Seawater desalination by forward-osmosis-assisted temperature swing solvent extraction

Abstract

This study investigates seawater desalination by a combined process consisting of forward osmosis and temperature swing solvent extraction (TSSE). While direct desalination by temperature swing solvent extraction suffers from low extraction efficiencies, forward osmosis enables to transfer water into a more favourable salt solution expanding the amount of suitable solvents and increasing the subsequent extraction yield. In this work, tie lines of aqueous two-phase systems consisting of the thermoresponsive solvent polypropylene glycol 600 and different salts (NaCl, Na2SO4, MgSO4) were determined to investigate the influence of salt type, concentration, and temperature on the liquid-liquid equilibrium. Based on these results, MgSO4 (20 wt%) was selected as draw solute achieving water fluxes up to 7.5 Lm−2 h−1 in forward osmosis against artificial seawater. The diluted draw solution is regenerated by TSSE achieving high extraction yields (0.38 kgwater/kgsolvent). Eventually, the extracted water is separated from the solvent by temperature-induced phase separation at low temperatures enabling the use of low-grade thermal energy sources. In comparison to the direct use of the thermoresponsive polymer as draw agent, the proposed process benefits from significantly higher water fluxes due to reduced internal concentration polarization when using a salt based draw solution.