Hollow fiber polyamide composite membrane and preparation method thereof

Abstract

The invention relates to the technical field of membrane separation, in particular to a hollow fiber polyamide composite membrane for seawater desalination, wastewater filtration, water purification treatment and the like and a preparation method of the hollow fiber polyamide composite membrane. The carboxymethyl cellulose can solve the problem that macropores in the surface of the hollow fiber support body are not beneficial to interfacial polymerization, and the hydrophilicity of the carboxymethyl cellulose is beneficial to improvement of adsorption of a water-phase monomer on the surface of the support body, so that the permeability of the polyamide composite membrane is improved.

Description

Technical Field

[0001] The present invention relates to the field of membrane separation technology, in particular to a hollow fiber polyamide composite membrane for applications such as seawater desalination, wastewater filtration, water purification, and the like, and a preparation method thereof. Background Art

[0002] Polyamide composite membrane is a high-performance separation membrane material, which is widely used in desalination, wastewater treatment, gas separation and food industry. Its unique chemical structure and physical properties make the polyamide composite membrane perform well in separation efficiency, mechanical strength and chemical stability. Polyamide membrane is usually prepared by interfacial polymerization, that is, a dense polyamide film is formed between two immiscible solutions. This film has excellent retention performance and flux, and can meet a variety of separation needs.

[0003] Hollow fiber membranes occupy an important position in the field of membrane separation due to their high specific surface area, good permeability and easy large-scale production. Using hollow fibers as supports to prepare polyamide composite membranes can significantly improve the packing density and separation efficiency of the membranes. In addition, the geometric structure of hollow fibers is conducive to the formation of a uniform membrane layer, improving the separation performance and stability of the membrane. Therefore, the preparation of polyamide composite membranes using hollow fibers has become one of the current research hotspots.

[0004] In the process of preparing polyamide composite membranes using hollow fiber supports, the large pore size on the surface of the hollow fiber support is not conducive to the coating of the aqueous monomer solution of interfacial polymerization. In order to solve this problem, the prior art coats a shielding layer on the surface of the support to cover the macroporous defects on the surface of the support, but the presence of the shielding layer easily leads to the problem of polyamide membrane layer falling off. There are also prior art attempts to inject paraffin through the inner tube of the hollow fiber first, use paraffin to fill the unevenness on the surface of the polyamide composite membrane, and then perform interfacial polymerization to form a polyamide composite membrane. However, this method has obvious defects: after the paraffin fills the inside of the hollow fiber, it takes a long time to immerse in a non-polar solvent to remove the paraffin after the polyamide composite membrane is formed, which will inevitably affect the polyamide membrane layer; secondly, under pressure, paraffin will pass through the membrane pores, resulting in unevenness on the surface of the support, resulting in a low yield; in addition, the presence of hydrophobic paraffin will affect the interfacial polymerization process, and the residual paraffin will affect the hydrophilicity of the polyamide composite membrane. Therefore, seeking a more effective preparation method to overcome these problems has become an urgent need for current research. Summary of the invention

[0005] Based on the above problems, the present invention provides a new solution, using carboxymethyl cellulose to replace the paraffin in the prior art, and optimizing the preparation method of the hollow fiber composite membrane, thereby well solving the above problems.

[0006] The present invention provides a method for preparing a hollow fiber polyamide composite membrane, which is characterized by comprising the following steps:

[0007] (1) preparing carboxymethyl cellulose gel;

[0008] (2) Connecting both ends of the hollow fiber support to a vacuum pump, immersing it in carboxymethyl cellulose gel while evacuating the vacuum, and maintaining the suction time for 1-5 minutes to allow the gel to enter the pores on the surface of the support;

[0009] (3) Stop vacuuming, remove the support from the gel, and tilt it to remove excess carboxymethyl cellulose gel on the surface of the hollow fiber support. Connect the two ends of the hollow fiber support to a gas heating device to pass heated high-temperature gas into the hollow fiber. The gas temperature is 60-80° C. and the drying time is 3-8 min.

[0010] (4) continuing to wipe the surface of the hollow fiber support body with a hot and wet flexible material to remove the carboxymethyl cellulose on the surface of the hollow fiber support body;

[0011] (5) successively immersing the treated outer surface of the hollow fiber support body in an aqueous monomer solution and an organic monomer solution to form a polyamide membrane, and drying the membrane;

[0012] (6) Deionized water is continuously introduced into the inner cavity of the dried hollow fiber support body to remove the carboxymethyl cellulose in the pores of the hollow fiber support body, thereby obtaining a hollow fiber polyamide composite membrane.

[0013] As a preferred embodiment, the carboxymethyl cellulose gel is composed of carboxymethyl cellulose and a cross-linking agent, wherein the concentration of the carboxymethyl cellulose is 4-6wt%, and the amount of the cross-linking agent is 3%-7% of the mass of the carboxymethyl cellulose.

[0014] As a preferred embodiment, the cross-linking agent is one of sodium tetraborate and N-hydroxymethyl acrylamide.

[0015] As a preferred embodiment, the purity of the carboxymethyl cellulose is not less than 99%, and the molecular weight ranges from 10,000 to 200,000 Da.

[0016] As a preferred embodiment, the hollow fiber support is an ultrafiltration membrane, made of one of polysulfone, polyethersulfone and polyacrylonitrile, with an inner diameter ranging from 0.5 to 2.0 mm, a wall thickness ranging from 0.1 to 0.5 mm, and a porosity of not less than 70%.

[0017] As a preferred embodiment, the vacuum degree during the vacuuming process is 0.05-0.2 MPa.

[0018] As a preferred embodiment, the high temperature gas is selected from one of air, nitrogen and inert gas, and the gas flow rate is 5-30 ml / min.

[0019] As a preferred embodiment, the flexible material is cloth or sponge, and the specific operation is to immerse the flexible material in warm water of 50-60° C., and then take it out and wring it dry.

[0020] As a preferred embodiment, the concentration of the aqueous monomer solution is 2-5wt%, the coating time is 2-5min, the concentration of the organic monomer solution is 0.1-0.5wt%, and the coating time is 0.5-2min.

[0021] As a preferred embodiment, the flow rate of continuous flushing with deionized water is 10-50 ml / min, the time is 20-60 min, and the water temperature is 50-60°C.

[0022] The present invention also provides a hollow fiber polyamide composite membrane prepared according to the method.

[0023] The hollow fiber polyamide composite membrane prepared by the invention is used in seawater desalination, wastewater filtration and water purification.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] First, the present invention utilizes carboxymethyl cellulose to alleviate the problem that the large pores on the surface of the hollow fiber support are not conducive to interfacial polymerization, and the hydrophilicity of carboxymethyl cellulose is conducive to improving the adsorption of water-phase monomers on the surface of the support, thereby improving the permeability of the polyamide composite membrane.

[0026] Secondly, compared with the use of paraffin to plug the pores in the prior art, the present invention uses carboxymethyl cellulose. On the one hand, there is no need to use organic solvents to soak the polyamide composite membrane for a long time, and only water is needed to remove it; on the other hand, even if a small amount of carboxymethyl cellulose remains, it is also beneficial to improve the overall hydrophilicity of the polyamide composite membrane.

[0027] In addition, the present invention uses vacuum suction to extract carboxymethyl cellulose, which prevents the carboxymethyl cellulose from occupying the entire hollow fiber lumen and reduces the time for subsequent removal. The surface of the hollow fiber support body is wiped with hot and wet flexible materials to remove the residual carboxymethyl cellulose to the greatest extent. DETAILED DESCRIPTION

[0028] Example 1

[0029] (1) Preparation of carboxymethyl cellulose gel:

[0030] A certain amount of high-purity carboxymethyl cellulose with an average molecular weight of 20,000 Da is prepared into an aqueous solution with a concentration of 5 wt%. Sodium tetraborate with a concentration of 5 wt% of the mass of carboxymethyl cellulose is added to the solution as a crosslinking agent, and the mixture is stirred and mixed to obtain a carboxymethyl cellulose gel.

[0031] (2) Preparation of hollow fiber support:

[0032] An ultrafiltration membrane made of polysulfone, with an inner diameter of 1.0 mm, a wall thickness of 0.2 mm, and a porosity of 72% was selected as the hollow fiber support.

[0033] (3) Impregnation gel:

[0034] Connect both ends of the hollow fiber support to a vacuum pump and adjust the vacuum degree to 0.1 MPa. While evacuating, immerse the hollow fiber support in the carboxymethyl cellulose gel prepared above and maintain the suction time for 3 minutes to ensure that the gel fully enters the pores on the support surface.

[0035] (4) Drying treatment:

[0036] Stop vacuuming, remove the support from the gel, and tilt it to remove excess gel on the surface. Connect both ends of the hollow fiber support to a gas heating device, and pass air heated to 70°C with a gas flow rate of 15 ml / min and a drying time of 5 min.

[0037] (5) Remove surface gel:

[0038] Use a cloth soaked in 55°C warm water, wring it out and wipe the surface of the hollow fiber support to remove the carboxymethyl cellulose on the surface of the hollow fiber.

[0039] (6) Formation of polyamide film:

[0040] The outer surface of the treated hollow fiber support body was first immersed in a 3wt% m-phenylenediamine monomer solution for 3 minutes, and then immersed in a 0.2wt% trimesoyl chloride monomer solution for 1 minute to form a polyamide composite membrane, and then dried.

[0041] (7) Removing gel from the pores:

[0042] Deionized water at 55° C. was continuously introduced into the inner cavity of the dried hollow fiber support body at a flow rate of 25 ml / min for 20 min to remove the carboxymethyl cellulose in the pores, thereby obtaining a hollow fiber polyamide composite membrane.

[0043] Example 2

[0044] In step (1), N-hydroxymethyl acrylamide is used to replace sodium tetraborate, and the remaining operations are consistent with the example.

[0045] Example 3

[0046] The continuous flushing time in step (7) is 60 minutes.

[0047] Comparative Example 1

[0048] In step (3) of impregnating the gel, the carboxymethyl cellulose gel is coated on the surface of the hollow fiber by wiping, and the number of coatings is 5. The remaining operations are consistent with the embodiment.

[0049] Comparative Example 2

[0050] In step (4) of drying, stop vacuuming, remove the support from the gel, and tilt it to remove excess gel on the surface. Then place the hollow fiber support in a 70°C oven for 5 minutes. The remaining operations are consistent with the embodiment.

[0051] Comparative Example 3

[0052] The difference between Comparative Example 3 and Example 1 is that step (5) of removing the surface gel is missing. The remaining operations are consistent with the example.

[0053] Comparative Example 4

[0054] The difference between Comparative Example 4 and Example 1 is that the continuous flushing time in step (7) is 5 minutes.

[0055] Comparative Example 5

[0056] The difference between Comparative Example 5 and Example 1 is that the continuous flushing time in step (7) is 120 minutes.

[0057] Penetration performance test

[0058] The above samples were tested for reverse osmosis performance under the following conditions: the test solution was 0.5% NaCl solution, the operating pressure was 0.5MPa, the room temperature, and the feed solution entered from the polyamide membrane layer (external pressure type). The test included: the initial flux and initial retention rate after 1h stabilization, and the flux and retention rate after 5 days of stable operation. Finally, the filtration was changed to internal pressure type, the operating pressure was changed to 2MPa, and after 5 days of continuous permeation, the membrane was changed to external pressure type to test the flux and retention rate again.

[0059] Table 1 Permeability results of hollow fiber polyamide composite membranes prepared by different methods

[0060] Initial flux / L / (m2.h) Initial retention rate / % Flux after 5 days / L / (m2.h) Retention rate after 5 days / % Flux after reverse osmosis / L / (m2.h) Rejection rate after reverse osmosis / % Example 1 53.1 95.6 55.6 95.2 66.2 94.0 Example 2 54.7 95.4 56.1 95.1 / / Example 3 55.9 95.5 57.1 95.2 / / Comparative Example 1 72.1 91.1 72.5 90.9 / / Comparative Example 2 60.1 93.0 60.2 93.1 / / Comparative Example 3 44.0 95.4 52.1 95.3 77.8 89.1 Comparative Example 4 22.8 95.9 54.8 95.5 / / Comparative Example 5 54.3 95.2 54.0 95.2 / /

[0061] It can be seen from the above examples and comparative examples that the method of the present invention can be used to prepare a hollow fiber polyamide composite membrane with relatively ideal flux and retention rate, and has high application potential.

[0062] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a hollow fiber polyamide composite membrane, characterized in that The following steps are involved: (1) preparing carboxymethyl cellulose gel; (2) Connecting both ends of the hollow fiber support to a vacuum pump, immersing it in carboxymethyl cellulose gel while evacuating the vacuum, and maintaining the suction time for 1-5 minutes to allow the gel to enter the pores on the surface of the support; (3) Stop vacuuming, remove the support from the gel, and tilt it to remove excess carboxymethyl cellulose gel on the surface of the hollow fiber support. Connect the two ends of the hollow fiber support to a gas heating device to pass heated high-temperature gas into the hollow fiber. The gas temperature is 60-80° C. and the drying time is 3-8 min. (4) continuing to wipe the surface of the hollow fiber support body with a hot and wet flexible material to remove the carboxymethyl cellulose on the surface of the hollow fiber support body; (5) successively immersing the treated outer surface of the hollow fiber support body in an aqueous monomer solution and an organic monomer solution to form a polyamide membrane, and drying the membrane; (6) Deionized water is continuously introduced into the inner cavity of the dried hollow fiber support body to remove the carboxymethyl cellulose in the pores of the hollow fiber support body, thereby obtaining a hollow fiber polyamide composite membrane.

2. The method according to claim 1, characterized in that The carboxymethyl cellulose gel is composed of carboxymethyl cellulose and a cross-linking agent, wherein the concentration of the carboxymethyl cellulose is 4-6wt%, and the amount of the cross-linking agent is 3%-7% of the mass of the carboxymethyl cellulose.

3. The method according to claim 1, characterized in that The cross-linking agent is one of sodium tetraborate and N-hydroxymethyl acrylamide.

4. The method according to claim 1, characterized in that The purity of carboxymethyl cellulose is not less than 99% and the molecular weight ranges from 10,000-200,000 Da.

5. The method according to claim 1, characterized in that The hollow fiber support is an ultrafiltration membrane, made of one of polysulfone, polyethersulfone and polyacrylonitrile, with an inner diameter ranging from 0.5 to 2.0 mm, a wall thickness ranging from 0.1 to 0.5 mm, and a porosity of not less than 70%.

6. The method according to claim 1, characterized in that The vacuum degree during the vacuuming process is 0.05-0.2MPa.

7. The method according to claim 1, characterized in that The high temperature gas is selected from one of air, nitrogen and inert gas, and the gas flow rate is 5-30 ml / min.

8. The method according to claim 1, characterized in that The flexible material is cloth or sponge. The specific operation is to immerse the flexible material in warm water of 50-60° C., then take it out and wring it dry.

9. The method according to claim 1, characterized in that The concentration of the aqueous monomer solution is 2-5wt%, the coating time is 2-5min, the concentration of the organic monomer solution is 0.1-0.5wt%, the coating time is 0.5-2min; the flow rate of continuous flushing with deionized water is 10-50ml / min, the time is 20-60min, and the water temperature is 50-60°C.

10. The hollow fiber polyamide composite membrane prepared according to the method of claim 1.