Seawater desalination plant

Abstract

To improve a recovery rate of fresh water.SOLUTION: A seawater desalination apparatus 100 includes: a mixing part 210 that generates a mixed liquid by mixing seawater and carbon dioxide; a generation part 220 that generates a solid-gas-liquid mixture containing carbon dioxide hydrate, a crystal chloride and salt water by cooling the mixed liquid to an eutectic temperature of the carbon dioxide hydrate and the crystal chloride; and a separation part 410 that separates the solid-gas-liquid mixture into carbon dioxide hydrate, a crystal chloride and salt water.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a seawater desalination apparatus for desalinating seawater. [Background technology]

[0002] Conventionally, the Multi-Stage Flash (MSF) method has been used as a technology for desalinating seawater. In the Multi-Stage Flash method, the process of heating seawater to evaporate (flash) the water, and then cooling and condensing the evaporated water (steam) is repeated multiple times. For this reason, the Multi-Stage Flash method requires an enormous amount of thermal energy for heating, and also requires an enormous amount of thermal energy to be wasted to the outside through condensation. Therefore, the Multi-Stage Flash method not only requires high heating costs, but is also a factor in global warming.

[0003] Therefore, a reverse osmosis (RO) method has been developed in which high-pressure seawater is passed through a reverse osmosis membrane (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-203258 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the reverse osmosis method, the recovery rate of fresh water is low at about 30%, so there is a demand for the development of technology that can improve the recovery rate of fresh water.

[0006] In view of the above problems, an object of the present invention is to provide a seawater desalination apparatus capable of improving the recovery rate of fresh water. [Means for solving the problem]

[0007] In order to solve the above problems, the seawater desalination apparatus according to the present invention is a method for producing seawater by 、 carbon dioxide , methane, ethane, propane, butane, ethylene, xenon, and krypton. A mixing section for mixing the above to generate a mixed liquid; gas Cool the mixture to the eutectic temperature of the hydrate and crystalline chloride. gas A generating unit for generating a solid-gas-liquid mixture containing hydrate, crystalline chloride, and salt water; gas A separation section for separating the hydrate, the crystalline chloride, and the salt water; a decomposition section that heats the gas hydrate separated by the separation section to decompose it into gas and water; Equipped with The production section exchanges heat between the mixed liquid and a refrigerant, and the decomposition section exchanges heat between the refrigerant that has been heat exchanged in the production section and the gas hydrate. do.

[0010] The seawater desalination apparatus may further include a return pump that returns the salt water separated by the separation section to the mixing section. In order to solve the above problems, another seawater desalination apparatus according to the present invention includes a mixing unit that mixes seawater with one or more gases selected from the group consisting of carbon dioxide, methane, ethane, propane, butane, ethylene, xenon, and krypton to produce a mixed liquid, a production unit that cools the mixed liquid to the eutectic temperature of the gas hydrate and the crystalline chlorides to produce a solid-gas-liquid mixture containing the gas hydrate, the crystalline chloride, and brine, a separation unit that separates the solid-gas-liquid mixture into the gas hydrate, the crystalline chloride, and brine, and a return pump that returns the brine separated by the separation unit to the mixing unit.

[0011] The seawater desalination apparatus may further include a freshwater supplying section that supplies freshwater having a lower sodium chloride concentration than saltwater to the separating section to form a freshwater layer above the saltwater layer. Effect of the Invention

[0012] According to the present invention, it is possible to improve the recovery rate of fresh water. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram illustrating a schematic configuration of a seawater desalination apparatus. [Diagram 2] FIG. 2 is a diagram illustrating a schematic configuration of a pre-processing unit. [Diagram 3] 2 is a diagram illustrating a schematic configuration of a mixing section, a generating section, and a refrigerant cooling section. FIG. [Figure 4] FIG. 2 is a diagram illustrating a schematic configuration of a separation unit. [Diagram 5] FIG. 2 is a diagram illustrating a schematic configuration of a decomposition unit. [Figure 6] FIG. 2 is a diagram illustrating a schematic configuration of a purification section. [Figure 7]FIG. 1 is a diagram showing the relationship between the sodium chloride concentration in salt water and the hydrate formation temperature and freezing temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. The dimensions, materials, and other specific values ​​shown in the embodiment are merely examples for facilitating understanding of the invention, and do not limit the present invention unless otherwise specified. In this specification and drawings, elements having substantially the same functions and configurations are given the same reference numerals to avoid repeated explanations, and elements not directly related to the present invention are not shown.

[0015] [Seawater desalination equipment 100] FIG. 1 is a diagram illustrating a schematic configuration of a seawater desalination apparatus 100. In FIG. 1, solid arrows indicate the flows of seawater, solid-gas-liquid mixture SL, carbon dioxide hydrate H, decomposition water, mixed gas MG, and circulating salt water J. In FIG. 1, dashed arrows indicate the flow of a refrigerant. As shown in FIG. 1, the seawater desalination apparatus 100 includes a pretreatment section 110, a mixing section 210, a generation section 220, a refrigerant cooling section 310, a separation section 410, a cold heat recovery section 510, a decomposition section 610, a purification section 710, and a central control section 810.

[0016] The pretreatment section 110 removes impurities from the seawater. The mixing section 210 mixes the seawater and the mixed gas MG. The refrigerant cooling section 310 cools the refrigerant. The production section 220 cools the mixed liquid of the seawater and the mixed gas MG with the refrigerant to produce carbon dioxide hydrate H and crystalline chlorides. The separation section 410 separates the carbon dioxide hydrate H and crystalline chlorides from the solid-gas-liquid mixture SL produced by the production section 220. The cold heat recovery section 510 recovers the cold heat of the carbon dioxide hydrate H with seawater and guides it to the mixing section 210. The decomposition section 610 decomposes the carbon dioxide hydrate H into water and the mixed gas MG. The purification section 710 produces decomposed water (water) obtained by decomposing the carbon dioxide hydrate H.

[0017] The central control unit 810 is composed of a semiconductor integrated circuit including a CPU (Central Processing Unit). The central control unit 810 reads out programs and parameters for operating the CPU itself from a ROM (Read Only Memory). The central control unit 810 manages and controls the entire seawater desalination apparatus 100 in cooperation with programs, a RAM (Random Access Memory) as a work area, and other electronic circuits.

[0018] The pretreatment section 110, the mixing section 210, the generation section 220, the refrigerant cooling section 310, the separation section 410, the cold heat recovery section 510, the decomposition section 610, and the purification section 710 will be described in detail below.

[0019] [Preprocessing unit 110] Fig. 2 is a diagram illustrating a schematic configuration of the pre-treatment unit 110. In Fig. 2, solid arrows indicate the flows of seawater and fresh water.

[0020] As shown in FIG. 2, the pretreatment section 110 includes a rough purification section 112 , a seawater boost pump 114 , a reverse osmosis membrane filter 116 , a clean water purification section 118 , and a seawater supply pipe 120 .

[0021] The rough purification unit 112 removes impurities from the seawater (for example, at a temperature between 2° C. and 28° C.) and sterilizes the seawater. The rough purification unit 112 is composed of, for example, a filter and a germicidal lamp.

[0022] The suction side of the seawater boost pump 114 is connected to the rough purification section 112. The discharge side of the seawater boost pump 114 is connected to the reverse osmosis membrane filter 116. The seawater boost pump 114 boosts the pressure of the seawater from which impurities have been removed and which has been sterilized by the rough purification section 112 (to, for example, 5.6 MPa) and guides it to the reverse osmosis membrane filter 116.

[0023] The reverse osmosis membrane filter 116 recovers a portion of the fresh water from the seawater (approximately 30% of the seawater guided from the seawater boost pump 114). The recovered fresh water is guided to the water purification section 118. The water purification section 118 purifies the fresh water. The purified fresh water is used as drinking water or the like.

[0024] Meanwhile, in the reverse osmosis membrane filter 116, the seawater from which some of the fresh water has been recovered (approximately 70% of the seawater led from the seawater boost pump 114) is depressurized to, for example, 2.7 MPa and led to the mixing section 210 through the seawater supply pipe 120. The seawater supply pipe 120 is a pipe that connects the reverse osmosis membrane filter 116 and the mixing section 210. A safety valve 124, a flow rate adjustment valve 126, and a cold heat recovery section 510 are provided in the seawater supply pipe 120.

[0025] [Mixing section 210] Fig. 3 is a diagram illustrating a schematic configuration of the mixing section 210, the generation section 220, and the refrigerant cooling section 310. In Fig. 3, solid arrows indicate the flows of seawater, the solid-gas-liquid mixture SL, the carbon dioxide hydrate H, the mixed gas MG, and the circulating salt water J. In Fig. 3, dashed arrows indicate the flow of the refrigerant.

[0026] As shown in FIG. 3, seawater is guided from the pretreatment section 110 to the mixing section 210 through the seawater supply pipe 120. The seawater is cooled by the cold heat recovery section 510, which will be described later, while passing through the seawater supply pipe 120. The mixed gas MG is guided from the separation section 410 to the mixing section 210 through the guest supply pipe 432. The mixed gas MG is composed of carbon dioxide and nitrogen. The carbon dioxide concentration in the mixed gas MG is, for example, 98 mol %, and the nitrogen concentration is, for example, 2 mol %. The circulating salt water J is guided from the separation section 410 to the mixing section 210 through the circulating salt water supply pipe 442.

[0027] The mixing section 210 mixes (gas-liquid mixes) seawater, the mixed gas MG, and the circulating salt water J (−13.6° C.). The mixing section 210 is, for example, a spiral line mixer (a mixer that rotates with the flow of the fluid to agitate the fluid). The mixing section 210 is configured with a mixer that distributes bubbles of the mixed gas MG (for example, microbubbles with a particle size of 10 μm to 40 μm) substantially evenly in the liquid phase (seawater). The pressure inside the mixing section 210 is, for example, 2.7 MPa. The mixing section 210 mixes seawater introduced from the seawater supply pipe 120, and the mixed gas MG and circulating salt water J introduced from the separation section 410 described later.

[0028] Then, carbon dioxide hydrate H is generated by the carbon dioxide contained in the mixed gas MG and the water contained in the seawater. The hydrate generation reaction between carbon dioxide and water (seawater) proceeds at the gas-liquid interface. Therefore, by the mixer 210 mixing the microbubbles of the mixed gas MG with seawater, the contact area between the mixed gas MG and seawater can be increased, and the hydrate generation reaction can be promoted.

[0029] In addition, the hydrate production reaction is an exothermic reaction. Therefore, by increasing the contact area between the mixed gas MG and the seawater in the mixer 210, the reaction heat can be efficiently removed. This makes it possible to accelerate the hydrate production reaction.

[0030] The amount of the circulating salt water J led from the separation section 410 is 5 to 10 times the amount of seawater led from the pretreatment section 110. In the production section 220 described later, when the solid-gas-liquid mixture SL produced in the mixing section 210 is cooled, carbon dioxide hydrate H is rapidly produced. Then, the amount of water in the solid-gas-liquid mixture SL decreases and the concentration of sodium chloride (NaCl) increases. When the concentration of sodium chloride reaches a eutectic concentration (for example, 24.5 mass%), sodium chloride dihydrate (NaCl·2H2O, crystalline chloride) precipitates. Therefore, by setting the amount of the circulating salt water J to 5 to 10 times the amount of seawater, the amount of solids produced in the production section 220 (carbon dioxide hydrate H and sodium chloride dihydrate) can be set to 10 mass% or less. This makes it possible to prevent clogging of the solid-gas-liquid mixture delivery pipe 222.

[0031] [Generation unit 220] The pressure in the generation section 220 is, for example, 2.7 MPa. The generation section 220 cools the solid-gas-liquid mixture SL introduced from the mixing section 210 through the solid-gas-liquid mixture delivery pipe 212 to the eutectic temperature of the carbon dioxide hydrate H and the crystalline chlorides (−9.6° C. when the pressure is 2.7 MPa). In this embodiment, the generation section 220 is a heat exchanger that exchanges heat between the solid-gas-liquid mixture SL and a refrigerant. The generation section 220 cools the solid-gas-liquid mixture SL and heats the refrigerant. The hydrate generation reaction in the solid-gas-liquid mixture SL is further advanced by being cooled by the generation section 220. As a result, the carbon dioxide hydrate H is further generated in the generation section 220. The solid-gas-liquid mixture SL containing the carbon dioxide hydrate H, the mixed gas MG, the crystalline chlorides, and the saltwater of the eutectic concentration, which is generated by the generation section 220, is introduced to the separation section 410 through the solid-gas-liquid mixture delivery pipe 222.

[0032] Moreover, the length of the production section 220 (the length through which the solid-gas-liquid mixture SL passes) is, for example, 60 m or more and 80 m or less. This makes it possible to make the production rate of carbon dioxide hydrate H 80% or more.

[0033] [Refrigerant cooling section 310] The refrigerant cooling unit 310 includes a buffer tank 320 , a refrigerant compressor 330 , a cooler 340 , and a buffer tank 350 .

[0034] The buffer tank 320 is connected to the generation unit 220 through a refrigerant return pipe 322. In addition, the buffer tank 320 is connected to the separation unit 410 through a refrigerant return pipe 324. The buffer tank 320 stores the gaseous refrigerant guided from the generation unit 220 through the refrigerant return pipe 322, and the gaseous refrigerant guided from the separation unit 410 through the refrigerant return pipe 324. By including the buffer tank 320, the refrigerant cooling unit 310 can suppress fluctuations in the suction pressure and the suction amount of the refrigerant compressor 330.

[0035] The suction side of the refrigerant compressor 330 is connected to the buffer tank 320. The discharge side of the refrigerant compressor 330 is connected to the cooler 340 and the decomposition unit 610. The refrigerant compressor 330 pressurizes the refrigerant. The refrigerant compressed by the refrigerant compressor 330 is guided to the cooler 340 and the decomposition unit 610.

[0036] The cooler 340 cools and condenses the refrigerant pressurized by the refrigerant compressor 330. The cooler 340 is configured, for example, by an air-cooling device or a water-cooling device.

[0037] The buffer tank 350 stores the refrigerant (liquid) condensed by the cooler 340 and the refrigerant (liquid) condensed in the decomposition unit 610. By including the buffer tank 350, the refrigerant cooling unit 310 can absorb pulsations (fluctuations in flow rate and pressure) of the refrigerant (liquid).

[0038] The refrigerant supply pipe 360 ​​connects the discharge side of the refrigerant compressor 330 and the decomposition section 610 (decomposition heat exchanger 622). A flow rate control valve 362 is provided in the refrigerant supply pipe 360. The central control unit 810 adjusts the opening degree of the flow rate control valve 362 so that carbon dioxide hydrate H is decomposed by the refrigerant passing through the decomposition heat exchanger 622 of the decomposition section 610 described later. The refrigerant return pipe 364 connects the decomposition section 610 and the buffer tank 350.

[0039] The refrigerant supply pipe 370 connects the buffer tank 350 and the generation unit 220. A flow rate control valve 372 is provided in the refrigerant supply pipe 370. The central control unit 810 adjusts the opening degree of the flow rate control valve 372 so that the temperature of the solid-gas-liquid mixture SL in the generation unit 220 becomes the eutectic temperature (−9.6° C.).

[0040] The refrigerant supply pipe 380 connects the buffer tank 350 and the separation unit 410. A flow rate control valve 382 is provided in the refrigerant supply pipe 380. The central control unit 810 adjusts the opening degree of the flow rate control valve 382 so that the temperature of the solid-gas-liquid mixture SL in a solid-gas-liquid separator 412 of the separation unit 410 described below becomes a eutectic temperature (for example, −9.6° C.).

[0041] [Separation section 410] Fig. 4 is a diagram illustrating a schematic configuration of separation unit 410. In Fig. 4, solid arrows indicate the flow of liquid and solid. In Fig. 4, dashed arrows indicate the flow of gas. In Fig. 4, dashed arrows indicate the flow of refrigerant. In Fig. 4, dashed lines indicate the interface between salt water and fresh water, and dashed lines indicate the interface between fresh water and gas phase (mixed gas MG).

[0042] As shown in FIG. 4, the separation section 410 includes a solid-gas-liquid separator 412, a heat exchanger 420, a dehydrator 422, a dryer 424, a circulation compressor 430, a return pump 440, a free water diluter 450, a dehydrator 460, and a guest supply section 470.

[0043] The solid-gas-liquid separator 412 is a cylindrical container. The solid-gas-liquid separator 412 includes a large diameter portion 412a, a reduced diameter portion 412b, and a small diameter portion 412c. The large diameter portion 412a is cylindrical. The reduced diameter portion 412b is continuous with the upper end of the large diameter portion 412a. The diameter of the reduced diameter portion 412b gradually decreases from the lower end to the upper end. The small diameter portion 412c is continuous with the upper end of the reduced diameter portion 412b. The small diameter portion 412c has a smaller diameter than the large diameter portion 412a.

[0044] The large diameter portion 412a is provided therein with a jet portion 414. The jet portion 414 jets out the mixed gas MG.

[0045] The solid-gas-liquid mixture delivery pipe 222 connects the generation section 220 to a portion below the ejection section 414 in the solid-gas-liquid separator 412 (large diameter section 412a). Therefore, the solid-gas-liquid mixture SL guided from the generation section 220 is supplied to the solid-gas-liquid separator 412 below the ejection section 414.

[0046] The heat exchanger 420 cools the solid-gas-liquid mixture SL in the large diameter section 412a to a eutectic temperature (e.g., −9.6° C.). The refrigerant supply pipe 380 and the refrigerant return pipe 324 are connected to the heat exchanger 420. The heat exchanger 420 exchanges heat between the refrigerant guided from the refrigerant cooling section 310 and the solid-gas-liquid mixture SL to cool the solid-gas-liquid mixture SL. This further promotes the hydrate production reaction in the large diameter section 412a. Then, when the temperature of the solid-gas-liquid mixture SL reaches the eutectic temperature (e.g., −9.6° C.), the concentration of sodium chloride in the salt water contained in the solid-gas-liquid mixture SL reaches the eutectic concentration (e.g., 24.5 mass%), and crystalline chloride precipitates.

[0047] In the solid-gas-liquid separator 412, the solid-gas-liquid mixture SL is separated into carbon dioxide hydrate H, crystalline chloride, and salt water due to the difference in specific gravity. Specifically, the specific gravity of the carbon dioxide hydrate H is 1.15. The specific gravity of salt water is 1.20. The specific gravity of the crystalline chloride is 1.69. As will be described later in detail, fresh water (specific gravity 1.0) having a lower sodium chloride concentration than salt water is supplied to the solid-gas-liquid separator 412. Therefore, the salt water and the fresh water are separated due to the difference in specific gravity. That is, in the solid-gas-liquid separator 412, a lower layer composed of salt water, an upper layer (-5°C or higher and 0°C or lower) composed of fresh water, and a gas phase located above the fresh water are formed. In this embodiment, the solid-gas-liquid separator 412 is designed so that the interface between the upper layer and the lower layer (indicated by a broken line in FIG. 4) is formed at the small diameter portion 412c.

[0048] Therefore, in the solid-gas-liquid separator 412, the carbon dioxide hydrate H rises to the interface (indicated by the dashed line in FIG. 4) between the fresh water (upper layer) and the salt water (lower layer), and the crystalline chloride sinks into the salt water. Approximately 4% of the carbon dioxide hydrate H is located in the fresh water (upper layer), and approximately 96% is located in the salt water. Also, since unreacted minute air bubbles adhere to the carbon dioxide hydrate H, the apparent specific gravity of the carbon dioxide hydrate H becomes lighter. Therefore, 4% or more of the carbon dioxide hydrate H is located in the upper layer (fresh water). The solid-gas-liquid separator 412 is designed so that the residence time of the carbon dioxide hydrate H in the upper layer is long enough to sufficiently replace the free water (produced water) of the carbon dioxide hydrate H from salt water to fresh water in the solid-gas-liquid separator 412.

[0049] Then, the position of the carbon dioxide hydrate H that has risen to the upper layer composed of fresh water is detected by a liquid level gauge (specific gravity meter) not shown, and the carbon dioxide hydrate H (-5°C or higher and 0°C or lower) located in the upper layer is guided to the free water diluter 450 through a discharge nozzle provided in the small diameter section 412c. Therefore, almost no salt water is accompanied by the carbon dioxide hydrate H, and only fresh water is accompanied. This makes it possible to reduce the sodium chloride concentration in the water generated in the subsequent decomposition section 610.

[0050] In addition to carbon dioxide, the mixed gas MG contains a small amount of nitrogen, which has an extremely low solubility in water. Therefore, as long as the carbon dioxide is within the conditions for the formation of carbon dioxide hydrate H (temperature, pressure, carbon dioxide content), carbon dioxide hydrate H will continue to be formed. In other words, carbon dioxide will be converted to carbon dioxide hydrate H until the carbon dioxide in the mixed gas MG falls to the critical fraction for hydrate formation.

[0051] The reaction for producing carbon dioxide hydrate H proceeds in the gas phase at the gas-liquid interface between carbon dioxide and water, and the produced carbon dioxide hydrate H is separated from the reaction field. Therefore, under the conditions for producing carbon dioxide hydrate H, carbon dioxide hydrate H is continuously precipitated, and the carbon dioxide concentration in the gas phase decreases to the hydrate production limit concentration.

[0052] The remaining carbon dioxide dissolves in equilibrium in the water that is not converted to carbon dioxide hydrate H. Therefore, when the saltwater is returned to the mixer 210 by the return pump 440 described below, the concentration of carbon dioxide dissolved in the saltwater converges to a dissolution concentration that is approximately in equilibrium with the hydrate formation limit concentration. Therefore, the carbon dioxide concentrations in the carbon dioxide hydrate H and the associated water discharged from the solid-gas-liquid separator 412 become this dissolution concentration.

[0053] Meanwhile, the precipitated crystalline chloride is dehydrated by the dehydrator 422. The crystalline chloride dehydrated by the dehydrator 422 is dried by the dryer 424. Meanwhile, the salt water generated by the dehydrator 422 is returned to the mixer 210 through the seawater supply pipe 120. The crystalline chloride (sodium chloride) dried by the dryer 424 is shipped as a product.

[0054] Further, a flow rate control valve 426 is provided in the pipe connecting the dehydrator 422 and the seawater supply pipe 120. The central control unit 810 adjusts the opening degree of the flow rate control valve 426 so as to maintain the dehydration efficiency of the dehydrator 422.

[0055] In addition, the unreacted mixed gas MG separated in the solid-gas-liquid separator 412 is returned to the mixer 210 and the ejection section 414 by the circulation compressor 430.

[0056] The suction side of the circulation compressor 430 is connected to an upper portion of the small diameter portion 412c in the solid-gas-liquid separator 412. The discharge side of the circulation compressor 430 is connected to a guest supply pipe 432 via an accumulator 430a. The guest supply pipe 432 is a pipe that connects the circulation compressor 430 and the mixer 210. The guest supply pipe 432 is provided with a flow control valve 434. The central control unit 810 adjusts the opening degree of the flow control valve 434 so that the total amount of the injected fresh water and the unreacted water in the seawater becomes positive (that is, so that all the water does not become carbon dioxide hydrate H in the solid-gas-liquid separator 412) in order to equalize the material balance in the solid-gas-liquid separator 412. The conversion rate of carbon dioxide hydrate H in the mixer 210, the generator 220, and the solid-gas-liquid separator 412 is 80% or more and 90% or less. Therefore, 10% to 20% of unreacted water is discharged together with the fresh water to the free water diluter 450. Therefore, the central control unit 810 adjusts the opening degree of the flow control valve 434 so that the specific gravity of the saltwater layer above the jetting part 414 in the solid-gas-liquid separator 412 becomes constant.

[0057] The guest circulation pipe 436 is a pipe that connects the portion between the flow rate control valve 434 and the mixer 210 in the guest supply pipe 432, and the jetting portion 414. A flow rate control valve 438 is provided in the guest circulation pipe 436.

[0058] Since the solid-gas-liquid mixture SL guided from the solid-gas-liquid mixture delivery pipe 222 is at the eutectic temperature (-9.6°C), if the carbon dioxide hydrate H continues to rise and reaches the freshwater layer (upper layer), the freshwater layer will freeze. Therefore, the mixed gas MG is supplied to the solid-gas-liquid separator 412 through the ejection part 414, thereby further promoting the reaction of producing carbon dioxide hydrate H in the solid-gas-liquid separator 412. This makes it possible to maintain the temperature of the freshwater layer at -5°C or higher and 0°C or lower by the reaction heat of the carbon dioxide hydrate H. The central control unit 810 adjusts the opening of the flow rate control valve 438 so that the amount of the mixed gas MG guided to the solid-gas-liquid separator 412 is an amount necessary to keep the temperature of the upper layer at -5°C or higher and 0°C or lower (for example, about 10%).

[0059] Incidentally, the carbon dioxide hydrate H located in the upper layer is partially decomposed, but is replenished with carbon dioxide hydrate H generated by the mixed gas MG supplied from the jetting portion 414.

[0060] In addition, the carbon dioxide hydrate H in the upper layer can be vibrated by ejecting the mixed gas MG from the ejection part 414. This makes it possible to promote the replacement of free water in the carbon dioxide hydrate H with fresh water.

[0061] Furthermore, salt water (for example, at -9.6°C) separated in the solid-gas-liquid separator 412 is returned to the mixing section 210 by a return pump 440. The suction side of the return pump 440 is connected to a lower side of the ejection section 414 in the solid-gas-liquid separator 412. The discharge side of the return pump 440 is connected to a circulating salt water supply pipe 442. The circulating salt water supply pipe 442 is a pipe that connects the return pump 440 and the mixing section 210. A flow rate control valve 444 is provided in the circulating salt water supply pipe 442. The central control section 810 adjusts the opening degree of the flow rate control valve 444 so that the amount of salt water (circulating salt water J) introduced to the mixing section 210 is between 5 and 10 times the amount of seawater.

[0062] The carbon dioxide hydrate H and the fresh water and salt water accompanying the carbon dioxide hydrate H are introduced to the free water diluter 450. The free water diluter 450 separates the carbon dioxide hydrate H from salt water with a low sodium chloride concentration (hereinafter referred to as "dilute salt water") due to the difference in specific gravity.

[0063] In the free water diluter 450, weak salt water accumulates in the upper layer. Also, a mixture of carbon dioxide hydrate H and free water (salt water with a high sodium chloride concentration (hereinafter referred to as "strong salt water")) accumulates in the lower layer.

[0064] A liquid level gauge (specific gravity meter) not shown detects the interface between the thin salt water layer and the layer of carbon dioxide hydrate H. The accumulated water discharge pump 452 (fresh water supply unit) sucks in the thin salt water so that the interface between the thin salt water layer and the layer of carbon dioxide hydrate H in the free water diluter 450 is at a constant position. A part of the sucked thin salt water is guided to the seawater supply pipe 120 through a three-way valve 454. The rest of the sucked thin salt water is returned to the free water diluter 450 and the solid-gas-liquid separator 412 through a connecting pipe 456 and a three-way valve 458.

[0065] On the other hand, the dehydrator 460 sucks the carbon dioxide hydrate H and free water, drops the pressure (about 0.07 MPa), and dehydrates them so that the interface between the thin saltwater layer and the carbon dioxide hydrate H layer in the free water diluter 450 is at a constant position. The dehydrator 460 reduces the produced water to 30 mass % or less, for example. The carbon dioxide hydrate H dehydrated by the dehydrator 460 is guided to the cold heat recovery unit 510. On the other hand, the produced water (thick saltwater) generated by the dehydrator 460 is returned to the seawater supply pipe 120. A flow rate control valve 462 is provided in the piping connecting the dehydrator 460 and the seawater supply pipe 120. The central control unit 810 adjusts the opening of the flow rate control valve 462 so as to maintain the dehydration efficiency of the dehydrator 460.

[0066] The guest supply unit 470 introduces the mixed gas MG to the solid-gas-liquid separator 412. The guest supply unit 470 includes a nitrogen cylinder 472, a carbon dioxide cylinder 474, a guest supply pipe 476, and a nitrogen supply pipe 478. The nitrogen cylinder 472 stores nitrogen. The carbon dioxide cylinder 474 stores carbon dioxide. The guest supply pipe 476 is a pipe that connects the carbon dioxide cylinder 474 to a portion of the small diameter portion 412c of the solid-gas-liquid separator 412 where a gas phase is formed. The guest supply pipe 476 is provided with a safety valve 476a and a flow rate control valve 476b. The nitrogen supply pipe 478 is a pipe that connects the nitrogen cylinder 472, the flow rate control valve 476b in the guest supply pipe 476, and the solid-gas-liquid separator 412. The nitrogen supply pipe 478 is provided with a safety valve 478a and a flow rate control valve 478b.

[0067] [Cold heat recovery section 510] The cold heat recovery section 510 exchanges heat between the dehydrated carbon dioxide hydrate H (-5°C or more and 0°C or less) and seawater (2°C or more and 28°C or less) passing through the seawater supply pipe 120. This cools the seawater to about 4°C. This makes it possible to reduce the cost required for cooling the seawater.

[0068] Furthermore, the carbon dioxide hydrate H is heated by the cold heat recovery section 510. As a result, a part of the carbon dioxide hydrate H is decomposed, and water (decomposition water) is generated. Therefore, the decomposition water functions as a lubricating liquid, and the carbon dioxide hydrate H can be easily transferred from the cold heat recovery section 510 to the decomposition section 610.

[0069] [Disassembly section 610] The decomposition section 610 heats the carbon dioxide hydrate H separated by the separation section 410 to decompose it into carbon dioxide and water. FIG. 5 is a diagram illustrating a schematic configuration of the decomposition section 610. In FIG. 5, solid arrows indicate the flows of the carbon dioxide hydrate H, carbon dioxide, and decomposed water. In FIG. 5, dashed arrows indicate the flow of the refrigerant. Also, in FIG. 5, the dashed line indicates the interface between the water and carbon dioxide.

[0070] As shown in FIG. 5, decomposition section 610 includes a decomposer 620 and a recovery compressor 630.

[0071] The decomposer 620 is a cylindrical container. A decomposition heat exchanger 622 is provided inside the decomposer 620. The decomposition heat exchanger 622 exchanges heat between the carbon dioxide hydrate H in the decomposer 620 and the refrigerant guided from the refrigerant cooling unit 310. The decomposition heat exchanger 622 heats the carbon dioxide hydrate H by the heat of condensation of the refrigerant.

[0072] The refrigerant from which heat has been removed (cooled) is temporarily stored in a liquid trap 366 and then returned to the buffer tank 350 through a refrigerant return pipe 364 .

[0073] On the other hand, the carbon dioxide hydrate H heated by the refrigerant is decomposed into carbon dioxide and water (decomposed water). Therefore, in the decomposer 620, the carbon dioxide forms an upper layer, and the decomposed water forms a lower layer.

[0074] The decomposed water retained in the lower layer of the decomposer 620 is led to the purification section 710.

[0075] Meanwhile, the carbon dioxide remaining in the upper layer of the decomposer 620 is guided to the guest supply pipe 432 of the separation section 410 by the recovery compressor 630. This allows the decomposition water to be degassed and the carbon dioxide dissolved in the decomposition water to be removed. In addition, by returning the carbon dioxide remaining in the upper layer of the decomposer 620 to the separation section 410, it becomes possible to recover 99% of the carbon dioxide.

[0076] The suction side of the recovery compressor 630 is connected to the upper part of the decomposer 620. The discharge side of the recovery compressor 630 is connected to a guest supply pipe 432 of the separation section 410. In addition, a flow rate adjustment valve 632 is provided in the piping connecting the recovery compressor 630 and the guest supply pipe 432 of the separation section 410.

[0077] [Refining Department 710] Fig. 6 is a diagram illustrating a schematic configuration of the purification section 710. In Fig. 6, solid arrows indicate the flow of water.

[0078] As shown in FIG. 6, the purification section 710 includes a decomposed water boost pump 712 , a reverse osmosis membrane filter 720 , a clean water purification section 722 , and a fresh water supply pump 726 .

[0079] The decomposition water boost pump 712 boosts the pressure of the decomposition water (water) generated in the decomposition heat exchanger 622 of the decomposition section 610, and guides it to the reverse osmosis membrane filter 720. The suction side of the decomposition water boost pump 712 is connected to the bottom of the decomposition heat exchanger 622. The discharge side of the decomposition water boost pump 712 is connected to the reverse osmosis membrane filter 720.

[0080] The reverse osmosis membrane filter 720 recovers fresh water with a salinity of 80 ppm or less from the decomposed water. The recovered fresh water is guided to the water purification section 722. The water purification section 722 purifies the fresh water. The purified fresh water is used as drinking water or the like.

[0081] Meanwhile, in the reverse osmosis membrane filter 720, a portion of the decomposed water from which fresh water has been recovered is returned as fresh water to the small diameter section 412c of the solid-gas-liquid separator 412 of the separation section 410 through the three-way valve 724 together with thin salt water discharged by the accumulated water discharge pump 452 (fresh water supply section). To be more specific, the suction side of the fresh water supply pump 726 (fresh water supply section) is connected to the three-way valve 724, and the discharge side is connected to the connection pipe 456 of the separation section 410. Therefore, the fresh water supply pump 726 guides a portion of the decomposed water from which fresh water has been recovered to the solid-gas-liquid separator 412 and the free water diluter 450.

[0082] The remainder of the decomposed water after the fresh water is recovered (between 3% and 5%) is discharged to the outside through the three-way valve 624.

[0083] [Startup method] Next, a method of starting up the seawater desalination apparatus 100 will be described. First, the central control unit 810 drives the seawater boost pump 114 of the pretreatment unit 110 to fill the solid-gas-liquid separator 412 with seawater (raw material). Then, when the seawater is filled up to a position slightly below the discharge nozzle provided in the small diameter unit 412c of the solid-gas-liquid separator 412, the central control unit 810 stops driving the seawater boost pump 114. Then, the central control unit 810 drives the return pump 440 to circulate the seawater in the solid-gas-liquid separator 412. In addition, the central control unit 810 drives the guest supply unit 470 to supply the mixed gas MG to the solid-gas-liquid separator 412, and drives the circulation compressor 430 to supply the mixed gas MG to the mixer 210.

[0084] Next, the central control unit 810 drives the refrigerant cooling unit 310. The central control unit 810 controls the refrigerant cooling unit 310 so that the carbon dioxide hydrate H is precipitated at a constant rate in the production unit 220, that is, so that the sodium chloride concentration in the saltwater increases at a constant rate.

[0085] When the temperature of the circulating salt water J reaches the eutectic temperature (-9.6°C) and the sodium chloride concentration of the circulating salt water J reaches 24.5 mass%, the central control unit 810 starts the seawater boost pump 114 and switches to normal operation. The central control unit 810 also switches the refrigerant cooling unit 310, the circulation compressor 430, and the return pump 440 to normal operation.

[0086] The specific gravity of seawater (raw material) is smaller than that of carbon dioxide hydrate H. Therefore, in the initial stage of the generation of carbon dioxide hydrate H, the seawater (raw material) rises to the top of the carbon dioxide hydrate H in the solid-gas-liquid separator 412. That is, in the initial stage of the generation of carbon dioxide hydrate H, the carbon dioxide hydrate H sinks to the bottom of the seawater (raw material). Meanwhile, in the solid-gas-liquid separator 412, as the hydrate generation reaction progresses, the water in the seawater (raw material) is converted to carbon dioxide hydrate H. Then, the salinity of the seawater in the solid-gas-liquid separator 412 increases. When the salinity of the seawater (salt water) in the solid-gas-liquid separator 412 reaches about 19% by mass (equilibrium temperature -4°C), the specific gravities of the seawater (salt water) and the carbon dioxide hydrate H become equal, and the carbon dioxide hydrate H is suspended in the seawater (salt water) in a one-phase state. As the hydrate production reaction progresses further, the salinity of the seawater (salt water) increases further, and the specific gravity of the seawater (salt water) increases. As a result, the seawater (salt water) settles below the carbon dioxide hydrate H. For this reason, the carbon dioxide hydrate H generated before the seawater (salt water) in the solid-gas-liquid separator 412 reaches the eutectic temperature is retained in the lower part of the solid-gas-liquid separator 412, thereby reducing the mixing of the carbon dioxide hydrate H into the circulating salt water J. Therefore, the discharge port for the circulating salt water J is preferably provided at a position approximately 20 volume % above the bottom of the solid-gas-liquid separator 412.

[0087] As described above, the seawater desalination apparatus 100 according to this embodiment can convert almost all of the seawater into freshwater by generating carbon dioxide hydrate H from seawater and decomposing the carbon dioxide hydrate H. Therefore, the seawater desalination apparatus 100 can improve the recovery rate of freshwater compared to a technology that employs only reverse osmosis. Therefore, the seawater desalination apparatus 100 can achieve zero emissions. In addition, the seawater desalination apparatus 100 can achieve almost zero liquid discharge (ZLD).

[0088] As described above, the seawater desalination apparatus 100 includes the mixing section 210, the production section 220, the refrigerant cooling section 310, and the separation section 410, and cools seawater and the mixed gas MG to produce carbon dioxide hydrate H and crystalline chloride in parallel. FIG. 7 is a diagram showing the relationship between the sodium chloride concentration in saltwater and the production temperature and freezing temperature of carbon dioxide hydrate H. In FIG. 7, the horizontal axis indicates the sodium chloride concentration [wt% (mass%)] in saltwater. In FIG. 7, the vertical axis indicates temperature [°C]. In FIG. 7, the solid line indicates the production temperature of carbon dioxide hydrate H, the dashed line indicates the freezing temperature, and the dashed line indicates the saturated solubility of sodium chloride. That is, the intersection of the solid line and the dashed line is the eutectic point (eutectic temperature) of carbon dioxide hydrate H and sodium chloride dihydrate, and the intersection of the dashed line and the dashed line is the eutectic point (eutectic temperature) of ice and sodium chloride dihydrate.

[0089] 7, regardless of the sodium chloride concentration, the production temperature of carbon dioxide hydrate H hydrate is higher than the freezing temperature (freezing point). Therefore, in the mixer 210, the production section 220, and the separation section 410, carbon dioxide hydrate H is produced in preference to freezing.

[0090] In addition, since the production temperature of carbon dioxide hydrate H hydrate is higher than the freezing temperature, the seawater desalination plant 100 can reduce the cooling load of the refrigerant by the refrigerant cooling unit 310 compared to the case where seawater is frozen.

[0091] As described above, the mixing section 210, the production section 220, and the separation section 410 are maintained at the eutectic temperature of the carbon dioxide hydrate H and the crystalline chlorides. Therefore, the seawater desalination apparatus 100 can produce the carbon dioxide hydrate H and the crystalline chlorides in parallel.

[0092] As described above, the seawater desalination apparatus 100 includes the seawater booster pump 114, the mixer 210, the generator 220, the separator 410, the decomposer 610, and the purifier 710. This enables the seawater desalination apparatus 100 to continuously produce fresh water and salt.

[0093] As described above, carbon dioxide and nitrogen (mixed gas MG) are non-flammable, so the seawater desalination plant 100 can be operated safely.

[0094] As described above, the seawater desalination apparatus 100 includes the refrigerant cooling section 310 and the decomposition heat exchanger 622. This allows the refrigerant to be cooled by the carbon dioxide hydrate H. This makes it possible to reduce the energy required to remove the heat of reaction of the carbon dioxide hydrate H. Furthermore, the decomposition section 610 can decompose the carbon dioxide hydrate H by the heat of the refrigerant. This allows the seawater desalination apparatus 100 to reduce the thermal energy required for decomposing the carbon dioxide hydrate H.

[0095] As described above, the seawater desalination apparatus 100 includes the cold heat recovery unit 510. This allows the seawater to be cooled by the carbon dioxide hydrate H. This allows the seawater desalination apparatus 100 to reduce the energy required to cool the seawater.

[0096] As described above, the seawater desalination plant 100 includes the return pump 440. This enables the seawater desalination plant 100 to improve the recovery rate of crystallized chlorides and decomposed water.

[0097] As described above, fresh water is supplied to the solid-gas-liquid separator 412. This allows the salt water associated with the carbon dioxide hydrate H to be replaced with fresh water. This makes it possible to reduce the sodium chloride concentration in the decomposition water obtained as a result of decomposing the carbon dioxide hydrate H.

[0098] As described above, the solid-gas-liquid separator 412 includes the small diameter portion 412c, and the interface between the fresh water and the salt water becomes the small diameter portion 412c. This makes it possible to suppress fluctuations in the composition of the carbon dioxide hydrate H and the free water (produced water) introduced to the free water diluter 450.

[0099] As described above, the pressure of the mixer 210, the generator 220, and the solid-gas-liquid separator 412 is 2.7 MPa. This allows inexpensive parts to be used, and the manufacturing cost of the seawater desalination apparatus 100 itself can be reduced.

[0100] Although the preferred embodiment of the present invention has been described above with reference to the accompanying drawings, it goes without saying that the present invention is not limited to such an embodiment. It is clear that a person skilled in the art can think of various modifications or alterations within the scope of the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

[0101] For example, in the above embodiment, the pretreatment unit 110 is provided with the reverse osmosis membrane filter 116. However, the reverse osmosis membrane filter 116 is not an essential component.

[0102] In the above embodiment, the generation section 220 cools the mixed liquid by the refrigerant cooled by the decomposition heat exchanger 622 exchanging heat with the carbon dioxide hydrate H. However, the decomposition of the carbon dioxide hydrate H and the cooling of the mixed liquid may be performed by different heat media or devices. For example, the decomposition of the carbon dioxide hydrate H may be performed by heat exchange with the combustion exhaust gas. Furthermore, the cooling of the mixed liquid may be performed by heat exchange with LNG.

[0103] In the above embodiment, the seawater desalination plant 100 includes the return pump 440. However, the return pump 440 is not an essential component.

[0104] In the above embodiment, fresh water is supplied to the solid-gas-liquid separator 412. However, fresh water does not have to be supplied to the solid-gas-liquid separator 412.

[0105] In the above embodiment, carbon dioxide is used as an example of the guest substance of the hydrate generated by the generation unit 220. However, the guest substance of the hydrate is not limited. For example, instead of or in addition to carbon dioxide, any one or more of methane, ethane, propane, butane, ethylene, xenon, and krypton may be used as the guest substance.

[0106] In the above embodiment, the separation unit 410 (solid-gas-liquid separator 412) separates the solid-gas-liquid mixture SL into carbon dioxide hydrate H, crystalline chlorides, and salt water based on the difference in specific gravity. However, the separation unit 410 (solid-gas-liquid separator 412) may be configured with a centrifuge. That is, the separation unit 410 (solid-gas-liquid separator 412) may centrifuge the solid-gas-liquid mixture SL into carbon dioxide hydrate H, crystalline chlorides, and salt water. This makes it possible to install the seawater desalination apparatus 100 inside a ship, etc. [Explanation of symbols]

[0107] 100 Seawater desalination equipment 210 Mixing section 220 Generation part 410 Separation section 440 Return Pump 452 Stagnant water discharge pump (fresh water supply section) 610 Disassembly section 726 Freshwater supply pump (freshwater supply section)

Claims

1. a mixing section for mixing seawater with one or more gases selected from the group consisting of carbon dioxide, methane, ethane, propane, butane, ethylene, xenon, and krypton to generate a mixed liquid; a generating unit that generates a solid-gas-liquid mixture containing the gas hydrate, the crystalline chloride, and salt water by cooling the mixed liquid to a eutectic temperature of the gas hydrate and the crystalline chloride; A separation section that separates the solid-gas-liquid mixture into the gas hydrate, the crystalline chloride, and the salt water; a decomposition section that heats the gas hydrate separated by the separation section to decompose the gas hydrate into gas and water; Equipped with The generating unit exchanges heat between the mixed liquid and a refrigerant, The decomposition section is a seawater desalination apparatus that exchanges heat between the refrigerant that has been heat exchanged in the production section and the gas hydrate.

2. The seawater desalination apparatus according to claim 1 , further comprising a return pump that returns the salt water separated by the separation section to the mixing section.

3. a mixing section for mixing seawater with one or more gases selected from the group consisting of carbon dioxide, methane, ethane, propane, butane, ethylene, xenon, and krypton to generate a mixed liquid; a generating unit that generates a solid-gas-liquid mixture containing the gas hydrate, the crystalline chloride, and salt water by cooling the mixed liquid to a eutectic temperature of the gas hydrate and the crystalline chloride; A separation section that separates the solid-gas-liquid mixture into the gas hydrate, the crystalline chloride, and the salt water; A return pump that returns the salt water separated by the separation section to the mixing section; A seawater desalination device comprising:

4. The seawater desalination apparatus according to claim 1 , further comprising a freshwater supply unit that supplies freshwater having a lower sodium chloride concentration than the saltwater to the separation unit to form a layer of freshwater above the layer of saltwater.

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