Preparation method of CFRO membrane for high-power salt concentration

Abstract

The invention relates to a preparation method of a CFRO membrane for high-power salt concentration, in particular to a preparation method of a double-skin polyamide RO membrane applied to CFRO. According to the invention, by constructing a double-skin layer structure, the comprehensive improvement of the membrane separation performance is realized. Compared with a traditional single-skin-layer membrane, the double-skin-layer structure has the advantages that the water flux is remarkably improved while the high rejection rate is kept through functional partition and gradient design. The high-pressure skin layer provides a stable separation barrier, and efficient interception of solute is ensured; the low-pressure skin layer reduces the water transmission resistance by optimizing the pore structure and the surface property, so that the membrane is gt; and high-flux operation can still be maintained under the pressure of 10Mpa.

Description

Technical Field

[0001] This invention relates to a method for preparing a CFRO membrane for high-concentration salt concentration, and more particularly to a method for preparing a double-skin polyamide RO membrane for use in CFRO. Background Technology

[0002] Polyamide reverse osmosis membranes (RO membranes) are currently the most widely used high-efficiency separation materials in the water treatment field. Their core technology involves forming an ultra-thin polyamide selective layer on the surface of a porous support through interfacial polymerization. This selective layer is generated by the rapid reaction of aqueous and oil-phase monomers at the oil-water interface, forming a dense structure with high cross-linking and low porosity, effectively retaining salts, organic matter, and microorganisms.

[0003] Concentrated Filtration Reverse Osmosis Membrane (CFRO) is a type of reverse osmosis membrane specifically designed for the separation of high-concentration solutions. Its core applications include seawater desalination, high-salinity wastewater treatment, and chemical material concentration. Unlike conventional reverse osmosis membranes, CFRO membranes require long-term stable operation under extreme pressures exceeding 10 MPa to overcome the osmotic pressure of high-concentration solutions and achieve efficient concentration. However, this high-pressure environment presents a dual challenge to membrane materials: on the one hand, the selective layer must possess sufficient mechanical strength to resist pressure-induced deformation; on the other hand, high flux must be maintained to reduce energy consumption. Traditional polyamide membranes are prone to selective layer compression and decreased porosity under pressures above 10 MPa, leading to a sharp decline in flux (typically reduced by 30%-50%), and even membrane structure damage, severely impacting separation efficiency and service life.

[0004] To improve the resilience of reverse osmosis membranes under high pressure, existing technologies generally employ increasing the thickness of the selective layer. This is achieved by extending the interfacial polymerization reaction time, applying multiple layers of polyamide, or introducing nanoparticle reinforcement layers to construct a denser structure that resists pressure-induced deformation. However, this strategy has significant limitations: First, increasing membrane thickness directly leads to a longer water transport path and reduced porosity, resulting in a sharp decline in flux. Second, an excessively thick selective layer is prone to localized stress concentration under high pressure, causing irreversible compression deformation, pore closure, or interlayer delamination, leading to fluctuations in rejection rate or even membrane failure. Furthermore, the multilayer coating process requires extremely high interfacial uniformity; even slight errors can introduce defects, reducing the overall performance stability of the membrane. Therefore, simply relying on thickening the selective layer cannot simultaneously meet the dual requirements of high pressure resilience and high flux for CFRO membranes. Performance breakthroughs are urgently needed through membrane structure innovation and process optimization. Summary of the Invention

[0005] To address the aforementioned issues, this invention applies the commonly used double-layer polyamide membrane for forward osmosis to reverse osmosis and optimizes its structure and preparation process, thereby improving the membrane flux and stability while ensuring membrane pressure resistance.

[0006] Specifically, the present invention provides a method for preparing a CFRO membrane for high-concentration salt concentration. The CFRO membrane includes a support layer, a high-pressure skin layer located on one side of the support layer, and a low-pressure skin layer located on the other side of the support layer. The preparation method includes the following steps: S1: Impregnating one side of the support layer with a solution containing aliphatic diamine and a polyacryl chloride solution in sequence facilitates interfacial polymerization to form a low-pressure skin layer; S2: The other side of the support layer is successively impregnated with a solution containing polyaniline and a solution containing polyacryl chloride to form a high-pressure skin layer by interfacial polymerization; S3: The entire support layer loaded with the low-pressure skin layer and the high-pressure skin layer is immersed in a solution of nitrous acid or its salt to undergo a diazotization reaction, thereby forming the CFRO membrane.

[0007] As a preferred embodiment, the support layer is selected from one of polysulfone, polyethersulfone, polyolefin, polytetrafluoroethylene or polyvinylidene fluoride, and has a pore size of 200-2000 nm.

[0008] As a preferred embodiment, in step S1, the aliphatic diamine is one or more of ethylenediamine, propylenediamine, butanediamine, and pentanediamine, and its concentration in the solution containing the aliphatic diamine is 0.5-5 wt%.

[0009] As a preferred embodiment, in step S1, the immersion time of the support layer in the aliphatic diamine solution and the polyacrylamide solution is 5-60s and 10-60s, respectively.

[0010] As a preferred embodiment, in step S2, the polyphenylene amine is selected from one or more of m-phenylenediamine and p-phenylenediamine, and its concentration in the solution containing aliphatic diamine is 1-5 wt%.

[0011] As a preferred embodiment, in step S2, the immersion time of the support layer in the polyaniline solution and the polyacrylamide solution is 20-30s and 30-60s, respectively.

[0012] As a preferred embodiment, in step S3, the concentration of nitrite or its salt solution is 0.1-1 wt%, and the pH is 2-5.

[0013] As a preferred embodiment, in step S3, the support layer loaded with the low-pressure skin layer and the high-pressure skin layer is immersed in nitrite or its salt solution for 5-60 seconds.

[0014] The present invention also provides a method for preparing a high-concentration CFRO membrane according to the above preparation method, wherein the CFRO membrane includes a support layer, a high-pressure skin layer located on one side of the support layer, and a low-pressure skin layer located on the other side of the support layer.

[0015] The CFRO membrane prepared by this invention can be used in high-rate reverse osmosis concentration with an operating pressure ≥10MPa.

[0016] Compared with the prior art, the present invention has the following beneficial effects: First, this invention achieves a comprehensive improvement in membrane separation performance by constructing a dual-skin structure. Compared to traditional single-skin membranes, the dual-skin structure, through functional partitioning and gradient design, significantly increases water flux while maintaining a high rejection rate. The high-pressure skin provides a stable separation barrier, ensuring efficient retention of solutes; the low-pressure skin, by optimizing pore structure and surface properties, reduces water transport resistance, enabling the membrane to maintain high-flux operation even under pressures >10 MPa.

[0017] Secondly, the high-pressure skin layer of this invention uses highly cross-linked aromatic monomers and polyacryl chlorides for interfacial polymerization to form a dense selective layer with high mechanical strength, which can resist membrane compression deformation caused by high pressure; the low-pressure skin layer uses aliphatic monomers with low cross-linking degree to react with polyacryl chlorides to generate a structure with high porosity and low surface energy, which not only reduces water transport resistance but also ensures membrane flux.

[0018] Furthermore, this invention, through a single diazotization reaction of the polyamide double skin layer, can simultaneously improve the crosslinking degree of the high-pressure skin layer and increase the flux to the low-pressure skin layer. Specifically, the high-pressure skin layer, employing aniline monomers, utilizes nitrous acid or its salt solution to react with the aromatic amine structure, thereby enhancing the membrane's crosslinking degree and thus its pressure resistance. Meanwhile, the low-pressure skin layer, employing small-molecule aliphatic diamines, can significantly increase the membrane flux under the action of nitrous acid or its salt solution. Attached Figure Description

[0019] Figure 1 SEM images of the surface of the membrane sample prepared in Example 1 (left - high pressure skin, right - low pressure skin). Detailed Implementation

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings involved in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention, and not all embodiments. Any other embodiments obtained by those skilled in the art based on these drawings without creative effort are within the protection scope of the present invention.

[0021] Example 1 Step 1: Preparation of the low-pressure cortex A PES support layer (polyethersulfone (PES) ultrafiltration membrane, average pore size 450 nm, thickness approximately 120 μm) was fixed in a frame. One side was immersed in an ethylenediamine aqueous solution (0.5 wt%) for 30 s, and then removed and the surface was purged with compressed air to remove excess solution. Immediately after immersion in a TMC n-hexane solution (1 wt%) for 45 s, an interfacial polymerization reaction occurred to form a low-pressure skin layer. The membrane was then dried internally in a 60 °C oven for 10 min.

[0022] Step 2: Preparation of the high-pressure skin The other side of the support layer was immersed in a 3 wt% aqueous solution of m-phenylenediamine for 20 seconds, the surface was purged, and then immersed in a 5 wt% TMC hexane solution for 60 seconds to form a high-pressure skin. It was rinsed with hexane and dried at 60°C for 10 minutes.

[0023] Step 3: Post-diazotization treatment The double-layer membrane was immersed in sodium nitrite solution (0.5 wt%, pH=3) for 30 s to undergo diazotization. After removal, it was rinsed with deionized water and vacuum dried at 80 °C for 2 h to obtain the CFRO membrane (its surface SEM characterization image is shown below). Figure 1 (As shown).

[0024] Example 2 Step 1: Preparation of the low-pressure cortex A PES support layer (polyethersulfone (PES) ultrafiltration membrane, average pore size 450 nm, thickness approximately 120 μm) was fixed in a frame. One side was immersed in an ethylenediamine aqueous solution (0.5 wt%) for 30 s, and then removed and the surface was purged with compressed air to remove excess solution. Immediately after immersion in a TMC n-hexane solution (1 wt%) for 45 s, an interfacial polymerization reaction occurred to form a low-pressure skin layer. The membrane was then dried internally in a 60 °C oven for 10 min.

[0025] Step 2: Preparation of the high-pressure skin The other side of the support layer was immersed in a 3 wt% aqueous solution of m-phenylenediamine for 20 seconds, the surface was purged, and then immersed in a 5 wt% TMC hexane solution for 60 seconds to form a high-pressure skin. It was rinsed with hexane and dried at 60°C for 10 minutes.

[0026] Step 3: Post-diazotization treatment The double-layer membrane was immersed in a potassium nitrite solution (0.7 wt%, pH=6) for 30 s to undergo a diazotization reaction. After removal, it was rinsed with deionized water and dried under vacuum at 80 °C for 2 h to obtain the CFRO membrane.

[0027] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that m-phenylenediamine is used instead of ethylenediamine in step (1), while the concentration is the same.

[0028] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that ethylenediamine is used instead of m-phenylenediamine in step (2), while the concentration is the same.

[0029] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that the diazotization reaction in step (3) is omitted.

[0030] Comparative Example 4 Step 1: Preparation of the low-pressure cortex A PES support layer (polyethersulfone (PES) ultrafiltration membrane, average pore size 450 nm, thickness approximately 120 μm) was fixed in a frame. One side was immersed in an ethylenediamine aqueous solution (0.5 wt%) for 30 s, and then removed and the surface was purged with compressed air to remove excess solution. Immediately after immersion in a TMC n-hexane solution (1 wt%) for 45 s, an interfacial polymerization reaction occurred to form a low-pressure skin layer. The membrane was then dried internally in a 60 °C oven for 10 min.

[0031] Step 2: Preparation of the high-pressure skin The side of the support layer loaded with the low-pressure skin was immersed in an aqueous solution of m-phenylenediamine (3wt%) for 20s, the surface was purged, and then immersed in a TMC n-hexane solution (5wt%) for 60s to form a high-pressure skin. It was rinsed with n-hexane and dried at 60°C for 10min.

[0032] Step 3: Post-diazotization treatment The double-layer membrane was immersed in a sodium nitrite solution (0.5 wt%, pH=3) for 30 s to undergo a diazotization reaction. After removal, it was rinsed with deionized water and dried under vacuum at 80 °C for 2 h to obtain the CFRO membrane.

[0033] Characterization The CFRO membrane samples obtained from the above embodiments and comparative examples were characterized, including the following: (1) Compression resistance test The physical properties of the film samples were tested using an Instron 5967 universal testing machine (load accuracy ±0.3%) at 23℃ and 45% humidity, with a testing rate of 0.8 mm / min. The characterization results are shown in Table 1. Table 1. Physical property test results of different embodiments and comparative samples. (2) Separation performance test The rejection rate and flux of the above samples to 3.5 wt% NaCl solution were tested at a pressure of 12 MPa and a temperature of room temperature. Samples were taken every 1 hour, and 10 samples were tested, and the average value was taken. The test results are shown in Table 2: Table 2. Separation performance test results of different embodiments and comparative samples. (3) Stability test The membrane sample was continuously run at 12 MPa pressure for 500 hours, and then its flux was tested. Samples were taken every 1 hour, and the average value of 10 samples was taken. Then the flux decay rate was calculated.

[0034] Table 3. Stability test results of different embodiments and comparative samples As can be seen from the examples and comparative examples, the CFRO membrane prepared by the present invention has significant improvements in both pressure resistance and separation performance, and has high application value.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A method for preparing a CFRO membrane for high-concentration salt concentration, characterized in that, The CFRO membrane includes a support layer, a high-pressure skin layer located on one side of the support layer, and a low-pressure skin layer located on the other side of the support layer, wherein the preparation method includes the following steps: S1: Impregnating one side of the support layer with a solution containing aliphatic diamine and a polyacryl chloride solution in sequence facilitates interfacial polymerization to form a low-pressure skin layer; S2: The other side of the support layer is successively impregnated with a solution containing polyaniline and a solution containing polyacryl chloride to form a high-pressure skin layer by interfacial polymerization; S3: The entire support layer loaded with the low-pressure skin and the high-pressure skin is immersed in a solution of nitrite or its salt to undergo a diazotization reaction, thereby forming the CFRO membrane.

2. The preparation method according to claim 1, characterized in that, The support layer is selected from one of polysulfone, polyethersulfone, polyolefin, polytetrafluoroethylene or polyvinylidene fluoride, and has a pore size of 200-2000 nm.

3. The preparation method according to claim 1, characterized in that, In step S1, the aliphatic diamine is one or more of ethylenediamine, propylenediamine, butanediamine, and pentanediamine, and its concentration in the solution containing the aliphatic diamine is 0.5-5 wt%.

4. The preparation method according to claim 1, characterized in that, In step S1, the immersion time of the support layer in the aliphatic diamine solution and the polyacrylamide solution is 5-60s and 10-60s, respectively.

5. The preparation method according to claim 1, characterized in that, In step S2, the polyphenylene aniline is selected from one or more of m-phenylenediamine and p-phenylenediamine, and its concentration in the solution containing aliphatic diamine is 1-5 wt%.

6. The preparation method according to claim 1, characterized in that, In step S2, the immersion time of the support layer in the polyaniline solution and the polyacrylamide solution is 20-30s and 30-60s, respectively.

7. The preparation method according to claim 1, characterized in that, In step S3, the concentration of nitrite or its salt solution is 0.1-1 wt%, and the pH is 2-5.

8. The preparation method according to claim 1, characterized in that, In step S3, the support layer loaded with the low-pressure skin and the high-pressure skin is immersed in nitrite or its salt solution for 5-60 seconds.

9. A method for preparing a high-concentration CFRO membrane according to claim 1, characterized in that, The CFRO membrane includes a support layer, a high-pressure skin layer located on one side of the support layer, and a low-pressure skin layer located on the other side of the support layer.

10. The application of the CFRO membrane according to claim 9 in high-rate reverse osmosis concentration, characterized in that, Its working pressure is ≥10MPa.

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