Study Of Oxidation-Adsorption Combined Desulfurization Of Fuel By Supported Silica Gel

With the increasing concerns about environmental issues, more stringent regulations for sulfur content in gasoline and diesel fuels have been enacted by countries and regions around the world. Therefore, the production of low-sulfur content fuels has been the dominant trend. Currently, the technology for the desulfurization of fuel can be divided into two major ways Hydrodesulfurization(HDS) and non–Hydrodesulfurization(NHDS). Considering the decreasing of octane number and increasing of operating costs in producing ultra clean fuels in HDS, some technologies of NHDS have attracted more and more attention. The Oxidation–adsorption combined desulfurization is a promising technology, for its higher desulfurization rate, better selectivity, less demanding requirement for the equipment .As a macroporous material, silica gel has large surface area and certain polarity towards surface hydroxyl group. These properties make silica gel have better selectivity for polarized molecules. Herein, we report cheap silica gel as the adsorbent in this adsorption process. Numerous investigations has been devoted to optimizing adsorbent and preparing conditions in adsorption desulfurization process , time and temperature in oxidation and calcination, type and mesh size of silica gel , adsorption temperature, velocity andloading metal oxides. Morevoer, physical properties of the adsorbent were demonstrated in this study. . Furthermore, our work is also found to provide a theoretical basis for the application of adsorption desulfurization in industry.Firstly, the oxidation process was investigated in this study, and our results show improved selectivity for sulfur compounds after oxidation. The desulfurization rate of , oxidized diesel could reach as high as 94.57%at 40oC in adsorption and 6.0h-1in velocity. It is 50.06% improvement compared to the untreated diesel. .To select a better adsorbent, two types of silica gel were performed individually as adsorbents in the combined desulfurization process. The result demonstrated SG-A had better desulfurization effect than SG-B under the condition of 80oC in adsorption and 6.0h-1 in velocity. The optimum desulfurization rate for SG-A and SG-B were 94.63% and 72.59%, respectively. The FT-IR analysis displayed stretching vibration of S=O at 1305 cm-1 after oxidation, which demonstrated that thiophene-type sulfidewere oxidized to sulfoxides or sulfones.The S=O adsorption peaks disappeared after the treatment of SG-A while this peak still existed after SG-B adsorption. This indicates SG-A has a better adsorption effect of sulfur containing compounds in oxidized diesel.The fixed bed investigation displayed that the desulfurization rate and velocity of diesel and gasoline fluctuate with the increasing temperature and velocity separately. The obtained maximum desulfurization rate of diesel was 94.57% at 40oC(adsorption) and6.0h-1(velocity) while highest desulfurization rate of gasoline was 62.72% under the condition of 100oC(adsorption) and 6.0h-1(velocity).In this study incipient wetness impregnation method was employed to load active metallic components on silica gel and the desulfurization effect of the loaded silica gel were detected. The result shows that the desulfurization rate increased greatly after loading of active metallic components on silica gel. The optimum desulfurization rate was 56.35% with 2.0 % CuO/SiO2,100oC(adsorption),6.0h-1(velocity), 10(oil /agent). This rate reached 52.96% when supported with 1.0% NiO under the same adsorption conditions as CuO/SiO2.Hydrocarbon compositions have a major impact on the adsorption desulfurization of gasoline and diesel ,the main sulfur-containing compound in the gasoline are thiophene-type sulfide,and there are a large number of aromatics and olefins.The unsaturated hydrocarbons occurred competitive adsorption with sulfur-containing compound; and reduces the selectivity of silica gel. The main sulfur-containing compound in diesel are benzothiophene, dibenzothiophene and derivatives, hydrocarbons in diesel oil are mainly saturated hydrocarbons, and they have little impact on the adsorption, so the using of silica gel for adsorption desulfurization in diesel oil can achieve good effect.The Adsorption desulfurization of three model compounds(Th, DBT, 4,6-DMDBT) showed the sulfur atom in thiophene has lower electron density, which makes it difficult for the attacktion of oxygen, thus the desulfurization rate was poor, while the electron density of sulfur atoms in dibenzothiophene and 4,6 - dimethyl dibenzothiophene are higher, therefore were prone to be oxidized which improves the selectivity of adsorption desulfurization by silica gel. The desulfurization rate for the three model compounds follows the Order of 4,6-DMDBT>DBT>Th. In the oxidation- adsorption desulfurization process,the electron density plays the dominant role, while the steric effects was a less important factor.The composition of hydrocarbon plays an important role in the adsorption desulfurization of gasoline and diesel. The main sulfur- compounds in gasoline are thiophene-type sulfide, the unsaturated hydrocarbons such as aromatics and olefins in gasoline will lead to the competitive adsorption, therefore lowering the desulfurization rate of gasoline by silica gel. The main sulfur-compounds in gasoline are benzothiophene, dibenzothiophene and derivatives, but the major hydrocarbons are saturated, the competitive adsorption was lower, thus the adsorption desulfurization rate of diesel by silica gel was better.