SAFT-VR modelling of the phase equilibrium of long-chain n-alkanes

The statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) has been used in the calculation of the phase equilibria for long-chain n-alkanes and their mixtures. We treat the molecules as chains formed from united-atom hard-sphere segments, with a square-well potential of variable attractive range to describe the dispersive forces. A empirical relationship derived in earlier work is used to determine the number of such segments in relation to the carbon number for each member of the homologous series. Simple linear relationships between the potential model parameters and molecular weight have been determined enabling predictions of the fluid phase equilibria of heavier n-alkane molecules for which no experimental data are available. This is illustrated by a comparison with simulation data from the literature for the coexisting densities of n- octatetracontane (C₄₈H₉₈). Additionally we have examined binary mixtures of methane + n-hexadecane (C₁₆H₃₄) and hexane + n-tetradecane (C₁₄H₃₀) with the SAFT-VR approach, using simple Lorentz-Berthelot combining rules to determine the unlike interaction parameters. This study will allow an extension of the SAFT-VR approach to polymeric systems.