Phase Equilibria for Systems Containing Hydrocarbons, Water, and Salt: An Extended Peng-Robinson Equation of State

The empirical success of the Peng-Robinson equation of state (PREOS) is unmatched; there is no other simple equation of state of the van der Waals form that has shown such wide and reliable applicability to the calculation of vapor-liquid equilibria (VLE) for systems containing light hydrocarbons, permanent gases, carbon dioxide, and hydrogen sulfide. PREOS is one of the great success stories of applied chemical engineering thermodynamics. However, PREOS also has limitations; two of these are addressed here. Theoretically based extensions are presented, first, for including water and aqueous hydrocarbon mixtures and, second, for including salt in water and in aqueous hydrocarbon mixtures. Illustrative examples show that these extensions are successful for VLE (where L refers to a hydrocarbon liquid dilute in water) and for VLE (where L refers to saline water). Because the extension to include water, based on SAFT (statistical association fluid theory), cannot properly take into account the well-documented hydrophobic effect, it is not yet possible to give a good representation of phase equilibria when one of the phases is a dilute aqueous liquid solution of hydrocarbon where the hydrophobic effect is significant.