Interpreting Gas-Saturation Vapor-Pressure Measurements Using Virial Coefficients Derived from Molecular Models

Abstract

We calculate virial coefficients of gas mixtures to demonstrate their use for interpreting gas-saturation measurements of the vapor pressure of low-volatility compounds. We obtain coefficients from molecular models, via calculation of Mayer integrals that rigorously connect the models and the coefficients. We examine He, CO2, N2, and SF6 as carrier gases, and n-C14H30 and n-C20H42 as prototype low-volatility compounds, considering both united-atom (UA) and explicit-hydrogen (EH) alkane models for them. Both the pure virial coefficients of every species and the cross-coefficients of each gas with n-C20H42 are calculated up to third order; cross-coefficients of SF6 with n-C14H30 and all EH-based coefficients are given only to second order. Using these coefficients, we calculate corrections to the vapor pressure of n-C20H42 at 323.15 K for all four carrier gases. With the corrections, the derived vapor pressures in He, CO2, and N2 carrier gases are in excellent agreement, resolving most of the variation observed in apparent vapor pressures when gas-phase nonideality is neglected. Results are less satisfactory for SF6 as the carrier gas. We also calculate corrections to vapor-pressure data for n-C14H30 at (283.15, 293.15, 303.15, and 313.15) K in an SF6 carrier gas.