Elsevier

Desalination

Volume 295, 1 June 2012, Pages 11-15
Desalination

Sea ice desalination under the force of gravity in low temperature environments

https://doi.org/10.1016/j.desal.2012.03.017Get rights and content

Abstract

In winter 2009 and 2010, 9000 m3 and 12,000 m3 of sea ice, respectively, were collected from Bohai Bay and stored in a desalination pool. In 2009, the experiment lasted 80 days, and produced 4500 m3 of desalinated water with the salinity of 0.8‰. By contrast, the experiment of 2010 lasted 90 days and yielded 6700 m3 of water with the salinity of 1.4‰. Experimental results showed that under the force of gravity and low temperature, brine pockets within the sea ice were gradually drained out and sea ice was therefore converted from saline ice to freshwater ice. In this process, ambient temperature significantly affected drainage volume and salinity. When temperature rose, drainage volume increased and salinity decreased. Soluble salts decreased at different magnitudes during the course of gravity-induced desalination, in which chlorides showed the largest magnitude of decrease and sulfates the lowest. pH increased as a result of increasing sodium ion concentration. Insulation measures yielded higher rate of freshwater production by extending the duration of desalination. Shortening the period of desalination while maintaining high yield is worthy of further attention in studying gravity-induced sea ice desalination.

Introduction

In the natural world, sea ice is mainly distributed in Polar Regions and high-altitude areas. China's Bohai Bay, an inland sea that is surrounded by land on three sides, is not high in altitude (37°07′–41°0′ N, 117°35′–121°10′ E). However, it does witness annual ice formation from November to March the following year under the influence of the northerly airstream of the Eurasia continent [1]. The annual ice volume of Bohai Bay is estimated to be 1–3 billion m3 [2].

With an area of 518,000 m2 and a population of 229 million, Bohai Bay is an important economic region in China. At the same time, it also experiences the severest form of water shortage with a per capita freshwater resource of less than 350 m3 per year. Land freshwater is no longer able to meet the demands of sustainable socioeconomic development in the region. Seeking for freshwater from the sea has become one of the key solutions to resolve shortage of freshwater.

Though seawater desalination with multi-stage flash (MSF) distillation and reverse osmosis (RO) has matured in the international community, both techniques pose high demands on energy. In the context of global warming, China, a resource-scarce country, must consider energy as a limiting factor when desalinating sea ice on a large scale. In order to reduce energy consumption, Shi et al. (2002) proposed using sea ice from Bohai Bay as a source of freshwater [3].

Sea ice desalination is a new type of seawater desalination. More precisely speaking, it utilizes freezing to desalinate. This method originated in the middle 17th century and was divided into natural freezing method and artificial freezing method depending on the mode of ice crystals formation [4], [5]. The former refers to the natural process of seawater freezing to form ice in a cold environment, whereas the latter utilizes refrigerants or cooling media to indirectly freeze seawater. Naturally frozen sea ice contains brine pockets – highly saline water trapped in the ice during the process of freezing – that fundamentally differentiate it from freshwater [6], [7]. On the other hand, cleansing, separation and other technical problems involving the use of artificially frozen sea ice remain unsolved, impeding its commercial development on a large scale [8], [9]. In recent years, researchers desalinated sea ice with centrifuges and successfully separated brine pockets from pure ice crystals.

To convert sea ice into freshwater, brine pockets in the ice must be removed. Brine pockets are liquid-phase substances whereas freshwater ice crystals are solids. The differences between liquid- and solid-phases could be utilized to remove brine pockets in an energy-conserving fashion. Since 1999, under the support of research projects of the Ministry of Science and Technology, the author has taken part in various sea ice desalination studies. This article aims to summarize gravity-induced sea ice desalination and empirical results.

Section snippets

The theory of gravity-induced sea ice desalination

Sea ice is the frozen state of seawater under cold temperatures. In freeze separation, when seawater freezes, water molecules crystallize to form ice crystals while inorganic salts and other organic materials remain in the seawater [5]. With decreasing temperature, freshwater ice crystals gradually gather at the water surface and freeze to form ice. A portion of highly saline water will be trapped inside the ice and form the so-called brine pockets (Fig. 1a). Hence, sea ice is a solid–liquid

Desalination pools

An inverted trapezoidal desalination pool (bottom: 63 m × 20 m, top: 83 m × 35 m, depth: 3.5 m) was made with a catchwater channel at the center of the pool and drainage well placed within the channel (Fig. 2). The collected sea ice was placed inside the desalination pool during the experiment in winter. It was mulched with a sheet of grass after the collection was over to prevent undesired melting of sea ice. With time and changes in temperature, brine pockets would drain from the ice under the force

Gravity-induced desalination experiment in 2009

Ice was collected from Jan. 10 to Feb. 9, 2009 and effectiveness of desalination observed from Jan. 13 to Apr. 9, 2009 (Fig. 3). Drainage volume, salinity and temperature were measured against time during the experiments.

From Fig. 4, it is clear that there are two characteristics in the changes of drainage volume with time. First, changes in drainage volume expressed three phases. Volume was high both at the initial (Jan. 13–Feb. 13) and final (Mar. 6–Apr. 9) stages of the experiment and

Conclusions

Sea ice was collected in winter. Under the force of gravity and rising temperature, brine in the sea ice (brine pockets) was naturally drained from ice. After a certain period, sea ice converted from saline ice into freshwater ice. Large-scale desalination would be possible if a large amount of sea ice could be collected in a desalination pool with a drainage function.

During gravity-induced sea ice desalination, the drained water includes remaining seawater on the surface of the sea ice, brine

Acknowledgements

This article was supported by science and technology projects of the Chinese government: project supported by State Key Laboratory of Earth Surface Processes and Resource Ecology (2010-TD-02); NSFC general program (No. 40801230), National Key Technology R&D Program (No. 2006BAB03A03, 2011AA100505) and the Emphasis Project of National High Technology Research and Development Program of China (No. 2006AA100206).

Ph.D. candidates Wu Zhizheng, Huang Shuqing, Master's candidate Bu Danyang and Zhang

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