A membrane, its preparation method, and its application in natural gas membrane decarbonization.

Abstract

This application discloses a membrane, its preparation method, and its application in natural gas membrane decarbonization. The membrane comprises a base membrane, a hydrophilic layer, and a hydrophilic functional coating. The hydrophilic layer is made of at least one material selected from ethanolamine, diethanolamine, diaminopropanol, diaminobenzoic acid, and 2,4-diaminobenzenesulfonic acid. The functional coating is made of at least one material selected from cellulose acetate, polyethylene glycol, chitosan, polyvinyl alcohol, and polylactic acid. Under heavy hydrocarbon pollution conditions, the swelling of the hydrophilic material is very slight, completely ensuring the integrity of the membrane surface and separation performance. The process is simple, the raw materials are inexpensive, and the reliability is high, exhibiting significant technical advantages.

Description

Technical Field

[0001] This application relates to a membrane, its preparation method, and its application in natural gas membrane decarbonization, belonging to the field of membrane separation. Background Technology

[0002] Natural gas extraction typically involves varying amounts of CO2, which not only reduces the calorific value of the natural gas, increases transportation load and costs, but also corrodes pipelines and equipment. Therefore, decarbonization treatment is essential in the early stages to meet national commercial gas standards. my country is rich in offshore natural gas resources, with the Bohai Sea, East China Sea, and northern South China Sea alone accounting for 15.6% of the country's total resources. Currently, the mature technology for natural gas decarbonization is the absorption tower process, which is widely used onshore. However, it shows significant shortcomings when applied to offshore natural gas decarbonization processes, such as large size, tall tower, limited operational flexibility, and severe impact on processing efficiency due to shaking. It also suffers from drawbacks such as mist entrainment, flooding, and leakage. In contrast, membrane separation has advantages such as low energy consumption, small footprint, simple process, and modular operation, and is expected to dominate the field of natural gas decarbonization on offshore platforms.

[0003] Natural gas extracted from offshore platforms is often under high pressure, and some gas fields have high CO2 content. Taking a gas field in the South China Sea as an example, the pressure of the raw natural gas extracted reaches 6.0–7.0 MPa, with a CO2 content as high as 42.67%, methane 46.12%, and the remainder being nitrogen, ethane, propane, butane, and heavy hydrocarbons. Therefore, when using membrane decarbonization of natural gas, the CO2 partial pressure on the feedstock side is as high as 2.0 MPa or more. Furthermore, because CO2 is a condensable gas with a high solubility coefficient within polyimide membrane materials, it is a gas that permeates rapidly through the membrane. Under these circumstances, the large-scale permeation of CO2 from the high-pressure side to the low-pressure side will form a typical Joule-Thomson process, inevitably leading to a significant throttling expansion and cooling effect.

[0004] Taking the natural gas produced from a gas field in Hainan as an example, if operated at 323K, the CO2 partial pressure difference across the membrane will exceed 2.0 MPa. Under these conditions, the throttling expansion caused by CO2 throttling will result in a temperature drop of nearly 20.0℃. On the one hand, the heavy hydrocarbons contained in the natural gas are "slow gases" permeating through the membrane, and their concentration in the membrane module is already far higher than the inlet value due to the large amount of CO2 permeate loss. At the same time, the accompanying cooling effect will further reduce the saturated vapor pressure of the heavy hydrocarbon components, making the heavy hydrocarbon content on the high-pressure side of the membrane module increasingly prone to supersaturation, resulting in varying degrees of pollution phenomena such as adsorption / condensation on the membrane surface.

[0005] In the process of preparing separation membranes, in order to obtain high permeation flux with practical application value, separation membranes are all asymmetric structures. Their dense separation layer is generally less than 0.1 micrometers thick. Such a thickness cannot be used to directly prepare a defect-free separation membrane by phase inversion method. Their surface often has a certain proportion of micro-defects, and a silicone rubber coating pore-blocking technology is required for post-treatment. Only after this treatment can the separation performance of the membrane approach or reach the intrinsic separation coefficient of the material. However, under the condition of heavy hydrocarbon adsorption and swelling, the silicone rubber pore-blocking part of the membrane surface will be destroyed, causing the defects on the separation membrane surface to reappear, resulting in a decrease in membrane separation performance or even loss of separation performance.

[0006] To avoid heavy hydrocarbon contamination of membrane modules, engineers often empirically raise the operating temperature of membrane modules to 60.0–70.0℃, and in some gas fields with high initial heavy hydrocarbon content, the operating temperature can even reach over 90.0℃. While this design can minimize heavy hydrocarbon contamination, its drawbacks are obvious: First, high-temperature operation places an additional heating energy burden on offshore platforms; second, prolonged high temperatures seriously threaten the stable operation of membrane modules, as the high-temperature resistance of the epoxy sealing heads cannot be guaranteed in the long term; and most seriously, it affects the separation performance of the membrane modules. For glassy polymer membranes, the gas permeation flux increases at high temperatures while the separation coefficient decreases, leading to an unnecessary and sharp increase in methane loss, thus impacting the economic competitiveness of membrane decarbonization processes. Summary of the Invention

[0007] To address the issue of membrane separation performance degradation caused by the destruction of silicone rubber plugs on the membrane surface under heavy hydrocarbon pollution, this patent proposes a coating method using hydrophilic materials to seal defect pores on the separation membrane surface. This method can significantly alleviate the problem of membrane performance degradation and has obvious technical advantages.

[0008] In response to the problem that the cooling phenomenon caused by CO2 throttling expansion and the large-scale CO2 permeation during the membrane decarbonization process of natural gas on offshore platforms leads to the supersaturation of heavy hydrocarbons in the membrane shell, which in turn causes the contamination and damage to the silicone rubber pores, resulting in a sharp decline in membrane separation performance, this invention proposes a membrane coating method that uses a hydrophilic material to replace the traditional silicone rubber.

[0009] According to one aspect of this application, a membrane is provided, comprising a base membrane, a hydrophilic layer, and a hydrophilic functional coating.

[0010] The material of the hydrophilic layer is selected from at least one of ethanolamine, diethanolamine, diaminopropanol, diaminobenzoic acid, and 2,4-diaminobenzenesulfonic acid;

[0011] The hydrophilic functional coating material is selected from at least one of cellulose acetate, polyethylene glycol, chitosan, polyvinyl alcohol, and polylactic acid.

[0012] The base film is selected from at least one of polyimide film, cellulose acetate film, and polysulfone film.

[0013] According to another aspect of this application, a method for preparing the above-mentioned membrane is provided. First, a hydrophilic modifier is used to hydrophilically modify the defect pores on the surface of the hollow fiber membrane contained in the membrane core, forming affinity anchors within the defect pores to facilitate the subsequent hydrophilic material coating and pore-blocking process. After the membrane core with hydrophilically modified pores is cleaned to remove residual modifying liquid from the surface, it is dried at high temperature to promote the hydrophilic modification process and fully remove excess residual liquid from the membrane pores. Then, the membrane core is immersed in a coating liquid prepared with hydrophilic material, and a vacuum is drawn at the epoxy end of the membrane core to drive the coating liquid into the membrane pores and combine with the affinity anchors within the membrane pores, forming a pore-blocking effect of the hydrophilic material. After the coating is completed, the residual coating liquid on the surface is cleaned to remove it, and then dried at high temperature to further promote the cross-linking of the hydrophilic material in the coating, forming a stable pore-blocking effect, so as to obtain a CO2 separation membrane that can withstand the damage caused by heavy hydrocarbon pollution common in natural gas membrane decarbonization processes.

[0014] Includes the following steps:

[0015] The substrate is immersed in a solution of solvent I containing a hydrophilic modifier, dried, and then immersed in a solution of solvent II containing a hydrophilic functional material and a crosslinking agent, dried to obtain the membrane.

[0016] The hydrophilic modifier is selected from at least one of ethanolamine, diethanolamine, diaminopropanol, diaminobenzoic acid, and 2,4-diaminobenzenesulfonic acid.

[0017] Solvent I is selected from at least one of methanol, ethanol, and propanol; ethanol is preferred.

[0018] In the solvent I solution containing the hydrophilic modifier, the concentration of the hydrophilic modifier is 0.01–10 wt%.

[0019] The vacuum degree of the impregnation I is 0 to 0.1 MPa;

[0020] The immersion time for I is 1 to 30 minutes;

[0021] The temperature of the drying process I is 50–120°C;

[0022] The drying time for step I is 1 to 120 minutes.

[0023] The hydrophilic functional material is selected from at least one of cellulose acetate, polyethylene glycol, chitosan, polyvinyl alcohol, and polylactic acid;

[0024] The crosslinking agent is selected from at least one of glutaraldehyde and boric acid;

[0025] Solvent II is selected from at least one of acetone, water, and ethanol; water is preferred.

[0026] In the solvent II solution containing the hydrophilic functional material and the crosslinking agent, the concentration of the hydrophilic functional material is 0.01–10 wt%.

[0027] In the solvent II solution containing the hydrophilic functional material and the crosslinking agent, the mass of the crosslinking agent is 5 to 25 wt% of the mass of the hydrophilic functional material.

[0028] The volume of the solvent II solution containing the hydrophilic functional material and the crosslinking agent is 2 to 5 times the volume of the bottom film;

[0029] The vacuum degree of the impregnation II is 0 to 0.1 MPa;

[0030] The immersion time for the second stage is 1 to 30 minutes;

[0031] The temperature of the impregnation II is 0–100°C;

[0032] The temperature of the drying II process is 1–100°C;

[0033] The drying time for step II is 1 to 120 minutes.

[0034] Drying I and Drying II are carried out in a high-temperature vacuum oven.

[0035] The drying processes I and II are preceded by cleaning.

[0036] The cleaning solution used for cleaning is selected from at least one of water, ethanol, and methanol;

[0037] The cleaning time is 1 to 30 minutes.

[0038] The specific steps are as follows:

[0039] According to the separation requirements, a certain number of bottom membranes are cast into a core module with a conventional end cap on one end and an epoxy seal on the other end; depending on the separation throughput, the number of membrane filaments can range from a few to hundreds of thousands.

[0040] The membrane core is immersed in the prepared hydrophilic modification solution. The modification solution is introduced into the defect pores on the membrane surface by vacuuming through the epoxy end. Then, the residual modification solution on the surface is washed away. After a period of modification reaction to ensure that a large number of affinity anchors appear in the membrane micropores, the membrane core is placed in a drying oven for high-temperature treatment to fully remove the residual liquid in the membrane micropores.

[0041] After the membrane core dries, it is immersed in a prepared hydrophilic coating solution. The temperature of the hydrophilic coating solution is maintained at a high level to facilitate solvent evaporation. Then, a vacuum is drawn at the epoxy end. This promotes the coating solution to penetrate into the defects and pores on the membrane surface, while simultaneously promoting the continuous evaporation of the high-temperature solvent to increase the concentration of hydrophilic material within the micropores, thereby achieving a pore-blocking effect. After a period of vacuuming, the change in vacuum degree is observed. A significant decrease in vacuum degree indicates that the defects and pores on the membrane surface have been fully blocked. Then, the membrane core is immersed in a cleaning solution while maintaining the vacuum degree to remove any residual coating solution from its surface.

[0042] After cleaning, the membrane core is placed in a high-temperature drying oven and heated at a certain temperature for a fixed time, so that the hydrophilic material in the membrane pores can fully cross-link and form a stable pore-blocking effect; at the same time, the liquid remaining in the membrane micropores and on the surface is fully removed, thus preparing a membrane core resistant to heavy hydrocarbon pollution.

[0043] According to another aspect of this application, an application of the above-described membrane is provided for membrane decarbonization of natural gas.

[0044] The beneficial effects that this application can produce include:

[0045] This invention provides a method for coating the bottom membrane of a CO2 separation membrane with a hydrophilic material. Unlike the traditional silicone rubber coating process, the hydrophilic material, due to its higher tolerance to heavy hydrocarbons, allows the CO2 separation membrane prepared by this invention to exhibit better long-term stability in natural gas decarbonization processes. With silicone rubber-coated CO2 separation membranes, when heavy hydrocarbons condense on the membrane surface, the silicone rubber within the micropores swells, compromising its surface integrity. As is well known, gas separation membranes must be foolproof to achieve their separation function; once surface defects are opened, the membrane's gas separation performance will drastically decrease or even be completely lost, causing serious economic losses. In contrast, the hydrophilic material plugging method proposed in this invention exhibits very slight swelling of the hydrophilic material under heavy hydrocarbon contamination conditions, completely preserving the membrane surface integrity and separation performance. The process is simple, uses inexpensive raw materials, and has high reliability, demonstrating significant technical advantages. Attached Figure Description

[0046] Figure 1 (a), (b), (c), and (d) are electron microscope images of uncoated polyvinyl alcohol films. Figure 1 (e), (f), (g), and (h) are electron microscope images of the polyvinyl alcohol film after coating. Among them, (a) and (e) have a scale of 100 μm, (b) and (f) have a scale of 10 μm, and (c), (d), (g), and (h) have a scale of 1 μm.

[0047] Figure 2a Photograph of a silicone rubber membrane soaked in petroleum ether for 20 minutes.

[0048] Figure 2b Photograph of a polyvinyl alcohol film soaked in petroleum ether for 30 days. Detailed Implementation

[0049] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0050] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0051] Example 1

[0052] Using a polyimide film as the base film, the base film is immersed in an ethanol solution containing 1 wt% ethanolamine for 10 minutes. The film is then placed in a forced-air oven and dried at 70 degrees Celsius for 2 hours. The dried modified film is then immersed in a 2% polyvinyl alcohol aqueous solution. A certain vacuum is applied inside the hollow tube of the film to drive the polyvinyl alcohol solution into the micro-defect pores on the film surface. The coating time is 15 minutes. The coated film is then placed in a forced-air oven and dried at 100 degrees Celsius for 2 hours to obtain a polyimide film with polyvinyl alcohol pore plugging.

[0053] Figure 1 (a), (b), (c), and (d) are electron microscope images of uncoated polyvinyl alcohol films. Figure 1 (e), (f), (g), and (h) are electron microscope images of the polyvinyl alcohol film after coating. Among them, (a) and (e) have a scale of 100 μm, (b) and (f) have a scale of 10 μm, and (c), (d), (g), and (h) have a scale of 1 μm.

[0054] As can be seen from the figure, the surface smoothness of the polyvinyl alcohol film after coating is significantly improved.

[0055] Comparative Test Example 1

[0056] The polyimide-based film was plugged with type 184 silicone rubber. The silicone rubber coating method was natural immersion. After the coating was heat-dried, a polyimide film with silicone rubber coating was obtained.

[0057] A comparative experiment was conducted on the solvent resistance of polyimide films with silicone rubber coating and polyimide films with polyvinyl alcohol coating.

[0058] External pressure swelling experiments were conducted on two types of membranes using petroleum ether. The specific experimental steps were as follows: the membrane was installed in the membrane module, both ends were sealed with epoxy, petroleum ether was injected into the outside of the membrane, and then the petroleum ether was pressurized. The results were observed to see if petroleum ether permeated through the inner pores of the hollow membrane.

[0059] The results showed that the polyimide membrane with silicone rubber coating was quickly swollen by petroleum ether because the silicone rubber blocking the pores was quickly dissolved, and defects appeared on the membrane surface. It was soon discovered that petroleum ether permeated through the hollow fiber membrane and flowed out from the inner tube.

[0060] A polyimide membrane with polyvinyl alcohol coating showed no petroleum ether permeation in its inner tube after a 48-hour continuous test at 2.0 MPa pressure. This indicates that the polyimide membrane with polyvinyl alcohol coating and pore plugging has excellent solvent resistance, i.e., resistance to heavy hydrocarbon contamination.

[0061] Comparative Test Example 2

[0062] The permeation separation performance of polyimide membranes with silicone rubber coating and polyimide membranes with polyvinyl alcohol were tested after contamination under extreme conditions.

[0063] A polyimide membrane with a silicone rubber coating was immersed in petroleum ether with a boiling point of 30.0–60.0℃ for 15 minutes, and then left to stand naturally in air for 30 minutes (25.0℃) to allow the free petroleum ether on the surface to evaporate before testing its gas permeation separation performance. The membrane contaminated with petroleum ether was then dried in a vacuum oven at 70.0℃ for 2 hours before its performance was tested again, yielding the permeation separation performance of the decontaminated membrane. Specific results are shown in Table 1.

[0064] Table 1. Changes in permeate separation performance of polyimide membranes after fouling (25.0℃, 0.50MPa)

[0065]

[0066] As shown in Table 1, the evolution of the gas permeation separation performance of the membranes reveals that after the polyimide membrane was contaminated with petroleum ether, the separation coefficient for CO2 / CH4 dropped drastically from the initial 35.5 to 16.3, nearly half of the initial value. After vacuum heating to remove the petroleum ether, the silicone rubber coating of the membrane was locally swollen and damaged by the petroleum ether, resulting in pore defects on the surface of the polyimide membrane. This led to faster gas permeation and a decrease in separation performance.

[0067] The polyimide film with silicone rubber coating described above and the polyimide film with polyvinyl alcohol obtained in Example 1 were impregnated in petroleum ether.

[0068] Figure 2a Photograph of a polyimide film with a silicone rubber coating soaked in petroleum ether for 20 minutes.

[0069] Figure 2b Photograph of a polyimide film containing polyvinyl alcohol after being immersed in petroleum ether for 30 days.

[0070] It can be seen that the polyimide membrane with silicone rubber coating swelled significantly within 20 minutes and peeled off from the petri dish. At the same time, it exhibited extremely poor mechanical properties, indicating that the polyimide membrane with silicone rubber coating could not withstand the swelling attack of petroleum ether. This is the reason why the polyimide membrane could not withstand heavy hydrocarbon pollution during the natural gas decarbonization process.

[0071] Polyimide films containing polyvinyl alcohol exhibited extremely good stability in petroleum ether. After 30 days of continuous immersion, the polyimide films containing polyvinyl alcohol did not show obvious swelling and could not be peeled off from the petri dish.

[0072] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A membrane, characterized in that, It consists of, in sequence, a base film, a hydrophilic layer, and a hydrophilic functional coating; The material of the hydrophilic layer is selected from at least one of ethanolamine, diethanolamine, diaminopropanol, diaminobenzoic acid, and 2,4-diaminobenzenesulfonic acid; The hydrophilic functional coating material is selected from at least one of cellulose acetate, polyethylene glycol, chitosan, polyvinyl alcohol, and polylactic acid.

2. The membrane according to claim 1, characterized in that, The base film is selected from at least one of polyimide film, cellulose acetate film, and polysulfone film.

3. A method for preparing the membrane according to claim 1 or 2, characterized in that, Includes the following steps: S1: Immerse the substrate in solvent I containing a hydrophilic modifier and dry it; S2: Immerse the membrane again in a solvent II solution containing hydrophilic functional material and crosslinking agent, and dry to obtain the membrane.

4. The preparation method according to claim 3, characterized in that, The hydrophilic modifier is selected from at least one of ethanolamine, diethanolamine, diaminopropanol, diaminobenzoic acid, and 2,4-diaminobenzenesulfonic acid. Solvent I is selected from at least one of methanol, ethanol, and propanol; In the solvent I solution containing the hydrophilic modifier, the concentration of the hydrophilic modifier is 0.01~10wt%.

5. The preparation method according to claim 3, characterized in that, The vacuum level in step S1 is 0~0.1MPa; The time for step S1 is 1~30 minutes; In step S1, the drying temperature is 50~120℃; In step S1, the drying time is 1~120 min.

6. The preparation method according to claim 3, characterized in that, The hydrophilic functional material is selected from at least one of cellulose acetate, polyethylene glycol, chitosan, polyvinyl alcohol, and polylactic acid; The crosslinking agent is selected from at least one of glutaraldehyde and boric acid; Solvent II is selected from at least one of acetone, water, and ethanol; In the solvent II solution containing the hydrophilic functional material and the crosslinking agent, the concentration of the hydrophilic functional material is 0.01~10wt%; In the solvent II solution containing the hydrophilic functional material and the crosslinking agent, the mass of the crosslinking agent is 5 to 25 wt% of the mass of the hydrophilic functional material.

7. The preparation method according to claim 3, characterized in that, The vacuum level in step S2 is 0~0.1MPa; The time for step S2 is 1~30 minutes; The temperature in step S2 is 0~100℃; In step S2, the drying temperature is 1~100℃; In step S2, the drying time is 1~120 min.

8. The preparation method according to claim 3, characterized in that, In steps S1 and S2, the product is cleaned before drying.

9. The preparation method according to claim 8, characterized in that, The cleaning solution used for cleaning is selected from at least one of water, ethanol, and methanol; The cleaning time is 1 to 30 minutes.

10. An application of the membrane according to any one of claims 1 or 2, characterized in that, Used for membrane decarbonization of natural gas.

Images