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 man-made 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 of artificial fibres often have an extremely low (bio)degradation rate. These nets therefore 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 getting into 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 fate of recovered waste fishing nets, including those containing polyamide 6, ranges from landfilling, incineration (possibly with heat recovery), regranulation and kneading to depolymerization. Regranulation and kneading is a recycling process in which waste plastic is melted (possibly with subsequent 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 monomeric components (ε-caprolactam in the case of the polymer being polyamide 6).

[0007] Mechanical recycling (also known as materials recycling or back-to-plastics recycling) refers to operations that aim to recover plastics by mechanical processes (crushing, washing, separating, drying, regranulating and kneading), thus generating recyclable materials that can be converted into plastic products and substitute for virgin plastics. Currently, most virgin plastics are produced from petrochemical feedstocks such as natural gas, coal or crude oil, which have never been used or processed before. In mechanical recycling processes, the polymer chains remain more or less intact. Mechanical recycling is a form of waste downcycling, since the quality and functionality of the recycled material is inferior 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 of applications. Virgin monomers are produced from petrochemical feedstocks such as natural gas, coal or crude oil that have never been used or processed before.

[0009] In 1938, Paul Schlack invented polyamide 6 (CAS 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 the ring-opening polymerization of ε-caprolactam in an inert atmosphere at a temperature of about 260° C.: [ka]

[0011] Methods for the production of 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 the preparation of 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)introduction of polyamide 6 into the depolymerization reactor) or in a continuous mode.

[0015] LADmitrieva, AA Speranskii, SAKrasavin and YNBychkov, “Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns”, Fibre Chemistry, pp. 229-241, March 1986, (translated from Khimicheskie Volokna, No. 4, pp. 5-12, July-August, 1985) is a literature review describing methods for depolymerizing polyamide 6 with and without 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 an article 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 in the absence of added catalyst with superheated steam at a temperature of about 250° C. to about 400° C. and at a pressure in the range of about 1 atm to about 100 atm and substantially 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 recycling of polyamide 6, the quality of the monomer ε-caprolactam obtained by these methods was particularly low. As a result, ε-caprolactam obtained by depolymerization of fishing nets containing polyamide 6 is only applied in less demanding applications such as engineering plastics and carpets (downcycling). If ε-caprolactam obtained from depolymerization of fishing nets is to be used in more demanding applications, it needs to be mixed with larger amounts of higher and purer grades of ε-caprolactam to hide the rather inferior quality of ε-caprolactam obtained from depolymerization of fishing nets. High-speed melt spinning of polyamide 6 to produce fine textile fibers requires high-quality ε-caprolactam as a raw material. Not only are high-quality ε-caprolactam grades for these applications extremely pure, but their properties should not change over time.

[0019] Taken together, prior art processes 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 methods are available for recovering high purity ε-caprolactam from fishing nets containing polyamide 6, despite the pressing need for such methods. There is an urgent need for high purity ε-caprolactam recovery methods that can replace virgin ε-caprolactam grades, especially for demanding applications such as high speed melt spinning during textile fiber production.

[0021] Furthermore, there is a need for a process that allows for the recovery of high purity ε-caprolactam from fishing nets containing polyamide 6 in an economically rational 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 processes that use virgin ε-caprolactam obtained by de novo synthesis, for example, by Beckmann rearrangement of cyclohexanone oxime. Additionally, there is a need for plants to produce high purity grades of ε-caprolactam from fishing net derived materials that contain polyamide 6.

[0023] Finally, in order to process the enormous amounts of polyamide 6-containing fishing nets that are discarded every year, there is a need for a method that allows the recovery of ε-caprolactam from polyamide 6-containing fishing nets 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 shortcomings 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, which can replace high purity virgin ε-caprolactam for all applications, including high speed melt spinning of polyamide 6 for the production of fine textile fibers.

[0026] It is a further object of the present invention to provide a process for the recovery of 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 process for recovering high purity grade ε-caprolactam in an economically responsible manner from fishing nets comprising polyamide 6. In particular, it is an object of the present invention to provide a process suitable for recovering high purity grade ε-caprolactam from fishing nets comprising polyamide 6, which 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 comprising 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, the present invention also aims to provide a method for reducing the environmental load of discarded fishing nets containing polyamide 6. It is also an object of the present invention to provide a plant for the production of high purity grade ε-caprolactam from material derived from fishing nets that contains 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 comprising 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 comprising polyamide 6 into said depolymerization section [B]; b) depolymerizing the material from fishing nets comprising polyamide 6 in the depolymerization section [B] at a temperature in the range of 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, the purification comprising: (i) extracting the crude ε-caprolactam with an organic solvent to obtain an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities; (ii) exchanging the organic solvent by at least partially replacing it with water, whereby an aqueous phase is obtained comprising water, ε-caprolactam and impurities having 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, whereby 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 combination of a particular sequence of process steps and process conditions according to the invention, i.e. the sequence of the depolymerization, recovery and purification steps described above, makes it possible to recover high grade ε-caprolactam in high yield and in a simple and economically rational manner from materials derived from fishing nets containing polyamide 6. The inventive process is economically rational and advantageous from several points of view. Firstly, the inventive process is suitable for a great variety of materials, e.g. from fishing net materials containing polyamide 6, which may differ in overall composition and / or polyamide 6 content. Secondly, the inventive process allows for an effective separation of ε-caprolactam from non-ε-caprolactam compounds, so that a high purity grade ε-caprolactam can be obtained, which can replace high purity virgin ε-caprolactam for all applications, including high speed melt spinning of polyamide 6 for the production of fine textile fibers. Thirdly, the inventive process is so effective that it allows for a high yield of ε-caprolactam to be obtained. Fourthly, the method of the present invention allows the recovery of ε-caprolactam on an industrial scale from fishing nets containing polyamide 6 in order to process the enormous quantities of fishing nets containing polyamide 6 that are currently discarded. Finally, the method of the present invention allows the production of ε-caprolactam by de novo synthesis of ε-caprolactam with a significantly lower carbon footprint compared to ε-caprolactam produced, for example, by Beckmann rearrangement of cyclohexanone oxime. The method of the present invention allows the efficient processing of material from fishing nets containing polyamide 6 and the reduction of the environmental burden of said products. In particular, the method of the present invention allows 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 derived from ecoinvent version 3.7.1; location: Europe).Product carbon footprint values ​​given 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 inventive method, the present invention also relates to a plant for producing purified ε-caprolactam from fishing nets containing polyamide 6, said plant comprising: Depolymerization section [B]; Collection section [C]; Purification Section [D] and Equipped with There is also provided a plant, said plant being configured to carry out the method of the invention.

[0034] The 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 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 forth in the dependent claims and are explained in more detail below.

[0035] Fishing nets containing polyamide 6 The method of the present invention uses a fishing net comprising polyamide 6 or a material derived therefrom as a starting material. A fishing net comprising polyamide 6 is typically a solid material, and in particular, a fishing net comprising polyamide 6 is usually a mesh-like structure formed by knotting together relatively thin threads comprising polyamide 6. A material derived from a fishing net comprising polyamide 6 as used herein means a material derived from a fishing net comprising polyamide 6, for example after crushing, washing, sorting, compression, pelleting, etc. A material derived from a fishing net comprising polyamide 6 as used herein also includes a fishing net comprising polyamide 6 that has not been pretreated, for example by crushing, washing, sorting, compression, pelleting, 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 "material derived from a fishing net comprising polyamide 6". As used in this disclosure and the claims, the singular forms "a," "an," and "the" include the plural forms, specifically meaning "one or more," unless the context clearly dictates otherwise.

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

[0037] Surfaces immersed in seawater are rapidly covered with marine organisms, termed marine biofouling or biofouling. Biofouling communities are a complex phenomenon resulting from several processes, the rate and extent of which is influenced by a multitude of physical, chemical and biological factors in close proximity to the surface, affecting most wetted surfaces and resulting 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, so cleaning and reapplication of paint is required frequently. In addition to economic problems, leaching of the copper or other biocides causes contamination of seawater and problems for non-target organisms.

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

[0040] Possible pre-treatment steps Before being subjected to step a) of the method of the present invention, the material from fishing nets containing polyamide 6 is preferably subjected to a pretreatment in a pretreatment section [A], in particular a size reduction in a mechanical size reduction section [β] and / or cleaning in a cleaning section [α]. This has the advantage that the material from fishing nets containing polyamide 6 entering 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 fishing nets containing polyamide 6 can be handled more easily.

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

[0042] (Discarded) polyamide 6-containing fishing nets can be mixed with all kinds of other materials such as rocks, metal materials (e.g. wires, chains, anchors), organic materials (e.g. dead fish and mussels) and other (marine) debris (e.g. ropes, (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 more than 200,000 square meters. Such huge nets, for example purse seines and nets set vertically in the water with a float attached to the top edge, a weight attached to the bottom edge and a series of loops through which a pursing cable passes, can be as long as 1.5 km and more than 150 m deep.

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

[0045] In some cases, before the mechanical crushing or crushing of the fishing nets containing polyamide 6, large metal fragments, rocks and other interfering materials that cause significant wear of the equipment used for mechanical crushing or crushing are removed. Preferably, non-polyamide 6-containing materials, such as polyethylene, polypropylene and polyamide 6,6-containing materials, such as fishing nets, are removed before or after the mechanical crushing or crushing of the fishing nets containing polyamide 6, as described further below. The removal of foreign bodies can be done mechanically or manually. The removal of these interfering materials has the advantage that the maintenance costs of the equipment used for mechanical crushing or crushing can be significantly reduced. Furthermore, the polyamide 6 content of the material obtained after mechanical crushing or crushing is higher than if the interfering materials are not removed. In particular, the removal of polyamide 6,6 is advantageous, since polyamide 6,6 interferes with the depolymerization of polyamide 6, blocks the reactor itself, reduces the recovery yield of ε-caprolactam and interferes with the subsequent purification of the recovered ε-caprolactam.

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

[0047] In some cases, foreign objects are separated from fishing nets containing mechanically crushed or crushed polyamide 6. For this purpose, various separation processes can be applied, 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, and the less dense materials float and are separated from the more dense, sinking materials. In practice, density separation is often performed by a series of density separation stages. For example, in one stage, high density materials such as rocks, sand and metals (including iron and lead) are separated, and in another stage, low density materials such as polyolefins, polypropylene and polyethylene are separated. Magnetic separation is a process that separates the components of a mixture by attracting the magnetic materials using magnets. 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, the fishing nets comprising polyamide 6 are cleaned by washing with a solvent, preferably water, before being fed into the depolymerization section [B]. Preferably, a cleaning agent is added to the solvent in a concentration range of 0-20% by weight relative to the solvent for improved cleaning efficiency. NaOH is the preferred cleaning agent. Preferably, an aqueous solution containing 0-10% by weight NaOH, even more preferably 0-5% by weight NaOH is used in the cleaning step. Preferably, the Cu-based antifouling coating is removed by washing with an aqueous solution containing 1-5% by weight NaOH, preferably 1.5-3% by weight NaOH, more preferably about 2% by weight NaOH. The enhanced cleaning effect of NaOH is most likely caused by the 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 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 since any (adherent) dirt is removed and therefore does not interfere with the next steps of the method of the invention.

[0050] Optionally, 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 into 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 dimensions 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 pieces). However, other shapes such as (imperfect) spheres are also possible. The dimensions of the pellets can be selected within wide limits. Usually, the 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 comprising washed and size-reduced polyamide 6, has the advantage of an increased bulk density, which reduces the costs of storage and transportation of intermediates if 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 the (automated) feeding to the depolymerization section [B].

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

[0055] Feeding process a) In step a) of the present invention, the possibly size-reduced and / or washed material from fishing nets comprising polyamide 6 is fed into a 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 transport and can 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 fed into the depolymerization section [B], for example by mechanical compression or by extrusion of the molten material followed by cooling and cutting to size. Again, this has the advantage that smaller volumes are required for intermediate storage and transport, which can facilitate feeding into the depolymerization section [B].

[0058] In a further preferred embodiment, the fishing nets comprising polyamide 6 are dried before being introduced into the depolymerization section [B], in particular after subjecting the fishing nets comprising polyamide 6 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 comprising ε-caprolactam and water obtained in the depolymerization section [B] is expected to contain more impurities).

[0059] The material from fishing nets comprising polyamide 6 is preferably fed to the depolymerization reactor(s) as a solid phase or as a melt. Preferably, the material from fishing nets comprising polyamide 6 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 input of the polyamide 6-containing fishing net derived material.

[0061] Depolymerization step b) In the depolymerization section [B], the material from the fishing nets containing polyamide 6 is depolymerized to form ε-caprolactam. The formed ε-caprolactam 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] In general, the rate of ε-caprolactam formation increases at higher temperatures. Temperatures below 400 ° C are preferred, since at temperatures above 400 ° C side reactions of polyamide 6 and reactions of impurities occur more frequently, leading to the formation of a more diverse set of impurities. Some of these impurities end up in the product stream containing ε-caprolactam that is discharged from the depolymerization reactor(s). In a preferred embodiment of the invention, the depolymerization of fishing nets containing polyamide 6 is carried out at temperatures in the range of 220 ° C to 340 ° C or 240 ° C to 325 ° C. Experiments have shown that this temperature range allows the production of particularly pure ε-caprolactam.

[0064] The pressure in the depolymerization section [B] may vary and may range from 1 kPa to 100 MPa, preferably from 10 kPa to 5 MPa, more preferably from 25 kPa to 2 MPa, and most preferably from 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 material from fishing nets containing polyamide 6 can be achieved in the presence or absence of a solvent. Preferably, the depolymerization of the material from fishing nets containing polyamide 6 is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, in particular superheated steam. Preferably, the depolymerization is completed in 0.1 to 24 hours, more preferably in 0.5 to 6 hours.

[0066] By feeding water as steam to the depolymerization reactor, a vapour stream comprising 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 fed to the fishing net-derived material comprising polyamide 6 in the depolymerisation section [B]. In a preferred embodiment, the depolymerisation in step b) is carried out in the presence of water and the stream comprising ε-caprolactam is a vapour stream comprising 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, 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 oligomers of ε-caprolactam 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 a pretreatment that remain stable, react or decompose under the depolymerization conditions. Thus, in the case where water is used as the solvent, the vapor stream removed from the depolymerization section [B] contains not only water and ε-caprolactam, but also impurities.

[0069] Preferably, superheated steam having a temperature between 100°C and 600°C is fed into the depolymerization reactor(s). Preferably, the superheated steam fed into 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 into the depolymerization reactor(s) is high enough so that no other heat input is required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam. In another preferred embodiment, the depolymerization section [B] is fed with superheated steam having a temperature in the range between 220°C and 575°C. In an even more preferred embodiment, the depolymerization section [B] is fed with superheated steam having a temperature in the range between 275°C and 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] comprises the material from the fishing net containing polyamide 6 and possibly the solvent, catalyst, additional factors and / or depolymerization agents. Thus, without additional measures, there would be an accumulation of material (often called "residual material") in the depolymerization section [B]. Preferably, a separate stream is discharged from the depolymerization section [B]. This has the advantage that the accumulation of material in the depolymerization section [B] is reduced or avoided. The additional stream can comprise impurities present in the material from the fishing net containing polyamide 6, polyamide that was not depolymerized, ε-caprolactam that was not evaporated, catalyst(s), and compounds formed under the depolymerization conditions, such as mono-, di- and / or triammonium phosphate in the case where phosphoric acid is used as the depolymerization catalyst. In a preferred embodiment, a stream comprising mono-, di- and / or triammonium phosphate is discharged from the depolymerization section [B]. Even more preferably, this stream, intermittently or continuously discharged from the 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.-%, most preferably 0.5 to 5 wt.-%.

[0071] The depolymerization of the fishing net-derived material comprising polyamide 6 in the presence of water vapor may be carried out in the presence of an additional depolymerization agent such as ammonia. The concentration of ammonia in the depolymerization section [B] may vary. Thus, when ammonia is present in the depolymerization section [B], the vapor stream removed from the depolymerization section [B] may not only contain 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. The acid catalyst may in particular 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 abovementioned acids, Al2O3 and SiO2, and combinations thereof. The base catalyst may for example be selected from the group consisting of 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 hydrogen carbonate and potassium hydrogen carbonate 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 the depolymerization, and in another embodiment, p-toluenesulfonic acid is used.

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

[0074] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction starts already at lower temperatures and can be carried out under atmospheric conditions. The appropriate concentration of catalyst used for the depolymerization of polyamide 6-derived materials to ε-caprolactam is known to the skilled person and can be easily determined by routine experiments. If the concentration of catalyst used is too low, the reaction rate will be slow. Conversely, if the concentration of catalyst used is too high, the reaction will be fast but the (one or more) side reactions will also increase. Furthermore, the catalyst cost will increase, which is economically disadvantageous. Preferably, the catalyst content is 0.01-100% by weight with respect to the polyamide 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1-50% by weight. The optimal catalyst concentration depends on the type of catalyst applied for the depolymerization of polyamide 6. In the case of the catalyst orthophosphoric acid, the preferred content is 0.1-25%, more preferably 1-20% by weight. 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 the polyamide 6-containing feedstock, i.e., material from fishing nets containing polyamide 6 and optionally a catalyst, is charged to a depolymerization reactor, and the 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. the material from fishing nets containing polyamide 6 and optionally a catalyst, is first fed into the depolymerization reactor. Subsequently, superheated steam is fed into the depolymerization reactor and ε-caprolactam is discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Then, the feeding of superheated steam into the depolymerization reactor is interrupted. After optionally removing the residual material from the depolymerization reactor, a new cycle is started by feeding the raw material (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, the residual material is not removed during every cycle.

[0077] In a particular advantageous embodiment, the depolymerization of polyamide 6 is carried out in a continuous mode. In the continuous mode, the material from the feedstock containing polyamide 6 (and optionally the catalyst) is continuously fed into the depolymerization reactor. At the same time, superheated water vapor is continuously fed into the depolymerization reactor and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream containing ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently fed into the depolymerization reactor. Furthermore, the residual material is continuously discharged from the depolymerization reactor. Preferably, the material from the fishing net containing polyamide 6 is fed as a melt. Preferably, the catalyst is fed 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 the feedstock containing polyamide 6 (and optionally catalyst) are intermittently fed into the depolymerization reactor, while superheated water vapor is continuously fed into 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 of 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 comprises 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, in the case where water is fed as a solvent to the depolymerization section [B], the stream comprising ε-caprolactam discharged from the depolymerization section [B] comprises 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 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 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 since it contains impurities such as polyamide 6 decomposition products or other impurities originating from non-polyamide 6 components of the material from the fishing net containing polyamide 6. The crudely purified ε-caprolactam recovered in step c) is an aqueous solution comprising water and ε-caprolactam, preferably comprising ε-caprolactam. Thus, the crudely purified ε-caprolactam recovered in the recovery section [C] requires further purification to obtain high purity ε-caprolactam. Thus, 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 product of the process of the present invention.

[0083] Preferably, the crude 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 pure ε-caprolactam.

[0085] Optionally, the crude ε-caprolactam is filtered before being fed 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] The purified ε-caprolactam is obtained from the 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-C 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-C 10A mixed extractant consisting of an aliphatic or alicyclic alcohol and a C5-C8 alkane or a C5-C8 cycloalkane. Particularly good purification results are achieved if the organic solvent for extracting the 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 the extraction of the crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohol and mixtures thereof. Even more preferably, the organic solvent for the extraction of the crude ε-caprolactam is selected from the group consisting of toluene, 1-octanol, 4-methyl-2-pentanol, 2-ethylhexanol and mixtures thereof. Preferably, the weight ratio of the organic solvent to ε-caprolactam is from 0.01:1 to 40:1, preferably from 0.05:1 to 10:1, more preferably from 0.1:1 to 5:1.

[0088] In some cases, the organic solvent for the extraction of crude ε-caprolactam is selected from the group consisting 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 lower 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 at the top of the column and the organic solvent is introduced at the bottom 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, the weight ratio of impurities to ε-caprolactam being 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 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, the weight ratio of impurities to ε-caprolactam being 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.

[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). If washing is carried out with an aqueous alkaline solution, the alkaline solution is preferably an aqueous solution containing an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably contains 0.5-2.0 wt. % sodium hydroxide or potassium hydroxide.

[0092] A person skilled in the art can determine by routine experimentation the amount of water or aqueous alkaline solution required for efficient washing of the organic phase containing the organic solvent, ε-caprolactam and impurities. Preferably, this amount is 0.1-5% by weight relative to the amount of organic solvent, excluding ε-caprolactam, dissolved in the organic phase to be washed. In another preferred embodiment, washing of the organic phase containing the organic solvent, ε-caprolactam and impurities with water or aqueous alkaline solution is carried out in a countercurrent driven washing column, in which the organic phase containing the organic solvent, ε-caprolactam and impurities is introduced into the lower part of the column and water or aqueous alkaline solution is 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. Usually, 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 process of the present invention, the obtained organic phase comprising the organic solvent, ε-caprolactam and impurities, optionally washed with water or aqueous alkaline solution, is solvent exchanged, 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 lower or higher boiling point than ε-caprolactam, the solvent exchange process being selected from a process based on back extraction (also known as re-extraction) with water and a process based on solvent switching distillation, whereby 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 may be a process based on back extraction with water, which results in an aqueous phase containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam. Preferably, this aqueous phase containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam is stripped and / or distilled to remove residual organic solvents. The amount of water used for back extraction of ε-caprolactam may vary, but the amount of water used is preferably 0.3-20 times, more preferably 0.4-10 times, and most preferably 0.5-6 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, in which the organic phase, comprising the organic solvent, ε-caprolactam and impurities, optionally washed, has a density lower than water, the organic phase is introduced into the lower part of the extraction column and water into the upper part of the extraction column. The stripping results in an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, as well as an organic solvent phase comprising impurities. The stripping results in an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, the weight ratio of impurities to ε-caprolactam being reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase comprising the organic solvent, ε-caprolactam and impurities before stripping. Thus, purer ε-caprolactam is obtained due to stripping. Preferably, the organic solvent phase comprising 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 higher density than water, the organic phase is introduced into the upper part of the extraction column and the water into the lower part of the extraction column. The stripping results in an aqueous phase containing water, ε-caprolactam and impurities having a lower or higher boiling point than ε-caprolactam and an organic solvent phase containing impurities. The stripping results in an aqueous phase containing water, ε-caprolactam and impurities having a lower or higher boiling point than ε-caprolactam, 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, due to the 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 may also be a process based on solvent-switching distillation, whereby the organic solvent is distilled off and water is introduced. In a preferred embodiment, the solvent exchange process is a process based on solvent-switching distillation 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 introduced. 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 aim of the azeotropic distillation is to remove the organic solvent and add water. Preferably, substantially all the organic solvent is removed. "Substantially all" in this context means that at least 90% by weight, preferably at least 95% by weight, 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, the water is added as a liquid. More preferably, the water in liquid state 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 condensation of 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] The organic phase comprising 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 comprising 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 part of the organic solvent removed by distillation at the top of the distillation column is introduced into the top of the distillation column as a liquid after condensation. The remaining organic phase comprising the organic solvent, ε-caprolactam and impurities is discharged from the first stage and introduced into the second stage. Due to the distillation in the first stage, the remaining organic phase comprising the organic solvent, ε-caprolactam and impurities has a different chemical composition than the organic phase comprising 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 contains a higher percentage by weight of ε-caprolactam and compounds having a higher boiling point than ε-caprolactam, and a lower percentage by weight of compounds having a lower boiling point than ε-caprolactam.

[0102] In a second step, the remaining organic phase, which contains the organic solvent, ε-caprolactam and impurities, is distilled of the remaining organic solvent and charged with water. 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, most preferably at least 98% by weight or 99% by weight of the organic solvent present in the remaining organic phase, including the organic solvent, ε-caprolactam and impurities, is removed. Preferably, water is added as a liquid. More preferably, water in liquid state 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 a continuous mode, may be used at each stage for the solvent exchange. The distillation column may contain trays, packing, or a combination thereof.

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

[0106] 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 solvent-switching distillation.

[0107] In step d)(iii) of the process of the present invention, the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam obtained by solvent exchange in step d)(ii) is distilled in order 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 a boiling point 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, most preferably less than 10 kPa. In general, 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 comprises the separation of low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) from ε-caprolactam and / or the separation of organic high-boiling impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. The distillation preferably comprises in a first step the separation of low-boiling impurities from ε-caprolactam as an overhead product and the production of 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 comprising 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 with a boiling point lower or higher than ε-caprolactam. Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol, more preferably and most 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 a particularly effective distillative removal of impurities with a boiling point lower and higher than 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 may be added as a solid, or as a slurry, or in the form of an aqueous solution to the aqueous phase containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam, so that a dilute aqueous solution is obtained. The skilled person can determine by routine experimentation the amount of oxidizing agent required for efficient oxidation of the aqueous phase containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam. The exact amount of oxidizing agent depends, inter alia, very largely on the composition of the material from waste fishing nets containing polyamide 6 used as raw material in the process of the invention. Preferably, the amount of oxidizing agent is 0.01 to 5% by weight 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 with a boiling point lower or higher than ε-caprolactam with an oxidizing agent, prior to the 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., the oxidizing agent being 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 the oxidizing agent 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 the 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 ε-caprolactam, used for oxidation with the oxidizing agent, can vary. Preferably, the aqueous solution used for oxidation comprises a weight-to-weight ratio of ε-caprolactam to water of 5:1 to 1:5, more preferably 3:1 to 1:3, 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 either by adding water or by removing water.

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

[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], adapted to carry out the above-mentioned 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, the plant is suitable for carrying out the method of the present invention, and it is to be understood that what is described in relation to the method of the present invention applies equally to the embodiments of the plant.

[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 tonnes / year of ε-caprolactam (i.e., can in principle produce said amounts of ε-caprolactam).

[0120] The plant of the invention is suitable for the production of 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 thus the plant, are adapted to carry out the process of the invention as described above.

[0121] Further, the plant of the present invention may comprise a pre-treatment section [A], which may comprise a mechanical size reduction section [β] for breaking the fishing nets comprising polyamide 6 into small pieces and / or a cleaning section [α] for washing the fishing nets comprising polyamide 6 and / or a densification section [γ] for increasing the bulk density of the fishing nets comprising nylon 6. Cleaning includes both washing and separation of foreign bodies from the fishing nets comprising polyamide 6. Separation of foreign bodies may be performed either manually (manual sorting) or mechanically (e.g. density separation and magnetic separation). Both manual and mechanical devices such as brushes may be useful in the cleaning process in the cleaning section [α]. Cleaning is preferably performed by additional friction effect. Various types of industrial cleaning systems are available on the market, such as high-speed friction cleaners. The mechanical size reduction section [β] comprises a device for mechanically breaking the fishing nets comprising polyamide 6 into small pieces. Non-limiting examples of this breaking device are cutters, shredders, mills, grinders and chippers. The densification section [γ] comprises equipment for the densification of the material constituting 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 compactor machines and presses, as well as equipment in which the feed is first melted and then solidified by cooling, such as, for example, single-screw and twin-screw extruders.

[0122] The depolymerization section [B] comprises one or more depolymerization reactors operated in sequence and / or in parallel. The fishing nets comprising polyamide 6 are fed to the reactor as a solid or as a melt, preferably as a melt. This feeding can be achieved 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 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 material from the fishing nets comprising polyamide 6, as well as possibly superheated steam and a catalyst. Furthermore, the depolymerization reactor is equipped with facilities for discharging the stream comprising ε-caprolactam and the 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 vapour 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 apparatuses, one or more solvent exchange apparatuses, an oxidation section and one or more distillation apparatuses, to which the crude purified ε-caprolactam is input and from which the high-purity ε-caprolactam is discharged.

[0129] The extractor is charged with crude ε-caprolactam and an organic solvent, and discharges an organic solvent, an organic phase containing ε-caprolactam and impurities, and an aqueous phase containing water and impurities. 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 stirred extraction column, such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulse column, a perforated plate column (static) column, a randomly packed (static) column, and a structured packing (SMVP) (static) column.

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

[0131] The solvent exchange apparatus for the process based on solvent switching distillation is selected from a perforated plate column 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 an apparatus for reflux. 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 at the top of the distillation column, and the aqueous phase containing water, ε-caprolactam and impurities with a boiling point 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 in sequence and / or in parallel. The oxidizing agent and the ε-caprolactam-aqueous phase containing water, ε-caprolactam and impurities are fed into the oxidation section. Usually, the oxidizing agent is fed as a solid, or as a slurry, or as an aqueous solution. In case potassium permanganate or sodium permanganate is applied as the oxidizing agent, the oxidation section also comprises a filtration section. The oxidation reactor can have any desired form. Preferred reactor types are stirred and unstirred reactors and packed column reactors. The oxidation reactor must comprise facilities for feeding the aqueous phase containing water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, and the oxidizing agent. In addition, the oxidation reactor must comprise facilities for discharging the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam and impurities, and the solid manganese(IV) oxide (MnO2) particles formed, if any. Preferably, the oxidation is carried out at a temperature in the range of 20° C. to 85° C. and under atmospheric conditions.

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

[0134] The distillation apparatus is charged with an ε-caprolactam-water phase containing water, ε-caprolactam and impurities and discharges high-purity ε-caprolactam, water and impurities (i.e. low-boiling organic impurities (having a boiling point lower than ε-caprolactam) and high-boiling organic impurities (having a boiling point higher than ε-caprolactam). The distillation apparatus is selected from a sieve-plate distillation column, a random-packed distillation column, a structured-packed distillation column, and horizontal and vertical (falling and rising) film evaporators. Preferably, the distillation column comprises a reboiler, a condenser and an apparatus for reflux. The distillation apparatus may be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure, preferably subatmospheric 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 organic high-boiling impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises in a first step the separation of water as an overhead product and the production of 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 ε-caprolactam, more preferably from 2 to 80 mmol per kg ε-caprolactam. This results in a particularly effective distillative removal of impurities with lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0137] The process of the present invention may be operated in a continuous, semi-continuous or batch manner. Thus, the plant of the present invention may also be configured to allow one or more of these operation 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 the sections described herein in one location. In some cases, the sections or parts thereof are located in two or more locations. In particular, the pretreatment section [A] may be located in a first location, whereas the depolymerization section [B], the recovery section [C] and the purification section [D] may be located in a second location. Similarly, the mechanical size reduction section [β] as part of the pretreatment section [A] may also be located in a first location, whereas the cleaning section [α] as part of the pretreatment section [A] may be located in a second location, whereas 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, which are all optionally 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], the recovery section [C] and the 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 invention provides as a new product ε-caprolactam obtained via depolymerization of polyamide 6, produced from material originating from fishing nets comprising polyamide 6 according to the method of the invention. This ε-caprolactam is advantageously characterized in particular by having a product carbon footprint of less than 2 kg CO2 equivalent per kg purified ε-caprolactam (based on data originating from ecoinvent version 3.7.1; location: Europe). The ε-caprolactam obtained according to the invention may also be called "purified ε-caprolactam". "Purified" as used herein means that the ε-caprolactam is produced from fishing nets comprising polyamide 6 according to the method of the invention, so that the ε-caprolactam is obtained in purified form. In this sense, the ε-caprolactam is obtained and purified from material originating from fishing nets comprising polyamide 6.

[0139] The process of the invention makes it possible to produce high-purity and therefore high-quality ε-caprolactam that meets the specifications for demanding applications, which at the same time is particularly economically friendly due to its reduced product carbon footprint and due to the use of waste materials as starting material. In a preferred embodiment, the ε-caprolactam obtained by the process of the 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 invention is also particularly economical and environmentally friendly, as is evident from 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 the "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 tonne 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. Quantification of the carbon footprint of a product can be done, for example, as described in the 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 external (preferably) accredited organizations that verify and certify 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 Beckmann rearrangement of cyclohexanone oxime in a case involving coal-based electricity and steam generation is 7.5 tons of CO2 equivalent per ton of ε-caprolactam (which is equivalent to 7.5 kg of CO2 equivalent 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 equivalent per tonne of ε-caprolactam (which is equivalent to 6.4 kg of CO2 equivalent 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 CO2 equivalent, most preferably 2 kg or less CO2 equivalent per kg ε-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). [Brief description of the drawings]

[0145] The invention will now be described with reference to the drawings which show particular embodiments of the invention. The invention, however, as defined in the claims and 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 carried out in the optional pre-treatment section [A], depolymerization section [B], recovery section [C] and purification section [D]. [Diagram 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 objects and by washing with a washing 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 fishing nets comprising polyamide 6 are first cleaned in a cleaning section [α] by removing foreign objects and by washing with a washing solvent, and then crushed in a mechanical size reduction section [β] to obtain cleaned and crushed pieces of fishing nets 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 objects and by washing with a solvent to obtain cleaned and crushed pieces of fishing nets comprising polyamide 6. [Diagram 3] FIG. 3 illustrates two embodiments of the purification section [D], in which crude ε-caprolactam is purified to obtain high purity ε-caprolactam. [Figure 3A] FIG. 3A illustrates one embodiment of the purification section [D] of the process of the 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 diagrammatically in Figure 1. The process is carried out in the following plant sections:

[0147] Optionally, the polyamide 6-containing fishing nets [1] are cleaned in a pretreatment section [A] by removal of foreign bodies and by washing with a washing solvent [2], thereby obtaining a contaminated washing solvent [3]. The polyamide 6-containing fishing nets are then crushed by mechanical size reduction. The cleaned and crushed polyamide 6-containing fishing nets [6] are discharged from the pretreatment section [A]. Optionally, the polyamide 6-containing fishing nets [1] are further cleaned in a pretreatment section [A] by removal of foreign bodies. The removal of foreign bodies can be performed before and / or after the crushing of the polyamide 6-containing fishing nets. Optionally, the cleaned and crushed nylon 6-containing fishing nets are densified before being depolymerized to ε-caprolactam in a depolymerization section [B] (not shown in FIG. 1).

[0148] The possibly 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 materials [8] are 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]. In addition, 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] FIG. 2A illustrates one embodiment of the pretreatment section [A'] (area enclosed by dashed lines), in which the polyamide 6-containing fishing net [1'] is first cleaned in the cleaning section [α'] by removing foreign bodies and by washing with a washing solvent [2'], thereby obtaining foreign bodies, contaminated washing solvent [3'] and a cleaned polyamide 6-containing fishing net [4']. The cleaned polyamide 6-containing fishing net [4'] is subsequently crushed in the 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 to ε-caprolactam in the depolymerization section [B].

[0152] FIG. 2B illustrates one embodiment of the pretreatment section [A″] (area enclosed by dashed lines), in which the fishing net comprising polyamide 6 [1″] is first crushed in a mechanical size reduction section [β″] to obtain crushed pieces of fishing net comprising polyamide 6 [5″]. The crushed pieces of fishing net comprising polyamide 6 [5″] are subsequently cleaned in a cleaning section [α″] by removing foreign objects and by washing with a washing solvent [2″] to obtain foreign objects, contaminated washing solvent [3″] and discharged cleaned and crushed pieces of fishing net comprising polyamide 6 “6″]. Optionally, the cleaned and crushed fishing net comprising nylon 6 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 purification section [D′″] (area surrounded 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-bearing 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 a lower or higher boiling point 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 a lower or higher boiling point than ε-caprolactam (not shown in FIG. 3A).

[0157] In the optional oxidation section [θ'''], the aqueous phase [22'''] containing water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam and 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 the distillation section [λ′″], the optionally oxidized ε-caprolactam-aqueous phase [24′″] containing water, ε-caprolactam and impurities, from which optionally 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 the distillation section [λ′″]. Optionally, prior to the distillation in the 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 FIG. 3A).

[0159] FIG. 3B illustrates one embodiment of 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-bearing 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 addition of water [19''''] to obtain an aqueous phase [22''''] containing the organic solvent [21''''] and water, ε-caprolactam and impurities having a lower or higher boiling point 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 a boiling point lower or higher than ε-caprolactam and 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 FIG. 3B) before being discharged from the oxidation section [θ''''].

[0164] In the distillation section [λ''''], the optionally oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam and impurities, from which the 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 the distillation section [λ'''']. Optionally, prior to the distillation in the distillation section [λ''''], an alkali metal hydroxide is administered to the optionally oxidized ε-caprolactam-aqueous phase [24''''] containing water, ε-caprolactam and impurities entering the distillation section [λ''''] (not shown in FIG. 3B). EXAMPLES

[0165] The following examples serve to more fully illustrate the invention, 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 Caprolactam for industrial use - Determination of the permanganate index of caprolactam - Spectroscopic method, revision of the first edition of ISO 8660; 1988, E290: ISO 7059-Caprolactam for industrial use-Determination of absorbance at a wavelength of 290 nm, 1982 Volatile bases (VB) ISO 8661-Caprolactam for industrial use-Determination of the volatile base content-Titration 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 grey color, then X grams of an aqueous solution of ε-caprolactam containing Y weight percent of ε-caprolactam (determined by refractive index) is added and the solution is back-titrated with 0.01N H2SO4 solution to a grey 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 = weight of sample (g) Y = concentration of ε-caprolactam (wt%)

[0169] The polyamide 6-containing pellets used in the examples and comparative experiments were made from discarded fishing nets. Pretreatment included removal of foreign bodies, 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 20-30 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 / min during the 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 about 20° C. to obtain a condensate containing ε-caprolactam and water.

[0172] The condensate, consisting of about 44 grams of ε-caprolactam with most of the remainder being water, was concentrated to an ε-caprolactam concentration of 49.7% by weight 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 shows 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 per kg of ε-caprolactam was then added. Water and impurities having a boiling point lower than ε-caprolactam were then removed as top product by distillation under reduced pressure in a distillation apparatus operated batchwise, and the pressure was reduced stepwise. ε-caprolactam was distilled at 300 Pa, while impurities having a higher boiling point compared to ε-caprolactam remained as bottom product in the distillation apparatus. 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 study 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, since 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 with respect 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 study 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 ε-caprolactam solution 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 study shows that the ε-caprolactam obtained from the depolymerization of polyamide 6, derived from discarded fishing nets containing polyamide 6 and purified by oxidation and distillation, is of low quality and cannot be used as is 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. The crude ε-caprolactam was then treated with 0.2 wt. % KMnO4 with respect 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 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:3 E290:0.08 VB: 0.43 mmol / kg Alkalinity: 0.29mmol / kg.

[0181] This comparative study 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 as is 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 being 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] The 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 extracts were concentrated to an ε-caprolactam concentration of about 40 wt% by distillation under vacuum conditions, 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%. The 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 about 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 addition of 75 mmol of aqueous sodium hydroxide solution 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, back-extraction, 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 about 65° C.). (This mixture, crude ε-caprolactam, is the mixture to be purified.)

[0187] The crude ε-caprolactam was extracted 11 times batchwise with toluene 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 wt. % by distillation under vacuum conditions. The resulting concentrated organic extracts were then extracted 4 times batchwise with water at a temperature of about 25° C. The total amount of water used was 1.79 grams of water per gram of combined organic extracts. The resulting combined aqueous ε-caprolactam solution was concentrated to an ε-caprolactam concentration of 52.2 wt. % 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, back-extraction and distillation, allows the production of 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 about 65° C.). Then, both concentrated aqueous ε-caprolactam solutions were added together. The resulting mixture, i.e., 1 / 3 of the crude ε-caprolactam, is 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 conditions. The resulting concentrated organic extracts were subsequently 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 of water 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: <0.02 mmol / kg Alkalinity: 0.1mmol / kg.

[0193] From this example 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, makes it possible to obtain 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. The process comprises the steps of: - mechanical removal of foreign objects from fishing nets, including 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 the washed pieces of fishing nets and the aqueous extract 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 extract; -Oxidation of concentrated aqueous extract with KMnO4; -Addition of caustic substances; and -Recovery of pure ε-caprolactam by vacuum distillation Includes:

[0195] Based on the figures for the consumption of raw materials, the carbon footprint of the refined ε-caprolactam is calculated and the utility of the above method is based on data originating 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] Although the 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. 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], - the collection section [C], and - Purification section [D] Equipped with The method comprises: a) feeding a material derived from a fishing net comprising polyamide 6 into said depolymerization section [B]; b) depolymerizing the material originating 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, (i) extracting the crude ε-caprolactam with an organic solvent to obtain an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities; (ii) exchanging the organic solvent by at least partially replacing it with water, whereby an aqueous phase is obtained comprising water, ε-caprolactam and impurities having 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, whereby 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; A method comprising:

2. 2. The process according to 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 from 1:2 to 1:15, and wherein in the extraction in step d)(i) both an aqueous phase and an organic phase are obtained.

3. Prior to the distillative removal in step d)(iii), said purification in step d) comprises 3. The method according to claim 1 or 2, further 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, said oxidizing agent being selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, in particular potassium permanganate.

4. 4. The process according to claim 2 or 3, wherein the water present in step b) is in the form of steam which is charged to the depolymerization section [B] in step b) as superheated steam having a temperature in the range from 220° C. to 575° C., preferably from 275° C. to 500° C.

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

6. 5. The process according to any one of claims 1 to 4, wherein the solvent exchange in step d)(ii) is a solvent-switching distillation based process.

7. 7. The process according to any one of claims 3 to 6, 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.

8. 8. The process according to any one of claims 3 to 7, 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.

9. The depolymerization in step b) is carried out in the absence or presence of a catalyst, the catalyst being selected from acid and base catalysts, the acid catalyst being 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, in particular orthophosphoric acid, and 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, in particular sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen carbonate and potassium hydrogen carbonate.

10. 10. The process according to any one of claims 1 to 9, wherein the depolymerization in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

11. 11. The method according to any one of claims 1 to 10, wherein prior to step a), said material from fishing nets comprising polyamide 6 is obtained as product of a pre-treatment in a pre-treatment section [A], in particular cleaning in a cleaning section [α] and / or mechanical size reduction in a mechanical size reduction section [β] and / or increasing bulk density in a densification section [γ].

12. - after step d)(i), the organic phase obtained in step d)(i) is washed with water or an aqueous alkaline solution, and / or - before said distillative removal in step d)(ii), an alkali metal hydroxide, preferably NaOH, is added to said aqueous phase, The method according to any one of claims 1 to 11.

13. 1. A plant for producing purified ε-caprolactam from a material derived from fishing nets comprising polyamide 6, said plant comprising: a depolymerization section [B], a collection section [C]; - purification section [D]; Equipped with The plant is configured to carry out the method according to any one of claims 1 to 12.

14. The plant, - a mechanical size reduction section [β], and - Cleaning section [α] and - possibly densification section [γ] 14. The plant of claim 13, further comprising a pre-treatment section [A] comprising:

15. Purified ε-caprolactam obtained via depolymerization of polyamide 6 derived from fishing nets containing polyamide 6 by the method according to any one of claims 1 to 12, said ε-caprolactam being obtained at a concentration of less than 2 kg CO per kg purified ε-caprolactam. ２ Refined ε-caprolactam with a significant product carbon footprint.

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