Patent Publication Number: US-2023159830-A1

Title: Systems and methods for recycling waste plastics

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/544,613, filed Dec. 7, 2021, which claims the benefit of U.S. Provisional Application No. 63/123,965, filed Dec. 10, 2020, the contents of each of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates generally to the recycling of waste plastics. Certain embodiments relate more specifically to systems and methods for pyrolyzing plastic feedstock to convert waste plastics into hydrocarbon based oil products. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The written disclosure herein describes illustrative embodiments that are non-limiting and non-exhaustive. Reference is made to certain of such illustrative embodiments that are depicted in the figures, in which: 
         FIG.  1    provides a schematic diagram of an embodiment of a plastic recycling system; and 
         FIG.  2    provides a schematic diagram of another embodiment of a plastic recycling system. 
     
    
    
     DETAILED DESCRIPTION 
     Certain embodiments of systems and methods described herein are configured for efficient recycling of waste plastics. Some systems and methods can quickly and simply convert waste plastics into one or more purified organic molecular species, which can be considered a hydrocarbon oil product. The hydrocarbon oil product may be readily stored, transported, and/or refined into fuel or other commercially relevant materials. 
     In some embodiments, waste plastic feedstock can be fed continuously through the systems disclosed herein. The feedstock may be pre-melted via a mixing, heating and compacting apparatus prior to introduction into a pyrolysis reactor, which then heats the pre-melted feedstock such that the feedstock transitions into a vapor (e.g., one or more gases) for further processing. In some instances, the vapor can be introduced into a condenser and directly contacted with a pH adjusted solution (or other process solution), which can, in some instances, absorb a portion of the vapor and condense another portion thereof. The condensed material can comprise one or more organic molecular species that can be termed herein as a hydrocarbon oil product. The hydrocarbon oil product can be separated from the other portions of the vapor that are absorbed into the pH adjusted solution, and thus the hydrocarbon oil product can be of a clean or purified quality such that it may be readily refined from a crude state. In other instances, other condensing apparatuses or methodologies may be used to condense out desirable products from the vapor discharged from the pyrolysis reactor or reactors. 
     In some instances, the feedstock may comprise waste plastics including an appreciable amount of halide compounds or heteroatoms from one or more sources of contamination and the system may be configured to recover a hydrocarbon oil product therefrom. The system may include a mixing, heating and compacting apparatus configured to receive a supply of the waste plastic feedstock and to output a densified melt of plastic material. An amendment comprising or consisting of alkaline earth oxides and/or hydroxides, oxides of iron, and/or oxides of aluminum may be mixed, heated and compacted with the waste plastic based feedstock in the mixing, heating and compacting apparatus to form a densified melt of plastic material including the amendment. The system may further comprise a pyrolysis reactor configured to receive the densified melt of plastic material including the amendment, pyrolyze the densified melt of plastic material including the amendment, and output a hydrocarbon gas stream and a solids residue stream, the solid residue stream including a substantial portion of the halide compounds or heteroatoms of the waste plastic based feedstock via interaction of the halide compounds or heteroatoms with the amendment. Additionally, one or more condensers may be provided to condense out a hydrocarbon oil product from the hydrocarbon gas stream output from the pyrolysis reactor. In some instances, the system may be configured to condense out a tars product from the hydrocarbon gas stream output from the pyrolysis reactor with a quenching apparatus. The system may be further configured to route the tars product to a secondary pyrolysis reactor to generate a secondary hydrocarbon gas stream and a secondary solids residue stream. The secondary hydrocarbon gas stream may be further processed and combined with an altered hydrocarbon gas stream output from the quenching apparatus for subsequent processing by the one or more condensers to recover a hydrocarbon oil product. 
     Various illustrative embodiments of inventive systems and methods will now be described. Advantages of the systems and methods, as well as features and steps thereof, respectively, will be apparent from the disclosure that follows, including the Figures. 
       FIG.  1    provides a schematic diagram of an embodiment of a plastic recycling system  10 . The plastic recycling system includes a heating system  12  that is configured to deliver heat to a plastic feedstock  14 . The heating system  12  can comprise any suitable heating mechanism, such as, for example, a combustion burner, a fluidized bed burner, a retort, or any other such heating system. In some instances, the heating system comprises a pyrolysis recovery unit (PRU). The PRU may include a dual screw feed mechanism to receive the plastic feedstock and simultaneously transport and pyrolyze the feedstock, and to output a hydrocarbon gas stream  16  and a solids residue stream  18 . The PRU may include multiple successive zones of heating along a length thereof 
     The plastic feedstock  14  can comprise waste plastics of one or more varieties (e.g., mixed plastics), and may include trace amounts of non-plastic contamination or impurities. For example, the impurities may be of an external nature (e.g., water, foodstuffs, labeling, soil, paper, or cellulose waste) or may result from internal amendments of the waste plastics, such as fillers, plasticizers and other amendments, introduced at the time of manufacture of the waste plastics (e.g., glass, metal, iron, bromine, and/or chlorine). The plastic feedstock  14  may be provided in a ground, chipped, or other form that can promote the transfer of heat thereto. 
     The plastic feedstock  14  may be fed to the system in a continuous manner. A feed apparatus can include bins, hoppers, conveyors, mixers, heaters and compactors designed to provide a continuous material feed. The feed apparatus may comprise a mixing, heating and compacting apparatus  20  and may include a compactor and a pre-melter, such as a mixer designed to receive the feedstock and output a continuous stream of densified plastic melt. In other instances, the feedstock may be fed directly into the heating system (e.g., PRU)  12  without being subjected to pre-melting. 
     The heat provided by the heating system (e.g., pyrolysis recovery unit)  12  can be sufficient to crack or depolymerize the plastic feedstock  14  and convert at least a portion thereof into a vapor. The vapor can include one or more gaseous organic species, one or more gaseous inorganic species, and/or one or more varieties of entrained particles. In particular, the vapor can include depolymerized non-polar organic gases, which may be desirable for collection and refinement, and which can be mixed with impurities. The organic gases can include, for example, one or more paraffins, olefins, naphthenes, aromatics, and/or other classes of hydrocarbon materials. The mixed-in impurities can include, for example, inorganic acids (e.g., hydrochloric acid, hydrobromic acid), entrained metals or metalloids (e.g., cadmium, iron, antimony); and/or organic acids (e.g., terephthalic acid). In some embodiments, the vapor may include additional molecular species, such as polar organic molecules, which may or may not be collected with the non-polar organic molecules. For example, the vapor can include one or more alcohols, ketones, ethers, phenols, carboxylic acids, or other polar organic molecules. 
     In some embodiments, the plastic feedstock may be heated under vacuum conditions, or under negative pressure. In other embodiments, the plastic feedstock may be heated under positive pressure. In still other or further embodiments, the plastic feedstock may be heated under atmospheric pressure conditions, or under any suitable combination of the foregoing (e.g., the pressure may be varied during a heating event). 
     The vapor can be delivered to a vapor treatment system  22  that effects a phase change of at least a portion of the vapor such that certain molecules transition from a gaseous state to a liquid state. The vapor treatment system  22  may also be referred to as a vapor treatment unit or a vapor treatment vessel. The vapor treatment system  22  may include a pH adjusted solution (or other process solution) that is used to effect the condensation. Moreover, the pH adjusted solution can be configured to absorb at least a portion of the impurities from the vapor. Embodiments of the solution can readily absorb organic acids, inorganic acids, metals, metalloids, and/or certain polar organic molecules. The term “pH adjusted solution” is used in a broad sense and includes solutions that are not pH neutral and that exhibit any or all of the various properties described herein. For example, a pH adjusted solution can be formulated to remove impurities from the vapor, and in further embodiments, can be immiscible with condensed oils so as to be readily separated therefrom. For example, in some embodiments, the pH adjusted solution can comprise an acidic solution, which may, in some cases, be strongly acidic. In further embodiments, the pH adjusted solution can comprise a buffered aqueous solution adjusted to a desired pH value. In various embodiment, the pH adjusted solution can have a pH value that is less than 7, less than about 6.5, less than about 6, less than about 5.5, less than about 5, less than about 4, or less than about 3. 
     The pH adjusted solution can include one or more chemical amendments of any suitable variety to achieve the desired properties of the solution. Such properties can include, for example, the ability to remove one or more impurities from the vapor and/or a high immiscibility with oil. Adjustment or optimization of one or more of foregoing properties may be achieved by altering the concentration of the one or more chemical amendments within the pH adjusted solution. For example, the presence, combination, and/or concentration of one or more materials within the pH adjusted solution can optimize removal of contaminants from the vapor as it interacts with the pH adjusted solution. In various embodiments, the pH adjusted solution can include strong and/or weak inorganic acids (e.g., hydrochloric acid, acetic acid), one or more pH buffer solutions (e.g., acetic acid+sodium acetate), one or more chelating agents (e.g., ethylenediaminetetraacetic acid (EDTA)), and/or one or more coagulants and/or flocculants (e.g., calcium hydroxide, polyacrylamide). 
     The vapor treatment system  22  can be configured to effect direct contact between the vapor received therein and the pH adjusted solution (or other process solution). For example, as further discussed below, in some embodiments, the pH adjusted solution may be sprayed into contact with the vapor, whereas in other embodiments, the vapor may be bubbled through the solution. The pH adjusted solution can absorb or dissolve portions of the vapor (e.g., organic acids, inorganic acids, metals, metalloids, and/or certain polar organic molecules). The pH adjusted solution also can be provided at a lower temperature than that of the vapor such that the solution condenses at least those portions of the vapor that are immiscible therein (e.g., non-polar organic molecules). 
     Those portions of the condensed vapor that are immiscible in the pH adjusted solution (i.e., the hydrophobic portions) can be readily separated from the solution. In some embodiments, the separation (or at least one or more stages thereof) takes place within the vapor treatment system, whereas in other embodiments, the separation (or at least one or more stages thereof) takes place within a separator  24  that is independent of the vapor treatment system  22 . 
     In some embodiments, the immiscible portions are removed from the vapor treatment system as a form of crude oil  26 . The crude oil  26  thus can have few or no impurities, as the impurities that were present in the plastic feedstock are dissolved or absorbed into the pH adjusted solution. In some embodiments, at least some of the dissolved or absorbed impurities can remain within the pH adjusted solution within the vapor treatment system  22 . For example, in some instances, after the pH adjusted solution has amassed the impurities, it may continue to be used within the vapor treatment system  22 , such that the impurities are not removed (at least not immediately) from the vapor treatment system  22 . In other or further embodiments, dissolved or absorbed impurities are removed from the vapor treatment system  22  separately from the oil  26 . 
     Certain classes of polar organic molecules may only partially (or at least partially) partition into the pH adjusted solution. For example, a portion of certain alcohols, ketones, ethers, phenols, carboxylic acids, and/or other polar organic molecules may partition into the pH adjusted solution and another portion thereof may partition into the crude oil. Accordingly, in some embodiments, crude oil that includes a portion of a species of polar organic molecules may be separated from a pH adjusted solution that contains another portion of the species of polar organic molecules. 
     The vapor may include portions that do not condense within the vapor treatment system  22  and are not absorbed by the pH adjusted solution. Such non-condensable gases  29  can be removed separately from the vapor treatment system  22 , and may be combusted or disposed of in any other suitable manner. In various embodiments, the vapor treatment system  22  may operate under vacuum conditions, or under negative pressure. In other embodiments, the vapor treatment  22  system may operate under positive pressure. In still other or further embodiments, the vapor treatment system  22  may operate under atmospheric pressure conditions, or under any suitable combination of the foregoing (e.g., the pressure may be varied during a condensing event). 
     The system can be well suited for quickly cracking or depolymerizing the plastic feedstock. For example, in some embodiments, heating of the plastic feedstock and conversion thereof into the vapor can be performed at high temperatures at which a variety of different molecular species may be gasified simultaneously. Such different molecular species might have different vaporization temperatures at a given pressure, and a temperature at which the plastic feedstock is heated can exceed this temperature for some or all of the molecular species. The molecular species can then be separated from each other when the vapor is delivered to the vapor treatment system, as previously described. Accordingly, the system can operate without the heating system slowly heating up and occasionally holding steady at various discreet temperature levels along the way so as to allow for individual molecular species to be gasified sequentially. It is to be appreciated, however, the system may also be used in an operational mode in which the heating system and the plastic feedstock progress through a series of sequential heating steps or levels, as just described. 
     The system  10  can include a vacuum system  30  that is configured to maintain a negative pressure within the heating system (e.g., PRU)  12  and within the vapor treatment system  22 . The vacuum system  30  can continuously evacuate gases from the heating system (e.g., PRU)  12  such that depolymerization of the plastic feedstock occurs in an oxygen-deprived or oxygen-free environment. The vacuum system  30  draws the vapor into the vapor treatment system  22 , where it is contacted by the pH adjusted solution, or non-PH adjusted solution, or otherwise processed by a condensing apparatus or device. The vacuum system  30  draws the non-condensable gases  29  from the vapor treatment system  22 , and may distribute them to a combustion unit or other suitable disposal device  32  (e.g., emissions control device). In some instances, the non-condensable gases  29  from the vapor treatment system  22  may be routed to a heat exchanger or other apparatus or system for energy recovering purposes. For instance, the non-condensable gases  29  from the vapor treatment system  22  may be routed to a heat exchanger to supply heat for other aspects of the systems and methods of processing waste plastics described herein, such as, for example, supplying heat to the mixing, heating and compacting apparatus  20  (e.g., pre-melt extruder) to assist in generating the densified plastic melt for introduction into the heating system (PRU)  12 . In other instances, the non-condensable gases  29  from the vapor treatment system  22  may be routed to a heat exchanger or other energy recovery device to supply energy for other unrelated purposes. 
     The system  10  may include a coalescer/separator  24  that receives an emulsion of condensed material from the vapor treatment system  22 . The emulsion can comprise crude oil that includes a small amount of the pH adjusted solution (or other process solution) entrained therein. The coalescer/separator  24  can be configured to separate the crude oil  26  from the pH adjusted solution (or other process solution) based on the difference in relative density between these materials. For example, the coalescer/separator  24  can comprise a settling tank that allows gravitational separation of the solution from the crude oil  26 . In other embodiments, the coalescer/separator  24  may comprise a centrifuge or other separator device. 
     The system  10  can further include a variety of sensor and control components (not shown). For example, the system  10  can include one or more pressure sensors and/or temperature sensors, which can provide to a controller various data regarding the operation of the heating system (e.g., PRU)  12  and the amount of heat being delivered to the feedstock. The sensors can communicate with a controller in any suitable manner, such as by wired or wireless connection. The controller can alter operational parameters of the heating system (e.g., PRU)  12  in response to data received from the sensors and/or as a result of other programming. 
     A master control system may be configured to communicate with the controller, and may also be configured to communicate with additional controllers that may each be dedicated to subsystems of the plastic recycling system. For example, separate subsystem controllers may be dedicated to the vapor treatment system  22  and the vacuum system  30 , respectively. In some embodiments, subsystem controllers may be situated locally (e.g., near the various subsystems with which they are associated), whereas the master control system may be situated in a supervisory station in which an operator can monitor the instantaneous status of the subsystems of the system  10  and make changes thereto as desired, whether onsite or offsite. 
     For the sake of convenience, subsystem controller(s) associated with a particular component may not be identified hereafter, nor will it be explicitly stated that a particular subsystem controller and/or the master control system is able to monitor and/or control the operation of a particular component of the plastic recycling system  10 , although such is understood. It is also noted that steps or control events discussed herein which can be effected by sub-controllers and/or the master control system may be embodied in machine-executable instructions that are to be executed by a general-purpose or special-purpose computer (or other electronic device). Alternatively, the steps or control events may be performed or instigated by hardware components that include specific logic for performing the steps or control events, or by a combination of hardware, software, and/or firmware. Some or all of the steps may be performed locally (e.g., via a subsystem controller) or remotely. 
     As the feedstock is heated in the heating system (e.g., PRU)  12 , the plastic feedstock eventually gasifies or vaporizes. The vapor can be introduced into the vapor treatment system  22  in any suitable manner. For example, in some embodiments, the vapor may be introduced into a condensing tower of a condenser substantially without altering a trajectory of the vapor. In other embodiments, the vapor may encounter a baffle upon entering the condensing tower. 
     Those portions of the vapor that are not condensed (i.e., non-condensable gases) may be passed to a caustic scrubber, which passes the remaining vapor through a caustic solution so as to chemically scrub the vapor (e.g., remove mercaptan sulfur species therefrom) and so as to neutralize trace levels of free inorganic acids. The remainder of the vapor may pass from the caustic scrubber through to an emissions control device (ECD)  32 . Any suitable vacuum system  30  may be used with the plastic recycling system  10  to move the vapor accordingly. 
     Any suitable emissions control device (ECD)  32  can be used with the plastic recycling system  10 . In some embodiments, the emissions control device  32  can comprise a burner or other combustion device. Exhaust  34  from the emissions control device  32  may be vented to atmosphere. In other embodiments, the hot exhaust  34  may instead be transferred to other portions of the plastic recycling system  10 . 
     The absorbed and condensed portions of the vapor may settle into a tank of the coalescer/separator  24  that includes one or more weirs. The pH adjusted solution (or other process solution), which retains the absorbed impurities, may facilitate coagulation of some contaminants which have a greater relative density than the condensed crude oil material  26 , and may settle to the bottom of the tank. Accordingly, the condensed crude oil  26  rises to the top of the tank and flows over the one or more weirs to be collected for further processing  36 , storage  38  or use. 
     In addition, downstream processing may be provided in some embodiments to purify the product streams discussed herein or fractionate said streams into specified hydrocarbon cuts. Process units for this purpose may include, but are not limited to, distillation, solvent extraction, adsorption, and catalyst treatment units. 
     In some instances, one or more supplemental condensers  40  may be provided to condense out light hydrocarbons  42  from the quenched pyrolysis gas  28 . The one or more streams of light hydrocarbons  42  may then be combined with the crude oil product  26 , or may be stored as separate hydrocarbon products. In some instances, at least a portion  43  of the light hydrocarbons  42  may be directed to the vapor treatment system  22  to assist in quenching the pyrolysis gases, namely, the hydrocarbon gas stream  16  output from the heating system (e.g., PRU)  12 . In some instances, at least a portion  43  of the light hydrocarbons  42  may be directed to the heating system (e.g., PRU)  12  to be reintroduced into the heating system (e.g., PRU)  12  for further processing. 
       FIG.  2    provides a schematic diagram of another embodiment of a plastic recycling system  100 , which is particularly well suited for producing one or more hydrocarbon oil products  126 ,  143  from waste plastic feedstock  114 . Similar to the aforementioned system, a mixing, heating and compacting apparatus (e.g., pre-melt extruder)  120  may be provided to receive a supply of waste plastic  114  and to output a densified melt of plastic material  121 . The system  100  may further include a pyrolysis recovery unit (PRU)  112  that is configured to receive the densified melt of plastic material  121  and pyrolyze the densified melt of plastic material  121  and output a primary hydrocarbon gas stream  116  and a primary solids residue stream  118 . The PRU  112  may comprise, for example, a dual screw feed mechanism within a reactor shell which is configured to simultaneously transport and heat the feedstock introduced into the PRU  112 . The PRU  112  may have multiple zones of heating along a length thereof to effectively heat the feedstock to a pyrolysis temperature as the material progresses from one end of the PRU  112  to the other. 
     The system  100  may further comprise a primary quenching apparatus  122  that is configured to receive the hydrocarbon gas stream  116  output from the PRU  112  and to condense out a tars product  123  for introduction into a supplemental tars pyrolysis reactor  160  for further processing, as discussed elsewhere herein. Similar to the PRU  112 , the supplemental tars pyrolysis reactor  160  may comprise a dual screw feed mechanism within a reactor shell which is configured to simultaneously transport and heat the material introduced into the supplemental tars pyrolysis reactor  160 . The supplemental tars pyrolysis reactor  160  may have multiple zones of heating along a length thereof to effectively heat the feedstock to a pyrolysis temperature as the material progresses from one end of the supplemental tars pyrolysis reactor  160  to the other. The PRU  112  and the supplemental tars pyrolysis reactor  160  may have different form factors, throughput capacities and/or different heating profiles. 
     The primary quenching apparatus  122  may also be configured to discharge an altered hydrocarbon gas stream  128  for further processing. A primary condenser and separator  140  may be provided to receive the altered hydrocarbon gas stream  128  from the primary quenching apparatus  122  and to condense out and separate a hydrocarbon oil product  126 . Additionally, a supplemental condenser and separator  141  may be provided to receive a discharged gas stream  142  from the primary condenser and separator  140  and to condense out and separate light hydrocarbons (e.g., methane, ethane, propane) to form a light hydrocarbon product  143  or to be combined with other product streams. The primary condenser and separator  140  and/or the supplemental condenser and separator  141  may be configured to direct a portion of the hydrocarbon oil product  126  and the light hydrocarbon product  143  upstream to the primary quenching apparatus  122  to assist in condensing out the tars product  123  from the hydrocarbon gas stream  116  output from the PRU  112 . In some instances, at least a portion of the hydrocarbon oil product  126  and/or at least a portion of the light hydrocarbon product  143  may be directed to the heating system (e.g., PRU)  12  to be reintroduced into the heating system (e.g., PRU)  12  for further processing. 
     Any remaining non-condensable gasses  129  may be processed as described in connection with the aforementioned system, including processing by an appropriate emissions control device  132  and under the influence of a vacuum system  130 . In some instances, the non-condensable gases  129  from the vapor treatment system  22  may be routed to a heat exchanger or other apparatus or system for energy recovering purposes. For instance, the non-condensable gases  129  from the vapor treatment system  22  may be routed to a heat exchanger to supply heat for other aspects of the systems and methods of processing waste plastics described herein, such as, for example, supplying heat to the mixing, heating and compacting apparatus  20  (e.g., pre-melt extruder) to assist in generating the densified plastic melt for introduction into the heating system (PRU)  12 . In other instances, the non-condensable gases  129  from the vapor treatment system  22  may be routed to a heat exchanger or other energy recovery device to supply energy for other unrelated purposes. 
     Although multiple stage condenser/separators are shown and described herein, it is appreciated that the systems and methods described herein are not limited to the use of multiple stage condensing and separating, and may, in other instances, include a single stage condensing and separating apparatus or employ other well know systems and techniques for condensing and separating out one or more desired products, such as through distillation techniques and the like. 
     The hydrocarbon oil product stream may be filtered and chilled by a product filter  150  and a product chiller  151  prior to product storage or transport. In addition, downstream processing may be provided in some embodiments to purify the product streams discussed herein or fractionate said streams into specified hydrocarbon cuts. 
     Process units for this purpose may include, but are not limited to, distillation, solvent extraction, adsorption, and catalyst treatment units. 
     As discussed above, the system  100  may comprise a primary quenching apparatus  122  that is configured to receive the hydrocarbon gas stream  116  output from the PRU  112  and to condense out a tars product  123  for routing to a tars pyrolysis reactor  160  for further processing. A supply of the tars product  123  may be temporarily stored in a tars tank  162  for subsequent processing or use. A portion of the tars product  123  may be bled off from the system  100  and routed via a tars bleed line  164  to an energy recovery unit or to an appropriate disposal system. At least some of the tars product  123  may be routed to the tars pyrolysis reactor  160  for pyrolysis of the tars product  123  into a secondary hydrocarbon gas stream  166  and a secondary solids residue stream  168 . The secondary solids residue stream  168  may be combined with the primary solids residue stream  118  and routed to an appropriate waste disposal system  169 . 
     Advantageously, the system  100  may further comprise a secondary quenching apparatus  170  that is configured to receive the secondary hydrocarbon gas stream  166  output from the tars pyrolysis reactor  160  and to condense out a supplemental hydrocarbon product  172 . The supplemental hydrocarbon product  172  may be combined with the tars product  123  output from the primary quenching apparatus  122  for reentry into the tars pyrolysis reaction  160  for further processing as discussed above. In this manner, a portion of the feedstock can be continuously refined by the tars pyrolysis reactor  160  to provide an exceptionally purified hydrocarbon based oil product. 
     The secondary quenching apparatus  170  may also be configured to discharge a supplemental altered hydrocarbon gas stream  174 . The supplemental altered hydrocarbon gas stream  174  may be combined with the altered hydrocarbon gas stream  128  from the primary quenching apparatus  122  and routed to the primary and secondary condenser and separators  140 ,  141  for further processing as discussed above. In addition, the secondary quenching apparatus  170  may be configured to receive a portion of the hydrocarbon oil product  126  and/or the light hydrocarbon product  143  from the condenser and separators  140 ,  141  to assist in condensing out the supplemental hydrocarbon product  172  from the secondary hydrocarbon gas stream  166  output from the tars pyrolysis reactor  160 . 
     In accordance with some aspects of the present invention, a supply of the waste plastic feedstock may include an appreciable amount of halide compounds (e.g., hydrogen chloride) or heteroatoms (e.g., sulfur, phosphorous) from one or more sources of contamination (e.g., halide containing plastics such as PVC, polymer additives, contaminants from food, soil, salt and other environmental sources). Advantageously, embodiments of the systems described herein may include an amendment feed arrangement  180  to enable the introduction of an amendment  181  to the feedstock to be pyrolyzed with the waste plastic material  114  and any contaminants therein. The amendment  181  may comprise, for example, alkaline earth oxides and/or hydroxides, oxides of iron, and/or oxides of aluminum. In particular, the amendment  181  may comprise, for example, at least one of: calcium oxide (CaO); calcium hydroxide (Ca(OH) 2 ); ferric oxide (Fe 2 O 3 ); and alumina (Al 2 O 3 ). The amendment  181  may be provided in the form of: a homogenous powder or particles; supported material on a solid substrate; or a slurry. For example, the amendment  181  may be provided in the form of a slurry having a solvent in the form of water, a hydrocarbon, or a hydrocarbon mixture. The amendment  181  may be provided in a concentration of between 1% and 10% by weight of the combined waste plastic feedstock and amendment, or between 3% and 4% by weight of the combined waste plastic feedstock and amendment, or about 3.5% by weight of the combined waste plastic feedstock and amendment. 
     Advantageously, the amendment  181  may be combined with the waste plastic based feedstock prior to mixing, heating and compacting of the supply of the waste plastic based feedstock  114  to form the densified melt of plastic material  121 . For example, the supply of the waste plastic based feedstock  114  and the amendment  181  may be mixed, compacted and heated to at least about 200 degrees Celsius prior to introduction in the pyrolysis reactor  112  to provide a particularly advantageous densified melt of plastic material having the amendment interspersed therein. In addition, the waste plastic based feedstock  114  and the amendment  181  may be fed through an airlock  182  to substantially reduce or eliminate oxygen from the waste plastic based feedstock  114  and the amendment  181 . In this manner, the densified melt of plastic material  121  may comprise the amendment intermixed throughout the densified melt of plastic material  121  prior to supplying of the densified melt of plastic material  121  to the pyrolysis reactor  112  under oxygen free or low oxygen conditions. Although the airlock  182  is illustrated schematically as a separate component or sub-system, the airlock  182  may be integrated into the mixing, heating and compacting apparatus (e.g., pre-melt extruder)  120 , when provided, or into the pyrolysis reactor  112  to provide oxygen free or low oxygen conditions within the pyrolysis reactor  112  during pyrolysis. 
     The amendment  181  may be mixed with the waste plastic based feedstock  114  prior to introduction into the mixing, heating and compacting apparatus (e.g., pre-melt extruder)  120 , or may be mixed with the waste plastic based feedstock within the mixing, heating and compacting apparatus (e.g., pre-melt extruder)  120  itself. In other less advantageous instances, the amendment  181  may be fed into the pyrolysis reactor  112  separate from the densified melt of plastic material  121  to be mixed with the plastic material within the pyrolysis reactor  112  itself. Still further, in embodiments where the mixing, heating and compacting apparatus (e.g., pre-melt extruder)  120  may be omitted, the amendment  181  may be combined with the waste plastic based feedstock  114  and may forgo pre-melting prior to introduction of the materials into the pyrolysis reactor  112 . 
     In any event, upon pyrolysis of the waste plastic feedstock  114  intermixed with the amendment  181 , the pyrolysis reactor  112  is configured to output the hydrocarbon gas stream  116  for processing (as discussed above) and the solids residue stream  118 . Advantageously, in such instances, the solid residue stream  118  includes a substantial portion of the halide compounds or heteroatoms of the waste plastic based feedstock  114  via interaction of the halide compounds or heteroatoms with the amendment  181 . As such, the addition of the amendment  181  can assist to provide an exceptionally purified hydrocarbon based oil product by effectively removing halide compounds and heteroatoms from the product streams. 
     In view of the foregoing, it will be appreciated that various methods of processing waste plastics may be provided. 
     According to one example embodiment, a method of processing waste plastics may be summarized as including: mixing, heating and compacting a supply of a waste plastic based feedstock including an appreciable amount of halide compounds or heteroatoms from one or more sources of contamination; providing an amendment comprising alkaline earth oxides and/or hydroxides, oxides of iron, and/or oxides of aluminum to be mixed, heated and compacted with the waste plastic based feedstock to form a densified melt of plastic material including the amendment; supplying the densified melt of plastic material including the amendment to a pyrolysis reactor; pyrolyzing the densified melt of plastic material including the amendment within the pyrolysis reactor to generate a hydrocarbon gas stream and a solids residue stream, the solid residue stream including a substantial portion of the halide compounds or heteroatoms of the waste plastic based feedstock via interaction of the halide compounds or heteroatoms with the amendment; and condensing out the hydrocarbon based product from the hydrocarbon gas stream output from the pyrolysis reactor. 
     Providing the amendment may include mixing the amendment with the waste plastic based feedstock prior to or during the mixing, heating and compacting of the supply of the waste plastic based feedstock to form the densified melt of plastic material, such that the densified melt of plastic material comprises the amendment intermixed throughout the densified melt of plastic material prior to supplying of the densified melt of plastic material including the amendment to the pyrolysis reactor. 
     The method may further include, prior to mixing, heating and compacting the supply of the waste plastic based feedstock and the amendment, passing the supply of the waste plastic based feedstock and the amendment through an airlock to substantially reduce or eliminate oxygen from the waste plastic based feedstock and the amendment. 
     The method may further include condensing out a tars product from the hydrocarbon gas stream output from the pyrolysis reactor with a quenching apparatus and routing the tars product to a secondary pyrolysis reactor distinct from the pyrolysis reactor to generate a secondary hydrocarbon gas stream and a secondary solids residue stream. The method may further include condensing out a hydrocarbon product from the secondary hydrocarbon gas stream output from the secondary pyrolysis reactor with a secondary quenching apparatus and routing the hydrocarbon product to the secondary pyrolysis reactor for further processing. Still further, the method may include: discharging an altered hydrocarbon gas stream from the quenching apparatus; discharging a supplemental altered hydrocarbon gas stream from the secondary quenching apparatus; and combining the altered hydrocarbon gas stream from the quenching apparatus and the supplemental altered hydrocarbon gas stream from the secondary quenching apparatus for further processing by one or more condensers. 
     Another example embodiment of a method of processing waste plastics may be summarized as including: pyrolyzing a supply of a waste plastic feedstock within a primary pyrolysis reactor to generate a primary hydrocarbon gas stream and a primary solids residue stream; condensing out a tars product from the primary hydrocarbon gas stream output from the primary pyrolysis reactor with a primary quenching apparatus; discharging a primary altered hydrocarbon gas stream from the primary quenching apparatus; pyrolyzing the tars product within a secondary pyrolysis reactor to generate a secondary hydrocarbon gas stream and a secondary solids residue stream; condensing out a supplemental hydrocarbon product from the secondary hydrocarbon gas stream output from the secondary pyrolysis reactor with a secondary quenching apparatus; discharging a supplemental altered hydrocarbon gas stream from the secondary quenching apparatus; combining the supplemental altered hydrocarbon gas stream from the secondary quenching apparatus with the primary altered hydrocarbon gas stream from the primary quenching apparatus to form a combined altered hydrocarbon gas stream; and condensing out a hydrocarbon oil product from the combined altered hydrocarbon gas stream with one or more condensers. The method may further include routing the supplemental hydrocarbon product from the secondary quenching apparatus to the secondary pyrolysis reactor for further processing. 
     Furthermore, the method may include: mixing, heating and compacting the supply of the waste plastic feedstock to form a densified melt of plastic material; and supplying the densified melt of plastic material to the primary pyrolysis reactor. In some instances, the method may further include providing an amendment comprising alkaline earth oxides and/or hydroxides, oxides of iron, and/or oxides of aluminum to be mixed, heated and compacted with the waste plastic based feedstock for introduction into the pyrolysis reactor. In yet other instances, the method may further include providing an amendment comprising alkaline earth oxides and/or hydroxides, oxides of iron, and/or oxides of aluminum to be combined with the waste plastic feedstock for pyrolysis treatment within the primary pyrolysis reactor without pre-melting. 
     It is appreciated that aspects of the systems and methods described herein may be applicable to treating mixed waste plastics comprising a variety of plastics types, or treating a mixed waste plastic feedstock that may be characterized by including predominantly one or more particular waste plastics, such as, for example, a feedstock of predominately waste polystyrene from which a monomer (i.e., styrene) product may be recovered. 
     It is also appreciated that aspects of the systems and methods described herein may include the aforementioned mixing, heating and compacting apparatus (e.g., pre-melt extruder)  20 ,  120  to receive a supply of waste plastic and to output a densified melt of plastic material, or, in other instances, the feedstock may be fed directly into the heating system or pyrolysis reactor (e.g., PRU)  12 ,  112  without being subjected to pre-melting. 
     Still further, it is appreciated that various product streams generated throughout the systems and methods described herein, may be routed back to the heating system or pyrolysis reactor (PRU)  12 ,  112  to be reintroduced into the heating system or pyrolysis reactor (PRU)  12 ,  112  for further processing, or may by routed elsewhere or sold as a separate product with or without additional conditioning of said product streams. 
     Additional components, features and functionality of the systems will be readily apparent to those of ordinary skill in the relevant art upon a review of the detailed schematic diagrams provided in the Figures. 
     Moreover, it will be understood by those having ordinary skill in the relevant art that changes may be made to the details of the embodiments described and illustrated herein without departing from the underlying principles presented herein. For example, any suitable combination of various embodiments, or the features thereof, is contemplated. For example, various embodiments may be configured to operate in one or more of a batch mode, a continuous batch mode, or a continuous mode. Other or further embodiments may include a condenser system and/or other components that are configured to operate under one or more of vacuum conditions, atmospheric pressure conditions, or positive pressure conditions. 
     Any methods disclosed herein comprise one or more steps or actions for performing the described method. The method steps and/or actions may be interchanged with one another. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and/or use of specific steps and/or actions may be modified. 
     Throughout this specification, any reference to “one embodiment,” “an embodiment,” or “the embodiment” means that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, the quoted phrases, or variations thereof, as recited throughout this specification are not necessarily all referring to the same embodiment. 
     Similarly, it should be appreciated that in the above description of embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure. This method of disclosure, however, is not to be interpreted as reflecting an intention that any claim require more features than those expressly recited in that claim. Rather, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. It will be apparent to those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles set forth herein. 
     U.S. Provisional Patent Application No. 63/123,965, filed Dec. 10, 2020, is incorporated herein by reference, in its entirety.