Improving the thermodynamic and economic efficiencies of desalination plants

Reverse osmosis, distillation, and freeze desalination processes are analyzed using the first and second-laws of thermodynamics with particular attention to the minimum separation work requirement and the flow exergy. The minimum work of general complete separation processes is first investigated by considering reversible processes for which entropy generation and exergy destruction are zero. Minimum work relations for complete separation of mixtures are obtained and presented in various convenient forms. These relations are later employed to develop the minimum separation work for incomplete separation of saline water solution encountered in desalination plants. The minimum work input is determined for various salinities of incoming saline water and outgoing brine and product water, and the results are tabulated and plotted. The minimum work values show that a lower and an upper limit for the minimum work exist at corresponding recovery ratios of 0% and 100%. The plots of the minimum work versus recovery ratio at various salinities of the incoming saline water also show that there is an optimum value of the recovery ratio which decreases with increasing salinity. The analysis of the minimum separation work is also performed for the reverse osmosis and distillation processes as well as the freeze desalination process. It is shown that the minimum separation work is independent of any hardware or process and thus the same for all three processes. Next, the exergy analysis of typical ideal and actual reverse osmosis desalination processes is conducted together with the discussion of the minimum separation work requirement. The exergy changes of major components are calculated and illustrated using exergy flow diagrams.