A method for recovering epsilon-caprolactam from multicomponent materials containing nylon 6.

Abstract

The present invention provides an improved process for recovering ε-caprolactam from multicomponent materials, in particular multilayer films, comprising nylon 6. Furthermore, the present invention provides a plant adapted to carry out the process of the present invention, and recovered ε-caprolactam obtainable by the process of the present invention, having a product carbon footprint of less than 2.5 kg CO2 equivalent per kg purified ε-caprolactam (based on Eco Invent version 3.7.1 (region: Europe) data).

Description

[Technical Field]

[0001] The present invention relates to a method for recovering ε-caprolactam from multicomponent materials, in particular multilayer films, containing nylon 6. Furthermore, the present invention relates to a plant adapted to carry out the method of the present invention, and to the recovered ε-caprolactam obtained by the method of the present invention. [Background technology]

[0002] In 1938, Paul Schlack invented nylon 6 (CAS number: 25038-54-4), also known as polyamide 6, poly(caprolactam), poly(hexano-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide), or poly[imino(1-oxohexane-1,6-diyl)].

[0003] Generally, nylon 6 is produced by the ring-opening polymerization of ε-caprolactam at a temperature of about 260°C under an inert atmosphere: [ka] is synthesized by

[0004] A method for producing virgin ε-caprolactam is described, for example, in “Ullmann's Encyclopedia of Industrial Chemistry”, (Germany), Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim, May 25, 2018, Chapter 25 “Caprolactam” (electronic version available at https: / / doi.org / 10.1002 / 14356007.a05_031.pub3).

[0005] A method for producing nylon 6 is described, for example, in "Ullmann's Encyclopedia of Industrial Chemistry," (Germany), Wiley-VCH Verlag 30 GmbH and Co. KGaA, Weinheim, January 15, 2013, Chapter "Polyamides" (electronic version available at https: / / doi.org / 10.1002 / 14356007.a21_179.pub3).

[0006] Nylon 6 is currently the world's most widely produced synthetic polyamide and possesses a variety of desirable properties for industrial applications, including high mechanical strength, particularly stiffness, hardness, and toughness; good electrical insulation; good dyeability; good processability; good UV light protection; good fatigue resistance; and good barrier properties against gases, flavors, hydrocarbon solvents, and low-polarity flavor compounds from food. Due to this array of favorable properties, nylon 6 is widely used in a wide variety of industries, including building and construction, consumer goods, electronics and electrical appliances, transportation, and packaging. However, a drawback of using nylon 6 is that it is more expensive than the polyolefins polyethylene and polypropylene. Meanwhile, polyolefins have limited industrial applicability due to their poor gas barrier properties, low temperature resistance, and difficulty in joining.

[0007] As a result, nylon 6 is widely used as a component in multicomponent materials, most often in combination with polyolefins. Multicomponent materials containing nylon 6 that are of particular concern are, for example, flexible modified or controlled atmosphere packaging films, typically used inside food packages to maintain a constant atmosphere to improve the shelf life of the packaged product. The majority of packaging films today are multilayer films, typically comprising 2 to 15 layers, although more layers are possible. An example of an 11-layer coextruded cast film is the following: nylon 6 / TIE / PE(1) / TIE / nylon 6 / EVOH / PA / TIE / PE(2) / PE(3) / PE(4), where TIE is a tie layer, EVOH is ethylene vinyl alcohol, PA is a polyamide other than nylon 6, and PE(1), PE(2), PE(3), and PE(4) are different grades of polyethylene. The typical purpose of using nylon 6 in such multicomponent materials is to impart mechanical strength properties and puncture resistance, and / or barrier properties to limit gas exchange and aroma loss. Multilayer films containing nylon 6 in particular make it possible to maintain an optimized modified atmosphere inside the package, in particular a constant oxygen concentration, which is important, for example, for preserving the freshness of perishable goods for a longer period of time. The use of multilayer films can enable cost reductions, since more expensive polymers such as nylon 6 can be partially replaced by cheaper polymers. Nylon 6 is also an excellent alternative to aluminum and other metal films.

[0008] Multicomponent materials containing two or more (polymer) components are attractive for industrial applications because, for example, alternating layering of two (or more) different components is a way to obtain composite materials with novel properties compared to the pure components. Combining several layers of different materials can improve the mechanical and physical properties of a film, including puncture resistance, tear resistance, and heat resistance, as well as its barrier properties to moisture and gases (especially oxygen). Because it is possible to continuously observe how the properties of a multicomponent material change when changing the components used and the thickness of the component layers used, new composite materials with desired properties can be developed relatively easily. In multicomponent materials, especially multilayer films, polyolefin layers are often combined with layers made of polar polymers such as nylon 6, nylon 66, EVOH (ethylene vinyl alcohol), or PVDC (polyvinylidene chloride). In addition, tie layers containing special adhesive polymers or bonding resins (e.g., polyurethanes and acid / anhydride-grafted polyolefins) are commonly required as intermediate layers between two dissimilar components that do not adhere well. As explained above, the use of nylon 6 in multi-component materials has become very popular to improve the mechanical and gas barrier properties of such materials. Another important reason why nylon 6 is often combined with other materials is to save on the cost of nylon 6, which is quite expensive.

[0009] Environmental concerns regarding the production and use of these multicomponent nylon 6-containing materials relate to the waste generated during manufacturing and after use. These concerns may be alleviated by recycling individual components, such as nylon 6, from multicomponent materials that are no longer in use or would otherwise be disposed of. As packaging materials, nylon 6-containing multilayer films are a particularly problematic source of waste. These films often contain large amounts of nylon 6. Therefore, the existence of a feasible method for recovering nylon 6 from these composite waste materials would not only benefit the environment, but also represent a valuable new source of nylon 6.

[0010] The same is true for the manufacturing waste generated in processes that use nylon 6, particularly in film production. For a limited number of applications, this waste can be recovered and reused as regrind. However, the optical and mechanical properties of the material are adversely affected by the regrind, and further degradation occurs with each cycle of reuse and processing. As a result, the reuse and processing cycle cannot be repeated as many times as desired.

[0011] In addition, nylon 6 recycled by currently available methods is generally not considered a "food-grade plastic," significantly limiting its range of applications.

[0012] Mechanical recycling (also known as material recycling, back-to-plastics recycling, or regrinding) refers to the recovery of plastics through mechanical processing (crushing, washing, separating, drying, re-granulating, and compounding) with the goal of producing recycled material that can be converted into plastic products to replace virgin plastics. Currently, most virgin plastics are produced from petrochemical feedstocks that have never been used or processed before, such as natural gas, coal, or crude oil. In the mechanical recycling process, the polymer chains remain largely intact. Mechanical recycling is a form of waste down-recycling, as the recycled material is of lower quality and functionality than the original material.

[0013] Depolymerization, or chemical recycling, is a technique that converts a polymer into its monomer components (ε-caprolactam in the case of nylon 6). Depending on the properties of the recovered monomer, it can either be used to replace virgin monomer for all applications or only for limited applications. Virgin monomer is produced from petrochemical feedstocks that have never been used or processed before, such as natural gas, coal, or crude oil.

[0014] Following the regrinding of nylon 6-containing materials, methods have been developed to depolymerize nylon 6 to its useful monomer, ε-caprolactam, which typically use as a starting material nylon 6 plastic article produced, for example, by injection molding, extrusion, spinning, or other processing steps.

[0015] The depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam: [ka] is.

[0016] Methods for the depolymerization of nylon 6 are known. Such processes may be conducted in a batchwise mode, a semi-continuous mode (typically involving batchwise (re)charging of nylon 6 into the depolymerization reactor), or a continuous mode. LA Dmitrieva, AA Speranskii, SA Krasavin, and YN Bychkov, "Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibers and Yarns," Fibre Chemistry, (Germany), March 1986, pp. 229-241 (translated from Khimicheskie Volokna, (Russia), July-August 1985, No. 4, pp. 5-12), is a literature review describing methods for depolymerizing nylon 6 with and without the use of a catalyst.

[0017] A.A. Ogale, "Depolymerization of Nylon 6: Some Kinetic Modeling Aspects," Journal of Applied Polymer Science, (USA), 1984, vol. 29, pp. 3947-3954 (available electronically at https: / / doi.org / 10.1002 / app.1984.070291227), is a paper that describes the depolymerization kinetics of nylon 6.

[0018] Generally, prior art chemical regeneration or recycling processes for the depolymerization of essentially pure nylon 6 to ε-caprolactam monomer involve a hydrolysis step at high temperature in the presence of water and a recycling step of the formed monomer by steam distillation. The hydrolysis step can be carried out either in the presence or absence of a catalyst. The catalyst can be either an acidic compound, such as phosphoric acid and boric acid, or a basic compound, such as potassium carbonate and sodium hydroxide.

[0019] US Patent No. 5,929,234 describes a method for recovering ε-caprolactam from waste materials containing polycaprolactam. Depolymerization is carried out in the absence of an added catalyst using superheated steam at a temperature of about 250°C to about 400°C and a pressure in the range of about 1 atm to about 100 atm, which is significantly lower than the saturated vapor pressure of water at the temperature at which a vapor stream containing ε-caprolactam is formed.

[0020] The reuse of ε-caprolactam from essentially pure nylon 6 has been known for a long time, either by recovering the extract of nylon 6 polymerization plants, recycling nylon 6 scrap (i.e. nylon 6 polymer containing little non-nylon 6 material) or products made of almost pure nylon 6 (e.g. fishing nets).

[0021] For example, German Patent Application Publication No. 4211609 discloses a method for recovering ε-caprolactam by thermal cleavage of polyamide 6 at 250-320°C in the presence of a small amount of potassium carbonate, and a method for purifying the recovered ε-caprolactam. German Patent Application Publication No. 4211609 describes that this method is applicable not only to pure polyamide 6 products, but also to polyamide 6 products containing glass fibers, polyamide 6 products containing fillers, and products containing impact modifiers. The term "polyamide 6" used in German Patent Application Publication No. 4211609 refers to pure polyamide 6 and preferably to polyamides containing more than 80% by weight of polyamide 6.

[0022] Chinese Patent Publication No. 103467378 discloses a method for producing ε-caprolactam by using recycled waste chinlon silk and surplus products, in which phosphoric acid is used as a depolymerization catalyst. Superheated steam is introduced into the reaction system during the depolymerization process.

[0023] U.S. Patent No. 5,869,654 discloses a method for recovering ε-caprolactam from polycaprolactam processing waste, where the polycaprolactam can be selected from waste nylon 6 polymer and / or oligomeric materials, such as yarn waste, chip waste, or extruder slag. The method comprises contacting the polycaprolactam waste, in the absence of an added catalyst, with superheated steam at a temperature of about 250°C to about 400°C and a pressure in the range of about 1.5 atm to about 100 atm, which is significantly below the saturated vapor pressure of water, at which a vapor stream containing ε-caprolactam is formed.

[0024] Japanese Patent Application Laid-Open No. 2000038471 discloses a method for depolymerizing nylon 6 resin derived from a molded body to obtain ε-caprolactam, particularly a method for depolymerizing automotive resin parts in which nylon 6 accounts for 90% by weight or more of all organic compounds in the molded body, such as nylon 6 fastener parts, electrical parts, body mechanism parts, and exterior parts.

[0025] Although methods for depolymerizing nearly pure waste nylon 6 into its useful monomer, ε-caprolactam, have been developed, recovery of ε-caprolactam from nylon 6-containing multicomponent materials remains a major challenge, especially those with a moderate to high nylon 6 content (e.g., 40% to 80% by weight). Recovery of ε-caprolactam from nylon 6-containing multicomponent materials with a low nylon 6 content (e.g., less than 40% by weight) is even more difficult. An example of a nylon 6-containing product with a moderate to high nylon 6 content is carpet. High-quality carpet typically contains face fibers containing up to 55% by weight of nylon 6; the remainder is backing (support) material.

[0026] Reclaiming ε-caprolactam from nylon 6-containing materials, both low and medium to high nylon 6 content, has not been commercially viable or successful based on existing methods.

[0027] The most frequently reported problem associated with the recovery of ε-caprolactam from nylon 6-containing materials with a moderate to high nylon 6 content is low recovery rates. These low recovery rates are due, on the one hand, to the difficulty of separating the formed ε-caprolactam from non-nylon 6 materials. On the other hand, low recovery rates are due to decomposition reactions leading to the production of ammonia and to the heterogeneity of the depolymerization mixture, whose high viscosity prevents effective mixing, resulting in localized overheating and increased consumption of expensive superheated steam (used as a discharging agent).

[0028] By incorporating a mechanical pretreatment step (e.g., scraping or grinding surface fibers from the carpet backing, followed by density separation such as float-sink separation and air sorting), it is possible to produce a treated carpet feedstock containing more than 90% by weight of nylon 6, which can be used in the depolymerization process of the prior art described above. However, this mechanical pretreatment is primarily applied to off-spec material (pre-consumer waste) within production facilities for carpet and other nylon 6-containing textiles. Furthermore, this pretreatment cannot be used for other non-carpet multicomponent materials containing nylon 6.

[0029] Currently, economically viable methods for recovering high-purity ε-caprolactam from multicomponent materials containing nylon 6 are not available, despite the urgent need for such methods. In particular, there is an urgent need for ε-caprolactam recovery methods for more demanding nylon 6-containing multicomponent materials, such as those with low to moderate nylon 6 content (e.g., 1% to 35% by weight), and for the recovery of ε-caprolactam from complex nylon 6-containing multicomponent products that cannot be pre-concentrated by simple mechanical pretreatment steps (e.g., scraping or grinding followed by density separation). Finally, there is a need for methods that enable the recovery of ε-caprolactam from nylon 6-containing multicomponent materials, such as multilayer films, where the nylon 6 layer is often sandwiched between non-nylon 6 components and therefore is inaccessible from the outer surface of the material.

[0030] Taken together, prior art methods for recovering ε-caprolactam from nylon 6-containing multicomponent materials are unable to produce high quality grades of ε-caprolactam that can be used to replace virgin grades of ε-caprolactam for demanding applications.

[0031] Currently, no method is available for recovering high-purity ε-caprolactam from nylon 6-containing multicomponent materials, despite the urgent need for such a method. In particular, there is an urgent need for a method for recovering high-purity ε-caprolactam that can replace virgin-grade ε-caprolactam for demanding applications such as high-speed melt spinning in textile fiber production.

[0032] Furthermore, there is a need for a process that allows for the recovery of high-purity ε-caprolactam in an economically reasonable manner from multicomponent materials containing nylon 6. The production cost of the recovered high-purity ε-caprolactam should be comparable to or lower than the production cost of high-purity virgin ε-caprolactam.

[0033] There is also a need to purify crude ε-caprolactam obtained by depolymerization of nylon 6-containing multicomponent materials without using oxidizing agents such as potassium permanganate (KMnO4) or adsorbents such as (activated) carbon and diatomaceous earth. These oxidizing agent and adsorbent-based techniques are very time-consuming and labor-intensive and generate solid waste.

[0034] Furthermore, there is a need to provide high purity grades of ε-caprolactam derived from nylon 6-containing multicomponent materials that have a significantly lower carbon footprint than ε-caprolactam obtained by new syntheses, such as the Beckmann rearrangement of virgin cyclohexanone oxime.

[0035] Finally, in order to process the enormous amounts of nylon 6-containing multicomponent materials that are discarded each year, a method is needed that allows for the recovery of ε-caprolactam from nylon 6-containing multicomponent materials on an industrial scale. Summary of the Invention

[0036] It is an object of the present invention to fulfill one or more of the above needs and to overcome the drawbacks associated with prior art methods.

[0037] In particular, the present invention aims to provide a method for recovering high-purity ε-caprolactam from multicomponent waste materials containing nylon 6 in an economical, reliable manner and with a high recovery rate. In particular, the present invention aims to provide a method suitable for recovering ε-caprolactam from nylon 6-containing multicomponent waste materials having a nylon 6 content of up to 35% by weight, and from complex nylon 6-containing multicomponent materials, in particular nylon 6-containing multilayer films.

[0038] It is a further object of the present invention to provide a method for recovering high purity ε-caprolactam from nylon 6-containing multicomponent materials that can replace high purity virgin ε-caprolactam for all applications, including high speed melt spinning of nylon 6 to produce fine textile fibers.

[0039] It is a further object of the present invention to provide a method for recovering high-purity grade ε-caprolactam from nylon 6-containing multicomponent waste materials, which method is characterized by a significantly lower carbon footprint than neo-synthetic methods, such as those for producing ε-caprolactam by Beckmann rearrangement of cyclohexanone oxime.

[0040] Therefore, a further object of the present invention is to provide a method for reducing the environmental impact of multi-component waste materials containing nylon 6 and products made from such materials. The above mentioned objects are solved in whole or at least in part by the methods of claims 1 and 15, the plant of claim 13 and / or the product of claim 14.

[0041] Surprisingly, it has been found that high-purity grade ε-caprolactam can be obtained in high yield from a nylon 6-containing multicomponent material by the following process of the present invention. The process of the present invention uses a plant comprising (A) a nylon 6 preconcentration section, (B) a depolymerization section, (C) a recovery section, and (D) a purification section. The process of the present invention comprises:

[0042] a) extracting the nylon 6-containing multicomponent material with one or more organic solvents in a nylon 6 preconcentration section to obtain a solid preconcentrated nylon 6-containing material enriched in nylon 6 relative to the nylon 6-containing multicomponent material; b) depolymerizing the pre-concentrated nylon 6-containing material in the presence of water in a depolymerization section to obtain a vapor stream comprising water and ε-caprolactam in a weight-to-weight ratio of 2:1 to 15:1; c) recovering crude ε-caprolactam from the vapor stream in a recovery section; d) purifying the crude ε-caprolactam in a purification section to obtain purified ε-caprolactam; Includes:

[0043] It was surprising that the combination of the special process steps and conditions according to the present invention, i.e., the above-defined series of nylon 6 preconcentration, depolymerization, recovery, and purification steps, makes it possible to recover high-grade ε-caprolactam from nylon 6-containing multicomponent materials in high yields in a simple, straightforward, and economically rational manner. The process of the present invention is economically rational and advantageous from several perspectives. First, the process of the present invention is suitable for a wide variety of nylon 6-containing multicomponent materials, which may differ, for example, in their overall composition, nylon 6 content, and / or nylon 6 distribution within the materials. Second, the process of the present invention enables the effective separation of nylon 6 from non-nylon 6 compounds so as to obtain high-purity grade ε-caprolactam. Third, the extraction of nylon 6 from nylon 6-containing multicomponent materials according to the present invention is so effective that it allows for high yields of ε-caprolactam. Finally, the method of the present invention allows for the production of ε-caprolactam with a significantly lower carbon footprint than ε-caprolactam produced by de novo synthesis of ε-caprolactam, for example, via the Beckmann rearrangement of cyclohexanone oxime. The method of the present invention allows for the efficient processing of discarded nylon 6-containing multicomponent materials and reduces the environmental impact of the product. Specifically, the method of the present invention allows for the production of purified ε-caprolactam with a carbon footprint of less than 2.5 kg of CO₂ equivalent per kg of purified ε-caprolactam, a significant improvement over the 7.5-6.5 kg of CO₂ equivalent per kg of ε-caprolactam traditionally associated with the production of "virgin" ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime (based on data from Ecoinvent version 3.7.1 (region: Europe)). Product carbon footprint values reported herein are based on Eco Invent version 3.7.1 and European region data, unless otherwise stated.

[0044] Next to the process of the present invention, the present invention further provides a chemical plant suitable for producing purified ε-caprolactam from a nylon 6-containing multicomponent material, comprising at least (A) a nylon 6 preconcentration section, (B) a depolymerization section, (C) a recovery section, and (D) a purification section, the plant and in particular the four sections being configured to carry out the process of the present invention.

[0045] The present invention further provides purified ε-caprolactam obtainable by the process of the present invention and having a product carbon footprint of less than 2.5 kg CO2 equivalent per kg (based on Eco Invent version 3.7.1 (region: Europe) data). Finally, the present invention provides the use of ε-caprolactam recovered from nylon 6-containing multicomponent materials to reduce the carbon footprint of an ε-caprolactam production plant, which also produces ε-caprolactam from the Beckmann rearrangement of at least cyclohexanone oxime. Advantageous embodiments of the invention are set out in the dependent claims and are described in more detail below.

[0046] <Nylon 6-containing multi-component material> The method of the present invention uses as a starting material a nylon 6-containing multicomponent material, which is typically a solid, particularly a solid body, net, sheet or film material.

[0047] Multicomponent materials contain two or more components. Multicomponent materials are often used as a means of combining the properties of different components into a single material. As used herein, the term "component" refers to a chemical entity. In a preferred embodiment, a nylon 6-containing multicomponent material contains nylon 6 and at least one non-nylon 6 polymer as components. Two or more components can exist as separate domains within the multicomponent material (e.g., in a sheath / core, sea-island, side-by-side, multilayer, or other configuration). Alternatively or additionally, a multicomponent material can contain a mixture of different components (e.g., in the form of a polymer blend). Mixtures of these two embodiments are also possible, such as in a multilayer film containing a layer of a single polymer and an additional layer of a polymer blend or polymer matrix filled with polymer filler particles. Typically, nylon 6 is present in a nylon 6-containing multicomponent material as a separate domain and / or as a mixture with at least one non-nylon 6 polymer.

[0048] Preferably, the components of a multicomponent material are at least two different polymers, one of which is or includes nylon 6. In one embodiment, at least two components of a multicomponent material comprise or consist of polymers. In this embodiment, the multicomponent material can include additional non-polymeric components. Preferably, however, greater than 80% by weight of the multicomponent material is made up of polymers. As used herein, the term "polymer" refers to its ordinary meaning, i.e., a large molecule (e.g., an oligomer) or macromolecule made of repeating subunits. As used herein, polymers may be synthetic, such as plastics including polypropylene, nylon 6, and polystyrene, or naturally occurring, such as starch or proteins. As used herein, the term "polymer" also includes homopolymers, copolymers, and polymer blends. Naturally and synthetically derived polymers are created by the polymerization of many small molecules known as monomers. Homopolymers are prepared from only one type of monomer. Copolymers are prepared from different types of monomers and may have a random or block structure. As used herein, "polymer blend" or "blend" means a composition of two or more polymers, usually obtained by extrusion. The blend may or may not be miscible and may or may not be phase separated.

[0049] In one embodiment, at least one component of the multicomponent material comprises or consists of nylon 6, and at least one other component of the multicomponent material comprises or consists of a polymer different from nylon 6. The polymer different from nylon 6 can be selected from the group consisting of polyolefins (especially polyethylene and polypropylene), polyethylene oxide, polypropylene oxide, polycaprolactone, polyamides (other than polyamide 6), polyesters, polyvinylidene fluoride, polyvinylidene chloride, polystyrene, polycarbonate, polymethyl methacrylate, ethylene acrylic acid copolymers, polyoxymethylene, ethylene vinyl alcohol, polyurethanes, and combinations thereof, particularly blends thereof. Particularly good results in terms of the purity and yield of the resulting ε-caprolactam can be achieved when the polymer different from nylon 6 is selected from the group consisting of polyolefins (especially polyethylene and polypropylene), polyamides (other than polyamide 6, particularly polyamide 6,6, polyamide 4,6, polyamide 6,10, and polyamide 12), and combinations thereof.

[0050] The multicomponent material used in the method of the present invention contains nylon 6. As used herein, the term "nylon 6" refers to polycaprolactam. As used herein, the term "nylon 6-containing multicomponent material" refers to a multicomponent material containing two or more components, one of which is or includes nylon 6. The weight fraction of nylon 6 in the nylon 6-containing multicomponent material used in the method of the present invention is not critical to the practice of the invention and can range from 0.5% to 99% by weight. However, a particular advantage of the method of the present invention is that it works very well with multicomponent materials containing only a low nylon 6 content compared to the nylon 6 content of over 80% required for the successful practical application of prior art methods. Thus, according to a particularly advantageous embodiment, the weight fraction of nylon 6 in the nylon 6-containing multicomponent material used in the method of the present invention is 1% to 75% by weight, more preferably 1% to 60% by weight, even more preferably 2% to 35% by weight, and most preferably 3% to 25% by weight. Unless otherwise specified, all weight percent values stated herein are always based on the total weight of the nylon 6-containing multicomponent material.

[0051] The nylon 6-containing multicomponent material can have any shape, such as (multifilament) yarn, chip, film, or any molded form. The shape of the nylon 6-containing multicomponent material used in the method of the present invention is not critical.

[0052] Preferably, the nylon 6-containing multicomponent material used in the method of the present invention is a multilayer material in which at least one layer comprises or consists of nylon 6. As used herein, the term "multilayer material" refers to a material constructed of different layers. These layers can differ in their relative position, thickness, and / or composition. These layers can be of the same type or different types. Preferably, the multilayer material includes at least one nylon 6-free layer. In a further preferred embodiment, the multilayer material comprises two skin layers as the outermost layers on both sides of the multilayer material, neither of which contains nylon 6, such that nylon 6 is contained in one of the layers sandwiched between the skin layers. Non-limiting examples of suitable polymers that can be used in the skin layers include polypropylene, polyethylene, polyethylene oxide, polycaprolactone, polyamides (other than nylon 6), polyesters, polyvinylidene fluoride, polyvinylidene chloride, polystyrene, polycarbonate, ethylene vinyl alcohol, polymethyl methacrylate, ethylene acrylic acid copolymers, polyoxymethylene, and blends of two or more thereof. A particular advantage of the method of the present invention is that it works with multilayer materials where the nylon 6 does not need to be exposed or accessible at the surface of the material.

[0053] The multilayer material can include a barrier layer as a non-nylon 6-containing skin layer or intermediate layer. The barrier layer can be formed from any material with barrier properties. The barrier layer can include or consist of organic materials, such as carbon; inorganic materials, such as metals, ceramics, and oxides; polymeric materials, or combinations thereof. Non-limiting examples of suitable polymers other than nylon 6 that can be used in or as a barrier layer include polyethylene terephthalate, ethylene vinyl alcohol, polyvinylidene chloride copolymers, polyamides (other than nylon 6), polyketones, blends of two or more of these, and blends of one or more of these with other polymers.

[0054] The multilayer material can include a cling or adhesive layer that connects at least one nylon 6-containing layer to other layers in the multilayer material. Non-limiting examples of suitable polymers that can be used as cling or adhesive layers include olefin block copolymers, such as propylene-based block copolymers sold under the trade name INTUNE™ (The Dow Chemical Company) and ethylene-based block copolymers sold under the trade name INFUSE™ (The Dow Chemical Company); polar ethylene copolymers, such as copolymers with vinyl acetate, acrylic acid, methyl acrylate, and ethyl acrylate; ionomers; maleic anhydride-grafted ethylene polymers and copolymers; polyurethane adhesives; blends of two or more of these; and blends containing one or more of these with other polymers.

[0055] In a preferred embodiment, the term "layer" as used herein refers to a "film." Even more preferably, the nylon 6-containing multicomponent material used in the method of the present invention is itself a film, i.e., a multilayer film containing at least one layer containing or consisting of nylon 6. Preferably, the multilayer film contains at least one layer containing or consisting of nylon 6 and at least one layer not containing nylon 6. The multilayer film can be produced by any film lamination and / or coextrusion technique. The term "film" as used herein is defined as a thin sheet having a thickness of less than 1 mm. The "layers" within a film may be very thin, such as in the case of nanolayers. The term "film" as used herein may be in a shape such as a profile, parison, tube, etc., that is not necessarily "flat" in the sense of being planar. In a preferred embodiment, the term "film" as used herein refers to a film consisting of polymeric material or containing more than 80% by weight of polymeric material. The use of multilayer films in the method of the present invention is advantageous because multilayer films are a common waste product. The majority of packaging films today are multilayer films. According to embodiments of particular practical importance, "multilayer film" as used herein is a packaging film, particularly a modified atmosphere packaging film, both of which are described in the background section above.

[0056] Due to the close connection between the film layers, prior to the present invention, it was very difficult to recover high-grade ε-caprolactam from a multilayer film containing nylon 6, especially when the nylon 6 content is less than 80% by weight as described above, and / or when at least one nylon 6-containing layer is sandwiched between two non-nylon 6-containing layers in the multilayer film, which may, but need not, be skin layers, and therefore not easily accessible. Therefore, multilayer films of the above type represent a particularly advantageous embodiment of the present invention when used as a nylon 6-containing multicomponent material in step a). Thus, specifically, the nylon 6-containing multicomponent material used in the method of the present invention can be a multilayer film comprising at least one layer containing or consisting of nylon 6 sandwiched (i.e., embedded) between two or more layers that do not contain nylon 6. The multilayer film can, of course, comprise additional layers, with or without nylon 6, located anywhere between, above, or below the aforementioned layers.

[0057] The method of the present invention has the advantage that, unlike prior art methods that are limited to nylon 6-containing fibers obtained, for example, from carpets that must be subjected to a mechanical pretreatment step, the method of the present invention is not so limited and can be very well applied, particularly to multilayer films. In certain embodiments, the nylon 6-containing multicomponent material used in the method of the present invention is not a carpet or a carpet-derived material. In other embodiments, the nylon 6-containing multicomponent material is not a textile or a textile-derived material. In other embodiments, the nylon 6-containing multicomponent material does not contain nylon 6 fibers.

[0058] <Possible pre-processing steps> The nylon 6-containing multicomponent-derived material used in step a) is obtained by extracting the nylon 6-containing multicomponent material with one or more solvents to obtain a solid pre-concentrated nylon 6-containing material that is enriched in nylon 6 relative to the nylon 6-containing multicomponent material, as described in more detail below. Before or after this pre-treatment, the material may advantageously be subjected to a further pre-treatment step, which is described below.

[0059] Prior to being subjected to the process step a) of the present invention, the nylon 6-containing multicomponent material preferably undergoes pretreatment, particularly a mechanical crushing step and / or a washing step. Therefore, a mechanical crushing section and / or a washing section may be provided before the nylon 6 preconcentration section in the method of the present invention. Preferably, the nylon 6-containing multicomponent material is fragmented into small pieces and then extracted with one or more organic solvents in the nylon 6 preconcentration section of step a). This mechanical pretreatment, i.e., mechanical comminution or fragmentation of the nylon 6-containing multicomponent material, can be achieved, for example, by cutting, chopping, grinding, milling, or chipping. In a preferred embodiment, the nylon 6-containing multicomponent material is introduced into step a) in the form of small pieces having an average length along its longest axis of 0.01 to 100 cm, preferably 0.05 to 10 cm, and most preferably 0.1 to 5 cm. Those skilled in the art can easily determine the average length along the longest axis of the particles used by first taking a representative sample of the particles, then measuring the length of the longest axis of each of these particles (e.g., 50 pieces), and finally calculating the average of all these individual measurements. The preferred particle dimensions can also be described in terms of average particle weight. Preferably, the nylon 6-containing multicomponent material particles have an average particle weight of 0.1 milligrams to 100 kilograms, preferably 1 milligram to 10 kilograms, more preferably 10 milligrams to 1 kilogram, and most preferably 10 milligrams to 100 grams. The use of nylon 6-containing multicomponent material particles having the aforementioned dimensions has the advantage of increased surface area and / or easier handling of the particles and / or mixing with the organic solvent added for extraction in the nylon 6 preconcentration section.

[0060] Optionally, the nylon 6-containing multicomponent material is cleaned before preconcentration of nylon 6 is carried out, which is advantageous because any (adherent) dirt that is removed will not consequently interfere with the next step of the method of the present invention.

[0061] As used herein, the term "cleaning" is defined as any process that removes non-nylon 6 materials that are attached to or intermingled with a nylon 6-containing multicomponent material. Cleaning is advantageous because any non-nylon 6 materials that are removed will not interfere with subsequent steps in the method of the present invention.

[0062] Optionally, the nylon 6-containing multicomponent material is cleaned by washing with a solvent, preferably water, before being introduced into the nylon 6 preconcentration section. This is advantageous because any (adherent) dirt is removed, which consequently does not interfere with the next step of the method of the present invention. Preferably, a detergent is added to the solvent in a concentration range of 0 to 30% by weight of the solvent to improve cleaning efficiency. In another preferred embodiment, the cleaning process includes a final rinse step using a detergent-free (clean) cleaning solvent to remove any remaining detergent and any existing dirt adhering to the nylon 6-containing multicomponent material. Preferably, the cleaning solvent is heated to further enhance the cleaning process. Optionally, the nylon 6-containing multicomponent material is dried after the cleaning step and before being introduced into the nylon 6 preconcentration section. This has the advantage that the solvent added for extraction in the preconcentration section is not diluted or contaminated by the washing solvent. Preferably, the washing is carried out under friction. Various types of industrial cleaning systems, such as rotary plastic washers and (high-speed) friction washers, are commercially available. Optionally, mechanical crushing and washing of the nylon 6-containing multicomponent material is combined, for example in a so-called wet crushing unit.

[0063] Optionally, prior to mechanical comminution or fragmentation of the nylon 6-containing multicomponent material, metal chips, rocks, and other obstructions that would cause heavy wear on the equipment used for mechanical comminution or fragmentation are removed. Preferably, non-nylon 6-containing materials, such as polyethylene, polypropylene, and nylon 6,6-containing materials, are removed prior to mechanical comminution or fragmentation of the nylon 6-containing multicomponent material, or thereafter, as described below. The removal of the obstructions can be performed mechanically or manually. The removal of the obstructions has the advantage of significantly reducing the maintenance costs of the equipment used for mechanical comminution or fragmentation. In addition, the nylon 6 content of the material obtained after mechanical comminution or fragmentation is higher than if the obstructions were not removed. Optionally, the obstructions are separated from the mechanically comminuted or fragmented nylon 6-containing multicomponent material. For this purpose, various separation processes, such as, but not limited to, density separation and magnetic separation, can be applied. In density separation, materials of different densities are placed in a liquid of intermediate density, in which lower-density materials float and separate from higher-density sinking materials. In practice, density separation is often performed through a series of density separation stages. For example, one stage separates and removes high-density materials such as rock, sand, and metals (including iron and lead), while another stage separates and removes low-density materials such as polyolefins, polypropylene, and polyethylene. Magnetic separation is a process for separating components of a mixture by using magnets to attract magnetic materials. A typical process for magnetic separation separates magnetic materials from non-magnetic materials. Removal of foreign matter in comminuted or fragmented nylon 6-containing multicomponent materials is advantageous because such materials may interfere with subsequent steps of the method of the present invention. Optionally, the nylon 6-containing multicomponent material, preferably after washing and crushing, is densified. Preferably, the densification of the nylon 6-containing multicomponent material is carried out by feeding it into a melt furnace (for example an extruder) or by compacting it in a (mechanical) compactor or agglomerator.Densification of the preferably cleaned and / or crushed nylon 6-containing multicomponent material has the advantage of increased bulk density, which reduces intermediate storage and transportation costs when pre-processing is carried out at a different location (see below).

[0064] In a melting furnace, the nylon 6-containing multicomponent material is melted. Preferably, the resulting polymer melt is filtered, which has the advantage of removing solid impurities. The melted and optionally filtered polymer melt is then cooled and preferably fed to a pelletizer, which cuts the product into pellets. The size and shape of the pellets (often called granules) can be selected within a wide range. Generally, pellets are cylindrical (resulting from thin strands chopped into small pieces). However, other shapes, such as (not perfect) spheres, are also possible. The size of the pellets can be selected within a wide range. Typically, pellets have a diameter in the range of 1 to 10 mm, preferably 2 to 7 mm, and more preferably 3 to 5 mm. In a preferred embodiment, the pellets have a length in the range of 1 to 50 mm, preferably 2 to 25 mm, and more preferably 3 to 15 mm.

[0065] Pelletization of preferably cleaned and crushed nylon 6-containing multicomponent material has the advantage of increasing the bulk density, which reduces intermediate storage and transportation costs when pretreatment is carried out at different locations (see below). Apart from the increased density, pelletization also provides other benefits, such as a homogeneous shape and structure of the material to be processed, which is convenient for (automatic) feeding into equipment applied for nylon 6 preconcentration by extraction with solvents.

[0066] The site where the pretreatment of the nylon 6-containing multicomponent material is carried out and the site where the nylon 6 preconcentration by solvent extraction is located can be the same. However, preferably, one or more of the pretreatment steps are carried out at different locations, for example, at a site specialized in the pretreatment of waste polymer materials and in particular nylon 6-containing multicomponent material. The nylon 6-containing multicomponent material pretreated at various locations can then be transported to the site where the nylon 6 preconcentration by solvent extraction is located.

[0067] Therefore, according to a particularly advantageous embodiment of the present invention, before step a), the material comprising the nylon 6-containing multicomponent material is subjected to a pretreatment and preconcentration in a pretreatment section [A] to obtain a material derived from the nylon 6-containing multicomponent material, in particular purification in a purification section and / or mechanical crushing in a mechanical crushing section and / or a densification section and extraction section.

[0068] <Nylon 6 preconcentration step a)> In step a) of the method of the present invention, a nylon 6-containing multicomponent material is extracted with one or more organic solvents in a nylon 6 preconcentration section to obtain a solid preconcentrated nylon 6-containing material that is enriched in nylon 6 relative to the nylon 6-containing multicomponent material. This is a key step in the method of the present invention, ensuring the high yield and high quality of ε-caprolactam obtainable using the present invention. This step is particularly important when a multilayer film is used as the starting material, especially when the nylon 6-containing layer is sandwiched between other polymer layers and therefore cannot be directly contacted for depolymerization. Preconcentration of the nylon 6-containing multicomponent material is also particularly important when a multicomponent material containing nylon 6 and at least one non-nylon 6 polymer is used as the starting material, especially when nylon 6 exists in the nylon 6-containing multicomponent material as a separate domain and / or as a mixture with at least one non-nylon 6 polymer and therefore cannot be directly contacted for depolymerization.

[0069] The extraction in the nylon 6 preconcentration section according to the present invention is typically carried out as follows: (i) adding one or more organic solvents to a nylon 6-containing multicomponent material; (ii) performing a phase separation to obtain a liquid extract phase comprising the solvent and dissolved components from the nylon 6-containing multicomponent material, and an at least partially solid phase comprising undissolved components of the nylon 6-containing multicomponent material and optionally the solvent; (iii) removing the solvent from the liquid extraction phase and, if present, from at least a partial solid phase to obtain two solid phases, one of which is a pre-concentrated nylon 6-containing material enriched in nylon 6 relative to the nylon 6-containing multicomponent material used as the starting material; Includes:

[0070] Step (i) can, of course, also involve treatment with a solvent, i.e., the solvent and the nylon 6-containing multicomponent material are contacted for a time and under conditions sufficient to dissolve the components to be extracted from the nylon 6-containing multicomponent material. The phase separation in step (ii) can be carried out by any suitable means known for solid / liquid phase separation. Suitable phase separation means include, for example, filters, centrifuges, and cyclones. The solvent removal in step (iii) is advantageously carried out by evaporation of the solvent, which leaves the dissolved components extracted from the nylon 6-containing multicomponent material as a residual solid precipitate. However, various techniques exist and can be used to enable the recovery of solvents from dissolved compounds. These techniques are well known to those skilled in the art and include cooling, evaporation, distillation, precipitation by adding a precipitant (a solvent for precipitating the dissolved polymer), and combinations thereof.

[0071] The terms "extraction," "extracted," and "extracting," as used herein, refer to a physical or chemical method of removing one or more components from a substrate by a solvent followed by solvent removal and recovery of the extracted components. In the methods of the present invention, one or more solvents may be used to extract nylon 6, in which case a pre-concentrated nylon 6-containing material is obtained as the remaining solid residue after removal of one or more solvents from an extraction phase containing a liquid solvent, and / or may be used to extract non-nylon 6 compounds from a nylon 6-containing multicomponent material, in which case the remaining undissolved nylon 6-containing multicomponent material, optionally after removal of any adhering solvent, is the pre-concentrated nylon 6-containing material referred to herein.

[0072] As used herein, the term "preconcentrated" refers to the extraction of a nylon 6-containing multicomponent material using one or more solvents followed by solvent removal to obtain a material enriched in nylon 6 compared to the nylon 6-containing multicomponent material used as the starting material. "Enriched in nylon 6" means enriched compared to the nylon 6-containing multicomponent material used as the starting material. Nylon 6 enrichment is defined as the nylon 6 content, in weight percent based on the total weight of polymers, in the solid nylon 6-containing material obtained after extraction compared to the nylon 6 content, in weight percent based on the total weight of polymers, in the nylon 6-containing multicomponent material used as the starting material. This enrichment depends highly on the proportion of non-nylon 6 compounds removed. The degree of enrichment in the preconcentrated nylon 6-containing material is advantageously 1.1 to 50, particularly 1.2 to 30, and more particularly 1.5 to 10. If the starting material has a nylon 6 content of 20 wt. % based on the total polymer content, and the pretreated nylon 6-containing phase has a nylon 6 content of 80 wt. % based on the total polymer content, the degree of nylon 6 enrichment is (80 wt. % divided by 20 wt. % =) 4.

[0073] The solvent used for extraction in the nylon 6 preconcentration section may be an organic solvent or an inorganic solvent. Preferably, the solvent is an organic solvent. Extraction of nylon 6-containing multicomponent materials with an organic solvent has the advantage that various components, particularly various polymers, can be separated from each other and, in particular, from the nylon 6 to be enriched due to their different solubilities in certain solvents.

[0074] Extraction typically involves (i) contacting the nylon 6-containing multicomponent material with one or more solvents, resulting in a liquid extract phase containing the solvent and dissolved compounds from the nylon 6-containing multicomponent material, and a typically solid or partially solid second phase consisting of undissolved nylon 6-containing multicomponent material, followed by (ii) phase separation and (iii) solvent removal to obtain a pre-concentrated nylon 6-containing material.

[0075] As explained above, depending on whether a solvent that dissolves (1) nylon 6 or (2) non-nylon 6 is used, a preconcentrated nylon 6-containing material will be obtained (1) after solvent removal from the liquid extraction phase containing dissolved nylon 6, or (2) from the solid or partially solid second phase consisting of the nylon 6-containing multicomponent material that remains undissolved. Two or more different, particularly complementary, extractions can also be combined.

[0076] In the case of two or more solvents, the treatments can be carried out simultaneously or sequentially. When two or more solvents are used, the solvents should be distinguished by their ability to dissolve nylon 6. In a particularly advantageous embodiment of the present invention, step a) comprises at least a two-stage procedure in which (i) the nylon 6-containing multicomponent material is subjected to a first extraction with a solvent capable of dissolving non-nylon 6 components, followed by (ii) treatment and extraction of the remaining undissolved nylon 6-containing multicomponent material from the first extraction with a solvent capable of dissolving nylon 6, thereby obtaining, after phase separation and solvent removal from the liquid extraction phase containing dissolved nylon 6 obtained from this second extraction, a pre-concentrated nylon 6-containing material. Proceeding in this manner is particularly advantageous when a multilayer film is used as the starting material, especially when the nylon 6-containing layer is covered, in particular sandwiched between other polymer layers, and therefore cannot come into direct contact with the solvent that dissolves nylon 6.

[0077] The choice of solvent and processing conditions (e.g., temperature, processing time, and amount of solvent relative to the amount of nylon 6-containing multicomponent material) can affect the effectiveness of the extraction. Depending on the solvent used, the extraction in step a) can be of the following types: - a preconcentration step in which non-nylon 6 compounds are preferentially extracted from the nylon 6-containing multicomponent material (the resulting preconcentrated nylon 6-containing material is the unextracted nylon 6-containing multicomponent material, optionally after removal of the adhering solvent); - a preconcentration step in which nylon 6 compounds are preferentially extracted from the nylon 6-containing multicomponent material (the preconcentrated nylon 6-containing material is thus obtained after removing the solvent from the liquid extraction phase comprising the solvent and dissolved nylon 6); and - Nylon 6 and non-nylon 6 compounds are preferentially extracted from nylon 6-containing multicomponent materials in a sequential manner, i.e., extractions in which a combination of extractions for non-nylon 6 compounds and nylon 6 are performed sequentially. It could be.

[0078] Extraction, in which non-nylon 6 compounds are preferentially extracted from a nylon 6-containing multicomponent material, can be carried out using a suitable solvent as a selective extractant for non-nylon 6 compounds. This solvent is one that readily dissolves non-nylon 6 compounds but has limited dissolution for nylon 6. The solvent must have high selectivity for dissolving non-nylon 6 compounds. In addition to the choice of solvent, the temperature at which the extraction is carried out, the processing time, and the amount of solvent relative to the amount of nylon 6-containing multicomponent material can also have a significant impact on extraction selectivity. Therefore, the selection of these parameters depends on the type of non-nylon 6-containing component to be selectively removed. Those skilled in the art can easily determine, through simple trial dissolution experiments, which solvent is effective for extracting the non-nylon 6-containing component of interest in a given nylon 6-containing multicomponent material. Any one or more of the organic solvents described below with respect to specific non-nylon 6 compounds can be used as the solvent in the process of the present invention.

[0079] Particularly useful for dissolving polyolefins (e.g., LD, LLD, and HD polyethylene, and polypropylene) are solvents selected from the group consisting of aliphatic hydrocarbons, naphthenic hydrocarbons, aromatic hydrocarbons, and mixtures thereof. These are obtained as boiling fractions in petroleum refineries to produce fuels such as gasoline and diesel. The aforementioned boiling fractions include paraffin wax, petroleum wax, and white spirit, each of which can be used as a solvent in accordance with the present invention. Pure solvents such as isomeric mixtures of xylene or toluene are also known to dissolve polyolefins. Chlorinated hydrocarbons such as tetrachloroethane can also be used to dissolve polyolefins.

[0080] A wide variety of solvents can be used to dissolve the polystyrenic polymer, including aromatic organic solvents. Preferably, benzene, toluene, xylene, and mixtures thereof are used as solvents to dissolve the polystyrenic polymer. Most preferably, xylene is used to dissolve the polystyrenic polymer.

[0081] To dissolve polyvinyl chloride, solvents such as tetrahydrofuran, cyclohexane, dioxane and methyl ethyl ketone (MEK) and mixtures thereof are preferably used.

[0082] Solvents that can be used to dissolve other polymeric non-nylon 6 compounds are known to those skilled in the art or can be easily identified by conducting trial dissolution experiments. Due to the versatility of the method of the present invention, when selecting a solvent for extraction in the nylon 6 preconcentration section, other factors can also be taken into consideration, such as stable commercial availability, ease of handling, health aspects, and cost of the solvent, among others.

[0083] Preferably, after extraction, in which the non-nylon 6 compounds are extracted with a solvent, the undissolved compounds are removed by phase separation from the mixture obtained in the nylon 6 preconcentration section. The extraction should be carried out in such a way that the largest proportion by weight of the undissolved compounds is nylon 6 during extraction with a solvent that preferentially dissolves the non-nylon 6 compounds. This undissolved material then corresponds to the nylon 6-enriched preconcentrated nylon 6-containing material, optionally after removal of the adhering solvent. The used solvent is usually recovered from the extraction phase and reused in the nylon 6 preconcentration section.

[0084] In some cases, two or more extractions are performed, with the non-nylon 6 compounds preferentially extracted. By selecting different solvents and / or other processing conditions (e.g., different extraction temperatures), non-nylon 6 compounds not removed in a previous extraction can be removed in additional extractions. After each extraction, a pretreated, typically solid or partially solid, nylon 6-containing phase is obtained that is richer in nylon 6 than the pre-concentrated nylon 6-containing material resulting from the previous extraction. With each extraction, progressively more non-nylon 6 compounds are extracted from the nylon 6-containing multicomponent material.

[0085] Alternatively, or as a complementary extraction to the extraction using a solvent that dissolves non-nylon 6 compounds described above, extraction in which nylon 6 is preferentially extracted from a nylon 6-containing multicomponent material can be carried out using a suitable solvent as an extractant selective for nylon 6. To preferentially extract nylon 6, a solvent is used that dissolves nylon 6 but has limited dissolution of non-nylon 6 compounds. In other words, a solvent with high selectivity for dissolving nylon 6 should be used. Similar to the selective dissolution of non-nylon 6 compounds, the selective dissolution of nylon 6 compounds can be affected by the choice of solvent, the temperature at which the extraction is carried out, the processing time, and the amount of solvent relative to the amount of nylon 6-containing multicomponent material.

[0086] Solvents known from prior art methods for extracting polyamides can be used in this nylon 6-specific extraction step. For example, EP 603434 A1 discloses suitable solvents for dissolving polyamides, including nylon 6. Such solvents include highly concentrated inorganic acids, formic acid, chloroacetic acid, phenol, cresol, alcoholic solutions of alkaline earth halides, aromatic alcohols such as phenylethanol and benzyl alcohol, as well as glycols, lactams, and lactones. U.S. Pat. No. 5,840,773 discloses that aliphatic alcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, pentanol, hexanol, 2-ethylbutanol, 4-methylpentanol, 1-heptanol, 2-heptanol, 4-heptanol, 2,4-dimethylpentanol, 1-octanol, 2-octanol, 2-ethylhexanol, 1-nonanol, 2-nonanol, n-decanol, n-undecanol, n-dodecanol, n-tridecanol, n-tetradecanol, n-pentadecanol, n-hexadecanol, n-octadecanol, and the like, as well as substituted monohydric and dihydric alcohols, such as 2-methoxy-1-ethanol and methylene glycol, can be used as solvents for dissolving polyamides such as nylon 6. In a preferred embodiment of U.S. Pat. No. 5,840,773, C1-C 12 Alcohols are used. In particular, methanol and ethanol are preferably used as the extractant. It is also conceivable that the extractant is a mixture of an aliphatic alcohol and water. U.S. Pat. No. 5,840,773 further discloses that an extraction time of 60 minutes has been found to be suitable for methanol extraction of polyamide-6, and that the extraction is preferably carried out in an autoclave at a temperature of about 135°C to about 140°C. Along these lines, U.S. Pat. No. 5,840,773 states that polyamide-6,6 is preferentially dissolved in methanol at temperatures of at least about 140°C. In Example IV of U.S. Pat. No. 5,840,773, an extraction temperature of about 160°C for polyamide-6,6 is applied.

[0087] In some cases, two or more extractions are performed in which nylon 6 is preferentially extracted. After each extraction, a pre-concentrated nylon 6-containing material is obtained that is richer in nylon 6 than the pre-concentrated nylon 6-containing material from the previous extraction. This is believed to be due to the fact that each extraction progressively reduces the amount of impurities in the pre-concentrated nylon 6-containing material, which typically include non-nylon 6 compounds.

[0088] After extraction, in which nylon 6 compounds are preferentially extracted with the solvent, the solvent is recovered. First, insoluble materials are removed from the liquid extraction phase containing the solvent and nylon 6, for example, by filtration or centrifugation. The extraction should be carried out in a way that the largest proportion by weight of the insoluble compounds are non-nylon 6 compounds. Second, the solvent is recovered from the nylon 6-containing liquid extraction phase to obtain a pre-concentrated solid nylon 6-containing material enriched in nylon 6. Any of the solvent removal techniques described above, such as cooling, evaporation, distillation, precipitation by adding a precipitant (a solvent for precipitating the dissolved polymer), and combinations thereof, can be carried out to separate the dissolved nylon 6-containing material from the solvent.

[0089] The extraction may also be a combination of sequential extractions using a solvent that preferentially extracts non-nylon 6 compounds and a solvent that preferentially extracts nylon 6. Such a combination of sequential extractions results in a nylon 6-containing phase that is even more enriched in nylon 6 than a nylon 6-containing phase that has been pretreated only once.

[0090] In an embodiment of the present invention, the nylon 6-containing multicomponent material is first extracted in a first nylon 6 preconcentration section with a solvent that preferentially extracts non-nylon 6 compounds from the nylon 6-containing multicomponent material to obtain a nylon 6-enriched preconcentrated nylon 6-containing material. In a second extraction, the nylon 6-enriched preconcentrated nylon 6-containing material is treated with a solvent that preferentially extracts nylon 6 compounds from the nylon 6-enriched preconcentrated nylon 6-containing material to obtain a further nylon 6-enriched preconcentrated nylon 6-containing material, which can then be used in step b). For example, in the first extraction, the non-nylon 6 compound LD polyethylene is removed from the nylon 6-containing multicomponent material by extraction with white spirit. In the second extraction, the nylon 6-enriched preconcentrated nylon 6-containing material is further extracted with methanol or aqueous methanol (e.g., water and MeOH in a weight ratio of 1:19) to obtain a further preconcentrated solid nylon 6-containing material after removal of the solvent (methanol, water).

[0091] According to another advantageous embodiment of the present invention, a nylon 6-containing multicomponent material is first treated in a first nylon 6 preconcentration section with a solvent that preferentially extracts nylon 6 compounds from the nylon 6-containing multicomponent material to obtain a nylon 6-rich preconcentrated nylon 6-containing material. In a second extraction, the nylon 6-rich preconcentrated nylon 6-containing material is treated with a solvent that preferentially extracts non-nylon 6 compounds from the nylon 6-rich preconcentrated nylon 6-containing material to obtain a nylon 6-rich preconcentrated nylon 6-containing material. For example, in the first extraction, nylon 6 is extracted from the nylon 6-containing multicomponent material with ethanol to form a nylon 6-rich nylon 6-containing phase after recovery of the ethanol. In the second extraction, non-nylon 6 components are further extracted from the nylon 6-rich preconcentrated nylon 6-containing material with toluene to obtain a nylon 6-rich nylon 6-containing phase.

[0092] The solvent extraction in the nylon 6 preconcentration section is carried out at a temperature of 0° C. to 350° C. (above which nylon 6 decomposes), more preferably 20° C. to 210° C., and most preferably 50° C. to 190° C. This temperature range promotes selective extraction of most solvents.

[0093] The time required for extraction in the extraction section, i.e., extraction with a solvent, can be easily determined by those skilled in the art through routine experimentation. The time allowed for extraction should preferably take into consideration the type of nylon 6-containing multicomponent material, the amount of nylon 6 contained therein, and its accessibility. The time required for extraction can range from several seconds to several hours. Preferably, the time required for extraction is longer than 5 seconds and shorter than 6 hours. More preferably, the time required for extraction is longer than 15 seconds and shorter than 2 hours.

[0094] As a result of the extraction in the nylon 6 preconcentration section, a solid preconcentrated nylon 6-containing material rich in nylon 6 is obtained. The term "solid" in this case refers to the state of the material at room temperature (20°C). At higher temperatures, the preconcentrated nylon 6-containing material may exist as a melt. The preconcentrated nylon 6-containing material is stable and may be stored for later use in step b) of the method of the present invention, or may be transported to a separate depolymerization section for this purpose. Thus, as described above for the other pretreatment steps, the site where nylon 6 preconcentration by solvent extraction is performed and the site where steps a) to d) of the method of the present invention are performed may be the same or different. Preferably, the pretreatment step and one or more of the preconcentration steps are performed at different locations, such as a site specialized in the pretreatment of waste polymeric materials and, in particular, nylon 6-containing multicomponent materials. The nylon 6-containing multicomponent material that has been pretreated and preconcentrated at various locations can then be transported to the site where one or more of steps a) to d) of the method of the present invention are performed.

[0095] <Depolymerization step b)> In step b) of the present invention, the preconcentrated nylon 6-containing material, which is rich in nylon 6 and obtained in the nylon 6 preconcentration section, is fed to a depolymerization section and depolymerized to form ε-caprolactam. The formed ε-caprolactam is discharged from the depolymerization section as an ε-caprolactam-containing stream. The depolymerization section comprises one or more depolymerization reactors operated in series and / or parallel.

[0096] In some cases, the pre-concentrated nylon 6-containing material is mechanically compressed to a smaller volume before being introduced into the depolymerization section. In particular, the pre-concentrated nylon 6-containing material can be compressed into dense particles before being introduced into the depolymerization section, for example, by mechanical compaction, or by extruding the molten material followed by cooling and cutting to size, or by forming droplets of the molten material which are then solidified by cooling. Such compression has the advantage of requiring a smaller volume for intermediate storage and transportation, and potentially facilitating injection into the depolymerization section.

[0097] In some cases, the pre-concentrated nylon 6-containing material is dried before being introduced into the depolymerization section. This has the advantage that less or no solvent is carried into the depolymerization section. Solvent carried into the depolymerization section may have a negative effect on the depolymerization process (e.g., a slower depolymerization reaction rate, greater catalyst consumption, the vapor stream containing ε-caprolactam and water obtained in the depolymerization section may contain more impurities, etc.).

[0098] The pre-concentrated nylon 6-containing material is preferably fed to the depolymerization reactor as a solid phase or as a melt, which can be accomplished by using an extruder, gear pump, or other means known to those skilled in the art. Feeding the pre-concentrated nylon 6-containing material to the depolymerization reactor can be achieved by continuous or intermittent injection of the nylon 6-rich pre-concentrated nylon 6-containing material.

[0099] In the depolymerization section, the preconcentrated nylon 6-containing material is depolymerized to form ε-caprolactam. This is accomplished by contacting the preconcentrated nylon 6-containing material with water, preferably in the steam state. By supplying water as steam to the depolymerization reactor, a vapor stream containing ε-caprolactam and water, the desired product of step b) of the present invention, can be obtained, optionally without further heating. A similarly desired weight-to-weight ratio of water to ε-caprolactam in this vapor stream of 2:1 to 15:1 can be achieved by adjusting the ratio of steam to preconcentrated nylon 6-containing material fed to the depolymerization section. During the depolymerization reaction, degradation products including linear and cyclic oligomers of ε-caprolactam may be formed. Additionally, the nylon 6-rich preconcentrated nylon 6-containing material feed stream may also contain other components, namely impurities such as non-nylon 6 compounds and residues of the solvent used in the nylon 6 preconcentration section, which remain stable, react, or decompose under the depolymerization conditions. Thus, the vapor stream withdrawn from the depolymerization section contains not only water and ε-caprolactam but also impurities.

[0100] The depolymerization reaction is preferably carried out at a temperature of at least 180°C but not exceeding 400°C. The preferred temperature range for the depolymerization reaction is 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C. Generally, the rate of ε-caprolactam formation increases at higher temperatures. Temperatures below 400°C are preferred because side reactions of nylon 6 and impurities occur more frequently at temperatures above 400°C, resulting in the formation of a more diverse and / or larger number of impurities. Some of these impurities ultimately end up in the ε-caprolactam-containing product stream discharged from the depolymerization reactor. In a preferred embodiment of the present invention, the depolymerization of the preconcentrated nylon 6-containing material is carried out at a temperature in the range of 220°C to 340°C or 240°C to 325°C. This temperature range results in the production of particularly pure ε-caprolactam after purification of the resulting crude product. The depolymerization of nylon 6 is accomplished under wet conditions, i.e., in the presence of water, preferably in the steam state, especially superheated steam.

[0101] Preferably, superheated steam having a temperature of 100°C to 600°C is introduced into the depolymerization reactor. Preferably, the superheated steam introduced into the depolymerization reactor has a temperature that is at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam introduced into the depolymerization reactor is high enough so that no additional heat input is required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam. In a preferred embodiment of the present invention, superheated steam having a temperature in the range of 220°C to 575°C is introduced into the depolymerization section. In an even more preferred embodiment of the present invention, superheated steam having a temperature in the range of 275°C to 500°C is introduced into the depolymerization section.

[0102] The depolymerization of the pre-concentrated nylon 6-containing material in the presence of water vapor can be carried out in the presence of an additional depolymerization agent such as ammonia, an amine, or an alcohol, such as methanol and ethanol.

[0103] Most preferably, the depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted) acid or base. The acid catalyst may be selected from the group consisting of orthophosphoric acid, p-toluenesulfonic acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids such as xylenesulfonic acid, 4-sulfoisophthalic acid, and other sulfonated aromatic hydrocarbons, solid acids, salts of the aforementioned acids, Al2O3, and SiO2, and combinations thereof. The base catalyst may be selected from the group consisting of alkalis such as alkali hydroxides, alkali salts, alkaline earth hydroxides, and earth salts, organic bases, and solid bases, and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali hydroxides, and alkali salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate are used. More preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid, and sodium hydroxide are used. In one particularly preferred embodiment, orthophosphoric acid is used as the depolymerization catalyst, and in another embodiment, p-toluenesulfonic acid is used.

[0104] Although the aforementioned (Lewis or Bronsted) acids or bases are called "catalysts," it is known that some of them are consumed during the depolymerization reaction, resulting in the formation of by-products. For example, it is known that phosphoric acid is consumed during the depolymerization of nylon 6 using steam, resulting in the production of phosphorus-containing by-products. Generally, these phosphorus-containing by-products are ineffective as catalysts and represent a loss of effective catalytic activity. Furthermore, some catalysts, such as H3PO4, tend to polymerize and (partially) lose their catalytic activity, especially at higher temperatures.

[0105] However, in another preferred embodiment of the present invention, no catalyst is used in the depolymerization of the pre-concentrated nylon 6-containing material, which has the advantage of being less costly (both in terms of catalyst and catalyst waste disposal), but higher temperatures (and pressures) are typically required.

[0106] The advantage of using a catalyst (particularly orthophosphoric acid) is that the depolymerization reaction begins at a low temperature and can be carried out under atmospheric conditions. The appropriate catalyst concentration used for the depolymerization of nylon 6 to ε-caprolactam is known to those skilled in the art and can be easily determined through routine experimentation. If the catalyst concentration is too low, the reaction rate is slow. Conversely, if the catalyst concentration is too high, the reaction is fast but side reactions increase. Furthermore, the cost of the catalyst increases, making this economically disadvantageous. Typically, the catalyst content is 0.01 to 100 wt. % relative to the nylon 6 contained in the depolymerization reactor. Preferably, the catalyst content is 0.1 to 50 wt. The optimal catalyst concentration varies depending on the type of catalyst used for the depolymerization of nylon 6. For orthophosphoric acid catalysts, the preferred content is 0.1 to 25 wt. %, more preferably 1 to 20 wt. For p-toluenesulfonic acid catalysts, the preferred content is 10 to 35 wt. %.

[0107] The depolymerization of nylon 6 can be carried out in a batch, semi-continuous, or continuous manner, all of which are well known to those skilled in the art. As used herein, the terms "batch," "semi-continuous," and "continuous" refer to a manner in which a nylon 6-containing feedstock, i.e., a pre-concentrated nylon 6-containing material enriched in nylon 6 (and optionally a catalyst), is introduced into a depolymerization reactor, and a manner in which a residue is discharged from the depolymerization reactor. Typically, the residue contains non-nylon 6 compounds, non-depolymerized nylon 6, ε-caprolactam, the catalyst, and decomposition products of these components.

[0108] In a preferred embodiment, the depolymerization of nylon 6 is carried out in a batch mode. In the batch mode, the feedstock, i.e., a pre-concentrated nylon 6-containing material enriched in nylon 6, and optionally a catalyst, are first charged to a depolymerization reactor. Superheated steam is then charged to the depolymerization reactor, and ε-caprolactam is released from the depolymerization reactor as a vapor stream containing ε-caprolactam and water. The charging of superheated steam to the depolymerization reactor is then discontinued. After the residue is removed from the depolymerization reactor, a new cycle is initiated by charging the feedstock (and optionally the catalyst) to the depolymerization reactor. In a preferred embodiment, the residue is not necessarily removed every cycle.

[0109] In a particularly advantageous embodiment, the depolymerization of nylon 6 is carried out in a continuous manner. In a continuous manner, a nylon 6-containing feedstock (and optionally a catalyst) is continuously introduced into a depolymerization reactor. Simultaneously, superheated steam is continuously introduced into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream comprising ε-caprolactam and water. In addition, residue is continuously discharged from the depolymerization reactor. Preferably, the nylon 6-containing feedstock (and optionally a catalyst) is introduced as a melt, solid pieces, a slurry, or a solution. More preferably, the nylon 6-containing feedstock (and optionally a catalyst) is introduced as a melt, a slurry, or a solution.

[0110] In a preferred embodiment, the depolymerization of nylon 6 is carried out in a semi-continuous manner. In a semi-continuous manner, a nylon 6-containing feedstock (and optionally a catalyst) is intermittently charged to a depolymerization reactor, while superheated steam is continuously charged to the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream comprising ε-caprolactam and water. Residue is intermittently discharged from the depolymerization reactor in the semi-continuous nylon 6 depolymerization.

[0111] The ε-caprolactam leaves the depolymerization section in the form of a vapor stream comprising water and ε-caprolactam in a weight-to-weight ratio of 1:1 to 50:1, preferably 2:1 to 10:1, more preferably 3:1 to 8:1. Preferably, the pressure of the ε-caprolactam in the vapor stream is 0.02 to 10 MPa, more preferably 0.08 to 1.5 MPa, and most preferably 0.1 to 0.5 MPa.

[0112] <Recovery step c)> In the recovery section, ε-caprolactam is recovered from the ε-caprolactam-containing vapor stream released from the depolymerization section, preferably by (partial) condensation of the vapor stream.

[0113] The ε-caprolactam-containing vapor stream discharged from the depolymerization section contains ε-caprolactam, water, and impurities. The ε-caprolactam can be separated from the remaining components of the vapor stream by passing the vapor stream from the depolymerization reactor, typically to an upper condenser (preferably a partial condenser), to obtain a condensate containing ε-caprolactam. Preferably, the ε-caprolactam is separated from the remaining components of the vapor stream by passing the product stream from the depolymerization reactor, preferably to an upper distillation column, which obtains a water-rich phase as an overhead product and an ε-caprolactam-rich phase as a bottom product.

[0114] The ε-caprolactam recovered in the recovery section is in a crude state because it contains impurities such as nylon 6 degradation products and other impurities derived from non-nylon 6 components of the multi-component starting material. The crude ε-caprolactam recovered in step c) comprises water and ε-caprolactam, preferably an aqueous solution containing ε-caprolactam. Thus, the crude ε-caprolactam recovered in the recovery section requires additional purification to produce high-purity ε-caprolactam. Therefore, "crude" as used herein can be defined as having a lower purity than the purified ε-caprolactam obtained as the product of the process of the present invention.

[0115] Preferably, the crude ε-caprolactam contains ε-caprolactam in the range of 6% to 95% by weight, more preferably 20% to 90% by weight, and most preferably 40% to 75% by weight, with the remainder being primarily water.

[0116] <Purification step d)> In step d), the crude ε-caprolactam obtained in the recovery section [C] is purified in the purification section [D] to produce highly pure ε-caprolactam. Optionally, the crude ε-caprolactam is filtered before being fed to the purification section, which ensures the removal of undissolved impurities that could interfere with further purification steps.

[0117] Purified ε-caprolactam is obtained from crude ε-caprolactam by first extracting the crude ε-caprolactam with an organic solvent in step (i), which results in an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam and impurities. The organic solvent used for the extraction of crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 Optionally, the organic solvent used for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, an alicyclic hydrocarbon, a halogenated hydrocarbon, and / or a C4-C 10and a C5-C8 alkane or C5-C8 cycloalkane. Particularly good results are achieved when the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC, methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol, and mixtures thereof. More preferably, the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohol, and mixtures thereof. Even more preferably, the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of toluene, 1-octanol, 4-methyl-2-pentanol, 2-ethylhexanol, and mixtures thereof. Typically, the weight ratio of the organic solvent to ε-caprolactam is from 0.01:1 to 40:1, preferably from 0.01:1 to 20:1, more preferably from 0.05:1 to 10:1, and most preferably from 0.1:1 to 5:1.

[0118] Optionally, the organic solvent for the extraction of crude ε-caprolactam is alkane C m H 2m+2 (m is 5 to 8), cycloalkane or C m H 2m (m is 5 to 8) to form a mixed extractant. Particularly good results are achieved when the alkane or cycloalkane is present in the mixed extractant in a weight ratio of 5 to 90%, preferably 25 to 75%, based on the total weight of the mixed extractant.

[0119] In an embodiment of the present invention in which the organic solvent has a lower density than the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in an extraction column operated in countercurrent, into which the crude ε-caprolactam to be purified is introduced at the top and the organic solvent at the bottom. The extraction results in an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam, and impurities, the weight ratio of impurities to ε-caprolactam in the organic phase being reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, the extraction results in ε-caprolactam with a higher purity than before the extraction.

[0120] In another preferred embodiment of the present invention, in which the organic solvent is denser than the crude ε-caprolactam, the extraction with the organic solvent in step d(i) is carried out in a countercurrent extraction column into which the crude ε-caprolactam to be purified is introduced at the bottom of the column and the organic solvent is introduced at the top. The extraction results in an aqueous phase containing water and impurities and an organic phase containing the organic solvent, ε-caprolactam, and impurities, the weight ratio of impurities to ε-caprolactam in the organic phase being reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, the extraction results in ε-caprolactam with a higher purity than before the extraction.

[0121] Optionally, the organic phase containing the organic solvent, ε-caprolactam, and impurities is washed with water or an aqueous alkaline solution before proceeding to step d(iii). When washing is carried out with an aqueous alkaline solution, the alkaline solution is preferably an aqueous solution containing an alkali metal hydroxide and / or alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably contains 0.5 to 2.0 wt. % sodium hydroxide or potassium hydroxide.

[0122] Those skilled in the art can determine by routine experimentation the amount of water or aqueous alkaline solution required for efficient washing of the organic phase containing the organic solvent, ε-caprolactam, and impurities. Typically, this amount is 0.1 to 5 wt. % based on the amount of organic solvent, excluding ε-caprolactam dissolved in the organic phase to be washed. Preferably, washing of the organic phase containing the organic solvent, ε-caprolactam, and impurities with water or aqueous alkaline solution is carried out in a wash column operated countercurrently, into which the organic phase containing the organic solvent, ε-caprolactam, and impurities is introduced at the bottom and water or aqueous alkaline solution is introduced at the top. As a result of washing, a washed organic phase containing the organic solvent, ε-caprolactam, and impurities and a residue phase are obtained. Typically, the residue phase contains water and ε-caprolactam, with only traces of impurities. As a result of washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase before washing.

[0123] Optionally, step d(iii) of the process of the present invention, in which purified ε-caprolactam is obtained by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam, is preceded by one or more treatment steps in order to reduce the energy usage of purification step d) of the process of the present invention, to increase the quality of the purified ε-caprolactam, and / or to simplify the distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam.

[0124] Optionally, the organic phase obtained according to step d)(ii) of the process of the present invention, which comprises the organic solvent, ε-caprolactam and impurities and which has been optionally washed with water or an aqueous alkaline solution, is subjected to solvent switching, in which the organic solvent in the organic phase comprising the organic solvent, ε-caprolactam and impurities is replaced with water to obtain an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than that of ε-caprolactam, the solvent switching step being selected from a step based on back-extraction (also known as re-extraction) with water and a step based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced. As used herein, the term "replaced" means that at least 60%, preferably at least 80%, and most preferably at least 90, 95 or 98% by weight of the organic solvent present in the organic phase comprising the organic solvent, ε-caprolactam and impurities is replaced with water.

[0125] Two solvent switching options are described below. Solvent Switching: First Choice: Optionally, the washed ε-caprolactam-solvent phase is subsequently re-extracted with water to obtain an ε-caprolactam-aqueous phase. Therefore, this ε-caprolactam-aqueous phase is preferably subjected to stripping and / or distillation to remove residual solvent. The amount of water used to recover ε-caprolactam is not critical, but the amount of water used is typically 0.5 to 20 times by weight relative to the amount of ε-caprolactam recovered. Preferably, the amount of water used is 0.75 to 10 times by weight, more preferably 1 to 5 times by weight.

[0126] The re-extraction with water can be advantageously carried out in an extraction column operated in countercurrent, in which the ε-caprolactam-solvent phase to be purified is introduced at the bottom of the column and water is introduced at the top of the column.As a result of the re-extraction, an ε-caprolactam-water phase and an impurity-containing solvent phase are produced.Usually, the impurity-containing solvent phase is reused, possibly after purification, preferably by distillation.

[0127] In another preferred embodiment, in which the optionally washed organic phase containing the organic solvent, ε-caprolactam, and impurities has a density lower than that of 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. Stripping results in an organic solvent phase containing impurities and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than that of ε-caprolactam, the weight ratio of impurities to ε-caprolactam being reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase containing the organic solvent, ε-caprolactam, and impurities before stripping. Stripping thus results in higher-purity ε-caprolactam. Preferably, the organic solvent phase containing impurities is reused, optionally after purification (preferably by distillation).

[0128] In another preferred embodiment, in which the organic phase containing the organic solvent, ε-caprolactam, and impurities is denser than water, the organic phase is introduced in the upper section of the extraction column, and water is introduced in the lower section of the extraction column. Stripping results in an organic solvent phase containing impurities, and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase containing the organic solvent, ε-caprolactam, and impurities before stripping. Thus, stripping results in ε-caprolactam with a higher purity than before stripping. Preferably, the organic solvent phase containing impurities is reused, optionally after purification (preferably by distillation).

[0129] The resulting ε-caprolactam-water phase, which may have been stripped and / or distilled to remove residual solvent, is concentrated by evaporation of water to give a concentrated aqueous ε-caprolactam phase, the ε-caprolactam content of which is typically between 50 and 99.9% by weight, based on the total weight of the phase.

[0130] Solvent Switching: Second Option: Optionally, instead of re-extraction with water, the organic solvent is evaporated from the optionally washed ε-caprolactam-solvent phase. Any suitable evaporation vessel, such as a column, can be used. Preferably, the evaporation is carried out in the presence of water. More preferably, the evaporation is carried out as an azeotropic distillation, in which case the organic solvent evaporates as an azeotrope. The evaporation results in an ε-caprolactam product. Typically, the ε-caprolactam product is an aqueous ε-caprolactam phase. The ε-caprolactam content of this aqueous ε-caprolactam phase is usually 50 to 99.9 wt. % based on the total phase.

[0131] The solvent switching process may be a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is added. In a preferred embodiment, the solvent switching process is a process based on solvent exchange distillation carried out as a single-stage process, in which the organic solvent is distilled off from the organic phase containing the organic solvent, ε-caprolactam, and impurities, and water is added. More preferably, the solvent switching is carried out as an azeotropic distillation with the addition of water, in which the organic solvent evaporates as an azeotropic mixture containing the organic solvent and water. The objective of azeotropic distillation is to remove the organic solvent and add water. Preferably, almost all of the organic solvent is removed. In this context, "almost all" means that at least 90%, preferably at least 95%, and most preferably at least 98 or 99% by weight of the organic solvent present in the organic phase containing the organic solvent, ε-caprolactam, and impurities is removed. Preferably, water is added as a liquid. More preferably, water is added in a liquid state as a reflux liquid to the upper part of the distillation column. Even more preferably, a portion of the water added as reflux liquid is obtained by condensing the azeotropic mixture distilled off in the distillation column. Any suitable vessel, such as a column, preferably a distillation column operated in a continuous manner, can be used for the solvent switching process. The distillation column can include a tray column, a packed column, or a combination thereof. In another preferred embodiment, the solvent swap distillation is carried out as a two-stage process: the first stage is a pre-concentration stage and the second stage is the actual solvent swap distillation.

[0132] The organic phase containing the organic solvent, ε-caprolactam, and impurities is introduced into the first stage. In the first stage, a first fraction of the organic solvent is removed by distillation from the organic phase containing the organic solvent, ε-caprolactam, and impurities in the upper part of the distillation column. Preferably, this distillation is carried out under reflux. Under reflux means that the organic solvent is introduced into the upper part of the distillation column in a liquid phase. More preferably, a portion of the organic solvent removed by distillation in the upper part of the distillation column is introduced into the upper part of the distillation column as a liquid after condensation. The remaining organic phase containing the organic solvent, ε-caprolactam, and impurities is discharged from the first stage and introduced into the second stage. Due to the distillation in the first stage, the chemical composition of the remaining organic phase containing the organic solvent, ε-caprolactam, and impurities is different from that of the organic phase containing the organic solvent, ε-caprolactam, and impurities introduced into the first stage. Generally, compared to the organic phase containing the organic solvent, ε-caprolactam and impurities charged in the first stage, the remaining organic phase containing the organic solvent, ε-caprolactam and impurities has a higher weight percent content of ε-caprolactam and compounds with boiling points higher than ε-caprolactam, and a lower weight percent content of compounds with boiling points lower than ε-caprolactam.

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

[0134] Any suitable vessel, such as a column, preferably a distillation column operated in a continuous manner, can be used for each stage of the solvent switch. The distillation column can include a tray column, a packed column, or a combination thereof.

[0135] The solvent exchange distillation (carried out as either a one-stage process or a two-stage process) results in an aqueous phase containing water, ε-caprolactam, impurities with lower or higher boiling points than ε-caprolactam, and optionally residual organic solvent. Preferably, the ε-caprolactam content of this aqueous phase is 25 to 99.9%, preferably 50 to 99.5%, and most preferably 85 to 99%, by weight of the total aqueous phase.

[0136] Thus, according to a particularly advantageous embodiment of the present invention, after extraction of crude ε-caprolactam in step d)(i), the purification in step d) also comprises a step of (ii) solvent switching based on solvent exchange distillation.

[0137] Optionally, the aqueous ε-caprolactam phase obtained according to the first or second solvent switching option step is subsequently subjected to oxidation and / or treatment with ion exchange resins and / or hydrogenation.

[0138] In a preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate, and / or hydrogen peroxide, is added to the aqueous ε-caprolactam phase. Most preferably, potassium permanganate is used as the oxidizing agent.

[0139] Preferably, the oxidizing agent is added to the aqueous ε-caprolactam phase in the form of an aqueous solution so that a dilute aqueous solution is obtained during purification by oxidation. The oxidizing agent may be added as a solid. The oxidizing agent may be added as a slurry. Those skilled in the art can determine the amount of oxidizing agent required for efficient oxidation of the aqueous ε-caprolactam phase by routine experimentation. The exact amount of oxidizing agent will depend, among other things, greatly on the composition of the nylon 6 stream fed to the depolymerization section of the process of the present invention. Preferably, the amount of oxidizing agent is 0.01 to 5% by weight relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0140] The temperature used for the oxidation of the aqueous solution in the process of the present invention may vary. Preferably, the oxidation of the aqueous solution with an oxidizing agent is carried out at a temperature in the range of 20° C. to 85° C., more preferably in the range of 30° C. to 80° C., and the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof, in particular potassium permanganate.

[0141] The length of time for oxidation with the oxidizing agent may vary. Preferably, in the process of the present invention, oxidation of the aqueous ε-caprolactam phase with the oxidizing agent is carried out for a period of from 1 minute to 24 hours, more preferably from 2 minutes to 6 hours, and most preferably from 5 minutes to 2 hours.

[0142] The concentration of ε-caprolactam in the aqueous ε-caprolactam phase used for oxidation with an oxidizing agent may vary. Preferably, the aqueous solution used for oxidation comprises ε-caprolactam and water in a weight-to-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-to-weight ratio of ε-caprolactam to water is adjusted prior to the addition of the oxidizing agent to the aqueous phase. Preferably, the weight-to-weight ratio of ε-caprolactam to water is adjusted by either adding or removing water.

[0143] When potassium permanganate and / or sodium permanganate are used as the oxidizing agent, solid manganese(IV) oxide (MnO) particles are formed as the reaction product. Those skilled in the art can determine, through routine experimentation, the optimal solid-liquid filtration procedure for efficiently removing the solid manganese(IV) oxide particles from the aqueous phase after oxidation. The use of filter aids such as activated carbon particles or diatomaceous earth to improve the filtration procedure is common in this regard.

[0144] In another preferred embodiment, the aqueous ε-caprolactam phase is treated with an ion exchange resin. Optionally, the aqueous ε-caprolactam phase is first treated with an acidic cation exchange resin, followed by a basic anion exchange resin. Optionally, the aqueous ε-caprolactam phase is first treated with a basic anion exchange resin, followed by an acidic cation exchange resin. Preferably, the cation exchange resin is sulfonated polystyrene or styrene-divinylbenzene copolymer, and the anion exchange resin is quaternary ammonium group-containing polystyrene or an exchange resin having secondary or tertiary amino groups. The treatment temperature of the aqueous ε-caprolactam solution is preferably 15°C to 100°C, more preferably 35°C to 70°C. To efficiently perform adsorption and separation of impurities on the cation exchange resin and the anion exchange resin, the concentration of the aqueous ε-caprolactam solution to be treated is preferably 5 to 90%, more preferably 5 to 70%.

[0145] Those skilled in the art can determine by routine experimentation the optimum (combination of) ion exchange resins and amounts of ion exchange resins for efficient removal of impurities from the aqueous ε-caprolactam phase and regeneration of the loaded ion exchange resin.

[0146] In another preferred embodiment, the aqueous ε-caprolactam phase is hydrogenated in the presence of a hydrogenation catalyst known per se. The hydrogenation can be advantageously carried out, for example, as described in EP-A-635487.

[0147] The hydrogenation temperature is generally between 20°C and 160°C. Generally, a temperature that is not too low is selected, because a low temperature increases the reaction time. Generally, the temperature is not too high, because a high temperature adversely affects the quality of ε-caprolactam. Therefore, the temperature is preferably between 70°C and 130°C, and most preferably between 80°C and 100°C.

[0148] The hydrogenation pressure may be 0.1 MPa to 15 MPa. High pressure is advantageous because it allows a large amount of hydrogen to be dissolved in the water-ε-caprolactam mixture. Since the impurity content is usually not high enough to require a large amount of hydrogen, excessively high pressure is not necessary. Ultra-high pressure also has the disadvantage of requiring expensive processing equipment. Therefore, the pressure is generally 0.3 MPa to 5 MPa.

[0149] The hydrogenation catalyst may be any known heterogeneous hydrogenation catalyst. Examples of such catalysts include ruthenium-aluminum oxide, rhodium-aluminum oxide, platinum on carbon, palladium on carbon, Raney nickel, nickel-silica, and nickel-aluminum oxide. Preferably, a nickel-containing catalyst is used.

[0150] Suitable nickel catalysts generally have a nickel content of 5-80% by weight based on the metal and support. In addition to nickel, the catalyst may contain several activators such as Zr, Mn, Cu, or Cr. The activator content is generally 1-20% by weight. When a heterogeneous catalyst containing palladium is used, the palladium content is generally 0.01-10% by weight.

[0151] The aqueous ε-caprolactam phase, which has been optionally subjected to oxidation and / or treatment with an ion exchange resin and / or hydrogenation, is optionally evaporated to remove water. Following oxidation and / or hydrogenation and / or evaporation of water, the aqueous ε-caprolactam phase is distilled to recover high-purity ε-caprolactam and a distillation residue.

[0152] In a preferred embodiment of the process of the present invention, prior to the distillative removal of step d)(iii), the aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than that of ε-caprolactam is a solvent replacement step in which the organic solvent is at least partially replaced with water, the step being selected from a process based on re-extraction with water and a process based on solvent replacement distillation in which the organic solvent is removed by distillation and water is introduced; and optionally followed by oxidation with an oxidizing agent selected from potassium permanganate, sodium permanganate and hydrogen peroxide; and / or optionally subsequent treatment with an acidic cation exchange resin and / or a basic anion exchange resin; and / or optionally followed by hydrogenation in the presence of a hydrogenation catalyst selected from Raney nickel, nickel-silica, nickel-aluminum oxide, ruthenium-aluminum oxide, rhodium-aluminum oxide, platinum on carbon and palladium on carbon. is obtained by

[0153] In step d)(iii) of the process of the present invention, purified ε-caprolactam is obtained by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam. Typically, the distillation of the optionally washed organic phase containing the organic solvent, ε-caprolactam, and impurities is carried out under reduced pressure. In an embodiment of the present invention, the distillation is carried out at a pressure of less than 50 kPa, preferably less than 20 kPa, more preferably less than 10 kPa. Preferably, the distillation temperature at the bottom of the distillation column is between 100°C and 200°C, more preferably between 110°C and 180°C. The distillation comprises separating low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) and / or high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam.

[0154] In a preferred embodiment of the present invention, an alkali metal hydroxide, preferably NaOH, is added to the organic phase prior to the distillative removal in step d)(ii). Preferably, the amount of NaOH added ranges from 0.5 to 150 mmol, more preferably and most preferably from 2 to 80 mmol per kg of ε-caprolactam. This results in a particularly effective distillative removal of impurities with lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0155] The purification of crude ε-caprolactam in the purification section according to the present invention typically comprises the following steps: (i) extracting crude ε-caprolactam with an organic solvent to obtain an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities; (ii) optionally a solvent switching step, in which the organic solvent is at least partially replaced with water to obtain an aqueous phase comprising water, ε-caprolactam, and impurities with a boiling point lower or higher than that of ε-caprolactam, said solvent switching step (ii) being selected from a process based on back-extraction with water and a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced; and (iii) obtaining purified ε-caprolactam by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam; Includes:

[0156] In a preferred embodiment of the present invention, the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified in the purification section separately from the crude ε-caprolactam recovered in the recovery section, which has the advantage that the carbon footprint of a plant for de novo production of ε-caprolactam from the Beckmann rearrangement of cyclohexanone oxime can be reduced by introducing the recycled ε-caprolactam according to the present invention.

[0157] The high purity ε-caprolactam obtained by the process of the present invention can be used to make nylon 6 using methods well known to those skilled in the art, which can then be used in any known material, including engineering materials, fibers, and films.

[0158] <Plant> The present invention further provides a plant capable of carrying out the above-described method of the present invention. Accordingly, all apparatus features specifically described below with respect to a plant also apply to specific embodiments of the method of the present invention, and vice versa. That is, the plant is suitable for carrying out the method of the present invention. It should therefore be understood that what has been described with respect to the method of the present invention applies equally to the plant embodiment.

[0159] The plant may correspond to a laboratory installation as 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 t / year of ε-caprolactam when operated full-time (i.e., is capable of producing that amount of ε-caprolactam in principle).

[0160] The plant of the present invention is suitable for producing purified ε-caprolactam from a nylon 6-containing multicomponent material and comprises at least four sections: (A) a nylon 6 preconcentration section, (B) a depolymerization section, (C) a recovery section, and (D) a purification section. These sections, and therefore the plant, are configured to carry out the process of the present invention described above.

[0161] Additionally, the plant may include a mechanical shredding section [O] for fragmenting the nylon 6-containing multicomponent material into small pieces, and / or a washing section [W] for washing the nylon 6-containing multicomponent material. The mechanical shredding section [O] includes equipment for mechanically fragmenting the nylon 6-containing multicomponent material into small pieces. Non-limiting examples of this fragmenting equipment include a cutter, shredder, grinder, pulverizer, or chipper.

[0162] The nylon 6 preconcentration section [A] comprises an extraction device in which the nylon 6-containing multicomponent material is extracted with one or more organic solvents. The extraction device can have any desired form, such as a mixer-settler extractor, an extraction column, and / or a centrifugal extraction device, and is optionally operated under pressure. The nylon 6-containing multicomponent material particles are preferably mixed with the organic solvent by means of a stirrer in the extraction device of the nylon 6 preconcentration section. The nylon 6 preconcentration section can comprise further units, in particular units that allow for the storage, recovery, and purification of one or more organic solvents used in this section, as well as units that allow for the collection and intermediate storage of the resulting mainly non-nylon 6-containing by-products formed in this section and the desired preconcentrated nylon 6-containing material.

[0163] The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or parallel. The nylon 6-containing multicomponent material is fed to the reactors as a solid or melt, preferably as a melt. This feeding can be accomplished by using an extruder, a gear pump, or other means known in the art.

[0164] During production, the depolymerization reactor is at least partially filled with nylon 6-containing feed, residual material, ε-caprolactam (and optionally catalyst). The depolymerization reactor can have any desired configuration. Preferred reactor types are stirred and unagitated bubble column reactors, stirred reactors, and extruder-type reactors.

[0165] The depolymerization reactor must be equipped with a facility for supplying the nylon 6-containing multicomponent material, superheated steam, and optionally a catalyst. In addition, the depolymerization reactor must be equipped with a facility for discharging a vapor stream containing ε-caprolactam and water, and residual materials.

[0166] Adequate contact between the steam and the reactor contents is essential for effective operation. Such contact can be achieved by a variety of means commonly known in the art. For example, multiple inlets can be used to distribute the steam throughout the material, e.g., using a steam distributor. Improved contact can be achieved by incorporating mechanical agitation into the reactor, e.g., using a combination of rotating paddles and stationary fins.

[0167] Depolymerization is usually complete in 0.5 to 6 hours. If hot superheated steam is not available at the production site, it must be produced systematically by superheating the steam available from the boiler in a so-called superheater.

[0168] The recovery section [C] comprises one or more condensers (preferably partial condensers) into which the vapor stream comprising ε-caprolactam and water is introduced. Such condensers (partial condensers) can have any desired form. Preferably, the condenser is a distillation column which gives a water-rich phase as overhead product and an ε-caprolactam-rich phase as bottom product.

[0169] The purification section [D] comprises one or more extraction devices, optionally one or more solvent switching devices, optionally an oxidation section, optionally a hydrogenation section, and one or more distillation devices, into which crude ε-caprolactam is input and from which high-purity ε-caprolactam is released.

[0170] The extraction device is fed with crude ε-caprolactam and an organic solvent, and discharges an organic phase containing the organic solvent, ε-caprolactam, and impurities, and an aqueous phase containing water and impurities. The extraction device is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the extraction device is a static or stirred extraction column, such as a KARR column, a SCHEIBEL column, a rotating disk contactor (RDC), a pulse column, a sieve tray (static) column, a randomly packed (static) column, and a structured packed (SMVP) (static) column.

[0171] The solvent switching device is charged with water and an organic phase containing the organic solvent, ε-caprolactam, and impurities, and discharged with an organic solvent and an ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities. The solvent switching device for the re-extraction-based process is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the device for re-extraction is a static or stirred extraction column, such as a KARR column, a SCHEIBEL column, a rotating disk contactor (RDC), a pulsed column, a sieve tray (static) column, a randomly packed (static) column, and a structured packed (static) column.

[0172] The solvent switching equipment for the process based on solvent exchange distillation is preferably selected from a sieve tray type distillation column, a randomly packed distillation column, and a structured packed distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser, and reflux equipment. The distillation column can be operated at atmospheric pressure, a pressure lower than atmospheric pressure, or a pressure higher than atmospheric pressure. Preferably, water is charged to the upper part of the distillation column, and an aqueous phase containing water, ε-caprolactam, and impurities with a boiling point lower or higher than ε-caprolactam is discharged from the lower part of the distillation column.

[0173] The oxidation section comprises one or more oxidation reactors operated in series and / or parallel. An oxidizing agent and an ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities are fed to the oxidation section. Typically, the oxidizing agent is fed as an aqueous solution or as a solid. When potassium permanganate or sodium permanganate is used as the oxidizing agent, the oxidation section further comprises a filtration section. The oxidation reactor can have any desired configuration. Preferred reactor types are stirred reactors, unstirred reactors, and packed column reactors. The oxidation reactor must be equipped with a facility for feeding the aqueous phase containing water, ε-caprolactam, and impurities with a boiling point lower or higher than that of ε-caprolactam, and the oxidizing agent. Additionally, the oxidation reactor must be equipped with a facility for discharging the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities, and optionally the solid manganese(IV) oxide (MnO) particles formed. Preferably, the oxidation is carried out at a temperature in the range of 20° C. to 85° C. under atmospheric conditions.

[0174] Possibly present solid manganese(IV) oxide (MnO2) particles can be removed by precipitation or solid-liquid filtration, preferably by solid-liquid filtration. The use of filter aids, such as activated carbon particles, to improve the filtration procedure is common. Filter systems suitable for separating solid manganese(IV) oxide particles are known to those skilled in the art. Such filter systems are fed with a suspension of solid manganese(IV) oxide particles and an oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities, and a filtered oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities. Generally, the solid manganese(IV) oxide particles are retained in the filter system. Preferably, such filter systems are operated in a semi-continuous manner, where the suspension and the filtered phase are continuously fed and discharged, while the separated solids are collected in the filter system. From time to time, the feeding of the suspension is interrupted, and the collected solids are removed from the filter system.

[0175] The hydrogenation section comprises one or more hydrogenation reactors operated in series and / or parallel. Any type of reactor in which hydrogenation is carried out in the presence of a solid catalyst can be used. Preferably, the hydrogenation reactor is a stirred tank reactor or a fixed bed reactor.

[0176] A gas stream containing hydrogen and an ε-caprolactam-water phase containing water, ε-caprolactam, and impurities are introduced into the hydrogenation section. A hydrogenated ε-caprolactam-water phase containing water, ε-caprolactam, and impurities, and optionally a purge gas stream containing hydrogen, are discharged from the hydrogenation section. Under normal operating conditions, the hydrogenation catalyst remains in the hydrogenation reactor and is replaced only after deactivation.

[0177] The distillation apparatus is fed with an ε-caprolactam-water phase containing water, ε-caprolactam, and impurities, and discharges high-purity ε-caprolactam, water, and impurities (i.e., low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) and high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam)). The distillation apparatus is selected from a sieve tray distillation column, a randomly packed distillation column, a structured packed distillation column, and a horizontal and vertical (downward and upward) film evaporator. Preferably, the distillation column is equipped with a reboiler, a condenser, and reflux equipment. The distillation apparatus can be operated at atmospheric pressure, subatmospheric pressure, or superatmospheric pressure, preferably subatmospheric pressure.

[0178] Preferably, the distillation comprises separating water, low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam), and / or high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises, in a first step, separating water as an overhead product and producing ε-caprolactam containing low-boiling and high-boiling impurities as a bottom product. In a second step, the low-boiling impurities are separated as an overhead product, and ε-caprolactam containing high-boiling impurities is obtained as a bottom product. In a third step, high-purity ε-caprolactam is separated as an overhead product, and a distillation residue containing ε-caprolactam and high-boiling impurities is produced as a bottom product. Optionally, the first and second steps are combined.

[0179] Preferably, prior to the distillative removal of water and impurities, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-water phase containing water, ε-caprolactam and impurities. Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol per kg of ε-caprolactam, more preferably from 2 to 80 mmol per kg of ε-caprolactam. This results in particularly effective distillative removal of impurities with lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0180] The process of the present invention can be carried out in a continuous, semi-continuous, or batch mode. Accordingly, the plant of the present invention can be configured to allow one or more of these modes of operation. In a preferred embodiment, the plant is configured to carry out the process of the present invention in a continuous or semi-continuous mode. However, discontinuous processes are also possible. For example, the plant of the present invention need not have all of the sections described herein located in one place. In particular, the nylon 6 preconcentration section can be located in a first location, while the depolymerization section, recovery section, and purification section can be located in a second location. Similarly, the mechanical shredding section can be located in a different location.

[0181] In a particularly advantageous embodiment of the present invention, the plant can include a purification section shared with a plant for the de novo synthesis of ε-caprolactam, which purification section is normally or actually used to purify crude ε-caprolactam produced by other processes, such as by the Beckmann rearrangement of cyclohexanone oxime. Thus, the present invention provides for the use of ε-caprolactam recovered from multilayer materials containing nylon 6 to reduce the carbon footprint of ε-caprolactam production plants. Proceeding in this manner also has the advantage that the highly sophisticated purification section of an existing plant producing ε-caprolactam by the Beckmann rearrangement reaction can be used in the process of the present invention. This is cost-effective, since it is not necessary to build a separate purification section for the process and plant of the present invention. It is surprising that recovered ε-caprolactam obtained by depolymerization of nylon 6 produced from nylon 6-containing multicomponent materials can be mixed and co-purified with crude ε-caprolactam produced by other processes, such as the Beckmann rearrangement of cyclohexanone oxime, without compromising the quality of the crude ε-caprolactam. In the applicant's view, this is only possible with the special combination of nylon 6 pre-concentration, depolymerization, and recovery steps of the present invention, which results in an ε-caprolactam product of exceptional purity that can be mixed and co-purified with ε-caprolactam produced by other processes, such as the Beckmann rearrangement, without any disadvantage in terms of the quality of the resulting product.

[0182] <product> The process of the present invention makes it possible to produce highly pure and therefore high quality ε-caprolactam which meets the properties for demanding applications while at the same time having a low product carbon footprint and being particularly economical due to the use of waste as starting material. In a preferred embodiment, the ε-caprolactam obtained by the process of the present invention has the following properties: PAN: up to 5 E290: max 0.05 VB: max 0.5mmol / kg Alkalinity: Max 0.1mmol / kg Acidity: up to 0.1mmol / kg The parameters and measurement methods are defined in the Examples section herein below.

[0183] The ε-caprolactam produced by the process of the present invention is particularly economical and environmentally friendly, as evidenced by its much lower carbon footprint compared to conventionally produced ε-caprolactam (e.g., by Beckmann rearrangement of cyclohexanone oxime).

[0184] The environmental impact of a product is commonly expressed as the "product carbon footprint". The carbon footprint of a product is defined as the total emissions caused by the formation of that product, expressed in tonnes of carbon dioxide equivalent per tonne of product. The carbon footprint of a product varies depending on, among other things, feed materials, auxiliary materials, energy consumption, energy source, production process and process efficiency. Quantification of the carbon footprint of a product can be done, for example, as described in European Standard EN ISO 14040:2006 ("Environmental management - Life cycle assessment - Principles and framework").

[0185] Product carbon footprint calculations may be performed either in-house or by an external (preferably) certified body that verifies and certifies product carbon footprint calculations, for example according to the LCA standard ISO 14040.

[0186] J. Hong and X. Xu ("Environmental impact assessment of caprolactam production - a case study in China"; J. of Cleaner Production 27, (Netherlands), 2012, 103-108; DOI: 10.1016 / j.jclepro.2011.12.037) report that the global warming potential of "virgin" e-caprolactam obtained by the Beckmann rearrangement of cyclohexanone oxime is 7.5 tons of CO2 equivalent per ton of e-caprolactam when coal-based electricity and steam generation are used. When natural gas-based electricity and steam generation are used, the global warming potential of virgin e-caprolactam from this e-caprolactam production method is reduced to 6.4 tons of CO2 equivalent per ton of e-caprolactam.

[0187] The product carbon footprint of the ε-caprolactam obtained by the method of the present invention is much lower than that of de novo synthesized, i.e., "virgin," ε-caprolactam. Preferably, the product carbon footprint of the ε-caprolactam obtained by the method of the present invention is less than 4 tonnes of CO₂ equivalent per tonne of ε-caprolactam, more preferably less than 3 tonnes of CO₂ equivalent per tonne of ε-caprolactam, even more preferably less than 2.5 tonnes of CO₂ equivalent per tonne of ε-caprolactam, and most preferably less than 2 tonnes of CO₂ equivalent per tonne of ε-caprolactam (based on data from Ecoinvent version 3.7.1 (region: Europe)). [Brief explanation of the drawings]

[0188] The present invention will now be described with reference to the drawings, which show certain embodiments of the invention. However, the invention as defined in the claims and generally as described herein should not be limited to the embodiments shown by way of example in the following figures.

[0189] [Figure 1] 1 is a schematic representation of the process of the present invention, including the processing steps carried out in the nylon 6 preconcentration section, depolymerization section, recovery section, and purification section. [Figure 2] 1 illustrates three embodiments of the nylon 6 preconcentration step a) of the process of the present invention, in which a nylon 6-containing multicomponent material is extracted with an organic solvent to obtain a pretreated nylon 6-containing phase enriched in nylon 6. [Figure 2a] FIG. 1 illustrates an embodiment of nylon 6 preconcentration step a) of the method of the present invention, which preferentially extracts non-nylon 6 compounds from a nylon 6-containing multicomponent material to obtain a nylon 6-enriched preconcentrated nylon 6-containing material. [Figure 2b] FIG. 1 illustrates an embodiment of nylon 6 preconcentration step a) of the method of the present invention, which preferentially extracts nylon 6 from a nylon 6-containing multicomponent material to obtain a nylon 6-enriched preconcentrated nylon 6-containing material. [Figure 2c] FIG. 1 illustrates an embodiment of nylon 6 preconcentration step a) of the process of the present invention, in which extraction of non-nylon 6 compounds from a nylon 6-containing multicomponent material is combined with extraction of nylon 6 from the resulting nylon 6-enriched nylon 6-containing phase to obtain a nylon 6-containing phase that is even more enriched in nylon 6.

[0190] [Detailed description of the drawings] The method of the present invention is shown schematically in Figure 1. The method comprises the following parts: - a nylon 6 preconcentration section [A] in which a nylon 6-containing multicomponent material [1] is treated with an (organic) solvent [2] to obtain a nylon 6-rich preconcentrated nylon 6-containing material [3] and a second phase [4] containing non-nylon 6 compounds; - Depolymerization section [B], where pretreated nylon 6 rich in nylon 6 [3] is depolymerized to ε-caprolactam, which is discharged as a product vapor stream [5] containing ε-caprolactam. In addition, residual materials [6] are also discharged. Superheated steam [7] and optionally a catalyst [8] are introduced into the depolymerization section [B]; - a recovery section [C] for recovering crude ε-caprolactam [9] from the ε-caprolactam-containing product vapor stream [5] discharged from the depolymerization section [B]; and - Refining section [D], which refines the crude ε-caprolactam [9] discharged from the recovery section [C] to produce high-purity ε-caprolactam

[10] . It is equipped with:

[0191] FIG. 2a shows an embodiment of extraction in a nylon 6 pre-concentration section [A'] in which a nylon 6-containing multicomponent material [1'] is treated with a solvent [2'] that preferentially extracts non-nylon 6 compounds from the nylon 6-containing multicomponent material [1'] to yield a nylon 6-rich pre-concentrated nylon 6-containing material [3'] and a second phase [4'] enriched in non-nylon 6 compounds.

[0192] FIG. 2b shows an embodiment of the extraction in a nylon 6 pre-concentration section [A″] in which a nylon 6-containing multicomponent material [1″] is treated with a solvent [2″] that preferentially extracts nylon 6 from the nylon 6-containing multicomponent material [1″] to yield a nylon 6-rich pre-concentrated nylon 6-containing material [3″] and a second phase [4″] enriched in non-nylon 6 compounds.

[0193] FIG. 2c shows an embodiment of extraction in a first nylon 6 pre-concentration section [A'''] in which a nylon 6-containing multicomponent material [1'''] is treated with a solvent [2'''] that preferentially extracts non-nylon 6 compounds from the nylon 6-containing multicomponent material [1'''] to obtain a nylon 6-rich pre-concentrated nylon 6-containing phase [3'''] and a second phase [4'''] enriched in non-nylon 6 compounds. In a second nylon 6 pre-concentration section [A''''], the nylon 6-rich pre-concentrated nylon 6-containing material [3'''] is treated with a second solvent [2''''] that preferentially extracts nylon 6 from the nylon 6-rich pre-concentrated nylon 6-containing material [3'''] to obtain a pre-treated nylon 6-containing phase [3'''] further enriched in nylon 6 and a further phase [4'''] enriched in non-nylon 6 compounds. [Example]

[0194] The present invention is illustrated by the following examples, which are not intended to be limiting.

[0195] The starting material used in Examples 1, 2, and 4 and the Comparative Example was a multilayer packaging film waste containing layers of polyethylene and nylon 6. The nylon 6 layer was buried in this used multilayer packaging film waste, i.e., sandwiched between other layers, and therefore not in sufficient contact.

[0196] The nylon 6 preconcentration step a) of multilayer packaging film waste can be carried out as described in EP 0 849 312 A or DE 10 2016 015 198 A1, where polyethylene is selectively dissolved in an organic solvent, such as white spirit (Sigma-Aldrich; CAS No. 68551-17-7) or methylcyclohexane, at elevated temperatures, resulting in a mixture containing undissolved nylon 6 and an organic solution containing the organic solvent and dissolved polyethylene. The undissolved nylon 6 material is obtained after separating the organic solution from the resulting mixture. The resulting undissolved nylon 6 material can be dried and optionally densified by melting under nitrogen, and subsequently converted into solid particles enriched in nylon 6 compared to the nylon 6-containing multicomponent starting material.

[0197] To determine the quality of the resulting ε-caprolactam, the following parameters were measured: the permanganate absorption number of ε-caprolactam was measured (PAN: ISO 8660 - Plastics - Determination of the permanganate absorption number of caprolactam - Spectroscopic analysis, 2nd edition ISO 8660; 2002). Furthermore, the absorbance at a wavelength of 290 nm was measured (E290: ISO 7059 - Industrial caprolactam - Determination of absorbance at a wavelength of 290 nm; 1982). In addition, the volatile base content was measured (Volatile base (VB): ISO 8661 - Industrial caprolactam - Determination of the volatile base content - Titration method after distillation, 1988). Finally, the alkalinity or acidity was measured to a value of 0.1% (weight / volume). エタノール) Methylene blue: 0.1% (weight / volume エタノール ) Methyl red is measured by titration (end point is gray) at 25°C using a 1:2 ratio of Tashiro indicator. A flask containing water and indicator is first titrated to a gray color, then X grams of an aqueous solution of ε-caprolactam containing Y weight percent ε-caprolactam (measured by refractive index) is added and the solution is titrated back to gray using 0.01N H2SO4 solution (if the solution is alkaline) or 0.01N NaOH solution (if the solution is acidic).

[0198] The alkalinity was then calculated using the following formula: Alkalinity (mmol / kg(ε-caprolactam)) = v*t*1000 / (X*Y) (where: v = volume of H2SO4 solution added (ml) t = normality of H2SO4 solution (=0.01N) X = sample weight (g) Y = concentration of ε-caprolactam (wt%) Obtained by.

[0199] The acidity was then calculated using the following formula: Acidity (mmol / kg(ε-caprolactam)) = v*t*1000 / (X*Y) (where: v = volume of NaOH solution added (ml) t = normality of NaOH solution (=0.01N) X = sample weight (g) Y = concentration of ε-caprolactam (wt%) Obtained by.

[0200] Higher purity quality ε-caprolactam can be used undiluted for all major polymerization applications and has the following properties: PAN: up to 5 E290: max 0.05 VB: max 0.5mmol / kg Alkalinity: Max 0.1mmol / kg Acidity: up to 0.1mmol / kg Satisfies all of the above.

[0201] [Example 1] <Depolymerization of nylon 6 and recovery of ε-caprolactam> The material used for depolymerization and further processing steps described below can be any nylon 6-containing multicomponent material or derivative thereof. The particular material used in the following examples was enriched in nylon 6 relative to the nylon 6-containing multicomponent starting material and was prepared as described above from multilayer packaging film waste.

[0202] This nylon 6-rich starting material was formed into pearl-shaped solid particles (diameter: 3-4 mm). The polyethylene content of this undissolved nylon 6 material was approximately 1 wt% as determined by TGA (thermogravimetric analysis).

[0203] In step b), 33.6 g of pearl-shaped solid particles and 9.8 g of 20 wt. % phosphoric acid were charged into a Premex high-pressure autoclave. The reactor contents were first heated under nitrogen, and then superheated steam was continuously injected at a rate of 4 g / min for 120 minutes of reaction. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, a steam flow was continuously released from the reactor.

[0204] In step c), crude ε-caprolactam was recovered from the resulting vapor by cooling it to 20°C. The condensate, which averaged 28.3 grams of ε-caprolactam and the remainder was mostly water, was concentrated by evaporation in a rotavap (rotary evaporator) operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) to an average ε-caprolactam concentration of 49.6 wt%. (The resulting mixture, crude ε-caprolactam, is the mixture to be purified.) Steps b) and c) were repeated five times, and the solution obtained by combining the five mixtures was used as the stock solution in the following examples and comparative experiments.

[0205] This example demonstrates that it is possible to obtain crude ε-caprolactam in good yield and without operational problems by depolymerization of nylon 6 derived from unwanted nylon 6 multilayer packaging film waste.

[0206] [Example 2] <Purification by extraction, caustic washing, re-extraction, oxidation and distillation> In step d)(i), 70 grams of crude ε-caprolactam obtained in Example 1 was extracted once with 100 grams and nine times with 50 grams of the solvent mixture MIBC / cyclohexane (50 wt.%:50 wt.%) at 25°C. The resulting ε-caprolactam phase containing the solvent mixture was combined in ten portions and washed with 7 grams of aqueous caustic solution (2 wt.%). Subsequently, the washed ε-caprolactam containing solvent mixture was extracted six times with 50 grams of water at 25°C. The resulting six portions of aqueous ε-caprolactam phase were combined. Water and the remaining solvent mixture were distilled off from the aqueous ε-caprolactam phase to obtain a concentrated ε-caprolactam solution with a water content of 50 wt.%. The resulting mixture was treated with 0.2 wt.% KMnO4 (relative to ε-caprolactam) at 50°C for 2 hours. The formed solid was then removed from the oxidation reaction product by filtration. Then, 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the resulting aqueous ε-caprolactam solution.

[0207] In step d)(ii), the distillative removal of water and impurities having a boiling point lower or higher than that of ε-caprolactam was carried out. Subsequently, water and impurities having a boiling point lower than that of ε-caprolactam were removed as the overhead product by vacuum distillation in a batchwise operated distillation unit. Finally, purified ε-caprolactam was recovered as the overhead product at 300 Pa, while impurities having a boiling point higher than that of ε-caprolactam were left as the bottom product in the distillation unit. The characteristics of this purified ε-caprolactam are: PAN: 5 E290: 0.023 VB: 0.167 Alkalinity: 0.044 It was.

[0208] From this example, it can be concluded that purified ε-caprolactam meeting the required property information for major polymerization applications can be obtained from small pieces of depolymerized waste multilayer packaging film by removing polyethylene by extraction and purifying by extraction with organic solvents, oxidation and distillation.

[0209] [Comparative Experiment 1] <Purification by distillation> To 35 grams of crude ε-caprolactam obtained in Example 1, 0.75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added. This mixture was then distilled according to the procedure described in Example 2. The characterization information of the purified ε-caprolactam was: PAN: 73.5 E290: 0.531 VB: 0.718 Acidity: 7.643 It was.

[0210] This comparative experiment shows that the quality of the ε-caprolactam obtained by depolymerization of small pieces of waste multilayer packaging film, followed by removal of polyethylene by extraction and purification by distillation is very poor, as it does not meet any of the property information required for major polymerization applications.

[0211] [Comparative Experiment 2] <Purification by oxidation and distillation> 35 grams of crude ε-caprolactam obtained in Example 1 was treated with 0.2 wt. % KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solids formed were then removed from the oxidation reaction product by filtration. 0.75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was then added to the resulting aqueous ε-caprolactam solution. This mixture was then distilled according to the procedure described in Example 2. The properties of purified ε-caprolactam are: PAN: 11.7 E290: 0.324 VB: 0.657 Acidity: 2.991 It was.

[0212] This comparative experiment shows that the quality of the ε-caprolactam obtained by depolymerization of small pieces of waste multilayer packaging film, removing the polyethylene therefrom by extraction, and purifying it by oxidation and distillation is very poor, as it does not meet any of the property information required for the main polymerization applications.

[0213] [Comparative Experiment 3] <Purification by extraction, caustic washing, and re-extraction> 70 grams of crude ε-caprolactam obtained in Example 1 was extracted once with 100 grams and nine times with 50 grams of the solvent mixture MIBC / cyclohexane (50% by weight:50% by weight) at 50°C. The resulting ε-caprolactam phase containing the solvent mixture in ten batches was combined and washed with 7 grams of aqueous caustic solution (2% by weight). The washed ε-caprolactam containing solvent mixture was then re-extracted six times with 50 g of water at 25°C. The resulting six batches of aqueous ε-caprolactam phase were combined. Water and the remaining solvent mixture were distilled off from the aqueous ε-caprolactam phase to obtain a concentrated ε-caprolactam solution with an ε-caprolactam content of 50.4% by weight. The characterization information of the purified ε-caprolactam was: PAN: 82 E290: 1.2 It was.

[0214] This comparative experiment shows that the quality of the ε-caprolactam obtained by depolymerization of small pieces of waste multilayer packaging film, followed by extraction to remove polyethylene, and purification by extraction and re-extraction is very poor, as it does not meet all the required property information for the main polymerization applications.

[0215] [Comparative Experiment 4] <Purification by extraction, caustic washing, re-extraction and oxidation> The concentrated ε-caprolactam solution obtained in Comparative Experiment 3, with an ε-caprolactam content of 50.4% by weight, was treated with 0.04% by weight of KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solids formed were then removed from the oxidation reaction product by filtration. The properties of the purified ε-caprolactam were: PAN: 83 E290: 3.01 It was.

[0216] This comparative experiment shows that the quality of the ε-caprolactam obtained by depolymerization of small pieces of waste multilayer packaging film, removing the polyethylene therefrom by extraction, and then purifying it by extraction, re-extraction and oxidation is very poor, as it does not meet all the required property information for the main polymerization applications.

[0217] [Example 3] <Calculation of product carbon footprint of purified ε-caprolactam> A continuous process according to the present invention for producing purified ε-caprolactam from multilayer packaging film waste containing a polyethylene layer and a nylon 6 layer was simulated. The nylon 6 content of the multilayer packaging film waste containing a polyethylene layer and a nylon 6 layer was 20 wt %.

[0218] The method comprises: - cutting multi-layer packaging film waste, including a polyethylene layer and a nylon 6 layer, into small pieces; - washing a small piece of multilayer packaging film waste comprising a polyethylene layer and a nylon 6 layer with water; - drying the washed pieces of multilayer packaging film waste comprising a polyethylene layer and a nylon 6 layer by centrifugation; - Extraction of polyethylene using volatile oil; - Separation of undissolved solid pre-concentrated nylon 6-containing material by centrifugation; - washing the undissolved solid pre-concentrated nylon 6-containing material with water; - Separation of the washed undissolved solid preconcentrated nylon 6-containing material and the aqueous extract by centrifugation; - Decolorization of volatile oil solutions containing polyethylene by treatment with activated carbon; - recovery of polyethylene from a solution of polyethylene-containing volatile oil by cooling and partial evaporation of the volatile oil; - Melting and pelletizing of recycled polyethylene; - Melting and pelletizing of pre-concentrated nylon 6-containing material from washed undissolved solids; - Depolymerization of nylon 6 in pelletized nylon 6-containing materials under the influence of H3PO4 and superheated steam; - Recovery of crude ε-caprolactam by partial condensation of the vapors released from the depolymerization reactor; - Evaporative concentration of crude ε-caprolactam to 80% by weight ε-caprolactam; - Countercurrent extraction of concentrated crude ε-caprolactam with benzene; - washing of organic extracts with dilute caustic solutions; - Countercurrent extraction of the washed organic extract with water; - Evaporative concentration of aqueous extracts; - oxidation of the concentrated extract with KMnO4 and subsequent filtration of the oxidation product to remove the solids formed; - Addition of caustic agents; - Recovery of pure ε-caprolactam by vacuum distillation Includes:

[0219] The main products of this process are polyethylene and pure ε-caprolactam. The recovery rate of pure ε-caprolactam is approximately 0.8 kg of pure ε-caprolactam per kg of nylon 6 in multilayer packaging film waste containing a polyethylene layer and a nylon 6 layer. The resulting by-product is incinerated for energy recovery.

[0220] The product carbon footprint of purified ε-caprolactam was calculated based on the raw material consumption, but the utility of the above method is based on data from Eco-Invent version 3.7.1. Allocation of the environmental impact of the products purified ε-caprolactam and polyethylene in the nylon 6 preconcentration section was based on the weight ratio of these products.

[0221] These results revealed that the product carbon footprint of purified ε-caprolactam obtained from multilayer packaging film containing a polyethylene layer and a nylon 6 layer is less than 2 tonnes of CO2 equivalent per tonne of ε-caprolactam (region: Europe).

[0222] [Example 4] <Depolymerization of nylon 6 and recovery of ε-caprolactam> The material used for depolymerization and further processing steps, as described below, was enriched in nylon 6 relative to the nylon 6-containing multicomponent starting material and was prepared as described above from multilayer packaging film waste. This nylon 6-rich starting material was formed into pearl-shaped solid particles (diameter: 3-4 mm). The polyethylene content of this undissolved nylon 6 material was approximately 1 wt. % as determined by TGA (thermogravimetric analysis).

[0223] In step b), 33.6 g of pearl-shaped solid particles and 9.8 g of 20 wt. % phosphoric acid were charged into a Premex high-pressure autoclave. The reactor contents were first heated under nitrogen, and then superheated steam was continuously injected at a rate of 2.7 g / min during the 135-minute reaction. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, a steam flow was continuously released from the reactor.

[0224] In step c), crude ε-caprolactam was recovered from the resulting vapor by cooling it to 20° C. The condensate consisted of 26.1 grams of ε-caprolactam, with the remainder being mostly water.

[0225] Steps b) and c) were repeated according to the same procedure, but this time 9.7 grams of 20% by weight phosphoric acid were charged into the Premex high-pressure autoclave, and the resulting condensate consisted of 26.9 grams of ε-caprolactam.

[0226] The two ε-caprolactam-containing condensates were concentrated by evaporation in a rotavap (rotary evaporator) operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) and then combined to an ε-caprolactam concentration of 79.6 wt. %. The resulting stock solution, or crude ε-caprolactam, is the mixture to be purified (see Example 5).

[0227] The properties of this crude ε-caprolactam are: PAN: 280 E290: 2.14 It was.

[0228] This example demonstrates that it is possible to obtain crude ε-caprolactam in good yield and without operational problems by depolymerization of nylon 6 derived from unwanted nylon 6 multilayer packaging film waste.

[0229] [Example 5] <Purification by extraction, re-extraction, oxidation and distillation> In step d)(i), 43.4 grams of crude ε-caprolactam obtained in Example 4 was diluted to 68.9% by weight by adding 6.7 g of water, and the resulting solution was extracted once with 77.5 grams of benzene and four times with 50 grams of benzene at 25°C. The resulting organic extracts were combined and concentrated by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) to an ε-caprolactam concentration of approximately 24.5% by weight. The concentrated extract containing ε-caprolactam was then extracted three times in a batchwise manner with 25 grams of water at a temperature of approximately 25°C. The resulting three aqueous ε-caprolactam phases were combined. Water and the remaining solvent mixture were distilled off from this aqueous ε-caprolactam phase to obtain a concentrated ε-caprolactam solution with a water content of 49.5% by weight. The characteristic information of the aqueous ε-caprolactam solution after re-extraction was as follows: PAN: 83 E290: 1.15 It was.

[0230] The resulting mixture was treated with 0.2 wt. % KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solid formed was then removed from the oxidation reaction product by filtration. After that, 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the aqueous ε-caprolactam solution after oxidation. This mixture was then distilled according to the procedure described in Example 2. The properties of the purified ε-caprolactam were: PAN: <5 E290: <0.05 VB: <0.5mmol / kg Alkalinity: <0.1mmol / kg Acidity: <0.1mmol / kg It was.

[0231] From this example, it can be concluded that purified ε-caprolactam meeting the required properties for major polymerization applications can be obtained from small pieces of depolymerized waste multilayer packaging film by extraction to remove polyethylene and purification by extraction with benzene, re-extraction with water, oxidation and distillation.

[0232] While the present invention has been illustrated with certain preferred embodiments, those skilled in the art will recognize that variations, modifications, including conversion to continuous operation modes, and improvements can be made while remaining within the scope and spirit of the invention. Accordingly, it is not intended that the invention be limited except as set forth in the appended claims.

Claims

1. 1. A process for recovering purified ε-caprolactam from a nylon 6-containing multicomponent material in an industrial-scale plant having a continuous operating capacity of at least 500 tons / year of ε-caprolactam, the process comprising: a nylon 6 depolymerization section comprising one or more depolymerization reactors operated in series and / or parallel; a recovery section comprising one or more condensers, and - a purification section comprising one or more extraction devices, one or more solvent switching devices, and one or more distillation devices; Equipped with and the method comprises: a) providing a pre-concentrated nylon 6-containing material obtained by extracting the nylon 6-containing multicomponent material with one or more organic solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 relative to the nylon 6-containing multicomponent material; b) depolymerizing the pre-concentrated nylon 6-containing material in the presence of water in a depolymerization section to obtain a vapor stream comprising water and ε-caprolactam in a weight-to-weight ratio of 2:1 to 15:1; c) recovering crude ε-caprolactam from the vapor stream in a recovery section; d) purifying the crude ε-caprolactam in a purification section to obtain purified ε-caprolactam; Including, The purification step comprises: (i) extracting crude ε-caprolactam with an organic solvent to obtain an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities; (ii) a solvent switching step, in which the organic solvent is at least partially replaced with water to obtain an aqueous phase comprising water, ε-caprolactam, and impurities with a boiling point lower or higher than that of ε-caprolactam, said solvent switching step (ii) being selected from a process based on back-extraction with water and a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced; and (iii) obtaining purified ε-caprolactam by distillative removal of impurities having a boiling point lower or higher than that of ε-caprolactam; Including, After step d)(i), washing the organic phase obtained in step d)(i) with water or an aqueous alkaline solution, step d)(ii) is followed by oxidation with an oxidizing agent selected from potassium permanganate, sodium permanganate and hydrogen peroxide; The nylon 6-containing multicomponent material is a thin sheet having a thickness of less than 1 mm, and is a multilayer packaging film containing at least one layer containing or consisting of nylon 6 and at least one layer not containing nylon 6, wherein the at least one layer containing or consisting of nylon 6 is sandwiched between two or more layers not containing nylon 6. method.

2. 10. The method of claim 1, wherein the weight fraction of nylon 6 in the nylon 6-containing multicomponent material ranges from 1 wt % to 75 wt % based on the total weight of the nylon 6-containing multicomponent material.

3. The extraction in step a) comprises: (i) adding one or more organic solvents to a nylon 6-containing multicomponent material; (ii) performing a phase separation to obtain a liquid extract phase comprising the solvent and dissolved components from the nylon 6-containing multicomponent material, and an at least partially solid phase comprising undissolved components of the nylon 6-containing multicomponent material and optionally the solvent; (iii) removing the solvent from the liquid extraction phase and, if present, from at least a partial solid phase to obtain two solid phases, one of which is a pre-concentrated nylon 6-containing material enriched in nylon 6 relative to the nylon 6-containing multicomponent material used as the starting material; 3. The method of claim 1 or 2, comprising:

4. 3. The method according to claim 1 or 2, wherein an organic solvent that preferentially dissolves non-nylon 6 compounds from the nylon 6-containing multicomponent material is used for the extraction in step a), and / or an organic solvent that preferentially dissolves nylon 6 from the nylon 6-containing multicomponent material is used in step a).

5. (i) prior to extraction in step a), the nylon 6-containing multicomponent material has undergone pretreatment; and / or (ii) the depolymerization of the pre-concentrated nylon 6-containing material in step b) is carried out at a temperature in the range of 180°C to 400°C; and / or (iii) the water present in step b) is in the form of water vapor; 3. The method according to claim 1 or 2.

6. The method described in claim 5, wherein the pretreatment is a washing step and / or a mechanical crushing step.

7. The method described in claim 5, wherein the water present in step b) is in the form of steam which is introduced into the depolymerization section in step b) as superheated steam at a temperature in the range of 220°C to 575°C.

8. The depolymerization of the pre-concentrated nylon 6-containing material in step b) is carried out in the absence or presence of a catalyst, the catalyst being selected from acid catalysts and base catalysts, the acid catalyst being orthophosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, salts of the aforementioned acids, Al ２ O ３ and SiO ２ and combinations thereof, and the base catalyst is selected from the group consisting of alkali hydroxides, alkali salts, alkaline earth hydroxides and alkaline earth salts, organic bases and solid bases, and combinations thereof.

9. The method of claim 8, wherein the acid catalyst is orthophosphoric acid.

10. The method of claim 8, wherein the base catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

11. - adding an alkali metal hydroxide before the distillative removal in step d)(iii), 3. The method according to claim 1 or 2.

12. The method of claim 11, wherein the alkali metal hydroxide is NaOH.

13. Before the distillative removal in step d)(iii), the aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than that of ε-caprolactam is a solvent replacement step in which the organic solvent is at least partially replaced by water, the step being chosen from processes based on re-extraction with water and processes based on solvent replacement distillation, in which the organic solvent is removed by distillation and water is introduced; and - optionally followed by treatment with an acidic cation exchange resin and / or a basic anion exchange resin; and / or optionally followed by hydrogenation in the presence of a hydrogenation catalyst selected from Raney nickel, nickel-silica, nickel-aluminum oxide, ruthenium-aluminum oxide, rhodium-aluminum oxide, platinum on carbon and palladium on carbon.

3. The method according to claim 1 or 2, wherein the compound is obtained by

14. 3. The process according to claim 1 or 2, wherein in the purification section used in step d), the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified together with the crude ε-caprolactam recovered in step c).

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