Preparation of Janus-based solid-liquid two-phase membranes for methane solubility and high recovery

Abstract

The application discloses a Janus-based solid-liquid two-phase membrane for efficiently recovering dissolved methane from anaerobic effluent and a preparation method thereof, and belongs to the technical field of wastewater treatment, methane recovery and environmental protection. The method first carries out oleophobic modification on a commercial polyvinylidene fluoride porous membrane (PVDF), and then prepares an oil-wet polyvinylidene fluoride membrane on the surface of the modified oleophobic membrane by an electrostatic spraying method to obtain a Janus membrane with one side being oil-wet and the other side being oleophobic. Further, silicon oil is injected into the oil-wet side of the Janus membrane to obtain a Janus-based solid-liquid two-phase membrane. Since the silicon oil has the characteristics of low surface energy, low volatility and high methane solubility, the prepared Janus-based solid-liquid two-phase membrane has excellent anti-wetting, anti-pollution and high methane recovery flux. In addition, the membrane can be stably operated in a dissolved methane recovery system through critical pressure test and water flow shear force test. Finally, the membrane has a very high methane recovery flux through a dissolved methane recovery experiment.

Description

Technical Field

[0001] This invention belongs to the field of dissolved methane recovery and membrane technology, specifically relating to the preparation of a Janus-based solid-liquid two-phase membrane, and the acquisition of a Janus-based solid-liquid two-phase membrane with high methane flux by controlling the structure and physicochemical properties of the Janus-based solid-liquid two-phase membrane. Background Technology

[0002] Anaerobic technology boasts advantages such as low energy consumption, small footprint, and low sludge production, while also generating methane-rich biogas, making it an optimal choice for sustainable wastewater treatment. However, anaerobic wastewater treatment technology faces a significant challenge: the methane produced during anaerobic treatment dissolves in the anaerobic effluent and is released into the environment. Reports indicate that in 2019, my country's wastewater treatment generated 254,000 to 2,313,000 tons of dissolved methane, the majority of which originated from anaerobic wastewater treatment. Direct discharge of dissolved methane with anaerobic effluent leads to increased greenhouse gas emissions and energy waste. Therefore, actively recovering dissolved methane from anaerobic effluent is crucial.

[0003] Membrane contactor technology enables efficient recovery of dissolved methane. Compared to traditional aeration and stripping technologies, it offers advantages such as a stable gas-liquid interface, large mass transfer area, independent operation of both phases, low energy consumption, small footprint, modularity, and ease of scale-up, leading to its widespread attention in recent years. However, membrane contactors often face many complex problems in actual operation, such as membrane wetting and membrane fouling, which ultimately result in reduced methane flux.

[0004] To address the challenges of membrane wetting, fouling, and low methane content in the recovered gas encountered by traditional membrane technologies, researchers have proposed using solid-liquid two-phase membranes—specifically, porous membranes infused with silicone oil—to recover dissolved methane. The innovation of this strategy lies in the fact that the low surface energy of silicone oil effectively avoids membrane wetting and fouling phenomena common in traditional membrane technologies. Furthermore, methane has a higher solubility in silicone oil than in water, thus promoting its transport from the feed phase to the extract phase under the influence of chemical potential. However, research has revealed that high methane mass transfer resistance and silicone oil leakage are the main factors limiting the further application of solid-liquid two-phase membranes.

[0005] To address the above problems, this invention discloses a solid-liquid two-phase membrane with a Janus structure. Silicone oil is infused into the oleophilic side of the Janus-based membrane, while the oleophobic properties of the oleophobic side prevent the silicone oil from seeping out, acting as a support and barrier, thereby enhancing the stability of the solid-liquid two-phase membrane. Simultaneously, methane mass transfer analysis revealed that the thickness of the oleophilic membrane is the main factor affecting methane mass transfer. Therefore, this invention optimizes the oleophilic membrane thickness by controlling the operating parameters during the preparation of the Janus-based solid-liquid two-phase membrane, ultimately achieving a high methane recovery flux. Summary of the Invention

[0006] The purpose of this invention is to address the problems of silicone oil leakage and high mass transfer resistance in existing solid-liquid two-phase membrane recovery of dissolved methane. By preparing a solid-liquid two-phase membrane with a Janus structure, silicone oil leakage is preferentially avoided, while high-throughput recovery of dissolved methane is achieved.

[0007] The present invention achieves its objective through the following technical solutions:

[0008] (1) Janus-based solid-liquid two-phase membranes were prepared sequentially by surface modification and electrostatic spraying.

[0009] The first step involves surface modification of commercial polyvinylidene fluoride (PVDF) porous membranes to obtain an oleophobic film. First, the PVDF membrane is rinsed with deionized water for 1 minute to remove surface contaminants. After rinsing, the PVDF membrane is immersed in 99.5% ethanol for 5 minutes to completely wet the pores. Next, the wetted PVDF membrane is placed in a sodium hydroxide solution of a certain concentration (5wt%-30wt%) and heated in a 60°C water bath for a certain time (0.5h-2h). After heating, the PVDF membrane is rinsed sequentially with 99.5% ethanol and then deionized water until all sodium hydroxide is removed. Specifically, the rinsed membrane is placed in a small beaker containing deionized water and sonicated for 1 minute. The pH of the deionized water after sonication is tested with pH paper; no color change on the pH paper indicates that the membrane is clean. Finally, the rinsed membrane is placed in a vacuum drying oven and dried at 80°C for 1 hour.

[0010] Using a pipette, 30 ml of n-hexane and 0.3 ml–2.1 ml of 1H,1H,2H,2H-perfluorodecyltrichlorosilane were sequentially transferred to disposable culture dishes. The dried membrane was then immersed in the culture dishes and refrigerated for 24 hours. After refrigeration, the membrane was rinsed sequentially with 99.5% ethanol and deionized water for 2 minutes each. The rinsed membrane was then heated in a 120°C oven for 1 hour.

[0011] Preferably, in the above steps, the sodium hydroxide concentration is 12 wt%, the sodium hydroxide activation time is 1 h, and the concentration of 1H,1H,2H,2H-perfluorodecyltrichlorosilane is 4 v / v.

[0012] The second step involves using an oleophobic modified PVDF membrane as a substrate and electrospraying an oleophilic PVDF membrane onto the membrane surface using an electrostatic spraying method, thereby obtaining a Janus-based solid-liquid two-phase membrane.

[0013] PVDF powder was added to a 100ml triangular-grouted conical flask containing N,N-dimethylamide (DMF). The solution was stirred continuously in a 50℃ water bath for 4-6 hours until the PVDF powder was completely dissolved. The solution was then placed in a vacuum drying oven and degassed for 1 hour at room temperature and -3 MPa. The degassed spinning solution was injected into a 10ml syringe and fixed to a syringe pump. Simultaneously, the oleophobic modified PVDF film from the previous step was smoothly adhered to the receiver. After setting the electrospinning parameters, the electrospinning machine was started. After electrospinning, the resulting Janus film was placed in a vacuum drying oven and dried for 24 hours until the solvent completely evaporated.

[0014] Preferably, the conditions for electro-injection are as follows: PVDF concentration of 5wt%, positive pressure of 8-10kV, negative pressure of -2kV, propulsion speed of 0.15mm / min, distance between needle and receiver of 10-15cm, translation distance of 80mm, receiver rotation speed of 400rpm, temperature of 30-35℃, and humidity of 35%-50%.

[0015] (2) Janus-based solid-liquid two-phase membranes were obtained by injecting silicone oil into the oleophilic side of the Janus membrane and their critical pressure was tested.

[0016] First, the Janus-based solid-liquid two-phase membrane was fixed on a disposable culture dish with the oleophilic layer facing upwards. A small amount of silicone oil was applied evenly to the surface of the oleophilic layer using a 1ml syringe. Under capillary action, the silicone oil gradually wetted the oleophilic layer and was firmly bound within the membrane pores. The culture dish was then vertically fixed and allowed to stand for 2-3 days. Excess silicone oil was removed from the membrane surface under gravity, successfully preparing the Janus-based solid-liquid two-phase membrane.

[0017] As attached Figure 2 and Figure 3 As shown, critical pressure and water flow shear force tests were conducted on Janus-based solid-liquid two-phase membranes using a vacuum pump and a peristaltic pump. The vacuum pump was started, and the pressure was gradually increased until water in the syringe penetrated the membrane and entered the glass container; the pressure reading at this point is the membrane's critical pressure. The water flow shear force was measured by calculating the loss rate of silicone oil on the membrane surface.

[0018] (3) Testing the methane recovery flux of Janus-based solid-liquid two-phase membrane

[0019] The first step is to prepare a methane solution of a specific concentration. Open the water inlet valve and introduce 20L of distilled water into the storage tank, then close the water inlet valve. Open the air inlet valve and introduce air at a flow rate of 20cm. 3 A certain amount of methane gas is introduced at a rate of / min, and the inlet valve is closed. An external insulation layer is added to control the temperature of the water storage tank at 25℃, and a magnetic stirrer is started to stir until the methane concentration in the water storage tank is uniform.

[0020] The second step is to start the dissolved methane recovery system. The Janus-based solid-liquid two-phase membrane is fixed in the membrane contactor. After completing the system's airtightness check, the outlet and inlet valves of the water storage tank are opened sequentially, and the peristaltic pump is started. Once the system is running stably, a 10ml water sample is taken from the sampling port into a 20ml sealed bottle using a syringe, and this moment is recorded as time 0. The water sample is placed in a constant-temperature shaking incubator and shaken for 1 hour. Then, the methane concentration in the headspace vial is measured using a gas chromatograph to obtain the methane concentration in the system at time 0. Samples are taken and tested every 2 hours until the methane concentration in the system no longer changes. The average methane recovery flux is calculated based on the methane concentration in the system at this moment. The above steps are repeated to investigate the dissolved methane recovery of Janus-based solid-liquid two-phase membranes with different thicknesses.

[0021] Preferably, the methane recovery flux is highest when the thickness of the oleophilic membrane in the Janus-based solid-liquid two-phase membrane is 20 micrometers, reaching 7.1 mol / (m³). 2 ·h).

[0022] Compared with existing methane recovery technologies, the Janus-based solid-liquid two-phase membrane proposed in this invention has the following advantages:

[0023] (1) In view of the membrane wetting and membrane fouling problems existing in the methane recovery process of traditional degassing membranes, the present invention effectively avoids the problems that limit the application of membrane technology, such as membrane wetting and membrane fouling, by fully hydrophobic modification and silicone oil injection.

[0024] (2) To address the problems of silicone oil leakage and low methane flux in existing solid-liquid two-phase membranes during methane recovery, this invention applies the Janus structure to the solid-liquid two-phase membrane, then injects silicone oil into the oleophilic layer, while the oleophobic layer acts as a support and barrier, effectively solving the silicone oil leakage problem. Furthermore, by reducing the thickness of the oleophilic layer, the methane mass transfer resistance is reduced, thereby solving the problem of low methane flux. Attached Figure Description

[0025] Figure 1 Critical pressure testing for Janus-based solid-liquid two-phase membranes

[0026] Figure 2 Water flow shear force test for Janus-based solid-liquid two-phase membranes

[0027] Figure 3 Methane recovery flux of Janus-based solid-liquid two-phase membranes at different thicknesses Detailed Implementation

[0028] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited thereto.

[0029] Example 1

[0030] Critical pressure tests were conducted on a conventional solid-liquid two-phase membrane and a Janus-based solid-liquid two-phase membrane with the same PVDF substrate and oleophilic layer thickness using a vacuum pressure testing device. Figure 1 As shown, the critical pressure of ordinary solid-liquid two-phase membranes is 0.048 MPa, meaning that silicone oil leakage will occur when the vacuum pressure exceeds 0.5 MPa. However, the Janus-based solid-liquid two-phase membrane did not leak even when the vacuum pressure exceeded 0.6 MPa, proving that the oleophobic layer of the Janus-based solid-liquid two-phase membrane plays a good role in supporting the membrane and blocking silicone oil.

[0031] Example 2

[0032] First, the mass of silicone oil injected was obtained by weighing the Janus membrane before and after injection. Then, the silicone oil-injected Janus-based solid-liquid two-phase membrane was continuously run in ultrapure water for 24 hours at a certain water flow rate. Finally, the TOC in the water was measured to deduce the amount of silicone oil lost. The silicone oil loss rate was then obtained by dividing the silicone oil loss by the amount of silicone oil injected onto the membrane. Experiments showed that the silicone oil loss rate was negligible. Figure 2 (The red part at the bottom of the bar chart) thus proves that Janus-based solid-liquid two-phase membranes have good resistance to water flow shear.

[0033] In summary, Janus-based solid-liquid two-phase membranes possess high critical pressure and resistance to water flow shear, effectively preventing silicone oil leakage and enabling long-term, stable operation.

[0034] Example 3

[0035] An oleophobic modified PVDF film was used as a substrate and fixed onto a receiver. By controlling the electrospraying time, oleophilic films of different thicknesses were prepared on the surface of the oleophobic film. Specifically, PVDF powder was added to a 100ml triangular ground glass conical flask containing N,N-dimethylamide (DMF) and stirred continuously in a 50℃ water bath for 4-6 hours until the PVDF powder was completely dissolved. The solution was then placed in a vacuum drying oven and degassed for 1 hour at room temperature and -3MPa. The degassed casting solution was injected into a 10ml syringe and fixed to a syringe pump. The electrospraying conditions were set as follows: PVDF concentration 5wt%, positive pressure 8-10kV, negative pressure -2kV, propulsion speed 0.15mm / min, needle-receiver distance 10-15cm, translational distance 80mm, receiver rotation speed 400rpm, temperature 30-35℃, and humidity 35%-50%. Janus membranes with oleophilic film thicknesses of 10 μm, 20 μm, 40 μm, 60 μm, and 80 μm were prepared by controlling the electro-spraying time. Finally, silicone oil was injected to obtain Janus-based solid-liquid two-phase membranes with different liquid film thicknesses. Further, a methane recovery experiment was conducted to test the average methane flux of Janus-based solid-liquid two-phase membranes of different thicknesses after 4 hours of operation. Figure 3 As shown, within the liquid film thickness range of 20-80 micrometers, the methane flux gradually increases as the liquid film thickness decreases, reaching its maximum at 20 micrometers, which is consistent with our theoretical analysis. This is because the resistance in the methane mass transfer process mainly comes from the feed liquid phase and the membrane phase. The resistance of the membrane phase is primarily affected by the membrane thickness; the smaller the thickness, the lower the methane mass transfer resistance, the larger the mass transfer coefficient, and the higher the methane flux.

[0036] In summary, this invention prepared a solid-liquid two-phase membrane with a Janus structure through chemical modification and electrostatic spraying technology, and proved that the membrane can ensure a high methane recovery flux without silicone oil leakage.

Claims

1. A method for preparing a Janus-based solid-liquid two-phase membrane for efficient recovery of dissolved methane, comprising combining oleophilic and oleophobic properties onto a single membrane through chemical modification and electrostatic spraying technology. The oleophilic layer, by injecting silicone oil with high methane solubility, enables the membrane to achieve high methane recovery and excellent anti-wetting properties, while the oleophobic layer acts as a support and barrier, allowing the Janus-based solid-liquid two-phase membrane to operate stably under certain pressure and hydraulic conditions. The method for preparing this Janus-based solid-liquid two-phase membrane for efficient recovery of dissolved methane is characterized by... This includes the following three aspects: (1) By using chemical modification technology, 1H,1H,2H,2H-perfluorodecyltrichlorosilane with low surface energy was grafted onto the surface of a commercial polyvinylidene fluoride porous membrane (PVDF) to obtain an oleophobic membrane. Then, 5wt% PVDF solution was electrosprayed onto the surface of the oleophobic modified membrane using electrostatic spraying technology to obtain an oleophilic-oleophobic Janus membrane. (2) A Janus-based solid-liquid two-phase membrane was obtained by injecting silicone oil into the oleophilic side of the Janus membrane. The stability of the Janus-based solid-liquid two-phase membrane was then tested using a vacuum pump and a peristaltic pump, including critical pressure and water flow shear force tests. (3) In the construction of the dissolved methane recovery system, the methane recovery of Janus-based solid-liquid two-phase membranes with different thicknesses was tested.

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