The electrolyte nonrandom two-liquid (NRTL) model proposed by Chen et al. (1982) is generalized to represent the excess Gibbs energy of aqueous multicomponent electrolyte systems. Using only binary parameters, the model correlates and predicts the deviation from ideality of aqueous multicomponent electrolyte systems over the entire range of temperature and concentration. SCOPERepresentation of the excess Gibbs energy and activity coefficients for aqueous electrolyte systems is a fundamental problem in the design and operation of many industrial processes. Recently, active research has led to several excess Gibbs energy models and activity coefficient models for aqueous electrolyte systems up to concentrations of 6 molal or higher. Among them are the models of Pitzer (1973), Meissner and Tester (1972). Bromley (1973), and Cruz and Renon (1 978).A local composition model was later proposed by Chen et al. (1982) to represent the excess Gibbs energy of single-solvent, single completely dissociated electrolyte systems over the entire range of temperature and concentration. The model proposed two fundamental assumptions about the liquid lattice structure of electrolyte systems:1. The like-ion repulsion assumption states that the local composition of cations around cations is zero (and likewise for anions around anions). This implies that the repulsive forces between ions of like charge are relatively large. The like-ion repulsion was justified on the basis that repulsive forces between ions of the same sign are very strong for neighboring species. The local electroneutrality assumption statesCorrespondence mnmrning this paper should be addressed to Chau-Chyun Chen 444March 1986 that the distribution of cations and anions around a central molecule is such that the net local ionic charge is zero. Local electroneutrality has been observed for interstitial molecules in salt crystals. The model further postulates the excess Gibbs energy to be the sum of two contributions, one resulting from long-range electrostatic forces between ions and the other from short-range forces between all the species. The Pitzer-Debye-Huckel equation (Pitzer, 1980) was applied to model the long-range contribution. The nonrandom two-liquid (NRTL) theory (Renon and Prausnitz, 1968) was adopted to account for the short-range contribution.The model was applied to obtain good data correlation results on many single-solvent, single completely dissociated electrolyte systems over a wide range of concentration and temperature. The model represents a basic framework that provides a continuous connection among limiting electrolyte systems, such as infinitely dilute electrolyte systems (where the model reduces to the Debye-Huckel model), pure molecular systems (where the model reduces to the NRTL model), and pure fused salts.The objectives of this work are to generalize this electrolyte NRTL model to represent the excess Gibbs energy of aqueous multicomponent electrolyte systems and to examine the behavior and performance of the Vol. 32, No. 3AIChE Journal

An electrolyte local composition model is developed for excess Gibbs energy, which is assumed to be the sum of two contributions, one resulting from long range electrostatic forces between ions and the other from short range forces between all the species. The validity of the model is demonstrated for systems emcompassing the entire range from molecular liquid to fused salt. and Cruz and Renon (1978). Of these, the Pitzer equation is especially useful. It has been applied successfully to represent data within experimental error from dilute solutions up to an ionic strength of six molal for both aqueous single strong electrolyte systems (Pitzer and Mayorga, 1973) and aqueous mixed strong electrolyte systems (Pitzer and Kim, 1974). Modified forms of the Pitzer equation have been used by Beutier and Renon (1978) and Edwards et al. (1978) as models for the ionic activity coefficients of aqueous weak electrolyte systems. The Pitzer equation was later extended (Chen et al., 1979) in a thermodynamically consistent manner to allow for molecular as well as ionic solutes in the aqueous systems. CHAU-CHYUN CHENPitzer's excess Gibbs energy equation is a virial expansion equation and is subject to all of the limitations of a virial model. The model parameters are arbitrary, temperaturedependent, and characteristic of the solvent. Binary parameters are empirical functions of ionic strength and ternary parameters are necessary at high ionic strength. The Pitzer equation can not be used for mixed solvent systems because its parameters are unknown functions of solvent composition. Therefore, although the Pitzer equation has been shown to be a convenient and accurate representation of aqueous electrolyte systems, a more versatile model is needed.In this study a new model is developed that does not have the disadvantages of a virial expression and that is applicable to a wide variety of electrolyte systems over the entire range of electrolyte concentrations and system temperatures. The kinds of systems that have been studied using the new model involve mixed electrolytes, mixed solvents, and partially dissociated electrolytes (Chen, 1980; Chen et al., 1980). It is anticipated that the model is also applicable to systems involving salt precipitation, immiscible liquid phases, and fused salt solutions. However, this paper, part one of a series, is limited to single solvent, single completely dissociated electrolyte systems in order to emphasize the model development, the physical interpretation of the model parameters, and the two critical assumptions on which the model is based. In part two of the series (Chen et al., 1982) the model is extended to multicomponent systems, including partially dissociated electrolytes which involve dissociation equilibria. CONCLUSIONS AND SIGNIFICANCEA new model has been developed for the excess Gibbs energy of electrolyte systems. It is based on the local composition concept and is designed to represent the properties of all kinds of electrolyte systems over the entire range of electrolyte concentra...

lates the vapor-liquid equilibrium and liquid-liquid equilibrium of mixedsolvent electrolyte systems over the entire range of temperature and concentrations.Correlation of experimental data for 47 single-solvent electrolyte systems and 33 mixed-solvent electrolyte systems demonstrates that the electrolyte NRTL model gives excellent representation of vapor-liquid equilibrium and liquid-liquid equilibrium of mixed-sol-vent electrolyte systems. Average errors of AP = 1 kPa, AT = 0.1 K, Ay = 0.01 in correlating vapor-liquid equilibrium data at atmospheric conditions, and Ax = 0.01 in correlating liquid-liquid equilibrium data are typical. Previous MethodsIn the late 1960's and the 1970's, several semiempirical models for representing the excess Gibbs energy of nonelectrolyte liquid solutions were developed with the local composition concept. Examples are the Wilson (1964) model, the NRTL model (Renon and Prausnitz, 1968), and the UNIQUAC model (Abrams and Prausnitz, 1975). These models are able to represent, with a reasonable number of binary adjustable parameters, the phase equilibrium of highly nonideal nonelectrolyte systems. Parameter RequirementsThe model binary adjustable parameters are associated with the solvent-solvent pairs, solvent-salt pairs, and salt-salt pairs.

The representation of phase equilibrium for amino acids, peptides, and proteins in solution is an important problem in the design and optimization of downstream processes for recovery of the biomolecules. This paper presents a molecular thermodynamic framework [1, 2] for the representation of the solubilities of amino acids and small peptides. With this framework, satisfactory results have been obtained in representing and predicting the solubilities of amino acids and small peptides in aqueous solution as functions of temperature, ionic strength, dipolar species concentrations, solvent compositions, and pH.

The semi-empirical Pitzer equation for modeling equilibrium in aqueous electrolyte systems has been extended in a thermodynamically consistent manner to allow for.molecular as well as ionic solutes. Under limiting conditions, the extended model reduces to the well-known Setschenow equation for the salting out effect of molecular solutes. To test the validity of the model, correlations of vapor-liquid equilibrium data were carried out for three systems: the hydrochloric acid aqueous solution at 298.15OK and concentrations up to 18 molal; the NH3-C02 aqueous solution studied by Van Krevelen et al. (1949) SCOPEThe use of modern process simulators for analysis and design of processes involving electrolytes has been greatly limited by the lack of adequate correlations for electrolyte thermodynamics. For most systems of industrial importance, empirical correlations are applicable only to one particular system, over a limited range of conditions. The empirical correlations do not provide a framework for treating new systems or for extending the range of existing data, because the nonidealities have not been accounted for in a general and consistent manner.Aqueous electrolyte systems are common in the chemical industry. One example of interest is the potassium carbonate aqueous solution used in the Hot Carbonate Process as the acid gas removal agent in coal conversion plants. The Hot Carbonate Process was developed by the U.S. Bureau of Mines as part of a program on the synthesis of liquid fuel from coal. It provides an economical chemical absorption process for removing large quantities of COz from synthesis gases (Riesenfeld and Kohl 1974).Knowledge of the equilibrium solubility of carbon dioxide in the potassium carbonate aqueous solution is essential to the design of the Hot Carbonate Process. A large amount of comprehensive physical data on the KzCO3-CO2 aqueous solution system is available in the literature. However, due to inability to properly correlate electrolyte thermodynamics, empirical equations or nomographs of the vapor-liquid equilibrium data have been used as basic design tools (Bocard and Mayland 1962, Maddox and Burns 1967, Mapstone 1966, Wen 1971.Recently, there have been a number of significant developments in the modeling of electrolyte behavior. Brornley (1973), Meissner and Tester (1972), Meissner and Kusik (1972), Pitzer and co-workers (1973a, 1973b, 1974, 1976), and Cruz and Renon (1978), presented models for calculating the mean ionic activity coefficients of many types of aqueous electrolytes. In addition, Edwards et al. (1975) proposed a thermodynamic framework to calculate equilibrium vapor-liquid compositions for aqueous solutions of one or more volatile weak electrolytes which involved activity coefficients of ionic species. Most recently, Beutier and Renon (1978) and Edwards et al. (1978) used simplified forms of the Pitzer equation to represent ionic activity coefficients.The purposes of this study are to 1) reformulate the Pitzer equation in a more rigorous way, to obtain a thermodyn...