Optimal Number of Effects for Multi Effect Evaporation Seawater Desalination Process Using Solar Thermal Energy-Plate Evaporators

Seawater desalination using Multi effect-plate evaporators and solar energy is one of very promising ways to deal with fresh water shortage and pollution in the same time. Indeed, falling film plate evaporators are becoming more used in the desalination applications. This is because of their multiple advantages compared to tubular evaporators such as high heat transfer rate and low investment cost. This study presents the methodology used for sizing and optimization of plate evaporators for seawater desalination unit using multi effect evaporation and solar thermal energy. The pilot plant unit operates with hot water provided by solar vacuum tubes. The unit was designed for a production capacity of 5 to 7 m3/day of fresh water obtained from evaporation of 28 to 30% of feed seawater. The modeling equations are given at steady-state conditions and they are based on mass, heat balance and heat transfer equations, and thermodynamic and physical properties of each stream. MATLAB programming software is used to resolve the developed algorithm. This work focuses more on studying and analyzing the impact of the number of effects on the total required heat transfer area, the required thermal energy and therefore on the specific cost of water. The obtained results show that it is more beneficial to use nine effects, which gives the key solution for minimizing water production cost. The optimal design shows that the total required heat transfer area of evaporators and the last condenser is approximately equal to 42 m2 and the thermal energy needed to drive the pilot unit is about 64 kW.