A method for recycling wastewater containing film-making diluent

Abstract

The present invention relates to a method for recycling wastewater containing membrane diluent, which belongs to the technical field of environmental water treatment, and comprises the following steps: (1) heat exchange and cooling; (2) pre-precipitation; (3) precipitation; (4) ultrafiltration cross-flow filtration; (5) ultrafiltration cleaning; (6) production water mixing and replenishment recycling. The advantages of the present invention are: (1) the wastewater is fully reused without adding chemical reagents; (2) the membrane diluent DPC is cooled, precipitated and recovered, and the wastewater is reused by ultrafiltration, which basically realizes the resource utilization and near-zero emission of the diluent wastewater; (3) the use of new water for production and the discharge of wastewater are reduced, which brings economic benefits to the enterprise and reduces environmental pollution.

Description

Technical Field

[0001] The present invention relates to a method for recycling wastewater containing membrane diluent, and in particular to a method and device for recycling wastewater containing membrane diluent in the production of thermal polyvinylidene fluoride (PVDF) ultrafiltration membrane, and belongs to the technical field of environmental water treatment for wastewater containing diluent in the preparation of polyvinylidene fluoride porous membranes using diphenyl carbonate (DPC) or diphenyl carbonate as the main component and thermally induced phase separation (TIPS). Background Art

[0002] Polyvinylidene fluoride (PVDF) has outstanding properties such as resistance to oxidation, UV resistance, acid and alkali corrosion, as well as high temperature resistance and stable chemical properties. It is a recognized membrane material with excellent performance. The ultrafiltration membrane products made from it also have the characteristics of high strength, corrosion resistance and long service life.

[0003] At present, the mainstream ultrafiltration membranes in the industry all use polyvinylidene fluoride (PVDF) as the membrane material. The production method of polyvinylidene fluoride (PVDF) ultrafiltration membrane is also constantly being optimized. The membrane making process of polyvinylidene fluoride (PVDF) ultrafiltration membrane is mainly divided into wet method (NIPS) and thermal method (thermally induced phase separation method, TIPS). Among them, the solvent used in the wet method (NIPS) is mainly N, N-dimethylformamide or N, N-dimethylacetamide, but the pore size distribution of the membrane prepared by this method is wide and the strength is poor. The thermal method (thermally induced phase separation method, TIPS) mainly uses the principle of high temperature compatibility and low temperature phase separation of polymer and diluent, and causes the polymer and diluent to separate by cooling to form a pore structure. The porous membrane prepared by the thermal method (thermally induced phase separation method, TIPS) has high porosity, uniform pore size distribution and high strength.

[0004] Chinese patent application CN200810147491.7 discloses a method for preparing a polyvinylidene fluoride porous membrane, which is prepared according to the following steps: 1) mixing a polyvinylidene fluoride resin with a diluent; placing the mixture in a high-temperature stirring kettle, heating the mixture to 160°C to 220°C to form a polymer homogeneous solution, and standing the mixture for degassing; 2) directly scraping the homogeneous solution onto a support net to form a flat membrane or spinning the homogeneous solution through a spinneret to form a hollow fiber membrane, and then immersing the obtained flat membrane or hollow fiber membrane into a coolant to cool the membrane-forming solution, so that the membrane-forming solution undergoes phase separation and finally solidifies into a membrane; 3) using an alcohol or ether extractant to extract the diluent from the obtained membrane to obtain a polyvinylidene fluoride porous flat membrane or a polyvinylidene fluoride porous hollow fiber membrane. Therefore, in the spinning process of hot polyvinylidene fluoride (PVDF) hollow fiber ultrafiltration membrane, especially when cooling in a water bath, a large amount of organic wastewater containing diphenyl carbonate (DPC) diluent will be generated. If it is not properly treated, it will cause water pollution. At the same time, if the production wastewater can be reused after treatment, it will significantly improve the water recovery rate of the enterprise, reduce production water consumption and final wastewater discharge. Diphenyl carbonate (DPC) is a white crystalline solid that is insoluble in water and soluble in organic solvents such as hot ethanol, benzene, ether, carbon tetrachloride, glacial acetic acid, etc. It can be decomposed by hot alkali. Diphenyl carbonate is mainly used as a synthetic raw material for engineering plastics such as polycarbonate and polyparahydroxybenzoate, and can also be used as a plasticizer and solvent for nitrocellulose. The diluent precipitates significantly after the water is below 50°C, and can slowly precipitate when left to stand.

[0005] At present, there are only patents for the treatment of wastewater produced by the wet process of hollow fiber membranes using dimethylacetamide as solvent. For example, Chinese invention patent application CN201310319712.5 discloses a method for treating wastewater generated in the production of ultrafiltration membranes, which adopts the "iron-carbon micro-electrolysis + Fenton + coagulation and sedimentation" process; Chinese invention patent application CN201710957710.7 discloses a recovery system and treatment method for dimethylacetamide in ultrafiltration membrane production wastewater, which uses a refining tower to recover dimethylacetamide (DMAC) and condensed water.

[0006] Therefore, developing a new water treatment and reuse process method and device for the recovery and utilization of diluent-containing wastewater from thermal PVDF ultrafiltration membranes, so that the diluent-containing wastewater from thermal PVDF ultrafiltration membranes can be fully reused and discharged at near-zero levels, has become a technical problem that needs to be urgently solved in this technical field. Summary of the invention

[0007] One of the purposes of the present invention is to provide a new type of water treatment and reuse device to maximize the reuse and treatment of wastewater containing membrane diluent in the production of TIPS method PVDF ultrafiltration membrane. The wastewater containing diluent has the characteristics of containing organic suspended matter, high COD and difficult biodegradation. Through the device of the present invention, the production water and discharge water of the membrane fiber workshop are reduced, the wastewater resource is realized, and good environmental and economic benefits are achieved.

[0008] The above-mentioned object of the present invention is achieved by the following technical solutions:

[0009] A recycling device for wastewater containing membrane diluent comprises a production / make-up water tank, an ultrafiltration device, an ultrafiltration water inlet pump, an inclined tube sedimentation tank, a pre-sedimentation tank, a plate heat exchanger, a wastewater pump, a wastewater collection tank, a cooling water bath and a circulating water make-up pump; the water inlet end of the production / make-up water tank is connected to the middle of the ultrafiltration device, the bottom of the ultrafiltration device is connected to the inclined tube sedimentation tank through the ultrafiltration water inlet pump, the middle and upper part of the ultrafiltration device is connected to the inclined tube sedimentation tank through a pipeline, the inclined tube sedimentation tank is connected to the pre-sedimentation tank, the pre-sedimentation tank is connected to the plate heat exchanger, the plate heat exchanger is connected to one end of the wastewater collection tank through the wastewater pump, the other end of the wastewater collection tank is connected to the water outlet end of the cooling water bath, and the water inlet end of the cooling water bath is connected to the production / make-up water tank through the circulating water make-up pump.

[0010] Preferably, the inclined tube sedimentation tank is divided into two areas, the right side is the water inlet mixing area, and the left side is the inclined tube sedimentation area, which are separated by a partition in the middle.

[0011] Preferably, the bottom of the ultrafiltration device is connected to the inclined tube sedimentation area on the left side of the inclined tube sedimentation tank through an ultrafiltration water inlet pump.

[0012] Preferably, the inclined tube sedimentation zone is provided with a honeycomb inclined tube, the water depth in the upper part of the inclined tube zone is 0.7-1m, the inclined tube inclination angle is 60 degrees, and the aperture is 50-80mm.

[0013] Preferably, the upper and middle portion of the ultrafiltration device is connected to the water inlet mixing zone on the right side of the inclined tube sedimentation tank through a pipeline.

[0014] Preferably, the inlet mixing zone on the right side of the inclined tube sedimentation tank is connected to the pre-sedimentation tank.

[0015] Preferably, the length-to-width ratio of the pre-sedimentation tank is 10-30:1, and the bottom slope is 2-5%.

[0016] Another object of the present invention is to provide a method for recycling wastewater containing membrane-making diluent.

[0017] The above-mentioned object of the present invention is achieved by the following technical solutions:

[0018] A method for recycling wastewater containing a membrane-making diluent comprises the following steps:

[0019] (1) Heat exchange and cooling: The wastewater containing diluent is collected through the wastewater collection tank. When the temperature of the collected wastewater containing diluent is higher than 45°C, it passes through the wastewater pump and is heat exchanged in the plate heat exchanger, and the water temperature is reduced to 40-43°C;

[0020] (2) Pre-precipitation: The cooled wastewater containing diluent enters the pre-precipitation tank from the side and bottom, where it stays for 2-4 hours, and is further cooled to precipitate the membrane-making diluent;

[0021] (3) Precipitation: The effluent from the pre-sedimentation tank enters the inlet mixing zone of the inclined tube sedimentation tank, the cross-flow concentrated water from the ultrafiltration device is mixed with the effluent from the pre-sedimentation tank, and then enters the inclined tube sedimentation zone of the inclined tube sedimentation tank to collect the precipitated membrane diluent;

[0022] (4) Ultrafiltration cross-flow filtration: The effluent from the upper part of the inclined tube sedimentation zone of the inclined tube sedimentation tank enters the ultrafiltration device through the ultrafiltration water inlet pump for cross-flow filtration;

[0023] (5) Ultrafiltration cleaning: When the pressure difference rises to 0.1-0.2Mpa, use 50-60℃ hot ethanol to soak and clean the ultrafiltration device + backwash: first drain the wastewater in the ultrafiltration device, manually prepare hot ethanol, add hot ethanol to the blocked ultrafiltration device through the water production end and soak the membrane fibers. After soaking, discharge the waste ethanol, connect pressurized clean water (tap water can be used) from the water production end to backwash and restore, and then continue to use;

[0024] (6) Produced water mixed with make-up water for recycling: The turbidity of the produced water from the ultrafiltration device is stably less than 1 NTU. The produced water enters the produced water / make-up water tank and is mixed with the make-up water for recycling.

[0025] Preferably, in step (4), the ultrafiltration device uses thermal PVDF hollow fiber ultrafiltration with an effective filtration area of ​​50m 2 , the filtration flux is 20-30LMH, the cross flow rate is 10-30%, and the ultrafiltration backwash frequency is 1-2 weeks.

[0026] Preferably, after step (6), step (7) of ethanol recovery is added: the ethanol after washing is mixed with the ethanol in the extraction workshop, distilled, and reused for washing and extraction.

[0027] Beneficial effects:

[0028] The advantages of the present invention are: 1) the wastewater is fully reused without adding chemical reagents; 2) the membrane diluent DPC is cooled and precipitated for recovery, and the wastewater is reused by ultrafiltration, which basically realizes the resource utilization and near-zero emission of the diluent wastewater; 3) the use of new production water and wastewater discharge are reduced, which brings economic benefits to the enterprise and reduces environmental pollution.

[0029] The present invention is further described below through the accompanying drawings and specific implementation examples, but the implementation of the present invention is not limited thereto. It should be understood that the embodiments described in this specification are only for explaining the invention, and are not intended to limit the patent of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic structural diagram of the device for recycling wastewater containing membrane-making diluent according to the present invention.

[0031] Description of main reference numerals:

[0032] 1 Water production / water replenishment tank 2 Ultrafiltration device

[0033] 3Ultrafiltration water inlet pump 4Inclined tube sedimentation tank

[0034] 5Pre-sedimentation tank 6Plate heat exchanger

[0035] 7 Wastewater pump 8 Wastewater collection tank

[0036] 9Cooling water bath 10Circulating water pump

[0037] Specific implementation cases

[0038] The components and parts in the following specific embodiments, unless otherwise specified, are all conventional components. Unless otherwise specified, the components and parts used in the present invention can be obtained commercially.

[0039] Example 1

[0040] like Figure 1As shown, it is a schematic structural diagram of a recycling device for wastewater containing membrane diluent of the present invention, wherein 1 is a water production / water replenishment water tank, 2 is an ultrafiltration device, 3 is an ultrafiltration water inlet pump, 4 is an inclined tube sedimentation tank, 5 is a pre-sedimentation tank, 6 is a plate heat exchanger, 7 is a wastewater pump, 8 is a wastewater collecting tank, 9 is a cooling water bath tank, and 10 is a circulating water replenishment pump; the recycling device for wastewater containing membrane diluent of the present invention comprises a water production / water replenishment water tank 1, an ultrafiltration device 2, an ultrafiltration water inlet pump 3, an inclined tube sedimentation tank 4, a pre-sedimentation tank 5, a plate heat exchanger 6, a wastewater pump 7, a wastewater collecting tank 8, a cooling water bath tank 9 and a circulating water replenishment pump 10; the inclined tube sedimentation tank 4 is divided into two areas, the right side is a water inlet mixing area, and the left side is an inclined tube sedimentation area, which are separated in the middle by a partition; the water inlet end of the water production / water replenishment water tank 1 is connected to the middle part of the ultrafiltration device 2 The bottom of the ultrafiltration device 2 is connected to the oblique tube sedimentation zone on the left side of the oblique tube sedimentation tank 4 through the ultrafiltration water inlet pump 3. The oblique tube sedimentation zone is provided with a honeycomb oblique tube. The water depth at the upper part of the oblique tube zone is 0.7-1m, the inclination angle of the oblique tube is 60 degrees, and the aperture is 50-80mm. The upper and middle parts of the ultrafiltration device 2 are connected to the water inlet mixing zone on the right side of the oblique tube sedimentation tank 4 through a pipeline. The water inlet mixing zone on the right side of the oblique tube sedimentation tank 4 is connected to the pre-sedimentation tank 5. The pre-sedimentation tank 5 is connected to the plate heat exchanger 6. The aspect ratio of the pre-sedimentation tank 5 is 10-30:1, and the bottom slope is 2-5%. The plate heat exchanger 6 is connected to one end of the wastewater collecting tank 8 through the wastewater pump 7. The other end of the wastewater collecting tank 8 is connected to the water outlet end of the cooling water bath tank 9. The water inlet end of the cooling water bath tank 9 is connected to the water production / water supply water tank 1 through the circulating water supply pump 10.

[0041] The operation process of the above-mentioned device for recycling wastewater containing membrane diluent of the present invention comprises the following steps:

[0042] (1) Heat exchange and cooling: The wastewater containing diluent is collected by the wastewater collecting tank 8. When the temperature of the collected wastewater containing diluent is higher than 45°C, it passes through the wastewater pump 7 and is heat exchanged by the plate heat exchanger 6, and the water temperature is reduced to 40-43°C;

[0043] (2) Pre-precipitation: The cooled wastewater containing diluent enters the pre-precipitation tank 5 from the side and bottom, where it stays for 2-4 hours and is further cooled to precipitate the membrane-forming diluent;

[0044] (3) Sedimentation: The effluent from the pre-sedimentation tank 5 enters the inlet mixing zone of the inclined tube sedimentation tank 4, and the cross-flow concentrated water from the ultrafiltration device 2 is mixed with the effluent from the pre-sedimentation tank 5, and then enters the inclined tube sedimentation zone of the inclined tube sedimentation tank 4. No coagulation and precipitation agent is added. A sedimentation zone is set at the bottom of the inclined tube sedimentation zone of the inclined tube sedimentation tank 4 to collect the precipitated membrane-forming diluent;

[0045] (4) Ultrafiltration cross-flow filtration: The effluent from the upper part of the inclined tube sedimentation zone of the inclined tube sedimentation tank 4 enters the ultrafiltration device 2 through the ultrafiltration inlet pump 3 for cross-flow filtration. The ultrafiltration device 2 adopts thermal PVDF hollow fiber ultrafiltration with an effective filtration area of ​​50m 2 , the filtration flux is 20-30LMH, and the cross-flow rate is 20% (the cross-flow rate can be 10-30%. Unlike traditional filtration, the present invention is designed to have a low frequency of ultrafiltration backwashing (1-2 weeks) and a longer filtration cycle according to the characteristics of the diluent);

[0046] (5) Ultrafiltration cleaning: When the pressure difference rises to 0.1-0.2Mpa, the ultrafiltration device 2 is cleaned. Unlike conventional ultrafiltration with sodium hypochlorite and acid-base cleaning, the present invention uses hot ethanol at 50-60°C to soak and clean the ultrafiltration device 2 + backwash: first drain the waste water in the ultrafiltration device, manually prepare hot ethanol, add the hot ethanol to the blocked ultrafiltration device through the water production end and soak the membrane fibers, discharge the waste ethanol after soaking, connect pressurized clean water (tap water) from the water production end to backwash and restore, and continue to use without adding other chemical agents;

[0047] (6) Produced water mixed with make-up water for recycling: The turbidity of the produced water from the ultrafiltration device 2 is stably less than 1 NTU, and the produced water enters the produced water / make-up water tank 1, where it is mixed with the make-up water and then reused;

[0048] (7) Ethanol recovery: The cleaned ethanol is mixed with the ethanol from the extraction plant, distilled, and reused for cleaning and extraction (the distillation part is not within the scope of the present invention).

[0049] Application of the device for recycling wastewater containing membrane diluent of the present invention:

[0050] Application Example 1

[0051] A certain ultrafiltration PVDF membrane workshop uses the TIPS process to produce hollow fiber ultrafiltration membranes. After the PVDF raw materials and diluents are melted at high temperature, they are formed by the spinneret and then enter the cooling water bath. The workshop has 6 membrane production lines. After preliminary collection, the wastewater containing diluents is about 1m 3 / h. According to the production process, the cooling water is recycled to a certain extent. When the turbidity of the cooling water bath water is significantly higher than 10NTU, it will affect the production quality of the membrane fibers and the wastewater must be discharged for treatment.

[0052] The wastewater containing diluent treated by the method and device of the present invention can be circulated and used for water replenishment until the ultrafiltration device 2 is blocked and cleaned. The wastewater containing diluent is discharged from the cooling water bath and passes through the wastewater collecting tank 8, the wastewater pump 7, and the plate heat exchanger 6 to enter the pre-sedimentation tank 5 for cooling and precipitation. The residence time is 4 hours. The length, width and height of the pre-sedimentation tank 5 are 15mx0.8mx0.5m. The effluent enters the water inlet mixing area on the right side of the inclined tube sedimentation tank 4, mixes with the reflux concentrated water from the ultrafiltration device 2, and enters the inclined tube sedimentation area of ​​the inclined tube sedimentation tank 4 for further precipitation. The residence time is 2-3 hours. The ultrafiltration device 2 adopts thermal PVDF hollow fiber ultrafiltration with an effective filtration area of ​​50m 2 , the filtration flux is 25LMH, the cross flow rate is 20%, and after about 10 days of operation, the TMP (Trans-Membrane Pressure, membrane equipment operating parameters) rises to 110kPa, and the ultrafiltration components are cleaned with ethanol offline, and the hot ethanol is 55-60°C. After the ultrafiltration is soaked and backwashed (about 1 hour), the produced water is continued to be used; according to statistics, after being treated by the method and system of the present invention, the water reuse rate is more than 95%, and only less than 5% of the ultrafiltration backwashing water and the cooling water bath cleaning water (subsequently entering the plant for ethanol distillation and reuse) is used, achieving near-zero discharge of wastewater containing diluent; the wastewater before and after treatment is shown in Table 1; CODcr is the chemical oxygen demand.

[0053] Table 1

[0054] Water Sample Temperature ℃ Turbidity NTU CODcr(mg / L) Wastewater containing diluent (before treatment) 42-45 14.6 9780 After the present invention is processed 24-30 0.1 4710

[0055] Application Example 2

[0056] A certain ultrafiltration PVDF membrane workshop uses the TIPS process to produce hollow fiber ultrafiltration membranes. The workshop has 12 membrane production lines. After the PVDF raw materials and diluents are melted at high temperature, they are formed by the spinneret and then enter the cooling water bath. After preliminary collection, the wastewater containing diluents is about 2m 3 / h. According to the production process, the cooling water is recycled to a certain extent. When the turbidity of the cooling water bath water is significantly higher than 10NTU, it will affect the production quality of the membrane fibers and must be discharged and treated.

[0057] The water temperature of the wastewater containing diluent collected by the wastewater collection tank 8 is 45-48°C. After entering the plate heat exchanger 6 through the wastewater pump 7 for liquid-liquid cooling to 40-43°C, it enters the pre-sedimentation tank 5 for preliminary precipitation and precipitation. The residence time is 3-4 hours. The size of the pre-sedimentation tank 5 is 15mx0.8mx0.8m. The effluent enters the inlet mixing area on the right side of the inclined tube sedimentation tank 4, mixes with the reflux concentrated water from the ultrafiltration device 2, and enters the inclined tube sedimentation area of ​​the inclined tube sedimentation tank 4 for further precipitation and precipitation. The residence time is 2-3 hours. The ultrafiltration device 2 adopts thermal PVDF hollow fiber ultrafiltration with an effective filtration area of ​​80m 2 , the filtration flux is 20LMH, the cross flow rate is 20%, the water is produced to the water production / water replenishment water tank 1, supplemented to the cooling water bath 9, and recycled; after running for 7 days, the TMP rises to about 110kPa, the ultrafiltration component of the ultrafiltration device 2 is offline ethanol cleaned, the hot alcohol is 55-60℃, and the ultrafiltration device 2 continues to use the produced water after soaking and backwashing (about 1 hour); according to statistics, after being treated by the method and device of the present invention, the water reuse rate is more than 95%, and only less than 5% of the ultrafiltration backwashing water and the cooling water bath cleaning water (which can continue to enter the plant for ethanol distillation) are used, achieving near-zero discharge of wastewater containing diluents; the wastewater before and after treatment is shown in Table 2; CODcr is the chemical oxygen demand.

[0058] Table 2

[0059] Water Sample Temperature ℃ Turbidity NTU CODcr mg / L Wastewater containing diluent (before treatment) 45-48 16.5 10160 After the present invention is processed 24-30 (Room temperature) 0.1 5230

[0060] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for recycling wastewater containing membrane diluent, comprising the following steps: (1) Heat exchange and cooling: The wastewater containing the membrane diluent is collected through the wastewater collection tank. When the temperature of the collected wastewater containing the membrane diluent is higher than 45°C, it passes through the wastewater pump and is heat exchanged in the plate heat exchanger, and the water temperature is reduced to 40-43°C; (2) Pre-precipitation: The wastewater containing the membrane-making diluent after cooling enters the pre-precipitation tank from the side and bottom, and stays there for 2-4 hours, and then further cools down to precipitate the membrane-making diluent; (3) Sedimentation: The effluent from the pre-sedimentation tank enters the inlet mixing zone of the inclined tube sedimentation tank. The cross-flow concentrated water from the ultrafiltration device is mixed with the effluent from the pre-sedimentation tank and then enters the inclined tube sedimentation zone of the inclined tube sedimentation tank to collect the precipitated membrane diluent. (4) Ultrafiltration cross-flow filtration: The effluent from the upper part of the inclined tube sedimentation zone of the inclined tube sedimentation tank enters the ultrafiltration device through the ultrafiltration water inlet pump for cross-flow filtration; (5) Ultrafiltration cleaning: When the pressure difference rises to 0.1-0.2MPa, use 50-60℃ hot ethanol to soak and clean the ultrafiltration device + backwash: first drain the wastewater in the ultrafiltration device, manually prepare hot ethanol, add hot ethanol to the blocked ultrafiltration device through the water production end and soak the membrane fibers, then discharge the waste ethanol after soaking, connect pressurized clean water from the water production end to backwash and restore, and then continue to use; (6) Produced water mixed with make-up water for recycling: The turbidity of the produced water from the ultrafiltration device is stably less than 1 NTU. The produced water enters the produced water / make-up water tank and is mixed with the make-up water before being reused; (7) Ethanol recovery: The ethanol after cleaning is mixed with the ethanol in the extraction workshop, distilled, and reused for cleaning and extraction.

2. The method for recycling wastewater containing membrane diluent according to claim 1, Features: In step (4), the ultrafiltration device uses thermal PVDF hollow fiber ultrafiltration.

3. The method for recycling wastewater containing membrane diluent according to claim 2, Features: In step (4), the effective filtration area of ​​the ultrafiltration device is 50m 2 .

4. The method for recycling wastewater containing membrane diluent according to claim 3, Features: In step (4), the filtration flux of the ultrafiltration device is 20-30LMH.

5. The method for recycling wastewater containing membrane diluent according to claim 4, Features: In step (4), the cross flow rate of the ultrafiltration device is 10-30%.

6. The method for recycling wastewater containing membrane diluent according to claim 5, Features: In step (4), the ultrafiltration backwash frequency of the ultrafiltration device is 1-2 weeks.

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