A systematic study of methanol + n-alkane vapor–liquid and liquid–liquid equilibria using the CK-SAFT and PC-SAFT equations of state

In this work, we present a systematic study of the ability of the SAFT equation of state with the dispersion term of Gross and Sadowski (PC-SAFT) and the dispersion term of Chen and Kreglewski (CK-SAFT) to model vapor–liquid (VLE) and liquid–liquid equilibria (LLE) for methanol + n-alkane systems. In addition to using pure component SAFT parameters derived from the standard method of fitting to liquid density and saturated vapor pressure, CK-SAFT with associating parameters determined from two different methods of molecular orbital quantum mechanic (MO) calculation, the Hartree–Fock (HF) and Becke-3LYP (B3), are also used to reproduce experimental data. One adjustable temperature-dependent binary interaction parameter, kij, is used to fit the calculated results to the experimental data. K- and PC-SAFT reproduce experimental VLE data for methanol + alkane systems quantitatively when kij is fit to experimental data and both qualitatively predict experimental results when kij = 0. For these systems, widely used cubic equations of state, like the Peng–Robinson or Soave, Redlch and Kwong, are incapable of quantitatively reproducing VLE for these highly non-ideal systems. MO-based CK-SAFT parameters show agreement with the experimental results at approximately 298 K, but at higher temperatures, no kij value can reproduce the experimental data without introducing liquid–liquid splitting. For VLE calculations, a correlation for predicting kij based on molecular weight and temperature is presented, which is also shown to provide reasonable estimates for the kij need to reproduce LLE data. LLE calculations show good agreement with experimental data for pentane through heptane. Calculations for alkanes longer than heptane show only qualitative agreement with experimental data.