Depolymerization process of plastic waste

Abstract

A thermal catalytic process for depolymerizing waste plastic materials to produce pyrolysis oil is disclosed, which includes two stirred-vessel depolymerization reactors and two condensation sections. The configuration also includes a char handling section. The process is highly efficient and versatile, producing a pyrolysis product in the form of an oil for a variety of end uses.

Description

[Technical field]

[0001] The present disclosure relates to the field of depolymerizing plastic waste to produce new products, including hydrocarbon oils, that have valuable and useful properties. In one aspect, the present disclosure relates to a process for converting plastics into liquid hydrocarbons, particularly liquid hydrocarbons that are used as hydrocarbon feedstocks. Technical Background

[0002] It is becoming increasingly recognised that waste plastic materials have a negative impact on the environment and, consequently, on the health of all forms of life.

[0003] One attempt to mitigate this impact is to recycle plastic materials from household and industrial waste, reintroducing some of them into the production cycle, which can lead to further positive outcomes such as reducing the use of fossil hydrocarbon sources to manufacture plastic products.

[0004] However, various factors suggest that this solution alone is insufficient to achieve sustainability goals: in fact, mechanical recycling of plastic materials generally produces low-quality materials, is relatively expensive and cumbersome, and is not suitable for some municipal waste streams that mix plastics into a variety of different materials.

[0005] As a result, most of the plastic waste is either used as a source of heat energy in plants such as incinerators or simply stored in landfills, which, as mentioned above, contributes to the production of CO 2 This will lead to the deterioration of the global environment due to the increased emission and release of hazardous chemicals.

[0006] In view of the above, numerous attempts have been made to efficiently reprocess waste plastic feedstocks into liquid hydrocarbon products that have useful properties, particularly valuable as fuels.

[0007] Thermocatalysis is the basic process of converting plastic waste into liquid fuels (pyrolysis products) by the effects of thermal and optionally catalytic decomposition under oxygen-free conditions. Plastic waste is first melted, usually in a stainless steel chamber under an inert purge gas (such as nitrogen gas). In a first thermal step, this chamber heats the molten material to a gaseous state, and in a subsequent thermocatalytic step, it decomposes to form variable-length hydrocarbon chains.

[0008] The pyrolysis gas is then condensed in one or more condensers to produce a hydrocarbon distillate (pyrolysis oil) containing linear and branched aliphatic, cyclic aliphatic and aromatic hydrocarbons. The resulting mixture is used for various applications depending on its composition, but in each case, the product consistency and quality requirements must be met. Also, due to the unstable composition of the plastic waste feedstock, it is difficult to satisfy the quality consistency of the recovered oil.

[0009] The use of one or more catalytic cracking stages may contribute to lowering the operating cracking temperature and target the product composition to the desired goal. According to US Patent No. 5,608,136, the amount of catalyst used ranges from 10% to 200% by weight relative to the amount of plastics processed. This significantly affects the cost per unit of recovered oil, and to make the costs sustainable, it is practically mandatory to provide a separate catalyst recovery section in the plant.

[0010] Industrial pyrolysis plants or thermocatalytic plants may have two pyrolysis chambers, known as twin chamber systems, operating in parallel at approximately equal speeds. When the two chambers simultaneously complete the pyrolysis of waste, it is necessary to wait for each chamber to cool before removing the unwanted carbonaceous char from the bottom interior of each chamber. In one variation, the chambers may be operated in an alternating mode, such that one chamber is under cleaning treatment while the other chamber is operating. However, this method reduces the amount of plastic processed per unit time.

[0011] In view of the above, it is an object of the present disclosure to provide a thermal catalytic decomposition process for plastic waste having high productivity, at the same time stable quality, high throughput of liquid hydrocarbons as pyrolysis products, smooth, continuous and economical operation, and the possibility of operating with low catalyst consumption. Summary of the Invention

[0012] Accordingly, one aspect of the present disclosure is a process for depolymerizing waste plastic material to produce pyrolysis products, comprising: (a) feeding a mixture comprising waste plastic material in an oxygen-free atmosphere into a feeding system comprising at least one screw extruder (1) heated at the melting temperature of said plastic material; (b) feeding the molten plastic material from the extruder to a first depolymerization reactor (2), which is a continuous stirred tank reactor maintained at a temperature range of 280-600°C and operating at a pressure range of 1-10 barg, where depolymerization proceeds in the first depolymerization reactor to form a gaseous effluent and a liquid effluent; (c) directing at least a portion of the liquid effluent produced in the first reactor (2) to a char handling section (6) and feeding a gaseous effluent from the reactor (2) to a condensation unit (3) producing a gas stream and a liquid stream; (d) directing the gas stream from the first condensing unit (3) to a second condensing unit (5) operating at a lower temperature than the first condensing unit and directing the liquid stream from the condensing unit (3) to a second depolymerization reactor (4) which is a continuous stirred tank reactor maintained at a temperature range of 280-600°C and operating at a pressure range of 1-10 barg, where depolymerization proceeds in the second depolymerization reactor (4) to form a gaseous effluent and a liquid effluent; (e) withdrawing the gaseous effluent from the depolymerization reactor (4) and feeding it to the second condensation unit (5) and recycling at least a portion of the liquid effluent from the depolymerization reactor (4) to the first depolymerization reactor (2), The process further comprises recovering pyrolysis products from a condensation unit (5) and wherein at least one of the depolymerization reactors (2) and (4) operates in the presence of a depolymerization catalyst. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic diagram of a thermal catalytic process plant. [Diagram 2] FIG. 2 is a schematic diagram of a first type handling unit. [Diagram 3] FIG. 11 is a schematic diagram of a second type of handling unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The process of the present disclosure is preferably carried out in a continuous mode.

[0015] In step (a), the loading system allows the loading of the feed waste plastic material into the reactor (2), preferably in a continuous mode. Care must be taken not to introduce an oxygen-containing atmosphere into the system. A barrier against potential oxygen-containing atmospheres can be obtained by various means, such as, for example, a nitrogen blanket or a vacuum system connected to the barrel of the extruder.

[0016] More specifically, the plastic mixed waste is loaded into the feeding system of the depolymerization reactor (2) by means of one or more hoppers connected in parallel, and the oxygen present in the atmosphere of the plastic waste is substantially removed in the hopper(s).

[0017] The process according to the present disclosure is very flexible and can feed various plastic waste compositions, such as heterogeneous mixtures of uneconomical waste plastic materials (called Plasmix in Italy) where polyolefins are the most abundant component, but with additional sorting steps. It is preferred to depolymerize mixed plastic waste with a polyolefins (PE and PP) content of 70 wt.% or more, especially when the pyrolysis products are recycled to the cracking / purification unit.

[0018] The waste plastic material preferably undergoes a pretreatment step, in which the waste plastic material is melted by heat and may be mixed with an additive, which may be an alkaline material. By melt pretreatment, a heterogeneous mixture of different types of waste plastics can be converted into a homogeneous plastic composite material. Therefore, this pretreatment is also preferred in the case of pyrolysis without the use of additives.

[0019] The heating temperature in the pretreatment stage is appropriately set according to the type and content of plastic contained in the waste plastic material so as to suppress thermal decomposition of the plastic material to be treated. Such a temperature is generally in the range of 100°C to 300°C, and preferably in the range of 150°C to 250°C. At temperatures around 300°C or higher, HCl is removed from the PVC resin in which it may be present.

[0020] When waste plastic materials are mixed with alkaline materials during melting / kneading pretreatment, the HCl-forming gas may be removed and neutralized through an exhaust system or may be trapped. To carry out the melting operation, a general kneader, screw extruder, etc. may be used. The plastic waste is preferably fed to the depolymerization reactor by an extruder.

[0021] The extruder melts the plastic waste, holds it at a high temperature (250-350°C) and injects it into the first depolymerization reactor (2). The extruder receives the plastic waste cut into small pieces in a feed hopper and conveys the flow through a melting section, which may heat the polymer by the combined action of the mixing energy and heat provided by a barrel heater.

[0022] Additives may optionally be incorporated into the melt to reduce the corrosiveness of the received plastic waste or to improve the conversion process in the reaction section.

[0023] Upon extrusion, one or more degassing steps can be foreseen to remove any residual moisture present in the product.

[0024] The melt stream may be filtered to remove solid impurities present in the plastic waste before being fed to the reactor (2).

[0025] Depending on the amount and particle size of solid impurities, melt filtration units manufactured in several designs can be applied.

[0026] It is preferable to use a self-cleaning melt filter that can operate for extended periods (days) without manual replacement of the filter element.

[0027] One of the preferred designs of melt filters is based on a circular perforated plate as the melt filtering element, holes (made by laser or by machining depending on the opening) in which solid contaminants accumulate. The accumulation of impurities can increase the differential pressure on the melt filter. To perform in-line cleaning of the filtering element, a rotating scraper removes the accumulated impurities and directs them to an outlet, which is briefly opened to remove contaminants arising in the process.

[0028] This cycle can be repeated several times (up to several days of operating time) without the need for manual intervention or the need to stop production for the time required to replace the filtration elements.

[0029] Another option, self-cleaning melt filters, is based on the application of a continuous filtering metal band through which the polymer stream passes. Impurities accumulate on the metal filter, causing a pressure increase, which pushes the clogged filtering band section out of the polymer flow path, after which a washing section is inserted.

[0030] This process is automatic and can be repeated over extended periods of time (up to several days) without the need for manual intervention or the need to stop production for the time required to replace the filtration elements.

[0031] Any extrusion system can be used, such as a single screw extruder, a twin screw extruder, a twin screw extruder with a gear pump, or a combination thereof.

[0032] In step (b), the depolymerization reactor (2) is preferably a stirred vessel operating in the temperature range of 300-550°C, more preferably 350-500°C.

[0033] The operating pressure is preferably maintained in the range of 2.0 to 8 barg, more preferably in the range of 2.5 to 7 barg.

[0034] In order to improve the fluidity of the material, a preferred embodiment is configured as follows: the waste plastic melt entering the reactor is premixed with a hydrocarbon oil, preferably the recycled oil from the first condensation unit, to promote the melt dissolution in the depolymerization reactor. In this case, the volume ratio of oil / molten material can be in the range of 0.1:1 to 1:1.

[0035] The depolymerization reactor (2) preferably has a cylindrical cross section and a circular bottom.

[0036] Preferably, the reactor has a mixer mounted on its vertical axis, equipped with a geared motor that allows the mixer blades to rotate and keep the system in a state of agitation. The design of the mixer and the power of the motor may vary depending on the content, volume and shape of the reactor. However, as a non-limiting example, a geared motor may be used with a power of 0.2 to 4 kW / m 3 , preferably 0.2 to 2 kW / m 3 , more preferably 0.3 to 1.5 kW / m 3 It is preferred to operate the reactor with a power input of

[0037] The reactor is heated by heat transfer via a molten salt stream heated to a temperature range of 300-570°C.

[0038] The supply circuit for the molten salt (not shown) is configured to prevent leakage of the molten salt. The molten salt is preferably molten solar salt consisting of a mixture of sodium nitrate and potassium nitrate, more preferably solar salt mixed in a weight ratio of 2:3 to 3:2. The solar salt in turn receives heat from a dedicated furnace, either electric or fuel-fed. In the latter case, a portion of the recovered oil from the condensation unit (5) may be used to feed the furnace. Alternatively or in combination, the heat may be generated by burning gaseous or liquid hydrocarbons. It is preferable to use gaseous hydrocarbons.

[0039] The heat associated with the molten salt is preferably transferred to the depolymerization reactor by circulating it through a jacket surrounding the entire reactor and / or by feeding the molten salt to an external heat exchanger, which will be described later.

[0040] In either case, the salt is circulated using a circulation pump. A series of fins ensures even distribution of the molten salt flow within the jacket and maximizes the heat exchange coefficient.

[0041] In the depolymerization process carried out in the reactor, the chain length is shortened, producing molecules with lower boiling points. This continuously progressing chain scission mechanism, especially close to the reactor wall, produces smaller and smaller molecules, some of which are gaseous at the operating temperature and pressure.

[0042] As a result, the composition in the reactor covers a wide range of hydrocarbons, from methane to heavier products, including both saturated hydrocarbons and olefins, with linear or highly branched structures. Some aromatic products and fused ring structures may also be present.

[0043] Materials that remain liquid under operating conditions serve to reduce the viscosity of the liquid material. Depending on the depolymerization process and the composition of the feed, the contents of reactor (2) can be defined as a coexistence of a liquid slurry phase (solids, especially carbonaceous and inorganic materials, dispersed in the liquid hydrocarbon mixture) and a gas phase.

[0044] At least a portion of the liquid slurry phase constitutes a liquid effluent which is withdrawn from the reactor, preferably from the bottom of the reactor, and sent to a char handling section (6) which is described in more detail hereinafter.

[0045] From an operational standpoint, the withdrawal of the slurry phase from the bottom of the reactor is preferably triggered by a density sensor which detects when the density of the liquid slurry reaches a predetermined value.

[0046] In certain preferred embodiments, a portion of the liquid slurry withdrawn from the reactor (2) is recirculated via a circulation pump (7) and, if desired, returned to the top of the reactor by an external heater (8). As mentioned above, the heat transferred to the external heater is preferably provided by a molten salt. This embodiment can improve both the homogeneity of the reactor contents and the reactor heating.

[0047] According to a preferred embodiment, the liquid slurry portion recycled to the reactor is withdrawn at a point different from the point at which the liquid slurry portion sent to the char handling section is withdrawn.

[0048] According to another preferred embodiment, both the liquid slurry portion recycled to the reactor and the liquid slurry portion sent to the char handling section are withdrawn from the same point and then split sequentially.

[0049] The division between the liquid slurry portion sent to the char handling section and the liquid slurry portion recycled to the reactor can be carried out either before or after the recirculation pump (7). In the latter embodiment, the liquid slurry is first fed into a dedicated vessel with a lower and an upper outlet point. The liquid portion led to the char handling section (6) is withdrawn in concentrated form from the lower outlet point, while the liquid portion recycled to the reactor (2) is withdrawn from the upper outlet point.

[0050] The gas phase of the reactor (2) constitutes the gaseous effluent that is sent to a condensation unit (3) for further processing.

[0051] The gaseous effluent contains a mixture of light hydrocarbons and may further contain some heavy hydrocarbons and entrained char particles. The gaseous effluent is conveyed from the top of the reactor to a condensation unit (3) which preferably operates at a pressure slightly lower than the reactor pressure.

[0052] The condensation unit (3) is more preferably designed as a scrubber in order to suppress the entrained char. The temperature of the condenser is selected so that the heavy hydrocarbons are condensed and the light hydrocarbons are released as a gas stream. The gas stream (H 2 and light hydrocarbons) are conveyed to another condensation unit (5) operating at a lower temperature than the condensation unit (3) that recovers the oil.

[0053] The operating temperature of the condensation unit (3) may vary in a wide range depending on the operating pressure. The temperature, relative to atmospheric pressure, may be between 20 and 200° C., more preferably between 50 and 200° C., particularly preferably between 60 and 180° C. If a higher operating pressure is selected, the temperature range may of course be modified.

[0054] In a particular embodiment, the condenser unit (3) is operated at about 80° C. and the condensed liquid is collected and analyzed by GC analysis.

[0055] The number of compounds is so large that the analytical results are reported by grouping the compounds obtained according to their internal residence time, taking a particular hydrocarbon as the standard. The results indicate that there is more than about 2% by weight of compounds with a residence time equal to or less than that of n-heptane, more than about 25% by weight of compounds with a residence time between that of n-heptane and that of n-dodecane, and a more abundant fraction of compounds (up to 70% by weight) with a residence time higher than that of n-dodecane but lower than that of n-octacosane, with the possibility of a smaller fraction of the higher residence times being present.

[0056] In a preferred configuration of the condensing unit (3), a dephlegmator (partial condenser) is mounted on top of the scrubber and operates at a lower temperature than the temperature in the tower. The condensate flows down by gravity as reflux for the scrubber. The dephlegmator may be mounted as a separate device or within the scrubber.

[0057] In another preferred configuration, a pump recirculates liquid collected at the bottom of the scrubber to the top of the column, where it is cooled in a dedicated heat exchanger before being injected into the top of the scrubber as reflux.

[0058] The hydrocarbon condensate, preferably having more than C7 carbon atoms, constitutes the liquid stream that is conveyed by a pump to the second depolymerization reactor (4).

[0059] The second depolymerization reactor (4) is preferably of the same type as the first depolymerization reactor, more preferably a continuous stirred tank reactor.

[0060] The depolymerization is carried out in the same temperature range, but preferably at a higher pressure than in the first reactor in order to limit the volatility of heavy hydrocarbons, specifically in the range of 2 to 10 barg, preferably 3 to 9 barg, more preferably 3 to 8 barg.

[0061] Reactor (4) is fed with the condensed effluent from reactor (2) and therefore contains less impurities and produces less char. Fresh catalyst is preferably fed to reactor (4) via conduit (15).

[0062] According to the present disclosure, the catalyst may be selected from active catalysts that act as depolymerization / decomposition catalysts in thermal catalytic processes. In particular, the catalyst may be selected from metal oxides, heteropolyacids, mesoporous silica, aluminosilicate catalysts (such as halloysite, kaolinite, etc.), and preferably zeolites. Among them, particularly preferred zeolites are synthetic Y-type zeolite and ZSM-5.

[0063] In a particularly preferred embodiment, the catalyst supply amount is at most 10% by weight, preferably at most 5% by weight, in particular at most 2% by weight, relative to the plastic waste supply amount.

[0064] In a preferred embodiment, the catalyst is injected into the second reactor as a powder dispersed in a hydrocarbon oil, preferably the liquid pyrolysis product (oil) obtained from the condensation unit (3) or (5), preferably the liquid pyrolysis product (oil) obtained from the condensation unit (3).

[0065] The catalyst slurry is preferably prepared in a pot (continuously stirred tank) into which the catalyst is poured from a dedicated silo in order to keep the concentration of catalyst in the slurry constant.

[0066] The pyrolysis oil with dispersed catalyst is preferably withdrawn from the condensation unit (3) in order to keep the level of the slurry in the pot constant. Once ready, the catalyst slurry can preferably be injected into the second reactor, preferably by means of a progressive cavity pump to keep its level constant.

[0067] The liquid effluent from reactor (4) is preferably a concentrated hydrocarbon slurry containing the depolymerization catalyst. This liquid effluent is discharged from the second reactor and returned to the first reactor via conduit (16). The same concentration control exercised in reactor (4) for withdrawing the slurry is preferably also applied to reactor (2).

[0068] The concentration of the slurry can be controlled by available methods such as gamma radiation measurement or Coriolis densitometer. If the operating pressure of the first reactor is lower than that of the second reactor, the light hydrocarbons of the slurry entering the first reactor are expected to vaporize and be extracted with the gaseous effluent produced in reactor (2).

[0069] The amount of the slurry recycled to the first reactor is preferably 5 to 40% by volume, more preferably 10 to 30% by volume, of the content of the second reactor.

[0070] In addition, in the reactor (4), a preferred embodiment is configured as follows: a portion of the liquid slurry withdrawn from the bottom of the reactor (4) is recirculated via a circulation pump (12) and returned to the top of the reactor by an external heater (13).

[0071] The gaseous effluent produced from the reactor (4) is conveyed to a condensation unit (5) for recovery of the pyrolysis products in the form of oil.

[0072] The condensation unit (5) preferably has a similar configuration to the condensation unit (3).

[0073] The operating conditions of the condensing unit (5) are preferably selected such that the condensing unit (5) operates at a lower operating temperature and pressure relative to the condensing unit (3).

[0074] In particular, the temperature may be in the range of 20-80° C., preferably 30-70° C. The pressure value is preferably lower than the pressure value of the condensing unit (3) in order to allow the non-condensable gases from the condensing unit (3) to enter the condensing unit (5) without further pressurization. The oil recovered from the condensing unit 5 is usually lighter than the oil recovered from the first condensing unit and may in particular have the following composition (GC measurement): Approximately 10-15% by weight of the portion whose retention time is less than or equal to that of n-heptane about 70-75 wt. % of the portion whose residence time is between the residence time of n-heptane and the residence time of n-dodecane; About 12-20 wt. % of a product having a retention time higher than that of n-dodecane but lower than that of n-octacosane; Trace compounds with longer residence times.

[0075] As mentioned above, the liquid effluent discharged from the pyrolysis reactor (2) and sent to the char handling section is concentrated in the form of a slurry withdrawn from the bottom of the reactor, preferably continuously discharged. By operating the pressurized reactor, the concentrated slurry can be easily discharged to the low pressure unit without the use of additional withdrawal equipment. As mentioned above, the slurry can be discharged from the bottom of the reactor or, if present, from a line or vessel behind the circulation pump (7).

[0076] From the viewpoint of process setting, it is preferable that the flow of the slurry stream is continuous. The char content in the slurry can be in the range of 10 to 50%, preferably 20 to 40% by weight.

[0077] A preferred char handling section is a first jacketed chamber (6a) equipped with an inlet conduit (6a-1) for feeding a char-containing slurry and a stirring system (6b) operable in a temperature range of 350-570°C, and further equipped with a conduit (6b-1) for withdrawing a gaseous effluent; means (6c) for withdrawing the concentrated slurry from said chamber (6a) and conveying it to a second chamber (6d) capable of maintaining said slurry in the same temperature range as said chamber (6a) and equipped with a degassing system for removing gaseous effluents and producing a dry char; a second stripping chamber (6d) for receiving the dried char, comprising a stirring system (6d-1), a char outlet (6d-2), a gas inlet (6d-3) located at the bottom of said chamber, and a gas outlet (6d-4) for removing the gaseous effluent stripped from the char; a means (6e) for receiving the dried char from the stripping chamber (6d) and transporting it to a third collection chamber (6f) and capable of maintaining the char at a temperature in the range of 60°C to 100°C; It is provided with a third collection chamber (6f) for receiving the dried char, equipped with an agitation system (6f-1) and an outlet for char disposal operated by a valve (6f-2).

[0078] The purpose of the char handling process is to separate and discard the non-depolymerizable solids in the plastic feed.

[0079] In a preferred embodiment, the hot slurry withdrawn from the reactor (2) is sent to a jacketed chamber (6a) operating in the temperature range of 350-570°C, where the depolymerization process can be carried out under the effect of heat transferred by the high temperature jacket and under stirring by a rotor (6b). The gaseous effluent produced leaves the chamber (6a) and is preferably sent to a condensation unit (3) to be treated together with the gaseous effluent of the first reactor (2). A filtering means is preferably installed in the gas outlet conduit to intercept entrained char particles. In a particular embodiment, the gaseous effluent is first condensed in a dedicated unit and then pumped back to the condensation unit (3) by a pump unit.

[0080] A thickened slurry (mud) is formed and is extracted from the bottom of chamber (6a) by a screw (hot) conveyor (6c) and conveyed to chamber (6d). The screw conveyor (6c) is preferably jacketed so that it can operate within the same temperature range as chamber (6a). In a more preferred embodiment, molten solar salt circulates within chamber (6a) and the jacket of the screw conveyor (6c).

[0081] The screw conveyor (6c) is preferably composed of two different conveyors connected to each other. The thickened slurry, preferably in the form of mud, is preferably extracted from the bottom of the chamber (6a) by a first elevating screw conveyor (6c-1), the lower end of which is integrated with the bottom of the chamber (6a) and the upper end of which is higher than the lower end and is connected to one end of a second screw conveyor (6c-2), the other end of which is connected to the top of the stripping chamber (6d) to feed the slurry. Both conveyors are jacketed and operate in the same temperature range as the chamber (6a).

[0082] As the mud is lifted from the bottom of the chamber (6a) and transported by a second conveyor to the stripping chamber (6d), it is gradually discharged as part of the liquid flows back into the chamber (6a) and part is converted into a gaseous effluent that is removed.

[0083] The system described above makes it possible to feed the stripping chamber (6d) with a char material containing a limited amount of liquid and preferably partially dried.

[0084] In the stripping chamber (6d), nitrogen is preferably injected into the bottom of the stripping chamber to remove volatile hydrocarbon products. Gas is withdrawn from the top of the stripping chamber (6d) and sent to a condensation unit (3), and the dry char is discharged from the bottom of the chamber, preferably through a ball valve, and enters another jacketed screw conveyor (6e), maintained at a temperature range of 60-100°C, which conveys the char directly to a third collection chamber (6f), preferably operating at approximately atmospheric pressure and at a temperature range of 60-100°C. In the collection chamber, the circulation of nitrogen gas is more preferably maintained by a gas inlet (6f-3) and a gas outlet (6f-4). Finally, the dry char is discharged through a valve (6f-2), preferably into a transfer vessel (6g) for handling.

[0085] The above disclosed process and apparatus for char removal and handling is very flexible and can be operated in a continuous mode without stopping the depolymerization in the reactor (2), and it also allows the installation of a lighter depolymerization plant since it is no longer necessary to operate reactors in parallel to perform the depolymerization process running continuously.

[0086] The char handling process is also very efficient in releasing a dry char with a typical composition by weight of about 20% exhaust catalyst, 50% inorganics, and about 30% carbon.

[0087] In another embodiment, according to FIG. 3, the char handling section comprises two or more jacketed chambers (17, 18) equipped with inlet conduits (17a, 18a) for feeding the char-containing slurry and agitation systems (17b, 18b) operable in a temperature range of 350° C. to 570° C., conduits (17c, 18c) for withdrawing the gaseous effluent and conduits (17d, 18d) for withdrawing the dried char which is sent to a conveying means; a means (19, 20) for conveying the char to a collection chamber (21) capable of reducing the temperature of the char leaving the pair of jacketed chambers (17, 18); It is provided with a mixing system (21a) and a collection chamber (21) for collecting the dried char, with an outlet for char disposal.

[0088] The discharged slurry is preferably conveyed in two twin devolatilization chambers.

[0089] Each devolatilization chamber preferably operates in alternating and opposing modes: in the first mode (accumulation), the chamber receives and collects the discharged slurry, and in the second mode (drying), the chamber provides a thermal treatment residence time during which the heavy oil is vaporized or further pyrolyzed into light oil.

[0090] Preferably, one devolatilization chamber is operating in accumulation mode while the other devolatilization chamber is operating in drying mode. Preferably, they are synchronized so that they can complete one mode and switch to the other mode at substantially the same time. This ensures a constant output of dry char over time.

[0091] At the end of the heat treatment, the char is preferably substantially dry.

[0092] The pyrolysis gases released from the devolatilization chamber are sent to a dedicated condenser (22) which operates similarly to the condenser (3).

[0093] Preferably, the pyrolysis vapors are suppressed and condensed at 80-100° C. The condensation temperature is selected to avoid the formation of wax. The condensed water is preferably recycled to the depolymerization reactor (2) via conduit (22a).

[0094] The non-condensable fraction is believed to be exhaust gas, primarily nitrogen with traces of entrained hydrocarbons, which is discharged via conduit 22b.

[0095] The devolatilization chamber is a jacketed stirred vessel. Preferably, the heat transfer fluid is solar salt, which can achieve high operating temperatures (500-550°C).

[0096] Removal of pyrolysis vapors from the devolatilization chamber is preferably facilitated by flushing with stripping nitrogen.

[0097] Preferably the devolatilisation chamber and dedicated condenser are connected and operate in the pressure range 1-3.0 barg.

[0098] After the char is completely dried, it is discharged onto specially designed jacketed screw conveyors (19, 20) for cooling the char.

[0099] The screw conveyor cools the dried char. The sensible heat of the carbon is preferably removed by cold water flowing through one or more of the body jacket, the screw shaft, and the threads (hollow threads).

[0100] The dried, cooled char is collected in a pre-chamber (21) before being discharged for disposal.

[0101] The pre-chamber is equipped with a jacket (cold water), an agitator (21a), and a bag filter (21b) to prevent powder entrainment. The filtered nitrogen gas is considered to be the exhaust gas.

[0102] The depolymerization process designed in this way is highly efficient since it can produce about 10 wt% pyrolysis gas, about 80 wt% pyrolysis oil, and about 10 wt% char.

[0103] As previously mentioned, the preferred use of the primary product of the disclosed pyrolysis process is as a hydrocarbon feedstock to partially replace oil feedstock in a cracking plant, although other uses such as fuels are also contemplated. Working Example

[0104] Example 1 The following experimental steps were carried out in a depolymerization apparatus consisting of two reactors connected in series, consisting of mechanically stirred vessels (and jackets for heating). The first reactor has an inlet for the plastic waste fed from the extruder and an outlet for the produced gas. The gas withdrawn from the reactor was sent to a condenser, from which non-condensed gas and pyrolysis oil were obtained. Thermocouples were placed in the reactors to monitor and record the temperature. The oil collected from the condensation unit was fed to a second depolymerization reactor, further equipped with an inlet for the catalyst feed. The catalyst was fed to the reactor as a solid slurry by mixing it with a portion of the same oil from the condensation section.

[0105] The second reactor is also equipped with an exit line for recycling a portion of the reactor contents to the first depolymerization reactor.

[0106] The plastic waste feedstock was analyzed for polyolefin content (97 wt%), other common polymers (PET, PS, PA, PU) and residuals containing trace amounts of inorganic contaminants.

[0107] The raw materials were homogenized and granulated before being filled into the hopper. The hopper should feed an extruder operating at a temperature of 290 ° C and discharging continuously at a rate of 4 kg / h into the depolymerization reactor. The first depolymerization reactor has a pressure of 3 barg and a temperature range of about 408 ° C with an average residence time of about 3 hours. The gas phase of the reactor was sent to a condensation unit formed by a cooling / washing tower operating at 80 ° C and a dephlegmator operating at 25 ° C. The oil stream was then added to the second vessel operating at 398 ° C and 5 barg. The average residence time in this case was about 105 minutes. In the second reactor, a sample of H-USY zeolite type (CBV 400 - CAS number 1318-02-1, Zeolyst International) was tested. The catalyst was fed to the pyrolyzer in an amount such that it had a proportion of 6% by weight relative to the mass of the reaction phase.

[0108] As above, approximately 10% by volume of the reactor contents was recycled to the first reactor, and the gas outlet from the second reactor was sent to a second condenser, also held at 25° C., where the pyrolysis oil was condensed. The non-condensable gas stream was sent to the exhaust.

[0109] The condensed oil was analyzed by GC-FID.

[0110] Since the number of compounds was so large, we used internal retention time standards for specific hydrocarbons, grouped the compounds according to their retention times, and reported the analytical results. The results are shown in Table 1.

[0111] Example 2 The reactor pressure was maintained at 6 barg, but the process settings were similar to those reported in Example 1. The average residence time in the second reactor was about 150 minutes. The results are shown in Table 1.

[0112] Comparative Example 3 Similar tests were carried out as described in Example 1, except that the gas stream from the first reactor was sent directly to a final condensation unit operating in a similar manner, and the results in Table 1 show that the quality of the oil composition is degraded.

[0113] Comparative Example 4 The conditions were the same as those reported in Example 1, except that the contents of the second reactor were not recycled to the first reactor. In order to maintain the same level of cracked oil quality, the residence time in the first reactor was 3.2 hours and the feed rate was reduced to 3.3 kg / h.

[0114] [Table 1]

Claims

1. 1. A process for depolymerizing waste plastic material to produce pyrolysis products, comprising: (a) feeding a mixture comprising waste plastic material in an oxygen-free atmosphere into a feeding system comprising at least one screw extruder (1) heated at the melting temperature of said waste plastic material; (b) feeding the molten waste plastic material from the extruder to a first depolymerization reactor (2), which is a continuous stirred tank reactor maintained at a temperature range of 280-600°C and operating at a pressure range of 1-10 barg, and allowing depolymerization to proceed in the first depolymerization reactor to form a gaseous effluent and a liquid effluent; (c) directing at least a portion of the liquid effluent produced in the first depolymerization reactor (2) to a char handling section (6) and feeding a gaseous effluent from the first depolymerization reactor (2) to a first condensation unit (3) producing a gas stream and a liquid stream; (d) directing the gas stream from the first condensing unit (3) to a second condensing unit (5) operating at a lower temperature than the first condensing unit (3) and directing the liquid stream from the first condensing unit (3) to a second depolymerization reactor (4) which is a continuous stirred tank reactor maintained at a temperature range of 280-600° C. and operating at a pressure range of 1-10 barg, wherein depolymerization proceeds in the second depolymerization reactor (4) to form a gaseous effluent and a liquid effluent; (e) withdrawing the gaseous effluent from the second depolymerization reactor (4) and feeding it to the second condensation unit (5) and recycling at least a portion of the liquid effluent from the second depolymerization reactor (4) to the first depolymerization reactor (2), further comprising recovering pyrolysis products from said second condensation unit (5); and wherein at least one of said first depolymerization reactor (2) and said second depolymerization reactor (4) operates in the presence of a depolymerization catalyst.

2. 2. The process of claim 1, wherein the first depolymerization reactor (2) is a stirred vessel operating at a temperature in the range of 300 to 550°C, more preferably 350 to 500°C, and under a pressure maintained in the range of 2.0 to 8 barg, more preferably 2.5 to 7 barg.

3. 2. The process according to claim 1, wherein the heating of the first depolymerization reactor (2) is carried out by heat transfer through a molten salt stream heated to a temperature range of 300° C. to 570° C. and circulating inside the reactor jacket.

4. 2. The process according to claim 1, wherein a part of the liquid effluent withdrawn from the bottom of the first depolymerization reactor (2) is recycled to the top of the first depolymerization reactor (2) via a circulation pump (7).

5. 2. The process according to claim 1, wherein the first condensation unit (3) is a wash tower in which heavy hydrocarbons are condensed and light hydrocarbons are discharged as a gas stream.

6. 2. The process according to claim 1, wherein the second depolymerization reactor (4) operates in the temperature range of 280-600° C. and at a higher pressure than the first depolymerization reactor (2), in particular in the pressure range of 2-10 barg.

7. 2. The process according to claim 1, wherein fresh depolymerization catalyst is fed to the second depolymerization reactor (4).

8. At least a portion of the liquid effluent produced in the first depolymerization reactor (2) is directed to a char handling section, the char handling section comprising: two or more jacketed chambers (17, 18) equipped with inlet conduits (17a, 18a) for feeding a char-containing slurry and an agitation system (17b, 18b) operable in the temperature range of 350°C to 570°C, with conduits (17c, 18c) for withdrawing a gaseous effluent and with conduits (17d, 18d) for withdrawing dried char which is sent to a conveying means; a means (19, 20) for conveying the char to a collection chamber (21) capable of reducing the temperature of the char leaving said two or more jacketed chambers (17, 18); 2. The process of claim 1, further comprising a collection chamber (21) for collecting the dried char, the collection chamber (21) being provided with an agitation system (21a) and an outlet for disposing of the char.

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