Process for recovering epsilon-caprolactam from fishing nets comprising polyamide 6

Abstract

The invention provides a process and plant for recovering purified ε-caprolactam from fishing nets comprising polyamide 6, wherein the plant comprises a depolymerization section [B], a recovery section [C] and a purification section [D]. The invention also provides purified ε-caprolactam having a particularly low product carbon footprint and obtained via depolymerization of polyamide 6 from fishing nets.

Description

[0001] This invention relates to a method for recovering ε-caprolactam from waste fishing nets containing polyamide 6. More specifically, this invention relates to a method for recovering ε-caprolactam from waste fishing nets containing polyamide 6, thereby obtaining high-quality ε-caprolactam. Background Technology

[0002] A fishing net is a net used for catching fish. A net is a device made of fibers woven into a mesh-like structure. Fishing nets are typically formed by weaving relatively fine threads. Modern nets are usually made of synthetic fibers (such as polyamide 6, polyamide 6,6, polyester, polypropylene, and polyethylene).

[0003] Fishing nets can be left behind or lost in the ocean by fishermen. These nets, known as ghost nets, pose a serious problem for fish and other animals. Generally, fishing nets made of synthetic fibers have an extremely low (bio)degradation rate. Therefore, these nets can remain in marine ecosystems for many years, leading to the accumulation of large quantities of ghost nets.

[0004] The latest assessment by the FAO and the United Nations Environment Programme (UNEP) indicates that approximately 640 million kilograms of fishing gear are abandoned, lost, or discarded each year.

[0005] In recent years, many initiatives have been launched to prevent ghost fishing gear from entering the environment, such as collecting abandoned fishing nets in harbors and even removing such nets from the seabed with the help of diving volunteers.

[0006] The final disposal of collected waste fishing nets, including those containing polyamide 6, ranges from landfill, incineration (optionally recovering heat), regranulation, and compounding to depolymerization. Regranulation and compounding is a recycling process that melts the waste plastic (and optionally subsequently filters to remove solid impurities), and then converts it into extrudates or directly injects it into molds. Depolymerization is a technique for converting the polymer into its monomeric components (ε-caprolactam in the case of polyamide 6).

[0007] Mechanical recycling (also known as material recycling or back-to-plastics recycling) refers to operations aimed at recovering plastics through mechanical processes (grinding, washing, separating, drying, regranulation, and blending), thereby producing recycled materials that can be converted into plastic products that can replace the virgin plastics. Currently, most virgin plastics are derived from petrochemical raw materials that have never been used or processed before, such as natural gas, coal, or crude oil. In mechanical recycling, the polymer chains remain more or less intact. Mechanical recycling is a degraded form of waste recycling because the recovered material has lower quality and functionality than the virgin material.

[0008] Depolymerization, or chemical recycling, is a technique that converts polymers into their monomeric components. The specifications of the recovered monomers determine whether they can replace the virgin monomers for all applications or only for limited applications. The virgin monomers are generated from petrochemical feedstocks such as natural gas, coal, or crude oil that have never been used or processed before.

[0009] In 1938, Paul Schlack invented polyamide 6 (CAS No.: 25038-54-4), also known as nylon 6, poly(caprolactam), poly(hexane-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide), or poly[imino(1-oxohexane-1,6-diyl)].

[0010] Generally, polyamide 6 (also known as nylon 6 or polycaprolactam) is synthesized by ring-opening polymerization of ε-caprolactam at a temperature of about 260°C in an inert atmosphere.

[0011]

[0012] The method for producing pristine ε-caprolactam is described, for example, in the chapter “Caprolactam” of Ullmann’s Encyclopedia of Industrial Chemistry (May 25, 2018), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, which is available electronically via https: / / doi.org / 10.1002 / 14356007.a05_031.pub3.

[0013] The method for producing polyamide 6 is described, for example, in the chapter “Polyamides” of Ullmann’s Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, which is available electronically via https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0014] The depolymerization of polyamide 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam.

[0015]

[0016] Methods for the depolymerization of polyamide 6 are known. Such methods can be operated in batch mode, in semi-continuous mode (typically in the case of batch (re)loading of polyamide 6 into the depolymerization reactor), or in continuous mode.

[0017] LADmitrieva, AASperanskii, SAKrasavin, and YNBychkov, “Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns”, Fibre Chemistry, pp. 229-241, March 1986 (translated from Khimicheskie Volokna, No. 4, pp. 5-12, July-August 1985), is a literature review describing methods for depolymerizing polyamide 6 with and without catalysts.

[0018] AAOgale, “Depolymerization of Nylon 6: Some Kinetic Modeling Aspects”, Journal of Applied Polymer Science, Vol. 29, 1984, pp. 3947-3954, which can be obtained electronically via https: / / doi.org / 10.1002 / app.1984.070291227, is a paper describing the depolymerization kinetics of polyamide 6.

[0019] US5929234 describes a method for recovering ε-caprolactam from polycaprolactam-containing waste materials. Depolymerization is carried out in the absence of an added catalyst using superheated steam at a temperature of about 250°C to about 400°C and a pressure in the range of about 1 atm to about 100 atm, substantially less than the water saturated vapor pressure at the temperature at which the ε-caprolactam-containing vapor stream is formed.

[0020] Depolymerization of fishing nets containing polyamide 6 has been practiced in the past. However, despite the long history of polyamide 6 recycling, the quality of the ε-caprolactam monomer obtained through this method has consistently been particularly poor. Therefore, ε-caprolactam obtained from the depolymerization of fishing nets containing polyamide 6 is only used in less demanding applications (degradation recycling), such as engineering plastics and carpets. If ε-caprolactam obtained from the depolymerization of fishing nets is used in more demanding applications, it needs to be blended with large quantities of higher-grade and purer ε-caprolactam to mask its particularly poor quality. High-speed melt spinning of polyamide 6 for producing fine textile fibers requires high-quality ε-caprolactam as a raw material. The high-quality ε-caprolactam grade used for such applications should not only be extremely pure but also possess properties that do not change over time.

[0021] In summary, existing technical methods for recovering ε-caprolactam from fishing nets containing polyamide 6 have failed to produce high-quality ε-caprolactam grades that can be used to replace the original ε-caprolactam grades for demanding applications.

[0022] Currently, there is no method available for recovering high-purity ε-caprolactam from fishing nets containing polyamide 6, although such a method is urgently needed. In particular, there is an urgent need for a high-purity ε-caprolactam recovery method that can replace the original ε-caprolactam grade for demanding applications, such as high-speed melt spinning during textile fiber production.

[0023] Furthermore, there is a need to allow for an economically viable method to recover high-purity ε-caprolactam from fishing nets containing polyamide 6. The production cost of the recovered high-purity ε-caprolactam should be similar to or lower than that of the original high-purity ε-caprolactam.

[0024] In addition, there is a need to provide high-purity grades of ε-caprolactam from fishing nets containing polyamide 6, with a carbon footprint significantly lower than that of ε-caprolactam produced by a method using pristine ε-caprolactam obtained, for example, via a Beckmann rearrangement of cyclohexanone oxime.

[0025] Furthermore, a factory is needed to produce high-purity grades of ε-caprolactam from materials used in fishing nets that are self-derived from polyamide 6.

[0026] Finally, there is a need for methods that allow for the industrial-scale recovery of ε-caprolactam from fishing nets containing polyamide 6 in order to process the large quantities of polyamide 6-containing fishing nets that are discarded each year. Summary of the Invention

[0027] The objective of this invention is to satisfy one or more of the requirements described above, and to overcome the disadvantages associated with prior art methods.

[0028] In particular, an object of the present invention is to provide a method for recovering high-purity ε-caprolactam from fishing nets containing polyamide 6. In this regard, another object of the present invention is to provide a method for recovering high-purity ε-caprolactam from fishing nets containing polyamide 6, which can replace high-purity virgin ε-caprolactam for all applications, including high-speed melt spinning of polyamide 6 for the production of fine textile fibers.

[0029] Another objective of the present invention is to provide a method for recovering high-purity grades of ε-caprolactam from fishing nets containing polyamide 6 on an industrial scale.

[0030] A further objective of this invention is to provide a method for recovering high-purity grades of ε-caprolactam from fishing nets containing polyamide 6 in an economical manner. Particularly, the objective of this invention is to provide a method suitable for recovering high-purity grades of ε-caprolactam from fishing nets containing polyamide 6, which does not exceed the production cost of the original high-purity ε-caprolactam.

[0031] Another objective of the present invention is to provide high-purity grades of ε-caprolactam from fishing nets containing polyamide 6, characterized by a significantly lower carbon footprint than ε-caprolactam produced by a method using pristine ε-caprolactam obtained, for example, via a Beckmann rearrangement of cyclohexanone oxime.

[0032] Therefore, another objective of the present invention is to provide a method for reducing the environmental burden of discarded fishing nets containing polyamide 6.

[0033] Another objective of this invention is to provide a plant for producing high-purity grades of ε-caprolactam from materials used in fishing nets that are derived from and contain polyamide 6.

[0034] One or more other objectives may become apparent from the remainder of this specification.

[0035] All, some, or at least one of the aforementioned objectives are achieved by the method of claim 1, the plant of claim 13, and the product of claim 15.

[0036] This invention provides a method for recovering purified ε-caprolactam from a material derived from a fishing net containing polyamide 6 in a factory, wherein the factory comprises:

[0037] -De-aggregation section [B],

[0038] -Reclaim section [C], and

[0039] -Purification section [D],

[0040] And the method includes the following steps:

[0041] a) The material derived from the fishing net containing polyamide 6 is loaded into the depolymerization section [B];

[0042] b) In the depolymerization zone [B], the material derived from the fishing net containing polyamide 6 is depolymerized at a temperature in the range of 180°C to 400°C to obtain a stream containing ε-caprolactam;

[0043] c) Discharge a stream containing ε-caprolactam from the depolymerization section [B], and recover crude ε-caprolactam from this stream in the recovery section [C]; and

[0044] d) Purify the crude ε-caprolactam in purification section [D] to obtain purified ε-caprolactam, wherein purification includes the following steps:

[0045] (i) Crude ε-caprolactam is extracted with an organic solvent to obtain an organic phase, wherein the organic phase contains an organic solvent, ε-caprolactam and impurities;

[0046] (ii) The solvent is converted by at least partially replacing the organic solvent with water to obtain an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, wherein the solvent conversion step (ii) is selected from methods based on water back-extraction and methods based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; and

[0047] (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation from the aqueous phase.

[0048] Surprisingly, according to the present invention, the combination of specific processing steps and conditions in a particular order—that is, the sequence of depolymerization, recovery, and purification steps as defined above—allows for the high-yield and direct, and economically viable, recovery of high-grade ε-caprolactam from materials derived from fishing nets containing polyamide 6. The method of the present invention is economically viable and advantageous from several perspectives. First, the method is suitable for a variety of materials derived from fishing nets containing polyamide 6, differing, for example, in their overall composition and / or their polyamide 6 content. Second, the method allows for the efficient separation of ε-caprolactam from non-ε-caprolactam compounds, resulting in the acquisition of high-purity grades of ε-caprolactam that can replace high-purity virgin ε-caprolactam for all applications, including high-speed melt spinning of polyamide 6 for the production of fine textile fibers. Third, the method is so effective that ε-caprolactam can be obtained in high yields. Fourth, the method allows for the industrial-scale recovery of ε-caprolactam from fishing nets containing polyamide 6 for the processing of currently discarded large quantities of polyamide 6-containing fishing nets. Finally, the method of the present invention allows for the production of ε-caprolactam with a significantly lower carbon footprint than that produced by de novo synthesis of ε-caprolactam, for example, via the Beckmann rearrangement of cyclohexanone oxime. The method of the present invention allows for the efficient processing of materials derived from fishing nets containing polyamide 6 and reduces the environmental burden of such products. In particular, the method of the present invention allows for the production of purified ε-caprolactam with a carbon footprint of less than 2 kg CO2 equivalent per kg of purified ε-caprolactam, a significant improvement compared to the 6.5 to 7.5 kg CO2 equivalent per kg of ε-caprolactam associated with the production of “raw” ε-caprolactam from the Beckmann rearrangement of cyclohexanone oxime (based on data derived from ecoinvent version 3.7.1; location: Europe). Unless otherwise stated, the product carbon footprint values ​​stated herein are based on data derived from ecoinvent version 3.7.1 and located in Europe.

[0049] In addition to the method of the present invention, the present invention also provides a plant for producing purified ε-caprolactam from a fishing net containing polyamide 6 in a plant, wherein the plant includes

[0050] De-aggregation section [B],

[0051] Reclaimed section [C],

[0052] Purification section [D], and

[0053] The factory is configured to carry out the method of the present invention.

[0054] The present invention also provides purified ε-caprolactam, which is obtained by depolymerizing polyamide 6 produced by a fishing net containing polyamide 6 according to the method of the invention, wherein the carbon footprint of the ε-caprolactam product is less than 2 kg CO2 equivalent per kg of purified ε-caprolactam (based on data derived from ecoinvent version 3.7.1; location: Europe).

[0055] Advantageous embodiments of the invention are specified in the dependent claims and are described in more detail below. Detailed Implementation

[0056] Fishing nets containing polyamide 6

[0057] The method of the present invention uses a fishing net containing polyamide 6 or a material derived from such a fishing net as a starting material. Fishing nets containing polyamide 6 are generally solid materials, and in particular, fishing nets containing polyamide 6 are generally meshes formed by weaving relatively fine polyamide 6 threads. In this document, material derived from a fishing net containing polyamide 6 means, for example, material derived from a fishing net containing polyamide 6 after processes such as crushing, washing, sorting, compaction, granulation, etc. In this document, material derived from a fishing net containing polyamide 6 also includes fishing nets containing polyamide 6 that have not undergone pretreatment such as crushing, washing, sorting, compaction, granulation, etc. The term "fishing net" as used in this specification also refers to "fishing net material," that is, such terms are used synonymously herein and may also be replaced by the term "material" derived from a fishing net containing polyamide 6. Unless the context clearly specifies otherwise, the singular forms "a" and "the" ("the") used in this specification and claims include the plural forms, especially in the sense of "one or more".

[0058] Fishing nets containing polyamide 6 and materials derived from them may contain a variety of compounds added during or after their polymerization and filament formation to achieve different property variations. These compounds include, for example, brighteners, hardeners, antistatic lubricants, colorants, brighteners, spinning agents, surface smoothers, antioxidants, UV stabilizers, and so on. The composition of fishing nets and materials derived from polyamide 6 also depends on their specific application. Therefore, fish farming nets, purse seines, and bottom nets have different chemical compositions.

[0059] The surface of a seawater-immersed vessel quickly becomes covered with marine organisms known as biofouling or marine biodegradation. Biofouling is a complex phenomenon caused by several processes, the rate and extent of which are influenced by numerous physical, chemical, and biological factors closest to the surface and affect most wetted surfaces, resulting in significant material costs. The accumulation of algae and barnacles increases drag on ships and damages aquaculture facilities and equipment used in aquaculture.

[0060] Currently, antifouling coatings are formulated with toxic copper or other biocides to prevent the growth of marine sessile organisms. Copper is an effective and still widely used biocide. However, its effectiveness is relatively short-lived, typically only a few months, thus requiring frequent cleaning and recoating. Besides economic factors, the leaching of copper or other biocides causes seawater pollution and problems with non-target organisms.

[0061] The method of the present invention has advantages over, for example, existing methods that are limited to relatively pure polyamide 6-containing materials, such as carpets and spinning waste containing PA6. The method of the present invention is not limited thereto and can be applied with great success to fishing nets containing polyamide 6 and any kind of material derived from fishing nets containing polyamide 6.

[0062] Possible preprocessing steps

[0063] Prior to step a) of the method of the present invention, the material derived from the fishing net containing polyamide 6 preferably undergoes pretreatment in the pretreatment section [A], particularly size reduction in the mechanical size reduction section [β] and / or cleaning in the cleaning section [α]. This has the advantage that the material derived from the fishing net containing polyamide 6, loaded into the depolymerization section [B], is less contaminated with non-polyamide 6 materials, which improves the yield and purity of ε-caprolactam produced in the chemical plant of the present invention. Another advantage is that the size-reduced fishing net containing polyamide 6 can be more easily disposed of.

[0064] In this document, the term "cleaning" is defined as any process of removing non-nylon 6 materials adhering to or mixed with a fishing net containing polyamide 6. Cleaning is advantageous because removing any non-nylon 6 materials will therefore not interfere with the next step of the method of the present invention.

[0065] Discarded fishing nets containing polyamide 6 can be mixed with various other materials, such as rocks, metallic materials (e.g., lines, chains, anchors), organic materials (e.g., dead fish and mussels), and other marine debris (e.g., ropes, polystyrene foam floats, and sinking pipes). Additionally, discarded fishing nets containing polyamide 6 can be mixed with non-polyamide 6 fishing nets, such as those made of polyamide 6,6, polyethylene terephthalate (PET), polypropylene (PP), or polyethylene (PE). Discarded fishing nets containing polyamide 6 can also be coated with metal-based antifouling coatings, such as copper, or contain non-metallic antifouling coatings.

[0066] The size of discarded fishing nets containing polyamide 6 largely depends on the specific application. Discarded fishing nets containing polyamide 6 range from a few square meters to more than 200,000 square meters. Such large nets are, for example, purse seines and nets set vertically in the water, with floats attached to the upper edge, weights attached to the lower edge, and a series of loops through which the haul-in line passes, and can be 1.5 km long and more than 150 m deep.

[0067] Preferably, in step a), prior to depolymerization in the depolymerization zone [B], the fishing net containing polyamide 6 is fragmented into pieces. This mechanical pretreatment, i.e., the mechanical crushing or fragmentation of the fishing net containing polyamide 6, can be achieved, for example, by cutting, tearing, grinding, abrading, and / or shaving. In a preferred embodiment, the fishing net containing polyamide 6 is loaded into step a) in the form of fragments ranging from 0.005 g to 100 kg, preferably from 0.01 g to 10 kg, and most preferably from 0.02 g to 1 kg. Using material fragments derived from the fishing net containing polyamide 6 with the aforementioned weight has the advantage that such fragments can be more easily disposed of and / or cleaned by solvent washing.

[0068] Optionally, large metal fragments, rocks, and other interfering materials that cause severe wear and tear on the plant used for mechanical crushing or fragmentation are removed before the mechanical crushing or fragmentation of the fishing net containing polyamide 6. Preferably, materials containing non-polyamide 6, such as materials containing polyethylene, polypropylene, and polyamide 6,6, such as fishing nets, are removed before or after the mechanical crushing or fragmentation of the fishing net containing polyamide 6. The removal of foreign materials can be done mechanically or manually. The removal of these interfering materials has the advantage of significantly reducing the maintenance costs of the plant used for mechanical crushing or fragmentation. Additionally, the polyamide 6 content of the material obtained after mechanical crushing or fragmentation is higher than that of the material without removing interfering materials. In particular, the removal of polyamide 6,6 is advantageous because it interferes with the depolymerization of polyamide 6, causing reactor clogging, reducing the recovery yield of ε-caprolactam, and interfering with the subsequent purification of the recovered ε-caprolactam.

[0069] Preferably, the fishing net containing polyamide 6 with a Cu-based antifouling coating is removed before the fishing net containing polyamide 6 is mechanically crushed or fragmented. More preferably, the fishing net with the Cu-based antifouling coating is washed in an additional separate washing step to remove the Cu-based antifouling coating. This washed material can then be added to a material with a similar composition.

[0070] Optionally, foreign materials are separated from the fishing net containing polyamide 6, which has been mechanically crushed or fragmented. Various separation methods can be applied for this purpose, including but not limited to density separation and magnetic separation. In density separation, materials of different densities are placed in a medium-density liquid, where the less dense materials float and separate from the more dense settling materials. In practice, density separation is typically accomplished through a series of density separation stages. For example, in one stage, high-density materials such as rocks, sand, and metals (including iron and lead) are separated, while in another stage, low-density materials such as polyolefins such as polypropylene and polyethylene are separated. Magnetic separation is a method of separating components of a mixture by using a magnet to attract magnetic materials. This method, typically used for magnetic separation, separates magnetic materials from non-magnetic materials. Removing foreign materials from the crushed or fragmented fishing net containing polyamide 6 is advantageous because such materials can interfere with the depolymerization of polyamide 6, reduce the recovery yield of ε-caprolactam, and interfere with the subsequent purification of the recovered ε-caprolactam.

[0071] Optionally, the fishing net containing polyamide 6 is cleaned by washing with a solvent, preferably water, before loading into the depolymerization zone [B]. Preferably, a detergent with a concentration in the range of 0 to 20% by weight relative to the solvent is added to the solvent to improve washing efficiency. NaOH is a preferred detergent. Preferably, an aqueous solution containing 0 to 10% by weight of NaOH is used in the washing step, more preferably 0 to 5% by weight of NaOH. Preferably, the Cu-based antifouling coating is removed by washing with an aqueous solution containing 1 to 5% by weight of NaOH, preferably 1.5 to 3% by weight of NaOH, more preferably about 2% by weight of NaOH. The enhanced washing effect of NaOH is most likely caused by the enhanced hydrolysis of molecules including biopolymers and non-biopolymers. In addition, NaOH hydrolyzed copolymers, such as polyethylene vinyl acetate (PEVA, also known as EVA), are known for use in Cu-based antifouling coatings. Preferably, the washing solvent is heated to further enhance the washing process. In another preferred embodiment, the washing process includes a final rinsing step using a (cleaning) washing solvent in the absence of detergent, in order to remove any residual detergent and stains adhering to the fishing net containing polyamide 6.

[0072] Washing is preferably performed under friction. Different types of industrial washing systems are available on the market, such as high-speed friction washers.

[0073] Washing fishing nets containing polyamide 6, especially those containing polyamide 6 that have been mechanically crushed or fragmented, is advantageous because it removes all (attached) dirt and thus does not interfere with subsequent steps of the method of the present invention.

[0074] Optionally, the fishing net containing polyamide 6 is dried after the cleaning step and before loading into the depolymerization section [B]. This has the advantages of reducing the weight of the cleaned fishing net containing polyamide 6 and preventing subsequent process steps from being affected by dilution or contamination from the washing solvent.

[0075] Optionally, a fishing net containing polyamide 6, preferably washed and reduced in size, is loaded into a furnace (e.g., an extruder). The fishing net containing polyamide 6 is melted in the furnace. Preferably, the resulting polymer melt is filtered. This has the advantage of removing solid impurities. The molten and optionally filtered polymer melt is then cooled and fed into a granulator. The granulator cuts the product into pellets. Preferably, the pellets or the molten and optionally filtered polymer melt is directly loaded into the depolymerization section [B].

[0076] The size and shape of the granules (also commonly referred to as pellets) can be selected within a wide range. Generally, the pellets are cylindrical (derived from fine strands cut into fragments). However, other shapes, such as (imperfect) spheres, are also possible. The size of the pellets can be selected within a wide range. Typically, the diameter of the pellets is in the range of 1 to 10 mm, preferably 2 to 7 mm, and more preferably 3 to 5 mm. In a preferred embodiment, the length of the pellets is in the range of 1 to 50 mm, preferably 2 to 25 mm, and more preferably 3 to 15 mm.

[0077] Granulation of preferably cleaned and reduced-size fishing nets containing polyamide 6 has the advantage of increased bulk density, which reduces intermediate storage and transportation costs when pre-treated at different locations (see below). In addition to increased density, granulation provides other benefits, such as facilitating the uniform shape and structure of the material to be treated in the (automated) feeding into the depolymerization zone [B].

[0078] The location of the pretreatment site for the fishing nets containing polyamide 6 and the location of the depolymerization section [B] may be the same. However, preferably, one or more operations in the pretreatment step are carried out at different locations, for example near a harbor where waste fishing nets containing polyamide 6 are collected and / or at a location specifically for pretreatment of waste fishing nets. The fishing nets containing polyamide 6 pretreated at each location can then be loaded into the depolymerization section [B] of the (chemical) plant of the present invention for producing purified ε-caprolactam from the material derived from the fishing nets containing polyamide 6.

[0079] Loading step a)

[0080] In step a) of the invention, material derived from a fishing net containing polyamide 6, optionally reduced in size and / or washed, is loaded into the depolymerization section [B]. The depolymerization section [B] comprises one or more depolymerization reactors operating in series and / or in parallel.

[0081] In one embodiment, the fishing net containing polyamide 6 is mechanically compressed to a smaller volume before being loaded into the depolymerization section [B]. This has the advantage of requiring a smaller volume for intermediate storage and transfer and also facilitates feeding to the depolymerization section [B].

[0082] In another embodiment, before being loaded into the depolymerization zone [B], the fishing net containing polyamide 6 is compressed into denser particles, for example by mechanical compaction or by extruding molten material, followed by cooling and cutting it to a certain size. This also has the advantage of a smaller volume required for intermediate storage and transport and facilitates feeding to the depolymerization zone [B].

[0083] In another preferred embodiment, the fishing net containing polyamide 6 is dried before being loaded into the depolymerization section [B], particularly after a cleaning step of the fishing net containing polyamide 6. This has the advantage of introducing less solvent or no solvent into the depolymerization section [B]. Solvents introduced into the depolymerization section [B] are expected to have a negative impact on the depolymerization process (e.g., reduced depolymerization reaction rate, higher catalyst consumption, higher energy consumption, and the vapor stream containing ε-caprolactam and water expected to be more impurities in the depolymerization section [B]).

[0084] Preferably, the material derived from the fishing net containing polyamide 6 is fed into the depolymerization reactor in solid phase or molten form. Preferably, the material derived from the fishing net containing polyamide 6 is fed in molten form. Feeding in molten form can be achieved using an extruder, gear pump, or other means known to those skilled in the art.

[0085] The material derived from the fishing net containing polyamide 6 can be fed into the depolymerization reactor via continuous or intermittent feeding of the material derived from the fishing net containing polyamide 6.

[0086] Depolymerization step b)

[0087] In the depolymerization zone [B], the material derived from the fishing net containing polyamide 6 is depolymerized to form ε-caprolactam. The formed ε-caprolactam is discharged from the depolymerization zone in the form of a stream containing ε-caprolactam.

[0088] The depolymerization of the material derived from the fishing net containing polyamide 6 is achieved in the depolymerization zone [B] by raising the temperature of the material derived from the fishing net containing polyamide 6 to at least 180°C but not higher than 400°C. The preferred temperature range for the depolymerization reaction is 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C.

[0089] Generally, the formation rate of ε-caprolactam increases at higher temperatures. Temperatures below 400°C are preferred because above 400°C, side reactions of polyamide 6 and the reaction of impurities occur more frequently, leading to the formation of a wider variety of impurities. Some of these impurities will ultimately be present in the ε-caprolactam-containing product stream discharged from the self-depolymerization reactor. In a preferred embodiment of the invention, the depolymerization of the fishing net containing polyamide 6 is carried out at a temperature in the range of 220°C to 340°C or 240°C to 325°C. Experiments have shown that this temperature range allows for the production of particularly pure ε-caprolactam.

[0090] The pressure in the depolymerization zone [B] can vary and can be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, and most preferably 50 kPa to 1 MPa. Experiments have shown that this pressure range allows for the production of particularly pure ε-caprolactam.

[0091] The depolymerization of materials derived from fishing nets containing polyamide 6 can be achieved with or without a solvent. Preferably, the depolymerization of materials derived from fishing nets containing polyamide 6 is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, especially superheated steam.

[0092] Preferably, the depolymerization will be completed within 0.1 hours to 24 hours, more preferably 0.5 hours to 6 hours.

[0093] Feeding water as steam into the depolymerization reactor allows for the optional production of a steam stream containing ε-caprolactam and water without further heating. The weight ratio of ε-caprolactam to water in this steam stream can be adjusted by modifying the amount of steam fed into the depolymerization section [B] derived from the fishing net containing polyamide 6. In a preferred embodiment, the depolymerization in step b) is carried out in the presence of water, and the stream containing ε-caprolactam is a steam stream containing ε-caprolactam and water in a weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10, and most preferably 1:3 to 1:8.

[0094] Preferably, the ε-caprolactam in the steam stream containing ε-caprolactam and water has a partial pressure of 5 kPa to 1 MPa, more preferably 10 kPa to 0.5 MPa, and most preferably 15 kPa to 0.1 MPa.

[0095] During the depolymerization reaction, decomposition products can be formed, including linear and cyclic oligomers of ε-caprolactam. Additionally, the feed stream derived from the material in the fishing net containing polyamide 6 may contain other components, i.e., impurities, such as non-polyamide 6 compounds and residues of solvents used in the pretreatment, which remain stable, react, or decompose under depolymerization conditions. Therefore, if water is used as the solvent, the vapor stream removed from the self-depolymerization zone [B] contains not only water and ε-caprolactam but also impurities.

[0096] Preferably, superheated steam with a temperature between 100°C and 600°C is charged into the depolymerization reactor. Preferably, the superheated steam charged into the depolymerization reactor has a temperature at least equal to the melting temperature of polyamide 6. Preferably, the superheated steam charged into the depolymerization reactor has a sufficiently high energy content so that no additional heat input is required for the depolymerization reaction and the evaporation of the ε-caprolactam formed. In another preferred embodiment, the depolymerization section [B] is charged with superheated steam with a temperature in the range of 220°C to 575°C. In a more preferred embodiment, the depolymerization section [B] is charged with superheated steam with a temperature in the range of 275°C to 500°C.

[0097] Generally, the mass of the vapor stream removed by the self-depolymerization section [B] is less than the mass of the total feed to the depolymerization section. The total feed to the depolymerization section [B] includes material derived from the fishing net containing polyamide 6 and optionally solvents, catalysts, additional reagents, and / or depolymerizing agents. Therefore, without any additional measures, there will be an accumulation of material (commonly referred to as "residual material") in the depolymerization section [B]. Preferably, the self-depolymerization section [B] discharges a separate stream. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization section [B]. When phosphoric acid is used as the depolymerization catalyst, the additional stream may contain impurities present in the material derived from the fishing net containing polyamide 6, undepolymerized polyamide, unevaporated ε-caprolactam, catalyst, and compounds formed under depolymerization conditions, such as monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate. In a preferred embodiment, the self-depolymerization section [B] discharges a stream containing monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate. More preferably, the stream discharged intermittently or continuously from the self-depolymerization section [B] contains 0.01 to 50% by weight, preferably 0.1 to 25% by weight, more preferably 0.5 to 10% by weight, and most preferably 0.5 to 5% by weight of monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate.

[0098] Depolymerization of materials derived from fishing nets containing polyamide 6 can be carried out in the presence of steam, in the presence of additional depolymerizing agents, such as ammonia. The concentration of ammonia in the depolymerization zone [B] can vary. Therefore, if ammonia is present in the depolymerization zone [B], the steam stream removed from the depolymerization zone [B] may contain not only ε-caprolactam and impurities, but also ammonia.

[0099] Most preferably, depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is Lewis or Brønsted. Acids or bases. Acid catalysts may be particularly selected from the group consisting of: phosphoric acid; p-toluenesulfonic acid; boric acid; sulfuric acid; organic acids; organic sulfonic acids, including xylenesulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons; solid acids; salts of the aforementioned acids; Al2O3; and SiO2; and combinations thereof. Base catalysts may be selected, for example, from the group consisting of: alkali metal hydroxides; alkali metal salts; alkaline earth metal hydroxides; and alkali metals, such as alkaline earth metal salts; organic bases and solid bases; and combinations thereof. Preferably, phosphoric acid, boric acid, organic acids, alkali metal hydroxides and alkali metal salts are used as catalysts. More preferably, phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate are used as catalysts. More preferably, phosphoric acid, p-toluenesulfonic acid, boric acid and sodium hydroxide are used as catalysts. In one particularly preferred embodiment, phosphoric acid is used as a catalyst for depolymerization; in another, p-toluenesulfonic acid is used.

[0100] However, in another preferred embodiment, the catalyst is not used for the depolymerization of materials derived from fishing nets containing polyamide 6. This has the advantage of lower cost (catalyst cost and catalyst waste disposal cost). However, it typically requires higher temperatures (and pressures).

[0101] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction begins at low temperatures and can be carried out under atmospheric conditions. The suitable concentration of the catalyst used for the depolymerization of materials derived from polyamide 6 into ε-caprolactam is known to those skilled in the art and can be easily determined by routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. Conversely, if the concentration of the catalyst used is too high, the reaction is rapid, and side reactions increase. Furthermore, the cost of the catalyst increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the polyamide 6 contained in the depolymerization reactor. More preferably, the catalyst content is 0.1 to 50% by weight. The most preferred catalyst concentration depends on the type of catalyst used for the depolymerization of polyamide 6. For the catalyst orthophosphoric acid, the preferred content is 0.1 to 25% by weight, and more preferably 1 to 20% by weight. For the catalyst p-toluenesulfonic acid, the preferred content is 10 to 35% by weight, and more preferably 15 to 30% by weight.

[0102] The depolymerization of polyamide 6 can be carried out in batch, semi-continuous, or continuous modes, all of which are known to those skilled in the art. As used herein, the terms “batch,” “semi-continuous,” and “continuous” refer to the mode in which polyamide 6-containing feedstock, i.e., material derived from a fishing net containing polyamide 6, and optionally a catalyst, are loaded into the depolymerization reactor, and the mode in which residual material is discharged from the self-depolymerization reactor.

[0103] In a preferred embodiment, the depolymerization of polyamide 6 is carried out in a batch mode. In the batch mode, the raw material, i.e., the material derived from a fishing net containing polyamide 6, and optionally a catalyst, are first charged into the depolymerization reactor. Subsequently, superheated steam is charged into the depolymerization reactor and ε-caprolactam is discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Next, the charging of superheated steam into the depolymerization reactor is stopped. After the residual material is optionally removed from the depolymerization reactor, a new cycle is started by charging the raw material (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, the residual material is not removed between cycles.

[0104] In a particular advantageous embodiment, the depolymerization of polyamide 6 is carried out in a continuous mode. In the continuous mode, material derived from a polyamide 6-containing raw material (and optionally a catalyst) is continuously charged into the depolymerization reactor. Simultaneously, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged into the depolymerization reactor. Additionally, residual material is continuously discharged from the depolymerization reactor. Preferably, the material derived from a fishing net containing polyamide 6 is charged in molten form. Preferably, the catalyst is charged in molten, slurry, or solution form.

[0105] In another preferred embodiment, the depolymerization of polyamide 6 is carried out in a semi-continuous mode. In the semi-continuous mode, material derived from a polyamide 6-containing raw material (and optionally a catalyst) is intermittently charged into the depolymerization reactor, while superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. The residual material is intermittently discharged from the depolymerization reactor in the semi-continuous mode of polyamide 6 depolymerization.

[0106] Recycling step c)

[0107] In the recovery section [C], ε-caprolactam is recovered from the stream containing ε-caprolactam discharged from the self-depolymerization section [B]. This stream contains ε-caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the stream containing ε-caprolactam.

[0108] Preferably, without loading the solvent into the depolymerization zone [B], the ε-caprolactam obtained by condensation is dissolved in water to obtain an ε-caprolactam-rich phase. This ε-caprolactam-rich phase also contains impurities.

[0109] Preferably, when water is used as a solvent in the depolymerization section [B], the stream containing ε-caprolactam discharged from the self-depolymerization section [B] contains ε-caprolactam, water, and impurities. The ε-caprolactam can be separated from the remaining components of the steam stream by feeding the steam stream into the self-depolymerization reactor, preferably from the top, to a (preferably partially) condenser, to obtain a condensate containing ε-caprolactam. Preferably, the ε-caprolactam is separated from the remaining components of the steam stream by feeding the product stream into the self-depolymerization reactor, preferably from the top, to a distillation column, thereby obtaining an aqueous phase as the top product and an ε-caprolactam-rich phase as the bottom product.

[0110] The ε-caprolactam recovered in recovery section [C] is crude because it contains impurities such as polyamide 6 decomposition products or other non-polyamide 6 components derived from materials containing polyamide 6 fishing nets. The crude ε-caprolactam recovered in step c) comprises water and ε-caprolactam, preferably an aqueous solution containing ε-caprolactam. Therefore, the crude ε-caprolactam recovered in recovery section [C] requires additional purification to obtain high-purity ε-caprolactam. Thus, "crude" as used herein can be defined as having lower purity compared to the purified ε-caprolactam obtained as a product of the method of the present invention, i.e., containing fewer impurities.

[0111] Preferably, the crude ε-caprolactam comprises 6 to 95% by weight, more preferably 20 to 90% by weight, and most preferably 35 to 80% by weight of ε-caprolactam. The remainder is primarily water.

[0112] Purification step d)

[0113] In step d), the crude ε-caprolactam obtained in the recovery section [C] is purified in the purification section [D] to obtain high-purity ε-caprolactam.

[0114] Optionally, the crude ε-caprolactam is filtered before loading the purification section [D]. Filtration ensures the removal of undissolved impurities that could otherwise hinder further purification.

[0115] Optionally, the oil is separated from the crude ε-caprolactam before loading into the purification section [D]. Oil separation ensures the removal of impurities that might otherwise hinder further purification.

[0116] Purified ε-caprolactam is obtained by first extracting crude ε-caprolactam with an organic solvent in step (i), thereby yielding an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The organic solvent used for extracting crude ε-caprolactam is preferably an aromatic hydrocarbon, aliphatic hydrocarbon, cycloaliphatic hydrocarbon, halogenated hydrocarbon, and / or C4-C4 hydrocarbon. 10 Aliphatic alcohols or cyclic aliphatic alcohols. Optionally, the organic solvent used for extracting crude ε-caprolactam is preferably a mixed extractant composed of aromatic hydrocarbons, aliphatic hydrocarbons, cyclic aliphatic hydrocarbons, halogenated hydrocarbons, and / or C4-C6 hydrocarbons. 10 The extract consists of aliphatic or cyclic aliphatic alcohols and C5-C8 alkanes or C5-C8 cycloalkanes. Particularly favorable results are achieved when the organic solvent used for extracting crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC methyl isobutyl methanol), 1-octanol, 2-ethylhexanol, and mixtures thereof. More preferably, the organic solvent used for extracting crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohols, and mixtures thereof. Even more preferably, the organic solvent used for extracting crude ε-caprolactam is selected from the group consisting of toluene, 1-octanol, 4-methyl-2-pentanol, 2-ethylhexanol, and mixtures thereof. Preferably, the weight ratio of the organic solvent to ε-caprolactam is from 0.01:1 to 40:1, more preferably from 0.05:1 to 10:1, and most preferably from 0.1:1 to 5:1.

[0117] Optionally, the organic solvent used for extracting crude ε-caprolactam is mixed with the following alkane: C m H 2m+2 Where m is 5 to 8; cycloalkanes or C m H 2m Where m is 5 to 8, a mixed extractant is formed. Particularly good results are achieved if alkanes or cycloalkanes are present in the mixed extractant at a weight of 5 to 90% by weight, preferably 25 to 75% by weight, of the total weight of the mixed extractant.

[0118] In another embodiment, the organic solvent has a lower density than the crude ε-caprolactam. Step d)(i) of the extraction using the organic solvent is carried out in a countercurrent extraction column, wherein the crude ε-caprolactam to be purified is introduced into the upper part of the column and the organic solvent is introduced into the lower part. The extraction produces an aqueous phase containing water and impurities, and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The extraction produces an organic phase containing the organic solvent, ε-caprolactam, and impurities, wherein the weight ratio of impurities to ε-caprolactam is lower than that in the crude ε-caprolactam. Therefore, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0119] In another embodiment of the invention, the organic solvent has a higher density than the crude ε-caprolactam. Step d)(i) of the extraction using the organic solvent is carried out in a countercurrent extraction column, wherein the crude ε-caprolactam to be purified is introduced into the lower part of the column and the organic solvent is introduced into the upper part. The extraction produces an aqueous phase containing water and impurities, and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The extraction produces an organic phase containing the organic solvent, ε-caprolactam, and impurities, wherein the weight ratio of impurities to ε-caprolactam is lower than that of the crude ε-caprolactam. Therefore, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0120] Optionally, the organic phase containing the organic solvent, ε-caprolactam, and impurities is washed with water or an alkaline aqueous solution before proceeding to step d)(ii). If washing with an alkaline aqueous solution, the alkaline solution is preferably an aqueous solution containing an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably contains 0.5 to 2.0% by weight of sodium hydroxide or potassium hydroxide.

[0121] Technicians can determine the amount of water or alkaline aqueous solution required for effectively washing an organic phase containing organic solvents, ε-caprolactam, and impurities using routine experiments. Preferably, this amount is between 0.1 and 5% by weight, relative to the amount of organic solvent excluding ε-caprolactam dissolved in the organic phase to be washed. In another preferred embodiment, washing of the organic phase containing organic solvents, ε-caprolactam, and impurities with water or an alkaline aqueous solution is performed in a countercurrent washing column, wherein the organic phase containing organic solvents, ε-caprolactam, and impurities is introduced at the bottom of the column and water or an alkaline aqueous solution is introduced at the top. The washing produces a washed organic phase containing organic solvents, ε-caprolactam, and impurities, and an aqueous phase containing residues. Typically, the aqueous phase containing residues contains water, ε-caprolactam, and impurities. Due to washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase before washing.

[0122] Subsequently, in step d)(ii) of the method according to the invention, the solvent of the obtained organic phase containing the organic solvent, ε-caprolactam and impurities, optionally washed with water or an alkaline aqueous solution, is converted, such that the organic solvent in the organic phase containing the organic solvent, ε-caprolactam and impurities is replaced by water, thereby obtaining an aqueous phase containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam, wherein the solvent conversion method is selected from a method based on water back-extraction (also known as re-extraction) and a method based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added.

[0123] As used herein, the term “displacement” means that at least 60% by weight, preferably at least 80% by weight, and most preferably at least 90% by weight, 95% by weight, or 98% by weight of the organic solvent present in an organic phase comprising the organic solvent, ε-caprolactam, and impurities is replaced by water.

[0124] Solvent conversion can be based on water back-extraction to obtain an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Preferably, the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is stripped and / or distilled to remove residual organic solvents. Although the amount of water used for back-extraction of ε-caprolactam can vary, the amount of water used is preferably 0.3 to 20 times, more preferably 0.4 to 10 times, and most preferably 0.5 to 6 times the weight of the recovered ε-caprolactam.

[0125] Preferably, back-extraction using water can be carried out in a countercurrent extraction column.

[0126] In another preferred embodiment, the organic phase, optionally washed and containing organic solvent, ε-caprolactam, and impurities, has a density lower than that of water. The organic phase is introduced into the lower part of the extraction column, and water is introduced into the upper part. Back-extraction produces an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, and an organic solvent phase containing the impurities. The weight ratio of impurities to ε-caprolactam in the aqueous phase containing water, ε-caprolactam, and impurities is lower than that in the organic phase containing organic solvent, ε-caprolactam, and impurities before back-extraction. Therefore, purer ε-caprolactam is obtained due to back-extraction. Preferably, the organic solvent phase containing impurities is optionally reused after purification (preferably by distillation).

[0127] In another preferred embodiment, the organic phase, comprising an organic solvent, ε-caprolactam, and impurities, has a density higher than that of water. The organic phase is introduced into the upper part of the extraction column, and water is introduced into the lower part. Back-extraction produces an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, and an organic solvent phase comprising the impurities. The weight ratio of impurities to ε-caprolactam in the aqueous phase comprising water, ε-caprolactam, and impurities is lower than that in the organic phase comprising the organic solvent, ε-caprolactam, and impurities before back-extraction. Therefore, due to back-extraction, the ε-caprolactam is purer than before back-extraction. Preferably, the organic solvent phase comprising impurities is optionally reused after purification (preferably by distillation).

[0128] Therefore, according to a particularly advantageous embodiment of the invention, after the extraction of crude ε-caprolactam in step d)(i), the purification in step d) further comprises step (ii) solvent conversion based on back-extraction with water.

[0129] The solvent conversion method can also be a solvent exchange distillation method, in which the organic solvent is distilled off and water is added. In a preferred embodiment, the solvent conversion method is a solvent exchange distillation method performed as a single-stage process, in which the organic solvent is distilled off from an organic phase containing the organic solvent, ε-caprolactam, and impurities, and water is added. More preferably, the solvent conversion is performed in the form of azeotropic distillation with the addition of water, in which case the organic solvent evaporates as an azeotropic mixture containing the organic solvent and water. The purpose of azeotropic distillation is to remove the organic solvent and add water. Preferably, substantially all of the organic solvent is removed. In this context, "substantially all" means removing at least 90% by weight, preferably at least 95% by weight, and most preferably at least 98% by weight or 99% by weight of the organic solvent present in the organic phase containing the organic solvent, ε-caprolactam, and impurities. Preferably, water is added in liquid form. More preferably, liquid water is added as reflux to the upper part of the distillation column. More preferably, a portion of the water added as reflux is obtained by condensing the azeotropic mixture distilled off in the distillation column.

[0130] Any suitable container can be used for solvent conversion methods, such as a column, preferably a distillation column operating in continuous mode. A distillation column may include trays, packing material, or a combination thereof.

[0131] In another preferred embodiment, solvent exchange distillation is performed as a two-stage process. The first stage is a pre-concentration stage, and the second stage is the actual solvent exchange distillation.

[0132] An organic phase containing organic solvent, ε-caprolactam, and impurities is charged into the first stage. In the first stage, a first fraction of the organic solvent is removed from the organic phase containing the organic solvent, ε-caprolactam, and impurities by distillation at the top of the distillation column. Preferably, this distillation is carried out under reflux. Under reflux means that the organic solvent, in liquid phase, is charged into the top of the distillation column. More preferably, a portion of the organic solvent removed at the top of the distillation column by distillation is condensed and then charged back into the top of the distillation column in liquid form. The remaining organic phase containing the organic solvent, ε-caprolactam, and impurities is discharged from the first stage and charged into the second stage. Due to the distillation in the first stage, the chemical composition of the remaining organic phase containing the organic solvent, ε-caprolactam, and impurities differs from the organic phase containing the organic solvent, ε-caprolactam, and impurities charged into the first stage. Generally speaking, compared to the organic phase containing organic solvents, ε-caprolactam, and impurities in the first stage, the remaining organic phase containing organic solvents, ε-caprolactam, and impurities contains a higher weight percentage of ε-caprolactam and compounds with boiling points higher than ε-caprolactam, and a lower weight percentage of compounds with boiling points lower than ε-caprolactam.

[0133] In the second stage, the remaining organic solvent is distilled off from the remaining organic phase containing the organic solvent, ε-caprolactam, and impurities, and then water is added. More preferably, in the second stage, solvent conversion is carried out by azeotropic distillation with the addition of water, in which case the organic solvent evaporates as an azeotropic mixture containing the organic solvent and water.

[0134] The purpose of azeotropic distillation is to remove organic solvents and add water. Preferably, substantially all organic solvents are removed. In this context, "substantially all" means removing at least 90% by weight, preferably at least 95% by weight, and most preferably at least 98% or 99% by weight of the organic solvent present in the remaining organic phase, which includes the organic solvent, ε-caprolactam, and impurities. Preferably, water is added in liquid form. More preferably, liquid water is added as reflux to the upper part of the distillation column. More preferably, a portion of the water added as reflux is obtained by condensing the azeotropic mixture distilled in the distillation column.

[0135] Any suitable container can be used for each stage of solvent conversion, such as a column, preferably a distillation column operating in continuous mode. A distillation column may include trays, packing material, or a combination thereof.

[0136] Solvent-changing distillation (performed as a single-stage or two-stage process) yields an aqueous phase comprising water, ε-caprolactam, and an aqueous phase with a boiling point below or above that of ε-caprolactam, and optionally residual organic solvent. Preferably, the ε-caprolactam content of this aqueous phase is between 25% by weight and 99.9% by weight relative to the total aqueous phase, more preferably between 50% by weight and 99.5% by weight, and most preferably between 85% by weight and 99% by weight.

[0137] Therefore, according to a particularly advantageous embodiment of the invention, after the extraction of crude ε-caprolactam in step d)(i), the purification in step d) further comprises step (ii): solvent conversion based on solvent exchange distillation.

[0138] In step d)(iii) of the method of the present invention, the aqueous phase containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam obtained by solvent conversion in step d)(ii) is distilled to remove impurities with boiling points lower or higher than ε-caprolactam from the aqueous phase.

[0139] Preferably, water is first evaporated from the aqueous phase, which contains water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. After evaporation, ε-caprolactam is distilled to recover high-purity ε-caprolactam. Preferably, distillation is carried out under reduced pressure. More preferably, distillation is carried out at a pressure less than 50 kPa, more preferably less than 20 kPa, and most preferably less than 10 kPa. Generally, the temperature is between 90°C and 210°C. Preferably, the temperature is between 100°C and 200°C, and more preferably between 110°C and 180°C. These temperatures refer to the temperature at the bottom of the distillation column where distillation is performed.

[0140] Distillation includes separating low-boiling-point organic impurities (having a boiling point lower than ε-caprolactam) from ε-caprolactam and / or separating high-boiling-point organic impurities (having a boiling point higher than ε-caprolactam) from ε-caprolactam. Distillation preferably includes: in a first step, separating low-boiling-point impurities from ε-caprolactam as a top product and producing ε-caprolactam containing high-boiling-point impurities as a bottom product. In a second step, separating high-purity ε-caprolactam as a top product and obtaining a distillation residue containing ε-caprolactam and high-boiling-point impurities as a bottom product.

[0141] In a preferred embodiment, prior to the distillation removal in step d)(iii), an alkali metal hydroxide, preferably NaOH, is added to the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol per kilogram of ε-caprolactam, more preferably and most preferably 2 to 80 mmol. Experiments have shown that the addition of an alkali metal hydroxide, especially NaOH, allows for particularly effective distillation removal of impurities with boiling points lower or higher than ε-caprolactam.

[0142] In another preferred embodiment, prior to the distillation removal in step d)(iii), an oxidant, such as potassium permanganate, sodium permanganate, and / or hydrogen peroxide, is added to the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Most preferably, potassium permanganate is used as the oxidant.

[0143] The oxidant can be added in solid, slurry, or aqueous solution form to an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam to obtain a diluted aqueous solution. A skilled technician can determine the amount of oxidant required to effectively oxidize the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam through routine experiments. The precise amount of oxidant depends particularly heavily on the composition of the material derived from waste fishing nets containing polyamide 6 used as feed in the method of the present invention. Preferably, the amount of oxidant is between 0.01% by weight and 5% by weight relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0144] In the method of the present invention, the temperature used for oxidizing the aqueous solution can be varied. Preferably, prior to the distillation removal in step d)(iii), the aqueous solution containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidant at a temperature in the range of 20°C to 85°C, more preferably in the range of 30°C to 80°C, wherein the oxidant is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof, especially potassium permanganate.

[0145] The duration of oxidation with an oxidant can vary. Preferably, the aqueous solution containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidant for 1 minute to 24 hours, more preferably 2 minutes to 6 hours, and most preferably 5 minutes to 2 hours.

[0146] The concentration of ε-caprolactam in the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam for oxidation with an oxidant can vary. Preferably, the aqueous solution for oxidation contains ε-caprolactam and water in a weight ratio of 5:1 to 1:5, more preferably 3:1 to 1:3, and most preferably 2:1 to 1:2. Optionally, the weight ratio of ε-caprolactam to water is adjusted before adding the oxidant to the aqueous phase. Preferably, the weight ratio of ε-caprolactam to water is adjusted by adding water or by removing water.

[0147] When potassium permanganate or sodium permanganate is used as the oxidant, manganese oxide (IV) (MnO2) solid particles are formed as the reaction product. Technicians can determine the optimal solid-liquid filtration procedure for effectively removing the manganese oxide (IV) solid particles from the aqueous phase after oxidation through routine experiments. In this regard, it is common practice to use filter aids such as activated carbon or diatomaceous earth particles to improve the filtration procedure.

[0148] The high-purity ε-caprolactam obtained according to the method of the present invention can be used to manufacture polyamide 6 using methods well known to those skilled in the art. This polyamide 6 can then be used in all known materials, including engineering materials, fibers, and membranes. This polyamide 6, derived from materials containing fishing nets containing polyamide 6, is particularly suitable for high-speed spinning applications, including garments containing spandex (also known as elastane).

[0149] factory

[0150] The present invention also provides a plant, namely a chemical plant, comprising a depolymerization section [B], a recovery section [C], and a purification section [D], configured to implement the methods of the present invention described above. All plant features specifically described below in conjunction with the plant also correspond to specific embodiments of the methods of the present invention, and vice versa. Therefore, the plant is suitable for implementing the methods of the present invention, and it should be understood that the descriptions already incorporated into the methods of the present invention also apply to the plant implementation.

[0151] The plant could be a laboratory facility as in the example. However, preferably, the plant is an industrial-scale plant. “Industrial-scale” means that the plant has a production capacity of at least 500 tons / year of ε-caprolactam when operating continuously (i.e., it is capable of producing that amount of ε-caprolactam in principle).

[0152] The plant of the present invention is adapted to produce purified ε-caprolactam from a fishing net containing polyamide 6 and comprises at least three sections: a depolymerization section [B], a recovery section [C], and a purification section [D]. These sections, and thus the plant, are configured to carry out the methods of the present invention described above.

[0153] Additionally, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [β] for fragmenting fishing nets containing polyamide 6 into fragments, and / or a cleaning section [α] for washing fishing nets containing polyamide 6 and / or a densification section [γ] for increasing the bulk density of fishing nets containing nylon 6. Cleaning includes washing and separating foreign materials from fishing nets containing polyamide 6. Separation of foreign materials may be performed manually (hand-picking) and mechanically (e.g., density separation and magnetic separation). Manual and mechanical devices, such as brushes, may facilitate the washing process in the cleaning section [α]. Washing is preferably carried out by additional frictional effects. Different types of industrial washing systems are commercially available, such as high-speed friction washers. The mechanical size reduction section [β] includes equipment for mechanically fragmenting fishing nets containing polyamide 6 into fragments. Non-limiting examples of this fragmentation equipment are cutters, shredders, grinders, mills, and cutting machines. The densification section [γ] includes equipment for densifying the material comprising nylon 6 fishing net. Densification of the material to obtain a high bulk density can be achieved through several techniques known to those skilled in the art. Notable examples of densification equipment include electric and hydraulic compactors and presses, as well as equipment in which the feed is first melted and then solidified by cooling, such as single-screw and twin-screw extruders.

[0154] The depolymerization section [B] comprises one or more depolymerization reactors operating in series and / or parallel. A fishing net containing polyamide 6 is fed into the reactor in solid or molten form, preferably molten. This feeding can be achieved using an extruder, gear pump, or other means known in the art.

[0155] During production, the depolymerization reactor is at least partially filled with a feedstock containing polyamide 6, residual material, ε-caprolactam (and optionally a catalyst). The depolymerization reactor can have any suitable form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors, and extruder-type reactors.

[0156] The depolymerization reactor must be equipped with mechanisms for feeding a feed material derived from a netting material containing polyamide 6, along with optional superheated steam and catalyst. Additionally, the depolymerization reactor must be equipped with mechanisms for discharging the stream containing ε-caprolactam and residual material.

[0157] Good contact between steam and reactor contents is essential for efficient operation. Such contact can be achieved through various means known to those skilled in the art. As an example, steam can be injected into the material using multiple inlets, such as a steam distributor. Further improved contact can be achieved by incorporating mechanical agitation within the reactor, such as using a combination of rotating blades and static fins.

[0158] Preferably, the depolymerization will be completed within 0.5 to 6 hours.

[0159] If high-temperature superheated steam is not available at the production site, it must be specially manufactured by superheating the steam obtained from the boiler in a so-called superheater.

[0160] The recovery section [C] may include one or more (preferably partial) condensers, wherein an ε-caprolactam-containing stream is fed in the form of a vapor stream containing ε-caprolactam and water. This (partial) condenser may have any desired form. Preferably, the condenser is a distillation column from which an aqueous phase as the top product and crude ε-caprolactam as the bottom product are obtained.

[0161] The purification section [D] includes one or more extraction devices, one or more solvent conversion devices, an oxidation section, and one or more distillation devices, into which crude ε-caprolactam is introduced and from which high-purity ε-caprolactam is discharged.

[0162] Crude ε-caprolactam and an organic solvent are introduced into the extraction device, and an organic phase containing the organic solvent, ε-caprolactam, and impurities, as well as an aqueous phase containing water and impurities, are discharged. The extraction device is selected from a mixed sedimentation extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the extraction device is a static or stirred extraction column, such as… tubular column Rotary disc tower (RDC), pulse tube, sieve plate (static) tube, random packing (static) tube, and structured packing (SMVP) (static) tube.

[0163] The solvent conversion device is filled with water and an organic phase containing organic solvent, ε-caprolactam, and impurities, and the organic solvent and an aqueous phase containing ε-caprolactam and impurities are discharged. The solvent conversion device used in the back-extraction-based method is selected from mixed sedimentation extractors, extraction columns, centrifugal extractors, and combinations thereof. Preferably, the back-extraction device is a static or stirred extraction column, such as… tubular column Rotary disc tower (RDC), pulse tube, sieve plate (static) tube, random packing (static) tube, and structured packing (static) tube.

[0164] Solvent switching equipment for solvent-based distillation methods is selected from sieve plate distillation columns, random packing distillation columns, and structured packing distillation columns. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation column can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure. Preferably, water is introduced into the upper part of the distillation column, and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is discharged from the lower part of the distillation column.

[0165] The oxidation section comprises one or more oxidation reactors operating in series and / or parallel. An oxidant and an aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities are charged into the oxidation section. Typically, the oxidant is charged in solid, slurry, or aqueous solution form. When potassium permanganate or sodium permanganate is used as the oxidant, the oxidation section also includes a filtration section. The oxidation reactor can have any suitable form. Preferred reactor types are stirred and unstirred reactors and packed column reactors. The oxidation reactor must be equipped with a mechanism for feeding an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, as well as the oxidant. Additionally, the oxidation reactor must be equipped with a mechanism for discharging the oxidized aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities, and optionally, the formed manganese(IV) (MnO2) solid particles. Preferably, oxidation is carried out at a temperature in the range of 20°C to 85°C under atmospheric conditions.

[0166] Manganese oxide (IV) (MnO2) solid particles, optionally present, can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to use filter aids, such as activated carbon particles or diatomaceous earth, to improve the filtration process. Filter systems suitable for separating manganese oxide (IV) solid particles are known to those skilled in the art. This filter system is loaded with a suspension of oxidized ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities, along with manganese oxide (IV) solid particles, and the filtered oxidized ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities is discharged. Generally, the manganese oxide (IV) solid particles remain in the filter system. Preferably, this filter system operates in a semi-continuous mode, thereby continuously loading and discharging the suspension and the filtered phase while collecting the separated solids in the filter system. Occasionally, the loading of the suspension is interrupted, and the collected solids are removed from the filter system.

[0167] The distillation apparatus is charged with an aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities, and high-purity ε-caprolactam, water, and impurities (i.e., low-boiling-point organic impurities (with boiling points lower than ε-caprolactam) and high-boiling-point organic impurities (with boiling points higher than ε-caprolactam)) are discharged. The distillation apparatus is selected from sieve plate distillation columns, random-packed distillation columns, structured-packed distillation columns, and horizontal and vertical (lift-type) thin-film evaporators. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation apparatus can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure, preferably at sub-atmospheric pressure.

[0168] Preferably, distillation includes separating water, low-boiling-point organic impurities (having a boiling point lower than ε-caprolactam), and / or high-boiling-point organic impurities (having a boiling point higher than ε-caprolactam) from ε-caprolactam. Preferably, distillation includes separating water as a top product in a first step and producing ε-caprolactam containing both low-boiling-point and high-boiling-point impurities as a bottom product. In a second step, low-boiling-point impurities are separated as a top product, and ε-caprolactam containing high-boiling-point impurities is obtained as a bottom product. In a third step, high-purity ε-caprolactam is separated as a top product, and a distillation residue containing ε-caprolactam and high-boiling-point impurities is produced as a bottom product. Optionally, the first and second steps are combined.

[0169] Preferably, before distillation to remove water and impurities, an alkali metal hydroxide, preferably NaOH, is added to the aqueous phase of oxidized ε-caprolactam containing water, ε-caprolactam, and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol per kilogram of ε-caprolactam, and more preferably 2 to 80 mmol per kilogram of ε-caprolactam. This makes it particularly effective in subsequent distillation to remove impurities with boiling points lower than and higher than ε-caprolactam.

[0170] The method of the present invention can be operated continuously, semi-continuously, or in batches. Therefore, the plant of the present invention can also be configured to allow one or more of these operating modes. In a preferred embodiment, the plant is configured to operate the method of the present invention continuously or semi-continuously. However, discontinuous methods are also possible. For example, the plant of the present invention does not necessarily contain all the sections described herein in one location. Optionally, such sections, or portions thereof, are located in two or more locations. In particular, the pretreatment section [A] may be located in a first location, while the depolymerization section [B], recovery section [C], and purification section [D] are located in a second location. Similarly, the mechanical size reduction section [β], which is part of the pretreatment section [A], may also be located in a first location, while the cleaning section [α], which is part of the pretreatment section [A], may be located in a second location, while the depolymerization section [B], recovery section [C], and purification section [D] are located in a third location. Optionally, the cleaning section [α] is split into two or more segments, all optionally located in different locations. For example, the first segment of the cleaning segment [α], which is part of the pretreatment segment [A], may be located at a first position; the mechanical size reduction segment [β], which is part of the pretreatment segment [A], may be located at a second position; and the second segment of the cleaning segment [α], which is part of the pretreatment segment [A], may be located at a third position, while the depolymerization segment [B], the recovery segment [C], and the purification segment [D] are located at a fourth position. The densification segment [γ] may be located at the same position as one or more other segments of the pretreatment segment [A], or it may be located at the same position as the depolymerization segment [B].

[0171] product

[0172] This invention provides a new product, ε-caprolactam, which is obtained by the method according to the invention via the depolymerization of polyamide 6 derived from a fishing net containing polyamide 6. Advantageously, this ε-caprolactam is characterized in particular by having a product carbon footprint of less than 2 kg CO2 equivalent per kg of purified ε-caprolactam (based on data derived from ecoinvent version 3.7.1; location: Europe). The ε-caprolactam obtained according to the invention may also be referred to as "purified ε-caprolactam". As used herein, "purified" means that the ε-caprolactam is produced by the method according to the invention from a fishing net containing polyamide 6, thereby obtaining the ε-caprolactam in purified form. In this sense, the ε-caprolactam is obtained from and purified from a fishing net containing polyamide 6.

[0173] The method of this invention allows for the production of high-purity and therefore high-quality ε-caprolactam, meeting specifications for demanding applications. Furthermore, this method is particularly economical due to its reduced carbon footprint and the use of waste as a starting material. In a preferred embodiment, the ε-caprolactam obtained by the method of this invention satisfies one or more of the following specifications, wherein parameters and measurement methods are defined in the Examples section below:

[0174] PAN: Maximum 5

[0175] E290: Maximum 0.05

[0176] VB: Maximum 0.5 mmol / kg

[0177] Alkalinity: Maximum 0.1 mmol / kg.

[0178] The ε-caprolactam produced by the method of this invention is particularly economical and environmentally friendly. Compared to conventionally produced ε-caprolactam (e.g., via the Beckmann rearrangement of cyclohexanone oxime), the ε-caprolactam produced by the method of this invention has a significantly lower carbon footprint.

[0179] The environmental impact of a product is generally described as its "carbon footprint." A product's carbon footprint is defined as the total emissions resulting from the formation of that product, expressed as the carbon dioxide equivalent per tonne of product. A product's carbon footprint is particularly dependent on raw materials, auxiliary materials, energy consumption, energy sources, production methods, and method efficiency. The quantification of a product's carbon footprint can be as described, for example, in the European standard EN ISO 14040:2006 (Environmental management—Life cycle assessment—Principles and framework).

[0180] Product carbon footprint calculations can be performed by internal or external (preferably) certified organizations. These organizations, for example, verify and certify product carbon footprint calculations based on the LCA standard ISO 14040.

[0181] J. Hong and X. Xu (“Environmental impact assessment of caprolactam production-a case study in China”; J. of Cleaner production 27(2012)103-108; DOI: 10.1016 / j.jclepro.2011.12.037) reported that, in the case of coal-based power and steam generation, the potential global warming impact of “primary” ε-caprolactam obtained via the Beckmann rearrangement of cyclohexanone oxime is 7.5 tons of CO2 equivalent per ton of ε-caprolactam (which is equal to 7.5 kg of CO2 equivalent per kilogram of ε-caprolactam). If natural gas-based power and steam generation is involved, the potential global warming impact of primary ε-caprolactam from ε-caprolactam production methods decreases to 6.4 tons of CO2 equivalent per ton of ε-caprolactam (which is equal to 6.4 kg of CO2 equivalent per kilogram of ε-caprolactam).

[0182] The carbon footprint of the ε-caprolactam product obtained by the method according to the invention is much lower than that of either resynthesized or “original” ε-caprolactam. Preferably, the carbon footprint of the ε-caprolactam product obtained by the method according to the invention is less than 4 kg of CO2 per kg of ε-caprolactam, more preferably less than 3 kg of CO2, and most preferably equal to or less than 2 kg of CO2 equivalent (based on data derived from ecoinvent version 3.7.1; location: Europe). Attached Figure Description

[0183] In the following description, the invention will be illustrated with reference to certain embodiments thereof. However, the invention is as defined in the claims and as generally described herein. It should not be limited to the embodiments shown in the following figures for illustrative purposes.

[0184] Figure 1 This is a schematic diagram of the method of the present invention, which includes processing steps performed in an optional pretreatment section [A], depolymerization section [B], recovery section [C], and purification section [D].

[0185] Figure 2 illustrates two implementations of the pretreatment section [A], in which the fishing net containing polyamide 6 is cleaned in the cleaning section [α] by removing foreign materials and washing with a washing solvent, and fragmented in the mechanical size reduction section [β] to obtain fragments of the cleaned and fragmented fishing net containing polyamide 6.

[0186] Figure 2A An embodiment of the pretreatment section [A] is described, wherein the fishing net containing polyamide 6 is first cleaned in the cleaning section [α] by removing foreign materials and washing with a washing solvent, and then fragmented in the mechanical size reduction section [β] to obtain fragments of the cleaned and fragmented fishing net containing polyamide 6.

[0187] Figure 2B An embodiment of the pretreatment section [A] is described, wherein the fishing net containing polyamide 6 is first fragmented in the mechanical size reduction section [β], and then cleaned in the cleaning section [α] by removing foreign materials and by washing with solvent to obtain cleaned and fragmented pieces of the fishing net containing polyamide 6.

[0188] Figure 3 illustrates two implementations of the purification section [D], in which crude ε-caprolactam is purified to obtain high-purity ε-caprolactam.

[0189] Figure 3A An embodiment of the purification section [D] of the method of the present invention is described, which includes an extraction section [γ], an optional washing section [δ], a back-extraction section [ε], an optional oxidation section [θ], and a distillation section [λ].

[0190] Figure 3B An embodiment of the purification section [D] of the method of the present invention is described, which includes an extraction section [γ], an optional washing section [δ], a solvent exchange distillation section [μ], an optional oxidation section [θ], and a distillation section [λ].

[0191] Detailed illustration

[0192] The method of the present invention is in Figure 1 The diagram is shown schematically. This method is implemented in the following factory sections:

[0193] Optionally, in the pretreatment section [A], the fishing net containing polyamide 6 is cleaned by removing foreign materials and by washing with a washing solvent [2] to obtain a contaminated washing solvent [3]. Then, the fishing net containing polyamide 6 is fragmented by mechanical dimensional reduction. The cleaned and fragmented fishing net containing polyamide 6 is discharged from the pretreatment section [A] [6]. Optionally, in the pretreatment section [A], the fishing net containing polyamide 6 is further cleaned by removing foreign materials [1]. The removal of foreign materials can be performed before and / or after fragmenting the fishing net containing polyamide 6. Optionally, the optionally cleaned and fragmented fishing net containing nylon 6 is densified and then depolymerized in the depolymerization section [B] to form ε-caprolactam ( Figure 1 (Not shown in the text).

[0194] In the depolymerization zone [B], a fishing net [6] containing polyamide 6, optionally cleaned and fragmented, is depolymerized into ε-caprolactam. The self-depolymerization zone [B] discharges a stream containing ε-caprolactam [7]. Additionally, residual material is discharged [8]. Optionally, superheated steam [9] and a catalyst

[10] are introduced into the depolymerization zone [B].

[0195] Crude ε-caprolactam is recovered from the stream [7] containing ε-caprolactam discharged from the self-depolymerization section [B] in the recovery section [C]

[11] . Alternatively, when water or superheated steam [9] is introduced into the depolymerization section [B], the aqueous phase is discharged from the recovery section [C]

[12] .

[0196] The crude ε-caprolactam discharged from the recovery section [C] was purified in the purification section [D]

[11] to obtain high-purity ε-caprolactam

[26] . Water and impurities were also discharged from the purification section [D]

[25] .

[0197] Figure 2A An embodiment of the pretreatment section [A'] (the area enclosed by dashed lines) is depicted, wherein the fishing net containing polyamide 6 is first cleaned in the cleaning section [α'] by removing foreign material and washing with a washing solvent [2'], thereby obtaining foreign material, contaminated washing solvent [3'], and a cleaned fishing net containing polyamide 6 [4']. Subsequently, the cleaned fishing net containing polyamide 6 [4'] is fragmented in the mechanical size reduction section [β'] to obtain clean and fragmented fragments of the fishing net containing polyamide 6 [6']. The cleaned and fragmented fragments are then discharged. Optionally, the cleaned and fragmented fishing net containing nylon 6 is densified and then depolymerized in the depolymerization section [B] to form ε-caprolactam ( Figure 2A (Not shown in the text).

[0198] Figure 2B The implementation of the pretreatment section [A”] (the area enclosed by the dashed line) is depicted, wherein the fishing net containing polyamide 6 [1”] is first fragmented in the mechanical size reduction section [β”] to obtain fragmented pieces of the fishing net containing polyamide 6 [5”]. Subsequently, in the cleaning section [α”], the fragmented pieces of the fishing net containing polyamide 6 are cleaned by removing foreign materials and by washing with a washing solvent [2”] to obtain discharged foreign materials, contaminated washing solvent [3”], and cleaned and fragmented pieces of the fishing net containing polyamide 6 [6”]. Optionally, the cleaned and fragmented fishing net containing nylon 6 is densified and then depolymerized into ε-caprolactam in the depolymerization section [B]. Figure 2B (Not shown in the text).

[0199] Figure 3AAn implementation depicting the purification section [D”'] (the area enclosed by a dashed line) includes the following sections:

[0200] In the extraction section [γ”'], crude ε-caprolactam [11”'] is extracted with an organic solvent [13”'] to obtain an aqueous phase [14”'] containing water and impurities and an organic phase [15”'] containing the organic solvent, ε-caprolactam, and impurities. Both phases are discharged from the extraction section [γ”'].

[0201] In an optional washing section [δ”'], the organic phase containing organic solvent, ε-caprolactam, and impurities is washed with water or an alkaline aqueous solution [16”'] to obtain an aqueous phase containing residue [17”'] and a washed organic phase containing organic solvent, ε-caprolactam, and impurities [18”']. Both phases are discharged from the washing section [δ”'].

[0202] In the back-extraction section [ε”'], an optionally washed organic phase containing organic solvent, ε-caprolactam, and impurities [18”'] is back-extracted with water [19”'] to obtain an organic solvent phase containing impurities [20”'] and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam [22”']. Both phases are discharged from the back-extraction section [ε”']. Optionally, residual organic solvent is removed by stripping and / or distillation of the aqueous phase [22”'] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Figure 3A (Not shown in the text).

[0203] In an optional oxidation zone [θ”'], an aqueous phase [22”'] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidizing agent [23”'] to obtain an oxidized ε-caprolactam aqueous phase [24”'] containing water, ε-caprolactam, and impurities. This phase is discharged from the oxidation zone [θ”']. Optionally, the oxidized ε-caprolactam aqueous phase [24”'] containing water, ε-caprolactam, and impurities is filtered to remove manganese oxide (IV) solid particles, after which it is discharged from the oxidation zone [θ”']. Figure 3A (Not shown in the text).

[0204] In the distillation section [λ”'], an optionally oxidized aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities [24”'] (from which manganese oxide (IV) solid particles have been optionally removed by filtration) is distilled to obtain water and impurities [25”'] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high-purity ε-caprolactam [26”']. All distillation products are discharged from the distillation section [λ”']. Optionally, prior to distillation in the distillation section [λ”'], an alkali metal hydroxide is fed to the optionally oxidized aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities [24”'] ( Figure 3A (Not shown in the text).

[0205] Figure 3B An implementation of a purification section [D””] (the area enclosed by a dashed line) comprising the following segments is described:

[0206] In the extraction section [γ””], crude ε-caprolactam [11””] is extracted with an organic solvent [13””] to obtain an aqueous phase [14””] containing water and impurities and an organic phase [15””] containing the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the extraction section [γ””].

[0207] In an optional washing section [δ””], the organic phase containing organic solvent, ε-caprolactam, and impurities is washed with water or an alkaline aqueous solution [16””] to obtain an aqueous phase containing residue [17””] and a washed organic phase containing organic solvent, ε-caprolactam, and impurities [18””]. Both phases are discharged from the washing section [δ””].

[0208] In the solvent-exchange distillation section [μ””], an optionally washed organic phase [18””] containing organic solvent, ε-caprolactam, and impurities is subjected to solvent-exchange distillation by adding water [19””] to obtain an organic solvent [21””] and an aqueous phase [22””] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Both distillation products are discharged from the solvent-exchange distillation section [μ””].

[0209] In an optional oxidation zone [θ””], an aqueous phase [22””] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidizing agent [23””] to obtain an oxidized ε-caprolactam aqueous phase [24””] containing water, ε-caprolactam, and impurities. This phase is discharged from the oxidation zone [θ””]. Optionally, the oxidized ε-caprolactam aqueous phase [24””] containing water, ε-caprolactam, and impurities is filtered to remove manganese oxide (IV) solid particles, after which it is discharged from the oxidation zone [θ””]. Figure 3B(Not shown in the text).

[0210] In the distillation section [λ””], an optionally oxidized aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities [24””] (from which manganese oxide (IV) solid particles have been optionally removed by filtration) is distilled to obtain water and impurities [25””] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high-purity ε-caprolactam [26””]. All distillation products are discharged from the distillation section [λ””]. Optionally, before the distillation section [λ””], an alkali metal hydroxide is fed into the optionally oxidized aqueous phase of ε-caprolactam containing water, ε-caprolactam, and impurities [24″″], which is then loaded into the distillation section [λ″″]. Figure 3B (Not shown in the text).

[0211] Example

[0212] The following examples are provided to illustrate the invention in more detail, particularly with respect to certain forms of the invention. However, these examples are not intended to limit the scope of the invention.

[0213] ε-caprolactam, suitable for all major polyamide 6 polymerization applications without the need for dilution with higher purity ε-caprolactam, meets all of the following specifications:

[0214]

[0215] These parameters and measurement methods are defined as follows:

[0216] PAN: ISO DIS 8660 Caprolactam for industrial use - Determination of permanganate index of caprolactam - Spectrometric method, revision of the first edition of ISO 8660; 1988.

[0217] E290: ISO 7059-Caprolactam for industrial use-determination ofabsorbance at a wavelength of 290nm, 1982,

[0218] Volatile bases (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile bases content - Titrimetric method after distillation, 1988.

[0219] Basicity of ε-caprolactam products: Basicity was determined by titration at 25°C using a Tashiro indicator, which is 0.1 wt. / v in a 1:2 ratio. 乙醇 % Methylene Blue: 0.1 wt. / v 乙醇 % Methyl red (which turns gray at the endpoint). First, titrate the solution in a flask containing water and indicator until gray, then add X grams of an aqueous solution of ε-caprolactam containing Y wt.% ε-caprolactam (determined by refractive index), and titrate the solution back to gray using 0.01N H₂SO₄ solution. The alkalinity is then given as follows:

[0220] Alkalinity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y)

[0221] in:

[0222] v = Volume of H2SO4 solution added (ml)

[0223] t = the equivalent concentration of the H₂SO₄ solution (= 0.01N)

[0224] X = Sample weight (g)

[0225] Y = ε-caprolactam concentration (wt.%)

[0226] The polyamide 6-containing pellets used in the examples and comparative experiments were made from discarded fishing nets. Pretreatment included removal of foreign material, washing, grinding, melting, and conversion into chips / pellets. The pellets were obtained from a fishing net recycling company in China.

[0227] The pellets are rod-shaped with an average diameter of about 3 mm and an average length of about 4 mm, and most of the pellets weigh between 20 and 30 mg.

[0228] The combination of pyrolysis gravimetric analysis (TGA) and qualitative information from differential scanning calorimetry (DSC) showed that the polyamide 6 content of the pellets was >98 wt.% (on a dry basis).

[0229] Example 1

[0230] Depolymerization of polyamide 6 and recovery of ε-caprolactam

[0231] 48 grams of polyamide 6-containing scrap / granules and 14 grams of 20 wt.% phosphoric acid were charged into an autoclave. First, the reactor contents were heated under nitrogen, followed by continuous injection of superheated steam at a rate of 4 g / min over a 120-minute reaction period. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, the steam stream was continuously vented from the reactor and cooled to approximately 20°C to obtain the ε-caprolactam and water condensate contained within.

[0232] The condensate, consisting of approximately 44 grams of ε-caprolactam and mostly water, was concentrated to an ε-caprolactam concentration of 49.7 wt.% by evaporation in a rotary evaporator (rotary evaporator) operating under vacuum (9.5 kPa; water bath temperature of approximately 65 °C). (This mixture, i.e., crude ε-caprolactam, is the mixture to be purified.)

[0233] The specifications for crude ε-caprolactam are as follows:

[0234] PAN: 16

[0235] E290: 2.33

[0236] This example demonstrates that crude ε-caprolactam can be obtained by depolymerizing polyamide 6 derived from discarded fishing nets containing polyamide 6. Due to its extremely poor quality, this crude ε-caprolactam cannot be used as is in all major polyamide 6 polymerization applications.

[0237] Comparative Experiment 1

[0238] Polyamide 6 was depolymerized, ε-caprolactam was recovered, and the product was purified by distillation.

[0239] Follow the procedure of Example 1. Then add 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam. Subsequently, remove water and impurities with boiling points lower than ε-caprolactam as the top product by vacuum distillation in a batch-run distillation apparatus, wherein the pressure is gradually reduced. ε-caprolactam is distilled at 300 Pa, while impurities with boiling points higher than ε-caprolactam are retained in the distillation apparatus as the bottom product. The specifications of the distilled ε-caprolactam are as follows:

[0240]

[0241] The comparative experiment showed that the ε-caprolactam obtained by depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by distillation was of extremely poor quality because it did not meet most of the required specifications for major polymerization applications.

[0242] Comparative Experiment 2

[0243] Depolymerization of polyamide 6, recovery of ε-caprolactam, and purification by oxidation

[0244] The procedure of Example 1 was followed. Crude ε-caprolactam was then treated with 0.2 wt.% KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solid formed was then removed from the oxidation product by filtration. The purified ε-caprolactam specifications were as follows:

[0245] PAN: 30

[0246] E290: 3.62

[0247] The comparative experiment showed that the ε-caprolactam obtained by depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by oxidation was of extremely poor quality and could not be used as is in all major polyamide 6 polymerization applications.

[0248] Comparative Experiment 3

[0249] The polyamide 6 was depolymerized, ε-caprolactam was recovered, and the polyamide was purified by oxidation and distillation.

[0250] For the oxidatively purified ε-caprolactam aqueous solution obtained in Comparative Experiment 2, 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam was first added, followed by distillation according to the procedure described in Comparative Experiment 1. The purified ε-caprolactam obtained was as follows:

[0251]

[0252] The comparative experiment showed that ε-caprolactam, obtained by depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by oxidation and distillation, was of poor quality and could not be used as is in all major polyamide 6 polymerization applications.

[0253] Comparative Experiment 4

[0254] The polyamide 6 was depolymerized, ε-caprolactam was recovered, and the polyamide was purified by oxidation, carbon treatment, and distillation.

[0255] Following the procedure in Example 1, crude ε-caprolactam was then treated with 0.2 wt.% KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The resulting oxidized solution was then treated with 0.4 wt.% powdered activated carbon at 50°C for 0.5 hours. The resulting solids and activated carbon particles were then removed from the ε-caprolactam aqueous solution by filtration. Subsequently, after adding 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam, the activated carbon-treated ε-caprolactam aqueous solution was distilled according to the procedure described in Comparative Experiment 1. The purified ε-caprolactam obtained was of the following specifications:

[0256]

[0257] The comparative experiment showed that ε-caprolactam obtained by depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by oxidation, carbonization and distillation was of poor quality and could not be used as is in all major polyamide 6 polymerization applications.

[0258] Example 2

[0259] Polyamide 6 is depolymerized, ε-caprolactam is recovered, and the product is purified by extraction, back-extraction, oxidation, and distillation.

[0260] Following the procedure of Example 1, the condensate, consisting of 44.5 g of ε-caprolactam and predominantly water, was concentrated to an ε-caprolactam concentration of 50.2 wt.% by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65 °C). (This mixture, i.e., crude ε-caprolactam, is the mixture to be purified.)

[0261] Crude ε-caprolactam was extracted ten times in batches with a solvent mixture of 4-methyl-2-pentanol (50 wt.%) and cyclohexane (50 wt.%) at approximately 25°C. The total amount of extraction solvent used was 8.05 g of 4-methyl-2-pentanol / cyclohexane per g of crude ε-caprolactam. The combined organic extracts were washed in batches with 7 g of 2 wt.% NaOH aqueous solution. The resulting washed organic extract was concentrated to an ε-caprolactam concentration of approximately 40 wt.% by distillation under vacuum, followed by the addition of fresh cyclohexane. The resulting mixture had an ε-caprolactam concentration of approximately 27 wt.%, and the weight ratio of the solvent mixture of 4-methyl-2-pentanol / cyclohexane was 50 wt.% : 50 wt.%. This mixture was extracted seven times in batches with water at approximately 25°C. The total amount of water used was 5.75 g of water per g of recovered ε-caprolactam.

[0262] The resulting aqueous solution of ε-caprolactam was concentrated to an ε-caprolactam concentration of 45.9 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature approximately 65 °C). The resulting mixture was treated with 0.04 wt.% KMnO4 relative to ε-caprolactam at 50 °C for 2 hours. The solids formed were then removed from the oxidation reaction products by filtration. After adding 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam, the ε-caprolactam in the resulting aqueous solution was further purified by distillation as described in Comparative Experiment 1. The purified ε-caprolactam obtained was as follows:

[0263]

[0264] Based on this experiment, it can be concluded that purified ε-caprolactam, meeting all the required specifications for major polymerization applications, can be obtained by depolymerizing polyamide 6 derived from discarded fishing nets containing polyamide 6 and purifying it through extraction, back-extraction, oxidation, and distillation.

[0265] Example 3

[0266] Polyamide 6 is depolymerized, ε-caprolactam is recovered, and the product is purified by extraction, back-extraction, and distillation.

[0267] Follow the procedure in Example 1.

[0268] The obtained aqueous solution of ε-caprolactam was concentrated to an ε-caprolactam concentration of 50.7 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of approximately 65 °C). (This mixture, i.e., crude ε-caprolactam, is the mixture to be purified.)

[0269] Crude ε-caprolactam was extracted 11 times in batches with toluene at approximately 25°C. The total amount of extraction solvent used was 12.5 g of toluene per gram of crude ε-caprolactam. The combined organic extracts were concentrated to an ε-caprolactam concentration of 30 wt.% by distillation under vacuum. Subsequently, the concentrated organic extract was extracted 4 times in batches with water at approximately 25°C. The total amount of water used was 1.79 g of water per gram of combined organic extract. The resulting combined aqueous solution of ε-caprolactam was concentrated to an ε-caprolactam concentration of 52.2 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature approximately 65°C).

[0270] After adding 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam, the ε-caprolactam in the obtained aqueous solution was further purified by distillation as described in Comparative Experiment 1. The purified ε-caprolactam obtained was of the following specifications:

[0271]

[0272] Based on this experiment, it can be concluded that purified ε-caprolactam, meeting all the required specifications for major polymerization applications, can be obtained from the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by extraction, back-extraction, and distillation.

[0273] Example 4

[0274] Polyamide 6 is depolymerized, ε-caprolactam is recovered, and the product is purified by extraction, back-extraction, and distillation.

[0275] Follow the procedure of Example 1 twice.

[0276] The two obtained aqueous ε-caprolactam solutions were concentrated to ε-caprolactam concentrations of 70.2 wt.% and 67.6 wt.%, respectively, by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of approximately 65 °C). Subsequently, two portions of the concentrated aqueous ε-caprolactam solution were added together. One-third of the crude ε-caprolactam from the resulting mixture was used for further purification.

[0277] Crude ε-caprolactam was diluted to 65.0 wt.% and extracted five times with benzene at approximately 25°C. The total amount of extraction solvent used was 10.7 g of benzene per gram of crude ε-caprolactam. The combined organic extracts were washed in batches with 3.1 g of 2 wt.% NaOH aqueous solution. The washed organic extracts were concentrated to an ε-caprolactam concentration of approximately 17 wt.% by distillation under vacuum. Subsequently, the concentrated organic extracts were extracted four times with water in batches at approximately 25°C. The total amount of water used was 1.19 g of water per gram of concentrated combined organic extracts. The combined aqueous solution of ε-caprolactam was concentrated to an ε-caprolactam concentration of 56.3 wt.% by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C).

[0278] After adding 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam, the ε-caprolactam in the obtained aqueous solution was further purified by distillation as described in Comparative Experiment 1. The purified ε-caprolactam obtained was of the following specifications:

[0279]

[0280] Based on this example, it can be concluded that purified ε-caprolactam, meeting all the required specifications for major polymerization applications, can be obtained from the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by extraction, back-extraction, and distillation.

[0281] Example 5

[0282] Calculate the carbon footprint of purified ε-caprolactam

[0283] A continuous method according to the present invention for producing purified ε-caprolactam from a fishing net containing polyamide 6 in a factory is simulated. The method includes:

[0284] - Mechanical removal of foreign materials from fishing nets containing polyamide 6;

[0285] - Cut the fishing net containing polyamide 6 into small pieces;

[0286] - Wash the fragments of the fishing net containing polyamide 6 with water;

[0287] - The washed fishing net containing polyamide 6 and the aqueous extract were separated by centrifugation;

[0288] -Dry the washed fragments of fishing nets containing polyamide 6;

[0289] - Melt and granulate the washed fragments of fishing net containing polyamide 6;

[0290] -Depolymerize polyamide 6 under the influence of H3PO4 and superheated steam;

[0291] - Partial condensation recovery of crude ε-caprolactone from the steam discharged from the self-degrading polymerization reactor

[0292] Amine (75 wt.% ε-caprolactam);

[0293] - Crude ε-caprolactam was concentrated by countercurrent extraction with toluene;

[0294] - Wash the organic extract with a diluted caustic alkali solution;

[0295] - Back-extraction of washed organic extracts using water countercurrent;

[0296] - Evaporation and concentration of aqueous extract;

[0297] - Aqueous extract concentrated by oxidation with KMnO4;

[0298] - Add caustic soda; and

[0299] - Pure ε-caprolactam was recovered by vacuum distillation.

[0300] The carbon footprint of purified ε-caprolactam is calculated based on the consumption value of raw materials, and the utility of the method described above is based on data derived from ecoinvent version 3.7.1.

[0301] The results showed that the carbon footprint of the purified ε-caprolactam product obtained from fishing nets containing polyamide 6 was less than 2.0 tCO2 equivalents / t ε-caprolactam (location: Europe).

[0302] Although the present invention has been described with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention, including (semi)continuous operation and commercial scale-up.

Claims

1. A method for recovering purified ε-caprolactam from a material derived from a fishing net containing polyamide 6 in a factory, wherein the factory comprises: De-aggregation section [B], Reclaimed section [C], and Purification section [D], The method includes the following steps: a) The material derived from the fishing net containing polyamide 6 is loaded into the depolymerization section [B]; b) In the depolymerization zone [B], the material derived from the fishing net containing polyamide 6 is depolymerized at a temperature in the range of 180°C to 400°C to obtain a stream containing ε-caprolactam; c) Discharging the stream containing ε-caprolactam from the depolymerization section [B], and recovering crude ε-caprolactam from the stream in the recovery section [C]; and d) Purify the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises the following steps: (i) Extracting the crude ε-caprolactam with an organic solvent to obtain an organic phase, wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities; (ii) The solvent is converted by at least partially replacing the organic solvent with water to obtain an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, wherein the solvent conversion step (ii) is selected from methods based on water back-extraction and methods based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; and (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation from the aqueous phase; and wherein... After step d)(i), the organic phase obtained in step d)(i) is washed with water or an alkaline aqueous solution, and Prior to the distillation removal in step d)(iii), an alkali metal hydroxide is added to the aqueous phase.

2. The method of claim 1, wherein the alkali metal hydroxide is NaOH.

3. The method of claim 1, wherein the depolymerization in step b) is carried out in the presence of water, wherein the stream containing ε-caprolactam is a vapor stream containing ε-caprolactam and water in a weight ratio of 1:2 to 1:15; and wherein in the extraction in step d)(i), an aqueous phase and an organic phase are obtained.

4. The method of any one of claims 1 to 3, wherein prior to the distillation removal in step d)(iii), the purification in step d) further comprises the following steps: Oxidation is carried out at a temperature ranging from 20°C to 85°C using an oxidizing agent in an aqueous solution, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof.

5. The method of claim 4, wherein the oxidant is potassium permanganate.

6. The method of claim 3, wherein the water present in step b) is in the form of steam, which is introduced into the depolymerization section in step b) as superheated steam having a temperature in the range of 220°C to 575°C [B].

7. The method of claim 6, wherein the superheated steam has a temperature in the range of 275°C to 500°C.

8. The method of any one of claims 1 to 3, wherein the solvent in step d)(ii) is converted to a method based on water back-extraction.

9. The method of any one of claims 1 to 3, wherein the solvent in step d)(ii) is converted to a solvent exchange distillation method.

10. The method according to any one of claims 1 to 3, wherein the organic solvent in step d)(i) is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof.

11. The method of claim 4, wherein the oxidation in step d) is carried out in an aqueous solution containing water and ε-caprolactam in a weight ratio of 5:1 to 1:

5.

12. The method of any one of claims 1 to 3, wherein the depolymerization in step b) is carried out in the absence or presence of a catalyst, wherein the catalyst is selected from acid catalysts and base catalysts, the acid catalyst being selected from the group consisting of: phosphoric acid, boric acid, sulfuric acid, organic acids, salts of the aforementioned acids, Al2O3 and SiO2, and combinations thereof; and the base catalyst being selected from the group consisting of: organic bases and solid bases, and combinations thereof.

13. The method of claim 12, wherein the acid catalyst is orthophosphoric acid.

14. The method of claim 12, wherein the acid catalyst is an organic sulfonic acid.

15. The method of claim 12, wherein the alkaline catalyst is selected from the group consisting of alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides, and alkaline earth metal salts.

16. The method of claim 12, wherein the alkaline catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

17. The method of any one of claims 1 to 3, wherein the depolymerization in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

18. The method of any one of claims 1 to 3, wherein prior to step a), the material derived from the fishing net containing polyamide 6 is obtained in the form of the product of the pretreatment in the pretreatment section [A].

19. The method of claim 18, wherein the pretreatment in the pretreatment section [A] includes cleaning in the cleaning section [α] and / or mechanical size reduction in the mechanical size reduction section [β] and / or bulk density increase in the densification section [γ].

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