The design and optimization for a closed-loop pressure retarded osmosis process

Pressure Retarded Osmosis (PRO) is an emerging renewable energy technique with potential for scale up and commercialization. It has been extensively investigated in the last decade as a new technique for power generation from osmotic energy. Both open-loop and closed-loop PRO have been suggested for power generation, differing in that the draw and feed solutions are disposed of after membrane treatment in the open-loop PRO process. The closed-loop PRO (CLPRO) process operates like a heating engine in which the diluted draw solution enters a special unit after leaving the membrane for regeneration and reuse. CLPRO uses a pure draw solution for process operation, hence reducing membrane fouling and cleaning requirements. Furthermore, there is no waste discharge to the environment and no pretreatment is required for the feed solution. CLPRO design requires optimizing the operating parameters to reduce the capital and operating costs of the process. Membrane area, feed pressure, draw solution, and draw solution flow rate are among the key operating parameters that significantly impact CLPRO performance. A computer model is developed in the current study to evaluate the impact of these parameters on CLPRO performance. The feed solution osmotic pressure was considered to be negligible because of the high purity of water produced from the regeneration unit. Simulation results revealed that feed pressure is the most influential parameter on the CLPRO performance, followed by the draw solution flow rate and membrane area. In fact, the membrane impact on the performance of CLPRO did not exceed 4%. The result of this study should be considered in the early stages of CLPRO design to reduce operating and capital costs.