The impact of uni-univalent electrolytes on (water + acetic acid + toluene) equilibria: Representation with electrolyte-NRTL model

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Highlights

  • Experimental LLE data for water + acetic acid + toluene + NaCl or KCl were reported.

  • The salting-out effect was detected; indicating the stronger effect of NaCl.

  • The electrolyte-NRTL model was adequately used to correlate the phase equilibria.

  • A good agreement was observed between calculated and experimental tie-lines.

Abstract

The presence of salts can significantly alter the (liquid + liquid) equilibrium and extraction process. In this work, a study was conducted on the (liquid + liquid) equilibria of (water + acetic acid + toluene + sodium chloride or potassium chloride) at temperatures (288.2, 298.2 and 313.2) K. This chemical system, irrespective of salt, is frequently used in (liquid + liquid) extraction investigations. The selected salt concentrations in initial aqueous solutions were (0.9 and 1.7) mol · L−1. The results show that salting-out effect of the salts was significant, so that an enhancement in the acetic acid distribution coefficient was achieved within (15.6 to 66.8)% with NaCl and within (2.5 to 37.6)% with KCl. Meantime, high separation factors were found at low temperatures and low solute concentrations. The electrolyte-NRTL model was satisfactorily used to correlate the phase equilibria. In this regard for each salt, the temperature dependent binary interaction parameters between components were calculated. The predicted tie-line mole fractions give root-mean square deviation (RMSD) values of only 0.0038 and 0.0045 for the systems containing NaCl and KCl, respectively.

Introduction

(Liquid + liquid) equilibrium (LLE) has a wide range of applications in many industrial processes. Water is the most common liquid components and the presence of electrolytes is inevitable in most water sources. An important feature of liquid systems with electrolytes is the fact that phase equilibria and thermodynamic properties are inherently altered due to different phenomena like salting out, ion pairing, acid–base reactions, hydrogen bonding and complexation [1].

Electrolytes may dissociate partially or completely in solutions. The degree of dissociation is greatly influenced by the solvents. An electrolyte system with completely dissociated electrolytes consists of cations, anions and solvent molecules. In the case of partial electrolyte dissociation, non-dissociated electrolyte molecules are also present. In addition to dissociation, electrolyte systems may exhibit association between cations and anions or between ions and solvent molecules to form ion pairs or complex ions [2]. Failure to recognize the partial dissociation of an electrolyte in each phase may lead to less satisfactory results in LLE calculations for aqueous-organic systems [2].

The solubility of an organic solute in aqueous solutions often decreases upon the addition of an electrolyte. When the electrolyte ions are solvated, a part of the water molecules becomes unavailable for the molecules of an organic solute, leading to salting-out of the organic solute. On the other hand, when a polar solvent is added to an aqueous salt solution, it captures water molecules leading the salting-in effect, which may be used to recover salts from concentrated aqueous solutions. The salt effect on (liquid + liquid) equilibrium has been extensively investigated during recent years [3], [4], [5].

Salting-out can be used to improve separation in processes such as extraction because of altering the distribution coefficient and driving force (involved in the mass transfer flux evaluation) [6], [7]. Increasing the amount of dissolved salts usually causes enhancement in extraction efficiency of a solute, but there will be a higher concentration of dissolved salt in the raffinate phase. Therefore, a lower salt concentration is usually preferred. This salt influence can be altered with different temperature applications.

Accordingly, relationships between chemical speciation and phase equilibrium in aqueous solutions can provide insight into structural phenomena such as solvation of ions and the change in ionic interactions. In this regard, Wang et al. [8] have developed a general speciation-based thermodynamic model for mixed-solvent electrolyte solutions. This model was shown to reproduce simultaneously (vapour + liquid) equilibria, (liquid + liquid) equilibria, speciation, caloric and volumetric properties of electrolytes in organic or mixed (organic + water) solvents. The model is valid for salts from infinite dilution to the fused salt limit. Also, the model is capable of representing phase equilibria in multicomponent inorganic systems containing two salts and water, or a salt, an acid and water [9]. Complex phase behaviour such as formation of multiple hydrated salts, double salts or the presence of eutectic points has been accurately represented.

Despite the reported investigations around the influence of electrolytes on LLE, there is a lack of study for the conventional chemical systems used in (liquid + liquid) extraction process studies. Furthermore, most of natural and industrial waters contain, to some extent, different soluble salts. The focus of this work is placed on the impact of uni-univalent salts NaCl and KCl on the equilibrium phase behaviour of the (water + acetic acid + toluene) system at different temperatures. This chemical system is frequently used in (liquid + liquid) extraction investigations [10], [11], [12]. The influence is discussed in terms of distribution coefficient and separation factor of components. The provided data are correlated with the electrolyte-NRTL model [13], [14], [15], with respect to perfect binary interactions. The model with the provided parameters represents the phase equilibrium behaviour of the system within the conditions used.

Section snippets

Materials

All the used materials were Merck products. Acetic acid and toluene were with mass fraction purity of more than 0.998 and 0.995, respectively, and were used without further purification. Sodium chloride and potassium chloride were analytical purity grade with mass fraction purity of more than 0.998 and 0.995, respectively, and dried in oven at 150 °C before use. Standard 0.1 mol · L−1 (±0.2%) sodium hydroxide solutions were used for titration. Deionised water with ionic conductivity <0.08 μS · cm−1

Experimental tie-line data

The tie-line data obtained for the quaternary systems of (water + acetic acid + toluene + NaCl or KCl) at different temperatures used are given in TABLE 2, TABLE 3, TABLE 4, TABLE 5. In a previous investigation [16], we reported LLE data for the ternary system of (water + acetic acid + toluene). The tie-line data show that acetic acid is more soluble in water; however, NaCl and KCl cause higher acetic acid concentrations in the organic phase, which in fact is due to the “salting out” phenomenon. This

Conclusions

LLE data for the quaternary systems of (water + acetic acid + toluene + sodium chloride or potassium chloride) were obtained at conventional temperatures of (288.2, 298.2, and 313.2) K. The consistency of experimental tie-lines at each temperature was confirmed by the Othmer–Tobias and Hand equations; showing a high level of agreement. The perfect (toluene + water) mole fraction range was considered. These data are beneficial in investigations on different aspects of extraction process.

According to the

Acknowledgement

The authors wish to acknowledge the Bu- Ali Sina University authorities due to financial support of this work.

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