Continuous extraction process for low melting copolyamides

Abstract

The present application relates to a kind of extraction processes of nylon 6 chip, especially a kind of continuous extraction method of low melting point copolyamide.Employing multi-stage extraction device to carry out continuous multiple extraction by chip, the interval of overall extraction temperature is 75-95 DEG C and gradually increases from top to bottom, chip is contacted with extraction liquid countercurrent in each stage extraction device, and dynamic displacement is generated to chip;The extraction liquid is the mixture of desalted water and alcohol additive.The present application realizes the continuous automatic extraction of low melting point copolyamide chip by improving process technology, solves the problem of chip sticking, and the problem of low efficiency and uneven quality of intermittent extraction.The extraction efficiency is increased, the water consumption of extraction is reduced, and the production is more stable.

Description

Technical Field

[0001] The invention relates to an extraction process for nylon 6 chips, in particular to a continuous extraction method for low-melting-point copolyamide. Background Art

[0002] In addition to its applications in conventional civilian yarns, industrial yarns, and industrial plastics, nylon 6 can also be modified through copolymerization to produce hot-melt yarns for special purposes. Generally, by copolymerizing PA6 monomer with other types of polyamide monomers (such as PA66 salt, PA1010 salt, PA12, PA11, etc.) in a certain ratio, you can get copolyamide slices with a lower melting point, also known as low-melting-point copolyamide slices. This slice can be used as a hot-melt adhesive, or for spinning hot-melt yarns for some special clothing occasions. Because PA6 monomer is added to the copolymerization component, the final prepared slice contains 1%-10% oligomers or monomers, which will affect the stability of the slice during high-speed spinning, so an extraction step is needed to remove the oligomers or monomers.

[0003] In the conventional nylon 6 chip extraction process, the chips are extracted in one or several static towers connected end to end. The chips flow downward by gravity, while the extraction water flows upward in a near-laminar flow. This countercurrent process completes the extraction. The extraction liquid is desalted water with a bath ratio of 1:1.1-1:1.2, heated by steam, and controlled at a temperature of 110°C-120°C. After dozens of hours of extraction, the oligomer content of the chips is reduced to the target value (i.e., <0.4%).

[0004] The characteristics of low-melting-point copolyamide chips for spinning are: the melting range is generally between 120°C and 165°C; chips may clump at temperatures above 85°C, and a certain degree of extrusion makes them more prone to caking. If low-melting-point copolyamide chips are extracted according to the extraction method for conventional PA6 chips, the chips will stick together or even melt, making production impossible. Even if the temperature is lowered for extraction, because the chips move downward in the extraction tower in a relatively static state, after a period of time, they will stick to each other. At the same time, the bottom chips are more likely to stick together under the pressure of the upper chips, which is not conducive to production. On the other hand, the use of desalted water as the extraction liquid and the lower temperature result in low extraction efficiency.

[0005] To address the unique characteristics of low-melting-point copolyamide chips, existing processes utilize intermittent extraction. Indirect extraction is performed in a stirred tank. First, low-melting-point copolyamide chips are placed in the tank. A predetermined amount of desalted water is added at a bath ratio of 1:1.2-1:1.5. The solution is heated to a desired temperature (generally <85°C to prevent sticking). The agitator is then activated to stir the chips (at a rate of 60-90 rpm) to enhance mass transfer. This agitation also creates a certain amount of displacement in the chips, preventing sticking from prolonged static standing. A stirrer with a large agitator blade, such as a paddle or folding blade type, is used to agitate the chips and enhance the flow of the extraction water. After 6-10 hours of extraction, the oligomer content in the extraction water has reached equilibrium, requiring a water exchange to further extract the remaining oligomers. After the water exchange, the temperature is raised again, and the extraction process continues with stirring. This cycle is repeated 3-4 times to complete the extraction process.

[0006] The existing indirect extraction method of low-melting-point copolyamide chips still has the following disadvantages:

[0007] 1. Due to the characteristics of the slices, a lower temperature is required for extraction. At the same time, the extraction liquid used is desalted water. The oligomer components of the low-melting-point copolyamide slices are poorly soluble in water at lower temperatures. These two reasons lead to low extraction efficiency.

[0008] 2. Since the extraction process requires multiple water changes, it consumes a lot of water, causes serious waste, is cumbersome to operate, has low production efficiency, and is difficult to automate.

[0009] 3. Intermittent production will lead to quality differences between different batches of products. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide a continuous extraction method for low-melting-point copolyamide, thereby improving production efficiency and production stability.

[0011] The present invention is achieved in that:

[0012] A continuous extraction method for low-melting-point copolyamide, wherein slices are subjected to continuous multiple extractions through multi-stage extraction devices connected from top to bottom. The overall extraction temperature range is 75°C to 95°C and increases step by step from top to bottom. The slices are in countercurrent contact with the extraction liquid in each stage of the extraction device, and the slices undergo dynamic displacement. The extraction liquid is a mixture of desalted water and an alcohol auxiliary agent.

[0013] Further:

[0014] The alcohol auxiliary agent is selected from ethanol, ethylene glycol, and polyethylene glycol.

[0015] The content of the alcohol auxiliary agent accounts for 0.1%-5% of the total mass of the extract.

[0016] Independent extraction water channels are set between each level of extraction devices, which are separated from the slice flow channel to prevent the flow of slices from affecting the direction of water.

[0017] The slices are extracted in the multi-stage extraction device for 20 hours to 24 hours to reduce the oligomer content to below 0.4%.

[0018] The number of extraction stages of the multi-stage extraction device is ≥3.

[0019] Each level of extraction device is equipped with a stirring mechanism, and the rotation speed is controlled at 25rpm-60rpm.

[0020] After the extraction is completed, the slices are cooled to 50℃-70℃ and then centrifuged and dried in a centrifugal dehydrator.

[0021] The multi-stage extraction device includes a plurality of extraction tanks connected up and down and interconnected; the extraction tanks include at least a primary extraction tank, a secondary extraction tank and a tertiary extraction tank from top to bottom, and each extraction tank is equipped with a heat preservation mechanism, a stirring mechanism, a liquid level gauge and a temperature sensor; the top of the primary extraction tank is provided with a slice feed port, and an extraction water overflow port with a filter is provided at 60%-80% of the tank body height; the lowest extraction tank is connected to the extraction water heating system, the extraction water storage tank, the extraction water preheating tank and the centrifugal dehydrator in sequence through a water circulation pipeline; the centrifugal dehydrator is at the highest position, the extraction water preheating tank is at the second height, and the primary extraction device and the extraction water storage tank are at the same height and at the third height.

[0022] Specifically:

[0023] The height relationship of each device satisfies the following conditions: when the system is filled with water, all pipeline valves are fully open, and the rotating equipment is in the closed state, when extraction water is added to the extraction water preheating tank, it will enter the extraction water storage tank through overflow and flow out from the overflow port of the first-level extraction tank.

[0024] The extraction water storage tank is connected to the extraction water heating system in sequence through a centrifugal pump and a filter, the extraction water heating system is connected to the lowest-level extraction tank in sequence through a temperature sensor, a flow meter, and an air-controlled valve, and the lowest-level extraction tank is connected to the centrifugal dehydrator in sequence through a discharge manual valve, a rotary valve, and a mud pump; an extraction liquid inlet is provided on the connecting pipeline between the centrifugal dehydrator and the extraction water preheating tank; the extraction water preheating tank is connected to the extraction water storage tank through an overflow port, and the extraction water storage tank is connected to the rotary valve through a hand valve; the filter is connected to the bottom of the lowest-level extraction tank through an air-controlled valve; an extraction liquid inlet is provided on the connecting pipeline between the extraction water preheating tank and the centrifugal dehydrator, and the extraction liquid inlet is connected to a desalted water addition branch and an alcohol auxiliary agent addition branch through a flow meter.

[0025] The stirring mechanism comprises a stirring shaft, on which a stirring blade is provided. The stirring blade is cylindrical, and the diameter of the cylinder is less than or equal to 3 cm.

[0026] The working principle of the present invention is as follows:

[0027] ① Set up a multi-stage extraction device connected from top to bottom, and by controlling the temperature of each stage of extraction, form an extraction system with gradually increasing temperature from top to bottom (the temperature range is 75℃-95℃. For example, if the extraction is three-stage, the temperature can be set from top to bottom as 75-85℃ for the first stage extraction, 80-90℃ for the second stage extraction, and 85-95℃ for the third stage extraction). The extraction water continuously enters the extraction device from the bottom and then overflows from the highest point. The temperature difference between the top and the bottom is used to stabilize the flow direction of the water. Independent extraction water channels are set between the extraction devices at each stage to prevent the flow of the slices from affecting the direction of the water.

[0028] The slices are introduced into the extraction system from the highest point, flowing downward. At the highest point, the oligomer concentration within the slices is highest, making extraction easier. Extraction can be performed at lower temperatures using a solution with a higher oligomer content. At the lowest point, the oligomer content within the slices is lower, requiring extraction at relatively higher temperatures using a solution with a lower oligomer content. The oligomer concentration distribution within the system is maintained, with the lowest concentration at the bottom and highest concentration at the top, thus achieving continuous extraction.

[0029] After extraction, the slices need to be cooled to 50-70°C and then centrifuged to obtain the final product. At this time, the extraction water from the extraction water storage tank is directly pumped and filtered to the bottom of the lowest extraction tank, which reduces the temperature at the bottom of the lowest extraction tank and cools the slices.

[0030] ② To prevent the slices from sticking together during the extraction process, a dynamic extraction method is used in each level of the extraction device. This disrupts the relative stillness of the slices flowing from top to bottom, allowing the slices to undergo a small relative displacement without affecting the flow of the extraction water, thus avoiding the occurrence of sticking. For example, a magnetic stirrer is installed in the extraction device, with the speed controlled at 25-60 rpm. This allows the slices to undergo a small relative displacement without affecting the flow of the extraction water, thus avoiding the occurrence of sticking. At the same time, the stirrer structure can also be improved, and the stirring blades can be changed to a cylindrical shape to reduce disturbance to the water flow.

[0031] ③ The present invention changes the extracting liquid from desalted water to a compound extracting liquid, which is prepared by mixing desalted water with an alcohol auxiliary agent. During the extraction process, the ratio of the slice feed mass to the extracting liquid feed mass (bath ratio) is 1:1.1-1.2.

[0032] Due to the poor hydrophilicity of the oligomers in the low-melting-point polyamide chips, mass transfer is poor during the extraction process. Furthermore, the diffused oligomers tend to adhere to the chip surface, becoming physically entangled with each other and causing the chips to stick together. This extraction solution, leveraging the principle of like dissolving like, enhances the dissolution of the oligomers within the chips, thereby improving extraction efficiency. The addition of additives can disrupt these entangled structures and reduce the occurrence of sticking. This allows the extraction temperature to be raised by 5-10°C, further improving extraction efficiency.

[0033] The present invention has the following advantages:

[0034] Through the improvement of process technology, continuous and automated extraction of low-melting-point copolyamide chips is achieved, solving the problem of chip adhesion and the low efficiency and uneven quality of intermittent extraction.

[0035] 1. Increased extraction efficiency: Under the same bath ratio, the present invention reduces the oligomer content in the slices from 4%-5% to below 0.4% after 20-24 hours of extraction; the previous method (intermittent extraction) requires 48-54 hours to reduce the oligomer content in the slices from 4%-5% to below 0.4%.

[0036] 2. Reduced water consumption for extraction: The present invention reduces the oligomer content in 1 ton of chips to below 0.4%, using approximately 1-1.2 tonnes of water. Conventional equipment and processes reduced the oligomer content to below 0.4% per ton of chips, using approximately 5-5.3 tonnes of water. The present invention uses only 20% of the original water consumption.

[0037] 3. Production stability: The present invention avoids the problem of slice sticking, which often requires cleaning and maintenance due to slice sticking in previous methods, affecting production. At the same time, the present invention has better slice quality uniformity and a higher level of automation. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0039] Figure 1 It is a structural schematic diagram of the present invention. DETAILED DESCRIPTION

[0040] like Figure 1 As shown, the system mainly includes the following parts: primary extraction tank A, secondary extraction tank B, tertiary extraction tank C, extraction water preheating tank D, extraction water storage tank E, heating system F, and 11 pipelines 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.

[0041] The primary extraction tank A is equipped with a magnetic stirrer A1 (in one embodiment, the stirring blades of an industrial magnetic stirrer are modified into cylindrical shapes. A stirring blade A12 is vertically mounted on the stirring shaft A11, and the diameter of the cylinder is 2 cm. The number of stirring blades is determined by the size of the container and can be arranged in parallel in multiple groups (the same applies below), a liquid level gauge A2, a slice inlet A3, an extraction water overflow port A4 (located between 60% and 80% of the tank height and equipped with a filter), a thermometer A5, a jacket A6, and an air-controlled valve A7 (for adding the jacket medium). The jacket medium is hot water (WH), which enters from the bottom and exits from the top. The primary extraction tank A and the secondary extraction tank B are connected by two pipelines. A manual valve 11 is installed on pipeline 1 to cut off the passage when necessary. Pipeline 1 is the passage for slices and extraction water between A and B. A manual valve 21 is installed on pipeline 2 to cut off the passage when necessary. In addition, a filter is installed at the connection between pipeline 2 and pipelines A and B to prevent slices from entering the pipeline. Pipeline 2 is the passage for extraction water between A and B.

[0042] Secondary extraction tank B is equipped with a magnetic stirrer B1, a level gauge B2, a thermometer B3, a jacket B4, and a pneumatic control valve B5 (for adding jacket medium). The jacket medium is hot water, entering from the bottom and exiting from the top. Secondary extraction tank B and tertiary extraction tank C are connected by two pipes. A manual valve 31 is installed on pipe 3 to cut off the passage when necessary. Pipe 3 is the passage for slices and extraction water between B and C. A manual valve 41 is installed on pipe 4 to cut off the passage when necessary. In addition, a filter is installed at the connection between pipe 4, B, and C to prevent slices from entering the pipe. Pipe 4 is the passage for extraction water between B and C.

[0043] The three-stage extraction tank C is equipped with: magnetic stirrer C1, liquid level gauge C2, thermometer C3, jacket C4, slice delivery water inlet C5, extraction water inlet C6, and air control valve C7 (for adding jacket medium). The jacket medium is hot water, which enters from the bottom and exits from the top.

[0044] The pipeline 5 (connecting the slice outlet at the bottom of the third-stage extraction tank C and the inlet of the centrifugal dehydrator 54) is equipped with: a discharge manual valve 51, a rotary valve 52 (for conveying slices), a mud pump 53 (for conveying slices), and a centrifugal dehydrator 54 (for separating slices from extraction water).

[0045] Extraction water preheating tank D is equipped with: D1: agitator, D2: thermometer, D3: air control valve (to control the amount of hot water), D4: jacket, D5: overflow port (at the top of the device, overflowing to E). The jacket medium is hot water, which enters from the bottom and exits from the top.

[0046] The extraction water storage tank E is equipped with: thermometer E1 and liquid level gauge E2.

[0047] The heating system F is equipped with: air-controlled valve F1 (for adding hot water) and air-controlled valve F2 (for adding cold water).

[0048] The pipeline 6 (connecting the bottom of the extraction water storage tank E and the rotary valve 52) is provided with a manual valve 61 (controlling the amount of slice delivery water).

[0049] Pipeline 7 (connecting the extraction liquid outlet of the centrifugal dehydrator 54 and the bottom of the extraction water preheating tank D) is equipped with: an air-controlled valve 71 (for adding desalted water WD), an air-controlled valve 72 (for adding additives), a flow meter 73 (for measuring the amount of desalted water added), a flow meter 74 (for measuring the amount of additives added), a flow meter 75 (for measuring the total flow), and a manual valve 76 (for cutting off when necessary).

[0050] Pipeline 8: connects the overflow port D5 and the top of the extraction water storage tank E.

[0051] The pipeline 9 (connecting the extraction liquid outlet of the extraction water storage tank E and the extraction liquid inlet of the heating system F) is provided with: a centrifugal pump 91 and a filter 92.

[0052] The pipeline 10 (connecting the heating system F and the three-stage extraction tank C) is provided with: a flow meter 101, an air control valve 102 (controlling the overflow water size), and a thermometer 103.

[0053] The pipeline 11 (connecting the outlet of the filter 92 and the bottom of the third-stage extraction tank C) is provided with: an air control valve 111 (controlling the amount of water for transporting slices).

[0054] Others: In the system, the centrifugal dehydrator 5-4 is at the highest position, the first-stage extraction device A, the extraction water tank E, and the extraction water preheating tank D are at the second highest level, with D being the highest, followed by E and A (both at the same height). The height relationship satisfies the following: when the system is filled with water, all pipeline valves are fully open, and the rotating equipment (referring to all equipment that requires motor drive, such as: agitator A1, agitator B2, agitator C1, agitator D1, rotary valve 52, mud pump 53, centrifugal pump 91, centrifugal dehydrator 54, etc.) is in the closed state, water is added to the extraction water preheating tank D, which will overflow into the extraction water storage tank E and flow out of the overflow port of the first-stage extraction tank A. When the system is in operation, except for the desalted water and additive addition port, the slice outlet, the slice inlet, and the overflow port of the first-stage extraction A, the other parts of the extraction system are not connected to the outside world and are in a sealed state.

[0055] Extraction process and operation method:

[0056] 1. System irrigation and extraction liquid preparation: The extraction liquid is mixed with desalted water and additives (one or more of ethanol, ethylene glycol, polyethylene glycol, etc.) in a certain proportion (the total amount of additives accounts for the total extraction liquid feed amount: 0.1%-5%).

[0057] Table 1 Extraction effect of different proportions of additives

[0058] Additive ratio Extraction time Oligomer content in slices before extraction Oligomer content in slices after extraction Extraction temperature Adhesion 0 36h 4.11％ 0.38％ 80℃ No bonding 0.05％ / 4.65％ / 85℃ Sticking, unable to complete extraction 0.20％ 24h 4.33％ 0.38％ 87℃ No bonding 1％ 22h 4.77％ 0.35％ 92℃ No bonding 3％ 21h 4.69％ 0.31％ 94℃ No bonding 5％ 20h 4.79％ 0.32％ 95℃ No bonding

[0059] As shown in Table 1, using the extraction apparatus described in the examples, a higher additive ratio leads to higher extraction temperatures and, for similar initial oligomer contents, shorter extraction times. However, above 5%, additive additions no longer significantly increase extraction temperature and significantly reduce extraction time. Therefore, for cost considerations, we selected a maximum additive ratio of 5%.

[0060] The specific process is as follows: Open valves 76, 71, and 72. Based on the measured values ​​of valves 73 and 74, the system controls the opening of valves 71 and 72, ensuring that the two are gravity-fed into the extraction water preheating tank D according to the established mass ratio. After agitation and mixing, the mixture overflows into the system. The system is filled until extract flows out of the overflow port A. At this point, according to the designed height, the liquid level in the extraction water storage tank E should be between 60% and 80% (as determined by level gauge E2), completing the system filling. Thereafter, the total extract feed flow rate is controlled to the established bath ratio (chip feed volume: extract feed volume = 1:1.1-1.2). The addition of additives helps improve extraction efficiency and reduce chip sticking.

[0061] 2. Circulation start: After the system is filled with water, start the centrifugal pump 91 and the mud pump 53. At this time, two water flows are formed in the system.

[0062] The first water flow direction:

[0063] Extraction liquid addition port → Pipeline 7 → Extraction water preheating tank D → Pipeline 8 → Extraction water storage tank E → Centrifugal pump 91 → Filter 92 → Heating system F → Tertiary extraction tank C → Pipelines 4 and 3 → Secondary extraction tank B → Pipelines 2 and 1 → Primary extraction tank A → Overflow outlet A4. The overflow outlet flow rate = Extraction liquid feed rate measured by flowmeter 75 = Flow rate measured by flowmeter 101. The extraction water inflow and outflow of the entire system remain balanced.

[0064] The system is powered by a centrifugal pump 91, which continuously replenishes extraction water to maintain a constant level in extraction water storage tank E. Consequently, the system overflows at the highest point downstream, which is the overflow port at the primary extraction tank A. Furthermore, a flow meter 101 is designed to control the flow of the extraction water in the first direction. This water is heated by a heating system F, providing a certain degree of warmth for each stage of extraction. Second water flow direction:

[0065]

[0066] The first water flow direction plays a major role in the extraction process. The extractant flows through each extraction stage, extracting the slices in reverse and ultimately overflowing the system. The second water flow direction circulates within the system, transporting the extracted slices without changing the total amount of water. The extractant water is transported from a low point to a high point, and then flows back into the system at the high point. During system operation, the system controls the opening of the air-controlled valve 111 by measuring the liquid level in the extractant water storage tank E, thereby achieving automatic control of the system's liquid level. Simultaneously, the system controls the opening of the air-controlled valve 102 using the measurement value of the flowmeter 101, thereby controlling the flow of extractant water entering the tertiary extraction tank C and ensuring temperature and liquid level stability in various parts of the system. This ensures that the system has two water flow directions. The temperature gradient then stabilizes the water flow in the overflow direction (i.e., the water tends to diffuse from high-temperature areas to low-temperature areas), preventing back-mixing and maintaining the extract concentration of the extractant water in the extraction tank at a low bottom and a high top.

[0067] 3. System heating: After the cycle is started, the system will heat up and keep warm according to the process requirements. The temperature of the extraction water preheating tank D and the extraction water storage tank E is maintained at 50-70°C. This temperature should not be too high because it will affect the transportation of subsequent slices, otherwise the slices will easily stick together under the centrifugal force of the centrifuge. The system controls the temperature of the extraction water preheating tank D by controlling the opening of the air control valve D3 according to the measurement value of the thermometer D2. Hot water (jacket) is selected as the heating medium to make the heating process more stable. The outside of the extraction water storage tank E is wrapped with insulation cotton, and the temperature is monitored by the thermometer E1. It does not have a heating jacket itself. The extraction water enters the heating system F through the centrifugal pump 91. The upper section of this heating system (conventional device) is heated by hot water, and the lower section is cooled by cooling water. The system controls the opening of F1 and F2 according to the measurement value of the thermometer 103, thereby realizing automatic temperature control. The purpose of using two-stage temperature control is to prevent unexpected factors from causing the extraction water temperature to overheat, which could cause large amounts of slices to stick together and damage the equipment. Hot water heating, compared to steam, also provides more stable heating. The extraction water temperature, heated by heating system F, is maintained at 80-90°C. Finally, each stage of the extraction apparatus is equipped with an insulation jacket, which is filled with hot water to maintain stable process temperatures and offset system heat loss. The process temperature requirements are: 75-85°C for the first stage extraction tank A, 80-90°C for the second stage extraction tank B, and 85-95°C for the third stage extraction tank C.

[0068] The extraction tank's temperature is precisely controlled through heat exchange with the medium in the jacket. Using the temperature indicated by a thermometer, the opening of the corresponding gas-controlled valves (A7, C7, B5, etc.) is controlled to adjust the flow of the heating medium, thereby achieving temperature control. Some temperature-sensitive areas, such as the extraction water storage tank E, are not jacketed. Furthermore, all equipment and piping in the system are designed with insulation to minimize heat loss.

[0069] 4. Feed extraction: Start the agitators A1 / B1 / C1 of each level of extraction and maintain the speed of 25-60r / min. The low-melting-point nylon 6 chips enter the first-level extraction tank A from A3 for preliminary extraction, and then enter the second-level extraction tank B through pipe 1 by gravity for the next stage of extraction, and then enter the third-level extraction tank C through pipe 3 to complete the final extraction process.

[0070] In the above process, the slices are stirred by the stirrer, which produces a certain displacement, which can prevent the sticking problem caused by long-term relative stillness; the additives added to the extraction water promote the mass transfer process and play a certain anti-sticking role.

[0071] It should be noted that A / B / C are conical-bottomed containers (conventional extraction devices). Pipes 1 and 3 are connected directly below A and B, respectively. Pipes 2 and 4 serve as diversion channels for the upward-flowing extraction water, preventing solid slices in pipes 1 and 3 from affecting the flow of water. After the extraction process is completed, the slices are transported via the flowing water at C5, through a rotary valve 52 and a slurry pump 53, to a centrifugal dehydrator 54 for centrifugal drying, completing the entire extraction process. Here, the rotary valve 52 serves to quantitatively transport the slices, the slurry pump 53 provides transport power, and the centrifugal dehydrator 54 separates the slices from the water, separating the extracted water for further reuse via pipe 7. Because the slices are subject to intense compression and collision during transport, to prevent sticking, the process requires maintaining the temperature of the extraction water preheating tank D between 50°C and 70°C. This allows the transport water temperature in the second water flow to also be controlled at 50°C to 70°C, allowing the extracted slices to be cooled during transport and preventing sticking during centrifugal drying.

[0072] 5. Normal Operation: During normal operation, the extraction water flows from bottom to top, the chips flow from top to bottom, and the transport water circulates stably within the system. The distribution of oligomers in the system is as follows: the chips gradually decrease from top to bottom until they meet the standard and are transported to the next process section, while the extraction water gradually increases from bottom to top until it reaches its maximum value and overflows. The oligomer content in the circulating water is low and stable.

[0073] Although the specific embodiments of the present invention are described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and are not intended to limit the scope of the present invention. Equivalent modifications and changes made by those skilled in the art in accordance with the spirit of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A continuous extraction method for low-melting-point copolyamide, characterized in that: The slices are extracted continuously multiple times through multi-stage extraction devices connected from top to bottom. The overall extraction temperature range is 75℃-95℃ and increases step by step from top to bottom. The slices are in countercurrent contact with the extraction liquid in each stage of extraction device, and the slices undergo dynamic displacement; the extraction liquid is a mixture of desalted water and alcohol auxiliary agents.

2. The continuous extraction method of low-melting-point copolyamide according to claim 1, characterized in that: The alcohol auxiliary agent is selected from ethanol, ethylene glycol, and polyethylene glycol.

3. The continuous extraction method of low-melting-point copolyamide according to claim 2, characterized in that: The content of the alcohol auxiliary agent accounts for 0.1%-5% of the total mass of the extract.

4. The continuous extraction method of low-melting-point copolyamide according to claim 1, characterized in that: Independent extraction water channels are provided between the extraction devices at each stage, which are separated from the slice flow channel so that the slices do not enter the extraction water channel.

5. The continuous extraction method of low-melting-point copolyamide according to claim 1, characterized in that: Each level of extraction device is equipped with a stirring mechanism, and the rotation speed is controlled at 25rpm-60rpm to make the slices produce dynamic displacement.

6. The continuous extraction method of low-melting-point copolyamide according to claim 1, characterized in that: After the extraction is completed, the slices are cooled to 50℃-70℃ and then centrifuged and dried in a centrifugal dehydrator.

7. The continuous extraction method of low-melting-point copolyamide according to claim 1, characterized in that: The multi-stage extraction device includes a plurality of extraction tanks connected up and down and interconnected; the extraction tanks include at least a primary extraction tank, a secondary extraction tank and a tertiary extraction tank from top to bottom, and each stage of the extraction tank is equipped with a heat preservation mechanism, a stirring mechanism, a liquid level gauge and a temperature sensor; the top of the primary extraction tank is provided with a slice feed port, and an extraction water overflow port with a filter is provided at 60%-80% of the tank body height; the lowest stage extraction tank is connected in sequence to an extraction water heating system, an extraction water storage tank, an extraction water preheating tank and a centrifugal dehydrator through a water circulation pipeline.

8. The continuous extraction method of low-melting-point copolyamide according to claim 7, characterized in that: The centrifugal dehydrator is at the highest position, the extraction water preheating tank is at the second height, the primary extraction device and the extraction water storage tank are at the same height and at the third height, and the height relationship of each device satisfies: when the system is filled with water, all pipeline valves are fully open, and the rotating equipment is in the closed state, extraction water is added to the extraction water preheating tank, and it will enter the extraction water storage tank through overflow and flow out from the overflow port of the primary extraction tank.

9. The continuous extraction method of low-melting-point copolyamide according to claim 7, characterized in that: The extraction water storage tank is connected to the extraction water heating system in sequence through a centrifugal pump and a filter, the extraction water heating system is connected to the lowest-level extraction tank in sequence through a temperature sensor, a flow meter, and an air-controlled valve, and the lowest-level extraction tank is connected to the centrifugal dehydrator in sequence through a discharge manual valve, a rotary valve, and a mud pump; an extraction liquid inlet is provided on the connecting pipeline between the centrifugal dehydrator and the extraction water preheating tank; the extraction water preheating tank is connected to the extraction water storage tank through an overflow port, and the extraction water storage tank is connected to the rotary valve through a hand valve; the filter is connected to the bottom of the lowest-level extraction tank through an air-controlled valve; an extraction liquid inlet is provided on the connecting pipeline between the extraction water preheating tank and the centrifugal dehydrator, and the extraction liquid inlet is connected to a desalted water addition branch and an alcohol auxiliary agent addition branch through a flow meter.

10. The continuous extraction method of low-melting-point copolyamide according to claim 7, characterized in that: The stirring mechanism comprises a stirring shaft, on which a stirring blade is provided. The stirring blade is cylindrical, and the diameter of the cylinder is less than or equal to 3 cm.

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