Exergy analysis of dual-stage nanofiltration seawater desalination

•Three dual stage NF seawater desalination processes were analyzed and compared by exgery analysis methods.•The main exergy destroyed were occurred in membrane element and concentration stream valves.•Pressure drop along and through the membrane was the main reason of exergy destroy in membrane element.•Concentration blending and energy recovery device could reduce the exergy destroy significantly.•The energy consumption was 2.09 kWh/m3, while the system recovery ratio was 42.78% under an optimized condition.

Exergy analysis is a powerful tool for determining the efficiency of processes that influence system performance. Thus, the exergy of dual-stage NF (nanofiltration) seawater desalination was analyzed. Three different processes were simulated by Dow's Reverse Osmosis System Analysis, and the exergies were compared. The results indicated that the main exergy destruction in the conventional process occurred in the membrane and concentration stream valves. To reduce the exergy and energy consumption, concentration blending and an energy recovery device were applied in the improved process, which reduced the specific energy consumption and enhanced the exergetic efficiency and recovery ratio. The calculated specific energy consumption was reduced to 2.09 kWh/m3, and the system recovery ratio was increased to reach 42.78% under the condition specified in this paper. Thus, the development of a novel energy-saving membrane module and an energy recovery device is important in reducing energy consumption in dual-stage NF seawater desalination.