| Selective adsorption desulfurization of fuel oils has already become one of the hottest research topics lately. However,no consensus has be reached in mechanism of adsorption desulfurization yet, especially in the presence of a large quantity of olefins and aromatics, the mechanism of selective and competitive adsorption has been reported rarely. The studies of adsorption behavior of sulfur compounds are of importance for guidance of solution to desulfurization technology. Depending on the former research, this paper takes modified zeolites as the absorbents, focusing on the current issues of selective adsorption desulfurization, to operate relatively theoretical and experimental study. Some valuable consequences have been observed in the experiment.Modified zeolites were prepared by liquid-phase ion-exchange method. Adsorbents were characterized by XRD, FT-IR, UV-DRS, ICP and TEM analysis, which make the preparing conditions associated with the desulfurizing properties of adsorbents. The results indicate that the desulfurizing properties of modified zeolites almost have no relationship with physical properties of Y zeolites pore volume, specific surface area, while related closely to the exchange degree of sodium ion or the mental-loadings of metal cation of the zeolites. For the same type cation, the higher the mental-loadings are, the better the desulfurization capacity is. XPS showed that metal cations in CeY zeolite calcined air atmosphere mainly were Ce3+. With the calcination temperature increasing, Ce4+ content increased gradually, through the TEM observation of up to 800℃, some of the cerium ions were clustered.The influence of various parameters on desulfurization of modified zeolited was investigated. The results showed that modified zeolites exhibit higher adsorptive selectivity desulfurization while oil adsorbents ratio on CeY, NiY and CuY is 14: 1, 20: 1 and 60: 1, desulfurization time 4 h, 5 h and 6 h, respectively. It was found that adsorption capacity of adsorbents in desulfurization was obviously different under the same adsorption condition. It follows the order CeY < NiY < CuY. The desulfurization speed of the adsorbents studied decreases as follows: CeY > NiY > CuY. The spend zeolites were regenerated with N2 at 400℃, and the regenerated zeolites were tested again for the desulfurization. The results showed that 95% of the sulfur capacity was recovered at breakthrough.The adsorption behavior of the organosulfur compounds are investigated by using fixed-bed adsorption, FT-IR and the Density functional theory (DFT) molecular simulation techniques. The fuels tested for desulfurization include commerical and model fuels. In addition, the treated and untreated fuels comprising the major refractory sulfur compounds were fully characterized by using the latest generation of SCD detectors in our laboratory, which are capable of distinguishing different sulfides. The adsorption capacities of each sulfur compounds on the zeolites are different but present a similar trend that benzothiophenes possess higher sulfur adsorption capacities than thiophene and that the refractory sulfur compounds with substituents have higher sulfur adsorption capacities than those without substituents except 4,6-dimethyl dibenzothiophen(4,6-DMDBT), which agrees well with the DFT calculation results.The influence of aromatics and olefins on the desulfurization is different for different modified zeolites. It has been found that the effect of olefins on desulfurization of the adsorbents decreases in the order of diolefins > cyclenes > linear-monenes for all the sorbents. The influence of olefin compounds on selective adsorption desulfurization is greater than aromatic hydrocarbon on CeY zeolites, the order is: the benzene < xylene < 1-octene≈1-hexylenes < tetrahydrobenzene < 1,5-hexadiene. To NiY and CuY zeolite, the influence of olefin and aromatic hydrocarbon is almost equal, the order: 1-octene≈1-hexylene≈1,5-hexadiene≈tetrahydrobenzene≈benzene< xylene < 1-octene.Desulfurization capacity can also be deeply affected by acidity of modified zeolites. In-situ infrared methods,which employs pyridine as the probe molecule, was used to evaluate the acidity of the modified zeolites. With the exchange of different cations, the modified zeolite acid strength and distribution changed (CeY zeolite B-acid value is 0.662 mmol/g, CuY zeolite B-acid value is 0.306 mmol/g). The sulfur compounds and olefins adsorbed on the protons may occur subsequently alkylation reactions, which plug the zeolite pore and then hider sulfide molecules access to super-cage to contact with the metal cation. Alkylation reactions are not good for selective adsorption desulfurization. Modified zeolites were passivated by different method, which change acid strength and the acid distribution. It reveals that desulfurization capacity on absorbent is closely related with B/L of adsorbents. |
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