Thermoplastic vinyl polymer depolymerization process

Abstract

Thermoplastic vinyl polymers are depolymerized in a non-backmixing tubular reactor in the presence of a depolymerization catalyst at mild temperatures (150 to 350°C). Hydrogen is preferably present to facilitate depolymerization. The depolymerization produces useful low molecular weight chemicals such as alkanes and alkenes that are useful as a feedstock for steam cracking to produce ethylene and other polyolefins, or else as waxes and greases.

Description

[0001] THERMOPLASTIC VINYL POLYMER DEPOLYMERIZATION PROCESS

[0002] This invention relates to methods for recovering useful hydrocarbon values from a polyolefin.

[0003] Polyolefins are produced on the scale of tens of millions of metric tons annually. A large proportion of that annual production goes into products such as packaging films, food wrap, bottles, packaging foams and toys among many others, that are either single-use or else have short useful lifetimes. The used products present a formidable disposal problem. Polyolefins also go into products that have long useful lives, such as pipes and wire and cable insulation. However, even these products eventually reach the end of their life cycles and need to be disposed of.

[0004] Despite extensive efforts to recycle these materials, recycling has proven to be at best an inadequate solution to the disposal problem. Much of the material ends up in landfills or in the environment, where due to its nonbiodegradability can persist for centuries or longer.

[0005] Polyolefins can be depolymerized through pyrolysis to produce lower molecular weight products that are either useful by themselves or else can be converted easily to useful materials. For example, depolymerization products having melting temperatures of about 35 to 85°C can be used as waxes. Lower molecular weight gaseous and liquid depolymerization products, especially alkanes and alkenes having up to about 24 carbon atoms, form a feedstock that can be fed into a cracker to produce polymerizable alkenes, in particular ethylene.

[0006] Polyolefin depolymerization is generally performed at very high temperatures, such as 400°C or higher. Energy requirements are high to produce the needed temperatures, so the carbon footprint is unfavorable.

[0007] Milder conditions can be used if hydrogen is supplied into the depolymerization reaction. It has been reported that the so-called hydrogen-assisted or hydrogenolysis process can be operated at temperatures of as low at 225 °C in the presence of a specific combination of depolymerization catalysts (Liu et al., Sci. Adv. 2001, 7 :eabf8283). This work was performed in batch-type Parr reactor under 30 bar hydrogen pressure.

[0008] A continuous process is wanted to implement this technology on an industrial scale. Attempts have been made to pyro lyze polyolefins continuously. Wallis et al., Polymer Degradation and Stability 92, 1721-20 (2007) describes thermal degradation of high-density polyethylene (HDPE) in an extruder at 400-425°C. Serrano et al., J. Analytical and Applied Pyrolysis V 58-59, 789-801 (1 Apr 2001) and Aguado et al., Catalyst Today Vol. 75 (2002), 257- 262 describe the thermal and catalytic degradation of low-density polyethylene (LDPE) in a “continuous screw kiln reactor” at temperatures of 400-450°C.

[0009] W02012 / 076890A describes hydrogenolysis of LLDPE in an extruder in the presence of a zeolite catalyst at temperatures of 210 to 375°C. Hydrogen (about 0.6-0.7 parts per 100 parts by weight of the polyolefin) is injected into the extruder downstream from a heat seal and vented from a downstream port. Gaseous decomposition products are vented with the hydrogen. Conversions to pyrolysis products are very low except in the presence of large amounts of catalyst at 375°C. Little of the hydrogen is consumed, as the vented hydrogen stream contains at most 13.5% gaseous decomposition products.

[0010] This invention is in one aspect a process for depolymerizing a starting thermoplastic vinyl polymer having a number average molecular weight of at least 20,000 g / mol, comprising continuously feeding the starting thermoplastic vinyl polymer and a depolymerization catalyst into a heating zone of a non-backmixing tubular reactor equipped with mixing elements, heating the starting thermoplastic vinyl polymer and the catalyst to a temperature of 150°C to 350°C in the heating zone to produce a mixture of heat-softened thermoplastic vinyl polymer and catalyst; continuously forwarding the mixture of heat-softened thermoplastic vinyl polymer and catalyst into and through a downstream digestion zone of the non-backmixing tubular reactor and digesting the mixture of heat-softened thermoplastic vinyl polymer and catalyst in the digestion zone for a digestion period of 2 to 60 minutes at a temperature of 150°C to 350°C to convert at least a portion of the heat-softened thermoplastic vinyl polymer into depolymerized species having molecular weights of 5000 g / mol or less and produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less; continuously discharging the product mixture from the non-backmixing tubular reactor.

[0011] The invention is a second aspect is a process for depolymerizing a starting thermoplastic vinyl polymer having a number average molecular weight of at least 20,000 g / mol, comprising continuously feeding the starting thermoplastic vinyl polymer and a depolymerization catalyst into a heating zone of a non-backmixing tubular reactor equipped with mixing elements, heating the starting thermoplastic vinyl polymer and the catalyst to a temperature of 150°C to 350°C in the heating zone to produce a mixture of heat-softened thermoplastic vinyl polymer and catalyst; continuously forwarding the mixture of heat-softened thermoplastic vinyl polymer and catalyst into and through a downstream hydrogen mixing zone of the non-backmixing tubular reactor while producing a melt seal between the mixing zone and the hydrogen injection zone and maintaining the temperature at 150°C to 350°C; continuously injecting 1 to 10 parts by weight hydrogen per 100 parts by weight of the thermoplastic vinyl polymer into the hydrogen mixing zone of the non-backmixing tubular reactor and combining the hydrogen into the mixture of heat-softened thermoplastic vinyl polymer and catalyst while maintaining a temperature of 150°C to 350°C to form a reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen; continuously forwarding the reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen into and through a downstream digestion zone of the non- backmixing tubular reactor without headspace and digesting the reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen at a temperature of 150°C to 350°C in the digestion zone for a digestion period of 2 to 120 minutes to convert at least a portion of the heat-softened thermoplastic vinyl polymer into depolymerized species having molecular weights of 5000 g / mol or less and produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less; and continuously discharging the product mixture from the non-backmixing tubular reactor.

[0012] The Figure is a schematic view of an apparatus suitable for continuous depolymerization of a thermoplastic vinyl polymer in accordance with the invention.

[0013] In this invention, the starting thermoplastic vinyl polymer and catalyst are fed into a heating zone of non-backmixing tubular reactor. By “non-backmixed” or “non-backmixing”, it is meant that material flowing through the tubular reactor moves generally in a downstream direction from the point(s) of introduction toward the point(s) of removal without “backwards” movement in an upstream direction (z.e., toward the points of introduction). By “tubular”, it is meant the reactor has an elongated internal cavity, preferably having a length to internal diameter ratio of at least 5, at least 10, at least 20, or any arbitrarily larger value such as up to 500, up to 100 or up to 50, the elongated internal cavity forming a flow path from one or more inlets to one or more outlets where the product mixture is discharged. The tubular reactor may have any arbitrary cross-sectional geometry, including circular, ellipsoidal, polygonal, “figure 8”-shaped, etc., and the cross- sectional geometry and area may or may not be constant along the entire length of the tubular reactor. When the internal cavity is not circular in cross-section, the diameter is taken as the diameter of a circle having the same area as the cross-sectional area of the internal cavity. If the internal cavity is not constant in cross-sectional area, the length to diameter ratio is taken as the entire length of the internal cavity of the tubular reactor, divided by its average cross-sectional area.

[0014] The non-backmixing tubular reactor has internal mixing elements, which may be active, static, or a combination of both active and static types. The internal mixing elements may be present in all of the heating, hydrogen mixing zone (in the second aspect of the invention) and digestion zones, or in any subset of those. Preferably, a rotating screw with forwarding elements comprises at least a portion of the internal mixing elements, and such rotating screw extends through at least the heating zone and (in the second aspect of the invention) preferably into the hydrogen mixing zone. In particularly preferred embodiments, a single-screw extruder, more preferably a twin-screw extruder, constitutes at least a portion of the tubular reactor. In such embodiments, the operation of the screw(s) provides mixing and motive force to move the reactants through the reactor from the various inlet(s) to the outlet(s).

[0015] The starting thermoplastic vinyl polymer is a polymer of at least one monomer that has a polymerizable vinyl (CH2-CHR-) group where R is alkyl, substituted alkyl, phenyl, substituted phenyl or preferably hydrogen. It has a number average molecular weight of at least 20,000 g / mol, as measured by gel permeation chromatography. The number average molecular weight can be any higher value so long as the starting vinyl polymer is thermoplastic; therefore, the number average molecular weight may be, for example, at least 30,000, at least 40,000 or at least 50,000 g / mol and in some embodiments is up to 500,000 or up to 250,000 g / mol. The thermoplastic vinyl polymer may be a polymer of a vinyl monomer that contains heteroatoms, such as vinyl chloride, vinylidene chloride, vinyl alcohol, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, hydroxyethyl acrylate, t-butyl acrylate, and methyl methacrylate. It is generally preferred that the starting thermoplastic vinyl polymer is a hydrocarbon, i.e., does not contain heteroatoms. Examples of thermoplastic vinyl polymers include polyolefins such as polyethylene (including high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and metallocene polyethylene), polypropylene, and ethylene-propylene copolymers; and vinyl aromatic polymers such as polystyrene and copolymers of styrene (random, block and / or graft) with an olefm such as ethylene. Waste polymers (post-consumer resin) of any one or more of the foregoing types are useful and are a preferred feedstock as the process of the invention allows the waste materials to be reclaimed and converted to useful chemicals.

[0016] The catalyst is any that catalyzes the depolymerization and / or (in the second aspect of the invention) the hydrodepolymerization of the starting thermoplastic vinyl polymer. Examples of useful catalysts include Fe-Cu-Mo-P on alumina; hydrides or other compounds of Group 3 through Group 12 elements (2016 IUPAC table of the elements), such as nickel, palladium, platinum, cobalt, rhodium, iridium, iron, ruthenium, titanium, zirconium, scandium, lanthanum, zirconium, hafnium, titanium, tantalum, vanadium, niobium, chromium, molybdenum or tungsten, preferably supported on an inorganic support such as an alumina or silica / alumina support; zeolite catalysts such as are described in PCT / GB / 2010 / 050932, WO 2012 / 076890 and CA2202941A, including natural zeolites such as chabazite, mordenite, erionite, faujasite and clinoptilolite and synthetic zeolites such as zeolite A, zeolite x, zeolite y, zeolite L, FAU-type zeolite, ZSM-5 (MFI), Beta (BEA) and zeolite omega; and nanoparticle platinum deposited on a tungstate-zirconia as described by Liu et al., Sei. Adv. 2021 :7: eabf8283. It is generally suitable to provide 1 to 25 parts by weight catalyst per 100 parts by weight of the starting thermoplastic vinyl polymer. A preferred minimum amount is at least 5 parts or at least 7.5 parts of catalyst per 100 parts of starting thermoplastic vinyl polymer, and a preferred maximum amount is up to 20 parts, up to 15 parts or up to 12.5 parts, on the same basis.

[0017] The starting thermoplastic vinyl polymer and catalyst may be fed into the heating zone separately or together. It is often convenient to mix the catalyst with the starting thermoplastic vinyl polymer and feed them together as a mixture into the tubular reactor.

[0018] The starting thermoplastic vinyl polymer and catalyst preferably are fed into the tubular reactor in the form of particulate solids. Waste thermoplastic vinyl polymer may be comminuted into particles for feeding into the tubular reactor. This can be done continuously in an upstream apparatus integrated into the process or performed separately.

[0019] The starting thermoplastic vinyl polymer and catalyst, if in solid form, can be fed into the tubular reactor using a gravimetric feeder or other device such as is readily available for feeding a particulate solid into an extruder for melt processing. It is within the scope of the invention to heat-soften the starting thermoplastic vinyl polymer prior to introducing it into the heating zone of the tubular reactor, and then introduce the heat-softened thermoplastic vinyl polymer into the heating zone of the tubular reactor using a suitable pump or injector.

[0020] The starting thermoplastic vinyl polymer and the catalyst are continuously moved through the heating zone and brought to a temperature of 150 to 350°C in the heating zone to produce a mixture of heat-softened thermoplastic vinyl polymer and catalyst. The temperature may be 150 to 325°C, 150 to 300°C, 200 to 300°C, 250 to 300°C or 275 to 300°C. The heating step may cause volatile components contained within the starting thermoplastic vinyl polymer and / or the catalyst to volatilize to produce gases such as water vapor. Such gases may be vented from the non- backmixing tubular reactor upstream of the hydrogen mixing zone (when present) and digestion zone.

[0021] In the first aspect of the invention, the resulting mixture of heat-softened thermoplastic vinyl polymer and catalyst are then continuously forwarded into and through the downstream digestion zone of the non-backmixing tubular reactor. The mixture of heat-softened thermoplastic vinyl polymer and catalyst is then digested in the digestion zone at a temperature of 150°C to 350°C to convert at least a portion of the heat-softened thermoplastic vinyl polymer into depolymerized species having molecular weights of 5000 g / mol or less and produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less. The length of the digestion zone and the linear rate of travel of the reaction mixture through the non-backmixed tubular reactor are selected together to provide a digestion period of 2 to 120 minutes, the digestion period in the first embodiment of the invention being the residence time of the mixture of heat-softened thermoplastic vinyl polymer and catalyst from the point the mixture attains a temperature of at least 150°C until the product mixture is discharged from the non- backmixed tubular reactor. The digestion period preferably is at least 10 minutes or at least 20 minutes. Longer residence times tend to lead to more complete conversion of starting thermoplastic vinyl polymer to depolymerized species and to produce lower molecular weight depolymerized species. Preferred digestion periods are up to 60 minutes or up to 40 minutes. The digestion zone of the non-backmixing tubular reactor preferably is operated without head space i.e., completely filled), to help prevent gaseous decomposition products from separating out of the reaction mixture during the digestion process. The digestion zone may contain mixing elements as before, which may be static (static mixing elements) and / or active (such as a rotating screw with mixing and / or forwarding elements).

[0022] In some embodiments, digestion in the digestion zone is continued to produce a product mixture that contains hydrocarbon species that have 1 to 30, preferably 2 to 30 carbon atoms, which preferably are mainly linear or branched alkanes and alkenes suitable for steam cracking in an ethylene cracker to produce ethylene (and by-products). The product mixture may, for example, contain linear and / or branched alkanes and alkenes having 1 to 24 or 2 to 24 carbon atoms. The product mixture may in addition contain catalyst and / or catalyst residues, and residual quantities of thermoplastic vinyl polymer having molecular weights of greater than 5,000 g / mol, which have not become completely digested in the digestion zone. Some chars may form.

[0023] In the second aspect of the invention, the resulting mixture of heat-softened thermoplastic vinyl polymer and catalyst is continuously forwarded from the heating zone into and through a hydrogen mixing zone of the non-backmixing tubular reactor prior to being further continuously forwarded into and through the digestion zone. The hydrogen mixing zone contains mixing elements, which may be active or static as before, which mixing elements preferably include a rotating screw with kneading and / or forwarding elements. Hydrogen is continuously injected into hydrogen mixing zone of the non-backmixing tubular reactor and combined into the mixture of heat-softened thermoplastic vinyl polymer and catalyst that is moving continuously through the hydrogen mixing zone to form a reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen. The temperature is maintained at 150°C to 350°C. The temperature may be 150°C to 300°C, 200°C to 300°C, 250°C to 325°C, 250°C to 300°C or 275°C to 300°C. 1 to 10 parts by weight, preferably 1 to 8, 1 to 6 or 1.5 to 4 parts, hydrogen are injected and combined with the mixture of heat-softenened thermoplastic vinyl polymer and catalyst per 100 parts by weight of the thermoplastic vinyl polymer. The resulting reaction mixture is then forwarded continuously into and through a digestion zone as described before. The digestion zone of the non-backmixing tubular reactor preferably is operated without head space (i.e., completely filled), to help prevent hydrogen and gaseous decomposition products from separating out of the reaction mixture during the digestion process. Digestion periods are as described before, the digestion period in the second aspect being the residence time of the mixture of heat-softened thermoplastic vinyl polymer, hydrogen and catalyst from the point the time the hydrogen is

[0024] - n- injected until the product mixture is discharged from the non-backmixed tubular reactor, or until unreacted hydrogen is separated from the product mixture, whichever occurs first.

[0025] It is preferred to form a melt seal upstream of the hydrogen mixing zone to maintain pressure in the hydrogen mixing zone and digestion zone and prevent hydrogen from flowing upstream. Such a melt seal should be downstream of any vent for removing vapors and other gases produced in the heating zone. The melt seal when present marks the end of the heating zone and the start of the hydrogen mixing zone. Preferably, the non-backing mixing tubular reactor includes internal structures, such as a rotating screw or screws that include suitable elements, for producing the melt seal. Such internal structures are well known and described, for example, in US 5,490,725 and DE 19819324C2.

[0026] As with the first aspect of the invention, digestion in the digestion zone is continued to produce a product mixture that contains hydrocarbon species that have 1 to 30, preferably 2 to 30 carbon atoms, which preferably are mainly linear or branched alkanes and alkenes suitable for steam cracking in an ethylene cracker to produce ethylene (and by-products). The product mixture may, for example, contain linear and / or branched alkanes and alkenes having 1 to 24 or 2 to 24 carbon atoms. The product mixture may in addition contain unreacted hydrogen, catalyst and / or catalyst residues, and residual quantities of thermoplastic vinyl polymer having molecular weights of greater than 5,000 g / mol, which have not become completely digested in the digestion zone. Some chars may form.

[0027] The product mixture produced in the digestion zone in either aspect of the invention is then continuously discharged from the non-backmixing tubular reactor. In some embodiments, all components of the product mixture, including gasses, liquids and solids, are discharged together through a single outlet. It is also within the scope of the invention to discharge gaseous components of the product mixture separately from liquid and / or solid components of the product mixture, through a separate outlet such as a vent. In the hydrodepolymerization process of the second aspect of the invention, unreacted hydrogen is not removed until the reaction mixture has resided in the digestion zone for the digestion period as specified above.

[0028] The product mixture preferably is separated into various components. Gaseous products of the depolymerization reaction are readily separated from solid and liquid components of the product mixture by venting or similar methods. Solid and liquid components of the product mixture are readily separated using solid-liquid separation devices of various types, which are well-known in the art. Hydrocarbons having up to about 20 carbon atoms that may be present as vapors can be condensed to separate them from more volatile materials such as unreacted hydrogen.

[0029] Hydrocarbons having 2 to about 20 carbon atoms recovered from the product mixture are useful as feedstock for steam cracking to produce ethylene and / or other alkenes.

[0030] Hydrocarbons having about 20 to 100 carbon atoms, especially 20 to 50 carbon atoms, that are recovered from the product mixture are useful as greases or waxes. Alternatively, such hydrocarbons can be further processed to reduce their molecular weights even more to form suitable feedstocks for steam cracking. Hydrogen produced in the steam cracking process can be used as the source of hydrogen in the second aspect of this invention.

[0031] Residual thermoplastic vinyl polymer can be processed again through the non-backmixing tubular reactor if desired to further reduce its molecular weight. The residual thermoplastic vinyl polymer can be fed as a melt or a solid into the non-backmixing tubular reactor.

[0032] Spent catalyst or catalyst residues can be recovered, regenerated (thermally by calcination or otherwise) if necessary and if desired recycled or reused in the process of the invention by introducing it into the heating zone of the non-backmixing tubular reactor.

[0033] Turning to the Figure, an exemplary apparatus 1 for performing continuous depolymerization of a thermoplastic vinyl polymer in accordance with the invention includes non- backmixing tubular reactor 20, which in the embodiment shown includes single- or twin-screw extruder 2 and static mixer 3, which are operated in tandem with extruder 2 feeding static mixer 3.

[0034] In alternative embodiments, a single- or twin-screw extruder constitutes the entire non- backmixing tubular reactor. In other alternative embodiments, the non-backmixing tubular reactor comprises two or more single- or twin-screw extruders operated in tandem, with an upstream extruder feeding one or more downstream extruders in series. In still other alternative embodiments, a progressive cavity pump constitutes all or a part of the non-backmixing tubular reactor. For example, a progressive cavity pump may feed one or more downstream static mixers.

[0035] Tubular reactor 20 in the Figure includes a port 10 for introducing the starting thermoplastic vinyl polymer into heating zone 28 of tubular reactor 20. Typically, a solids feeder 4 is provided to transport and / or meter the starting thermoplastic vinyl polymer from a storage vessel or other upstream apparatus (not shown) into tubular reactor 20. Solid depolymerization catalyst is conveniently mixed with the starting thermoplastic vinyl polymer and fed together with the starting thermoplastic vinyl polymer through port 10 into heating zone 28 of tubular reactor 20. In alternative embodiments, depolymerization catalyst is introduced separately into heating zone 28 of tubular reactor 20 via a separate port.

[0036] Non-backmixing tubular reactor 20 is equipped with heating apparatus to maintain the mixture of starting thermoplastic vinyl polymer, catalyst and (in the second embodiment of the invention) hydrogen with temperature ranges as described before. In the particular embodiment shown in the Figure, extruder 2 is fitted with a heating jacket or other heating apparatus which during operation brings the starting thermoplastic vinyl polymer introduced through port 10 at a temperature as described before. Extruder 2 may have multiple, independently operably heating zones arranged sequentially along its length. In a preferred process, the starting thermoplastic vinyl polymer is introduced into tubular reactor 20 through port 10 in the form of a solid which is heated to the aforementioned temperature and heat-softened within extruder 2 upstream of hydrogen injection port 11. However, it is within the scope of the invention to introduce the starting thermoplastic vinyl polymer in the form of a melt, i.e., the starting thermplatic vinyl polymer is heat-softened prior to introduction into tubular reactor 20.

[0037] Optional vent 5 preferably is provided to remove volatiles (such as water vapor) from the starting thermoplastic vinyl polymer and / or catalyst from tubular reactor, upstream of hydrogen injection port 11.

[0038] In the preferred hydrodepolymerization process shown in the Figure, hydrogen is introduced into hydrogen mixing zone 29 of tubular reactor 20 via port hydrogen injection port 11. Hydrogen injection port 11 and hydrogen mixing zone 29 are located downstream of port 10, at a point along the length of tubular reactor 20 at which the starting thermoplastic vinyl polymer is heat-softened. In this embodiment, hydrogen mixing zone 29 is located within extruder 2, the rotating screw or screws (with attached elements) of which mix the hydrogen into the heat-softened thermoplastic vinyl polymer and catalyst forwarded into hydrogen mixing zone 29 from heating zone 28. Metering apparatus such as mass flow controller 6 optionally but preferably provides control over mass flow rates.

[0039] Preferably, extruder 2 includes elements that produce melt seal 25 upstream of hydrogen injection port 11 and downstream of port 10 and vent 5 (when present) to prevent hydrogen from traveling upstream from hydrogen injection port 11. In the particular embodiment shown in the Figure, hydrogen mixing zone 29 is bounded by melt seal 25 on the upstream side and extends along the length of tubular reactor 20 in a downstream direction until the injected hydrogen has become mixed into the mixture of heat-softened thermoplastic vinyl polymer and catalyst. The approximate location of this point is indicated by the dotted line identified by reference numeral 21 in the Figure.

[0040] In the particular embodiment shown in the Figure, digestion zone 26 starts at the end of hydrogen mixing zone 29 (at the point indicated by by the dotted line identified by reference numeral 21) and extends downstream through the entire remaining length of extruder 2 and the entire length of static mixer 3. The temperature in the digestion zone is maintained as indicated before. At least a portion of the heat-softened thermoplastic vinyl polymer is converted in digestion zone 26 into depolymerized species having molecular weights of 5000 g / mol or less to produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less.

[0041] In the embodiment shown in The Figure, optional valve 7 resides at an outlet end of tubular reactor 20. Valve 7 restricts the flow of the product mixture out of tubular reactor 20, thereby regulating the rate of flow of product mixture through and out of digestion zone 26 and thereby providing a means for adjusting digestion period. By providing backpressure, optional valve 7 also ensures that digestion zone 26 is completely filled during operation, without headspace, so hydrogen and gases produced in the depolymerization reaction remain mixed into the liquid components. In lieu of valve 7, tubular reactor 20 may have a constricted outlet that performs a like function.

[0042] In the embodiment shown in the Figure, the entire product mixture, including liquid depolymerization products, gaseous depolymerization products, residual hydrogen (if any), catalyst, any unreacted starting thermoplastic vinyl polymer and by-products (if any), is discharged from tubular reactor 20 through outlet 9. In this embodiment, the product mixture is introduced into various equipment for separation into solid, liquid and gaseous components and recovery of useful products and catalyst. In the particular embodiment shown, the entire product mixture is discharged into separator 8 where solid and higher-boiling liquids are separated from gases and lower-boiling liquids. The solid and liquid components are withdrawn through solid / liquid product outlet 15 and may be further separated in optional subsequent apparatus that is not shown. Gases are transferred to condenser 12 where less volatile reaction products such as C6-C20 hydrocarbons are condensed to form a liquid product stream that is withdrawn through liquid removal port 14. This liquid product stream may be recombined with liquids withdrawn from separator 8 via solid / liquid product outlet 15. Gases that do not volatilize in condenser 12 are transferred to knockout vessels 17 for further separation and purification. Gaseous reaction products are withdrawn from the system via line 13. Any liquid depolymerization products recovered in knock-out vessels 17 can be combined with the liquid product stream that is withdrawn through liquid removal port 14 and / or through solid / liquid product outlet 15. Residual hydrogen (if any) may be separated from gaseous hydrocarbon products.

[0043] The following examples are provided to illustrate the invention and are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

[0044] Examples 1-14, 4A, 12A and 13A

[0045] In Examples 1-14, virgin high-density polyethylene (HDPE) is processed through an apparatus as generally shown in the Figure. Extruder 2 is a Thermo Fisher Scientific Process parallel co-rotating intermesh 11 -mm diameter twin screw extruder with clamshell barrel. The extruder length is 44 L / D that includes 40 L / D barrel and a 4 L / D barrel extension. Extruder 2 has a feed port 10 (in heating zone 1, which is water-cooled), several separate downstream heating zones (zones 2-8) and a heated die. Vent 5 is positioned in heating zone 4. A MiniTwin MTO gravimetric feeder supplies starting polymer / catalyst mixture into port 10. A SIERRA SMARTRAK 100 mass flow controller supplies hydrogen through hydrogen injection port 11, which is located in hydrogen mixing zone 29 of extruder 2.

[0046] Static mixer 3 is a heated, 0.5-inch outside diameter, 23.75 inch long, 32 element Kenics type tube mixer.

[0047] Valve 7 is a Swagelok SS-18RF8 high temperature valve that provides back pressure to the system, to keep digestion zone 26 full and control residence time.

[0048] Heating zone 1 is cooled to 10°C. Heating zones 2-5 are set at 160°C, 180°, 180°C, and 180°C, respectively. The remaining heating zones, static mixer 3 and valve 7 are set at 250-300°C, as indicated in the following Table 1 . The screw speed is set at 600 rpm.

[0049] The thermoplastic vinyl polymer samples are manually premixed with catalyst powders as indicated in Table 1 below, hand shaken, loaded into the gravimetric feeder and fed continuously into extruder 2 via port 10 at a rate of 100 g / hour until the apparatus is filled with polymer. A melt seal is produced between vent 5 and hydrogen injection port 11. Then, hydrogen is injected continuously into extruder 2 via hydrogen injection port 11 and mixed into the mixture of heat- softened thermoplastic vinyl polymer and catalyst in hydrogen mixing zone 29. The amount of hydrogen fed is as indicated in the following Table. The resulting reaction mixture is then continuously transferred by operation of the screws into digestion zone 26, which extends through the entire length of static mixer 3. Valve 7 is set to establish a digestion period of approximately 19 to 22 minutes, calculated as the residence time of the reactor contents as it travels from hydrogen injection port 11 to outlet 9. This produces a product mixture which exits continuously through outlet 9 and flows into 2-inch diameter glass separator 8. Solids and liquids are removed from separator 8 through solid / liquid product outlet 15. Gasses are passed through water-cooled condenser 12 in which residual low-boiling liquid hydrocarbon products are condensed and removed from the system through liquid outlet 14. The remaining gasses are passed through glass knock-out pots 17, removed through gas outlet 13 and captured for analysis.

[0050] The catalysts used in these examples are as follows;

[0051] Catalyst A: An HY (FAU)-zeolite sold as CBV 720 by Zeolyst International, having an SiCh / AhCh ratio of 30 and 780 m2 / g surface area.

[0052] Catalyst B: 1% platinum deposited onto Catalyst A.

[0053] Catalyst C: A 50 / 50 mixture of Catalyst A and a platinum on tungstenated zirconia. Zirconia is tungstenated according to the method described in Liu et al., Sci. Adv. 2001, 7 :eabf8283, and 0.5 wt-% platinum is deposited onto the tungstenated zirconia.

[0054] Examples 4A, 12A and 13A are performed in the same manner as Examples 4, 12 and 13, respectively, except the static mixer is removed from the system. Product mixture is discharged from extruder 2 directly into separator 8, reducing the digestion period to approximately 4 to 5 minutes.

[0055] Conversion is measured by weighing the solids exiting outlet 9 and comparing with the amount of polymer fed into extruder 1 , adjusting for the weight of catalyst on the assumption that all catalyst passes through the apparatus and is removed as a solid. The converted materials are all room temperature liquids or gasses having molecular weights mainly below 500 g / mol. Table 1 — HDPE Depolymerizations

[0056] *The static mixer is removed for these experiments, reducing the digestion period to about 4-5 minutes.

[0057] The data in Table 1 illustrates the effect of temperature, choice of catalyst and amount and residence time on conversion. Increasing temperature from 250°C to 275°C and then to 300°C increases conversion significantly, as can be seen by comparing Example 8 with Examples 5 and 2, Example 9 with Examples 6 and 3 and Example 10 with Examples 7 and 4. The lower conversions at lower temperature can be improved by increasing digestion period. Decreasing the amount of catalyst reduces conversion under these conditions; the lower conversions when using less catalyst again can be improved by increasing digestion period. Adding hydrogen increases conversion when supplied in an amount up to 3% by weight based on the weight of starting resin. As shown by Example 14, further increasing the amount of hydrogen to 5% provides no further benefit in conversion (compare to Examples 3 and 4). The mixture of zeolite and platinum on tungstenated zirconia catalyst (Catalyst C) in these experiments produces by far the highest conversion at nearly 90% (Example 12).

[0058] Examples 15-20

[0059] Post-consumer HDPE (KWR 101-150 from KW Plastics, “PCR HDPE”) and postconsumer LDPE (Avangard 150, from Avangard Innovations, “PCR LDPE”) are separately processed in the same manner as described for Examples 1-14. Resin, catalyst type and amount, amount of hydrogen, digestion period and conversion are as indicated in Table 2. The converted materials are all room temperature liquids or gasses having molecular weights mainly below 500 g / mol.

[0060] Table 2 - Post-Consumer Resin Depolymerizations

[0061] The data in Table 2 shows that post-consumer polyethylene resins are successfully depolymerized in accordance with the invention. As before, adding hydrogen in amounts from 1 to 3% significantly increases conversion. Higher conversions are readily obtained with longer residence times. Example 21

[0062] Virgin LDPE is processed in the same manner as described for Examples 4A, 12A and 13 A. Catalyst type and amount, amount of hydrogen, digestion period and conversion are as indicated in Table 3. The converted materials are all room temperature liquids or gasses having molecular weights mainly below 500 g / mol.

[0063] Table 3 - Virgin LDPE Depolymerizations

[0064] Examples 22-23

[0065] Virgin polypropylene is processed in the same manner as described for Examples 4A, 12A and 13 A. Catalyst type and amount, amount of hydrogen, digestion period and conversion are as indicated in Table 4. The converted materials are all room temperature liquids or gasses having molecular weights mainly below 500 g / mol.

[0066] Table 4 - Virgin Polypropylene Depolymerizations

Claims

CLAIMS:

1. A process for depolymerizing a starting thermoplastic vinyl polymer having a number average molecular weight of at least 20,000 g / mol, comprising continuously feeding the starting thermoplastic vinyl polymer and a depolymerization catalyst into a heating zone of a non-backmixing tubular reactor equipped with mixing elements, heating the starting thermoplastic vinyl polymer and the catalyst to a temperature of 150 to 35O°C in the heating zone to produce a mixture of heat- softened thermoplastic vinyl polymer and catalyst; continuously forwarding the mixture of heat-softened thermoplastic vinyl polymer and catalyst into and through a downstream digestion zone of the non-backmixing tubular reactor and digesting the mixture of heat-softened thermoplastic vinyl polymer and catalyst in the digestion zone for a digestion period of 2 to 120 minutes at a temperature of 150°C to 350°C to convert at least a portion of the heat-softened thermoplastic vinyl polymer into depolymerized species having molecular weights of 5000 g / mol or less and produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less; and continuously discharging the product mixture from the non-backmixing tubular reactor.

2. The process of claim 1 wherein hydrogen is injected into the mixture of heat-softened thermoplastic vinyl polymer and catalyst.

3. A process for depolymerizing a starting thermoplastic vinyl polymer having a number average molecular weight of at least 20,000 g / mol, comprising continuously feeding the starting thermoplastic vinyl polymer and a depolymerization catalyst into a heating zone of a non-backmixing tubular reactor equipped with mixing elements, heating the starting thermoplastic vinyl polymer and the catalyst to a temperature of 150°C to 350°C in the heating zone to produce a mixture of heat-softened thermoplastic vinyl polymer and catalyst; continuously forwarding the mixture of heat-softened thermoplastic vinyl polymer and catalyst into and through a downstream hydrogen mixing zone of the non-backmixing tubularreactor while producing a melt seal between the mixing zone and the hydrogen injection zone and maintaining the temperature at 150°C to 350°C; continuously injecting 1 to 10 parts by weight hydrogen per 100 parts by weight of the thermoplastic vinyl polymer into hydrogen mixing zone of the non-backmixing tubular reactor and combining the hydrogen into the mixture of heat-softened thermoplastic vinyl polymer and catalyst while maintaining a temperature of 150°C to 350°C to form a reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen; continuously forwarding the reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen into and through a downstream digestion zone of the non- backmixing tubular reactor under plug flow conditions and digesting the reaction mixture comprising the heat-softened thermoplastic vinyl polymer, catalyst and hydrogen at a temperature of 150°C to 35O°C in the digestion zone for a digestion period of 2 to 120 minutes to convert at least a portion of the heat-softened thermoplastic vinyl polymer into depolymerized species having molecular weights of 5000 g / mol or less and produce a product mixture comprising the depolymerized species having molecular weights of 5000 g / mol or less; and continuously discharging the product mixture from the non-backmixing tubular reactor.

4. The process of claim 3 wherein 1.5 to 4 parts by weight hydrogen per 100 parts by weight of the thermoplastic vinyl polymer are injected into hydrogen mixing zone of the non- backmixing tubular reactor.

5. The process of any preceding claim wherein the starting thermoplastic vinyl polymer is a hydrocarbon.

6. The process of claim 5 wherein the starting thermoplastic vinyl polymer is a polyolefin.

7. The process of claim 6 wherein the polyolefin is polyethylene or polypropylene.

8. The process of any preceding claim, wherein the non-backmixing tubular reactor comprises a single-screw or twin-screw extruder.

9. The process of any preceding claim, wherein the non-backmixing tubular reactor comprises a single-screw or twin-screw extruder and a static mixer in tandem.

10. The process of any preceding claim wherein the digestion period is 10 to 60 minutes.

11. The process of any preceding claim wherein the depolymerization catalyst comprises a zeolite, a nanoparticle platinum deposited on a tungstenated zirconia catalyst, or a mixture thereof.

12. The process of any claim wherein the depolymerization catalyst is present in an amount of 5 to 20 parts by weight per 100 parts by weight of starting thermoplastic vinyl polymer.

13. A depolymerized product obtained in the process of any of claims 1-11.

14. The depolymerized product of claim 13 which comprises linear or branched alkanes and alkenes having 1 to 24 carbon atoms.

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