Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes
AbstractIncreasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies driven by different combinations of heat, work, and chemical energy. This paper develops a consistent basis for comparing the energy consumption of such technologies using Second Law efficiency. The Second Law efficiency for a chemical separation process is defined in terms of the useful exergy output, which is the minimum least work of separation required to extract a unit of product from a feed stream of a given composition. For a desalination process, this is the minimum least work of separation for producing one kilogram of product water from feed of a given salinity. While definitions in terms of work and heat input have been proposed before, this work generalizes the Second Law efficiency to allow for systems that operate on a combination of energy inputs, including fuel. The generalized equation is then evaluated through a parametric study considering work input, heat inputs at various temperatures, and various chemical fuel inputs. Further, since most modern, large-scale desalination plants operate in cogeneration schemes, a methodology for correctly evaluating Second Law efficiency for the desalination plant based on primary energy inputs is demonstrated. It is shown that, from a strictly energetic point of view and based on currently available technology, cogeneration using electricity to power a reverse osmosis system is energetically superior to thermal systems such as multiple effect distillation and multistage flash distillation, despite the very low grade heat input normally applied in those systems.
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Mistry, K.H.; Lienhard, J.H., V. Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes. Entropy 2013, 15, 2046-2080.
Mistry KH, Lienhard JH, V. Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes. Entropy. 2013; 15(6):2046-2080.Chicago/Turabian Style
Mistry, Karan H.; Lienhard, John H., V. 2013. "Generalized Least Energy of Separation for Desalination and Other Chemical Separation Processes." Entropy 15, no. 6: 2046-2080.