Seawater Desalination Method Based On Freezing Process And The Micro-structure Of Seawater Ice

The shortage of freshwater resources and abundant seawater resources make the seawater desalination industry develop rapidly.National Seawater Utilization 13 th Five-Year Plan puts forward the goal of exceeding 2.2 million tons per day in the total scale of seawater desalination at the end of the 13 th Five-Year Plan.The increasing import of LNG also brings an opportunity to solve the problem of high cost of cold source in the process of seawater desalination based on freezing process,and it makes the seawater desalination technology based on freezing process have a practical application background.In this paper,the seawater desalination technology based on the freezing process is mainly studied in two parts: the FSCD experiment based on the freezing process of refrigerator;and the FSCD experiment based on the freezing process of experimental device.Two experiments were conducted to study the effects of soaking time,soaking solution temperature and ice-liquid ratio(ice-liquid ratio is the ratio of initial mass of ice samples to soaking solution's mass)on desalination effect of FSCD process by using DW-HL388 refrigerator and LNG-based seawater freezing ice-making experimental device.Seawater desalination based on freezing process is mainly a process of removing salts from brine pockets in sea ice and the surface of ice crystals.Therefore,it is important to study the morphology,size and distribution of brine pockets in sea ice for seawater desalination based on freezing process.In this paper,the micromechanism of seawater desalination based on freezing process is studied as follows:(1)Taking Ice made from NaCl solution with different mass concentration as the object,microstructure images are obtained by using microscopic imaging equipment and processed by image processing.Porosity and pore size of NaCl brine ice with different mass concentration are studied.(2)The morphology,size and distribution of brine pockets and air bubbles at different heights of the same artificial sea ice sample were studied by micro-CT and ImageJ image processing software.The main conclusions of the experiment are as follows:1)The salt removal efficiency and TDS removal efficiency of FSCD process are positively correlated with soaking time and soaking solution temperature,while the ice yield rate is negatively correlated with soaking time and soaking solution temperature.When the temperature of the soaking solution is 27?,the highest ice yield rate of FSCD process based on the refrigerator and the experimental device is 15.35% and 7.98%.Therefore,when it takes raw seawater as the soaking solution,it is not suitable for using FSCD process in summer.2)When the temperature of soaking solution is 11?,the salt removal efficiency and TDSremoval efficiency of FSCD process based on the refrigerator with 1:4 ice-liquid ratio is larger than that with 1:3 ice-liquid ratio and 1:5 ice-liquid ratio,the lowest and the highest salt removal efficiency of the FSCD process is 89.41% and 92.99%.3)The salt removal efficiency and TDS removal efficiency of FSCD process based on the experimental device increase with the increase of ice-liquid ratio,but the ice yield rate is opposite.When the temperature of soaking solution is 11?,the lowest and the highest salt removal efficiency of the FSCD process is 92.14% and 95.41%.4)At the same ice yield rate,compared with the FSCD process based on the refrigerator,the FSCD process based on the experimental device has higher salt removal efficiency and lower salinity of water product.5)Microscopic observation showed that the porosity,number of pores per unit area and pore area of NaCl brine ice samples with 6% mass concentration were larger than that of NaCl brine ice with 3% mass concentration.6)The analysis of the image obtained by micro-CT shows that:The upper part of the ice sample contains the least brine pockets and air bubbles,while the area of the brine pockets and air bubbles in the lower part accounts for the largest proportion.The area ratio of brine pockets in the middle and lower part of the ice sample show an overall upward trend.The distribution range of equivalent diameter of brine pockets and air bubbles increases approximately as the slice position moves down.The proportion of brine pockets and air bubbles with equivalent diameter less than 200 um is the largest.