JP7627153B2 - Seawater Desalination Plant - Google Patents

Description

The present invention relates to a seawater desalination device that desalinates seawater.

Conventionally, the Multi-Stage Flash (MSF) method has been used as a technology for desalinating seawater. In the Multi-Stage Flash method, seawater is heated to evaporate (flash), and the evaporated water (steam) is cooled and condensed, a process that is repeated multiple times. As a result, the Multi-Stage Flash method requires an enormous amount of thermal energy for heating, and also wastes an enormous amount of thermal energy to the outside due to condensation. Therefore, the Multi-Stage Flash method not only requires high heating costs, but is also a factor in global warming.

As a result, the reverse osmosis (RO) method has been developed, in which high-pressure seawater is passed through a reverse osmosis membrane (see, for example, Patent Document 1).

JP 2020-203258 A

The reverse osmosis method described above has a low freshwater recovery rate of around 30%. Therefore, there is a need to develop technology that can improve the freshwater recovery rate.

In view of these problems, the present invention aims to provide a seawater desalination device that can improve the recovery rate of freshwater.

In order to solve the above problems, the seawater desalination apparatus of 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 chloride 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 gas hydrate, the crystalline chloride, and brine ; and a decomposition unit that heats the gas hydrate separated by the separation unit to decompose it into gas and water, wherein the production unit exchanges heat between the mixed liquid and a refrigerant, and the decomposition unit exchanges heat between the refrigerant heat-exchanged in the production unit and the gas hydrate .

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.

The seawater desalination apparatus may also include a freshwater supply section that supplies freshwater having a lower sodium chloride concentration than saltwater to the separation section to form a layer of freshwater above the saltwater layer.

The present invention makes it possible to improve the recovery rate of freshwater.

FIG. 1 is a diagram illustrating a schematic configuration of a seawater desalination apparatus. FIG. 2 is a diagram illustrating a schematic configuration of a pre-processing unit. 2 is a diagram illustrating a schematic configuration of a mixing section, a generating section, and a refrigerant cooling section. FIG. FIG. 2 is a diagram illustrating a schematic configuration of a separation unit. FIG. 2 is a diagram illustrating a schematic configuration of a decomposition unit. FIG. 2 is a diagram illustrating a schematic configuration of a purification section. FIG. 1 is a diagram showing the relationship between the sodium chloride concentration in salt water and the hydrate formation temperature and freezing temperature.

The preferred embodiment of the present invention will be described in detail below with reference to the attached drawings. The dimensions, materials, and other specific values shown in the embodiment are merely examples to facilitate 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 duplicated explanations, and elements not directly related to the present invention are not illustrated.

[Seawater desalination device 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.

The pretreatment section 110 removes impurities from the seawater. The mixing section 210 mixes seawater and mixed gas MG. The refrigerant cooling section 310 cools the refrigerant. The production section 220 cools the mixed liquid of seawater and mixed gas MG with a refrigerant to produce carbon dioxide hydrate H and crystalline chlorides. The separation section 410 separates 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 mixed gas MG. The purification section 710 produces decomposed water (water) obtained by decomposing the carbon dioxide hydrate H.

The central control unit 810 is composed of a semiconductor integrated circuit including a CPU (Central Processing Unit). The central control unit 810 reads 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.

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

[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.

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

The rough purification unit 112 removes impurities from seawater (e.g., 2°C to 28°C) and sterilizes the seawater. The rough purification unit 112 is composed of, for example, a filter and a germicidal lamp.

The suction side of the seawater boost pump 114 is connected to the crude 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 pressurizes (to, for example, 5.6 MPa) the seawater that has been sterilized and has had impurities removed by the crude purification section 112, and directs it to the reverse osmosis membrane filter 116.

The reverse osmosis membrane filter 116 recovers a portion of the fresh water from the seawater (approximately 30% of the seawater delivered from the seawater boost pump 114). The recovered fresh water is delivered to the water purification section 118. The water purification section 118 purifies the fresh water. The purified fresh water is used as drinking water, etc.

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. The seawater supply pipe 120 is provided with a safety valve 124, a flow rate adjustment valve 126, and a cold heat recovery section 510.

[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.

As shown in FIG. 3, seawater is guided from the pretreatment section 110 to the mixing section 210 through the seawater supply pipe 120. In addition, the seawater is cooled by the cold heat recovery section 510 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.

The mixing section 210 mixes (gas-liquid mixing) seawater, mixed gas MG, and 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 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 with the mixed gas MG and circulating salt water J introduced from the separation section 410 described below.

As a result, carbon dioxide hydrate H is produced by the carbon dioxide contained in the mixed gas MG and the water contained in the seawater. The hydrate production reaction between carbon dioxide and water (seawater) proceeds at the gas-liquid interface. Therefore, by the mixing section 210 mixing the microbubbles of the mixed gas MG with seawater, the contact area between the mixed gas MG and the seawater can be increased, making it possible to promote the hydrate production reaction.

The hydrate formation reaction is an exothermic reaction. Therefore, by increasing the contact area between the mixed gas MG and seawater in the mixing section 210, the reaction heat can be efficiently removed. This makes it possible to promote the hydrate formation reaction.

In addition, 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 generation 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.2H ₂ O, 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 (carbon dioxide hydrate H and sodium chloride dihydrate) produced in the generation section 220 can be set to 10 mass% or less. This makes it possible to prevent the solid-gas-liquid mixture delivery pipe 222 from being blocked.

[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 further progresses in the solid-gas-liquid mixture SL 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.

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 allows the production rate of carbon dioxide hydrate H to be 80% or more.

[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 .

The buffer tank 320 is connected to the generation unit 220 through the refrigerant return pipe 322. The buffer tank 320 is also connected to the separation unit 410 through the refrigerant return pipe 324. The buffer tank 320 stores the gaseous refrigerant led from the generation unit 220 through the refrigerant return pipe 322, and the gaseous refrigerant led 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 suction volume of the refrigerant compressor 330.

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 section 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 section 610.

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

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

The refrigerant supply pipe 360 connects the discharge side of the refrigerant compressor 330 to the decomposition section 610 (decomposition heat exchanger 622). A flow control valve 362 is provided on the refrigerant supply pipe 360. The central control unit 810 adjusts the opening of the flow 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 below. The refrigerant return pipe 364 connects the decomposition section 610 to the buffer tank 350.

The refrigerant supply pipe 370 connects the buffer tank 350 and the generation unit 220. A flow rate control valve 372 is provided on the refrigerant supply pipe 370. The central control unit 810 adjusts the opening 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).

The refrigerant supply pipe 380 connects the buffer tank 350 and the separation unit 410. A flow rate control valve 382 is provided on the refrigerant supply pipe 380. The central control unit 810 adjusts the opening of the flow rate control valve 382 so that the temperature of the solid-gas-liquid mixture SL in the solid-gas-liquid separator 412 of the separation unit 410 (described later) becomes the eutectic temperature (e.g., -9.6°C).

[Separation unit 410]
Fig. 4 is a diagram illustrating a schematic configuration of the 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 the gas phase (mixed gas MG).

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.

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.

A jet portion 414 is provided inside the large diameter portion 412a. The jet portion 414 jets out the mixed gas MG.

The solid-gas-liquid mixture delivery pipe 222 connects the generation section 220 to the lower part of 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 lower part of the ejection section 414 in the solid-gas-liquid separator 412.

The heat exchanger 420 cools the solid-gas-liquid mixture SL in the large diameter section 412a to the 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 generation 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.

Then, 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 carbon dioxide hydrate H is 1.15. The specific gravity of salt water is 1.20. The specific gravity of crystalline chloride is 1.69. As will be described in detail later, fresh water (specific gravity 1.0) with a lower sodium chloride concentration than salt water is supplied to the solid-gas-liquid separator 412. Therefore, salt water and fresh water are separated due to the difference in specific gravity. In other words, 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 dashed line in FIG. 4) is formed at the small diameter portion 412c.

Therefore, in the solid-gas-liquid separator 412, the carbon dioxide hydrate H floats to the interface between the fresh water (upper layer) and the salt water (lower layer) (shown by the dashed line in FIG. 4), 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 tiny 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 (associated water) of the carbon dioxide hydrate H from salt water to fresh water in the solid-gas-liquid separator 412.

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 located in the upper layer (-5°C or higher and 0°C or lower) is guided to the free water diluter 450 through a discharge nozzle provided in the small diameter section 412c. Therefore, the carbon dioxide hydrate H is accompanied by almost no salt water, but only fresh water. This makes it possible to reduce the sodium chloride concentration of the water generated in the subsequent decomposition section 610.

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.

The reaction to form carbon dioxide hydrate H proceeds in the gas phase at the gas-liquid interface between carbon dioxide and water, and the formed carbon dioxide hydrate H is separated from the reaction site. Therefore, under the conditions for forming carbon dioxide hydrate H, carbon dioxide hydrate H is continuously precipitated, and the carbon dioxide concentration in the gas phase decreases to the limit concentration for hydrate formation.

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 mixing section 210 by the return pump 440 described below, the carbon dioxide concentration 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 associated water discharged from the solid-gas-liquid separator 412 are this dissolution concentration.

Meanwhile, the settled 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 produced by the dehydrator 422 is returned to the mixing section 210 through the seawater supply pipe 120. The crystalline chloride (sodium chloride) dried by the dryer 424 is shipped as a product.

A flow control valve 426 is provided in the piping connecting the dehydrator 422 and the seawater supply pipe 120. The central control unit 810 adjusts the opening of the flow control valve 426 so as to maintain the dehydration efficiency of the dehydrator 422.

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

The suction side of the circulation compressor 430 is connected to the upper part of the small diameter section 412c in the solid-gas-liquid separator 412. The discharge side of the circulation compressor 430 is connected to the guest supply pipe 432 via the accumulator 430a. The guest supply pipe 432 is a pipe that connects the circulation compressor 430 and the mixing section 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 injected fresh water and the unreacted water in the seawater is positive (i.e., so that not all the water becomes 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 mixing section 210, the generation section 220, and the solid-gas-liquid separator 412 is 80% or more and 90% or less. Therefore, 10% to 20% of the unreacted water is discharged together with the fresh water to the free water diluter 450. Therefore, the central control unit 810 adjusts the opening of the flow control valve 434 so that the specific gravity of the saltwater layer above the ejection section 414 in the solid-gas-liquid separator 412 is constant.

The guest circulation pipe 436 is a pipe that connects the flow rate control valve 434 in the guest supply pipe 432 to the mixing section 210 and the ejection section 414. A flow rate control valve 438 is provided in the guest circulation pipe 436.

Since the solid-gas-liquid mixture SL led 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, by supplying the mixed gas MG to the solid-gas-liquid separator 412 through the ejection part 414, the reaction of producing carbon dioxide hydrate H in the solid-gas-liquid separator 412 is further promoted. As a result, the temperature of the freshwater layer can be maintained 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 control valve 438 so that the amount of mixed gas MG led to the solid-gas-liquid separator 412 is the amount necessary to keep the temperature of the upper layer at -5°C or higher and 0°C or lower (for example, about 10%).

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 ejection part 414.

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

The saltwater (e.g., -9.6°C) separated in the solid-gas-liquid separator 412 is returned to the mixing section 210 by the return pump 440. The suction side of the return pump 440 is connected below the ejection section 414 in the solid-gas-liquid separator 412. The discharge side of the return pump 440 is connected to a circulating saltwater supply pipe 442. The circulating saltwater supply pipe 442 is a pipe that connects the return pump 440 and the mixing section 210. The circulating saltwater supply pipe 442 is provided with a flow control valve 444. The central control unit 810 adjusts the opening of the flow control valve 444 so that the amount of saltwater (circulating saltwater J) introduced to the mixing section 210 is between 5 and 10 times the amount of seawater.

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") based on the difference in specific gravity.

In the free water diluter 450, weak salt water remains 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")) remains in the lower layer.

A liquid level gauge (specific gravity meter) not shown detects the interface between the thin saltwater layer and the carbon dioxide hydrate H layer. The stagnant water discharge pump 452 (fresh water supply unit) draws in the thin saltwater so that the interface between the thin saltwater layer in the free water diluter 450 and the carbon dioxide hydrate H layer remains at a constant position. A portion of the drawn in thin saltwater is guided to the seawater supply pipe 120 through the three-way valve 454. The remainder of the drawn in thin saltwater is returned to the free water diluter 450 and the solid-gas-liquid separator 412 through the connecting pipe 456 and the three-way valve 458.

On the other hand, the dehydrator 460 sucks in the carbon dioxide hydrate H and the free water, reduces the pressure (to about 0.07 MPa), and dehydrates the carbon dioxide hydrate H and the free water so that the interface between the thin saltwater layer in the free water diluter 450 and the carbon dioxide hydrate H layer 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 section 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 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 control valve 462 so as to maintain the dehydration efficiency of the dehydrator 460.

The guest supply unit 470 guides 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 the location where the gas phase is formed in the small diameter portion 412c of the solid-gas-liquid separator 412. 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.

[Cold heat recovery section 510]
The cold heat recovery section 510 exchanges heat between the dehydrated carbon dioxide hydrate H (-5°C or higher and 0°C or lower) and seawater (2°C or higher and 28°C or lower) passing through the seawater supply pipe 120. This causes the seawater to be cooled to about 4°C. Therefore, the cost required for cooling the seawater can be reduced.

The carbon dioxide hydrate H is also heated by the cold heat recovery section 510. This causes a portion of the carbon dioxide hydrate H to decompose, producing water (decomposition water). The decomposition water therefore functions as a lubricating liquid, facilitating the transfer of the carbon dioxide hydrate H from the cold heat recovery section 510 to the decomposition section 610.

[Decomposition unit 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.

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

The decomposer 620 is a cylindrical container. Inside the decomposer 620, a decomposition heat exchanger 622 is provided. 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 section 310. The decomposition heat exchanger 622 heats the carbon dioxide hydrate H by the heat of condensation of the refrigerant.

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

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.

The decomposition water that accumulates in the lower layer of the decomposer 620 is led to the purification section 710.

Meanwhile, the carbon dioxide remaining in the upper layer of the decomposer 620 is guided by the recovery compressor 630 to the guest supply pipe 432 of the separation section 410. 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 is possible to recover 99% of the carbon dioxide.

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 the guest supply pipe 432 of the separation section 410. In addition, a flow control valve 632 is provided in the piping connecting the recovery compressor 630 and the guest supply pipe 432 of the separation section 410.

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

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

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.

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 led to the water purification section 722. The water purification section 722 purifies the fresh water. The purified fresh water can be used as drinking water, etc.

Meanwhile, in the reverse osmosis membrane filter 720, a portion of the decomposition 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 the thin salt water discharged by the retained 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 decomposition water from which fresh water has been recovered to the solid-gas-liquid separator 412 and the free water diluter 450.

In addition, the remaining decomposition water after fresh water is recovered (3% to 5%) is discharged to the outside through the three-way valve 624.

[How to start]
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 portion 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 mixing unit 210.

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 generation unit 220, that is, so that the sodium chloride concentration in the salt water increases at a constant rate.

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% by 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.

The specific gravity of seawater (raw material) is smaller than that of carbon dioxide hydrate H. Therefore, in the early 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. In other words, in the early stage of the generation of carbon dioxide hydrate H, the carbon dioxide hydrate H sinks to the bottom of the seawater (raw material). Meanwhile, as the hydrate generation reaction progresses in the solid-gas-liquid separator 412, 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 formation reaction progresses further, the salinity of the seawater (salt water) increases further, and the specific gravity of the seawater (salt water) increases. This causes the seawater (salt water) to sink to the bottom of 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 bottom of the solid-gas-liquid separator 412, reducing the mixing of the carbon dioxide hydrate H into the circulating salt water J. Therefore, the outlet for the circulating salt water J should be located at a position about 20% by volume above the bottom of the solid-gas-liquid separator 412.

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).

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 the 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 shows the sodium chloride concentration [wt% (mass%)] in saltwater. In FIG. 7, the vertical axis shows temperature [°C]. In FIG. 7, the solid line shows the production temperature of carbon dioxide hydrate H, the dashed line shows the freezing temperature, and the dashed line shows the saturated solubility of sodium chloride. In other words, 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.

As shown in FIG. 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 mixing section 210, the production section 220, and the separation section 410, carbon dioxide hydrate H is produced in preference to freezing.

In addition, since the production temperature of carbon dioxide hydrate H hydrate is higher than the freezing temperature, the seawater desalination apparatus 100 can reduce the cooling load of the refrigerant by the refrigerant cooling unit 310 compared to freezing seawater.

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.

As described above, the seawater desalination apparatus 100 includes a seawater boost pump 114, a mixing section 210, a generating section 220, a separating section 410, a decomposing section 610, and a purifying section 710. This enables the seawater desalination apparatus 100 to continuously produce fresh water and salt.

In addition, as described above, carbon dioxide and nitrogen (mixed gas MG) are non-flammable. This allows the seawater desalination device 100 to be operated safely.

As described above, the seawater desalination apparatus 100 is equipped with a refrigerant cooling section 310 and a 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 using the heat of the refrigerant. This allows the seawater desalination apparatus 100 to reduce the thermal energy required to decompose the carbon dioxide hydrate H.

As described above, the seawater desalination apparatus 100 is also equipped with a cold heat recovery section 510. This allows seawater to be cooled by carbon dioxide hydrate H. This allows the seawater desalination apparatus 100 to reduce the energy required to cool seawater.

As described above, the seawater desalination apparatus 100 is also equipped with a return pump 440. This allows the seawater desalination apparatus 100 to improve the recovery rate of crystalline chlorides and decomposed water.

Furthermore, as described above, fresh water is supplied to the solid-gas-liquid separator 412. This allows the associated water of the carbon dioxide hydrate H to be replaced from salt water to 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.

As described above, the solid-gas-liquid separator 412 has a small diameter portion 412c, and the interface between the fresh water and 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) that are led to the free water diluter 450.

As described above, the mixing section 210, the generation section 220, and the solid-gas-liquid separator 412 are all at 2.7 MPa. This allows inexpensive parts to be used, reducing the manufacturing cost of the seawater desalination device 100 itself.

Although the preferred embodiment of the present invention has been described above with reference to the attached 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 come up with various modified or revised examples within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

For example, in the above embodiment, the pretreatment unit 110 is provided with a reverse osmosis membrane filter 116. However, the reverse osmosis membrane filter 116 is not a required component.

In the above embodiment, the generation unit 220 cools the mixed liquid with 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 equipment. 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.

In the above embodiment, the seawater desalination apparatus 100 is provided with a return pump 440. However, the return pump 440 is not a required component.

In the above embodiment, a configuration in which fresh water is supplied to the solid-gas-liquid separator 412 is given as an example. However, fresh water does not have to be supplied to the solid-gas-liquid separator 412.

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, there is no limitation on the guest substance of the hydrate. 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.

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. In other words, 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 device 100 on board a ship, etc.

100 Seawater desalination apparatus 210 Mixing section 220 Generation section 410 Separation section 440 Return pump 452 Accumulated water discharge pump (freshwater supply section)
610 Decomposition section 726 Fresh water supply pump (fresh water supply section)

Claims (4)

a mixing section 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 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 . 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. a mixing section 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 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: 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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