Process for recovering epsilon-caprolactam from a multicomponent material containing nylon 6

Abstract

The present invention provides an improved process for the recovery of epsilon-caprolactam from a multi-component material, in particular a multi-layer film, containing nylon 6. Furthermore, the present invention provides: a plant configured to carry out the process of the present invention; and the recovered epsilon-caprolactam obtainable by the process of the present invention having a product carbon footprint of less than 2.5 kg CO2 equivalent per kg of purified epsilon-caprolactam (based on data from ecoinvent version 3.7.1 ; location: Europe).

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for recycling epsilon-caprolactam derived from a nylon 6 containing multicomponent material, in particular a multilayer film. Furthermore, the present invention relates to a plant configured to carry out the process of the present invention and to the recycled epsilon-caprolactam obtainable by the process of the present invention. BACKGROUND

[0002] In 1938, Paul Schlack invented nylon 6 (CAS No.: 25038-54-4), also known as polyamide 6, poly(hexamide), poly(hexamethylene-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide) or poly[imidazolyl(l-oxohexane-l,6-diyl)]. In general, nylon 6 is synthesized by ring-opening polymerization of epsilon-caprolactam at a temperature of about 260 °C in an inert atmosphere:

[0003]

[0004] Processes for producing raw epsilon-caprolactam are described, for example, in Chapter 25 “Caprolactam” of Ullmann’s Encyclopedia of Industrial Chemistry (May 25, 2018), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, which is available electronically via https: / / doi.org / 10.1002 / 14356007.a05_031.pub3.

[0005] Processes for producing nylon 6 are described, for example, in the section “Polyamides” of Ullmann’s Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, which is available electronically via https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0006] Today, Nylon 6 is the most produced synthetic polyamide in the world and has a number of properties that are advantageous for industrial applications, including high mechanical strength, especially stiffness, hardness and toughness; good electrical insulation properties; good dyeability; good processability; good UV light protection; good fatigue resistance; and good barrier function against gases, aromatics, hydrocarbon solvents and low-polarity aromatic substances from food. This advantageous set of properties is the reason why Nylon 6 is widely used in very different industries such as building, consumer goods, electronics, transportation and packaging. However, a disadvantage of using Nylon 6 is that it is more expensive than polyolefin polyethylene and polypropylene. Polyolefins in turn have the disadvantage that their industrial applicability is more limited due to their poor gas barrier function, their low temperature resistance and their difficult adhesion properties.

[0007] Therefore, Nylon 6 is widely used as a component in multi-component materials, in most cases in combination with polyolefins. In particular, relevant Nylon 6-containing multi-component materials are, for example, flexible gas-modifying or gas-controlling packaging films, which are often used to maintain a certain atmosphere inside a food-containing package to improve the shelf life of the packaged product. Most of today's packaging films are multi-layer films, which usually comprise two to 15 layers, but can also comprise more layers. An example of an 11-layer co-extruded film is the following: Nylon 6 / TIE / PE(l) / 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(l), PE(2), PE(3) and PE(4) are different grades of polyethylene. The typical purpose of using Nylon 6 in such multi-component materials is to impart mechanical strength and puncture resistance properties and / or barrier properties to limit gas exchange and aroma loss. Nylon 6-containing multi-layer films in particular allow the maintenance of an optimized gas modification inside the package, especially a certain oxygen concentration that is critical, for example, to keep the freshness of fresh products for a longer period of time. The use of multi-layer films enables cost reduction because cheaper polymers can partially replace more expensive polymers, such as Nylon 6. Nylon 6 is also an excellent alternative to aluminum and other metal films.

[0008] Multi-component materials comprising more than one (polymeric) component are attractive in industrial applications because, for example, the alternating layering of two (or more) different components is a way to obtain a composite material with new properties compared to the pure components. The combination of several different material layers can, for example, improve the mechanical and physical properties of the film, including puncture resistance, tear resistance and heat resistance as well as moisture and gas (especially oxygen) barrier properties. Since the properties of multi-component materials can be continuously monitored to see how they change when the components used and the thickness of the layers of the components used are changed, it is relatively easy to develop novel composite materials with the desired properties.

[0009] In multicomponent materials, especially multilayer films, polyolefin layers are often combined with layers composed of polar polymers, such as nylon 6, nylon 66, EVOH (ethylene-vinyl alcohol) or PVDC (polyvinylidene chloride). In addition, tie layers comprising special adhesive polymers or tie resins, such as polyurethanes and acid / anhydride grafted polyolefins, are often required as intermediate layers between two different components that cannot adhere well to each other. As set forth above, the use of nylon 6 in multicomponent materials has become quite common in order to improve the mechanical and gas barrier properties of such materials. Another important reason for the combination of nylon 6 with other materials is the considerable cost savings of the cost of nylon 6.

[0010] An environmental problem associated with the production and use of such multicomponent materials containing nylon 6 is the waste generated during the process and after consumption. Such problems can be mitigated by recycling the individual components, such as nylon 6, from multicomponent materials that are no longer used or discarded. As packaging materials, multilayer films containing nylon 6 are a particularly relevant source of waste. Such waste often contains a large amount of nylon 6. Therefore, if there is a feasible method to recover nylon 6 from such composite waste materials, this would not only be beneficial to the environment, but also provide a new source of nylon 6 with economic value.

[0011] The above applies to in-process waste generated in processes using nylon 6, especially film production processes. For a limited number of applications, such waste can be recycled and reused as regrind. However, the optical and mechanical properties of the material are adversely affected by regrind, and further deterioration occurs each time the cycle of reuse and processing is repeated. Therefore, the cycle of reuse and processing cannot be repeated as often as desired.

[0012] In addition, nylon 6 recycled according to existing methods is generally not included in “food-grade plastics”, which limits the application to a large extent.

[0013] Mechanical recycling (also referred to as material recycling or back-to-plastics recycling or regrind) refers to an operation aimed at recovering plastics through mechanical processes (grinding, washing, separation, drying, regranulation and compounding), thereby generating a cycle of recyclates that can be converted into plastic products that replace the original plastics. Currently, most original plastics are produced from petrochemical feedstocks that have never been used or processed before, such as natural gas, coal or crude oil. During mechanical recycling, the polymer chains remain more or less intact. Mechanical recycling is a form of downcycling of waste, as the recovered material has lower quality and functionality than the original material.

[0014] Depolymerization or chemical recycling is a technique to convert polymers into their monomeric components (in case of nylon 6, ε-caprolactam). The specification of the recovered monomer determines whether it can be substituted for virgin monomer for all applications or only a limited amount of applications. Virgin monomers are produced from petrochemical feedstocks that have never been used or processed before, such as natural gas, coal or crude oil.

[0015] Immediately after regrinding the nylon 6 containing material, a process has been developed to depolymerize the nylon 6 into its valuable monomer, ε-caprolactam. Such a process typically uses a nylon 6 plastic article as a starting material, which is produced for example by injection molding, extrusion, spinning or other processing steps.

[0016] Depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring opening polymerization of ε-caprolactam:

[0017]

[0018] Processes for the depolymerization of nylon 6 are known. Such processes can be run in batch mode, in semi-continuous mode (typically with batch (re)charging of the depolymerization reactor with nylon 6) or in continuous mode.

[0019] L. A. Dmitrieva, A. A. Speranskii, S. A. Krasavin and Y. N. Bychkov, “Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns”, Fibre Chemistry, pages 229-241, March 1986, (translated from Khimicheskie Volokna, No. 4, pages 5-12, July-August 1985) is a literature review describing processes for depolymerizing nylon 6 with and without catalysts

[0020] A. A. Ogale, “Depolymerization of Nylon 6: Some Kinetic Modeling Aspects”, Journal of Applied Polymer Science, Vol. 29, 1984, pages 3947-3954, which is available electronically at https: / / doi.org / 10.1002 / app.1984.070291227, is a paper describing the kinetics of the depolymerization of nylon 6.

[0021] Generally, the prior art of chemical regeneration or recycling processes to depolymerize substantially pure nylon 6 into ε-caprolactam monomer involves a hydrolytic degradation step at high temperature in the presence of water and a step of reclaiming the formed monomer by steam distillation. The hydrolytic degradation step can be performed in the presence and in the absence of a catalyst. Acidic compounds such as phosphoric acid and boric acid as well as basic compounds such as potassium carbonate and sodium hydroxide can be used as catalysts.

[0022] US5929234 describes a process for recovering ε-caprolactam from poly- caprolactam-containing waste material. Depolymerization is carried out in the absence of added catalyst using superheated steam at a temperature of from about 250°C to about 400°C and at a pressure in the range of from about 1 atm to about 100 atm and substantially less than the saturated vapor pressure of water at the temperature at which the ε-caprolactam-containing vapor stream is formed.

[0023] It has long been known to regenerate ε-caprolactam from substantially pure nylon 6 by recycling extraction liquor in a nylon 6 polymerization plant, recycling nylon 6 scrap (i.e. nylon 6 polymer substantially free of non-nylon 6 material) or from products made from almost pure nylon 6 such as fishing nets.

[0024] For example, DE4211609 discloses a process for recovering ε-caprolactam by pyrolysis of polyamide 6 in the presence of a small amount of potassium carbonate at 250-320°C and a process for purifying the recovered ε-caprolactam. DE4211609 describes a process which is applicable not only to pure polyamide 6 products but also to polyamide 6 products containing glass fibers and fillers and to products containing impact modifiers. "Polyamide 6" as used in DE4211609 means pure polyamide 6 as well as polyamides which are preferably greater than 80% by weight of polyamide 6.

[0025] CN103467378 discloses a process for producing ε-caprolactam by using recycled waste nylon filament and residues in which phosphoric acid is used as a catalyst for depolymerization. Superheated steam is charged into the reaction system during the depolymerization process.

[0026] US5869654 discloses a process for recovering ε-caprolactam from poly- caprolactam processing waste wherein the poly-caprolactam can be selected from the group consisting of scrap nylon 6 polymer material and / or oligomeric material such as yarn waste, shred waste or extrusion residue. The process comprises the step of contacting the poly-caprolactam waste with superheated steam in the absence of added catalyst at a temperature of from about 250°C to about 400°C and at a pressure in the range of from about 1.5 atm to about 100 atm and substantially less than the saturated vapor pressure of water at the temperature at which the ε-caprolactam-containing vapor stream is formed.

[0027] JP 2000038471 discloses a method for depolymerizing nylon 6 resin from a molded body to obtain ε-caprolactam, in particular depolymerizing automotive resin parts such as nylon 6 fastening parts, electrical parts, body mechanism parts and exterior parts, wherein the nylon 6 accounts for 90 wt.% or more of the total organic compounds in the molded body.

[0028] Although methods have been developed for depolymerizing almost pure waste nylon 6 into its valuable monomer ε-caprolactam, recycling ε-caprolactam from nylon 6-containing multi-component materials, in particular such materials having a medium to high nylon 6 content (e.g. 40 wt.% to 80 wt.%), remains a great challenge. Recycling ε-caprolactam from nylon 6-containing multi-component materials having a low nylon 6 content (e.g. less than 40 wt.%) is even more difficult. Nylon 6-containing products having a medium to high nylon 6 content are for example carpets. High quality carpets typically comprise up to 55 wt.% surface fibers containing nylon 6. The remainder is backing (support) material.

[0029] Based on the prior art methods, recycling ε-caprolactam from nylon 6-containing materials having a low and even medium to high nylon 6 content is not commercially feasible or has not been successful.

[0030] The most reported problem with respect to recycling ε-caprolactam from nylon 6-containing materials having a medium to high nylon 6 content is the low recovery yield. On the one hand, this low recovery yield is attributed to the difficulty to separate the formed ε-caprolactam from non-nylon 6 materials. On the other hand, the low recovery yield is attributed to decomposition reactions leading to the production of ammonia and to the heterogeneity of the depolymerization mixture, since the high viscosity of the depolymerization mixture prevents efficient mixing, so that local overheating and high consumption of expensive overheated steam (used as stripping agent) occur.

[0031] The introduction of a mechanical pre-treatment step (e.g. scraping the surface fibers from the backing of a carpet, or grinding, followed by density separation, such as sink-float separation and wind-sifting) allows to produce processed carpet material feedstocks containing more than 90 wt.% of nylon 6, which can be used in the prior art depolymerization methods described above. However, this mechanical pre-treatment is mainly applied to off-spec materials (pre-consumer waste) inside the production facilities of carpets and other nylon 6-based fabrics. It cannot be transferred to other non-carpet nylon 6-containing multi-component materials.

[0032] Currently, no process is available to recover high purity epsilon-caprolactam from nylon 6-containing multicomponent materials in an economically reasonable way, although such a process is urgently needed. In particular, there is an urgent need for epsilon-caprolactam recovery processes that work on more demanding nylon 6-containing multicomponent materials, such as on multicomponent materials with low to moderately high nylon 6 content (e.g. 1 wt.% to 35 wt.%), and on epsilon-caprolactam recovery from compounded nylon 6-containing multicomponent products that cannot be pre-concentrated by simple mechanical pre-treatment steps such as scraping or milling followed by density separation. Finally, there is a need for processes that allow the recovery of epsilon-caprolactam from nylon 6-containing multicomponent materials such as multilayer films, where the nylon 6 layer is typically sandwiched between non-nylon 6 components and thus not accessible from the outer surface of such materials.

[0033] Prior art processes for the recovery of epsilon-caprolactam from nylon 6-containing multicomponent materials fail to produce high quality epsilon-caprolactam grades that can be used to replace virgin epsilon-caprolactam grades for high demand applications.

[0034] Currently, no process is available to recover high purity epsilon-caprolactam from nylon 6-containing multicomponent materials, although such a process is urgently needed. In particular, there is an urgent need for high purity epsilon-caprolactam recovery processes that can replace virgin epsilon-caprolactam grades for high demand applications such as high speed melt spinning during textile fiber production.

[0035] Furthermore, there is a need for processes that allow the recovery of high purity epsilon-caprolactam from nylon 6-containing multicomponent materials in an economically reasonable way. The production cost of the recovered high purity epsilon-caprolactam should be similar or lower than the production cost of virgin high purity epsilon-caprolactam.

[0036] Moreover, there is a need to purify crude epsilon-caprolactam obtained by depolymerization of nylon 6-containing multicomponent materials without the use of oxidizing agents such as potassium permanganate (KMn04) or adsorbents such as (activated) carbon and diatomaceous earth. Techniques based on such oxidizing agents and adsorbents are rather laborious and generate solid waste.

[0037] Furthermore, there is a need to provide high purity grades of epsilon-caprolactam from nylon 6-containing multicomponent materials that have a significantly lower carbon footprint than epsilon-caprolactam obtained by new synthesis, for example by Beckmann rearrangement of virgin cyclohexanone oxime.

[0038] Finally, there is a need for processes that allow the recovery of epsilon-caprolactam from nylon 6-containing multicomponent materials on an industrial scale in order to process the large amounts of nylon 6-containing multicomponent materials that are discarded every year. SUMMARY

[0039] The object of the present invention is to meet one or more of the needs described above and to overcome the drawbacks associated with the prior art processes.

[0040] In particular, the object of the present invention is to provide a process for the recovery of high purity epsilon-caprolactam from a nylon 6 containing multi-component waste material in an economically reliable way and with a high recovery yield. In particular, the object of the present invention is to provide a process suitable for the recovery of epsilon-caprolactam from a nylon 6 containing multi-component waste material having a nylon 6 content of at most 35 wt.% and from a compounded nylon 6 containing multi-component material, in particular a multi-layer film containing nylon 6.

[0041] The object of the present invention is also to provide a process for the recovery of high purity epsilon-caprolactam from a nylon 6 containing multi-component material which can replace high purity virgin epsilon-caprolactam for all applications, including high speed melt spinning of nylon 6 for fine textile fiber production.

[0042] Another object of the present invention is a process for the recovery of high purity grade epsilon-caprolactam from a nylon 6 containing multi-component waste material characterized by a significantly lower carbon footprint than processes for the production of epsilon-caprolactam by new synthesis, for example by Beckmann rearrangement of cyclohexanone oxime.

[0043] Therefore, the present invention also aims to provide a process for reducing the environmental burden of a nylon 6 containing multi-component waste material and of products made therefrom.

[0044] All or at least some of the aforementioned objects are solved by the process of claims 1 and 15, the plant of claim 13 and / or the product of claim 14.

[0045] Surprisingly, it was found that high purity grade epsilon-caprolactam can be obtained from a nylon 6 containing multi-component material in high yield by the following process of the present invention. The process of the present invention utilizes a plant comprising (A) a nylon 6 pre-concentration section, (B) a depolymerization section, (C) a recovery section, and (D) a purification section. The process of the present invention comprises the following steps:

[0046] a) extracting the nylon 6 containing multi-component material with one or more organic solvents in the nylon 6 pre-concentration section to obtain a solid pre-concentrated nylon 6 containing material enriched in nylon 6 compared to the nylon 6 containing multi-component material;

[0047] b) depolymerizing the pre-concentrated nylon 6 containing material in the presence of water in the depolymerization section to obtain a vapor stream comprising water and epsilon-caprolactam in a weight ratio of 2: 1 to 15: 1;

[0048] c) recovering crude epsilon-caprolactam from the vapor stream in the recovery section; and

[0049] d) purifying the crude e-caprolactam in a purification section to obtain purified e- caprolactam.

[0050] Surprisingly, the specific order of process steps and process conditions according to the present application, i.e. the order of the nylon 6 pre-concentration, depolymerization, recovery and purification steps as defined above, allows for the recovery of high grade e-caprolactam from nylon 6 containing multi-component materials in high yield and in a straightforward and economically reasonable manner. The process of the present application is economically reasonable and advantageous from several perspectives. First, the process of the present application is suitable for a wide variety of nylon 6 containing multi-component materials which can differ, for example, in their overall composition, in their nylon 6 content and / or in the distribution of nylon 6 within the material. Second, the process of the present application allows for an efficient separation of nylon 6 from non-nylon 6 compounds such that a high purity grade e-caprolactam can be obtained. Third, the extraction of nylon 6 from nylon 6 containing multi-component materials by the process of the present application is so efficient that e-caprolactam can be obtained in high yield. Finally, the process of the present application allows for the production of e-caprolactam with a significantly lower carbon footprint than e-caprolactam produced by the new synthesis of e-caprolactam, for example, by the Beckmann rearrangement of cyclohexanone oxime. The process of the present application allows for the efficient processing of waste nylon 6 containing multi-component materials and reduces the environmental burden of such products. In particular, the process of the present application allows for the production of purified e-caprolactam with a carbon footprint of less than 2.5 kg CO2 equivalent per kg of purified e-caprolactam, which is a significant improvement compared to 7.5 to 6.5 kg CO2 equivalent per kg of e-caprolactam associated with the production of “virgin” e-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime (based on data from ecoinvent version 3.7.1; location: Europe). Unless otherwise stated, values for product carbon footprint stated herein are based on data from ecoinvent version 3.7.1 and location is Europe.

[0051] In line with the process of the present application, the present application also provides a chemical plant suitable for the production of purified e-caprolactam from nylon 6 containing multi-component materials, which chemical plant comprises at least (A) a nylon 6 pre- concentration section, (B) a depolymerization section, (C) a recovery section, and (D) a purification section, wherein the plant and especially such four sections are configured to carry out the process of the present application.

[0052] The present invention also provides a purified epsilon-caprolactam, which is obtainable by the process of the present invention and which has a product carbon footprint of less than 2.5 kg CO2 equivalent per kg of purified epsilon-caprolactam (based on data from ecoinvent version 3.7.1 ; location: Europe). Finally, the present invention provides a use of epsilon-caprolactam recovered from a multi-component material containing nylon 6 for reducing the carbon footprint of an epsilon-caprolactam production plant which produces at least also epsilon-caprolactam from Beckmann rearrangement of cyclohexanone oxime.

[0053] Advantageous embodiments of the present invention are indicated in the dependent claims and are explained in more detail below. DETAILED DESCRIPTION

[0054] Detailed description of preferred embodiments

[0055] Multi-component material containing nylon 6

[0056] The process of the present invention uses a multi-component material containing nylon 6 as starting material. The multi-component material containing nylon 6 is typically in solid state, in particular a solid block, web, sheet or film material.

[0057] A multi-component material comprises two or more components. It is typically used as a means to combine properties of different components into a single material. The term "component" as used herein refers to a chemical substance. In a preferred embodiment, the multi-component material containing nylon 6 comprises nylon 6 and at least one non-nylon 6 polymer as components. The two or more components can be present in the multi-component material as separate domains (e.g. in a sheath-core arrangement, island-in-the-sea arrangement, side-by-side arrangement, multi-layer arrangement or other arrangement). Alternatively or additionally, the multi-component material can comprise a mixture of different components (e.g. in the form of a polymer blend). A mixture of such two embodiments is also possible, e.g. in case a multi-layer film comprises one layer of a single polymer and an additional layer of a polymer blend or polymer matrix filled with polymer-filled particles. Typically, the nylon 6 is present in the multi-component material containing nylon 6 in the form of separate domains and / or in the form of a mixture with at least one non-nylon 6 polymer.

[0058] Preferably, the components of the multi-component material are at least two different polymers, one of which is or comprises nylon 6. In one embodiment, at least two components of the multi-component material comprise or consist of a polymer. In this embodiment, the multi-component material can comprise other non-polymer components. Preferably, however, greater than 80 wt.% of the multi-component material consists of polymers. As used herein, the term "polymer" refers to the term in its ordinary sense, i.e., a macromolecule (e.g., oligomer) or megamolecule consisting of repeating subunits. As used herein, a polymer can have a synthetic origin, such as a plastic, including polypropylene, nylon 6, and polystyrene, or a natural origin, such as starch or a protein. As used herein, the term "polymer" also includes homopolymers, copolymers, and polymer blends. Polymers of natural and synthetic origin are produced by polymerization of many small molecules called monomers. Homopolymers are made from only one type of monomer. Copolymers are made from different types of monomers, which can be arranged in a random or block configuration. As used herein, "polymer blend" or "blend" refers to a composition of two or more polymers and is typically obtained by extrusion. The blend can or can not be miscible, it can or can not be phase separated.

[0059] In one embodiment, at least one component of the multi-component material comprises or consists of nylon 6, and at least one other component of the multi-component 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 (except polyamide 6), polyesters, polyvinylidene fluoride, polyvinylidene chloride, polystyrene, polycarbonate, polymethyl methacrylate, ethylene-co-acrylic acid copolymer, polyoxymethylene, ethylene-vinyl alcohol, polyurethane, and combinations thereof, especially blends. Especially good results in terms of the purity and yield of ε-caprolactam obtained can be achieved when the polymer different from nylon 6 is selected from the group consisting of polyolefins (especially polyethylene and polypropylene), polyamides (except polyamide 6, especially polyamide 6,6, polyamide 4,6, polyamide 6,10, polyamide 12), and combinations thereof.

[0060] The multi-component material employed in the process of the present application comprises nylon 6. The term "nylon 6" as used herein refers to polycaprolactam. The term "nylon 6-containing multi-component material" as used herein refers to a multi-component material comprising two or more components, one of the components being or comprising nylon 6. The weight fraction of nylon 6 in the nylon 6-containing multi-component material used in the process of the present application is not critical to the practice of the present application and can range from 0.5 wt.% to 99 wt.%. However, in contrast to the more than 80 wt.% content of nylon 6 required for the prior art processes which have been practically successfully applied, a particular advantage of the process of the present application is that it is effective with multi-component materials containing only a lower content of nylon 6. Thus, according to a particular advantageous embodiment, the weight fraction of nylon 6 in the nylon 6-containing multi-component material used in the process of the present application is from 1 wt.% to 75 wt.%, more preferably from 1 wt.% to 60 wt.%, more preferably from 2 wt.% to 35 wt.% and most preferably from 3 wt.% to 25 wt.%. All wt.% values described herein are always based on the total weight of the nylon 6-containing multi-component material, if not indicated otherwise.

[0061] The nylon 6-containing multi-component material can have any shape, including (multi-filament) yarns, chips, films or any shaped form. The shape of the nylon 6-containing multi-component material used in the process of the present application is not critical.

[0062] Preferably, the nylon 6-containing multi-component material used in the process of the present application is a multi-layer material, wherein at least one layer comprises or consists of nylon 6. The term "multi-layer material" as used herein refers to a material organized in different layers. The relative position, thickness and / or composition of such layers can be different. Such layers can belong to the same type or to different types. Preferably, the multi-layer material contains at least one layer which does not comprise nylon 6. In another preferred embodiment, the multi-layer material comprises two skin layers on opposite sides of the multi-layer material as outermost layers, wherein both skin layers do not comprise nylon 6, and thus, the nylon 6 is contained in one layer sandwiched between the skin layers. Non-limiting examples of suitable polymers which can be used in the skin layers include polypropylene, polyethylene, polyethylene oxide, polycaprolactone, polyamide (other than nylon 6), polyester, polyvinylidene fluoride, polyvinylidene chloride, polystyrene, polycarbonate, ethylene-vinyl alcohol, polymethyl methacrylate, ethylene-co-acrylic acid copolymer, polyoxymethylene and blends of two or more of these. A particular advantage of the process of the present application is that it is effective with multi-layer materials wherein the nylon 6 does not need to be exposed or accessible at the surface of the material.

[0063] The multilayer material can contain a barrier layer as a skin layer or intermediate layer containing non-nylon 6. The barrier layer can be formed from any material having barrier properties. The barrier layer can comprise or consist of an organic material, such as carbon; an inorganic material, such as a metal, ceramic, oxide; a polymeric material; or a combination 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 copolymer, polyamide (other than nylon 6), polyketone, a blend of two or more of these; and a blend of one or more of these with other polymers.

[0064] The multilayer material can comprise a tie layer or adhesive layer connecting 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 a tie layer or adhesive layer include: olefin block copolymers, such as propylene-based block copolymers sold under the trade name INTUNE TM (available from The Dow Chemical Company) and ethylene-based block copolymers sold under the trade name INFUSE TM (available from 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; a blend of two or more of these; and a blend of one or more of these with other polymers.

[0065] In preferred embodiments, a "layer" as used herein refers to a "film". More preferably, the nylon 6-containing multicomponent material used in the method of the present application is itself a film, i.e. a multilayer film containing at least one layer comprising or consisting of nylon 6. Preferably, the multilayer film contains at least one layer comprising or consisting of nylon 6 and at least one layer not comprising nylon 6. The multilayer film can be produced by any film lamination and / or co-extrusion technique. The term "film" as used herein is defined as a thin sheet having a thickness of less than 1 mm. The "layer" in a film can be very thin, as in the case of a nanolayer. The "film" as used herein can be in the form of a shape that is not necessarily "flat" in the planar sense, such as a profile, a parison, a tube, etc. In preferred embodiments, the term "film" as used herein refers to a film consisting of or comprising more than 80 wt.% of polymeric material. The use of a multilayer film in the method of the present application is advantageous because it is a common waste product. Most packaging films today are multilayer films. According to practically especially relevant embodiments, "multilayer film" as used herein is a packaging film, especially a modified atmosphere packaging film, both of which are described in the background section above.

[0066] Due to the tight connections between the film layers, prior to the present invention, it was extremely difficult to recover high-grade epsilon-caprolactam from a multilayer film containing nylon 6, especially when the nylon 6 content was below 80 wt.%, as indicated above, and / or when at least one layer containing nylon 6 was sandwiched between two non-nylon 6 containing layers in the multilayer film, which can but not necessarily be the surface layers, and thus less accessible. Such types of multilayer films are thus used as the nylon 6 containing multi-component material in step a) in a particularly advantageous embodiment of the present invention. Thus, in particular, the nylon 6 containing multi-component material used in the method of the present invention can be a multilayer film, which contains at least one layer comprising or consisting of nylon 6, which is sandwiched, i.e. embedded, between two or more layers not comprising nylon 6. Of course, the multilayer film can comprise further layers, either containing or not containing nylon 6, at any position between such layers, above or below such layers.

[0067] The method of the present invention has the advantage over methods of the prior art, which are for example limited to nylon 6 containing fibers derived from carpets, which need to be subjected to a mechanical pre-processing step, that the method of the present invention is not limited to this and can in particular also be applied very successfully to multilayer films. In a particular embodiment, the nylon 6 containing multi-component material used in the method of the present invention is not a carpet or a material derived from a carpet. In another embodiment, it is not a fabric or a material derived from a fabric. In another embodiment, it does not comprise nylon 6 fibers.

[0068] Possible pre-treatment steps

[0069] The material derived from the nylon 6 containing multi-component material used in step a) has been obtained by extraction of the nylon 6 containing multi-component material with one or more solvents to obtain a solid pre-concentrated nylon 6 containing material, which is enriched in nylon 6 compared to the nylon 6 containing multi-component material. This is described in more detail below. Prior to or after this pre-treatment, the material can advantageously be subjected to further pre-treatment steps, which are described below.

[0070] Prior to carrying out step a) of the inventive process, the nylon 6 containing multi- component material is preferably subjected to a pre-treatment, in particular a mechanical size reduction and / or a washing step. Thus, a mechanical size reduction and / or a washing section can precede the nylon 6 pre-concentration section in the inventive process. Preferably, the nylon 6 containing multi-component material is shredded into pieces, before being extracted in step a) in the nylon 6 pre-concentration section with one or more organic solvents. This mechanical pre-treatment of the nylon 6 containing multi-component material, i.e. the mechanical comminution or shredding, can be achieved, for example, by cutting, shredding, grinding, milling, chopping. In a preferred embodiment, the nylon 6 containing multi-component material is charged to step a) in the form of pieces, such pieces having a length along the longest axis of the piece of on average 0.01 to 100 cm, preferably 0.05 to 10 cm and most preferably 0.1 to 5 cm. The skilled person can easily determine the average length of the pieces used along the longest axis by first taking a representative sample of the pieces, then measuring the length of the longest axis of each of such pieces, for example 50 pieces, and finally calculating the average of all such individual measurements. The preferred particle size can also be described in terms of average particle weight. Preferably, the average particle weight of the pieces of the nylon 6 containing multi-component material is 0.1 mg to 100 kg, preferably 1 mg to 10 kg, more preferably 10 mg to 1 kg and most preferably 10 mg to 100 g. The use of pieces of the nylon 6 containing multi-component material having the aforementioned size has the advantage that the surface area is increased and / or the pieces can be more easily handled and / or mixed with the organic solvent which is added in the nylon 6 pre-concentration section for extraction.

[0071] Optionally, the nylon 6 containing multi-component material is cleaned prior to the nylon 6 pre-concentration. This is advantageous, because all (adhesive) stains removed will thus not interfere with the next steps of the inventive process.

[0072] As used herein, the term "cleaning" is defined as any process which removes non-nylon 6 material adhering to or mixed with the nylon 6 containing multi-component material. Cleaning is advantageous, because all non-nylon 6 material removed will thus not interfere with the next steps of the inventive process.

[0073] Optionally, the multi-component material comprising nylon 6 is cleaned by washing with a solvent, preferably water, before being charged into the nylon 6 pre-concentration section. This is advantageous because all (adhesive) dirt is removed which therefore does not interfere with the next steps of the process of the present application. Preferably, a detergent is added to the solvent in a concentration in the range of 0 to 30 wt% relative to the solvent to improve the washing efficiency. In another preferred embodiment, the washing process comprises a final rinsing step with (cleaning) washing solvent without detergent in order to remove residual detergent and dirt present which is attached to the multi-component material comprising nylon 6. Preferably, the washing solvent is heated to further facilitate the washing process. Optionally, the multi-component material comprising nylon 6 is dried after the cleaning step and before being charged into the nylon 6 pre-concentration section. This has the advantage that the solvent added for the extraction is not diluted or contaminated with the washing solvent. The washing is preferably performed under friction. Different types of industrial washing systems are available on the market, such as rotating plastic washers and (high speed) friction washers. Optionally, the mechanical size reduction of the multi-component material comprising nylon 6 and the washing are combined in e.g. a so-called wet crusher.

[0074] Optionally, before the mechanical comminution or shredding of the multi-component material comprising nylon 6, metal fragments, rocks and other interfering materials which cause severe wear of the equipment used for the mechanical comminution or shredding are removed. Preferably, before or after the mechanical comminution or shredding of the multi-component material comprising nylon 6, non-nylon 6 containing materials, such as polyethylene, polypropylene and nylon 6,6 containing materials, are removed as further described below. The removal of foreign materials can be done mechanically or manually. The removal of such interfering materials has the advantage that the maintenance costs of the equipment used for the mechanical comminution or shredding can be reduced to a large extent. In addition, the nylon 6 content of the material obtained after the mechanical comminution or shredding is higher than without removal of interfering materials.

[0075] Optionally, foreign materials are separated from the mechanically comminuted or shredded multi-component material comprising nylon 6. For this purpose, various separation methods can be applied, including but not limited to density separation and magnetic separation. In density separation, materials of different densities are placed in a liquid of intermediate density, where the less dense materials float up and separate from the more dense sinking materials. In practice, density separation is often done through a series of density separation stages. For example, in one stage high density materials such as rocks, sand and metals (including iron and lead) are separated, while in another stage low density materials such as polyolefin polypropylene and polyethylene are separated. Magnetic separation is a method of separating components of a mixture by using magnets to attract magnetic materials. This method is often used to split magnetic materials from non-magnetic materials. The removal of foreign materials from the comminuted or shredded multi-component material comprising nylon 6 is advantageous because such materials can interfere with the subsequent steps of the process of the present application.

[0076] Optionally, the preferably washed and size-reduced nylon 6-containing multi-component material is densified. Preferably, the densification of the nylon 6-containing multi-component material is performed by charging to a melting furnace, such as an extruder, or by compacting in a (mechanical) compactor or in a briquetting machine. The densification of the preferably cleaned and / or size-reduced nylon 6-containing multi-component material has the advantage of an increased bulk density, which reduces the costs for intermediate storage and transport in case of pre-treatment at different locations (see below).

[0077] In the melting furnace, the nylon 6-containing multi-component material is melted. Preferably, the resulting polymer melt is filtered. This has the advantage of removing solid impurities. Subsequently, the melted and optionally filtered polymer melt is cooled and preferably fed to a pelletizer. The pelletizer cuts the product into pellets. The size and shape of the pellets, which are also commonly referred to as granules, can be chosen within a broad range. Generally, the shape of the pellets is cylindrical (resulting from cutting short strands into pieces). However, other shapes, such as (non-perfect) spheres, are also possible. The size of the pellets can be chosen within a broad range. Typically, the diameter of the pellets is in the range of 1 to 10 mm, preferably 2 to 7 mm, more preferably 3 to 5 mm. In a preferred embodiment, the length of the pellets is in the range of 1 to 50 mm, preferably 2 to 25 mm, more preferably 3 to 15 mm.

[0078] The pelletization of the preferably cleaned and size-reduced nylon 6-containing multi-component material has the advantage of an increased bulk density, which reduces the costs for intermediate storage and transport in case of pre-treatment at different locations (see below). In addition to the increased density, the pelletization also provides other benefits, such as a uniform shape and structure of the material to be processed, which facilitates the (automatic) feeding into the equipment applied for the pre-concentration of the nylon 6 by extraction with a solvent.

[0079] The site where the pre-treatment of the nylon 6-containing multi-component material is performed and the site where the pre-concentration of the nylon 6 by extraction with a solvent is performed can be the same. Preferably, however, one or more of the pre-treatment steps are performed at a different location, for example at a location specialized in pre-treating waste polymeric material and in particular nylon 6-containing multi-component material. The nylon 6-containing multi-component material that has been pre-treated at the respective location can then be shipped to the site where the pre-concentration of the nylon 6 by extraction with a solvent is performed.

[0080] Accordingly, in accordance with a particular advantageous embodiment of the present application, prior to step a), the material comprising the nylon 6-containing multi-component material is pre-treated and pre-concentrated in a pre-treatment section [A], in particular cleaned in a cleaning section, and / or mechanically size-reduced in a mechanical size-reduction section and / or densified in a densification section and an extraction section, to obtain a material derived from the nylon 6-containing multi-component material.

[0081] Pre-concentration of the nylon 6 step a)

[0082] In step a) of the process of the present application, the nylon 6 containing multi- component material is extracted in a nylon 6 pre-concentration section with one or more organic solvents to obtain a solid state pre-concentrated nylon 6 containing material, which is enriched in nylon 6 compared to the nylon 6 containing multi-component material. This is an important step of the process of the present application, which ensures a high yield and high quality of e-caprolactam that can be obtained by the present application. This step is especially important when a multi-layer film is used as starting material, especially when the nylon 6 containing layer is sandwiched between other polymer layers and thus not directly accessible for depolymerization. This step is also especially important when a multi-component material comprising nylon 6 and at least one non-nylon 6 polymer is used as starting material, especially when the nylon 6 is present in the nylon 6 containing multi-component material in the form of separate domains and / or in the form of a mixture with the at least one non-nylon 6 polymer and thus not directly accessible for depolymerization. The pre-concentration of the nylon 6 containing multi-component material is also especially important when the nylon 6 is present in the form of a mixture with the at least one non-nylon 6 polymer.

[0083] The extraction in the nylon 6 pre-concentration section according to the present application typically comprises the following steps:

[0084] (i) adding one or more organic solvents to the nylon 6 containing multi- component material;

[0085] (ii) performing a phase separation to obtain a liquid extraction phase comprising the solvent and dissolved components from the nylon 6 containing multi-component material and at least a partially solid phase comprising the non-dissolved components of the nylon 6 containing multi-component material and optionally solvent;

[0086] (iii) removing the solvent from the liquid extraction phase and, if present therein, also from the at least partially solid phase to obtain two solid phases, one of which is a pre-concentrated nylon 6 containing material, which is enriched in nylon 6 compared to the nylon 6 containing multi-component material used as starting material.

[0087] Of course, step (i) also includes the treatment with the solvent, i.e. the solvent and the nylon 6 containing multi-component material are brought into contact under conditions sufficient to allow the dissolution of the components of the nylon 6 containing multi-component material to be extracted for a sufficient time. The phase separation in step (ii) can be performed by any suitable means known for the separation of solid / liquid phases. Suitable phase separation means are for example filters, centrifuges, cyclones. The removal of the solvent in step (iii) is advantageously performed by solvent evaporation, whereby the dissolved components extracted from the nylon 6 containing multi-component material are obtained in the form of a remaining solid precipitate. However, various techniques exist and can be used, which enable the recovery of the solvent from the dissolved compounds. Such techniques are known to the skilled person and include cooling, evaporation, distillation, precipitation by addition of a precipitant for the solvent of the dissolved polymer and combinations thereof.

[0088] The term "extraction", "extracted" or "extracting" as used herein refers to a physical or chemical process by which one or more components are removed from a substrate with the aid of a solvent, followed by removal of the solvent and recovery of the extracted component(s). One or more solvents can be used in the process of the present application to extract nylon 6, in which case the pre-concentrated nylon 6 containing material is obtained in the form of a remaining solid residue after removal of the one or more solvents from the liquid solvent containing extraction phase, and / or to extract non-nylon 6 compounds from the multi-component material containing nylon 6, in which case the undissolved multi-component material containing nylon 6 remaining after removal of any adhering solvent is the pre-concentrated nylon 6 containing material referred to herein.

[0089] The term "pre-concentration" as used herein refers to the extraction of the multi-component material containing nylon 6 with one or more solvents followed by removal of the solvent to obtain a material enriched in nylon 6 compared to the multi-component material containing nylon 6 used as starting material. "Enriched in nylon 6" refers to enrichment compared to the multi-component material containing nylon 6 used as starting material. The enrichment in nylon 6 is defined as the content of nylon 6 in wt.% based on the total weight of polymers in the solid nylon 6 containing material obtained after extraction compared to the content of nylon 6 in wt.% based on the total weight of polymers in the multi-component material containing nylon 6 used as starting material. This enrichment is largely dependent on the fraction of non-nylon 6 compounds removed. The degree of enrichment in the pre-concentrated nylon 6 containing material is advantageously from 1.1 to 50, in particular from 1.2 to 30, and more particularly from 1.5 to 10. In case the starting material has a content of nylon 6 of 20 wt.% based on the total polymer content and the pre-treated nylon 6 containing phase has a content of nylon 6 of 80 wt.% based on the total polymer content, the degree of enrichment of nylon 6 is (80 wt.% divided by 20 wt.% =) 4.

[0090] The solvent used for extraction in the nylon 6 pre-concentration section can be an organic solvent or an inorganic solvent. Preferably, the solvent is an organic solvent. Extraction of the multi-component material containing nylon 6 with an organic solvent has the advantage that different components, in particular different polymers, can be separated from each other and in particular from the nylon 6 to be enriched by their different solubilities in certain solvents.

[0091] Extraction typically involves (i) contacting the multi-component material containing nylon 6 with one or more solvents such that a liquid extraction phase comprising the solvent and dissolved compounds from the multi-component material containing nylon 6 and a second phase, typically solid or partially solid, consisting of the undissolved multi-component material containing nylon 6 is obtained, followed by (ii) phase separation and (iii) solvent removal to obtain the pre-concentrated nylon 6 containing material.

[0092] As explained above, depending on whether (1) a nylon 6 dissolving solvent or (2) a non-nylon 6 dissolving solvent is used, (1) a pre-concentrated nylon 6 containing material is obtained after removal of the solvent from the liquid extraction phase comprising dissolved nylon 6 or (2) from a solid or partially solid second phase consisting of the undissolved remaining nylon 6 containing multi-component material. Two or more different, especially complementary, extractions can also be combined.

[0093] In case more than one solvent is used, the treatment can be performed simultaneously or sequentially. If more than one solvent is used, the solvents should differ in their ability to dissolve nylon 6. In a particular advantageous embodiment of the present application, step a) comprises at least a two-step procedure, wherein (i) first the nylon 6 containing multi-component material is extracted with a solvent capable of dissolving non-nylon 6 components and subsequently (ii) the undissolved remaining nylon 6 containing multi-component material from the first extraction is treated and extracted with a solvent capable of dissolving nylon 6, thereby obtaining a pre-concentrated nylon 6 containing material after phase separation, after removal of the solvent from the liquid extraction phase comprising dissolved nylon 6 obtained from the second extraction. This way is especially advantageous when a multi-layer film is used as starting material, especially when the nylon 6 containing layer is covered, especially sandwiched between other polymer layers and thus not directly accessible for the nylon 6 dissolving solvent.

[0094] By selecting the solvent and treatment conditions (e.g. temperature, length of treatment and amount of solvent relative to the amount of the nylon 6 containing multi-component material), the extraction effect can be influenced. Depending on the solvent used, the extraction in step a) can be of the following types:

[0095] - a pre-concentration step wherein non-nylon 6 compounds are preferentially extracted from the nylon 6 containing multi-component material (then the pre-concentrated nylon 6 containing material obtained after optional removal of any adhering solvent is an unextracted nylon 6 containing multi-component material;

[0096] - a pre-concentration step wherein nylon 6 compounds are preferentially extracted from the nylon 6 containing multi-component material (then the pre-concentrated nylon 6 containing material is obtained after removal of the solvent from the liquid extraction phase comprising solvent and dissolved nylon 6); and

[0097] - a combination of preferential extraction of nylon 6 and non-nylon 6 compounds from the nylon 6 containing multi-component material in a sequential manner, i.e. sequential extraction of non-nylon 6 compounds and nylon 6.

[0098] Extraction can be performed with solvents suitable as selective extractants for non-nylon 6 compounds, wherein the non-nylon 6 compounds are preferentially extracted from the nylon 6-containing multi-component material. Such solvents are solvents in which the non-nylon 6 compounds are readily dissolved, while the dissolution of nylon 6 in the solvent is limited. Such solvents must have a high selectivity for the dissolution of non-nylon 6 compounds. In addition to the selection of the solvent, the temperature at which the extraction is performed, the treatment time and the amount of solvent relative to the amount of the nylon 6-containing multi-component material can also have a significant influence on the extraction selectivity. Thus, the selection of such parameters depends on the type of non-nylon 6-containing component that must be selectively removed. The skilled person can easily determine which solvents are effective in extracting the relevant non-nylon 6-containing component in a given nylon 6-containing multi-component material by simple test dissolution experiments.

[0099] Any one or more of the organic solvents mentioned below with respect to specific non-nylon 6 compounds can be used as the solvent in the process of the present application.

[0100] Solvents, for example selected from the group consisting of aliphatic hydrocarbons, cycloalkanes, aromatic hydrocarbons and mixtures thereof, are especially suitable for dissolving polyolefins, such as LD, LLD and HD polyethylene and polypropylene. They are obtained as boiling fractions in the processing of petroleum for the production of fuels, such as petrol and diesel. The aforementioned boiling fractions include solid paraffin, petroleum wax and petroleum solvent, each of which can be used as a solvent according to the present application. It is also known that isomeric mixtures of xylene or pure solvent such as toluene dissolve polyolefins. Chlorinated hydrocarbons, such as tetrachloroethane, can also be used to dissolve polyolefins.

[0101] A variety of solvents including aromatic organic solvents can be used to dissolve polystyrene polymers. Preferably, benzene, toluene, xylene and mixtures thereof are used as solvents to dissolve polystyrene polymers. Most preferably, xylene is used to dissolve polystyrene polymers.

[0102] Solvents tetrahydrofuran, cyclohexane, dioxane and methyl ethyl ketone (MEK) and mixtures thereof are preferably used to dissolve polyvinyl chloride.

[0103] Solvents that can be used to dissolve other (polymeric) non-nylon 6 compounds are known to the skilled person or can be easily identified by performing test dissolution experiments. When selecting a solvent for extraction in the nylon 6 pre-concentration section, the flexibility of the process of the present application allows other factors to be taken into account, including, among others, stable commercial availability, ease of disposal, health profile and solvent cost.

[0104] After extraction of the non-nylon 6 compounds with a solvent, removal of the undissolved compounds is achieved by phase separation from the mixture obtained in the nylon 6 pre-concentration section. The extraction should be performed in such a way that after extraction with a solvent that preferentially dissolves the non-nylon 6 compounds, the maximum weight ratio of undissolved compounds should be nylon 6. Optionally, after removal of all adhering solvent, the undissolved material then represents a pre-concentrated nylon 6 containing material enriched in nylon 6. The solvent used is typically recovered from the extraction phase and reused in the nylon 6 pre-concentration section.

[0105] Optionally, the extraction that preferentially extracts the non-nylon 6 compounds is performed more than once. By selecting another solvent and / or other processing conditions (e.g. another extraction temperature), non-nylon 6 compounds that were not removed in a previous extraction can be removed in an additional extraction. After each extraction, a pre-treated, typically solid or partially solid, nylon 6 containing phase is obtained that is more concentrated in nylon 6 than the pre-concentrated nylon 6 containing material obtained from the previous extraction. In each extraction, more and more non-nylon 6 compounds are extracted from the multi-component material containing nylon 6.

[0106] Alternatively or as a complementary extraction to the extraction with a non-nylon 6 dissolving solvent described above, an extraction can be performed that preferentially extracts nylon 6 from the multi-component material containing nylon 6 with a solvent that is suitable for use as a selective extractant for nylon 6. To preferentially extract nylon 6, a solvent is used that dissolves nylon 6 but the dissolution of non-nylon 6 compounds is limited in the solvent. In other words, a solvent should be used that has a high selectivity for the dissolution of nylon 6. As with the selective dissolution of non-nylon 6 compounds, the selective dissolution of nylon 6 compounds can also be influenced by the solvent, the temperature at which the extraction is performed, the processing time and the amount of solvent relative to the amount of the multi-component material containing nylon 6.

[0107] Solvents known from prior art methods for the extraction of polyamides can be used for this nylon 6 specific extraction step. For example, EP 603434 discloses suitable solvents for dissolving polyamides including nylon 6. The solvents include concentrated inorganic acids, formic acid, chloroacetic acid, phenol, cresol, alcoholic solutions of alkaline earth metal halides, aromatic alcohols such as benzyl alcohol and phenethyl alcohol, and glycols, lactams and lactones. US 5840773 discloses solvents for dissolving polyamides such as nylon 6 aliphatic alcohols, for example methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tertiary butanol, amyl alcohol, hexanol, 2-ethyl butanol 4-methyl pentanol, 1-heptanol, 2-heptanol, 4-heptanol, 2,4-dimethyl pentanol, 1-octanol, 2-octanol, 2-ethyl hexanol, 1-nonanol, 2-nonanol, n-decanol, n-undecanol, n-dodecanol, n-tridecanol, n-tetradecanol, n-pentadecanol, n-hexadecanol, n-octadecanol and the like; and substituted mono- and dihydric alcohols such as 2-methoxy-1-ethanol and methyl glycol. In the preferred embodiment of US 5840773, C1-C12 Alcohols. In particular, methanol and ethanol are preferred as extractants. The extractant can be a mixture of an aliphatic alcohol and water. US 5840773 further discloses that an extraction time of 60 minutes has been found suitable for methanol extraction of polyamide 6, wherein the extraction is preferably carried out in an autoclave at a temperature of about 135°C to about 140°C. In line with this thinking, US 5840773 mentions that polyamide 6,6 is preferably dissolved in methanol at a temperature of at least about 140°C. In Example IV of US 5840773, a polyamide 6,6 extraction temperature of about 160°C is applied.

[0108] Optionally, the extraction for preferential extraction of nylon 6 is carried out more than once. After each extraction, a pre-concentrated nylon 6 containing material is obtained, which is more concentrated in nylon 6 than the pre-concentrated nylon 6 containing material resulting from the previous extraction. This appears to be due to the gradual decrease in the amount of impurities in the pre-concentrated nylon 6 containing material, which typically comprises non-nylon 6 compounds, by each extraction.

[0109] After the extraction for preferential extraction of nylon 6 compounds with a solvent, the solvent is recovered. Thereby, first the undissolved material is removed from the liquid extraction phase containing the solvent and the nylon 6, for example by filtration or centrifugation. The extraction should be carried out in such a way that the maximum amount of undissolved compounds is non-nylon 6 compounds. Second, the solvent is recovered from the liquid extraction phase containing the nylon 6 to obtain a pre-concentrated solid nylon 6 containing material enriched in nylon 6. Any of the above described solvent removal techniques can be performed to separate the dissolved nylon 6 containing material from the solvent, such techniques including cooling, evaporation, distillation, precipitation by addition of a precipitant for the solvent of the dissolved polymer, and combinations thereof.

[0110] The extraction can also be a combination of sequential extractions with a solvent for preferential extraction of non-nylon 6 compounds and a solvent for preferential extraction of nylon 6. By this combination of sequential extractions, a nylon 6 containing phase is obtained which is more concentrated in nylon 6 than a nylon 6 containing phase which has been pre-treated only once.

[0111] In an embodiment of the present application, the multi-component material comprising nylon 6 is first treated with a solvent that preferentially extracts non-nylon 6 compounds from the multi-component material comprising nylon 6 in a first nylon 6 pre-concentration section to obtain a pre-concentrated nylon 6- enriched material comprising nylon 6. In a second extraction, the pre-concentrated nylon 6-enriched material is treated with a solvent that preferentially extracts nylon 6 compounds from the pre-concentrated nylon 6-enriched material to obtain a pre-concentrated nylon 6-further enriched material comprising nylon 6. This can subsequently be used in step b). For example, in the first extraction, the non-nylon 6 compound LD polyethylene is removed from the multi-component material comprising nylon 6 by extraction with a petroleum solvent. In the second extraction, the pre-concentrated nylon 6-enriched material is further extracted with methanol or a methanol aqueous solution (e.g. water to MeOH in a weight ratio of 1 :19), whereby a more pre-concentrated solid material comprising nylon 6 is obtained after removal of the solvent (methanol, water).

[0112] According to another advantageous embodiment of the present application, the multi-component material comprising nylon 6 is first treated with a solvent that preferentially extracts nylon 6 compounds from the multi-component material comprising nylon 6 in a first nylon 6 pre-concentration section to obtain a pre-concentrated nylon 6-enriched material comprising nylon 6. In a second extraction, the pre-concentrated nylon 6-enriched material is treated with a solvent that preferentially extracts non-nylon 6 compounds from the pre-concentrated nylon 6-enriched material to obtain a pre-concentrated nylon 6-further enriched material comprising nylon 6. For example, in the first extraction, the nylon 6 is extracted from the multi-component material comprising nylon 6 with ethanol to form a nylon 6-enriched phase comprising nylon 6 after recovery of the ethanol. In the second extraction, the non-nylon 6 components are further extracted from the pre-concentrated nylon 6-enriched material with toluene, whereby a nylon 6-further enriched phase comprising nylon 6 is obtained.

[0113] The extraction with a solvent in the nylon 6 pre-concentration section is performed at a temperature between 0 °C and 350 °C (at higher temperatures, the nylon 6 decomposes), more preferably between 20 °C and 210 °C and most preferably between 50 °C and 190 °C. Within this temperature range, the selective extraction of the majority of the solvent is enhanced.

[0114] The time required for the extraction in the extraction section, i.e. the extraction with the solvent, can easily be determined by the skilled person by routine experiments. The time allowed for the extraction should preferably take into account the type of the multi-component material comprising nylon 6 and the amount of nylon 6 contained therein and its accessibility. The time required for the extraction can range from a few seconds to several hours. Preferably, the time required for the extraction is greater than 5 seconds and less than 6 hours. More preferably, the time required for the extraction is greater than 15 seconds and less than 2 hours.

[0115] Due to the extraction in the nylon 6 pre-concentration section, a solid-state pre-concentrated nylon 6 containing material enriched in nylon 6 is obtained. In this regard, the term "solid-state" refers to the state of the material at room temperature (20°C). At higher temperatures, the pre-concentrated nylon 6 containing material can also exist in the form of a melt. The pre-concentrated nylon 6 containing material is stable and can be stored for future use in step b) of the process of the present application or transported to a depolymerization section at a different location for this purpose. Thus, similar to the explanations above for the other pre-treatment steps, the place where the nylon 6 pre-concentration by extraction with a solvent takes place and the place where steps a) to d) of the process of the present application are performed can be the same or different. Preferably, one or more of the pre-treatment and pre-concentration steps are performed at a different location, e.g. at a location dedicated for pre-treating waste polymeric material and in particular multi-component material containing nylon 6. The multi-component material containing nylon 6 that is pre-treated and has been pre-concentrated at various locations can then be shipped to the place where one or more of steps a) to d) of the process of the present application are performed.

[0116] Depolymerization step b)

[0117] In step b) of the present application, the pre-concentrated nylon 6 containing material enriched in nylon 6 and obtained in the nylon 6 pre-concentration section is charged to a depolymerization section and depolymerized to form ε-caprolactam. The ε-caprolactam formed is discharged from the depolymerization section in the form of an ε-caprolactam containing stream. The depolymerization section comprises one or more depolymerization reactors operated in series and / or in parallel.

[0118] Optionally, the pre-concentrated nylon 6 containing material is mechanically compressed to a smaller volume prior to being charged to the depolymerization section. In particular, the pre-concentrated nylon 6 containing material can be compressed to particles of increased density prior to being charged to the depolymerization section, e.g. by mechanical compaction or by extruding the molten material followed by cooling and cutting it to size, or by forming droplets of the molten material which are subsequently solidified by cooling. Such compression has the advantage of requiring less volume for intermediate storage and transport, and can facilitate the feeding to the depolymerization section.

[0119] Optionally, the pre-concentrated nylon 6 containing material is dried prior to being charged to the depolymerization section. This has the advantage of introducing less or no solvent into the depolymerization section. The introduction of solvent into the depolymerization section can adversely affect the depolymerization process (e.g. the depolymerization reaction rate is reduced, the catalyst consumption is higher, the vapor stream comprising ε-caprolactam and water resulting from the depolymerization section can contain more impurities).

[0120] Preferably, the pre-concentrated nylon 6 containing material is fed to the depolymerization reactor as a solid phase or as a melt. Feeding as a melt can be achieved by using an extruder, a gear pump or other means known to the skilled person.

[0121] The feed of the pre-concentrated nylon 6 containing material to the depolymerization reactor can be achieved by a continuous or batch-wise supply of the pre-concentrated nylon 6 containing material enriched in nylon 6.

[0122] In the depolymerization section, the pre-concentrated nylon 6 containing material is depolymerized to form ε-caprolactam. This is achieved by contacting the pre-concentrated nylon 6 containing material with water, preferably in the form of steam. Feeding the water in the form of steam to the depolymerization reactor allows to obtain the desired steam stream comprising ε-caprolactam and water as product of step b) according to the present application, optionally without further heating. The desired weight ratio of water to ε-caprolactam in this steam stream of 2:1 to 15:1 can be obtained by adjusting the ratio of steam to pre-concentrated nylon 6 containing material fed into the depolymerization section. During the depolymerization reaction, decomposition products can be formed, including linear oligomers of ε-caprolactam and cyclic oligomers of ε-caprolactam. In addition, the feed stream of the pre-concentrated nylon 6 containing material enriched in nylon 6 can also contain other components, i.e. impurities, such as non-nylon 6 compounds and residues of solvents applied in the nylon 6 pre-concentration section, which such components remain stable, react or decompose under the depolymerization conditions. Thus, the steam stream removed from the depolymerization section comprises not only water and ε-caprolactam, but also impurities.

[0123] The depolymerization reaction is preferably carried out at a temperature of at least 180°C but not higher than 400°C. A 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. In general, the ε-caprolactam formation rate increases at higher temperatures. Temperatures below 400°C are preferred because at temperatures above 400°C side reactions of nylon 6 and reactions of impurities occur more frequently which would lead to the formation of more diverse and / or more various impurities. Part of such impurities would end up in the ε-caprolactam containing product stream discharged from the depolymerization reactor. In a preferred embodiment of the present application, the depolymerization of the pre-concentrated 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 allows to obtain an especially pure ε-caprolactam after purification of the obtained crude product.

[0124] The depolymerization of the nylon 6 is achieved under wet conditions, i.e. in the presence of water, preferably in the form of steam, especially superheated steam.

[0125] Preferably, superheated steam with a temperature between 100°C and 600°C is charged into the depolymerization reactor. Preferably, the superheated steam charged into the depolymerization reactor has a temperature at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam charged into the depolymerization reactor is high enough that no additional heat input is required for the depolymerization reaction and the evaporation of the ε-caprolactam formed. In a preferred embodiment of the invention, the depolymerization section is charged with superheated steam with a temperature in the range of 220°C to 575°C. In a more preferred embodiment of the invention, the depolymerization section is charged with superheated steam with a temperature in the range of 275°C to 500°C.

[0126] The depolymerization of nylon 6-containing materials pre-concentrated in the presence of vapor can be carried out in the presence of additional depolymerizing agents, such as ammonia, amines, or alcohols including methanol and ethanol.

[0127] Most preferably, depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is Lewis or Boehringer Ingelheim. Acids or bases. Acid catalysts may be particularly selected from the group consisting of: phosphoric acid; p-toluenesulfonic acid; boric acid; sulfuric acid; organic acids; organic sulfonic acids, including xylenesulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons; solid acids; salts of the aforementioned acids; Al2O3; and SiO2; and combinations thereof. Base catalysts may be selected, for example, from the group consisting of: alkali metal hydroxides; alkali metal salts; alkaline earth metal hydroxides; and alkali metals, such as alkaline earth metal salts; organic bases and solid bases; and combinations thereof. Preferably, phosphoric acid, boric acid, organic acids, alkali metal hydroxides and alkali metal salts are used as catalysts. More preferably, phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate are used. More preferably, phosphoric acid, p-toluenesulfonic acid, boric acid and sodium hydroxide are used. In one particularly preferred embodiment, phosphoric acid is used as a catalyst for depolymerization, and in another, p-toluenesulfonic acid is used.

[0128] Although the aforementioned (Lewis or Bowenst) acids or bases are referred to as "catalysts," it is known that several catalysts are consumed during depolymerization reactions, resulting in the formation of byproducts. For example, it is well known that phosphoric acid is consumed during the steam depolymerization of nylon 6, thereby producing phosphorus-containing byproducts. Generally, such phosphorus-containing byproducts render the catalyst ineffective and represent a loss of valuable catalytic activity. Furthermore, especially at higher temperatures, several catalysts, such as H3PO4, tend to polymerize and (partially) lose their catalytic activity.

[0129] However, in another preferred embodiment of the invention, the catalyst is not used for the depolymerization of the pre-concentrated nylon 6-containing material. This has the advantage of lower cost (catalyst cost and catalyst waste disposal cost). However, it typically requires higher temperatures (and pressures).

[0130] The use of the catalyst (and in particular orthophosphoric acid) is such that the depolymerization reaction has already started at low temperature and can be carried out under atmospheric conditions. Suitable concentrations of catalyst for depolymerizing nylon 6 into ε-caprolactam are known to the skilled person and can easily be determined by routine experimentation. If the concentration of the catalyst used is too low, the reaction rate is slow. Conversely, if the concentration of the catalyst used is too high, the reaction is fast and the side reactions increase. Furthermore, the cost of the catalyst increases, which is economically disadvantageous. Generally, the catalyst content is between 0.01 and 100 wt.%, relative to the nylon 6 contained in the depolymerization reactor. Preferably, the catalyst content is between 0.1 and 50 wt.%. The most preferred catalyst concentration depends on the type of catalyst used for the depolymerization of nylon 6. For the catalyst orthophosphoric acid, a preferred content is between 0.1 and 25 wt.%, more preferably between 1 and 20 wt.%. The preferred content of the catalyst p-toluenesulfonic acid is between 10 and 35 wt.%.

[0131] The depolymerization of nylon 6 can be carried out in batch mode, in semi-continuous mode or in continuous mode, all of which modes are known to the skilled person. As used herein, the terms "batch mode", "semi-continuous mode" and "continuous mode" refer to the mode of charging the depolymerization reactor with the nylon 6 containing raw material, i.e. the pre-concentrated nylon 6 containing material enriched in nylon 6 (and optionally catalyst), and the mode of discharging the residual material from the depolymerization reactor. Generally, the residual material comprises non-nylon 6 compounds, undepolymerized nylon 6, ε-caprolactam, catalyst and decomposition products of such components.

[0132] In a preferred embodiment, the depolymerization of nylon 6 is carried out in batch mode. In batch mode, the raw material, i.e. the pre-concentrated nylon 6 containing material enriched in nylon 6, and optionally catalyst, is first charged into the depolymerization reactor. Subsequently, superheated steam is charged into the depolymerization reactor and ε-caprolactam is discharged from the depolymerization reactor in the form of a steam stream comprising ε-caprolactam and water. Next, the charging of superheated steam into the depolymerization reactor is interrupted. After removal of the residual material from the depolymerization reactor, a new cycle is started by charging the raw material (and optionally catalyst) into the depolymerization reactor. In a preferred embodiment, the residual material is not removed between individual cycles.

[0133] In a particular advantageous embodiment, the depolymerization of the nylon 6 is carried out in a continuous mode. In the continuous mode, the nylon 6 containing feedstock (and optionally catalyst) is continuously charged into the depolymerization reactor. At the same time, superheated steam is continuously charged into the depolymerization reactor and ε-caprolactam is continuously withdrawn from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. In addition, residual material is continuously withdrawn from the depolymerization reactor. Preferably, the nylon 6 containing feedstock (and optionally catalyst) is charged in the form of a melt, solid pieces, slurry or solution. More preferably, the nylon 6 containing feedstock (and optionally catalyst) is charged in the form of a melt, slurry or solution.

[0134] In a preferred embodiment, the depolymerization of the nylon 6 is carried out in a semi-continuous mode. In the semi-continuous mode, the nylon 6 containing feedstock (and optionally catalyst) is intermittently charged into the depolymerization reactor, while superheated steam is continuously charged into the depolymerization reactor and ε-caprolactam is continuously withdrawn from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Residual material is intermittently withdrawn from the depolymerization reactor in the semi-continuous mode of the depolymerization of the nylon 6.

[0135] The ε-caprolactam leaves the depolymerization section as a steam stream comprising water and ε-caprolactam in a weight ratio of 1 : 1 to 50: 1, preferably 2: 1 to 10: 1, more preferably 3: 1 to 8: 1. Preferably, the ε-caprolactam in the steam stream has a pressure of 0.02 to 10 MPa, more preferably 0.08 to 1.5 MPa or most preferably 0.1 to 0.5 MPa.

[0136] Recovery step c)

[0137] In the recovery section, ε-caprolactam is recovered from the ε-caprolactam containing steam stream withdrawn from the depolymerization section. Preferably, this recovery is carried out by (partial) condensation of the steam stream.

[0138] The ε-caprolactam containing steam stream withdrawn from the depolymerization section comprises ε-caprolactam, water and impurities. Preferably, the ε-caprolactam is separated from the remaining components of the steam stream by sending the product stream from the depolymerization reactor, preferably from the top, to a distillation column, thereby obtaining a water rich phase as top product and an ε-caprolactam rich phase as bottom product. Preferably, the ε-caprolactam is separated from the remaining components of the steam stream by sending the product stream from the depolymerization reactor, preferably from the top, to a distillation column, thereby obtaining a water rich phase as top product and an ε-caprolactam rich phase as bottom product.

[0139] The e-caprolactam recovered in the recovery section is crude material, as it contains impurities such as nylon 6 decomposition products or other impurities stemming from non-nylon 6 components of the multi-component starting material. The crude e-caprolactam recovered in step c) comprises water and e-caprolactam, preferably it is an aqueous solution comprising e-caprolactam. Thus, the crude e-caprolactam recovered in the recovery section needs additional purification to obtain high purity e-caprolactam. Thus, "crude" as used herein can be defined as lower purity compared to the purified e-caprolactam obtained as product of the process of the present application.

[0140] Preferably, the crude e-caprolactam comprises e-caprolactam in the range of 6 wt.% to 95 wt.%, more preferably 20 wt.% to 90 wt.% and most preferably 40 wt.% to 75 wt.%. The remainder is mainly water.

[0141] Purification step d)

[0142] In step d), the crude e-caprolactam obtained in the recovery section [C] is purified in the purification section [D] to obtain high purity e-caprolactam.

[0143] Optionally, the crude e-caprolactam is filtered prior to charging the purification section. Filtration ensures removal of undissolved impurities, which might otherwise hinder the further purification process.

[0144] The purified e-caprolactam is obtained from the crude e-caprolactam by first extracting the crude e-caprolactam with an organic solvent in step (i), whereby an aqueous phase and an organic phase comprising the organic solvent, e-caprolactam and impurities are obtained. The organic solvent by which the crude e-caprolactam is extracted is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 aliphatic alcohol or a cycloaliphatic alcohol. Optionally, the organic solvent by which the crude e-caprolactam is extracted is preferably a mixed extractant consisting of an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10aliphatic or cycloaliphatic alcohol and a C5-C8 alkane or a C5-C8 cycloalkane. Especially good results are achieved if the organic solvent used for the extraction of the crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol and mixtures thereof. More preferably, the organic solvent used for the extraction of the crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohols and mixtures thereof. More preferably, the organic solvent used for the extraction of the crude ε-caprolactam is selected from the group consisting of toluene, 1-octanol, 4-methyl-2-pentanol, 2-ethylhexanol and mixtures thereof. Generally, the weight ratio of the organic solvent to ε-caprolactam is in the range of 0.01 : 1 to 40: 1, preferably 0.01 : 1 to 20: 1, more preferably 0.05: 1 to 10: 1 and most preferably 0.1 : 1 to 5: 1.

[0145] Optionally, the organic solvent used for the extraction of the crude ε-caprolactam is mixed with an alkane: m H 2m+2 wherein m is 5 to 8; a cycloalkane; or a C m H 2m wherein m is 5 to 8, so that a mixed extractant is formed. Especially good results are achieved if the alkane or cycloalkane is present in the mixed extractant in the range of 5 to 90 wt.-% and preferably 25 to 75 wt.-% of the total weight of the mixed extractant.

[0146] In an embodiment of the present application, wherein the organic solvent has a lower density than the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in a countercurrently operated extraction column, wherein the crude ε-caprolactam to be purified is introduced at the top of the column and the organic solvent is introduced at the bottom. The extraction results in an aqueous phase and an organic phase comprising the organic solvent, ε-caprolactam and impurities, which have a lower weight ratio of impurities to ε-caprolactam than the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0147] In another preferred embodiment of the present application, wherein the organic solvent has a higher density than the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in a countercurrently operated extraction column, wherein the crude ε-caprolactam to be purified is introduced at the lower part of the column and the organic solvent is introduced at the upper part. The extraction results in an aqueous phase comprising water and impurities and an organic phase comprising the organic solvent, ε-caprolactam and impurities, which have a lower weight ratio of impurities to ε-caprolactam than the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Thus, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0148] Optionally, the organic phase comprising organic solvent, ε-caprolactam and impurities is washed with water or with an aqueous base solution prior to entering step d)(iii). If the washing is performed with an aqueous base solution, the basic solution is preferably an aqueous solution comprising an alkali hydroxide and / or an alkali carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali hydroxide solution preferably comprises 0.5 to 2.0 wt.% of sodium hydroxide or potassium hydroxide.

[0149] The skilled person can determine the amount of water or aqueous base solution required for an effective washing of the organic phase comprising organic solvent, ε-caprolactam and impurities by routine experimentation. Typically, this amount is between 0.1 and 5 wt.% relative to the amount of organic solvent excluding ε-caprolactam dissolved in the organic phase to be washed. Preferably, the washing of the organic phase comprising organic solvent, ε-caprolactam and impurities with water or an aqueous base solution is performed in a countercurrent running washing column, wherein the organic phase comprising organic solvent, ε-caprolactam and impurities is introduced at the bottom of the column and water or the aqueous base solution is introduced at the top. The washing results in a washed organic phase comprising organic solvent, ε-caprolactam and impurities and a residual containing phase. Typically, the residual containing phase contains water and ε-caprolactam and only some traces of impurities. Due to the washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase prior to the washing.

[0150] Optionally, prior to step d)(iii) of the process of the present application, the step d)(iii) is preceded by one or more process steps to reduce the energy consumption of the purification step d) of the process of the present application, to enhance the quality of the purified ε-caprolactam, and / or to simplify the distillative removal of the impurities having a boiling point lower or higher than ε-caprolactam.

[0151] Optionally, according to step d)(ii) of the process of the present application, the solvent of the obtained organic phase comprising organic solvent, ε-caprolactam and impurities, optionally washed with water or with an aqueous base solution, is exchanged, whereby the organic solvent in the organic phase comprising organic solvent, ε-caprolactam and impurities is replaced by water, and whereby an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is obtained, and wherein the solvent exchange process is selected from a process based on a water back extraction and a process based on a solvent swing distillation, wherein the organic solvent is distilled off and water is charged. The term "replacement" as used herein means that at least 60 wt.%, preferably at least 80 wt.% and most preferably at least 90 wt.%, 95 wt.% or 98 wt.% of the organic solvent present in the organic phase comprising organic solvent, ε-caprolactam and impurities is replaced by water.

[0152] Two solvent exchange alternatives are described below.

[0153] Solvent exchange: first alternative:

[0154] Optionally, the optionally washed ε-caprolactam solvent phase is then extracted with water to obtain an ε-caprolactam aqueous phase. Therefore, preferably, this ε-caprolactam aqueous phase is stripped and / or distilled to remove residual solvent. Although the amount of water used to recover ε-caprolactam is not critical, the amount of water used is typically 0.5 to 20 times the weight of the recovered ε-caprolactam. Preferably, it is 0.75 to 10 times the weight, more preferably 1 to 5 times the weight.

[0155] Water re-extraction can be advantageously carried out in a countercurrent extraction column, with the ε-caprolactam solvent phase to be purified introduced at the bottom of the column and water introduced at the top. Re-extraction produces an aqueous phase of ε-caprolactam and a solvent phase containing impurities. Typically, the solvent phase containing impurities is reused, preferably after purification by distillation.

[0156] In another preferred embodiment, the organic phase, optionally washed and containing organic solvent, ε-caprolactam, and impurities, has a density lower than that of water. The organic phase is introduced into the lower part of the extraction column, and water is introduced into the upper part. Back-extraction produces an organic solvent phase containing impurities and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. The weight ratio of impurities to ε-caprolactam in this aqueous phase is lower than that in the organic phase containing organic solvent, ε-caprolactam, and impurities before back-extraction. Therefore, purer ε-caprolactam is obtained due to back-extraction. Preferably, the organic solvent phase containing impurities is reused after purification (preferably by distillation).

[0157] In another preferred embodiment, the organic phase, comprising an organic solvent, ε-caprolactam, and impurities, has a density higher than that of water. The organic phase is introduced into the upper part of the extraction column, and water is introduced into the lower part. Back-extraction produces an organic solvent phase containing impurities and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam. The weight ratio of impurities to ε-caprolactam in the aqueous phase is lower than that in the organic phase containing the organic solvent, ε-caprolactam, and impurities before back-extraction. Therefore, due to back-extraction, the ε-caprolactam is purer than before back-extraction. Preferably, the organic solvent phase containing impurities is optionally reused after purification (preferably by distillation).

[0158] The resulting aqueous phase of ε-caprolactam is concentrated by selectively stripping and / or distilling water to remove residual solvent, thereby obtaining a concentrated aqueous phase of ε-caprolactam. The ε-caprolactam content of this aqueous phase of ε-caprolactam is typically between 50 and 99.9 wt.% relative to the intact phase.

[0159] Solvent conversion: Second alternative:

[0160] Optionally, the organic solvent is evaporated from the optionally washed ε-caprolactam solvent phase, instead of re-extracting with water. Any suitable evaporation vessel can be used, such as a column. Preferably, the evaporation is performed in the presence of water. More preferably, the evaporation is performed in the form of azeotropic distillation, in which case the organic solvent is evaporated as an azeotropic mixture. The evaporation results in an ε-caprolactam product. Typically, the ε-caprolactam product is an ε-caprolactam water phase. The ε-caprolactam content of this ε-caprolactam water phase is typically between 50 and 99.9 wt.% relative to the complete phase.

[0161] The solvent conversion process can also be a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is charged. In a preferred embodiment, the solvent conversion process is a process based on solvent exchange distillation, which is performed as a single stage process, in which the organic solvent is distilled off from the organic phase comprising organic solvent, ε-caprolactam and impurities, and water is charged. More preferably, the solvent conversion is performed in the form of azeotropic distillation in the presence of water, in which case the organic solvent is evaporated as an azeotropic mixture comprising organic solvent and water. The purpose of the azeotropic distillation is to remove the organic solvent and to charge water. Preferably, substantially all of the organic solvent is removed. By "substantially all" in this context is meant that at least 90 wt.%, preferably at least 95 wt.% and most preferably at least 98 wt.% or 99 wt.% of the organic solvent present in the organic phase comprising organic solvent, ε-caprolactam and impurities is removed. Preferably, the water is charged in liquid form. More preferably, water in liquid form is added as reflux to the upper part of the distillation column. More preferably, part of the water added as reflux is obtained by condensation of the azeotropic mixture distilled off in the distillation column.

[0162] Any suitable vessel can be used for the solvent conversion process, such as a column, preferably a distillation column operated in continuous mode. The distillation column can comprise trays, packing or a combination thereof.

[0163] In another preferred embodiment, the solvent exchange distillation is performed as a two-stage process. The first stage is a pre-concentration stage and the second stage is the actual solvent exchange distillation.

[0164] The organic phase comprising organic solvent, ε-caprolactam and impurities is charged to a first stage. In the first stage, a first fraction of organic solvent is removed by distillation from the organic phase comprising organic solvent, ε-caprolactam and impurities at the upper part of the distillation column. Preferably, this distillation is performed under reflux. Under reflux means that the organic solvent in liquid phase is charged to the upper part of the distillation column. More preferably, a part of the organic solvent removed by distillation at the upper part of the distillation column is, after condensation, charged in liquid form to the upper part of the distillation column. The remaining organic phase comprising organic solvent, ε-caprolactam and impurities is discharged from the first stage and charged to a second stage. Due to the distillation in the first stage, the chemical composition of the remaining organic phase comprising organic solvent, ε-caprolactam and impurities is different from the organic phase comprising organic solvent, ε-caprolactam and impurities charged to the first stage. In general, the remaining organic phase comprising organic solvent, ε-caprolactam and impurities contains a higher weight percentage amount of ε-caprolactam and compounds having a boiling point higher than ε-caprolactam, and a lower weight percentage of compounds having a boiling point lower than ε-caprolactam, compared to the organic phase comprising organic solvent, ε-caprolactam and impurities charged to the first stage.

[0165] In the second stage, the remaining organic solvent is distilled from the remaining organic phase comprising organic solvent, ε-caprolactam and impurities, and water is charged. More preferably, in the second stage, the solvent shift is performed in the form of an azeotropic distillation with the addition of water, in which case the organic solvent evaporates as an azeotropic mixture comprising organic solvent and water.

[0166] Any suitable vessel can be used for the stages of solvent exchange, such as a column, preferably a distillation column operated in continuous mode. The distillation column can comprise trays, packing or a combination thereof.

[0167] The solvent exchange distillation, performed as a single stage process or as a two stage process, results in a water phase comprising water, ε-caprolactam and compounds having a boiling point lower or higher than ε-caprolactam, and optionally residual organic solvent. Preferably, the ε-caprolactam content of this water phase is between 25 and 99.9 weight %, more preferably between 50 and 99.5 weight %, and most preferably between 85 and 99 weight %, relative to the entire water phase.

[0168] Accordingly, in a particular advantageous embodiment according to the present application, the purification in step d) further comprises, after the extraction of the crude ε-caprolactam in step d)(i), a step (ii) of solvent shift based on solvent exchange distillation.

[0169] Optionally, the water phase comprising ε-caprolactam obtained according to the first or second solvent shift alternative is subsequently oxidized and / or treated with an ion exchange resin and / or hydrogenated.

[0170] In a preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide is added to the aqueous phase of ε-caprolactam. Most preferably, potassium permanganate is used as oxidizing agent.

[0171] Preferably, the oxidizing agent is added to the aqueous phase of ε-caprolactam in form of an aqueous solution, so that a diluted aqueous solution is obtained during purification by oxidation. The oxidizing agent can also be added in solid form. The oxidizing agent can also be added in form of a slurry. The skilled person can determine by routine experimentation the amount of oxidizing agent required for effective oxidation of the aqueous phase of ε-caprolactam. The precise amount of oxidizing agent depends, inter alia, to a large extent on the composition of the stream of nylon 6 fed into the depolymerization section in this process of the present application. Preferably, the amount of oxidizing agent is between 0.01 wt.% and 5 wt.% relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0172] The temperature used for the oxidation of the aqueous solution in the process of the present application can vary. Preferably, the aqueous solution is oxidized with an oxidizing agent at a temperature in the range of 20°C to 85°C, more preferably in the range of 30°C to 80°C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, in particular potassium permanganate.

[0173] The length of time used for oxidation with the oxidizing agent can vary. Preferably, the aqueous phase of ε-caprolactam is oxidized with the oxidizing agent in the process of the present application for 1 minute to 24 hours, more preferably for 2 minutes to 6 hours, and most preferably for 5 minutes to 2 hours.

[0174] The concentration of ε-caprolactam in the aqueous phase of ε-caprolactam used for oxidation with the oxidizing agent can vary. Preferably, the aqueous solution used for oxidation comprises ε-caprolactam and water in a weight ratio of 5:1 to 1:5, more preferably of 3:1 to 1:3, and most preferably of 2:1 to 1:2. Optionally, the weight ratio of ε-caprolactam to water is adapted prior to adding the oxidizing agent to the aqueous phase. Preferably, the weight ratio of ε-caprolactam to water is adapted by adding water or by removing water.

[0175] In case potassium permanganate and / or sodium permanganate is used as oxidizing agent, manganese (IV) oxide (Mn02) solid particles are formed as reaction product. The skilled person can determine by routine experimentation the most preferred solid-liquid filtration procedure for effective removal of manganese (IV) oxide solid particles from the aqueous phase after oxidation. In this regard, it is common practice to use a filter aid, such as activated carbon particles or diatomaceous earth, to improve the filtration procedure.

[0176] In another preferred embodiment, the aqueous ε-caprolactam phase is treated with ion exchange resins. Optionally, the aqueous ε-caprolactam phase is first treated with an acidic cation exchange resin and subsequently with a basic anion exchange resin. Optionally, the aqueous ε-caprolactam phase is first treated with a basic anion exchange resin and subsequently with an acidic cation exchange resin. Preferably, the cation exchange resin is a sulfonated polystyrene or a styrene-divinylbenzene copolymer and the anion exchange resin is a polystyrene containing quaternary ammonium groups or an exchange resin having secondary or tertiary amine groups. The treatment temperature of the aqueous ε-caprolactam solution can be preferably between 15°C and 100°C, more preferably between 35°C and 70°C. In order to effectively perform the adsorptive separation of the impurities with the cation exchange resin and the anion exchange resin, the concentration of the aqueous ε-caprolactam solution to be treated can be preferably between 5% and 90%, more preferably between 5% and 70%.

[0177] The skilled person can determine the most preferred ion exchange resin and the amount of ion exchange resin (combination) for effective removal of impurities from the aqueous ε-caprolactam phase and regeneration of the loaded ion exchange resin by routine experimentation.

[0178] In another preferred embodiment, the aqueous ε-caprolactam phase is hydrogenated in the presence of a hydrogenation catalyst known per se. The hydrogenation can advantageously be performed as described in, for example, EP 635 487.

[0179] The hydrogenation temperature is generally between 20°C and 160°C. In general, a temperature that is not too low will be chosen, because at low temperatures the reaction time is longer. The temperature will generally not be too high, because high temperatures have a negative effect on the quality of the ε-caprolactam. The temperature is therefore preferably between 70°C and 130°C, and most preferably between 80°C and 100°C.

[0180] The hydrogenation pressure can be between 0.1 MPa and 15 MPa. A high pressure is advantageous, because it makes it possible to dissolve a larger amount of hydrogen in the water-ε-caprolactam mixture. Since the amount of impurities is generally not too high, a large amount of hydrogen is not required, so that a high pressure is not necessary. Very high pressures also have the disadvantage that expensive process equipment is required. The pressure is therefore generally between 0.3 MPa and 5 MPa.

[0181] The hydrogenation catalyst can be any known heterogeneous hydrogenation catalyst. Examples of such catalysts are ruthenium on alumina, rhodium on alumina, platinum on carbon, palladium on carbon, Raney nickel, nickel on silica and nickel on alumina. Preferably, a catalyst containing nickel is used.

[0182] A suitable nickel catalyst generally has a nickel content of between 5 wt.% and 80 wt.% relative to the metal and the carrier. In addition to nickel, the catalyst can contain some activator, such as Zr, Mn, Cu or Cr. The activator content is generally between 1 wt.% and 20 wt.%.

[0183] If a heterogeneous catalyst comprising palladium is used, the palladium content will generally be between 0.01 wt.% and 10 wt.%.

[0184] Optionally, water is evaporated from the aqueous ε-caprolactam phase, optionally the aqueous ε-caprolactam phase is oxidized and / or treated with an ion exchange resin and / or hydrogenated. After 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.

[0185] In a preferred embodiment of the process of the present application, the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is obtained prior to distillation removal in step d)(iii) by:

[0186] the organic solvent is at least partially replaced with water, and wherein the solvent replacement step is selected from one method based on re-extraction with water and one method based on solvent replacement distillation, wherein the organic solvent is distilled off and water is charged; and

[0187] optionally followed by oxidation with an oxidizing agent selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide; and / or

[0188] optionally followed by treatment with an acidic cation exchange resin and / or a basic anion exchange resin; and / or

[0189] optionally followed by hydrogenation in the presence of a hydrogenation catalyst selected from the group consisting of Raney nickel, nickel on silica, nickel on alumina, ruthenium on alumina, rhodium on alumina, platinum on carbon and palladium on carbon.

[0190] In step d)(iii) of the process of the present application, purified ε-caprolactam is obtained by distillation removal of impurities having a boiling point lower or higher than ε-caprolactam. Typically, the distillation of the optionally washed organic phase comprising organic solvent, ε-caprolactam and impurities is carried out under reduced pressure. In embodiments of the present application, the distillation is carried out at a pressure of less than 50 kPa, preferably less than 20 kPa and more preferably less than 10 kPa. Preferably, the distillation temperature at the bottom of the distillation column is between 100 °C and 200 °C and more preferably between 110 °C and 180 °C. The distillation comprises separation of low boiling organic impurities (having a boiling point lower than ε-caprolactam) from ε-caprolactam and / or separation of organic high boiling impurities (having a boiling point higher than ε-caprolactam) from ε-caprolactam.

[0191] In a preferred embodiment of the present application, in step d)(ii) prior to the distillative removal, an alkali hydroxide, preferably NaOH, is added to the organic phase. Preferably, the amount of NaOH added is in the range of 0.5 to 150 mmol, more preferably and most preferably 2 to 80 mmol per kg of ε-caprolactam. This allows for an especially efficient distillative removal of impurities having a boiling point lower than and higher than ε-caprolactam in the subsequent distillation.

[0192] The purification of the crude ε-caprolactam in the purification section according to the present application typically comprises the following steps:

[0193] (i) extraction of the crude ε-caprolactam with an organic solvent, thereby obtaining an aqueous phase and an organic phase, and wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities;

[0194] (ii) optionally, conversion of the solvent by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam, and wherein the solvent conversion step (ii) is selected from one method based on a water back-extraction and one method based on a solvent exchange distillation, wherein the organic solvent is distilled off and water is charged; and

[0195] (iii) obtaining purified ε-caprolactam by distillative removal of impurities having a boiling point lower than or higher than ε-caprolactam.

[0196] In a preferred embodiment of the present application, the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified in the purification section, with the exception of the crude ε-caprolactam recovered in the recovery section. This has the advantage that by introducing recycled ε-caprolactam according to the present application, the carbon footprint of the plant for the renewed production of ε-caprolactam from cyclohexanone oxime can be reduced.

[0197] The high purity ε-caprolactam obtained according to the process of the present application can be used to manufacture Nylon 6 using methods well known to the skilled person. The Nylon 6 can subsequently be used in all known materials, including engineering materials, fibers and films.

[0198] Plant

[0199] The present application also provides a plant in which the process of the present application as described above can be carried out. Thus, all plant features specifically described below in connection with the plant also apply as specific embodiments of the process of the present application, and vice versa. That is, the plant is suitable for carrying out the process of the present application. Thus, it is to be understood that what has been described in connection with the process of the present application applies equally to the plant embodiments.

[0200] The plant can correspond to a laboratory setup in an example. Preferably, however, the plant is an industrial scale plant. By "industrial scale" it is meant that the plant has a production capacity of at least 500 tons / year of ε-caprolactam (i.e. in principle is able to produce this amount of ε-caprolactam) if operated continuously.

[0201] The plant of the present application is suitable for producing purified ε-caprolactam from a nylon 6 containing multi-component material and comprises at least the following four sections: (A) a nylon 6 pre-concentration section, (B) a depolymerization section, (C) a recovery section and (D) a purification section. Such sections and thus the plant are configured to carry out the method of the present application as described above.

[0202] In addition, the plant can comprise a mechanical size reduction section [O] to shred the nylon 6 containing multi-component material into pieces and / or a washing section [W] to wash the nylon 6 containing multi-component material. The mechanical size reduction section [O] comprises equipment for mechanically shredding the nylon 6 containing multi-component material into pieces. Non-limiting examples of such shredding equipment are a cutter, a shredder, a grinder, a mill or a chipper.

[0203] The nylon 6 pre-concentration section [A] comprises an extraction equipment, wherein the nylon 6 containing multi-component material is extracted with one or more organic solvents. The extraction equipment can have any desirable form, such as a mixer-settler extractor, an extraction column and / or a centrifugal extraction equipment, and optionally is operated under pressure. Preferably in the extraction equipment of the nylon 6 pre-concentration section, the pieces of the nylon 6 containing multi-component material are mixed with the organic solvent by means of a stirrer. The nylon 6 pre-concentration section can comprise further units, in particular units allowing for the storage, recovery and purification of the one or more organic solvents applied in this section, and for the collection and intermediate storage of the main non-nylon 6 containing by-products and the desired nylon 6 containing material pre-concentrated formed in this section.

[0204] The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or in parallel. The nylon 6 containing multi-component material is fed to the reactor in solid form or in melt form, preferably in melt form. This feeding can be achieved by using an extruder, a gear pump or other means known in the art.

[0205] During production, the depolymerization reactor is at least partially filled with the nylon 6 containing raw material, residual material, ε-caprolactam (and optionally catalyst). The depolymerization reactor can have any desirable form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors and extruder type reactors.

[0206] The depolymerization reactor must be equipped with means for feeding the multi-component material containing nylon 6, superheated steam and optionally catalyst. In addition, the depolymerization reactor must be equipped with means for discharging the stream of steam comprising ε-caprolactam and water and residual material.

[0207] Good contact between the steam and the reactor contents is essential for efficient operation. Such contact can be achieved by various means generally known in the art. As an example, the steam can be sparged into the material using a plurality of inlets, for example using a steam distributor. Improved contact can be achieved by including mechanical agitation in the reactor, for example using a combination of rotating paddles and static fins.

[0208] Typically, depolymerization will be completed in 0.5 to 6 hours.

[0209] If superheated steam at high temperature is not available at the production site, it must be produced ad hoc by superheating steam obtained from a boiler in a so-called superheater.

[0210] The recovery section [C] comprises one or more (partial) condensers into which a stream of steam comprising ε-caprolactam and water is charged. The (partial) condenser can have any desirable form. Preferably, the condenser is a distillation column from which a water-rich phase is obtained as top product and a ε-caprolactam-rich phase as bottom product.

[0211] The purification section [D] comprises one or more pieces of extraction equipment, optionally one or more solvent conversion equipment, optionally an oxidation section, optionally a hydrogenation section and one or more pieces of distillation equipment into which crude ε-caprolactam is charged and from which high purity ε-caprolactam is discharged.

[0212] The extraction equipment is selected from mixer-settler extractors, extraction columns, centrifugal extractors and combinations thereof. Preferably, the extraction equipment is a static or agitated extraction column, such as a column, a rotating disc contactor (RDC), a pulse column, a sieve tray (static) column, a random packed (static) column and a structured packing (SMVP) (static) column.

[0213] The solvent conversion equipment is selected from mixer-settler extractors, extraction columns, centrifugal extractors and combinations thereof. Preferably, the re-extraction equipment is a static or agitated extraction column, such as a column, Rotating disc contactor (RDC), pulse column, sieve tray (static) column, random packing (static) column and structured packing (static) column.

[0214] The solvent conversion apparatus for the process based on solvent swing distillation is preferably selected from the group consisting of sieve tray distillation column, random packing distillation column and structured packing distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser and a reflux apparatus. The distillation column can be operated at atmospheric pressure, sub-atmospheric pressure or super-atmospheric pressure. Preferably, water is charged to the upper part of the distillation column and a water phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is discharged from the lower part of the distillation column.

[0215] The oxidation section comprises one or more oxidation reactors operated in series and / or in parallel. An oxidizing agent and an ε-caprolactam water phase comprising water, ε-caprolactam and impurities are charged to the oxidation section. Typically, the oxidizing agent is charged in the form of an aqueous solution or in solid form. In case potassium permanganate or sodium permanganate is used as oxidizing agent, the oxidation section further comprises a filtration section. The oxidation reactors can have any desired form. Preferred reactor types are stirred and unstirred reactors and packed column type reactors. The oxidation reactors must be equipped with means for charging the water phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and the oxidizing agent. In addition, the oxidation reactors must be equipped with means for discharging the oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities and optionally formed manganese (IV) oxide (Mn02) solid particles. Preferably, the oxidation is carried out at a temperature in the range of 20°C to 85°C and under atmospheric conditions.

[0216] The optional manganese (IV) oxide (Mn02) solid particles can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to use filter aids, such as activated carbon particles, to improve the filtration procedure. Filter systems suitable for separating manganese (IV) oxide solid particles are known to the skilled person. The filter system is charged with a suspension of the oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities and manganese (IV) oxide solid particles and the filtered oxidized ε-caprolactam water phase comprising water, ε-caprolactam and impurities is discharged. Generally, the manganese (IV) oxide solid particles remain in the filter system. Preferably, the filter system is operated in a semi-continuous mode, wherein the suspension is continuously charged and the filtered phase is continuously discharged, while the separated solids are collected in the filter system. Occasionally, the suspension is charged intermittently and the collected solids are removed from the filter system.

[0217] The hydrogenation section comprises one or more hydrogenation reactors operated in series and / or in parallel. All types of reactors for hydrogenation in the presence of a solid catalyst can be employed. Preferably, the hydrogenation reactors are stirred tank reactors or fixed bed reactors.

[0218] A hydrogen-containing gas stream and an epsilon-caprolactam aqueous phase comprising water, epsilon-caprolactam and impurities are charged to the hydrogenation section. From the hydrogenation section, a hydrogenated epsilon-caprolactam aqueous phase comprising water, epsilon-caprolactam and impurities and optionally a hydrogen-containing flushing gas stream is discharged. Under normal operating conditions, the hydrogenation catalyst remains in the hydrogenation reactor and is only replaced after deactivation.

[0219] An epsilon-caprolactam aqueous phase comprising water, epsilon-caprolactam and impurities is charged to the distillation apparatus and high purity epsilon-caprolactam, water and impurities (i.e. low boiling point organic impurities (having a boiling point lower than epsilon-caprolactam) and high boiling point organic impurities (having a boiling point higher than epsilon-caprolactam)) are discharged. The distillation apparatus is selected from a sieve tray distillation column, a random packed distillation column, a structured packed distillation column and a horizontal and vertical (elevating) thin film evaporator. Preferably, the distillation column is equipped with a reboiler, a condenser and a reflux apparatus. The distillation apparatus can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure, preferably at subatmospheric pressure.

[0220] Preferably, the distillation comprises separating water, low boiling point organic impurities (having a boiling point lower than epsilon-caprolactam) and / or high boiling point organic impurities (having a boiling point higher than epsilon-caprolactam) from epsilon-caprolactam. Preferably, the distillation comprises in a first step separating water as top product and producing epsilon-caprolactam containing low boiling point impurities and high boiling point impurities as bottom product. In a second step, low boiling point impurities are separated as top product and epsilon-caprolactam containing low boiling point impurities is obtained as bottom product. In a third step, high purity epsilon-caprolactam is separated as top product and a distillation residue comprising epsilon-caprolactam and high boiling point impurities is produced as bottom product. Optionally, the first step and the second step are combined.

[0221] Preferably, prior to removal of water and impurities by distillation, an alkali metal hydroxide, preferably NaOH, is added to the oxidized epsilon-caprolactam aqueous phase comprising water, epsilon-caprolactam and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol per kg of epsilon-caprolactam and more preferably 2 to 80 mmol per kg of epsilon-caprolactam. This allows for an especially efficient distillation removal of impurities having a boiling point lower than and higher than epsilon-caprolactam in the subsequent distillation.

[0222] The process of the present application can be operated in continuous, semi-continuous or batch mode. Thus, the plant of the present application can also be configured to allow one or more of such modes of operation. In a preferred embodiment, the plant is configured to operate the process of the present application in continuous or semi-continuous mode. However, non-continuous processes are also possible. For example, the plant of the present application does not necessarily contain all the sections described herein at one location. In particular, the nylon 6 pre-concentration section can be located at a first location, while the depolymerization section, the recovery section and the purification section are located at a second location. Similarly, the mechanical size reduction section can also be located at a different site.

[0223] In a particularly advantageous embodiment of the present application, the plant can comprise as purification section a purification section shared with a plant for the synthesis of fresh epsilon-caprolactam, wherein the purification section is typically or actually used for purifying crude epsilon-caprolactam produced by other processes, such as by Beckmann rearrangement of cyclohexanone oxime. In this way, the present application provides the use of epsilon-caprolactam recovered from a multilayer material containing nylon 6 for reducing the carbon footprint of an epsilon-caprolactam production plant. This way of proceeding also has the advantage that highly sophisticated purification sections of existing plants for the production of epsilon-caprolactam by Beckmann rearrangement reaction can be used in the process of the present application. This is cost-effective, as no separate purification section needs to be built for the process and plant of the present application. Surprisingly, the recovered epsilon-caprolactam obtained from a multicomponent material containing nylon 6 by depolymerization can be mixed and purified together with crude epsilon-caprolactam produced by other processes, such as by Beckmann rearrangement of cyclohexanone oxime, without reducing the quality of the epsilon-caprolactam. In the view of the applicant, this is only possible by the special combination of the nylon 6 pre-concentration, depolymerization and recovery steps of the present application, which results in an exceptionally pure epsilon-caprolactam product, which can be mixed and purified together with epsilon-caprolactam produced by other processes, such as by Beckmann rearrangement, without a disadvantage in terms of quality of the resulting product.

[0224] Product

[0225] The process of the present application allows to produce epsilon-caprolactam of high purity and thus of high quality, which meets the specifications for high demanding applications and at the same time is particularly economically friendly due to its reduced product carbon footprint and the use of waste material as starting material. In a preferred embodiment, the epsilon-caprolactam obtained by the process of the present application meets one or more of the following specifications, wherein the parameters and measurement methods are defined in the example section below:

[0226] PAN: max. 5

[0227] E290: max. 0.05

[0228] VB: max. 0.5 mmol / kg

[0229] Basicity: max. 0.1 mmol / kg

[0230] Acidity: max. 0.1 mmol / kg

[0231] The ε-caprolactam produced by the process of the present application is particularly economic and environmentally friendly. Compared to ε-caprolactam produced conventionally (e.g. by Beckmann rearrangement of cyclohexanone oxime), the ε-caprolactam produced by the process of the present application has a significantly lower carbon footprint.

[0232] The environmental impact of a product is generally expressed as "product carbon footprint". The product carbon footprint is defined as the total emissions caused by the formation of that product, expressed as tons of carbon dioxide equivalent per ton of product in tons. The product carbon footprint is determined, inter alia, by raw materials, auxiliary materials, energy consumption, energy sources, production method and method efficiency. The quantification of the product carbon footprint can be carried out as described, for example, in European Standard EN ISO 14040:2006 (Environmental management - Life cycle assessment - Principles and framework).

[0233] The product carbon footprint calculation can be done by an internal or external (preferred) certified organization. Such organizations verify and certify product carbon footprint calculations based on, for example, the LCA standard ISO 14040.

[0234] J. Hong and X. Xu ("Environmental impact assessment of caprolactam production - a case study in China"; J. of Cleaner production 27 (2012) 103-108; DOI: 10.1016 / j.jclepro.2011.12.037) report that, in case of coal-based electricity and steam production, the potential impact on global warming of "raw" ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime is 7.5 tons CO2 equivalent per ton of ε-caprolactam. If natural gas-based electricity and steam production is involved, the potential impact on global warming of raw ε-caprolactam in the ε-caprolactam production process would decrease to 6.4 tons CO2 equivalent per ton of ε-caprolactam.

[0235] The product carbon footprint of the e-caprolactam obtained according to the process of the present application is much lower than in newly synthesized or "virgin" e-caprolactam. Preferably, the product carbon footprint of the e-caprolactam obtained according to the process of the present application is less than 4 tons CO2 equivalent per ton e-caprolactam, more preferably less than 3 tons CO2 equivalent per ton e-caprolactam, more preferably less than 2.5 tons CO2 equivalent per ton e-caprolactam, most preferably less than 2 tons CO2 equivalent per ton e-caprolactam (based on data derived from ecoinvent version 3.7.1 ; location: Europe). BRIEF DESCRIPTION OF DRAWINGS

[0236] In the following, the present application will be described with reference to the accompanying drawings, which depict certain embodiments of the present application. The present application, however, is not limited to the embodiments shown in the drawings for illustrative purposes. It should not be restricted to the embodiments shown in the following drawings.

[0237] Figure 1 Schematic representation of the process of the present application comprising the process steps performed in the nylon 6 pre-concentration section, the depolymerization section, the recovery section and the purification section.

[0238] Figure 2 represents three embodiments of the nylon 6 pre-concentration step a) of the process of the present application, wherein the nylon 6 containing multi-component material is extracted with an organic solvent to obtain a pre-treated nylon 6 containing phase enriched in nylon 6.

[0239] Figure 2a Embodiment of the nylon 6 pre-concentration step a) of the process of the present application is described, wherein non-nylon 6 compounds are preferentially extracted from the nylon 6 containing multi-component material to obtain a pre-concentrated nylon 6 containing material enriched in nylon 6.

[0240] Figure 2b Embodiment of the nylon 6 pre-concentration step a) of the process of the present application is described, wherein nylon 6 is preferentially extracted from the nylon 6 containing multi-component material to obtain a pre-concentrated nylon 6 containing material enriched in nylon 6.

[0241] Figure 2c Embodiment of the nylon 6 pre-concentration step a) of the process of the present application is described, wherein the extraction of non-nylon 6 compounds from the nylon 6 containing multi-component material is combined with the extraction of nylon 6 from the resulting nylon 6 containing phase, the latter being enriched in nylon 6 to result in a nylon 6 phase, thereby further enriching the nylon 6.

[0242] BRIEF DESCRIPTION OF DRAWINGS

[0243] The process of the present application is schematically shown in Figure 1 The process comprises the following sections:

[0244] - a nylon 6 pre-concentration section [A] in which a multi-component material [1] comprising nylon 6 is treated with a (organic) solvent [2] to obtain a pre-concentrated nylon 6 containing material [3] enriched in nylon 6 and a second phase [4] comprising non-nylon 6 compounds;

[0245] - a depolymerization section [B] in which the pre-treated nylon 6 enriched in nylon 6 [3] is depolymerized into ε-caprolactam, which is discharged as a ε-caprolactam containing product vapor stream [5]. In addition, residual material [6] is discharged. Superheated steam [7] and optionally a catalyst [8] are charged to the depolymerization section [B];

[0246] - a recovery section [C] in which crude ε-caprolactam [9] is recovered from the ε-caprolactam containing product vapor stream [5] discharged from the depolymerization section [B]; and

[0247] - a purification section [D] in which the crude ε-caprolactam [9] discharged from the recovery section [C] is purified to obtain high purity ε-caprolactam

[10] .

[0248] Figure 2a An embodiment of the extraction in a nylon 6 pre-concentration section [A'] is depicted, in which a multi-component material [1'] comprising nylon 6 is treated with a solvent [2'] that preferentially extracts non-nylon 6 compounds from the multi-component material [1'] comprising nylon 6 to obtain a pre-concentrated nylon 6 containing material [3'] enriched in nylon 6 and a second phase [4'] enriched in non-nylon 6 compounds.

[0249] Figure 2b An embodiment of the extraction in a nylon 6 pre-concentration section [A"] is depicted, in which a multi-component material [1"] comprising nylon 6 is treated with a solvent [2"] that preferentially extracts nylon 6 from the multi-component material [1"] comprising nylon 6 to obtain a pre-concentrated nylon 6 containing material [3"] enriched in nylon 6 and a second phase [4"] enriched in non-nylon 6 compounds.

[0250] Figure 2c An embodiment of the extraction in a first nylon 6 pre-concentration section [A'" ] is depicted, in which a multi-component material [1"'] comprising nylon 6 is treated with a solvent [2"'] that preferentially extracts non-nylon 6 compounds from the multi-component material [1"'] comprising nylon 6 to obtain a pre-treated nylon 6 containing phase [3"'] enriched in nylon 6 and a second phase [4"'] enriched in non-nylon 6 compounds. In a second nylon 6 pre-concentration section [A""], the pre-concentrated nylon 6 containing material [3"'] enriched in nylon 6 is treated with a second solvent [2"" ] that preferentially extracts nylon 6 from the pre-concentrated nylon 6 containing material [3"'] enriched in nylon 6 to obtain a pre-treated nylon 6 containing phase [3"" ] further enriched in nylon 6 and another phase [4"" ] enriched in non-nylon 6 compounds.

[0251] The application is illustrated by the following examples, but is not intended to be limited thereby.

[0252] Example

[0253] The starting material used in Examples 1 and 2 and 4 and in the comparative examples was a waste multilayer packaging film comprising polyethylene layers and nylon 6 layers. The nylon 6 layers were embedded in the waste multilayer packaging films used, i.e. sandwiched between other layers, and were therefore not easily accessible.

[0254] The nylon 6 pre-concentration step a) of the waste multilayer packaging film can be carried out as described in EP 0849312 or DE 102016015198, wherein polyethylene is selectively dissolved at elevated temperature in an organic solvent, such as petroleum solvent (Sigma-Aldrich; CAS No. 68551-17-7) or methylcyclohexane, whereby the resulting mixture comprises undissolved nylon 6 and an organic solution comprising the organic solvent and the dissolved polyethylene. The undissolved nylon 6 material is obtained after separation of the organic solution from the resulting mixture. The resulting undissolved nylon 6 material can be dried and optionally densified by melting under nitrogen and subsequent conversion into solid particles enriched in nylon 6 compared to the nylon 6 containing multicomponent starting material.

[0255] To determine the quality of the obtained ε-caprolactam, the following parameters were measured: the potassium permanganate absorption number (PAN: ISO 8660 - Plastics - Determination of permanganate absorption number of caprolactam - Spectometric method, second edition ISO 8660; 2002) of the ε-caprolactam was determined. In addition, the absorbance at a wavelength of 290 nm (E290: ISO 7059 - Caprolactam for industrial use - Determination of absorbance at a wavelength of 290 nm; 1982) was measured. Further, the volatile base content (volatile bases (VB): ISO 8661 - Caprolactam for industrial use - Determination of volatile bases content - Titrimetric method after distillation, 1988) was determined. Finally, the water content (water (W): ISO 7058 - Caprolactam for industrial use - Determination of water content - Karl-Fischer method; 1982) was determined. 乙醇 % methylene blue: 0.1 wt. / v 乙醇Titrated in % methyl red to determine alkalinity or acidity, which is grey at the end point. First the flask containing water and indicator is titrated to grey, then X grams of an aqueous ε-caprolactam solution containing Y wt.% ε-caprolactam (as determined by refractive index) is added and the solution is titrated back to grey using a 0.01 N H2SO4 solution (in case the solution is basic) or a 0.01 N NaOH solution (in case the solution is acidic).

[0256] The alkalinity is then given by:

[0257] Alkalinity (mmol / Kg ε-caprolactam) = v * t * 1000 / (X * Y)

[0258] where:

[0259] v = volume of H2SO4 solution added (ml)

[0260] t = equivalent concentration of H2SO4 solution (= 0.01 N)

[0261] X = sample weight (g)

[0262] Y = ε-caprolactam concentration (wt. %)

[0263] The acidity is then given by:

[0264] Acidity (mmol / Kg ε-caprolactam) = v * t * 1000 / (X * Y)

[0265] where:

[0266] v = volume of NaOH solution added (ml)

[0267] t = equivalent concentration of NaOH solution (= 0.01 N)

[0268] X = sample weight (g)

[0269] Y = ε-caprolactam concentration (wt. %)

[0270] The ε-caprolactam which can be used for all major polymerization applications without dilution by more pure quality ε-caprolactam meets all the following specifications:

[0271] PAN: max. 5

[0272] E290: max. 0.05

[0273] VB: max. 0.5 mmol / kg

[0274] Alkalinity: max. 0.1 mmol / kg

[0275] Acidity: max. 0.1 mmol / kg

[0276] Example 1

[0277] Depolymerization of nylon 6 and recovery of ε-caprolactam.

[0278] The material used for depolymerization and other processing steps as described below can be any multi-component material containing nylon 6 or a derivative thereof. The nylon 6 of the specific material used in the following example is enriched compared to the multi-component starting material containing nylon 6 and was prepared from a waste multi-layer packaging film as described above.

[0279] The starting material enriched in nylon 6 was shaped into pearl-like solid particles (diameter: 3 to 4 mm). The polyethylene content in the undissolved nylon 6 material was about 1 wt.%, as determined by TGA (Thermogravimetric Analysis).

[0280] In step b), 33.6 g of the pearl-like solid particles and 9.8 grams of 20 wt.% phosphoric acid were charged into a Premex autoclave. First, the reactor contents were heated under nitrogen, then superheated steam was continuously injected at a rate of 4 grams per minute over a 120 minute reaction period. The temperature and pressure in the reactor were maintained at 260 °C and 0.11 MPa, respectively. During the reaction, a stream of steam was continuously removed from the reactor.

[0281] In step c), to recover crude ε-caprolactam from the obtained steam, the steam was cooled to 20 °C. The condensate consisting of an average of 28.3 grams of ε-caprolactam, the rest being mostly water, was concentrated to an ε-caprolactam concentration of an average of 49.6 wt.% by evaporation in a rotary evaporator running under vacuum (9.5 kPa; water bath temperature of about 65 °C). (The resulting mixture crude ε-caprolactam is the mixture to be purified.)

[0282] Steps b) and c) were repeated five times. The five mixtures were combined and the resulting solution was used as stock solution in the following examples and comparative experiments.

[0283] This example shows that crude ε-caprolactam can be obtained from discarded nylon 6 waste multi-layer packaging film by depolymerization in good yield and without running problems.

[0284] Example 2

[0285] Purification by extraction, caustic wash, re-extraction, oxidation and distillation.

[0286] In step d)(i), 70 g of the crude ε-caprolactam obtained in Example 1 was extracted once with 100 g of the solvent mixture MIBC / cyclohexane (50 wt.%:50 wt.%) at 25°C and nine times with 50 g of this solvent mixture. The solvent mixtures containing the ε-caprolactam phase obtained in the ten extractions were combined and washed with 7 g of aqueous caustic (2 wt.%). Subsequently, the washed ε-caprolactam-containing solvent mixture was extracted six times with 50 g of water at 25°C. The ε-caprolactam water phases obtained in the six extractions were combined. The ε-caprolactam water phase was distilled off from the water and residual solvent mixture, whereby a concentrated ε-caprolactam solution with a water content of 50 wt.% was obtained. The resulting mixture was treated with 0.2 wt.% KMnO4 relative to ε-caprolactam at 50°C for 2 hours. Subsequently, the solids formed were removed from the oxidized product by means of filtration. Subsequently, 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the resulting ε-caprolactam aqueous solution.

[0287] In step d)(ii), a distillative removal of water and impurities with a boiling point lower or higher than ε-caprolactam was performed. Subsequently, water and impurities with a boiling point lower than ε-caprolactam were removed as top product by distillation under reduced pressure in a batch operated distillation apparatus. Finally, purified ε-caprolactam was recovered as top product at 300 Pa, while impurities with a boiling point higher than ε-caprolactam were retained in the distillation apparatus as bottom product. The specifications of the purified ε-caprolactam were:

[0288] PAN: 5

[0289] E290: 0.023

[0290] VB: 0.167

[0291] Basicity: 0.044

[0292] From this example, it can be concluded that purified ε-caprolactam, which meets the required specifications for major polymerization applications, can be obtained from depolymerized waste multilayer packaging film flakes by removing polyethylene from such flakes by extraction and purification by extraction with an organic solvent, oxidation and distillation.

[0293] Comparative Experiment 1

[0294] Purification by distillation.

[0295] An aqueous solution of 0.75 mmol of sodium hydroxide per kg of ε-caprolactam was added to 35 g of the crude ε-caprolactam obtained in Example 1. Subsequently, this mixture was distilled according to the procedure described in Example 2. The specifications of the purified ε-caprolactam were:

[0296] PAN: 73.5

[0297] E290: 0.531

[0298] VB: 0.718

[0299] Acidity: 7.643

[0300] This comparative experiment shows that the quality of ε-caprolactam obtained from waste multilayer packaging film chips by depolymerization removing polyethylene by extraction and purified by oxidation and distillation is extremely poor because it does not meet any of the required specifications in major polymerization applications.

[0301] Comparative Experiment 2

[0302] Purification by oxidation and distillation.

[0303] The 35 g of crude ε-caprolactam obtained in Example 1 was treated with 0.2 wt.% KMn04relative to ε-caprolactam at 50 °C for 2 hours. The formed solid was then removed from the oxidized reaction product by filtration. Then 0.75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the resulting ε-caprolactam aqueous solution. This mixture was then distilled according to the procedure described in Example 2. The specifications of the purified ε-caprolactam were:

[0304] PAN: 11.7

[0305] E290: 0.324

[0306] VB: 0.657

[0307] Acidity: 2.991

[0308] This comparative experiment shows that the quality of ε-caprolactam obtained from waste multilayer packaging film chips by depolymerization removing polyethylene by extraction and purified by oxidation and distillation is extremely poor because it does not meet any of the required specifications in major polymerization applications.

[0309] Comparative Experiment 3

[0310] Purification by extraction, caustic wash and re-extraction.

[0311] The 70 g of crude ε-caprolactam obtained in Example 1 was extracted once with 100 g of solvent mixture MIBC / cyclohexane (50 wt.%: 50 wt.%) at 50 °C and nine times with 50 g of this solvent mixture. The ε-caprolactam containing solvent mixtures obtained in the ten extractions were combined and washed with 7 g of aqueous caustic (2 wt.%). Then, the washed ε-caprolactam containing solvent mixture was re-extracted six times with 50 g of water at 25 °C. The ε-caprolactam aqueous phases obtained in the six re-extractions were combined. The ε-caprolactam aqueous phase was distilled off from the water and residual solvent mixture, thereby obtaining a concentrated ε-caprolactam solution with an ε-caprolactam content of 50.4 wt.%. The specifications of the purified ε-caprolactam were:

[0312] PAN: 82

[0313] E290: 1.2

[0314] This comparative experiment shows that the quality of the epsilon-caprolactam obtained from waste multilayer packaging film flakes where the polyethylene was removed by extraction and the waste multilayer packaging film flakes were purified by extraction and re-extraction is extremely poor because it does not meet all the required specifications in major polymerization applications.

[0315] Comparative experiment 4

[0316] purified by extraction, caustic wash, re-extraction and oxidation.

[0317] A concentrated epsilon-caprolactam solution with an epsilon-caprolactam content of 50.4 wt.% obtained in comparative experiment 3 was treated with 0.04 wt.% KMn04relative to epsilon-caprolactam at 50°C for 2 hours. Subsequently, the formed solids were removed from the oxidized reaction product by means of filtration. The specifications of the purified epsilon-caprolactam were:

[0318] PAN: 83

[0319] E290: 3.01

[0320] This comparative experiment shows that the quality of the epsilon-caprolactam obtained from waste multilayer packaging film flakes where the polyethylene was removed by extraction and the waste multilayer packaging film flakes were purified by extraction, re-extraction and oxidation is extremely poor because it does not meet all the required specifications in major polymerization applications.

[0321] Example 3

[0322] Calculating the product carbon footprint of the purified epsilon-caprolactam

[0323] A continuous process according to the application for producing purified epsilon-caprolactam from waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer was simulated. Such waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer had a nylon 6 content of 20 wt.%.

[0324] The process comprises:

[0325] - cutting the waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer into small pieces;

[0326] - washing the small pieces of waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer with water;

[0327] - drying the washed small pieces of waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer by centrifugation;

[0328] - extracting the polyethylene with a petroleum solvent;

[0329] - separating the undissolved solid pre-concentrated nylon 6 containing material by centrifugation;

[0330] - washing the dissolved solid pre-concentrated nylon 6 containing material with water;

[0331] - separating the washed undissolved solid pre-concentrated nylon 6 containing material and the aqueous extract by centrifugation;

[0332] - decolorizing the polyethylene containing petroleum solvent solution by treatment with activated carbon;

[0333] - recovering polyethylene from the polyethylene containing petroleum solvent solution by cooling and partial evaporation of the petroleum solvent;

[0334] - melting and pelletizing the recovered polyethylene;

[0335] - melting and pelletizing the washed undissolved solid pre-concentrated nylon 6 containing material;

[0336] - depolymerizing the nylon 6 in the pelletized nylon 6 containing material under the influence of H3PO4 and superheated steam;

[0337] - recovering crude ε-caprolactam by partial condensation of the steam exiting the depolymerization reactor;

[0338] - evaporative concentration of the crude ε-caprolactam to 80 wt.% ε-caprolactam;

[0339] - countercurrent extraction of the concentrated crude ε-caprolactam with benzene;

[0340] - washing the organic extract with a diluted caustic solution;

[0341] - countercurrent extraction of the washed organic extract with water;

[0342] - evaporative concentration of the aqueous extract;

[0343] - oxidation of the concentrated extract with KMnO4 and subsequent filtration of the oxidized reaction product to remove the formed solids;

[0344] - addition of caustic;

[0345] - recovery of pure ε-caprolactam by vacuum distillation.

[0346] The main products of this process are polyethylene and pure ε-caprolactam. The recovery yield of pure ε-caprolactam is about 0.8 kg pure ε-caprolactam per kg of nylon 6 in the waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer. The by-products obtained are valorized by energy recovery by incineration.

[0347] The product carbon footprint of purified ε-caprolactam was calculated based on the consumption values of raw materials, and the utility of the method described above is based on data derived from ecoinvent version 3.7.1. The distribution of environmental impacts between the product purified ε-caprolactam and polyethylene in the Nylon 6 pre-concentration section is based on the weight ratio of such products.

[0348] The results show that the product carbon footprint of purified ε-caprolactam obtained from a multilayer packaging film comprising a polyethylene layer and a Nylon 6 layer is less than 2 tons CO2 equivalent per ton ε-caprolactam (location: Europe).

[0349] Example 4

[0350] Depolymerization of Nylon 6 and recovery of ε-caprolactam.

[0351] The Nylon 6 material used for depolymerization and other processing steps as described below is enriched compared to the Nylon 6 containing multi-component starting material, and it is prepared from waste multilayer packaging film as described above.

[0352] The Nylon 6 enriched starting material is shaped into pearl-like solid particles (diameter: 3 to 4 mm). The polyethylene content in the undissolved Nylon 6 material is about 1 wt.%, as determined by TGA (Thermogravimetric Analysis).

[0353] In step b), 33.6 g of the pearl-like solid particles and 9.8 grams of 20 wt.% phosphoric acid are charged into the Premex autoclave. First, the reactor contents are heated under nitrogen, then superheated steam is continuously injected at a rate of 2.7 grams per minute over a reaction period of 135 minutes. The temperature and pressure in the reactor are maintained at 260 °C and 0.11 MPa, respectively. During the reaction, a stream of steam is continuously withdrawn from the reactor.

[0354] In step c), to recover crude ε-caprolactam from the obtained steam, the steam is cooled to 20 °C. The condensate consists of 26.1 grams of ε-caprolactam, the rest being mostly water.

[0355] Steps b) and c) are repeated by following the same procedure, except that now 9.7 grams of 20 wt.% phosphoric acid are charged into the Premex autoclave and the obtained condensate consists of 26.9 grams of ε-caprolactam.

[0356] The two ε-caprolactam containing condensates are concentrated by evaporation in a rotary evaporator (rotary evaporator) running under vacuum (9.5 kPa; water bath temperature of about 65 °C), and thereafter combined to an ε-caprolactam concentration of 79.6 wt.%. The resulting stock solution of crude ε-caprolactam is the mixture to be purified (see Example 5).

[0357] The specifications of the crude ε-caprolactam are:

[0358] PAN: 280

[0359] E290: 2.14

[0360] This example shows that crude ε-caprolactam can be obtained from depolymerized nylon 6 derived from discarded nylon 6 waste multilayer packaging film in good yield and without running problems.

[0361] Example 5

[0362] Purification by extraction, re-extraction, oxidation and distillation.

[0363] In step d)(i) the 43.4 grams of crude ε-caprolactam obtained in example 4 were diluted to 68.9 wt.% by adding 6.7 grams of water, the obtained solution was extracted once with 77.5 grams of benzene at 25 °C and four times with 50 grams of benzene. The resulting organic extracts were combined and concentrated to an ε-caprolactam concentration of about 24.5 wt.% by evaporation in a rotary evaporator running under vacuum (9.5 kPa; water bath temperature of about 65 °C). Subsequently, the concentrated extract containing ε-caprolactam was portion-wise extracted three times with 25 grams of water at a temperature of about 25 °C. The three resulting aqueous ε-caprolactam phases were combined. The aqueous ε-caprolactam phase was distilled off from the water and residual solvent mixture, thereby obtaining a concentrated ε-caprolactam solution with a water content of 49.5 wt.%. The specifications of the aqueous ε-caprolactam solution after re-extraction were:

[0364] PAN: 83

[0365] E290: 1.15

[0366] The resulting mixture was treated with 0.2 wt.% KMn04 relative to ε-caprolactam at 50 °C for 2 hours. Subsequently the formed solids were removed from the oxidized product by means of filtration. Subsequently 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the oxidized aqueous ε-caprolactam solution. Subsequently this mixture was distilled according to the procedure described in example 2. The specifications of the purified ε-caprolactam were:

[0367] PAN: <5

[0368] E290: <0.05

[0369] VB: <0.5 mmol / kg

[0370] Basicity: <0.1 mmol / kg

[0371] Acidity: <0.1 mmol / kg

[0372] According to this embodiment, purified ε-caprolactam, which complies with the required specifications for the main polymerization application, can be obtained from depolymerized waste multilayer packaging film flakes, by removing the polyethylene from such flakes by extraction and purification by extraction with benzene, re-extraction with water, oxidation and distillation.

[0373] While the application has been described by way of several preferred embodiments, it will be recognized by those skilled in the art that changes, including modifications, can be made within the scope and spirit of the application, including modifications to convert to a continuous mode of operation, and improvements. Therefore, it is intended that the application not be limited to the particular embodiments described, but that the application will include all embodiments falling within the scope of the appended claims.

Claims

1. A method for recovering purified ε-caprolactam derived from multi-component materials containing nylon 6 in an industrial-scale plant, said plant having an ε-caprolactam production capacity of at least 500 tons / year under continuous operation, said plant comprising: The nylon 6 depolymerization section comprises one or more depolymerization reactors operating in series and / or parallel. The recycling section includes one or more condensers, and The purification section includes one or more extraction devices, one or more solvent conversion devices, and one or more distillation devices. And the method described therein includes the following steps: a) Providing a solid pre-concentrated nylon 6 material obtained by extraction with one or more organic solvents from a pre-concentrated nylon 6-containing multi-component material, which is enriched in nylon 6 compared to the pre-concentrated nylon 6-containing multi-component material; b) Depolymerize the pre-concentrated nylon 6-containing material in the depolymerization zone in the presence of water to obtain a vapor stream containing water and ε-caprolactam in a weight ratio of 2:1 to 15:

1. c) Recover crude ε-caprolactam from the steam stream in the recovery section; and d) Purify the crude ε-caprolactam in the purification section to obtain purified ε-caprolactam, wherein The purification process includes the following steps: (i) The crude ε-caprolactam is extracted with an organic solvent to obtain an aqueous phase and an organic phase, wherein the organic phase contains the organic solvent, ε-caprolactam and impurities; (ii) The solvent is converted by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, wherein said solvent conversion step (ii) is selected from... A method based on water back-extraction and a method based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; as well as (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation; After step d)(i), the organic phase obtained in step d)(i) is washed with water or with an alkaline aqueous solution; Step d)(ii) is then oxidized by an oxidant selected from potassium permanganate, sodium permanganate and hydrogen peroxide; Prior to the distillation removal in step d)(iii), an alkali metal hydroxide is added; The multi-component material containing nylon 6 is a multi-layer packaging film, which is a sheet with a thickness of less than 1 mm, and contains at least one layer containing nylon 6 or composed of nylon 6 and at least one layer not containing nylon 6, wherein the at least one layer containing nylon 6 or composed of nylon 6 is sandwiched between two or more layers not containing nylon 6.

2. The method of claim 1, wherein the weight fraction of nylon 6 in the multi-component material containing nylon 6 is in the range of 1 wt.% to 75 wt.% based on the total weight of the multi-component material containing nylon 6.

3. The method of claim 1, wherein the weight fraction of nylon 6 in the multi-component material containing nylon 6 is in the range of 1 wt.% to 60 wt.% based on the total weight of the multi-component material containing nylon 6.

4. The method of claim 1, wherein the weight fraction of nylon 6 in the multi-component material containing nylon 6 is in the range of 2 wt.% to 35 wt.% based on the total weight of the multi-component material containing nylon 6.

5. The method of claim 1, wherein the weight fraction of nylon 6 in the multi-component material containing nylon 6 is in the range of 3 wt.% to 25 wt.% based on the total weight of the multi-component material containing nylon 6.

6. The method of any one of claims 1 to 5, wherein the extraction in step a) comprises the following steps: (i) Adding one or more organic solvents to the multi-component material containing nylon 6; (ii) Perform phase separation to obtain a liquid extract phase and at least a partial solid phase, the liquid extract phase comprising a solvent and a dissolved component from the multi-component material containing nylon 6, the at least a partial solid phase comprising an undissolved component of the multi-component material containing nylon 6 and an optional solvent; (iii) Remove the solvent from the liquid extraction phase, and if present therein, also remove the solvent from the at least part of the solid phase to obtain two solid phases, one of which is the pre-concentrated nylon 6-containing material, which is enriched in nylon 6 compared to the multi-component nylon 6-containing material used as a starting material.

7. The method of any one of claims 1 to 5, wherein the organic solvent used for extraction in step a) preferentially dissolves non-nylon 6 compounds from the multi-component material containing nylon 6, and / or the organic solvent used in step a) preferentially dissolves nylon 6 from the multi-component material containing nylon 6.

8. The method of any one of claims 1 to 5, wherein (i) The multi-component material containing nylon 6 has been pretreated prior to the extraction in step a); and / or (ii) wherein 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) wherein the water present in step b) is in the form of steam.

9. The method of any one of claims 1 to 5, wherein (i) Prior to the extraction in step a), the multi-component material containing nylon 6 has been subjected to washing and / or mechanical dimensional reduction steps; and / or (ii) wherein the depolymerization of the pre-concentrated nylon 6-containing material in step b) is carried out at a temperature in the range of 200°C to 350°C; and / or (iii) wherein the water present in step b) is in the form of steam, which is superheated steam having a temperature in the range of 220°C to 575°C, and is introduced into the depolymerization section in step b).

10. The method of claim 8, wherein (ii) The depolymerization of the pre-concentrated nylon 6-containing material in step b) is carried out at a temperature in the range of 220°C to 340°C; and / or (iii) wherein the water present in step b) is introduced into the depolymerization section in step b) in the form of steam, which is superheated steam having a temperature in the range of 275°C to 500°C.

11. The method of any one of claims 1 to 5, wherein the depolymerization of the pre-concentrated nylon 6-containing material in step b) is carried out in the absence or presence of a catalyst, wherein the catalyst is selected from acid catalysts and base catalysts, the acid catalyst being selected from the group consisting of: phosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, salts of the aforementioned acids, Al2O3 and SiO2 and combinations thereof; and the base catalyst being selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof.

12. The method of claim 11, wherein the acid catalyst is orthophosphoric acid; and the base catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

13. The method of any one of claims 1 to 5, wherein NaOH is added before the distillation removal in step d)(iii).

14. The method of any one of claims 1 to 5, wherein prior to the distillation removal in step d)(ii), an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is obtained by: The organic solvent is at least partially replaced with water, and the solvent replacement step is selected from a method based on water-based re-extraction and a method based on solvent replacement distillation, wherein the organic solvent is distilled off and water is added; and Optionally, it may then be treated with an acidic cation exchange resin and / or a basic anion exchange resin; and / or Optionally, it is then hydrogenated in the presence of a hydrogenation catalyst selected from Razor nickel, nickel / silica, nickel / alumina, ruthenium / alumina, rhodium / alumina, platinum / carbon, and palladium / carbon.

15. The method of any one of claims 1 to 5, wherein in the purification section used in step d), the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is purified together with the crude ε-caprolactam recovered in step c).

Images