Method for the recovery of epsilon-caprolactam from fishing nets containing polyamide 6

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a method for the recovery of ε-caprolactam from waste fishing nets comprising polyamide 6. More particularly, the present invention relates to a method for the recovery of ε-caprolactam from waste fishing nets comprising polyamide 6, which method results in high quality ε-caprolactam. [Background technology]

[0002] A fishing net is a net used for catching fish. A net is a device made from fibers woven in a grid-like structure. Fishing nets are usually mesh-like structures formed by knotting together relatively thin threads. Modern nets are usually made from artificial fibers (such as polyamide 6, polyamide 6,6, polyester, polypropylene, and polyethylene).

[0003] Fishing nets can be abandoned or lost in the seas and oceans by fishermen. These nets, known as ghost nets, cause major problems for fish and other animals. In general, fishing nets made from artificial fibers often have an extremely low (biodegradation) rate. Therefore, these nets remain in the marine ecosystem for many years, leading to the accumulation of huge amounts of ghost nets.

[0004] The latest estimates by FAO and the United Nations Environment Programme (UNEP) suggest that the amount of abandoned, lost and discarded fishing gear is around 640 million kilograms per year.

[0005] In recent years, many efforts have been launched to prevent ghost gear from entering the environment, such as collecting discarded fishing nets in ports and even removing them from the seabed with the help of volunteer divers.

[0006] The fates of recovered waste fishing nets, including those containing polyamide 6, range from landfilling, incineration (possibly with heat recovery), re-granulation and compounding, to depolymerization. Re-granulation and compounding is a recycling process in which waste plastic is melted (possibly followed by filtration to remove solid impurities) and then converted into extrudates or injected directly into molds. Depolymerization is a technique in which the polymer is converted into its monomer components (ε-caprolactam in the case of polyamide 6).

[0007] Mechanical recycling (also known as materials recycling or back-to-plastics recycling) refers to operations aimed at recovering plastics through mechanical processes (crushing, washing, separating, drying, re-granulating, and kneading) to produce recyclable materials that can be converted into plastic products and serve as a substitute for virgin plastics. Currently, most virgin plastics are produced from petrochemical feedstocks, such as natural gas, coal, or crude oil, that have never been used or processed before. In the mechanical recycling process, the polymer chains remain more or less intact. Mechanical recycling is a form of waste downcycling, as the recycled material is of inferior quality and functionality to the original material.

[0008] Depolymerization, or chemical recycling, is a technique in which polymers are converted into their monomeric components. The specifications of the recovered monomers determine whether they can replace virgin monomers for all applications or only limited amounts. Virgin monomers are produced from petrochemical feedstocks such as natural gas, coal, or crude oil that have not been previously used or processed.

[0009] In 1938, Paul Schlack invented polyamide 6 (CAS number: 25038-54-4), also known as nylon 6, poly(caprolactam), poly(hexano-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide) or poly[imino(1-oxohexane-1,6-diyl)].

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

[0011] Methods for producing virgin ε-caprolactam are described, for example, in the chapter "Caprolactam" in Ullmann's Encyclopedia of Industrial Chemistry (May 25, 2018), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, available electronically via https: / / doi.org / 10.1002 / 14356007.a05_031.pub3.

[0012] Methods for producing polyamide 6 are described, for example, in the chapter "Polyamides" in Ullmann's Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, available electronically via https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0013] The depolymerization of polyamide 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam: [ka]

[0014] Processes are known for the depolymerization of polyamide 6. Such processes can be operated in a batchwise operating mode, a semi-continuous mode (generally involving batchwise (re)charging of polyamide 6 into the depolymerization reactor), or a continuous mode.

[0015] L.A. Dmitrieva, A.S. Peranskii, S.A. Krasavin and Y.N. Bychkov, “Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibers and Yarns,” Fiber Chemistry, pp. 229-241, March 1986, (translated from Khimicheskie Volokna, No. 4, pp. 5-12, July-August, 1985) is a literature review describing methods for depolymerizing polyamide 6 with and without the use of catalysts.

[0016] AAOgale, “Depolymerization of Nylon 6: Some Kinetic Modeling Aspects,” Journal of Applied Polymer Science, vol. 29, 1984, pp. 3947-3954, available electronically via https: / / doi.org / 10.1002 / app.1984.070291227, is a paper describing the depolymerization kinetics of polyamide 6.

[0017] No. 5,929,234 describes a process for recovering ε-caprolactam from polycaprolactam-containing waste. Depolymerization is carried out using superheated steam in the absence of an added catalyst 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, which is substantially below the saturated vapor pressure of water at the temperature at which an ε-caprolactam-containing vapor stream is formed.

[0018] Depolymerization of fishing nets containing polyamide 6 has been carried out in the past. However, despite the long history of polyamide 6 recycling, the quality of the monomer ε-caprolactam obtained by these methods has been particularly low. As a result, ε-caprolactam obtained by depolymerization of fishing nets containing polyamide 6 is only applicable to less demanding applications such as engineering plastics and carpets (downcycling). If ε-caprolactam obtained from fishing net depolymerization is to be used in more demanding applications, it must be blended with larger amounts of higher and purer grades of ε-caprolactam to mask the rather inferior quality of the ε-caprolactam obtained from fishing net depolymerization. High-speed melt spinning of polyamide 6 to produce fine textile fibers requires high-quality ε-caprolactam as a raw material. High-quality ε-caprolactam grades for these applications should not only be extremely pure, but their properties should not change over time.

[0019] Taken together, prior art methods for recovering ε-caprolactam from fishing nets containing polyamide 6 are unable to produce high quality ε-caprolactam grades that can be used to replace virgin ε-caprolactam grades for demanding applications.

[0020] Currently, no method is available for recovering high-purity ε-caprolactam from fishing nets containing polyamide 6, despite the urgent need for such a method. In particular, there is an urgent need for a high-purity ε-caprolactam recovery method that can replace virgin ε-caprolactam grades for demanding applications such as high-speed melt spinning during textile fiber production.

[0021] Furthermore, there is a need for a method that allows for the recovery of high-purity ε-caprolactam from fishing nets containing polyamide 6 in an economically reasonable manner. The production cost of the recovered high-purity ε-caprolactam should be in the same price range as or lower than the production cost of high-purity virgin ε-caprolactam.

[0022] Furthermore, there is a need to provide high-purity grade ε-caprolactam from fishing nets containing polyamide 6 that has a significantly lower carbon footprint than ε-caprolactam produced by methods that use virgin ε-caprolactam obtained by de novo synthesis, for example, via the Beckmann rearrangement of cyclohexanone oxime. Additionally, there is a need for plants to produce high-purity grade ε-caprolactam from fishing net-derived materials containing polyamide 6.

[0023] Finally, in order to process the enormous amount of fishing nets containing polyamide 6 that are discarded every year, there is a need for a method that allows for the recovery of ε-caprolactam from fishing nets containing polyamide 6 on an industrial scale. Summary of the Invention

[0024] It is an object of the present invention to fulfill one or more of the above-mentioned needs and to overcome the drawbacks associated with prior art methods.

[0025] In particular, it is an object of the present invention to provide a method for recovering high purity ε-caprolactam from fishing nets comprising polyamide 6. In this regard, it is a further object of the present invention to provide a method for recovering high purity ε-caprolactam from fishing nets comprising polyamide 6 that can replace high purity virgin ε-caprolactam for all applications, including high speed melt spinning of polyamide 6 for the production of fine textile fibers.

[0026] It is a further object of the present invention to provide a process for recovering high purity grade ε-caprolactam from fishing nets containing polyamide 6 on an industrial scale.

[0027] It is also an object of the present invention to provide a method for recovering, in an economically responsible manner, high-purity grade ε-caprolactam from fishing nets containing polyamide 6. In particular, it is an object of the present invention to provide a method suitable for recovering high-purity grade ε-caprolactam from fishing nets containing polyamide 6, which method does not exceed the production costs of high-purity virgin ε-caprolactam.

[0028] It is a further object of the present invention to provide high purity grade ε-caprolactam from fishing nets containing polyamide 6, characterized by a significantly lower carbon footprint than ε-caprolactam produced by a process using virgin ε-caprolactam obtained by a novel synthesis, for example, by Beckmann rearrangement of cyclohexanone oxime.

[0029] Therefore, another object of the present invention is to provide a method for reducing the environmental impact of discarded fishing nets containing polyamide 6. It is also an object of the present invention to provide a plant for producing high purity grade ε-caprolactam from material derived from fishing nets containing polyamide 6. One or more further objects may become apparent from the remainder of the description.

[0030] All, some or at least one of the above mentioned objects are solved by a method according to claim 1, a plant according to claim 13 and a product according to claim 15.

[0031] The present invention relates to a method for recovering purified ε-caprolactam from a material of fishing net origin containing polyamide 6 in a plant, said plant comprising: - Depolymerization section [B], - Recovery Section [C], and -Refining Section [D] Equipped with The method comprises: a) feeding a material derived from a fishing net containing polyamide 6 into the depolymerization section [B]; b) depolymerizing the material from fishing nets comprising polyamide 6 in the depolymerization section [B] at a temperature ranging from 180°C to 400°C to obtain a stream comprising ε-caprolactam; c) discharging a stream containing the ε-caprolactam from the depolymerization section [B] and recovering crude ε-caprolactam from the stream in the recovery section [C]; d) purifying the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises: (i) extracting the partially purified ε-caprolactam with an organic solvent to obtain an organic phase, the organic phase containing the organic solvent, ε-caprolactam, and impurities; (ii) exchanging the solvent by at least partially replacing the organic solvent with water, resulting in an aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam, the solvent exchange step (ii) being selected from a method based on back-extraction with water and a method based on solvent-switching distillation, in which the organic solvent is distilled off and water is introduced; (iii) obtaining purified ε-caprolactam by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam from the aqueous phase; Including, obtaining purified ε-caprolactam; The present invention provides a method comprising:

[0032] It was surprising that the specific sequence of processing steps and process conditions, i.e., the sequence of the depolymerization, recovery, and purification steps described above, combined according to the present invention, makes it possible to recover high-grade ε-caprolactam from polyamide 6-containing fishing net-derived materials in high yield and in a simple and economically rational manner. The present method is economically rational and advantageous from several perspectives. First, it is suitable for a wide variety of materials derived from polyamide 6-containing fishing net materials, which may differ in overall composition and / or polyamide 6 content. Second, it allows for the effective separation of ε-caprolactam from non-ε-caprolactam compounds, so as to obtain high-purity grade ε-caprolactam that can replace high-purity virgin ε-caprolactam for a variety of applications, including high-speed melt spinning of polyamide 6 for the production of fine textile fibers. Third, the present method is so effective that it can obtain ε-caprolactam in high yield. Fourth, the method of the present invention enables the industrial-scale recovery of ε-caprolactam from polyamide 6-containing fishing nets, allowing for the treatment of the enormous amount of polyamide 6-containing fishing nets currently discarded. Finally, the method of the present invention enables the de novo synthesis of ε-caprolactam to produce ε-caprolactam with a significantly lower carbon footprint than ε-caprolactam produced, for example, by the Beckmann rearrangement of cyclohexanone oxime. The method of the present invention enables the efficient treatment of materials derived from polyamide 6-containing fishing nets and reduces the environmental impact of the 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 equivalents per kg purified ε-caprolactam, which is a significant improvement compared to the 6.5-7.5 kg CO2 equivalents per kg ε-caprolactam associated with the production of "virgin" ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime (based on data from ecoinvent version 3.7.1; location: Europe).Product carbon footprint values ​​presented herein are based on data derived from ecoinvent version 3.7.1 and with Europe as the location, unless otherwise stated.

[0033] Next to the method of the invention, the invention also relates to a plant for producing purified ε-caprolactam from fishing nets containing polyamide 6, said plant comprising: Depolymerization section [B] and Recovery section [C] and Purification Section [D] and Equipped with Also provided is a plant, said plant being configured to carry out the method of the invention.

[0034] The present invention also provides purified ε-caprolactam obtained via depolymerization of polyamide 6 produced from polyamide 6-containing fishing nets, according to the method of the present invention, said ε-caprolactam having a product carbon footprint of less than 2 kg CO2 equivalents per kg purified ε-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). Advantageous embodiments of the invention are set out in the dependent claims and are explained in more detail below.

[0035] Fishing net containing polyamide 6 The method of the present invention uses a polyamide 6-containing fishing net or a material derived therefrom as a starting material. A polyamide 6-containing fishing net is typically a solid material, and in particular, a polyamide 6-containing fishing net is usually a mesh-like structure formed by kneading together relatively thin threads comprising polyamide 6. As used herein, a polyamide 6-containing fishing net-derived material refers to a polyamide 6-containing fishing net-derived material after, for example, crushing, washing, screening, compressing, pelletizing, etc. As used herein, a polyamide 6-containing fishing net-derived material also includes a polyamide 6-containing fishing net that has not been pre-treated, for example, by crushing, washing, screening, compressing, pelletizing, etc. The present disclosure uses the term "fishing net" to also refer to "fishing net material(s)," i.e., these terms are used interchangeably herein and can also be replaced by the term "polyamide 6-containing fishing net-derived material." As used in this disclosure and the claims, the singular forms "a," "an," and "the" include the plural forms, particularly in the sense of "one or more," unless the context clearly dictates otherwise.

[0036] Polyamide 6-containing fishing nets and materials derived from them can contain a wide variety of compounds added during their polymerization, during thread formation, or afterward to achieve various property variations. These compounds include, for example, brighteners, curing agents, antistatic lubricants, colorants, gloss agents, spin finishes, surface smoothing agents, antioxidants, UV stabilizers, and the like. The composition of fishing nets and materials derived from polyamide 6-containing fishing nets also depends on their exact use. Thus, fish farming nets, purse seine nets, and bottom trawl nets have different chemical compositions.

[0037] Surfaces immersed in seawater are rapidly covered with marine organisms, a phenomenon known as marine biofouling or biofouling. Biofouling is a complex phenomenon resulting from several processes, the rate and extent of which are influenced by numerous physical, chemical, and biological factors intimately connected to the surface. It affects most wet surfaces and results in significant financial costs. The accumulation of algae and barnacles increases the frictional resistance of ships and destroys equipment used for conservation and aquaculture.

[0038] Today, antifouling paints are formulated with toxic copper or other biocides to prevent the growth of sessile marine organisms. Copper is an effective and still widely used biocide. However, its effectiveness is relatively short, often only a few months, requiring frequent cleaning and reapplication of the paint. In addition to economic problems, leaching of the copper or other biocides can cause contamination of seawater and problems for non-target organisms.

[0039] The method of the present invention has the advantage that, unlike prior art methods which are limited to fairly pure polyamide 6-containing materials, such as carpets, and spinning waste containing PA6, the method of the present invention is not so limited and can in particular also be applied very successfully to polyamide 6-containing fishing nets and any type of material derived from polyamide 6-containing fishing nets.

[0040] Possible pre-treatment steps Before being subjected to step a) of the method of the present invention, the polyamide 6-containing fishing net material is preferably subjected to pretreatment in the pretreatment section [A], in particular size reduction in the mechanical size reduction section [β] and / or cleaning in the cleaning section [α]. This has the advantage that the polyamide 6-containing fishing net material fed to the depolymerization section [B] is less contaminated with non-polyamide 6 material, improving the yield and purity of the ε-caprolactam produced in the chemical plant of the present invention. Another advantage is that the size-reduced polyamide 6-containing fishing net can be handled more easily.

[0041] As used herein, the term "cleaning" is defined as any process that removes non-nylon 6 materials that are attached to or mixed with fishing nets that contain polyamide 6. Cleaning is advantageous because any removed non-nylon 6 materials will not thereby disturb the next steps of the method of the present invention.

[0042] (Discarded) polyamide 6-containing fishing nets can be mixed with all kinds of other materials, such as rocks, metal materials (e.g., wire, chain, anchors), organic materials (e.g., dead fish and mussels), and other (marine) debris (e.g., rope, (Styrofoam) flotsam, and sink lines). Furthermore, (discarded) polyamide 6-containing fishing nets can be mixed with non-polyamide 6 fishing nets made of polyamide 6,6, polyethylene terephthalate (PET), polypropylene (PP), or polyethylene (PE). (Discarded) polyamide 6-containing fishing nets can also be coated with antifouling coatings based on metals, such as copper, or with metal-free antifouling coatings.

[0043] The size of fishing nets containing (discarded) polyamide 6 depends highly on the exact application. Fishing nets containing (discarded) polyamide 6 range from a few square meters to over 200,000 square meters. Such huge nets, for example purse seines and nets set vertically in the water with flotation devices attached to the top edge, weights attached to the bottom edge and a series of loops through which pursing cables pass, can be as long as 1.5 km and over 150 m deep.

[0044] Preferably, the polyamide 6-containing fishing net is broken into small pieces before being depolymerized in the depolymerization section [B] in step a). This mechanical pretreatment, i.e., mechanical crushing or shredding of the polyamide 6-containing fishing net, can be achieved, for example, by cutting, shredding, milling, grinding, and / or chipping. In a preferred embodiment, the polyamide 6-containing fishing net is introduced into step a) in the form of small pieces having a weight ranging from 0.005 grams to 100 kg, preferably from 0.01 grams to 10 kg, and most preferably from 0.02 grams to 1 kg. Using small pieces of polyamide 6-containing material derived from the fishing net having the above-mentioned weight has the advantage that the pieces can be more easily handled and / or cleaned by washing with a solvent.

[0045] In some cases, prior to mechanical crushing or shredding of polyamide 6-containing fishing nets, large metal fragments, rocks, and other interfering materials that cause significant wear on the equipment used for mechanical crushing or shredding are removed. Preferably, non-polyamide 6-containing materials, such as polyethylene, polypropylene, and polyamide 6,6-containing materials, are removed before or after mechanical crushing or shredding of polyamide 6-containing fishing nets, as described further below. Removal of these interfering materials can be performed mechanically or manually. Removal of these interfering materials has the advantage of significantly reducing the maintenance costs of the equipment used for mechanical crushing or shredding. Furthermore, the polyamide 6 content of the material obtained after mechanical crushing or shredding is higher than if the interfering materials were not removed. Removal of polyamide 6,6 is particularly advantageous because polyamide 6,6 interferes with the depolymerization of polyamide 6, causing blockages in the reactor itself, reducing the recovery yield of ε-caprolactam, and interfering with the subsequent purification of the recovered ε-caprolactam.

[0046] Preferably, the fishing net (comprising polyamide 6) having the Cu-based antifouling coating is also removed prior to mechanically crushing or shredding the fishing net comprising polyamide 6. More preferably, the fishing net having the Cu-based antifouling coating is washed in an additional, separate washing step to remove the Cu-based antifouling coating. This washed net material can then be added to a material having a similar composition.

[0047] In some cases, foreign matter is separated from fishing nets containing mechanically crushed or shredded polyamide 6. Various separation processes 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 liquid of intermediate density, with the lower density materials floating and separated from the higher density materials, which sink. In practice, density separation is often performed through a series of density separation stages. For example, in one stage, high-density materials such as rock, sand, and metals (including iron and lead) are separated, while in another stage, low-density materials such as polyolefins, polypropylene, and polyethylene are separated. Magnetic separation is a process in which components of a mixture are separated by using magnets to attract magnetic materials. A process typically used for magnetic separation separates non-magnetic materials from magnetic materials. Removal of foreign matter in fishing nets containing crushed or shredded polyamide 6 is advantageous because foreign matter in fishing nets containing crushed or shredded polyamide 6 can interfere with the depolymerization of polyamide 6, reduce the recovery yield of ε-caprolactam, and interfere with the subsequent purification of the recovered ε-caprolactam.

[0048] Optionally, polyamide 6-containing fishing nets are cleaned by washing with a solvent, preferably water, before being introduced into the depolymerization section [B]. Preferably, a cleaning agent is added to the solvent at a concentration ranging from 0 to 20% by weight relative to the solvent for improved cleaning efficiency. NaOH is the preferred cleaning agent. Preferably, an aqueous solution containing 0 to 10% by weight NaOH, even more preferably 0 to 5% by weight NaOH, is used in the cleaning process. Preferably, Cu-based antifouling coatings are removed by washing with an aqueous solution containing 1 to 5% by weight NaOH, preferably 1.5 to 3% by weight NaOH, and more preferably about 2% by weight NaOH. The enhanced cleaning effect of NaOH is most likely caused by enhanced hydrolysis of molecules, including biopolymers and non-biopolymers. Furthermore, NaOH is known to hydrolyze copolymers used in Cu-based antifouling coatings, such as polyethylene-vinyl acetate (PEVA, also known as EVA). Preferably, the washing solvent is heated to further enhance the cleaning process. In another preferred embodiment, the washing process includes a final rinse step with a detergent-free (clean) washing solvent to remove detergent residues and any existing dirt adhering to the fishing net comprising polyamide 6.

[0049] The cleaning is preferably carried out under friction. Various types of industrial cleaning systems are available on the market, such as high speed friction cleaners. Washing of fishing nets comprising polyamide 6, in particular fishing nets comprising mechanically crushed or shredded polyamide 6, is advantageous because any (adherent) dirt is removed and therefore does not interfere with the subsequent steps of the method of the invention.

[0050] In some cases, the polyamide 6-containing fishing nets are dried after the cleaning step and before being fed into the depolymerization section [B]. This has the advantage that the weight of the cleaned polyamide 6-containing fishing nets is reduced and the next process steps are not subject to dilution or contamination by the cleaning solvent.

[0051] The optionally, preferably washed and size-reduced fishing nets containing polyamide 6 are fed into a melting furnace (e.g., an extruder). In the melting furnace, the fishing nets containing polyamide 6 are melted. Preferably, the resulting polymer melt is filtered. This has the advantage that solid impurities are removed. The melted and optionally filtered polymer melt is then cooled and fed to a pelletizer. The pelletizer cuts the product into pellets. Preferably, the pellets or the melted and optionally filtered polymer melt are fed directly into the depolymerization section [B].

[0052] The size and shape of the pellets (often also called granules) can be selected within wide limits. Generally, pellets are cylindrical (derived from thin strands chopped into small pieces). However, other shapes, such as (imperfect) spheres, are also possible. The size of the pellets can be selected within wide limits. Usually, pellets have a diameter ranging from 1 to 10 mm, preferably from 2 to 7 mm, more preferably from 3 to 5 mm. In a preferred embodiment, the pellets have a length ranging from 1 to 50 mm, preferably from 2 to 25 mm, more preferably from 3 to 15 mm.

[0053] Pelletization of fishing nets, preferably containing washed and size-reduced polyamide 6, has the advantage of increased bulk density, which reduces the costs of storage and transportation of intermediates when pre-processing is carried out at a different location (see below). Apart from the increased density, pelletization also offers other advantages, such as a uniform shape and structure of the material to be processed, which is advantageous for (automated) feeding into the depolymerization section [B].

[0054] The location where the pretreatment of the polyamide 6-containing fishing nets takes place and the location where the depolymerization section [B] is located can be the same. However, preferably, one or more of the pretreatment steps take place in different locations, for example near a port where the waste polyamide 6-containing fishing nets are collected and / or at a location specialized in the pretreatment of waste fishing nets. The polyamide 6-containing fishing nets pretreated in various locations can then be fed into the depolymerization section [B] of the (chemical) plant of the present invention in order to produce purified ε-caprolactam from the polyamide 6-containing fishing net-derived material.

[0055] Feeding process a) In step a) of the present invention, the optionally size-reduced and / or washed material from fishing nets comprising polyamide 6 is fed into depolymerization section [B], which comprises one or more depolymerization reactors operated in series and / or in parallel.

[0056] In one embodiment, the fishing nets comprising polyamide 6 are mechanically compressed to a smaller volume before being introduced into the depolymerization section [B]. This has the advantage that a smaller volume is required for intermediate storage and transportation, which may also facilitate introduction into the depolymerization section [B].

[0057] In another embodiment, the fishing nets comprising polyamide 6 are compressed into particles with increased density before being introduced into the depolymerization section [B], for example by mechanical compression or by extrusion of the molten material followed by cooling and cutting to size. This again has the advantage that a smaller volume is required for intermediate storage and transport, which can facilitate introduction into the depolymerization section [B].

[0058] In a further preferred embodiment, the fishing nets containing polyamide 6 are dried before being introduced into the depolymerization section [B], in particular after the fishing nets containing polyamide 6 have been subjected to a cleaning process. This has the advantage that less solvent or no solvent is introduced into the depolymerization section [B]. The solvent introduced into the depolymerization section [B] is expected to have a negative effect on the depolymerization process (e.g., a reduced depolymerization reaction rate, increased catalyst consumption, increased energy consumption, and the vapor stream containing ε-caprolactam and water obtained in the depolymerization section [B] is expected to contain more impurities).

[0059] The polyamide 6-containing fishing net-derived material is preferably fed to the depolymerization reactor(s) as a solid phase or as a melt. Preferably, the polyamide 6-containing fishing net-derived material is introduced as a melt. Feeding as a melt can be achieved by using an extruder, a gear pump, or other means known to those skilled in the art.

[0060] The feeding of the polyamide 6-containing fishing net-derived material to the depolymerization reactor(s) can be achieved by continuous or intermittent addition of the polyamide 6-containing fishing net-derived material.

[0061] Depolymerization step b) In the depolymerization section [B], the fishing net-derived material containing polyamide 6 is depolymerized to form ε-caprolactam, which is discharged from the depolymerization section [B] as an ε-caprolactam-containing stream.

[0062] Depolymerization of the polyamide 6-containing fishing net-derived material is achieved in depolymerization section [B] by increasing the temperature of the polyamide 6-containing fishing net-derived material to a temperature of 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.

[0063] Generally, the rate of ε-caprolactam formation increases at higher temperatures. Temperatures below 400°C are preferred because side reactions of polyamide 6 and impurities occur more frequently at temperatures above 400°C, resulting in the formation of a more diverse set of impurities. Some of these impurities end up in the ε-caprolactam-containing product stream discharged from the depolymerization reactor(s). In a preferred embodiment of the present invention, the depolymerization of polyamide 6-containing fishing nets is carried out at temperatures ranging from 220°C to 340°C or from 240°C to 325°C. Experiments have shown that this temperature range allows for the production of particularly pure ε-caprolactam.

[0064] The pressure in the depolymerization section [B] may vary and may be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, and most preferably 50 kPa to 1 MPa. Experiments have shown that this pressure range allows the production of particularly pure ε-caprolactam.

[0065] The depolymerization of the polyamide 6-containing fishing net-derived material can be achieved in the presence or absence of a solvent. Preferably, the depolymerization of the polyamide 6-containing fishing net-derived material is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, particularly superheated steam. Preferably, the depolymerization is completed in 0.1 to 24 hours, more preferably in 0.5 to 6 hours.

[0066] By supplying water as steam to the depolymerization reactor, a vapor stream containing ε-caprolactam and water can be obtained, optionally without further heating. The weight-to-weight ratio of ε-caprolactam to water in this steam stream can be adjusted by modifying the amount of steam supplied to the fishing net-derived material containing polyamide 6 in 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 vapor stream containing ε-caprolactam and water in a weight-to-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.

[0067] Preferably, the ε-caprolactam in the vapour stream comprising ε-caprolactam and water has a partial pressure of from 5 kPa to 1 MPa, more preferably from 10 kPa to 0.5 MPa, most preferably from 15 kPa to 0.1 MPa.

[0068] During the depolymerization reaction, degradation products may be formed, including linear and cyclic oligomers of ε-caprolactam. Furthermore, the feed stream of fishing net-derived material containing polyamide 6 may also contain other components, i.e., impurities such as non-polyamide 6 compounds, and residues of solvent(s) applied in pretreatment that remain stable, react, or decompose under the depolymerization conditions. Thus, when water is used as the solvent, the vapor stream removed from depolymerization section [B] contains not only water and ε-caprolactam, but also impurities.

[0069] Preferably, superheated steam having a temperature of 100°C to 600°C is fed to the depolymerization reactor(s). Preferably, the superheated steam fed to the depolymerization reactor(s) has a temperature of at least the melting temperature of polyamide 6. Preferably, the energy content of the superheated steam fed to the depolymerization reactor(s) is sufficiently high so that no other heat input is required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam. In another preferred embodiment, depolymerization section [B] is fed with superheated steam having a temperature in the range of 220°C to 575°C. In an even more preferred embodiment, depolymerization section [B] is fed with superheated steam having a temperature in the range of 275°C to 500°C.

[0070] Generally, the mass of the vapor stream removed from the depolymerization section [B] is less than the mass of the total feed to the depolymerization section. The total feed to the depolymerization section [B] includes the fishing net-derived material containing polyamide 6, as well as optionally a solvent, a catalyst, additional factors, and / or a depolymerization agent. Therefore, without additional measures, there would be an accumulation of material (often referred to as "residual material") in the depolymerization section [B]. Preferably, a separate stream is discharged from the depolymerization section [B]. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization section [B]. The additional stream can contain impurities present in the fishing net-derived material containing polyamide 6, undepolymerized polyamide, unevaporated ε-caprolactam, catalyst(s), and compounds formed under the depolymerization conditions, such as mono-, di-, and / or triammonium phosphate when phosphoric acid is used as the depolymerization catalyst. In a preferred embodiment, a stream containing mono-, di-, and / or triammonium phosphate is discharged from the depolymerization section [B]. Even more preferably, this stream intermittently or continuously discharged from depolymerization section [B] comprises mono-, di- and / or triammonium phosphate in a weight fraction of 0.01 to 50 wt. %, preferably 0.1 to 25 wt. %, more preferably 0.5 to 10 wt. %, and most preferably 0.5 to 5 wt. %.

[0071] The depolymerization of the fishing net-derived material containing polyamide 6 in the presence of water vapor can be carried out in the presence of an additional depolymerization agent, such as ammonia. The concentration of ammonia in the depolymerization section [B] can vary. Thus, if ammonia is present in the depolymerization section [B], the vapor stream removed from the depolymerization section [B] can contain not only ε-caprolactam and impurities, but also ammonia.

[0072] Most preferably, the depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted) acid or base. Acid catalysts may be selected from the group consisting of orthophosphoric 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 from the group consisting of, for example, alkali hydroxides, alkali salts, alkaline earth hydroxides and alkalis, such as earth salts, organic bases and solid bases, and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali hydroxides and alkali salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate are used as catalysts. Even more preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid, and sodium hydroxide are used as catalysts. In one particularly preferred embodiment, orthophosphoric acid is used as the catalyst for depolymerization, and in another embodiment, p-toluenesulfonic acid is used.

[0073] However, in another preferred embodiment, no catalyst is used for the depolymerization of fishing net-derived materials containing polyamide 6. This has the advantage of lower costs (both for the catalyst and for disposal of catalyst waste), but typically requires higher temperatures (and pressures).

[0074] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction begins at a lower temperature and can be carried out under atmospheric conditions. The appropriate catalyst concentration for the depolymerization of polyamide 6-derived materials to ε-caprolactam is known to those skilled in the art and can be easily determined by routine experimentation. If the catalyst concentration is too low, the reaction rate will be slow. Conversely, if the catalyst concentration is too high, the reaction will be fast, but the occurrence of one or more side reactions will also increase. Furthermore, the catalyst cost will increase, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100 wt. % relative to the polyamide 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1 to 50 wt. The optimal catalyst concentration depends on the type of catalyst used for the depolymerization of polyamide 6. In the case of the catalyst orthophosphoric acid, the preferred content is 0.1 to 25 wt. %, more preferably 1 to 20 wt. %. The content of the catalyst p-toluenesulfonic acid is preferably 10 to 35% by weight, and more preferably 15 to 30% by weight.

[0075] The depolymerization of polyamide 6 can be carried out in batch mode, semi-continuous mode, or continuous mode, all of which are known to those skilled in the art. As used herein, the terms "batch mode," "semi-continuous mode," and "continuous mode" refer to a mode in which polyamide 6-containing feedstock, i.e., material derived from fishing nets containing polyamide 6, and optionally a catalyst, are introduced into a depolymerization reactor, and residual material is discharged from the depolymerization reactor.

[0076] In a preferred embodiment, the depolymerization of polyamide 6 is carried out in batch mode. In batch mode, the raw material, i.e., material derived from fishing nets containing polyamide 6 and optionally a catalyst, is first introduced into a depolymerization reactor. Subsequently, superheated steam is introduced into the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a vapor stream containing ε-caprolactam and water. Then, the introduction of superheated steam into the depolymerization reactor is discontinued. Optionally, after removing residual material from the depolymerization reactor, a new cycle is started by introducing the raw material (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, residual material is not removed between all cycles.

[0077] In a particularly advantageous embodiment, the depolymerization of polyamide 6 is carried out in a continuous mode. In the continuous mode, a material derived from a feedstock containing polyamide 6 (and optionally a catalyst) is continuously introduced into a depolymerization reactor. At the same time, superheated steam is continuously introduced into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream containing ε-caprolactam and water. Optionally, a catalyst is continuously or intermittently introduced into the depolymerization reactor. Furthermore, residual material is continuously discharged from the depolymerization reactor. Preferably, the material derived from fishing nets containing polyamide 6 is introduced as a melt. Preferably, the catalyst is introduced as a melt, a slurry, or a solution.

[0078] In another preferred embodiment, the depolymerization of polyamide 6 is carried out in semi-continuous mode. In semi-continuous mode, materials from a feedstock containing polyamide 6 (and optionally a catalyst) are intermittently added to a depolymerization reactor, while superheated steam is continuously added to the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream comprising ε-caprolactam and water. Residual materials are intermittently discharged from the depolymerization reactor in semi-continuous mode for polyamide 6 depolymerization.

[0079] Recovery process c) In the recovery section [C], ε-caprolactam is recovered from the ε-caprolactam-containing stream discharged from the depolymerization section [B]. This stream contains ε-caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the ε-caprolactam-containing stream.

[0080] Preferably, if no solvent is introduced into the depolymerization section [B], the ε-caprolactam obtained by condensation is dissolved in water to obtain an ε-caprolactam-rich phase, which also contains impurities.

[0081] Preferably, when water is introduced into the depolymerization section [B] as a solvent, the ε-caprolactam-containing stream discharged from the depolymerization section [B] contains ε-caprolactam, water, and impurities. ε-caprolactam can be separated from the remaining components of the vapor stream by sending the vapor stream from the depolymerization reactor, preferably from the top, to a (preferably partial) condenser to obtain a condensate containing ε-caprolactam. Preferably, ε-caprolactam is separated from the remaining components of the vapor stream by sending the product stream from the depolymerization reactor, preferably from the top, to a distillation column, from which a water-rich phase is obtained as an overhead product and an ε-caprolactam-rich phase is obtained as a bottom product.

[0082] The ε-caprolactam recovered in the recovery section [C] is crudely purified because it contains impurities such as polyamide 6 degradation products or other impurities derived from non-polyamide 6 components of the polyamide 6-containing fishing net-derived material. The crudely purified ε-caprolactam recovered in step c) is an aqueous solution containing water and ε-caprolactam, preferably ε-caprolactam. Therefore, the crudely purified ε-caprolactam recovered in the recovery section [C] requires further purification to obtain high-purity ε-caprolactam. Therefore, as used herein, "crudely purified" can be defined as being of lower purity, i.e., containing fewer impurities, than the purified ε-caprolactam obtained as the product of the method of the present invention.

[0083] Preferably, the partially purified ε-caprolactam contains ε-caprolactam in the range of 6 to 95% by weight, more preferably 20 to 90% by weight, and most preferably 35 to 80% by weight, with the remainder being mainly water.

[0084] Purification process d) In step d), the crude ε-caprolactam obtained in the recovery section [C] is purified in the purification section [D] to obtain highly purified ε-caprolactam.

[0085] Optionally, the crude ε-caprolactam is filtered before being introduced into the purification section [D]. Filtration ensures the removal of undissolved impurities that may otherwise interfere with the further purification process.

[0086] Optionally, oil is separated from the crude ε-caprolactam before it is fed to the refining section [D]. Oil separation ensures the removal of impurities that could otherwise interfere with the further refining process.

[0087] Purified ε-caprolactam can be obtained from crude ε-caprolactam by first extracting the crude ε-caprolactam with an organic solvent in step (i), thereby obtaining an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The organic solvent for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, an alicyclic hydrocarbon, a halogenated hydrocarbon, and / or a C4 to C6 hydrocarbon. 10 In some cases, the organic solvent for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, an alicyclic hydrocarbon, a halogenated hydrocarbon, and / or a C4 to C6 10The extractant mixture is composed of an aliphatic or alicyclic alcohol and a C5-C8 alkane or a C5-C8 cycloalkane. Particularly good purification results are achieved when the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC, methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol, and mixtures thereof. More preferably, the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohol, and mixtures thereof. Even more preferably, the organic solvent 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 0.01:1 to 40:1, preferably 0.05:1 to 10:1, more preferably 0.1:1 to 5:1.

[0088] In some cases, the organic solvent for the extraction of crude ε-caprolactam is a mixture of an alkane, C ... m H 2m+2 (wherein m is 5 to 8), a cycloalkane or C m H 2m (wherein m is 5 to 8). Particularly good results are achieved if the alkane or cycloalkane is present in the mixed extractant in an amount ranging from 5 to 90% by weight, preferably from 25 to 75% by weight, of the total weight of the mixed extractant.

[0089] In another embodiment, in which the organic solvent has a density lower than that of the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in a countercurrent-driven extraction column, in which 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 of the column. The extraction results in an aqueous phase containing water and impurities and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The extraction results in an organic phase containing the organic solvent, ε-caprolactam, and impurities, and the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, as a result of this extraction, the ε-caprolactam is purer than before the extraction.

[0090] In another embodiment of the present invention, in which the organic solvent has a higher density than the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in a countercurrent-driven extraction column, in which 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 of the column. The extraction results in an aqueous phase containing water and impurities and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The extraction results in an organic phase containing the organic solvent, ε-caprolactam, and impurities, and the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Therefore, as a result of this extraction, the ε-caprolactam is purer than before the extraction.

[0091] Optionally, the organic phase containing the organic solvent, ε-caprolactam, and impurities is washed with water or an aqueous alkaline solution before entering step d)(ii). When washing is carried out with an aqueous alkaline solution, the alkaline solution is preferably an aqueous solution containing an alkali metal hydroxide and / or alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably contains 0.5 to 2.0 wt. % sodium hydroxide or potassium hydroxide.

[0092] Those skilled in the art can determine by routine experimentation the amount of water or aqueous alkali solution required for efficient washing of the organic phase containing the organic solvent, ε-caprolactam, and impurities. Preferably, this amount is 0.1 to 5 wt. % based on the amount of organic solvent, excluding ε-caprolactam, dissolved in the organic phase to be washed. In another preferred embodiment, washing of the organic phase containing the organic solvent, ε-caprolactam, and impurities with water or aqueous alkali solution is carried out in a countercurrent-driven wash column, with the organic phase containing the organic solvent, ε-caprolactam, and impurities introduced into the lower part of the column and the water or aqueous alkali solution introduced into the upper part of the column. The washing results in a washed organic phase containing the organic solvent, ε-caprolactam, and impurities, and an aqueous residue-containing phase. Typically, the aqueous residue-containing phase contains water, ε-caprolactam, and impurities. As a result of washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase before washing.

[0093] Subsequently, according to step d)(ii) of the method of the present invention, the obtained organic phase comprising the organic solvent, ε-caprolactam and impurities, and optionally washed with water or aqueous alkali solution, is subjected to solvent exchange, in which the organic solvent in the organic phase comprising the organic solvent, ε-caprolactam and impurities is replaced by water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than that of ε-caprolactam, the solvent exchange process being selected from a process based on back-extraction with water (also known as re-extraction) and a process based on solvent-switching distillation, in which the organic solvent is distilled off and water is introduced.

[0094] The term "displaced" as used herein means that at least 60% by weight, preferably at least 80% by weight, and most preferably at least 90, 95 or 98% by weight of the organic solvent present in the organic phase comprising the organic solvent, ε-caprolactam and impurities is replaced by water.

[0095] Solvent exchange can be a process based on aqueous back-extraction, which results in 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 solvent. The amount of water used for back-extraction of ε-caprolactam can vary, but the amount of water used is preferably 0.3 to 20 times, more preferably 0.4 to 10 times, and most preferably 0.5 to 6 times by weight relative to the recovered ε-caprolactam. Preferably, the water back-extraction is carried out in a countercurrent driven extraction column.

[0096] In another preferred embodiment, the organic phase containing the organic solvent, ε-caprolactam, and impurities, optionally washed, 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 of the extraction column. Stripping results in an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than those of ε-caprolactam, as well as an organic solvent phase containing impurities. Stripping results in an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than those of ε-caprolactam, with the weight ratio of impurities to ε-caprolactam being reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase containing the organic solvent, ε-caprolactam, and impurities before stripping. Thus, purer ε-caprolactam is obtained as a result of stripping. Preferably, the organic solvent phase containing impurities is reused, optionally after purification (preferably by distillation).

[0097] In another preferred embodiment, in which the organic phase containing the organic solvent, ε-caprolactam, and impurities has a density higher than water, the organic phase is introduced into the upper part of the extraction column, and water is introduced into the lower part of the extraction column. Stripping results in an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than those of ε-caprolactam, as well as an organic solvent phase containing impurities. Stripping results in an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than those of ε-caprolactam, with the weight ratio of impurities to ε-caprolactam being reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase containing the organic solvent, ε-caprolactam, and impurities before stripping. Thus, after stripping, the ε-caprolactam is purer than before stripping. Preferably, the organic solvent phase containing impurities is reused, optionally after purification (preferably by distillation).

[0098] Thus, according to a particularly advantageous embodiment of the invention, after extraction of the crude ε-caprolactam in step d)(i), the purification in step d) also comprises a step of (ii) solvent exchange based on back-extraction with water.

[0099] The solvent exchange process can also be based on solvent-switching distillation, whereby the organic solvent is distilled off and water is added. In a preferred embodiment, the solvent exchange process is based on solvent-switching distillation, which is carried out as a single-stage process, whereby the organic solvent is distilled off from the organic phase containing the organic solvent, ε-caprolactam, and impurities, and water is added. More preferably, the solvent exchange is carried out as an azeotropic distillation with the addition of water, in which case the organic solvent is evaporated 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. "Substantially all" in this context means that 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 organic phase containing the organic solvent, ε-caprolactam, and impurities is removed. Preferably, water is added as a liquid. More preferably, liquid water is added as a reflux to the top of the distillation column. Even more preferably, part of the water added as a reflux is obtained by condensing the azeotropic mixture distilled off in the distillation column.

[0100] Any suitable vessel, such as a column, preferably a distillation column operated in a continuous mode, can be used for the solvent exchange process. The distillation column can contain trays, packing, or a combination thereof. In another preferred embodiment, the solvent-switching distillation is carried out as a two-stage process: the first stage is a pre-concentration stage and the second stage is the actual solvent-switching distillation.

[0101] An organic phase containing the organic solvent, ε-caprolactam, and impurities is introduced 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 at the top of the distillation column. Preferably, this distillation is carried out under reflux. Under reflux means that the organic solvent in the liquid phase is introduced into the top of the distillation column. More preferably, a portion of the organic solvent removed by distillation at the top of the distillation column is condensed and then introduced into the top of the distillation column as a liquid. The remaining organic phase containing the organic solvent, ε-caprolactam, and impurities is discharged from the first stage and introduced into the second stage. After the distillation in the first stage, the remaining organic phase containing the organic solvent, ε-caprolactam, and impurities has a chemical composition different from that of the organic phase containing the organic solvent, ε-caprolactam, and impurities introduced into the first stage. Generally, compared to the organic phase containing organic solvent, ε-caprolactam and impurities charged in the first stage, the remaining organic phase containing organic solvent, ε-caprolactam and impurities will contain a higher percentage by weight of ε-caprolactam and compounds having a boiling point higher than ε-caprolactam, and a lower percentage by weight of compounds having a boiling point lower than ε-caprolactam.

[0102] In the second step, the remaining organic solvent is distilled off from the remaining organic phase containing the organic solvent, ε-caprolactam, and impurities, and water is added. More preferably, in the second step, the solvent exchange is carried out as an azeotropic distillation with the addition of water, in which case the organic solvent is evaporated as an azeotropic mixture containing the organic solvent and water.

[0103] The purpose of the azeotropic distillation is to remove the organic solvent and add water. Preferably, substantially all of the organic solvent is removed. "Substantially all" in this context means that 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 remaining organic phase containing the organic solvent, ε-caprolactam, and impurities is removed. Preferably, water is added as a liquid. More preferably, liquid water is added as reflux to the top of the distillation column. Even more preferably, part of the water added as reflux is obtained by condensation of the azeotropic mixture distilled off in the distillation column.

[0104] Any suitable vessel, such as a column, preferably a distillation column operated in continuous mode, can be used at each stage for solvent exchange. The distillation column can contain trays, packing, or a combination thereof.

[0105] The solvent-switching distillation (carried out as either a one-stage or two-stage process) results in an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, and optionally containing residual organic solvent. Preferably, the ε-caprolactam content of this aqueous phase is 25 to 99.9 wt. %, more preferably 50 to 99.5 wt. %, and most preferably 85 to 99 wt. %, based on the total aqueous phase.

[0106] Thus, according to a particularly advantageous embodiment of the present invention, after extraction of the crude ε-caprolactam in step d)(i), the purification in step d) also comprises a step of (ii) solvent exchange based on solvent-switching distillation.

[0107] In step d)(iii) of the process of the present invention, the aqueous phase obtained by solvent exchange in step d)(ii) and containing water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is distilled to remove impurities having a boiling point lower or higher than ε-caprolactam from said aqueous phase.

[0108] Preferably, water is first evaporated from the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. After evaporation of water, ε-caprolactam is distilled to recover high-purity ε-caprolactam. Preferably, the distillation is carried out under reduced pressure. Even more preferably, the 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. Generally, the temperature is between 90°C and 210°C. Preferably, the temperature is between 100°C and 200°C, more preferably between 110°C and 180°C. These temperatures refer to the temperature at the bottom of the distillation column in which the distillation is carried out.

[0109] The distillation involves separating low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) from ε-caprolactam and / or separating high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. The distillation preferably involves, in a first step, separating low-boiling impurities from ε-caprolactam as an overhead product and producing ε-caprolactam containing high-boiling impurities as a bottom product. In a second step, high-purity ε-caprolactam is separated as an overhead product, and a distillation residue containing ε-caprolactam and high-boiling impurities is obtained as a bottom product.

[0110] In a preferred embodiment, prior to the distillative removal in step d)(iii), an alkali metal hydroxide, preferably NaOH, is added to the aqueous phase containing water, ε-caprolactam, and impurities having a boiling point lower or higher than that of ε-caprolactam. Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol, more preferably from 2 to 80 mmol, per kg of ε-caprolactam. Experiments have shown that the addition of an alkali metal hydroxide, in particular NaOH, allows for particularly effective distillative removal of impurities having a boiling point lower and higher than that of ε-caprolactam.

[0111] In another preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide, is added to the aqueous phase containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam prior to the distillative removal in step d)(iii). Most preferably, potassium permanganate is used as the oxidizing agent.

[0112] The oxidizing agent can be added as a solid, a slurry, or an aqueous solution to the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, so as to obtain a dilute aqueous solution. Those skilled in the art can determine the amount of oxidizing agent required for efficient oxidation of the aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam through routine experimentation. The exact amount of oxidizing agent will depend greatly, inter alia, on the composition of the polyamide 6-containing waste fishing net material used as the raw material in the method of the present invention. Preferably, the amount of oxidizing agent is 0.01 to 5 wt. % relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0113] The temperature used for the oxidation of the aqueous solution in the process of the present invention can vary. Preferably, the oxidation of the aqueous solution containing water, ε-caprolactam and impurities having a boiling point lower or higher than that of ε-caprolactam with an oxidizing agent prior to distillative removal in step d)(iii) is carried out at a temperature in the range of 20°C to 85°C, more preferably in the range of 30°C to 80°C, and the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, in particular potassium permanganate.

[0114] The length of time used for oxidation with an oxidizing agent can vary. Preferably, the oxidation of an aqueous solution containing water, ε-caprolactam, and impurities having a boiling point lower or higher than that of ε-caprolactam with an oxidizing agent is carried out for 1 minute to 24 hours, more preferably 2 minutes to 6 hours, and most preferably 5 minutes to 2 hours.

[0115] The concentration of ε-caprolactam in the aqueous phase containing water, ε-caprolactam, and impurities with a boiling point lower or higher than that of ε-caprolactam, used for oxidation with an oxidizing agent, can vary. Preferably, the aqueous solution used for oxidation contains a weight-to-weight ratio of ε-caprolactam to water of 5:1 to 1:5, more preferably 3:1 to 1:3, and most preferably 2:1 to 1:2. Optionally, the weight-to-weight ratio of ε-caprolactam to water is adapted before adding the oxidizing agent to the aqueous phase. Preferably, the weight-to-weight ratio of ε-caprolactam to water is adapted by either adding or removing water.

[0116] When potassium permanganate or sodium permanganate is used as the oxidizing agent, solid manganese(IV) oxide (MnO2) particles are formed as the reaction product. Those skilled in the art can determine, through routine experimentation, the optimal solid-liquid filtration procedure for efficiently removing the solid manganese(IV) oxide particles from the aqueous phase after oxidation. It is common practice in this regard to use filter aids such as activated carbon or diatomaceous earth particles to improve the filtration procedure.

[0117] The high-purity ε-caprolactam obtained by the process of the present invention can be used to produce polyamide 6 using methods well known to those skilled in the art. This polyamide 6 can then be used in all known materials, including engineering materials, fibers, and films. This polyamide 6 produced from fishing net-derived materials containing polyamide 6 is particularly suitable for high-speed spinning applications, including garments containing spandex (also known as elastane).

[0118] plant The present invention also provides a plant, i.e., a chemical plant, comprising a depolymerization section [B], a recovery section [C], and a purification section [D], configured to carry out the above-described process of the present invention. All plant features specifically described below in relation to the plant also correspond to specific embodiments of the method of the present invention, and vice versa. Thus, it is understood that the plant is suitable for carrying out the method of the present invention, and that what is described in relation to the method of the present invention equally applies to the plant embodiments.

[0119] The plant can be a laboratory facility as in the examples. Preferably, however, the plant is an industrial-scale plant. By "industrial-scale" is meant that the plant, if in full operation, has a production capacity of at least 500 tons / year of ε-caprolactam (i.e., can in principle produce the above amounts of ε-caprolactam).

[0120] The plant of the present invention is suitable for producing purified ε-caprolactam from fishing nets containing polyamide 6 and comprises at least three sections: a depolymerization section [B], a recovery section [C] and a purification section [D]. These sections, and therefore the plant, are configured to carry out the above-described method of the present invention.

[0121] Furthermore, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [β] for breaking down the polyamide 6-containing fishing net into small pieces and / or a cleaning section [α] for washing the polyamide 6-containing fishing net and / or a densification section [γ] for increasing the bulk density of the nylon 6-containing fishing net. Cleaning includes both washing and separating foreign matter from the polyamide 6-containing fishing net. The separation of foreign matter can be performed manually (hand sorting) or mechanically (e.g., density separation and magnetic separation). Both manual and mechanical devices, such as brushes, can be used in the cleaning process in the cleaning section [α]. Cleaning is preferably performed by additional friction. Various types of industrial cleaning systems, such as high-speed friction washers, are available on the market. The mechanical size reduction section [β] includes a device for mechanically breaking down the polyamide 6-containing fishing net into small pieces. Non-limiting examples of this shredding device include cutters, shredders, mills, grinders, and chippers. The densification section [γ] comprises equipment for densifying the material that makes up the fishing net, including nylon 6. Densification to obtain a material with a higher bulk density can be carried out by 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.

[0122] The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or in parallel. The polyamide 6-containing fishing nets are fed to the reactors as a solid or melt, preferably as a melt. This feeding can be accomplished by an extruder, a gear pump, or other means known in the art.

[0123] During production, the depolymerization reactor is at least partially filled with polyamide 6-containing feedstock, residual materials, ε-caprolactam (and optionally a catalyst). The depolymerization reactor can have any desired configuration. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors, and extruder-type reactors.

[0124] The depolymerization reactor must be equipped with facilities for feeding the fishing net-derived material containing polyamide 6, as well as optionally superheated steam and a catalyst. Furthermore, the depolymerization reactor is equipped with facilities for discharging the ε-caprolactam-containing stream and residual material.

[0125] Good contact between the steam and the reactor contents is essential for effective operation. Such contact can be achieved by a variety of means known to those skilled in the art. As an example, multiple inlets can be used to sparge the steam through the material, for example, using a steam distributor. Further improved contact can be achieved by including mechanical agitation within the reactor, for example, using a combination of rotating paddles and stationary fins. Preferably, the depolymerization is completed in 0.5 to 6 hours.

[0126] If hot superheated steam is not available at the production site, hot superheated steam must be created intentionally by superheating available steam from a boiler in a so-called superheater.

[0127] The recovery section [C] may comprise one or more (preferably partial) condensers into which the ε-caprolactam-containing stream in the form of a vapor stream comprising ε-caprolactam and water is introduced. Such (partial) condensers may have any desired form. Preferably, the condenser is a distillation column from which a water-rich phase is obtained as overhead product and crude ε-caprolactam is obtained as bottom product.

[0128] The purification section [D] comprises one or more extraction devices, one or more solvent exchange devices, an oxidation section, and one or more distillation devices, to which the crude ε-caprolactam is input and from which the highly purified ε-caprolactam is discharged.

[0129] Crude ε-caprolactam and an organic solvent are introduced into an extractor, and an organic phase containing the organic solvent, ε-caprolactam, and impurities, and an aqueous phase containing water and impurities are discharged. The extractor is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the extractor is a static or agitated extractor, such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulse column, a sieve plate column (static), a randomly packed column (static), or a structured packing (SMVP) (static) column.

[0130] An organic phase containing water, an organic solvent, ε-caprolactam, and impurities is introduced into the solvent exchanger, and an ε-caprolactam-aqueous phase containing the organic solvent, water, ε-caprolactam, and impurities is discharged. The solvent exchanger for the stripping-based process is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the stripping-based apparatus is a static or agitated extraction column, such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulse column, a sieve-plate column (static), a randomly packed column (static), or a structured-packed column (static).

[0131] The solvent exchange apparatus for the process based on solvent switching distillation is selected from a sieve tray distillation column, a randomly packed distillation column, and a structured packed distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation column can be operated at atmospheric pressure, a pressure lower than atmospheric pressure, or a pressure higher than atmospheric pressure. Preferably, water is charged to the top 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 bottom of the distillation column.

[0132] The oxidation section comprises one or more oxidation reactors operated sequentially and / or in parallel. An oxidizing agent and an ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities are introduced into the oxidation section. Typically, the oxidizing agent is introduced as a solid, a slurry, or an aqueous solution. When potassium permanganate or sodium permanganate is used as the oxidizing agent, the oxidation section also comprises a filtration section. The oxidation reactor can have any desired configuration. Preferred reactor types are stirred and unstirred reactors and packed column reactors. The oxidation reactor must be equipped with a facility for supplying the aqueous phase containing water, ε-caprolactam, and impurities with a boiling point lower or higher than that of ε-caprolactam, as well as the oxidizing agent. Furthermore, the oxidation reactor must be equipped with a facility for discharging the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities, as well as any solid manganese(IV) oxide (MnO) particles that may have formed. Preferably, the oxidation is carried out at a temperature in the range of 20° C. to 85° C. and under atmospheric conditions.

[0133] Solid manganese(IV) oxide (MnO2) particles, if present, can be removed by settling or solid-liquid filtration, preferably solid-liquid filtration. It is common practice to use filter aids, such as activated carbon particles or diatomaceous earth, to improve the filtration procedure. Filter systems suitable for separating solid manganese(IV) oxide particles are known to those skilled in the art. Such a filter system is fed with a suspension of an oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities, and solid manganese(IV) oxide particles, and the filtered oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities is discharged. Generally, the solid manganese(IV) oxide particles are retained within the filter system. Preferably, such a filter system is operated in a semi-continuous mode, with the suspension and filtered phase continuously fed and discharged, while the separated solids are collected in the filter system. From time to time, the feeding of the suspension is interrupted, and the collected solids are removed from the filter system.

[0134] An ε-caprolactam-water phase containing water, ε-caprolactam, and impurities is input to the distillation apparatus, and high-purity ε-caprolactam, water, and impurities (i.e., low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) and high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam)) are discharged. The distillation apparatus is selected from a sieve tray distillation column, a randomly packed distillation column, a structured packed distillation column, and horizontal and vertical (falling and rising) film evaporators. Preferably, the distillation column is equipped with a reboiler, a condenser, and a device for reflux. The distillation apparatus can be operated at atmospheric pressure, a pressure lower than atmospheric pressure, or a pressure higher than atmospheric pressure, preferably a pressure lower than atmospheric pressure.

[0135] Preferably, the distillation comprises the separation of water, low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam), and / or high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises, in a first step, separating water as an overhead product and producing ε-caprolactam containing low-boiling and high-boiling impurities as a bottom product. In a second step, the low-boiling impurities are separated as an overhead product, and ε-caprolactam containing high-boiling impurities is obtained as a bottom product. In a third step, high-purity ε-caprolactam is separated as an overhead product, and a distillation residue containing ε-caprolactam and high-boiling impurities is produced as a bottom product. Optionally, the first and second steps are integrated.

[0136] Preferably, prior to the distillative removal of water and impurities, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities. Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol per kg of ε-caprolactam, more preferably from 2 to 80 mmol per kg of ε-caprolactam. This results in particularly effective distillative removal of impurities with boiling points lower and higher than ε-caprolactam in the subsequent distillation.

[0137] The process of the present invention can be operated continuously, semi-continuously, or in a batch mode. Accordingly, the plant of the present invention can also be configured to enable one or more of these operating modes. In a preferred embodiment, the plant is configured to operate the process of the present invention in a continuous or semi-continuous manner. However, discontinuous processes are also possible. For example, the plant of the present invention need not contain all of the sections described herein in a single location. In some cases, sections, or portions thereof, are located in two or more locations. In particular, the pretreatment section [A] can be located in a first location, while the depolymerization section [B], the recovery section [C], and the purification section [D] can be located in a second location. Similarly, the mechanical size reduction section [β] as part of the pretreatment section [A] can also be located in a first location, while the cleaning section [α] as part of the pretreatment section [A] can be located in a second location, while the depolymerization section [B], the recovery section [C], and the purification section [D] are located in a third location. In some cases, the cleaning section [α] is divided into two or more segments, optionally all located in different locations. For example, the first segment of the cleaning section [α] as part of the pretreatment section [A] can be located at a first position, the mechanical size reduction section [β] as part of the pretreatment section [A] can be located at a second position, while the second segment of the cleaning section [α] as part of the pretreatment section [A] can be located at a third position, while the depolymerization section [B], recovery section [C] and purification section [D] are located at a fourth position. The densification section [γ] can be located at the same position as one or more other segments of the pretreatment section [A] or can be located at the same position as the depolymerization section [B].

[0138] product The present invention provides a new product: ε-caprolactam obtained through the depolymerization of polyamide 6, produced from polyamide 6-containing fishing net material according to the method of the present invention. This ε-caprolactam is advantageously characterized, in particular, by having a product carbon footprint of less than 2 kg of CO2 equivalents per kg of purified ε-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). The ε-caprolactam obtained according to the present invention may also be referred to as "purified ε-caprolactam." As used herein, "purified" means that the ε-caprolactam is produced from polyamide 6-containing fishing net material according to the method of the present invention, resulting in the ε-caprolactam being obtained in purified form. In this sense, the ε-caprolactam is obtained and purified from polyamide 6-containing fishing net material.

[0139] The process of the present invention makes it possible to produce highly pure and therefore high-quality ε-caprolactam that meets the specifications for demanding applications, while at the same time the process is particularly economically friendly due to its reduced product carbon footprint and the use of waste as starting material. In preferred embodiments, the ε-caprolactam obtained by the process of the present invention meets one or more of the following specifications, the parameters and measurement methods being defined as in the Examples section hereinbelow: PAN: max 5 E290: Maximum 0.05 VB: max. 0.5mmol / kg Alkalinity: Max 0.1mmol / kg.

[0140] The ε-caprolactam produced by the method of the present invention is also particularly economical and environmentally friendly, as evidenced by its much lower carbon footprint compared to classically produced ε-caprolactam (e.g., by Beckmann rearrangement of cyclohexanone oxime).

[0141] The environmental impact of a product is commonly expressed as its "product carbon footprint." The carbon footprint of a product is defined as the total emissions caused by the formation of that product, expressed as tons of carbon dioxide equivalent per ton of product. The carbon footprint of a product depends, among other things, on raw materials, auxiliary materials, energy consumption, energy source, production process, and process efficiency. Quantifying a product's carbon footprint can be done, for example, as described in European Standard EN ISO 14040:2006 ("Environmental management - Life cycle assessment - Principles and framework").

[0142] The product carbon footprint calculation may be performed in-house or by an external (preferably) accredited organization that verifies and certifies the product carbon footprint calculation, for example according to the LCA standard ISO 14040.

[0143] 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 the potential global warming impact of "virgin" ε-caprolactam obtained via the Beckmann rearrangement of cyclohexanone oxime when coal-based electricity and steam generation are involved is 7.5 tons of CO2 equivalents per ton of ε-caprolactam (which is equivalent to 7.5 kg of CO2 equivalents per kg of ε-caprolactam). When natural gas-based electricity and steam generation are involved, the potential impact of virgin ε-caprolactam on global warming of the ε-caprolactam production process is reduced to 6.4 tonnes of CO2 equivalents per tonne of ε-caprolactam (which is equivalent to 6.4 kg of CO2 equivalents per kg of ε-caprolactam).

[0144] The product carbon footprint of the ε-caprolactam obtained according to the method of the present invention is much lower than that of de novo synthesized, i.e., "virgin" ε-caprolactam. The product carbon footprint of the ε-caprolactam obtained in the method of the present invention is less than 4 kg, more preferably less than 3 kg, and most preferably 2 kg or less of CO equivalents per kg of ε-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). [Brief explanation of the drawings]

[0145] The present invention will now be described with reference to the drawings which illustrate particular embodiments of the invention. However, the invention as defined in the claims and as generally described herein should not be limited to the embodiments shown for illustrative purposes in the following figures. [Figure 1] FIG. 1 is a schematic diagram of the process of the present invention, including the processing steps performed in the optional pretreatment section [A], depolymerization section [B], recovery section [C] and purification section [D]. [Figure 2] FIG. 2 illustrates two embodiments of the pre-treatment section [A], where the fishing net comprising polyamide 6 is cleaned in the cleaning section [α] by removing foreign matter and by washing with a cleaning solvent, and crushed in the mechanical size reduction section [β] to obtain cleaned and crushed pieces of fishing net comprising polyamide 6. [Figure 2A] FIG. 2A illustrates one embodiment of the pre-treatment section [A], where the fishing net comprising polyamide 6 is first cleaned in the cleaning section [α] by removing foreign matter and by washing with a cleaning solvent, and then crushed in the mechanical size reduction section [β] to obtain cleaned and crushed pieces of fishing net comprising polyamide 6. [Figure 2B]FIG. 2B illustrates one embodiment of the pre-treatment section [A], where fishing nets comprising polyamide 6 are first crushed in a mechanical size reduction section [β] and then cleaned in a cleaning section [α] by removing foreign matter and by washing with a solvent to obtain cleaned and crushed pieces of fishing nets comprising polyamide 6. [Figure 3] FIG. 3 illustrates two embodiments of the purification section [D], in which crude ε-caprolactam is purified to obtain highly purified ε-caprolactam. [Figure 3A] FIG. 3A illustrates one embodiment of the purification section [D] of the process of the present invention, comprising an extraction section [γ], an optional wash section [δ], a back-extraction section [ε], an optional oxidation section [θ], and a distillation section [λ]. [Figure 3B] FIG. 3B illustrates one embodiment of the purification section [D] of the process of the present invention, comprising an extraction section [γ], an optional wash section [δ], a solvent-switching distillation section [μ], an optional oxidation section [θ], and a distillation section [λ].

[0146] [Detailed description of the drawings] The process of the present invention is illustrated schematically in Figure 1. The process is carried out in the following plant sections:

[0147] Optionally, the polyamide 6-containing fishing net [1] is cleaned in a pre-treatment section [A] by removing foreign matter and by washing with a washing solvent [2], thereby obtaining a contaminated washing solvent [3]. The polyamide 6-containing fishing net is then crushed by mechanical size reduction. The cleaned and crushed polyamide 6-containing fishing net [6] is discharged from the pre-treatment section [A]. Optionally, the polyamide 6-containing fishing net [1] is further cleaned in the pre-treatment section [A] by removing foreign matter. The removal of foreign matter can occur before and / or after crushing the polyamide 6-containing fishing net. Optionally, the cleaned and crushed nylon 6-containing fishing net is densified before being depolymerized into ε-caprolactam in a depolymerization section [B] (not shown in FIG. 1).

[0148] The optionally cleaned and shredded polyamide 6-containing fishing nets [6] are depolymerized to ε-caprolactam in the depolymerization section [B]. A stream [7] containing ε-caprolactam is discharged from the depolymerization section [B]. Additionally, residual material [8] is discharged. Optionally, superheated steam [9] and a catalyst

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

[0149] Crude ε-caprolactam

[11] is recovered in the recovery section [C] from the ε-caprolactam-containing stream [7] discharged from the depolymerization section [B]. Furthermore, if water or superheated steam [9] is introduced into the depolymerization section [B], an aqueous phase

[12] is discharged from the recovery section [C].

[0150] The crude ε-caprolactam

[11] discharged from the recovery section [C] is purified in the purification section [D] to obtain high-purity ε-caprolactam

[26] . Water and impurities

[25] are also discharged from the purification section [D].

[0151] 2A illustrates one embodiment of a pretreatment section [A'] (area enclosed by a dashed line), in which a polyamide 6-containing fishing net [1'] is first cleaned in a cleaning section [α'] by removing foreign matter and washing with a washing solvent [2'], thereby obtaining a fishing net containing foreign matter, contaminated washing solvent [3'], and a cleaned polyamide 6-containing fishing net [4']. The cleaned polyamide 6-containing fishing net [4'] is then crushed in a mechanical size reduction section [β'] to obtain cleaned and crushed pieces of polyamide 6-containing fishing net [6']. The cleaned and crushed pieces are then discharged. Optionally, the cleaned and crushed nylon 6-containing fishing net is densified (not shown in FIG. 2A ) before being depolymerized into ε-caprolactam in a depolymerization section [B].

[0152] FIG. 2B illustrates one embodiment of the pretreatment section [A″] (area enclosed by a dashed line), in which a polyamide 6-comprising fishing net [1″] is first crushed in a mechanical size reduction section [β″] to obtain crushed pieces of polyamide 6-comprising fishing net [5″]. The crushed pieces of polyamide 6-comprising fishing net [5″] are subsequently cleaned in a cleaning section [α″] by removing foreign matter and washing with a washing solvent [2″] to obtain foreign matter, contaminated washing solvent [3″], and discharged cleaned and crushed pieces of polyamide 6-comprising fishing net “6″”. Optionally, the cleaned and crushed nylon 6-comprising fishing net is densified (not shown in FIG. 2B ) before being depolymerized to ε-caprolactam in a depolymerization section [B].

[0153] FIG. 3A illustrates one embodiment of the purification section [D′″] (area enclosed by dashed lines) comprising the following sections:

[0154] In the extraction section [γ'''], the 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'''].

[0155] In the optional washing section [δ'''], the organic phase [15'''] containing the organic solvent, ε-caprolactam and impurities is washed with water or an aqueous alkaline solution [16'''] to obtain an aqueous residue-containing phase [17'''] and a washed organic phase [18'''] containing the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ'''].

[0156] In the back-extraction section [ε'''], the optionally washed organic phase [18'''] containing organic solvent, ε-caprolactam, and impurities 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 (not shown in Figure 3A).

[0157] In the optional oxidation section [θ'''], the aqueous phase [22'''] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, from which any residual organic solvents present have been removed by stripping and / or distillation, is oxidized with an oxidizing agent [23'''] to obtain an oxidized ε-caprolactam-aqueous phase [24'''] containing water, ε-caprolactam, and impurities. This phase is discharged from the oxidation section [θ''']. Optionally, the oxidized ε-caprolactam-aqueous phase [24'''] containing water, ε-caprolactam, and impurities is filtered to remove solid manganese(IV) oxide particles (not shown in Figure 3A) before being discharged from the oxidation section [θ'''].

[0158] In distillation section [λ'''], the optionally oxidized ε-caprolactam-aqueous phase [24'''] containing water, ε-caprolactam, and impurities, from which solid manganese(IV) oxide particles have optionally been removed by filtration, is distilled to obtain water and impurities [25'''] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high-purity ε-caprolactam [26''']. All of the distillation products are discharged from distillation section [λ''']. Optionally, prior to distillation in distillation section [λ'''], an alkali metal hydroxide is administered to the optionally oxidized ε-caprolactam-aqueous phase [24'''] containing water, ε-caprolactam, and impurities (not shown in Figure 3A).

[0159] FIG. 3B illustrates one embodiment of the purification section [D''''] (area enclosed by dashed lines) comprising the following sections:

[0160] In the extraction section [γ''''], the 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''''].

[0161] In the optional washing section [δ''''], the organic phase [15''''] containing the organic solvent, ε-caprolactam and impurities is washed with water or an aqueous alkaline solution [16''''] to obtain an aqueous residue-containing phase [17''''] and a washed organic phase [18''''] containing the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ''''].

[0162] In the solvent-switching distillation section [μ''''], the optionally washed organic phase [18''''] containing the organic solvent, ε-caprolactam and impurities is solvent-switched distilled by adding water [19''''] to obtain an aqueous phase [22''''] containing the organic solvent [21''''] and water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam. Both distillation products are discharged from the solvent-switching distillation section [μ''''].

[0163] In the optional oxidation section [θ''''], the aqueous phase [22''''] containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, from which any residual organic solvents present have been removed by stripping and / or distillation, is oxidized with an oxidizing agent [23''''] to obtain an oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam, and impurities. This phase is discharged from the oxidation section [θ'''']. Optionally, the oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam, and impurities is filtered to remove solid manganese(IV) oxide particles (not shown in Figure 3B) before being discharged from the oxidation section [θ''''].

[0164] In distillation section [λ''''], the optionally oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam, and impurities, from which solid manganese(IV) oxide particles have been removed by filtration, is distilled to obtain water and impurities [25''''] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high-purity ε-caprolactam [26'''']. All of the distillation products are discharged from distillation section [λ'''']. Optionally, prior to distillation in distillation section [λ''''], an alkali metal hydroxide is administered to the optionally oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam, and impurities that enters distillation section [λ''''] (not shown in Figure 3B). [Example]

[0165] The following examples serve to illustrate the present invention in more detail, particularly with respect to certain aspects of the invention, but are not intended to limit the disclosure.

[0166] ε-Caprolactam that can be used for all major polyamide 6 polymerization applications without dilution with purer qualities of ε-Caprolactam meets all of the following specifications: PAN: max 5 E290: Maximum 0.05 VB: max. 0.5mmol / kg Alkalinity: Max 0.1mmol / kg. The parameters and measurement methods are defined as follows:

[0167] PAN:ISO DIS 8660 Industrial caprolactam - Determination of the permanganate index of caprolactam - Spectroscopic method, revision of 1st edition ISO 8660; 1988, E290: ISO 7059 - Industrial caprolactam - Determination of absorbance at a wavelength of 290 nm, 1982 Volatile bases (VB) ISO 8661 - Industrial caprolactam - Determination of the volatile base content - Titrimetric method after distillation, 1988.

[0168] Alkalinity of ε-caprolactam product: Alkalinity is gray at its end point, 0.1 wt / v in a 1:2 ratio エタノール % Methylene Blue: 0.1 wt / v エタノール It is determined by titration at a temperature of 25°C using Tashiro indicator with % methyl red. First, a flask containing water and indicator is titrated to a gray color, then X grams of an aqueous solution of ε-caprolactam containing Y weight percent ε-caprolactam (determined by refractive index) is added and the solution is back-titrated with 0.01N H2SO4 solution to a gray color. The alkalinity is then given by: Alkalinity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y) During the ceremony, v = volume of H2SO4 solution added (ml) t = normality of H2SO4 solution (=0.01N) X = sample weight (g) Y = concentration of ε-caprolactam (wt%)

[0169] The polyamide 6-containing pellets used in the examples and comparative experiments were prepared from discarded fishing nets. Pretreatment included removal of foreign matter, washing, crushing, melting, and conversion into chips / pellets. The pellets were obtained from a fishing net recycling company in China. The pellets were rod-shaped, with an average diameter of about 3 mm and an average length of about 4 mm, and most pellets weighed 20 to 30 mg.

[0170] Thermogravimetric analysis (TGA) combined with qualitative information from differential scanning calorimetry (DSC) revealed that the polyamide 6 content of the pellets was greater than 98 wt% (on a dry basis).

[0171] [Example 1] Depolymerization of polyamide 6 and recovery of ε-caprolactam 48 grams of chips / pellets containing polyamide 6 and 14 grams of 20 wt% phosphoric acid were charged into a Premex high-pressure autoclave. The reactor contents were first heated under nitrogen, followed by continuous injection of superheated steam at a rate of 4 grams / minute during a 120-minute reaction. 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 discharged from the reactor and cooled to approximately 20°C, yielding a condensate containing ε-caprolactam and water.

[0172] The condensate, consisting of about 44 grams of ε-caprolactam, with the remainder mostly water, was concentrated to an ε-caprolactam concentration of 49.7 wt. % by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was about 65° C.). (This mixture, crude ε-caprolactam, is the mixture to be purified.) The specifications of the crude ε-caprolactam were as follows: PAN:16 E290:2.33

[0173] This example demonstrates that crude ε-caprolactam can be obtained by depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6. Due to its extremely low quality, this crude ε-caprolactam cannot be directly used in all major polyamide 6 polymerization applications.

[0174] [Comparative Experiment 1] Depolymerization of polyamide 6, recovery of ε-caprolactam, and distillation purification The procedure of Example 1 was followed. 75 mmol of aqueous sodium hydroxide solution per kg of ε-caprolactam was then added. Water and impurities with a boiling point lower than ε-caprolactam were then removed as the top product by distillation under reduced pressure in a distillation apparatus operated batchwise, and the pressure was reduced in stages. ε-caprolactam was distilled at 300 Pa, while impurities with a boiling point higher than ε-caprolactam remained in the distillation apparatus as the bottom product. The specifications of the distilled ε-caprolactam were as follows: PAN:5 E290:0.09 VB: 0.34 mmol / kg Alkalinity: 0.25mmol / kg.

[0175] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of polyamide 6, derived from discarded fishing nets containing polyamide 6 and purified by distillation, is extremely low, as it does not meet most of the specifications required for major polymerization applications.

[0176] [Comparative Experiment 2] Depolymerization of polyamide 6, recovery of ε-caprolactam, and purification by oxidation The procedure of Example 1 was followed. The crude ε-caprolactam was then treated with 0.2 wt. % KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solids formed were then removed from the oxidized reaction product by filtration. The specifications of the purified ε-caprolactam were as follows: PAN:30 E290:3.62

[0177] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by oxidation is too low to be used directly in all major polyamide 6 polymerization applications.

[0178] [Comparative Experiment 3] Depolymerization of polyamide 6, recovery of ε-caprolactam, and purification by oxidation and distillation The aqueous solution of ε-caprolactam purified by oxidation obtained in Comparative Experiment 2 was then distilled, after addition of 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam, according to the procedure described in Comparative Experiment 1. The specifications of the obtained purified ε-caprolactam were as follows: PAN:3 E290:0.09 VB: 0.84 mmol / kg Alkalinity: 0.35mmol / kg.

[0179] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of polyamide 6, derived from discarded fishing nets containing polyamide 6 and purified by oxidation and distillation, is low and cannot be used directly for all major polyamide 6 polymerization applications.

[0180] [Comparative Experiment 4] Depolymerization of polyamide 6, recovery of ε-caprolactam, and purification by oxidation, carbon treatment, and distillation The procedure of Example 1 was followed. Subsequently, the crude ε-caprolactam was 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 formed solids and activated carbon particles were then removed from the aqueous ε-caprolactam solution by filtration. This aqueous ε-caprolactam solution treated with activated carbon was then distilled according to the procedure described in Comparative Experiment 1 after adding 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam. The specifications of the resulting purified ε-caprolactam were as follows: PAN:3 E290:0.08 VB: 0.43 mmol / kg Alkalinity: 0.29mmol / kg.

[0181] This comparative experiment shows that the ε-caprolactam obtained from the depolymerization of polyamide 6, derived from discarded fishing nets containing polyamide 6 and purified by oxidation, carbon treatment, and distillation, is of low quality and cannot be used directly for all major polyamide 6 polymerization applications.

[0182] [Example 2] Depolymerization of polyamide 6, recovery of ε-caprolactam and purification by extraction, stripping, oxidation and distillation. The procedure of Example 1 was followed. The condensate, consisting of 44.5 grams of ε-caprolactam, with the remainder mostly water, was concentrated to an ε-caprolactam concentration of 50.2 wt. % by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was about 65° C.). (This mixture, crude ε-caprolactam, is the mixture to be purified.)

[0183] Crude ε-caprolactam was extracted ten times in a batchwise manner with a solvent mixture of 4-methyl-2-pentanol (50 wt%) / cyclohexane (50 wt%) at a temperature of about 25°C. The total amount of extraction solvent used was 8.05 grams of 4-methyl-2-pentanol / cyclohexane per gram of crude ε-caprolactam. The combined organic extracts were washed batchwise with 7 grams of 2 wt% aqueous NaOH solution. The resulting washed organic extract was concentrated to an ε-caprolactam concentration of about 40 wt% by distillation under vacuum, and then fresh cyclohexane was added. The ε-caprolactam concentration of the resulting mixture was about 27 wt%, and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50 wt%:50 wt%. This mixture was extracted seven times in a batchwise manner with water at a temperature of about 25°C. The total amount of water used was 5.75 grams of water per gram of ε-caprolactam recovered.

[0184] The resulting aqueous ε-caprolactam solution was then concentrated to an ε-caprolactam concentration of 45.9 wt. % by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was approximately 65°C). The resulting mixture was treated with 0.04 wt. % KMnO4 with respect to ε-caprolactam at 50°C for 2 hours. The formed solids were then removed from the oxidized reaction product by filtration. The ε-caprolactam in the resulting aqueous solution was further purified by distillation as described in Comparative Experiment 1 after the addition of 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam. The specifications of the resulting purified ε-caprolactam were as follows: PAN:1.5 E290:0.032 VB: 0.082 mmol / kg Alkalinity: 0.1mmol / kg.

[0185] From this experiment, it can be concluded that the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by extraction, stripping, oxidation, and distillation can yield purified ε-caprolactam that meets all the specifications required for major polymerization applications.

[0186] [Example 3] Depolymerization of polyamide 6, recovery of ε-caprolactam and purification by extraction, stripping and distillation. The procedure of Example 1 was followed. The resulting aqueous ε-caprolactam solution was concentrated to an ε-caprolactam concentration of 50.7 wt. % by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was approximately 65°C). (This mixture, crude ε-caprolactam, is the mixture to be purified.)

[0187] The crude ε-caprolactam was extracted 11 times with toluene in a batchwise manner at a temperature of about 25°C. The total amount of extraction solvent used was 12.5 grams of toluene per gram of crude ε-caprolactam. The combined organic extracts were concentrated to an ε-caprolactam concentration of 30% by weight by distillation under vacuum. Subsequently, the resulting concentrated organic extracts were extracted four times with water in a batchwise manner at a temperature of about 25°C. The total amount of water used was 1.79 grams of water per gram of combined organic extract. The resulting combined aqueous ε-caprolactam solution was concentrated to an ε-caprolactam concentration of 52.2% by weight by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was about 65°C).

[0188] The resulting ε-caprolactam in aqueous solution was further purified by distillation after addition of 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam as described in Comparative Experiment 1. The specifications of the resulting purified ε-caprolactam were as follows: PAN:4 E290:0.04 VB: 0.2 mmol / kg Alkalinity: 0.1mmol / kg.

[0189] From this experiment, it can be concluded that the depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by extraction, stripping, and distillation can yield purified ε-caprolactam that meets all the specifications required for major polymerization applications.

[0190] [Example 4] Depolymerization of polyamide 6, recovery of ε-caprolactam and purification by extraction, stripping and distillation. The procedure of Example 1 was followed twice. Both resulting aqueous ε-caprolactam solutions were concentrated to ε-caprolactam concentrations of 70.2 wt. % and 67.6 wt. %, respectively, by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was approximately 65° C.). Then, both concentrated aqueous ε-caprolactam solutions were added together. One-third of the resulting mixture, i.e., crude ε-caprolactam, was used for further purification.

[0191] The crude ε-caprolactam was diluted to 65.0 wt % and extracted five times with benzene in a batchwise manner at a temperature of about 25°C. The total amount of extraction solvent used was 10.7 grams of benzene per gram of crude ε-caprolactam. The combined organic extracts were washed batchwise with 3.1 grams of 2 wt % aqueous NaOH solution. The resulting washed organic extracts were concentrated to an ε-caprolactam concentration of about 17 wt % by distillation under vacuum. Subsequently, the resulting concentrated organic extracts were extracted four times with water in a batchwise manner at a temperature of about 25°C. The total amount of water used was 1.19 grams per gram of concentrated combined organic extracts. The resulting combined aqueous ε-caprolactam solution was concentrated to an ε-caprolactam concentration of 56.3 wt % by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature was about 65°C).

[0192] The resulting ε-caprolactam in aqueous solution was further purified by distillation after addition of 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam as described in Comparative Experiment 1. The specifications of the resulting purified ε-caprolactam were as follows: PAN:3 E290:0.01 VB:<0.02mmol / kg Alkalinity: 0.1mmol / kg.

[0193] From this example, it can be concluded that depolymerization of polyamide 6 derived from discarded fishing nets containing polyamide 6 and purified by extraction, stripping and distillation can yield purified ε-caprolactam that meets all the specifications required for major polymerization applications.

[0194] [Example 5] Calculation of the carbon footprint of purified ε-caprolactam In a plant, a continuous process according to the invention for producing purified ε-caprolactam from fishing nets containing polyamide 6 was simulated. - mechanical removal of foreign objects from fishing nets containing polyamide 6; -Cutting fishing nets containing polyamide 6 into small strips; Washing of fishing net strips containing polyamide 6 with water; - Separation by centrifugation of washed pieces of fishing nets and aqueous extracts containing polyamide 6; - Drying of washed strips of fishing nets containing polyamide 6; - Melting and pelletizing of washed pieces of fishing nets containing polyamide 6; - Depolymerization of polyamide 6 under the influence of H3PO4 and superheated water vapor; - recovery of crude ε-caprolactam (75% by weight of ε-caprolactam) by partial condensation of the vapors discharged from the depolymerization reactor; -Countercurrent extraction of concentrated crude ε-caprolactam with toluene; -Washing of organic extracts with diluted caustic solutions; -Countercurrent back-extraction of the washed organic extract with water; -Evaporative concentration of aqueous extracts; -Oxidation of concentrated aqueous extract with KMnO4; - Addition of caustic substances; and -Recovery of pure ε-caprolactam by vacuum distillation It included.

[0195] Based on the figures for raw material consumption, the carbon footprint of purified ε-caprolactam was calculated and the utility of the above method is based on data derived from ecoinvent version 3.7.1.

[0196] The results revealed that the product carbon footprint of purified ε-caprolactam obtained from fishing nets containing polyamide 6 was less than 2.0 tonnes of CO2 equivalent per tonne of ε-caprolactam (location Europe).

[0197] While 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 can be made therein without departing from its spirit and scope, including (semi-)continuous operation and scale-up to industrial scale.

Claims

1. 1. A method for recovering purified ε-caprolactam from a material derived from fishing nets comprising polyamide 6 in a plant, said plant comprising: - depolymerization section [B], - Recovery section [C], and - Purification section [D] Equipped with The method comprises: a) introducing a material derived from a fishing net containing polyamide 6 into the depolymerization section [B]; b) depolymerizing the material derived from fishing nets comprising polyamide 6 in the depolymerization section [B] at a temperature ranging from 180°C to 400°C so as to obtain a stream comprising ε-caprolactam; c) discharging a stream containing the ε-caprolactam from the depolymerization section [B] and recovering crude ε-caprolactam from the stream in the recovery section [C]; d) purifying the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises: (i) extracting the partially purified ε-caprolactam with an organic solvent to obtain an organic phase, the organic phase containing the organic solvent, ε-caprolactam, and impurities; (ii) exchanging the organic solvent by at least partially replacing it with water, resulting in an aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam, the solvent exchange step (ii) being selected from a method based on back-extraction with water and a method based on solvent-switching distillation, in which the organic solvent is distilled off and water is introduced; (iii) obtaining purified ε-caprolactam by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam from the aqueous phase; Including, obtaining purified ε-caprolactam; Including, - after step d)(i), the organic phase obtained in step d)(i) is washed with water or an aqueous alkaline solution, and - before the distillative removal in step d)(iii), an alkali metal hydroxide is added to the aqueous phase, method.

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

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

4. Prior to the distillative removal in step d)(iii), said purification in step d) comprises 4. The method of any one of claims 1 to 3, also comprising the step of oxidizing with an oxidizing agent in an aqueous solution at a temperature in the range of 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.

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

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

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

8. 4. The method according to any one of claims 1 to 3, wherein the solvent exchange in step d)(ii) is a method based on solvent-switching distillation.

9. 2. The method of claim 1, wherein the organic solvent in step d)(i) is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof.

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

5.

11. The depolymerization in step b) is carried out in the absence or presence of a catalyst, and the catalyst is selected from acid catalysts and base catalysts, and the acid catalyst is selected from orthophosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, salts of the aforementioned acids, Al ２ O ３ and SiO ２ and combinations thereof, wherein the base catalyst is selected from the group consisting of alkali hydroxides, alkali salts, alkaline earth hydroxides and alkaline earth salts, organic bases and solid bases, and combinations thereof.

12. 12. The method of claim 11, wherein the acid catalyst is orthophosphoric acid.

13. 12. The method of claim 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.

14. 4. The process according to claim 1, wherein the depolymerization in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

15. 4. The method according to any one of claims 1 to 3, wherein prior to step a), said material from fishing nets comprising polyamide 6 is obtained as a product of a pretreatment in a pretreatment section [A].

16. 16. The method according to claim 15, wherein the pre-treatment in the pre-treatment section [A] is cleaning in a cleaning section [α], and / or mechanical size reduction in a mechanical size reduction section [β], and / or increasing the bulk density in a densification section [γ].

Images