Molecular thermodynamics for fluids at low and high densities. Part II: Phase equilibria for mixtures containing components with large differences in molecular size or potential energy

Abstract

The molecular-thermodynamic correlation presented in Part I for pure fluids is extended t o mixtures; this extension is particularly useful for mixtures of components with large differences in molecular size or potential energy. The novel feature of this correlation is a separation of the high-density and low-density contributions to the residual Helmholtz energy; this separation allows use of separate mixing rules for each density regime, guided by theoretically defined boundary conditions. Such flexibility is necessary to represent phase equilibria in highly asymmetric fluid mixtures such as water-hydrocarbon systems. Phase equilibria are correlated for a variety of binary and multicomponent mixtures, including a petroleum reservoir fluid containing carbon dioxide and including a mixture of hydrogen and coal-derived liquids.

An Approach to Calculate Thermodynamic Properties of Mixtures Including Propane, n-Butane, and Isobutane

“…(5) d i represents the hard-sphere diameter and is a temperature-dependent function and ψ i is the mole fraction of component i. In our case we have used the equation developed by Cotterman et al [52]:…”

“…For the dispersion term we have used an equation by Cotterman et al [52] and one-dimensional VDW theory for mixtures [26]:…”

A correlation for thermodynamic properties of heavy fossil-fuel fractions

“…A modified perturbed-hard-chain equation of state, recently developed by Cotterman et al [Cotterman et al (1986a), Cotterman and Prausnitz (1986b)], is used as the basis of the correlation.…”

“…Using the mixing rules presented by Cotterman and Prausnitz (1986b), the equation of state used in this work can be extended to mixtures. To use these mixing rules, the three pure-component equation-of-state parameters are defined in terms of the five parameters shown in Equations 1 and 2.. (kq , k11 , and k-8 ) are also needed to calculate mixture properties.…”

“…The volume of a molecular segment ( v) is set at 10 cm 31mole. To determine r for a fraction, the correlated value of v is divided by v. For pure well-defined components including light gases, we use previously published values [Cotterman et al (1986a), Cotterman and Prausnitz (1986b To determine a correlation for k 1 for light-gas/heavy-fraction equilibria, interaction parameters, we used experimental VLE data for binaries containing H,, CH4 , CO,. C,H6, and H,S with model compounds and fossil-fuel fractions.…”