An equation of state (EoS) suitable for describing associating
fluids is presented. The equation
combines the simplicity of a cubic equation of state (the
Soave−Redlich−Kwong), which is used
for the physical part and the theoretical background of the
perturbation theory employed for
the chemical (or association) part. The resulting EoS (Cubic Plus
Association) is not cubic with
respect to volume and contains five pure compound parameters which are
determined using
vapor pressures and saturated liquid densities. Excellent
correlations of both vapor pressures
and saturated liquid volumes are obtained for primary-alcohols (from
methanol up to 1-tridecanol), phenol, tert-butyl alcohol, triethylene glycol, and
water. Moreover, excellent prediction
of saturated liquid volumes may be obtained from parameters which have
been estimated by
regressing only vapor pressures. Finally, we suggest a method for
reducing the number of
adjustable parameters for alcohols to three while maintaining the good
correlation of vapor
pressures and saturated liquid volumes. We investigate the
possibility of using the homomorph
approach for estimating the EoS parameters and explain the problems
observed. The estimated
pure compound parameters have been tested in the prediction of second
virial coefficients with
satisfactory results.
Two equations of state, the cubic plus association (CPA) and the statistical associating fluid theory (SAFT), which account explicitly for the effect of hydrogen bonding on the thermodynamic properties of associating fluids using the perturbation theory of Wertheim (J. Stat. Phys. 1986, 42, 459, 477), are applied to predict the phase equilibrium of pure water, n-alkanes, and 1-alkenes as well as the low-and high-pressure phase equilibrium of water/hydrocarbon mixtures. The pure compound parameters for the two equations are estimated by fitting experimental vapor pressure and saturated liquid density data that cover a very wide temperature range from approximately the triple point to very close to the critical point. One temperature-independent binary interaction parameter is calculated for each of the mixtures examined. The analysis of the results shows that the increased complexity of SAFT over CPA does not offer any improvement in modeling highly nonideal fluid behavior, at least for the systems examined here.
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