High-pressure-resistant flat sheet membrane, preparation method and application

Abstract

The invention discloses a high-pressure-resistance flat sheet membrane as well as a preparation method and application thereof. The flat sheet membrane is prepared by taking a porous non-woven fabric as a carrier, dissolving polysulfone, quaternized polysulfone (cationic polyelectrolyte) and sulfonated polysulfone (anionic polyelectrolyte) in an organic solvent to form a membrane casting solution and carrying out a phase inversion method, and has a density gradient spongy pore structure. Wherein the quaternized polysulfone is prepared by reacting chloromethylated polysulfone with a trimethylamine solution, and the content of polysulfone in the membrane casting solution is 5%-15%, the content of quaternized polysulfone is 3%-10%, and the content of sulfonated polysulfone is 2%-8%. After the membrane is pre-pressed at the high pressure of 1MPa for 30 minutes, the water flux attenuation rate is lower than 10%, and the membrane is expected to be used for preparing a high-pressure reverse osmosis membrane. The preparation process comprises the steps of cationic polyelectrolyte synthesis, membrane casting solution preparation and flat sheet membrane forming.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite reverse osmosis membranes, and in particular relates to a high-pressure-resistant flat membrane, a preparation method and applications. Background Art

[0002] Research on composite reverse osmosis membranes, both domestically and internationally, has focused on elucidating their separation mechanisms, modifying existing membrane materials, and developing new, high-performance membrane materials. However, the selection and performance improvement of flat membranes, which serve as the reverse osmosis base membrane, have been neglected. With the rapid development of composite reverse osmosis technology in recent years, researchers have increasingly recognized that the performance of composite membranes is closely related not only to the properties of the ultrathin functional layer but also to the performance of the base membrane. The structure of the base membrane is closely related to its performance, and the key to improving base membrane performance lies in the precise control of the membrane microstructure. Reverse osmosis membranes operate at high pressures, placing even higher demands on the pressure resistance of the flat membranes used as the reverse osmosis base membrane. Summary of the Invention

[0003] The purpose of the present invention is to improve the pressure resistance and flux performance of a flat membrane.

[0004] The purpose of the present invention is to adopt the following technical solutions to achieve:

[0005] A high-pressure-resistant flat membrane comprises a porous non-woven fabric carrier and a polyelectrolyte complex separation layer formed thereon;

[0006] The separation layer is prepared by a phase inversion method using a casting solution containing anionic polyelectrolytes and cationic polyelectrolytes, and has a density gradient sponge-like pore structure;

[0007] The cationic polyelectrolyte is quaternized polysulfone, the anionic polyelectrolyte is sulfonated polysulfone, and the water flux difference of the flat membrane after pre-pressing at 1 MPa and 0.1 MPa for 30 minutes does not exceed 50 L / m 2 ·h.

[0008] Preferably, the composition of the separation layer comprises, by mass percentage, 5%-15% polysulfone, 3%-10% quaternized polysulfone, 2%-8% sulfonated polysulfone, 0-5% small molecule additives, and 75%-90% organic solvent.

[0009] Preferably, the small molecule additive is one or more of acetic acid, water, sodium chloride, and lithium chloride.

[0010] Preferably, the organic solvent is one or more of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0011] Preferably, the porous non-woven fabric is polyester or polypropylene non-woven fabric.

[0012] Based on the same inventive concept, the present invention also provides a method for preparing a high-pressure-resistant flat membrane, comprising the following steps:

[0013] Preparation of cationic polyelectrolyte: Chloromethylated polysulfone and trimethylamine solution are mixed in a mass ratio of 1:6-1:10, reacted at 36-45°C for 30-90 minutes, washed with pure water and then dried to obtain quaternized polysulfone;

[0014] Preparation of casting solution: stirring and dissolving an organic solvent, polysulfone, the quaternized polysulfone, the sulfonated polysulfone and a small molecule additive, filtering and degassing to obtain a casting solution;

[0015] Flat membrane forming: using porous non-woven fabric as a carrier, coating the casting solution at a speed of 2-7 m / min under the conditions of ambient temperature 19-23°C and humidity 50-70 RH%, and solidifying in a coagulation bath at 18-25°C and a rinsing water tank at 35-45°C to obtain the flat membrane.

[0016] Preferably, the preparation of the cationic polyelectrolyte includes: a trimethylamine solution concentration of 3%-10%, a drying temperature of 45-60° C., and a drying time of 6-12 hours.

[0017] Preferably, the stirring and dissolving temperature in the preparation of the casting solution is 70-85° C., and the dissolving time is 20-28 hours.

[0018] Preferably, the coagulation bath and the rinsing water tank in the flat membrane forming process are both made of pure water.

[0019] Based on the same inventive concept, the present invention also provides an application of the high-pressure-resistant flat membrane in a water treatment system.

[0020] Preferably, the water treatment system includes a drinking water purification system, a wastewater treatment system, a seawater desalination system, a high-salt wastewater treatment system or a high-pressure industrial separation system.

[0021] Preferably, the flat membrane can operate at an operating pressure above 1 MPa.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention introduces a polyelectrolyte complex (quaternized polysulfone and sulfonated polysulfone) with a rigid structure into the flat membrane preparation system, utilizing the Coulomb force interaction between anionic and cationic polyelectrolytes to significantly improve the pressure resistance of the flat membrane. Test data show that the prepared high-pressure-resistant flat membrane can still maintain excellent water flux (for example, 283 L / m in Example 1) after 30 minutes of high-pressure pre-pressing at 1 MPa. 2 h), and the flux difference with the flux after 0.1MPa low pressure preloading shall not exceed 50L / m 2h, demonstrating excellent pressure stability, meeting the requirements for the preparation of high-pressure reverse osmosis membranes. By precisely controlling the casting solution composition (5-15% polysulfone, 3-10% quaternized polysulfone, 2-8% sulfonated polysulfone) and phase inversion process parameters (coagulation bath temperature 18-25°C, rinse temperature 35-45°C), a flat membrane with a density-gradient sponge-like pore structure was successfully constructed. This unique microstructure ensures both high mechanical strength and excellent water permeability.

[0024] The preparation process of the present invention has good repeatability and industrial scale-up potential. The cationic polyelectrolyte synthesis reaction time can be shortened to 30 minutes (Example 5), the flat membrane speed can reach 7m / min (Example 18), and it is applicable to various carrier materials such as polyester or polypropylene non-woven fabrics. In addition, by adjusting the ratio of polysulfone to polyelectrolyte and the type and content of small molecule additives (such as lithium chloride), the separation performance of the membrane can be flexibly controlled, making it promising for high-pressure seawater desalination (such as Example 10 with a flux of 366L / m 2 h), can also be used for low-pressure wastewater treatment, with significant cost advantages and broad market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the preparation of the cationic polyelectrolyte of the present invention;

[0026] Figure 2 This is an electron microscope magnified view of the cross-sectional structure of a high-pressure-resistant flat membrane of the present invention. DETAILED DESCRIPTION

[0027] The technical solution is further described below with reference to the accompanying drawings and specific embodiments to facilitate understanding of the content of the present invention.

[0028] Reverse osmosis membranes generally consist of two parts: a base membrane and an interfacial modification layer. The flat membrane in this invention refers to the base membrane of the reverse osmosis membrane. The flat membrane is obtained by performing an interfacial polymerization reaction on the surface of the base membrane to form an interfacial modification layer. The flat membrane can be used in high-pressure reverse osmosis systems and applied in the field of water treatment.

[0029] The present invention provides a high-pressure-resistant flat membrane, which includes a porous non-woven fabric carrier and a polyelectrolyte complex separation layer formed thereon; the separation layer is prepared by a phase inversion method from a casting solution containing an anionic polyelectrolyte and a cationic polyelectrolyte, and has a density gradient sponge-like pore structure; wherein the cationic polyelectrolyte is a quaternized polysulfone, the anionic polyelectrolyte is a sulfonated polysulfone, and the water flux difference of the flat membrane after pre-pressing at 1MPa and 0.1MPa for 30 minutes does not exceed 50L / m 2 ·h.

[0030] The flat membrane comprises, by mass percentage, 5%-15% of polysulfone, 3%-10% of quaternized polysulfone, 2%-8% of sulfonated polysulfone, 0%-5% of small molecule additives, and 75%-90% of organic solvent.

[0031] The small molecule additive is one or more of acetic acid, water, sodium chloride, and lithium chloride.

[0032] The organic solvent is one or more of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0033] The porous non-woven fabric is polyester or polypropylene non-woven fabric.

[0034] A polyelectrolyte complex is a complex formed by the Coulombic interaction of two differently charged anionic and cationic polyelectrolytes. This complex exhibits large dimensions, a relatively rigid structure, and a rich and easily controllable structure. This invention leverages the rigidity of polyelectrolyte complexes by incorporating them into a flat-plate membrane fabrication system, leveraging the interactions between the anionic and cationic polyelectrolytes to enhance the membrane's pressure resistance. Through microstructural design and meticulously controlled fabrication processes, a flat-plate membrane with excellent pressure resistance and a cross-sectional structure characterized by density-gradient, sponge-like pores is fabricated.

[0035] Based on the same inventive concept, the present invention also provides a method for preparing a high-pressure-resistant flat membrane, comprising the following steps:

[0036] 1. Preparation of cationic polyelectrolyte

[0037] A certain amount of chloromethylated polysulfone is placed in a trimethylamine solution (concentration 3%-10%), the mass ratio of chloromethylated polysulfone to trimethylamine solution is 1:6-1:10, the temperature is controlled at 36°C-45°C, and the mixture is stirred for 30-90 minutes. The modified polymethylated polysulfone is then washed with pure water and dried at 45-60°C for 6h-12h to obtain quaternized polysulfone for standby use. The process is as follows: Figure 1 shown.

[0038] 2. Preparation of casting solution

[0039] Sulfonated polysulfone is used as the anionic polyelectrolyte. A certain amount of polysulfone, quaternized polysulfone, sulfonated polysulfone, and a small molecule additive are dissolved in an organic solvent. The small molecule additive is one or more of acetic acid, water, sodium chloride, and lithium chloride. The organic solvent is one or more of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. The weight of the polysulfone is 5-15%, the weight of the quaternized polysulfone is 3%-10%, the weight of the sulfonated polysulfone is 2%-8%, the weight of the small molecule additive is 0-5%, and the weight of the organic solvent is 75%-90%.

[0040] Under stirring, the organic solvent, polysulfone, quaternized polysulfone, sulfonated polysulfone and small molecule additive are added to a stirring and dissolving kettle in batches in the order of stirring and dissolving at a constant temperature of 70-85°C for 20-28 hours; the dissolved casting solution is filtered and refined, vacuum-enhanced degassing is performed, and the solution is set aside.

[0041] 3. Flat membrane preparation

[0042] A high-pressure-resistant flat membrane is produced using a porous nonwoven fabric as a carrier using a flat-plate film-forming machine. The film-forming environment is maintained at a temperature of 19-23°C, a humidity of 50-70% RH, and a production speed of 2-7 m / min. Both the coagulation bath and the rinse tank are filled with pure water. The coagulation bath temperature is 18-25°C, and the rinse tank temperature is 35-45°C. The nonwoven fabric is either polyester or polypropylene.

[0043] The specific embodiment process and data are as follows:

[0044] Example 1

[0045] A certain amount of chloromethylated polysulfone was placed in a trimethylamine solution (5wt%), wherein the mass ratio of chloromethylated polysulfone to trimethylamine solution was 1:8, the reaction temperature was controlled at 40°C, and the reaction time was 60min. The modified polymethylated polysulfone was then washed with pure water and dried at 60°C for 6h to obtain quaternized polysulfone for standby use.

[0046] Under stirring, dimethylacetamide, polysulfone, quaternized polysulfone, sulfonated polysulfone, and acetic acid are added in batches to a stirring dissolution kettle and stirred and dissolved at 80°C for 24 hours. The mass fraction of dimethylacetamide is 80%, the mass fraction of polysulfone is 10%, the mass fraction of quaternized polysulfone is 4%, the mass fraction of sulfonated polysulfone is 3%, and the mass fraction of acetic acid is 3%. The dissolved casting solution is filtered and refined, vacuum-enhanced degassing is performed, and then set aside.

[0047] A porous polyester non-woven fabric was used as a carrier, and the film-forming environment temperature was controlled at 21°C, humidity 55 RH%, and a preparation speed of 6 m / min. Both the coagulation bath and the rinsing water tank were pure water, the coagulation bath temperature was 20°C, and the rinsing water tank temperature was 32°C. A flat film-forming device was used to form a high-pressure flat film. The cross-sectional structure of the high-pressure flat film is enlarged as shown in FIG. Figure 2 As shown, the cross section of the flat membrane has a density gradient sponge-like pore structure with the pore size gradually increasing from top to bottom, which has good pressure resistance and is an ideal membrane pore structure.

[0048] The high pressure-resistant flat membrane prepared above was pre-pressed at 1 MPa and 0.1 MPa for 30 min respectively, and then the water flux was tested at 0.05 MPa, which were 283 L / m 2 h and 297L / m 2·h, indicating that the prepared flat membrane has good pressure resistance.

[0049] Examples 2-6

[0050] The specific implementation is consistent with that of Example 1, except for the differences shown in Table 1 below:

[0051] Table 1 Preparation process of quaternized polysulfone

[0052]

[0053]

[0054] Examples 7-13

[0055] The specific implementation is consistent with that of Example 1, except for the differences shown in Table 2 below:

[0056] Table 2 Different casting solution preparation processes

[0057]

[0058] Examples 14-19

[0059] The specific implementation is consistent with that of Example 1, except for the differences shown in Table 3 below:

[0060] Table 3 Different flat membrane preparation processes

[0061] Example 14 Example 15 Example 16 Example 17 Example 18 Example 19 Ambient temperature (℃) 20 19 22 21 23 22 Ambient humidity (RH%) 64 70 50 68 57 51 Preparation speed (m / min) 2 6 6 5 7 4 Coagulation bath temperature (℃) 20 22 18 23 21 25 Rinse water tank temperature (℃) 45 37 40 35 42 38 Non-woven fabric type Polypropylene Polyester Polyester Polypropylene Polyester Polypropylene

[0062] Table 4: Pressure resistance test data of Examples 1-19:

[0063]

[0064] Pressure resistance test method: The high pressure resistance flat membrane prepared above is pre-pressed at 1MPa and 0.1MPa for 30min respectively, and then a water flux test is performed at 0.05MPa. If the fluxes of the two tests are closer, the pressure resistance of the membrane is better. The present invention introduces a polyelectrolyte complex (quaternary ammonium polysulfone + sulfonated polysulfone) and utilizes the Coulomb force between anionic / cationic polyelectrolytes to form a rigid cross-linked structure, thereby enhancing the mechanical strength of the flat membrane. The test data of Examples 1-19 show that after 1MPa high pressure pre-pressing, the water flux still maintains a high level (such as Example 1: 283L / m 2 h), and the flux difference with low pressure (0.1MPa) is ≤50L / m 2 h (as in Example 1, the difference is 14 L / m 2h), demonstrating the membrane structure's excellent stability under high pressure. Compared to conventional polysulfone flat membranes, the flat membranes of the present invention exhibit significantly improved pressure resistance and are expected to be used in the preparation of high-pressure reverse osmosis membranes for water treatment applications such as desalination.

[0065] The present invention optimizes separation performance by finely regulating the membrane microstructure. By regulating the composition of the casting solution (such as 2%-8% sulfonated polysulfone and 0-5% small molecule additives) and the phase inversion process (solidification bath temperature 18-25°C, rinsing temperature 35-45°C), a dense layer-support layer structure with a pore size gradient distribution is formed. The gradient sponge-like pore structure can optimize separation performance; the density gradient structure (such as Example 10) still maintains a high water flux (366L / m 2 h), while reducing membrane compaction effects.

[0066] The present invention further optimizes the solvent system, specifically using a mixed solvent such as dimethylacetamide / N-methylpyrrolidone (Examples 7-13) to improve the polymer solubility and phase separation rate. The pore connectivity can be controlled by small molecule additives (such as lithium chloride, Examples 11-12) to improve the permeation efficiency (Example 12 flux 353L / m 2 ·h).

[0067] The synthesis process of the cationic polyelectrolyte of the present invention is controllable. By optimizing parameters such as trimethylamine concentration (3%-10%) and reaction temperature (36-45°C) (Table 1), the degree of substitution of the quaternized polysulfone is ensured to be uniform. According to the comparison between Examples 5 and 6, the reaction time is shortened to 30 minutes (Example 5) and the pressure resistance (flux 276 L / m 2 h), significantly reducing energy consumption; leveraging the robust bond between the nonwoven carrier (polyester / polypropylene) and the separation layer (Examples 14-19), the risk of delamination in industrial applications is avoided. The present invention exhibits excellent compatibility for continuous production: flat-plate film production speeds of 2-7 m / min (Examples 14-19) and precise control of ambient temperature and humidity (19-23°C, 50-70% RH) make it suitable for large-scale roll-to-roll production.

[0068] The present invention adapts to different pressure requirements (such as low-pressure wastewater treatment or high-pressure seawater desalination) by adjusting the polysulfone / polyelectrolyte ratio (such as polysulfone 5%-15%). Example 10 (polysulfone 5%) has a flux of 366 L / m at 1 MPa. 2 h, suitable for high-salt wastewater treatment; Example 11 (polysulfone 15%) can be used as a base membrane for the preparation of high-pressure reverse osmosis membranes, achieving a wider range of application scenarios. The sulfonated polysulfone of the present invention only requires 2%-8% (Examples 8-9), with significant economic benefits.

[0069] The present invention also provides an application of the high-pressure-resistant flat membrane in a water treatment system.

[0070] The water treatment system includes a drinking water purification system, a wastewater treatment system, a seawater desalination system, a high-salinity wastewater treatment system or a high-pressure industrial separation system. Preferably, the flat membrane can operate at an operating pressure of 1 MPa or above.

[0071] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention.

Claims

1. A high pressure-resistant flat membrane, characterized in that: The invention comprises a porous nonwoven fabric carrier and a polyelectrolyte complex separation layer formed thereon; The separation layer is prepared by a phase inversion method using a casting solution containing anionic polyelectrolytes and cationic polyelectrolytes, and has a density gradient sponge-like pore structure; The cationic polyelectrolyte is quaternized polysulfone, the anionic polyelectrolyte is sulfonated polysulfone, and the water flux difference of the flat membrane after pre-pressing at 1 MPa and 0.1 MPa for 30 minutes does not exceed 50 L / m 2 ·h.

2. The high pressure-resistant flat membrane according to claim 1, characterized in that: The separation layer comprises, by mass percentage, 5%-15% of polysulfone, 3%-10% of quaternized polysulfone, 2%-8% of sulfonated polysulfone, 0%-5% of small molecule additives, and 75%-90% of organic solvent.

3. The high pressure-resistant flat membrane according to claim 2, characterized in that: The small molecule additive is one or more of acetic acid, water, sodium chloride, and lithium chloride.

4. The high pressure-resistant flat membrane according to claim 2, characterized in that: The organic solvent is one or more of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

5. The high pressure-resistant flat membrane according to claim 1, characterized in that: The porous non-woven fabric is polyester or polypropylene non-woven fabric.

6. A method for preparing the high pressure-resistant flat membrane according to any one of claims 1 to 5, characterized in that: The following steps are involved: Preparation of cationic polyelectrolyte: Chloromethylated polysulfone and trimethylamine solution are mixed in a mass ratio of 1:6-1:10, reacted at 36-45°C for 30-90 minutes, washed with pure water and then dried to obtain quaternized polysulfone; Preparation of casting solution: stirring and dissolving an organic solvent, polysulfone, the quaternized polysulfone, the sulfonated polysulfone and a small molecule additive, filtering and degassing to obtain a casting solution; Flat membrane forming: using porous non-woven fabric as a carrier, coating the casting solution at a speed of 2-7 m / min under the conditions of ambient temperature 19-23°C and humidity 50-70 RH%, and solidifying in a coagulation bath at 18-25°C and a rinsing water tank at 35-45°C to obtain the flat membrane.

7. The preparation method according to claim 6, characterized in that The preparation of the cationic polyelectrolyte comprises the following steps: the concentration of the trimethylamine solution is 3%-10%, the drying temperature is 45-60° C., and the drying time is 6-12 hours.

8. The preparation method according to claim 6, characterized in that The stirring and dissolving temperature in the preparation of the casting solution is 70-85° C., and the dissolving time is 20-28 hours.

9. The preparation method according to claim 6, characterized in that The coagulation bath and the rinsing tank in the flat film forming process both use pure water.

10. Use of the high pressure-resistant flat membrane according to any one of claims 1 to 5 in a water treatment system.

11. The use according to claim 10, characterized in that The water treatment system includes a drinking water purification system, a wastewater treatment system, a seawater desalination system, a high-salt wastewater treatment system or a high-pressure industrial separation system.

12. The use according to claim 10, characterized in that The flat membrane can operate at an operating pressure above 1 MPa.

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