A method for preparing a polypropylene hollow fiber degassing membrane with a superhydrophobic surface

Abstract

This invention discloses a method for preparing a gas separation membrane, specifically a method for densifying and hydrophobically modifying the outer surface of a hollow fiber membrane. The invention primarily involves mixing isotactic polypropylene granules in a mixer and heating them to 150-200°C. The granules are then stretched and extruded into a membrane through a spinneret while in a molten state. The membrane is then cooled for 1-3 minutes in a cooling chamber using volatile alkanes or alcohols as atomizing cooling media. Finally, the hollow fiber membrane fibers are stretched and drawn onto rollers, automatically wound onto the rollers to form the original membrane fibers. These fibers are then subjected to post-treatment processes such as overnight soaking in deionized water and washing to obtain the hollow fiber degassing membrane. The advantages of this invention are that it eliminates the need for introducing other components (the alcohol solvent is fully evaporated without residue), avoids material compatibility and stability issues between different components, and provides a long-lasting and reliable hydrophobic enhancement effect.

Description

Technical Field

[0001] This invention relates to a method for preparing a gas separation membrane, specifically involving the densification and hydrophobic modification of the outer surface of a hollow fiber membrane to prevent leakage and failure when the liquid comes into contact with the membrane. This method belongs to the field of gas-liquid separation membrane preparation and can achieve efficient removal or recycling of gas components in the liquid, enhance the processing efficiency of the degassing membrane, and reduce its usage cost. Background Technology

[0002] Membrane-based dissolved gas removal technology has been widely used in fields such as water deoxygenation (O2) and carbon dioxide (CO2) removal. Compared with traditional vacuum degassing, blower degassing, and chemical degassing processes, degassing membrane modules can achieve water-gas separation without the need for hydrodynamic dispersion of the aqueous solution. At the same time, the membrane module is woven from thousands of hollow fiber membrane clusters, and the membrane and the feed liquid have an extremely high contact area and a stable gas-liquid separation interface. This feature significantly improves the gas mass transfer efficiency, giving membrane degassing technology excellent separation efficiency and energy consumption advantages.

[0003] In membrane degassing, the feed liquid is on one side of the membrane while the gas is on the other. The membrane material acts as a support, allowing the feed liquid and gas to contact each other through the micropores on both sides of the membrane. This is achieved by controlling the gas pressure and allowing the gas to enter the gas phase from the feed liquid. Simultaneously, the membrane material also acts as an effective "barrier," preventing the liquid from directly penetrating the micropores to the gas side and causing leakage. Therefore, constructing hydrophobic surfaces and microporous structures to prevent direct permeation of the feed liquid through the membrane has become a fundamental principle in the design of degassing membrane materials. Currently, the mainstream degassing membranes on the market all use aliphatic polypropylene. Polypropylene is a non-polar plastic with high crystallinity, possessing good chemical stability, acid and alkali resistance, high hydrophobicity, and strong thermal stability, while also being inexpensive, leading to extensive research both domestically and internationally. However, polypropylene is prone to uneven stress during thermoforming, making it difficult to control pore size and resulting in an excessively wide pore size distribution. This is especially true in hollow fiber membrane morphology, where the pore size range is large (0.05-2 micrometers), and pores exist on both the inner and outer surfaces of the membrane fibers. When used for liquid degassing, a large amount of liquid can penetrate to the other side of the membrane fibers, posing a risk of membrane failure during actual operation and severely affecting the stability of the degassing process and the membrane's service life. Therefore, further improving the hydrophobicity of polypropylene hollow fiber degassing membranes and strengthening their barrier effect on the feed liquid is a key technical challenge in the research and development and manufacturing of this type of product. Summary of the Invention:

[0004] This invention addresses the problems of excessively wide pore size distribution and insufficient surface hydrophobicity in polypropylene hollow fiber degassing membranes by providing a simple method for preparing degassing membranes with highly hydrophobic and dense membrane surface structures. This method is based on the principle that the molecular chains of polypropylene are not easily extended at low temperatures, inhibiting pore formation. During the hot stretching and forming process of the hollow fiber membrane filaments, after the filaments are extruded from the spinneret, the outer surface of the filaments is rapidly cooled to form a dense structure, while the inner surface remains normally porous (no pores on the outer surface, pores on the inner surface), thus enhancing the hydrophobicity of the outer surface of the degassing membrane. To achieve the ideal cooling effect, this invention uses low-boiling-point, easily volatile alkanes or alcohols as the cooling medium. After atomization, these compounds are directly blown onto the outer surface of the polypropylene membrane filaments. Upon contact with the high-temperature outer surface of the membrane filaments, the cooling medium rapidly evaporates, carrying away a large amount of heat and rapidly cooling the outer surface of the membrane filaments. This method requires no complex post-processing and has good scalability. The technical solution of this invention is as follows: A method for preparing a polypropylene hollow fiber degassing membrane with a superhydrophobic surface, comprising the following steps:

[0005] (1) Mix isotactic polypropylene granules in a mixer and heat to 150-200℃ to obtain the polymer material for hollow fiber membranes. The hot-melt polymer material is stretched and extruded into a film through a spinneret. The stretching rate is controlled by adjusting the frequency of the roller drive motor, and the stretching speed is controlled at 1-4 m / min. The discharge rate is controlled by controlling the extrusion pressure, and the discharge rate is controlled at 30-80 g / min.

[0006] (2) The spinneret outlet is connected to the cooling chamber. The atomizer atomizes the liquid cooling medium in the cooling tank and inputs it to the lower end of the cooling chamber. Under the high temperature outside the membrane fibers, the cooling medium vaporizes and is then drawn into the cooling tank by the vacuum pump through the upper port to be cooled to a liquid state. It is then recycled through the atomizer under mixing and stirring conditions. The residence time of the hollow fiber membrane in the cooling chamber is 1-3 minutes; the vacuum degree of the vacuum pump is maintained at 0.01-0.1 Bar (1 Bar = 10⁻⁶ bar). 5 Pa); the cooling tank temperature is set to -10-5℃; the atomization rate of the atomizer is set to 1-10 g / min; the cooling medium is one or a mixture of volatile alkane or alcohol compounds, wherein the alkane includes n-hexane, n-butane, and cyclohexane, and the alcohol includes ethanol, propanol, isopropanol, and butanol.

[0007] (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the membrane fibers are removed from the rollers, they are soaked in deionized water overnight for cleaning and then tested.

[0008] Preferably, the melting temperature of polypropylene in step (1) above is set to 160-180℃;

[0009] Preferably, in step (1) above, the membrane fiber stretching speed is 2-3 m / min and the discharge rate is 50-70 g / min;

[0010] Preferably, the cooling medium in step (2) above is a mixture of volatile alkanes and alcohols, specifically a mixture of ethanol: hexane in a ratio of 6:1 to 3:1.

[0011] Preferably, the temperature of the cooling tank in step (2) above is -5 to 0℃;

[0012] Preferably, the atomization rate of the atomizer in step (2) above is set to 5-8 g / min;

[0013] As a supplementary explanation, among the cooling medium, alcohols have high molecular polarity and weak interaction with non-polar polypropylene materials. After atomization, alcohol molecules are difficult to adsorb onto the membrane fiber surface. On the other hand, alkanes have low molecular polarity and easily combine with the membrane fiber surface, which can quickly remove heat. However, alkanes are flammable and explosive and are not suitable for large-scale use. Therefore, this invention uses a mixture of alcohols and alkanes as the cooling medium to balance production quality and safety.

[0014] Beneficial effects:

[0015] Compared with existing degassing membrane material preparation technologies, the technical features of this invention are as follows:

[0016] (1) By utilizing the characteristic of rapid crystallization of polypropylene polymer at low temperature, the pore-forming behavior in the stretching and molding of hollow fiber membrane fibers is suppressed, the pore size of the outer surface of the membrane fibers is reduced, and the porous characteristics of the inner surface of the membrane fibers are retained, so that the hollow fiber membrane has both good air permeability and hydrophobicity, and prevents leakage of the liquid under long-term pressurized cross-flow operation conditions.

[0017] (2) Compared with the direct cold air purging method, the present invention uses volatile compounds as cooling medium to directly purge the outer surface of hollow fiber membrane filaments. The atomized cooling medium can uniformly contact the outside of the membrane filaments and rapidly vaporize and carry away heat. The heat transfer process is transformed from the "gas-solid" mode to the "liquid-solid" mode, which significantly improves the mass transfer efficiency, further increases the membrane filament cooling rate, can significantly reduce the membrane pore size, and increase hydrophobicity.

[0018] (3) In the prior art, in order to enhance the hydrophobicity of the degassing membrane, post-treatment methods such as membrane surface modification and coating are commonly used. It is often necessary to introduce a third hydrophobic component on the membrane surface. However, the present invention does not require the introduction of other components (the alcohol solvent leaves no residue after sufficient evaporation), thus avoiding the material compatibility and stability problems between different components. The hydrophobicity enhancement effect is long-lasting and reliable. Attached Figure Description

[0019] Figure 1 Schematic diagram of the device for implementing the present invention Detailed Implementation

[0020] The present invention will be described below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. Experimental methods not specifically described in the embodiments generally use conventional conditions and conditions described in the manual, or conditions recommended by the manufacturer. The general equipment, materials, reagents, etc., used are all commercially available unless otherwise specified. The raw materials used in the following embodiments and comparative examples are all commercially available.

[0021] Example 1

[0022] (1) Isotactic polypropylene granules are mixed in a mixer and heated to 180°C to serve as the polymer material for hollow fiber membranes. The hot-melt polymer material is stretched and extruded into a film through a spinneret. The stretching rate is controlled by adjusting the frequency of the roller drive motor, and the stretching speed is controlled at 2 m / min. The discharge rate is controlled by controlling the extrusion pressure, and the discharge rate is controlled at 50 g / min.

[0023] (2) The spinneret outlet is connected to the cooling chamber, where the hollow fiber membrane resides for 1 minute; the vacuum pump maintains a vacuum level of 0.01 Bar (1 Bar = 10⁻⁶). 5 Pa); the cooling tank temperature is set to -10℃; the atomization rate of the atomizer is set to 1g / min; the cooling medium is a volatile alcohol solution, and the alcohol solvent is a mixture of propanol and isobutanol in a mass ratio of 1:2.

[0024] (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the membrane fibers are removed from the rollers, they are soaked in deionized water overnight for cleaning and then tested.

[0025] As a control, the cooling system was directly shut off, designated as Control Group 1 (room temperature cooling); or the cooling medium in the cooling system was removed, i.e., only cold air was used to blow the surface of the hollow fiber membrane, designated as Control Group 2 (cold air blowing). All other preparation conditions were the same as in Example 1. The results are shown in the table below:

[0026] Key structural parameters and performance of hollow fiber degassing membranes

[0027]

[0028] The data shows that although cold air purging can reduce the surface aperture of the membrane and improve hydrophobicity to some extent, the effect of purging with a cooling medium is significantly better, while the membrane gas permeability is partially lost.

[0029] Example 2

[0030] (1) Isotactic polypropylene granules are mixed in a mixer and heated to 200°C to serve as the polymer material for hollow fiber membranes. The hot-melt polymer material is stretched and extruded into a film through a spinneret. The stretching rate is controlled by adjusting the frequency of the roller drive motor, and the stretching speed is controlled at 4 m / min. The discharge rate is controlled by controlling the extrusion pressure, and the discharge rate is controlled at 60 g / min.

[0031] (2) The spinneret outlet is connected to the cooling chamber, where the hollow fiber membrane resides for 2 minutes; the vacuum pump maintains a vacuum level of 0.05 Bar (1 Bar = 10⁻⁶). 5 Pa); the cooling tank temperature is set to 0℃; the atomization rate of the atomizer is set to 4g / min; the cooling medium is an alcohol / alkane mixed solution, specifically a mixture of ethanol and n-hexane in a mass ratio of 4:1.

[0032] (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the membrane fibers are removed from the rollers, they are soaked in deionized water overnight for cleaning and then tested.

[0033] The outer surfaces of the hollow fiber membranes prepared in Examples 1 and 2 were subjected to 24-hour continuous pressurized water immersion treatment to examine the water resistance of the membrane surface. As a control, the cooling system was directly shut off, designated as Control Group 3 (room temperature cooling); or the cooling medium in the cooling system was removed, i.e., only cold air was used to blow away the surface of the hollow fiber membrane, designated as Control Group 4 (cold air blowing). All other preparation conditions were the same as in Example 2, and tests were conducted under the same conditions. The test results are shown in the table below:

[0034] Anti-wetting properties of the outer surface of hollow fiber degassing membrane

[0035]

[0036] The above results show that, after water pressurization wetting treatment, the outer surface of the hollow fiber degassing membrane (including Example 1 and Example 2) prepared by the present invention still has good hydrophobicity.

[0037] Example 3

[0038] (1) Isotactic polypropylene granules are mixed in a mixer and heated to 200°C to serve as the polymer material for hollow fiber membranes. The hot-melt polymer material is stretched and extruded into a film through a spinneret. The stretching rate is controlled by adjusting the frequency of the roller drive motor, and the stretching speed is controlled at 4 m / min. The discharge rate is controlled by controlling the extrusion pressure, and the discharge rate is controlled at 70 g / min.

[0039] (2) The spinneret outlet is connected to the cooling chamber, where the hollow fiber membrane resides for 2 minutes; the vacuum pump maintains a vacuum level of 0.07 Bar (1 Bar = 10⁻⁶). 5 Pa); the cooling tank temperature is set to 0℃; the atomization rate of the atomizer is set to 7g / min; the cooling medium is an alcohol / alkane mixed solution, specifically a mixture of isopropanol and n-butane in a mass ratio of 3:1.

[0040] (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the membrane fibers are removed from the rollers, they are soaked in deionized water overnight for cleaning and then tested.

[0041] The hollow fiber membrane fibers prepared in Examples 1, 2, and 3 were packed into membrane modules for feed deoxygenation treatment. The permeate-side vacuum was 0.1 MPa, the feed temperature was 80°C, the feed flow rate was 60 L / h, and the feed pressure was 0.1 MPa. As a control, the cooling system was directly shut off, designated as Control Group 5 (room temperature cooling); or the cooling medium in the cooling system was removed, i.e., only cold air was used to blow the surface of the hollow fiber membrane, designated as Control Group 6 (cold air blowing). All other preparation conditions were the same as in Example 3. The test results are shown in the table below:

[0042] Long-term operational stability of hollow fiber degassing membrane

[0043]

[0044]

[0045] The results show that control group 5 experienced significant gas permeability loss. This is because the feed liquid gradually seeped into the pores on the membrane surface, leading to a decrease in the gas-liquid interface area and inhibiting oxygen escape. In contrast, the hollow fiber degassing membrane prepared by this invention maintained stable oxygen permeability even under long-term operating conditions.

[0046] Example 4

[0047] (1) Isotactic polypropylene granules are mixed in a mixer and heated to 200°C to serve as the polymer material for hollow fiber membranes. The hot-melt polymer material is stretched and extruded into a film through a spinneret. The stretching rate is controlled by adjusting the frequency of the roller drive motor, and the stretching speed is controlled at 4 m / min. The discharge rate is controlled by controlling the extrusion pressure, and the discharge rate is controlled at 75 g / min.

[0048] (2) The spinneret outlet is connected to the cooling chamber, where the hollow fiber membrane resides for 3 minutes; the vacuum pump maintains a vacuum level of 0.1 Bar (1 Bar = 10⁻⁶). 5Pa); the cooling tank temperature is set to 5℃; the atomization rate of the atomizer is set to 10g / min; the cooling medium is an alcohol / alkane mixed solution, specifically a mixture of butanol and cyclohexane in a mass ratio of 2:1.

[0049] (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the membrane fibers are removed from the rollers, they are soaked in deionized water overnight for cleaning and then tested.

[0050] During the preparation processes of Examples 1, 2, 3, and 4, the temperature changes of the outer surface of the hollow fiber membrane fibers before and after entering the cooling chamber were recorded. As a control, the cooling system was directly shut off, designated as Control Group Seven (room temperature cooling); or the cooling medium in the cooling system was removed, i.e., only cold air was used to blow the surface of the hollow fiber membrane, designated as Control Group Eight (cold air blowing). All other preparation conditions were the same as in Example 3. The test results are shown in the table below:

[0051] Temperature change of the outer surface of hollow fiber membrane before and after cooling

[0052]

[0053]

[0054] The results show that the temperature of the membrane fibers decreased significantly after using the cooling medium, indicating that the heat transfer efficiency was greatly improved compared with room temperature cooling or cold air blowing.

Claims

1. A method for preparing a polypropylene hollow fiber degassing membrane with a superhydrophobic surface, characterized in that, Includes the following steps: (1) Mix isotactic polypropylene granules in a mixer and heat them to 150-200℃ to serve as hollow fiber membrane polymer material. The hot-melt polymer material is extruded and stretched into a film through a spinneret. The stretching speed is controlled at 1-4 m / min and the discharge rate is controlled at 30-80 g / min. (2) The spinneret outlet is connected to the cooling chamber, and the hollow fiber membrane passes through the cooling chamber; the cooling medium is atomized and enters through the lower end of the cooling chamber, and the cooling medium is drawn away by the vacuum pump connected to the upper port of the cooling chamber; the hollow fiber membrane passes through the cooling chamber for 1-3 min; the vacuum degree of the vacuum pump is maintained at 0.01-0.1 Bar; the temperature of the cooling medium when it enters the cooling chamber is set to -10℃-5℃; the atomization rate of the cooling medium is 1-10 g / min; the cooling medium is one or a mixture of several of the following: n-hexane, n-butane, cyclohexane, ethanol, propanol, isopropanol, or butanol; (3) After passing through the cooling chamber, the hollow fiber membrane fibers are automatically wound onto the rollers under the stretching and traction action of the rollers to form the original membrane fibers. After the original membrane fibers are removed from the rollers, they are soaked in deionized water overnight to clean them and obtain hollow fiber degassing membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the polypropylene melting temperature is set to 160-180℃.

3. The preparation method according to claim 1, characterized in that, In step (1), the membrane fiber stretching speed is 2-3 m / min and the discharge rate is 50-70 g / min.

4. The preparation method according to claim 1, characterized in that, In step (2), the cooling medium is a mixture of ethanol and n-hexane in a ratio of 6:1 to 3:

1.

5. The preparation method according to claim 1, characterized in that, In step (2), the temperature of the cooling medium entering the cooling chamber is set to -5 to 0℃.

6. The preparation method according to claim 1, characterized in that, In step (2), the atomization rate of the atomizer is set to 5-8 g / min.

7. The preparation method according to claim 1, characterized in that, In step (2), the vacuum pump outlet is connected to the atomizing device of the cooling medium.

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