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Research Papers: Energy Systems Analysis

Deliberate Salinization of Seawater for Desalination of Seawater

[+] Author and Article Information
Francisco J. Arias

Department of Fluid Mechanics,
University of Catalonia,
ESEIAAT C/Colom 11,
Barcelona 08222, Spain

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received May 1, 2017; final manuscript received September 7, 2017; published online October 17, 2017. Assoc. Editor: Esmail M. A. Mokheimer.

J. Energy Resour. Technol 140(3), 032004 (Oct 17, 2017) (5 pages) Paper No: JERT-17-1192; doi: 10.1115/1.4038053 History: Received May 01, 2017; Revised September 07, 2017

The basis of a novel method for seawater desalination is outlined. In this work, pressure-retarded osmosis (PRO) energy is obtained and used posteriorly for the reverse osmosis (RO) process for seawater desalination. Although PRO process coupled with an RO process has been studied in the past, however, in this work, there is a fundamental difference. Instead of bringing river or wastewaters with low salinity to the coast to be mixed with the seawater to run the PRO process, here is the seawater which is deliberately salinized. This technique has one important consequence, namely, that it is no longer required to be in places where rivers or wastewaters flow into the sea. This important difference eliminates this until now somehow paradoxical requirement if one considers that regions needing desalination are generally poor of water resources. On the other hand, it is not a coincidence that regions needing desalination plants are also regions with rich open salt deposits in the neighborhood; high evaporation, high concentration of salt deposits, and the need for freshwater are all of them directly correlated. Therefore, the idea proposed in the paper is consistent with the problem. The high evaporation in the region which is causing the need for desalination also is creating the solution to do this by using the salt deposits created. The economic feasibility of this method is preliminarily assessed in terms of the thermodynamic limits of extractable energy and then with the cost of the salt required to obtain this energy which is compared with the price from electrical grid. It was found that in order to reduce the amount of salt required for the process, and to make the cost of energy competitive, it is necessary to direct the hypersaline draw solution (draw solution) in a cyclic loop and to have the highest possible volume fraction for the nonsalinized solution (feed solution). Additional R&D is required to explore the possibilities of this concept.

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Topics: Cycles , Seawater , Water
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Figures

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Fig. 1

Sketch of the proposed PRO process driven by deliberately pouring salt into seawater

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Fig. 2

Specific Gibbs free energy of mixing for waters of different sources as a function of the feed volume fraction, ϕ. Considering the following sources of waters: seawater (SW, 0.6 M NaCl), water molarity 3.0 (3.0 M NaCl), and saturated (saturated, 6.0 M NaCl).

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Fig. 3

Sketch of the modified deliberated hypersaline PRO cycle. This process is intended to allow a reduction of the amount of salt per kilowatt-hour needed in comparison with the most simple and direct process depicted in Fig. 1.

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Fig. 4

Left axis: specific Gibbs free energy for mixing, and right axis: specific Gibbs free energy per dollar required for the mixing of water from different sources as a function of the feed volume fraction, ϕ, considering the following sources of water: seawater (SW, 0.6 M NaCl), saturated (saturated, 6.0 M NaCl), and 3.0 M NaCl

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Fig. 5

Evolution of the price of the salt in brine in the U.S. Courtesy United States Geological Survey, Salt Statistics and Information, 2017 [19].

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Fig. 6

Evolution trend comparison between salt in brine and electricity in the U.S.

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