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

Abstract

The present invention provides a process for recovering epsilon-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane in a plant, wherein the plant comprises a separation section [B], a depolymerization section [C], a recovery section [D] and a purification section [E]. The invention further provides a device suitable for carrying out the above method and the polyether polyurethanes and epsilon-caprolactam obtained by the method of the invention.

Description

Technical Field

[0001] The present invention relates to a method for recovering ε-caprolactam from a material comprising polyamide 6 and polyether polyurethane. More specifically, the present invention relates to a method for recovering both purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane on an industrial scale. Background Art

[0002] ε-caprolactam is an important organic chemical raw material, mainly used in the production of polyamide 6 (PA6), also known as nylon 6 (N6) or polycaprolactam. 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, i.e., a mixture of sulfuric acid and SO3, or by gas-phase Beckmann rearrangement of cyclohexanone oxime in the presence of a solid catalyst. Usually, this type of ε-caprolactam is called "native ε-caprolactam". The cyclohexanone oxime required to form (native) ε-caprolactam can be prepared from cyclohexanone, which is mainly produced from benzene (for example, see "Ullmann's Encyclopedia of Industrial Chemistry" (2018) "Caprolactam" chapter; https: / / doi.org / 10.1002 / 14356007.a05_031.pub3). Most of this benzene comes from non-renewable fossil resources such as petroleum and coal.

[0003] Initially, pure PA6 was used to produce textiles, including stockings. Later, in order to improve the properties of fabrics, various blended fabrics were developed and produced. Especially polyether polyurethane has been widely used in various clothing. The benefits of polyether polyurethane are its significant strength and elasticity, as well as its ability to recover its original shape after stretching and a faster drying speed than ordinary fabrics. The improved elasticity of the fabric is usually obtained by applying a blended yarn blended by PA6 fiber and polyurethane fiber. Examples of clothes containing fabric blends of PA6 and polyether polyurethane include stretchable stockings, underwear and sportswear. The moisture permeability and waterproofness of the fabric can be obtained by treating (e.g., coating) the surface of PA6 fiber with polyurethane resin. These surface-treated yarns are usually used to make, for example, raincoats, clothes suitable for cold weather and ski suits. The polyether polyurethane used to produce clothing is also commonly referred to as spandex (Spandex), Lycra (Lycra) or elastic fibers.

[0004] The recycling of PA6 enables the conservation of fossil resources and can add value to the circular economy. Mechanical recycling of waste PA6 is the process of transferring waste PA6 into secondary raw materials or into products in which (preferably) the chemical structure of the material undergoes only minimal changes. Conversion of PA6 and depolymerization of PA6 are two forms of chemical recycling of PA6. In the conversion recycling process, PA6 is broken down into raw materials similar to petroleum or natural gas, which can replace newly extracted fossil raw materials. The resulting products can be used to produce chemicals including the monomer ε-caprolactam. In the depolymerization recycling process, PA6 is broken down into its monomer building block ε-caprolactam. The PA6 depolymerization recycling process is developed entirely for clean and fairly pure waste PA6 products. Mechanical recycling always results in downgraded recycling, so the properties of products made from mechanically recycled PA6 are always inferior to those made from virgin PA6. However, chemical recycling of waste PA6-containing materials may also allow the production of high-purity ε-caprolactam with properties similar to those of virgin-prepared ε-caprolactam, which can then be converted into high-grade PA6. Unfortunately, chemical recycling of waste PA6-containing materials is often hampered by the presence of impurities in the waste PA6 materials. Typically, due to the presence of these impurities, the quality of the produced ε-caprolactam is inferior compared to virgin ε-caprolactam.

[0005] Compared with the depolymerization of fairly pure PA6, the depolymerization of blends of PA6 and polyether polyurethane has many disadvantages. These disadvantages include, among other things, clogging of pipes and other plant parts by the polyether polyurethane and its decomposition products, a poorer quality of the produced ε-caprolactam due to the presence of the polyether polyurethane decomposition products, a (strongly) reduced ε-caprolactam recovery, poisoning of the depolymerization catalyst and, therefore, a higher consumption of the depolymerization catalyst. Another disadvantage of adding the blend of PA6 and polyether polyurethane to the depolymerization reactor is that the polyether polyurethane is destroyed. Therefore, it is not possible to recycle the polyether polyurethane, which is more valuable than PA6.

[0006] Virgin spandex is produced by a dry or wet spinning process starting with a spandex spinning solution consisting of polyurethane in a suitable solvent such as dimethylacetamide or dimethylformamide.

[0007] Direct recycling of normal (pure) elastane waste fibers, for example from yarn production, by dissolving in the spinning solvent employed has proven unsuccessful due to the high viscosity which prevents further processing.

[0008] US6830715B1 describes a method for producing spandex yarn from a spinning solution using recycled spandex material, which overcomes the above-mentioned viscosity problem by adding a secondary aliphatic amine to a mixture of (cut) spandex and spinning solvent. The dissolution of the (cut) spandex is achieved at a temperature of 60°C to 150°C.

[0009] In the past, several attempts to recycle blends of PA 6 and polyether polyurethanes have been described.

[0010] JP2011088943A describes a pretreatment method for separating polyether polyurethane from a PA6 product containing polyether polyurethane and subsequently depolymerizing the residual PA6. The pretreatment comprises heating the PA6 product containing polyether polyurethane together with a solvent containing cyclic amide at a temperature of 80°C to the boiling point of the solvent. JP2011088943A indicates that the amount of cyclic amide compound in the cyclic amide compound solvent is preferably 50% by weight or more, more preferably 85% by mass or more. JP2011088943A further indicates that the cyclic amide compound solvent may contain components other than the cyclic amide compound, such as water and an organic solvent, and water is particularly preferred from the viewpoint of operability. The experiment described in the patent was carried out at a temperature of 110°C with 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 over a period of 2 hours. As a result of this treatment, the polyether polyurethane was partially decomposed and dissolved in the solvent. Thus, the resulting PA6 product is depolymerized, the polyether polyurethane has been removed from the product and the product is separated from the solution by filtration. However, JP2011088943A does not mention the recovery of the partially decomposed polyether polyurethane. The fate of the solvent used is also not described.

[0011] WO 2013032408A1 describes a method for recycling polyamide fibers including polyamide 6 and polyamide 6,6 including polyamide and spandex. Spandex fibers are removed from elastomeric fabrics by a method comprising controlled thermal degradation of spandex, a controlled washing treatment to remove spandex or its degradation products from high purity polyamide by using a suitable and sustainable solvent, preferably ethanol, and a final step of removing excess solvent from the polyamide fibers. The temperature used during the heat treatment of the polyamide fibers (including polyamide 6 fibers) is in the range of 150°C-220°C, preferably 190°C-216°C, and preferably during a time period of 0.5 to 4 hours. Thereafter, the heat-treated fabric is washed with preferably ethanol at a temperature in the range of 5°C to 78°C to remove spandex and its degradation components. WO2013032408A1 does not mention the recovery of spandex and its degradation components from the washing solvent.

[0012] In view of the above, there is no process for recovering purified ε-caprolactam and polyether polyurethanes from materials including polyamide 6 and polyether polyurethanes on an industrial scale to date. The reason why a recycling process for recovering purified ε-caprolactam and polyether polyurethanes from materials including polyamide 6 and polyether polyurethanes on an industrial scale has not been achieved is that in all methods for bringing the elastic fibers into solution, it is necessary to apply high solution temperatures, usually well above 100° C. However, at these temperatures, the elastic fibers begin to degrade and only inferior materials are obtained after recovery. Therefore, the fate of the recovered elastic fibers is often limited to incineration and land filling. Another disadvantage of the dissolution process at high temperatures is the formation of degradation products of the applied solvents, which may hinder the recovery of polyamide 6 and polyether polyurethanes and the recycling of the applied solvents.

[0013] From the perspective of reducing carbon dioxide emissions, it is of great significance to recycle materials including polyamide 6 and polyether polyurethane to recover purified ε-caprolactam and polyether polyurethane.

[0014] The raw material price of virgin polyether polyurethane is high, usually even more than twice the price of virgin PA6. Therefore, there is a great economic incentive to recover and reuse (recycle) waste polyether polyurethane and PA6.

[0015] A problem with prior art methods is that they consume a large amount of energy to recover the dissolved elastic fibers from the solution, since the dissolved elastic fibers are usually obtained by removing the solvent by evaporation.

[0016] Another disadvantage of the prior art is that no suitable recovery and reuse (recycling) strategy is provided or known for recovering and reusing (recycling) the solvents of polyether polyurethane and PA6 from materials including polyamide 6 and polyether polyurethane.

[0017] Finally, there is a need for a process allowing the recovery of pure ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane on an industrial scale in order to deal with the large amounts of materials comprising polyamide 6 and polyether polyurethane that are wasted every year. Summary of the invention

[0018] It is an object of the present invention to meet one or more of the above needs and to overcome the disadvantages associated with prior art approaches.

[0019] In particular, an object of the present invention is to provide a method for recovering ε-caprolactam from a material comprising polyamide 6 and polyether polyurethane. Another object of the present invention is to provide a method for recovering both purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane.

[0020] A further object of the present invention is to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane on an industrial scale.

[0021] It is also an object of the present invention to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane in an economical and efficient manner. In this respect, in particular, it is an object of the present invention to provide a process which is suitable for recovering both purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane and which does not exceed the production costs of virgin high-purity ε-caprolactam and polyether polyurethane.

[0022] A further object of the present invention is to provide a process for recovering both purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane, said process being characterized by a significantly lower carbon footprint than processes for producing virgin polyether polyurethanes, e.g. by reaction of polyether polyols and diisocyanate monomers and for producing ε-caprolactam by novel syntheses, e.g. by Beckmann rearrangement of cyclohexanone oxime.

[0023] Therefore, the present invention also aims to provide a method for reducing the environmental burden of discarded materials comprising polyamide 6 and polyether polyurethane.

[0024] One or more additional objects may become apparent from the remainder of the specification.

[0025] All or at least some of the above objects are solved or at least alleviated to a large extent by the method of claim 1.

[0026] Therefore, the present invention provides a method for recovering ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane in a plant, wherein the plant comprises

[0027] - separation section [B],

[0028] - depolymerization section [C],

[0029] - Recovery Sector [D], and

[0030] - purification segment [E],

[0031] And wherein the method comprises the following steps:

[0032] a) charging the material comprising polyamide 6 and polyether polyurethane into the separation section [B];

[0033] b) in the separation section [B], separating the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream by selectively dissolving the polyether polyurethane in an organic solvent at a temperature below 100°C, preferably at a temperature in the range of 0°C to 100°C, more preferably at a temperature in the range of 10°C to 90°C, even more preferably at a temperature in the range of 10°C to 80°C and most preferably at a temperature in the range of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane;

[0034] c.1) discharging the polyamide 6-rich stream from the separation section [B] and feeding the polyamide 6-rich stream to a depolymerization section [C], wherein the polyamide 6 content of the polyamide 6-rich stream is at least 85% by weight, based on dry weight;

[0035] c.2) depolymerizing the polyamide 6 in the polyamide 6-rich stream at a temperature in the range of 180° C. to 400° C. in the depolymerization section [C], such that a stream comprising ε-caprolactam is obtained, and discharging the obtained stream comprising ε-caprolactam from the depolymerization section [C];

[0036] c.3) recovering crude ε-caprolactam from the stream comprising ε-caprolactam in the recovery section [D];

[0037] c.4) purifying the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification comprises the following steps:

[0038] (i) extracting the crude ε-caprolactam with an organic solvent so as to obtain an aqueous phase and an organic phase, and wherein the organic phase comprises the organic solvent, the ε-caprolactam and impurities;

[0039] (ii) switching the solvent by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam; and

[0040] (iii) removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation to obtain purified ε-caprolactam;

[0041] d.1) recovering polyether polyurethane from the polyether polyurethane-rich stream in the separation section [B]; and

[0042] d.2) discharging the recovered polyether polyurethane from the separation section [B], wherein the polyether polyurethane content of the discharged stream is at least 85% by weight, based on the dry weight.

[0043] Surprisingly, the combination of a particular sequence of processing steps and process conditions according to the invention, i.e. the sequence of separation, depolymerization and recovery steps described above, allows the recovery of ε-caprolactam from materials comprising polyamide 6 and polyether polyurethanes in high yield and in a direct and economically sound manner. The process of the invention is economically sound and advantageous from several perspectives. Firstly, the process of the invention is applicable to a wide variety of materials comprising polyamide 6 and polyether polyurethanes, which materials may differ, for example, in their overall composition, such as their polyamide 6 content. Secondly, the process of the invention allows the effective separation of polyamide 6 from polyether polyurethane compounds, so that high-grade polyamide 6 can be obtained. Thirdly, the process of the invention is very efficient, so that the obtained polyamide 6 can be depolymerized into high-grade ε-caprolactam in high yield. Fourthly, the process of the invention allows the recovery of polyether polyurethanes, which can be used again to replace polyether polyurethanes produced by de novo synthesis, for example by reaction of polyether polyols and diisocyanate monomers. Fifthly, the process of the invention allows the recovery of ε-caprolactam from materials comprising polyamide 6 and polyether polyurethanes on an industrial scale, so as to treat large amounts of materials comprising polyamide 6 and polyether polyurethanes that are currently wasted. Finally, the process of the present invention allows the production of ε-caprolactam with a significantly lower carbon footprint compared to ε-caprolactam produced by de novo synthesis of ε-caprolactam (e.g., by Beckmann rearrangement of cyclohexanone oxime). The process of the present invention allows the efficient processing of materials including polyamide 6 and polyether polyurethanes and reduces the environmental burden of the materials. In particular, the process of the present invention allows the production of polyether polyurethanes with a carbon footprint of less than 1 kg CO2 / kg polyether polyurethane, which is a significant improvement compared to the 4.8 kg CO2 / kg polyether polyurethane associated with the production of "virgin" polyether polyurethanes obtained by chemical synthesis. In particular, the process of the present invention allows the production of purified ε-caprolactam with a carbon footprint of less than 3 kg CO2 / kg purified ε-caprolactam, which is a significant improvement compared to the 6.4 to 7.5 kg CO2 / kg ε-caprolactam associated with the production of "virgin" ε-caprolactam obtained from Beckmann rearrangement of cyclohexanone oxime.

[0044] In addition to the process of the invention, the present invention also provides a plant for producing purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane, in particular an industrial-scale plant, wherein the chemical plant comprises:

[0045] - Optionally, a pre-treatment section [A],

[0046] - separation section [B],

[0047] - depolymerization section [C],

[0048] - Recovery Sector [D], and

[0049] - optionally, a purification section [E], and

[0050] Wherein the chemical plant is configured to carry out the method of the present invention.

[0051] The present invention also provides ε-caprolactam obtained by depolymerization of polyamide 6 produced from a material comprising polyamide 6 and polyether polyurethane according to the process of the present invention, wherein the product carbon footprint of the ε-caprolactam is less than 3.0 kg CO2 / kg purified ε-caprolactam.

[0052] The present invention also provides a polyether polyurethane obtained by separation from a material comprising polyamide 6 and a polyether polyurethane according to the method of the present invention, wherein the product carbon footprint of the polyether polyurethane is less than 1.0 kg CO2 / kg polyether polyurethane.

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

[0054] Materials including polyamide 6 and polyether polyurethane

[0055] The method of the present invention uses a material comprising polyamide 6 and polyether polyurethane as a starting material. Typically, the material comprising polyamide 6 and polyether polyurethane is a solid material, specifically a fiber, a yarn (i.e., fiber strands spun together) or a fiber-based fabric, wherein the fiber is a blend of PA6 fiber and polyurethane fiber, or wherein the fiber is obtained by coating the surface of PA6 fiber with a polyether polyurethane resin.

[0056] The material comprising polyamide 6 and polyether polyurethane can be of pre-consumer and post-consumer origin. The material comprising nylon 6 and polyether polyurethane can be a mixture of different materials comprising nylon 6 and polyether polyurethane, or a mixture of one or more materials comprising nylon 6 and polyether polyurethane with one or more different materials.

[0057] The material comprising polyamide 6 and polyether polyurethane can include additional components. Such components can be added, for example, during polymerization, fiber formation or thereafter, to achieve desired property changes. These compounds include, for example, brighteners, hardeners, antistatic lubricants, colorants, brighteners, spin finishes, surface smootheners, antioxidants, UV stabilizers, etc. The amount of these additional components depends on the application of the material comprising polyamide 6 and polyether polyurethane.

[0058] The weight ratio of polyamide 6 to polyether polyurethane in the material including polyamide 6 and polyether polyurethane can vary. Preferably, the weight ratio of polyamide 6 to polyether polyurethane ranges from about 1:1 to about 100:1, most preferably from about 3:1 to about 20:1.

[0059] Polyurethanes are produced by reacting isocyanates containing two or more isocyanate groups per molecule with polyols containing an average of two or more hydroxyl groups per molecule in the presence of a catalyst or by activation with ultraviolet light. The main ingredients for preparing polyurethanes are diisocyanates, triisocyanates and polyols. The diisocyanate monomer methylene diphenyl diisocyanate (MDI) is the most commonly used isocyanate in the production of polyurethanes for clothing applications.

[0060] Polyether polyurethanes are made by reacting isocyanates with polyether polyols. Polyether polyols are usually made by polymerizing a cyclic ether compound onto an initiator compound. The most commonly used cyclic ethers in the production of polyether polyols are ethylene oxide, propylene oxide, and 1,4-butylene oxide or tetrahydrofuran (for the production of poly(butylene oxide) polyols). Poly(butylene oxide) polyols are primarily used in applications that require highly hydrophobic properties.

[0061] The term "about" or "approximately" is used in conjunction with the numerical values ​​herein to indicate that the numerical values ​​may be subject to measurement errors, which typically cause the numerical values ​​to vary by no more than ±5%. The numerical values ​​disclosed herein with the term "about" or "approximately" are also intended to be so, i.e., disclosed without the term "about". Further, the numerical ranges described herein are intended to include and disclose each value within the 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. Specifically, the ranges of different or identical "preference levels" are specifically compatible with each other. As used in the present disclosure and claims, unless the context clearly provides otherwise, the singular forms "one", "a kind of" and "the" include plural forms, particularly in the sense of "one or more".

[0062] Possible preprocessing steps

[0063] Before being subjected to step a) of the method of the invention, the material comprising polyamide 6 and polyether polyurethane may be pretreated in a pretreatment section [A], in particular size reduction in a mechanical size reduction section and / or cleaning in a cleaning section. Preferably, the method of the invention is carried out in a plant further comprising a pretreatment section [A], and wherein before step a), the material comprising polyamide 6 and polyether polyurethane is subjected to pretreatment in the pretreatment section [A], in particular cleaning in a cleaning section and / or mechanical size reduction in a mechanical size reduction section. This has the advantage that the product comprising polyamide 6 and polyether polyurethane loaded into the separation section [B] is less contaminated by foreign materials, which improves the yield and purity of ε-caprolactam and polyether polyurethane produced in the plant of the invention configured to carry out the method of the invention. Another advantage is that the size-reduced product comprising polyamide 6 and polyether polyurethane can be handled more easily.

[0064] The size and shape of the (waste) products comprising polyamide 6 and polyether polyurethanes depends largely on the exact application. The (waste) products comprising polyamide 6 and polyether polyurethanes range from individual fibers and yarns, optionally in the form of spools, to (waste) woven fabrics and (used) garments.

[0065] (Waste) products comprising polyamide 6 and polyether polyurethanes may be contaminated with various types of dirt (eg with mud, oil, paint or grease).

[0066] (Waste) products including polyamide 6 and polyether polyurethane may be mixed with a range of other materials such as rock, glass, metallic materials, organic waste and other polymer waste (e.g. polyamide 6,6, polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE)).

[0067] Preferably, the material comprising polyamide 6 and polyether polyurethane is broken into fragments before separation in the separation section [B] in step b). Such mechanical pretreatment of the material comprising polyamide 6 and polyether polyurethane, i.e. mechanical comminution or crushing, can be achieved, for example, by cutting, punching, chopping, grinding, crushing and / or cutting. In a preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is charged to the separation section [B] of step a) in the form of fragments. The use of fragments consisting of the material comprising polyamide 6 and polyether polyurethane has the advantage that the fragments can be more easily handled and / or cleaned by washing with a solvent. In a preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is charged to the separation section [B] of step a) in the form of fragments, the fragments having an average length of 1 mm to 100 m, preferably 3 mm to 1 m, and most preferably 1 cm to 50 cm along the longest axis of the fragments. The average length along the longest axis of the employed fragments can be easily determined by a skilled person by first taking a representative sample of the fragments, then measuring the length of the longest axis of each of these fragments (e.g., 50 fragments), and finally calculating the average of all these individual measurements. The preferred particle size can also be described in terms of average particle weight. Preferably, the average particle weight of the fragments consisting of a material comprising polyamide 6 and polyether polyurethane is from 0.01 grams to 25 kg, preferably from 0.05 grams to 1 kg, and most preferably from 0.1 grams to 100 grams. Experiments have shown that fragments consisting of a material comprising polyamide 6 and polyether polyurethane having the above-mentioned size or weight dimensions are particularly suitable for treatment in the method of the present invention and / or for cleaning by washing with a solvent.

[0068] Optionally, large metal fragments, rocks and other interfering materials that cause severe wear on the equipment used for mechanical crushing or crushing are removed before the mechanical crushing or crushing of the material including polyamide 6 and polyether polyurethane. Preferably, foreign materials including but not limited to materials including polyethylene, polypropylene and polyamide 6,6 are also removed before the mechanical crushing or crushing of the material including polyamide 6 and polyether polyurethane. The removal of foreign materials can be carried out mechanically or manually. The advantage of removing these interfering materials is that the maintenance costs of the equipment used for mechanical crushing or crushing can be greatly reduced. In addition, the polyamide 6 and polyether polyurethane content of the material obtained after mechanical crushing or crushing 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 polyamide 6, reduces the recovery rate of ε-caprolactam, and interferes with the subsequent purification of the recovered ε-caprolactam.

[0069] Optionally, foreign materials are separated from materials including polyamide 6 and polyether polyurethane that have been mechanically crushed or broken. To this end, various separation methods can be applied, including but not limited to density separation and magnetic separation. In density separation, materials of different densities are placed in a medium-density liquid, wherein the material with lower density floats and is separated from the sinking material with higher density. In practice, density separation is usually completed by a series of density separation stages. For example, in one stage, high-density materials such as rocks, sand and metals (including iron and lead) are separated, and in another stage, low-density materials such as polyolefins, polypropylene and polyethylene are separated. Magnetic separation is a method for separating the components of a mixture by using a magnet to attract a magnetic material. The method preferably used for magnetic separation separates non-magnetic materials from magnetic materials. It is advantageous to remove foreign materials in the crushed or broken product materials including polyamide 6 and polyether polyurethane, because such materials can interfere with the separation of polyamide 6 and polyether polyurethane, the depolymerization of polyamide 6, reduce the recovery rate of ε-caprolactam and / or interfere with the subsequent purification of the recovered ε-caprolactam.

[0070] Optionally, the material comprising polyamide 6 and polyether polyurethane is cleaned by washing with a solvent (preferably water) before being loaded into the separation section [B]. Preferably, a detergent in a concentration range of 0 to 20 wt % relative to the solvent is added to the solvent to improve the washing efficiency. NaOH is a preferred detergent. Even more preferably, an aqueous solution containing 0 to 10 wt % NaOH is used in the washing step, still more preferably 0 to 5 wt % NaOH. The enhanced washing effect of NaOH is likely caused by the enhanced hydrolysis of molecules (including biopolymers and non-biopolymers). Preferably, the washing solvent is heated to further enhance the washing process. In another preferred embodiment, the washing process comprises a final rinsing step with a detergent-free (clean) washing solvent to remove detergent residues and dirt present adhering to the material comprising polyamide 6 and polyether polyurethane.

[0071] Washing is preferably carried out under friction.Different types of industrial friction application washing systems are available on the market, like rotary plastics washers and (high speed) friction washers.

[0072] Washing the material comprising polyamide 6 and polyether polyurethane, in particular mechanically comminuted or crushed material comprising polyamide 6 and polyether polyurethane is advantageous, since any (adherent) dirt is removed and thus does not interfere with the following steps of the method of the invention.

[0073] Optionally, the material comprising polyamide 6 and polyether polyurethane is dried after the cleaning step and before being charged to the separation section [B]. This has the advantage that the weight of the cleaned material comprising polyamide 6 and polyether polyurethane is reduced and the next step is not affected by dilution or contamination by washing solvents.

[0074] The location where the pretreatment of the material comprising polyamide 6 and polyether polyurethane is carried out and the location where the separation section [B] is located may be the same. However, preferably, one or more of the pretreatment steps are carried out at a location different from the location of the separation section [B], for example, near a plant where waste materials comprising polyamide 6 and polyether polyurethane are collected and / or at a location dedicated to the pretreatment of materials comprising polyamide 6 and polyether polyurethane. A polyamide 6-rich stream can then be obtained in the separation section [B] from the material comprising polyamide 6 and polyether polyurethane that has been pretreated at a different location.

[0075] The location where the separation of the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream is carried out and the location where the depolymerization section [C] is located may be the same. However, preferably, the separation of the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream is carried out at a location different from the location of the depolymerization section [C], for example, near a plant for producing virgin polyether polyurethane and / or at a location dedicated to processing and / or distilling organic solvents. The location where the pretreatment of the material comprising polyamide 6 and polyether polyurethane is carried out and the location where the depolymerization section [C] is located may 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 a plant for collecting waste materials comprising polyamide 6 and polyether polyurethane and / or at a location dedicated to pretreatment of materials comprising polyamide 6 and polyether polyurethane.

[0076] Loading step a)

[0077] In step a) of the present invention, the material comprising polyamide 6 and polyether polyurethane, optionally pretreated in pretreatment section [A], is fed to separation section [B].

[0078] In one embodiment, the material comprising polyamide 6 and polyether polyurethane is mechanically compressed into a smaller volume before being loaded into the separation section [B]. This has the advantage that a smaller volume is required for intermediate storage and transportation, and can also facilitate administration to the separation section [B].

[0079] In another preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is dried before being charged to the separation section [B], in particular after subjecting the polyamide 6 and polyether polyurethane material to a cleaning step. This has the advantage that less or no solvent is introduced into the separation section [B]. Solvents introduced into the separation section [B], in particular water, may have a negative impact on the separation process (e.g. the dissolution rate of polyether polyurethane from the material comprising polyamide 6 and polyether polyurethane in the organic solvent is reduced).

[0080] The material comprising polyamide 6 and polyether polyurethane is preferably fed into the dissolution vessel in solid phase.

[0081] Feeding the material comprising polyamide 6 and polyether polyurethane to the separation section [B] (eg, to the polyether polyurethane dissolution section) may be achieved by continuously or intermittently dosing the material comprising polyamide 6 and polyether polyurethane.

[0082] Separation step b)

[0083] In separation section [B], the material comprising polyamide 6 and polyether polyurethane is separated to form a polyamide 6-rich stream and a polyether polyurethane-rich stream.

[0084] Preferably, the process of the present invention employs a separation section [B] comprising a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ], and comprises the following steps in the separation section:

[0085] b.1) charging an organic solvent and a material comprising polyamide 6 and polyether polyurethane into the dissolving section [α];

[0086] b.2) dissolving the polyether polyurethane from the material comprising polyamide 6 and polyether polyurethane in an organic solvent in the dissolving section [α], so that a polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6 are obtained, and the obtained stream is discharged from the dissolving section [α];

[0087] b.3) feeding a second solvent and the polyether polyurethane-rich stream comprising the organic solvent and the dissolved polyether polyurethane into the precipitation section [γ], so that the polyether polyurethane is precipitated from the mixture comprising the organic solvent and the second solvent;

[0088] b.4) recovering the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharging the precipitated polyether polyurethane from the precipitation section [γ];

[0089] b.5) ​​discharging the mixture comprising the organic solvent and the second solvent, from which the precipitated polyether polyurethane has been recovered in step b.4), from the precipitation section [γ] and charging the mixture to the solvent distillation section [δ];

[0090] b.6) charging a third solvent and the stream comprising undissolved polyamide 6 to the washing section [β];

[0091] b.7) washing the stream comprising undissolved polyamide 6 with the third solvent in the washing section [β], so that a stream rich in polyamide 6 and a mixture comprising the organic solvent and the third solvent is obtained;

[0092] b.8) discharging the polyamide 6-rich stream from the washing section [β];

[0093] b.9) discharging the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β] and charging the mixture to the solvent distillation section [δ];

[0094] b.10) separating the organic solvent from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and discharging the second solvent, the third solvent and the separated organic solvent from the solvent distillation section [δ].

[0095] Optionally, the second solvent (see step b.3)) may partly or completely be a mixture comprising an organic solvent and a third solvent obtained in step b.7) from the washing section [β].

[0096] Preferably, the separation section [B] used in the process of the present invention comprises a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ]. Such a separation section [B] is particularly suitable for carrying out the separation of a material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream according to the process of the present invention. Surprisingly, when the process steps b.1) to b.10) are carried out in a separation section [B] comprising a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ], the separation of the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream is particularly effective. Steps b.1) to b.10) carried out in a preferred separation section [B] are further explained below. Steps b.1) to b.10) can replace process steps a) and b) in the process of the present invention. Step b.1) thus corresponds to step a).

[0097] As used herein, a "stream enriched in..." refers to a stream that contains a greater amount of an enriched component than another stream obtained in the same process step. A stream enriched in polyamide 6 thus contains more polyamide 6 than a stream enriched in polyether polyurethane. In contrast, a stream enriched in polyether polyurethane contains more polyether polyurethane than a stream enriched in polyamide 6.

[0098] In particular, the polyamide 6-rich stream has a weight ratio of polyamide 6 to polyether polyurethane that is higher than the weight ratio of polyamide 6 to polyether polyurethane of the material comprising polyamide 6 and polyether polyurethane. Preferably, the polyamide 6-rich stream has a weight ratio of polyether polyurethane to polyamide 6 that is at most one-half, more preferably at most one-third of the weight ratio of polyether polyurethane to polyamide 6 of the material comprising polyamide 6 and polyether polyurethane.

[0099] The polyether polyurethane-rich stream has a polyether polyurethane to polyamide 6 weight ratio that is higher than the polyether polyurethane to polyamide 6 weight ratio of the material comprising polyamide 6 and polyether polyurethane.

[0100] Preferably, the polyether polyurethane-rich stream has a weight ratio of polyether polyurethane to polyamide 6 that is at least twice, more preferably at least three times, the weight ratio of polyether polyurethane to polyamide 6 of the material comprising polyamide 6 and polyether polyurethane.

[0101] Loading separation section step (step b.1)):

[0102] In the separation section [B], an organic solvent and a material including polyamide 6 and polyether polyurethane are charged into the dissolution section [α].

[0103] The charging of the material comprising polyamide 6 and polyether polyurethane into the dissolution section [α] is the charging of the material comprising polyamide 6 and polyether polyurethane optionally pretreated in the pretreatment section [A] as mentioned in step a) of the present invention.

[0104] The organic solvent charged into the dissolving section [α] may be any organic solvent in which the polyether polyurethane can be dissolved. Preferably, the organic solvent combines a (very) good solubility for the polyether polyurethane with a (very) poor solubility for polyamide 6. According to another preferred embodiment, the organic solvent 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.

[0105] The organic solvent charged to the dissolution section [α] may be a separated organic solvent (i.e. a recycled organic solvent or an organic solvent from within the process which has been recovered in the solvent distillation section), a fresh organic solvent (i.e. a solvent from outside the process) or a combination of a separated organic solvent and a fresh organic solvent.

[0106] The organic solvent charged to the dissolution section [α] preferably has a low water content. Even more preferably, the organic solvent is dried (preferably by distillation or application of a drying agent, such as an inorganic salt such as Na2SO4, zeolite, silica and alumina) before being charged to the dissolution section [α]. Drying does not necessarily have to be complete. Typically, after drying the organic solvent, less than 1 wt.%, preferably less than 0.1 wt.%, more preferably less than 0.02 wt.% of water remains.

[0107] Dissolution step (step b.2)):

[0108] In the dissolution section [α], the polyether polyurethane is dissolved from the material comprising polyamide 6 and the polyether polyurethane in the organic solvent charged to the dissolution section [α] in step b.1). A polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6 discharged from the dissolution section [α] are thereby obtained.

[0109] Preferably, dissolution of the polyether polyurethane will be complete within 0.1 hour to 24 hours, more preferably within 0.5 hour to 6 hours. Complete means that the dissolution has reached a plateau, with no substantial further dissolution occurring at a later time point.

[0110] The temperature in the dissolution zone [α] may vary and is preferably below 120°C, more preferably below 110°C, even more preferably below 100°C, and most preferably below 80°C. Preferably, the temperature in the dissolution zone [α] may be in the range of 0°C to 100°C, more preferably 10°C to 90°C, even more preferably 10°C to 80°C, and most preferably 20°C to 75°C. Experiments have shown that these temperature ranges cause the polyether polyurethane to dissolve particularly completely. This has the advantage that the polyether polyurethane present in the material comprising polyamide 6 and the polyether polyurethane can be dissolved. Another advantage of these temperatures is that the degradation of the polyether polyurethane is minimized or even completely prevented.

[0111] The amount of solvent (expressed in tons) in the dissolution section [α] can vary and can be 0.5 to 100 times, preferably 1 to 60 times, more preferably 2 to 20 times, and most preferably 3 to 10 times the amount of polyether polyurethane (expressed in tons) in the dissolution section [α]. A small amount of solvent reduces the dissolution rate of the polyether polyurethane and may even result in incomplete dissolution of the polyether polyurethane. A large amount of solvent gives the polyether polyurethane a high dissolution rate, but recycling the used organic solvent requires more energy and cost. The technician can determine the optimal ratio of solvent to polyether polyurethane or a material comprising nylon 6 and polyether polyurethane by routine testing.

[0112] The dissolution section [α] may comprise one or more dissolution vessels operating in series or in parallel. Preferably, these vessels are equipped with additional equipment (e.g. agitators) to increase friction and thereby reduce the dissolution time.

[0113] Precipitation step (step b.3)):

[0114] The polyether polyurethane-rich stream comprising the organic solvent and the dissolved polyether polyurethane discharged from the dissolution section [α] and the second solvent are fed to the precipitation section [γ] so that the polyether polyurethane is precipitated from the mixture comprising the organic solvent and the second solvent.

[0115] Preferably, the second solvent is a non-solvent (also referred to as an anti-solvent) for the polyether polyurethane. The addition of the second solvent reduces the solubility of the polyether polyurethane dissolved in the solution, which results in precipitation of the precipitated polyether polyurethane particles. The second solvent can be any solvent. Preferably, the second solvent is water. The advantage of water as a second solvent is that it is an environmentally friendly solvent.

[0116] In a preferred embodiment, the second solvent is a mixture comprising an organic solvent and a third solvent obtained from the washing section [β] in step b.7). In another preferred embodiment, the second solvent consists of or is a portion of a mixture comprising an organic solvent and a third solvent obtained from the washing section [β] in step b.7). "A portion of a mixture" as used herein is defined as any fraction between 0 and 100 wt.%, preferably between 10 and 100 wt.%, more preferably between 25 and 100 wt.%, even more preferably between 75 and 100 wt.%. This has the advantage that less fresh solvent is used for the precipitation of the polyether polyurethane. Another advantage is that less solvent needs to be distilled in the solvent distillation section [δ], which results in lower energy costs, smaller equipment size, lower investment costs and a more environmentally friendly process.

[0117] The temperature in the precipitation section [γ] can vary and can be in the range of 0° C. to 150° C., preferably 10° C. to 100° C., more preferably 15° C. to 80° C. and most preferably 20° C. to 60° C. Experiments have shown that these temperature ranges allow particularly complete precipitation of the dissolved polyether polyurethane.

[0118] The amount of the second solvent charged to precipitation section [γ] can vary and can be in the range of 0.1 wt.% to 500 wt.%, preferably 0.2 wt.% to 100 wt.%, more preferably 0.5 wt.% to 50 wt.% and most preferably 1 wt.% to 25 wt.% compared to the organic solvent charged to precipitation section [γ].

[0119] Small amounts of the second solvent reduce the degree of precipitation of the polyether polyurethane. Large amounts of the second solvent complicate the recovery of the precipitated polyether polyurethane and the organic solvent. Large amounts of the second solvent also require more energy and cost to recover the precipitated polyether polyurethane and the organic solvent. The technician will be able to find the optimal ratio of the second solvent to the polyether polyurethane-rich stream for the specific solvent and equipment used by performing simple precipitation tests.

[0120] Polyether polyurethane recovery step (step b.4):

[0121] In the polyether polyurethane recovery step, the precipitated polyether polyurethane is recovered from a mixture including an organic solvent and a second solvent, and the precipitated polyether polyurethane is discharged from the precipitation section [γ]. There are several suitable methods for recovering the precipitate, including but not limited to filtration, centrifugation and decantation. Preferably, filtration is used to recover the polyether polyurethane. For filtration, a solution containing the precipitate is loaded into a filter so that the precipitate is expected to remain on the filter when the liquid passes through the filter. When centrifugation is used as a recovery method, the solution containing the precipitate is rapidly rotated, causing the solid precipitate to settle (assuming that the density of the solid precipitate is higher than the density of the liquid). Compacted precipitates can also be obtained by pouring out the liquid. In decantation, the liquid layer is poured out or sucked out of the precipitate.

[0122] Optionally, the recovered precipitate is washed with a solvent before being discharged from the precipitation section [γ]. Preferably, the solvent used for washing the recovered precipitate is the organic solvent of step b.1) or the second solvent.

[0123] Optionally, the recovered precipitate, optionally washed with a solvent, is dried before being discharged from precipitation section [γ]. In a preferred embodiment, the precipitated polyether polyurethane discharged from precipitation section [γ] in step b.4) is reused in the production of textiles, optionally in combination with fresh polyether polyurethane.

[0124] Step of discharging the solvent mixture (step b.5)):

[0125] The mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered in step b.4) is discharged from the precipitation section [γ] and fed to the solvent distillation section [δ]. Optionally, the solvent resulting from washing the recovered precipitate is also discharged from the precipitation section [γ] and fed to the solvent distillation section [δ].

[0126] Load the wash step (step b.6)):

[0127] In the separation section [B], the third liquid solvent and the stream comprising undissolved polyamide 6 are fed to the washing section [β]. The stream comprising undissolved polyamide 6 fed to the washing section [β] may comprise, in addition to undissolved polyamide 6, an organic solvent. The purpose of the washing section [β] is to recover the organic solvent present in the stream comprising undissolved polyamide 6, thereby obtaining a stream rich in polyamide 6 containing less organic solvent than the stream comprising undissolved polyamide 6.

[0128] Preferably, the third solvent has good solubility for organic solvents and poor solubility for polyamide 6. The third solvent may be any solvent. Preferably, the second solvent is water. The advantage of water as the third solvent is that it is an environmentally friendly solvent.

[0129] The amount of the third solvent charged to washing section [β] may vary and may 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 comprising undissolved nylon 6 charged to washing section [β].

[0130] Washing step (step b.7)):

[0131] In the washing section [β], the stream comprising undissolved polyamide 6 is washed with a third solvent, so that a stream rich in polyamide 6 and a mixture comprising the organic solvent and the third solvent is obtained.

[0132] The temperature in the washing section [β] can vary and may 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 lead to particularly complete recovery of the organic solvent from the stream comprising undissolved polyamide 6.

[0133] The skilled person can determine by routine experimentation the amount of third solvent and the optimal temperature required to effectively wash the stream comprising undissolved polyamide 6.

[0134] The washing of the stream comprising undissolved polyamide 6 with the third solvent can be carried out in various apparatuses, all of which are known to the skilled person.

[0135] Step of discharging a polyamide 6-rich stream (step b.8)):

[0136] A polyamide 6-rich stream is discharged from the washing section [β]. The polyamide 6-rich stream comprises a third solvent and optionally an organic solvent. Preferably, the polyamide 6-rich stream has an organic solvent content of less than 25 wt.%, preferably less than 10 wt.%, more preferably less than 2 wt.%, and most preferably less than 0.2 wt.%, relative to the total weight of the polyamide 6-rich stream. Optionally, the polyamide 6-rich stream is dried before being discharged.

[0137] Preferably, the polyamide 6 content in the resulting polyamide 6-rich stream from washing section [β] is at least 85 wt.-%, more preferably at least 90 wt.-%, and most preferably at least 95 wt.-%, based on dry weight.

[0138] Preferably, the polyamide 6 content in the resulting polyamide 6-rich stream from separation section [B] is at least 85 wt-%, more preferably at least 90 wt-%, even more preferably at least 95 wt-%, and most preferably at least 98 wt-%, based on dry weight.

[0139] The determination of the polyamide 6 content is a routine activity and can be carried out by various methods, all of which are known to the skilled person. Preferably, the polyamide 6 content in the polyamide 6-rich stream is determined by thermogravimetric analysis (TGA) and / or by DSC (e.g. by method ISO 11357-3).

[0140] As used herein, "dry weight basis" refers to the amount of an ingredient expressed as a percentage relative to the total dry weight of the composition, wherein "dry weight" is the weight of a substance after removing water and / or other liquids from the substance by drying until the weight is constant.

[0141] Step of discharging a mixture comprising an organic solvent and a third solvent (step b.9)):

[0142] A mixture comprising an organic solvent and a third solvent is discharged from the washing section [β]. Typically, the organic solvent and / or the third solvent are valuable compounds, and recycling and reusing them has great economic and environmental advantages. The mixture comprising the organic solvent and the third solvent is charged to the solvent distillation section [δ]. Optionally, the mixture comprising the organic solvent and the third solvent is first charged to another section and then to the solvent distillation section [δ]. In a preferred embodiment, the mixture comprising the organic solvent and the third solvent is first partially or completely charged to the precipitation section [γ].

[0143] Solvent separation step (step b.10)):

[0144] In the solvent distillation section [δ], the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered and the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] are separated by distillation. Thereafter, the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered and the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] are referred to as a combined feed to the solvent distillation section [δ].

[0145] The separated organic solvent, the second solvent and the third solvent as well as the residue are discharged from the solvent distillation section [δ]. In a preferred embodiment, the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is fed to 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 reused in the precipitation section [γ] and / or the washing section [β], optionally after being separated from each other.

[0146] Separation by distillation is used to separate liquids from non-volatile solids and to separate liquids with different boiling points.Distillation is a conventional activity and can be carried out in a variety of apparatuses, all of which are known to the skilled person.

[0147] The mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered optionally further contains (dissolved) polyether polyurethane, (dissolved) polyamide 6 and degradation products of the organic solvent and / or the second solvent. The mixture comprising the organic solvent and the third solvent optionally further contains (dissolved) polyether polyurethane, (dissolved) polyamide 6 and degradation products of the organic solvent and / or the third solvent.

[0148] In general, separation of three different liquids by distillation is more complicated than separation of two different liquids by distillation, requires more energy, and requires more equipment. Therefore, it is desirable to limit the number of different solvents in the method of the present invention to two. In a preferred embodiment of the present invention, the second solvent and the third solvent are identical solvents. In a more preferred embodiment, the second solvent and the third solvent are water. Using water as solvent is advantageous because it is cheap, available in large quantities, non-explosive and environmentally friendly.

[0149] In a preferred embodiment of the present invention, the organic solvent is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidone such as N-methyl-2-pyrrolidone (NMP, C5H9NO), 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).

[0150] In another preferred embodiment, the organic solvent is selected from N,N-dimethylformamide (DMF, (CH3)2NC(=O)H), N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2), 1,4-dioxane (dioxane, C4H8O2), N-alkyl-2-pyrrolidone such as N-methyl-2-pyrrolidone (NMP, C5H9NO), tetrahydrofuran (THF; also known as 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.

[0151] When only N,N-dimethylacetamide (DMAc) and water are present in the combined feed to the solvent distillation section [δ], the difference in volatility between the two is large and separation by distillation will be quite easy. In such cases, the continuous separation of the combined feed to the solvent distillation section [δ] can be achieved in a single distillation column operated in a continuous mode. In such a distillation column, water is distilled from the top of the column in a gaseous state, and N,N-dimethylacetamide (DMAc) is discharged from the bottom of the column in a liquid state. When non-volatile compounds (such as polyether polyurethane and polyamide 6) are present in the combined feed to the solvent distillation section [δ] in addition to N,N-dimethylacetamide (DMAc) and water, the solvent distillation layout required for continuous separation into N,N-dimethylacetamide (DMAc), water and non-volatile compounds is more extensive. This type of direct layout for separation by distillation includes two distillation columns operated in series. A feed consisting of N,N-dimethylacetamide (DMAc), water and non-volatile compounds (in a liquid state) is charged to the first distillation column. Water is distilled from the top of the first distillation column in a gaseous state, and a mixture of N,N-dimethylacetamide (DMAc) and non-volatile compounds (in a liquid or slurry form) is discharged from the bottom of the first distillation column. The bottom stream of the first column is charged to the second distillation column. N,N-dimethylacetamide (DMAc) is distilled from the top of the second distillation column in a gaseous state, and non-volatile compounds are discharged from the bottom of the second distillation column. In practice, the mixture of N,N-dimethylacetamide (DMAc) and non-volatile compounds can be discharged from the bottom of the second distillation column to reduce the risk of plugging of the second distillation column. An extrusion (column) unit for recovering N,N-dimethylacetamide (DMAc) from the bottom stream of the second distillation column can be added to increase the overall recovery rate of N,N-dimethylacetamide (DMAc).

[0152] As an alternative to separation by distillation of two distillation columns operated in series, a single distillation column with a side draw operated in continuous mode can be selected. In such a distillation column, water is distilled in the gaseous state from the top of the distillation column, N,N-dimethylacetamide (DMAc) is discharged as a liquid side stream, and non-volatile compounds are discharged from the bottom of the second distillation column. Optionally, the bottom stream is fed to an extrusion (column) unit.

[0153] As an alternative to charging the feed in the distillation column in the liquid state, the charging into the first distillation column can be carried out in the gaseous state. In this case, the combined feed to the solvent distillation section [δ], except for the non-volatile compounds, is evaporated in a feed evaporator before being charged to the first distillation column. The non-volatile compounds are discharged from the feed evaporator. The advantage of this alternative is that less fouling occurs in the distillation column. Optionally, the squeeze column can be combined with the feed evaporator. Typically, process streams comprising N,N-dimethylacetamide (DMAc) contain a certain amount of acetic acid and dimethylamine due to, for example, the degradation of N,N-dimethylacetamide (DMAc). This is also the case for the combined feed to the solvent distillation section [δ].

[0154] The boiling point of pure dimethylamine at atmospheric pressure is about 7° C. Dimethylamine is very soluble in water.

[0155] The boiling point of pure acetic acid at atmospheric pressure is approximately 118°C.

[0156] However, acetic acid and N,N-dimethylacetamide (DMAc) form a high boiling azeotrope (composition of about 21 wt.% acetic acid and about 79 wt.% N,N-dimethylacetamide at atmospheric pressure) and have an atmospheric boiling point of about 171°C, which is only slightly higher than the atmospheric boiling point of N,N-dimethylacetamide. A high boiling azeotrope (or constant boiling mixture) is a mixture of two liquids having a boiling point higher than the boiling points of the two individual liquids (in pure form).

[0157] If the pressure is reduced, the azeotropic point of the mixture of acetic acid and N,N-dimethylacetamide shifts to a higher concentration of the low-boiling-point component, which is acetic acid.

[0158] At 6.7 kPa, a high boiling azeotrope consisting of about 30 wt.% acetic acid and about 70 wt.% N,N-dimethylacetamide is formed, and the boiling point of the high boiling azeotrope is about 93°C.

[0159] At 1.3 kPa, a high boiling azeotrope of composition of about 42 wt.% acetic acid and about 58 wt.% N,N-dimethylacetamide is formed, the boiling point of which is about 75°C. Preferably, the degradation products are removed from the solvent used again in the process. This has the advantage that the accumulation of degradation products is prevented. In a preferred embodiment, the degradation products of the organic solvent are removed before the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is charged to the dissolution section [α] in step b.1). However, N,N-dimethylacetamide (DMAc) and acetic acid cannot be separated by simple distillation due to the formation of a maximum azeotrope.

[0160] Several techniques for breaking azeotropes in distillation are known to those skilled in the art.

[0161] One group of techniques is based on the addition of another compound that changes the molecular interactions and eliminates the highest azeotrope of N,N-dimethylacetamide-acetic acid. Chlorobenzene, toluene, ethylbenzene and xylene are examples of compounds that break this highest azeotrope. Adding one of these compounds to a mixture of N,N-dimethylacetamide and acetic acid results in the formation of a new azeotrope of this compound and acetic acid that can be removed by evaporation. Subsequently, the resulting mixture of this compound and acetic acid is separated.

[0162] Another group of technologies, generally referred to as pressure swing distillation, is based on the fact that the highest azeotrope is pressure-dependent. In such systems, two distillation columns are operated at different pressure levels. A feed mixture of N,N-dimethylacetamide and acetic acid is fed to a first distillation column operated at a certain pressure. The bottom stream of this first distillation column is fed to a second distillation column. The bottom stream of the second distillation column is fed to the first distillation column. As top products of the distillation columns, high-purity N,N-dimethylacetamide and acetic acid are obtained. The main advantage of pressure swing distillation is the absence of foreign compounds.

[0163] Finally, there is a group of technologies that convert acetic acid to another component (eg, salt formation due to addition of caustic) or selectively adsorb acetic acid from a mixture of N,N-dimethylacetamide and acetic acid.

[0164] When, in addition to N,N-dimethylacetamide (DMAc), water and non-volatile compounds such as polyether polyurethanes and polyamide 6, acetic acid and dimethylamine are also present in the combined feed to the solvent distillation section [δ], the required solvent distillation layout for the continuous separation into N,N-dimethylacetamide (DMAc), water, non-volatile compounds, acetic acid and dimethylamine is even more extensive.

[0165] The optimum layout depends largely on the composition and volume flow of the combined feeds to the solvent distillation section [δ]. The layout may contain a combination of the various separation solutions described above.

[0166] If the fraction of acetic acid in the combined feed to the solvent distillation section [δ] is rather low, the solvent distillation section [δ] may comprise two distillation columns operated in series, followed by a neutralizer evaporation step. A feed consisting of N,N-dimethylacetamide (DMAc), water, non-volatile compounds, acetic acid and dimethylamine is charged to a first distillation column. Water and dimethylamine are distilled in the gaseous state from the top of the first distillation column, and a mixture of N,N-dimethylacetamide (DMAc), acetic acid and non-volatile compounds (in the form of a liquid or slurry) is discharged from the bottom of the first distillation column. The water vapor from the top of the first distillation column is condensed. The resulting liquid phase contains water and dissolved dimethylamine. In a dimethylamine stripping unit, dimethylamine can be removed from this liquid phase, thereby obtaining water that is (almost) free of dimethylamine. The bottom stream of the first column is charged to a second distillation column. N,N-dimethylacetamide (DMAc) is distilled from the top of the second distillation column in a gaseous state, and a mixture of N,N-dimethylacetamide (DMAc), acetic acid and non-volatile compounds is discharged from the bottom of the second distillation column. The bottom stream of the second column is charged to a neutralizer evaporation unit. A base, such as an aqueous NaOH solution, is charged to the neutralizer evaporation unit to neutralize the acetic acid. Due to evaporation, gaseous N,N-dimethylacetamide (DMAC) is discharged from the neutralizer evaporation unit. If present, water is also evaporated. A mixture of neutralized acetic acid (containing salts of acetic acid) and non-volatile compounds is discharged from the bottom of the neutralizer evaporation unit. In practice, a mixture of N,N-dimethylacetamide (DMAc), neutralized acetic acid (containing salts of acetic acid) and non-volatile compounds can be discharged from the bottom of the neutralizer evaporation unit to reduce the risk of blockage of the neutralizer evaporation unit.

[0167] N,N-dimethylacetamide (DMAc) recovered by distillation is discharged from the solvent distillation section [δ] and charged to the dissolution section [α].

[0168] Preferably, the water recovered in the solvent distillation section [δ] is reused in the process according to the invention, optionally after removal of dimethylamine. Preferably, water is used as the second solvent and / or as the third solvent.

[0169] Load step c.1)

[0170] In step c.1) of the present invention, the resulting polyamide 6-rich stream is fed to a depolymerization section [C], wherein the polyamide 6 content in the polyamide 6-rich stream is at least 85% by weight, preferably at least 90% by weight, based on dry weight. This has the advantage that the polyamide 6-rich stream fed to the depolymerization section [C] is less contaminated with foreign materials, which increases 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. Other advantages are that the polyamide 6-rich stream will require less depolymerization agent in the depolymerization section [C], will generate less waste, and will require less energy. And finally, the polyamide 6-rich stream will reduce operating problems, such as fouling and clogging of equipment. The depolymerization section [C] comprises one or more depolymerization reactors operated in series and / or in parallel.

[0171] Optionally, the polyamide 6-rich stream is mechanically compressed to a smaller volume before being fed to the depolymerization section [C]. This has the advantage that a smaller volume is required for intermediate storage and transport and can also facilitate dosing to the depolymerization section [C].

[0172] Optionally, the polyamide 6-rich stream is compressed into granules of increased density before being fed into the depolymerization section [C]. This can be achieved, for example, by mechanical compaction or by extruding the molten material by means of a die, followed by cooling and cutting to size. Compressing the polyamide 6-rich stream into granules has the advantage of increased bulk density, which reduces the costs of intermediate storage and transportation. In addition to the increased density, granulation also provides other benefits, such as a uniform shape and structure, which facilitates (automatic) feeding into the depolymerization section [C].

[0173] Optionally, the polyamide 6-rich stream is fed into a furnace (e.g., an extruder). In the furnace, the polyamide 6-rich stream is melted. Preferably, the resulting polymer melt is filtered. This has the advantage of removing solid impurities. The molten and optionally filtered polymer melt is cooled and then fed to a pelletizer. The pelletizer cuts the product into pellets. These pellets are fed into a depolymerization section [C].

[0174] The size and shape of the pellets (also referred to as granules) can be selected within a wide range. In general, the shape of the pellets is cylindrical (derived from the strands cut into pieces). However, other shapes such as (imperfect) spheres are also possible. The size of the pellets can be selected within a wide range. The diameter of the pellets can range from 1 to 10 mm, preferably from 2 to 5 mm, more preferably from 3 to 5 mm. In a preferred embodiment, the length of the pellets ranges from 1 to 50 mm, preferably from 2 to 25 mm, and more preferably from 5 to 15 mm.

[0175] Optionally, the polyamide 6-rich stream discharged from separation section [B] is dried before being fed to depolymerization section [C]. This has the advantage that less or no solvent is introduced in depolymerization section [C]. Solvent introduced in depolymerization section [C] can have a negative impact on the depolymerization process (e.g., reduced depolymerization reaction rate, higher catalyst consumption, higher energy consumption, and the vapor stream comprising ε-caprolactam and water obtained in depolymerization section [C] may contain more impurities).

[0176] The polyamide 6-rich stream is preferably fed to the depolymerization reactor in the solid phase or in the form of a melt. Preferably, the polyamide 6-rich stream is charged in the form of a melt. Feeding in the form of a melt can be achieved by using an extruder, a gear pump or other means known to the skilled person.

[0177] Feeding the polyamide 6-rich stream to the depolymerization reactor can be achieved by continuously or intermittently dosing the polyamide 6-rich stream.

[0178] Depolymerization step c.2)

[0179] In the depolymerization section [C], the polyamide 6-rich stream is depolymerized to form ε-caprolactam. The ε-caprolactam formed is discharged from the depolymerization section [C] as a stream comprising ε-caprolactam.

[0180] Depolymerization of the polyamide 6-rich stream is achieved by increasing the temperature of the polyamide 6-rich stream to a temperature of at least 180° C. but not higher than 400° C. The preferred temperature range for the depolymerization reaction is 200° C. to 350° C., more preferably 220° C. to 340° C., and most preferably 240° C. to 325° C. Experiments have shown that within these temperature ranges, depolymerization is particularly effective and side reactions and impurity reactions of polyamide 6 and ε-caprolactam occur less frequently.

[0181] In general, the rate of formation of ε-caprolactam increases at elevated temperatures. Temperatures below 400° C. are preferred, since at temperatures above 400° C., side reactions and impurity reactions of polyamide 6 and ε-caprolactam occur more frequently, which will lead to the formation of a greater variety of impurities. A portion of these impurities will eventually enter the product stream comprising ε-caprolactam discharged from the depolymerization section [C]. In a preferred embodiment, the depolymerization of the polyamide 6-rich stream is carried out at a temperature in the range of 220° C. to 340° C. or 240° C. to 325° C. Experiments have shown that this temperature range allows the production of particularly pure ε-caprolactam.

[0182] The pressure in the depolymerization section [C] may vary and may be in the range of 1 kPa to 100 MPa, preferably 10 kPa to 5 MPa, more preferably 25 kPa to 2 MPa, most preferably 50 kPa to 1 MPa. This pressure range allows the production of particularly pure ε-caprolactam.

[0183] The depolymerization of the polyamide 6-rich stream can be achieved in the presence or absence of a solvent. Preferably, the depolymerization of the polyamide 6-rich stream is achieved in the presence of water as solvent. In this case, the water is preferably in the form of steam, in particular superheated steam.

[0184] Preferably, the depolymerization will be complete within 0.1 hour to 24 hours, more preferably within 0.5 hour to 6 hours.

[0185] Feeding water in the form of steam to the depolymerization reactor allows obtaining a steam stream comprising ε-caprolactam and water, optionally without further heating. The weight ratio of ε-caprolactam to water in the steam stream can be adjusted by varying the amount of steam fed to the polyamide 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, so that the stream comprising ε-caprolactam is a steam stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50, preferably 1:2 to 1:15, more preferably 1:2 to 1:10 and most preferably 1:3 to 1:8. Within these ranges, a particularly economical and efficient process can be carried out.

[0186] Preferably, the ε-caprolactam in the vapor stream comprising ε-caprolactam and water has a 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.

[0187] During the depolymerization reaction, decomposition products may be formed, which decomposition products include linear oligomers of ε-caprolactam and cyclic oligomers of ε-caprolactam. In addition, the feed stream of the polyamide 6-rich stream may also contain other components, i.e. impurities, such as non-polyamide 6 compounds and solvent residues that remain stable, react or decompose under the depolymerization conditions applied in the pretreatment section and / or separation section. Therefore, in the case of using water as solvent in the depolymerization section [C], the vapor stream removed from the depolymerization section [C] includes not only water and ε-caprolactam, but also impurities.

[0188] Preferably, superheated steam having a temperature between 100° C. and 600° C. is fed to the depolymerization reactor. Preferably, the temperature of the superheated steam fed to the depolymerization reactor is at least the melting temperature of polyamide 6. Preferably, the energy content of the superheated steam fed to the depolymerization reactor is sufficiently high that no further heat input is required for the depolymerization reaction and for evaporation of the ε-caprolactam formed. In another preferred embodiment, superheated steam having a temperature in the range of 220° C. to 575° C. is fed to the depolymerization section [C]. In an even more preferred embodiment, superheated steam having a temperature in the range of 275° C. to 500° C. is fed to the depolymerization section [C].

[0189] Typically, the mass of the vapor stream removed from the depolymerization section [C] is less than the mass of the total feed to the depolymerization section. The total feed to the depolymerization section [C] comprises a stream rich in polyamide 6 and optionally solvents, catalysts, further reagents and / or depolymerizing agents. Thus, without any further measures, there will be an accumulation of material (generally referred to as 'residual material') in the depolymerization section [C]. Preferably, another stream is discharged from the depolymerization section [C]. This has the advantage of reducing or avoiding the accumulation of material in the depolymerization section [C]. The discharged further stream may include impurities present in the stream rich in polyamide 6, undepolymerized polyamide, unevaporated ε-caprolactam, catalysts and compounds formed under depolymerization conditions, such as monoammonium phosphate, diammonium phosphate and / or triammonium phosphate in the case where phosphoric acid is used as a depolymerization catalyst. In a preferred embodiment, a stream comprising monoammonium phosphate, diammonium phosphate and / or triammonium phosphate is discharged from the depolymerization section [C]. Even more preferably, the stream intermittently or continuously discharged from the depolymerization section [C] comprises a weight fraction of monoammonium phosphate, diammonium phosphate and / or triammonium phosphate of 0.01 to 50 wt.%, preferably 0.1 to 25 wt.%, more preferably 0.5 to 10 wt.%, most preferably 0.5 to 5 wt.%.

[0190] The depolymerization of the polyamide 6-rich stream in the presence of steam can be carried out in the presence of a further depolymerizing agent, such as ammonia. The concentration of ammonia in the depolymerization section [C] can vary. Thus, in the presence of ammonia in the depolymerization section [C], the vapor stream removed from the depolymerization section [C] comprises not only ε-caprolactam and impurities, but also ammonia.

[0191] Most preferably, the depolymerization is carried out in the presence of a catalyst. Preferably, the catalyst used is a (Lewis or Bronsted) acid or base. The acid catalyst may be specifically selected from the group consisting of orthophosphoric acid, p-toluenesulfonic acid, boric acid, sulfuric acid, organic acids, organic sulfonic acids (including xylenesulfonic acid, 4-sulfoisophthalic acid and other sulfonated aromatic hydrocarbons), solid acids, salts of the above acids, Al2O3 and SiO2 and combinations thereof. The base catalyst may, for example, be selected from the group consisting of alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts, organic bases and solid bases and combinations thereof. Preferably, orthophosphoric acid, boric acid, organic acids, alkali metal hydroxides and alkali metal salts are used as catalysts. More preferably, orthophosphoric acid, sodium phosphate, potassium phosphate, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate and potassium bicarbonate are used as catalysts. Still more preferably, orthophosphoric acid, p-toluenesulfonic acid, boric acid and sodium hydroxide are used as catalysts. In a particularly preferred embodiment, orthophosphoric acid is used as catalyst for the depolymerization, in another embodiment p-toluenesulfonic acid is used.

[0192] However, in another preferred embodiment, no catalyst is used for the depolymerization of the polyamide 6-rich stream. This has the advantage of lower costs (for disposal of the catalyst and catalyst waste). However, higher temperatures (and pressures) are generally required.

[0193] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction has already started at a lower temperature and can be carried out under atmospheric conditions. The suitable concentration of the catalyst for depolymerizing polyamide 6 into ε-caprolactam is known to the technician and can be easily determined by routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. On the contrary, if the concentration of the catalyst used is too high, the reaction is very fast, but the side reactions will also increase. In addition, the catalyst cost increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the polyamide 6 contained in the depolymerization reactor. Even more preferably, the catalyst content is 0.1 to 50% by weight. The optimal catalyst concentration depends on the type of catalyst used for the depolymerization of polyamide 6. For the catalyst orthophosphoric acid, the preferred content is 0.1 to 25% by weight, and more preferably 1 to 20% by weight. The preferred content of the catalyst p-toluenesulfonic acid is 10 to 35% by weight, and more preferably 15 to 30% by weight.

[0194] The depolymerization of polyamide 6 can be carried out in batch mode, semi-continuous mode or continuous mode, all of which are known to the skilled person. The terms "batch mode", "semi-continuous mode" and "continuous mode" as used herein refer to a mode in which a polyamide 6-containing feedstock (i.e., a polyamide 6-rich stream) and an optional catalyst are charged to a depolymerization reactor, and a mode in which residual materials are discharged from the depolymerization reactor.

[0195] In a preferred embodiment, the depolymerization of polyamide 6 is carried out in batch mode. In batch mode, the raw material, i.e., a stream rich in polyamide 6 and an optional catalyst, is initially charged to the depolymerization reactor. Subsequently, superheated steam is charged to the depolymerization reactor, and ε-caprolactam is discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Next, the charging of superheated steam to the depolymerization reactor is interrupted. After optionally removing residual material from the depolymerization reactor, a new cycle is started by charging the raw material (and optional catalyst) to the depolymerization reactor. In a preferred embodiment, residual material is not removed between each cycle.

[0196] In a particularly advantageous embodiment, the depolymerization of polyamide 6 is carried out in a continuous mode. In the continuous mode, a feedstock containing polyamide 6 (and optionally a catalyst) is continuously charged to a depolymerization reactor. At the same time, superheated steam is continuously charged to the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged to the depolymerization reactor. In addition, residual material is continuously discharged from the depolymerization reactor. Preferably, the polyamide 6-rich stream is charged in the form of a melt. Preferably, the catalyst is charged in the form of a melt, a slurry or a solution.

[0197] In another preferred embodiment, the depolymerization of polyamide 6 is carried out in a semi-continuous mode. In the semi-continuous mode, the feedstock containing polyamide 6 (and optional catalyst) is intermittently charged to the depolymerization reactor, while superheated steam is continuously charged to the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. In the semi-continuous mode of depolymerization of polyamide 6, residual materials are intermittently discharged from the depolymerization reactor.

[0198] Recovery step c.3)

[0199] In recovery section [D], ε-caprolactam is recovered from the stream comprising ε-caprolactam discharged from depolymerization section [C]. The stream comprises ε-caprolactam and impurities. Preferably, the recovery is performed by (partial) condensation of the stream comprising ε-caprolactam.

[0200] Preferably, the ε-caprolactam obtained by condensation is dissolved in water without charging a solvent into the depolymerization section [C], thereby obtaining a phase rich in ε-caprolactam. The phase rich in ε-caprolactam also comprises impurities.

[0201] Preferably, in case water is charged as solvent to the depolymerization section [C], the stream comprising ε-caprolactam discharged from the depolymerization section [C] comprises ε-caprolactam, water and impurities. The ε-caprolactam can be separated from the remaining components of the vapor stream by conveying the vapor stream from the depolymerization reactor (preferably the top) to a (preferably partial) condenser to obtain a condensate containing ε-caprolactam. Preferably, the ε-caprolactam is separated from the remaining components of the vapor stream by conveying the product stream from the depolymerization reactor (preferably the top) to a distillation column, from which a water-rich phase is obtained as top product and an ε-caprolactam-rich phase is obtained as bottom product.

[0202] The ε-caprolactam recovered in the recovery section [D] is crude, as it contains impurities, such as polyamide 6 decomposition products and / or other impurities caused by (decomposition products of) non-polyamide 6 components of the material comprising polyamide 6 and polyether polyurethane and / or reactions with impurities. The crude ε-caprolactam recovered in step c.3) comprises water and ε-caprolactam, preferably the crude ε-caprolactam is an aqueous solution comprising ε-caprolactam. Therefore, the crude ε-caprolactam recovered in the recovery section [D] requires additional purification to produce high-purity ε-caprolactam. Therefore, "crude" as used herein can be defined as having a lower purity, i.e. containing more impurities, than the purified ε-caprolactam obtained after purification in the purification section [E].

[0203] Preferably, the crude ε-caprolactam comprises in the range of 6 to 95 wt%, more preferably 20 to 90 wt%, and most preferably 35 to 80 wt% ε-caprolactam. The remainder is mainly water.

[0204] Purification step c.4)

[0205] The method of the present invention is further carried out in a factory, the factory further comprising

[0206] - purification segment [E],

[0207] And wherein the method further comprises the following steps:

[0208] c.4) purifying the crude ε-caprolactam obtained in the recovery section [D] in a purification section [E] to obtain purified ε-caprolactam, wherein the purification comprises the following steps:

[0209] (i) extracting the crude ε-caprolactam with an organic solvent to obtain an aqueous phase and an organic phase, and wherein the organic phase comprises the organic solvent, the ε-caprolactam and impurities;

[0210] (ii) optionally, switching the solvent by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam. The solvent switching step c.4) (ii) can be specifically selected from a method based on stripping with water and a method based on solvent exchange distillation, wherein the organic solvent is distilled off and water is charged;

[0211] (iii) obtaining purified ε-caprolactam by removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation.

[0212] In step c.4), the crude ε-caprolactam obtained in the recovery section [D] is purified in a purification section [E] to obtain high-purity ε-caprolactam.

[0213] Optionally, the crude ε-caprolactam is filtered before being charged to the purification section [E]. Filtration ensures the removal of undissolved impurities which might otherwise hamper the further purification process.

[0214] Purified ε-caprolactam is obtained from crude ε-caprolactam by first extracting crude ε-caprolactam with an organic solvent in step (i), thereby obtaining an aqueous phase and an organic phase comprising organic solvent, ε-caprolactam and impurities. The organic solvent used for extracting crude ε-caprolactam is preferably an aromatic hydrocarbon, a halogenated hydrocarbon and / or a C4-C10 aliphatic or C4-C10 cycloaliphatic alcohol. Optionally, the organic solvent used for extracting crude ε-caprolactam is preferably a mixed extractant, i.e. it can be composed of one or more organic solvents and optionally another diluent. Preferably, one or more organic solvents are independently selected from aromatic hydrocarbons, halogenated hydrocarbons and / or C4-C10 aliphatic or cycloaliphatic alcohols, and C5-C8 alkanes or C5-C8 cycloalkanes. Particularly good purification results can be obtained if the organic solvent used to extract the crude ε-caprolactam is selected from the group consisting of cyclohexane, benzene, toluene, dichloromethane, chloroform, trichloroethane, 4-methyl-2-pentanol (also known as MIBC, methyl isobutyl carbinol), 1-octanol, 2-ethylhexanol and mixtures thereof. More preferably, the organic solvent used to extract the crude ε-caprolactam is selected from the group consisting of benzene, toluene, cyclohexane, alcohols and mixtures thereof. Still more preferably, the organic solvent used to extract 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 the ε-caprolactam is from 0.01:1 to 50:1, preferably from 0.05:1 to 20:1, more preferably from 0.1:1 to 10:1, and most preferably from 0.1:1 to 5:1.

[0215] In another embodiment, the extraction with the organic solvent in step c.4) (i) is carried out in an extraction column operated in countercurrent, 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. The extraction produces an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam and impurities.

[0216] Optionally, the organic phase comprising organic solvent, ε-caprolactam and impurities is washed with water or an alkaline aqueous solution before entering step c.4)(ii). If washing is carried out with an alkaline aqueous solution, the alkaline solution is preferably an aqueous solution comprising an alkali metal hydroxide and / or an alkali metal carbonate, preferably sodium hydroxide or potassium hydroxide. The alkali metal hydroxide solution preferably comprises 0.5 wt.% to 2.0 wt.% of sodium hydroxide or potassium hydroxide.

[0217] The skilled person can determine by routine experiments the amount of water or alkaline aqueous solution required for efficient washing of the organic phase comprising organic solvent, ε-caprolactam and impurities. Preferably, the amount is between 0.1 vol.% and 5 vol.% relative to the amount of organic solvent in the organic phase to be washed.

[0218] In another preferred embodiment, washing the organic phase comprising organic solvent, ε-caprolactam and impurities with water or an alkaline aqueous solution is carried out in a countercurrently operated washing column, whereby the organic phase comprising organic solvent, ε-caprolactam and impurities is introduced at the bottom of the column and water or an alkaline aqueous solution is introduced at the top of the column. The washing produces a washed organic phase comprising organic solvent, ε-caprolactam and impurities and a phase comprising the residue. The phase comprising the residue then contains water, impurities and ε-caprolactam.

[0219] In a preferred embodiment, the optionally washed organic phase comprising organic solvent, ε-caprolactam and impurities is then extracted with water to obtain an ε-caprolactam-water phase. Preferably, the ε-caprolactam-water phase is then stripped and / or distilled to remove residual solvent. Although the amount of water used to recover ε-caprolactam can vary, the amount of water used is 0.5 to 20 times, preferably 0.75 to 10 times, and more preferably 1 to 5 times by weight of the ε-caprolactam recovered.

[0220] In another preferred embodiment, the extraction with water is carried out in an extraction column operated in countercurrent, the ε-caprolactam-containing phase to be purified is introduced at the bottom of the column, and water is introduced at the top of the column. The extraction produces an ε-caprolactam-water phase and a solvent phase comprising impurities. Typically, the solvent phase comprising impurities is reused, optionally after purification (preferably by distillation).

[0221] The ε-caprolactam-water phase, optionally stripped and / or distilled to remove residual solvents, is concentrated by evaporating water so that a concentrated ε-caprolactam-water phase is obtained. The ε-caprolactam content of the concentrated ε-caprolactam-water phase is generally between 60 and 99.9 wt.% relative to the entire phase.

[0222] In another preferred embodiment, the organic solvent is evaporated from an organic phase comprising the organic solvent, ε-caprolactam and impurities which is optionally washed instead of extracted with water. Any suitable evaporation vessel, such as a tower, can be used. Preferably, the evaporation is carried out in the presence of water. More preferably, the evaporation is carried out in the form of azeotropic distillation, in which case the organic solvent evaporates in the form of an azeotropic mixture. The evaporation produces an ε-caprolactam product. Preferably, the ε-caprolactam product is an ε-caprolactam aqueous phase. The ε-caprolactam content of the ε-caprolactam aqueous phase is typically between 40wt.% and 99.9wt.% relative to the entire phase.

[0223] In another preferred embodiment, an oxidizing agent, such as potassium permanganate, sodium permanganate and / or hydrogen peroxide, is added to the ε-caprolactam-water phase prior to the distillative removal in step c.4) (ii) of the process of the invention. Most preferably, potassium permanganate is used as oxidizing agent.

[0224] Preferably, the oxidant is added to the ε-caprolactam-water phase in the form of a solid, a slurry or an aqueous solution, so that a dilute aqueous solution is obtained during the purification by oxidation. The skilled person can determine the amount of oxidant required for efficient oxidation of the ε-caprolactam-water phase by routine experiments. The exact amount of oxidant depends to a great extent on the composition of the polyamide 6-rich stream fed to the depolymerization section in the process according to the invention. Preferably, the amount of oxidant is between 0.01 and 5% by weight relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0225] The temperature used for oxidizing the aqueous solution in the process of the present invention may vary. Preferably, the oxidation of the aqueous solution with an oxidizing agent is carried out at a temperature in the range of 20° C. to 85° C., more preferably in the range of 30° C. to 80° C., prior to the distillative removal in step c.4) (ii), wherein the oxidizing agent is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof, in particular potassium permanganate.

[0226] The length of time for oxidation with the oxidizing agent can vary. Preferably, in the process of the present invention, oxidation of the ε-caprolactam-water phase with 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, prior to distillative removal in step c.4)(ii).

[0227] The concentration of ε-caprolactam in the ε-caprolactam-water phase used for oxidation with the oxidant can vary. Preferably, the aqueous solution used for oxidation comprises a weight ratio of ε-caprolactam to water of 5:1 to 1:5, more preferably 3:1 to 1:3, and most preferably 2:1 to 1:2. Optionally, the weight ratio of ε-caprolactam to water is adjusted before the oxidant is added to the aqueous phase. Preferably, the weight ratio of ε-caprolactam to water is adjusted by adding water or by removing water.

[0228] In the case of using potassium permanganate and / or sodium permanganate as oxidants, solid manganese (IV) oxide (MnO2) particles are formed as reaction products. The best solid-liquid filtration procedure for efficiently removing the solid manganese (IV) oxide particles from the aqueous phase after oxidation can be determined by the skilled person through routine experimentation. In this regard, it is common practice to use filter aids such as activated carbon or diatomaceous earth particles to improve the filtration procedure.

[0229] Optionally the aqueous ε-caprolactam phase is hydrogenated and, if the aqueous ε-caprolactam phase is hydrogenated, this is preferably done in the presence of a hydrogenation catalyst known per se. The hydrogenation can be carried out as described, for example, in EP635487.

[0230] Optionally, water is evaporated from the optionally hydrogenated ε-caprolactam aqueous phase. Preferably, water is evaporated from the optionally hydrogenated ε-caprolactam aqueous phase. After hydrogenation and / or evaporation of water, the ε-caprolactam aqueous phase is distilled to recover high-purity ε-caprolactam and a distillation residue.

[0231] In step c.4)(iii) of the process of the present invention, the optionally washed organic phase comprising organic solvent, ε-caprolactam and impurities is subsequently distilled. Preferably, the distillation of the optionally washed organic phase comprising organic solvent, ε-caprolactam and impurities is carried out under reduced pressure. In one embodiment, the distillation is carried out at a pressure of less than 350 kPa, preferably less than 50 kPa, more preferably less than 20 kPa and most preferably less than 10 kPa. Preferably, the distillation temperature at the bottom of the distillation column is between 100°C and 200°C, and more preferably between 110°C and 180°C. The distillation comprises the separation of low-boiling organic impurities (boiling point lower than ε-caprolactam) and / or the separation of high-boiling organic impurities (boiling point higher than ε-caprolactam) from ε-caprolactam.

[0232] In a preferred embodiment, an alkali metal hydroxide, preferably NaOH, is added to the phase comprising ε-caprolactam before the distillative removal in step c.4) (iii). Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol / kg ε-caprolactam, and more preferably 2 to 80 mmol / kg ε-caprolactam. This allows particularly efficient distillative removal of impurities having lower and higher boiling points than ε-caprolactam.

[0233] In another preferred embodiment, in addition to the crude ε-caprolactam recovered in the recovery section, crude ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime is also purified in the purification section. This has the advantage that the carbon footprint of a plant for the de novo production of ε-caprolactam by the Beckmann rearrangement of cyclohexanone oxime can be reduced by introducing the recycled ε-caprolactam according to the invention.

[0234] The high-purity ε-caprolactam obtained according to the process of the invention can be used to prepare polyamide 6 using methods well known to the skilled person. Polyamide 6 can then be used in all known materials, including engineering materials, fibers and membranes.

[0235] Recovery of polyether polyurethane (steps d.1) and d.2)):

[0236] The polyether polyurethane-rich stream obtained during separation step b) is used for recovering the polyether polyurethane in separation section [B], wherein the polyether polyurethane-rich stream is a solution comprising solvent and polyether polyurethane (step d.1)).

[0237] In step d.2) of the process of the invention, the recovered polyether polyurethane is discharged from separation section [B]. Optionally, the recovered polyether polyurethane is dried before being discharged from separation section [B].

[0238] Steps d.1) and d.2) can be replaced by steps b.3) and b.4). Therefore, a preferred embodiment of the method of the present invention is performed in a plant comprising:

[0239] - a separation section [B], said separation section comprising a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ],

[0240] - a depolymerization segment [C], and

[0241] - Recovery section [D],

[0242] And includes the following steps:

[0243] a) charging a material comprising polyamide 6 and polyether polyurethane into the separation section [B];

[0244] b.1) charging an organic solvent and a material comprising polyamide 6 and polyether polyurethane into the dissolving section [α] of the separation section [B];

[0245] b.2) dissolving the polyether polyurethane from the material comprising polyamide 6 and polyether polyurethane in an organic solvent in the dissolving section [α], so that a polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6 are obtained, and the obtained stream is discharged from the dissolving section [α];

[0246] b.3) feeding a second solvent and the polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane to the precipitation section [γ] of the separation section [B], so that the polyether polyurethane is precipitated from the mixture comprising organic solvent and second solvent;

[0247] b.4) recovering the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharging the precipitated polyether polyurethane from the precipitation section [γ];

[0248] b.5) ​​discharging the mixture comprising the organic solvent and the second solvent from which the precipitated polyether polyurethane has been recovered in step b.4) from the precipitation section [γ] and charging the mixture to the solvent distillation section [δ] of the separation section [B];

[0249] b.6) charging a third solvent and the stream comprising undissolved polyamide 6 to the washing section [β] of the separation section [B];

[0250] b.7) washing the stream comprising undissolved polyamide 6 with the third solvent in the washing section [β], so that a stream rich in polyamide 6 and a mixture comprising the organic solvent and the third solvent is obtained;

[0251] b.8) discharging the polyamide 6-rich stream from the washing section [β];

[0252] b.9) discharging the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β] and charging the mixture to the solvent distillation section [δ] of the separation section [B];

[0253] b.10) separating the organic solvent from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and discharging the second solvent, the third solvent and the separated organic solvent from the solvent distillation section [δ].

[0254] c.1) discharging the polyamide 6-rich stream from the separation section [B] and feeding the polyamide 6-rich stream to a depolymerization section [C], wherein the polyamide 6-rich stream has a polyamide 6 content of at least 85% by weight, preferably at least 90% by weight, based on dry weight;

[0255] c.2) depolymerizing the polyamide 6 in the polyamide 6-rich stream at a temperature in the range of 180° C. to 400° C. in the depolymerization section [C], such that a stream comprising ε-caprolactam is obtained, and discharging the obtained stream comprising ε-caprolactam from the depolymerization section [C];

[0256] c.3) recovering crude ε-caprolactam from the stream comprising ε-caprolactam in the recovery section [D].

[0257] factory

[0258] The present invention also provides a factory in which the above method of the present invention can be implemented. The factory is a chemical factory. Therefore, all factory features described below in conjunction with the factory also correspond to specific embodiments of the method of the present invention, and vice versa. Therefore, the factory is suitable for implementing the method of the present invention, and it should be understood that what has been described in conjunction with the method of the present invention is equally applicable to the factory embodiment.

[0259] The plant may be a laboratory setting as in the examples. Preferably, however, the plant is an industrial scale plant. "Industrial scale" means that the ε-caprolactam production capacity (i.e. the amount of ε-caprolactam that can be produced in principle) of the plant, if it were operating, is at least 500 tonnes / year.

[0260] The plant of the present invention is suitable for producing purified ε-caprolactam and polyether polyurethane from materials including polyamide 6 and polyether polyurethane, and comprises at least the following three sections: a separation section [B], a depolymerization section [C] and a recovery section [D], and further optionally a pretreatment section [A] and an optional purification section [E]. These sections and therefore the plant are configured to carry out the above-mentioned method of the present invention.

[0261] In addition, the plant of the present invention may include a pretreatment section [A], which may include a mechanical size reduction section for breaking the material comprising polyamide 6 and polyether polyurethane into fragments and / or a cleaning section for washing the material comprising polyamide 6 and polyether polyurethane. Cleaning comprises washing the material comprising polyamide 6 and polyether polyurethane and separating foreign materials from the material. The separation of foreign materials can be carried out manually (hand picking) and mechanically (e.g. density separation and magnetic separation). Manual and mechanical devices, such as brushes, can assist washing in the cleaning section. Washing is preferably carried out by additional friction. Different types of industrial washing systems are available on the market, such as rotary plastic washers and (high-speed) friction washers. The mechanical size reduction section includes equipment for mechanically breaking the material comprising polyamide 6 and polyether polyurethane into fragments. Non-limiting examples of such breaking equipment are cutters, punches, shredders, mills, grinders or chippers.

[0262] The separation section [B] may comprise the following four sections: a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ]. These sections and therefore the plant are configured to carry out the above-described method of the invention.

[0263] The dissolving section [α] comprises one or more dissolving units operating in series and / or in parallel, and one or more separation units operating in series and / or in parallel. The optionally cleaned and / or crushed fragments consisting of a material comprising polyamide 6 and polyether polyurethane are fed to the dissolving unit in the form of a solid or a melt, preferably a solid. The optionally cleaned and / or crushed fragments consisting of a material comprising polyamide 6 and polyether polyurethane are optionally dried before being fed to the dissolving section [α]. The dissolving unit is equipped with an inlet for loading the separated organic solvent and fresh organic solvent. The dissolving unit is equipped with one or more outlets for discharging the mixture obtained in the dissolving unit. The mixture comprises undissolved polyamide 6 and an organic solvent in which polyether polyurethane is dissolved. Preferably, the dissolving unit is closed. This has the advantage of reducing the emission of organic solvents to the environment. Preferably, the dissolving unit is equipped with a device for controlling the temperature in the unit (e.g., a steam coil, steam tracing, electric tracing). Optionally, the temperature of the separated organic solvent and the fresh organic solvent is adjusted (eg in a heat exchanger) before charging into the dissolution section [α].

[0264] Good contact between the organic solvent and the material including polyamide 6 and polyether polyurethane is essential for effective operation. Such contact can be achieved by various means known in the art. Improved contact can be achieved by mechanical friction. Mechanical friction can then be achieved, for example, by stirring with a combination of rotating paddles and static fins.

[0265] Suitable dissolution units for dissolving polymers in organic solvents are known to the person skilled in the art. A stirred vessel is an example of such a dissolution unit.

[0266] In the separation unit, the mixture discharged from the dissolution unit is separated into a polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6. The polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane is discharged through a discharge line and fed to a precipitation section [γ]. The stream comprising undissolved polyamide 6 is discharged through a discharge line and fed to a washing section [β]. Advantageously, the amount of organic solvent in the stream comprising undissolved polyamide 6 is low to facilitate washing in the washing section [β].

[0267] Suitable separation units for separating (undissolved) polymer and organic solvent are known to the person skilled in the art. Centrifuges and filters are examples of such separation units.

[0268] Preferably, the dissolution unit and the separation unit are combined in one piece of equipment. This allows for process intensification.

[0269] The washing section [β] comprises a device for washing a stream comprising undissolved polyamide 6 with a third solvent to obtain a stream rich in polyamide 6 and a mixture comprising an organic solvent and a third solvent. The purpose of the washing section [β] is to remove the organic solvent to a large extent. Optionally, the washing section [β] comprises a device for drying the stream rich in polyamide 6. The mixture comprising the organic solvent and the third solvent is discharged from the washing section [β] and charged to the solvent distillation section [δ]. In this context, "charging to the solvent distillation section [δ]" also includes indirect means, i.e., additional steps / sections in the middle. Preferably, the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] is completely or partially charged to the precipitation section [γ] before being charged to the solvent distillation section [δ].

[0270] Optionally, after drying, a polyamide 6-rich stream is discharged from the washing section [β] and fed to the depolymerization section [C].

[0271] Good contact between the third solvent and the polyamide 6-rich stream is essential for efficient operation. This contact can be achieved by various means known to the skilled person. One means is a (continuous) centrifuge, into which the polyamide 6-rich stream is fed and into which the third solvent is charged as a wash solvent.

[0272] The distillation section [δ] comprises one or more distillation columns operated in series and / or in parallel. The distillation columns can be operated in batch, semi-continuous or continuous mode. The choice between these operating modes depends largely on the scale of operation. In general, batch operation of distillation columns is more suitable for processing small volume feed streams. Continuous operation of distillation columns is more suitable for processing large volume feed streams.

[0273] The exact layout of the distillation section [δ] also depends on the nature of the organic solvent, the second solvent and the third solvent.

[0274] A mixture comprising an organic solvent and a second solvent from which a precipitated polyether polyurethane has been recovered and a mixture comprising an organic solvent and a third solvent discharged from the washing section [β] are charged into the distillation section [δ]. The separated organic solvent is discharged from the distillation section [δ] and charged to the dissolution section [α]. The second solvent and the third solvent are discharged from the distillation section [δ] and optionally charged to the precipitation section [γ] and the washing section [β], respectively. The residue is discharged from the distillation section [δ]. Optionally, the residue is incinerated to recover energy. Finally, the degradation products of the used solvent are discharged from the distillation section [δ]. Acetic acid and dimethylamine are examples of degradation products of the organic solvent N,N-dimethylacetamide (DMAc).

[0275] The depolymerization section [C] comprises one or more depolymerization reactors operated in series and / or in parallel. The polyamide 6-rich stream is fed into the reactor in the form of a solid or a melt, preferably a melt. This feeding can be achieved by using an extruder, a gear pump or other means known in the art.

[0276] During production, the depolymerization reactor is at least partially filled with feedstock containing polyamide 6, residual material, ε-caprolactam (and optionally catalyst). The depolymerization reactor can have any desired form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors and extruder type reactors.

[0277] The depolymerization reactor must be equipped with means for feeding a polyamide 6-rich stream, superheated steam and optionally a catalyst. In addition, the depolymerization reactor must be equipped with means for discharging a vapor stream comprising ε-caprolactam and water, and residual materials.

[0278] Good contact between the steam and the reactor contents is essential for efficient operation. Such contact can be achieved by various means known to the skilled person. As an example, steam can be sprayed through the material using multiple inlets provided by, for example, a steam distributor. Improved contact can also be achieved by including mechanical agitation in the reactor. Mechanical agitation can be achieved, for example, by using a combination of rotating paddles and static fins.

[0279] Preferably, the depolymerization is completed within 0.5 to 6 hours.

[0280] If high-temperature superheated steam is not available at the production location, superheated steam must be produced intentionally by superheating the available steam in the boiler in a so-called superheater.

[0281] The recovery section [D] comprises one or more (preferably partial) condensers, into which a vapour stream comprising ε-caprolactam and water is charged. Such (partial) condensers may have any desired form. Preferably, the condenser is a distillation column, from which a water-rich phase is obtained as top product and crude ε-caprolactam is obtained as bottom product.

[0282] The purification section [E] comprises one or more extraction devices, one or more distillation devices, optionally one or more solvent switching devices, an optional oxidation section and an optional hydrogenation section, crude ε-caprolactam is charged into said device or said section and high-purity ε-caprolactam is discharged from said device or said section.

[0283] The crude ε-caprolactam and the organic solvent are charged into an extraction device, and an organic phase comprising the organic solvent, ε-caprolactam and impurities and an aqueous phase comprising water and impurities are discharged. The extraction device is selected from a mixer-settler extractor, an extraction column and a centrifugal extractor and a combination thereof. Preferably, the extraction device is a static or stirred extraction column, such as tower, towers, rotating disk contactors (RDC), pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (SMVP) (static) towers.

[0284] Water and an organic phase comprising an organic solvent, ε-caprolactam and impurities are fed into a solvent switching device, and a phase comprising an organic solvent and an ε-caprolactam-water phase comprising water, ε-caprolactam and impurities are discharged. The solvent switching device for the method based on stripping is selected from a mixer-settler extractor, an extraction column and a centrifugal extractor and combinations thereof. Preferably, the device for stripping is a static or stirred extraction column, such as tower, towers, rotating disk contactors (RDC), pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (static) towers.

[0285] The solvent switching device for the process based on solvent exchange distillation is selected from a sieve plate distillation column, a random packing distillation column and a structured packing distillation column. Preferably, the distillation column is equipped with a reboiler, a condenser and a device for reflux. The distillation column can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure. Preferably, water is charged to the upper part of the distillation column, and the aqueous phase comprising water, ε-caprolactam and impurities with a boiling point lower or higher than ε-caprolactam is discharged from the lower part of the distillation column.

[0286] The oxidation section optionally present comprises one or more oxidation reactors operated in series and / or in parallel. The oxidant and the ε-caprolactam-water phase comprising water, ε-caprolactam and impurities are charged into the oxidation section. The oxidant can be charged in the form of a solid, a slurry or an aqueous solution. Preferably, the oxidant is charged in the form of an aqueous solution. In the case of applying potassium permanganate or sodium permanganate as an oxidant, 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 must be equipped with facilities for feeding an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam and an oxidant. In addition, the oxidation reactor must be equipped with facilities for discharging the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities and optionally formed solid manganese oxide (IV) (MnO2) particles. Typically, the oxidation is carried out at a temperature and atmospheric conditions ranging from 20°C to 85°C.

[0287] The solid manganese oxide (IV) (MnO2) particles that are optionally present can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to use filter aids such as activated carbon particles and diatomaceous earth to improve the filtration procedure. Filtration systems suitable for separating solid manganese oxide (IV) particles are known to the technician. Into this filtration system, a suspension of an oxidized ε-caprolactam-water phase including water, ε-caprolactam and impurities and solid manganese oxide (IV) particles is loaded, and the filtered oxidized ε-caprolactam-water phase including water, ε-caprolactam and impurities is discharged. Typically, the solid manganese oxide (IV) particles are retained in the filtration system. Typically, this filtration system is operated in a semi-continuous mode, so that the suspension and the filtered phase are continuously loaded and continuously discharged, respectively, while the separated solids are collected in the filtration system. The loading of the suspension is interrupted from time to time, and the collected solids are removed from the filtration system.

[0288] Purification of crude ε-caprolactam to obtain purified ε-caprolactam may comprise hydrogenation using a heterogeneous catalyst, in which case the plant will comprise a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

[0289] The hydrogenation section includes one or more hydrogenation reactors operated in series and / or in parallel. Hydrogenation can be carried out in a three-phase system (gas, liquid, solid), which is composed of an ε-caprolactam aqueous mixture, gaseous hydrogen and a heterogeneous hydrogenation catalyst. Alternatively, hydrogenation can be carried out in a two-phase system (liquid, solid), which is composed of an ε-caprolactam aqueous mixture completely or partially saturated with hydrogen and a heterogeneous hydrogenation catalyst. The dissolution of hydrogen in a water-ε-caprolactam mixture can be carried out by any method known to those skilled in the art. Preferably, the mixture is contacted with hydrogen in an absorber or a mixer, wherein a constant hydrogen pressure is maintained. Close contact between hydrogen and the mixture will ensure that hydrogen is dissolved in the mixture. This process is preferably carried out continuously. Subsequently, the mixture containing hydrogen is contacted with a hydrogenation catalyst, for example in a separate reactor.

[0290] The heterogeneous catalyst can be contacted with the hydrogen-containing reaction mixture in various ways. The hydrogenation can be carried out, for example, in a stirred tank reactor, wherein the catalyst particles are suspended in the mixture to be hydrogenated (slurry phase method). In this slurry phase method, the catalyst particles and the purified mixture must be separated in another method step after the hydrogenation reaction, for example by filtering. Preferably, the catalyst comprises palladium or nickel.

[0291] Alternatively, the hydrogenation can be carried out in a fixed bed reactor, in which the catalyst is fixed in the reactor, so that an additional step for separating the catalyst and the reaction mixture can be omitted. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0292] The hydrogenation temperature is typically between 20° C. and 160° C. The hydrogenation pressure is typically between 0.1 and 15 MPa.

[0293] The ε-caprolactam-water phase including water, ε-caprolactam and impurities is charged into a distillation apparatus, and high-purity ε-caprolactam, water and impurities (i.e., low-boiling organic impurities with a boiling point lower than that of ε-caprolactam and high-boiling organic impurities with a boiling point higher than that of ε-caprolactam) are discharged. The distillation apparatus is selected from a sieve plate distillation column, a random packing distillation column, a structured packing distillation column, a horizontal and vertical (descending and ascending) film evaporator. Preferably, the distillation column is equipped with a reboiler, a condenser and a device for reflux. The distillation apparatus can be operated at atmospheric pressure, subatmospheric pressure or superatmospheric pressure, preferably at subatmospheric pressure.

[0294] Preferably, the distillation comprises separating water, low-boiling organic impurities (having a lower boiling point than ε-caprolactam) from ε-caprolactam and / or separating high-boiling organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. Thus, the distillation comprises separating water from ε-caprolactam as a top product in a first step, while removing ε-caprolactam containing low-boiling impurities and high-boiling impurities as a bottom product. In a second step, low-boiling impurities are removed from ε-caprolactam as a top product, while 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, while a distillation residue comprising ε-caprolactam and high-boiling impurities is left as a bottom product. Optionally, the first step and the second step are combined.

[0295] Typically, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-water phase comprising water, ε-caprolactam and impurities before distilling off water and impurities. Preferably, the amount of NaOH added is in the range of 0.5 to 100 mmol / kg ε-caprolactam, more preferably 2 to 25 mmol / kg ε-caprolactam. This allows particularly efficient distillation removal of impurities having lower and higher boiling points than ε-caprolactam in the subsequent distillation.

[0296] The method of the present invention can be operated in a continuous, semi-continuous or batch mode. Therefore, the plant of the present invention can also be configured to allow one or more of these operating modes. In a preferred embodiment, the plant is configured to operate the method of the present invention in a continuous or semi-continuous manner. However, a discontinuous method is also possible. For example, the plant of the present invention does not need to contain all the sections described herein in one position. Specifically, the pretreatment section [A] can be located at a first position, while the separation section [B], depolymerization section [C], recovery section [D] and purification section [E] are located at a second position. Similarly, a mechanical size reduction section as part of the pretreatment section [A] can also be located at a first position, while a cleaning section as part of the pretreatment section [A] can be located at a second position. Then the separation section [B], depolymerization section [C], recovery section [D] and purification section [E] can even be located at a third position.

[0297] Optionally, the cleaning section is divided into two or more sections, which are optionally all located at different positions. For example, a first section of the cleaning section as part of the pretreatment section [A] can be located at a first position, a mechanical size reduction section as part of the pretreatment section [A] can be located at a second position, and a second section of the cleaning section as part of the pretreatment section [A] can be located at a third position. The separation section [B], the depolymerization section [C], the recovery section [D], and the purification section [E] are located at a fourth position.

[0298] Optionally, the separation section [B] may be located at a position different from the positions of the pretreatment section [A], the depolymerization section [C], the recovery section [D] and the purification section [E]. Preferably, the separation section [B] may be located near the position where the recovered polyether polyurethane is reused.

[0299] product

[0300] The process of the invention allows the production of ε-caprolactam and polyether polyurethanes which meet the specifications for demanding applications and at the same time the process is particularly environmentally friendly due to its reduced product carbon footprint and the use of waste as starting materials.

[0301] In a preferred embodiment, the ε-caprolactam obtained by the process of the present invention meets one or more of the following specifications, wherein the parameters and measurement methods are defined in the Examples section below:

[0302] PAN: Maximum 5

[0303] E290: 0.05 max.

[0304] VB: Maximum value 0.5mmol / kg

[0305] Alkalinity: Maximum value 0.1mmol / kg.

[0306] In a preferred embodiment, the polyether polyurethane obtained by the process of the present invention can be reused in the production of textiles as such or in combination with virgin polyether polyurethane.

[0307] The ε-caprolactam produced by the process of the invention is also particularly economical and environmentally friendly. This is evident because the carbon footprint of the ε-caprolactam produced by the process of the invention is much lower than that of conventionally produced ε-caprolactam (e.g. by Beckmann rearrangement of cyclohexanone oxime).

[0308] The polyether polyurethanes produced by the process of the present invention are also particularly economical and environmentally friendly. Again, this is evident because the carbon footprint of the polyether polyurethanes produced by the process of the present invention is much lower than that of conventionally produced polyether polyurethanes (e.g., by the reaction of polyether polyols and diisocyanate monomers).

[0309] The environmental impact of a product is usually expressed as the carbon footprint of the product or simply as 'carbon footprint'. The carbon footprint of a product is defined as the total emissions caused by the formation of the product, which is expressed in tons of carbon dioxide equivalent / ton of product. The carbon footprint of a product depends in particular on raw materials, auxiliary materials, energy consumption, energy, production methods and process efficiency. The quantification of the carbon footprint of a product can be carried out as described in, for example, the European standard EN ISO 14040:2006 ("Environmental management-Life cycle assessment-Principles and framework").

[0310] The product carbon footprint calculation can be performed internally or by an external (preferably) certified organization. These organizations verify and certify product carbon footprint calculations based on, for example, the LCA standard ISO 14040.

[0311] J. Hong and X. Xu (“Environmental impact assessment of caprolactam production–a case study in China”); J. of Cleaner Production 27 (2012) 103-108; DOI: 10.1016 / j.jclepro.2011.12.037) reported that the global warming potential of “virgin” ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime in the context of coal-based electricity and steam generation is 7.5 tonnes of CO2 equivalent / tonne of ε-caprolactam (which is equivalent to 7.5 kg CO2 equivalent / kg ε-caprolactam). In the case involving natural gas based electricity and steam generation, the potential impact of virgin ε-caprolactam on global warming for the ε-caprolactam production process would drop to 6.4 tonnes CO2 equivalent / tonne ε-caprolactam (which is equivalent to 6.4 kg CO2 equivalent / kg ε-caprolactam).

[0312] The product carbon footprint of the ε-caprolactam obtained according to the process of the present invention is much lower than the product carbon footprint of de novo synthesized or "virgin" ε-caprolactam. The product carbon footprint of the ε-caprolactam obtained according to the process of the present invention is less than 5.0 kg, preferably less than 4.0 kg, and most preferably less than 3.0 kg CO2 equivalent / kg ε-caprolactam.

[0313] NM van der Velden, MK Patel and JG (Table 7 below: “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) reports that the global warming potential of virgin elastane production is equivalent to 4.836 kg CO2 equivalent / kg elastane.

[0314] The product carbon footprint of the polyether polyurethane obtained according to the method of the present invention is much lower than the product carbon footprint of de novo synthesized or "virgin" polyether polyurethane. The product carbon footprint of the polyether polyurethane obtained according to the method of the present invention is less than 3.0 kg, preferably less than 2.0 kg CO2, and most preferably less than 1.0 kg CO2 equivalent / kg polyether polyurethane. BRIEF DESCRIPTION OF THE DRAWINGS

[0315] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, which depict certain embodiments of the present invention. However, the present invention is defined in the claims and as generally described herein. It should not be limited to the embodiments shown in the following drawings for illustrative purposes.

[0316] Figure 1 Schematic diagram of a process according to the present invention for recovering purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane, the process comprising treatment steps carried out in an optional pretreatment section [A], a separation section [B], a depolymerization section [C], a recovery section [D] and an optional purification section [E].

[0317] Figure 2An example of a separation section [B] is shown, in which a material comprising polyamide 6 and polyether polyurethane is separated to obtain a polyamide 6-rich stream and a polyether polyurethane-rich stream. The example consists of a dissolution section [α], a washing section [β], a precipitation section [γ] and a solvent distillation section [δ].

[0318] Detailed description with drawings

[0319] The method of the present invention is Figure 1 The method is implemented in the following plant sections:

[0320] In an optional pretreatment section [A], the material comprising polyamide 6 and polyether polyurethane [1] is cleaned and / or shredded by mechanical size reduction to obtain discharged cleaned and / or shredded fragments [2] consisting of the material comprising polyamide 6 and polyether polyurethane. Optionally, in the pretreatment section [A], in which the material comprising polyamide 6 and polyether polyurethane [1] is cleaned by removing foreign materials and / or by washing with a washing solvent [3], discharged foreign materials and contaminated washing solvent [4] are obtained. The removal of foreign materials can be carried out before and / or after washing with the washing solvent [3]. The cleaning can be carried out before and / or after shredding the material comprising polyamide 6 and polyether polyurethane [1]. Optionally, the material comprising polyamide 6 and polyether polyurethane [2] is dried after the cleaning step and before being charged to the separation section [B];

[0321] In separation section [B], the optionally cleaned and / or shredded fragments [2] consisting of a material comprising polyamide 6 and polyether polyurethane are separated into a polyamide 6-rich stream

[13] and a polyether polyurethane-rich stream. Preferably, the separation of the material comprising polyamide 6 and polyether polyurethane [2] into a polyamide 6-rich stream

[13] and a polyether polyurethane-rich stream is achieved by selective dissolution of the polyether polyurethane in an organic solvent. Optionally, fresh organic solvent is fed to separation section [B] via line [6]. Optionally, a second solvent is fed to separation section [B] via line [9], which second solvent is used to precipitate the polyether polyurethane from the polyether polyurethane-rich stream, so that a precipitated polyether polyurethane

[10] is obtained. Optionally, the precipitated polyether polyurethane

[10] is dried before being discharged from separation section [B]. Optionally, a third solvent is fed to the separation section [B] via the pipeline

[12] , said third solvent being used for washing the polyamide 6-rich stream

[13] before being discharged from the separation section [B]. Optionally, the polyamide 6-rich stream

[13] is dried before being discharged from the separation section [B]. Optionally, the separation section [B] comprises a section (preferably a solvent distillation section) for separating the second solvent and the third solvent

[15] and the residue

[16] discharged from the separation section [B];

[0322] In the depolymerization section [C], the polyamide 6-rich stream

[13] is depolymerized so that a stream

[19] comprising ε-caprolactam is obtained which is discharged from the depolymerization section [C]. In addition, residual material

[20] is discharged. Superheated steam

[17] and optionally a catalyst

[18] are fed to the depolymerization section [C]. Optionally, the polyamide 6-rich stream

[13] is melted before being fed to the depolymerization section [C];

[0323] In the recovery section [D], crude ε-caprolactam

[21] is recovered from the stream

[19] comprising ε-caprolactam discharged from the depolymerization section [C]. In addition, an aqueous phase

[22] is discharged from the recovery section [C];

[0324] In the optional purification section [E], the crude ε-caprolactam

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

[24] . Water and impurities

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

[0325] Figure 2 An example of a separation section [B] (area surrounded by a dashed line) is depicted, in which a material comprising polyamide 6 and polyether polyurethane is separated to obtain a polyamide 6-rich stream and a polyether polyurethane-rich stream. This example includes the following sections:

[0326] In the dissolution section [α], the polyether polyurethane is dissolved in an organic solvent from the optionally cleaned and / or shredded chips [2] consisting of a material comprising polyamide 6 and polyether polyurethane to obtain a polyether polyurethane-rich stream [7] comprising organic solvent and dissolved polyether polyurethane and a stream [8] comprising undissolved polyamide 6. These are discharged from the dissolution section [α]. The organic solvent is the separated organic solvent [5], to which fresh organic solvent [6] is optionally added;

[0327] In the precipitation section [γ], the second solvent [9] and the polyether polyurethane-rich stream [7] comprising the organic solvent and the dissolved polyether polyurethane are mixed so that the polyether polyurethane is precipitated from the mixture comprising the organic solvent and the second solvent. The precipitated polyether polyurethane is recovered from the mixture comprising the organic solvent and the second solvent (e.g., by filtration or centrifugation). The recovered precipitated polyether polyurethane

[10] is discharged from the precipitation section [γ]. Optionally, the precipitated polyether polyurethane

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

[11] is discharged from the precipitation section [γ] and the mixture is fed to the solvent distillation section [δ];

[0328] In the washing section [β], the stream [8] comprising undissolved polyamide 6 is washed with a third solvent

[12] to obtain a stream

[13] rich in polyamide 6 and a mixture

[14] comprising an organic solvent and a third solvent. The stream

[13] rich in polyamide 6 is discharged from the washing section [β]. Optionally, the stream

[13] rich in polyamide 6 is dried after the washing step and before being discharged from the washing section [β]. The mixture

[14] comprising an organic solvent and a third solvent is discharged from the washing section [β] and optionally charged to a solvent distillation section [δ];

[0329] In the solvent distillation section [δ], the mixture

[11] comprising the organic solvent and the second solvent, and optionally the mixture

[14] comprising the organic solvent and the third solvent, from which the precipitated polyether polyurethane has been recovered, is distilled to obtain a separated organic solvent [5], a second solvent and a third solvent

[15] , and a residue

[16] . Optionally, both the second solvent and the third solvent are water, in which case the separated organic solvent [5] is a dried organic solvent. The separated organic solvent [5] is charged to the dissolution section [α].

[0330] Examples

[0331] The following examples are used to explain the present invention in more detail, particularly with respect to certain forms of the present invention. However, the examples are not intended to limit the present disclosure.

[0332] ε-Caprolactam, which can be used in all major polymerization applications without dilution with purer ε-caprolactam, meets all of the following specifications:

[0333]

[0334] The parameters and measurement methods are defined as follows:

[0335] PAN: ISO DIS 8660 - Plastics - Determination of the permanganate index of caprolactam - Spectroscopic method, revision of the first edition of ISO 8660; 1988,

[0336] E290: ISO 7059 - Caprolactam for industrial use - Determination of absorbance at a wavelength of 290 nm.

[0337] Volatile base (VB) ISO 8661 - Caprolactam for industrial use - Determination of volatile base content - Titration after distillation.

[0338] Basicity of ε-caprolactam product: measured by using Tashiro indicator at 0.1wt. / v at 25°C 乙醇 % Methylene blue: 0.1wt. / v 乙醇The basicity is determined by titrating the flask containing water and indicator in a 1:2 ratio with % methyl red, with a gray end point. The flask containing water and indicator is first titrated to gray, then X grams of an aqueous solution of ε-caprolactam containing Y wt.% ε-caprolactam (as determined by refractive index) is added and the solution is titrated back to gray using 0.01N H2SO4 solution. The basicity is then given by:

[0339] Alkalinity (mmol / kg ε-caprolactam) = v*t*1000 / (X*Y)

[0340] in:

[0341] v = volume of H2SO4 solution added (ml)

[0342] t = normal concentration of H2SO4 solution (= 0.01N)

[0343] X = sample weight (g)

[0344] Y = concentration of ε-caprolactam (wt.%)

[0345] Example 1

[0346] Pretreatment and separation of polyamide 6 and polyether polyurethane

[0347] The used polyamide 6 waste fabric containing polyether polyurethane had a polyether polyurethane content of 20.2 + / - 0.4 wt.% (as measured by differential scanning calorimetry (DSC) ISO 11357-3_-130°C to 300°C_10°C / min_dt 1.00 sec). The color of the used nylon 6 waste fabric containing polyether polyurethane (spandex) was purple.

[0348] The polyamide 6 waste fabric containing polyether polyurethane is cut into small pieces, each small piece is 5 to 20 cm 2 150.4 g of this cut material was dissolved in 800 g of DMAc at 70° C. for 1 hour under stirring. The resulting solution was vacuum filtered through a heated double-walled Buchner funnel at 70° C. to obtain Filtrate 1. The remaining undissolved material was further treated three times with 400 g of DMAc each time and stirred at 70° C. for 1 hour to obtain Filtrates 2 to 4.

[0349] At a temperature of 65°C, the undissolved material on the Buchner funnel was washed with 325 g of water to obtain filtrate 5, and then the undissolved material was dried and then weighed. The five filtrates were combined to obtain a precipitate and a clear solution. The precipitate was filtered out and washed with 200 g of water, and then dried and then weighed. The weight fraction of the dried precipitate was 20 wt.% of the total weight of the dried precipitate and the dried undissolved material, and the total weight was almost equal to the weight of the starting material.

[0350] Analysis by differential scanning calorimetry (DSC) revealed that the polyether polyurethane content of the dry precipitate was approximately 100 wt.% (no polyamide 6 detected). Based on DSC analysis, the nylon 6 content of the dried, undissolved material was approximately 100 wt.% (no polyether polyurethane detected).

[0351] The recycled polyether polyurethane precipitate can be reused in the production of textiles as such or in combination with virgin polyether polyurethane.

[0352] This example shows that polyether polyurethane and polyamide 6 can be separated from polyamide 6 waste fabric containing polyether polyurethane by selective extraction, precipitation of dissolved polyether polyurethane and washing of undissolved polyamide 6. After drying, the recovered precipitate consists of almost pure polyether polyurethane, and the polyether polyurethane content of the undissolved material is much lower than that of the polyamide 6 waste fabric containing polyether polyurethane used as the starting material.

[0353] Example 2

[0354] Pretreatment and separation of polyamide 6 and polyether polyurethane

[0355] The used polyamide 6 waste fabric containing polyether polyurethane had a polyether polyurethane content of 8.15 + / - 0.05 wt.% (as measured by differential scanning calorimetry (DSC) ISO 11357-3_-130°C to 300°C_10°C / min_dt 1.00 sec). The used nylon 6 waste fabric containing polyether polyurethane (spandex) had a salmon pink color.

[0356] The procedure of Example 1 was followed, except that now 254.1 g of polyamide 6 waste fabric containing polyether polyurethane was dissolved in 1200 g of DMAc and 475 g of DMAc and 350 g of water were used to wash the remaining undissolved material on the Buchner funnel after filtration. The precipitate was filtered out and washed with 200 g of water and then dried. The undissolved material on the Buchner funnel was washed with 1000 g of water, then also dried, and then weighed. The weight fraction of the dried precipitate was 8 wt.% of the total weight of the dried precipitate and the dried undissolved material, which was almost equal to the weight of the starting material.

[0357] Analysis by differential scanning calorimetry (DSC) revealed that the polyether polyurethane content of the dry precipitate was about 100 wt.% (no polyamide 6 was detected). The polyamide 6 content and polyether polyurethane content of the dried undissolved material were about 99 wt.% and less than 1 wt.%, respectively (DSC analysis).

[0358] The recycled polyether polyurethane precipitate can be reused in the production of textiles as such or in combination with virgin polyether polyurethane.

[0359] This example shows that polyether polyurethane and polyamide 6 can be separated from polyamide 6 waste fabric containing polyether polyurethane by selective extraction, precipitation of dissolved polyether polyurethane and washing of undissolved polyamide 6. After drying, the recovered precipitate consists of almost pure polyether polyurethane, and the polyether polyurethane content of the undissolved material is much lower than that of the starting material polyamide 6 waste fabric containing polyether polyurethane.

[0360] Example 3

[0361] Depolymerization of polyamide 6 and recovery of ε-caprolactam

[0362] Before the dried undissolved material obtained in Example 1 was charged into the depolymerization reactor, the material was first densified. The dried undissolved material was melted at 237° C. under nitrogen and forced through a perforated metal plate. The obtained 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.

[0363] 48 grams of these pellets containing polyamide 6 and 14 grams of 20 wt.% phosphoric acid were charged into a Premex autoclave. First, the reactor contents were heated under nitrogen, and then superheated steam was continuously injected at a rate of 4 grams per minute during a reaction period of 120 minutes. The temperature and pressure in the reactor were maintained at 260° C. and 0.11 MPa, respectively. During the reaction, a vapor stream was continuously discharged from the reactor and cooled to about 20° C., thereby obtaining a condensate including ε-caprolactam and water.

[0364] The condensate consisting of 41 g of ε-caprolactam (most of the remainder is water) is concentrated by evaporation in a rotary evaporator (rotary evaporator) operated under vacuum (9.5 kPa; water bath temperature is about 65° C.) to an ε-caprolactam concentration of 51.5 wt.%. (This mixture is crude ε-caprolactam, which is the mixture to be purified.)

[0365] The specifications of crude ε-caprolactam are:

[0366] PAN: 389

[0367] E290: 3.42

[0368] This example shows that crude ε-caprolactam can be obtained by depolymerization of polyamide 6 obtained by extraction separation from polyamide 6 waste fabric containing polyether polyurethane.

[0369] Example 4

[0370] Depolymerization of polyamide 6 and recovery of ε-caprolactam

[0371] The procedure of Example 3 was followed, except that now 48 grams of pellets made from the dried undissolved material obtained in Example 2 and 14 grams of 20 wt. % phosphoric acid were charged to the Premex autoclave.

[0372] The condensate consisting of 33 g of ε-caprolactam (most of the remainder is water) is concentrated by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature is about 65° C.) to an ε-caprolactam concentration of 64.6 wt.%. (This mixture is crude ε-caprolactam, which is the mixture to be purified.)

[0373] The specifications of crude ε-caprolactam are:

[0374] PAN: 214

[0375] E290: 2.68

[0376] This example shows that crude ε-caprolactam can be obtained by depolymerization of polyamide 6 obtained by extraction separation from polyamide 6 waste fabric containing polyether polyurethane.

[0377] Comparative Experiment 1

[0378] Depolymerization of polyether polyurethane

[0379] The procedure of Example 3 was followed except that now 37.6 grams of polyether polyurethane fibers (the same material contained in the waste fabric used in Examples 1 and 2) and 14 grams of 20 wt. % phosphoric acid were charged to the Premex autoclave.

[0380] 10 minutes after the start of the injection of superheated steam, the experiment had to be stopped because the line for discharging the vapor stream comprising ε-caprolactam and water was clogged with insoluble material.

[0381] From this comparative experiment it can be concluded that depolymerization of polyamide 6 waste fabric containing polyether polyurethane causes operational problems without removing the polyether polyurethane prior to depolymerization.

[0382] Comparative Experiment 2

[0383] Depolymerization of polyamide 6 waste fabrics containing polyether polyurethane and recovery of ε-caprolactam

[0384] The procedure of Example 3 was followed, except that now 48 grams of pellets made from polyamide 6 waste fabric containing polyether polyurethane with a polyether polyurethane content of 20.2 + / - 0.4 wt.% (the same raw material used in Example 1) and 14 grams of 20 wt.% phosphoric acid were charged into the Premex autoclave.

[0385] The condensate consisting of 26 g of ε-caprolactam (most of the remainder is water) is concentrated by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature is about 65° C.) to an ε-caprolactam concentration of 47.0 wt.%. (This mixture is crude ε-caprolactam, which is the mixture to be purified.)

[0386] The specifications of crude ε-caprolactam are:

[0387] PAN: 352

[0388] E290: 3.60

[0389] Observations:

[0390] ●The condensate obtained before concentration does contain an unknown precipitate

[0391] ●After the depolymerization experiment, it was found that the inner wall of the Premex autoclave was fouled

[0392] From this comparative experiment, it can be concluded that depolymerization of polyamide 6 waste fabric containing polyether polyurethane without removing the polyether polyurethane before depolymerization causes operational problems. Another observation is that the ε-caprolactam yield in this comparative experiment is much lower than the yield in Example 3 (pretreated polyamide 6 waste fabric containing polyether polyurethane as feed), where the ε-caprolactam yield is defined as the ratio of the weight of ε-caprolactam in the condensate to the weight of polyamide 6 in the feed charged to the Premex autoclave.

[0393] Comparative Experiment 3

[0394] Purification by distillation

[0395] 75 mmol of aqueous sodium hydroxide solution / kg of ε-caprolactam was then added to a portion of the crude ε-caprolactam obtained in Comparative Experiment 2. The mixture was then distilled by reducing the pressure in steps. ε-caprolactam was distilled at 300 Pa.

[0396] The specifications of distilled ε-caprolactam are:

[0397]

[0398] This comparative experiment shows that the quality of the ε-caprolactam obtained by depolymerization of polyamide 6 waste fabric containing polyether polyurethane (without pretreatment in a separation section in which the material comprising polyamide 6 and polyether polyurethane is separated into a polyamide 6-rich stream and a polyether polyurethane-rich stream) and subsequent concentration and purification by distillation is very poor, since it does not meet any of the specifications required for major polymerization applications.

[0399] Comparative Experiment 4

[0400] Purification by distillation

[0401] 75 mmol of aqueous sodium hydroxide solution / kg of ε-caprolactam were then added to a portion of the crude ε-caprolactam obtained in Example 3. The mixture was then distilled by reducing the pressure in steps. ε-caprolactam was distilled at 300 Pa.

[0402] The specifications of distilled ε-caprolactam are:

[0403]

[0404] From this comparative experiment it can be concluded that purification of crude ε-caprolactam by distillation alone is not sufficient to meet the specifications required for major polymerization applications.

[0405] Comparative Experiment 5

[0406] Purification by permanganate treatment and distillation

[0407] The crude ε-caprolactam obtained in Example 3 was treated with 0.2 wt. % KMnO4, relative to ε-caprolactam, at 50° C. for 2 hours. The solid formed was then removed from the oxidized reaction product by filtration.

[0408] The ε-caprolactam in the oxidized reaction product was further purified by distillation as described in Comparative Example 4 after adding 75 mmol of aqueous sodium hydroxide solution / kg of ε-caprolactam.

[0409] The specifications of distilled ε-caprolactam are:

[0410]

[0411] From this comparative experiment it can be concluded that purification of crude ε-caprolactam by permanganate treatment followed by distillation is not sufficient to meet all specifications required for the main polymerization applications as quantified above.

[0412] Example 5

[0413] Purification by extraction, oxidation and distillation

[0414] A portion of the crude ε-caprolactam obtained in Example 4 was further concentrated by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature was about 65° C.) to an ε-caprolactam concentration of 37.9 wt.%. (This mixture is the concentrated crude ε-caprolactam, which is the mixture to be purified.)

[0415] The concentrated crude ε-caprolactam was extracted batchwise 11 times with a solvent mixture of 4-methyl-2-pentanol (50 wt.%) / cyclohexane (50 wt.%) at a temperature of about 25° C. The total amount of extraction solvent used was 3.22 g of 4-methyl-2-pentanol / cyclohexane per g of concentrated crude ε-caprolactam.

[0416] The concentrated extract was batch-extracted 7 times with water at a temperature of about 25° C. The total amount of water used was 5.67 g of water per g of ε-caprolactam recovered.

[0417] The combined aqueous extracts were treated with 0.1 wt. % KMnO4 relative to ε-caprolactam at 50° C. for 2 hours. The solid formed was then removed from the oxidized reaction product by filtration.

[0418] The resulting mixture was concentrated by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature of about 65° C.) to an ε-caprolactam concentration of 49.7 wt.%. The ε-caprolactam in the resulting concentrated aqueous solution was further purified by distillation as described in Comparative Example 4 after adding 75 mmol aqueous sodium hydroxide solution / kg ε-caprolactam.

[0419] The specifications of distilled ε-caprolactam are:

[0420]

[0421] From this experiment it can be concluded that purified ε-caprolactam meeting the specifications required for major polymeric applications can be obtained from depolymerized polyamide 6 waste fabric containing polyether polyurethane from which the polyether polyurethane has been removed by extraction and has been purified by extraction, oxidation and distillation.

[0422] The examples show that by the claimed combination of steps, primary caprolactam can be produced even in the case of colored waste fabrics. The waste fabrics used are so-called 'post-consumer waste', i.e. the waste fabrics used in the experiments have been used by consumers and then disposed of. Generally speaking, 'post-consumer waste' is dirtier (contains more impurities / dirt) than post-industrial waste and is therefore a particularly challenging starting material. The claimed method, by its special combination of steps, works well even in the case of such challenging starting materials.

[0423] Experiment 6

[0424] Calculation of the carbon footprint of purified ε-caprolactam and polyether polyurethane

[0425] A continuous process according to the invention for producing purified ε-caprolactam and polyether polyurethane from polyamide 6 waste fabric containing polyether polyurethane was simulated. The process comprises:

[0426] - cutting polyamide 6 waste fabric containing polyether polyurethane into small pieces;

[0427] - Selective extraction of polyether polyurethane with DMAc;

[0428] - separation of undissolved polyamide 6 and a polyether polyurethane-rich stream by centrifugation;

[0429] - washing the undissolved polyamide 6 with water;

[0430] - separation of the washed undissolved polyamide 6 and the aqueous extract by centrifugation;

[0431] - precipitating the polyether polyurethane from the polyether polyurethane-rich stream by adding the aqueous extract obtained above;

[0432] - separating the precipitated polyether polyurethane from the DMAc-water mixture by filtration;

[0433] - recovering DMAc and water from the DMAc-water mixture obtained above;

[0434] - Dry filtered polyether polyurethane;

[0435] - drying the washed undissolved polyamide 6;

[0436] - melting and pelletizing the washed undissolved polyamide 6;

[0437] - Depolymerization of polyamide 6 under the action of H3PO4 and superheated steam;

[0438] - recovery of crude ε-caprolactam (80 wt.% ε-caprolactam) by partial condensation of the vapors exiting the depolymerization reactor;

[0439] - extracting the concentrated crude ε-caprolactam with toluene in countercurrent;

[0440] - washing the organic extract with a dilute caustic solution;

[0441] - extracting the washed organic extract with water in countercurrent;

[0442] - Treat with KMnO4 and filter the solid formed;

[0443] - Concentrating the aqueous extract by evaporation;

[0444] - Adding caustic soda;

[0445] - Recovery of pure ε-caprolactam by vacuum distillation.

[0446] The carbon footprint of purified ε-caprolactam and polyether polyurethane was calculated based on the consumption data of raw materials and the utility of the above method was based on data derived from ecoinvent version 3.7.1. The environmental impact distribution between the purified ε-caprolactam and polyether polyurethane products in the pretreatment section and the separation section was based on the weight ratio of these products.

[0447] The results revealed that the product carbon footprint of purified ε-caprolactam obtained from polyamide 6 waste fabric containing polyether polyurethane was less than 3.0 ton CO2 equivalent / ton ε-caprolactam and less than 1.0 ton CO2 equivalent / ton polyether polyurethane.

[0448] While the invention has been described with reference to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications may be made therein, including (semi)continuous operation and scaling up to commercial scale, without departing from the spirit and scope of the invention.

Claims

1. A method for recovering ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane in a plant, wherein the plant comprises - separation section [B], - depolymerization section [C], - Recovery Sector [D], and - purification segment [E], And wherein the method comprises the following steps: a) charging a material comprising polyamide 6 and polyether polyurethane into the separation section [B]; b) in the separation section [B], by, at a temperature below 100° C., selectively dissolving the polyether polyurethane in an organic solvent, preferably at a temperature in the range of 0°C to 100°C, more preferably at a temperature in the range of 10°C to 90°C, even more preferably at a temperature in the range of 10°C to 80°C and most preferably at a temperature in the range of 20°C to 75°C, separating the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane; c.1) discharging the polyamide 6-rich stream from the separation section [B] and feeding the polyamide 6-rich stream to a depolymerization section [C], wherein the polyamide 6 content of the polyamide 6-rich stream is at least 85% by weight, based on dry weight; c.2) depolymerizing the polyamide 6 in the polyamide 6-rich stream at a temperature in the range of 180° C. to 400° C. in the depolymerization section [C], such that a stream comprising ε-caprolactam is obtained, and discharging the obtained stream comprising ε-caprolactam from the depolymerization section [C]; c.3) recovering crude ε-caprolactam from the stream comprising ε-caprolactam in the recovery section [D]; c.4) purifying the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification comprises the following steps: (i) extracting the crude ε-caprolactam with an organic solvent so as to obtain an aqueous phase and an organic phase, and wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities; (ii) switching the solvent by at least partially replacing the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam and impurities having a boiling point lower or higher than ε-caprolactam; and (iii) removing impurities having a boiling point lower or higher than that of ε-caprolactam by distillation to obtain purified ε-caprolactam; d.1) recovering polyether polyurethane from the polyether polyurethane-rich stream in the separation section [B]; and d.2) discharging the recovered polyether polyurethane from the separation section [B], wherein the polyether polyurethane content of the discharged stream is at least 85% by weight, based on the dry weight.

2. The method according to claim 1, wherein the separation section [B] comprises: - dissolution segment [α], - washing section [β], - precipitation section [γ], and - solvent distillation section [δ], and wherein the method comprises the following steps in the separation section [B]: b.1) charging an organic solvent and a material comprising polyamide 6 and polyether polyurethane into the dissolving section [α]; b.2) dissolving the polyether polyurethane from the material comprising polyamide 6 and polyether polyurethane in an organic solvent in the dissolving section [α], so that a polyether polyurethane-rich stream comprising organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6 are obtained, and the obtained stream is discharged from the dissolving section [α]; b.3) feeding a second solvent and the polyether polyurethane-rich stream comprising the organic solvent and the dissolved polyether polyurethane into the precipitation section [γ], so that the polyether polyurethane is precipitated from the mixture comprising the organic solvent and the second solvent; b.4) recovering the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharging the precipitated polyether polyurethane from the precipitation section [γ]; b.5) ​​discharging the mixture comprising the organic solvent and the second solvent, from which the precipitated polyether polyurethane has been recovered in step b.4), from the precipitation section [γ] and charging the mixture to the solvent distillation section [δ]; b.6) charging a third solvent and the stream comprising undissolved polyamide 6 to the washing section [β]; b.7) washing the stream comprising undissolved polyamide 6 with the third solvent in the washing section [β], so that a stream rich in polyamide 6 and a mixture comprising the organic solvent and the third solvent is obtained; b.8) discharging the polyamide 6-rich stream from the washing section [β]; b.9) discharging the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β] and charging part or all of the mixture to the solvent distillation section [δ]; b.10) separating the organic solvent from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and discharging the second solvent, the third solvent and the separated organic solvent from the solvent distillation section [δ].

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 to the dissolution section [α] in step b.1).

4. The method according to any one of claims 1 to 3, wherein (i) drying the polyamide 6-rich stream discharged from the separation section [B] before feeding it to the depolymerization section [C]; and / or (ii) the depolymerization in step c.2) is carried out in the presence of water, so that the stream comprising ε-caprolactam is a vapor stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50, specifically 1:2 to 1:15, 1:2 to 1:10 or 1:3 to 1:8; and / or (iii) charging the depolymerization section [C] with superheated steam having a temperature ranging from 220°C to 575°C, specifically from 275°C to 500°C.

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

6. The method according to any one of claims 1 to 5, wherein the plant further comprises - Pre-processing section [A], And wherein before step a), said material comprising polyamide 6 and polyether polyurethane is subjected to a pretreatment in said pretreatment section [A], in particular to cleaning in a cleaning section and / or to mechanical size reduction in a mechanical size reduction section.

7. The method according to any one of claims 1 to 6, wherein the organic solvent is dimethylacetamide.

8. The method according to any one of claims 1 to 7, wherein the second solvent and the third solvent are the same solvent, more preferably water.

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

10. The process according to any one of claims 1 to 9, wherein the second solvent and the third solvent discharged from the solvent distillation section [δ] are reused in the precipitation section [γ] and / or the washing section [β], optionally after separation from one another.

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

Images