Method and system for depolymerizing waste plastics

Abstract

A continuous flow process and system for depolymerizing plastic. A heterogeneous mixture of plastic particles, solvent, and catalyst is continuously pumped through a heating zone at a flow rate just sufficiently high to maintain a particle velocity sufficiently high to maintain the plastic particles suspended. The temperature of the heterogeneous mixture is raised in the heating zone and maintained in a holding zone to complete depolymerization of the mixture to a homogeneous solution containing liquefied reaction products. The homogeneous solution is cooled to solidify and precipitate a solid reaction product. The solid reaction product is separated from the solvent to be recycled. The solvent is recycled for reuse as a component of the heterogeneous mixture.

Description

[0001] This application is a divisional application of the Chinese patent application with application number 202180010602.3 and invention name “Method and system for depolymerizing waste plastics”. Technical Field

[0002] This application claims priority to U.S. Provisional Application No. 62 / 964,948, filed on January 23, 2020, which is hereby incorporated by reference. Background Art

[0003] The present invention generally relates to the depolymerization of resins, plastics or polymers. More specifically, it relates to the depolymerization of waste plastics in a continuous process.

[0004] Plastics are conventionally depolymerized in large reaction vessels, usually equipped with heating jackets and agitators. The depolymerization reaction is isolated in the vessel until the depolymerization is complete. After depolymerization, the vessel is emptied and then refilled. Each batch is heated to accelerate depolymerization and then cooled to produce a viable feedstock for new polymers. Batch processes typically take 20 minutes to 800 minutes. Continuous operation is simulated by emptying and refilling a set of reaction vessels in a cyclic manner. The constant need to fill, heat, cool, empty, and repeat wastes energy and requires additional equipment to maintain the illusion of actual continuous flow in parallel batch processes. Summary of the invention

[0005] A method for depolymerizing plastics embodying features of the present invention comprises: (a) continuously flowing a mixture containing solid plastic particles in a solvent through a pipeline in a heating chamber at a particle velocity sufficient to maintain the plastic particles suspended in the solvent and prevent the plastic particles from agglomerating and clogging the pipeline; and (b) transferring heat through the pipeline in the heating chamber to heat the mixture to a reaction temperature, thereby initiating depolymerization of the plastic particles in the solvent into a homogeneous solution containing a liquefied reaction product.

[0006] A system for continuous depolymerization of plastics embodying features of the invention includes a pump operated at a pump flow rate and a pipeline, the pump continuously feeding a heterogeneous mixture through the pipeline at a particle velocity, the heterogeneous mixture comprising plastic particles in a solvent. A heating zone increases the temperature of the heterogeneous mixture flowing through the pipeline to a reaction temperature of at least 150° C. Conversion of the heterogeneous mixture containing plastic particles into a homogeneous solution containing liquefied reaction products (including monomers, dimers, oligomers and / or reaction byproducts) begins in the heating zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a block diagram of a system for depolymerizing plastics embodying features of the present invention.

[0008] Figure 2 It shows that a certain volume of plastic Figure 1 Flowchart of the progression of a system undergoing the depolymerization process. DETAILED DESCRIPTION

[0009] Systems and methods for depolymerizing plastics are shown in Figure 1 and 2 The systems and methods can be used for a variety of plastics such as, but not limited to, PET, modified PET, PET blends, PEN, PBT, PET-G, PLA, PGA, PLGA, PEF, copolyesters, polycarbonates, polyamides (nylons), polyurethanes, and combinations and blends. Plastics are depolymerized into the following substances, but not limited to: (bis(2-hydroxyethyl)terephthalate (BHET), dimethyl terephthalate (DMT), terephthalic acid (TA), (bis(2-hydroxyethyl)naphthalate (BHEN), (bis(2-hydroxyethyl)furoate (BHEF); their respective oligomers, acids, half esters or mixed esters. In addition, chemically useful compounds such as dioctyl terephthalate (DOTP), diisobutyl terephthalate (DITP), dibutyl terephthalate (DBTP), bisphenol A (BPA), lactic acid esters, bis(2-hydroxyethyl)terephthalamide (BHETA) and other terephthalamides.

[0010] Solid plastic particles of waste polyester material in the form of flakes, fines, granules, granules, oatmeal, chunks, chunks and / or powder are mixed with solvents and catalysts in a mixer 10 to produce a heterogeneous mixture 12. The mixer 10 can use an agitator (such as a propeller 13, a stirrer or other agitator) or a recycled solvent for mixing. Alternatively, the mixture can be pre-mixed. Examples of solvents are, but are not limited to, ethylene glycol (EG), diethylene glycol (DEG), glycol ethers, methanol, ethanol, propanol, butanol, 2-ethylhexanol, tetramethylcyclobutanediol (CBDM), cyclohexanedimethanol (CHDM), alcohols, ethanolamines, ionic liquids, polar protic solvents, polar aprotic solvents, and water. Examples of suitable catalysts include, but are not limited to, zinc salts, zinc acetate; zinc chloride; titanium salts; manganese salts; magnesium salts; sodium hydroxide; potassium hydroxide; 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD); 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU); magnesium acetate; 4-dimethylaminopyridine (DMAP); amines; trialkylamines; and combinations of these catalysts. The heterogeneous mixture 12 is pumped through a series of connected lines, such as pipes or tubes, by pump 14. Agitators, augers, or extruders are not required to propel the mixture through the system. Pump 14 is operated at a sufficiently large flow rate to move the mixture 12 through the system at a particle velocity that is large enough to maintain the particles suspended in the solvent and prevent the particles from agglomerating and plugging the lines. By operating continuously without stopping, pump 14 causes the heterogeneous mixture to flow through the system at a steady rate, which allows the conversion of the plastic to the liquefied product to vary with the position within the system rather than with time (as in a batch system).

[0011] An optional preheating heat exchanger (preheater) 16 is used to preheat the heterogeneous mixture 12. The preheater 16 may heat the heterogeneous mixture 12 by a heat source such as flame, steam, hot oil, or a circulating heat transfer fluid. Alternatively, a hot homogeneous solution containing liquefied products after the depolymerization reaction may be used in the preheater 16 to transfer heat to the heterogeneous mixture and cool itself down in the process.

[0012] The preheated heterogeneous mixture 12 ' continuously flows into and passes through the downstream heating chamber 18, where depolymerization begins. The heating chamber 18 can be implemented as a reactor heat exchanger that raises the temperature of the heterogeneous mixture to a reaction temperature of at least 150°C. The heterogeneous mixture is heated by a heat source 20 in the reaction heat exchanger 18. The heat source 20 can directly heat the heterogeneous mixture using microwave radiation, direct flame, electric heating pipe, induction heating pipe, geothermal, magnetostrictive thermoelectric or ohmic mode (as some examples). Or the heat source 20 can indirectly heat the heterogeneous mixture by directly heating the heat transfer fluid outside the heating chamber 18. Examples of suitable transfer fluids are hot oil, hot fluids, molten salts and steam. The heated heat transfer fluid is then pumped through the pipeline containing the heterogeneous mixture in the heating chamber 18. Heat is transferred from the heat transfer fluid to the heterogeneous mixture to start depolymerization. The heterogeneous mixture flowing through the heating chamber 18 is not in direct contact with the heat transfer fluid.

[0013] The holding tube 22 after the heating chamber 18 maintains the reaction temperature for at least one minute to complete the conversion of the heterogeneous mixture containing plastics to a homogeneous solution 24 containing liquefied products. The holding tube 22 can be implemented by an insulating reel or coil of a pipe or tube or as a jacketed pipe or container. Alternatively, the holding tube can be a part of the heating chamber, rather than an independent component. The reaction is completed in the holding tube. The homogeneous solution leaving contains solvent, spent catalyst and depolymerized plastic in the form of liquefied reaction products, which typically include monomers, oligomers and / or a small amount of by-products (e.g., half esters, half amides, mixed esters, mixed amides) from the reaction.

[0014] The homogeneous solution 24 is continuously pumped through the optional preheat heat exchanger 16 to cool itself and preheat the incoming heterogeneous mixture 12. The back pressure regulator 26 maintains the system pressure (eg, 100 psi to 400 psi) above the vapor pressure of the solvent at the reaction temperature.

[0015] After flowing through back pressure regulator 26 , homogeneous solution 24 flows through an optional cooling heat exchanger (cooler) 28 which uses chilled water or other chilled heat transfer fluid from a cooling reservoir 30 to remove any excess heat not recovered by preheater 16 .

[0016] After the solution has cooled, it is poured into a precipitation or crystallization tank and cooled until the liquefied product is precipitated as a solid reaction product 34. The solvent is then decanted, filtered, centrifuged, or distilled from the solid reaction product. The solid reaction product may then be filter pressed to further separate it from any remaining solvent. Decanting, filtering, centrifuging, or distilling the solvent is followed by pressurizing the solid reaction product 34 in the solution 24 to separate the solvent 36. Figure 1 This is indicated by separator 32.

[0017] The separated solvent 36 is recycled back in the mixer 10 for reuse. Optional solvent cleaning, purification or regeneration steps may be required to remove reaction pollutants from the solvent supplied to the subsequent heterogeneous mixture 12. Reaction pollutants may include particles, ionic salts, anions, cations, spent catalysts, dyes, adhesives, components from blends, fillers and / or decomposed solvents. Pollutant removal 42 can occur by passing the separated solvent 36 through a filter and / or adsorbent (such as activated carbon, ion exchange resins, diatomaceous earth, sand, zeolite, clay, silicon dioxide, aluminum oxide, oxides, size exclusion and / or tangential flow filtration). The pollutant removal 42 of the solvent 36 can be an online or offline process. Pollutant removal 42 can occur in the separation solvent step 36 or the homogeneous solution step 24.

[0018] Thus, the system moves the heterogeneous mixture 12 through four zones: Z1 - a cold entry zone, where the mixture is fed into the system by a pump 14; Z2 - a preheating zone, where the mixture is heated in a preheater 16; Z3 - a heating zone, where the mixture is heated to raise its temperature to the reaction temperature; and Z4 - a holding zone, where the mixture is held at the reaction temperature to complete the conversion of the heterogeneous mixture into a homogeneous solution 24. The homogeneous solution 24 is moved through a cooling zone Z5, where the homogeneous solution is cooled in a cooler 28, or cooled by transferring heat to the incoming heterogeneous mixture 12 in the preheater 16. The pump 14 maintains a continuous flow through the system, which ensures that the particle velocity of the heterogeneous mixture is large enough to keep the particles suspended. In this way, plastic particles do not settle in the pipeline and clog the system.

[0019] The size of the plastic particles pumped through the system can vary, but they are typically between 0.1 μm and 20,000 μm in at least one dimension. In order to maintain the particles in suspension, the flow rate of the pump 14 is set to ensure that the particle velocity through the system is at least 20 cm / s. Particle velocities exceeding 20 cm / s or 30 cm / s provide a safety margin. The pump flow rate is set equal to the product of the desired particle velocity and the cross-sectional area of ​​the pipeline (pipe or tube) through which the mixture is pumped. Lower particle velocities are possible if a mixer is installed in the pipeline between the pump 14 and the regulator 26.

[0020] In the heating zone Z3, the heating chamber 18 raises the temperature to the reaction temperature or higher to start the depolymerization reaction, which is completed in the holding zone Z4. The length L of the holding pipe 22 in the holding zone Z4 depends on its cross-sectional area A, the flow rate Q of the pump and the holding time T required to complete the reaction at the reaction temperature: L=QT / A. The holding time can be in the range of 5 minutes to 10 minutes or even 1 minute to 60 minutes. The diameter of the pipeline passing through the zone is 1 cm to 10 cm, but can be as large as 100 cm. If a jacketed pipeline is used, the diameter of the jacket can range from 1.1 to 5.0 times the diameter of the inner pipeline through which the mixture is pumped.

Claims

1. A continuous flow method for depolymerizing plastics, the method comprising: (a) providing a mixture containing solid plastic particles in a solvent; (b) preheating the mixture by flowing the mixture through a heating channel in a preheating heat exchanger; (c) continuously flowing the mixture through the pipeline in the heating chamber at a particle velocity sufficient to keep the plastic particles suspended in the solvent and prevent the plastic particles from agglomerating and plugging the pipeline, the particle velocity being at least 30 cm / s; (d) transferring heat through the line in the heating chamber to heat the mixture to a reaction temperature of at least 150° C. to begin depolymerizing the plastic particles in the solvent into a homogeneous solution comprising a liquefied reaction product, and maintaining the mixture at the reaction temperature for at least one minute; (e) flowing the homogeneous solution through a cooling channel of the preheating heat exchanger after the homogeneous solution leaves the heating chamber, wherein the homogeneous solution is cooled while transferring heat to the mixture flowing through the heating channel in the preheating heat exchanger; (f) separating the solid reaction product from the solvent by one or more of decantation, filtration, centrifugation, pressurization and distillation, wherein the plastic consists of PET, and The solvent is composed of one or more of the following: ethylene glycol, methanol and water.

2. The continuous flow process of claim 1, wherein the liquefaction reaction product comprises oligomers.

3. The continuous flow method of claim 1, wherein the liquefaction reaction product comprises a monomer or a dimer.

4. The continuous flow method of claim 1 , wherein the liquefaction reaction product comprises one or more of the following: bis(2-hydroxyethyl)terephthalate, dimethyl terephthalate, terephthalic acid, bis(2-hydroxyethyl)naphthalate, bis(2-hydroxyethyl)furoate and their respective oligomers, acids, half esters, mixed esters; dioctyl terephthalate, diisobutyl terephthalate, dibutyl terephthalate, bisphenol A, lactic acid esters and bis(2-hydroxyethyl)terephthalamide.

5. The continuous flow process of claim 1, further comprising maintaining a system pressure above the vapor pressure of the solvent at the reaction temperature to prevent evaporation of the solvent.

6. The continuous flow process of claim 1, wherein step (a) further comprises mixing the solid plastic particles and the solvent with a catalyst to form the mixture.

7. The continuous flow process of claim 6, wherein the catalyst consists of one or more of: Zinc salts, titanium salts, manganese salts, magnesium salts, sodium hydroxide, potassium hydroxide, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,8-diazabicyclo[5.4.0]undec-7-ene, 4-dimethylaminopyridine and amines.

8. The continuous flow method of claim 7, wherein the zinc salt consists of one or more of: Zinc acetate and zinc chloride.

9. The continuous flow process of claim 7, wherein the magnesium salt is magnesium acetate.

10. The continuous flow method of claim 7, wherein the titanium salt consists of one or more of: Titanium tetraisopropoxide and titanium tetra-n-butoxide.

11. The continuous flow process of claim 7, wherein the amine is a trialkylamine.

12. The continuous flow method of claim 1, further comprising: (g) cooling the homogeneous solution in a cooling heat exchanger to a temperature below 50°C.

13. The continuous flow method of claim 12, further comprising: (h) allowing the cooled homogeneous solution to stand at room temperature for 0.5 hour to 100 hours to solidify the liquefied reaction product into the solid reaction product.

14. The continuous flow method of claim 1, further comprising: (i) reusing the solvent separated from the solid reaction product in the process.

15. The continuous flow method of claim 1, further comprising: (j) separating the solvent from the reaction product; (k) removing contaminants from the solvent by filtration, adsorbents, or a combination thereof; as well as (1) Reusing the solvent in the process by mixing solid plastic particles in the reused solvent to form the mixture.

16. The continuous flow method of claim 15, wherein the adsorbent consists of one or more of: Activated carbon, ion exchange resins, diatomaceous earth, sand, zeolites, clays and oxides.

17. The continuous flow process of claim 16, wherein the oxide consists of one or more of: silicon dioxide and aluminum oxide.

18. The continuous flow process of claim 1, wherein the plastic particles have a size in at least one dimension of 0.1 μm to 20,000 μm.

19. The continuous flow process of claim 1, wherein the solid plastic particles are in the form of one or more of the following: flakes, fines, granules, chunks, and powder.

20. The continuous flow process of claim 1, wherein the solid plastic particles are in the form of one or more of the following: flakes, fines, granules, chunks, and powder.

21. The continuous flow process of claim 1 wherein in step (d) the mixture is heated indirectly in the lines of the heating chamber by pumping a hot heat transfer fluid through the lines.

22. A system for continuous depolymerization of plastics, comprising: A pump that operates at a certain flow rate; a pipeline through which the pump continuously supplies a heterogeneous mixture comprising solid plastic particles in a solvent at a particle velocity of at least 30 cm / s; a cold entry zone in which the mixture is fed into the system by the pump; a heating zone that increases the temperature of the heterogeneous mixture flowing through the pipeline to a reaction temperature of at least 150° C., wherein conversion of the heterogeneous mixture containing the solid plastic particles into a homogeneous solution containing a liquefied reaction product begins in the heating zone; a preheating zone in which the mixture is preheated by flowing the mixture through a heating channel in a preheating heat exchanger, and after the homogeneous solution leaves the heating zone, flowing the homogeneous solution through a cooling channel of the preheating heat exchanger, wherein the homogeneous solution is cooled while transferring heat to the mixture flowing through the heating channel in the preheating heat exchanger; a holding tube that receives the heated heterogeneous mixture from the heating zone and maintains the reaction temperature for a holding time of at least one minute at a flow rate that completely converts the heterogeneous mixture containing the solid plastic particles into the homogeneous solution containing the liquefied reaction product; as well as a separator that separates the solvent from the solid reaction product by one or more of decantation, filtration, centrifugation, pressurization, and distillation, wherein the plastic consists of PET, and The solvent is composed of one or more of the following: ethylene glycol, methanol and water.

23. The system of claim 22, wherein the retaining tube is an insulating tube.

24. The system of claim 23, wherein the length of the holding tube is long enough to ensure complete conversion of the heterogeneous mixture into the homogeneous solution containing the liquefied reaction product.

25. The system of claim 24, wherein the holding time in the holding tube is between 1 minute and 60 minutes.

26. The system of claim 24, wherein the holding time in the holding tube is between 5 minutes and 10 minutes.

27. The system of claim 22, further comprising a mixer upstream of the heating zone, the mixer agitating the heterogeneous mixture using an agitator or a recirculating solvent.

28. The system of claim 22, wherein the heterogeneous mixture comprises the solid plastic particles, the solvent, and a catalyst.

29. The system of claim 22, wherein the solid plastic particles have a size in at least one dimension of 0.1 μm to 20,000 μm.

30. The system of claim 22, wherein the pump maintains the flow rate such that a particle velocity of the solid plastic particles exceeds 30 cm / s.

31. The system of claim 22, wherein the flow rate is set equal to the product of a desired particle velocity and a cross-sectional area of ​​the pipeline.

32. The system of claim 22, wherein the preheat heat exchanger indirectly preheats the heterogeneous mixture with the homogeneous solution containing the liquefied reaction product to reduce a retention time of the heterogeneous mixture in the heating zone and cool the homogeneous solution.

33. The system of claim 22, further comprising a reactor heat exchanger in the heating zone that raises the temperature of the heterogeneous mixture to the reaction temperature.

34. The system of claim 33, wherein a heat source is configured to heat a heat transfer fluid flowing through the heterogeneous mixture in the reactor heat exchanger to transfer heat to the heterogeneous mixture.

35. The system of claim 22, further comprising a back pressure regulator downstream of the heating zone, the back pressure regulator maintaining a system pressure above the vapor pressure of the solvent at the reaction temperature.

36. The system of claim 22, further comprising a cooler downstream of the heating zone and comprising a cooling heat exchanger, the homogeneous solution on one side of the heat exchanger being indirectly cooled by a cold liquid on the other side of the heat exchanger, wherein the cooler reduces the temperature of the homogeneous solution to below 50°C.

37. The system of claim 22, wherein the separator comprises a precipitation or crystallization tank, wherein the liquefied reaction product in the homogeneous solution solidifies into a solid reaction product and precipitates.

38. The system of claim 22, further comprising a contaminant removal section downstream of the separator, wherein the contaminant removal section removes reaction contaminants from the solvent, and wherein the contaminant removal section comprises a filter or an adsorbent.

39. The system of claim 38, wherein the filter comprises a size exclusion filter or a tangential flow filter.

40. The system of claim 38, wherein the adsorbent consists of one or more of: Activated carbon, ion exchange resins, diatomaceous earth, sand, zeolites, clays and oxides.

41. The system of claim 40, wherein the oxide consists of one or more of: silicon dioxide and aluminum oxide.

42. The system of claim 22, comprising a preheater upstream of the heating zone and a holding tube after the heating zone; wherein the preheater, the heating zone, and the holding tube each comprise a heat exchanger.

43. The system of claim 42, wherein each heat exchanger is a single shell and tube, multi-shell and tube, shell and coil, tube-in-tube, jacketed tube, plate, plate and shell, or plate and frame heat exchanger.

44. The system of claim 22, comprising a preheater upstream of the heating zone and a holding tube after the heating zone; wherein the preheater, the heating zone, and the holding tube comprise multiple sections of jacketed tubing having a jacket surrounding an inner tube, wherein the jacketed tubing is connected.

45. The system of claim 44, wherein the inner diameter of the inner tube in the jacketed tubing is 1 cm to 100 cm, and the diameter of the jacket is 1.1 to 5.0 times the diameter of the inner tube.

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