External-degrees-of-freedom parameter in the UNIFAC-FV model. Comments Academic Article uri icon


  • Sir: Recently, Oishi and Prausnitz (1978) have proposed a modification to the UNIFAC group- contribution model to allow estimation of solvent activities in polymer solutions. Their new model (UNIFAC-FV) requires the specification of two parameters for the computation of the additional free-volume term. The first parameter, b, which arises from the evaluation of the reduced volume is set equal to 1.28. The second parameter, 3cl, describes the total number of external degrees of freedom per solvent molecule. As pointed out by Beret and Prausnitz (19751, c1= 1.0 for argon-like molecules; for all other molecules c1> 1. Values for the number of external degrees of freedom for several fluids were obtained by Beret and Prausnitz from PVT and vapor pressure data. These values vary between 1.0 for small molecules (hydrogen, methane, etc.) to 4.52 for 9-(2-~ henylethyl)-heptadecane. In their …

publication date

  • January 1, 1982