Phase equilibria of imidazolium ionic liquids and the refrigerant gas, 1,1,1,2-tetrafluoroethane (R-134a)
Introduction
Ionic liquids have found increasing application with hydrofluorocarbon refrigerant gases both in separations, and engineering applications. Ionic liquids have been shown to aid in the separation of refrigerants and intermediates that often have very similar physical and chemical properties, including breaking azeotropes [1]. Ionic liquids can dramatically improve the efficiency of absorption refrigeration processes where the IL absorbs the refrigerant gas in one stage and releases the high-pressure gas with heat. Current liquid solvents with even low volatility are not optimal as often bulky equipment is needed to remove their contamination of the high-pressure gas. Non-volatile ionic liquids in these systems with refrigerants may help solve these problems [2].
A quantitative model of the high-pressure phase behavior and equilibria data is essential to design these and other IL/refrigerant applications. The phase behavior determines the conditions (temperature and pressure) of the transitions in equilibria, whether vapor–liquid, liquid–liquid, etc. Phase equilibria data quantify the solubility of each component in each phase in these phases. Moreover, a model to correlate and to predict data would be highly useful for design. In this investigation, the vapor–liquid equilibrium, vapor–liquid–liquid equilibrium, and mixture critical points have been measured for the refrigerant, 1,1,1,2-tetrafluoroethane (R-134a) with ionic liquids containing the 1-hexyl-3-methyl-imidazolium cation ([HMIm]) and anions of bis(trifluoromethylsulfonyl)amide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) (see Fig. 1). The temperatures investigated were 25 °C, 50 °C, 75 °C and pressures up to approximately 143 bar. These new experimental data with previous data from our group allow comparison of the effect of both the anion and the alkyl-chain length of the cation on the phase behavior and equilibrium. The Peng–Robinson equation of state with van der Waals 2-parameter mixing rule has also been employed to correlate the data.
Section snippets
Background
We have recently investigated the global phase behavior (pressure–temperature projection) of binary mixtures of R-134a and [EMIm][Tf2N], [HMIm][Tf2N], [BMIm][PF6], [HMIm][PF6], and [HMIm][BF4] between approximately 0 °C and 105 °C and pressures to 330 bar [3], [4]. In all cases, the phase behavior is classified with reasonably certainty as Type V according to the classification of Scott and van Konynenburg [5]. Type V systems are characterized by regions of vapor–liquid–liquid equilibrium between
Phase equilibria
The solubility of the refrigerant in the ionic liquids at various temperatures and pressures were measured in a static equilibrium apparatus. Details of the apparatus are described in Ren and Scurto [23] and an overview will be given here. The system consists of a custom-built high-pressure windowed equilibrium cell, a high-pressure and high-precision syringe pump (Teledyne-Isco, Inc., 100DM) filled with R-134a, a water bath and high-precision temperature measurement (using ITS-90 scale and
Modeling
The Peng–Robinson equation of state [29] with van der Waals 2-parameter mixing rules were selected to model and correlate the phase equilibria data. The Peng–Robinson EoS is given as:where am and bm are the mixture attractive and co-volume parameters, respectively, withThe mixture parameters, am and bm, are computed with the van der Waals 2-parameter mixing rules (vdW-2) given as
Results and discussion
The vapor–liquid equilibrium, vapor–liquid–liquid equilibrium and mixture critical points of 1,1,1,2-tetrafluoroethane (R-134a) and the ionic liquids [HMIm][Tf2N], [HMIm][BF4], and [HMIm][PF6] were measured at 25 °C, 50 °C, and 75 °C and pressures up to 143 bar. In addition, the volume expansion and the liquid molar volume were measured for each IL at each isotherm. The results are listed in Table 2. The experimental data of IL/R-134a data were correlated by the Peng–Robinson EoS with van der Waals
Conclusions
The phase equilibria of 1,1,1,2-tetrafluoroethane (R-134a) with [HMIm][Tf2N], [HMIm][PF6], and [HMIm][BF4], were measured at 25 °C, 50 °C, and 75 °C and pressures up to 143 bar. As these systems are Type V by the classification scheme of Scott and van Konynenburg, regions of multiphase equilibria exist at the higher temperatures, viz. VLE, LLE, and VLLE. The anion has a more pronounced effect on the solubility of R-134a in the ILs increasing in the order of [BF4] < [PF6] < < [Tf2N]. The mixture critical
Acknowledgements
This work was supported by the DOT:KU Transportation Research Institute (TRI) (DOT# DT0S59-06-G-00047). The author appreciates the support of the DuPont Young Professor Award.
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