Energy Utilization of Brine from an Msf Desalination Plant by Pressure Retarded Osmosis

Seawater desalination plants discharge brine which is at higher salt concentration than the supplied seawater feed. Diluting this brine by mixing it with the lower concentration feed (seawater) will reduce the environmental hazards associated with the high salt concentration of the disposed brine. Energy can be generated from the dilution process in which two streams of different salt concentrations are mixed. Different methods have been proposed to generate energy by mixing two streams of different salt concentrations. This includes pressure retarded osmosis (PRO), reverse electrodialysis (RED) and hydrocratic generation (HG). However, these methods have not been used on a large commercial scale and are still undergoing development. In this paper, the potential of energy generation from applying PRO to the disposed brine blown down from a Multi-Stage Flash (MSF) plant is estimated. In the PRO process, the feed solution, a low salinity stream, and the draw solution, a high salinity stream, are pumped to the opposing sides of semi-permeable membranes. Water permeates through the semipermeable membrane, from low hydrostatic pressure stream (feed solution) to the higher hydrostatic pressure stream (draw solution), due to the osmotic pressure difference. This increases the mass flow rate of pressurized draw stream, and energy is then obtained by depressurizing the draw stream through a hydro turbine. A differential model is developed for the PRO system that takes into consideration the concentration variation of both the feed stream and the draw stream along the PRO membrane. Available reverse osmosis (RO) membranes characteristics currently on the market are used to make the calculation. Membrane characteristics, especially water permeability coefficient, and flow conditions such as mass transfer coefficient significantly affect the PRO plant performance. An improvement in the membranes could significantly increase the energy produced by PRO plant.