Conversion of waste plastics to petrochemicals

Abstract

To provide a process and system for converting waste plastic.SOLUTION: The method includes supplying waste plastic to a melting tank where the waste plastic is heated to form molten plastic. The molten plastic is withdrawn from the melting tank and fed to a pyrolysis reactor. In the pyrolysis reactor, the molten plastic is heated to a pyrolysis temperature to produce a pyrolysis oil product and a liquid pitch product. The pyrolysis oil is then separated into a pyrolysis gas fraction, a light pyrolysis oil fraction, a medium pyrolysis oil fraction, and a heavy pyrolysis oil fraction.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE Embodiments of the present disclosure generally relate to the conversion of waste plastics into petrochemicals, fuels, and other useful intermediates and products. [Background technology]

[0002] Environmental concerns regarding the need to recover and recycle plastic waste are growing rapidly, however, plastic pyrolysis technology is in the early stages of development in the industry. Summary of the Invention [Means for solving the problem]

[0003] SUMMARY OF THE INVENTION Embodiments herein relate to thermochemical processes and systems useful for converting waste plastics into petrochemicals, fuels, and other intermediate or end products.

[0004] In one aspect, embodiments herein relate to a process for converting waste plastics. The process includes feeding waste plastics to a melting tank, where the waste plastics are heated to form molten plastics. The molten plastics are withdrawn from the melting tank and fed to a pyrolysis reactor. In the pyrolysis reactor, the molten plastics are heated to pyrolysis temperatures to produce a pyrolysis oil product and a liquid pitch product. The pyrolysis oil is then separated into a pyrolysis gas fraction, a light pyrolysis oil fraction, a medium pyrolysis oil fraction, and a heavy pyrolysis oil fraction.

[0005] In another aspect, embodiments herein relate to a system for pyrolyzing waste plastics. The system includes a melting tank having an inlet configured to receive a feed stream containing waste plastics from a waste plastics feed system. The melting tank also includes a heating system configured to heat the waste plastics from a feed temperature to a melting temperature to produce molten plastics, and an outlet configured to discharge the molten plastics. The system also includes a pyrolysis reactor having an inlet configured to receive the molten plastics and a heating system configured to heat the molten plastics to a pyrolysis temperature, as well as a first outlet for recovering pyrolysis oil. The pyrolysis reactor further includes a second outlet for recovering a pitch product. The system further includes a control system configured to control the heating system to limit the temperature of the molten plastics to a temperature for producing the pitch product (below a temperature at which significant char or coke would form). [Brief explanation of the drawings]

[0006] [Figure 1] 1 illustrates a simplified process flow diagram of a system according to one or more embodiments disclosed herein. [Figure 2] 1 illustrates a simplified process flow diagram of a system according to one or more embodiments disclosed herein. [Figure 3] 1 illustrates a simplified process flow diagram of a system according to one or more embodiments disclosed herein. DETAILED DESCRIPTION OF THE INVENTION

[0007] FIELD OF THE INVENTION Embodiments herein relate to thermochemical processes that convert waste plastics into useful petrochemicals, fuels, and other intermediate or end products. Embodiments herein also relate to the design and control of pyrolysis reactors.

[0008] Polymers that can be pyrolyzed to form waste plastic pyrolysis oil can include thermoplastic resins, thermosetting resins, and elastomers. For example, waste materials that can be pyrolyzed to form waste plastic pyrolysis oil include polystyrene, polypropylene, polyphenylene sulfide, polyphenylene oxide, polyethylene, polyetherimide, polyetheretherketone, polyoxymethylene, polyethersulfone, polycarbonate, polybenzimidazole, polylactic acid, nylon, and acrylic polymers (e.g., polymethyl methacrylate (PMMA)), among many other thermoplastic resins. Waste plastic pyrolysis oil useful herein can also be formed from various unsaturated or saturated elastomers and rubbers known in the art, such as polybutadiene, isoprene, styrene-butadiene, ethylene vinyl acetate, and others. While embodiments herein may be robust enough to process some amounts of heteroatom-containing polymers, including those listed above and others known in the art, the heteroatom content of the resulting waste plastic pyrolysis oil should typically be less than 2 wt%, e.g., less than 1 wt% or less than 0.5 wt%.

[0009] Waste plastics can be converted into pyrolysis oil using a system according to embodiments herein. The system may include a melting tank and a pyrolysis reactor as basic components. The system may also include a waste plastics feeding system, a pyrolysis oil separation system, and one or more heating systems, as described below.

[0010] Generally, the waste plastic feed system is a system configured to provide a waste plastic feed to a melting tank and is not particularly limited, as many different configurations can be used. In some embodiments, the waste plastic feed system can include a feed hopper that can be filled with a quantity of waste plastic, such as chips, pellets, flakes, attenuated fiber, shredded plastic, and other waste plastic forms that may be received from a recycler or other waste plastic supplier. The feed hopper can be fluidly connected to a screw conveyor or other means for metering the waste plastic to the melting tank. In some embodiments, the waste plastic feed system can include an extruder, which can be single-screw or twin-screw, that heats and partially or completely melts the waste plastic through viscous dissipation of mechanical energy supplied by a motor to a rotating screw.

[0011] It is generally desirable to limit the amount of water and oxygen fed to the melt tank and pyrolysis reactor. The screw conveyor and / or feed hopper or associated flow lines may be connected to a hot nitrogen supply system so that the polymer is heated to a temperature sufficient to remove water but not so high as to melt the polymer, thereby venting the nitrogen stream containing water vapor and displaceable oxygen.

[0012] The dried waste plastic may then be fed to a melting tank, where it is heated to a temperature sufficient to melt the plastic but low enough to avoid significant conversion of the waste plastic. This is because it is preferable to control the reaction and resulting reaction products due to conversion in the pyrolysis reactor. In some embodiments, the waste plastic may be heated in an extruder or fed to the melting tank in a heated, partially or completely molten state. The waste plastic may then be heated or further heated in the melting tank to a temperature in the range of about 200°C to about 375°C, e.g., about 300°C. The temperature of the molten plastic should be high enough to melt the plastic and provide a molten plastic of the desired viscosity to facilitate transport between unit operations, but low enough to limit or avoid conversion of the plastic in the melting tank, as described above.

[0013] The melting tank may be an agitated tank. The tank may include an inlet for receiving a feed stream containing waste plastics from a waste plastics feed system and an outlet for conveying or discharging the resulting molten plastics. The melting tank may also include a steam outlet for venting gases produced in the melting tank or released from the waste plastics, such as those that may be entrained in the feed or that may result from heating the waste plastics. The melting tank vessel may include a heating system for heating the waste plastics from the feed temperature to a melting temperature. The heating system may include one or both of an external jacket and an internal coil that provide heat for melting the waste plastics.

[0014] After melting the waste plastic, the molten plastic may be fed to a pyrolysis reactor for converting the molten plastic into a pyrolysis oil product and a pitch product. The pyrolysis reactor may include an inlet configured to receive the molten plastic, a first outlet for recovering the pyrolysis oil, and a second outlet for recovering the pitch product. The pyrolysis reactor may also include a heating system configured to heat the molten plastic from the inlet temperature to a pyrolysis temperature. The molten plastic may be heated in the pyrolysis reactor to a temperature ranging from about 350°C to about 700°C, e.g., from about 370°C (700°F) to about 675°C (1250°F), e.g., from 350°C (662°F) to 550°C (1022°F), at a pressure ranging from about 0.3 barg (4 psig) to about 1.4 barg (20 psig), e.g., about 0.4 barg (6 psig).

[0015] Upon heating, the molten plastic may be broken down into short-chain petroleum hydrocarbons, which can be recovered and separated in a separation system. It is desirable for embodiments herein to produce a pitch product, thus limiting the amount of coke or char formed from the thermal decomposition of plastic. Heating of the molten plastic in the pyrolysis reactor may be limited or controlled so that in some embodiments, less than 1 wt. %; in other embodiments, less than 0.5 wt. %; and in still other embodiments, less than 0.2 wt. % of the waste plastic is converted to coke or char. Limiting char formation may provide significantly longer reactor run times and the production of more valuable pitch products. A control system may be provided to control the pyrolysis reactor heating system to provide a heating profile or heating temperature that favors the production of pitch products and is below temperatures that would result in significant char or coke formation.

[0016] In some embodiments, the pyrolysis reactor heating system may include a preheating zone and a reaction zone. The preheating zone may be configured to heat the molten plastic to a first pyrolysis temperature, targeting a low conversion rate, e.g., 5-20 wt% conversion, e.g., 10 wt% conversion (conversion rates herein are wt% unless otherwise specified). After the preheating zone, the molten plastic may be processed in a reaction section to convert a significant portion of the plastic into pyrolysis oil. The pyrolysis temperature and residence time may target a reaction zone conversion rate in the range of 30 wt% to 80 wt%, e.g., 70 wt% conversion. The overall conversion rate of the waste plastic to pyrolysis gas and pyrolysis oil may be, for example, greater than 80 wt%, greater than 85 wt%, greater than 90 wt%, or even up to 95 wt% or 97 wt%, in some embodiments, with the remainder of the waste plastic being recovered from the pyrolysis reactor as a pitch product.

[0017] The pyrolysis oil recovered from the pyrolysis reactor may be fed to a separation system for separating the pyrolysis reaction effluent into two or more fractions. The separation system may include one or more distillation columns. A pre-flash tank may also be provided for separating the pyrolysis reactor effluent into a liquid feed and a vapor feed that are fed to the distillation column. In some embodiments, the pyrolysis reaction products recovered from the pyrolysis reactor may be separated into a pyrolysis gas fraction, a light pyrolysis oil fraction, a medium pyrolysis oil fraction, and a heavy pyrolysis oil fraction based on boiling point.

[0018] To facilitate mixing and melting of the polymer, embodiments herein may include a flow splitter disposed intermediate the melt tank outlet and the pyrolysis reactor inlet and configured to split the molten plastic into a first portion and a second portion. The first portion may be fed to the pyrolysis reactor inlet. The second portion may be fed to a mixing system configured to mix the second portion of the molten plastic with the waste plastic. The mixing system may be configured as part of a waste plastic feeding system, such as by mixing the waste plastic solids with the molten plastic with a screw conveyor; in other embodiments, the mixing system may be disposed upstream of the melt tank inlet and downstream of the waste plastic feeding system.

[0019] In some embodiments, the pyrolysis reaction effluent may be quenched intermediate the recovery and separation system from the pyrolysis reactor. For example, in some embodiments, medium or heavy pyrolysis oil may be used as a quenching medium to reduce the temperature of the pyrolysis reactor effluent and slow or stop reactions that might otherwise occur during transport to the separation system. In this manner, excessive pyrolysis of plastics and excessive production of pyrolysis gases may be avoided, resulting in higher liquid pyrolysis oil recovery rates. Thus, the systems herein may include a flow splitter (which may be a single tee or pipe / valve) for dividing the medium or heavy pyrolysis oil into first and second portions, and a quenching system for mixing one of the portions with the pyrolysis reaction product recovered from the pyrolysis reactor.

[0020] To enhance the transportability of the molten plastics recovered from the melting tank, the process herein may include mixing the molten plastics with a portion of the heavy or medium pyrolysis oil. For example, a flow splitter may be used to split the heavy pyrolysis oil, and a portion of the heavy pyrolysis oil may be recycled as a diluent and mixed with the molten plastics in a mixing system (which may be, for example, a floatie, static mixer, or small stirred tank). In other embodiments, for example, a flow splitter may be used to split the medium pyrolysis oil, and a portion of the medium pyrolysis oil may be recycled as a diluent and mixed with the molten plastics in the mixing system. The mixing system may be located downstream of the melting tank and upstream of the pyrolysis reactor. In some embodiments, the mixing system is located intermediate the melting tank and the molten plastic flow splitter.

[0021] In some embodiments, a heat exchanger is also provided in the flow line transporting the molten plastic from the melt tank to the pyrolysis reactor. The heat exchanger may be configured to heat or maintain the temperature of the molten plastic to maintain its flowability to downstream systems. Heat may also be required after mixing with the recycled medium or heavy pyrolysis heat to maintain the molten plastic at the desired temperature.

[0022] As outlined above, the systems herein may include a melting tank with a heating jacket and heating coils, a nitrogen heater, and a molten plastic heat exchanger. Heat may be provided to each of them using a heat exchange medium. The heat exchange medium may be heated, for example, using a combustion or electric heating system. Fuel may be supplied to the combustion heating system, may be an external fuel supply, or may be an internal fuel supply, such as pyrolysis gas recovered from a separation system. The heat exchange medium may be provided via a closed loop system, where the heat exchange medium is circulated from the combustion heater to each respective user at a desired rate and then returned to the combustion heater for continued heating, circulation, and use.

[0023] For systems processing chlorine- and other halogen-containing polymers, such as PVC, embodiments herein may include a chemical injection system to provide reagents that react with the halogens and facilitate their removal. Inorganic and organic chlorides and other halides (fluorides, bromides) are commonly present in all types of plastics, typically as additives, binders, flame retardants, and residual catalysts, but are not limited to these. These halides are partially removed by dissociation in the melt tank to form chlorides, fluorides, or hydrogen bromides, which are recovered as vent gases from the melt tank or melt vapor separator and treated using a chloride removal system. Remaining halogens may be removed by in-situ alkaline chemical injection. Chemical injection may be performed into the polymer melt, for example, upstream of the pyrolysis reactor or directly into the pyrolysis reactor. Alkaline chemicals, such as, but not limited to, anhydrous lime, caustic soda, aluminum oxide, calcium carbonate, or magnesium oxide, are typically used for this purpose. For example, a slurry-phase solution of alkaline chemicals is prepared and injected into a pyrolysis reactor, where a temperature ranging from 500°F to 1000°F and a residence time ranging from 5 minutes to 60 minutes is provided to react the organic halides with the alkaline chemicals to form salts, thereby removing them from the pyrolysis product oil and gas. The resulting alkaline salts and unreacted alkaline chemicals are extracted from the bottom of the pyrolysis reactor along with the pitch.

[0024] In some embodiments, the alkaline chemical injection system may include a lime supply system for forming a lime slurry. The lime slurry may be formed, for example, by mixing lime with a portion of the heavy or medium pyrolysis oil. The lime slurry may then be fed to a mixing system for mixing the lime slurry with the molten plastic. The lime mixing system may be located upstream of the pyrolysis reactor and, in some embodiments, downstream of the molten plastic flow splitter. In the flow line and pyrolysis reactor, the lime may react with chlorine contained in the molten plastic to produce calcium chloride, which may be recovered from the pyrolysis reactor along with the pitch product. Other alkaline chemical injection reagents may be used in a similar manner.

[0025] A simplified process flow chart of a thermochemical process according to embodiments herein is shown in Figure 1. A plastic waste feed 10 may be fed to a pyrolysis reaction zone 20 for conversion of plastics to pyrolysis oil, which may include various hydrocarbons, such as light (C1-C4) hydrocarbons, naphtha range hydrocarbons, diesel range hydrocarbons, and heavier hydrocarbons, even pitch. The pitch may be recovered as a by-product stream 12, and the remaining conversion products may be recovered as pyrolysis oil 14. The pyrolysis oil 14 may then be fed to a distillation and separation zone 22 for separation of the pyrolysis oil into various hydrocarbon fractions, such as the aforementioned light hydrocarbons 15, naphtha 16, diesel 18, and heavy oil fraction 19. Some or all of the light hydrocarbons 15, for example, C1 or C1-C2 or C1 through C3, C4, C5, C6, or C7 hydrocarbons, may be recycled to the thermal cracking reaction zone 20 for use as fuel for burners (not shown) and / or heaters (also not shown) associated with the reactor. Heavier products may also be recycled to the thermal cracking reaction zone 20 as needed or desired; for example, naphtha, diesel, or heavy oil fractions may be fed to the thermal cracking reaction zone 20 and used as a diluent or quench stream, as described further below.

[0026] In some embodiments, the process equipment for the pyrolysis unit may be modularized. Process modules may be transported by truck or rail and assembled on-site. Modularization allows for strict quality control to be achieved under factory conditions. Modularization also ensures that cost and schedule requirements are more predictable and that overruns are minimized.

[0027] Embodiments herein rely on the thermochemical decomposition (pyrolysis) of plastic feedstock to produce various grades of gas and liquid products. Solid plastic particles are introduced into a melting tank and then, successively, into reactors. Each of these vessels is thermally heated. In the case of the melting tank, heat is provided by a circulating high-temperature thermal fluid stream. The reactors are thermally heated by a series of gas-fired burners that impinge burner flue gases directly onto the vessel walls, which may be jacketed with a low-melting-point metal. The metal inside the jacket melts to provide uniform heating.

[0028] The thermal mechanism of the process is non-catalytic in nature, breaking polymer bonds in the plastic feedstock by thermal scission.

[0029] The degree of pyrolysis is related to the temperature of the melt tank and reactor. Generally, the higher the temperature, the more pyrolysis of the feedstock plastic occurs and the lower the molecular weight of the product (i.e., the polymer chains are broken down into smaller fragments with fewer carbon atoms). Therefore, the process temperature used must strike a balance between being too high (resulting in more gassing and a lighter, less viscous liquid product) and being too low (resulting in less gassing and a heavier, more viscous liquid product).

[0030] Varying the composition of the feedstock plastics affects the preferred temperatures for operating the melt tank and reactor. In particular, increasing the amount of PVC and high-density PP and PE in the feedstock requires increased temperatures. Similarly, decreasing the amount of PVC in the feedstock and increasing the amount of low-density PP and PE allows for the use of lower process temperatures.

[0031] After melting and pyrolysis, embodiments herein then use fractionation to separate the gases and liquids into desired product streams. Embodiments herein may provide the following list of products: (i) combustible pyrolysis gases; (ii) lighter liquid products with physical and chemical properties similar to untreated natural naphtha or gasoline; (iii) heavier liquid products with physical and chemical properties similar to untreated diesel and heavy oil; and (iv) a heavy liquid pitch product that can be blended with asphaltenes from other refinery processes.

[0032] One advantage of embodiments herein is their simplicity and flexibility for dealing with variations in plastic feedstock composition and volume. Additionally, because process embodiments herein do not require the addition of catalysts or additives, the process requires only fairly simple and basic process controls (i.e., temperature) to manage product yield and properties.

[0033] If necessary, process temperatures can be adjusted to reflect changes in the composition of the plastic feedstock. At the same time, the amount of PVC and PET in the plastic feedstock should be minimized. PVC plastic is more difficult to melt and decompose; and the decomposition products release chlorine gas (which is vented from the melt tank and then removed by contact with water). Residual chlorine can also ultimately contaminate the liquid product stream, which is undesirable. PET decomposition tends to release oxygenates into the product stream, which is also an undesirable contaminant.

[0034] Referring now to FIG. 2, a simplified process flow diagram of a system for converting plastic waste according to embodiments herein is shown. Shredded plastic 10 may be transferred to a feed area located near the top of a melting tank 40. The plastic is then directed into the melting tank 40 through a feed auger (not shown). In some embodiments, the feed auger may be replaced by or further include an extruder, and the plastic feed may be heated by the action of the extruder. Prior to being fed into the melting tank, the shredded plastic may be combined with a portion of molten plastic 42 recycled from the bottom of the melting tank. The combined plastic feed is then heated in the melting tank, for example, to a temperature of 232°C to 343°C (450°F to 650°F), e.g., 300°C (573°F). The tank may be heated using a high-temperature thermal fluid circulated through an external tank jacket (not shown) and internal coils (not shown) of the melting tank. Alternatively, the tank may be heated by an electric heating element or by hot flue gas circulating in the heating jacket. The molten plastic may be thoroughly mixed by an internal melt tank agitator (not shown).

[0035] In some embodiments, the molten polymer 41 may then be withdrawn from the melt tank 40, pumped, and further heated against a high-temperature thermal fluid in a melt heat exchanger (not shown) and fed to a melt vapor separator (not shown). The feed temperature to the separator may be set by controlling the amount of high-temperature thermal fluid to the melt heat exchanger. Vapors produced in the melt tank may be recovered as off-gas separated from the molten polymer. In some embodiments, the heavy pyrolysis liquid product 76B and / or the medium pyrolysis liquid product 74B may be used as a diluent, recycled and added to the molten plastic in the melt tank or added at an appropriate location upstream of the pyrolysis reactor 44. This diluent also supplements the total heat duty required to melt the solid plastic by providing heat directly to the plastic melt. The concentration of the recycled diluent is varied to optimize heat input and maintain molten plastic flow.

[0036] The first portion 43 of the molten plastic may then be directed to the inlet of a pyrolysis reactor 44 where it may be further heated and pyrolyzed. The second portion 42 of the pumped molten plastic may be recycled back to the melt tank, for example after being heated in a melt heat exchanger, and combined with fresh plastic from the supply package.

[0037] Vent gases 45 recovered from the melt tank and / or melt vapor separator (not shown) may be combined and directed to a chloride removal system. The chloride removal system may include a caustic or water wash tower (not shown) to completely neutralize the hydrogen chloride or to produce a hydrochloric acid by-product. Chlorides may be present in the vent gas stream and may be removed prior to combining with the overhead vapor 68 from separation tower 70 to overhead condenser 72.

[0038] The molten plastic 43 may be slowly fed to a feed distributor in the pyrolysis reactor, where the molten plastic is heated on the downflow inner wall with the aid of an upper pyrolysis reactor agitator (not shown). In some embodiments, the inner wall may be tapered. As described above, an alkaline reagent may also be introduced into the pyrolysis reactor. The molten plastic may be heated to a temperature in the range of, for example, about 700°F to about 1250°F, e.g., 662°F to 1022°F, at a pressure in the range of about 4 psig to about 20 psig, e.g., about 6 psig, by hot flue gases from a gas burner circulating through the outer jacket. As it is heated, the molten plastic may be cracked into short-chain petroleum hydrocarbons, which may be separated in the distillation and separation section 22.

[0039] The pyrolysis reactor according to embodiments herein may be divided into two zones: a first section and a second section. The first section may include a preheating zone targeting a low conversion rate, such as 5-20%, e.g., 10%, and a reaction zone targeting a conversion rate of 30-80%, e.g., 40-70%, e.g., about 60% (by weight). The second section is a liquid product maximization zone operating at the temperatures described above, where final conversion is completed with a longer residence time. The temperature of the second section may be controlled to avoid coking and favor the production of a liquid pitch product. A lower pyrolysis reactor agitator (not shown) mixes the pitch product and removes solids from the reactor walls and bottom.

[0040] In some embodiments, the separation system may include one or more distillation columns. In some embodiments, the separation system 22 may include a preflash drum and one or more distillation columns, among other components. In some embodiments, the pyrolysis vapor product 46 is withdrawn from the pyrolysis reactor 44 and quenched in a pyrolysis reactor vapor quench mixer with a portion of the cooled medium pyrolysis cut liquid 74a recovered from a side draw 74 of the separation column 70. The quenched pyrolysis vapor 48 is then sent to a preflash drum (not shown) before being fed to the separation column 70. To prevent buildup, a hot pitch product 50 from the bottom of the reactor is removed and sent to a pitch drum (not shown) via a pyrolysis reactor bottom auger (not shown). The pitch product from the pyrolysis reactor may be cooled by a slipstream (not shown) of heavy pyrolysis cut liquid from the preflash drum (not shown), which also reduces the viscosity of the pitch product somewhat. The pitch drum may be cooled by ambient air to a temperature below about 400°F but above the softening point.

[0041] The quenched pyrolysis product 48 from the top of the pyrolysis reactor 44 may be fed to a pre-flash drum (not shown) in the distillation and separation zone 22. The heavy pyrolysis cut liquid that drops from the pre-flash drum may be pumped and combined with the bottom 76 of the separation tower 70. A portion of the heavy pyrolysis cut liquid from this pre-flash drum may also be sent back to the pitch drum (not shown) to cool and reduce the viscosity of the pitch product from the pyrolysis reactor, as described above. The top of the flash drum is supplied as feed to the separation tower, which may consist of a valve tray in the top section, a packed bed in the middle section, and a baffle tray in the bottom section. The basic purpose of the tower is to separate the pyrolysis reactor effluent 48 into pyrolysis gas 78, a light pyrolysis cut 80, a medium pyrolysis cut 74, and a heavy pyrolysis cut 76.

[0042] The entire overhead vapor 68 from the separation column 70 is combined with the treated vent gas stream 45 from the caustic drum (not shown), which is then partially condensed against cooling water in an overhead condenser before entering the reflux drum 72. A light pyrolysis liquid cut from the reflux drum is then pumped by the light pyrolysis cut pump, with a portion 80R being sent back to the top of the column as reflux. The remaining light pyrolysis products 80 are recovered as product, and in some embodiments, an antioxidant is injected into this stream. In some embodiments, the light pyrolysis products may be withdrawn as a side draw from the top tray of the separation column; in such embodiments, the column may operate at total reflux.

[0043] The light hydrocarbons and non-condensables 78 from the reflux drum 72 are sent to a pyrolysis gas compressor package (not shown) where they are compressed and then cooled. The cooled pyrolysis gas is then sent to a pyrolysis gas accumulator (not shown) where it may be mixed with fuel gas make-up, such as may be needed during start-up and shutdown. The off-gas from the accumulator is then sent to a thermal fluid package (not shown) and the pyrolysis reactor burners (not shown) for use as fuel gas.

[0044] The medium pyrolysis cut product is withdrawn from the central section of separation tower 70 as a side draw 74. The medium pyrolysis product 74 is then pumped, and a portion 74a is recycled back to the separation tower to wash the vapor feed before it is cooled. A medium pyrolysis cut cooler (not shown) may cool the remaining remainder, and a portion of the remaining remainder may be recycled back to the pyrolysis reactor vapor quench mixer to be used as quench liquid for the pyrolysis reactor vapor. The remaining portion may be recovered as product, and antioxidants may be injected into this stream.

[0045] The column reboiler duty may be provided by a hot thermal fluid circulating in a reboiler (not shown). The heavy pyrolysis cut product 76 from the column bottom, which is a mixture of heavy hydrocarbons, may be pumped and then combined with the heavy pyrolysis cut from the preflash drum and cooled in a heavy pyrolysis cut cooler (not shown) before being sent to storage.

[0046] As noted above, a portion 76B of the heavy pyrolysis cut product 76 and / or a portion 76B of the medium pyrolysis cut 74 may be recycled back to the melting tank or an appropriate location in the melting section and mixed with molten plastics recovered from the melting tank 40.

[0047] The pyrolysis gas 78, or a portion thereof, may be used to fire the burners of a high-temperature thermal fluid heating system (not shown) and the pyrolysis reactor burners (not shown). However, the raw pyrolysis gas may contain olefins and diolefins. In some embodiments, the raw pyrolysis gas may be cooled and fractionated to recover all compounds heavier than C2 or C3. In other embodiments, the entire raw pyrolysis gas product stream 78 may be compressed and recovered as a product.

[0048] As noted above, systems according to embodiments herein may be modular. In some embodiments, the system may be constructed to include three or more modules, such as (1) a melting tank and associated equipment, (2) a pyrolysis reactor and associated equipment, and (3) a separation section and associated equipment. Various other storage tanks, compressors, thermal fluid heating systems, and other parts of the system may also be included in one or more additional modules. The modules may also be designed to be secured to a foundation with anchor bolts. Additionally, the modules may be provided with appropriate connection points for utilities, such as power, cooling water, natural gas, feedstock introduction, products, etc.

[0049] Referring now to FIG. 3 , a simplified process flow diagram of a feed system, melt tank, and vent gas separator according to an embodiment of the present disclosure is shown. Waste plastic 302, such as shredded plastic (which may be provided in chips, chunks, pellets, fibers, or other readily flowing forms), may be fed to a feed hopper 304. A small, continuous stream of heated nitrogen 306 is fed to the feed hopper 304 to remove moisture associated with the feed waste plastic and also prevent oxygen (air) from entering the melt tank 308. For example, the nitrogen feed may be heated to a temperature greater than 60° C. or greater than 100° C. using high-temperature thermal fluid 330A in a nitrogen heater 307. A wet nitrogen stream 312 is exhausted from the feed hopper 304, and in some embodiments, may be vented to the atmosphere. The dried waste plastic is then directed to the melt tank 308 through a feed auger 310, which may be used to meter the shredded plastic via flow line 311 into the melt tank at a desired rate.

[0050] Prior to being fed to melt tank 308, shredded plastic 311 is combined with a portion of recycled molten plastic 316, which may be a portion of plastic melted in melt tank 308 and recovered from its bottom via flow line 318. The combined plastic feed 317 is then heated in melt tank 308 to a temperature sufficient to melt the waste plastic, for example, in the range of 250°C to 350°C, for example, 300°C, so that the molten plastic has sufficient fluidity (low enough viscosity) to be drawn off via flow line 318 and transported to downstream equipment.

[0051] The melter tank 308 may be raised in temperature using high-temperature thermal fluid 330 from a thermal fluid package 328. As shown in FIG. 3, the melter tank may be raised in temperature by circulating a portion 330C of the high-temperature thermal fluid through an external melter tank jacket 308J and a portion 330B of the high-temperature thermal fluid through an internal heating coil 308C. The use of thermal fluid 330 limits the maximum surface temperature anywhere in the melter tank, which may help minimize or eliminate coke formation in the melter tank 308. The molten plastic is mixed within the melter tank 308 by an internal melter tank agitator 320.

[0052] The molten plastic withdrawn from the melt tank via flow line 318 is then further heated in melt heat exchanger 322 against high temperature thermal fluid 330D, for example to a temperature in the range of 325°C to 375°C, e.g., 350°C. The resulting heated polymer melt may be split into a portion 316 that is recycled to melt tank 308, as described above, and a polymer melt portion 324 that may be fed downstream to melt vapor separator 340 and downstream processing (not shown), such as a pyrolysis reactor for converting waste polymer melt to lighter hydrocarbons.

[0053] In some embodiments, a solvent, such as heavy oil or a medium or heavy cut of pyrolysis oil produced in a downstream pyrolysis reactor, may be supplied via flow line 342 and mixed with the molten polymer 318 withdrawn from the melting tank 308. Recycled pyrolysis oil can help melt the plastic, which is the first step toward successful pyrolysis in the reactor. Plastics are non-Newtonian liquids with high viscosity. Heating and maintaining uniformity in the tank is extremely difficult. Plastic viscosity varies significantly with the molecular weight of the plastic polymer, shear rate due to agitators and other mechanical stresses, and temperature. To limit the wide variation in plastic rheology below the desired melting temperature, a suitable solvent 342 can be mixed with the molten plastic in the melting tank 308. The solvent 342 can come from either the liquid product from the pyrolysis process (in-house solvent) or an external source. In some embodiments, the solvent is provided at a flow rate of 10-30% (by weight) of the fresh waste plastic 311 feed rate. As an added benefit, the use of the solvent reduces the plastic viscosity and improves heat transfer in the melt tank.

[0054] As described above, the polymer melt is heated in exchanger 322 before entering melt vapor separator 340. The feed temperature to melt vapor separator 340 is set by controlling the amount of hot thermal fluid to melt heat exchanger 322, which may be a spiral heat exchanger. Depending on the polymer being processed, lime 344 may be injected as a slurry phase into melt vapor separator 340 to remove inorganic and organic chlorides from the molten plastic feed. Calcium oxide reacts with chlorides in the reactor to form calcium chloride, which is then removed from the reactor along with the pitch. Molten plastic recovered from the bottom of separator 340 via flow line 346 is then directed to a pyrolysis reactor (not shown) where it is heated, pyrolyzed, and processed as described above with respect to FIGS. 1 and 2.

[0055] Vent gas 348 from the melt tank and vent gas 350 from the melt vapor separator may be combined and directed to a vent gas cooler (not shown). The cooled vent gas may be sent to a wax oil separator (not shown). Condensed hydrocarbons / wax oils are separated from the cooled vent gas in the wax oil separator drum. The liquid hydrocarbons / wax oils may be sent to a separation tower downstream of the pyrolysis reactor to recover hydrocarbons. Chlorides present in the combined vent gas stream may be removed with a water wash and then combined with overhead vapors from a separation tower downstream of the pyrolysis reactor. Lime added to the reactor also removes difficult-to-remove organic chlorides, resulting in very low ppm chloride levels in the pyrolysis product.

[0056] Lime or other alkaline injection into the reactor may be used to meet product chloride specifications. Chlorides associated with feeds, such as those derived from PVC, are both organic and inorganic chlorides. While some of the inorganic and organic chlorides dissociated at melting tank conditions are removed with the melting tank vapors, other chlorides that are more difficult to remove react with alkaline reagents, such as CaO (lime), in the pyrolysis reactor to form salts, such as calcium chloride (CaCl). The resulting salts and unreacted alkaline reagents, e.g., CaCl and unreacted CaO, are removed from the bottom of the pyrolysis reactor along with the pitch. In some embodiments, anhydrous lime powder is mixed with pyrolysis oil, such as a heavy cut product, and the resulting lime slurry is pumped to the pyrolysis reactor via flow line 344. The lime (alkaline reagent) injection rate can be adjusted to achieve the desired chlorine reduction in the pyrolysis product.

[0057] As described above, a thermal fluid package 328 may be used to supply heated thermal fluid to each of the nitrogen heater 307, melter tank coil 308C, melter tank jacket 308J, and melt heat exchanger 322. After heat exchange in each unit, the thermal fluid may be returned to the thermal fluid package for reheating and reuse in the heat exchange loop. The thermal fluid package 328 may be, for example, a fired heat exchanger that may be fueled by one or more fuels, which in some embodiments may be a light hydrocarbon fraction 360 recovered from a separation column downstream of the pyrolysis reactor or an external fuel supply 361. The fuel may be combusted with air 362 to produce exhaust gases 364. The fired heat exchanger used to heat the thermal fluid may be a stand-alone unit or may be configured as part of a larger fired heater used to heat multiple process or utility streams in the complex. In other embodiments, the thermal fluid may be heated by an electric heater.

[0058] In some embodiments, the entire pyrolysis unit is designed to consume a portion of the pyrolysis gas product within the unit itself once normal operating conditions are reached. This pyrolysis gas would be used to fire the burners of the high-temperature thermal fluid heating system 328 and the pyrolysis reactor burners. However, depending on the overall process, the raw pyrolysis gas may contain olefins and diolefins, resulting in a flame profile that could be environmentally problematic. Alternatively, the raw pyrolysis gas can be purified by cooling and fractionation to recover all compounds heavier than C1, C2, or C3. A further option is to compress the entire raw pyrolysis gas product stream (e.g., to approximately 27 barg) and then transport the compressed stream as product. In that case, the heating requirements of the pyrolysis skid could be met entirely using imported natural gas.

[0059] As described above, embodiments herein relate to processes and systems for the conversion of waste polymeric materials into useful petrochemicals, fuels, and other intermediates and products. Advantageously, embodiments herein produce pitch products rather than tar or coke, and operate at temperatures and conversion rates that enable the production of high-quality pyrolysis oil.

[0060] Unless otherwise defined, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which these systems, devices, methods, processes, and compositions belong.

[0061] The singular forms "a," "an," "one," "the," and "the above" include plural referents unless the context clearly dictates otherwise.

[0062] As used in this specification and the appended claims, the words "comprise," "have," and "contain," and all grammatical variations thereof, are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

[0063] "Optionally" or "optionally" means that the subsequently described event or circumstance may or may not occur. The description includes cases where the event or circumstance occurs and cases where it does not occur.

[0064] When the word "approximately" or "about" is used, this term may mean that the value may vary by up to ±10%, up to 5%, up to 2%, up to 1%, up to 0.5%, up to 0.1%, or up to 0.01%.

[0065] Ranges may be expressed as from about one particular value to about the other particular value, inclusive of those values. When such a range is expressed, it is to be understood that another embodiment is from the one particular value to the other particular value, along with all particular values ​​and combinations thereof within that range.

[0066] While the present disclosure includes only a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will recognize that other embodiments may be devised that do not depart from the scope of the present disclosure, which scope, therefore, should be limited only by the appended claims.

Claims

1. A process for converting waste plastics, comprising: feeding waste plastics into a melting tank; Heating the waste plastic in the melting tank to form a molten plastic; withdrawing the molten plastic from the melting tank and feeding the molten plastic into a pyrolysis reactor; heating the hot molten plastic to a pyrolysis temperature in the pyrolysis reactor to produce a pyrolysis oil product and a liquid pitch product; and Separating the pyrolysis oil product into a pyrolysis gas fraction, a light pyrolysis oil fraction, a medium pyrolysis oil fraction, and a heavy pyrolysis oil fraction. A process involving:

2. 10. The process of claim 1, wherein the waste plastic is provided to the melting tank as partially or fully melted waste plastic recovered from an extruder, and the process further comprises partially or fully melting the waste plastic in the extruder.

3. 10. The process of claim 1, further comprising controlling the temperature of the pyrolysis reactor to limit the temperature of the hot molten plastic below a temperature at which char or coke would form.

4. dividing the heated molten plastic into a first portion and a second portion; mixing the waste plastic with the second portion upstream of the melting tank; and feeding said first portion to said pyrolysis reactor.

10. The process of claim 1 further comprising:

5. 10. The process of claim 1, further comprising contacting the waste plastic with nitrogen at a temperature sufficient to remove water from the waste plastic before delivering the waste plastic to the melting tank.

6. dividing the medium pyrolysis oil fraction into a first portion and a second portion; and mixing the first portion of the medium pyrolysis oil with the heated molten plastic, the mixing occurring downstream of the melting tank and upstream of the pyrolysis reactor; 10. The process of claim 1 further comprising:

7. dividing the medium pyrolysis oil fraction into a third portion; and quenching the pyrolysis oil recovered from the pyrolysis reactor with the third portion of the medium pyrolysis oil.

7. The process of claim 6, further comprising:

8. dividing the heavy pyrolysis oil fraction into a first portion and a second portion; and mixing the first portion of the heavy pyrolysis oil with the heated molten plastic, the mixing occurring downstream of the melting tank and upstream of the pyrolysis reactor; 10. The process of claim 1 further comprising:

9. 10. The process of claim 1, further comprising providing heat to one or both of the melting tank or the pyrolysis reactor directly or indirectly by combustion of the pyrolysis gas fraction.

10. 2. The process of claim 1, further comprising mixing an alkaline reagent with the heated molten plastic downstream of the melting tank and upstream of the pyrolysis reactor, reacting the alkaline reagent with chlorine contained in the heated molten plastic to form a calcium salt, and recovering the calcium salt with the liquid pitch product.

11. 10. The process of claim 1 further comprising withdrawing a vapor stream from the melting tank and optionally treating the vapor stream to remove halogens contained therein.

12. 1. A system for pyrolyzing waste plastics, comprising: an inlet configured to receive a feed stream comprising waste plastics from a waste plastics feed system; a heating system configured to heat the waste plastic from a supply temperature to a melting temperature to produce molten plastic; an outlet configured to discharge said molten plastic; a melting tank having an inlet configured to receive the molten plastic; a heating system configured to heat the molten plastic to a pyrolysis temperature; a first outlet for recovering pyrolysis oil; a second outlet for recovering the pitch product; and a control system configured to control the heating system to limit the temperature of the molten plastic to a temperature for producing the pitch product. a pyrolysis reactor having A system including:

13. The system of claim 12 , wherein the waste plastics supply system includes an extruder for partially or melting the waste plastics.

14. a flow line disposed intermediate the melter tank outlet and the pyrolysis reactor inlet, the flow line configured to divide the molten plastic into a first portion that is fed to the pyrolysis reactor inlet and a second portion; and a mixing system configured to mix the waste plastic with the second portion of the molten plastic upstream of the inlet and downstream of the waste plastic supply system; The system of claim 12 further comprising:

15. The waste plastic feeding system includes a feeding hopper and a screw conveyor, a nitrogen supply and a nitrogen supply line for providing nitrogen to at least one of the feed hopper and the screw conveyor; and a nitrogen heater configured to heat the nitrogen to a temperature greater than 100°C The system of claim 12 further comprising:

16. 13. The system of claim 12, further comprising a separation system configured to separate the pyrolysis oil into two or more fractions.

17. 17. The system of claim 16, wherein the separation system is configured to separate the pyrolysis oil into a pyrolysis gas fraction, a light pyrolysis oil fraction, a medium pyrolysis oil fraction, and a heavy pyrolysis oil fraction.

18. 18. The system of claim 17, further comprising a flow line and a mixing system configured to supply a portion of the medium pyrolysis oil fraction or a portion of the heavy pyrolysis oil fraction and mix it with the molten plastic, the mixing system being located downstream of the melting tank outlet and upstream of the pyrolysis reactor inlet.

19. 18. The system of claim 17, further comprising a quenching system configured to quench the pyrolysis oil recovered via the first outlet with a portion of the medium pyrolysis oil fraction.

20. 18. The system of claim 17, further comprising a flow line configured to supply the pyrolysis gas fraction as fuel to a heater.

21. 21. The system of claim 20, wherein the heater is configured to provide heated heat exchange fluid to the melter tank heating system, a heat exchanger located downstream of the melter tank outlet and upstream of the pyrolysis reactor inlet, and a nitrogen heater.

22. The system of claim 12 , wherein the melter tank heating system includes an outer jacket disposed around an inner melter tank vessel and one or more heating coils disposed within the inner melter tank vessel.

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