The correlation and estimation of vapour pressures IV. Extrapolation of vapour pressures and estimation of critical pressures by the principle of corresponding states using two reference fluids with non-spherical molecules

Abstract

Application of the principle of corresponding states is today usually based on two reference fluids, one of which is a simple fluid with acentric factor ω = 0 and the other a fluid with ω = 0.4. It is shown here that the use of two real fluids, such as propane and octane, or benzene and pentafluorotoluene with ω ≈ 0.2 and ω ≈ 0.4, to provide the reference lines gives better estimates of critical pressures from observed vapour pressures in the range 10 to 200 kPa, and the accuracy is similar to that obtained by use of the method of constrained extrapolation proposed in an earlier paper in this series. The two-real-fluid method also gives better correlation of observed vapour pressures throughout the range of reduced temperatures 0.5 to 1.0. Whichever pair of reference fluids is chosen, the principle of corresponding states is superior for both estimation and correlation to comparison with the behaviour of a single reference fluid.