(257af) A New Quartic Equation of State for Real Chain Fluids

A new quartic equation of state (EOS) is constructed from molecular simulation and vapor-liquid coexistence data for n-alkanes up through C₈. Molecular simulation data [1] for the repulsive-plus-covalent bonding compressibility factor contribution are adequately fit to a recently proposed expression [2] for the athermal compressibility component Z_Ath versus packing fraction in terms of three substance-dependent parameters. Z_Ath is subtracted from experimental saturated vapor and liquid compressibility factor coordinates to obtain three points on the dispersion compressibility factor Z_Dis versus packing fraction profile (including the ideal gas limit Z_Dis = 0 as packing fraction → 0). An overall quartic in molar volume EOS is obtained by using an SRK-type attractive term with two temperature-dependent substance specific adjustable parameters k and a for which multiple suitable pairs are possible for simultaneous description of the three data points. The optimal set of k and a at each temperature is found that most closely reproduces the vapor-liquid coexistence condition (equality of vapor and liquid fugacity coefficients). Note that this procedure results in an exact match of the experimental vapor pressure and good-to-excellent results for saturated liquid and vapor molar volume. Another key feature of the proposed model is that the definition of packing fraction avoids use of a temperature-dependent hard-sphere diameter. The new quartic EOS model also gives a good description of compressed liquid Z versus packing fraction isotherms for the chain fluids studied. Correlations are presented for the EOS parameters in terms of molar mass, first ionization potential and molar polarizability. Finally, several Illustrative examples are given.

References

[1]  J. Cui and J. R. Elliott Jr., Phase Diagrams for a Multistep Potential Model of n-Alkanes by Discontinuous Molecular Dynamics and Thermodynamic Perturbation Theory, J. Chem. Phys., 116, 8625-8631 (2002).

[2] A. S. Gow and R. B. Kelly, Twenty-One New Theoretically Based Cubic Equations of State for Athermal Hard-Sphere Chain Pure Fluids and Mixtures, AIChE J., 61, 1677-1690 (2015).