Desulfurization of Hydrocarbon Fuels at Ambient Conditions Using Supported Silver Oxide-Titania Sorbents
Type of Degreedissertation
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Sulfur in refined fuels is considered a significant cause for atmospheric pollution such as acid rain and smog. Sulfur is also a poison for electrocatalysts in fuel cells and catalysts in hydrocarbon refining and reformation processes. Thus sulfur removal is essential for large scale production of transportation fuels as well as in smaller scales for mobile and stationary fuel cell and reforming applications. Hydro desulfurization (HDS) is the most prevalent desulfurization technology used currently. Several alternative technologies have been reported to be effective in sulfur removal from liquids such as catalytic oxidation, biological sulfur removal and membrane separation. The presented work focuses on the formulation, optimization and mechanistic investigations of adsorptive desulfurization adsorbents for liquid fuels at ambient conditions. Dispersed silver oxides on supports such as TiO2, γ-Al2O3 and SiO2 were observed to be effective desulfurizing agents for refined fuels at ambient conditions. Among the supports, TiO2 was found to be the most stable. Using titanium oxide of varying surface characteristics, it was determined that sulfur capacity corresponded to the specific surface area. Increasing the Ag loading on the support was observed to decrease dispersion and simultaneously decrease the sulfur capacity. At 4 Wt.% Ag loading, the sulfur capacity of the sorbent was 6.3 mgS/g for JP5 fuel containing 1172 ppmw sulfur. The sorbent composition was thermally regenerated (450⁰C) to 10 cycles using air as a stripping medium. Variation in desulfurization efficiency between JP5, JP8 and a lighter fraction of JP5 was established and correlated to the variation in sulfur speciation of the fuels. Lower concentration of trimethyl benzothiophenes in the lighter fraction JP5 resulted in the highest sulfur capacity demonstrated by Ag/TiO2. These studies on performance, effects of composition, fuel chemistry and regeneration procedures are presented in Chapter III. With the composition and performance of the sorbent established, synthesis procedures were optimized considering impregnation, drying and calcination stages. The effect of synthesis conditions on the sulfur capacity was correlated to the resulting pore structure and dispersion of Ag (Chapter IV). Incipient wetness among the various impregnation techniques resulted in the highest sulfur capacity. Calcination temperatures above 500 ⁰C were observed to degrade the pore structure and thus lower the sulfur capacity of the sorbent. Characterization techniques such as BET surface area measurements, oxygen chemisorption, temperature programmed reduction (TPR), ultraviolet spectroscopy were used to study the adsorbent composition. The variation in the oxidation state of Ag with weight loading was determined using TPR and thermogravimetry. At 4% Ag loading approximately 28% of the deposited Ag was found to exist as the oxide. Lowering the metal loading significantly increased the dispersion. These dispersed Ag oxides were observed to be stable to temperatures of 550⁰C. UV spectroscopy showed absorption bands representing oxides of Ag while bands representing metallic Ag were absent. It was therefore concluded that a majority of the Ag at the adsorption interface existed in the oxide phase. This indicated an alternative mechanism of sulfur removal compared to other transition metal based sorbents where the active material is considered to be the metal ion. Several aspects to be considered during the scale-up of adsorption units such as bed configuration, liquid face velocity and bed temperature and the effect on sulfur capacity was addressed as well. Having established the composition of the sorbent with respect to the oxidation state of Ag present, the dispersion of Ag and pore structure, several studies were carried out to determine the mechanism of sulfur removal in these materials (Chapter V & VI). Variation in desulfurization efficiency between sulfur aromatics varying in structure aspects such as aromaticity and presence of side chains were linked to the chemistry of the active center. These studies established that the active centers were acidic in nature. Probe molecules were used to poison the active centers and subjected to desulfurization studies. Surface complexes formed from the probe molecules were also identified using IR spectroscopy. These experiments indicated that the surface group responsible for the sulfur capacity was single or geminal hydroxyl groups. Equilibrium isotherms were also established for thiophene, benzothiophene, dibenzothiophene and 46 dimethyl dibenzothiophene at 22, 40 and 60⁰C and fitted to Langmuir, Freundlich and Fritz-Schlunder models (Chapter VII). The adsorption data followed the Langmuir model indicating that sulfur removal was effected by associative physical adsorption.