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

Abstract

The invention provides a process and a plant for recovering purified epsilon-caprolactam from fishing nets comprising nylon 6, wherein the plant comprises a depolymerization section [B], a recovery section [C] and a purification section [D]. The invention also provides purified epsilon-caprolactam having a particularly low product carbon footprint and obtained by depolymerizing the nylon 6 of fishing nets. In a second preferred embodiment, purification of the distilled caprolactam is effected by crystallization during concentration. The crystalline caprolactam resulting from the concentration is generally pure enough for direct use. After crystallization, it can be necessary to purify the mother liquor, for example by recycling it to the aqueous solution before extraction with alkyl phenol. The mother liquor can be purified, for example, by distillation.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for the recovery of ε-caprolactam from a material derived from waste fishing nets comprising nylon 6. More specifically, the present invention relates to a process for the recovery of ε-caprolactam from a material derived from waste fishing nets comprising nylon 6, whereby a high quality ε-caprolactam is obtained. BACKGROUND

[0002] Fishing nets are nets used for fishing. A net is a device made of fibers woven in a grid-like structure. Fishing nets are usually meshes formed by braiding relatively thin threads. Modern nets are usually made of man-made fibers such as nylon 6 nylon 6,6, polyester, polypropylene and polyethylene.

[0003] Fishing nets can be left or lost in the ocean by fishermen. These nets, called ghost nets, pose a serious problem for fish and other animals. In general, the (bio)degradation rate of fishing nets made of man-made fibers is usually very low. As a result, these nets will remain in the marine ecosystem for many years, leading to a build-up of a large amount of ghost nets.

[0004] Recent assessments by the FAO and the United Nations Environment Programme (UNEP) indicate that the amount of fishing gear that is discarded, lost and abandoned each year is approximately 640 million kilograms.

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

[0006] The end-destinations of the collected waste fishing nets, including fishing nets comprising nylon 6, range from landfill, incineration (optionally with heat recovery), re-pelletization and compounding to depolymerization. Re-pelletization and compounding are recycling processes in which waste plastics are melted (and optionally subsequently filtered to remove solid impurities) and then converted into an extrudate or directly into a mold. Depolymerization is a technique in which a polymer is converted into its monomeric components (ε-caprolactam in case of nylon 6).

[0007] Mechanical recycling (also referred to as material recycling or back-to-plastics recycling) refers to the operation of recovering plastics from mechanical processes (grinding, washing, separation, drying, re-pelletization and compounding) whereby a recyclate is produced that can be converted into a plastic product that replaces the original plastic. Currently, most original plastics are produced from petrochemical feedstocks that were never used or treated before, such as natural gas, coal or crude oil. During mechanical recycling, the polymer chains more or less remain intact. Mechanical recycling is a form of downcycling of waste, as the recovered material has a lower quality and functionality than the original material.

[0008] Depolymerization or chemical recycling is a technique to convert polymers into their monomeric components. The specification of the recovered monomers determines whether they can be substituted for virgin monomers for all or only a limited amount of applications. Virgin monomers are produced from petroleum chemical feedstocks that have never been used or processed before, such as natural gas, coal, or crude oil.

[0009] Nylon 6 (CAS No. 25038-54-4), also known as N6, polyamide 6, PA 6, poly(caprolactam), poly(hexane-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide), or poly[imidazolyl(l-oxohexane-1,6-diyl)] was invented in 1938 by Paul Schlack.

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

[0011]

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

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

[0014] Depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam:

[0015]

[0016] Processes for depolymerization of nylon 6 are known. Such processes can be operated in batch mode, in semi-continuous mode (typically with batch (re)charging of the depolymerization reactor with nylon 6) or in continuous mode.

[0017] L. A. Dmitrieva, A. A. Speranskii, S. A. Krasavin and Y. N. Bychkov, "Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns", Fibre Chemistry, pages 229-241, March 1986, (translated from Khimicheskie Volokna, No. 4, pages 5-12, July-August 1985) is a review article describing processes for depolymerization of nylon 6 with and without catalyst.

[0018] A. A. Ogale, "Depolymerization of Nylon 6: Some Kinetic Modeling Aspects", Journal of Applied Polymer Science, Vol. 29, 1984, pages 3947-3954, which is available electronically via https: / / doi.org / 10.1002 / app.1984.070291227, is a paper describing the kinetics of depolymerization of nylon 6.

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

[0020] Depolymerization of fishing nets comprising nylon 6 has been practiced in the past. Many purification techniques and combinations thereof have been practiced to purify the crude epsilon caprolactam obtained from these depolymerization reactions. Even oxidizing agents like potassium permanganate (KMn04) are often used to form manganese (IV) oxide (Mn02) particles as reaction products. Removing these Mn02particles requires solid-liquid filtration and is a rather cumbersome and laborious task. Other applications use adsorption based techniques, wherein solid adsorbents like (activated) carbon and diatomaceous earth are used as adsorbents. These techniques also produce a lot of waste and are a rather cumbersome and laborious task, in addition, the quality of the monomeric epsilon caprolactam obtained by these methods is still particularly poor, despite the long history of nylon 6 recycling.

[0021] Therefore, the epsilon caprolactam obtained by depolymerization of fishing nets comprising nylon 6 is only applied in less demanding applications (downcycling), like engineering plastics and carpets. If the epsilon caprolactam obtained from depolymerization of fishing nets is used in more demanding applications, it needs to be blended with a lot of higher grade and purer grades of epsilon caprolactam in order to mask the rather poor quality of the epsilon caprolactam obtained from depolymerization of fishing nets. High speed melt spinning of nylon 6 for the production of fine textile fibers requires high quality epsilon caprolactam as a raw material. The high quality epsilon caprolactam grade for these applications should not only be extremely pure, but also its properties should not change over time.

[0022] In summary, the prior art methods for recycling epsilon caprolactam from fishing nets comprising nylon 6 fail to produce a high quality epsilon caprolactam grade that can be used to replace virgin epsilon caprolactam grades for high demanding applications.

[0023] Currently, no method is available for recycling high purity epsilon caprolactam from fishing nets comprising nylon 6, despite the urgent need for such a method. In particular, there is an urgent need for a high purity epsilon caprolactam recycling method that can replace virgin epsilon caprolactam grades for high demanding applications, like high speed melt spinning during textile fiber production.

[0024] In addition, there is a need for a method that allows recycling high purity epsilon caprolactam from fishing nets comprising nylon 6 in an economically reasonable way. The production costs of the recycled high purity epsilon caprolactam should be similar or lower than the production costs of virgin high purity epsilon caprolactam.

[0025] In addition, there is a need to provide a high purity grade epsilon caprolactam from fishing nets comprising nylon 6 that has a significantly lower carbon footprint than epsilon caprolactam produced by a method that uses virgin epsilon caprolactam obtained by new synthesis via, for example, Beckmann rearrangement of cyclohexanone oxime.

[0026] Moreover, there is a need to purify the crude epsilon caprolactam obtained by depolymerization of fishing nets comprising nylon 6 without using oxidizing agents such as potassium permanganate (KMn04) or adsorbents such as (activated) carbon and diatomaceous earth. The techniques based on these oxidizing agents and adsorbents are rather laborious and generate solid waste.

[0027] Moreover, there is a need for a plant for producing high purity grade epsilon-caprolactam from materials derived from fishing nets comprising nylon 6.

[0028] Finally, there is a need for a process allowing the recovery of epsilon-caprolactam from fishing nets comprising nylon 6 on an industrial scale, so as to process the large amounts of fishing nets comprising nylon 6 that are discarded every year. SUMMARY

[0029] The object of the present invention is to meet one or more of the needs described above and to overcome or alleviate the drawbacks associated with the prior art processes.

[0030] In particular, the object of the present invention is to provide a process for recovering high purity epsilon-caprolactam from materials derived from fishing nets comprising nylon 6. In this regard, another object of the present invention is to provide a process for recovering high purity epsilon-caprolactam from materials derived from fishing nets comprising nylon 6, which can replace high purity virgin epsilon-caprolactam for all applications, including high speed melt spinning of nylon 6 for fine textile fiber production.

[0031] Another object of the present invention is to provide a process for recovering high purity grade epsilon-caprolactam from materials derived from fishing nets comprising nylon 6 on an industrial scale.

[0032] The object of the present invention is also to provide a process for recovering high purity grade epsilon-caprolactam from materials derived from fishing nets comprising nylon 6 in an economically advantageous manner. In particular, the object of the present invention is to provide a process suitable for recovering high purity grade epsilon-caprolactam from materials derived from fishing nets comprising nylon 6, which does not exceed the production cost of virgin high purity epsilon-caprolactam.

[0033] The object of the present invention is also to provide a process for purifying crude epsilon-caprolactam obtained by depolymerization of materials derived from fishing nets comprising nylon 6, which does not generate solid waste.

[0034] Another object of the present invention is to provide high purity grade epsilon-caprolactam from materials derived from fishing nets comprising nylon 6, characterized by a significantly lower carbon footprint than epsilon-caprolactam produced by a process using virgin epsilon-caprolactam obtained by new synthesis via, for example, Beckmann rearrangement of cyclohexanone oxime.

[0035] It is therefore also an object of the present invention to provide a method of reducing the environmental burden of waste fishing nets comprising nylon 6.

[0036] It is also an object of the present invention to provide a plant for producing high purity grades of epsilon-caprolactam from material derived from fishing nets comprising nylon 6.

[0037] One or more further objects can become apparent from the remainder of the present specification.

[0038] All, some or at least one of the aforementioned objects is / are achieved by the method according to claim 1, the plant according to claim 13 and the product according to claim 15.

[0039] The present invention provides a method for recovering purified epsilon-caprolactam in a plant from material derived from fishing nets comprising nylon 6, wherein the plant comprises:

[0040] - a depolymerization section [B],

[0041] - a recovery section [C], and

[0042] - a purification section [D],

[0043] and wherein the method comprises the following steps:

[0044] a) charging material derived from fishing nets comprising nylon 6 into the depolymerization section [B];

[0045] b) depolymerizing the material derived from fishing nets comprising nylon 6 material in the depolymerization section [B] at a temperature in the range of 180 °C to 400 °C, preferably 200 °C to 350 °C, more preferably 220 °C to 340 °C and most preferably 240 °C to 325 °C, so as to obtain a stream comprising epsilon-caprolactam;

[0046] c) discharging the stream comprising epsilon-caprolactam from the depolymerization section [B] and recovering crude epsilon-caprolactam from the stream in the recovery section [C]; and

[0047] d) purifying the crude epsilon-caprolactam in the purification section [D] to obtain purified epsilon-caprolactam, wherein the purification comprises the following steps:

[0048] (i) extracting the crude epsilon-caprolactam with an organic solvent, thereby obtaining an organic phase, and wherein the organic phase comprises the organic solvent, epsilon-caprolactam and impurities; and

[0049] (iv) obtaining purified epsilon-caprolactam by crystallizing epsilon-caprolactam from a solution comprising epsilon-caprolactam and impurities at a temperature of 10 to 95 °C.

[0050] Surprisingly, the specific order of process steps and process conditions according to the present application, i.e. the order of the depolymerization, recovery and purification steps as defined above, allows for the recovery of high grade epsilon-caprolactam from fishnets comprising nylon 6 in high yield and in a straightforward and economically reasonable manner. The process of the present application is economically reasonable and advantageous from several perspectives. First, the process of the present application is suitable for a variety of fishnet materials comprising nylon 6 which can differ, for example, in their overall composition and / or their nylon 6 content. Second, the process of the present application allows for an efficient separation of epsilon-caprolactam from non-epsilon-caprolactam compounds such that a high purity grade of epsilon-caprolactam can be obtained which can replace high purity virgin epsilon-caprolactam for all applications, including high speed melt spinning of nylon 6 for fine textile fiber production. Third, the process of the present application is so efficient that epsilon-caprolactam can be obtained in high yield. Fourth, the process of the present application allows for the recovery of epsilon-caprolactam from fishnets comprising nylon 6 on an industrial scale in order to process the currently large amounts of fishnets comprising nylon 6 which are wasted. Finally, the process of the present application allows for the production of epsilon-caprolactam with a carbon footprint which is significantly lower than epsilon-caprolactam produced by re-synthesis of epsilon-caprolactam, for example, via Beckmann rearrangement of cyclohexanone oxime. The process of the present application allows for the efficient processing of fishnets comprising nylon 6 and reduces the environmental burden of such products. In particular, the process of the present application allows for the production of purified epsilon-caprolactam with a carbon footprint of less than 2 kg CO2 equivalent per kg of purified epsilon-caprolactam which is a significant improvement compared to the production of "virgin" epsilon-caprolactam obtained from Beckmann rearrangement of cyclohexanone oxime which is associated with 6.5 to 7.5 kg CO2 equivalent per kg of epsilon-caprolactam (based on data derived from ecoinvent version 3.7.1 ; location: Europe). Unless otherwise stated, the values for product carbon footprint stated herein are based on data derived from ecoinvent version 3.7.1 and the location is Europe.

[0051] In line with the process of the present application, the present application also provides a plant for producing purified epsilon-caprolactam from fishnets comprising nylon 6 in the plant, wherein the plant comprises:

[0052] a depolymerization section [B],

[0053] a recovery section [C],

[0054] a purification section [D], and

[0055] wherein the plant is configured for carrying out the process of the present application.

[0056] The present invention also provides purified epsilon-caprolactam, which is obtained via depolymerization of nylon 6 produced according to the method of the present invention from a fishing net comprising nylon 6, wherein the product carbon footprint of epsilon-caprolactam is less than 2 kg CO2 equivalent per kg of purified epsilon-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe).

[0057] Advantageous embodiments of the present invention are indicated in the dependent claims and are explained in more detail below. DETAILED DESCRIPTION

[0058] Detailed description of preferred embodiments

[0059] Fishing nets comprising nylon 6

[0060] The method of the present invention uses a fishing net comprising nylon 6 or a fishing net material derived from such a fishing net as starting material. Fishing nets comprising nylon 6 are typically solid materials, in particular fishing nets comprising nylon 6 are typically nets formed by braiding relatively thin threads comprising nylon 6. As used herein, material derived from a fishing net comprising nylon 6 means material derived from a fishing net comprising nylon 6 after e.g. shredding, washing, sorting, densification, etc. The present disclosure uses the term "fishing net" also in reference to "fishing net material", i.e. these terms are used synonymously herein. As used in the present disclosure and the claims, the singular form "a", "an", and "the" include plural references unless the context clearly dictates otherwise, in particular in the sense of "one or more".

[0061] Fishing nets comprising nylon 6 can contain a variety of compounds added during or after polymerization, thread formation to achieve different property changes. These compounds include, for example, brighteners, stiffeners, antistatic lubricants, colorants, brighteners, spin finish, surface smoothing agents, antioxidants, UV stabilizers, etc. The composition of the fishing net also depends on its exact application. Thus, fish farming nets, purse seines and bottom trawl nets have different chemical compositions.

[0062] Surfaces immersed in seawater are rapidly covered by marine organisms known as marine biofouling or biofouling. The biofouling community is a complex phenomenon caused by several processes, the rate and extent of which are influenced by many physical, chemical and biological factors closest to the surface and affect most wetted surfaces, resulting in huge financial costs. The accumulation of algae and barnacles increases the resistance of ships and destroys the protection of aquaculture and equipment used therein.

[0063] Nowadays, antifouling paints are formulated with toxic copper or other biocides to prevent the growth of marine sessile organisms. Copper is an effective and still widely used biocide. However, its effectiveness is relatively short, usually only a few months, so frequent cleaning and repainting of the paint is required. In addition to economic factors, leaching of copper or other biocides causes problems of pollution of the sea water and non-target organisms.

[0064] The process of the present invention has the advantage over processes of the prior art, which are for example restricted to rather pure nylon 6 containing materials, such as carpets and spinning waste comprising PA 6, that the process of the present invention is not restricted to this and can in particular also be applied with great success to any kind of fishing nets comprising nylon 6.

[0065] Possible pre-treatment steps

[0066] Before carrying out step a) of the process of the present invention, the material containing the fishing net comprising nylon 6 is subjected to a pre-treatment in a pre-treatment section [A] to obtain a material derived from the fishing net comprising nylon 6, in particular a cleaning in a cleaning section [a] and / or a mechanical size reduction in a mechanical size reduction section [b] and / or a densification in a densification section [g]. This has the advantage that the fishing net comprising nylon 6 that is charged into the depolymerization section [B] is less contaminated with non-nylon 6 materials, which improves the yield and purity of the e-caprolactam produced in the chemical plant of the present invention. Another advantage is that the size-reduced and / or densified fishing net comprising nylon 6 can be more easily disposed of.

[0067] As used herein, the term "cleaning" is defined as any process that removes non-nylon 6 materials that are adhered to or mixed with the multi-component material containing nylon 6. Cleaning is advantageous because any non-nylon 6 materials that are removed will thus not interfere with the next steps of the process of the present invention.

[0068] The (waste) fishing net comprising nylon 6 can be mixed with various other materials, such as rocks, metallic materials (e.g. lines, chains, anchors), organic materials (e.g. dead fish and mussels), and other (marine) litter (e.g. ropes, (polystyrene foam) floats, and sink lines). In addition, the (waste) fishing net comprising nylon 6 can be mixed with non-nylon 6 fishing nets, such as those made of nylon 6,6, polyethylene terephthalate (PET), polypropylene (PP), or polyethylene (PE). The (waste) fishing net comprising nylon 6 can also be coated with an antifouling coating, for example based on a metal such as copper, or containing a non-metal.

[0069] The size of the (discarded) fishing net comprising nylon 6 depends to a large extent on the exact application. The (discarded) fishing net comprising nylon 6 is in the range of a few square meters to more than 200,000 square meters. Such huge nets are for example purse seines and nets which are set vertically in the water, with a float attached to the upper edge, a weight attached to the lower edge and with a series of rings through which the net is drawn, which can be 1.5 km long and more than 150 m deep.

[0070] Preferably, the fishing net comprising nylon 6 is fragmented into pieces before depolymerization in step a) in the depolymerization section [B]. This mechanical pre-treatment, i.e. the mechanical comminution or fragmentation of the fishing net comprising nylon 6 can be achieved for example by cutting, shredding, milling, grinding and / or cutting. In a preferred embodiment, the fishing net comprising nylon 6 is charged to step a) in the form of pieces having a weight in the range of 0.005 gram to 100 kg, preferably 0.01 gram to 10 kg and most preferably 0.02 gram to 1 kg. The use of pieces of the fishing net comprising nylon 6 having the aforementioned weights has the advantage that such pieces can be more easily disposed of and / or cleaned by washing with a solvent.

[0071] Optionally, before the mechanical comminution or fragmentation of the fishing net comprising nylon 6, large metal pieces, rocks and other interfering materials which cause severe wear of the equipment used for the mechanical comminution or fragmentation are removed. Preferably, before or after the mechanical comminution or fragmentation of the fishing net comprising nylon 6, materials comprising non-nylon 6, such as materials comprising polyethylene, polypropylene and nylon 6,6, such as fishing nets, are removed, as further described below. The removal of the foreign materials can be done mechanically or manually. The removal of these interfering materials has the advantage that the maintenance costs of the equipment used for the mechanical comminution or fragmentation can be reduced to a large extent. In addition, the nylon 6 content of the material obtained after the mechanical comminution or fragmentation is higher if the interfering materials are not removed. In particular, the removal of nylon 6,6 is advantageous because it interferes with the depolymerization of nylon 6, clogs the reactor itself, reduces the recovery yield of ε-caprolactam and interferes with the subsequent purification of the recovered ε-caprolactam.

[0072] Preferably, also the fishing net comprising nylon 6 having a Cu-based antifouling coating is removed before the mechanical comminution or fragmentation of the fishing net comprising nylon 6. More preferably, the fishing net having a Cu-based antifouling coating is washed in an additional separate washing step so that the Cu-based antifouling coating is removed. This washed clean material can then be added to materials having a similar composition.

[0073] Optionally, foreign materials are separated from the mechanically comminuted or fragmented fishnet comprising nylon 6. For this purpose, various separation methods 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, wherein the less dense materials float up and separate from the more dense settling materials. In practice, density separation is often done through a series of density separation stages. For example, in one stage, high density materials such as rocks, sand and metals (including iron and lead) are separated, while in another stage, low density materials such as polyolefin polypropylene and polyethylene are separated. Magnetic separation is a method of separating components of a mixture by using magnets to attract magnetic materials. This method is often used to split magnetic materials from non-magnetic materials. Removal of foreign materials from the comminuted or fragmented fishnet comprising nylon 6 is advantageous, because such materials can interfere with depolymerization of the nylon 6, reduce the recovery yield of ε-caprolactam, and interfere with subsequent purification of the recovered ε-caprolactam.

[0074] Optionally, the fishnet comprising nylon 6 is cleaned by washing with a solvent, preferably water, before being charged into the depolymerization section [B]. Preferably, a detergent is added to the solvent in a concentration ranging from 0 to 20 wt% relative to the solvent to improve the washing efficiency. NaOH is a preferred detergent. Preferably, an aqueous solution containing 0 to 10 wt% NaOH is used in the washing step, more preferably 0 to 5 wt% NaOH. Preferably, the Cu-based antifouling coating is removed by washing with an aqueous solution containing 1 to 5 wt% NaOH, preferably 1.5 to 3 wt% NaOH, more preferably about 2 wt% NaOH. The enhanced washing effect of NaOH is most likely caused by enhanced hydrolysis of molecules including biopolymers and non-biopolymers. Furthermore, NaOH is known to hydrolyze copolymers such as polyethylene-vinyl acetate (PEVA, also known as EVA) used for Cu-based antifouling coatings. Preferably, the washing solvent is heated to further enhance the washing process. In another preferred embodiment, the washing process includes a final rinsing step with (cleaning) washing solvent without detergent, in order to remove residual detergent and smudges present attached to the fishnet comprising nylon 6.

[0075] The washing is preferably carried out under friction. Different types of industrial washing systems are available on the market, such as high-speed friction pads.

[0076] Washing the fishnet comprising nylon 6, especially the mechanically comminuted or fragmented fishnet comprising nylon 6, is advantageous because all (adherent) smudges are removed and thus do not interfere with the subsequent steps of the process of the present application.

[0077] Optionally, the fishnet comprising nylon 6 is dried after the cleaning step and before charging the depolymerization section [B]. This has the advantage that the weight of the cleaned fishnet comprising solvent nylon 6 is reduced and that the next process step is not affected by dilution or contamination with the washing solvent.

[0078] Optionally, the preferably washed and size-reduced fishnet comprising nylon 6 is charged to a melting furnace, such as an extruder. The fishnet comprising nylon 6 is melted in the melting furnace. Preferably, the resulting polymer melt is filtered. This has the advantage that solid impurities are removed. The melted and optionally filtered polymer melt is subsequently allowed to cool and is fed to a pelletizer. The pelletizer cuts the product into pellets. The pellets or the melted and optionally filtered polymer melt are directly charged to the depolymerization section [B].

[0079] The size and shape of the pellets, often also referred to as granules, can be chosen within wide limits. Generally, the shape of the pellets is cylindrical (derived from cutting the fine strands into pieces). However, other shapes, such as (non-perfect) spheres are also possible. The size of the pellets can be chosen within wide limits. Typically, the diameter of the pellets is in the range of 1 to 10 mm, preferably 2 to 7 mm, more preferably 3 to 5 mm. In a preferred embodiment, the length of the pellets is in the range of 1 to 50 mm, preferably 2 to 25 mm, more preferably 3 to 15 mm.

[0080] Pelletizing of the preferably cleaned and size-reduced fishnet comprising nylon 6 has the advantage of increased bulk density, which reduces the costs for intermediate storage and transport in case of pre-treatment at different locations (see below). In addition to the increased density, pelletizing also provides other benefits, such as a uniform shape and structure of the material to be processed in the depolymerization section [B] facilitating (automated) feeding.

[0081] The location where the pre-treatment of the fishnet comprising nylon 6 is performed and the location where the depolymerization section [B] is located can be the same. Preferably, however, one or more of the pre-treatment steps are performed at a different location, for example near a harbor where the waste fishnet comprising nylon 6 is collected and / or at a location specialized in pre-treating waste fishnets. The fishnet comprising nylon 6 pre-treated at the various locations can subsequently be charged to the depolymerization section [B] of the (chemical) plant of the present invention for producing purified ε-caprolactam from fishnet comprising nylon 6.

[0082] Thus, according to a particular advantageous embodiment of the present invention, prior to step a), the material comprising fishnet comprising nylon 6 is pre-treated in a pre-treatment section [A], in particular cleaned in a cleaning section [a] and / or mechanically size-reduced in a mechanical size-reduction section [β] and / or densified in a densification section [γ], to obtain a material derived from fishnet comprising nylon 6.

[0083] Charging step a)

[0084] In step a) of the present application, the fishnet comprising nylon 6, which has optionally been reduced in size and / or washed and / or melted and resolidified, is charged into a depolymerization section [B]. The depolymerization section [B] comprises one or more depolymerization reactors, which are operated in series and / or in parallel.

[0085] In one embodiment, the fishnet comprising nylon 6 is mechanically compressed to a smaller volume prior to being charged into the depolymerization section [B]. This has the advantage that less volume is required for intermediate storage and transport and can also facilitate the feeding into the depolymerization section [B].

[0086] In another embodiment, the fishnet comprising nylon 6 is compressed to particles of increased density prior to being charged into the depolymerization section [B], for example by mechanical compaction or by extruding the molten material, followed by cooling and cutting to size. Again, this has the advantage that less volume is required for intermediate storage and transport and can facilitate the feeding into the depolymerization section [B].

[0087] In another preferred embodiment, the fishnet comprising nylon 6 is dried prior to being charged into the depolymerization section [B], in particular after a cleaning step of the fishnet comprising nylon 6. This has the advantage that less or no solvent is introduced into the depolymerization section [B]. It is expected that solvents introduced into the depolymerization section [B] have a negative impact on the depolymerization process (e.g. reduced depolymerization reaction rate, higher catalyst consumption, higher energy consumption, and it is expected that the resulting vapor stream comprising ε-caprolactam and water in the depolymerization section [B] contains more impurities).

[0088] The (material derived from) fishnet comprising nylon 6 is preferably fed to the depolymerization reactor in solid phase form or in the form of a melt. Preferably, the fishnet comprising nylon 6 is charged in the form of a melt. Feeding in the form of a melt can be achieved by using an extruder, a gear pump or other means known to the skilled person.

[0089] The (material derived from) fishnet comprising nylon 6 to the depolymerization reactor can be achieved by continuous or batchwise feeding of the fishnet comprising nylon 6.

[0090] Depolymerization step b)

[0091] In the depolymerization section [B], the material derived from the fishnet comprising nylon 6 is depolymerized to form ε-caprolactam. The ε-caprolactam formed is discharged from the depolymerization section in the form of a stream comprising ε-caprolactam.

[0092] The depolymerization of the fishnet comprising nylon 6 is achieved in the depolymerization section [B] by increasing the temperature of the fishnet comprising nylon 6 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 preferred 220°C to 340°C and most preferred 240°C to 325°C.

[0093] Generally, the rate of ε-caprolactam formation increases at higher temperatures. Temperatures below 400°C are preferred, because at temperatures above 400°C side reactions of nylon 6 and reactions of impurities occur more frequently, which will lead to the formation of a more diverse range of impurities. Part of these impurities will end up in the product stream comprising ε-caprolactam that is discharged from the depolymerization reactor. In a preferred embodiment of the present application, the depolymerization of the fishnet comprising nylon 6 is carried out at a temperature in the range of 220°C to 340°C or 240°C to 325°C. This temperature range allows the production of especially pure ε-caprolactam.

[0094] The pressure in the depolymerization section [B] can vary and can be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, most preferably 50 kPa to 1 MPa. This pressure range allows the production of especially pure ε-caprolactam.

[0095] The depolymerization of (the material derived from) the fishnet comprising nylon 6 can be achieved in the presence or absence of a solvent. Preferably, the depolymerization of (the material derived from) the fishnet comprising nylon 6 is achieved in the presence of water as solvent. In this case, the water is preferably in the form of steam, especially superheated steam.

[0096] Preferably, the depolymerization will be completed within 0.1 hour to 24 hours, more preferably 0.5 hour to 6 hours.

[0097] Feeding water to the depolymerization reactor in the form of steam allows to obtain a stream comprising ε-caprolactam and water, optionally without further heating. The weight ratio of ε-caprolactam to water in this stream can be adjusted by modifying the amount of steam fed to the fishnet comprising nylon 6 in the depolymerization section [B]. In a preferred embodiment, the depolymerization in step b) is carried out in the presence of water, wherein the stream comprising ε-caprolactam is a stream comprising steam of ε-caprolactam and water in a weight ratio of 1 : 1 to 1 : 50, preferably 1 : 2 to 1 : 15, more preferably 1 : 2 to 1 : 10 and most preferably 1 : 3 to 1 : 8.

[0098] Preferably, the ε-caprolactam in the stream comprising ε-caprolactam and water has a partial pressure of 0.1 kPa to 1 MPa, more preferably 0.3 kPa to 0.5 MPa and most preferably 1 kPa to 0.1 MPa.

[0099] During the depolymerization reaction, decomposition products can form, including linear oligomers of ε-caprolactam and cyclic oligomers of ε-caprolactam. In addition, the feed stream comprising the fishing nets of nylon 6 can also contain other components, i.e. impurities, such as non-nylon 6 compounds and residues of solvents applied in the pre-treatment, which components remain stable, react or decompose under the depolymerization conditions. Thus, in case water is used as solvent, the vapor stream removed from the depolymerization section [B] comprises not only water and ε-caprolactam, but also impurities.

[0100] Preferably, superheated steam having a temperature between 100 °C and 600 °C is charged to the depolymerization reactor. Preferably, the superheated steam charged to the depolymerization reactor has a temperature of at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam charged to the depolymerization reactor is sufficiently high so that no further heat input is required for carrying out the depolymerization reaction and for evaporating the formed ε-caprolactam. In another preferred embodiment, the depolymerization section [B] is charged with superheated steam having a temperature in the range of 220 °C to 575 °C. In a more preferred embodiment, the depolymerization section [B] is charged with superheated steam having a temperature in the range of 275 °C to 500 °C. In another preferred embodiment, a part of the heat input required for carrying out the depolymerization reaction and for evaporating the formed ε-caprolactam is introduced via the walls of the depolymerization reactor.

[0101] 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 fishing nets containing nylon 6 and optionally solvent, catalyst, additional reagents and / or depolymerization agent. Thus, without any additional measures, there will be an accumulation of material, generally referred to as ‘residual material’, in the depolymerization section [B]. Preferably, a further stream is discharged from the depolymerization section [B]. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization section [B]. In case phosphoric acid is used as depolymerization catalyst, the additional stream can comprise impurities present in the fishing nets containing nylon 6, undepolymerized nylon 6, undepolymerized ε-caprolactam, catalyst and compounds formed under the depolymerization conditions, such as monoammonium phosphate, diammonium phosphate and / or triammonium phosphate. In a preferred embodiment, a stream comprising monoammonium phosphate, diammonium phosphate and / or triammonium phosphate is discharged from the depolymerization section [B]. More preferably, this stream discharged from the depolymerization section [B] comprises monoammonium phosphate, diammonium phosphate 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.-%.

[0102] The depolymerization of the fishnet comprising nylon 6 in the presence of steam can be performed in the presence of an additional depolymerization agent, such as ammonia. The concentration of ammonia in depolymerization section [B] can vary. Thus, in case ammonia is present in depolymerization section [B], the steam stream removed from depolymerization section [B] comprises not only ε-caprolactam and impurities, but can also comprise ammonia.

[0103] Most preferably, the depolymerization is performed in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted acid or base. The acid catalyst can be especially selected from the group consisting of orthophosphoric acid; p-toluenesulfonic acid; boric acid; sulfuric acid; organic acids; organic sulfonic acids, including xylene sulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons; solid acids; salts of the foregoing acids; AI2O3; and SiO2; and combinations thereof. The base catalyst can for example be selected from the group consisting of alkali metal hydroxides; alkali metal salts; alkaline earth metal hydroxides; and alkali metal, such as alkaline earth metal salts; organic bases and solid bases; and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali metal hydroxides and alkali metal salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate are used as catalysts. More preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid and sodium hydroxide are used as catalysts. In a particularly preferred embodiment, orthophosphoric acid is used as catalyst for the depolymerization, in another p-toluenesulfonic acid is used.

[0104] However, in another preferred embodiment, no catalyst is used for the depolymerization of the fishnet comprising nylon 6. This has the advantage of lower costs (cost of catalyst and cost of disposal of catalyst waste). However, typically higher temperatures (and pressures) are required compared to the depolymerization of the fishnet comprising nylon 6 performed in the presence of a catalyst.

[0105] The advantage of using a catalyst (and in particular orthophosphoric acid) is that the depolymerization reaction has already started at low temperature and can be carried out under atmospheric conditions. Suitable concentrations of catalysts for depolymerizing nylon 6 into ε-caprolactam are known to the skilled person and can easily be determined by routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. Conversely, if the concentration of the catalyst used is too high, the reaction is fast and the side reactions increase. Furthermore, the catalyst costs increase, which is economically disadvantageous. Preferably, the catalyst content is between 0.01 and 100% by weight, relative to the nylon 6 contained in the depolymerization reactor. More preferably, the catalyst content is between 0.1 and 50% by weight. The most preferred catalyst concentration depends on the type of catalyst used for the depolymerization of nylon 6. For the catalyst orthophosphoric acid, a preferred content is between 0.1 and 25% by weight and more preferably between 1 and 20% by weight. The preferred content of the catalyst p-toluenesulfonic acid is between 10 and 35% by weight and more preferably between 15 and 30% by weight.

[0106] The depolymerization of nylon 6 can be carried out in batch mode, in semi-continuous mode or in continuous mode, all of which modes are known to the skilled person. The terms "batch mode", "semi-continuous mode" and "continuous mode" as used herein refer to the mode of charging the depolymerization reactor with the nylon 6 containing raw material, i.e. the fishing net comprising nylon 6 and optionally the catalyst, and the mode of discharging the residual material from the depolymerization reactor.

[0107] In a preferred embodiment, the depolymerization of nylon 6 is carried out in batch mode. In batch mode, the raw material, i.e. the fishing net comprising nylon 6, and optionally the catalyst, is first charged into the depolymerization reactor. Subsequently, superheated steam is charged into the depolymerization reactor and ε-caprolactam is discharged from the depolymerization reactor in the form of a steam stream comprising ε-caprolactam and water. Next, the charging 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 charging the raw material (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, the residual material is not removed between individual cycles.

[0108] In a particular advantageous embodiment, the depolymerization of nylon 6 is carried out in continuous mode. In continuous mode, the nylon 6 containing raw material (and optionally the catalyst) is continuously charged into the depolymerization reactor. At the same time, superheated steam is continuously charged into the depolymerization reactor and ε-caprolactam is continuously discharged from the depolymerization reactor in the form of a steam stream comprising ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged into the depolymerization reactor. In addition, the residual material is continuously discharged from the depolymerization reactor. Preferably, the fishing net comprising nylon 6 is charged in the form of a melt. Preferably, the catalyst is charged in the form of a melt, a slurry or a solution.

[0109] In another preferred embodiment, the nylon 6 depolymerization is carried out in a semi-continuous mode. In the semi-continuous mode, the nylon 6 containing feedstock (and optionally the catalyst) is intermittently charged to the depolymerization reactor, while superheated steam is continuously charged to the depolymerization reactor, and the e-caprolactam is continuously withdrawn from the depolymerization reactor as a stream comprising e-caprolactam and water. Residual material is intermittently withdrawn from the depolymerization reactor in the semi-continuous mode of nylon 6 depolymerization.

[0110] Recovery step c)

[0111] In recovery section [C], e-caprolactam is recovered from the stream comprising e- caprolactam withdrawn from depolymerization section [B]. This stream comprises e- caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the stream comprising e-caprolactam.

[0112] Preferably, the e-caprolactam obtained by condensation is dissolved in water, thereby obtaining an e-caprolactam rich phase, without charging a solvent to depolymerization section [B]. This e-caprolactam rich phase also comprises impurities.

[0113] Preferably, in case water is charged as solvent to depolymerization section [B], the stream comprising e-caprolactam withdrawn from depolymerization section [B] comprises e-caprolactam, water and impurities. The water can be charged in liquid form or in vapor form. Preferably, the water is charged in vapor form. The e-caprolactam can be separated from the stream comprising e-caprolactam withdrawn from depolymerization section [B] by sending this vapor or gas stream from the depolymerization reactor, preferably from the top, to a (preferably partial) condenser, to obtain a condensate comprising e-caprolactam. Preferably, the e-caprolactam is separated from the remaining components of the vapor stream by sending the product stream from the depolymerization reactor, preferably from the top, to a distillation column, thereby obtaining a water rich phase as top product and an e-caprolactam rich phase as bottom product.

[0114] The e-caprolactam recovered in recovery section [C] is a crude material, as it contains impurities such as nylon 6 decomposition products or other impurities originating from non-nylon 6 components (of the decomposition products) of the nylon 6 containing fishing net. The crude e-caprolactam recovered in step c) comprises water and e-caprolactam, preferably it is an aqueous solution comprising e-caprolactam. Therefore, the crude e-caprolactam recovered in recovery section [C] needs additional purification to obtain high purity e-caprolactam. Therefore, “crude” as used herein can be defined as having a lower purity, i.e. containing more impurities, compared to the purified e-caprolactam obtained as product of the process of the present invention.

[0115] Preferably, the crude e-caprolactam comprises e-caprolactam in the range of 6 to 95 wt.%, more preferably 20 to 90 wt.% and most preferably 35 to 80 wt.%. The remainder is mainly water.

[0116] Purification step d)

[0117] In step d), the crude ε-caprolactam obtained in recovery section [C] is purified in purification section [D] to obtain ε-caprolactam of high purity.

[0118] Optionally, the crude ε-caprolactam is filtered prior to charging purification section [D]. Filtration ensures removal of unsolved impurities which might otherwise hinder the further purification process.

[0119] Optionally, the crude ε-caprolactam is separated from oil prior to charging purification section [D]. Oil separation ensures removal of impurities which might otherwise hinder the further purification process.

[0120] Purified ε-caprolactam is obtained from crude ε-caprolactam by first extracting the crude ε-caprolactam with an organic solvent in step (i), whereby an aqueous phase and an organic phase comprising the organic solvent, ε-caprolactam and impurities are obtained. The organic solvent with which the crude ε-caprolactam is extracted is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 aliphatic alcohol or a cycloaliphatic alcohol. Optionally, the organic solvent with which the crude ε-caprolactam is extracted is preferably a mixed extractant consisting of an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 aliphatic alcohol or a cycloaliphatic alcohol and a C5-C8 alkane or a C5-C8 cycloalkane. Especially good results are achieved if the organic solvent used for extracting the crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol and mixtures thereof. More preferably, the organic solvent used for extracting the crude ε-caprolactam is selected from the group consisting of benzene, toluene, an alcohol and mixtures thereof. More preferably, the organic solvent used for extracting 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 0.01 : 1 to 40 : 1, preferably 0.05 : 1 to 15 : 1, more preferably 0.1 : 1 to 7 : 1 and most preferably 0.1 : 1 to 5 : 1.

[0121] Optionally, the organic solvent used for extracting the crude ε-caprolactam is mixed with an alkane: m H 2m+2 wherein m is 5 to 8; a cycloalkane; or a C m H 2mwherein m is 5 to 8, such that a mixed extractant is formed. Particularly good results are achieved if the alkane or cycloalkane is present in the mixed extractant in the range of 5 to 90 % by weight and preferably 25 to 75 % by weight of the total weight of the mixed extractant.

[0122] In another embodiment, wherein 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 an extract column operated in counter current, wherein the crude ε-caprolactam to be purified is introduced at the upper part of the column and the organic solvent is introduced at the lower part. The extraction results in an aqueous phase comprising water and impurities and an organic phase comprising the organic solvent, ε-caprolactam and impurities. The extraction results in an organic phase comprising the organic solvent, ε-caprolactam and impurities, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, due to this extraction, the ε-caprolactam is more pure than before the extraction.

[0123] In another embodiment of the present application, wherein 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 an extract column operated in counter current, wherein the crude ε-caprolactam to be purified is introduced at the lower part of the column and the organic solvent is introduced at the upper part. The extraction results in an aqueous phase comprising water and impurities and an organic phase comprising the organic solvent, ε-caprolactam and impurities. The extraction results in an organic phase comprising the organic solvent, ε-caprolactam and impurities, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, due to this extraction, the ε-caprolactam is more pure than before the extraction.

[0124] Optionally, the organic phase comprising the organic solvent, ε-caprolactam and impurities is washed with water or with an aqueous alkaline solution before entering step d)(iv). If the washing is carried out with an aqueous alkaline solution, the alkaline solution is preferably an aqueous solution comprising an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably comprises 0.5 to 2.0 % by weight of sodium hydroxide or potassium hydroxide.

[0125] The skilled person can determine the amount of water or aqueous base solution required to effectively wash the organic phase comprising organic solvent, ε-caprolactam and impurities by routine experimentation. Preferably, this amount is between 0.1 and 5 wt% relative to the amount of organic solvent excluding ε-caprolactam dissolved in the organic phase to be washed. In another preferred embodiment, washing the organic phase comprising organic solvent, ε-caprolactam and impurities with water or aqueous base solution is carried out in a countercurrently operated washing column, wherein the organic phase comprising organic solvent, ε-caprolactam and impurities is introduced at the bottom of the column and water or aqueous base solution is introduced at the top. The washing results in a washed organic phase comprising organic solvent, ε-caprolactam and impurities and an aqueous phase comprising residuals. Typically, the aqueous phase comprising residuals comprises water, ε-caprolactam and impurities. Due to the washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase prior to the washing.

[0126] Optionally, according to step d)(ii) of the process of the present application, the solvent of the obtained organic phase comprising organic solvent, ε-caprolactam and impurities, optionally washed with water or with aqueous base, is converted, whereby the organic solvent in the organic phase comprising organic solvent, ε-caprolactam and impurities is replaced by water, and whereby an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is obtained, and wherein the solvent conversion process is selected from a process based on water stripping (also referred to as re-extraction) and a process based on solvent exchange distillation, wherein the organic solvent is distilled off and water is charged.

[0127] The term "replacement" as used herein means that at least 60 wt%, preferably at least 80 wt% and most preferably at least 90 wt%, 95 wt% or 98 wt% of the organic solvent present in the organic phase comprising organic solvent, ε-caprolactam and impurities is replaced by water.

[0128] The solvent conversion can be a process based on water stripping, whereby an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is obtained. Preferably, this aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is stripped and / or distilled to remove residual organic solvent. Although the amount of water used for stripping ε-caprolactam can vary, the amount of water used is preferably 0.3 to 20 times, more preferably 0.4 to 10 times and most preferably 0.5 to 5 times the weight of ε-caprolactam recovered.

[0129] Preferably, the stripping with water can be carried out in a countercurrently operated extraction column.

[0130] In another preferred embodiment, wherein the organic phase comprising organic solvent, ε-caprolactam and impurities has a lower density than water, the organic phase is introduced in the lower part of the extraction column and water is introduced in the upper part of the extraction column. The back extraction results in an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and an organic solvent phase comprising impurities. The back extraction 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 in the aqueous phase being lower compared to the weight ratio of impurities to ε-caprolactam in the organic phase comprising organic solvent, ε-caprolactam and impurities before the back extraction. Thus, due to the back extraction, a purer ε-caprolactam is obtained. Preferably, the organic solvent phase comprising impurities is reused, optionally after purification, preferably by distillation.

[0131] In another preferred embodiment, wherein the organic phase comprising organic solvent, ε-caprolactam and impurities has a higher density than water, the organic phase is introduced in the upper part of the extraction column and water is introduced in the lower part of the extraction column. The back extraction results in an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and an organic solvent phase comprising impurities. The back extraction 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 in the aqueous phase being lower compared to the weight ratio of impurities to ε-caprolactam in the organic phase comprising organic solvent, ε-caprolactam and impurities before the back extraction. Thus, due to the back extraction, ε-caprolactam is purer than before the back extraction. Preferably, the organic solvent phase comprising impurities is reused, optionally after purification, preferably by distillation.

[0132] Thus, according to a particular advantageous embodiment of the present application, the purification in step d) further comprises a step (ii) a) solvent switch based on back extraction with water after the extraction of the crude ε-caprolactam in step d) (i).

[0133] The solvent conversion process can also be a process based on solvent exchange distillation, wherein the organic solvent is distilled off and water is charged. In a preferred embodiment, the solvent conversion process is a process based on solvent exchange distillation, which is performed as a single stage process, wherein the organic solvent is distilled off from the organic phase comprising the organic solvent, the ε-caprolactam and the impurities, and water is charged. More preferably, the solvent conversion is performed in the form of azeotropic distillation with the addition of water, in which case the organic solvent evaporates as an azeotropic mixture comprising the organic solvent and water. The purpose of the azeotropic distillation is to remove the organic solvent and to add water. Preferably, substantially all of the organic solvent is removed. By "substantially all" in this context is meant that at least 90 wt.%, preferably at least 95 wt.% and most preferably at least 98 wt.% or 99 wt.% of the organic solvent present in the organic phase comprising the organic solvent, the ε-caprolactam and the impurities is removed. Preferably, the water is added in liquid form. More preferably, the water in liquid form is added as reflux to the upper part of the distillation column. More preferably, part of the water added as reflux is obtained by condensation of the azeotropic mixture distilled off in the distillation column.

[0134] Any suitable vessel can be used for the solvent conversion process, such as a column, preferably a distillation column, operated in continuous mode. The distillation column can comprise trays, packing or a combination thereof.

[0135] In another preferred embodiment, the solvent exchange distillation is performed as a two-stage process. The first stage is a pre-concentration stage and the second stage is the actual solvent exchange distillation.

[0136] The organic phase comprising the organic solvent, the ε-caprolactam and the impurities is charged to 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, the ε-caprolactam and the impurities in the upper part of the distillation column. Preferably, this distillation is performed under reflux. By under reflux is meant that the organic solvent in liquid form is charged to the upper part of the distillation column. More preferably, part of the organic solvent removed by distillation in the upper part of the distillation column is, after condensation, charged to the upper part of the distillation column in liquid form. The remaining organic phase comprising the organic solvent, the ε-caprolactam and the impurities is discharged from the first stage and charged to the second stage. Due to the distillation in the first stage, the chemical composition of the remaining organic phase comprising the organic solvent, the ε-caprolactam and the impurities is different from the organic phase comprising the organic solvent, the ε-caprolactam and the impurities charged to the first stage. In general, the remaining organic phase comprising the organic solvent, the ε-caprolactam and the impurities contains a higher weight percentage amount of ε-caprolactam and compounds having a boiling point higher than ε-caprolactam, and a lower weight percentage of compounds having a boiling point lower than ε-caprolactam than the organic phase comprising the organic solvent, the ε-caprolactam and the impurities charged to the first stage.

[0137] In the second stage, the remaining organic phase comprising the remaining organic solvent, the ε-caprolactam and the impurities is distilled for removal of the remaining organic solvent and water is charged. More preferably, in the second stage, the solvent shift is performed in the form of azeotropic distillation with the addition of water, in which case the organic solvent evaporates as an azeotropic mixture comprising the organic solvent and water.

[0138] The purpose of the azeotropic distillation is the removal of the organic solvent and the addition of water. Preferably, substantially all of the organic solvent is removed. "Substantially all" in this context means that at least 90 wt.-%, preferably at least 95 wt.-% and most preferably at least 98 wt.-% or 99 wt.-% of the organic solvent present in the remaining organic phase comprising the organic solvent, the ε-caprolactam and the impurities is removed. Preferably, the water is added in liquid form. More preferably, the water is added as reflux to the upper part of the distillation column in liquid form. More preferably, part of the water added as reflux is obtained by condensation of the azeotropic mixture distilled out of the distillation column.

[0139] Any suitable vessel can be used for the stages of the solvent shift, such as a column, preferably a distillation column operated in continuous mode. The distillation column can comprise trays, packing or a combination thereof.

[0140] The solvent exchange distillation, performed as a single stage process or as a two stage process, results in a water phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and optionally residual organic solvent. Preferably, the ε-caprolactam content of this water phase is between 25 wt.-% and 99.9 wt.-%, more preferably between 50 wt.-% and 99.5 wt.-% and most preferably between 85 wt.-% and 99 wt.-% relative to the complete water phase.

[0141] Accordingly, in a particular advantageous embodiment of the present application, the purification in step d) further comprises step (ii) b) solvent shift based on solvent exchange distillation after the extraction of the crude ε-caprolactam in step d) (i).

[0142] Optionally, step d) (iii) of the process according to the present application, prior to the crystallization in step d) (iv), impurities having a boiling point lower or higher than ε-caprolactam are removed by distillation under vacuum conditions, thereby obtaining a phase comprising ε-caprolactam and impurities. The obtained phase comprising ε-caprolactam and impurities is purer, i.e. contains less impurities, than the phase charged to step d) (iii).

[0143] In step d)(iii) of the process of the application, the organic phase comprising the organic solvent, the ε-caprolactam and the impurities obtained by the extraction in step d)(i), optionally washed with water or with an aqueous base, is distilled to remove the organic solvent and the impurities having a boiling point lower or higher than ε-caprolactam from the organic phase. Preferably, the distillation is carried out under reduced pressure. More preferably, the distillation is carried out at a pressure lower than 80 kPa, more preferably lower than 20 kPa and most preferably lower than 10 kPa. Preferably, the temperature is between 90°C and 210°C and more preferably between 1 10°C and 180°C. These temperatures refer to the temperature in the bottom of the distillation column where the distillation is carried out.

[0144] Alternatively, in step d)(iii) of the process of the application, the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam obtained by the solvent switch in step d)(ii) is distilled to remove the impurities having a boiling point lower or higher than ε-caprolactam from the aqueous phase.

[0145] Preferably, water is first evaporated from the aqueous phase comprising water, ε-caprolactam and impurities having 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. More preferably, the distillation is carried out at a pressure lower than 50 kPa, more preferably lower than 20 kPa and most preferably lower than 10 kPa. Preferably, the temperature is between 100°C and 200°C and more preferably between 1 10°C and 180°C. These temperatures refer to the temperature in the bottom of the distillation column where the distillation is carried out.

[0146] The distillation comprises separating low boiling point organic impurities (having a boiling point lower than ε-caprolactam) from ε-caprolactam and / or separating organic high boiling point impurities (having a boiling point higher than ε-caprolactam) from ε-caprolactam. The distillation preferably comprises, in a first step, separating low boiling point impurities from ε-caprolactam as a top product and producing ε-caprolactam containing high boiling point impurities as a bottom product. In a second step, high purity ε-caprolactam is separated as a top product and a distillation residue comprising ε-caprolactam and high boiling point impurities is obtained as a bottom product.

[0147] According to a particular advantageous embodiment of the application, the purification in step d) further comprises a step (iii) of distillation under vacuum conditions to remove impurities having a boiling point lower or higher than ε-caprolactam prior to the crystallization in step d)(iv).

[0148] In a preferred embodiment, prior to the distillative removal in step d)(iii), an alkali hydroxide, preferably NaOH, is added to the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol, more preferred and most preferred 2 to 80 mmol per kg of ε-caprolactam. Experiments have shown that the addition of an alkali hydroxide, especially NaOH, allows an especially effective distillative removal of impurities having a boiling point lower and higher than ε-caprolactam.

[0149] In another preferred embodiment, prior to the distillative removal in step d)(iii), an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide, is added to the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Most preferred, potassium permanganate is used as oxidizing agent.

[0150] The oxidizing agent can be added to the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam in solid form, in slurry form or in form of an aqueous solution in order to obtain a diluted aqueous solution. The skilled person can determine the amount of oxidizing agent required for the effective oxidation of the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam by routine experiments. The precise amount of oxidizing agent depends, inter alia, to a large extent on the composition of the waste fishing nets comprising nylon 6 used as feed material in the process of the present application. Preferably, the amount of oxidizing agent is between 0.01 wt.-% and 5 wt.-% relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0151] The temperature used for the oxidation of the aqueous solution in the process of the present application can vary. Preferably, the aqueous solution comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is oxidized with an oxidizing agent selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, especially potassium permanganate, at a temperature in the range of 20 °C to 85 °C, more preferred in the range of 30 °C to 80 °C, prior to the distillative removal in step d)(iii).

[0152] The length of time for the oxidation with the oxidizing agent can vary. Preferably, the aqueous solution comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is oxidized with the oxidizing agent for 1 minute to 24 hours, more preferred for 2 minutes to 6 hours and most preferred for 5 minutes to 2 hours.

[0153] The concentration of ε-caprolactam in the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam for oxidation with an oxidizing agent can vary. Preferably, the aqueous solution for oxidation comprises ε-caprolactam and water in a weight ratio of 5:1 to 1 :5, more preferably 3:1 to 1 :3 and most preferably 2:1 to 1 :2. Optionally, the weight ratio of ε-caprolactam to water is adapted prior to adding the oxidizing agent to the aqueous phase. Preferably, the weight ratio of ε-caprolactam to water is adapted by adding water or by removing water.

[0154] In case potassium permanganate or sodium permanganate is used as oxidizing agent, manganese (IV) oxide (Mn02) solid particles are formed as reaction product. The skilled person can determine by routine experimentation the most preferred solid-liquid filtration procedure for efficient removal of manganese (IV) oxide solid particles from the aqueous phase after oxidation. In this regard, it is common practice to use a filter aid, such as activated carbon or diatomite particles, to improve the filtration procedure.

[0155] In a preferred embodiment of the present application, the aqueous solution comprising water, ε-caprolactam and impurities is hydrogenated in the presence of a hydrogenation catalyst prior to crystallization in step d)(iv). The hydrogenation catalyst can be any known heterogeneous hydrogenation catalyst. Examples of such catalysts are ruthenium on alumina, rhodium on alumina, platinum on carbon, palladium on carbon, Raney nickel, nickel on silicon and nickel on alumina. Preferably, a nickel containing catalyst is utilized. Suitable nickel catalysts typically have a nickel content between 5 wt.% and 80 wt.% relative to the metal and the support. In addition to nickel, the catalyst can contain some activator, such as Zr, Mn, Cu or Cr. The activator content is typically between 1 wt.% and 20 wt.%. If a palladium containing heterogeneous catalyst is used, the palladium content will typically be between 0.01 wt.% and 10 wt.%.

[0156] The heterogeneous catalyst can be contacted with the hydrogen containing reaction mixture in various ways. The hydrogenation can for example take place in a stirred tank reactor, wherein the catalyst particles are suspended in the mixture to be purified (slurry phase process). In another embodiment, the hydrogenation is carried out in a fixed bed reactor, wherein the catalyst is immobilized in the reactor.

[0157] The hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising the aqueous ε-caprolactam mixture, gaseous hydrogen and the heterogeneous hydrogenation catalyst. Alternatively, the hydrogenation can be carried out in a two-phase system (liquid, solid) comprising the aqueous ε-caprolactam mixture which is fully or partially hydrogen saturated and the heterogeneous hydrogenation catalyst. The dissolution of hydrogen in the water-ε-caprolactam mixture to obtain a fully or partially hydrogen saturated mixture can be achieved by any method known to the person skilled in the art.

[0158] The hydrogenation temperature is generally between 20 and 160°C. The hydrogenation pressure is generally between 0.1 and 15 MPa.

[0159] The hydrogenation of the water-ε-caprolactam mixture is carried out to hydrogenate unsaturated compounds present in the impure ε-caprolactam. The presence of these unsaturated compounds is disadvantageous because they can impair the physical-mechanical properties of the nylon 6 prepared by polymerization of ε-caprolactam. The saturated compounds formed by hydrogenation do not adversely affect these physical-mechanical properties of the nylon 6, in addition, these compounds are more easily removed in the steps following the hydrogenation step, for example in a distillation step and / or a crystallization step.

[0160] In step d)(iv) of the process of the present application, the purified ε-caprolactam is obtained by crystallization of ε-caprolactam from a solution comprising ε-caprolactam and impurities at a temperature of 10 to 95°C, more preferably at a temperature of 20 to 85°C.

[0161] The solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is an organic phase comprising an organic solvent, ε-caprolactam and impurities, which is optionally washed with water or with an aqueous base, which is obtained by extraction in step d)(i). Preferably, the solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, which is obtained by solvent switching in step d)(ii). More preferably, the solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is a phase comprising ε-caprolactam and impurities, which is obtained by distillation under vacuum in step d)(iii).

[0162] Preferably, the ε-caprolactam crystallization process in step d)(iv) comprises the following steps:

[0163] 1. feeding the solution comprising ε-caprolactam and impurities to a crystallizer;

[0164] 2. setting conditions in the crystallizer so that ε-caprolactam crystals and mother liquor are formed;

[0165] 3. separating the ε-caprolactam crystals from the mother liquor;

[0166] 4. recycling the mother liquor.

[0167] More preferably, the ε-caprolactam crystallization process in step d)(iv) comprises the following steps:

[0168] 1. feeding the solution comprising ε-caprolactam and impurities to a crystallizer;

[0169] 2. Set conditions in a crystallizer such that ε-caprolactam crystals and mother liquor are formed;

[0170] 3. Feed the stream from the crystallizer to a separator, where ε-caprolactam crystals are separated from the mother liquor;

[0171] 4. Recycle the mother liquor.

[0172] Crystallization can be applied to produce ε-caprolactam. It is mainly used for its purification potential and / or product recovery to increase yield. All crystallization processes are based on the formation of a solid crystalline phase from a liquid. In a preferred embodiment, the crystallization in step d)(iv) is performed by solution crystallization or melt crystallization.

[0173] The term solution crystallization is used for the crystallization of a compound from a solution comprising the (impure) compound and an auxiliary solvent added. The auxiliary solvent is water or a non-aqueous solvent. In case the auxiliary solvent is water, the amount of water in the solution can be chosen in a wide range, preferably the amount of water is in the range of 0.5 to 25 wt.%, more preferably 1 to 8 wt.%. Preferably, the crystallization temperature is in the range of 20 to 70 °C, more preferably 30 to 65 °C. The purified ε-caprolactam is recovered from a slurry concentration preferably in the range of 5 to 75 wt.%, more preferably in the range of 10 to 70 wt.%, most preferably in the range of 15 to 50 wt.%. In case the auxiliary solvent is a non-aqueous solvent, the amount of non-aqueous solvent in the solution can be chosen in a wide range, preferably the amount of water is in the range of 5 to 95 wt.%, more preferably 10 to 90 wt.%, most preferably 30 to 70 wt.%. Preferably, the crystallization temperature is in the range of 20 to 70 °C, more preferably 30 to 65 °C. The purified ε-caprolactam is recovered from a slurry concentration preferably in the range of 5 to 75 wt.%, more preferably in the range of 10 to 70 wt.%, most preferably in the range of 15 to 50 wt.%. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, iso-octane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetra-chloro-methane, chloroform or chloroethane), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate), and mixtures of these solvents. Among these, cyclohexane is preferred.

[0174] Solution crystallization is usually performed at atmospheric pressure, but can be performed under reduced pressure or under pressurized conditions. In case of solution crystallization, the product is recovered by evaporative crystallization, wherein the solvent is evaporated, or by cooling crystallization, wherein cooling is obtained by direct cooling, indirect cooling or vacuum cooling, or by a combination of these methods. After the crystallization step, the crystals formed and the mother liquor are separated by, for example, sedimentation, filtration and / or centrifugation. Optionally, the resulting crystals are washed with, for example, a cleaning solvent. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in the form of crystals.

[0175] The term melt crystallization is defined in a narrow sense as crystallization of a compound from a solution containing the (impure) compound without the use of an auxiliary solvent. The term melt crystallization is defined in a broader sense as well to crystallization from a solution containing a low solvent concentration. Preferably, the solvent concentration in the solution is less than 25 wt.%, more preferably less than 10 wt.% and most preferably less than 5 wt.%. Herein, we will use the broader definition of melt crystallization unless explicitly mentioned otherwise. The compound crystals obtained by melt crystallization are separated from the mother liquor and optionally washed with a melt of pure compound material. Optionally, the crystallization-separation sequence is repeated several times. Finally, the optionally washed crystals are melted and discharged as a melt or removed mechanically.

[0176] Preferably, the solvent is present in the mixture in the crystallizer, but crystallization can also be performed without a solvent. Many solvents for ε-caprolactam are suitable. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform or chloroethane), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate). Preferably, water and aromatic hydrocarbons are used as solvent, as these solvents yield large crystals. Most preferably water is used as solvent. The solvent will act as a freezing point depressant for the melt in the crystallizer.

[0177] Generally, melt crystallization requires less energy than solution crystallization, however, the operation on an industrial scale can be more challenging.

[0178] Melt crystallization as used herein means especially layer melt crystallization or suspension melt crystallization. The two process methods for melt crystallization are characterized by (1) the formation of a crystal layer on the heat exchanger wall (layer melt crystallization) and (2) the growth of crystals in suspension (suspension melt crystallization). Generally, the operation of a process for the growth of a crystal layer on the wall of a heat exchanger is referred to as layer melt crystallization. First, a melt is charged into a crystallizer, then a crystal layer is grown on the cooled heat exchanger surface, followed by discharge of the remaining melt containing impurities rejected by the growing crystals from the crystallizer, after which the melt crystal layer is melted and the purified product is recovered. The purification efficiency can be further improved by, for example, melting, also referred to as partial melting, i.e. slightly heating the crystal layer to near its melting temperature, so that the impure mother liquor trapped and adhering is discharged. The layer melt crystallization process is operated in a batch mode. Well-known examples of processes based on layer melt crystallization are the ProABD process of BEFS Prokem and the Sulzer Chemtech process.

[0179] Layer melt crystallization can be performed in a static or dynamic mode. In static crystallization mode, crystals grow from stagnant melt onto a cooled surface. In static mode, the desired compound is batch crystallized from the stagnant melt in a closed vessel on the heat exchanger wall. Such crystallization is characterized by a low crystal growth rate, and thus a long residence (or batch) time. Preferably, the crystallization time is in the range of 1 hour to 75 hours, more preferably 2 hours to 50 hours, most preferably 4 hours to 24 hours. After the crystallization step, the remaining melt is drained. Subsequently, a melt phase is optionally introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or removed mechanically.

[0180] Generally, dynamic crystallization is performed in a shell-and-tube heat exchanger, where the melt is circulated downwardly towards a cooled surface, on which the compound crystallizes. Generally, the melt is pumped through the tubes, and crystals grow inside the tubes while a cooling medium flows through the shell. The thickness of the crystal layer increases in real time. After a certain period of time, the circulation of the melt is stopped and the remaining melt is drained. Dynamic layer crystallization is similar to stagnant layer crystallization, which is also performed in batch mode. In contrast to the stagnant mode, the crystal growth rate is higher in the dynamic mode, and thus the crystallization time is shorter. Preferably, the crystallization time is in the range of 0.05 hours to 12 hours, more preferably 0.1 hours to 6 hours, most preferably 0.3 hours to 3 hours. Subsequently, a melt phase is optionally introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or removed mechanically.

[0181] Suspension melt crystallization can be performed in batch or continuous mode. For suspension melt crystallization, the melt is cooled below its saturation temperature, and crystals start to grow (optionally after addition of crystal nuclei). The crystal growth rate is controlled by the super-saturation temperature of the melt. Suspension melt crystallization can be performed in any exchanger or vessel type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is performed in a wiped surface crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of crystals of the desired compound and mother liquor is packed into a so-called washing column. In the washing column, the mother liquor is drained from the crystals, which are then optionally washed with purified compound material.

[0182] After the ε-caprolactam crystallization step, a mother liquor is obtained which comprises ε-caprolactam in addition to impurities. Methods to recover ε-caprolactam from such mother liquors are well known to the skilled person. Moreover, due to these recovery methods, almost all ε-caprolactam present in the mother liquor can be recovered and converted into high purity ε-caprolactam. In case of multiple stage crystallization, one possible solution is to recycle the mother liquor in a counter current fashion, i.e. to charge the mother liquor obtained in the n-th crystallization stage to the feed of the (n-1)-th crystallization stage. In general, the mother liquor obtained from the 1-st crystallization stage is charged to an upstream (purification) unit of the process or to a dedicated mother liquor processing unit, e.g. based on distillation or crystallization. After the ε-caprolactam crystallization, it can be necessary to purify the obtained mother liquor (or a fraction thereof) by e.g. recycling it to a crude ε-caprolactam aqueous solution which is extracted with an organic solvent. Alternatively, the mother liquor can be purified by e.g. distillation, before being charged to the ε-caprolactam crystallization step.

[0183] The high purity ε-caprolactam obtained according to the process of the present application can be used to manufacture nylon 6 using methods well known to the skilled person. This nylon 6 can subsequently be used in all known materials, including engineering materials, fibers and films. This nylon 6 derived from fishing nets comprising nylon 6 is especially suitable for high speed spinning applications, including garments containing spandex (also known as elastane).

[0184] Plant

[0185] The present application also provides a plant, i.e. a chemical plant, comprising a depolymerization section [B], a recovery section [C] and a purification section [D], which plant is configured to carry out the process of the present application as described above. All plant features specifically described below in connection with the plant also correspond to specific embodiments of the process of the present application, and vice versa. Thus, the plant is suitable for carrying out the process of the present application, and it is understood that what has been described in connection with the process of the present application equally applies to the plant embodiments.

[0186] The plant can be a laboratory setup in the examples. Preferably, however, the plant is an industrial scale plant. By "industrial scale" is meant that the plant has a production capacity of at least 500 tons / year of ε-caprolactam (i.e. in principle is able to produce that amount of ε-caprolactam) if operated continuously.

[0187] The plant of the present application is suitable for producing purified ε-caprolactam from materials derived from fishing nets comprising nylon 6 and comprises at least the following three sections: a depolymerization section [B], a recovery section [C] and a purification section [D]. These sections and thus the plant are configured to carry out the process of the present application as described above.

[0188] Additionally, the plant of the present application can comprise a pre-treatment section [A] which can comprise a mechanical size reduction section [β] to fragment the material comprising the nylon 6 containing fishing nets into pieces and / or a cleaning section [α] to clean the material comprising the nylon 6 containing fishing nets and / or a densification section [γ] to obtain a material with increased bulk density. Cleaning comprises washing and separating foreign material from the fishing nets comprising nylon 6. Separating foreign material can be done manually (hand picking) and mechanically (e.g. density separation and magnetic separation). Manual and mechanical means, like brushes, can aid the washing process in the cleaning section [α]. Washing is preferably performed by an additional rubbing effect. Different types of industrial washing systems are available on the market, like high speed rubbing pads. The mechanical size reduction section [β] comprises equipment for mechanically fragmenting the fishing nets comprising nylon 6 into pieces. Non-limiting examples of such fragmentation equipment are cutting machines, shredders, grinders, mills and chippers. The densification section [γ] comprises equipment for densifying the material comprising the fishing nets containing nylon 6, which material is optionally fragmented and / or cleaned. Densification to obtain a material with higher bulk density can be performed by several techniques known to the person skilled in the art. Well-known examples of densification equipment include electric and hydraulic compaction machines and presses, as well as equipment where the feed is first melted and then solidified by cooling, such as single- and twin-screw extruders.

[0189] In a preferred embodiment of the present application, the plant further comprises a pre-treatment section [A] comprising an extraction section [ω], and optionally a mechanical size reduction section [β], and optionally a cleaning section [α], and optionally a densification section [γ].

[0190] In a more preferred embodiment of the present application, the plant further comprises a pre-treatment section [A] comprising a mechanical size reduction section [β] and a cleaning section [α], and optionally a densification section [γ].

[0191] The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or in parallel. The fishing nets comprising nylon 6 are fed to the reactor in solid form or in the form of a melt, preferably in the form of a melt. This feeding can be achieved by using an extruder, a gear pump or other means known in the art.

[0192] During production, the depolymerization reactor is at least partially filled with the raw material containing nylon 6, residual material, ε-caprolactam (and optionally catalyst). The depolymerization reactor can have any desired form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors and extruder type reactors.

[0193] The depolymerization reactor must be equipped with means for feeding the fishing net comprising nylon 6 and optionally superheated steam and catalyst. In addition, the depolymerization reactor is equipped with means for discharging the stream comprising ε-caprolactam and residual material.

[0194] Good contact between the steam and the reactor contents is essential for efficient operation. Such contact can be achieved by various means known to the skilled person. As an example, the steam can be injected into the material using a plurality of inlets, for example using a steam distributor. Further improved contact can be achieved by including mechanical agitation in the reactor, for example using a combination of rotating paddles and static fins.

[0195] Preferably, the depolymerization will be completed within 0.5 to 6 hours.

[0196] If superheated steam at high temperature is not available at the production site, it must be produced ad hoc by superheating steam obtained from a boiler in a so-called superheater.

[0197] The recovery section [C] can comprise one or more (partial) condensers into which a stream comprising ε-caprolactam in the form of a stream of steam comprising ε-caprolactam and water is charged. This (partial) condenser can have any desirable form. Preferably, the condenser is a distillation column from which a water-rich phase is obtained as top product and crude ε-caprolactam as bottom product.

[0198] The purification section [D] can comprise one or more pieces of extraction equipment, one or more pieces of solvent conversion equipment, an oxidation section, a hydrogenation section, one or more pieces of distillation equipment and a crystallization section into which crude ε-caprolactam is charged and high purity ε-caprolactam is discharged.

[0199] The extraction equipment is selected from mixer-settler extractors, extraction columns, centrifugal extractors and combinations thereof. Preferably, the extraction equipment is a static or agitated extraction column, such as columns, rotating disc contactors (RDC), pulse columns, sieve tray (static) columns, random packed (static) columns and structured packed (SMVP) (static) columns.

[0200] The solvent conversion equipment is selected from mixer-settler extractors, extraction columns, centrifugal extractors and combinations thereof. Preferably, the stripping equipment is a static or agitated extraction column, such as columns, Rotating disc contactor (RDC), pulse column, sieve tray (static) column, random packing (static) column and structured packing (static) column.

[0201] The solvent conversion apparatus for the process based on solvent swing distillation is selected from a sieve tray distillation column, a random packing distillation column and a structured packing distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser and a reflux apparatus. The distillation column can be operated at atmospheric pressure, sub-atmospheric pressure or super-atmospheric pressure. Preferably, water is charged to the upper part of the distillation column and a water phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is withdrawn from the lower part of the distillation column.

[0202] The oxidation section comprises one or more oxidation reactors operated in series and / or in parallel. An oxidizing agent and an ε-caprolactam water phase comprising water, ε-caprolactam and impurities are charged to the oxidation section. Typically, the oxidizing agent is charged in solid form, in slurry form or in aqueous solution. In case potassium permanganate or sodium permanganate is used as oxidizing agent, the oxidation section further comprises a filtration section. The oxidation reactors can have any desired form. Preferred reactor types are stirred and non-stirred reactors and packed column type reactors. The oxidation reactors must be equipped with means for charging the water phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and the oxidizing agent. In addition, the oxidation reactors must be equipped with means for withdrawing the oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities and optionally the formed manganese (IV) oxide (Mn02) solid particles. Preferably, the oxidation is carried out at a temperature in the range from 20 °C to 85 °C and under atmospheric conditions.

[0203] The optionally present manganese (IV) oxide (Mn02) solid particles can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to use filter aids, such as activated carbon particles or diatomaceous earth, to improve the filtration procedure. Filter systems suitable for the separation of manganese (IV) oxide solid particles are known to the skilled person. The filter system is charged with a suspension of the oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities and the manganese (IV) oxide solid particles and the filtered oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities is withdrawn. In general, the manganese (IV) oxide solid particles are retained in the filter system. Preferably, the filter system is operated in a semi-continuous mode whereby the suspension is continuously charged and the filtered phase is continuously withdrawn while the separated solids are collected in the filter system. Occasionally, the suspension is charged intermittently and the collected solids are removed from the filter system.

[0204] The purification of the crude ε-caprolactam in step d) to obtain purified ε-caprolactam can comprise a hydrogenation with a heterogeneous catalyst, in which case the plant will comprise a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0205] The hydrogenation section comprises one or more hydrogenation reactors operating in series and / or in parallel. The hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising an aqueous ε-caprolactam mixture, gaseous hydrogen and a heterogeneous hydrogenation catalyst. Alternatively, the hydrogenation can be carried out in a two-phase system (liquid, solid) comprising an aqueous ε-caprolactam mixture which is fully or partially hydrogen-saturated and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in the aqueous-ε-caprolactam mixture can be carried out by any method known to the person skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or a mixer which maintains a constant hydrogen pressure. Sufficient contact between hydrogen and the mixture will ensure the dissolution of hydrogen in the mixture. This method is preferably carried out continuously. The hydrogen-containing mixture is then contacted with the hydrogenation catalyst, for example in a separate reactor.

[0206] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. The hydrogenation can take place, for example, in a stirred tank reactor, in which the catalyst particles are suspended in the mixture to be hydrogenated (slurry phase process). In this slurry phase process, the catalyst particles must be separated from the purified mixture in an additional process step after the hydrogenation reaction, for example by filtration. Preferably, the catalyst comprises palladium or nickel.

[0207] Alternatively, the hydrogenation can be carried out in a fixed bed reactor, in which the catalyst is fixed in the reactor, so that an additional step of separating the catalyst from the reaction mixture can be omitted. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0208] The hydrogenation temperature is generally between 20 and 160°C. The hydrogenation pressure is generally between 0.1 and 15 MPa.

[0209] A distillation apparatus is charged with an aqueous ε-caprolactam phase comprising water, ε-caprolactam and impurities, and high-purity ε-caprolactam, water and impurities (i.e. low-boiling organic impurities (with a boiling point lower than that of ε-caprolactam) and high-boiling organic impurities (with a boiling point higher than that of ε-caprolactam)) are discharged. The distillation apparatus is selected from a sieve tray distillation column, a packed distillation column, a structured packed distillation column and a horizontal and vertical (elevating) thin-film evaporator. Preferably, the distillation column is equipped with a reboiler, a condenser and a reflux apparatus. The distillation apparatus can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure, preferably at subatmospheric pressure.

[0210] Preferably, the distillation comprises separating water, low-boiling organic impurities (having a lower boiling point than ε-caprolactam) and / or high-boiling organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises separating water in a first step as an overhead product and producing ε-caprolactam containing low-boiling impurities 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 comprising ε-caprolactam and high-boiling impurities is produced as a bottom product. Optionally, the first and second steps are combined.

[0211] Preferably, prior to removal of water and impurities by distillation, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam aqueous phase comprising water, ε-caprolactam and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol per kg of ε-caprolactam and more preferably 2 to 80 mmol per kg of ε-caprolactam. This allows for an especially efficient distillation removal of impurities having a boiling point lower than and higher than ε-caprolactam in the subsequent distillation.

[0212] The crystallization section comprises one or more crystallizers operated in series and / or in parallel. In general, the crystallization section also comprises several vessels to store the (intermediate) product stream and / or fresh and used washing liquids. Crystallization of ε-caprolactam can be performed by solution crystallization or melt crystallization, as described before.

[0213] In case of solution crystallization, ε-caprolactam is recovered by evaporative crystallization, wherein the solvent is evaporated, or by cooling crystallization, wherein cooling is obtained by direct cooling, indirect cooling or vacuum cooling, or by a combination of these methods. After the crystallization step in the crystallizer, the formed crystals are separated from the mother liquor by, for example, sedimentation in a settler, filtration in a filter and / or centrifugation in a centrifuge. Optionally, the crystallizer is equipped with a stirrer and / or one or more baffles.

[0214] Optionally, the obtained crystals are washed with, for example, a cleaning solvent. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in the form of crystals.

[0215] The auxiliary solvent is water or a non-aqueous solvent. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetra-chloromethane, chloroform or chloroethane), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate), and mixtures of these solvents. In general, the auxiliary solvent is recovered and reused in the crystallization process.

[0216] Solution crystallization is usually carried out at atmospheric pressure, but can be carried out under reduced pressure or under pressurized conditions.

[0217] The melt crystallization of ε-caprolactam can be accomplished by layer melt crystallization, in which a crystal layer containing ε-caprolactam is formed on the wall of a heat exchanger, or by suspension melt crystallization, in which crystals containing ε-caprolactam are grown in a suspension.

[0218] Preferably, a solvent is present in the mixture in the melt crystallizer, but melt crystallization can also be carried out without a solvent. Many solvents for ε-caprolactam are suitable. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as tetra- chloromethane, chloroform or chloroethane), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate). Preferably, water and aromatic hydrocarbons are used as solvent, since these solvents produce large crystals. Most preferably water is used as solvent. The solvent will act as a freezing point depressant for the melt in the crystallizer.

[0219] Layer melt crystallization:

[0220] First, the melt is charged into the crystallizer, then a crystal layer is grown on the cooled heat exchanger surface, then the remaining melt containing the impurities rejected by the growing crystals is discharged from the crystallizer, after which the melt crystal layer is melted and the purified product is recovered. The purification efficiency can be further improved by, for example, melting, also called partial melting, in which the crystal layer is slightly heated to near its melting temperature, so that the trapped and adhering impure mother liquor is discharged. The layer melt crystallization process is operated in a batch mode. Well-known examples of processes based on layer melt crystallization are the ProABD process of BEFS Prokem and the Sulzer Chemtech process.

[0221] Layer melt crystallization can be carried out in a static or dynamic mode. In the static crystallization mode, crystals are grown from a stagnant melt onto a cooled surface. In the static mode, the desired compound is batch crystallized from a stagnant melt in a closed vessel on the wall of a heat exchanger. Such crystallization is characterized by a low growth rate of the crystals, and thus a long residence (or batch) time. Preferably, the crystallization time is in the range of 1 hour to 75 hours, more preferably 2 hours to 50 hours, most preferably 4 hours to 24 hours. After the crystallization step, the remaining melt is discharged. Subsequently, a melting phase is optionally introduced to remove impurities adhering to the crystals or trapped in the crystals. Finally, the crystals are completely melted and discharged or removed mechanically.

[0222] Suspension melt crystallization:

[0223] The suspension melt crystallization of ε-caprolactam can be carried out in batch or continuous mode. In terms of suspension melt crystallization, the melt is cooled below its saturation temperature and ε-caprolactam crystals start to grow (optionally after addition of crystal nuclei). The growth rate of the crystals is controlled by the super-saturation temperature of the melt. The suspension melt crystallization can be carried out in any exchanger type or vessel type crystallizer which allows cooling of the melt. Preferably, the suspension melt crystallization is carried out in a wiped surface crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of ε-caprolactam crystals and mother liquor is charged into a so-called washing column. In the washing column, the mother liquor is drained from the ε-caprolactam crystals, which are then optionally washed with purified ε-caprolactam.

[0224] The process of the present application can be operated in continuous, semi-continuous or batch mode. Thus, the plant of the present application can also be configured to allow one or more of these modes of operation. In a preferred embodiment, the plant is configured to operate the process of the present application in continuous or semi-continuous mode. However, non-continuous processes are also possible. For example, the plant of the present application does not necessarily contain all the sections described herein at one location. In particular, the pre-treatment section [A] can be located at a first location, while the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a second location. Similarly, the mechanical size reduction section [β] as part of the pre-treatment section [A] can also be located at the first location, while the cleaning section [α] as part of the pre-treatment section [A] can be located at a second location, while the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a third location. Optionally, the cleaning section [α] is split into two or more subsections, which are optionally located at different locations. For example, a first subsection of the cleaning section [α] as part of the pre-treatment section [A] can be located at a first location, the mechanical size reduction section [β] as part of the pre-treatment section [A] can be located at a second location, while a second subsection of the cleaning section [α] as part of the pre-treatment section [A] can be located at a third location, while the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a fourth location. Optionally, the depolymerization section [B] is located at a different location than the pre-treatment section [A] and / or the recovery section [C] and the purification section [D].

[0225] Product

[0226] The present invention provides the new product epsilon-caprolactam, which is obtained according to the method of the present invention via depolymerization of nylon 6 resulting from (the material of) a fishing net comprising nylon 6. Advantageously, this epsilon-caprolactam is characterized, inter alia, by having a product carbon footprint of less than 2 kg CO2 equivalent per kg of purified epsilon-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). The epsilon-caprolactam obtained according to the present invention can also be referred to as "purified epsilon-caprolactam". "Purified" as used herein means that the epsilon-caprolactam is produced from a fishing net comprising nylon 6 according to the method of the present invention, thereby making the epsilon-caprolactam obtained in purified form. In this sense, the epsilon-caprolactam is obtained from a fishing net comprising nylon 6 and purified.

[0227] The method of the present invention allows for the production of high purity and thus high quality epsilon-caprolactam, which meets the specifications for high demanding applications, while the method is economically particularly friendly due to its reduced product carbon footprint and the use of waste material as starting material. In a preferred embodiment, the epsilon-caprolactam obtained by the method of the present invention meets one or more of the following specifications, wherein the parameters and measurement methods are defined in the example section below:

[0228] PAN: max. 5

[0229] E290: max. 0.05

[0230] VB: max. 0.5 mmol / kg

[0231] Basicity: max. 0.1 mmol / kg.

[0232] The epsilon-caprolactam produced by the method of the present invention is also economically and environmentally particularly friendly. Compared to traditionally produced epsilon-caprolactam (e.g. by Beckmann rearrangement of cyclohexanone oxime), the carbon footprint of the epsilon-caprolactam produced by the method of the present invention is significantly lower.

[0233] The environmental impact of a product is generally expressed as 'product carbon footprint'. The product carbon footprint is defined as the total emissions caused by the formation of that product, expressed as tons of carbon dioxide equivalent per ton of product. The product carbon footprint is determined, inter alia, by the raw materials, auxiliary materials, energy consumption, energy sources, production method and method efficiency. The quantification of the product carbon footprint can be performed as described in, for example, European Standard EN ISO 14040:2006 (Environmental management - Life cycle assessment - Principles and framework).

[0234] The product carbon footprint calculation can be done by an internal or external (preferred) certified organization. These organizations validate and certify product carbon footprint calculations based on e.g. LCA standard ISO 14040.

[0235] 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) report that the potential impact on global warming of "raw" e-caprolactam obtained via Beckmann rearrangement of cyclohexanone oxime is 7.5 tons of CO2 equivalent per ton of e-caprolactam (which equals 7.5 kg of CO2 equivalent per kg of e-caprolactam) in case coal-based electricity and steam generation is involved. If natural gas-based electricity and steam generation is involved, the potential impact on global warming of raw e-caprolactam in the e-caprolactam production process will decrease to 6.4 tons of CO2 equivalent per ton of e-caprolactam (which equals 6.4 kg of CO2 equivalent per kg of e-caprolactam).

[0236] The product carbon footprint of the e-caprolactam obtained according to the process of the present application is much lower than in resynthesized or "raw" e-caprolactam. Preferably, the product carbon footprint of the e-caprolactam obtained according to the process of the present application is less than 4 kg, more preferably less than 3 kg and most preferably equal to or less than 2 kg of CO2 equivalent per kg of e-caprolactam (based on data from ecoinvent version 3.7.1; location: Europe). BRIEF DESCRIPTION OF DRAWINGS

[0237] In the following, the present application is described with reference to the accompanying drawings, which depict certain embodiments of the present application. The present application, however, is not limited to the embodiments depicted in the drawings for illustrative purposes. It should not be restricted to the embodiments shown in the following drawings.

[0238] Figure 1 For a schematic representation of the process of the present application, the process comprises processing steps performed in an optional pre-treatment section [A], a depolymerization section [B], a recovery section [C] and a purification section [D].

[0239] Figure 2 represents two embodiments of the pre-treatment section [A] in which the fishing nets comprising nylon 6 are cleaned in a cleaning section [a] by removing extraneous material and by washing with a washing solvent and fragmented in a mechanical size reduction section [b] to obtain fragments of cleaned and fragmented fishing nets comprising nylon 6.

[0240] Figure 2A An embodiment of the pre-treatment section [A] is depicted, wherein the fishnet comprising nylon 6 is first cleaned in a cleaning section [a] by removing foreign material and by washing with a washing solvent, and subsequently fragmented in a mechanical size reduction section [β] to obtain fragments of the cleaned and fragmented fishnet comprising nylon 6.

[0241] Figure 2B An embodiment of the pre-treatment section [A] is depicted, wherein the fishnet comprising nylon 6 is first fragmented in a mechanical size reduction section [β] and subsequently cleaned in a cleaning section [a] by removing foreign material and by washing with a solvent to obtain fragments of the cleaned and fragmented fishnet comprising nylon 6.

[0242] Figure 3 represents two embodiments of the purification section [D], wherein the crude ε-caprolactam is purified to obtain high purity ε-caprolactam.

[0243] Figure 3A An embodiment of the purification section [D] of the process of the present invention is depicted, which process comprises an extraction section [γ], an optional washing section [δ], an optional back-extraction section [ε], an optional distillation section [θ] and a crystallization section [λ].

[0244] Figure 3B An embodiment of the purification section [D] of the process of the present invention is depicted, which purification section [D] comprises an extraction section [γ], an optional washing section [δ], an optional solvent exchange distillation section [μ], an optional distillation section [θ] and a crystallization section [λ].

[0245] Figure legend

[0246] The process of the present invention is schematically shown in Figure 1 . The process is carried out in the following plant sections:

[0247] Optionally, the fishnet comprising nylon 6 [1] is cleaned in a pre-treatment section [A] by removing foreign material and by washing with a washing solvent [2], whereby a contaminated washing solvent [3] is obtained. Subsequently, the fishnet comprising nylon 6 is fragmented by mechanical size reduction. The cleaned and fragmented fishnet comprising nylon 6 [6] is discharged from the pre-treatment section [A]. Optionally, the fishnet comprising nylon 6 [1] is additionally cleaned in the pre-treatment section [A] by removing foreign material. The removal of foreign material can be carried out before and / or after fragmentation of the fishnet comprising nylon 6. Optionally, the cleaned and fragmented fishnet comprising nylon 6 is densified, after which it is depolymerized in a depolymerization section [B] into ε-caprolactam (not shown in Figure 1

[0248] ​In the depolymerization zone [B], a optionally cleaned and fragmented fishing net containing nylon 6 [6] is depolymerized into ε-caprolactam. The stream containing ε-caprolactam is discharged from the depolymerization zone [B] [7]. Additionally, residual material is discharged [8]. Optionally, superheated steam [9] and catalyst

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

[0249] Crude ε-caprolactam is recovered from the stream containing ε-caprolactam discharged from the depolymerization section [B] in the recovery section [C] [7]

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

[12] .

[0250] The crude ε-caprolactam discharged from the recovery section [C] was purified in the purification section [D]

[11] to obtain high-purity ε-caprolactam

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

[25] .

[0251] Figure 2A The implementation of the pretreatment section [A′] (the area enclosed by the dashed line) is depicted, wherein the fishing net containing nylon 6 is first cleaned in the cleaning section [α′] by removing foreign material and washing with a washing solvent [2′], thereby obtaining foreign material, contaminated washing solvent [3′], and a cleaned fishing net containing nylon 6 [4′]. Subsequently, the cleaned fishing net containing nylon 6 [4′] is fragmented in the mechanical size reduction section [β′] to obtain clean and fragmented fragments of the fishing net containing nylon 6 [6′]. The clean and fragmented fragments are then discharged. Optionally, the cleaned and fragmented fishing net containing nylon 6 is densified and then depolymerized in the depolymerization section [B] to form ε-caprolactam ( Figure 2A (Not shown in the text).

[0252] Figure 2B An embodiment of the pretreatment section [A″] (the area enclosed by the dashed line) is depicted, wherein the fishing net containing nylon 6 [1″] is first fragmented in the mechanical size reduction section [β″] to obtain fragmented nylon 6 fishing net fragments [5″]. Subsequently, in the cleaning section [α″], the fragmented nylon 6 fishing net fragments [5″] are cleaned by removing foreign materials and washing with a washing solvent [2″] to obtain discharged foreign materials, contaminated washing solvent [3″], and cleaned and fragmented nylon 6 fishing net fragments [6″]. Optionally, the cleaned and fragmented nylon 6 fishing net is densified and then depolymerized into ε-caprolactam in the depolymerization section [B]. Figure 2B (Not shown in the text).

[0253] Figure 3A An implementation depicting the purification section [D″′] (the area enclosed by a dashed line) includes the following sections:

[0254] In an optional washing section [δ''] the organic phase [15''] comprising organic solvent, ε-caprolactam and impurities is washed with water or an aqueous base [16''] to obtain a water phase [17''] comprising residuals and a washed organic phase [18''] comprising organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ''].

[0255] In an optional washing section [δ''] the organic phase [15''] comprising organic solvent, ε-caprolactam and impurities is washed with water or an aqueous base [16''] to obtain a water phase [17''] comprising residuals and a washed organic phase [18''] comprising organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ''].

[0256] In an optional stripping section [ε''] the optionally washed organic phase [18''] comprising organic solvent, ε-caprolactam and impurities is stripped with water [19''] to obtain an organic solvent phase [20''] comprising impurities and a water phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Both phases are discharged from the stripping section [ε'']. Optionally, residual organic solvent is removed from the water phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam by stripping and / or distillation (not shown in Figure 3A ).

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

[0258] Optionally, the oxidized ε-caprolactam water phase (not shown in Figure 3A ) comprising water, ε-caprolactam and impurities is filtered to remove manganese (IV) oxide solids before feeding to the next section (not shown in

[0259] Optionally, the water phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam from which optional residual organic solvent has been removed by stripping and / or distillation is hydrogenated with hydrogen in the presence of a heterogeneous catalyst to obtain a hydrogenated ε-caprolactam water phase (not shown in Figure 3A ) comprising water, ε-caprolactam and impurities before feeding to the next section (not shown in

[0260] In an optional distillation section [θ''], the aqueous phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is distilled to remove impurities having a boiling point lower or higher than ε-caprolactam and optionally organic solvent or water, and to thereby obtain a distilled ε-caprolactam phase [24'']. All distillation products are discharged from the distillation section [θ'']. Optionally, prior to distillation in the distillation section [θ''], a base metal hydroxide [23''] is fed to the aqueous ε-caprolactam phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam.

[0261] In a crystallization section [λ''], the optionally distilled ε-caprolactam phase [24''] is crystallized to remove impurities [25''] from ε-caprolactam and to thereby obtain high purity ε-caprolactam [26'']. All crystallization products are discharged from the crystallization section [λ'']. Optionally, prior to crystallization in the crystallization section [λ''] ( Figure 3A a solvent is charged into the crystallization section [λ''] (not shown).

[0262] Figure 3B An embodiment of the purification section [D''] (area enclosed by the dashed line) comprising the following sections is depicted:

[0263] In an extraction section [γ''], the crude ε-caprolactam [11''] is extracted with an organic solvent [13''] to obtain an aqueous phase [14''] comprising water and impurities and an organic phase [15''] comprising organic solvent, ε-caprolactam and impurities. Both phases are discharged from the extraction section [γ''].

[0264] In an optional washing section [δ''], the organic phase [15''] comprising organic solvent, ε-caprolactam and impurities is washed with water or a basic aqueous solution [16''] to obtain an aqueous phase [17''] comprising residuals and a washed organic phase [18''] comprising organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ''].

[0265] In an optional solvent exchange distillation section [μ''], the optionally washed organic phase [18''] comprising organic solvent, ε-caprolactam and impurities is subjected to solvent exchange distillation by addition of water [19''] to obtain an organic solvent [21''] and an aqueous phase [22''] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Both distillation products are discharged from the solvent exchange distillation section [μ''].

[0266] Optionally, the aqueous ε-caprolactam phase [22'"] comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam, from which optional residual organic solvent has been removed by stripping and / or distillation, is oxidized with an oxidizing agent [22'"] to obtain an oxidized aqueous ε-caprolactam phase comprising water, ε-caprolactam and impurities [22'"] Figure 3B

[0267] Optionally, the oxidized aqueous ε-caprolactam phase comprising water, ε-caprolactam and impurities is filtered to remove manganese (IV) oxide solid particles, before being charged to the next section Figure 3B

[0268] Optionally, the aqueous ε-caprolactam phase [22'"] comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam, from which optional residual organic solvent has been removed by stripping and / or distillation, is hydrogenated with hydrogen in the presence of a heterogeneous catalyst [22'"] to obtain a hydrogenated aqueous ε-caprolactam phase comprising water, ε-caprolactam and impurities, before being charged to the next section Figure 3B

[0269] In the optional distillation section [Θ'"], the aqueous ε-caprolactam phase [22'"] comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam is distilled to remove impurities having a boiling point lower than or higher than ε-caprolactam and optionally organic solvent or water, and thereby obtain a distilled ε-caprolactam phase [24'"]. All distillation products are withdrawn from the distillation section [Θ'"]. Optionally, prior to distillation in the distillation section [Θ'"], a base metal hydroxide [23'"] is fed to the aqueous ε-caprolactam phase [22'"] comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam.

[0270] In the crystallization section [λ'"], the optionally distilled ε-caprolactam phase [24'"] is crystallized to remove impurities [25'"] from ε-caprolactam and thereby obtain high purity ε-caprolactam [26'"]. The crystallization product is withdrawn from the crystallization section [λ'"]. Optionally, prior to crystallization in the crystallization section [λ'"] Figure 3B

[0271] Example

[0272] The following examples serve to illustrate the present application in more detail, in particular with regard to certain forms of the present application. However, the examples are not intended to limit the present disclosure.

[0273] The ε-caprolactam which can be used for all major Nylon 6 polymerization applications without dilution with purer quality ε-caprolactam meets all the following specifications:

[0274] PAN: max 5 ​​​​

[0275] E290: max. 0.05

[0276] VB: max. 0.5 mmol / kg

[0277] Basicity: max. 0.1 mmol / kg

[0278] Acidity: max. 0.1 mmol / kg

[0279] Such parameters and measurement methods are defined as follows:

[0280] PAN: ISO DIS 8660 - Plastics - Determination of permanganate index of caprolactam - Spectrometric method, revision of the first edition ISO 8660; 1988,

[0281] E290: ISO 7059 - caprolactam for industrial use - determination of absorbance at a wavelength of 290 nm,

[0282] volatile bases (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile bases content - Titrimetric method after distillation.

[0283] Basicity of the ε-caprolactam product: by titration in 0.1 wt. / v 乙醇 % methylene blue: 0.1 wt. / v 乙醇 % methyl red (which is grey at the end point) in a 1 :2 ratio of 0.1 wt. / v

[0284] Basicity is then given by:

[0285] Basicity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y)

[0286] where:

[0287] v = volume of H2SO4 solution added (ml)

[0288] t = equivalent concentration of H2SO4 solution (= 0.01 N)

[0289] X = sample weight (g)

[0290] Y = ε-caprolactam concentration (wt. %)

[0291] The acidity is subsequently given by:

[0292] Acidity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y)

[0293] where:

[0294] v = volume of NaOH solution added (ml)

[0295] t = molar concentration of NaOH solution (= 0.01 N)

[0296] X = sample weight (g)

[0297] Y = ε-caprolactam concentration (wt. %)

[0298] The pellets comprising nylon 6 used in the examples and comparative experiments were made from discarded fishing nets. The pre-treatment included removal of foreign material, cleaning, grinding, melting and conversion into crumb / pellets. The pellets were obtained from a fishing net recycling company in China.

[0299] The pellets were rod-like and had an average diameter of about 3 mm and an average length of about 4 mm, and the weight of most of the pellets was between 20 and 30 mg.

[0300] The combination of thermogravimetric analysis (TGA) and qualitative information from differential scanning calorimetry (DSC) showed that the nylon 6 content of the pellets was > 98 wt. % (on a dry basis).

[0301] Example 1

[0302] Depolymerization and recovery of ε-caprolactam from nylon 6

[0303] Forty-eight grams of crumb / pellets comprising nylon 6 and 14 grams of 20 wt. % phosphoric acid were charged to an autoclave. First, the reactor contents were heated under nitrogen, and then superheated steam was continuously injected at a rate of 4 grams per minute during a 120 minute reaction. The temperature and pressure in the reactor were maintained at 260 °C and 0.11 MPa, respectively. During the reaction, a stream of vapors was continuously removed from the reactor and cooled to about 20 °C, thereby obtaining a condensate comprising ε-caprolactam and water.

[0304] The condensate consisting of approximately 39.9 grams of ε-caprolactam, the remainder mostly water, was concentrated to an ε-caprolactam concentration of 50.6 wt.% by evaporation in a rotary evaporator (rotary evaporator) operating under vacuum (9.5 kPa; water bath temperature of approximately 65 °C). (This mixture crude ε-caprolactam is the mixture to be purified.)

[0305] The specifications of the crude ε-caprolactam were:

[0306] PAN: 272

[0307] E290: 3.66

[0308] This example shows that crude ε-caprolactam can be obtained by depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6. Due to the extremely poor quality, this crude ε-caprolactam cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0309] Comparative Experiment 1

[0310] Depolymerization of nylon 6, recovery of ε-caprolactam and distillation purification

[0311] The procedure of Example 1 was followed. The condensate was concentrated to an ε-caprolactam concentration of 49.7 wt.% by evaporation. Subsequently, 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added. Subsequently, water and impurities with a boiling point lower than ε-caprolactam were removed as top product by distillation under reduced pressure in a batch operated distillation apparatus, wherein the pressure was gradually reduced. The ε-caprolactam was distilled at 300 Pa, while impurities with a boiling point higher than ε-caprolactam were retained in the distillation apparatus as bottom product. The specifications of the distilled ε-caprolactam were:

[0312]

[0313] This comparative experiment shows that the quality of ε-caprolactam obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by distillation is extremely poor, as it does not meet any of the required specifications for major polymerization applications.

[0314] Comparative Experiment 2

[0315] Depolymerization of nylon 6, recovery of ε-caprolactam and purification by oxidation

[0316] The procedure of Example 1 was followed. Subsequently, the crude ε-caprolactam was treated with 0.2 wt.% KMn04relative to ε-caprolactam at 50 °C for 2 hours. Subsequently, the formed solid was removed from the oxidized reaction product by filtration. The specifications of the purified ε-caprolactam were:

[0317] PAN: 127

[0318] E290: 4.37

[0319] This comparative experiment shows that the quality of the ε-caprolactam obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by oxidation is extremely poor and cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0320] Comparative Experiment 3

[0321] Depolymerization of nylon 6, recovery of ε-caprolactam and purification by oxidation and distillation

[0322] Subsequently, the ε-caprolactam water solution obtained in Comparative Experiment 2 was distilled according to the procedure described in Comparative Experiment 1 after addition of 75 mmol sodium hydroxide per kg ε-caprolactam in water. The specifications of the purified ε-caprolactam obtained were:

[0323]

[0324] This comparative experiment shows that the quality of the ε-caprolactam obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by oxidation and distillation is poor and cannot be used as ε-caprolactam for all major nylon 6 polymerization applications.

[0325] Comparative Experiment 4

[0326] Depolymerization of nylon 6, recovery of ε-caprolactam and purification by oxidation, carbon treatment and distillation

[0327] The procedure of Example 1 was followed. Subsequently, the crude ε-caprolactam was treated with 0.2 wt.% KMnO4 relative to ε-caprolactam at 50 °C for 2 hours. Subsequently, the resulting oxidized solution was treated with 0.4 wt.% powdered activated carbon at 50 °C for 0.5 hours. Subsequently, the formed solids and activated carbon particles were removed from the ε-caprolactam water solution by filtration. Subsequently, this activated carbon treated ε-caprolactam water solution was distilled according to the procedure described in Comparative Experiment 1 after addition of 75 mmol sodium hydroxide per kg ε-caprolactam in water. The specifications of the purified ε-caprolactam obtained were:

[0328] PAN: 6

[0329] E290: 0.40

[0330] VB: 0.72 mmol / kg

[0331] Basicity: 0.30 mmol / kg.

[0332] This comparative experiment shows that the quality of the epsilon-caprolactam obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by oxidation, carbon treatment and distillation is poor and cannot be used as epsilon-caprolactam for all major nylon 6 polymerization applications.

[0333] Comparative experiment 5

[0334] Nylon 6 was depolymerized, epsilon-caprolactam was recovered and purified by extraction, back extraction and distillation.

[0335] The procedure of example 1 was followed.

[0336] The obtained aqueous epsilon-caprolactam solution was concentrated to an epsilon- caprolactam concentration of 36.4 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of about 65 °C). (This mixture crude epsilon-caprolactam is the mixture to be purified.)

[0337] The crude epsilon-caprolactam was batch extracted ten times 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 8.02 grams of 4-methyl-2-pentanol / cyclohexane per gram of crude epsilon-caprolactam. The resulting organic extract was concentrated to an epsilon-caprolactam concentration of about 40 wt.% by distillation under vacuum, followed by the addition of fresh cyclohexane. The resulting mixture had an epsilon-caprolactam concentration of about 24 wt.% and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50 wt.%: 50 wt.%. This mixture was batch extracted 5 times with water at a temperature of about 25 °C. The total amount of water used was 18 grams of water per gram of recovered epsilon-caprolactam.

[0338] The obtained aqueous epsilon-caprolactam solution was further purified by distillation after the addition of 75 mmol sodium hydroxide aqueous solution per kg of epsilon- caprolactam as described in comparative experiment 1. The specifications of the obtained purified epsilon-caprolactam were:

[0339] PAN: 6

[0340] E290: 0.10

[0341] VB: 0.18 mmol / kg

[0342] Basicity: 0.15 mmol / kg.

[0343] This comparative experiment shows that the quality of the epsilon-caprolactam obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by extraction, back extraction and distillation is poor and cannot be used as epsilon-caprolactam for all major nylon 6 polymerization applications.

[0344] Example 2

[0345] The nylon 6 was depolymerized, the epsilon-caprolactam was recovered and purified by extraction, back extraction, distillation and crystallization.

[0346] The procedure of Example 1 was followed. The condensate consisting of about 40 grams of epsilon-caprolactam, the rest mostly water, was concentrated to an epsilon-caprolactam concentration of 36.1 wt.% by evaporation in a rotary evaporator operating under vacuum (9.5 kPa; water bath temperature of about 65 °C). (This mixture is the crude epsilon-caprolactam to be purified.)

[0347] The crude epsilon-caprolactam was extracted in batches ten times 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 8.09 grams of 4-methyl-2-pentanol / cyclohexane per gram of crude epsilon-caprolactam. The resulting organic extract was concentrated by distillation under vacuum to an epsilon-caprolactam concentration of about 40 wt.% followed by the addition of fresh cyclohexane. The resulting mixture had an epsilon-caprolactam concentration of about 25 wt.% and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50 wt.%: 50 wt.%. This mixture was extracted in batches seven times with water at a temperature of about 25 °C. The total amount of water used was 5.65 grams of water per gram of recovered epsilon-caprolactam.

[0348] The epsilon-caprolactam in the obtained aqueous solution was further purified by distillation as described in Comparative Experiment 1 after the addition of 75 mmol of aqueous sodium hydroxide solution per kilogram of epsilon-caprolactam.

[0349] Distilled water was subsequently added to the distilled epsilon-caprolactam to obtain a mixture with an epsilon-caprolactam concentration of 91.4 wt.%. This aqueous epsilon-caprolactam solution was introduced in a crystallization apparatus at a temperature of 52 °C. The aqueous epsilon-caprolactam solution was cooled to 40 °C and some seed crystals were added to the mixture. Thereafter, the mixture was allowed to cool to 30 °C, which temperature was maintained for 30 minutes. The crystallized epsilon-caprolactam was recovered by filtration and washed with an 85 wt.% aqueous epsilon-caprolactam solution. The specifications of the obtained purified epsilon-caprolactam were:

[0350] PAN: 2

[0351] E290: 0.03

[0352] VB: 0.04 mmol / kg

[0353] Acidity: 0.05 mmol / kg.

[0354] According to this embodiment, it can be concluded that purified ε-caprolactam, which meets all the required specifications for major polymerization applications, can be obtained from depolymerization of nylon 6 derived from discarded fishing nets containing nylon 6 and purified by extraction, back-extraction, distillation and crystallization.

[0355] Example 3

[0356] Depolymerization of nylon 6, recovery of ε-caprolactam and purification by extraction, back-extraction, distillation and crystallization.

[0357] The procedure of Example 1 was followed twice.

[0358] The two obtained aqueous ε-caprolactam solutions were concentrated by evaporation in a rotary evaporator running under vacuum (9.5 kPa; water bath temperature of about 65 °C) to ε-caprolactam concentrations of 70.2 wt.% and 67.6 wt.% respectively. Subsequently, the two concentrated aqueous ε-caprolactam solutions were added together. Two thirds of the resulting mixture of crude ε-caprolactam was used for further purification.

[0359] The crude ε-caprolactam was diluted to 65.0 wt.% and batchwise extracted with benzene 5 times at a temperature of about 25 °C. The total amount of extraction solvent used was 10.5 grams of benzene per gram of crude ε-caprolactam. The combined organic extracts were batchwise washed with 3.1 grams of a 2 wt.% aqueous NaOH solution. The resulting washed organic extracts were concentrated by distillation under vacuum to an ε-caprolactam concentration of about 17 wt.%. Subsequently, the resulting concentrated organic extracts were batchwise extracted with water 5 times at a temperature of about 25 °C. The total amount of water used was 1.57 grams of water per gram of concentrated combined organic extracts. The resulting combined aqueous ε-caprolactam solution was concentrated by evaporation in a rotary evaporator running under vacuum (9.5 kPa; water bath temperature of about 65 °C) to an ε-caprolactam concentration of 48.8 wt.%.

[0360] Subsequently, 75 mmol of aqueous sodium hydroxide per kilogram of ε-caprolactam was added to the concentrated ε-caprolactam solution. Subsequently, water and impurities with a boiling point lower than ε-caprolactam were removed as top products by distillation under reduced pressure in a batch operated distillation apparatus. Finally, the distilled ε-caprolactam was recovered as top product at 300 Pa, while impurities with a boiling point higher than ε-caprolactam were retained in the distillation apparatus as bottom product.

[0361] The distilled ε-caprolactam was introduced in a crystallization apparatus at a temperature of 52°C and deionized water was added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt.%. The ε-caprolactam aqueous solution was cooled to 40°C and some seed crystals were added to the mixture. Thereafter, the mixture was continued to cool to 30°C, at which temperature it was kept for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt.% ε-caprolactam aqueous solution. The specifications of the obtained purified ε-caprolactam were:

[0362] PAN: 2

[0363] E290: 0.03

[0364] VB: 0.1 mmol / kg

[0365] Basicity: 0.01 mmol / kg.

[0366] From this example, it can be concluded that purified ε-caprolactam, which complies with all the required specifications for main polymeric applications, can be obtained from the depolymerization of polyamide 6 derived from discarded fishing nets comprising polyamide 6 and purified by extraction, back-extraction, distillation and crystallization.

[0367] Example 4

[0368] Calculation of the carbon footprint of the purified ε-caprolactam

[0369] A continuous process for producing purified ε-caprolactam from fishing nets comprising polyamide 6 in a factory according to the present application was simulated. The process comprises:

[0370] - mechanical removal of foreign materials from the fishing nets comprising polyamide 6;

[0371] - cutting the fishing nets comprising polyamide 6 into small pieces;

[0372] - washing the pieces of fishing nets comprising polyamide 6 with water;

[0373] - separating the washed fishing nets comprising polyamide 6 and the aqueous extract by centrifugation;

[0374] - drying the pieces of washed fishing nets comprising polyamide 6;

[0375] - melting and pelletizing the pieces of washed fishing nets comprising polyamide 6;

[0376] - depolymerizing the polyamide 6 under the influence of H3PO4 and superheated steam;

[0377] - recovering the raw ε-caprolactam (80 wt.% ε-caprolactam) by partial condensation of the steam discharged from the depolymerization reactor;

[0378] - countercurrent extraction of the concentrated crude ε-caprolactam with benzene;

[0379] - washing the organic extract with a diluted caustic solution;

[0380] - countercurrent back extraction of the washed organic extract with water;

[0381] - evaporation of the concentrated aqueous extract;

[0382] - addition of caustic soda;

[0383] - distillative removal of light and heavy substances by vacuum distillation; and

[0384] - recovery of pure ε-caprolactam by melt crystallization at a temperature of 61 °C.

[0385] The carbon footprint of the purified ε-caprolactam was calculated based on the consumption values of the raw materials, and the utility of the process described above is based on data derived from ecoinvent version 3.7.1.

[0386] The results show that the product carbon footprint of the purified ε-caprolactam obtained from a fishing net containing polyamide 6 is less than 2 tons of CO2 equivalent per ton of ε-caprolactam (location Europe).

[0387] Although the application has been described with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the application, including (semi) continuous operation and commercial scale-up.

Claims

1. A process for recovering purified ε-caprolactam in a plant from a material selected from the group consisting of materials derived from fishing nets comprising nylon 6, wherein the plant comprises: a depolymerization section [B], a recovery section [C], and a purification section [D], and wherein the process comprises the following steps: a) charging a material derived from fishing nets comprising nylon 6 into the depolymerization section [B]; b) depolymerizing the material derived from fishing nets comprising nylon 6 material in the depolymerization section [B] at a temperature in the range of 180 °C to 400 °C so as to obtain a stream comprising ε-caprolactam; and d) purifying the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises the following steps: (i) extracting the crude ε-caprolactam with an organic solvent, thereby obtaining an organic phase, and wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities; and (ii) a) solvent switch based on counter-extraction with water, or (ii) b) solvent switch based on solvent exchange distillation; (iii) distillative removal of impurities having a boiling point lower or higher than ε-caprolactam under vacuum conditions; and (iv) obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution comprising ε-caprolactam and impurities at a temperature in the range of 10 to 95 °C; wherein after step d)(i) the organic phase obtained in step d)(i) is washed with water or with an aqueous base.

2. The process according to claim 1, wherein b) depolymerizing the material derived from fishing nets comprising nylon 6 material in the depolymerization section [B] at a temperature in the range of 200 °C to 350 °C so as to obtain a stream comprising ε-caprolactam.

3. The process according to claim 1, wherein b) depolymerizing the material derived from fishing nets comprising nylon 6 material in the depolymerization section [B] at a temperature in the range of 220 °C to 340 °C so as to obtain a stream comprising ε-caprolactam.

4. The process according to claim 1, wherein b) depolymerizing the material derived from fishing nets comprising nylon 6 material in the depolymerization section [B] at a temperature in the range of 240 °C to 325 °C so as to obtain a stream comprising ε-caprolactam.

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

6. The process according to claim 5, wherein the water present in step b) is in the form of steam, which is charged into the depolymerization section [B] in step b) in the form of superheated steam having a temperature in the range of 220 °C to 575 °C. ​ c) discharging the stream comprising ε-caprolactam from the depolymerization section [B] and recovering crude ε-caprolactam from the stream in the recovery section [C]; ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 7. The process of claim 5, wherein the water present in step b) is in the form of steam, which is charged into the depolymerization section [B] in step b) in the form of superheated steam having a temperature in the range of 275 °C to 500 °C.

8. The process of any one of claims 1 to 7, wherein the organic solvent in step d)(i) is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol and mixtures thereof.

9. The process of any one of claims 1 to 7, wherein the purification of the crude ε-caprolactam in step d) to obtain purified ε-caprolactam comprises: an oxidation step with an oxidizing agent in an aqueous solution at a temperature in the range of 20 °C to 85 °C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, and wherein the oxidation is carried out in an aqueous solution comprising water and ε-caprolactam in a weight ratio of 5:1 to 1 :5; and / or a hydrogenation step using a heterogeneous catalyst.

10. The process of claim 9, wherein the oxidizing agent is potassium permanganate; and / or the catalyst comprises nickel or palladium.

11. The process of any one of claims 1 to 7, wherein the depolymerization of the material derived from a fishing net comprising nylon 6 in step b) is carried out in the absence or in the presence of a catalyst, wherein the catalyst is selected from an acid catalyst and a base catalyst, the acid catalyst is selected from the group consisting of orthophosphoric acid, boric acid, sulfuric acid, organic acids, salts of the foregoing acids, AI2O3 and SiO2 and combinations thereof; and the base catalyst is selected from the group consisting of organic bases and solid bases and combinations thereof.

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

13. The process of claim 11, wherein the base catalyst is selected from the group consisting of alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts and combinations thereof.

14. The process of claim 11, wherein the base catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate and combinations thereof.

15. The process of claim 11, wherein the depolymerization of the material derived from a fishing net comprising nylon 6 in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

16. The process of any one of claims 1 to 7, wherein the solution comprising ε- caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further comprises water.

17. The process of any one of claims 1 to 7, wherein the solution comprising ε- caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further comprises more than 1 wt% water.

18. The process of any one of claims 1 to 7, wherein prior to step a) the material derived from a fishing net comprising nylon 6 is pretreated in a pretreatment section [A] to obtain the material derived from a fishing net comprising nylon 6.

19. Process according to any one of claims 1 to 7, wherein prior to step a) the material comprising fishing nets containing nylon 6 is cleaned in a cleaning section [a] and / or mechanically size-reduced in a mechanical size-reduction section [β] and / or densified in a densification section [γ] to obtain a material derived from fishing nets containing nylon 6.

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