Patent Publication Number: US-11649403-B2

Title: Multi-step process for conversion of waste plastics to hydrocarbon liquids

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application also claims the benefit of U.S. Provisional Application No. 63/148,707 filed on Feb. 12, 2021. 
    
    
     FIELD OF INVENTION 
     The present invention relates to the conversion of plastic and or blends of plastic and biomass, especially from non-recyclable waste streams, to useful hydrocarbon products. More specifically, the present invention relates to conversion of non-recyclable plastic and biomass materials using hydrothermal treatment, steam cracking and coking, so as to enable broad control and optimization of quantities and qualities of the resulting hydrocarbon products. 
     BACKGROUND OF THE INVENTION 
     Thermal conversion of plastics to liquid fuels has been a subject of environmental and commercial interest for several decades. China&#39;s move to stop their import of most solid waste from foreign countries including plastics, paper products, and textiles, etc. sparked renewed interest in the commercial development of such thermal conversion systems. There is a need for environmentally sound disposal of personal protective equipment (PPE), as well as other plastics, especially thin films materials (typically polyethylene or polypropylene) ranging from large sheeting use in agricultural and construction to plastic bags to packaging/wrapping. 
     Small scale commercial systems, mostly of indirectly heated pyrolytic design, operating at or near ambient pressure, have been built for conversion of plastic to hydrocarbon fuels. These are mainly in developing countries for liquid fuel and or power generation purposes. However, the quality of the product hydrocarbon liquids is well below that of most crude oils, let alone refined petroleum products. 
     The last decade has seen significant interest and investigation into the application of hydrothermal treatment for waste conversion, including conversion of plastics to liquid and gas phase hydrocarbon fuels. The invention disclosed herein teaches an advantageous approach to converting plastics and other hydrocarbon solid waste to liquids and utilizing hydrothermal treatment combined with other more conventional and well established petroleum refining processes. 
     SUMMARY OF THE INVENTION 
     The present invention provides an integrated system for the optimal conversion of plastic waste to useful hydro-carbonaceous products, and, more specifically, the conversion of plastic waste using a system that enables broad control and optimization of quantities and qualities of the products produced. In some embodiments, the steps the process include: (a) preparation of the plastic or mixed plastics and biomass feedstock for the hydrothermal liquefaction step by cleaning and sizing the process feedstock, (b) hydrothermal treatment of the feedstock, which initiates depolymerization, (c) steam cracking of the hydrothermally activated and de-polymerizing molten plastics or plastics and biomass feedstock, (d) separation of the depolymerized and cracked feedstock into product or by-product streams, (e) recycling of a portion of the heavy oil stream and (f)) treatment of residual heavy oils in a coker, the vapors from which may be condensed, refined, and stored. 
     Products obtained from the thermal treatments described above yield liquid and gas phase hydrocarbons in the non-condensable, naphtha (gasoline and solvents), middle distillate (#2 diesel oil and kerosine), heavy fuel oil (boiler fuel oil and marine diesel), and heavy residual oil boiling point ranges. Coke is a dry carbon product that will be formed in the steam cracker and coker, especially if plastics with intercalated carbon black are in the feedstock. 
     In the following description of the present invention, it is understood that the term “plastics” may refer to single polymer plastics, thermal plastics, crosslinked polymer plastics, mixed plastics, or plastics and biomass feedstocks, or any combination thereof, especially as would be obtained from solid waste streams. The above listed materials or combinations thereof are typically non-recyclables derived from solid waste streams and will be referred to herein as “feedstock” or “feedstocks”. The term “blendable”, as used herein, designates distillates that can be blended with commercial petroleum fuels such as #2 diesel oil, Jet A, etc. Middle distillates directly from the process of the invention can be used for marine diesel, boiler fuel, home heating oil, etc. Middle distillate fractions may be further refined using post-production filtration and distillation, to make them suitable as stand-alone “drop in” fuels for many applications including as a stand-alone #2 diesel oil. 
     It should also be noted that middle distillate hydrocarbon products from conversion of plastics by the present invention are comprised of the same molecular species, within the same boiling point range, as refined petroleum derived hydrocarbon fuels. The main difference in composition is a relatively greater abundance of olefins and fewer naphthenes in the plastic derived middle distillates as compared to those refined from petroleum. 
     It should also be noted that plastics derived middle distillates are not fatty acid methyl ester (FAME) biodiesels, the proportion of which can blended into petroleum #2 diesel fuel is limited in many jurisdictions to 10% or less. Plastic derived middle distillates can be refined to be fully compliant with ASTM D 975 for diesel fuel. 
     The process also includes utility support systems, depending on feedstock characteristics and desired hydrocarbon product suite. These support or auxiliary systems include: (a) a high and low pressure hydrocarbon vent gas collection system, which delivers gas either for process heating or in an exhaust thermal oxidizer, (b) an air pollution control system with flue gas recycling and a baghouse, (c) a water management system for recycling and effluent wastewater discharge treatment, and (d) a coolant cooling and recirculation system. 
     The described components and utility support systems, along with other incidental but essential subsystems common to commercial facilities, such as a process monitoring and control systems, achieves plastics recycling to useful products of higher quality and lower costs than current state of the art systems. That is, the waste plastics conversion process of the present invention results in products of higher quality, at high process efficiency, and reduced costs compared to current thermal conversion processes. 
     The advantages compared to the current standard for plastics conversion by conventional low pressure indirect pyrolysis, or via supercritical hydrothermal plants include: (a) lower capital cost than supercritical plants for hydrothermal treatment of plastics, (b) lower operation and maintenance costs than supercritical hydrothermal treatment of plastics, (c) higher yield than supercritical hydrothermal treatment of plastics, and (d) higher quality fuel than conventional low pressure indirect pyrolysis treatment of plastics plants. 
     The overall process of the present invention achieves the conversion of plastics, plastics and biomass, or biomass to useful products of higher quality and lower costs than current systems. In addition, the present invention provides the flexibility to produce various converted hydrocarbons according to market demand. Using the present invention product hydrocarbons across the entire boiling point range from non-condensables to heavy fuel and residual oils qualify as ultra-low sulfur fuels, as well as low carbon intensity fuels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates the three process units or components of the present invention and their relationship to each other. 
         FIG.  2    illustrates the subsystem components of the hydrothermal system. 
         FIG.  3    illustrates the components of the steam cracking system. 
         FIG.  4    illustrates the components of the coker system. 
         FIG.  5    provides the flow diagrams for the water management system. 
         FIG.  6    provides the flow diagrams for the fuel gas system. 
         FIG.  7    provides the flow diagram for the coolant system. 
         FIG.  8    provides the flow diagram for the flue gas recycle and exhaust management system. 
     
    
    
     It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. The Figures are provided for the purpose of illustrating one or more embodiments of the invention with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention of the present invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without some of these specific details. Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than as limitations on the invention. That is, the following description provides examples, and the accompanying drawings show various examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are merely intended to provide examples of the invention rather than to provide an exhaustive list of all possible implementations of the invention. 
     Specific embodiments of the invention will now be further described by the following, non-limiting examples which will serve to illustrate various features. The examples are intended merely to facilitate an understanding of ways in which the invention may be practiced and to further enable those of skill in the art to practice the invention. Accordingly, the examples should not be construed as limiting the scope of the invention. In addition, reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     The descriptions herein use the term diesel to refer to a range of middle distillate hydrocarbons, because #2 diesel oil boiling range hydrocarbon is the most common and valuable product of plastics to liquid fuels conversion. Along with non-condensable hydrocarbon gasses, other liquid phase fractions can be produced by application of the present invention, including but not limited to naphtha, middle distillate (kerosene, jet fuel, home heating oil), and heavy fuel and residual oils (marine diesel, boiler fuels and bunker oil), as well as lube oils and waxes. Likewise, the descriptions herein use heavy fuel oil to refer to heavy distillates. The following invention disclosure narrative is best understood in light of the attached diagrams, and the illustrative descriptions and examples provided, which comprise a component thereof. 
     With reference to  FIG.  1   , shown is one embodiment of the method. A hydrothermal system  1  prepares the waste feedstock  4  by hydrothermal activation and initiates depolymerization. The hydrothermal system  1  operates in a subcritical pressure and temperature range. A steam cracking system  2  takes the hydrothemally activated and depolymerizing molten plastics  5  and steam cracks them and separates them into by-product streams  7 ,  8 ,  9 . A portion of the heavy fuel oil  6  produced is recycled. Cracked vapors  10  from the coker system  3  may be refined and stored in the products storage systems of this stage  2 . A coker system  3  cokes the net heavy fuel  6  leaving the steam cracking system  2  and produces a coke by-product  11  along with cracked vapors  10  which are sent back to the steam cracking system  2  for separation and refining. 
     An exemplary hydrothermal system  1  is shown in  FIG.  2   . Waste plastics  4  are reduced to a size range suitable for aqueous suspension and transport in a feedstock sizing system  12 . Cleaning and sizing processes can include dry shredding, extrusion, metal removal and/or grinding and washing. The optimal procedure varies by feedstock. Waste plastic  4  should be understood to signify not only used plastics and mixed plastic waste feedstock, but also mixed waste feedstocks that include biomass and inorganic material. The sized waste feedstock  17  is stored in a hopper and fed into the suspension and pressurization system  13  and mixed with recycled process water  18 . Sized feedstock is added until the desired water to plastics ratio in the suspension is achieved for the specific application. A recirculation pump may be used to recirculate the resulting water-plastic slurry, to improve mixing and help maximize suspension density. The plastic-water suspension is discharged to a progressive cavity pump in which the pressure of the aqueous suspension of plastics is raised to the range of between 500 psia and 1,000 psia. 
     The pressurized aqueous suspension  19  then enters the hydrothermal depolymerization system  14 , wherein the suspension is first preheated against hot recycle process water  20  in a heat exchanger. This heating brings the plastics close to their melting temperature. The aqueous plastics suspension exiting the hot water heat exchanger is then heated to 500° F.+/−25° F. against a flue gas stream  21  exiting the mineral solids dryer  16 . This heating achieves hydrothermal activation and initiates de-polymerization. The resulting molten de-polymerizing plastics in aqueous suspension is then ducted to a detention tank to provide residence time to further initial de-polymerization. Non-condensables  22  in the hydrothermal detention vessel are vented to the high-pressure gas manifold  102 . Cooled process water  23  exiting the hot water heat exchanger, and flue gas  24  exiting the flue gas heat exchanger then passes to the recycle process water tank  88  and flue gas cooler  142 , respectively. 
     The molten depolymerizing plastics in aqueous suspension  25  is transferred to the multiphase separation system  15 . The multiphase separation system  15  first reduces the pressure of the suspension by 50 to 200 psia across a pressure let-down valve. The optimal pressure drop depends on the average waste characterization of the feedstock  4  to provide a pressure let-down sufficient to achieve high pressure multiphase separation by density differential in a separator vessel. Non-condensables  26  at the operating conditions in the freeboard of the multi-phase separator vessel are vented to the high pressure gas manifold  102 . The hydrocarbon rich phase  5 , which includes interspersed water, is transferred to the steam cracking system  2  after a portion of the separated aqueous phase is blended back into the molten de-polymerizing plastics stream. The majority of the aqueous phase  20  is recycled to the hot water heat exchanger in the hydrothermal depolymerization system  14 . 
     The mineral sludge  28  at the base of the multi-phase separator vessel is discharged through mineral handling system  16 , which may include a dryer, a cooler and storage. Hot flue gas  29  from the downstream of the steam cracker  36  is used to dry the mineral solids. A glycol-water coolant  32 , from the cooler  116 , may be used to cool the solids in an indirect contact augered cooler with the discharge  30  sent back to the cooler  116 . Dried mineral solids  33  may be landfilled. 
     The steam cracking system  2 , an example of which is shown in  FIG.  3   , includes several subsystems. One skilled in the art will recognize that the steam cracker described herein can be structurally similar to, and provide the functions of, a visbreaking furnace. Molten de-polymerizing plastics  5  with interspersed and additional high pressure water from the multiphase separation system  15  are transferred across a pressure let-down valve  35  resulting in steam generation in the 175 to 200 psia pressure range. This stream of steam and molten de-polymerizing plastics  41  which may include un-vaporized interspersed water enters a steam cracking fired heater  36 . 
     The temperature of the steam and hydrocarbon rich multi-phase mixture is raised from below 400° F. to over 800° F. at the exit of the steam cracker  36 . In one embodiment of the invention, the fired heater is outfitted with a low NOx burner that incorporates the premixing of recycle flue gas  44 , air  42  and recycled product fuel gases  43  from the high pressure fuel gas storage tank  104  to provide process heat to drive the endothermic reactions. The product mixture of heavy hydrocarbon liquids, coke particles, cracked hydrocarbon vapors and water gas  46  are ducted to a multiphase separation and quenching system  37 . In the multiphase separation and quenching system  37  a venturi quench, is cooled by recirculating heavy fuel oil to 700° F. prior to entering a vapor-liquid separator. Heavy fuel oil  6  is discharged through the base of the separator with a portion sent across a pressure let-down valve  60 . The reduced pressure heavy oil  66  is fed into the coker  61 . The balance of the heavy oil is cooled in a heat exchanger and pumped across the venturi exchanger quenching the multi-phase phase mixture from above 800° F. to 700° F. or below and condensing the heavy fuel oil out of the multi-phase mixture fed. A glycol-water coolant  48 , from the dry cooler  116 , is used to cool the recirculating heavy fuel oil in a shell and tube heat exchanger with the discharge  47  sent back to the dry cooling  116 . 
     The vapor steam  50  exiting the separator contains species in the diesel boiling range and lower. This vapor stream  50  is ducted to a diesel condensing system  38 , comprised of an indirect parallel plate condenser in which the temperature is reduced to 300° F. or lower, thereby condensing out diesel boiling range hydrocarbons. A glycol-water coolant  52 , from the cooler  116 , is used as coolant in the parallel plate heat exchanger, with the discharge  51  sent back to the cooler  116 . The multi-phase mixture exiting the condenser is fed into a vapor liquid knock-out drum. Product diesel oil exiting the base of the drum is let down in pressure with the vapor-liquid stream produced separated in a second knock-out drum. Vapors  53  exiting this drum are ducted to the low-pressure fuel gas manifold  100 , while the liquid product  7  is sent to the diesel product storage tank. The gas vent  106  from the diesel storage tank is also ducted to the low-pressure fuel gas manifold  100 . 
     Vapors  54  leaving the diesel condenser system  38  are fed to a naphtha-water condenser system  39 , which includes a parallel plate indirect condenser in which the temperature is reduced to 77° F. or lower thereby condensing out naphtha boiling range hydrocarbons and the bulk of the water vapor entering. Glycol-water coolant  55 ,  56  is used to cool the condenser. A chiller may be deployed to enable the condenser to operate at or below 77° F. 
     Dewatering chemicals are added to the multi-phase mixture exiting the condenser, which is fed into a vertical oil, water, and vapor separator. Water  92  is removed from the separator base and recycled to the water management system&#39;s recycle process water tank  88 . Non-condensable vapors  57  exiting overhead from the separator are ducted to the high-pressure fuel gas storage tank  104 . Product naphtha floating on top of the water layer of the separator bottom is then let down in pressure with the vapor-liquid stream produced separated in a knock-out drum. Vapors  59  exiting this drum are ducted to the low pressure fuel gas manifold  100  while the liquid product  8  is sent to a naphtha product storage tank. The gas vent  107  from the naphtha storage tank is also ducted to the low-pressure fuel gas manifold  100 . 
     The coker system  3 , an example of which is shown in  FIG.  4   , includes several components. The portion of the heavy fuel oil product  6  that is split for final processing in the coker system  3  is first let down in pressure by a pressure let-down valve  60  and then fed to a coker unit  61 . In one embodiment of the invention, the coker unit  61  may be an, at near ambient pressure, indirectly fired, augered kiln. In one embodiment of the invention, the coker unit  61  is heated with a low NOx burner that incorporates the premixing of recycle flue gas  69 , air  67  and recycled product fuel gases  68  from the high-pressure fuel gas storage tank  104 . This provides process heat to drive the endothermic reactions of pyrolytic coking of the heavy fuel oil. In another embodiment of the invention, the coker unit is built with the inlet at a base elevation and the exit higher than the inlet. This configuration facilitates the heavy oil to be coked dry by operating the kiln with an exit temperature of the coker solids above 1000° F. 
     The coke  71  exits the coker  61  into an augered cooler  62  and is cooled indirectly with a glycol-water mixture  72 ,  73  provided from the cooler  116 . The vapors  74  exiting the coker  61  at or slightly below the coke exit temperature are quenched to 700° F. in the coker quench separator  63 , condensing the heavy oil fraction  78 . A heat exchanger, cooled by a glycol-water mixture  75 ,  76  provided from the cooler  116 , cools the recirculating heavy fuel oil. A low-pressure pump may be provided to generate the head required to operate the venturi quench. A portion of the heavy oil  79  split off and pumped to high pressure for recycle to either the hydrothermal system  1  or the steam cracking system  2  for further de-polymerization and or steam cracking. The net product fuel oil is withdrawn from the discharge of the low pressure pump. 
     Naphtha and lighter hydrocarbons  80  exiting the coker quench separator  63  are ducted to the blendable liquids condenser  64 . In one embodiment of the invention, the blendable liquids condenser  64  may be a parallel plate heat exchanger. This heat exchanger is cooled to 77° F. by a glycol water mixture  81 ,  82  from the cooler  116 . A chiller may be deployed to enable the condenser to operate at or below 77° F. The multiphase mixture of diesel and lighter hydrocarbons  83  are separated in a vapor liquid separator  65  producing a blendable liquid bottoms product  85  that is directed to a liquid blend tank. The overhead vapors  84  are ducted to the low pressure fuel gas manifold  100 . 
     In addition to the three primary steps of the process, there are four major auxiliary systems. An exemplary water management system is shown in  FIG.  5   . The water management system includes several subsystems. In embodiments of the invention that utilize a low moisture content plastic waste feedstock, make-up water  90  may be required, corresponding to a net consumption of water, predominantly in the steam cracker  36 . Such low moisture content waste plastics may include polyethylene and/or polypropylene and/or polystyrene. A make-up metering system  87  may be used to achieve water balance. In embodiments of the inventions where the feedstock contains significant biomass, there may be a net generation of water and make-up water  90  may not be required. The water management system includes a recycle process water tank  88 . This tank  88  will receive recycle process water  23  from the high-pressure separator  15  after it flows through the hot water heat exchanger in the hydrothermal depolymerization system  14 . Said tank  88  will also receive recycle process water  92  from the low pressure naphtha-water condenser  39 , in addition to any make-up water  91  required. Vent gases  93  from said tank is ducted to the high pressure vent gas manifold  102 . To maintain water quality, a portion of the process water  94  is sent to an effluent treatment system  89 . 
     The water management system includes an effluent treatment system  89 , the overall configuration of which depends on the characteristics of the waste plastic feedstock. In one embodiment of the invention, the main function of the effluent treatment system  89  may be to remove dissolved hydrocarbon systems, while in other embodiments an aerobic and anaerobic treatment may be required to achieve effluent compliance standards. In addition it may be technically and economically feasible to generate green substitute natural gas via anaerobic digestion in the effluent treatment system  89 . This is especially the case with higher percentage contents of biopolymers versus waste plastics. The effluent treatment system  89  produces clean water  96  for discharge and produce vent gases  95  of similar energy content to the fuel gas generated  57 . A dried sludge  97  is also produced. 
     An exemplary fuel gas management system is shown in  FIG.  6   . A low-pressure vent gas manifold  100  providing low pressure fuel gas  108  to a storage tank  101  for fuel gas surge control. Fuel gas  109  from the storage tank  101  is sent to a thermal oxidizer  103 . Vent gases  77 ,  106 ,  107  from the heavy fuel oil, diesel and naphtha storage tanks are collected in the manifold  100  along with let-down vent gases  53 ,  59  from the diesel and naphtha knock-out drums. 
     A high-pressure vent gas manifold  102  educts high pressure vent gases  110  through the heavy fuel oil venturi quench nozzle in the multiphase product quenching system  37 . A high-pressure gas storage tank  104  stores gases  57  venting from the non-condensables, naphtha and water condenser system  39 . Gases  43 ,  68  from the storage tank  104  fuel the steam cracker burner  36  and the coker  61 , with the balance of the gas  111  burned in the thermal oxidizer  103 . The thermal oxidizer  103  uses air  113  blended with recycle flue gas  112  to cleanly burn both high-pressure fuel gas  111  and low-pressure fuel gas  109  with low NOx emissions. 
     Thermal oxidizer flue gas  114  and coker burner flue gas  70  are combined with the flue gas  45  from the steam cracker  36  and used for mineral solids drying and aqueous plastics suspension pre-heating as described. In embodiments producing large volumes of gas, excess heat or gas may be used to generate electricity. Electrical generation may be accomplished using excess fuel gas  111  directly in reciprocating engines, or by recovering the heat from flue gas  114  to power a Rankine cycle or organic Rankine cycle system. In embodiments using a Rankine cycle, a portion of the hot flue gas  114  from the thermal oxidizer  103  is ducted to a heat recovery steam generator, which provides steam for electrical generation. In embodiments using an organic Rankine cycle, the flue gas cooler  142  includes an oil heat exchanger for transferring waste heat prior to the cooling of the flue gas 
     An exemplary cooler system is shown in  FIG.  7   . Warmed coolants  30 ,  47 ,  51 ,  55 ,  72 ,  75 ,  81 ,  117 ,  119  exit the mineral solids cooler  16 , the carbon solids cooler  62 , the heavy fuel oil quench cooler  37 , the coker multiphase quench  63 , the diesel condenser  38 , the naphtha-water condenser  39 , the blendable liquid condenser  81  and the flue gas cooler  142  and are cooled in a cooler  116 . 
     In one embodiment of the invention an air-cooled dry cooler is used. Cooled water  32 ,  48 ,  52 ,  56 ,  73 ,  76 ,  82 ,  118 ,  120  leaving the cooler  116  and are recycled to their respective coolers. To achieve control and capture of condensing of the naphtha and blendable liquids, the glycol-water coolant is cooled to 35° F. or lower in a chiller. Ambient air  121 ,  122  may be used to operate the cooler. 
     An exemplary air pollution control system is shown in  FIG.  8   . The air pollution control system is only included if flue gas requires cleaning to be compliant with applicable air emission regulations. Flue gases  45 ,  70 ,  114  generated in the steam cracker fired heater  36 , the coker  61 , and the thermal oxidizer  103 , are combined into a single flue gas stream  29 . After exiting the hydrothermal depolymerization system  13 , the combined flue gas  24  enters the flue gas cooler  142  where it is cooled to approximately 300° F. Cooled flue gas  131  enters the bag-house entrance duct  125 . Sorbent injectors  124 , inject sorbents  133 , such as activated carbon and acid gas capture reagents, into the entrance duct  125 . Flue gas  134  enters the fabric filter baghouse  126 , where reacted air pollution control solids and any other particulates  140  are removed. After being cooled and stored in a designated system  127  the collected air pollution control collected solids  141  may be disposed in a landfill. The clean product gases  135  exiting the baghouse  126  are pass through a draft fan  128 . Flue gas  136  leaving the draft fan  128  enters a flue gas splitter  129 . A portion of flue gas  138  is recycled for low NOx, burner operations. The remaining flue gas  137  is released to the atmosphere through an exhaust stack  130 . 
     In embodiments producing large volumes of gas, excess heat or gas may be used to generate electricity. Electrical generation may be accomplished using excess fuel gas  111  directly in reciprocating engines, or by recovering the heat from flue gas  114  to power a Rankine cycle or organic Rankine cycle generating system. In embodiments using a Rankine cycle, a portion of the hot flue gas  114  from the thermal oxidizer  103  is ducted to a heat recovery steam generator, which provides steam for electrical generation. In embodiments using organic Rankine cycle, the flue gas cooler  142  includes a heat exchanger between the entering flue gas and an oil heating system, upstream of the cooler. 
     Certain embodiments of this invention arise mainly from the composition of the plastic, or mixed plastic and biomass waste, its morphology, and local factors ranging from quantities of waste available to the product mix desired in local markets. The latter consideration can be affected by seasonal and or temporal market demands as to how these factors can best be incorporated into an optimal processing configuration. 
     In any embodiments of this invention, the yield of different hydrocarbon fractions can be controlled by variation of feed flow rate, waste composition fed, water to waste ratio, operating temperatures, pressures, and residences times in the various processes. These conditions may be altered to achieve optimal product ratios on daily, weekly, and/or seasonal bases. 
     Waste plastics that were produced by free radical addition polymerization of olefinic monomers such as ethylene and propylene do not typically decompose significantly under subcritical hydrothermal conditions. However, with the subsequent steam cracking of the partially depolymerizing plastics, problematic plastics can be depolymerized thermally in the steam cracking and/or coker process. With the three processes included, the yield of middle distillates and the quality of the heavy fuel oil is improved. This occurs from both the additional pyrolytic cracking, and by the separation of a dry carbon or coke product. Thus, the flexibility in the three-step thermal processing taught in this patent application supports the goals of those focused on achieving circular economy for plastics management. 
     Catalysts may also be used to address the problem of insufficient depolymerization. In one embodiment of the invention, a cracking catalyst, such as bentonite, may be used in the hydrothermal depolymerization system or in the steam cracking system. Catalyst may be most suitable in the processing of the aforementioned free radical addition produced polymers from vinyl monomers. The spent catalyst may be removed in the mineral sludge exiting the high-pressure separator downstream in the hydrothermal treatment step. Alternatively, where a catalyst is required in the steam cracking process, the catalyst exits the system in the carbon or coke by-product. 
     Oxygen containing feedstocks require special consideration. As the percentage of oxygen content in the waste increases, whether from the percentage of polyethylene terephthalate or from the percentage of a condensation polymer such as nylon, the percentage of chain length reduction that can be achieved in subcritical hydrothermal treatment increases. One impact of this is the generation of oxygenated hydrocarbons. These commonly have solubilities in water that are typically an order of magnitude or two or more than hydrocarbon whose elemental composition is carbon and hydrogen atoms only. The main impact is that these water-soluble oxygenated hydrocarbons build up in the water recirculation loop. 
     In embodiments that process only hydrocarbon polymers, the circulating process water will contain increased levels of the light hydrocarbons formed from thermal depolymerization. For this case, wastewater treatment of the dissolved hydrocarbons can be readily achieved by activated carbon absorption and activate oxygen polishing steps. 
     In embodiments that treat biopolymers and biomass, the water treatment system must handle dissolved oxygenated compounds. The auxiliary water management system for such a processing facility must have a combination of aerobic and anaerobic treatments to remove such compounds. 
     The optimal process used in the subsystems of this invention will change with the scale of the facility. This disclosure of the invention describes the use of simple condensation schemes for product separation, including by direct contact methods of quenching and by indirect condensing using typically cost effective parallel plate condensers. One skilled in the art should appreciate that there is an economy of scale above which such basic separations techniques may be replaced by distillation columns similar to those in petroleum refineries for product separation and refining. Nominally, 1,000 barrels of oil to be distilled is considered a rough threshold for such economy of scale considerations. Similar substitutions may be applicable to the steam cracking and coking processes, that also have analogous large-scale petrochemical processes. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the above-described embodiments, which are presented in this description for purposes of illustration and not of limitation. The specification and drawings are not intended to limit the exclusionary scope of this patent document. It is noted that various equivalents for the particular embodiments discussed in this description may practice the invention as well. That is, while the present invention has been described in conjunction with specific embodiments, it is evident that any alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims. The fact that a product, process or method exhibits differences from one or more of the above-described exemplary embodiments does not mean that the product or process is outside the scope (literal scope and/or other legally-recognized scope) of the following claims.