Method for recycling nylon 6 solid material and regenerating caprolactam monomer through depolymerization

Abstract

The invention relates to a method for depolymerizing and regenerating caprolactam monomers from recovered nylon 6 solid materials, and relates to the technical field of depolymerizing and regenerating the recovered nylon 6 solid materials, the method comprises the following steps: passing recovered nylon 6 solid particles through a feeding device, and degassing to obtain a nylon 6 high polymer melt; adding the nylon 6 high polymer melt into a depolymerization kettle, and carrying out catalytic depolymerization reaction under the action of a catalyst and superheated steam to obtain a first depolymerization product and mixed gas; the mixed gas enters a concentration tower to be concentrated, and a first caprolactam monomer concentrated solution is obtained; the first caprolactam monomer concentrated solution enters a dehydrating tower to be dehydrated, then alkali liquor is added, and a second caprolactam monomer concentrated solution is obtained. According to the method disclosed by the invention, depolymerization recovery is carried out under a mild condition, so that the technical process is simplified, the method is environment-friendly, the high-purity caprolactam monomer is continuously recovered in a large-capacity manner, the physical and chemical properties of the caprolactam monomer reach GB / T13254-2017 superior product indexes, and the quality requirements of producing high-quality nylon 6 slices are met.

Description

Technical Field

[0001] The invention relates to the technical field of recycling nylon 6 solid material by depolymerization and regeneration, and in particular to a method for recycling nylon 6 solid material by depolymerization and regenerating caprolactam monomer. Background Art

[0002] Nylon 6 (also known as polyamide 6 or nylon 6) is a polymer compound synthesized from caprolactam through a series of chemical reactions such as hydrolysis ring opening, addition polymerization and condensation polymerization. With its excellent physical and chemical properties, such as high strength and toughness, self-lubricating properties, good hot processing properties and excellent corrosion resistance, nylon 6 has been widely used in the fields of films, castings and fiber products.

[0003] With the large-scale consumption of nylon 6 products, the solid waste generated is also increasing. These wastes are difficult to recycle and treat due to their stable physical and chemical properties, and are difficult to degrade naturally. If not properly managed, they will pose a major threat to the ecological environment.

[0004] At present, the recycling and treatment methods of nylon 6 solid waste mainly include energy recovery, physical recovery and chemical recovery. Although energy recovery can release the molecular bond energy in polymers through incineration, it is easy to produce nitrogen oxides in the process, causing secondary pollution, so this method is gradually being eliminated. Physical recovery is to remove small molecules from the recovered solid waste through solvent extraction, and then wash, dry, and melt granulate it into nylon 6 chips. Although this method is simple and easy to operate, it has high requirements for the purity of the raw materials, otherwise the quality of the recycled nylon 6 chips will be greatly reduced, so its application is subject to certain restrictions. Chemical recovery, also known as tertiary recovery, is a more thorough and more valuable recovery method. It uses heat energy and chemical reagents to cause the recovered nylon 6 solid waste to undergo a depolymerization reaction to generate caprolactam monomer and a small amount of by-products. Subsequently, through separation and purification, high-concentration and high-purity caprolactam monomer can be obtained. This method not only fundamentally solves the problem of environmental pollution, but also realizes the recycling of resources.

[0005] Chemical recycling, as a closed-loop, sustainable and high-quality green recycling method, has made significant progress in this field. According to the reaction conditions or mechanisms of chemical recycling, it can be roughly divided into cracking method, hydrolysis method, alcoholysis method, aminolysis method, supercritical method and ionic liquid method. However, existing recycling methods often have many challenges, such as depolymerization reaction under high temperature and high pressure conditions, complex process and many by-products, and lengthy post-processing separation and purification process of depolymerization products. These problems lead to high manufacturing costs of equipment, harsh operating conditions, high energy consumption and environmental pollution, making it difficult to achieve continuous and industrial large-scale application. Therefore, it is an urgent problem to improve and optimize the existing recycling methods to reduce costs, simplify processes, improve efficiency and environmental protection. Summary of the invention

[0006] To solve the above problems, the present invention provides a method for depolymerizing and regenerating caprolactam monomer from recycled nylon 6 solid material.

[0007] The technical solution provided by the present invention is as follows: The present invention provides a method for depolymerizing and regenerating caprolactam monomer from recycled nylon 6 solid material, and the method comprises the following steps: Feeding the recycled nylon 6 solid granular material through a feeding device and degassing to obtain a nylon 6 polymer melt; Adding the nylon 6 polymer melt into a depolymerization kettle, and carrying out a catalytic depolymerization reaction under the action of a catalyst and superheated steam to obtain a first depolymerization product and a mixed gas; Feeding the mixed gas into a concentration tower for concentration to obtain a first concentrated solution of caprolactam monomer; Feeding the first concentrated solution of caprolactam monomer into a dehydration tower for dehydration, and then adding an alkali solution to obtain a second concentrated solution of caprolactam monomer; Removing impurities with boiling points lower than that of caprolactam from the second concentrated solution of caprolactam monomer through a pre-distillation tower to obtain a first regenerated caprolactam monomer solution; Removing impurities with boiling points higher than that of caprolactam from the first caprolactam monomer solution through a distillation tower to obtain a second regenerated caprolactam monomer solution.

[0008] Further, the recycled nylon 6 solid particles include at least one of solid particle foams made from fishing net filaments, carpet filaments or waste filaments in spinning and nylon 6 waste slices.

[0009] Further, the feeding device includes a primary metering screw and a secondary melting screw. The primary metering screw includes a loss-in-weight metering scale and a feeding screw, and the secondary melting screw includes a melting extruder with a feeding section, a melting section, a degassing section and a discharging section.

[0010] Further, the depolymerization kettle is a two-stage structure depolymerization kettle. The upper part of the two-stage structure depolymerization kettle is not provided with a heat preservation jacket. The lower part of the two-stage structure depolymerization kettle includes a diphenyl heat medium heat preservation jacket, a superheated steam distributor and a stirrer. The superheated steam distributor includes a plurality of circular tubes with radially arranged sieve holes, and the stirrer includes a six-blade semi-tube disk turbine structure.

[0011] Further, the operating condition parameters of the depolymerization kettle include: an operating pressure of 0.003 - 0.03 MPa and an operating temperature of 240 - 260 °C.

[0012] Further, the catalyst includes phosphoric acid; calculated by weight percentage, the proportion of the catalyst in the depolymerization kettle is 10 - 30%.

[0013] Furthermore, the mass ratio of the nylon 6 polymer melt to the superheated steam is 1:(1 - 3), the residence time of the nylon 6 polymer melt is 3 - 5 hours, and the cracking rate of the nylon 6 polymer melt is ≥98%; the mixed gas is the superheated steam and the second depolymerization product stripped from the nylon 6 polymer melt, and the weight proportion of the second depolymerization product in the mixed gas is 25 - 50%.

[0014] Furthermore, the concentration tower is a first structured packing tower, and a first external condenser is arranged at the top of the first structured packing tower; the operating condition parameters of the first structured packing tower include: pressure 0 - 0.02 MPa, temperature 90 - 110 °C, top reflux ratio 0.2 - 0.5; by weight percentage, the water content of the first caprolactam monomer concentrated solution is 40 - 70%; The dehydration tower is a second structured packing tower, a second external condenser is arranged at the top of the second structured packing tower, and a first self - circulating falling - film evaporator is arranged at the bottom of the second structured packing tower; the operating condition parameters of the second structured packing tower include: pressure - 0.085 - - 0.095 MPa, temperature 110 - 120 °C, top reflux ratio 0.5 - 0.8; by weight percentage, the water content of the second caprolactam monomer concentrated solution is <1.5%; The alkali solution includes at least one of a sodium hydroxide solution with a weight percentage of 30 - 40% and a potassium hydroxide solution with a weight percentage of 30 - 40%; the weight proportion of the alkali solution in the second caprolactam monomer concentrated solution is 0.1 - 0.3%.

[0015] Furthermore, the pre - rectification tower is a third structured packing tower, a first internal condenser is arranged at the top of the third structured packing tower, and a second self - circulating falling - film evaporator is arranged at the bottom of the third structured packing tower; the operating condition parameters of the third structured packing tower include: pressure ≤ - 0.099 MPa, temperature 115 - 125 °C, and the top reflux is total reflux; the bottom fraction of the pre - rectification tower obtains a caprolactam monomer solution with a purity of more than 98.5 wt%. The rectification tower is a fourth structured packing tower, a second internal condenser is arranged at the top of the fourth structured packing tower, and a third self - circulating falling - film evaporator is arranged at the bottom of the fourth structured packing tower; the operating condition parameters of the rectification tower include: pressure ≤ - 0.099 MPa, temperature 120 - 130 °C, top reflux ratio 0.3 - 0.6; the top fraction of the rectification tower obtains a caprolactam monomer solution with a purity of more than 99.5 wt%.

[0016] Further, the second recycled caprolactam monomer solution is separated from high-boiling impurities in the second recycled caprolactam monomer solution through a wiped-film evaporator, and low-boiling components are recycled, so that the caprolactam monomer yield of the recovered nylon 6 solid pellet material is ≥86 wt%; the operating condition parameters of the wiped-film evaporator include: pressure ≤ -0.095 MPa, temperature 130-140 °C; the wiped-film evaporator is a fixed scraper type wiped-film evaporator, and the cylinder body of the fixed scraper type wiped-film evaporator is provided with a steam heating jacket.

[0017] The above technical solutions provided by the embodiments of the present invention have at least the following advantages compared with the prior art: The present invention provides a method for depolymerizing and recycling caprolactam monomer from recycled nylon 6 solid material. The present invention performs depolymerization and recycling under mild conditions, realizing simplified process flow, environmental friendliness, continuous and large-capacity recovery of high-purity caprolactam monomer, and the physical and chemical properties reach the first-class product index of GB / T13254-2017, meeting the quality requirements for producing high-quality nylon 6 chips. Specifically: (1) In this solution, the sources of the recycled nylon 6 solid material are diverse, including both waste chips generated during the nylon 6 polymerization process, defective filaments generated during spinning production, and recyclable solid materials such as fishing net filaments, carpet filaments, and cord fabrics made of nylon 6.

[0018] (2) The depolymerization reaction conditions in this solution are mild and environmentally friendly. The depolymerization kettle operates under slightly positive pressure, with a pressure of only 0.003-0.03 MPa. At the same time, the operating temperature is almost lower than the normal boiling point of caprolactam. Using common phosphoric acid as a catalyst, through the coordinated action with superheated steam, the cracking rate is greatly improved, and the discharge of solid waste residue is reduced.

[0019] (3) The process flow for recycling caprolactam monomer from the recycled nylon 6 solid material in this solution is simplified, and it can be continuously and large-capacity applied in production. First, through a two-stage feeding device, continuous metering and melting feeding of the recycled solid material are realized into the depolymerization kettle. Then, a steam mixture is obtained through the mild reaction of the depolymerization kettle, and then through processes such as concentration, dehydration, pre-distillation, and rectification. Finally, high-purity caprolactam monomer is obtained. The whole process is short, continuous, highly operable, and conducive to device scaling-up and industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present invention, and are used together with the specification to explain the principles of the present invention.

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 It is a flow chart of the method for depolymerizing and recycling caprolactam monomer from recycled nylon 6 solid material provided by the present invention; Figure 2 It is a simplified device flow diagram of the method for depolymerizing and recycling caprolactam monomer from recycled nylon 6 solid material provided by the present invention; Figure 2 In it: E1 - primary metering screw, E2 - secondary melting screw, E3 - depolymerization kettle, E4 - concentration tower, E5 - dehydration tower, E6 - pre - rectification tower, E7 - rectification tower, E8 - wiped - film evaporator; S1 - recycled nylon 6 solid material, S2 - degassing, S3 - catalyst, S4 - superheated steam, S5 - cracking residue, S6 - condensed water, S7 - lye, S8 - condensed water, S9 - non - condensable gas, S10 - caprolactam monomer, S11 - evaporation residue. Specific embodiments

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0024] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0025] The present invention provides a method for depolymerizing and recycling caprolactam monomer from recycled nylon 6 solid material, and the method includes the following steps: Feed the recycled nylon 6 solid pellet material through a feeding device and degas it to obtain a nylon 6 polymer melt; Add the nylon 6 polymer melt into a depolymerization kettle, and carry out a catalytic depolymerization reaction under the action of a catalyst and superheated steam to obtain a first depolymerization product and a mixed gas; Feed the mixed gas into a concentration tower for concentration to obtain a first concentrated caprolactam monomer solution; Feed the first concentrated caprolactam monomer solution into a dehydration tower for dehydration, and then add lye to obtain a second concentrated caprolactam monomer solution; The second caprolactam monomer concentrate is subjected to pre-distillation in a pre-distillation column to remove impurities with boiling points lower than that of caprolactam, obtaining a first regenerated caprolactam monomer solution; The first caprolactam monomer solution is subjected to distillation in a distillation column to remove impurities with boiling points higher than that of caprolactam, obtaining a second regenerated caprolactam monomer solution.

[0026] The present invention provides a method for recycling and regenerating caprolactam monomers by depolymerizing nylon 6 solid materials. The present invention conducts depolymerization and recycling under mild conditions, achieving a simplified process flow, environmental friendliness, continuous, large-capacity recovery of high-purity caprolactam monomers. The physical and chemical properties meet the first-class product indicators of GB / T13254-2017, meeting the quality requirements for producing high-quality nylon 6 chips.

[0027] In the present invention, the first depolymerization product is a mixture, including the main product caprolactam and other by-products. A part of it enters the mixture through the stripping of hot steam, and a part remains in the depolymerization kettle.

[0028] In some specific embodiments, the recycled nylon 6 solid particles include at least one of solid particle foams made from fishing net filaments, carpet filaments or waste filaments generated in spinning, and nylon 6 waste slices.

[0029] In some specific embodiments, the feeding device includes a primary metering screw and a secondary melting screw. The primary metering screw includes a loss-in-weight scale and a feeding screw, and the secondary melting screw includes a melting extruder with a feeding section, a melting section, a degassing section and a discharging section.

[0030] In some specific embodiments, the depolymerization kettle is a two-stage structure depolymerization kettle. The upper part of the two-stage structure depolymerization kettle is not provided with a heat preservation jacket. The lower part of the two-stage structure depolymerization kettle includes a diphenyl heat medium heat preservation jacket, a superheated steam distributor and a stirrer. The superheated steam distributor includes several radially arranged round tubes with sieve holes, and the stirrer includes a six-blade semi-tube disc turbine structure.

[0031] In some specific embodiments, the operating condition parameters of the depolymerization kettle include: the operating pressure is 0.003~0.03 MPa, and the operating temperature is 240~260 °C.

[0032] In some specific embodiments, the catalyst includes phosphoric acid; calculated by weight percentage, the proportion of the catalyst in the depolymerization kettle is 10~30%.

[0033] In some specific embodiments, the mass ratio of the nylon 6 polymer melt to the superheated steam is 1:(1 - 3), the residence time of the nylon 6 polymer melt is 3 - 5 hours, and the cracking rate of the nylon 6 polymer melt is ≥98%; the mixed gas is the superheated steam and the second depolymerization product stripped from the nylon 6 polymer melt, and the weight proportion of the second depolymerization product in the mixed gas is 25 - 50%.

[0034] In some specific embodiments, the concentration tower is a first structured packing tower, and a first external condenser is arranged at the top of the first structured packing tower; the operating condition parameters of the first structured packing tower include: pressure 0 - 0.02 MPa, temperature 90 - 110 °C, top reflux ratio 0.2 - 0.5; by weight percentage, the water content of the first caprolactam monomer concentrated solution is 40 - 70%; The dehydration tower is a second structured packing tower, a second external condenser is arranged at the top of the second structured packing tower, and a first self - circulating falling - film evaporator is arranged at the bottom of the second structured packing tower; the operating condition parameters of the second structured packing tower include: pressure - 0.085 - 0.095 MPa, temperature 110 - 120 °C, top reflux ratio 0.5 - 0.8; by weight percentage, the water content of the second caprolactam monomer concentrated solution is <1.5%, specifically the water content of the caprolactam solution after adding and subtracting liquid, and it is also the water content of the bottom discharge of the second structured packing tower; The alkali solution includes at least one of a sodium hydroxide solution with a weight percentage of 30 - 40% and a potassium hydroxide solution with a weight percentage of 30 - 40%; the weight proportion of the alkali solution in the second caprolactam monomer concentrated solution is 0.1 - 0.3%.

[0035] In some specific embodiments, the pre - rectifying tower is a third structured packing tower, a first internal condenser is arranged at the top of the third structured packing tower, and a second self - circulating falling - film evaporator is arranged at the bottom of the third structured packing tower; the operating condition parameters of the third structured packing tower include: pressure ≤ - 0.099 MPa, temperature 115 - 125 °C, and the top reflux is total reflux; the bottom fraction of the pre - rectifying tower obtains a caprolactam monomer solution with a purity of more than 98.5 wt%; The rectifying tower is a fourth structured packing tower, a second internal condenser is arranged at the top of the fourth structured packing tower, and a third self - circulating falling - film evaporator is arranged at the bottom of the fourth structured packing tower; the operating condition parameters of the rectifying tower include: pressure ≤ - 0.099 MPa, temperature 120 - 130 °C, top reflux ratio 0.3 - 0.6; the top fraction of the rectifying tower obtains a caprolactam monomer solution with a purity of more than 99.5 wt%.

[0036] In some specific embodiments, the second recycled caprolactam monomer solution is separated from high-boiling impurities in the second recycled caprolactam monomer solution through a wiped-film evaporator, and low-boiling components are recycled, so that the caprolactam monomer yield of the recovered nylon 6 solid pellet material is ≥86 wt%; the operating condition parameters of the wiped-film evaporator include: pressure ≤ -0.095 MPa, temperature 130 - 140 °C; the wiped-film evaporator is a fixed scraper type wiped-film evaporator, and the cylinder body of the fixed scraper type wiped-film evaporator is provided with a steam heating jacket.

[0037] In some specific embodiments, the flow chart of the method for depolymerizing and recycling caprolactam monomer from the recovered nylon 6 solid material provided by the present invention is as Figure 1 shown, and the simplified device flow chart adopted is as Figure 2 shown, Figure 2 wherein: E1 - primary metering screw, E2 - secondary melting screw, E3 - depolymerization kettle, E4 - concentration tower, E5 - dehydration tower, E6 - pre-distillation tower, E7 - distillation tower, E8 - wiped-film evaporator; S1 - recovered nylon 6 solid material, S2 - degassing, S3 - catalyst, S4 - superheated steam, S5 - cracking residue, S6 - condensate, S7 - lye, S8 - condensate, S9 - non-condensable gas, S10 - caprolactam monomer, S11 - evaporation residue.

[0038] It should be noted that for the raw materials involved in the method for depolymerizing and recycling caprolactam monomer from the recovered nylon 6 solid material provided by the present invention, if there is no special limitation or specific description, commercially available products can be directly used or prepared by oneself according to the preparation process disclosed in the prior art; meanwhile, for the operation steps and condition parameters involved, if there is no special limitation or specific description, they can be prepared according to the preparation process of the existing method for depolymerizing and recycling caprolactam monomer from the recovered nylon 6 solid material or by using existing equipment, and the present invention document will not elaborate one by one.

[0039] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The experimental methods without specific conditions noted in the following embodiments are usually determined according to national standards. If there is no corresponding national standard, they are carried out according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0040] Example 1 This example provides a method for depolymerizing and recycling caprolactam monomer from the recovered nylon 6 solid material, including the following steps: (1) 1500 kg of nylon 6 solid pellet foam made from recycled fishing net filaments is fed through the primary metering screw and the secondary melting screw feeding device to form a nylon 6 polymer melt, and air, oil agent and other volatile components in the melt are removed through the degassing section of the secondary melting twin screw; (2) The nylon 6 polymer melt undergoes a catalytic depolymerization reaction in the depolymerization kettle under the synergistic action of a catalyst and superheated steam, obtaining a vapor mixture composed of a depolymerization product containing caprolactam monomer and superheated steam. The operating conditions of the depolymerization kettle are as follows: pressure 0.02 MPa, temperature 250 °C, phosphoric acid is used as the catalyst, and its mass ratio in the depolymerization kettle is 20%. The mass ratio of the nylon 6 polymer melt to superheated steam is 1:2. The residence time of the nylon 6 polymer melt is 4 hours, the cracking rate is ≥98.3%, and the mass ratio of the depolymerization product in the vapor mixture is 33%; (3) The vapor mixture enters the concentration tower for concentration and dehydration to obtain a monomer concentrate with a water content of 50%. The operating conditions of the concentration tower are: pressure 0.01 MPa, temperature 100 °C, and top reflux ratio 0.4; (4) The obtained monomer concentrate reduces the water content to less than 1.5% in the dehydration tower to obtain a monomer solution, and a sodium hydroxide solution with a mass concentration of 30% is added, accounting for 0.2% of the mass of the monomer solution. Its operating conditions are: pressure -0.090 MPa, temperature 115 °C, and top reflux ratio 0.7; (5) The monomer solution removes impurities with boiling points lower than that of caprolactam through a pre - rectification tower, and a caprolactam monomer solution with a purity of over 98.5% is obtained from the bottom fraction. Its operating conditions are: pressure -0.099 MPa, temperature 125 °C, and total reflux at the top; (6) The caprolactam monomer solution further removes impurities with boiling points higher than that of caprolactam through a rectification tower, and caprolactam monomer with a purity of over 99.5% is obtained from the top fraction. Its operating conditions are: pressure -0.099 MPa, temperature 130 °C, and top reflux ratio 0.5; (7) A wiped - film evaporator realizes the separation of high - boiling - point impurities in the monomer solution, enables the reuse of low - boiling - point components in the monomer solution, improves the monomer recovery rate, and makes the caprolactam monomer recovery rate of the recycled nylon 6 solid material as high as over 86.6%. Its operating conditions are: pressure -0.095 MPa, temperature 140 °C.

[0041] The caprolactam obtained by the method for recycling and regenerating caprolactam monomer from nylon 6 solid material as described above is subjected to purity detection. The concentration of caprolactam monomer is 99.6%, meeting the first - class product index of the GB / T13254 - 2017 standard and meeting the quality requirements for producing high - quality nylon 6 chips.

[0042] Example 2 This example provides a method for recycling and regenerating caprolactam monomer from nylon 6 solid material, including the following steps: (1) 1500 kg of nylon 6 solid particle foam made from recycled carpet yarns is fed through a primary metering screw and a secondary melting screw feeding device to form a nylon 6 polymer melt, and volatile components such as air and oil agent in the melt are removed through the degassing section of the secondary melting twin screws. (2) In the depolymerization kettle, the nylon 6 polymer melt undergoes a catalytic depolymerization reaction under the synergistic action of a catalyst and superheated steam to obtain a steam mixture composed of a depolymerization product containing caprolactam monomer and superheated steam. The operating conditions of the depolymerization kettle are as follows: the pressure is 0.03 MPa, the temperature is 260 °C, phosphoric acid is used as the catalyst, and its mass ratio in the depolymerization kettle is 20%. The mass ratio of the nylon 6 polymer melt to superheated steam is 1:3. The residence time of the nylon 6 polymer melt is 5 hours, the cracking rate is ≧98.4%, and the mass ratio of the depolymerization product in the steam mixture is 25%. (3) The steam mixture enters the concentration tower for concentration and dehydration to obtain a monomer concentrate with a water content of 70%. The operating conditions of the concentration tower are as follows: the pressure is 0.02 MPa, the temperature is 110 °C, and the top reflux ratio is 0.5. (4) The obtained monomer concentrate is dehydrated in the dehydration tower to reduce the water content to less than 1.5% to obtain a monomer solution, and a sodium hydroxide solution with a mass concentration of 30% is added, accounting for 0.3% of the mass of the monomer solution. The operating conditions are as follows: the pressure is -0.095 MPa, the temperature is 110 °C, and the top reflux ratio is 0.8. (5) The monomer solution is passed through a pre-distillation tower to remove impurities with boiling points lower than that of caprolactam, and a caprolactam monomer solution with a purity of more than 98.5% is obtained from the bottom fraction. The operating conditions are as follows: the pressure is -0.100 MPa, the temperature is 120 °C, and the top reflux is total reflux. (6) The caprolactam monomer solution is further passed through a distillation tower to remove impurities with boiling points higher than that of caprolactam, and caprolactam monomer with a purity of more than 99.5% is obtained from the top fraction. The operating conditions are as follows: the pressure is -0.100 MPa, the temperature is 125 °C, and the top reflux ratio is 0.6. (7) The wiped film evaporator realizes the separation of high-boiling impurities in the monomer solution, and the low-boiling components in the monomer solution are recycled to improve the monomer yield, so that the caprolactam monomer yield of the recycled nylon 6 solid material is as high as more than 86.9%. The operating conditions are as follows: the pressure is -0.098 MPa, and the temperature is 135 °C.

[0043] The caprolactam obtained by the method for depolymerizing and regenerating caprolactam monomer from a recycled nylon 6 solid material is subjected to purity detection. The concentration of caprolactam monomer is 99.7%, reaching the first-class product index of the GB / T 13254-2017 standard and meeting the quality requirements for producing high-quality nylon 6 chips.

[0044] Example 3 This example provides a method for depolymerizing and regenerating caprolactam monomer from recycled nylon 6 solid material, including the following steps: (1) 1500 kg of waste nylon 6 bright chips are fed through a primary metering screw and a secondary melting screw feeding device to form a nylon 6 polymer melt, and volatile components such as air and sizing agent in the melt are removed through the degassing section of the secondary melting twin-screw; (2) The nylon 6 polymer melt undergoes a catalytic depolymerization reaction in a depolymerization kettle under the synergistic action of a catalyst and superheated steam to obtain a steam mixture composed of a depolymerization product containing caprolactam monomer and superheated steam. The operating conditions of the depolymerization kettle are: pressure 0.003 MPa, temperature 240 °C, phosphoric acid is used as the catalyst, and its mass ratio in the depolymerization kettle is 20%, the mass ratio of the nylon 6 polymer melt to superheated steam is 1:1, the residence time of the nylon 6 polymer melt is 3 hours, the cracking rate ≥ 98.7%, and the mass ratio of the depolymerization product in the steam mixture is 50%; (3) The steam mixture enters a concentration tower for concentration and dehydration to obtain a monomer concentrate with a water content of 40%. The operating conditions of the concentration tower are: pressure 0 MPa, temperature 190 °C, and top reflux ratio 0.2; (4) The obtained monomer concentrate is dehydrated in a dehydration tower to reduce the water content to less than 1.5% to obtain a monomer solution, and a 40% sodium hydroxide solution is added, accounting for 0.1% of the mass of the monomer solution. The operating conditions are: pressure -0.090 MPa, temperature 115 °C, and top reflux ratio 0.5; (5) The monomer solution is passed through a pre-distillation tower to remove impurities with boiling points lower than that of caprolactam, and a caprolactam monomer solution with a purity of more than 98.5% is obtained at the bottom fraction. The operating conditions are: pressure -0.100 MPa, temperature 120 °C, and top reflux is total reflux; (6) The caprolactam monomer solution is further passed through a distillation tower to remove impurities with boiling points higher than that of caprolactam, and a caprolactam monomer with a purity of more than 99.5% is obtained at the top fraction. The operating conditions are: pressure -0.100 MPa, temperature 125 °C, and top reflux ratio 0.3; (7) A wiped film evaporator is used to separate high-boiling impurities in the monomer solution, and low-boiling components in the monomer solution are recycled to improve the monomer yield, so that the caprolactam monomer yield of the recycled nylon 6 solid material is as high as more than 90.1%. The operating conditions are: pressure -0.099 MPa, temperature 130 °C.

[0045] The caprolactam obtained by the above method for depolymerizing and regenerating caprolactam monomer from recycled nylon 6 solid material is subjected to purity detection. The concentration of caprolactam monomer is 99.8%, reaching the first-class product index of the GB / T13254-2017 standard and meeting the quality requirements for producing high-quality nylon 6 chips.

[0046] Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that the mass ratio of the catalyst in the depolymerization kettle is 10%.

[0047] This example provides a method for recycling and regenerating caprolactam monomer from nylon 6 solid materials, including the following steps: (1) 1500 kg of nylon 6 solid particle foam materials made from recycled fishing net filaments are formed into a nylon 6 high-polymer melt through a primary metering screw and a secondary melting screw feeding device, and volatile components such as air and oil agent in the melt are removed through the degassing section of the secondary melting twin screw; (2) In the depolymerization kettle, the nylon 6 high-polymer melt undergoes a catalytic depolymerization reaction under the synergistic action of a catalyst and superheated steam to obtain a steam mixture composed of a depolymerization product containing caprolactam monomer and superheated steam. The operating conditions of the depolymerization kettle are: pressure 0.02 MPa, temperature 250 °C, phosphoric acid is used as the catalyst, and its mass ratio in the depolymerization kettle is 10%, the mass ratio of the nylon 6 high-polymer melt to superheated steam is 1:2, the residence time of the nylon 6 high-polymer melt is 4 hours, the cracking rate is ≧ 90.6%, and the mass ratio of the depolymerization product in the steam mixture is 33%; (3) The steam mixture enters the concentration tower for concentration and dehydration to obtain a monomer concentrate with a water content of 50%. The operating conditions of the concentration tower are: pressure 0.01 MPa, temperature 100 °C, and the top reflux ratio is 0.4; (4) The obtained monomer concentrate is dehydrated in the dehydration tower to reduce the water content to less than 1.5% to obtain a monomer solution, and a sodium hydroxide solution with a mass concentration of 30% is added, accounting for 0.2% of the mass of the monomer solution. Its operating conditions are: pressure -0.090 MPa, temperature 115 °C, and the top reflux ratio is 0.7; (5) The monomer solution is passed through a pre-distillation tower to remove impurities with boiling points lower than that of caprolactam, and a caprolactam monomer solution with a purity of more than 98.5% is obtained from the bottom fraction. Its operating conditions are: pressure -0.099 MPa, temperature 125 °C, and the top reflux is total reflux; (6) The caprolactam monomer solution is further passed through a distillation tower to remove impurities with boiling points higher than that of caprolactam, and a caprolactam monomer with a purity of more than 99.5% is obtained from the top fraction. Its operating conditions are: pressure -0.099 MPa, temperature 130 °C, and the top reflux ratio is 0.5; (7) A wiped film evaporator is used to separate high-boiling impurities in the monomer solution, and low-boiling components in the monomer solution are recycled to improve the monomer yield, so that the caprolactam monomer yield of the recycled nylon 6 solid material is as high as more than 78.4%. Its operating conditions are: pressure -0.095 MPa, temperature 140 °C.

[0048] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that the mass ratio of the catalyst in the depolymerization kettle is 30%.

[0049] This example provides a method for recycling caprolactam monomers by depolymerizing and regenerating nylon 6 solid materials, which includes the following steps: (1) 1500 kg of nylon 6 solid particle foam made from recycled fishing net filaments is fed through a primary metering screw and a secondary melting screw feeding device to form a nylon 6 polymer melt, and volatile components such as air and oil agent in the melt are removed through the degassing section of the secondary melting twin-screw. (2) In the depolymerization kettle, the nylon 6 polymer melt undergoes a catalytic depolymerization reaction under the synergistic action of a catalyst and superheated steam to obtain a steam mixture composed of a depolymerization product containing caprolactam monomers and superheated steam. The operating conditions of the depolymerization kettle are: pressure 0.02 MPa, temperature 250 °C, phosphoric acid is used as the catalyst, and its mass ratio in the depolymerization kettle is 30%, the mass ratio of the nylon 6 polymer melt to superheated steam is 1:2, the residence time of the nylon 6 polymer melt is 4 hours, the cracking rate is ≥ 98.8%, and the mass ratio of the depolymerization product in the steam mixture is 33%. (3) The steam mixture enters a concentration tower for concentration and dehydration to obtain a monomer concentrate with a water content of 50%. The operating conditions of the concentration tower are: pressure 0.01 MPa, temperature 100 °C, and top reflux ratio 0.4. (4) The obtained monomer concentrate is dehydrated in a dehydration tower to reduce the water content to less than 1.5% to obtain a monomer solution, and a sodium hydroxide solution with a mass concentration of 30% is added, accounting for 0.2% of the mass of the monomer solution. The operating conditions are: pressure -0.090 MPa, temperature 115 °C, and top reflux ratio 0.7. (5) The monomer solution is passed through a pre-distillation tower to remove impurities with boiling points lower than that of caprolactam, and a caprolactam monomer solution with a purity of over 98.5% is obtained from the bottom fraction. The operating conditions are: pressure -0.099 MPa, temperature 125 °C, and top reflux is total reflux. (6) The caprolactam monomer solution is further passed through a distillation tower to remove impurities with boiling points higher than that of caprolactam, and a caprolactam monomer with a purity of over 99.5% is obtained from the top fraction. The operating conditions are: pressure -0.099 MPa, temperature 130 °C, and top reflux ratio 0.5. (7) A wiped film evaporator is used to separate high-boiling impurities in the monomer solution, and the low-boiling components in the monomer solution are recycled to improve the monomer yield, so that the caprolactam monomer yield of the recycled nylon 6 solid material is as high as over 86.9%. The operating conditions are: pressure -0.095 MPa, temperature 140 °C.

[0050] The operating process parameters of the depolymerization kettle in Examples 1 to 3 and Comparative Examples 1 to 2, as well as the yields of the final caprolactam monomers, were statistically calculated as shown in Table 1 below.

[0051] Table 1

[0052] As can be seen from Table 1, after the proportion of the catalyst added to the depolymerization kettle reaches a certain content, the yield of the catalytic cracking product does not increase significantly.

[0053] The various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be construed as a rigid limitation on the scope of the present invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and individual values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and individual numbers within the range, such as 1, 2, 3, 4, 5, and 6, which applies regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0054] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer, characterized in that: The method comprises the following steps: The recovered nylon 6 solid particles are passed through a feeding device and degassed to obtain a nylon 6 polymer melt; Adding nylon 6 polymer melt into a depolymerization kettle, performing catalytic depolymerization reaction under the action of a catalyst and superheated steam to obtain a first depolymerization product and a mixed gas; The mixed gas enters a concentration tower for concentration to obtain a first caprolactam monomer concentrated liquid; The first caprolactam monomer concentrated solution is placed in a dehydration tower for dehydration, and then an alkali solution is added to obtain a second caprolactam monomer concentrated solution; The second caprolactam monomer concentrate is subjected to a pre-distillation tower to remove impurities below the boiling point of caprolactam to obtain a first regenerated caprolactam monomer solution; The first caprolactam monomer solution is passed through a distillation tower to remove impurities with a boiling point greater than that of caprolactam, thereby obtaining a second regenerated caprolactam monomer solution.

2. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The recycled nylon 6 solid particles include at least one of solid particle foams made from fishing net yarns, carpet yarns or waste yarns produced during spinning, and nylon 6 waste chips.

3. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The feeding device comprises a primary metering screw and a secondary melting screw. The primary metering screw comprises a weight loss metering scale and a feeding screw. The secondary melting screw comprises a melting extruder having a feeding section, a melting section, a degassing section and a discharging section.

4. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The depolymerization kettle is a two-stage depolymerization kettle, the upper part of which is not provided with a heat-insulating jacket, and the lower part of which comprises a biphenyl heat medium heat-insulating jacket, a superheated steam distributor and an agitator, wherein the superheated steam distributor comprises a plurality of radially sieve-opened circular tubes, and the agitator comprises a six-blade half-tube disc turbine structure.

5. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1 or 4, characterized in that: The working condition parameters of the depolymerization kettle include: operating pressure of 0.003-0.03MPa and operating temperature of 240-260°C.

6. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The catalyst includes phosphoric acid; in terms of weight percentage, the catalyst accounts for 10-30% in the depolymerization kettle.

7. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The mass ratio of the nylon 6 polymer melt to the superheated steam is 1:(1-3), the residence time of the nylon 6 polymer melt is 3-5 hours, and the cracking rate of the nylon 6 polymer melt is ≧98%; the mixed gas is the second depolymerization product of superheated steam and the nylon 6 polymer melt stripping, and the weight proportion of the second depolymerization product in the mixed gas is 25-50%.

8. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The concentration tower is a first structured packing tower, and a first external condenser is arranged on the top of the first structured packing tower; the working condition parameters of the first structured packing tower include: pressure 0-0.02MPa, temperature 90-110°C, tower top reflux ratio 0.2-0.5; in terms of weight percentage, the water content of the first caprolactam monomer concentrate is 40-70%; The dehydration tower is a second structured packing tower, the top of the second structured packing tower is provided with a second external condenser, and the bottom of the second structured packing tower is provided with a first self-circulating falling film evaporator; the working condition parameters of the second structured packing tower include: pressure -0.085~-0.095MPa, temperature 110~120℃, tower top reflux ratio 0.5~0.8; in terms of weight percentage, the water content of the second caprolactam monomer concentrate is <1.5%; The alkali solution includes at least one of a sodium hydroxide solution with a weight percentage of 30-40% and a potassium hydroxide solution with a weight percentage of 30-40%; the weight percentage of the alkali solution in the second caprolactam monomer concentrated solution is 0.1-0.3%.

9. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The pre-distillation tower is a third structured packing tower, the top of the third structured packing tower is provided with a first built-in condenser, and the bottom of the third structured packing tower is provided with a second self-circulating falling film evaporator; the working condition parameters of the third structured packing tower include: pressure ≦-0.099MPa, temperature 115-125°C, and the top reflux is total reflux; the bottom fraction of the pre-distillation tower obtains a caprolactam monomer solution with a purity of more than 98.5wt%; The distillation tower is a fourth structured packing tower, a second built-in condenser is arranged at the top of the fourth structured packing tower, and a third self-circulating falling film evaporator is arranged at the bottom of the fourth structured packing tower; the working condition parameters of the distillation tower include: pressure ≦-0.099MPa, temperature 120~130℃, and top reflux ratio 0.3~0.6; the top fraction of the distillation tower obtains a caprolactam monomer solution with a purity of more than 99.5wt%.

10. The method for recovering nylon 6 solid material and depolymerizing and regenerating caprolactam monomer according to claim 1, characterized in that: The second regenerated caprolactam monomer solution is separated by a scraped film evaporator to separate high boiling point impurities in the second regenerated caprolactam monomer solution, and the low boiling point components are reused, so that the caprolactam monomer yield of the recovered nylon 6 solid particles is ≥86wt%; The working condition parameters of the scraped film evaporator include: pressure ≦-0.095MPa, temperature 130-140°C; the scraped film evaporator is a fixed scraped film evaporator, and the cylinder of the fixed scraped film evaporator is provided with a steam heating jacket.

Images