Multicomponent phase equilibrium calculations for water-methanol-alkane mixtures

Prediction of phase equilibrium for multicomponent systems containing assoc iating compounds (e.g., water and alcohols) is essential in a number of eng ineering applications (e.g., environmental technology, gas hydrate inhibiti on) and at the same time represents one of the most stringent tests for a t hermodynamic model. Conventional models (e.g., cubic equations of state and excess Gibbs free energy models) provide rapid and often reliable estimate s of phase equilibrium in many cases but extension to multicomponent system s, especially those containing water is often troublesome. On the other han d, novel association equations of state perform considerably better but are slower compared to conventional models. Furthermore, the extension of seve ral of them to cross-associating systems (e.g,, water-alcohols) exhibits pr oblems, in this work, the performance of two well-known conventional models (SRK and NRTL) is compared for multicomponent systems to a recently propos ed association equation of state both in terms of accuracy of predictions a nd timing. The proposed model incorporates the Wertheim chemical associatio n theory (employed previously in models such as SAFT) and the SRK equation. The model is applied in this work to multicomponent systems in such a way that the inclusion of the Wertheim theory does not give execution times muc h higher than conventional models. The model yields very satisfactory predi ctions of multicomponent equilibria for aqueous (both vapor-liquid and liqu id-liquid equilibria) systems containing methanol, gases and hydrocarbons, which are moreover, as will be demonstrated in this work, considerably bett er compared to SRK and NRTL. (C) 1999 Elsevier Science B.V. All rights rese rved.