A freezing seawater desalination system and method

Abstract

This invention discloses a frozen seawater desalination system and method. The frozen seawater desalination system includes a crystallizer, which comprises an inlet pipe, a feed end, a discharge end, and refrigeration pipes. The crystallizer is divided into multiple sections from the feed end to the discharge end, and the refrigeration pipes in each section form a refrigeration pipe group. The refrigeration capacity of each refrigeration pipe group is independently controlled. Seawater is transported into the crystallizer; the temperature in each section of the crystallizer is controlled to decrease in a gradient from the feed end to the discharge end, and the temperature of each section is controlled to adapt to the freezing temperature of the seawater passing through that section; ice crystals are separated from the upper end of the crystallizer, and saline water is discharged from the lower end, thus achieving seawater desalination. The system's water output rate can reach over 80%, and the salt content of the ice crystals is less than 400 mg / L, meeting the requirements for industrial water.

Description

Technical Field

[0001] This invention belongs to the field of seawater desalination, and specifically relates to a frozen seawater desalination system and method. Background Technology

[0002] Seawater has a high salinity and cannot be used directly for industrial production; desalination is necessary. This is especially important in regions like the Middle Eastern deserts, where water is more expensive than electricity, making desalination a valuable research area. There are two main methods for industrial-scale seawater desalination: distillation and membrane methods. Distillation uses high temperatures to produce fresh water, resulting in high energy consumption and low yield. Membrane methods use membrane filtration, but involve high investment costs, difficult membrane cleaning, short lifespan, and the use of chemicals for cleaning, leading to secondary pollution. Both methods suffer from high production costs.

[0003] In recent years, seawater desalination via freezing has attracted widespread attention. Ice is a monolithic mineral and cannot coexist with other substances. Therefore, during the crystallization process, water removes impurities to maintain its purity. Seawater desalination via freezing utilizes this principle, freezing high-salinity seawater through a series of processes, causing ice crystals to release salt and transform into freshwater. However, traditional seawater desalination via freezing only utilizes a single temperature point on the solid-liquid phase diagram of seawater, resulting in low yield and high salinity of the freshwater.

[0004] Seawater desalination methods using freezing include direct contact, vacuum, and indirect methods. Chinese patent CN104803433B discloses a method for freezing and concentrating saline wastewater. The freezing and concentrating device includes a crystallizer, compressor, air cooler, water cooler, and heat exchanger. In the crystallizer, cold brine and liquid refrigerant directly contact and exchange heat, separating ice crystals and brine. Because the refrigerant is recycled, the system must be strictly sealed; otherwise, leaks can lead to refrigerant accumulation and safety hazards. Furthermore, although refrigerant and water are immiscible, incomplete separation can contaminate the freshwater, rendering it unusable for industrial applications. Chinese patent CN105600859 discloses a frozen seawater desalination crystallizer, which primarily achieves seawater freezing and crystallization through continuous heat exchange between seawater inside the crystallization tube and the refrigerant outside. Its disadvantage is low heat transfer efficiency due to heat transfer through the tube walls, resulting in a low system output rate. Summary of the Invention

[0005] This invention aims to address the shortcomings of existing technologies by providing a frozen seawater desalination system and method. The system controls the temperature difference of seawater within the crystallizer to change gradually in a stepwise manner. Seawater can complete the freezing, ice crystal growth, purification, and separation processes within the crystallizer. The system operates continuously, has a high water output rate, and produces freshwater with low salinity, making it suitable for industrial applications.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A frozen seawater desalination system includes a crystallizer, which includes an inlet pipe, a feed end, a discharge end, and refrigeration pipes. Further, the crystallizer is divided into multiple sections from the feed end to the discharge end, and the refrigeration pipes in each section form a refrigeration pipe group. The refrigeration capacity of each refrigeration pipe group is independently controlled.

[0008] The freezing temperature of brine is related to its concentration. When the brine concentration is below a certain value, its freezing temperature decreases with increasing concentration; conversely, when the concentration exceeds this value, the freezing temperature increases with increasing concentration. Freshwater has a low freezing temperature, while brine has a high freezing temperature; the higher the brine concentration, the higher the freezing temperature. Controlling the occurrence of seawater desalination in the liquid-solid coexistence zone is crucial for ensuring the yield and salinity of the desalinated water. This invention divides the crystallizer into multiple sections from the feed end to the discharge end. Each section contains refrigeration tubes forming a refrigeration tube group, and the refrigeration capacity of each group is independently controlled. This controls the temperature of the crystallizer to decrease gradually from the feed end to the discharge end, resulting in a step-like gradual change in temperature difference within the crystallizer. Seawater crystallizes in each section of the crystallizer. This system has a high yield and low salt content in the ice crystals, meeting the requirements for industrial water.

[0009] Furthermore, the refrigeration tube assembly includes multiple refrigeration tube loops, with the number of refrigeration tube loops in each segment increasing sequentially from the feed end to the discharge end of the crystallizer. By adjusting the number of refrigeration tube loops in each segment of the crystallizer, the cooling capacity of each segment of the crystallizer is adjusted. When the number of refrigeration tube loops is large, the cooling capacity of that segment is large and the temperature is low; when the number of refrigeration tube loops is small, the cooling capacity of that segment is relatively small and the temperature is relatively high. This controls the temperature in the crystallizer to decrease in a gradient from the feed end to the discharge end.

[0010] Furthermore, the diameter of the cooling pipe in each section increases sequentially from the feed end to the discharge end of the crystallizer; by adjusting the diameter of the cooling pipe in each section of the crystallizer, the cooling capacity of each section of the crystallizer is adjusted; when the diameter of the cooling pipe is large, the cooling capacity of that section is large and the temperature is low; when the diameter of the cooling pipe is small, the cooling capacity of that section is relatively small and the temperature is relatively high; thus controlling the temperature in the crystallizer to decrease in a gradient from the feed end to the discharge end.

[0011] Furthermore, the diameter of the refrigeration pipe is 25mm to 100mm.

[0012] Furthermore, the number of refrigeration pipe groups is ≥3, and the spacing between adjacent refrigeration pipe groups is >100mm.

[0013] Furthermore, the water inlet pipe extends into the crystallizer, and the water inlet pipe is provided with multiple small water inlet holes. The axial distance between the small water inlet holes from top to bottom is 100mm-200mm, and the circumferential spacing is <100mm. The small water inlet holes are arranged in different layers in a staggered manner, and the diameter of the small water inlet holes is 20-80mm. Through the small water inlet holes on the water inlet pipe, seawater enters the crystallizer from different heights and directions at different flow rates. Part of the seawater directly contacts the ice crystals and freezes, while part of the seawater is frozen into ice crystals during the downward flow. The seawater impacts the ice crystals, releasing the heat of solidification and preventing the ice crystals from freezing completely.

[0014] Furthermore, it also includes a solid-liquid separator, wherein the inlet of the solid-liquid separator is higher than the outlet, and its inclination rate is 0.5 to 1%.

[0015] Preferably, the refrigerant is hexafluoropropylene or perfluorohexanone, the refrigerant cooling intensity is greater than 400 kJ / kg, the refrigerant has the characteristics of low viscosity, high specific heat, non-corrosiveness, good safety performance, good low-temperature fluidity, large cooling and thermal conductivity, high refrigeration efficiency, and can be recycled.

[0016] Preferably, the crystallizer and the cooling pipes are made of stainless steel.

[0017] Based on the same inventive concept, the present invention also provides a method for desalination of frozen seawater, comprising the following steps:

[0018] 1) Transport seawater into the crystallizer;

[0019] 2) Control the temperature in the crystallizer to decrease in a gradient from the feed end to the discharge end, and control the temperature of each section to adapt to the solidification temperature of the seawater passing through that section.

[0020] 3) Separate ice crystals from the top of the crystallizer and discharge salt water from the bottom.

[0021] Preferably, before the seawater enters the crystallizer, it is pre-cooled to below 15°C through a cooling pipe outside the crystallizer.

[0022] Preferably, when the number of refrigeration tube groups is equal to 3, the temperatures of each section from the feed end to the discharge end in the crystallizer are 0±5℃, -15±5℃, and -30±5℃, respectively; the temperatures of each section from the feed end to the discharge end in the crystallizer are the seawater temperature at the feed end, the seawater temperature at the middle position of the crystallizer, and the seawater temperature at the discharge end, respectively.

[0023] Preferably, when the number of refrigeration pipe groups is equal to 3, the seawater temperature in the crystallizer increases uniformly from the discharge end to the middle position of the crystallizer; the seawater temperature increases non-uniformly from the middle position of the crystallizer to the feed end, and the temperature difference gradually decreases. Utilizing the gradually decreasing temperature difference within the crystallizer, ice crystals move upwards, resulting in solid-liquid separation and ice crystal purification; while the continuously decreasing temperature downwards promotes ice crystal formation, allowing for sufficient purification and formation time. Adjusting the ice crystals to a suitable size reduces their salinity and increases the system's water output rate.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] (1) This invention divides the crystallizer into multiple sections and independently controls the cooling capacity of each section, controlling the temperature to decrease gradually from the feed end to the discharge end within the crystallizer, maintaining a stable temperature gradient within the crystallizer, and enabling seawater to crystallize in each section of the crystallizer. This solves the problem that traditional freezing methods can only achieve single-point freezing. The system has a water output rate of over 80%, and the freshwater salinity is less than 400 mg / L, meeting the requirements for industrial water use.

[0026] (2) By using the small inlet holes on the inlet pipe of the crystallizer, seawater enters the crystallizer from different directions and heights at different flow rates, impacting the ice crystals, releasing the heat of solidification, preventing the ice crystals from freezing as a whole, and improving the system's water output rate. Attached Figure Description

[0027] Figure 1 This is a process flow diagram of the frozen seawater desalination system of the present invention;

[0028] Figure 2 This is a schematic diagram of the refrigeration pipe assembly;

[0029] In the diagram, 1-lift pump; 11-seawater pipe; 2-industrial water tank; 3-solid-liquid separator; 4-crystallizer; 41-inlet pipe; 42-refrigeration pipe assembly; 421-refrigeration pipe circuit; 43-grating overflow port; 44-temperature sensor; 45-saltwater outlet; 46-feed end; 47-discharge end; 5-compressor; 6-refrigerant regulating valve Detailed Implementation

[0030] The present invention will be described in detail below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other. For ease of description, the words "upper," "lower," "left," and "right" appearing below only indicate that they are consistent with the upper, lower, left, and right directions of the drawings themselves, and do not limit the structure.

[0031] Example 1

[0032] like Figure 1 , Figure 2As shown, a frozen seawater desalination system includes a crystallizer 4, a solid-liquid separator 3, a compressor 5, three refrigerant regulating valves 6, a booster pump 1, and an industrial water tank 2. The crystallizer 4 includes an inlet pipe 41, three refrigerant pipe groups 42, a grid overflow port 43, a temperature sensor 44, a saltwater outlet 45, a feed end 46, and a discharge end 47.

[0033] The crystallizer 4 is divided into three sections from the feed end to the discharge end. Each section is equipped with a refrigeration tube group 42. Each refrigeration tube group 42 is connected to a refrigerant regulating valve 6. The cooling capacity of each refrigeration tube group 42 is controlled independently.

[0034] From the feed end to the discharge end of crystallizer 4, the diameters of the refrigeration tubes in the three refrigeration tube groups are 25mm, 75mm and 100mm respectively, and the distance between adjacent refrigeration tube groups is 150mm.

[0035] The water inlet pipe 41 is provided with multiple water inlet holes. The axial distance between the water inlet holes from top to bottom is 100mm, and the circumferential spacing is 80mm. The water inlet holes are arranged in different layers in a staggered manner, and the diameter of each water inlet hole is 20mm.

[0036] The method for treating seawater using a frozen seawater desalination system includes the following steps:

[0037] Take 1m 3 Seawater (salinity 35‰) enters crystallizer 4 via lift pump 1 and coastal water pipe 11. Before entering crystallizer 4, the seawater is pre-cooled to below 15°C via refrigeration pipe outside crystallizer 4.

[0038] Pre-cooled seawater is continuously added from the water inlet pipe 4 at the top of the crystallizer, and enters the crystallizer 4 from different directions and heights at different flow rates through the water inlet holes on the water inlet pipe 41.

[0039] Three refrigerant regulating valves 6 are opened to control the temperature to decrease in a gradient from the feed end to the discharge end of the crystallizer. The seawater temperature at the feed end of the crystallizer is 0±5℃, the seawater temperature at the middle position of the crystallizer is -15±5℃, and the seawater temperature at the discharge end is -30±5℃. The opening of the refrigerant regulating valves 6 is adjusted to control the seawater temperature in the crystallizer to increase uniformly from the discharge end to the middle position of the crystallizer, with a temperature difference of 2℃. The seawater temperature in the crystallizer to increase non-uniformly from the middle position of the crystallizer to the feed end is controlled to gradually decrease the temperature difference. In this embodiment, 12 small segments are set between the middle position of the crystallizer and the feed end of the crystallizer. The seawater temperature difference between the first and third segments is 2℃, the seawater temperature difference between the fourth and sixth segments is 1.5℃, the seawater temperature difference between the seventh and ninth segments is 1℃, the seawater temperature difference between the tenth and eleventh segments is 0.5℃, and the seawater temperature difference between the twelfth segment is 0℃.

[0040] Seawater crystallizes in a crystallizer to produce saltwater and ice crystals. The ice crystals move upwards, and the salt inside the ice crystals melts and precipitates out. The saltwater flows downwards until it freezes back into ice crystals, which then move upwards again. This cycle continues, and the ice crystals are continuously purified while the seawater is continuously concentrated.

[0041] When the ice crystal size is ≤50mm×50mm×50mm, the ice crystal enters the solid-liquid separator 3 through the grid overflow port 43 for solid-liquid separation. The ice crystal enters the industrial water tank 2, and the saline water is returned to the crystallizer 4 for further processing.

[0042] The refrigerant absorbs heat through the refrigeration pipe assembly 42 and is then sent to the compressor 5. The compressed refrigerant is then recycled.

[0043] The seawater stays in the crystallizer for 1 hour. The ice crystals produced at the top of the crystallizer contain 380 mg / L of salt, and the salinity of the salt water at the bottom is 180‰. It can be calculated that the freshwater production rate is 80.8%.

[0044] Example 2

[0045] like Figure 1 , Figure 2 As shown, a frozen seawater desalination system includes a crystallizer 4, a solid-liquid separator 3, a compressor 5, three refrigerant regulating valves 6, a booster pump 1, and an industrial water tank 2. The crystallizer 4 includes an inlet pipe 41, three refrigerant pipe groups 42, a grid overflow port 43, a temperature sensor 44, a saltwater outlet 45, a feed end 46, and a discharge end 47.

[0046] The crystallizer 4 is divided into three sections from the feed end to the discharge end. Each section is equipped with a refrigeration tube group 42. Each refrigeration tube group 42 is connected to a refrigerant regulating valve 6. The cooling capacity of each refrigeration tube group 42 is controlled independently.

[0047] The diameter of the refrigeration tubes in the three refrigeration tube groups 42 is 50mm, and the distance between adjacent refrigeration tube groups is 150mm. From the feed end to the discharge end of the crystallizer 4, the number of refrigeration tube loops in the three refrigeration tube groups are 1, 3, and 5 respectively. The water inlet pipe 41 is provided with multiple water inlet holes. The axial distance between the water inlet holes from top to bottom is 100mm, and the circumferential spacing is 80mm. The water inlet holes are arranged in different layers in a staggered manner, and the diameter of the water inlet holes is 20mm.

[0048] The method for treating seawater using a frozen seawater desalination system includes the following steps:

[0049] Take 1m 3 Seawater (salinity 35‰) enters crystallizer 4 via lift pump 1 and coastal water pipe 11. Before entering crystallizer 4, the seawater is pre-cooled to below 10°C via refrigeration pipe outside crystallizer 4.

[0050] Pre-cooled seawater is continuously added from the water inlet pipe 4 at the top of the crystallizer, and enters the crystallizer 4 from different directions and heights at different flow rates through the water inlet holes on the water inlet pipe 41.

[0051] Three refrigerant regulating valves 6 are opened to control the temperature to decrease in a gradient from the feed end to the discharge end of the crystallizer. The seawater temperature at the feed end of the crystallizer is 0±5℃, the seawater temperature at the middle position of the crystallizer is -15±5℃, and the seawater temperature at the discharge end is -30±5℃. The opening of the refrigerant regulating valves 6 is adjusted to control the seawater temperature in the crystallizer to increase uniformly from the discharge end to the middle position of the crystallizer, with a temperature difference of 2℃. The seawater temperature in the crystallizer to increase non-uniformly from the middle position of the crystallizer to the feed end is controlled to gradually decrease the temperature difference. In this embodiment, nine small segments are set between the middle position of the crystallizer and the feed end of the crystallizer. The seawater temperature difference in the first to fifth segments is 2℃, the seawater temperature difference in the sixth segment is 1.5℃, the seawater temperature difference in the seventh segment is 1℃, the seawater temperature difference in the eighth segment is 0.5℃, and the seawater temperature difference in the ninth segment is 0℃.

[0052] Seawater crystallizes in a crystallizer to produce saltwater and ice crystals. The ice crystals move upwards, and the salt inside the ice crystals melts and precipitates out. The saltwater flows downwards until it freezes back into ice crystals, which then move upwards again. This cycle continues, and the ice crystals are continuously purified while the seawater is continuously concentrated.

[0053] When the ice crystal size is ≤50mm×50mm×50mm, the ice crystal enters the solid-liquid separator 3 through the grid overflow port 43 for solid-liquid separation. The ice crystal enters the industrial water tank 2, and the saline water is returned to the crystallizer 4 for further processing.

[0054] The refrigerant absorbs heat through the refrigeration pipe assembly 42 and is then sent to the compressor 5. The compressed refrigerant is then recycled.

[0055] The seawater stays in the crystallizer for 1.2 hours. The ice crystals produced at the top contain 300 mg / L of salt, and the concentrated brine discharged at the bottom contains 190‰ of salinity. The freshwater production rate is calculated to be 81.7%.

[0056] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A frozen seawater desalination system, comprising a crystallizer (4), said crystallizer (4) including an inlet pipe (41), a feed end, a discharge end, and a refrigeration pipe, characterized in that, The crystallizer (4) is divided into multiple sections from the feed end to the discharge end. The cooling tubes in each section form a cooling tube group (42), and the cooling capacity of each cooling tube group (42) is independently controlled. The refrigeration tube assembly (42) includes multiple refrigeration tube loops (421), and the number of refrigeration tube loops (421) in each section increases sequentially from the feed end to the discharge end of the crystallizer (4). The diameter of the cooling pipe in each section increases sequentially from the feed end to the discharge end of the crystallizer (4), thereby controlling the temperature in the crystallizer to decrease in a gradient from the feed end to the discharge end. The water inlet pipe (41) extends into the crystallizer (4). The water inlet pipe (41) is provided with multiple water inlet holes. The axial distance between the water inlet holes from top to bottom is 100mm-200mm, and the circumferential spacing is <100mm. The water inlet holes are arranged in different layers in a staggered manner. The diameter of the water inlet holes is 20-80mm.

2. The frozen seawater desalination system according to claim 1, characterized in that, The diameter of the refrigeration pipe is 25mm to 100mm.

3. The frozen seawater desalination system according to claim 1, characterized in that, The number of the refrigeration tube groups (42) is ≥3, and the distance between adjacent refrigeration tube groups is >100mm.

4. The frozen seawater desalination system according to claim 1, characterized in that, It also includes a solid-liquid separator (3), the inlet of which is higher than the outlet, and its inclination rate is 0.5 to 1%.

5. A method for desalinating frozen seawater, characterized in that, The process, using the frozen seawater desalination system according to any one of claims 1-4, includes the following steps: 1) Transport seawater into the crystallizer (4); 2) Control the temperature in the crystallizer (4) to decrease in a gradient from the feed end to the discharge end, and control the temperature of each section to adapt to the solidification temperature of the seawater passing through that section. 3) Separate ice crystals from the top of the crystallizer (4) and discharge salt water from the bottom.

6. The method for desalination of frozen seawater according to claim 5, characterized in that, When the number of refrigeration tube groups (42) is equal to 3, the temperatures of each section from the feed end to the discharge end in the crystallizer (4) are 0±5℃, -15±5℃, and -30±5℃, respectively. The temperatures of each section from the feed end to the discharge end in the crystallizer (4) are the seawater temperature at the feed end, the seawater temperature at the middle position of the crystallizer, and the seawater temperature at the discharge end, respectively.

7. The method for desalination of frozen seawater according to claim 5, characterized in that, When the number of refrigeration tubes (42) is equal to 3, the seawater temperature in the crystallizer (4) increases uniformly from the discharge end to the middle position of the crystallizer; the seawater temperature in the crystallizer (4) increases non-uniformly from the middle position of the crystallizer to the feed end, and the temperature difference gradually decreases.

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