Modeling phase equilibria and speciation in mixed-solvent electrolyte systems: II. Liquid–liquid equilibria and properties of associating electrolyte solutions

A comprehensive mixed-solvent electrolyte model has been extended to calculate liquid– liquid equilibria in water–organic salt systems. Also, it has been applied to calculate phase equilibria and speciation in strongly associating systems such as sulfuric acid/oleum (H2SO4+SO3+H2O) in the entire concentration range. The model combines an excess Gibbs energy model with detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer–Debye–Hückel expression, a short-range term expressed by the UNIQUAC model and a middle-range term of a second-virial-coefficient type for specific ionic interactions. The model accurately represents the thermodynamic behavior of systems ranging from infinite dilution in water to pure acids and beyond, e.g. in mixtures of H2SO4 and SO3. In particular, the model has been shown to predict speciation that is consistent with spectroscopic measurements for the H2SO4+H2O system. In addition, vapor– liquid and liquid– liquid equilibria can be accurately reproduced in mixtures that show complex phase behavior. D 2005 Elsevier B.V. All rights reserved.