High-Pressure Vapor-Liquid Equilibrium Data for a Binary Mixture: the Poynting Factor Correction

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Consider a binary mixture composed of -hexane and -heptane. This Demonstration computes and plots the isobaric vapor-liquid equilibrium diagram, the equilibrium curve, the relative volatility, the -value, the gas-phase fugacity coefficients using the Soave–Redlich–Kwong equation of state (EOS), and the Poynting correction factor for user-specified values of the pressure. The Demonstration takes into account deviation from ideal behavior in the gas phase by including gas phase fugacity coefficients. Thus high-pressure VLE data can be obtained. This Demonstration allows for the Poynting factor correction. Snapshots present calculations at a pressure approximately equal to 303.975 kPa.


VLE data is computed using the following formula (the gamma-phi-poy equation):

, where and are the vapor and liquid mole fractions of component , is the total pressure, is the saturation vapor pressure of component , is the vapor-phase fugacity coefficient, and is the liquid-phase activity coefficient, where and are determined using the Soave–Redlich–Kwong equation of state and the Wilson model, respectively; is the liquid molar volume of molecule determined using the Rackett equation.


Contributed by: Housam Binous (March 2011)
Open content licensed under CC BY-NC-SA



For more information about pressure effects, the Rackett equation, and the Pitzer–Curl method, see

[1] M. F. Doherty and M. F. Malone, Conceptual Design of Distillation Systems, New York: McGraw-Hill, 2001.

[2] B. E. Poling, J. M. Prausnitz, and J. P. O'Connell, The Properties of Gases and Liquids, 5th ed., New York: McGraw-Hill, 2004.

For more information about the SRK equation of state and the Wilson activity coefficients, see

[3] S. I. Sandler, Chemical and Engineering Thermodynamics, New York: John Wiley & Sons, 1999.

[4] J. R. Elliot and C. T. Lira, Introductory Chemical Engineering Thermodynamics, Upper Saddle River, NJ: Prentice Hall, 1999.

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