Liquid Phase Desulfurization of Hydrocarbon Fuels under Ambient Conditions using Regenerable Mixed Oxide Supported Silver Adsorbents
Type of Degreedissertation
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Sulfur derivatives are major contaminants in hydrocarbon fuels. Sulfur emission from fuel is a major environmental concern and many countries around the world are enacting laws to limit it. In addition, the development of fuel cell systems is restricted because of the demand of ultra low sulfur fuels as pre-reformate streams. Therefore the removal of sulfur from hydrocarbon feed streams is essential. Conventional hydrodesulfurization (HDS) process in the refinery is efficient in removing most of the sulfur from crude oils. However, the process becomes expensive for producing ultra low sulfur fuels. Among several alternative processes, adsorptive desulfurization has shown to be a promising process for the intended applications. This dissertation discusses the aspects of liquid fuel desulfurization using regenerable oxide adsorbents at ambient conditions. In this work, the development of an adsorptive desulfurization process using mixed oxide supported silver oxides along with the corresponding characterization analyses and the mechanisms involved have been presented. The TiO2–Al2O3 and TiO2–SiO2 mixed oxides were formulated by dispersing titanium precursor on high surface area Al2O3 and SiO2 supports. The mixed oxides were subsequently impregnated with AgNO3 followed by calcination. The resulting formulation yielded highly dispersed titania and silver oxide phases that had promising sulfur adsorption capacities (~10 mg S/g adsorbent) and lowered exit sulfur threshold (<75 ppbw). The adsorbents were effective toward a wide variety of commercial (off-road and ultra low sulfur diesels) and logistic (JP5 and JP8 jet fuels) fuels. The mixed oxides also provided seats for more silver loading (up to ~12 wt% Ag on TiO2–Al2O3), consequently increasing sulfur adsorption capacities. The adsorbent retained its capacity after multiple cycles of regeneration in air. The variations in sulfur adsorption capacities for different fuel blends were also established. The presence of thiophenic molecules with methyl groups created steric hindrances for sulfur adsorption. The adsorbent formulation, performance, regeneration and variations of sulfur species are discussed in chapter III. The promising performance of mixed oxide supported silver adsorbents called for a detailed analysis regarding the active sites involved and the effect of surface acidity. The Ag/TiO2–Al2O3 adsorbent was characterized via fixed bed continuous adsorption (breakthrough) experiments, N2 physisorption, X-ray diffraction (XRD), UV-vis spectroscopy, Raman spectroscopy, O2 chemisorption, NH3 adsorption, and infrared (IR) spectroscopy (chapter IV). The mesoporous adsorbent demonstrated enhanced capacity through higher surface area, greater TiO2 (<4 nm) and Ag dispersions (~23% for 10 wt% Ag loading on TiO2–Al2O3). TiO2 and Ag dispersion resulted in 89% (compared to TiO2) and 91% (compared to TiO2–Al2O3) increase in sulfur capacity, respectively. Anatase TiO2 dispersion on Al2O3 also allowed increase in adsorbent activity (3.27 eV band gap). The synergistic effect of TiO2–Al2O3 resulted in higher surface acidity (~14 cc/g NH3 uptake at P = 800 mm). Infrared spectra of the adsorbent samples treated with probe molecules (Ammonia, 2-lutidine, trimethyl chlorosilane, and thiophene) revealed the presence of surface acid sites. These acid sites were primarily responsible for silver incorporation (Lewis acid sites) and sulfur adsorption (surface hydroxyl groups). Having established the active surface sites, the adsorption mechanisms for aliphatic and aromatic sulfur compounds onto Ag/TiO2–Al2O3 adsorbents were investigated via complimentary breakthrough experiments and IR spectroscopy (chapter V). The mesoporous mixed oxide supported silver adsorbent demonstrated effective adsorption capacities for different organosulfur compounds. However, the selectivity varied for different sulfur species as well as non-sulfur aromatics. The adsorbent had higher breakthrough capacities for sulfur aliphatics than sulfur aromatics. The presence of non-sulfur aromatics negatively affected the sulfur adsorption capacities (~18% loss in capacity for Ag/TiO2–Al2O3). Infrared spectra of adsorbent samples treated with different sulfur molecules were acquired and investigated. Organosulfur adsorption on Ag/TiO2–Al2O3 was primarily attributed to the surface hydroxyl groups (via hydrogen/σ bonding) and the surface bound silver oxides (via π bonding). The presence of non-sulfur aromatics (benzene) reduced the sulfur adsorption capacity by occupying the π interaction sites. Finally, a density functional theory (DFT) study was carried out to estimate the adsorption selectivity toward different sulfur and non sulfur species. For the cluster model, a silver atom was placed on a titania (anatase) matrix. A hybrid DFT method (B3LYP) was applied for geometry optimization, frequency analysis, and single point energy calculations using LANL2DZ and 6-31G(d)+SDD basis sets. The organosulfur compounds (thiophene, benzothiophene, dibenzothiophene, 4,6-dimethyldibenzopthiophene) and non-sulfur aromatics (quinoline, benzofuran, benzene, naphthalene) species were adsorbed on Ag–TiO2 clusters. The adsorption on Ag–TiO2 demonstrated higher adsorption energies; and the adsorption orientation was π-preferred. Attached benzene rings in the sulfur heterocycles increased adsorption energies. The adsorbent affinity toward heterocycles with different functional groups followed the order: quinoline>benzothiophene>benzofuran. The computational calculations were in good agreement with the experimental results.