Salt effect on (liquid + liquid) equilibrium of (water + tert-butanol + 1-butanol) system: Experimental data and correlation

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Abstract

(Liquid + liquid) equilibrium data for the quaternary systems (water + tert-butanol + 1-butanol + KBr) and (water + tert-butanol + 1-butanol + MgCl2) were experimentally determined at T = 293.15 K and T = 313.15 K. For mixtures with KBr, the overall salt concentrations were 5 and 10 mass percent; for mixtures with MgCl2, the overall salt concentrations were 2 and 5 mass percent. The experimental results were used to estimate molecular interaction parameters for the NRTL activity coefficient model, using the Simplex minimization method and a concentration-based objective function. The correlation results are extremely satisfactory, with deviations in phase compositions below 1.7%.

Introduction

Separation by solvent extraction becomes increasingly more difficult as the tie-line becomes parallel to the solvent axis, as shown by a solutropic solution. The addition of a salt to such systems introduces ionic forces that affect the equilibrium, modifying the tie-line, sometimes even to the extent of eliminating the solutrope. When the ions are solvated, some of the water become 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, nucleic acids and others.

This work is a continuation of previous studies on the (liquid + liquid) equilibria of (water + solvent + solvent + salt) systems, determined in our 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 were 1-butanol, 3-methyl-1-butanol and 1-pentanol, while the salts 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. In this work, experimental data for the ternary system (water + tert-butanol + 1-butanol) and the quaternary system (water + tert-butanol + 1-butanol + salt) were determined at T = 293.15 K and T = 313.15 K. The salts were potassium bromide and magnesium chloride. The mixtures containing KBr were studied at overall 5% and 10% mass, while the mixtures containing MgCl2 were studied at overall 2% and 5% mass. The ternary and quaternary (liquid + liquid) equilibrium data were used for the estimation of the binary interaction parameters of the NRTL [4] activity coefficient model, using the Simplex [5] method for minimization of a concentration-based objective function [6]. With these parameters, the experimental data were correlated.

Section snippets

Experimental

All the reagents, 1-butanol, tert-butanol, KBr and MgCl2, were of analytical grade (Merck) and were used without further purification. Water was bidistilled before utilization.

Experiments were carried out in equilibrium cells, such as those suggested by Stragevitch [7] and described elsewhere [1]. The cell temperature was regulated by a thermostatic bath (Tecnal TE-184), accurate to ±0.01 K.

The overall mixture was prepared directly inside the cell and the components were weighed on an analytical

Experimental results and discussion

The experimental (liquid + liquid) equilibrium data for the ternary, salt-free system (water + tert-butanol + 1-butanol) and the quaternary system (water + tert-butanol + 1-butanol + salt) are shown in TABLE 1, TABLE 2 as mass fractions. The same data appear in FIGURE 1, FIGURE 2. In figure 1, the salt effect of KBr (5% and 10% mass) and MgCl2 (2% and 5% mass) on the ternary system at T = 293.15 K is shown. From this figure, it can be seen that the effect of MgCl2 is far stronger than that of KBr. Indeed, for

Thermodynamic model

The concept of the local composition, introduced by Wilson [9], basically establishes that the composition of the system in the neighborhood of a given molecule is not the same that the ‘bulk’ composition, because of the intermolecular forces. The model NRTL – non-random, two-liquid – by Renon and Prausnitz [4] for the activity coefficient 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 [7]; the procedure is based on the Simplex method proposed by Nelder and Mead [5], and consists in the minimization of a concentration-based objective function, S [6], [7].S=kDjMiN-1xijkI,exp-xijkI,calc2+xijkII,exp-xijkII,calc2.Here, D is the number of data sets, N and M are the number of components and tie-lines in each data set; the superscripts I and II refer to the two liquid phases in equilibrium, while the superscripts

Conclusion

Experimental (liquid + liquid) equilibrium data were determined for the ternary system (water + tert-butanol + 1-butanol) and for the quaternary system {water + tert-butanol + 1-butanol + salt (KBr or MgCl2)}. The results show that, for the organic phase, the effect of MgCl2 is greater than that of KBr; on the other hand, for the aqueous phase, the addition of KBr has a negative effect on the extraction of tert-butanol, while the addition of MgCl2 extracts all the tert-butanol.

Binary interaction parameters

Acknowledgment

The financial support from 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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This paper was presented at VI Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design (EQUIFASE), Foz do Iguaçu, 2002.

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