Structural optimization of osmosis processes for water and power production in desalination applications

Osmotic pressure is a separation barrier during the reverse osmosis (RO) desalination operation. Hydraulic pressure must be inserted on the membrane surface to overcome the osmotic pressure and to facilitate the desalination process. Pressure retarded osmosis (PRO) is a renewable source for power production. Power generation through PRO is achieved through salinity gradient generated across an asymmetric membrane. In this study, we seek an optimal integrated system of RO and PRO for seawater desalination and power generation. A mixed integer nonlinear programming (MINLP) model is developed to find optimal osmosis processes arrangement with auxiliary equipment, and their optimal operation conditions for seawater desalination and power generation. Several case studies were analyzed to show the applicability of the proposed model. In general, the results show that the operation of RO can be a feasible source of salinity gradient for power generation by PRO technology while producing desalinated water streams. The process layouts show arrangement of RO and PRO stages with recycled streams in order to increase the salinity gradient in the PRO stages. This is an undesirable condition for the RO operation and it is suggested from our results that high performance PRO membrane with high water permeation and salt rejection may eliminate this condition.