Abstract
The performance of the classic Peng–Robinson (PR) and the modern Cubic-Plus-Association (CPA) equations of state were evaluated in modeling isobaric and isothermal vapor–liquid equilibria (VLE) of binary mixtures of carboxylic acids (formic, acetic, propanoic or butanoic) + water. Two functionalities of the alpha term were tested in PR, the original term proposed by Soave and the Matthias–Copeman term specially developed for modeling polar compounds. Within the Soave functionality, two generalized forms of the acentric factor were studied, the original general form and the Robinson and Peng modification for values of the acentric factor larger than 0.491. In addition, the case of PR with fitted parameters from saturated properties (as commonly obtained for modern equations of state) was also evaluated. VLE calculations without the use of a binary interaction parameter are in general more accurate with the modern CPA due to the association term; however, when a binary interaction parameter is used, the performance of the PR versions studied here are on average similar to those of CPA, and in some cases even superior. The original alpha function used in the PR equation and the original generalized form of the acentric factor are the best options for modeling organic acids + water systems when the binary interaction parameter is available. Temperature-dependent binary interaction parameters are provided as a database in modeling these complex systems.
Highlights
Carboxylic acids are important commodity chemicals due to their versatile applications
Thermodynamic modeling of carboxylic acids with water is challenging since the systems exhibit strong non-ideal behavior due the presence of self- and crossassociation as well as polar interactions [1,2,3,4,5,6]
Young et al [16] have shown in their comparison of 20 alpha functions that MC is one of the best for modeling pure component properties of water and carboxylic acids
Summary
List of Symbols a Attraction parameter (MPa·L2·mol−2) a0 Characteristic parameter in CPA (MPa·L2·mol−2) b Repulsive parameter (L·mol−1) c1 Characteristic parameter in CPA (–) g Radial distribution function (–) kij Binary interaction parameter (–) MC Mathias–Copeman parameter (–) N Number of data points P Bubble pressure (MPa) Pc Critical pressure (MPa) Pv Vapor pressure (MPa) R Universal gas constant (J·mol−1·K−1) T Temperature (K) Tc Critical temperature (K) Tr Reduced temperature (–) v Molar volume (L·mol−1) x Liquid mole fraction (–) XA Monomer fraction at site A (–) y Vapor mole fraction (–). Greek Letters α Alpha function (–) βAB Association volume in CPA (–) Δ Average deviation (–) ΔAB Association strength (L·mol−1) εAB Association energy (L) η Reduced density (–) θ Property ρ Density (kg·L−1) ω Acentric factor (–)
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