PET (Polyethylene Terephthalate) recovery process

Abstract

The invention relates to the technical field of plastic recovery, in particular to a PET (Polyethylene Terephthalate) recovery process, which comprises the steps of melting and primary devolatilization, coarse filtration, liquid phase tackifying and enhanced melt devolatilization, melt refining, pelletizing and intermediate product preparation, solid phase polycondensation, efficient nitrogen purification and product cooling. Liquid-phase tackifying is introduced as a core hub, deep removal of impurities and precise homogenization of viscosity in a molten state are achieved, an optimized NPU system is combined, and finally the high-quality, low-acetaldehyde and ultra-clean food-grade PET slices are produced. The scheme not only ensures that the product is comparable with a raw material on key indexes such as viscosity, purity, color and luster, but also greatly reduces the production cost through process optimization, enhances the market competitiveness of regenerated PET, and provides an industrial solution with both technical advancement and economic feasibility for plastic recycling economy.

Description

Technical Field

[0001] This invention relates to the field of plastic recycling technology, specifically a PET recycling process. Background Technology

[0002] The PET bottle-to-bottle recycling process involves sorting, crushing, and deep cleaning waste PET beverage bottles to remove impurities. The bottles are then melt-extruded and granulated, and subjected to a high-temperature, high-vacuum solid-state polycondensation (SSP) reaction to improve their intrinsic viscosity and purity. The final product is recycled PET chips that meet food safety standards and are used to manufacture new bottles in a closed-loop cycle.

[0003] Currently, there are some problems with physical recycling of PET bottles. The mainstream process includes: PET clean sheets → vacuum impurity removal → screw melting → filtration → pelletizing → solid phase thickening (SSP) → finished product, which relies on vacuum impurity removal machine + SSP for impurity removal; or PET clean sheets → screw melting → filtration → pelletizing → SSP → finished product, which relies only on SSP for impurity removal.

[0004] For example, Chinese Patent CN117597222A discloses a method for recycling packaging waste made of PET or polyolefin (PO) and at least a second article made of PET or PO, wherein the packaging waste is referred to as the first article and can be decolorized according to the "lock and key" principle. The method includes the following steps: (a) pre-sorting the first and second articles, (b) pre-washing the first and second articles, (c) pulverizing the articles to form flakes, (d) washing the articles in a first washing step or a strong washing step, (e) dehydrating and drying the flakes in a first drying step, (f) sorting the flakes, (g) extrusion, (h) solid-state polycondensation (SSP) in the case of PET waste, or (i) purification in the case of PO waste.

[0005] To concentrate the first product, it is separated from the second product by optical sorting, and after separation, it is further decolorized in decolorization step (j) using a reagent called a "key". After step (j), the first decolorized product is temporarily stored in a storage tank (m). After storage, the first product is fed into separate steps (g) and (h) or (i).

[0006] It can be seen that the above patent is a representative process for PET bottle-to-bottle recycling using solid-phase thickening.

[0007] However, including the aforementioned patents, the impurity removal process in existing technologies is carried out in a solid state. Although a certain impurity removal effect is achieved, it is still not thorough enough (such as odor substances and acetaldehyde precursors) and the removal efficiency is limited.

[0008] Secondly, fluctuations in the intrinsic viscosity (IV) of raw material fragments are directly transmitted to the SSP process, resulting in poor viscosity stability of the final product (IV fluctuations often exceed ±0.02 dl / g).

[0009] Meanwhile, adding color masterbatch before melting results in short mixing time and uneven dispersion, affecting the color consistency of the product; the melt filtration accuracy is usually only 40-60μm, which is insufficient, leading to more black spots and making it difficult to meet the requirements of high-end applications.

[0010] To address the above problems, this invention provides a PET recycling process. Summary of the Invention

[0011] The purpose of this invention is to provide a PET recycling process to solve the problems mentioned in the background art.

[0012] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a PET recycling process, comprising the following steps:

[0013] S1, Melting and Primary Deviation: Clean PET fragments are melted through a twin-screw extruder. The twin-screw extruder is equipped with at least three vacuum extraction ports along the material direction. Multi-stage vacuum devolatilization is carried out during the melting process to remove water vapor and most of the volatile small molecule organic matter to obtain the melt.

[0014] S2, coarse filtration, the melt passes through the filter screen of the multi-station automatic screen changer to intercept mechanical impurities and unmelted materials;

[0015] S3, Liquid phase thickening and enhanced melt devolatilization: The melt enters a dedicated liquid phase thickening reactor. Under high vacuum and high temperature conditions, the melt is continuously stretched into a film by a stirrer at 4-6 rpm to renew the surface. The melt stays for 30-50 minutes, and the resulting melt IV is homogenized and increased to 0.65-0.78 dL / g.

[0016] S4, melt refining: After thickening, the melt is pressurized by a gear pump and then filtered through a candle wick precision filter with a filtration accuracy of 10-20μm to completely remove fine impurities and obtain an ultra-clean melt.

[0017] S5, pelletizing and intermediate product preparation: The ultra-clean melt enters the pelletizing unit, is cast into strips by the casting head, cooled in a water tank, cut by the pelletizer, dehydrated and dried and vibrated sieve to obtain cylindrical, uniform basic slices;

[0018] S6, solid-state polycondensation: The base slices enter the continuous SSP system. First, in a hot nitrogen fluidized bed, the amorphous slices are heated to above the glass transition temperature to increase their crystallinity to >35% and prevent subsequent adhesion. Then, in a vertical preheater, the slices are preheated to 200-210℃ using heated circulating nitrogen. The preheated slices are then placed in a two-stage series reactor and undergo polycondensation at 210-220℃ under high-purity nitrogen purging for 10-20 hours until the slices reach the target bottle-grade requirements (IV).

[0019] S7, high-efficiency nitrogen purification, the reaction tail gas containing moisture, acetaldehyde and ethylene glycol discharged during the operation of the two-stage series reactor enters the high-efficiency NPU system for purification and recovery.

[0020] S8, Product Cooling: The hot PET chips after the reaction are cooled to below 60°C by filtered ambient air in the cooling hopper, thus obtaining the finished PET chips.

[0021] In a more optimized manner, in step S1, the melting temperature is 265-275℃, and the three-stage vacuum extraction port includes,

[0022] First stage: Removal of volatile organic compounds at atmospheric pressure;

[0023] Level 2: Absolute pressure ≤ 15 kPa;

[0024] Level 3: Absolute pressure ≤ 5 kPa.

[0025] In a more optimized manner, in step S2, the filter screen precision is 40 μm, and the resulting melt IV is 0.60-0.63 dL / g.

[0026] In a more optimized manner, in step S3, the high vacuum and high temperature environment is an absolute pressure of 100-500 Pa and a temperature of 280-285℃.

[0027] In a more optimized manner, the high vacuum and high temperature environment was 200 Pa absolute pressure, 283 °C temperature, and 5 rpm stirring rate. The resulting melt IV was homogenized and increased to 0.72 ± 0.01 dl / g.

[0028] In a more optimized manner, in step S6, the target bottle grade requires an IV of 0.80-0.84 dL / g.

[0029] In a more optimized manner, the high-efficiency NPU system includes a cyclone separator for removing sliced ​​powder entrained in the gas, an energy-saving heat exchanger that uses hot exhaust gas to preheat the cold nitrogen entering the reactor, a two-stage spray scrubbing tower, a demister, a molecular sieve drying tower, a precision filter, a centrifugal fan, and a heater.

[0030] In a more optimized two-stage spray scrubbing tower, the first stage uses an ethylene glycol aqueous solution sprayed at room temperature to initially absorb organic matter; the second stage uses low-temperature chilled water sprayed at 5°C to condense and deeply capture volatile organic compounds such as acetaldehyde.

[0031] In a more optimized manner, the molecular sieve adsorption drying tower lowers the dew point of the gas to below -40°C, and then removes residual dust through a 5μm precision filter. After purification, the acetaldehyde content in the nitrogen is <5 ppm and the dew point is <-40°C. The nitrogen is then pressurized by a centrifugal fan and heated by an electric heater before being returned to the SSP system for recycling.

[0032] The present invention also provides a PET recycling system with low impurity content for any of the above-mentioned PET recycling processes, comprising a twin-screw extruder, a first-stage filtration device, a liquid phase thickening reactor, a melt gear pump, a second-stage precision filtration device, a pelletizing unit, a continuous SSP system, and a high-efficiency NPU system connected in sequence.

[0033] The first-stage filtration device is a multi-station screen changer with a 40μm filter screen;

[0034] The liquid-phase viscosity-enhancing reactor is equipped with a high-vacuum system and a surface-renewing stirrer;

[0035] The second-stage precision filtration device is a candle wick filter;

[0036] The continuous SSP system includes a crystallizer, a preheater, two-stage series reactors, and a cooler connected in sequence.

[0037] The high-efficiency NPU system forms a closed-loop nitrogen circuit with the SSP's preheater and reactor.

[0038] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0039] (1) This invention has achieved a fundamental breakthrough in the effect of impurity removal and purification. Through the synergistic effect of "multi-stage screw devolatilization" and "high vacuum enhanced devolatilization in liquid phase thickening vessel", the system removes small molecule organic matter such as acetaldehyde and odor monomers in the molten state; combined with the two-stage filtration of "40μm coarse filtration + 20μm fine filtration", the number of solid impurities and black spots is reduced by more than 70% compared with the traditional process, and the purity of the product reaches the high-end standard of food grade.

[0040] (2) This invention solves the industry problem of unstable viscosity of recycled PET. The liquid phase viscosity-enhancing reactor, as the core control unit, can accurately stabilize the intrinsic viscosity (IV) of the melt within a narrow range of ±0.01 dL / g, providing a uniform raw material basis for subsequent SSP, so that the IV fluctuation of the final product is reduced by more than 50% compared with the traditional process, and the consistency of product quality is significantly improved.

[0041] (3) This invention has significant advantages in terms of energy consumption and overall cost. Liquid phase thickening undertakes the main thickening task, which greatly shortens the SSP reaction time and nitrogen consumption; the high-efficiency NPU system replaces the high-energy-consuming catalytic combustion with "two-stage spray washing + molecular sieve drying" and integrates energy-saving heat exchange design, which reduces the total energy consumption of the system by about 35-40%. The flat layout of the equipment reduces the requirements of the plant, simplifies maintenance, and significantly reduces the overall investment and operating costs.

[0042] (4) This invention enhances the flexibility of production operations and the adaptability of raw materials. The twin-screw multi-stage vacuum design relaxes the stringent requirements on the moisture content of raw material fragments; the liquid phase thickening reactor also serves as an ideal color matching and homogenization unit, where color masterbatch can be added and excellent dispersion effect can be obtained, significantly improving the product color difference problem and enhancing the robustness of the process.

[0043] (5) This invention introduces liquid-phase thickening as the core, achieving deep removal of impurities and precise viscosity homogenization in the molten state. Combined with an optimized NPU system, it ultimately produces high-quality, low-acetaldehyde, and ultra-clean food-grade PET chips. This solution not only ensures that the product is comparable to virgin materials in key indicators such as viscosity, purity, and color, but also significantly reduces production costs through process optimization, enhancing the market competitiveness of recycled PET and providing an industrial solution for the plastic circular economy that combines technological advancement and economic feasibility. Attached Figure Description

[0044] Figure 1 This is a process flow diagram of the liquid phase thickening section of the present invention;

[0045] Figure 2 This is a process flow diagram of the SSP section and the high-efficiency NPU system of the present invention. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] The present invention will be described in detail with reference to the design embodiments.

[0048] 1. System configuration: The entire system is divided into three major modules: front-end melt purification, mid-stage liquid phase thickening, and back-end SSP and gas purification. The modules are closely connected through melt pipelines, slice conveying systems and nitrogen circuits to achieve continuous production.

[0049] 2. Pre-melting purification.

[0050] Raw material: Purchased PET bottle flakes with a moisture content of <0.8%.

[0051] Melt devolatilization: A Φ133 co-rotating twin-screw extruder with three vacuum pumping stations is used. The melt temperature is 270±5℃. The vacuum system progressively reduces the volatile content in the melt to below 0.2%.

[0052] Coarse filtration: A dual-column automatic screen changer with a 40μm filter screen is used, and the screen change pressure differential is set to 12 MPa.

[0053] 3. Mid-stage liquid phase viscosity enhancement and enhanced devolatilization.

[0054] like Figure 1 As shown, the melt enters 30 m 3 Vertical liquid phase viscosity-enhancing reactor. Operating parameters: absolute pressure set at 250 Pa, temperature set at 283 °C, stirring speed at 5 rpm, average residence time at 45 minutes.

[0055] Process monitoring: The inlet melt IV (approximately 0.62 dL / g) was monitored by an online viscometer. By fine-tuning the vacuum and temperature inside the reactor, the outlet melt IV was stably controlled at 0.72±0.01 dL / g.

[0056] Volatilization effect: This stage can remove an additional 60% of residual acetaldehyde and most of the small odor molecules, significantly improving the odor level of the melt.

[0057] 4. Melt refining and pelletizing.

[0058] Fine filtration: The melt is sent to the candle filter via a gear pump (outlet pressure approximately 12 MPa), using a 20μm precision filter element.

[0059] Pelletizing: The melt is cast into strips, cooled by water, pelletized, dehydrated by cyclone, dried in a fluidized bed, and sieved to obtain cylindrical basic slices with an IV of 0.72 dL / g and an acetaldehyde content of <10 ppm.

[0060] 5. Back-end SSP and high-efficiency NPU.

[0061] like Figure 2 As shown, the base slices are fed into the SSP system at a flow rate of 3 tons / hour. In the fluidized bed crystallizer, they are treated with hot nitrogen at 160°C to achieve a crystallinity of 40%.

[0062] Preheating and reaction: The slices were heated to 210°C in a preheater and then entered a two-stage reactor in series (design reaction temperature 215°C), with a total residence time of approximately 16 hours. After the reaction, the IV of the slices increased to 0.82 dL / g.

[0063] After the reaction, the hot slices are cooled to below 45°C with clean air and then automatically metered and packaged.

[0064] During this period, the efficient NPU operation process is as follows:

[0065] The reaction tail gas (80℃, containing approximately 500 ppm acetaldehyde) is first de-dusted by a cyclone separator; it then enters a plate-type energy-saving heat exchanger, where it exchanges heat with cold nitrogen gas (approximately 30℃) from a circulating fan to recover energy; the tail gas then enters a two-stage spray tower: the first stage sprays a 25% ethylene glycol aqueous solution (25℃); the second stage sprays 5℃ chilled water. After spraying, the gas temperature drops to approximately 10℃, and the acetaldehyde content drops to <10 ppm; after passing through a demister, the gas enters a molecular sieve adsorption tower (dual-tower switching, dew point ≤-50℃), and then passes through a 5μm filter; the purified high-purity nitrogen gas is pressurized by a centrifugal fan, heated to the required temperature by an electric heater, and then sent to the preheater and reactor respectively.

[0066] The main indicators of the PET slices produced in this embodiment after testing are as follows:

[0067] Intrinsic viscosity (IV): 0.82 ± 0.005 dL / g;

[0068] Acetaldehyde content: < 1.0 ppm;

[0069] Number of black spots: < 3 per gram;

[0070] Color value (b*): < 1.2 (can stably produce bright color chips).

[0071] To verify the effectiveness of the embodiments, the present invention provides comparative examples for verification.

[0072] The embodiment is the above-mentioned snare integrated process, which includes a complete process of "twin-screw multi-stage devolatilization → 40μm coarse filtration → liquid phase thickening and enhanced devolatilization → 20μm fine filtration → pelletizing → SSP equipped with high-efficiency NPU".

[0073] The comparative example used a process of "twin-screw melting (single-stage vacuum) → 60μm filtration → pelletizing → SSP equipped with catalytic combustion NPU". The SSP reaction time was extended to achieve a similar final viscosity.

[0074] The examples and comparative examples used the same batch of PET bottle flakes obtained through the same pretreatment (sorting, washing, and drying), with an initial intrinsic viscosity (IV) range of 0.58-0.64 dL / g. The production capacity was designed and calculated based on a continuous production line scale of 3.1 tons / hour, with a target intrinsic viscosity (IV) of 0.80 ± 0.02 dL / g.

[0075] The results are shown in Table 1.

[0076] Table 1. Test Results of Examples and Comparative Examples

[0077]

[0078] As shown in Table 1, the IV stability of the product in this embodiment is improved by over 80%, black spots are reduced by over 70%, and acetaldehyde content is reduced by over 80%. Liquid phase thickening undertakes the main thickening task, shortening the SSP time by about 20%. The NPU uses spraying instead of catalytic combustion, reducing the total system energy consumption by about 34%. It has high tolerance for raw material moisture content, and colorants can be added in the liquid phase thickening reactor with excellent dispersion uniformity. The equipment layout is compact, saving about 25% in investment. The NPU system has a simple structure and low maintenance cost.

[0079] The above description is merely an optional embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A PET recycling process, characterized in that, Includes the following steps: S1, Melting and Primary Deviation: Clean PET fragments are melted through a twin-screw extruder. The twin-screw extruder is equipped with at least three vacuum extraction ports along the material direction. Multi-stage vacuum devolatilization is carried out during the melting process to remove water vapor and most of the volatile small molecule organic matter to obtain the melt. S2, coarse filtration, the melt passes through the filter screen of the multi-station automatic screen changer to intercept mechanical impurities and unmelted materials; S3, Liquid phase thickening and enhanced melt devolatilization: The melt enters a dedicated liquid phase thickening reactor. Under high vacuum and high temperature conditions, the melt is continuously stretched into a film by a stirrer at 4-6 rpm to renew the surface. The melt stays for 30-50 minutes, and the resulting melt IV is homogenized and increased to 0.65-0.78 dL / g. S4, melt refining: After thickening, the melt is pressurized by a gear pump and then filtered through a candle wick precision filter with a filtration accuracy of 10-20μm to completely remove fine impurities and obtain an ultra-clean melt. S5, pelletizing and intermediate product preparation: The ultra-clean melt enters the pelletizing unit, is cast into strips by the casting head, cooled in a water tank, cut by the pelletizer, dehydrated and dried and vibrated sieve to obtain cylindrical, uniform basic slices; S6, solid-state polycondensation: The base slices enter the continuous SSP system. First, in a hot nitrogen fluidized bed, the amorphous slices are heated to above the glass transition temperature to increase their crystallinity to >35% and prevent subsequent adhesion. Then, in a vertical preheater, the slices are preheated to 200-210℃ using heated circulating nitrogen. The preheated slices are then placed in a two-stage series reactor and undergo polycondensation at 210-220℃ under high-purity nitrogen purging for 10-20 hours until the slices reach the target bottle-grade requirements (IV). S7, high-efficiency nitrogen purification, the reaction tail gas containing moisture, acetaldehyde and ethylene glycol discharged during the operation of the two-stage series reactor enters the high-efficiency NPU system for purification and recovery. S8, Product Cooling: The hot PET chips after the reaction are cooled to below 60°C by filtered ambient air in the cooling hopper, thus obtaining the finished PET chips.

2. The PET recycling process according to claim 1, characterized in that: In step S1, the melting temperature is 265-275℃, and the three-stage vacuum extraction port includes... First stage: Removal of volatile organic compounds at atmospheric pressure; Level 2: Absolute pressure ≤ 15 kPa; Level 3: Absolute pressure ≤ 5 kPa.

3. The PET recycling process according to claim 1, characterized in that: In step S2, the filter screen accuracy is 40 μm, and the resulting melt IV is 0.60-0.63 dL / g.

4. The PET recycling process according to claim 1, characterized in that: In step S3, the high vacuum and high temperature environment is an absolute pressure of 100-500 Pa and a temperature of 280-285℃.

5. A PET recycling process according to claim 4, characterized in that: The high vacuum and high temperature environment was 200 Pa absolute pressure, 283 °C temperature, and 5 rpm stirring rate. The resulting melt IV was homogenized and increased to 0.72 ± 0.01 dl / g.

6. The PET recycling process according to claim 1, characterized in that: In step S6, the target bottle grade requires an IV of 0.80-0.84 dL / g.

7. The PET recycling process according to claim 1, characterized in that: The high-efficiency NPU system includes a cyclone separator for removing sliced ​​powder entrained in the gas, an energy-saving heat exchanger that uses hot waste gas to preheat the cold nitrogen entering the reactor, a two-stage spray scrubbing tower, a demister, a molecular sieve drying tower, a precision filter, a centrifugal fan, and a heater.

8. A PET recycling process according to claim 7, characterized in that: In the two-stage spray scrubbing tower, the first stage uses an ethylene glycol aqueous solution sprayed at room temperature to initially absorb organic matter; the second stage uses low-temperature chilled water sprayed at 5°C to condense and deeply capture volatile organic compounds such as acetaldehyde.

9. A PET recycling process according to claim 7, characterized in that: The molecular sieve adsorption drying tower lowers the dew point of the gas to below -40°C. Then, the gas passes through a 5μm precision filter to remove residual dust. After purification, the acetaldehyde content in the nitrogen is <5ppm and the dew point is <-40°C. The nitrogen is then pressurized by a centrifugal fan and heated by an electric heater before being returned to the SSP system for recycling.

10. A PET recycling system with low impurity content, used to implement any one of the PET recycling processes as described in claims 1-9, characterized in that: It includes a twin-screw extruder, a first-stage filtration unit, a liquid phase thickening reactor, a melt gear pump, a second-stage precision filtration unit, a pelletizer, a continuous SSP system, and a high-efficiency NPU system connected in sequence. The first-stage filtration device is a multi-station screen changer with a 40μm filter screen; The liquid phase thickening reactor is equipped with a high vacuum system and a surface-renewing stirrer; The second-stage precision filtration device is a wick filter; The continuous SSP system includes a crystallizer, a preheater, two-stage series reactors, and a cooler connected in sequence. The high-efficiency NPU system forms a closed-loop nitrogen circuit with the SSP's preheater and reactor.

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