High pressure vapour-liquid equilibrium measurements for R116 and ethane with perfluorohexane and perfluorooctane.

Abstract

Isothermal binary vapour-liquid equilibrium data are presented for gas –liquid systems for R116 and ethane with perfluorohexane and perfluorooctane. There are no data in the open literature for these systems except for ethane with perfluorooctane. These high pressure VLE (HPVLE) measurements form part of the ongoing work by the Thermodynamics Research Unit at the University of KwaZulu-Natal. The work involves, partly, generating a database of thermophysical properties of fluorochemicals. These can be used to develop thermodynamic models. The performance of these models in correlating data can be compared to predictive models. They are also useful to study the interactions between hydrocarbons and perfluorocarbons. This is important for solvent screening in flue gas treatments and absorption processes. HPVLE Measurements were performed over a temperature range of 273 – 313 K and pressures up to 5 MPa. The R116 or ethane binary systems were measured with perfluorohexane and perfluorooctane at four or five isotherms, at temperatures both above and below the critical temperature of the lighter component. The measurements were undertaken using the “static-analytic” type apparatus with the sampling of the phases done using a rapid-online sample-injector (ROLSI™). The expanded uncertainties (95% confidence level) in the temperature, pressure and liquid and vapour composition were estimated as 0.09 K, 0.02 MPa and 0.015 and 0.007 mole fraction, respectively. The VLE data were correlated with the Peng-Robinson equation of state with the classical one-fluid mixing rule and/or Peng-Robinson equation of state (EoS) containing the Mathias Copeman alpha function, Wong Sandler mixing rules with the Non-Random Two-Liquid local composition model. The HPVLE data comprising R116 were described well with the Peng-Robinson EoS and the classical one-fluid mixing rule. The relative deviations in pressure and composition were within 1%. The HPVLE data involving ethane were better represented with the more complex Peng-Robinson EoS with the Wong-Sandler mixing rule. Relative deviations in pressure and composition were within 2%. Although acceptable average absolute relative deviations and bias values in pressure and vapour compositions were obtained, systematic overestimations or underestimations of the experimental vapour compositions were observed. Both thermodynamic models provide good representation of the critical regions.