Polyvinyl alcohol ultrafiltration membrane as well as preparation method and application thereof

Abstract

The invention belongs to the field of polyether amine production, and relates to a polyvinyl alcohol ultrafiltration membrane as well as a preparation method and application thereof. The preparation method of the polyvinyl alcohol ultrafiltration membrane comprises the following steps: 1) mixing polyvinyl alcohol with water, a plasticizer, polyethylene glycol and SiO2, heating and fully stirring to obtain a polyvinyl alcohol solution, and drying and curing the polyvinyl alcohol solution in a mold to form a membrane; 2) coating the surface of the membrane with a sodium alginate aqueous solution, and drying to obtain a composite membrane; and 3) contacting the composite membrane with a cross-linking agent solution to carry out cross-linking reaction, then washing with alcohol to remove the cross-linking agent solution left on the surface, and drying to obtain the polyvinyl alcohol ultrafiltration membrane. By using the polyvinyl alcohol ultrafiltration membrane provided by the invention, 2, 2-methylmorpholine in wastewater can be recovered on the premise of low cost and low energy consumption, and direct biochemical treatment of the treated wastewater is realized.

Description

Technical Field

[0001] The invention relates to a polyvinyl alcohol ultrafiltration membrane and a preparation method thereof, and a technology for separating and recovering 2,2-methylmorpholine as an impurity in wastewater generated in the production process of polyetheramines by using the polyvinyl alcohol ultrafiltration membrane and effectively reducing the COD of the wastewater. Technical Background

[0002] Polyetheramine is a kind of polymer with a polyether structure as the main chain and an amine group as the terminal active functional group. Due to its unique structure, as a kind of functional chemical product, amino-terminated polyether plays an important role in many fields, and is mainly used in epoxy resin curing agent, polyurethane (polyurea) industry, gasoline cleaning dispersant and other fields. At present, there are two methods for producing polyetheramine: leaving group method and hydroamination method. Among them, the process route of hydroamination method is the most advanced, and the product quality produced is the most stable, and it is more in line with environmental protection requirements. Therefore, it has become the main production method of polyetheramine industrialization at home and abroad. The hydroamination method can be divided into two production processes: batch method and continuous method. Compared with the batch method, the continuous production process has continuous feeding and discharging, the ratio of liquid nitrogen to polyether can be adjusted arbitrarily, the reaction materials have a short residence time in the reactor, there are few side reactions, the product quality is stable, the production cost is low, and it is easy to realize automatic control, which has become the main process production of polyetheramine at present.

[0003] In the process of producing polyetheramines by hydroamination, a wastewater containing 2,2-methylmorpholine is generated. The current treatment measures for this wastewater are to decompose morpholine through treatment with sulfuric acid, sodium hydroxide, hydrogen peroxide and other agents, reduce the COD of the wastewater and then send it to biochemical treatment. In order to reduce the generation of 2,2-methylmorpholine, it is necessary to add an additional distillation separation process to pre-remove the dipropylene glycol in the raw material polyether for producing polyetheramines and reduce it to a mass fraction of less than 0.2%.

[0004] In fact, 2,2-methylmorpholine is an important intermediate for the synthesis of pesticide fungicides such as caprolactam, morpholin, propiconazole and antifungal drugs. It can also be used to prepare rubber vulcanization accelerators, rust inhibitors, preservatives, detergents, descaling agents, analgesics, local anesthetics, fruit preservatives, textile printing and dyeing auxiliaries, etc. In the process of producing artificial fibers, 2,2-methylmorpholine is used as a solvent, which has the advantages of being non-toxic, pollution-free, and having high product quality. In particular, the wet properties and strength of artificial fibers produced by this method are better than those of ordinary viscose fibers, and the temperature performance exceeds that of nylon. Many performance indicators are similar to those of cotton fibers, and are called "green artificial fibers."

[0005] Therefore, recovering 2,2-methylmorpholine from wastewater is a solution that is both environmentally friendly and can increase the added value of the device. Summary of the invention

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

[0007] The present invention also provides a method for recovering 2,2-methylmorpholine as an impurity in wastewater produced by polyetheramines. By using the self-made polyvinyl alcohol ultrafiltration membrane, 2,2-methylmorpholine in the wastewater can be recovered under the premise of low cost and low energy consumption, and direct biochemical treatment of the treated wastewater can be achieved.

[0008] In order to achieve the above technical effects, the present invention adopts the following technical solutions:

[0009] In one aspect, the present invention provides a method for preparing a polyvinyl alcohol ultrafiltration membrane, comprising the steps of:

[0010] 1) mixing polyvinyl alcohol with water, a plasticizer, polyethylene glycol and SiO2, heating and stirring to obtain a polyvinyl alcohol solution, placing the solution in a mold to dry and solidify to form a film;

[0011] 2) coating the sodium alginate aqueous solution on the surface of the membrane and drying it to obtain a composite membrane;

[0012] 3) contacting the composite membrane with a cross-linking agent solution to carry out a cross-linking reaction, then washing with alcohol to remove the cross-linking agent solution remaining on the surface, and drying to obtain the polyvinyl alcohol ultrafiltration membrane.

[0013] In one embodiment, the mass ratio of the polyvinyl alcohol to water in step 1) is 1:10 to 20, such as 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 proportion of the plasticizer is 10-12%, for example 10%, 10.5%, 11%, 11.5%, 12%, etc., preferably 11-12%, the mass proportion of the polyethylene glycol is 29-40%, for example 29%, 30%, 32%, 34%, 36%, 38%, 40%, etc., preferably 35-40%, the mass proportion of the SiO2 is 4-8%, for example 4%, 5%, 6%, 7%, 8%, etc., preferably 4-6%.

[0015] In one embodiment, the molecular weight of the polyethylene glycol in step 1) is 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 particle size of the SiO2 particles in step 1) is 0.5 to 20 um, for example, 0.5, 1, 3, 5, 7, 10, 12, 15, 18, 20 um, etc., preferably 5 to 10 um.

[0018] In one embodiment, the heating in step 1) is fully stirred, and the heating temperature is 90-100°C, for example, 90, 92, 94, 96, 98, 100°C, etc., preferably 90-95°C; the stirring time is 1-1.5h, for example, 1, 1.1, 1.2, 1.3, 1.4, 1.5h, etc.

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

[0020] In one embodiment, the membrane in step 1) has a thickness of 1 to 2 nm, for example, 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 to 5 wt%, such as 2, 2.5, 3, 3.5, 4, 4.5, 5 wt%, etc., preferably 2 to 4 wt%.

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

[0023] In one embodiment, the cross-linking 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 crosslinker solution in step 3) has a crosslinker concentration of 30-40 wt%, such as 30, 32, 34, 36, 38, 40 wt%, etc.;

[0025] Preferably, the cross-linking agent solution further contains 0.5-1 wt % sulfuric acid, such as 0.5, 0.6, 0.7, 0.8, 0.9, 1 wt %, etc.;

[0026] Preferably, the cross-linking agent solution is an alcohol solution of the cross-linking agent, wherein the solvent alcohol is preferably ethanol.

[0027] In one embodiment, step 3) the composite membrane is contacted with a crosslinking agent solution for a crosslinking reaction, and the contact method is not limited, for example, it can be conventional means such as immersion and pouring, and the crosslinking reaction temperature is 40-80° C., for example, 40, 45, 50, 55, 60, 65, 70, 75, 80° C., etc., and the time is 5-8h, for example, 5, 6, 7, 8h, etc.;

[0028] After the reaction is completed, the alcohol washing is preferably performed with ethanol or isopropanol;

[0029] After the reaction is completed, the drying temperature and time are not limited, for example, it can be 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 the thickness of the ultrafiltration membrane is 1 to 2 mm, preferably 1.5 to 2 mm;

[0032] The breaking elongation of the ultrafiltration membrane is 200 to 500%, preferably 400 to 500%.

[0033] The invention also provides a membrane component made of the polyvinyl alcohol ultrafiltration membrane.

[0034] On the other hand, the present invention also provides the use of the polyvinyl alcohol ultrafiltration membrane and membrane assembly in the field of organic wastewater treatment.

[0035] In an optional application, the above-mentioned polyvinyl alcohol ultrafiltration membrane and membrane assembly can recover 2,2-methylmorpholine in wastewater at low cost and low energy consumption, and realize direct biochemical treatment of the treated wastewater.

[0036] Specifically, the present invention provides a method for recovering 2,2-methylmorpholine in wastewater, the steps comprising:

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

[0038] S2: The wastewater is separated by a membrane device made of the 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 in step S1 has a 2,2-methylmorpholine content of about 10,000 to 15,000 ppm and an organic COD of about 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, 7, etc. The neutralization is conventional acid-base neutralization, for example, alkaline wastewater can be neutralized to a suitable pH range by acid, and acidic wastewater can also be adjusted to a pH value by alkaline solution;

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

[0043] Optionally, the wastewater containing 2,2-methylmorpholine is ammonia-containing wastewater in a polyetheramine production system, more specifically, ammonia-containing wastewater obtained from the top of a dehydration tower in a hydroamination polyetheramine production system, wherein the wastewater contains 2,2-methylmorpholine as an impurity, with a content of about 10000 to 15000 ppm, and organic matter of about 15000 to 25000 ppm;

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

[0045] In one embodiment, the membrane device 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, in step S2, the wastewater is subjected to the separation process of the membrane device at an operating temperature of 100 to 120° C., preferably 100 to 110° C.;

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

[0048] The wastewater treatment process provided by the present invention has the advantage that after the wastewater is neutralized, it is only preheated to 100-120°C, and a low-energy membrane treatment method can be used to remove organic matter such as 2,2-methylmorpholine in the wastewater produced in the production of polyetheramines, and the COD content of the wastewater is reduced from 15000ppm to below 3000ppm. The obtained 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 operation, conventional purification methods such as distillation operation can be used to separate the removed 2,2-methylmorpholine and other organic substances to obtain recovered 2,2-methylmorpholine.

[0050] The invention provides a method for reducing 2,2-methylmorpholine in polyetheramine wastewater. Compared with the prior art, the method can effectively reduce the energy consumption of the device, has low cost and significantly reduces the impurity content of 2,2-methylmorpholine. DETAILED DESCRIPTION

[0051] The following examples are used to further illustrate the embodiments of the present invention, and all the examples are operated according to the operating conditions of the above technical solution, and the purpose is to better understand the content of the present invention. Therefore, the examples given do not limit the protection scope of the present invention.

[0052] In the embodiments and comparative examples of the present invention, the sources of the main raw materials used are as follows. Unless otherwise specified, other raw materials and reagents were purchased from commercial sources:

[0053] Among them, ammonia-containing wastewater, i.e. wastewater containing 2,2-methylmorpholine, comes from the production equipment of polyetheramines by the hydroamination process.

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

[0055] The gas chromatography conditions were as follows: column box temperature: 35°C for 8 min, then heated to 100°C at a rate of 5°C / min, then heated to 200°C at a rate of 10°C / min and held for 5 min; column flow rate: 1.5 ml / min; inlet temperature: 240°C; detector temperature: 300°C; split ratio: 5:1; tail gas flow rate: 50 ml / min.

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

[0057] The moisture content was measured by a moisture meter.

[0058] Preparation Example 1

[0059] Preparation of polyvinyl alcohol ultrafiltration membrane 1:

[0060] 1) Polyvinyl alcohol and water are mixed in a mass ratio of 1:15, and then plasticizer urea, polyethylene glycol with a molecular weight of 10,000, and SiO2 with a particle size of 5um are added, and heated to 90°C and stirred for 1 hour to obtain a clear and transparent polyvinyl alcohol solution, wherein the content of plasticizer urea is 12wt%, the content of polyethylene glycol is 40wt%, and the content of SiO2 is 4wt% based on the total mass of polyvinyl alcohol and water. The prepared polyvinyl alcohol solution is poured into a horizontally placed stainless steel mold, dried and cured at 65°C for 8 hours, and a membrane with a thickness of 2nm is obtained;

[0061] 2) coating a 2 wt % sodium alginate aqueous solution on the surface of the membrane and drying it at room temperature to obtain a composite membrane;

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

[0063] Preparation Example 2

[0064] 1) Polyvinyl alcohol and water are mixed in a mass ratio of 1:15, and then trimethyl phosphate as a plasticizer, polyethylene glycol with a molecular weight of 15,000, and SiO2 with a particle size of 10 um are added, and heated to 90°C and stirred for 1 hour to obtain a clear and transparent polyvinyl alcohol solution, wherein the content of trimethyl phosphate as a plasticizer is 10wt%, the content of polyethylene glycol is 40wt%, and the content of SiO2 is 8wt% based on the total mass of polyvinyl alcohol and water. The prepared polyvinyl alcohol solution is poured into a horizontally placed stainless steel mold, and dried and cured at 65°C for 8 hours to obtain a membrane with a thickness of 1.5 nm;

[0065] 2) coating a 4 wt % sodium alginate aqueous solution on the surface of the membrane and drying it at room temperature to obtain a composite membrane;

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

[0067] Preparation Example 3

[0068] 1) Polyvinyl alcohol and water are mixed in a mass ratio of 1:15, and then plasticizer polycaprolactam, polyethylene glycol with a molecular weight of 50,000, and SiO2 with a particle size of 20 um are added, and heated to 90°C and stirred for 1 hour to obtain a clear and transparent polyvinyl alcohol solution, based on the total mass of polyvinyl alcohol and water, wherein the plasticizer polycaprolactam content is 12wt%, the polyethylene glycol content is 29wt%, and the SiO2 content is 8wt%. The prepared polyvinyl alcohol solution is poured into a horizontally placed stainless steel mold, dried and cured at 65°C for 8 hours, and a membrane with a thickness of 2nm is obtained;

[0069] 2) coating a 2 wt % sodium alginate aqueous solution on the surface of the membrane and drying it at room temperature to obtain a composite membrane;

[0070] 3) The composite membrane was immersed in an ethanol solution of maleic anhydride as a cross-linking agent with a concentration of 40wt%, which contained 0.8wt% sulfuric acid, and was taken out after cross-linking reaction at 40°C for 8h, and then rinsed with ethanol to remove the unreacted cross-linking agent, and dried at room temperature to obtain a polyvinyl alcohol ultrafiltration membrane 3; the thickness of the ultrafiltration membrane was 2mm; and the elongation at break of the ultrafiltration membrane was 500%.

[0071] Example 1

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

[0073] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, and the wastewater is neutralized to neutrality. The temperature of the neutralization tank is controlled at 40°C, the operating pressure is normal pressure, and 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 sent to the membrane device to remove impurities such as 2,2-methylmorpholine. The filtration time of the material in the membrane module is 2 hours. The removed wastewater is cooled to below 40°C and pumped to the biochemical system for wastewater treatment.

[0075] The membrane device comprises a membrane assembly made of the polyvinyl alcohol ultrafiltration membrane 1 prepared in Preparation Example 1 of the present invention, and the number of membranes connected in parallel in the membrane assembly is required to be 10 layers.

[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 example is ammonia-containing wastewater obtained from the top of a dehydration tower in a hydroamination polyetheramine production system, which contains 14800 ppm of 2,2-methylmorpholine and 25000 ppm of organic matter COD, wherein the organic matter mainly includes ammonia.

[0079] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, and the wastewater is neutralized to neutrality. The temperature of the neutralization tank is controlled at 45°C, the operating pressure is normal pressure, and 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 sent to the membrane device to remove impurities such as 2,2-methylmorpholine. The filtration time of the material in the membrane module is 2.5 hours. The removed wastewater is cooled to below 40°C and pumped to the biochemical system for wastewater treatment.

[0081] The membrane device comprises a membrane assembly made of the polyvinyl alcohol ultrafiltration membrane 2 prepared in Preparation Example 2 of the present invention, and the number of membranes connected in parallel in the membrane assembly is required to 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 example is ammonia-containing wastewater obtained from the top of a dehydration tower in a hydroamination polyetheramine production system, which contains 13500 ppm 2,2-methylmorpholine and 23500 ppm organic matter COD, wherein the organic matter mainly includes ammonia.

[0085] The ammonia-containing wastewater is pumped into a neutralization tank for acid-base neutralization, and the wastewater is neutralized to neutrality. The temperature of the neutralization tank is controlled at 42°C, the operating pressure is normal pressure, and 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 sent to the membrane device to remove impurities such as 2,2-methylmorpholine. The filtration time of the material in the membrane module is 1.5 hours. The removed wastewater is cooled to below 40°C and pumped to the biochemical system for wastewater treatment.

[0087] The membrane device comprises a membrane assembly made of the polyvinyl alcohol ultrafiltration membrane 3 prepared in Preparation Example 3 of the present invention, and the number of membranes connected in parallel in the membrane assembly is required to 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 directly sent to Fenton treatment without being treated by a membrane module. Its product analysis indicators are shown in Table 1, and the wastewater treatment cost is shown in Table 2.

[0091] Comparative Example 2

[0092] Referring to the method of Example 1, the difference is that the polyvinyl alcohol ultrafiltration membrane 1 prepared in Example 1 is replaced with the pervaporation separation membrane of Zhejiang Huiyong, and 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] Referring to the method of Preparation Example 1, the difference is that: SiO2 is not added in step 1), and other operations and conditions remain unchanged to obtain a membrane;

[0095] The above-mentioned membrane was used to replace the polyvinyl alcohol ultrafiltration membrane 1 in Example 1, 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 effects of treated wastewater in the embodiments 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 embodiments 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 in that the steps include: 1) mixing polyvinyl alcohol with water, a plasticizer, polyethylene glycol and SiO2, heating and stirring to obtain a polyvinyl alcohol solution, placing the solution in a mold to dry and solidify to form a film; 2) coating the sodium alginate aqueous solution on the surface of the membrane and drying it to obtain a composite membrane; 3) contacting the composite membrane with a cross-linking agent solution to carry out a cross-linking reaction, then washing with alcohol to remove the cross-linking agent solution remaining on the surface, and drying to obtain the polyvinyl alcohol ultrafiltration membrane.

2. The preparation method according to claim 1, characterized in that: Step 1) The mass ratio of polyvinyl alcohol to water is 1:10-20, preferably 1:15-20; Preferably, based on the total mass of the polyvinyl alcohol solution and water, the mass proportion of the plasticizer is 10-12%, preferably 11-12%, the mass proportion of the polyethylene glycol is 29-40%, preferably 35-40%, and the mass proportion of the SiO2 is 4-8%, preferably 4-6%; and / or Step 1) the molecular weight of the polyethylene glycol is 10,000 to 50,000; and / or Step 1) the plasticizer is selected from one or more of urea, trimethyl phosphate, and polycaprolactam; and / or Step 1) The SiO2 particles have a particle size of 0.5 to 20 um, preferably 5 to 10 um; and / or Step 1) the heating is fully stirred, the heating temperature is 90-100°C, preferably 90-95°C; the stirring time is 1-1.5h; and / or In step 1), the drying and curing is performed at a temperature of 60 to 70° C., preferably 65 to 70° C., and for a time of 6 to 8 hours, preferably 7 to 8 hours.

3. The preparation method according to claim 1, characterized in that: Step 2) the sodium alginate aqueous solution has a concentration of 2 to 5 wt%, preferably 2 to 4 wt%; and / or Step 2) the drying is carried out at a temperature of 20-25° C. for 10-15 hours.

4. The preparation method according to claim 1, characterized in that: Step 3) the cross-linking agent is selected from one or more of formaldehyde, epoxy resin, maleic anhydride, citric acid, glutaraldehyde, preferably maleic anhydride; and / or Step 3) the cross-linking agent solution, the cross-linking agent concentration is 30-40wt%; Preferably, the cross-linking agent solution further contains 0.5-1wt% sulfuric acid; Preferably, the cross-linking agent solution is an alcohol solution of the cross-linking agent, wherein the solvent alcohol is preferably ethanol; and / or Step 3) the cross-linking reaction is carried out at a temperature of 40-80°C and a time of 5-8h; After the reaction is completed, the alcohol washing is preferably performed with ethanol or isopropanol.

5. A polyvinyl alcohol ultrafiltration membrane prepared by the method according to any one of claims 1 to 4; Preferably, the thickness of the ultrafiltration membrane is 1 to 2 mm, preferably 1.5 to 2 mm; Preferably, the elongation at break of the ultrafiltration membrane is 200-500%, preferably 400-500%.

6. A membrane assembly made of the polyvinyl alcohol ultrafiltration membrane according to claim 5.

7. Use of the polyvinyl alcohol ultrafiltration membrane according to claim 5 and the membrane assembly according to claim 6 in the field of organic wastewater treatment.

8. A method for recovering 2,2-methylmorpholine from wastewater, characterized in that the steps include: S1: neutralize the wastewater containing 2,2-methylmorpholine to a pH value of 5 to 7; S2: The wastewater is separated by a membrane device made of the polyvinyl alcohol ultrafiltration membrane described in claim 5 to obtain recovered 2,2-methylmorpholine and treated wastewater.

9. The recycling method according to claim 8, characterized in that: The wastewater containing 2,2-methylmorpholine in step S1, wherein the content of 2,2-methylmorpholine is about 10000-15000 ppm; The organic matter includes ammonia, 2,2-methylmorpholine; and / or In the neutralization step S1, the temperature is controlled below 50° C., preferably 40-45° C., and the operating pressure is normal pressure.

10. The recycling method according to claim 8, characterized in that: The membrane device in step S2 requires that the membrane assembly comprises 8 to 10 layers of membranes in parallel, preferably 9 to 10 layers; and / or Step S2: the wastewater is subjected to the separation process of the membrane device, and the operating temperature is 100-120°C, preferably 100-110°C; The filtration time in the membrane module is 1.5 to 3 hours, preferably 2 to 2.5 hours.

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