Formulation of Seawater Flow Exergy Using Accurate Thermodynamic Data

Seawater is a complex electrolyte solution of water and salts with sodium chloride as the major constituent. However, the thermodynamic properties of seawater are considerably different from those of aqueous sodium chloride solution. In the literature, exergy analyses of seawater desalination systems have sometimes modeled seawater by sodium chloride solutions of equivalent salt content or salinity; however, such matching does not bring all important properties of the two solutions into agreement. Further, some published studies attempt to represent sodium chloride solutions as a specific model for an ideal mixture of liquid water and solid sodium chloride, which is shown to have serious shortcomings. In this paper, the most up-to-date thermodynamic properties of seawater are compared with those of aqueous sodium chloride solution as well as the ideal mixture model. The flow exergy is calculated using various models and the results are compared. In addition, the minimum desalination work required to produce a unit mass of fresh water from seawater of varying salinity is calculated using these models. A difference of about 50% in the minimum desalination work is observed when calculating separation work using the ideal mixture model. This consequently shows that exergy analysis and the second law efficiency calculations performed using the ideal mixture model in question are comparatively far from the actual values. NOMENCLATURE af specific flow exergy J kg c specific heat at constant pressure J kg K G Gibbs energy J g specific Gibbs energy J kg h specific enthalpy J kg M molecular weight g mol m molality mol kg m mass flow rate kg s mw mass of water kg ms mass of salt kg n number of species in mixture P Pressure Pa R gas constant J kg K s specific entropy J kg K SA absolute salinity kg kg T temperature C w mass fraction Wmin minimum separation work J kg x mole fraction Greek Symbols φ osmotic coefficient γ activity coefficient μ chemical potential J kg ν number of ions Subscripts 0 environmental dead state s Salt w pure water Superscripts * restricted dead state ● standard state Copyright © 2010 by ASME 1