A method for preparing a composite tubular membrane

Abstract

The application relates to a preparation method of a composite tubular membrane, comprising the following steps: respectively preparing a casting solution and a supporting base material; stirring the casting solution for 8-12 hours, vacuumizing and standing for 8-12 hours for defoaming; performing one-time coating on the inner wall of the rotating supporting base material by using a coating doctor blade, solidifying the casting solution into a membrane in a solidifying bath after uniform casting, and drying for 22-26 hours; performing two-time coating on the inner wall of the rotating supporting base material by using a coating doctor blade, solidifying the casting solution into a membrane in a solidifying bath after uniform casting, and drying for 22-26 hours, so as to obtain the composite tubular membrane. The composite tubular membrane prepared by the method can guarantee the uniformity and integrity of coating due to two-time coating.

Description

Technical Field

[0001] This application relates to the field of tubular membranes, and in particular to a method for preparing a composite tubular membrane. Background Technology

[0002] Tubular membranes are a type of membrane module suitable for membrane separation technologies such as ultrafiltration, microfiltration, nanofiltration, and reverse osmosis. Their advantages include wide flow channels, turbulent flow of the feed solution within the tubes, lower requirements for feed pretreatment precision, and ease of cleaning—both chemically and mechanically. Tubular membrane modules also exhibit low pressure loss, allowing for longer flow channels and potentially higher filtration efficiency. Therefore, they are widely used in the food, beverage, chemical, microelectronics, paper, and pharmaceutical industries.

[0003] Specific applications of tubular membranes include high-concentration wastewater treatment, such as landfill leachate treatment; special chemical wastewater treatment, such as coking, leather tanning, and textile wastewater treatment; oil-water separation, such as oilfield reinjection water, washing, and dairy wastewater treatment; material separation, such as tea beverage and fruit juice clarification and concentration; filtration of food and biopharmaceutical fermentation broths, such as the recovery of emulsified oil and electrophoretic paint; and medium- and low-concentration wastewater treatment, such as electronic wastewater treatment and municipal wastewater treatment.

[0004] Tubular membranes are a novel type of separation membrane. Depending on the application, different organic film layers of various materials are coated onto a porous tubular support substrate. The porous material provides support, and the separation effect is primarily determined by the surface organic film layer. The organic film layer is an asymmetric membrane, consisting of an extremely thin (approximately 0.1–1 μm) active layer and a porous layer with a sponge-like or finger-like structure. The support tube can be a metal tube, a non-woven fabric tube, a sintered polymer tube, a ceramic tube, or other tubular materials.

[0005] Chinese Patent Application No. 201910057590.4 discloses a method for preparing a composite organic tubular membrane. The preparation steps involve using a polymer as a substrate, preparing a support layer casting solution and a functional layer casting solution by mixing the polymer, solvent, and additives. The support layer casting solution and the functional layer casting solution are degassed separately, and then simultaneously coated onto a non-woven fabric rolled-up tubular substrate. The membrane is then solidified in a coagulation bath to form the organic tubular membrane. Although this method uses two casting solutions for coating, the coating process is still completed in a single step, thus it cannot guarantee the uniformity and integrity of the coating. Summary of the Invention

[0006] This application provides a method for preparing a composite tubular membrane to solve the technical problem in related technologies where coating is still completed in one step, which cannot guarantee the uniformity and integrity of the coating.

[0007] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0008] The casting solution and the supporting substrate were prepared separately.

[0009] Stir the casting solution for 8-12 hours, and then allow it to stand under vacuum for 8-12 hours to remove bubbles.

[0010] The coating is applied once to the inner wall of the rotating support substrate using a coating scraper. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and dries for 22-26 hours.

[0011] A secondary coating is applied to the inner wall of a rotating support substrate using a coating scraper. After the casting liquid is evenly distributed, it solidifies into a film in a coagulation bath and is dried for 22-26 hours to obtain a composite tubular membrane.

[0012] In some embodiments, the coating scraper has a cavity, one end of which is a closed structure and the other end is an open structure, and is connected to an external device for supplying casting liquid. The side wall of the coating scraper has an outlet hole that communicates with the cavity.

[0013] In some embodiments, the sidewall of the coating scraper is provided with at least one groove, in which a thickness control ring is engaged.

[0014] In some embodiments, the specific steps of applying a coating to the inner wall of the rotating support substrate using a coating scraper include:

[0015] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. The distance that the support substrate moves forward by rotating 360 degrees is less than the width of the coating scraper.

[0016] In some embodiments, the specific steps of performing secondary coating on the inner wall of the rotating support substrate using a coating scraper include:

[0017] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. The distance that the support substrate moves forward by rotating 360 degrees is less than the width of the coating scraper.

[0018] In some embodiments, the casting solution comprises a polymeric material, a pore-forming agent, and a solvent.

[0019] In some embodiments, the polymeric material is a combination of at least one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, and polyamide;

[0020] The pore-forming agent is at least one or a combination of two of polyvinylpyrrolidone and polyethylene glycol;

[0021] The solvent is at least one or a combination of two of dimethylformamide and dimethylacetamide.

[0022] In some embodiments, the water temperature of the coagulation bath is 45-60 degrees Celsius.

[0023] The beneficial effects of the technical solution provided in this application include:

[0024] This application provides a method for preparing a composite tubular membrane. The method involves a first coating on the inner wall of a rotating support substrate using a coating tool. After the casting solution is uniformly cast, the membrane solidifies in a coagulation bath and is dried for 22-26 hours. A second coating is then performed on the inner wall of the rotating support substrate using the same coating tool. After the casting solution is uniformly cast, the membrane solidifies in a coagulation bath and is dried for another 22-26 hours to obtain the composite tubular membrane. Because a two-stage rotational coating is used, and the rotational coating is a cast coating process, the uniformity and integrity of the coating can be guaranteed. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 Flowcharts provided for embodiments of this application;

[0027] Figure 2 This is a schematic diagram of the structure of the coating scraper provided in the embodiments of this application;

[0028] Figure 3 This is a schematic diagram of the coating scraper and supporting substrate provided in an embodiment of this application.

[0029] In the diagram: 1. Coating scraper; 2. Cavity; 3. Liquid outlet; 4. Groove; 5. Thickness control ring. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0031] It should be noted that, unless specific conditions are specified in the embodiments, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products. Furthermore, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, solution B, or a solution that simultaneously satisfies A and B. In addition, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, such a combination should be considered non-existent and not within the scope of protection claimed by this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0032] This application provides a method for preparing a composite tubular membrane, which solves the technical problem in related technologies where coating is still completed in one step and cannot effectively guarantee the uniformity and integrity of the coating.

[0033] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0034] S1. Prepare casting solution and supporting substrate respectively;

[0035] S2. Stir the casting solution for 8-12 hours, and then vacuum and allow it to stand for 8-12 hours to remove bubbles;

[0036] S3. Apply a coating to the inner wall of the rotating support substrate once using a coating scraper. After the casting liquid is evenly cast, solidify it in the coagulation bath and dry for 22-26 hours.

[0037] S4. Apply a second coating to the inner wall of the rotating support substrate using a coating scraper. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and dries for 22-26 hours to obtain a composite tubular membrane.

[0038] The supporting substrate is made of polymer sintered tube, which has advantages such as high pressure resistance, high water permeability, simple production process, and low cost. The winding length of the supporting tube is determined according to the requirements of the membrane module, and membrane tubes of any length can be produced continuously.

[0039] In this application, the use of two-stage coating and rotary casting coating methods can ensure the uniformity and integrity of the coating, while also ensuring its throughput and retention rate.

[0040] In the new design of the coating scraper 1 provided in this application, the coating scraper 1 has a cavity 2, one end of which is a closed structure and the other end is an open structure, and is connected to the external equipment for supplying casting liquid. The side wall of the coating scraper 1 has a liquid outlet hole 3 that communicates with the cavity 2.

[0041] Compared to the traditional method of directly inserting the coating scraper 1 into the substrate and applying the film by pulling, the one-time coating method of this application involves first placing the coating scraper 1 on the inner wall of the supporting substrate, driving the supporting substrate to rotate and move forward along the axis. The driving body can be a belt or rubber roller as in the prior art. During this process, the tank storing the casting liquid injects the casting liquid into the cavity 2 through the pipe, and the liquid flows to the inner wall of the supporting substrate in a casting manner through the liquid outlet 3.

[0042] This ensures that all parts of the inner wall of the substrate come into contact with the casting liquid through a casting process, guaranteeing the uniformity and consistency of the film layer.

[0043] It should be understood that in order to ensure that the coating scraper 1 can coat the casting liquid onto the inner wall of the support substrate, when the support substrate rotates once, the distance the support substrate moves forward is less than the width coated by the coating scraper 1.

[0044] To facilitate control of the coating thickness, the side wall of the coating scraper 1 is provided with at least two grooves 4, and a thickness control ring 5 is engaged in the groove 4. Preferably, only two grooves 4 are provided, and each groove 4 is engaged with a thickness control ring 5.

[0045] Because of the presence of the thickness control ring 5, the distance that the supporting substrate advances is less than the width of the coating by the coating rod 1, which is actually less than the distance between the two thickness control rings 5 ​​on the coating rod 1.

[0046] The thickness control ring 5 can be made of elastic plastic. It is ring-shaped and has a notch. Users can enlarge the notch to make it easy to fit into the groove 4. If the thickness control ring 5 needs to be replaced, simply replace it with another thickness control ring 5 with the same inner diameter but a different outer diameter.

[0047] The thickness control ring 5 will abut against the side wall of the supporting substrate, so that there is a certain gap between the liquid outlet 3 and the inner wall of the supporting substrate. This gap is the coating thickness. Different thickness control rings 5 ​​will correspond to different coating thicknesses.

[0048] The thickness control ring 5 is manufactured mechanically, so errors are unavoidable in the actual production process. This means that there will be a certain error between the axis of the thickness control ring 5 and the axis of the supporting substrate. Therefore, in the traditional pull coating method, the thickness of the film layer will be inconsistent.

[0049] Compared to the traditional method of using a thickness control ring to control the coating thickness, in this application, since a casting coating method is used, the positions of the thickness control ring 5 and the coating scraper 1 are fixed. Therefore, during spin coating, the coating thickness can be guaranteed to be consistent and is not affected by errors generated during the manufacturing stage of the thickness control ring.

[0050] Of course, the coating process is repeated once during the second coating. It should be understood that when the coating is applied three times, although the coating integrity is improved, i.e. the initial bubble pressure is increased, the filtration flux decreases significantly, and the practicality deteriorates.

[0051] Furthermore, the casting solution comprises a polymer material, a pore-forming agent, and a solvent.

[0052] Specifically, the polymer material is a combination of at least one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, and polyamide;

[0053] The pore-forming agent is at least one or a combination of two of polyvinylpyrrolidone and polyethylene glycol;

[0054] The solvent is at least one or a combination of two of dimethylformamide and dimethylacetamide.

[0055] Furthermore, the water temperature of the coagulation bath is between 45 and 60 degrees Celsius.

[0056] The present invention will be described in detail below through embodiments.

[0057] Example 1:

[0058] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0059] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 17:10:10:63. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0060] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and is dried for 24 hours.

[0061] The coating scraper is placed on the inner wall of the support substrate again, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it is solidified in the coagulation bath and dried for 24 hours to obtain a composite tubular membrane.

[0062] Example 2:

[0063] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0064] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 16:10:10:64. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0065] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and is dried for 24 hours.

[0066] The coating scraper is placed on the inner wall of the support substrate again, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it is solidified in the coagulation bath and dried for 24 hours to obtain a composite tubular membrane.

[0067] Example 3:

[0068] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0069] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 15:10:10:65. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0070] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and is dried for 24 hours.

[0071] The coating scraper is placed on the inner wall of the support substrate again, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it is solidified in the coagulation bath and dried for 24 hours to obtain a composite tubular membrane.

[0072] Comparative Example 1:

[0073] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0074] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 17:10:10:63. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0075] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and is dried for 24 hours to obtain a tubular membrane.

[0076] Comparative Example 2:

[0077] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0078] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 17:10:10:63. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0079] The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and is dried for 24 hours.

[0080] The second time, the coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. The film is solidified in the coagulation bath and dried for 24 hours.

[0081] The coating scraper was placed on the inner wall of the support substrate for the third time, and the support substrate was driven to rotate and move forward along the axis. After the casting liquid was evenly cast, it was solidified in the coagulation bath to form a film, and dried for 24 hours to obtain a tubular membrane.

[0082] Comparative Example 3:

[0083] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0084] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 17:10:10:63. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0085] The prepared casting liquid is squeezed into the inner wall of the support substrate. Using a conventional scraper, the scraper is pulled from the bottom to the top of the support substrate to coat the film. The film is then solidified in a coagulation bath and dried for 24 hours.

[0086] Comparative Example 4:

[0087] A method for preparing a composite tubular membrane is provided, comprising the following steps:

[0088] Polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP) additive, polyethylene glycol 200 (PEG) and dimethylformamide (DMF) solvent were physically mixed. The mass ratio of PVDF, PVP, PEG, and DMF was 17:10:10:63. The mixture was stirred at 80°C for 12 hours to obtain a casting solution. The casting solution was then allowed to stand under vacuum for 10 hours to remove bubbles.

[0089] The above-prepared casting liquid is squeezed into the inner wall of the support substrate. Using a conventional scraper, the scraper is pulled from the bottom to the top of the support substrate to coat the film. The film is then solidified in a coagulation bath and dried for 24 hours.

[0090] The prepared casting liquid is squeezed into the inner wall of the support substrate again. Using a traditional scraper, the scraper is pulled from the bottom to the top of the support substrate to coat the film. The film is then solidified in the coagulation bath and dried for 24 hours.

[0091] The performance data of Examples 1-3 and Comparative Examples 1-4 are shown in Table 1 below:

[0092]

[0093]

[0094] The initial bubble point test method involves sealing one end of a tubular composite membrane and immersing it in alcohol. Nitrogen gas is then applied from the other side of the composite tubular membrane, and the bubbling on the membrane surface is observed. The pressure corresponding to the appearance of the first string of bubbles on the membrane surface is recorded, which is the initial bubble pressure of the membrane.

[0095] Internal pressure flux refers to the initial internal pressure flux of a membrane when one end of a composite tubular membrane is closed and pure water enters from the other side of the membrane tube, at a pressure of 0.1 MPa and a water temperature of 25°C, by measuring the volume of pure water that permeates through the membrane in 1 minute.

[0096] As shown in Table 1 above, the overall comprehensive data of Examples 1-3 are better than those of Comparative Examples 1-4, mainly due to the following reasons:

[0097] 1. As shown in Examples 1-3, with the decrease in the concentration of the polymer polyvinylidene fluoride (mass fraction from 17% to 15%), the initial bubble point of the membrane decreases (from 0.1 MPa to 0.08 MPa), and the flux increases slightly (from 1085 L / (m³)). 2 *h) increased to 1230L / (m 2 *h)), the average pore size increased (from 0.028 μm to 0.030 μm);

[0098] 2. As can be seen from Comparative Example 1 and Example 1, after two coatings using spin coating, the initial bubble pressure of the membrane significantly increased (from 0.01 MPa to 0.1 MPa, an increase of 10 times), while the flux and average pore size decreased slightly (flux decreased from 1200 L / (m²)). 2 *h to 1085L / (m 2 *h, with an average pore size ranging from 0.032μm to 0.028μm, it can be seen that under the condition that the flux does not change much, the initial bubble pressure increases significantly, the coating defects decrease, the average pore size decreases slightly, and the rejection rate increases. The secondary coating has better performance and is more practical than the primary coating.

[0099] 3. As can be seen from Comparative Example 2 and Example 1, after the third coating, the initial bubble point of the membrane increased (from 0.1 MPa to 0.15 MPa), and the average pore size decreased (from 0.028 μm to 0.015 μm). However, the flux decreased significantly (from 1085 L / (m³)). 2 *h decreased to 700 L / (m 2 *h), practicality decreases;

[0100] 4. As shown in Comparative Examples 3 and 1, compared to the traditional coating method, the spin coating method, while maintaining a relatively small change in flux, increases the bubble pressure from 0.005 MPa to 0.01 MPa, while slightly decreasing the flux and average pore size. The flux increases from 1250 L / (m²) 2 *h) decreased to 1200L / (m 2 *h), with an average pore size ranging from 0.035μm to 0.032μm, so spin coating can produce films with superior performance.

[0101] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the patent protection scope of the present invention.

Claims

1. A method for preparing a composite tubular membrane, characterized in that, Includes the following steps: The casting solution and the supporting substrate were prepared separately. Stir the casting solution for 8-12 hours, and then allow it to stand under vacuum for 8-12 hours to remove bubbles. The coating is applied once to the inner wall of the rotating support substrate using a coating scraper. The side wall of the coating scraper has at least two grooves, and a thickness control ring is engaged in the groove. After the casting liquid is evenly cast, it solidifies into a film in the coagulation bath and dries for 22-26 hours. A secondary coating is applied to the inner wall of a rotating support substrate using a coating scraper. After the casting solution is evenly distributed, it solidifies into a film in a coagulation bath and is dried for 22-26 hours to obtain a composite tubular membrane.

2. The method for preparing the composite tubular membrane as described in claim 1, characterized in that: The coating scraper has a cavity, one end of which is a closed structure and the other end is an open structure, which is connected to the external equipment for supplying casting liquid. The side wall of the coating scraper has a liquid outlet hole that communicates with the cavity.

3. The method for preparing the composite tubular membrane as described in claim 1, characterized in that, The specific steps for applying a coating to the inner wall of the rotating support substrate using a coating scraper include: The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. The distance that the support substrate moves forward by rotating 360 degrees is less than the width of the coating scraper.

4. The method for preparing the composite tubular membrane as described in claim 1, characterized in that, The specific steps for performing secondary coating on the inner wall of the rotating support substrate using a coating scraper include: The coating scraper is placed on the inner wall of the support substrate, and the support substrate is driven to rotate and move forward along the axis. The distance that the support substrate moves forward by rotating 360 degrees is less than the width of the coating scraper.

5. The method for preparing the composite tubular membrane as described in claim 1, characterized in that: The casting solution comprises polymeric materials, pore-forming agents, and solvents.

6. The method for preparing the composite tubular membrane as described in claim 5, characterized in that: The polymer material is a combination of at least one or more of polyvinylidene fluoride, polysulfone, polyethersulfone, and polyamide; The pore-forming agent is at least one or a combination of two of polyvinylpyrrolidone and polyethylene glycol; The solvent is at least one or a combination of two of dimethylformamide and dimethylacetamide.

7. The method for preparing the composite tubular membrane as described in claim 1, characterized in that: The water temperature of the coagulation bath is 45-60 degrees Celsius.

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