(Liquid + liquid) equilibrium of (water + 2-propanol + 1-butanol + salt) systems at T = 313.15 K and T = 353.15 K: Experimental data and correlation

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Abstract

(Liquid + liquid) equilibrium data for the quaternary systems (water + 2-propanol + 1-butanol + potassium bromide) and (water + 2-propanol + 1-butanol + magnesium chloride) were measured at T = 313.15 K and T = 353.15 K. The overall salt concentrations were 5 and 10 mass percent. Ternary (liquid + liquid) equilibrium data for the salt-free system (water + 2-propanol + 1-butanol) were also determined and found to be in good agreement with data from the literature. The NRTL model for the activity coefficient was used to correlate the data. New interaction parameters were estimated, using the Simplex minimization method and a concentration-based objective function. The results are very satisfactory, with root mean square deviations between experimental and calculated compositions of both phases being less than 0.5%.

Introduction

Aqueous solutions containing salts are of increasing importance and influence on separation processes in chemical engineering. The electrolyte influence must be considered both in process design and operation, because it can significantly change the equilibrium composition. Aqueous (liquid + liquid) equilibrium is the result of intermolecular forces, mainly of the hydrogen-bonding type; addition of a salt to such systems introduces ionic forces that affect the thermodynamic equilibrium. When the ions are solvated, some of the water becomes unavailable for the solution, and the organic solvent is “salted out” from the aqueous phase. This salt effect may be used for removing organic components from water. The salt effect is also important in biological separation processes, such as purification of proteins, enzymes, and others.

The separation by solvent extraction becomes increasingly more difficult as the tie-line becomes parallel to the solvent axis, as shown in solutropic solutions. By adding an adequate salt, the mutual solubility of the mixture can be significantly changed, in order to modify the slope of the tie-lines, even to the extent of eliminating the solutrope.

This work is a continuation of previous studies on the (liquid + liquid) equilibria of (water + solvent + solvent + salt) systems, determined in this laboratory. Aznar et al. [1] and Santos et al. [2] determined experimental data for six quaternary systems of the (water + ethanol + alcohol + salt) type. The alcohols used were 1-butanol, 3-methyl-1-butanol, and 1-pentanol, and the salts used were sodium chloride, sodium acetate, calcium chloride, potassium chloride, potassium sulfate, and potassium bromide. Santos et al. [3] studied the effect of the addition of sodium chloride and sodium acetate on the (water + 1-butanol + acetone) system. Pereira and Aznar [4] studied the effect of the addition of potassium bromide and magnesium chloride on the (water + 1-butanol + tert-butanol) system.

In this work, (liquid + liquid) equilibrium data of the quaternary systems (water + 2-propanol + 1-butanol + salt) were determined at T = 313.15 K and T = 353.15 K, using of 5% and 10% mass of potassium bromide and magnesium chloride in the overall mixture. Experimental data for the ternary, salt-free system were also determined at both temperatures, and those at T = 353.15 K were found in good agreement with those published by Morozov et al. [5]. The results are also compared with salt-free data published by Aicher et al. [6] at T = 293.15 K and T = 333.15 K.

The experimental (liquid + liquid) equilibrium data were correlated with the NRTL model by Renon and Prausnitz [7] for the activity coefficient. The procedure is based on the modified Simplex method by Nelder and Mead [8], by minimization of a concentration-based objective function.

Section snippets

Experimental

All the reagents, 2-propanol, 1-butanol, potassium bromide, and magnesium chloride, were of analytical grade (Merck) with purity >99.5%, confirmed by gas chromatography, and were used without further purification. Water was distilled twice before utilization.

Experiments were carried out in equilibrium cells, such as those suggested by Stragevitch [9] and described elsewhere [1]. The cell temperature was controlled by a thermostatic bath (Tecnal TE-184), with an uncertainty of ±0.01 K.

The overall

Experimental results and discussion

The experimental (liquid + liquid) equilibrium data for the ternary, salt-free system (water + 1-butanol + 2-propanol) and the quaternary systems (water + 1-butanol + 2-propanol + salt) are shown in TABLE 1, TABLE 2 as mass fractions. The same data are plotted in FIGURE 1, FIGURE 2. The salt-free data determined at T = 353.15 K were found in good agreement with those published by Morozov et al. [5]. For comparison, in both figures appear also salt-free data published by Aicher et al. [6] at T = 293.15 K and T = 

Thermodynamic model

The concept of the local composition, introduced by Wilson [14], basically establishes that the composition of the system in the neighborhood of a given molecule is not the same as that of the ‘bulk’ composition, because of the intermolecular forces. The activity coefficient model NRTL – non-random, two-liquid – by Renon and Prausnitz [7] is based on the local composition concept, and it is applicable for partially miscible systems. In order to take into account the salt effect on (liquid + 

Parameter estimation

The estimation was performed using the Fortran code TML-LLE 2.0 [9]; the procedure is based on the Simplex method [7], and consists in the minimization of the objective function S [15].S=kDjMiN-1xijkI,exp-xijkI,calc2+xijkII,exp-xijkII,calc2.Here, D is the number of data sets, M is the number of data points and N is the number of components in each data set; the superscripts I and II refer to the two liquid phases in equilibrium, while the superscripts ‘exp’ and ‘calc’ refer to the

Conclusion

Electrolyte (liquid + liquid) equilibrium data of the quaternary systems (water + 2-propanol + 1-butanol + KBr) and (water + 2-propanol + 1-butanol + MgCl2) were experimentally determined at T = 313.15 K and T = 353.15 K by chromatographic and gravimetric analysis. The effect of the salt addition on the original ternary systems was observed by the increase of the size of the two-phase region. Experimental data for the ternary, salt-free system were also obtained, and those determined at T = 353.15 K were found in

Acknowledgment

The financial support of Fundação de Apoio à Pesquisa do Estado de São Paulo, FAPESP, through projects 99/08028-2 and 99/02062-4, is gratefully acknowledged.

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Part of this paper was presented in Portuguese at XIV Brazilian Congress of Chemical Engineering (COBEQ), Natal, 2002.

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