Application of critical-region scaling to pure-component equations of state

Abstract

A critical scaling method for equations of state developed by Fox at the National Bureau of Standards has been extended to predict thermodynamic properties and all of the first and second derivatives of the chemical potential and density. This is done by using a new damping function and developing expressions which avoid all numerical evaluations of the derivatives needed in calculating thermodynamic properties. Using this procedure, an analytical 32-constant equation of state was scaled in the critical region for pure light hydrocarbons and for carbon dioxide. The BWRS equation was scaled for heavier hydrocarbons. Calculations of the saturated vapor and liquid densities of methane in the region from 175 K to the critical temperature of 190.5 K were improved, and the mean absolute error decreased from 0.153 without scaling to 0.031 with scaling. Other derivative properties were also improved in the critical region for several of the lighter pure hydrocarbons.