Vapor-liquid equilibria of linear and branched alkanes from perturbation theory

Abstract

The vapor-liquid equilibrium properties of linear and branched alkanes with up to eight carbon atoms have been determined by means of a simple perturbation theory. An accurate equation of state for the reference system is combined with a mean field treatment of the perturbation term, along with a reasonable set of potential parameters. Experimental trends of the critical properties of alkanes are described qualitatively by the theory. In particular, the maximum in the critical pressure and density of linear alkanes as a function of the number of carbon atoms is reproduced and explained on a simple basis. The effect of branching on the critical properties is considered and it is found that the decrease in the critical temperature with branching is correctly predicted. With a few exceptions for some substances, other general trends in critical pressure and density are predicted and explained. The effect of branching on the principle of corresponding states is also studied, in particular on the coexistence densities and vapor pressures. It is found that branching reduces the slope of the vapor pressure curve and makes the coexistence curve narrower when these magnitudes are represented in a corresponding states plot. It is shown that a simple mean field theory is able to describe qualitatively the variation in the critical properties of linear and branched alkanes. © 1998 American Institute of Physics.