On exergy calculations of seawater with applications in desalination systems

Abstract

Exergy analysis is a powerful diagnostic tool in thermal systems performance evaluation. The use of such an analysis in seawater desalination processes is of growing importance to determine the sites of the highest irreversible losses. In the literature, exergy analyses of seawater desalination systems have sometimes modeled seawater as sodium chloride solutions of equivalent salt content or salinity; however, such matching does not bring all important properties of the two solutions into agreement. Furthermore, a common model that represents seawater as an ideal mixture of liquid water and solid sodium chloride may have serious shortcomings. Therefore, in this paper, the most up-to-date thermodynamic properties of seawater, as needed to conduct an exergy analysis, are given as correlations and tabulated data. The effect of the system properties as well as the environment dead state on the exergy and flow exergy variation is investigated. In addition, an exergy analysis for a large MSF distillation plant is performed using plant operating data and results previously published using the above-mentioned ideal mixture model. It is demonstrated that this ideal mixture model gives flow exergy values that are far from the correct ones. Moreover, the second law efficiency differs by about 80% for some cases.