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...

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...

No abstract

Thermodynamic property computations using equations of state first require computation of the density root. Since higher level calculations such as single-stage flash, distillation and data regression are usually performed iteratively, properties are often demanded at conditions where the appropriate density root does not exist. A strategy of returning suitable pseudoproperties under such conditions is proposed. It has been successfully used in ASPEN (Advanced System for Process Engineering), a general process simulator developed at the Massachusetts Institute of Technology.

A new algorithm was developed for the solution of the equations that describe multicomponent, multistage separation processes operating at steady state. The algorithm is based on the use of newly defined energy and volatility parameters as the primary successive approximation variables. A third parameter was defined for each stage as a unique combination of the liquid and vapor phase rates and the temperature, and the quasi‐Newton method of Broyden was employed to iterate on these parameters. The exceptional stability of the new algorithm in very difficult cases, as well as its efficiency in easy cases, are demonstrated using a variety of example problems.