 600oC. The possibility of using Mo/H-ZSM-5 catalysts for the aromatisation of propane was also evaluated at 530oC, with consideration of three variables, namely, the molybdenum loading, the reaction temperature and %XRD crystallinity. The results indicated that impregnation H-ZSM-5 catalysts with molybdenum led to lower iv propane aromatisation activities. This lower activity was attributed to the lower Brønsted acid sites in the Mo/H-ZSM-5. The activities of the catalysts could be improved by operation at higher temperatures, but the rate of deactivation was also improved at higher temperatures. In line with the observations from the conversion of methane, higher activities were observed when the %XRD crystallinity of the catalyst was 61%.