A method for supercritical CO2 assisted removal of nylon 6 extractable oligomers and simultaneous catalytic conversion

Abstract

The application discloses a method for removing extractable oligomers of nylon 6 with the aid of supercritical CO2 and synchronously catalytically converting, and belongs to the technical field of post-processing of high polymer materials. The method is characterized in that supercritical CO2 fluid containing sulfonic acid catalyst and methanol / water composite entrainer is used to dynamically extract nylon 6 material, so that cyclic dimers are opened in the supercritical fluid phase to be converted into caprolactam and linear oligomers; the extraction temperature is 50-100 DEG C; the pressure is 15-35 MPa; and the extraction time is 30-240 min. The method integrates extraction and catalytic conversion in the same system, and simultaneously realizes efficient removal of extractable oligomers and resourceful conversion of cyclic dimers under low-temperature conditions. The total content of extractable oligomers in the final product is less than or equal to 0.5 wt%, and the content of cyclic dimers is less than or equal to 0.2 wt%.

Description

Technical Field

[0001] This invention relates to the field of polymer material post-processing technology, specifically to a method for supercritical CO2-assisted removal of extractable nylon 6 oligomers and simultaneous catalytic conversion. Background Technology

[0002] Nylon 6 (PA6) is an engineering plastic synthesized by the ring-opening polymerization of caprolactam. It possesses outstanding advantages such as wear resistance, high strength, chemical corrosion resistance, and ease of processing, and is widely used in fibers, films, and engineering plastics. However, the ring-opening polymerization of caprolactam is a reversible equilibrium reaction with a polymerization equilibrium conversion rate of approximately 90%. The product contains about 10 wt% extractable oligomers, mainly including caprolactam monomer (CL) and cyclic oligomers, of which cyclic dimers (CD) account for about 90% of the total cyclic oligomer content. CD has a melting point of 348℃, far higher than that of nylon 6, and is stable and not easily ring-opened. During processing, it easily precipitates and clogs the channels, leading to filament breakage during spinning. This significantly reduces the mechanical properties and appearance quality of the final product, making large-scale melt spinning of nylon 6 currently difficult.

[0003] Currently, the main industrial method for removing extractable oligomers from nylon 6 is hot water extraction. At 90-100℃, hot water is used to extract caprolactam monomers and some oligomers, and the removal is achieved through multi-stage countercurrent or intermittent washing. This process is mature and simple to operate, but it has the following drawbacks: (1) CD has extremely poor water solubility, and the removal rate is usually less than 50%, and residual CD is prone to clogging the spinneret; (2) The extraction time is long, usually requiring 6-10 hours, and the energy consumption is high; (3) The wastewater discharge is large, with 5-10 tons of wastewater containing oligomers generated per ton of chips, which puts great pressure on environmental protection; (4) The upper limit of water temperature under normal pressure is 100℃, and it is impossible to increase the solubility and diffusion rate of CD by raising the temperature.

[0004] Supercritical CO2 extraction technology provides a new technical route for the removal of oligomers from nylon 6. Venema et al. (Journal of High Resolution Chromatography, 1993, 16: 522-524) reported the extraction of caprolactam and its oligomers from nylon 6 using CO2 containing 7.5% (m / m) methanol as a supercritical fluid at 60°C, demonstrating that the supercritical CO2 + methanol system can effectively extract oligomers. Chinese patent CN104086788A discloses a method for supercritical carbon dioxide extraction of caprolactam monomers from nylon 6 chips, which extracts nylon 6 chips with low monomer content at 70-110°C and 15-25 MPa. Chinese patent CN1175921A discloses a method for extracting unwanted substances from polymer products using supercritical fluids, involving the use of supercritical CO2 to remove unreacted monomers, oligomers, etc. from polymers. However, the supercritical CO2 extraction technology mentioned above is mainly for the removal of caprolactam monomers or the extraction of general oligomers. For cyclic dimers (CD) that are stable and have a melting point as high as 348°C, the removal efficiency is still limited. Moreover, the extracted oligomers (especially CD) are difficult to utilize effectively and are usually treated as waste, resulting in resource waste.

[0005] On the other hand, for the most stable cyclic dimer (CD) among the extractable oligomers of nylon 6, existing technologies have included studies on separating and purifying it before catalytic ring-opening and repolymerization. For example, sulfonic acid catalysts are used under heating conditions to convert CD into polymerizable monomers or linear oligomers, which are then prepolymerized and polycondensed to prepare recycled nylon 6. However, this technical route requires pre-separation and purification of CD, and the ring-opening reaction is carried out in an independent aqueous system. The processes are independent of each other, resulting in a long process flow, large equipment investment, and limited production efficiency.

[0006] Therefore, in view of the above problems, the present invention provides a method for supercritical CO2-assisted removal of extractable oligomers of nylon 6 and simultaneous catalytic conversion, so as to solve the problems of low removal efficiency of cyclic dimers and long process flow caused by the need for pre-separation and purification before catalytic conversion in the existing nylon 6 extractable oligomer removal process. Summary of the Invention

[0007] The purpose of this invention is to provide a method for supercritical CO2-assisted removal of extractable oligomers from nylon 6 and simultaneous catalytic conversion. This method integrates supercritical CO2 dynamic extraction with sulfonic acid-catalyzed ring-opening conversion into the same supercritical fluid system. Under low-temperature conditions of 50-100°C, supercritical CO2 fluid containing a sulfonic acid catalyst is used to dynamically extract nylon 6 material, allowing the cyclic dimers in nylon 6 to be extracted into the supercritical fluid phase and simultaneously undergo ring-opening conversion within the fluid phase. This achieves efficient removal of extractable oligomers and resource-based conversion of cyclic dimers in nylon 6 within the same device, avoiding the cumbersome process of pre-separation and purification of CD, shortening the process flow, reducing equipment investment and energy consumption, and ensuring that the final nylon 6 material contains ≤0.5wt% total extractable oligomers and ≤0.2wt% cyclic dimers.

[0008] The objective of this invention is achieved through the following technical solution: A method for supercritical CO2-assisted removal of extractable nylon 6 oligomers and simultaneous catalytic conversion includes the following steps: S1. Preparation: Place the nylon 6 material containing extractable oligomers into a supercritical reactor. The extractable oligomers contain at least caprolactam and cyclic dimers.

[0009] S2. Preparation of supercritical fluid: Sulfonic acid catalyst and entrainer are added to CO2 to form a supercritical CO2 fluid containing the catalyst; the entrainer is a mixture of methanol and water.

[0010] S3. Simultaneous Extraction-Conversion: Under the conditions of temperature 50~100℃ and pressure 15~35 MPa, the nylon 6 material is dynamically extracted with the supercritical CO2 fluid containing the catalyst obtained in step S2 for 30~240 min. During the dynamic extraction process, the supercritical CO2 fluid containing the catalyst flows continuously through the nylon 6 material at a flow rate of 0.5~5 L / min. The cyclic dimers in the nylon 6 material are extracted into the supercritical fluid phase and converted into caprolactam and linear oligomers under the action of sulfonic acid catalyst.

[0011] S4. Separation: After extraction, vacuum separation is performed to obtain nylon 6 material with the cyclic dimer removed and a fluid phase containing the ring-opening product.

[0012] Preferably, in step S1, the extractable oligomers further include at least one of cyclic trimers, cyclic tetramers, cyclic pentamers, cyclic hexamers, cyclic heptamers, cyclic octamers, and cyclic nonamers. In the equilibrium product of nylon 6 polymerization, caprolactam accounts for approximately 90 wt% of the total extractable oligomers, cyclic dimers account for approximately 90% of the total cyclic oligomers, and the remainder consists of cyclic trimers to cyclic nonamers. The cyclic dimers have a melting point of 348°C, which is much higher than that of nylon 6. They are stable and do not easily undergo ring-opening, making them the most difficult component to process among the extractable oligomers of nylon 6.

[0013] Preferably, in step S1, the nylon 6 material is in granular form. Granular form facilitates uniform flow of the supercritical fluid in the material bed, ensuring sufficient contact between the fluid and the material during dynamic extraction and improving mass transfer efficiency.

[0014] Preferably, in step S2, the mass ratio of methanol to water in the entrainer is 95:5 to 50:50. Water molecules can effectively swell the nylon 6 matrix, promoting the diffusion and migration of cyclic dimers from the polymer interior to the surface, while methanol has good solubility for cyclic dimers. The synergistic effect of the two ensures both the efficiency of cyclic dimer migration from the matrix interior and its solubility stability in the supercritical fluid phase.

[0015] Preferably, in step S2, the mass ratio of methanol to water in the entrainer is 70:30 to 50:50. Within this ratio range, the swelling effect of water on the nylon 6 matrix and the dissolving effect of methanol on the cyclic dimer are optimally balanced, resulting in optimal removal efficiency and ring-opening conversion rate of the cyclic dimer.

[0016] Preferably, in step S2, the sulfonic acid catalyst is one of p-toluenesulfonic acid, methanesulfonic acid, or trifluoromethanesulfonic acid, and the amount of catalyst used is 0.1wt% to 5wt% of the mass of the cyclic dimer. The sulfonic acid catalyst can effectively catalyze the ring-opening reaction of the cyclic dimer at a relatively low temperature, converting the stable cyclic dimer into caprolactam and linear oligomers, and the ring-opening product has polymerizable activity.

[0017] Preferably, in step S2, the sulfonic acid catalyst is p-toluenesulfonic acid. P-Toluenesulfonic acid is a better choice due to its suitable acidity and good catalytic selectivity; it ensures high catalytic activity while having fewer side reactions and producing a purer product.

[0018] Preferably, in step S3, the dynamic extraction is carried out at a temperature of 60-80℃ and a pressure of 20-30 MPa for a time of 60-120 min. Under these preferred conditions, supercritical CO2 has a good diffusion coefficient and solubility, enabling it to complete the extraction and conversion of cyclic dimers in a short time, thus achieving high processing efficiency.

[0019] Preferably, in step S3, the dynamic extraction is carried out under periodically pulsating pressure conditions, with pressure increasing and decreasing at a rate of 0.5 to 2 MPa / min within the range of 15 to 35 MPa, and each pressure-depressurization cycle lasting 5 to 15 minutes. The periodic pressure pulsation causes regular changes in the density and solubility of the supercritical fluid. During the pressure-increasing phase, it enhances the fluid's ability to dissolve cyclic dimers, while during the depressurization phase, it promotes repeated permeation and extraction of the fluid within the micropores of the nylon 6 matrix, generating a "breathing"-like enhanced mass transfer effect. This further promotes the diffusion and migration of cyclic dimers from the interior of the nylon 6 matrix into the fluid phase, thereby improving the extraction efficiency.

[0020] Preferably, in step S3, the ring-opening conversion rate of the cyclic dimer reaches 50%~90%. The cyclic dimer is in a molecularly dispersed state in the supercritical CO2 fluid phase, and its contact efficiency with the sulfonic acid catalyst is much higher than that of the system where the solid powder is dispersed in the aqueous phase. Therefore, a high ring-opening conversion rate can be achieved at a lower temperature.

[0021] Preferably, in step S4, the depressurization separation includes reducing the system pressure to atmospheric pressure, causing the supercritical CO2 to convert into a gaseous state and be recycled. After depressurization, the supercritical CO2 changes from a supercritical state to a gaseous state, automatically separating from the ring-opening products and entrainers. The collected CO2 gas can be recycled back to the extraction process, achieving green production.

[0022] Preferably, in step S4, the total content of extractable oligomers in the nylon 6 material after cyclic dimer removal is ≤0.5 wt%, and the content of cyclic dimers is ≤0.2 wt%. After treatment by the method of the present invention, the caprolactam monomer in the nylon 6 material is efficiently removed, and the content of the most difficult-to-remove cyclic dimers is also reduced to below 0.2 wt%. The resulting nylon 6 material can be directly used for subsequent spinning or injection molding processes, significantly reducing the problems of fiber breakage and clogging caused by CD precipitation.

[0023] Preferably, in step S4, after separation, the ring-opening product in the fluid phase containing the ring-opening product is recycled as a raw material to the nylon 6 polymerization process. The caprolactam and linear oligomers in the ring-opening product have polymerizable activity; recycling them to the nylon 6 polymerization process enables complete resource utilization of the extractable oligomers of nylon 6, avoiding waste emissions and reducing raw material consumption.

[0024] Preferably, in step S4, the nylon 6 material to which the cyclic dimer has been removed is vacuum dried at 60°C to constant weight to obtain the final product.

[0025] Due to the application of the above technical solution, the present invention has the following beneficial effects compared with the prior art: 1. This invention integrates supercritical CO2 dynamic extraction and sulfonic acid catalytic ring-opening conversion into the same supercritical fluid system, achieving efficient removal of extractable oligomers and simultaneous catalytic conversion of cyclic dimers in nylon 6 in the same device, avoiding the pre-separation and purification of cyclic dimers and shortening the process flow.

[0026] 2. This invention achieves the extraction and ring-opening transformation of cyclic dimers at a low temperature of 50~100℃. The reaction temperature is lower than that of existing catalytic ring-opening technologies, which reduces the energy consumption of the process.

[0027] 3. This invention utilizes supercritical CO2 fluid phase as reaction medium. The cyclic dimer is in a molecularly dispersed state in the fluid phase, and its contact efficiency with sulfonic acid catalyst is better than the solid-liquid contact mode in the existing aqueous system. A high ring-opening conversion rate can be obtained with a lower catalyst dosage.

[0028] 4. This invention uses a methanol / water composite entrainer. Water molecules swell the nylon 6 matrix to promote the outward migration of cyclic dimers, while methanol enhances the solubility of cyclic dimers by supercritical CO2. The synergistic effect of the two results in the final nylon 6 material having a total extractable oligomer content of ≤0.5wt% and a cyclic dimer content of ≤0.2wt%.

[0029] 5. In this invention, CO2 can be recycled and reused, and the ring-opening products can be used as raw materials in the nylon 6 polymerization process, realizing the full-component resource utilization of extractable oligomers and avoiding waste emissions. Detailed Implementation

[0030] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific implementation schemes are now described in detail.

[0031] The present invention will be further described below with reference to embodiments, but the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0032] Example 1 This embodiment provides a method for supercritical CO2-assisted removal of extractable nylon 6 oligomers and simultaneous catalytic conversion, including the following steps: S1. Preparation: Place 50g of unextracted nylon 6 granules (total extractable oligomer content 13.47wt%, including caprolactam 10.24wt%, cyclic dimers 1.85wt%, and other oligomers 1.38wt%) into a supercritical reactor.

[0033] S2. Preparation of supercritical fluid: Sulfonic acid catalyst and entrainer are added to CO2 to form a supercritical CO2 fluid containing the catalyst. In this embodiment, the entrainer is a mixture of 180g of deionized water and 20g of methanol (methanol to water mass ratio of 10:90), the sulfonic acid catalyst is p-toluenesulfonic acid, and the amount of catalyst used is 2.5wt% of the mass of the cyclic dimer (based on 50g of raw material in this embodiment, the cyclic dimer content is 1.85wt%, and the amount of catalyst used is approximately 0.023g).

[0034] S3. Simultaneous Extraction-Conversion: Nylon 6 granules were dispersed with an entrainer and transferred to a 500 mL autoclave. The autoclave was heated to 50°C and held at that temperature for 0.5 h. Then, it was purged three times with CO2 to remove air from the autoclave. Subsequently, CO2 was introduced to 25 MPa. Under these conditions (50°C, 25 MPa), the nylon 6 material was dynamically extracted using supercritical CO2 fluid containing a catalyst for 120 min. The supercritical CO2 fluid containing the catalyst was continuously flowed through the nylon 6 material at a flow rate of 0.5–5 L / min. During the extraction process, the cyclic dimers in the nylon 6 material were extracted into the supercritical fluid phase and, under the action of a sulfonic acid catalyst, underwent ring-opening conversion into caprolactam and linear oligomers.

[0035] S4. Separation: After extraction, the mixture is cooled to room temperature in a water bath and slowly depressurized to atmospheric pressure, allowing the supercritical CO2 to be converted into a gaseous state and recycled. The granules and extract are removed from the autoclave, filtered, and the granules are washed three times with anhydrous ethanol and dried under vacuum at 60°C to constant weight to obtain the final product.

[0036] Example 2 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0037] The difference between this embodiment and the above embodiment 1 is that in step S2, the entrainer is a mixture of 100g deionized water and 100g methanol (the mass ratio of methanol to water is 50:50), and the rest is the same as in embodiment 1.

[0038] Example 3 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0039] The difference between this embodiment and the above embodiment 1 is that in step S3, the extraction time is 90 min, and the rest is the same as in embodiment 1.

[0040] Example 4 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0041] The difference between this embodiment and the above embodiment 1 is that in step S3, the temperature is raised to 90°C (i.e., the extraction temperature is 90°C), while the rest is the same as in embodiment 1.

[0042] Example 5 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0043] The difference between this embodiment and the above embodiment 1 is that in step S3, dynamic extraction is carried out under the condition of periodic pressure pulsation, with pressure increasing and decreasing at a rate of 0.5 to 2 MPa / min in the range of 15 to 35 MPa, and the cycle of each pressure increase-decrease cycle is 5 to 15 min. The rest is the same as in embodiment 1.

[0044] Example 6 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0045] The difference between this embodiment and the above embodiment 1 is that: in step S2, the mass ratio of methanol to water in the entrainer is 95:5, the catalyst is methanesulfonic acid, and the amount of catalyst is 0.1 wt% of the mass of the cyclic dimer; in step S3, the extraction temperature is 60°C and the extraction time is 240 min; the rest are the same as in embodiment 1.

[0046] Example 7 This embodiment is based on the above embodiment 1, and the similarities with the above embodiment 1 will not be repeated.

[0047] The difference between this embodiment and the above embodiment 1 is that: in step S2, the mass ratio of methanol to water in the entrainer is 70:30, the catalyst is trifluoromethanesulfonic acid, the amount of catalyst is 5wt% of the mass of the cyclic dimer, and in step S3, the extraction temperature is 80℃, the pressure is 20MPa, and the extraction time is 60min. All other aspects are the same as in embodiment 1.

[0048] Comparative Example 1 This comparative example is based on Example 1 above, and the similarities with Example 1 above will not be repeated.

[0049] The difference between this comparative example and Example 1 above is that in step S2, the entrainer is 200g of deionized water and 0g of methanol (i.e., pure water, with a methanol to water mass ratio of 0:100), while the rest is the same as in Example 1.

[0050] Comparative Example 2 This comparative example is based on Example 1 above, and the similarities with Example 1 above will not be repeated.

[0051] The difference between this comparative example and Example 1 above is that in step S2, the entrainer is 0g of deionized water and 200g of methanol (i.e., pure methanol, with a methanol to water mass ratio of 100:0), while the rest is the same as in Example 1.

[0052] Comparative Example 3 This comparative example is based on Example 1 above, and the similarities with Example 1 above will not be repeated.

[0053] The difference between this comparative example and Example 1 above is that in step S3, supercritical CO2 is not introduced, that is, dynamic extraction is not carried out in the supercritical CO2 system. The rest is the same as in Example 1.

[0054] Comparative Example 4 This comparative example is based on Example 1 above, and the similarities with Example 1 above will not be repeated.

[0055] The difference between this comparative example and Example 1 above is that in step S3, the extraction temperature is 120°C and the pressure is 35 MPa, while the rest are the same as in Example 1.

[0056] The content of extractable oligomers in the final products obtained in each embodiment and comparative example was tested using the following methods: The content of extractable oligomers was tested using high performance liquid chromatography (HPLC). The instrument was a Waters 2489 HPLC system with an Agilent C18 column (250 × 4.6 mm), an injection volume of 10 μL, a flow rate of 0.8 mL / min, a detection wavelength of 220 nm, a column temperature of 35 °C, and a gradient elution system of phosphate buffer (pH=3) and methanol as the mobile phase.

[0057] Take an appropriate amount of the final product, weigh it as m1, extract it with methanol using a Soxhlet extractor for 12 hours, carefully remove the sample, vacuum dry it to constant weight, and weigh it as m2. (m1-m2) / m1×100% is the total content of extractable oligomers. After making up the volume of the extract, inject it into the instrument for analysis. Use the standard sample to create an external standard curve for quantitative analysis of caprolactam and cyclic dimers.

[0058] The test results for each embodiment and comparative example are shown in Table 1.

[0059] Table 1

[0060] As shown in Table 1, the caprolactam removal rate of each embodiment reached over 96%, indicating that the method of the present invention has excellent removal effect on caprolactam; the removal rate of cyclic dimers reached over 89%, with Examples 2 and 4 reaching 95.14% and 96.22% respectively, indicating that the method of the present invention also has significant removal and conversion effects on stable and difficult-to-remove cyclic dimers.

[0061] A comparison of Example 1 and Comparative Example 1 shows that when the methanol content is low (10%), the removal rate of cyclic dimers in both Example 1 and the pure water system (Comparative Example 1) is 90.81%, indicating that the addition of a small amount of methanol does not produce a significant synergistic gain, and the swelling effect of water is dominant at this point. A comparison of Example 2 with Comparative Examples 1 and 2 shows that when the methanol to water mass ratio is 50:50, the removal rate of cyclic dimers (95.14%) is significantly higher than that in the pure water system (90.81%) and the pure methanol system (93.51%), confirming a synergistic effect between the swelling effect of water and the dissolving effect of methanol in the methanol / water composite entrainer. Comparative Example 2 uses pure methanol as an entrainer, which has a certain dissolving ability for cyclic dimers, but lacks the swelling effect of water, resulting in a high residue of other oligomers (0.15 wt%).

[0062] A comparison of Examples 1 and 4 shows that when the extraction temperature is increased from 50°C to 90°C, the removal rate of cyclic dimers increases from 90.81% to 96.22%, indicating that appropriately increasing the temperature is beneficial to improving the diffusion coefficient and solubility of supercritical CO2, thereby enhancing the extraction efficiency. However, the present invention can obtain an acceptable high removal rate (90.81%) at a lower temperature of 50°C, demonstrating the advantages of low-temperature operation.

[0063] A comparison of Examples 2 and 5 shows that, under the same entrainer ratio (50:50), after introducing periodic pressure pulsation, the removal rate of cyclic dimers further increased from 95.14% to 95.68%, indicating that periodic pressure pulsation, through a "breathing" effect, promoted the diffusion and migration of cyclic dimers from the interior of the nylon 6 matrix to the fluid phase, further enhancing the mass transfer effect.

[0064] As shown in Comparative Example 3, without the introduction of supercritical CO2, the removal rate of cyclic dimers was only 41.62%, and the total content of extractable oligomers was as high as 3.56 wt%, which was far lower than the removal effect of each example. This indicates that the dynamic extraction effect of supercritical CO2 is essential for the efficient removal of cyclic dimers.

[0065] As shown in Comparative Example 4, although a slightly higher removal rate (96.76%) was observed at 120℃, this temperature exceeded the 50-100℃ low-temperature extraction range specified in this invention, resulting in higher energy consumption and the risk of thermal oxidative degradation of the nylon 6 matrix. This invention achieves a cyclic dimer removal rate of ≥89% at a low temperature of 50-100℃, with a minimum removal rate as low as 0.07wt%, realizing low-temperature, high-efficiency, and integrated extraction and simultaneous catalytic conversion.

[0066] This invention utilizes supercritical CO2 fluid containing sulfonic acid catalyst for dynamic extraction at low temperatures of 50~100℃, achieving efficient removal of extractable oligomers and simultaneous catalytic conversion of cyclic dimers in nylon 6. The effect is optimal when the methanol to water mass ratio is 50:50, which can reduce the content of cyclic dimers in the final product to below 0.09wt% and the total content of extractable oligomers to below 0.44wt%, which is superior to existing technical solutions.

[0067] In summary, this invention integrates supercritical CO2 dynamic extraction and sulfonic acid catalytic ring-opening conversion into the same supercritical fluid system. Under low-temperature conditions of 50-100°C, supercritical CO2 fluid containing a sulfonic acid catalyst is used to dynamically extract nylon 6 material, allowing the cyclic dimers in nylon 6 to be extracted into the supercritical fluid phase and simultaneously undergo ring-opening conversion within the fluid phase. This achieves efficient removal of extractable oligomers and resource-based conversion of cyclic dimers in the same device. This method avoids the cumbersome process of pre-separating and purifying cyclic dimers, shortens the process flow, and reduces equipment investment and energy consumption. Simultaneously, the swelling effect of water and the dissolving effect of methanol in the methanol / water composite entrainer synergistically complement each other, combined with the efficient catalysis of the sulfonic acid catalyst in the fluid phase, resulting in a final nylon 6 material with a total extractable oligomer content ≤0.5wt% and a cyclic dimer content ≤0.2wt%. In addition, the CO2 separated by vacuum can be recycled, and the ring-opening product can be used as a raw material in the nylon 6 polymerization process, realizing the full-component resource utilization of extractable oligomers, which has good industrial application prospects and environmental benefits.

[0068] The embodiments described above are merely illustrative of more specific and detailed implementations of the present invention, and should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for supercritical CO2-assisted removal of extractable oligomers of nylon 6 and simultaneous catalytic conversion, characterized in that, Includes the following steps: S1. Preparation: Place the nylon 6 material containing extractable oligomers into a supercritical reactor. The extractable oligomers contain at least caprolactam and cyclic dimers. S2. Preparation of supercritical fluid: Sulfonic acid catalyst and entrainer are added to CO2 to form a supercritical CO2 fluid containing the catalyst; the entrainer is a mixture of methanol and water. S3. Simultaneous extraction-conversion: Under the conditions of temperature 50~100℃ and pressure 15~35 MPa, the nylon 6 material is dynamically extracted with the supercritical CO2 fluid containing the catalyst obtained in step S2 for 30~240 min; during the dynamic extraction process, the supercritical CO2 fluid containing the catalyst flows continuously through the nylon 6 material at a flow rate of 0.5~5 L / min. The cyclic dimers in nylon 6 material were extracted into the supercritical fluid phase and, under the action of sulfonic acid catalyst, were ring-opened and converted into caprolactam and linear oligomers; S4. Separation: After extraction, vacuum separation is performed to obtain nylon 6 material with the cyclic dimer removed and a fluid phase containing the ring-opening product.

2. The method according to claim 1, characterized in that, In step S2, the mass ratio of methanol to water in the entrainer is 95:5 to 50:

50.

3. The method according to claim 1, characterized in that, In step S2, the sulfonic acid catalyst is one of p-toluenesulfonic acid, methanesulfonic acid or trifluoromethanesulfonic acid, and the amount of catalyst used is 0.1wt% to 5wt% of the mass of the cyclic dimer.

4. The method according to claim 1, characterized in that, In step S3, the dynamic extraction is carried out under periodic pressure pulsation, with the pressure increased and decreased at a rate of 0.5 to 2 MPa / min within the range of 15 to 35 MPa, and the cycle of each pressure increase-depressurization cycle is 5 to 15 min.

5. The method according to claim 1, characterized in that, In step S3, the ring-opening conversion rate of the cyclic dimer reaches 50%~90%.

6. The method according to claim 1, characterized in that, In step S4, the total content of extractable oligomers in the nylon 6 material after removing the cyclic dimers is ≤0.5 wt%, and the content of cyclic dimers is ≤0.2 wt%.

7. The method according to claim 1, characterized in that, In step S4, after the fluid phase containing the ring-opening product is separated, the ring-opening product is recycled as a raw material in the nylon 6 polymerization section.

8. The method according to claim 1, characterized in that, In step S1, the extractable oligomer further includes at least one of cyclic trimers to cyclic nonamers.

9. The method according to claim 1, characterized in that, In step S3, the nylon 6 material is in granular form.

10. The method according to claim 1, characterized in that, In step S4, the pressure reduction separation includes reducing the system pressure to atmospheric pressure, so that the supercritical CO2 is converted into a gaseous state and recycled.

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