Extraction of benzene, ethylbenzene, and xylenes from n-heptane using binary mixtures of [4empy][Tf2N] and [emim][DCA] ionic liquids

doi:10.1016/j.fluid.2014.07.034

Fluid Phase Equilibria

Volume 380, 25 October 2014, Pages 1-10

https://doi.org/10.1016/j.fluid.2014.07.034 Get rights and content

Highlights

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Extraction of aromatics from n-heptane employing mixed ILs was studied.
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Extractive properties of mixed ILs were between those of pure ILs.
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The highest extractive properties were achieved in the separation of benzene.
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LLE data of pseudoternary systems were measured and fitted to the NRTL model.
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The [4empy][Tf₂N] + [emim][DCA] mixture was revealed as a replacement of sulfolane.

Abstract

Benzene, toluene, ethylbenzene, and xylenes (BTEX) are mainly obtained by liquid–liquid extraction from pyrolisis and reformer gasolines using sulfolane as solvent. The use of ionic liquids (ILs) as replacements of sulfolane has been extensively studied, because an industrial process of aromatic extraction using ILs could have fewer process steps and lower operating costs as a result of the exceptional properties of ILs. Nevertheless, pure ILs studied so far have not simultaneously shown physical and extractive properties comparable to those of sulfolane. In our recent work, we studied the use of 1-ethyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide ([4empy][Tf₂N]) and 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]) binary IL mixtures as toluene extraction solvent from its mixtures with n-heptane, 2,3-dimethylpentane, and cyclohexane. The {[4empy][Tf₂N] + [emim][DCA]} mixture showed better extractive properties than those of sulfolane, whereas their densities and viscosities were similar to the sulfolane values. In this work, we have studied the performance of {[4empy][Tf₂N] + [emim][DCA]} IL mixtures in the liquid–liquid extraction of benzene, ethylbenzene, o-xylene, m-xylene, and p-xylene from n-heptane at 313.2 K. To evaluate the applicability of the IL mixtures as extraction solvent of BTEX, liquid–liquid equilibria (LLE) experiments have also been performed using sulfolane under the same conditions. A {[4empy][Tf₂N] + [emim][DCA]} mixture with a [4empy][Tf₂N] mole fraction of 0.3 has been revealed as a potential replacement of sulfolane in the liquid–liquid extraction of BTEX, according to its extractive and physical properties.

Introduction

Pyrolisis and reformer gasolines are the most important sources for benzene, toluene, ethylbenzene, and xylenes (BTEX) [1]. Although the amount of BTEX in these streams are between 25 and 60 wt%, the production of pure aromatics by distillation is technically not feasible as a result of the similar boiling points of aromatic, paraffinic, and naphthenic hydrocarbons and the presence of azeotropic mixtures [1], [2]. Because of this, the liquid–liquid extraction is employed at industrial scale to separate BTEX from pyrolisis and reformer gasolines [1], [3].

The Udex process was the traditional method of liquid–liquid extraction of aromatics, using diethylene glycol and triethylene glycol as solvents at 150 °C and 9 bar [1]. This process was replaced by the Sulfolane process developed by Shell and UOP, operating at 100 °C and 2 bar. Major drawbacks of the Sulfolane process are the high energy consumption in solvent regeneration due to the high boiling point of the sulfolane and the need of a wash column to recover the sulfolane dissolved in the raffinate phase [4].

Taking into account the properties of ILs, the drawbacks of the Sulfolane process could be overcome using an IL-based solvent in aromatic extraction. As a consequence of the non-volatile nature of ILs, solvent regeneration could be achieved by stripping or simple flash distillation at mild conditions [5]. IL recovery and recycling could reduce the environmental and economic impact of the process [6]. Solvent recovery column from raffinate could not be necessary using an IL-based solvent, because of the IL negligible solubility in hydrocarbons forming the raffinate phase [7]. In addition, the extraction column using ILs could operate at temperatures close to room temperature (30–50 °C) and atmospheric pressure, reducing operating costs and energy consumption compared with those of the Sulfolane process [3], [8].

An IL-based solvent would be considered as an alternative solvent to sulfolane if it showed higher aromatic distribution ratios and aromatic/aliphatic selectivities than the sulfolane values, together with suitable physical properties. However, these requirements have not been fully achieved to date by pure ILs [2], [8]. For that reason, we have proposed the use of binary IL mixtures as aromatic extraction solvents to obtain mixed solvents with extractive and thermophysical properties intermediate between those of pure ILs forming the mixture [9], [10], [11], [12], [13].

In our recent work, we studied the performance of {[4empy][Tf₂N] + [emim][DCA]} IL mixtures in the liquid–liquid extraction of toluene from n-heptane, 2,3-dimethylpentane, and cyclohexane at 313.2 K [11]. The pure [4empy][Tf₂N] was selected because this IL had showed high values of toluene distribution ratios in the separation of toluene from n-heptane [14], whereas the pure [emim][DCA] had exhibited toluene/n-heptane selectivities substantially higher than those of sulfolane [15], low dynamic viscosities [15], and adequate thermal stability [16]. The {[4empy][Tf₂N] + [emim][DCA]} IL mixtures showed higher extractive properties than those of sulfolane in the separation of toluene from alkanes and their densities and viscosities were similar to sulfolane values. This IL mixture also exhibited thermal stability, specific heats, and surface tensions between the values of the pure ILs [11], [13]. To confirm the applicability of the IL mixture in the liquid–liquid extraction of BTEX from non-aromatic hydrocarbons, in this work we have studied the liquid–liquid extraction of benzene, ethylbenzene, o-xylene, m-xylene, and p-xylene from n-heptane using {[4empy][Tf₂N] + [emim][DCA]} mixtures as solvents.

Section snippets

Materials

[4empy][Tf₂N] and [emim][DCA] ILs were purchased from Iolitec GmbH (Germany) with a mass fraction purity higher than 0.98. Halides mass fractions in ILs were less than 2 × 10⁻² and water mass fractions were lower than 2 × 10⁻³. Benzene, ethylbenzene, o-xylene, m-xylene, p-xylene, n-heptane, and sulfolane were supplied by Sigma-Aldrich (USA) with the specifications showed in Table 1. All chemicals were used as received without further purification and they were stored in a desiccator in their

Screening with {[4empy][Tf₂N] + [emim][DCA]} binary IL mixtures as aromatic extraction solvents over the whole range of composition

To select the most suitable composition in the {[4empy][Tf₂N] + [emim][DCA]} IL mixture, we have performed an initial screening employing pure [4empy][Tf₂N] and [emim][DCA] ILs and their binary mixtures in the whole range of compositions as extraction solvents at 313.2 K and 0.1 MPa. In the LLE experiments of the screening, hydrocarbon feeds formed by a 10% of aromatic hydrocarbon in mass basis (i.e. benzene, ethylbenzene, o-xylene, m-xylene, or p-xylene) and a 90% of n-heptane were used. In Table 2

Conclusions

In this work, we have studied the liquid–liquid extraction of benzene, ethylbenzene, o-xylene, m-xylene, and p-xylene from n-heptane using binary mixtures of [4empy][Tf₂N] and [emim][DCA] ILs at 313.2 K and 0.1 MPa. Initially, we have studied the performance of {[4empy][Tf₂N] + [emim][DCA]} IL mixtures over the whole range of composition in the separation of aromatics from n-heptane. Aromatic distribution ratios and aromatic/n-heptane selectivities have been intermediate between those of the pure

Acknowledgments

The authors are grateful to the Ministerio de Economía y Competitividad (MINECO) of Spain and the Comunidad Autónoma de Madrid for financial support of Projects CTQ2011–23533 and S2009/PPQ–1545, respectively. Marcos Larriba thanks Ministerio de Educación, Cultura y Deporte for awarding him an FPU grant (Reference AP–2010–0318), and Pablo Navarro thanks MINECO for awarding him an FPI grant (Reference BES–2012–052312).

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Extraction of benzene, ethylbenzene, and xylenes from n-heptane using binary mixtures of [4empy][Tf2N] and [emim][DCA] ionic liquids

Highlights

Abstract

Introduction

Section snippets

Materials

Screening with {[4empy][Tf2N] + [emim][DCA]} binary IL mixtures as aromatic extraction solvents over the whole range of composition

Conclusions

Acknowledgments

Chem. Eng. Res. Des.

Fluid Phase Equilib.

Chem. Eng. J.

Fluid Phase Equilib.

Sep. Purif. Technol.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Fluid Phase Equilib.

Industrial Aromatic Chemistry

Ind. Eng. Chem. Res.

Fuel Process. Technol.

Chem. Eng. Prog.

J. Phys. Chem. B

Extraction of benzene, ethylbenzene, and xylenes from n-heptane using binary mixtures of [4empy][Tf₂N] and [emim][DCA] ionic liquids

Screening with {[4empy][Tf₂N] + [emim][DCA]} binary IL mixtures as aromatic extraction solvents over the whole range of composition