Process for recovering epsilon-caprolactam and polyether polyurethane from a material 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 present invention further provides an apparatus suitable for carrying out the above process as well as the polyether polyurethane and epsilon-caprolactam obtained by the process of the present invention.

Description

Technical Field

[0001] This invention relates to a method for recovering ε-caprolactam from a material comprising polyamide 6 and polyether polyurethane. More specifically, this invention relates to a method for recovering purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane on an industrial scale. Background Technology

[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 the liquid-phase Beckmann rearrangement of cyclohexanone oxime in the presence of fuming sulfuric acid, i.e., a mixture of sulfuric acid and SO3, or by the gas-phase Beckmann rearrangement of cyclohexanone oxime in the presence of a solid catalyst. This type of ε-caprolactam is typically referred to as "primary ε-caprolactam." The cyclohexanone oxime required to form (primary) ε-caprolactam can be prepared from cyclohexanone, which is primarily produced from benzene (see, for example, the 'Caprolactam' section of Ullmann's Encyclopedia of Industrial Chemistry (2018); https: / / doi.org / 10.1002 / 14356007.a05_031.pub3). This benzene is largely derived from non-renewable fossil resources such as petroleum and coal.

[0003] Initially, pure PA6 was used to produce textiles, including stockings. Later, to improve fabric properties, a wide variety of blended fabrics were developed and produced. Polyether polyurethane, in particular, has been widely used in various garments. The benefits of polyether polyurethane include its significant strength and elasticity, its ability to recover its original shape after stretching, and its faster drying speed than ordinary fabrics. The improved elasticity of fabrics is typically achieved by using blended yarns made from PA6 and polyurethane fibers. Examples of clothing containing PA6 and polyether polyurethane blends include stretch stockings, underwear, and sportswear. The breathability and water resistance of fabrics can be achieved by treating (e.g., coating) the surface of PA6 fibers with polyurethane resin. These surface-treated yarns are commonly used to manufacture, for example, raincoats, clothing suitable for cold weather, and skiwear. Polyether polyurethane used in garment production is also commonly referred to as Spandex, Lycra, or elastic fiber.

[0004] Recycling PA6 enables the conservation of fossil resources and adds value to the circular economy. Mechanical recycling of waste PA6 is the process of transferring waste PA6 to secondary raw materials or (preferably) to products in which the chemical structure of the material undergoes only minimal change. PA6 conversion and PA6 depolymerization are two forms of PA6 chemical recycling. In conversion recycling, PA6 is broken down into petroleum or natural gas-like feedstocks that can replace freshly extracted fossil feedstocks. The resulting products can be used to produce chemicals containing the monomer ε-caprolactam. In depolymerization recycling, PA6 is broken down into its monomer building blocks, ε-caprolactam. The PA6 depolymerization recycling process was developed specifically for clean and relatively pure waste PA6 products. Mechanical recycling always results in downgraded recycling, therefore the properties of products derived from mechanically recycled PA6 are always inferior to those derived 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 ε-caprolactam, which can then be converted into higher-grade PA6. Unfortunately, the 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 to that of virgin ε-caprolactam.

[0005] Compared to the depolymerization of relatively pure PA6, the depolymerization of blends of PA6 and polyether polyurethane has several disadvantages. These disadvantages include, in particular, the clogging of pipes and other equipment parts by the polyether polyurethane and its decomposition products; the poor quality of the generated ε-caprolactam due to the presence of polyether polyurethane decomposition products, which (severely) reduces ε-caprolactam recovery; poisoning of the depolymerization catalyst; and therefore, higher consumption of the depolymerization catalyst. Another disadvantage of adding blends of PA6 and polyether polyurethane to the depolymerization reactor is the degradation of the polyether polyurethane. Therefore, it is impossible to achieve the recycling of polyether polyurethane, which is more valuable than PA6.

[0006] The virgin elastic fibers are produced by dry or wet spinning processes, which begin with an elastic fiber spinning solution composed of a suitable solvent containing polyurethane (such as dimethylacetamide or dimethylformamide).

[0007] Because the high viscosity prevented further processing, directly dissolving normal (pure) elastic fiber waste fibers in the spinning solvent used for example to recycle them from yarn production proved unsuccessful.

[0008] US6830715B1 describes a method for producing elastic fiber yarn from a spinning solution using recycled elastic fiber material, the method overcoming the aforementioned viscosity problem by adding a secondary aliphatic amine to a mixture of (cut) elastic fibers and a spinning solvent. The dissolution of the (cut) elastic fibers is achieved at a temperature between 60°C and 150°C.

[0009] In the past, several attempts have been described to recycle blends of PA6 and polyether polyurethane.

[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 involves heating the PA6 product containing polyether polyurethane together with a solvent containing a cycloamide at a temperature of 80°C to the boiling point of the solvent. JP2011088943A indicates that the amount of the cycloamide compound in the solvent is preferably 50% by weight or more, more preferably 85% by weight or more. JP2011088943A further indicates that the solvent may contain components other than the cycloamide compound, such as water and organic solvents, and that water is particularly preferred from a workability point of view. The experiment described in the patent was conducted at 110°C over a period of 2 hours using an aqueous solution containing 50% and 85% by weight of N-methylpyrrolidone, 85% by weight of 2-pyrrolidone, or 85% by weight of 2-piperidine as solvents. The result of this treatment is that the polyether polyurethane partially decomposes and dissolves in the solvent. Therefore, the resulting 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, nor does it describe the fate of the solvent used.

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

[0012] Given the above, to date, there is no method for recovering purified ε-caprolactam and polyether polyurethane from materials including polyamide 6 and polyether polyurethane on an industrial scale. The reason for not realizing a recycling method for recovering purified ε-caprolactam and polyether polyurethane from materials including polyamide 6 and polyether polyurethane on an industrial scale is that all methods of introducing elastic fibers into the solution require the application of high solution temperatures, typically well above 100°C. However, at these temperatures, the elastic fibers begin to degrade, and only inferior material is obtained after recycling. Therefore, the fate of the recovered elastic fibers is often limited to incineration and landfill. Another disadvantage of the high-temperature dissolution process is the formation of degradation products of the applied solvent, which may hinder the recovery of polyamide 6 and polyether polyurethane, as well as the recycling of the applied solvent.

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

[0014] The raw material price of virgin polyether polyurethane is relatively high, often more than twice the price of virgin PA6. Therefore, recycling and reusing (reusing) waste polyether polyurethane and PA6 has significant economic incentives.

[0015] The problem with existing methods is that they consume a lot of energy to recover dissolved elastic fibers from the solution, since dissolved elastic fibers are usually obtained by removing the solvent through evaporation.

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

[0017] Finally, there is a need for methods that allow for the industrial-scale recovery of pure ε-caprolactam and polyether polyurethane from materials including polyamide 6 and polyether polyurethane in order to address the large quantities of materials including polyamide 6 and polyether polyurethane wasted each year. Summary of the Invention

[0018] The purpose of this invention is to satisfy one or more of the above-mentioned requirements and overcome the disadvantages associated with existing technical methods.

[0019] Specifically, 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 purified ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane.

[0020] Another object of the present invention is to provide a method for recovering purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane on an industrial scale.

[0021] The object of this invention is to provide a method for recovering purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane in an economical and efficient manner. Specifically, the object of this invention is to provide a method suitable for recovering purified ε-caprolactam and polyether polyurethane from materials comprising polyamide 6 and polyether polyurethane, without exceeding the production cost of virgin high-purity ε-caprolactam and polyether polyurethane.

[0022] Another object of the present invention is to provide a method for recovering and purifying both ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane, the method being characterized by a significantly lower carbon footprint than methods for producing virgin polyether polyurethane, for example, through the reaction of polyether polyols and diisocyanate monomers, and for producing ε-caprolactam through novel synthesis, for example, through the Beckmann rearrangement of cyclohexanone oxime.

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

[0024] One or more additional purposes may become apparent from the remainder of the instruction manual.

[0025] The method of claim 1 largely solves or at least alleviates all or at least some of the aforementioned objectives.

[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 factory, wherein the factory includes

[0027] -Separation section [B],

[0028] -De-aggregation section [C],

[0029] -Reclaim section [D], and

[0030] -Purification section [E],

[0031] And the method described therein includes the following steps:

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

[0033] b) In the separation section [B], the material comprising polyamide 6 and polyether polyurethane is separated 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 in the range of 0°C to 100°C, more preferably in the range of 10°C to 90°C, even more preferably in the range of 10°C to 80°C, and most preferably in the range of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane.

[0034] c.1) Discharge the polyamide 6-rich stream from the separation section [B] and load the polyamide 6-rich stream into the depolymerization section [C], wherein the polyamide 6 content in the polyamide 6-rich stream is at least 85% by weight based on dry weight;

[0035] c.2) In the depolymerization zone [C], the polyamide 6 in the polyamide 6-rich stream is depolymerized at a temperature ranging from 180°C to 400°C, so as to obtain a stream containing ε-caprolactam, and the obtained stream containing ε-caprolactam is discharged from the depolymerization zone [C].

[0036] c.3) Recover crude ε-caprolactam from the feed stream containing ε-caprolactam in the recovery section [D];

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

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

[0039] (ii) By replacing the organic solvent at least partially with water, an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam is obtained; and

[0040] (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation;

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

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

[0043] Surprisingly, combining the specific sequence of processing steps and conditions according to the invention—namely, the sequence of the separation, depolymerization, and recovery steps described above—allows for the recovery of ε-caprolactam from materials comprising polyamide 6 and polyether polyurethane in a high-yield, direct, and economically viable manner. The method of the invention is economically sound and advantageous from several perspectives. First, the method is applicable to a wide variety of materials comprising polyamide 6 and polyether polyurethane, which may differ, for example, in their overall composition (e.g., their polyamide 6 content). Second, the method allows for the efficient separation of polyamide 6 from the polyether polyurethane compound, thereby yielding higher polyamide 6. Third, the method is highly efficient, enabling the obtained polyamide 6 to be depolymerized into higher ε-caprolactam in high yield. Fourth, the method allows for the recovery of polyether polyurethane, which can be reused to replace polyether polyurethane produced by de novo synthesis, for example, through the reaction of polyether polyols and diisocyanate monomers. Fifth, the method allows for the industrial-scale recovery of ε-caprolactam from materials comprising polyamide 6 and polyether polyurethane to address the currently wasted large quantities of materials comprising polyamide 6 and polyether polyurethane. Finally, compared to ε-caprolactam produced via de novo synthesis (e.g., via the Beckmann rearrangement of cyclohexanone oxime), the method of the present invention allows for the production of ε-caprolactam with a significantly lower carbon footprint. The method of the present invention allows for the efficient processing of materials comprising polyamide 6 and polyether polyurethanes, and reduces the environmental burden of said materials. Specifically, the method of the present invention allows for the production of polyether polyurethanes with a carbon footprint of less than 1 kg CO2 / kg, a significant improvement compared to the 4.8 kg CO2 / kg associated with the production of “native” polyether polyurethanes obtained through chemical synthesis. Specifically, the method of the present invention allows for the production of purified ε-caprolactam with a carbon footprint of less than 3 kg CO2 / kg, a significant improvement compared to the 6.4 to 7.5 kg CO2 / kg ε-caprolactam associated with the production of “native” ε-caprolactam obtained from the Beckmann rearrangement of cyclohexanone oxime.

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

[0045] -Optionally, preprocessing section [A],

[0046] -Separation section [B],

[0047] -De-aggregation section [C],

[0048] -Reclaim section [D], and

[0049] -Optionally, purify segment [E], and

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

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

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

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

[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 fibers, yarns (i.e., strands of fibers spun together) or fiber-based fabrics, wherein the fibers are a blend of PA6 fibers and polyurethane fibers, or wherein the fibers are obtained by coating the surface of PA6 fibers with polyether polyurethane resin.

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

[0057] Materials including polyamide 6 and polyether polyurethane may include additional components. These components may be added, for example, during or after polymerization, fiber formation, to achieve desired property changes. These compounds include, for example, brightening agents, curing agents, antistatic lubricants, colorants, brightening agents, spin finish agents, surface smoothers, antioxidants, UV stabilizers, etc. The amount of these additional components depends on the application of the material including polyamide 6 and polyether polyurethane.

[0058] The weight ratio of polyamide 6 to polyether polyurethane in materials comprising 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, and most preferably from about 3:1 to about 20:1.

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

[0060] Polyether polyurethanes are produced by reacting isocyanates with polyether polyols. Polyether polyols are typically prepared by polymerizing cyclic ether compounds onto initiator compounds. The most commonly used cyclic ethers in polyether polyol production are ethylene oxide, propylene oxide, and 1,4-epoxybutane or tetrahydrofuran (used to produce poly(epoxybutane) polyols). Poly(epoxybutane) polyols are primarily used in applications requiring highly hydrophobic properties.

[0061] The use of the terms “about” or “approximately” in conjunction with numerical values ​​herein indicates that the values ​​may be affected by measurement errors, which typically cause the values ​​to vary by no more than ±5%. Numerical values ​​disclosed herein with the terms “about” or “approximately” are also intended to be disclosed without the term “approximately.” Furthermore, the numerical ranges described herein are intended to encompass and disclose every value within that range. All upper and lower endpoints of ranges of the same parameters disclosed herein may be combined with each other. All ranges of different parameters disclosed herein may be combined with each other. Specifically, ranges of different or the same “preference levels” are specifically compatible with each other. As used in this disclosure and claims, unless the context clearly specifies otherwise, the singular forms “an,” “a,” and “described” include the plural forms, particularly in the sense of “one or more.”

[0062] Possible preprocessing steps

[0063] Prior to step a) of the method of the present invention, the material comprising polyamide 6 and polyether polyurethane may be pretreated in a pretreatment section [A], specifically in a mechanical size reduction section and / or in a cleaning section. Preferably, the method of the present invention is carried out in a plant further comprising a pretreatment section [A], wherein prior to step a), the material comprising polyamide 6 and polyether polyurethane is pretreated in the pretreatment section [A], specifically in a cleaning section and / or 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 present invention configured to carry out the method of the present invention. Another advantage is that the size-reduced product comprising polyamide 6 and polyether polyurethane can be processed more easily.

[0064] The size and shape of (waste) products including polyamide 6 and polyether polyurethane depend largely on the exact application. The range of (waste) products including polyamide 6 and polyether polyurethane extends from individual fibers and yarns (optionally in spool form) to (waste) woven fabrics and (used) garments.

[0065] Discarded products, including polyamide 6 and polyether polyurethane, may be contaminated with various types of dirt (e.g., dirt, oil, paint, or grease).

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

[0067] Preferably, the material comprising polyamide 6 and polyether polyurethane is broken into fragments before separation in the separation section [B] in step b). This mechanical pretreatment of the material comprising polyamide 6 and polyether polyurethane, i.e., mechanical crushing or breaking, can be achieved, for example, by cutting, stamping, shredding, grinding, milling, and / or cutting. In a preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is loaded into the separation section [B] of step a) in the form of fragments. The advantage of using fragments composed of the material comprising polyamide 6 and polyether polyurethane is that the fragments can be more easily handled and / or cleaned by solvent washing. In a preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is loaded into the separation section [B] of step a) in the form of fragments, the fragments having an average length of 1 mm to 100 m along the longest axis of the fragment, preferably 3 mm to 1 m, and most preferably 1 cm to 50 cm. A technician can readily determine the average length along the longest axis of the fragments used by first obtaining a representative sample of the fragments, then measuring the length of the longest axis of each of these fragments (e.g., 50 fragments), and finally calculating the average of all these individual measurements. Preferred particle size can also be described by average particle weight. Preferably, the average particle weight of fragments composed of materials comprising polyamide 6 and polyether polyurethane is from 0.01 g to 25 kg, preferably from 0.05 g to 1 kg, and most preferably from 0.1 g to 100 g. Experiments have shown that fragments composed of materials comprising polyamide 6 and polyether polyurethane having the above-described dimensions or weight dimensions are particularly suitable for processing in the methods of the present invention and / or for cleaning by solvent washing.

[0068] Optionally, large metal fragments, rocks, and other interfering materials that cause severe wear on the equipment used for mechanical grinding or crushing are removed before the materials comprising polyamide 6 and polyether polyurethane are mechanically ground or crushed. Preferably, foreign materials comprising, but not limited to, polyethylene, polypropylene, and polyamide 6,6 are also removed before the materials comprising polyamide 6 and polyether polyurethane are mechanically ground or crushed. 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 grinding or crushing can be significantly reduced. Additionally, the polyamide 6 and polyether polyurethane content of the material obtained after mechanical grinding or crushing is higher than in the case where interfering materials have not been removed. The removal of polyamide 6,6 is particularly advantageous because it interferes with the depolymerization of 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 pulverized or crushed. Various separation methods can be applied for this purpose, including but not limited to density separation and magnetic separation. In density separation, materials of different densities are placed in a medium-density liquid, where the lower-density material floats and separates from the higher-density material that sinks. In practice, density separation is typically accomplished through a series of density separation stages. For example, in one stage, high-density materials such as rocks, sand, and metals (including iron and lead) are separated, while in another stage, low-density materials such as polyolefins, polypropylene, and polyethylene are separated. Magnetic separation is a method of separating components of a mixture by using a magnet to attract magnetic materials. Methods preferably used for magnetic separation decouple non-magnetic materials from magnetic materials. Removing foreign materials from pulverized or crushed product materials including polyamide 6 and polyether polyurethane is advantageous 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 recovered ε-caprolactam.

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

[0071] Washing is preferably performed under friction. Different types of industrial friction washing systems are available on the market, such as rotary plastic washers and (high-speed) friction washers.

[0072] Washing materials comprising polyamide 6 and polyether polyurethane, specifically mechanically crushing or breaking down materials comprising polyamide 6 and polyether polyurethane, is advantageous because it removes any (adhered) dirt and thus does not interfere with subsequent steps of the method of the present invention.

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

[0074] The location of the pretreatment zone for materials comprising polyamide 6 and polyether polyurethane and the location of the separation section [B] may be the same. However, preferably, one or more pretreatment steps in the pretreatment process are performed at a location different from the location of the separation section [B], for example, near a plant that collects waste materials comprising polyamide 6 and polyether polyurethane and / or at a location specifically for 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 polyamide 6 and polyether polyurethane material that has already been pretreated at a different location.

[0075] The location of the separation of the material comprising polyamide 6 and polyether polyurethane into a polyamide 6-rich stream and a polyether polyurethane-rich stream can be the same as the location of the depolymerization section [C]. However, preferably, the separation of the material comprising 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 that produces virgin polyether polyurethane and / or at a location specifically for processing and / or distilling organic solvents. The location of the pretreatment of the material comprising polyamide 6 and polyether polyurethane can be the same as the location of the depolymerization section [C]. However, preferably, one or more pretreatment steps in the pretreatment process are carried out at a location different from the location of the depolymerization section [C], for example, near a plant that collects waste material comprising polyamide 6 and polyether polyurethane and / or at a location specifically for the pretreatment of material comprising polyamide 6 and polyether polyurethane.

[0076] Loading step a)

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

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

[0079] In another preferred embodiment, the material comprising polyamide 6 and polyether polyurethane is dried before being loaded into the separation section [B], particularly after the polyamide 6 and polyether polyurethane material has undergone a cleaning step. This has the advantage that less or no solvent is introduced into the separation section [B]. Solvents introduced into the separation section [B], particularly water, can negatively impact the separation process (e.g., a reduced dissolution rate of the polyether polyurethane from the material comprising polyamide 6 and polyether polyurethane in organic solvents).

[0080] The material comprising polyamide 6 and polyether polyurethane is preferably fed into the dissolving container in solid form.

[0081] Feeding materials comprising polyamide 6 and polyether polyurethane into the separation section [B] (e.g., into the polyether polyurethane dissolution section) can be achieved by continuously or intermittently feeding materials comprising polyamide 6 and polyether polyurethane.

[0082] Separation step b)

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

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

[0085] b.1) An organic solvent and a material including polyamide 6 and polyether polyurethane are loaded into the dissolution section [α];

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

[0087] b.3) The second solvent and the polyether-rich stream comprising the organic solvent and dissolved polyether polyurethane are fed into the precipitation section [γ], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent.

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

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

[0090] b.6) The third solvent and the feed stream including undissolved polyamide 6 are loaded into the washing section [β];

[0091] b.7) The stream containing undissolved polyamide 6 is washed in the washing section [β] with the third solvent to obtain a stream rich in polyamide 6 and a mixture comprising an organic solvent and a third solvent;

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

[0093] b.9) Discharge the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β], and load the mixture into the solvent distillation section [δ];

[0094] b.10) The organic solvent is separated from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and the second solvent, the third solvent and the separated organic solvent are discharged from the solvent distillation section [δ].

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

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

[0097] As used herein, "a stream rich in..." refers to a stream containing a greater amount of enriched component than another stream obtained in the same process steps. A stream rich in polyamide 6 therefore contains more polyamide 6 than a stream rich in polyether polyurethane. Conversely, a stream rich in polyether polyurethane contains more polyether polyurethane than a stream rich in polyamide 6.

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

[0099] The polyether polyurethane-rich stream has a higher polyether polyurethane to polyamide 6 weight ratio than the polyether polyurethane to polyamide 6 weight ratio of materials containing both polyamide 6 and polyether polyurethane.

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

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

[0102] In the separation section [B], organic solvents and materials including polyamide 6 and polyether polyurethane are loaded into the dissolution section [α].

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

[0104] The organic solvent incorporated into the dissolving section [α] can be any organic solvent in which the polyether polyurethane is soluble. Preferably, the organic solvent combines (very) good solubility for the polyether polyurethane with (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-pyrrolidones such as N-methyl-2-pyrrolidone (NMP, C5H9NO), tetrahydrofuran (THF; also known as oxopentane, C4H8O), and combinations thereof. Even more preferably, the organic solvent is selected from N,N-dimethylformamide and dimethylacetamide. Most preferably, the organic solvent is dimethylacetamide.

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

[0106] The organic solvent incorporated into the dissolving section [α] preferably has a low water content. Even more preferably, the organic solvent is dried before being incorporated into the dissolving section [α] (preferably by distillation or by applying a desiccant such as inorganic salts like Na₂SO₄, zeolite, silica, and alumina). Drying is not necessarily 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] Dissolving step (step b.2):

[0108] In the dissolution section [α], polyether polyurethane is dissolved from the material comprising polyamide 6 and polyether polyurethane in the organic solvent introduced into the dissolution section [α] in step b.1). This yields a polyether polyurethane-rich stream comprising the organic solvent and dissolved polyether polyurethane, and a stream comprising undissolved polyamide 6, discharged from the dissolution section [α].

[0109] Preferably, the dissolution of the polyether polyurethane is completed within 0.1 to 24 hours, more preferably within 0.5 to 6 hours. "Completion" means that the dissolution has reached a plateau, and no substantial further dissolution occurs at subsequent time points.

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

[0111] The amount of solvent (in tons) in the dissolution zone [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 (in tons) in the dissolution zone [α]. A small amount of solvent reduces the dissolution rate of the polyether polyurethane and may even lead to incomplete dissolution. A large amount of solvent gives the polyether polyurethane a high dissolution rate; however, recycling used organic solvents requires more energy and is more costly. Technicians can determine the optimal ratio of solvent to polyether polyurethane or materials including nylon 6 and polyether polyurethane through routine testing.

[0112] The dissolving section [α] may include one or more dissolving vessels operating in series or parallel. Preferably, these vessels are equipped with additional devices (e.g., a stirrer) to enhance friction, thereby reducing dissolving time.

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

[0114] The polyether polyurethane-rich feed stream, which includes organic solvent and dissolved polyether polyurethane, discharged from the dissolution section [α], and the second solvent are loaded into the precipitation section [γ], so that the polyether polyurethane precipitates from the mixture including organic solvent and the second solvent.

[0115] Preferably, the second solvent is a non-solvent (also known as an anti-solvent) for polyether polyurethane. The addition of this second solvent reduces the solubility of the polyether polyurethane dissolved in the solution, leading to the precipitation of the polyether polyurethane particles. The second solvent can be any solvent. Preferably, the second solvent is water. Water has the advantage of being an environmentally friendly solvent.

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

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

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

[0119] A small amount of secondary solvent reduces the precipitation of polyether polyurethane. A large amount of secondary solvent complicates the recovery of precipitated polyether polyurethane and organic solvents. A large amount of secondary solvent also requires more energy and cost to recover the precipitated polyether polyurethane and organic solvents. Technicians will be able to find the optimal ratio of secondary solvent to polyether polyurethane-rich feed for the specific solvent and equipment used by conducting simple precipitation tests.

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

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

[0122] Optionally, the recovered precipitate is washed with a solvent before being discharged from the sedimentation zone [γ]. Preferably, the solvent used to wash the recovered precipitate is the organic solvent of step b.1) or a second solvent.

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

[0124] Discharging the solvent mixture (step b.5):

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

[0126] Incorporate the washing process (step b.6):

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

[0128] Preferably, the third solvent has good solubility in organic solvents but poor solubility in polyamide 6. The third solvent can be any solvent. Preferably, the second solvent is water. Water as the third solvent has the advantage of being an environmentally friendly solvent.

[0129] The amount of the third solvent incorporated into the washing section [β] can vary and can range from 10 wt.% to 1000 wt.%, preferably 20 wt.% to 500 wt.%, more preferably 50 wt.% to 400 wt.%, and most preferably 100 wt.% to 250 wt.%, compared to the feed stream containing undissolved nylon 6 incorporated into the washing section [β].

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

[0131] In the washing section [β], the stream containing undissolved polyamide 6 is washed with a third solvent, thereby obtaining a stream rich in polyamide 6 and a mixture comprising an organic solvent and a third solvent.

[0132] The temperature in the washing section [β] can vary and can be in the range of 0°C to 150°C, preferably 10°C to 100°C, more preferably 15°C to 80°C, and most preferably 20°C to 60°C. Experiments have shown that these temperature ranges result in particularly complete recovery of the organic solvent from the feed stream, which includes undissolved polyamide 6.

[0133] Technicians can determine the amount of a third solvent and the optimal temperature required for effective washing of a stream containing undissolved polyamide 6 through routine experiments.

[0134] Washing of the stream containing undissolved polyamide 6 with a third solvent can be carried out in a variety of apparatuses, all of which are known to those skilled in the art.

[0135] Discharge of the polyamide 6-rich stream (step b.8):

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

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

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

[0139] The determination of polyamide 6 content is a routine activity and can be performed by a variety of methods, all of which are known to those skilled in the art. Preferably, the polyamide 6 content in a 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, “based on dry weight” means the content of an ingredient as a percentage of the total dry weight of the composition, where “dry weight” is the weight of the substance after water and / or other liquids have been removed from the substance by drying until the weight is constant.

[0141] The discharge includes a mixture of organic solvents and a third solvent (step b.9):

[0142] The mixture comprising the organic solvent and the third solvent is discharged from the washing section [β]. Typically, the organic solvent and / or the third solvent are valuable compounds, and their recovery and reuse offer significant economic and environmental advantages. The mixture comprising the organic solvent and the third solvent is then loaded into the solvent distillation section [δ]. Optionally, the mixture comprising the organic solvent and the third solvent is first loaded into another section and then into the solvent distillation section [δ]. In a preferred embodiment, the mixture comprising the organic solvent and the third solvent is first partially or completely loaded into the precipitation section [γ].

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

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

[0145] The separated organic solvent, second solvent, and third solvent, along with residues, are discharged from the solvent distillation section [δ]. In a preferred embodiment, the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is loaded into the dissolving section [α] in step b.1). In another preferred embodiment, the second and third solvents discharged from the solvent distillation section [δ] are optionally reused in the precipitation section [γ] and / or washing section [β] after separation from each other.

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

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

[0148] Generally, separating three different liquids by distillation is more complex, requires more energy, and necessitates more equipment compared to separating two different liquids by distillation. Therefore, 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 invention, the second and third solvents are the same solvent. In a more preferred embodiment, the second and third solvents are water. Using water as a solvent is advantageous because it is inexpensive, readily available, 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), and tetrahydrofuran (THF; also known as oxopentane, C4H8O). In a more preferred embodiment, the organic solvent is N,N-dimethylacetamide (DMAc, CH3C(=O)N(CH3)2).

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

[0151] When the combined feed of the solvent distillation section [δ] contains only N,N-dimethylacetamide (DMAc) and water, the volatility difference between the two is significant, and separation by distillation is relatively straightforward. In such cases, continuous separation of the combined feed of the solvent distillation section [δ] can be achieved in a single distillation column operating in continuous mode. In such a distillation column, water is distilled off as a gas from the top of the column, and N,N-dimethylacetamide (DMAc) is discharged as a liquid from the bottom of the column. When the combined feed of the solvent distillation section [δ] also contains non-volatile compounds (such as polyether polyurethane and polyamide 6) 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. A direct layout for this type of separation by distillation involves two distillation columns operating in series. The feed (in liquid form) consisting of N,N-dimethylacetamide (DMAc), water, and non-volatile compounds is charged into the first distillation column. Water is distilled in a gaseous state from the top of the first distillation column, and a mixture of N,N-dimethylacetamide (DMAc) and non-volatile compounds (in liquid or slurry form) is discharged from the bottom of the first distillation column. The bottom stream of the first column is then fed into a second distillation column. N,N-dimethylacetamide (DMAc) is distilled in a gaseous state from the top of the second distillation column, 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 blockage. A squeeze (column) unit for recovering N,N-dimethylacetamide (DMAc) from the bottom stream of the second distillation column can be added to improve the overall recovery rate of N,N-dimethylacetamide (DMAc).

[0152] As an alternative to separation via distillation of two distillation columns operating in series, a single distillation column with a side stream can be selected, operating in continuous mode. In such a column, water is distilled as a gas from the top of the column, 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 into a squeeze (column) unit.

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

[0154] Pure dimethylamine has a boiling point of approximately 7°C at atmospheric pressure. Dimethylamine is highly soluble in water.

[0155] Pure acetic acid has a boiling point of approximately 118°C at atmospheric pressure.

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

[0157] If the pressure decreases, the azeotropic point of the mixture of acetic acid and N,N-dimethylacetamide shifts to the higher concentration of the lower boiling point component (which is acetic acid).

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

[0159] At 1.3 kPa, a high-boiling azeotrope is formed, consisting of approximately 42 wt.% acetic acid and approximately 58 wt.% N,N-dimethylacetamide, with a boiling point of approximately 75°C. Preferably, the degradation products are removed from the solvent reused in the method. This has the advantage of preventing the accumulation of degradation products. In a preferred embodiment, the degradation products of the organic solvent are removed before the separated organic solvent, discharged from the solvent distillation section [δ] in step b.1), is loaded into the dissolution section [α] in step b.1). However, due to the formation of the highest azeotrope, N,N-dimethylacetamide (DMAc) and acetic acid cannot be separated by simple distillation.

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

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

[0162] Another set of techniques, often referred to as pressure swing distillation, is based on the fact that the highest azeotrope is pressure-dependent. In such systems, two distillation columns operate at different pressure levels. A feed mixture of N,N-dimethylacetamide and acetic acid is charged into a first distillation column operating at a given pressure. The bottom stream from this first distillation column is then fed into a second distillation column. The bottom stream from the second distillation column is then fed into the first distillation column. High-purity N,N-dimethylacetamide and acetic acid are obtained as the top products of the distillation columns. A major advantage of pressure swing distillation is the absence of foreign compounds.

[0163] Finally, there is a set of techniques for converting acetic acid into another component (e.g., forming a salt by adding a caustic alkali) or selectively adsorbing acetic acid from a mixture of N,N-dimethylacetamide and acetic acid.

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

[0165] The optimal layout depends largely on the combination of feed composition and volumetric flow rate in the solvent distillation section [δ]. The layout can contain combinations of the various separation solutions described above.

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

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

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

[0169] Loading step c.1)

[0170] In step c.1) of the present invention, the resulting polyamide 6-rich stream is fed into the 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 into the depolymerization section [C] is less contaminated by foreign materials, which improves the yield and purity of ε-caprolactam produced in the plant of the present invention configured to carry out the method of the present invention. Other advantages are that less depolymerizing agent is required in the depolymerization section [C], less waste is generated, and less energy is required. Finally, the polyamide 6-rich stream reduces operational problems such as equipment scaling and clogging. The depolymerization section [C] comprises one or more depolymerization reactors operating in series and / or parallel.

[0171] Optionally, the polyamide 6-rich stream is mechanically compressed to a smaller volume before being fed into the depolymerization section [C]. This has the advantage of requiring a smaller volume for intermediate storage and transportation, and also facilitates feeding into the depolymerization section [C].

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

[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 into a pellet mill. The pellet mill cuts the product into pellets. These pellets are then loaded into a depolymerization section [C].

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

[0175] Optionally, the polyamide 6-rich stream discharged from the separation section [B] is dried before being fed into the depolymerization section [C]. This has the advantage of introducing less or no solvent into the depolymerization section [C]. Solvents introduced into the depolymerization section [C] can negatively impact the depolymerization process (e.g., reduced depolymerization rate, higher catalyst consumption, higher energy consumption, and the vapor stream obtained in the depolymerization section [C] may contain more impurities, including ε-caprolactam and water).

[0176] Preferably, the polyamide 6-rich stream is fed into the depolymerization reactor in solid or melt form. Preferably, the polyamide 6-rich stream is fed in melt form. Melt feeding can be achieved using an extruder, a gear pump, or other means known to those skilled in the art.

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

[0178] Depolymerization step c.2)

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

[0180] The depolymerization of the polyamide 6-rich stream is achieved by raising the temperature of the polyamide 6-rich stream to 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 depolymerization is particularly effective within these temperature ranges, and side reactions and impurity reactions of polyamide 6 and ε-caprolactam occur less frequently.

[0181] Typically, the formation rate of ε-caprolactam increases with increasing temperature. Temperatures below 400°C are preferred because above 400°C, side reactions and impurity reactions between polyamide 6 and ε-caprolactam occur more frequently, leading to the formation of a more diverse range of impurities. Some of these impurities will eventually enter the product stream containing ε-caprolactam discharged from the depolymerization section [C]. In a preferred embodiment, the depolymerization of the polyamide 6-rich stream is carried out at a temperature ranging from 220°C to 340°C or from 240°C to 325°C. Experiments have shown that this temperature range allows for the production of particularly pure ε-caprolactam.

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

[0183] The depolymerization of the polyamide 6-rich stream can be achieved with or without a solvent. Preferably, the depolymerization of the polyamide 6-rich stream is achieved in the presence of water as a solvent. In this case, the water is preferably in the form of steam, particularly superheated steam.

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

[0185] Feeding water as steam into the depolymerization reactor allows for the optional production of a steam stream comprising ε-caprolactam and water without further heating. The weight ratio of ε-caprolactam to water in this steam stream can be adjusted by varying the amount of steam fed into the polyamide 6-rich stream in the depolymerization section [C]. In a preferred embodiment, depolymerization in step c.2) is carried out in the presence of water, such that the ε-caprolactam-containing stream 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 cost-effective method can be employed.

[0186] Preferably, the ε-caprolactam in the steam 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, including linear oligomers and cyclic oligomers of ε-caprolactam. Additionally, the feed stream rich in polyamide 6 may also contain other components, i.e., impurities, such as non-polyamide 6 compounds and solvent residues that remain stable, react, or decompose under depolymerization conditions and are used in the pretreatment and / or separation sections. Therefore, when water is used as a 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 with a temperature between 100°C and 600°C is introduced into the depolymerization reactor. Preferably, the temperature of the superheated steam introduced into the depolymerization reactor is at least the melting temperature of polyamide 6. Preferably, the energy content of the superheated steam introduced into the depolymerization reactor is high enough that no other heat input is required for the depolymerization reaction and evaporation of the formed ε-caprolactam. In another preferred embodiment, superheated steam with a temperature range of 220°C to 575°C is introduced into the depolymerization section [C]. In an even more preferred embodiment, superheated steam with a temperature range of 275°C to 500°C is introduced into the depolymerization section [C].

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

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

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

[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 cost (for the disposal of catalyst and catalyst waste). However, higher temperatures (and pressures) are typically required.

[0193] The advantage of using a catalyst (especially orthophosphoric acid) is that the depolymerization reaction begins at a lower temperature and can be carried out under atmospheric conditions. The appropriate concentration of the catalyst used to depolymerize polyamide 6 to ε-caprolactam is known to those skilled in the art and can be readily determined through routine experiments. If the concentration of the catalyst used is too low, the reaction rate is slow. Conversely, if the concentration of the catalyst used is too high, the reaction is rapid, but side reactions also increase. Furthermore, the cost of the catalyst increases, which is economically disadvantageous. Preferably, the catalyst content is 0.01 to 100% by weight relative to the 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, semi-continuous, or continuous modes, all of which are known to those skilled in the art. As used herein, the terms “batch,” “semi-continuous,” and “continuous” refer to the mode in which a feedstock containing polyamide 6 (i.e., a polyamide 6-rich stream) and, optionally, a catalyst are loaded into the depolymerization reactor, and the mode in which residual material is discharged from the depolymerization reactor.

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

[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 into the depolymerization reactor. Simultaneously, superheated steam is continuously charged into the depolymerization reactor, and ε-caprolactam is continuously discharged from the depolymerization reactor as a steam stream comprising ε-caprolactam and water. Optionally, the catalyst is continuously or intermittently charged into the depolymerization reactor. Additionally, residual material is continuously discharged from the depolymerization reactor. Preferably, the polyamide 6-rich stream is charged in the form of a melt. Preferably, the catalyst is charged in the form of a melt, slurry, or solution.

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

[0198] Recycling step c.3)

[0199] In the recovery section [D], ε-caprolactam is recovered from the feed stream containing ε-caprolactam discharged from the self-depolymerization section [C]. This feed stream contains ε-caprolactam and impurities. Preferably, this recovery is carried out by (partial) condensation of the feed stream containing ε-caprolactam.

[0200] Preferably, without solvent being introduced into the depolymerization zone [C], the ε-caprolactam obtained by condensation is dissolved in water to obtain an ε-caprolactam-rich phase. This ε-caprolactam-rich phase also includes impurities.

[0201] Preferably, when water is used as a solvent in the depolymerization section [C], the feed stream discharged from the depolymerization section [C] containing ε-caprolactam comprises ε-caprolactam, water, and impurities. 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 condenser (preferably partially) to obtain a condensate containing ε-caprolactam. Preferably, ε-caprolactam is separated from the remaining components of the vapor stream by conveying the product stream from the depolymerization reactor (preferably the top) to a distillation column, from which a water-rich phase as the top product and a ε-caprolactam-rich phase as the bottom product are obtained.

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

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

[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 section [E],

[0207] And the method further includes the following steps:

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

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

[0210] (ii) Optionally, the solvent is switched by replacing at least part of the organic solvent with water, thereby obtaining an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam. Solvent switching step c.4)(ii) may specifically be selected from methods based on back-extraction with water and methods based on solvent exchange distillation, wherein the organic solvent is distilled off and water is added;

[0211] (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation.

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

[0213] Optionally, the crude ε-caprolactam is filtered before being loaded into the purification section [E]. Filtration ensures the removal of undissolved impurities that could otherwise hinder further purification.

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

[0215] In another embodiment, the extraction with an organic solvent in step c.4)(i) is carried out in a countercurrent extraction column, thereby introducing the crude ε-caprolactam to be purified at the top of the column and introducing the organic solvent at the bottom of the column. The extraction produces an aqueous phase and an organic phase, the organic phase comprising the organic solvent, ε-caprolactam, and impurities.

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

[0217] Technicians can determine the amount of water or alkaline aqueous solution required for efficient washing of an organic phase, including organic solvents, ε-caprolactam, and impurities, through routine experiments. Preferably, this amount is between 0.1 vol.% and 5 vol.% relative to the amount of organic solvent in the organic phase to be washed.

[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 countercurrent washing column, whereby the organic phase comprising organic solvent, ε-caprolactam, and impurities is introduced at the bottom of the column, and water or an alkaline aqueous solution is introduced at the top of the column. The washing produces a washed organic phase comprising organic solvent, ε-caprolactam, and impurities, and a phase comprising residues. The phase comprising residues then contains water, impurities, and ε-caprolactam.

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

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

[0221] The ε-caprolactam-aqueous phase, optionally stripped and / or distilled to remove residual solvent, is concentrated by evaporating water to obtain a concentrated ε-caprolactam aqueous phase. The ε-caprolactam content of this concentrated ε-caprolactam aqueous phase is typically between 60 wt.% and 99.9 wt.% relative to the whole phase.

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

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

[0224] Preferably, the oxidant is added to the ε-caprolactam-aqueous phase in the form of a solid, slurry, or aqueous solution, so that a diluted aqueous solution is obtained during purification by oxidation. A skilled technician can determine the amount of oxidant required for efficient oxidation of the ε-caprolactam-aqueous phase through routine experiments. The exact amount of oxidant depends largely, particularly on the composition of the polyamide 6-rich feed stream into the depolymerization section in the method of the present invention. Preferably, the amount of oxidant is between 0.01 and 5% by weight relative to the amount of ε-caprolactam dissolved in the aqueous phase to be oxidized.

[0225] In the method of the present invention, the temperature used for oxidizing the aqueous solution can be varied. Preferably, prior to the distillation removal in step c.4)(ii), the oxidation of the aqueous solution with an oxidant is carried out at a temperature ranging from 20°C to 85°C, more preferably from 30°C to 80°C, wherein the oxidant is selected from the group consisting of potassium permanganate, sodium permanganate, and hydrogen peroxide, and combinations thereof, particularly potassium permanganate.

[0226] The duration of oxidation with the oxidant can vary. Preferably, in the method of the present invention, the ε-caprolactam-aqueous phase is oxidized with the oxidant for 1 minute to 24 hours, more preferably 2 minutes to 6 hours, and most preferably 5 minutes to 2 hours, prior to the distillation removal in step c.4)(ii).

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

[0228] When potassium permanganate and / or sodium permanganate are used as oxidants, solid manganese oxide (IV) (MnO2) particles are formed as reaction products. Technicians can determine the optimal solid-liquid filtration procedure for the efficient removal of solid manganese oxide (IV) particles from the aqueous phase after oxidation through routine experiments. In this regard, it is common practice to improve the filtration procedure using filter aids such as activated carbon or diatomaceous earth particles.

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

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

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

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

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

[0234] The high-purity ε-caprolactam obtained according to the method of the present invention can be used to prepare polyamide 6 using methods well known to those skilled in the art. 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 to recover 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 method of the present invention, the recovered polyether polyurethane is discharged from the separation section [B]. Optionally, the recovered polyether polyurethane is dried before being discharged from the 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 carried out in a factory comprising:

[0239] - Separation section [B], which includes a dissolution section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ].

[0240] -The unpacking section [C], and

[0241] -Reclaimed section [D],

[0242] And it includes the following steps:

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

[0244] b.1) The organic solvent and the material comprising polyamide 6 and polyether polyurethane are loaded into the dissolution section [α] of the separation section [B];

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

[0246] b.3) The second solvent and the polyether-rich stream comprising the organic solvent and dissolved polyether polyurethane are fed into the precipitation section [γ] of the separation section [B], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent.

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

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

[0249] b.6) The third solvent and the feed stream including undissolved polyamide 6 are loaded into the washing section [β] of the separation section [B];

[0250] b.7) The stream containing undissolved polyamide 6 is washed in the washing section [β] with the third solvent to obtain a stream rich in polyamide 6 and a mixture comprising an organic solvent and a third solvent;

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

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

[0253] b.10) The organic solvent is separated from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and the second solvent, the third solvent and the separated organic solvent are discharged from the solvent distillation section [δ].

[0254] c.1) Discharge the polyamide 6-rich stream from the separation section [B] and load the polyamide 6-rich stream into the 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;

[0255] c.2) In the depolymerization section [C], the polyamide 6 in the polyamide 6-rich stream is depolymerized at a temperature ranging from 180°C to 400°C, so as to obtain a stream containing ε-caprolactam, and the obtained stream containing ε-caprolactam is discharged from the depolymerization section [C].

[0256] c.3) Recover crude ε-caprolactam from the feed stream containing ε-caprolactam in the recovery section [D].

[0257] factory

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

[0259] The plant may be a laboratory setting as in the example. However, preferably, the plant is an industrial-scale plant. "Industrial-scale" means that, if operated continuously, the plant's ε-caprolactam production capacity (i.e., the amount of ε-caprolactam that it can theoretically produce) is at least 500 tons per year.

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

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

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

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

[0264] Good contact between organic solvents and materials including polyamide 6 and polyether polyurethanes is essential for efficient operation. This contact can be achieved by various means known in the art. Improved contact can be achieved through mechanical friction. Mechanical friction can then be achieved, for example, by stirring using a combination of a rotating paddle and static fins.

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

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

[0267] Separation units suitable for separating (undissolved) polymers and organic solvents are known to those skilled in the art. Centrifuges and filters are examples of such separation units.

[0268] Preferably, the dissolution unit and the separation unit are combined into one device. This intensifies the process.

[0269] The washing section [β] includes apparatus for washing the stream containing undissolved polyamide 6 with a third solvent to obtain a polyamide 6-rich stream 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] includes apparatus for drying the polyamide 6-rich stream. The mixture comprising the organic solvent and the third solvent is discharged from the washing section [β] and loaded into the solvent distillation section [δ]. In this document, "loaded into the solvent distillation section [δ]" also includes indirect means, i.e., additional intermediate steps / sections. Preferably, the mixture comprising the organic solvent and the third solvent discharged from the washing section [β] is completely or partially loaded into the precipitation section [γ] before being loaded into the solvent distillation section [δ].

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

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

[0272] A distillation section [δ] comprises one or more distillation columns operating in series and / or parallel. These columns can operate in batch, semi-continuous, or continuous modes. The choice between these modes depends largely on the scale of the operation. Generally, batch operation of distillation columns is better suited for handling small-volume feed streams. Continuous operation of distillation columns is better suited for handling large-volume feed streams.

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

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

[0275] The depolymerization section [C] comprises one or more depolymerization reactors operating in series and / or parallel. A stream rich in polyamide 6 is fed into the reactor in solid or melt form, preferably melt. This feeding can be achieved 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 a feedstock containing polyamide 6, residual material, ε-caprolactam (and optionally a catalyst). The depolymerization reactor can have any desired form. Preferred reactor types are stirred and unstirred bubble column reactors, stirred reactors, and extruder-type reactors.

[0277] The depolymerization reactor must be equipped with facilities for feeding a stream rich in polyamide 6, superheated steam, and optionally a catalyst. Additionally, the depolymerization reactor must be equipped with facilities for discharging the steam stream, including ε-caprolactam and water, as well as residual materials.

[0278] Good contact between steam and the reactor contents is essential for efficient operation. This contact can be achieved through various means known to those skilled in the art. For example, steam can be injected through the material using multiple inlets provided, for instance, by a steam distributor. Improved contact can also be achieved by incorporating mechanical agitation into the reactor. Mechanical agitation can be achieved, for example, by using a combination of rotating paddles and static fins.

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

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

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

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

[0283] Crude ε-caprolactam and an organic solvent are loaded into an extraction device, and an organic phase comprising the organic solvent, ε-caprolactam, and impurities, as well as an aqueous phase comprising water and impurities, is discharged. The extraction device is selected from mixed settling extractors, extraction columns, and centrifugal extractors, as well as combinations thereof. Preferably, the extraction device is a static or stirred extraction column, such as... tower, Towers, rotary contactor (RDC) towers, pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (SMVP) (static) towers.

[0284] A solvent switching device is used to load water and an organic phase including organic solvents, ε-caprolactam, and impurities, and to discharge the phase including the organic solvents and the ε-caprolactam-aqueous phase including water, ε-caprolactam, and impurities. The solvent switching device for the back-extraction-based method is selected from mixed sedimentation extractors, extraction columns, and centrifugal extractors, as well as combinations thereof. Preferably, the device for back-extraction is a static or stirred extraction column, such as... tower, Towers, rotary contactor (RDC) towers, pulse towers, sieve plate (static) towers, random packing (static) towers, and structured packing (static) towers.

[0285] Solvent switching equipment for solvent exchange distillation methods is selected from sieve plate distillation columns, random-packed distillation columns, and structured-packed distillation columns. Preferably, the distillation column is equipped with a reboiler, a condenser, and a reflux device. The distillation column can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure. Preferably, water is introduced into the upper part of the distillation column, and the aqueous phase, including water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam, is discharged from the lower part of the distillation column.

[0286] The optional oxidation section comprises one or more oxidation reactors operating in series and / or parallel. An oxidant and an ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities are charged into the oxidation section. The oxidant can be charged in solid, slurry, or aqueous solution form. Preferably, the oxidant is charged in aqueous solution form. When potassium permanganate or sodium permanganate is used as the oxidant, the oxidation section also includes a filtration section. The oxidation reactor can have any desired form. Preferred reactor types are stirred and unstirred reactors and packed tower reactors. The oxidation reactor must be equipped with facilities for feeding an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam, as well as the oxidant. Additionally, the oxidation reactor must be equipped with facilities for discharging the oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities, as well as optionally formed solid manganese oxide (IV) (MnO2) particles. Typically, oxidation is carried out under temperature and atmospheric conditions ranging from 20°C to 85°C.

[0287] Optional solid manganese oxide (IV) (MnO2) particles can be removed by sedimentation or by solid-liquid filtration, preferably by solid-liquid filtration. It is common practice to improve the filtration process using filter aids such as activated carbon particles and diatomaceous earth. Filtration systems suitable for separating solid manganese oxide (IV) particles are known to those skilled in the art. This filtration system is loaded with a suspension of oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities, and solid manganese oxide (IV) particles, and the filtered oxidized ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities is discharged. Typically, the solid manganese oxide (IV) particles remain in the filtration system. Typically, this filtration system is operated in a semi-continuous mode, such that the suspension and the filter phase are continuously loaded and 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] Purifying crude ε-caprolactam to obtain purified ε-caprolactam may include hydrogenation using a heterogeneous catalyst, in which case the plant will include a hydrogenation section. Preferably, the catalyst comprises nickel or palladium.

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

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

[0291] Alternatively, hydrogenation can be carried out in a fixed-bed reactor, where the catalyst is fixed within the reactor, eliminating the need for additional steps to separate the catalyst and the reaction mixture. Preferably, the fixed bed consists of a supported palladium or nickel catalyst.

[0292] Hydrogenation temperatures typically range from 20°C to 160°C. Hydrogenation pressures typically range from 0.1 MPa to 15 MPa.

[0293] The distillation apparatus is charged with an ε-caprolactam-aqueous phase comprising water, ε-caprolactam, and impurities, and discharged with high-purity ε-caprolactam, water, and impurities (i.e., low-boiling-point organic impurities with boiling points below ε-caprolactam and high-boiling-point organic impurities with boiling points above ε-caprolactam). The distillation apparatus is selected from sieve plate distillation columns, random-packed distillation columns, structured-packed distillation columns, and horizontal and vertical (descending and ascending) film evaporators. Preferably, the distillation column is equipped with a reboiler, a condenser, and equipment for reflux. The distillation apparatus can operate at atmospheric pressure, sub-atmospheric pressure, or extra-atmospheric pressure, preferably at sub-atmospheric pressure.

[0294] Preferably, distillation includes separating water, low-boiling-point organic impurities (having a lower boiling point than ε-caprolactam), and / or separating high-boiling-point organic impurities (having a higher boiling point than ε-caprolactam) from ε-caprolactam. Therefore, distillation includes separating water from ε-caprolactam as a top product in a first step, while removing ε-caprolactam containing both low-boiling-point and high-boiling-point impurities as a bottom product. In a second step, low-boiling-point impurities are removed from ε-caprolactam as a top product, resulting in ε-caprolactam containing high-boiling-point impurities as a bottom product. In a third step, high-purity ε-caprolactam is separated as a top product, while a distillation residue including ε-caprolactam and high-boiling-point impurities remains as a bottom product. Optionally, the first and second steps are combined.

[0295] Typically, before distillation to remove water and impurities, an alkali metal hydroxide, preferably NaOH, is added to the oxidized ε-caprolactam-aqueous phase, which comprises water, ε-caprolactam, 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 makes it particularly effective in the subsequent distillation to remove impurities with boiling points lower and higher than ε-caprolactam.

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

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

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

[0299] product

[0300] The method of the present invention allows for the production of ε-caprolactam and polyether polyurethane to meet the specifications of demanding applications, and is particularly environmentally friendly due to its reduced product carbon footprint and use of waste as starting material.

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

[0302] PAN: Maximum value 5

[0303] E290: Maximum value 0.05

[0304] VB: Maximum value 0.5 mmol / kg

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

[0306] In a preferred embodiment, the polyether polyurethane obtained by the method of the present invention can be reused in the production of textiles, either in this way or in combination with virgin polyether polyurethane.

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

[0308] The polyether polyurethane produced by the method of this invention is also particularly economical and environmentally friendly. This is also evident because the polyether polyurethane produced by the method of this invention has a much lower carbon footprint compared to conventionally produced polyether polyurethanes (e.g., through the reaction of polyether polyols and diisocyanate monomers).

[0309] The environmental impact of a product is typically expressed as its carbon footprint, or simply 'carbon footprint'. A product's carbon footprint is defined as the total emissions generated by the product's formation, expressed in tons of CO2 equivalent per ton of product. A product's carbon footprint depends particularly on raw materials, auxiliary materials, energy consumption, energy, production methods, and method efficiency. The quantification of a product's carbon footprint can be performed as described, for example, in the European standard EN ISO 14040:2006 ("Environmental management – ​​Life cycle assessment – ​​Principles and framework").

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

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

[0312] The carbon footprint of the ε-caprolactam product obtained by the method according to the invention is significantly lower than that of the de novo synthesized or “native” ε-caprolactam product. The carbon footprint of the ε-caprolactam product obtained by the method according to the 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] NMvan der Velden, MKPatel and JG (Table 7 below, “LCA benchmarking study on textiles made of cotton, polyester, nylon, acrylamide, or elastane,” *International Journal of Life Cycle Assessment* (2014) 19:331–356; DOI:10.1007 / s11367-013-0626-9) reports that the potential impact of virgin elastic fiber production on global warming is equivalent to 4.836 kg CO2 equivalent / kg elastic fiber.

[0314] The carbon footprint of the polyether polyurethane product obtained by the method according to the present invention is significantly lower than that of de novo synthesized or "native" polyether polyurethane products. The carbon footprint of the polyether polyurethane product obtained by the method according to the present invention is less than 3.0 kg, preferably less than 2.0 kg CO2, and most preferably less than 1.0 kg CO2 equivalent / kg polyether polyurethane. Attached Figure Description

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

[0316] Figure 1 This is a schematic diagram of the method of the present invention for recovering and purifying ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane, the method comprising processing steps 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 embodiment of a separation section [B] is shown, in which materials comprising polyamide 6 and polyether polyurethane are separated to obtain a polyamide 6-rich stream and a polyether polyurethane-rich stream. This embodiment consists of a dissolving section [α], a washing section [β], a precipitation section [γ], and a solvent distillation section [δ].

[0318] Detailed description of the attached figures

[0319] The method of the present invention is in Figure 1 The diagram illustrates the method in the following factory sections:

[0320] In an optional pretreatment section [A], the material [1] comprising polyamide 6 and polyether polyurethane is cleaned and / or crushed by mechanical dimensional reduction to obtain discharged clean and / or crushed fragments [2] consisting of the material comprising polyamide 6 and polyether polyurethane. Optionally, in the pretreatment section [A] in which the material [1] comprising polyamide 6 and polyether polyurethane is cleaned by removing foreign material and / or by washing with a washing solvent [3], discharged foreign material and contaminated washing solvent [4] are obtained. Removal of foreign material may be performed before and / or after washing with washing solvent [3]. Cleaning may be performed before and / or after crushing the material [1] comprising polyamide 6 and polyether polyurethane. Optionally, the material [2] comprising polyamide 6 and polyether polyurethane is dried after the cleaning step and before being loaded into the separation section [B].

[0321] In the separation section [B], optionally cleaned and / or broken fragments [2] composed of materials including 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 [2] comprising polyamide 6 and polyether polyurethane 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 introduced into the separation section [B] via line [6]. Optionally, a second solvent is introduced into the separation section [B] via line [9], the second solvent being used to precipitate the polyether polyurethane from the polyether polyurethane-rich stream, thereby obtaining precipitated polyether polyurethane

[10] . Optionally, the precipitated polyether polyurethane

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

[12] , the third solvent being used to wash the polyamide 6-rich stream

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

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

[15] discharged from the separation section [B] and the residue

[16] .

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

[13] is depolymerized, resulting in a stream

[19] containing ε-caprolactam discharged from the depolymerization zone [C]. Additionally, residual material

[20] is discharged. Superheated steam

[17] and optionally a catalyst

[18] are introduced into the depolymerization zone [C]. Optionally, the polyamide 6-rich stream

[13] is melted before being introduced into the depolymerization zone [C].

[0323] In the recovery section [D], crude ε-caprolactam is recovered from the feed stream

[19] containing ε-caprolactam discharged from the self-depolymerization section [C]

[21] . Additionally, an aqueous phase is discharged from the recovery section [C]

[22] .

[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 embodiment of a separation section [B] (the area enclosed by the dashed line) is depicted, in which materials comprising polyamide 6 and polyether polyurethane are separated to obtain a polyamide 6-rich stream and a polyether polyurethane-rich stream. This embodiment includes the following sections:

[0326] In the dissolution section [α], polyether polyurethane is dissolved in an organic solvent from optionally cleaned and / or broken fragments [2] consisting of a material comprising polyamide 6 and polyether polyurethane, to obtain a polyether polyurethane-rich stream [7] comprising the organic solvent and dissolved polyether polyurethane, and a stream comprising undissolved polyamide 6 [8]. 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], a second solvent [9] and a stream [7] containing organic solvent and dissolved polyether polyurethane are mixed, such that the polyether polyurethane precipitates from the mixture containing the organic solvent and the second solvent. The precipitated polyether polyurethane is recovered from the mixture containing the organic solvent and the second solvent (e.g., by filtration or centrifugation). The recovered precipitated polyether polyurethane

[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

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

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

[12] to obtain a polyamide 6-rich stream

[13] and a mixture

[14] comprising an organic solvent and a third solvent. The polyamide 6-rich stream

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

[13] 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 loaded into the solvent distillation section [δ].

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

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

[14] comprising an organic solvent and a third solvent, is distilled to obtain separated organic solvent [5], second solvent and third solvent

[15] , and residue

[16] . Optionally, the second solvent and the third solvent are both water, in which case the separated organic solvent [5] is a dry organic solvent. The separated organic solvent [5] is then loaded into the dissolving section [α].

[0330] Example

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

[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 permanganate index of caprolactam — Spectroscopic method, a revision of ISO 8660, 1988.

[0336] E290: ISO 7059 - Industrial caprolactam - Determination of absorbance at 290 nm.

[0337] Volatile bases (VB) ISO 8661 - Industrial caprolactam - Determination of volatile base content - Titration after distillation.

[0338] Alkalinity of ε-caprolactam products: determined by using a Tashiro indicator at 25°C at a concentration of 0.1 wt. / v. 乙醇 % Methylene Blue: 0.1 wt. / v 乙醇Alkalinity was determined by titration with 1:2 ratio of % methyl red, with an endpoint of gray. First, a flask containing water and indicator was titrated to gray. Then, X grams of an aqueous solution of ε-caprolactam containing Y wt.% ε-caprolactam (as determined by refractive index) was added, and the solution was titrated back to gray using 0.01N H₂SO₄ solution. The alkalinity was then given as follows:

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

[0340] in:

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

[0342] t = the equivalent concentration of the 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 polyether polyurethane content of used waste polyamide 6 fabric containing polyether polyurethane is 20.2 ± 0.4 wt.% (as determined by differential scanning calorimetry (DSC) ISO 11357-3 -130°C to 300°C -10°C / min -dt 1.00 s). The color of used waste nylon 6 fabric containing polyether polyurethane (Spandex) is purple.

[0348] Waste polyamide 6 fabric containing polyether polyurethane was cut into small pieces, each piece ranging from 5 to 20 cm in size. 2 Within the specified range. 150.4 g of this cutting material was dissolved in 800 g of DMAc at 70°C for 1 hour with stirring. The resulting solution was then vacuum filtered through a heated double-walled Buchner funnel at 70°C to obtain filtrate 1. The remaining undissolved material was further processed three times, each time with 400 g of DMAc and stirred at 70°C for 1 hour, to obtain filtrates 2 to 4.

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

[0350] Analysis by differential scanning calorimetry (DSC) revealed that the dry precipitate contained approximately 100 wt.% polyether polyurethane (polyamide 6 was not detected). Based on DSC analysis, the dry, undissolved material contained approximately 100 wt.% nylon 6 (polyether polyurethane was not detected).

[0351] The recycled polyether polyurethane precipitate can be reused in textile production, either in this way or in combination with virgin polyether polyurethane.

[0352] This example demonstrates that polyether polyurethane and polyamide 6 can be separated from waste fabrics containing polyamide 6 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 content of polyether polyurethane in the undissolved material is much lower than that in the waste fabrics containing polyamide 6 and containing polyether polyurethane used as starting material.

[0353] Example 2

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

[0355] The polyether polyurethane content of the used waste polyamide 6 fabric containing polyether polyurethane is 8.15 + / - 0.05 wt.% (as determined by differential scanning calorimetry (DSC) ISO 11357-3 -130℃ to 300℃ -10℃ / min -dt 1.00 s). The color of the used waste nylon 6 fabric containing polyether polyurethane (Spandex) is salmon pink.

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

[0357] Analysis by differential scanning calorimetry (DSC) revealed that the dry precipitate contained approximately 100 wt.% polyether polyurethane (polyamide 6 was not detected). The dry, undissolved material contained approximately 99 wt.% polyamide 6 and less than 1 wt.% polyether polyurethane (DSC analysis).

[0358] The recycled polyether polyurethane precipitate can be reused in textile production, either in this way or in combination with virgin polyether polyurethane.

[0359] This example demonstrates that polyether polyurethane and polyamide 6 can be separated from waste fabric containing polyamide 6 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 content of polyether polyurethane in the undissolved material is much lower than that in the starting material, waste fabric containing polyamide 6.

[0360] Example 3

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

[0362] Before loading the dried, undissolved material obtained in Example 1 into the depolymerization reactor, the material was first densified. The dried, undissolved material was melted at 237°C under nitrogen atmosphere, and forced through a perforated metal plate. The resulting strands were cooled to room temperature and cut into granules. The resulting granules had a diameter of 3 mm and a length of 1 cm.

[0363] 48 grams of these polyamide 6 granules and 14 grams of 20 wt.% phosphoric acid were charged into a Premex autoclave. First, the reactor contents were heated under nitrogen, followed by continuous injection of superheated steam at a rate of 4 grams per minute over a 120-minute reaction period. The temperature and pressure in the reactor were maintained at 260°C and 0.11 MPa, respectively. During the reaction, the steam stream was continuously discharged from the reactor and cooled to approximately 20°C, yielding a condensate comprising ε-caprolactam and water.

[0364] The condensate, consisting of 41 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 51.5 wt.% by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65 °C). (This mixture is crude ε-caprolactam and is the mixture to be purified.)

[0365] The specifications for crude ε-caprolactam are as follows:

[0366] PAN: 389

[0367] E290: 3.42

[0368] This example demonstrates that crude ε-caprolactam can be obtained by depolymerizing polyamide 6, which is obtained by extraction and separation from waste fabric containing polyamide 6 and polyether polyurethane.

[0369] Example 4

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

[0371] Follow the procedure of Example 3, except that 48 grams of granules made from the dry, undissolved material obtained in Example 2 and 14 grams of 20 wt.% phosphoric acid are now loaded into the Premex autoclave.

[0372] The condensate, consisting of 33 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 64.6 wt.% ε-caprolactam by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is crude ε-caprolactam and is the mixture to be purified.)

[0373] The specifications for crude ε-caprolactam are as follows:

[0374] PAN: 214

[0375] E290: 2.68

[0376] This example demonstrates that crude ε-caprolactam can be obtained by depolymerizing polyamide 6, which is obtained by extraction and separation from waste fabric containing polyamide 6 and polyether polyurethane.

[0377] Comparative Experiment 1

[0378] Depolymerization of polyether polyurethane

[0379] Follow the procedure of Example 3, except that 37.6 grams of polyether polyurethane fiber (the same material contained in the waste fabric used in Examples 1 and 2) and 14 grams of 20 wt.% phosphoric acid are now loaded into the Premex autoclave.

[0380] The experiment had to be stopped 10 minutes after the injection of superheated steam began because the pipeline used to discharge the steam stream, which included ε-caprolactam and water, was blocked by insoluble material.

[0381] The comparative experiment shows that depolymerizing waste polyamide 6 fabrics containing polyether polyurethane without removing the polyether polyurethane before depolymerization will cause operational problems.

[0382] Comparative Experiment 2

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

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

[0385] The condensate, consisting of 26 g of ε-caprolactam (with the majority of the residue being water), was concentrated to a concentration of 47.0 wt.% ε-caprolactam by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is crude ε-caprolactam and is the mixture to be purified.)

[0386] The specifications for crude ε-caprolactam are as follows:

[0387] PAN: 352

[0388] E290: 3.60

[0389] Observation results:

[0390] ●The condensate obtained before concentration does indeed contain unknown precipitates.

[0391] ● Following the depolymerization experiment, fouling was found on the inner wall of the Premex autoclave.

[0392] This comparative experiment concludes that depolymerizing waste polyamide 6 fabrics containing polyether polyurethane without removing the polyether polyurethane before depolymerization will cause operational problems. Another observation is that the ε-caprolactam yield in this comparative experiment was significantly lower than that in Example 3 (using pretreated waste polyamide 6 fabrics containing polyether polyurethane as feed), where the ε-caprolactam yield was defined as the ratio of the weight of ε-caprolactam in the condensate to the weight of polyamide 6 in the feed to the Premex autoclave.

[0393] Comparative Experiment 3

[0394] Purification by distillation

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

[0396] The specifications for distilled ε-caprolactam are as follows:

[0397]

[0398] The comparative experiment showed that the quality of ε-caprolactam obtained by depolymerizing waste fabric containing polyamide 6 and polyether polyurethane (without pretreatment in the separation section, which separates the polyamide 6 and polyether polyurethane materials into a polyamide 6-rich stream and a polyether polyurethane-rich stream) and then by distillation concentration and purification was very poor, as it did not meet any of the specifications required for the main polymerization applications.

[0399] Comparative Experiment 4

[0400] Purification by distillation

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

[0402] The specifications for distilled ε-caprolactam are as follows:

[0403]

[0404] This comparative experiment leads to the conclusion that crude ε-caprolactam purified by distillation alone is insufficient 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 resulting solid was then removed from the oxidation reaction products by filtration.

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

[0409] The specifications for distilled ε-caprolactam are as follows:

[0410]

[0411] This comparative experiment leads to the conclusion that purifying crude ε-caprolactam by permanganate treatment followed by distillation is insufficient to meet all the specifications required for major polymerization applications as described above.

[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 to a concentration of 37.9 wt.% by evaporation in a rotary evaporator operated under vacuum (9.5 kPa; water bath temperature approximately 65°C). (This mixture is concentrated crude ε-caprolactam and is the mixture to be purified.)

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

[0416] The concentrated extract was extracted seven times with water at approximately 25°C. The total amount of water used was 5.67 g water / g recovered ε-caprolactam.

[0417] The combined aqueous extracts were treated with 0.1 wt.% KMnO4 relative to ε-caprolactam at 50 °C for 2 hours. The resulting solids were then removed from the oxidation reaction products by filtration.

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

[0419] The specifications for distilled ε-caprolactam are as follows:

[0420]

[0421] This experiment leads to the conclusion that purified ε-caprolactam meeting the specifications required for major polymerization applications can be obtained from polyamide 6 waste fabric containing polyether polyurethane, from which the depolymerization of polyether polyurethane has been removed by extraction, and has been purified by extraction, oxidation and distillation.

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

[0423] Experiment 6

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

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

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

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

[0428] - Separate undissolved polyamide 6 and polyether-rich polyurethane streams by centrifugation;

[0429] - Wash undissolved polyamide 6 with water;

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

[0431] - The polyether polyurethane is precipitated from the polyether polyurethane-rich stream by adding the aqueous extract obtained above.

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

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

[0434] -Dried and filtered polyether polyurethane;

[0435] -Dry and wash the undissolved polyamide 6;

[0436] - Melt and granulate the washed, undissolved polyamide 6;

[0437] - Polyamide 6 is depolymerized under the action of H3PO4 and superheated steam;

[0438] - Crude ε-caprolactam (80 wt.% ε-caprolactam) is recovered by partial condensation of the vapor discharged from the depolymerization reactor;

[0439] - Crude ε-caprolactam was concentrated by countercurrent extraction with toluene;

[0440] - Wash the organic extract with a diluted caustic alkali solution;

[0441] - Organic extracts washed with countercurrent water extraction;

[0442] - The solid formed by treatment with KMnO4 and filtration;

[0443] -Evaporate and concentrate the aqueous extract;

[0444] - Add caustic soda;

[0445] - Pure ε-caprolactam was recovered by vacuum distillation.

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

[0447] The results revealed that the carbon footprint of purified ε-caprolactam obtained from waste polyamide 6 fabric containing polyether polyurethane was less than 3.0 tCO2 equivalents / ton ε-caprolactam and less than 1.0 tCO2 equivalents / ton polyether polyurethane.

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

Claims

1. A method for recovering ε-caprolactam and polyether polyurethane from a material comprising polyamide 6 and polyether polyurethane in a plant, wherein said plant includes -Separation section [B], -De-aggregation section [C], -Reclaim section [D], and -Purification section [E], The separation section [B] includes: -Dissolution zone [α], - Washing section [β], -precipitation zone [γ], and - Solvent distillation section [δ], And the method described therein includes the following steps: a) The material comprising polyamide 6 and polyether polyurethane is loaded into the separation section [B]; b) In the separation section [B], the material comprising polyamide 6 and polyether polyurethane is separated 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 of 20°C to 75°C, wherein the polyether polyurethane-rich stream is a solution comprising the organic solvent and the polyether polyurethane. The method includes the following steps in the separation section [B]: b.1) An organic solvent and a material including polyamide 6 and polyether polyurethane are loaded into the dissolution section [α]; b.2) In the dissolution zone [α], the polyether polyurethane is dissolved from the material comprising polyamide 6 and polyether polyurethane in an organic solvent, such that a polyether polyurethane-rich stream comprising the organic solvent and dissolved polyether polyurethane and a stream comprising undissolved polyamide 6 are obtained, and the obtained stream is discharged from the dissolution zone [α]. b.3) The second solvent and the polyether-rich stream comprising the organic solvent and dissolved polyether polyurethane are fed into the precipitation section [γ], such that the polyether polyurethane precipitates from the mixture comprising the organic solvent and the second solvent. b.4) Recover the precipitated polyether polyurethane from the mixture comprising the organic solvent and the second solvent, and discharge the precipitated polyether polyurethane from the precipitation section [γ]. b.5) ​​Discharge the mixture comprising the organic solvent and the second solvent from the precipitation section [γ] from which the precipitated polyether polyurethane has been recovered in step b.4), and load the mixture into the solvent distillation section [δ]. b.6) The third solvent and the feed stream including undissolved polyamide 6 are loaded into the washing section [β]; b.7) The stream containing undissolved polyamide 6 is washed in the washing section [β] with the third solvent to obtain a stream rich in polyamide 6 and a mixture comprising an organic solvent and a third solvent; b.8) Discharge the polyamide 6-rich stream from the washing section [β]; b.9) Discharge the mixture comprising the organic solvent and the third solvent obtained in step b.7) from the washing section [β], and partially or completely load the mixture into the solvent distillation section [δ]. b.10) The organic solvent is separated from the second solvent and the third solvent by distillation in the solvent distillation section [δ], and the second solvent, the third solvent and the separated organic solvent are discharged from the solvent distillation section [δ]. c.1) Discharge the polyamide 6-rich stream from the separation section [B] and load the polyamide 6-rich stream into the depolymerization section [C], wherein the polyamide 6 content in the polyamide 6-rich stream is at least 85% by weight based on dry weight. c.2) In the depolymerization section [C], the polyamide 6 in the polyamide 6-rich stream is depolymerized at a temperature ranging from 180°C to 400°C, so as to obtain a stream containing ε-caprolactam, and the obtained stream containing ε-caprolactam is discharged from the depolymerization section [C]. c.3) Recover crude ε-caprolactam from the feed stream containing ε-caprolactam in the recovery section [D]; c.4) Purify the crude ε-caprolactam obtained in the recovery section [D] in the purification section [E] to obtain purified ε-caprolactam, wherein the purification includes the following steps: (i) Extracting the crude ε-caprolactam with an organic solvent to obtain an aqueous phase and an organic phase, wherein the organic phase comprises the organic solvent, ε-caprolactam and impurities; (ii) By replacing the organic solvent at least partially with water, an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points below or above ε-caprolactam is obtained; and (iii) Purified ε-caprolactam is obtained by removing impurities with boiling points lower or higher than ε-caprolactam through distillation; Prior to the distillation removal in step c.4) (iii), an alkali metal hydroxide is added to an aqueous phase comprising water, ε-caprolactam, and impurities with boiling points lower or higher than ε-caprolactam; d.1) Recovering polyether polyurethane from the polyether-rich stream in the separation section [B]; and d.2) The recovered polyether polyurethane is discharged from the separation section [B], wherein the polyether polyurethane content in the discharged stream is at least 85% by weight based on dry weight.

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

3. The method according to any one of claims 1 or 2, wherein (i) The polyamide 6-rich stream discharged from the separation section [B] is dried before being loaded into the depolymerization section [C]; and / or (ii) The depolymerization in step c.2) is carried out in the presence of water, such that the feed stream comprising ε-caprolactam is a vapor feed stream comprising ε-caprolactam and water in a weight ratio of 1:1 to 1:50; and / or (iii) Introduce superheated steam with a temperature range of 220°C to 575°C into the depolymerization section [C].

4. The method according to claim 3, wherein (ii) The depolymerization in step c.2) is carried out in the presence of water, such that the feed stream comprising ε-caprolactam is a vapor feed stream comprising ε-caprolactam and water in a weight ratio of 1:2 to 1:15; and / or (iii) Introduce superheated steam with a temperature range of 275°C to 500°C into the depolymerization section [C].

5. The method according to claim 3, wherein (ii) The depolymerization in step c.2) is carried out in the presence of water, such that the feed stream comprising ε-caprolactam is a vapor feed stream comprising ε-caprolactam and water in a weight ratio of 1:2 to 1:

10.

6. The method according to claim 3, wherein (ii) The depolymerization in step c.2) is carried out in the presence of water, such that the feed stream comprising ε-caprolactam is a vapor feed stream comprising ε-caprolactam and water in a weight ratio of 1:3 to 1:

8.

7. The method of claim 1, wherein the recovered polyether polyurethane discharged from the precipitation section [γ] in step b.4) is reused in the production of textiles.

8. The method according to any one of claims 1 or 2, wherein the plant further comprises -Preprocessing section [A], Furthermore, prior to step a), the material comprising polyamide 6 and polyether polyurethane is pretreated in the pretreatment section [A].

9. The method of claim 8, wherein prior to step a), the material comprising polyamide 6 and polyether polyurethane is subjected to cleaning in a cleaning section and / or mechanical dimensional reduction in a mechanical dimensional reduction section.

10. The method according to any one of claims 1 or 2, wherein the organic solvent is dimethylacetamide.

11. The method of claim 1, wherein the second solvent and the third solvent are the same solvent.

12. The method of claim 11, wherein the second solvent and the third solvent are water.

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

14. The method of claim 1, 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].

15. The method of claim 14, 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] after being separated from each other.

16. The method of claim 2, wherein the degradation products of the organic solvent are removed before the separated organic solvent discharged from the solvent distillation section [δ] in step b.10) is loaded into the dissolution section [α] in step b.1).

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