Steady-state analysis of four effects evaporation desalination process using thermal solar energy

This paper presents a methodology and design of a pilot scale desalination unit combined with solar energy and using four multiple effect (MED) with a vertical falling film type. The pilot plant unit operates with a hot water provided by a solar field of solar vacuum tubes and used as heating medium for the first effect and hot utility for the heat exchangers. This pilot unit was designed for a production capacity of 7m3/d of fresh water obtained from evaporation of 25% of seawater inside the four effects. The modeling equations are given at steady-state conditions and based on mass and heat balances, heat transfer equations, thermodynamic and physical properties of each stream. MATLAB programming is used for resolution of the algorithm. The impact of important factors such as the diameter of tubes, the heating medium temperature and top brine temperature on the performance ratio, heat transfer coefficients and heat transfer area in each effect has been studied. The results show that for a range of outside tube diameter between 20 and 40mm and various tube length (L= 3, 3.5, and 4.0m) the heat transfer coefficient of the first effect increases as the diameter decreases and also when the length increases or number of tubes decreases, however, the heat transfer coefficient in the other effects (2, 3, and 4) shows an opposite behavior. From this analysis, several suggestions and criteria for vertical falling film evaporators design and optimization could be done.