Salt production system using RO membrane

Abstract

To provide a salt production system with high efficiency using an RO membrane, which appropriately combines the RO membrane with an NF membrane to improve the problem of clogging caused by a scale (SO4) of the RO membrane present in seawater in the salt production system.SOLUTION: In the salt production system, seawater is first passed through an NF membrane to remove most of a scale (SO4) present in the seawater. Then, the seawater is concentrated using an RO membrane, and is further concentrated using a BC permeation unit employing the RO membrane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a salt production system that uses a reverse osmosis membrane method and a brine concentration method for seawater. [Background technology]

[0002] Conventional salt production methods include, for example, the reduced pressure salt production apparatus of Patent Document 1 and a method using an RO membrane and an NF membrane (Patent Document 2). Meanwhile, a salt production method has been studied that combines the reverse osmosis membrane method using an RO membrane with the brine concentration method using a semi-transparent membrane (Patent Document 3). However, the semi-transparent membrane becomes clogged with scale (SO4) present in seawater, making it difficult to put this method into practical use as a salt production method. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-359523 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-156173 [Patent Document 3] U.S. Patent No. 9,085,471 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention was made in consideration of the above circumstances, and aims to solve the problem of clogging of translucent membranes due to scale (SO4) present in seawater, and to provide a salt production system that is significantly more efficient in salt production than conventional production methods.

[0005] The inventors noticed that the NF membrane used in the salt production method disclosed in Patent Document 2 can remove approximately 98% of scale, and by appropriately combining this salt production method with an RO membrane, they developed a salt production system with significantly higher salt production efficiency. Furthermore, this salt production system provides salt with adjusted Ca and Mg contents, which is highly palatable and has consistent quality. [Means for solving the problem]

[0006] The invention according to claim 1 relates to a salt production system for obtaining salt from seawater, characterized in that the following steps 1 to 6 are carried out in order. (Step 1) Seawater is passed through an NF membrane to remove SO4 (Step 2) A step of concentrating the seawater that has permeated the NF membrane by a reverse osmosis membrane method, in which the seawater that has permeated the NF membrane is concentrated by passing it through an RO membrane to obtain a first concentrated water having a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by passing it through an RO membrane to obtain a second concentrated water having a Baume gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water using a vacuum concentrator to obtain a third concentrated water having a Baume specific gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, in which the third concentrated water is subjected to salt production using a salt production steam boiler. (Step 6) A step of further refining the salt obtained by the salt processing, in which the salt obtained by the salt processing is separated into salt and mineral water using a centrifugal dehydrator, thereby further refining salt.

[0007] The invention of claim 2 relates to the salt production system according to claim 1, characterized in that the average pore size of the NF membrane in step 1 is 10 nm or less.

[0008] The invention according to claim 3 relates to the salt production system according to any one of claims 1 and 2, characterized in that the seawater is deep-sea water. [Effects of the Invention]

[0009] According to the invention of claim 1, by carrying out the following steps 1 to 6 in order to create a salt production system using seawater, salt production can be performed with significantly higher efficiency than conventional salt production. This also has the effect of realizing a reduction in the amount of energy consumed in conventional salt production processes. (Step 1) Seawater is passed through an NF membrane to remove SO4 (Step 2) A step of concentrating the seawater that has permeated the NF membrane by a reverse osmosis membrane method, in which the seawater that has permeated the NF membrane is concentrated by passing it through an RO membrane to obtain a first concentrated water having a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by passing it through an RO membrane to obtain a second concentrated water having a Baume gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water using a vacuum concentrator to obtain a third concentrated water having a Baume specific gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, in which the third concentrated water is subjected to salt production using a salt production steam boiler. (Step 6) A step of further refining the salt obtained by the salt processing, in which the salt obtained by the salt processing is separated into salt and mineral water using a centrifugal dehydrator, thereby further refining salt.

[0010] According to the invention of claim 2, the average pore size of the NF membrane is limited to 10 nm or less, which allows for salt production with significantly higher efficiency than conventional methods, and also has the effect of realizing a further reduction in energy consumption compared to conventional salt production processes.

[0011] According to the invention of claim 3, by using deep seawater as seawater, it is possible to provide a salt production system capable of producing salt containing a large amount of mineral components. Deep sea water is seawater generally found at depths of 200 meters or more, and is low in bacteria and viruses, and contains many types of minerals in addition to Ca and Mg. The reason deep sea water contains so many of these components is because the water temperature is lower than that of surface water, and there is also less fluctuation in water temperature. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a flow diagram of the salt production system according to the present invention. [Figure 2] FIG. 2 is a schematic diagram of the equipment used in steps 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] The salt produced by the salt production system according to the present invention will now be described. The salt produced by the salt production system of the present invention is salt obtained from seawater, and has an adjusted Mg content per 100g of 3100mg to 5000mg, and an adjusted Ca content per 100g of 800mg to 3300mg. The salt concentration in seawater varies depending on the location, but is approximately 3.5%, and the magnesium in the salt exists in the form of MgCl2 and MgSO4. The proportion of Mg itself is around 4% of the total salt content. Calcium exists in the form of CaSO4, and the proportion of calcium itself is around 2% of the total salt content. This content makes the salt highly palatable. Deep ocean water is preferred as the seawater used to obtain such salt.

[0014] A mode (embodiment) for carrying out the salt production system according to the present invention will be described for each process with reference to the drawings. Figure 1 shows a flow diagram of the salt production system, and Figure 2 shows a schematic diagram of the equipment used in processes 1 to 3.

[0015] (Process 1) In step 1, seawater (1) is passed through an NF membrane (2) to remove SO4. Raw seawater with a Baume gravity of 3.5 is converted to seawater with a Baume gravity of 3.0.

[0016] NF (Nano Filtration) membranes are semi-transparent membranes with an average pore size of 10 nm or less, preferably 8 nm or less, and more preferably 5 nm or less. They allow water, low-molecular-weight organic substances, monovalent ions, etc. to pass through, but are difficult to pass medium- to high-molecular-weight organic substances, multivalent ions, etc. One example of a substance that does not easily permeate the NF membrane (2) is SO4. Because SO4 does not easily permeate the NF membrane (2), by passing seawater (1) through the NF membrane (2), the NF membrane permeate obtained has a significantly reduced SO4 content. Generally, Ca combines with SO4 to precipitate as CaSO4, but since the NF membrane permeate water has a low SO4 content, Ca cannot combine with SO4 and remains in the concentrated water in an ionic state even during the subsequent concentration process, etc. This allows for the production of salt and mineral water with a high Ca content.

[0017] Seawater is supplied to the NF membrane 2 by a motor and an NF membrane treatment pump. The NF membrane treatment pump can be any pump capable of supplying seawater at the required pressure, such as a multi-stage pump or a plunger pump.

[0018] The NF membrane tank (21) in which the NF membrane (2) is filled is, for example, a cylindrical tank having a diameter of 8 inches and a length of 1.6 m, and a flow rate of 74 m3 per 24 hours. 3 Two or more tanks having the above structure are arranged in series or in parallel for use. The NF membrane (2) is a synthetic polymer composite membrane or a cross-linked polyamide composite membrane, and is packed in the NF membrane tank (21) in the form of hollow fibers.

[0019] By passing seawater (1) through the NF membrane (2) twice, it is possible to remove SO4 that was not completely removed in the first pass, and it is possible to more reliably prevent calcium from precipitating in the subsequent concentration process. Even if the salt is passed through the NF membrane twice, the Mg and Ca contents do not decrease. Therefore, even if the salt is passed through the NF membrane twice, it can be obtained as a salt with a high Mg and Ca content and a highly palatable taste. The water may be passed through the NF membrane (2) two or more times. By passing the water through the membrane two or more times, SO4 can be removed more reliably, and the Mg and Ca contents are not reduced. Therefore, salt with high Ca and Mg contents and mineral water with high Ca content can be obtained.

[0020] In step 1, seawater with a Baume gravity of 3.0 is obtained from raw seawater with a Baume gravity of 3.5. The seawater is stored in the permeate storage tank (3).

[0021] The Baume gravity value (hereinafter sometimes referred to as "Be'") is a unit used to indicate the concentration of concentrated water in seawater treatment processes, etc., and the higher the Baume gravity value, the higher the concentration.

[0022] (Process 2) Step 2 is a step of concentrating the seawater (permeate) that has permeated the NF membrane (2) by a reverse osmosis membrane method, in which the seawater that has permeated the NF membrane (2) is concentrated by passing it through an RO membrane (4) to obtain a first concentrate having a Baume gravity value of 7.0 or more and 12.0 or less.

[0023] RO (Reverse Osmosis) membranes are semi-transparent membranes with an average pore size of approximately 0.5 nm, allowing water to pass through but limiting the permeation of other molecules, making them suitable for concentrating seawater.

[0024] The reverse osmosis membrane method is a method of concentrating seawater using a reverse osmosis membrane that only allows water and mineral components below a certain size to pass through. Specifically, seawater is pressured to pass through a reverse osmosis membrane, and is separated into permeate that has passed through the membrane and concentrated water that has been reduced in water content.

[0025] Seawater that has permeated the NF membrane (2) is supplied to the RO membrane (4) by an electric motor and an RO membrane treatment pump. The RO membrane treatment pump can be any pump that can supply seawater at the required pressure, and similar to the NF membrane treatment pump, a multi-stage pump, a plunger pump, or the like can be used.

[0026] The RO membrane tank (41) in which the RO membrane (4) is filled is, for example, a cylindrical tank having a diameter of 8 inches and a length of 1.6 m, and a flow rate of 74 m3 per 24 hours. 3 Three or more tanks each having the above structure are arranged in series or parallel for use. The RO membrane (4) is a cross-linked wholly aromatic polyamide composite membrane, and is packed in the RO membrane tank (41) in the form of hollow fibers.

[0027] Deep seawater is preferred as the seawater 1 supplied to the RO membrane 4. This is because deep seawater is rich in various mineral components, and therefore the salt obtained by this embodiment also contains a large amount of various mineral components. After multiple passes through the first NF membrane (2), the effluent may be concentrated by an RO membrane (4). Even if the salt is passed through the NF membrane (2) multiple times, the Mg and Ca contents do not decrease, and furthermore, SO4 can be removed more reliably, preventing the precipitation of Ca in the subsequent concentration step. Therefore, salt with high Ca and Mg contents can be obtained.

[0028] In step 2, a first concentrate having a Baume gravity of 7.0 or more and 12.0 or less is obtained from seawater having a Baume gravity of 3.0 that has permeated the NF membrane (2).

[0029] (Step 3) Step 3 is a step of further concentrating the first concentrated water by passing the first concentrated water through an RO membrane (5) to obtain a second concentrated water having a Baume gravity value of 13.0 or more and 20.0 or less.

[0030] In step 3, an apparatus (BC permeation apparatus) used in the brine concentration method, which is known as a method for concentrating saltwater, is used.

[0031] FIG. 2 shows a four-stage BC permeation device (51) in which four BC permeation devices (51) are connected in series. The BC permeation device (51) has an RO membrane (5) and a first chamber (52) and a second chamber (53) separated by the RO membrane (5). The first concentrate from step 2 is supplied to the first chamber (52), and the first chamber (52) is pressurized, causing the water contained in the first concentrate to permeate through the RO membrane and transfer to the second chamber (53). The first concentrate concentrated in the first chamber is recovered as the second concentrate. The BC permeation device (51) may be one unit, or a plurality of units may be connected in series.

[0032] The concentrated water coming out of the first chamber of the first-stage BC permeation device (51) is sent to each first chamber (52) in order to be supplied to the first chamber (52) of the second-stage BC permeation device (51) and concentrated there. The permeated water coming out of the second chamber (53) of the fourth-stage BC permeation device (51) at the rear end is sent to the second chamber (53) of the second-stage BC permeation device (51). The permeated water coming out of the second chamber (53) of the first-stage, second-stage, and third-stage BC permeation devices (51) is sent to the NF permeated water storage tank (3). By configuring the permeated water recovery path in this way, it is possible to recover the permeated water without using a pump.

[0033] The RO membrane (5) in step 3 is a semi-transparent membrane with an average pore size of about 0.5 nm, and produces a second concentrate with a higher concentration from the first concentrate that has permeated the RO membrane (4) in step 2 as raw water, and also discharges diluted water with a salt concentration of about 6.5%. The diluted water is returned to the NF permeate storage tank (3). The RO membrane (4) in step 2 and the RO membrane (5) in step 3 may be semi-transparent membranes having the same average pore size, or may be semi-transparent membranes having different average pore sizes.

[0034] Brine (highly salty water) such as the first concentrate that has permeated the RO membrane (4) in step 2 is supplied to the RO membrane (5) by an RO membrane treatment pump. The RO membrane treatment pump may be any pump that can supply brine at the required pressure, and may be a multi-stage pump, a plunger pump, or the like, as with the NF membrane treatment pump.

[0035] The BC permeation device (51) is, for example, a cylindrical device with a diameter of 10 inches and a length of 1.6 m, and a flow rate of 14 m3 per 24 hours. 3Four of the above devices are used in series. 3 If more water is to be discharged, a BC permeation device (51) is installed. The RO membrane (5) is a hollow fiber type made of cellulose triacetate and is packed in the BC permeation device (51).

[0036] By concentrating the brine twice or more times using the RO membrane (5), a more concentrated brine can be obtained.

[0037] From the first concentrated water concentrated in step 2, a second concentrated water having a Baume gravity value of 13.0 or more and 20.0 or less is obtained in step 3. The second concentrated water is stored in a concentrated water storage tank (6).

[0038] In the salt production apparatus of the present invention, approximately 98% of the scale (SO4) is removed by the NF membrane (2) in step 1, so the frequency of clogging of the RO membrane (5) in step 3 is low. However, when the salt production apparatus is stopped, it is necessary to perform maintenance by washing the RO membrane (5) with water using a cleaning system.

[0039] (Step 4) Step 4 is a step of further concentrating the second concentrated water obtained in step 3, using a vacuum concentrator (7) on the second concentrated water to obtain a third concentrated water having a Baume gravity value of 24.0 or more. The vacuum concentrator (7) may be, for example, a heat pump type.

[0040] Each step from process 1 to process 4 is controlled by a level sensor and operates automatically.

[0041] The vacuum concentrator (7) will be explained. The role of the vacuum concentrator (7) in this salt production apparatus is to reduce the pressure of the second concentrated water while heating it at a temperature of 80°C or less, to obtain a third concentrated water having a Baume gravity of 24.0 or more. The reason for setting the heating temperature of the second concentrated water to 80°C or less is that, although heating the second concentrated water to a temperature above 80°C shortens the concentration time, it is possible that trace components contained in the second concentrated water, such as umami components such as amino acids and sugars, may be altered, which could adversely affect the taste of the salt produced, and is therefore undesirable. The degree of pressure reduction for the second concentrated water is not particularly limited, but may be reduced to -94 KPa or less, preferably about -98 to -100 KPa. If the pressure is reduced to more than -94 KPa, it will take a long time to concentrate the second concentrated water, which is not practical. By cooling the water vapor generated in the process of concentrating the second concentrated water, distilled water can be recovered, and the recovered distilled water can be used as water for producing pharmaceuticals or alcohol.

[0042] In step 4, a third concentrate having a Baume gravity value of 24.0 or more is obtained from the second concentrate that has permeated the RO membrane (5). The third concentrate is stored in a water storage tank (8).

[0043] When Be' is 24.0 or more, the content of Na in the third concentrated water decreases and the contents of components such as Mg and Ca increase, i.e., the contents of Mg and Ca in the total mineral components increase.

[0044] (Step 5) Step 5 is a step of obtaining salt from the third concentrated water obtained in step 4, and is a step of producing salt from the third concentrated water using a salt-making steam boiler (9). For example, four salt-making steam boilers (9) each having a capacity of 900 L are used. Alternatively, for example, one salt-making steam boiler (9) having a capacity of 3600 L is used.

[0045] Step 5 yields brine with a Be' of 29.5%. For example, 360 kg of brine can be obtained from one salt production steam boiler (9) with a capacity of 900 L. Also, for example, 1440 kg of brine can be obtained from one salt production steam boiler (9) with a capacity of 3600 L.

[0046] (Step 6) Step 6 is a step in which the salt product obtained in step 5 is separated into salt and water (bittern) using a centrifugal dehydrator (10) for further salt production. For example, the centrifugal force generated by rotation at 1000-1200 rpm provides high dehydration efficiency, separating salt and water (bittern). For example, 200 kg of salt and 160 kg of bittern can be obtained from the 360 ​​kg of brine. Also, for example, 800 kg of salt and 640 kg of bittern can be obtained from the 1440 kg of brine.

[0047] In step 6, further refined salt and bittern are obtained.

[0048] Example 1 Table 1 shows the data when the order of permeation through the NF membrane and RO membrane is reversed. JPEG2025188002000002.jpg41151 Permeation through an NF membrane followed by permeation through an RO membrane is represented as (NF membrane → RO membrane), and permeation through an RO membrane followed by permeation through an NF membrane is represented as (RO membrane → NF membrane). Table 1 shows the following results: (1) In both (NF membrane → RO membrane) and (RO membrane → NF membrane), the Na, K, Mg, and Ca contents are higher than those in the raw water. (2) However, the Na, K, Mg, and Ca contents are higher in the (NF membrane → RO membrane) than in the (RO membrane → NF membrane) case. (3) The Baume scale (RO membrane → NF membrane) is 4.5, but (NF membrane → RO membrane) is higher at 6.4. For these reasons, (NF membrane → RO membrane) is more preferable than (RO membrane → NF membrane).

[0049] <Example 2> Table 2 shows test examples for the salt production system of the present invention. Table 2 shows data for seawater after steps 1, 2, and 3. JPEG2025188002000003.jpg54165 The permeation pressures in steps 1, 2, and 3 are as follows: Process 1 (NF membrane tank): 2Mpa or less Process 2 (RO membrane tank): 8Mpa or less Process 3 (BC permeation device): 7Mpa or less

[0050] Table 2 shows the following results: (1) After step 1, the Baume scale has decreased by 0.6. (2) After step 2, the Na, K, Mg, and Ca contents had increased to about twice the amount in the raw water, and the Baume degree had also increased to 8.2. (3) After step 3, the Na, K, Mg, and Ca contents have doubled compared to step 2, and the Baume scale has risen to 14.9. In this test, the BC permeation device (51) has four stages, but if it were to have six stages, the Baume scale would rise to 17-18. (Even with three stages, the Baume scale would rise to 10-12.) <Comparison of the present invention with other processes> For comparison with the present invention, the order of the NF membrane tank and RO membrane tank of the present invention was reversed, and the concentration was carried out in the order of RO membrane tank → NF membrane tank → BC permeation device, and also in the case of concentrating in the order of RO membrane tank → BC permeation device without using the NF membrane tank of the present invention. The results were as follows. 1. When concentrating in the order of RO membrane tank → NF membrane tank → BC permeation device, the Baume degree after the BC permeation device only rose to 7-8. 2. When concentration was performed in the order of RO membrane tank → BC permeation device, clogging occurred in the first and second stages of the BC permeation device, making it impossible to perform concentration work.

[0051] By combining steps 1, 2, and 3 in this way, the Baume degree can be increased, making it possible to produce salt without the energy and effort required in the subsequent steps 4 to 6. Conventionally, salt was produced by performing steps 4 to 6 without performing step 3 (BC permeation device), which required a lot of energy and effort. In the present invention, by combining the NF membrane, which can remove approximately 98% of scale as described above, with the RO membrane of the reverse osmosis method and the RO membrane of the BC permeation device, we have been able to obtain a salt production system with significantly higher salt production efficiency. [Industrial Applicability]

[0052] The salt obtained by the present invention is used as table salt. In particular, since the salt obtained has a highly palatable taste, it can be suitably used in a variety of dishes. Furthermore, the bittern obtained as a by-product according to the present invention can be suitably used as an additive for drinking water, a health food, or a plant activator for agricultural use. [Explanation of symbols]

[0053] 1 Seawater, raw seawater 2 NF membrane 21 NF membrane tank 3 Permeate storage tank 4. RO membrane 41 RO membrane tank 5. RO membrane 51 BC permeation device 52 Room 1 53 Room 2 6. Concentrated water storage tank 7. Vacuum concentration device 8. Water Tank 9. Salt steam boiler 10 Centrifugal dehydrator

Claims

1. A salt production system for obtaining salt from seawater, characterized by carrying out the following steps 1 to 6 in order: (Step 1) Seawater is passed through an NF membrane to separate SO ４ The process of removing (Step 2) A step of concentrating the seawater that has permeated the NF membrane by a reverse osmosis membrane method, in which the seawater that has permeated the NF membrane is concentrated by passing it through an RO membrane to obtain a first concentrated water having a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by passing it through an RO membrane to obtain a second concentrated water having a Baume gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water, using a vacuum concentrator to obtain a third concentrated water having a Baume gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, which involves producing salt from the third concentrated water using a salt-making steam boiler. (Step 6) A step of further refining the salt produced by using a centrifugal dehydrator to separate the salt from mineral water, thereby further refining the salt.

2. The salt production system described in claim 1, characterized in that the average pore size of the NF membrane in step 1 is 10 nm or less.

3. The salt production system according to any one of claims 1 and 2, characterized in that the seawater is deep seawater.

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