Application of low temperature soaking liquid in combined freezing-based desalination processes in summer

R

salt removal efficiency (%)

C_ip

salinity of the ice product (%)

C₀

salinity of the raw seawater (%)

R_iy

ice yield rate (%)

M_ip

mass of ice product (g)

M_0i

mass of crushed ice (g)

t_s

soaking time (min)

t_c

centrifugation time (min)

r_c

centrifugation rotation rate (r/min)

t_g

the melting time of gravity-induced desalination (min)

P_br

brine reject proportion (%)

M_br

mass of brine reject (g)

R_is

the mass ratio of ice to soaking liquid

T_s

soaking liquid temperature (°C)

Salt water accounts for 97.5% of global water resources, of which the remaining is freshwater. Freshwater mostly covers mountainous and polar regions in the form of ice/snow, only 0.3% of which is available for human use (Youssef et al. 2014). In the past few decades, population growth, accelerated urbanization and global climate change have considerably increased the demand for water in China. Water shortages are restricting further development of the economy, especially in coastal provinces. Developing alternative water resources is an option to cope with the water crisis (Zhu et al. 2019). Desalination could be an effective approach to improve access to freshwater so as to solve the freshwater crisis by processing the saline water plentifully available on the Earth's surface (Yang et al. 2003). In recent years, China has been energetically developing the desalination industry. The goal set by the Chinese ‘13th Five-Year Plan for National Seawater Utilization’ is that the total scale of seawater desalination will reach 2.2 million tons per day at the end of 2020. In the case of the desalination industry developing rapidly, the development objective of industrialized desalination should be large scale, low cost and low energy consumption.

Desalination technologies can be categorized into two groups. One is thermal (phase-change) desalination, such as multi-stage flash (MSF), multi-effect distillation (MED) and freezing desalination (FD) and the other is membrane desalination, involving reverse osmosis (RO), electro-dialysis (ED and EDR), etc. (Al-Karaghouli & Kazmerski 2013). Facing the main obstacles of industrial desalination, several favorable points of freezing method have gradually become attractive and recognized. Compared with membrane-based desalination, the freezing method does not require complex pretreatment and membrane cleaning processes, which is economical and reduces the environmental pollution of the discharged concentrated brine caused by chemical addition (Kalista et al. 2018). Because the latent heat of ice melting (334.7 kJ/kg at 1 atm) is only 1/7 of the water vaporization heat (2,259.4 kJ/kg at 100 °C, 1 atm), the freezing method consumes less energy, in theory, in comparison with the distillation method. While FD has potential, it is still limited to apply in real industrial production. In practical terms, industrial waste heat can be used in thermal desalination, whereas the refrigeration cycle used to generate and maintain a low-temperature environment involves the consumption of high-grade energy, such as electricity. In addition, even if the freezing process is very slow, concentrated saline is inevitably trapped to form brine pockets inside the ice block (Huang 2010). Therefore, high-purity ice cannot be produced directly with just the freezing method. Therefore, reasonable selection of a cold source and low salt removal efficiency are two major obstacles for the application of the freezing method in industrial production.

Many scholars attempted to explore economical technologies for which natural cold resources or waste industrial cold energy can be utilized to reduce the cost of the freezing method. Hasan & Louhi-Kultanen (2016) proposed that natural freezing may become a sustainable and energy-saving purification technology in cold regions with temperatures lower than 0 °C. However, a natural cold source is greatly influenced by regions and seasons. ‘LNG cold energy’ is one type of waste energy released from the re-gasification of liquefied natural gas (LNG). The volume of LNG is only 1/600 of NG (natural gas) at standard atmospheric pressure and −162 °C, making it easier to transport and store. However, LNG must be re-gasified before being supplied in gaseous form in LNG terminals, and the gasification process absorbs approximately 837 kJ/kg energy. LNG terminals are generally built near harbors and the cold energy is usually directly discharged into seawater. With the continuous expansion scale of the LNG terminals, this traditional mode has already caused both cold pollution on adjacent waters and enormous energy loss. LNG cold energy is now considered to be a high-quality energy source to cool media due to its characteristics of low temperature, high energy density and the blooming global LNG market (Wang & Chung 2012). If this kind of energy can be effectively utilized, it may achieve the purpose of energy saving, emission reduction and economic efficiency. China is the second largest importing country of liquefied natural gas in the world. The LNG receiving stations along the coastal areas provide amounts of potential low-cost cold sources for freezing-based desalination. Antonelli (1983) proposed a seawater desalination process with N-butane as secondary refrigerant and LNG as the cold source. Mounting numbers of studies have subsequently shown that LNG cold energy has the potential to support the energy requirement of freezing desalination and minimize the overall energy consumption (Wang & Chung 2012; Cao et al. 2015; Chang et al. 2016).

The research objects are mainly divided into natural sea ice and artificial sea ice according to different cold sources. During the freezing process, some concentrated solution was trapped into the ice and existed in the form of brine pockets. Due to the repeated freeze-partial melting process that occurs during natural sea ice formation, more brine drains from the ice body, and the salinity of artificial sea ice is much higher. For the purpose of decreasing sea ice salinity, several post-treatment techniques such as gravity, centrifugation, watering and soaking are employed to separate brine pockets from ice. Wakatsuchi & Kawamura (1987) studied the formation processes of brine channels in young sea ice. The results showed that the formation of brine channels is largely caused by the brine expulsion and gravity drainage in sea ice desalination. Ma (2006) studied the influences of the gravity-induced desalination process on natural sea ice and proved that the placement angle, environmental temperature and melting time had impacts on the salt removal efficiency of sea ice. Gu et al. (2012, 2013) conducted gravity-induced desalination experiments with natural sea ice from Bohai Bay for three consecutive years. The results showed that freshwater product with salinity lower than 1‰ could be obtained. They pointed out that the ambient temperature has significant impact on the gravity-induced process, and future research of freezing-based desalination should pay more attention to increasing freshwater production or shortening the duration. Luo et al. (2010) froze brackish water by the unidirectional-freezing method. After the ice-water mixture was centrifuged for 8 minutes at 800 r/min, the salt removal efficiency increased from 73.38 to 90.60% with the increase of the sample concentration ranging from 1,320 to 8,350 ppm. The method of crushing ice and centrifugation (CIAC) helped to remove large amounts of solute from the ice by breaking the brine pockets. Therefore, the combination of freezing and CIAC was proved to be more efficient than the simply freezing method. Tan et al. (2015) studied centrifugal desalination of natural sea ice collected from Bohai Bay. Separation factor, separation time, viscosity, and ambient temperature were analyzed through theoretical analysis and single factor experiments. Experimental results showed that the ice salinity can be decreased below 1‰ at the separation factor of 1,100 for 1-minute centrifuging. Xu et al. (2003) collected two kinds of sea ice from Changxing Island, with salinity of 0.264 and 0.558%, respectively. In the experiments, they mainly studied the effects of soaking time, soaking liquid mass and soaking liquid temperature on the soaking desalination method and the combined soaking and centrifugal desalination method. The highest salt removal efficiency of the latter method could reach 91%, which met the requirement for irrigation water, surface water and drinkable water. It proved that the salt removal efficiency of combined soaking and centrifugal desalination method was higher than that of single soaking. Yang et al. (2017, 2018, 2020) conducted seawater desalination experiments on the ice flakes produced by designed experimental setup. Combined freezing, gravity-induced and centrifugal desalination (FGCD), combined freezing, watering and centrifugal desalination (FWCD) and combined freezing, microwaving and centrifugal desalination (FMCD) processes were employed for ameliorating the salt removal efficiency of the ice.

Soaking treatment is effective to enhance the ice purity since soaking liquid can decrease the viscosity of brine adhering to the ice as well as accelerate the melting process of some ice. As the ice soaked in the seawater, the salt in the brine pockets transferred to the soaking medium under osmotic pressure. Moreover, it facilitated the concentrated solution separated from the ice by overcoming the surface tension and viscous force when centrifugal force is applied to the ice. In our previous work (Dong 2019), we studied the influence of soaking-related parameters, such as soaking time, soaking medium temperature and the ice to soaking medium ratio, on the desalination effect of the FSCD process. The raw seawater taken from Bohai Bay was for ice making and used as soaking liquid. As the natural seawater temperature varies with seasons, the soaking liquid temperature was controlled at 27 °C to simulate the average seawater temperature in summer. No samples treated with the soaking process had an ice yield rate higher than 16% under the soaking liquid at 27 °C, which indicated that it was not suitable to desalinate sea water with the FSCD process in summer.

Aiming at the above-mentioned problems of the FSCD process, the objective of this paper is to increase the ice yield rate in summer while maintaining high salt removal efficiency. The desalination effect of FSCD and FSGCD processes employed in summer were investigated with low-temperature soaking liquid. Finally, the process performance in this study was compared with other desalination processes published previously by our research group.

The seawater that served for the experiments conducted in this paper was taken from Bohai Bay in winter, with a salinity of 2.9%. The DW-HL388 refrigerator (cooling temperature range from −10 to −86 °C) was used as the ice producer. In the centrifugal desalination procedure, the ice sample was centrifuged with the TD5F type filtering centrifuge whose rotation rate can be regulated from 0 to 4,000 r/min. A TE3102S analytical balance, with the maximum measuring value of 3,000 g and an accuracy of 10 mg, was used for measuring the mass of seawater and ice. The salinity of the raw seawater and the desalted water were measured by an AZ8371 salinity meter with a measuring range of 0–70 ppt and an accuracy of ±1%.

For the centrifugal process, centrifugation time and rotation rate have been proved to be the main factors directly affecting the desalination effect (Luo et al. 2010). Therefore, these two factors were researched respectively with the rotation rate set from 500 to 4,000 r/min and the centrifugation time varied from 1 to 5 min in this paper. For the soaking process, the impact of soaking time on the desalination effect was studied. In the FSCD process, the tested soaking time was from 1 to 15 minutes and the centrifugation time and rotation rate were determined according to the above centrifugal process experiments.

In the FSGCD process (Figure 7), the artificially frozen ice was produced and crushed in the same way as the FSCD experiments. The crushed ice was soaked in seawater with an initial temperature of 5 °C for 1 min, and the mass ratio of the ice to soaking liquid was 1:3. In the gravity-induced desalination procedure, the ice was melted gradually in a thermostatic chamber. The temperature of the thermostatic chamber was set at 30 °C to simulate the ambient temperature in summer. The brine drained to the beakers induced by gravity, as shown in Figure 8. The gravity-induced desalination process lasted from 5 to 60 min for the 12 samples in sequence. Then the remaining ice was centrifuged at 2,000 r/min for 2 min by the filter centrifuge. Finally, the ice product was melted to measure the mass and salinity, and the salt removal efficiency and ice yield rate were calculated by Equations (1) and (2).

For investigating the effect of the rotation rate on the FSCD process, experiments were carried out at the soaking time of 3 min, centrifugation time of 2 min and rotation rate varied from 500 to 4,000 r/min. Eight ice samples were named from Case I-1(1) to Case I-1(8). The experimental data and desalination results for the effect of the rotation rate are listed in Table 1.

Based on the data in Table 1, the relationship between the salt removal efficiency and ice yield rate to the rotation rate is shown in Figure 9. It can be found that as the rotation rate increases from 500 to 4,000 r/min, the salt removal efficiency grows from 42.55 to 91.17%, while the ice yield rate decreases from 71.36 to 57.53%. The salt removal efficiency is positively correlated with the rotation rate, while the ice yield rate is negatively correlated with the rotation rate.

The sea ice centrifugal desalination process could be mainly considered as the dehydration process. Since salt exists on the surface of ice particles in the form of brine, the ice purity can be enhanced with more saline water removed by centrifugal force. The separating factor, an important index to measure the inertial centrifugal force, is proportional to the square of the rotation rate. Therefore, the faster the rotation rate, the more the brine is removed per unit time. As the rotation rate increases to a certain extent, almost all brine is separated from ice in a short time. It can be seen from Figure 9 that when the rotating rate reached more than 3,000 r/min, no obvious change was observed in the salt removal efficiency and ice yield rate after 2 minutes centrifugal treatment.

Five seawater ice samples were taken, namely Case I-2(1) to Case I-2(5). The soaking time was kept at 3 min. In the centrifugal desalination procedure, the effect of centrifugation time was observed in the range of 1–5 min. It can be seen from the curve in Figure 9 that the salt removal efficiency and the ice yield rate changed slightly when the rotation rate was higher than 2,000 r/min. Thus, the rotation rate of 2,000 r/min was selected in this part of the experiment, considering the relationship between production efficiency and energy consumption. Based on the data in Table 2, which presents the experimental conditions and results, the salt removal efficiency and ice yield rate as a function of centrifugation time for FSCD process are plotted in Figure 10.

It can be observed from Figure 10 that the increase in centrifugation time is helpful to improve the salt removal efficiency, from 84.21 to 88.66%, with a range change of only about 4%. In the meantime, the ice yield rate gradually decreased from 63.40 to 56.50%, with a change of about 7%. Overall, trends in both the salt removal efficiency and ice yield rate are relatively slow.

More brine is separated from the ice with the centrifugation time extension when the rotation rate is constant. Due to the exposure of brine pockets caused by ice crushing, most of the concentrated water can be discharged quickly in a short time by the centrifugal method. However, because of the high degree of sea ice crushing, the small size and large specific surface area of sea ice led to the increase of the brine attachment area (Zhang 2008). It can be seen that the growth rate of the salt removal efficiency has slowed down when the centrifugation time is more than 2 minutes. Therefore, it implies that after 2 minutes centrifugation time, continuously prolonging the centrifugation time cannot improve the desalination effect significantly.

The work in the sections ‘Effect of rotation rate’ and ‘Effect of centrifugation time’ above assists in selecting more reasonable centrifugal parameters. In order to reduce the energy consumption as much as possible under the premise of achieving a better desalination effect, the rotation rate of 2,000 r/min and centrifugation time of 2 min were chosen with reference to the data in Tables 1 and 2. Fifteen ice samples (Case I-3(1) to Case I-3(15)) were soaked for 1–15 minutes, respectively. The experimental conditions and results are presented in Table 3.

The relationship of the salt removal efficiency and the ice yield rate to the soaking time is presented in Figure 11. Figure 11 shows that the salt removal efficiency could reach more than 85% after soaking for 1 minute, and it goes up from 85.17 to 89.21% with the prolongation of the soaking time. When the soaking time was increased from 5 to 15 min, the salt removal efficiency shows a slower growth trend, with an increase of only 1.49%. However, the ice yield rate drops from 55.92 to 38.58% due to more sample ice melting in the soaking liquid. It indicates that adding soaking time is helpful to ameliorate desalination effects, whereas a constant prolongation of soaking time would lose the practical application significance of desalination. So, the soaking time should be selected depending on the different usage and quality requirement of water.

From the above experiments, it can be seen that the salt removal efficiency of the FSCD process can reach more than 85% in a short time when the initial temperature of the soaking liquid was set at 5 °C. However, even if the soaking time is extended from 1 to 15 minutes, and the other conditions remain unchanged, the salt removal efficiency cannot exceed 90%. The gravity-induced process is helpful for improving the desalination effect of the centrifugal process (Yang et al. 2017, 2019). As ice melts naturally under gravity, drainage channels were formed by connecting the pores with the outside environment. The concentrated brine and the melt water drained along the channels, resulting in the discharge of salt. As ambient air can be taken as the natural heat source in the ice melting process, there is no additional energy consumption by adding the gravity-induced method. Our previous experimental study proved that the salt removal efficiency increases with the increase of gravity-induced brine drainage proportion in the FGCD process (Yang et al. 2016, 2017). Therefore, we tried to combine the soaking treatment and gravity-induced method to form the FSGCD process. The melting time of the gravity-induced process was investigated.

The gravity-induced desalination process lasted from 5 to 60 min for 12 sea ice samples (Case II-1(1) to Case II-1(12)). The experimental conditions and results are listed in Table 4. The changing trend between the salt removal efficiency and ice yield rate along with the gravity-induced melting time is plotted in Figure 12. Experimental results show that with the melting time increasing from 5 to 60 min, the salt removal efficiency increases from 88.14 to 97.03%, while the ice yield rate decreases from 53.79 to 20.44%. The salt removal efficiency is negatively related to the ice yield rate. Comparing the data in Table 4 with that in Table 3, it can be found that the salt removal efficiency of the FSGCD process is obviously higher than that of the FSCD process at the same ice yield rate.

The maximum salt removal efficiency of the FGCD (Yang et al. 2017) that was published previously could reach 97.58%. Since the ice production method of the FSGCD experiments in this paper and the ambient temperature are different from previous research (Yang et al. 2017), the FGCD experiments were carried out under the same conditions as the FSGCD, except for eliminating the soaking process. In the present FGCD experiments, ten ice samples were taken and marked Case II-2(1) to Case II-2(10), respectively. During the gravity-induced melting process, the brine drainage proportions were changed from about 5 to 50% for the ten samples through regulating the melting time. The brine reject proportion of each sample was determined by measuring the weight of the beaker which contained the brine and was calculated using Equation (3). The experimental conditions and results are listed in Table 5.

Based on the data in Tables 3–6, the variations of the salt removal efficiency to the ice yield rate are plotted in Figure 13. The relationships of the melting time to the ice yield rate for the experimented desalination processes are presented in Figure 14, in which the y-coordinate ‘t’ represents the gravity-induced melting time for the FGCD process; the soaking time for the FSCD process; and the soaking time plus the gravity-induced melting time for the FSGCD process. Since melting mainly happens in the gravity-induced and soaking processes rather than in the centrifugal process, the melting time is zero for the FCD process.

It can be concluded that the salt removal efficiencies of the FGCD, FSCD and FSGCD processes are all higher than that of the FCD process. This proves that the gravity-induced process and soaking process are helpful to ameliorate desalination effects. In the FSCD experiments, the salt removal efficiencies of all samples soaked in 27 °C seawater are higher than 90%, but the highest ice yield rate is only 15.35%. When the seawater with temperature reduced to 5 °C is used as the soaking liquid, the ice yield rate is significantly improved, while the salt removal efficiency of each sample is in the range of 84.21–89.21%. FSGCD experiments were also carried out with soaking liquid of 5 °C. The salt removal efficiency of the FSGCD process is more than 90%, and the maximum is up to 97.03%. Under the same ice yield rate, the salt removal efficiency of the FSGCD process is higher than that of the FSCD process. When the ice yield rate is lower than 33.74%, the salt removal efficiency of the FSGCD process is only less than 1% lower than that of the FGCD process, while the operation time required for FSGCD is at least 20 minutes shorter than that of the FGCD. Since the gravity-induced desalination experiments were carried out in ambient air at 30 °C, the duration of the FGCD process will be longer if it is adopted at low environment temperatures. Therefore, the work in the paper could provide an alternative to the FSCD process in summer application. The suitable method can be selected according to the different usage and water quality requirements in practical application.

The FSCD and FSGCD processes in the study are not only relatively easy to carry out and control but also improve the desalination effect. These characteristics may make them available in practical application. The meaningful point of the processes discussed above is the waste cold utilization rather than the freshwater production scale. Energy consumption of the FGCD method was briefly calculated and proved to be acceptable in previously published work (Yang et al. 2017). The power consumption of the refrigeration cycle is negligible if using the industrial waste cold energy or LNG cold energy. As the natural heat energy of air or seawater could be used, the energy consumed for ice-melting in the gravity-induced process could also be ignored. Therefore, the centrifugation energy consumption was the main additional energy consumption of the FGCD process. The analysis on energy consumption is similar in the study of this paper. As low-temperature soaking liquid can be obtained through melting pure ice products, recovering cold energy from cold concentrated brine or even directly cooled with the same cold resource of the freezing process, the centrifugation energy consumption is still the major cause of total energy consumption. Therefore, it can be predicted without further analysis that the total energy consumption is acceptable as it is similar to the FGCD process. Because the work of the paper focused on basic research ameliorating the desalination effect rather than the ice-making method using waste cold energy, the LNG cold energy or other low-cost cold resources were not utilized in the experiments. If the optimized processes were used in real industrial projects, the cold energy utilization plan, initial investment and maintenance cost should also be considered comprehensively.

In this paper, soaking treatment was applied in FSCD and FSGCD processes to desalinate sea ice. The low-temperature seawater was used as soaking liquid. The effects of rotation rate, centrifugation time and soaking time on the FSCD process were investigated. To optimize the desalination effect and application range of the FSCD process, FSGCD experiments were also carried out to investigate the influence of melting time of the gravity-induced process in ambient air with a temperature of 30 °C. The desalination effects of the two processes were tested and compared with that of other processes researched by our previous work. The conclusions are summarized as follows:

• (1)

  The ice yield rate is significantly improved by the utilization of low-temperature soaking liquid. In real industrial practice, the low-temperature seawater can be obtained through melting pure ice products or recovering cold energy from cold concentrated brine.

• (2)

  For the FSCD process, it is helpful to raise the salt removal efficiency with the increase of rotation rate, centrifugation time and soaking time. However, the salt removal efficiency of the FSCD process cannot reach up to 90%.

• (3)

  For the FSGCD process, the salt removal efficiency can be effectively improved by prolonging the melting time of the gravity-induced process. The salt removal efficiency of each sample is higher than 90%, and the maximum is up to 97.03%.

• (4)

  By discussing the performance comparison among different processes, it is indicated that the FSCD and FSGCD processes have the potential of practical application. As the study mainly paid attention to basic research ameliorating the desalination effect, practical application and economic feasibility would still require extensive research and development.

The authors appreciate the financial support from the National Natural Science Foundation of China (No. 52070012), Beijing Municipal Science & Technology Commission under the project number Z151100001415007 and Sinopec Gas Company with the contract number 314037. We would like to also thank the experts and our research team colleagues at BUCEA, SEI and Sinopec Gas Company for useful discussions, in particular Pu Hongbing, Wang Peng, Wang Baoqing, Wei Jie, Li Fengqi and Li Ming.

All relevant data are included in the paper or its Supplementary Information.