CONFORMAL SOLUTION METHOD WITH THE HARD CONVEX BODY EXPANSION THEORY FOR PREDICTING VAPOR-LIQUID EQUILIBRIA

[摘要] Like the hard sphere expansion (HSE) theory, the hard convex body expansion (HCBE) theory separates any residual thermodynamic property into a contribution from molecular repulsion, which is calculated directly from a hard convex body (HCB) equation of state, and other contributions from molecular attraction, which are obtained by the corresponding states principle (CSP) using pure reference fluids. The HSE theory yields good agreement with the experimental thermodynamic data for light hydrocarbon mixture systems. However, there is a limit to molecular size and shape difference in mixtures where the intermolecular repulsion can be represented by hard sphere mixture. A HCB equation of state developed by Naumann and Leland (1984) is applicable to pure components and their mixtures. The HCB equation of state for a pure component is characterized by two dimensionless geometrical parameters, $alpha$ and $ausp{-1},$ which are combinations of three molecular dimensions of a convex body--volume(V), surface area(S), and mean radius(R). Two dimensionless geometrical parameters are determined directly from Pitzer;;s acentric factor. The molecular volume is evaluated by equating the HCB equation of state to the optimal repulsion evaluated by the expansion method. The surface area and the mean radius are obtained from known dimensionless geometrical parameters and molecular volume. Four kinds of convex bodies are considered in this work. These are prolate spherocylinders, oblate spherocylinders, prolate ellipsoids, and oblate ellipsoids. Better results for the vapor-liquid equilibrium constants (K-values) for mixtures containing molecules as nonspherical as n-decane in the prolate models and cyclohexane in the oblate models are obtained with this method than with ordinary equations of state using empirical mixing rules. This HCBE theory can also be applied to predict thermodynamic properties of pure components using two reference fluids as in the Lee-Kesler method (1975).

[发布机构] Rice University