Method for recovering epsilon-caprolactam from multi-component material containing nylon 6

Abstract

The present invention provides a process for recovering purified epsilon-caprolactam from a nylon 6-containing multi-component material, comprising the steps of a) providing a pre-concentrated nylon 6-containing material by extraction, b) depolymerizing, c) recovering and d) purifying comprising extraction, back extraction, distillation and crystallization.

Description

Technical Field

[0001] The present invention relates to a method for recovering ε-caprolactam from a material derived from a multicomponent material containing nylon 6, in particular a multilayer film. More particularly, the present invention relates to a method for recovering ε-caprolactam from a material derived from a multicomponent material containing nylon 6, thereby obtaining high-quality ε-caprolactam. Further, the present invention relates to a plant configured to implement the method of the present invention and to the recovered ε-caprolactam obtainable by the method of the present invention. Background Art

[0002] In 1938, Paul Schlack invented nylon 6 (CAS No. 25038-54-4), also known as polyamide 6, PA6, N6, polycaprolactam, poly(hexane-6-lactam), poly(6-aminocaproic acid), poly(hexamethylene adipamide) or poly[imino(1-oxohexane-1,6-diyl)].

[0003] Typically, nylon 6 is synthesized by ring-opening polymerization of ε-caprolactam at a temperature of about 260°C in an inert atmosphere:

[0004]

[0005] Methods for producing native ε-caprolactam are described, for example, in the chapter "Caprolactam" in Ullmann's Encyclopedia of Industrial Chemistry (May 25, 2018), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, which is available in electronic form at https: / / doi.org / 10.1002 / 14356007.a05_031.pub3.

[0006] Processes for producing nylon 6 are described, for example, in the chapter “Polyamides” in Ullmann’s Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Verlag, Weinheim, Germany, which is available in electronic form at https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0007] Today, nylon 6 is the synthetic polyamide with the largest production volume in the world and has various favorable industrial application properties, including high mechanical strength, especially stiffness, hardness and toughness, good electrical insulation properties, good dyeability, good processability, excellent UV protection, good fatigue resistance and good barrier function to gases, aromas, hydrocarbon solvents and low-polarity aromatic substances from food. This series of favorable properties is the reason why nylon 6 is widely used in different industries such as building and construction, consumer goods, electronics and electrical, transportation and packaging. However, the disadvantage of using nylon 6 is that it is more expensive than polyolefins (polyethylene and polypropylene). In turn, the disadvantages of polyolefins are that their industrial applicability is more limited due to their poor gas barrier function, low temperature resistance and difficult bonding properties.

[0008] Therefore, nylon 6 is widely used as a component in multi-component materials, in most cases in combination with polyolefins. Particularly relevant multi-component materials containing nylon 6 are, for example, flexible modified atmosphere or controlled atmosphere packaging films, which are often used to maintain a specific atmosphere within a food-containing package to increase the shelf life of the packaged product. Most packaging films today are multilayer films, which typically include two to 15 layers, but may also include more layers. An example of an 11-layer coextruded cast film is as follows: nylon 6 / TIE / PE(1) / TIE / nylon 6 / EVOH / PA / TIE / PE(2) / PE(3) / PE(4), TIE being the tie layer, EVOH being ethylene-vinyl alcohol, PA being a polyamide other than nylon 6, and PE(1), PE(2), PE(3) and PE(4) being different grades of polyethylene. The typical purpose of using nylon 6 in such multi-component materials is to impart mechanical strength and puncture resistance and / or barrier properties to limit gas exchange and aroma loss. Multilayer films containing nylon 6 in particular allow maintaining an optimized modified atmosphere inside the package, in particular a certain oxygen concentration, which is key, for example, to maintain the freshness of fresh products over a longer period of time. The use of multilayer films can enable cost reductions, since cheaper polymers can partially replace more expensive polymers, such as nylon 6. Nylon 6 is also an excellent substitute for aluminum and other metal films.

[0009] Multicomponent materials comprising more than one (polymer) component are attractive for industrial applications, since, for example, the alternating layering of two (or more) different components is a way to obtain composite materials with new properties compared to the pure components. The combination of several layers of different materials 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 it is possible to continuously monitor how the properties of multicomponent materials change when changing the components employed and the thickness of the constituent layers employed, new composite materials with desired properties can be developed relatively easily.

[0010] In multi-component materials, especially in multi-layer films, polyolefin layers are often combined with layers consisting of polar polymers, such as nylon 6, nylon 66, EVOH (ethylene-vinyl alcohol) or PVDC (polyvinylidene chloride). In addition, a tie layer comprising a special adhesive polymer or a tie resin (e.g., polyurethane and acid / anhydride grafted polyolefin) is often required as an intermediate layer between two different components that are poorly bonded. As explained above, the use of nylon 6 in multi-component materials has become very common in order to improve the mechanical and gas barrier properties of such materials. Another important reason why nylon 6 is often combined with other materials is the cost savings of saving the rather expensive nylon 6.

[0011] Environmental concerns regarding the production and use of these multi-component materials containing nylon 6 relate to the waste generated during the process and after consumption. These issues can be mitigated by recycling individual components (such as nylon 6) from multi-component materials that are no longer used or are disposed of. Multi-layer films containing nylon 6 are a particularly relevant waste source as packaging materials. These typically contain significant amounts of nylon 6. Therefore, if there were a feasible method to recover nylon 6 from these composite wastes, this would not only benefit the environment but also provide an economically valuable new source of nylon 6.

[0012] The same applies to processes using nylon 6, especially process waste generated during membrane production. For a limited number of applications, this waste can be recovered and used again when regrinded. However, the optical and mechanical properties of the material are negatively affected by regrinding, and further degradation occurs each time the cycle of re-use and processing is repeated. Therefore, the cycle of re-use and processing cannot be repeated as frequently as desired.

[0013] In addition, nylon 6 recycled according to existing methods is generally not rated as "food grade plastic", which greatly limits its application.

[0014] Mechanical recycling (also known as material recycling or return to plastic recycling) refers to operations aimed at recovering plastics through mechanical processes (grinding, washing, separation, drying, regranulation and compounding) to produce recyclates that can be transformed into plastic products in order to replace virgin plastics. Currently, most virgin plastics are produced from petrochemical feedstocks such as natural gas, coal or crude oil that have never been used or processed before. During mechanical recycling, the polymer chains remain more or less intact. Mechanical recycling is a form of waste downcycling, as the recycled material is of lower quality and functionality than the virgin material.

[0015] Depolymerization or chemical recycling is a technology that converts a polymer into its monomer components (in the case of nylon 6, ε-caprolactam). The specifications of the recycled monomer determine whether it can replace virgin monomer in all applications or only in limited quantities. Virgin monomer is produced from petrochemical feedstocks such as natural gas, coal or crude oil that have never been used or processed before.

[0016] In addition to mechanical recycling of materials containing nylon 6, processes have also been developed for depolymerizing nylon 6 to its valuable monomer ε-caprolactam. These typically use nylon 6 plastics as starting material, which are produced, for example, by injection molding, extrusion, spinning or other processing steps.

[0017] The depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam:

[0018]

[0019] Processes for depolymerizing nylon 6 are known. Such processes can be operated in a batch mode of operation, in a semi-continuous mode (typically nylon 6 is (re)charged batchwise into the depolymerization reactor) or in a continuous mode.

[0020] LA Dmitrieva, AA Speranskii, S A Krasavin and Y N Bichkov, "Regeneration of ε-Caprolactam From Wastes In the Manufacture of Polycaproamide Fibres and Yarns", Fibre Chemistry, pp. 229-241, March 1986 (translated from Khimicheskie Volokna, No. 4, pp. 5-12, July-August 1985) is a literature review that describes processes for depolymerizing nylon 6 with and without catalysts.

[0021] AAO Gale, "Depolymerization of Nylon 6: Some Kinetic Modeling Aspects," Journal of Applied Polymer Science, Vol. 29, 1984, pp. 3947-3954, is a paper describing the depolymerization kinetics of nylon 6, which is available electronically at https: / / doi.org / 10.1002 / app.1984.070291227.

[0022] In general, the prior art chemical regeneration or recycling process for depolymerizing substantially pure nylon 6 to ε-caprolactam monomers comprises a hydrolytic degradation step at elevated temperature in the presence of water and a step of recovering the monomers formed by steam distillation. The hydrolytic degradation step can be carried out in the presence or absence of a catalyst. As catalysts, both acidic compounds such as phosphoric acid and boric acid and alkaline compounds such as potassium carbonate and sodium hydroxide can be used.

[0023] US5929234 describes a process for recovering ε-caprolactam from waste containing polycaprolactam. The depolymerization is carried out without the addition of a catalyst with superheated steam at a temperature of about 250°C to about 400°C and a pressure in the range of about 1 atm to about 100 atm and substantially below the saturated vapor pressure of water at a temperature at which a vapor stream containing ε-caprolactam is formed.

[0024] The recovery of ε-caprolactam generally requires substantially pure nylon 6 as a raw material. Therefore, methods for recovering ε-caprolactam by recycling the extract in nylon 6 polymerization plants, recycling nylon 6 waste (i.e. nylon 6 polymer substantially free of non-nylon 6 materials) or products made from nearly pure nylon 6 (e.g. fishing nets) have been known for a long time.

[0025] For example, DE4211609 discloses a method for recovering ε-caprolactam by thermal cracking of polyamide 6 at 250-320°C in the presence of a small amount of potassium carbonate and a method for purifying the recovered ε-caprolactam. DE4211609 describes that the method is applicable not only to pure polyamide 6 products, but also to polyamide 6 products containing glass fibers and containing fillers and products containing impact modifiers. "Polyamide 6" used in DE4211609 refers to pure polyamide 6 as well as polyamides, preferably based on polyamide 6 to more than 80% by weight.

[0026] CN103467378 discloses a method for producing ε-caprolactam by using recycled waste nylon yarns and scraps, wherein phosphoric acid is used as a depolymerization catalyst. During the depolymerization process, superheated steam is charged into the reaction system.

[0027] US5869654 discloses a method for recovering ε-caprolactam from polycaprolactam processing waste, wherein the polycaprolactam may be selected from waste nylon 6 polymer and / or oligomer materials, such as yarn waste, chip waste or extruder slag. The method comprises the steps of contacting the polycaprolactam waste with superheated steam having a temperature of about 250°C to about 400°C and a pressure in the range of about 1.5 atm to about 100 atm and substantially below the saturated vapor pressure of water at a temperature where a vapor stream containing ε-caprolactam is formed without adding a catalyst.

[0028] JP2000038471 discloses a method for depolymerizing nylon 6 resin from a molded body to obtain ε-caprolactam, particularly depolymerizing automotive resin parts such as nylon 6 fastener parts, electrical parts, body mechanism parts and exterior parts in which nylon 6 accounts for 90% by weight or more of the total organic compounds in the molded body.

[0029] Although processes have been developed for depolymerizing nearly pure waste nylon 6 to its valuable monomer ε-caprolactam, the recovery of ε-caprolactam from multi-component materials containing nylon 6, especially materials with moderate to high nylon 6 content (e.g., 40 wt.% to 80 wt.%), remains a great challenge. The recovery of ε-caprolactam from multi-component materials containing nylon 6 with low nylon 6 content (e.g., less than 40 wt.%) is even more difficult. Products containing nylon 6 with moderate to high nylon 6 content are, for example, carpets. High-quality carpets typically include up to 55 wt.% of face fibers containing nylon 6. The remainder is backing (support) material.

[0030] Based on existing methods, the recovery of ε-caprolactam from nylon 6 containing materials having low and even moderate to high nylon 6 content is not commercially feasible or successful.

[0031] The most frequently reported problem in connection with the recovery of ε-caprolactam from nylon 6-containing materials with a moderate to high nylon 6 content is the low recovery. This low recovery is due, on the one hand, to the difficulty in separating the ε-caprolactam formed from the non-nylon 6 material. On the other hand, the low recovery is due to the decomposition reaction leading to the production of ammonia and to the inhomogeneity of the depolymerization mixture, since effective mixing is prevented due to the high viscosity of the depolymerization mixture, resulting in local overheating and high consumption values ​​of expensive superheated steam (used as stripping agent).

[0032] The introduction of a mechanical pretreatment step (e.g., scraping the face fibers or grinding from the carpet backing followed by density separation, such as sedimentation-floatation separation and air screening) allows the production of a feedstock of processed carpet material containing over 90 wt.% nylon 6 that can be used in the above-described prior art deagglomeration processes. However, this mechanical pretreatment is primarily used for off-spec materials (pre-consumer waste) within carpet and other nylon 6-based textile production facilities. It is also not transferable to other non-carpet multicomponent materials containing nylon 6.

[0033] Currently, despite a pressing need for methods for recovering high purity ε-caprolactam from nylon 6-containing multicomponent materials in an economically sound manner, no such methods are available. In particular, there is a pressing need for ε-caprolactam recovery methods that can be used with more demanding nylon 6-containing multicomponent materials, such as materials having low to moderately high nylon 6 content (e.g., 1 wt.% to 35 wt.%), as well as for recovering ε-caprolactam from complex nylon 6-containing multicomponent products that cannot be pre-concentrated by simple mechanical pretreatment steps such as shaving or grinding followed by density separation. Finally, there is a need for methods that allow for the recovery of ε-caprolactam from nylon 6-containing multicomponent materials, such as multilayer membranes, where the nylon 6 layers are typically sandwiched between non-nylon 6 components and are therefore not accessible from the outer surfaces of these materials.

[0034] In summary, prior art processes for recovering ε-caprolactam from multi-component materials containing nylon 6 are unable to produce high quality ε-caprolactam grades that can be used to replace virgin ε-caprolactam grades for demanding applications.

[0035] Currently, there are no available methods for recovering high purity ε-caprolactam from multicomponent materials containing nylon 6, despite a pressing need for such methods. In particular, there is a pressing need for high purity ε-caprolactam recovery methods that can replace virgin ε-caprolactam grades for use in demanding applications such as high speed melt spinning during textile fiber production.

[0036] Further, there is a need for a process that allows for the recovery of high purity ε-caprolactam in an economically sound manner from a multicomponent material containing nylon 6. The production cost of the recovered high purity ε-caprolactam should be approximately the same as or lower than the production cost of virgin high purity ε-caprolactam.

[0037] There is also a need to purify crude ε-caprolactam obtained by depolymerization of multicomponent materials containing nylon 6 without using oxidants such as potassium permanganate (KMnO4) or adsorbents such as (activated) carbon and diatomaceous earth. Technologies based on these oxidants and adsorbents are rather laborious and generate solid waste.

[0038] Furthermore, there is a need to provide high purity grades of ε-caprolactam from multicomponent materials containing nylon 6, with a carbon footprint significantly lower than ε-caprolactam produced by processes using virgin ε-caprolactam obtained by new synthesis (e.g., by Beckmann rearrangement of cyclohexanone oxime).

[0039] Finally, there is a need for a process that allows 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 wasted each year. Summary of the invention

[0040] It is an object of the present invention to meet one or more of the above needs and to overcome or mitigate the disadvantages associated with prior art approaches.

[0041] In particular, the object of the present invention is to provide a method for recovering high purity ε-caprolactam from multi-component waste containing nylon 6 in an economically efficient manner and at a high recovery rate. In particular, the object of the present invention is to provide a method suitable for recovering high purity grades of ε-caprolactam from materials derived from multi-component waste containing nylon 6, which method does not exceed the production costs of virgin high purity ε-caprolactam. In particular, the object of the present invention is to provide a method suitable for recovering ε-caprolactam from multi-component waste containing nylon 6 having a nylon 6 content of up to 35 wt.% and from composite multi-component materials containing nylon 6, in particular multilayer films containing nylon 6.

[0042] The object of the present invention is also to provide a process for recovering high-purity ε-caprolactam from materials originating from multicomponent waste containing nylon 6, which high-purity ε-caprolactam can replace high-purity virgin ε-caprolactam for all applications including high-speed melt spinning of nylon 6 for the production of thin textile fibers.

[0043] A further object of the present invention is to provide a process for recovering high purity grades of ε-caprolactam from multi-component waste containing nylon 6, said process being characterized by a significantly lower carbon footprint compared to processes for producing ε-caprolactam by new synthesis, for example by Beckmann rearrangement of cyclohexanone oxime.

[0044] Therefore, the present invention also aims to provide a method for reducing the environmental burden of multi-component waste containing nylon 6 and products prepared from the waste.

[0045] A further object of the present invention is to provide a process for recovering high purity grades of ε-caprolactam from materials originating from multi-component waste containing nylon 6 on an industrial scale.

[0046] The object of the present invention is also to provide a process for purifying crude ε-caprolactam obtained by depolymerizing materials originating from multi-component waste containing nylon 6, said process not generating solid waste.

[0047] One or more additional objects may become apparent from the remainder of the specification.

[0048] All, some or at least one of the above objects are solved by the method according to claim 1 , the plant according to claim 13 and the product according to claim 15 .

[0049] The present invention provides a method for recovering purified ε-caprolactam from a material derived from a multicomponent material containing nylon 6 in a plant, wherein the plant comprises

[0050] - depolymerization section [B],

[0051] - Recovery section [C], and

[0052] - purification segment [D],

[0053] And wherein the method comprises the following steps:

[0054] a) providing a pre-concentrated nylon 6-containing material, the pre-concentrated nylon 6-containing material having been obtained by extracting a multi-component material containing nylon 6 with one or more solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 as compared to the multi-component material containing nylon 6, and charging the pre-concentrated material derived from the multi-component material containing nylon 6 to the depolymerization section [B];

[0055] b) depolymerizing said material derived from a multicomponent material containing nylon 6 at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C in said depolymerization section [B], such that a stream comprising ε-caprolactam is obtained;

[0056] c) discharging the stream comprising ε-caprolactam from the depolymerization section [B] and recovering crude ε-caprolactam from the stream in the recovery section [C]; and

[0057] d) purifying the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises the following steps:

[0058] (i) extracting the crude ε-caprolactam with an organic solvent to obtain an organic phase, and wherein the organic phase comprises the organic solvent, the ε-caprolactam and impurities; and

[0059] (iv) obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution comprising ε-caprolactam and impurities at a temperature of 10 to 95°C.

[0060] Surprisingly, the combination of the particular sequence of processing steps and process conditions according to the invention, i.e. the sequence of pre-concentration, depolymerization, recovery and specific purification steps defined above, allows the recovery of high-grade ε-caprolactam from materials derived from multi-component materials containing nylon 6 in high yield and in a direct and economically reasonable manner. The process of the invention is economically reasonable and advantageous from several perspectives. Firstly, the process of the invention is applicable to a variety of materials derived from multi-component materials containing nylon 6, which may differ, for example, in their overall composition and / or their nylon 6 content. Secondly, the process of the invention allows the effective separation of ε-caprolactam from non-ε-caprolactam compounds, making it possible to obtain high-purity grades of ε-caprolactam, which can replace high-purity virgin ε-caprolactam for all applications, including high-speed melt spinning of nylon 6 for the production of thin textile fibers. Thirdly, the process of the invention is very efficient, making it possible to obtain ε-caprolactam in high yield. Fourth, the method of the present invention allows for the recovery of ε-caprolactam from multi-component materials including nylon 6 on an industrial scale in order to treat the large amounts of multi-component materials including nylon 6 that are currently wasted. Finally, the method of the present invention allows for the production of ε-caprolactam at a significantly lower carbon footprint than ε-caprolactam produced by de novo synthesis of ε-caprolactam (e.g., by Beckmann rearrangement of cyclohexanone oxime). The method of the present invention allows for the efficient processing of materials derived from multi-component materials containing nylon 6 and reduces the environmental burden of the products. Specifically, the method of the present invention allows for the production of purified ε-caprolactam at a carbon footprint of less than 2.5 kg CO2 / kg purified ε-caprolactam, which is a significant improvement over the 6.4 to 7.5 kg CO2 / kg ε-caprolactam associated with the production of "virgin" ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime.

[0061] In addition to the process of the present invention, the present invention also provides a plant for producing purified ε-caprolactam from a material derived from a multicomponent material containing nylon 6 in a plant, wherein the plant comprises

[0062] Depolymerization section [B],

[0063] Recovery section [C],

[0064] Purification segment [D].

[0065] The plant is configured to carry out the method of the invention.

[0066] The present invention also provides purified ε-caprolactam obtained by depolymerization of nylon 6 produced from a material derived from a multicomponent material containing nylon 6 according to the process of the present invention, wherein the product carbon footprint of the ε-caprolactam is less than 2.5 kg CO2 / kg purified ε-caprolactam.

[0067] Advantageous embodiments of the invention are indicated in the dependent claims and are explained in more detail below. DETAILED DESCRIPTION

[0068] Multi-component materials containing nylon 6

[0069] The method of the present invention uses as a starting material a multi-component material containing nylon 6 or a multi-component material derived therefrom. The multi-component material containing nylon 6 is usually a solid, in particular a solid body, a web, a sheet or a film material.

[0070] As used in the present disclosure and claims, the singular forms "a," "an," and "the" include plural referents, particularly in the sense of "one or more," unless the context clearly dictates otherwise. Similarly, "a material" as used herein may particularly refer to one or more materials.

[0071] Multi-component materials include two or more components. They are generally used as a means of combining the properties of different components into a single material. The term "component" used herein refers to a chemical substance. In a preferred embodiment, a multi-component material containing nylon 6 includes nylon 6 and at least one non-nylon 6 polymer as a component. Two or more components can exist as separate domains in the multi-component material (for example, in a sheath and core configuration, an island-on-the-sea configuration, a side-by-side configuration, a multilayer configuration or other configurations). Alternatively or additionally, the multi-component material may include a mixture of different components (for example, in the form of a polymer blend). Moreover, a mixture of these two embodiments is also possible, such as in the case of a multilayer film including a single polymer layer and a polymer blend filled with polymer filler particles or an additional layer of a polymer matrix. Typically, nylon 6 is present in a multi-component material containing nylon 6 in the form of a separate domain and / or a mixture with at least one non-nylon 6 polymer.

[0072] Preferably, the components of the multi-component material are at least two different polymers, one of which is or includes nylon 6. In one embodiment, at least two components of the multi-component material include or consist of polymers. In this embodiment, the multi-component material may include additional non-polymer components. However, preferably, more than 80wt.% of the multi-component material is composed of polymers. The term "polymer" used herein refers to the usual meaning of the term, i.e., a macromolecule (e.g., oligomer) or a polymer composed of repeating subunits. The polymer used herein may be of synthetic origin, such as plastics comprising polypropylene, nylon 6, and polystyrene, or of natural origin, such as starch or protein. The term "polymer" used herein also includes homopolymers, copolymers, and polymer blends. Polymers of natural and synthetic origin are produced by the polymerization of many small molecules (referred to as monomers). Homopolymers are prepared from only one type of monomer. Copolymers are prepared from different types of monomers, which may be in random or block configurations. "Polymer blends" or "blends" used herein refer to compositions of two or more polymers, and are generally obtained by extrusion. A blend may or may not be miscible, it may or may not be phase separated.

[0073] 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 other than nylon 6. The polymer other than nylon 6 may be selected from the group consisting of polyolefins (especially polyethylene and polypropylene), polyethylene oxide, polypropylene oxide, polycaprolactone, polyamide (except polyamide 6), polyester, polyvinylidene fluoride, polyvinylidene chloride, polystyrene, polycarbonate, polymethyl methacrylate, ethylene-co-acrylic acid copolymer, polyoxymethylene, ethylene-vinyl alcohol, polyurethane and combinations thereof, especially blends thereof. Particularly good results can be obtained in terms of the purity and yield of the ε-caprolactam obtained when the polymer other than 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.

[0074] The multi-component material used in the method of the present invention comprises nylon 6. The term "nylon 6" as used herein refers to polycaprolactam. The term "multi-component material containing nylon 6" as used herein refers to a multi-component material comprising two or more components, one of which is or comprises nylon 6. The weight fraction of nylon 6 in the multi-component material containing nylon 6 used in the method of the present invention is not critical to the implementation of the present invention and may range from 0.5 wt.% to 99 wt.%. However, a particular advantage of the method of the present invention is that it is also very suitable for multi-component materials containing only a lower content of nylon 6 than the nylon 6 content of more than 80 wt.% required for the successful application of the methods of the prior art in practice. Therefore, according to a particularly advantageous embodiment, the weight fraction of nylon 6 in the multi-component material containing nylon 6 used in the method of the present invention is 1 wt.% to 75 wt.%, more preferably 1 wt.% to 60 wt.%, even more preferably 2 wt.% to 35 wt.%, and most preferably 3 wt.% to 25 wt.%. If not otherwise indicated, all wt. % values ​​stated herein are always based on the total weight of the nylon 6-containing multicomponent material.

[0075] The multicomponent material containing nylon 6 may have any shape, including (multi-filament) yarns, pieces, films or any molded forms. The shape of the multicomponent material containing nylon 6 used in the process of the present invention is not critical.

[0076] Preferably, the multi-component material containing nylon 6 used in the method of the present invention is a multilayer material, wherein at least one layer includes nylon 6 or consists of it. The term "multilayer material" as used herein refers to a material organized into different layers. The layers may differ in their relative positions, thicknesses and / or compositions. The layers may be of the same or different types. Preferably, the multilayer material contains at least one layer that does not include nylon 6. In a further preferred embodiment, the multilayer material includes two skin layers as the outermost layers on opposite sides of the multilayer material, wherein neither of the two skin layers includes nylon 6, so the nylon 6 is included in one of the layers sandwiched between the skin layers. Non-limiting examples of suitable polymers that can be used for the skin layers include polypropylene, polyethylene, polyethylene oxide, polycaprolactone, polyamide (except 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 method of the present invention is that it can work with multilayer materials, wherein nylon 6 does not need to be exposed or accessible on the surface of the material.

[0077] The multilayer material may contain a barrier layer as a surface layer or an intermediate layer that does not contain nylon 6. The barrier layer may be formed of any material having barrier properties. The barrier layer may include or consist of: an organic material such as carbon; an inorganic material such as a metal, a ceramic, an oxide; a polymer material; or a combination thereof. Non-limiting examples of suitable polymers other than nylon 6 that may be used for or as a barrier layer include: polyethylene terephthalate, ethylene-vinyl alcohol, polyvinylidene chloride copolymers, polyamides (not nylon 6), polyketones, blends of two or more of these; and blends including one or more of these with other polymers.

[0078] The multilayer material may include 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 tie layers or adhesive layers include: olefin block copolymers, such as those sold under the trade name INTUNE TM Propylene-based block copolymers sold by The Dow Chemical Company and under the trade name INFUSE TM Ethylene-based block copolymers sold by The Dow Chemical Company; polar ethylene copolymers, such as copolymers with vinyl acetate, acrylic acid, methyl acrylate and ethyl acrylate; ionomers; maleic anhydride grafted ethylene polymers and copolymers; polyurethane adhesives; blends of two or more of these; and blends including one or more of these with other polymers.

[0079] In a preferred embodiment, the "layer" used herein refers to a "film". Even more preferably, the multi-component material containing nylon 6 used in the method of the present invention is itself a film, that is, a multilayer film containing at least one layer comprising nylon 6 or consisting of it. Preferably, the multilayer film contains at least one layer comprising nylon 6 or consisting of it and at least one layer not comprising nylon 6. The multilayer film can be produced by any film lamination and / or coextrusion technology. The term "film" used herein is defined as a thin sheet with a thickness of less than 5 mm. The "layer" in the film can be very thin, as in the case of nanolayers. The "film" used herein can be in the form of a shape that is not necessarily "flat" in the sense of a plane, such as a profile, a parison, a tube, etc. In a preferred embodiment, the term "film" used herein refers to a film composed of or including more than 80 wt.% of a polymer material. It is advantageous to use a multilayer film in the method of the present invention because it is a common waste. Most packaging films today are multilayer films. According to an embodiment that is actually particularly relevant, the "multilayer film" used herein is a packaging film, in particular a modified atmosphere packaging film, both of which are described in the background section above.

[0080] Due to the close connection between the film layers, before the present invention, it was very difficult to recover high-grade ε-caprolactam from multilayer films containing nylon 6, especially if the nylon 6 content is lower than 80 wt.% as indicated above and / or if at least one nylon 6-containing layer is sandwiched between two layers not containing nylon 6, which can but need not be surface layers in the multilayer film and are therefore not very accessible. Therefore, these types of multilayer films present a particularly advantageous embodiment of the present invention when used as a multicomponent material containing nylon 6 in step a). Therefore, in particular, the multicomponent material containing nylon 6 used in the method of the present invention can be a multilayer film, the multilayer film containing at least one layer comprising or consisting of nylon 6, which layer is sandwiched (i.e. embedded) between two or more layers not comprising nylon 6. The multilayer film may of course include additional layers with or without nylon 6 anywhere between, on top of or below these layers.

[0081] The method of the invention has the advantage that, unlike prior art methods which are limited, for example, to nylon 6-containing fibers derived from carpets which need to be subjected to mechanical pre-processing steps, the method of the invention is not so limited and can in particular also be applied very successfully to multilayer films. In a particular embodiment, the nylon 6-containing multicomponent material used in the method of the invention is not a carpet or a material derived from a carpet. In another embodiment, it is not a textile or a material derived from a textile. In another embodiment, it does not comprise nylon 6 fibers.

[0082] Possible preprocessing steps

[0083] The material used in step a) originating from a multicomponent material containing nylon 6 has been obtained by extracting the multicomponent material containing nylon 6 with one or more solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 compared to the multicomponent material containing nylon 6. This is described in more detail below. Before or after this pretreatment, the material may advantageously be subjected to further pretreatment steps, which are described below.

[0084] Before being subjected to step a) of the process of the invention, the multicomponent material containing nylon 6 or the preconcentrated nylon 6-containing material is preferably subjected to a pretreatment, in particular a mechanical size reduction, washing and / or densification step. Thus, in the process of the invention, the mechanical size reduction, washing and / or densification section may precede (preferably) and / or follow the nylon 6-preconcentration by extraction. Preferably, before being subjected to step a) of the process, the multicomponent material containing nylon 6 is broken into fragments before being extracted with one or more solvents in the nylon 6-preconcentration. This mechanical pretreatment of the multicomponent material containing nylon 6, i.e. mechanical comminution or crushing, can be achieved, for example, by cutting, chopping, grinding, crushing or cutting. In a preferred embodiment, the multicomponent material containing nylon 6 is charged to the nylon 6-preconcentration in the form of fragments having an average length of 0.01 to 100 cm, preferably 0.05 to 10 cm, and most preferably 0.1 to 5 cm along the longest axis of the fragments. The average length along the longest axis of the employed fragments can be readily determined by a skilled person by first obtaining a representative sample of the fragments, then measuring the length of the longest axis of each of these fragments (e.g., 50 fragments), and finally calculating the average of all these individual measurements. The preferred particle size can also be described in terms of average particle weight. Preferably, the average particle weight of the fragments of the nylon 6-containing multicomponent material is from 0.1 mg to 100 kg, preferably from 1 mg to 10 kg, more preferably from 10 mg to 1 kg, and most preferably from 10 mg to 100 g. Using fragments of the nylon 6-containing multicomponent material having the above-mentioned sizes has the advantage that the surface area is increased and / or the fragments can be more easily handled and / or mixed with the organic solvent added for extraction in the nylon 6-preconcentration.

[0085] Optionally, the multicomponent material containing nylon 6 is cleaned before the nylon 6 preconcentration. This is advantageous, since any (adherent) dirt that is removed will thus not interfere with the next step of the process of the invention.

[0086] As used herein, the term "cleaning" is defined as any process that removes non-nylon 6 material that adheres to or is mixed with a multi-component material containing nylon 6. Cleaning is advantageous because any non-nylon 6 material that is removed will thereby not interfere with the next step of the method of the present invention.

[0087] Optionally, the multi-component material containing nylon 6 is cleaned by washing with a solvent, preferably water. Preferably, a detergent in a concentration range of 0 to 30% by weight relative to the solvent is added to the solvent to improve the washing efficiency. In another preferred embodiment, the washing process comprises a final rinsing step with a (clean) washing solvent without detergent to remove detergent residues and dirt that adhere to the multi-component material containing nylon 6. Preferably, the washing solvent is heated to further enhance the washing process. Washing is advantageous because any (adherent) dirt removed will therefore not interfere with the next step of the method of the present invention. Optionally, after the cleaning step and before nylon 6-preconcentration by extraction with an organic solvent, the multi-component material containing nylon 6 is dried. This has the advantage that the solvent added for extraction is not diluted or contaminated by the washing solvent. Washing is preferably carried out under friction. Different types of industrial washing systems are available on the market, such as rotary plastic washers and (high-speed) friction washers. Optionally, mechanical size reduction and washing of the multi-component material containing nylon 6 are combined in, for example, a so-called wet crusher.

[0088] Optionally, prior to mechanical comminution or crushing of the multi-component material containing nylon 6, metal fragments, rocks and other interfering materials that cause severe wear on the equipment used for the mechanical comminution or crushing are removed. Preferably, before or after mechanical comminution or crushing of the multi-component material containing nylon 6, as further described below, materials that do not include nylon 6, such as polyethylene, polypropylene and materials that include nylon 6,6 are removed. The removal of foreign materials can be performed mechanically or manually. The advantage of removing these interfering materials is that the maintenance costs of the equipment used for mechanical comminution or crushing can be greatly reduced. In addition, the nylon 6 content of the material obtained after mechanical comminution or crushing is higher than when the interfering materials are not removed.

[0089] Optionally, foreign materials are separated from the multi-component material containing nylon 6 that has been mechanically crushed or broken. To this end, 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 medium-density liquid, wherein the material with lower density floats and is separated from the sinking material with higher density. In practice, density separation is usually completed by a series of density separation stages. For example, in one stage, high-density materials such as rocks, sand and metals (including iron and lead) are separated, and in another stage, low-density materials such as polyolefins, polypropylene and polyethylene are separated. Magnetic separation is a method for separating the components of a mixture by using a magnet to attract magnetic materials. The method commonly used for magnetic separation separates magnetic materials from non-magnetic materials. It is advantageous to remove foreign materials from the crushed or broken multi-component material containing nylon 6 because such materials interfere with the subsequent steps of the method of the present invention.

[0090] Optionally, the preferably washed and size-reduced multi-component material containing nylon 6 is densified. Preferably, the densification of the multi-component material containing nylon 6 is done by charging into a furnace (e.g., an extruder) or by compacting in a (mechanical) compactor. Densification of the preferably cleaned and / or size-reduced multi-component material containing nylon 6 has the advantage of an increased bulk density, which reduces the costs of intermediate storage and transportation in case the pretreatment is carried out at a different location (see below).

[0091] In a furnace, a multi-component material containing nylon 6 is melted. Preferably, the resulting polymer melt is filtered. This has the advantage of removing solid impurities. The molten and optionally filtered polymer melt is then cooled and preferably fed to a pelletizer. The pelletizer cuts the product into pellets. The size and shape of the pellets (also referred to as granules) can be selected within a wide range. In general, the shape of the pellets is cylindrical (derived from thin strands cut into pieces). However, other shapes such as (imperfect) spheres are also possible. The size of the pellets can be selected within a wide range. Typically, the diameter of the pellets can range from 1 to 10 mm, preferably 2 to 7 mm, and more preferably 3 to 5 mm. In a preferred embodiment, the length of the pellets ranges from 1 to 50 mm, preferably 2 to 25 mm, and more preferably 3 to 15 mm.

[0092] The pelletization of the preferably cleaned and size-reduced multicomponent material containing nylon 6 has the advantage of an increased bulk density, which reduces the costs of intermediate storage and transportation in the case where the pretreatment is carried out at a different location (see below). In addition to the increased density, pelletization also provides other benefits, such as a uniform shape and structure of the material to be treated, which facilitates the (automatic) feeding into an apparatus for nylon 6 preconcentration by extraction with a solvent.

[0093] The location where the pretreatment of the multi-component material containing nylon 6 is carried out and the location where the nylon 6 preconcentration is carried out by solvent extraction can be the same. However, preferably, one or more of the pretreatment steps are carried out at different locations, for example at a location dedicated to pretreatment of waste polymer materials, in particular multi-component materials containing nylon 6. The multi-component material containing nylon 6 pretreated at the different location can then be transported to the location where the nylon 6 preconcentration is carried out by solvent extraction.

[0094] Therefore, according to a particularly advantageous embodiment of the present invention, prior to step a), the substance comprising the multicomponent material containing nylon 6 is subjected to a pretreatment and preconcentration in a pretreatment section [A] to obtain a material derived from the multicomponent material containing nylon 6, in particular a cleaning in a cleaning section [α] and / or a mechanical size reduction section [β], a mechanical size reduction and / or densification section [γ] and an extraction section [ω].

[0095] Pre-concentration of multi-component materials containing nylon 6

[0096] In step a) of the process of the invention, a material derived from a multicomponent material containing nylon 6 is provided, which material has been obtained by extracting the multicomponent material containing nylon 6 with one or more (especially organic) solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 compared to the multicomponent material containing nylon 6. This material is fed into a depolymerization section [B]. The previous pre-concentration of the multicomponent material containing nylon 6 is an important prerequisite for the starting material of the process of the invention, since it ensures a high yield and a high quality of the ε-caprolactam obtainable according to the invention. The pre-concentration of the multicomponent material containing nylon 6 is particularly important when a multi-layer membrane is used as starting material, in particular when the layer containing nylon 6 is sandwiched between other polymer layers and is therefore not directly accessible for depolymerization. The pre-concentration of the multicomponent material containing nylon 6 is also particularly important when a multicomponent material comprising nylon 6 and at least one non-nylon 6 polymer is used as starting material, in particular when nylon 6 is present in the multicomponent material containing nylon 6 as a separate domain and / or in a mixture with at least one non-nylon 6 polymer and is therefore not directly accessible for depolymerization.

[0097] According to the present invention, pre-concentrating the multi-component material containing nylon 6 by extraction to obtain a material derived from the multi-component material containing nylon 6 to be fed into the depolymerization section [B] generally comprises the following steps:

[0098] (i) adding one or more solvents to a multi-component material containing nylon 6, wherein an organic solvent is particularly used;

[0099] (ii) performing phase separation to obtain a liquid extract phase comprising a solvent and components dissolved from the multi-component material containing nylon 6 and at least a portion of a solid phase comprising undissolved components of the multi-component material containing nylon 6 and optionally a solvent;

[0100] (iii) removing the solvent from the liquid extract phase and, if present, also from at least a portion of the solid phase to obtain two solid phases, one of which is a pre-concentrated nylon 6-containing material enriched in nylon 6 compared to the nylon 6-containing multicomponent material used as the starting material.

[0101] Step (i) further comprises treating with a solvent, i.e., contacting the solvent and the multi-component material containing nylon 6 for a sufficient time and under conditions sufficient to dissolve the components to be extracted from the multi-component material containing nylon 6. The phase separation in step (ii) can be carried out by any known means suitable for solid / liquid phase separation. Suitable phase separation means are, for example, filters, centrifuges, cyclone separators or settlers. The solvent removal in step (iii) is advantageously carried out by evaporation of the solvent, thereby obtaining the dissolved components extracted from the multi-component material containing nylon 6 as the remaining solid precipitate. However, there are and can be used various techniques that enable the recovery of solvents from dissolved compounds. These techniques are known to the skilled person and include cooling, evaporation, distillation, precipitation by adding a precipitant (a solvent for precipitating the dissolved polymer) and combinations thereof.

[0102] As used herein, the terms "extraction", "extracted" or "extracting" refer to a physical or chemical process of removing one or more components from a matrix by a solvent, followed by removal of the solvent and recovery of the extracted components. The one or more solvents can be used in the process of the present invention to: extract nylon 6, in which case a pre-concentrated nylon 6-containing material is obtained as the remaining solid residue after removal of the one or more solvents from the liquid solvent-containing extraction phase; and / or extract non-nylon 6 compounds from a multi-component material containing nylon 6, in which case, optionally after removal of any adhering solvent, the remaining undissolved multi-component material containing nylon 6 is the pre-concentrated nylon 6-containing material referred to herein.

[0103] The term "pre-concentration" as used herein refers to the extraction of a multi-component material containing nylon 6 with one or more solvents, followed by the removal of the solvents, to obtain a material enriched in nylon 6 compared to the multi-component material containing nylon 6 used as the starting material. "Enriched in nylon 6" means enriched compared to the multi-component material containing nylon 6 used as the starting material. The enrichment of nylon 6 is defined as the content of nylon 6 (in wt.%) based on the total weight of polymers in the solid material containing nylon 6 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 the starting material. This enrichment depends largely 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. If the nylon 6 content of the starting material is 20 wt.% based on the total polymer content, and the nylon 6 content of the pretreated nylon 6-containing phase is 80 wt.% based on the total polymer content, then the enrichment in nylon 6 is (80 wt.% divided by 20 wt.% =)4.

[0104] 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. The advantage of using an organic solvent to extract a multi-component material containing nylon 6 is that different components, especially different polymers, can be separated from each other, especially from the nylon 6 to be enriched, by their different solubilities in certain solvents.

[0105] Extraction generally involves (i) contacting the nylon 6-containing multi-component material with one or more solvents so as to obtain a liquid extract phase comprising the solvent and compounds dissolved from the nylon 6-containing multi-component material and a second phase, typically solid or partially solid, consisting of undissolved nylon 6-containing multi-component material, followed by (ii) phase separation and (iii) solvent removal to obtain a pre-concentrated nylon 6-containing material.

[0106] As explained above, depending on whether (1) a nylon 6-dissolving solvent or (2) a non-nylon 6-dissolving solvent is used, the pre-concentrated nylon 6-containing material is obtained (1) after removal of the solvent from a 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 multicomponent material. It is also possible to combine two or more different, in particular complementary, extractions.

[0107] In the case of more than one solvent, the treatment can be carried out simultaneously or sequentially. If more than one solvent is used, the solvents should be distinguished for their ability to dissolve nylon 6. In a particularly advantageous embodiment of the invention, step a) comprises at least a two-step procedure, wherein (i) the multi-component material containing nylon 6 is first extracted with a solvent capable of dissolving non-nylon 6 components, and subsequently, (ii) the undissolved remaining multi-component material containing nylon 6 from the first extraction is treated and extracted with a solvent capable of dissolving nylon 6, whereby after phase separation, a pre-concentrated nylon 6-containing material is obtained after removing the solvent from the liquid extraction phase comprising dissolved nylon 6 obtained from the second extraction. It is particularly advantageous to proceed in this way when using a multilayer membrane as starting material, in particular when the nylon 6-containing layer is covered, in particular sandwiched between other polymer layers, and is therefore not directly accessible to the nylon 6-dissolving solvent.

[0108] The effect of the extraction can be influenced by the choice of solvent and treatment conditions (e.g., temperature, treatment length and amount of solvent relative to the amount of multicomponent material containing nylon 6). Depending on the solvent used, the extraction in step a) can be of the following type:

[0109] a pre-concentration step in which non-nylon 6 compounds are preferentially extracted from the nylon 6-containing multi-component material (the pre-concentrated nylon 6-containing material obtained is then, optionally after removal of any adhering solvent, an unextracted nylon 6-containing multi-component material);

[0110] a pre-concentration step in which nylon 6 compounds are preferentially extracted from the multi-component nylon 6-containing material (a pre-concentrated nylon 6-containing material is then obtained after removing the solvent from a liquid extract phase comprising the solvent and dissolved nylon 6); and

[0111] - Preferentially extracting nylon 6 and non-nylon 6 compounds from a multi-component material containing nylon 6 in a sequential manner, i.e. performing a combination of extraction of non-nylon 6 compounds and nylon 6 sequentially.

[0112] The extraction in which non-nylon 6 compounds are preferentially extracted from a multi-component material containing nylon 6 can be carried out with solvents that are suitable as selective extractants for non-nylon 6 compounds. These are solvents in which non-nylon 6 compounds are easily dissolved, while the solubility of nylon 6 in the solvents is limited. These solvents must be highly selective for the dissolution of non-nylon 6 compounds. In addition to the choice of solvent, the temperature at which the extraction is carried out, the treatment time and the amount of solvent relative to the amount of the multi-component material containing nylon 6 will also have a major impact on the extraction selectivity. Therefore, the selection of these parameters depends on the type of components that do not contain nylon 6 that should be selectively removed. The technician can easily determine which solvents are effective in extracting the relevant non-nylon 6 components in a given multi-component material containing nylon 6 by simple test dissolution experiments.

[0113] Any one or more of the organic solvents mentioned below with respect to specific non-nylon 6 compounds may be used as the solvent in the process of the present invention.

[0114] For dissolving polyolefins (e.g., LD, LLD and HD polyethylene and polypropylene), solvents such as those selected from the group consisting of aliphatic hydrocarbons, cycloalkanes, aromatic hydrocarbons and mixtures thereof are particularly useful. They are obtained as boiling fractions in oil processing for the production of fuels such as gasoline and diesel. The above-mentioned boiling fractions include paraffin, petroleum wax and white spirit, each of which can be used as solvent according to the present invention. Isomer mixtures of xylene or pure solvents such as toluene are also known to dissolve polyolefins. Chlorinated hydrocarbons such as tetrachloroethane can also be used to dissolve polyolefins.

[0115] In order to dissolve the polystyrene polymer, various solvents including aromatic organic solvents can be used. Preferably, benzene, toluene, xylene and mixtures thereof are used as solvents to dissolve the polystyrene polymer. Most preferably, xylene is used to dissolve the polystyrene polymer.

[0116] To dissolve the polyvinyl chloride, preference is given to using the solvents tetrahydrofuran, cyclohexane, dioxane and methyl ethyl ketone (MEK) and mixtures thereof.

[0117] Solvents that can be used to dissolve other (polymer) non-nylon 6 compounds are known to the skilled person or can be easily identified by performing test dissolution experiments. The flexibility of the process of the present invention allows other factors to be taken into account when selecting a solvent for extraction in the nylon 6-preconcentration section, including inter alia stable commercial availability, ease of operation, health aspects, and stability and cost of the solvent.

[0118] After extraction, in which non-nylon 6 compounds are preferentially extracted with a solvent, undissolved compounds are removed from the mixture obtained in the nylon 6-preconcentration section by phase separation. The extraction should be performed in such a way that the largest proportion of undissolved compounds by weight should be nylon 6 when extracting with a solvent that preferentially dissolves non-nylon 6 compounds. Optionally, after removal of any adhering solvent, the undissolved material then represents a preconcentrated nylon 6-containing material enriched in nylon 6. The employed solvent is usually recovered from the extraction phase and reused in the nylon 6-preconcentration section.

[0119] Optionally, the extraction in which non-nylon 6 compounds are preferentially extracted is performed more than once. Non-nylon 6 compounds that were not removed in the previous extraction can be removed in the additional extraction by selecting another solvent and / or other process conditions (e.g., another extraction temperature). After each extraction, a pretreated, generally solid or partially solid, nylon 6-containing phase is obtained that is more enriched 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 gradually extracted from the nylon 6-containing multi-component material.

[0120] Alternatively or as a complement to the extraction with the above-described non-nylon 6 dissolving solvent, an extraction in which nylon 6 is preferentially extracted from a multi-component material containing nylon 6 can be performed with a solvent suitable as a selective extractant for nylon 6. In order to preferentially extract nylon 6, solvents are used that dissolve nylon 6, while the dissolution of non-nylon 6 in these solvents is limited. In other words, solvents that are highly selective for the dissolution of nylon 6 should be used. As with the selective dissolution of non-nylon 6 compounds, the selective dissolution of nylon 6 compounds can also be affected by the choice of solvent, the temperature at which the extraction is performed, the treatment time, and the amount of solvent relative to the amount of multi-component material containing nylon 6.

[0121] Solvents known from prior art methods for extracting polyamides can be used for this nylon 6 specific extraction step. For example, EP603434 discloses suitable solvents for dissolving polyamides containing nylon 6. The solvents include concentrated mineral acids, formic acid, chloroacetic acid, phenols, cresols, alcoholic solutions of alkaline earth halides, aromatic alcohols such as phenylethanol and benzyl alcohol, as well as diols, lactams and lactones. US5840773 discloses a solvent for dissolving polyamides such as nylon 6, aliphatic alcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, amyl alcohol, hexanol, 2-ethylbutanol, 4-methylpentanol, 1-heptanol, 2-heptanol, 4-heptanol, 2,4-dimethylpentanol, 1-octanol, 2-octanol, 2-ethylhexanol, 1-nonanol, 2-nonanol, n-decanol, n-undecyl alcohol, n-dodecanol, n-tridecanol, n-tetradecanol, n-pentadecanol, n-hexadecanol, n-octadecanol, etc., and substituted monohydric alcohols and dihydric alcohols, such as 2-methoxy-1-ethanol and methylene glycol. In a preferred embodiment of US5840773, C1-C 12 Alcohol. In particular, methanol and ethanol are preferably used as extractants. The extractant may be a mixture of aliphatic alcohols and water. US5840773 further discloses that an extraction time of 60 minutes has been found to be 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. Along these lines, US5840773 mentions that polyamide 6,6 preferentially dissolves in methanol at a temperature of at least about 140°C. In Example IV of US5840773, a polyamide 6,6 extraction temperature of about 160°C is applied.

[0122] Optionally, the extraction in which nylon 6 is preferentially extracted is performed more than once. After each extraction, a pre-concentrated nylon 6-containing material is obtained, which is more enriched in nylon 6 than the pre-concentrated nylon 6-containing material obtained from the previous extraction. This appears to be caused by the fact that the amount of impurities in the pre-concentrated nylon 6-containing material, which typically includes non-nylon 6 compounds, is gradually reduced by each extraction.

[0123] After extraction in which nylon 6 compounds are preferentially extracted with a solvent, the solvent is recovered. In doing so, first, undissolved materials are removed from a liquid extract phase containing solvent and nylon 6, such as by filtration or centrifugation. The extraction should be performed in a manner in which the largest proportion of undissolved compounds by weight are non-nylon 6 compounds. Second, the solvent is recovered from the liquid extract phase containing nylon 6 to obtain a pre-concentrated nylon 6-containing solid material enriched in nylon 6. Any of the above-mentioned solvent removal techniques may be performed to separate the dissolved nylon 6-containing material from the solvent, including cooling, evaporation, distillation, precipitation by adding a precipitant (a solvent for precipitating the dissolved polymer), and combinations thereof.

[0124] The extraction may also be a combination of sequential extractions using a solvent that preferentially extracts non-nylon 6 compounds and a solvent that preferentially extracts nylon 6. By this combination of sequential extractions, a nylon 6-containing phase is obtained that is even more enriched in nylon 6 than a nylon 6-containing phase pretreated only once.

[0125] In an embodiment of the present invention, the multi-component material containing nylon 6 is first extracted in the first nylon 6-preconcentration section with a solvent that preferentially extracts non-nylon 6 compounds from the multi-component material containing nylon 6 to obtain a pre-concentrated nylon 6-containing material enriched in nylon 6. In a second extraction, the pre-concentrated nylon 6-containing material enriched in nylon 6 is treated with a solvent that preferentially extracts nylon 6 compounds from the pre-concentrated nylon 6-containing material enriched in nylon 6 to obtain a pre-concentrated nylon 6-containing material further enriched in nylon 6. It can then be used in step a). For example, in the first extraction, non-nylon 6 compounds LD polyethylene are removed from the multi-component material containing nylon 6 by extraction with white solvent oil. In the second extraction, the pre-concentrated nylon 6-containing material enriched in nylon 6 is further extracted with methanol or methanol-water solution (e.g., a weight ratio of water to MeOH of 1:19), thereby obtaining a further pre-concentrated nylon 6-containing solid material after removing the solvent (methanol, water).

[0126] According to another advantageous embodiment of the present invention, the multi-component material containing nylon 6 is first treated in a first nylon 6-preconcentration section with a solvent that preferentially extracts nylon 6 compounds from the multi-component material containing nylon 6 to obtain a preconcentrated nylon 6-containing material enriched in nylon 6. In a second extraction, the preconcentrated nylon 6-containing material enriched in nylon 6 is treated with a solvent that preferentially extracts non-nylon 6 compounds from the preconcentrated nylon 6-containing material enriched in nylon 6 to obtain a preconcentrated nylon 6-containing material further enriched in nylon 6. For example, in the first extraction, nylon 6 is extracted from the multi-component material containing nylon 6 with ethanol to form a nylon 6-enriched nylon 6-containing phase after recovering the ethanol. In the second extraction, the preconcentrated nylon 6-containing material enriched in nylon 6 is further extracted with toluene, thereby obtaining a nylon 6-containing phase further enriched in nylon 6.

[0127] The extraction with solvent in the nylon 6-preconcentration section is carried out at a temperature between 0° C. and 350° C. (at higher temperatures, nylon 6 decomposes), more preferably between 20° C. and 210° C., and most preferably between 50° C. and 190° C. In this temperature range, the selective extraction of most solvents is enhanced.

[0128] The time required for extraction (i.e., extraction with a solvent) in the extraction section can be readily determined by a skilled person through routine experimentation. The time allowed for extraction should preferably take into account the type of multi-component material containing nylon 6 and the amount of nylon 6 included therein and its accessibility. The time required for extraction can range from a few seconds to a few hours. Preferably, the time required for extraction is greater than 5 seconds and less than 6 hours. More preferably, the time required for extraction is greater than 15 seconds and less than 2 hours.

[0129] As a result of the extraction in the nylon 6-preconcentration section, a solid preconcentrated nylon 6-containing material enriched in nylon 6 is obtained. In this regard, the term "solid" refers to the state of the material at room temperature (20° C.). At higher temperatures, the preconcentrated nylon 6-containing material can also be present as a melt. The preconcentrated nylon 6-containing material is stable and can be stored for future use in step a) of the method of the present invention, or transported to a depolymerization section at a different location for this purpose. Therefore, similar to what is explained above for the other pretreatment steps, the location where nylon 6-preconcentration is performed by extraction with a solvent and the location where steps a) to d) of the method of the present invention are performed can be the same or different. Preferably, one or more of the pretreatment and preconcentration steps are performed at different locations, for example at a location dedicated to pretreating waste polymer materials, in particular multi-component materials containing nylon 6. The multi-component material containing nylon 6, which has been pretreated and preconcentrated at different locations, can then be transported to the location where one or more of steps a) to d) of the method of the present invention are performed.

[0130] Loading step a)

[0131] In step a) of the present invention, a material derived from a multi-component material containing nylon 6, which has been obtained by extracting the multi-component material containing nylon 6 with one or more solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 compared to the multi-component material containing nylon 6, is fed to a depolymerization section [B]. Optionally, before the extraction, the multi-component material containing nylon 6, from which the material derived from the multi-component material containing nylon 6 was obtained after the extraction, has been subjected to size reduction and / or washing and / or melting and resolidification. The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or in parallel.

[0132] In one embodiment, the material derived from the multicomponent material containing nylon 6 is mechanically compressed into a smaller volume before being loaded into the depolymerization section [B]. This has the advantage that a smaller volume is required for intermediate storage and transportation, and can also facilitate administration to the depolymerization section [B].

[0133] In another embodiment, the material derived from the multi-component material containing nylon 6 is compressed into particles of increased density before being charged to the depolymerization section [B], for example by mechanical compaction or by extrusion of the molten material, followed by cooling and cutting to size. Again, this has the advantage that the volume required for intermediate storage and transportation is smaller and administration to the depolymerization section [B] can be facilitated.

[0134] In a further preferred embodiment, the material originating from the multicomponent material containing nylon 6 is dried before being charged to the depolymerization section [B]. This has the advantage that less or no solvent is introduced into the depolymerization section [B]. The solvent introduced in the depolymerization section [B] is expected to have a negative impact on the depolymerization process (e.g., reduced depolymerization reaction rate, higher catalyst consumption, higher energy consumption, formation of impurities due to reaction between solvent and ε-caprolactam and / or the vapor stream comprising ε-caprolactam and water obtained in the depolymerization section [B] is expected to contain more impurities).

[0135] The material derived from the multi-component material containing nylon 6 is preferably fed to the depolymerization reactor in the solid phase or in the form of a melt. Preferably, the material derived from the multi-component material containing nylon 6 is charged in the form of a melt. Feeding in the form of a melt can be achieved by using an extruder, a gear pump or other means known to the skilled person.

[0136] The feeding of the material derived from the multi-component material containing nylon 6 to the depolymerization reactor can be achieved by continuously or intermittently dosing the material derived from the multi-component material containing nylon 6.

[0137] Depolymerization step b)

[0138] In the depolymerization section [B], the material originating from the multicomponent material containing nylon 6 is depolymerized to form ε-caprolactam. The ε-caprolactam formed is discharged from the depolymerization section [B] as a stream comprising ε-caprolactam.

[0139] Depolymerization of the material derived from the multi-component material containing nylon 6 is achieved by increasing the temperature of the material derived from the multi-component material containing nylon 6 in the depolymerization zone [B] to a temperature of at least 180° C. but not higher than 400° C. The preferred temperature range for the depolymerization reaction is 200° C. to 350° C., more preferably 220° C. to 340° C., and most preferably 240° C. to 325° C.

[0140] In general, the rate of formation of ε-caprolactam increases at elevated temperatures. Temperatures below 400°C are preferred because at temperatures above 400°C, side reactions of nylon 6 and impurity reactions occur more frequently, which will result in the formation of a greater variety of impurities. A portion of these impurities will eventually enter the product stream comprising ε-caprolactam discharged from the depolymerization reactor. In a preferred embodiment of the present invention, the depolymerization of the material derived from the multicomponent material containing nylon 6 is carried out at a temperature in the range of 220°C to 340°C or 240°C to 325°C. This temperature range allows the production of particularly pure ε-caprolactam.

[0141] The pressure in the depolymerization section [B] may vary and may be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, most preferably 50 kPa to 1 MPa. This pressure range allows the production of particularly pure ε-caprolactam.

[0142] The depolymerization of the material derived from the multi-component material containing nylon 6 can be achieved in the presence or absence of a solvent. Preferably, the depolymerization of the material derived from the multi-component material containing nylon 6 is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, in particular superheated steam.

[0143] Preferably, the depolymerization will be complete within 0.1 hour to 24 hours, more preferably within 0.5 hour to 6 hours.

[0144] Feeding water in the form of steam to the depolymerization reactor allows obtaining a vapor stream comprising ε-caprolactam and water, optionally without further heating. The weight ratio of ε-caprolactam to water in the vapor stream can be adjusted by varying the amount of steam fed to the material originating from the multicomponent material containing nylon 6 in the depolymerization section [B]. In a preferred embodiment, the depolymerization in step b) is carried out in the presence of water, whereby the stream comprising ε-caprolactam is a vapor stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10 and most preferably 1:3 to 1:8.

[0145] Preferably, the ε-caprolactam in the vapor stream comprising ε-caprolactam and water has a partial pressure of 0.1 kPa to 1 MPa, more preferably 0.3 kPa to 0.5 MPa, and most preferably 1 kPa to 0.1 MPa.

[0146] During the depolymerization reaction, decomposition products may be formed, which decomposition products include linear oligomers of ε-caprolactam and cyclic oligomers of ε-caprolactam. In addition, the feed stream of the material derived from the multicomponent material containing nylon 6 may also contain other components, i.e. impurities, such as non-nylon 6 compounds that remain stable, react or decompose under the depolymerization conditions and residues of the solvent used in the pretreatment. Therefore, in the case of using water as solvent, the vapor stream removed from the depolymerization section [B] includes not only water and ε-caprolactam, but also impurities.

[0147] Preferably, superheated steam having a temperature between 100°C and 600°C is fed to the depolymerization reactor. Preferably, the temperature of the superheated steam fed to the depolymerization reactor is at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam fed to the depolymerization reactor is sufficiently high that no further heat input is required for carrying out the depolymerization reaction and evaporating the ε-caprolactam formed. In another preferred embodiment, the depolymerization section [B] is fed with superheated steam having a temperature in the range of 220°C to 575°C. In an even more preferred embodiment, the depolymerization section [B] is fed with superheated steam having a temperature in the range of 275°C to 500°C. In another preferred embodiment, a portion of the heat input required for carrying out the depolymerization reaction and evaporating the ε-caprolactam formed is introduced through the wall of the depolymerization reactor.

[0148] Typically, the mass of the vapor stream removed from the depolymerization section [B] is less than the mass of the total feed to the depolymerization section. The total feed to the depolymerization section [B] comprises material derived from the multi-component material containing nylon 6 and optionally solvents, catalysts, additional reagents and / or depolymerizing agents. Therefore, without any additional measures, there will be an accumulation of material (usually referred to as 'residual material') in the depolymerization section [B]. Preferably, another stream is discharged from the depolymerization section [B]. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization section [B]. The additional stream may include impurities present in the multi-component material containing nylon 6, undepolymerized nylon 6, unevaporated ε-caprolactam, catalysts and compounds formed under depolymerization conditions, such as monoammonium phosphate, diammonium phosphate and / or triammonium phosphate in the case where phosphoric acid is used as a depolymerization catalyst. In a preferred embodiment, a stream comprising monoammonium phosphate, diammonium phosphate and / or triammonium phosphate is discharged from the depolymerization section [B]. Even more preferably, the stream intermittently or continuously discharged from the depolymerization section [B] comprises a weight fraction of 0.01 to 50 wt.-%, preferably 0.1 to 25 wt.-%, more preferably 0.5 to 10 wt.-%, most preferably 0.5 to 5 wt.-% monoammonium phosphate, diammonium phosphate and / or triammonium phosphate.

[0149] The depolymerization of the material derived from the multicomponent material containing nylon 6 in the presence of steam may be carried out in the presence of an additional depolymerizing agent such as ammonia. The concentration of ammonia in the depolymerization section [B] may vary. Thus, in the presence of ammonia in the depolymerization section [B], the vapor stream removed from the depolymerization section [B] includes not only ε-caprolactam and impurities, but also ammonia.

[0150] Most preferably, the depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted) acid or base. The acid catalyst may be specifically selected from the group consisting of orthophosphoric acid, p-toluenesulfonic acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids (including xylenesulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons), solid acids, salts of the above acids, Al2O3 and SiO2 and combinations thereof. The base catalyst may, for example, be selected from the group consisting of alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides, alkaline earth metal salts, organic bases and solid bases and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali metal hydroxides and alkali metal salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate are used as catalysts. Still more preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid and sodium hydroxide are used as catalysts. In a particularly preferred embodiment, orthophosphoric acid is used as catalyst for the depolymerization, in another embodiment p-toluenesulfonic acid is used.

[0151] However, in another preferred embodiment, no catalyst is used to depolymerize the material derived from the multi-component material containing nylon 6. This has the advantage of lower costs (for disposal of catalyst and catalyst waste). However, higher temperatures (and pressures) are generally required compared to depolymerization of the material derived from the multi-component material containing nylon 6 carried out in the presence of a catalyst.

[0152] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction already starts at a relatively low temperature and can be carried out under atmospheric conditions. The suitable concentration of the catalyst for depolymerizing nylon 6 into ε-caprolactam is known to the technician and can be easily determined by routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. On the contrary, if the concentration of the catalyst used is too high, the reaction is very fast, but side reactions will also increase. In addition, the catalyst cost increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the nylon 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1 to 50% by weight. The optimal catalyst concentration depends on the type of catalyst used for the depolymerization of nylon 6. For the catalyst orthophosphoric acid, the preferred content is 0.1 to 25% by weight, and more preferably 1 to 20% by weight. The preferred content of the catalyst p-toluenesulfonic acid is 10 to 35% by weight, and more preferably 15 to 30% by weight.

[0153] The depolymerization of nylon 6 can be carried out in a batch mode, a semi-continuous mode or a continuous mode, all of which are known to the skilled person. The terms "batch mode", "semi-continuous mode" and "continuous mode" used herein refer to a mode in which a raw material containing nylon 6, i.e., a material derived from a multi-component material containing nylon 6 and an optional catalyst are charged into a depolymerization reactor, and a mode in which a residual material is discharged from the depolymerization reactor.

[0154] In a preferred embodiment, the depolymerization of nylon 6 is carried out in a batch mode. In the batch mode, the raw materials, i.e., the materials derived from the multi-component material containing nylon 6 and the optional catalyst are initially charged into the depolymerization reactor. Subsequently, superheated steam is charged into the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Next, the charging of superheated steam into the depolymerization reactor is interrupted. After optionally removing residual materials from the depolymerization reactor, a new cycle is started by charging the raw materials (and optional catalyst) into the depolymerization reactor. In a preferred embodiment, the residual materials are not removed between each cycle.

[0155] In a particularly advantageous embodiment, the depolymerization of nylon 6 is carried out in a continuous mode. In the continuous mode, the raw material containing nylon 6 (and optionally the catalyst) is continuously charged into the depolymerization reactor. At the same time, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged into the depolymerization reactor. In addition, the residual material is continuously discharged from the depolymerization reactor. Preferably, the material derived from the multi-component material containing nylon 6 is charged in the form of a melt. Preferably, the catalyst is charged in the form of a melt, a slurry or a solution.

[0156] In another preferred embodiment, the depolymerization of nylon 6 is carried out in a semi-continuous mode. In the semi-continuous mode, the feedstock containing nylon 6 (and optional catalyst) is intermittently charged into the depolymerization reactor, while superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. In the semi-continuous mode of nylon 6 depolymerization, the residual material is intermittently discharged from the depolymerization reactor.

[0157] Recovery step c)

[0158] In recovery section [C], ε-caprolactam is recovered from the stream comprising ε-caprolactam discharged from depolymerization section [B]. The stream comprises ε-caprolactam and impurities. Preferably, the recovery is performed by (partial) condensation of the stream comprising ε-caprolactam.

[0159] Preferably, the ε-caprolactam obtained by condensation is dissolved in water without charging a solvent into the depolymerization section [B], thereby obtaining a phase rich in ε-caprolactam. The phase rich in ε-caprolactam also comprises impurities.

[0160] Preferably, in case water is charged as solvent to the depolymerization section [B], the stream comprising ε-caprolactam discharged from the depolymerization section [B] comprises ε-caprolactam, water and impurities. The water may be charged in the form of liquid or steam. Preferably, the water is charged in the form of steam. ε-caprolactam may be separated from the stream comprising ε-caprolactam discharged from the depolymerization section [B] by conveying this vapor or gaseous stream from the depolymerization reactor (preferably the top) to a (preferably partial) condenser to obtain a condensate containing ε-caprolactam. Preferably, ε-caprolactam is separated from the remaining components of the vapor stream by conveying the product stream from the depolymerization reactor (preferably the top) to a distillation column, from which a water-rich phase as top product and an ε-caprolactam-rich phase as bottom product are obtained.

[0161] The ε-caprolactam recovered in the recovery section [C] is crude, in that it contains impurities, such as nylon 6 decomposition products or other impurities resulting from (decomposition products / reactions of) non-nylon 6 components of the material derived from the multi-component material containing nylon 6. The crude ε-caprolactam recovered in step c) comprises water and ε-caprolactam, preferably the crude ε-caprolactam is an aqueous solution comprising ε-caprolactam. Therefore, the crude ε-caprolactam recovered in the recovery section [C] requires additional purification to produce high-purity ε-caprolactam. Therefore, "crude" as used herein may be defined as being lower in purity, i.e. containing more impurities, than the purified ε-caprolactam obtained as a product of the process of the present invention.

[0162] Preferably, the crude ε-caprolactam comprises in the range of 6 to 95 wt%, more preferably 20 to 90 wt%, and most preferably 35 to 80 wt% ε-caprolactam. The remainder is mainly water.

[0163] Purification step d)

[0164] In step d), the crude ε-caprolactam obtained in the recovery section [C] is purified in a purification section [D] to obtain high-purity ε-caprolactam.

[0165] Optionally, the crude ε-caprolactam is filtered before being charged to the purification section [D]. Filtration ensures the removal of undissolved impurities which might otherwise hamper the further purification process.

[0166] Optionally, oil is separated from the crude ε-caprolactam before charging to the purification section [D]. The separation of the oil ensures the removal of impurities which might otherwise hinder the further purification process.

[0167] Purified ε-caprolactam is obtained from crude ε-caprolactam by first extracting the crude ε-caprolactam with an organic solvent in step (i), thereby obtaining an aqueous phase and an organic phase comprising the organic solvent, ε-caprolactam and impurities. The organic solvent used for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 Optionally, the organic solvent used for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, an aliphatic hydrocarbon, a cycloaliphatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C 10 A mixed extractant consisting of an aliphatic or cycloaliphatic alcohol and a C5-C8 alkane or a C5-C8 cycloalkane. If the organic solvent used to extract 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 a mixture thereof, a particularly good purification result can be obtained. More preferably, the organic solvent used to extract the crude ε-caprolactam is selected from the group consisting of benzene, toluene, alcohol and a mixture thereof. Still more preferably, the organic solvent used to extract the crude ε-caprolactam is selected from the group consisting of toluene, 1-octanol, 4-methyl-2-pentanol, 2-ethylhexanol and a mixture thereof. Preferably, the weight ratio of the organic solvent to the ε-caprolactam is 0.01:1 to 40:1, preferably 0.05:1 to 15:1, and more preferably 0.1:1 to 7:1.

[0168] Optionally, the organic solvent used to extract the crude ε-caprolactam is mixed with an alkane C wherein m is 5 to 8. m H 2m+2 , cycloalkanes or Cm H 2m Particularly good results are obtained if the alkane or cycloalkane is present in the mixed extractant in an amount ranging from 5 to 90% by weight, and preferably from 25 to 75% by weight, based on the total weight of the mixed extractant.

[0169] In a preferred embodiment in which the organic solvent has a lower density than the crude ε-caprolactam, the extraction with the organic solvent in step d)(i) is carried out in an extraction column operated in countercurrent, whereby the crude ε-caprolactam to be purified is introduced at the upper part of the column and the organic solvent is introduced at the lower part of the column. The extraction results in an aqueous phase comprising water and impurities, and an organic phase comprising the organic solvent, ε-caprolactam and impurities, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Therefore, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0170] In another preferred embodiment of the present invention, in which the density of the organic solvent is higher than that of the crude ε-caprolactam, the extraction with the organic solvent in step d) (i) is carried out in an extraction column operated in countercurrent, whereby 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 of the column. The extraction results in an aqueous phase comprising water and impurities, and an organic phase comprising the organic solvent, ε-caprolactam and impurities, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the crude ε-caprolactam. Therefore, due to this extraction, the ε-caprolactam is purer than before the extraction.

[0171] Optionally, the organic phase comprising organic solvent, ε-caprolactam and impurities is washed with water or an alkaline aqueous solution before entering step d) (iv). If washing is carried out with an alkaline aqueous solution, the alkaline solution is preferably an aqueous solution comprising an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably comprises 0.5 to 2.0 wt. % sodium hydroxide or potassium hydroxide.

[0172] The technician can determine the amount of water or alkaline aqueous solution required for efficient washing of the organic phase including organic solvent, ε-caprolactam and impurities by routine experiments. Preferably, in the case of ε-caprolactam dissolved in the organic phase to be washed, the amount is between 0.1 and 5 weight % relative to the amount of organic solvent, excluding the ε-caprolactam dissolved in the organic phase to be washed. In another preferred embodiment, washing the organic phase including organic solvent, ε-caprolactam and impurities with water or alkaline aqueous solution is carried out in a countercurrent washing tower, whereby the organic phase including organic solvent, ε-caprolactam and impurities is introduced at the bottom of the tower, and water or alkaline aqueous solution is introduced at the top of the tower. Washing produces an organic phase including organic solvent, ε-caprolactam and impurities and an aqueous phase including residues. Typically, the aqueous phase including residues includes water, ε-caprolactam and impurities. Due to washing, the impurity content of the washed organic phase is reduced compared to the impurity content of the organic phase before washing.

[0173] Optionally, according to step d)(ii) of the process of the present invention, the obtained organic phase comprising organic solvent, ε-caprolactam and impurities, optionally washed with water or with an alkaline aqueous solution, is subjected to solvent switching, wherein the organic solvent in the organic phase comprising organic solvent, ε-caprolactam and impurities is replaced by water, and thereby 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 switching process is selected from a process based on back extraction (also called re-extraction) with water and a process based on solvent exchange distillation, thereby distilling off the organic solvent and charging water.

[0174] The term "replaced" 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.

[0175] The solvent switch may be a process based on back extraction with water, wherein an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is obtained. Preferably, the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is stripped and / or distilled to remove residual organic solvent. Although the amount of water used for the back extraction of ε-caprolactam may vary, the amount of water used is preferably 0.3 to 20 times, more preferably 0.4 to 10 times, and most preferably 0.5 to 5 times by weight based on the recovered ε-caprolactam.

[0176] Preferably, the stripping with water is carried out in an extraction column operated countercurrently.

[0177] In another preferred embodiment in which the organic phase comprising organic solvent, ε-caprolactam and impurities, which has been optionally washed, has a lower density than water, the organic phase is introduced at the lower part of the extraction column, and water is introduced at the upper part of the extraction column. The back extraction produces an organic solvent phase comprising impurities, and produces an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase comprising organic solvent, ε-caprolactam and impurities before back extraction. Therefore, a purer ε-caprolactam is obtained due to back extraction. Preferably, the organic solvent phase comprising impurities is reused optionally after purification (preferably by distillation).

[0178] In another preferred embodiment in which the organic phase comprising organic solvent, ε-caprolactam and impurities has a higher density than water, the organic phase is introduced at the upper part of the extraction column, and water is introduced at the lower part of the extraction column. The back extraction produces an organic solvent phase comprising impurities, and produces an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, wherein the weight ratio of impurities to ε-caprolactam is reduced compared to the weight ratio of impurities to ε-caprolactam in the organic phase comprising organic solvent, ε-caprolactam and impurities before back extraction. Therefore, due to back extraction, ε-caprolactam is purer than before back extraction. Preferably, the organic solvent phase comprising impurities is optionally reused after purification (preferably by distillation).

[0179] Therefore, according to a particularly advantageous embodiment of the present invention, after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises the step of (ii) a) a solvent switch based on back extraction with water.

[0180] The solvent switching process can also be a process based on solvent exchange distillation, whereby the organic solvent is distilled off and water is charged. In a preferred embodiment, the solvent switching process is a process based on solvent exchange distillation performed in the form of a single-stage process, whereby the organic solvent is distilled off from the organic phase comprising the organic solvent, ε-caprolactam and impurities, and water is charged. More preferably, the solvent switching is performed in the form of azeotropic distillation with the addition of water, in which case the organic solvent evaporates in the form of an azeotropic mixture comprising the organic solvent and water. The purpose of the azeotropic distillation is to remove the organic solvent and add water. Preferably, substantially all of the organic solvent is removed. In this context, "substantially all" means that at least 90% by weight, preferably at least 95% by weight, and most preferably at least 98 or 99% by weight of the organic solvent present in the organic phase comprising the organic solvent, ε-caprolactam and impurities is removed. Preferably, water is added in the form of a liquid. More preferably, water in liquid form is added to the upper part of the distillation column in the form of reflux. Even more preferably, a portion of the water added in the form of reflux is obtained by condensation of the azeotropic mixture distilled in the distillation column.

[0181] Any suitable vessel may be used for the solvent switch process, such as a column, preferably a distillation column operated in a continuous mode. The distillation column may comprise trays, packing or a combination thereof.

[0182] In another preferred embodiment, the solvent exchange distillation is carried out as a two-stage process. The first stage is a pre-concentration stage and the second stage is the actual solvent exchange distillation.

[0183] The organic phase comprising organic solvent, ε-caprolactam and impurities is charged to the first stage. In the first stage, a first fraction of the organic solvent is removed by distillation from the organic phase comprising organic solvent, ε-caprolactam and impurities located at the upper part of the distillation column. Preferably, the distillation is carried out 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 charged to the upper part of the distillation column in the form of a liquid after condensation. The remaining organic phase comprising organic solvent, ε-caprolactam and impurities is discharged from the first stage and charged to the second stage. Due to the distillation in the first stage, the chemical composition of the remaining organic phase comprising organic solvent, ε-caprolactam and impurities is different from the organic phase comprising organic solvent, ε-caprolactam and impurities charged to the first stage. Typically, the remaining organic phase comprising organic solvent, ε-caprolactam and impurities contains a higher amount (in weight percentage) of ε-caprolactam and compounds with a higher boiling point than ε-caprolactam and a lower weight percentage of compounds with a lower boiling point than ε-caprolactam than the organic phase comprising organic solvent, ε-caprolactam and impurities charged to the first stage.

[0184] In the second stage, the remaining organic solvent is distilled off from the remaining organic phase comprising the organic solvent, ε-caprolactam and impurities, and water is charged. More preferably, in the second stage, the solvent switch is carried out in the form of azeotropic distillation with addition of water, in which case the organic solvent evaporates in the form of an azeotropic mixture comprising the organic solvent and water.

[0185] Any suitable vessel may be used in each stage of the solvent switch, such as a column, preferably a distillation column operated in a continuous mode. The distillation column may comprise trays, packing or a combination thereof.

[0186] The solvent exchange distillation (performed as a single-stage process or a two-stage process) produces an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, and optionally residual organic solvent. Preferably, the ε-caprolactam content of the aqueous phase is between 25 and 99.9% by weight, more preferably between 50 and 99.5% by weight, and most preferably between 85 and 99% by weight, relative to the total aqueous phase.

[0187] Therefore, according to a particularly advantageous embodiment of the present invention, after the extraction of the crude ε-caprolactam in step d)(i), the purification in step d) further comprises a step of (ii) b) a solvent switch based on a solvent exchange distillation.

[0188] Optionally, according to step d)(iii) of the process of the present invention, impurities having a boiling point lower or higher than that of ε-caprolactam are removed by distillation under vacuum conditions before the crystallization in step d)(iv), thereby obtaining a phase comprising ε-caprolactam and impurities. The phase comprising ε-caprolactam and impurities obtained is purer, i.e. contains fewer impurities, than the phase charged to step d)(iii).

[0189] In step d)(iii) of the process of the present invention, the organic phase comprising organic solvent, ε-caprolactam and impurities obtained by the extraction in step d)(i), optionally washed with water or with an alkaline aqueous solution, is distilled to remove the organic solvent and impurities having a boiling point lower or higher than ε-caprolactam from the organic phase. Preferably, the distillation is carried out under reduced pressure. Even more preferably, the distillation is carried out at a pressure lower than 80 kPa, more preferably lower than 20 kPa and most preferably lower than 10 kPa. Preferably, the temperature is between 90° C. and 210° C., and more preferably between 110° C. and 180° C. These temperatures refer to the temperatures in the bottom of the distillation column where the distillation is carried out.

[0190] Alternatively, in step d)(iii) of the process of the present invention, the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam obtained by the solvent switch in step d)(ii) is distilled to remove impurities having a boiling point lower or higher than ε-caprolactam from the aqueous phase.

[0191] Preferably, water is first evaporated from an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. After the evaporation of water, the ε-caprolactam is distilled to recover high-purity ε-caprolactam. Preferably, the distillation is carried out under reduced pressure. Even more preferably, the distillation is carried out at a pressure lower than 50 kPa, more preferably lower than 20 kPa and most preferably lower than 10 kPa. Preferably, the temperature is between 100° C. and 200° C., and more preferably between 110° C. and 180° C. These temperatures refer to the temperatures in the bottom of the distillation column where the distillation is carried out.

[0192] The distillation comprises separating low-boiling organic impurities (having a lower boiling point than ε-caprolactam) from ε-caprolactam and / or separating high-boiling organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. The distillation preferably comprises: in a first step, separating low-boiling impurities from ε-caprolactam as a top product, and producing high-boiling impurities containing ε-caprolactam as a bottom product. In a second step, separating high-purity ε-caprolactam as a top product, and obtaining a distillation residue comprising ε-caprolactam and high-boiling impurities as a bottom product.

[0193] Therefore, according to a particularly advantageous embodiment of the present invention, the purification in step d) further comprises a step of (iii) distilling under vacuum conditions to remove impurities having a boiling point lower or higher than ε-caprolactam, before the crystallization in step d)(iv).

[0194] Optionally, according to step d)(iii) of the process of the invention, impurities with a boiling point lower or higher than ε-caprolactam are removed by distillation under vacuum conditions after the crystallization in step d)(iv), thereby obtaining a phase comprising ε-caprolactam and impurities.

[0195] Therefore, according to a particularly advantageous embodiment of the present invention, after the crystallization in step d) (iv), the purification in step d) further comprises a step of (iii) distilling under vacuum conditions to remove impurities having a boiling point lower or higher than ε-caprolactam.

[0196] Optionally, according to step d)(iii) of the process of the invention, impurities with boiling points lower or higher than ε-caprolactam are removed by distillation under vacuum conditions before and after the crystallization in step d)(iv), thereby obtaining a phase comprising ε-caprolactam and impurities.

[0197] Therefore, according to a particularly advantageous embodiment of the present invention, the purification in step d) further comprises, before and after the crystallization in step d)(iv), a step of (iii) distilling under vacuum conditions to remove impurities having a boiling point lower or higher than ε-caprolactam.

[0198] In a preferred embodiment, an alkali metal hydroxide, preferably NaOH, is added to the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam before the distillative removal in step d)(iii). Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol, more preferably and most preferably 2 to 80 mmol / kg ε-caprolactam. Experiments have shown that the addition of an alkali metal hydroxide, in particular NaOH, is particularly effective for the distillative removal of impurities having a boiling point lower and higher than ε-caprolactam.

[0199] In another preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide, is added to the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam prior to the distillative removal in step d)(iii). Most preferably, potassium permanganate is used as the oxidizing agent.

[0200] The oxidant can be added to the aqueous phase including water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam in the form of a solid, a slurry or an aqueous solution so that a diluted aqueous solution is obtained. The skilled person can determine the amount of oxidant required for the efficient oxidation of the aqueous phase including water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam by routine experiments. Among other things, the exact amount of oxidant depends to a large extent on the composition of the material derived from the multi-component material containing nylon 6 used as feed in the process of the present invention. Preferably, the amount of oxidant is between 0.01 and 5 weight % relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0201] The temperature used for oxidizing the aqueous solution in the process of the present invention may vary. Preferably, the oxidation of the aqueous solution comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam with an oxidizing agent is carried out at a temperature in the range of 20° C. to 85° C., more preferably in the range of 30° C. to 80° C., prior to the distillative removal in step d) (iii), 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.

[0202] The length of time for oxidation with an oxidizing agent can vary. Preferably, the aqueous solution comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is oxidized with an oxidizing agent for 1 minute to 24 hours, more preferably 2 minutes to 6 hours, and most preferably 5 minutes to 2 hours.

[0203] The concentration of ε-caprolactam in the aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam oxidized with the oxidant may vary. Preferably, the aqueous solution used for oxidation comprises a weight ratio of ε-caprolactam to water of 5:1 to 1:5, more preferably 3:1 to 1:3 and most preferably 2:1 to 1:2. Optionally, the weight ratio of ε-caprolactam to water is adjusted before the oxidant is added to the aqueous phase. Preferably, the weight ratio of ε-caprolactam to water is adjusted by adding water or by removing water.

[0204] In the case of using potassium permanganate or sodium permanganate as the oxidant, solid manganese (IV) oxide (MnO2) particles are formed as the reaction product. The best solid-liquid filtration procedure for efficiently removing the solid manganese (IV) oxide particles from the aqueous phase after oxidation can be determined by the skilled person through routine experimentation. In this regard, it is common practice to use filter aids such as activated carbon or diatomaceous earth particles to improve the filtration procedure.

[0205] In a preferred embodiment of the present invention, before crystallization in step d)(iv), a solution comprising a solvent, ε-caprolactam and impurities is hydrogenated in the presence of a hydrogenation catalyst. Preferably, the solvent is water. 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 made of nickel is used. Suitable nickel catalysts typically have a nickel content between 5 and 80 wt.% relative to the metal and the carrier. In addition to nickel, the catalyst may also contain some activators, such as Zr, Mn, Cu or Cr. The activator content is typically between 1 and 20 wt.%. If a palladium-containing heterogeneous catalyst is used, the palladium content will typically be between 0.01 and 10 wt.%.

[0206] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. The hydrogenation can be carried out, for example, in a stirred tank reactor, wherein the catalyst particles are suspended in the mixture to be purified (slurry phase process). In another embodiment, the hydrogenation is carried out in a fixed bed reactor, wherein the catalyst is fixed in the reactor.

[0207] The hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising an aqueous ε-caprolactam mixture, gaseous hydrogen and a heterogeneous hydrogenation catalyst. Alternatively, the hydrogenation can be carried out in a two-phase system (liquid, solid) comprising an aqueous ε-caprolactam mixture fully or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in the water-ε-caprolactam mixture to obtain a mixture fully or partially saturated with hydrogen can be carried out by any method known to those skilled in the art.

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

[0209] The hydrogenation of the water-ε-caprolactam mixture is to hydrogenate the unsaturated compounds present in the impure ε-caprolactam. The presence of these unsaturated compounds is disadvantageous because they impair the physical and mechanical properties of nylon 6 prepared by polymerizing ε-caprolactam. The saturated compounds formed by hydrogenation do not have an adverse effect on these physical and mechanical properties of nylon 6, and these compounds are more easily removed in, for example, a distillation step and / or a crystallization step after the hydrogenation step.

[0210] In step d)(iv) of the process of the present invention, purified ε-caprolactam is obtained by crystallizing ε-caprolactam from a solution comprising ε-caprolactam and impurities at a temperature of 10 to 95°C, more preferably at a temperature of 20 to 85°C.

[0211] The solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is an organic phase comprising an organic solvent, ε-caprolactam and impurities obtained by extraction in step d)(i), said organic phase optionally being washed with water or with an alkaline aqueous solution. Preferably, the solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than that of ε-caprolactam obtained by solvent switching in step d)(ii). More preferably, the solution comprising ε-caprolactam and impurities from which the purified ε-caprolactam is obtained by crystallization is a phase comprising ε-caprolactam and impurities obtained by distillation under vacuum conditions in step d)(iii).

[0212] Preferably, the ε-caprolactam crystallization process in step d(iv) comprises the following steps:

[0213] 1. Feeding a solution comprising ε-caprolactam and impurities into a crystallizer;

[0214] 2. setting conditions in the crystallizer so that ε-caprolactam crystals and mother liquor are formed;

[0215] 3. Separating ε-caprolactam crystals and mother liquor;

[0216] 4. Recirculate the mother liquor.

[0217] Crystallization can be applied to the production of ε-caprolactam. It is mainly used for its purification potential and / or product recovery to increase yield. All crystallization processes are based on the formation of a solid crystalline phase from a liquid. In a preferred embodiment, the crystallization in step d) (iv) is carried out by solution crystallization or melt crystallization.

[0218] The term solution crystallization is used to crystallize a compound from a solution comprising the (impure) compound and to which an auxiliary solvent is added. The auxiliary solvent is water or a non-aqueous solvent. If the auxiliary solvent is water, the amount of water in the solution can be selected in a wide range, preferably the amount of water is in the range of 0.5 to 25 wt.%, more preferably 1 to 9 wt.%. The crystallization temperature can be selected in a wide range, preferably the crystallization temperature range is 10 to 95 ° C, more preferably 20 to 85 ° C, even more preferably the crystallization temperature range is 20 to 70 ° C, most preferably 30 to 65 ° C. Crystallization purified ε-caprolactam is recovered from a slurry preferably ranging from 5 to 75 wt.%, more preferably ranging from 10 to 70 wt.%, most preferably ranging from 15 to 50 wt.%. If the auxiliary solvent is a non-aqueous solvent, the amount of the non-aqueous solvent in the solution can be selected in a wide range, preferably the amount of the non-aqueous solvent ranges from 5 to 95 wt.%, more preferably 10 to 90 wt.%, most preferably 30 to 70 wt.%. The crystallization temperature can be selected in a wide range, preferably the crystallization temperature range is 10 to 95°C, more preferably 20 to 85°C, even more preferably the crystallization temperature range is 20 to 70°C, most preferably 30 to 65°C. Crystallization purified ε-caprolactam is recovered from a slurry preferably having a slurry concentration of 5 to 75 wt.%, more preferably 10 to 70 wt.%, most preferably 15 to 50 wt.%. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform or ethyl chloride), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate), and mixtures of these solvents. Among them, cyclohexane is preferred.

[0219] Solution crystallization is usually carried out at atmospheric pressure, but can also be carried out under reduced or pressurized conditions. In the case of solution crystallization, the product is crystallized by evaporative crystallization in which the solvent is evaporated or by cooling crystallization in which cooling is obtained by direct cooling, indirect cooling or vacuum cooling or by a combination of these methods. After the crystallization step, the formed crystals and mother liquor are separated by, for example, sedimentation, filtration and / or centrifugation. Optionally, the obtained crystals are washed with, for example, a solvent or a solvent mixture with a low impurity content. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in the form of crystals.

[0220] The narrow definition of the term melt crystallization is to crystallize a compound from a solution comprising the (impure) compound without using an auxiliary solvent. In the broader definition of the term, melt crystallization is also applicable to crystallization from a solution containing a low solvent concentration. Preferably, the solvent concentration in the solution is less than 25wt%, more preferably less than 10wt.%, most preferably less than 5wt.%. Here, unless otherwise explicitly mentioned, the broader definition of melt crystallization will be used. The compound crystals obtained by melt crystallization are separated from the mother liquor and optionally washed with a melt of pure compound material. Optionally, the crystallization-separation sequence is repeated several times. Finally, the optionally washed crystals are melted and discharged or mechanically removed in the form of a melt.

[0221] Preferably, the solvent is present in the mixture in the crystallizer, but crystallization can also be carried out without a solvent. Many solvents for ε-caprolactam are suitable. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform or ethyl chloride), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate). Preferably water and aromatic hydrocarbons are used as solvents because these solvents produce large crystals. As solvent, water is most preferred. The solvent will serve as a freezing point depressant for the melt in the crystallizer.

[0222] In general, melt crystallization requires less energy than solution crystallization, however, it can be more challenging to operate on an industrial scale.

[0223] Melt crystallization used herein specifically refers to layer melt crystallization or suspension melt crystallization. The characteristics of these two types of melt crystallization technology methods are that (1) a crystal layer is formed on the heat exchanger wall (layer melt crystallization) and (2) crystals grown in a suspension (suspension melt crystallization). In general, the operation of the method for the crystal layer grown on the heat exchanger wall is called layer melt crystallization. First, the melt is loaded into a crystallizer, and then a crystal layer is grown on the cooled heat exchanger surface, followed by discharging the remaining melt containing impurities discharged from the growing crystals from the crystallizer, and then the crystal layer is melted and the purified product is recovered. The purification efficiency can be further improved by, for example, sweating (also known as partial melting), that is, gently heating the crystal layer to close to its melting temperature, thereby discharging the trapped and adhered impure mother liquor. The layer melt crystallization method is operated in batch mode. Well-known examples of methods based on layer melt crystallization are the ProABD method of BEFS Prokem and the Sulzer Chemtech method.

[0224] Layer melt crystallization can be carried out in static or dynamic mode. In static crystallization mode, crystals grow from a stagnant melt onto a cooling surface. In static mode, the desired compound is crystallized in batches on the heat exchanger wall from a stagnant melt in a closed container. This type of crystallization is characterized by a low growth rate of the crystals and the long residence (or batch) time caused thereby. Preferably, the crystallization time ranges from 1 hour to 75 hours, more preferably from 2 hours to 50 hours, and most preferably from 4 hours to 24 hours. After the crystallization step, the remaining melt is discharged. Then, a sweating phase is optionally introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and discharged or mechanically removed.

[0225] In general, dynamic crystallization is carried out in a shell and tube heat exchanger, whereby the melt circulates downward along a cooling surface where the compound crystallizes. In general, the melt is pumped through the tube, and the crystal grows inside the tube while the cooling medium flows outside the tube. The thickness of the crystal layer increases over time. After a certain period of time, the circulation of the melt is stopped and the remaining melt is discharged. Like stagnant layer crystallization, dynamic layer crystallization is also carried out in batch mode. Compared with the stagnant mode, the crystal growth rate in the dynamic mode is higher, and therefore the crystallization time is shorter. Preferably, the crystallization time ranges from 0.05 hours to 12 hours, more preferably from 0.1 hours to 6 hours, and most preferably from 0.3 hours to 3 hours. Then, a sweating phase is optionally introduced to remove impurities adhering to the crystal or trapped in the crystal. Finally, the crystal is completely melted and discharged or mechanically removed.

[0226] Suspension melt crystallization can be carried out in batch mode or in continuous mode. In the case of suspension melt crystallization, the melt is cooled below its saturation temperature, and the crystal begins to grow (optionally, after adding nuclei). The growth rate of the crystal is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be carried out in any exchanger type or container type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is carried out in a scraped surface crystallizer. Optionally, after crystallization, the mixture of the obtained crystal and mother liquor is separated by filtration. Optionally, after crystallization, the mixture of the obtained crystal and mother liquor of the desired compound is loaded into a so-called washing tower. In the washing tower, the mother liquor is filtered or discharged from the crystal, and the crystal is then usually washed with a purified compound material.

[0227] After the ε-caprolactam crystallization step, a mother liquor is obtained which includes ε-caprolactam in addition to impurities. Methods for recovering ε-caprolactam from such mother liquors are well known to those skilled in the art. And due to these recovery methods, almost all of the ε-caprolactam present in the mother liquor can be recovered and converted into high-purity ε-caprolactam. In the case of multi-stage crystallization, a possible solution is to recycle the mother liquor in a countercurrent manner, that is, the mother liquor obtained in the nth crystallization stage is charged into the feed of the (n-1)th crystallization stage. In general, the mother liquor obtained from the 1st crystallization stage is charged into an upstream (purification) unit or a dedicated mother liquor processing unit (e.g., based on distillation or crystallization) of the method. After the ε-caprolactam crystallization, it may be necessary to purify it by, for example, recycling the obtained mother liquor (or a portion thereof) to any previous stage in the method. Alternatively, the mother liquor can be purified, for example, by distillation, before being recharged into the ε-caprolactam crystallization step.

[0228] The high purity ε-caprolactam obtained according to the process of the present invention can be used to prepare nylon 6 using methods well known to the skilled person. The nylon 6 can then be used in all known materials, including engineering materials, fibers and films. The nylon 6 produced from materials derived from multi-component materials containing nylon 6 is particularly suitable for high-speed spinning applications, including garments containing spandex (also known as elastic fiber).

[0229] factory

[0230] The present invention also provides a plant configured to carry out the above-mentioned method of the present invention, i.e. a chemical plant, said plant comprising a depolymerization section [B], a recovery section [C] and a purification section [D]. All plant features described below in detail in connection with the plant also correspond to specific embodiments of the method of the present invention, and vice versa. Thus, said plant is suitable for carrying out the method of the present invention, and it should be understood that what has been described in connection with the method of the present invention applies equally to the plant embodiment.

[0231] The plant may be a laboratory setting as in the examples. Preferably, however, the plant is an industrial scale plant. "Industrial scale" means that the ε-caprolactam production capacity (i.e. the amount of ε-caprolactam that can be produced in principle) of the plant, if it were operating, is at least 500 tonnes / year.

[0232] The plant of the present invention is suitable for producing purified ε-caprolactam from a material derived from a multicomponent material containing nylon 6, and comprises at least the following three sections: a depolymerization section [B], a recovery section [C] and a purification section [D]. These sections and therefore the plant are configured to carry out the above-described process of the present invention.

[0233] In addition, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [β] for breaking the substance including the multi-component material containing nylon 6 into pieces and / or a cleaning section [α] for cleaning the substance including the multi-component material containing nylon 6 and / or a densification section [γ] for obtaining a material with an increased bulk density. Cleaning comprises washing the substance including the multi-component material containing nylon 6 and separating foreign materials from the substance. The separation of foreign materials can be performed manually (hand picking) and mechanically (e.g. density separation and magnetic separation). Manual and mechanical devices, such as brushes, can assist the washing process in the cleaning section [α]. Washing is preferably carried out by additional friction. Different types of industrial washing systems are available on the market, such as high-speed friction washers. The mechanical size reduction section [β] includes equipment for mechanically breaking the substance including the multi-component material containing nylon 6 into pieces. Non-limiting examples of such breaking equipment are cutters, shredders, mills, grinders and chippers. The densification section [γ] comprises equipment for densifying a mass comprising a multicomponent material containing nylon 6, optionally crushed and / or cleaned. Densification can be performed by several techniques known to those skilled in the art to obtain a material with a higher bulk density. Well-known examples of densification equipment include electric and hydraulic compactors and presses, as well as equipment in which the feed is first melted and then solidified by cooling, such as single-screw and twin-screw extruders.

[0234] The pretreatment section [A] further comprises an extraction section [ω] from which a material derived from the multi-component material containing nylon 6 is discharged. The extraction section [ω] is charged with a substance comprising a multi-component material containing nylon 6, which is optionally subjected to size reduction and / or cleaning and / or densification. In the extraction section [ω], a solid pre-concentrated material containing nylon 6 enriched in nylon 6 compared to the multi-component material containing nylon 6 is obtained by extracting the substance comprising the multi-component material containing nylon 6 with one or more solvents as described above. In addition, a solid pre-concentrated material not containing nylon 6 enriched in non-nylon 6 compounds compared to the multi-component material containing nylon 6 is obtained by extracting the substance comprising the multi-component material containing nylon 6 with one or more organic solvents.

[0235] The extraction section [ω] comprises means configured to carry out one or more extraction steps of the following types:

[0236] - pre-concentration, wherein non-nylon 6 compounds are preferentially extracted from the nylon 6-containing multi-component material (the pre-concentrated nylon 6-containing material obtained is then, optionally after removal of any adhering solvent, an unextracted nylon 6-containing multi-component material);

[0237] - pre-concentration, wherein nylon 6 compounds are preferentially extracted from the multi-component nylon 6-containing material (and a pre-concentrated nylon 6-containing material is then obtained after removing the solvent from a liquid extract phase comprising the solvent and dissolved nylon 6); and

[0238] - Preferentially extracting nylon 6 and non-nylon 6 compounds from a multi-component material containing nylon 6 in a sequential manner, i.e. performing a combination of extraction of non-nylon 6 compounds and nylon 6 sequentially.

[0239] The selection of solvents and processing conditions (eg, temperature, length of treatment, and amount of solvent relative to the amount of multicomponent material containing nylon 6) have been described previously.

[0240] The extraction section [ω] may comprise one or more extraction devices, one or more solvent recovery and purification devices, one or more solid-liquid separation devices, one or more drying devices and one or more densification devices.

[0241] The extraction device is selected from a (optionally stirred) mixing extractor, an extraction column, a centrifugal extractor and a combination thereof. Preferably, the substance including the multi-component material containing nylon 6 is charged into the extraction device and treated with one or more solvents. The resulting liquid-solid mixture is separated into a solid phase (with an adhering liquid) and a liquid phase in a solid-liquid separation device. Preferably, the solid-liquid separation is performed by settling in a settler, filtering in a filter and / or centrifuging in a centrifuge. In a centrifugal extractor, extraction and solid-liquid separation are completed in one device.

[0242] Optionally, the obtained solid material is washed, for example with a cleaning solvent. Optionally, the extraction-separation sequence is repeated several times.

[0243] One or more solvent recovery and purification devices are used to recover the solvent from the liquid phase obtained after the solid-liquid separation and optionally purify the solvent. Preferably, the recovery of the solvent is accomplished by evaporation followed by condensation of the vapors, whereby the dissolved material will precipitate. Preferably, the solvent recovery is carried out in a distillation device and / or a (vacuum) dryer equipped with a condenser operating in a continuous, batch or semi-continuous mode.

[0244] Preferably, the solvent is reused in a closed circuit, optionally after distillation.

[0245] Optionally, the resulting precipitate is washed with, for example, a cleaning solvent.

[0246] Preferably, both the solid pre-concentrated nylon 6-free material enriched in non-nylon 6 compounds and the solid pre-concentrated nylon 6-containing material enriched in nylon 6 are dried before being discharged from the extraction section [ω].

[0247] In a preferred embodiment of the present invention, the plant further comprises a pretreatment section [A] comprising an extraction section [ω] and an optional mechanical size reduction section [β], as well as an optional cleaning section [α] and an optional densification section [γ].

[0248] In a more preferred embodiment of the present invention, the plant further comprises a pre-treatment section [A] comprising a mechanical size reduction section [β] and a cleaning section [α] and optionally a densification section [γ] and an extraction section [ω].

[0249] The depolymerization section [B] comprises one or more depolymerization reactors operated in series and / or in parallel. The material derived from the multi-component material containing nylon 6 is fed into the reactor in the form of a solid or a melt, preferably a melt. This feeding can be achieved by using an extruder, a gear pump or other means known in the art.

[0250] During production, the depolymerization reactor is at least partially filled with feedstock containing nylon 6, residual material, ε-caprolactam (and optionally a catalyst). The depolymerization reactor may have any desired form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors and extruder type reactors.

[0251] The depolymerization reactor must be equipped with means for feeding the material originating from the multicomponent material containing nylon 6 and optionally superheated steam and a catalyst. In addition, the depolymerization reactor is equipped with means for discharging a stream comprising ε-caprolactam and residual materials.

[0252] Good contact between the steam and the reactor contents is essential for efficient operation. Such contact can be achieved by various means known to the skilled person. As an example, multiple inlets can be used, such as using a steam distributor to spray steam through the material. Even further improved contact can be achieved by including mechanical agitation in the reactor, such as using a combination of rotating paddles and static fins.

[0253] Preferably, depolymerization will be complete within 0.5 to 6 hours.

[0254] If high-temperature superheated steam is not available at the production location, superheated steam must be produced intentionally by superheating the available steam in the boiler in a so-called superheater.

[0255] The recovery section [C] may comprise one or more (preferably partial) condensers, into which a stream comprising ε-caprolactam is charged in the form of a vapour stream comprising ε-caprolactam and water. Such (partial) condensers may have any desired form. Preferably, the condenser is a distillation column, from which a water-rich phase is obtained as top product and crude ε-caprolactam is obtained as bottom product.

[0256] The purification section [D] may comprise one or more extraction devices, one or more solvent switching devices, an oxidation section, a hydrogenation section, one or more distillation devices and a crystallization section, into which crude ε-caprolactam is charged and from which high purity ε-caprolactam is discharged.

[0257] The crude ε-caprolactam and the organic solvent are charged into the extraction device, and the organic phase including the organic solvent, ε-caprolactam and impurities and the aqueous phase including water and impurities are discharged. The extraction device is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor and a combination thereof. Preferably, the extraction device is a static or stirred extraction column, such as tower, towers, rotating disk contactors (RDC), pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (SMVP) (static) towers.

[0258] The solvent switching device is charged with water and an organic phase comprising an organic solvent, ε-caprolactam and impurities, and the organic solvent and the ε-caprolactam-water phase comprising water, ε-caprolactam and impurities are discharged. The solvent switching device for the method based on stripping is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor and a combination thereof. Preferably, the device for stripping is a static or stirred extraction column, such as tower, towers, rotating disk contactors (RDC), pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (static) towers.

[0259] The solvent switching device for the process based on solvent exchange distillation is selected from a sieve plate distillation column, a random packing distillation column and a structured packing distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser and a device for reflux. The distillation column can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure. Preferably, water is charged to the upper part of the distillation column, and the aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam is discharged from the lower part of the distillation column.

[0260] The oxidation section comprises one or more oxidation reactors operated in series and / or in parallel. The oxidant and the ε-caprolactam-water phase comprising water, ε-caprolactam and impurities are charged into the oxidation section. Typically, the oxidant is charged in the form of a solid, a slurry or an aqueous solution. In the case of applying potassium permanganate or sodium permanganate as the oxidant, the oxidation section further comprises a filtration section. In the filtration section, the solid MnO2 particles formed are separated from the oxidized ε-caprolactam-water phase. The oxidation reactor may have any desired form. Preferred reactor types are stirred and non-stirred reactors and packed tower reactors. The oxidation reactor must be equipped with facilities for feeding an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower than or higher than ε-caprolactam and an oxidant. In addition, the oxidation reactor must be equipped with facilities for discharging the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities and optionally formed solid manganese oxide (IV) (MnO2) particles. Preferably, the oxidation is carried out at a temperature and atmospheric conditions ranging from 20°C to 85°C.

[0261] The solid manganese (IV) oxide (MnO2) particles which are optionally present can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to improve the filtration procedure using filter aids such as activated carbon particles or diatomaceous earth. Filtration systems suitable for separating solid manganese (IV) oxide particles are known to the skilled person. Into this filtration system is loaded a suspension of an oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities and solid manganese (IV) oxide particles, and the filtered oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities is discharged. Typically, the solid manganese (IV) oxide particles remain in the filtration system. Preferably, this filtration system is operated in a semi-continuous mode, whereby the suspension and the filtered phase are continuously loaded and continuously discharged, while the separated solids are collected in the filtration system. The loading of the suspension is interrupted from time to time, and the collected solids are removed from the filtration system.

[0262] Purification of the crude ε-caprolactam to obtain purified ε-caprolactam in step d) may comprise hydrogenation using a heterogeneous catalyst, in which case the plant will comprise a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0263] The hydrogenation section includes one or more hydrogenation reactors operated in series and / or in parallel. Hydrogenation can be carried out in a three-phase system (gas, liquid, solid), which is composed of an ε-caprolactam aqueous mixture, gaseous hydrogen and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid), which is composed of an ε-caprolactam aqueous mixture completely or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in a water-ε-caprolactam mixture can be carried out by any method known to those skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or a mixer, wherein a constant hydrogen pressure is maintained. Close contact between hydrogen and the mixture will ensure that hydrogen is dissolved in the mixture. This process is preferably carried out continuously. Subsequently, the mixture containing hydrogen is contacted with a hydrogenation catalyst, for example in a separate reactor.

[0264] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. The hydrogenation can be carried out, for example, in a stirred tank reactor, wherein the catalyst particles are suspended in the mixture to be hydrogenated (slurry phase method). In this slurry phase method, the catalyst particles and the purified mixture must be separated in another method step after the hydrogenation reaction, for example by filtering. Preferably, the catalyst comprises palladium or nickel.

[0265] Alternatively, the hydrogenation can be carried out in a fixed bed reactor, in which the catalyst is fixed in the reactor, so that an additional step for separating the catalyst and the reaction mixture can be omitted. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0266] The hydrogenation temperature is typically between 20° C. and 160° C. The hydrogenation pressure is typically between 0.1 and 15 MPa.

[0267] The ε-caprolactam-water phase including water, ε-caprolactam and impurities is charged into a distillation apparatus, and high-purity ε-caprolactam, water and impurities (i.e., low-boiling organic impurities (lower boiling point than ε-caprolactam) and high-boiling organic impurities (higher boiling point than ε-caprolactam)) are discharged. The distillation apparatus is selected from a sieve plate distillation column, a random packing distillation column, a structured packing distillation column, and a horizontal and vertical (descending and ascending) film evaporator. Preferably, the distillation column is equipped with a reboiler, a condenser and a device for reflux. The distillation apparatus can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure, preferably at subatmospheric pressure.

[0268] Preferably, the distillation comprises separating water, low-boiling organic impurities (having a lower boiling point than ε-caprolactam) and / or high-boiling organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises: in a first step, separating out from water as a top product, and producing low-boiling impurities and high-boiling impurities containing ε-caprolactam as a bottom product. In a second step, separating out low-boiling impurities as a top product, and obtaining high-boiling impurities containing ε-caprolactam as a bottom product. In a third step, separating out high-purity ε-caprolactam as a top product, and producing a distillation residue comprising ε-caprolactam and high-boiling impurities as a bottom product. Optionally, the first step and the second step are combined.

[0269] Preferably, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities before distilling off water and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol / kg ε-caprolactam, and more preferably 2 to 80 mmol / kg ε-caprolactam. This allows particularly efficient distillation removal of impurities having lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0270] The crystallization section comprises one or more crystallizers operated in series and / or in parallel. In general, the crystallization section also comprises several containers for storing (intermediate) product streams and / or fresh and used washing liquids. The crystallization of ε-caprolactam can be carried out by solution crystallization or melt crystallization as described above.

[0271] In the case of solution crystallization, the ε-caprolactam is recovered by evaporative crystallization in which the solvent evaporates or by cooling crystallization in which cooling is obtained by direct cooling, indirect cooling or vacuum cooling or by a combination of these methods. After the crystallization step in the crystallizer, the formed crystals and the mother liquor are separated, for example, by settling in a settler, filtering in a filter and / or centrifuging in a centrifuge. Optionally, the crystallizer is equipped with a stirrer and / or one or more baffles.

[0272] Optionally, the obtained crystals are washed with, for example, a cleaning solvent or a saturated aqueous solution of ε-caprolactam. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in the form of crystals.

[0273] The auxiliary solvent is water or a non-aqueous solvent. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propyl alcohol, n-butyl alcohol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform or ethyl chloride), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate), and mixtures of these solvents. In general, the auxiliary solvent is recovered and reused in the crystallization process.

[0274] Solution crystallization is usually carried out at atmospheric pressure, but can also be carried out under reduced or increased pressure conditions.

[0275] The melt crystallization of ε-caprolactam can be achieved by layer melt crystallization, in which a layer of crystals containing ε-caprolactam is formed on the heat exchanger wall, or by suspension melt crystallization, in which crystals containing ε-caprolactam grow in suspension.

[0276] Preferably, the solvent is present in the mixture in the melt crystallizer, but melt crystallization can also be carried out in the absence of a solvent. Many solvents for ε-caprolactam are suitable. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform or ethyl chloride), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate). Preferably water and aromatic hydrocarbons are used as solvents because these solvents produce large crystals. As solvent, water is most preferred. The solvent will serve as a freezing point depressant for the melt in the crystallizer.

[0277] Layer melt crystallization:

[0278] First, the melt is charged into a crystallizer, then a crystal layer is grown on a cooled heat exchanger surface, the remaining melt containing impurities discharged from the growing crystals is subsequently discharged from the crystallizer, and the crystal layer is then melted and the purified product is recovered. The purification efficiency can be further improved by, for example, sweating (also known as partial melting), i.e., gently heating the crystal layer to close to its melting temperature, thereby discharging trapped and adhering impure mother liquor. The layer melt crystallization process is operated in batch mode. Well-known examples of processes based on layer melt crystallization are the ProABD process of BEFS Prokem and the SulzerChemtech process.

[0279] Layer melt crystallization can be carried out in static or dynamic mode. In static crystallization mode, crystals grow from a stagnant melt onto a cooling surface. In static mode, the desired compound is crystallized in batches on the heat exchanger wall from a stagnant melt in a closed container. This type of crystallization is characterized by a low growth rate of the crystals and the long residence (or batch) time caused thereby. Preferably, the crystallization time ranges from 1 hour to 75 hours, more preferably from 2 hours to 50 hours, and most preferably from 4 hours to 24 hours. After the crystallization step, the remaining melt is discharged. Then, a sweating phase is optionally introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and discharged or mechanically removed.

[0280] In general, dynamic crystallization is carried out in a shell and tube heat exchanger, whereby the melt circulates downward along a cooling surface where the compound crystallizes. In general, the melt is pumped through the tube, and the crystal grows inside the tube while the cooling medium flows outside the tube. The thickness of the crystal layer increases over time. After a certain period of time, the circulation of the melt is stopped and the remaining melt is discharged. Like stagnant layer crystallization, dynamic layer crystallization is also carried out in batch mode. Compared with the stagnant mode, the crystal growth rate in the dynamic mode is higher, and therefore the crystallization time is shorter. Preferably, the crystallization time ranges from 0.05 hours to 12 hours, more preferably from 0.1 hours to 6 hours, and most preferably from 0.3 hours to 3 hours. Then, a sweating phase is optionally introduced to remove impurities adhering to the crystal or trapped in the crystal. Finally, the crystal is completely melted and discharged or mechanically removed.

[0281] Suspension melt crystallization:

[0282] The suspension melt crystallization of ε-caprolactam can be carried out in batches or in a continuous mode. In the case of suspension melt crystallization, the melt is cooled below its saturation temperature, and the ε-caprolactam crystals begin to grow (optionally, after adding nuclei). The growth rate of the crystals is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be carried out in any exchanger type or container type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is carried out in a scraped surface crystallizer. Optionally, after crystallization, the mixture of the obtained crystals and mother liquor is separated by filtration. Optionally, after crystallization, the mixture of the obtained ε-caprolactam crystals and mother liquor is loaded into a so-called washing tower. In the washing tower, the mother liquor is filtered or discharged from the ε-caprolactam crystals, and then the ε-caprolactam crystals are usually washed with purified ε-caprolactam.

[0283] The method of the present invention can be operated in a continuous, semi-continuous or batch mode. Therefore, the plant of the present invention can also be configured to allow one or more of these operating modes. In a preferred embodiment, the plant is configured to operate the method of the present invention in a continuous or semi-continuous manner. However, a discontinuous method is also possible. For example, the plant of the present invention does not need to contain all the sections described herein in one position. Specifically, the pretreatment section [A] can be located at a first position, while the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a second position. Similarly, the mechanical size reduction section [β] as part of the pretreatment section [A] can also be located at a first position, while the cleaning section [α] as part of the pretreatment section [A] can be located at a second position, and the extraction section [ω] as part of the pretreatment section [A] can be located at a third position, and the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a fourth position. Optionally, the cleaning section [α] is divided into two or more sections, and the two or more sections are optionally all located at different positions. For example, the first section of the cleaning section [α] as part of the pretreatment section [A] can be located at a first position, the mechanical size reduction section [β] as part of the pretreatment section [A] can be located at a second position, and the second section of the cleaning section [α] as part of the pretreatment section [A] can be located at a third position, and the extraction section [ω] as part of the pretreatment section [A] is located at a fourth position, and the depolymerization section [B], the recovery section [C] and the purification section [D] are located at a fifth position. Optionally, the extraction section [ω] as part of the pretreatment section [A] is divided into two or more sections, and the two or more sections are optionally all located at different positions. Optionally, the extraction section [ω] as part of the pretreatment section [A] is located at the same position as the depolymerization section [B]. Optionally, the depolymerization section [B] is located at a different position than the pretreatment section [A] and / or the recovery section [C] and the purification section [D].

[0284] product

[0285] The present invention provides a novel product ε-caprolactam obtained by depolymerization of nylon 6 produced from a material derived from a multi-component material containing nylon 6 according to the process of the present invention. The advantageous feature of the ε-caprolactam is in particular that the product carbon footprint is less than 2.5 kg CO2 / kg purified ε-caprolactam. The ε-caprolactam obtained according to the present invention may also be referred to as "purified ε-caprolactam". "Purified" as used herein means that the ε-caprolactam is produced from a material derived from a multi-component material containing nylon 6 according to the process of the present invention, which enables it to be obtained in a purified form. In this sense, the ε-caprolactam is obtained and purified from a material derived from a multi-component material containing nylon 6.

[0286] The process of the invention allows the production of high purity and therefore high quality ε-caprolactam meeting the specifications for demanding applications and at the same time the process is particularly environmentally friendly due to its reduced product carbon footprint and the use of waste as starting materials. In a preferred embodiment, the ε-caprolactam obtained by the process of the invention meets one or more of the following specifications, wherein the parameters and measurement methods are defined as in the Examples section below:

[0287] PAN: Maximum 5

[0288] E290: Maximum value 0.05

[0289] VB: Maximum value 0.5mmol / kg

[0290] Alkalinity: Maximum value 0.1mmol / kg

[0291] Acidity: Maximum value 0.1mmol / kg.

[0292] The ε-caprolactam produced by the process of the invention is also particularly economical and environmentally friendly. This is evident because the carbon footprint of the ε-caprolactam produced by the process of the invention is much lower than that of conventionally produced ε-caprolactam (e.g. by Beckmann rearrangement of cyclohexanone oxime).

[0293] The environmental impact of a product is often expressed as a 'product carbon footprint'. The carbon footprint of a product is defined as the total emissions caused by the formation of the product, expressed in tonnes of carbon dioxide equivalent per tonne of product. The carbon footprint of a product depends inter alia on raw materials, auxiliary materials, energy consumption, energy sources, production methods and process efficiency. The quantification of the product carbon footprint can be carried out as described, for example, in the European standard EN ISO 14040:2006 ("Environmental management - Life cycle assessment - Principles and framework").

[0294] The product carbon footprint calculation can be performed internally or by an external (preferably) certified organization. These organizations verify and certify product carbon footprint calculations based on, for example, the LCA standard ISO 14040.

[0295] J. Hong and X. Xu (“Environmental impact assessment of caprolactam production–a case study in China”); J. of Cleaner Production 27 (2012) 103-108; DOI: 10.1016 / j.jclepro.2011.12.037) reported that the global warming potential of “virgin” ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime in the context of coal-based electricity and steam generation is 7.5 tonnes of CO2 equivalent / tonne of ε-caprolactam (which is equivalent to 7.5 kg CO2 equivalent / kg ε-caprolactam). In the case involving natural gas based electricity and steam generation, the potential impact of virgin ε-caprolactam on global warming for the ε-caprolactam production process would drop to 6.4 tonnes CO2 equivalent / tonne ε-caprolactam (which is equivalent to 6.4 kg CO2 equivalent / kg ε-caprolactam).

[0296] The product carbon footprint of the ε-caprolactam obtained according to the process of the present invention is much lower than the product carbon footprint of de novo synthesized or "virgin" ε-caprolactam. The product carbon footprint of the ε-caprolactam obtained in the process of the present invention is less than 4 kg, more preferably less than 3 kg, and most preferably equal to or less than 2.5 kg CO2 equivalent / kg ε-caprolactam. BRIEF DESCRIPTION OF THE DRAWINGS

[0297] In the following, the present invention is described with reference to the accompanying drawings, which depict certain embodiments of the present invention. However, the present invention is defined in the claims and as generally described herein. It should not be limited to the embodiments shown in the following drawings for illustrative purposes.

[0298] Figure 1 is a schematic diagram of the process of the present invention comprising processing steps performed in an optional or off-site pretreatment section [A], a depolymerization section [B], a recovery section [C] and a purification section [D].

[0299] FIG. 2 shows two embodiments of a pretreatment section [A], wherein a substance comprising a multi-component material containing nylon 6 is cleaned in a cleaning section [α] by removing foreign materials and washing with a washing solvent, and is crushed in a mechanical size reduction section [β] to obtain fragments of a cleaned and crushed substance comprising a multi-component material containing nylon 6, and then the fragments are extracted with a solvent in an extraction section [ω] to obtain a pre-concentrated material derived from the multi-component material containing nylon 6.

[0300] Figure 2A An embodiment of a pretreatment section [A'] is depicted, wherein a substance comprising a multi-component material containing nylon 6 is first cleaned in a cleaning section [α'] by removing foreign material and washing with a washing solvent, and then crushed in a mechanical size reduction section [β'] to obtain fragments of cleaned and crushed substance comprising a multi-component material containing nylon 6, and then the fragments are extracted with a solvent in an extraction section [ω'] to obtain a material derived from the multi-component material containing nylon 6.

[0301] Figure 2B An embodiment of a pretreatment section [A"] is depicted, wherein a substance comprising a multi-component material containing nylon 6 is first crushed in a mechanical size reduction section [β"] and then cleaned in a cleaning section [α"] by removing foreign material and washing with a solvent to obtain cleaned and crushed fragments of the substance comprising a multi-component material containing nylon 6, and then the fragments are extracted with a solvent in an extraction section [ω"] to obtain a material derived from the multi-component material containing nylon 6.

[0302] FIG3 shows two examples of a purification section [D], wherein crude ε-caprolactam is purified to obtain high-purity ε-caprolactam.

[0303] Figure 3A An embodiment of a purification section [D"'] of the process of the present invention is depicted, comprising an extraction section [γ"'], an optional washing section [δ"'], an optional stripping section [ε"'], an optional distillation section [θ"'] and a crystallization section [λ"'].

[0304] Figure 3B An embodiment of a purification section [D""] of the process of the present invention is depicted, comprising an extraction section [γ""], an optional wash section [δ""], an optional solvent exchange distillation section [μ""], an optional distillation section [θ""] and a crystallization section [λ""].

[0305] Detailed description with drawings

[0306] The method of the present invention is Figure 1 The method is implemented in the following plant sections:

[0307] Optionally, in the pretreatment section [A], the multi-component material [1] containing nylon 6 or a substance comprising the material is cleaned by removing foreign materials and washing with a washing solvent [2], thereby obtaining foreign materials and a contaminated washing solvent [3]. Optionally, the multi-component material [1] containing nylon 6 or a substance comprising the material is crushed by mechanical size reduction in the pretreatment section [A]. Optionally, foreign materials are removed after crushing. The optionally cleaned and crushed multi-component material containing nylon 6 or a substance comprising the material is charged to the extraction section. Optionally, the fragments of the optionally cleaned and crushed multi-component material containing nylon 6 or a substance comprising the material are densified before being charged to the extraction section. In the extraction section, the multi-component material containing nylon 6 is extracted with a solvent [4], thereby discharging a material not containing nylon 6 [5]. Further, a material [6] derived from the multi-component material containing nylon 6 enriched in nylon 6 is discharged from the pretreatment section [A] and charged to the depolymerization section [B]. Optionally, the material [6] derived from the multicomponent material containing nylon 6 is densified before being depolymerized to ε-caprolactam in the depolymerization zone [B].

[0308] In the depolymerization section [B], the optionally densified material [6] originating from the multicomponent material containing nylon 6 is depolymerized to ε-caprolactam. A stream [7] comprising ε-caprolactam is discharged from the depolymerization section [B]. In addition, residual material [8] is discharged. Optionally, superheated steam [9] and a catalyst

[10] are fed to the depolymerization section [B].

[0309] In the recovery section [C], crude ε-caprolactam

[11] is recovered from the stream [7] comprising ε-caprolactam discharged from the depolymerization section [B]. In addition, in the case where water or superheated steam [9] is fed to the depolymerization section [B], an aqueous phase

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

[0310] The crude ε-caprolactam

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

[26] . Water and impurities

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

[0311] Figure 2AAn embodiment of a pretreatment section [A'] (area surrounded by a dotted line) is depicted, wherein a multi-component material [1'] containing nylon 6 is first cleaned in a cleaning section [α'] by removing foreign materials and washing with a washing solvent [2'], thereby obtaining a foreign material and contaminated washing solvent [3'] and a cleaned substance [a'] including a multi-component material containing nylon 6. Subsequently, the cleaned multi-component material containing nylon 6 [a'] is crushed in a mechanical size reduction section [β'] to obtain cleaned and crushed fragments of a multi-component material containing nylon 6 [c']. The cleaned and crushed fragments of a multi-component material containing nylon 6 are then discharged from the mechanical size reduction section [β'] and charged to an extraction section [ω']. Optionally, the cleaned and crushed fragments of a multi-component material containing nylon 6 are densified before being charged to the extraction section [ω']. In the extraction section [ω'], the cleaned and crushed fragments of the multi-component material containing nylon 6, which are optionally densified, are extracted with an organic solvent [4'], thereby obtaining a material [6'] derived from the multi-component material containing nylon 6 and a material [5'] not containing nylon 6, both of which are discharged from the extraction section [ω']. The material [6'] derived from the multi-component material containing nylon 6 is charged to the depolymerization section [B]. Optionally, the material [6'] derived from the multi-component material containing nylon 6 is densified ( Figure 2A not shown).

[0312] Figure 2BAn embodiment of a pretreatment section [A"] (area surrounded by a dotted line) is depicted, wherein a multi-component material containing nylon 6 [1"] is first crushed in a mechanical size reduction section [β"] to obtain crushed fragments [b"] of the multi-component material containing nylon 6. Subsequently, the crushed fragments [b"] of the multi-component material containing nylon 6 are cleaned in a cleaning section [α"] by removing foreign material and washing with a washing solvent [2"] to obtain foreign material and contaminated washing solvent [3"] and cleaned and crushed fragments [c"] of the multi-component material containing nylon 6. The cleaned and crushed fragments [c"] of the multi-component material containing nylon 6 are then discharged from the cleaning section [α"] and charged into an extraction section [ω"]. Optionally, the cleaned and crushed fragments [c"] of the multi-component material containing nylon 6 are densified before being charged into the extraction section [ω"]. In the extraction section [ω"], the optionally densified cleaned and crushed fragments of the multi-component material containing nylon 6 are extracted with an organic solvent [4"] to obtain a material [6"] derived from the multi-component material containing nylon 6 and a material [5"] not containing nylon 6, both of which are discharged from the extraction section [ω"]. The material [6"] derived from the multi-component material containing nylon 6 is charged to the depolymerization section [B]. Optionally, the material [6"] derived from the multi-component material containing nylon 6 is densified ( Figure 2B not shown).

[0313] Figure 3A An example of a purification segment [D"'] (area surrounded by dashed lines) is depicted, comprising the following segments:

[0314] In the extraction section [γ"'], the crude ε-caprolactam [11"'] is extracted with an organic solvent [13"'] to obtain an aqueous phase [14"'] comprising water and impurities and an organic phase [15"'] comprising the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the extraction section [γ"'].

[0315] In the optional washing section [δ"'], the organic phase [15"'] comprising the organic solvent, ε-caprolactam and impurities is washed with water or an alkaline aqueous solution [16"'] to obtain an aqueous phase [17"'] comprising the residue and a washed organic phase [18"'] comprising the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ"'].

[0316] In the optional stripping section [ε"'], the optionally washed organic phase [18"'] comprising organic solvent, ε-caprolactam and impurities is stripped with water [19"'] to obtain an organic solvent phase [20"'] comprising impurities and an aqueous phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Both phases are discharged from the stripping section [ε"']. Optionally, residual organic solvent is removed from the aqueous phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam by stripping and / or distillation. Figure 3A not shown).

[0317] Optionally, an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, from which any residual organic solvent has been removed by stripping and / or distillation, is oxidized with an oxidizing agent [22"'] to obtain an oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities ( Figure 3A not shown).

[0318] Optionally, the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities is filtered to remove solid manganese (IV) oxide particles ( Figure 3A not shown).

[0319] Optionally, the aqueous phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, from which any residual organic solvent has been removed by stripping and / or distillation, is hydrogenated with hydrogen in the presence of a heterogeneous catalyst before being charged to the next section to obtain a hydrogenated ε-caprolactam-water phase ( Figure 3A not shown).

[0320] In the optional distillation section [θ"'], the aqueous phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is distilled to remove impurities having a boiling point lower or higher than ε-caprolactam and optional organic solvent or water, thereby obtaining a distilled ε-caprolactam phase [24"']. All distillation products are discharged from the distillation section [θ"']. Optionally, an alkali metal hydroxide [23"'] is dosed into the ε-caprolactam-water phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam before the distillation in the distillation section [θ"'].

[0321] In the crystallization section [λ"'], the optionally distilled ε-caprolactam phase [24"'] is crystallized to remove impurities [25"'] from the ε-caprolactam, thereby obtaining high-purity ε-caprolactam [26"']. All crystallized products are discharged from the crystallization section [λ"']. Optionally, a solvent is charged to the crystallization section [λ"'] before the crystallization of the ε-caprolactam ( Figure 3A not shown).

[0322] Figure 3B An example of a purification segment [D""] (area surrounded by dashed lines) is depicted, comprising the following segments:

[0323] In the extraction section [γ""], the crude ε-caprolactam [11""] is extracted with an organic solvent [13""] to obtain an aqueous phase [14""] comprising water and impurities and an organic phase [15""] comprising the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the extraction section [γ""].

[0324] In the optional washing section [δ""], the organic phase [15""] comprising the organic solvent, ε-caprolactam and impurities is washed with water or an alkaline aqueous solution [16""] to obtain an aqueous phase [17""] comprising the residue and a washed organic phase [18""] comprising the organic solvent, ε-caprolactam and impurities. Both phases are discharged from the washing section [δ""].

[0325] In the optional solvent exchange distillation section [μ""], the optionally washed organic phase [18""] comprising organic solvent, ε-caprolactam and impurities is subjected to solvent exchange distillation by adding water [19""] to obtain an organic solvent [21""] and an aqueous phase [22""] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. Both distillation products are discharged from the solvent exchange distillation section [μ""].

[0326] Optionally, an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, from which any residual organic solvent has been removed by stripping and / or distillation, is oxidized with an oxidizing agent [22""] to obtain an oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities ( Figure 3B not shown).

[0327] Optionally, the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities is filtered to remove solid manganese (IV) oxide particles ( Figure 3B not shown).

[0328] Optionally, the aqueous phase [22""] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, from which any residual organic solvent has been removed by stripping and / or distillation, is hydrogenated with hydrogen in the presence of a heterogeneous catalyst before being charged to the next section to obtain a hydrogenated ε-caprolactam-water phase comprising water, ε-caprolactam and impurities ( Figure 3B not shown).

[0329] In the optional distillation section [θ""], the aqueous phase [22"'] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam is distilled to remove impurities having a boiling point lower or higher than ε-caprolactam and optional organic solvent to obtain a distilled ε-caprolactam phase [24""']. All distillation products are discharged from the distillation section [θ""]. Optionally, an alkali metal hydroxide [23""] is dosed into the ε-caprolactam-water phase [22""] comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam before the distillation in the distillation section [θ""].

[0330] In the crystallization section [λ""], the optionally distilled ε-caprolactam phase [24""] is crystallized to remove impurities [25""] from the ε-caprolactam, thereby obtaining high-purity ε-caprolactam [26""]. All crystallized products are discharged from the crystallization section [λ""]. Optionally, before the crystallization of the ε-caprolactam, a solvent is charged to the crystallization section [λ""] ( Figure 3B not shown).

[0331] Examples

[0332] The following examples are used to explain the present invention in more detail, particularly with respect to certain forms of the present invention. However, the examples are not intended to limit the present disclosure.

[0333] ε-Caprolactam, which can be used in all major nylon 6 polymerization applications without dilution with purer qualities of ε-caprolactam, meets all of the following specifications:

[0334] PAN: Maximum 5

[0335] E290: Maximum value 0.05

[0336] VB: Maximum value 0.5mmol / kg

[0337] Alkalinity: Maximum value 0.1mmol / kg

[0338] Acidity: Maximum value 0.1mmol / kg

[0339] The parameters and measurement methods are defined as follows:

[0340] PAN: ISO DIS 8660 - Plastics - Determination of the permanganate index of caprolactam - Spectroscopic method, revision of the first edition of ISO 8660; 1988,

[0341] E290: ISO 7059 - Caprolactam for industrial use - Determination of absorbance at a wavelength of 290 nm,

[0342] Volatile base (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile base content - Titration after distillation.

[0343] Basicity of ε-caprolactam product: measured by using Tashiro indicator at 0.1wt. / v at 25°C 乙醇 % Methylene blue: 0.1wt. / v 乙醇 Basicity is determined by a 1:2 ratio titration with 0.01N H2SO4 solution (if the solution is alkaline) or 0.01N NaOH solution (if the solution is acidic). The flask containing the water and indicator is first titrated to a gray color, then X grams of an aqueous solution of ε-caprolactam containing Y wt.% ε-caprolactam (as determined by refractive index) is added and the solution is titrated back to a gray color using a 0.01N H2SO4 solution (if the solution is alkaline) or a 0.01N NaOH solution (if the solution is acidic).

[0344] The alkalinity is then given by:

[0345] Alkalinity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y)

[0346] in:

[0347] v = volume of H2SO4 solution added (ml)

[0348] t = normal concentration of H2SO4 solution (= 0.01N)

[0349] X = sample weight (g)

[0350] Y = concentration of ε-caprolactam (wt.%)

[0351] The acidity is then given by:

[0352] Acidity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y)

[0353] in:

[0354] v = volume of NaOH solution added (ml)

[0355] t = molar concentration of NaOH solution (=0.01N)

[0356] X = sample weight (g)

[0357] Y = concentration of ε-caprolactam (wt.%)

[0358] The starting material used in Examples 1, 2 and 3 and Comparative Examples was a waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer. The nylon 6 layer was embedded in the used waste multilayer packaging film by being sandwiched between other layers and was therefore not well accessible.

[0359] The nylon 6-preconcentration step a) of the waste multilayer packaging film is carried out by selectively dissolving polyethylene in an organic solvent, such as white mineral spirits (Sigma-Aldrich; CAS No. 68551-17-7) or methylcyclohexane, at elevated temperature, whereby the resulting mixture comprises undissolved nylon 6 and an organic solution comprising the organic solvent and the dissolved polyethylene. After separation of the organic solution from the resulting mixture, the undissolved nylon 6 material is obtained. The resulting undissolved nylon 6 material can be dried and optionally densified by melting under nitrogen and subsequently converted into solid particles enriched in nylon 6 compared to the multicomponent starting material containing nylon 6.

[0360] Example 1

[0361] Depolymerization of nylon 6 and recovery of ε-caprolactam.

[0362] The material used for depolymerization and further processing steps as described below can be any multicomponent material containing nylon 6 or a derivative thereof. The specific material used in the following examples is enriched in nylon 6 compared to the multicomponent starting material containing nylon 6 and was prepared from waste multilayer packaging film as described above.

[0363] The nylon 6-enriched starting material was formed 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 DSC (differential scanning calorimetry). The color of the nylon 6 starting material was yellow-green.

[0364] 48.1 g of pearl solid particles and 15.0 g of 20 wt.% phosphoric acid were charged into a Premex autoclave. First, the reactor contents were heated under nitrogen, and then superheated steam was continuously injected at a rate of 4 g per minute during a reaction period of 120 minutes. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, a vapor stream was continuously discharged from the reactor.

[0365] The obtained vapor stream was cooled to 20° C. in order to recover crude ε-caprolactam from the vapor stream. The condensate consisting of 34.7 g of ε-caprolactam (most of the remainder was water) was concentrated by evaporation in a rotary evaporator (rotary evaporator) operated under vacuum (9.5 kPa; water bath temperature was about 65° C.) to an ε-caprolactam concentration of about 54.4 wt.%. (The obtained mixture is crude ε-caprolactam, which is the mixture to be purified.)

[0366] The specifications of crude ε-caprolactam are:

[0367] PAN: 396

[0368] E290: 3.13

[0369] This example shows that crude ε-caprolactam can be obtained in good yields by depolymerization of nylon 6 originating from discarded nylon 6 scrap multilayer packaging films. Due to very poor quality, this crude ε-caprolactam cannot be used as such in all major nylon 6 polymerization applications.

[0370] Comparative Experiment 1

[0371] Depolymerization of nylon 6, recovery of ε-caprolactam, extraction, alkali washing and stripping.

[0372] The depolymerization and recovery procedures of Example 1 were followed.

[0373] 70 grams of crude ε-caprolactam obtained in Example 1 were extracted once with 100 grams of a solvent mixture 4-methyl-2-pentanol (50 wt.%) / cyclohexane (50 wt.%) at 25°C and extracted nine times with 50 grams of the solvent mixture. The ten resulting ε-caprolactam phases comprising the solvent mixture were combined and washed with 7 grams of a caustic aqueous solution (2 wt.%). The washed organic extract was concentrated to an ε-caprolactam concentration of about 40 wt.% by distillation under vacuum conditions, and then fresh cyclohexane was added. The ε-caprolactam concentration of the resulting mixture was about 25 wt.%, and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50 wt.%:50 wt.%. Subsequently, the washed and concentrated solvent mixture comprising ε-caprolactam was extracted six times with 50 grams of water at 25°C. The six resulting ε-caprolactam aqueous phases were combined. The combined aqueous phases were concentrated by evaporation in a rotary evaporator (rotavapor) operated under vacuum (9.5 kPa; water bath temperature of about 65° C.) to an ε-caprolactam concentration of 50.4 wt.%. The specifications of the concentrated ε-caprolactam aqueous solution obtained were:

[0374] PAN:82

[0375] E290:1.17

[0376] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of nylon 6 originating from discarded nylon 6 scrap multilayer packaging films and purified by extraction, alkali washing and stripping is very poor as it does not meet any of the specifications required for major polymerization applications.

[0377] Comparative Experiment 2

[0378] Depolymerization of nylon 6, recovery of ε-caprolactam, extraction, alkali washing, stripping and oxidation.

[0379] The aqueous solution of ε-caprolactam purified by stripping obtained in Comparative Experiment 1 was then treated with 0.04 wt.% KMnO4 relative to ε-caprolactam at 50°C for 2 hours. The solid formed was then removed from the oxidized reaction product by filtration. The specifications of the purified ε-caprolactam were:

[0380] PAN:83

[0381] E290:3.01

[0382] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of nylon 6 originating from discarded nylon 6 scrap multilayer packaging films and purified by extraction, alkali washing, stripping and oxidation is very poor as it does not meet any of the specifications required for major polymerization applications.

[0383] Comparative Experiment 3

[0384] Depolymerization of nylon 6, recovery and distillation purification of ε-caprolactam.

[0385] The procedure of Example 1 was followed.

[0386] 75 mmol of an aqueous sodium hydroxide solution / kg of ε-caprolactam was added to the obtained crude ε-caprolactam.

[0387] Subsequently, water and impurities with a lower boiling point than ε-caprolactam are removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, at 300 Pa, the distilled ε-caprolactam is recovered as top product, while impurities with a higher boiling point than ε-caprolactam remain in the distillation apparatus as bottom product. The specifications of the distilled ε-caprolactam are:

[0388] PAN: 74

[0389] E290: 0.53

[0390] VB: 0.72mmol / kg

[0391] Acidity: 7.64mmol / kg

[0392] This comparative experiment shows that the quality of ε-caprolactam obtained from the depolymerization of nylon 6 originating from discarded nylon 6 scrap multilayer packaging films and purified by distillation is very poor, as it does not meet any of the specifications required for major polymerization applications.

[0393] Comparative Experiment 4

[0394] Depolymerization of nylon 6, recovery of ε-caprolactam and purification by distillation and crystallization.

[0395] The procedure of Example 1 was followed.

[0396] 75 mmol of an aqueous sodium hydroxide solution / kg of ε-caprolactam was added to the obtained crude ε-caprolactam.

[0397] Subsequently, water and impurities with a lower boiling point than ε-caprolactam are removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, at 300 Pa, the distilled ε-caprolactam is recovered as top product, while impurities with a higher boiling point than ε-caprolactam remain in the distillation apparatus as bottom products.

[0398] Distilled water is then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 92.3 wt.%. The aqueous ε-caprolactam is introduced into a crystallization apparatus at a temperature of 52°C. The aqueous ε-caprolactam is cooled to 40°C, under which crystals begin to form. The mixture is then further cooled to 30°C and maintained for 30 minutes. The crystallized ε-caprolactam is recovered by filtration and washed with an 85 wt.% ε-caprolactam aqueous solution. The specifications of the purified ε-caprolactam obtained are:

[0399] PAN: 4

[0400] E290: 0.07

[0401] VB: <0.02mmol / kg

[0402] Alkalinity: 0.21mmol / kg

[0403] This comparative experiment shows that the ε-caprolactam obtained from the depolymerization of nylon 6 originating from discarded nylon 6 scrap multilayer packaging films and purified by distillation and crystallization is of poor quality since it does not meet all of the specifications required for major polymerization applications.

[0404] Example 2

[0405] Purification by extraction, stripping, distillation and crystallization.

[0406] 70 grams of crude ε-caprolactam obtained in Example 1 were extracted once with 100 grams of a solvent mixture of 4-methyl-2-pentanol (50 wt.%) / cyclohexane (50 wt.%) at 25°C and extracted nine times with 50 grams of the solvent mixture. The obtained organic extract was concentrated to an ε-caprolactam concentration of about 40 wt.% by distillation under vacuum conditions, and then fresh cyclohexane was added. The ε-caprolactam concentration of the obtained mixture was about 25 wt.%, and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50 wt.%:50 wt.%. The mixture was extracted 7 times in batches with 50 grams of water at a temperature of about 25°C. The 7 aqueous phases obtained were combined. The combined aqueous phase was concentrated to an ε-caprolactam concentration of 47 wt.% by evaporation in a rotary evaporator (rotary evaporator) operated under vacuum (9.5 kPa; water bath temperature was about 65°C). The specifications of the concentrated ε-caprolactam aqueous solution obtained are:

[0407] PAN: 150

[0408] E290: 2.15

[0409] 75 mmol of aqueous sodium hydroxide solution / kg of ε-caprolactam is then added to the concentrated ε-caprolactam solution. Subsequently, water and impurities with a boiling point lower than that of ε-caprolactam are removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, at 300 Pa, the distilled ε-caprolactam is recovered as a top product, while impurities with a higher boiling point than ε-caprolactam remain in the distillation apparatus as bottom products. The specifications of the distilled ε-caprolactam are:

[0410] PAN: 16

[0411] E290: 0.18

[0412] Distilled water is then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt.%. The aqueous ε-caprolactam is introduced into a crystallization apparatus at a temperature of 52°C. The aqueous ε-caprolactam is cooled to 40°C and 2 mg of seed crystals are added to the mixture. The mixture is then further cooled to 30°C and maintained for 30 minutes. The crystallized ε-caprolactam is recovered by filtration and washed with an 85 wt.% ε-caprolactam aqueous solution. The specifications of the purified ε-caprolactam obtained are:

[0413] PAN: 3

[0414] E290: 0.03

[0415] VB: 0.05mmol / kg

[0416] Alkalinity: 0.02mmol / kg.

[0417] From this experiment it can be concluded that purified ε-caprolactam meeting all specifications required for major polymerization applications can be obtained from the depolymerization of nylon 6 derived from discarded nylon 6 scrap multilayer packaging film and purified by extraction, stripping, distillation and crystallization.

[0418] Example 3

[0419] Depolymerization, recovery, purification by extraction, stripping, distillation and crystallization.

[0420] The depolymerization and recovery procedures of Example 1 were followed.

[0421] 34 g of crude ε-caprolactam obtained in Example 1 were extracted once with 78 g of benzene at 25° C. and four times with 50 g of benzene. The resulting organic extract was concentrated to an ε-caprolactam concentration of about 25 wt.% by distillation under vacuum conditions. The mixture was extracted three times with 25 g of water at a temperature of about 25° C. The specifications of the ε-caprolactam aqueous solution after stripping were:

[0422] PAN:83

[0423] E290:1.15

[0424] 75 mmol of aqueous sodium hydroxide solution / kg of ε-caprolactam is then added to the concentrated ε-caprolactam solution. Subsequently, water and impurities with a lower boiling point than ε-caprolactam are removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, at 300 Pa, the distilled ε-caprolactam is recovered as a top product, while impurities with a higher boiling point than ε-caprolactam remain in the distillation apparatus as a bottom product.

[0425] Distilled water is then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt.%. The aqueous ε-caprolactam is introduced into a crystallization device at a temperature of 52°C. The aqueous ε-caprolactam is cooled to 40°C and some seed crystals are added to the mixture. The mixture is then further cooled to 30°C and kept for 30 minutes. The crystallized ε-caprolactam is recovered by filtration and washed with an 85 wt.% ε-caprolactam aqueous solution. The specifications of the purified ε-caprolactam obtained meet all of the specifications required for the main polymerization applications.

[0426] From this experiment it can be concluded that purified ε-caprolactam meeting all specifications required for major polymerization applications can be obtained from the depolymerization of nylon 6 derived from discarded nylon 6 scrap multilayer packaging film and purified by extraction, stripping, distillation and crystallization.

[0427] Experiment 4

[0428] Calculation of the product carbon footprint of purified ε-caprolactam.

[0429] A continuous process according to the invention for producing purified ε-caprolactam on an industrial scale from waste multilayer packaging films comprising a polyethylene layer and a nylon 6 layer was simulated. These waste multilayer packaging films comprising a polyethylene layer and a nylon 6 layer had a nylon 6 content of 20 wt.%.

[0430] The method comprises:

[0431] - cutting the waste multilayer packaging film comprising a polyethylene layer and a nylon 6 layer into small pieces;

[0432] - washing small pieces of waste multilayer packaging film comprising polyethylene layers and nylon 6 layers with water;

[0433] - washing small pieces of waste multilayer packaging film comprising polyethylene layers and nylon 6 layers by centrifugal drying;

[0434] - Extraction of polyethylene with white spirit;

[0435] - pre-concentrated nylon 6-containing material by centrifugation to separate undissolved solids;

[0436] - washing the undissolved solid pre-concentrated nylon 6 containing material with water;

[0437] - pre-concentrating the nylon 6-containing material and the aqueous extract by centrifugation of the washed undissolved solids;

[0438] - Decolorization of white mineral spirits solutions including polyethylene by treatment with activated carbon;

[0439] - recovering polyethylene from a white mineral spirit solution comprising polyethylene by cooling and partially evaporating the white mineral spirit;

[0440] - Melting and pelletizing the recycled polyethylene;

[0441] - Melting and pelletizing the washed undissolved solid pre-concentrated nylon 6 containing material;

[0442] - depolymerizing nylon 6 in a pelletized material containing nylon 6 under the action of H3PO4 and superheated steam;

[0443] - recovery of crude ε-caprolactam (80 wt.% ε-caprolactam) by partial condensation of the vapors exiting the depolymerization reactor;

[0444] - extracting the concentrated crude ε-caprolactam with toluene in countercurrent;

[0445] - counter-extracting the washed organic extract with water;

[0446] - Concentrating the aqueous extract by evaporation;

[0447] - Removal of lights and heavies by vacuum distillation; and

[0448] - Recovery of pure ε-caprolactam by melt crystallization at a temperature of 61°C.

[0449] The carbon footprint of purified ε-caprolactam was calculated based on the consumption data of raw materials, and the utility of the above method was based on data derived from ecoinvent version 3.7.1.

[0450] The results revealed that the product carbon footprint of purified ε-caprolactam obtained from a multi-component material containing nylon 6 was less than 2.5 tons of CO2 equivalent / ton of ε-caprolactam (location: Europe).

[0451] The main products of the process are polyethylene and pure ε-caprolactam. The recovery rate of pure ε-caprolactam is about 0.8 kg of pure ε-caprolactam per kg of nylon 6 in the waste multilayer packaging film comprising polyethylene layer and nylon 6 layer. The obtained by-products are incinerated with energy recovery.

[0452] The product carbon footprint of purified ε-caprolactam was calculated based on the consumption data of raw materials and the utility of the above method was based on data derived from ecoinvent version 3.7.1. The environmental impact distribution between purified ε-caprolactam and polyethylene products in the nylon 6-preconcentration section was based on the weight ratio of these products.

[0453] The results revealed that the product carbon footprint of purified ε-caprolactam obtained from a multilayer packaging film comprising a polyethylene layer and a nylon 6 layer was less than 2.5 tonnes of CO2 equivalent per tonne of ε-caprolactam (location: Europe).

Claims

1. A method for recovering purified ε-caprolactam from a material derived from a multicomponent material containing nylon 6 in a plant, wherein the plant comprises - depolymerization section [B], - Recovery section [C], and - purification segment [D], And wherein the method comprises the following steps: a) providing a pre-concentrated nylon 6-containing material, the pre-concentrated nylon 6-containing material having been obtained by extracting a multi-component material containing nylon 6 with one or more solvents to obtain a solid pre-concentrated nylon 6-containing material enriched in nylon 6 as compared to the multi-component material containing nylon 6, and charging the pre-concentrated material derived from the multi-component material containing nylon 6 to the depolymerization section [B]; b) depolymerizing said material derived from a multicomponent material containing nylon 6 at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C in said depolymerization section [B], such that a stream comprising ε-caprolactam is obtained; c) discharging the stream comprising ε-caprolactam from the depolymerization section [B] and recovering crude ε-caprolactam from the stream in the recovery section [C]; as well as d) purifying the crude ε-caprolactam in the purification section [D] to obtain purified ε-caprolactam, wherein the purification comprises the following steps: (i) extracting the crude ε-caprolactam with an organic solvent to obtain an organic phase, and wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities; and (ii) a) solvent switching based on back extraction with water; or (ii) b) solvent switching based on solvent exchange distillation; as well as (iii) distilling under vacuum conditions to remove impurities having a boiling point lower or higher than ε-caprolactam; and (iv) obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution comprising ε-caprolactam and impurities at a temperature of 10 to 95° C.; and wherein the multi-component material containing nylon 6 is or includes a) a multilayer film comprising at least one layer comprising or consisting of nylon 6 and at least one layer comprising or consisting of a polyolefin, b) a multilayer film comprising at least one layer comprising or consisting of nylon 6 and at least two layers comprising or consisting of polyolefins, wherein the at least one layer comprising or consisting of nylon 6 is sandwiched between two or more layers comprising or consisting of polyolefins, and / or c) A multi-component material comprising nylon 6 and at least one polyolefin, wherein the nylon 6 is present in the multi-component material containing nylon 6 as separate domains and / or in a mixture with the at least one polyolefin.

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

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

4. The process according to any one of claims 1 to 3, wherein the organic solvent in step d)(i) is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol and mixtures thereof.

5. The process according to any one of claims 1 to 4, wherein in step d) the purification of crude ε-caprolactam to obtain purified ε-caprolactam comprises - a step of oxidation with an oxidizing agent in aqueous solution at a temperature ranging from 20° C. to 85° C., wherein the oxidizing agent is chosen from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, in particular potassium permanganate, and wherein the oxidation is carried out in an aqueous solution comprising water and ε-caprolactam in a weight ratio of 5:1 to 1:5; and / or - a hydrogenation step with a heterogeneous catalyst, wherein the catalyst preferably comprises nickel or palladium.

6. The method according to any one of claims 1 to 5, wherein the depolymerization of the material derived from the multi-component material containing nylon 6 in step b) is carried out in the absence or presence of a catalyst, wherein the catalyst is selected from an acid catalyst and a base catalyst, the acid catalyst is selected from the group consisting of orthophosphoric acid, boric acid, sulfuric acid, organic acid, organic sulfonic acid, solid acid, salts of the above acids, Al2O3 and SiO2 and combinations thereof, in particular orthophosphoric acid, and the base catalyst is selected from the group consisting of alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof, in particular sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate, preferably the depolymerization of the material derived from the multi-component material containing nylon 6 in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

7. The process according to any one of claims 1 to 6, wherein the solution comprising ε-caprolactam and impurities from which ε-caprolactam is crystallized in step d)(iv) further comprises water, preferably more than 1 wt.% water.

8. The method according to any one of claims 1 to 10, wherein before step a), the substance comprising the multicomponent material containing nylon 6 is subjected to a pretreatment in a pretreatment section [A] to obtain a material derived from the multicomponent material containing nylon 6, in particular a cleaning in a cleaning section [α] and / or a mechanical size reduction section [β], a mechanical size reduction and / or densification section [γ] and an extraction section [ω].

9. The process according to any one of claims 1 to 8, wherein after step d)(i), the organic phase obtained in step d)(i) is washed with water or with an alkaline aqueous solution.

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