Multiple-effect evaporation desalination systems: thermal analysis

Seawater desalination by parallel feed multiple-effect evaporation has a si mple layout in comparison with other multiple-effect or multistage desalina tion systems. Several operating configurations are analyzed, including the parallel flow (MEE-P), the parallel/cross flow (MEE-PC), and systems combin ed with thermal (TVC) or mechanical (MVC) vapor compression. All models tak e into account dependence of the stream physical properties on temperature and salinity, thermodynamic losses, temperature depression in the vapor str eam caused by pressure losses and the presence of non-condensable gases, an d presence of the flashing boxes. Analysis was performed as a function of t he number of effects, the heating steam temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapor con densate and the brine blowdown. Results are presented as a function of para meters controlling the unit product cost, which include the specific heat t ransfer area, the thermal performance ratio, the specific power consumption , the conversion ratio, and the specific flow rate of the cooling water. Th e thermal performance ratio of the TVC and specific power consumption of th e MVC are found to decrease at higher heating steam temperatures. Also, an increase of the heating steam temperature drastically reduces the specific he;rt transfer area. Results indicate better performance for the MEE-PC sys tem; however, the MEE-P has a similar thermal performance ratio and simpler design and operating characteristics. The conversion ratio is found to dep end on the: brine flow configuration and to be independent of the vapor com pression mode.