Alkylation Desulfurization Of FCC Gasoline By Reactive Distillation For Producing Clean Fuel

FCC gasoline is the main blending component in the Chinese gasoline pool, and the quality of the gasoline pool is strongly affected by the high sulfur compounds in FCC gasoline. The stringent environmental requirements push refineries to develop new technology to remove sulfur compounds in FCC gasoline. Hydrodesulfurization is one of the most important and popular technologies for removing sulfur in gasoline. However, the saturation of olefins in the hydrodesulfurization process always leads to the problem of the octane loss. Another problem is the consumption of hydrogen is high and unbearable for refineries. Therefore, developing a new technology for efficiently removing sulfur in gasoline without losing its octane number is meaningful and excepted.This paper focused on the research of the catalytic alkylation desufuirzation process of FCC gasoline by reactive distillation. At the beginning, the alkylation reaction was investigated for the enrichment of sulfur compounds in light fractions to heavy fractions. Two types of the alkylation catalysts were prepared, one was compound solid acid catalyst for high temperature alkylation and the other was resin supported catalyst for low temperature alkylation. As for solid acid catalyst, SiO2-Al2O3 was select as its supporter and the optimal Si to Si+Al ratio is 0.7; the loading amount of solid acid is 60% and the mass ratio of polyphosphoric acid to phosphoric acid is 2. The optimized calcination temperature for preparing the catalyst is 500℃-550℃. At the conditions, the total acid quantity of the solid acid catalyst reaches 0.32-0.33mmol/g, and the strong Broensted acid sites give the main activity for the alkylation reactions. As for resin supported catalyst, pocket sulfonic acid resin was selected for preparing the catalyst. The total acidity of the resin is 5.33 mmol/g. The method of loading AlCl3 on the surface of resin can effectively improve the reactive stability of the catalyst. The feasible reaction temperature of the resin catalyst is 120℃. Results showed that the optimal Al loading amount is 3.48%, and the catalyst could be reused for several times with a stable 90% conversion of thiophenic sulfur compounds.The two catalysts were tested in a batch reactor. Based on the batch reactor result, a continuous reactive distillation column was setup and the two catalysts were investigated for obtaining the optimal conditions. The optimal reflux ratios for both of the two catalysts were all 1.5. The reaction temperatures for resin supported catalyst and compound solid acid catalyst are 100-120℃and 140-160℃, respectively. At optimal conditions, the sulfur content in the distillate of the reactive distillation column was depressed to very low level, and its octane number only dropped about 0.1-0.2 units compared with the feedstock. The yield of the gasoline fraction (<170℃) reaches 85% and its sulfur was lower than 30mg/L. This low sulfur fraction was an ideal blending compound for blending clean gasoline product.1000h pilot experimental result proved that the stability of the catalysts was satisfied. Blending the distillate with reformed gasoline in a ratio of 9:1 by volume, the sulfur content of the blend could meet the requirement of the national IV gasoline emission standard, which improved the clean gasoline resource utilization and allocation level of refineries.Kinetics of catalytic alkylation of various sulfur compounds in gasoline were also investigated. These sulfur compounds include thiophene (T),2-methyl thiophene(2-MT), 3-methyl thiophene (3-MT) and 2,4-dimethyl thiophene (2,4-MT). In the case of compound solid acid catalyst, conversion rates of all sulfides reached at a maximum value at 160℃. The reaction rates of them follow the order 2-MT>3-MT>T>2,4-MT. The result also showed that alkylation of 3-MT or T was a temperature sensitive reaction, while alkylation of 2-MT was not sensitive to the reaction temperature. Therefore, higher conversion of 3-MT and T can be obtained by increasing the reaction temperature. As for resin supported catalyst, the activated energy and pre-exponential factor for alkylation reaction of various sulfur compounds were also determined. The alkylation kinetics equations for T,2-MT,3-MT and 2,4-MT are ln(CT0/CT)=4.26×106. exp(-44.13/RT)-t, In(C2MT0/C2MT)=2.27×104. exp(-28.17/RT)·t, ln(C3MT0/C3MT)=7.52×105. exp(-38.54/RT)-t and ln(C2,4-DMT0/C2,4-DMT)=6.86×104 exp(-31.75/RT)·t, respectively.Density functional theory (DFT) method was used to study the reaction mechanism of the alkylation of thiophenic compounds and 2-methyl-2-butylene. The geometry structures of reactants, intermediates, transition states and products were optimized, and frequency analysis was applied to confirm the reliability of the structures. Based on that, a reasonable reaction mechanism was also proposed. Finally, steady state simulation was carried out by Aspen Plus in order to provide some useful information for the industrial process of FCC gasoline alkylation reactive distillation. The properties of the reaction substances in the process were estimated on the basic of various modules provided by Aspen plus. Then, a RadFrac module was employed to simulate the process of reactive distillation process. The simulation results are in fair line with the experimental results of reactive distillation column. The above research results provide detailed information for the further research and industrial utilization of the technology.