(98h) Explicit Phase Equilibrium Calculations from Cubic Equations of State

Equations of state may be used to calculate pure component vapor-liquid equilibrium properties such as vapor pressure, heat of vaporization, liquid density, and vapor density. The standard approach requires coupling the EOS with a phase equilibrium criterion such as free energy, chemical potential, or fugacity. The resulting equations are nonlinear and must be solved by numerical methods.

An alternative approach is applicable to cubic EOS such as the commonly used Soave-Redlich-Kwong and Peng-Robinson equations. Equilibrium properties may be explicitly expressed as power series in reduced temperature or related functions. These results are more convenient than numerical calculations, but are subject to truncation error in many practical situations.

Modifications or alternatives to these power series methods were developed. These included simplified versions of the power series which minimized truncation error, a generalized version of the Antoine equation for vapor pressure, and a new expression for vapor pressure which showed reasonable accuracy over the temperature range from the triple point to the critical point. All results were expressed as universal functions of temperature and the acentric factor. The deviation between these results and the exact equilibrium property predictions from the EOS were illustrated by generalized graphs.