Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination

Advances in highly permeable reverse osmosis (RO) membranes have enabled desalting operations, in which it is practically feasible for the applied pressure to approach the osmotic pressure of the exit brine stream. However, energy cost remains a major contributor to the total cost of water produced by RO membrane desalination. Reduction of the overall cost of water production represents a major challenge and, in the present work, various elements of water production cost are evaluated from the viewpoint of optimization, with respect to various costs (energy, membrane area and permeability, brine management, and pressure drop), as well as the important thermodynamic cross-flow constraint, utilization of energy recovery devices, and operational feed and permeate flow rate constraints. More specifically, in the present study, an approach to the optimization of product water recovery at pressures that approach the osmotic pressure of the exit brine stream is presented via several simple RO process models that utilize highly permeable membranes. The results suggest that it is indeed feasible to refine RO processes to target for operation under the condition of minimum energy consumption, while considering the constraint imposed by the osmotic pressure, as specified by the thermodynamic cross-flow restriction. Although it is shown that multistage RO provides energy savings, this is at the expense of greater membrane area cost. Overall, as process costs above energy costs are added, the operational point for achieving minimum water production cost shifts to higher recoveries. Although the newer generation of RO membranes can allow high recovery operations at lower pressures, limitations due to mineral scaling and fouling impose additional constraints. The incorporation of these phenomena in the optimization approach is the subject of ongoing research.