Forward Osmosis Desalination Using Thermoresponsive Ionic Liquids: Bench-Scale Demonstration and Cost Analysis

Forward osmosis (FO) desalination using thermoresponsive ionic liquid (IL)-water mixtures is a promising technology for treating nontraditional water sources. However, its demonstration has primarily been at the lab-scale, with water flux and recovery values that are not representative of realistic applications. In this work, the performance of tetrabutyl-phosphonium trifluoroacetate (P₄₄₄₄TFA), as well as a new dual draw of P₄₄₄₄TFA with tetrabutyl-ammonium trifluoroacetate (N₄₄₄₄TFA) is characterized. The dual draw combines the higher osmolality of one IL with the lower critical solution temperature (LCST) of the second IL to outperform its constituents at the same total concentration of IL in water (70 wt %). Experiments were first performed in a lab-scale coupon tester to understand the effects of draw osmotic pressure and viscosity on water flux through the membrane. Bench-scale experiments were then performed in an element tester with a 1 m² membrane area to evaluate the performance of IL-based FO for the desalination of produced water feed from oil and gas. Specifically, 10 kg of IL-water draw solution was used with 3 kg of real produced water feed, resulting in water recoveries of 60% with initial and final water fluxes of 14 LMH and 3 LMH, respectively. The bench-scale experimental results were used as inputs for a cost analysis, yielding a levelized cost of water (LCOW) of $1.18 per m³. This reveals the potential of IL-based draw solutions for cost-effective desalination of challenging feedwaters using FO.