Elsevier

Desalination

Volume 430, 15 March 2018, Pages 159-164
Desalination

Sea ice desalination under gravity using microwave heating

https://doi.org/10.1016/j.desal.2017.12.054Get rights and content

Highlights

  • A new and efficient sea ice gravity desalination method is proposed.

  • Microwave technology is first applied to sea ice desalination.

  • The entire desalination process was completed in merely a few minutes.

Abstract

Gravitational desalination is an important sea ice desalination method. It essentially makes use of the different freezing points of a brine and pure water to promote the separation of the brine and fresh water ice crystals. The ambient temperature influences the ice temperature, which leads to a long desalination cycle and a low production efficiency. Aiming to solve the above-mentioned problems, experiments were designed considering different microwave powers, salinities, and sea ice masses and the physical properties of sea ice and microwave heating characteristics. The results show that the desalination rate of sea ice is the highest after 2–4 min; the desalination rate reaches up to 90% at a loss rate of < 40% when sea ice with a salinity of 5‰ is heated for 8 min in a 600 W microwave. Heavier sea ices need more time to completely desalinate. At lower microwave energies, sea ice can melt brines with higher salinities.

Introduction

Sea ice desalination is an important method of seawater desalination and has been studied since the 1960s[1], [2], [3]. Sea ice discharges most of the salts in the formation process; hence, the overall salinity of sea ice is lower than that of the original seawater. Residual sea ice salts exist in the form of “brine pockets”. Current sea ice desalination methods include gravity desalination, centrifugal desalination, extrusion desalination, controlled temperature freeze–thaw desalination, soaking desalination, and spray freezing desalination. These methods can be divided into two categories based on different salt melting processes. One method is slight melting under external heat conduction. The basic principle of this method is that brine pockets are heated up to melt ice crystals at ambient temperature. The gravity effect leads to the formation of vertical “drainage channels” among the pockets and to the discharge of salt through the channels[4], [5]. However, this method generally requires a desalination cycle of one to two months[6] and its continuous production is poor. The other method is based on a mechanical process causing the direct exposure of the brine pockets. Centrifugation[7], soaking[8], and other associated processes are then used to achieve the separation of brines and fresh water ice crystals. However, most of the brine pockets are very small; the method therefore requires a high degree of sea ice crushing. In addition, there are also higher requirements with respect to the power of equipment and apertures of filter meshes. However, this desalination process is characterized by relatively higher energy consumption and lower actual production. The long desalination cycle and low production efficiency limit the actual production and application of sea ice desalination. Therefore, it is key to identify a rapid and efficient desalination method to promote the industrialization of sea ice desalination. Microwave heating technology has many advantages and can theoretically solve the above-mentioned problems.

Microwave heating technology utilizes microwaves to produce a high-frequency electromagnetic field, which initiates polar molecule vibrations at high speeds, resulting in molecular levels of friction producing a lot of heat[9]. The microwave heating technology is characterized by a high heating speed, selective heating, and strong penetration ability[10]. Based on these characteristics, it is applied in the food industry[11], [12], material synthesis[13], [14], and ceramic calcination[15], [16], [17]. In theory, differences in dielectric properties exist between the components of sea ice; sea ice is heat selective[18]. The dielectric constant of freshwater ice crystals is low. They almost absorb no microwave energy, manifested as strong penetrability of microwaves. However, the brine pockets have a high dielectric constant and high absorption capacity with respect to the microwave energy[19]. Under microwave heating, brines rapidly warm up to melt the cell wall and accelerate the formation of saline channels. This study aims to explore the effects of the sea ice salinity, microwave power, and sea ice mass on the sea ice desalination efficiency.

Section snippets

Experiment design

Four factors (microwave power, heating time, original sea ice salinity, and sea ice weight) were considered for the experiments. Single-factor experiments were conducted. Thus, there are four parallel experiments for each single experimental condition.

Microwave heating sea ice desalination

Fig. 2 shows that the desalination and loss rates of the raw sea ice with a salinity of 5.0‰ notably increased with increasing heating time in the 600 W microwave. The desalination rate increased by 65.6% in the 2–6 min period. The loss rate increased by 23.3%. In the period of 6–10 min, the desalination rate increased by 20.0% and the loss rate increased by 17.3%. After 8 min of microwave heating, the desalination and loss rates were 89.7% and 36.8%, respectively. Compared with the traditional sea

Conclusions

The brine pockets that are directly heated by microwaves can shorten the formation time of drainage channels during the desalinating process. Sea ices with different salinities can achieve desalination through microwave heating.

  • (1)

    Compared with traditional sea ice gravity desalination, a sea ice salinity of < 1‰ can be achieved in only 4–6 min using microwave heating; the desalination time is greatly shortened. During the microwave heating of the sea ices with a salinity of 5.0‰ at 600 W, the salt

Acknowledgements

This study was supported by The National Key Research and Development Program of China (2017YFA0604903) and State Key Laboratory of Earth Surface Processes and Resource Ecology (2017-ZY-04) in BNU, China. Special thanks to teachers who giving help in the process of this article writing, colleagues and undergraduates from regional geography laboratory who complete a large number of experiments.

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