Method for recovering epsilon-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials

Abstract

The present invention provides a process for recovering ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials in a plant, the plant comprising a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E].

Description

[Technical Field]

[0001] The present invention relates to a process for recovering ε-caprolactam from nylon 6 and polyether polyurethane-containing materials. More particularly, the present invention relates to a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale. [Background technology]

[0002] In 1938, Paul Schlack invented nylon 6 (CAS number: 25038-54-4), also known as polyamide 6, PA6, N6, polycaprolactam, poly(hexano-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide) or poly[imino(1-oxohexane-1,6-diyl)]. Generally, nylon 6 is synthesized by ring-opening polymerization of ε-caprolactam in an inert atmosphere at a temperature of about 260°C:

[0003] [ka]

[0004] It is well known that ε-caprolactam can be prepared by the liquid-phase Beckmann rearrangement of cyclohexanone oxime in the presence of oleum, a mixture of sulfuric acid and sulfuric acid, or by the vapor-phase Beckmann rearrangement of cyclohexanone oxime in the presence of a solid catalyst. Generally, this type of ε-caprolactam is referred to as "virgin ε-caprolactam." The cyclohexanone oxime required for the formation of (virgin) ε-caprolactam can be prepared from cyclohexanone, which is primarily produced from benzene. Most of this benzene is derived from non-renewable fossil resources such as petroleum and coal.

[0005] Processes for producing virgin ε-caprolactam are described, for example, in the Chapter "Caprolactam" of Ullmann's Encyclopedia of Industrial Chemistry (May 25, 2018), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, available electronically via 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 GmbH & Co. KGaA, Weinheim, Germany, available electronically via https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0007] Initially, pure nylon 6 was used to produce textiles such as stockings. Later, various blended fabrics were developed and produced to improve the properties of the fabric. In particular, polyether polyurethanes have been incorporated into a wide range of nylon 6-based garments. The advantages of polyether polyurethanes are their remarkable strength and elasticity, as well as their ability to return to their original shape after stretching and drying more quickly than conventional fabrics. Improved elasticity of fabrics is often achieved by applying blended yarns made by blending nylon 6 fibers with polyurethane fibers. Examples of garments containing blended nylon 6 and polyether polyurethane fabrics include stretch stockings, underwear, and sportswear. Treating (e.g., coating) the surface of nylon 6 fibers with polyurethane resin can impart breathability and waterproofness to the fabric. These surface-treated yarns are often used in the manufacture of raincoats, winter clothing, and skiwear, for example. Polyether polyurethanes used in garment production are often also called spandex, Lycra, or elastane.

[0008] Recycling nylon 6 allows for the preservation of fossil resources and adds value to the circular economy. Mechanical recycling of waste nylon 6 involves converting it into secondary raw materials or into products that (preferably) only minimally change the chemical structure of the material. Nylon 6 conversion and nylon 6 depolymerization are two forms of nylon 6 chemical recycling. In the conversion recycling process, nylon 6 is broken down into oily or gaseous feedstocks that can replace newly extracted fossil feedstocks. The resulting products can be used to produce chemicals, including the monomer ε-caprolactam. In the depolymerization recycling process, nylon 6 is broken down into its monomer building block, ε-caprolactam. The depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam.

[0009] [ka]

[0010] Generally, prior art chemical regeneration or recycling processes for depolymerizing essentially pure nylon 6 to ε-caprolactam monomer involve hydrolytic decomposition at high temperatures in the presence of water and recovery of the formed monomer by steam distillation.

[0011] Nylon 6 depolymerization recycling processes are well developed for clean, fairly pure waste nylon 6 material. Because mechanical recycling always results in downcycling, the properties of products made from mechanically recycled nylon 6 are always lower than those made from virgin nylon 6. However, chemical recycling of waste nylon 6-containing materials can also produce high-purity epsilon-caprolactam with properties similar to those of virgin epsilon-caprolactam, which can then be converted to high-grade nylon 6. Unfortunately, chemical recycling of waste nylon 6-containing materials is often hindered by the presence of impurities in the waste nylon 6 material. As a result of these impurities, the quality of the epsilon-caprolactam produced is generally inferior to that of virgin epsilon-caprolactam.

[0012] Depolymerization of nylon 6 and polyether polyurethane blends has many disadvantages compared to depolymerization of fairly pure nylon 6. These disadvantages include, among others, clogging of piping and other equipment components by polyether polyurethane and its degradation products, poor quality of the produced ε-caprolactam due to the presence of polyether polyurethane degradation products, (strongly) reduced ε-caprolactam recovery, and poisoning of the depolymerization catalyst, resulting in increased depolymerization catalyst consumption. Another disadvantage of adding a nylon 6 and polyether polyurethane blend to a depolymerization reactor is the destruction of the polyether polyurethane. Therefore, recycling of the polyether polyurethane, which is more valuable than nylon 6, is impossible.

[0013] Virgin elastane fibers are produced by a dry or wet spinning process starting with an elastane spinning solution composed of polyurethane in a suitable solvent such as dimethylacetamide or dimethylformamide.

[0014] Direct recycling of normal (pure) elastane waste fibers, e.g., from yarn production, is carried out by dissolving them in used spinning solvents, which proved unsuccessful because their high viscosity prevented further processing.

[0015] US Patent No. 6,830,715 (B1) describes a method for producing elastane yarns from a spinning solution using recycled elastane material, which overcomes the aforementioned viscosity problem by adding a secondary aliphatic amine to a mixture of (chopped) elastane fibers and a spinning solvent. Dissolution of the (chopped) elastane fibers is achieved at temperatures between 60°C and 150°C. In the past, several attempts have already been described to recycle blends of nylon 6 and polyether polyurethane.

[0016] JP 2011088943(A) describes a pretreatment method for separating polyether polyurethane from a nylon 6 product containing polyether polyurethane, followed by depolymerization of the remaining nylon 6. The pretreatment involves heating the nylon 6 product containing polyether polyurethane with a cyclic amide-containing solvent at a temperature between 80°C and the boiling point of the solvent. JP 2011088943(A) also describes that the amount of cyclic amide compound in the cyclic amide compound solvent is preferably 50% by weight or more, more preferably 85% by weight or more. JP 2011088943(A) further describes that the cyclic amide compound solvent may contain components other than the cyclic amide compound, such as water and an organic solvent, and that water is particularly preferred from the standpoint of operability. The experiments described in the patent were conducted at 110°C for 2 hours using aqueous solutions containing 50 and 85% by weight of N-methylpyrrolidone, 85% by weight of 2-pyrrolidone, or 85% by weight of 2-piperidine as the solvent. As a result of this treatment, the polyether polyurethane is partially decomposed and dissolved in the solvent. As a result, the polyether polyurethane is removed, the solution is separated by filtration, and the resulting nylon 6 product is depolymerized. However, JP 2011088943(A) does not mention the recovery of the partially decomposed polyether polyurethane. It also does not mention the fate of the solvent used.

[0017] WO 2013032408(A1) describes a method for recycling polyamide fibers, including polyamide 6 and polyamide 6,6, including polyamide and spandex. Spandex fibers are removed from elastomeric fabrics by a process consisting of controlled thermal degradation of the spandex, a controlled washing process to remove the spandex or its degradation products from the high-purity polyamide using a suitable, sustainable solvent, preferably ethanol, and a final step to remove excess solvent from the polyamide fibers. The temperature range used during the heat treatment of polyamide fibers (such as polyamide 6 fibers) is 150°C to 220°C, preferably 190°C to 216°C, for a period of 0.5 to 4 hours. The heat-treated fabric is then washed, preferably with ethanol, at temperatures ranging from 5°C to 78°C to remove the spandex and its decomposed components. WO 2013032408(A1) does not describe the recovery of spandex and its decomposed components from the washing solvent.

[0018] In light of the above, to date, no industrial-scale process exists for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials. The lack of an industrial-scale recycling process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials lies in the fact that all processes for bringing elastane into solution require high solution temperatures, typically well above 100°C. However, at these temperatures, decomposition of the elastane begins, resulting in only low-quality material after recovery. As a result, the fate of recovered elastane is often limited to incineration and landfilling. Another drawback of high-temperature dissolution processes is the formation of decomposition products of the applied solvent, which can hinder the recovery of nylon 6 and polyether polyurethane and the recycling of the applied solvent.

[0019] Environmental concerns regarding the production and use of these nylon 6 and polyether polyurethane-containing materials relate to waste generated during processing and post-consumer use. These concerns can be alleviated by recycling individual components, such as nylon 6 and polyether polyurethane, from nylon 6 and polyether polyurethane-containing materials that are no longer in use or disposed of. As textiles, nylon 6 and polyether polyurethane-containing materials are a particularly relevant source of waste. These often contain significant amounts of nylon 6 and polyether polyurethane. Therefore, if a viable process existed to recover nylon 6 and polyether polyurethane from these composite waste streams, this would not only benefit the environment but also provide an economically valuable new source of nylon 6 and polyether polyurethane.

[0020] In view of reducing carbon dioxide emissions, it is very important to recycle nylon 6 and polyether polyurethane-containing materials, thereby recovering purified ε-caprolactam and polyether polyurethane.

[0021] There is also a need to provide high purity grades of ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials that have a significantly lower carbon footprint than ε-caprolactam produced by a process that uses virgin ε-caprolactam obtained by de novo synthesis, for example, by Beckmann rearrangement of cyclohexanone oxime, and polyether polyurethane produced by de novo synthesis of polyether polyurethane.

[0022] The raw material cost of virgin polyether polyurethane is high, typically more than twice as high as that of virgin nylon 6. Therefore, there is a significant economic incentive to recover and reuse (recycle) discarded both polyether polyurethane and nylon 6.

[0023] A problem with prior art processes is that they consume a large amount of energy to recover the dissolved elastane from the solution, as the dissolved elastane is typically obtained by removing the solvent by evaporation.

[0024] There is also a need to purify crude ε-caprolactam obtained by depolymerization of polyether polyurethane from nylon 6 and polyether polyurethane-containing materials without using oxidizing agents such as potassium permanganate (KMnO4) or adsorbents such as activated carbon and diatomaceous earth. These oxidizing and adsorbent-based techniques are very labor-intensive and produce solid waste.

[0025] Another drawback of the prior art is that no suitable recovery and reuse (recycle) strategy is provided or known for the solvent(s) used for the recovery and reuse (recycle) of polyether polyurethane and nylon 6 from nylon 6 and polyether polyurethane-containing materials.

[0026] Currently, no process is available for recovering high-purity ε-caprolactam from nylon 6 and polyether polyurethane-containing materials, despite the urgent need for such a process. In particular, there is an urgent need for a high-purity ε-caprolactam recovery process that can replace virgin ε-caprolactam grades for demanding applications such as high-speed melt spinning during the production of textile fibers.

[0027] Finally, due to the vast amount of nylon 6 and polyether polyurethane-containing materials processed each year, including materials that are discarded, a process is needed that can recover pure ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale. Summary of the Invention

[0028] It is an object of the present invention to fulfill one or more of the above-mentioned needs and to overcome or mitigate disadvantages associated with prior art methods. In particular, it is an object of the present invention to provide a process for recovering ε-caprolactam from nylon 6 and polyether polyurethane-containing materials. It is also an object of the present invention to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials.

[0029] It is a further object of the present invention to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale.

[0030] It is also an object of the present invention to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials in an economically responsible manner. In this regard, it is a particular object of the present invention to provide a process suitable for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials that does not exceed the cost of producing high-purity virgin ε-caprolactam and polyether polyurethane.

[0031] It is also an object of the present invention to provide a process for purifying crude ε-caprolactam obtained by depolymerization of nylon 6 and polyether polyurethane containing materials that does not produce solid waste.

[0032] It is a further object of the present invention to provide a process for producing virgin polyether polyurethane, e.g., by reaction of polyether polyol with diisocyanate monomer, and a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials, characterized by a significantly lower carbon footprint than ε-caprolactam, e.g., by de novo synthesis via Beckmann rearrangement of cyclohexanone oxime.

[0033] It is also an object of the present invention to provide a process for recovering high purity ε-caprolactam from nylon 6 and polyether polyurethane containing materials which can replace high purity virgin ε-caprolactam for all applications such as high speed melt spinning of nylon 6 to produce thin textile fibers. Therefore, the present invention also aims to provide a process that reduces the environmental impact of waste nylon 6 and polyether polyurethane-containing materials. One or more additional objects may become apparent from the remainder of the description.

[0034] All or at least some of the above mentioned objectives are solved or at least significantly alleviated by the process according to claim 1, the plant according to claim 13 and the products according to claims 14 and 15.

[0035] The present invention provides a process for recovering ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing material in a plant, the plant comprising - a separation section [B]; - depolymerization section [C]; -Recovery section [D] and a purification section [E], Here, the process includes:

[0036] a) feeding a nylon 6 and polyether polyurethane containing material into a separation section [B]; b) separating the nylon 6- and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream in a separation section [B] by selectively dissolving the polyether polyurethane in an organic solvent at a temperature below 100°C, preferably at a temperature in the range of 0°C to 100°C, more preferably at a temperature in the range of 10°C to 90°C, even more preferably at a temperature in the range of 10°C to 80°C, and most preferably at a temperature in the range of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising an organic solvent and polyether polyurethane; c.1) discharging a nylon 6-rich stream from the separation section [B] and introducing the nylon 6-rich stream into a depolymerization section [C], wherein the nylon 6-rich stream has a nylon 6 content of at least 85 wt.% on a dry weight basis; c.2) depolymerizing nylon 6 in the nylon 6-rich stream in a depolymerization section [C] at a temperature in the range of 180°C to 400°C, preferably 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C, thereby obtaining an ε-caprolactam-containing stream, and discharging the obtained ε-caprolactam-containing stream from the depolymerization section [C]; c.3) recovering crude ε-caprolactam from the ε-caprolactam-containing stream in a recovery section [D]; c.4) Purifying the crude ε-caprolactam obtained in the recovery section [D] in a purification section [E] to obtain purified ε-caprolactam,

[0037] (i) extracting crude ε-caprolactam with an organic solvent, thereby obtaining an aqueous phase and an organic phase, the organic phase containing the organic solvent, ε-caprolactam and impurities, preferably the organic solvent being selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol and mixtures thereof, and optionally washing the obtained organic phase with water or an aqueous alkali solution; (ii) optionally, a step of switching the solvent by at least partially replacing the organic solvent with water or an aqueous solution, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than that of ε-caprolactam, the solvent switching step being selected from a process based on back-extraction with water and a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced; (iii) optionally removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation to obtain purified ε-caprolactam; (iv) obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C;

[0038] d.1) recovering polyether polyurethane from the polyether polyurethane rich stream in separation section [B]; d.2) Discharging the recovered polyether polyurethane from separation section [B], wherein the polyether polyurethane content of the discharged stream is at least 85 wt. % on a dry weight basis.

[0039] Surprisingly, it has been found that the combination of a special series of processing steps and process conditions according to the present invention, i.e., the order of separation, depolymerization, recovery, and purification steps defined above, allows for the recovery of ε-caprolactam from nylon 6 and polyether polyurethane-containing materials in a simple and economically rational manner with high yields. The process of the present invention is economically rational and advantageous from several perspectives. First, the process of the present invention is suitable for a wide variety of materials derived from nylon 6 and polyether polyurethane-containing materials, which may differ, for example, in their overall composition and / or their polyether polyurethane content and / or their nylon 6 content. Second, the process of the present invention allows for the effective separation of nylon 6 from polyether polyurethane compounds, resulting in the production of high-grade nylon 6. Third, the process of the present invention is effective enough to obtain ε-caprolactam in high yields. Fourth, the process of the present invention allows for the recovery of polyether polyurethane, which can be reused to replace polyether polyurethanes produced by de novo synthesis, for example, by the reaction of polyether polyols with diisocyanate monomers. Fifth, the process of the present invention enables the effective separation of ε-caprolactam from non-ε-caprolactam compounds, thereby producing a high-purity grade of ε-caprolactam that can replace high-purity virgin ε-caprolactam for all applications, including high-speed melt spinning of nylon 6 to produce thin textile fibers. Finally, the process of the present invention enables the de novo synthesis of ε-caprolactam to produce ε-caprolactam with a significantly lower carbon footprint than ε-caprolactam produced, for example, by the Beckmann rearrangement of cyclohexanone oxime. The process of the present invention enables the efficient processing of nylon 6 and polyether polyurethane-containing materials, reducing the environmental impact of the product.In particular, the process of the present invention allows for the production of polyether polyurethanes having a carbon footprint of less than 1 kg CO2 per kg polyether polyurethane, which is a significant improvement over the 4.8 kg CO2 per kg polyether polyurethane associated with the production of "virgin" polyether polyurethanes obtained by chemical synthesis. In particular, the process of the present invention allows for the production of purified ε-caprolactam having a carbon footprint of less than 3 kg CO2 per kg purified ε-caprolactam, which is a significant improvement over the 6.4-7.5 kg CO2 per kg ε-caprolactam associated with the production of "virgin" ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime.

[0040] Next to the process of the present invention, the present invention also provides a plant for producing purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing material, the plant comprising: optionally a pre-treatment section [A]; Separation section [B]; Nylon 6 depolymerization section [C], an ε-caprolactam recovery section [D]; an ε-caprolactam purification section [E]; A chemical plant is configured to carry out the process of the present invention.

[0041] The present invention also provides purified ε-caprolactam obtained by separation from nylon 6 and polyether polyurethane-containing material by the process of the present invention, wherein the ε-caprolactam has a product carbon footprint of less than 3.0 kg CO2 equivalents per kg of purified ε-caprolactam.

[0042] The present invention also provides polyether polyurethane obtained by separation from nylon 6 and polyether polyurethane-containing materials by the process of the present invention, wherein the polyether polyurethane has a product carbon footprint of less than 1.0 kg CO2 equivalent per kg of recovered polyether polyurethane. Advantageous embodiments of the invention are set out in the dependent claims and are explained in more detail below.

[0043] Nylon 6 and polyether polyurethane containing materials The process of the present invention uses nylon 6 and polyether polyurethane-containing materials as starting materials. The nylon 6 and polyether polyurethane-containing materials can be (can be used as) nylon 6 and polyether polyurethane-containing products or materials derived therefrom. The nylon 6 and polyether polyurethane-containing materials can be nylon 6 and polyether polyurethane-containing materials, i.e., multiple nylon 6 and polyether polyurethane-containing materials or a mixture thereof. Typically, the nylon 6 and polyether polyurethane-containing materials are solid materials, particularly fibers, yarns (i.e., strands of fibers spun together), or fiber-based fabrics, whereby the fibers are blends of nylon 6 fibers and polyurethane fibers, or the fibers are obtained by coating the surface of nylon 6 fibers with a polyether polyurethane resin.

[0044] The nylon 6 and polyether polyurethane-containing material can be of both pre-consumer and post-consumer origin. The nylon 6 and polyether polyurethane-containing material can be a mixture of different nylon 6 and polyether polyurethane-containing materials or a mixture containing one or more nylon 6 and polyether polyurethane-containing materials with one or more different materials.

[0045] Nylon 6 and polyether polyurethane-containing materials can contain additional ingredients. Such ingredients can be added, for example, during polymerization, during fiber formation, or afterward to achieve desired property variations. These compounds can include, for example, optical brighteners, curing agents, antistatic lubricants, colorants, optical brighteners, spin finishes, surface smoothing agents, antioxidants, UV stabilizers, and the like. The amount of these additional ingredients depends on the application of the nylon 6 and polyether polyurethane-containing materials.

[0046] The weight to weight ratio of nylon 6 to polyether polyurethane in the nylon 6 and polyether polyurethane-containing material can vary. Preferably, the weight to weight ratio of nylon 6 to polyether polyurethane is from about 1:1 to about 100:1, most preferably from about 3:1 to about 20:1.

[0047] Polyurethanes are produced by reacting an isocyanate containing two or more isocyanate groups per molecule with a polyol containing an average of two or more hydroxyl groups per molecule in the presence of a catalyst or by activating it with ultraviolet light. The main components for making polyurethanes are diisocyanates and triisocyanates and polyols. The diisocyanate monomer methylene diphenyl diisocyanate (MDI) is the isocyanate most commonly used to produce polyurethanes for clothing applications.

[0048] Polyether polyurethanes are made by the reaction of isocyanates with polyether polyols. Polyether polyols are generally made by polymerizing cyclic ether compounds onto initiator compounds. The most commonly used cyclic ethers for producing polyether polyols are ethylene oxide, propylene oxide, and 1,4-butylene oxide or tetrahydrofuran (to produce poly(butylene oxide) polyols). Poly(butylene oxide) polyols are primarily used in applications requiring highly hydrophobic properties.

[0049] The use of the term "ca." or "about" in connection with numerical values ​​herein indicates that the numerical value may be subject to measurement error, typically varying the numerical value by no more than ±5%. Numeric values ​​disclosed herein with the term "ca." or "about" are also meant to be disclosed, i.e., without the term "about." Furthermore, numerical ranges described herein are meant to include and disclose each and every value within that range. All upper and lower endpoints of ranges for the same parameter disclosed herein are combinable with each other. All ranges for different parameters disclosed herein are combinable with each other. In particular, ranges for different or the same "preferred level" are particularly compatible with each other. As used in this disclosure and claims, the singular forms "a," "an," and "the" include the plural form "one or more," unless the context clearly dictates otherwise.

[0050] Possible pre-treatment steps Before being subjected to step a) of the inventive process, the nylon 6 and polyether polyurethane-containing material can be subjected to pretreatment in the pretreatment section [A], particularly size reduction in the mechanical size reduction section [λ] and / or cleaning in the cleaning section [ω]. Pretreatment can be carried out at a location different from the location of the separation section [B]. Preferably, however, the inventive process is carried out in a plant further comprising a pretreatment section [A], and prior to step a), the nylon 6 and polyether polyurethane-containing material is subjected to pretreatment in the pretreatment section [A], particularly cleaning in the cleaning section [ω] and / or mechanical size reduction in the mechanical size reduction section [λ]. This has the advantage that the nylon 6 and polyether polyurethane-containing material entering the separation section [B] is less contaminated with foreign matter, improving the yield and purity of the ε-caprolactam and polyether polyurethane produced in the inventive plant configured to carry out the inventive process. Another advantage is that the size-reduced nylon 6 and polyether polyurethane-containing material can be more easily handled.

[0051] The nylon 6 and polyether polyurethane-containing material is preferably derived from or is a used or waste nylon 6 and polyether polyurethane-containing material. The size and shape of the material will depend greatly on the exact application from which it is derived. The nylon 6 and polyether polyurethane-containing material that can be used in accordance with the present invention ranges from individual fibers and yarns, possibly present in bobbins, to woven fabrics and garments. Preferably, the nylon 6 and polyether polyurethane-containing material is a waste or used nylon 6 and polyether polyurethane-containing material.

[0052] In particular, discarded or used nylon 6 and polyether polyurethane containing materials can become contaminated with various types of dirt (e.g., mud, oil, paint or grease).

[0053] (Waste) nylon 6 and polyether polyurethane containing materials can be mixed with a whole range of other materials such as rock, glass, metal materials, organic waste and other polymeric materials (e.g., polyamide 6,6, polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE)).

[0054] Preferably, the nylon 6 and polyether polyurethane-containing material is fragmented into small pieces before being separated in the separation section [B] of step b). This mechanical pretreatment, i.e., mechanical comminution or fragmentation of the nylon 6 and polyether polyurethane-containing material, can be achieved, for example, by cutting, punching, crushing, milling, grinding, and / or chipping. In a preferred embodiment, the nylon 6 and polyether polyurethane-containing material is introduced into the separation section [B] of step a) in the form of fragmented small pieces. The use of fragmented small pieces has the advantage that the small pieces can be more easily handled and / or cleaned by washing with a solvent. In a preferred embodiment, the small pieces have, on average, a length along their longest axis of 1 mm to 100 m, preferably 3 mm to 1 m, and most preferably 1 cm to 50 cm. One skilled in the art can easily determine the average length along the longest axis of the small pieces used by first taking a representative sample of the small pieces, then measuring the length of the longest axis of each of these small pieces (e.g., 50 pieces), and finally calculating the average of all these individual measurements. Preferred particle sizes can also be described in terms of average particle weight. Preferably, the nylon 6 and polyether polyurethane-containing material particles have an average particle weight of 0.01 grams to 25 kg, preferably 0.05 grams to 1 kg, and most preferably 0.1 grams to 100 grams. Experiments have shown that nylon 6 and polyether polyurethane-containing material particles of the above-mentioned size or weight dimensions are particularly well suited for handling and / or cleaning by solvent washing in the process of the present invention.

[0055] Optionally, prior to mechanical crushing or fragmenting the nylon 6- and polyether polyurethane-containing material, large metal chips, rocks, and other interfering materials that cause severe wear on the equipment used for mechanical crushing or fragmentation are removed. Preferably, foreign matter, such as materials containing polyethylene, polypropylene, and polyamide 6,6, is also removed prior to mechanical crushing or fragmenting the nylon 6- and polyether polyurethane-containing material. Removal of the foreign matter can be performed mechanically or manually. Removing these interfering materials has the advantage of significantly reducing the maintenance costs of the equipment used for mechanical crushing or fragmentation. Furthermore, the nylon 6 and polyether polyurethane content of the material obtained after mechanical crushing or fragmentation is higher than if the interfering materials were not removed. In particular, removal of polyamide 6,6 is advantageous because it interferes with the depolymerization of nylon 6, reduces the recovery yield of ε-caprolactam, and interferes with the subsequent purification of the recovered ε-caprolactam. As used herein, the term "foreign matter" refers to non-nylon 6 and non-polyether polyurethane materials or compounds.

[0056] In some cases, contaminants are separated from mechanically crushed or fragmented nylon 6 and polyether polyurethane-containing materials. Various separation processes are applicable for this purpose, including, but not limited to, density separation and magnetic separation. In density separation, materials of different densities are placed in a liquid of intermediate density, where the lower-density materials float and are separated from the higher-density sinking materials. In practice, density separation is often carried out through a series of density separation stages. For example, in one stage, high-density materials such as rock, sand, and metals (such as iron and lead) are separated, while in another stage, low-density materials such as polyolefin polypropylene and polyethylene are separated. Magnetic separation is a process in which components of a mixture are separated by using a magnet to attract magnetic materials. In the process preferably used for magnetic separation, non-magnetic materials are separated from magnetic materials. Removal of foreign matter in the ground or fragmented nylon 6 and polyether polyurethane-containing material is advantageous because such material can interfere with the separation of nylon 6 and polyether polyurethane, the depolymerization of nylon 6, reduce the recovery yield of ε-caprolactam, and / or interfere with the subsequent purification of the recovered ε-caprolactam.

[0057] Optionally, particularly in the pretreatment step described above, the nylon 6 and polyether polyurethane-containing material is cleaned by washing with a solvent, preferably at least water, before being introduced into the separation section [B]. The solvent used herein can be a single solvent or a mixture of different solvents. Preferably, a detergent is added to the solvent in a concentration range of 0 to 20% by weight relative to the solvent to improve the cleaning efficiency. NaOH is a preferred detergent. Even more preferably, an aqueous solution containing 0 to 10% by weight NaOH, and even more preferably an aqueous solution containing 0 to 5% by weight NaOH, is used in the washing step. The enhanced cleaning effect of NaOH is likely caused by improved hydrolysis of molecules such as biopolymers and non-biopolymers. Preferably, the washing solvent is heated to further enhance the washing process. In another preferred embodiment, the washing process includes a final rinse step using a detergent-free (clean) washing solvent to remove detergent residues and existing soils adhering to the nylon 6 and polyether polyurethane-containing material.

[0058] Cleaning is preferably carried out under friction. Various types of industrial friction application cleaning systems are commercially available, such as rotary plastic washers and (high speed) friction washers.

[0059] Washing of nylon 6 and polyether polyurethane containing materials, especially nylon 6 and polyether polyurethane containing materials that have been mechanically comminuted or fragmented, is advantageous because any (adherent) soiling is removed so that it does not disrupt the next step in the process of the invention.

[0060] Optionally, the nylon 6 and polyether polyurethane-containing material is dried after the cleaning step before being introduced into separation section [B], which has the advantage that the weight of the cleaned nylon 6 and polyether polyurethane-containing material is reduced and the next step is not affected by dilution or contamination by the cleaning solvent.

[0061] The location where the pretreatment of the nylon 6 and polyether polyurethane-containing material is carried out and the location where the separation section [B] is located can be the same. However, preferably, one or more of the pretreatment steps are carried out at a location different from the location of the separation section [B], for example, near a plant where the (used or discarded) nylon 6 and polyether polyurethane-containing material is collected and / or at a location dedicated to the pretreatment of the nylon 6 and polyether polyurethane-containing material. A nylon 6-rich stream can then be obtained in the separation section [B] from the already pretreated nylon 6 and polyether polyurethane-containing material.

[0062] The location where the separation of the nylon 6- and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream is carried out can be the same as the location where the depolymerization section [C] is located. However, preferably, the separation of the nylon 6- and polyether polyurethane-containing material into a nylon-6-rich stream and a polyether polyurethane-rich stream is carried out at a location different from the location of the depolymerization section [C], for example, near the plant where virgin polyether polyurethane is produced and / or at a location specialized in the handling and / or distillation of organic solvents.

[0063] The location where the pretreatment of nylon 6 and polyether polyurethane-containing material is carried out and the location where the depolymerization section [C] is located can be the same. However, preferably, one or more of the pretreatment steps are carried out at a location different from the location of the depolymerization section [C], for example, near the plant where waste nylon 6 and polyether polyurethane-containing material is collected and / or at a location dedicated to the pretreatment of nylon 6 and polyether polyurethane-containing material.

[0064] Feeding process a) In step a) of the process of the present invention, the nylon 6 and polyether polyurethane containing material, optionally pretreated in pretreatment section [A], is fed to separation section [B].

[0065] In one embodiment, the nylon 6 and polyether polyurethane containing material is mechanically compressed to a smaller volume before being introduced into separation section [B], which has the advantage of requiring less volume for intermediate storage and transportation, and also facilitating administration into separation section [B].

[0066] In another preferred embodiment, the nylon 6 and polyether polyurethane-containing material is dried before being introduced into the separation section [B], especially after the nylon 6 and polyether polyurethane material has been subjected to a cleaning process. This has the advantage that less or no solvent is introduced into the separation section [B] together with the material. The solvent, especially water, introduced into the separation section [B] can have a negative effect on the separation process (e.g., a decrease in the dissolution rate of the polyether polyurethane from the nylon 6 and polyether polyurethane-containing material in the organic solvent is obtained).

[0067] The nylon 6 and polyether polyurethane containing material is preferably fed as a solid phase to the dissolution section [α], in particular to the corresponding dissolution vessel(s) contained therein.

[0068] The feeding of the nylon 6 and polyether polyurethane-containing material to the separation section [B] (e.g., feeding to the polyether polyurethane dissolving section) can be achieved by continuous or intermittent addition of the nylon 6 and polyether polyurethane-containing material.

[0069] Separation step b) In separation section [B], the nylon 6 and polyether polyurethane containing material is separated to form a nylon 6 rich stream and a polyether polyurethane rich stream.

[0070] Preferably, the process of the present invention uses a separation section [B] comprising a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ], the separation section comprising the following steps: b.1) introducing an organic solvent and a nylon 6 and polyether polyurethane-containing material into a dissolution section [α]; b.2) in the dissolving section [α], dissolving polyether polyurethane in an organic solvent from a nylon 6 and polyether polyurethane-containing material, thereby obtaining a polyether polyurethane-rich stream containing the organic solvent and the dissolved polyether polyurethane and a stream containing undissolved nylon 6, and discharging the obtained streams from the dissolving section [α]; b.3) introducing the second solvent and the polyether polyurethane-rich stream comprising the organic solvent and the dissolved polyether polyurethane into a precipitation section [γ] such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent; b.4) recovering the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent and discharging the precipitated polyether polyurethane from a precipitation section [γ]; b.5) ​​discharging the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane of step b.4) was recovered from the precipitation section [γ] and introducing this mixture into the solvent distillation section [δ];

[0071] b.6) introducing a stream comprising a third solvent and undissolved nylon 6 into a washing section [β]; b.7) in a washing section [β], washing the stream containing undissolved nylon 6 with a third solvent, thereby obtaining a nylon 6-rich stream and a mixture containing an organic solvent and the third solvent; b.8) discharging the nylon 6 rich stream from the washing section [β]; b.9) the mixture comprising the organic solvent and the third solvent obtained in step b.7) is discharged from the washing section [β], and this mixture is partially or completely introduced into the solvent distillation section [δ]; b.10) Separating the organic solvent from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and discharging the second solvent, the third solvent and the separated organic solvent from the solvent distillation section [δ].

[0072] Optionally, the second solvent (see step b.3)) may be a mixture comprising partly or completely an organic solvent and a third solvent obtained in step b.7) from washing section [β].

[0073] Preferably, the separation section [B] used in the process of the present invention comprises a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ]. Such a separation section [B] is particularly suitable for separating a nylon 6- and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream by the process of the present invention. Surprisingly, the separation of a nylon 6- and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream is particularly effective when process steps b.1) to b.10) are performed in the separation section [B], which comprises a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ]. Steps b.1) to b.10) performed in the preferred separation section [B] are further described below. Steps b.1) to b.10) can be substituted for process steps a) and b) of the process of the present invention. Step b.1) thereby corresponds to step a).

[0074] As used herein, a "rich stream" is a stream that contains a greater amount of an enriched component than another stream obtained from the same process step. Thus, a nylon 6-rich stream will contain more nylon 6 than a polyether polyurethane-rich stream. In contrast, a polyether polyurethane-rich stream will contain more polyether polyurethane than a nylon 6-rich stream.

[0075] In particular, the nylon 6-rich stream has a weight to weight ratio of nylon 6 to polyether polyurethane that is higher than the weight to weight ratio of nylon 6 to polyether polyurethane of the nylon 6 and polyether polyurethane-containing material.

[0076] Preferably, the nylon 6-rich stream has a weight to weight ratio of polyether polyurethane to nylon 6 that is at least two times lower, more preferably at least three times lower, than the weight to weight ratio of polyether polyurethane to nylon 6 of the nylon 6 and polyether polyurethane-containing material.

[0077] The polyether polyurethane rich stream has a weight to weight ratio of polyether polyurethane to nylon 6 that is higher than the weight to weight ratio of polyether polyurethane to nylon 6 of the nylon 6 and polyether polyurethane containing material.

[0078] Preferably, the polyether polyurethane rich stream has a weight to weight ratio of polyether polyurethane to nylon 6 that is at least two times higher, more preferably at least three times higher, than the weight to weight ratio of polyether polyurethane to nylon 6 of the nylon 6 and polyether polyurethane containing material. Feeding into the separation section (step b.1)): In the separation section [B], the organic solvent and the nylon 6 and polyether polyurethane containing material are introduced into the dissolution section [α].

[0079] Feeding the nylon 6 and polyether polyurethane-containing material into the dissolution section [α] is equivalent to feeding the nylon 6 and polyether polyurethane-containing material, which has optionally been pretreated in the pretreatment section [A] as described in step a) of the present invention.

[0080] The organic solvent introduced into the dissolution section [α] can be any organic solvent capable of dissolving polyether polyurethane, particularly at temperatures below 100°C, particularly within 24 hours or less than 6 hours. The organic solvent used herein can also refer to an organic solvent or a mixture of liquid compositions containing more than 60% by volume, preferably more than 70%, 80%, or 90% by volume, of an organic solvent, capable of dissolving polyether polyurethane at temperatures below 100°C, preferably between 0°C and 100°C, more preferably between 10°C and 90°C, even more preferably between 10°C and 80°C, and most preferably between 20°C and 75°C. Preferably, the organic solvent combines good, particularly very good, solubility for polyether polyurethane with poor, particularly very poor, solubility for nylon 6. These properties can be tested in a simple dissolution test using nylon 6 and a polyether polyurethane-containing material or a mixture of nylon 6 and polyether polyurethane. According to a preferred embodiment, the organic solvent comprises or is selected from N,N-dimethylformamide (DMF, (CH)NC(=O)H), dimethylacetamide (DMAc, CHC(=O)N(CH)), 1,4-dioxane (dioxane, CHO), N-alkyl-2-pyrrolidone, e.g., N-methyl-2-pyrrolidone (NMP, CHNO), tetrahydrofuran (THF; also known as oxolane, CHO), and combinations thereof. Even more preferably, the organic solvent is selected from N,N-dimethylformamide and dimethylacetamide. Most preferably, the organic solvent is dimethylacetamide.

[0081] The organic solvent input to the dissolution section [α] can be a separated organic solvent (i.e., a recycled organic solvent or an organic solvent from inside the process recovered in the solvent distillation section), a fresh organic solvent (i.e., a solvent from outside the process), or a combination of a separated organic solvent and a fresh organic solvent.

[0082] The organic solvent introduced into the dissolving section [α] preferably has a low water content. Even more preferably, the organic solvent is dried (preferably by distillation or application of a drying agent such as an inorganic salt such as NaSO, zeolite, silica, and alumina) before being introduced into the dissolving section [α]. Drying does not necessarily have to be carried out to completion. Typically, it is acceptable for the water remaining after drying of the organic solvent to be less than 1% by weight, preferably less than 0.1% by weight, and more preferably less than 0.02% by weight.

[0083] Dissolution step (step b.2): In the dissolution section [α], polyether polyurethane is dissolved from the nylon 6 and polyether polyurethane-containing material in the organic solvent introduced into the dissolution section [α] in step b.1), thereby obtaining a polyether polyurethane-rich stream containing the organic solvent and dissolved polyether polyurethane, and a stream containing undissolved nylon 6, both of which are discharged from the dissolution section [α].

[0084] Preferably, dissolution of the polyether polyurethane is complete in 0.1 to 24 hours, more preferably 0.5 to 6 hours, where complete means that dissolution has reached a plateau and no substantial further dissolution occurs at any time thereafter.

[0085] The temperature in the dissolution section [α] may vary, preferably below 120°C, more preferably below 110°C, even more preferably below 100°C, and most preferably below 80°C. Preferably, the temperature in the dissolution section [α] may range from 0°C to 100°C, more preferably from 10°C to 90°C, even more preferably from 10°C to 80°C, and most preferably from 20°C to 75°C. These experiments have shown that these temperature ranges result in particularly complete dissolution of the polyether polyurethane while maintaining its integrity in the organic solvent. This has the advantage that the polyether polyurethane present in the nylon 6 and polyether polyurethane-containing material can be dissolved while maintaining its integrity.

[0086] The amount of solvent (expressed by weight, especially in tons) in the dissolution section [α] may vary, ranging from 0.5 to 100 times, preferably 1 to 60 times, more preferably 2 to 20 times, and most preferably 3 to 10 times the amount of polyether polyurethane (expressed by weight, especially in tons) in the dissolution section [α]. A small amount of solvent can slow the dissolution rate of the polyether polyurethane, resulting in too little or incomplete dissolution of the polyether polyurethane. A large amount of solvent can increase the dissolution rate of the polyether polyurethane, but requires more energy and cost to recover the organic solvent used. Those skilled in the art can determine the optimal ratio of solvent to polyether polyurethane or nylon 6 and polyether polyurethane-containing material through routine testing.

[0087] The dissolution section [α] may comprise one or more dissolution vessels operated in series or parallel, preferably equipped with additional devices to increase friction, such as stirrers, mixers or agitators, which shorten dissolution times.

[0088] Precipitation step (step b.3): The polyether polyurethane-rich stream containing the organic solvent and dissolved polyether polyurethane discharged from the dissolution section [α] and the second solvent is introduced into the precipitation section [γ] where they are mixed, and as a result, the polyether polyurethane is precipitated from the mixture containing the organic solvent and the second solvent.

[0089] Preferably, the second solvent is a non-solvent (also known as an anti-solvent) for polyether polyurethane, i.e., a liquid in which polyether polyurethane is not soluble and causes the polyether polyurethane to precipitate. The addition of this second solvent reduces the solvent power of the polyether polyurethane dissolved in the solution, thereby causing the precipitated polyether polyurethane particles to precipitate. The second solvent can be any solvent. Preferably, the second solvent is an aqueous solution or water, which has the advantage of being an environmentally friendly solvent.

[0090] In a preferred embodiment, the second solvent consists of or is a part of the mixture containing the organic solvent and the third solvent obtained in step b.7) from the washing section [β]. As used herein, "part of or is a mixture" is defined as any fraction between 1 and 100% by weight, preferably between 10 and 100% by weight, more preferably between 25 and 100% by weight, and even more preferably between 75 and 100% by weight. This has the advantage that less fresh solvent is required for precipitation of the polyether polyurethane. Another advantage is that less solvent needs to be distilled in the solvent distillation section [δ], resulting in lower energy costs, smaller equipment size, lower investment costs, and a more environmentally friendly process.

[0091] The temperature in the precipitation section [γ] may vary and may range from 0° C. to 150° C., preferably from 10° C. to 100° C., more preferably from 15° C. to 80° C., and most preferably from 20° C. to 60° C. Experiments have shown that these temperature ranges result in particularly complete precipitation of the dissolved polyether polyurethane.

[0092] The amount of the second solvent introduced into the precipitation section [γ] may vary and may be in the range of 0.1% to 500% by weight, preferably 0.2% to 100% by weight, more preferably 0.5% to 50% by weight, and most preferably 1% to 25% by weight, compared to the organic solvent introduced into the precipitation section [γ].

[0093] If the second solvent is used in a small amount, the degree of precipitation of the polyether polyurethane decreases. If the second solvent is used in a large amount, the recovery of the precipitated polyether polyurethane and the organic solvent becomes complicated. In addition, if the second solvent is used in a large amount, more energy and cost are required to recover the precipitated polyether polyurethane and the organic solvent. Those skilled in the art will be able to find the optimal ratio of the second solvent to the polyether polyurethane-rich stream for the specific solvent and equipment used by conducting a simple precipitation test trial.

[0094] Polyether polyurethane recovery process (process b.4): In the polyether polyurethane recovery process, the precipitated polyether polyurethane is recovered from the mixture containing the organic solvent and the second solvent and discharged from the precipitation section [γ]. Several suitable methods for recovering the polyether polyurethane precipitate include, but are not limited to, filtration, centrifugation, and decantation. Filtration is preferably used to recover the polyether polyurethane. For filtration, the solution containing the precipitate is introduced into a filter, whereby the precipitate is expected to remain on the filter while the liquid passes through. When using centrifugation as a recovery method, the solution containing the precipitate is rapidly spun, thereby settling a solid precipitate (which is assumed to have a higher density than the liquid). Compacted precipitate can also be obtained by pouring the liquid. In decantation, the liquid layer is poured or suctioned from the precipitate.

[0095] Optionally, the recovered precipitate is washed with a solvent before being discharged from precipitation section [γ]. Preferably, the solvent used to wash the recovered precipitate is either the organic solvent of step b.1) or the second solvent.

[0096] The recovered precipitate, optionally washed with a solvent, is optionally dried before being discharged from precipitation section [γ]. Advantageously, it has been found that the precipitated and recovered polyether polyurethane discharged from precipitation section [γ] in step b.4) of the process of the present invention is of high quality and can be reused, optionally in combination with fresh polyether polyurethane, in the production of textiles, which is a preferred embodiment of the present invention.

[0097] Discharge solvent mixture step (step b.5)): The mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane was recovered in step b.4) is discharged from the precipitation section [γ] and fed to the solvent distillation section [δ]. Optionally, the solvent resulting from washing the recovered precipitate is also discharged from the precipitation section [γ] and fed to the solvent distillation section [δ].

[0098] Input to the washing process (process b.6): In the separation section [B], the stream containing the third liquid solvent and undissolved nylon 6 is fed to the washing section [β]. The stream containing undissolved nylon 6 fed to the washing section [β] may also contain an organic solvent next to the undissolved nylon 6. The purpose of the washing section [β] is to recover the organic solvent present in the stream containing undissolved nylon 6, thereby obtaining a nylon 6-rich stream containing less organic solvent than the stream containing undissolved nylon 6.

[0099] Preferably, the third solvent has good solubility in organic solvents and low solubility in nylon 6. The third solvent can be any solvent. Preferably, the second solvent is water or an aqueous solution. Water as the third solvent has the advantage of being an environmentally friendly solvent. The third and second solvents can be of the same composition, which has the advantage of simplifying the process by reducing the amounts of different components required.

[0100] The amount of the third solvent fed to the washing section [β] may vary and may be in the range of 10% to 1000% by weight, preferably 20% to 500% by weight, more preferably 50% to 400% by weight, and most preferably 100% to 250% by weight, relative to the stream containing undissolved nylon 6 fed to the washing section [β].

[0101] Cleaning step (step b.7)): In the washing section [β], the stream containing undissolved nylon 6 is washed with a third solvent, thereby obtaining a nylon 6-rich stream and a mixture containing an organic solvent and the third solvent.

[0102] The temperature in the washing section [β] can vary and can be in the range of 0° C. to 150° C., preferably 10° C. to 100° C., more preferably 15° C. to 80° C., and most preferably 20° C. to 60° C. Experiments have shown that these temperature ranges allow particularly complete recovery of organic solvent from streams containing undissolved nylon 6. One skilled in the art can determine by routine experimentation the amount of third solvent and the optimum temperature required for efficient washing of the stream containing undissolved nylon 6. Washing the stream containing undissolved nylon 6 with a third solvent can be carried out in a variety of devices, all of which are known to those skilled in the art.

[0103] Discharge process of nylon 6 rich stream (step b.8): A nylon 6-rich stream is discharged from the washing section [β]. This nylon 6-rich stream contains a third solvent and, optionally, an organic solvent. Preferably, the organic solvent content of the nylon 6-rich stream is less than 25 wt %, preferably less than 10 wt %, more preferably less than 2 wt %, and most preferably less than 0.2 wt %, based on the total weight of the nylon 6-rich stream. Optionally, the nylon 6-rich stream is dried before being discharged.

[0104] Typically, the resulting nylon 6-rich stream contains solid nylon 6-rich material. The term "solid" in this respect refers to the state of the material at room temperature (20°C). At higher temperatures, the nylon 6-rich material can also exist as a melt. The nylon 6-rich material is stable and can be stored before being introduced into the depolymerization section in step c.1) of the process of the present invention, or transported to the depolymerization section at a different location for this purpose.

[0105] Preferably, the nylon 6 content of the nylon 6-rich stream obtained from washing section [β] is at least 85 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %, based on dry weight.

[0106] Preferably, the polyamide 6 content of the polyamide 6-rich stream obtained from separation section [B] is at least 85 wt. %, more preferably at least 90 wt. %, even more preferably at least 95 wt. %, and most preferably at least 98 wt. %, based on dry weight.

[0107] Determining the nylon 6 content is a routine activity and can be done by a variety of methods, all of which are known to those skilled in the art. Preferably, the polyamide 6 content of the nylon 6-rich stream is determined by thermogravimetric analysis (TGA) and / or differential scanning calorimetry (DSC) (e.g., the method of ISO 11357-3).

[0108] As used herein, the phrase "dry weight basis" refers to the content of an ingredient expressed as a percentage of the total dry weight of the composition, where "dry weight" is the weight of a substance after water and / or other liquids have been removed from the substance by drying until a constant weight is reached. Discharge step of the mixture comprising the organic solvent and the third solvent (step b.9)):

[0109] A mixture containing an organic solvent and a third solvent is discharged from the washing section [β]. Generally, the organic solvent and / or the third solvent are valuable compounds, and their recovery and reuse have significant economic and environmental benefits. Therefore, in a preferred embodiment of the present invention, the mixture containing an organic solvent and a third solvent is fed to the solvent distillation section [δ]. Optionally, the mixture containing an organic solvent and a third solvent is first fed and used as a second solvent in another section, particularly a precipitation section, before being fed to the solvent distillation section [δ]. In a preferred embodiment, the mixture containing an organic solvent and a third solvent is first partially or completely fed to the precipitation section [γ] and used as a second solvent. This allows for particularly efficient use and reuse of solvents in the process of the present invention.

[0110] Solvent separation step (step b.10)): In the solvent distillation section [δ], the different solvents used in the process of the present invention are separated and recovered, allowing them to be advantageously reused. In particular, the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane is recovered and the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] are separated by distillation in the solvent distillation section [δ]. In the following, the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane is recovered and the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] are referred to as the combined feed to the solvent distillation section [δ].

[0111] The solvent distillation section [δ] discharges the separated organic solvent, the second solvent, the third solvent, and the residue. In a preferred embodiment, the separated organic solvent discharged from the solvent distillation section [δ] of step b.10) is fed to the dissolution section [α] of step b.1). In another preferred embodiment, the second solvent and the third solvent discharged from the solvent distillation section [δ] are reused in the precipitation section [γ] and / or the washing section [β], optionally after being separated from each other.

[0112] Separation by distillation is used to separate liquids from non-volatile solids and to separate liquids with different boiling points. Distillation is a common activity and can be carried out in a variety of devices, all of which are known to those skilled in the art.

[0113] The mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane was recovered optionally also comprises (dissolved) polyether polyurethane, (dissolved) nylon 6, colorants, additives, decomposition products of the organic solvent and / or the second solvent. The mixture comprising the organic solvent and the third solvent optionally also comprises (dissolved) polyether polyurethane, (dissolved) nylon 6, and decomposition products of the organic solvent and / or the third solvent.

[0114] Generally, the separation of three different liquids by distillation is more complicated, requires more energy, and requires more equipment than the separation of two different liquids by distillation. Therefore, it is particularly advantageous to limit the number of different solvents in the process of the present invention to two by using the same solvent as the second and third solvents. In a preferred embodiment of the present invention, the second and third solvents are the same solvent. In a more preferred embodiment, the second and third solvents are water or an aqueous solution. The use of water as a solvent is advantageous because it is cheap, abundantly available, non-explosive, and environmentally friendly.

[0115] In a preferred embodiment of the present invention, the organic solvent is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidone, e.g., N-methyl-2-pyrrolidone (NMP, C5H9NO), tetrahydrofuran (THF; also known as oxolane, C4H8O). In a more preferred embodiment, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2).

[0116] In another preferred embodiment, the organic solvent is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidone, such as N-methyl-2-pyrrolidone (NMP, CH9NO), and tetrahydrofuran (THF; also known as oxolane, C4H8O), and both the second solvent and the third solvent are water. In a more preferred embodiment of the present invention, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), and both the second solvent and the third solvent are water. The boiling point of N,N-dimethylacetamide (DMAc) at atmospheric pressure is approximately 165°C. The boiling point of water at atmospheric pressure is 100°C.

[0117] It has been found that when only DMAc and water are present in the solvent distillation section [δ] from the combined feed, the difference in volatility between the two is large, making separation by distillation quite easy. In such circumstances, continuous separation of the combined feed to the solvent distillation section [δ] can be achieved with a single distillation column operated in continuous mode. In such a distillation column, water is distilled in gaseous form from the top of the column, and DMAc is withdrawn in liquid form from the bottom of the column. If nonvolatile compounds (such as polyether polyurethane and nylon 6) are also present in the combined feed to the solvent distillation section [δ] in addition to DMAc and water, the solvent distillation configuration required for continuous separation into DMAc, water, and nonvolatile compounds becomes more extensive. A straightforward configuration for this type of separation by distillation involves two distillation columns operated in series. The feed (liquid state) consisting of DMAc, water, and nonvolatile compounds is introduced into the first distillation column. Water is distilled in gaseous form from the top of the first distillation column, and the mixture of DMAc and nonvolatile compounds is withdrawn (in liquid state or as a slurry) from the bottom of the first distillation column. The bottom flow of the first column is fed to a second distillation column. DMAc is distilled in a gaseous state from the top of the second distillation column, and non-volatile compounds are withdrawn from the bottom of the second distillation column. In practice, to reduce the risk of plugging the second distillation column, a mixture of DMAc and non-volatile compounds can be withdrawn from the bottom of the second distillation column. To improve the overall recovery of DMAc, a squeeze (column) unit can be added to recover DMAc from the bottom stream of the second distillation column.

[0118] As an alternative to distillative separation in two distillation columns operated in series, one can choose a distillation column with a side draw operated in continuous mode. In such a distillation column, water is distilled in gaseous form from the top of the distillation column, DMAc is discharged as a liquid side stream, and non-volatile compounds are withdrawn from the bottom of the second distillation column. Optionally, the bottom flow is fed to a compression (column) unit.

[0119] As an alternative to introducing the feed into the distillation column in a liquid state, the feed into the first distillation column can be carried out in a gaseous state. In such a case, the combined feed to the solvent distillation section [δ], excluding non-volatile compounds, is evaporated in a feed evaporator before being introduced into the first distillation column. The non-volatile compounds are discharged from the feed evaporator. This alternative has the advantage of reducing the occurrence of fouling in the distillation column. Optionally, a squeeze column can be combined with the feed evaporator. Typically, the DMAc-containing process flow contains a certain amount of acetic acid and dimethylamine, for example, due to decomposition of DMAc. This is also the case for the combined feed to the solvent distillation section [δ]. The boiling point of pure dimethylamine at atmospheric pressure is about 7°C. Dimethylamine is very soluble in water. The boiling point of pure acetic acid at atmospheric pressure is approximately 118°C.

[0120] However, acetic acid and DMAc form a high-boiling azeotrope (composition at atmospheric pressure of about 21% acetic acid and about 79% N,N-dimethylacetamide by weight), with an atmospheric boiling point of about 171°C, which is slightly higher than the atmospheric boiling point of N,N-dimethylacetamide. A high-boiling azeotrope (or constant-boiling mixture) is a mixture of two liquids that has a boiling point higher than the boiling points of the two individual liquids (in their pure forms).

[0121] As the pressure is reduced, the azeotropic point of a mixture of acetic acid and N,N-dimethylacetamide shifts toward a higher concentration of the lower boiling component (acetic acid). At 6.7 kPa, a high-boiling azeotrope having a composition of about 30% by weight acetic acid and about 70% by weight N,N-dimethylacetamide is formed with a boiling point of about 93°C.

[0122] A high-boiling azeotrope with a composition of about 42% by weight of acetic acid and about 58% by weight of N,N-dimethylacetamide at 1.3 kPa is formed at a boiling point of about 75°C. Preferably, the decomposition products are removed from the solvent reused in the process. This has the advantage of preventing the accumulation of decomposition products. In a preferred embodiment, the decomposition products of the organic solvent are removed before the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is introduced into the dissolution section [α] in step b.1). However, DMAc and acetic acid cannot be separated by simple distillation due to the formation of a maximum azeotrope. Those skilled in the art know several techniques for breaking azeotropes in distillation.

[0123] One group of techniques is based on adding another compound that alters molecular interactions and eliminates the maximum azeotrope of N,N-dimethylacetamide-acetic acid. Chlorobenzene, toluene, ethylbenzene, and xylene are examples of compounds that disrupt this maximum azeotrope. Adding one of these compounds to a mixture of N,N-dimethylacetamide and acetic acid forms a new azeotrope of that compound with acetic acid, which can be removed by evaporation. The resulting mixture of that compound and acetic acid is then separated.

[0124] Another group of techniques, often referred to as pressure swing distillation, is based on the pressure dependence of the maximum azeotrope. In such systems, two distillation columns are operated at different pressure levels. A feed mixture of N,N-dimethylacetamide and acetic acid is introduced into the first distillation column, which operates at a specific pressure. The bottoms of the first distillation column are introduced into the second distillation column. The bottoms of the second column are introduced into the first distillation column. High-purity N,N-dimethylacetamide and acetic acid are obtained as the top product of the distillation columns. The main advantage of pressure swing distillation is the absence of contaminants.

[0125] Finally, there are a range of techniques that convert acetic acid to another component (e.g., salt formation by addition of caustic) or selectively adsorb acetic acid from a mixture of N,N-dimethylacetamide and acetic acid.

[0126] If, besides N,N-dimethylacetamide (DMAc), water and non-volatile compounds (such as polyether polyurethane and polyamide 6), acetic acid and dimethylamine are also present in the composite feed to the solvent distillation section [δ], the arrangement of solvent distillation required for the continuous separation into DMAc, water, non-volatile compounds, acetic acid and dimethylamine is further extended.

[0127] The optimum configuration is highly dependent on both the composition and the volumetric flow rate of the composite feed to the solvent distillation section [δ]. The configuration may include a combination of the various separation solutions mentioned above.

[0128] If the fraction of acetic acid in the combined feed to the solvent distillation section [δ] is significantly smaller, the solvent distillation section [δ] can comprise two distillation columns operated in series, followed by a neutralizer evaporation step. A feed consisting of DMAc, water, non-volatile compounds, acetic acid, and dimethylamine is introduced into the first distillation column. Water and dimethylamine are distilled from the top of the first distillation column in the gaseous state, and a mixture of DMAc, acetic acid, and non-volatile compounds is withdrawn (in the liquid state or as a slurry) from the bottom of the first distillation column. The water vapor from the top of the first distillation column is condensed. The resulting liquid phase contains water and dissolved dimethylamine. A dimethylamine stripping unit can remove the dimethylamine from this liquid phase, thereby obtaining water (almost) free of dimethylamine. The bottom flow from the first column is introduced into the second distillation column. DMAc is distilled in a gaseous state from the top of the second distillation column, and a mixture of DMAc, acetic acid, and non-volatile compounds is withdrawn from the bottom of the second distillation column. The bottom flow of the second column is introduced into a neutralizer-evaporation unit. A base, such as an aqueous NaOH solution, is introduced into the neutralizer-evaporation unit to neutralize the acetic acid. Through evaporation, gaseous DMAc is discharged from the neutralizer-evaporation unit. Water, if present, is also evaporated. A mixture of neutralized acetic acid (acetic acid-containing salts) and non-volatile compounds is discharged from the bottom of the neutralizer-evaporation unit. In practice, to reduce the risk of clogging of the neutralizer-evaporation unit, the mixture of DMAc, neutralized acetic acid (acetic acid-containing salts), and non-volatile compounds can be withdrawn from the bottom of the neutralizer-evaporation unit. The DMAc recovered by distillation is discharged from the solvent distillation section [δ] and introduced into the dissolution section [α].

[0129] Preferably, the water recovered in the solvent distillation section [δ] is reused in the process of the present invention, optionally after removal of dimethylamine. Preferably, water is used as the second solvent and / or the third solvent.

[0130] Feeding process c.1) In step c.1) of the present invention, the resulting nylon 6-rich stream is fed to a depolymerization section [C], where the nylon 6 content of the nylon 6-rich stream is at least 85% by weight, preferably at least 90% by weight, on a dry weight basis. This has the advantage that the nylon 6-rich stream fed to the depolymerization section [C] is less contaminated with foreign matter, thereby improving the yield and purity of ε-caprolactam produced in a plant of the present invention configured to carry out the process of the present invention. Another advantage is that the nylon 6-rich stream requires less depolymerization agent, less waste, and less energy in the depolymerization section [C]. Finally, the nylon 6-rich stream is less likely to cause operational problems such as equipment fouling and clogging. The depolymerization section [C] comprises one or more depolymerization reactors operated in series and / or parallel.

[0131] Optionally, the nylon 6-rich stream is mechanically compressed to a smaller volume before being fed to the depolymerization section [C], which has the advantage of requiring less volume for intermediate storage and transportation and facilitating dosing in the depolymerization section [C].

[0132] Optionally, the nylon 6-rich stream is compressed into particles of increased density before being fed into the depolymerization section [C]. This can be achieved, for example, by mechanical compression or by extruding the molten material through a die, followed by cooling and cutting it to size. Compressing the nylon 6-rich stream into particles has the advantage of increasing bulk density, thereby reducing intermediate storage and transportation costs. Apart from the density increase, pelletization also offers other advantages, such as a uniform shape and structure that is favorable for (automated) feeding into the depolymerization section [C].

[0133] Optionally, the nylon 6-rich stream is fed to a blast furnace (e.g., an extruder). In the blast furnace, the nylon 6-rich stream is melted. Preferably, the resulting polymer melt is filtered, which has the advantage of removing solid impurities. The melted and optionally filtered polymer melt is cooled and then fed to a pelletizer to obtain pellets. These pellets are fed to a depolymerization section [C].

[0134] The size and shape of the pellets (often also called granules) can be selected within wide limits. Generally, pellets are cylindrical (derived from thin strands that are chopped into small pieces). However, other shapes, such as (imperfect) spheres, are also possible. The size of the pellets can be selected within wide limits. The pellets can have a diameter ranging from 1 to 10 mm, preferably from 2 to 5 mm, more preferably from 3 to 5 mm. In a preferred embodiment, the pellets have a length ranging from 1 to 50 mm, preferably from 2 to 25 mm, more preferably from 5 to 15 mm.

[0135] In some cases, the nylon 6-rich stream discharged from the separation section [B] is dried before being fed to the depolymerization section [C]. This has the advantage that less or no solvent is introduced into the depolymerization section [C]. The solvent introduced into the depolymerization section [C] may have a negative effect on the depolymerization process (e.g., it may result in a slower depolymerization reaction rate, increased catalyst consumption, increased energy consumption, and the vapor stream containing ε-caprolactam and water obtained in the depolymerization section [C] may contain more impurities).

[0136] The nylon 6-rich stream is preferably fed to the depolymerization reactor(s) as a solid or melt. Preferably, the nylon 6-rich stream is introduced as a melt. Melt feeding can be achieved by using an extruder, a gear pump, or other means known to those skilled in the art. Feeding the nylon 6-rich stream to the depolymerization reactor(s) can be accomplished by continuous or intermittent injection of the nylon 6-rich stream.

[0137] Depolymerization step c.2) In the depolymerization section [C], the nylon 6-rich stream is depolymerized to form ε-caprolactam, which is discharged from the depolymerization section [C] as an ε-caprolactam-containing stream.

[0138] Depolymerization of the nylon 6-rich stream is achieved in the depolymerization section [C] by increasing the temperature of the nylon 6-rich stream from at least 180°C to a temperature of up to 400°C. A preferred temperature range for the depolymerization reaction is 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C. Experiments have shown that depolymerization is particularly effective in these temperature ranges, resulting in less side reactions of polyamide 6 and ε-caprolactam and reactions of impurities.

[0139] Generally, the rate of ε-caprolactam formation increases at higher temperatures. Temperatures below 400°C are preferred because side reactions of nylon 6 and impurities occur more frequently at temperatures above 400°C, resulting in the formation of a more diverse set of impurities. Some of these impurities become the ε-caprolactam-containing product stream discharged from the depolymerization reactor section [C]. In a preferred embodiment of the present invention, the depolymerization of the nylon 6-rich stream is carried out at temperatures ranging from 220°C to 340°C or 240°C to 325°C. This temperature range allows for the production of particularly pure ε-caprolactam.

[0140] The pressure in the depolymerization section [C] 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, and most preferably 50 kPa to 1 MPa. This pressure range allows the production of particularly pure ε-caprolactam.

[0141] The depolymerization of the nylon 6-rich stream can be accomplished in the presence or absence of a solvent. Preferably, the depolymerization of the nylon 6-rich stream is accomplished in the presence of water as a solvent. In this case, the water is preferably in the form of steam, especially superheated steam. Preferably, the depolymerization is completed in 0.1 to 24 hours, more preferably in 0.5 to 6 hours.

[0142] By supplying water as steam to the depolymerization reactor, a vapor stream containing ε-caprolactam and water can be obtained, optionally without further heating. The weight-to-weight ratio of ε-caprolactam to water in this vapor stream can be adjusted by changing the amount of steam supplied to the nylon 6-rich stream in the depolymerization section [C]. In a preferred embodiment, the depolymerization in step c.2) is carried out in the presence of water, and the ε-caprolactam-containing stream is a vapor stream containing ε-caprolactam and water in a weight-to-weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10, and most preferably 1:3 to 1:8. Within these ranges, a particularly economically efficient process can be performed.

[0143] Preferably, the ε-caprolactam in the vapor stream comprising ε-caprolactam and water has a partial pressure of from 0.1 kPa to 1 MPa, more preferably from 0.3 kPa to 0.5 MPa, most preferably from 1 kPa to 0.1 MPa.

[0144] During the depolymerization reaction, degradation products, including oligomers of ε-caprolactam, may be formed. Furthermore, feed streams of nylon 6-derived materials, including multicomponent materials, may also contain other components, i.e., impurities such as non-nylon 6 compounds, and residue(s) of solvents applied in pretreatments that remain stable, react, or decompose under the depolymerization conditions. Thus, if water is used as the solvent, the vapor stream removed from the depolymerization section [C] contains not only water and ε-caprolactam, but also impurities.

[0145] Preferably, superheated steam having a temperature of 100°C to 600°C is fed into the depolymerization reactor(s). Preferably, the superheated steam fed into the depolymerization reactor(s) has a temperature of at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam fed into the depolymerization reactor(s) is sufficiently high so that no other heat input is required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam. In another preferred embodiment, the depolymerization section [C] 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 [C] is fed with superheated steam having a temperature in the range of 275°C to 500°C. In another preferred embodiment, part of the heat input required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam is introduced through the wall of the depolymerization reactor(s).

[0146] Generally, the mass of the vapor stream removed from the depolymerization section [C] is less than the mass of the total feed to the depolymerization section. The total feed to the depolymerization section [C] includes the nylon 6-rich stream and, optionally, solvent, catalyst, additional agent, and / or depolymerization agent. Therefore, unless additional measures are taken, accumulation of material (often referred to as "residual material") will occur in the depolymerization section [C]. Preferably, a separate stream is discharged from the depolymerization section [C]. This has the advantage of reducing or avoiding accumulation of material in the depolymerization section [C]. When phosphoric acid is used as the depolymerization catalyst, the additional stream can contain impurities present in the nylon 6-rich stream, non-depolymerized nylon 6, non-vaporized ε-caprolactam, catalyst(s), and compounds formed under the depolymerization conditions, such as mono-, di-, and / or triammonium phosphate. In a preferred embodiment, a stream containing mono-, di-, and / or triammonium phosphate is discharged from the depolymerization section [C]. Even more preferably, this stream intermittently or continuously discharged from the depolymerization section [C] comprises mono-, di- and / or triammonium phosphates in a fraction of 0.01 to 50% by weight, preferably 0.1 to 25% by weight, more preferably 0.5 to 10% by weight, most preferably 0.5 to 5% by weight.

[0147] The depolymerization of the nylon 6-rich stream in the presence of steam can be carried out in the presence of an additional depolymerization agent, such as ammonia. The concentration of ammonia in the depolymerization section [C] can vary. Therefore, if ammonia is present in the depolymerization section [C], the vapor stream removed from the depolymerization section [C] can contain not only ε-caprolactam and impurities but also ammonia.

[0148] Most preferably, the depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted) acid or base. The acid catalyst may be selected from the group consisting of orthophosphoric acid, p-toluenesulfonic acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids including xylenesulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons, solid acids, salts of the aforementioned acids, Al2O3 and SiO2, and combinations thereof. The base catalyst may be selected from the group consisting of, for example, alkali hydroxides, alkali salts, alkaline earth hydroxides and alkaline earth salts, organic bases and solid bases, and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali hydroxides and alkali salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, or potassium bicarbonate are used as catalysts. Even more preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid, and sodium hydroxide are used as catalysts. In one particularly preferred embodiment, orthophosphoric acid is used as the catalyst for depolymerization, and in another embodiment, p-toluenesulfonic acid is used.

[0149] However, in another preferred embodiment, no catalyst is used in the depolymerization of the nylon 6-rich stream. This has the advantage of lower costs (both in terms of catalyst and disposal of catalyst waste). However, higher temperatures (and pressures) are typically required compared to depolymerization of the nylon 6-rich stream in the presence of a catalyst.

[0150] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction can be initiated at a lower temperature and can be carried out under atmospheric conditions. The suitable catalyst concentration used for the depolymerization of nylon 6 to ε-caprolactam is known to those skilled in the art and can be easily determined by routine experimentation. If the catalyst concentration is too low, the reaction rate will be slow. Conversely, if the catalyst concentration is too high, the reaction will be fast, but side reactions (or reactions) will increase. This will also increase the catalyst cost, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100 wt% relative to the nylon 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1 to 50 wt%. 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 wt%, more preferably 1 to 20 wt%. For the catalyst p-toluenesulfonic acid, the preferred content is 10 to 35 wt%, more preferably 15 to 30 wt%.

[0151] The depolymerization of nylon 6 can be carried out in batch mode, semi-continuous mode, or continuous mode, all of which are known to those skilled in the art. As used herein, the terms "batch mode," "semi-continuous mode," and "continuous mode" refer to a mode in which a nylon 6-containing feedstock, i.e., a nylon 6-rich stream, and optionally a catalyst, are charged to a depolymerization reactor, and residual materials are discharged from the depolymerization reactor.

[0152] In a preferred embodiment, nylon 6 depolymerization is carried out in batch mode. In batch mode, the feedstock, i.e., nylon 6-derived material including multicomponent materials, and optionally a catalyst, are first charged to a depolymerization reactor. Superheated steam is then charged to the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. The charge of superheated steam to the depolymerization reactor is then discontinued. Optionally, after removing residual materials from the depolymerization reactor, a new cycle is initiated by charging the feedstock (and optionally the catalyst) to the depolymerization reactor. In a preferred embodiment, residual materials are not removed between cycles.

[0153] In a particularly advantageous embodiment, the depolymerization of nylon 6 is carried out in a continuous mode. In the continuous mode, a nylon 6-containing feedstock (and optionally a catalyst) is continuously introduced into a depolymerization reactor. Simultaneously, superheated steam is continuously introduced into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream containing ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently introduced into the depolymerization reactor. Additionally, residual materials are continuously discharged from the depolymerization reactor. Preferably, the nylon 6-rich stream is introduced as a melt. Preferably, the catalyst is introduced as a melt, a slurry, or a solution.

[0154] In another preferred embodiment, nylon 6 depolymerization is conducted in a semi-continuous mode. In the semi-continuous mode, nylon 6-containing feedstock (and optional catalyst) is intermittently charged to a depolymerization reactor, superheated steam is continuously charged to the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a vapor stream comprising ε-caprolactam and water. Residual materials are intermittently discharged from the depolymerization reactor in the semi-continuous mode of nylon 6 depolymerization.

[0155] Recovery process c.3) In the recovery section [D], ε-caprolactam is recovered from the ε-caprolactam-containing stream discharged from the depolymerization section [C]. This stream contains ε-caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the ε-caprolactam-containing stream.

[0156] Preferably, when no solvent is added to the depolymerization section [C], the ε-caprolactam obtained by the condensation dissolves in water, thereby obtaining an ε-caprolactam-rich phase, which also contains impurities.

[0157] Preferably, when water is introduced into the depolymerization section [C] as a solvent, the ε-caprolactam-containing stream discharged from the depolymerization section [C] contains ε-caprolactam, water, and impurities. Water can be introduced as a liquid or in the form of vapor. Preferably, water is introduced in the form of vapor. ε-caprolactam can be separated from the ε-caprolactam-containing stream discharged from the depolymerization section [C] by sending this vapor or gas stream (preferably partially) from the depolymerization reactor, preferably at the top, to a condenser to obtain a condensate containing ε-caprolactam. Preferably, ε-caprolactam is separated from the remaining components of the vapor stream by sending the product stream from the depolymerization reactor, preferably from the top, to a distillation column, from which a water-rich phase is obtained as an overhead product and an ε-caprolactam-rich phase is obtained as a bottom product.

[0158] The ε-caprolactam recovered in the recovery section [D] is crude because it contains impurities such as nylon 6 degradation products or other impurities derived from (decomposition products of) non-nylon 6 components of the material derived from the nylon 6-rich stream fed to the depolymerization section [C]. The crude ε-caprolactam recovered in step c.3) contains water and ε-caprolactam, and is preferably an aqueous solution containing ε-caprolactam. Therefore, the crude ε-caprolactam recovered in the recovery section [D] requires further purification to obtain high-purity ε-caprolactam. Therefore, as used herein, "crude" can be defined as having a lower purity, i.e., containing more impurities, than the purified ε-caprolactam obtained as the product of the process of the present invention.

[0159] Preferably, the crude ε-caprolactam contains ε-caprolactam in the range of 6 to 95% by weight, more preferably 20 to 90% by weight, and most preferably 35 to 80% by weight, with the remainder being mainly water. Purification process c.4) In step c.4), the crude ε-caprolactam obtained in the recovery section [D] is purified in the purification section [E] to obtain highly purified ε-caprolactam.

[0160] Optionally, the crude ε-caprolactam is filtered before being introduced into the purification section [E]. Filtration ensures the removal of undissolved impurities that may otherwise interfere with the further purification process.

[0161] Optionally, oil is separated from the crude ε-caprolactam before it is fed to the refining section [D]. Separating the oil ensures the removal of impurities that may otherwise interfere with the further refining process.

[0162] Preferably, the purified ε-caprolactam is obtained by purifying the crude ε-caprolactam obtained in the recovery section [D] of the purification section [E] by crystallizing the ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C, more preferably at a temperature of 20°C to 85°C.

[0163] The process of the present invention is carried out in a plant, wherein the plant comprises a purification section [E], wherein the purification comprises: c.4)(iv) A step of obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C.

[0164] Preferably, the process of the present invention is carried out in a plant, wherein the plant comprises a purification section [E], wherein the purification comprises: c.4)(i) extracting crude ε-caprolactam with an organic solvent, thereby obtaining an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities; c.4)(ii) optionally switching the solvent by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam, the solvent switching step (ii) being selected from a process based on back-extraction with water and a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced; and c.4)(iii) obtaining purified ε-caprolactam by distilling off impurities having a boiling point lower or higher than ε-caprolactam.

[0165] Preferably, this is followed by: c.4)(iv) A process for obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C.

[0166] Highly pure ε-caprolactam can be obtained from crude ε-caprolactam by first extracting the crude ε-caprolactam with an organic solvent in step c.4)(i), thereby obtaining an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam, and impurities. The organic solvent for extracting the crude ε-caprolactam is preferably an aromatic hydrocarbon, a halogenated hydrocarbon, and / or a C4-C6 hydrocarbon. 10 Aliphatic or C4-C 10 Optionally, the organic solvent from which the crude ε-caprolactam is extracted may preferably be a mixed extractant, i.e., comprise one or more organic solvents and optionally a further diluent. Preferably, the one or more organic solvents are aromatic hydrocarbons, halogenated hydrocarbons and / or C4-C6 10 The organic solvents are independently selected from aliphatic or alicyclic alcohols, and C5-C8 alkanes or C5-C8 cycloalkanes. Particularly good purification results are achieved when the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC, methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol, and mixtures thereof. More preferably, the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of benzene, toluene, cyclohexane, alcohols, and mixtures thereof. Even more preferably, the organic solvent for extracting crude ε-caprolactam is selected from the group consisting of toluene, cyclohexane, 1-octanol, 2-ethylhexanol, and mixtures thereof. Preferably, the weight ratio of organic solvent to ε-caprolactam is from 0.01:1 to 50:1, preferably from 0.05:1 to 20:1, more preferably from 0.1:1 to 10:1, and most preferably from 0.1:1 to 5:1.

[0167] In another embodiment, the extraction with organic solvent in step c.4)(i) is carried out in a countercurrent extraction column, whereby the crude ε-caprolactam to be purified is introduced at the top of the column and the organic solvent is introduced at the bottom of the column, resulting in an aqueous phase and an organic phase containing the organic solvent, ε-caprolactam and impurities.

[0168] Optionally, the organic phase containing the organic solvent, ε-caprolactam, and impurities is washed with water or an aqueous alkaline solution before entering step c.4)(ii). When washing is carried out with an aqueous alkaline solution, the alkaline solution is preferably an aqueous solution containing an alkali metal hydroxide and / or alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably contains 0.5% to 2.0% by weight of sodium hydroxide or potassium hydroxide.

[0169] Those skilled in the art can determine by routine experimentation the amount of water or aqueous alkaline solution required for efficient washing of the organic phase containing the organic solvent, ε-caprolactam, and impurities. Preferably, this amount is 0.1% to 5% by volume relative to the amount of organic solvent in the organic phase to be washed.

[0170] In another preferred embodiment, washing the organic phase containing the organic solvent, ε-caprolactam, and impurities with water or an aqueous alkaline solution is carried out in a countercurrent washing column, whereby the organic phase containing the organic solvent, ε-caprolactam, and impurities is introduced into the bottom of the column, and water or an aqueous alkaline solution is introduced into the top of the column. Washing results in a washed organic phase containing the organic solvent, ε-caprolactam, and impurities, and a residue-containing phase. The residue-containing phase then contains water, impurities, and ε-caprolactam.

[0171] In a preferred embodiment, the organic phase containing the organic solvent, ε-caprolactam, and optionally washed impurities is then extracted with water, thereby obtaining an ε-caprolactam-aqueous phase. Preferably, this ε-caprolactam-aqueous phase is then stripped and / or distilled to remove residual solvent. The amount of water used to recover ε-caprolactam can vary, but is typically 0.5 to 20 times, preferably 0.75 to 10 times, and more preferably 1 to 5 times the weight of the recovered ε-caprolactam.

[0172] In another preferred embodiment, the extraction with water is carried out in a countercurrent extraction column, the purified ε-caprolactam-containing phase being introduced into the bottom of the column and water into the top. The extraction results in an ε-caprolactam-water phase and an impurity-containing solvent phase. The impurity-containing solvent phase is usually reused, optionally after purification, preferably by distillation.

[0173] The ε-caprolactam-aqueous phase, optionally stripped and / or distilled to remove residual solvent, is concentrated by evaporation of water to obtain a concentrated aqueous ε-caprolactam phase, the ε-caprolactam content of which is typically between 60% and 99.9% by weight, based on the total weight of the phase.

[0174] In another preferred embodiment, the organic solvent is evaporated from the organic phase containing the organic solvent, ε-caprolactam, and impurities, and optionally washed instead of being extracted with water. Any suitable evaporation vessel, such as a column, can be used. Preferably, the evaporation is carried out in the presence of water. More preferably, the evaporation is carried out as an azeotropic distillation, in which case the organic solvent is evaporated as an azeotropic mixture. The evaporation yields an ε-caprolactam product. Preferably, the ε-caprolactam product is an aqueous ε-caprolactam phase. The ε-caprolactam content of this aqueous ε-caprolactam phase is typically 40% to 99.9% by weight based on the total weight of the phase.

[0175] In another preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide, is added to the ε-caprolactam-aqueous phase prior to distillative removal in step c.4)(iii) of the inventive process. Most preferably, potassium permanganate is used as the oxidizing agent.

[0176] Preferably, the oxidizing agent is added to the ε-caprolactam-aqueous phase as a solid, as a slurry, or in the form of an aqueous solution, resulting in a dilute aqueous solution during purification by oxidation. Those skilled in the art can determine the amount of oxidizing agent required for efficient oxidation of the ε-caprolactam-aqueous phase through routine experimentation. The exact amount of oxidizing agent is highly dependent, inter alia, on the composition of the polyamide 6-rich stream fed to the depolymerization section in this process of the present invention. Preferably, the amount of oxidizing agent is 0.01 to 5 wt. % relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0177] The temperature used for the oxidation of the aqueous solution in the process of the present invention can vary. Preferably, the oxidation of the aqueous solution with an oxidizing agent prior to distillative removal in step c.4)(iii) is carried out at a temperature in the range of 20°C to 85°C, more preferably in the range of 30°C to 80°C, wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide, and combinations thereof, especially potassium permanganate.

[0178] The length of time used for oxidation with the oxidizing agent can vary. Preferably, in the process of the present invention, oxidation of the ε-caprolactam-aqueous phase with the oxidizing agent prior to distillative removal in step c.4)(iii) is carried out for a period of from 1 minute to 24 hours, more preferably from 2 minutes to 6 hours, and most preferably from 5 minutes to 2 hours.

[0179] The concentration of ε-caprolactam in the ε-caprolactam-water phase used for oxidation with the oxidizing agent can vary. Preferably, the aqueous solution used for oxidation comprises a weight-to-weight ratio of ε-caprolactam to water of 5:1 to 1:5, more preferably 3:1 to 1:3, and most preferably 2:1 to 1:2. Optionally, the weight-to-weight ratio of ε-caprolactam to water is adapted before adding the oxidizing agent to the aqueous phase. Preferably, the weight-to-weight ratio of ε-caprolactam to water is adapted by either adding or removing water.

[0180] When potassium permanganate and / or sodium permanganate are used as oxidizing agents, solid manganese(IV) oxide (MnO) particles are formed as the reaction product. Those skilled in the art can determine, through routine experimentation, the optimal solid-liquid filtration procedure for efficiently removing the solid manganese(IV) oxide particles from the aqueous phase after oxidation. The use of filter aids such as activated carbon or diatomaceous earth particles to improve the filtration procedure is common practice in this regard.

[0181] The aqueous ε-caprolactam phase is optionally hydrogenated, preferably in the presence of a hydrogenation catalyst known per se. The hydrogenation can be carried out, for example, as described in EP 635487.

[0182] In a preferred embodiment of the present invention, prior to the crystallization in step c.4)(iv), the aqueous solution containing water, ε-caprolactam, and impurities is hydrogenated in the presence of a hydrogenation catalyst. The hydrogenation catalyst may be any known heterogeneous hydrogenation catalyst. Examples of such catalysts include ruthenium on aluminum oxide, rhodium on aluminum oxide, platinum on activated carbon, palladium on activated carbon, Raney nickel, nickel on silica, and nickel on aluminum oxide. Preferably, a nickel-containing catalyst is used. Suitable nickel catalysts generally have a nickel content of 5 to 80 wt. % based on the metal and support. In addition to nickel, the catalyst may contain several activators, such as Zr, Mn, Cu, or Cr. The activator content is generally 1 to 20 wt. When a palladium-containing heterogeneous catalyst is used, the palladium content is generally 0.01 to 10 wt. %.

[0183] 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 in which 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 in which the catalyst is fixed in the reactor.

[0184] The hydrogenation can be carried out in a three-phase system (gas, liquid, solid) containing 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) containing an aqueous ε-caprolactam mixture fully or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. Dissolution of hydrogen in a water-ε-caprolactam mixture to obtain a mixture fully or partially saturated with hydrogen can be carried out by any process known to those skilled in the art. The hydrogenation temperature is generally 20° C. to 160° C. The hydrogenation pressure is generally 0.1 to 15 MPa.

[0185] Hydrogenation of the water-ε-caprolactam mixture is carried out to hydrogenate the unsaturated compounds present in the impure ε-caprolactam. The presence of these unsaturated compounds is disadvantageous because they can impair the physico-mechanical properties of nylon 6 produced by polymerization of ε-caprolactam. The saturated compounds formed by hydrogenation do not adversely affect these physico-mechanical properties of nylon 6, and furthermore, these compounds are more easily removed, for example, in distillation and / or crystallization steps following the hydrogenation step.

[0186] Preferably, water is evaporated from the optionally hydrogenated aqueous ε-caprolactam phase. After hydrogenation and / or evaporation of the water, the aqueous ε-caprolactam phase is distilled to recover high-purity ε-caprolactam and a distillation residue.

[0187] In step c.4)(iii) of the process of the present invention, the optionally washed organic phase comprising the organic solvent, ε-caprolactam and impurities is then distilled. Preferably, the distillation of the optionally washed organic phase comprising the organic solvent, ε-caprolactam and impurities is carried out under reduced pressure. In one embodiment, the distillation is carried out at a pressure of less than 350 kPa, preferably less than 50 kPa, more preferably less than 20 kPa, and most preferably less than 10 kPa. Preferably, the distillation temperature at the bottom of the distillation column is between 100°C and 200°C, more preferably between 110°C and 180°C. The distillation comprises the separation of low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) and / or high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam.

[0188] In a preferred embodiment, an alkali metal hydroxide, preferably NaOH, is added to the ε-caprolactam-containing phase prior to the distillative removal in step c.4)(iii). Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol per kg of ε-caprolactam, more preferably from 2 to 80 mmol per kg of ε-caprolactam. Experiments have shown that the addition of an alkali metal hydroxide, in particular NaOH, allows for particularly effective distillative removal of impurities with boiling points lower and higher than ε-caprolactam.

[0189] In another preferred embodiment, the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified in the purification section [E], separately from the crude ε-caprolactam recovered in the recovery section. This has the advantage that the carbon footprint of a plant producing ε-caprolactam de novo from the Beckmann rearrangement of cyclohexanone oxime can be reduced by introducing recycled ε-caprolactam according to the present invention.

[0190] In step c.4)(iv) of the process of the present invention, the solution containing ε-caprolactam and impurities is crystallized to obtain purified ε-caprolactam at a temperature between 10°C and 95°C.

[0191] Preferably, the solution containing ε-caprolactam and impurities crystallized in step c.4)(iv) is crude ε-caprolactam obtained in recovery section [D].

[0192] More preferably, the solution comprising ε-caprolactam and impurities from which purified ε-caprolactam is obtained by crystallization in step c.4)(iv) is an organic phase comprising an organic solvent, ε-caprolactam and impurities, optionally washed with water or an aqueous alkaline solution, and obtained by extraction in step c.4)(i).

[0193] Even more preferably, the solution comprising ε-caprolactam and impurities from the purified ε-caprolactam obtained by crystallization in step c.4)(iv) is an aqueous phase comprising water, ε-caprolactam and impurities with a lower or higher boiling point than the ε-caprolactam obtained by switching the solvent in step c.4)(ii).

[0194] Most preferably, the solution comprising ε-caprolactam and impurities from the purified ε-caprolactam obtained by crystallization in step c.4)(iv) is a phase comprising ε-caprolactam and impurities obtained by distillation under vacuum conditions in step c.4)(iii).

[0195] Preferably, the ε-caprolactam crystallization process in step c.4)(iv) comprises the following steps: 1. A solution containing ε-caprolactam and impurities is fed into a crystallizer; 2. The conditions of the crystallizer are set so that ε-caprolactam crystals and a mother liquor are formed; 3. Separating the ε-caprolactam crystals from the mother liquor; 4. The mother liquor is recycled.

[0196] Crystallization can be applied to the production of ε-caprolactam. It is mainly used for its purification ability to increase the yield and / or for product recovery. All crystallization processes are based on the formation of a solid crystalline phase from a liquid. In a preferred embodiment, the crystallization in step c.4)(iv) is carried out by either solution crystallization or melt crystallization.

[0197] Optionally, according to step c.4)(iii) of the process of the invention, prior to the crystallization in step c.4)(iv), impurities having a boiling point lower or higher than that of ε-caprolactam are removed by distillation under vacuum conditions, thereby obtaining a phase comprising ε-caprolactam and impurities.

[0198] Thus, according to a particularly advantageous embodiment of the invention, the purification in step c.4) also comprises a step c.4)(iii) of distillative removal under vacuum conditions of impurities with a boiling point lower or higher than that of ε-caprolactam, prior to the crystallization in step c.4)(iv).

[0199] Optionally, according to step c.4)(iii) of the process of the present invention, after the crystallization in step c.4)(iv), impurities having a boiling point lower or higher than that of ε-caprolactam are removed by distillation under vacuum conditions, thereby obtaining a phase comprising ε-caprolactam and impurities.

[0200] Thus, according to a particularly advantageous embodiment of the invention, after the crystallization of step c.4)(iv), the purification in step c.4) also comprises a step c.4)(iii) of distillative removal under vacuum conditions of impurities with a boiling point lower or higher than that of ε-caprolactam.

[0201] Optionally, according to step c.4)(iii) of the process of the invention, impurities having a boiling point lower or higher than that of ε-caprolactam are removed by distillation under vacuum conditions both before and after the crystallization in step c.4)(iv), thereby obtaining a phase comprising ε-caprolactam and impurities.

[0202] Thus, according to a particularly advantageous embodiment of the invention, both before and after the crystallization in step c.4)(iv), the purification in step c.4) also comprises a step c.4)(iii) of distillative removal under vacuum conditions of impurities with a boiling point lower or higher than that of ε-caprolactam.

[0203] The term solution crystallization, including the (impure) compound, is used to describe the crystallization of a compound from a solution to which a co-solvent has been added. The co-solvent can be water or a non-aqueous solvent. When the co-solvent is water, the amount of water in the solution can be selected within a wide range, preferably 0.5 to 25% by weight, more preferably 1 to 9% by weight. The crystallization temperature can be selected within a wide range, preferably 10 to 95°C, more preferably 20 to 85°C, even more preferably 20 to 70°C, and most preferably 30 to 65°C. The crystallized and purified ε-caprolactam is recovered from the slurry at a slurry concentration preferably in the range of 5 to 75% by weight, more preferably 10 to 70% by weight, and most preferably 15 to 50% by weight. When the co-solvent is a non-aqueous solvent, the amount of the non-aqueous solvent in the solution can be selected within a wide range, preferably 5 to 95% by weight, more preferably 10 to 90% by weight, and most preferably 30 to 70% by weight. The crystallization temperature can be selected within a wide range, preferably 10 to 95°C, more preferably 20 to 85°C, even more preferably 20 to 70°C, and most preferably 30 to 65°C. The crystallized and purified ε-caprolactam is recovered from the slurry at a slurry concentration of preferably 5 to 75% by weight, more preferably 10 to 70% by weight, and most preferably 15 to 50% by weight. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, and cyclohexane), alcohols (such as methanol, ethanol, n-propanol, and n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, and p-xylene), ammonia, chlorinated hydrocarbons (such as tetrachloromethane, chloroform, and ethyl chloride), ketones (such as acetone and methyl ethyl ketone), and esters (such as ethyl acetate), as well as mixtures of these solvents. Among these, cyclohexane is preferred.

[0204] Solution crystallization is usually carried out under atmospheric pressure, but may also be carried out under reduced pressure or pressure. In solution crystallization, the product is crystallized by evaporation crystallization, whereby the solvent is evaporated, or by cooling crystallization, whereby cooling is achieved by direct cooling, indirect cooling, or vacuum cooling, or a combination of these methods. After the crystallization process, the formed crystals and mother liquor are separated, for example, by settling, filtration, and / or centrifugation. Optionally, the obtained crystals are washed, for example, with a solvent or solvent mixture with a low impurity content. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained as crystals.

[0205] The narrow definition of the term melt crystallization is the crystallization of a compound from a solution containing the (impure) compound without the use of a cosolvent. A broader definition of the term melt crystallization also applies to crystallization from a solution containing a low solvent concentration. Preferably, the solvent concentration in the solution is less than 25% by weight, more preferably less than 10% by weight, and most preferably less than 5% by weight. Unless otherwise specified, the broader definition of melt crystallization is used herein. The compound crystals obtained by melt crystallization are separated from the mother liquor and optionally washed with a melt of the pure compound material. The purification efficiency can be further improved, for example, by sweating, i.e., partial melting, i.e., gently heating the crystal layer to near its melting temperature to induce the discharge of trapped, adherent impure mother liquor. Optionally, the crystallization-separation sequence is repeated several times. Finally, the optionally washed crystals are melted and discharged as a melt or mechanically removed.

[0206] Preferably, a 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 include water, alkanes (e.g., n-hexane, n-heptane, isooctane, cyclohexane), alcohols (e.g., methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (e.g., benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (e.g., tetrachloromethane, chloroform, or ethyl chloride), ketones (e.g., acetone or methyl ethyl ketone), and esters (e.g., ethyl acetate). Water and aromatic hydrocarbons are preferably used as solvents because they produce large crystals. Water is the most preferred solvent. The solvent acts as a freezing point depressant for the melt in the crystallizer. Generally, melt crystallization requires less energy than solution crystallization, but can be more difficult to operate on an industrial scale.

[0207] As used herein, melt crystallization specifically refers to layer melt crystallization or suspension melt crystallization. These two types of melt crystallization technological processes are characterized by (1) the formation of a crystal layer on the heat exchanger wall (layer melt crystallization) and (2) crystals growing in suspension (suspension melt crystallization). Generally, the operation of a process for growing a crystal layer on the heat exchanger wall is called layer melt crystallization. First, the melt is introduced into a crystallizer, and a crystal layer grows on the cooled heat exchanger surface. Next, the remaining melt, including impurities removed from the growing crystals, is discharged from the crystallizer. The crystal layer is then melted, and the purified product is recovered. Purification efficiency can be further improved, for example, by sweating, i.e., by gently heating the crystal layer to near its melting temperature, causing the discharge of trapped, adherent, impure mother liquor. The layer melt crystallization process is operated in a batch mode. Well-known examples of processes based on layer melt crystallization are the ProABD process by BEFS Prokem and the Sulzer Chemtech process.

[0208] Layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization, crystals grow from a stagnant melt onto a cooling surface. In static mode, the desired compound is crystallized batchwise from a stagnant melt in a closed vessel onto the heat exchanger wall. This type of crystallization is characterized by a slow crystal growth rate and a long residence (or batch) time. Preferably, the crystallization time ranges from 1 to 75 hours, more preferably from 2 to 50 hours, and most preferably from 4 to 24 hours. After the crystallization process, the remaining melt is drained. Optionally, a sweating phase is then introduced to remove impurities attached to or trapped within the crystals. Finally, the crystals are completely melted and drained or mechanically removed.

[0209] Dynamic crystallization is typically carried out in a tube-and-shell heat exchanger, whereby the melt circulates beneath a cooling surface where the compound crystallizes. Typically, the melt is pumped through a tube, and crystals grow inside the tube while a cooling medium flows around the outside of the tube. The thickness of the crystal layer increases over time. After a certain time, the melt circulation is stopped and the remaining melt is drained. Dynamic layer crystallization is similar to stagnant layer crystallization, which is also performed in a batch mode. The crystal growth rate is higher in dynamic mode compared to stagnant mode, resulting in a shorter crystallization time. Preferably, the crystallization time ranges from 0.05 to 12 hours, more preferably from 0.1 to 6 hours, and most preferably from 0.3 to 3 hours. A sweating phase is then optionally introduced to remove impurities attached to or trapped within the crystals. Finally, the crystals are completely melted and drained or mechanically removed.

[0210] Suspension melt crystallization can be carried out in either batch or continuous mode. In suspension melt crystallization, the melt is cooled below its saturation temperature, and crystals begin to grow (optionally after the addition of nuclei). The crystal growth rate is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be carried out in any exchanger-type or vessel-type crystallizer capable of cooling the melt. Preferably, suspension melt crystallization is carried out in a scraper-type surface crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of crystals of the desired compound and mother liquor is introduced into a so-called wash tower. In the wash tower, the mother liquor is filtered or drained from the crystals, and then the crystals are generally washed with purified compound material.

[0211] After the ε-caprolactam crystallization step, a mother liquor is obtained that still contains ε-caprolactam next to impurities. Methods for recovering ε-caprolactam from this type of mother liquor are well known to those skilled in the art. As a result of 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, one possible solution is to recycle the mother liquor countercurrently, i.e., to feed the mother liquor obtained in the nth crystallization step into the feed of the (n-1)th crystallization step. Typically, the mother liquor obtained from the first crystallization step is fed to an upstream (purification) unit of the process or to a dedicated mother liquor processing unit (e.g., based on distillation or crystallization). After ε-caprolactam crystallization, it may be necessary to purify the obtained mother liquor (or a portion thereof), for example, by recycling it to any previous stage in the process. Alternatively, the mother liquor can be purified, for example, by distillation, before being reintroduced into the ε-caprolactam crystallization step.

[0212] The high-purity ε-caprolactam obtained by the process of the present invention can be used to make nylon 6 using processes well known to those skilled in the art. This nylon 6 can then be used in all known materials, such as engineering materials, fibers, and films. This nylon 6 produced from nylon 6 and polyether polyurethane-containing materials according to the present invention is particularly suitable for high-speed spinning applications, such as garments containing polyether polyurethane (also known as elastane). plant

[0213] The present invention also provides a plant, i.e., a chemical plant, comprising a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E], which is configured to carry out the above-described process of the present invention. All plant features specifically described in connection with the above-described process also apply to the plant of the present invention described hereinafter, and vice versa. It is therefore understood that the plant is suitable for carrying out the process of the present invention, and that what has been described in connection with the process of the present invention applies equally to the plant embodiment.

[0214] The plant can be a laboratory facility, as in the examples. Preferably, however, the plant is an industrial-scale plant. By "industrial scale" is meant that the plant, when constantly operating, has a production capacity of at least 500 tons / year of ε-caprolactam (i.e., can, in principle, produce the same in that amount).

[0215] The plant of the present invention is suitable for producing purified ε-caprolactam from nylon 6 and polyether polyurethane-containing materials and includes at least four sections: a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E]. These sections, and the plant, are configured to carry out the process of the present invention described above.

[0216] Optionally, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [λ] for fragmenting the nylon 6 and polyether polyurethane-containing material into small pieces, and / or a cleaning section [ω] for cleaning the nylon 6 and polyether polyurethane-containing material or the fragmented pieces obtained therefrom. Cleaning includes both cleaning and separating foreign matter from the nylon 6 and polyether polyurethane-containing material. Separation of foreign matter can be performed both manually (hand picking) and mechanically (e.g., density separation and magnetic separation). Both manual and mechanical devices, such as brushes, can assist in cleaning in the cleaning section [ω]. Cleaning is preferably achieved by additional friction. Various types of industrial cleaning systems, such as rotary plastic washers and (high-speed) friction washers, are commercially available. The mechanical size reduction section [λ] includes a device for mechanically fragmenting the nylon 6 and polyether polyurethane-containing material into small pieces. Non-limiting examples of this fragmentation device include a cutter, puncher, shredder, mill, grinder, or chipper.

[0217] The separation section [B] can comprise four sections: a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ]. These sections, and the plant thereby, are configured to carry out the process of the present invention as described above.

[0218] The dissolution section [α] comprises one or more dissolution units operated in series and / or parallel and one or more separation units operated in series and / or parallel. The optionally cleaned and / or fragmented pieces of nylon 6 and polyether polyurethane-containing material are fed to the dissolution unit(s) as a solid or as a melt, preferably as a solid. The optionally cleaned and / or fragmented pieces of nylon 6 and polyether polyurethane-containing material are optionally dried before being fed to the dissolution section [α]. The dissolution unit(s) are equipped with inlets for introducing the separated organic solvent and fresh organic solvent. The dissolution unit(s) are equipped with one or more outlets for discharging the mixture obtained in the dissolution unit(s). This mixture comprises undissolved nylon 6 and the organic solvent in which the polyether polyurethane is dissolved. Preferably, the dissolution unit(s) are closed. This has the advantage of reducing the discharge of organic solvents into the environment. Preferably, the dissolution unit(s) are equipped with a means for controlling the temperature within the unit(s) (e.g., steam coils, steam tracing, electrical tracing). Optionally, the temperature of the separated organic solvent and the fresh organic solvent is adjusted (e.g., in a heat exchanger) before being introduced into the dissolving section [α].

[0219] Intimate contact between the organic solvent and the nylon 6 and polyether polyurethane-containing material is essential for effective operation. Such contact can be achieved by a variety of means commonly known in the art. Improved contact can be achieved through mechanical friction, which can be achieved, for example, by agitation using a combination of rotating paddles and stationary fins. Dissolving units suitable for dissolving polymers in organic solvents are known to those skilled in the art. A stirred vessel is one example of such a dissolving unit.

[0220] In the separation unit(s), the mixture discharged from the dissolution unit(s) is separated into a polyether polyurethane-rich stream containing the organic solvent and dissolved polyether polyurethane, and a stream containing undissolved nylon 6. The polyether polyurethane-rich stream containing the organic solvent and dissolved polyether polyurethane is discharged through a discharge line and fed to the precipitation section [γ]. The stream containing undissolved nylon 6 is discharged through a discharge line and fed to the washing section [β]. To facilitate washing in the washing section [β], it is advantageous to have a small amount of organic solvent in the stream containing undissolved nylon 6.

[0221] Separation unit(s) suitable for separating the (undissolved) polymer and the organic solvent are known to those skilled in the art. Centrifuges and filters are examples of such separation units. Preferably, the lysis unit and the separation unit are combined in one device, which results in process intensification.

[0222] The washing section [β] includes a device for washing the stream containing undissolved nylon 6 with a third solvent to obtain a nylon 6-rich stream and a mixture containing an organic solvent and the third solvent. The purpose of the washing section [β] is to remove the organic solvent to a large extent. Optionally, the washing section [β] includes a device for drying the nylon 6-rich stream. The mixture containing the organic solvent and the third solvent is discharged from the washing section [β] and introduced into the solvent distillation section [δ]. As used herein, "introduced into the solvent distillation section [δ]" also includes indirect means, i.e., additional steps / sections in between. Preferably, the mixture containing the organic solvent and the third solvent discharged from the washing section [β] is fully or partially introduced into the precipitation section [γ] before being introduced into the solvent distillation section [δ]. Optionally, after drying, the nylon 6 rich stream is discharged from the washing section [β] and introduced into the depolymerization section [C].

[0223] Intimate contact between the third solvent and the nylon 6-rich stream is essential for effective operation. Such contact can be achieved by a variety of means known to those skilled in the art. One means is a (continuous) centrifuge into which the nylon 6-rich stream is fed and into which the third solvent is introduced as a wash solvent.

[0224] The distillation section [δ] contains one or more distillation columns operated in series and / or parallel. The distillation columns can be operated in batch, semi-continuous, or continuous modes. The choice between these modes of operation depends largely on the scale of operation. Generally, batch operation of distillation columns is more suitable for processing small feed streams. Continuous operation of distillation columns is more suitable for processing large feed streams. The exact placement of the distillation section [δ] also depends on the nature of the organic solvent, the second solvent and the third solvent.

[0225] The distillation section [δ] is fed with the mixture containing the organic solvent and the second solvent from which the precipitated polyether polyurethane was recovered, as well as the mixture containing the organic solvent and the third solvent discharged from the washing section [β]. The separated organic solvent is discharged from the distillation section [δ] and fed to the dissolution section [α]. The second solvent and the third solvent are discharged from the distillation section [δ] and optionally fed to the precipitation section [γ] and the washing section [β], respectively. The residue is discharged from the distillation section [δ]. Optionally, the residue is incinerated, thereby recovering energy. Finally, the decomposition products of the used solvent are discharged from the distillation section [δ]. Acetic acid and dimethylamine are examples of decomposition products of the organic solvent DMAc.

[0226] The depolymerization section [C] comprises one or more depolymerization reactors operated in series and / or parallel. The nylon 6-rich stream is fed to the reactors as a solid or as a melt, preferably as a melt. This feeding can be accomplished by using an extruder, a gear pump, or other means known in the art.

[0227] During production, the depolymerization reactor is at least partially filled with nylon 6-containing feedstock, residual materials, ε-caprolactam (and optionally catalyst). The depolymerization reactor can have any desired configuration. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors, and extruder-type reactors.

[0228] The depolymerization reactor must be equipped with facilities for feeding the nylon-6-rich stream, optionally superheated steam, and optionally a catalyst, and further equipped with facilities for discharging the ε-caprolactam-containing stream and residual materials.

[0229] Good contact between the steam and the reactor contents is essential for effective operation. Such contact can be achieved by a variety of means known to those skilled in the art. As an example, the steam can be sparged through the material using multiple inlets, e.g., using a steam distributor. Further improved contact can be achieved by including mechanical agitation within the reactor, e.g., using a combination of rotating paddles and static fins. Preferably, the depolymerization is completed in 0.5 to 6 hours.

[0230] If superheated steam at high temperatures is not available at the production site, it must be made fit for purpose by superheating available steam from the boiler in a so-called superheater.

[0231] The recovery section [D] may comprise one or more (preferably partial) condensers into which the ε-caprolactam-containing stream in the form of a vapor stream comprising ε-caprolactam and water is introduced. Such (partial) condensers may have any desired configuration. Preferably, the condenser is a distillation column from which a water-rich phase is obtained as an overhead product and crude ε-caprolactam is obtained as a bottom product.

[0232] The purification section [E] may include one or more extraction units, one or more solvent switchers, an oxidation section, a hydrogenation section, one or more distillation units and a crystallization section, into which crude ε-caprolactam is input and from which highly purified ε-caprolactam is output.

[0233] The extractor is charged with crude ε-caprolactam and an organic solvent, and discharges an organic phase containing the organic solvent, ε-caprolactam, and impurities, and an aqueous phase containing water and impurities. The extractor is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the extractor is a static or stirred extractor, such as a KARR column, a SCHEIBEL column, a rotating disk contactor (RDC), a pulse column, a sieve tray (static) column, a randomly packed (static) column, and a structured packed (SMVP) (static) column.

[0234] The solvent switcher is fed with an organic phase containing water and an organic solvent, ε-caprolactam, and impurities, and discharges a solvent phase containing impurities and an ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities. Solvent switchers for stripping-based processes are selected from mixer-settler extractors, extraction columns, centrifugal extractors, and combinations thereof. Preferably, the stripping device is a static or stirred extraction column, such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulse column, a sieve tray (static) column, a randomly packed (static) column, and a structured packed (static) column.

[0235] The solvent switching device for the solvent exchange distillation-based process is selected from a sieve tray distillation column, a randomly packed distillation column, and a structured packed distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation column can be operated at atmospheric, subatmospheric, or superatmospheric pressure. Preferably, water is charged to the top of the distillation column, and an aqueous phase containing water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam is discharged from the bottom of the distillation column.

[0236] The optional oxidation section includes one or more oxidation reactors operated in series and / or parallel. An oxidizing agent and an ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities are fed into the oxidation section. Typically, the oxidizing agent is fed as a solid, a slurry, or an aqueous solution. When potassium permanganate or sodium permanganate is used as the oxidizing agent, the oxidation section also includes a filtration section. The oxidation reactor can have any desired configuration. Preferred reactor types are stirred and unstirred reactors and packed column reactors. The oxidation reactor must be equipped with a facility for feeding an aqueous phase containing water, ε-caprolactam, impurities with a boiling point lower or higher than that of ε-caprolactam, and the oxidizing agent. Furthermore, the oxidation reactor must be equipped with a facility for discharging the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, impurities, and optionally formed solid manganese oxide (IV) (MnO) particles. Preferably, the oxidation is carried out at a temperature in the range of 20° C. to 85° C. and under atmospheric conditions.

[0237] Any solid manganese(IV) oxide (MnO) particles present can be removed by settling or solid-liquid filtration, preferably solid-liquid filtration. The use of filter aids such as activated carbon particles or diatomaceous earth to improve the filtration procedure is common practice. Filtration systems suitable for separating solid manganese(IV) oxide particles are known to those skilled in the art. Such filtration systems are fed with a suspension of an oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities, as well as solid manganese(IV) oxide particles, and the filtered oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities is discharged. Generally, the solid manganese(IV) oxide particles are retained within the filtration system. Preferably, such filtration systems operate in a semi-continuous mode, whereby the suspension and the filtrate phase are continuously fed and discharged, and the separated solids are collected in the filtration system. From time to time, the feeding of the suspension is interrupted, and the collected solids are removed from the filtration system.

[0238] In step c.4), the purification of crude ε-caprolactam to obtain purified ε-caprolactam optionally comprises heterogeneously catalyzed hydrogenation, in which case the plant comprises a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0239] The hydrogenation section includes one or more hydrogenation reactors operated in series and / or parallel. Hydrogenation can be carried out in a three-phase system (gas, liquid, solid) containing an aqueous ε-caprolactam mixture, gaseous hydrogen, and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid) containing an aqueous ε-caprolactam mixture fully or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. Dissolving hydrogen in the water-ε-caprolactam mixture can be carried out by any process known to those skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or a mixer where a constant hydrogen pressure is maintained. Intensive contact of hydrogen with the mixture ensures that the hydrogen dissolves in the mixture. Such a process is preferably carried out continuously. The hydrogen-containing mixture is then contacted with a hydrogenation catalyst, for example, in a separate reactor.

[0240] 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 in which the catalyst particles are suspended in the mixture to be hydrogenated (slurry phase process). In such a slurry phase process, the catalyst particles and the purified mixture must be separated in an additional process step after the hydrogenation reaction, for example by filtration. Preferably, the catalyst comprises palladium or nickel.

[0241] Alternatively, the hydrogenation can be carried out in a fixed bed reactor in which the catalyst is fixed within the reactor, thereby eliminating the additional step of separating the catalyst and reaction mixture. Preferably, the fixed bed consists of a supported palladium or nickel catalyst. The hydrogenation temperature is generally 20° C. to 160° C. The hydrogenation pressure is generally 0.1 to 15 MPa.

[0242] The distillation apparatus is charged with an optionally stripped and / or concentrated and / or optionally oxidized and / or hydrogenated ε-caprolactam-water phase containing water, ε-caprolactam and impurities, and discharges high-purity ε-caprolactam, water and impurities (i.e., low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam) and high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam)). The distillation apparatus is selected from a sieve tray distillation column, a randomly packed distillation column, a structured packed distillation column, and horizontal and vertical (falling and rising) film evaporators. Preferably, the distillation column is equipped with a reboiler, a condenser and a reflux device. The distillation apparatus can be operated at atmospheric, subatmospheric or superatmospheric pressure, preferably subatmospheric pressure.

[0243] Preferably, the distillation comprises separating water, low-boiling organic impurities (having a boiling point lower than that of ε-caprolactam), and / or high-boiling organic impurities (having a boiling point higher than that of ε-caprolactam) from ε-caprolactam. Preferably, the distillation comprises, in a first step, separating water as an overhead product and producing ε-caprolactam containing low-boiling and high-boiling impurities as a bottom product. In a second step, the low-boiling impurities are separated as an overhead product, and ε-caprolactam containing high-boiling impurities is obtained as a bottom product. In a third step, high-purity ε-caprolactam is separated as an overhead product, and a distillation residue containing ε-caprolactam and high-boiling impurities is produced as a bottom product. Optionally, the first and second steps are combined.

[0244] Preferably, before distilling off the water and impurities, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities. Preferably, the amount of NaOH added ranges from 0.5 to 100 mmol per kg of ε-caprolactam, more preferably from 2 to 80 mmol per kg of ε-caprolactam. This results in particularly effective distillative removal of impurities with lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0245] The crystallization section comprises one or more crystallizers operated in series and / or parallel. Typically, the crystallization section also comprises several vessels for storing (intermediate) product streams and / or fresh and used wash solutions. The crystallization of ε-caprolactam can be carried out by either solution crystallization or melt crystallization, as described above.

[0246] In the case of solution crystallization, ε-caprolactam is recovered by evaporation crystallization, in which the solvent evaporates, or by cooling crystallization, whereby cooling is obtained by direct cooling, indirect cooling, vacuum cooling, or a combination of these methods. After the crystallization step in the crystallizer, the formed crystals and mother liquor are separated, for example, by settling in a settler, filtration in a filter, and / or centrifugation in a centrifuge. Optionally, the crystallizer is equipped with a stirrer and / or one or more baffles. Optionally, the obtained crystals are washed, for example with clean solvent. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained as crystals.

[0247] Co-solvent is water or non-aqueous solvent.Examples of non-aqueous solvent 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 tetrachloromethane, chloroform or ethyl chloride), ketones (such as acetone or methyl ethyl ketone) and esters (such as ethyl acetate), and the mixture of these solvents.Generally, co-solvent is recovered and reused in the crystallization process. Solution crystallization is usually carried out under atmospheric pressure, but may also be carried out under reduced pressure or under increased pressure.

[0248] The melt crystallization of ε-caprolactam can be carried out either by layer melt crystallization, in which a layer of ε-caprolactam-containing crystals is formed on the heat exchanger walls, or by suspension melt crystallization, in which ε-caprolactam-containing crystals grow in a suspension.

[0249] Preferably, a solvent is present in the mixture in the melt crystallizer, but melt crystallization can also be carried out without a solvent. Many solvents for ε-caprolactam are suitable. Examples of suitable solvents include water, alkanes (e.g., n-hexane, n-heptane, isooctane, cyclohexane), alcohols (e.g., methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (e.g., benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (e.g., tetrachloromethane, chloroform, or ethyl chloride), ketones (e.g., acetone or methyl ethyl ketone), and esters (e.g., ethyl acetate). Water and aromatic hydrocarbons are preferably used as solvents because they produce large crystals. Water is the most preferred solvent. The solvent acts as a freezing point depressant for the melt in the crystallizer.

[0250] Layer melt crystallization: First, the melt is introduced into a crystallizer, where a crystal layer grows on the cooled heat exchanger surface. The remaining melt, including impurities removed from the growing crystals, is then removed from the crystallizer. The crystal layer is then melted, and the purified product is recovered. Purification efficiency can be further improved, for example, by sweating, or partial melting, gently heating the crystal layer to near its melting temperature, causing the trapped, sticky, impure mother liquor to drain. The layer melt crystallization process is operated in a batch mode. Well-known examples of processes based on layer melt crystallization are the ProABD process by BEFS Prokem and the Sulzer Chemtech process.

[0251] Layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization, crystals grow from a stagnant melt onto a cooling surface. In static mode, the desired compound is crystallized batchwise from a stagnant melt in a closed vessel onto the heat exchanger wall. This type of crystallization has a slow crystal growth rate, resulting in a long residence (or batch) time. Preferably, the crystallization time ranges from 1 to 75 hours, more preferably from 2 to 50 hours, and most preferably from 4 to 24 hours. After the crystallization process, the remaining melt is drained. Optionally, a sweating phase is then introduced to remove impurities attached to or trapped within the crystals. Finally, the crystals are completely melted and drained or mechanically removed.

[0252] Melt crystallization of suspension: Suspension melt crystallization of ε-caprolactam can be carried out either in batch mode or in continuous mode. During suspension melt crystallization, the melt is cooled below its saturation temperature, and (optionally, after the addition of seeds) ε-caprolactam crystals begin to grow. The crystal growth rate is controlled by the supersaturation temperature of the melt. Suspension melt crystallization can be carried out in any exchanger-type or vessel-type crystallizer capable of cooling the melt. Preferably, suspension melt crystallization is carried out in a scraper-type surface crystallizer. Optionally, after crystallization, the resulting mixture of crystals and mother liquor is separated by filtration. Optionally, after crystallization, the resulting mixture of ε-caprolactam crystals and mother liquor is introduced into a so-called wash tower. In the wash tower, the mother liquor is drained from the ε-caprolactam crystals, and then, optionally, the ε-caprolactam crystals are washed with purified ε-caprolactam.

[0253] The process of the present invention can be operated continuously, semi-continuously, or batchwise. Accordingly, the plant of the present invention can be configured to operate in one or more of these modes of operation. In a preferred embodiment, the plant is configured to operate the process of the present invention in a continuous or semi-continuous manner. However, discontinuous processes are also possible. For example, the plant of the present invention need not include all of the sections described herein in a single location. In particular, the pretreatment section [A] can be located in a first location, and the separation section [B], depolymerization section [C], recovery section [D], and purification section [E] can be located in a second location. Similarly, the mechanical size reduction section [λ] as part of the pretreatment section [A] can also be located in a first location, the cleaning section [ω] as part of the pretreatment section [A] can also be located in a second location, and the separation section [B], depolymerization section [C], recovery section [D], and purification section [E] can be located in a third location. Optionally, the cleaning section [ω] is divided into two or more segments, all of which are located in different locations. For example, the first segment of the cleaning section [ω] as part of the pretreatment section [A] can be located in a first position, the mechanical size reduction section [λ] as part of the pretreatment section [A] can be located in a second position, the second segment of the cleaning section [ω] as part of the pretreatment section [A] can be located in a third position, and the separation section [B], depolymerization section [C], recovery section [D], and purification section [E] can be located in a fourth position. Optionally, the cleaning section [ω] as part of the pretreatment section [A] is divided into two or more segments that are all located in different positions. Optionally, the cleaning section [ω] as part of the pretreatment section [A] is located in the same position as the separation section [B]. Optionally, the separation section [B], depolymerization section [C], and recovery section [D] are located in different positions from the pretreatment section [A] and the purification section [E].Optionally, the depolymerization section [C] and the recovery section [D] are located at different locations from the pretreatment section [A] and the separation section [B].

[0254] product The present invention provides both ε-caprolactam and polyether polyurethane obtained by depolymerization of nylon 6 and polyether polyurethane-containing materials by the process of the present invention as new products that meet the specifications for demanding applications. At the same time, the process is particularly environmentally friendly due to the reduced carbon footprint of the product and the use of waste materials as starting materials. The ε-caprolactam obtained by the process of the present invention is advantageously characterized, in particular, by having a product carbon footprint of less than 3 kg of CO2 per kg of purified ε-caprolactam. The ε-caprolactam obtained by the process of the present invention may also be referred to as "purified ε-caprolactam." As used herein, "purified" means that the ε-caprolactam is produced from nylon 6 and polyether polyurethane-containing materials by the process of the present invention and is thus obtained in a purified form. In this sense, the ε-caprolactam is obtained and purified from nylon 6 and polyether polyurethane-containing materials.

[0255] The process of the present invention can produce high-purity, and therefore high-quality, ε-caprolactam, which meets the specifications for demanding applications, while the process is particularly economically friendly due to the reduced carbon footprint of the product and the use of waste as starting material. In preferred embodiments, the ε-caprolactam obtained by the process of the present invention meets one or more of the following specifications, where these parameters and measurement methods are defined as in the Examples section hereinbelow: PAN: max 5 E290: Maximum 0.05 VB: max. 0.5mmol / kg Alkalinity: Max 0.1mmol / kg Acidity: max. 0.1mmol / kg.

[0256] The ε-caprolactam produced by the process of the present invention is also particularly economical and environmentally friendly, as evidenced by the much lower carbon footprint of the ε-caprolactam produced by the process of the present invention compared to classically produced ε-caprolactam (e.g., by Beckmann rearrangement of cyclohexanone oxime).

[0257] The environmental impact of a product is commonly expressed as its "carbon footprint." The carbon footprint of a product is defined as the total emissions caused by the formation of that product, expressed as tons of carbon dioxide equivalent per ton of product. The carbon footprint of a product depends, among other things, on feedstocks, auxiliary materials, energy consumption, energy source, production process, and process efficiency. Quantification of a product's carbon footprint can be performed, for example, as described in European Standard EN ISO 14040:2006 ("Environmental management - Life cycle assessment - Principles and framework").

[0258] The calculation of the carbon footprint of a product can be done both in-house or (preferably) by external accredited organizations that verify and certify the carbon footprint calculation of a product, for example according to the LCA standard ISO 14040.

[0259] J. Hong and X. Xu ("Environmental impact assessment of caprolactam production—a case study in China"; J. of Cleaner production 27(2012)103-108; DOI:10.1016 / j.jclepro.2011.12.037) reported that the potential global warming impact of "virgin" ε-caprolactam obtained via the Beckmann rearrangement of cyclohexanone oxime when accompanied by coal-based electricity and steam generation is 7.5 tCO2-eq per ton of ε-caprolactam (equivalent to 7.5 kg CO2-eq per kg of ε-caprolactam). When accompanied by natural gas-based electricity and steam generation, the potential impact of virgin ε-caprolactam on global warming for the ε-caprolactam production process is reduced to 6.4 tonnes CO2 equivalent per tonne of ε-caprolactam (equivalent to 6.4 kg CO2 equivalent per kg of ε-caprolactam).

[0260] The product carbon footprint of ε-caprolactam obtained according to the process of the present invention is much lower than that of de novo synthesized, i.e., "virgin" ε-caprolactam: the product carbon footprint of ε-caprolactam obtained by the process of the present invention is less than 4 kg CO2 equivalent per kg ε-caprolactam, more preferably less than 3 kg CO2 equivalent per kg ε-caprolactam, and most preferably less than 3.0 kg CO2 equivalent per kg ε-caprolactam.

[0261] The polyether polyurethanes produced by the process of the present invention are also particularly economically and environmentally friendly, as evidenced again by their much lower carbon footprint compared to polyether polyurethanes classically produced, for example, by the reaction of polyether polyols with diisocyanate monomers.

[0262] NM van der Velden, MK Patel and JG Vogtlander (Table 7 of "LCA benchmarking study on textiles made of cotton, polyester, nylon, acryl, or elastane," Int J Life Cycle Assess (2014) 19:331–356; DOI: 10.1007 / s11367-013-0626-9) reported that the potential impact of virgin elastane production on global warming is equivalent to 4.836 kg of CO2 equivalent per kg of elastane.

[0263] The product carbon footprint of polyether polyurethanes obtained according to the process of the present invention is much less than that of de novo synthesized or "virgin" polyether polyurethanes. The product carbon footprint of polyether polyurethanes obtained according to the process of the present invention is less than 3.0 kg, preferably less than 2.0 kg, and most preferably less than 1.0 kg CO equivalent per kg of polyether polyurethane.

[0264] The present invention will now be described with reference to the drawings which show particular embodiments of the invention. However, the present invention, as defined in the claims and generally described herein, should not be limited to the embodiments shown for illustrative purposes in the following figures. [Brief explanation of the drawings]

[0265] [Figure 1] 1 is a schematic diagram of the process of the present invention for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing material. The process optionally includes processing steps in a pretreatment section [A], a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E]. [Figure 2]Two embodiments of the pre-treatment section [A] are shown, in which the nylon-6 and polyether polyurethane-containing material is cleaned in the cleaning section [ω] by removing foreign matter and washing with a washing solvent, and then fragmented in the mechanical size reduction section [λ] to obtain cleaned and fragmented pieces of the nylon-6 and polyether polyurethane-containing material. Figure 2A shows an embodiment of the pre-treatment section [A], in which the nylon-6 and polyether polyurethane-containing material is first cleaned in the cleaning section [ω] by removing foreign matter and washing with a washing solvent, and then fragmented in the mechanical size reduction section [λ] to obtain cleaned and fragmented pieces of the nylon-6 and polyether polyurethane-containing material. Figure 2B shows an embodiment of the pre-treatment section [A], in which the nylon-6 and polyether polyurethane-containing material is first fragmented in the mechanical size reduction section [λ], and then cleaned in the cleaning section [ω] by removing foreign matter and washing with a solvent, to obtain cleaned and fragmented pieces of the nylon-6 and polyether polyurethane-containing material. [Figure 3] 1 shows an embodiment of a separation section [B] in which nylon 6 and polyether polyurethane-containing material are separated to obtain a nylon 6-rich stream and recovered polyether polyurethane. The plant used in this embodiment includes a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ]. [Figure 4]Two embodiments of the purification section [E] are shown, in which crude ε-caprolactam is purified to obtain high-purity ε-caprolactam. Figure 4A shows an embodiment of the purification section [E] of the process of the present invention, which includes an extraction section [K], an optional washing section [L], an optional back-extraction section [M], an optional stripping and concentration section [N], an optional distillation section [O], and a crystallization section [P]. Figure 4B shows an embodiment of the purification section [E] of the process of the present invention, which includes an extraction section [K], an optional washing section [L], an optional solvent exchange distillation section [R], an optional stripping and concentration section [N], an optional distillation section [O], and a crystallization section [P].

[0266] Detailed Description of the Drawings The process of the present invention is shown schematically in Figure 1. The process is carried out in the following plant sections: optional pretreatment section [A], separation section [B], depolymerization section [C], recovery section [D] and purification section [E].

[0267] Optionally, in a pre-treatment section [A], the nylon 6 and polyether polyurethane-containing material

[0301] is cleaned and / or fragmented by mechanical size reduction to obtain discharged cleaned and / or fragmented pieces of nylon 6 and polyether polyurethane-containing material

[0304] . Optionally, in a pre-treatment section [A], the nylon 6 and polyether polyurethane-containing material

[0301] is cleaned by removal of foreign matter and / or washing with a washing solvent

[0302] to obtain discharged foreign matter and contaminated washing solvent

[0303] . Removal of foreign matter can occur before and / or after washing with the washing solvent

[0302] . Cleaning can occur before and / or after fragmentation of the nylon 6 and polyether polyurethane-containing material

[0301] . Optionally, the cleaned and / or fragmented pieces of nylon 6 and polyether polyurethane-containing material

[0304] are dried before being introduced into the separation section [B]. Optionally, the cleaned and / or shredded pieces of nylon 6 and polyether polyurethane containing material are densified before being introduced into separation section [B].

[0268] In the separation section [B], optionally purified and / or fragmented pieces of nylon 6 and polyether polyurethane-containing material

[0304] are separated to obtain a nylon-6 rich stream

[0311] and recovered polyether polyurethane

[0308] , which are discharged from the separation section [B]. The nylon 6 rich stream

[0311] is input to the depolymerization section [C]. Optionally, the nylon 6 rich stream

[0311] and / or recovered polyether polyurethane

[0308] are dried before being discharged from the separation section [B]. Optionally, the nylon 6 rich stream

[0311] is densified before being depolymerized to ε-caprolactam in the depolymerization section [C]. The separation section [B] includes the following sections: a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ] (not shown in FIG. 1 ). An organic solvent

[0305] is introduced into the separation section [B] to dissolve the polyether polyurethane. A second solvent

[0306] is introduced into the separation section [B] to precipitate the dissolved polyether polyurethane. A third solvent

[0307] is introduced into the separation section [B] to wash the undissolved nylon 6. After distillation separation, the second and third solvents

[0309] and residue

[0310] are discharged from the separation section [B].

[0269] The optionally dried and / or densified nylon 6-rich stream

[0311] is depolymerized to ε-caprolactam in the depolymerization section [C]. An ε-caprolactam-containing stream

[0315] is discharged from the depolymerization section [C]. Also, residual materials

[0314] are discharged. Optionally, superheated steam

[0312] and a catalyst

[0313] are introduced into the depolymerization section [C].

[0270] Crude ε-caprolactam

[0317] is recovered from the ε-caprolactam-containing stream

[0315] discharged from the depolymerization section [C]. The crude ε-caprolactam

[0317] is discharged from the recovery section [D] and fed to the purification section [E]. In addition, when water or superheated steam

[0312] is fed to the depolymerization section [C] or when water (not shown in Figure 1) is fed to the recovery section [D], an aqueous phase

[0316] is discharged from the recovery section [D].

[0271] The crude ε-caprolactam

[0317] discharged from the recovery section [D] is purified in the purification section [E] to obtain high-purity ε-caprolactam

[0319] . Water and impurities

[0318] are also discharged from the purification section [E].

[0272] FIG. 2A shows a pretreatment section [embodiment A] (area enclosed by a dashed line), in which nylon 6 and polyether polyurethane-containing material

[31] is first cleaned in a cleaning section [ω] by removing foreign matter and washing with a washing solvent

[32] , thereby obtaining foreign matter and contaminated washing solvent

[33] and a cleaned nylon 6 and polyether polyurethane-containing material

[34] . The cleaned nylon 6 and polyether polyurethane-containing material

[34] is then fragmented in a mechanical size reduction section [λ] to obtain cleaned and fragmented pieces of nylon 6 and polyether polyurethane-containing material

[35] . The cleaned and fragmented nylon 6 and polyether polyurethane-containing material

[35] is then discharged. Optionally, the cleaned and fragmented pieces of nylon 6 and polyether polyurethane-containing material

[35] are densified before being separated into a nylon 6-rich stream and recovered polyether polyurethane in a separation section [B] (not shown in FIG. 2A).

[0273] FIG. 2B shows an embodiment of the pretreatment section [A] (area enclosed by a dashed line), in which the nylon 6 and polyether polyurethane-containing material

[41] is first fragmented in a mechanical size reduction section [λ] to obtain fragmented pieces of nylon 6 and polyether polyurethane-containing material

[42] . The fragmented pieces of nylon 6 and polyether polyurethane-containing material

[42] are then cleaned in a cleaning section [ω] by removing foreign matter and washing with a washing solvent

[43] to obtain foreign matter and contaminated washing solvent

[44] , and the cleaned fragmented pieces of nylon 6 and polyether polyurethane-containing material

[45] are then discharged. Optionally, the cleaned fragmented pieces of nylon 6 and polyether polyurethane-containing material

[45] are densified before being separated into a nylon 6-rich stream and recovered polyether polyurethane in a separation section [B] (not shown in FIG. 2B).

[0274] 3 shows an embodiment of the separation section [B] (area enclosed by dashed lines) in which the nylon 6 and polyether polyurethane-containing material is separated to obtain a nylon 6-rich stream and recovered polyether polyurethane. This embodiment includes a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ].

[0275] In the dissolving section [α], the polyether polyurethane is dissolved in an organic solvent from the optionally cleaned and / or fragmented pieces of polyamide 6 and polyether polyurethane-containing material

[0101] to obtain a polyether polyurethane-rich stream

[0104] containing the organic solvent and the dissolved polyether polyurethane, and a stream

[0105] containing undissolved nylon 6, which are discharged from the dissolving section [α]. The organic solvent is the separated organic solvent

[0102] , to which fresh organic solvent

[0103] is optionally added.

[0276] In the precipitation section [γ], the second solvent

[0106] is mixed with the polyether polyurethane-rich stream

[0104] containing the organic solvent and dissolved polyether polyurethane, thereby causing the polyether polyurethane to precipitate from the mixture containing the organic solvent and the second solvent. The precipitated polyether polyurethane is recovered from the mixture containing the organic solvent and the second solvent (e.g., by filtration or centrifugation). The recovered precipitated polyether polyurethane is discharged from the precipitation section [γ] as recovered polyether polyurethane

[0107] . Optionally, the recovered polyether polyurethane

[0107] is dried after the recovery step before being discharged from the precipitation section [γ]. The mixture containing the organic solvent and the second solvent

[0108] from which the precipitated polyether polyurethane has been recovered is discharged from the precipitation section [γ] and introduced into the solvent distillation section [δ].

[0277] In the washing section [β], the stream

[0105] containing undissolved nylon 6 is washed with a third solvent

[0109] to obtain a nylon 6-rich stream

[0110] and a mixture

[0111] containing an organic solvent and a third solvent. The nylon 6-rich stream

[0110] is discharged from the washing section [β]. Optionally, the nylon 6-rich stream

[0110] is dried after the washing step before being discharged from the washing section [β]. Optionally, the nylon 6-rich stream

[0110] is densified (not shown in FIG. 3 ) before being depolymerized into ε-caprolactam in the depolymerization section [C]. The mixture

[0111] containing the organic solvent and the third solvent is discharged from the washing section [β] and optionally introduced into the solvent distillation section [δ].

[0278] In the solvent distillation section [δ], the mixture

[0108] containing the organic solvent and the second solvent from which the precipitated polyether polyurethane was recovered, and optionally the mixture

[0111] containing the organic solvent and the third solvent, are distilled to obtain a separated organic solvent

[0102] , the second solvent and the third solvent

[0112] , and a residue

[0113] . Optionally, the second solvent and the third solvent are both water, in which case the separated organic solvent

[0102] is a dry organic solvent. The separated organic solvent

[0102] is introduced into the dissolution section [α]. FIG. 4A shows one embodiment of the purification section [E] (area enclosed by dashed lines) which includes the following sections:

[0279] In the extraction section [K], the crude ε-caprolactam

[0201] is extracted with an organic solvent

[0202] to obtain an aqueous phase

[0203] containing water and impurities and an organic phase

[0204] containing the organic solvent, ε-caprolactam and impurities. Both of these phases are discharged from the extraction section [K].

[0280] In an optional washing section [L], the organic phase

[0204] containing the organic solvent, ε-caprolactam and impurities is washed with water or an aqueous alkaline solution

[0205] to obtain a residual aqueous solution-containing phase

[0206] and a washed organic phase

[0207] containing the organic solvent, ε-caprolactam and impurities. Both of these phases are discharged from the washing section [L].

[0281] In an optional back-extraction section [M], the optionally washed organic phase

[0207] containing the organic solvent, ε-caprolactam and impurities is back-extracted with water

[0208] to obtain an organic solvent phase

[0209] containing impurities and an aqueous phase

[0210] containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam. Both of these phases are discharged from the back-extraction section [M].

[0282] In the optional stripping and concentration section [N], residual organic solvent and water are removed by stripping and / or distillation from the aqueous phase containing water, ε-caprolactam, and impurities having a boiling point lower or higher than that of ε-caprolactam

[0210] to obtain a stripped and concentrated aqueous phase

[0212] containing residual organic solvent and water

[0211] , and impurities having a boiling point lower or higher than that of ε-caprolactam. Both of these phases are discharged from the stripping and concentration section [N].

[0283] Optionally, the aqueous phase containing water, ε-caprolactam, and impurities having a boiling point lower or higher than that of ε-caprolactam, from which any residual organic solvent and water have been removed by stripping and / or distillation, is oxidized with an oxidizing agent to obtain an oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam, and impurities (not shown in FIG. 4A).

[0284] Optionally, the oxidized ε-caprolactam-water phase containing water, ε-caprolactam, and impurities is filtered to remove solid manganese(IV) oxide particles before being introduced into the next section (not shown in FIG. 4A).

[0285] In the optional hydrogenation section, the stripped and concentrated aqueous phase

[0212] containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam is hydrogenated with hydrogen in the presence of a heterogeneous catalyst to obtain a hydrogenated ε-caprolactam-aqueous phase containing water, ε-caprolactam and impurities, which is then fed to the next section (not shown in FIG. 4A).

[0286] In an optional distillation section [O], the stripped and concentrated aqueous phase

[0212] containing water, ε-caprolactam, and impurities having boiling points lower or higher than ε-caprolactam is distilled to remove impurities

[0214] having boiling points lower or higher than ε-caprolactam, and optionally organic solvents or water (not shown in FIG. 4A), thereby obtaining a distilled ε-caprolactam phase

[0215] . All distillation products are discharged from the distillation section [O]. The distilled ε-caprolactam phase

[0215] is introduced into the crystallization section [P]. Optionally, prior to distillation in the distillation section [O], an alkali metal hydroxide

[0213] is introduced into the stripped and concentrated aqueous phase

[0212] containing water, ε-caprolactam, and impurities having boiling points lower or higher than ε-caprolactam.

[0287] In the crystallization section [P], the optionally distilled ε-caprolactam phase

[0215] is crystallized at a temperature between 10°C and 95°C to remove impurities

[0217] from the ε-caprolactam, thereby obtaining high-purity ε-caprolactam

[0218] . All of the crystalline product is discharged from the crystallization section [P]. Optionally, a solvent

[0216] is introduced into the crystallization section [P] prior to crystallization. FIG. 4B shows one embodiment of the purification section [E] (area enclosed by dashed lines) which includes the following sections:

[0288] In the extraction section [K], the crude ε-caprolactam

[0401] is extracted with an organic solvent

[0402] to obtain an aqueous phase

[0403] containing water and impurities and an organic phase

[0404] containing the organic solvent, ε-caprolactam and impurities. Both of these phases are discharged from the extraction section [K].

[0289] In an optional washing section [L], the organic phase

[0404] containing the organic solvent, ε-caprolactam and impurities is washed with water or an aqueous alkaline solution

[0405] to obtain a residual aqueous solution-containing phase

[0406] and a washed organic phase

[0407] containing the organic solvent, ε-caprolactam and impurities. Both of these phases are discharged from the washing section [L].

[0290] In an optional solvent exchange distillation section [R], the optionally washed organic phase

[0407] containing the organic solvent, ε-caprolactam and impurities is solvent exchange distilled by adding water

[0408] to obtain a stream

[0409] containing the organic solvent and an aqueous phase

[0410] containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam. Both of these products are discharged from the solvent exchange distillation section [R].

[0291] In the optional stripping and concentration section [N], residual organic solvent and water are removed by stripping and / or distillation from the aqueous phase

[0410] containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam to obtain a stripped and concentrated aqueous phase

[0412] containing residual organic solvent and water

[0411] , and impurities with boiling points lower or higher than ε-caprolactam. Both of these phases are discharged from the stripping and concentration section [N].

[0292] Optionally, the aqueous phase

[0410] containing water, ε-caprolactam and impurities having boiling points lower or higher than ε-caprolactam, from which any residual organic solvent and water have been removed by stripping and / or distillation, is oxidized with an oxidizing agent to obtain an oxidized ε-caprolactam-aqueous phase containing water, ε-caprolactam and impurities (not shown in FIG. 4B).

[0293] Optionally, the oxidized ε-caprolactam-water phase containing water, ε-caprolactam, and impurities is filtered to remove solid manganese(IV) oxide particles before being introduced into the next section (not shown in FIG. 4B).

[0294] In the optional hydrogenation section, the stripped and concentrated aqueous phase

[0412] containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam is hydrogenated with hydrogen in the presence of a heterogeneous catalyst to obtain a hydrogenated ε-caprolactam-aqueous phase containing water, ε-caprolactam and impurities, which is then fed to the next section (not shown in FIG. 4B).

[0295] In an optional distillation section [O], the stripped and concentrated aqueous phase

[0412] containing water, ε-caprolactam, and impurities having boiling points lower or higher than ε-caprolactam is distilled to remove impurities

[0414] having boiling points lower or higher than ε-caprolactam, and optionally organic solvents or water (not shown in Figure 4B), thereby obtaining a distilled ε-caprolactam phase

[0415] . All distillation products are discharged from the distillation section [O]. The distilled ε-caprolactam phase

[0415] is charged to the crystallization section [P]. Optionally, prior to distillation in the distillation section [O], an alkali metal hydroxide

[0413] is charged to the stripped and concentrated aqueous phase

[0412] containing water, ε-caprolactam, and impurities having boiling points lower or higher than ε-caprolactam.

[0296] In the crystallization section [P], the optionally distilled ε-caprolactam phase

[0415] is crystallized at a temperature between 10°C and 95°C to remove impurities

[0417] from the ε-caprolactam, thereby obtaining high-purity ε-caprolactam

[0418] . All of the crystalline product is discharged from the crystallization section [P]. Optionally, a solvent

[0416] is introduced into the crystallization section [P] prior to crystallization. [Example]

[0297] The following examples serve to illustrate the present invention in more detail, particularly with respect to certain aspects of the invention, but are not intended to limit the disclosure.

[0298] ε-Caprolactam that can be used in all major nylon 6 polymerization applications without dilution with purer qualities of ε-Caprolactam meets all of the following specifications: PAN: max 5 E290: Maximum 0.05 VB: max. 0.5mmol / kg Alkalinity: Max 0.1mmol / kg Acidity: Max 0.1mmol / kg The parameters and measurement methods are defined as follows:

[0299] PAN:ISODIS 8660-Plastics-Determination of permanganate index of caprolactam-Spectrometric method, revision of first edition ISO 8660;1988, E290: ISO 7059 Caprolactam for industrial use, absorbance at wavelength 290 nm;

[0300] Volatile bases (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile base content - Titrimetric method after distillation. Alkalinity of ε-caprolactam product: The alkalinity was measured using Tashiro indicator at a temperature of 25°C, with a value of 0.1 wt / v. Ethanol % Methylene Blue: 0.1 wt / v EthanolThe % methyl red is determined by titrating with methyl red in a 1:2 ratio, which has a gray endpoint. A flask containing water and indicator is first titrated to a gray color, then X grams of an aqueous ε-caprolactam solution containing Y weight percent ε-caprolactam (determined by refractive index) is added, and the solution is titrated back to a gray color using 0.01N H2SO4 solution (if the solution is alkaline) or 0.01N NaOH solution (if the solution is acidic).

[0301] The alkalinity is then obtained by: Alkalinity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y) During the ceremony, v = volume of H2SO4 solution added (ml) t = normality of H2SO4 solution (=0.01N) X = sample weight (g) Y = ε-caprolactam concentration (wt%) is.

[0302] The acidity is then obtained by: Acidity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y) During the ceremony, v = volume of NaOH solution added (ml) t = molar concentration of NaOH solution (=0.01N) X = sample weight (g) Y = ε-caprolactam concentration (wt%) is.

[0303] Example 1 Pretreatment and separation of nylon 6 and polyether polyurethane. The polyether polyurethane content of the post-consumer nylon 6 waste fabric was 20.2±0.4% by weight (determined by differential scanning calorimetry (DSC) method ISO 11357-3, 130°C to 300°C, 10°C / min, dt 1.00 sec). The color of the post-consumer nylon 6 waste fabric was purple.

[0304] Nylon 6 waste fabric containing polyether polyurethane was used, each 5 to 20 cm 2 The resulting solution was vacuum filtered through a heated double-walled Buchner funnel at 70°C with stirring to obtain Filtrate 1. The remaining undissolved material was treated three more times, each time with 400 grams of DMAc at 70°C with stirring for one hour, to obtain Filtrates 2-4.

[0305] The undissolved material on the Buchner funnel was washed with 325 grams of water at 65°C, thereby obtaining filtrate 5, which was then dried and weighed. The five filtrates were combined, thereby obtaining a precipitate and a clear solution. The precipitate was filtered off, washed with 200 g of water, then dried and weighed. The total weight of the dried precipitate and dried undissolved material was approximately equal to the weight of the starting material.

[0306] Analysis by differential scanning calorimetry (DSC) revealed that the polyether polyurethane content of the dried precipitate was approximately 100 wt % (no polyamide 6 was detected).Based on DSC analysis, the nylon 6 content of the dried undissolved material was approximately 100 wt % (no polyether polyurethane was detected).

[0307] The recovered polyether polyurethane precipitate can be reused as is or in combination with virgin polyether polyurethane in the production of textiles.

[0308] This example demonstrates that polyether polyurethane and nylon 6 can be separated from nylon 6 waste fabric containing polyether polyurethane by selective extraction, precipitation of dissolved polyether polyurethane, and washing of undissolved nylon 6. After drying, the recovered precipitate consisted of almost pure polyether polyurethane, and the nylon 6 content of the dried undissolved material was nearly 100 wt %.

[0309] Example 2 Pretreatment and separation of nylon 6 and polyether polyurethane. The polyether polyurethane content of the post-consumer nylon 6 waste fabric was 8.15 + / - 0.05 wt% (determined by differential scanning calorimetry (DSC) method ISO 11357-3 - 130°C to 300°C, 10°C / min, dt 1.00 sec). The color of the post-consumer nylon 6 waste fabric was salmon pink.

[0310] The procedure of Example 1 was followed, except that 254.1 grams of polyether polyurethane-containing nylon 6 waste fabric was dissolved in 1200 grams of DMAc, and after filtration, 475 grams of DMAc and 350 grams of water were used to wash the remaining undissolved material on the Buchner funnel. The precipitate was filtered off, washed with 200 g of water, and then dried. The undissolved material on the Buchner funnel was washed with 1000 grams of water, then similarly dried and then weighed. The weight fraction of the dried precipitate was 8 wt % of the total weight of the dried precipitate and the dry undissolved material, which was approximately the same as the weight of the starting material.

[0311] Differential scanning calorimetry (DSC) analysis revealed that the polyether polyurethane content of the dried precipitate was approximately 100% by weight (no nylon 6 was detected). The nylon 6 and polyether polyurethane contents of the dried, undissolved material were approximately 99% by weight and less than 1% by weight, respectively (DSC analysis).

[0312] The recovered polyether polyurethane precipitate can be reused as is or in combination with virgin polyether polyurethane in the production of textiles.

[0313] This example demonstrates that polyether polyurethane and nylon 6 can be separated from nylon 6 waste fabric containing polyether polyurethane by selective extraction, precipitation of the dissolved polyether polyurethane, and washing of the undissolved nylon 6. After drying, the recovered precipitate consists of nearly pure polyether polyurethane, and the polyether polyurethane content of the undissolved material is much lower than that of the starting material, nylon 6 waste fabric containing polyether polyurethane. The resulting polyether polyurethane is of high quality and can be used directly or in combination with virgin polyether polyurethane in demanding follow-up applications such as fiber spinning.

[0314] Example 3 Depolymerization of nylon 6 and recovery of ε-caprolactam. The dry, non-dissolved material obtained in Example 1 was first densified before being charged into the depolymerization reactor. The dry, non-dissolved material was melted at 237°C under nitrogen and passed through a perforated metal plate. The resulting strand was cooled to room temperature and cut into pellets. The diameter and length of the resulting pellets were 3 mm and 1 cm, respectively.

[0315] 33.6 grams of these nylon 6-containing pellets and 9.5 grams of 20 wt % phosphoric acid were charged into a Premex high-pressure autoclave. The reactor contents were first heated under nitrogen, and then superheated steam was continuously injected at a rate of 2.7 grams / minute during the 120-minute reaction. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, the steam stream was continuously discharged from the reactor and cooled to about 20°C, thereby obtaining ε-caprolactam and water containing condensate.

[0316] The condensate, consisting of 25.7 grams of ε-caprolactam with the remainder being mostly water, was concentrated by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) to an ε-caprolactam concentration of 57.3 wt% (this mixture, crude ε-caprolactam, is the mixture to be purified). The specifications of the crude ε-caprolactam were as follows: PAN:353 E290:2.91

[0317] This example demonstrates that crude ε-caprolactam can be obtained in good yield without operational problems by depolymerization of nylon 6 obtained by extraction and separation from polyether polyurethane-containing nylon 6 waste fabric.

[0318] Example 4 Depolymerization of nylon 6 and recovery of ε-caprolactam. 48 grams of pellets made from the dry, undissolved material from Example 2 and 14 grams of 20 wt % phosphoric acid were charged into a Premex high pressure autoclave, otherwise the procedure of Example 3 was followed.

[0319] The condensate, consisting of 41 grams of ε-caprolactam with the remainder being mostly water, was concentrated by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) to an ε-caprolactam concentration of 63.1 wt% (this mixture, crude ε-caprolactam, is the mixture to be purified).

[0320] The specifications of the crude ε-caprolactam were as follows: PAN:274 E290:2.70 This example demonstrates that crude ε-caprolactam can be obtained in good yield without operational problems by depolymerization of nylon 6 obtained by extraction and separation from polyether polyurethane-containing nylon 6 waste fabric.

[0321] Comparative experiment 1 Depolymerization of nylon 6 and polyether polyurethane. The procedure of Example 3 was otherwise followed, except that 37.6 grams of polyether polyurethane fiber (the same material contained in the waste fabric used in Example 1) and 14 grams of 20 wt % phosphoric acid were loaded into a Premex high-pressure autoclave.

[0322] Ten minutes after the start of the injection of superheated steam, the experiment had to be stopped because the line for discharging the vapor stream containing ε-caprolactam and water became clogged with insoluble material.

[0323] From this comparative experiment, it can be concluded that depolymerization of nylon 6 waste fabric containing polyether polyurethane without removing the polyether polyurethane before depolymerization may lead to operational problems.

[0324] Comparative experiment 2 Depolymerization of waste nylon 6 fabric containing polyether polyurethane and recovery of ε-caprolactam The procedure of Example 3 was otherwise followed, except that 48 grams of pellets made from polyether polyurethane-containing nylon 6 waste fabric (the same feedstock as used in Example 1) having a polyether polyurethane content of 20.2 + / - 0.4 wt% and 14 grams of 20 wt% phosphoric acid were loaded into a Premex high-pressure autoclave.

[0325] The condensate, consisting of 26 grams of ε-caprolactam with the remainder being mostly water, was concentrated by evaporation in a rotavap operated under vacuum (9.5 kPa; water bath temperature approximately 65°C) to an ε-caprolactam concentration of 47.0 wt% (this mixture, crude ε-caprolactam, is the mixture to be purified).

[0326] The specifications of the crude ε-caprolactam were as follows: PAN:352 E290:3.60 observation: The condensate obtained before concentration contained an unknown precipitate. After this depolymerization experiment, contamination of the inner wall of the Premex high-pressure autoclave was observed.

[0327] Again, this comparative experiment shows that depolymerization of nylon 6 waste fabric containing polyether polyurethane without removing the polyether polyurethane prior to depolymerization can lead to operational problems. Another observation is that the yield of ε-caprolactam in this comparative experiment is much lower than that in Example 3 (nylon 6 waste fabric containing pretreated polyether polyurethane as feed), whereby the yield of ε-caprolactam is defined as the ratio of the weight of ε-caprolactam in the condensate to the weight of nylon 6 in the feed charged to the Premex high-pressure autoclave.

[0328] Comparative experiment 3 Purification by distillation. Then, 75 mmol of aqueous sodium hydroxide solution per kg of ε-caprolactam was added to the crude ε-caprolactam obtained in Comparative Experiment 2. This mixture was then distilled by gradually reducing the pressure. The ε-caprolactam was distilled at 300 Pa. The specifications of the distilled ε-caprolactam were as follows: PAN:31 E290:3.08 VB: 2.45 mmol / kg Alkalinity: 3.13mmol / kg.

[0329] This comparative experiment shows that the quality of ε-caprolactam obtained by depolymerization of nylon 6 waste fabric containing polyether polyurethane (without pretreatment in a separation section where nylon 6 and polyether polyurethane-containing material are separated into a nylon 6-rich stream and a polyether polyurethane-rich stream) and then concentrated and purified by distillation is very poor, as it does not meet any of the specifications required for major polymerization applications.

[0330] Comparative experiment 4 Depolymerization of waste nylon 6 fabric containing polyether polyurethane and recovery of ε-caprolactam, followed by purification by permanganate treatment and distillation. A nylon 6 waste fabric containing polyether polyurethane (the same feedstock as used in Example 2) with a polyether polyurethane content of 8.15 + / - 0.05 wt % was used in this comparative experiment.

[0331] The nylon 6 and polyether polyurethane pretreatment and separation procedures of Example 1 were followed. The recovered polyether polyurethane precipitate can be reused in textile production, either as is or in combination with virgin polyether polyurethane. The depolymerization and recovery procedures of Example 3 were followed.

[0332] The resulting crude ε-caprolactam was treated with 0.2 wt % KMnO4 relative to ε-caprolactam for 2 hours at 50° C. The formed solid was then removed from the oxidation reaction product by filtration.

[0333] The ε-caprolactam in the oxidized reaction product was further purified by distillation as described in Comparative Example 3 after adding 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam.

[0334] The specifications of the distilled ε-caprolactam were as follows: PAN:3 E290:0.12 VB: 1.45 mmol / kg Alkalinity: 1.96mmol / kg

[0335] From this comparative experiment, it can be concluded that the purification of crude ε-caprolactam by successful permanganate treatment with distillation is not sufficient to meet all the specifications required for the major polymerization applications quantified above.

[0336] Example 5 Depolymerization of waste nylon 6 fabric containing polyether polyurethane and recovery of ε-caprolactam, followed by extraction, stripping, distillation, and purification by crystallization. A nylon 6 waste fabric containing polyether polyurethane (the same feedstock as used in Example 2) with a polyether polyurethane content of 8.15 + / - 0.05 wt% was used in this example.

[0337] The nylon 6 and polyether polyurethane pretreatment and separation procedures of Example 1 were followed. The recovered polyether polyurethane precipitate can be reused in textile production, either as is or in combination with virgin polyether polyurethane. The depolymerization and recovery procedures of Example 3 were followed.

[0338] The resulting crude ε-caprolactam (76 grams) was extracted once with 100 grams of water and then extracted nine times with 50 grams of a solvent mixture (4-methyl-2-pentanol (50 wt%) / cyclohexane (50 wt%)) at 25°C. The resulting 10 ε-caprolactam phases containing the solvent mixture were combined and then concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; water bath temperature was approximately 65°C) to an ε-caprolactam concentration of approximately 40 wt%, followed by the addition of fresh cyclohexane. The ε-caprolactam concentration of the resulting mixture was approximately 25 wt%, and the weight ratio of the solvent mixture (4-methyl-2-pentanol / cyclohexane) was 50 wt%:50 wt%. The concentrated ε-caprolactam containing solvent mixture was then extracted eight times with 50 grams of water at 25°C. The resulting eight aqueous ε-caprolactam phases were combined. The combined aqueous phases were concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; water bath temperature was approximately 65° C.) to an ε-caprolactam concentration of 47.1 wt %. The specifications of the resulting concentrated aqueous ε-caprolactam solution were as follows:

[0339] PAN:108 E290:1.58 To the obtained concentrated aqueous solution of ε-caprolactam, 75 mmol of aqueous sodium hydroxide solution per 1 kg of ε-caprolactam was added.

[0340] Water and impurities having a boiling point lower than that of ε-caprolactam were then removed as the overhead product by distillation under reduced pressure in a batchwise operated distillation apparatus. Finally, the distilled ε-caprolactam was recovered as the overhead product at 300 Pa, while the impurities having a boiling point higher than that of ε-caprolactam remained as the bottom product of the distillation apparatus.

[0341] Distilled water was then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt %. This aqueous ε-caprolactam was introduced into a crystallizer at a temperature of 52°C. The aqueous ε-caprolactam was cooled to 40°C, and then 0.016 grams of seeds were added to the mixture. The mixture was then cooled to 30°C and held for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt % aqueous ε-caprolactam solution. The specifications of the obtained purified ε-caprolactam were as follows:

[0342] PAN:2 E290:0.02 VB:<0.01mmol / kg Alkalinity: 0.03mmol / kg

[0343] 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 nylon 6 waste fabric containing polyether polyurethane and purified by extraction, stripping, distillation, and crystallization. All the above examples demonstrate that the present invention can produce good products from dyed feedstocks, especially as they exist when derived from discarded textiles.

[0344] Example 6 Pretreatment and separation of nylon 6 and polyether polyurethane, depolymerization of nylon 6 and recovery of ε-caprolactam, purification by extraction, stripping, distillation and crystallization. The nylon 6 and polyether polyurethane pretreatment and separation procedures of Example 1 were followed. The recovered polyether polyurethane precipitate can be reused in textile production, either as is or in combination with virgin polyether polyurethane. The depolymerization and recovery procedures of Example 3 were followed.

[0345] The resulting crude ε-caprolactam, 36 grams, was extracted once with 68 grams and four times with 50 grams of benzene at 25°C. The resulting organic extracts were combined and concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; water bath temperature was about 65°C) to an ε-caprolactam concentration of about 25 wt%. This mixture was batch extracted about twice with about 25 g of water at a temperature of about 25°C. The specifications of the aqueous ε-caprolactam solution after back-extraction were as follows:

[0346] PAN:132 E290:1.55

[0347] Next, 75 mmol of aqueous sodium hydroxide per kg of ε-caprolactam was added to the concentrated ε-caprolactam solution. Water and impurities with boiling points lower than ε-caprolactam were then removed as the overhead product by distillation under reduced pressure in a batchwise distillation apparatus. Finally, the distilled ε-caprolactam was recovered as the overhead product at 300 Pa, while the impurities with higher boiling points than ε-caprolactam remained as the bottom product of the distillation apparatus.

[0348] Distilled water was then added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4 wt%. This aqueous ε-caprolactam was introduced into a crystallizer at a temperature of 52°C. The aqueous ε-caprolactam was cooled to 40°C, and several seeds were added to the mixture. The mixture was then cooled to 30°C and held for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt% aqueous ε-caprolactam solution. The specifications of the resulting purified ε-caprolactam met all the requirements for major polymerization applications.

[0349] This example demonstrates that polyether polyurethane and nylon 6 can be separated from waste nylon 6 fabric containing polyether polyurethane by selective extraction, precipitation of dissolved polyether polyurethane, and washing of undissolved nylon 6. It can also be concluded from this experiment that purified ε-caprolactam meeting all specifications required for major polymerization applications can be obtained by depolymerization of nylon 6 derived from discarded polyether polyurethane-containing nylon 6, and purified by extraction, stripping, distillation, and crystallization.

[0350] Experiment 7 Calculation of the carbon footprint of purified ε-caprolactam and polyether polyurethane. A continuous process according to the present invention for producing purified ε-caprolactam and polyether polyurethane from waste nylon 6 fabrics containing polyether polyurethane was simulated. The polyether polyurethane content of these waste nylon 6 fabrics containing polyether polyurethane was 20 wt %, with the remainder being primarily nylon 6.

[0351] This process included: -Cutting waste nylon 6 fabric containing polyether polyurethane into small pieces; -Selective extraction of polyether polyurethanes with DMAc; - Separating the non-dissolved nylon 6 and polyether polyurethane rich stream by centrifugation; - washing the non-dissolved nylon 6 with water; - separating the washed non-dissolved nylon 6 from the aqueous extract by centrifugation; - Precipitating polyether polyurethane from the polyether polyurethane-rich stream by adding the aqueous extract obtained above; - separating the precipitated polyether polyurethane by filtration from the DMAc-water mixture; recovering DMAc and water from the DMAc-water mixture obtained above; -Drying the filtered polyether polyurethane; - drying the washed non-dissolved nylon 6; - Melting and pelletizing the washed non-dissolved nylon 6; - Depolymerization of nylon 6 under the influence of H3PO4 and superheated steam; -recovering crude ε-caprolactam (80 wt. % ε-caprolactam) by partial condensation of the vapors discharged from the depolymerization reactor; - carrying out a countercurrent extraction of concentrated crude ε-caprolactam with toluene; - washing the organic extract with diluted caustic solution; - countercurrent back-extraction of the washed organic extract with water; - concentrating the aqueous extract by evaporation; -Adding caustic substances; - distillative removal of light and heavy ends by vacuum distillation; and Recovery of pure ε-caprolactam by melt crystallization at a temperature of -61°C.

[0352] The carbon footprints of purified ε-caprolactam and polyether polyurethane were calculated based on the raw material consumption figures, and the utility of the above process was based on data from ecoinvent version 3.7.1. The distribution of environmental impact between the products of the pretreatment and separation sections, purified ε-caprolactam and polyether polyurethane, was based on the weight ratio of these products.

[0353] The results show that the product carbon footprint of purified ε-caprolactam obtained from polyamide 6 waste fabric containing polyether polyurethane is less than 3.0 tonnes of CO2 equivalent per tonne of ε-caprolactam and less than 1.0 tonnes of CO2 equivalent per tonne of polyether polyurethane (European region).

[0354] Although the present invention has been described with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof, including (semi-)continuous operation and scaling up to commercial scale.

Claims

1. 1. A process for recovering ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane containing material in a plant, said plant comprising: a separation section [B]; a depolymerization section [C]; - a collection section [D]; - a purification section [E], wherein the process comprises: a) introducing the nylon 6 and polyether polyurethane containing material into the separation section [B]; b) separating the nylon 6 and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream in the separation section [B] by selectively dissolving the polyether polyurethane in an organic solvent at a temperature of less than 100°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and polyether polyurethane; c. 1) discharging the nylon 6-rich stream from the separation section [B] and introducing the nylon 6-rich stream into the depolymerization section [C], wherein the nylon 6 content of the nylon 6-rich stream is at least 85 wt% on a dry weight basis; c. 2) depolymerizing the nylon 6 in the nylon 6-rich stream in the depolymerization section [C] at a temperature in the range of 180°C to 400°C, thereby obtaining an ε-caprolactam-containing stream, and discharging the obtained ε-caprolactam-containing stream from the depolymerization section [C]; c.3) recovering crude ε-caprolactam from the ε-caprolactam-containing stream in the recovery section [D]; c. 4) purifying the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification comprises: (i) extracting the crude ε-caprolactam with an organic solvent, thereby obtaining an aqueous phase and an organic phase, the organic phase containing the organic solvent, ε-caprolactam and impurities; (ii) switching the solvent by at least partially replacing the organic solvent with water or an aqueous solution, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam, the solvent switching step being selected from a process based on back-extraction with water and a process based on solvent exchange distillation, in which the organic solvent is distilled off and water is introduced; (iii) obtaining purified ε-caprolactam by distilling off impurities having a boiling point lower or higher than that of ε-caprolactam; (iv) obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C; d. 1) recovering polyether polyurethane from said polyether polyurethane-rich stream in said separation section [B]; d.2) discharging the recovered polyether polyurethane from separation section [B], wherein the polyether polyurethane content of the discharged stream is at least 85 wt.% on a dry weight basis; The process includes:

2. The process of claim 1, wherein the organic solvent is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof.

3. The process of claim 1, wherein the resulting organic phase is washed with water or an aqueous alkaline solution.

4. 2. The process of claim 1, The separation section [B] comprises: - a dissolution section [α]; a washing section [β]; - precipitation section [γ], a solvent distillation section [δ], The process comprises, in the separation section [B]: b. 1) introducing an organic solvent and the nylon 6 and polyether polyurethane-containing material into the dissolution section [α]; b.2) in the dissolving section [α], dissolving the polyether polyurethane from the nylon 6 and polyether polyurethane-containing material in an organic solvent, thereby obtaining a polyether polyurethane-rich stream containing the organic solvent and the dissolved polyether polyurethane and a stream containing undissolved nylon 6, and discharging the obtained streams from the dissolving section [α]; b.3) introducing a second solvent and the polyether polyurethane-rich stream comprising an organic solvent and dissolved polyether polyurethane into the precipitation section [γ], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent; b.4) recovering the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent and discharging the precipitated polyether polyurethane from the precipitation section [γ]; b.5) ​​discharging the mixture of the polyether polyurethane precipitated in step b.4) containing the recovered organic solvent and second solvent from the precipitation section [γ] and introducing the mixture into the solvent distillation section [δ]; b. 6) introducing a stream comprising a third solvent and undissolved nylon 6 into a washing section [β]; b.7) in the washing section [β], washing the stream containing undissolved nylon 6 with the third solvent, thereby obtaining a nylon 6-rich stream and a mixture containing an organic solvent and a third solvent; b.8) discharging the nylon 6-rich stream from the washing section [β]; b.9) Discharging the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β] and partially or completely introducing the mixture into the solvent distillation section [δ]; b.10) separating an organic solvent from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and discharging the second solvent, the third solvent, and the separated organic solvent from the solvent distillation section [δ]; The process includes:

5. 5. The process according to claim 4, wherein the separated organic solvent discharged from the solvent distillation section [δ] of step b.10) is introduced into the dissolution section [α] of step b.1).

6. The process according to any one of claims 1 to 5, (i) the nylon 6-rich stream discharged from the separation section [B] is dried and / or densified before being fed to the depolymerization section [C]; and / or (ii) the depolymerization in step c.2) is carried out in the presence of water, whereby the ε-caprolactam-containing stream is a vapor stream comprising ε-caprolactam and water in a weight-to-weight ratio of 1:1 to 1:50; and / or (iii) superheated steam having a temperature in the range of 220°C to 575°C is introduced into the depolymerization section [C]; (iv) The depolymerization in step c.2) is carried out in the absence or presence of a catalyst, and the catalyst is selected from an acid catalyst and a base catalyst, and the acid catalyst is selected from orthophosphoric acid, boric acid, sulfuric acid, an organic acid, an organic sulfonic acid, a solid acid, a salt of the aforementioned acid, Al ２ O ３ and SiO ２ and combinations thereof, wherein the base catalyst is selected from the group consisting of alkali hydroxides, alkali salts, alkaline earth hydroxides and alkaline earth salts, organic bases and solid bases, and combinations thereof.

7. The process of claim 6, wherein the acid catalyst is orthophosphoric acid.

8. The process of claim 6, wherein the base catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate.

9. The process of claim 6, wherein the depolymerization in step c.2) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

10. 6. The process according to any one of claims 4 to 5, wherein in step b.4), the recovered polyether polyurethane discharged from the precipitation section [γ] is reused in the production of textiles.

11. The plant, - Pre-processing section [A] further comprising 6. The process according to any one of claims 1 to 5, wherein prior to step a), the nylon 6 and polyether polyurethane containing material is subjected to a pretreatment in the pretreatment section [A].

12. The process of claim 11, wherein the pretreatment is cleaning in a cleaning section [ω] and / or mechanical size reduction in a mechanical size reduction section [λ].

13. 5. The process of claim 4, wherein the organic solvent is dimethylacetamide.

14. The process of claim 13, wherein the second solvent and the third solvent are the same solvent.

15. The process of claim 14, wherein the second solvent and the third solvent are aqueous solutions or water.

16. 5. The process according to claim 4, wherein the second solvent in step b.3) is a mixture comprising the organic solvent and a third solvent partially or completely obtained from washing section [β] in step b.7).

17. 5. The process according to claim 4, wherein the second solvent and the third solvent discharged from the solvent distillation section [δ] are reused in the precipitation section [γ] and / or the washing section [β].

18. The process described in claim 17, wherein the second solvent and the third solvent discharged from the solvent distillation section [δ] are separated from each other and then reused in the precipitation section [γ] and / or the washing section [β].

19. 6. The process of claim 5, wherein decomposition products of the organic solvent are removed prior to introducing the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) into the dissolution section [α] in step b.1).

20. 6. The process of any one of claims 1 to 5, wherein the solution comprising ε-caprolactam and impurities from which ε-caprolactam is crystallized in step c.4)(iv) also comprises water.

21. The process of claim 20, wherein the solution containing ε-caprolactam and impurities from which ε-caprolactam is crystallized in step c.4)(iv) also contains more than 1 wt.% water.

Images