PREDICTING THE HIGH-PRESSURE PHASE-EQUILIBRIA OF WATER PLUS N-ALKANESUSING A SIMPLIFIED SAFT THEORY WITH TRANSFERABLE INTERMOLECULAR INTERACTION PARAMETERS

The high-pressure phase equilibria of water + n-alkane mixtures are ch aracterized by vapor-liquid critical lines which first exhibit a tempe rature minimum and then extend to temperatures above the critical poin t of pure water; this so-called ''gas-gas'' coexistence is a consequen ce of the large degree of immiscibility of the two components. We use a simplified version of the SAFT equation of state, which is based on the thermodynamic perturbation theory of Wertheim for associating flui ds: the original SAFT equation of state treats the molecules as chains of Lennard-Jones segments while the simplified SAFT-HS equation treat s molecules as chains of hard-sphere segments with van der Waals inter actions. The water molecules are modeled as spherical repulsive cores with four association sites which mediate the hydrogen-bonding interac tions. It turns out that a simple relationship for the parameters of t he various mixtures can be used with the SAFT-HS treatment. The nonsph erical nature of the alkanes is incorporated into the theory by treati ng the molecules as chains formed from united-atom spherical segments. The parameters for the pure components of the water + n-butane mixtur e are fitted to the critical points of each component; the strength an d range of the hydrogen-bonding interaction between water molecules we re obtained in a separate study by fitting to the vapor pressure and s aturated liquid density of pure water. The parameters for the unlike i nteractions are fitted to the minimum of the high-pressure gas-liquid critical line of the water + n-butane mixture. We use a simple relatio nship between the number of segments in the united-atom chain models o f the n-alkanes and the number of carbon atoms to predict the properti es of mixtures of water with other n-alkane homologues without the rec ourse to further fitting. The phase equilibria of the mixtures obtaine d using this transferable interaction parameter approach are in excell ent agreement with the experimental data even though the parameters ar e fitted to just one mixture. The water + methane system is the except ion to this: the pure component parameters have to be refitted to the anomalous critical point of methane, as does the unlike mean-field int eraction. We also predict that the type III phase behavior exhibited b y water + n-alkane mixtures persists even for very long n-alkane chain s, i.e., water + n-eicosane.