Hybrid electrodialysis reverse osmosis system design and its optimization for treatment of highly saline brines

Abstract

The demand is rising for desalination technologies to treat highly saline brines arising from hydraulic fracturing processes and inland desalination. Interest is growing in the use of electrical desalination technologies for this application. The hybridization of electrodialysis (ED) with reverse osmosis (RO) allows high salinities (beyond the range of RO alone) to be reached while avoiding the operation of ED with a low conductivity diluate stream. Such hybrid systems have been experimentally investigated for concentrates from brackish and seawater desalination. However, progress is required in the modelling and optimization of hybrid systems at higher concentrations. A novel hybrid arrangement of counterflow ED systems with reverse osmosis is presented to concentrate a saline feed at 120 ppt. The system is considered from the perspective of efficiency, membrane productivity and the levelised cost of water, with emphasis on the optimisation of current density. In contrast to brackish ED systems, membrane resistances are found to dominate diluate and concentrate resistances at high salinity. The current density found to minimise LCW (levelised cost of water) is significantly greater than the current density found to maximise efficiency, indicating the high current capital cost of ED per unit membrane area and poor membrane transport properties relative to RO. Finally, performance at high recoveries is found to be limited by high stream-to-stream concentration differences, increasing water transport via osmosis, decreasing efficiency and increasing the LCW.

Keywords:

• Electrodialysis
• Reverse osmosis
• Hybrid
• Brine concentration Hydraulic fracturing

Acknowledgements

The first author would like to acknowledge support from the US Department of State via the Fulbright Science and Technology Program, support from the MIT Martin Fellowship for Sustainability and support from IDA via the Channabasappa Memorial Scholarship. This research program is supported by King Fahd University of Petroleum and Minerals through the Center for Clean Water and Clean Energy at MIT and KFUPM.

Notes

‡ More rigorously, according to a capital asset pricing model, the rate of return should be consistent with the covariance of the project’s cash flows with the returns of the overall financial markets. As a benchmark, as of 31 January 2013 three-year annualized returns on the S&P500 index were 14·14% (Standard and Poor, 2013).

§ Though efficiency exhibits an optimum, membrane productivity is monotonic in current density in this model as there is no competing factor reducing productivity at higher current density. Concentration polarization would constitute such a competing factor and its effect would be seen at high current densities, see Fig. 2.