| Light oil refers to a kind of hydrocarbon compound with a boiling point below 200?,which has been widely used as fuel,catalytic reforming,chemical raw materials for ammonia,ethylene,methanol and synthetic liquid fuels.Mercaptan is the most common sulfur compound in light fuel.Its existence not only affects the quality of oil,but also generates SOx after combustion,which will cause air pollution.On the other hand,C4 hydrocarbons will form azeotrope with methanol in the MTBE production and then will affect the yield and octane number of alkylated oil.In view of the problems of the traditional separation process of mercaptan and methanol from the light oil,such as high equipment requirements,complex operation conditions,high energy consumption and incomplete separation,it is urgent to develop a new technology of energy saving and environmental protection for the removal of mercaptan and methanol.In this paper,butyl mercaptan-hexane and methanol-hexane were used as model oil,respectively.Ionic liquid was applied to the removal of mercaptan and methanol in light fuel.Based on the selected ionic liquids,the effect of extraction conditions on the extraction efficiency of mercaptan/methanol was systematically investigated.The extraction mechanism was discussed and Merox process enhancement was achieved by using the composite extractants.In view of the removal process of mercaptans from light oil,1-butly mercaptan-n-hexane system was chosen as model oil.The ionic liquid[Bmim][Ac]with the best selectivity and solubility was screened out based on COSMO-RS method.The effects of the mass ratio of ionic liquids[Bmim][Ac]to model oil,extraction time,extraction temperature and the number of cycles on the 1-butyl mercaptan extraction efficiency were systematically investigated by single factor experiments.The optimal extraction conditions of the system was confirmed,including the mass ratio of[Bmim][Ac]:model oil at 1:20,extraction time at 11h and extraction temperature at 25?.Under this condition,the single-stage extraction rate of 1-butyl mercaptan reached 95.8%,and the content of 1-butyl mercaptan was less than 10 ppm in the oil phase.The extraction system could be recycled 5 times with no dramatic degradation in the extraction performance.Furthermore,'H NMR was performed to investigate the interaction bewtwen[Bmim][Ac]and 1-butyl mercaptan.It can be seen that the application of ionic liquid[Bmim][Ac]as an extractant in the process of mercaptan gained the good extraction results due to hydrogen bonding interaction,which can also replace the traditional process of removal of mercaptan.In order to study the extraction mechanism of[Bmim][Ac],density functional theory and natural bond orbit method were applied to investigate interaction between anion and cation,interaction between 1-butyl mercaptan and[Bmim][Ac],charge distribution and orbital interaction in the system.The results indicated that the hydrogen bonds exist in ionic liquid[Bmim][Ac],which is one of the reasons for the thermodynamic stability of[Bmim][Ac].It is[Ac]-plays a vital role when[Bmim][Ac]interacting with 1-butyl mercaptan,and the binding energy is-438.70kJ/mol.According to the result of the charge distribution and relatedsecond-order perturbation energies E(2),it is found that there is an obvious charge transfer in the position of the hydrogen bond between[Bmim][Ac]and 1-butyl mercaptan,and there is a strong interaction between anti bonding orbital ?*(C3-H6)and ?*(S46-H47).By capering the the formation types of hydrogen bonds between[Bmim][Ac]and 1-butyl mercaptanl,it's found that a more stable red-shift hydrogen bond S46-H47...O32 with anion[Ac]-was formed,which weakens the interaction between anion[Ac]-and C3-H6 on the imidazole ring,resulting in blue shift of hydrogen bond.It is known that the hydrogen bonding interaction exist in the ionic liquid[Bmim][Ac]as well as ionic liquid[Bmim][Ac]and mercaptan.The mercaptan can be extracted from n-hexane to the[Bmim][Ac]due to the stable hydrogen bonding interaction.In the study of lye/ionic liquid as a composite extractant extractant to strengthen Merox mercaptan removing process,it is found that NaOHaq and ionic liquid[Bmim][Ac]exhibited the best performance.In order to strengthen the Merox process,the mercaptan removal experiments using alkali/ionic liquids as composite extractants were carried out.The results showed that the mercaptan removal performance of NaOH aqueous solution/ionic liquid[Bmim][Ac]composite extractant was better than that of NaOH aqueous solution alone.Under the condition of the mass ratio of moii:mNaOH:mIL=20:20:0.5,extraction time at 2 min,and extraction temperature at 25?,the single-stage extraction rate of 1-butyl mercaptan reached 96.3%.Under the same conditions,the extraction rate was only 87.7%when using NaOH aqueous alone.Furthermore,the result of molecular surface charge density and excess enthalpy indicated that there existed strong hydrogen bond in the mixed system which contribute most to the excess enthalpy.The ionic liquid with the best selectivity and solubility selected by COSMO-RS method was used for separation of methanol/alkanes.The results showed that ionic liquid[Bmim][DBP]is a good extractant separation of alkane/methanol system owing to its high selectivity and good thermodynamic stability.The optimal extraction conditions of the system was confirmed,including the mass ratio of[Bmim][DBP]:model oil at 1:2,extraction time at 60 min and extraction temperature at 25?.Under this condition,the single-stage extraction rate of methanol reached 99.65%.The extraction system could be recycled 6 times with no dramatic degradation in the extraction performance.By comparing the ionic liquid before and after methanol mixing by 1H NMR,it was found that there was hydrogen bond between[Bmim][DBP]and methanol,which was the main reason why ionic liquid[Bmim][DBP]had good extraction demethanol performance.The density functional theory and the natural orbit method were used to analyze interaction between anion and cation,interaction between methanol and[Bmim][DBP],charge distribution and orbital interaction in the system.The calculation results showed that hydrogen bonds also exist in ionic liquid[Bmim][DBP].When[Bmim][DBP]interacts with methanol,the cation[Bmim]+ and anion[DBP]-can both form hydrogen bond with methanol at the same time.The hydrogen bond position indicates that methanol is both a hydrogen bond donor and a hydrogen bond acceptor.According to the result of the charge distribution and relatedsecond-order perturbation energies E(2),it is found that there is an obvious charge transfer in the position of the hydrogen bond between[Bmim][DBP]and methanol,and there is a strong interaction between LP(060)and ?*(C13-H15),LP(027)and ?*(O60-H61)as well.Thus,the effect of[Bmim][DBP]on the separation of methanol is also affected by the hydrogen bonding interaction. |
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