HIGH-PRESSURE VAPOR-LIQUID EQUILIBRIA FOR ASYMMETRIC MIXTURES

An equation of state has been developed for asymmetric mixtures, based on perturbed-hard-chain theory and the local-composition concept.; Perturbed-hard-chain theory is based on the statistical-mechanical perturbation theory proposed by Zwanzig and developed by Barker, Henderson, and others. The Helmholtz energy of a fluid is calculated as the sum of a hard-sphere part (given by the expression of Carnahan and Starling for hard spheres), an attractive part (given by the expression of Alder et al. for square-well molecules), and a term to account for contributions due to density-dependent rotational and vibrational degrees of freedom (using a modification of an idea of Prigogine). To account for polar interactions, we assume a specific temperature dependence for the effective square-well depth. The perturbation expansion of the Helmholz energy in powers of reciprocal temperature is summed by approximating all higher-order terms with the method of Barker and Henderson.; To extend an equation of state to mixtures, a local-composition model has been developed based on a modification of the quasi-chemical theory of Guggenheim for liquid-state activity coefficients. The new model can be applied to any equation of state of the van der Waals form at all fluid densities. As suggested by quasi-chemical theory, we incorporate nonrandomness corrections for thermodynamic properties. We calculate the compositions of all species in the immediate neighborhood of a given molecule from the overall composition and from Boltzmann factors using an energy characteristic of the interaction between the possible types of molecular pairs. Unlike in previous treatments, that energy is here calculated from the equation of state, without the introduction of new adjustable binary parameters. Significant improvement over the random-mixing models is shown for vapor-liquid equilibria for several asymmetric mixtures.; To obtain data for asymmetric systems (in particular, aqueous-hydrocarbon systems), we have designed and built a high-pressure phase-equilibrium apparatus for pressures to 2000 bar at 400(DEGREES)C. The static-equilibrium cell was fabricated of INCONEL 718 for strength and corrosion resistance, and it has Bridgman seals.