Impact of polymer molecular weight on the efficiency of temperature swing solvent extraction for desalination of concentrated brines

Abstract

Thermally-responsive non-ionic polymers can be used for temperature swing solvent extraction to desalinate concentrated brines. We determined how polypropylene glycols [PPG] with number average molecular weights (Mn) of 425, 725 or 1000 g/mol affected the quantity and quality of the extracted water, and overall process energy requirements using model 10 wt% NaCl (~100 g/L TDS) and field derived (82.1 g/L TDS) concentrated brines. The amount of water recovered and its salinity was highest for PPG425 (lowest Mn) and lowest for PPG1000 (highest Mn). PPG725 (intermediate Mn) provided the best balance between water recovery and salt rejection. PPG725 produced a water with low salinity (<1.6 wt% NaCl) from a field derived brine using a 20:1 Solvent:Feed ratio, with an overall energy consumption of 49 ± 13 kWh/m3treatedwater. This is comparable with mechanical vapor compression crystallizers (>50 kWh/m3treatedwater). The influent feed salinity at 20:1 S:F did not affect the water extraction efficiency, and NaCl(s) forms in the raffinate, suggesting the potential for near zero liquid discharge desalination in this process. Overall, PPG725 is a promising polymer for desalination of highly concentrated brines. Improving the efficiency of the water-polymer separation during thermal regeneration would further improve the attractiveness of this approach.