Modeling phase diagrams of systems containing ionic liquids used in different applications

In this work, we present the application of the PCSAFT equation of state to model phase diagrams of systems containing ILs. The first part of this work 
consists on an experimental section including vapor-liquid equilibrium (VLE) measurement for binary systems containing {ILs+CO2} and {ILs+ pyridine}. In a second part, the experimental data were correlated with the PC-SAFT model and accurate prediction was obtained for the two binary systems.


Introduction
Ionic liquids (ILs) [1], organic salts with melting points around or below ambient temperature, have been used as green solvents, owing to their interesting physicochemical properties: negligible vapour pressure, high chemical and thermal stabilities, and recyclability. Moreover, they are able to dissolve a wide range of organic or inorganic substances, and it is possible to adjust some of their properties such as polarity or miscibility by employing different cation-anion combinations [2][3][4][5][6]. For all these reasons, ILs are being postulated as promising alternative solvents for a number of technological applications in the context of green processes such as the absorption of carbon dioxide and the extractive desulfurization of diesel oil.
Thermodynamic properties of ILs can be obtained by experimental techniques and/or simulations. The main advantage of the use of an equation of state versus the other techniques is the speed and efficiency in which these calculations are performed. An equation of state such as PC-SAFT (Perturbed Chain-Statistical Associating Fluid Theory) [7] based on statistical mechanics offers several advantages. The first advantage is that each of the approximations made in the development of SAFT such as the chain and association terms has been verified from molecular simulation results. In this way, the range of applicability and the shortcomings of each term in the equation of state have been assessed. Moreover, the SAFT parameters have a physical meaning. The PC-SAFT is a useful tool in which the effects of molecular structure on the thermodynamic properties can be separated and quantified. For example, non-ideal contributions such as chain length and/or molecular shape, molecular association and polar interactions can be introduced in the development of the equation.
In this work, we present the application of the PC-SAFT equation of state to model phase diagrams of systems containing ILs. The first part of this work consists on an experimental section including vaporliquid equilibrium (VLE) measurement for binary systems containing {ILs+CO 2 } and {ILs+ pyridine}. In a second part, the experimental data were correlated with the PC-SAFT model and accurate prediction was obtained for the two binary systems.

Materials
Carbon dioxide was purchased from Messer with a purity of 0.99 in mass fraction. The ILs studied in this work:

VLE measurements for {CO2 + ILs}
Bubble point pressures of the systems {CO 2 + IL} were measured using a high-pressure variable-volume visual cell (Top Industry, S.A.) shown in Figure 1. The technique used to carry out phase equilibrium measurements was based on a synthetic method which avoids sampling and analyses of the phases.  8: Calibrated pressure sensor (0 < P < 340bar); 9: Sapphire window; 10: Video camera; 11: Monitor.

VLE measurements for {pyridine + ILs}
VLE measurements of {pyridine + ILs} have been performed in a glass cell by using a static method. The apparatus is shown schematically in Figure 2. This apparatus can be applied for the measurement of reliable isothermal P-x data up to 298 K and 40 kPa.  (1) where res a is the residual Helmholtz free energy of the system. The superscripts hc, disp and assoc refer to a reference hard chain contribution, a dispersion contribution and an associating contribution, respectively. Equations are as follows: 1

Results
The PC-SAFT parameters of pure ILs were determined using experimental densities measured in our laboratory. Carbon dioxide was modelled as a non-associating substance and represented by three molecular parameters: m , and B k . The values of these three parameters are taken from the literature [9]. The PC-SAFT parameters of pure ILs were determined considering them as self-associating compounds. The three non-associating parameters ( m , , B k ) and two self-associating parameters ( ij AB and ij AB k ) were obtained by a fitting procedure on pure-component data. Parameters of pyridine were obtained from literature [10]. Results for molecular parameters of carbon dioxide, ILs and pyridine with absolute average deviation (AAD %) on density are provided in Table 1

Conclusion
Phase diagrams of systems containing {ILs+CO 2 } and {ILs+ pyridine} were studied in a wide range of temperatures and pressures. It was found that ILs may have a good capacity for CO 2 absorption and fuels desulfurization. A thermodynamic model based on the PC-SAFT EoS was used with success in the correlation of the measured VLE data. The model provides a good description of phase diagrams of these mixtures.