Measurement And Thermodynamic Modeling Of Vapor-Liquid Equilibria For Systems Containing Methanol, Ethanol, Water And Ionic Liquids

Ionic Liquids (ILs) are one kind of novel ionic compounds, whose properties are between that of "molecular solvent" and "inorganic electrolyte". With respect to the "solvent" property, ILs exhibit excellent solubility with most of the polar solvents; With regard to the "electrolyte" property, ILs represent the ability of generating the salt effect in the polar solvents. Besides, the negligible vapor pressure and the good thermal stability of ILs indicat their safety in operation and ease of regeneration. The liquid feature of ILs at ambient temperature means their convenience in storage and transportation in a process industry. Therefore, ILs can be used as a new kind of solvents in the field of cleaner production and process development toward energy saving and emission reduction.The prerequisite for the industrial application of ILs is that their thermodynamic properties and thermodynamic models are available so as to facilitate the design of the process technology. In order for screening some suitable ILs as salt extraction solvents for the distillation separation of ethanol from its mixture with water and methanol, a series of vapor-liquid equilibrium (VLE) data for systems containing methanol, ethanol, water, and an ionic liquid (IL) were measured, the affinity between different ILs and solvents was analyzed, and some candidate ILs were recommended accordingly. In addition, the experimental data also lay the foundation for the development of thermodynamic models. The main objective of this thesis is to develop a group contribution based thermodynamic model so as to achieve a cross prediction of some thermodynamic properties, e.g. density, boiling temperature, critical properties and vapor pressure and so forth, on the basis of limited available experimental data of pure ILs.The major work and innovative results of this thesis are as follows:(1) Five hydrophilic ILs have been designed and synthesized, namely, triethylmethylammonium dimethylphosphate ([N1222][DMP]), 1-methylimidazolium hydrochloride ([MIm][C1]), mono(2-hydroxyethyl) ammonium tetrafluoroborate ([HMEA][BF4]), di(2-hydroxyethyl) ammonium tetrafluoroborate ([HDEA][BF4]), and tri(2-hydroxyethyl) ammonium tetrafluoroborate ([HTEA][BF4]). The structures of five ILs are confirmed by 1H NMR and 13C NMR, and the corresponding purities of ILs are estimated as x≥99.4%; the melting points and thermal stability are characterized by the thermal analysis (namely, TQ DSC and DTA). All five ILs have the melting points below 75℃, and the decomposition temperatures above 200℃, with the exception of [MIm][C1], whose decomposition temperature is approximately 144～150℃. In summary, the five ILs synthesized are of high purity, good thermal stability, and good solubility with polar solvents like methanol, ethanol and water, and applicability as potential entrainers in the extractive distillation process.(2) Vapor pressure data were first measured using a non-analytical quasi-static ebulliometer for 15 binary systems and 7 ternary systems containing methanol, ethanol, water, and one of the five ILs described above at varying temperatures and IL contents. The NRTL model of nonelectrolyte solution with three parameters version was employed to correlate the vapor pressure data. The correlated deviations for IL-containing binary and ternary systems in terms of an overall average absolute relative deviation (AARD) were 1.43% and 0.92%, respectively, and the binary parameters can be used to predict the vapor pressures of the corresponding ternary systems with an overall AARD of 1.41%. The excellent correlated and predicted results justify the applicability of the NRTL model for the representation of VLE data of IL-containing systems.(3) Based on the experimental data above for 15 binary systems containing methanol, ethanol, water, and one of the five ILs, the effect of temperature, ILs type and their concentrations on the activity coefficients of different solvents were analyzed. The results showed that (Ⅰ) all ILs studied can depress the vapor pressure of three solvents studied but to different contents depending on the nature of ILs and the polarity of solvents; (Ⅱ) for all solvents studied, their activity coefficients always increase with temperature and approach unity eventually; (III) three binary systems containing ethanol and one of three hydroxyethyl ammonium-based ILs, namely, [HMEA][BF4], [HDEA][BF4], and [HTEA][BF4], show a positive deviation from the Raoult's law (γ>1.0), while the other 12 binary systems show a negative one (γ<1.0); the higher the IL concentration is, the higher the nonideality of the solution is; the binary systems with the largest positive and negative deviation were observed for{ethanol+[HTEA][BF4]} and {water+[N1222][DMP]}, respectively, suggesting the different affinity between different ILs and the solvents; (IV) for a specified IL at the same temperature and IL concentration, the activity coefficients of three solvents are in order of water<methanol<ethanol, indicating the strongest affinity between IL and water, and the weakest affinity between IL and ethanol.(4) Isobaric vapor-liquid equilibria (VLE) for the azeotrope mixture {water+ethanol} and close boiling mixture{methanol+ethanol} in the presence of [N1222][DMP] at 101.33 kPa were predicted by the NRTL model on the basis of fitted NRTL binary parameters, respectively. The results indicate that the relative volatiles of ethanol to water or methanol were both enhanced with the addition of [N1222][DMP], especially when the IL content was up to w≥30%, the azeotrope of{water+ethanol} was even removed completely, and the methanol component in {methanol+ethanol} mixture was gradually reversed from a light component to a heavy one in the ethanol-rich region due to the salt effect of [N1222][DMP]. Therefore, the hardly separating mixture of {water+ethanol+methanol} can be sufficiently facilitated at a specified content of IL, whereby both methanol and water are separated from the bottom of the distillation tower. By comparing the effect of the five ILs on the isobaric VLE of {water+ ethanol}, we observed that only [MIm][C1] showed a notable salting-out effect on ethanol in the water-rich region, indicating the largest affinity between IL [MIm][Cl] and water, while in the ethanol-rich region, all ILs showed a salting-out effect on ethanol and followed the order of [HTEA][BF4]>[HDEA][BF4]>[N1222][DMP]≥[MIm][C1]> [HMEA][BF4], suggesting that [HTEA][BF4] can most sufficiently facilitate the distillation separation for the azeotrope mixture of {water+ethanol}.(5) A new group contribution (GC) equation of state (EOS) model, viz the GC-PT model, was proposed for the prediction of thermodynamic properties of pure ILs based on combination of the GC concept with the cubic Patel-Teja EOS. A series of 47 group contribution increments (viz.ΔTb,ΔTc,Δpc, andΔVc), including 7 new ionic groups, were obtained through correlating a large number of density data of varying types of ILs at ambient temperatures and atmospheric pressure, and the resulting correlative deviation in terms of AARD was 4.4%; The group increments thus determined were applicable for both the GC-PT model and the Valderrama density correlations (namely, VSD, VSY, and LGM), and the minimum calculated deviation was observed for the GC-PT model. In addition, the GC-PT model along with the group increments given above can be used to predict the densities of different kinds of ILs at varying temperatures and pressures, which justified the reasonability of the group increments and the applicability of the GC-PT model.(6) The vapor pressures of 5 imidazolium-based ILs at different temperatures were predicted by the GC-PT model and compared with the corresponding experimental data and the estimated values taken from the references. The results indicate that the predicted results by the GC-PT model are generally of 1-3 order-of-magnitude difference with the experimental values, which, however, are superior to that predicted by the COSMO-RS model and the Clausius-Clapeyron equation. Further, the predicted results by the GC-PT model show a better regularity than the experimental ones. In addition, the vapor pressures of 6 ILs at T=298.15K were also predicted by the GC-PT model, the resulting vapor pressures of the ILs are all lower than 10-7 kPa at ambient temperature, which are extremely low and can be deemed as nonvolatile. It can be concluded that the GC-PT model proposed in this work can reasonably predicted the thermodynamic properties of different ILs, such as the density, the normal boiling temperature, the critical properties and the vapor pressures.