Preparation method of pervaporation membrane for recovering DMF (Dimethyl Formamide) from low-concentration DMF wastewater

Abstract

The invention discloses a preparation method of a pervaporation membrane for recovering DMF (Dimethyl Formamide) from low-concentration DMF wastewater. The pervaporation membrane is a membrane obtained by blending polyvinyl alcohol and chitosan and crosslinking with polyacrylic acid. The invention also discloses a preparation method of the pervaporation membrane, which specifically comprises the following steps: firstly preparing an aqueous solution of polyvinyl alcohol and polyacrylic acid, simultaneously preparing an acetic acid aqueous solution of chitosan, mixing and stirring according to a certain mass ratio, preparing a membrane by a solution casting method, and finally carrying out heat treatment to obtain the required pervaporation membrane. According to the pervaporation membrane, DMF in a low-concentration DMF aqueous solution can be effectively separated out, DMF recycling is achieved, the permeation flux is improved, meanwhile, the mechanical performance and heat stability of the membrane can be effectively improved through the chemical crosslinking and heat treatment method, the swelling resistance of the membrane is enhanced, and long-time operation can be achieved.

Description

Technical Field

[0001] The present invention relates to a pervaporation membrane for recovering DMF from low-concentration DMF wastewater, and also relates to a preparation method of the above pervaporation membrane for recovering DMF from low-concentration DMF wastewater. Background Art

[0002] N,N-dimethylformamide (DMF) is an excellent organic solvent and a widely used chemical raw material, commonly used in industries such as medicine, petrochemical, agricultural production, and synthetic leather. In the wet polyurethane synthetic leather process, DMF finally exists in the form of waste liquid with a mass fraction of 10-25%, however, the maximum allowable mass concentration of DMF in surface water in China is 25 mg·L -1 . According to statistics, nearly 100 million tons of DMF wastewater is generated by the synthetic leather industry in China every year. If it can be recycled, it can not only protect the environment, but also save production raw materials and reduce waste.

[0003] The pervaporation membrane separation technology utilizes the different affinities and mass transfer resistances of the membrane for different components in the feed liquid, and has obvious competitive advantages compared with other separation methods with the advantages of low energy consumption and high efficiency. However, due to the strong solvation ability of DMF, it may cause some membrane materials to swell or dissolve, thus affecting the structure and performance of the membrane and reducing the pervaporation separation performance. Therefore, the DMF / water system limits the research and application of pervaporation membrane materials.

[0004] Pervaporation membranes can be divided into inorganic membranes and organic membranes. Organic membranes have the disadvantages of easy swelling, poor thermal stability and mechanical properties, but their low cost, simple preparation and good chemical stability have attracted extensive attention. Generally speaking, organic membranes can play the complementary and synergistic effects of the properties between different polymers through blending modification to increase the permeation flux while improving the separation efficiency, or chemical cross-linking and heat treatment methods can help reduce the swelling of the membrane, improve the pervaporation separation effect and reduce the cost, which will be more helpful to reduce energy consumption.

[0005] D.A. Devi et al. (Separation and Purification Technology 51 (2006) 104–111) used cross-linked polyvinyl alcohol (PVA) and polyacrylic acid (PAAc) blend membranes and applied them to the dehydration of DMF aqueous solutions. They studied the separation performance of the membranes at DMF mass concentrations of 50 wt% - 98 wt%, but did not conduct extended applicability studies on low-concentration DMF water systems. Chinese Patent CN 112588118A disclosed a composite membrane preparation method and its separation application for separating DMF and water using polyvinyl alcohol and polyacrylic acid as raw materials doped with modified silica, but this membrane has high swelling and poor stability in low-concentration DMF water systems. S.Das et al. (Desalination 197 (2006) 106 - 116) separated DMF aqueous solutions through polyurethane urea (PUU) and polymethyl methacrylate (PMMA) membranes, and studied the flux and selectivity at different DMF mass concentrations (5 - 80%) to explore the applicability of this membrane for recovering pure DMF. The membrane preparation process is relatively complex, and in low-concentration DMF water systems, the separation performance of the membrane needs to be improved. Currently, existing membranes mainly focus on the separation research of DMF water in a larger concentration range, and there are problems such as poor membrane stability, low permeation flux, and low pervaporation separation performance in low-concentration DMF water systems. Summary of the Invention

[0006] Technical Problem: Aiming at the problems of poor stability, low permeation flux, and low pervaporation separation performance of the organic membranes for separating DMF and water in the prior art, the present invention provides a polyacrylic acid cross-linked modified polyvinyl alcohol / chitosan membrane that can selectively recover DMF from low-concentration DMF wastewater, and at the same time provides a preparation method for the above-mentioned membrane.

[0007] Technical Solution: A method for preparing a pervaporation membrane for recovering DMF from low-concentration DMF wastewater according to the present invention: The pervaporation membrane is prepared by chemical cross-linking and heat treatment. By selecting biocompatible polyvinyl alcohol and chitosan as the membrane matrix and using polyacrylic acid as the cross-linking agent, the final pervaporation membrane is obtained after heat treatment of the prepared membrane. The specific preparation steps are as follows:

[0008] Step 1. First, prepare an aqueous solution of polyvinyl alcohol, and add polyacrylic acid to the aqueous solution of polyvinyl alcohol to form a mixed solution; at the same time, prepare an aqueous acetic acid solution, and add chitosan to the aqueous acetic acid solution to form an acetic acid aqueous solution of chitosan.

[0009] Step 2. Mix and stir the above two aqueous solutions according to the mass ratio and defoam to obtain a casting solution.

[0010] Step 3. Use the solution casting method to prepare the casting solution into a membrane.

[0011] Step 4. Heat-treat the membrane prepared in Step 3 to obtain a pervaporation membrane crosslinked by polyvinyl alcohol, chitosan, and polyacrylic acid.

[0012] The mass concentration of DMF in the low-concentration DMF wastewater is 5 wt% - 30 wt%.

[0013] In Step 1, the degree of polymerization of polyvinyl alcohol is 1500 - 1800, and the degree of alcoholysis is 50% - 99%;

[0014] In Step 1, the molecular weight of polyacrylic acid is 2000.

[0015] The degree of deacetylation of chitosan is ≥85%, and the viscosity is 100 - 400 mPa·s.

[0016] In Step 1, prepare an aqueous solution of polyvinyl alcohol and polyacrylic acid. Specifically: Add polyvinyl alcohol and deionized water to a container, heat and stir at 80 - 90°C for 2 - 4 h until the solution is homogeneous and transparent to obtain an aqueous solution of polyvinyl alcohol with a mass concentration of 2 wt%. Take polyacrylic acid and add it to the aqueous solution of polyvinyl alcohol, and stir the solution to obtain an aqueous solution mixed with polyvinyl alcohol and polyacrylic acid, where polyacrylic acid is 1 - 15 wt% of polyvinyl alcohol.

[0017] In Step 1, prepare an acetic acid aqueous solution of chitosan. Specifically: Prepare a solution with an acetic acid mass concentration of 1 - 4% using glacial acetic acid and deionized water, use it as a solvent to dissolve chitosan, and stir at room temperature for 4 - 6 h until the chitosan is completely dissolved to obtain an acetic acid aqueous solution of chitosan with a mass concentration of 1 - 4%.

[0018] In Step 2, mix the aqueous solution of polyvinyl alcohol and polyacrylic acid with the acetic acid aqueous solution of chitosan in a mass ratio of 3:1 - 1:3, stir evenly for 3 - 4 h, and perform ultrasonic degassing for 1 - 2 h to obtain a casting solution.

[0019] In Step 4, the heat treatment temperature is 120°C - 160°C, and the heat treatment time is 2 - 4 h.

[0020] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages:

[0021] 1. The polyvinyl alcohol / chitosan-polyacrylic acid-polypropylene acid membrane of the present invention realizes the preferential selectivity for DMF at low concentrations (5wt%-30wt%), thereby improving the permeation flux and separation coefficient of the membrane when separating low-concentration DMF aqueous solutions, enhancing the separation performance of the membrane, effectively solving the problem of low pervaporation separation performance in existing research, and is expected to realize the recovery of DMF in synthetic leather wastewater; by cross-linking polyacrylic acid with polyvinyl alcohol and chitosan and performing heat treatment, the mechanical strength of the membrane is improved, the swelling resistance of the membrane is enhanced, and the thermal stability and chemical stability of the membrane are improved, enabling long-term operation and extending the service life of the membrane.

[0022] 2. The preparation process of the present invention is simple, green and pollution-free, with low preparation costs, has the potential for treating industrial synthetic leather wastewater containing DMF, and realizes the low-energy and efficient separation of DMF and water. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a pervaporation experimental device diagram of the present invention. Among them: 1 - raw material bottle; 2 - feed pump; 3 - membrane module; 4 - constant temperature device; 5 - vacuum gauge; 6 - permeate collection bottle; 7 - cold trap; 8 - drying tower; 9 - safety bottle; 10 - vacuum degree adjustment valve; 11 - vacuum pump; 12 - retentate collection bottle. DETAILED DESCRIPTION OF THE INVENTION

[0024] The polyvinyl alcohol / chitosan-polyacrylic acid membrane in the present invention is used for pervaporation separation of DMF / H2O solutions, and its device flow chart is as shown in the appendix Figure 1 shown. A stainless steel pervaporation cell is used, and the membrane is supported on a stainless steel porous plate, with an effective membrane area of 10 cm 2 . A DMF / H2O feed solution with a certain concentration (DMF mass concentration of 5-30wt%) is sent into the membrane module through a feed pump. The membrane module is placed in an oven to maintain a constant temperature. The vacuum pump is turned on and the vacuum degree is displayed by a vacuum gauge. After the operation is stable, tests are carried out. The permeate passes through the membrane, vaporizes on the back side of the membrane and is condensed and collected in the cold trap. The mass of the collected permeate is accurately weighed, and the composition of the permeate is analyzed by an Abbe refractometer. The pervaporation performance evaluation of the membrane is comprehensively evaluated by the permeation flux and the separation coefficient.

[0025] The present invention forms a crosslinked network structure by crosslinking polyacrylic acid with polyvinyl alcohol / chitosan. By adjusting the content of the crosslinking agent, the membrane effectively realizes the preferential selectivity for DMF at low concentrations of DMF, making the membrane more likely to bind to DMF and increasing the adsorption content of DMF. Secondly, a polyvinyl alcohol / chitosan-polyacrylic acid membrane is prepared by a chemical crosslinking modification method. Polyvinyl alcohol and chitosan play a complementary role in performance, and the addition of polyacrylic acid promotes the formation of a crosslinked network structure in the composite membrane. The polyvinyl alcohol membrane has a high crystallinity and a low permeation flux, while the chitosan membrane has a low crystallinity and a high flux. When the ratio of polyvinyl alcohol to chitosan is 1:1 and the content of polyacrylic acid is 4 wt%, the membrane has a large permeation flux and a large separation factor for DMF during pervaporation separation of low-concentration DMF aqueous solution. Finally, after heat treatment, the polyvinyl alcohol / chitosan-polyacrylic acid membrane can effectively increase the mechanical strength of the membrane, reduce the swelling degree of the membrane, and improve the thermal stability and chemical stability of the membrane, enabling long-term operation. The following further illustrates the technical solution of the present invention in combination with specific examples.

[0026] A method for recovering DMF by pervaporation membrane from low-concentration DMF wastewater in the present invention is as follows: The pervaporation membrane is prepared by chemical crosslinking and heat treatment. By selecting biocompatible polyvinyl alcohol and chitosan as the membrane matrix and using polyacrylic acid as the crosslinking agent, the final pervaporation membrane is obtained after heat treatment of the prepared membrane. The specific preparation steps are as follows:

[0027] Step 1. First, prepare an aqueous solution of polyvinyl alcohol, and add polyacrylic acid to the aqueous solution of polyvinyl alcohol to form a mixed solution; at the same time, prepare an aqueous solution of acetic acid, and add chitosan to the aqueous solution of acetic acid to form an acetic acid aqueous solution of chitosan.

[0028] Step 2. Mix and stir the above two aqueous solutions according to the mass ratio and defoam to obtain a casting solution.

[0029] Step 3. Use the solution casting method to prepare the casting solution into a membrane.

[0030] Step 4. Heat-treat the membrane prepared in Step 3 to obtain a pervaporation membrane crosslinked by blending polyvinyl alcohol and chitosan with polyacrylic acid.

[0031] Example 1

[0032] The preparation method of the pervaporation membrane for separating DMF and water in synthetic leather wastewater in the present invention includes the following steps:

[0033] Weigh the polyvinyl alcohol, polyacrylic acid and water, and add them into a 250 ml round-bottom flask. Stir at 90 °C for 2 h until the polyvinyl alcohol and polyacrylic acid are completely dissolved to obtain a 2 wt% aqueous solution of polyvinyl alcohol, where the polyacrylic acid is 6 wt% of the polyvinyl alcohol; weigh glacial acetic acid and add it to water to obtain a 2 wt% aqueous acetic acid solution, and add it to the weighed chitosan, stir for 4 h until completely dissolved to obtain a 2 wt% aqueous acetic acid solution of chitosan; mix and stir the aqueous solution of polyvinyl alcohol and the aqueous acetic acid solution of chitosan to obtain a mixed solution, and the mass ratio of polyvinyl alcohol to chitosan in the mixed solution is 1:1; use the solution casting method to prepare a film, volatilize the solvent in a fume hood for 48 h, peel off the film and heat-treat it in an oven at 120 °C for 2 h to obtain a cross-linked film of PVA / CS-PAA6.

[0034] The film was subjected to pervaporation separation of DMF / aqueous solution (DMF 20 wt%, water 80 wt%). The pressure on the feed side was kept at atmospheric pressure, the permeate side was evacuated, the temperature of the feed liquid was 20 °C, and the pervaporation performance was tested. The permeation flux of the film was 675.93 g / (m 2 ·h), and the separation factor was 2.08.

[0035] Example 2

[0036] Weigh the polyvinyl alcohol, polyacrylic acid and water, and add them into a 250 ml round-bottom flask. Stir at 90 °C for 2 h until the polyvinyl alcohol and polyacrylic acid are completely dissolved to obtain a 2 wt% aqueous solution of polyvinyl alcohol, where the polyacrylic acid is 8 wt% of the polyvinyl alcohol; weigh glacial acetic acid and add it to water to obtain a 2 wt% aqueous acetic acid solution, and add it to the weighed chitosan, stir for 4 h until completely dissolved to obtain a 2 wt% aqueous acetic acid solution of chitosan; mix and stir the aqueous solution of polyvinyl alcohol and the aqueous acetic acid solution of chitosan to obtain a mixed solution, and the mass ratio of polyvinyl alcohol to chitosan in the mixed solution is 1:1; use the solution casting method to prepare a film, volatilize the solvent in a fume hood for 48 h, peel off the film and heat-treat it in an oven at 120 °C for 2 h to obtain a cross-linked film of PVA / CS-PAA8.

[0037] The film was subjected to pervaporation separation of DMF / aqueous solution (DMF 10 wt%, water 90 wt%). The pressure on the feed side was kept at atmospheric pressure, the permeate side was evacuated, the temperature of the feed liquid was 20 °C, and the pervaporation performance was tested. The permeation flux of the film was 674.35 g / (m 2 ·h), and the separation factor was 1.47.

[0038] Example 3

[0039] Weigh the polyvinyl alcohol, polyacrylic acid and water and add them to a 250 ml round-bottom flask. Stir at 90 °C for 2 h until the polyvinyl alcohol and polyacrylic acid are completely dissolved to obtain a 2 wt% aqueous solution of polyvinyl alcohol, where the polyacrylic acid is 4 wt% of the polyvinyl alcohol; weigh glacial acetic acid and add it to water to obtain a 2 wt% aqueous acetic acid solution, and add it to the weighed chitosan. Stir for 4 h until completely dissolved to obtain a 2 wt% aqueous acetic acid solution of chitosan; mix and stir the aqueous solution of polyvinyl alcohol and the aqueous acetic acid solution of chitosan to obtain a mixed solution, and the mass ratio of polyvinyl alcohol to chitosan in the mixed solution is 1:1; use the solution casting method to prepare a film, volatilize the solvent in a fume hood for 48 h, peel off the film and heat-treat it in an oven at 120 °C for 2 h to obtain a cross-linked film of PVA / CS-PAA4.

[0040] Perform pervaporation separation of the DMF / aqueous solution (DMF 20 wt%, water 80 wt%) on the film. Keep the pressure on the feed side at atmospheric pressure, evacuate the permeate side, and the temperature of the feed liquid is 20 °C. Perform pervaporation performance testing to obtain the permeation flux of the film as 631.43 g / (m 2 ·h), and the separation factor is 4.24.

[0041] Example 4

[0042] Weigh the polyvinyl alcohol, polyacrylic acid and water and add them to a 250 ml round-bottom flask. Stir at 90 °C for 2 h until the polyvinyl alcohol and polyacrylic acid are completely dissolved to obtain a 2 wt% aqueous solution of polyvinyl alcohol, where the polyacrylic acid is 4 wt% of the polyvinyl alcohol; weigh glacial acetic acid and add it to water to obtain a 2 wt% aqueous acetic acid solution, and add it to the weighed chitosan. Stir for 4 h until completely dissolved to obtain a 2 wt% aqueous acetic acid solution of chitosan; mix and stir the aqueous solution of polyvinyl alcohol and the aqueous acetic acid solution of chitosan to obtain a mixed solution, and the mass ratio of polyvinyl alcohol to chitosan in the mixed solution is 1:1; use the solution casting method to prepare a film, volatilize the solvent in a fume hood for 48 h, peel off the film and heat-treat it in an oven at 120 °C for 2 h to obtain a cross-linked film of PVA / CS-PAA4.

[0043] Perform pervaporation separation of the DMF / aqueous solution (DMF 20 wt%, water 80 wt%) on the film. Keep the pressure on the feed side at atmospheric pressure, evacuate the permeate side, and the temperature of the feed liquid is 50 °C. Perform pervaporation performance testing to obtain the permeation flux of the film as 975.51 g / (m 2 ·h), and the separation factor is 2.26.

Claims

1. A method for recovering DMF pervaporation membrane from low-concentration DMF waste water, characterized in that: The pervaporation membrane is prepared by chemical crosslinking and heat treatment. Biocompatible polyvinyl alcohol and chitosan are selected as membrane matrices, polyacrylic acid is used as a crosslinking agent, and heat treatment is performed after the membrane is prepared to obtain the final pervaporation membrane. The specific steps of its preparation are as follows: Step 1. First, prepare a polyvinyl alcohol aqueous solution, add polyacrylic acid to the polyvinyl alcohol aqueous solution to prepare a mixed solution; and simultaneously prepare an acetic acid aqueous solution, add chitosan to the acetic acid aqueous solution to prepare an acetic acid aqueous solution of chitosan; Step 2. Mix the above two aqueous solutions according to the mass ratio, stir and degas to obtain a casting solution; Step 3. preparing a film from the casting solution by a solution casting method; Step 4. Heat-treating the membrane prepared in step 3 to obtain a pervaporation membrane cross-linked with polyvinyl alcohol and chitosan blended with polyacrylic acid.

2. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that The mass concentration of DMF in the low-concentration DMF wastewater is 5wt%-30wt%.

3. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, In step 1, the degree of polymerization of polyvinyl alcohol is 1500-1800, and the degree of alcoholysis is 50%-99%.

4. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, In step 1, the molecular weight of polyacrylic acid is 2000.

5. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, The deacetylation degree of chitosan is ≥85%, and the viscosity is 100-400mPa.s.

6. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, In step 1, an aqueous solution of polyvinyl alcohol and polyacrylic acid is prepared, specifically, polyvinyl alcohol and deionized water are added to a container, heated and stirred at 80-90° C. for 2-4 hours until the solution is uniform and transparent to obtain an aqueous solution of polyvinyl alcohol, wherein the mass concentration of polyvinyl alcohol is 2wt%, polyacrylic acid is added to the aqueous solution of polyvinyl alcohol, and the solution is stirred to obtain an aqueous solution of polyvinyl alcohol and polyacrylic acid, wherein the polyacrylic acid is 1-15wt% of the polyvinyl alcohol.

7. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, In step 1, an acetic acid aqueous solution of chitosan is prepared, specifically: glacial acetic acid and deionized water are used to prepare a solution with an acetic acid mass concentration of 1-4%, and the solution is used as a solvent to dissolve chitosan, and the solution is stirred at room temperature for 4 to 6 hours until the chitosan is completely dissolved to obtain an acetic acid aqueous solution of chitosan with a mass concentration of 1-4%.

8. The method for recovering DMF pervaporation membrane from low-concentration DMF waste water according to claim 1, characterized in that, In step 2, the aqueous solution of polyvinyl alcohol and polyacrylic acid is mixed with the acetic acid aqueous solution of chitosan at a mass ratio of 3:1 to 1:3, stirred evenly for 3 to 4 hours, and subjected to ultrasonic degassing for 1 to 2 hours to obtain a casting solution.

9. The method for recovering DMF from low-concentration DMF waste water by pervaporation membrane according to claim 1, characterized in that, In step 4, the heat treatment temperature is 120°C-160°C, and the heat treatment time is 2 to 4 hours.

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