What will it take to get to 250,000 ppm brine concentration via ultra-high pressure reverse osmosis? And is it worth it?

Abstract

The possibility of ultra-high pressure reverse osmosis (UHPRO) to replace thermal brine concentration promises to reduce up to 50 % of both energy consumption and capital cost. However, commercially available reverse osmosis (RO) membranes lose up to 50 % of their water permeability when operated up to 200 bar. Higher temperature operations such as in Middle East seawater desalination and brine mining further exacerbate compaction due to softening of the polysulfone support membrane. Assuming that pressure and temperature-related compaction issues can be resolved through innovations in membrane materials and/or module designs, this study asks the questions, “What will it take to get to 250,000 ppm via ultra-high pressure reverse osmosis? And is it worth it?” Herein we comprehensively examine a three stage RO process beginning with seawater RO (SWRO) to high-pressure RO (HPRO) and ultra-high-pressure RO (UHPRO). We elucidate the influence of thermophysical properties, hydraulic pressure, permeate flux, water permeability, feed spacer design, concentration polarization (CP), salt rejection and stage design on recovery and specific energy consumption (SEC). Our analysis of high rejection (HR) and high flux (HF) RO membranes reveals that while HF RO membranes boast superior water permeability and lower trans-membrane (hydraulic) pressure (TMP), they require higher operating flux, which leads to severe CP and operating pressures of 300 to 400 bar to meet the targeted brine concentration of ~250 g/L TDS. In addition, membrane compaction drives up the TMP, SEC, and capital cost of UHPRO systems. To bring the applied pressure below 300 bar, compaction-resistant UHPRO membranes and mass-transfer enhancing feed spacers will be required. The findings herein show that it will take time, money and technological innovation to enable UHPRO at commercial scale, and it may or may not be worth it. Further, this work makes clear the technological refinements needed to enable UHPRO membrane brine concentration.