To get better correlation results of vapor liquid equilibrium (VLE) of mixture systems

it is usually necessary to select the suitable mixing rule. The modifications of force parameter a have obtained good results in both theory and application. However

the research on co-volume parameter b is less and usually empirical. A new modified method for co-volume b was presented. In the revised mixing rule

the molecular shape factors obtained from Leach et al were introduced to amend co-volume b on the basis of Mie potential-energy and London dispersion theory. Thus

the interactive parameter lij in co-volume b was dependent on the molecular shape factors θ

φ

and critical parameters of the components. Without experimental data

molecular shape factors and interactive parameter lij could be obtained

which could be used to reflect the actual situations of mixture systems. The VLE calculation results for 16 kinds of different systems were given by using modified co-volume parameter b in MHV1/LCVM/HV mixing rules combined with PR EOS and NRTL models

and the results were compared with the results of using b ∑xibi in MHV1/LCVM/HV mixing rule combined with PR EOS and NRTL models. The models with the modified co-volume b were better than those with b ∑xibi to some extent.