A system and method for treating impurities in a fischer-tropsch tail wax

Abstract

This invention discloses a system and method for treating impurities in Fischer-Tropsch tail wax, comprising the following steps: heating the Fischer-Tropsch tail wax in a storage tank to a completely melted state; conveying the liquid Fischer-Tropsch tail wax in the storage tank to an inorganic membrane filtration unit via a feed pump, filtering and separating impurities in the liquid Fischer-Tropsch tail wax to obtain Fischer-Tropsch tail wax permeate; passing the flushing liquid from the backwashing system into the inorganic membrane filtration unit to clean the inorganic membrane; and discharging the filtered impurities and flushing liquid through a settling tank; wherein the pipelines used for conveying Fischer-Tropsch tail wax between the storage tank, feed pump, and inorganic membrane filtration unit are all equipped with heat tracing. This invention can effectively remove trace impurities from solid or semi-solid Fischer-Tropsch tail wax, resulting in a total metal content of less than 5 PPM in the treated tail wax.

Description

Technical Field

[0001] This invention relates to the field of deep processing technology of Fischer-Tropsch tail wax, specifically to a system and method for treating impurities in Fischer-Tropsch tail wax. Background Technology

[0002] Fischer-Tropsch synthesis is a process that uses syngas obtained from coal-based or natural gas-based sources as raw material to synthesize liquid fuels and other chemical products, primarily paraffinic hydrocarbons, in a catalytic system. Fischer-Tropsch wax is one of the main products of the Fischer-Tropsch synthesis reaction. In the Fischer-Tropsch synthesis process, iron-based catalysts are the most industrially valuable due to their low cost, high activity, and good selectivity. However, the particle size of the iron-based catalysts used ranges from 1 to 200 μm. Furthermore, catalyst breakage due to abrasion during the reaction results in ultrafine catalyst particles mixed in with the final wax product, reaching submicron levels.

[0003] Refined Fischer-Tropsch tail wax is the bottom product after hydrorefining and atmospheric / vacuum cleaning of Fischer-Tropsch products. Cracking Fischer-Tropsch tail wax is the bottom product after hydrocracking and atmospheric / vacuum cleaning of refined Fischer-Tropsch tail wax. It consists of straight-chain hydrocarbons with more than 18 carbon atoms, a melting point greater than 30°C, and is solid or semi-solid at room temperature. It also contains submicron-sized impurities. Refined and cracked Fischer-Tropsch tail waxes are typically further processed to effectively utilize resources, such as for the preparation of high-melting-point waxes, cracking to produce gasoline and diesel, and isomerization to produce high-end lubricating oil base oils. However, trace impurities in Fischer-Tropsch tail waxes can reduce the activity, selectivity, and lifespan of catalysts used in further processing. Therefore, it is necessary to remove trace impurities from refined and cracked tail waxes before further processing. These submicron-sized impurities cannot be adsorbed by bleaching clay, and existing impurity removal methods such as centrifugation, bag filters, plate and frame filters, and belt filters are ineffective in filtering them. Therefore, there is an urgent need to develop a deep impurity removal technology for Fischer-Tropsch synthesis tail waxes.

[0004] CN101623574 discloses a filtration method for heavy wax produced during Fischer-Tropsch synthesis. This method involves feeding the heavy wax extracted from the Fischer-Tropsch synthesis reactor into a high-gradient magnetic filter, where the iron-based catalyst is adsorbed and removed before proceeding to the next stage. While this method uses a high-gradient magnetic filter to adsorb and remove the iron-based catalyst from the heavy wax, and is simple and easy to operate, the adsorption and impurity removal effect is not very good. Nanoscale particles and non-magnetic impurities are difficult to remove, and the Fischer-Tropsch tail wax is solid or semi-solid at room temperature, making effective filtration of its interior impossible. Summary of the Invention

[0005] In view of this, the main objective of the present invention is to provide a system and method for treating impurities in Fischer-Tropsch tail wax, which can effectively remove trace impurities from solid or semi-solid Fischer-Tropsch tail wax, so that the total metal content in the treated tail wax is less than 5 PPM.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: a system for treating impurities in Fischer-Tropsch tail wax, comprising:

[0007] The storage tank is equipped with a heating unit to heat the Fischer-Tropsch tail wax contained inside the tank and keep it in a molten state;

[0008] A feed pump, connected to the outlet of the storage tank via a pipeline, is used to transport molten Fischer-Tropsch tail wax;

[0009] An inorganic membrane filtration unit is connected to the feed pump via a pipeline and has multiple parallel inorganic membrane tubes that allow molten Fischer-Tropsch tail wax to pass through simultaneously. The molten Fischer-Tropsch tail wax is filtered through the inorganic membrane to obtain Fischer-Tropsch tail wax permeate.

[0010] The backwashing unit is connected to the inorganic membrane filtration unit and is used to rinse the inorganic membrane in the inorganic membrane filtration unit to obtain a rinsing solution.

[0011] The settling tank is connected to the inorganic membrane filtration unit and the storage tank via pipelines, and is used to filter impurities and discharge rinsing liquid;

[0012] The pipelines used to transport Fischer tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit are all equipped with heat tracing devices.

[0013] The inorganic membrane tube includes a first-stage membrane, a second-stage membrane, and a third-stage membrane stacked together to form a wedge-shaped structure for the Fischer-Tropsch tail wax to pass through sequentially. The filtration area of ​​the first-stage membrane is less than that of the second-stage membrane, which is less than that of the third-stage membrane. The pore size of the first-stage membrane is 50-500 nm, the pore size of the second-stage membrane is 500-800 nm, and the pore size of the third-stage membrane is greater than 800 nm. The pore size of the inorganic membrane increases stepwise, effectively preventing the membrane tube from becoming clogged.

[0014] According to the system of the present invention, the flushing fluid in the backwashing unit is an organic n-alkane with less than 18 carbon atoms.

[0015] According to the system of the present invention, the backwashing fluid of the backwashing unit enters the inorganic membrane filtration unit in the opposite direction to the flow of Fischer-Tropsch tail wax.

[0016] According to the system of the present invention, the inorganic membrane is a ceramic membrane or a metal membrane, etc.

[0017] According to the system of the present invention, the heat tracing device heats the pipes, pumps and inorganic membranes that transport Fischer tail wax in the system, and can be a device such as steam heat tracing, electric heat tracing tape and electric heat tracing pipe.

[0018] Another aspect of the present invention provides a method for treating impurities in Fischer-Tropsch tail wax, comprising the following steps:

[0019] Heat the Fischer tail wax in the storage tank until it is completely melted;

[0020] The Fischer-Tropsch tail wax liquid in the storage tank is transported to the inorganic membrane filtration unit by the feed pump, where impurities in the Fischer-Tropsch tail wax liquid are filtered and separated to obtain Fischer-Tropsch tail wax permeate.

[0021] The backwashing fluid in the backwashing system is passed into the inorganic membrane filtration unit to clean the inorganic membrane; and the filtered impurities and backwashing fluid are discharged through a settling tank; wherein, the pipelines used to transport Fischer-Tropsch tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit are all equipped with heat tracing.

[0022] According to the method of the present invention, the Fischer-Tropsch tail wax is heated to 50-300°C, preferably 80-200°C, in the storage tank, and the pipeline used for conveying the Fischer-Tropsch tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit is provided with a heat tracing temperature of 100-300°C, preferably 80-200°C.

[0023] According to the method of the present invention, the clarified liquid obtained from the backwashing unit is recycled after distillation.

[0024] According to the method of the present invention, the pressure inside the inorganic membrane filtration unit is 0.05–2.0 MPa. ， The preferred pressure is 0.5-1.0 MPa. The higher the pressure, the greater the filtration speed and the larger the throughput.

[0025] According to the method of the present invention, preferably, the pore size of the first-stage membrane is 100-300 nm, more preferably 100-200 nm, the pore size of the second-stage membrane is 500-600 nm, and the pore size of the third-stage membrane is 800-900 nm; the pressure is 0.5-1.0 MPa. ， Increased pressure can improve filtration speed and throughput, but excessive pressure can reduce membrane life. Therefore, the best filtration effect is achieved when an appropriate pressure is used in conjunction with an inorganic membrane with stepped pore size.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. The method of the present invention can process high-melting-point solid or semi-solid Fischer-Tropsch tail wax, keeping it in a molten state during the processing, making it easy to operate in a liquid state, and making the removal of solid or semi-solid impurities a reality.

[0028] 2. The processing system of the present invention can operate continuously and stably, and is simple to operate; in order to prevent impurities from depositing on the surface and clogging the membrane tube of the inorganic membrane filtration unit, the outer wall of the inorganic membrane tube can be backwashed with rinsing liquid, and the clear liquid on the inner wall can be circulated and settled, and the circulation volume can be controlled and adjusted, making full use of resources.

[0029] 3. The filtration effect of the method of this invention is good, and the total metal content in the treated sample is less than 5 PPM. In order to solve the problem of blockage by nano-impurities during membrane filtration, a wedge-shaped inorganic membrane is used, and the size of the contact surface gradually increases. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a system for treating impurities in Fischer-Tropsch tail wax, as exemplified by the present invention.

[0031] Figure 2 This is a schematic diagram illustrating the working principle of an inorganic membrane filtration unit.

[0032] Figure 3 for Figure 2 A schematic diagram of the structure of an inorganic membrane.

[0033] Figure 4 This is a schematic diagram showing the particle distribution of impurities in the untreated Fischer-Tropsch tail wax raw material.

[0034] Labeling description: Storage tank 1, feed pump 2, inorganic membrane filtration unit 3, backwashing unit 4, settling tank 5, inorganic membrane tube 3.1, first-stage membrane 3.11, second-stage membrane 3.12, third-stage membrane 3.13. Detailed Implementation

[0035] Embodiments of the present invention are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0036] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0037] Unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing" should be interpreted broadly. For example, they can refer to fixed connections or detachable connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and connections within two components or interactions between two components. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0038] The main idea of ​​this invention is to provide a method for treating impurities in Fischer-Tropsch tail wax, comprising the following steps:

[0039] Heat the Fischer tail wax in the storage tank until it is completely melted;

[0040] The Fischer-Tropsch tail wax liquid in the storage tank is transported to the inorganic membrane filtration unit by the feed pump, where impurities in the Fischer-Tropsch tail wax liquid are filtered and separated to obtain Fischer-Tropsch tail wax permeate.

[0041] The backwashing fluid in the backwashing system is passed into the inorganic membrane filtration unit to clean the inorganic membrane; and the filtered impurities and backwashing fluid are discharged through a settling tank; wherein, the pipelines used to transport Fischer-Tropsch tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit are all equipped with heat tracing.

[0042] To illustrate the implementation process of the above processing method, refer to... Figure 1 This invention provides a system for treating impurities in Fischer-Tropsch tail wax, comprising:

[0043] Storage tank 1 is equipped with a heating unit for heating the Fischer-Tropsch tail wax contained inside the storage tank and keeping it in a molten state;

[0044] Feed pump 2 is connected to the outlet of the storage tank via a pipeline and is used to transport molten Fischer-Tropsch tail wax;

[0045] The inorganic membrane filtration unit 3 is connected to the feed pump 2 through a pipeline and has multiple parallel inorganic membrane tubes 3.1 that allow molten Fischer-Tropsch tail wax to pass through simultaneously. After the molten Fischer-Tropsch tail wax passes through the inorganic membrane filtration, Fischer-Tropsch tail wax permeate (i.e., the filtered product) is obtained.

[0046] The backwashing unit 4 is connected to the inorganic membrane filtration unit 3, and the backwashing liquid therein enters the inorganic membrane filtration unit 3 in the opposite direction to the flow of Fischer-Tropsch tail wax.

[0047] The settling tank 5 is connected to the inorganic membrane filtration unit 3 and the storage tank 1 through pipelines, and is used to filter impurities and discharge rinsing liquid;

[0048] The pipelines used for conveying Fischer-Tropsch tail wax between the storage tank 1, the feed pump 2, and the inorganic membrane filter unit 3 are all equipped with heat tracing devices.

[0049] Figure 2 This is a schematic diagram of the working principle of the inorganic membrane filtration unit. The raw material is refined tail oil or cracked tail oil. The liquid (concentrate) containing a small amount of unfiltered high-concentration impurity particles at the end of the inorganic membrane tube 3.1 is discharged or circulated to the storage tank 1.

[0050] like Figure 2 and Figure 3 As shown, the inorganic membrane tube 3.1 includes a first-stage membrane, a second-stage membrane, and a third-stage membrane stacked together to form a wedge-shaped structure for the simultaneous passage of Fischer-Tropsch tail wax. The filtration area of ​​the first-stage membrane is less than that of the second-stage membrane, which is less than that of the third-stage membrane. The pore size of the first-stage membrane is 50-500 nm, the pore size of the second-stage membrane is 500-800 nm, and the pore size of the third-stage membrane is greater than 800 nm.

[0051] The heat tracing described in this article refers to heating the pipelines transporting Fischer tail wax in the system. Heat tracing methods can include steam tracing, electric heating tape, and electric heat tracing pipes. The heat tracing medium can be hot water, steam, heat transfer fluid, or electricity. Figure 1 The structure is not specifically illustrated, but existing technical structures can be referenced. Similarly, the heating device for the storage tank... Figure 1 The diagram is not specifically illustrated, but its purpose is to heat the raw materials in the storage tank to a certain heating temperature (50-300℃). Those skilled in the art can understand its structure.

[0052] Preferably, the inorganic membrane filtration unit has multiple inorganic membrane tubes connected in parallel, for example... Figure 1 The three shown are for illustrative purposes only.

[0053] Preferably, the flushing solution in the backwashing unit is an organic n-alkane with less than 18 carbon atoms. Further, the clarified solution obtained from the backwashing unit is recycled after distillation.

[0054] Preferably, the inorganic membrane is a ceramic membrane.

[0055] Preferably, the Fischer-Tropsch tail wax is heated to 50-300°C in the storage tank, and the pipeline used to transport the Fischer-Tropsch tail wax between the storage tank, the feed pump, and the inorganic membrane filtration unit is heated to a temperature of 100-300°C. The pressure in the inorganic membrane filtration unit is 0.05-2.0 MPa; those skilled in the art can easily understand how this pressure is achieved, and will not be described in detail here.

[0056] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the following references are made to the appendix. Figure 1-3The system and method flow of the present invention will be further described in detail with reference to specific embodiments.

[0057] Example 1

[0058] refer to Figure 1 A method for treating impurities in Fischer-Tropsch tail wax includes the following steps:

[0059] The cracked Fischer tailings wax feedstock in storage tank 1 is heated to a molten state at a temperature of 80°C.

[0060] The Fischer-Tropsch tail wax liquid in the storage tank is transported to the inorganic membrane filtration unit 3 through the feed pump 2, where impurities in the Fischer-Tropsch tail wax liquid are filtered and separated to obtain the Fischer-Tropsch tail wax permeate product.

[0061] The backwashing fluid (using diesel fraction from indirect coal liquefaction) in the backwashing system is introduced into the inorganic membrane filtration unit in the opposite direction to the Fischer-Tropsch tail wax flow to clean the inorganic membrane; and the filtered impurities and backwashing fluid are discharged through a settling tank; the pipelines used to transport the Fischer-Tropsch tail wax between the storage tank, feed pump, and inorganic membrane filtration unit are all equipped with electric heat tracing pipes for heat tracing at a temperature of 80°C. The pressure inside the inorganic membrane filtration unit is 0.5 MPa.

[0062] refer to Figure 2 and Figure 3 The inorganic membrane filtration unit 3 includes three inorganic membrane tubes 3.1. Each inorganic membrane tube 3.1 contains an inorganic membrane consisting of a first-stage membrane 3.11, a second-stage membrane 3.12, and a third-stage membrane 3.12 stacked together to form a wedge-shaped structure for the Fischer-Tropsch tail wax to pass through sequentially. The filtration area of ​​the first-stage membrane is less than that of the second-stage membrane, which is less than that of the third-stage membrane. The pore size of the first-stage membrane is 250 nm, the pore size of the second-stage membrane is 600 nm, and the pore size of the third-stage membrane is 900 nm.

[0063] Fischer-Tropsch tail wax enters the inorganic membrane in the order of small pores-medium pores-large pores (i.e., first-stage membrane 3.11-second-stage membrane 3.12-third-stage membrane 3.12). Impurities remain on the small pores, and blockage only blocks the small pores, not the entire membrane tube. The flushing fluid enters the inorganic membrane in the opposite direction of the Fischer-Tropsch tail wax, from the large pores-medium pores-small pores, and flushes away the blockage impurities.

[0064] Figure 1 The inorganic membrane tubes 3.1 are connected in parallel at the top and bottom positions. The Fischer-Tropsch tail wax liquid flows through each of the parallel inorganic membrane tubes 3.1 at the same time. When an inorganic membrane tube is severely blocked and the pressure difference is large, it needs to be isolated for backwashing until there is no pressure difference.

[0065] Example 2

[0066] Using Fischer-Tropsch refined tail wax as raw material, the processing steps are similar to those in Example 1, except that: the adjustment parameters are: heating temperature and tracing temperature are 150°C, the pressure in the inorganic membrane filtration unit is 2.0 MPa, the pore size of the first-stage membrane is 200 nm, the pore size of the second-stage membrane is 550 nm, and the pore size of the third-stage membrane is 800 nm.

[0067] Example 3

[0068] Using Fischer-Tropsch cracking tail wax as raw material, the processing steps are similar to those in Example 1, except that: the heating temperature and tracing temperature are adjusted to 120°C, the pressure inside the inorganic membrane filtration unit is 1.0 MPa, the pore size of the first-stage membrane is 200 nm, the pore size of the second-stage membrane is 500 nm, and the pore size of the third-stage membrane is 800 nm.

[0069] Comparative Example 1

[0070] Using Fischer-Tropsch cracking tail wax as raw material, the processing steps are similar to those in Example 1, except that: the filtration area of ​​the first-stage membrane = the filtration area of ​​the second-stage membrane = the filtration area of ​​the third-stage membrane; and the pore size of the first-stage membrane, the second-stage membrane, and the third-stage membrane are all 600 nm.

[0071] Figure 4 This is a schematic diagram of the particle distribution of impurities in the untreated Fischer-Tropsch tail wax feedstock. Table 1 below shows the ICP elemental analysis data of the tail oil before and after treatment by the methods of Examples 1-3 and Comparative Example 1 of this invention.

[0072] Table 1

[0073]

[0074]

[0075] As shown in Table 1, the methods of Examples 1-3 of this invention can effectively remove impurities from cracked Fischer-Tropsch tailings feedstock or refined Fischer-Tropsch tailings feedstock, resulting in a total metal content of less than 5 PPM in the treated tailings. Among these, Example 3 of this invention exhibits the best treatment effect due to its optimal setting of membrane tube pore size and pressure parameters, as can be seen from the permeate data after filtration in Example 3. Comparative Example 1 shows a poorer effect in removing impurities, which is attributed to the larger pore size of the membrane tube, allowing fine particles to pass through and enter the permeate.

[0076] This invention innovatively utilizes the combination of devices within each unit and has developed optimized settings for process parameters. Although the specific structures of each device are not described in detail, those skilled in the art can understand their specific structures based on their respective functional descriptions, so they will not be described in detail here.

[0077] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is impossible to exhaustively list all embodiments here. All obvious variations or modifications derived from the technical solutions of the present invention are within the spirit and scope of the present invention.

Claims

1. A system for treating impurities in Fischer-Tropsch tail wax, characterized in that: include: The storage tank is equipped with a heating unit to heat the Fischer-Tropsch tail wax contained inside the tank and keep it in a molten state; A feed pump, connected to the outlet of the storage tank via a pipeline, is used to transport molten Fischer-Tropsch tail wax; An inorganic membrane filtration unit is connected to the feed pump via a pipeline and has multiple parallel inorganic membrane tubes that allow molten Fischer-Tropsch tail wax to pass through simultaneously. The molten Fischer-Tropsch tail wax is filtered through the inorganic membrane tubes to obtain Fischer-Tropsch tail wax permeate. The backwashing unit is connected to the inorganic membrane filtration unit and is used to rinse the inorganic membrane in the inorganic membrane filtration unit to obtain a rinsing solution. The settling tank is connected to the inorganic membrane filtration unit and the storage tank via pipelines, and is used to filter impurities and discharge rinsing liquid; The pipelines used to transport Fischer tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit are all equipped with heat tracing devices. The inorganic membrane tube includes a first-stage membrane, a second-stage membrane, and a third-stage membrane stacked together to form a wedge structure for the sequential passage of Fischer-Tropsch tail wax. The filtration area of ​​the first-stage membrane is less than that of the second-stage membrane, which is less than that of the third-stage membrane. The pore size of the first-stage membrane is 50-500 nm, the pore size of the second-stage membrane is 500-800 nm, and the pore size of the third-stage membrane is greater than 800 nm.

2. The impurity processing system according to claim 1, characterized in that: The backwashing solution of the backwashing unit enters the inorganic membrane filtration unit in the opposite direction to the flow of Fischer-Tropsch tail wax.

3. The impurity processing system according to claim 1 or 2, characterized in that: The inorganic membrane is a ceramic membrane or a metal membrane.

4. The impurity processing system according to claim 1, characterized in that: The backwashing fluid in the backwashing unit is an organic n-alkanes with less than 18 carbon atoms.

5. The impurity processing system according to claim 1, characterized in that: The heat tracing device heats the pipes, pumps, and inorganic membranes that transport Fischer tail wax in the system, and is selected from one or more of steam heat tracing, electric heat tracing tape, and electric heat tracing pipe.

6. A method for treating impurities in Fischer-Tropsch tail wax, utilizing the system described in any one of claims 1-5, characterized in that: Includes the following steps: Heat the Fischer tail wax in the storage tank until it is completely melted; The Fischer-Tropsch tailings liquid in the storage tank is transported to the inorganic membrane filtration unit by a feed pump to filter and separate the impurities in the Fischer-Tropsch tailings liquid to obtain Fischer-Tropsch tailings permeate. The backwashing fluid in the backwashing system is passed into the inorganic membrane filtration unit to clean the inorganic membrane; and the filtered impurities and backwashing fluid are discharged through a settling tank; wherein, the pipelines used to transport Fischer-Tropsch tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit are all equipped with heat tracing.

7. The method for treating impurities according to claim 6, characterized in that: The Fischer-Tropsch tail wax is heated to 50-300°C in the storage tank, and the pipeline used to transport the Fischer-Tropsch tail wax between the storage tank, the feed pump and the inorganic membrane filtration unit is heated to a temperature of 100-300°C.

8. The method for treating impurities according to claim 6 or 7, characterized in that: The pressure inside the inorganic membrane filtration unit is 0.05–2.0 MPa.

9. The method for treating impurities according to claim 6, characterized in that: The clarified liquid obtained from the backwashing unit is recycled after distillation.

10. The method for treating impurities according to claim 6, characterized in that: The first-stage membrane has a pore size of 100-300 nm, the second-stage membrane has a pore size of 500-600 nm, the third-stage membrane has a pore size of 800-900 nm, and the pressure is 0.5-1.0 MPa.

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