Study on sea ice desalination technology

Desalination 245 (2009) 146–154 Study on sea ice desalination technology Lixin Xiea, Jia Maa, Fang Chengb*, Pingli Lia, Jie Liua, Weibin Chenc, Shichang Wanga a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b Tianjin Institute of Urban Construction, Tianjin 300384, China Tel. +86 (22) 2740 4347; Fax: +86 (22) 8789 1367; email: [email protected] c National Marine Environmental Monitoring Center, Dalian 116023, China Received 18 September 2007; Accepted 29 June 2008 Abstract In winter in high-latitude oceans, there is a great amount of sea ice, which is being recognized as a new resource of fresh water. Different desalination methods including sea ice solid-state desalination and sea ice melt-water desalination were compared. In a centrifugal desalination experiment, rotation speed was one of the major controlling parameters. The best desalination result in terms of salinity was obtained at a rotational speed of 2000 rpm for a duration of 2–3 min. When sea ice was desalinated by slight-melting, environment temperature and time were the dominant factors. By using the reverse osmosis process, the total dissolved solids of sea ice melt-water could reach about 100 mg/L. The results indicate that methods of sea ice desalination could decrease sea ice salinity from 3‰–8‰ to 0.2‰–2‰ at a reasonable cost. Keywords: Sea ice; Natural freezing; Desalination; Centrifuge; Reverse osmosis 1. Introduction fresh water is a serious threat to the socio- Water shortage has become a big problem in economic development of the world [1]. More many parts of the world. Following an increase in and more people begin to pay attention to the population and consumption, the fast growth of ocean, which covers about 70% surface of our the economy and the serious situation of water earth and contains the largest resource of fresh pollution, the conflict between water supply and water. Desalination technologies, which contain demand is aggravated day by day. The scarcity of distillation, reverse osmosis (RO) freezing desali- nation and others using alternative energy, can be *Corresponding author. utilized to produce fresh water for industry, 0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.06.016  L. Xie et al. / Desalination 245 (2009) 146–154 147 Table 1 Sea ice melt water quality and China water standards (in mg/L) Item One-piece ice Cumulate ice Neaped ice Irrigation water Drinking water Surface water Sulfate 588 390 346 — 250 250 Chloride 4133 3903 2605 250 250 250 Nitrate 3.2 3.5 3.1 — 20 10 Nitrite 0.01 0.043 0.027 — 0.05 0.1 Total hardness 253 208 133 — 450 — Total salt 7650 6650 3660 1000–2000 — — pH 7.35 7.00 7.26 5.5–8.5 6.5–8.5 6.5–8.5 agriculture and human consumption [2–6]. The grown ice in the form of a so-called brine pocket. freezing method, especially natural freezing Further desalination may occur when the sea ice desalination, is a good prospect in high latitude grows and melts, which can reduce the salinity to coastline areas due to its low energy consump- a lower level [9]. Thus, sea ice is composed of ice tion, few corrosion and scaling problems, and crystals, brine pockets, air pockets and a bit of little requirement for complex pretreatment. solid impurities, which is different from fresh The Circum-Bohai region on the coast of the water ice. Bohai Sea in northern China is an important The salt from sea ice mainly exists in the brine economic zone. Because of its dry weather and pocket; the salinity of sea ice is usually about little rivers, one of the biggest problems it faces 3‰–8‰, which is much lower than seawater now is the serious shortage of fresh water supply. (about 3.5%). It is seen that the sea ice formation However, there are large areas of sea ice in Bohai process is actually a natural desalination process, Sea formed by the cold wave activities of Siberia which can save lots of investment and reduce the in winter. Research has shown that in cold cost when the sea ice is used as a desalination winters the amount of sea ice in the Bohai Sea, material. which could be used as a freezing desalination The sea ice melt-water in the Bohai Sea area resource, is over 4.0×1011 m3 [7,8]. Therefore, was analyzed and the results are shown in Table 1 exploiting and utilizing the sea ice in the Bohai [10]. The “one-piece ice” was formed in a rela- Sea would be of great significance to relieve the tively quiet sea surface, which normally had a serious water shortage in the surrounding areas. large area and was thought to be one layer theo- retically. When the one-piece ice was broken, accumulated and frozen again, the “cumulate ice” 2. Basic mechanism of freezing desalination was formed which had more layers. The national technology standard of the People’s Republic of China for Ice is formed when the temperature is below drinking water (GB5749-2005), surface water the freezing point, but most salt ionic species (GB3838-2002) and irrigation water (GB5084- contained in seawater are so large that it is diffi- 1992) are also listed for comparison. From cult to incorporate them into the ice crystal lat- Table 1 it can be found that although most of the tice. So the salts contained in seawater are salt and impurities were lost in the sea ice forma- transferred into the underlying ocean and only tion process, the total salt, chloride and sulfate in parts of them are retained in the volume of newly sea ice were still not up to the requirements for 148 L. Xie et al. / Desalination 245 (2009) 146–154 irrigation and drinking. Therefore, how to effec-  m  tively remove the brine in sea ice is the most L   1  P   100% (2) important research topic for sea ice desalination.  mF  In this paper methods of sea ice desalination including solid-state desalination and liquid-state where mP is the mass of produced sea ice melt- desalination were studied in different scales in water (kg), and mF the mass of feed sea ice melt- detail. Centrifugal desalination and slight-melting water (kg). desalination are the typical methods of solid-state desalination while RO is the main method of 3.2. Results and discussion liquid-state desalination. The results are shown in Figs. 1–7. Effects of centrifugal rotation speed and centrifugal time on 3. Centrifugal desalination Sea ice is mainly composed of brine pockets and ice crystals, and the salts are mostly con- tained in the liquid brine. When centrifugal force is applied to sea ice, the brine can be drained from the ice as the force difference. So the brine is separated from the sea ice which can then be melted into fresh water. 3.1. Experiment Bench-scale experiments were carried out for investigating the effects of centrifugal rotational speed and centrifugal time on salt rejection of this Fig. 1. Variation of TDS over centrifugal time and cen- method at an ambient temperature of 5EC and ice trifugal rotation speed. temperature !2.5EC. At first, the sea ice was broken into particles and about 50 g samples of 6 mm were put into the centrifuge every time. After a period of centrifugal treatment, the TDS and the sulfate concentration of sea ice melt water were tested. The salt rejection was calculated by the formula:  C  R  1  P   100% (1)  CF  where CP is the TDS of produced sea ice melt- water (mg/L) and CF is the TDS of feed sea ice melt-water (mg/L). The loss factor is: Fig. 2. Variation of TDS over centrifugal rotation speed.  L. Xie et al. / Desalination 245 (2009) 146–154 149 the TDS and sulfate are indicated in Figs. 1–4. In were open, and the brine was easily drained out. Fig. 1, it is seen that the TDS of sea ice was After centrifuged at a certain speed, most of the reduced with the increase of centrifugal time, and brine was drained out but the residue was difficult the reduce trend was slowed down after about to remove because of the closed channel. So the 2 min. The TDS in high centrifugal rotation speed trend was slowed down. was higher than the lower one. Figs. 5–7 indicate the effect of centrifugal The sulfate concentration had a similar situa- rotation speed and centrifugal time on the salt tion, shown in Figs. 3 and 4. This was because rejection and loss factor. In Figs. 5 and 6, it is the salt, no matter TDS or sulfate, mainly existed shown that both the salt rejection and loss factor in the brine pocket of sea ice. When the sea ice were increased with centrifugal time, but the was broken into particles, some brine pockets trends were also slowed down after 2 min, as seen Fig. 4. Variation of the sulfate over centrifugal rotation Fig. 3. Variation of the sulfate over centrifugal time and speed. centrifugal rotation speed. Fig. 5. Variation of salt rejection over centrifugal time Fig. 6. Variation of salt rejection and loss of sea ice mass and centrifugal rotation speed. over centrifugal time. 150 L. Xie et al. / Desalination 245 (2009) 146–154 in Figs. 1–4. In Fig. 7, it can be seen that when 4.1. Experiment the centrifugal rotation speed was below The experiment was performed in the equip- 2000 rpm, the salt rejection and the loss of sea ice ment shown in Fig. 8, with dimensions of 600× mass increased gradually with centrifugal rotation 600×1200 mm. The ice was placed so that the speed. When the centrifugal rotation speed was vertical was aligned with the direction in which 2000 rpm and centrifugal time was 2–3 min, the the ice grew, and the size of every sea ice sample salt rejection could reach 85% and the loss of sea was 100×100×100 mm. ice mass was about 20%. When centrifugal rota- The experiment began when the sea ice began tion and time were increased, the total salt, to melt, and the salinity of melt-water was tested sulfate, salt rejection and loss factor of sea ice every hour. The experiments were performed were nearly constant. This means that most of the under different temperatures and experiment brine had been drained out, and a long time and times, which had an important effect on the high rotation speed were not necessary for better salinity of sea ice and brine draining from the sea water quality. ice [12]. The TDS of sea ice at a special time (Csea ice) was calculated by the following formulas: 4. Slight-melting desalination The freezing point of brine is lower than the M t  C   m  c  M sea ice  Csea ice (3) one of fresh water, so when the ambient tem- perature is kept a little higher than the melting point of sea ice, the brine pocket increases in size M t  m  M sea ice (4) because the area near the brine melted faster than the fresh water ice. As a consequence, the brine where Mt is the mass of sea ice before the pockets become connected with other ones, and experiment, C the TDS of sea ice before the finally brine channels are formed [11]. Then the experiment, m the mass of melt-water collected brine in liquid pockets can be drained out, and the every hour, c the TDS of melt-water collected sea ice is desalted. every hour and Mseaice the mass of sea ice at this moment. Fig. 8. Equipment for slight-melting desalination. Fig. 7. Variation of salt rejection and loss of sea ice mass 1 thermotank, 2 temperature controller, 3 sea ice, 4 grid- over centrifugal rotation speed. structure vessel, 5 supporter, 6 vessel.  L. Xie et al. / Desalination 245 (2009) 146–154 151 4.2. Results and discussion 14th hour, the change of salinity of sea ice melt- water was small. This was because most of the The experimental results are shown in Figs. 9– brine had been drained out during the first 14 h, 11. Compared to the centrifugal desalination and the mass of fresh water melted from sea ice method, slight-melting desalination can achieve began to be more than the brine. As a result, the a lower salinity. It is shown in Fig. 9 that the salinity of sea ice could not obviously be reduced relationship between the loss of sea ice and the while the loss of sea ice was increased greatly. slight-melting time was nearly linear. This was As shown in Fig.11, when the environmental because the loss of sea ice was mainly caused by temperature was 2.5EC, the decrease of salinity its melting, which had a direct relationship with was a little faster than that at ambient tempera- time, which is also a difference with the cen- tures of 1.5EC and 2.0EC from the beginning of trifugal desalination method. slight-melting to the 5th hour. But in the later In Fig.10, it was observed that the salinity of hours, the decrease at the ambient temperature sea ice melt-water decreased gradually with time 2.5EC was much less. The salinity of sea ice at during the 17-h experiment. Starting from the 2.5EC became the highest after 14 h. This was due to the salt that was mainly contained in the brine pocket. High temperature easily resulted to a high speed melting of sea ice near the brine and made the pocket connected to drain out the brine. However, the ice in the outer surface was also melting faster at a higher temperature at the same time, which made the sea ice lose a lot of fresh water. Thus, the sea ice TDS at a high tempera- ture was slowly reduced and even became the highest from these two aspects. So a very high temperature had no benefit for the desalination process. In industrial applications a suitable tem- perature (near 2.0EC) and operation time is important for the quality of desalination. Fig. 9. Variation of loss of sea ice mass over time. Fig. 11. Variation of TDS of sea ice over experimental Fig. 10. Variation TDS of sea ice melt-water over time. temperature. 152 L. Xie et al. / Desalination 245 (2009) 146–154 5. Desalination by reverse osmosis membrane system was studied. The results are shown in Figs. 13 and 14. It was concluded that RO is one of the widest used desalination the quality of the outlet water from the RO methods in the world, with many advantages [13]. membrane system was the best one among the Desalination of sea ice melt water by RO belongs three methods of sea ice desalination, with salt to the liquid-state desalina-tion method. rejection being more than 90%. Sea ice was collected in winter and piled into The TDS of outlet water in RO membrane a cement pool. The stored sea ice melted into a system decreased when the operating pressure solution when the weather became warmer. The was less than 1.5 MPa but increased later. In the ultrafiltration (UF)–RO integrated membrane RO process, water flux is proportional to the system was adopted to treat the melt-water of sea operating pressure while the permeation flux of ice. Because the salinity of sea ice becomes lower solute (salt) was related with the concentration in the freezing process, so the operation condition difference of solution near the two sides of the and requirements of this system are not stricter membrane. Under a lower operation pressure, the than seawater RO. water flux increased with increase of pressure and the permeation driver of salt is the salt concen- 5.1. Experiment tration difference near the membrane surface, so permeation of salt was little. As a result, the TDS The process flow diagram of sea ice melt- of the outlet water was reduced gradually. With a water desalination with an integrated membrane further increase of operating pressure, the con- system is shown in Fig. 12. The feed water of this centration polarization became serious, resulting system was the melting water of sea ice collected in a concentration of inorganic salt near the sur- in winter. Twenty-two UF membrane modules face of the membrane being increased with more (UF-PAN4040) were purchased from Tianjin salt permeating to the outlet. As a result, the TDS XinSanwei Membrane Technology; their cut of outlet water in the RO membrane system molecular weight (CMW) was 50,000. The RO increased gradually. membrane was Hydranautics CPA3. A single membrane housing containing six modules was used in this system. 6. Economic analysis of desalination methods of sea ice 5.2. Results and discussion The cost of product water from these desali- The effect of operating pressure on the salt nation methods of sea ice varied significantly rejection and TDS (of outlet water from a RO from each other. A preliminary economic analysis Fig. 12. Flow program of sea ice melt-water desalination with an integrated membrane system.  L. Xie et al. / Desalination 245 (2009) 146–154 153 Fig. 13. Variation of salt rejection (%) over operating Fig. 14. Variation of TDS of outlet water flux over pressure in a RO membrane system. operating pressure in a RO membrane system. Table 2 Economic analysis of three sea ice desalination processes (in yuan/m3) Desalination method Raw Energy Chemical Labor Depreciation Equipment Water ice consumption costs costs charge maintenance costs costs Centrifugal 2.6 1.2 — 0.4 1.8 0.3 6.3 Slight-melting 3.6 0.3 — — — — 3.9 Reverse osmosis 2.9 1.1 0.3 0.8 0.5 0.1 5.7 from each other significantly. A preliminary 7. Conclusions economic analysis for centrifugal desalination, slight-melting and RO was carried out and results Three different kinds of sea ice desalination are shown in Table 2. The total cost was com- methods were investigated, and the results indi- posed of the cost of sea ice collecting from the cate that sea ice desalination is a great prospect— Bohai Sea, sea ice transportation, energy con- not only in bench scale but also for industry. The sumption of the desalination process, chemical production of the different methods is compared costs, depreciation charge, equipment main- in Table 3 where it is shown that among the three tenance and labor costs. It was found that the methods, the quality and salt rejection of RO quality of product water of centrifugal desali- outlet water were the best, and the slight-melting nation and RO desalination was better, but the desalination was the worst for these parameters. total costs were higher than for slight-melting However, the slight-melting desalination had its desalination. Although the product water quality own special advantages; for example, no complex of slight-melting desalination was not so good equipment, easy to be realized, lower cost, easy and the loss of sea ice mass was high, the cost of operation, etc. Therefore, it is suited for situations product water was the lowest among the three which do not require very high water quality but methods. do need lower investment. 154 L. Xie et al. / Desalination 245 (2009) 146–154 Table 3 Advances in sea water desalination technologies, Production qualities of the different methods Desalination, 221 (2008) 47–69. [3] E. Mathioulakis, V. Belessiotis and E. Delyannis, Desalination TDS, Salt Loss Desalination by using alternative energy: Review and methods mg/L rejection, % ratio, % state-of-the-art, Desalination, 203 (2007) 346–365. Centrifugal 500 80–90 20–30 [4] Y. Dreizin, A. Tenne and D. Hoffman, Integrating Slight-melting 1000 80 >30% large scale seawater desalination plants within RO 100–350 >90 60 Israel’s water supply system, Desalination, 220 (2008) 132–149. [5] B.M. Hamieh and J.R. Beckman, Seawater desali- nation using dewvaporation technique: experimental Centrifugal desalination and the RO method and enhancement work with economic analysis, produce higher water quality with greater effi- Desalination, 195 (2006) 14–25. ciency and capacity, which are was suited for [6] L. Zhang, L. Xie, H.-L. Chen and C.-J. Gao, Progress situations requiring high-quality water. The and prospects of seawater desalination in China, different methods have different advantages and Desalination, 182 (2005) 13–18. [7] P.J. Shi, Y.D. Fan and S. Ha, Calculating gross sea disadvantages; therefore, their selection should be ice resource using AVHRR and MODIS data, J. Nat. according to purpose and requirements. A hybrid Res., 17(2) (2002) 138–143. process may be also a useful choice. [8] W. Gu, P.J. Shi and Y. Liu, The characteristics of As an exploratory investigation, most of the temporal and spatial distribution of negative accu- experiments for centrifugal and slight-melting mulated temperature in Bohai Sea and North Yellow were carried out at bench scale while the RO Sea, J. Nat. Res., 17(2) (2002) 168–173. experiment was at pilot scale. But the data found [9] W.F. Weeks and S.F. Ackley, The growth, structure during the research can be useful for application and properties of sea ice, in: N. Untersteiner, The in industrial processes. Pilot-scale equipment and Geophysics of Sea Ice, Martinus Nijhoff, Dordrecht, experimentsshould be carried out based on these 1986, pp. 9–164. [10] X.R. Xu, W.B. Chen, X.M. Liu, G.G. Zhang, Z.S. studies. Lin and D.M. Guan, Quality status of the water Through economic analyses, it is seen that melting from sea ice in the sea area near the Chang- producing fresh water from sea ice in the Bohai xing Island, Mar. Environ. Sci., 22(2) (2003) 33–36. Sea of China every winter proved to be a feasible [11] K.M. Golden, H. Eicken, A.L. Heaton, J. Miner, D.J. approach to alleviate the pressure of fresh water Pringle and J. Zhu, Thermal evolution of perme- shortages. ability and microstructure in sea ice, Geophys. Res. Lett., 34 (2007) L16501. [12] G.F.N. Cox and W.F. Weeks, Brine drainage and initial salt entrapment in sodium chloride ice. RREL References Research Report 345, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, [1] UN Water, Coping With Water Scarcity—Challenge 1975, p. 46. of the 21st Century, http://www.unwater.org/wwd07/ [13] C. Fritzmann, J. Lowenberg, T. Wintegens and T. downloads/ documents/ escarcity.pdf, 2007. Melin, State-of-the-art of reverse osmosis, Desali- [2] A.D. Khawaji, I.K. Kutubkhanah and J.-M. Wie, nation, 216 (2007) 1–76.