Process for the semi-continuous production of polyamides and polyamide products

Abstract

The application relates to a process for semi-continuous production of polyamide and a polyamide product, comprising the following steps: adding a batch of polyamide salt liquid for polymerization in a reaction kettle, after discharging is completed, water vapor containing a depolymerization auxiliary agent is introduced into the reaction kettle, the residual material is subjected to on-line depolymerization, the residual material is depolymerized to form a prepolymer with a molecular weight of 2000-4000, wherein the mass concentration of the depolymerization auxiliary agent in the water vapor containing the depolymerization auxiliary agent is 0.1%-10%, the depolymerization auxiliary agent is selected from organic monobasic acids, and the boiling point of the depolymerization auxiliary agent is 100 DEG C-180 DEG C; then the next batch of polyamide salt liquid is directly added into the reaction kettle, and polymerization and on-line depolymerization are sequentially and circularly carried out. The process provided by the application can obviously improve product quality, improve production efficiency and material utilization rate, effectively reduce the damage risk of the reaction kettle, and improve production safety.

Description

Technical Field

[0001] This application relates to the field of polyamide production technology, and in particular to a semi-continuous process for producing polyamide and a polyamide product. Background Technology

[0002] Polyamide, commonly known as nylon, is a general term for polymers whose main molecular chain repeating units contain amide groups. It is also the most produced, most diverse, and most widely used of the five major general-purpose engineering plastics. Nylon 66 (PA66), as the earliest industrialized polyamide product, is widely used in mechanical parts, electronic components, high-end fabrics, and other fields.

[0003] Currently, there are two production processes for PA66: continuous and batch. Among them, the batch process has advantages such as simple operation, low equipment investment, and convenient specification switching. According to the content disclosed in Chinese patent CN203833856U, the mainstream batch process for producing polyamide involves: salting in a salt storage tank to obtain polyamide brine; concentration in an evaporator to meet polymerization requirements; simultaneous polymerization in multiple high-pressure reactors to produce polyamide products in batches; and the addition of additives to the high-pressure reactors as needed using an additive injector.

[0004] Although the production equipment mentioned in the patent can significantly improve the flexibility of large-scale polyamide production, it still has many problems. On the one hand, after each batch of production, high molecular weight materials inevitably remain in the reactor. As the number of production batches increases, the residual materials will not only gradually accumulate and the molecular weight will become larger and larger, but they will also undergo dry burning and carbonization, eventually leading to yellowing of the product, an increase in the number of black spots, cross-linking gelation, and uneven distribution of the product's molecular weight. All of these affect the product quality, especially causing great trouble for PA66 in downstream civilian yarn applications. Normally, the proportion of molecular weight greater than 5w in the product does not exceed 6%, but after multiple batches of production, the proportion of molecular weight greater than 5w in the product exceeds 20%. These high molecular weight substances will increase the probability of yarn breakage during spinning and traction, and the spinnability will decrease significantly. Therefore, it is necessary to stop the high-pressure reactor for cleaning, which increases the production cost.

[0005] On the other hand, the temperature difference between the autoclave and the material during each batch transfer is about 100℃~150℃. This high temperature difference will cause significant stress changes, which will damage the autoclave. Long-term production will increase the damage to the autoclave body, eventually leading to the autoclave rupture and causing a safety accident. At the same time, the high temperature difference during each batch transfer will also increase the amine content in the wastewater, increase the chemical oxygen demand (COD), increase the cost of wastewater treatment, and because the amine loss changes, the formula ratio needs to be adjusted, which will significantly affect the product stability. Summary of the Invention

[0006] Therefore, it is necessary to provide a semi-continuous process for producing polyamide and a polyamide product to address the above problems. The process provided in this application can significantly improve product quality, increase production efficiency and material utilization, effectively reduce the risk of reactor damage, and improve production safety.

[0007] A semi-continuous process for producing polyamide includes the following steps:

[0008] A batch of polyamide salt solution is added to the reactor for polymerization. After the discharge is completed, water vapor containing a depolymerization aid is introduced into the reactor to depolymerize the residual material online, so that the residual material depolymerizes to form a prepolymer with a molecular weight of 2000~4000. The mass concentration of the depolymerization aid in the water vapor containing the depolymerization aid is 0.1%~10%. The depolymerization aid is selected from organic monobasic acid and the boiling point of the depolymerization aid is 100℃~180℃.

[0009] Then, the next batch of polyamide salt solution is added directly to the reactor, and the polymerization and online depolymerization are carried out in a cycle.

[0010] In one embodiment, the depolymerization aid is selected from at least one of formic acid, acetic acid, propionic acid, and oxalic acid.

[0011] In one embodiment, the depolymerization aid has a mass concentration of 0.1% to 5% in the water vapor containing the depolymerization aid.

[0012] In one embodiment, after the material is discharged, the temperature inside the reactor is 30°C to 60°C higher than the temperature of the water vapor containing the depolymerization aid.

[0013] In one embodiment, the temperature of the water vapor containing the depolymerization aid is 210°C to 260°C.

[0014] In one embodiment, the pressure of the water vapor containing the depolymerization aid is 1.5 MPaG to 4.5 MPaG.

[0015] In one embodiment, when water vapor containing a depolymerization aid is introduced into the reactor, the orifice diameter of the vaporization nozzle is 6 mm to 14 mm.

[0016] In one embodiment, the online depolymerization time is 5 min to 60 min.

[0017] In one embodiment, the polymerization process includes: first, prepolymerization is carried out at 1.5 MPaG~3.5 MPaG and 220℃~250℃, and then pressure relief polymerization is carried out simultaneously during the process of heating to 260℃~280℃. When the pressure drops to within 0.05 MPaG, final polymerization is carried out at 280℃~320℃ by vacuum exhaust or micro-positive pressure exhaust.

[0018] A polyamide product produced by a semi-continuous polyamide production process as described above, wherein the mass fraction of polymers with a molecular weight greater than 50,000 in the polyamide product is less than 7% within twenty batches of semi-continuous production.

[0019] The process described in this application utilizes steam from an organic monocarboxylic acid with a boiling point of 100℃~180℃ for online depolymerization after each batch of polymerization. This depolymerizes the residual material in the reactor into prepolymers of a specific molecular weight, which can directly participate in the next batch of polymerization. Furthermore, the depolymerization aid can also act as a capping agent. Compared to traditional batch processes that require shutdown and cleaning, this process eliminates the need to drain the cleaning solution, thus improving product quality and production efficiency while effectively utilizing the residual material in the reactor. Simultaneously, the introduction of steam helps regulate the temperature difference within the reactor, reducing stress and amine loss on the reactor body when the next batch of brine enters the reaction vessel. This solves the problem of reactor damage in traditional batch processes for polyamide production. Detailed Implementation

[0020] To facilitate understanding of this application, it will be described in more detail below. However, it should be understood that this application can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0021] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular implementations or embodiments only and is not intended to be limiting of this application. In this application, when numerical ranges are involved, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0022] This application provides a semi-continuous process for producing polyamide, comprising the following steps:

[0023] A batch of polyamide salt solution is added to the reactor for polymerization. After the discharge is completed, water vapor containing a depolymerization aid is introduced into the reactor to depolymerize the residual material online, so that the residual material depolymerizes to form a prepolymer with a molecular weight of 2000~4000. The mass concentration of the depolymerization aid in the water vapor containing the depolymerization aid is 0.1%~10%. The depolymerization aid is selected from organic monobasic acid and the boiling point of the depolymerization aid is 100℃~180℃.

[0024] Then, the next batch of polyamide salt solution is added directly to the reactor, and the polymerization and online depolymerization are carried out in a cycle.

[0025] It should be noted that the polyamide salt solution is obtained by a salt-forming reaction using diacid and diamine as raw materials and pure water as solvent; this application does not limit the mass concentration of the polyamide salt solution, but preferably 40wt%~70wt%.

[0026] The process described in this application utilizes steam containing a specific depolymerization aid to depolymerize the residual high-molecular-weight material in the reactor online after each batch of polymerization. To more clearly illustrate the depolymerization principle, this application uses PA66 as an example. The depolymerization aid is an organic monocarboxylic acid with a boiling point of 100℃~180℃. Hydrogen ions on the monofunctional groups in the depolymerization aid attack the oxygen on the carbonyl group of the amide bond, forming a protonated ketone, increasing the positive charge of the carbonyl group. Simultaneously, oxygen atoms with lone pairs of electrons in the water act as nucleophiles, attacking the carbon atom, changing its sp2 hybridization to sp3 hybridization, forming a tetrahedral intermediate. Proton transfer then occurs, breaking the carbon-nitrogen bond, thereby depolymerizing the residual material in the reactor to form a prepolymer of a specific molecular weight, which directly participates in the next batch of polymerization. Compared to traditional batch processes that require shutdown for cleaning, the process provided in this application does not require draining the cleaning solution, which not only improves product quality and production efficiency but also allows for the effective utilization of residual material in the reactor.

[0027] In the process provided in this application, the depolymerization aid is an organic monocarboxylic acid with a boiling point of 100℃~180℃. It can also be used as a capping agent to participate in the next batch of polymerization reaction. This not only reduces the types of aids used and saves the time of adding similar functional aids in the next batch of polymerization steps, improving production efficiency and stability, but also avoids poor synergy or antagonistic effects of multiple aids, which is conducive to further improving product quality.

[0028] In addition, since steam itself has a certain amount of heat, when steam is introduced into the reactor, it is also beneficial to controllably adjust the temperature difference change inside the reactor and achieve a more gradual temperature difference transition. This effectively reduces the stress on the reactor body and the impact of amine loss when the next batch of salt solution enters the reactor, thus solving the problem of easy damage to the reactor in the traditional batch process for producing polyamide.

[0029] Therefore, the semi-continuous polyamide production process provided in this application can effectively solve many problems such as yellowing of product color, increased black spots, cross-linking gelation, and damage to the autoclave after multiple production processes through a specific online depolymerization step, thereby improving product quality and production efficiency, and thus improving the situation of filament breakage and difficulty in traction during polyamide spinning production.

[0030] It should be noted that conventional depolymerization aids are not necessarily suitable as end-capping agents, and vice versa. Through long-term and in-depth research on depolymerization aids and end-capping agents, organic monocarboxylic acids with boiling points of 100℃~180℃ have been selected as depolymerization aids and end-capping agents for subsequent polymerization reactions, preventing cross-linking and excessively high molecular weights. Choosing organic monocarboxylic acids with boiling points of 100℃~180℃ not only ensures complete vaporization of the depolymerization aid, facilitating the depolymerization of residual materials to form prepolymers with molecular weights of 2000~4000, but also avoids excessively high temperatures in the water vapor containing the depolymerization aid, which helps regulate the temperature difference within the reactor and reduces the risk of damage to the reactor.

[0031] It is understandable that organic monocarboxylic acids with boiling points of 100℃ to 180℃ are usually liquid at room temperature and have good compatibility with water-based solvents; the boiling points of organic monocarboxylic acids include, but are not limited to, any one of 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, and 180℃ or any range between two of them.

[0032] In one embodiment of this application, the depolymerization aid includes, but is not limited to, at least one of formic acid, acetic acid, propionic acid, and oxalic acid, preferably acetic acid.

[0033] It is understandable that formic acid has a boiling point of approximately 100.8℃, acetic acid approximately 117.9℃, propionic acid approximately 141.1℃, and oxalic acid approximately 150℃.

[0034] In one embodiment of this application, the mass concentration of the depolymerization aid in the water vapor containing the depolymerization aid includes, but is not limited to, any one of 0.1%, 0.5%, 1%, 5%, and 10%, or a range between any two, preferably 0.1% to 5%. By controlling the concentration of the depolymerization aid in the water vapor, it is beneficial to ensure that the residual material is fully depolymerized to form a prepolymer with a molecular weight of 2000 to 4000, enabling the subsequent polymerization and reuse of the prepolymer. Since the depolymerization aid also acts as a capping agent, excessive use should be avoided. Too high a concentration of the depolymerization aid will result in a lower degree of polymerization in the final product, thus affecting product quality, and will also corrode components such as the reactor, pipes, and seals.

[0035] It should be noted that using water as a dispersion carrier for the depolymerization aid has two advantages: firstly, it provides oxygen atoms with lone pairs of electrons, promoting polymer depolymerization; secondly, it helps reduce the concentration of acidic vapors, minimizing damage to equipment.

[0036] In one embodiment of this application, by adjusting the temperature inside the reactor after the discharge is 30°C to 60°C higher than the temperature of the water vapor containing the depolymerization aid, it is beneficial to further alleviate the temperature difference generated when the next batch of salt solution enters the reactor, reduce the stress of the reactor body and the impact of amine loss due to the temperature difference, and thus effectively solve the problem of easy damage to the reactor when producing polyamide in the traditional batch process.

[0037] It is understood that the temperature difference between the reactor temperature and the water vapor containing the depolymerization aid after the discharge is completed includes, but is not limited to, any one of 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃ or any range between the two.

[0038] In one embodiment of this application, the temperature of the water vapor containing the depolymerization aid is preferably 210°C to 260°C, including but not limited to any one of 210°C, 220°C, 230°C, 240°C, 250°C, and 260°C, or a range between any two, and more preferably 210°C to 240°C.

[0039] In one embodiment of this application, the pressure of the water vapor containing the depolymerization aid is preferably 1.5 MPaG to 4.5 MPaG, including but not limited to any one of 1.5 MPaG, 2 MPaG, 2.5 MPaG, 3 MPaG, 3.5 MPaG, 4 MPaG, and 4.5 MPaG, or a range between any two, and more preferably 2 MPaG to 3 MPaG.

[0040] By synergistically controlling the concentration, temperature, and pressure of the water vapor containing the depolymerization aid, it is beneficial to further ensure that the residual materials in the reactor can be fully converted into prepolymers with a molecular weight of 2000~4000 through online depolymerization, which can enable the polyamide products produced by this process to maintain a stable molecular weight, thereby improving product quality.

[0041] In one embodiment of this application, when water vapor containing depolymerization aid is introduced into the reactor, the orifice diameter of the vaporization nozzle is preferably 6mm to 14mm. This is beneficial for optimizing the flow rate of water vapor containing depolymerization aid, controlling the formation of stable and uniform droplets, and enabling it to fully contact the residual materials in the reactor, thereby ensuring the smooth progress of the online depolymerization process and improving production efficiency.

[0042] In one embodiment of this application, the online depolymerization time is preferably 5 min to 60 min, including but not limited to any one of 5 min, 10 min, 20 min, 40 min, and 60 min or any range between two, and more preferably 10 min to 30 min.

[0043] It is understood that this application does not impose restrictions on the polymerization process and control conditions, and conventional processes and preparation conditions can be used.

[0044] In one embodiment of this application, the polymerization process includes: firstly, prepolymerization is carried out at 1.5 MPaG~3.5 MPaG and 220℃~250℃, and then pressure relief polymerization is carried out simultaneously during the process of heating to 260℃~280℃. When the pressure drops to within 0.05 MPaG, final polymerization is carried out at 280℃~320℃ by vacuum exhaust or micro-positive pressure exhaust.

[0045] It should be noted that this application does not limit the apparatus for implementing a semi-continuous polyamide production process. Conventional equipment combinations can be used. For example, in polyamide production, an adipic acid suspension is first prepared using a high-speed shear pump, and then amine is introduced in stages through multiple hexamethylenediamine feed pipes to undergo a salt formation reaction, resulting in a salt solution with a mass fraction of 30% to 65%, which is then transferred to a salt solution storage tank for later use. The salt solution is then concentrated to a mass fraction of 70% to 95% using evaporators connected in series or parallel. Series evaporators are preferred to achieve cascaded utilization, thereby improving evaporation efficiency. Finally, the salt solution is transported through pipelines to multiple high-pressure reactors. Inside, it is preferred to use three or more parallel high-pressure reactors, maintaining the pressure at 1.5MPaG~3.5MPaG and gradually raising the temperature to 220℃~250℃ for high-pressure prepolymerization. Then, while raising the temperature to 260℃~280℃, depressurization polymerization is carried out simultaneously, and the pressure is reduced to below 0.05MPaG. Finally, at 280℃~320℃, final polymerization is carried out by vacuum exhaust or micro-positive pressure exhaust to obtain the polyamide product. After the high-pressure reactor discharge is completed, an aqueous solution containing depolymerization aid is transported through the pipeline of a steam cleaner, and the liquid is turned into steam through a high-pressure vaporization nozzle and introduced into the high-pressure reactor for online depolymerization.

[0046] It is understood that the apparatus used to implement the semi-continuous polyamide production process described in this application can be simply an addition of a steam cleaner to the reactor components of a traditional polyamide production plant, without the need for additional cleaning liquid discharge components. This achieves a comprehensive improvement in production efficiency and safety, as well as improved product quality, at a relatively low modification cost.

[0047] This application also provides a polyamide product obtained by the semi-continuous polyamide production process described above. In the semi-continuous polyamide production process provided in this application, online depolymerization is added between two batches of polyamide salt solution polymerization steps. Low-concentration water vapor containing organic monobasic acid with a specific boiling point is used to strictly control the molecular weight of the prepolymer formed by the depolymerization of residual materials to be between 2000 and 4000. This effectively controls the proportion of high molecular weight polymers in each batch of polyamide products and improves product quality. In particular, in the semi-continuous production of less than twenty batches in this application, the mass fraction of polymers with a molecular weight greater than 50,000 in the polyamide product is less than 7%, achieving high-quality production.

[0048] The following specific embodiments will further illustrate the semi-continuous polyamide production process and the polyamide product. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0049] Example 1

[0050] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Acetic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.7 MPaG, and the temperature was raised to 240°C for high-pressure prepolymerization. Then the temperature was raised to 270°C, and pressure relief polymerization was carried out during the process, with the pressure reduced to 0.03 MPaG. Finally, final polymerization was carried out at 285°C through a vacuum device. The total reaction time was 2 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0051] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with a diameter of 10mm, acetic acid vapor at a temperature of 225℃, a pressure of 2.5MPaG, and a mass concentration of 4.5% is introduced to depolymerize the residual material online for 20 minutes. Then, the temperature inside the reactor drops to 250℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃ before the next batch of polymerization reaction begins.

[0052] Example 2

[0053] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Acetic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.7 MPaG, and the temperature was raised to 240°C for high-pressure prepolymerization. Then the temperature was raised to 270°C, and pressure relief polymerization was carried out during the process, with the pressure reduced to 0.03 MPaG. Finally, final polymerization was carried out at 285°C through a vacuum device. The total reaction time was 2 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0054] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with a diameter of 10mm, acetic acid vapor at a temperature of 215℃, a pressure of 2.0MPaG, and a mass concentration of 0.1% is introduced to depolymerize the residual material online for 20 minutes. Then, the temperature inside the reactor drops to 245℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃ before the next batch of polymerization reaction begins.

[0055] Example 3

[0056] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Acetic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.7 MPaG, and the temperature was raised to 240°C for high-pressure prepolymerization. Then the temperature was raised to 270°C, and pressure relief polymerization was carried out during the process, with the pressure reduced to 0.03 MPaG. Finally, final polymerization was carried out at 285°C through a vacuum device. The total reaction time was 2 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0057] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with a diameter of 10mm, acetic acid vapor at a temperature of 245℃, a pressure of 3.5MPaG, and a mass concentration of 0.1% is introduced to depolymerize the residual material online for 20 minutes. Then, the temperature inside the reactor drops to 255℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃ before the next batch of polymerization reaction begins.

[0058] Example 4

[0059] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Formic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.75 MPaG, and the temperature was raised to 245°C for high-pressure prepolymerization. Then the temperature was raised to 275°C, and depressurization polymerization was carried out during the process, with the pressure reduced to 0.02 MPaG. Finally, final polymerization was carried out at 280°C through a vacuum device. The total reaction time was 2.2 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0060] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with an orifice diameter of 8mm, formic acid water vapor at a temperature of 215℃, a pressure of 1.95MPaG, and a mass concentration of 0.5% is introduced to depolymerize the residual material online for 20 minutes. Then, the temperature inside the reactor drops to 240℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃, and the next batch of polymerization reaction is carried out.

[0061] Example 5

[0062] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Propionic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.78 MPaG, and the temperature was raised to 248°C for high-pressure prepolymerization. Then the temperature was raised to 268°C, and depressurization polymerization was carried out during the process, with the pressure reduced to 0.03 MPaG. Finally, final polymerization was carried out at 282°C through a vacuum device. The total reaction time was 2.1 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0063] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with a diameter of 12mm, propionic acid water vapor at a temperature of 215℃, a pressure of 1.9MPaG, and a mass concentration of 0.5% is introduced to depolymerize the residual material online for 20 minutes. Then, the temperature inside the reactor drops to 240℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃, and the next batch of polymerization reaction is carried out.

[0064] Example 6

[0065] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Oxalic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.76 MPaG, and the temperature was raised to 245°C for high-pressure prepolymerization. Then the temperature was raised to 270°C, and depressurization polymerization was carried out during the process, with the pressure reduced to 0.02 MPaG. Finally, final polymerization was carried out at 285°C through a vacuum device. The total reaction time was 1.9 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0066] After the material is discharged, the temperature inside the reactor is 275℃. Using a vaporization nozzle with a diameter of 10mm, oxalic acid water vapor at a temperature of 220℃, a pressure of 2.2MPaG, and a mass concentration of 0.5% is introduced to depolymerize the residual material online for 30 minutes. Then, the temperature inside the reactor drops to 245℃, and the next batch of salt solution is introduced. The temperature inside the reactor drops to 200℃ before the next batch of polymerization reaction begins.

[0067] Comparative Example 1

[0068] Salt formation reaction was carried out in a brine storage tank to obtain a salt solution with a mass fraction of 60%. The salt solution was concentrated to 85% by an evaporator and then piped to three parallel high-pressure reactors. Acetic acid end-capping agent was added through an additive injector, allowing the three high-pressure reactors to carry out polymerization reactions simultaneously. The pressure was maintained at 1.7 MPaG, and the temperature was raised to 240°C for high-pressure prepolymerization. Then the temperature was raised to 270°C, and pressure relief polymerization was carried out during the process, with the pressure reduced to 0.03 MPaG. Finally, final polymerization was carried out at 285°C through a vacuum device. The total reaction time was 2 hours. Each reactor yielded approximately 400 kg of product, and each batch yielded approximately 1200 kg of product.

[0069] After the material is discharged, the temperature inside the reactor is 275°C. Then, the next batch of salt solution is introduced, and the temperature inside the reactor drops to 220°C. Acetic acid end-capping agent is then added through the additive injector to continue the next batch of polymerization reaction.

[0070] Comparative Example 2

[0071] The difference between Comparative Example 2 and Example 1 is that benzoic acid was used as a depolymerization aid. Benzoic acid, which is solid at room temperature, was dissolved in water to prepare an aqueous solution of the same mass concentration. After vaporization, online depolymerization was carried out under the same temperature and pressure conditions. However, benzoic acid is not the end-capping agent required in the production process. When introducing the next batch of salt solution, the cleaning liquid needs to be drained before the next batch of polymerization can proceed, and acetic acid end-capping agent needs to be added.

[0072] Comparative Example 3

[0073] The difference between Comparative Example 3 and Example 1 is that boric acid was used as a depolymerization aid. Boric acid, which is solid at room temperature, was dissolved in water to prepare an aqueous solution of the same mass concentration. After vaporization, online depolymerization was carried out under the same temperature and pressure conditions. However, boric acid has no end-capping effect. When introducing the next batch of salt solution, the washing liquid needs to be drained before the next batch of polymerization can proceed, and acetic acid end-capping agent needs to be added.

[0074] Comparative Example 4

[0075] The difference between Comparative Example 4 and Example 1 is that phosphoric acid was used as a depolymerization aid. Phosphoric acid, which is solid at room temperature, was dissolved in water to prepare an aqueous solution of the same mass concentration. After vaporization, online depolymerization was carried out under the same temperature and pressure conditions. However, phosphoric acid has no end-capping effect; when introducing the next batch of salt solution, the washing liquid needs to be drained before the next batch of polymerization can proceed, and acetic acid end-capping agent needs to be added.

[0076] Comparative Example 5

[0077] The difference between Comparative Example 5 and Example 1 is that the mass concentration of acetic acid in the acetic acid water vapor is 11%.

[0078] Comparative Example 6

[0079] The difference between Comparative Example 6 and Example 1 is that the mass concentration of acetic acid in the acetic acid water vapor is 0.01%.

[0080] The molecular weight of the prepolymers formed during the preparation process of Examples 1 to 6 and Comparative Examples 2 to 6 was determined, and the specific results are shown in Table 1.

[0081] Table 1

[0082]

[0083] The color, black spots, amine content, chemical oxygen demand (COD), and molecular weight distribution (PD) of the first, tenth, and twentieth batches of products prepared in all examples and comparative examples were tested. Relevant data during downstream spinning were also tested. The specific results are shown in Tables 2, 3, 4, and 5.

[0084] Table 2

[0085]

[0086] Table 3

[0087]

[0088] Table 4

[0089]

[0090] Table 5

[0091]

[0092] As can be seen from Tables 2, 3, 4 and 5, depolymerization of residual materials in the reactor under appropriate conditions using depolymerization aids can significantly improve production stability and resin product quality, and perform excellently in downstream spinning processes, reducing the occurrence of fiber breakage, fiber drift and pore blockage.

[0093] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0094] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A process for the semi-continuous production of a polyamide, characterized in that, Includes the following steps: A batch of polyamide salt solution is added to a reactor for polymerization. The polymerization process includes: first, prepolymerization is carried out at 1.5 MPaG~3.5 MPaG and 220℃~250℃; then, during the process of heating to 260℃~280℃, pressure-relief polymerization is carried out simultaneously; when the pressure drops to below 0.05 MPaG, final polymerization is carried out at 280℃~320℃ by vacuum exhaust or micro-positive pressure exhaust; after the material is discharged, water vapor containing depolymerization aid is introduced into the reactor to depolymerize the residual material in the reactor online, so that the residual material depolymerizes to form a prepolymer with a molecular weight of 2000~4000. The mass concentration of the depolymerization aid in the water vapor containing the depolymerization aid is 0.1%~10%, and the depolymerization aid is selected from at least one of formic acid, acetic acid, propionic acid, and oxalic acid, and the boiling point of the depolymerization aid is 100℃~180℃. Then, the next batch of polyamide salt solution is added directly to the reactor, and the polymerization and online depolymerization are carried out in a cycle.

2. The process for the semi-continuous production of polyamide according to claim 1, characterized in that, The water vapor containing the depolymerization aid has a mass concentration of 0.1% to 5%.

3. The process for the semi-continuous production of polyamide according to claim 1, characterized in that, After the material is discharged, the temperature inside the reactor is 30°C to 60°C higher than the temperature of the water vapor containing the depolymerization aid.

4. The semi-continuous polyamide production process according to claim 1 or claim 3, characterized in that, The temperature of the water vapor containing the depolymerization aid is 210℃~260℃.

5. The semi-continuous polyamide production process according to claim 1, characterized in that, The pressure of the water vapor containing the depolymerization aid is 1.5 MPaG to 4.5 MPaG.

6. The semi-continuous polyamide production process according to claim 1, characterized in that, When water vapor containing depolymerization aid is introduced into the reactor, the orifice diameter of the vaporization nozzle is 6mm~14mm.

7. The semi-continuous polyamide production process according to claim 1, characterized in that, The online depolymerization time is 5 to 60 minutes.

8. A polyamide product obtained by a semi-continuous polyamide production process according to any one of claims 1 to 7, characterized in that, Within twenty batches of semi-continuous production, the mass fraction of polymers with a molecular weight greater than 50,000 in the polyamide product is less than 7%.

Images