A method for improving equations of state near the critical point

Abstract

Accurate nonanalytic equations of state have been developed for pentane and carbon dioxide for use as reference equations in the critical region. This was done using a method originated by Fox which transforms any analytic equation of state into a nonanalytic equation of state. For this study, a 32 constant analytic BWR equation of state developed by Jacobson and Stewart was transformed into a nonanalytic equation of state. The nonanalytic equation of state is more accurate in the critical region for the prediction of the PVT properties of carbon dioxide than is a Schmidt-Wagner analytic equation of state developed at the National Bureau of Standards. The nonanalytic equation of state is slightly less accurate over the whole PVT surface than the Schmidt-Wagner equation of state, but the predicted nonanalytic surface makes a smooth transition into the classical region. This is an improvement over the scaled fundamental equation of state which breaks down at a small distance away from the critical point.