Chen Yao1,2, Dong Yanhui1,2, Zhang Shengli1,2
Department of Chemistry, Jinan University; Institute of Nanochemistry, Jinan University, 510632, China)
Received on Sept. 7, 2006; Supported by Foundation of Ministry of Education (No.2002247), Foundation of Jinan University (No.640071) and Foundation of Guangdong province (No.2003C33101).
AbstractLiquidliquid equilibrium tie line data were determined for the quaternary system of water + ethanol + diisopropyl ether + heptane at 298.15 K and ambient pressure. The experimental liquidliquid equilibrium results have been successfully correlated by a modified and an extended UNIQUAC models both with ternary and quaternary parameters in addition to binary ones.
Keywords Liquidliquid equilibria, Oxygenated compounds, Ternary and quaternary mixtures, Modified and extended UNIQUAC models
In recent years there has been growing interest in the use of oxygenate additives to improve gasoline performance as anti-knocking agents and reduction air pollution. Reformulated gasoline includes certain oxygenated compounds such as alcohols and ethers. These are commonly methanol, ethanol, propanol, and butanol as well as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), diisopropyl ether (DIPE) and tert-amyl methyl ether (TAME). DIPE is an ideal fuel additive to replace MTBE because of its high octane number and lower vapor pressure. To assess the solubility of DIPE in the alcohol aqueous with hydrocarbon, we continue to study the phase behavior of liquid alcohol aqueous mixtures of DIPE with gasoline-substitution hydrocarbons. Here we report liquidliquid equilibrium (LLE) measurements on one quaternary system water + ethanol + DIPE + heptane at 298.15 K. The experimental LLE data were correlated by means of the modified UNIQUAC and extended UNIQUAC models[1,2]including both ternary and quaternary parameters coming from multicomponent intermolecular interactions, in addition to binary parameters. The constituent ternary systems of water + ethanol + heptane, water + ethanol + DIPE, water + DIPE + heptane, were used to obtain ternary parameters for accurate representation of the quaternary LLE system studied in this work. The binary parameters of miscible binary mixtures of constituents of the ternary and quaternary systems were obtained from vaporliquid equilibrium data[5-8and those of immiscible mixtures were obtained from mutual solubility data [3,9]
DIPE was provided by the Tianjin Kermel Chemical Reagents Development Center, with a mass fraction of 99.5 % at the fewest. Ethanol was supplied by Guangzhou Chemical Reagent Factory, with a minimum mass fraction of 99.5 %. -Heptane was supplied by the Guangzhou Chemical Reagent Factory, with a minimum mass fractions of 99.7 %. All chemicals were used without further purification. For GC analysis, appreciable peaks have not been detected. Water was distilled twice.
2.2 Apparatus and Procedures
Ternary and quaternary LLE measurements were carried out at 298.15 ± 0.01 K. The quaternary mixtures were prepared by mixing stepwise the binary DIPE + -heptane mixtures whose compositions are , , and with water, and then ethanol to cover the two-phase regions. The values of , , and are approximate 0.25, 0.50, and 0.75, respectively, indicating the mole fraction of DIPE in the binary DIPE + -heptane mixtures. About 70 cm of each mixture was poured into the equilibrium glass cell placed in a thermostated water bath. The mixture was then stirred vigorously by magnetic stirrer for 3 h and then allowed to settle for 3 h, which was sufficient to separate into two liquid phases. Dry nitrogen gas was used to prevent contamination from moisture in the headspace of the equilibrium cell. Samples, withdrawn from upper and lower phases in the cell by a microsyringe, were analyzed by a gas chromatograph (GC-14C) equipped with a thermal conductivity detector. Each component of the ternary and quaternary mixtures was separated clearly, using a stainless steel column (2 m long, 3 mm i.d.) packed with Porapak QS. The temperatures of the injection system and detector system were all set at513.15 K. The initial temperature and final temperature of the oven was kept at 483.15 K and 453.15 K, respectively. The hydrogen flow rates for both the separation and reference columns were set at 1.1 cm―. The peak areas of the components, detected with a chromatopac (N2000), were calibrated with prepared known mixtures by mass. The mass of each component of the mixture was determined from the calibration and converted to mole fraction. Three analyses were done for each sample to obtain a mean value with a reproducibility of better than 0.1%. The accuracy of the measurements was estimated within ±0.001 in mole fraction.
2.3 Experimental results
Tables 1 and 2 show experimental LLE data for the water + ethanol + heptane and water + ethanol + DIPE + heptane mixtures.
Table 1 Equilibrium phase compositions in mole fraction for the ternary mixtures of water (1) + ethanol (2) + heptane (3) at 298.15 K
|Organic phase||Aqueous phase|
Table 2 Equilibrium phase compositions in mole fraction for the quaternary mixtures of water (1) + ethanol (2) + DIPE (3) + heptane (4) at 298.15 K
|Organic phase||Aqueous phase|
|water +ethanol+DIPE + (1- – – ) heptane }|
3. CALCULATION PROCEDURE AND RESULTS
3.1 Calculation procedure
We have used the modified UNIQUACand extended UNIQUACmodels with binary and additional ternary and quaternary parameters for an accurate description of the experimental quaternary LLE data and constituent ternary data as well as binary VLE and mutual solubility data.
The binary parameter defined by the binary energy parameter ji is expressed as
where ji can be obtained from binary experimental phase equilibrium data, and was set to 1 for the extended UNIQUAC and 0.65 for the modified UNIQUAC.
The binary energy parameters for the miscible mixtures were obtained from the VLE data reduction using the following thermodynamic equations
where , , , and? are the total pressure, the liquid-phase mole fraction, the vapor-phase mole fraction, and the activity coefficient, respectively. The pure component vapor pressure, , was calculated by using the Antoine equation with coefficients taken from the literatures[11,12]. The liquid molar volume, , was obtained by a modified Rackett equation[13. The fugacity coefficient,, was calculated by the Eqn.(3). The pure and cross second virial coefficients, B, were estimated by the method of Hayden and OConnell. The binary energy parameters for the partially miscible mixtures were obtained by solving the following thermodynamic equations simultaneously.
and ( I, II = two liquid phases ) (5)
The ternary and quaternary LLE calculations were carried out using the Eqns.(4) and (5). For the ternary systems of type 1 having a plait point, two-parameter UNIQUAC models predict generally larger solubility envelope than the experimental one. It is necessary to correlate ternary and quaternary LLE using ternary and quaternary parameters in addition to binary ones. The additional ternary parameterijkwas obtained by fitting the model to the ternary experimental LLE data and the quaternary parameterijklwas determined from the quaternary experimental LLE data using a simplex method by minimizing the objective function:
F = (6)
where min means minimum values, = 1 to 3 for ternary mixtures or =1 to 4 for quaternary mixtures, = phases and II, = 1,2,… (no. of tie lines), = 2ni, and = (the liquid-phase mole fraction).
3.2 Calculation results
Table 3 presents the constituent binary energy parameters of the modified and extended UNIQUAC models. Table 4 shows the ternary parameters obtained in fitting the modified and extended UNIQUAC models to the experimental ternary LLE systems, and root-mean-square deviation of the mole fraction of tie lines between the experimental and calculated results for the ternary LLE systems. It seems that the modified UNIQUAC model with the only binary parameters predicts the ternary LLEs more successfully than the extended UNIQUAC model, and these models can give a much more accurate representation for the ternary LLEs by including the ternary parameters in addition to the binary ones. Figure 1 compares the experimental and correlated liquidliquid equilibrium results of three boundary ternary systems making up the quaternary system at = 298.15 K. It shows good agreement between the experimental values and those correlated using additional ternary parameters in Figure 1. The quaternary system exhibits type 2 quaternary liquidliquid behavior, which are composed of two ternary liquidliquid equilibrium for the mixtures (water + ethanol + DIPE) and (water + ethanol + -heptane) classified as type 1, and one ternary liquidliquid equilibrium for the mixtures (water + DIPE + -heptane) as type 2.
Table 4 summarizes the quaternary LLE results predicted by the modified UNIQUAC and extended UNIQUAC models with the binary and ternary parameters, together with root-mean-square deviations. The root-mean-square (r.m.s.) deviations predicted using the binary and ternary parameters are slightly large for the water + ethanol + DIPE + -heptane system, but both models can describe accurately the quaternary experimental LLE data by the correlation involving the additional quaternary parameters, and the correlated r.m.s. is 1.88 mol% for the modified UNIQUAC model and 4.13 mol% for the extended UNIQUAC model.
Table 3Calculated results of binary phase equilibrium data reduction
|System (1+2)||T /K||No. of data points||Model||Energy parameters||Ref.|
|ethanol + water||298.15||10||
|ethanol + heptane||298.15||19||II||107.23
|ethanol + DIPE||344.17||II||24.30
|DIPE + heptane||342.35~363.45||10||II||244.92
|DIPE + water||298.15||MS||II||1590.60
|heptane + water||298.15||MS||II||1884.20
Modified UNIQUAC model;
Extended UNIQUAC model;
Table 4Calculated results for ternary liquid-liquid equilibrium at 298.15 K
|water + ethanol + -heptane||13||0.2730||0.9130||0.2883||4.78||0.55||this work|
|water + ethanol + DIPE||0.4224||1.4848||1.7460||1.78||1.42|||
|water + DIPE + -heptane||11||0.0270||0.3098||0.2238||0.51||0.22|||
Number of tie lines.
Modified UNIQUAC model
Extended UNIQUAC model.
Predicted results using only binary parameters.
Correlated results using binary and ternary parameters.
Root-mean-square deviation (mol%).
Table 5.Calculated Results for Quaternary LiquidLiquid Equilibria at 298.15 K
Number of data points.
Modified UNIQUAC model.
Extended UNIQUAC model.
Predicted results using binary parameters alone.
Correlated results using binary and quaternary parameters.
Figure 1 Experimental and calculated (liquid + liquid) equilibria of three ternary mixtures making up of (water + ethanol + DIPE + heptane) at = 298.15 K. ●- – -●, Experimental tie line; ——, correlated by the modified UNIQUAC model with binary and ternary parameters taken from Tables 3 and 4.
The quaternary LLE of the water + ethanol + DIPE + heptane system were measured at 298.15 K in this work. The experimental quaternary liquidliquid equilibrium data were successfully correlated by using both models including binary, ternary and quaternary parameters. The quaternary liquidliquid equilibrium results calculated by the modified UNIQUAC model are more suitable agreement with the experimental results.