Water and salts recovery from desalination brines: An exergy evaluation

Exergy analyses are becoming important tools in identifying the less efficient components of plants through the evaluation of the energy lost in entropy production. In this work, this type of analysis was considered for desalination plants based on membrane operations, with specific emphasis on membrane distillation and membrane crystallization for the improvement of the water recovery factor and the recovery of valuable salts from produced brines. The state of the art in the field was firstly presented and discussed. In all literature works, the thermal supply was the highest contributor to exergy losses. However, the use of waste heat or renewable energies can significantly improve the exergy efficiency of the process, as the feed is heated by the available source and enters the process already at the desired temperature. Furthermore, it resulted that the gain for the sale of salts produced by membrane crystallization was able to cover the desalination costs, even without waste heat availability. Then, the exergy analysis of membrane distillation (MD) and membrane crystallization (MCr) working on four different feeds was experimentally carried out. Commercial modules equipped with capillary polypropylene membranes (0.2 µm pore size) were used at fixed feed temperature and flow rate. A high exergy efficiency (72%) was obtained with the Synthetic RO feed (79.9 g/L), together with a high water recovery factor (79%) and brine concentration (around 476 g/L).