Method for recovering ε-caprolactam and polyether polyurethane from a material containing nylon 6 and polyether polyurethane

Abstract

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

Description

Technical Field

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

Background Art

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

[0003]

Chemical Formula

[0004] It is well known that ε-caprolactam can be prepared by liquid-phase Beckmann rearrangement of cyclohexanone oxime in the presence of fuming sulfuric acid or a mixture of sulfuric acid and SO3, or by gas-phase Beckmann rearrangement of cyclohexanone oxime in the presence of a solid catalyst. Generally, this type of ε-caprolactam is called "virgin ε-caprolactam". The cyclohexanone oxime required for the formation of (virgin) ε-caprolactam can be prepared mainly from cyclohexanone produced from benzene. Most of this benzene is derived from non-renewable fossil resources such as petroleum and coal.

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

[0006] The process for producing nylon 6 is described, for example, in the chapter "Polyamides" of Ullmann’s Encyclopedia of Industrial Chemistry (January 15, 2013), Wiley-VCH Verlag GmbH&Co.KGaA, Weinheim, Germany, which is electronically available via, for example, https: / / doi.org / 10.1002 / 14356007.a21_179.pub3.

[0007] Initially, pure nylon 6 was used in the production of fabrics such as stockings. Later, various blended fabrics have been developed and produced to improve the properties of the fabric. In particular, polyether polyurethane is incorporated into a wide range of nylon 6-based clothing. The advantages of polyether polyurethane are its remarkable strength and elasticity, as well as its ability to stretch and return to its original shape after drying faster than ordinary fabrics. In many cases, by applying a blended yarn made by blending nylon 6 fibers and polyurethane fibers, an improvement in the elasticity of the fabric can be obtained. Examples of clothing containing blended fabrics of nylon 6 and polyether polyurethane include stretch stockings, underwear, and sportswear. By processing (e.g., coating) the surface of nylon 6 fibers with a polyurethane resin, the moisture permeability and waterproofness of the fabric can be obtained. These surface-treated yarns are often used in the production of, for example, raincoats, cold-proof clothing, and skiwear. Polyether polyurethane used in the production of clothing is often also called spandex, Lycra, or elastane.

[0008] Recycling nylon 6 enables the conservation of fossil resources and can add value to the circular economy. Mechanical recycling of waste nylon 6 includes processes that convert waste nylon 6 into secondary raw materials or into products where the chemical structure of the material changes (preferably) only minimally. The conversion and depolymerization of nylon 6 are two forms of the chemical recycling of nylon 6. In the conversion recycling process, nylon 6 is decomposed into an oily or gaseous feedstock that can replace newly extracted fossil feedstocks. The resulting product can be used to produce chemicals containing the monomer ε-caprolactam. In the depolymerization recycling process, nylon 6 is decomposed into its monomer building block ε-caprolactam. The depolymerization of nylon 6 to ε-caprolactam is the reverse reaction of the ring-opening polymerization of ε-caprolactam.

[0009]

Chem.

[0010] Generally, prior art chemical recycling or recycling processes that depolymerize essentially pure nylon 6 into ε-caprolactam monomers include a decomposition step by hydrolysis at high temperature in the presence of water and a regeneration step of the monomers formed by steam distillation.

[0011] The depolymerization recycling process of nylon 6 is well developed for clean and fairly pure waste nylon 6 materials. Due to mechanical recycling always resulting in downcycling, the properties of products made from mechanically recycled nylon-6 are always lower than those made from virgin nylon 6. However, the chemical recycling of waste nylon 6-containing materials can also enable the production of high-purity ε-caprolactam having properties similar to those of virgin ε-caprolactam, which can later be converted to high-grade nylon 6. Unfortunately, the chemical recycling of waste nylon 6-containing materials is often hindered by the presence of impurities in the waste nylon 6 materials. Generally, as a result of the presence of these impurities, the quality of the produced ε-caprolactam is inferior compared to virgin ε-caprolactam.

[0012] The depolymerization of blends of nylon 6 and polyether polyurethane has many drawbacks compared to the depolymerization of fairly pure nylon 6. These drawbacks include, among others, clogging of pipes and other equipment parts by polyether polyurethane and its decomposition products, poor quality of the produced ε-caprolactam due to the presence of decomposition products of polyether polyurethane, a (strongly) reduced ε-caprolactam recovery rate, poisoning of the depolymerization catalyst, and thus an increased consumption of the depolymerization catalyst. Another drawback of adding a blend of nylon 6 and polyether polyurethane to the depolymerization reactor is that the polyether polyurethane is destroyed. Thus, recycling of polyether polyurethane, which is more valuable than nylon 6, is not possible.

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

[0014] Direct recycling of normal (pure) elastane waste fibers, for example from yarn production, is done by dissolving them in the used spinning solvent, which has been shown to be unsuccessful because further processing is hindered by the high viscosity.

[0015] U.S. Patent No. 6,830,715 (B1) describes a method for producing elastane yarns from a spinning solution using a recycled elastane material that overcomes the aforementioned viscosity problem by adding a secondary aliphatic amine to a mixture of (cut) elastane fibers and a spinning solvent. The dissolution of the (cut) elastane fibers is achieved at a temperature of 60°C to 150°C. Several attempts to recycle blends of nylon 6 and polyether polyurethane have already been described in the past.

[0016] JP-A-2011088943 describes a pretreatment method for separating polyether polyurethane from a nylon 6 product containing polyether polyurethane and then depolymerizing the remaining nylon 6. The pretreatment includes heating a nylon 6 product containing polyether polyurethane with a cyclic amide-containing solvent at a temperature from 80°C to the boiling point of the solvent. JP-A-2011088943 describes that the amount of the cyclic amide compound in the cyclic amide compound solvent is preferably 50% by weight or more, more preferably 85% by mass or more. Further, JP-A-2011088943 describes that the cyclic amide compound solvent may contain components other than the cyclic amide compound such as water and organic solvents, and water is particularly preferred from the viewpoint of operability. The experiments described in the above patent were carried out at a temperature of 110°C for 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 a solvent. As a result of this treatment, the polyether polyurethane is partially decomposed and dissolved in the solvent. As a result, the polyether polyurethane is removed and the solution is separated by filtration, and the obtained nylon 6 product is depolymerized. However, JP-A-2011088943 does not mention the recovery of the partially decomposed polyether polyurethane. The fate of the solvent used is also not described.

[0017] International Publication No. 2013 / 032408 (A1) describes a method for recycling polyamide fibers comprising polyamide 6 and polyamide 6,6 containing polyamide and spandex. The spandex fibers are removed from the elastomeric fabric by a process consisting of controlled thermal decomposition of spandex, a controlled washing process for removing spandex or its decomposition products from the high-purity polyamide using a suitable and sustainable solvent, preferably ethanol, and a final step for removing excess solvent from the polyamide fibers. The temperature range used during the heat treatment of the polyamide fibers (such as polyamide 6 fibers) is from 150°C to 220°C, preferably from 190°C to 216°C, preferably for 0.5 to 4 hours. Thereafter, the heat-treated fabric is preferably washed with ethanol to remove spandex and its decomposition components at a temperature in the range of 5°C to 78°C. International Publication No. 2013 / 032408 (A1) does not describe the recovery of spandex and its decomposed components from the washing solvent.

[0018] In view of the above, to date, there is no process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale. The reason why a recycling process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale has not been realized lies in the fact that in all processes for solubilizing elastane, it is necessary to apply a high solution temperature which usually far exceeds 100°C. However, at these temperatures, the decomposition of elastane starts and only low-quality materials can be obtained after recovery. As a result, the fate of the recovered elastane is often limited to incineration and landfill. Another drawback of the dissolution process at high temperatures is the formation of decomposition products of the applied solvent, which can thereby prevent the recovery of nylon-6 and polyether polyurethane and the recycling of the applied solvent.

[0019] Environmental concerns regarding the production and use of these nylon 6 and polyether polyurethane-containing materials relate to the waste generated during processing and after consumer use. These concerns can be mitigated by recycling individual components such as nylon 6 and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials that are no longer in use or disposed of. As fabrics, nylon 6 and polyether polyurethane-containing materials are particularly relevant sources of waste. These often contain significant amounts of nylon 6 and polyether polyurethane. Therefore, if there is a viable process for recovering nylon 6 and polyether polyurethane from these composite wastes, this will not only benefit the environment but also provide a new economically valuable source of nylon 6 and polyether polyurethane.

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

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

[0022] The raw material price of virgin polyether polyurethane is high, generally more than twice that of virgin nylon 6. Therefore, there is a great economic incentive for the recovery and reuse (recycling) of both discarded polyether polyurethane and nylon 6.

[0023] Problems with prior art processes typically involve consuming large amounts of energy to recover dissolved elastane from a solution because the dissolved elastane is obtained by removing the solvent by evaporation.

[0024] Also, it is necessary to purify the crude ε-caprolactam obtained by depolymerization of polyether polyurethane from nylon 6 and polyether polyurethane-containing materials without using oxidizing agents such as potassium permanganate (KMnO4) or adsorbents such as (activated) carbon and diatomaceous earth. Technologies based on these oxidizing agents and adsorbents are very laborious and produce solid waste.

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

[0026] Currently, a process for recovering high-purity ε-caprolactam from nylon 6 and polyether polyurethane-containing materials is not available despite the urgent need for such a process. In particular, a high-purity ε-caprolactam recovery process that can replace virgin ε-caprolactam grades is urgently needed for demanding applications such as high-speed melt spinning during the production of textile fibers.

[0027] Finally, there is a need for a process that can recover pure ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials on an industrial scale to process the vast amounts of nylon 6 and polyether polyurethane-containing materials, including materials discarded each year. SUMMARY OF THE INVENTION

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

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

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

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

[0032] A further object of the present invention is to provide a process for producing virgin polyether polyurethane by reacting, for example, a polyether polyol with a diisocyanate monomer, and a process for recovering both purified ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials, which have a significantly lower carbon footprint than ε-caprolactam by a new synthesis, for example, by the Beckmann rearrangement of cyclohexanone oxime.

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

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

[0035] The present invention provides a process for recovering ε-caprolactam and polyether polyurethane from nylon 6 and polyether polyurethane-containing materials in a plant, the plant comprising - a separation section [B], - a depolymerization section [C], - a recovery section [D], - a purification section [E], wherein the process comprises the following:

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

[0037] (i) A process of extracting crude ε-caprolactam with an organic solvent, whereby an aqueous phase and an organic phase are obtained, the organic phase containing the organic solvent, ε-caprolactam, and impurities. Preferably, the organic solvent is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof. Optionally, the obtained organic phase is washed with water or an aqueous alkali solution. (ii) Optionally, a process of switching the solvent by at least partially replacing the organic solvent with water or an aqueous solution, whereby an aqueous phase containing water, ε-caprolactam, and impurities having a lower or higher boiling point than ε-caprolactam is obtained. The solvent switching process is selected from a process based on back-extraction with water and a process based on solvent exchange distillation in which the organic solvent is distilled off and water is introduced. (iii) Optionally, a process of obtaining purified ε-caprolactam by distilling off impurities having a lower or higher boiling point than ε-caprolactam. (iv) A process of obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C. The process includes the above steps.

[0038] d.1) A process of recovering polyether polyurethane from the polyether polyurethane-rich stream in separation section [B]; d.2) A process of discharging the recovered polyether polyurethane from separation section [B], wherein the polyether polyurethane content of the discharged stream is at least 85% by weight on a dry weight basis. The process includes the above steps.

[0039] Surprisingly, by combining a special series of processing steps and process conditions according to the present invention, namely the order of the separation, depolymerization, recovery and purification steps defined above, ε-caprolactam can be recovered from nylon 6 and polyether polyurethane-containing materials in a high yield and in a simple and economically reasonable way. The process of the present invention is economically reasonable and advantageous from several viewpoints. First, the process of the present invention is suitable for a wide variety of materials derived from nylon 6 and polyether polyurethane-containing materials which may differ, for example, in their overall composition and / or their polyether polyurethane content and / or their nylon 6 content. Second, in the process of the present invention, it becomes possible to effectively separate nylon 6 from the polyether polyurethane compound, and as a result, high-grade nylon 6 can be obtained. Third, the process of the present invention is effective enough to obtain ε-caprolactam in a high yield. Fourth, in the process of the present invention, it becomes possible to recover polyether polyurethane which can be reused to replace polyether polyurethane produced by de novo synthesis, for example, by the reaction of polyether polyol and diisocyanate monomer. Fifth, in the process of the present invention, it becomes possible to effectively separate ε-caprolactam from non-ε-caprolactam compounds, whereby a high-purity grade of ε-caprolactam can be obtained which can replace high-purity virgin ε-caprolactam for all applications including high-speed melt spinning of nylon 6 for producing thin textile fibers. Finally, the process of the present invention makes it possible to produce ε-caprolactam having a significantly lower carbon footprint compared to ε-caprolactam produced by de novo synthesis, for example, by the Beckmann rearrangement of cyclohexanone oxime. The process of the present invention can efficiently process nylon 6 and polyether polyurethane-containing materials and reduce the environmental impact of the product.In particular, in the process of the present invention, it is possible to produce polyether polyurethane having a carbon footprint of less than 1 kg of CO2 per 1 kg of polyether polyurethane, which represents a significant improvement compared to 4.8 kg of CO2 per 1 kg of polyether polyurethane associated with the production of "virgin" polyether polyurethane obtained by chemical synthesis. In particular, in the process of the present invention, it is possible to produce purified ε-caprolactam having a carbon footprint of less than 3 kg of CO2 per 1 kg of purified ε-caprolactam, which represents a significant improvement compared to 6.4 - 7.5 kg of CO2 per 1 kg of ε-caprolactam associated with the production of "virgin" ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime.

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

[0041] The present invention also provides purified ε-caprolactam obtained by separation from nylon 6 and polyether polyurethane-containing materials by the process of the present invention, wherein the ε-caprolactam has a product carbon footprint of less than 3.0 kg of CO2 equivalent per 1 kg of purified ε-caprolactam.

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

[0043] Nylon 6 and polyether polyurethane-containing material The process of the present invention uses, as starting material, a nylon 6 and polyether polyurethane-containing material. The nylon 6 and polyether polyurethane-containing material can be (can be used as) a nylon 6 and polyether polyurethane-containing product or a material derived therefrom. The nylon 6 and polyether polyurethane-containing material can be a nylon 6 and polyether polyurethane-containing material, i.e. a plurality of nylon 6 and polyether polyurethane-containing materials or a mixture thereof. Typically, the nylon 6 and polyether polyurethane-containing material is a solid material, in particular a fibre, a yarn (i.e. a strand of fibres spun together) or a fibre-based fabric, whereby the fibres are a blend of nylon 6 fibres and polyurethane fibres or the fibres are obtained by coating the surface of the nylon 6 fibres with a polyether polyurethane resin.

[0044] The nylon 6 and polyether polyurethane-containing material can have both pre-consumer and post-consumer origins. The nylon 6 and polyether polyurethane-containing material may be a mixture of different nylon 6 and polyether polyurethane-containing materials, or a mixture comprising one or more nylon 6 and polyether polyurethane-containing materials and one or more different materials.

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

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

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

[0048] Polyether polyurethane is produced by the reaction of an isocyanate and a polyether polyol. Polyether polyols are generally produced by polymerizing cyclic ether compounds on an initiator compound. The cyclic ethers most commonly used in the production of polyether polyols are ethylene oxide, propylene oxide and 1,4-butylene oxide or tetrahydrofuran (for producing poly(butylene oxide) polyol). Poly(butylene oxide) polyol is mainly used in applications where highly hydrophobic properties are required.

[0049] In connection with the numerical values in this specification, the use of the terms "ca." or "about" typically indicates that the numerical value may be affected by measurement errors that vary the numerical value by up to ±5%. Numerical values disclosed herein with the terms "ca." or "about" also mean that they are disclosed without the term "about". Further, the numerical ranges described herein are meant to include every value and any value within that range. All upper and lower endpoints of the ranges of the same parameter disclosed herein can be combined with each other. All ranges of different parameters disclosed herein can be combined with each other. In particular, ranges of different or the same "preferred levels" are particularly compatible with each other. As used in this disclosure and the claims, the singular forms "a", "an", and "the" include the plural form "one or more" unless the context clearly indicates otherwise.

[0050] Possible pretreatment steps Before being subjected to step a) of the process of the present invention, the nylon 6 and polyether polyurethane-containing material can be pre-treated in a pre-treatment section [A], in particular size-reduced in a mechanical size reduction section [λ] and / or cleaned in a cleaning section [ω]. The pre-treatment can be carried out at a location different from where the separation section [B] is located. However, preferably, the process of the present invention is carried out in a plant further comprising a pre-treatment section [A], and before step a), the nylon 6 and polyether polyurethane-containing material is subjected to pre-treatment in the pre-treatment section [A], in particular cleaning in the cleaning section [ω] and / or mechanical size reduction in the mechanical size reduction section [λ]. This has the advantage that the nylon 6 and polyether polyurethane-containing material fed into the separation section [B] is less contaminated with foreign matter and improves the yield and purity of ε-caprolactam and polyether polyurethane produced in a plant of the present invention configured to carry out the process of the present invention. Another advantage is that the size-reduced nylon 6 and polyether polyurethane-containing material can be handled more easily.

[0051] The nylon 6 and polyether polyurethane-containing material is preferably used, used or waste nylon 6 and polyether polyurethane-containing material, or derived therefrom. The dimensions and shape of the materials depend very much on the exact application from which they are derived. The nylon 6 and polyether polyurethane-containing materials that can be used according to the present invention range, in some cases, from individual fibers and yarns present in bobbins to woven fabrics and clothing. Preferably, the nylon 6 and polyether polyurethane-containing material is a material comprising waste nylon 6 and polyether polyurethane-containing material or used nylon 6 and polyether polyurethane-containing material.

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

[0053] (Discarded) Nylon 6 and polyether polyurethane-containing materials can be mixed with the full range of other materials such as rock, glass, metallic materials, organic waste and other polymer litters (e.g., polyamide 6,6, polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE)).

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

[0055] Optionally, prior to mechanical grinding or fragmentation of the nylon 6 and polyether polyurethane-containing material, large metal pieces, rocks and other interfering materials that cause severe wear of the apparatus used for mechanical grinding or fragmentation are removed. Preferably, but not limited to these, foreign substances such as materials including polyethylene, polypropylene and polyamide 6,6 are also removed prior to mechanical grinding or fragmentation of the nylon 6 and polyether polyurethane-containing material. The removal of foreign substances can be carried out mechanically or manually. Removing these interfering materials has the advantage of significantly reducing the maintenance costs of the apparatus used for mechanical grinding or fragmentation. Furthermore, the nylon 6 and polyether polyurethane content of the material obtained after mechanical grinding or fragmentation is higher than when the interfering materials are not removed. In particular, the removal of polyamide 6,6 is 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 substance" means a non-nylon 6 and non-polyether polyurethane material or compound.

[0056] In some cases, foreign substances are separated from mechanically ground or fragmented nylon 6 and polyether polyurethane-containing materials. For this purpose, various separation processes including, but not limited to, density separation and magnetic separation are applicable. In density separation, materials of different densities are placed in a liquid of intermediate density, where the low-density materials float and are separated from the higher-density sedimenting materials. In practice, density separation is often carried out by a series of density separation steps. For example, in one step, high-density materials such as rocks, sand and metals (such as iron and lead) are separated, and in another step, low-density materials such as polyolefin polypropylene and polyethylene are separated. Magnetic separation is a process of separating the components of a mixture by using a magnet to attract magnetic materials. In a process preferably used for magnetic separation, non-magnetic materials are separated from magnetic materials. The removal of foreign substances in the ground or fragmented nylon 6 and polyether polyurethane-containing materials 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, especially in the above-mentioned pretreatment step, the nylon 6 and polyether polyurethane-containing material is cleaned by washing with a solvent, preferably at least water, before being introduced into the separation section [B]. The solvent used herein can be a single solvent or a mixture of different solvents. Preferably, in order to improve the washing efficiency, a cleaning agent in the range of 0 to 20% by weight based on the solvent is added to the solvent. NaOH is a preferred cleaning agent. Even more preferably, an aqueous solution containing 0 to 10% by weight of NaOH, and even more preferably an aqueous solution containing 0 to 5% by weight of NaOH is used in the washing step. The enhancement of the washing effect of NaOH is probably caused by the improvement of the hydrolysis of molecules such as biopolymers and non-biopolymers. Preferably, the washing solvent is heated to further improve the washing process. In another preferred embodiment, the washing process includes a final rinsing step using a cleaning agent-free (clean) washing solvent to remove the residue of the cleaning agent adhering to the nylon 6 and polyether polyurethane-containing material and the existing dirt.

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

[0059] The washing of the nylon 6 and polyether polyurethane-containing material, especially the mechanically ground or fragmented nylon 6 and polyether polyurethane-containing material, is advantageous because any (adherent) dirt is removed, so that the subsequent steps of the process of the present invention are not disturbed.

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

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

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

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

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

[0065] In one embodiment, the nylon 6 and polyether polyurethane-containing material is mechanically compressed to a smaller volume before being introduced into separation section [B]. This has the advantage that less volume is required for intermediate storage and transportation, and dosing into separation section [B] can also be facilitated.

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

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

[0068] The supply of the nylon 6 and polyether polyurethane-containing material to separation section [B] (for example, the supply to the polyether polyurethane dissolution section) can be achieved by continuous or intermittent feeding of the nylon 6 and polyether polyurethane-containing material.

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

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

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

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

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

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

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

[0076] Preferably, the nylon 6 rich stream has a weight - to - weight ratio of polyether polyurethane to nylon 6 that is at least 2 times lower, more preferably at least 3 times lower, than the weight - to - weight ratio of polyether polyurethane to nylon 6 in the nylon 6 and polyether polyurethane - containing material.

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

[0078] Preferably, the polyether polyurethane rich stream has a weight - to - weight ratio of polyether polyurethane to nylon 6 that is at least 2 times higher, more preferably at least 3 times higher, than the weight - to - weight ratio of polyether polyurethane to nylon 6 in the nylon 6 and polyether polyurethane - containing material. Feeding into the separation section (step b.1)): In the separation section [B], an organic solvent, as well as the nylon 6 and polyether polyurethane - containing material, are fed into the dissolution section [α].

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

[0080] The organic solvent fed into the dissolution section [α] can be any organic solvent that can dissolve polyether polyurethane, particularly at a temperature below 100 °C, particularly in less than 24 hours or less than 6 hours. The organic solvent used herein can also mean a mixture of an organic solvent or a liquid composition containing more than 60% by volume, preferably more than 70% by volume, 80% by volume or 90% by volume of 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, can dissolve polyether polyurethane. Preferably, the organic solvent combines good, particularly very good solubility in polyether polyurethane with poor, particularly very poor solubility in nylon 6. These properties can be tested by a simple dissolution test using a nylon 6 and polyether polyurethane-containing material or a mixture of nylon 6 and polyether polyurethane. According to a preferred embodiment, the organic solvent comprises or is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), 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 oxolane, 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 introduced into the dissolution section [α] can be the separated organic solvent (i.e., the recycled organic solvent from inside the process recovered in the solvent distillation section or an organic solvent), a fresh organic solvent (i.e., a solvent from outside the process), or a combination of the separated organic solvent and the fresh organic solvent.

[0082] The organic solvent introduced into the dissolution section [α] preferably has a low water content. Even more preferably, the organic solvent is dried (preferably by distillation or coating with a desiccant such as inorganic salts like Na2SO4, zeolite, silica, and alumina) before being introduced into the dissolution section [α]. The drying does not necessarily have to be completed. Typically, the water remaining after drying the organic solvent is acceptable if it is less than 1 wt%, preferably less than 0.1 wt%, and more preferably less than 0.02 wt%.

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

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

[0085] The temperature of the dissolution section [α] may vary, preferably less than 120°C, more preferably less than 110°C, even more preferably less than 100°C, and most preferably less than 80°C. Preferably, the temperature in the dissolution section [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. These experiments show that within these temperature ranges, a particularly complete dissolution of the polyether polyurethane is achieved while maintaining its integrity in the organic solvent. This has the advantage that the polyether polyurethane present in the nylon 6 and polyether polyurethane-containing material can be dissolved while maintaining its integrity.

[0086] The amount of the solvent (by weight, especially expressed in tons) in the dissolution section [α] may vary and can be in the range of 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 the polyether polyurethane (by weight, especially expressed in tons) in the dissolution section [α]. If the amount of the solvent is small, the dissolution rate of the polyether polyurethane decreases, and the dissolution of the polyether polyurethane may be too low or incomplete. When there is a large amount of the solvent, the dissolution rate of the polyether polyurethane becomes fast, but more energy and cost are required to recover the organic solvent used. A person skilled in the art can determine the optimal ratio of the solvent to the polyether polyurethane or the nylon 6 and polyether polyurethane-containing material through routine tests.

[0087] The dissolution section [α] may include one or more dissolution vessel containers operated in series or in parallel. Preferably, these vessel containers are equipped with additional devices for improving friction, such as stirrers, mixers or agitators for shortening the dissolution time.

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

[0089] Preferably, the second solvent is a non-solvent (also known as an anti-solvent) for the polyether polyurethane, i.e., a liquid in which the polyether polyurethane is not soluble and which causes the polyether polyurethane to precipitate. The addition of this second solvent reduces the solvent power of the solution for the dissolved polyether polyurethane, thereby causing precipitation of the precipitated 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 or is a part of the mixture containing the organic solvent and the third solvent obtained in step b.7) from the washing section [β]. As used herein, "consisting of or being a mixture" is defined as any fraction from 1 to 100 wt%, preferably from 10 to 100 wt%, more preferably from 25 to 100 wt%, and even more preferably from 75 to 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, a smaller apparatus size, lower investment costs, and a more environmentally friendly process.

[0091] The temperature in the precipitation section [γ] may 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 precipitation of the dissolved polyether polyurethane.

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

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

[0094] Polyether polyurethane recovery step (step b.4): In the polyether polyurethane recovery step, the precipitated polyether polyurethane is recovered from the mixture containing the organic solvent and the second solvent and discharged from the precipitation section [γ]. There are several suitable methods for recovering the polyether polyurethane precipitate, including but not limited to filtration, centrifugation, and decantation. Preferably, filtration is used to recover the polyether polyurethane. For filtration, the solution containing the precipitate is fed into a filter, and it is expected that the precipitate will remain on the filter while the liquid passes through the filter. When centrifugation is used as the recovery method, the solution containing the precipitate is rapidly rotated to precipitate the solid sediment (assuming the solid sediment has a higher density than the liquid). The compressed precipitate can also be obtained by pouring off the liquid. In decantation, the liquid layer is poured or suctioned from the precipitate.

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

[0096] Optionally, the recovered precipitate washed with a solvent is optionally dried before being discharged from the precipitation section [γ]. Advantageously, in step b.4) of the method of the present invention, the precipitated and recovered polyether polyurethane discharged from the precipitation section [γ] is of high quality and has been found to be optionally reusable in the production of fabrics, which is a preferred embodiment of the present invention, in combination with fresh polyether polyurethane.

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

[0098] Feed to the washing step (step b.6)): In the separation section [B], a stream containing the third liquid solvent and undissolved nylon 6 is fed into the washing section [β]. The stream containing undissolved nylon 6 fed into the washing section [β] can also contain an organic solvent adjacent to the undissolved nylon 6. The purpose of the washing section [β] is to recover the organic solvent present in the stream containing undissolved nylon 6, whereby a nylon 6-rich stream containing less organic solvent than the stream containing undissolved nylon 6 is obtained.

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

[0100] The amount of the third solvent introduced into the washing section [β] can vary and can be in the range of 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 stream containing undissolved nylon 6 introduced into the washing section [β].

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

[0102] The temperature of 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 within these temperature ranges, the organic solvent is particularly completely recovered from the stream containing undissolved nylon 6. One skilled in the art can determine the amount of the third solvent and the optimal temperature required for efficient washing of the stream containing undissolved nylon 6 through routine experiments. Washing the stream containing undissolved nylon 6 with the third solvent can be carried out with various devices, all of which are known to those skilled in the art.

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

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

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

[0106] Preferably, the polyamide 6 content of the polyamide 6-rich stream obtained 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 the dry weight.

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

[0108] As used herein, the expression "on a dry weight basis" refers to the content of a component expressed as a percentage of the total dry weight of the composition, where "dry weight" is the weight of the substance after removing water and / or other liquids from the substance by drying until the weight becomes constant. Discharge step of the mixture containing the organic solvent and the third solvent (step b.9)):

[0109] From the washing section [β], a mixture containing the organic solvent and the third solvent is discharged. Generally, the organic solvent and / or the third solvent are valuable compounds, and recovery and reuse have great economic and environmental advantages. Therefore, in a preferred embodiment of the present invention, the mixture containing the organic solvent and the third solvent is fed into the solvent distillation section [δ]. Optionally, the mixture containing the organic solvent and the third solvent is first fed and used as the second solvent in another section, particularly in the precipitation section, before being fed into the solvent distillation section [δ]. In a preferred embodiment, the mixture containing the organic solvent and the third solvent is first fed partially or completely into the precipitation section [γ] and used as the second solvent. This represents a particularly resourceful use and reuse of solvents in the process of the present invention.

[0110] Solvent separation step (step b.10)): In the solvent distillation section [δ], the different solvents used in the process of the present invention are separated and recovered again. Thereby, they can be advantageously reused. In particular, the mixture containing the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered, as well as the mixture containing the organic solvent and the third solvent discharged from the washing section [β], are separated by distillation in the solvent distillation section [δ]. Hereinafter, the mixture containing the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered, as well as the mixture containing the organic solvent and the third solvent discharged from the washing section [β], are referred to as the composite feed to the solvent distillation section [δ].

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

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

[0113] The mixture containing the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered may optionally also contain (dissolved) polyether polyurethane, (dissolved) nylon 6, a colorant, an additive, the organic solvent, and / or decomposition products of the second solvent. The mixture containing the organic solvent and the third solvent may optionally also contain (dissolved) polyether polyurethane, (dissolved) nylon 6, and decomposition products of the organic solvent and / or the third solvent.

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

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

[0116] In another preferred embodiment, the organic solvent is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidone, for example N-methyl-2-pyrrolidone (NMP, C5H9NO), tetrahydrofuran (THF; also known as oxolane, C4H8O), and the second solvent and the third solvent are both water. In a more preferred embodiment of the present invention, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), and the second solvent and the third solvent are both water. The boiling point of N,N-dimethylacetamide (DMAc) at atmospheric pressure is about 165 °C. The boiling point of water at atmospheric pressure is 100 °C.

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

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

[0119] As an alternative to feeding the feedstock to the distillation column in the liquid state, the feed to the first distillation column can be carried out in the gaseous state. In such a case, except for non-volatile compounds, the combined feed to the solvent distillation section [δ] is evaporated in a feed evaporator before being fed to the first distillation column. The non-volatile compounds are discharged from the feed evaporator. This alternative has the advantage of less contamination occurring in the distillation column. Optionally, the extraction column can be combined with the feed evaporator. Generally, DMAc including the process flow contains a certain amount of acetic acid and dimethylamine, for example, due to the decomposition of DMAc. This is also the case for the combined feed to the solvent distillation section [δ]. The boiling point of pure dimethylamine at atmospheric pressure is about 7°C. Dimethylamine dissolves very well in water. The boiling point of pure acetic acid at atmospheric pressure is about 118°C.

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

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

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

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

[0124] Often, another group of techniques called pressure swing distillation is based on the fact that the maximum azeotrope mixture is pressure-dependent. In such a system, two distillation columns are operated at different pressure levels. A feed mixture of N,N-dimethylacetamide and acetic acid is introduced into a first distillation column operated at a specific pressure. The bottom flow of the first distillation column is introduced into a second distillation column. The bottom flow of the second column is introduced into the first distillation column. High-purity N,N-dimethylacetamide and acetic acid are obtained as the top products of the distillation columns. The main advantage of pressure swing distillation is the absence of foreign substances.

[0125] Finally, there is a series of techniques for converting acetic acid into another component (e.g., salt formation by addition of a caustic substance) or selectively adsorbing acetic acid from a mixture of N,N-dimethylacetamide and acetic acid.

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

[0127] The optimal arrangement depends very strongly on both the composition and the volumetric flow rate of the combined feed to the solvent distillation section [δ]. The arrangement may include various combinations of the separation solutions described above.

[0128] When the fraction of acetic acid in the composite feed to the solvent distillation section [δ] is quite low, the solvent distillation section [δ] can include two distillation columns operated in series, followed by a neutralizer evaporation step. A feed consisting of DMAc, water, non-volatile compounds, acetic acid and dimethylamine is introduced into the first distillation column. Water and dimethylamine are distilled from the top of the first distillation column in a gaseous state, and a mixture of DMAc, acetic acid and non-volatile compounds is withdrawn from the bottom of the first distillation column (in a liquid state or as a slurry). The water vapor from the top of the first distillation column is condensed. The resulting liquid phase contains water and dissolved dimethylamine. In the dimethylamine stripping unit, dimethylamine can be removed from this liquid phase, whereby water containing (almost) no dimethylamine is obtained. The bottom flow of the first column is introduced into the second distillation column. DMAc is distilled from the top of the second distillation column in a gaseous state, and a mixture of DMAc, acetic acid and non-volatile compounds is withdrawn from the bottom of the second distillation column. The bottom flow of the second column is introduced into the neutralizer evaporation unit. A base, such as an aqueous solution of NaOH, is introduced into the neutralizer evaporation unit to neutralize acetic acid. By evaporation, gaseous DMAc is discharged from the neutralizer evaporation unit. Water also evaporates if present. A mixture of neutralized acetic acid (acetic acid-containing salt) and non-volatile compounds is discharged from the bottom of the neutralizer-evaporation unit. In practice, in order to reduce the risk of clogging the neutralizer-evaporation unit, a mixture of DMAc, neutralized acetic acid (acetic acid-containing salt) and non-volatile compounds can be withdrawn from the bottom of the neutralizer-evaporation unit. The DMAc recovered by distillation is discharged from the solvent distillation section [δ] and introduced into the dissolution section [α].

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

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

[0131] 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 that less volume is required for intermediate storage and transportation, and dosing into the depolymerization section [C] can also be facilitated.

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

[0133] Optionally, a nylon 6 rich stream is fed into a smelting furnace (for example, an extruder). In the smelting furnace, the nylon 6 rich stream is melted. Preferably, the obtained polymer melt is filtered. This has the advantage that solid impurities are removed. The melted and optionally filtered polymer melt is cooled and then fed to a pelletizer to obtain pellets. These pellets are fed into the depolymerization section [C].

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

[0135] 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 that less or no solvent is introduced into the depolymerization section [C]. The solvent introduced into the depolymerization section [C] can have a negative impact on the depolymerization process (for example, resulting in a decrease in the depolymerization reaction rate, an increase in the consumption of the catalyst, an increase in energy consumption, and the vapor stream containing ε-caprolactam and water obtained in the depolymerization section [C] can contain more impurities).

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

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

[0138] The depolymerization of the nylon 6 rich stream is achieved by raising the temperature of the nylon 6 rich stream in the depolymerization section [C] from at least 180 °C to a temperature of 400 °C or less. 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, resulting in fewer side reactions of polyamide 6 and ε-caprolactam and reactions of impurities.

[0139] Generally, the rate of formation of ε-caprolactam increases at higher temperatures. Temperatures above 400 °C are not preferred because side reactions of nylon 6 and reactions of impurities occur more frequently, resulting in the formation of a more diverse set of impurities. Some of these impurities end up in the ε-caprolactam-containing product stream discharged from the depolymerization reactor section [C]. In a preferred embodiment of the present invention, the depolymerization of the nylon 6 rich stream is carried out at a temperature in the range of 220 °C to 340 °C or 240 °C to 325 °C. In this temperature range, particularly pure ε-caprolactam can be produced.

[0140] The pressure in the depolymerization section [C] can vary 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. In this pressure range, the production of particularly pure ε-caprolactam becomes possible.

[0141] The depolymerization of the nylon 6 rich stream can be achieved in the presence or absence of 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, especially superheated steam. Preferably, the depolymerization is completed in 0.1 hours to 24 hours, more preferably in 0.5 hours to 6 hours.

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

[0143] Preferably, the ε-caprolactam in the vapor stream containing 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.

[0144] During the depolymerization reaction, decomposition products containing oligomers of ε-caprolactam can be formed. Furthermore, the feed stream of materials derived from nylon 6 containing multi-component materials may also contain other components, i.e., impurities such as non-nylon 6 compounds, and residues of the solvent(s) applied in the pretreatment that remain stable, react, or decompose under the depolymerization conditions. Therefore, when water is used as a solvent, the vapor stream removed from the depolymerization section [C] contains not only water and ε-caprolactam but also impurities.

[0145] Preferably, superheated steam having a temperature of 100°C to 600°C is introduced into the depolymerization reactor(s). Preferably, the superheated steam introduced into the depolymerization reactor(s) has a temperature of at least the melting temperature of nylon 6. Preferably, the energy content of the superheated steam introduced into the depolymerization reactor(s) is sufficiently high so that no other heat input is required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam. In another preferred embodiment, superheated steam having a temperature in the range of 220°C to 575°C is introduced into the depolymerization section [C]. In an even more preferred embodiment, the depolymerization section [C] is introduced with superheated steam having a temperature in the range of 275°C to 500°C. In another preferred embodiment, part of the heat input required to carry out the depolymerization reaction and evaporate the formed ε-caprolactam is introduced through the walls of the depolymerization reactor(s).

[0146] Generally, the mass of the vapor stream removed from the depolymerization section [C] is less than the mass of the total feed to the depolymerization section. The total feedstock to the depolymerization section [C] includes a nylon 6 rich stream and optionally a solvent, catalyst, additional agent, and / or depolymerizing agent. Thus, without any additional measures, there will be an accumulation of material (often referred to as "residual material") in the depolymerization section [C]. Preferably, another stream is discharged from the depolymerization section [C]. This has the advantage that the accumulation of material in the depolymerization section [C] is reduced or avoided. The additional stream can include, when phosphoric acid is used as the depolymerization catalyst, the nylon 6 rich stream, non-depolymerized nylon 6, non-evaporated ε-caprolactam, the catalyst(s), and impurities present in compounds formed under depolymerization conditions such as mono-, di-, and / or triammonium phosphate. In a preferred embodiment, the stream containing mono-, di-, and / or triammonium phosphate is discharged from the depolymerization section [C]. Even more preferably, this stream discharged intermittently or continuously from the depolymerization section [C] contains 0.01 - 50 wt%, preferably 0.1 - 25 wt%, more preferably 0.5 - 10 wt%, and most preferably 0.5 - 5 wt% fraction of mono-, di-, and / or triammonium phosphate.

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

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

[0149] However, in another preferred embodiment, no catalyst is used for the depolymerization of the nylon 6 rich stream. This has the advantage of low cost (both for the catalyst and for the disposal of catalyst waste). However, higher temperatures (and pressures) are usually required compared to the depolymerization of the nylon 6 rich stream carried out in the presence of a catalyst.

[0150] The advantages of using a catalyst (especially orthophosphoric acid) are that the depolymerization reaction starts at a much lower temperature and can be carried out under atmospheric conditions. The preferred concentration of the catalyst used for the depolymerization of nylon 6 to ε-caprolactam is known to those skilled in the art and can be easily determined by routine experiments. If the concentration of the catalyst used is too low, the reaction rate will be slow. Conversely, if the concentration of the catalyst used is too high, the reaction is fast, but side reactions (if any) will also increase. Also, the catalyst cost will increase, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight based on 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 applied to the depolymerization of nylon 6. In the case of the catalyst orthophosphoric acid, the preferred content is 0.1 to 25% by weight, more preferably 1 to 20% by weight. The preferred content of the catalyst p-toluenesulfonic acid is 10 to 35% by weight, and the more preferred content is 15 to 30% by weight.

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

[0152] In a preferred embodiment, the depolymerization of nylon 6 is carried out in batch mode. In batch mode, the feedstock, i.e., the nylon 6-derived material containing the multi-component material, and optionally the catalyst, are first charged into the depolymerization reactor. Then, superheated steam is charged into the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Next, the charging of superheated steam into the depolymerization reactor is interrupted. Optionally, after removing the 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, the residual material is not removed between each cycle.

[0153] In a particular advantageous embodiment, the depolymerization of nylon 6 is carried out in continuous mode. In continuous mode, the nylon 6-containing feedstock (and optionally the catalyst) is continuously charged into the depolymerization reactor. At the same time, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. Optionally, the catalyst is charged into the depolymerization reactor continuously or intermittently. Further, the residual material is continuously discharged from the depolymerization reactor. Preferably, the nylon 6 rich stream is charged as a melt. Preferably, the catalyst is charged as a melt, slurry or solution.

[0154] In another preferred embodiment, the depolymerization of nylon 6 is carried out in semi-continuous mode. In semi-continuous mode, the nylon 6-containing feedstock (and optionally the catalyst) is intermittently charged into the depolymerization reactor, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream containing ε-caprolactam and water. The residual material is intermittently discharged from the depolymerization reactor in the semi-continuous mode of nylon 6 depolymerization.

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

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

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

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

[0159] Preferably, the crude ε-caprolactam contains ε-caprolactam in the range of 6 to 95% by weight, more preferably 20 to 90% by weight, and most preferably 35 to 80% by weight. The balance is mainly water. 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.

[0160] Optionally, the crude ε-caprolactam is filtered before being fed into the purification section [E]. Filtration reliably removes undissolved impurities that could otherwise interfere with further purification processes.

[0161] Optionally, the oil is separated from the crude ε-caprolactam before being fed into the purification section [D]. Separating the oil reliably removes impurities that could otherwise interfere with further purification processes.

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

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

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

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

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

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

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

[0169] A person skilled in the art can determine, by routine experiments, the amount of water or aqueous alkali solution necessary for the efficient washing of the organic phase containing the organic solvent, ε-caprolactam and impurities. Preferably, this amount is 0.1 vol% to 5 vol% based on the amount of the organic solvent in the organic phase to be washed.

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

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

[0172] In another preferred embodiment, the extraction with water is carried out in a countercurrent extraction column, and the ε-caprolactam-containing phase to be purified is introduced at the bottom of the column, and the water is introduced at the top. By extraction, an ε-caprolactam-water phase and a solvent phase containing impurities are obtained. Usually, the solvent phase containing impurities is preferably reused by distillation, optionally after purification.

[0173] The ε-caprolactam-water phase, optionally stripped and / or distilled to remove the residual solvent, is concentrated by evaporation of water, thereby obtaining a concentrated aqueous ε-caprolactam phase. The ε-caprolactam content of this concentrated aqueous ε-caprolactam phase is usually 60% to 99.9% by weight based on the whole phase.

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

[0175] In another preferred embodiment, before the distillation removal in step c.4)(iii) of the process of the present invention, an oxidizing agent, 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 oxidizing agent.

[0176] Preferably, the oxidizing agent is added to the ε-caprolactam-aqueous phase as a solid, as a slurry, or in the form of an aqueous solution, so that a dilute aqueous solution is obtained during purification by oxidation. A person skilled in the art can determine the amount of the oxidizing agent required for efficient oxidation of the ε-caprolactam-aqueous phase by routine experiments. The exact amount of the oxidizing agent depends very much, inter alia, on the composition of the polyamide 6 rich stream fed into the depolymerization section in this process of the present invention. Preferably, the amount of the oxidizing agent is 0.01 to 5% by weight based on the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

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

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

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

[0180] When potassium permanganate and / or sodium permanganate are used as the oxidizing agent, solid manganese(IV) oxide (MnO2) particles are formed as the reaction product. A person skilled in the art can determine the optimal solid-liquid filtration procedure for efficiently removing the solid manganese(IV) oxide particles from the aqueous phase after oxidation through routine experiments. The use of a filter aid such as activated carbon or diatomaceous earth particles to improve the filtration procedure is a common practice in this regard.

[0181] When the aqueous ε-caprolactam phase is hydrogenated, preferably in the presence of a hydrogenation catalyst known per se, the aqueous ε-caprolactam phase is optionally hydrogenated. Hydrogenation can be carried out, for example, as described in European Patent No. 635487.

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

[0183] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. The hydrogenation may be carried out, for example, in a stirred tank reactor in which the catalyst particles are suspended in the mixture to be purified (slurry phase process). In another embodiment, the hydrogenation is carried out in a fixed bed reactor in which the catalyst is fixed in the reactor.

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

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

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

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

[0188] In a preferred embodiment, before the distillation removal in step c.4)(iii), an alkali metal hydroxide, preferably NaOH, is added to the ε-caprolactam-containing phase. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol per kg of ε-caprolactam, more preferably 2 to 80 mmol per kg of ε-caprolactam. Experiments have shown that the addition of an alkali metal hydroxide, particularly NaOH, enables the specific and effective distillation removal of impurities having a lower boiling point and a higher boiling point than ε-caprolactam.

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

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

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

[0192] 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, optionally washed with water or an aqueous alkali solution, and obtained by extraction in step c.4)(i).

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

[0194] Most preferably, the solution containing ε-caprolactam and impurities from 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).

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

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

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

[0198] Accordingly, according to a particular advantageous embodiment of the present invention, before the crystallization in step c.4)(iv), the purification in step c.4) also includes step c.4)(iii) of removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation under vacuum conditions.

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

[0200] Accordingly, according to a particular advantageous embodiment of the present invention, after the crystallization of step c.4)(iv), the purification in step c.4) also includes step c.4)(iii) of removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation under vacuum conditions.

[0201] Optionally, according to step c.4)(iii) of the process of the present invention, in both the front and the back of the crystallization in step c.4)(iv), impurities having a boiling point lower or higher than that of ε-caprolactam are removed by distillation under vacuum conditions. Thereby, a phase containing ε-caprolactam and impurities is obtained.

[0202] Accordingly, according to a particular advantageous embodiment of the present invention, in both the front and the back of the crystallization in step c.4)(iv), the purification in step c.4) also includes step c.4)(iii) of removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation under vacuum conditions.

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

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

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

[0206] Preferably, a solvent is present in the mixture within the crystallization apparatus, although crystallization can also be carried out without a solvent. Many solvents are suitable for ε-caprolactam. Examples of suitable solvents are water, alkanes (n-hexane, n-heptane, isooctane, cyclohexane, etc.), alcohols (methanol, ethanol, n-propanol, n-butanol, etc.), aromatic hydrocarbons (benzene, toluene, o-xylene, m-xylene, p-xylene, etc.), ammonia, chlorinated hydrocarbons (tetrachloromethane, chloroform or ethyl chloride, etc.), ketones (acetone or methyl ethyl ketone, etc.) and esters (ethyl acetate, etc.). These solvents result in large crystals, so preferably water and aromatic hydrocarbons are used as the solvent. Most preferably, the solvent is water. The solvent acts as a freezing point depressant for the melt within the crystallization apparatus. Generally, melt crystallization requires less energy than solution crystallization, but the operation on an industrial scale can be more difficult.

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

[0208] Layered melt crystallization can be carried out in either a static mode or a dynamic mode. In the static crystallization mode, crystals grow on the cooling surface from a stagnant melt. In the static mode, the desired compound is crystallized batchwise on the heat exchanger wall from a stagnant melt in a closed vessel container. This type of crystallization is characterized by a slow crystal growth rate and a long residence (or batch) time as a result. 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 residual melt is discharged. Then, optionally, a sweating phase introduced to remove impurities adhering to or trapped within the crystals is introduced. Finally, the crystals are completely melted and discharged or mechanically removed.

[0209] Generally, dynamic crystallization is carried out in a tube and shell heat exchanger, whereby the melt circulates below the cooling surface where the compound crystallizes. Generally, the melt is pumped through the tube, the crystals grow inside the tube, and the cooling medium flows outside the tube. The thickness of the crystal layer increases with time. After a certain time, the circulation of the melt is stopped and the residual melt is discharged. Dynamic layer crystallization is similar to the stagnant layer crystallization also carried out in a batch mode. The crystal growth rate is higher in the dynamic mode compared to the stagnant mode, and as a result, the crystallization time is shorter. Preferably, the crystallization time ranges from 0.05 hour to 12 hours, more preferably from 0.1 hour to 6 hours, and most preferably from 0.3 hour to 3 hours. Then, optionally, a sweating phase introduced to remove impurities adhering to or trapped within the crystals is introduced. Finally, the crystals are completely melted and discharged or mechanically removed.

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

[0211] After the ε-caprolactam crystallization step, a mother liquor still containing ε-caprolactam in addition to impurities is obtained. Methods for recovering ε-caprolactam from this type of mother liquor are well known to those skilled in the art. As a result of these recovery methods, almost all of the ε-caprolactam present in the mother liquor can be recovered and converted into high-purity ε-caprolactam. One possible solution in the case of multi-stage crystallization is to recycle the mother liquor in a countercurrent manner, i.e., the mother liquor obtained in the nth crystallization stage is introduced into the feed of the (n - 1)th crystallization stage. Generally, the mother liquor obtained from the first crystallization stage is introduced into an 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 the obtained mother liquor (or a part thereof) by recycling it, for example, to any previous stage in the process. Alternatively, the mother liquor can be purified, for example, by distillation before being re-introduced into the ε-caprolactam crystallization process.

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

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

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

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

[0216] Optionally, the plant of the present invention can include a pretreatment section [A], which can include a mechanical size reduction section [λ] for fragmenting the nylon 6 and polyether polyurethane-containing material into small pieces, and / or a cleaning section [ω] for cleaning the nylon 6 and polyether polyurethane-containing material or the fragmented small pieces obtained therefrom. Cleaning includes both cleaning and separating foreign substances from the nylon-6 and polyether polyurethane-containing material. Separation of foreign substances can be performed both manually (handpicking) and mechanically (e.g., density separation and magnetic separation). Both manual devices such as brushes and mechanical devices can assist in the cleaning in the cleaning section [ω]. The cleaning is preferably performed by an additional frictional effect. Various types of industrial cleaning systems such as rotary plastic washers and (high-speed) friction washers are commercially available. The mechanical size reduction section [λ] includes a device for mechanically fragmenting the nylon 6 and polyether polyurethane-containing material into small pieces. Non-limiting examples of such fragmentation devices are cutters, punchers, shredders, mills, grinders, or chippers.

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

[0218] The dissolution section [α] includes one or more dissolution units operated in series and / or in parallel, and one or more separation units operated in series and / or in parallel. Small pieces of nylon 6 and polyether polyurethane-containing materials that have been cleaned and / or fragmented, if any, are fed to the dissolution unit(s), preferably as solids, or as melts, preferably as solids. Small pieces of nylon 6 and polyether polyurethane-containing materials that have been cleaned and / or fragmented, if any, are optionally dried before being fed to the dissolution section [α]. The dissolution unit(s) has an inlet for introducing a separated organic solvent and a fresh organic solvent. The dissolution unit(s) has one or more outlets for discharging the mixture obtained in the dissolution unit(s). This mixture contains undissolved nylon 6 and an organic solvent in which the polyether polyurethane is dissolved. Preferably, the dissolution unit(s) is closed. This has the advantage that the discharge of the organic solvent into the environment is reduced. Preferably, the dissolution unit(s) has means (e.g., steam coils, steam tracing, electrical tracing) for controlling the temperature within the unit(s). Optionally, the temperature of the separated organic solvent and the fresh organic solvent is adjusted (e.g., in a heat exchanger) before being introduced into the dissolution section [α].

[0219] Sufficient contact between the organic solvent and the nylon 6 and polyether polyurethane-containing materials is essential for effective operation. Such contact can be achieved by various means generally known in the art. Improvement of contact can be achieved by mechanical friction. Mechanical friction can be achieved, for example, by agitation by a combination of a rotating paddle and a stationary fin. Dissolution units suitable for dissolving the polymer in the organic solvent are known to those skilled in the art. A stirred vessel is an example of such a dissolution unit.

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

[0221] Separation unit(s) 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. Preferably, the dissolution unit and the separation unit are combined in one device. This brings about an enhancement of the process.

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

[0223] It is essential for effective operation that the third solvent and the nylon 6 rich stream are in sufficient contact. Such 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 stream is fed and the third solvent is introduced as a washing solvent.

[0224] The distillation section [δ] includes one or more distillation columns operated in series and / or in parallel. The distillation columns can be operated in batch mode, semi - continuous mode or continuous mode. The choice between these operating modes depends greatly on the scale of operation. Generally, batch operation of the distillation column is more suitable for the treatment of small supply streams. Continuous operation of the distillation column is more suitable for the treatment of large supply streams. The exact arrangement of the distillation section [δ] also depends on the properties of the organic solvent, the second solvent and the third solvent.

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

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

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

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

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

[0230] If superheated steam at high temperature is not available at the production site, it is necessary to make it suitable for the purpose by superheating the available steam from a boiler in a so-called superheater.

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

[0232] The purification section [E] can 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, into which crude ε-caprolactam is introduced and high-purity ε-caprolactam is discharged.

[0233] Into the extraction device, crude ε-caprolactam and an organic solvent are introduced, and an organic phase containing the organic solvent, ε-caprolactam, and impurities, as well as an aqueous phase containing water and impurities, are discharged. The extraction device is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the extraction device is a static extraction column or a stirred extraction column such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulsed column, a sieve tray (static) column, an irregular packing (static) column, and a regular packing (SMVP) (static) column.

[0234] Into the solvent switching device, an organic phase containing water and an organic solvent, ε-caprolactam, and impurities are introduced, and a solvent phase containing impurities and an ε-caprolactam-water phase containing water, ε-caprolactam, and impurities are discharged. The solvent switching device for a process based on back-extraction is selected from a mixer-settler extractor, an extraction column, a centrifugal extractor, and combinations thereof. Preferably, the device for back-extraction is a static extraction column or a stirred extraction column such as a KARR® column, a SCHEIBEL® column, a rotating disk contactor (RDC), a pulsed column, a sieve tray (static) column, an irregular packing (static) column, and a regular packing (static) column.

[0235] The solvent switching device for a process based on solvent exchange distillation is selected from a sieve tray distillation column, a random packing distillation column, and a structured packing distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser, and a device for reflux. The distillation column can be operated at atmospheric pressure, sub-atmospheric pressure, or super-atmospheric pressure. Preferably, water is introduced into the upper part of the distillation column, and an aqueous phase containing water, ε-caprolactam, and impurities having a lower or higher boiling point than ε-caprolactam is discharged from the lower part of the distillation column.

[0236] The optional oxidation section includes one or more oxidation reactors operated in series and / or in parallel. An oxidizing agent and an ε-caprolactam-water phase containing water, ε-caprolactam, and impurities are introduced into the oxidation section. Usually, the oxidizing agent is introduced as a solid, as a slurry, or as an aqueous solution. When potassium permanganate or sodium permanganate is applied as the oxidizing agent, the oxidation section also includes a filtration section. The oxidation reactor can have any desired form. Preferred reactor types are stirred and non-stirred reactors and packed tower reactors. The oxidation reactor needs to be equipped with facilities for supplying an aqueous phase containing water, ε-caprolactam, and impurities having a lower or higher boiling point than ε-caprolactam, and the oxidizing agent. Furthermore, the oxidation reactor needs to be equipped with facilities for discharging an oxidized ε-caprolactam-water phase containing water, ε-caprolactam, and impurities, and optionally formed solid manganese(IV) oxide (MnO2) particles. Preferably, the oxidation is carried out at a temperature in the range of 20 °C to 85 °C and under atmospheric conditions.

[0237] Any randomly present solid manganese(IV) oxide (MnO₂) particles can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. The use of a filter aid such as activated carbon particles or diatomaceous earth to improve the filtration procedure is a common practice. Filtration systems suitable for the separation of solid manganese(IV) oxide particles are known to those skilled in the art. Such a filtration system is charged with a suspension of an ε-caprolactam-water phase containing water, ε-caprolactam and impurities, as well as solid manganese(IV) oxide particles, and a filtered ε-caprolactam-water phase containing water, ε-caprolactam and impurities is discharged. Generally, the solid manganese(IV) oxide particles are retained within the filtration system. Preferably, such a filtration system operates in semi-continuous mode, whereby the suspension and the filtered phase are continuously charged and continuously discharged, and the separated solids are collected in the filtration system. Occasionally, the charging of the suspension is interrupted and the collected solids are removed from the filtration system.

[0238] In step c.4), the purification of the crude ε-caprolactam to obtain a purified ε-caprolactam may optionally include hydrogenation with a heterogeneous catalyst, in which case the plant includes a hydrogenation section. Preferably, the catalyst contains nickel or palladium.

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

[0240] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. Hydrogenation may be carried out, for example, in a stirred tank reactor in which the catalyst particles are suspended in the mixture to be hydrogenated (slurry phase process). In such a slurry phase process, the catalyst particles and the purified mixture need to be separated, for example, by filtration, in an additional process step after the hydrogenation reaction. Preferably, the catalyst contains palladium or nickel.

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

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

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

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

[0245] The crystallization section includes one or more crystallizers operated in series and / or in parallel. Generally, the crystallization section also includes several vessel containers for storing (intermediate) product streams and / or fresh and used cleaning liquids. Crystallization of ε-caprolactam can be carried out by either solution crystallization or melt crystallization as described above.

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

[0247] The auxiliary solvent is water or a non-aqueous solvent. Examples of non-aqueous solvents include alkanes (n-hexane, n-heptane, isooctane, cyclohexane, etc.), alcohols (methanol, ethanol, n-propanol, n-butanol, etc.), aromatic hydrocarbons (benzene, toluene, o-xylene, m-xylene, p-xylene, etc.), ammonia, chlorinated hydrocarbons (tetrachloromethane, chloroform or ethyl chloride, etc.), ketones (acetone or methyl ethyl ketone, etc.) and esters (ethyl acetate, etc.), and mixtures of these solvents. Generally, the auxiliary solvent is recovered and reused in the crystallization process. Solution crystallization is usually carried out at atmospheric pressure, but it may also be carried out under reduced pressure or increased pressure.

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

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

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

[0251] Layered melt crystallization can be carried out in either a static mode or a dynamic mode. In the static crystallization mode, crystals grow on the cooling surface from a stagnant melt. In the static mode, the desired compound is crystallized batchwise on the heat exchanger wall from a stagnant melt in a closed vessel container. This type of crystallization has a slow crystal growth rate, and as a result, its residence (or batch) time is long. 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 residual melt is discharged. 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.

[0252] Melt crystallization of suspension: The suspension melt crystallization of ε-caprolactam can be carried out in either a batch mode or a continuous mode. By 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. The suspension melt crystallization can be carried out in any exchanger type or vessel container type crystallizer capable of cooling the melt. Preferably, the suspension melt crystallization is carried out in a scraper-type surface crystallizer. Optionally, after crystallization, the mixture of the obtained crystals and the mother liquor is separated by filtration. Optionally, after crystallization, the mixture of the obtained ε-caprolactam crystals and the mother liquor is introduced into a so-called washing tower. In the washing tower, the mother liquor is discharged from the ε-caprolactam crystals, and then, optionally, the ε-caprolactam crystals are washed with purified ε-caprolactam.

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

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

[0255] In the process of the present invention, high-purity, and thus high-quality, ε-caprolactam can be produced, which meets the specifications for demanding applications. At the same time, this process is particularly economically friendly for reducing the carbon footprint of its products and using waste as starting materials. In a preferred embodiment, the ε-caprolactam obtained by the process of the present invention meets one or more of the following specifications, where these parameters and measurement methods are defined as in the following example section of this specification: PAN: maximum 5 E290: maximum 0.05 VB: maximum 0.5 mmol / kg Alkalinity: maximum 0.1 mmol / kg Acidity: maximum 0.1 mmol / kg.

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

[0257] The environmental impact of a product is generally expressed as the "carbon footprint of the product". The carbon footprint of a product is defined as the total emissions caused by the formation of that product, which is expressed in tons of carbon dioxide equivalent per ton of product. The carbon footprint of a product depends, inter alia, on feedstock, auxiliary materials, energy consumption, energy source, production process and process efficiency. The quantification of the carbon footprint of a product can be carried out, for example, as described in the European standard EN ISO 14040:2006 ("Environmental management-Life cycle assessment-Principles and framework").

[0258] The calculation of the carbon footprint of a product can be carried out both in-house or by an external (preferably) accredited organization. These organizations verify and certify the calculation of the carbon footprint of a product, for example, based on the LCA standard ISO 14040.

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

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

[0261] The polyether polyurethane produced by the process of the present invention is also particularly economically and environmentally friendly. Again, this is evident from the fact that the carbon footprint of the polyether polyurethane produced by the process of the present invention is much lower compared to polyether polyurethanes classically produced, for example, by the reaction of a polyether polyol and a diisocyanate monomer.

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

[0263] The product carbon footprint of the polyether polyurethane obtained according to the process of the present invention is much less than that of de novo synthesized polyether polyurethane or "virgin" polyether polyurethane. The product carbon footprint of the polyether polyurethane obtained according to the process of the present invention is less than 3.0 kg, preferably less than 2.0 kg of CO2, and most preferably less than 1.0 kg of CO2 per kg of polyether polyurethane in terms of CO2 equivalent.

[0264] Hereinafter, the present invention will be described with reference to the drawings showing specific embodiments of the present invention. However, the present invention is defined in the claims and is as generally described herein. This should not be limited to the embodiments shown for illustrative purposes in the following figures.

Brief Description of the Drawings

[0265]

Figure 1

Figure 2

Figure 3

Figure 4

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

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

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

[0304] to be discharged. Optionally, in the pretreatment section [A] where the nylon 6 and polyether polyurethane-containing material

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

[0302] , the discharged foreign matter and contaminated washing solvent

[0303] are obtained. Removal of foreign matter can be performed before and / or after washing with a washing solvent

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

[0301] . Optionally, the cleaned and / or fragmented small pieces

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

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

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

[0311] and the recovered polyether polyurethane

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

[0311] is fed into the depolymerization section [C]. Optionally, the nylon-6 rich stream

[0311] and / or the recovered polyether polyurethane

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

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

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

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

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

[0309] and the residue

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

[0269] Optionally, the dried and / or densified nylon-6 rich stream

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

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

[0314] is discharged. Optionally, superheated steam

[0312] and a catalyst

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

[0270] Crude ε-caprolactam

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

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

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

[0312] is fed into the depolymerization section [C], or when water (not shown in FIG. 1) is fed into the recovery section [D], the aqueous phase

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

[0271] The crude ε-caprolactam

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

[0319] in the purification section [E]. Water and impurities

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

[0272] FIG. 2A shows an embodiment of the pretreatment section [A (the region enclosed by the dashed line)]. The nylon 6 and polyether polyurethane-containing material

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

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

[33] as well as the cleaned nylon 6 and polyether polyurethane-containing material

[34] . Next, the cleaned nylon 6 and polyether polyurethane-containing material

[34] is fragmented in the mechanical size reduction section [λ] to obtain cleaned and fragmented small pieces

[35] of the nylon 6 and polyether polyurethane-containing material. Then, the cleaned and fragmented nylon 6 and polyether polyurethane-containing material

[35] is discharged. Optionally, the cleaned and fragmented small pieces

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

[0273] Figure 2B shows an embodiment of the pretreatment section [A] (the region surrounded by the dashed line), where the nylon 6 and polyether polyurethane-containing material

[41] is first fragmented in the mechanical size reduction section [λ] to obtain fragmented pieces

[42] of the nylon 6 and polyether polyurethane-containing material. Next, the fragmented pieces

[42] of the nylon 6 and polyether polyurethane-containing material are cleaned in the cleaning section [ω] by removing foreign matter and washing with a washing solvent

[43] , obtaining foreign matter and contaminated washing solvent

[44] , and obtaining cleaned and fragmented pieces

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

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

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

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

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

[0101] to obtain a polyether polyurethane-rich stream

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

[0105] containing undissolved nylon 6. These are discharged from the dissolution section [α]. The organic solvent is the separated organic solvent

[0102] , to which optionally fresh organic solvent

[0103] is added.

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

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

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

[0107] . Optionally, the recovered polyether polyurethane

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

[0108] containing the recovered organic solvent and the second solvent from which the polyether polyurethane has precipitated is discharged from the precipitation section [γ] and fed into the solvent distillation section [δ].

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

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

[0109] to obtain a nylon 6-rich stream

[0110] and a mixture

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

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

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

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

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

[0278] In the solvent distillation section [δ], a mixture containing an organic solvent and a second solvent from which the precipitated polyether polyurethane has been recovered

[0108] , and optionally a mixture containing an organic solvent and a third solvent

[0111] are distilled to obtain a separated organic solvent

[0102] , the second solvent and the third solvent

[0112] , and a residue

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

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

[0102] is fed into the dissolution section [α]. Figure 4A shows one embodiment (the region enclosed by the dashed line) of the purification section [E] including the following sections.

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

[0201] is extracted with an organic solvent

[0202] to obtain an aqueous phase

[0203] containing water and impurities and an organic phase

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

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

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

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

[0206] and a washed organic phase

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

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

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

[0208] to obtain an organic solvent phase

[0209] containing impurities and an aqueous phase

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

[0282] In any stripping and concentration section [N], the residual organic solvent and water are removed by stripping and / or distillation from an aqueous phase containing water, ε-caprolactam, and impurities

[0210] having a boiling point lower or higher than that of ε-caprolactam, to obtain a stripped and concentrated aqueous phase

[0212] containing the residual organic solvent and water

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

[0283] Optionally, an aqueous phase containing water, ε-caprolactam, and impurities

[0210] having a boiling point lower or higher than that of ε-caprolactam, from which any residual organic solvent and water have been removed by stripping and / or distillation, is oxidized with an oxidizing agent to obtain an oxidized ε-caprolactam-water phase containing water, ε-caprolactam, and impurities (not shown in Figure 4A).

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

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

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

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

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

[0214] having a boiling point lower or higher than that of ε-caprolactam and optionally an organic solvent or water (not shown in Figure 4A), whereby a distilled ε-caprolactam phase

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

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

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

[0212] containing water, ε-caprolactam, and impurities having a boiling point lower or higher than that of ε-caprolactam.

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

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

[0217] from the ε-caprolactam, whereby high-purity ε-caprolactam

[0218] is obtained. All of the crystal products are discharged from the crystallization section [P]. Optionally, before the crystallization, a solvent

[0216] is introduced into the crystallization section [P]. Figure 4B shows an embodiment (the region surrounded by the dashed line) of the purification section [E] including the following sections.

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

[0401] is extracted with an organic solvent

[0402] to obtain an aqueous phase

[0403] containing water and impurities and an organic phase

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

[0289] In any washing section [L], an organic phase

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

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

[0406] and a washed organic phase

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

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

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

[0408] to obtain a stream

[0409] containing the organic solvent and an aqueous phase

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

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

[0410] containing water, ε-caprolactam, and impurities having a lower or higher boiling point than ε-caprolactam to obtain a residual organic solvent and water

[0411] and a stripped and concentrated aqueous phase

[0412] containing water, ε-caprolactam, and impurities having a lower or higher boiling point than ε-caprolactam. Both of these phases are discharged from the stripping and concentration section [N].

[0292] Optionally, the aqueous phase

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

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

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

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

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

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

[0414] having a boiling point lower or higher than that of ε-caprolactam and optionally an organic solvent or water (not shown in Figure 4B), whereby a distilled ε-caprolactam phase

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

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

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

[0412] containing water, ε-caprolactam and impurities having a boiling point lower or higher than that of ε-caprolactam.

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

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

[0417] from ε-caprolactam, whereby high-purity ε-caprolactam

[0418] is obtained. All crystallization products are discharged from the crystallization section [P]. Optionally, a solvent

[0416] is added to the crystallization section [P] before crystallization.

Examples

[0297] The following examples serve to explain the present invention in more detail with respect to specific forms of the present invention. However, the examples are not intended to limit the present disclosure.

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

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

[0300] Volatile base (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile base content - Titration method after distillation. Alkalinity of ε-caprolactam product: Alkalinity is measured using a tashiro indicator at a temperature of 25 °C with 0.1 wt / v Ethanol % methylene blue: 0.1 wt / v EthanolIt is determined by titrating methyl red at a ratio of 1:2. The end point is gray. First, titrate the flask containing water and the indicator until it turns gray, then add X grams of an aqueous ε-caprolactam solution containing Y weight % of ε-caprolactam (determined by refractive index), and titrate this solution back to gray using a 0.01N H2SO4 solution (when the solution is alkaline) or a 0.01N NaOH solution (when the solution is acidic).

[0301] Then, the alkalinity is obtained as follows. Alkalinity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) Where v = volume of H2SO4 solution added (ml) t = normality of H2SO4 solution (= 0.01N) X = weight of sample (g) Y = ε-caprolactam concentration (weight %) is.

[0302] Then, the acidity is obtained as follows. Acidity (mmol / kg ε-caprolactam) = v * t * 1000 / (X * Y) Where v = volume of NaOH solution added (ml) t = molar concentration of NaOH solution (= 0.01N) X = weight of sample (g) Y = ε-caprolactam concentration (weight %) is.

[0303] Example 1 Pretreatment and separation of nylon 6 and polyether polyurethane. The polyether polyurethane content of nylon 6 waste cloth containing used polyether polyurethane was 20.2 + / - 0.4 weight %. (Determined by differential scanning calorimetry (DSC) method ISO11357-3_-130 °C to 300 °C_10 °C / min_dt1.00 second). The color of the nylon 6 waste cloth containing used polyether polyurethane was purple.

[0304] Nylon 6 waste cloth containing polyether polyurethane was cut into small pieces in the range of 5 - 20 cm each. 2 150.4 grams of this cut material was dissolved in 800 grams of DMAc while stirring at a temperature of 70 °C for 1 hour. The resulting solution was vacuum filtered through a heated double - wall Büchner funnel at a temperature of 70 °C, thereby obtaining filtrate 1. The remaining undissolved material was further treated 3 times, each time with 400 grams of DMAc while stirring at a temperature of 70 °C for 1 hour, thereby obtaining filtrates 2 - 4.

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

[0306] Analysis by differential scanning calorimetry (DSC) revealed that the polyether polyurethane content of the dried precipitate was approximately 100 wt% (polyamide 6 was not detected). Based on the DSC analysis, the nylon 6 content of the dried undissolved material was approximately 100 wt% (polyether polyurethane was not detected).

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

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

[0309] Example 2 Pretreatment and separation of nylon 6 and polyether polyurethane. The polyether polyurethane content of nylon 6 waste cloth containing used polyether polyurethane was 8.15 + / - 0.05 wt% (determined by differential scanning calorimetry (DSC) method ISO11357-3_-130 °C to 300 °C_10 °C / min_dt1.00 s). The color of the nylon 6 waste cloth containing used polyether polyurethane was salmon pink.

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

[0311] Analysis by differential scanning calorimetry (DSC) revealed that the polyether polyurethane content of the dry precipitate was about 100 wt% (nylon 6 was not detected). The nylon 6 content and polyether polyurethane content of the dry undissolved material were about 99 wt% and less than 1 wt%, respectively (DSC analysis).

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

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

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

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

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

[0317] This example shows that by depolymerizing nylon 6 obtained by extraction and separation from polyether polyurethane-containing nylon 6 waste cloth, crude ε-caprolactam can be obtained in a good yield without operational problems.

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

[0319] The condensate composed of 41 grams of ε-caprolactam and the majority of the remainder being water was concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; water bath temperature was about 65 °C) to a ε-caprolactam concentration of 63.1 wt%. (This mixture is the crude ε-caprolactam, which is the mixture to be purified.)

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

[0321] Comparative Experiment 1 Depolymerization of nylon 6 and polyether polyurethane. 37.6 grams of polyether polyurethane fiber (the same material as contained in the waste cloth used in Example 1) and 14 grams of 20 wt% phosphoric acid were charged into a Premex autoclave, and otherwise the procedure of Example 3 was followed.

[0322] Ten minutes after starting the injection of superheated steam, the experiment had to be stopped because the line for discharging the steam stream containing ε-caprolactam and water was clogged by insoluble materials.

[0323] From this comparative experiment, it can be concluded that if the depolymerization of nylon 6 waste cloth containing polyether polyurethane is carried out without removing the polyether polyurethane before depolymerization, operational problems can be brought about.

[0324] Comparative Experiment 2 Depolymerization of Nylon 6 Waste Cloth Containing Polyether Polyurethane and Recovery of ε-Caprolactam 48 grams of pellets (the same feedstock as used in Example 1) made from nylon 6 waste cloth containing polyether polyurethane having a polyether polyurethane content of 20.2 + / - 0.4 wt% and 14 grams of 20 wt% phosphoric acid were charged into a Premex autoclave, and otherwise the procedure of Example 3 was followed.

[0325] A condensate composed of 26 grams of ε-caprolactam and the majority of the remainder being water was concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; water bath temperature was about 65 °C) to bring the ε-caprolactam concentration to 47.0 wt%. (This mixture is the crude ε-caprolactam, which is the mixture to be purified).

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

[0327] Here too, this comparative experiment shows that depolymerizing nylon 6 waste fabric containing polyether polyurethane without removing the polyether polyurethane before depolymerization causes operational problems. Another observation is that the yield of ε-caprolactam in this comparative experiment is much lower than that in Example 3 (nylon 6 waste fabric containing pretreated polyether polyurethane as the feedstock), and thereby, the yield of ε-caprolactam is defined as the ratio of the weight of ε-caprolactam in the condensate to the weight of nylon 6 in the feedstock introduced into the Premex high-pressure autoclave.

[0328] Comparative Experiment 3 Purification by distillation. Next, 75 mmol of aqueous sodium hydroxide solution per 1 kg of ε-caprolactam was added to the crude ε-caprolactam obtained in Comparative Experiment 2. Then, this mixture was distilled by gradually reducing the pressure. ε-Caprolactam was distilled at 300 Pa. The specifications of the distilled ε-caprolactam were as follows: PAN: 31 E290: 3.08 VB: 2.45 mmol / kg Alkalinity: 3.13 mmol / kg.

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

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

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

[0332] The obtained crude ε-caprolactam was treated with 0.2% by weight of KMnO4 with respect to ε-caprolactam at 50 °C for 2 hours. Then, the formed solid was removed from the oxidation reaction product by filtration.

[0333] ε-Caprolactam in the oxidized reaction product was further purified by distillation as described in Comparative Example 3 after adding 75 mmol of aqueous sodium hydroxide solution per 1 kg of ε-caprolactam.

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

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

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

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

[0338] 76 grams of the obtained crude ε-caprolactam was extracted once with 100 grams, and extracted 9 times at 25 °C with 50 grams of a solvent mixture of 4-methyl-2-pentanol (50% by weight) / cyclohexane (50% by weight). The 10 ε-caprolactam phases obtained containing the solvent mixture were combined and then concentrated by evaporation using a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; the water bath temperature was about 65 °C) so that the ε-caprolactam concentration became about 40% by weight, and then fresh cyclohexane was added. The ε-caprolactam concentration of the obtained mixture was about 25% by weight, and the weight ratio of the solvent mixture 4-methyl-2-pentanol / cyclohexane was 50% by weight:50% by weight. Then, the concentrated ε-caprolactam containing the solvent mixture was extracted 8 times at 25 °C with 50 grams of water. The 8 obtained aqueous ε-caprolactam phases were combined. The combined aqueous phase was concentrated by evaporation in a rotary evaporator (rotavap) operated under vacuum (9.5 kPa; the water bath temperature was about 65 °C) to a ε-caprolactam concentration of 47.1% by weight. The specifications of the obtained concentrated aqueous ε-caprolactam solution were as follows.

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

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

[0341] Next, distilled water was added to the distilled ε-caprolactam to obtain a mixture having an ε-caprolactam concentration of 91.4% by weight. This aqueous ε-caprolactam was introduced into a crystallization apparatus at a temperature of 52 °C. The aqueous ε-caprolactam was cooled to 40 °C, and then 0.016 grams of seed was added to the mixture. Thereafter, the cooling of the mixture was continued until it dropped to 30 °C, and this was held for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85% by weight aqueous ε-caprolactam solution. The specifications of the obtained purified ε-caprolactam were as follows.

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

[0343] From this experiment, it can be concluded that purified ε-caprolactam that meets all the specifications required for major polymerization applications can be obtained from the depolymerization of nylon 6 derived from nylon 6 waste cloth containing polyether polyurethane and purified by extraction, back-extraction, distillation, and crystallization. All of the above examples demonstrated that the present invention can produce good products from coloring feedstocks, especially when derived from discarded fabrics.

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

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

[0346] PAN: 132 E290: 1.55

[0347] Next, an aqueous sodium hydroxide solution of 75 mmol per kg of ε-caprolactam was added to the concentrated ε-caprolactam solution. Then, water and impurities having a lower boiling point than ε-caprolactam were removed as the top product by distillation under reduced pressure in a batch-operated distillation apparatus. Finally, the distilled ε-caprolactam was recovered as the top product at 300 Pa, while impurities having a higher boiling point than ε-caprolactam remained as the bottom product of the distillation apparatus.

[0348] Next, distilled water was added to the distilled ε-caprolactam to obtain a mixture with an ε-caprolactam concentration of 91.4% by weight. This aqueous ε-caprolactam was introduced into a crystallization apparatus at a temperature of 52°C. The aqueous ε-caprolactam was cooled to 40°C, and several seeds were added to the mixture. Then, the cooling of the mixture was continued until it dropped to 30°C, and this was held for 30 minutes. The crystallized ε-caprolactam was recovered by filtration and washed with an 85% by weight aqueous solution of ε-caprolactam. The specifications of the obtained purified ε-caprolactam met all the requirements for major polymerization applications.

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

[0350] Experiment 7 Calculation of the carbon footprint of purified ε-caprolactam and polyether polyurethane. The continuous process according to the present invention for producing purified ε-caprolactam and polyether polyurethane from nylon 6 waste cloth containing polyether polyurethane was simulated. The polyether polyurethane content of these nylon 6 waste cloths containing polyether polyurethane was 20% by weight, and the remainder was mainly nylon 6.

[0351] This process included the following: - Cutting the nylon 6 waste cloth containing polyether polyurethane into small pieces; - Selectively extracting the polyether polyurethane with DMAc; - Separating the undissolved nylon 6 and the polyether polyurethane-rich stream by centrifugation; - Washing the undissolved nylon 6 with water; - Separating the washed undissolved nylon 6 and the aqueous extract by centrifugation; - Precipitating polyether polyurethane from the polyether polyurethane-rich stream by adding the aqueous extract obtained above; - Separating the precipitated polyether polyurethane by filtration from the DMAc-water mixture; - Recovering DMAc and water from the DMAc-water mixture obtained above; - Drying the filtered polyether polyurethane; - Drying the washed undissolved nylon 6; - Melting and pelletizing the washed undissolved nylon 6; - Depolymerizing nylon 6 under the influence of H3PO4 and superheated steam; - Recovering crude ε-caprolactam (80 wt% ε-caprolactam) by partial condensation of the vapor discharged from the depolymerization reactor; - Performing countercurrent extraction of the concentrated crude ε-caprolactam with toluene; - Washing the organic extract with a dilute caustic solution; - Performing countercurrent back-extraction of the washed organic extract with water; - Evaporating and concentrating the aqueous extract; - Adding a caustic substance; - Performing light and heavy distillation removals by vacuum distillation; and - Recovering pure ε-caprolactam by melt crystallization at a temperature of 61°C.

[0352] The carbon footprints of the purified ε-caprolactam and polyether polyurethane were calculated based on the numerical values of the raw material consumption, and the usefulness of the above process was based on the data derived from the ecoinvent version 3.7.1. The distribution of the environmental impacts between the purified ε-caprolactam and polyether polyurethane, which are the products in the pretreatment section and the separation section, was based on the weight ratio of these products.

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

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

Claims

1. A process for recovering ε-caprolactam and polyether polyurethane from a nylon 6 and polyether polyurethane-containing material in a plant, said plant comprising: - a separation section [B]; - a depolymerization section [C]; - a recovery section [D]; - a purification section [E], wherein said process comprises: a) feeding said nylon 6 and polyether polyurethane-containing material into said separation section [B]; b) selectively dissolving said 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, in said separation section [B], to separate said nylon 6 and polyether polyurethane-containing material into a nylon 6-rich stream and a polyether polyurethane-rich stream, wherein said polyether polyurethane-rich stream is a solution comprising said organic solvent and polyether polyurethane; c.1) discharging said nylon 6-rich stream from said separation section [B] and feeding said nylon 6-rich stream into said depolymerization section [C], wherein the nylon 6 content of said nylon 6-rich stream is at least 85% by weight on a dry weight basis; c.2) depolymerizing said nylon 6 in said nylon 6-rich stream in said depolymerization section [C] at a temperature in the range of 180 °C to 400 °C, preferably 200 °C to 350 °C, more preferably 220 °C to 340 °C, and most preferably 240 °C to 325 °C, thereby obtaining an ε-caprolactam-containing stream and discharging said obtained ε-caprolactam-containing stream from said depolymerization section [C]; c.3) recovering crude ε-caprolactam from said ε-caprolactam-containing stream in said recovery section [D]; c.4) purifying said crude ε-caprolactam obtained in said recovery section [D] in said purification section [E] to obtain purified ε-caprolactam, said purification comprising (i) A step of extracting the crude ε-caprolactam with an organic solvent, whereby an aqueous phase and an organic phase are obtained, the organic phase containing the organic solvent, ε-caprolactam and impurities, and preferably, the organic solvent being selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol and mixtures thereof, and optionally, washing the obtained organic phase with water or an aqueous alkali solution; (ii) A step of switching the solvent by at least partially replacing the organic solvent with water or an aqueous solution, whereby an aqueous phase containing water, ε-caprolactam and impurities having a lower or higher boiling point than ε-caprolactam is obtained, the solvent switching step being a process based on back-extraction with water and a process selected from a process based on solvent exchange distillation in which the organic solvent is distilled off and water is introduced; (iii) A step of obtaining purified ε-caprolactam by distilling off impurities having a lower or higher boiling point than ε-caprolactam; (iv) A step including obtaining purified ε-caprolactam by crystallizing ε-caprolactam from a solution containing ε-caprolactam and impurities at a temperature of 10°C to 95°C; d.1) A step of recovering polyether polyurethane from the polyether polyurethane-rich stream in the separation section [B]; d.2) A step of discharging the recovered polyether polyurethane from the separation section [B], the polyether polyurethane content of the discharged stream being at least 85% by weight on a dry weight basis; A process comprising.

2. The process according to claim 1, wherein the separation section [B] is - a dissolution section [α]; - a washing section [β]; - a precipitation section [γ]; - including a solvent distillation section [δ], the process being in the separation section [B], b.1) A step of introducing an organic solvent, and the nylon 6 and polyether polyurethane-containing material into the dissolution section [α]; b.2) In the dissolution section [α], polyether polyurethane is dissolved in an organic solvent from the nylon 6 and polyether polyurethane-containing material, whereby a polyether polyurethane-rich stream containing the organic solvent and the dissolved polyether polyurethane and a stream containing undissolved nylon 6 are obtained, and discharging the obtained stream from the dissolution section [α]; b.3) Feeding the second solvent and the polyether polyurethane-rich stream containing the organic solvent and the dissolved polyether polyurethane into the precipitation section [γ] such that the polyether polyurethane precipitates from a mixture containing the organic solvent and the second solvent; b.4) Recovering the precipitated polyether polyurethane from the mixture containing the organic solvent and the second solvent and discharging the precipitated polyether polyurethane from the precipitation section [γ]; b.5) Discharging the mixture containing the organic solvent and the second solvent from which the polyether polyurethane precipitated in step b.4) was recovered from the precipitation section [γ] and feeding the mixture into the solvent distillation section [δ]; b.6) Feeding a stream containing the third solvent and undissolved nylon 6 into the washing section [β]; b.7) In the washing section [β], washing the stream containing undissolved nylon 6 with the third solvent to thereby obtain a nylon 6-rich stream and a mixture containing the organic solvent and the third solvent; b.8) Discharging the nylon 6-rich stream from the washing section [β]; b.9) Discharging the mixture containing the organic solvent and the third solvent obtained in step b.7) from the washing section [β] and feeding the mixture partially or completely into the solvent distillation section [δ]; b.10) In the solvent distillation section [δ], separating the organic solvent from the second solvent and the third solvent by distillation and discharging the second solvent, the third solvent and the separated organic solvent from the solvent distillation section [δ], A process comprising.

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

4. The process 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 fed into the depolymerization section [C]; and / or (ii) the depolymerization in step c.2) is carried out in the presence of water, whereby the ε-caprolactam-containing stream is a vapor stream containing ε-caprolactam and water in a weight-to-weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10, and most preferably 1:3 to 1:8; and / or (iii) superheated steam having a temperature in the range of 220°C to 575°C, particularly 275°C to 500°C, is fed into the depolymerization section [C]; (iv) In step c.2), the depolymerization is carried out in the absence or presence of a catalyst, the catalyst being selected from an acid catalyst and a base catalyst, the acid catalyst being selected from orthophosphoric acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids, solid acids, salts of the aforementioned acids, Al ２ O ３ and SiO ２ , and combinations thereof, particularly selected from the group consisting of orthophosphoric acid, the base catalyst being selected from alkali hydroxides, alkali salts, alkaline earth hydroxides and alkaline earth salts, organic bases and solid bases, and combinations thereof, particularly selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate, preferably a process carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

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

6. The plant further comprises - a pretreatment section [A], and before step a), the nylon 6 and polyether polyurethane-containing material is subjected to pretreatment in the pretreatment section [A], particularly cleaning in the cleaning section [ω] and / or mechanical size reduction in the mechanical size reduction section [λ]. The process according to any one of claims 1 to 5.

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

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

9. The process 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 optionally separated from each other and then reused in the precipitation section [γ] and / or the washing section [β].

10. ​ The process according to any one of claims 3 to 9, wherein the decomposition product of the organic solvent is removed before the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is introduced into the dissolution section [α] in step b.1). **Claim 11** The process according to any one of claims 1 to 10, wherein the solution containing ε-caprolactam and impurities, from which ε-caprolactam is crystallized in step c.4)(iv), preferably also contains water in an amount exceeding 1% by weight.

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