In this context, we introduce a novel solvent-driven water extraction (SDWE) process that employs dimethyl ether (DME) to selectively extract water from hypersaline brines, enabling both scaling-resistant brine concentration and fractional crystallization of targeted salts. Unlike traditional solvent extraction systems that extract specific solutes, SDWE uses DME to remove water from the aqueous phase. This reduction in water activity drives supersaturation in the remaining brine, leading to the selective precipitation of sparingly soluble salts, a process that can be tuned to isolate metals such as samarium or cobalt. Hence, SDWE provides a powerful mechanism for indirect selective separations through phase-controlled crystallization, rather than through direct solute extraction.
DME is particularly well-suited for this application due to its distinctive physicochemical properties. Its low dielectric constant promotes strong selectivity for water over ionic solutes, with salt rejection exceeding three orders of magnitude. Simultaneously, its high volatility and low boiling point (269 K at 1 atm) enable solvent regeneration using ultra-low-grade heat sources below 50 °C, temperatures commonly available from industrial waste streams or ambient thermal gradients. Together, these properties make DME an effective and regenerable solvent that circumvents the high energy costs and scaling issues associated with conventional separation technologies.
The SDWE system comprises two main stages. First, in the liquid–liquid extraction step, DME is contacted with the hypersaline feed brine in a pressurized separator. Water is selectively transferred into the DME-rich phase, while salts and other solutes remain in the aqueous phase. Depending on the brine composition and the extent of water removal, this step can induce fractional crystallization of targeted solutes, such as calcium sulfate or metal sulfates, by driving supersaturation in the brine. Second, the water-laden DME stream undergoes thermal regeneration in a multi-stage concentrator that uses controlled flashing and heat exchange to achieve vapor–liquid (VLE) or vapor–liquid–liquid equilibrium (VLLE). This enables the separation of water and recovery of >99% of the DME for reuse.
To evaluate the feasibility of SDWE, we develop a comprehensive computational framework that integrates detailed thermodynamic modeling with process-level optimization. Our thermodynamic model combines the extended UNIQUAC (eUNIQUAC) activity coefficient model with the virial equation of state, enabling accurate prediction of the complex phase behavior in DME–water–salt mixtures. These models capture the effects of salting-out, boiling point elevation, and three-phase equilibrium, all of which are essential to predicting the performance of the extraction and regeneration steps. The process model simulates a multi-stage thermal regeneration system, allowing us to examine how key variables, such as interstage flash pressure, heat source temperature, and the number of concentration stages, affect system efficiency, DME recovery, and cost. Our simulations reveal that optimal performance is achieved at interstage flash pressures between 0.4–0.5 bar and heat source temperatures between 323–373 K, with higher thermodynamic efficiency at the lower end of this range. At a source temperature of 323 K and interstage pressure of 0.489 bar, the system achieves a thermodynamic (Second Law) efficiency of 20.5%, a brine concentration of 5.5 mol/L NaCl, and >99% DME recovery. These conditions enable solvent regeneration with minimal external energy input while maintaining high separation performance.
One of the key advantages of SDWE lies in its inherent resistance to scaling and fouling. As water extraction occurs in a physically separate liquid–liquid phase system, the critical mass transfer processes are decoupled from heat exchangers and membranes, which are the typical sites of fouling in thermal or pressure-driven systems. This decoupling significantly reduces the operational risks and maintenance costs associated with high-salinity brine processing, especially in applications involving scale-prone ions like calcium or magnesium.
In addition to the technical performance, we conduct a techno-economic analysis (TEA) to estimate the capital and operating costs of the system. Capital costs are modeled based on the specific heat exchanger area required for solvent regeneration, while operating costs include thermal and electrical energy consumption, pumping work, and make-up solvent requirements. Assuming waste heat is freely available, we estimate a specific cost of $1.93 per cubic meter of water recovered, making SDWE economically competitive with leading commercial desalination and zero-liquid discharge (ZLD) technologies. Moreover, the modularity of the system and its compatibility with low-grade heat sources make it highly adaptable for decentralized or off-grid applications.