Preparation method of bio-based nylon 4T salt

Abstract

The invention provides a preparation method of bio-based nylon 4T salt, and relates to the technical field of nylon synthesis. The preparation method comprises the following steps: mixing 1, 4-butanediamine with an alcohol-containing aqueous solution to form a reaction base solution, and then adding terephthalic acid in batches for neutralization reaction to generate a soluble nylon 4T salt intermediate product; and adding a precipitant to carry out alcohol precipitation, crystallizing and precipitating the product, and carrying out solid-liquid separation and washing to obtain the nylon 4T salt. According to the preparation method, the caking phenomenon caused by local supersaturation and instantaneous heat release is effectively avoided through batch charging, the homogeneous phase stability of a reaction system is maintained, and the controllability and safety of the process are improved; meanwhile, crystals obtained through alcohol precipitation crystallization are uniform in grain size and easy to separate, and a high-purity and low-moisture bio-based nylon 4T salt product can be obtained in cooperation with mild drying conditions.

Description

Technical Field

[0001] This invention relates to the field of nylon synthesis technology, and in particular to a method for preparing a bio-based nylon 4T salt. Background Technology

[0002] Nylon 4T (PA4T) is a high-performance semi-aromatic polyamide material produced by polycondensation reaction of 1,4-butanediamine (DBA) and terephthalic acid (PTA). Due to its high crystallinity, excellent high-temperature resistance, mechanical strength, ultra-low water absorption, good dimensional stability, and lead-free solderability, it has been widely used in the electronics, automotive, and high-end industrial fields.

[0003] Bio-based nylon 4T refers to its key monomer, 1,4-butanediamine, which originates from biomass fermentation rather than a complete petroleum-based production chain, aligning with green and sustainable development principles. However, PA4T's melting point is higher than its decomposition temperature, making direct melt processing impossible. Therefore, current commercial products often use it by copolymerizing with PA66 and / or PA6 to lower the melting point, enabling practical applications. Industrially, PA4T typically employs a three-stage production process: salt formation, prepolymerization, and final polymerization. Nylon 4T salt, as a polycondensation precursor, ensures that DBA and PTA participate in the reaction in an equimolar ratio, reducing side reactions and facilitating polymerization process control and product quality stability. Furthermore, compared to the volatile and somewhat toxic 1,4-butanediamine, nylon 4T salt is easier to store and transport, offering higher safety and significantly reducing storage and logistics costs.

[0004] Currently, the synthesis of nylon salts mainly employs aqueous neutralization, organic solvent, or solvent-free methods. A common process involves neutralizing a diacid with a diamine in an equimolar ratio in an aqueous or organic phase, followed by crystallization, filtration, and drying to obtain high-purity nylon salt crystals. Specific methods include aqueous solution-crystallization, organic solvent-crystallization, and solvent-free salt formation.

[0005] For example, CN118496104A discloses a green preparation method for bio-based semi-aromatic nylon 5X salt. This method involves adding a bio-based pentanediamine aqueous solution dropwise to an aromatic diacid suspension under inert gas protection, with enhanced mass transfer via ultrasound and / or microwave. After cooling and crystallization, seed crystals are added for rinsing and filtration, and finally, the solution is dried to obtain refined salt. CN113698288A discloses a method and apparatus for the continuous preparation of powdered nylon salt, using spray drying technology to rapidly evaporate the solvent from a pH-adjusted nylon salt solution, directly obtaining the powdered product. CN101456804B proposes a neutralization reaction in organic solvents such as N-methylpyrrolidone (NMP) or dimethylformamide (DMF), followed by centrifugation and washing of the precipitate to obtain the semi-aromatic nylon salt. CN115725072A discloses a method for preparing nylon dry salt, in which a diacid and a diamine are directly fed into a twin-screw extruder, mixed and reacted under solvent-free conditions, and then extruded to obtain nylon dry salt with a water content of less than 20 wt%.

[0006] However, the above methods have significant limitations when applied to the synthesis of nylon 4T salt. First, terephthalic acid is poorly soluble in water at room temperature and pressure, with a solubility of only 1.01 g / 100 g in methanol. Adding 1,4-butanediamine solution dropwise to the PTA suspension results in poor mass transfer, and the generated nylon salt may encapsulate unreacted PTA, affecting product purity. Second, while using organic solvents such as NMP or DMF can improve solubility, subsequent solvent recovery is energy-intensive, increasing production costs and posing environmental pollution risks. Third, when using cooling or evaporation crystallization for purification, the solubility of nylon 4T salt in water changes only slightly with temperature (from 60℃ to 5℃, its solubility decreases only from 59.32 g / 100 g water to 41.18 g / 100 g water), resulting in limited crystal precipitation and high energy consumption. Finally, although the solvent-free salt formation method eliminates the crystallization process and is easy to operate, the volatility of 1,4-butanediamine makes it difficult to accurately control the raw material ratio, which can easily lead to low purity of nylon salts. At the same time, its volatility and toxicity also increase the safety hazards to the health of operators and the environment.

[0007] In summary, existing nylon salt preparation processes struggle to simultaneously meet multiple requirements, including reaction efficiency, product purity, process safety, and environmental friendliness, when used for the synthesis of bio-based nylon 4T salts. Therefore, there is an urgent need to develop a green synthesis process that is efficient, economical, environmentally friendly, and capable of stably producing high-yield, high-purity nylon 4T salts.

[0008] In view of this, the present invention is hereby proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing bio-based nylon 4T salt, which has the technical advantages of simple process flow, no need for high temperature and high pressure equipment, and suitability for large-scale production.

[0010] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: This invention provides a method for preparing bio-based nylon 4T salt, the method comprising the following steps: (A) 1,4-Butanediamine is mixed with an aqueous solution containing alcohol to form a reaction base solution. Terephthalic acid is added in batches to the reaction base solution to carry out a neutralization reaction, generating a soluble nylon 4T salt intermediate product. (B) Add a precipitant to the intermediate product solution of nylon 4T salt to perform alcohol precipitation, so that nylon 4T salt crystallizes out. Separate the precipitated crystals from the solid and liquid and wash them to obtain nylon 4T salt.

[0011] Furthermore, the alcohol in the alcohol-containing aqueous solution has a mass fraction of 30% to 40% wt, and its mass is 7 to 7.5 times that of 1,4-butanediamine. Preferably, when adding terephthalic acid to the reaction base liquid in batches, the amount of terephthalic acid added in each batch is 10-15% of the total amount of terephthalic acid, and the next batch is added only after the reaction system has become clear after each batch is added. Preferably, the temperature of the neutralization reaction is controlled at 40~60℃, and the pH value at the end of the reaction is 7.2~7.5.

[0012] Furthermore, the precipitant is selected from one or more of methanol, ethanol, isopropanol, or n-butanol.

[0013] Preferably, when adding the precipitant, the precipitant is introduced into the bottom of the solution in the form of a gas-liquid mixture by a nitrogen-blowing agitator to enhance mass transfer and maintain an inert atmosphere; Preferably, the reaction temperature for alcohol precipitation is 20~40℃ and the reaction time is 1.0~2.0 h.

[0014] Furthermore, the steps of solid-liquid separation and washing of the precipitated crystals include: After the alcohol precipitation is completed, the mixture is allowed to stand for 0.5 to 1.0 h to allow the solid and liquid to separate into layers. The supernatant is discharged, and the bottom suspension is sent to a rotary vacuum filter press for filtration. The filter cake is washed with alcohol 1 to 2 times to obtain nylon 4T salt.

[0015] Furthermore, the preparation method further includes: recovering the organic solvent components from the filtrate and / or washing liquid containing organic solvents generated during solid-liquid separation and washing of the crystals precipitated in step (B) via membrane separation, and returning at least a portion of the recovered organic solvents to the step of recycling in the alcohol precipitation and / or washing step.

[0016] Furthermore, the preparation method further includes: a step of vacuum drying the nylon 4T salt from step (B) to obtain refined nylon 4T salt; Preferably, the vacuum drying temperature is 40~60℃ and the time is 6~12 hours.

[0017] This invention provides a bio-based nylon 4T salt production system for implementing the above-described preparation method. The production system includes a neutralization reactor, an alcohol precipitation tank, a rotary vacuum filter, a vacuum drying oven, a primary membrane separation unit, a secondary membrane separation unit, an alcohol-water storage tank, and a recycled alcohol storage tank, wherein: A neutralization reactor is used to mix 1,4-butanediamine with an alcohol-containing aqueous solution and to accept the addition of terephthalic acid in batches for neutralization reaction; The alcohol precipitation tank is connected to the neutralization reactor via a pipeline and is used to receive the neutralized solution and add a precipitant to crystallize it out. A rotary vacuum filter is connected to the bottom outlet of the alcohol precipitation tank for filtering and washing the precipitated crystals with alcohol. The nylon 4T salt obtained from the discharge end of the rotary vacuum filter is manually transferred to a vacuum drying oven for vacuum drying. The primary membrane separation unit and the secondary membrane separation unit are respectively connected to the filtrate outlet and the alcohol washing liquid outlet of the rotary vacuum filter, and are used for the stepwise separation of liquids containing organic solvents; The alcohol-water storage tank is connected to the permeate outlet of the primary membrane separation unit and is used to store a dilute alcohol-water solution that can be reused to prepare the reaction base solution. The recycled alcohol storage tank is connected to the concentrate outlet of the secondary membrane separation unit to store high-purity recovered alcohol and supply it to the alcohol precipitation tank and / or rotary vacuum filter for recycling.

[0018] Furthermore, the alcohol precipitation tank is equipped with a nitrogen-blowing agitator, which introduces alcohol precipitant from the recycled alcohol storage tank or externally supplied in the form of a gas-liquid mixture through a small hole at the bottom of the hollow shaft. Preferably, the alcohol precipitation tank is provided with a U-shaped tube on the outside, one end of which is connected to the upper part of the side wall of the tank, and the other end is provided with multiple discharge outlets with shut-off valves distributed along the height direction, for selectively discharging the supernatant after settling. Preferably, the production system further includes a fresh alcohol storage tank, which is connected to the inlet of the alcohol precipitation tank for replenishing the precipitant.

[0019] Furthermore, the neutralization reactor is equipped with a rotary valve and a weighing module for controlling the batch addition of terephthalic acid.

[0020] Furthermore, a transfer pump is provided between the alcohol precipitation tank and the rotary vacuum filter for transferring high-concentration suspension; Preferably, the conveying pump is a screw pump, which is suitable for conveying thick slurries with a solid content of 30~40wt%.

[0021] Furthermore, the neutralization reactor is equipped with a rotary valve and a weighing module for controlling the batch addition of terephthalic acid.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for preparing bio-based nylon 4T salt. The method involves pre-mixing 1,4-butanediamine with an alcohol-containing aqueous solution to form a reaction substrate, and then adding terephthalic acid in batches for neutralization. This effectively avoids the localized supersaturation and instantaneous exothermic concentration caused by a single addition, maintaining the homogeneous stability of the reaction system and preventing premature product precipitation or agglomeration, thus improving the controllability and safety of the reaction process. Simultaneously, the nylon 4T salt generated in this application exists in the solution as a soluble intermediate, providing a uniform precursor system for subsequent crystallization steps. Furthermore, after the neutralization reaction, this application uses a precipitant to perform alcohol precipitation, allowing the nylon 4T salt to selectively crystallize. The resulting crystals have a relatively uniform particle size and are easy to filter and separate, complementing the solid-liquid separation and washing processes. Verification shows that this method can obtain nylon 4T salt products with high purity and low water content.

[0023] The present invention provides a bio-based nylon 4T salt production system, which integrates key unit equipment such as neutralization reaction, alcohol precipitation crystallization, solid-liquid separation, drying, and solvent recovery, realizing continuous and integrated operation of the entire process from raw material feeding to product refining. Specifically, the sequential connection between the neutralization reactor and the alcohol precipitation tank ensures the stable transfer of the neutralization reaction solution to the crystallization stage; the direct connection between the rotary vacuum filter and the front-end alcohol precipitation tank reduces intermediate material exposure, which is beneficial for maintaining product stability.

[0024] In particular, by connecting the primary and secondary membrane separation units to the filtrate and washing liquid outlets respectively, the waste liquid containing organic solvents is separated in stages, enabling efficient recovery of alcohol components of different concentrations. Low-concentration permeate enters the alcohol-water storage tank and can be reused to prepare the reaction base solution, while high-purity concentrate is stored in the reuse alcohol storage tank and returned to the alcohol precipitation or washing process for recycling, significantly reducing solvent consumption and waste emissions. The system has a reasonable structural layout, clear material flow, and combines operational stability with resource utilization efficiency, making it suitable for large-scale green production of high-quality nylon 4T salt. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 The synthesis flow chart of the bio-based nylon 4T salt production system provided in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the structure of the nitrogen blowing agitator provided in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the structure of the U-shaped tube, the external connector of the alcohol precipitation tank, provided in Embodiment 1 of the present invention.

[0027] Icons: 1-Reaction vessel; 1a-Inlet of the first neutralization reaction vessel; 1b-Inlet of the second neutralization reaction vessel; 1c-Hopper; 1d-Rotary valve; 1e-Terephthalic acid inlet; 2-Alcohol precipitation tank; 2a-Inlet of nylon 4T salt intermediate product; 2b-Nitrogen blowing agitator; 2c-Nitrogen vent; 2d-Regulating valve; 2e-U-tube; 2f-Alcohol precipitation outlet; 2e1-First connection outlet; 2e2-Second connection outlet; 2e3-Third connection outlet; 3-Screw pump; 4-Vacuum filter; 5-Vacuum drying oven; 6-First-stage membrane separation unit; 7-Second-stage membrane separation unit; 8-Alcohol-water storage tank; 9-Fresh alcohol storage tank; 10-Recycled alcohol storage tank. Detailed Implementation

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

[0029] According to one aspect of the present invention, a method for preparing a bio-based nylon 4T salt, the method comprising the following steps: (A) 1,4-Butanediamine is mixed with an aqueous solution containing alcohol to form a reaction base solution. Terephthalic acid is added in batches to the reaction base solution to carry out a neutralization reaction, generating a soluble nylon 4T salt intermediate product. (B) Add a precipitant to the intermediate product solution of nylon 4T salt to perform alcohol precipitation, so that nylon 4T salt crystallizes out. Separate the precipitated crystals from the solid and liquid and wash them to obtain nylon 4T salt.

[0030] The present invention provides a method for preparing bio-based nylon 4T salt. This method involves pre-mixing 1,4-butanediamine with an alcohol-containing aqueous solution to form a reaction substrate, and then adding terephthalic acid in batches for neutralization reaction. This method can effectively avoid the phenomenon of local supersaturation and instantaneous exothermic concentration caused by one-time addition, maintain the homogeneous and stable state of the reaction system, thereby preventing premature precipitation or agglomeration of products and improving the controllability and safety of the reaction process.

[0031] Meanwhile, the nylon 4T salt generated in this application exists in solution as a soluble intermediate, providing a uniform precursor system for subsequent crystallization steps. Furthermore, after the neutralization reaction, this application involves adding a precipitant to the solution for alcohol precipitation, allowing the nylon 4T salt to selectively crystallize out. The resulting crystals have a relatively uniform particle size and are easy to filter and separate, complementing the solid-liquid separation and washing processes. Verification has shown that the preparation method of this application can obtain nylon 4T salt products with high purity and low water content. Moreover, the method has a simple process flow, mild operating conditions, and does not require high-temperature or high-pressure equipment, making it suitable for large-scale production.

[0032] Therefore, this invention breaks through the traditional reaction path of "acid dissolved in water followed by amine dropwise addition," proposing a reverse control strategy of first constructing a homogeneous reaction environment and then gradually introducing the acid component. Combined with a closed-loop system of alcohol precipitation crystallization and stepwise membrane recovery, it achieves systematic optimization from reaction kinetics to process engineering. The bio-based nylon 4T salt preparation method provided by this invention not only solves the problems of localized exothermics and agglomeration, but also constructs a novel bio-based nylon salt synthesis route that combines safety, efficiency, and sustainability.

[0033] Note: In this application, bio-based nylon 4T refers to the key monomer 1,4-butanediamine, which is derived from the biomass fermentation process.

[0034] In a preferred embodiment of the present invention, the alcohol in the alcohol-containing aqueous solution has a mass fraction of 30% to 40%, and its mass is 7 to 7.5 times that of 1,4-butanediamine. As an optional implementation, the alcohol-containing aqueous solution has an alcohol mass fraction of 30% to 40%, for example, it can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%, or any value between 30% and 40%; and the mass of the alcohol-containing aqueous solution is 7 to 7.5 times the mass of 1,4-butanediamine, for example, it can be 7.0, 7.1, 7.2, 7.3, 7.4, or 7.5 times, or any value between 7 and 7.5 times.

[0035] In a preferred embodiment of the present invention, when adding terephthalic acid to the reaction base liquid in batches, the amount of terephthalic acid added in each batch is 10% to 15% of the total amount of terephthalic acid, and the next batch is added only after the reaction system has been clarified after each batch is added. As an optional implementation method, the amount of terephthalic acid added in each batch is 10% to 15% of the total amount of terephthalic acid, for example, it can be 10%, 11%, 12%, 13%, 14% or 15%, or any value between 10% and 15%; and after each batch is added, the reaction system is allowed to become clear before the next batch is added.

[0036] It should be noted that this application, by controlling the mass fraction of alcohol in the alcohol-containing aqueous solution to 30-40% and setting its amount to 7-7.5 times the mass of 1,4-butanediamine, can ensure the full dissolution of 1,4-butanediamine while adjusting the polarity and dielectric constant of the reaction system. This facilitates the gradual and uniform participation of terephthalic acid in the neutralization reaction, avoiding local precipitation or reaction lag caused by unsuitable solvent polarity, thereby maintaining the clarity and stability of the reaction solution. Under these solvent conditions, by adding 10-15% of the total amount of terephthalic acid in each batch, and allowing the reaction system to return to a clear state before adding the next batch, a gradual equilibrium of the acid-base reaction can be achieved, effectively mitigating concentrated exothermic reactions and drastic pH fluctuations, and improving the controllability and safety of the reaction process.

[0037] In a preferred embodiment of the present invention, the temperature of the neutralization reaction is controlled at 40~60°C, and the pH value at the reaction endpoint is 7.2~7.5.

[0038] In a preferred embodiment, the neutralization reaction temperature is controlled within the range of 40~60℃, which can accelerate the reaction rate and avoid the volatilization of components or side reactions caused by excessively high temperature. When the pH value at the reaction endpoint reaches 7.2~7.5, it indicates that the neutralization reaction is close to complete, and the obtained nylon 4T salt intermediate product remains in a stable dissolved state in the solution, laying a good foundation for subsequent efficient crystallization.

[0039] As an optional implementation, the temperature of the neutralization reaction is controlled at 40~60℃, for example, it can be 40℃, 45℃, 50℃, 55℃ or 60℃, or any value between 40~60℃; the pH value at the reaction endpoint is 7.2~7.5, for example, it can be 7.2, 7.3, 7.4 or 7.5, or any value between 7.2~7.5.

[0040] In a preferred embodiment of the present invention, the precipitant is selected from one or more of methanol, ethanol, isopropanol or n-butanol.

[0041] Preferably, when adding the precipitant, the precipitant is introduced into the bottom of the solution in the form of a gas-liquid mixture by a nitrogen-blowing agitator 2b to enhance mass transfer and maintain an inert atmosphere; As a preferred embodiment, this application selects one or more of methanol, ethanol, isopropanol, or n-butanol as the precipitant for alcohol precipitation, which can effectively reduce the solubility of nylon 4T salt in the reaction system and promote its uniform and controllable crystallization. These lower alcohols are miscible with water, have good dilution ability for the mother liquor, and are less likely to introduce impurity ions, which is beneficial for obtaining high-purity crystalline products. During the addition of the precipitant, the precipitant is introduced from the bottom of the solution in the form of a gas-liquid mixture using a nitrogen-blowing stirrer 2b. This allows for in-situ stirring while dispersing the precipitant, enhancing mass transfer efficiency. Furthermore, the nitrogen atmosphere maintains an inert atmosphere in the system, reducing the risk of oxidation of raw materials or products under high temperature or exposure conditions and improving product stability.

[0042] In a preferred embodiment of the present invention, the reaction temperature for alcohol precipitation is 20~40℃ and the reaction time is 1.0~2.0 h.

[0043] In a preferred embodiment, the temperature of the alcohol precipitation process is controlled at 20-40°C and the time is controlled at 1.0-2.0 hours. This ensures a moderate crystallization rate, avoids excessively fine crystals or inclusions caused by rapid precipitation, and facilitates the formation of crystals with more uniform particle size that are easy to filter and separate, thereby improving the efficiency of subsequent solid-liquid separation and product yield.

[0044] As an optional implementation, the reaction temperature of the alcohol precipitation is 20~40℃, for example, it can be 20℃, 25℃, 30℃, 35℃ or 40℃, or any value between 20~40℃; the reaction time is 1.0~2.0 h, for example, it can be 1.0 h, 1.5 h or 2.0 h, or any value between 1.0~2.0 h.

[0045] In a preferred embodiment of the present invention, the steps of solid-liquid separation and washing of the precipitated crystals include: after the alcohol precipitation is completed, the mixture is allowed to stand for 0.5 to 1.0 h to allow the solid and liquid to separate into layers, the upper clear liquid is discharged, the bottom suspension is sent to a rotary vacuum filter press for filtration, and the filter cake is washed with alcohol 1 to 2 times to obtain nylon 4T salt.

[0046] In a preferred embodiment, after alcohol precipitation, the crystallized slurry is allowed to settle naturally for 0.5 to 1.0 hours to achieve solid-liquid stratification. This effectively increases the solid concentration of the bottom suspension and reduces the liquid load in subsequent filtration processes. After draining the supernatant, the high-concentration bottom suspension is sent to a rotary vacuum filter press for filtration. This improves filtration efficiency, reduces the liquid content of the filter cake, and prevents fine crystals from being lost in the large amount of mother liquor, thereby increasing product yield.

[0047] In a preferred embodiment of the present invention, the preparation method further includes: recovering the organic solvent components in the filtrate and / or washing liquid containing organic solvent generated during solid-liquid separation and washing of the crystals precipitated in step (B) by membrane separation, and returning at least a portion of the recovered organic solvent to the step of recycling in the alcohol precipitation and / or washing step.

[0048] In a preferred embodiment, this application utilizes membrane separation technology to recover the organic solvent components in the filtrate and / or washing liquid containing organic solvents generated during the solid-liquid separation and washing process of precipitated crystals. This effectively separates the solvent from water, reducing the organic content in the waste liquid. At least a portion of the recovered organic solvent is recycled back to the alcohol precipitation or washing steps, reducing the consumption of fresh solvent, lowering production costs, alleviating the pressure on subsequent wastewater treatment, and enhancing the greenness and sustainability of the process. This solvent recovery and recycling method is closely integrated with the preparation process, is simple to operate, and is suitable for continuous industrial operation.

[0049] In a preferred embodiment of the present invention, the preparation method further includes: a step of vacuum drying the nylon 4T salt in step (B) to obtain refined nylon 4T salt, wherein the vacuum drying temperature is 40~60℃ and the time is 6~12 hours.

[0050] As an optional implementation, the vacuum drying temperature is 40~60℃, for example, it can be 40℃, 45℃, 50℃, 55℃ or 60℃, or any value between 40~60℃; the drying time is 6~12 hours, for example, it can be 6 hours, 8 hours, 10 hours or 12 hours, or any value between 6~12 hours.

[0051] Preferably, the green preparation method of the bio-based nylon 4T salt described in this application includes the following steps: (a) Mix 1,4-butanediamine with an aqueous alcohol solution containing 30%-40%wt of alcohol to form a reaction base solution; (b) Add terephthalic acid powder in batches in a molar ratio with 1,4-butanediamine, and control the reaction temperature at 40~60℃ until the solution is clear and the pH value reaches 7.2~7.5 to obtain nylon 4T salt solution; (c) Transfer the obtained nylon 4T salt solution to alcohol precipitation tank 2, add alcohol for alcohol precipitation, the reaction temperature is 20~40℃, the time is 1.0~2.0 h, so that nylon 4T salt crystallizes out; (d) After the alcohol precipitation is completed, let it stand for 0.5~1.0h to allow the solid and liquid to separate into layers. Discard the upper clear liquid and send the bottom suspension into a rotary vacuum filter press for filtration. Wash the filter cake with alcohol 1~2 times to obtain nylon 4T salt. (e) The nylon 4T salt is vacuum dried at 40~60℃ for 6~12 h to obtain refined nylon 4T salt; (f) The alcohol-containing filtrate and / or alcohol washing liquid generated in step d are sent to the primary membrane separation unit 6 to obtain an alcohol-water solution with a concentration of 30%-40%wt, part of which is recycled to step a; the remaining liquid is further processed through the secondary membrane separation unit 7 to obtain an alcohol solution with a purity of ≥95%, which is recycled to steps c and d.

[0052] If described in terms of preparation stages, the green preparation method of the bio-based nylon 4T salt of this application can be summarized as follows: A. Neutralization Section: First, an aqueous alcohol solution and 1,4-butanediamine are added sequentially to neutralization reactor 1. The aqueous alcohol solution contains 30%-40%wt alcohol, preferably 30%-35%wt, and its mass is 7-7.5 times, preferably 7-7.2 times, the mass of 1,4-butanediamine. Using the aqueous alcohol solution as a base is to pre-saturate the aqueous phase, reducing the amount of alcohol added in the subsequent alcohol precipitation stage and allowing the nylon salt formed in the solution to precipitate out more quickly. The initial temperature of reactor 1 is set not to exceed 40℃, and the stirring speed is 200-300 rpm to ensure uniform mixing.

[0053] Next, terephthalic acid was added to neutralization reactor 1 in batches, in an equimolar ratio to 1,4-butanediamine. Each batch consisted of 1 / 10 to 1 / 15 of the total amount of terephthalic acid. After each batch of terephthalic acid was added, the solution was allowed to clarify before adding the next batch. The purpose of adding the terephthalic acid in batches was to prevent the neutralization reaction from releasing a large amount of heat, which could lead to the cyclization of unreacted butanediamine. The reaction temperature was controlled at 40-60℃ during this process, also to prevent the cyclization of butanediamine.

[0054] The endpoint of the neutralization reaction can be identified by the clarity of the solution or by detecting the pH value at the endpoint, which is 7.2-7.5.

[0055] B. Alcohol precipitation section: The obtained nylon 4T salt solution was transferred to alcohol precipitation tank 2, and alcohol was added for alcohol precipitation. The reaction temperature was set at 20-40℃, and the reaction time was about 1.0-2.0h.

[0056] The amount of alcohol added is 2.1-2.7 times, preferably 2.2-2.6 times, the initial mass of the alcohol-water solution. The added alcohol can be one or more of methanol, ethanol, isopropanol, and n-butanol. To enhance the mass transfer between the alcohol and the nylon 4T salt solution, nitrogen gas is used as a carrier to transport the alcohol to the alcohol precipitation tank 2. Uniform mixing with the existing nylon 4T salt solution in a gas-liquid mixture state reduces the crystallization time.

[0057] C. Alcohol washing and filtration section: After the alcohol precipitation reaction is complete and no more crystals precipitate in the vessel, let the suspension stand for 0.5-1.0 hours to allow the solids to settle to the bottom of the tank. The excess clear liquid on the upper layer is then transported to the membrane separation section through the side outlet.

[0058] The remaining suspension at the bottom of the tank is then transported to a rotary vacuum filter press for filtration to obtain nylon 4T salt. The filtrate can then be separated by membrane separation to obtain reusable alcohol. To facilitate transport by screw pump 3, the solid content of the remaining suspension is 30%-40%wt.

[0059] Next, the nylon 4T salt is washed with alcohol 1-2 times in a rotary vacuum filter press. The amount of alcohol used is 0.8-2.0 times the mass of the nylon 4T crude salt. The residual impurities are removed by filtration. The resulting solid enters the drying section, and the filtrate can be separated by membrane to obtain reusable alcohol solution.

[0060] D. Drying Section: Finally, after ethanol washing, the Nylon 4T salt is dried in a vacuum oven at 40-60℃ for 6-12 hours to obtain refined Nylon 4T salt.

[0061] E. Membrane separation section: The resulting alcohol-containing filtrate and washing liquid are separated into a 30%-40% wt alcohol-water solution by a primary membrane separation unit 6. A portion of this solution can be reused in the neutralization process, while the remaining portion is further separated by a secondary membrane separation unit 7 to obtain an alcohol solution with a purity of over 95%, which can then be reused in the alcohol precipitation and alcohol washing filtration processes. The membranes used in this separation unit are hydrophobic, such as polydimethylsiloxane (PDMS) membranes. This allows for mother liquor reuse, reduces alcohol consumption, lowers subsequent wastewater treatment costs, and aligns with the "green production" concept.

[0062] According to one aspect of the present invention, a bio-based nylon 4T salt production system for carrying out the above-described preparation method is provided, the production system comprising a neutralization reactor 1, an alcohol precipitation tank 2, a rotary vacuum filter 4, a vacuum drying oven 5, a primary membrane separation device 6, a secondary membrane separation device 7, an alcohol-water storage tank 8, and a recycled alcohol storage tank 10, wherein: Neutralization reactor 1 is used to mix 1,4-butanediamine with an alcohol-containing aqueous solution and to accept the addition of terephthalic acid in batches for neutralization reaction; The alcohol precipitation tank 2 is connected to the neutralization reactor 1 via a pipeline and is used to receive the neutralized solution and add a precipitant for crystallization. The rotary vacuum filter 4 is connected to the bottom outlet of the alcohol precipitation tank 2 and is used to filter and wash the precipitated crystals with alcohol. The vacuum drying oven 5 is connected to the discharge end of the rotary vacuum filter 4 and is used to dry the obtained nylon 4T salt. The primary membrane separation device 6 and the secondary membrane separation device 7 are respectively connected to the filtrate outlet and the alcohol washing liquid outlet of the rotary vacuum filter 4, and are used for the stepwise separation of liquids containing organic solvents. The alcohol-water storage tank 8 is connected to the permeate outlet of the primary membrane separation device 6 and is used to store a dilute alcohol-water solution that can be reused to prepare the reaction base solution. The recycled alcohol storage tank 10 is connected to the concentrate outlet of the secondary membrane separation unit 7 to store high-purity recovered alcohol and supply it to the alcohol precipitation tank 2 and / or the rotary vacuum filter 4 for recycling.

[0063] The present invention provides a bio-based nylon 4T salt production system, which integrates key unit equipment such as neutralization reaction, alcohol precipitation crystallization, solid-liquid separation, drying, and solvent recovery, realizing continuous and integrated operation from raw material feeding to product refining. Specifically, the sequential connection of the neutralization reaction vessel 1 and the alcohol precipitation tank 2 ensures the stable transfer of the neutralization reaction solution to the crystallization stage.

[0064] In particular, by connecting the primary membrane separation unit 6 and the secondary membrane separation unit 7 to the filtrate and washing liquid outlets respectively, the waste liquid containing organic solvents is separated in stages, enabling efficient recovery of alcohol components of different concentrations. The low-concentration permeate enters the alcohol-water storage tank 8 and can be reused to prepare the reaction base solution, while the high-purity concentrate is stored in the reuse alcohol storage tank 10 and returned to the alcohol precipitation or washing process for recycling, significantly reducing solvent consumption and waste emissions. This system has a reasonable structural layout, clear material flow, and combines operational stability with resource utilization efficiency, making it suitable for large-scale green production of high-quality nylon 4T salt.

[0065] In a preferred embodiment of the present invention, the alcohol precipitation tank 2 is equipped with a nitrogen blowing agitator 2b, which introduces alcohol precipitant from the recycled alcohol storage tank 10 or externally supplied material into the tank in the form of a gas-liquid mixture through a small hole at the bottom of the hollow shaft. In a preferred embodiment, this application includes a nitrogen-blowing agitator 2b in the alcohol precipitation tank 2. This agitator allows alcohol precipitants from the recycled alcohol storage tank 10 or externally supplied materials to be introduced into the reaction system as a gas-liquid mixture through small holes at the bottom of the hollow shaft. This structure achieves uniform dispersion and efficient mixing simultaneously with the addition of the precipitant, enhancing mass transfer and preventing severe precipitation caused by localized overconcentration. Furthermore, the continuous introduction of nitrogen maintains an inert atmosphere within the tank, reducing the risk of oxidation of raw materials or intermediate products during crystallization and improving product stability.

[0066] In a preferred embodiment of the present invention, the alcohol precipitation tank 2 is provided with a U-shaped tube 2e on the outside, one end of which is connected to the upper part of the side wall of the tank body, and the other end is provided with a plurality of liquid discharge outlets with shut-off valves distributed along the height direction, for selectively discharging the upper clear liquid after static settling. As a preferred embodiment, the alcohol precipitation tank 2 is equipped with a U-shaped tube 2e, one end of which is connected to the upper part of the side wall of the tank, and the other end is provided with multiple discharge outlets distributed along the height direction and equipped with shut-off valves. After static settling, the appropriate discharge point can be selected according to the liquid-solid stratification position to achieve precise discharge of the upper clear liquid, reduce product loss, and improve yield.

[0067] In a preferred embodiment of the present invention, the production system further includes a fresh alcohol storage tank 9, which is connected to the inlet of the alcohol precipitation tank 2 for replenishing the precipitant.

[0068] In a preferred embodiment, the fresh alcohol storage tank 9 can replenish the alcohol precipitation tank 2 with fresh precipitant, ensuring the continuity of precipitant supply and process stability, especially suitable for situations where alcohol recovery is temporarily insufficient or solvent purity needs to be adjusted. The synergistic effect of these structures improves the operational controllability, separation efficiency, and system adaptability of the alcohol precipitation process.

[0069] In a preferred embodiment of the present invention, the neutralization reactor 1 is equipped with a rotary valve 1d and a weighing module for controlling the batch addition amount of terephthalic acid.

[0070] In a preferred embodiment of the present invention, a transfer pump is provided between the alcohol precipitation tank 2 and the rotary vacuum filter 4 for transferring high-concentration suspension; Preferably, the conveying pump is a screw pump 3, which is suitable for conveying thick slurry with a solid content of 30~40wt%.

[0071] The technical solution of the present invention will be further described below with reference to the embodiments.

[0072] Example 1 Figure 1 The synthesis flow chart of the bio-based nylon 4T salt production system provided in Embodiment 1 of the present invention.

[0073] See Figure 1 A bio-based nylon 4T salt production system, comprising a neutralization reactor 1, an alcohol precipitation tank 2, a rotary vacuum filter 4, a vacuum drying oven 5, a primary membrane separation unit 6, a secondary membrane separation unit 7, an alcohol-water storage tank 8, and a recycled alcohol storage tank 10, wherein: Neutralization reactor 1 is used to mix 1,4-butanediamine with an alcohol-containing aqueous solution and to accept the addition of terephthalic acid in batches for neutralization reaction; The alcohol precipitation tank 2 is connected to the neutralization reactor 1 via a pipeline and is used to receive the neutralized solution and add a precipitant for crystallization. The rotary vacuum filter 4 is connected to the bottom outlet of the alcohol precipitation tank 2 and is used to filter and wash the precipitated crystals with alcohol. The vacuum drying oven 5 is connected to the discharge end of the rotary vacuum filter 4 and is used to dry the obtained nylon 4T salt. The primary membrane separation device 6 and the secondary membrane separation device 7 are respectively connected to the filtrate outlet and the alcohol washing liquid outlet of the rotary vacuum filter 4, and are used for the stepwise separation of liquids containing organic solvents. The alcohol-water storage tank 8 is connected to the permeate outlet of the primary membrane separation device 6 and is used to store a dilute alcohol-water solution that can be reused to prepare the reaction base solution. The recycled alcohol storage tank 10 is connected to the concentrate outlet of the secondary membrane separation unit 7 to store high-purity recovered alcohol and supply it to the alcohol precipitation tank 2 and / or the rotary vacuum filter 4 for recycling.

[0074] The bio-based nylon 4T salt production system provided in this embodiment integrates key unit equipment such as neutralization reaction, alcohol precipitation crystallization, solid-liquid separation, drying, and solvent recovery, realizing continuous and integrated operation from raw material feeding to product refining. Specifically, the sequential connection between the neutralization reactor 1 and the alcohol precipitation tank 2 ensures the stable transfer of the neutralization reaction solution to the crystallization stage; the direct connection between the rotary vacuum filter 4 and the front-end alcohol precipitation tank 2 and the rear-end vacuum drying oven 5 reduces intermediate material exposure, which is beneficial for maintaining product stability. In particular, the primary membrane separation device 6 and the secondary membrane separation device 7 are connected to the filtrate and washing liquid outlets respectively, implementing cascade separation of waste liquid containing organic solvents, which can efficiently recover alcohol components of different concentrations: the low-concentration permeate enters the alcohol-water storage tank 8 and can be reused to prepare the reaction base solution, while the high-purity concentrate is stored in the reuse alcohol storage tank 10 and returned to the alcohol precipitation or washing process for recycling, significantly reducing solvent consumption and waste emissions. The system has a reasonable structure and layout, clear material flow, and combines operational stability with resource utilization efficiency, making it suitable for large-scale green production of high-quality nylon 4T salt.

[0075] As a preferred embodiment of this example, the alcohol precipitation tank 2 is equipped with a nitrogen blowing agitator 2b, which introduces alcohol precipitant from the recycled alcohol storage tank 10 or externally supplied material into the tank in the form of a gas-liquid mixture through a small hole at the bottom of the hollow shaft. Figure 2 This is a schematic diagram of the structure of the nitrogen blowing stirrer 2b provided in Embodiment 1 of the present invention.

[0076] The main structure of the nitrogen-blowing agitator 2b inside the alcohol precipitation tank 2 is a hollow shaft with a series of nitrogen blowing holes at the bottom, the holes having a diameter of 2-5mm, as shown in the details. Figure 2As shown. Its working principle is to use a high-speed nitrogen gas flow to form a bubble cluster at the bottom of the liquid, and rely on the turbulence generated by the rising bubbles to achieve mixing. It is particularly suitable for systems that are highly shear-sensitive, flammable, explosive, or require a strictly oxygen-free environment. Using this type of stirrer can avoid electrostatic sparks generated by mechanical shearing while providing an oxygen-free environment, effectively reducing the possibility of combustion and explosion of alcohols.

[0077] As a preferred embodiment of this example, the alcohol precipitation tank 2 is provided with a U-shaped tube 2e on the outside, one end of which is connected to the upper part of the side wall of the tank, and the other end is provided with multiple discharge outlets with shut-off valves distributed along the height direction, which are used to selectively discharge the upper clear liquid after static settling. Figure 3 This is a schematic diagram of the structure of the U-shaped tube 2e, the external connector of the alcohol precipitation tank 2 provided in Embodiment 1 of the present invention.

[0078] Figure 3 One end of the U-shaped tube 2e of the external connecting part of the alcohol precipitation tank 2 is connected to the regulating valve 2d, and the other end has three outlets connected to the shut-off valve from top to bottom (first connecting outlet 2e1, second connecting outlet 2e2, and third connecting outlet 2e3). Its main function is to indicate the solution level and the position of the supernatant after settling. At the same time, the three outlets connected to different positions can flexibly adjust the discharge rate of supernatant in the alcohol precipitation tank 2.

[0079] In a preferred embodiment of this invention, the production system further includes a fresh alcohol storage tank 9, which is connected to the inlet of the alcohol precipitation tank 2 and is used to replenish the precipitant.

[0080] In a preferred embodiment of this invention, the neutralization reactor 1 is equipped with a rotary valve 1d and a weighing module for controlling the batch addition of terephthalic acid.

[0081] In a preferred embodiment of this invention, a transfer pump is provided between the alcohol precipitation tank 2 and the rotary vacuum filter 4 for transferring high-concentration suspension; the transfer pump is a screw pump 3, which is suitable for conveying thick slurry with a solid content of 30~40wt%.

[0082] The working process of the bio-based nylon 4T salt production system described in this embodiment is as follows: (a) Preparation of the reaction base solution and batch addition for neutralization: 1,4-Butanediamine was mixed with an alcohol-containing aqueous solution and added to a reaction vessel to form the initial reaction base solution. Subsequently, terephthalic acid powder was added to the system in batches, and a neutralization reaction was carried out under stirring. After each batch was added, the reaction system was allowed to return to a clear state before adding the next batch to avoid localized supersaturation and instantaneous exothermic reactions that could lead to agglomeration or side reactions. The entire reaction was carried out at 40–60°C until all raw materials were added and the reaction was complete, yielding a clear, soluble nylon 4T salt intermediate solution.

[0083] (II) Transfer to the crystallization unit: After the neutralization reaction is complete, the resulting clear solution is transferred to a crystallization tank by gravity flow in preparation for the next step of alcohol precipitation crystallization.

[0084] (III) Inert atmosphere replacement and precipitant introduction: To ensure operational safety and prevent the formation of an explosive mixture between flammable alcohol vapors and air, nitrogen gas is first introduced into the crystallization tank to displace the existing gas, and then discharged through the vent. Subsequently, the alcohol precipitant is mixed with high-pressure nitrogen gas and introduced into the solution from the bottom of the tank in a gas-liquid two-phase flow. This method achieves uniform dispersion of the precipitant and enhances mass transfer efficiency through bubble agitation, promoting a uniform crystallization process.

[0085] (iv) Alcohol precipitation, crystallization, and sedimentation: Stir continuously at 20–40°C for 1.0–2.0 hours to allow nylon 4T salt to gradually crystallize out. After stopping stirring, let stand for 0.5–1.0 hours to allow the system to settle naturally, forming a clear solid-liquid layer: the upper layer is a clear alcohol-water mixture, and the lower layer is a high-concentration crystal suspension.

[0086] (v) Selective discharge of the supernatant: By installing drain outlets at different heights on the side wall of the tank, corresponding valves can be flexibly opened according to the actual liquid-solid interface position to accurately discharge the upper clear liquid and reduce product entrainment losses. The discharged clear liquid is mainly a low-concentration alcohol-water solution, which is sent to the primary membrane separation unit 6 for preliminary recovery treatment.

[0087] (vi) High-concentration suspension transportation and solid-liquid separation: The remaining high-concentration crystal suspension (solid content of about 30% to 40 wt%) at the bottom of the tank is transported to the rotary vacuum filter 4 by a positive displacement pump for filtration to achieve effective separation of crystals and mother liquor.

[0088] (vii) Crystal washing and drying: During the filtration process, the filter cake is washed 1-2 times with an alcohol solvent to remove residual impurities and obtain nylon 4T salt. Subsequently, the washed salt is transferred to a vacuum drying device and dried at 40-60°C for 6-12 hours to finally obtain a high-purity, low-moisture nylon 4T refined salt product.

[0089] (viii) Solvent recovery and recycling: Waste liquids containing organic solvents generated during filtration and washing processes are collected and treated separately: Some of the low-concentration waste liquid enters the first-stage membrane separation unit 6, and is recovered to obtain a dilute alcohol aqueous solution with a mass fraction of 30% to 40%, which can be used to prepare the reaction base liquid for the next batch. Another portion of the liquid is further purified by the secondary membrane separation unit 7 to obtain high-purity recovered alcohol with a purity of ≥95%, which is returned to the alcohol precipitation or washing process for recycling; the wastewater generated after separation is discharged into the sewage treatment system for compliant disposal after testing and meeting the standards.

[0090] Example 2 This embodiment provides a method for preparing nylon 4T salt based on the bio-based nylon 4T salt production system described in Example 1. Through the coordinated operation of each unit device, a fully integrated operation from neutralization reaction to product purification and solvent recovery is achieved. The devices and their connections are as follows: Figures 1-3 As shown, the specific process flow is as follows: (1) Batch feeding and controlled neutralization reaction are carried out in neutralization reactor 1: 4707 g of a 30 wt% aqueous alcohol solution was added to neutralization reactor 1 through inlet 1a. Then, 676.7 g (approximately 7.67 mol) of 1,4-butanediamine with a purity of 98.017% was added through inlet 1b of the second neutralization reactor. The initial reaction temperature was set to 30°C, and stirring was started to ensure thorough mixing and the formation of a homogeneous reaction mixture.

[0091] A total of 1250 g of terephthalic acid powder, approximately 7.67 mol, was introduced into the system through hopper 1c. The powder was added in batches controlled by rotary valve 1d and fed into the neutralization reactor 1 through terephthalic acid inlet 1e. Each batch consisted of 125 g, for a total of 10 batches. After each batch was added, the reaction system was allowed to return to a clear state before the next batch was added to avoid excessively high local concentrations that could lead to instantaneous exothermic reactions or side reactions.

[0092] The temperature was controlled to not exceed 55°C throughout the reaction process. When all materials were added and the solution remained transparent and clear, the pH value of the system was measured to be 7.25, indicating that the neutralization reaction was complete and a soluble nylon 4T salt intermediate product solution was generated.

[0093] (2) Transfer to alcohol precipitation tank 2 and carry out alcohol precipitation crystallization under an inert atmosphere: The clarified nylon 4T salt intermediate product solution is transported from the outlet of reactor 1 to alcohol precipitation tank 2 through a pipeline, and enters the tank through nylon 4T salt intermediate product inlet 2a.

[0094] To ensure operational safety, nitrogen gas is introduced into the alcohol precipitation tank 2 before adding alcohol to replace the air inside the tank, preventing the risk of combustion and explosion caused by contact between flammable alcohol vapors and oxygen. The gas after replacement is discharged through the nitrogen vent 2c.

[0095] Subsequently, 10.4 kg of anhydrous methanol from either the fresh methanol storage tank 9 or the recycled methanol storage tank 10 was introduced into the solution from the bottom of the tank as a gas-liquid mixture under high-pressure nitrogen pressure via a nitrogen-blowing agitator 2b. This introduction method not only achieved uniform dispersion of the precipitant but also enhanced mass transfer efficiency through the turbulence effect generated by rising bubbles, promoting the uniform precipitation of nylon 4T salt.

[0096] After stirring continuously for 0.5 hours at 35°C, stirring was stopped, and the crystallized slurry was allowed to stand for 1 hour to achieve solid-liquid separation: the upper layer was a methanol-water clear liquid, and the lower layer was a high-concentration crystal suspension.

[0097] (3) Selectively discharge the supernatant to reduce the load on subsequent treatments: After settling, open regulating valve 2d and select an appropriate drain point through the multiple connecting outlets on U-tube 2e to discharge the supernatant. For example, when the liquid-solid interface is at the middle height, the second connecting outlet 2e2 can be opened for precise drainage to minimize product entrainment loss.

[0098] The discharged clear liquid is mainly a low-concentration alcohol-water mixture, which is transported to the primary membrane separation unit 6 for preliminary separation by a centrifugal pump.

[0099] (4) High-concentration suspension transportation and solid-liquid separation: The high-concentration crystal suspension remaining at the bottom of the alcohol precipitation tank 2, with a solid content of approximately 30%–40 wt%, is discharged through the alcohol precipitation outlet 2f and then transported to the rotary vacuum filter 4 by the screw pump 3. This pump type has good self-priming capability and adaptability to thick slurries, ensuring stable transport of materials with high solid content.

[0100] In the rotary vacuum filter 4, the crystals are filtered and washed 1 to 2 times with alcohol detergent to remove residual mother liquor and impurities, thus obtaining nylon 4T salt.

[0101] (5) Vacuum drying to obtain the final product: The washed Nylon 4T salt was output from the discharge end of the rotary vacuum filter 4 and manually transferred to the vacuum drying oven 5. It was dried at 40°C for 6 hours to finally obtain white granular Nylon 4T refined salt product with a total yield of about 1766 g.

[0102] (6) Waste liquid treatment and solvent cascade recycling: Waste liquids containing organic solvents generated during filtration and washing processes are collected separately: The filtrate and washing liquid from the rotary vacuum filter 4 are first sent to the primary membrane separation unit 6, where a dilute alcohol-water solution with a mass fraction of 30% to 40% is obtained on the permeate side and stored in the alcohol-water storage tank 8 for use in preparing the reaction base liquid for the next batch. The concentrated liquid continues to enter the secondary membrane separation unit 7 for deep purification to obtain high-purity recovered alcohol with a purity of ≥95%, which is stored in the reuse alcohol storage tank 10 and returned to the alcohol precipitation tank 2 or the rotary vacuum filter 4 for recycling; the final wastewater is discharged into the sewage treatment system for compliant disposal after passing the test.

[0103] Testing showed that the yield of the nylon 4T refined salt product obtained in this embodiment reached 98.6%, the purity was as high as 99.5%, and the moisture content was 0.27%, which fully meets the quality requirements of monomer salt for the polycondensation process of high-performance semi-aromatic polyamides such as PA4T.

[0104] The results show that by deeply coupling a specific chemical process path with dedicated production equipment, this invention constructs a green synthesis route that is controllable in reaction, uniform in crystallization, efficient in separation, resource-recyclable, and inherently safe, and has good prospects for industrial application.

[0105] Example 3 Except for the use of "40 wt% alcohol-water solution" in step (1), this embodiment is the same as in embodiment 2.

[0106] Testing revealed that the yield of the nylon 4T refined salt product obtained in this embodiment reached 96.8%, with a purity of 99.4% and a moisture content of 0.28%, fully meeting the quality requirements of monomer salts for high-performance semi-aromatic polyamides such as PA4T polycondensation processes. Therefore, within the limited range of 30% to 40%, even under boundary conditions, this application can still achieve stable salt formation and effective crystallization.

[0107] Example 4 Except for step (1), in which “terephthalic acid is added in 7 batches, with the first 6 batches each containing 187.5 g (i.e., 15% of the total amount), and the last batch is used to make up the remaining amount”, the rest of this embodiment is the same as in embodiment 2.

[0108] Testing revealed that the yield of the nylon 4T refined salt product obtained in this embodiment reached 97.1%, with a purity of 99.2% and a moisture content of 0.25%, fully meeting the quality requirements of monomer salts for the polycondensation process of high-performance semi-aromatic polyamides such as PA4T. Therefore, the addition of 10% to 15% of the raw material per batch in this application is a reasonable operating range under the premise of ensuring reaction safety and product quality.

[0109] Comparative Example 1 The difference between this comparative example and Example 2 is that, instead of using a batch addition method, 1250 g (approximately 7.67 mol) of terephthalic acid powder was rapidly added to the reaction system in one go. The remaining process steps are the same as in Example 2.

[0110] Observations revealed that a large amount of terephthalic acid agglomerated in neutralization reactor 1 of this comparative example, hindering the normal operation of the stirrer. The temperature inside the reactor rose sharply, even exceeding 60°C, and the resulting liquid was yellowish, suggesting that butanediamine underwent a side reaction. After the neutralization reaction was completed, the liquid in neutralization reactor 1 was transferred to alcohol precipitation tank 2. It was found that a certain amount of powder was suspended in the liquid, rather than a clear solution, suggesting the presence of unreacted terephthalic acid residue.

[0111] Further testing revealed that the yield of the nylon 4T refined salt product obtained in this comparative example reached 89.8%, with a purity of only 92.5% and a moisture content of 0.25%, lower than 0.40%. Adding terephthalic acid in a single step is not conducive to obtaining high-purity nylon 4T salt, as it contains terephthalic acid that could not be removed by alcohol washing. In actual operation, the rapid rise in reactor temperature and abnormal stirring are both detrimental to equipment maintenance and use.

[0112] In summary, the method and system described in this invention can operate stably within the defined parameter range and obtain products with high yield and high purity, fully meeting the needs of industrial-scale production.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a bio-based nylon 4T salt, characterized in that, The preparation method includes the following steps: (A) 1,4-Butanediamine is mixed with an aqueous solution containing alcohol to form a reaction base solution. Terephthalic acid is added to the reaction base solution in batches to carry out a neutralization reaction, generating a soluble nylon 4T salt intermediate product. (B) Add a precipitant to the intermediate product solution of nylon 4T salt to perform alcohol precipitation, so that nylon 4T salt crystallizes out. Separate the precipitated crystals from the solid and liquid and wash them to obtain nylon 4T salt.

2. The method for preparing bio-based nylon 4T salt according to claim 1, characterized in that, The alcohol-containing aqueous solution has an alcohol mass fraction of 30%~40%wt, and its mass is 7~7.5 times that of 1,4-butanediamine. Preferably, when adding terephthalic acid to the reaction base liquid in batches, the amount of terephthalic acid added in each batch is 10-15% of the total amount of terephthalic acid, and the next batch is added only after the reaction system has become clear after each batch is added. Preferably, the temperature of the neutralization reaction is controlled at 40~60℃, and the pH value at the end of the reaction is 7.2~7.

5.

3. The method for preparing bio-based nylon 4T salt according to claim 1, characterized in that, The precipitant is selected from one or more of methanol, ethanol, isopropanol or n-butanol; Preferably, when adding the precipitant, the precipitant is introduced into the bottom of the solution in the form of a gas-liquid mixture using a nitrogen-blowing agitator; Preferably, the reaction temperature for alcohol precipitation is 20~40℃ and the reaction time is 1.0~2.0 h.

4. The method for preparing bio-based nylon 4T salt according to claim 1, characterized in that, The steps of solid-liquid separation and washing of the precipitated crystals include: After the alcohol precipitation is completed, the mixture is allowed to stand for 0.5 to 1.0 h to allow the solid and liquid to separate into layers. The supernatant is discharged, and the bottom suspension is sent to a rotary vacuum filter press for filtration. The filter cake is washed with alcohol 1 to 2 times to obtain nylon 4T salt.

5. The method for preparing bio-based nylon 4T salt according to claim 1, characterized in that, The preparation method further includes: recovering the organic solvent components from the filtrate and / or washing liquid containing organic solvent generated during solid-liquid separation and washing of the crystals precipitated in step (B) via membrane separation, and returning at least a portion of the recovered organic solvent to the step of recycling in the alcohol precipitation and / or washing step.

6. The method for preparing bio-based nylon 4T salt according to claim 1, characterized in that, The preparation method further includes: vacuum drying the nylon 4T salt from step (B) to obtain refined nylon 4T salt; Preferably, the vacuum drying temperature is 40~60℃ and the time is 6~12h.

7. The method for preparing bio-based nylon 4T salt according to any one of claims 1 to 6, characterized in that, The production system for preparing bio-based nylon 4T salt includes a neutralization reactor (1), an alcohol precipitation tank (2), a rotary vacuum filter (4), a vacuum drying oven (5), a primary membrane separation unit (6), a secondary membrane separation unit (7), an alcohol-water storage tank (8), and a recycled alcohol storage tank (10), wherein: Neutralization reactor (1) is used to mix 1,4-butanediamine with an alcohol-containing aqueous solution and to accept terephthalic acid added in batches for neutralization reaction; The alcohol precipitation tank (2) is connected to the neutralization reactor (1) through a pipeline and is used to receive the neutralized solution and add a precipitant for crystallization. The rotary vacuum filter (4) is connected to the bottom outlet of the alcohol precipitation tank (2) for filtering and washing the precipitated crystals with alcohol. The nylon 4T salt obtained from the discharge end of the rotary vacuum filter (4) is manually transferred to the vacuum drying oven (5) for vacuum drying; The primary membrane separation device (6) and the secondary membrane separation device (7) are respectively connected to the filtrate outlet and alcohol washing liquid outlet of the rotary vacuum filter (4) for step separation of liquids containing organic solvents; The alcohol-water storage tank (8) is connected to the permeate outlet of the primary membrane separation device (6) and is used to store a dilute alcohol-water solution that can be reused to prepare the reaction base solution. The alcohol storage tank (10) is connected to the concentrate outlet of the secondary membrane separation unit (7) to store high-purity recovered alcohol and supply it to the alcohol precipitation tank (2) and / or the rotary vacuum filter (4) for recycling.

8. The method for preparing bio-based nylon 4T salt according to claim 7, characterized in that, The alcohol precipitation tank (2) is equipped with a nitrogen blowing agitator (2b), which introduces alcohol precipitant from the recycled alcohol storage tank (10) or externally supplied in the form of a gas-liquid mixture through a small hole at the bottom of the hollow shaft. Preferably, the alcohol precipitation tank (2) is provided with a U-shaped tube (2e) on the outside, one end of which is connected to the upper part of the side wall of the tank, and the other end is provided with multiple discharge outlets with shut-off valves distributed along the height direction, for selectively discharging the upper clear liquid after static settling; Preferably, the production system further includes a fresh alcohol storage tank (9), which is connected to the inlet of the alcohol precipitation tank (2) for replenishing the precipitant.

9. The method for preparing bio-based nylon 4T salt according to claim 7, characterized in that, The neutralization reactor (1) is equipped with a rotary valve (1d) and a weighing module for controlling the batch addition of terephthalic acid.

10. The method for preparing bio-based nylon 4T salt according to claim 7, characterized in that, A transfer pump is provided between the alcohol precipitation tank (2) and the rotary vacuum filter (4) for transferring high-concentration suspension; Preferably, the delivery pump is a screw pump (3).

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