Elsevier

Fluid Phase Equilibria

Volume 258, Issue 2, 15 September 2007, Pages 179-185
Fluid Phase Equilibria

Vapor–liquid equilibrium data for the hexafluoroethane + carbon dioxide system at temperatures from 253 to 297 K and pressures up to 6.5 MPa

https://doi.org/10.1016/j.fluid.2007.06.012Get rights and content

Abstract

Isothermal vapor–liquid equilibrium data have been measured for the hexafluoroethane (R-116) + carbon dioxide (R-744) binary system at six temperatures between 253.29 and 296.72 K, and at pressures from 1.051 to 6.45 MPa. The three remarkable peculiarities of this system are: (1) the existence of azeotropic behavior and critical azeotropy, (2) the splitting of phase envelope into two parts at high temperatures and (3) the proximity of bubble and dew curves in the whole range of compositions at all temperatures investigated. The experimental method used in this work to measure VLE data is of the “static-analytic” type, taking advantage of two pneumatic capillary samplers (Rolsi™, Armines’ patent) developed in the CEP/TEP laboratory. The data were obtained with uncertainties within ±0.01 K, ±0.0015 MPa and ±1.6% for molar compositions.

The isothermal P, x, y data are well represented with the Peng–Robinson equation of state using the Mathias–Copeman alpha function and the Wong–Sandler mixing rules involving the NRTL model.

Introduction

R-116 (hexafluoroethane) and R-744 (carbon dioxide) are two very interesting fluids for industry. Their common peculiarity is their low critical temperatures (see Table 1); the critical temperature of CO2 is 304.20 K, while that of R-116 is 293.04 K. Moreover, this system presents azeotropic behavior, indicating strong interaction between the two species, in particular a strong repulsive interaction. Azeotropic and even quasi-azeotropic behavior are of utmost interest not only to refrigeration industry but also to all industries involved in supercritical fluid extraction. In fact supercritical fluids are employed in many industrial applications, such as in food and pharmaceutical industries, in biotechnologies and for the development of new materials. Utilization of fluids with low critical points that are also safe for the environment is generally highly preferred. But there is unfortunately a drawback, as the critical properties are very low, the coefficient of performance (COP) is not as high as could be wished.

Moreover, refrigeration industry needs new fluids to replace ozone-destroying refrigerants like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Their use, production and distribution are ruled by the modification of the 1987 Montreal Protocol. CFCs are prohibited since 1996 for countries-members while the deadline for HCFCs which have lower ozone depletion potential is 2030. The GWP of CO2 is 1 and the GWP of R-116 is around 9000. An azeotropic mixture of R-116 and R-744 could be very interesting for industry of refrigeration as the azeotrope position is at high R-744 composition. Consequently, we present new vapor–liquid equilibrium data for temperature above and below the critical temperature of R-116.

The TEP laboratory has already published numerous VLE data for various mixtures with R32, R227ea, propane or CO2 [1], [2], [3], [4], [5], [6]. In this work, VLE data are presented for the R-116–R-744 system at four temperatures below the R-116 critical temperature (253.29, 273.27, 283.24 and 291.22 K) and two above (294.22 and 296.72 K). This system corresponds to type I according to the classification of van Konynenburg and Scott [7].

The experimental results are fitted using the Peng–Robinson equation of state (PR EoS). Finally, we present the pure component PT lines along with the predicted mixture critical line using the parameters adjusted to our binary VLE data and the azeotropic line as function of temperature.

Section snippets

Materials

R-744 was obtained from Aldrich with a certified purity higher than 99.9 vol%. R-116 was purchased from DEHON (France) and has a certified purity higher than 99.99 vol%. No further purification was performed before use.

Apparatus

The apparatus used in this work is based on a static-analytic method with liquid and vapor phase sampling. This apparatus is similar to that described by Laugier and Richon [8] and Valtz et al. [1], [2].

The equilibrium cell is immersed inside a temperature regulated liquid bath.

Correlations

The critical temperatures (TC), critical pressures (PC) for each of the two pure components are provided in Table 1. Our experimental VLE data are correlated by means of homemade software thermosoft, developed by Armines/Ecole des Mines de Paris. We have used the PR EoS [10] together with the Mathias–Copeman alpha function [11] for accurate representation of the vapor pressures of each component. Mathias–Copeman alpha function (Eq. (1)) has three ci adjustable parameters; it was especially

Vapor pressures data

These data were measured for R-116 at 20 temperatures from 243.33 to 292.22 K and used to adjust Mathias–Copeman parameters (see Table 2). Experimental and calculated vapor pressures values are reported in Table 3. The absolute relative deviation observed is less than 0.08% for R-116. Concerning the R-744, the Mathias–Copeman parameters are taken from ref [15] (see Table 2).

Vapor–liquid equilibrium data for the R-116–R-744 mixture

The experimental and calculated VLE data are reported in Table 4, Table 5, Table 6 and plotted in Fig. 1. The adjusted

Conclusion

In this paper we present VLE data for the system R-116 + R-744 at six temperatures. We used a “static-analytic” method to obtain our experimental data. We chose the Peng–Robinson EoS, with the Mathias–Copeman alpha function and the Wong–Sandler mixing rules involving the NRTL model to fit the experimental data.

The experimental results are given with following uncertainties: ±0.01 K, ±0.0015 MPa and ±1.6% for vapor and liquid mole fractions. For some temperatures above the critical temperature of

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