Elsevier

Fluid Phase Equilibria

Volume 227, Issue 2, 25 January 2005, Pages 275-281
Fluid Phase Equilibria

Vapour–liquid equilibrium measurements and correlation for the pentafluoroethane (R125) + n-butane (R600) system

https://doi.org/10.1016/j.fluid.2004.11.023Get rights and content

Abstract

Mixtures formed by hydrocarbons (HC) and hydrofluorocarbons (HFC) are considered promising possible substitutes for the chlorinated refrigerants and for HFC with high global warming potential (GWP). Following our studies on this kind of systems, vapour–liquid equilibria (VLE) experimental data for the R125 + R600 system were measured at temperatures 278.15 and 298.15 K by means of a static analytical method, with preliminary recirculation of the mixture to get a faster equilibrium. The composition of both phases in equilibrium was measured by a gas-chromatographic method. In the experimental temperature range, the system shows a strong positive deviation from the Raoult's law, even if it does not present an azeotropic composition. The VLE data were correlated by different equations of state involving various mixing rules, enabling a comparable analysis of their correlating ability of strongly non-ideal systems.

Introduction

In the search for alternative refrigerants with zero ozone depletion potential (ODP) and low global warming potential (GWP), mixtures formed by hydrocarbons (HC) and hydrofluorocarbons (HFC) seem to be very interesting. Following our studies on vapour–liquid equilibria (VLE) of several HC + HFC systems, such as 1,1,1,2-tetrafluoroethane (R134a) + isobutane (R600a), R600a + 1,1,1,3,3,3-hexafluoropropane (R236fa) [1], 1,1,1-trifluoroethane (R143a) + propane (R290), R290 + R134a [2], R290 + R245fa [3], R290 + R236fa [4], R290 + 1,1,1,2,3,3-hexafluoropropane (R236ea) [5], R600a + 1,1,1,3,3-pentafluoropropane (R245fa) [6], R290 + 1,1,1,2,3,3,3-heptafluoropropane (R227ea) [7], pentafluoropropane (R125) + R290 [8], difluoromethane (R32) + R290 [9] and R32 + n-butane (R600) [10], and on liquid–liquid equilibria (LLE) of R32 + R290 [11], R32 + R600 [12] and R125 + R600 system [13], vapour–liquid equilibria (VLE) data for this last system were measured at temperatures of 278.15 and 298.15 K and presented in this work. The experimental data were used for a comparative analysis of the correlating ability of few equations of state with various mixing rules for this strongly non-ideal system.

Section snippets

Materials

R125 (pentafluoroethane, C2HF5) was provided by Solvay, with a declared purity >99.9% and R600 (n-butane, C4H10) by Aldrich with a stated purity >99%. After the elimination of the non-condensable gases, neither thermal conductivity nor flame ionization (FID) detectors of a gas chromatograph identified any impurities; a Porapak Q column with a length of 2 m and an external diameter of 1/8 in. was used. All samples were used with no further purification.

Apparatus

VLE measurements were performed using the

Results and discussion

The VLE (TPxy) were measured at two isotherms (T = 278.15 and 298.15 K) and the experimental data are presented in Table 1 and Fig. 1. Within this temperature range the liquid phase is completely miscible and zeotropic over the whole composition range but with a strong positive deviation from the Raoult's law. For this system, the appearance of a partial miscibility of liquid phase was observed at temperatures below 216 K [13]. In [13], 24 solubility PTx data for the R125 + R600 system were

Conclusions

Twenty six vapour–liquid equilibria experimental data were measured at two temperatures (278.15 and 298.15 K). The data show a strong positive deviation from the Raoult's law, even if the system is zeotropic.

Several models were considered to represent the system behaviour. The RKS, the PR and the CSD EoS, together with the classical mixing rule, did not prove to work successfully. An improvement was found using a composition dependent mixing rule with the same EoS. The best results were obtained

Acknowledgments

Mauro Scattolini is gratefully acknowledged for his help in the measurements. Roman Stryjek is indebted to CNR-ITC in Padova for financial support during his visit.

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