Improving seawater desalination and seawater desalination brine management

Abstract

Water scarcity is an increasing problem globally. Seawater desalination is increasingly being relied upon as a means of mitigating the problem of water scarcity. However, seawater desalination has costs associated with it: capital costs, cost of energy to desalinate and environmental costs from the discharge of high salinity brine. Efficient and cost-effective seawater desalination and desalination brine management systems are necessary to make seawater desalination a sustainable scalable process. This work seeks to improve seawater desalination and seawater desalination brine management in several ways. For the first time, the thermophysical properties of seawater have been characterized as a function of pressure across the full desalination operating regimes of temperature, salinity and pressure. Functions that allow accurate thermodynamic least work of desalination and seawater flow exergy analysis have been developed.

The least work of desalination, brine concentration and salt production was investigated and the performance of state-of-the-art brine concentrators and crystallizers was calculated. Hybrid designs of reverse osmosis (RO) and electrodialysis (ED) were proposed to be integrated with a crystallizer to concentrate desalination brine more efficiently. The RO-ED-crystallizer concept was applied to two separate applications: (a) salt production from seawater and (b) zero brine discharge seawater desalination. A parametric analysis to minimize the specific cost of salt production and water production was conducted. Parameters varied were: the ratio of seawater to RO brine in the ED diluate channel, ED current density, ED diluate outlet salinity, electricity, water and salt prices, and RO recovery by adding a high pressure RO (HPRO) stage. Results showed that significant cost reductions could be achieved in RO-ED systems by increasing the ED current density from 300 A/m² to 600 A/m².

Increasing RO brine salinity by using HPRO and operating at 120 bar pressure reduced salt production costs while increasing water production costs. Transport properties of monovalent selective ED (MSED) membranes were also experimentally obtained for sodium chloride, significantly improving the accuracy of modeling MSED brine concentration systems. MSED cell pairs transported only about ~~50% the water but nearly as much salt as a standard ED cell pair, while having twice the average membrane resistance.