Study On Gasoline Adsorptive Desulfurization By Carbonaceous Materials

Removal of sulfur containing compounds in fuels is one of the key processes in production of clean fuels. As a kind of non-hydrogenation desulfurization approach, adsorptive desulfurization has gained attention worldwidely. By this means, the sulfur containing compounds can be removed from fuels under ambient temperature and pressure. Many adsorbents show adsorptive removal performance of thiophenic compounds, while most researches focus on screening of materials and only minor efforts have been put on study of interactions of adsorbates and adsorbents, and influences of competitive compounds on adsorption process. In this paper, carbon materials were chosen as adsorption materials and model gasoline chosen as treatment object to explore the adsorptive desulfurization process. The change of surface chemistry of carbon materials by oxidation and its influence on adsorption performance for removal of thiophene, influence of model adsorption competitive compounds on thiophene removal and mechanism, and both thermodynamic and kinetics character of the process were investigated systematically. And a new adsorption carbonaceous material was synthesized for primarily exploring of its adsorptive desulfurization. The desulfurization performance of these materials for FCC gasoline was evaluated as well.Firstly, various carbonaceous materials were screened and coconut activated carbon was chosen for further study. Ammonium persulfate and ozone were used as oxidizing agent to modify coconut activated carbon by changing its surface chemistry. The optimized condition was verified by evaluation of capacity for removal of thiophene in thiophene-cyclohexane solution as model gasoline. Results show that after oxidation by ammonium persulfate and ozone, the static equilibrium adsorption capacity of activated carbon, counted by sulfur capacity per unit mass, has increased from 8.47 mg-S/g to 15.52 and 21.50 mg-S/g, respectively. And dynamic breakthrough adsorption capacity has increased from 2.03 mg-S/g to 8.10 and 14.21 mg-S/g, respectively. With cyclohexene as olefin model compound, toluene as aromatic model compound, and cyclohexane, n-heptane and n-decane as paraffin model compounds, the competitive effects of these compounds for thiophene on carbon surfaces were investigated. Results show that olefins and aromatics strongly competitive with thiophene on carbon surfaces, and aromatics show more severe effect, which makes the adsorption capacity of thiophene reduce enormously. However, the competitive effect of paraffins is weak, which hardly influences thiophene adsorption capacity. The pore structure and surface chemistry of activated carbons was characterized by N2 adsorption, Boehm titration, FT-IR, and XPS. Results show that the influence of ammonium persulfate treatment on pore structure is greater than that of ozonation. Both the two treatments increase oxygen-containing functional groups on carbon surfaces considerably with close number of phenolic groups and more carboxyl groups for ozonation. Correlation of thiophene adsorption performance with physical and chemical properties of activated carbon indicates that the surface chemistry plays an important role in the process of thiphene adsorption, from which it can be deduced that the oxygen-containing functional groups introduced by oxidation is the main reason for thiophene adsorption capacity boosting.The interactions of activated carbon with adsorbate molecules were analyzed within microscopic domain. The chemical hardness in HSAB theory of compounds in model gasoline system was calculated by quantum chemistry method. The chemical hardness of these compounds shows good correlation with results of competitive adsorption experiments, which render the chemical hardness as an acceptable criterion for competitive adsorption ability. The analysis of local chemical hardness of activated carbon also elucidates the change in local chemical hardness can adopt it with adsorbate molecules to increase the interaction strength to improve the adsorption selectivity and capacity. The isotherm of thiophene adsorption on activated carbon fits well with Longmuir model. And analysis of kinetically functions indicates that thiophene adsorption is an exothermic, entropy-reducing, and spontaneous process. Oxidation can improve the affinity of carbon surface with thiophene molecules. Investigation of adsorption dynamics also shows that the process can be described with pseudo-second-order dynamic equation. The intra-particle diffusion is one of the rate-determining steps of the total process, and the adsorption rate is also controlled by external diffusion processes.The ozone-oxidized carbon, with comparatively better thiophene adsorptive removal capacity, was chosen for treatment of FCC gasoline. Optimized conditions were determined by breakthrough experiments. Under optimized conditions, the initial effluent of treated FCC gasoline with sulfur content of 796μg/g is reduced to 18μg/g. The saturated activated carbon can still reduced the initial effluent sulfur content to 45μg/g even after 3 cycles of regeneration. Phenolic weak acidic cation exchange resin was chosen as carbon precursor to prepare metal-loaded spherical activated carbon, and its adsorptive desulfurization performance was investigated primarily. The treatment results for model gasoline show that this type of actived carbon can reduce the competitive effects for thiophene adsorption with altered adsorptive interactions between thiophene and adsorbent. The treatment for FCC gasoline shows its good performance for removal of thiophenic compounds.