Optimization of parallel/cross flow multi-effect desalination system with flue gas pre-heating considering thermo-vapor compression coupling

Abstract

To reduce the energy consumption of low-temperature multi-effect distillation (LT-MED), a new scheme was proposed by combining the thermo-vapor compression (TVC) and utilizing the low-grade heat of flue gas in the thermal power generating unit for feed brine preheating. For further improvement, the concept of the mass fraction is especially proposed and considered. Taking the gained output ratio (GOR) and water production cost as the objective functions, the effects of temperature difference, TVC positions were investigated by thermodynamic and economic analysis. The optimal effect number was selected for the LT-MED system under the lowest investment. The results indicated that GOR increased and the water production cost decreased with the backward movement of TVC. The lowest water production cost was obtained under the conditions that N = 5 and TVC was at the 4th effect with the mass fraction Rmf is 0. Thermodynamic, economic, and exergy analysis was performed on the selected systems and the control group. It was concluded that the new proposed system was superior to the others in both the water production and the water production cost. GOR increased from 9.67 to 10.94 and the water production cost fell from 3.13 $/m3 to 2.84 $/m3.