Exergy analysis of water purification and desalination: A study of exergy model approaches

In the literature, several exergy analysis approaches have been proposed to investigate desalination processes. It is not clear, however, which approach is the most appropriate or indeed whether all approaches are valid. The objective of this paper is to review the various methods and to critically assess their assumptions, limitations, advantages and disadvantages. The main focus of this work is the chemical exergy term. Several exergy calculation models were examined and compared using a dataset from the literature. In addition, an accurate approach to calculate the chemical exergy of electrolyte solutions, based on the Pitzer equations, was proposed. The models assessed were: (1) the ideal mixture model (NaCl and water), (2) the ideal mixture model (seawater salt and water), (3) the Sharqawy seawater functions, and (4) the electrolyte solution model (Pitzer equations, NaCl and water), (5) the model used by Drioli et al. and (6) the dissociated ion approach (NaCl and water). Four of the six approaches produced very similar results. Moreover, one other exergy calculation method was found to have serious limitations. The findings presented here show that the choice of exergy model can have a significant impact on the results obtained and that considerable care must be taken to select the most suitable approach.