The need for sustainable desalination processes is growing as reverse osmosis (RO) systems generate large volumes of concentrated brine, often discharged. Efficient recovery of water and salts from these streams is essential to reduce environmental impact and progress towards zero liquid discharge (ZLD) [1]. This research explores multistage counter-current liquid-liquid extraction (LLX) using thermoresponsive polymers as an energy-efficient alternative to conventional separation methods.
The study examines the performance of Dehypon® LS54 and PPG 400 in selectively extracting water from NaCl solutions at 25°C, based on RO reject brine compositions (7wt% NaCl). After measuring ternary equilibrium data over a wide range of concentrations in polymer – water – NaCl systems, the developed ternary diagrams could be applied for graphical stage determination. A minimum solvent-to-feed ratio (S/F) of 1.4 and four equilibrium stages are required to achieve the desired salt concentration in the raffinate (surpassing 20 wt.% NaCl).
In addition to these theoretical predictions, 4-stage cross-current batch extractions were conducted using Dehypon® LS54 to explore whether multistage approaches can indeed remove additional water from aqueous systems. The raffinate concentration increased from 7 wt.% to 12 wt.%, confirming the feasibility of the process. However, the cross-current approach is less effective than counter-current operation.
Therefore, the focus shifted to implementing a Rousselet-Robatel 4-stage counter-current centrifuge, expecting to improve water recovery efficiency. The continuous counter-current configuration is anticipated to provide more effective phase separation, enhancing overall extraction performance.
Preliminary results support the feasibility of multistage counter-current LLX with thermoresponsive polymers for brine management. Ongoing experiments aim to validate continuous operation in the centrifuge and to explore the use of PPG 400, comparing it with Dehypon® LS54. The integration of this extraction process with RO could offer an energy-efficient and sustainable approach for industrial desalination applications aiming at ZLD.