Thermoeconomic design of a multi-effect evaporation mechanical vapor compression (MEE–MVC) desalination process

Abstract

This work presents thermoeconomic design for a multi-effect evaporation–mechanical vapor compression (MEE–MVC) desalination process. Exergy and thermoeconomic mathematical models of the considered process units are developed and presented in this work. The design data of an existing MEE–MVC (1500 m 3/day) desalination process is used for the present analysis. The effect of using external steam to initiate the evaporation process is investigated. The MEE–MVC without external steam is investigated under different operating conditions. The developed Visual Design and Simulation (VDS) package is utilized as a powerful tool for the present analysis. The energy analysis shows that the thermal performance ratio of the considered system with external steam is 8% less than that the system without external steam. Thermoeconomic analysis shows the unit product cost is 29% higher in the system of external steam. The unit product cost of the desalted water at the normal operation (without external steam) is calculated by 1.7 $/m 3. For system without external steam, by reducing the pressure ratio of the vapor compressor from 1.35 to 1.15, the capital cost of the compressor is reduced by 16%. The specific power consumption is also reduced by 50%. Sequentially, the unit product cost is reduced from 1.7 to 1.24 $/m 3 (27% reduction). Thermoeconomic results show that, reducing the splitter ratio of the brine re-circulation flow rate from 0.5 to 0.25 while the compression ratio is specified by 1.15, the unit product cost decreases to 1.21 $/m 3. Using the design condition of the considered MEE–MVC desalination plant, thermoeconomic results show that the minimum unit product cost is obtained at three evaporators. As the unit product cost at two evaporators and three evaporators are almost the same and due to the complexity and maintenance requirements, the system of two evaporators is preferable. By increasing the capacity of the considered process from 1500 m 3/day to 5000 m 3/day, the results show that the unit product cost at two and three evaporators are almost the same. Design calculations show that increasing the required capacity of the considered system, the unit product cost decreases.