Elsevier

Fluid Phase Equilibria

Volume 303, Issue 1, 15 April 2011, Pages 103-110
Fluid Phase Equilibria

Effect of mono-, di- and tri-ethanolammonium tetrafluoroborate protonic ionic liquids on the volatility of water, ethanol, and methanol

https://doi.org/10.1016/j.fluid.2011.01.009Get rights and content

Abstract

Vapor pressure data were measured for nine binary systems containing water, ethanol, or methanol with one of three protonic ionic liquids (PILs), viz. mono-, di- and tri-ethanolammonium tetrafluoroborate ([HMEA][BF4], [HDEA][BF4], and [HTEA][BF4]), at varying temperatures and PIL-contents using a quasi-static ebulliometer. The vapor pressure data were correlated by NRTL model with an overall average absolute relative deviation (AARD) of 0.0175. It is showed that the effect of PILs on the vapor pressure lowering of solvents follows the order of [HMEA][BF4] > [HDEA][BF4] > [HTEA][BF4], and the vapor pressure lowering degree follows the order of water > methanol > ethanol. Besides, the activity coefficients of solvent for binary system {solvent + PIL} at fixed PIL mole fraction of 0.10 were calculated using the regressed NRTL parameters. The results indicate that three PILs can give rise to a negative deviation from the Raoult's law for water and methanol and a positive deviation for ethanol to a varying degree, leading to the variation of relative volatility of a solvent.

Research highlights

Vapor pressure data for nine binary systems {solvent + ionic liquid} were measured. ► Water, ethanol, methanol, and ethanolammonium-based tetrafluoroborate were studied. ► The vapor pressure data can be well correlated by NRTL model. ► The activity coefficients of solvents were calculated by NRTL model. ► The vapor pressure lowering degree follows the order of water > methanol > ethanol.

Introduction

Separation of water and methanol from ethanol is of essential importance in the food industry, as methanol is an inevitable companion of ethanol produced via fermentation processes [1]. Such separation is generally achieved through distillation in industry. However, the separation efficiency is greatly restricted by the low relative volatility between methanol and ethanol and the occurrence of azeotropic phenomenon for the {water + ethanol} mixture. To facilitate the separation for close boiling or azeotropic mixtures, special distillation (e.g., extractive or salt distillation) is often used in which an entrainer (a solvent or salt) is employed to increase the relative volatility of one component and make the separation more efficient. However, salt distillation has some drawbacks, such as the limited salt solubility (e.g., calcium chloride, potassium acetate or their solutions with glycol) in the lean water composition range, which may precipitate in the tower and build up the plates and pipelines. For solving these problems, new substitutes need to be explored. As an organic molten salt with dual functions of both solvent and salt, ionic liquids (ILs) might be one of such candidates, which have drawn much interest in recent years due to their unique structure and properties [2]. From the point of view of structure and composition, ILs can be recognized as a kind of transition substances from a typical electrolyte to non-electrolyte, which provide us an opportunity to study the thermodynamics of electrolyte and non-electrolyte solutions in a continuum approach. From the point of view of applications, ILs can be used as the alternative green solvents besides water and supercritical fluids due to their unique properties, e.g., non-volatility and non-flammability [3]. These properties are largely attributed to their ionic attributes and electrostatic interaction among ionic species as well as short range interaction among organic groups.

In recent years, the industrial applications of ILs as entrainers for extractive distillation have been proposed by Gmehling and Krummen [4] and Arlt et al. [5], [6]. Many studies have been carried out for the salt effect of ILs on the vapor–liquid equilibria (VLE) of the mixtures with azeotropic point or close boiling temperature. e.g., {propanol + water} [7], [8], [9], [10], {ethylacetate + ethanol} [11], [12], [13], and {water + ethanol} [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26] mixtures. On this basis, the intermolecular interactions between IL and solvent can be assessed, and appropriate entrainers for extractive distillation can be screened out. Among various ILs reported [27], the ILs with hydroxyl-substituted quaternary ammonium cations and tetrafluoroborate are noteworthy for their hydrophilicity, and more importantly their ease of production and low cost for industrial applications [28]. Therefore, the vapor liquid equilibrium data were measured for some binary systems containing water, ethanol or methanol with a series of ethanolammonium tetrafluoroborate PILs.

In this paper, the effect of three PILs, viz. [HMEA][BF4], [HDEA][BF4], and [HTEA][BF4], on the vapor pressure of water, ethanol, and methanol at differing temperatures and PIL-contents were investigated using a quasi-static ebulliometer. The experimental vapor pressure data for binary systems were correlated using the nonelectrolyte NRTL model. The vapor pressure depression of binary systems at PIL mole fraction of 0.050 in the temperature range of (303.15–373.15 K) was calculated by interpolating the experimental data. Furthermore, the activity coefficients of solvents for binary system {solvent + PIL} at fixed PIL mole fraction of 0.10 were calculated using the regressed NRTL parameters at varying temperatures.

The structures of the ethanolammonium PILs studied in this work are shown in Fig. 1.

Section snippets

Materials

The chemical reagents used in this work were distilled-deionized water, ethanol, methanol, monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), and tetrafluoroboric acid (HBF4). MEA, DEA, and TEA (Beijing Yili Chemical Co., Ltd., China) with the purity of w  0.990, HBF4 (Tianjin No. 3 Chemical Reagent Factory, China) with the mass fraction content of HBF4 not less than 40% (aqueous solution), were all AR grade commercial products and used without further purification. Ethanol and

Hydrolysis of PILs

It is known that [BF4] or [PF6] anion in an IL tends to be hydrolyzed [34] in the presence of water, which exists as a trace contaminant due to the highly hygroscopic nature of ‘dry’ ILs. The hydrolysis of [BF4] produces [BF3OH] and HF, and [BF3OH] can further hydrolyze and produce more HF [35]. The influence of hydrolysis of [BF4] on VLE have been studied by Zhang et al. [36].

In order to confirm the hydrolysis stability of PILs (viz. [HMEA][BF4], [HDEA][BF4], and [HTEA][BF4]) in water,

Conclusions

PILs [HMEA][BF4], [HDEA][BF4], and [HTEA][BF4] can reduce the vapor pressure of water, methanol and ethanol but to different extents due to the affinity difference between a given PIL and different solvents. The effect of PILs on the vapor pressure lowering follows the order of [HMEA][BF4] > [HDEA][BF4] > [HTEA][BF4] for water, methanol, and ethanol, and the vapor pressure lowering of the solvent follows the order of water > methanol > ethanol. The vapor pressure data of nine binary systems can be well

Acknowledgments

The authors are grateful to the financial support from National Science Foundation of China (21076005) and Research Fund for the Doctoral Program of Higher Education of China (20090010110001).

References (39)

  • Z. Lei et al.

    Fluid Phase Equilib.

    (2006)
  • M. Doker et al.

    Fluid Phase Equilib.

    (2005)
  • J. Zhao et al.

    Fluid Phase Equilib.

    (2006)
  • X. Jiang et al.

    J. Chem. Thermodyn.

    (2007)
  • J. Wang et al.

    Fluid Phase Equilib.

    (2007)
  • N. Calvar et al.

    Fluid Phase Equilib.

    (2007)
  • Y. Ge et al.

    J. Chem. Thermodyn.

    (2008)
  • J. Wang et al.

    J. Chem. Thermodyn.

    (2009)
  • R. Kato et al.

    Fluid Phase Equilib.

    (2005)
  • J. Zhu et al.

    Chem. Eng. J.

    (2009)
  • Y. Xu et al.

    J. Chem. Thermodyn.

    (2004)
  • S.P. Verevkin et al.

    Fluid Phase Equilib.

    (2005)
  • X.Q. Kong

    Ind. Miner. Process.

    (2004)
  • J. Ranke et al.

    Chem. Rev.

    (2007)
  • H. Zhao

    Chem. Eng. Commun.

    (2006)
  • J. Gmehling, M. Krummen, Separation of aromatic hydrocarbons from non-aromatic hydrocarbons, comprises using a...
  • M. Seiler et al.

    AlChE J.

    (2004)
  • L. Zhang et al.

    J. Chem. Eng. Data

    (2007)
  • A.V. Orchillés et al.

    J. Chem. Eng. Data

    (2008)
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