Depolymerization recovery method of PET / PTT (polyethylene terephthalate / polytrimethylene terephthalate) blended fabric

Abstract

The invention belongs to the technical field of polyester recovery, and relates to a PET / PTT blended fabric depolymerization recovery method, a basic catalyst and dimethyl carbonate are adopted, and the basic catalyst is composed of a strong basic catalyst and a weak basic catalyst in a mass ratio of 2: 8-8: 2; the strong alkaline catalyst is one or more of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium methoxide, potassium methoxide, sodium ethoxide and potassium ethoxide, and the weak alkaline catalyst is one or more of potassium bicarbonate, sodium bicarbonate and sodium dihydrogen phosphate; the depolymerization recovery method disclosed by the invention is good in separation effect, the separation difficulty and the product loss are reduced, and the product purity is high.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polyester recycling, and relates to a depolymerization and recycling method for PET / PTT blended fabric. BACKGROUND

[0002] Polyethylene terephthalate (PET) and polytrimethylene terephthalate (PTT) are two common polyester materials. PET / PTT side-by-side fibers have a heat shrinkage difference that gives them a self-curling effect, unique softness and elasticity, and are widely used in high-performance textiles such as sportswear and high-end home textiles. With the growth of global textile consumption, PET / PTT fiber waste is difficult to recycle due to its complex homogeneous structure, causing serious environmental problems.

[0003] Conventional chemical recycling methods such as hydrolysis, ethylene glycolysis, and methanolysis all face difficulties in physical separation when recycling PET / PTT fibers. In the depolymerization process, the mixture of ethylene glycol and propylene glycol increases the complexity of separation, and linear oligomers (such as dimers or trimers) are easily formed due to the presence of chemical equilibrium. In addition, traditional alcoholysis reactions have a reverse reaction, resulting in low reaction efficiency.

[0004] To overcome the conversion limitations of traditional alcoholysis reactions, carbonic acid dimethyl ester depolymerization technology has been developed in recent years. Carbonic acid dimethyl ester, as a green solvent and trapping agent, reacts with the diols produced during depolymerization to form cyclic carbonates, driving the reaction in the positive direction. For example, patent CN117986114B discloses a method for depolymerizing PET using carbonic acid dimethyl ester as a solvent in the presence of N-methylpyrrolidone. The depolymerization products include ethylene glycol, dimethyl terephthalate, and ethylene carbonate. Dimethyl terephthalate can be crystallized and purified for separation, and ethylene carbonate can be used as a high-value chemical for battery electrolyte or solvent. However, in practice, the depolymerization products also include a large amount of linear oligomers. For example, as shown in the results of the literature (Depolymerization of Polyester Fibers with Dimethyl Carbonate-Aided Methanolysis [J]. ACS Materials Au, 2024, 4(3): 11.), the cyclization reaction dominated by the formation of ethylene carbonate is inhibited, and 2-hydroxyethyl methyl carbonate and 2,5-dioxane adipate dimethyl ester linear oligomers are also generated from ethylene glycol units in the later stage of the reaction. A large amount of linear oligomers can cause the depolymerization products to be highly viscous, interfering with the separation of the depolymerization products and resulting in poor separation efficiency.

[0005] When using carbonic acid dimethyl ester depolymerization technology to depolymerize PET / PTT blended fabric, the above problems still exist, which makes the separation of the products more difficult and reduces the separation efficiency. These limit the further industrial application of carbonic acid dimethyl ester depolymerization technology.

[0006] Therefore, it is of great significance to study a method for directional depolymerization and recycling of blended fabrics to solve the above problems. SUMMARY

[0007] The purpose of the present application is to solve the problems in the prior art and provide a depolymerization and recycling method for PET / PTT blended fabrics.

[0008] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] A depolymerization and recycling method for PET / PTT blended fabrics, which adopts an alkaline catalyst and dimethyl carbonate, the alkaline catalyst is composed of a strong alkaline catalyst and a weak alkaline catalyst with a mass ratio of 2:8 to 8:2; the strong alkaline catalyst is one or more of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium methoxide, potassium methoxide, sodium ethoxide and potassium ethoxide; and the weak alkaline catalyst is one or more of potassium bicarbonate, sodium bicarbonate and sodium dihydrogen phosphate.

[0010] When the dimethyl carbonate is used to depolymerize the PET / PTT blended fabric under the action of the strong alkaline catalyst, a nucleophilic ester exchange reaction occurs between the dimethyl carbonate and the glycol units to generate linear oligomers A with double carbonate end groups, and a thermodynamically driven end group backbite ring-opening reaction also occurs during the nucleophilic ester exchange reaction to generate ethylene carbonate; similarly, a nucleophilic ester exchange reaction occurs between the dimethyl carbonate and the propylene glycol units to generate linear oligomers B with double carbonate end groups, and a thermodynamically driven end group backbite ring-opening reaction also occurs during the nucleophilic ester exchange reaction to generate trimethylene carbonate.

[0011] The depolymerization product of the prior art has a high content of linear oligomers (10-15 wt%), and the present application has found that the root cause of this phenomenon is that the strong alkalinity of the strong alkaline catalyst inhibits the thermodynamically driven end group backbite ring-opening reaction, and the selectivity of the catalytic reaction is reduced.

[0012] To solve this problem, the present application regulates the alkalinity of the catalytic system by using a weak alkaline catalyst, which maintains the reaction system in a relatively mild alkaline range through an acid-base conjugate pair (such as H2PO4 - / HPO4 2- or HCO3 - / CO3 2- ), thereby reducing the alkalinity of the strong alkaline catalyst, avoiding uncontrolled chain cleavage, allowing the end group backbite ring-opening reaction to proceed normally, thereby promoting the generation of cyclic substances, ethylene carbonate and trimethylene carbonate, reducing the content of linear oligomers with double carbonate end groups in the product, and reducing the difficulty of separation and product loss.

[0013] The equation of the depolymerization reaction of dimethyl carbonate with the blended fabric under the action of a strong alkaline catalyst is as follows:

[0014]

[0015]

[0016] As a preferred technical solution:

[0017] The PET / PTT blended fabric and the dimethyl carbonate have a mass ratio of 1:1 to 1:3.

[0018] The alkaline catalyst has a mass of 0.1wt% to 1.0wt% of the mass of the PET / PTT blended fabric.

[0019] The PET / PTT blended fabric and the dimethyl carbonate have a mass ratio of 1:1 to 1:3.

[0020] The depolymerization reaction is divided into two stages; the first stage has a temperature of 150 to 170°C and a time of 30 to 60 minutes, and the second stage has a temperature of 160 to 200°C and a time of 60 to 120 minutes.

[0021] After obtaining the crude product and before separating different substances in the crude product according to the physical properties, dimethyl carbonate is added to the crude product for cooling. Since the reaction temperature of the depolymerization system is relatively high, part of the system is in the gas phase, making it difficult to continuously transport and separate the material. In addition, it directly affects the separation of components in the next separation unit. For example, part of the dimethyl terephthalate in the gas phase will mix with the dimethyl carbonate through the primary separation tower, causing the transport channel to be blocked. Therefore, cooling is needed. Using dimethyl carbonate for cooling will not introduce other component substances, reducing the separation pressure. In addition, adding dimethyl carbonate for cooling reduces the dynamic viscosity of the cooled material, which can be continuously transported by a pipeline.

[0022] The added amount of dimethyl carbonate added to the crude product is 20 to 80wt% of the mass of the crude product. Adding dimethyl carbonate in this range makes the dynamic viscosity of the material suitable, which can be directly transported to the separation tower by a metering pump. Excessive addition of dimethyl carbonate will increase the energy consumption of subsequent separation.

[0023] The depolymerization recycling method of the PET / PTT blended fabric as described above, the crude product comprises dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with bis-carbonate end groups; wherein the yield of ethylene carbonate is ≥90%, the yield of trimethylene carbonate is ≥90%; the yield of linear oligomer with bis-carbonate end groups is <5%. The yield of ethylene carbonate is calculated relative to the molar amount of ethylene glycol units in the raw material; the yield of trimethylene carbonate is calculated relative to the molar amount of propylene glycol units in the raw material; the yield of linear oligomer with bis-carbonate end groups is calculated relative to the total molar amount of propylene glycol units and ethylene glycol units in the raw material.

[0024] The specific steps for separating different substances in the crude product according to physical properties in the depolymerization recycling method of the PET / PTT blended fabric as described above are as follows:

[0025] (a) separating the solid phase component and the liquid phase component from the crude product by centrifugation;

[0026] (b) obtaining refined dimethyl terephthalate by flash evaporation or sublimation from the solid phase component;

[0027] (c) sequentially passing the liquid phase component through a first separation column, a second separation column and a third separation column to separate the substances in the liquid phase component; wherein dimethyl carbonate is separated and collected at the top of the first separation column, ethylene carbonate is separated and collected at the top of the second separation column, trimethylene carbonate is separated and collected at the top of the third separation column, the remaining dimethyl terephthalate is separated and collected from the side line of the third separation column, and linear oligomer with bis-carbonate end groups is separated and collected at the bottom of the third separation column;

[0028] The feed temperature of the first separation column is 80-120°C, the top temperature is 65-90°C, the bottom temperature is 100-150°C, the pressure is 1-10 kPa, the number of plates is 5-10, and the reflux ratio is 1.5-4;

[0029] The feed temperature of the second separation column is 140-170°C, the top temperature is 180-200°C, the bottom temperature is 220-250°C, the pressure is 0.1-0.5 atm, the reflux ratio is 5-10, and the number of plates is 20-30;

[0030] The feed temperature of the third separation column is 150-180°C, the top temperature is 190-210°C, the side line extraction temperature is 220-240°C, the bottom temperature is 240-260°C, the pressure is 0.05-0.2 atm, the reflux ratio is 8-20, and the number of plates is 30-40.

[0031] The liquid phase component contains dimethyl carbonate (boiling point 90°C), dimethyl terephthalate (boiling point 288°C, flash point 153°C); ethylene carbonate (boiling point 248°C, flash point 160°C), trimethylene carbonate (boiling point 255.2°C, flash point 148.6°C); and linear oligomer with double carbonate end groups (boiling point > 300°C). The primary separation tower is mainly used for separating dimethyl carbonate in the system, that is, dimethyl carbonate is collected at the top of the tower at a lower temperature; other components are transported from the bottom of the tower to the secondary separation tower, and under vacuum conditions, the boiling points of the components are reduced, and the boiling point of ethylene carbonate is lower than that of other components (7-52°C), at this time, the separated component at the top of the tower is ethylene carbonate, and the mixture of trimethylene carbonate, dimethyl terephthalate and linear oligomer with double carbonate end groups is at the bottom of the tower; the mixture from the bottom of the secondary separation tower enters the tertiary separation tower under vacuum conditions (0.05-0.2 atm), and the boiling points are reduced, and trimethylene carbonate has the lowest boiling point (255.2°C), which is easy to evaporate from the top of the tower, dimethyl terephthalate (288°C) is the intermediate component, which is extracted through a side line; and the linear oligomer with double carbonate end groups mixture (> 300°C) remains at the bottom.

[0032] The purity of the separated dimethyl terephthalate is ≥98%, the purity of the separated ethylene carbonate is 98-99.99%, and the purity of the separated trimethylene carbonate is 98-99.5%.

[0033] The PET / PTT blended fabric needs to be subjected to soaping and drying and crushing treatment before being blended with other substances; after soaping and drying, the water content of the PET / PTT blended fabric is <0.5wt%, and the size of the crushed PET / PTT blended fabric is 20×20-80×80mm. 2 .

[0034] The mass ratio of the PET fiber to the PTT fiber in the PET / PTT blended fabric is 90:10-50:50.

[0035] Advantages:

[0036] The PET / PTT blended fabric depolymerization and recovery method has good separation effect, reduces separation difficulty and product loss, and has high product purity. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 A depolymerization and recovery schematic diagram of a PET / PTT blended fabric;

[0038] Wherein, 1-dimethyl carbonate, 2-ethylene carbonate, 3-trimethylene carbonate, 4-dimethyl terephthalate, 5-first separation tower, 6-second separation tower, 7-third separation tower, 8-reboiler, 9-linear oligomer. DETAILED DESCRIPTION

[0039] The application will be further described below in connection with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not used to limit the scope of the application. Furthermore, it should be understood that after reading the content of the application, those skilled in the art can make various modifications or changes to the application, and these equivalent forms also fall within the scope of the appended claims.

[0040] The performance indicators in the examples and comparative examples of the application involve the following test methods:

[0041] The calculation formula of the yield of ethylene carbonate, the yield of trimethylene carbonate, and the yield of linear oligomer with double carbonate end groups is as follows:

[0042] ;

[0043] ;

[0044] ;

[0045] Wherein, m0 is the mass of the blended fabric (g), m1 is the mass of ethylene carbonate (g), m2 is the mass of trimethylene carbonate (g), m3 is the mass of linear oligomer A (g), m4 is the mass of linear oligomer B (g), σ is the proportion of polyester in the blended fabric, N1 is the molar mass of the PET repeating unit (mol / g), N2 is the molar mass of the PTT repeating unit (mol / g), N3 is the molar mass of ethylene carbonate (mol / g), N4 is the molar mass of trimethylene carbonate (mol / g), N3' is the molar mass of linear oligomer A (mol / g), and N4' is the molar mass of linear oligomer B (mol / g).

[0046] The purity of trimethylene carbonate and the purity of ethylene carbonate are tested by gas chromatography, the gas column is selected as TraceGOLD TG-35MS (30m*0.25mm*0.25μm, intermediate polarity phase), the injection port temperature is 260℃, the split ratio is 20:1, the detector temperature is 270℃, the column temperature is 100℃, holding for 0min, programmed temperature rising 10℃ / min to 120℃, holding for 0min, programmed temperature rising 20℃ / min to 220℃, holding for 6min, the carrier gas is high-purity nitrogen, the flow rate is 5mL / min, and the injection amount is 0.4μL. The purity fitting is performed by the internal standard method, and the contents of trimethylene carbonate and ethylene carbonate are determined by the internal standard curve method.

[0047] The purity of dimethyl terephthalate: using an Agilent gas chromatograph, a hydrogen flame ionization detector (FID) and a chromatographic data processing workstation are configured, the purity of the collected dimethyl terephthalate is quantitatively analyzed, and the purity fitting is carried out by the internal standard method. The dimethyl terephthalate content is determined by the internal standard curve method, dipropyl phthalate is selected as the internal standard substance, chloroform is used as the solvent, and the peak area is quantified. Test conditions: carrier gas is high-purity nitrogen (≥99.95%), split ratio is 15:1, chromatographic injection port temperature is set to 280°C, column temperature is 60°C, total flow rate is 3.7 mL / min, and chromatographic column flow rate is 0.7 mL / min.

[0048] Moisture content test: the moisture content of the textile is tested according to GB / T 9995-1997.

[0049] The PET / PTT blended fabric of each of the following examples and comparative examples is a fabric woven from PET / PTT parallel fibers.

[0050] Example 1

[0051] A depolymerization and recycling method of a PET / PTT blended fabric, the specific steps are as follows:

[0052] (1) The PET / PTT blended fabric with a mass ratio of PET to PTT of 90:10 is subjected to soaping drying and crushing treatment; after soaping drying, the moisture content of the PET / PTT blended fabric is 0.2wt%, and the size of the PET / PTT blended fabric after crushing treatment is 20x20mm. 2 ;

[0053] (2) Sodium hydroxide, dimethyl carbonate, potassium bicarbonate and the PET / PTT blended fabric are jointly added into a first depolymerization kettle with a temperature of 150°C for reaction for 60min, then the materials in the first depolymerization kettle are transported into a second depolymerization kettle with a temperature of 160°C for continuous reaction for 120min to obtain a crude product, and then dimethyl carbonate is added for cooling; wherein the mass ratio of the PET / PTT blended fabric to the first added dimethyl carbonate is 1:1; the mass ratio of sodium hydroxide to potassium bicarbonate is 5:5; the total mass of sodium hydroxide and potassium bicarbonate is 0.3wt% of the mass of the PET / PTT blended fabric;

[0054] The crude product includes dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and double carbonate end group linear oligomer; wherein the yield of ethylene carbonate is 90%, the yield of trimethylene carbonate is 90%; the yield of double carbonate end group linear oligomer is 4.1%; the addition amount of dimethyl carbonate added to the crude product is 20wt% of the mass of the crude product;

[0055] (3) Separate different substances in the crude product based on their physical properties;

[0056] (3.1) The crude product is separated into solid and liquid phases by centrifugation;

[0057] (3.2) The solid component is flash-evaporated or sublimated to obtain purified dimethyl terephthalate;

[0058] (3.3) such as Figure 1 As shown, the liquid phase component is sequentially passed through a primary separation column 5, a secondary separation column 6, and a tertiary separation column 7 to separate the various substances in the liquid phase component. Specifically, dimethyl carbonate 1 is separated and collected at the top of the primary separation column 5, ethylene carbonate 2 is separated and collected at the top of the secondary separation column 6, trimethylene carbonate 3 is separated and collected at the top of the tertiary separation column 7, residual dimethyl terephthalate 4 is separated and collected from the side stream of the tertiary separation column 7, and linear oligomer 9 with dicarbonate end groups is separated and collected from the bottom of the tertiary separation column 7. The liquid phase component in the separation column is vaporized and refluxed through a reboiler 8.

[0059] The feed temperature of the primary separation tower is 80°C, the top temperature is 65°C, the bottom temperature is 100°C, the pressure is 1 kPa, the number of trays is 5, and the reflux ratio is 1.5.

[0060] The feed temperature of the secondary separation tower is 140°C, the top temperature is 180°C, the bottom temperature is 220°C, the pressure is 0.1 atm, the number of trays is 20, and the reflux ratio is 5.

[0061] The feed temperature of the three-stage separation tower is 150°C, the top temperature is 190°C, the side stream temperature is 220°C, the bottom temperature is 240°C, the pressure is 0.05 atm, the number of trays is 30, and the reflux ratio is 8.

[0062] The purity of the dimethyl terephthalate, ethylene carbonate, and trimethylene carbonate obtained in the final separation was 99.5%.

[0063] Comparative Example 1

[0064] A method for depolymerization and recycling of PET / PTT blended fabrics is basically the same as in Example 1, except that the mass ratio of sodium hydroxide to potassium bicarbonate in step (2) is 9:2.

[0065] The yields of ethylene carbonate and trimethylene carbonate were 62.0% and 68.4%, respectively; the yields of linear oligomers with dicarbonate end groups were 15.7%.

[0066] The purity of the dimethyl terephthalate obtained after the final separation is 98.5%, the purity of the ethylene carbonate obtained after the separation is 92.9%, and the purity of the trimethylene carbonate obtained after the separation is 91.6%.

[0067] Comparing the comparative example 1 and the example 1, it can be found that the yield of the ethylene carbonate and the trimethylene carbonate is significantly reduced, the content of the linear oligomer is greatly increased, and the purity of the ethylene carbonate and the trimethylene carbonate is reduced, which is mainly caused by the fact that the addition amount of the potassium bicarbonate is too small, the alkalinity of the reaction system is reduced, and the alkalinity of the reaction system is still strong, which leads to the increase of the linear oligomer in the depolymerization product, the yield of the ethylene carbonate and the trimethylene carbonate is greatly lost, and a small amount of the linear oligomer existing in the separation process is mixed into the product, which leads to the decrease of the purity.

[0068] Comparative example 2

[0069] A depolymerization and recycling method of a PET / PTT blended fabric, which is basically the same as the example 1, and the only difference is that the mass ratio of the sodium hydroxide to the potassium bicarbonate in the step (2) is 2:9.

[0070] The yield of the ethylene carbonate is 72%, the yield of the trimethylene carbonate is 77%, and the yield of the linear oligomer with the double carbonate end group is 4.4%;

[0071] The purity of the dimethyl terephthalate obtained after the final separation is 99.0%, the purity of the ethylene carbonate obtained after the separation is 99.0%, and the purity of the trimethylene carbonate obtained after the separation is 99.5%.

[0072] Comparing the comparative example 2 and the example 1, it can be found that the yield of the ethylene carbonate and the trimethylene carbonate is significantly reduced, which is mainly caused by the fact that the alkalinity of the reaction system is reduced too much, the chain breaking reaction of the nucleophilic attack of the dimethyl carbonate to the ester bond is weakened, and the yield is greatly reduced.

[0073] Example 2

[0074] A depolymerization and recycling method of a PET / PTT blended fabric, and the specific steps are as follows:

[0075] (1) The PET / PTT blended fabric with the mass ratio of the PET to the PTT being 80:20 is subjected to a soaping and drying treatment and a crushing treatment; after the soaping and drying treatment, the water content of the PET / PTT blended fabric is 0.3wt%, and the size of the PET / PTT blended fabric after the crushing treatment is 30x40mm 2 ;

[0076] (2) potassium carbonate, dimethyl carbonate, sodium bicarbonate and PET / PTT blended fabric are jointly added into a first depolymerization kettle with a temperature of 155°C to react for 50 minutes, then the materials in the first depolymerization kettle are transported into a second depolymerization kettle with a temperature of 170°C to continue to react for 100 minutes to obtain a crude product, and then dimethyl carbonate is added for cooling; wherein the mass ratio of PET / PTT blended fabric to the first added dimethyl carbonate is 1:1.5; the mass ratio of potassium carbonate to sodium bicarbonate is 2:8; the total mass of potassium carbonate and sodium bicarbonate is 0.1wt% of the mass of PET / PTT blended fabric;

[0077] The crude product comprises dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with double carbonate end groups; wherein the yield of ethylene carbonate is 91%, the yield of trimethylene carbonate is 92%; the yield of linear oligomer with double carbonate end groups is 4.2%; the added amount of dimethyl carbonate added into the crude product is 30wt% of the mass of the crude product;

[0078] (3) different substances in the crude product are separated according to physical properties;

[0079] (3.1) the crude product is separated into solid phase components and liquid phase components by centrifugation;

[0080] (3.2) the solid phase components are obtained by flash evaporation or sublimation to obtain fine dimethyl terephthalate;

[0081] (3.3) the liquid phase components are sequentially subjected to a first separation tower, a second separation tower and a third separation tower, so that each substance in the liquid phase components is separated; wherein dimethyl carbonate is separated and collected at the top of the first separation tower, ethylene carbonate is separated and collected at the top of the second separation tower, trimethylene carbonate is separated and collected at the top of the third separation tower, residual dimethyl terephthalate is separated and collected at the side line of the third separation tower, and linear oligomer with double carbonate end groups is separated and collected at the bottom of the third separation tower;

[0082] The feed temperature of the first separation tower is 90°C, the top temperature is 70°C, the bottom temperature is 110°C, the pressure is 2kPa, the number of plates is 6, and the reflux ratio is 2;

[0083] The feed temperature of the second separation tower is 145°C, the top temperature is 185°C, the bottom temperature is 225°C, the pressure is 0.2atm, the number of plates is 24, and the reflux ratio is 6;

[0084] The feed temperature of the third separation tower is 155°C, the top temperature is 195°C, the side line extraction temperature is 225°C, the bottom temperature is 245°C, the pressure is 0.1atm, the number of plates is 32, and the reflux ratio is 10;

[0085] The purity of the dimethyl terephthalate obtained by the final separation is 99.5%, the purity of the ethylene carbonate obtained by the separation is 98%, and the purity of the trimethylene carbonate obtained by the separation is 98%.

[0086] Example 3

[0087] A depolymerization recycling method of a PET / PTT blended fabric, and the specific steps are as follows:

[0088] (1) The PET / PTT blended fabric with a mass ratio of PET to PTT of 70:30 is subjected to soaping drying and crushing treatment; after soaping drying, the water content of the PET / PTT blended fabric is 0.2wt%, and the size of the PET / PTT blended fabric after crushing treatment is 60x60mm 2 ;

[0089] (2) Sodium methoxide, dimethyl carbonate, sodium dihydrogen phosphate and the PET / PTT blended fabric are jointly added into a first depolymerization kettle with a temperature of 160℃ for reaction for 40min, then the materials in the first depolymerization kettle are transported into a second depolymerization kettle with a temperature of 180℃ for continuous reaction for 90min to obtain a crude product, and then dimethyl carbonate is added for cooling; wherein the mass ratio of the PET / PTT blended fabric to the first added dimethyl carbonate is 1:2; the mass ratio of sodium methoxide to sodium dihydrogen phosphate is 6:4; the total mass of sodium methoxide and sodium dihydrogen phosphate is 0.5wt% of the mass of the PET / PTT blended fabric;

[0090] The crude product includes dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with bis-carbonate end groups; wherein the yield of ethylene carbonate is 91%, the yield of trimethylene carbonate is 93%, and the yield of linear oligomer with bis-carbonate end groups is 2%; the added amount of dimethyl carbonate added into the crude product is 80wt% of the mass of the crude product;

[0091] (3) Different substances in the crude product are separated according to the physical properties;

[0092] (3.1) The crude product is separated into solid phase components and liquid phase components by centrifugation;

[0093] (3.2) The solid phase components are obtained by flash evaporation or sublimation to obtain fine dimethyl terephthalate;

[0094] (3.3) sequentially passing the liquid phase components through a first separation column, a second separation column and a third separation column, so as to separate each substance of the liquid phase components; wherein dimethyl carbonate is separated and collected at the top of the first separation column, ethylene carbonate is separated and collected at the top of the second separation column, trimethylene carbonate is separated and collected at the top of the third separation column, residual dimethyl terephthalate is separated and collected at the side line of the third separation column, and the linear oligomer with double carbonate end groups is separated and collected at the bottom of the third separation column;

[0095] The feed temperature of the first separation column is 100°C, the top temperature is 75°C, the bottom temperature is 120°C, the pressure is 5 kPa, the plate number is 7, and the reflux ratio is 2.5;

[0096] The feed temperature of the second separation column is 150°C, the top temperature is 190°C, the bottom temperature is 230°C, the pressure is 0.3 atm, the plate number is 28, and the reflux ratio is 7;

[0097] The feed temperature of the third separation column is 160°C, the top temperature is 200°C, the side line extraction temperature is 230°C, the bottom temperature is 250°C, the pressure is 0.15 atm, the plate number is 36, and the reflux ratio is 14;

[0098] The purity of the dimethyl terephthalate obtained after final separation is 99.9%, the purity of the ethylene carbonate obtained after separation is 99.9%, and the purity of the trimethylene carbonate obtained after separation is 99%.

[0099] Example 4

[0100] A depolymerization and recycling method of a PET / PTT blended fabric, and the specific steps are as follows:

[0101] (1) The PET / PTT blended fabric with a mass ratio of PET to PTT of 60:40 is subjected to soaping and drying and crushing treatment; after soaping and drying, the water content of the PET / PTT blended fabric is 0.4wt%, and the size of the PET / PTT blended fabric after crushing treatment is 50×50mm 2 ;

[0102] (2) Potassium ethoxide, dimethyl carbonate, potassium bicarbonate and the PET / PTT blended fabric are jointly added into a first depolymerization kettle with a temperature of 165°C for reaction for 35min, then the materials in the first depolymerization kettle are transported into a second depolymerization kettle with a temperature of 190°C for continuous reaction for 80min to obtain a crude product, and then dimethyl carbonate is added for cooling; wherein the mass ratio of the PET / PTT blended fabric to the first added dimethyl carbonate is 1:2.5; the mass ratio of potassium ethoxide to potassium bicarbonate is 4:6; and the total mass of potassium ethoxide and potassium bicarbonate is 0.7wt% of the mass of the PET / PTT blended fabric;

[0103] The crude product comprises dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with bis-carbonate end groups; the yield of ethylene carbonate is 94%, the yield of trimethylene carbonate is 90%, and the yield of linear oligomer with bis-carbonate end groups is 3.7%; the added amount of dimethyl carbonate added to the crude product is 50wt% of the mass of the crude product;

[0104] (3) Separating different substances in the crude product according to physical properties;

[0105] (3.1) Separating the solid phase component and the liquid phase component from the crude product by centrifugation;

[0106] (3.2) Obtaining refined dimethyl terephthalate by flash evaporation or sublimation from the solid phase component;

[0107] (3.3) Separating the liquid phase component by sequentially passing through a first separation tower, a second separation tower and a third separation tower, so as to separate each substance in the liquid phase component; dimethyl carbonate is separated and collected at the top of the first separation tower, ethylene carbonate is separated and collected at the top of the second separation tower, trimethylene carbonate is separated and collected at the top of the third separation tower, the remaining dimethyl terephthalate is separated and collected from the side line of the third separation tower, and the linear oligomer with bis-carbonate end groups is separated and collected at the bottom of the third separation tower;

[0108] The feed temperature of the first separation tower is 110°C, the top temperature is 80°C, the bottom temperature is 130°C, the pressure is 8kPa, the number of plates is 8, and the reflux ratio is 3;

[0109] The feed temperature of the second separation tower is 160°C, the top temperature is 195°C, the bottom temperature is 240°C, the pressure is 0.4atm, the number of plates is 22, and the reflux ratio is 8;

[0110] The feed temperature of the third separation tower is 170°C, the top temperature is 205°C, the side line extraction temperature is 235°C, the bottom temperature is 255°C, the pressure is 0.2atm, the number of plates is 38, and the reflux ratio is 15;

[0111] The purity of the dimethyl terephthalate obtained after final separation is 99.9%, the purity of the ethylene carbonate obtained after separation is 99.99%, and the purity of the trimethylene carbonate obtained after separation is 99%.

[0112] Example 5

[0113] A depolymerization recycling method for a PET / PTT blended fabric, the specific steps are as follows:

[0114] (1) PET / PTT blended fabric with mass ratio of PET to PTT of 50:50 is subjected to soaping drying and breaking treatment; the water content of the PET / PTT blended fabric after soaping drying is 0.1wt%, and the size of the PET / PTT blended fabric after breaking treatment is 20x20mm 2 ;

[0115] (2) A mixture of sodium hydroxide and potassium hydroxide (mass ratio 1:1), a mixture of dimethyl carbonate, sodium bicarbonate and sodium dihydrogen phosphate (mass ratio 1:1), and PET / PTT blended fabric are added into a first depolymerization kettle with a temperature of 170°C for reaction for 30 min, then the materials in the first depolymerization kettle are transported into a second depolymerization kettle with a temperature of 200°C for further reaction for 60 min to obtain a crude product, and then dimethyl carbonate is added for cooling; wherein the mass ratio of the PET / PTT blended fabric to the first added dimethyl carbonate is 1:3; the mass ratio of the mixture of sodium hydroxide and potassium hydroxide to the mixture of sodium bicarbonate and sodium dihydrogen phosphate is 8:2; the total mass of the mixture of sodium hydroxide and potassium hydroxide and the mixture of sodium bicarbonate and sodium dihydrogen phosphate is 1.0wt% of the mass of the PET / PTT blended fabric;

[0116] The crude product comprises dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with bis-carbonate end groups; wherein the yield of ethylene carbonate is 93%, the yield of trimethylene carbonate is 95%, and the yield of linear oligomer with bis-carbonate end groups is 2.2%; the added amount of dimethyl carbonate added into the crude product is 40wt% of the mass of the crude product;

[0117] (3) Different substances in the crude product are separated according to physical properties;

[0118] (3.1) The crude product is subjected to centrifugal separation to obtain solid phase component and liquid phase component;

[0119] (3.2) The solid phase component is subjected to flash evaporation or sublimation to obtain refined dimethyl terephthalate;

[0120] (3.3) The liquid phase component is sequentially subjected to a first separation column, a second separation column and a third separation column to separate different substances in the liquid phase component; wherein dimethyl carbonate is separated and collected at the top of the first separation column, ethylene carbonate is separated and collected at the top of the second separation column, trimethylene carbonate is separated and collected at the top of the third separation column, residual dimethyl terephthalate is separated and collected at the side line of the third separation column, and linear oligomer with bis-carbonate end groups is separated and collected at the bottom of the third separation column;

[0121] The feed temperature of the first separation column is 120°C, the top temperature is 90°C, the bottom temperature is 150°C, the pressure is 10kPa, the plate number is 10, and the reflux ratio is 4.

[0122] The feed temperature of the second separation column was 170°C, the overhead temperature was 200°C, the bottom temperature was 250°C, the pressure was 0.5 atm, the number of plates was 30, and the reflux ratio was 10;

[0123] The feed temperature of the third separation column was 180°C, the overhead temperature was 210°C, the side draw temperature was 240°C, the bottom temperature was 260°C, the pressure was 0.05 atm, the number of plates was 40, and the reflux ratio was 20;

[0124] The purity of the dimethyl terephthalate obtained by the final separation was 99.9%, the purity of the ethylene carbonate obtained by the separation was 99.99%, and the purity of the trimethylene carbonate obtained by the separation was 99.5%.

Claims

1. A depolymerization recycling method of a PET / PTT blended fabric using an alkaline catalyst and dimethyl carbonate, characterized by, The basic catalyst is composed of a strong basic catalyst and a weak basic catalyst in a mass ratio of 2:8-8:2; the strong basic catalyst is one or more of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium methoxide, potassium methoxide, sodium ethoxide and potassium ethoxide; and the weak basic catalyst is one or more of potassium bicarbonate, sodium bicarbonate and sodium dihydrogen phosphate.

2. The depolymerization recycling method of a PET / PTT blended fabric according to claim 1, characterized in that, The mass ratio of the PET / PTT blended fabric to dimethyl carbonate is 1:1-1:

3.

3. The depolymerization recycling method of a PET / PTT blended fabric according to claim 1, characterized in that, The mass of the basic catalyst is 0.1wt%-1.0wt% of the mass of the PET / PTT blended fabric.

4. The depolymerization recycling method of a PET / PTT blended fabric according to any one of claims 1 to 3, characterized in that, The specific process is as follows: the basic catalyst, dimethyl carbonate and the PET / PTT blended fabric are mixed, then a depolymerization reaction is performed to obtain a crude product, and then different substances in the crude product are separated according to physical properties.

5. The depolymerization recycling method of a PET / PTT blended fabric according to claim 4, characterized in that, The depolymerization reaction is divided into two stages; the temperature of the first stage is 150-170°C, and the time is 30-60min; the temperature of the second stage is 160-200°C, and the time is 60-120min.

6. The depolymerization recycling method of a PET / PTT blended fabric according to claim 4, characterized in that, After obtaining the crude product, dimethyl carbonate is added to the crude product for cooling before separating different substances in the crude product according to physical properties.

7. The depolymerization recycling method of a PET / PTT blended fabric according to claim 6, characterized in that, The added amount of dimethyl carbonate added to the crude product is 20-80wt% of the mass of the crude product.

8. The depolymerization recycling method of a PET / PTT blended fabric according to claim 4, characterized in that, The crude product includes dimethyl terephthalate, dimethyl carbonate, ethylene carbonate, trimethylene carbonate and linear oligomer with bis-carbonate end groups; the yield of ethylene carbonate is ≥90%, the yield of trimethylene carbonate is ≥90%, and the yield of linear oligomer with bis-carbonate end groups is <5%.

9. The depolymerization recycling method of a PET / PTT blended fabric according to claim 8, characterized in that, The specific steps of separating different substances in the crude product according to physical properties are as follows: (a) centrifuging the crude product to separate solid-phase components and liquid-phase components; (b) obtaining dimethyl terephthalate by flash evaporation or sublimation of the solid-phase components; (c) sequentially passing the liquid-phase components through a first-stage separation tower, a second-stage separation tower and a third-stage separation tower to separate the substances in the liquid-phase components; wherein dimethyl carbonate is separated and collected at the top of the first-stage separation tower, ethylene carbonate is separated and collected at the top of the second-stage separation tower, trimethylene carbonate is separated and collected at the top of the third-stage separation tower, residual dimethyl terephthalate is separated and collected from the side line of the third-stage separation tower, and linear oligomer with bis-carbonate end groups is separated and collected at the bottom of the third-stage separation tower; the feed temperature of the first-stage separation tower is 80-120°C, the top temperature is 65-90°C, the bottom temperature is 100-150°C, the pressure is 1-10kPa, the number of plates is 5-10, and the reflux ratio is 1.5-4; the feed temperature of the second-stage separation tower is 140-170°C, the top temperature is 180-200°C, the bottom temperature is 220-250°C, the pressure is 0.1-0.5atm, the reflux ratio is 5-10, and the number of plates is 20-30; the feed temperature of the third-stage separation tower is 150-180°C, the top temperature is 190-210°C, the side line extraction temperature is 220-240°C, the bottom temperature is 240-260°C, the pressure is 0.05-0.2atm, the reflux ratio is 8-20, and the number of plates is 30-40.

10. The depolymerization recycling method of a PET / PTT blended fabric according to claim 9, characterized in that, The purity of the separated dimethyl terephthalate is ≥98%, the purity of the separated ethylene carbonate is 98-99.99%, and the purity of the separated trimethylene carbonate is 98-99.5%. The purity of the separated dimethyl terephthalate is ≥98%, the purity of the separated ethylene carbonate is 98-99.99%, and the purity of the separated trimethylene carbonate is 98-99.5%. The purity of the separated dimethyl terephthalate is ≥98%, the purity of the separated ethylene carbonate is 98-99.99%, and the purity of

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