Short Fundamental Equations of State for 20 Industrial Fluids

Abstract

In a preceding project, functional forms for “short” Helmholtz energy equations of state for typical nonpolar and weakly polar fluids and for typical polar fluids were developed using simultaneous optimization. In this work, the coefficients of these short forms for the equations of state have been fitted for the fluids acetone, carbon monoxide, carbonyl sulfide, decane, hydrogen sulfide, 2-methylbutane (isopentane), 2,2-dimethylpropane (neopentane), 2-methylpentane (isohexane), krypton, nitrous oxide, nonane, sulfur dioxide, toluene, xenon, hexafluoroethane (R-116), 1,1-dichloro-1-fluoroethane (R-141b), 1-chloro-1,1-difluoroethane (R-142b), octafluoropropane (R-218), 1,1,1,3,3-pentafluoropropane (R-245fa), and fluoromethane (R-41). The 12 coefficients of the equations of state were fitted to substance specific data sets. The results show that simultaneously optimized functional forms can be applied to other fluids out of the same class of fluids for which they were optimized without significant loss of accuracy. The high numerical stability of the functional forms resulted in successful fits for fluids that previously could not be described by accurate empirical equations of state. For R-41, it is shown that the accuracies can be increased further by fitting the temperature exponents in addition to the coefficients of the equation of state, provided that highly accurate experimental data are available. Typical uncertainties of properties calculated using the new equations are 0.2 % in density, 1 % to 2 % in heat capacity and liquid-phase speed of sound, and 0.2 % in vapor pressure. Where data are available, uncertainties in vapor-phase sound speeds are generally less than 0.1 %.