Studies On The Adsorption And Oxidation-Extraction (Adsorption) For Ultra-deep Desulfurization Of Diesel Fuels

Ultra-deep desulfurization of fuel oils has become an environmentally urgent subject worldwide due to very stringent environmental regulations that limit the sulfur levels in diesel fuels to less than 15 ppm. Besides, the fuels with ultra-low level of sulfur or free of sulfur are required for hydrogen fuel cell if its hydrogen is produced from the fuels. Although conventional hydrodesulfurization (HDS) can effectively remove the majority of sulfur-containing compounds, it is difficult to achieve the ultra-deep desulfurization. A number of alternative technologies including adsorption, oxidation, extraction and bioprocessing are being explored to realize the ultra-deep desulfurization. The adsorption and selective catalytic oxidation combined with extraction (adsorption) is one of the most promising ultra-deep desulfurization methods. This thesis focuses on the researches of selective adsorption and oxidation-extraction (adsorption) to challenge the ultra-deep desulfurization of diesel.The adsorption of dibenzothiophene present in diesel on the modified activated carbons and the main factors affecting adsorption capacity of dibenzothiophene of the activated carbons are systematically investigated. In the second part of the thesis, the selective oxidation of diesel is investigated using a catalyst assembled in emulsion. The sulfides present in diesel can be oxidized into corresponding sulfones and the sulfones can then be readily separated from the diesel using an extractant (or adsorbent). The ultra-deep desulfurization of diesel (less than 0.1 ppm) can be approached based on this work.The result from adsorption of dibenzothiophene on modified activated carbons shows that different modification methods have great effect on sulfur removal of activated carbons. The effect of the different oxidation modifications on the adsorption capacities of activated carbons follows order: concentrated H2SO4, concentrated HNO3 > (NH4)3S2O8 > H2O2 (30%) > KMnO4. The activated carbons modified by concentrated H2SO4 have the largest sulfur removal among the investigated modification methods. The adsorption capacity of sulfur on the activated carbon modified by concentrated H2S04 increases from 0.0240 to 0.0529 g sulfur / g adsorbent. The sulfur removal for the diesel on the activated carbon modified byconcentrated H2SO4 increases from 23.3 % to 32.5 % as compared with the unmodified activated carbon, indicating that the modification of activated carbons with H2SO4 is favorable for the adsorption of sulfides present hi diesel. The relationship between the porous structures and the surface properties of the activated carbons and the adsorption capacities of dibenzothiophene on the activated carbons is investigated using low temperature nitrogen adsorption, base titration, FTBR, and molecule adsorption with different sizes and polarities. The result shows that the increase of adsorption capacity of dibenzothiophene on the modified activated carbons is mainly related to the increase of the mesoporous volumes and the amount of acidic surface oxygen groups.The selectively catalytic oxidation of the sulfur-containing compounds including 4,6-dimethyldibenzothiophene shows that a [(C18H37)2N+(CH3)2]3 [PW12O40] catalyst assembled in emulsion droplets (water in oil) behaves the nature of homogeneous-like catalyst. The catalyst can completely oxidize the sulfur-containing compounds into sulfones with H2O2 as the oxidant under mild conditions (30℃ and ambient pressure). The selectivity for sulfides is 100 % and the efficiency of H2O2 utilization is more than 96 %. The catalyst (more than 90 %) can be separated from the diesel through demulsification and centrifugation. The sulfones can be selectively separated from the diesel using an extraction method. The sulfur level of the desulfurized diesel can be lowered from about 500 ppm to 0.1 ppm without changing the properties of the diesel.