Polyvinyl alcohol ultrafiltration membrane, preparation method and application thereof

Abstract

The application belongs to the field of polyetheramine production and relates to a polyvinyl alcohol ultrafiltration membrane and a preparation method and application thereof.The preparation method of the polyvinyl alcohol ultrafiltration membrane is as follows: 1) polyvinyl alcohol, water, a plasticizer, polyethylene glycol and SiO2 are mixed, heated and fully stirred to obtain a polyvinyl alcohol solution, the solution is placed in a mold for drying and solidification to form a film; 2) a sodium alginate aqueous solution is coated on the surface of the film, and the composite film is dried to obtain a composite membrane; 3) the composite membrane is contacted with a crosslinking agent solution to perform a crosslinking reaction, then alcohol is used to remove the residual crosslinking agent solution on the surface, and the polyvinyl alcohol ultrafiltration membrane is obtained after drying. The polyvinyl alcohol ultrafiltration membrane can be used to recover 2,2-methylmorpholine in wastewater at low cost and low energy consumption, and the treated wastewater can be directly subjected to biochemical treatment.

Description

Technical Field

[0001] This invention relates to a polyvinyl alcohol ultrafiltration membrane and its preparation method, as well as a technology for using the polyvinyl alcohol ultrafiltration membrane to separate and recover 2,2-methylmorpholine, an impurity generated in wastewater during the production of polyetheramine, and to effectively reduce the COD of the wastewater. Technical Background

[0002] Polyetheramines are polymers with a polyether backbone and amine groups as terminal active functional groups. Due to their unique structure, terminal amino polyethers play an important role in multiple fields as functional chemical products, primarily in epoxy resin curing agents, polyurethane (polyurea) industries, and gasoline detergent dispersants. Currently, there are two methods for producing polyetheramines: the leaving group method and the hydroamination method. The hydroamination method is the most advanced, produces the most stable product quality, and is more environmentally friendly, thus becoming the main production method for polyetheramine industrialization both domestically and internationally. The hydroamination method can be further divided into batch and continuous processes. Compared to the batch method, the continuous production process involves continuous feeding and discharging, allows for arbitrary adjustment of the liquid nitrogen to polyether ratio, has a short residence time of reactants in the reactor, fewer side reactions, stable product quality, lower production costs, and is easier to automate, making it the current main process for polyetheramine production.

[0003] In the process of producing polyetheramine via hydroamylation, wastewater containing 2,2-methylmorpholine is generated. Currently, this wastewater is treated with sulfuric acid, sodium hydroxide, and hydrogen peroxide to decompose the morpholine, reducing the COD of the wastewater before it is sent for biological treatment. Furthermore, to reduce the generation of 2,2-methylmorpholine, an additional distillation separation process is needed to pre-remove dipropylene glycol from the polyether feedstock used in polyetheramine production, reducing its mass fraction to below 0.2%.

[0004] In fact, 2,2-methylmorpholine is an important intermediate in the synthesis of fungicides such as clopyralid, morpholine, and propiconazole, as well as antifungal drugs. It can also be used to prepare rubber vulcanization accelerators, rust inhibitors, preservatives, detergents, scale removers, analgesics, local anesthetics, fruit preservatives, and textile printing and dyeing auxiliaries. Using 2,2-methylmorpholine as a solvent in the production of synthetic fibers offers advantages such as being non-toxic, pollution-free, and producing high-quality products. In particular, the synthetic fibers produced by this method have superior wet properties and strength compared to ordinary viscose fibers, and their temperature performance exceeds that of nylon. Many performance indicators are similar to cotton fibers, earning it the title of "green synthetic fiber."

[0005] Therefore, recovering 2,2-methylmorpholine from wastewater is an environmentally friendly solution that can also increase the added value of the equipment. Summary of the Invention

[0006] In view of this, the present invention provides a self-made polyvinyl alcohol ultrafiltration membrane that can be used to separate and recover impurities in wastewater.

[0007] This invention also provides a method for recovering 2,2-methylmorpholine, an impurity, from wastewater in the production of polyetheramine. By using the self-made polyvinyl alcohol ultrafiltration membrane, 2,2-methylmorpholine in the wastewater can be recovered at low cost and low energy consumption, and the treated wastewater can be directly biochemically treated.

[0008] To achieve the above technical effects, the present invention adopts the following technical solution:

[0009] On one hand, the present invention provides a method for preparing a polyvinyl alcohol ultrafiltration membrane, the steps of which include:

[0010] 1) Mix polyvinyl alcohol with water, plasticizer, polyethylene glycol, and SiO2, heat and stir thoroughly to obtain a polyvinyl alcohol solution, place it in a mold and dry and solidify it to form a film;

[0011] 2) Coat the membrane surface with an aqueous solution of sodium alginate and dry to obtain a composite membrane;

[0012] 3) The composite membrane is brought into contact with a crosslinking agent solution to carry out a crosslinking reaction, and then the residual crosslinking agent solution on the surface is removed by washing with alcohol and dried to obtain the polyvinyl alcohol ultrafiltration membrane.

[0013] In one embodiment, the mass ratio of polyvinyl alcohol to water in step 1) is 1:10 to 20, for example, 1:10, 1:12, 1:14, 1:16, 1:18, 1:20, etc., preferably 1:15 to 20;

[0014] Preferably, based on the total mass of the polyvinyl alcohol solution and water, the mass percentage of the plasticizer is 10-12%, for example, 10%, 10.5%, 11%, 11.5%, 12%, etc., preferably 11-12%; the mass percentage of the polyethylene glycol is 29-40%, for example, 29%, 30%, 32%, 34%, 36%, 38%, 40%, etc., preferably 35-40%; and the mass percentage of the SiO2 is 4-8%, for example, 4%, 5%, 6%, 7%, 8%, etc., preferably 4-6%.

[0015] In one embodiment, the polyethylene glycol in step 1) has a molecular weight of 10,000-50,000, such as 10,000, 20,000, 30,000, 40,000, 50,000, etc.

[0016] In one embodiment, the plasticizer in step 1) is selected from one or more of urea, trimethyl phosphate, and polycaprolactam.

[0017] In one embodiment, the SiO2 particles in step 1) have a particle size of 0.5 to 20 μm, such as 0.5, 1, 3, 5, 7, 10, 12, 15, 18, 20 μm, etc., preferably 5 to 10 μm.

[0018] In one embodiment, the heating and stirring in step 1) are carried out at a temperature of 90-100°C, such as 90, 92, 94, 96, 98, 100°C, preferably 90-95°C; and the stirring time is 1-1.5h, such as 1, 1.1, 1.2, 1.3, 1.4, 1.5h.

[0019] In one embodiment, the drying and curing in step 1) is carried out at a temperature of 60-70°C, preferably 65-70°C, for a time of 6-8 hours, preferably 7-8 hours.

[0020] In one embodiment, the membrane in step 1) has a thickness of 1 to 2 nm, such as 1, 1.2, 1.4, 1.6, 1.8, 2 nm, etc.

[0021] In one embodiment, the sodium alginate aqueous solution in step 2) has a concentration of 2-5 wt%, such as 2, 2.5, 3, 3.5, 4, 4.5, 5 wt%, etc., preferably 2-4 wt%.

[0022] In one embodiment, the drying process in step 2) is not limited by temperature or time. For example, it can be air-dried at room temperature or dried at a temperature of 20-25°C for 10-15 hours.

[0023] In one embodiment, the crosslinking agent in step 3) is selected from one or more of formaldehyde, epoxy resin, maleic anhydride, citric acid, glutaraldehyde, etc., preferably maleic anhydride.

[0024] In one embodiment, the crosslinking agent solution in step 3) has a crosslinking agent concentration of 30-40 wt%, such as 30, 32, 34, 36, 38, 40 wt%.

[0025] Preferably, the crosslinking agent solution further contains 0.5-1 wt% sulfuric acid, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1 wt%.

[0026] Preferably, the crosslinking agent solution is an alcoholic solution of the crosslinking agent, wherein the solvent alcohol is preferably ethanol.

[0027] In one embodiment, in step 3), the composite film is brought into contact with the crosslinking agent solution to carry out a crosslinking reaction. The contact method is not limited, and can be conventional means such as soaking or pouring. The crosslinking reaction is carried out at a temperature of 40-80°C, such as 40, 45, 50, 55, 60, 65, 70, 75, 80°C, and for a time of 5-8 hours, such as 5, 6, 7, 8 hours.

[0028] After the reaction is complete, the alcohol washing is preferably performed using ethanol or isopropanol.

[0029] After the reaction is complete, the drying process can be carried out at any temperature and for any time; for example, it can be air-dried at room temperature.

[0030] In another aspect, the present invention also provides a polyvinyl alcohol ultrafiltration membrane prepared by the above method.

[0031] The ultrafiltration membrane has a porous structure and a thickness of 1-2 mm, preferably 1.5-2 mm.

[0032] The elongation at break of the ultrafiltration membrane is 200-500%, preferably 400-500%.

[0033] The present invention also provides a membrane module made from the polyvinyl alcohol ultrafiltration membrane.

[0034] On the other hand, the present invention also provides the application of the above-mentioned polyvinyl alcohol ultrafiltration membrane and membrane module in the field of organic wastewater treatment.

[0035] In one alternative application, the aforementioned polyvinyl alcohol ultrafiltration membrane and membrane module can recover 2,2-methylmorpholine from wastewater at low cost and low energy consumption, and achieve direct biochemical treatment of the treated wastewater.

[0036] Specifically, the present invention provides a method for recovering 2,2-methylmorpholine from wastewater, the steps of which include:

[0037] S1: Neutralize the wastewater containing 2,2-methylmorpholine to a pH of 5-7;

[0038] S2: The wastewater is separated by a membrane device made of polyvinyl alcohol ultrafiltration membrane of the present invention to obtain recovered 2,2-methylmorpholine and treated wastewater.

[0039] In one embodiment, the wastewater containing 2,2-methylmorpholine described in step S1 has a 2,2-methylmorpholine content of approximately 10,000 to 15,000 ppm and an organic matter COD of approximately 20,000 to 25,000 ppm.

[0040] The organic matter mainly includes ammonia.

[0041] In one embodiment, step S1 neutralizes the wastewater to a pH value of 5 to 7, such as 5, 6, or 7. The neutralization is a conventional acid-base neutralization. For example, alkaline wastewater can be neutralized with acid to a suitable pH range, and acidic wastewater can also have its pH value adjusted with alkaline solution.

[0042] In specific operation, the neutralization temperature is controlled below 50°C, preferably 40-45°C, and the operating pressure is atmospheric pressure.

[0043] Optionally, the wastewater containing 2,2-methylmorpholine is ammonia-containing wastewater from the polyetheramine production system, more specifically, ammonia-containing wastewater obtained from the top of the dehydration tower in the hydroamicization polyetheramine production system, which contains the impurity 2,2-methylmorpholine at a content of approximately 10,000 to 15,000 ppm and organic matter at approximately 15,000 to 25,000 ppm.

[0044] When using the ammonia-containing wastewater from the above-mentioned polyetheramine production system as raw material, a specific operation of step S1 is as follows: First, the dipropylene glycol pre-removal distillation process of the polyether raw material is omitted in the polyetheramine production process. The ammonia-containing wastewater obtained from the top of the dehydration tower is sent to the neutralization tank for acid-base neutralization. The temperature of the neutralization tank is controlled below 50°C, preferably 40-45°C, and the operating pressure is atmospheric pressure. A 2-5 wt% H2SO4 aqueous solution is continuously introduced into the tank to neutralize the pH to 5-7.

[0045] In one embodiment, the membrane device described in step S2 requires that the membrane assembly therein comprises 8 to 10 layers of membranes connected in parallel, preferably 9 to 10 layers.

[0046] In one embodiment, the wastewater from step S2 undergoes the separation process of the membrane device at an operating temperature of 100–120°C, preferably 100–110°C.

[0047] The filtration time in the membrane module is 1.5 to 3 hours, preferably 2 to 2.5 hours.

[0048] The advantages of the wastewater treatment process provided by this invention are that after neutralizing the wastewater, it is only preheated to 100-120°C, and low-energy membrane treatment can be used to remove organic matter such as 2,2-methylmorpholine from the wastewater in the production of polyetheramine, reducing the COD content of the wastewater from 15,000 ppm to below 3,000 ppm. The resulting wastewater can be directly subjected to biochemical treatment, which can effectively reduce the cost of wastewater treatment to 40% of the original cost.

[0049] Based on the above operations, conventional purification methods, such as distillation, can be used to separate the removed organic compounds, such as 2,2-methylmorpholine, to obtain recovered 2,2-methylmorpholine.

[0050] This invention provides a method for reducing 2,2-methylmorpholine in polyetheramine wastewater. Compared with the prior art, this method can effectively reduce the energy consumption of the equipment, is low in cost, and significantly reduce the content of 2,2-methylmorpholine impurities. Detailed Implementation

[0051] The following examples provide a more detailed description of the implementation of the present invention. All examples are operated under the conditions described above, and their purpose is to provide a better understanding of the present invention. Therefore, the examples given do not limit the scope of protection of the present invention.

[0052] The main raw materials used in the various embodiments and comparative examples of this invention are sourced as follows. Unless otherwise specified, other raw materials and reagents were obtained through commercially available channels:

[0053] Among them, the ammonia-containing wastewater contains 2,2-methylmorpholine and originates from the production unit that produces polyetheramine by the hydroamylation method.

[0054] The main analytical methods used in the embodiments and comparative examples of this invention are as follows:

[0055] The gas chromatography conditions were as follows: column oven temperature: 35℃ for 8 min, then ramped to 100℃ at a rate of 5℃ / min, and then ramped to 200℃ at a rate of 10℃ / min for 5 min; column flow rate: 1.5 ml / min; injection port temperature: 240℃; detector temperature: 300℃; split ratio: 5:1; make-up gas flow rate: 50 ml / min.

[0056] The contents of 2,2-methylmorpholine and ammonia in the material were determined by gas chromatography.

[0057] Moisture content was measured using a moisture meter.

[0058] Preparation Example 1

[0059] Preparation of polyvinyl alcohol ultrafiltration membrane 1:

[0060] 1) Polyvinyl alcohol (PVA) and water were mixed at a mass ratio of 1:15. Then, urea (plasticizer), polyethylene glycol (molecular weight 10000), and SiO2 (particle size 5 μm) were added. The mixture was heated to 90°C and stirred thoroughly for 1 hour to obtain a clear and transparent PVA solution. Based on the total mass of PVA and water, the plasticizer urea content was 12 wt%, polyethylene glycol content was 40 wt%, and SiO2 content was 4 wt%. The prepared PVA solution was poured into a horizontally placed stainless steel mold and dried and cured at 65°C for 8 hours to obtain a film with a thickness of 2 nm.

[0061] 2) A 2wt% sodium alginate aqueous solution was coated onto the surface of the membrane and dried at room temperature to obtain a composite membrane;

[0062] 3) The composite membrane was immersed in an ethanol solution of maleic anhydride as a crosslinking agent with a concentration of 30 wt% and containing 0.5 wt% sulfuric acid. After the crosslinking reaction was carried out at 80°C for 5 h, it was taken out and rinsed with ethanol to remove unreacted crosslinking agent. It was then air-dried at room temperature to obtain polyvinyl alcohol ultrafiltration membrane 1. The thickness of the ultrafiltration membrane was 2 mm and the elongation at break of the ultrafiltration membrane was 500%.

[0063] Preparation Example 2

[0064] 1) Polyvinyl alcohol (PVA) and water were mixed at a mass ratio of 1:15. Then, trimethyl phosphate (PTP), polyethylene glycol (PEG) with a molecular weight of 15,000, and SiO2 with a particle size of 10 μm were added. The mixture was heated to 90°C and stirred thoroughly for 1 hour to obtain a clear and transparent PVA solution. Based on the total mass of PVA and water, the content of PTP was 10 wt%, PEG was 40 wt%, and SiO2 was 8 wt%. The prepared PVA solution was poured into a horizontally placed stainless steel mold and dried and cured at 65°C for 8 hours to obtain a film with a thickness of 1.5 nm.

[0065] 2) A 4 wt% sodium alginate aqueous solution was coated onto the surface of the membrane and dried at room temperature to obtain a composite membrane;

[0066] 3) The composite membrane was immersed in an ethanol solution of maleic anhydride as a crosslinking agent with a concentration of 35 wt% and containing 1 wt% sulfuric acid. After the crosslinking reaction was carried out at 80°C for 5 h, the membrane was removed and rinsed with ethanol to remove unreacted crosslinking agent. The membrane was then air-dried at room temperature to obtain polyvinyl alcohol ultrafiltration membrane 2. The thickness of the ultrafiltration membrane was 1.5 mm and the elongation at break of the ultrafiltration membrane was 400%.

[0067] Preparation Example 3

[0068] 1) Polyvinyl alcohol (PVA) and water were mixed at a mass ratio of 1:15. Then, polycaprolactam (PVC), polyethylene glycol (PEG) with a molecular weight of 50,000, and SiO2 with a particle size of 20 μm were added. The mixture was heated to 90°C and stirred thoroughly for 1 hour to obtain a clear and transparent PVA solution. Based on the total mass of PVA and water, the contents of PVC were 12 wt%, PEG 29 wt%, and SiO2 8 wt%. The prepared PVA solution was poured into a horizontally placed stainless steel mold and dried and cured at 65°C for 8 hours to obtain a film with a thickness of 2 nm.

[0069] 2) A 2wt% sodium alginate aqueous solution was coated onto the surface of the membrane and dried at room temperature to obtain a composite membrane;

[0070] 3) The composite membrane was immersed in an ethanol solution of maleic anhydride as a crosslinking agent with a concentration of 40 wt% and containing 0.8 wt% sulfuric acid. After the crosslinking reaction was carried out at 40°C for 8 hours, it was taken out and rinsed with ethanol to remove unreacted crosslinking agent. It was then air-dried at room temperature to obtain polyvinyl alcohol ultrafiltration membrane 3. The thickness of the ultrafiltration membrane was 2 mm and the elongation at break of the ultrafiltration membrane was 500%.

[0071] Example 1

[0072] The raw material used in this embodiment is ammonia-containing wastewater obtained from the top of the dehydration tower in the hydroammoniation method polyetheramine production system. It contains 10,000 ppm of 2,2-methylmorpholine and 20,000 ppm of organic matter COD. The organic matter mainly includes ammonia.

[0073] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, neutralizing the wastewater to neutral. The temperature of the neutralization tank is controlled at 40℃, and the operating pressure is atmospheric pressure. A 2wt% H2SO4 aqueous solution is continuously introduced into the tank to neutralize the pH to 7.

[0074] The wastewater is then heated to 105°C and fed into a membrane unit to remove impurities such as 2,2-methylmorpholine. The filtration time in the membrane module is 2 hours. After removal, the wastewater is cooled to below 40°C and pumped to a biological treatment system for further treatment.

[0075] The membrane device comprises a membrane module made of polyvinyl alcohol ultrafiltration membrane 1 prepared in Preparation Example 1 of the present invention, wherein the membrane module requires 10 membranes to be connected in parallel.

[0076] The analysis indicators of the treated wastewater are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0077] Example 2

[0078] The raw material used in this embodiment is ammonia-containing wastewater obtained from the top of the dehydration tower in the hydroammoniation method polyetheramine production system. It contains 14,2-methylmorpholine at a content of 14,800 ppm and organic matter COD of 25,000 ppm. The organic matter mainly includes ammonia.

[0079] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, neutralizing the wastewater to neutral. The temperature of the neutralization tank is controlled at 45°C, and the operating pressure is atmospheric pressure. A 1.5wt% H2SO4 aqueous solution is continuously introduced into the tank to neutralize the pH to 7.

[0080] The wastewater is then heated to 110°C and fed into a membrane unit to remove impurities such as 2,2-methylmorpholine. The filtration time in the membrane module is 2.5 hours. After removal, the wastewater is cooled to below 40°C and pumped to a biological treatment system for further treatment.

[0081] The membrane device comprises a membrane module made of the polyvinyl alcohol ultrafiltration membrane 2 prepared by Preparation Example 2 of the present invention, wherein the membrane module requires that the number of membranes connected in parallel be 9 layers.

[0082] The analysis indicators of the treated wastewater are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0083] Example 3

[0084] The raw material used in this embodiment is ammonia-containing wastewater obtained from the top of the dehydration tower in the hydroammoniation process of polyetheramine production system. It contains 13,2-methylmorpholine at a content of 13,500 ppm and has an organic matter COD of 23,500 ppm. The organic matter mainly includes ammonia.

[0085] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, neutralizing the wastewater to neutral. The temperature of the neutralization tank is controlled at 42℃, and the operating pressure is atmospheric pressure. A 5wt% H2SO4 aqueous solution is continuously introduced into the tank to neutralize the pH to 7.

[0086] The wastewater is then heated to 108°C and fed into a membrane unit to remove impurities such as 2,2-methylmorpholine. The filtration time in the membrane module is 1.5 hours. After removal, the wastewater is cooled to below 40°C and pumped to a biological treatment system for further treatment.

[0087] The membrane device comprises a membrane module made of the polyvinyl alcohol ultrafiltration membrane 3 prepared in Preparation Example 3 of the present invention, wherein the membrane module requires that the number of membranes connected in parallel be 8 layers.

[0088] The analysis indicators of the treated wastewater are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0089] Comparative Example 1

[0090] This comparative example uses the same wastewater raw material as Example 1, which is sent directly to Fenton treatment without being treated by membrane modules. The product analysis indicators are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0091] Comparative Example 2

[0092] The method is the same as in Example 1, except that the polyvinyl alcohol ultrafiltration membrane 1 prepared in Example 1 is replaced with a pervaporation separation membrane from Zhejiang Huiyong. Other operations and conditions remain unchanged. The product analysis indicators are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0093] Comparative Example 3

[0094] The method is the same as in Example 1, except that SiO2 is not added in step 1), while other operations and conditions remain unchanged, and a film is obtained.

[0095] The polyvinyl alcohol ultrafiltration membrane 1 in Example 1 was replaced with the above-mentioned membrane, and other operations and conditions remained unchanged. The product analysis indicators are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0096] Table 1 Comparison of the treatment effects on wastewater in the examples and comparative examples.

[0097] 2,2-Methylmorpholine concentration (ppm) COD content (ppm) Example 1 2100 3500 Example 2 2820 3652 Example 3 2625 3425 Comparative Example 1 10000 15000 Comparative Example 2 8200 12054 Comparative Example 3 7350 11523

[0098] Table 2 Comparison of wastewater treatment costs in the examples and comparative examples.

[0099]

[0100]

[0101] As can be seen from the table, the new wastewater treatment process has a good effect on reducing 2,2-methylmorpholine in wastewater, and the steam consumption cost is reduced by about 40%.

Claims

1. A method for preparing a polyvinyl alcohol ultrafiltration membrane, characterized by the steps of The method comprises the following steps: 1) mixing polyvinyl alcohol, water, plasticizer, polyethylene glycol and SiO2, and heating and stirring to obtain a polyvinyl alcohol solution, which is dried and solidified in a mold to form a film; 2) coating a sodium alginate aqueous solution on the surface of the film, and drying to obtain a composite film; 3) contacting the composite film with a crosslinking agent solution to perform a crosslinking reaction, then washing the surface with alcohol to remove residual crosslinking agent solution, and drying to obtain the polyvinyl alcohol ultrafiltration membrane.

2. The production method according to claim 1, characterized by, In step 1), the mass ratio of the polyvinyl alcohol to water is 1:10-20; and / or In step 1), the molecular weight of the polyethylene glycol is 10,000-50,000; and / or In step 1), the plasticizer is selected from one or more of urea, trimethyl phosphate and polycaprolactam; and / or In step 1), the particle size of the SiO2 is 0.5-20 um; and / or In step 1), the heating and stirring are performed at a temperature of 90-100℃ for 1-1.5 h; and / or In step 1), the drying and solidification are performed at a temperature of 60-70℃ for 6-8 h.

3. The method of claim 2, wherein, The mass ratio of the polyvinyl alcohol to water is 1:15-20.

4. The preparation method according to claim 2, characterized in that, The particle size of the SiO2 is 5-10 um.

5. The preparation method according to claim 2, characterized in that, The heating and stirring are performed at a temperature of 90-95℃.

6. The preparation method according to claim 2, characterized in that, The drying and solidification are performed at a temperature of 65-70℃ for 7-8 h.

7. The preparation method according to claim 1, characterized in that, In step 1), the mass ratio of the plasticizer is 10-12%, the mass ratio of the polyethylene glycol is 29-40%, and the mass ratio of the SiO2 is 4-8% based on the total mass of the polyvinyl alcohol solution and water.

8. The production method according to claim 7, characterized by, The mass ratio of the plasticizer is 11-12%, the mass ratio of the polyethylene glycol is 35-40%, and the mass ratio of the SiO2 is 4-6%.

9. The method of claim 1, wherein, In step 2), the concentration of the sodium alginate aqueous solution is 2-5 wt%; and / or In step 2), the drying is performed at a temperature of 20-25℃ for 10-15 h.

10. The method of claim 9, wherein, The concentration of the sodium alginate aqueous solution is 2-4 wt%.

11. The method of claim 1, wherein, In step 3), the crosslinking agent is selected from one or more of formaldehyde, epoxy resin, maleic anhydride, citric acid and glutaraldehyde; and / or In step 3), the concentration of the crosslinking agent solution is 30-40 wt%; and / or In step 3), the crosslinking reaction is performed at a temperature of 40-80℃ for 5-8 h. The alcohol washing is performed after the reaction is completed.

12. The method of claim 11, wherein, The alcohol washing is performed using ethanol or isopropanol.

13. The method of claim 1, wherein, In step 3), the crosslinking agent solution contains 0.5-1 wt% of sulfuric acid.

14. The method of claim 1, wherein, In step 3), the crosslinking agent solution is an alcohol solution of the crosslinking agent.

15. The method of claim 14, wherein, In the alcohol solution, the solvent alcohol is ethanol.

16. A polyvinyl alcohol ultrafiltration membrane prepared by the method of any one of claims 1-15.

17. The polyvinyl alcohol ultrafiltration membrane according to claim 16, characterized in that, The thickness of the ultrafiltration membrane is 1-2 mm.

18. The polyvinyl alcohol ultrafiltration membrane according to claim 17, characterized by The thickness of the ultrafiltration membrane is 1.5-2 mm.

19. The polyvinyl alcohol ultrafiltration membrane according to claim 16, wherein, The elongation at break of the ultrafiltration membrane is 200-500%.

20. The polyvinyl alcohol ultrafiltration membrane according to claim 19, characterized in that, The elongation at break of the ultrafiltration membrane is 400-500%.

21. A membrane module prepared from the polyvinyl alcohol ultrafiltration membrane of claim 16.

22. The use of the polyvinyl alcohol ultrafiltration membrane of claim 16 or the membrane module of claim 21 in the treatment of organic wastewater.

23. A method for recovering 2,2-methylmorpholine from wastewater, characterized by the steps of The method comprises the following steps: S1: neutralizing the wastewater containing 2,2-methylmorpholine to pH 5-7; S2: separating the wastewater by a membrane device made of the polyvinyl alcohol ultrafiltration membrane according to claim 16 to obtain recovered 2,2-methylmorpholine and treated wastewater.

24. The recycling method of claim 23, wherein, The wastewater containing 2,2-methylmorpholine according to step S1, wherein the content of 2,2-methylmorpholine is 10000-15000 ppm; the organic matters include ammonia, 2,2-methylmorpholine; and / or The neutralization according to step S1 is controlled at a temperature below 50℃ and an operating pressure of normal pressure.

25. The recycling method of claim 24, wherein, The neutralization according to step S1 is controlled at a temperature of 40-45℃.

26. The recycling method of claim 23, wherein, The membrane device according to step S2 requires that the membrane assembly therein contains 8-10 layers of parallelly connected membranes; and / or The separation process of the wastewater by the membrane device according to step S2 is controlled at a temperature of 100-120℃. The filtration time in the membrane assembly is 1.5-3h.

27. The recycling method of claim 26, wherein, The membrane assembly therein contains 9-10 layers of parallelly connected membranes.

28. The recycling method of claim 26, wherein, The separation process is controlled at a temperature of 100-110℃.

29. The recycling method of claim 26, wherein, The filtration time in the membrane assembly is 2-2.5h.

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