Molecular design targets and optimization of low-temperature thermal desalination systems

Abstract

Directional solvent extraction (DSE) is an emerging membrane-free liquid-liquid extraction process to desalinate water using low-grade heat. Several unique features make DSE a potentially disruptive desalination technology: 1) it is thermally driven and utilizes low-grade heat; 2) it does not require the use of membranes; 3) there are opportunities to intensify, modularize and customize the process; 4) there is a vast solvent molecular design space. This work establishes a technoeconomic modeling framework to simultaneously optimize and heat integrate the DSE process. We perform rapid bottom-up screening to predict the energy intensity and levelized cost of water (LCOW) of organic acid and ionic liquid directional solvents (DS) in an optimized DSE process. Likewise, we perform top-down analysis to set continuous solvent property targets necessary to realize a LCOW of less than $0.5/m3. LCOW is most sensitive to the solubility of the DS in water and thermoresponsive ability (a.k.a. yield) of the solvent, i.e., the change in the solubility of water in the DS with temperature. Despite their lower cost, organic acids have a small thermoresponsive ability and LCOW of at least $1.3/m3. In contrast, we set modest quantitative thermophysical property targets for ionic liquid DS to achieve below $0.5/m3 LCOW.