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Abstract
The water energy environment nexus is an important issue in today’s development of desalination for potable water production. Engineers and scientists in desalination are striving not only for more energy efficient technology, they are concern with the environment impact in terms of CO2 emissions and discharge of chemical laden brine into the sea. The complexity of available technologies dictates engineers and scientists to be aware of the environmental impact from both fundamental sciences and technology processes. The thermodynamics limit for desalination is known to have an energy consumption of 0·78 to 1·09 kWhe per m3, but practical desalination processes are operated at several folds higher than the thermodynamic limit due to irreversible losses incurred in removing dissolved salts. Although recent advances in membrane technology has an energetic benchmark of 3·5–5 kWhe m−3 or about 14·5 kWh_pe (primary energy) per m3, the high water production rates and protection of membranes have resulted in large dosage of chemicals and acids being used in the pretreatment of seawater feed. This resulted in the excessive discharge of chemically laden brine from RO plants. Despite the high but low exergy latent energy consumed for evaporation process, the multiple re-use of latent energy have managed to lower their specific energy consumption to 22 kWh-pe m−3 at a high gain output ratio (GOR). In this paper, the state-of-art of adsorption desalination over the past decade is presented along with a hybridization of the adsorption and nano-filtration (AD-NF) processes to the conventional MED processes. Their integration achieves an energetic efficiency comparable to those from RO processes. Incorporation of AD-NF is a result of recent innovations in selective removal of covalence salts within the feed prior to its introduction to the MED; It permits the top brine temperatures of MED to operate beyond the present limit of 65°C. Dissolved ions in the heated brine are known to be responsible ‘soft-scales’ formation, namely Ca++, Mg++, etc., but when removed partially prior to supplying to the MED, it enables the TBT to be raised up to 125°C without excessive scaling on the heat transfer surfaces. Coupled with an AD cycle to the bottom of MED stage, it lowers the stage temperature (LBT) to below that of the ambient, typically as low as 10°C; The low evaporative temperature retards scaling and fouling on the tube surfaces despite the higher saline concentration at LBT stage. We proposed an AD-NF-MED cycle where their integration can achieve an energetic efficiency that rivals the best available energetic efficiency in desalination and yet retaining the cycle robustness in handling large fluctuations in feed water quality, caused either by the high salinity or the harmful algae (HAB), etc.
Acknowledgements
The authors wish to thank the National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Programme (CRP Award No. NRF CRP9-2011-05 (R265-000-466-281)), King Abdullah University of Science & Technology (KAUST), KSA, under the project no. 7000000411, and the World Class University (WCU) program of Korea (R-33-2009-000-10101660), hosted by Jeju National University (JNU), Korea.