A gravity-induced desalination technology effectively improving the freshwater production efficiency of freeze desalination

Abstract

Low desalination efficiency and water production efficiency limit the commercial application of freeze desalination (FD) technology. In this study, three freezing temperatures were set to carry out single-stage progressive freezing desalination of brackish water in southern Xinjiang, China. The ice were melted using conventional methods and gravity-induced desalination (GD), and the salt migration law during the icing and melting process was studied. The effect of GD technology on the improvement of the output of frozen desalinated produced water was quantitatively analyzed using scenario simulation. Experimental results showed that salts accumulated towards concentrated water during FD process. The ice total dissolved solids (TDS) increased with decreasing freezing temperature, which was linearly and positively correlated with the raw water TDS. GD efficiency was significantly affected by the salt concentration of ice crystals. In the first stage (the first 450 mL), ice-melt water was salt explosive dissolution, with salt drained mass accounted for more than 85% of the total salt mass. During GD process, exponential decay of ice-melt water TDS (TDSI) was observed with increasing ice-melt water amount. Through the simulation of the two exponential equation scenarios, it was found that Scenario II (mixed water TDSI as the target value) had significantly higher water yield and production rate than Scenario I (TDSI as the target value), with an increase of more than 23%. The algebraic relationship between TDSI of mixed water and the amount of ice-melt water was derived. GD technology can effectively improve the freshwater output efficiency of the FD process, which is manifested by the fact that the ice crystals of the FD process cannot produce freshwater using conventional melting. Scenario II can produce freshwater as high as 69.83% of the total amount of the GD ice-melt water. The GD technology performed increased multiplicatively in WPdv (decrease in water production) and WPRdv (decrease in water production rate) when the TDSI of the mixed water demanded by the user was low. When FD and GD are combined to desalinate salt water, lowering the freezing temperature can significantly increase the freshwater yield, breaking the limitation of freezing temperature for FD. Therefore, an automated, energy-saving freeze desalination-gravity desalination device capable of realizing the collection of the product water according to the users' needs was conceptualized. This study provides new ideas and algorithms for accurately assessing the efficiency of water production in the combined desalination of saltwater by FD and GD.