Quaternary (liquid + liquid) equilibrium data for the extraction of toluene from alkanes using the ionic liquid [EMim][MSO4]

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Highlights

  • EMim[MSO4] was proposed as solvent for the extraction of toluene from alkanes.

  • The quaternary system {heptane + cyclohexane + toluene + [EMim][MSO4]} was evaluated.

  • The extraction of toluene would be facilitated in the presence of one alkane.

  • Experimental LLE data were successfully correlated with the NRTL model.

Abstract

(Liquid + liquid) equilibrium (LLE) studies for the extraction of aromatics from alkanes present in the petroleum fractions are important to develop theoretical/semiempirical (liquid + liquid) equilibrium models, which are used in the design of extraction processes. In this work, the ionic liquid 1-ethyl-3-methylimidazolium methylsulfate, [EMim][MSO4], was evaluated as potential solvent for the separation of toluene from heptane and cyclohexane. The LLE data for the quaternary system {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][MSO4] (4)} were experimentally determined at T = 298.15 K and atmospheric pressure. Moreover, the LLE data for the ternary systems {heptane or cyclohexane (1) + toluene (2) + [EMim][MSO4] (3)} were also determined. Solute distribution ratios and selectivities were calculated and analysed in order to evaluate the capability of the ionic liquid to accomplish the separation target. A comparison between the solute distribution ratios and selectivities for the quaternary and the ternary systems was also made. Finally, the experimental tie-line data were correlated with the NRTL model.

Introduction

The interest of ionic liquids (ILs) has been boosted in the last years in the chemical industry because of their unique properties. Wide liquid range, chemical stability at high temperatures, simple recovery and negligible vapour pressure are some of the properties which make the ionic liquids very attractive for their application as alternative solvents [1], [2]. Moreover, a large number of cations and anions that can form the ILs are available, so the ionic liquids can be designed for a specific application.

In the separation processes of aromatic hydrocarbons from petroleum fractions by (liquid + liquid) extraction, conventional volatile organic solvents such as sulfolane or ethylene glycols are used. These solvents require additional distillation steps to separate and purify the extraction solvent and as a result, additional investments and energy consumption are required. Therefore, ILs have received great interest as promising solvents to replace conventional extractants in this field of separations [3].

Due to the large number of compounds present in the current industrial petroleum streams, (liquid + liquid) equilibrium (LLE) studies for systems with more than three components are necessary and important in the development of theoretical and semi-empirical LLE models to use in the design of extraction processes. A large number of researches focused on ternary (liquid + liquid) extraction of aromatics from aliphatic hydrocarbons using ILs have been published [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Nevertheless, similar investigations for quaternary systems are really scarce [17], [18], [19]. According to Ferreira et al. [16], ILs containing cations with shorter alkyl chains and anions such as [MSO4], [ESO4] and [N(CN)2] are preferred for aromatic extraction processes.

Therefore, in this work, the ionic liquid 1-ethyl-3-methylimidazolium methylsulfate, [EMim][MSO4], was investigated as solvent for the extraction of toluene from heptane and cyclohexane. This ionic liquid was selected because it can be easily and efficiently prepared under ambient conditions [20], and it shows lower toxicity than those ionic liquids with long alkyl chain length of the cation and halogen-based anion [21]. Furthermore, alkylsulfate-based ILs have been previously considered as suitable solvent for the extraction of aromatics from alkanes [4], [11], [13].

Thus, the LLE data for the quaternary system {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][MSO4] (4)} were determined at T = 298.15 K and atmospheric pressure. Solute distribution ratios and selectivities were calculated from the LLE data and compared with those of the ternary systems {heptane or cyclohexane (1) + toluene (2) + [EMim][MSO4] (3)} also determined in this article. The LLE of the ternary system {heptane (1) + cyclohexane (2) + [EMim][MSO4] (3)} was carried out for a complete characterization of the quaternary system. Phase diagrams were plotted and the experimental LLE data were correlated by the Non-Random Two Liquid (NRTL) thermodynamic model [22].

Section snippets

Materials

The ionic liquid [EMim][MSO4] was supplied by IoLiTec GmbH. Cyclohexane, heptane and toluene were purchased from VWR Prolabo. The mass fraction purities as well as the density and refractive index of pure chemicals at T = 298.15 K together with available literature data [23], [24], [25], [26] are presented in table 1. Prior to the experiments, the ionic liquid was subjected to vacuum (P = 0.2 Pa) at moderate temperature (T = 343 K) for at least 48 h to reduce the initial water content and other volatile

Experimental LLE data

The experimental LLE data for the ternary systems {heptane or cyclohexane (1) + toluene (2) + [EMim][MSO4] (3)} and {heptane (1) + cyclohexane (2) + [EMim][MSO4] (3)} are reported in table 2 and those for the quaternary system {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][MSO4] (4)} at T = 298.15 K and atmospheric pressure in table 3. The triangular diagrams for the ternary and the quaternary systems are shown in figures 2(a) and (b) and 3, respectively. In the case of the quaternary system, to avoid

Conclusions

The LLE of the quaternary system {heptane (1) + cyclohexane (2) + toluene (3) + [EMim][MSO4] (4)} and of the ternary systems {heptane or cyclohexane (1) + toluene (2) + [EMim][MSO4] (3)} and {heptane (1) + cyclohexane (2) + [EMim][MSO4] (3)} were investigated at T = 298.15 K and atmospheric pressure.

The selectivity values for the [EMim][MSO4] ionic liquid for the ternary systems as well as for the quaternary system were especially high because of the reduced amounts of alkanes detected in the lower phase. Thus,

Acknowledgments

The authors are grateful to Ministerio de Economía y Competitividad (Spain) for the financial support via the project CTQ 2010-18147. S.C. is also thankful to Xunta de Galicia for her predoctoral grant (Plan I2C) and E.G. for financial support through the Ángeles Alvariño Program. N.C. acknowledges the financial support from Fundação para a Ciência e a Tecnologia (Portugal) through her postdoctoral grant (SFRH/BPD/37775/2007).

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