A method for cleaning a nylon 6 devolatilizer

Abstract

The present application relates to the technical field of high polymer material production, and discloses a cleaning method for a nylon 6 devolatilizer, which comprises the following steps: 1) injecting a caprolactam aqueous solution into the devolatilizer for cleaning; 2) injecting one or more of a hydroxyl compound, a condensate of the hydroxyl compound and an ester of the hydroxyl compound for cleaning; 3) injecting an acetic acid solution or a phosphoric acid solution for cleaning; and 4) injecting water into the devolatilizer for cleaning; after injecting different cleaning agents each time, the cleaning agent is repeatedly vaporized and condensed on the inner surface of the devolatilizer. The cleaning method has four stages, and different cleaning agents and cleaning parameters are used in each stage, so that the cleaning efficiency can be significantly improved, the residual substances on the inner surface of the devolatilizer can be effectively removed, and the cleaning cost is relatively low.

Description

Technical Field

[0001] This invention relates to the field of polymer material production technology, and in particular to a method for cleaning a nylon 6 devolatilizer. Background Technology

[0002] Nylon 6 (PA6) possesses excellent properties such as high mechanical strength, good crystallinity, corrosion resistance, and wear resistance, and is widely used in various fields including the automotive industry, rail transportation, film packaging, electronics, and textile fibers. The production process of Nylon 6 often requires a devolatilization step to remove monomers, oligomers, and molecular chain growth reactions. On large-scale continuous industrial production lines for PA6, the devolatilizer is a key piece of equipment. When quality deteriorates over time and fails to meet product quality requirements, or when abnormal production shutdowns occur, cleaning agents are often used to remove residues from the Nylon 6 devolatilizer.

[0003] For polymer production plants, the goal is to quickly and efficiently clean the interior of the devolatilizer with as little cleaning agent as possible, while also ensuring the cleaning agent is easily removable and leaves minimal residue inside the devolatilizer. Existing cleaning technologies suffer from drawbacks such as large agent consumption, long cleaning times, high energy consumption, and the need for secondary or even multiple cleaning cycles. Furthermore, some cleaning agents pose safety hazards, such as explosiveness, when discharged. Therefore, there is an urgent need to develop a safe, low-cost, efficient, and low-consumption cleaning method for nylon 6 devolatilizers. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a cleaning method for nylon 6 devolatilizers. The method of this invention designs the cleaning process into four stages, with each stage using different cleaning agents and parameters, thereby significantly improving cleaning efficiency. It effectively removes residual unreacted caprolactam, some oligomers generated during the reaction, and thermal decomposition or crosslinking reaction products from the inner surface of the devolatilizer, while maintaining low cleaning costs.

[0005] The specific technical solution of the present invention is: a cleaning method for nylon 6 devolatilizers, which includes the following steps:

[0006] 1) Use caprolactam aqueous solution as cleaning agent A; inject it into the devolatilizer on the inner surface where there are residual nylon 6 polymerization reaction residues to wash away residual nylon 6, unreacted caprolactam and some oligomers generated by the reaction; the cleaning temperature is 80-120℃ and the pressure is 0.1~1MPa.

[0007] 2) Use one or more of the following as cleaning agent B: hydroxyl compound, condensate of hydroxyl compound, and ester of hydroxyl compound; inject it into the devolatilizer to wash away residual caprolactam cyclic oligomers; the cleaning temperature is 60-260℃ and the pressure is 0.1~1MPa.

[0008] 3) Use acetic acid solution or phosphoric acid solution as cleaning agent C, inject it into the devolatilizer, and degrade the residual cyclic oligomers and / or devolatilization reaction pyrolysis products into caprolactam or linear oligomers; the cleaning temperature is 50-100℃ and the pressure is 0.1~0.5MPa.

[0009] 4) Use deionized water as cleaning agent D and inject it into the devolatilizer to wash away residual cleaning agent and residues (such as caprolactam, methanol, phosphoric acid, and caprolactam and linear oligomers generated after phosphoric acid cleaning); the cleaning temperature is 50-100℃.

[0010] Because the residual material in the devolatilizer consists of caprolactam, nylon 6, nylon 6 oligomers (including linear and cyclic oligomers), as well as by-reaction products and nylon 6 decomposition products accompanying the devolatilization process, and the different components have significant differences in physical properties, this invention divides the cleaning process into four stages:

[0011] (1) In the first stage, the devolatilizer is cleaned with caprolactam aqueous solution. In particular, the residues on the inner wall of the devolatilizer that come into contact with non-flowing materials during the devolatilization reaction, as well as the caprolactam residues in various parts of the devolatilizer, can be carried away by the caprolactam aqueous solution. Some linear oligomers and cyclic oligomers can also be dissolved in the caprolactam aqueous solution. A large amount of residual substances in the devolatilizer can be removed through the first cleaning. The cleaning agent A mixed with the residues enters the extract concentration device for processing and can be recycled and reused.

[0012] (2) In the second stage, hydroxyl compounds, condensates of hydroxyl compounds, and esters of hydroxyl compounds (such as methanol, ethylene glycol, and triethylene glycol) are used to clean the residual caprolactam cyclic dimers and some nylon 6 in the devolatilizer. The residual caprolactam cyclic dimers in the nylon 6 polymerization reactor appear as white powder, especially under certain conditions as flaky β crystals with a melting point as high as 345°C. They cannot be completely dissolved in caprolactam aqueous solution, but they can be dissolved in hot solvents such as methanol.

[0013] (3) In the third stage, acetic acid solution or phosphoric acid solution is used to clean the residual nylon 6 and the cyclic polymers, thermal cracking or cross-linking reaction products formed by the nylon 6 devolatilization reaction in the devolatilization device. Acetic acid or phosphoric acid has a strong destructive effect on the amide bonds of the nylon 6 molecular chain.

[0014] (4) Clean the residual cleaning agents A, B and C from the first three stages with deionized water. Cleaning agents A, B and C have good solubility in hot water and can be carried away by a large amount of hot water without remaining in the devolatilizer.

[0015] Because the residual compounds in the devolatilizer consist of cyclic caprolactam, linear and cyclic nylon 6 polymers with vastly different degrees of polymerization, by-products of unconventional condensation reactions such as nylon 6 crosslinking or thermal decomposition, and small amounts of other substances, the different components vary greatly in density, solubility, melting and boiling points, and chemical stability. It is difficult to remove the residues using a single cleaning agent in a single step. Therefore, a multi-stage cleaning process is required, and the overall principle should be to proceed from easy to difficult removal, to recycle the cleaning agent as much as possible, and to minimize the amount of cleaning agent residue in the reactor so as not to affect subsequent use. Therefore, the first stage uses caprolactam aqueous solution to clean the devolatilizer because caprolactam has good solubility in hot water. Residual caprolactam in various parts of the devolatilizer can be carried away by the caprolactam aqueous solution. Simultaneously, some linear and cyclic oligomers can also dissolve in the caprolactam aqueous solution. Furthermore, after the first stage cleaning, the caprolactam solution containing residues discharged from the discharge port can be filtered and then enter the extraction concentrate unit for recycling and reuse in the reaction system to produce Nylon 6, maximizing resource utilization. After the first stage cleaning, a small amount of caprolactam, used as the first-stage cleaning agent, remains in the devolatilizer. Cyclic oligomers that are poorly soluble in caprolactam aqueous solution, especially cyclic dimers, and some Nylon 6, particularly the plate-like β-crystalline cyclic dimers with melting points as high as 345℃ under certain conditions, cannot be dissolved in the first-stage cleaning agent caprolactam aqueous solution. However, they can dissolve in hot solvents such as methanol. Therefore, these difficult-to-remove substances can be eluted by adding hydroxyl alcohols or their esters. However, the cleaning agents used in the first and second stages have limited ability to dissolve and decompose nylon 6. Acetic acid or phosphoric acid, which have a strong destructive effect on the amide bonds of the nylon 6 molecular chain, are used as the third cleaning agent. At a certain temperature, these solutions decompose and remove nylon 6 residues, thermal decomposition products, or cross-linking reaction products formed during the nylon 6 devolatilization reaction, which are difficult to wash off. After the first three stages of cleaning, various substances remaining in the devolatilization reactor from the nylon 6 devolatilization reaction can be washed away. However, the residue of the cleaning agents themselves can affect the next stage of use of the devolatilization reactor. Therefore, a large amount of heated deionized water, which can dissolve the cleaning agents from the first three stages, is added for rinsing.

[0016] As can be seen from the above, the order of the cleaning agents in the four stages is specially designed and cannot be changed. Otherwise, it would be difficult to remove all residual components from the devolatilizer and ensure that no cleaning agent residue remains when the devolatilizer is used in the next stage. If the order of the cleaning agents in the second or third stage is changed from that in the first stage, the first cleaning agent will contain residues of the second / third cleaning agents. These residues cannot be removed by simple methods such as filtration, extraction, or distillation, making it difficult to recycle the first cleaning agent.

[0017] In this process, after each injection of a different cleaning agent, the cleaning agent is controlled to repeatedly vaporize and condense on the inner surface of the devolatilizer.

[0018] Preferably, the nylon 6 polymerization residue includes caprolactam, nylon 6, oligomers, and by-products and decomposition products of nylon 6 accompanying the devolatilization process.

[0019] Preferably, the hydroxyl compound, the condensate of the hydroxyl compound, and the ester of the hydroxyl compound are selected from one or two of methanol, ethylene glycol, triethylene glycol, glycerol, pentaerythritol, methyl oxalate, and acetate.

[0020] Preferably, in step 1), the cleaning time does not exceed 48 hours, and the amount of cleaning fluid A injected is 4 / 5 to 1 / 10 of the devolatilizer capacity; in step 2), the cleaning time does not exceed 24 hours, and the amount of cleaning fluid B injected is 1 / 2 to 1 / 30 of the devolatilizer capacity; in step 3), the cleaning time does not exceed 24 hours, and the amount of cleaning fluid C injected is 1 / 10 to 1 / 30 of the devolatilizer capacity.

[0021] Preferably, in step 1), the devolatilizer is provided with a film-forming element that can support the flow of material forming a film, a material chamber, and a material distribution component connected to the material chamber or providing flowing material to the film-forming element; the inner wall surface includes the inner wall surface of the devolatilizer, the outer wall surface of the film-forming element, the surface of the material chamber and the surface of the material distribution component, and the inner surface of the pipe communicating with the devolatilizer.

[0022] In a further preferred embodiment, the devolatilizer adopts a vertical tower structure, and the tower contains film-forming elements installed vertically from top to bottom. The film-forming elements are rod-shaped bodies with an aspect ratio greater than 60:1. The material flows from top to bottom to the bottom of the devolatilizer after being distributed by the material distribution component.

[0023] Preferably, the devolatilizer is provided with a feed inlet, a devolatilization outlet, and a discharge outlet. The discharge outlet is located at the bottom of the devolatilizer. After the material flows in from the feed inlet, it enters the material chamber. After being distributed, it flows along the outer wall of the film-forming element while being devolatilized. After reaching the bottom of the devolatilizer, it flows out from the discharge outlet. During the devolatilization process, the substances volatilized from the material are extracted through the devolatilization outlet.

[0024] Preferably, the feed inlet is provided with a feed stop valve and a first discharge port on the feed channel. The first discharge port is used to discharge the cleaning agent, relieve pressure, and discharge the residue in the feed channel. The devolatilizer is provided with a cleaning agent inlet. The discharge outlet is provided with a second discharge port and a discharge stop valve on the discharge channel. The residue in the vessel after cleaning the devolatilizer can be discharged from the vessel through the second discharge port. The cleaning agent inlet is located on the devolatilization channel of the devolatilization port. The pipe connected to the devolatilization port is provided with a devolatilization port stop valve and is located downstream of the cleaning agent inlet.

[0025] Preferably, after the cleaning agent enters the devolatilizer, the devolatilization port shut-off valve, feed shut-off valve, and discharge shut-off valve on the pipeline connected to the devolatilization port are in the closed state, so that the devolatilizer is in a closed state. The pressure inside the devolatilizer is controlled by adjusting the temperature of the devolatilizer, so that the cleaning agent repeatedly condenses and vaporizes on the inner surface. When the pressure inside the vessel is too high, the devolatilization channel and / or the feed shut-off valve and the first discharge port are opened to release air and pressure. According to the pressure change inside the vessel, the devolatilization channel and / or the feed shut-off valve and the first discharge port are kept in an intermittently open state or a normally open state, so that the cleaning agent can vaporize and evaporate and flow out of the devolatilizer.

[0026] Preferably, the film-forming element has a sleeve structure inside that allows the heat medium to flow through. During cleaning, the temperature of the heat medium inside the film-forming element is controlled to be lower than the temperature of the deodorizer, so that the vapor formed by the cleaning agent condenses on the outer wall of the film-forming element and washes away the residue.

[0027] Preferably, the devolatilization port on the devolatilizer is connected to a condenser, and a collector is located below the condenser. The vaporized cleaning agent and the eluent are discharged into the condenser through the devolatilization port on the devolatilizer at a set cleaning temperature. The vaporized material is condensed by the condenser and enters the collector below for further separation and recycling. The residue that is not carried away by the vaporization of the cleaning agent and remains at the bottom of the devolatilizer is discharged from the second discharge port.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] The method of this invention designs the cleaning process into four stages, and each stage uses different cleaning agents and cleaning parameters, thereby significantly improving cleaning efficiency. It can effectively wash away residual unreacted caprolactam and some oligomers generated by the reaction, as well as thermal decomposition or cross-linking reaction products on the inner surface of the devolatilizer. Moreover, the cleaning agent is a common and inexpensive material, resulting in low cleaning cost. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a nylon 6 devolatilizer and its connected channels.

[0031] Among them, 1 is the feed inlet, 11 is the first discharge port, 12 is the feed shut-off valve, 2 is the devolatilizer, 21 is the inner wall of the devolatilizer, 3 is the material chamber, 31 is the film plate, 4 is the film forming element, 41 is the outer wall of the film forming element, 5 is the discharge port, 51 is the second discharge port, 52 is the discharge shut-off valve, 6 is the devolatilizer port, 61 is the cleaning agent inlet, 62 is the devolatilizer port shut-off valve, 7 is the condenser, and 8 is the collector. Detailed Implementation

[0032] The present invention will be further described below with reference to embodiments.

[0033] A method for cleaning a nylon 6 devolatilizer includes the following steps:

[0034] 1) Use caprolactam aqueous solution as cleaning agent A; inject it into the devolatilizer on the inner surface where there are residual nylon 6 polymerization reaction residues to wash away residual nylon 6, unreacted caprolactam and some oligomers, thermal decomposition or cross-linking reaction products generated by the reaction; the cleaning temperature is 80-120℃ and the pressure is 0.1-1MPa.

[0035] 2) Use one or more of the following as cleaning agent B: hydroxyl compound, condensate of hydroxyl compound, and ester of hydroxyl compound; inject it into the devolatilizer to wash away residual caprolactam cyclic oligomers; the cleaning temperature is 60-260℃ and the pressure is 0.1-1MPa.

[0036] 3) Use acetic acid solution or phosphoric acid solution as cleaning agent C, inject it into the devolatilizer, and degrade the residual cyclic oligomers and / or devolatilization reaction pyrolysis products into caprolactam or linear oligomers; the cleaning temperature is 50-100℃ and the pressure is 0.1-1MPa.

[0037] 4) Use water as cleaning agent D and inject it into the devolatilizer to wash away residual cleaning agent and residual reactants (such as caprolactam, methanol, phosphoric acid, and caprolactam and linear oligomers generated after phosphoric acid cleaning); the cleaning temperature is 50-100℃.

[0038] In this process, after each injection of a different cleaning agent, the cleaning agent is controlled to repeatedly vaporize and condense on the inner surface of the devolatilizer.

[0039] In some implementations, the nylon 6 polymerization residues include caprolactam, nylon 6, oligomers, and by-products and decomposition products of nylon 6 that accompany the devolatilization process.

[0040] In some implementations, the hydroxyl compound, the condensate of the hydroxyl compound, and the ester of the hydroxyl compound are selected from one or two of methanol, ethylene glycol, triethylene glycol, glycerol, pentaerythritol, methyl oxalate, and acetate.

[0041] In some implementation cases, in step 1), the cleaning time does not exceed 48 hours, and the amount of cleaning fluid A injected is 4 / 5 to 1 / 10 of the devolatilizer capacity; in step 2), the cleaning time does not exceed 24 hours, and the amount of cleaning fluid B injected is 1 / 2 to 1 / 30 of the devolatilizer capacity; in step 3), the cleaning time does not exceed 24 hours, and the amount of cleaning fluid C injected is 1 / 10 to 1 / 30 of the devolatilizer capacity.

[0042] In some implementation cases, in step 1), the devolatilizer 2 is provided with a film-forming element 4 that can support the flow of material forming a film, a material chamber 3, and a material distribution component connected to the material chamber or providing flowing material to the film-forming element; the inner wall surface includes the inner wall surface 21 of the devolatilizer, the outer wall surface 41 of the film-forming element, the surface of the material chamber and the surface 31 of the material distribution component, and the inner surface of the pipe communicating with the devolatilizer.

[0043] In some implementations, the film-forming element is a rod-shaped body with an aspect ratio greater than 60:1, which is vertically fixed from top to bottom. The material is distributed from the top by the material distribution component and then flows from top to bottom to the bottom of the devolatilizer.

[0044] In some implementation cases, the devolatilizer is provided with a feed inlet 1, a devolatilization port 6, and a discharge port 5. The discharge port is located at the bottom of the devolatilizer. After the material flows in from the feed inlet, it enters the material chamber. After being distributed, it flows along the outer wall of the film-forming element while being devolatilized. After reaching the bottom of the devolatilizer, it flows out from the discharge port. During the devolatilization process, the substances volatilized from the material are extracted through the devolatilization port.

[0045] In some implementation examples, the feed inlet is equipped with a feed shut-off valve 12 and a first discharge port 11. The first discharge port is used to discharge the cleaning agent, relieve pressure, and discharge the residue in the feed inlet. The devolatilizer is equipped with a cleaning agent inlet 61. The discharge outlet is equipped with a second discharge port 51 and a discharge shut-off valve 52. The residue in the vessel after cleaning the devolatilizer can be discharged from the vessel through the second discharge port. The cleaning agent inlet is located on the devolatilization channel of the devolatilization port. The pipe connected to the devolatilization port is equipped with a devolatilization port shut-off valve 62 and is located downstream of the cleaning agent inlet.

[0046] In some implementation cases, after the cleaning agent enters the devolatilizer, the devolatilization port shut-off valve, feed shut-off valve, and discharge shut-off valve on the pipeline connected to the devolatilization port are in the closed state, so that the devolatilizer is in a closed state. The pressure inside the devolatilizer is controlled by adjusting the temperature of the devolatilizer, so that the cleaning agent repeatedly condenses and vaporizes on the inner surface. When the pressure inside the vessel is too high, the devolatilization channel and / or feed shut-off valve and the first discharge port are opened to release air and pressure. According to the pressure change inside the vessel, the devolatilization channel and / or feed shut-off valve and the first discharge port are kept in an intermittently open state or a normally open state, so that the cleaning agent can vaporize and evaporate and flow out of the devolatilizer.

[0047] In some implementations, the film-forming element is provided with a sleeve structure that allows the heat medium to flow through. During cleaning, the temperature of the heat medium inside the film-forming element is controlled to be lower than the temperature of the devolatilizer, so that the vapor formed by the cleaning agent condenses on the outer wall of the film-forming element and washes away the residue.

[0048] In some implementation cases, a condenser 7 is connected to the external devolatilization port of the devolatilizer, and a collector 8 is located below the condenser. The vaporized cleaning agent and the eluent are discharged into the condenser through the devolatilization port of the devolatilizer at a set cleaning temperature. After being condensed by the condenser, the vaporized material enters the collector below and is then separated and recycled. The residue that is not carried away by the vaporization of the cleaning agent and remains at the bottom of the devolatilizer is discharged from the second discharge port.

[0049] Example 1

[0050] like Figure 1 The diagram shows a devolatilizer 2 (specifically a falling film devolatilizer). The top of the falling film devolatilizer has a feed inlet 1, and the feed inlet channel has a first discharge port 11 and a feed shut-off valve 12. The falling film devolatilizer has a material chamber 3, and the bottom plate of the material chamber is a film-forming plate 31. The film-forming element 4 is vertically installed through the film-forming plate 31 and has a gap between it and the film-forming plate. The bottom of the falling film devolatilizer has a discharge port 5, and the discharge port channel has a second discharge port 51 and a discharge shut-off valve 52. The side of the falling film devolatilizer has a devolatilization port, and the devolatilization port channel has a cleaning agent inlet 61 and a devolatilization port shut-off valve 62. A condenser 7 and a collector 8 are connected to the outside of the devolatilization channel.

[0051] Molten nylon 6 enters the material chamber of the falling film devolatilizer, then passes through the pores to the outer wall surface 41 of the film-forming element for a devolatilization and thickening reaction. It then flows out from the bottom outlet, with the volatiles drawn off from the devolatilization port 6 and condensed in the condenser 7 before entering the collector. After the reaction, polymers, oligomers, byproducts, and pyrolysis products accompanying chain growth remain in the material chamber, on the inner wall surface 21 of the devolatilizer, and on the outer wall surface of the film-forming element. Some areas on the inner wall surface 21 of the devolatilizer have thin layers of material of varying thickness, ranging from blackish to yellowish to light white, primarily composed of caprolactam monomers, oligomers, and pyrolysis products. Some areas on the outer wall surface of the film-forming element have yellow or white residue, mainly composed of nylon, with trace amounts of oligomers and pyrolysis products. The following cleaning steps are used:

[0052] (1) Reduce the temperature of the falling film devolatilizer to below 80°C. Inject a caprolactam aqueous solution with a volume equal to 1 / 2 of the reactor's volume into the falling film devolatilizer through the cleaning agent injection port 61. Raise its temperature to 100°C. Close the feed shut-off valve 12, the devolatilization port shut-off valve 62, and the discharge shut-off valve 52. The water vapor containing caprolactam will fully contact the inner wall 21 of the devolatilizer, the outer wall 41 of the film-forming element, and the bottom surface of the film-forming plate 31 to remove residues. Monitor the pressure inside the falling film devolatilizer and adjust the gauge pressure inside the devolatilizer to not exceed 1 MPa by adjusting the temperature and / or the switching of the feed shut-off valve 12 and the devolatilization port shut-off valve 62. After 24 hours of cleaning, the falling film devolatilizer was cooled to 80°C. The devolatilization port shut-off valve 62 and the discharge shut-off valve 52 were closed. The residue inside the vessel after cleaning the falling film devolatilizer was discharged from the vessel through the second discharge port 61, and then the discharge port was closed, completing the first stage cleaning of the falling film devolatilizer used for the devolatilization reaction of nylon 6. After the first stage cleaning, the inner wall surface 21 of the devolatilizer and the outer wall surface of the film-forming element below the caprolactam solution immersion level were cleaned, leaving only stains. The residue on the inner wall surface 21 of the devolatilizer and the outer wall surface of the film-forming element above the caprolactam solution immersion level was significantly reduced. A thin layer of material remained where the residue was previously thicker, and its main components were various oligomers. The thickness of the nylon residue on the outer wall surface of the film-forming element was significantly reduced.

[0053] (2) Inject 1 / 10 of the volume of ethylene glycol into the falling film devolatilizer through the cleaning agent injection port 61, raise its temperature to 200°C, and close the feed shut-off valve 12, the devolatilization port shut-off valve 62, and the discharge shut-off valve 52. The ethylene glycol vapor will fully contact the inner wall 21 of the devolatilizer, the outer wall 41 of the film-forming element, and the bottom surface of the film plate 31 to remove residues. Monitor the pressure inside the falling film devolatilizer and adjust the gauge pressure inside the devolatilizer to not exceed 1 MPa by adjusting the temperature and / or the switching of the feed shut-off valve 12 and the devolatilization port shut-off valve 62. After cleaning for 16 hours, keep the feed shut-off valve 12 and the discharge shut-off valve 52 closed and the devolatilization port shut-off valve 62 open, and evaporate most of the ethylene glycol to the condenser 7, where it will condense and flow into the collector 8. Two hours later, the stop valve 62 of the devolatilization port is closed, and the residue after cleaning the falling film devolatilizer is discharged from the reactor through the second discharge port 51 or the discharge valve 52, completing the second stage cleaning of the falling film devolatilizer used for the devolatilization reaction of nylon 6. After the second stage cleaning, most of the residue on the inner wall surface 21 of the devolatilizer and the outer wall surface of the film-forming element is cleaned, with only some areas retaining material or stains and residual cleaning agent from the first and second stage cleaning.

[0054] (3) Inject acetic acid solution with a volume of 1 / 15 of the reactor's contents into the falling film devolatilizer through the cleaning agent injection port 61, raise its temperature to 80°C, and close the feed shut-off valve 12, the devolatilization port shut-off valve 62, and the discharge shut-off valve 52. The acetic acid solution vapor will fully contact the inner wall 21 of the devolatilizer, the outer wall 41 of the film-forming element, and the bottom surface of the film-forming plate 31 to remove residues. Monitor the pressure inside the falling film devolatilizer and adjust the gauge pressure inside the devolatilizer to not exceed 1 MPa by adjusting the temperature and / or the switching of the feed shut-off valve 12 and the devolatilization port shut-off valve 62. After cleaning for 16 hours, raise the temperature of the devolatilizer to 100°C, keep the feed shut-off valve 12 and the discharge shut-off valve 52 closed, and the devolatilization port shut-off valve 62 open. Distill off a portion of the acetic acid solution to the condenser 7, where it will condense and flow into the collector 8. Two hours later, the devolatilization port shut-off valve 62 was closed, and the temperature was lowered to 80°C. The residue after cleaning the falling film devolatilizer was discharged from the reactor through the second discharge port 51 or the discharge valve 52, completing the third stage cleaning of the falling film devolatilizer used for the devolatilization reaction of nylon 6. After the third stage cleaning, there was no visible residue on the inner wall 21 of the devolatilizer and the outer wall of the film-forming element, with only some areas showing traces of material or cleaning agent.

[0055] (4) Inject deionized water into the falling film devolatilizer through the cleaning agent injection port 61, maintain the temperature inside the devolatilizer at 80°C, close the feed shut-off valve 12, the devolatilization port shut-off valve 62 and the discharge shut-off valve 52, and ensure that the deionized water fully contacts the inner wall surface 21 of the devolatilizer, the outer wall surface 41 of the film-forming element, and the bottom surface of the film-forming plate 31 to remove the cleaning agent remaining from the aforementioned cleaning stage. Keep the devolatilization port shut-off valve 62 slightly open, soak for 8 hours, and then cool to room temperature. Discharge the cleaning solution from the second discharge port 51 to the outside of the vessel, completing the final cleaning of the falling film devolatilizer used for the devolatilization reaction of nylon 6. At this time, there are no nylon 6 devolatilization reaction residues or cleaning agent in the devolatilizer.

[0056] Comparative Example 1

[0057] Compared to Example 1, only the first stage + fourth stage of Example 1 is used for cleaning.

[0058] Compared to Example 1, Comparative Example 1 reduced steps (2) and (3) of Example 1. The results showed that after cleaning in steps (1) and (4) of Example 1, the inner wall surface 21 of the devolatilizer and the outer wall surface of the film-forming element that were not soaked in the caprolactam solution still had obvious devolatilization reaction residues, only the thickness of the residues was reduced and the residue area was smaller than before cleaning. This is because some oligomers (such as cyclic dimers) and complex structural cleavage products brought about by side reactions during devolatilization are difficult to dissolve in caprolactam solution, and even more difficult to be washed off by deionized water. Extending the cleaning time (exceeding the upper limit of 48 hours in the first stage) and increasing the cleaning temperature (exceeding the upper limit of 120 °C in the first stage) can improve the cleaning effect, but it is still impossible to completely remove some residues.

[0059] Comparative Example 2

[0060] Compared with Example 1, only the second stage + fourth stage of Example 1 is used for cleaning.

[0061] Compared to Example 1, Comparative Example 2 reduced steps (1) and (3) of Example 1. The results showed that after cleaning via steps (2) and (4) of Example 1, the amount of devolatilization residue on the inner wall 21 of the devolatilizer and the outer wall of the film-forming element was only reduced, with a relatively large residual amount. This is because high-temperature ethylene glycol can only dissolve and remove caprolactam, linear nylon 6 molecular chains, and a small amount of oligomers, but it has no good cleaning ability for cyclic dimers, cyclic trimers, and by-products with higher content in the oligomers. Deionized hot water can only remove a small amount of small nylon molecule compounds and the cleaning agent ethylene glycol, and it also has no significant cleaning effect on the nylon 6 devolatilization residue that is difficult to remove with ethylene glycol. Its cleaning effect is significantly different from that of Example 1.

[0062] Comparative Example 3

[0063] Compared to Example 1, only the third stage + fourth stage cleaning is used for cleaning.

[0064] Compared to Example 1, Comparative Example 3 reduced steps (1) and (3) of Example 1. The results showed that after cleaning via steps (2) and (4) of Example 1, the amount of devolatilization residue on the inner wall 21 of the devolatilizer and the outer wall of the film-forming element was only reduced, with a relatively large residual amount. This is because high-temperature ethylene glycol can only dissolve and remove caprolactam, linear nylon 6 molecular chains, and a small amount of oligomers, but it has no good cleaning ability for cyclic dimers, cyclic trimers, and by-products with higher content in the oligomers. Deionized hot water can only remove a small amount of small nylon molecule compounds and the cleaning agent ethylene glycol, and it also has no significant cleaning effect on the nylon 6 devolatilization residue that is difficult to remove with ethylene glycol. Its cleaning effect is significantly different from that of Example 1.

[0065] Comparative Example 4

[0066] Compared to Example 1, the cleaning sequence of the first to third stages is replaced (ACB) before proceeding to the fourth stage.

[0067] Comparative Example 5

[0068] Compared to Example 1, the cleaning sequence of the first to third stages is replaced (BAC) before proceeding to the fourth stage.

[0069] Comparative Example 6

[0070] Compared to Example 1, the cleaning sequence of the first to third stages is replaced (BCA) before proceeding to the fourth stage.

[0071] Comparative Example 7

[0072] Compared to Example 1, the cleaning sequence of the first to third stages is replaced (CAB) before proceeding to the fourth stage.

[0073] Comparative Example 8

[0074] Compared to Example 1, the cleaning sequence of the first to third stages is replaced (CBA) before proceeding to the fourth stage.

[0075] Analysis of Comparative Examples 4-8: Compared to Example 1, Comparative Examples 4-8 reversed the cleaning order of the first three stages. The results showed that after changing the cleaning order, a small amount of thin material remained on the inner wall of the devolatilizer and the outer wall of the film-forming element, indicating that a small amount of devolatilization reaction residue was not cleaned; grayish-yellow stains also remained on each wall surface. In Comparative Examples 4-8, the thickness of the residue and the number of stains varied among the different comparisons, and the cleaning effect was weaker than that of Example 1. This is because cleaning agent A is an aqueous solution of caprolactam, which mainly removes caprolactam residue and some oligomers remaining after the devolatilization reaction. Residues on the inner wall of the devolatilizer that come into contact with non-flowing materials during the devolatilization reaction, as well as residual caprolactam throughout the devolatilizer, can be carried away by the caprolactam aqueous solution. Some linear and cyclic oligomers can also be dissolved in the caprolactam aqueous solution. For cleaning residual caprolactam cyclic dimers and some nylon 6 within the devolatilizer, cleaning agent B, ethylene glycol, due to its molecular polarity, can promote the conversion of cyclic dimers from a high-melting-point crystal form to a relatively easily detachable low-melting-point crystal form. It has no decomposition effect on nylon 6, but its strong penetrating power can reduce the surface energy between the nylon molecular chains and the metal wall at high temperatures, promoting their detachment. Cleaning agent C, acetic acid solution, has a strong destructive effect on nylon 6 and the cyclic polymers formed by the nylon 6 devolatilization reaction, as well as the amide bonds in thermal decomposition or cross-linking reaction products. Relatively speaking, the order of components from easiest to hardest to detach from the inner wall of the devolatilization reactor and the outer surface of the film-forming element is roughly as follows: caprolactam, linear oligomers, nylon 6, partially cyclic oligomers, cyclic dimers, pyrolysis products, and gels generated by cross-linking reactions. The order of the cleaning agents is specifically designed, generally following the principle of cleaning the components that are easiest to remove. This cannot be changed; otherwise, it would be difficult to remove all residual components from the devolatilization reactor and ensure that no cleaning agent residue remains when using the devolatilization reactor in the next stage, and it would also be difficult to remove cleaning agent residues. If the order of the cleaning agents in the second or third stage is reversed compared to the first stage, the first cleaning agent will contain residues from the second / third stage. These residues cannot be removed by simple methods such as filtration, extraction, or distillation, making it difficult to recycle the first cleaning agent.

Claims

1. A method for cleaning a nylon 6 devolatilizer, characterized in that: Includes the following steps: 1) Use caprolactam aqueous solution as cleaning agent A; It is injected into the devolatilizer on the inner surface where there are residual nylon 6 polymerization devolatilization reaction products, in order to wash away residual nylon 6, unreacted caprolactam, oligomers generated by the reaction, thermal decomposition or cross-linking reaction products; the cleaning temperature is 80-120℃ and the pressure is 0.1-1MPa; 2) Use one or more of the following as cleaning agent B: hydroxyl compounds, condensates of hydroxyl compounds, and esters of hydroxyl compounds; Inject it into the devolatilizer to wash away residual caprolactam cyclic oligomers; the washing temperature is 60-260℃ and the pressure is 0.1-1MPa; 3) Use acetic acid solution or phosphoric acid solution as cleaning agent C, inject it into the devolatilizer, and degrade the residual cyclic oligomers and / or devolatilization reaction pyrolysis products into caprolactam or linear oligomers; the cleaning temperature is 50-100℃ and the pressure is 0.1-0.5MPa; 4) Use deionized water as cleaning agent D and inject it into the devolatilizer to wash away residual cleaning agent and residual reactants; the cleaning temperature is 50-100℃; After each injection of a different cleaning agent, the cleaning agent is controlled to repeatedly vaporize and condense on the inner surface of the devolatilizer.

2. The cleaning method as described in claim 1, characterized in that: The residues from the nylon 6 polymerization devolatilization reaction include caprolactam, nylon 6, oligomers, by-products from the devolatilization process, and nylon 6 decomposition products.

3. The cleaning method as described in claim 1, characterized in that: The hydroxyl compound, condensate of the hydroxyl compound, and ester of the hydroxyl compound are selected from one or two of methanol, ethylene glycol, triethylene glycol, glycerol, pentaerythritol, methyl oxalate, and acetate.

4. The cleaning method as described in claim 2 or 3, characterized in that: In step 1), the cleaning time shall not exceed 48 hours, and the amount of cleaning solution A injected shall be 4 / 5 to 1 / 10 of the capacity of the devolatilizer; In step 2), the cleaning time shall not exceed 24 hours, and the amount of cleaning fluid B injected shall be 1 / 2 to 1 / 30 of the capacity of the devolatilizer; In step 3), the cleaning time shall not exceed 24 hours, and the amount of cleaning fluid C injected shall be 1 / 10 to 1 / 30 of the capacity of the devolatilizer.

5. The cleaning method as described in claim 1, characterized in that: In step 1), the devourer is provided with a film-forming element that can support the flow of material to form a film, a material chamber, and a material distribution component connected to the material chamber or providing flowing material to the film-forming element; The inner surface includes the inner wall of the devolatilizer, the outer wall of the film-forming element, the surface of the material chamber and the surface of the material distribution component, and the inner surface of the pipe communicating with the devolatilizer.

6. The cleaning method as described in claim 5, characterized in that: The film-forming element is a rod-shaped body with an aspect ratio greater than 60:1, which is vertically fixed from top to bottom. The material is distributed from the top by the material distribution component and then flows from top to bottom to the bottom of the devolatilizer.

7. The cleaning method as described in claim 5, characterized in that: The devolatilizer is equipped with a feed inlet, a devolatilization outlet, and a discharge outlet. The discharge outlet is located at the bottom of the devolatilizer. After the material flows in from the feed inlet, it enters the material chamber. After being distributed, it flows along the outer wall of the film-forming element while being devolatilized. After reaching the bottom of the devolatilizer, it flows out from the discharge outlet. During the devolatilization process, the substances volatilized from the material are extracted through the devolatilization outlet.

8. The cleaning method as described in claim 7, characterized in that: The feed inlet is equipped with a feed stop valve and a first discharge port. The first discharge port is used to discharge the cleaning agent, relieve pressure, and discharge the residue in the feed inlet. The devolatilizer is equipped with a cleaning agent inlet. The discharge outlet is equipped with a second discharge port and a discharge stop valve. The residue in the vessel after cleaning the devolatilizer can be discharged from the vessel through the second discharge port. The cleaning agent inlet is located on the devolatilization channel of the devolatilization port. The pipe connected to the devolatilization port is equipped with a devolatilization port stop valve and is located downstream of the cleaning agent inlet.

9. The cleaning method as described in claim 8, characterized in that: When the cleaning agent enters the devolatilizer, the devolatilization port shut-off valve, feed shut-off valve, and discharge shut-off valve are closed, making the devolatilizer a closed system. The pressure inside the devolatilizer is controlled by adjusting the temperature, causing the cleaning agent to repeatedly condense and vaporize on the inner surface. When the pressure inside the vessel is too high, the devolatilization channel and / or feed shut-off valve and the first discharge port are opened to release air and pressure. Depending on the pressure changes inside the vessel, the devolatilization channel and / or feed shut-off valve and the first discharge port are kept in an intermittent or normally open state, allowing the cleaning agent to vaporize and flow out of the devolatilizer.

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