Preparation and Application of a Highly Stable Underwater Superoleophobic Silicon Carbide Ceramic Film

Abstract

This invention discloses the preparation and application of a highly stable underwater superoleophobic silicon carbide ceramic membrane. The preparation involves first placing an unoxidized silicon carbide ceramic membrane in a muffle furnace and sintering it at 500–900°C for 25–45 minutes to obtain a post-oxidized silicon carbide ceramic membrane. Then, the post-oxidized silicon carbide ceramic membrane is placed in a plasma cleaner and subjected to plasma modification at a vacuum of 300–800 mT for 5–120 minutes. Experiments have shown that the method of this invention produces a silicon carbide ceramic membrane that not only possesses excellent underwater superoleophobic properties but also retains these properties for a long time with high stability, showing great promise for application in oil-water separation. Furthermore, the preparation method of this invention is simple, uses mild conditions, is low-cost, and easily scalable.

Description

Technical Field

[0001] This invention relates to the preparation and application of a highly stable underwater superoleophobic silicon carbide ceramic membrane, belonging to the field of silicon carbide ceramic membrane technology. Background Technology

[0002] Oil-water mixtures or emulsions are widely present in industries such as petroleum, chemical, and machinery, posing significant hazards to the ecological environment and human health, and impacting production processes. While common physical and chemical methods for treating oil-water mixtures have some effect on oil-water separation, they each have their own limitations, such as requiring large amounts of energy, having low separation accuracy, complex processes, and expensive equipment.

[0003] Membrane separation technology refers to the selective separation of mixtures of molecules with different particle sizes at the molecular level when passing through a semi-permeable membrane. Compared with conventional separation methods, membrane separation technology can be carried out at room temperature, and the process involves no phase change, has high single-stage separation efficiency, and is flexible and simple. It is currently widely used in the field of oil-water separation.

[0004] Although membrane separation technology has been widely used in oil-water separation, membrane fouling during the filtration process remains a major technical challenge. In oil-water separation, problems such as filter cake clogging caused by oil droplets of different sizes and membrane pore blockage severely affect the separation performance of ceramic membranes. Regular physical cleaning of the filter membrane can eliminate reversible fouling and restore some of its separation performance. When the membrane flux drops to 40%–50% of the initial flux, chemical cleaning of the filter membrane is required to eliminate irreversible fouling; however, after repeated cleaning, the membrane structure will inevitably be damaged. Compared with polymer membrane materials, ceramic membranes have relatively high mechanical strength and can withstand higher backwashing pressures; in addition, ceramic membranes have excellent stability, can withstand acid and alkali corrosion, and can work in relatively harsh chemical environments. To fundamentally solve the membrane fouling problem, in addition to using ceramic membranes with longer service life to replace organic membranes, modifying the surface of ceramic membranes to improve their anti-fouling ability in oil-water separation has become a hot topic of research in recent years.

[0005] In addition to the common properties of inorganic membranes, silicon carbide ceramic membranes possess superior properties such as resistance to strong acids and alkalis, high thermal shock resistance, and a near-zero contact angle with water. Their significant advantage lies in their ability to operate stably for extended periods under harsh conditions during oil-water separation, making them internationally recognized as an ideal material for next-generation membrane product research. Currently, silicon carbide ceramic membrane modification mainly involves sol-gel methods, hydrothermal methods, atomic layer deposition (ALD), and chemical vapor deposition (CVD). However, these modification methods suffer from drawbacks such as reduced membrane pore size, complex modification processes, and limitations in large-scale applications. Patent CN201710507897.0 discloses an oil-water separation method based on a silicon carbide ceramic membrane. The method first ultrasonically disperses oily wastewater, then adjusts its pH to between 1 and 13. Next, it measures the zeta potential and particle size of the oily wastewater at different pH values ​​in one-unit increments. Based on the inflection point of the zeta potential change and the particle size, a silicon carbide ceramic membrane with an appropriate pore size is selected for filtration. Although this method pretreats the filtered oil-water emulsion, it does not modify the silicon carbide ceramic membrane and does not mention the relevant situation of subsequent membrane fouling issues. Patent CN202210025969.9 discloses a one-step method for controlling the pore structure and surface properties of silicon carbide films. This method first thoroughly mixes SiC powder with a sintering aid, and then synergistically controls the pore structure and surface wettability of the SiC film by controlling the molding pressure and sintering conditions. The prepared SiC film has an adjustable porosity of 13%–48%, an adjustable pore size of 0.17 μm–1 μm, and an adjustable dynamic water initial contact angle of 12.01°–66.8° and an adjustable underwater oil contact angle of 120.3°–155.1°. Although this method can improve the oil-repellent effect of silicon carbide ceramic films by introducing sintering aids to control the amount of SiO2 generated, it suffers from problems such as complex operation, the introduction of other impurities, and an insignificant underwater oil-repellent effect.

[0006] Plasma technology emerged in the 1960s and has since become ubiquitous in human life. Its application in materials science is particularly significant, using plasma to modify surfaces for higher performance—a crucial method in current materials research and development. Plasma modification offers several advantages: ① It's a dry process, meeting current energy-saving and environmental protection requirements; ② It has no special requirements for the materials being processed, making it universally applicable; ③ It only modifies the surface layer, without damaging the substrate. Therefore, plasma technology has a promising future compared to traditional modification techniques. However, current plasma modification techniques for inorganic materials suffer from a time-effect defect: the modified effect quickly disappears with prolonged exposure. Only by maintaining the material in an inert atmosphere or rapidly combining it with the substrate can optimal modification results be achieved. This defect hinders the application of plasma modification technology in the modification of silicon carbide ceramic films. Summary of the Invention

[0007] In view of the above-mentioned problems in the existing technology, the purpose of this invention is to provide a method for preparing and applying a highly stable underwater superoleophobic silicon carbide ceramic film.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a highly stable underwater superoleophobic silicon carbide ceramic film includes the following steps:

[0010] a) Place the unoxidized silicon carbide ceramic film into a muffle furnace and oxidize and sinter it at 500-900℃ for 25-45 minutes to obtain a post-oxidized silicon carbide ceramic film.

[0011] b) The post-oxidized silicon carbide ceramic membrane is placed in a plasma cleaner and subjected to plasma modification for 5 to 120 minutes at a vacuum of 300 to 800 mT (i.e., millitor, 1 mT = 0.001 T (torr)) to obtain the high-stability underwater superoleophobic silicon carbide ceramic membrane.

[0012] In one embodiment, step a) involves first heating the temperature to 500–900°C at a heating rate of 5–10°C / min, and then performing oxidation sintering at 500–900°C for 25–45 minutes.

[0013] In a preferred embodiment, step a) involves first heating the temperature to 900°C at a rate of 10°C / min, and then performing oxidation sintering at 900°C for 25–45 minutes.

[0014] In one embodiment, in step b), the power of the plasma cleaner is 15-20W, preferably 18W.

[0015] In one implementation scheme, step b) involves plasma modification for 5 to 120 minutes at a vacuum of 700 to 750 mT.

[0016] In one implementation scheme, in step b), the plasma modification time is 60–120 minutes.

[0017] Application of the highly stable underwater superoleophobic silicon carbide ceramic membrane prepared by the above preparation method in oil-water separation.

[0018] One implementation scheme involves oil-water separation under pressures ranging from -0.01 MPa to -0.03 MPa.

[0019] Compared with the prior art, the present invention has the following significant advantages:

[0020] This invention involves first subjecting the silicon carbide ceramic membrane to oxidation sintering treatment, followed by plasma modification. This process not only endows the prepared silicon carbide ceramic membrane with excellent underwater superoleophobic properties, but also ensures that these properties are retained for a long time and exhibit high stability, making it a promising candidate for oil-water separation. Furthermore, the preparation method of this invention is simple, mild, and inexpensive, making it easy to scale up. Attached Figure Description

[0021] Figure 1 This is a SEM image of the post-oxidized silicon carbide ceramic film prepared in Example 1 of the present invention;

[0022] Figure 2 This is a SEM image of the highly stable underwater superoleophobic silicon carbide ceramic membrane prepared in Example 1 of the present invention;

[0023] Figure 3 The elemental distribution diagram is shown for the post-oxidized silicon carbide ceramic film prepared in Example 1 of the present invention.

[0024] Figure 4 The elemental distribution diagram is shown for the highly stable underwater superoleophobic silicon carbide ceramic film prepared in Example 1 of the present invention.

[0025] Figure 5 This is a diagram showing the underwater oil contact angle of the silicon carbide ceramic membrane in Embodiment 2 of the present invention.

[0026] Figure 6 The figures show the turbidity of the oil-water mixture before filtration and the filtrate after filtration with the silicon carbide ceramic membrane in an application example of the present invention. In the figures: (a) is the oil-water mixture before filtration, (b) is the filtrate after filtration with the silicon carbide ceramic membrane after oxidation, and (c) is the filtrate after filtration with the high-stability underwater superoleophobic silicon carbide ceramic membrane.

[0027] Figure 7 The graph shows the oil-water separation flux changes of the post-oxidized silicon carbide ceramic membrane and the high-stability underwater superoleophobic silicon carbide ceramic membrane after 540 minutes of operation in the application examples of the present invention.

[0028] Figure 8 The graph shows the changes in oil-water separation flux of the post-oxidized silicon carbide ceramic membrane and the highly stable underwater superoleophobic silicon carbide ceramic membrane after 3 months in the application examples of this invention. Detailed Implementation

[0029] The technical solution of the present invention will be further described in detail and completely below with reference to the embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer.

[0030] Example 1

[0031] a) Place the unoxidized silicon carbide ceramic film into a muffle furnace, heat it to 900°C at a heating rate of 10°C / min, and oxidize and sinter it at 900°C for 30 minutes to obtain the post-oxidized silicon carbide ceramic film.

[0032] b) The post-oxidized silicon carbide ceramic membrane was placed in a plasma cleaner (power 18W) and subjected to plasma modification for 60 minutes under a vacuum of 725±25mT to obtain a highly stable underwater superoleophobic silicon carbide ceramic membrane.

[0033] Figure 1 This is a SEM image of the post-oxidized silicon carbide ceramic film prepared in this embodiment; Figure 2 This is a SEM image of the highly stable underwater superoleophobic silicon carbide ceramic film prepared in this embodiment; (The image is from...) Figure 1 and Figure 2 It can be seen that the silicon carbide ceramic film modified by plasma did not undergo significant changes compared with the post-oxidized silicon carbide ceramic film (before plasma modification), and the pore structure of the film remained intact.

[0034] Figure 3 The elemental distribution diagram of the post-oxidized silicon carbide ceramic film prepared in this embodiment is shown. Figure 4 This is an elemental distribution diagram of the highly stable underwater superoleophobic silicon carbide ceramic film prepared in this embodiment; (The remaining text appears to be a fragment and requires further context for accurate translation.) Figure 3 and Figure 4 It is evident that, compared to the post-oxidized silicon carbide ceramic membrane (before plasma modification), the silicon carbide ceramic membrane modified by plasma has an increased oxygen content on its surface. This increased oxygen content is beneficial for improving the membrane's hydrophilicity, increasing the underwater oil contact angle, and enhancing its antifouling performance.

[0035] Example 2

[0036] The difference between this embodiment and Embodiment 1 is that the oxidation sintering temperatures are 500℃ and 900℃ respectively; the plasma modification time is 30 minutes; and all other contents are the same as described in Embodiment 1.

[0037] The hydrophilicity in air and oleophobicity underwater of the prepared post-oxidized silicon carbide ceramic film (before plasma modification) and the plasma-modified silicon carbide ceramic film were tested in terms of immersion time and underwater oil contact angle. Shorter immersion time indicates better hydrophilicity, while a larger underwater oil contact angle indicates better underwater oleophobicity, better oil retention, and better antifouling performance. An underwater oil contact angle exceeding 150° is considered superoleophobic underwater. Detailed test results are shown in Tables 1 and 2. Figure 5 As shown.

[0038] Table 1 Immersion time of silicon carbide ceramic membrane

[0039] Unmodified Newly modified 3 days later 7 days later Unoxidized silicon carbide ceramic film ＞10s 4.22±0.08s ＞10s ＞10s 500℃ silicon carbide ceramic film ＞10s 3.33±0.60s 8.80±2.47s ＞10s 900℃ silicon carbide ceramic membrane 2.50±0.42s 0.73±0.19s 1.33±0.14s 1.33±0.14s

[0040] In the table: Unoxidized silicon carbide ceramic film refers to the initial silicon carbide ceramic film that has not undergone oxidation sintering treatment; 500℃ silicon carbide ceramic film refers to silicon carbide ceramic film obtained by oxidation sintering treatment at 500℃; 900℃ silicon carbide ceramic film refers to silicon carbide ceramic film obtained by oxidation sintering treatment at 900℃; Unmodified refers to silicon carbide ceramic film that has not undergone plasma modification; Newly modified refers to silicon carbide ceramic film that has just undergone plasma modification; After 3 days refers to silicon carbide ceramic film that has undergone plasma modification and been placed for 3 days; After 7 days refers to silicon carbide ceramic film that has undergone plasma modification and been placed for 7 days.

[0041] As shown in Table 1, plasma modification effectively reduced the immersion time and improved the hydrophilicity of unoxidized silicon carbide ceramic films, silicon carbide ceramic films at 500℃, and silicon carbide ceramic films at 900℃, indicating that plasma modification can effectively improve the hydrophilicity of silicon carbide ceramic films. However, the hydrophilicity of unoxidized silicon carbide ceramic films decreased sharply after 3 days of plasma modification, and the hydrophilicity of 500℃ silicon carbide ceramic films also deteriorated over time, with a significant decrease after 7 days. The hydrophilicity of 900℃ silicon carbide ceramic films could be maintained for more than 7 days after plasma modification, indicating that the oxidation sintering treatment before plasma modification had a significant impact on extending the retention time of hydrophilicity, and the oxidation sintering temperature also had a certain influence on the retention time of hydrophilicity, with the best effect achieved by oxidation sintering treatment at 900℃.

[0042] Table 2 Underwater oil contact angle of silicon carbide ceramic membrane

[0043] Unmodified Newly modified 3 days later 7 days later Unoxidized silicon carbide ceramic film 96.410° 158.390° 142.694° 115.396° 500℃ silicon carbide ceramic film 152.238° 160.097° 147.167° 148.972° 900℃ silicon carbide ceramic membrane 160.238° 168.605° 168.521° 168.959°

[0044] Figure 5 This is a diagram showing the underwater oil contact angle of the silicon carbide ceramic membrane in this embodiment.

[0045] From Table 2 and Figure 5 It is evident that plasma modification effectively improved the underwater oil contact angle and underwater oleophobicity of unoxidized silicon carbide ceramic films, silicon carbide ceramic films at 500℃, and silicon carbide ceramic films at 900℃, indicating that plasma modification can effectively improve the underwater oleophobicity of silicon carbide ceramic films. However, the underwater oleophobicity of unoxidized silicon carbide ceramic films could not be maintained after plasma modification, and the underwater oil contact angle decreased over time. The underwater oil contact angle of the 500℃ silicon carbide ceramic film also gradually decreased over time after plasma modification, while the underwater oil contact angle of the 900℃ silicon carbide ceramic film could be maintained for more than 7 days after plasma modification. This indicates that the oxidation sintering treatment before plasma modification had a significant impact on extending the retention time of underwater oleophobicity, and the oxidation sintering temperature also had a certain influence on the retention time of underwater oleophobicity, with the best effect achieved by oxidation sintering treatment at 900℃.

[0046] Example 3

[0047] The difference between this embodiment and Embodiment 1 is that the plasma modification time is 5 minutes, 15 minutes, 30 minutes, 60 minutes, and 120 minutes, respectively; all other contents are the same as described in Embodiment 1.

[0048] The underwater oil contact angle of the plasma-modified silicon carbide ceramic film was tested using a contact angle meter, and the test results are shown in Table 3.

[0049] Table 3. Underwater oil contact angle of silicon carbide ceramic films with different plasma modification times.

[0050] Plasma modification time 5 minutes 15 minutes 30 minutes 60 minutes 120 minutes underwater oil contact angle 141.057° 152.238° 168.605° 172.197° 171.650°

[0051] As shown in Table 3, plasma modification time has a certain impact on the underwater oleophobicity of silicon carbide ceramic films, with the best modification effect observed under the condition of 60 minutes.

[0052] Example 4

[0053] The difference between this embodiment and Embodiment 1 is that the vacuum degree during plasma modification is 325±25mT, 525±25mT, and 725±25mT, respectively; all other contents are the same as described in Embodiment 1.

[0054] The underwater oil contact angle of the plasma-modified silicon carbide ceramic membrane was tested using a contact angle meter, and the test results are shown in Table 4.

[0055] Table 4. Underwater oil contact angle of silicon carbide ceramic films with different plasma-modified vacuum levels.

[0056] Plasma-modified vacuum 325±25mT 525±25mT 725±25mT underwater oil contact angle 150.600° 154.789° 172.197°

[0057] As shown in Table 4, the vacuum degree of plasma modification has a certain influence on the underwater oleophobicity of silicon carbide ceramic films, with the best modification effect under the condition of 725±25mT.

[0058] Application example: Application of silicon carbide ceramic membranes in oil-water separation

[0059] 1. The post-oxidized silicon carbide ceramic membrane prepared in Example 1 (before plasma modification) and the plasma-modified silicon carbide ceramic membrane (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane) were used for oil-water separation under pressures of -0.01 MPa, -0.02 MPa, and -0.03 MPa, respectively, for a running time of 120 minutes. The turbidity of the oil-water mixture before filtration was 1029 NTU. The turbidity of the oil-water mixture and the filtrate after filtration through the silicon carbide ceramic membrane were tested, and the test results are shown in Table 5. Figure 6 As shown.

[0060] Table 5. Turbidity of filtrate after filtration through silicon carbide ceramic membranes under different pressures.

[0061] Operating pressure Unmodified Plasma modification -0.01MPa 54.126NTU 2.840NTU -0.02MPa 177.000NTU 4.910NTU -0.03MPa 178.206NTU 6.460NTU

[0062] In the table: Unmodified refers to the post-oxidized silicon carbide ceramic film before plasma modification, and plasma modified refers to the silicon carbide ceramic film after plasma modification (i.e., high-stability underwater superoleophobic silicon carbide ceramic film).

[0063] Figure 6 The figures show the turbidity of the oil-water mixture before filtration and the filtrate after filtration through the silicon carbide ceramic membrane. In the figures, (a) is the oil-water mixture before filtration, (b) is the filtrate after filtration through the post-oxidized silicon carbide ceramic membrane, and (c) is the filtrate after filtration through the plasma-modified silicon carbide ceramic membrane (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane).

[0064] Combined with Table 5 and Figure 6 It is evident that the plasma-modified silicon carbide ceramic membrane has a significant oil removal effect on oil-water mixtures. The filtrate filtered through this membrane is very clear, indicating that plasma modification can effectively improve the oil removal effect of the silicon carbide ceramic membrane.

[0065] 2. The post-oxidized silicon carbide ceramic membrane prepared in Example 1 and the plasma-modified silicon carbide ceramic membrane (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane) were run for 540 minutes respectively, and the changes in oil-water separation flux were compared. The results are shown in […]. Figure 7 As shown.

[0066] Figure 7 This is a graph showing the oil-water separation flux changes of the silicon carbide ceramic membrane before plasma modification and the silicon carbide ceramic membrane after plasma modification (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane) after 540 minutes of operation. Figure 7 It is evident that, in terms of long-term flux retention, the plasma-modified silicon carbide ceramic membrane is significantly superior to the unmodified post-oxidized silicon carbide ceramic membrane. This indicates that the plasma-modified silicon carbide ceramic membrane not only has a significantly better degreasing effect than the post-oxidized silicon carbide ceramic membrane, but also exhibits higher stability.

[0067] 3. The antifouling effects of the post-oxidized silicon carbide ceramic membrane prepared in Example 1 and the plasma-modified silicon carbide ceramic membrane (i.e., high-stability underwater superoleophobic silicon carbide ceramic membrane) before and after oil-water separation were evaluated. The results are shown in Table 6.

[0068] Table 6. Underwater oil contact angles before and after filtration

[0069] silicon carbide ceramic membrane Before filtration After filtration After cleaning Unmodified 160.238° 126.406° 133.206° Plasma modification 172.197° 150.430° 156.727°

[0070] In the table: Unmodified refers to the post-oxidized silicon carbide ceramic film before plasma modification, and plasma modified refers to the silicon carbide ceramic film after plasma modification (i.e., high-stability underwater superoleophobic silicon carbide ceramic film).

[0071] As shown in Table 6, the underwater oil contact angle of the post-oxidized silicon carbide ceramic membrane was 160.238° before filtration, 126.406° after filtration, and 133.206° after cleaning. The underwater oil contact angle after cleaning was less than 150°, indicating that the post-oxidized silicon carbide ceramic membrane no longer possesses underwater superoleophobicity. On the other hand, the underwater oil contact angle of the plasma-modified silicon carbide ceramic membrane was 172.197° before filtration, 150.430° after filtration, and 156.727° after cleaning. The underwater oil contact angle after cleaning was still more than 150°, indicating that the plasma-modified silicon carbide ceramic membrane still possesses underwater superoleophobicity after cleaning. This suggests that the plasma-modified silicon carbide ceramic membrane maintains better underwater oleophobicity and has better antifouling performance.

[0072] 4. The separation effects of the post-oxidized silicon carbide ceramic membrane prepared in Example 1 and the plasma-modified silicon carbide ceramic membrane (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane) were compared after 3 months (with intermittent operation for 60 hours within 3 months). The results are shown in Table 7 and... Figure 8 As shown.

[0073] Table 7 Turbidity before and after filtration

[0074] initial turbidity Unmodified Plasma modification 1029NTU 45.570NTU 5.068NTU

[0075] In the table: Unmodified refers to the post-oxidized silicon carbide ceramic film before plasma modification, and plasma modified refers to the silicon carbide ceramic film after plasma modification (i.e., high-stability underwater superoleophobic silicon carbide ceramic film).

[0076] Figure 8 The graph shows the changes in oil-water separation flux of post-oxidized silicon carbide ceramic membrane and high-stability underwater superoleophobic silicon carbide ceramic membrane after 3 months.

[0077] Combined with Table 7 and Figure 8 It can be seen that the plasma-modified silicon carbide ceramic membrane (i.e., the high-stability underwater superoleophobic silicon carbide ceramic membrane) still has a high flux after 3 months, and can significantly reduce the turbidity of the filtered filtrate. This indicates that the plasma-modified silicon carbide ceramic membrane has a significant oil removal effect and high stability.

[0078] In summary, this invention, by first subjecting the silicon carbide ceramic membrane to oxidation sintering and then plasma modification, not only enables the prepared silicon carbide ceramic membrane to possess excellent underwater superoleophobic properties, but also ensures that these properties are retained for a long time and exhibit high stability, thus showing great promise for application in the field of oil-water separation. Furthermore, the preparation method of this invention is simple, the conditions are mild, the raw materials are readily available, the cost is low, and it is easy to scale up.

[0079] Finally, it should be noted that the above are only some preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a highly stable underwater superoleophobic silicon carbide ceramic film, characterized in that, Includes the following steps: a) Place the unoxidized silicon carbide ceramic film into a muffle furnace, first heat it to 900°C at a heating rate of 10°C / min, and then oxidize and sinter it at 900°C for 25 to 45 minutes to obtain the post-oxidized silicon carbide ceramic film. b) The post-oxidized silicon carbide ceramic membrane is placed in a plasma cleaner and subjected to plasma modification for 60-120 minutes under a vacuum of 700-750 mT to obtain the high-stability underwater superoleophobic silicon carbide ceramic membrane.

2. The preparation method according to claim 1, characterized in that: In step b), the power of the plasma cleaner is 15-20 W.

3. The application of the highly stable underwater superoleophobic silicon carbide ceramic membrane prepared by the preparation method according to any one of claims 1 to 2 in oil-water separation.

4. The application according to claim 3, characterized in that: Oil-water separation is performed under pressures ranging from -0.01 MPa to -0.03 MPa.

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