Method for recovering polyethylene in mixed plastics and applications thereof

Abstract

The application provides a method for recycling polyethylene in mixed plastics and application thereof, and belongs to the technical field of waste plastic recycling.The method provided by the application uses a paraffin with a carbon number of 12-20 as a main solvent, a chain ether as a cosolvent, and selectively dissolves polyethylene in mixed plastics in the presence of supercritical carbon dioxide, so that separation of polyethylene and polyester is realized.The method has the advantages of high recovery rate, high purity and low yellowness.

Description

Technical Field

[0001] This invention belongs to the field of waste plastic recycling technology, specifically relating to a method for recycling polyethylene from mixed plastics and its application. Background Technology

[0002] In the field of mixed plastic recycling, there is an urgent need to separate polyethylene from polyester (PET). However, traditional dissolution separation technologies used for this purpose face significant challenges. Existing methods mainly rely on alkanes for dissolution, typically using alkanes with fewer than 10 carbon atoms. However, alkanes with fewer than 10 carbon atoms have low boiling points, significantly increasing the risk of accidents. Furthermore, dissolving polyethylene with long-chain alkanes (C12-C20) suffers from inherent defects, resulting in low efficiency. C18-C20 alkanes, in particular, are semi-solid or very viscous liquids at low temperatures, exhibiting significant molecular diffusion resistance and making it difficult to effectively penetrate the highly crystalline regions of polyethylene. This necessitates dissolution processes at temperatures above 150°C, leading to substantial energy consumption. Additionally, the high boiling points, difficulty in removal, and tendency to leave residues of C12-C20 alkanes result in high solvent residues in the separated polyethylene, reducing its purity.

[0003] Patent application WO2022219034A1 discloses a process for extracting polyethylene from waste plastics. The process involves dissolving polyethylene in waste plastics using C4-C10 hydrocarbons at 135-200 °C and 0.1-5 MPa, then separating the solution from the insoluble matter, and finally extracting polyethylene from the solution. However, this process uses low-boiling-point solvents, posing a high level of technological hazard. Summary of the Invention

[0004] To address the technical problem of poor penetration of C12-C20 long-chain alkanes into polyethylene in existing technologies, this invention provides a method for recovering polyethylene from mixed plastics. The recovery method provided by this invention uses C12-C20 alkanes as the main solvent and a specific type of chain ether as a co-solvent, effectively separating polyethylene from mixed plastics and offering advantages such as high recovery rate, high purity, and low yellowing.

[0005] Another object of the present invention is to provide an application of the above-described method.

[0006] Another object of the present invention is to provide polyethylene obtained by the above method.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] A method for recycling polyethylene from mixed plastics includes the following steps:

[0009] Dissolution: The mixed plastic particles are contacted with a solvent in supercritical carbon dioxide and kept at a temperature of 110~150℃ for 30~90 minutes to dissolve the polyethylene in the solvent, resulting in a first fluid with uniform solid-liquid mixing.

[0010] Filtration: The first fluid is filtered to obtain the second fluid;

[0011] Deviation: The second fluid is sequentially subjected to flash devolatilization and extrusion devolatilization to obtain recycled polyethylene;

[0012] in:

[0013] The hybrid plastic particles contain polyester;

[0014] Based on a total volume of 100 parts, the solvent comprises 80-95 parts of alkanes and 5-20 parts of ethers;

[0015] The alkanes have 12 to 20 carbon atoms;

[0016] The general chemical formula of the chain ether is R1-O-R2, wherein R1 and R2 are independently selected from alkyl groups having 1 to 4 carbon atoms, and R1 and R2 are not both methyl groups.

[0017] A method for separating polyethylene and polyester includes the following steps:

[0018] Dissolution: The mixed plastic particles containing polyethylene and polyester are contacted with a solvent in supercritical carbon dioxide and kept at a temperature of 110~150℃ for 30~90min to dissolve the polyethylene in the solvent, thus obtaining a first fluid with uniform solid-liquid mixing.

[0019] Filtration: The first fluid is filtered to obtain the second fluid;

[0020] Deviation: The second fluid is sequentially subjected to flash devolatilization and extrusion devolatilization to obtain recycled polyethylene;

[0021] in:

[0022] Based on a total volume of 100 parts, the solvent comprises 80-95 parts of alkanes and 5-20 parts of ethers;

[0023] The alkanes have 12 to 20 carbon atoms;

[0024] The general chemical formula of the chain ether is R1-O-R2, wherein R1 and R2 are independently selected from alkyl groups having 1 to 4 carbon atoms, and R1 and R2 are not both methyl groups.

[0025] Specifically, the alkane can be a straight-chain alkane or a branched-chain alkane, the ether can be a straight-chain ether or a branched-chain ether, and the alkyl can be a straight-chain alkyl or a branched-chain alkyl.

[0026] Specifically, the alkanes can be 80 parts, 85 parts, 90 parts, or 95 parts, and the ethers can be 5 parts, 10 parts, 15 parts, or 20 parts.

[0027] Preferably, the pressure is 8~20 MPa, within which high purity, high recovery rate, and low yellowness can be achieved. Specifically, the pressure is 8 MPa, 9 MPa, 10 MPa, 11 MPa, 12 MPa, 13 MPa, 14 MPa, 15 MPa, 16 MPa, 17 MPa, 18 MPa, 19 MPa, or 20 MPa.

[0028] More preferably, the pressure is 15-18 MPa. Within this range, the method offers better purity, recovery rate, yellowness, and cost.

[0029] Specifically, the temperatures are 110 ℃, 120 ℃, 130 ℃, 140 ℃, and 150 ℃.

[0030] Specifically, the heat preservation time is 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, and 90 min.

[0031] Preferably, the heat preservation time is 40-50 minutes, which can balance high purity, high recovery rate, low yellowness and low cost.

[0032] Specifically, the feeding ratio of the mixed plastic particles to the solvent is 1g:1~5mL, specifically 1:1, 1:3, or 1:5.

[0033] The polyester described in this invention is a conventional polyester in the art, especially polyethylene terephthalate or polybutylene terephthalate. The recycling method of this invention has a good separation effect on polyethylene and the above-mentioned polyester.

[0034] Specifically, the particle size of the mixed plastic particles is controlled between 0.5 and 5 cm. This size range allows for a suitable recycling rate.

[0035] Specifically, the moisture content of the mixed plastic particles is no higher than 1 wt%. A moisture content within this range ensures that the yellowness of the resulting recycled polyethylene is within a suitable range.

[0036] Specifically, the dissolution includes the following steps: contacting the mixed plastic particles with a solvent in a reaction vessel, introducing carbon dioxide until the pressure inside the reaction vessel is not less than 8 MPa, so that the carbon dioxide reaches a supercritical state, heating to raise the temperature inside the reaction vessel to 110~150℃, and keeping it at the temperature to dissolve the polyethylene in the solvent, thereby obtaining a first fluid with uniform solid-liquid mixing.

[0037] More specifically, carbon dioxide must be used to replace the residual oxygen in the reactor before it is introduced to avoid high-temperature oxidation.

[0038] Specifically, the filtration uses a fully automatic meshless filter with a 150-mesh filter.

[0039] Specifically, the flash devolatilization includes rapidly feeding the first fluid through a connecting pipe into the top of a flash tower with an internal pressure of 0.1 MPa to -0.1 MPa, and collecting solid particles, melt and / or fluid through a collection device at the bottom of the flash tower.

[0040] Specifically, the extrusion devolatilization includes: passing the solid particles, melt and / or fluid collected by flash evaporation into a screw extruder equipped with a vacuum devolatilization system through a connecting pipe for vacuum devolatilization.

[0041] More specifically, the screw extruder has a processing temperature of 160~300 ℃ and a vacuum degree of 10~50 kPa.

[0042] The present invention also provides the application of the above-described recycling method in the recycling of waste plastics containing polyethylene and polyester.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The recovery method of the present invention has the advantages of high purity, high recovery rate and low yellowness. Detailed Implementation

[0045] The present invention is further illustrated below with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used, unless otherwise specified, are all commercially available from the conventional market. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention are within the scope of protection claimed by the present invention.

[0046] The reagents and products used in the various embodiments and comparative examples of this invention are described below:

[0047] PET / PE composite film: PE and PET are used to form a film with a thickness of 50 μm and a PE layer accounting for 50 wt% by a two-stage film casting process;

[0048] C11: n-Undecane, CAS No. 1120-21-4, purchased from Beijing Innocare Technology Co., Ltd.;

[0049] C12: n-Dodecane, CAS No. 112-40-3, purchased from Beijing Innocare Technology Co., Ltd.;

[0050] C14: n-Tetradecane, CAS No. 629-59-4, purchased from Beijing Innocare Technology Co., Ltd.;

[0051] C16: n-Hexadecane, CAS No. 544-76-3, purchased from Beijing Innocare Technology Co., Ltd.;

[0052] C16 isomer 1: 2,2,4,4,6,8,8-heptamethylnonane, CAS No. 4390-04-9, purchased from Aladdin Biochemical Technology Co., Ltd.

[0053] C16 isoform 2: 2-methylpentadecane, CAS No. 60908-77-2, purchased from Shaoyuan Chemical Technology (Shanghai) Co., Ltd.;

[0054] C18: n-Octadecane, CAS No. 593-45-3, purchased from Beijing Innocare Technology Co., Ltd.;

[0055] C20: n-eicosane, CAS No. 112-95-8, purchased from Beijing Innocare Technology Co., Ltd.;

[0056] C21: n-Ticosane, CAS No. 629-94-7, purchased from Beijing Innocare Technology Co., Ltd.;

[0057] Dimethyl ether: CAS No. 115-10-6, purchased from Beijing Innocare Technology Co., Ltd.;

[0058] Diethyl ether: CAS number, purchased from Beijing Innocare Technology Co., Ltd.;

[0059] Propyl ether: CAS No. 111-43-3, purchased from Beijing Innocare Technology Co., Ltd.;

[0060] Diisobutyl ether: CAS No. 628-55-7, purchased from Beijing Innocare Technology Co., Ltd.;

[0061] Butyl ether: CAS No. 142-96-1, purchased from Beijing Innocare Technology Co., Ltd.;

[0062] Pentyl ether: CAS No. 110-62-3, purchased from Beijing Innocare Technology Co., Ltd.;

[0063] The PE was purchased from Maoming Petrochemical, and its grade was LLDPE DFDA-7042.

[0064] The PET was purchased from Sinopec Yizheng Chemical Fiber, and its grade is PET FG600.

[0065] The performance testing methods and standards obtained by the automated methods of the various embodiments and comparative examples of this invention are as follows:

[0066] 1. Purity determination method:

[0067] In this invention, the purity of polyethylene is determined by a comprehensive method combining Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The specific steps are as follows:

[0068] Sample preparation: Randomly select multiple sample points from the obtained polyethylene product, grind the samples into powder with a particle size of less than 100 μm, mix them evenly and set aside.

[0069] FTIR: Take approximately 1 mg of the prepared sample powder and grind it thoroughly with 100 mg of potassium bromide (KBr) powder in an agate mortar until homogeneous. Press the mixture using a tablet press for 30 seconds to form a transparent thin film. Place the film into the sample cell of the FTIR spectrometer and run it in the wavenumber range of 4000–400 cm⁻¹. - ¹ An infrared spectrum of the sample was obtained by scanning within the range. Analysis of characteristic peaks in the spectrum: The characteristic absorption peak of polyethylene is mainly located at 2916 cm⁻¹. -1 2848 cm -1 (CH stretching vibration) and 1466 cm -1 720cm -1 (CH bending vibration).

[0070] TGA: Weigh approximately 10 mg of the prepared sample powder and place it in the crucible of the TGA analyzer. Under a nitrogen atmosphere, heat the sample from 30 °C to 600 °C at a heating rate of 10 °C / min. Record the thermogravimetric curve (TG curve) and differential thermogravimetric curve (DTG curve) of the sample. The solvent's weight loss temperature is <350 °C, while the decomposition temperature range of polyethylene is typically 375~550 °C.

[0071] The purity of recycled PE is assessed in two ways: 1) The degree of PE / PET mixing: Taking FTIR test results as an example, the purity is judged based on the characteristic functional groups in the infrared spectrum. If only the characteristic peaks of polyethylene appear in the spectrum, and there are no other plastics (such as PET at 1720 cm⁻¹), the purity is determined by the presence of these characteristic peaks. -1 Characteristic peak, 1640~1660 cm -1 and 1540 cm -1 If the characteristic absorption peaks of amide bonds and other impurities are present, it indicates that there are no other obvious plastics or impurities in the sample. 2) Solvent content in recycled PE: Calculated based on the weight loss rate of the sample in TGA within the temperature range of 375~550 ℃, using the following formula;

[0072]

[0073] 2. PE Recovery Rate: If a significant amount of solvent remains in the recycled PE, its recovery quality needs to be calculated. Therefore, the purity of the recycled PE is characterized by the weight loss rate of DSC at 375~550 ℃, and its recovery rate is further calculated.

[0074]

[0075] 3. Yellowness: The sample is hot-pressed into a uniform 1 mm thin sheet using a flat vulcanizing machine at a temperature of 180~190 ℃. The sheet is then cut to a suitable size for Lab value measurement. The test is performed using a spectrophotometer with the D65 / 10° conditioned reflectance method. Calibration is required before testing, and a standard white plate must be placed behind the sample during the test.

[0076] Examples and Comparative Examples

[0077] The methods in each embodiment and comparative example include the following steps:

[0078] (0) Pretreatment: The mixed plastics are initially screened to remove large impurities such as metal, wood, and glass; the mixed plastics are crushed into smaller mixed plastic particles by a crusher, and the particle size is controlled between 0.5 and 5 cm; the crushed mixed plastic particles are sent to a washing device and washed with an aqueous solution containing surfactants to remove dirt, dust and other impurities from the surface of the mixed plastic particles; after washing, dehydration is carried out, which can be done by centrifugal dehydration or filtration dehydration, so that the moisture content of the mixed plastic particles is reduced to 1 wt% or less.

[0079] (1) Dissolution: The pretreated mixed plastic particles are fed into the reactor and solvent is added. After the addition, carbon dioxide is used to purge the reactor to replace the residual oxygen and avoid high-temperature oxidation. Then carbon dioxide gas is introduced until a specific dissolution pressure is reached to achieve the supercritical state, and the reactor is heated to raise the temperature to a specific dissolution temperature. The temperature is maintained for a specific dissolution time. Polyethylene in the mixed plastic dissolves in the solvent under the action of supercritical carbon dioxide and exists in liquid form, while PET does not dissolve or undergoes slight swelling under the conditions and still exists in solid phase, resulting in a first fluid with uniform solid-liquid mixing.

[0080] The dissolution temperature, dissolution pressure and dissolution time are shown in Table 1. The ratio of mixed plastic particles to solvent is 1:5 (g / mL).

[0081] (2) Automatic filtration and purification: The first fluid is filtered through a 150-mesh fully automatic meshless filter to obtain the second fluid. The solid phase of PET is filtered out. The second fluid is a high-pressure fluid of high-purity single polyethylene resin and solvent mixture.

[0082] (3) Flash Evaporation: High-pressure fluid is rapidly fed into the top of the flash tower through a connecting pipe. The pressure inside the flash tower is pre-controlled at 0.1 MPa ~ -0.1 MPa. When the mixed fluid enters the flash tower, due to the sharp drop in pressure, the polyethylene dissolved in the solvent will quickly precipitate out, forming solid particles, melt and / or fluid, while some of the solvent exists in gaseous form. The solid particles, melt and / or fluid are collected by the collection device at the bottom of the flash tower.

[0083] (4) Extrusion devolatilization: The solid particles, melt and / or fluid are fed into a twin-screw extruder equipped with a multi-stage vacuum devolatilization system through a connecting pipe for vacuum devolatilization, further removing residual solvents from the polyethylene to obtain recycled polyethylene.

[0084] The twin-screw extruder has eight temperature zones, with processing temperatures set at 190, 210, 210, 230, 230, 210, 210, and 190 degrees Celsius, an initial vacuum of 100 Pa, and a rotational speed of 100 rpm.

[0085] Recycled polyethylene can be modified and processed as needed, while gaseous organic solvents are fed into a subsequent condensation and recovery device through negative pressure vacuum, where they are recycled and reused through simple filtration, adsorption, and other methods.

[0086] The parameters for each embodiment and comparative example are shown in Table 1:

[0087] Table 1

[0088]

[0089] Continued from Table 1

[0090]

[0091] Continued from Table 1

[0092]

[0093] Continued from Table 1

[0094]

[0095] Continued from Table 1

[0096]

[0097] As can be seen from the above embodiments, the recycling method of the present invention has the advantages of high recovery rate, high polyethylene purity and low yellowness. Its recovery rate is not less than 90%, the purity of the recycled polyethylene prepared is not less than 95%, and the yellowness is not higher than 3.

[0098] Comparative Example 1 uses dimethyl ether as a co-solvent. Under the dissolution conditions of this invention, dimethyl ether is prone to generating free radicals, resulting in excessive yellowness of the final polyethylene.

[0099] Comparative Example 2 used pentylenetetrazine as a cosolvent. On the one hand, pentylenetetrazine has a weak carry-out effect on long-chain alkanes, leaving a large amount of long-chain alkanes in the recycled polyethylene that cannot be removed. On the other hand, pentylenetetrazine has insufficient solubilizing effect on long-chain alkanes, resulting in a reduced recovery rate.

[0100] Comparative Example 3 used n-undecane as the main solvent, but the recovery rate was insufficient, indicating that short-chain alkanes with fewer than 12 carbon atoms cannot effectively dissolve polyethylene in the system of this invention and are not suitable for the system of this invention.

[0101] Comparative Example 4 used n-docosane as the main solvent, and the recovery rate and purity decreased, indicating that long-chain alkanes with more than 20 carbon atoms cannot effectively dissolve polyethylene in the system of the present invention, and cannot be effectively removed, and are not suitable for the system of the present invention.

[0102] The amount of chain ether added in Comparative Example 5 was too small, resulting in a weak effect on the dissolution and carry-out of long-chain alkanes, leading to a decrease in recovery rate and purity.

[0103] In Comparative Example 6, the amount of chain ether added was too large, and the amount of long-chain alkanes was insufficient, resulting in weak dissolution of polyethylene. Furthermore, the excessive chain ether in the system of this invention easily generates too many free radicals, causing the recycled polyethylene product to turn yellowish.

[0104] The recovery rate was reduced because the dissolution temperature of Comparative Example 7 was too low.

[0105] The dissolution temperature of Comparative Example 8 was too high, resulting in an excessively high yellowness.

[0106] The dissolution pressure of Comparative Example 9 was too low, and the carbon dioxide in the system did not reach the supercritical state. The dissolution-promoting and carry-out effects on long-chain alkanes were weak, resulting in a decrease in recovery rate and purity.

[0107] The dissolution time of Comparative Example 10 was too short, resulting in insufficient recovery rate.

[0108] The dissolution time of Comparative Example 11 was too long, resulting in a yellowish tint to the recycled polyethylene product.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for recycling polyethylene from mixed plastics, characterized in that, Includes the following steps: Dissolution: The mixed plastic particles are contacted with a solvent in supercritical carbon dioxide and kept at a temperature of 110~150℃ for 30~90 minutes to dissolve the polyethylene in the solvent, resulting in a first fluid with uniform solid-liquid mixing. Filtration: The first fluid is filtered to obtain the second fluid; Deviation: The second fluid is sequentially subjected to flash devolatilization and extrusion devolatilization to obtain recycled polyethylene; in: The hybrid plastic is composed of polyethylene and polyester; Based on a total volume of 100 parts, the solvent is composed of 80-95 parts of alkanes and 5-20 parts of ethers; The alkanes have 12 to 20 carbon atoms; The general chemical formula of the chain ether is R1-O-R2, wherein R1 and R2 are independently selected from alkyl groups having 1 to 4 carbon atoms, and R1 and R2 are not both methyl groups.

2. A method for separating polyethylene and polyester, characterized in that, Includes the following steps: Dissolution: The hybrid plastic particles composed of polyethylene and polyester are contacted with a solvent in supercritical carbon dioxide and kept at a temperature of 110~150℃ for 30~90min to dissolve the polyethylene in the solvent, thus obtaining a first fluid with uniform solid-liquid mixing. Filtration: The first fluid is filtered to obtain the second fluid; Deviation: The second fluid is sequentially subjected to flash devolatilization and extrusion devolatilization to obtain recycled polyethylene; in: Based on a total volume of 100 parts, the solvent is composed of 80-95 parts of alkanes and 5-20 parts of ethers; The alkanes have 12 to 20 carbon atoms; The general chemical formula of the chain ether is R1-O-R2, wherein R1 and R2 are independently selected from alkyl groups having 1 to 4 carbon atoms, and R1 and R2 are not both methyl groups.

3. The method according to claim 1 or 2, characterized in that, The alkanes include straight-chain alkanes and / or branched-chain alkanes, the ethers include straight-chain ethers and / or branched-chain ethers, and the alkyl groups include straight-chain alkyl groups and / or branched-chain alkyl groups.

4. The method according to claim 1 or 2, characterized in that, The heat preservation time is 40-50 minutes.

5. The method according to claim 1 or 2, characterized in that, The feeding ratio of the mixed plastic particles to the solvent is 1g:1~5mL, the particle size of the mixed plastic particles is 0.5~5 cm, and the moisture content of the mixed plastic particles is not higher than 1wt%.

6. The method according to claim 1 or 2, characterized in that, The dissolution process includes the following steps: contacting the mixed plastic particles with a solvent in a reactor, introducing carbon dioxide until the pressure inside the reactor is not less than 8 MPa, so that the carbon dioxide reaches a supercritical state, heating the reactor to raise the temperature to 110~150℃, and keeping it at this temperature to dissolve the polyethylene in the solvent, thereby obtaining a first fluid with a uniform solid-liquid mixture.

7. The method according to claim 6, characterized in that, The pressure is 8~20MPa.

8. The method according to claim 1 or 2, characterized in that, The flash devolatilization process involves rapidly feeding the first fluid through a connecting pipe into the top of a flash tower with an internal pressure of 0.1 MPa to -0.1 MPa, and collecting solid particles and / or fluid through a collection device at the bottom of the flash tower.

9. The method according to claim 1 or 2, characterized in that, The extrusion devolatilization includes: passing the solid particles and / or fluid collected by flash devolatilization into a screw extruder equipped with a vacuum devolatilization system through a connecting pipe for vacuum devolatilization. The processing temperature of the screw extruder is 160~300 ℃ and the vacuum degree is 10~50 kPa.

10. The application of the method according to any one of claims 1 to 9 in the recycling of waste plastics composed of polyethylene and polyester.

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