Experimental and predicted phase equilibria and excess properties for systems with ionic liquids

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Abstract

Vapor–liquid equilibria (VLE) for the binary systems methylcyclohexane–toluene and 1-octene–n-octane and the ternary systems with the ionic liquid 1-hexyl-3-methyl-imidazolium bis(trifluoro-methylsulfonyl)imide [HMIM]+[BTI] as entrainer were measured. The addition of the ionic liquid leads to an increase of the separation factors. Excess enthalpies (HE) were measured for methylcyclohexane and n-octane in the same ionic liquid as well. Furthermore activity coefficients at infinite dilution (γ) were determined for a few solutes in various ionic liquids such as 1-hexyl-3-methyl-imidazolium bis(trifluoro-methylsulfonyl)imide [HMIM]+[BTI], 1-hexyl-3-methyl-imidazolium trifluoro-methanesulfonate [HMIM]+[OTF], 1-methyl-1-octyl-pyrrolidinium bis(trifluoromethyl-sulfonyl)imide [MOPYR]+[BTI], 1-methyl-1-butyl-pyrrolidinium trifluoromethanesulfonate [MBPYR]+ [OTF] and 1-ethyl-3-methyl-imidazolium ethyl sulfate [EMIM]+[EtOSO3]. The γ-values were used to determine the selectivities Sij=γi/γj and the capacities ki=1/γi at infinite dilution. The experimental data with [R1R2IM]+[BTI] were compared with the predicted results using mod. UNIFAC (Do). The predicted results are in good agreement with the experimental data.

Introduction

Since a few years there is an increasing interest in ionic liquids (IL). This has lead to an exponential increase of publications about ionic liquids [1]. Due to the fact that the properties of the ionic liquids can be adjusted by variation of the cation and the anion, the chain length of the alkyl rests and the substituents, theoretically up to 1018 possible ionic liquids are designable [2]. Therefore, ionic liquids are also referred as designer solvents, hence their properties can be optimized for a specific task. The use of ionic liquids offers several advantages. They have a negligible vapor pressure, which leads to negligible solvent emissions. So volatile organic compounds (VOCs) could replace organic solvents in various industrial applications resulting in a decreasing VOC-emission. The simple regeneration of ionic liquids is another advantage, because volatile components can be removed easily. Ionic liquids are characterized by a low melting point <373.15 K and a wide liquid range [3]. They show high solubility for both polar and non-polar organic and inorganic substances. Furthermore, ionic liquids are suitable solvents for biphasic chemical processes. The biphasic acid scavenging utilizing ionic liquid-process (BASIL) from BASF is the first large-scale process forming an ionic liquid [4], [5]. During the reaction a product phase is formed by the ionic liquid.

Since the measurement of the huge number of feasible systems with ionic liquids is very time-consuming and expensive, the development of a reliable prediction method would be most desirable. Kato and Gmehling [6] and Nebig et al. [7] have shown that modified UNIFAC (Dortmund) is suitable to describe the thermodynamic behavior of systems with ionic liquids. But up to now the parameter matrix for ionic liquids is still limited. Therefore, work is going on to extend the existing parameter matrix.

In this work it was investigated if ionic liquids can be used as alternative entrainers for the separation of aliphatics from aromatics or alkanes from alkenes by extractive distillation. Extractive distillation is often used when the separation factor shows values close to unity. Entrainers should influence the separation factor allowing the separation of the system. The separation of the binary systems methylcyclohexane–toluene and 1-octene–octane are investigated in this work. Besides binary vapor–liquid equilibria (VLE) data experimental VLE data of ternary systems as well as γ and HE-data with ionic liquids are presented. Additionally activity coefficients of the separation problem ethanol and water in various ionic liquids were measured, to show how the separation factor is influenced by the presence of the selected ionic liquids.

The structures of the cations and anions of the ionic liquids investigated in this work are shown in Fig. 1.

The new experimental data together with the data stored in the Dortmund Data Bank (DDB) [8] will be used for the extension of the group interaction parameter matrix of mod. UNIFAC (Do) for ionic liquids.

Section snippets

Materials and purities

All components in this work, except the ionic liquids, were used without degassing. The chemicals were dried over molecular sieves and the final purities were checked by gas chromatography (HP 6890 with Chemstation Rev. B.02.01-SR1). The water content was controlled for every compound including ionic liquids by Karl Fischer titration [9]. For all components investigated the water concentration determined with this method was less than 100 ppm. The purity, the supplier and the water content of

VLE

Isothermal VLE (xy) measurements were carried out with a static apparatus. In this case a new headspace sampler (HS-sampler) from Agilent Technologies (G1888 Network Headspace sampler) was used. The underlying analytical technique is called as “static headspace gas chromatography”. The term “headspace” refers to the vapor space above the liquid sample placed in a vial. The experimental procedure is described in detail by Liebert [10]. A 70-sample vial tray is located on top of the HS-sampler.

Results and discussion

The results for the excess enthalpies of the systems methylcyclohexane + [HMIM]+[BTI] at 363.15 K and 1617 kPa and n-octane + [HMIM]+[BTI] at 363.15 K and 1548 kPa are listed in Table 2. In graphical form these data are also shown together with the predicted results using the group contribution method mod. UNIFAC (Do) in Fig. 2, Fig. 3. As can be seen the predicted results are in good agreement with the experimental findings. However, the miscibility gap is not described exactly.

The values for the

Conclusion

In this work the suitability of ionic liquids as entrainer for various separation problems (aliphatics–aromatics, alkanes–alkenes, ethanol–water) are investigated. HE-data were determined for methylcyclohexane and n-octane with [HMIM]+[BTI] at 363.15 K. Additionally activity coefficients at infinite dilution were measured for 11 systems consisting of methylcyclohexane, toluene, ethanol and water with different ionic liquids in the temperature range 303.15–353.15 K. Isothermal VLE-data were

Acknowledgements

The authors thank R. Bölts for technical assistance and Merck GmbH for supplying the ionic liquids. We are also grateful to Deutsche Forschungsgemeinschaft (DFG) for financial support of this study.

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