This binary mixture presents a peculiar behavior: indeed, the azeotrope, which is present at low pressure ( -butanol at ), disappears at high pressures (e.g. at ).

The Demonstration plots gas-phase fugacity coefficients for user-specified values of the pressure (expressed in psi).

A comparison with the results obtained using ASPEN (http://www.aspentech.com/) for is also shown for both the equilibrium curve and the -value for -butanol (cyan dots come from ASPEN).

In order to use the Hayden–O'Connell method, which takes into account deviations from ideal gas-phase behavior at moderate and high pressures, one has to compute second-order virial coefficients , , and or ) versus temperature. The Demonstration computes and plots these latter coefficients and compares the results obtained in the present study with the data (red dots) calculated by a Fortran program kindly provided by Professor J. P. O'Connell.