(247b) Solvent-Driven Water Extraction from Hypersaline Brines: Thermodynamics of the Dimethyl Ether System

Solvent-driven water extraction (SDWE) has promising applications in high-salinity brine desalination, including zero-liquid discharge processing for industrial wastewaters, and resource recovery, such as the extraction of lithium and rare earth elements from solution mining leachate. SDWE uses an organic solvent to extract water from hypersaline brines, while rejecting ionic solutes. Despite its potential to desalinate hypersaline and contaminated brines, the water recovery and energy consumption achievable by SDWE remain poorly understood. In this study, we develop thermodynamic models for the liquid-liquid extraction of water from high-salinity brines using dimethyl ether (DME) to quantify and analyze the performance of SDWE.

DME is a polar aprotic organic solvent that is partially miscible with water. The high volatility of DME, which has a vapor pressure of 5.9 bar at 298 K, allows for its rapid separation from water-DME mixtures after extraction, while its low polarity minimizes the solubility of electrolytes, such as sodium chloride (NaCl), in the DME-rich phase. DME is contacted with saline water in a multistage liquid-liquid separator (LLS) operating at 6 bar and 298 K. Water from the saline feed stream dissolves into the organic stream, while ionic salts remain in the aqueous stream, producing a DME-rich mixture containing extracted water and a high salinity concentrated brine stream. The water-laden DME-rich stream can be readily separated by heating or flashing, yielding purified water and regenerated DME.

We begin by developing an excess Gibbs free energy model for water-DME-NaCl mixtures based on LIQUAC, which combines the UNIQUAC activity coefficient model for short-range interaction with a Pitzer-like electrolyte solution model for middle- and long-range interactions. A computational platform is then built combining LIQUAC with the Peng-Robinson Stryjek-Vera (PRSV) equation of state to perform isofugacity calculations and determine equilibrium compositions in aqueous-organic-electrolyte mixtures. Water-DME interaction energy parameters are estimated through the non-linear regression of published liquid-liquid, vapor-liquid, and vapor-liquid-liquid phase equilibrium (LLE, VLE, and VLLE, respectively) data, while osmotic coefficient data is used to regress LIQUAC parameters for water-Na⁺ and water-Cl^-. DME-Na⁺ and DME-Cl^- interaction energy parameters are estimated for the first time using metaheuristic global optimization techniques to regress recently measured LLE and VLLE data for water-DME-NaCl mixtures across NaCl concentrations from 0.0 to 6.0 mol kg^-1.

A counter-current, multistage LLS model is developed to explore the water recovery achievable as a function of feed salinity, DME to feed water flow rate ratio, and the number of ideal stages. Our analysis demonstrates the potential for DME, with an initial DME to feed water ratio of 3.0 and 3 ideal stages, to achieve a water recovery of 52% with an initial NaCl mole fraction of 0.02, which corresponds to a salinity of 1.1 mol kg^-1, approximately double that of seawater. For a hypersaline brine with an initial NaCl mole fraction of 0.05 or a salinity of 2.9 mol kg^-1, roughly five times seawater salinity, liquid-liquid extraction using DME is able to achieve a water recovery ratio of 45% with an initial DME to feed water ratio of 3.0 and 3 ideal stages. We conclude by optimizing the countercurrent multistage LLS system to maximize water recovery and DME-phase water content for a range of feed salinities.

Our work highlights the potential for solvent-driven water extraction to desalinate or concentrate hypersaline brines for zero-liquid discharge and resource recovery applications. Using fluid phase equilibrium data, we regress DME-Na⁺ and DME-Cl^- interaction parameters for the first time. By combining equilibrium calculations with a multistage liquid-liquid extraction model, we demonstrate that water recoveries exceeding 50% can be achieved for brines that are twice as saline as seawater.