Equation of state for square-well fluids: development of a coordination number model

A new expression for the coordination number of square-well fluids is developed. This expression meets the low density and close-packed density boundary conditions. On the basis of this model, a new equation of state (EOS) is introduced. This EOS needs the square-well potential function parameters—that is, R, σ, and ε/k—as input parameters. When these parameters are used as adjustable parameters, the EOS accurately describes the thermodynamic properties of normal fluids. The average absolute error for calculating the saturated vapor pressure, liquid density, vapor volume and enthalpy of vaporization of pure fluids was found to be 6.85, 3.95, 8.68, and 4.75%, respectively. Also, for a wide range of temperature and pressure, the compressed liquid density of methane and 1,1-difluoroethane were predicted accurately. The average absolute error was found to be 4.13 and 1.43% for methane and 1,1-difuoroethane, respectively.