Multiple-effect evaporation desalination systems: thermal analysis

Seawater desalination by parallel feed multiple-effect evaporation has a simple layout in comparison with other multiple-effect or multistage desalination systems. Several operating configurations are analyzed, including the parallel flow (MEE–P), the parallel/cross flow (MEE–PC), and systems combined with thermal (TVC) or mechanical (MVC) vapor compression. All models take into account dependence of the stream physical properties on temperature and salinity, thermodynamic losses, temperature depression in the vapor stream caused by pressure losses and the presence of non-condensable gases, and presence of the flashing boxes. Analysis was performed as a function of the number of effects, the heating steam temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapor condensate and the brine blowdown. Results are presented as a function of parameters controlling the unit product cost, which include the specific heat transfer area, the thermal performance ratio, the specific power consumption, the conversion ratio, and the specific flow rate of the cooling water. The thermal performance ratio of the TVC and specific power consumption of the MVC are found to decrease at higher heating steam temperatures. Also, an increase of the heating steam temperature drastically reduces the specific heat transfer area. Results indicate better performance for the MEE–PC system; however, the MEE–P has a similar thermal performance ratio and simpler design and operating characteristics. The conversion ratio is found to depend on the brine flow configuration and to be independent of the vapor compression mode.

Mathematical models are developed for the parallel feed MEE systems with/without vapor compression. In light of the results, analysis, and discussion, the following conclusions are made: The thermal performance ratio for the thermal vapor compression systems is higher at low top brine temperatures and a larger number of effects. The thermal performance ratio for the MEE–PC/TVC system is higher than for the MEE–P/TVC system. The specific power consumption for both systems decreases at higher temperatures for the brine blowdown and upon reduction in the difference of the saturation temperature of the compressed vapor and the brine blowdown temperature. The specific power consumption for the MEE–PC/MVC system is lower than for the MEE–P/MVC system. The specific heat transfer area for both systems decreases drastically at higher operating temperatures. The specific heat transfer area for the MEE–PC/MVC system is lower than for the MEE–P/MVC system. The conversion ratio is independent of the vapor compression mode. The conversion ratio for the MEE–P/TVC or MEE–P/MVC system decreases at higher operating temperatures. The conversion ratio for the MEE–PC/TVC or MEE–PC/MVC systems is independent of the top brine temperature. However, an increase in the brine blowdown temperature reduces the conversion ratio for both systems.