Method for preparing dihydroxy ethyl terephthalate from waste textiles
By using a countercurrent extraction system of weakly polar aromatic hydrocarbons and long-chain alcohols, the problem of efficient removal of multi-component dyes from waste textiles was solved, resulting in high-purity BHET and improving the quality of recycled polyester.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- ZHEJIANG HENGYI PETROCHEMICAL RES INST CO LTD
- Filing Date
- 2026-04-27
- Publication Date
- 2026-05-26
Smart Images
Figure SMS_1
Abstract
Description
Technical Field
[0001] This application relates to the field of polyester recycling technology, and in particular to a method for preparing dihydroxyethyl terephthalate from waste textiles. Background Technology
[0002] Chemical recycling of waste textiles is a key path to achieving sustainable development in the textile industry. Polyester fibers, primarily polyethylene terephthalate (PET), account for over 60% of textiles. Their chemical recycling typically involves alcoholysis to depolymerize PET into dihydroxyethyl terephthalate (BHET) monomers, which are then purified and repolymerized into high-quality recycled polyester. However, waste textiles come from diverse sources, often being blends of natural fibers (such as cotton, linen, silk, and wool) and chemical fibers (such as polyester, nylon, and acrylic), and containing impurities such as dyes, auxiliaries (such as flame retardants and waterproofing agents), and finishing agents. These impurities remain in the BHET aqueous solution after alcoholysis, resulting in high solution color and numerous impurities, severely affecting the whiteness and color of the recycled polyester.
[0003] Currently, the decolorization of BHET solutions is a core technological bottleneck in the chemical recycling of waste textiles. Traditional decolorization methods struggle to simultaneously achieve efficient removal of multiple dyes (such as disperse dyes, vat dyes, and azo dyes), avoid BHET monomer loss, adapt to complex dye mixture systems, and meet environmental and economic requirements. Therefore, developing a highly efficient, selective decolorization technology applicable to multi-component dye systems is crucial for promoting the high-value recycling of waste textiles. Summary of the Invention
[0004] This application provides a method for preparing dihydroxyethyl terephthalate from waste textiles. This method can efficiently and selectively prepare dihydroxyethyl terephthalate from waste textiles, and the preparation method is also applicable to multi-component dye systems, making it suitable for widespread application.
[0005] This application provides a method for preparing dihydroxyethyl terephthalate from waste textiles, comprising:
[0006] Ethylene glycol was used to perform an alcoholysis reaction with polyester textiles to obtain a mixture to be separated.
[0007] The mixture to be separated was subjected to N-stage countercurrent extraction using a mixed solvent including a weakly polar aromatic solvent and an alcohol solvent to separate the phases into an organic phase and an aqueous phase including dihydroxyethyl terephthalate.
[0008] The alcohol solvent has ≥4 carbon atoms and ≥1 N.
[0009] In the method described above, the weakly polar aromatic solvent includes toluene and / or xylene; and / or, the alcohol solvent includes n-octanol, isoamyl alcohol, and isooctanol.
[0010] In the method described above, the volume ratio of the weakly polar aromatic solvent to the alcohol solvent in the mixed solvent is (1-5):1.
[0011] In the method described above, the temperature during the countercurrent extraction process is 50-75°C, and the volume ratio of the mixed solvent to the mixture to be separated is 1:(3-10).
[0012] In the method described above, the temperature during the alcoholysis reaction is 180-220°C.
[0013] The method described above further includes performing an M-stage thermal filtration process on the product obtained from the alcoholysis reaction to obtain a mixture to be separated, where M ≥ 1.
[0014] In the method described above, the temperature of the M-level thermal filtration process is ≥100°C.
[0015] The method described above further includes distilling the organic phase.
[0016] The method described above further includes crystallizing the aqueous phase to obtain the dihydroxyethyl terephthalate.
[0017] The method for preparing dihydroxyethyl terephthalate from waste textiles provided in this application adopts a synergistic mixed solvent extraction system, which can achieve high-efficiency extraction of dye molecules with various structures and low polarity, thereby obtaining a high-purity, low-color BHET solution, providing a raw material guarantee for the production of high-quality recycled polyester. Detailed Implementation
[0018] Exemplary embodiments will be described in detail below. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.
[0019] Achieving highly selective extraction of both low-polarity dyes (such as disperse dyes) and polar dyes (such as azo dyes and vat dyes) is a key problem that needs to be solved. This application designs a countercurrent extraction method using a mixed solvent system composed of a weakly polar aromatic solvent and a long-chain alcohol solvent, which enables highly efficient and selective extraction of various dyes from the alcoholysis liquid of waste textiles.
[0020] This invention provides a method for preparing dihydroxyethyl terephthalate from waste textiles, comprising: subjecting ethylene glycol to an alcoholysis reaction with polyester textiles to obtain a mixture to be separated; performing N-stage countercurrent extraction on the mixture to be separated using a mixed solvent comprising a weakly polar aromatic solvent and an alcohol solvent to separate the phases to obtain an organic phase and an aqueous phase comprising dihydroxyethyl terephthalate; wherein the alcohol solvent has ≥4 carbon atoms and ≥1 N.
[0021] Specifically, ethylene glycol is subjected to an alcoholysis reaction with polyester textiles to obtain a crude BHET and a mixture of ethylene glycol to be separated. Then, a mixed solvent including a weakly polar aromatic solvent and an alcohol solvent with ≥4 carbon atoms is used to perform countercurrent extraction on the mixture to be separated. The fresh mixed solvent is contacted with the low-dye-concentration BHET mixture to be separated (the mixture to be separated after at least one extraction), while the nearly saturated mixed solvent (the mixed solvent after at least one extraction) is contacted with the high-dye-concentration BHET mixture to be separated (the mixture to be separated), thereby maximizing the utilization rate of the solvent. In the countercurrent extraction process, various dyes enter the mixed solvent to form an organic phase, and the BHET without various dyes forms an aqueous phase.
[0022] The weakly polar aromatic solvent of this invention is a well-known weakly polar aromatic solvent in the art. The main function of the weakly polar aromatic solvent is as a "hydrophobic dye trap". Its benzene ring structure has a strong π-π conjugation with the aromatic skeleton of most dye molecules (especially disperse dyes and azo dyes), which can effectively dissolve and extract nonpolar and weakly polar dyes with strong hydrophobicity and large molecular weight. The alcohol solvent with ≥4 carbon atoms is a long-chain alcohol, which has the following characteristics: 1) Hydrogen bond association: The hydroxyl group (-OH) at the end of its molecule can act as a hydrogen bond donor or acceptor, forming hydrogen bonds with polar functional groups such as amino (-NH2), imino (-NH-), and carbonyl (C=O) contained in the dye molecule, which greatly enhances the extraction ability for polar dyes containing these groups; 2) Promoting phase separation: Long-chain alcohols have lower solubility in water than short-chain alcohols (such as methanol and ethanol), which helps to form a clear and rapid phase separation with the aqueous BHET solution after extraction, reducing mutual entrainment. Furthermore, this invention mixes weakly polar aromatic solvents of different polarities with alcohol solvents with ≥4 carbon atoms, which can precisely "fine-tune" the overall polarity (or solubility parameter δ) of the entire mixed solvent, making it closer to the average solubility parameter of common dye mixtures in waste textiles, thereby improving the efficiency of countercurrent extraction.
[0023] This invention employs countercurrent extraction using a mixed solvent comprising a weakly polar aromatic solvent and an alcohol solvent. The polarity of the mixed solvent matches the dye but differs significantly from that of the BHET monomer (which is strongly hydrophilic). Therefore, it can selectively extract dye molecules with minimal co-extraction loss of BHET (<1%), ensuring a high yield of the final product and significant economic benefits. Furthermore, this method exhibits universally applicable and highly efficient decolorization capabilities for dye mixtures, effectively addressing the technical challenges posed by the diverse sources and mixed colors of waste textiles, and providing a reliable solution for large-scale processing of mixed waste textiles. The method of this invention has a concise process flow and also offers energy-saving and consumption-reducing advantages. Countercurrent extraction maximizes the extraction capacity of the mixed solvent, has a strong mass transfer driving force, and can achieve extremely high decolorization rates with a relatively small amount of mixed solvent.
[0024] In some implementations, a 2-3 stage countercurrent extraction process can be used.
[0025] In this invention, the mixture to be separated and the mixed solvent can be continuously pumped into a high-efficiency centrifugal extractor for countercurrent extraction.
[0026] The system obtained after countercurrent extraction (a liquid-liquid two-phase system after thorough mixing and mass transfer) can be fed into a phase separation device (such as a settling tank or centrifuge). Due to the density difference and the excellent immiscibility of the solvent system, the system will rapidly separate into two layers: the upper layer (organic phase): a dark mixed solvent rich in the extracted dyes; the lower layer (aqueous phase): a deeply decolorized BHET aqueous solution. The two phases are precisely separated through interface detection and control.
[0027] In some embodiments of the present invention, the weakly polar aromatic solvent may include toluene and / or xylene.
[0028] Alcohol solvents may include at least one of n-octanol, isoamyl alcohol, and isooctanol.
[0029] In this embodiment, toluene and xylene are preferred as weakly polar aromatic solvents. Their boiling points (toluene 110°C, xylene 138-144°C) create a significant temperature difference with long-chain alcohol solvents (such as n-octanol 196°C), which is beneficial for subsequent distillation separation. Long-chain alcohol solvents such as n-octanol and isoamyl alcohol have low solubility in water due to their molecular chain length (C5-C8) (for example, n-octanol has a water solubility of only 0.1 g / 100 mL at 20°C), which can effectively reduce entrainment losses with the aqueous phase.
[0030] In some embodiments of the present invention, when the volume ratio of weakly polar aromatic solvent to alcohol solvent in the mixed solvent is (1-5):1, the content of alcohol solvent and weakly polar aromatic solvent is appropriate, and it has the best extraction effect on various dyes.
[0031] In one specific embodiment, during countercurrent extraction, the temperature is 50-75°C, and the volume ratio of the mixture to be separated to the mixed solvent is 1:(3-10). In this embodiment, controlling the temperature within the range of 50-75°C ensures sufficient dissolution of the dye by the solvent (for example, the solubility of disperse dyes in toluene at 65°C is 40% higher than at room temperature), while avoiding the risk of BHET degradation due to high temperatures (BHET begins to thermally decompose above 100°C). Preferably, the operating temperature of 60-65°C reduces the solvent viscosity to the optimal mass transfer conditions (the viscosity of toluene at 65°C is 0.56 mPa·s, a decrease of 35% compared to room temperature), while maintaining the high stability of BHET in the aqueous phase (half-life > 8h). When the volume ratio of the mixed solvent to the mixture to be separated (oil-water volume ratio O / A) is 1:(3-10), the contact area between the mixed solvent and the mixture to be separated is maximized, while avoiding phase separation difficulties caused by excessive mixed solvent.
[0032] In one specific embodiment, the alcoholysis reaction is carried out at a temperature of 180-220°C, which allows the alcoholysis reaction between ethylene glycol and PET to reach the optimal kinetic conditions.
[0033] In some implementations, zinc acetate catalyst can be added to the alcoholysis reaction to reduce energy consumption while avoiding excessive carbonization of PET fibers.
[0034] In one specific embodiment, the product obtained from the alcoholysis reaction is further subjected to M-stage thermal filtration to obtain a mixture to be separated, where M ≥ 1. In this embodiment, M-stage (e.g., 2-3 stages) thermal filtration can effectively remove mechanical impurities, catalyst residues, lint, or solid particles generated from the degradation of other non-PET components from the product obtained from the alcoholysis reaction.
[0035] Furthermore, the thermal filtration temperature is controlled at ≥100℃, preferably 120-150℃, so that the porosity of the filter medium (such as stainless steel sintered mesh) is maintained at 80-90%, while avoiding the crystallization and precipitation of BHET at low temperatures.
[0036] In one specific embodiment, the organic phase is further subjected to distillation. In this embodiment, solvents such as toluene and n-octanol in the organic phase are separated from dye residues through distillation. For example, using an atmospheric distillation column, the separation efficiency of toluene (boiling point 110°C) and n-octanol (boiling point 196°C) reaches over 98%. The recovered solvent can be directly returned to the extraction process for recycling, greatly reducing raw material consumption and operating costs. The dye residue discharged from the bottom of the distillation column is concentrated dye residue, which is small in volume and has a dye concentration that can reach 20-30 times that of the original alcoholysis liquid, facilitating subsequent harmless treatment or resource utilization.
[0037] In one specific embodiment, the process further includes crystallizing the aqueous phase to obtain dihydroxyethyl terephthalate (BHET). In this embodiment, the decolorized BHET aqueous solution is subjected to cooling crystallization (e.g., cooling from 75°C to 15°C) to precipitate crude BHET crystals, which are then further purified by recrystallization or washing. The crystallization process employs a gradient cooling method to control the BHET crystal growth rate at 0.1-0.3 mm / h, thereby obtaining a high-purity product (purity > 99.5%) with uniform particle size distribution (D50 = 150-200 μm), meeting the requirements for fiber polymerization. In some embodiments, the crystallization process can be carried out in a crystallizer.
[0038] The method of this invention is concise, operates at low temperatures, and consumes far less energy than high-temperature molecular distillation decolorization. The solvent can be recycled in a closed loop, resulting in virtually no wastewater discharge and no solid waste generation (except for concentrated dyes), making it environmentally friendly and in line with green chemistry principles. Furthermore, for BHET solutions derived from mixed waste textiles, this method can consistently achieve a total dye removal rate of over 98%, and reduce the solution color from over 1000 to below 30, far superior to the effects of single solvents (70-85%) and activated carbon adsorption (85-95%).
[0039] The present invention will be further described below with reference to specific embodiments.
[0040] Example 1
[0041] The method for preparing dihydroxyethyl terephthalate from waste textiles in this embodiment includes:
[0042] Waste military green polyester camouflage uniforms were collected, crushed, and washed. They were then subjected to alcoholysis with ethylene glycol at 190°C under the catalysis of zinc acetate. After the reaction was completed, the mixture was hot filtered at a temperature greater than 100°C to obtain a dark military green BHET / EG solution (the mixture to be separated).
[0043] 200L of the above mixture to be separated was heated in a mixing vessel and kept at a constant temperature of 65°C; toluene and n-octanol were mixed at a volume ratio of 3:1 to obtain a mixed solvent, totaling 40L (i.e., oil-water ratio O / A = 1:5); the preheated solution of the mixture to be separated and the mixed solvent were continuously fed into a three-stage centrifugal extractor for countercurrent extraction;
[0044] The mixture after countercurrent extraction was fed into a centrifuge for rapid phase separation; the upper organic phase, rich in dye, was subjected to laboratory distillation, with recoveries of toluene and n-octanol of 99.1% and 98.7%, respectively; the lower aqueous phase containing BHET was fed into a crystallizer for crystallization to obtain BHET.
[0045] Example 2
[0046] The method for preparing dihydroxyethyl terephthalate from waste textiles in this embodiment is basically the same as that in Example 1, except that:
[0047] The materials used are waste colored polyester clothing from diverse sources (a mixture of red, blue, black, and other colors), and the mixture to be separated is dark in color, almost black.
[0048] The volume of the mixture to be separated was 300 L, and the temperature of the countercurrent extraction was 60°C. A mixed solvent of 50 L (O / A = 1 : 6) was prepared by mixing toluene and isoamyl alcohol at a volume ratio of 2:1. Two-stage countercurrent extraction was used.
[0049] Example 3
[0050] The method for preparing dihydroxyethyl terephthalate from waste textiles in this embodiment is basically the same as that in Example 1, except that:
[0051] The volume ratio of weakly polar aromatic hydrocarbons to alcohols in the mixed solvent is 6:1.
[0052] Example 4
[0053] The method for preparing dihydroxyethyl terephthalate from waste textiles in this embodiment is basically the same as that in Example 1, except that:
[0054] Use 10L of mixed solvent (oil / water ratio of 1:20).
[0055] Comparative Example 1
[0056] The method for preparing dihydroxyethyl terephthalate from waste textiles in this comparative example is basically the same as that in Example 1, except that:
[0057] 40L of monotoluene was used as the extractant.
[0058] Comparative Example 2
[0059] The method for preparing dihydroxyethyl terephthalate from waste textiles in this comparative example is basically the same as that in Example 1, except that:
[0060] 40L of single n-octanol was used as the extraction solvent.
[0061] Comparative Example 3
[0062] The method for preparing dihydroxyethyl terephthalate from waste textiles in this comparative example is basically the same as that in Example 1, except that:
[0063] A direct extraction method was adopted, in which the preheated solution to be separated and the mixed solvent were mixed and mass transferred in a single extraction device. The extracted mixture was then fed into a centrifuge for rapid phase separation. The upper organic phase, rich in dye, was subjected to laboratory distillation, with recoveries of 89.3% and 91.1% for toluene and n-octanol, respectively. The lower aqueous phase containing BHET was then fed into a crystallizer for crystallization to obtain BHET.
[0064] Results and Analysis:
[0065] The colorimetry and color value of the aqueous phases containing BHET obtained in the examples and comparative examples were tested using a colorimeter and a colorimeter, respectively. The dye removal rate was calculated by comparing the colorimetry of the aqueous phase containing BHET with the colorimetry of the mixture to be separated: ((colorimetry of the mixture to be separated - colorimetry of the aqueous phase containing BHET) / colorimetry of the mixture to be separated * 100%). The BHET content in the aqueous phase containing BHET was analyzed by high-performance liquid chromatography (HPLC), and the BHET loss rate was calculated. The results are shown in Table 1.
[0066] Table 1
[0067]
[0068] Note: ① In Example 1, the color of the mixture to be separated was 720, L value was 65.3, a value was -4.6, and b value was 13.9; ② In Example 2, the color of the mixture to be separated was 1470, L value was 56.31, a value was 6.36, and b value was 16.90.
[0069] As can be seen from Table 1, the method for preparing dihydroxyethyl terephthalate from waste textiles in this embodiment of the invention exhibits superior decolorization depth and BHET selectivity. Specifically, by comparing the examples with Comparative Examples 1 and 2, it is clear that the single solvent is inferior to the mixed solvent system of this embodiment in terms of both decolorization depth and BHET selectivity. Examples 1 and 2 show that countercurrent extraction can further improve both decolorization depth and BHET selectivity, fully demonstrating the efficient synergy between the mixed solvent and countercurrent extraction in this method.
[0070] Furthermore, by comparing the results of Example 1 and Example 3, it can be seen that when the volume ratio of weakly polar aromatic hydrocarbons to alcohols is in the range of (1-5):1, the decolorization depth and the selectivity for BHET are better.
[0071] The results of comparing Example 1 and Example 4 show that when the oil-water ratio is in the range of 1:(3-10), the decolorization depth and the selectivity for BHET are better.
[0072] Therefore, this invention provides a mixed solvent synergistic extraction method specifically for decolorizing BHET solutions in the chemical recycling of waste textiles. This method, through binary solvent system design and process optimization, successfully removes complex dye systems while ensuring a high recovery rate of BHET monomers. This technology boasts significant advantages such as high decolorization efficiency, good selectivity, energy saving and environmental friendliness, and ease of industrialization. It provides a practical solution to overcome the technical bottlenecks in the high-value recycling of waste textiles and is expected to accelerate the transformation of China's textile industry towards a green, circular, and sustainable direction.
[0073] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described and shown above, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.
Claims
1. A method for preparing dihydroxyethyl terephthalate from waste textiles, characterized in that, include: Ethylene glycol is subjected to an alcoholysis reaction with polyester textiles to obtain a mixture to be separated. The mixture to be separated was subjected to N-stage countercurrent extraction using a mixed solvent including a weakly polar aromatic solvent and an alcohol solvent to separate the phases into an organic phase and an aqueous phase including dihydroxyethyl terephthalate. The alcohol solvent has ≥4 carbon atoms and ≥1 N.
2. The method according to claim 1, characterized in that, The weakly polar aromatic solvents include toluene and / or xylene.
3. The method according to claim 1 or 2, characterized in that, The alcohol solvent includes at least one of n-octanol, isoamyl alcohol, and isooctanol.
4. The method according to claim 3, characterized in that, In the mixed solvent, the volume ratio of the weakly polar aromatic solvent to the alcohol solvent is (1-5):
1.
5. The method according to claim 1, characterized in that, In the countercurrent extraction process, the temperature is 50-75℃, and the volume ratio of the mixed solvent to the mixture to be separated is 1:(3-10).
6. The method according to claim 1, characterized in that, The alcoholysis reaction is carried out at a temperature of 180-220℃.
7. The method according to claim 1 or 6, characterized in that, It also includes performing M-stage thermal filtration on the product obtained from the alcoholysis reaction to obtain a mixture to be separated, where M≥1.
8. The method according to claim 7, characterized in that, The temperature of the M-level thermal filtration process is ≥100℃.
9. The method according to claim 1, characterized in that, It also includes distillation of the organic phase.
10. The method according to claim 1 or 9, characterized in that, It also includes crystallizing the aqueous phase to obtain the dihydroxyethyl terephthalate.