A seawater desalination system

Abstract

The present invention relates to a seawater desalination system, which includes a coarse filtration device, a pretreatment membrane filter and a reverse osmosis device; the coarse filtration device is used to remove large particle impurities in seawater; the pretreatment membrane filter is used to remove tiny particles; the reverse osmosis device is used for desalination; the pretreatment membrane filter includes a clamping plate and a membrane stack, the membrane stack is composed of a plurality of membrane bag units, each membrane bag unit includes two flat membranes and an inner lining spacer between the two flat membranes, and the two flat membranes form a bag-type structure; the flat membrane includes a filter substrate, the filter substrate is a polymer membrane material containing a negatively charged microporous framework material; the inner lining spacer includes a polymer material spacer, and a hydrophilic polymer containing a nano-catalytic material is coated on the surface of the polymer material spacer. The present invention is provided with a pretreatment membrane filter, so that the RO membrane can desalinate stably and efficiently for a long time; the pretreatment membrane filter has the characteristics of high water flux, anti-fouling and self-cleaning, and does not need frequent cleaning and maintenance, so it can take into account the efficiency of fresh water production.

Description

Technical Field

[0001] The invention relates to the technical field of seawater desalination, and in particular to a seawater desalination system. Background Art

[0002] The reverse osmosis desalination process is one of the most commonly used desalination technologies. The basic process flow of the reverse osmosis desalination process usually includes: removing large particles such as sand, algae, silt, colloids and other impurities in seawater through coarse filtration to promote the normal operation of the subsequent membrane desalination system; then, the pre-treated seawater is pumped into the reverse osmosis system at high pressure. In the reverse osmosis unit, pressurized seawater is forced through a semi-permeable membrane. Since the membrane only allows water molecules to pass through and blocks salt and other dissolved solids, fresh water is produced on one side of the membrane, while saline wastewater (also called concentrated water or sewage) is concentrated on the other side.

[0003] Among them, the core component of the RO desalination process is the reverse osmosis membrane. The pore size of the reverse osmosis (RO) membrane is very small, usually only between 0.0001 and 0.001 microns (i.e. 1-10 angstroms, Å). This extremely small pore size enables the reverse osmosis membrane to effectively block dissolved salts, organic matter, bacteria, viruses and other tiny particles in the water, and only allows water molecules and some very small molecules to pass through. Precisely because of its very small pore size and fine structure, it is easy to be contaminated and blocked during use, and often requires frequent flushing and maintenance, which causes the reverse osmosis membrane to be easily damaged under the pressure of repeated flushing, so its durability is poor. At present, the cost of RO membrane, flushing and maintenance is the main cost expenditure of the RO desalination process.

[0004] The existing technology uses a coarse filtration process (generally sand filtration or security filter) to pre-treat the taken seawater, and then transports the seawater to the RO membrane for reverse osmosis desalination. However, at this time, the impurity content in the inlet water of the RO membrane is still high, which makes the RO membrane easily blocked and causes the reverse osmosis pressure to increase. This is mainly because the sand filter and ordinary security filter used in the coarse filtration process of the existing technology have the problem of unstable impurity removal effect, and the ordinary security filter cannot degrade organic matter close to the membrane surface. Therefore, it has poor resistance to pollution impact, short cleaning cycle, and low filtration membrane flux. It is not only difficult to ensure the long-term stable operation of the subsequent reverse osmosis membrane desalination system, but also seriously reduces the water production efficiency of the seawater membrane desalination process. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a seawater desalination system, in which a pretreatment membrane filter is arranged after coarse filtration. By optimizing the pretreatment membrane filter, impurities (including organic matter, bacteria, viruses and other tiny particles) in seawater can be removed efficiently and stably, and the organic matter can be degraded during the membrane filtration process, thereby improving the anti-pollution impact ability of the pretreatment membrane filter, increasing the pretreatment impurity removal effect and filtration efficiency, and providing the RO desalination device with continuous, stable and clean water quality, which is beneficial to improving the durability of the RO desalination device and promoting the long-term stable operation of the seawater desalination system.

[0007] (II) Technical solution

[0008] In a first aspect, the present invention provides a seawater desalination system, which comprises: a coarse filtration device, a pretreatment membrane filter and a reverse osmosis device; the coarse filtration device is used to remove large particle impurities in seawater; the pretreatment membrane filter is used to remove tiny particles; the reverse osmosis device is used for desalination;

[0009] Among them, the pretreatment membrane filter includes a clamping plate and a membrane stack, the membrane stack is composed of a number of membrane bag units, each membrane bag unit includes two flat membranes and an inner lining mesh between the two flat membranes, and the two flat membranes constitute a bag-type structure; the flat membrane includes a filter substrate, and the filter substrate is a polymer membrane material containing a negatively charged microporous framework material; the inner lining mesh includes a polymer material mesh, and a hydrophilic polymer containing a nano-catalytic material is coated on the surface of the polymer material mesh.

[0010] According to a preferred embodiment of the present invention, the preparation method of the filter substrate is: a negatively charged microporous framework material and a polymer are mixed and dissolved in an organic solvent (dichloromethane, decahydronaphthalene or chloroform) to obtain a casting solution, and a filter substrate is obtained after phase inversion casting; wherein the negatively charged microporous framework material accounts for 1-20% of the mass of the filter substrate; the preparation method of the negatively charged microporous framework material is as follows:

[0011] (1) adding 2,5-dicyanophenol, 1,3-propane sultone and potassium carbonate into N,N-dimethylacetamide in a molar ratio of 1:1.1-1.5:3 for dispersion, reacting at 110-130° C. for 10-15 hours to obtain potassium 3-(p-dicyanophenoxy)propane sulfonate;

[0012]

[0013] (2) Dispersing potassium 3-(p-dicyanophenoxy)propane sulfonate and 2,5-dicyanophenol in trifluoromethanesulfonic acid at a molar ratio of 1:2-3, and continuing the reaction at 60-80°C for 4-8 hours;

[0014] (3) Separate the precipitate by centrifugation, collect and wash the precipitate, and dry it to obtain a negatively charged microporous framework material.

[0015] The negatively charged microporous framework material prepared by the invention has good compatibility with polymers, so that the negatively charged microporous framework material can be firmly combined and evenly dispersed in the filtering substrate.

[0016] According to a preferred embodiment of the present invention, the polymer is at least one of polyethylene, polypropylene, polysulfone, polyethersulfone and polyvinylidene fluoride.

[0017] According to a preferred embodiment of the present invention, the flat membrane is modified by spraying a composite photocatalyst on one side facing outward, and the modification method is: preheating the filter substrate of the flat membrane at 100-120°C, dispersing the composite photocatalyst in anhydrous ethanol, spraying the dispersion on the one side facing outward of the flat membrane, and drying.

[0018] Wherein, the composite photocatalyst is TiO 2 Composite photocatalyst, gC 3 N 4 At least one of (graphite phase carbon nitride) based composite photocatalyst, Bi-based composite photocatalyst, transition metal oxide composite photocatalyst, MOF (metal organic framework) based composite photocatalyst, quantum dot composite photocatalyst, etc.

[0019] According to a preferred embodiment of the present invention, the preparation method of the composite photocatalyst is: 4-amino-3,5-dicyclohexyltriazole (see the following structure), aminophthalide and silver nitrate are dispersed in a mixed solution of acetonitrile and ethanol in a volume ratio of 1:1 at a molar ratio of 1:1-1.2:1, reacted for 48-72 hours to obtain a silver-triazole-aminophthalide complex, and the obtained complex is mixed with a titanate (such as magnesium titanate, barium titanate, etc.) to obtain a composite photocatalyst.

[0020]

[0021] According to a preferred embodiment of the present invention, the polymer material separator is a polyvinyl chloride or polypropylene separator; the preparation method of the lining separator is:

[0022] (1) Dispersing cobalt salt and iron salt in water at a molar ratio of 1:1-1.5, adjusting the pH to 8-9, transferring to a high-pressure reactor, and conducting a hydrothermal reaction at 120-150° C. for 16-24 hours. After the reaction is completed, adding graphene oxide (5-10% of the mass of the metal salt) and stirring for 1.5-5 hours, filtering, and drying to obtain a metal-graphene oxide layered nanocatalytic material;

[0023] (2) dispersing the metal-graphene oxide layered nanocatalytic material and the hydrophilic polymer in hot water (making the concentration of the hydrophilic polymer be 10-15 wt%), stirring until there are no particles, and preparing a coating slurry;

[0024] (3) Applying the coating slurry to the surface of the polymer material separator, and after drying, obtaining the lining separator. The coating method includes any one of spraying, brushing, spin coating, dipping, etc.

[0025] According to a preferred embodiment of the present invention, in step (2), the metal-graphene oxide layered nano-catalytic material accounts for 0.1-5% of the mass of the hydrophilic polymer. The hydrophilic polymer can hydrophilically modify the inner lining mesh and act as an adhesive, thereby firmly adhering the metal-graphene oxide layered nano-catalytic material to the surface of the polymer material mesh.

[0026] According to a preferred embodiment of the present invention, the coarse filtration device includes a sand filter and a filter element filter; the filter element of the filter element filter is a melt-blown filter element or a wire-wound filter element; the coarse filtration device removes particles ≥ 5 microns; including sand, algae, some fungi, etc. The porosity of the filter substrate of the flat membrane is 40-80%, and impurities with a particle size of 0.5-5 microns are intercepted, such as high molecular organic matter, bacteria, viruses and other tiny particles. The reverse osmosis device is used to remove dissolved salts to achieve the effect of desalination of seawater.

[0027] After desalination, the brine can be further concentrated, and the concentrate contains high concentrations of salt and other pollutants and needs to be properly handled. This can be discharged directly into the ocean (subject to environmental regulations), or further processed to recover valuable resources or reduce environmental impact. Fresh water produced by reverse osmosis may need to be mineralized, that is, minerals such as calcium and magnesium ions are added in appropriate amounts to improve the taste of the water (if preparing drinking water) and prevent corrosion to pipes. In order to ensure the safety of drinking water, ultraviolet irradiation or the addition of a small amount of disinfectant such as sodium hypochlorite may be used for final disinfection.

[0028] Preferably, the present invention further comprises a seawater intake device, a backwashing device for cleaning the pretreatment membrane filter and the reverse osmosis membrane, and the like.

[0029] (III) Beneficial effects

[0030] 1. The present invention further removes impurities from the effluent of the coarse filtration device by arranging a pretreatment membrane filter between the coarse filtration device and the reverse osmosis device, and removes impurities of 0.5-5 microns, including organic matter, bacteria, viruses and other tiny particles, etc., which can effectively protect the RO membrane of the reverse osmosis device, enable the RO membrane to desalinate stably and efficiently for a long time, increase the service life of the membrane, and reduce the cost of cleaning, maintenance and replacement.

[0031] 2. The pretreatment membrane filter designed by the present invention has a flat membrane containing a negatively charged microporous frame material, which is used to increase the water permeation channel and membrane flux in the membrane. The negative charge can prevent fine particles from entering the membrane pores and causing fouling; the outer side of the flat membrane has a composite photocatalyst, which produces photocatalytic degradation of organic matter entering the membrane pores, so that the flat membrane has a self-cleaning function, which can not only improve the water quality of the effluent, but also maintain a high water flux. Titanate is mixed in the composite photocatalyst to provide self-condensation sites, so that difficult-to-degrade pollutants can be aggregated into large particles on the membrane surface, further improving the filtration effect. The surface of the lining screen is coated with a hydrophilic polymer containing nano-catalytic materials, which not only improves the pollution resistance of the screen, but also further oxidizes and mineralizes the difficult-to-remove pollutants, improves the filtration effect, and ensures the filtration speed.

[0032] The seawater desalination system of the present invention greatly improves the cleanliness of water entering the RO membrane by arranging a pretreatment membrane filter before the RO device, effectively prolongs the service life of the RO membrane system and improves the effluent water quality. The specially designed pretreatment membrane filter has the characteristics of high water flux, anti-fouling and self-cleaning, and does not require frequent cleaning and maintenance, so it can take into account the efficiency of fresh water production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of the composition of the seawater desalination system of the present invention;

[0034] Figure 2 It is a schematic diagram of the structure of the membrane bag unit in the pretreatment membrane filter of the present invention. DETAILED DESCRIPTION

[0035] In order to better explain the present invention and facilitate understanding, the present invention is described in detail below through specific implementation methods.

[0036] like Figure 1 As shown, the seawater desalination system of the present invention includes a coarse filtration device, a pretreatment membrane filter and a reverse osmosis device. The coarse filtration device includes a sand filter and a filter element filter; the filter element of the filter element filter is a melt-blown filter element or a wire-wound filter element; the coarse filtration device removes particles ≥5 microns; including sand, algae, some fungi, etc. The porosity of the filter substrate of the pretreatment membrane filter is 40-80%, and impurities with a particle size of 0.5-5 microns are retained, such as high molecular organic matter, bacteria (bacteria less than 5 microns), viruses and other tiny particles. The reverse osmosis device is mainly used for desalination and smaller molecules to produce fresh water. The RO membrane is a reverse osmosis membrane with a pore size of 0.0001 to 0.001μm.

[0037] In addition, a cleaning device is provided for cleaning the membrane assembly of the pretreatment membrane filter. However, the pretreatment membrane filter designed and optimized in the present invention has strong anti-fouling and self-cleaning capabilities, and the upstream water is coarsely filtered, so there is no need to frequently start the cleaning device.

[0038] The pretreatment membrane filter includes a clamping plate and a membrane stack, the membrane stack is composed of a plurality of membrane bag units, and the membrane bag unit includes two flat membranes 11 and a liner spacer 12 (such as Figure 2 As shown), two flat membranes 11 form a bag-like structure.

[0039] In the present invention, a pretreatment membrane filter is provided between the coarse filtration device and the reverse osmosis device to filter impurities of 0.5-5 microns. The fine impurities in this particle size range are mainly some organic polymer colloids, bacteria, viruses and other tiny particles, etc. They are small in size and usually have negative charges. They are easily adsorbed into the membrane pores, resulting in membrane pore contamination and blockage, decreased membrane flux, etc. The pretreatment membrane filter provided in the present invention transfers the contamination / blocking pressure originally on the RO membrane of the reverse osmosis device to the pretreatment membrane filter.

[0040] In order to improve the anti-pollution ability of the pretreatment membrane filter, maintain effective membrane flux, protect the RO membrane and improve the final water quality, while taking into account the fresh water production efficiency of the seawater desalination system, the present invention has modified the membrane bag unit of the pretreatment membrane filter as follows:

[0041] First, the filtering substrate of the flat membrane 11 is a polymer membrane material containing a negatively charged microporous framework material; in some embodiments, a composite photocatalyst is further coated on the outer side thereof.

[0042] Specifically, the preparation method of the filter substrate of the flat membrane 11 is: a negatively charged microporous framework material is mixed with a polymer and dissolved in an organic solvent, and a filter substrate is obtained after phase inversion casting; wherein the negatively charged microporous framework material accounts for 1-20% (preferably 8-12%) of the mass of the filter substrate. Wherein, the polymer is at least one of polyethylene, polypropylene, polysulfone, polyethersulfone and polyvinylidene fluoride. The organic solvent is selected according to the properties of the polymer material. For example, if the polymer is polyethylene, an organic solvent such as dichloromethane, decahydronaphthalene or chloroform can be used. The organic solvent is preferably able to dissolve the polymer well and prepare the casting solution. When the polymer is polysulfone, the organic solvent can be N-methylpyrrolidone, dimethyl sulfoxide, dimethylacetamide, etc.

[0043] Among them, phase inversion refers to the process of causing phase separation inside the casting liquid by changing environmental conditions (such as temperature, humidity or introducing non-solvents), and then solidifying to form a membrane with a specific pore structure, including but not limited to the immersion precipitation method (Wet Phase Inversion), in which the coated casting liquid is quickly immersed in a coagulation bath containing a large amount of non-solvent (usually water); Dry-Induced Phase Separation (DIPS), in which the casting liquid is coated on a substrate and exposed to the air; Thermally Induced Phase Separation (TIPS): Applicable to thermosensitive polymer systems, the polymer and solvent are made miscible by heating, and then cooled down to induce phase separation. As the temperature decreases, the polymer precipitates and solidifies into a membrane.

[0044] In the above scheme, the negatively charged microporous framework material can be negatively charged MOFs, COFs, mesoporous silicon-based materials, zeolites and zeolite-like materials in the prior art.

[0045] In the present invention, preferably, the preparation method of the negatively charged microporous framework material is as follows:

[0046] (1) adding 2,5-dicyanophenol, 1,3-propane sultone and potassium carbonate into N,N-dimethylacetamide in a molar ratio of 1:1.1-1.5:3 for dispersion, reacting at 110-130° C. for 10-15 hours to obtain potassium 3-(p-dicyanophenoxy)propane sulfonate;

[0047] (2) Dispersing potassium 3-(p-dicyanophenoxy)propane sulfonate and 2,5-dicyanophenol in trifluoromethanesulfonic acid at a molar ratio of 1:2-3, and continuing the reaction at 60-80°C for 4-8 hours;

[0048] (3) Separate the precipitate by centrifugation, collect and wash the precipitate, and dry it to obtain a negatively charged microporous framework material. The negatively charged microporous framework material has the properties of an organic substance and can be well mixed and compatible with a polymer when preparing a casting solution. In the prepared membrane material, the negatively charged microporous framework material has good dispersion uniformity, thereby enabling the negatively charged microporous framework material to be firmly combined and evenly dispersed in the filter substrate.

[0049] Negatively charged microporous framework materials are a type of porous solid materials with permanent microporous structures and negatively charged surfaces. These materials contain micropores inside, which provide a huge specific surface area, and the skeleton or pore wall of the material carries a negative charge. This type of material can effectively remove heavy metal ions or other pollutants in water. Due to the negative charge characteristics of its surface, it is particularly suitable for capturing positively charged harmful substances. At the same time, the negative charge can prevent pollutants from entering the membrane pores and causing fouling. Adding a certain proportion of negatively charged microporous framework materials to the filter substrate of the flat membrane can effectively increase the water channels in the membrane and improve the permeation flux.

[0050] Furthermore, in order to improve the self-cleaning ability and water quality of the pretreatment membrane filter, the present invention further modifies the flat membrane 11 of the pretreatment membrane filter, especially the surface of the flat membrane 11 facing outward. The modification method can be a coating method, which can preheat the filter substrate of the flat membrane at 100-120°C, disperse the composite photocatalyst in anhydrous ethanol (concentration of 3-10wt%), spray the dispersion on the side of the flat membrane 11 facing outward, and complete it after drying.

[0051] Among them, the composite photocatalyst is a self-prepared TiO 2 / silver composite photocatalyst, the preparation process is: 4-amino-3,5-dicyclohexyltriazole, aminophthalide and silver nitrate are dispersed in a mixed solution of acetonitrile and ethanol in a volume ratio of 1:1 according to a molar ratio of 1:1-1.2:1, heated at 35-50°C for 48-72h to obtain a silver-triazole-aminophthalide complex, and the obtained complex is mixed with a titanate (such as magnesium titanate, barium titanate, etc.) to obtain a composite photocatalyst. In other embodiments, the composite photocatalyst can use other composite photocatalysts, such as TiO 2 Composite photocatalyst, gC 3 N 4 At least one of (graphite phase carbon nitride) based composite photocatalyst, Bi based composite photocatalyst, transition metal oxide composite photocatalyst, MOFs (metal organic framework) based composite photocatalyst, quantum dot composite photocatalyst, etc. TiO 2 Based composite photocatalysts including TiO 2 Composite with precious metals (such as Pt, Pd, Au), TiO 2 / Semiconductor composite, TiO 2 / carbon material composite, etc. Among them, gC 3 N 4 It is a non-metallic semiconductor photocatalyst that responds to visible light. It is compatible with other materials (such as TiO 2 、MoS 2, CdS, etc.), can not only expand its light absorption range, but also improve the charge transfer properties, thereby enhancing the ability to degrade organic matter. In addition, there are BiOX (X = Cl, Br, I), Fe 2 O 3 (Iron Oxide), Co 3 O 4 (cobalt oxide), MOFs, CdS, CdSe and other quantum dots, when they are combined with TiO 2 , gC 3 N 4 Or when compounded with other materials, the photocatalytic activity can be significantly improved.

[0052] The composite photocatalyst is sprayed on the outward side of the flat membrane 11 to photocatalytically degrade organic pollutants, making the flat membrane 11 self-cleaning, which can not only improve the quality of water production, but also maintain the water flux of the flat membrane 11. The titanate mixed in the composite photocatalyst provides self-agglomeration sites, allowing those pollutants or flocculants that are difficult to degrade to self-aggregate into larger particles on the membrane surface, further improving the filtration effect.

[0053] The inner lining screen 12 comprises a polymer screen, and a hydrophilic polymer containing a nano-catalytic material is coated on the surface of the polymer screen. The polymer screen is a polyvinyl chloride or polypropylene screen. The nano-catalytic material is a transition metal composite oxide supported by layered graphene oxide, for example, iron-cobalt oxide supported by layered graphene oxide. In this case, the preparation method of the inner lining screen 12 is as follows:

[0054] (1) Dispersing water-soluble cobalt salt and iron salt in water at a molar ratio of 1:1-1.5, adding ammonia water to adjust the pH to 8-9, transferring to a high-pressure reactor, and performing a hydrothermal reaction at 120-150° C. for 16-24 hours. After the reaction is completed, adding 5-10% of the mass of the metal salt (commercially available product) of graphene oxide and stirring the reaction for 1.5-5 hours, filtering, and drying to obtain a metal-graphene oxide layered nanocatalytic material;

[0055] (2) dispersing the metal-graphene oxide layered nanocatalytic material and a hydrophilic polymer (selected from at least one of polyvinyl pyrrolidone, polyacrylic acid, and polyacrylamide) in hot water at 40-60° C. (making the concentration of the hydrophilic polymer 10-15 wt %), and stirring until completely dissolved to prepare a coating slurry;

[0056] Among them, the metal-graphene oxide layered nano-catalytic material accounts for 0.1-5% of the mass of the hydrophilic polymer. The hydrophilic polymer plays a bonding role and can firmly adhere the metal-graphene oxide layered nano-catalytic material to the surface of the polymer material separator.

[0057] (3) Apply the coating slurry to the surface of the polymer material separator, and after drying, prepare the lining separator. The coating method includes any one of spraying, brushing, spin coating, dipping, etc. The polymer material separator mainly plays the role of a support frame, which is used to support the two flat membranes 11 to maintain the bag-like structure. The pore size of the polymer material separator is 0.5-2cm. The hydrophilic polymer improves the hydrophilicity of the lining separator 12, which can improve the anti-fouling performance of the lining separator 12 and improve the water flux. The nanocatalytic material can improve the self-cleaning ability of the lining separator 12, promote the decomposition of organic matter accumulated during long-term use into small molecules, and can perform nano-confined reactions on micro-pollutants that are difficult to remove in water, further improving the water quality of the flat membrane; wherein, graphene oxide as an electron transfer medium can promote the reduction and regeneration of cobalt and iron, effectively improve the stability of its performance in oxidizing micro-pollutants, and maintain the high water flux of the pretreatment membrane filter.

[0058] In order to further clarify the scheme of the present invention and its technical advancement, the following is a description in conjunction with specific embodiments and technical effects.

[0059] Example 1

[0060] The present embodiment provides a seawater desalination system, which includes a coarse filtration device, a pretreatment membrane filter and a reverse osmosis device. The coarse filtration device includes a sand filter and a filter element filter; the filter element of the filter element filter is a wire-wound filter element; the coarse filtration device removes particles ≥5 microns; including sand, algae, some fungi, etc. The pretreatment membrane filter is used to remove impurities of 0.5-5 microns, such as high molecular organic matter, bacteria (bacteria less than 5 microns), viruses and other tiny particles. The reverse osmosis device is mainly used for desalination and smaller molecules to produce fresh water. The RO membrane is a reverse osmosis membrane with a pore size of 0.0001 to 0.001 μm.

[0061] The pretreatment membrane filter includes a clamping plate and a membrane stack, and the membrane stack is composed of a plurality of membrane bag units, each of which is composed of two flat membranes 11 lined with a spacer 12. The seawater is divided into multiple streams and enters different membrane bag units for filtration, which can effectively ensure the treatment efficiency and the stable operation of the entire system.

[0062] The flat membrane 11 includes a filter substrate, which is polyethylene containing 1wt% of negatively charged microporous framework material. The porosity of the flat membrane 11 is 40%. The lining screen 12 includes a polyvinyl chloride screen, and polyvinyl pyrrolidone containing nano-catalytic material (accounting for 0.1wt% of the mass of polyvinyl pyrrolidone) is coated on the surface of the polyvinyl chloride screen.

[0063] The preparation method of the flat membrane 11 is as follows:

[0064] (1) 2,5-Dicyanophenol, 1,3-propane sultone and potassium carbonate were dispersed in N,N-dimethylacetamide at a molar ratio of 1:1.1:3, and reacted at 120°C for 10 hours to obtain 3-(p-dicyanophenoxy)propane sulfonate potassium; then 3-(p-dicyanophenoxy)propane sulfonate potassium and 2,5-dicyanophenol were dispersed in trifluoromethanesulfonic acid at a molar ratio of 1:2, and the reaction was continued at 60°C for 6 hours; the precipitate was separated by centrifugation, the precipitate was collected, washed, and dried to obtain a negatively charged microporous framework material.

[0065] (2) The negatively charged microporous framework material and polyethylene are dissolved in NMP to prepare a casting solution, and the prepared casting solution is evenly cast on a glass plate and a smooth substrate to ensure uniform thickness. The cast film is allowed to stand in the air for a period of time, allowing part of the solvent to evaporate naturally to initially form a solidified film layer. The membrane material is quickly immersed in water, and NMP quickly diffuses into the coagulation bath, while water diffuses into the casting solution; after the phase transfer treatment is completed, the membrane material is dried at room temperature to obtain the filter substrate of the flat membrane 11. The double-layer filter substrate is stacked together to obtain a flat membrane 11 to improve the strength of the flat membrane 11. The porosity of the flat membrane 11 is 40%.

[0066] The polymer material separator in the lining separator 12 is a polyvinyl chloride separator, and the preparation method of the lining separator 12 is as follows:

[0067] Cobalt nitrate and ferric nitrate were dispersed in water in a molar ratio of 1:1, ammonia water was added to adjust the pH to 8, and then transferred to a high-pressure reactor for hydrothermal reaction at 120°C for 24 hours. After the reaction, graphene oxide (5% of the mass of the metal salt) was added and stirred for 3 hours, and then filtered and dried to obtain metal-graphene oxide layered nanocatalytic materials. Metal-graphene oxide layered nanocatalytic materials and polyvinyl pyrrolidone were dispersed in 60°C hot water, the nanocatalytic material accounted for 0.1wt% of the mass of PAM, and stirred until completely dissolved to obtain a coating slurry, in which the concentration of polyvinyl pyrrolidone was 10wt%. The polyvinyl chloride screen was immersed in the coating slurry for 30 minutes and then taken out. After drying, the lining screen 11 was obtained.

[0068] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The seawater quality entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 190LMH, the turbidity dropped to 0.61NTU, COD dropped to 2.2mg / L, and no fouling occurred during the pretreatment process.

[0069] Example 2

[0070] This embodiment further modifies the outer side of the flat membrane 11 based on the embodiment 1. The method for modifying the flat membrane 11 in this embodiment is as follows:

[0071] The composite photocatalyst is sprayed on the outward side of the flat film 11. The TiO 2 / Metal silver composite photocatalyst, the preparation method of which is: 4-amino-3,5-dicyclohexyltriazole, aminophthalide and silver nitrate (molar ratio 1:1.2:1) are reacted in a mixed solution of acetonitrile and ethanol (volume ratio 1:1) for 48 hours to obtain a silver-triazole-aminophthalide complex, and then the obtained complex is mixed with magnesium titanate at a mass ratio of 1:0.3 to obtain a composite photocatalyst. The flat film prepared in step (2) of Example 1 is preheated to 100°C, the composite photocatalyst is dispersed in anhydrous ethanol at a concentration of 3wt%, the dispersion is sprayed on the side of the flat film facing outward, and after drying, a modified flat film 11 is obtained. The other parts of this embodiment are the same as Example 1.

[0072] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 190LMH, the turbidity dropped to 0.14NTU, COD dropped to 1.2mg / L, and no fouling occurred during the pretreatment process.

[0073] Example 3

[0074] This embodiment provides a seawater desalination system, which includes a coarse filtration device, a pretreatment membrane filter and a reverse osmosis device. The coarse filtration device includes a sand filter and a filter element filter; the filter element of the filter element filter is a melt-blown filter element; the coarse filtration device removes particles ≥5 microns; including sand, algae, some fungi, etc. The pretreatment membrane filter is used to remove impurities of 0.5-5 microns, such as high molecular organic matter, bacteria (bacteria less than 5 microns), viruses and other tiny particles. The reverse osmosis device is mainly used for desalination and smaller molecules to produce fresh water. The RO membrane is a reverse osmosis membrane with a pore size of 0.0001 to 0.001 μm.

[0075] The pretreatment membrane filter includes a clamping plate and a membrane stack, and the membrane stack is composed of a plurality of membrane bag units, each of which is composed of two flat membranes 11 lined with a spacer 12. The seawater is divided into multiple streams and enters different membrane bag units for filtration.

[0076] The flat membrane 11 includes a filter substrate, which is polysulfone containing 10wt% negatively charged microporous framework material, and the porosity of the flat membrane 11 is 60%. The lining mesh 12 includes a polypropylene mesh, and the surface of the polypropylene mesh is coated with polyacrylic acid containing nano-catalytic material (accounting for 2.5wt% of the mass of the polyacrylic acid).

[0077] The preparation method of the flat membrane 11 is as follows:

[0078] (1) 2,5-Dicyanophenol, 1,3-propane sultone and potassium carbonate were dispersed in N,N-dimethylacetamide at a molar ratio of 1:1.3:3, and reacted at 130°C for 12 hours to obtain 3-(p-dicyanophenoxy)propane sulfonate potassium; then 3-(p-dicyanophenoxy)propane sulfonate potassium and 2,5-dicyanophenol were dispersed in trifluoromethanesulfonic acid at a molar ratio of 1:2, and the reaction was continued at 70°C for 6 hours; the precipitate was separated by centrifugation, the precipitate was collected, washed, and dried to obtain a negatively charged microporous framework material.

[0079] (2) The negatively charged microporous framework material and polysulfone are dissolved in NMP to prepare a casting solution, and the prepared casting solution is evenly cast on a glass plate and a smooth substrate to ensure uniform thickness. The cast film is allowed to stand in the air for a period of time, allowing part of the solvent to evaporate naturally to initially form a solidified film layer. The membrane material is quickly immersed in water, and NMP quickly diffuses into the coagulation bath, while water diffuses into the casting solution; after the phase transfer treatment is completed, the membrane material is dried at room temperature to obtain a filter substrate for the flat membrane 11. The double-layer filter substrate is stacked together to obtain a flat membrane 11 to improve the strength of the flat membrane 11.

[0080] (3) 4-amino-3,5-dicyclohexyltriazole, aminophthalide and silver nitrate (molar ratio 1:1.2:1) were reacted in a mixed solution of acetonitrile and ethanol (volume ratio 1:1) for 60 hours to obtain a silver-triazole-aminophthalide complex, and the obtained complex was mixed with magnesium titanate at a mass ratio of 1:0.4 to obtain a composite photocatalyst. The flat film prepared according to step (2) of Example 1 was preheated to a surface temperature of about 120° C., the composite photocatalyst was dispersed in anhydrous ethanol at a concentration of 3 wt%, and the dispersion was sprayed on the outer side of the flat film. After drying, a modified flat film 11 was obtained.

[0081] The polymer material separator in the lining separator 12 is a polypropylene separator. The preparation method of the lining separator 12 is as follows: cobalt nitrate and iron nitrate are dispersed in water at a molar ratio of 1:1.5, ammonia water is added to adjust the pH to 8.5, and the mixture is transferred to a high-pressure reactor for hydrothermal reaction at 150°C for 24 hours. After the reaction is completed, graphene oxide (8% of the mass of the metal salt) is added and stirred for 4 hours, and then filtered and dried to obtain a metal-graphene oxide layered nanocatalytic material. The metal-graphene oxide layered nanocatalytic material and polyacrylic acid are dispersed in 40°C hot water, the nanocatalytic material accounts for 2.5wt% of the mass of the polyacrylic acid, and stirred until completely dissolved to obtain a coating slurry, in which the concentration of polyacrylic acid is 12wt%. The polypropylene separator is immersed in the coating slurry for 30 minutes and then taken out. After drying, the lining separator 11 is obtained.

[0082] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 196LMH, the turbidity dropped to 0.11 NTU, COD dropped to 0.92 mg / L, and no fouling occurred during the pretreatment process.

[0083] Example 4

[0084] The difference between this embodiment and embodiment 3 is that the composite photocatalyst used in modifying the outer side of the flat film 11 is different. The composite photocatalyst used in this embodiment is TiO 2 / gC 3 N 4 Composite photocatalyst, gC 3 N 4 With TiO 2 The composite mass ratio is 1:0.3. The flat film 11 is prepared according to the method of Example 3 and the outer surface of the flat film 11 is coated with TiO 2 / gC 3 N 4 The other parts of this embodiment are the same as those of embodiment 3.

[0085] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 195LMH, the turbidity dropped to 0.21NTU, COD dropped to 1.6mg / L, and no fouling occurred during the pretreatment process.

[0086] Example 5

[0087] This embodiment is the same as embodiment 2, except that the content of negatively charged microporous framework material in the filter substrate of the flat membrane 11 is increased to 20%, and the porosity of the flat membrane is adjusted to 80%. The surface of the lining screen 12 is coated with polyvinyl pyrrolidone containing nano-catalytic material, and the nano-catalytic material accounts for 5wt% of the mass of polyvinyl pyrrolidone. The nano-catalytic material is prepared according to the method of embodiment 1. The outer surface of the flat membrane 11 is sprayed with a composite photocatalyst after preheating at 110°C. The preparation method of the composite photocatalyst is referred to embodiment 2, and the composite photocatalyst is dispersed in anhydrous ethanol at a concentration of 2.5wt% and then surface sprayed. The other parts of this embodiment are the same as embodiment 2.

[0088] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 199LMH, the turbidity dropped to 0.12NTU, COD dropped to 0.97mg / L, and no fouling occurred during the pretreatment process.

[0089] Example 6

[0090] The embodiment is the same as the embodiment 2, except that the content of the negatively charged microporous framework material in the filter substrate of the flat membrane 11 is adjusted to 0.5wt%, and the other conditions are the same as the embodiment 2. The seawater is first treated by the coarse filtration device and then treated by the pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter is: turbidity is 32.5 NTU, COD is 20.8 mg / L; the effluent flux of the pretreatment membrane filter is 173LMH, the turbidity is reduced to 0.27NTU, COD is reduced to 1.9mg / L, and no fouling occurs during the pretreatment process.

[0091] Example 7

[0092] The difference between Example 7 and Example 2 is that the content of the negatively charged microporous framework material in the filtering substrate of the flat membrane 11 is adjusted to 8 wt %, and the other conditions are the same as those of Example 2.

[0093] The seawater was first treated by a coarse filtration device and then by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity was 32.5 NTU, COD was 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 192LMH, the turbidity dropped to 0.13NTU, COD dropped to 1.05mg / L, and no fouling occurred during the pretreatment process.

[0094] Comparative Example 1

[0095] Compared with Example 1, this comparative example does not incorporate negatively charged microporous framework materials when preparing the flat membrane 11. The seawater is first treated by a coarse filtration device and then treated by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter is: turbidity is 32.5 NTU, COD is 20.8 mg / L; the effluent flux of the pretreatment membrane filter is 130LMH, the turbidity drops to 0.75NTU, COD drops to 3.7mg / L, and the pretreatment membrane filter and the reverse osmosis device are unstable.

[0096] Comparative Example 2

[0097] Compared with Example 1, in this comparative example, when preparing the inner lining screen 12, only polyvinyl pyrrolidone with a concentration of 10 wt% is coated, and no nano-catalytic material is contained.

[0098] The seawater was first treated by a coarse filtration device and then treated by a pretreatment membrane filter. The quality of the seawater entering the pretreatment membrane filter was: turbidity of 32.5 NTU, COD of 20.8 mg / L; the effluent flux of the pretreatment membrane filter was 180LMH, and the turbidity of the produced water was 0.82 NTU, COD of 3.3 mg / L. In this comparative example, the membrane filter had a poor effect on removing pollutants, and the pretreatment membrane filter and the reverse osmosis device were unstable.

[0099] In addition, in the desalination systems of Examples 1-7, the pretreatment membrane filters can operate stably for more than 20 days without cleaning, and the pretreatment membrane filters of Examples 2, 3, and 5 can continue to operate stably for more than 25 days, and during this process, the operating pressure of the downstream reverse osmosis device is stable, and the effluent water quality is good and stable, and can meet the drinking water standard after disinfection. In contrast, the desalination pretreatment membrane filters of Comparative Examples 1-2 have a significant increase in operating pressure and a 20% decrease in water flux after about 5-7 days of operation, which means that they need to be shut down for flushing, and the operating pressure of the downstream reverse osmosis device is unstable during operation, and the effluent water quality fluctuates greatly.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A seawater desalination system, characterized in that: It includes: Coarse filtration units, pretreatment membrane filters, and reverse osmosis units; The coarse filtration device is used to remove large particles of impurities in seawater; The pretreatment membrane filter is used to remove tiny particles; The reverse osmosis device is used for desalination; The pretreatment membrane filter comprises a clamping plate and a membrane stack, wherein the membrane stack is composed of a plurality of membrane bag units, each of which comprises two flat membranes and an inner lining mesh between the two flat membranes, and the two flat membranes form a bag-type structure; the flat membrane comprises a filter substrate, which is a polymer membrane material containing a negatively charged microporous framework material; the inner lining mesh comprises a polymer material mesh, and a hydrophilic polymer containing a nano-catalytic material is coated on the surface of the polymer material mesh; The preparation method of the filter substrate is as follows: a negatively charged microporous framework material and a polymer are mixed and dissolved in an organic solvent to obtain a casting solution, and a filter substrate is obtained after phase conversion casting; The negatively charged microporous framework material accounts for 1-20% of the mass of the filter substrate; the preparation method of the negatively charged microporous framework material is as follows: (1) adding 2,5-dicyanophenol, 1,3-propane sultone and potassium carbonate into N,N-dimethylacetamide in a molar ratio of 1:1.1-1.5:3 for dispersion, reacting at 110-130° C. for 10-15 hours to obtain potassium 3-(p-dicyanophenoxy)propane sulfonate; (2) Dispersing potassium 3-(p-dicyanophenoxy)propane sulfonate and 2,5-dicyanophenol in trifluoromethanesulfonic acid at a molar ratio of 1:2-3, and continuing the reaction at 60-80°C for 4-8 hours; (3) Separate the precipitate by centrifugation, collect and wash the precipitate, and dry it to obtain a negatively charged microporous framework material.

2. The seawater desalination system according to claim 1, characterized in that: The polymer is at least one of polyethylene, polypropylene, polysulfone, polyethersulfone and polyvinylidene fluoride.

3. The seawater desalination system according to claim 1, characterized in that: The flat membrane is modified by spraying a composite photocatalyst on the outward side thereof. The modification method comprises: preheating the filter substrate of the flat membrane at 100-120° C., dispersing the composite photocatalyst in anhydrous ethanol, spraying the dispersion on the outward side of the flat membrane, and drying the dispersion.

4. The seawater desalination system according to claim 3, characterized in that: The preparation method of the composite photocatalyst is as follows: 4-amino-3,5-dicyclohexyltriazole, aminophthalide and silver nitrate are dispersed in a mixed solution of acetonitrile and ethanol in a volume ratio of 1:1 according to a molar ratio of 1:1-1.2:1, reacted for 48-72 hours to obtain a silver-triazole-aminophthalide complex, and the obtained complex is mixed with titanate to obtain a composite photocatalyst.

5. The seawater desalination system according to claim 1, characterized in that: The polymer material separator is a polyvinyl chloride or polypropylene separator; the preparation method of the lining separator is: (1) Dispersing cobalt salt and iron salt in water at a molar ratio of 1:1-1.5, adjusting the pH to 8-9, transferring the mixture into a high-pressure reactor, and conducting a hydrothermal reaction at 120-150° C. for 16-24 hours. After the reaction, adding graphene oxide accounting for 5-10% of the mass of the metal salt and stirring for 1.5-5 hours, filtering and drying the mixture to obtain a metal-graphene oxide layered nanocatalytic material; (2) dispersing the metal-graphene oxide layered nanocatalytic material and the hydrophilic polymer in hot water, stirring until no particles are present, to prepare a coating slurry; (3) The coating slurry is coated on the surface of the polymer material separator, and after drying, the lining separator is obtained.

6. The seawater desalination system according to claim 5, characterized in that: In step (2), the metal-graphene oxide layered nanocatalytic material accounts for 0.1-5% of the mass of the hydrophilic polymer; the hydrophilic polymer is at least one of polyvinyl pyrrolidone, polyacrylic acid, and polyacrylamide.

7. The seawater desalination system according to claim 1, characterized in that: The coarse filtration device includes a sand filter and a filter element filter; the filter element of the filter element filter is a melt-blown filter element or a wire-wound filter element; the coarse filtration device removes particles ≥5 microns.

8. The seawater desalination system according to claim 1, characterized in that: The porosity of the filtering substrate of the flat membrane is 40-80%, and impurities with a particle size of 0.5-5 microns are retained.

9. The seawater desalination system according to claim 1, characterized in that: The seawater desalination system also includes a salt water concentration device and a fresh water regulating and disinfecting device.

Images