A purification device for crude ethylene carbonate product

Abstract

The utility model belongs to lithium battery technical field, concretely is a kind of purification device of ethylene carbonate crude product. Including the reaction kettle, material processing tank and distillation tank that are sequentially arranged, the bottom of reaction kettle is connected to the top of material processing tank by material flow pipe, the bottom of material processing tank is connected to the top of distillation tank by first discharge pipe;Material processing tank is laterally detachably provided with oil film, and the cavity below the oil film is filled with water-absorbing molecular sieve, and the bottom of material processing tank is also provided with molecular sieve outlet.This application carries out precision filtration to crude product by setting oil film and water-absorbing molecular sieve in material processing tank, realizes the effective dehydration of crude product, effectively improves product purity by molecular distillation, and improves purification efficiency.

Description

Technical Field

[0001] This utility model belongs to the field of lithium battery technology, specifically a purification device for crude ethylene carbonate. Background Technology

[0002] Ethylene carbonate (VEC), a key additive in lithium-ion battery electrolytes, effectively improves battery cycle stability and high-temperature performance. Currently, the main methods for synthesizing ethylene carbonate include transesterification, the epoxide-butene and carbon dioxide addition method, and the phosgene method. Among these, the transesterification method is widely used industrially; it uses 3,4-dihydroxy-1-butene and dialkyl carbonate as raw materials to generate ethylene carbonate through transesterification. The epoxide-butene and carbon dioxide addition method is simple to operate and has a high yield, but the raw material epoxy compounds are unstable, difficult to obtain, and carbon dioxide activation requires high pressure and a highly efficient catalyst. As for the phosgene method for synthesizing ethylene carbonate, this method is limited due to the toxicity of phosgene, and in accordance with the development of green environmental protection concepts, the phosgene method has been abandoned. The purification methods for ethylene ethylene carbonate (VEC) in industry typically involve adding polymerization inhibitors and then distilling the product through a distillation column to obtain a VEC product with a purity greater than 99.95%. However, this method has drawbacks. The presence of water in the crude product leads to the decomposition of VEC, and many compounds with similar boiling points are present in the crude product. To ensure the purity of the target product, the distillation process requires discarding 25-40% of the middle fraction (containing effective components), resulting in reduced equipment utilization and increased production costs. In particular, when the crude product contains trace amounts of water, the hydrolysis reaction of VEC is accelerated during high-temperature distillation, which not only reduces the product yield but also generates byproducts.

[0003] Therefore, there is an urgent need to develop a purification device to solve the problems of VEC's easy decomposition in water and the difficulty in separating distillation impurities in industrial VEC purification. Utility Model Content

[0004] To address the problems mentioned above, this invention provides a purification device for crude ethylene carbonate. By setting up an oil film, molecular sieve, and dust cover inside the material processing tank to filter the crude product, effective dehydration of the crude product is achieved, thus improving purification efficiency.

[0005] The present invention adopts the following technical solution:

[0006] A purification apparatus for crude ethylene carbonate includes a reaction vessel, a material processing tank, and a distillation tank arranged sequentially, wherein:

[0007] The reactor has a feed inlet at the top and a first discharge outlet at the bottom. The first discharge outlet is connected to the top of the material processing tank through a material flow pipe. The material processing tank has a second discharge outlet at the bottom. The second discharge outlet is connected to the top of the distillation tank through the first discharge pipe.

[0008] An oil film is detachably installed horizontally inside the material handling tank, and the cavity below the oil film is filled with a water-absorbing molecular sieve.

[0009] Furthermore, a molecular sieve outlet is provided at the bottom of the material processing tank; a dust cover is provided on the second outlet; a first ball valve is installed on the side of the first discharge pipe near the material processing tank; and a flow rate valve is installed on the side of the first discharge pipe near the distillation tank.

[0010] Furthermore, a first heating sleeve is fitted on the outside of the distillation tank, and a condenser is installed inside the distillation tank. The upper and lower parts of the condenser are respectively provided with a condensate outlet and a condensate inlet that are connected to the outside.

[0011] The lower part of the distillation tank is equipped with a second discharge pipe. One end of the second discharge pipe extends into the distillation tank and is connected to the outer wall of the condenser tube, so that the condensed liquid on the outer wall of the condenser tube flows down into the second discharge pipe.

[0012] Furthermore, a vacuum pump is installed at the top of the distillation tank.

[0013] Furthermore, the condenser is a vertical tube that extends from the bottom to the top of the distillation tank. A collecting ring is provided at the lower end of the outer wall of the condenser to collect the condensed liquid. A discharge nozzle is formed on the collecting ring and extends into the second discharge pipe.

[0014] Furthermore, the portion of the second discharge pipe located outside the distillation tank is connected to at least one sealed receiving tank, and the lower side wall of the sealed receiving tank has a finished product discharge port.

[0015] Furthermore, at least two parallel sealed receiving tanks are provided on the second discharge pipe, and a second ball valve is provided on the feed pipe of each sealed receiving tank.

[0016] Furthermore, the bottom of the distillation tank is connected to the heavy component receiving tank via a third discharge pipe, and the lower part of the heavy component receiving tank is provided with an impurity outlet.

[0017] Furthermore, a second heating sleeve is fitted on the outside of the reactor, and a reaction stirrer is installed inside the reactor.

[0018] Furthermore, the location where the oil film is installed inside the material handling tank is provided with a stepped structure, and the oil film is detachably fixed to the stepped structure.

[0019] Compared with the prior art, the beneficial effects of this utility model are as follows:

[0020] (1) This application employs molecular distillation and water-absorbing molecular sieve dehydration, which effectively improves product purity and purification efficiency. The use of oil film precision filtration and water-absorbing molecular sieve dehydration effectively reduces the impact of strong alkali and moisture on VEC distillation purification. The final purification stage uses molecular distillation, which solves the problem of high-temperature decomposition of materials through high-vacuum short-path distillation, effectively improving product purity from 99.95% to 99.99%, reducing energy loss, and greatly reducing the problem of distillation reuse, thereby improving production efficiency.

[0021] (2) In this application, a vertically extending condenser is installed inside the distillation tank. When the steam is cooled and liquefied on the outer wall of the condenser, the liquid flows naturally down the surface of the vertical tube. The collecting ring plate receives the liquid flowing down from all directions through the ring structure, forming a continuous condensate and guiding it to the discharge nozzle. The discharge nozzle extends into the inlet of the second discharge pipe, so that the condensate directly enters the second discharge pipe.

[0022] (3) In this application, the bottom of the distillation tank is connected to the heavy component receiving tank through the third discharge pipe. After the effective components are distilled in the distillation tank, the high-boiling-point heavy components accumulated at the bottom are introduced into the heavy component receiving tank through the third discharge pipe. Through the design of the physically isolated heavy component receiving tank, the heavy component processing and the main distillation process can be carried out simultaneously, thereby improving the effective running time of the distillation process. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the purification apparatus provided in one embodiment of this application;

[0025] Figure 2 This is a schematic diagram of the material handling tank structure of a purification apparatus provided in an embodiment of this application;

[0026] Figure 3 A schematic diagram of the distillation tank structure of a purification apparatus provided in an embodiment of this application;

[0027] Wherein: 1-Reaction vessel, 11-Inlet, 12-First outlet, 13-Reaction stirrer, 14-Material flow pipe, 2-Material processing tank, 21-Oil film, 22-Water-absorbing molecular sieve, 23-Step structure, 24-Molecular sieve outlet, 25-Dust cover, 26-Second outlet, 3-Distillation tank, 31-Condenser, 32-Condensate inlet, 33-Condensate outlet, 34-Vacuum pump, 35-Collecting ring plate, 4-First outlet pipe, 41-First ball valve, 42-Flow rate valve, 5-Second outlet pipe, 6-Sealed collection tank, 61-Second ball valve, 62-Finished product outlet, 7-Third outlet pipe, 8-Heavy component receiving tank, 81-Impurity outlet. Detailed Implementation

[0028] The technical solutions of the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this utility model, and not all embodiments. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this utility model.

[0029] The following is in conjunction with the appendix Figure 1 To be continued Figure 3 The present invention will be described in detail with specific embodiments.

[0030] like Figure 1-3As shown, this utility model provides a purification device for crude ethylene carbonate, comprising a reaction vessel 1, a material processing tank 2, and a distillation tank 3 arranged sequentially. The reaction vessel 1 has an inlet 11 at its top and a first outlet 12 at its bottom, which is connected to the top of the material processing tank 2 via a material flow pipe 14. The material processing tank 2 has a second outlet 26 at its bottom, which is connected to the top of the distillation tank 3 via a first outlet pipe 4. An oil film 21 is detachably arranged horizontally inside the material processing tank 2, and a water-absorbing molecular sieve 22 is filled in the cavity below the oil film 21. A molecular sieve outlet 24 is also provided at the bottom of the material processing tank 2. A dust cover 25 covers the second outlet 26. A first ball valve 41 is installed on the side of the first outlet pipe 4 near the material processing tank 2, and a flow rate valve 42 is installed on the side of the first outlet pipe 4 near the distillation tank 3. In this application, the oil film 21 refers to a porous filter medium with hydrophobic properties, specifically a polytetrafluoroethylene microporous membrane. The mechanical interception of large particulate impurities and preliminary separation of moisture are achieved through pore size control. The water-absorbing molecular sieve 22 refers to an adsorbent material with a high specific surface area, which captures residual moisture through physical adsorption. In this application, the water-absorbing molecular sieve 22 is randomly arranged in the physical treatment tank 2. The dust cover 25 refers to a protective structure covering the second discharge port 26, specifically a stainless steel mesh cover, to prevent external dust from entering the processed material. The first ball valve 41 is used to cut off or open the channel from the material treatment tank 2 to the distillation tank 3. The flow rate valve 42 is a device for regulating fluid flow rate, which can be implemented using a needle valve, but is not limited to this. The flow rate valve 42 controls the flow rate of the material entering the distillation tank 3 by adjusting its opening degree.

[0031] Specifically, in reactor 1, 3,4-dihydroxy-1-butene and dialkyl carbonate are reacted with sodium hydroxide as a catalytic catalyst. After the reaction, the crude VEC product is obtained by filtration and flows into material processing tank 2 by gravity. Oil film 21 traps suspended particles and intercepts some liquid water, while the remaining material permeates to the water-absorbing molecular sieve layer 22 for deep dehydration. After dehydration, the material is protected by dust cover 25 and discharged through first ball valve 41, with the flow rate valve 42 regulating the flow rate into distillation tank 3. The water-absorbing molecular sieve 22 can be replaced through bottom molecular sieve outlet 24 when saturated. The removable and washable oil film 21 maintains filtration efficiency. The combined design of water-absorbing molecular sieve 22 and oil film 21 extends the continuous operation cycle of the device and facilitates maintenance. The pretreated material is purified by distillation in distillation tank 3 through boiling point differences, avoiding decomposition reactions caused by residual moisture and reducing the impurity load during the distillation stage. This application achieves the stepwise removal of water through the synergistic effect of oil film 21 and water-absorbing molecular sieve 22. The combination of physical interception and adsorption significantly reduces the impurity handling pressure of the distillation system.

[0032] This application employs molecular distillation and dehydration using a water-absorbing molecular sieve 22, effectively improving product purity and purification efficiency. The use of a precision filtration film 21 and dehydration with the water-absorbing molecular sieve 22 effectively reduces the impact of strong alkali and moisture on VEC distillation purification. The final purification stage utilizes molecular distillation, employing high-vacuum short-path distillation to solve problems such as high-temperature decomposition of materials, effectively increasing product purity from 99.95% to 99.99%, reducing energy loss, and significantly minimizing the need for distillation reuse, thus improving production efficiency.

[0033] Furthermore, a first heating sleeve is fitted on the outside of the distillation tank 3. The first heating sleeve is an annular jacket structure wrapped around the outside of the distillation tank. Specifically, it can be achieved by winding electric heating wire or circulating heat transfer oil, and is used to uniformly heat the outside of the distillation tank 3. A condenser tube 31 is provided inside the distillation tank 3. The upper and lower parts of the condenser tube 31 are respectively provided with a condensate outlet 33 and a condensate inlet 32 ​​that are connected to the outside, forming a circulation channel for the cooling medium. A second discharge pipe 5 is provided at the lower part of the distillation tank 3. One end of the second discharge pipe 5 extends into the distillation tank 3 and is connected to the outer wall of the condenser tube 31, so that the condensed liquid on the outer wall of the condenser tube 31 flows down into the second discharge pipe 5. In this application, the second discharge pipe 5 and the condenser pipe 31 can be directly connected or indirectly connected. Direct connection can be achieved by welding a stainless steel pipe to the outer wall of the condenser pipe 31; indirect connection can be achieved by connecting through an intermediate connector, such as by non-contact or contact connection between the second discharge pipe 5 and the collection ring plate 35 fixed on the outer wall of the condenser pipe 31; the second discharge pipe 5 is used to directly discharge the condensed liquid.

[0034] The first heating jacket controls the distillation temperature through external heating to prevent localized overheating that could lead to the decomposition of ethylene carbonate. A cooling medium is continuously circulated through the condenser tube 31, entering through the lower condensate inlet 32 ​​and absorbing heat from the steam. The heated medium then exits through the upper condensate outlet 33, creating counter-current heat exchange to enhance condensation efficiency. The condensed liquid product flows naturally down the outer wall of the condenser tube 31 and is directly discharged into the sealed collection tank 6 through the second discharge pipe 5, preventing condensate from dripping back into the distillation tank 3 and causing impurities to enter.

[0035] Furthermore, a vacuum pump 34 is installed at the top of the distillation tank 3. The vacuum pump 34 can be a rotary vane vacuum pump or a Roots vacuum pump to create a reduced-pressure distillation environment. This device connects the vacuum pump 34 to the top of the distillation tank 3, reducing the pressure inside the tank to a set range, thereby lowering the boiling point of the mixture below a safe temperature. Specifically, when the vacuum pump 34 is activated, the internal pressure of the distillation tank 3 gradually decreases, and the boiling point of the mixture inside the tank decreases accordingly. At this time, the target product, ethylene ethylene carbonate, vaporizes at a lower temperature. Simultaneously, due to the increased volatility difference of different components in the low-pressure environment, low-boiling-point components and high-boiling-point impurities are more easily separated. The vaporized target product is condensed by the condenser 31 to form a liquid, which flows along the outer wall into the second discharge pipe 5, and finally enters the sealed receiving tank 6 for collection. This application actively regulates the pressure through the vacuum pump 34 to achieve efficient distillation under conditions below conventional temperatures, avoiding the decomposition of the target product and enhancing the component separation effect by utilizing pressure differences.

[0036] In the preferred embodiment, see Figure 3 The condenser tube 31 is a vertical tube extending from the bottom to the top of the distillation tank 3. A vertical tube refers to a tubular structure arranged along the longitudinal axis of the distillation tank 3, utilizing gravity to allow the condensed liquid to flow downwards along the tube wall. A collecting ring plate 35 is installed at the lower end of the outer wall of the condenser tube 31 to collect the condensed liquid. The collecting ring plate 35 can be fixed to the condenser tube 31 by welding, but is not limited to this method. A discharge nozzle is formed on the collecting ring plate 35, extending into the second discharge pipe 5. The collecting ring plate 35 is an annular component surrounding the outer wall of the condenser tube 31, forming a continuous guiding surface around the tube body to collect condensed liquid flowing in all directions. The collecting ring plate 35 can have a certain angle of inclination on the condenser tube 31 to guide the condensed liquid towards the discharge nozzle. The discharge nozzle is an extension or gathering part located at the edge of the collecting ring plate 35, ensuring that the liquid can flow directly into the second discharge pipe 5 from the discharge nozzle, avoiding dripping. When the vapor liquefies upon encountering cold on the outer wall of the condenser tube 31, the liquid flows naturally downwards along the surface of the vertical tube. The collecting ring 35, through its annular structure, collects liquid flowing from all directions, forming a continuous condensate that is guided to the discharge nozzle. The discharge nozzle extends into the inlet of the second discharge pipe 5, allowing the condensate to directly enter the second discharge pipe 5. During this process, the length of the vertical pipe covers the vertical height of the working area of ​​the distillation tank 3, ensuring that condensate from different locations can be effectively collected.

[0037] For further details, please refer to [link / reference]. Figure 3The portion of the second discharge pipe 5 located outside the distillation tank 3 is connected to at least one sealed receiving tank 6. A finished product discharge port 62 is provided on the lower side wall of the sealed receiving tank 6. The sealed receiving tank 6 is used to collect the finished ethylene ethylene. Preferably, at least two parallel sealed receiving tanks 6 are provided on the second discharge pipe 5, and a second ball valve 61 is provided on the feed pipe of each sealed receiving tank 6. The second ball valve 61 is an opening and closing control component installed on the feed pipe, and can be a ball valve with manual or automatic adjustment function. Its function is to continuously collect the finished product material by controlling the feeding state of each sealed receiving tank 6.

[0038] Furthermore, the bottom of the distillation tank 3 is connected to the heavy component receiving tank 8 via a third discharge pipe 7. The lower part of the heavy component receiving tank 8 is equipped with an impurity outlet 81. The third discharge pipe 7 refers to the pipe used to transport the high-boiling-point heavy components from the bottom of the distillation tank. Specifically, it can be implemented using a corrosion-resistant metal pipe, and its inner wall can be coated with polytetrafluoroethylene to enhance chemical corrosion resistance. After the effective components are distilled in the distillation tank 3, the high-boiling-point heavy components accumulated at the bottom are introduced into the heavy component receiving tank 8 through the third discharge pipe 7. Once it is confirmed that the heavy components have completely settled, the operator opens the valve of the impurity outlet 81 to allow solid impurities in the heavy components to be preferentially discharged. This application, through the design of the physically isolated heavy component receiving tank 8, allows the processing of heavy components to be carried out simultaneously with the main distillation process, increasing the effective operating time of the rectification process.

[0039] Furthermore, a second heating sleeve is fitted around the outside of the reactor 1, and a reaction stirrer 13 is installed inside the reactor 1. The second heating sleeve forms a stable heat transfer interface by wrapping around the outer wall of the reactor 1. During the heating process, the overall temperature of the reaction system is maintained within a preset range by adjusting the power of the heat source or the temperature of the heat transfer medium, thus avoiding local overheating or temperature fluctuations that could lead to the decomposition of ethylene carbonate. When the reaction stirrer 13 is in operation, the blade shearing action breaks up the material stratification, promotes the uniform mixing of the liquid reactants and the solid catalyst, and accelerates the transfer of heat within the reaction system, eliminating the difference in reaction efficiency caused by temperature gradients.

[0040] For further details, please refer to [link / reference]. Figure 2Inside the material handling tank 2, a stepped structure 23 is provided at the location where the oil film 21 is installed. The oil film 21 is detachably fixed to the stepped structure 23. The stepped structure 23 refers to an annular protrusion extending circumferentially along the inner wall of the tank. Specifically, it can be implemented by welding an annular support plate or stamping a flange. This structure forms a planar support surface to support the edge of the oil film. The detachable fixing means that the oil film 21 and the stepped structure 23 are reversibly connected by a mechanical connection, which can be achieved by snap-fit ​​clamps or bolts. During installation, the oil film 21 covers the annular plane of the stepped structure 23 in a flat manner. The edge of the oil film 21 is evenly pressed against the stepped surface by the clamps. When the liquid flows over the oil film 21, the continuous support surface formed by the stepped structure 23 can resist the liquid impact force. When it is necessary to replace the oil film 21, the old oil film can be removed as a whole by releasing the clamps or bolts. The new oil film is aligned with the stepped structure 23 through the positioning holes and then re-fixed without disassembling other internal parts of the tank. In some embodiments, the surface of the stepped structure 23 is processed with anti-slip textures to enhance the friction at the edge of the oil film, such as cross-grid patterns or concentric circle patterns.

[0041] Specifically, the pore size of the oil film 21 is 0.4-0.5 micrometers, preferably 0.45 micrometers. The pore size of the oil film 21 refers to the maximum passing size of the pores on the surface of the filter membrane, which can be achieved using a polytetrafluoroethylene (PTFE) microporous membrane. This material has stable chemical inertness and controllable pore size distribution characteristics. The selective retention function refers to the separation of solid suspended matter and large molecular impurities in liquid materials through physical sieving. This can be achieved by adjusting the film formation process parameters to control the porosity and pore size distribution.

[0042] When the crude ethylene carbonate product passes through the oil film 21, solid particles larger than 0.5 micrometers are completely blocked, while substances with a size in the range of 0.4-0.5 micrometers are partially retained. Since the dynamic diameter of water molecules is approximately 0.28 micrometers, this pore size range allows water to pass freely but effectively intercepts colloidal particles encapsulating water, reducing the influent load of the subsequent water-absorbing molecular sieve 22 adsorption stage. Simultaneously, ethylene carbonate molecules, due to their much smaller size than the membrane pores, maintain high permeability, avoiding the loss of effective components. This application achieves pre-separation of free water from the crude product, reducing the risk of ethylene carbonate hydrolysis during subsequent high-temperature distillation. Furthermore, the oil film 21 reduces the workload of the water-absorbing molecular sieve 22 in the material processing tank 2, extends the regeneration cycle, and improves the overall operational stability of the purification process.

[0043] The present invention has been further described above with reference to specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the substance and scope of the present invention. Various modifications made by those skilled in the art to the above embodiments after reading this specification are all within the scope of protection of the present invention.

Claims

1. A purification apparatus for crude ethylene carbonate, characterized in that, It includes a reaction vessel, a material handling tank, and a distillation tank arranged in sequence, wherein: The reactor has a feed inlet at the top and a first discharge outlet at the bottom. The first discharge outlet is connected to the top of the material processing tank through a material flow pipe. The material processing tank has a second discharge outlet at the bottom. The second discharge outlet is connected to the top of the distillation tank through the first discharge pipe. An oil film is detachably arranged horizontally inside the material handling tank, and the cavity below the oil film is filled with a water-absorbing molecular sieve.

2. The purification apparatus for crude ethylene carbonate according to claim 1, characterized in that, The bottom of the material processing tank is also provided with a molecular sieve outlet; the second discharge port is covered with a dust cover, a first ball valve is installed on the side of the first discharge pipe near the material processing tank, and a flow rate valve is installed on the side of the first discharge pipe near the distillation tank.

3. The purification apparatus for crude ethylene carbonate according to claim 1, characterized in that, The outer side of the distillation vessel is fitted with a first heating sleeve, and the inside of the distillation vessel is provided with a condenser tube. The upper and lower parts of the condenser tube are respectively provided with a condensate outlet and a condensate inlet that communicate with the outside. The lower part of the distillation tank is provided with a second discharge pipe. One end of the second discharge pipe extends into the distillation tank and is connected to the outer wall of the condenser tube, so that the condensed liquid on the outer wall of the condenser tube flows down into the second discharge pipe.

4. The purification apparatus for crude ethylene carbonate according to claim 3, characterized in that, A vacuum pump is installed at the top of the distillation tank.

5. The purification apparatus for crude ethylene carbonate product according to claim 3, characterized in that, The condenser is a vertical tube that extends from the bottom to the top of the distillation tank. A collection ring is provided at the lower end of the outer wall of the condenser for collecting condensed liquid. A discharge nozzle is formed on the collection ring and extends into the second discharge pipe.

6. The purification apparatus for crude ethylene carbonate according to claim 3, characterized in that, The portion of the second discharge pipe located outside the distillation tank is connected to at least one sealed receiving tank, and the lower side wall of the sealed receiving tank has a finished product discharge port.

7. The purification apparatus for crude ethylene carbonate according to claim 6, characterized in that, At least two parallel sealed receiving tanks are provided on the second discharge pipe, and a second ball valve is provided on the feed pipe of each sealed receiving tank.

8. The purification apparatus for crude ethylene carbonate according to claim 1, characterized in that, The bottom of the distillation tank is connected to the heavy component receiving tank via a third discharge pipe, and the lower part of the heavy component receiving tank is provided with an impurity outlet.

9. The purification apparatus for crude ethylene carbonate according to claim 1, characterized in that, The reactor is fitted with a second heating sleeve on the outside, and a reaction stirrer is installed inside the reactor.

10. The purification apparatus for crude ethylene carbonate according to claim 1, characterized in that, The material handling tank has a stepped structure at the location where the oil film is installed, and the oil film is detachably fixed to the stepped structure.

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