A liquid semi-closed cycle seawater desalination system

Abstract

The present invention discloses a liquid semi-closed cycle seawater desalination system, which includes a dust collector, a fan, a dew point evaporation tower, a regenerative device, a heater, a fresh water storage tank, a feed liquid pump, a feed liquid tank and a concentrated brine pump. The dust collector is connected to the air inlet of the dew point evaporation tower through a pipeline via the fan. The regenerative device is connected to the dew point evaporation tower through a pipeline. The heater is arranged between the dew point evaporation tower and the regenerative device to heat the solution. A fresh water storage tank is arranged on the regenerative device to collect condensed water. The regenerative device is connected to the feed liquid tank through the feed liquid pump. The concentrated brine in the dew point evaporation tower is discharged through the concentrated brine pump from the liquid inlet. Through the mutual cooperation between the dew point evaporation tower and the regenerative device, the air mass flow rate in the upper and lower regions of the dew point evaporation tower changes, and different liquid-gas mass flow ratios are achieved at different position nodes of a single dew point evaporation tower, thereby increasing the matching degree of the gas-liquid operation line and improving the thermal efficiency of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of seawater desalination, and particularly to a liquid semi-closed cycle seawater desalination system. Background Art

[0002] The problem of water resource shortage has become a key factor restricting the sustainable development of human society. According to the prediction of the United Nations Water Resources Development Report, by 2050, the total global demand for fresh water will increase by 20% - 30%, and more than 2 billion people will live in countries or regions with highly scarce water resources.

[0003] Seawater desalination is an open-source and incremental technology for water resource utilization and is considered one of the most effective ways to solve the problem of water resource shortage. Currently, the mainstream seawater desalination technologies are mainly multi-stage flash evaporation, multi-effect evaporation, and reverse osmosis method. However, the existing technologies have problems such as high consumption of fossil energy and large investment costs. Therefore, there is an urgent need for a new type of low-cost seawater desalination technology. The dew point evaporation technology is a new type of evaporation technology. By simulating the natural rain cycle, it uses the moisture-carrying capacity of air to produce fresh water. The dew point evaporation technology can achieve evaporation at normal pressure and low temperature, so the requirements for equipment manufacturing and installation are low, and it has the advantages of low manufacturing cost and convenient later maintenance. It is especially suitable for being driven by low-grade renewable energy and can effectively avoid the disadvantages of high consumption of fossil energy. However, compared with conventional seawater desalination technologies, the water production efficiency and thermal efficiency of the dew point evaporation technology are relatively low. Summary of the Invention

[0004] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. In this part, as well as in the abstract and title of the present application, some simplifications or omissions may be made to avoid obscuring the purpose of this part, the abstract, and the title. However, such simplifications or omissions cannot be used to limit the scope of the present invention.

[0005] The technical problem to be solved by the present invention is how to improve the water production efficiency and thermal efficiency of seawater desalination using the dew point evaporation technology.

[0006] To solve the above technical problem, the present invention provides the following technical solution: A liquid semi-closed cycle seawater desalination system, which includes a dust collector, a fan, a dew point evaporation tower, a heat recovery device, a heater, a fresh water storage tank, a feed liquid pump, a feed liquid tank, and a concentrated brine pump. The dust collector is connected to the air inlet of the dew point evaporation tower through a pipeline via the fan. The heat recovery device is connected to the dew point evaporation tower through a pipeline. The heater is arranged between the dew point evaporation tower and the heat recovery device to heat the solution. A fresh water storage tank is arranged on the heat recovery device to collect condensed water. The heat recovery device is connected to the feed liquid tank through the feed liquid pump. The concentrated brine in the dew point evaporation tower is discharged through the concentrated brine pump from the liquid outlet.

[0007] As a preferred embodiment of the liquid semi-closed cycle seawater desalination system of the present invention, the following is provided: The regenerative device includes a secondary regenerator and a primary regenerator. The air inlet of the secondary regenerator is connected to the air outlet of the dew point evaporation tower, the air outlet of the secondary regenerator is connected to the air inlet of the primary regenerator. An air extraction port is provided in the middle of the dew point evaporation tower, and the air extraction port is connected to the gas pipeline connecting the secondary regenerator and the primary regenerator through a pipeline. The liquid inlet of the primary regenerator is connected to the feed liquid pump, the liquid outlet of the primary regenerator is connected to the liquid inlet of the secondary regenerator, and the secondary regenerator is connected to the liquid inlet of the dew point evaporation tower through a heater.

[0008] As a preferred embodiment of the liquid semi-closed cycle seawater desalination system of the present invention, the following is provided: It further includes a brine discharge valve and a brine reflux valve. The concentrated brine pump discharges the concentrated brine through the brine discharge valve, and the concentrated brine pump is connected to the feed liquid tank through the brine reflux valve. By controlling the opening and closing of the brine discharge valve and the brine reflux valve, the circulation of brine in the system can be adjusted to an open cycle or a semi-closed cycle according to the changes in the air outlet temperature of the air outlet of the secondary regenerator and the air outlet temperature of the air extraction port. The semi-closed cycle can reduce the overall entropy increase of the system by partial brine reflux and improve the thermal efficiency of the system.

[0009] As a preferred embodiment of the liquid semi-closed cycle seawater desalination system of the present invention, the following is provided: The dew point evaporation tower is a packed tower. The packing area in the packed tower is divided into upper and lower regions, and the air extraction port is located in the middle of the two packing regions to facilitate the adjustment of the air mass flow rate in the upper and lower regions of the dew point evaporation tower.

[0010] As a preferred embodiment of the liquid semi-closed cycle seawater desalination system of the present invention, the following is provided: After the concentrated brine is refluxed to the feed liquid tank through the brine reflux valve, the concentration of the circulating brine in the system is lower than 15%, avoiding the decrease in the evaporation rate in the dew point evaporation tower due to the too high concentration of the circulating brine in the system and further reducing the overall thermal efficiency of the system.

[0011] The beneficial effects of the present invention are as follows:

[0012] 1. By providing an air extraction port in the middle position of the dew point evaporation tower and extracting part of the air from the middle of the dew point evaporation tower, the air mass flow rate in the upper and lower regions of the dew point evaporation tower changes, and different liquid-gas mass flow ratios are achieved at different position nodes of a single dew point evaporation tower, thereby increasing the matching degree of the gas-liquid operating line and improving the thermal efficiency of the system;

[0013] 2. Only by using a single dew point evaporation tower with a packed tower structure can the adjustment of the liquid-gas flow ratio at different position nodes in the dew point evaporation tower be realized, simplifying the overall process of the system, effectively reducing the number of internal components and the number of tower bodies of the tower, and reducing the cost of equipment manufacturing;

[0014] 3. By partially recycling the concentrated brine flowing out of the dew point evaporation tower, the temperature difference between the air extracted from the dew point evaporation tower and the air at the outlet of the secondary recuperator is eliminated, further reducing the entropy increase of the system and improving the thermal efficiency of the system. Description of the Drawings

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. Among them:

[0016] Figure 1 It is a schematic process flow diagram of a liquid semi-closed cycle seawater desalination system.

[0017] Figure 2 It is a temperature-enthalpy operation curve graph of a multi-stage dew point evaporation concentration system.

[0018] Figure 3 It is a comparative curve graph of the thermal efficiency of a conventional system and a two-stage system.

[0019] Figure 4 It is a curve graph of the air temperature at the extraction point and the injection point changing with the reflux ratio.

[0020] Figure 5 It is a curve graph of the reflux ratio and the thermal efficiency change of a liquid semi-closed cycle seawater desalination system.

[0021] In the figure: 1. Dust collector; 2. Fan; 3. Dew point evaporation tower; 301. Air inlet; 302. Air extraction outlet; 303. Air outlet; 304. Liquid inlet; 305. Liquid outlet; 4. Secondary recuperator; 5. Primary recuperator; 6. Feed liquid tank; 7. Feed liquid pump; 8. Heater; 9. Concentrated brine pump; 901. Brine discharge valve; 902. Brine reflux valve; 10. Fresh water storage tank. Detailed Embodiments

[0022] To make the above objects, features, and advantages of the present invention more obvious and understandable, the detailed embodiments of the present invention will be described in detail below with reference to the drawings of the specification.

[0023] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0024] Secondly, the so-called "one embodiment" or "embodiment" herein refers to specific features, structures, or characteristics that may be included in at least one implementation manner of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or selectively exclusive embodiments from other embodiments.

[0025] Embodiment 1

[0026] Referring to Figures 1 to 3 , which is the first embodiment of the present invention. This embodiment provides a liquid semi-closed cycle seawater desalination system, including a dust collector 1, a fan 2, a dew point evaporation tower 3, a regenerative device, a heater 8, a fresh water storage tank 10, a feed liquid pump 7, a feed liquid tank 6, and a concentrated brine pump 9. The dust collector 1 is connected to the air inlet 301 of the dew point evaporation tower 3 through a pipeline via the fan 2. The regenerative device is connected to the dew point evaporation tower 3 through a pipeline. The heater 8 is arranged between the dew point evaporation tower 3 and the regenerative device to heat the solution. A fresh water storage tank 10 is arranged on the regenerative device to collect condensed water. The regenerative device is connected to the feed liquid tank 6 through the feed liquid pump 7. The concentrated brine in the dew point evaporation tower 3 is discharged through the concentrated brine pump 9 from the liquid outlet 305.

[0027] After the outside air enters the system through the air intake, it will enter the dust collector 1. The dust collector 1 removes the solid impurities contained in the air. The purified air will be sucked by the fan 2 and enter the dew point evaporation tower 3 through the air inlet 301 at the bottom of the dew point evaporation tower 3. The air flows from the bottom to the top of the dew point evaporation tower 3, makes full contact with seawater, undergoes heat and mass transfer, the air is heated up, the seawater is cooled down, the water evaporates and is carried away by the air. The high-temperature and high-humidity air enters the regenerative device, and the latent heat in the air is recovered by the incoming seawater that enters the regenerative device through the feed liquid tank 6 via the feed liquid pump 7, realizing the preheating of the seawater. At the same time, the water in the air condenses to produce condensed water and flows into the fresh water storage tank 10. A heater 8 is installed between the dew point evaporation tower 3 and the regenerative device. The seawater preheated in the regenerative device is heated by the heater 8 and then enters the dew point evaporation tower 3 through the liquid inlet 304, makes contact with the air in the dew point evaporation tower 3 and undergoes heat and mass transfer. The concentrated brine in the dew point evaporation tower 3 flows out through the liquid outlet 305 at the bottom and is discharged through the concentrated brine pump 9. The system cooperates with the dew point evaporation tower 3 and the regenerative device to improve the thermal efficiency of the system. And because the heat source temperature required by this system is relatively low, the heat required for operation can be provided by low-grade waste heat or renewable energy such as solar energy and geothermal energy, thereby reducing the consumption of fossil energy.

[0028] The regenerative device includes a secondary regenerator 4 and a primary regenerator 5. The air inlet of the secondary regenerator 4 is communicated with the air outlet 303 of the dew point evaporation tower 3. The air outlet of the secondary regenerator 4 is communicated with the air inlet of the primary regenerator 5. An air extraction port 302 is arranged in the middle of the dew point evaporation tower 3. The air extraction port 302 is communicated with the gas pipeline connecting the secondary regenerator 4 and the primary regenerator 5 through a pipeline. The liquid inlet of the primary regenerator 5 is communicated with the feed liquid pump 7. The liquid outlet of the primary regenerator 5 is communicated with the liquid inlet of the secondary regenerator 4. The secondary regenerator 4 is communicated with the liquid inlet 304 of the dew point evaporation tower 3 through a heater 8.

[0029] In a conventional dew point evaporation system, the differences in the gas-liquid temperature-enthalpy operating lines will lead to a relatively large increase in the system entropy, resulting in a low system thermal efficiency. As Figure 2 and Figure 3 shown, by setting multiple evaporators and regenerators and adjusting the liquid-gas mass flow ratio in different-stage evaporators and regenerators through solution shunting, the liquid operating line can be made to better fit the saturated air curve, improving the system thermal efficiency, but its structure is relatively complex.

[0030] The regenerative device in this embodiment mainly consists of a secondary regenerator 4 and a primary regenerator 5. After seawater enters the feed liquid tank 6, it is transported to the primary regenerator 5 through the feed liquid pump 7. The liquid outlet of the primary regenerator 5 is communicated with the liquid inlet of the secondary regenerator 4. The seawater flows out of the primary regenerator 5 and enters the secondary regenerator 4. It flows out of the liquid outlet of the secondary regenerator 4, is heated by the heater 8, and then enters the dew point evaporation tower 3 from the liquid inlet 304, where it directly contacts the air entering the dew point evaporation tower 3 and undergoes heat and mass transfer.

[0031] The high-temperature and high-humidity air at the top of the dew point evaporation tower 3 flows out from the air outlet 303 and enters the secondary regenerator 4. The seawater entering the secondary regenerator 4 recovers part of the latent heat in the air and preheats the incoming seawater at the same time. At this time, the high-temperature and high-humidity air cools down and generates some condensed water. The cooled air then flows out of the secondary regenerator 4 and mixes with the humid and hot air extracted from the dew point evaporation tower 3 by the air extraction outlet 302, so that the air flow rate in the secondary regenerator 4 is always less than that in the primary regenerator 5, that is, the liquid-gas mass flow ratio of the secondary regenerator 4 is always greater than that of the primary regenerator 5. At the same time, part of the air is extracted from the middle of the dew point evaporation tower 3, and the branched flow of the air makes the air mass flow rate in the upper and lower regions of the dew point evaporation tower 3 also change, thereby improving the matching degree of the gas-liquid operating line in the system, improving the system thermal efficiency, and the overall structure and process of the system are simpler. Only a single evaporator is used, reducing the manufacturing cost. The air flowing into the primary regenerator 5 exchanges heat further with the low-temperature seawater entering the primary regenerator 5. The air temperature drops further and more condensed water is generated. At the same time, the waste heat in the air is continuously recovered to reduce the system energy consumption. Subsequently, the air is directly discharged, and the fresh water generated by the primary regenerator 5 and the secondary regenerator 4 is collected in the fresh water storage tank 10.

[0032] Embodiment 2

[0033] Refer to Figures 1 to 5 , which is the second embodiment of the present invention, and this embodiment is based on the previous embodiment.

[0034] It also includes a brine discharge valve 901 and a brine reflux valve 902. The concentrated brine pump 9 discharges the concentrated brine through the brine discharge valve 901, and the concentrated brine pump 9 is connected to the feed liquid tank 6 through the brine reflux valve 902.

[0035] Under normal operating conditions, it is difficult to keep the air extraction temperature in the middle of the dew point evaporation tower 3 equal to the air temperature at the air outlet 303 of the secondary regenerator 4, and the mixing of gases at different temperatures will lead to an increase in entropy. The reflux of concentrated brine in the system can eliminate the temperature difference between the air extraction temperature in the middle of the dew point evaporation tower 3 and the air temperature at the air outlet 303 of the secondary regenerator 4. As Figure 4 shown, when the reflux ratio reaches Figure 4 , the air extraction temperature of the dew point evaporation tower 3 is equal to the air temperature at the outlet of the secondary regenerator 4. The temperature difference between the air extraction temperature of the dew point evaporation tower 3 and the air temperature at the outlet of the secondary regenerator 4 decreases, and the entropy increase of the system will also decrease accordingly. Therefore, as Figure 5 shown, the thermal efficiency of the system under this working condition will also increase with the increase of the reflux ratio. When the reflux ratio reaches Figure 4 , the thermal efficiency of the system also reaches the maximum and remains unchanged.

[0036] Therefore, in this embodiment, the concentrated brine at the bottom of the dew point evaporation tower 3 flows out from the liquid outlet 305 and is divided into two paths by the concentrated brine pump 9. One path is discharged through the brine discharge valve 901, and the other part enters the feed liquid tank 6 through the brine reflux valve 902. During the operation of the system, if the temperature of the air outlet 303 of the secondary regenerator 4 is greater than or equal to the temperature of the air outlet 303 of the air extraction port 302, the brine discharge valve 901 is opened and the brine reflux valve 902 is closed, and all the concentrated brine is discharged from the system through the brine discharge valve 901. At this time, the brine circulation is an open cycle. If the temperature of the air outlet 303 of the secondary regenerator 4 is less than the temperature of the air outlet 303 of the air extraction port 302, both the brine discharge valve 901 and the brine reflux valve 902 are opened. Part of the concentrated brine is discharged from the system through the brine discharge valve 901, and the other part flows into the feed liquid tank 6 through the brine reflux valve 902, mixes with the raw seawater and then re-enters the system for circulation. At this time, the brine circulation is a semi-closed cycle, with part of the concentrated brine refluxing and part being discharged from the system. At this time, the brine reflux valve 902 can be gradually adjusted to increase the brine reflux flow rate, thereby increasing the temperature of the air outlet 303 of the secondary regenerator 4, and gradually reducing the temperature difference between the air at the outlet of the secondary regenerator 4 and the air extracted from the air extraction port 302 until it is zero, so that the entropy increase during the mixing of the two air streams decreases, reducing the overall entropy increase of the system, so as to achieve the effect of further improving the thermal efficiency of the system.

[0037] The dew point evaporation tower 3 is a packed tower. The packing area in the packed tower is divided into upper and lower regions, and the air extraction port 302 is located in the middle of the two packing regions.

[0038] Since part of the wet air flows out from the air extraction port 302 in the middle of the dew point evaporation tower 3, a packed tower is selected as the dew point evaporation tower 3. The air extraction port 302 is in the middle of the two packing regions. After part of the air in the dew point evaporation tower 3 is extracted from the air extraction port 302, the air flow rates in the upper and lower packing regions of the dew point evaporation tower 3 are different. The air flow rate in the upper packing region is always less than the dry air flow rate in the lower packing region, that is, the liquid-gas mass flow ratio in the upper packing region is always greater than that in the lower packing region, thereby improving the matching degree of the gas-liquid operating line and the thermal efficiency of the system.

[0039] After the concentrated brine returns to the feed liquid tank 6 through the brine reflux valve 902, the concentration of the circulating brine in the system is lower than 15%.

[0040] When the salinity of the circulating seawater in the system is greater than 15%, the thermal efficiency of the brine semi-closed cycle is lower than that of the brine open cycle under some conditions. Therefore, the brine concentration in the system needs to be maintained within 15% to avoid the decrease of the specific heat capacity of the brine and the saturated vapor pressure of the water vapor due to the gradual increase of the salinity in the system, resulting in the decrease of the evaporation rate in the dew point evaporation tower 3 and the overall thermal efficiency of the system.

[0041] During operation, external air enters the system through the air suction port and then enters the dust collector 1. The dust collector 1 removes the solid impurities contained in the air. The air after removal is sucked by the fan 2 and enters the dew point evaporation tower 3 through the air inlet 301 at the bottom of the dew point evaporation tower 3. The air flows from the bottom to the top of the dew point evaporation tower 3 and comes into direct contact with seawater, resulting in heat and mass transfer. The air is heated up, the seawater is cooled down, and the water evaporates and is carried away by the air. The high-temperature and high-humidity air at the top of the dew point evaporation tower 3 flows out from the air outlet 303 and enters the secondary recuperator 4. The seawater entering the secondary recuperator 4 recovers part of the latent heat in the air and also preheats the incoming seawater. At this time, the high-temperature and high-humidity air is cooled down and some condensed water is generated;

[0042] The cooled air then flows out of the secondary recuperator 4 and mixes with the humid and hot air extracted from the dew point evaporation tower 3 through the air extraction port 302, so that the air flow rate in the secondary recuperator 4 is always less than that in the primary recuperator 5, that is, the liquid-gas mass flow ratio of the secondary recuperator 4 is always greater than that of the primary recuperator 5. At the same time, part of the air is extracted from the middle of the dew point evaporation tower 3, and the branched air flow makes the air mass flow rate in the upper and lower regions of the dew point evaporation tower 3 also change, thereby improving the matching degree of the gas-liquid operating line in the system, increasing the system thermal efficiency, and simplifying the overall structure and working process of the system; The air flowing into the primary recuperator 5 further exchanges heat with the low-temperature seawater that enters the primary recuperator 5 from the feed liquid tank 6 through the feed liquid pump 7. The air temperature further drops and more condensed water is generated. At the same time, the waste heat in the air is continuously recovered to reduce the system energy consumption. Subsequently, the air is directly discharged. The fresh water generated by the primary recuperator 5 and the secondary recuperator 4 is collected in the fresh water storage tank 10;

[0043] Meanwhile, the seawater preheated in the secondary regenerator 4 is heated by the heater 8, and then enters the dew point evaporation tower 3 through the liquid inlet 304, where it undergoes heat and mass transfer with the air and flows to the bottom of the dew point evaporation tower 3. It flows out through the liquid outlet 305 and is divided into two paths by the concentrated brine pump 9. One path is discharged through the brine discharge valve 901, and the other part enters the feed liquid tank 6 through the brine reflux valve 902; during the operation of the system, if the temperature of the air outlet 303 of the secondary regenerator 4 is greater than or equal to the temperature of the air outlet 303 of the air extraction port 302, the brine discharge valve 901 is opened and the brine reflux valve 902 is closed, and all the concentrated brine is discharged from the system through the brine discharge valve 901. At this time, the brine circulation is an open cycle; if the temperature of the air outlet 303 of the secondary regenerator 4 is less than the temperature of the air outlet 303 of the air extraction port 302, both the brine discharge valve 901 and the brine reflux valve 902 are opened. Part of the concentrated brine is discharged from the system through the brine discharge valve 901, and the other part flows into the feed liquid tank 6 through the brine reflux valve 902, mixes with the raw seawater and then re-enters the system for circulation. At this time, the brine circulation is a semi-closed cycle, with part of the concentrated brine refluxing and part being discharged from the system; at this time, the brine reflux valve 902 can be gradually adjusted to increase the brine reflux flow rate, thereby increasing the temperature of the air outlet 303 of the secondary regenerator 4, and gradually reducing the temperature difference between the air at the outlet of the secondary regenerator 4 and the air extracted from the air extraction port 302 until it is zero, so that the entropy increase during the mixing of the two air streams decreases, reducing the overall entropy increase of the system, in order to achieve the effect of further improving the thermal efficiency of the system.

[0044] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A liquid semi-closed cycle seawater desalination system, characterized in that: Comprising, a dust collector, a fan, a dew point evaporation tower, a regenerative device, a heater, a fresh water storage tank, a feed liquid pump, a feed liquid tank and a concentrated brine pump. The dust collector is connected to the air inlet of the dew point evaporation tower through a pipeline via the fan. The regenerative device is connected to the dew point evaporation tower through a pipeline. The heater is arranged between the dew point evaporation tower and the regenerative device to heat the solution. A fresh water storage tank is arranged on the regenerative device to collect condensed water. The regenerative device is connected to the feed liquid tank via the feed liquid pump. The concentrated brine in the dew point evaporation tower is discharged through the concentrated brine pump from the liquid outlet; The regenerative device includes a secondary regenerator and a primary regenerator. The air inlet of the secondary regenerator is connected to the air outlet of the dew point evaporation tower. The air outlet of the secondary regenerator is connected to the air inlet of the primary regenerator. An air extraction port is arranged in the middle of the dew point evaporation tower. The air extraction port is connected to the gas pipeline connecting the secondary regenerator and the primary regenerator through a pipeline. The liquid inlet of the primary regenerator is connected to the feed liquid pump. The liquid outlet of the primary regenerator is connected to the liquid inlet of the secondary regenerator. The secondary regenerator is connected to the liquid inlet of the dew point evaporation tower via the heater; It further includes a brine discharge valve and a brine reflux valve. The concentrated brine pump discharges the concentrated brine through the brine discharge valve. The concentrated brine pump is connected to the feed liquid tank via the brine reflux valve; After the concentrated brine flows back to the feed liquid tank through the brine reflux valve, the concentration of the circulating brine in the system is lower than 15%; If the air outlet temperature of the secondary regenerator is less than the air outlet temperature of the air extraction port, both the brine discharge valve and the brine reflux valve are opened. Part of the concentrated brine is discharged from the system through the brine discharge valve, and the other part flows into the feed liquid tank through the brine reflux valve and mixes with the raw seawater and then re-enters the system for circulation. At this time, the brine circulation is a semi-closed cycle, with part of the concentrated brine flowing back and part being discharged from the system. At this time, the brine reflux amount can be gradually increased by gradually increasing the brine reflux valve, so as to increase the air outlet temperature of the secondary regenerator and gradually reduce the temperature difference between the air at the air outlet of the secondary regenerator and the air extracted from the air extraction port until it is zero; If the air outlet temperature of the secondary regenerator is greater than or equal to the air outlet temperature of the air extraction port, the brine discharge valve is opened and the brine reflux valve is closed. All the concentrated brine is discharged from the system through the brine discharge valve. At this time, the brine circulation is an open cycle.

2. The liquid semi-closed cycle seawater desalination system according to claim 1, wherein: The dew point evaporation tower is a packed tower. The packing area in the packed tower is divided into upper and lower regions. The air extraction port is located in the middle of the two packing regions.

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