Method for recovering epsilon-caprolactam from fishing nets comprising nylon 6

Abstract

The present invention relates to a method for recovering epsilon-caprolactam from fishing nets comprising nylon 6. The invention provides a method and a plant for recovering purified epsilon-caprolactam from a fishing net comprising nylon 6, wherein the plant comprises a depolymerization section [B], a recovery section [C] and a purification section [D]. The invention also provides purified [epsilon]-caprolactam which has a particularly low product carbon footprint and is obtained by depolymerizing nylon 6 of a fishing net. In a second preferred embodiment, the purification of the distilled caprolactam is carried out by crystallization in the concentration process. The crystalline caprolactam produced by the concentration is generally sufficiently pure for direct use. After crystallization, it may be necessary to purify the mother liquor by recycling the mother liquor to the aqueous solution, for example, prior to extraction with alkylphenol. The mother liquor may be purified, for example, by distillation.

Description

[0001] This application is a divisional application of Chinese patent application No. 202380019028.7, filed on January 27, 2023, entitled "Method for recovering ε-caprolactam from a fishing net containing nylon 6". Technical Field

[0002] This invention relates to a method for recovering ε-caprolactam from materials derived from discarded fishing nets containing nylon 6. More specifically, this invention relates to a method for recovering ε-caprolactam from materials derived from discarded fishing nets containing nylon 6, thereby obtaining high-quality ε-caprolactam. Background Technology

[0003] 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 nylon 6, polyester, polypropylene, and polyethylene).

[0004] 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. As a result, these nets remain in marine ecosystems for many years, leading to the accumulation of large quantities of ghost nets.

[0005] The latest assessment by the FAO and the United Nations Environment Programme (UNEP) indicates that approximately 640 million kilograms of fishing gear are discarded, lost, or abandoned each year.

[0006] 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.

[0007] The final destinations for collected waste fishing nets, including those containing nylon 6, range from landfill, incineration (optionally utilizing heat recovery), 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 monomer components (ε-caprolactam in the case of nylon 6).

[0008] Mechanical recycling (also known as material recycling or back-to-plastics recycling) refers to the process of 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 such as natural gas, coal, or crude oil that have never been used or processed before. During 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.

[0009] 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 or only 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.

[0010] In 1938, Paul Schlack invented Nylon 6 (CAS No.: 25038-54-4), also known as N 6, polyamide 6, PA 6, poly(caprolactam), poly(hexane-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide) or poly[imino(1-sideoxyhexane-1,6-diyl)].

[0011] Generally, nylon 6 (also known as polyamide 6 or polycaprolactam) is synthesized by ring-opening polymerization of ε-caprolactam at a temperature of about 260°C in an inert atmosphere. 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.

[0012] The method for producing Nylon 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.

[0013] The depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam. Methods for the depolymerization of nylon 6 are known. Such methods can be operated in batch mode, in semi-continuous mode (typically in the case of batch (re)loading nylon 6 into the depolymerization reactor), or in continuous mode.

[0014] LA Dmitrieva, AA Speranskii, SA Krasavin, and YN Bychkov, “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 nylon 6 with and without catalysts.

[0015] 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 nylon 6.

[0016] 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 at a pressure in the range of about 1 atm to about 100 atm and substantially less than the saturated vapor pressure of water at the temperature at which the ε-caprolactam-containing vapor stream is formed.

[0017] Depolymerization of fishing nets containing nylon 6 has been practiced in the past. Numerous purification techniques and combinations thereof have been implemented to purify the crude ε-caprolactam obtained from these depolymerization reactions. Oxidants such as potassium permanganate (KMnO4) are even frequently used to form manganese oxide (IV) (MnO2) particles as reaction products. Removing these MnO2 particles requires solid-liquid filtration, which is a rather tedious and laborious process. Other applications employ adsorption-based techniques, where solid adsorbents such as (activated) carbon and diatomaceous earth are used as adsorbents. These techniques, which generate significant amounts of waste, are also quite tedious and laborious. Furthermore, despite the long history of nylon 6 recycling, the quality of the monomeric ε-caprolactam obtained through these methods remains particularly poor.

[0018] Therefore, ε-caprolactam obtained from the depolymerization of fishing nets containing nylon 6 is only suitable for less demanding applications (degradation cycles), such as engineering plastics and carpets. If ε-caprolactam obtained from the depolymerization of fishing nets is used for more demanding applications, it needs to be blended with large quantities of higher-grade and purer ε-caprolactam to mask the rather poor quality of the ε-caprolactam obtained from the depolymerization of fishing nets. High-speed melt spinning of nylon 6 for producing fine textile fibers requires high-quality ε-caprolactam as a raw material. The high-quality ε-caprolactam grade used for these applications should not only be extremely pure, but its properties should also not change over time.

[0019] In summary, prior art methods for recovering ε-caprolactam from fishing nets containing nylon 6 have failed to produce high-quality ε-caprolactam grades that can be used to replace the original ε-caprolactam grades for demanding applications.

[0020] Currently, there is no method available for recovering high-purity ε-caprolactam from fishing nets containing nylon 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.

[0021] Furthermore, there is a need to allow for an economically viable method to recover high-purity ε-caprolactam from fishing nets containing nylon 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.

[0022] In addition, there is a need to provide high-purity grades of ε-caprolactam from fishing nets containing nylon 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.

[0023] Furthermore, it is necessary to purify the crude ε-caprolactam obtained through the depolymerization of a fishing net containing nylon 6 without using oxidants such as potassium permanganate (KMnO4) or adsorbents such as (activated) carbon and diatomaceous earth. Techniques based on these oxidants and adsorbents are quite laborious and generate solid waste.

[0024] Furthermore, a plant is needed to produce high-purity grades of ε-caprolactam from materials derived from fishing nets containing nylon 6.

[0025] Finally, there is a need for methods that allow for the industrial-scale recycling of ε-caprolactam from fishing nets containing nylon 6 in order to process the large quantities of nylon 6-containing fishing nets that are discarded each year. Summary of the Invention

[0026] The objective of this invention is to satisfy one or more of the requirements described above and to overcome or mitigate the disadvantages associated with prior art methods.

[0027] In particular, an object of the present invention is to provide a method for recovering high-purity ε-caprolactam from materials derived from fishing nets containing nylon 6. In this regard, another object of the present invention is to provide a method for recovering high-purity ε-caprolactam from materials derived from fishing nets containing nylon 6, which can replace high-purity virgin ε-caprolactam for all applications, including high-speed melt spinning of nylon 6 for the production of fine textile fibers.

[0028] Another objective of this invention is to provide a method for recovering high-purity grades of ε-caprolactam from materials derived from fishing nets containing nylon 6 on an industrial scale.

[0029] A further objective of this invention is to provide a method for recovering high-purity ε-caprolactam from materials derived from fishing nets containing nylon 6 in an economical manner. In particular, the objective of this invention is to provide a method suitable for recovering high-purity ε-caprolactam from materials derived from fishing nets containing nylon 6, without exceeding the production cost of the original high-purity ε-caprolactam.

[0030] The present invention also aims to provide a method for purifying crude ε-caprolactam obtained by depolymerization of a material derived from a fishing net containing nylon 6, without generating solid waste.

[0031] Another objective of the present invention is to provide high-purity grades of ε-caprolactam from materials derived from fishing nets containing nylon 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 nylon 6.

[0033] Another objective of this invention is to provide a plant for producing high-purity grades of ε-caprolactam from materials derived from fishing nets containing nylon 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 materials derived from fishing nets containing nylon 6 in a factory, wherein the factory comprises: -De-aggregation section [B], -Reclaim section [C], and -Purification section [D], The method includes the following steps: a) The material derived from the fishing net containing nylon 6 is loaded into the depolymerization section [B]; b) In the depolymerization zone [B], the material derived from the fishing net material containing nylon 6 is depolymerized at a temperature in the range of 180°C to 400°C, preferably 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C, in order to obtain a stream containing ε-caprolactam; 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 d) Purify the crude ε-caprolactam in purification section [D] to obtain purified ε-caprolactam, wherein purification includes the following steps: (i) Extracting crude ε-caprolactam with an organic solvent to obtain an organic phase comprising an organic solvent, ε-caprolactam, and impurities; and (iv) Purified ε-caprolactam was obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10 to 95 °C.

[0037] 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, economically viable recovery of high-grade ε-caprolactam from fishing nets containing nylon 6. The method of the present invention is economically viable and advantageous from several perspectives. First, the method is suitable for a variety of fishing net materials containing nylon 6, differing, for example, in their overall composition and / or their nylon 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 nylon 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 nylon 6 for the processing of currently discarded large quantities of nylon 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 ε-caprolactam produced, for example, via the Beckmann rearrangement of cyclohexanone oxime. The method of the present invention allows for the efficient processing of fishing nets containing nylon 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 from ecoinvent version 3.7.1; location: Europe). Unless otherwise stated, the carbon footprint values ​​of the products stated herein are based on data from ecoinvent version 3.7.1, and are located in Europe.

[0038] Following the method described herein, the present invention also provides a plant for producing purified ε-caprolactam from fishing nets containing nylon 6 within the plant, wherein the plant comprises: De-aggregation section [B], Reclaimed section [C], Purification section [D], and The factory is configured to carry out the method of the present invention.

[0039] The present invention also provides purified ε-caprolactam, which is obtained by depolymerizing nylon 6 produced from a fishing net containing nylon 6 using the method according to 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 from ecoinvent version 3.7.1; location: Europe).

[0040] Advantageous embodiments of the invention are indicated in the dependent claims and are described in more detail below. Detailed Implementation

[0041] Detailed description of preferred embodiments Fishing nets containing Nylon 6 The method of this invention uses fishing nets containing nylon 6 or fishing net materials derived from such nets as starting materials. Fishing nets containing nylon 6 are generally solid materials, and in particular, fishing nets containing nylon 6 are generally meshes formed by weaving relatively fine nylon 6 threads. As used herein, materials derived from fishing nets containing nylon 6 mean, for example, materials derived from fishing nets containing nylon 6 after processes such as crushing, washing, sorting, densification, etc. The term "fishing net" is used in this disclosure and also refers to "fishing net material," i.e., these terms are used synonymously herein. Unless the context clearly requires otherwise, as used in this disclosure and the claims, the singular forms "a" and "the" include the plural forms, particularly in the sense of "one or more."

[0042] Fishing nets containing nylon 6 may contain a variety of compounds added during or after their polymerization and yarn 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 the fishing net also depends on its specific application. Therefore, fish farming nets, purse seines, and bottom nets have different chemical compositions.

[0043] The surface of a seawater-immersed vessel quickly becomes covered with marine organisms known as biofouling or marine biofouling. 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 of the wetted surface, resulting in significant material costs. The accumulation of algae and barnacles increases drag on ships and damages aquaculture facilities and equipment used in them.

[0044] 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.

[0045] The method of the present invention has advantages over prior art methods, which are limited, for example, to relatively pure nylon 6-containing materials, such as carpets and spinning waste containing PA 6. The method of the present invention is not limited thereto and can be applied with great success to any kind of fishing net containing nylon 6.

[0046] Possible preprocessing steps Prior to step a) of the method of the present invention, the material containing the fishing net comprising nylon 6 undergoes pretreatment in the pretreatment section [A] to obtain materials derived from the fishing net comprising nylon 6, particularly cleaning in the cleaning section [α] and / or mechanical size reduction in the mechanical size reduction section [β] and / or densification section [γ]. This has the advantage that the fishing net comprising nylon 6 loaded into the depolymerization section [B] contains less contamination with non-nylon 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 and / or densified fishing net comprising nylon 6 can be more easily disposed of.

[0047] As used herein, the term "cleaning" is defined as any process of removing non-nylon 6 materials adhering to or mixed with a multi-component material containing nylon 6. Cleaning is advantageous because any non-nylon 6 material removed will therefore not interfere with the next step of the method of the present invention.

[0048] Discarded fishing nets containing nylon 6 can be mixed with various other materials, such as rocks, metals (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 nylon 6 can be mixed with non-nylon 6 fishing nets, such as those made of nylon 6,6, polyethylene terephthalate (PET), polypropylene (PP), or polyethylene (PE). Discarded fishing nets containing nylon 6 can also be coated with antifouling coatings, for example, based on metals such as copper, or containing non-metallic antifouling coatings.

[0049] The size of discarded fishing nets containing Nylon 6 largely depends on the specific application. Discarded fishing nets containing Nylon 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, which can be 1.5 km long and more than 150 m deep.

[0050] Preferably, the fishing net containing nylon 6 is fragmented into pieces, and then depolymerized in the depolymerization zone [B] in step a). This mechanical pretreatment, i.e., the mechanical crushing or fragmentation of the fishing net containing nylon 6, can be achieved, for example, by cutting, tearing, grinding, abrading, and / or shaving. In a preferred embodiment, the fishing net containing nylon 6 is loaded into step a) in the form of fragments weighing 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 fragments of the fishing net containing nylon 6 with the aforementioned weight has the advantage that such fragments can be more easily disposed of and / or cleaned by solvent washing.

[0051] Optionally, large metal fragments, rocks, and other interfering materials that cause severe wear on the equipment used for mechanical crushing or fragmentation are removed before the mechanical crushing or fragmentation of the fishing net containing nylon 6. Preferably, materials containing non-nylon 6, such as materials containing polyethylene, polypropylene, and nylon 6,6, such as fishing nets, are removed before or after the mechanical crushing or fragmentation of the fishing net containing nylon 6. The removal of foreign materials can be done mechanically or manually. The removal of these interfering materials has the advantages of significantly reducing the maintenance costs of the equipment used for mechanical crushing or fragmentation. In addition, the nylon 6 content of the material obtained after mechanical crushing or fragmentation is higher than that without removing the interfering materials. In particular, the removal of nylon 6,6 is advantageous because it interferes with the depolymerization of nylon 6, causing the reactor to clog itself, reducing the recovery yield of ε-caprolactam, and interfering with the subsequent purification of the recovered ε-caprolactam.

[0052] Preferably, the fishing net containing nylon 6 with a Cu-based antifouling coating is removed before mechanical shredding or fragmentation. More preferably, the fishing net with a 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.

[0053] Optionally, foreign materials are separated from the fishing net containing nylon 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 nylon 6 is advantageous because such materials can interfere with the depolymerization of nylon 6, reduce the recovery yield of ε-caprolactam, and interfere with the subsequent purification of the recovered ε-caprolactam.

[0054] Optionally, the fishing net containing nylon 6 is cleaned by washing with a solvent, preferably water, before loading into the depolymerization section [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 nylon 6.

[0055] Washing is preferably performed under friction. Different types of industrial washing systems are available on the market, such as high-speed friction washers.

[0056] Washing fishing nets containing nylon 6, especially those containing nylon 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.

[0057] Optionally, the fishing net containing nylon 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 solvent-based nylon 6 and preventing subsequent process steps from being affected by dilution or contamination from the washing solvent.

[0058] Optionally, a preferably washed and reduced-size fishing net containing nylon 6 is loaded into a furnace (e.g., an extruder). The fishing net containing nylon 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. The pellets or the molten and optionally filtered polymer melt is directly loaded into the depolymerization section [B].

[0059] 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, 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, more preferably 3 to 15 mm.

[0060] Granulation of cleaned and reduced-size fishing nets containing nylon 6 offers 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 processed in the (automated) feeding to the depolymerization zone [B].

[0061] The locations of the pretreatment sites for fishing nets containing nylon 6 and the depolymerization section [B] can be the same. However, preferably, one or more of the pretreatment steps are carried out at different locations, for example near a harbor where discarded fishing nets containing nylon 6 are collected and / or at a dedicated pretreatment site for discarded fishing nets. The fishing nets containing nylon 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 fishing nets containing nylon 6.

[0062] Therefore, according to a particular advantageous embodiment of the invention, prior to step a), the material containing the fishing net containing nylon 6 is pretreated in the pretreatment section [A], particularly in the cleaning section [α] and / or in the mechanical size reduction section [β] and / or the densification section [γ], to obtain a material derived from the fishing net containing nylon 6.

[0063] Loading step a) In step a) of the invention, a fishing net containing nylon 6, optionally reduced in size and / or washed and / or melted and re-cured, is loaded into the depolymerization section [B]. The depolymerization section [B] comprises one or more depolymerization reactors operating in series and / or in parallel.

[0064] In one embodiment, the fishing net containing nylon 6 is mechanically compressed into 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].

[0065] In another embodiment, before being loaded into the depolymerization section [B], the fishing net containing nylon 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 section [B].

[0066] In another preferred embodiment, the fishing net containing nylon 6 is dried before being loaded into the depolymerization section [B], particularly after a cleaning step. 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]).

[0067] Preferably, the material (derived from) the fishing net containing nylon 6 is fed into the depolymerization reactor in solid or molten form. Preferably, the fishing net containing nylon 6 is fed in molten form. Molten feeding can be achieved using an extruder, gear pump, or other components known to those skilled in the art.

[0068] The feed of the depolymerization reactor (derived from) fishing nets containing nylon 6 can be achieved by continuous or intermittent feeding of fishing nets containing nylon 6.

[0069] Depolymerization step b) In the depolymerization section [B], the material derived from the fishing net containing nylon 6 is depolymerized to form ε-caprolactam. The formed ε-caprolactam is discharged from the depolymerization section in the form of a stream containing ε-caprolactam.

[0070] The depolymerization of the fishing net containing nylon 6 is achieved in the depolymerization zone [B] by raising the temperature of the fishing net containing nylon 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.

[0071] Generally, the formation rate of ε-caprolactam increases at higher temperatures. Temperatures below 400°C are preferred because above 400°C, side reactions and impurities of nylon 6 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 depolymerization reactor. In a preferred embodiment of the invention, the depolymerization of the fishing net containing nylon 6 is carried out at a temperature in the range of 220°C to 340°C or 240°C to 325°C. This temperature range allows for the production of particularly pure ε-caprolactam.

[0072] 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. This pressure range allows for the production of particularly pure ε-caprolactam.

[0073] The depolymerization of the fishing net (material) (derived from) nylon 6 can be achieved with or without a solvent. Preferably, the depolymerization of the fishing net (material) (derived from) nylon 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.

[0074] Preferably, the depolymerization will be completed within 0.1 hours to 24 hours, more preferably 0.5 hours to 6 hours.

[0075] 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 nylon 6-containing fishing net in the depolymerization section [B]. 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.

[0076] Preferably, the ε-caprolactam in the steam stream containing ε-caprolactam and water has a partial pressure of 0.1 kPa to 1 MPa, more preferably 0.3 kPa to 0.5 MPa, and most preferably 1 kPa to 0.1 MPa.

[0077] During the depolymerization reaction, decomposition products can be formed, including linear and cyclic oligomers of ε-caprolactam. Additionally, the feed stream of the fishing net containing nylon 6 may contain other components, i.e., impurities, such as non-nylon 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 depolymerization zone [B] will contain not only water and ε-caprolactam but also impurities.

[0078] 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 the melting temperature of nylon 6. Preferably, the energy content of the superheated steam charged into the depolymerization reactor is high enough 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. In another preferred embodiment, a portion of the heat input required for the depolymerization reaction and the evaporation of the ε-caprolactam formed is introduced through the wall of the depolymerization reactor.

[0079] Generally, the mass of the vapor stream removed from the depolymerization zone [B] is less than the mass of the total feed to the depolymerization zone. The total feed to the depolymerization zone [B] includes a net containing nylon 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 zone [B]. Preferably, another stream is discharged from the depolymerization zone [B]. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization zone [B]. When phosphoric acid is used as the depolymerization catalyst, the additional stream may contain impurities present in the net containing nylon 6, undepolymerized nylon 6, unevaporated ε-caprolactam, catalyst, and compounds formed under depolymerization conditions, such as monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate. In a preferred embodiment, a stream containing monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate is discharged from the depolymerization zone [B]. More preferably, this stream discharged intermittently or continuously from the 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.

[0080] Depolymerization of fishing nets containing nylon 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.

[0081] Most preferably, depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is Lewis or Brenstein. 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 a particularly preferred embodiment, phosphoric acid is used as a catalyst for depolymerization, and in another, p-toluenesulfonic acid is used.

[0082] However, in another preferred embodiment, no catalyst is used in the depolymerization of fishing nets containing nylon 6. This has the advantage of lower cost (catalyst cost and catalyst waste disposal cost). However, higher temperatures (and pressures) are typically required compared to depolymerization of fishing nets containing nylon 6 in the presence of a catalyst.

[0083] 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 nylon 6 to 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 catalyst cost increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the nylon 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 nylon 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.

[0084] The depolymerization of nylon 6 can be carried out in batch, semi-continuous, or continuous mode, all of which are known to those skilled in the art. As used herein, “batch,” “semi-continuous,” and “continuous” refer to the mode in which nylon 6 feedstock, i.e., a fishing net containing nylon 6 and optionally a catalyst, is loaded into the depolymerization reactor, and the mode in which residual material is discharged from the depolymerization reactor.

[0085] In a preferred embodiment, the depolymerization of nylon 6 is carried out in a batch mode. In the batch mode, the raw material, namely a fishing net containing nylon 6, and optionally a catalyst, are first loaded into the depolymerization reactor. Subsequently, superheated steam is loaded into the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Next, the loading of superheated steam into the depolymerization reactor is interrupted. After optionally removing residual material from the depolymerization reactor, a new cycle is started by loading the raw material (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, residual material is not removed between cycles.

[0086] In a particular advantageous embodiment, the depolymerization of nylon 6 is carried out in a continuous mode. In the continuous mode, a nylon 6-containing feedstock (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 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 fishing net containing nylon 6 is charged in molten form. Preferably, the catalyst is charged in molten, slurry, or solution form.

[0087] In another preferred embodiment, the depolymerization of nylon 6 is carried out in a semi-continuous mode. In the semi-continuous mode, a nylon 6-containing feedstock (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. Residual material is intermittently discharged from the depolymerization reactor in the semi-continuous mode of nylon 6 depolymerization.

[0088] Recycling step c) In the recovery section [C], ε-caprolactam is recovered from the stream containing ε-caprolactam discharged from the 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.

[0089] Preferably, without loading the solvent into the depolymerization zone [B], the ε-caprolactam obtained by condensation is dissolved in water, thereby obtaining an ε-caprolactam-rich phase. This ε-caprolactam-rich phase also contains impurities.

[0090] Preferably, when water is used as a solvent in the depolymerization zone [B], the stream containing ε-caprolactam discharged from the depolymerization zone [B] contains ε-caprolactam, water, and impurities. The water may be introduced in liquid or vapor form. Preferably, the water is introduced in vapor form. The ε-caprolactam can be separated from the stream containing ε-caprolactam discharged from the depolymerization zone [B] by feeding this vapor or stream from the depolymerization reactor, preferably from the top, to a condenser (preferably partially), to obtain a condensate containing ε-caprolactam. Preferably, the ε-caprolactam is separated from the remaining components of the vapor stream by feeding the product stream from the 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.

[0091] The ε-caprolactam recovered in recovery section [C] is crude because it contains impurities such as nylon 6 decomposition products or other impurities derived from non-nylon 6 components (decomposition products) of fishing nets containing nylon 6. 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 than the purified ε-caprolactam obtained as a product of the method of the present invention, i.e., containing more impurities.

[0092] 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.

[0093] Purification step d) 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.

[0094] 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.

[0095] 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.

[0096] 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 to extract 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. 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 15:1, more preferably from 0.1:1 to 7:1, and most preferably from 0.1:1 to 5:1.

[0097] 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.

[0098] 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 of the crude ε-caprolactam. Therefore, due to this extraction, the ε-caprolactam is purer than before extraction.

[0099] In another embodiment of the invention, wherein the organic solvent has a higher density than the crude ε-caprolactam, the extraction using the organic solvent in step d)(i) 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.

[0100] 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)(iv). 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.

[0101] 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 that removes 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.

[0102] Optionally, in step d)(ii) of the method according to the invention, the solvent of the obtained organic phase, which is optionally washed with water or an alkaline aqueous solution and contains organic solvent, ε-caprolactam and impurities, is replaced with water, thereby obtaining an aqueous phase containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam, wherein the solvent replacement 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.

[0103] As used herein, the term "displacement" means the replacement of 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 an organic solvent present in an organic phase comprising an organic solvent, ε-caprolactam, and impurities, with water.

[0104] Solvent conversion can be based on water back-extraction, thereby obtaining an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Preferably, this 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 5 times the weight of the recovered ε-caprolactam.

[0105] Preferably, back-extraction using water can be carried out in a countercurrent extraction column.

[0106] 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 reused after purification (preferably by distillation).

[0107] 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 the impurities is optionally reused after purification (preferably by distillation).

[0108] Therefore, according to a particular advantageous embodiment of the invention, after the extraction of crude ε-caprolactam in step d)(i), the purification in step d) further includes step (ii)a) solvent conversion based on back-extraction with water.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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 by distillation from the organic phase containing the organic solvent, ε-caprolactam, and impurities in the upper part 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 upper part of the distillation column. More preferably, a portion of the organic solvent removed by distillation in the upper part of the distillation column is condensed and charged into the upper part 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.

[0113] 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 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.

[0114] 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.

[0115] 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.

[0116] Solvent-change distillation (performed as a single-stage or two-stage process) produces 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 intact aqueous phase, more preferably between 50% by weight and 99.5% by weight, and most preferably between 85% by weight and 99% by weight.

[0117] Therefore, according to a particular advantageous embodiment of the invention, after the extraction of crude ε-caprolactam in step d)(i), the purification in step d) further includes step (ii)b): solvent conversion based on solvent exchange distillation.

[0118] Optionally, in step d)(iii) of the method according to the invention, prior to crystallization in step d)(iv), impurities with boiling points below or above ε-caprolactam are removed by distillation under vacuum conditions, thereby obtaining a phase containing ε-caprolactam and the impurities. The obtained phase containing ε-caprolactam and the impurities is purer than the phase loaded in step d)(iii), i.e., contains fewer impurities.

[0119] In step d)(iii) of the method of the present invention, distillation is performed on an organic phase containing organic solvent, ε-caprolactam, and impurities obtained by extraction in step d)(i) and optionally washed with water or an alkaline aqueous solution, to remove the organic solvent and impurities with boiling points lower or higher than ε-caprolactam from the organic phase. Preferably, distillation is carried out under reduced pressure. More preferably, distillation is carried out at a pressure of less than 80 kPa, more preferably less than 20 kPa, and most preferably less than 10 kPa. Preferably, the temperature is between 90°C and 210°C, and more preferably between 110°C and 180°C. These temperatures refer to the temperature at the bottom of the distillation column in which distillation is performed.

[0120] Alternatively, 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.

[0121] Preferably, water is first evaporated from an aqueous phase containing 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 of less than 50 kPa, more preferably less than 20 kPa, and most preferably less than 10 kPa. 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.

[0122] 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.

[0123] Therefore, according to a particular advantageous embodiment of the invention, prior to crystallization in step d)(iv), the purification in step d) further comprises step (iii): removing impurities with boiling points below or above ε-caprolactam by distillation under vacuum conditions.

[0124] 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.

[0125] 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.

[0126] 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 using conventional experiments. The precise amount of oxidant depends particularly heavily on the composition of the waste fishing net containing nylon 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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 the effective removal of 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.

[0131] In a preferred embodiment of the invention, prior to crystallization in step d)(iv), an aqueous solution containing water, ε-caprolactam, and impurities is hydrogenated in the presence of a hydrogenation catalyst. The hydrogenation catalyst can be any known heterogeneous hydrogenation catalyst. Examples of such catalysts are ruthenium / alumina, rhodium / alumina, platinum / carbon, palladium / carbon, Raneynickel, nickel / silicon, and nickel / alumina. Preferably, a nickel-containing catalyst is used. Suitable nickel catalysts typically have a nickel content between 5 wt.% and 80 wt.% relative to the metal and support. In addition to nickel, the catalyst may contain some activator, such as Zr, Mn, Cu, or Cr. The activator content is generally between 1 wt.% and 20 wt.%. If a palladium-containing heterogeneous catalyst is used, the palladium content is generally between 0.01 wt.% and 10 wt.%.

[0132] Heterogeneous catalysts can be contacted with hydrogen-containing reaction mixtures in various ways. Hydrogenation can occur, for example, in a stirred tank reactor, where catalyst particles are suspended in the mixture to be purified (slurry-liquid phase process). In another embodiment, hydrogenation is carried out in a fixed-bed reactor, where the catalyst is fixed in the reactor.

[0133] Hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising an aqueous ε-caprolactam mixture, gaseous hydrogen, and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid) comprising a fully or partially hydrogen-saturated aqueous ε-caprolactam mixture and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in the water-ε-caprolactam mixture can be achieved by any method known to those skilled in the art to obtain a fully or partially hydrogen-saturated mixture.

[0134] Hydrogenation temperatures are typically between 20 and 160°C. Hydrogenation pressures are typically between 0.1 and 15 MPa.

[0135] Hydrogenation of the water-ε-caprolactam mixture is performed to hydrogenate unsaturated compounds present in impure ε-caprolactam. The presence of these unsaturated compounds is disadvantageous because they can weaken the physical and mechanical properties of nylon 6 prepared by polymerizing ε-caprolactam. Saturated compounds formed by hydrogenation do not adversely affect these physical and mechanical properties of nylon 6, and these compounds are more readily removed after the hydrogenation step in, for example, distillation and / or crystallization steps.

[0136] In step d)(iv) of the method of the present invention, purified ε-caprolactam is obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10 to 95°C, more preferably at a temperature of 20 to 85°C.

[0137] The purified ε-caprolactam solution obtained by crystallization, comprising ε-caprolactam and impurities, is an organic phase comprising an organic solvent, ε-caprolactam, and impurities, optionally washed with water or an alkaline aqueous solution, obtained by extraction in step d)(i). Preferably, the purified ε-caprolactam solution obtained by crystallization, comprising ε-caprolactam and impurities, is an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, obtained by solvent conversion in step d)(ii). More preferably, the purified ε-caprolactam solution obtained by crystallization, comprising ε-caprolactam and impurities, is a phase comprising ε-caprolactam and impurities, obtained by distillation under vacuum conditions in step d)(iii).

[0138] Preferably, the ε-caprolactam crystallization process in step d)(iv) includes the following steps: 1. Feed the solution containing ε-caprolactam and impurities into the crystallizer; 2. Set conditions in the crystallizer to form ε-caprolactam crystals and mother liquor; 3. Separate ε-caprolactam crystals from the mother liquor; 4. Recycle the mother liquor.

[0139] More preferably, the ε-caprolactam crystallization process in step d)(iv) includes the following steps: 1. Feed the solution containing ε-caprolactam and impurities into the crystallizer; 2. Set conditions in the crystallizer to form ε-caprolactam crystals and mother liquor; 3. The feed from the crystallizer is fed to the separator, where the ε-caprolactam crystals are separated from the mother liquor; 4. Recycle the mother liquor.

[0140] Crystallization can be applied to the production of ε-caprolactam. It is primarily used for its purification potential and / or product recovery to increase yield. All crystallization processes are based on the formation of a solid crystalline phase from a liquid. In a preferred embodiment, the crystallization in step d)(iv) is carried out by solution crystallization or melt crystallization.

[0141] The term solution crystallization is used for the crystallization of a compound from a solution containing the (impure) compound and an auxiliary solvent. The auxiliary solvent is water or a non-aqueous solvent. If the auxiliary solvent is water, the amount of water in the solution can be selected within a wide range, preferably from 0.5 to 25 wt.%, more preferably from 1 to 8 wt.%. Preferably, the crystallization temperature is in the range of 20 to 70°C, more preferably from 30 to 65°C. The purified ε-caprolactam is crystallized from a slurry with a concentration preferably in the range of 5 to 75 wt.%, more preferably from 10 to 70 wt.%, and most preferably from 15 to 50 wt.%. If the auxiliary solvent is a non-aqueous solvent, the amount of non-aqueous solvent in the solution can be selected within a wide range, preferably from 5 to 95 wt.%, more preferably from 10 to 90 wt.%, and most preferably from 30 to 70 wt.%. Preferably, the crystallization temperature is in the range of 20 to 70°C, more preferably from 30 to 65°C. Purified ε-caprolactam is recovered and crystallized from a slurry concentration preferably in the range of 5 to 75 wt.%, more preferably in the range of 10 to 70 wt.%, and most preferably in the range of 15 to 50 wt.%. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform, or chloroethane), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate), as well as mixtures of these solvents. Cyclohexane is preferred.

[0142] Solution crystallization is typically carried out at atmospheric pressure, but can be performed under reduced or increased pressure. In the case of solution crystallization, the product is recovered by evaporative crystallization, in which the solvent is evaporated, or by cooling crystallization, in which cooling is achieved through direct cooling, indirect cooling, or vacuum cooling, or a combination of these methods. After the crystallization step, the formed crystals and mother liquor are separated by, for example, deposition, filtration, and / or centrifugation. Optionally, the obtained crystals are washed with, for example, a cleaning solvent. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in crystalline form.

[0143] The term melt crystallization is narrowly defined as the crystallization of a compound from a solution containing the (impure) compound without the use of an auxiliary solvent. A broader definition of melt crystallization also applies to crystallization from solutions containing low solvent concentrations. Preferably, the solvent concentration in the solution is less than 25 wt%, more preferably less than 10 wt.%, and most preferably less than 5 wt.%. Here, unless explicitly stated otherwise, the broader definition of melt crystallization will be used. The compound crystals obtained by melt crystallization are separated from the mother liquor and optionally washed with a melt of pure compound material. Optionally, the crystallization-separation sequence is repeated several times. Finally, the washed crystals are optionally melted and discharged as a melt or removed mechanically.

[0144] Preferably, the solvent is present in the mixture in the crystallizer, but crystallization can also be carried out without a solvent. Many solvents used for ε-caprolactam are suitable. Examples of suitable solvents include water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform, or chloroethane), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate). Preferably, water and aromatic hydrocarbons are used as solvents because these solvents produce large crystals. Most preferably, water is used as the solvent. The solvent will act as a depressant for the freezing point of the melt in the crystallizer.

[0145] Generally speaking, melt crystallization requires less energy than solution crystallization; however, industrial-scale operation can be more challenging.

[0146] As used herein, melt crystallization specifically refers to layer melt crystallization or suspension melt crystallization. The two types of techniques used for melt crystallization are characterized by (1) forming a crystal layer on the heat exchanger wall (layer melt crystallization) and (2) growing crystals in a suspension (suspension melt crystallization). Generally, the operation of the method for growing a crystal layer on the heat exchanger wall is called layer melt crystallization. First, a melt is loaded into a crystallizer, then a crystal layer is grown on the cooled heat exchanger surface, followed by the removal of the remaining melt containing impurities repelled by the grown crystals from the crystallizer, after which the crystal layer is melted and the purified product is recovered. Purification efficiency can be further improved, for example, by melting, also known as partial melting, i.e., slightly heating the crystal layer to near its melting temperature, thereby removing trapped and adhered impurities from the mother liquor. Layer melt crystallization methods are operated in batch mode. Well-known examples of layer melt crystallization methods are BEFS Prokem's ProABD process and Sulzer Chemtech's process.

[0147] Layer-by-layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization mode, crystals are grown from the stagnant melt onto a cooled surface. In static mode, the desired compound is batch-crystallized from the stagnant melt in a self-sealing container on the heat exchanger wall. This type of crystallization is characterized by a low crystal growth rate and therefore a long residence (or batch) time. Preferably, the crystallization time is in the range of 1 hour to 75 hours, more preferably 2 hours to 50 hours, and most preferably 4 hours to 24 hours. After the crystallization step, the remaining melt is drained. Subsequently, optionally, a molten phase is introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or removed mechanically.

[0148] Generally, dynamic crystallization takes place in a shell-and-tube heat exchanger, where the molten material circulates downward toward a cooling surface where the compound crystallizes. Typically, the molten material is pumped through the tubes, and crystals grow inside the tubes while a cooling medium flows through the shell. The thickness of the crystal layer increases in real time. After a certain period, circulation of the molten material is stopped and the remaining molten material is discharged. Dynamic layer crystallization is similar to stagnant layer crystallization, which is also carried out in batch mode. Compared to stagnant mode, the crystal growth rate is higher in dynamic mode, and therefore the crystallization time is shorter. Preferably, the crystallization time is in the range of 0.05 hours to 12 hours, more preferably 0.1 hours to 6 hours, and most preferably 0.3 hours to 3 hours. Subsequently, a molten phase is optionally introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and discharged or mechanically removed.

[0149] Suspension melt crystallization can be performed in batch or continuous mode. In suspension melt crystallization, the melt is cooled below its saturation temperature, and crystal growth begins (optionally, after the addition of crystal nuclei). The crystal growth rate is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be performed in any exchanger-type or container-type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is performed in a scraper-type crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of crystals of the desired compound and mother liquor is loaded into a so-called washing column. In the washing column, mother liquor is discharged from the crystals, and the crystals are subsequently washed, optionally, with purified compound material.

[0150] Following the ε-caprolactam crystallization step, a mother liquor containing ε-caprolactam, excluding impurities, is obtained. Methods for recovering ε-caprolactam from such mother liquor are well known to those skilled in the art. Moreover, due to these recovery methods, almost all of the ε-caprolactam present in the mother liquor can be recovered and converted into high-purity ε-caprolactam. In the case of multi-stage crystallization, a possible solution is to recycle the mother liquor in a countercurrent manner, i.e., to feed the mother liquor obtained in the nth crystallization stage into the feed of the (n-1)th crystallization stage. Generally, the mother liquor obtained from the first crystallization stage is fed into an upstream (purification) unit of the process or a dedicated mother liquor processing unit (e.g., based on distillation or crystallization). After ε-caprolactam crystallization, it may be necessary to purify the obtained mother liquor (or a portion thereof) by, for example, recycling it to an aqueous solution of crude ε-caprolactam extracted with an organic solvent. Alternatively, the mother liquor may be purified, for example, by distillation, before being fed into the ε-caprolactam crystallization step.

[0151] The high-purity ε-caprolactam obtained by the method according to the present invention can be used to manufacture nylon 6 using methods well known to those skilled in the art. This nylon 6 can then be used in all known materials, including engineered materials, fibers, and membranes. This nylon 6, produced from fishing nets containing nylon 6, is particularly suitable for high-speed spinning applications, including garments containing spandex (also known as elastane).

[0152] factory 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.

[0153] 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).

[0154] The plant of the present invention is suitable for producing purified ε-caprolactam from materials derived from fishing nets containing nylon 6 and comprising 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.

[0155] Additionally, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [β] for fragmenting the material containing the fishing net containing nylon 6 into fragments and / or a cleaning section [α] for cleaning the material containing the fishing net containing nylon 6 and / or a densification section [γ] for obtaining a material with increased bulk density. Cleaning includes washing and separating foreign material from the fishing net containing nylon 6. Separation of foreign material can be done manually (hand-picking) and mechanically (e.g., density separation and magnetic separation). Manual and mechanical devices, such as brushes, can 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 the fishing net containing nylon 6 into fragments. Non-limiting examples of this fragmentation equipment are cutters, shredders, grinders, mills, and cutting machines. The densification section [γ] comprises densifying a material containing a fishing net made of nylon 6, which may optionally be fragmented and / or cleaned. Densification of a material with a high bulk density can be achieved using several techniques known to those skilled in the art. Well-known 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.

[0156] In a preferred embodiment of the invention, the plant further includes a pretreatment section [A], which includes an extraction section [ω], an optional mechanical size reduction section [β], an optional cleaning section [α], and an optional densification section [γ].

[0157] In a more preferred embodiment of the invention, the plant further includes a pretreatment section [A], which includes a mechanical size reduction section [β] and a cleaning section [α], and optionally a densification section [γ].

[0158] The depolymerization section [B] comprises one or more depolymerization reactors operating in series and / or parallel. A fishing net containing nylon 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 components known in the art.

[0159] During production, the depolymerization reactor is at least partially filled with a feedstock containing nylon 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.

[0160] The depolymerization reactor must be equipped with a feeding device for a feed containing nylon 6, as well as optional superheated steam and catalyst. Additionally, the depolymerization reactor must be equipped with a device for discharging the stream containing ε-caprolactam and residual material.

[0161] 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.

[0162] Preferably, the depolymerization will be completed within 0.5 to 6 hours.

[0163] 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.

[0164] The recovery section [C] may include one or more (preferably partial) condensers, wherein a stream containing ε-caprolactam is introduced in the form of a vapor stream containing ε-caprolactam and water. This (partial) condenser may have any suitable 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.

[0165] The purification section [D] may include one or more extraction devices, one or more solvent conversion devices, an oxidation section, a hydrogenation section, one or more distillation devices, and a crystallization section, into which crude ε-caprolactam is introduced and high-purity ε-caprolactam is discharged.

[0166] The extraction apparatus is loaded with crude ε-caprolactam and an organic solvent, and discharged as an organic phase containing the organic solvent, ε-caprolactam, and impurities, and an aqueous phase containing water and impurities. The extraction apparatus is selected from mixed sedimentation extractors, extraction columns, centrifugal extractors, and combinations thereof. Preferably, the extraction apparatus is a static or stirred extraction column, such as a KARR. tubular column, SCHEIBEL Rotary disc tower (RDC), pulse tube, sieve plate (static) tube, random packing (static) tube, and structured packing (SMVP) (static) tube.

[0167] The solvent conversion device is filled with water and an organic phase containing organic solvent, ε-caprolactam, and impurities, and discharged with the organic solvent and an aqueous phase containing ε-caprolactam and impurities. The solvent conversion device used for 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 a KARR. tubular column, SCHEIBEL Rotary disc tower (RDC), pulse tube, sieve plate (static) tube, random packing (static) tube, and structured packing (static) tube.

[0168] 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.

[0169] 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.

[0170] 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, and a suspension of 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 are retained in the filter system. Preferably, this filter system is operated 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 suspension is intermittently loaded, and the collected solids are removed from the filter system.

[0171] The purification of crude ε-caprolactam in step d) to obtain purified ε-caprolactam may include hydrogenation with a heterogeneous catalyst, in which case the plant will include a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0172] The hydrogenation section comprises one or more hydrogenation reactors operating in series and / or parallel. Hydrogenation can be carried out in a three-phase system (gas, liquid, solid) containing an aqueous ε-caprolactam mixture, gaseous hydrogen, and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid) containing a fully or partially hydrogen-saturated aqueous ε-caprolactam mixture and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in the water-ε-caprolactam mixture can be achieved by any method known to those skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or mixer maintaining a constant hydrogen pressure. Sufficient contact between the hydrogen and the mixture ensures that the hydrogen dissolves in the mixture. This method is preferably carried out continuously. The hydrogen-containing mixture is then contacted with the hydrogenation catalyst, for example, in a separate reactor.

[0173] Heterogeneous catalysts can be contacted with hydrogen-containing reaction mixtures in various ways. Hydrogenation can occur, for example, in a stirred tank reactor, where catalyst particles are suspended in the mixture to be hydrogenated (a slurry-phase process). In this slurry-phase process, the catalyst particles must be separated from the purified mixture in an additional process step, such as by filtration, after the hydrogenation reaction. Preferably, the catalyst comprises palladium or nickel.

[0174] Alternatively, hydrogenation can be carried out in a fixed-bed reactor, where the catalyst is fixed within the reactor, eliminating the need for an additional step of separating the catalyst from the reaction mixture. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0175] Hydrogenation temperatures are typically between 20 and 160°C. Hydrogenation pressures are typically between 0.1 and 15 MPa.

[0176] 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, condenser, and reflux device. The distillation apparatus can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure, preferably at sub-atmospheric pressure.

[0177] 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.

[0178] 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.

[0179] The crystallization section contains one or more crystallizers operating in series and / or parallel. Generally, the crystallization section also includes containers for storing (intermediate) product streams and / or fresh and used wash liquids. ε-caprolactam can be crystallized by solution crystallization or melt crystallization, as previously described.

[0180] In the case of solution crystallization, ε-caprolactam is recovered by evaporative crystallization in which the solvent is evaporated, or by cooling crystallization in which cooling is achieved by direct cooling, indirect cooling, or vacuum cooling, or by a combination of these methods. Following the crystallization step in the crystallizer, the formed crystals are separated from the mother liquor by, for example, sedimentation in a settling tank, filtration in a filter, and / or centrifugation in a centrifuge. Optionally, the crystallizer is equipped with a stirrer and / or one or more baffles.

[0181] Optionally, the obtained crystals are washed with, for example, a cleaning solvent. Optionally, the crystallization-separation sequence is repeated several times. A product in crystalline form is obtained.

[0182] The auxiliary solvent is water or a non-aqueous solvent. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform, or chloroethane), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate), as well as mixtures of these solvents. Generally, the auxiliary solvent is recovered and reused during the crystallization process.

[0183] Solution crystallization usually takes place at atmospheric pressure, but it can be carried out under reduced or increased pressure.

[0184] ε-caprolactam can be melt-crystallized by layer-by-layer melting crystallization where a crystal layer containing ε-caprolactam is formed on the heat exchanger wall, or by suspension-by-suspension melting crystallization where crystals containing ε-caprolactam are grown in a suspension.

[0185] Preferably, the solvent is present in the mixture in the molten crystallizer, but melt crystallization can also be carried out without a solvent. Many solvents used for ε-caprolactam are suitable. Examples of suitable solvents include water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform, or chloroethane), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate). Preferably, water and aromatic hydrocarbons are used as solvents because these solvents produce large crystals. Most preferably, water is used as the solvent. The solvent will act as a freezing point depressant for the melt in the crystallizer.

[0186] Layer melting and crystallization: First, the molten material is loaded into a crystallizer, and then a crystal layer is grown on the surface of a cooled heat exchanger. The remaining molten material, containing impurities repelled by the self-grown crystals, is then discharged from the crystallizer. The crystal layer is then melted, and the purified product is recovered. Purification efficiency can be further improved, for example, by partial melting, which involves slightly heating the crystal layer to near its melting temperature, thereby removing trapped and adhered impurities from the mother liquor. Layer-melt crystallization methods are operated in batch mode. Well-known examples of layer-melt crystallization methods include BEFS Prokem's ProABD process and Sulzer Chemtech's process.

[0187] Layer-by-layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization mode, crystals are grown from the stagnant melt onto a cooled surface. In static mode, the desired compound is batch-crystallized from the stagnant melt in a self-sealing container on the heat exchanger wall. This type of crystallization is characterized by a low crystal growth rate and therefore a long residence (or batch) time. Preferably, the crystallization time is in the range of 1 hour to 75 hours, more preferably 2 hours to 50 hours, and most preferably 4 hours to 24 hours. After the crystallization step, the remaining melt is drained. Subsequently, optionally, a molten phase is introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or removed mechanically.

[0188] Suspension melt crystallization: Suspension melt crystallization of ε-caprolactam can be carried out in batch or continuous mode. In suspension melt crystallization, the melt is cooled below its saturation temperature, and ε-caprolactam crystals begin to grow (optionally, after the addition of crystal nuclei). The crystal growth rate is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be carried out in any exchanger-type or container-type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is carried out in a scraper-type crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of ε-caprolactam crystals and mother liquor is loaded into a so-called washing column. In the washing column, the mother liquor is discharged from the ε-caprolactam crystals, and subsequently, optionally, the ε-caprolactam crystals are washed with purified ε-caprolactam.

[0189] 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. In particular, the pretreatment section [A] may be located at a first location, while the depolymerization section [B], recovery section [C], and purification section [D] are located at a second location. Similarly, the mechanical size reduction section [β], which is part of the pretreatment section [A], may also be located at the first location, while the cleaning section [α], which is part of the pretreatment section [A], may be located at the second location, while the depolymerization section [B], recovery section [C], and purification section [D] are located at a third location. Optionally, the cleaning section [α] may be split into two or more segments, all optionally located at 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. Optionally, the depolymerization segment [B] may be located at a position different from that of the pretreatment segment [A] and / or the recovery segment [C] and the purification segment [D].

[0190] product This invention provides a new product, ε-caprolactam, which is obtained by the method according to the invention via the depolymerization of nylon 6 produced from a fishing net (material) containing nylon 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 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 from a fishing net containing nylon 6 by the method according to the invention, thereby obtaining the ε-caprolactam in purified form. In this sense, ε-caprolactam is obtained and purified from a fishing net containing nylon 6.

[0191] The method of this invention allows for the production of high-purity and therefore high-quality ε-caprolactam, meeting specifications for demanding applications. Furthermore, the 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: PAN: Maximum 5 E290: Maximum 0.05 VB: Maximum 0.5 mmol / kg Alkalinity: Maximum 0.1 mmol / kg.

[0192] The ε-caprolactam produced by the method of this invention is also 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.

[0193] The environmental impact of a product is generally described as its 'product 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 depends particularly on its raw materials, auxiliary materials, energy consumption, energy source, 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).

[0194] Product carbon footprint calculations can be performed by internal or external (preferably) certified organizations. These organizations verify and certify product carbon footprint calculations based on, for example, the LCA standard ISO 14040.

[0195] 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 production, 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 tonne 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 production is involved, the potential global warming impact of primary ε-caprolactam in ε-caprolactam production methods decreases to 6.4 tons of CO2 equivalent per tonne of ε-caprolactam (which is equal to 6.4 kg of CO2 equivalent per kilogram of ε-caprolactam).

[0196] 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 from ecoinvent version 3.7.1; location: Europe).

[0197] To further illustrate certain aspects of the invention, the invention also provides the following non-limiting embodiments: Implementation Method 1: A method for recovering purified ε-caprolactam in a factory from materials selected from the group consisting of materials derived from fishing nets containing nylon 6, wherein the factory comprises: De-aggregation section [B], Recycling section [C], and Purification section [D], And the method described therein includes the following steps: a) The material derived from the fishing net containing nylon 6 is loaded into the depolymerization section [B]; b) In the depolymerization zone [B], the material derived from the fishing net material containing nylon 6 is depolymerized at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C, in order 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; and (iv) Purified ε-caprolactam was obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10 to 95 °C.

[0198] Implementation Method 2: The method of Implementation Method 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.

[0199] Implementation Method 3, as in Implementation Method 1 or 2, wherein prior to the crystallization in step d)(iv), the purification in step d) further comprises the following steps: (iv) Remove impurities with boiling points below or above ε-caprolactam by distillation under vacuum conditions.

[0200] Implementation 4, the method of Implementation 2 or 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, preferably 275°C to 500°C [B].

[0201] Embodiment 5: The method of any one of Embodiments 1 to 4, wherein after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises the following steps: (ii)a) Solvent conversion based on water back-extraction.

[0202] Embodiment 6: The method of any one of Embodiments 1 to 4, wherein after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises the following steps: (ii)b) Solvent conversion based on solvent exchange distillation.

[0203] Embodiment 7: The method of any one of Embodiments 1 to 6, 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.

[0204] Embodiment 8: The method of any one of Embodiments 1 to 7, wherein step d) purifying crude ε-caprolactam to obtain purified ε-caprolactam comprises: The oxidation step involves oxidation in an aqueous solution with an oxidizing agent at a temperature ranging from 20°C to 85°C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, particularly potassium permanganate, and wherein the oxidation is carried out in an aqueous solution containing water and ε-caprolactam in a weight ratio of 5:1 to 1:5; and / or The hydrogenation step uses a heterogeneous catalyst, wherein the catalyst preferably comprises nickel or palladium.

[0205] Embodiment 9: The method of any one of Embodiments 1 to 8, wherein the depolymerization of the material derived from the fishing net containing nylon 6 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, wherein the acid catalyst is selected from the group consisting of: phosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, salts of the aforementioned acids, Al2O3 and SiO2 and combinations thereof, especially phosphoric acid; and the base catalyst is selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof, especially sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate, preferably carried out in the absence of a catalyst or in the presence of phosphoric acid.

[0206] Embodiment 10, the method of any one of Embodiments 1 to 9, wherein the solution containing ε-caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further contains water, preferably more than 1 by weight.

[0207] Implementation 11: The method of any one of Implementations 1 to 10, wherein prior to step a), the material containing the fishing net containing nylon 6 is pretreated in a pretreatment section [A], particularly by cleaning in a cleaning section [α] and / or by mechanical size reduction in a mechanical size reduction section [β] and / or a densification section [γ], to obtain a material derived from the fishing net containing nylon 6.

[0208] Embodiment 12: The method of any one of Embodiments 1 to 11, wherein after step d)(i), the organic phase obtained in step d)(i) is washed with water or with an alkaline aqueous solution.

[0209] Embodiment 13: A plant for producing purified ε-caprolactam from a material derived from a fishing net containing nylon 6, wherein the plant comprises: De-aggregation section [B], Reclaimed section [C], Purification section [D], and The plant is configured to implement the methods defined in any one of embodiments 1 to 12.

[0210] Implementation 14, a factory as described in Implementation 13, wherein the factory further includes a pre-processing section [A], which includes: The mechanical size reduction segment [β], and Clean section [α], and Optional densification segment [γ].

[0211] Embodiment 15: A purified ε-caprolactam, obtained by depolymerizing nylon 6 derived from a material containing nylon 6 fishing nets, according to the method defined in any one of Embodiments 1 to 12, wherein the ε-caprolactam has a product carbon footprint of less than 2 kg CO2 equivalent per kg of purified ε-caprolactam.

[0212] To further illustrate certain aspects of the present invention, the present invention also specifically provides the following non-limiting implementations: Implementation Scheme 1: A method for recovering purified ε-caprolactam in a factory from materials selected from the group consisting of materials derived from fishing nets containing nylon 6, wherein the factory comprises: De-aggregation section [B], Recycling section [C], and Purification section [D], And the method described therein includes the following steps: a) The material derived from the fishing net containing nylon 6 is loaded into the depolymerization section [B]; b) In the depolymerization zone [B], the material derived from the fishing net material containing nylon 6 is depolymerized at a temperature in the range of 180°C to 400°C in order to obtain a stream containing ε-caprolactam; c) Discharge the stream containing ε-caprolactam from the depolymerization section [B], and recover crude ε-caprolactam from the stream in the recovery section [C]; The crude ε-caprolactam recovered in step c) comprises 6 to 95% by weight of ε-caprolactam; 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; and (ii)a) Solvent conversion based on water back-extraction, or (ii)b) Solvent conversion based on solvent exchange distillation; (iii) Removal of impurities with boiling points below or above ε-caprolactam by distillation under vacuum conditions; and (iv) Purified ε-caprolactam was obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10 to 95 °C; After step d)(i), the organic phase obtained in step d)(i) is washed with water or with an alkaline aqueous solution.

[0213] Implementation Scheme 2, the method of Implementation Scheme 1, wherein the depolymerization in step b) is carried out at a temperature in the range of 200°C to 350°C.

[0214] Implementation scheme 3, the method of implementation scheme 1, wherein the depolymerization in step b) is carried out at a temperature in the range of 220°C to 340°C.

[0215] Implementation Scheme 4, the method of Implementation Scheme 1, wherein the depolymerization in step b) is carried out at a temperature in the range of 240°C to 325°C.

[0216] Implementation Scheme 5, the method of Implementation Scheme 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.

[0217] Implementation Scheme 6, the method of Implementation Scheme 5, 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].

[0218] Implementation Scheme 7, the method of Implementation Scheme 6, wherein the superheated steam has a temperature in the range of 275°C to 500°C.

[0219] Implementation Scheme 8: The method of any one of Implementation Schemes 1 to 7, 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.

[0220] Implementation Scheme 9: The method of any one of Implementation Schemes 1 to 7, wherein step d) purifying crude ε-caprolactam to obtain purified ε-caprolactam comprises: The oxidation step involves oxidation in an aqueous solution with an oxidizing agent at a temperature ranging from 20°C to 85°C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof, and wherein the oxidation is carried out in an aqueous solution containing water and ε-caprolactam in a weight ratio of 5:1 to 1:5; and / or The hydrogenation step uses a heterogeneous catalyst.

[0221] Implementation Scheme 10, the method of Implementation Scheme 9, wherein the oxidant is potassium permanganate; and / or the catalyst comprises nickel or palladium.

[0222] Implementation Scheme 11: The method of any one of Implementation Schemes 1 to 7, wherein the depolymerization of the material derived from the fishing net containing nylon 6 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, wherein the acid catalyst is 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 is selected from the group consisting of: organic bases and solid bases and combinations thereof.

[0223] Implementation Scheme 12, the method of Implementation Scheme 11, wherein the acid catalyst is an organic sulfonic acid.

[0224] Implementation Scheme 13, the method of Implementation Scheme 11, wherein the acid catalyst is orthophosphoric acid.

[0225] Implementation Scheme 14, the method of Implementation Scheme 11, wherein the base catalyst is selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides, and alkaline earth metal salts.

[0226] Implementation Scheme 15, the method of Implementation Scheme 11, wherein the base catalyst is selected from the group consisting of: sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

[0227] Implementation Scheme 16, the method of Implementation Scheme 11, wherein the depolymerization of the material derived from the fishing net containing nylon 6 in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

[0228] Implementation Scheme 17, the method of any one of Implementation Schemes 1 to 7, wherein the solution containing ε-caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further contains water.

[0229] Implementation Scheme 18, the method of Implementation Scheme 17, wherein the solution containing ε-caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further contains more than 1% by weight of water.

[0230] Implementation Scheme 19, the method of any one of Implementation Schemes 1 to 7, wherein prior to step a), the material containing the fishing net containing nylon 6 is pretreated in the pretreatment section [A] to obtain material derived from the fishing net containing nylon 6.

[0231] Implementation Scheme 20: The method of any one of Implementation Schemes 1 to 7, wherein prior to step a), the material containing the fishing net containing nylon 6 is cleaned in the cleaning section [α] and / or mechanically reduced in the mechanical size reduction section [β] and / or densification section [γ] to obtain material derived from the fishing net containing nylon 6. Attached Figure Description

[0232] In the following description, the invention is illustrated with reference to the accompanying drawings, which depict certain embodiments of the invention. 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 drawings for illustrative purposes.

[0233] 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].

[0234] Figures 2A-2B Two embodiments of the pretreatment section [A] are shown, wherein the fishing net containing nylon 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 nylon 6.

[0235] Figure 2A An embodiment of the pretreatment section [A] is described, wherein the fishing net containing nylon 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 nylon 6.

[0236] Figure 2BAn embodiment of the pretreatment section [A] is described, wherein the fishing net containing nylon 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 nylon 6.

[0237] Figures 3A-3B Two embodiments of the purification section [D] are shown, wherein crude ε-caprolactam is purified to obtain high-purity ε-caprolactam.

[0238] Figure 3A An embodiment of the purification section [D] of the method of the present invention is described, the method comprising an extraction section [γ], an optional washing section [δ], an optional back-extraction section [ε], an optional distillation section [θ], and a crystallization section [λ].

[0239] 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], an optional solvent exchange distillation section [μ], an optional distillation section [θ], and a crystallization section [λ].

[0240] Detailed Explanation of the Diagram The method of the present invention is in Figure 1 The diagram is shown schematically. This method is implemented in the following factory sections: Optionally, in the pretreatment section [A], the fishing net containing nylon 6 is cleaned by removing foreign materials and by washing with a washing solvent [2], thereby obtaining a contaminated washing solvent [3]. Then, the fishing net containing nylon 6 is fragmented by mechanical dimensional reduction. The cleaned and fragmented fishing net containing nylon 6 is discharged from the pretreatment section [A] [6]. Optionally, in the pretreatment section [A], the fishing net containing nylon 6 is further cleaned by removing foreign materials [1]. The removal of foreign materials may be performed before and / or after fragmenting the fishing net containing nylon 6. 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 1 (Not shown in the text).

[0241] In the depolymerization zone [B], a optionally cleaned and fragmented fishing net containing nylon 6 [6] is depolymerized into ε-caprolactam. The stream containing ε-caprolactam is discharged from the depolymerization zone [B] [7]. Additionally, residual material is discharged [8]. Optionally, superheated steam [9] and catalyst

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

[0242] Crude ε-caprolactam is recovered from the stream containing ε-caprolactam discharged from the depolymerization section [B] in the recovery section [C] [7]

[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] .

[0243] 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] .

[0244] Figure 2A The implementation of the pretreatment section [A'] (the area enclosed by the dashed line) is depicted, wherein the fishing net containing nylon 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 nylon 6 [4']. Subsequently, the cleaned fishing net containing nylon 6 [4'] is fragmented in the mechanical size reduction section [β'] to obtain clean and fragmented fragments of the fishing net containing nylon 6 [6']. The clean 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).

[0245] Figure 2B An embodiment of the pretreatment section [A''] (the area enclosed by dashed lines) is depicted, wherein the fishing net containing nylon 6 [1''] is first fragmented in the mechanical size reduction section [β''] to obtain fragmented pieces of the fishing net containing nylon 6 [5'']. Subsequently, in the cleaning section [α''], the fragmented pieces of the fishing net containing nylon 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 nylon 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).

[0246] Figure 3A An implementation depicting the purification section [D'''] (the area enclosed by dashed lines) includes the following sections: 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'''].

[0247] 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'''].

[0248] In an optional back-extraction section [ε'''], an organic phase [18'''] containing organic solvent, ε-caprolactam, and impurities, optionally washed, is back-extracted with water [19'''] to obtain an organic solvent phase [20'''] containing impurities and an aqueous phase [22'''] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. Both phases are discharged from the back-extraction [ε''']. Optionally, residual organic solvent is removed by stripping and / or distillation from 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).

[0249] Optionally, an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidizing agent [22'''], from which optional residual organic solvent has been removed by stripping and / or distillation, to obtain an oxidized aqueous phase containing water, ε-caprolactam, and impurities (ε-caprolactam). Figure 3A (Not shown in the text).

[0250] Optionally, the oxidized ε-caprolactam aqueous phase, containing water, ε-caprolactam, and impurities, is filtered to remove manganese oxide (IV) solid particles before being loaded into the next section. Figure 3A (Not shown in the text).

[0251] Optionally, an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is hydrogenated in the presence of a heterogeneous catalyst [22'''], from which optional residual organic solvents have been removed by stripping and / or distillation, to obtain a hydrogenated ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities, which is then loaded into the next section ( Figure 3A (Not shown in the text).

[0252] In an optional distillation section [θ'''], an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is distilled [22'''] to remove impurities with boiling points lower or higher than ε-caprolactam and optional organic solvents or water, thereby obtaining a distilled ε-caprolactam phase [24''']. All distillation products are discharged from the distillation section [θ''']. Optionally, prior to distillation in the distillation section [θ'''], an alkali metal hydroxide [23'''] is fed to the ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam [22'''].

[0253] In the crystallization zone [λ'''], the optionally distilled ε-caprolactam phase [24'''] is crystallized to remove impurities from the ε-caprolactam [25'''] and thereby obtain high-purity ε-caprolactam [26''']. All crystallized products are discharged from the crystallization zone [λ''']. Optionally, the solvent is pre-loaded into the crystallization zone [λ'''] ( Figure 3A (Not shown in the text).

[0254] Figure 3B The following implementation describes the purification section [D''''] (the area enclosed by dashed lines) containing the following segments: 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''''].

[0255] 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''''].

[0256] In an optional solvent-exchange distillation section [μ''''], an organic phase [18''''] containing organic solvent, ε-caprolactam, and impurities is optionally washed 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''''].

[0257] Optionally, an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is oxidized with an oxidizing agent [22''''], from which optional residual organic solvent has been removed by stripping and / or distillation, to obtain an oxidized aqueous phase containing water, ε-caprolactam, and impurities (ε-caprolactam). Figure 3B (Not shown in the text).

[0258] Optionally, the oxidized ε-caprolactam aqueous phase, containing water, ε-caprolactam, and impurities, is filtered to remove manganese oxide (IV) solid particles before being loaded into the next section. Figure 3B (Not shown in the text).

[0259] Optionally, an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is hydrogenated in the presence of a heterogeneous catalyst [22''''], from which optional residual organic solvents have been removed by stripping and / or distillation, to obtain a hydrogenated ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities, which is then loaded into the next section ( Figure 3B (Not shown in the text).

[0260] In an optional distillation section [θ''''], an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is distilled [22''''] to remove impurities with boiling points lower or higher than ε-caprolactam and optional organic solvents or water, thereby obtaining a distilled ε-caprolactam phase [24'''']. All distillation products are discharged from the distillation section [θ'''']. Optionally, prior to distillation in the distillation section [θ''''], an alkali metal hydroxide [23''''] is fed to the ε-caprolactam aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam [22''''].

[0261] In the crystallization zone [λ''''], the optionally distilled ε-caprolactam phase [24'''] is crystallized to remove impurities from the ε-caprolactam [25''''] and thereby obtain high-purity ε-caprolactam [26'''']. The crystallized product is discharged from the crystallization zone [λ'''']. Optionally, the solvent is pre-loaded into the crystallization zone [λ''''] ( Figure 3B (Not shown in the text).

[0262] Example The following embodiments are provided to illustrate the invention in more detail, particularly with respect to certain forms of the invention. However, the embodiments are not intended to limit this disclosure.

[0263] ε-caprolactam, which can be used in all major nylon 6 polymerization applications without dilution with higher purity ε-caprolactam, meets all of the following specifications: PAN: Maximum 5 E290: Maximum 0.05 VB: Maximum 0.5 mmol / kg Alkalinity: Maximum 0.1 mmol / kg Acidity: Maximum 0.1 mmol / kg These parameters and measurement methods are defined as follows: PAN: ISO DIS 8660 - Plastics - Determination of permanganate index of caprolactam - Spectrometric method, a revision of the first edition of ISO 8660; 1988. E290: ISO 7059-caprolactam for industrial use-determination ofabsorbance at a wavelength of 290 nm, Volatile bases (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile bases content - Titrimetric method after distillation.

[0264] Basicity of ε-caprolactam products: determined by measuring the basicity at 25°C using a Tashiro indicator at a 1:2 ratio of 0.1 wt. / v. 乙醇 %Methylene Blue: 0.1 wt. / v 乙醇 Alkalinity is determined by titration with % methyl red (which is gray at the endpoint). First, titrate 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 0.01 N H₂SO₄ solution (if the solution is alkaline) or 0.01 N NaOH solution (if the solution is acidic).

[0265] The alkalinity is then given as follows: Alkalinity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) in: v = Volume of H2SO4 solution added (ml) t = equivalent concentration of H2SO4 solution (= 0.01 N) X = Sample weight (g) Y = ε-caprolactam concentration (wt.%) The acidity is then given as follows: Acidity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) in: v = Volume of NaOH solution added (ml) t = molar concentration of NaOH solution (=0.01 N) X = Sample weight (g) Y = ε-caprolactam concentration (wt.%) The nylon 6-containing pellets used in the examples and comparative experiments were made from discarded fishing nets. Pretreatment included removal of foreign material, cleaning, grinding, melting, and conversion into scrap / granules. The pellets were sourced from a fishing net recycling company in China.

[0266] 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.

[0267] The combination of pyrolysis gravimetric analysis (TGA) and qualitative information from differential scanning calorimetry (DSC) showed that the nylon 6 content of the pellets was > 98 wt.% (on a dry basis).

[0268] Example 1 Depolymerization of Nylon 6 and recovery of ε-caprolactam 48 g of nylon 6-containing fragments / granules and 14 g 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, thereby obtaining the ε-caprolactam and water condensate contained within.

[0269] The condensate, consisting of approximately 39.9 g of ε-caprolactam and predominantly water, was concentrated to an ε-caprolactam concentration of 50.6 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture of crude ε-caprolactam is the mixture to be purified.) The specifications for crude ε-caprolactam are as follows: PAN: 272 E290: 3.66 This example demonstrates that crude ε-caprolactam can be obtained by depolymerizing nylon 6 derived from discarded fishing nets containing nylon 6. Due to its extremely poor quality, this crude ε-caprolactam is unsuitable for use in all major nylon 6 polymerization applications.

[0270] Comparative Experiment 1 The process involved depolymerizing nylon 6, recovering ε-caprolactam, and purifying it by distillation. The procedure of Example 1 was followed. The condensate was concentrated to an ε-caprolactam concentration of 49.7 wt.% by evaporation. Then, 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam was added. Subsequently, water and impurities with boiling points lower than ε-caprolactam were removed as the top product by vacuum distillation in a batch-run distillation apparatus, wherein the pressure was gradually reduced. ε-caprolactam was distilled at 300 Pa, while impurities with boiling points higher than ε-caprolactam were retained as the bottom product in the distillation apparatus. The specifications of the distilled ε-caprolactam were as follows: PAN: 46 E290: 0.69 VB: 0.82 mmol / kg Acidity: 4.20 mmol / kg.

[0271] This comparative experiment shows that the ε-caprolactam obtained from the depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by distillation is of extremely poor quality because it does not meet any of the required specifications for the main polymerization applications.

[0272] Comparative Experiment 2 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying the nylon through oxidation. The procedure of Example 1 was followed. Crude ε-caprolactam was then treated at 50°C with 0.2 wt.% KMnO4 relative to ε-caprolactam for 2 hours. The solid formed was then removed from the oxidation product by filtration. The purified ε-caprolactam was as follows: PAN: 127 E290: 4.37 This comparative experiment shows that the ε-caprolactam obtained by depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by oxidation is of extremely poor quality and cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0273] Comparative Experiment 3 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying the nylon through oxidation and distillation. Subsequently, after adding 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam, the oxidatively purified ε-caprolactam aqueous solution obtained in Comparative Experiment 2 was distilled according to the procedure described in Comparative Experiment 1. The purified ε-caprolactam obtained was as follows: PAN: 9 E290: 0.37 VB: 0.72 mmol / kg Acidity: 1.24 mmol / kg.

[0274] This comparative experiment shows that ε-caprolactam obtained from the depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by oxidation and distillation is of poor quality and cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0275] Comparative Experiment 4 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying it through oxidation, carbon treatment, and distillation. 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 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 by filtration from the ε-caprolactam aqueous solution. 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 as follows: PAN: 6 E290: 0.40 VB: 0.72 mmol / kg Alkalinity: 0.30 mmol / kg.

[0276] This comparative experiment shows that ε-caprolactam obtained from the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by oxidation, carbonization and distillation is of poor quality and cannot be used as ε-caprolactam for all major polyamide 6 polymerization applications.

[0277] Comparative Experiment 5 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying it through extraction, back-extraction, and distillation.

[0278] Follow the procedure of Example 1.

[0279] The obtained aqueous solution of ε-caprolactam was concentrated to an ε-caprolactam concentration of 36.4 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of approximately 65°C). (This mixture of crude ε-caprolactam is the mixture to be purified.) 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 8.02 g of 4-methyl-2-pentanol / cyclohexane per gram of crude ε-caprolactam. The resulting 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 24 wt.%, and the weight ratio of 4-methyl-2-pentanol / cyclohexane in the solvent mixture was 50 wt.% : 50 wt.%. This mixture was then extracted five times in batches with water at approximately 25°C. The total amount of water used was 18 g of water per gram of recovered ε-caprolactam.

[0280] 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: PAN: 6 E290: 0.10 VB: 0.18 mmol / kg Alkalinity: 0.15 mmol / kg.

[0281] This comparative experiment shows that ε-caprolactam obtained from the depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by extraction, back-extraction and distillation is of poor quality and cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0282] Example 2 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying it through extraction, back-extraction, distillation, and crystallization.

[0283] The procedure of Example 1 was followed. A condensate consisting of approximately 40 g of ε-caprolactam, with the remainder being predominantly water, was concentrated to an ε-caprolactam concentration of 36.1 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of approximately 65°C). (This mixture of crude ε-caprolactam is the mixture to be purified.) 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 8.09 g of 4-methyl-2-pentanol / cyclohexane per gram of crude ε-caprolactam. The resulting 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 25 wt.%, and the weight ratio of 4-methyl-2-pentanol / cyclohexane in the solvent mixture was 50 wt.% : 50 wt.%. This mixture was then extracted seven times in batches with water at approximately 25°C. The total amount of water used was 5.65 g of water per gram of recovered ε-caprolactam.

[0284] 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.

[0285] Distilled water was then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt.%. This aqueous ε-caprolactam solution was introduced into a crystallization apparatus at 52°C. The aqueous ε-caprolactam solution was cooled to 40°C, and some seed crystals were added to the mixture. The mixture was then further cooled to 30°C and held at this temperature for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt.% aqueous ε-caprolactam solution. The purified ε-caprolactam obtained was of the following specifications: PAN: 2 E290: 0.03 VB: 0.04 mmol / kg Acidity: 0.05 mmol / kg.

[0286] Based on this embodiment, it can be concluded that purified ε-caprolactam, meeting all the required specifications for major polymerization applications, can be obtained from the depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by extraction, back-extraction, distillation, and crystallization.

[0287] Example 3 The process involves depolymerizing nylon 6, recovering ε-caprolactam, and purifying it through extraction, back-extraction, distillation, and crystallization.

[0288] Follow the procedure of Example 1 twice.

[0289] 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. Two-thirds of the crude ε-caprolactam from the resulting mixture was used for further purification.

[0290] Crude ε-caprolactam was diluted to 65.0 wt.% and extracted five times with benzene in batches at approximately 25°C. The total amount of extraction solvent used was 10.5 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 resulting 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 five times with water in batches at approximately 25°C. The total amount of water used was 1.57 g of water per gram of concentrated combined organic extracts. The resulting combined ε-caprolactam aqueous solution was concentrated to an ε-caprolactam concentration of 48.8 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature approximately 65°C).

[0291] Subsequently, 75 mmol of sodium hydroxide aqueous solution per kilogram of ε-caprolactam was added to the concentrated ε-caprolactam solution. Water and impurities with boiling points lower than ε-caprolactam were then removed as the top product by vacuum distillation in a batch-run distillation apparatus. Finally, the distilled ε-caprolactam was recovered as the top product at 300 Pa, while impurities with boiling points higher than ε-caprolactam were retained as the bottom product in the distillation apparatus.

[0292] Distilled ε-caprolactam was introduced into a crystallization apparatus at 52°C, and deionized water was added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt.%. The ε-caprolactam aqueous solution was cooled to 40°C, and some seed crystals were added to the mixture. The mixture was then further cooled to 30°C and held at this temperature for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt.% ε-caprolactam aqueous solution. The purified ε-caprolactam obtained was of the following specifications: PAN: 2 E290: 0.03 VB: 0.1 mmol / kg Alkalinity: 0.01 mmol / kg.

[0293] Based on this embodiment, 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, distillation, and crystallization.

[0294] Example 4 Calculation of the carbon footprint of purified ε-caprolactam A continuous method according to the present invention for producing purified ε-caprolactam from a self-contained polyamide 6 fishing net in a factory is simulated. The method includes: - Mechanical removal of foreign materials from fishing nets containing polyamide 6; - Cut the fishing net containing polyamide 6 into small pieces; - Wash the fragments of the fishing net containing polyamide 6 with water; - The washed fishing net containing polyamide 6 and the aqueous extract were separated by centrifugation; -Dry the washed fragments of fishing nets containing polyamide 6; - Melt and granulate the washed fragments of fishing net containing polyamide 6; -Depolymerize polyamide 6 under the influence of H3PO4 and superheated steam; - Crude ε-caprolactam (80 wt.% ε-caprolactam) was recovered by partial condensation of the steam discharged from the depolymerization reactor; - Crude ε-caprolactam was concentrated by countercurrent extraction with benzene; - Wash the organic extract with a diluted caustic alkali solution; - Back-extraction of washed organic extracts using water countercurrent; - Evaporation and concentration of aqueous extract; - Add caustic soda; -Removal of light and heavy substances through vacuum distillation; and - Pure ε-caprolactam was recovered by melt crystallization at a temperature of 61°C.

[0295] The carbon footprint of purified ε-caprolactam is calculated based on the consumption of raw materials, and the utility of the method described above is based on data from ecoinvent version 3.7.1.

[0296] The results showed that the carbon footprint of the purified ε-caprolactam product obtained from fishing nets containing polyamide 6 was less than 2 tonnes of CO2 equivalent per tonne of ε-caprolactam (location: Europe).

[0297] 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 in a plant from materials selected from the group consisting of materials derived from fishing nets containing nylon 6, wherein the plant comprises: De-aggregation section [B], Reclaimed section [C], and Purification section [D], And the method described therein includes the following steps: a) The material derived from the fishing net containing nylon 6 is loaded into the depolymerization section [B]; b) In the depolymerization zone [B], the material derived from the fishing net material containing nylon 6 is depolymerized at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C, in order 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; and (iv) Purified ε-caprolactam was obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10 to 95 °C.

2. 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.

3. The method of claim 1 or 2, wherein prior to the crystallization in step d)(iv), the purification in step d) further comprises the following steps: (iii) Remove impurities with boiling points lower or higher than ε-caprolactam by distillation under vacuum conditions.

4. The method of claim 2 or 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, preferably 275°C to 500°C [B].

5. The method of any one of claims 1 to 4, wherein after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises the following steps: (ii)a) Solvent conversion based on water back-extraction.

6. The method of any one of claims 1 to 4, wherein after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises the following steps: (ii)b) Solvent conversion based on solvent exchange distillation.

7. The method of any one of claims 1 to 6, 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.

8. The method of any one of claims 1 to 7, wherein step d) purifying crude ε-caprolactam to obtain purified ε-caprolactam comprises: The oxidation step involves oxidation in an aqueous solution with an oxidizing agent at a temperature ranging from 20°C to 85°C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, particularly potassium permanganate, and wherein the oxidation is carried out in an aqueous solution containing water and ε-caprolactam in a weight ratio of 5:1 to 1:5; and / or The hydrogenation step uses a heterogeneous catalyst, wherein the catalyst preferably comprises nickel or palladium.

9. The method of any one of claims 1 to 8, wherein the depolymerization of the material derived from the fishing net containing nylon 6 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, organic sulfonic acids, salts of the aforementioned acids, Al2O3 and SiO2 and combinations thereof, especially phosphoric acid; and the base catalyst being selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof, especially sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate, preferably carried out in the absence of a catalyst or in the presence of phosphoric acid.

10. The method of any one of claims 1 to 9, wherein the solution containing ε-caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further contains water, preferably more than 1 by weight.

11. The method of any one of claims 1 to 10, wherein prior to step a), the material comprising the fishing net containing nylon 6 is pretreated in a pretreatment section [A], particularly by cleaning in a cleaning section [α] and / or by mechanical dimensional reduction in a mechanical dimensional reduction section [β] and / or a densification section [γ], to obtain a material derived from the fishing net comprising nylon 6.

12. The method of any one of claims 1 to 11, wherein after step d)(i), the organic phase obtained in step d)(i) is washed with water or with an alkaline aqueous solution.

13. A plant for producing purified ε-caprolactam from a material derived from a fishing net comprising nylon 6, said plant comprising: De-aggregation section [B], Reclaimed section [C], Purification section [D], and The plant is configured to implement the methods as defined in any one of claims 1 to 12.

14. The plant of claim 13, wherein the plant further comprises a pretreatment section [A], which includes: The mechanical size reduction segment [β], and Clean section [α], and Optional densification segment [γ].

15. A purified ε-caprolactam, obtained by depolymerizing nylon 6 derived from a material containing nylon 6 fishing nets, according to the method defined in any one of claims 1 to 12, wherein the ε-caprolactam has a product carbon footprint of less than 2 kg CO2 equivalent per kg of purified ε-caprolactam.

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