Capacity of ionic liquids [EMim][NTf2] and [EMpy][NTf2] for extraction of toluene from mixtures with alkanes: Comparative study of the effect of the cation
Highlights
► EMimNTf2 and EMpyNTf2 were studied as solvents to extract toluene from alkanes. ► Liquid–liquid equilibrium data were measured at 298.15 K for 6 ternary systems. ► Selectivity and solute distribution ratio were calculated and compared. ► The influence of the structure of the cation of the ionic liquid was analyzed. ► Experimental data were satisfactorily correlated using NRTL model.
Introduction
Extraction of aromatics from refinery products such as naphtha, kerosene, and fuel has a potential importance in the petrochemical industry [1]. There are some conventional organic solvents such as sulfolane, dimethylsulfoxide, propilene carbonate, and glycols, that can be used for the extraction of aromatics [2], [3], [4], [5]. Nevertheless, these solvents are frequently flammable, volatile and toxic, and its recovery is costly and difficult. An environmentally friendly alternative to replace traditional organic solvents in liquid–liquid extraction processes are the ionic liquids (ILs) [6].
The negligible volatility of ILs makes promising their application in extraction processes. Besides, most of ILs have many attractive properties such as non-flammability under ambient conditions, easy to recycle, chemical and thermal stability, large liquid range, and can be tailored to a specific liquid–liquid extraction [7].
In recent years, some works on the extraction of aromatics from aliphatic hydrocarbons using ILs have been reported [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. However, there are few works published of liquid–liquid equilibrium (LLE) with a comparative study between ILs with different cations [19], [20], [21], [22].
This paper is a continuation of our previous works on extraction of aromatic compounds from their mixtures with alkanes using ILs [23], [24], [25], [26], [27]. In this case, two ILs with different cation and the same substituent and anion have been selected: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMim][NTf2] and 1-ethyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide, [EMpy][NTf2] and therefore, the difference between an imidazolium and a pyridinium cation can be studied. Besides, these ionic liquids with [NTf2]− anion are also chosen for their relatively low viscosity and their easy availability. LLE data have been experimentally determined for the ternary systems {heptane or cyclohexane + toluene + [EMim][NTf2] or [EMpy][NTf2]} and {heptane + cyclohexane + [EMim][NTf2] or [EMpy][NTf2]} at 298.15 K and atmospheric pressure. Due to the fact that the composition of the petrochemical products is very complex since they are formed by multiple substances, the study of systems with more than three components should be made in the future and so, the composition of the real mixtures could be approached. Therefore, in this work we have chosen these ternary systems to study the quaternary system for each IL in the future.
Finally, the experimental LLE data was correlated by the NonRandom Two Liquid (NRTL) and the UNIversal QUAsiChemical (UNIQUAC) thermodynamic models.
Section snippets
Chemicals
Heptane, toluene and cyclohexane were supplied by Sigma–Aldrich with purities greater than 99.5%, 99.9% and 99.9% respectively. The ILs [EMim][NTf2] and [EMpy][NTf2] were acquired at IoLiTec and their purities, water content by mass, ww, and concentration of halides (F−, Cl− and Br−), whalide, are presented in Table 1. ILs were subjected to vacuum (P = 0.2 Pa) at moderate temperature (T = 343 K) until the density value is constant, to reduce the water content and volatile compounds to negligible
Experimental liquid–liquid equilibrium data
The experimental LLE data for the ternary systems studied at 298.15 K {heptane or cyclohexane (1) + toluene (2) + [EMim][NTf2] or [EMpy][NTf2] (3)} and {heptane (1) + cyclohexane (2) + [EMim][NTf2] or [EMpy][NTf2] (3)} are reported in Table 3 and the corresponding triangular diagrams are shown in Fig. 1. As expected, the tie-line ends corresponding to the upper phase lie on the edge heptane–toluene, cyclohexane–toluene or heptane–cyclohexane, since ionic liquid was not detected in that phase. As can be
Conclusions
LLE data for the ternary systems {heptane or cyclohexane (1) + toluene (2) + [EMim][NTf2] or [EMpy][NTf2] (3)} and {heptane (1) + cyclohexane (2) + [EMim][NTf2] or [EMpy][NTf2] (3)} were experimentally determined at 298.15 K and atmospheric pressure. Solute distribution ratios and selectivities were calculated and compared with those reported in literature. All systems showed selectivity values higher than the unity, hence, these ionic liquids could be used for extraction processes. Nevertheless,
List of symbols
- ww
water content by mass
- whalide
concentration of halides
- P
pressure
- T
temperature
- ρ
density
- nD
refractive index
- β
solute distribution ratio
- S
selectivity
- x
mole fraction
- w
mass fraction
- α
non-randomness parameter in the NRTL model
- ri; qi
van der Waals parameters of the UNIQUAC model
- σx
root-mean-square of the composition
- Δβ
mean error of the solute distribution ratio
- M
number of tie-lines
- N
number of components in the mixture
Acknowledgments
N. Calvar is grateful for the scholarship from Fundação para a Ciência e a Tecnologia (FCT, Portugal) (ref. SFRH/BDP/37775/2007), LSRE financing by FEDER/POCI/2010, and E. Gómez to the Xunta de Galicia (Ángeles Alvariño Programme) for financial support. Authors also thank to the Ministerio de Ciencia e Innovación (Spain) for financial support through the project CTQ2010-18147 and to the Xunta de Galicia through the project INCITE 09314258PR.
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