JP2008100220A - Method for producing freshwater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing freshwater having the quality of drinking water from salt water in high efficiency and at a high recovery rate. <P>SOLUTION: The method for producing freshwater from salt water by using a freshwater production apparatus provided with nanofiltration membrane units arranged in series in two or more stages and a desalination unit comprises the steps of: warming the salt water to be supplied to the first-stage nanofiltration membrane unit by using a heating unit; making the warmed salt water pass through the first-stage nanofiltration membrane unit; making the nanofiltrate from the first-stage nanofiltration membrane unit pass through the second-stage nanofiltration membrane unit; making the salt-concentrated water from the second-stage nanofiltration membrane unit or the succeeding unit pass through the desalination unit to produce desalted water; and mixing the desalted water with the nanofiltrate from the second-stage nanofiltration membrane unit or the succeeding unit to produce freshwater. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention produces water using a fresh water generator having a high recovery rate and a two-stage or more nanofiltration membrane unit device and a desalination unit device suitable for desalinating salt water to obtain fresh water. Regarding the method.

  In recent years, technologies for obtaining fresh water such as industrial water and drinking water from seawater have been developed, and the conventional evaporation methods, membrane separation methods typified by reverse osmosis membranes, and systems that combine them attract attention. It has become like this. Now, in the examination of this fresh water generation method, the technical focus is how to produce high quality water at low cost.

  As a study for lowering the water production cost, in Patent Document 1, the nanofiltration membrane with low operation pressure was arranged in two stages to obtain fresh water from seawater. The permeated water quality is 1000 mg / L in terms of evaporation residue (TDS), significantly exceeding the water quality standard value (500 mg / L), and sufficient drinking water quality is not obtained. In Patent Document 2, nanofiltration membranes are arranged in two stages, and the two-stage nanofiltration permeated water is treated by an evaporation method or a reverse osmosis membrane separation method, so that sufficient drinking water quality can be obtained. However, the recovery rate indicating how much water is produced from the salt water taken is significantly reduced, and the water production cost is greatly increased.

  Therefore, efforts are made to increase the recovery rate, but when operating at a high recovery rate, the concentration of the solute component on the concentrated water side of the reverse osmosis membrane increases, depositing a scale component with low solubility in water, In addition, turbid components contained in the supplied water are deposited on the membrane surface, causing problems such as a reduction in membrane life and permeate quality.

  And, the seawater supplied to the reverse osmosis membrane is pretreated with a nanofiltration membrane in advance, the scale component is removed to some extent, and a method for increasing the recovery rate has been developed and disclosed in Patent Documents 3 and 4, In Patent Documents 3 and 4, since the permeated water in the first stage of the nanofiltration membrane is processed by the reverse osmosis membrane separation method or the evaporation method, sufficient drinking water quality water quality can be obtained, but the recovery rate in the nanofiltration section ( 30-50%), and the recovery rate (40-60%) at the latter stage of nanofiltration, the overall recovery rate (-30%) does not increase as a result. Absent. Furthermore, in Patent Document 5, the permeated water of one stage of nanofiltration membrane is supplied to the reverse osmosis membrane, and conventionally the concentrated water of the reverse osmosis membrane to be discarded is further processed by the evaporation method to increase the amount of water produced, Since about 50% of the TDS of seawater remains in the permeated water of the nanofiltration membrane, an osmotic pressure of 1 to 2 MPa is generated when supplied to the reverse osmosis membrane. It is described as 1.5 to 2.5 MPa, and since a sufficient effective pressure cannot be obtained, a sufficient amount of permeated water cannot be obtained with a reverse osmosis membrane, resulting in a problem that the recovery rate does not increase sufficiently. was there.

JP 2005-524520 A JP 2006-21106 A JP 2003-170165 A Japanese translation of PCT publication No. 2003-507183 WO99 / 16714 pamphlet

  The object of the present invention is to solve the above-mentioned problems of the prior art, treat salt water in two or more stages in a nanofiltration membrane treatment process with low operating pressure, and concentrate water in the second and subsequent nanofiltration membrane treatment processes. It is to provide a fresh water generation method capable of obtaining drinking water-quality fresh water with a high recovery rate by treating the water in the desalting unit treatment step.

  In order to achieve the above object, the fresh water production method of the present invention is a method for producing fresh water from salt water by a fresh water production device in which nanofiltration membrane unit devices are arranged in series in two or more stages and further a desalination unit device is arranged. Then, the salt water before being supplied to the first-stage nanofiltration membrane unit apparatus is warmed using a heating device, and the heated salt water is passed through the first-stage nanofiltration membrane unit apparatus, and the first-stage nanofiltration membrane apparatus is passed through. The nanofiltration water from the membrane unit device is passed through the second-stage nanofiltration membrane unit device, and the nanofiltration concentrated water from the second-stage nanofiltration membrane unit device is passed through the desalination unit device to remove the desalted water. In addition, the demineralized water and the nanofiltration water from the second-stage nanofiltration membrane unit apparatus are mixed to obtain fresh water.

  According to the water production method of the present invention, salt water is treated in two or more stages in a nanofiltration membrane treatment process with low operating pressure, and the concentrated water in the second and subsequent nanofiltration membrane treatment processes is treated in a desalting unit. By combining these, high-quality fresh water that can be drunk can be produced at a high recovery rate and low cost.

  FIG. 1, FIG. 2, FIG. 3, and FIG. 4 are process schematic diagrams each showing a preferred embodiment of the desalting method of the present invention. In each figure, the same or equivalent components are denoted by the same reference numerals, and redundant description is omitted.

  In FIG. 1, the salt water taken is removed from the turbidity contained in the salt water by a turbidity removal device, and then the turbidity water that is the turbidity salt water is heated by the heating device 25, and the nanofiltration first stage pump 1 is supplied to the first-stage nanofiltration membrane module 2 in the first-stage nanofiltration membrane unit device 21. The nanofiltration water taken out from the first-stage nanofiltration membrane unit device 21 is supplied to the second-stage nanofiltration membrane module 4 in the second-stage nanofiltration membrane unit device 22 by the nanofiltration second-stage pump 3. In addition, the nanofiltration concentrated water discharged from the first-stage nanofiltration membrane unit device 21 is discharged out of the system.

  The nanofiltration concentrated water output from the second-stage nanofiltration membrane unit device 22 is supplied to the desalting unit device 23. Desalinated water is taken out from the desalting unit device 23, and further concentrated water is taken out.

  The nanofiltration water taken out from the second-stage nanofiltration membrane unit device 22 and the desalted water taken out from the desalination unit device 23 are mixed and taken out as fresh water.

  Here, salt water is water containing a significant concentration of scale components such as calcium ions, magnesium ions, bicarbonate ions, carbonate ions, sulfate ions, sodium ions, chloride ions, etc., for example, TDS concentration Is water exceeding 1,500 ppm, which is water called seawater, bay water or brine, but is not limited thereto. The TDS concentration refers to the evaporation residue concentration, and the evaporation residue includes sodium ion, calcium ion, magnesium ion, chloride ion, carbonate ion, sulfate ion and the like.

  In addition, fresh water refers to water that has a drinking water quality obtained by removing a considerable amount of ions contained in salt water. For example, the evaporation residue concentration is less than 500 ppm, and the chloride ion concentration is less than 200 ppm. Is water obtained by mixing nanofiltration water after the second stage, reverse osmosis membrane permeated water which is desalted water treated by the desalting unit device, and distilled water.

  The turbidity removal device is not particularly limited. For example, a contaminant removal device, a sand settling device, a bactericidal agent addition device, an agglomeration device, a pH adjustment device, a precipitation device, a pressurized flotation device, a decarbonation device, a sand filtration device. A device in which any one or more of pretreatment devices such as a membrane filtration device using a microfiltration membrane, an ultrafiltration membrane or the like is appropriately combined is preferably used.

  Although it does not restrict | limit especially about the heating apparatus 25 for heating turbidity water, A heat exchanger, such as a plate type, a double tube type, a multi-tube cylindrical type, and a spiral tube type, is mentioned. The heat source of the heat exchanger is not particularly limited as long as it has an amount of heat capable of warming the turbidity water, but is not limited to heated steam transported from a steam supply device, multistage flash distillation, multiple effect type Examples include waste heat generated from the desalination unit device 23 which is a distillation unit selected from the group consisting of distillation and vapor compression distillation.

  As the nanofiltration first-stage pump 1 and nanofiltration second-stage pump 3 for supplying water to the nanofiltration membrane unit device, various types of pumps such as a centrifugal pump, a turbine pump, and a plunger pump may be used. However, the present invention is not limited to these.

  For the nanofiltration membranes constituting the first-stage nanofiltration membrane module 2 and the second-stage nanofiltration membrane module 4, for example, the material is polyamide, polypiperazine amide, polyesteramide, or water-soluble vinyl polymer. The membrane structure has a dense layer on at least one side of the membrane, and has fine pores with gradually larger pore diameters from the dense layer to the inside of the membrane or the membrane on the other side. Those having a very thin separation functional layer (composite membrane) formed of another material on the dense layer of the asymmetric membrane can be used. However, a composite membrane is preferred for high water production during low-pressure operation, and among them, polyamide composite membranes and piperazine polyamide composite membranes are more preferable from the viewpoints of the amount of permeated water and chemical resistance.

  The first-stage nanofiltration membrane module 2 and the second-stage nanofiltration membrane module 4 have a flat membrane on both sides of a spiral element or a plate-type support plate in which a flat membrane-like membrane is wrapped around the water collecting pipe. Plate-and-frame type elements that are modularized by laminating objects at regular intervals through spacers, tubular elements using tubular membranes, and hollow fiber membranes that bundle hollow fiber membranes and store them in a case A single element or a plurality of elements are connected in series and accommodated in a pressure vessel. The element may be in any form, but it is preferable to use a spiral element from the viewpoint of operability and compatibility. The number of elements can be arbitrarily set according to the membrane performance. When spiral type elements are used, it is preferable that the number of elements loaded in one module is arranged in the order of 4 to 6 in series.

  Here, the nanofiltration membrane unit device is a nanofiltration membrane module arranged in series in one or more stages, piping for supplying saltwater to the nanofiltration membrane module, and saltwater by each stage nanofiltration membrane module. It is nanofiltered by a pipe for passing the concentrated nanofiltration concentrated water to the subsequent nanofiltration membrane module, a pipe for taking out the nanofiltration concentrated water from the last nanofiltration membrane module, and the nanofiltration membrane module at each stage. It is comprised with the piping which takes out nano filtration water, respectively. At this time, a plurality of nanofiltration membrane modules may be arranged in parallel at each stage. In the present invention, the first-stage nanofiltration membrane unit device, the first-stage nanofiltration membrane unit device, the second-stage nanofiltration membrane unit device, and the third-stage nanofiltration membrane unit device are referred to in this order. And Furthermore, when the second-stage nanofiltration membrane unit device is referred to, all or part of the nanofiltration membrane unit device from the second-stage nanofiltration membrane unit device to the last-stage nanofiltration membrane unit device is used. It means that.

  Further, different types of nanofiltration membrane elements may be used for each stage of nanofiltration membrane unit device or for each stage of nanofiltration membrane module constituting the nanofiltration membrane unit device.

  The first-stage nanofiltration membrane unit device 21 has a first-stage nanofiltration membrane module 2 including a nanofiltration membrane that removes scale components in the turbidity water obtained from the above-described turbidity removal apparatus. The filtration membrane unit device 21 is operated so that the recovery rate is in the range of 50 to 90%. In addition, the recovery rate of the first stage nanofiltration membrane unit device 21 should be as high as possible, but if it exceeds 90%, deposition of scale components such as calcium sulfate occurs on the nanofiltration membrane surface and the membrane is blocked. Therefore, it is not preferable. Further, the lower limit of the recovery rate is preferably up to 50%. When the recovery rate is lower than 50%, even if the recovery rate after the second stage is operated at a high recovery rate of 80 to 90%, when combined with the recovery rate of the first-stage nanofiltration membrane unit device 21, it is at most 40 to 45%. The recovery rate is only about a level, and the entire system cannot obtain fresh water at a high recovery rate. The more preferable range of the recovery rate of the nanofiltration membrane unit device is 60 to 90%, and more preferably 65 to 85%.

  There is no particular limitation as long as the nanofiltration membrane unit apparatus has an evaporation residue concentration elimination performance of 50 to 97% and a bivalent ion elimination performance of 80% or more. The exclusion performance was calculated by the following formula.

  Exclusion Performance (%) = (1−Nano Filtration Water Concentration / ((Supply Water Concentration + Nano Filtration Concentrated Water Concentration) / 2)) × 100

  The recovery rate is the ratio of the treated water flow rate to the supply water flow rate to each unit device. For example, in the nanofiltration membrane unit device, the nanofiltration water for the supply water, and in the desalination unit device 23, the dewatering for the supply water. It is the ratio of the flow rate of salt water and fresh water to salt water in the desalting method of the present invention.

  The operating pressure of the first-stage nanofiltration membrane unit device 21 is appropriately selected depending on the type of membrane, the quality and temperature of salt water, the recovery rate, etc., but is usually 0.1 to 7 MPa, preferably 0.5 to 4 MPa. The range is selected.

  Moreover, by adjusting the temperature of the turbidity water to be supplied to the first-stage nanofiltration membrane unit device 21 to warm water, the water permeability of the nanofiltration membrane is improved and the operating pressure of the first-stage nanofiltration membrane unit device 21 is reduced. Further, the temperature can be further lowered as compared with the case where the reverse osmosis membrane unit device is used instead of the nanofiltration membrane unit device without adjusting the temperature. This is a distillation unit that can reduce energy consumption and is selected from the group consisting of multi-stage flash distillation, multi-effect distillation, and vapor compression distillation as a heat source for adjusting the temperature of the turbid water. It is preferable to use waste heat generated from the desalting unit device 23 because energy consumption used for temperature control can be reduced.

  Normally, when the salt water temperature fluctuates annually, in order to keep the nanofiltration water flow rate of the first-stage nanofiltration membrane unit device 21 constant, when the water temperature decreases, the supply pressure of the turbid water is increased. When the water temperature rises, complicated operation control is performed such as lowering the supply pressure of the turbid water or reducing the number of first-stage nanofiltration membrane modules 2 used in the first-stage nanofiltration membrane unit device 21. On the other hand, if the water temperature can be controlled to be constant throughout the year using the heating device 25, the flow rate of nanofiltration water can be kept constant, so the supply pressure of salt water is constant throughout the year. The operation control of the first-stage nanofiltration membrane unit device 21 such as the control of the supply pressure of the turbid water and the change of the number of the first-stage nanofiltration membrane modules 2 can be simplified, and the operation cost can be reduced. This Can. Similarly, in the nanofiltration membrane unit devices subsequent to the second-stage nanofiltration membrane unit device 22 after the first-stage nanofiltration membrane unit device 21, since the complicated operation control accompanying the water temperature fluctuation can be simplified, the operating cost is reduced. Can be reduced. Furthermore, since the water permeability of the nanofiltration membrane and reverse osmosis membrane is kept constant throughout the year, the membrane separation performance and membrane unit performance at the time of design are easily predicted, which is preferable.

  About the water temperature which controls the temperature of the turbidity water supplied to the 1st step | paragraph nanofiltration membrane unit apparatus 21 with the heating apparatus 25, Preferably it is the range of 30-60 degreeC, More preferably, it is the range of 35-45 degreeC. When using a heating device to keep the turbid water at a constant water temperature throughout the year, it must be at least above the maximum salt water temperature that fluctuates annually. On the other hand, the water permeability of nanofiltration membranes and reverse osmosis membranes generally improves as the water temperature increases, but conversely, the rejection rate decreases and the amount of evaporation residue of nanofiltration water and reverse osmosis membrane permeation water increases. Considering the temperature durability of the membrane element and reverse osmosis membrane element, it is preferable to adjust the temperature of the turbidity water in the range of 35 to 45 ° C.

Next, the second-stage nanofiltration membrane unit device 22 may be in the same range as the configuration of the nanofiltration membrane module 2 of the first-stage nanofiltration membrane unit device 21. The operating pressure is appropriately selected depending on the type of membrane, the quality and temperature of the salt water, the recovery rate, and the like, but is usually selected in the range of 0.1 to 7 MPa, preferably 0.5 to 4 MPa. Further, the temperature of the nanofiltration water of the first-stage nanofiltration membrane unit apparatus 21 supplied to the second-stage nanofiltration membrane unit apparatus 22 may be adjusted. The water permeability of the nanofiltration membrane is improved by further increasing the temperature of the water supplied to the second-stage nanofiltration membrane unit device, and the operation pressure of the second-stage nanofiltration membrane module 4 can be further reduced.

And the desalination unit apparatus 23 processes the nanofiltration concentration water after the 2nd step | paragraph nanofiltration membrane unit apparatus 22, prepares desalination water, and mixes with the nanofiltration water after the 2nd step | paragraph nanofiltration membrane unit apparatus 22. Thus, it is prepared into fresh water that satisfies the drinking water quality. Examples of the desalting unit device 23 include a multistage flash distillation (MSFD) unit, a multi-effect distillation (MED) unit, a vapor compression distillation (VCD) unit, a distillation unit device combining these, and a reverse osmosis membrane unit device. However, it is not limited to these.

  Distilled water with very few impurities is prepared from the distillation unit apparatus, while blowdown, which is concentrated water, is discharged. Nanofiltration concentrated water after the second-stage nanofiltration membrane unit device 22 in which scale components such as calcium ions, magnesium ions, bicarbonate ions, carbonate ions, sulfate ions are reduced as compared with salt water as supply water to the distillation unit device. By using this, it becomes possible to raise the maximum brine temperature (TBT) of the heat drive type fresh water generation unit apparatus 23 compared with a prior art, and it can aim at the improvement of a recovery rate, and the improvement of energy efficiency. From the reverse osmosis membrane unit device, reverse osmosis membrane permeated water with very few impurities subjected to reverse osmosis treatment is prepared, and reverse osmosis membrane concentrated water as concentrated water is discharged. By using the nanofiltration concentrated water after the second-stage nanofiltration membrane unit device 22 whose scale components are reduced as compared with the salt water as the supply water to the reverse osmosis membrane unit device, the recovery rate can be increased more than the conventional technology. Since no precipitation of scale components occurs, energy efficiency can be improved.

  Here, the reverse osmosis membrane unit device includes an RO pump 11 for supplying water to the reverse osmosis membrane unit device and a reverse osmosis membrane module 12. As the RO pump, various types of pumps such as a centrifugal pump, a turbine pump, and a plunger pump can be used, but the RO pump is not particularly limited thereto. For the nanofiltration membrane constituting the reverse osmosis membrane module 12, for example, as a material, polyamide-based, polypiperazine amide-based, polyester amide-based, or a water-soluble vinyl polymer crosslinked can be used, The membrane structure has a dense layer on at least one side of the membrane, and has fine pores with gradually increasing pore diameters from the dense layer to the inside of the membrane or the membrane on the other side (asymmetric membrane), or such an asymmetric membrane. A material having a very thin separation functional layer (composite film) formed of another material on the dense layer can be used. However, a composite membrane is preferred for high water production during low-pressure operation, and among them, polyamide composite membranes and piperazine polyamide composite membranes are more preferable from the viewpoints of the amount of permeated water and chemical resistance. In addition, the reverse osmosis membrane module 12 is a spiral type element in which a flat membrane membrane is wound around a water collecting pipe, or a plate type support plate with flat membranes stretched on both sides at regular intervals via spacers. One or more plate-and-frame elements stacked and modularized, tubular elements using tubular membranes, and hollow fiber membrane elements bundled with hollow fiber membranes and housed in a case It is constructed by connecting in series. The element may be in any form, but it is preferable to use a spiral element from the viewpoint of operability and compatibility. The number of elements can be arbitrarily set according to the membrane performance. When spiral type elements are used, it is preferable that the number of elements loaded in one module is arranged in the order of 6 to 8 in series. In addition, the reverse osmosis membrane unit device is configured such that the reverse osmosis membrane modules 12 are arranged in series in one or more stages, and the reverse osmosis membrane concentrated water separated and concentrated by the reverse osmosis membrane modules in each stage is used as the reverse osmosis in the latter stage. It is configured by passing water through the membrane module. At this time, a plurality of reverse osmosis membrane modules may be arranged in parallel at each stage.

  The operating pressure of the reverse osmosis membrane unit device is appropriately selected depending on the type of membrane, the quality and temperature of the water supplied to the reverse osmosis membrane unit device, the recovery rate of the reverse osmosis membrane unit device, etc. , Preferably 3 to 7 MPa.

  The nanofiltration concentrated water after the second-stage nanofiltration membrane unit device 22 which is the supply water to the desalination unit device 23 has a reduced evaporation residue amount and a concentration of scale components compared to the saltwater or turbidity water. The desalted water can be obtained at a higher recovery rate than when the deturbed water is processed by the desalting unit device 23. Here, the recovery rate in the case where the desalination unit device 23 is a reverse osmosis membrane unit device is the upper limit of the operating pressure due to the amount of evaporation residue of the reverse osmosis membrane concentrated water becoming too high and the osmotic pressure becoming too high. The upper limit of the recovery rate is reached, or the upper limit of the recovery rate is such that the concentration of the scale component of the reverse osmosis membrane concentrated water increases and the precipitation of the scale component occurs. However, the upper limit of the recovery rate of the reverse osmosis membrane unit device is generally determined due to the amount of evaporation residue from the quality of the nanofiltration concentrated water after the second-stage nanofiltration membrane unit device 22. On the other hand, the recovery rate when the desalting unit device 23 is a distillation unit device has an upper limit determined by the recovery rate at which the concentration of the blowdown scale component increases and the scale component precipitates due to the principle of the distillation method. . Therefore, in the present invention, it is preferable in terms of the recovery rate to select a distillation unit device as the desalting unit device 23.

  The fresh water generator of FIG. 2 bypasses the nanofiltration concentrated water of the second-stage nanofiltration membrane unit device 22 and the heating device 25 and the nanofiltration membrane unit devices 21 and 22 as supply water to the desalination unit device 23. 1 is the same as the fresh water generator shown in FIG. 1 except that water mixed with turbid water is used.

  Here, the turbid water in which both the heating device and the nanofiltration membrane unit device are bypassed is turbid in the salt water by the turbidity device having the same configuration as the turbidity device for supplying to the first-stage nanofiltration membrane unit device 21. The turbidity water from which the quality has been removed may be used, and the turbidity is different from the turbidity device for supplying the first-stage nanofiltration membrane unit device 21, and the supply water to the desalting unit device 23 The turbidity water which removed the turbidity in salt water with the turbidity apparatus comprised from the pre-processing apparatus required in order to obtain sufficient water quality may be sufficient.

  3, the nanofiltration water obtained from the second-stage nanofiltration membrane unit device 22 is converted into a third-stage nanofiltration that is composed of a nanofiltration third-stage pump 7 and a third-stage nanofiltration membrane module 8. Water is passed through the membrane unit device 24, and the nanofiltration concentrated water discharged from the second-stage nanofiltration device unit 22 and the third-stage nanofiltration unit device 24 are discharged as supply water to the desalination unit device 23. Using the nanofiltration concentrated water to be mixed with the turbidity water bypassing the nanofiltration unit device, the nanofiltration water prepared by the third-stage nanofiltration membrane unit device 24, and the desalting unit Except for mixing with demineralized water prepared from the device 23 to obtain fresh water, the configuration is exactly the same as the fresh water generator illustrated in FIG.

  4 uses the nanofiltration supply flow rate adjustment valve 9 and the nanofiltration bypass flow rate adjustment valve 10 to convert the heated turbid water supplied to the first stage nanofiltration membrane unit device 21 to 1 The configuration is exactly the same as that of the fresh water generator illustrated in FIG. 2 except that the water supplied to the stage nanofiltration membrane unit device 21 is divided into the water supplied to the desalting unit device 23. Here, the feed water to the desalination unit device 23 is the second-stage nanofiltration membrane after the temperature-controlled turbidity water passes through the nanofiltration bypass flow rate adjustment valve 10 to bypass the nanofiltration membrane unit device. It is water mixed with nanofiltration concentrated water after the unit device 22.

In the desalinator constructed as shown in FIG. 1, the present invention is implemented as follows.
For example, seawater (salt water) is treated with a turbidity device, and the turbidized seawater is heated to a predetermined temperature by a heating device 25 and then pressurized to an operating pressure by a nanofiltration first-stage pump 1 to produce a first-stage nano Water is passed through the filtration membrane unit device 21 and separated into nanofiltration water and nanofiltration concentrated water. This first-stage nanofiltration membrane unit device 21 has nanofiltration membrane modules 2 arranged in series in one or more stages, and nanofiltration concentrated water separated and concentrated by the nanofiltration membrane modules in each stage It is configured by passing water through the filtration membrane module. At this time, a plurality of nanofiltration membrane modules may be arranged in parallel at each stage. Further, when the nanofiltration membrane module 2 is arranged in series in two or more stages, a booster pump may be provided in the middle of each stage. For example, the nanofiltration concentrated water of the first-stage nanofiltration membrane module may be boosted using a booster pump before being supplied to the second-stage nanofiltration membrane module. At this time, the first-stage nanofiltration membrane unit device 21 is operated such that the recovery rate is 50 to 95%, the evaporation residue removal performance is 50 to 95%, and the 2-load ion removal performance is 80% or more. Is done.

Here, calcium sulfate, which is a scale forming component contained in seawater, is most problematic when the above-described operating conditions of the first-stage nanofiltration membrane unit device 21 are satisfied. Calcium sulfate is precipitated mainly as a dihydrate (CaSO ₄ .2H ₂ O), and the precipitation limit concentration is reported to be about 3,280 mg / L according to the “Membrane Usage Handbook” (written by Haruhiko Ohya). From this data, the precipitation limit calcium ion concentration is about 965 mg / L, and the sulfate ion concentration is 2,315 mg / L.

  On the other hand, the seawater (the sea near Japan, assuming a salt concentration of 3.5% by weight) contains about 400 mg / L calcium ions and about 2,300 mg / L sulfate ions. In the separation method, scale precipitation does not occur unless the calcium concentration on the treated water side (concentrated water side) exceeds the precipitation limit concentration.

  Therefore, when the recovery rate of the first-stage nanofiltration membrane unit apparatus 21 is in the range of 50 to 90%, particularly when the ion removal performance of the first-stage nanofiltration membrane unit apparatus 21 is 90%, In order to prevent precipitation, the concentration of calcium ions on the concentrated water side is preferably in a range not exceeding 965 mg / L, which is the precipitation limit value of calcium sulfate. In order to satisfy this, the first-stage nanofiltration membrane unit device The recovery rate of 21 is preferably about 63% or less.

In addition, as a method for preventing scale deposition of the first-stage nanofiltration membrane unit device 21, a scale formation inhibitor may be added in advance to seawater that is salt water. The scale formation inhibitor forms a complex with a metal ion (Ca ^2+, etc.) in the scale component so as to solubilize it. For example, a polymer or a monomer composed of organic ions or inorganic ions may be used. it can. Specifically, chemicals such as sodium hexametaphosphate and tetrasodium ethylenediaminetetraacetate can be used. In this case, the recovery rate of the first-stage nanofiltration membrane unit device 21 can be 63% or more.

  Next, the nanofiltration water obtained by the first-stage nanofiltration membrane unit device 21 is pressurized by the nanofiltration second-stage pump so that the operation pressure is higher than the osmotic pressure, and the second-stage nanofiltration membrane unit. The water is passed through the device 22 and separated into nanofiltration water and nanofiltration concentrated water. The second-stage nanofiltration membrane unit device 22 includes nanofiltration membrane modules 4 arranged in series in one or more stages, and the nanofiltration concentrated water separated and concentrated by the nanofiltration membrane modules in each stage It is configured by passing water through the filtration membrane module. At this time, a plurality of nanofiltration membrane modules may be arranged in parallel at each stage. Further, when the nanofiltration membrane module 4 is arranged in series in two or more stages, a booster pump may be provided in the middle of each stage. For example, the nanofiltration concentrated water of the first-stage nanofiltration membrane module may be boosted using a booster pump before being supplied to the second-stage nanofiltration membrane module. At this time, the second-stage nanofiltration membrane unit device 22 was controlled so that the recovery rate was in the range of 50 to 95%.

  The nanofiltration concentrated water obtained by the second-stage nanofiltration membrane unit device 22 is supplied to the multistage flash (MSFD) unit 5 to produce distilled water, and the nanofiltration water of the second-stage nanofiltration membrane unit device 22 is produced. To make fresh water.

  The supply water to the multistage flash distillation apparatus is obtained by using the nanofiltration concentrated water of the second stage nanofiltration membrane unit apparatus 22 in which scale components such as calcium ions, magnesium ions, bicarbonate ions, carbonate ions, sulfate ions are reduced as compared with salt water. By using it, the maximum brine temperature (TBT) of the multistage flash distillation (MSFD) unit 5 can be increased as compared with the prior art, and the recovery rate and the energy efficiency can be improved.

  Further, the waste water generated from the multistage flash distillation (MSFD) unit 5 is used to control the water supplied to the first stage nanofiltration membrane unit device 21 within a desired temperature range (for example, 35 to 45 ° C.). Therefore, the first-stage nanofiltration membrane unit device 21 can stably produce a certain amount of nanofiltration water.

  Here, paying attention to the evaporation residue amount as the quality of the nanofiltration water of the second-stage nanofiltration membrane unit device 22, the evaporation residue amount of salt water is 35000 mg / L, and the first-stage nanofiltration membrane unit device 21 removes 90%. Assuming 85% removal by the stage nanofiltration membrane unit device 22, the concentration of evaporation residue of nanofiltration water is 525 mg / L, and evaporation residue on the basis of drinking water (for example, evaporation residue amount of 500 mg / L or less) Although exceeding the quantity, the drinking water standard can be satisfied by mixing the distilled water produced by the multistage flash distillation (MSFD) unit 5.

  In the present invention, an energy recovery device may be installed in the nanofiltration membrane unit device. Examples of the energy recovery device include, but are not limited to, a Pelton turbine, a turbocharger, a work exchanger, a pressure exchanger, and the like.

<Example 1>
Fresh water was produced by supplying seawater having a salinity of 3.5% by weight to the fresh water generator shown in FIG.

In addition, the turbidity removing device was constituted by arranging in parallel 12 elements in which a polyacrylonitrile ultrafiltration membrane having a nominal pore diameter of 0.01 μm was made into a hollow fiber and bundled to a membrane area of 12 m ² .

  In order to warm the turbidized salt water, an apparatus for heating by heat exchange using the waste heat from the heat-driven water making unit device 23 as a heat source was arranged.

The first-stage nanofiltration membrane unit device 21 has ^two modules in which four membrane elements having a membrane area of 7 m ² are placed in a pressure vessel in parallel, and one similar module is arranged in the subsequent stage. The concentrated water from the book was boosted with a booster pump and introduced into the subsequent module. The membrane performance of the nanofiltration membrane is as follows. When a sodium sulfate aqueous solution having a temperature of 30 ° C. and a concentration of 2000 mg / l is supplied at 0.45 MPa, the sulfate ion removal rate is 90% and the membrane permeation flux is 1.12 m ³ / m ^2. / D. When a sodium chloride aqueous solution having a temperature of 30 ° C. and a concentration of 2000 mg / l was supplied at 0.45 MPa, the chloride ion removal rate was 85% and the membrane permeation flux was 0.89 m ³ / m ² / d. .

Next, the second-stage nanofiltration membrane unit device 22 has ^two modules in which four membrane elements having a membrane area of 7 m ² are put in a pressure vessel in parallel, and a similar module is arranged in the subsequent stage. The concentrated water from the two modules was boosted with a booster pump and introduced into the subsequent module. The membrane performance of the nanofiltration membrane was as follows. When a sodium sulfate aqueous solution having a temperature of 30 ° C. and a concentration of 2000 mg / l was supplied at 0.45 MPa, the sulfate ion removal rate was 99% and the membrane permeation flux was 1.05 m ³ / m ^2. / D. When a sodium chloride aqueous solution having a temperature of 30 ° C. and a concentration of 2000 mg / l was supplied at 0.45 MPa, the chloride ion removal rate was 95% and the membrane permeation flux was 0.73 m ³ / m ² / d. .

  In addition, a multistage flash distillation apparatus (MSFD) is used for the heat driven fresh water generation unit apparatus 23, the maximum brine temperature (TBT) is set to 130 ° C., and the flow rate into the apparatus (second stage nanofiltration membrane unit apparatus) 22) and the ratio of the amount of distilled water was 1: 1.

  Using this fresh water generator, the turbid water obtained by the turbidity module was heated to a predetermined temperature (35 to 45 ° C.) and then passed through the first-stage nanofiltration membrane unit device 21, and the operating pressure was 4.0 MPa. To the former module, and the concentrated water of the former module is boosted to 6.0 MPa and supplied to the latter module so that the recovery rate in the nanofiltration membrane unit device 21 is 60%. As a result, nanofiltration water having an evaporation residue concentration of 0.082% by weight was obtained.

  Thereafter, the nanofiltration water is boosted to 1.5 MPa by the nanofiltration second-stage pump 3 and supplied to the front-stage module of the second-stage nanofiltration unit device 22, and the concentrated water of the front-stage module is reduced to 2.0 MPa. Pressurize and supply to the module in the subsequent stage, and operate so that the permeate recovery rate is 55%. The concentrated water is passed through a multistage flash distillation apparatus to produce distilled water for 50% of the concentrated water volume. did. The entire apparatus recovered 47%, and the evaporation residue concentration of the mixed water was 458 mg / L, and drinking water quality fresh water could be obtained at a high recovery rate.

<Comparative Example 1>
Instead of the nanofiltration membrane module 4 of the second-stage nanofiltration membrane unit device 22, a fresh water producing device similar to that of Example 1 was configured and water was formed except that a reverse osmosis membrane module was used.

In the reverse osmosis membrane module, four elements having a membrane area of 7 m ² were placed in a pressure vessel and arranged in the same configuration as in Example 1. The performance of the reverse osmosis membrane was as follows. When a sodium chloride aqueous solution having a water temperature of 30 ° C. and a concentration of 2000 mg / l was supplied at 7.5 MPa, the chloride ion removal rate was 99.5% and the membrane permeation flux was 0.93 m ^3. / M ² / d.

  Using this fresh water generator, the first-stage nanofiltration water is boosted to 1.5 MPa by the nanofiltration second-stage pump 3 and supplied to the previous module of the reverse osmosis membrane module. Was increased to 2.0 MPa and supplied to the latter module, the permeated water recovery rate was 31%, and the concentrated water was passed through a multistage flash distillation device to produce distilled water for 50% of the concentrated water volume. did. The evaporation residue concentration of the mixed water was 24 mg / L, and drinking water quality fresh water was obtained. The recovery rate was 39% for the entire apparatus.

<Comparative example 2>
A fresh water generator similar to Comparative Example 1 was used. Since the recovery rate of the entire fresh water generator is 47%, the first-stage nanofiltration water is boosted to 2.1 MPa by the nanofiltration second-stage pump 3 and supplied to the module before the reverse osmosis membrane module. Then, the concentrated water of the former module was boosted to 2.8 MPa and supplied to the latter module. The recovery rate of the permeated water in the reverse osmosis membrane was 55%, and the concentrated water was passed through a multistage flash distillation apparatus to produce distilled water for 50% of the concentrated water amount. The concentration of evaporation residue in the mixed water was 28 mg / L, and drinking water quality fresh water was obtained, but the power consumption increased because the supply pressure of the second-stage nanofiltration membrane unit increased 1.4 times. However, the water production cost increased.

It is a process flow figure showing one embodiment of a desalination process concerning the present invention method. It is a process flow figure showing another embodiment of a fresh water generation process concerning the present invention method. It is a process flow figure showing another embodiment of a fresh water generation process concerning the present invention method. It is a process flow figure showing another embodiment of a fresh water generation process concerning the present invention method.

Explanation of symbols

1: Nanofiltration first stage pump 2: First stage nanofiltration membrane module 3: Nanofiltration second stage pump 4: Second stage nanofiltration membrane module 5: Multistage flash distillation (MSFD) unit 7: Nanofiltration third stage Pump 8: Third stage nanofiltration membrane module 9: Nanofiltration supply flow rate adjustment valve 10: Nanofiltration bypass flow rate adjustment valve 11: RO pump 12: Reverse osmosis membrane module 21: First stage nanofiltration membrane unit device 22: Two stages Eye nanofiltration membrane unit device 23: Desalination unit device 24: Third stage nanofiltration membrane unit device 25: Heating device

Claims (8)

  A method for producing fresh water from salt water using a desalination apparatus in which nanofiltration membrane unit devices are arranged in series in two or more stages and further a desalination unit device is provided, and a first-stage nanofiltration membrane using a heating device The salt water before being supplied to the unit device is warmed, the heated salt water is passed through the first stage nanofiltration membrane unit device, and the nanofiltration water from the first stage nanofiltration membrane unit device is passed through the second stage nanofiltration membrane. Water is passed through the unit device, and the nanofiltration concentrated water from the second-stage nanofiltration membrane unit device is passed through the desalting unit device to produce desalted water, and the desalted water and the second-stage nanofiltration are further filtered. A fresh water generation method characterized by mixing with nano-filtrated water from the membrane unit apparatus and thereafter to obtain fresh water.   After the salt water before being supplied to the heating device is mixed with the nanofiltration concentrated water after the second stage nanofiltration membrane unit device before being supplied to the desalting unit device, The fresh water generation method according to claim 1, wherein water is passed.   After mixing a part of the salt water warmed using the heating device with the nanofiltration concentrated water from the second-stage nanofiltration membrane unit device before being supplied to the desalination unit device, the desalination unit The fresh water generation method according to claim 1, wherein water is passed through the apparatus.   The said desalination unit apparatus is a distillation unit selected from the group which consists of multistage flash distillation, multiple effect distillation, and vapor compression distillation, The fresh water production in any one of Claims 1-3 characterized by the above-mentioned. Method.   Using the waste heat generated from the desalination unit device as a heat source of the heating device, the salt water before being supplied to the first-stage nanofiltration membrane unit device is warmed, and then the first-stage nanofiltration membrane unit device The fresh water generation method according to claim 4, wherein water is passed through the water.   The fresh water generation method according to any one of claims 1 to 5, wherein the temperature of the salt water heated by using the heating device is controlled within a range of 35 to 45 ° C.   The fresh water generation method according to claim 1, wherein the salt water is seawater.   The fresh water obtained by mixing the nanofiltration water from the second-stage nanofiltration membrane unit and the demineralized water from the demineralization unit device satisfies the drinking water quality. The fresh water generation method in any one of.

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