Vapor-Liquid Equilibrium Measurement and Modeling for Sytems Containing Ionic Liquids
|School||Beijing University of Chemical Technology|
|Keywords||ionic liquid vapor-liquid equilibrium NRTL UNIFAC mean spherical approximation perturbation theory group contribution model|
Ionic liquids （ILs） as a new kind of solvent have many unique attributes, e.g. negligible vapor pressure, tunable solubility for both polar and non-polar substances, good thermal and chemical stability and electrolytic property, which make them attractive and potential being used as a benign medium and/or solvent in chemical reaction and separation processes. In order to promote their industrial applications in separation processes, phase equilibrium data for IL-containing systems is of vital importance, however, which is extremely scarce heretofore. Toward this end, vapor-liquid equilibrium （VLE） data were measured and their modeling work was carried out in this paper.At present, the price of ILs is extremely high in comparison with traditional molecular solvents, which to a large extent limits its commercialization. In order to reduce the price of ILs, it is required that the production and separation process is as simple as possible along with a high yield for the production process of ILs. Among various ILs known heretofore, the ILs with such anions as halides or alkylphosphates seem more competitive since they can be produced via a one step reaction with very high yield using N-methylimidazole and trialkylphosphate or alkyl halides as feedstocks. Such kind of ILs prepared in this thesis include 1-methyl-3-methylimidazolium dimethylphosphate （[MMIM][DMP]）, -ethyl-3-methylimidazolium diethylphosphate （[EMIM][DEP]）, 1-butyl-3-methylimidazoliumdibutylphosphate （[BMIM][DBP]）, l-butyl-3-methylimidazolium chloride （[BMIM][Cl]） and l-butyl-3-methylimidazolium bromide （[BMIM][Br]）. Besides, 1-butyl-3-methylimidazolium hexafluorophosphate （[BMIM][PF6]） was also prepared via a metathesis reaction for comparison with the above ILs. The ILs with alkylphosphate anion show some notable merits, namely ease of preparation and purification, high reaction yield（>90%）, low cost being about 30-40% of other conventional ILs, as well as their low toxicity to the environment, therefore, they are important and promising ILs with great potential to be used in some industrial applications. In this regard, the influence of theses ILs on the VLE of water, ethanol and methanol was studied in detail.Isobaric VLE data for ethanol-water systems containing an IL, viz, [MMIM][DMP], [EMIM][DEP], [BMIM][Cl], [BMIM][Br] or [BMIM][PF6] at different IL contents（from about 10wt% to 30wt%） were measured at atmospheric pressure（101.32kPa） by a dual circulation vapor-liquid equilibrium still （CP-I type）, and the effect of ILs on the VLE behavior of ethanol-water system was discussed briefly. The equilibrium compositions of the volatile components in both vapor and liquid phase were analyzed by gas chromatography, while the IL content in the liquid phase was determined with gravimetric method. The results indicated that all ILs studied showed a salting out effect for ethanol, giving rise to an enhancement of relative volatility of ethanol and even to an elimination of the azeotropic phenomenon at specific IL content. For [BMIM]+ series ILs, their salting out effect on ethanol follows the order of [BMIM][Cl] > [BMIM][Br] > [BMIM][PF6], and for the phosphate-type ILs, the salting out effect on ethanol follows the order of [MMIM][DMP]>[EMIM][DEP].Vapor pressure data were measured for nine binary systems containing water, methanol or ethanol with an IL, viz. [MMIM][DMP], [EMIM][DEP] or [BMIM][DBP] and four ternary systems, namely ethanol+methanol+[MMIM] [DMP], ethanol+methanol+[EMIM][DEP], ethanol+methanol+[BMIM][DBP], and ethanol+water+[MMIM][DMP] at varying temperature and IL mass percent using a quasi-static method. The experimental results of binary systems indicated that the ILs studied can lower the vapor pressure of solvents due to the affinity between IL and the solvent, while the lowering degree depended on the type and content of IL involved. Based on the vapor pressure data of binary systems interpolated at mole fraction of 0.05 for IL component, the influence of different ILs on the vapor pressure of different solvent was elucidated. It was showed that the effect of ILs on the vapor pressure lowering followed the order [MMIM][DMP]>[EMIM][DEP]>[BMIM][DBP] for water, while a reverse order was found for methanol and ethanol, suggesting that the "ionic" characteristic of ILs dominates in water, while the "molecule" characteristic prevails in organic solvent.The vapor pressure data of binary systems containing ILs studied were correlated by traditional NRTL model of non-electrolyte solution. The results showed that the NRTL model is applicable for the correlation of binary VLE data with overall average absolute relative deviation （ARD） being 0.89% and the maximum ARD within 2%. The fitted binary NRTL parameters can be used to predict the vapor pressure of the ternary system ethanol-water-[MMIM][DMP] with ARD of 2.8%. In order to check the reliability of the model parameters, the infinite dilution activity coefficients of water, ethanol and methanol at 353.15K in [MMIM][DMP] were predicted based on the binary NRTL parameters and the predicted values were found being qualitatively consistent with experimental ones, indicating that the NRTL model can be used to correlate and predict the VLE data of the systems containing ILs. In order to have a better representation for the ternary VLE behavior, the binary NRTL parameters for IL-solvent pair were finely readjusted in terms of the experimental vapor pressure data for ternary systems, on this basis, the isothermal VLE data for ethanol-methanol-[MMIM][DMP], ethanol-methanol-[EMIM][DEP], ethanol-methanol-[BMIM][DBP] and ethanol-water-[MMIM][DMP] systems at 320 K and IL mass fraction of 50% were predicted. The results indicated that①All ILs show a salting-out effect for ethanol and the salt effect follows the order [EMIM][DEP]> [MMIM][DMP]>[BMIM][DBP] for ethanol+methanol system.②The methanol component in the ethanol+methanol system is converted from a lighter component to a heavier one due to its stronger affinity to the IL involved.③The azeotropic phenomenon in the ethanol+water system can be completely removed under certain content of ILs. In conclusion, the addition of IL is helpful for the separation of methanol and water from their ethanol solutions.Based on the mean spherical approximation （MSA） integral equation theory, perturbation theory and group contribution concept, a new molecular thermodynamic model was established for the representation of activity coefficient of solvent in the IL-containing systems. In this model, the chemical potential arising from long range electrostatic interaction between ionic species, and the middle range interaction between ion and dipolar molecules were accounted for using mean spherical approximation （MSA） and perturbation theory, respectively, while the short range interaction contribution arising from dispersion and induction effects etc. among various groups of ILs and solvent molecules was represented using the UNIFAC group contribution model. The performance of the new model was preliminarily tested for the correlation of vapor pressure data for IL-containing binary systems, and good correlation accuracy was obtained. Moreover, the contribution of the above three kind of interaction on the activity coefficient of solvent as well as their dependence on the temperature and liquid composition was analyzed using the regressed model parameters, which laid a foundation for the further improvement of the model.