Improving the Thermodynamic and Economic Efficiencies of Desalination Plants: Minimum Work Required for Desalination and Case Studies of Four Working Plants;Final rept

摘要

Reverse osmosis, distillation, and freeze desalination processes were analyzed using the first and second-laws of thermodynamics with particular attention to the minimum separation work requirement and the flow energy. The minimum work for complete separation was investigated by first considering reversible processes for which entropy generation and energy destruction are zero. Minimum work relations for complete separation of mixtures were obtained and presented in various convenient forms. These relations were later employed to develop the minimum separation work for incomplete separation of saline water solution encountered in desalination plants. The minimum work input was determined for various salinities of incoming saline water and outgoing brine and product water, and the results were tabulated and plotted. Next, the energy analysis of typical ideal and actual desalination processes was conducted together with the discussion of the minimum separation work requirement. The energy changes of major components were calculated and illustrated using energy flow diagrams for four desalination systems using actual plant operation data. Three systems were part of a brackish water desalination plant in California that incorporates RO (reverse osmosis), NF (nanofiltration), and EDR (electrodialysis reversal) units. Each unit produces about one million gallons of fresh water per day. The fourth plant is located near the city of Al-Jubail at the Arabian Gulf coast. This MSF (multi-stage flash) plant consists of 40 distillation units, and each unit consists of 22 flashing stages. The plant is capable of producing distilled water at a rate of 230 million gallons per day. Energy flow rates were evaluated throughout the plant, and the energy flow diagrams were prepared.