The Study Of Extractive Distillation Deep Desulfurization Process For FCC Gasoline

In order to control the pollution resulted from vehicle exhaust the production of gasoline with ultra-low sulfur content has become an urgent mission for the oil refining industries in the 21st century. Extractive distillation (ED) deep desulfurization of FCC gasoline has many potential advantages over the other desulfurization techniques, such as mild process conditions, low loss of octane number, low energy consumed, environmentally friendly, and etc. The study of extraction/extractive distillation deep desulfurization is of significantly economic and social interesting.Studies in this work focused on selection and performance evaluation of the solvents for extraction desulfurization of FCC and modeling gasolines, optimization for continuous extraction/extractive distillation process of FCC gasoline, measurement and correlation of liquid-liquid, vapor-liquid phase equilibrium of three sulfides (thiophene, n-butyl mercaptan and n-dibutyl sulfide in FCC gasoline)– solvent, and simulation and analysis of FCC gasoline ED desulfurization process.In the experimental study of extraction desulfurization, FCC gasoline and modeling gasolines which were prepared by respectively added mercaptan, sulfide, thiophene into C6～C8, used as feeds, the performance of the four selected solvents (sulfolane, dimethyl sulfoxide, tetraethylene glycol and diethylene glycol) were investigated, and finally sulfolane was considered as a best solvent for FCC gasoline extraction distillation desulfurization. The experimental results showed that using sulfolane as solvent, under the conditions of reflux ratio 4, solvent/oil ratio 0.3, the FCC gasoline desulfurization percentage reached to 88.5%, the desulfurization percentage increased with the increasing of solvent/oil ratio as solvent/oil ratio< 0.3, and the desulfurization percentage increased slightly with the increasing of solvent/oil ratio as solvent/oil ratio > 0.3 and a suitable solvent/oil ratio could be in a range of 0.3 ~ 0.8. Further experimental results of FCC gasoline medium fraction ED desulfurization-dearomation with sulfolane indicated that under the conditions of reflux ratio 4 and solvent/oil ratio 0.55, in the raffinate, the sulfur content could be reduced to less than 30μg/g, the benzene content less than 0.5% and the aromatics content less than 3%. The results of stability experiments showed that the recovered sulfolane with a good chemical stability can be reused.Liquid-liquid equilibria (LLE) data of seven ternary systems of sulfide - octane - solvent at 40,50 and 60℃under atmospheric pressure were measured using an equilibria cell,the systems included: 1) thiophene- octane - dimethyl sulfoxide, 2) thiophene - octane - sulfolane, 3) thiophene- octane - teraethylene glycol, 4) n-butyl mercaptan -octane -sulfolane, 5) n-butyl mercaptan- octane - teraethylene glycol, 6) n-dibutyl sulfide- octane- sulfolane, 7) n-dibutyl sulfide- octane- teraethylene glycol. NRTL and UNIQUAC models were used to correlate and predict the LLE data of the ternary systems. The model parameters were estimated using the simplex optimization method and the quasi-Newton method with an objective function of mass fraction deviation square and the rule of minimum Gibbs free energy in a closed thermodynamic equilibrium system. The results indicated that NRTL model was better than UNIQUAC model, and the average absolute deviation of thiophene mass percentage is ca 0.0050, n-butyl mercaptan ca 0.0400, and n-dibutyl sulfide ca 0.0342. This implies that NRTL model is more suitable for describing the Liquid-liquid equilibria of the ternary systems studied.Vapour-liquid equilibria (VLE) data of four binary systems of the sulfides-octane/ sulfolane were measured using a dual-circulating VLE cell, and the systems included: 1) thiophene- octane, 2) thiophene-sulfolane, 3) n-butyl mercaptan-octane, 4) n-butyl mercaptan -sulfolane. var Laar, Wilson, NRTL and UNIQUAC models were employed to correlate the VLE data and predict phase composition. Prediction of the UNIQUAC model was better than those of the other three models, and average mass fraction absolute deviations of thiophene and n-butyl mercaptan are respectively ca 0.0614 and ca 0.0505. The VLE data of the four systems were satisfied with Herrington thermodynamic consistency.The SRK-KD and NRTL equations were used to simulate the ED process of FCC gasoline desulfurization using Pro II software. The inter-action parameters used in the simulation were determined using NRTL and UNIFAC activity coefficient models with the experimental VLE and LLE data. The simulated and the experimental results of the ED process were in fairly agreetment. Optimal operation conditions of FCC gasoline extraction distillation desulfurization were obtained as follows: theory stage number NT 10～15, solvent/oil ratio 0.5, reflux ratio 4, solvent feeded in the 4th tray, and oil feeded in the 3rd or 4th tray from bottom. Based on the experimental and simulated results, a combined deep desulfurization process of ED- HDS was suggusted.