Study the Vapor-Liquid Equilibrium and Excess Volume for the Binary Mixtures of Aqueous Solutions of Two Industrial Petroleum Solvents and Aromatic Hydrocarbon

Isobaric vapor-liquid equilibrium data were determined at 101.33 kPa for the binary mixtures of water + sulfolane and water + tetraethylene glycol. Calculations of the non ideality of the vapor phase were made with the second virial coefficients obtained from the Hayden-O’Connell method. The boiling points and vapor-phase compositions reported were well correlated by the Wilson, NRTL, and UNIQUAC models. In addition, the densities of binary mixtures of water + sulfolane, water + tetraethylene glycol, and benzene + tetraethylene glycol were determined over the entire concentration range at 298.15K and atmospheric pressure. Excess volumes were calculated for each data point. All mixtures exhibit negative excess volumes with a minimum which occurs at for the benzene + tetraethylene glycol system; it is shifted toward the water-rich region for the water + sulfolane and water + tetraethylene glycol systems. The experimental excess volumes were correlated using the Redlich-Kister equation.


Introduction
Solvent extraction is one of the most important methods to produce high-purity aromatic extracts from catalytic reformates. In recent years, sulfolane or tetraethylene glycol has been employed more and more in new or improved extraction processes. Therefore, it is necessary to have complete thermodynamic data for these systems. Several researchers [1][2][3][4] have studied the vapor-liquid and liquid-liquid equilibria for mixtures containing an aromatic and sulfolane or tetraethylene glycol. Herskowitz and Gottlieb [5] reported the activity coefficients of the solvent in the water + tetraethylene glycol system by an isopiestic method at 298.15 K.
For the volumetric properties, excess volumes for a nitrile + sulfolane [6,7] and an aromatic hydrocarbon + sulfolane [2,8] have been well documented for a reasonable range of temperature. To our knowledge, for the systems involving tetraethylene glycol, only aqueous solutions and solutions with HFC-134a have been studied [9]. Lepori and Mollica [10] have reported the volumetric properties of dilute aqueous solutions of tetraethylene glycol, and some density data for this system have been given by Muller and Rasmussen [11].
In this work, the isobaric vapor-liquid equilibrium data for the mixtures of water + sulfolane and water + tetraethylene glycol were measured at 101.33kPa, and the excess volumes of water + sulfolane, water + tetraethylene glycol, and benzene + tetraethylene glycol systems were reported at 298.15K, covering the whole concentration range.

Experimental SectionChemicals
Pure benzene, sulfolane and tetraethylene glycol were obtained from Fluka. All the chemicals were purified by distillation (sulfolane and tetraethylene glycol distilled under vacuum), and their middle fractions were collected. Then they were dried using type 0.5nm molecular sieves and the water content was found to be<0.01 mass%, as determined with a Mitsubishi moisture meter (Model CA-05). To minimize the contact of these deliquescent reagents with moist air, all the purified chemicals were kept in sealed bottles in desiccators. Doubly distilled water was used in all solutions.The purity was tested by GLC, which indicated a minimum purity of 99.8 mol%, and no appreciable peaks of impurities were detected.The densities and refractive indices of the purified chemicals are reported in Table 1 in comparison with the literature data [12,13] at 298.15K and atmospheric pressure.

Vapor-Liquid Equilibrium Measurements
An inclined ebulliometer with a pumplike stirrer developed by Zhou et al. [14] and described by Yu et al. [3] was used. It is a recirculation type, in which both liquid and vapor phase recirculate continuously, and the equilibrium compositions of both phases can be determined. The ebulliometer was operated at atmospheric pressure (101.33kPa). The steady state was usually reached after 1.5h of operation. The temperature in the equilibrium chamber was measured using a mercury thermometer. The uncertainty in the temperature measurement is±0.05K.
The compositions of the sampled liquid phase and vapor phase were analyzed by measuring their densities at 298.15K with a vibrating tube densimeter, after calibration with gravimetrically prepared standard solutions. The accuracy of the equilibrium composition measurements was±0.0003 mole fraction. Since there is a large difference between the boiling points of water and of tetraethylene glycol, the amount of tetraethylene glycol present in the vapor phase was so small that is was not detected by the densimeter. Therefore, the vapor-phase composition was not measured for the water + tetraethylene glycol system.

Density Measurements
An Anton Paar (Graz, Austria) DMA60 oscillating tube densimeter equipped with a density measuring cell (DMA602) was used to measure densities of pure components and binary mixtures at 298.15K. The temperature of the U-shaped tube was checked continuously using a calibrated digital thermometer (Anton Paar DT100-20) with an accuracy of±0.01K. The system was maintained at constant temperature to within±0.005K by means of a Hetotherm thermostated water bath (Heto, Type CB7). All the measurements were carried out at atmospheric pressure.
The density determination is based on measuring the period of oscillation of a vibrating U-shaped hollow tube that is filled with the sample. The relation between the period τ and density ρ is where A and B are temperature-dependent constants determined by calibration with doubly distilled water and dry air. The precision for the measured period tis 1×10 -6 s, which leads to a precision in densities and excess molar volumes of ± 5×10 -5 g. cm -3 and ± 3×10 -3 cm 3 .mol -1 , respectively.
The apparatus, the mixture standard sample preparation, and the procedure of density measurements have been described elsewhere [3,15].
The density ρ of the binary solutions was used to calculate the excess molar volume V E (cm 3 .mol -1 ), according to: Where x i , is the mole fraction, M i is the molar mass, and r i , is the density of component i.

Recent Advances in Petrochemical Science
Isobaric vapor-liquid equilibria were measured at 101.33kPa for water + sulfolane and water + tetraethylene glycol systems. The results are shown in Tables 2 & 3, respectively. The liquidphase activity coefficients g i , were calculated with Wheref i , and f sa t are the fugacity coefficients of component i in the mixture and pure vapor, respectively, T is the equilibrium temperature, P is the pressure, P sat i is the saturated vapor pressure, V L i is the saturated liquid molar volume, x i is the liquidphase molar fraction, and y i is the vapor-phase molar fraction of component i.   [3], are listed in Table 5. The value of the solvation parameter is taken as zero for the water + sulfolane system and 1.55 for the water + tetraethylene glycol system. The liquid molar volumes were calculated from the Hankinson-Brobst-Thomson equation [16].
The calculated activity coefficients for the water + sulfolane mixture are also listed in Table 2. The experimental vaporliquid equilibrium data for the water+sulfolane system were shown to be thermodynamically consistent by using a Herington analysis (Herington, 1951) with (D-J)<10 and the test described by Fredenslund et al. [23]. The activity coefficients of water in tetraethylene glycol were calculated by neglecting the amount of tetraethylene glycol present in the vapor phase. The values of g i for the water + tetraethylene glycol system are <1. This is in agreement with the result of Herskowitz and Gottlieb [5] with an isopiestic method at 298.15K.
The equation of the Pure Vapor Pressure,   Where N is the number of experimental points, w=0.5 for the water + sulfolane system, and w=0 for the water + tetraethylene glycol system because the vapor-phase composition for the latter system was not measured in this work. The regression results are shown in Table 7. The deviations in vapor-phase compositions and equilibrium temperatures are reasonably small, and this indicates that all three activity coefficient models are suitable to represent the binary experimental data. Where N is the number of experimental points.
The comparison between the experimental activity coefficients and the prediction by the NRTL model for the systems of water + sulfolane and water + tetraethylene glycol is shown in Figure  1 & 2. The experimental densities and excess molar volumes for the binary mixtures of water + sulfolane, water + tetraethylene glycol, and benzene + tetraethylene glycol at 298.15K are given in Table 8. The excess molar volume as a function of composition is graphically represented in Figure 3. The measured excess molar volumes VE were fitted to the following expression by a Maximum Likelihood Principle method:   Coefficients a j are given in Table 9 along with the standard deviation σ for each system, defined as ( )

Recent Advances in Petrochemical Science
Where m is the number of adjustable parameters a j in equation 11. The superscripts calc and exp refer to the values calculated by using equations 11 and 2, respectively. The literature excess volumes for the water+tetraethylene glycol system reported by Muller and Rasmussen [11] are also included in Figure 3. Good agreement was seen between the literature values and the present data.
It can be seen from Table 8 that, for all cases, the excess volumes are negative. For the benzene + tetraethylene glycol system, the minimum excess molar volume occurs at x ≅ 0.5, and it shifted toward the water-rich region for the water + sulfolane and water + tetraethylene glycol systems, as shown in Figure 3. Excess molar volumes for aqueous tetraethylene glycol solutions are smaller than those for the other two systems studied; this can be interpreted as an increase in the order of the system due to the formation of hydrogen bonds between water and tetraethylene glycol molecules.   , 1991)). The curves were calculated with eq 2.

Conclusion
Vapor-liquid equilibria of two binary mixtures of water + sulfolane and water + tetraethylene glycol were measured at 101.33kPa and tested by using the Herington analysis [28] and the method of Frendenslund et al. [23]. Analysis of the experimental vapor-liquid equilibrium data for the two binary systems (by using the Wilson, NRTL, and UNIQUAC models) shows that all three models generally give satisfactory results. Densities and excess volumes for three mixtures of water + sulfolane, water + tetraethylene glycol, and benzene + tetraethylene glycol were determined at 298.15K. The excess volumes for the three binary systems were successfully correlated using the Redlich-Kister equation [29,30].