Brazilian Journal of Chemical Engineering
Print version ISSN 0104-6632
Braz. J. Chem. Eng. vol.18 no.2 São Paulo June 2001
PREDICTION OF ELECTROLYTEVAPOR-LIQUID EQUILIBRIUM BY UNIFAC-DORTMUND
M.Aznar1* and A.S.Telles2
1Faculdade de Engenharia Química, Universidade Estadual de Campinas, C.P. 6166,
CEP 13081-970, Campinas - SP, Brazil
2Escola de Química, Universidade Federal do Rio de Janeiro, C.P. 68542,
CEP 21949-900, Rio de Janeiro - RJ, Brazil
(Received: October 10, 2000 ; Accepted: April 23,2001)
Abstract - The modified UNIFAC-Dortmund group contribution model is used for the correlation and prediction of salt effects in binary solvent-salt and ternary mixed solvent-salt systems. The long-range electrostatic interaction contribution, usually represented by a Debye-Hückel term, was empirically dropped. Previously published parameters for interactions between solvent groups (CH2,OH, CH3OH, H2O and CH3CO) were used, and group interactions between ions (Li+, Na+, K+, Ca+2, Cl-, Br-, NO3- and ACE-) and between ions and solvent groups have been estimated. The data base includes 29 binary and 56 ternary systems, used in part for the calculation of group interactions and in part for the testing of predictions.
Keywords: thermodynamics, vapor-liquid equilibrium, electrolytes,prediction, UNIFAC.
Prediction of vapor-liquid equilibrium in electrolyte systems is of particular importance in the design and operation of extractive or azeotropic distillation. In these processes, the addition of a strong, nonvolatile electrolyte, i.e., a salt, modifies the relative volatility of the components through the salt effect. There are several gE models proposed inthe literature for description and/or prediction of the salt effect. The basic idea is to combine a local composition gE model for the activity coefficient, used for the representation of the short-range interactions between ions and solvents, with a Debye-Hückel term (Robinson and Stokes, 1970) for the long-range ion-ion electrostatic interactions. Chen et al. (1982) and Chen and Evans (1986)combined the NRTL model and the Pitzer-Debye-Hückel (Pitzer, 1980) model for solvent-salt mixtures. Haghtalab and Vera (1988) presented a similar approach, using the original Debye-Hückel term. Liu et al. (1989) combined a three-parameter Wilson equation (Renon and Prausnitz, 1969) with an extended Debye-Hückel term for solvent-salt mixtures. Sander et al. (1986) presented a model combining amodified UNIQUAC model with a Debye-Hückel term for description of the salt effect on the vapor-liquid equilibrium of water-solvent mixtures. Macedo et al. (1990) modified the Sander model by using a modified Debye-Hückel term. Kikic et al. (1991) extended the Sander model, replacing UNIQUAC with UNIFAC, generating a predictive model. In another approach, Dahl and Macedo (1992) neglected the Debye-Hückelterm, at the suggestion of Mock et al. (1986) and Cardoso and O'Connell (1987). These authors showed that the Debye-Hückel term had no effect on vapor-liquid equilibrium behavior, although it could be essential in aqueous-phase ionic activity coefficient calculations. Aznar et al. (1994) followed the approach of Dahl and Macedo and tested the Wilson model (1964) in place of UNIFAC.
In this work,the modified UNIFAC-Dortmund model for short-range activity coefficients (Weidlich and Gmehling, 1987; Gmehling et al., 1993) is used for the correlation and prediction of experimental vapor-liquid equilibrium data in electrolyte systems without a Debye-Hückel long-range term. The data includes 29 binary solvent-salt and 56 ternary mixed solvent-salt systems. New group interaction parameters...