Phase stability criteria and fluid-phase equilibria in strong-electrolyte systems

Abstract

Although the presence of salts can significantly affect the fluid-phase equilibria, phase stability and equilibrium calculations remain challenging due to the nonlinearity of thermodynamic models and to the negligible amounts of ions that can be present in some phases. To address this, we introduce a new variable transformation and present the first formal proof of a stability criterion for strong (fully-dissociated) electrolyte solutions based on the tangent-plane distance under the electroneutrality constraint. The criterion can also be recast based on duality theory, yielding two alternative formulations with/without reformulation in the volume-composition space. We use these theoretical results to extend the Helmholtz free Energy Lagrangian Dual (HELD) algorithm (Pereira et al., Comput. Chem. Eng. 36 (2012) 99) to strong electrolyte mixtures. The resulting HELD2.0 algorithm provides reliable calculations of the nonideal phase behaviour of mixtures of organic molecules and water with alkali halide salts for a wide range of thermodynamic states.