CROSS-REFERENCE TO RELATED APPLICATION

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

The invention relates generally to the depolymerization of resin, plastic, or polymer. More particularly, it relates to the depolymerization of waste plastic in a continuous process.

Plastic is conventionally depolymerized in large reaction vessels usually equipped with a heating jacket and an agitator. The depolymerization reaction is sequestered in the vessel until depolymerization is complete. After depolymerization the vessel is emptied and then refilled. Each batch is heated to speed up depolymerization and then cooled to produce viable raw material for new polymers. The batch process typically takes between 20 min and 800 min. Continuous operation is simulated by sequentially emptying and refilling a group of reaction vessels in round-robin fashion. 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 a parallel batch process.

SUMMARY

A process embodying features of the invention for depolymerizing plastic comprises: (a) continuously flowing a mixture containing solid plastic particles in a solvent through a line in a heating chamber at a particle speed great enough to maintain the plastic particles suspended in the solvent and prevent the plastic particles from agglomerating and clogging the line; and (b) transferring heat through the line in the heating chamber to heat the mixture to a reaction temperature to start the depolymerization of the plastic particles in the solvent into a homogeneous solution including a liquefied reaction product.

A system embodying features of the invention for the continuous depolymerization of plastic comprises a pump operating at a pump flow rate and a line through which the pump continuously feeds a heterogeneous mixture including particles of plastic in a solvent at a particle speed. A heating zone raises the temperature of the heterogeneous mixture flowing through the line to a reaction temperature of at least 150°C. The conversion of the heterogeneous mixture containing the plastic particles into a homogeneous solution containing a liquefied reaction product including monomer, dimer, oligomers and/or reaction side-products is started in the heating zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. l is a block diagram of a system embodying features of the invention for depolymerizing plastic.

FIG. 2 is a flowchart showing the progression of a volume of plastic undergoing a depolymerization process in the system of FIG. 1.

DETAILED DESCRIPTION

A system and a process for depolymerizing plastic are shown in FIGS. 1 and 2. The system and process may be used with various plastics such as, but not limited to, PET, modified PET, PET blends, PEN, PBT, PET-G, PLA, PGA, PLGA, PEF, copolyesters, polycarbonates, polyamides (Nylon), polyurethanes and combinations and blends. Depolymerization of the plastic into the following, but not limited to: (bis (2 -hydroxy ethyl) terephthalate (BHET), dimethyl terephthalate (DMT), terephthalic acid (TA), (bis (2- hydroxyethyl) naphthalate (BHEN), (bis (2-hydroxyethyl) Furanoate (BHEF) their respective oligomers, acids, half-esters, or mixed esters. Additionally, chemically useful compounds such as dioctyl terephthalate (DOTP), diisobutyl terephthalate (DITP), dibutyl terephthalate (DBTP), bisphenol A (BPA), lactates, bis (2-hydroxyethyl) ter ephthal amide (BHETA), and other terephthal amides.

Solid plastic particles of waste polyester material, in the form of flakes, fines, grain, granules, granola, lumps, chunks, and/or powder, are mixed with a solvent and a catalyst in a mixer 10 to produce a heterogeneous mixture 12. The mixer 10 can use an agitator, such as a propeller 13, stirrer, or other agitator or a recirculating solvent to do the mixing. Or the mixture can be premixed. Examples of solvents are, but not limited to, ethylene glycol (EG), diethylene glycol (DEG), glycol ethers, methanol, ethanol, propanol, butanol, 2-ethyl hexanol, tetramethyl cyclobutanediol (CBDM), cyclohexanedimethanol (CHDM), alcohols, ethanol amine, ionic liquids, polar protic solvents, polar aprotic solvents, and water. Examples of suitable catalysts include but 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); amine; trialkyl amine; and combinations of those catalysts. The heterogeneous mixture 12 is pumped through a series of connected lines, such as tubes or pipes, by a pump 14. No agitator, auger, or extruder is needed to advance the mixture through the system. The pump 14 operates at a flow rate great enough to move the mixture 12 through the system with a particle speed great enough to maintain the particles suspended in the solvent and to prevent the particles from agglomerating and clogging the lines. By operating continuously without stopping, the pump 14 flows the heterogeneous mixture through the system at a steady rate that makes the conversion of plastic into liquified product a function of position within the system rather than a function of time — as in batch systems.

An optional preheating heat exchanger (preheater) 16 is used to preheat the heterogeneous mixture 12. The preheater 16 can heat the heterogeneous mixture 12 by a heat source, such as a flame, steam, hot-oil or a circulated heat transfer fluid. Or the hot homogeneous solution containing the liquified product after the depolymerization reaction can be used in the preheater 16 to transfer heat to the heterogeneous mixture and, in the process, cool itself down.

The preheated heterogeneous mixture 12' flows continuously into and through a downstream heating chamber 18 in which depolymerization starts. The heating chamber 18 may be realized 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 in the reaction heat exchanger 18 by a heat source 20. The heat source 20 may directly heat the heterogeneous mixture with microwave radiation, direct flame, electrically heated pipe, inductively heated pipe, geothermal, magnon-drag thermoelectricity, or ohmically, as a few examples. Or the heat source 20 may indirectly heat the heterogeneous mixture by directly heating a heat transfer fluid external to the heating chamber 18. Examples of suitable transfer fluids are hot oil, a thermal fluid, a molten salt, and steam. The heated heat transfer fluid is then pumped past the line 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 contacted directly by the heat transfer fluid.

A hold 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 plastic to a homogeneous solution 24 containing the liquified product. The hold tube 22 may be realized by an insulated spool or coil of pipe or tube or as a jacketed pipe or vessel. Or the hold tube can be part of the heating chamber rather than a stand-alone component. The reaction is completed in the hold tube. The exiting homogeneous solution contains the solvent, the spent catalyst, and depolymerized plastic in the form of a liquefied reaction product that typically includes monomers, oligomers, and/or minor side-products from the reaction (e.g. half-esters, half-amides, mixed esters, mixed amides).

The homogeneous solution 24 is pumped continuously through the optional preheating heat exchanger 16 to cool itself and preheat the incoming heterogeneous mixture 12. A backpressure regulator 26 maintains a system pressure, e.g., 100 psi to 400 psi, above the vapor pressure of the solvent at the reaction temperature.

After flowing through the backpressure regulator 26, the homogeneous solution 24 flows through an optional chilling heat exchanger (chiller) 28 that uses cold water or other cooling heat transfer fluid from a chilled reservoir 30 to remove any excess heat that the preheater 16 did not reclaim.

After the solution is cooled, it is poured into precipitation or crystallization tanks and cooled until the liquefied product precipitates as a solid reaction product 34. The solvent is then decanted, filtered, centrifuged or distilled away from the solid reaction product. The solid reaction product may be subsequently filter-pressed to further separate it from any remaining solvent. The decanting, filtration, centrifugation or distillation of solvent, followed by the pressing to separate the solid reaction product 34 in the solution 24 from the solvent 36 is represented in FIG. 1 by a separator 32.

The separated solvent 36 is recirculated back to the mixer 10 for reuse. An optional solvent cleaning, purification or regeneration step may be required to remove reaction contaminants from the solvent feeding the subsequent heterogeneous mixture 12. Reaction contaminants may include particulate, ionic salts, anions, cations, spent catalyst, dyes, adhesives, components from blends, fillers and/or decomposed solvent. Contamination removal 42 may occur by passing the separated solvent 36 through filters and/or over sorbents such as activated charcoal, ion exchange resin, diatomaceous earth, sand, zeolites, clay, silica, alumina, oxides, size exclusion and/or tangential flow filtration. Contamination removal 42 of solvent 36 may be an in-line or off-line process. Contamination removal 42 may occur at the separated solvent step 36 or at the homogeneous solution step 24.

Thus, the system moves the heterogeneous mixture 12 through four zones: Z1 - a cold entry zone in which the mixture is fed into the system by the pump 14; Z2 - a preheating zone in which the mixture is heated in the preheater 16; Z3 - a heating zone in which the mixture is heated to raise its temperature to the reaction temperature; and Z4 - a hold zone in which the mixture is maintained at the reaction temperature to complete the conversion of the heterogeneous mixture into the homogeneous solution 24. The homogeneous solution 24 is moved through a cooling zone Z5 in which the homogeneous solution is cooled in the chiller 28 or by the transfer of heat to the incoming heterogeneous mixture 12 in the preheater 16. The pump 14 maintains a continuous flow rate through the system that ensures a particle speed of the heterogeneous mixture great enough to keep the particles in suspension. In that way the plastic particles do not settle in the lines and clog the system.

The size of plastic particles pumped through the system can vary, but they are typically between 0.1 pm and 20,000 pm in at least one dimension. To maintain the particles in suspension, the flow rate of the pump 14 is set to ensure a particle speed of at least 20 cm/s through the system. Particle speeds above 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 speed and the cross-sectional area of the lines (pipes or tubes) through which the mixture is pumped. If mixers are installed in the lines between the pump 14 and the regulator 26, lower particle speeds are possible.

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 hold zone Z4. The length L of the hold tube 22 in the hold zone Z4 depends on its cross-sectional area A, the pump’s flow rate Q, and the hold time T required at the reaction temperature to complete the reaction: L = QT/A. The hold time can range from 5 min to 10 min or even from 1 min to 60 min. The diameter of the lines running through the zones is 1 cm to 10 cm, but can be as great as 100 cm. If jacketed piping is used, the diameter of the jacket may range from 1.1 to 5.0 times the diameter of the inner pipe through which the mixture is pumped.