Liquid extraction of acetic acid from aqueous solutions with various solvents has been reviewed by several researchers (Correa et al., 1989; Torul et al., 1992; Fahim et al., 1997; Aljimaz et al., 2000; Tamura et al., 2000). In addition, some advantages of high boiling separating agents have recently been reported (Ulusoy and Dramur, 1981; Tatli et al., 1987; Dramur and Tatli, 1993) and some quaternary systems have been investigated (Ruiz et al., 1985; Nagata, 1986 and 1993). In this research the mixtures of cyclohexanol and cyclohexyl acetate were used as mixed solvent in order to determine liquid-liquid equilibria (LLE) data for the water-acetic acid- cyclohexanol-cyclohexyl acetate quaternary system. LLE data for water-acetic acid-cyclohexyl acetate and water-acetic acid-cyclohexanol ternaries had been presented previously (Sayar et al.,1991:Tatli at al., 2000). It is seen that cyclohexanol has high distribution coefficient and low separation factor values, on the other hand cyclohexyl acetate shows lower distribution coefficient and higher separation factor values when the LLE data for water-acetic acid-cyclohexanol and water-acetic acid-cyclohexyl acetate ternary systems are compared. In this study our aim was to determine the LLE data of quaternary systems by using the mixture of this solvents in different composition and to show the trend of the distribution coefficients and separation factors for this quaternary systems. The mixed solvent of cyclohexanol and cyclohexyl acetate, which consists of completely miscible components, was prepared in three compositions (10%, 50% and 90%).

Acetic acid (100 mass %), cyclohexanol (reagent grade,99 mass %) and cyclohexyl acetate (reagent grade,99 mass %) were furnished by Merck. The purity of the chemicals was checked on the basis of their refractive indexes and densities at 293±0.20 K and their boiling points at 760±2 mmHg. Refractive indexes were measured with an Abbé-Hilger refractometer; its stated accuracy is ±0.0001 n_D. Densities were measured with a Westphal balance. Boiling point measurements were obtained by using a Fischer boiling point apparatus. The physical properties measured are listed in Table 1, along with (Weast, 1990) values from the literature.

Deionized water was further distilled before use.

Data for the solubility curve of the quaternary systems were determined by cloud point method. Solubility curve data determinations were made in an equilibrium cell equipped with a magnetic stirrer and an isothermal fluid jacket. The temperature of the mixture was regulated by a thermostated bath with an accuracy of ±0.2 K. The inner temperature of the cell was measured within an accuracy of ±0.1 K by a certified Fischer thermometer.

The cell, designed to contain a solution of 50-200 cm3, was filled with homogeneous water+acetic acid mixtures prepared by weighing. An electronic Sauter balance with an accuracy of ±0.1 mg was used. The mixed solvent was added by means of an automatic microburet with an accuracy of ±0.005 cm3 . The end point was determined by observing the transition from a homogeneous to a heterogeneous mixture. This pattern was convenient for providing the water-rich side of the curves. The data for the solvent-rich side of the curves were therefore obtained by titrating homogeneous acetic acid+mixed solvent with water until turbidity had appeared. Composition determinations were accurate to ±0.0005 weight fraction.

The mutual solubilities of water and mixed solvent were determined by applying a synthetic method. A weighed amount of a substance was introduced into the cell; the other was added until a permanent heterogeneity had been observed. An ultraaccurate titrator of ±0.001 cm³ was used.

The tie-line data determinations were obtained by using the equilibrium apparatus described above. Six different mixtures within the heterogeneous gap were prepared for the three study sets. The cell was filled with each of these mixtures and vigorously stirred for 1 h under isothermal conditions. After the stirrer was turned off, the contents were immediately introduced into the vertical settler, also equipped with an isothermal jacket. After the complete separation of the phases, a suitable amount of each layer was withdrawn for analysis. The acid contents of the samples were determined by volumetric titration with 0.1 N NaOH solutions to the ethanolic phenolphthalein end point. Several check determinations on known samples showed that the accuracy of the method was within ±0.001 of the weight fraction.

The mixed solvent was used in three compositions. Representation of the mixed solvent composition was characterized by a value of M which was defined as

The measured values for solubility curves and experimental mutual solubilities for the water-acetic acid-mixed solvent (cyclohexyl acetate-cyclohexanol) systems are reported in Table 2 (for M = 0.1, M = 0.5 and M = 0.9). The tie-line compositions for the water-acetic acid-mixed solvent systems are given in Table 3. The experimental tie-line compositions and solubility values of the systems for M = 0.1, M = 0.5 and M = 0.9 are plotted in Figures 1-3, respectively. The tetrahedral representation of the solubility surface of the quaternary system obtained from the experimental data is shown in Figure 4. The data for M = 1 and M = 0 are taken from the literature (Sayar et al., 1991; Tatlý et al., 2000).

The reliability of experimentally measured tie-line data can be ascertained by applying Othmer-Tobias and Hand Equations (Othmer and Tobias, 1942; Brandani and Ross, 1985). The Othmer-Tobias and Hand Correlation equations are given as equations (2) and (3), respectively.

The correlations are shown in Figures 5 and 6. The correlation coefficients and correlation factor (R²) values were determined by the least-squares method and are given in Table 4. A correlation factor (R²) close to 1 suggests a high degree of consistency of the related data.

In Figure 7 selectivity diagrams on a solvent-free basis are plotted for M = 0.1, M = 0.5 and M = 0.9.

Distribution coefficients, D_i, for acetic acid (i = 2) and water (i = 1) and separation factors, S, were determined as follows;

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