Predicting the high-pressure phase equilibria of binary mixtures of n-alkanes using the SAFT-VR approach

The phase behavior of selected alkane binary mixtures is studied using SAFT -VR, a version of the statistical associating fluid theory for potentials o f variable attractive range (SAFT). We treat the n-alkane molecules as chai ns formed from united-atom hard-sphere segments with square-well potentials of variable range to describe the attractive interactions. We use a simple relationship between the number of carbon atoms in the n-alkane molecule a nd the number of segments in the united atom chains in order to predict the phase behavior of n-butane with other n-alkanes. The calculated vapor pres sures and saturated liquid densities of the pure components are fitted to e xperimental data from the triple point to the critical point. These optimiz ed parameters are rescaled by the respective experimental critical points a nd used to determine the critical lines and phase behavior of the mixtures. We use the Lorentz-Berthelot combining rule for the unlike interactions. W e predict the phase behavior of n-butane + n-alkane binary mixtures, concen trating mainly on the critical region. The gas-liquid critical lines predic ted by SAFT-VR for the n-alkane mixtures are in excellent agreement with th e experimental data, and improve significantly on the results obtained with the simpler SAFT-HS approach where the attractive interactions are treated at the mean-field level.