Perturbed hard chain theory for fluid mixtures: Thermodynamic properties for mixtures in natural gas and petroleum technology

Perturbed‐hard‐chain theory for pure fluids, proposed previously by Beret, is modified slightly to yield better pure‐component results. More important, it is extended to multicomponent mixtures. The perturbed‐hard‐chain theory is a synthesis of the polymer solution theories of Flory and Prigogine and the perturbed‐hard‐sphere theories of Alder and Barker and Henderson. The resulting equation of state is applicable to simple as well as complex molecules (for example, heptane, naphthalene, polystyrene). It can be used to calculate both gas and liquid phase properties. Extension of perturbed‐hard‐chain theory to mixtures is based on a one‐fluid model without, however, making the usual assumption of random mixing. The perturbed‐hard‐chain theory has been applied to most fluids commonly encountered in natural‐gas and petroleum refining operations. The theory gives good agreement with experiment for pure‐component and fluid‐mixture properties including vapor pressures, liquid densities, enthalpies, and K factors. Molecular parameters have been obtained for forty‐five pure components and for more than sixty binary mixtures. Ternary and higher mixtures require no additional parameters.