Elsevier

Fluid Phase Equilibria

Volume 454, 25 December 2017, Pages 78-90
Fluid Phase Equilibria

Effect of cation on the solubility of ethane in three bis(fluorosulfonyl)imide ([FSI]) based low viscosity ionic liquids

https://doi.org/10.1016/j.fluid.2017.08.011Get rights and content

Highlights

  • Solubility of ethane in three bis(fluorosulfonyl)imide ([FSI]) based ionic liquids were measured.

  • Three piperidinium, pyrrolidinium and ammonium based cations were tested.

  • Solubility data were correlated with Peng-Robinson EoS using three different mixing rules.

  • Henry's law constants of C2H6 absorption were estimated at different temperatures.

  • Selectivity of these ILs towards CO2 over C2H6 were calculated at 303.15 K and 343.15 K.

Abstract

The solubility of ethane (C2H6) in three ionic liquids (ILs) with the same anion, [C3C1pip][FSI], [C3C1pyrr][FSI] and [N1223][FSI], was measured at (303.15, 323.15, and 343.15) K and at pressures up to 1.1 MPa. Experimental data were correlated with Peng-Robinson (PR) equation of state using three different mixing rules: (i) van der Waals one (vdW1, based on a single binary interaction parameter), (ii) van der Waals two (vdW2, based on two binary interaction parameters), and (iii) Wong-Sandler mixing rules combined with NRTL model. Henry's law constants, enthalpy and entropy for the absorption of ethane in these ILs were also estimated. No significant change was found in the values of Henry's law constants of C2H6 in these three ILs which indicates a negligible effect of cations of these ILs on C2H6 solubility. The selectivity towards carbon dioxide (CO2) over C2H6 for these ILs was also reported. [FSI]-based ILs seem to have higher selectivity than all the other ILs, except for [C4C1Im][BF4] and possibly [C4C1Im][PF6], which may qualify them as promising solvents for CO2 removal from natural gas streams.

Introduction

One of the biggest concerns in present time is the acceleration of global warming due to anthropogenic emissions of carbon dioxide (CO2), a major greenhouse gas (GHG), most of which is being produced by the combustion of fossil fuels [1]. One of the most technologically advanced method of capturing CO2 for mitigating emission from large stationary point sources is to capture the carbon dioxide using a liquid solvent, and sequester it into a specific geological formation such as an oil reservoir, depleted gas reservoir or salt water aquifer [1]. This short-term strategy for the reduction of carbon emission is known as carbon capture and storage (CCS). Physical, chemical (reactive) or hybrid solvents are used to capture CO2 [2]. Among these solvents, alkanolamine based chemical solvents are more efficient to capture carbon dioxide from less concentrated and low-pressure gas streams (post-combustion) [3]. However, CO2 capture by amine based solvent is still not economical due to high regeneration cost [4], [5], and solvent degradation [5]. Consequently, the most important research challenge nowadays in investigating CO2 absorption is to find a cost-effective and environmentally friendly liquid solvent [6].

Ionic liquids (ILs), commonly known as green solvents due to their special properties (e.g. non-flammable, negligible vapor pressure, high thermal stability, etc. [7], [8]), have received enormous research emphasis recently as an alternative to reactive solvents as they require much less energy for regeneration. The major three criteria for a good CO2 absorbent are: high removal efficiency, high selectivity towards CO2 and low energy requirement for regeneration. Usually, CO2 is captured from gas streams that contain not only CO2 but also other gases, such as N2O, O2, SO2, H2S, CH4, C2H6 and other hydrocarbons. It is, therefore, important to research the solubility of other gasses in the ionic liquids along with CO2. However, very limited research was done on the selectivity of ILs towards CO2 over other gases, until now. This study focuses on the determination of the solubility of ethane (C2H6) in ILs for applications in gas sweetening. Table 1 presents the list of ILs in which the solubility of ethane was measured, in addition to some important parameters such as the Henry's law constant (KH), enthalpy (ΔH) and entropy (ΔS) of solvation. Thermodynamic models used for data correlation are also listed in Table 1.

In a previous study, our research group [9] published data related to the solubility and thermodynamic modeling of CO2 absorption in three promising low-viscosity [FSI] based ILs with three structurally different cations ((i) N-propyl-n-methyl-piperidinium ([C3C1pip]+), (ii) N-propyl-n-methyl-pyrrolidinium ([C3C1pyrr]+) and (iii) N-propyl-n-methyl-n,n-dimethyl-ammonium ([N1223]+)). However, the solubility of ethane in these ILs was not previously reported in the literature. In this study, the solubility in these solvents was measured, and thermodynamic modeling of ethane (C2H6) in these three promising ILs, are also reported. The solubility of C2H6 in ILs was measured by the gravimetric method at three different temperatures (303.15, 323.15 and 343.15) K and at pressures up to 1.1 MPa. Experimental data were correlated with the well-known Peng-Robinson (PR) equation of state with three different mixing rules: (i) van der Waals 1 (vdW1, single binary interaction parameter), (ii) van der Waals 2 (vdW2, double binary interaction parameters) and (iii) Wong-Sandler mixing rules combined with NRTL model. Henry's law constants (KH) for ethane in ILs were estimated at these three temperatures. With the resulting values of KH, the enthalpy (ΔH) and entropy (ΔS) of solvation were estimated. Finally, the selectivity of these ILs towards carbon dioxide absorption (CO2) over C2H6 was evaluated.

Section snippets

Materials

The structures, suppliers, purities and water contents of all chemicals used (ILs and ethane) in the present study are listed in Table 2. According to some studies [31], [32], water content has a significant effect on the thermo-physical and transportation properties (e.g. density, viscosity etc.) of IL. Information about water content for these ILs was collected from the vendors and reported in Table 2. Densities of these ILs were measured and published by Tagiuri et al. [9]. Since the water

Model correlation

In this study, the solubility data of ethane in ILs were correlated by Peng-Robinson equation of state (PR EoS) [33]. The mathematical form of PR EoS is shown in Eq. (1); where am and bm are the parameters of intermolecular attractive force and van der Waals co-volume factor, respectively.P=RTvbmam(T)v(v+bm)+bm(vbm)

The mixture parameters am and bm can be estimated from pure component parameters using different mixing rules [34]. In this present study, van der Waals quadratic mixing rules

Verification of solubility data measurement

Two isotherm experiments were performed at 323.15 K to estimate the solubility of C2H6 into [C4C1Im][PF6] (which is commonly known as [bmim][PF6]) and compared with Anthony et al. [10]. The comparison between the results is shown in the form of P-x plots in Fig. 1 (solubility data are reported in Table S1 in supplementary information). In the present study, the estimated solubility data for C2H6 in [C4C1Im][PF6] were found to be in good agreement with those published by Anthony et al. [10].

Solubility data

All

Conclusion

The solubility of C2H6 in the ionic liquids studied decreased in the following order: [N1223][FSI] > [C3C1pip][FSI] > [C3C1pyrr][FSI]. However, the difference in solubility was very small because of structural similarities in all three ionic liquids. The estimated Henry's law constants were therefore also quite similar. Solubility data were correlated with PR EoS using three different mixing rules: van der Waals one (vdW1), van der Waals two (vdW2) and Wong-Sandler mixing rules combined with

Acknowledgements

Western Economic Diversification, Enterprise Saskatchewan, The Petroleum Technology Research Centre, The Natural Science and Engineering Research Council (NSERC), and The Faculty of Graduate Studies and Research at the University of Regina are acknowledged for funding and allocation of scholarships.

References (43)

  • M. Althuluth et al.

    Removal of small hydrocarbons (ethane, propane, butane) from natural gas streams using the ionic liquid 1-ethyl-3 -methylimidazolium tris(pentafluoroethyl)trifluorophosphate

    J. Supercrit. Fluids

    (2014)
  • S. Vitu et al.

    Predicting the phase equilibria of CO2 + hydrocarbon systems with the PPR78 model (PR EOS and kij calculated through a group contribution method)

    J. Supercrit. Fluids

    (2008)
  • J.A.P. Coutinho et al.

    Binary interaction parameters for nonpolar systems with cubic equations of state: a theoretical approach 1. CO2/hydrocarbons using SRK equation of state

    Fluid Phase Equilibria

    (1994)
  • M.-T. Lee et al.

    Prediction of mixture vapor–liquid equilibrium from the combined use of Peng–Robinson equation of state and COSMO-SAC activity coefficient model through the Wong–Sandler mixing rule

    Fluid Phase Equilibria

    (2007)
  • K. Sumida et al.

    Carbon dioxide capture in metal–organic frameworks

    Chem. Rev.

    (2012)
  • H. Lepaumier et al.

    New amines for CO2 capture. I. Mechanisms of amine degradation in the presence of CO2

    Indus. Eng. Chem. Res.

    (2009)
  • A.V. Rayer et al.

    Heats of absorption of CO2 in aqueous solutions of tertiary amines: N-methyldiethanolamine, 3-dimethylamino-1-propanol, and 1-dimethylamino-2-propanol

    Indus. Eng. Chem. Res.

    (2014)
  • Z. Lei et al.

    Gas solubility in ionic liquids

    Chem. Rev.

    (2014)
  • J.L. Anthony et al.

    Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate

    J. Phys. Chem. B

    (2002)
  • J.L. Anthony et al.

    Anion effects on gas solubility in ionic liquids

    J. Phys. Chem. B

    (2005)
  • J.L. Anderson et al.

    Solubility of CO2, CH4, C2H6, C2H4, O2, and N2 in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide: comparison to other ionic liquids

    Accounts Chem. Res.

    (2007)
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