The calculation of critical points of fluid mixtures-effect of improved pure component critical point representation

Abstract

The poor representation of mixture critical volumes by cubic equations of state is well known and has been widely attributed to the fact that the pure component critical compressibility calculated from the equation of state (designated as ζ ci) is, in general, not equal to the experimental critical compressibility (designated as Z ci expt) of most fluids. This work examines the behavior of two and three-constant cubic equations of state in the critical region and demonstrates that simply setting ζ ci equal to Z ci expt does not necessarily lead to more accurate prediction of mixture critical points. The improvement in the pure component critical point representation creates the necessity for an additional mixing rule which is not inherent in the usual mixing rules for the equation of state constants. The critical region behavior of two cubic equations of state was studied in this work. These equations, namely the Peng-Robinson and the Teja-Patel, were chosen as representative of two and three-constant equations of state.