Performance Optimization of Mechanical Vapor Compression Desalination System Using a Water-Injected Twin-Screw Compressor

Abstract

In this study, the thermal and operational characteristics of a 400 m3/day mechanical vapor compression desalination (MVCD) system that uses a water-injected twin-screw compressor have been studied and presented. A mathematical model of the MVCD system has been developed including mass and energy conservation equations, heat transfer equations, as well as thermophysical correlations. The effects of the MVCD system design and operation parameters on the system performance are analyzed and discussed. The effect of different boiling-point elevation correlations on the specific area is investigated. The brine and distillate preheaters' areas are studied as a function of inlet seawater temperature. The effect of the injection pressure on system performance is studied. Results show that the optimal injection point is close to the beginning of the compression process. Using this optimum injection pressure, the reduction in power consumption was found to be about 7.3% for high compression ratios. The effects of the brine and feed salinity on system performance are also analyzed. It is found that the specific heat transfer area strongly depends on the brine salinity, especially at temperature differences less than 6 °C. It increases by 44% and 32% at a temperature difference of 4 and 6 °C, respectively. The compressor inlet volume flowrate increases by 9% when the brine salinity increases from 50,000 to 150,000 ppm at all brine boiling temperatures considered. The feed-to-distillate ratio increases rapidly with rising feed salinity, while it decreases with rising brine salinity.