Process for recovering epsilon-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane

Abstract

The invention relates to a method for recovering epsilon-caprolactam and polyether polyurethanes from materials comprising nylon 6 and polyether polyurethanes. The invention provides a method for recovering epsilon-caprolactam and polyether polyurethane from a material comprising nylon 6 and polyether polyurethane in a plant, wherein the plant comprises a separation section [B], a depolymerization section [C], a recovery section [D] and a purification section [E].

Description

[0001] This application is a divisional application of Chinese patent application No. 202380053243.9, filed on July 11, 2023, entitled "Method for recovering ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane". Technical Field

[0002] This invention relates to a method for recovering ε-caprolactam from a material comprising nylon 6 and polyether polyurethane. More specifically, this invention relates to a method for recovering purified ε-caprolactam and polyether polyurethane from a material comprising nylon 6 and polyether polyurethane on an industrial scale. Background Technology

[0003] In 1938, Paul Schlack invented Nylon 6 (CAS No.: 25038-54-4), also known as polyamide 6, PA6, N6, polycaprolactam, poly(hexyl-6-lactam), poly(6-aminohexanoic acid), poly(hexamethylene adipamide) or poly[imino(1-oxohexane-1,6-diyl)].

[0004] Typically, nylon 6 is synthesized by ring-opening polymerization of ε-caprolactam at approximately 260°C in an inert atmosphere. As is well known, ε-caprolactam can be prepared by the liquid-phase Beckmann rearrangement of cyclohexanone oxime in the presence of fuming sulfuric acid, i.e., a mixture of sulfuric acid and SO3, or by the gas-phase Beckmann rearrangement of cyclohexanone oxime in the presence of a solid catalyst. This type of ε-caprolactam is typically referred to as "primary ε-caprolactam." The cyclohexanone oxime required to form (primary) ε-caprolactam can be prepared from cyclohexanone, which is primarily produced from benzene. This benzene is largely derived from non-renewable fossil resources such as petroleum and coal.

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

[0006] The method for producing Nylon 6 is described, for example, in the "Polyamide" chapter of the Ullmann Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Publishers GmbH, Weinheim, Germany, which is available electronically at https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0007] Initially, pure nylon 6 was used to produce textiles, including stockings. Later, to improve fabric properties, a wide variety of blended fabrics were developed and produced. In particular, polyether polyurethane has been widely used in various nylon 6-based garments. The benefits of polyether polyurethane include its significant strength and elasticity, its ability to recover its original shape after stretching, and its faster drying speed than ordinary fabrics. The improved elasticity of fabrics is typically achieved by using blended yarns made from nylon 6 fibers and polyurethane fibers. Examples of clothing containing blends of nylon 6 and polyether polyurethane include stretch stockings, underwear, and sportswear. The breathability and water resistance of fabrics can be achieved by treating (e.g., coating) the surface of nylon 6 fibers with polyurethane resin. These surface-treated yarns are commonly used to manufacture, for example, raincoats, clothing suitable for cold weather, and skiwear. Polyether polyurethane used in garment production is also commonly referred to as Spandex, Lycra, or elastic fiber.

[0008] Recycling nylon 6 enables the conservation of fossil resources and adds value to the circular economy. Mechanical recycling of waste nylon 6 involves processes that convert waste nylon 6 into secondary raw materials or (preferably) products with minimal changes to their chemical structure. The conversion and depolymerization of nylon 6 are two forms of chemical recycling. In conversion recycling, nylon 6 is broken down into petroleum- or natural gas-like feedstocks that can replace freshly extracted fossil feedstocks. The resulting products can be used to produce chemicals including the monomer ε-caprolactam. In depolymerization recycling, 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. Typically, existing chemical regeneration or recycling methods for depolymerizing substantially pure nylon 6 into ε-caprolactam monomers include a hydrolytic degradation step in the presence of water at elevated temperatures and a step of recovering the monomers formed by steam distillation.

[0009] The depolymerization and recycling process for nylon 6 was developed specifically for clean and relatively pure waste nylon 6 materials. Mechanical recycling always results in downgrading, and therefore the properties of products made from mechanically recycled nylon 6 are always inferior to those made from virgin nylon 6. However, chemical recycling of waste nylon 6-containing materials may also allow the production of high-purity ε-caprolactam with properties similar to those of virgin ε-caprolactam, which can then be converted into higher-grade nylon 6. Unfortunately, the chemical recycling of waste nylon 6-containing materials is often hampered by the presence of impurities in the waste nylon 6 materials. Typically, due to the presence of these impurities, the quality of the produced ε-caprolactam is inferior to that of virgin ε-caprolactam.

[0010] Compared to the depolymerization of relatively pure nylon 6, the depolymerization of blends of nylon 6 and polyether polyurethane has several disadvantages. These disadvantages include, in particular, the clogging of pipes and other equipment parts by the polyether polyurethane and its decomposition products; the poor quality of the generated ε-caprolactam due to the presence of polyether polyurethane decomposition products, which (severely) reduces ε-caprolactam recovery; poisoning of the depolymerization catalyst; and therefore, higher consumption of the depolymerization catalyst. Another disadvantage of adding blends of nylon 6 and polyether polyurethane to the depolymerization reactor is the degradation of the polyether polyurethane. Therefore, it is impossible to achieve the recycling of polyether polyurethane, which is more valuable than nylon 6.

[0011] The virgin elastic fibers are produced by dry or wet spinning processes, which begin with an elastic fiber spinning solution composed of a suitable solvent containing polyurethane (such as dimethylacetamide or dimethylformamide).

[0012] Because the high viscosity prevented further processing, directly dissolving normal (pure) elastic fiber waste fibers in the spinning solvent used for example to recycle them from yarn production proved unsuccessful.

[0013] US6830715B1 describes a method for producing elastic fiber yarn from a spinning solution using recycled elastic fiber material, the method overcoming the aforementioned viscosity problem by adding a secondary aliphatic amine to a mixture of (cut) elastic fibers and a spinning solvent. The dissolution of the (cut) elastic fibers is achieved at a temperature between 60°C and 150°C.

[0014] In the past, several attempts have been described to recycle blends of nylon 6 and polyether polyurethane.

[0015] JP2011088943A describes a pretreatment method for separating polyether polyurethane from a nylon 6 product containing polyether polyurethane, and subsequently depolymerizing the residual nylon 6. The pretreatment involves heating the nylon 6 product containing polyether polyurethane together with a solvent containing a cycloamide at a temperature of 80°C to the boiling point of the solvent. JP2011088943A indicates that the amount of the cycloamide compound in the solvent is preferably 50% by weight or more, more preferably 85% by weight or more. JP2011088943A further indicates that the solvent may contain components other than the cycloamide compound, such as water and organic solvents, and that water is particularly preferred from a workability point of view. The experiment described in the patent was conducted at 110°C over a period of 2 hours using an aqueous solution containing 50% and 85% by weight of N-methylpyrrolidone, 85% by weight of 2-pyrrolidone, or 85% by weight of 2-piperidine as solvents. The result of this treatment is that the polyether polyurethane partially decomposes and dissolves in the solvent. Therefore, the resulting nylon 6 product was depolymerized, the polyether polyurethane was removed from the product, and the product was separated from the solution by filtration. However, JP2011088943A does not mention the recovery of the partially decomposed polyether polyurethane, nor does it describe the fate of the solvent used.

[0016] WO 2013032408A1 describes a method for recycling polyamide fibers comprising polyamide 6 and polyamide 6,6 comprising polyamide and spandex. The method involves removing spandex fibers from the elastomer fabric by: controlled thermal degradation of spandex; a controlled washing treatment to remove spandex or its degradation products from high-purity polyamide using a suitable and 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 the polyamide fibers (containing polyamide 6 fibers) is 150°C–220°C, preferably 190°C–216°C, and preferably over a period of 0.5 to 4 hours. The heat-treated fabric is then washed with preferably ethanol at a temperature ranging from 5°C to 78°C to remove spandex and its degradation components. WO 2013032408A1 does not mention the recovery of spandex and its degradation components from the washing solvent.

[0017] Given the above, to date, there is no method for recovering purified ε-caprolactam and polyether polyurethane from materials including nylon 6 and polyether polyurethane on an industrial scale. The reason for not realizing a recycling method for recovering purified ε-caprolactam and polyether polyurethane from materials including nylon 6 and polyether polyurethane on an industrial scale is that all methods that introduce elastic fibers into the solution require the application of high solution temperatures, typically well above 100°C. However, at these temperatures, the elastic fibers begin to degrade, and only inferior material is obtained after recycling. Therefore, the fate of the recovered elastic fibers is often limited to incineration and landfill. Another disadvantage of the high-temperature dissolution process is the formation of degradation products of the applied solvent, which may hinder the recovery of nylon 6 and polyether polyurethane, as well as the recycling of the applied solvent.

[0018] The environmental issues surrounding the production and use of these materials, including nylon 6 and polyether polyurethane, involve waste generated during and after consumption. These issues can be mitigated by recycling individual components (such as nylon 6 and polyether polyurethane) from materials that are no longer used or have been disposed of. Materials including nylon 6 and polyether polyurethane are a particularly relevant source of waste, especially as textiles. These typically contain significant amounts of nylon 6 and polyether polyurethane. Therefore, if a viable method were available to recover nylon 6 and polyether polyurethane from these composite wastes, it would not only benefit the environment but also provide an economically valuable new source of nylon 6 and polyether polyurethane.

[0019] From the perspective of reducing carbon dioxide emissions, recycling materials including nylon 6 and polyether polyurethane, thereby recovering purified ε-caprolactam and polyether polyurethane, is of great significance.

[0020] Furthermore, high-purity grades of ε-caprolactam and polyether polyurethane are required from materials including nylon 6 and polyether polyurethane, with a significantly lower carbon footprint than ε-caprolactam produced by methods using native ε-caprolactam obtained through novel synthesis (e.g., via the Beckmann rearrangement of cyclohexanone oxime) and polyether polyurethane produced through de novo synthesis of polyether polyurethane.

[0021] The raw material price of virgin polyether polyurethane is relatively high, often more than twice that of virgin nylon 6. Therefore, recycling and reusing (reusing) waste polyether polyurethane and nylon 6 has significant economic incentives.

[0022] The problem with existing methods is that they consume a lot of energy to recover dissolved elastic fibers from the solution, since dissolved elastic fibers are usually obtained by evaporating to remove the solvent.

[0023] Furthermore, it is necessary to purify the crude ε-caprolactam obtained from the depolymerization of polyether polyurethane from materials including nylon 6 and polyether polyurethane, without using oxidants such as potassium permanganate (KMnO4) or adsorbents such as (activated) carbon and diatomaceous earth. Techniques based on these oxidants and adsorbents are quite laborious and generate solid waste.

[0024] Another drawback of the prior art is that no suitable recycling strategy is provided or known for recovering and reusing (recycling) solvents of polyether polyurethane and nylon 6 from materials including nylon 6 and polyether polyurethane.

[0025] Currently, while there is an urgent need for methods to recover high-purity ε-caprolactam from materials including nylon 6 and polyether polyurethanes, no such methods are available. Specifically, there is an urgent need for high-purity ε-caprolactam recovery methods that can replace virgin ε-caprolactam grades for demanding applications such as high-speed melt spinning during textile fiber production.

[0026] Finally, there is a need for methods that allow for the industrial-scale recovery of pure ε-caprolactam and polyether polyurethane from materials including nylon 6 and polyether polyurethane in order to address the large quantities of materials including nylon 6 and polyether polyurethane wasted each year. Summary of the Invention

[0027] The purpose of this invention is to satisfy one or more of the above-mentioned requirements and to overcome or mitigate the disadvantages associated with prior art methods.

[0028] Specifically, an object of the present invention is to provide a method for recovering ε-caprolactam from a material comprising nylon 6 and polyether polyurethane. Another object of the present invention is to provide a method for recovering purified ε-caprolactam and polyether polyurethane from a material comprising nylon 6 and polyether polyurethane.

[0029] Another object of the present invention is to provide a method for recovering purified ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane on an industrial scale.

[0030] The object of this invention is to provide a method for recovering purified ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane in an economical and efficient manner. Specifically, the object of this invention is to provide a method suitable for recovering purified ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane, without exceeding the production cost of virgin high-purity ε-caprolactam and polyether polyurethane.

[0031] The present invention also aims to provide a method for purifying crude ε-caprolactam, which is obtained by depolymerizing a material comprising nylon 6 and polyether polyurethane, and the method does not generate solid waste.

[0032] Another object of the present invention is to provide a method for recovering and purifying both ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane, the method being characterized by a significantly lower carbon footprint than methods for producing virgin polyether polyurethane, for example, through the reaction of polyether polyols and diisocyanate monomers, and for producing ε-caprolactam through novel synthesis, for example, through the Beckmann rearrangement of cyclohexanone oxime.

[0033] The object of the present invention is to provide a method for recovering high-purity ε-caprolactam from materials including nylon 6 and polyether polyurethane, wherein the high-purity ε-caprolactam can replace high-purity virgin ε-caprolactam in all applications, including high-speed melt spinning of nylon 6 for the production of thin textile fibers.

[0034] Therefore, the present invention also aims to provide a method for reducing the environmental burden of discarded materials including nylon 6 and polyether polyurethane.

[0035] One or more additional purposes may become apparent from the remainder of the instruction manual.

[0036] The method of claim 1, the plant of claim 13, and the product of claims 14 and 15 largely solve or at least alleviate all or at least some of the foregoing objectives.

[0037] This invention provides a method for recovering ε-caprolactam and polyether polyurethane from a material comprising nylon 6 and polyether polyurethane in a factory, wherein said factory includes -Separation section [B], -De-aggregation section [C], -Reclaim section [D], and -Purification section [E], And the method described therein includes the following steps: a) The material comprising nylon 6 and polyether polyurethane is loaded into the separation section [B]; b) In the separation section [B], the material comprising nylon 6 and polyether polyurethane is separated into a nylon 6-rich stream and a polyether polyurethane-rich stream by selectively dissolving the polyether polyurethane in an organic solvent at a temperature below 100°C, preferably in the range of 0°C to 100°C, more preferably in the range of 10°C to 90°C, even more preferably in the range of 10°C to 80°C, and most preferably in the range of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane. c.1) Discharge the nylon 6-rich stream from the separation section [B] and load the nylon 6-rich stream into the depolymerization section [C], wherein the nylon 6 content in the nylon 6-rich stream is at least 85% by weight based on dry weight. c.2) In the depolymerization zone [C], the nylon 6 in the nylon 6-rich stream is depolymerized at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C, so as to obtain a stream containing ε-caprolactam, and the obtained stream containing ε-caprolactam is discharged from the depolymerization zone [C]. c.3) Recover crude ε-caprolactam from the feed stream containing ε-caprolactam in the recovery section [D]; c.4) Purify the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification includes the following steps: (i) The crude ε-caprolactam is extracted with an organic solvent to obtain an aqueous phase and an organic phase, wherein the organic phase comprises the organic solvent, the ε-caprolactam, and impurities, and preferably the organic solvent is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof; and The obtained organic phase may be washed with water or an alkaline aqueous solution. (ii) Optionally, the solvent is switched 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 below or above ε-caprolactam, wherein the solvent switching is selected from a process based on water back-extraction and a process based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; and (iii) Optionally, purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation; (iv) Purified ε-caprolactam was obtained 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 the polyether-rich stream in the separation section [B]; and d.2) The recovered polyether polyurethane is discharged from the separation section [B], wherein the polyether polyurethane content in the discharged stream is at least 85% by weight based on dry weight.

[0038] Surprisingly, combining the specific sequence of processing steps and conditions according to the invention—namely, the sequence of the separation, depolymerization, recovery, and purification steps described above—allows for the recovery of ε-caprolactam from materials comprising nylon 6 and polyether polyurethane in a high-yield, direct, and cost-effective manner. The method of the invention is economically sound and advantageous from several perspectives. First, the method is applicable to a variety of materials derived from nylon 6 and polyether polyurethane, which may differ, for example, in their overall composition and / or their polyether polyurethane content and / or their nylon 6 content. Second, the method allows for the efficient separation of nylon 6 from the polyether polyurethane compound, thereby yielding a higher grade of nylon 6. Third, the method is highly efficient, enabling the high-yield recovery of ε-caprolactam. Fourth, the method allows for the recovery of polyether polyurethane, which can be reused to replace polyether polyurethane produced de novo, for example, through the reaction of polyether polyols and diisocyanate monomers. Fifth, the method of the present invention allows for the efficient separation of ε-caprolactam from non-ε-caprolactam compounds, enabling the acquisition of high-purity grades of ε-caprolactam that can replace high-purity virgin ε-caprolactam in all applications, including high-speed melt spinning of nylon 6 for the production of thin textile fibers. Finally, compared to ε-caprolactam produced via de novo synthesis (e.g., via the Beckmann rearrangement of cyclohexanone oxime), the method of the present invention allows for the production of ε-caprolactam with a significantly lower carbon footprint. The method of the present invention allows for the efficient processing of materials comprising nylon 6 and polyether polyurethanes and reduces the environmental burden of said products. Specifically, the method of the present invention allows for the production of polyether polyurethanes with a carbon footprint of less than 1 kg CO2 / kg polyether polyurethane, a significant improvement compared to the 4.8 kg CO2 / kg polyether polyurethane associated with the production of “virgin” polyether polyurethanes obtained through chemical synthesis. Specifically, the method of the present invention allows for the production of purified ε-caprolactam with a carbon footprint of less than 3 kg CO2 / kg, which is a significant improvement compared to the 6.4 to 7.5 kg CO2 / kg ε-caprolactam associated with the production of “native” ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime.

[0039] In addition to the method of the present invention, the present invention also provides a plant for producing purified ε-caprolactam and polyether polyurethane from materials comprising nylon 6 and polyether polyurethane, wherein the plant comprises: -Optionally, preprocessing section [A], -Separation section [B], -Nylon 6 depolymerization section [C], -ε-caprolactam recovery zone [D], -ε-caprolactam purification region [E], and The chemical plant described therein is configured to carry out the method of the present invention.

[0040] The present invention also provides purified ε-caprolactam obtained by means of the method according to the invention from materials comprising nylon 6 and polyether polyurethane, wherein the carbon footprint of the purified ε-caprolactam is less than 3.0 kg CO2 / kg.

[0041] The present invention also provides a method according to the invention for obtaining a polyether polyurethane by separating it from a material comprising nylon 6 and a polyether polyurethane, wherein the product carbon footprint of the polyether polyurethane is less than 1.0 kg CO2 equivalent / kg recycled polyether polyurethane.

[0042] Advantageous embodiments of the invention are indicated in the dependent claims and explained in more detail below. Detailed Implementation

[0043] Materials including nylon 6 and polyether polyurethane The method of this invention uses a material comprising nylon 6 and polyether polyurethane as a starting material. The material comprising nylon 6 and polyether polyurethane can be a product comprising nylon 6 and polyether polyurethane or a material derived therefrom. The material comprising nylon 6 and polyether polyurethane can be a variety of materials comprising nylon 6 and polyether polyurethane or a mixture thereof. Typically, the material comprising nylon 6 and polyether polyurethane is a solid material, specifically fibers, yarns (i.e., strands of fibers spun together), or fiber-based fabrics, wherein the fibers are a blend of nylon 6 fibers and polyurethane fibers, or wherein the fibers are obtained by coating the surface of nylon 6 fibers with a polyether polyurethane resin.

[0044] Materials comprising nylon 6 and polyether polyurethane can be of pre- or post-consumer origin. Materials comprising nylon 6 and polyether polyurethane can be mixtures of different materials comprising nylon 6 and polyether polyurethane, or mixtures of one or more materials comprising nylon 6 and polyether polyurethane with one or more different materials.

[0045] Materials including nylon 6 and polyether polyurethane may include additional components. These components may be added, for example, during or after polymerization, fiber formation, to achieve desired property changes. These compounds include, for example, brightening agents, curing agents, antistatic lubricants, colorants, brightening agents, spin finish agents, surface smoothers, antioxidants, UV stabilizers, etc. The amount of these additional components depends on the application of the material including nylon 6 and polyether polyurethane.

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

[0047] Polyurethane is produced by reacting isocyanates containing two or more isocyanate groups per molecule with polyols containing an average of two or more hydroxyl groups per molecule in the presence of a catalyst or by activation with ultraviolet light. The main components in the preparation of polyurethane are diisocyanates, triisocyanates, and polyols. Methylene diphenyl diisocyanate (MDI), a diisocyanate monomer, is the most commonly used isocyanate in the production of polyurethane for clothing applications.

[0048] Polyether polyurethanes are produced by reacting isocyanates with polyether polyols. Polyether polyols are typically prepared by polymerizing cyclic ether compounds onto initiator compounds. The most commonly used cyclic ethers in polyether polyol production are ethylene oxide, propylene oxide, and 1,4-epoxybutane or tetrahydrofuran (used to produce poly(epoxybutane) polyols). Poly(epoxybutane) polyols are primarily used in applications requiring highly hydrophobic properties.

[0049] The use of the terms “about” or “approximately” in conjunction with numerical values ​​herein indicates that the values ​​may be affected by measurement errors, which typically cause the values ​​to vary by no more than ±5%. Numerical values ​​disclosed herein with the terms “about” or “approximately” are also intended to be disclosed without the term “approximately.” Furthermore, the numerical ranges described herein are intended to encompass and disclose every value within that range. All upper and lower endpoints of ranges of the same parameters disclosed herein may be combined with each other. All ranges of different parameters disclosed herein may be combined with each other. Specifically, ranges of different or the same “preference level” are specifically compatible with each other. As used in this disclosure and claims, unless the context clearly specifies otherwise, the singular forms “an,” “a,” and “described” include the plural forms, particularly in the sense of “one or more.”

[0050] Possible preprocessing steps Prior to step a) of the method of the present invention, the material comprising nylon 6 and polyether polyurethane may be pretreated in a pretreatment section [A], specifically in a size reduction section [λ] and / or in a cleaning section [ω]. The pretreatment may be performed in a location different from the location of the separation section [B]. However, preferably, the method of the present invention is carried out in a plant further comprising a pretreatment section [A], wherein prior to step a), the material comprising nylon 6 and polyether polyurethane is pretreated in the pretreatment section [A], specifically in a cleaning section [ω] and / or in a size reduction section [λ]. This has the advantage that the material comprising nylon 6 and polyether polyurethane loaded into the separation section [B] is less contaminated by foreign materials, which improves the yield and purity of ε-caprolactam and polyether polyurethane produced in the plant of the present invention configured to carry out the method of the present invention. Another advantage is that the size-reduced material comprising nylon 6 and polyether polyurethane can be processed more easily.

[0051] Materials comprising nylon 6 and polyether polyurethane are preferably derived from or derived from used or discarded materials comprising nylon 6 and polyether polyurethane. The size and shape of the material depend largely on the exact application from which it originates. The range of materials comprising nylon 6 and polyether polyurethane that can be used according to the invention is from separated fibers and yarns, optionally in spool form, to woven fabrics and garments. Preferably, materials comprising nylon 6 and polyether polyurethane are discarded or used materials comprising nylon 6 and polyether polyurethane.

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

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

[0054] Preferably, the material comprising nylon 6 and polyether polyurethane is broken into fragments before separation in the separation section [B] in step b). This mechanical pretreatment of the material comprising nylon 6 and polyether polyurethane, i.e., mechanical crushing or breaking, can be achieved, for example, by cutting, stamping, shredding, grinding, milling, and / or cutting. In a preferred embodiment, the material comprising nylon 6 and polyether polyurethane is loaded into the separation section [B] of step a) in the form of broken fragments. The advantage of using broken fragments is that the fragments can be more easily handled and / or cleaned by washing with a solvent. In a preferred embodiment, the fragments have an average length of 1 mm to 100 m along the longest axis of the fragment, preferably 3 mm to 1 m, and most preferably 1 cm to 50 cm. A person skilled in the art can easily determine the average length along the longest axis of the fragments used by first obtaining a representative sample of the fragments, then measuring the length of the longest axis of each of these fragments (e.g., 50 fragments), and finally calculating the average of all these individual measurements. Preferred particle size can also be described by average particle weight. Preferably, the average particle weight of the fragments comprising nylon 6 and polyether polyurethane is from 0.01 g to 25 kg, more preferably from 0.05 g to 1 kg, and most preferably from 0.1 g to 100 g. Experiments have shown that fragments of materials comprising nylon 6 and polyether polyurethane having the above-mentioned size or weight dimensions are particularly suitable for processing in the method of the present invention and / or for cleaning by solvent washing.

[0055] Optionally, large metal fragments, rocks, and other interfering materials that cause severe wear on the equipment used for mechanical crushing or pulverizing are removed before the materials comprising nylon 6 and polyether polyurethane are mechanically crushed or pulverized. Preferably, foreign materials comprising, but not limited to, polyethylene, polypropylene, and polyamide 6,6 are also removed before the materials comprising nylon 6 and polyether polyurethane are mechanically crushed or pulverized. The removal of foreign materials can be carried out mechanically or manually. The advantage of removing these interfering materials is that the maintenance costs of the equipment used for mechanical crushing or pulverizing can be significantly reduced. Additionally, the nylon 6 and polyether polyurethane content of the material obtained after mechanical crushing or pulverization is higher than in the case where interfering materials have not been removed. The removal of polyamide 6,6 is particularly advantageous because it interferes with the depolymerization of nylon 6, reduces the recovery rate of ε-caprolactam, and interferes with the subsequent purification of the recovered ε-caprolactam. As used herein, the term "foreign material" refers to non-nylon 6 and non-polyether polyurethane materials or compounds.

[0056] Optionally, foreign materials are separated from materials including nylon 6 and polyether polyurethane that have been mechanically crushed or broken. Various separation methods can be applied for this purpose, including but not limited to density separation and magnetic separation. In density separation, materials of different densities are placed in a medium-density liquid, where the lower-density material floats and separates from the higher-density material that sinks. In practice, density separation is typically accomplished through a series of density separation stages. For example, in one stage, high-density materials such as rocks, sand, and metals (including iron and lead) are separated, while in another stage, low-density materials such as polyolefins, polypropylene, and polyethylene are separated. Magnetic separation is a method of separating components of a mixture by using a magnet to attract magnetic materials. Methods preferably used for magnetic separation decouple non-magnetic materials from magnetic materials. Removing foreign materials from crushed or broken materials including nylon 6 and polyether polyurethane is advantageous because such materials can interfere with the separation of nylon 6 and polyether polyurethane, the depolymerization of nylon 6, reduce the recovery rate of ε-caprolactam, and / or interfere with the subsequent purification of the recovered ε-caprolactam.

[0057] Optionally, specifically in the pretreatment steps described above, the materials comprising nylon 6 and polyether polyurethane are cleaned by washing with a solvent (preferably at least water) before being loaded into the separation section [B]. The solvent used herein may be a single solvent or a mixture of different solvents. Preferably, a detergent in the range of 0 to 20% by weight relative to the solvent concentration is added to the solvent to improve washing efficiency. NaOH is a preferred detergent. Even more preferably, an aqueous solution containing 0 to 10% by weight of NaOH is used in the washing step, and still more preferably 0 to 5% by weight of NaOH. The enhanced washing effect of NaOH is likely due to enhanced hydrolysis of molecules (including 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 rinsing step with a detergent-free (clean) washing solvent to remove detergent residues and present dirt adhering to the materials comprising nylon 6 and polyether polyurethane.

[0058] Washing is preferably performed under friction. Different types of industrial friction washing systems are available on the market, such as rotary plastic washers and (high-speed) friction washers.

[0059] Washing materials comprising nylon 6 and polyether polyurethane, specifically mechanically crushing or breaking down materials comprising nylon 6 and polyether polyurethane, is advantageous because it removes any (adhered) dirt and thus does not interfere with subsequent steps of the method of the present invention.

[0060] Optionally, the material comprising nylon 6 and polyether polyurethane is dried after the cleaning step and before being loaded into the separation section [B]. The advantage of doing so is that the weight of the cleaned material comprising nylon 6 and polyether polyurethane is reduced, and the next step is not affected by dilution or contamination by washing solvents.

[0061] The location of the pretreatment zone for materials comprising nylon 6 and polyether polyurethane can be the same as the location of the separation section [B]. However, preferably, one or more pretreatment steps are performed at a location different from the location of the separation section [B], for example, near a plant that collects (used or discarded) materials comprising nylon 6 and polyether polyurethane and / or at a location specifically for the pretreatment of materials comprising nylon 6 and polyether polyurethane. A nylon 6-rich stream can then be obtained in the separation section [B] from the pretreated material comprising nylon 6 and polyether polyurethane.

[0062] The location of the separation of the material comprising nylon 6 and polyether polyurethane into a nylon 6-rich stream and a polyether polyurethane-rich stream can be the same as the location of the depolymerization section [C]. However, preferably, the separation of the material comprising nylon 6 and polyether polyurethane 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 a plant that produces virgin polyether polyurethane and / or at a location that specifically processes and / or distills organic solvents.

[0063] The location of the pretreatment zone for materials including nylon 6 and polyether polyurethane can be the same as the location of the depolymerization zone [C]. However, preferably, one or more pretreatment steps in the pretreatment process are performed at a location different from the location of the depolymerization zone [C], for example, near a plant that collects waste materials including nylon 6 and polyether polyurethane and / or at a location specifically for the pretreatment of materials including nylon 6 and polyether polyurethane.

[0064] Loading step a) In step a) of the method of the present invention, materials comprising nylon 6 and polyether polyurethane, which have optionally been pretreated in the pretreatment section [A], are loaded into the separation section [B].

[0065] In one embodiment, the material comprising nylon 6 and polyether polyurethane is mechanically compressed to a smaller volume before being loaded into the separation section [B]. This has the advantage of requiring a smaller volume for intermediate storage and transportation, and also facilitates loading into the separation section [B].

[0066] In another preferred embodiment, the material comprising nylon 6 and polyether polyurethane is dried before being loaded into the separation section [B], particularly after the nylon 6 and polyether polyurethane material has undergone a cleaning step. This has the advantage that less or no solvent is introduced into the separation section [B] along with the material. Solvents introduced into the separation section [B], particularly water, can negatively impact the separation process (e.g., a reduced dissolution rate of the polyether polyurethane from the material comprising nylon 6 and polyether polyurethane in organic solvents).

[0067] The material comprising nylon 6 and polyether polyurethane is preferably fed in solid form into the dissolution section [α], specifically the corresponding dissolution container contained therein.

[0068] Feeding materials comprising nylon 6 and polyether polyurethane into the separation section [B] (e.g., feeding into the polyether polyurethane dissolution section) can be achieved by continuously or intermittently feeding materials comprising nylon 6 and polyether polyurethane.

[0069] Separation step b) In the separation section [B], materials comprising nylon 6 and polyether polyurethane are separated to form a nylon 6-rich stream and a polyether polyurethane-rich stream.

[0070] Preferably, the method of the present invention employs a separation section [B] comprising a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ], and includes the following steps in the separation section: b.1) An organic solvent and a material including nylon 6 and polyether polyurethane are loaded into the dissolution section [α]; b.2) In the dissolution zone [α], the polyether polyurethane is dissolved from the material comprising nylon 6 and polyether polyurethane in an organic solvent, such that a polyether polyurethane-rich stream comprising the organic solvent and dissolved polyether polyurethane and a stream comprising undissolved nylon 6 are obtained, and the obtained stream is discharged from the dissolution zone [α]. b.3) The second solvent and the polyether-rich stream comprising the organic solvent and dissolved polyether polyurethane are fed into the precipitation section [γ], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent. b.4) Recover the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharge the precipitated polyether polyurethane from the precipitation section [γ]. b.5) ​​Discharge the mixture comprising the organic solvent and the second solvent from the precipitation section [γ] from which the precipitated polyether polyurethane has been recovered in step b.4), and load the mixture into the solvent distillation section [δ]. b.6) The third solvent and the feed stream including undissolved nylon 6 are loaded into the washing section [β]; b.7) The stream containing undissolved nylon 6 is washed in the washing section [β] with the third solvent to obtain a stream rich in nylon 6 and a mixture comprising an organic solvent and a third solvent; b.8) Discharge the nylon 6-rich stream from the washing section [β]; b.9) Discharge the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β], and partially or completely load the mixture into the solvent distillation section [δ]. b.10) The organic solvent is separated from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and the second solvent, the third solvent and the separated organic solvent are discharged from the solvent distillation section [δ].

[0071] Optionally, the second solvent (see step b.3) may be partly or entirely a mixture of an organic solvent and a third solvent obtained from the washing section [β] in step b.7).

[0072] Preferably, the separation section [B] used in the method of the present invention includes 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 material comprising nylon 6 and polyether polyurethane into a nylon 6-rich stream and a polyether polyurethane-rich stream according to the method of the present invention. Surprisingly, the separation of the material comprising nylon 6 and polyether polyurethane into a nylon 6-rich stream and a polyether polyurethane-rich stream is particularly effective when method steps b.1) to b.10) are performed in the separation section [B] comprising the dissolution section [α], the washing section [β], the precipitation section [γ], and the solvent distillation section [δ]. Steps b.1) to b.10) performed in the preferred separation section [B] are further explained below. Steps b.1) to b.10) can replace method steps a) and b) in the method of the present invention. Step b.1) thus corresponds to step a).

[0073] As used herein, "a stream rich in..." refers to a stream containing a greater amount of enriched component than another stream obtained in the same process steps. A stream rich in nylon 6 therefore contains more nylon 6 than a stream rich in polyether polyurethane. Conversely, a stream rich in polyether polyurethane contains more polyether polyurethane than a stream rich in nylon 6.

[0074] Specifically, the nylon 6-rich stream has a higher weight ratio of nylon 6 to polyether polyurethane than the material comprising nylon 6 and polyether polyurethane.

[0075] Preferably, the nylon 6-rich feed stream has a polyether polyurethane to nylon 6 weight ratio that is at most one-half, more preferably at most one-third, of the polyether polyurethane to nylon 6 weight ratio of the material comprising nylon 6 and polyether polyurethane.

[0076] The polyether polyurethane-rich stream has a higher polyether polyurethane to nylon 6 weight ratio than the material containing both nylon 6 and polyether polyurethane.

[0077] Preferably, the polyether polyurethane-rich feed stream has a polyether polyurethane to nylon 6 weight ratio that is at least twice, more preferably at least three times, that of the material comprising nylon 6 and polyether polyurethane.

[0078] Loading the separated section (step b.1): In the separation section [B], organic solvents and materials including nylon 6 and polyether polyurethane are loaded into the dissolution section [α].

[0079] Loading materials comprising nylon 6 and polyether polyurethane into the dissolution section [α] is loading materials comprising nylon 6 and polyether polyurethane that have optionally been pretreated in the pretreatment section [A] as mentioned in step a) of the present invention.

[0080] The organic solvent incorporated into the dissolving section [α] can be any organic solvent in which the polyether polyurethane can be dissolved, specifically at temperatures below 100°C and specifically in less than 24 hours or less than 6 hours. The organic solvent used herein can also refer to mixtures of organic solvents or liquid compositions comprising greater than 60 vol.-%, preferably greater than 70 vol.-%, 80 vol.-%, or 90 vol.-%, wherein the mixture is capable of dissolving the polyether polyurethane at temperatures below 100°C, preferably at temperatures ranging from 0°C to 100°C, more preferably at temperatures ranging from 10°C to 90°C, even more preferably at temperatures ranging from 10°C to 80°C, and most preferably at temperatures ranging from 20°C to 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 materials comprising nylon 6 and polyether polyurethane or mixtures of nylon 6 and polyether polyurethane. According to preferred embodiments, the organic solvent includes or is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidones such as N-methyl-2-pyrrolidone (NMP, C5H9NO), tetrahydrofuran (THF; also known as oxopentane, C4H8O), 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 loaded into the dissolution section [α] can be a separated organic solvent (i.e., a recycled organic solvent or an organic solvent from within the process that has been 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 incorporated into the dissolving section [α] preferably has a low water content. Even more preferably, the organic solvent is dried before being incorporated into the dissolving section [α] (preferably by distillation or by applying a desiccant such as inorganic salts like Na₂SO₄, zeolite, silica, and alumina). Drying does not necessarily have to be complete. Generally, after drying the organic solvent, it is acceptable for less than 1 wt.%, preferably less than 0.1 wt.%, and more preferably less than 0.02 wt.% of water to remain.

[0083] Dissolving step (step b.2): In the dissolution zone [α], polyether polyurethane is dissolved from the material comprising nylon 6 and polyether polyurethane in the organic solvent introduced into the dissolution zone [α] in step b.1). This yields a polyether polyurethane-rich stream comprising the organic solvent and dissolved polyether polyurethane, and a stream comprising undissolved nylon 6, discharged from the dissolution zone [α].

[0084] Preferably, the dissolution of the polyether polyurethane is completed within 0.1 to 24 hours, more preferably within 0.5 to 6 hours. "Completion" means that the dissolution has reached a plateau, and no substantial further dissolution occurs at subsequent time points.

[0085] The temperature in the dissolution zone [α] can vary, and is 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 zone [α] can be in the range of 0°C to 100°C, more preferably 10°C to 90°C, even more preferably 10°C to 80°C, and most preferably 20°C to 75°C. Experiments have shown that these temperature ranges cause polyether polyurethanes to dissolve particularly completely while maintaining their integrity in organic solvents. This has the advantage that polyether polyurethanes present in materials including nylon 6 and polyether polyurethanes can dissolve while maintaining their integrity.

[0086] The amount of solvent in the dissolution zone [α] (expressed by weight, specifically in tons) can vary and can be 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 in the dissolution zone [α] (expressed by weight, specifically in tons). A small amount of solvent reduces the dissolution rate of the polyether polyurethane and may result in too little or incomplete dissolution. A large amount of solvent gives the polyether polyurethane a high dissolution rate; however, recycling used organic solvents requires more energy and is more costly. Technicians can determine the optimal ratio of solvent to polyether polyurethane or materials including nylon 6 and polyether polyurethane through routine testing.

[0087] The dissolving section [α] may include one or more dissolving containers operating in series or in parallel. Preferably, these containers are equipped with additional devices to enhance friction, such as stirrers, mixers, or agitators, which reduces dissolving time.

[0088] Precipitation step (step b.3): The polyether polyurethane-rich stream, which includes organic solvent and dissolved polyether polyurethane, and a second solvent, discharged from the dissolution section [α], is loaded into the precipitation section [γ], where they are mixed, causing the polyether polyurethane to precipitate from the mixture including the organic solvent and the second solvent.

[0089] Preferably, the second solvent is a non-solvent (also called an anti-solvent) for the polyether polyurethane, i.e., a liquid in which the polyether polyurethane is insoluble and causes precipitation. The addition of this second solvent reduces the solubility of the polyether polyurethane dissolved in the solution, leading to the precipitation of the polyether polyurethane particles. 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 a portion of, or the mixture of, the organic solvent and the third solvent obtained from the washing section [β] in step b.7). As used herein, “a portion of or the mixture” is defined as any fraction between 1 and 100 wt.%, preferably between 10 and 100 wt.%, more preferably between 25 and 100 wt.%, and even more preferably between 75 and 100 wt.%. This has the advantage of requiring less fresh solvent for the 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 method.

[0091] The temperature in the precipitation zone [γ] 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 the dissolved polyether polyurethane to precipitate particularly completely.

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

[0093] A small amount of secondary solvent reduces the precipitation of polyether polyurethane. A large amount of secondary solvent complicates the recovery of precipitated polyether polyurethane and organic solvents. A large amount of secondary solvent also requires more energy and cost to recover the precipitated polyether polyurethane and organic solvents. Technicians will be able to find the optimal ratio of secondary solvent to polyether polyurethane-rich feed for the specific solvent and equipment used by conducting simple precipitation tests.

[0094] Polyether polyurethane recycling steps (step b.4): In the polyether polyurethane recovery step, the precipitated polyether polyurethane is recovered from a mixture comprising an organic solvent and a second solvent, and the precipitated polyether polyurethane is discharged from the precipitation section [γ]. Several suitable methods exist for recovering polyether polyurethane precipitates, including but not limited to filtration, centrifugation, and decantation. Preferably, filtration is used to recover polyether polyurethane. For filtration, the solution containing the precipitate is loaded into a filter so that the precipitate is expected to remain on the filter as the liquid passes through it. When centrifugation is used as a recovery method, the solution containing the precipitate is rapidly rotated, causing the solid precipitate to settle (assuming the density of the solid precipitate is higher than the density of the liquid). The compacted precipitate can also be obtained by pouring out the liquid. In decantation, the liquid layer is poured out or aspirated from the precipitate.

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

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

[0097] Discharging the solvent mixture (step b.5): The mixture comprising the organic solvent and a second solvent from which the precipitated polyether polyurethane has been recovered in step b.4) is discharged from the precipitation section [γ] and the mixture is then loaded into the solvent distillation section [δ]. Optionally, the solvent generated from washing the recovered precipitate is also discharged from the precipitation section [γ] and loaded into the solvent distillation section [δ].

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

[0099] Preferably, the third solvent has good solubility in organic solvents but poor 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 have the same composition, which has the advantage of simplifying the method by reducing the amount of different components required.

[0100] The amount of the third solvent incorporated into the washing section [β] can vary and can range from 10 wt.% to 1000 wt.%, preferably 20 wt.% to 500 wt.%, more preferably 50 wt.% to 400 wt.%, and most preferably 100 wt.% to 250 wt.%, compared to the feed stream containing undissolved nylon 6 incorporated into the washing section [β].

[0101] Washing steps (step b.7): In the washing section [β], the stream containing undissolved nylon 6 is washed with a third solvent, resulting in a stream rich in nylon 6 and a mixture comprising the 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 result in particularly complete recovery of organic solvents from the feed stream, which includes undissolved nylon 6.

[0103] Technicians can determine the amount of a third solvent and the optimal temperature required for effective washing of a stream containing undissolved nylon 6 through routine experiments.

[0104] Washing of the stream containing undissolved nylon 6 with a third solvent can be carried out in a variety of apparatuses, all of which are known to those skilled in the art.

[0105] Discharge of the nylon 6-rich flow (step b.8): The nylon 6-rich stream is discharged from the washing section [β]. This nylon 6-rich stream comprises a third solvent and optionally an organic solvent. Preferably, the organic solvent content of the nylon 6-rich stream is less than 25 wt.%, more preferably less than 10 wt.%, more preferably less than 2 wt.%, and most preferably less than 0.2 wt.%, relative to the total weight of the nylon 6-rich stream. Optionally, the nylon 6-rich stream is dried before discharge.

[0106] Typically, the obtained nylon 6-rich stream comprises a nylon 6-rich solid material. In this regard, the term "solid" 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 or transported to depolymerization sections at different locations for this purpose prior to the loading of the depolymerization section in step c.1) of the method of the present invention.

[0107] Preferably, the nylon 6 content in the resulting nylon 6-rich stream from the washing section [β] is at least 85% by weight based on dry weight, more preferably at least 90% by weight, and most preferably at least 95% by weight.

[0108] Preferably, the polyamide 6 content in the resulting polyamide 6-rich stream from the separation section [B] is at least 85% by weight, more preferably at least 90% by weight, even more preferably at least 95% by weight, and most preferably at least 98% by weight, based on dry weight.

[0109] The determination of nylon 6 content is a routine activity and can be performed by a variety of methods, all of which are known to those skilled in the art. Preferably, the polyamide 6 content in a nylon 6-rich stream is determined by thermogravimetric analysis (TGA) and / or by differential scanning calorimetry (DSC) (e.g., by method ISO 11357-3).

[0110] As used herein, “based on dry weight” means the content of an ingredient as a percentage of the total dry weight of the composition, where “dry weight” is the weight of the substance after water and / or other liquids have been removed from the substance by drying until the weight is constant.

[0111] Discharge of the mixture including the organic solvent and the third solvent (step b.9): The mixture comprising the organic solvent and the third solvent is discharged from the washing section [β]. Typically, the organic solvent and / or the third solvent are valuable compounds, and their recovery and reuse offer significant economic and environmental advantages. In a preferred embodiment of the invention, the mixture comprising the organic solvent and the third solvent is therefore loaded into the solvent distillation section [δ]. Optionally, the mixture comprising the organic solvent and the third solvent is first loaded into another section and used in said other section, particularly as a second solvent in the precipitation section, and then loaded into the solvent distillation section [δ]. In a preferred embodiment, the mixture comprising the organic solvent and the third solvent is first partially or completely loaded into the precipitation section [γ] and used as a second solvent. This provides particularly full use and reuse of the solvent in the method of the invention.

[0112] Solvent separation step (step b.10): In the solvent distillation section [δ], the different solvents used in the method of the present invention are separated and recovered again. This allows them to be advantageously reused. Specifically, in the solvent distillation section [δ], a mixture of organic solvent and a second solvent from which the precipitated polyether polyurethane has been recovered is separated by distillation, as well as a mixture of organic solvent and a third solvent discharged from the washing section [β]. Thereafter, the mixture of organic solvent and a second solvent from which the precipitated polyether polyurethane has been recovered, and the mixture of organic solvent and a third solvent discharged from the washing section [β], are referred to as the combined feed of the solvent distillation section [δ].

[0113] The separated organic solvent, second solvent, and third solvent, along with residues, are discharged from the solvent distillation section [δ]. In a preferred embodiment, the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is loaded into the dissolving section [α] in step b.1). In another preferred embodiment, the second and third solvents discharged from the solvent distillation section [δ] are optionally reused in the precipitation section [γ] and / or washing section [β] after separation from each other.

[0114] Separation by distillation is used to separate liquids from non-volatile solids and to separate liquids with different boiling points. Distillation is a routine activity and can be carried out in a variety of apparatuses, all of which are known to those skilled in the art.

[0115] The mixture of precipitated polyether polyurethane, from which the organic solvent and the second solvent have been recovered, optionally also contains (dissolved) polyether polyurethane, (dissolved) nylon 6, colorant, additive, organic solvent and / or degradation products of the second solvent. The mixture of organic solvent and a third solvent optionally also contains (dissolved) polyether polyurethane, (dissolved) nylon 6, and degradation products of the organic solvent and / or the third solvent.

[0116] Generally, separating three different liquids by distillation is more complex, requires more energy, and necessitates more equipment compared to separating two different liquids by distillation. Therefore, the ability to limit the number of different solvents in the method of the present invention to two by using the same solvents as the second and third solvents is particularly advantageous. In a preferred embodiment of the 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. Using water as a solvent is advantageous because it is inexpensive, readily available, non-explosive, and environmentally friendly.

[0117] 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 such as N-methyl-2-pyrrolidone (NMP, C5H9NO), and tetrahydrofuran (THF; also known as oxopentane, C4H8O). In a more preferred embodiment, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2).

[0118] 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, C5H9NO), tetrahydrofuran (THF; also known as oxopentane, C4H8O), and the second and third solvents are both water. In a more preferred embodiment of the invention, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), and the second and third solvents are both water. N,N-dimethylacetamide (DMAc) has a boiling point of approximately 165°C at atmospheric pressure. Water has a boiling point of 100°C at atmospheric pressure.

[0119] When the combined feed of the solvent distillation section [δ] contains only DMAc and water, the volatility difference between the two is significant, and separation by distillation is found to be relatively easy. In such cases, continuous separation of the combined feed of the solvent distillation section [δ] can be achieved in a single distillation column operating in continuous mode. In such a distillation column, water is distilled as a gas from the top of the column, and DMAc is discharged as a liquid from the bottom. When the combined feed of the solvent distillation section [δ] contains non-volatile compounds (such as polyether polyurethane and nylon 6) in addition to DMAc and water, the solvent distillation layout required for continuous separation of DMAc, water, and non-volatile compounds is more extensive. A direct layout for this type of separation by distillation involves two distillation columns operating in series. A feed consisting of DMAc, water, and non-volatile compounds (in liquid form) is charged into the first distillation column. Water is distilled as a gas from the top of the first distillation column, and a mixture of DMAc and non-volatile compounds (in liquid or slurry form) is discharged from the bottom of the first distillation column. The bottom stream from the first column is charged into the second distillation column. DMAc is distilled as a gas from the top of the second distillation column, while non-volatile compounds are discharged from the bottom. In practice, a mixture of DMAc and non-volatile compounds can be discharged from the bottom of the second distillation column to reduce the risk of blockage. A squeeze (column) unit can be added to recover DMAc from the bottom stream of the second distillation column to improve the overall recovery rate of DMAc.

[0120] As an alternative to separation via distillation of two distillation columns operating in series, a single distillation column with a side stream can be selected, operating in continuous mode. In such a column, water is distilled as a gas from the top of the column, DMAc is discharged as a liquid side stream, and the non-volatile compounds are discharged from the bottom of the second distillation column. Optionally, the bottom stream is fed into a squeeze (column) unit.

[0121] As an alternative to charging the feed into the distillation column in a liquid state, the first distillation column can be charged in a gaseous state. In this case, the combined feed of the solvent distillation section [δ], except for volatile compounds, is evaporated in the feed evaporator before being charged into the first distillation column. Non-volatile compounds are discharged from the feed evaporator. The advantage of this alternative is that less fouling occurs in the distillation column. Optionally, a squeeze column can be combined with the feed evaporator. Typically, process streams including DMAc contain a certain amount of acetic acid and dimethylamine due to, for example, the degradation of DMAc. The same is true for the combined feed of the solvent distillation section [δ].

[0122] Pure dimethylamine has a boiling point of approximately 7°C at atmospheric pressure. Dimethylamine is highly soluble in water.

[0123] Pure acetic acid has a boiling point of approximately 118°C at atmospheric pressure.

[0124] However, acetic acid and DMAc form a high-boiling azeotrope (at atmospheric pressure, composed of approximately 21 wt.% acetic acid and approximately 79 wt.% N,N-dimethylacetamide) with a boiling point of approximately 171 °C at atmospheric pressure, which is only slightly higher than the boiling point of N,N-dimethylacetamide at atmospheric pressure. A high-boiling azeotrope (or constant-boiling-point mixture) is a mixture of two liquids whose boiling points are higher than the boiling points of the two individual liquids (in their pure forms).

[0125] If the pressure decreases, the azeotropic point of the mixture of acetic acid and N,N-dimethylacetamide shifts to the higher concentration of the lower boiling point component (which is acetic acid).

[0126] At 6.7 kPa, a high-boiling azeotrope is formed with a composition of about 30 wt.% acetic acid and about 70 wt.% N,N-dimethylacetamide, the boiling point of which is about 93°C.

[0127] At 1.3 kPa, a high-boiling azeotrope is formed, consisting of approximately 42 wt.% acetic acid and approximately 58 wt.% N,N-dimethylacetamide, with a boiling point of approximately 75°C. Preferably, the degradation products are removed from the solvent reused in the method. This has the advantage of preventing the accumulation of degradation products. In a preferred embodiment, the degradation products of the organic solvent are removed before the separated organic solvent, discharged from the solvent distillation section [δ] in step b.1), is loaded into the dissolution section [α] in step b.1). However, due to the formation of the highest azeotrope, DMAc and acetic acid cannot be separated by simple distillation.

[0128] Several techniques for destroying azeotropes during distillation are known to those skilled in the art.

[0129] One set of techniques is based on adding another compound that alters molecular interactions and eliminates the highest azeotrope of N,N-dimethylacetamide-acetic acid. Chlorobenzene, toluene, ethylbenzene, and xylene are examples of compounds that disrupt this highest azeotrope. Adding one of these compounds to a mixture of N,N-dimethylacetamide and acetic acid results in the formation of a new azeotrope between the compound and acetic acid, which can be removed by evaporation. Subsequently, the resulting mixture of the compound and acetic acid is separated.

[0130] Another set of techniques, often referred to as pressure swing distillation, is based on the fact that the highest azeotrope is pressure-dependent. In such systems, two distillation columns operate at different pressure levels. A feed mixture of N,N-dimethylacetamide and acetic acid is charged into a first distillation column operating at a given pressure. The bottom stream from this first distillation column is then fed into a second distillation column. The bottom stream from the second distillation column is then fed into the first distillation column. High-purity N,N-dimethylacetamide and acetic acid are obtained as the top products of the distillation columns. A major advantage of pressure swing distillation is the absence of foreign compounds.

[0131] Finally, there is a set of techniques for converting acetic acid into another component (e.g., forming a salt by adding a caustic alkali) or selectively adsorbing acetic acid from a mixture of N,N-dimethylacetamide and acetic acid.

[0132] When acetic acid and dimethylamine are also present in the combined feed of the solvent distillation section [δ] in addition to N,N-dimethylacetamide (DMAc), water, and non-volatile compounds (such as polyether polyurethane and polyamide 6), the solvent distillation layout required for continuous separation into DMAc, water, non-volatile compounds, acetic acid, and dimethylamine can be further extended.

[0133] The optimal layout depends largely on the combination of feed composition and volumetric flow rate in the solvent distillation section [δ]. The layout can contain combinations of the various separation solutions described above.

[0134] If the acetic acid fraction in the combined feed of the solvent distillation section [δ] is relatively low, the solvent distillation section [δ] may comprise two distillation columns operating in series, followed by a neutralizer evaporation step. A feed consisting of DMAc, water, a non-volatile compound, acetic acid, and dimethylamine is charged into the first distillation column. Water and dimethylamine are distilled off as gases from the top of the first distillation column, and a mixture of DMAc, acetic acid, and the non-volatile compound (in liquid or slurry form) is discharged 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. In a dimethylamine stripping unit, dimethylamine can be removed from this liquid phase to obtain (virtually) dimethylamine-free water. The bottom stream from the first column is charged into the second distillation column. DMAc is distilled off as gases from the top of the second distillation column, and a mixture of DMAc, acetic acid, and the non-volatile compound is discharged from the bottom of the second distillation column. The bottom stream from the second column is charged into the neutralizer evaporation unit. An alkali, such as an aqueous solution of NaOH, is introduced into the neutralizer evaporation unit to neutralize acetic acid. Gaseous DMAc is discharged from the neutralizer evaporation unit due to evaporation. Water, if present, will also evaporate. The mixture of neutralized acetic acid (containing acetic acid salts) and non-volatile compounds is discharged from the bottom of the neutralizer evaporation unit. In practice, the mixture of DMAc, neutralized acetic acid (containing acetic acid salts), and non-volatile compounds can be discharged from the bottom of the neutralizer evaporation unit to reduce the risk of blockage.

[0135] The DMAc recovered through distillation is discharged from the solvent distillation section [δ] and loaded into the dissolution section [α].

[0136] Preferably, the water recovered in the solvent distillation section [δ] is optionally reused in the method of the present invention after the removal of dimethylamine. Preferably, water is used as a second solvent and / or as a third solvent.

[0137] Loading step c.1) In step c.1) of the present invention, the resulting nylon 6-rich stream is fed into the depolymerization section [C], wherein the nylon 6 content in the nylon 6-rich stream is at least 85% by weight, preferably at least 90% by weight, based on dry weight. This has the advantage that the nylon 6-rich stream fed into the depolymerization section [C] is less contaminated by foreign materials, which improves the yield and purity of ε-caprolactam produced in the plant of the present invention configured to carry out the method of the present invention. Other advantages are that less depolymerizing agent is required in the depolymerization section [C], less waste is generated, and less energy is required. Finally, the nylon 6-rich stream reduces operational problems such as equipment scaling and clogging. The depolymerization section [C] comprises one or more depolymerization reactors operating in series and / or parallel.

[0138] Optionally, the nylon 6-rich stream is mechanically compressed to a smaller volume before being fed into the depolymerization section [C]. This has the advantage of requiring a smaller volume for intermediate storage and transportation, and also facilitates delivery to the depolymerization section [C].

[0139] Optionally, the nylon 6-rich stream is compressed into denser granules before being fed into the depolymerization section [C]. This can be achieved, for example, by mechanical compaction or by extruding the molten material using a die, followed by cooling and cutting to size. Compressing the nylon 6-rich stream into granules has the advantage of increased bulk density, which reduces intermediate storage and transportation costs. In addition to increased density, granulation provides other benefits such as uniform shape and structure, which facilitates (automatic) feeding into the depolymerization section [C].

[0140] Optionally, a nylon 6-rich stream is fed into a furnace (e.g., an extruder). In the furnace, the nylon 6-rich stream is melted. Preferably, the resulting polymer melt is filtered. This has the advantage of removing solid impurities. The molten and optionally filtered polymer melt is cooled and then fed into a pellet mill to obtain pellets. These pellets are then loaded into a depolymerization section [C].

[0141] The size and shape of the pellets (also called granules) can be selected within a wide range. Generally, the pellets are cylindrical (derived from fine strands cut into pieces). However, other shapes, such as (imperfect) spheres, are also possible. The size of the pellets can be selected within a wide range. The diameter of the pellets can range from 1 to 10 mm, preferably 2 to 5 mm, and more preferably 3 to 5 mm. In a preferred embodiment, the length of the pellets ranges from 1 to 50 mm, preferably 2 to 25 mm, and more preferably 5 to 15 mm.

[0142] Optionally, the nylon 6-rich stream discharged from the separation section [B] is dried before being fed into the depolymerization section [C]. This has the advantage of introducing less or no solvent into the depolymerization section [C]. Solvents introduced into the depolymerization section [C] can negatively impact the depolymerization process (e.g., leading to a lower depolymerization rate, higher catalyst consumption, higher energy consumption, and the vapor stream obtained in the depolymerization section [C] may contain more impurities, including ε-caprolactam and water).

[0143] Preferably, the nylon 6-rich stream is fed into the depolymerization reactor in solid or melt form. Preferably, the nylon 6-rich stream is fed in melt form. Melt feeding can be achieved using an extruder, a gear pump, or other means known to those skilled in the art.

[0144] Feeding a nylon 6-rich stream into the depolymerization reactor can be achieved by continuously or intermittently feeding the nylon 6-rich stream.

[0145] Depolymerization step c.2) In the depolymerization zone [C], the nylon 6-rich stream is depolymerized to form ε-caprolactam. The formed ε-caprolactam is discharged from the depolymerization zone [C] as a stream containing ε-caprolactam.

[0146] The depolymerization of the nylon 6-rich stream is achieved by raising the temperature of the nylon 6-rich stream in the depolymerization zone [C] to a temperature of at least 180°C but not exceeding 400°C. The preferred temperature range for the depolymerization reaction is 200°C to 350°C, more preferably 220°C to 340°C, and most preferably 240°C to 325°C. Experiments have shown that depolymerization is particularly effective within these temperature ranges, and side reactions and impurity reactions of polyamide 6 and ε-caprolactam occur less frequently.

[0147] Typically, the formation rate of ε-caprolactam increases with increasing temperature. Temperatures below 400°C are preferred because above 400°C, side reactions and impurity reactions of nylon 6 occur more frequently, leading to the formation of a more diverse range of impurities. Some of these impurities will eventually enter the product stream containing ε-caprolactam discharged from the depolymerization reactor section [C]. In a preferred embodiment of the invention, the depolymerization of the nylon 6-rich stream is carried out at a temperature ranging from 220°C to 340°C or from 240°C to 325°C. This temperature range allows for the production of particularly pure ε-caprolactam.

[0148] The pressure in the depolymerization zone [C] can be varied and can be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, and most preferably 50 kPa to 1 MPa. This pressure range allows for the production of particularly pure ε-caprolactam.

[0149] The depolymerization of the nylon 6-rich stream can be achieved with or without a solvent. Preferably, the depolymerization of the nylon 6-rich stream is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, particularly superheated steam.

[0150] Preferably, the depolymerization will be completed within 0.1 hours to 24 hours, more preferably within 0.5 hours to 6 hours.

[0151] Feeding water as steam into the depolymerization reactor allows for the optional production of a steam stream comprising ε-caprolactam and water without further heating. The weight ratio of ε-caprolactam to water in this steam stream can be adjusted by varying the amount of steam fed into the nylon 6-rich stream in the depolymerization section [C]. In a preferred embodiment, depolymerization in step c.2) is carried out in the presence of water, thereby producing a steam stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10, and most preferably 1:3 to 1:8. Within these ranges, a particularly cost-effective method can be employed.

[0152] Preferably, the ε-caprolactam in the steam stream comprising ε-caprolactam and water has a partial pressure of 0.1 kPa to 1 MPa, more preferably 0.3 kPa to 0.5 MPa, and most preferably 1 kPa to 0.1 MPa.

[0153] During the depolymerization reaction, decomposition products may be formed, which include oligomers of ε-caprolactam. Additionally, the feed stream from materials comprising multi-component materials including nylon 6 may contain other components, i.e., impurities, such as non-nylon 6 compounds that remain stable, react, or decompose under depolymerization conditions, and residues of solvents used in the pretreatment. Therefore, when water is used as the solvent, the vapor stream removed from the depolymerization section [C] includes not only water and ε-caprolactam, but also impurities.

[0154] Preferably, superheated steam with a temperature between 100°C and 600°C is introduced into the depolymerization reactor. Preferably, the temperature of the superheated steam introduced into the depolymerization reactor is at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam introduced into the depolymerization reactor is high enough that no other heat input is required for the depolymerization reaction and evaporation of the ε-caprolactam formed. In another preferred embodiment, superheated steam with a temperature range of 220°C to 575°C is introduced into the depolymerization section [C]. In an even more preferred embodiment, superheated steam with a temperature range of 275°C to 500°C is introduced into the depolymerization section [C]. In another preferred embodiment, a portion of the heat input required for the depolymerization reaction and evaporation of the ε-caprolactam formed is introduced through the wall of the depolymerization reactor.

[0155] Typically, 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 a nylon 6-rich stream and optional solvents, catalysts, additional reagents, and / or depolymerizing agents. Therefore, without any additional measures, an accumulation of material (often referred to as 'residual material') will occur in the depolymerization section [C]. Preferably, another stream is discharged from the depolymerization section [C]. This has the advantage of reducing or avoiding material accumulation in the depolymerization section [C]. The additional stream may include impurities present in the nylon 6-rich stream, undepolymerized nylon 6, unevaporated ε-caprolactam, catalysts, and compounds formed under depolymerization conditions, such as monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate when phosphoric acid is used as the depolymerization catalyst. In a preferred embodiment, a stream comprising monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate is discharged from the depolymerization section [C]. Even more preferably, the material stream discharged intermittently or continuously from the depolymerization section [C] comprises 0.01 to 50% by weight, preferably 0.1 to 25% by weight, more preferably 0.5 to 10% by weight, and most preferably 0.5 to 5% by weight of monoammonium phosphate, diammonium phosphate, and / or triammonium phosphate.

[0156] The depolymerization of a nylon 6-rich stream in the presence of steam can be carried out in the presence of another depolymerizing agent, such as ammonia. The concentration of ammonia in the depolymerization zone [C] can vary. Therefore, in the presence of ammonia in the depolymerization zone [C], the steam stream removed from the depolymerization zone [C] includes not only ε-caprolactam and impurities, but also ammonia.

[0157] Most preferably, depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Brønsted) acid or base. Acid catalysts can specifically be selected from the group consisting of: phosphoric acid, p-toluenesulfonic acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids (including xylenesulfonic acid, 4-sulfoisophthalic acid, and other sulfonated aromatic hydrocarbons), solid acids, salts of the above acids, Al₂O₃, and SiO₂, and combinations thereof. Base catalysts can, for example, be selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases, and solid bases, and combinations thereof. Preferably, phosphoric acid, boric acid, organic acids, alkali metal hydroxides, and alkali metal salts are used as catalysts. More preferably, phosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate are used as catalysts. Still more preferably, phosphoric acid, p-toluenesulfonic acid, boric acid, and sodium hydroxide are used as catalysts. In a particularly preferred embodiment, orthophosphoric acid is used as the depolymerization catalyst, while in another embodiment, p-toluenesulfonic acid is used.

[0158] However, in another preferred embodiment, no catalyst is used for the depolymerization of the nylon 6-rich stream. This has the advantage of lower cost (for the disposal of catalysts and 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.

[0159] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction begins at a lower temperature and can be carried out under atmospheric conditions. The appropriate concentration of the catalyst used to depolymerize nylon 6 to ε-caprolactam is known to those skilled in the art and can be readily determined through routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. Conversely, if the concentration of the catalyst used is too high, the reaction is rapid, but side reactions also increase. Furthermore, the cost of the catalyst increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the nylon 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1 to 50% by weight. The optimal catalyst concentration depends on the type of catalyst used for the depolymerization of nylon 6. For the catalyst orthophosphoric acid, the preferred content is 0.1 to 25% by weight, and more preferably 1 to 20% by weight. For the catalyst p-toluenesulfonic acid, the preferred content is 10 to 35% by weight, and more preferably 15 to 30% by weight.

[0160] The depolymerization of nylon 6 can be carried out in batch, semi-continuous, or continuous modes, all of which are known to those skilled in the art. As used herein, the terms “batch,” “semi-continuous,” and “continuous” refer to the mode in which the feedstock containing nylon 6 (i.e., a nylon 6-rich stream) and optionally a catalyst are loaded into the depolymerization reactor, and the mode in which residual material is discharged from the depolymerization reactor.

[0161] In a preferred embodiment, the depolymerization of nylon 6 is carried out in a batch mode. In the batch mode, the feedstock, i.e., a material derived from a multi-component material including nylon 6 and optionally a catalyst, is initially charged into the depolymerization reactor. Subsequently, superheated steam is charged into the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Next, the charging of superheated steam into the depolymerization reactor is interrupted. After optionally removing residual material from the depolymerization reactor, a new cycle is started by charging the feedstock (and optionally the catalyst) into the depolymerization reactor. In a preferred embodiment, residual material is not removed between each cycle.

[0162] In a particularly advantageous embodiment, the depolymerization of nylon 6 is carried out in a continuous mode. In the continuous mode, a feedstock containing nylon 6 (and optionally a catalyst) is continuously charged into the depolymerization reactor. Simultaneously, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged into the depolymerization reactor. Additionally, residual material is continuously discharged from the depolymerization reactor. Preferably, the nylon 6-rich stream is charged in the form of a melt. Preferably, the catalyst is charged in the form of a melt, slurry, or solution.

[0163] In another preferred embodiment, the depolymerization of nylon 6 is carried out in a semi-continuous mode. In the semi-continuous mode, a feedstock containing nylon 6 (and optionally a catalyst) is intermittently fed into the depolymerization reactor while superheated steam is continuously fed into the reactor, and ε-caprolactam is continuously discharged from the reactor as a steam stream comprising ε-caprolactam and water. In the semi-continuous mode of nylon 6 depolymerization, residual material is intermittently discharged from the depolymerization reactor.

[0164] Recycling step c.3) In the recovery section [D], ε-caprolactam is recovered from the feed stream containing ε-caprolactam discharged from the self-depolymerization section [C]. This feed stream contains ε-caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the feed stream containing ε-caprolactam.

[0165] Preferably, without solvent being introduced into the depolymerization zone [C], the ε-caprolactam obtained by condensation is dissolved in water to obtain an ε-caprolactam-rich phase. This ε-caprolactam-rich phase also includes impurities.

[0166] Preferably, when water is used as a solvent in the depolymerization section [C], the feed stream containing ε-caprolactam discharged from the depolymerization section [C] comprises ε-caprolactam, water, and impurities. Water may be introduced in liquid or vapor form. Preferably, water is introduced in vapor form. ε-caprolactam can be separated from the feed stream containing ε-caprolactam discharged from the depolymerization section [C] by conveying the vapor or gaseous feed stream from the depolymerization reactor (preferably the top) to a condenser (preferably partially) to obtain a condensate containing ε-caprolactam. Preferably, ε-caprolactam is separated from the remaining components of the vapor feed stream by conveying the product feed stream from the depolymerization reactor (preferably the top) to a distillation column, from which an aqueous-rich phase as the top product and an ε-caprolactam-rich phase as the bottom product are obtained.

[0167] The ε-caprolactam recovered in recovery section [D] is crude because it contains impurities such as nylon 6 decomposition products or other impurities arising from non-nylon 6 components (decomposition products) of materials derived from the nylon 6-rich feed stream loaded into depolymerization section [C]. The crude ε-caprolactam recovered in step c.3) comprises water and ε-caprolactam, preferably an aqueous solution comprising ε-caprolactam. Therefore, the crude ε-caprolactam recovered in recovery section [D] requires additional purification to produce high-purity ε-caprolactam. Thus, “crude” as used herein can be defined as having lower purity than purified ε-caprolactam obtained as a product of the method of the present invention, i.e., containing more impurities.

[0168] Preferably, the crude ε-caprolactam comprises 6 to 95% by weight, more preferably 20 to 90% by weight, and most preferably 35 to 80% by weight of ε-caprolactam. The remainder is primarily water.

[0169] Purification step 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 high-purity ε-caprolactam.

[0170] Optionally, the crude ε-caprolactam is filtered before being loaded into the purification section [E]. Filtration ensures the removal of undissolved impurities that could otherwise hinder further purification.

[0171] Optionally, the oil is separated from the crude ε-caprolactam before loading into the purification section [D]. Oil separation ensures the removal of impurities that might otherwise hinder further purification.

[0172] Preferably, the purified ε-caprolactam is obtained by purifying the crude ε-caprolactam obtained in the recovery section [D] by crystallizing the ε-caprolactam from a solution containing ε-caprolactam and impurities in the purification section [E] at a temperature of 10°C to 95°C, more preferably at a temperature of 20°C to 85°C.

[0173] The method of the present invention is carried out in a factory, wherein the factory includes a purification section [E], wherein purification includes the following steps: c.4)(iv) Purified ε-caprolactam is obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C.

[0174] Preferably, the method of the present invention is carried out in a factory, wherein the factory includes a purification section [E], wherein purification further includes the following steps: c.4)(i) Extract crude ε-caprolactam with an organic solvent to obtain an aqueous phase and an organic phase, wherein the organic phase comprises an organic solvent, ε-caprolactam and impurities; c.4)(ii) Optionally, the solvent is switched by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, wherein the solvent switching step (ii) is selected from a process based on water back-extraction and a process based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; and c.4)(iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation.

[0175] Preferably, after this: c.4)(iv) Purified ε-caprolactam is obtained by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C.

[0176] High-purity ε-caprolactam is obtained from crude ε-caprolactam by first extracting it with an organic solvent in step c.4)(i), thereby obtaining an aqueous phase and an organic phase comprising the organic solvent, ε-caprolactam, and impurities. The organic solvent used for extracting crude ε-caprolactam is preferably an aromatic hydrocarbon, a halogenated hydrocarbon, and / or a C4-C4 hydrocarbon. 10 Aliphatic or C4-C 10 Cyclic aliphatic alcohols. Optionally, the organic solvent used for extracting crude ε-caprolactam is preferably a mixed extractant, i.e., it may consist of one or more organic solvents and optionally an additional diluent. Preferably, the one or more organic solvents are independently selected from aromatic hydrocarbons, halogenated hydrocarbons, and / or C4-C4 hydrocarbons. 10 Aliphatic or cycloaliphatic alcohols, and C5-C8 alkanes or C5-C8 cycloalkanes. Particularly good purification results can be obtained if the organic solvent used for extraction of crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC, methyl isobutyl methanol), 1-octanol, 2-ethylhexanol, and mixtures thereof. More preferably, the organic solvent used for extraction of crude ε-caprolactam is selected from the group consisting of benzene, toluene, cyclohexane, alcohols, and mixtures thereof. Still more preferably, the organic solvent used for extraction of crude ε-caprolactam is selected from the group consisting of toluene, cyclohexane, 1-octanol, 2-ethylhexanol, and mixtures thereof. Preferably, the weight ratio of the organic solvent to ε-caprolactam is 0.01:1 to 50:1, more preferably 0.05:1 to 20:1, more preferably 0.1:1 to 10:1, and most preferably 0.1:1 to 5:1.

[0177] In another embodiment, the extraction with an organic solvent in step c.4)(i) is carried out in a countercurrent extraction column, thereby introducing the crude ε-caprolactam to be purified at the top of the column and introducing the organic solvent at the bottom of the column. The extraction produces an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam, and impurities.

[0178] Optionally, the organic phase, comprising the organic solvent, ε-caprolactam, and impurities, is washed with water or an alkaline aqueous solution before proceeding to step c.4) (ii). If washing with an alkaline aqueous solution, the alkaline solution is preferably an aqueous solution comprising an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably comprises 0.5 wt.% to 2.0 wt.% sodium hydroxide or potassium hydroxide.

[0179] Technicians can determine the amount of water or alkaline aqueous solution required for efficient washing of an organic phase, including organic solvents, ε-caprolactam, and impurities, through routine experiments. Preferably, this amount is between 0.1 vol.% and 5 vol.% relative to the amount of organic solvent in the organic phase to be washed.

[0180] In another preferred embodiment, washing the organic phase comprising organic solvent, ε-caprolactam, and impurities with water or an alkaline aqueous solution is carried out in a countercurrent washing column, whereby the organic phase comprising organic solvent, ε-caprolactam, and impurities is introduced at the bottom of the column, and water or an alkaline aqueous solution is introduced at the top of the column. The washing produces a washed organic phase comprising organic solvent, ε-caprolactam, and impurities, and a phase comprising residues. The phase comprising residues then contains water, impurities, and ε-caprolactam.

[0181] In a preferred embodiment, the organic phase, optionally washed with water, comprising the organic solvent, ε-caprolactam, and impurities, is then extracted to obtain an ε-caprolactam-aqueous phase. Preferably, the ε-caprolactam-aqueous phase is then stripped and / or distilled to remove residual solvent. Although the amount of water used to recover ε-caprolactam can vary, the amount of water used is 0.5 to 20 times, preferably 0.75 to 10 times, and more preferably 1 to 5 times, by weight of the recovered ε-caprolactam.

[0182] In another preferred embodiment, extraction with water is carried out in a countercurrent extraction column, with the ε-caprolactam-containing phase to be purified introduced at the bottom of the column and water introduced at the top. The extraction produces an ε-caprolactam-aqueous phase and a solvent phase including impurities. Typically, the solvent phase including impurities is optionally reused after purification (preferably by distillation).

[0183] The ε-caprolactam-aqueous phase, optionally stripped and / or distilled to remove residual solvent, is concentrated by evaporating water to obtain a concentrated ε-caprolactam aqueous phase. The ε-caprolactam content of this concentrated ε-caprolactam aqueous phase is typically between 60 wt.% and 99.9 wt.% relative to the whole phase.

[0184] In another preferred embodiment, the organic solvent, comprising the organic solvent, ε-caprolactam, and impurities, is evaporated from an organic phase, optionally washed rather than extracted with water. Any suitable evaporation vessel, such as a column, can be used. Preferably, evaporation is carried out in the presence of water. More preferably, evaporation is carried out in the form of azeotropic distillation, in which case the organic solvent evaporates as an azeotropic mixture. Evaporation produces an ε-caprolactam product. Preferably, the ε-caprolactam product is an aqueous phase of ε-caprolactam. The ε-caprolactam content of this aqueous phase is typically between 40 wt.% and 99.9 wt.% relative to the entire phase.

[0185] In another preferred embodiment, prior to the distillation removal in step c.4)(iii) of the method of the invention, an oxidant, such as potassium permanganate, sodium permanganate, and / or hydrogen peroxide, is added to the ε-caprolactam-aqueous phase. Most preferably, potassium permanganate is used as the oxidant.

[0186] Preferably, the oxidant is added to the ε-caprolactam-aqueous phase in the form of a solid, slurry, or aqueous solution, so that a diluted aqueous solution is obtained during purification by oxidation. A skilled technician can determine the amount of oxidant required for efficient oxidation of the ε-caprolactam-aqueous phase through routine experiments. The exact amount of oxidant depends largely, particularly on the composition of the polyamide 6-rich feed stream into the depolymerization section in the method of the present invention. Preferably, the amount of oxidant is between 0.01 and 5% by weight relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0187] In the method of the present invention, the temperature used for oxidizing the aqueous solution can be varied. Preferably, prior to the distillation removal in step c.4)(iii), the oxidation of the aqueous solution with an oxidant is carried out at a temperature ranging from 20°C to 85°C, more preferably from 30°C to 80°C, wherein the oxidant is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof, particularly potassium permanganate.

[0188] The duration of oxidation with an oxidant can vary. Preferably, in the method of the present invention, prior to the distillation removal in step c.4)(iii), the ε-caprolactam-aqueous phase is oxidized with an oxidant for 1 minute to 24 hours, more preferably 2 minutes to 6 hours, and most preferably 5 minutes to 2 hours.

[0189] The concentration of ε-caprolactam in the aqueous phase used for oxidation with an oxidant can vary. Preferably, the aqueous solution used for oxidation comprises an ε-caprolactam to water weight ratio of 5:1 to 1:5, more preferably 3:1 to 1:3, and most preferably 2:1 to 1:2. Optionally, the ε-caprolactam to water weight ratio is adjusted before the oxidant is added to the aqueous phase. Preferably, the ε-caprolactam to water weight ratio is adjusted by adding water or by removing water.

[0190] When potassium permanganate and / or sodium permanganate are used as oxidants, solid manganese oxide (IV) (MnO2) particles are formed as reaction products. Technicians can determine the optimal solid-liquid filtration procedure for the efficient removal of solid manganese oxide (IV) particles from the aqueous phase after oxidation through routine experiments. In this regard, it is common practice to improve the filtration procedure using filter aids such as activated carbon or diatomaceous earth particles.

[0191] Optionally, the aqueous phase of ε-caprolactam is hydrogenated, and if the aqueous phase of ε-caprolactam is hydrogenated, it is preferably carried out in the presence of a hydrogenation catalyst known per se. Hydrogenation can be carried out, for example, as described in EP635487.

[0192] In a preferred embodiment of the invention, prior to crystallization in step c.4)(iv), an aqueous solution comprising water, ε-caprolactam, and impurities is hydrogenated in the presence of a hydrogenation catalyst. The hydrogenation catalyst can be any known heterogeneous hydrogenation catalyst. Examples of such catalysts are ruthenium on alumina, rhodium on alumina, platinum on carbon, palladium on carbon, Raney nickel, nickel on silica, and nickel on alumina. Preferably, a nickel-containing catalyst is used. Suitable nickel catalysts typically have a nickel content between 5 and 80 wt.% relative to the metal and support. In addition to nickel, the catalyst may also contain activators such as Zr, Mn, Cu, or Cr. The activator content is typically between 1 and 20 wt.%. If a palladium-containing heterogeneous catalyst is used, the palladium content will typically be between 0.01 and 10 wt.%.

[0193] Heterogeneous catalysts can be contacted with hydrogen-containing reaction mixtures in various ways. Hydrogenation can be carried out, for example, in a stirred tank reactor, where catalyst particles are suspended in the mixture to be purified (slurry phase method). In another embodiment, hydrogenation is carried out in a fixed-bed reactor, where the catalyst is fixed in the reactor.

[0194] Hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising an aqueous mixture of ε-caprolactam, gaseous hydrogen, and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid) comprising an aqueous mixture of ε-caprolactam fully or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. Dissolving hydrogen in a water-ε-caprolactam mixture to obtain a mixture fully or partially saturated with hydrogen can be carried out by any method known to those skilled in the art.

[0195] Hydrogenation temperatures typically range from 20°C to 160°C. Hydrogenation pressures typically range from 0.1 to 15 MPa.

[0196] Hydrogenation of the water-ε-caprolactam mixture is for the purpose of hydrogenating unsaturated compounds present in impure ε-caprolactam. The presence of these unsaturated compounds is detrimental because they impair the physicomechanical properties of nylon 6 prepared by polymerizing ε-caprolactam. Saturated compounds formed by hydrogenation do not adversely affect these physicomechanical properties of nylon 6, and these compounds are more easily removed, for example, in distillation and / or crystallization steps following the hydrogenation step.

[0197] Preferably, water is evaporated from the optionally hydrogenated aqueous phase of ε-caprolactam. After hydrogenation and / or evaporation of water, the aqueous phase of ε-caprolactam is distilled to recover high-purity ε-caprolactam and distillation residue.

[0198] In step c.4) (iii) of the method of the present invention, the organic phase, optionally washed, comprising organic solvent, ε-caprolactam, and impurities, is subsequently distilled. Preferably, the distillation of the organic phase, optionally washed, comprising organic solvent, ε-caprolactam, and impurities, is carried out under reduced pressure. In one embodiment, the distillation is carried out at a pressure below 350 kPa, preferably below 50 kPa, more preferably below 20 kPa, and most preferably below 10 kPa. Preferably, the distillation temperature at the bottom of the distillation column is between 100°C and 200°C, and more preferably between 110°C and 180°C. The distillation includes the separation of low-boiling-point organic impurities (boiling point below ε-caprolactam) and / or the separation of high-boiling-point organic impurities (boiling point above ε-caprolactam) from ε-caprolactam.

[0199] In a preferred embodiment, prior to the distillation removal in step c.4)(iii), an alkali metal hydroxide, preferably NaOH, is added to the phase comprising ε-caprolactam. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol / kg ε-caprolactam, and more preferably in the range of 2 to 80 mmol / kg ε-caprolactam. Experiments have shown that the addition of an alkali metal hydroxide, particularly NaOH, is particularly effective in distilling away impurities with boiling points lower and higher than ε-caprolactam.

[0200] In another preferred embodiment, in addition to the crude ε-caprolactam recovered in the recovery section, the crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified in the purification section [E]. 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 the recycled ε-caprolactam according to the invention.

[0201] In step c.4)(iv) of the method of the present invention, the solution comprising ε-caprolactam and impurities is crystallized at a temperature of 10°C to 95°C to obtain purified ε-caprolactam.

[0202] Preferably, the solution containing ε-caprolactam and impurities that crystallizes in step c.4)(iv) is crude ε-caprolactam obtained in the recovery section [D].

[0203] More preferably, the solution containing ε-caprolactam and impurities from which purified ε-caprolactam is obtained by crystallization in step c.4)(iv) is an organic phase containing an organic solvent, ε-caprolactam and impurities obtained by extraction in step c.4)(i), said organic phase being optionally washed with water or with an alkaline aqueous solution.

[0204] Even more preferably, the solution containing ε-caprolactam and impurities obtained by crystallization in step c.4)(iv) is an aqueous phase containing water, ε-caprolactam and impurities with boiling points lower or higher than ε-caprolactam obtained by solvent switching in step c.4)(ii).

[0205] Most preferably, the purified ε-caprolactam obtained by crystallization in step c.4)(iv) is a phase containing ε-caprolactam and impurities obtained by distillation under vacuum conditions in step c.4)(iii).

[0206] Preferably, the ε-caprolactam crystallization process in step c.4)(iv) includes the following steps: 1. Feed the solution containing ε-caprolactam and impurities into the crystallizer; 2. Set conditions in the crystallizer to form ε-caprolactam crystals and mother liquor; 3. Separate ε-caprolactam crystals from the mother liquor; 4. Recycle the mother liquor.

[0207] Crystallization can be applied to the production of ε-caprolactam. It is primarily used due to its purification potential and / or product recovery to improve yield. All crystallization processes are based on the formation of a solid crystalline phase from a liquid. In a preferred embodiment, the crystallization in step c.4)(iv) is carried out by solution crystallization or melt crystallization.

[0208] Optionally, in step c.4)(iii) of the method according to the invention, impurities with boiling points lower or higher than ε-caprolactam are removed by distillation under vacuum conditions prior to crystallization in step c.4)(iv), thereby obtaining a phase comprising ε-caprolactam and the impurities.

[0209] Therefore, according to a particularly advantageous embodiment of the invention, prior to crystallization in step c.4)(iv), the purification in step c.4) further includes the step of distilling under vacuum to remove impurities with boiling points lower or higher than ε-caprolactam.

[0210] Optionally, in step c.4)(iii) of the method according to the invention, impurities with boiling points lower or higher than ε-caprolactam are removed by distillation under vacuum after crystallization in step c.4)(iv), thereby obtaining a phase comprising ε-caprolactam and the impurities.

[0211] Therefore, according to a particularly advantageous embodiment of the invention, after crystallization in step c.4)(iv), the purification in step c.4) further includes the step of distilling under vacuum to remove impurities with boiling points lower or higher than ε-lactam.

[0212] Optionally, in step c.4)(iii) of the method according to the invention, impurities with boiling points lower or higher than ε-caprolactam are removed by distillation under vacuum conditions before and after crystallization in step c.4)(iv), thereby obtaining a phase comprising ε-caprolactam and the impurities.

[0213] Therefore, according to a particularly advantageous embodiment of the invention, before and after crystallization in step c.4)(iv), the purification in step c.4)(iii) further includes the step of distilling under vacuum to remove impurities with boiling points lower or higher than ε-caprolactam.

[0214] The term solution crystallization is used to crystallize a compound from a solution comprising the (impure) compound and wherein an auxiliary solvent has been added. The auxiliary solvent is water or a non-aqueous solvent. If the auxiliary solvent is water, the amount of water in the solution can be selected within a wide range, preferably from 0.5 to 25 wt.%, more preferably from 1 to 9 wt.%. The crystallization temperature can be selected within a wide range, preferably from 10°C to 95°C, more preferably from 20°C to 85°C, even more preferably from 20°C to 70°C, and most preferably from 30°C to 65°C. The crystallized purified ε-caprolactam is recovered from a slurry with a slurry concentration preferably from 5 to 75 wt.%, more preferably from 10 to 70 wt.%, and most preferably from 15 to 50 wt.%. If the auxiliary solvent is a non-aqueous solvent, the amount of non-aqueous solvent in the solution can be selected within a wide range, preferably from 5 to 95 wt.%, more preferably from 10 to 90 wt.%, and most preferably from 30 to 70 wt.%. The crystallization temperature can be selected within a wide range, preferably from 10°C to 95°C, more preferably from 20°C to 85°C, even more preferably from 20°C to 70°C, and most preferably from 30°C to 65°C. The crystallized and purified ε-caprolactam is recovered from a slurry with a slurry concentration preferably from 5 to 75 wt.%, more preferably from 10 to 70 wt.%, and most preferably from 15 to 50 wt.%. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform, or ethyl chloride), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate), as well as mixtures of these solvents. Cyclohexane is preferred.

[0215] Solution crystallization is typically carried out at atmospheric pressure, but can also be carried out under reduced or increased pressure. In the case of solution crystallization, the product is crystallized by evaporative crystallization in which the solvent is evaporated, or by cooling crystallization in which cooling is achieved through direct cooling, indirect cooling, or vacuum cooling, or a combination of these methods. After the crystallization step, the resulting crystals and mother liquor are separated by, for example, sedimentation, filtration, and / or centrifugation. Optionally, the obtained crystals are washed with, for example, a solvent or a solvent mixture with low impurity content. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in crystalline form.

[0216] 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 an auxiliary solvent. In a broader definition, melt crystallization also applies to crystallization from solutions containing low solvent concentrations. Preferably, the solvent concentration in the solution is less than 25 wt.%, more preferably less than 10 wt.%, and most preferably less than 5 wt.%. Here, unless explicitly stated otherwise, the broader definition of melt crystallization will be used. The compound crystals obtained by melt crystallization are separated from the mother liquor and optionally washed with a melt of pure compound material. Purification efficiency can be further improved, for example, by sweating (also known as partial melting), i.e., gently heating the crystal layer to near its melting temperature, thereby removing trapped and adhered 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.

[0217] Preferably, the solvent is present in the mixture in the crystallizer, but crystallization can also be carried out without a solvent. Many solvents are suitable for ε-caprolactam. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform, or ethyl chloride), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate). Water and aromatic hydrocarbons are preferred as solvents because these solvents produce large crystals. Water is the most preferred solvent. The solvent will act as a freezing point lowering agent for the melt in the crystallizer.

[0218] Generally, melt crystallization requires less energy than solution crystallization; however, industrial-scale operations can be more challenging.

[0219] The melt crystallization used in this paper specifically refers to layer melt crystallization or suspension melt crystallization. These two types of melt crystallization techniques are characterized by (1) the formation of a crystal layer on the heat exchanger wall (layer melt crystallization) and (2) the growth of crystals in a suspension (suspension melt crystallization). Generally, the operation of the method for growing a crystal layer on the heat exchanger wall is called layer melt crystallization. First, a melt is loaded into a crystallizer, then a crystal layer is grown on the cooled heat exchanger surface. Next, the remaining melt containing impurities discharged during crystal growth is discharged from the crystallizer, and then the crystal layer is melted and the purified product is recovered. Purification efficiency can be further improved, for example, by sweating (also known as partial melting), which involves gently heating the crystal layer to near its melting temperature, thereby removing trapped and adhered impurities from the mother liquor. Layer melt crystallization methods are operated in batch mode. Well-known examples of layer melt crystallization-based methods are the ProABD method of BEFS Prokem and the Sulzer Chemtech method.

[0220] Layer-by-layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization mode, crystals grow from a stagnant melt onto a cooled surface. In static mode, the desired compound crystallizes in batches from a stagnant melt in a closed container onto the wall of a heat exchanger. This type of crystallization is characterized by a low crystal growth rate and the resulting long residence (or batching) time. Preferably, the crystallization time ranges from 1 hour to 75 hours, more preferably from 2 hours to 50 hours, and most preferably from 4 hours to 24 hours. After the crystallization step, the remaining melt is drained. Then, optionally, a sweating phase is introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or mechanically removed.

[0221] Generally, dynamic crystallization takes place in a shell-and-tube heat exchanger, whereby the melt circulates downwards along a cooling surface, where the compound crystallizes. Typically, the melt is pumped through the tubes, and crystals grow inside the tubes while the cooling medium flows outside. The thickness of the crystal layer increases over time. After a certain period, the circulation of the melt is stopped, and the remaining melt is discharged. Like stagnant layer crystallization, dynamic layer crystallization is also carried out in a batch mode. Compared to the stagnant mode, the crystal growth rate in the dynamic mode is higher, and therefore the crystallization time is shorter. Preferably, the crystallization time ranges from 0.05 hours to 12 hours, more preferably from 0.1 hours to 6 hours, and most preferably from 0.3 hours to 3 hours. Then, optionally, a sweating phase is introduced to remove impurities adhering to or trapped within the crystals. Finally, the crystals are completely melted and discharged or mechanically removed.

[0222] Suspension melt crystallization can be carried out in 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 container-type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is carried out in a scraper-type 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 fed into a so-called washing tower. In the washing tower, the mother liquor is filtered or discharged from the crystals, and the crystals are then typically washed with purified compound material.

[0223] Following the ε-caprolactam crystallization step, a mother liquor containing ε-caprolactam in addition to impurities is obtained. Methods for recovering ε-caprolactam from such mother liquor are well known to those skilled in the art. And due to these recovery methods, almost all of the ε-caprolactam present in the mother liquor can be recovered and converted into high-purity ε-caprolactam. In the case of multi-stage crystallization, one possible solution is to recycle the mother liquor in a countercurrent manner, i.e., to feed the mother liquor obtained in the nth crystallization stage into the feed of the (n-1)th crystallization stage. Generally, the mother liquor obtained from the first crystallization stage is fed into the upstream (purification) unit of the process or a dedicated mother liquor processing unit (e.g., based on distillation or crystallization). After ε-caprolactam crystallization, it may be necessary to purify it, for example, by recycling the obtained mother liquor (or a portion thereof) to any previous stage of the process. Alternatively, the mother liquor can be purified, for example, by distillation, before being re-loaded into the ε-caprolactam crystallization step.

[0224] The high-purity ε-caprolactam obtained by the method according to the invention can be used to prepare nylon 6 using methods well known to those skilled in the art. This nylon 6 can then be used in all known materials, including engineered materials, fibers, and membranes. The nylon 6 produced according to the invention from materials comprising nylon 6 and polyether polyurethane is particularly suitable for high-speed spinning applications, including garments containing polyether polyurethane (also known as elastic fibers).

[0225] factory The present invention also provides a plant configured to carry out the methods described above, namely a chemical plant, comprising a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E]. All plant features specifically described in connection with the methods described above also apply to the plant of the present invention described below, and vice versa. Therefore, the plant is suitable for carrying out the methods of the present invention, and it should be understood that the descriptions already provided in connection with the methods of the present invention are equally applicable to the plant embodiments.

[0226] The plant may be a laboratory setting as in the example. However, preferably, the plant is an industrial-scale plant. "Industrial-scale" means that, if operated continuously, the plant's ε-caprolactam production capacity (i.e., the amount of ε-caprolactam that it can theoretically produce) is at least 500 tons per year.

[0227] The plant of the present invention is suitable for producing purified ε-caprolactam from materials including nylon 6 and polyether polyurethane, and comprises at least four sections: a separation section [B], a depolymerization section [C], a recovery section [D], and a purification section [E]. These sections, and therefore the plant, are configured to carry out the methods of the present invention described above.

[0228] Optionally, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section [λ] for crushing materials comprising nylon 6 and polyether polyurethane into fragments and / or a cleaning section [ω] for washing materials comprising nylon 6 and polyether polyurethane or the crushed fragments obtained therefrom. Cleaning includes washing materials comprising nylon 6 and polyether polyurethane and separating foreign materials from said materials. Separation of foreign materials may be performed manually (hand-picking) and mechanically (e.g., density separation and magnetic separation). Manual and mechanical devices, such as brushes, may assist washing in the cleaning section [ω]. Washing is preferably carried out by additional frictional action. Different types of industrial washing systems are available on the market, such as rotary plastic washers and (high-speed) friction washers. The mechanical size reduction section [λ] includes equipment for mechanically crushing materials comprising nylon 6 and polyether polyurethane into fragments. Non-limiting examples of such crushing equipment are cutters, punches, shredders, mills, grinders, or chippers.

[0229] The separation section [B] may include the following four sections: a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ]. These sections, and therefore the plant, are configured to carry out the methods of the present invention described above.

[0230] The dissolving section [α] comprises one or more dissolving units operating in series and / or parallel, and one or more separating units operating in series and / or parallel. Fragments of material comprising nylon 6 and polyether polyurethane, optionally cleaned and / or crushed, are fed into the dissolving unit in solid or melt form, preferably solid. The fragments of material comprising nylon 6 and polyether polyurethane, optionally cleaned and / or crushed, are optionally dried before being fed into the dissolving section [α]. The dissolving unit is equipped with inlets for loading the separated organic solvent and fresh organic solvent. The dissolving unit is equipped with one or more outlets for discharging the mixture obtained in the dissolving unit. This mixture comprises undissolved nylon 6 and organic solvent in which polyether polyurethane is dissolved. Preferably, the dissolving unit is closed. This has the advantage of reducing the emission of organic solvent into the environment. Preferably, the dissolving unit is equipped with means for controlling the temperature in the control unit (e.g., steam coil, steam tracing, electric tracing). Optionally, the temperature of the separated organic solvent and fresh organic solvent is adjusted (e.g., in a heat exchanger) before being loaded into the dissolving section [α].

[0231] Good contact between organic solvents and materials including nylon 6 and polyether polyurethanes is essential for efficient operation. This contact can be achieved by various means known in the art. Improved contact can be achieved through mechanical friction. Mechanical friction can then be achieved, for example, by stirring using a combination of a rotating paddle and static fins.

[0232] Dissolution units suitable for dissolving polymers in organic solvents are known to those skilled in the art. A stirred vessel is an example of such a dissolution unit.

[0233] In the separation unit, the mixture discharged from the dissolution unit is separated into a polyether-rich stream comprising organic solvent and dissolved polyether polyurethane, and a stream comprising undissolved nylon 6. The polyether-rich stream comprising organic solvent and dissolved polyether polyurethane is discharged through a discharge line and loaded into a settling section [γ]. The stream comprising undissolved nylon 6 is discharged through a discharge line and loaded into a washing section [β]. Advantageously, the amount of organic solvent in the stream comprising undissolved nylon 6 is low to facilitate washing in the washing section [β].

[0234] Separation units suitable for separating (undissolved) polymers and organic solvents are known to those skilled in the art. Centrifuges and filters are examples of such separation units.

[0235] Preferably, the dissolution unit and the separation unit are combined into one device. This intensifies the process.

[0236] The washing section [β] includes apparatus for washing the stream containing undissolved nylon 6 with a third solvent to obtain a nylon 6-rich stream and a mixture comprising the 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 apparatus for drying the nylon 6-rich stream. The mixture comprising the organic solvent and the third solvent is discharged from the washing section [β] and loaded into the solvent distillation section [δ]. In this document, "loaded into the solvent distillation section [δ]" also includes indirect means, i.e., additional intermediate steps / sections. Preferably, the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] is completely or partially loaded into the precipitation section [γ] before being loaded into the solvent distillation section [δ].

[0237] Optionally, after drying, the nylon 6-rich stream is discharged from the washing section [β] and loaded into the depolymerization section [C].

[0238] Good contact between the third solvent and the nylon 6-rich feed stream is essential for efficient operation. This contact can be achieved by various means known to those skilled in the art. One means is a (continuous) centrifuge into which the nylon 6-rich feed stream is fed and the third solvent is introduced as a washing solvent.

[0239] A distillation section [δ] comprises one or more distillation columns operating in series and / or parallel. These columns can operate in batch, semi-continuous, or continuous modes. The choice between these modes depends largely on the scale of the operation. Generally, batch operation of distillation columns is better suited for handling small-volume feed streams. Continuous operation of distillation columns is better suited for handling large-volume feed streams.

[0240] The exact layout of the distillation zone [δ] also depends on the properties of the organic solvent, the second solvent, and the third solvent.

[0241] A mixture of an organic solvent and a second solvent from which the precipitated polyether polyurethane has been recovered is charged into the distillation section [δ], along with a mixture of an organic solvent and a third solvent discharged from the washing section [β]. The separated organic solvent is discharged from the distillation section [δ] and charged into the dissolving section [α]. The second and third solvents are discharged from the distillation section [δ] and optionally charged into the precipitation section [γ] and washing section [β], respectively. The residue is discharged from the distillation section [δ]. Optionally, the residue is incinerated to recover energy. Finally, the degradation products of the solvents used are discharged from the distillation section [δ]. Acetic acid and dimethylamine are examples of degradation products of the organic solvent DMAc.

[0242] The depolymerization section [C] comprises one or more depolymerization reactors operating in series and / or parallel. A nylon 6-rich stream is fed into the reactor in solid or melt form, preferably melt. This feeding can be achieved using an extruder, gear pump, or other means known in the art.

[0243] During production, the depolymerization reactor is at least partially filled with feedstock containing nylon 6, residual material, ε-caprolactam (and optionally a catalyst). The depolymerization reactor can have any desired form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors, and extruder-type reactors.

[0244] The depolymerization reactor must be equipped with facilities for feeding a nylon 6-rich feed stream, optional superheated steam, and optional catalyst. Additionally, the depolymerization reactor must be equipped with facilities for discharging the feed stream, including ε-caprolactam, and residual materials.

[0245] Good contact between steam and reactor contents is essential for efficient operation. This contact can be achieved through various means known to those skilled in the art. For example, multiple inlets can be used, such as a steam distributor to inject steam through the material. Further improved contact can be achieved by incorporating mechanical agitation into the reactor, such as using a combination of rotating paddles and static fins.

[0246] Preferably, depolymerization will be completed within 0.5 to 6 hours.

[0247] If high-temperature superheated steam is not available at the production site, superheated steam must be intentionally produced by superheating the available steam in the boiler in a so-called superheater.

[0248] The recovery section [D] may include one or more (preferably partial) condensers into which a feed stream comprising ε-caprolactam is introduced in the form of a vapor stream comprising ε-caprolactam and water. Such a (partial) condenser may have any desired form. Preferably, the condenser is a distillation column from which a water-rich phase is obtained as the top product, and crude ε-caprolactam is obtained as the bottom product.

[0249] The purification section [E] may include one or more extraction devices, one or more solvent switching devices, an oxidation section, a hydrogenation section, one or more distillation devices, and a crystallization section, wherein crude ε-caprolactam is loaded into the devices or the section and high-purity ε-caprolactam is discharged from the devices or the section.

[0250] Crude ε-caprolactam and an organic solvent are introduced into the extraction device, and an organic phase comprising the organic solvent, ε-caprolactam, and impurities, as well as an aqueous phase comprising water and impurities, is discharged. The extraction device is selected from mixed settling extractors, extraction columns, centrifugal extractors, and combinations thereof. Preferably, the extraction device is a static or stirred extraction column, such as a KARR. Tower, SCHEIBEL Towers, rotary contactor (RDC) towers, pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (SMVP) (static) towers.

[0251] The solvent switching device is filled with water and an organic phase comprising organic solvent, ε-caprolactam, and impurities, and discharged with a solvent phase comprising impurities and an ε-caprolactam-aqueous phase comprising water and impurities. The solvent switching device for the back-extraction-based method is selected from mixed sedimentation extractors, extraction columns, centrifugal extractors, and combinations thereof. Preferably, the device for back-extraction is a static or stirred extraction column, such as a KARR. Tower, SCHEIBEL Towers, rotary contactor (RDC) towers, pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (static) towers.

[0252] Solvent switching equipment for solvent exchange distillation methods is selected from sieve plate distillation columns, random-packed distillation columns, and structured-packed distillation columns. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation column can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure. Preferably, water is introduced into the upper part of the distillation column, and the aqueous phase, including water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, is discharged from the lower part of the distillation column.

[0253] The optional oxidation section comprises one or more oxidation reactors operating in series and / or parallel. An oxidant and an ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities are charged into the oxidation section. Typically, the oxidant is charged in solid, slurry, or aqueous solution form. When potassium permanganate or sodium permanganate is used as the oxidant, the oxidation section also includes a filtration section. The oxidation reactor can have any desired form. Preferred reactor types are stirred and unstirred reactors and packed tower reactors. The oxidation reactor must be equipped with facilities for feeding an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, as well as the oxidant. Additionally, the oxidation reactor must be equipped with facilities for discharging the oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities, as well as optionally formed solid manganese(IV) oxide (MnO2) particles. Preferably, oxidation is carried out under temperature and atmospheric conditions ranging from 20°C to 85°C.

[0254] Optional solid manganese oxide (IV) (MnO2) particles can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to improve the filtration process using filter aids such as activated carbon particles or diatomaceous earth. Filtration systems suitable for separating solid manganese oxide (IV) particles are known to those skilled in the art. This filtration system is loaded with a suspension of oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities, and solid manganese oxide (IV) particles, and discharged from the system after filtration of the filtered oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities. Typically, the solid manganese oxide (IV) particles remain in the filtration system. Preferably, this filtration system is operated in a semi-continuous mode, whereby the suspension and filter phase are continuously loaded and discharged, while the separated solids are collected in the filtration system. The loading of the suspension is interrupted from time to time, and the collected solids are removed from the filtration system.

[0255] Purifying crude ε-caprolactam in step c.4) to obtain purified ε-caprolactam optionally includes hydrogenation using a heterogeneous catalyst, in which case the plant will include a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0256] The hydrogenation section comprises one or more hydrogenation reactors operating in series and / or parallel. Hydrogenation can be carried out in a three-phase system (gas, liquid, solid) comprising an aqueous mixture of ε-caprolactam, gaseous hydrogen, and a multiphase hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid) comprising an aqueous mixture of ε-caprolactam fully or partially saturated with hydrogen and a multiphase hydrogenation catalyst. The dissolution of hydrogen in the water-ε-caprolactam mixture can be carried out by any method known to those skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or mixer, wherein a constant hydrogen pressure is maintained. Close contact between the hydrogen and the mixture ensures that the hydrogen dissolves in the mixture. This process is preferably carried out continuously. The hydrogen-containing mixture is then contacted with the hydrogenation catalyst, for example, in a separate reactor.

[0257] Heterogeneous catalysts can be contacted with hydrogen-containing reaction mixtures in various ways. Hydrogenation can be carried out, for example, in a stirred tank reactor, where catalyst particles are suspended in the mixture to be hydrogenated (slurry phase method). In this slurry phase method, the catalyst particles and the purified mixture must be separated in a further process step after the hydrogenation reaction, for example by filtration. Preferably, the catalyst comprises palladium or nickel.

[0258] Alternatively, hydrogenation can be carried out in a fixed-bed reactor, where the catalyst is fixed within the reactor, eliminating the need for additional steps to separate the catalyst and the reaction mixture. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0259] Hydrogenation temperatures typically range from 20°C to 160°C. Hydrogenation pressures typically range from 0.1 to 15 MPa.

[0260] The distillation apparatus is charged with an ε-caprolactam-aqueous phase, optionally stripped and / or concentrated and / or optionally oxidized and / or hydrogenated, comprising water, ε-caprolactam, and impurities, and discharged with high-purity ε-caprolactam, water, and impurities (i.e., low-boiling-point organic impurities (boiling point below ε-caprolactam) and high-boiling-point organic impurities (boiling point above ε-caprolactam)). The distillation apparatus is selected from sieve plate distillation columns, random-packed distillation columns, structured-packed distillation columns, and horizontal and vertical (descending and ascending) film evaporators. Preferably, the distillation column is equipped with a reboiler, a condenser, and equipment for reflux. The distillation apparatus can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure, preferably at sub-atmospheric pressure.

[0261] Preferably, distillation comprises separating water, low-boiling-point organic impurities (having a lower boiling point than ε-caprolactam), and / or high-boiling-point organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. Preferably, distillation comprises: in a first step, separating water as a top product and producing low-boiling-point impurities containing ε-caprolactam and high-boiling-point impurities as a bottom product; in a second step, separating the low-boiling-point impurities as a top product and obtaining high-boiling-point impurities containing ε-caprolactam as a bottom product; and in a third step, separating high-purity ε-caprolactam as a top product and producing a distillation residue comprising ε-caprolactam and high-boiling-point impurities as a bottom product. Optionally, the first and second steps are combined.

[0262] Preferably, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities before distillation to remove water and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol / kg ε-caprolactam, and more preferably in the range of 2 to 80 mmol / kg ε-caprolactam. This makes the removal of impurities with lower and higher boiling points than ε-caprolactam particularly efficient in subsequent distillations.

[0263] The crystallization section includes one or more crystallizers operating in series and / or parallel. Generally, the crystallization section also includes several containers for storing (intermediate) product streams and / or fresh and used washing solutions. Crystallization of ε-caprolactam can be carried out by solution crystallization or melt crystallization as described above.

[0264] In the case of solution crystallization, ε-caprolactam is recovered by evaporative crystallization in which the solvent is evaporated, or by cooling crystallization in which cooling is achieved by direct cooling, indirect cooling, or vacuum cooling, or by a combination of these methods. Following the crystallization step in the crystallizer, the formed crystals and mother liquor are separated by, for example, settling in a settling tank, filtration in a filter, and / or centrifugation in a centrifuge. Optionally, the crystallizer is equipped with a stirrer and / or one or more baffles.

[0265] Optionally, the resulting crystals are washed with, for example, a cleaning solvent. Optionally, the crystallization-separation sequence is repeated several times. The product is obtained in crystalline form.

[0266] The auxiliary solvent is water or a non-aqueous solvent. Examples of non-aqueous solvents include alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform, or ethyl chloride), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate), as well as mixtures of these solvents. Generally, the auxiliary solvent is recovered and reused during the crystallization process.

[0267] Solution crystallization usually takes place under atmospheric pressure, but it can also be carried out under reduced or increased pressure.

[0268] ε-caprolactam melt crystallization can be achieved through layer melt crystallization in which a layer of ε-caprolactam-containing crystals is formed on the heat exchanger wall, or through suspension melt crystallization in which ε-caprolactam-containing crystals are grown in a suspension.

[0269] Preferably, the solvent is present in the mixture in the melt crystallizer, but melt crystallization can also be carried out without a solvent. Many solvents are suitable for ε-caprolactam. Examples of suitable solvents are water, alkanes (such as n-hexane, n-heptane, isooctane, cyclohexane), alcohols (such as methanol, ethanol, n-propanol, n-butanol), aromatic hydrocarbons (such as benzene, toluene, o-xylene, m-xylene, p-xylene), ammonia, chlorinated hydrocarbons (such as carbon tetrachloride, chloroform, or ethyl chloride), ketones (such as acetone or methyl ethyl ketone), and esters (such as ethyl acetate). Water and aromatic hydrocarbons are preferred as solvents because these solvents produce large crystals. Water is the most preferred solvent. The solvent will act as a freezing point lowering agent for the melt in the crystallizer.

[0270] Layer melting and crystallization: First, the melt is loaded into a crystallizer, and then a crystal layer is grown on the surface of a cooled heat exchanger. Next, the remaining melt containing impurities discharged during crystal growth is discharged from the crystallizer, and then the crystal layer is melted and the purified product is recovered. Purification efficiency can be further improved, for example, by sweating (also known as partial melting), which involves gently heating the crystal layer to near its melting temperature, thereby removing trapped and adhered impurities from the mother liquor. Layer-melt crystallization methods are operated in batch mode. Well-known examples of layer-melt crystallization methods are BEFS Prokem's ProABD method and the SulzerChemtech method.

[0271] Layer-by-layer melt crystallization can be carried out in either static or dynamic mode. In static crystallization mode, crystals grow from a stagnant melt onto a cooled surface. In static mode, the desired compound crystallizes in batches from a stagnant melt in a closed container onto the wall of a heat exchanger. This type of crystallization is characterized by a low crystal growth rate, which results in a longer residence (or batching) time. Preferably, the crystallization time ranges from 1 hour to 75 hours, more preferably from 2 hours to 50 hours, and most preferably from 4 hours to 24 hours. After the crystallization step, the remaining melt is drained. Then, optionally, a sweating phase is introduced to remove impurities adhering to or trapped in the crystals. Finally, the crystals are completely melted and drained or mechanically removed.

[0272] Suspension melt crystallization: Suspension melt crystallization of ε-caprolactam can be carried out in a batch or continuous manner. In the case of suspension melt crystallization, the melt is cooled below its saturation temperature, and ε-caprolactam 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 container-type crystallizer that allows cooling of the melt. Preferably, suspension melt crystallization is carried out in a scraper-type 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 fed into a so-called washing tower. In the washing tower, the mother liquor is discharged from the ε-caprolactam crystals, and then the ε-caprolactam crystals are optionally washed with purified ε-caprolactam.

[0273] The method of the present invention can be operated continuously, semi-continuously, or in batches. Therefore, the plant of the present invention can also be configured to allow one or more of these operating modes. In a preferred embodiment, the plant is configured to operate the method of the present invention continuously or semi-continuously. However, discontinuous methods are also possible. For example, the plant of the present invention does not need to include all the sections described herein in one location. Specifically, the pretreatment section [A] can be located at a first location, while the separation section [B], depolymerization section [C], recovery section [D], and purification section [E] are located at a second location. Similarly, the mechanical size reduction section [λ], which is part of the pretreatment section [A], can also be located at the first location, while the cleaning section [ω], which is part of the pretreatment section [A], can be located at the second location, while the separation section [B], depolymerization section [C], recovery section [D], and purification section [E] are located at a third location. Optionally, the cleaning section [ω] is divided into two or more segments, which are optionally all located at different locations. For example, the first segment of the cleaning segment [ω], which is part of the pretreatment segment [A], may be located at a first position; the mechanical size reduction segment [λ], which is part of the pretreatment segment [A], may be located at a second position; the second segment of the cleaning segment [ω], which is part of the pretreatment segment [A], may be located at a third position; and the separation segment [B], depolymerization segment [C], recovery segment [D], and purification segment [E] may be located at a fourth position. Optionally, the cleaning segment [ω], which is part of the pretreatment segment [A], may be divided into two or more segments, all of which may be located at different positions. Optionally, the cleaning segment [ω], which is part of the pretreatment segment [A], may be located at the same position as the separation segment [B]. Optionally, the separation segment [B], depolymerization segment [C], and recovery segment [D] may be located at different positions from the pretreatment segment [A] and purification segment [E]. Optionally, the depolymerization section [C] and the recycling section [D] are located at different locations from the pretreatment section [A] and the separation section [B].

[0274] product The method of the present invention provides ε-caprolactam and polyether polyurethane obtained by depolymerization of a material comprising nylon 6 and polyether polyurethane as new products, which meet the specifications of demanding applications, and are particularly environmentally friendly due to their reduced product carbon footprint and use of waste as starting materials. A particularly advantageous feature of the ε-caprolactam obtained by the method of the present invention is that the product carbon footprint is less than 3 kg CO2 / kg purified ε-caprolactam. The ε-caprolactam obtained according to the present invention can also be referred to as "purified ε-caprolactam". As used herein, "purified" means that the ε-caprolactam is produced by the method according to the present invention from a material comprising nylon 6 and polyether polyurethane, thus obtaining it in a purified form. In this sense, ε-caprolactam is obtained and purified from a material comprising nylon 6 and polyether polyurethane.

[0275] The method of the present invention allows for the production of high-purity and therefore high-quality ε-caprolactam to meet the specifications of demanding applications, and is particularly economical due to its reduced product carbon footprint and use of waste as starting material. In a preferred embodiment, the ε-caprolactam obtained by the method of the present invention meets one or more of the following specifications, wherein the parameters and measurement methods are defined in the Examples section below: PAN: Maximum value 5 E290: Maximum value 0.05 VB: Maximum value 0.5 mmol / kg Alkalinity: Maximum value 0.1 mmol / kg Acidity: Maximum value 0.1 mmol / kg.

[0276] ε-caprolactam produced by the method of the present invention is also particularly economical and environmentally friendly. This is evident because the carbon footprint of ε-caprolactam produced by the method of the present invention is much lower than that of conventionally produced ε-caprolactam (e.g., via the Beckmann rearrangement of cyclohexanone oxime).

[0277] The environmental impact of a product is typically expressed as its 'product carbon footprint'. A product's carbon footprint is defined as the total emissions generated by the product's formation, expressed in tons of CO2 equivalent per ton of product. A product's carbon footprint depends particularly on raw materials, auxiliary materials, energy consumption, energy, production methods, and method efficiency. The quantification of a product's carbon footprint can be performed as described, for example, in the European standard EN ISO 14040:2006 ("Environmental management – ​​Life cycle assessment – ​​Principles and framework").

[0278] Product carbon footprint calculations can be performed internally or by an externally (preferably) certified organization. These organizations validate and certify product carbon footprint calculations based on, for example, the LCA standard ISO 14040.

[0279] 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, in the case of coal-based electricity and steam generation, the potential impact of “native” ε-caprolactam obtained via the Beckmann rearrangement of cyclohexanone oxime on global warming is 7.5 tg CO2 equivalent / t ε-caprolactam (which is equivalent to 7.5 kg CO2 equivalent / kg ε-caprolactam). In cases involving natural gas-based electricity and steam generation, the potential impact of virgin ε-caprolactam from ε-caprolactam production methods on global warming would decrease to 6.4 tCO2 equivalents / t ε-caprolactam (which is equivalent to 6.4 kg CO2 equivalents / kg ε-caprolactam).

[0280] The carbon footprint of ε-caprolactam obtained by the method according to the invention is significantly lower than that of de novo synthesized or “native” ε-caprolactam. The carbon footprint of ε-caprolactam obtained by the method of the invention is less than 4 kg, more preferably less than 3 kg, and most preferably equal to or less than 3.0 kg CO2 equivalents / kg ε-caprolactam.

[0281] The polyether polyurethane produced by the method of this invention is also particularly economical and environmentally friendly. This is also evident because the polyether polyurethane produced by the method of this invention has a much lower carbon footprint compared to conventionally produced polyether polyurethanes (e.g., through the reaction of polyether polyols and diisocyanate monomers).

[0282] NM van der Velden, MK Patel and JG Vogtländer (Table 7 below: “LCA benchmarking study on textiles made of cotton, polyester, nylon, acryl, or elastane”, International Journal of Life Cycle Assessment (2014) 19:331–356; DOI: 10.1007 / s11367-013-0626-9) reported that the potential impact of virgin elastic fiber production on global warming is equivalent to 4.836 kg CO2 equivalent / kg elastic fiber.

[0283] The carbon footprint of the polyether polyurethane product obtained by the method according to the present invention is significantly lower than that of de novo synthesized or "native" polyether polyurethane products. The carbon footprint of the polyether polyurethane product obtained by the method according to the present invention is less than 3.0 kg, preferably less than 2.0 kg CO2, and most preferably less than 1.0 kg CO2 equivalent / kg polyether polyurethane. Attached Figure Description

[0284] The invention is described below with reference to the accompanying drawings, which depict certain embodiments of the invention. However, the invention is as defined in the claims and as generally described herein. It should not be limited to the embodiments shown in the following drawings for illustrative purposes.

[0285] Figure 1 This is a schematic diagram of the method of the present invention for recovering and purifying ε-caprolactam and polyether polyurethane from materials including nylon 6 and polyether polyurethane, the method comprising processing steps in optional pretreatment section [A], separation section [B], depolymerization section [C], recovery section [D] and purification section [E].

[0286] Figures 2A-2B Two embodiments of the pretreatment section [A] are shown, wherein the material comprising nylon 6 and polyether polyurethane is cleaned in the cleaning section [ω] by removing foreign materials and washing with a washing solvent, and broken in the mechanical size reduction section [λ] to obtain clean and broken fragments of the material comprising nylon 6 and polyether polyurethane.

[0287] Figure 2AAn embodiment of a pretreatment section [A] is described, wherein the material comprising nylon 6 and polyether polyurethane is first cleaned in a cleaning section [ω] by removing foreign materials and washing with a washing solvent, and then crushed in a mechanical size reduction section [λ] to obtain clean and crushed fragments of the material comprising nylon 6 and polyether polyurethane.

[0288] Figure 2B An embodiment of a pretreatment section [A] is described, wherein the material comprising nylon 6 and polyether polyurethane is first crushed in a mechanical size reduction section [λ] and then cleaned in a cleaning section [ω] by removing foreign material and washing with solvent to obtain clean and crushed fragments of the material comprising nylon 6 and polyether polyurethane.

[0289] Figure 3 An embodiment of a separation section [B] is shown, in which materials comprising nylon 6 and polyether polyurethane are separated to obtain a nylon 6-rich stream and recycled polyether polyurethane. The plant used in this embodiment includes a dissolving section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ].

[0290] Figures 4A-4B Two examples of the purification section [E] are shown, in which crude ε-caprolactam is purified to obtain high-purity ε-caprolactam.

[0291] Figure 4A An embodiment of the purification section [E] of the method of the present invention is described, the purification section comprising 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].

[0292] Figure 4B An embodiment of the purification section [E] of the method of the present invention is described, the purification section comprising 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].

[0293] Detailed description of the attached figures The method of the present invention is in Figure 1 The method is illustrated schematically in the following plant sections: optional pretreatment section [A], separation section [B], depolymerization section [C], recovery section [D], and purification section [E].

[0294] In an optional pretreatment section [A], the material

[301] comprising nylon 6 and polyether polyurethane is cleaned and / or crushed by mechanical dimensional reduction to obtain discharged clean and / or crushed fragments of the material comprising nylon 6 and polyether polyurethane

[304] . Optionally, in the pretreatment section [A] where the material

[301] comprising nylon 6 and polyether polyurethane is cleaned by removing foreign material and / or by washing with a washing solvent

[302] , discharged foreign material and contaminated washing solvent

[303] are obtained. Removal of foreign material may be performed before and / or after washing with the washing solvent

[302] . Cleaning may be performed before and / or after crushing the material

[301] comprising nylon 6 and polyether polyurethane. Optionally, the clean and / or crushed fragments of the material comprising nylon 6 and polyether polyurethane

[304] are dried before being loaded into the separation section [B]. Optionally, the cleaned and / or broken fragments of materials including nylon 6 and polyether polyurethane are densified before being loaded into the separation section [B].

[0295] In the separation section [B], fragments

[304] of the material, optionally cleaned and / or crushed, comprising nylon 6 and polyether polyurethane, are separated to obtain a nylon 6-rich stream

[311] and recovered polyether polyurethane

[308] discharged from the separation section [B]. The nylon 6-rich stream

[311] is loaded into the depolymerization section [C]. Optionally, the nylon 6-rich stream

[311] and / or recovered polyether polyurethane

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

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

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

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

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

[309] and the residue

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

[0296] The optionally dried and / or densified nylon 6-rich stream

[311] is depolymerized to ε-caprolactam in the depolymerization section [C]. The stream containing ε-caprolactam is discharged from the depolymerization section [C]

[315] . Additionally, residual material is discharged

[314] . Optionally, superheated steam

[312] and catalyst

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

[0297] Crude ε-caprolactam

[317] is recovered from the feed stream

[315] containing ε-caprolactam discharged from the self-depolymerization section [C]. The crude ε-caprolactam

[317] is discharged from the recovery section [D] and loaded into the purification section [E]. Additionally, water or superheated steam

[312] is loaded into the depolymerization section [C] or ( Figure 1 When water is loaded into the recovery section [D] (not shown), the aqueous phase

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

[0298] In the purification section [E], the crude ε-caprolactam

[317] discharged from the recovery section [D] is purified to produce high-purity ε-caprolactam

[319] . Water and impurities

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

[0299] Figure 2A An embodiment of a pretreatment section [A] (the area enclosed by dashed lines) is depicted, in which a material

[31] comprising nylon 6 and polyether polyurethane is first cleaned in a cleaning section [ω] by removing foreign material and washing with a washing solvent

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

[33] and clean material comprising nylon 6 and polyether polyurethane

[34] . Subsequently, the clean material comprising nylon 6 and polyether polyurethane

[34] is crushed in a mechanical size reduction section [λ] to obtain clean and crushed material comprising nylon 6 and polyether polyurethane

[35] . The clean and crushed material comprising nylon 6 and polyether polyurethane

[35] is then discharged. Optionally, the fragments

[35] of the clean and crushed material comprising nylon 6 and polyether polyurethane are densified before being separated into a nylon 6-rich stream and recovered polyether polyurethane in a separation section [B]. Figure 2A (Not shown in the image).

[0300] Figure 2B An embodiment of a pretreatment section [A] (the area enclosed by dashed lines) is depicted, in which a material

[41] comprising nylon 6 and polyether polyurethane is first crushed in a mechanical size reduction section [λ] to obtain fragments of the material comprising nylon 6 and polyether polyurethane

[42] . Subsequently, the fragments of the material comprising nylon 6 and polyether polyurethane

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

[43] to obtain foreign materials and contaminated washing solvent

[44] , as well as clean and crushed fragments of the material comprising nylon 6 and polyether polyurethane

[45] . The clean and crushed fragments of the material comprising nylon 6 and polyether polyurethane are then discharged

[45] . Optionally, the clean and crushed fragments of the material comprising nylon 6 and polyether polyurethane

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

[0301] Figure 3 An example of a separation section [B] (the area enclosed by a dashed line) is shown, in which materials comprising nylon 6 and polyether polyurethane are separated to obtain a nylon 6-rich stream and recovered polyether polyurethane. This example includes the following sections: a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ].

[0302] In the dissolution section [α], polyether polyurethane is dissolved in an organic solvent from optionally cleaned and / or crushed fragments of material comprising polyamide 6 and polyether polyurethane

[101] to obtain a polyether polyurethane-rich stream

[104] comprising the organic solvent and dissolved polyether polyurethane, and a stream comprising undissolved nylon 6

[105] . These are discharged from the dissolution section [α]. The organic solvent is the separated organic solvent

[102] , to which fresh organic solvent

[103] is optionally added.

[0303] In the precipitation section [γ], a second solvent

[106] and a polyether-rich stream

[104] comprising an organic solvent and dissolved polyether polyurethane are mixed, causing the polyether polyurethane to precipitate from the mixture comprising the organic solvent and the second solvent. The precipitated polyether polyurethane is recovered from the mixture comprising 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

[107] . Optionally, the recovered polyether polyurethane

[107] is dried after the recovery step and before being discharged from the precipitation section [γ]. The mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered is discharged from the precipitation section [γ]

[108] , and the mixture is loaded into the solvent distillation section [δ].

[0304] In the washing section [β], the stream

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

[109] to obtain a nylon 6-rich stream

[110] and a mixture

[111] comprising an organic solvent and a third solvent. The nylon 6-rich stream

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

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

[110] is densified (not shown in 3) in the depolymerization section [C] before depolymerization to ε-caprolactam. The mixture

[111] comprising an organic solvent and a third solvent is discharged from the washing section [β] and optionally loaded into the solvent distillation section [δ].

[0305] In the solvent distillation section [δ], a mixture

[108] of the polyether polyurethane from which the precipitate has been recovered, comprising an organic solvent and a second solvent, and optionally a mixture

[111] comprising an organic solvent and a third solvent, is distilled to obtain separated organic solvent

[102] , second solvent and third solvent

[112] , and residue

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

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

[102] is then loaded into the dissolving section [α].

[0306] Figure 4A An embodiment of the purification section [E] (the area enclosed by dashed lines) is depicted, which includes the following sections: In the extraction section [K], crude ε-caprolactam

[201] is extracted with an organic solvent

[202] to obtain an aqueous phase

[203] comprising water and impurities and an organic phase

[204] comprising the organic solvent, ε-caprolactam, and impurities. Both phases are discharged from the extraction section [K].

[0307] In an optional washing section [L], the organic phase comprising the organic solvent, ε-caprolactam, and impurities is washed with water or an alkaline aqueous solution

[205]

[204] to obtain an aqueous phase comprising residues

[206] and a washed organic phase comprising the organic solvent, ε-caprolactam, and impurities

[207] . Both phases are discharged from the washing section [L].

[0308] In an optional back-extraction section [M], an organic phase

[207] , optionally washed, comprising organic solvent, ε-caprolactam, and impurities, is back-extracted with water

[208] to obtain an organic solvent phase

[209] comprising impurities and an aqueous phase

[210] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam. Both phases are then discharged from the back-extraction section [M].

[0309] In an optional stripping and concentration section [N], residual organic solvent and water are removed from an aqueous phase

[210] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam by stripping and / or distillation to obtain residual organic solvent and water

[211] and an aqueous phase

[212] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, which has been stripped and concentrated. Both phases are discharged from the stripping and concentration section [N].

[0310] Optionally, an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, which has been optionally desorbed from it by stripping and / or distillation, is oxidized with an oxidizing agent to obtain an oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities. Figure 4A (Not shown in the image).

[0311] Optionally, the oxidized ε-caprolactam-aqueous phase, comprising water, ε-caprolactam, and impurities, is filtered before being loaded into the next section to remove solid manganese oxide (IV) particles. Figure 4A (Not shown in the image).

[0312] In an optional hydrogenation section, the stripped and concentrated aqueous phase, comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam

[212] , is hydrogenated with hydrogen in the presence of a heterogeneous catalyst before being loaded into the next section to obtain a hydrogenated ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities. Figure 4A (Not shown in the image).

[0313] In an optional distillation section [O], the stripped and concentrated aqueous phase

[212] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam is distilled to remove impurities with boiling points below or above ε-caprolactam

[214] and optional organic solvent or water ( Figure 4A (not shown in the image), thereby obtaining a distilled ε-caprolactam phase

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

[215] is loaded into the crystallization section [P]. Optionally, before distillation in the distillation section [O], an alkali metal hydroxide

[213] is added to a stripped and concentrated aqueous phase

[212] comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam.

[0314] In the crystallization zone [P], the optionally distilled ε-caprolactam phase

[215] is crystallized at a temperature of 10°C to 95°C to remove impurities from the ε-caprolactam

[217] , thereby obtaining high-purity ε-caprolactam

[218] . All crystallized products are discharged from the crystallization zone [P]. Optionally, a solvent

[216] is loaded into the crystallization zone [P] prior to crystallization.

[0315] Figure 4B An embodiment of the purification section [E] (the area enclosed by dashed lines) is depicted, which includes the following sections: In the extraction section [K], crude ε-caprolactam

[401] is extracted with an organic solvent

[402] to obtain an aqueous phase

[403] comprising water and impurities and an organic phase

[404] comprising the organic solvent, ε-caprolactam, and impurities. Both phases are discharged from the extraction section [K].

[0316] In an optional washing section [L], the organic phase comprising the organic solvent, ε-caprolactam, and impurities is washed with water or an alkaline aqueous solution

[405]

[404] to obtain an aqueous phase comprising residues

[406] and a washed organic phase comprising the organic solvent, ε-caprolactam, and impurities

[407] . Both phases are then discharged from the washing section [L].

[0317] In an optional solvent exchange distillation section [R], an organic phase

[407] , optionally washed and comprising organic solvent, ε-caprolactam, and impurities, is subjected to solvent exchange distillation by adding water

[408] to obtain a stream comprising organic solvent

[409] and an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam

[410] . Both products are discharged from the solvent exchange distillation section [R].

[0318] In an optional stripping and concentration section [N], residual organic solvent and water are removed from an aqueous phase

[410] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam by stripping and / or distillation to obtain residual organic solvent and water

[411] and an aqueous phase

[412] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, which has been stripped and concentrated. Both phases are discharged from the stripping and concentration section [N].

[0319] Optionally, an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, which has been optionally desorbed from it by stripping and / or distillation, is oxidized with an oxidizing agent to obtain an oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities. Figure 4B (Not shown in the image).

[0320] Optionally, the oxidized ε-caprolactam-aqueous phase, comprising water, ε-caprolactam, and impurities, is filtered before being loaded into the next section to remove solid manganese oxide (IV) particles. Figure 4B (Not shown in the image).

[0321] In an optional hydrogenation section, the stripped and concentrated aqueous phase, comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam

[412] , is hydrogenated with hydrogen in the presence of a heterogeneous catalyst before being loaded into the next section to obtain a hydrogenated ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities. Figure 4B (Not shown in the image).

[0322] In an optional distillation section [O], the stripped and concentrated aqueous phase

[412] comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam is distilled to remove impurities with boiling points below or above ε-caprolactam

[414] and optional organic solvent or water ( Figure 4B(not shown in the image), thereby obtaining a distilled ε-caprolactam phase

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

[415] is loaded into the crystallization section [P]. Optionally, before distillation in the distillation section [O], an alkali metal hydroxide

[413] is added to a stripped and concentrated aqueous phase

[412] comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam.

[0323] In the crystallization zone [P], the optionally distilled ε-caprolactam phase

[415] is crystallized at a temperature of 10°C to 95°C to remove impurities from the ε-caprolactam

[417] , thereby obtaining high-purity ε-caprolactam

[418] . All crystallized products are discharged from the crystallization zone [P]. Optionally, a solvent

[416] is loaded into the crystallization zone [P] prior to crystallization.

[0324] Example The following examples are provided to explain the invention in more detail, particularly with respect to certain forms of the invention. However, these examples are not intended to limit this disclosure.

[0325] ε-caprolactam, which can be used in all major nylon 6 polymerization applications without dilution with higher purity ε-caprolactam, meets all of the following specifications: PAN: Maximum value 5 E290: Maximum value 0.05 VB: Maximum value 0.5 mmol / kg Alkalinity: Maximum value 0.1 mmol / kg Acidity: Maximum value 0.1 mmol / kg The parameters and measurement methods are defined as follows: PAN: ISO DIS 8660 — Plastics — Determination of permanganate index of caprolactam — Spectroscopic method, a revision of ISO 8660, 1988. E290: ISO 7059 - Industrial caprolactam - Determination of absorbance at 290 nm. Volatile bases (VB) ISO 8661 - Industrial caprolactam - Determination of volatile base content - Titration after distillation.

[0326] Alkalinity of ε-caprolactam products: determined by using a Tashiro indicator at 25°C at a concentration of 0.1 wt. / v. 乙醇 %Methylene Blue: 0.1 wt. / v 乙醇Alkalinity is determined by titration with 1:2 ratio of methyl red, with an endpoint of gray. First, titrate a flask containing water and indicator to gray. Then, add X grams of an aqueous solution of ε-caprolactam containing Y wt.% ε-caprolactam (as determined by refractive index), and titrate the solution back to gray using 0.01 N H₂SO₄ solution (if the solution is alkaline) or 0.01 N NaOH solution (if the solution is acidic).

[0327] The alkalinity is then given as follows: Alkalinity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) in: v = Volume of added H2SO4 solution (ml) t = equivalent concentration of H2SO4 solution (= 0.01 N) X = Sample weight (g) Y = Concentration of ε-caprolactam (wt.%) The acidity is then given by the following: Acidity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) in: v = Volume of NaOH solution added (ml) t = molar concentration of NaOH solution (= 0.01 N) X = Sample weight (g) Y = Concentration of ε-caprolactam (wt.%) Example 1 Pretreatment and separation of Nylon 6 and polyether polyurethane.

[0328] The polyether polyurethane content of the used nylon 6 waste fabric containing polyether polyurethane is 20.2 + / - 0.4 wt.% (as determined by differential scanning calorimetry (DSC) ISO11357-3 -130℃ to 300℃_10℃ / min_dt 1.00 s). The color of the used nylon 6 waste fabric containing polyether polyurethane (Spandex) is purple.

[0329] Waste nylon 6 fabric containing polyether polyurethane was cut into small pieces, each piece ranging from 5 to 20 cm in size. 2Within the specified range. 150.4 g of this cutting material was dissolved in 800 g of DMAc at 70°C for 1 hour with stirring. The resulting solution was then vacuum filtered through a heated double-walled Buchner funnel at 70°C to obtain filtrate 1. The remaining undissolved material was further processed three times, each time with 400 g of DMAc and stirred at 70°C for 1 hour, to obtain filtrates 2 to 4.

[0330] At 65°C, undissolved material on the Buchner funnel was washed with 325 g of water to obtain filtrate 5. The undissolved material was then dried and weighed. The five filtrates were combined to obtain a precipitate and a clear solution. The precipitate was filtered off and washed with 200 g of water, then dried and weighed. The total weight of the dried precipitate and the dried undissolved material was almost equal to the weight of the starting material.

[0331] Differential scanning calorimetry (DSC) analysis revealed that the dry precipitate contained approximately 100 wt.% polyether polyurethane (polyamide 6 was not detected). Based on DSC analysis, the dry, undissolved material contained approximately 100 wt.% nylon 6 (polyether polyurethane was not detected).

[0332] The recycled polyether polyurethane precipitate can be reused in textile production, either in this way or in combination with virgin polyether polyurethane.

[0333] This example demonstrates that polyether polyurethane and nylon 6 can be separated from waste nylon 6 fabrics containing polyether polyurethane by selective extraction, precipitation of dissolved polyether polyurethane, and washing of undissolved nylon 6. After drying, the recovered precipitate consists of almost pure polyether polyurethane, and the dried undissolved material contains almost 100 wt.% nylon 6.

[0334] Example 2 Pretreatment and separation of Nylon 6 and polyether polyurethane.

[0335] The polyether polyurethane content of the used nylon 6 waste fabric containing polyether polyurethane is 8.15 + / - 0.05 wt.% (as determined by differential scanning calorimetry (DSC) ISO11357-3 -130℃ to 300℃_10℃ / min_dt 1.00 s). The color of the used nylon 6 waste fabric containing polyether polyurethane (Spandex) is salmon pink.

[0336] Following the procedure of Example 1, except that 254.1 g of waste nylon 6 fabric containing polyether polyurethane was now dissolved in 1200 g of DMAc and, after filtration, washed with 475 g of DMAc and 350 g of water, the remaining undissolved material on the Buchner funnel was removed. The precipitate was filtered off and washed with 200 g of water, and then dried. The undissolved material on the Buchner funnel was washed with 1000 g of water, then dried, and then weighed. The weight fraction of the dried precipitate was 8 wt.% of the total weight of the dried precipitate and the dried undissolved material, which was almost equal to the weight of the starting material.

[0337] Differential scanning calorimetry (DSC) analysis revealed that the dry precipitate contained approximately 100 wt.% polyether polyurethane (Nylon 6 was not detected). The dry, undissolved material contained approximately 99 wt.% Nylon 6 and less than 1 wt.% polyether polyurethane (DSC analysis).

[0338] The recycled polyether polyurethane precipitate can be reused in textile production, either in this way or in combination with virgin polyether polyurethane.

[0339] This example demonstrates that polyether polyurethane and nylon 6 can be separated from waste nylon 6 fabric containing polyether polyurethane through selective extraction, precipitation of dissolved polyether polyurethane, and washing of undissolved nylon 6. Upon drying, the recovered precipitate consists of almost pure polyether polyurethane, and the content of polyether polyurethane in the undissolved material is significantly lower than that in the starting material, the waste nylon 6 fabric containing polyether polyurethane. The obtained polyether polyurethane is of high quality and can be used, either alone or in combination with virgin polyether polyurethane, for demanding subsequent applications, including spinning.

[0340] Example 3 Depolymerization of Nylon 6 and recovery of ε-caprolactam.

[0341] Before loading the dried, undissolved material obtained in Example 1 into the depolymerization reactor, the material was first densified. The dried, undissolved material was melted at 237°C under nitrogen atmosphere, and forced through a perforated metal plate. The resulting strands were cooled to room temperature and cut into pellets. The resulting pellets had a diameter of 3 mm and a length of 1 cm.

[0342] 33.6 g of nylon 6 granules and 9.5 g of 20 wt.% phosphoric acid were charged into a Premex autoclave. The reactor contents were first heated under nitrogen, followed by continuous injection of superheated steam at a rate of 2.7 g / min over a 120-minute reaction period. The temperature and pressure in the reactor were maintained at 260 °C and 0.11 MPa, respectively. During the reaction, the steam stream was continuously discharged from the reactor and cooled to approximately 20 °C, yielding a condensate comprising ε-caprolactam and water.

[0343] The condensate, consisting of 25.7 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 57.3 wt.% ε-caprolactam by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is crude ε-caprolactam and is the mixture to be purified.) The specifications for crude ε-caprolactam are as follows: PAN: 353 E290: 2.91 This example demonstrates that crude ε-caprolactam can be obtained in good yield and without operational problems by depolymerization of nylon 6, which is obtained by extraction and separation from nylon 6 waste fabric containing polyether polyurethane.

[0344] Example 4 Depolymerization of Nylon 6 and recovery of ε-caprolactam.

[0345] Follow the procedure of Example 3, except that 48 grams of granules made from the dry, undissolved material obtained in Example 2 and 14 grams of 20 wt.% phosphoric acid are now loaded into the Premex autoclave.

[0346] The condensate, consisting of 41 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 63.1 wt.% ε-caprolactam by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is crude ε-caprolactam and is the mixture to be purified.) The specifications for crude ε-caprolactam are as follows: PAN: 274 E290: 2.70 This example demonstrates that crude ε-caprolactam can be obtained in good yield and without operational problems by depolymerization of nylon 6, which is obtained by extraction and separation from nylon 6 waste fabric containing polyether polyurethane.

[0347] Comparative Experiment 1 Depolymerization of Nylon 6 and polyether polyurethane.

[0348] Follow the procedure of Example 3, 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 are now loaded into the Premex autoclave.

[0349] The experiment had to be stopped 10 minutes after the injection of superheated steam began because the pipeline used to discharge the steam stream, which included ε-caprolactam and water, was blocked by insoluble material.

[0350] The comparative experiment shows that depolymerizing waste nylon 6 fabrics containing polyether polyurethane without removing the polyether polyurethane before depolymerization will cause operational problems.

[0351] Comparative Experiment 2 Depolymerization and ε-caprolactam recovery of waste nylon 6 fabrics containing polyether polyurethane.

[0352] Following the procedure of Example 3, except that 48 grams of granules made from nylon 6 waste fabric containing polyether polyurethane at a content of 20.2 + / - 0.4 wt.% (the same raw material used in Example 1) and 14 grams of 20 wt.% phosphoric acid are now loaded into a Premex autoclave.

[0353] The condensate, consisting of 26 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 47.0 wt.% ε-caprolactam by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is crude ε-caprolactam and is the mixture to be purified.) The specifications for crude ε-caprolactam are as follows: PAN: 352 E290: 3.60 Observation results: ●The condensate obtained before concentration does indeed contain unknown precipitates. ● Following the depolymerization experiment, fouling was found on the inner wall of the Premex autoclave. This comparative experiment again demonstrates that depolymerizing waste nylon 6 fabrics containing polyether polyurethane without removing the polyether polyurethane before depolymerization presents operational problems. Another observation is that the ε-caprolactam yield in this comparative experiment was significantly lower than that in Example 3 (using pretreated waste nylon 6 fabrics containing polyether polyurethane as feed), where the ε-caprolactam yield was defined as the ratio of the weight of ε-caprolactam in the condensate to the weight of nylon 6 in the feed to the Premex autoclave.

[0354] Comparative Experiment 3 Purification is achieved through distillation.

[0355] Then, 75 mmol of sodium hydroxide aqueous solution / kg ε-caprolactam was added to the crude ε-caprolactam obtained in Comparative Experiment 2. The mixture was then distilled by stepwise pressure reduction. The ε-caprolactam was distilled at 300 Pa.

[0356] The specifications for distilled ε-caprolactam are as follows: PAN: 31 E290: 3.08 VB: 2.45 mmol / kg Alkalinity: 3.13 mmol / kg.

[0357] The comparative experiment showed that the quality of ε-caprolactam obtained by depolymerizing nylon 6 waste fabric containing polyether polyurethane (without pretreatment in the separation section, which separates the nylon 6 and polyether polyurethane materials into a nylon 6-rich stream and a polyether polyurethane-rich stream), and subsequently by distillation concentration and purification, was very poor because it did not meet any of the specifications required for the main polymerization applications.

[0358] Comparative Experiment 4 Depolymerization and ε-caprolactam recovery of waste nylon 6 fabrics containing polyether polyurethane, via permanganate treatment Purification is achieved through physicochemical processes and distillation.

[0359] In this comparative experiment, waste nylon 6 fabric containing polyether polyurethane with a polyether polyurethane content of 8.15 + / - 0.05 wt.% (the same raw material used in Example 2) was used.

[0360] Follow the pretreatment and separation procedures for nylon 6 and polyether polyurethane as in Example 1. The recovered polyether polyurethane precipitate can be reused in textile production, either directly or in combination with virgin polyether polyurethane. Follow the depolymerization and recycling procedures as in Example 3.

[0361] The obtained crude ε-caprolactam was treated with 0.2 wt.% KMn relative to ε-caprolactam. The mixture was treated at 50°C for 2 hours. The resulting solids were then removed from the oxidation reaction products by filtration.

[0362] The ε-caprolactam in the oxidation reaction product was further purified by distillation as described in Comparative Example 3 after the addition of 75 mmol of aqueous sodium hydroxide solution / kg ε-caprolactam.

[0363] The specifications for distilled ε-caprolactam are as follows: PAN: 3 E290: 0.12 VB: 1.45 mmol / kg Alkalinity: 1.96 mmol / kg This comparative experiment leads to the conclusion that purifying crude ε-caprolactam by permanganate treatment followed by distillation is insufficient to meet all the specifications required for major polymerization applications as described above.

[0364] Example 5 Depolymerization and ε-caprolactam recovery of waste nylon 6 fabrics containing polyether polyurethane, through extraction and back-extraction. Purification is achieved through extraction, distillation, and crystallization.

[0365] In this example, waste nylon 6 fabric containing polyether polyurethane with a polyether polyurethane content of 8.15 + / - 0.05 wt.% (the same raw material used in Example 2) was used.

[0366] Follow the pretreatment and separation procedures for nylon 6 and polyether polyurethane as in Example 1. The recovered polyether polyurethane precipitate can be reused in textile production, either directly or in combination with virgin polyether polyurethane. Follow the depolymerization and recycling procedures as in Example 3.

[0367] The obtained 76 g of crude ε-caprolactam was extracted once at 25 °C with a solvent mixture of 100 g of 4-methyl-2-pentanol (50 wt.%) / cyclohexane (50 wt.%) and nine times with 50 g of the same solvent mixture. The ten resulting ε-caprolactam phases, including the solvent mixture, were combined and subsequently concentrated by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65 °C) to an ε-caprolactam concentration of approximately 40 wt.%, followed by the addition of fresh cyclohexane. The resulting mixture had an ε-caprolactam concentration of approximately 25 wt.%, and the 4-methyl-2-pentanol / cyclohexane weight ratio was 50 wt.% : 50 wt.%. Subsequently, the concentrated solvent mixture including ε-caprolactam was extracted eight times with 50 g of water at 25 °C. The eight resulting ε-caprolactam aqueous phases were combined. The combined aqueous phases were concentrated to an ε-caprolactam concentration of 47.1 wt.% by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). The specifications of the obtained concentrated ε-caprolactam aqueous solution are as follows: PAN: 108 E290: 1.58 Add 75 mmol of sodium hydroxide aqueous solution / kg ε-caprolactam to the obtained concentrated ε-caprolactam aqueous solution.

[0368] Subsequently, water and impurities with boiling points lower than ε-caprolactam were removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, at 300 Pa, the distilled ε-caprolactam was recovered as a top product, while impurities with higher boiling points than ε-caprolactam were retained in the distillation apparatus as bottom products.

[0369] 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 crystallization apparatus at 52°C. The aqueous ε-caprolactam was cooled to 40°C, and 0.016 g of seed crystals were added to the mixture. The mixture was then further 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 purified ε-caprolactam obtained was of the following specifications: PAN: 2 E290: 0.02 VB: < 0.01 mmol / kg Alkalinity: 0.03 mmol / kg This experiment leads to the conclusion that purified ε-caprolactam, meeting all specifications required for major polymerization applications, can be obtained from the depolymerization of nylon 6 derived from waste nylon 6 fabrics containing polyether polyurethane, and purified by extraction, back-extraction, distillation, and crystallization.

[0370] All the examples above demonstrate that the present invention is capable of producing good products from colored raw materials, particularly colored raw materials derived from waste textiles, as presented.

[0371] Example 6 Pretreatment and separation of nylon 6 and polyether polyurethane, depolymerization of nylon 6 and recovery of ε-caprolactam, and extraction Purification is achieved through extraction, back-extraction, distillation, and crystallization.

[0372] Follow the pretreatment and separation procedures for nylon 6 and polyether polyurethane as in Example 1. The recovered polyether polyurethane precipitate can be reused in textile production, either directly or in combination with virgin polyether polyurethane. Follow the depolymerization and recycling procedures as in Example 3.

[0373] The obtained 36 g of crude ε-caprolactam was extracted once with 68 g of benzene and four times with 50 g of benzene at 25 °C. The resulting organic extracts were combined and concentrated by evaporation to approximately 25 wt.% ε-caprolactam concentration in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65 °C). This mixture was then extracted twice with 25 g of water in batches at approximately 25 °C. The specifications of the aqueous ε-caprolactam solution after back-extraction were as follows: PAN: 132 E290: 1.55 Then, 75 mmol of sodium hydroxide aqueous solution / kg ε-caprolactam was added to the concentrated ε-caprolactam solution. Subsequently, water and impurities with boiling points lower than ε-caprolactam were removed as top products by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, the distilled ε-caprolactam was recovered as a top product at 300 Pa, while impurities with higher boiling points than ε-caprolactam were retained in the distillation apparatus as bottom products.

[0374] 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 crystallization apparatus at 52°C. The aqueous ε-caprolactam was cooled to 40°C, and some seed crystals were added to the mixture. The mixture was then further cooled to 30°C and held for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85 wt.% aqueous solution of ε-caprolactam. The purified ε-caprolactam obtained met all specifications required for the main polymerization applications.

[0375] This example demonstrates that polyether polyurethane and nylon 6 can be separated from waste nylon 6 fabrics containing polyether polyurethane through selective extraction, precipitation of dissolved polyether polyurethane, and washing of undissolved nylon 6. Furthermore, this experiment concludes that purified ε-caprolactam meeting all specifications required for major polymerization applications can be obtained by depolymerizing nylon 6 from discarded waste nylon 6 fabrics containing polyether polyurethane and purified by extraction, back-extraction, distillation, and crystallization.

[0376] Experiment 7 Calculation of the carbon footprint of purified ε-caprolactam and polyether polyurethane.

[0377] A continuous process according to the present invention for producing purified ε-caprolactam and polyether polyurethane from nylon 6 waste fabrics containing polyether polyurethane was simulated. These nylon 6 waste fabrics containing polyether polyurethane had a polyether polyurethane content of 20 wt.%, with the remainder being primarily nylon 6.

[0378] The method includes: - Cut the waste nylon 6 fabric containing polyether polyurethane into small pieces. - Selective extraction of polyether polyurethane using DMAc; - Separate undissolved nylon 6 and polyether-rich polyurethane stream by centrifugation; - Wash the undissolved nylon 6 with water; - Separate the washed undissolved nylon 6 and aqueous extract by centrifugation; - The polyether polyurethane is precipitated from the polyether polyurethane-rich stream by adding the aqueous extract obtained above. - Separate the precipitated polyether polyurethane from the DMAc-water mixture by filtration; -Recover DMAc and water from the DMAc-water mixture obtained above; -Dried and filtered polyether polyurethane; -Dry and wash the undissolved nylon 6; - Melt and granulate the washed, undissolved nylon 6; - Nylon 6 is depolymerized under the action of H3PO4 and superheated steam; - Crude ε-caprolactam (80 wt.% ε-caprolactam) was recovered by partial condensation of the vapor discharged from the depolymerization reactor; - Crude ε-caprolactam was concentrated by countercurrent extraction with toluene; - Wash the organic extract with a diluted caustic alkali solution; - Organic extracts washed with countercurrent water extraction; -Evaporate and concentrate the aqueous extract; - Add caustic soda; - Light and heavy substances are removed by vacuum distillation; and - Pure ε-caprolactam was recovered by melt crystallization at a temperature of 61°C.

[0379] The carbon footprint of purified ε-caprolactam and polyether polyurethane was calculated based on raw material consumption data, and the utility of the above method was based on data from ecoinvent version 3.7.1. The environmental impact distribution between the purified ε-caprolactam and polyether polyurethane products in the pretreatment and separation sections was based on the weight ratio of these products.

[0380] The results revealed that the carbon footprint of purified ε-caprolactam obtained from waste polyamide 6 fabric containing polyether polyurethane was less than 3.0 tCO2 equivalents / t ε-caprolactam and less than 1.0 tCO2 equivalents / t polyether polyurethane (location: Europe).

[0381] Although the 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, including (semi-)continuous operation and commercial-scale upgrades, can be made therein without departing from the spirit and scope of the invention.

Claims

1. A method for recovering ε-caprolactam and polyether polyurethane from a material comprising nylon 6 and polyether polyurethane in a plant, wherein said plant includes -Separation section [B], -De-aggregation section [C], -Reclaim section [D], and -Purification section [E], And the method described therein includes the following steps: a) The material comprising nylon 6 and polyether polyurethane is loaded into the separation section [B]; b) In the separation section [B], the material comprising nylon 6 and polyether polyurethane is separated into a nylon 6-rich stream and a polyether polyurethane-rich stream by selectively dissolving the polyether polyurethane in an organic solvent at a temperature below 100°C, preferably in the range of 0°C to 100°C, more preferably in the range of 10°C to 90°C, even more preferably in the range of 10°C to 80°C, and most preferably in the range of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane. c.1) Discharge the nylon 6-rich stream from the separation section [B] and load the nylon 6-rich stream into the depolymerization section [C], wherein the nylon 6 content in the nylon 6-rich stream is at least 85% by weight based on dry weight. c.2) In the depolymerization zone [C], the nylon 6 in the nylon 6-rich stream is depolymerized at a temperature ranging from 180°C to 400°C, preferably from 200°C to 350°C, more preferably from 220°C to 340°C, and most preferably from 240°C to 325°C, so as to obtain a stream containing ε-caprolactam, and the obtained stream containing ε-caprolactam is discharged from the depolymerization zone [C]. c.3) Recover crude ε-caprolactam from the feed stream containing ε-caprolactam in the recovery section [D]; c.4) Purify the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification includes the following steps: (i) The crude ε-caprolactam is extracted with an organic solvent to obtain an aqueous phase and an organic phase, wherein the organic phase comprises the organic solvent, ε-caprolactam, and impurities, and preferably the organic solvent is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof; and The obtained organic phase may be washed with water or an alkaline aqueous solution. (ii) The solvent is switched 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 below or above ε-caprolactam, wherein the solvent switching is selected from a process based on water back-extraction and a process based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added; and (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation; (iv) Purified ε-caprolactam was obtained 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 the polyether-rich stream in the separation section [B]; and d.2) The recovered polyether polyurethane is discharged from the separation section [B], wherein the polyether polyurethane content in the discharged stream is at least 85% by weight based on dry weight.

2. The method according to claim 1, wherein the separation segment [B] comprises: -Dissolution zone [α], - Washing section [β], -precipitation zone [γ], and - Solvent distillation section [δ], And the method described therein includes the following steps in the separation section [B]: b.1) An organic solvent and a material including nylon 6 and polyether polyurethane are loaded into the dissolution section [α]; b.2) In the dissolution zone [α], the polyether polyurethane is dissolved from the material comprising nylon 6 and polyether polyurethane in an organic solvent, such that a polyether polyurethane-rich stream comprising the organic solvent and dissolved polyether polyurethane and a stream comprising undissolved nylon 6 are obtained, and the obtained stream is discharged from the dissolution zone [α]. b.3) The second solvent and the polyether-rich stream comprising the organic solvent and dissolved polyether polyurethane are fed into the precipitation section [γ], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent. b.4) Recover the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharge the precipitated polyether polyurethane from the precipitation section [γ]. b.5) ​​Discharge the mixture comprising the organic solvent and the second solvent from the precipitation section [γ] from which the precipitated polyether polyurethane has been recovered in step b.4), and load the mixture into the solvent distillation section [δ]. b.6) The third solvent and the feed stream including undissolved nylon 6 are loaded into the washing section [β]; b.7) The stream containing undissolved nylon 6 is washed in the washing section [β] with the third solvent to obtain a stream rich in nylon 6 and a mixture comprising an organic solvent and a third solvent; b.8) Discharge the nylon 6-rich stream from the washing section [β]; b.9) Discharge the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β], and partially or completely load the mixture into the solvent distillation section [δ]. b.10) The organic solvent is separated from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and the second solvent, the third solvent and the separated organic solvent are discharged from the solvent distillation section [δ].

3. The method according to claim 2, wherein the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is loaded into the dissolution section [α] in step b.1).

4. The method according to any one of claims 1 to 3, wherein (i) The nylon 6-rich stream discharged from the separation section [B] is dried and / or densified before being loaded into the depolymerization section [C]; and / or (ii) The depolymerization in step c.2) is carried out in the presence of water, such that the feed stream comprising ε-caprolactam is a vapor feed stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50, specifically 1:2 to 1:15, 1:2 to 1:10, or 1:3 to 1:8; and / or (iii) Introduce superheated steam at a temperature range of 220°C to 575°C, specifically 275°C to 500°C, into the depolymerization zone [C]. (iv) The depolymerization in step c.2) is carried out in the absence of a catalyst or in the presence of phosphoric acid, preferably in the absence of a catalyst or in the presence of phosphoric acid, wherein the catalyst is selected from acid and base catalysts, wherein the acid catalyst is selected from the group consisting of: phosphoric acid, boric acid, sulfuric acid, organic acid, organic sulfonic acid, solid acid, salts of the above acids, Al2O3 and SiO2 and combinations thereof, especially phosphoric acid, and wherein the base catalyst is selected from the group consisting of: alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof, especially sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate.

5. The method according to any one of claims 2 to 4, wherein the recovered polyether polyurethane discharged from the precipitation section [γ] in step b.4) is reused in the production of textiles.

6. The method according to any one of claims 1 to 5, wherein the plant further comprises -Preprocessing section [A], Furthermore, prior to step a), the material comprising nylon 6 and polyether polyurethane is subjected to pretreatment in the pretreatment section [A], specifically to cleaning in the cleaning section [ω] and / or to mechanical dimensional reduction in the mechanical dimensional reduction section [λ].

7. The method according to any one of claims 1 to 6, wherein the organic solvent is dimethylacetamide, and optionally, wherein the second solvent and the third solvent are the same solvent, preferably an aqueous solution or water.

8. The method according to any one of claims 1 to 7, wherein the second solvent in step b.3) is partly or entirely the mixture comprising the organic solvent and the third solvent obtained from the washing section [β] in step b.7).

9. The method according to any one of claims 1 to 8, 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], optionally after being separated from each other, and reused in the precipitation section [γ] and / or the washing section [β].

10. The method according to any one of claims 3 to 9, wherein the degradation products of the organic solvent are removed before the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is loaded into the dissolution section [α] in step b.1).

11. The method according to any one of claims 1 to 10, wherein the solution comprising ε-caprolactam and impurities from which ε-caprolactam crystallizes in step c.4)(iv) further comprises water, preferably more than 1% by weight.

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