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.

Exergy Analysis of a Two-Pass Reverse Osmosis (RO) Desalination Unit with and without an Energy Recovery Turbine (ERT) and Pressure Exchanger (PX)

“…Tables 4-6 show the calculated thermodynamic properties for all of the numbered streams in Figures 2-4, respectively. It is worth mentioning that of all streams the exergy rates for RO models (standalone, with ERT and with PX) are positive since they are above the dead state condition; this result is in agreement with those of Sharqawy et al [19]. On the other hand, these values could be end with negative values even though they are above the dead state condition if the seawater was assumed to be as an ideal mixture [28,29].…”

“…In the RO process, the streams are pure water and seawater. The physical and chemical exergy of the water and seawater streams is calculated by correlations suggested and validated (with a maximum deviation of ±1.5%) by Sharqawy et al [18,19]. The physical exergy (ePH) of the fluid stream is:…”

“…All the above studies of desalination exergy analysis assumed that the seawater is an ideal mixture; however, in reality, it is a highly electrolytic substance Sharqawy et al [18]. The considerable difference between the two assumptions was demonstrated by Sharqawy et al [19] by comparing their results with those of Kahraman and Cengel for the same MSF desalination unit [20]. Guirguis [21] studied a seawater reverse osmosis desalination plant with an energy recovery turbine (ERT) using efficiency and economic study analysis with respect to the pressure exchanger (PX).…”

“…The overall RO exergy efficiency is defined as the ratio of the minimum separation work required to the total input exergy [19,26]:…”

Exergetic Analysis of an Integrated Tri-Generation Organic Rankine Cycle

“…The values of enthalpy and entropy for water and seawater in the desalination plant based on experimental thermodynamic properties can be obtained from [26] and [27] respectively.…”