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

Abstract

Access to clean, freshwater is an ever-growing concern for modern society as it is critical to ensure human health, protect threatened ecosystems, and promote economic growth and prosperity. While desalination is a promising pathway to meet global water demands, modern desalination processes remain energy-intensive. 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 engineering design space. Previous work includes success demonstration of a batch bench-scale DSE process, molecular simulations to understand solvent performance, and heat integration analysis of a single-stage pseudo-steady state DSE process. In this work, we propose a mathematical modeling framework for simultaneous technoeconomic optimization and heat integration of the DSE process. Using the framework, we perform rapid bottom-up screening to predict the performance of known organic acid and ionic liquid directional solvents in an optimized DSE process. We then use the optimization framework to identify continuous solvent property targets necessary to realize a levelized cost of water (LCOW) of less than \$0.50/m$^3$. Specifically, we find the thermoresponsive ability of the solvent and solubility of the solvent in water at a reference temperature are the most influential properties over the cost of the DSE process. Fatty acids are unable to achieve the LCOW goal due to the low thermoresponsive ability of the solvent. However, ionic liquids hold promise. Most importantly, we find a need to engineer ionic liquids with a lower solubility in saline reject.