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

Abstract

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, k ij, is used to fit the calculated results to the experimental data. Both CK- and PC-SAFT reproduce experimental VLE data for methanol + alkane systems quantitatively when k ij is fit to experimental data and both qualitatively predict experimental results when k ij = 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 k ij value can reproduce the experimental data without introducing liquid–liquid splitting. For VLE calculations, a correlation for predicting k ij based on molecular weight and temperature is presented, which is also shown to provide reasonable estimates for the k ij 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.