4.3 Applications of ionic liquids in azeotrope mixture distillation

The set of parameters for a given equation of state model univocally defines a global phase diagram and, consequently, evolution of phase behavior for binary mixture in a wide range of temperatures and pressures. Global phase diagram for model systems of components explores that binary mixture of industrial refrigerants with imidazolium (IL)-based ionic liquids does not experience azeotropic behavior in the practicable range of parameters.

Distribution of critical points for major components of refrigerants [25–29] and hypothetical critical parameters of a number of imidazole-based ionic liquids is depicted in Figure 10.

It shall be noted that at subcritical temperatures, IL may undergo thermal decomposition that causes uncertainty in assessment of critical parameters. Usually, the available values of critical points of ionic liquids are derived from lowtemperature regions having extremely small saturation pressures. Computational procedures for finding model parameters are unstable. This can lead to errors in the predictions of phase equilibria at high temperatures.

Thanks to undetectable vapor pressure, ILs are considered as potential environmentally friendly candidates for replacing conventional solvents. The selective solubility properties of the ILs appeared for particular components of the mixtures and can be treated as extraction media for separation processes. Moreover, the increase of efficiency for absorption processes is promoted thanks to nonvolatile feature of ionic liquids acting as absorbents.

## Figure 10.

Critical point temperatures and molar volumes of representative components from [25–29].

The information on breaking of azeotrope is crucial for the separation of different industrial mixtures and is intensively discussed in the literature. The proposed existing expressions for identification of azeotrope behavior generally are empirical.

To describe and predict the phase behavior of the mixture, the equation of state models is more suitable for calculation of the thermodynamic properties.

The calculations of phase equilibria for system imidazolium-based ionic liquid and refrigerants R134a and R1234yf are performed. The Redlich-Kwong one-fluid model equation of state was selected due to simplicity, few parameters to be estimated, existing robust computational algorithms for obtaining the derivations of different thermodynamic features, as well as a large amount of existing data on binary interacting parameters in the existing literature.

The phase behavior of imidazolium-based ionic liquids С8H11N3F6S2O4 ([EMIm] [Tf2N]) and C10H19N2BF4 ([HMIm][BF4]) with refrigerants R134а and R1234yf was evaluated using the parameters regressed at the low-pressure experimental data. It is noted that the variation of anionic group leads to the shift of critical point of ILs and obviously impact intermolecular interactions between ionic liquids and molecules of refrigerants. To this end the phase behavior pattern is also impacted. Variation in the k12 interaction coefficient shifts the position of a specific point on the global phase diagram. For R1234yf-[HMIm][BF4] system, the position of the specific point at different values k12 = 0.1, 0, + 0.1 demonstrates a tendency to transition from azeotrope to zeotrope state or vice versa. The binary interaction parameters k12 and l12 for R134a-Il blends were restored from experimental data provided by Ren et al. [28, 29] with Pareto-based method described in Section 3.

Positive value of binary interaction coefficient k12 can report on III-type of phase

Allocation of characteristics points on the global phase diagram for R134a-R1234yf mixture with ionic liquids.

Azeotrope-Breaking Potential of Binary Mixtures in Phase Equilibria Modeling

DOI: http://dx.doi.org/10.5772/intechopen.83769

The responses to the environmental challenges require development and application of new environmentally friendly working media. As one of the promising media for chemical and refrigeration industry, the mixtures of conventional refrig-

Tremendously growing amount of data and development of the data science provides new basis for estimation of the model parameters which influence the

behavior for the system studied. The calculations were performed in MATLAB software on the base of the algorithms proposed by Michelsen-Mollerup [23].

erants with ionic liquids are considered. This requires reliable data on

description and further prediction of the different physical processes.

thermophysical properties and phase behavior of the mixture.

Azeotrope-breaking in the R134a-R1234yf mixture at ionic liquid addition.

5. Conclusions

85

Figure 12.

Figure 11.

In case a specific point is located in the northern or southern quadrants of the diagrams depicted in Figure 11, the azeotropic phenomenon is expected to appear in the binary mixture. The pattern of specific point location for ionic liquid [HMIm] [BF4] + refrigerants R134a (R1234yf) systems is also provided in Figure 11.

It is considered that the azeotrope definitely appears in the R134a-R1234yf system. The literature search provides lack of experimental data for this system. The boundaries presented in Figure 11 for the R1234yf-[HMIm][BF4] mixture practically coincide. The addition of the ionic liquid to azeotropic mixture leads to azeotrope-breaking that is demonstrated by a change of phase envelope for the R134a-R1234yf-[HMIm][BF4] mixture in Figure 12.

Azeotrope-Breaking Potential of Binary Mixtures in Phase Equilibria Modeling DOI: http://dx.doi.org/10.5772/intechopen.83769

Figure 11. Allocation of characteristics points on the global phase diagram for R134a-R1234yf mixture with ionic liquids.

Figure 12. Azeotrope-breaking in the R134a-R1234yf mixture at ionic liquid addition.

Positive value of binary interaction coefficient k12 can report on III-type of phase behavior for the system studied. The calculations were performed in MATLAB software on the base of the algorithms proposed by Michelsen-Mollerup [23].
