Wagner Equation Predicting Entire Curve for Pure Fluids from Limited VLE Data: Error Dependency Upon Data Interval & Fully-Determined Case

Abstract

The predictive error in vapor pressure of limited-data Wagner constants relative to that of entire-curve constants is studied for eleven data intervals. Good precision is assumed for data inputs, four digits in the mantissa of Ln Pv,r and five digits for Tr. An algebraic solution for the fully-determined case based on only four data points is used to estimate Wagner constants. Seventy-two species are used to assess the impact of the location of the two interior points and the location and width of the limited-data interval upon the error in predicted Pv,r due to data imprecision. Hydrogen, helium, R152a, and water are used to assess error due to Wagner imperfection and compare predictive capability of the algebraic fully-determined and regressed over-determined approaches. The results indicate that limited VLE data of good precision from reduced temperature intervals with a width ≥ 0.1 and a lower bound ≤ 0.6 can generally provide reasonable VLE predictions over the entire two-phase curve for pure substances, with average error of approximately 1%. It is shown that the algebraic, fully-determined solution presented is a viable tool for investigating the extensibility of limited-data Wagner constants.