Thermodynamic properties of liquid–liquid immiscible mixtures of seawater and directional solvents for the energy and exergy evaluation of DSE desalination systems

Directional Solvent Extraction (DSE) is a promising desalination process that can produce fresh water from saline water using low-temperature heat sources. Amines, fatty acids, and ionic liquids were proposed as potential directional solvents—these solvents and seawater form liquid–liquid binary mixtures in a two-phase immiscible system. The energy and exergy analyses of the DSE desalination processes require accurate and consistent thermodynamic properties of these liquid–liquid mixtures. This paper presents a systematic framework for evaluating the thermodynamic properties of two-phase, liquid–liquid immiscible binary solutions consisting of seawater and a potential directional solvent. The property prediction framework includes two main steps: (1) a fundamental Gibbs free energy equation is utilized to evaluate the thermodynamic properties of the pure liquid solvent, while pure water and seawater properties were determined using existing correlations; and (2) The Non-Random Two-Liquid (NRTL) excess Gibbs energy model was used membranes to capture deviations of water–solvent mixtures from ideal solution behavior. The thermodynamic properties of two-phase immiscible mixtures of seawater and octanoic acid as a directional solvent were then determined using the methodology described in this paper. Finally, the thermodynamic properties of liquid–liquid immiscible mixtures of octanoic acid and seawater were used to analyze a basic example of a DSE desalination system.