WO2025247791A1

WO2025247791A1 - Process for the depolymerization of plastic waste material

Info

Publication number: WO2025247791A1
Application number: PCT/EP2025/064421
Authority: WO
Inventors: Daniel F. White; Nicolò ARICH DE FINETTI; Ivano CAPOLUNGO
Original assignee: Basell Poliolefine Italia SRL
Current assignee: Basell Poliolefine Italia SRL
Priority date: 2024-05-27
Filing date: 2025-05-26
Publication date: 2025-12-04
Anticipated expiration: 2026-11-27

Abstract

The present disclosure relates to the field of depolymerization of plastic waste material into new products, comprising hydrocarbon oil, which have valuable and useful properties. In one aspect, the present disclosure relates to a process for converting plastics to liquid hydrocarbons, in particular to be used as hydrocarbon feedstock.

Description

TITLE

PROCESS FOR THE DEPOLYMERIZATION OF PLASTIC WASTE MATERIAL

FIELD OF THE INVENTION

[0001] The present disclosure relates to the field of depolymerization of plastic waste material into new products, comprising hydrocarbon oil, which have valuable and useful properties. In one aspect, the present disclosure relates to a process for converting plastics to liquid hydrocarbons, in particular to be used as hydrocarbon feedstock.

BACKGROUND OF THE INVENTION

[0002] The awareness that waste plastic materials have a negative impact on the environment and, as a consequence on the health of any form of life, is rapidly increasing.

[0003] One of the attempts to mitigate the impact is constituted by the recycling of plastic materials coming from domestic and industrial waste which allows a part of these materials to be reintroduced into the production cycle. This would involve further positive results such as lower use of fossil hydrocarbon sources to produce plastic items.

[0004] However, various factors indicate that this solution alone would not be sufficient for reaching the sustainability targets. In fact, mechanical recycling of plastic materials produces substances with usually lower quality, is relatively costly and burdensome and not applicable to certain urban waste in which plastic is mixed to various different materials.

[0005] As a consequence, a large part of plastic waste is either used as a source of thermal energy in plants such as incinerators, or simply stored in landfills which, as mentioned, contribute to degrade the earth environment by raising the CO2 emissions and by releasing hazardous chemicals.

[0006] In view of the above, numerous attempts have been made in the past to efficiently reprocess a feedstock of waste plastics back into a liquid hydrocarbon product that has valuable and useful properties either as a fuel, or preferably, as a feedstock for a cracker. [0007] Thermal depolymerization is a basic process whereby plastic waste material is converted to liquid fuel (pyrolytic product) by effect of thermal, and optionally catalytic, degradation in the absence of oxygen. Plastic waste is typically first melted within a stainless steel chamber under an inert purging gas, such as nitrogen or methane. Then, in the same or a different chamber, further heat, and optionally a catalyst, is provided in order to crack the polymer molecules of the molten material to a gaseous state which is made of relatively short hydrocarbon chains.

[0008] Hot pyrolytic gases are then condensed in one or more condensers to yield a hydrocarbon distillate comprising straight and branched chain aliphatic, cyclic aliphatic and aromatic hydrocarbons (pyrolytic oil).

[0009] Upon removal of generated short chain hydrocarbons, the depolymerization reactor becomes progressively enriched in solid materials comprising carbonaceous residues and inorganic fraction dispersed in a liquid hydrocarbon medium (char containing slurry). In batch technologies, the feeding of plastic material is stopped and depolymerization may be continued until when only a solid residue is left in the reactor. At that point, the depolymerization is concluded and the powdery residue is mechanically removed from the reactor.

[0010] In order to have a continuous process, the above solution is not viable and therefore the char slurry has to be either continuously or periodically withdrawn from the reactor and dried in a dedicated char handling section to obtain a liquid fraction, optionally reintroduced into the depolymerization reactor, and a dried powdery fraction that can be disposed.

[0011] As disclosed in WO2022/13633, the char containing slurry is subject to a series of steps at high temperature aimed at vaporizing the total amount of liquid fraction. As a consequence, there is a remarkable total heat duty of the char handling section especially in view of the fact that the heat transfer, when solids are involved, is less efficient.

[0012] In view of the above, it is an object of the present disclosure to provide a plastic waste depolymerization process in which the step of drying the char containing slurry is less demanding from the heat duty perspective while maintaining continuous and reliable operability and scalability. SUMMARY OF THE INVENTION

[0013] It is therefore an aspect of the present disclosure a process for depolymerizing waste plastic material and producing a pyrolytic product, wherein said process comprises the following steps:

(a) feeding the waste plastic material into a depolymerization reactor (2), which is operated in continuous mode, maintained at a temperature ranging from 280 to 600°C and operated under a pressure ranging from 2.0 to 10 barg in which depolymerization takes place thereby forming a gaseous effluent and a liquid effluent comprising a char containing hydrocarbon slurry;

(b) discharging at least a portion of the char containing hydrocarbon slurry from the reactor and directing it to a char handling section (6) and feeding the gaseous effluent from reactor (2) to a condensation unit (3); said process being characterized by the fact that in the char handling section, the char containing hydrocarbon slurry discharged from reactor (2) undergoes a centrifugation treatment capable of removing at least 50% wt of its liquid hydrocarbon portion.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic view of the depolymerization process plant

Fig.2 is a schematic view of the centrifugation section of the depolymerization process plant Fig. 3 is a schematic view of the char drying section of the depolymerization process plant

DETAILED DESCRIPTION OF THE INVENTION

[0014] Preferably, the whole process is carried out in a continuous mode.

[0015] Preferably, the feeding of the mixture comprising waste plastic materials, is carried out using a feeding system comprising at least one screw extruder (1) which generates a molten plastic material feed. In stage (a) a charging system allows charging, preferably in continuous mode, waste plastic materials to be fed, into the reactor (2). Care should be taken for not introducing oxygen containing atmosphere into the system. The barrier to the potentially oxygen-containing atmosphere can be obtained in different ways such as nitrogen blanketing or vacuum system connected to a barrel of the extruder.

[0016] More specifically, the plastic waste mixture, is charged into the feeding system of the depolymerization reactor (2) by means of a hopper, or two or more hoppers in parallel, and the oxygen present in the atmosphere of the plastic waste material is substantially displaced for example by means of nitrogen purge.

[0017] The process according to the present disclosure is very flexible and can be fed with a wide range of plastic waste composition including, as an example, heterogeneous mixtures of waste plastic materials in which polyolefins are the most abundant component but for which a further sorting step is no longer economical. A preferred feedstock, especially when the pyrolytic product is to be recirculated back to a cracking/refining unit, is a plastic waste mixture in which the polyolefin (PE and PP) content is equal to or higher than 70%wt.

[0018] The waste plastic material preferably undergoes a pre-treatment stage in which it is melted by heat and possibly mixed with an additive which can be an alkaline material. By the melting pre-treatment, a non-uniform mixture of different kinds of waste plastics can be transformed into a mass of uniform plastic composite. Therefore, this pre-treatment is also preferable for the case in which the pyrolytic decomposition is performed without additives.

[0019] The heating temperature in the pre-treatment stage is appropriately set to a temperature in accordance with the kind and content of the plastic contained in the waste plastic material such that pyrolytic decomposition of the plastic material to be treated is inhibited. Such a temperature is, in general, within a range of 100°C to 300°C, and preferably, 150°C to 250°C. At a temperature close to 300°C or more, elimination of HC1 from the PVC resin possibly present, takes place.

[0020] The HC1 forming gas can be either removed via a venting system and successively neutralized or trapped if the waste plastic material is mixed with an alkaline material during the melting/kneading pre-treatment. For performing the melting operation, ordinary kneaders, extruders with a screw and the like are applicable. Plastic waste is preferably fed to the depolymerization reactor by means of an extruder.

[0021] The extruder melts the plastic scrap, brings it at high temperature (250-350°C) and injects it into the first depolymerization reactor (2). The extruder may receive the plastic scrap cut in small pieces into the feed hopper, convey the stream in the melting section and heat the polymer by combined action of mixing energy and heat supplied by barrel heaters.

[0022] Additives can be optionally incorporated in the melt aiming at reducing corrosivity of plastic scrap or to improve pyrolytic products yield in the reaction section.

[0023] During the extrusion, one or more degassing steps can be foreseen to remove residual humidity present in the product. [0024] Before being fed to the reactor (2), the melt stream can be filtered by in order to remove solid impurities present in the plastic waste.

[0025] Several design of melt filtration units can be applied, depending on amount and particle size of the solid impurities.

[0026] It is preferred using self-cleaning melt filters that can be operated for long time (several days) without manual intervention to replace filtration elements.

[0027] One preferred design of melt filter is based on a circular perforated plate as melt filtration element, holes by laser or by machining according to openings, where solid contaminant are accumulated. Accumulation of impurities may increase differential pressure across the melt filter. In order to perform the in line cleaning of the filtration element, a rotating scraper removes the accumulated impurities and guides them to a discharge port, that is opened for short time to purge out of the process contaminated material.

[0028] This cycle can be repeated several times (up to operation time of several days) without manual intervention or need to stop production for the time needed to replace the filtration element. [0029] Another option of self-cleaning melt filter is based on the application of continuous filtering metal bands through which polymer flow is passed. Impurities are accumulated on the metal filter generating an increases of pressure. Accordingly, the clogged filtering band section is pushed out of the polymer passage area and clean section is then inserted.

[0030] This process is automatic and allows to operate for long time (up to several days) without manual intervention or need to stop production for the time needed to replace the filtration element.

[0031] Any extrusion systems can be applied, as single screw extruders, twin screw extruders, twin screw extruders with gear pump, or combination of the above.

[0032] The depolymerization reactor (2) is preferably an agitated vessel operated at temperature preferably ranging from 300 to 550 and more preferably from 350 to 500°C.

[0033] The operative pressure is preferably kept in the range 2.5 to 8 barg, more preferably in the range 3.0 to 7 barg.

[0034] By adopting the above mentioned conditions, the melt viscosity of the reactor content is suitable for being homogeneously mixed by the stirring device. In general, the melt viscosity measured at a temperature of 400°C ranges from 0.1 to 250 cP, particularly from 1 to 100 cP and even more particularly from 5 to 50 cP. [0035] It has to be noted that the above values of melt viscosity are referred to the reactor content per se, without addition of any viscosity decreasing agent.

[0036] However, if desired, it is possible to premix, preferably in a dedicated vessel, the molten mass of waste plastics entering the reactor with hydrocarbon oil, preferably recirculated oil coming from the condensation unit, in order to promote melt dissolution into the depolymerization reactor. In this case the volumetric ratio oil/molten mass can range from 0.1 :1 to 1: 1.

[0037] The depolymerization reactor (2) preferably has a cylindrical section, preferably with a rounded bottom. In an alternative embodiment, the bottom of the reactor has a conical or truncated conical shape.

[0038] Preferably, it has a mixer installed in the vertical axis of the reactor, completed with a gear motor which allows the blades of the mixer rotating in order to maintain the system in stirred state. The design of the mixer and the power of the motor can vary in respect of the reactor content, volume and shape, however, as a non-limiting example, it is preferred to operate the reactor with a power input ranging from 0.2 to 4 kW/m³, preferably 0.2-2 kW/m³ and more preferably from 0.3 to 1.5 kW/m³.

[0039] The reactor can be jacketed and at least part of the heat can be provided via heating fluid circulating in the jacket. However, according to a preferred embodiment, the reactor is provided with a recycling circuit by which part of the liquid effluent discharged from the reactor (2), is recirculated back to the reactor (2) via a recycling circuit comprising a centrifugal pump (4) and a shell and tube heat exchanger (5).

[0040] In a preferred embodiment, at least 80%, preferably more than 85% and especially more than 90% the total of heat demand of the step (b) is provided through the shell and tube heat exchanger (5). Accordingly, the heat provided to the reactor content by the reactor walls is less than 10%, preferably less than 5% and more preferably absent. As a consequence, the reactor (2) does not strictly need jacketed walls for heating the reactor content.

[0041] Heat to the external heat exchanger (5), as well as to the reactor walls, can be provided by any heat transfer fluid suitable to operate at the depolymerization temperature or above. Preferably, solar salt or synthetic oils are used. The use of molten solar salt heated to a temperature ranging from 300°C to 570°C is highly preferred. [0042] Preferably, the requested heat is exchanged via the shell and tube heat exchanger (5) by letting the liquid effluent from the reactor (2) flowing inside the tubes of the heat exchanger while the heat transfer fluid flows process side in the shell.

[0043] The feeding circuit (not shown) of the molten salt is constructed in such a way to prevent molten salt leakage. The molten salt is molten solar salt preferably constituted by a mixture of sodium nitrate and potassium nitrate, even more preferably in a weight ratio ranging from 2:3 to 3:2. The solar salt receives in turns heat from a dedicated furnace that may be either electric or be fed with fuel. Preferably, the furnace is electric and more preferably electricity comes from renewable sources. In case of use of fuel based furnace, part of the recovered oil from the condensation unit (3) may be used to feed the furnace.

[0044] Heat transfer fluid, particularly molten salt, is circulated into the heat exchanger by the use of a circulation pump.

[0045] Any available shell and tube heat exchanger can be used which can be sized according to the ordinary knowledge of the skilled in the art. In a preferred embodiment, a single shell /single tube pass heat exchanger is used.

[0046] It is also possible to operate with two shell and tube heat exchangers configured either in series or in parallel.

[0047] Preferably, within the tubes of the heat exchanger the slurry flows with a velocity ranging from 3-10 m/sec more preferably 5-8 m/sec.

[0048] From the operative point of view, the withdrawal of the slurry phase from the bottom of the reactor is preferably triggered by density sensors detecting the density of the liquid slurry reaching a predetermined value.

[0049] There are no limitations in terms of type of centrifugal pump (4) that can be used, any centrifugal pump commercially available having suitable features to pump the liquid effluent having the above mentioned characteristics can be used.

[0050] According to a preferred embodiment, the stream of the char containing hydrocarbon slurry recirculated to the reactor is withdrawn from a point of the reactor different from the point of the withdrawal of the char containing hydrocarbon slurry stream sent to the char handling section.

[0051] According to another preferred embodiment, both streams are withdrawn from the same point and then successively split. [0052] The split between the two streams can take place either before or after the centrifugal pump (4). In this latter embodiment, the slurry is first fed to a dedicated vessel equipped with a lower and upper exit point. The stream directed to char handling section (6) is withdrawn in a concentrated form from the lower exit point while the stream to be recycled to the reactor (2) is withdrawn from the upper exit point.

[0053] As mentioned, the liquid effluent is comprised of a slurry of solid materials dispersed in a liquid hydrocarbon medium (char containing hydrocarbon slurry). In view of process set-up the flow of the slurry stream is preferably continuous. The char content in the slurry discharged from the reactor (2) may range from 10 to 65% preferably from 20 to 55%wt.

[0054] As previously mentioned, in the char handling section, the char containing hydrocarbon slurry discharged from the reactor, undergoes a centrifugation treatment capable of removing at least 50% wt of its liquid hydrocarbon portion.

[0055] Such a treatment is preferably carried out in a centrifugation section (6a) which is located upstream the char drying section.

[0056] The temperature of the char containing hydrocarbon slurry subject to the centrifugation treatment ranges from room temperature to 600°C, preferably from 100 to 550°C, more preferably from 150 to 500°C and especially from 250 to 450°C.

[0057] The viscosity of the char containing hydrocarbon slurry to be centrifugated measured at a temperature of 200°C ranges from 1 to 100 cP, particularly from 1 to 50 cP and even more particularly from 1 to 30 cP.

[0058] Preferably, the composition and temperature of the char containing slurry withdrawn from the reactor and subject to the centrifugation treatment is such that its viscosity meets the above defined range.

[0059] As mentioned, the centrifugation treatment must be carried out under conditions such that at least 50% wt, preferably at least 60%wt, more preferably at least 70%wt and especially at least 75%wt of liquid hydrocarbon portion is removed from the char containing slurry.

[0060] The centrifugation technique is well known in the art and the skilled person can easily determine the specific conditions for obtaining the requested extent of separation.

[0061] In particular, suitable centrifuges are known in the art and available on the market.

[0062] Preferably, the centrifuge is a decanter centrifuge and more preferably is a vertical decanter centrifuge suitable to work at the requested pressure and temperature. [0063] This type of centrifuge allows separation of the liquid phase from the slurry and its continuous draining. As a result, a cake of the solid portion is separately obtained.

[0064] In one embodiment, the char containing hydrocarbon slurry discharged from the reactor (2) is directly fed to the centrifuge, which is preferably a decanter centrifuge.

[0065] The separated liquid is preferably sent to a further depolymerization stage. Preferably, when two depolymerization reactor are present, the liquid is sent to the second depolymerization reactor, in order to produce shorter chain hydrocarbons.

[0066] In a preferred embodiment, which is shown in Fig. 1, part of the liquid separated from the centrifuge (6a) is sent to a collecting vessel (6b) from which, through a pump (6c) and line (6d) is sent to a further depolymerization stage, preferably in a second depolymerization reactor. Another portion of separated liquid is conveyed through line (6e) to merge with line (13) containing the hydrocarbon slurry discharged from the reactor (2). In this way, the char containing slurry is diluted before entering the centrifuge.

[0067] In a preferred embodiment, a cooler is installed on line (6e) so that the liquid entering line (13) lowers the temperature of the char containing slurry discharged from the reactor (2). The lower temperature may have the beneficial effect of reducing the depolymerization and therefore the risk of fouling the discharge line (13).

[0068] It is also possible, in a further embodiment, to dilute the char containing hydrocarbon slurry discharged from the reactor (2) with a light hydrocarbon oil, for example that deriving from condensing the gas coming out from the top of the condensation column (3). The so diluted slurry is then sent to the centrifuge, which is preferably a decanter centrifuge.

[0069] In an additional embodiment, the wet cake resulting from the centrifugation stage can be subject to a further centrifugation step in order to further remove the liquid fraction. It is also possible, before carrying out the further centrifugation step, to mix the cake with a certain aliquot of the light hydrocarbon oil described above. The use of the light hydrocarbon oil may have the effect of facilitating the removal of the heavy oil fraction present in the char containing hydrocarbon slurry discharged from the reactor. As a result, the removal of the liquid phase from the final wet cake is expected to be less demanding.

[0070] Gaseous effluents possibly generated in the centrifuge (6a) and in the collecting vessel (6b) are sent via lines (6g) and (6h) to a condensing section. [0071] As disclosed above, the solid or semisolid material discharged from the centrifuge is preferably in the form of a wet cake which is sent to the drying stage of the char handling section. According to a preferred embodiment, the wet cake resulting from the centrifugation stage can be subject to a drying step carried out keeping the wet cake in motion for example through the use of specific devices, such as paddle driers, which allow to further remove the liquid fraction from the cake.

In another preferred embodiment, the drying section set-up is as that disclosed in Fig. 3 which is also disclosed in WO2022/13633, the relevant part of which is incorporated by reference.

[0072] The char dryer of the char handling section (6) is typically operated at nearly atmospheric pressure and higher temperature (with respect to the pyrolizer) in order to promote the separation between char and volatiles.

[0073] With reference to Fig.3 , the char handling section may comprise two or more jacketed chambers (17, 18), provided with inlet conduit (17a, 18a) for feeding the char containing slurry and stirring system (17b, 18b), capable to be operated at a temperature ranging from 350 to 570°C and further provided with a conduit (17c; 18c) for the withdrawal of a gaseous effluent and with a conduit (17d, 18d) for withdrawing a dried char to be sent to a conveyor mean;

[0074] means (19, 20) for conveying the char to a collecting chamber (21) said means (19, 20) being capable to lower the temperature of the char exiting the couple of jacketed chambers (17, 18);

[0075] a collecting chamber (21) receiving the dry char provided with stirring system (21a), an outlet for char disposal.

[0076] The discharged slurry is preferably conveyed in two twin devolatilization chambers.

[0077] Each devolatilization chambers preferably work in alternating and opposite modes. In the first mode (accumulation), the chamber receives and collects the discharged slurry. In the second mode (drying) the chamber provides residence time to a thermal treatment during which the heavy oil is either vaporized or further pyrolyzed in light oil.

[0078] Preferably, when one devolatilization chamber works in the accumulation mode, the other works in the drying mode. Preferably, they are synchronized so that they complete one mode at, substantially, the same time and can swap to the other mode. This ensures a constant output of dried char along the time.

[0079] At the end of the thermal treatment, the char is preferably substantially dried. [0080] Pyrolytic gases relieved by devolatilization chambers are conveyed to a dedicated condensation section (22), which operates in analogy with condensation section (3). Vapors coming from line (6h) are also fed to condensation section (22).

[0081] Preferably, pyrolytic vapors are suppressed and condensed at 80-100°C. The condensation temperature is selected in order to avoid the formation of wax. The condensate is preferably recycled via conduit (22a) to the depolymerization reactor (2).

[0082] The non-condensable fraction consists mainly of nitrogen entraining few hydrocarbons and is considered as waste gas exiting via conduit 22b.

[0083] The devolatilization chambers are agitated and jacketed vessels. Preferably, the heat transfer fluid is solar salt, by means of which high operating temperatures may be reached (500- 550°C).

[0084] The removal of the pyrolytic vapors from devolatilization chambers is preferably enhanced by flushing of stripping nitrogen.

[0085] Preferably, devolatilization chambers and dedicated condensation section are connected and operated at a pressure ranging from 1 to 3.0 barg.

[0086] When char is fully dried it is discharged into a jacketed screw conveyor (19, 20) especially designed for cooling the char.

[0087] The screw conveyors cool down the dried char. Char sensible heat is preferably removed by means of cold water flowing through one or more of body jacket, screw shaft and screw flight (hollow flight).

[0088] The dried and cold char is collected in a prechamber (21) before the discharge for disposal.

[0089] The prechamber is endowed with jacket (cold water), agitator (21a) and bag filter (21b) which prevents the entrainment of powder. The filtered nitrogen is considered as waste gas.

[0090] According to the present disclosure, the gaseous phase of the reactor (2) constitutes the gaseous effluent which is sent to the condensation unit (3) for further treatment.

[0091] The condensation unit (3) may be either a distillation unit or only a condensation unit. [0092] The gaseous effluent comprises a mixture of light hydrocarbons which may also include some heavy hydrocarbons and char particles entrained.

[0093] In the simple condensation unit the condenser (3) is preferably designed as a scrubber column in order to suppress the entrained char. The condenser temperature is selected in such a way that the heavy hydrocarbons are condensed and the light hydrocarbons are released as gaseous stream. The gaseous stream (H2 and light hydrocarbons) is preferably conveyed to a further condensation unit (not shown), preferably working at a temperature lower than the condensation unit (3), from which oil is recovered.

[0094] The operative temperature of the condensing unit (3) may vary in a wide range also depending on the operative pressure. The temperature, referred to atmospheric pressure, can be from 20°C to 200°C more preferably from 40 to 100°C and especially from 50 to 90°C. The temperature range can be of course different when a higher operating pressure is chosen.

[0095] In a preferred setup of the simple condensation unit (3) a dephlegmator (partial condenser) is installed on top of the scrubber and works at a temperature lower than that inside the column. The condensate flows down as reflux for the scrubber by virtue of gravity. The dephlegmator can either be installed as a separate piece of equipment or inside the scrubber.

[0096] When the condensation unit (3) is a distillation unit, it is preferably designed in a way to combine a scrubber zone and a distillation zone which are preferably located in the same column.

[0097] In a preferred design, the lower portion of the column is the scrubber zone where a liquid stream, preferably recirculated from the bottom of said column, flows downward in countercurrent with the gaseous effluent, coming from reactor (2) which is fed to the lower part of the column and is directed upward.

[0098] The distillation zone is preferably located in the upper portion of the column where a thermal gradient is established between the cold liquid stream coming from condensation unit (5) and the hot gaseous effluent coming up from the scrubber zone.

[0099] In a preferred embodiment, the distillation is based on the use of packing material, so is a packed distillation column.

[0100] Preferably, the distillation unit (3) is endowed with three or more equilibrium stages, preferably four or more equilibrium stages, especially from 5 to 20 equilibrium stages. The increased number of equilibrium stages improves the separation of components by their respective boiling points.

[0101] The hydrocarbon condensate, preferably having more than C7 carbon atoms, constitutes the liquid stream which is sent either to further processing or to a second depolymerization reactor. [0102] A further depolymerization reactor can also be present. If present, the second depolymerization reactor is preferably of the same type as the first one and more preferably a continuously stirred tank reactor equipped with the same recycling circuit which, by virtue of centrifugal pump and shell and tube heat exchanger, provides heat to the depolymerization stage. [0103] The second reactor may be connected either in series (sequential) or in parallel to the first reactor. The sequential setup is preferred.

[0104] It will be also apparent that one or more reactors can be equipped with one or more additional recycling circuits each of which provided with centrifugal pump and heat exchanger.

[0105] Depolymerization takes place in the same range of temperatures but, in order to limit the volatility of the heavy hydrocarbons, it is preferably operated at a pressure higher than the first reactor and in particular in the range from 3 to 10 barg, preferably from 3 to 9 barg and more preferably from 3 to 8 barg.

[0106] According to the present disclosure, the depolymerization step (b) can take place in the presence of a catalyst. This latter can be selected from those active as depolymerization/cracking catalysts in thermocatalytic processes. In particular, it can be selected from metal oxides, heteropolyacids, mesoporous silica, aluminosilicates catalysts, such as halloysite and kaolinite, and preferably from zeolites. Among them, particularly preferred zeolites are synthetic Y-type zeolite and ZSM-5.

[0107] In a particularly preferred embodiment, the amount of catalyst feed is not more than 10% preferably not more than 5% and especially not more than 2% wt with respect to the plastic waste feed.

[0108] In a preferred embodiment, the catalyst is injected into the reactor as powder dispersed into a hydrocarbon oil preferably the liquid pyrolytic product (oil) obtained from condensation unit (3).

[0109] Preferably, the catalyst slurry is prepared in a pot, continuously stirred vessel where the catalyst is poured from a dedicated silo in order to keep constant the concentration of the catalyst in the slurry.

[0110] The pyrolytic oil dispersing the catalyst is preferably withdrawn from the condensation unit (3) in order to keep constant the slurry level in the pot. Once ready the catalyst slurry can be injected, preferably by means of a progressive cavity pump in order to keep its level constant. [0111] The liquid effluent coming from reactor (2) is preferably a highly concentrated hydrocarbon slurry which, if used, also contains the depolymerization catalyst.

[0112] In another embodiment, the catalyst may be fed either in the plastic waste feedstock pre-treatment stage or, more preferably, added to the extruder where it becomes mixed with the molten feedstock.

[0113] When a second depolymerization reactor is present, its gaseous effluent can be conveyed to the condensation/distillation unit (3). The gaseous fraction coming from unit (3) can then be sent to a further condensing unit for recovering the pyrolytic oil.

[0114] Preferably, the operative conditions of the condensation unit associated to the second reactor are selected in a way that it has a lower operating temperature and pressure with respect to condensation unit (3).

[0115] In particular, the temperature may range from 20 to 80°C and preferably from 30 to 70°C. The pressure value should preferably be lower than that of condensation unit (3) so as to allow incondensable gases from unit (3) to enter second condensation unit without further pressurization.

[0116]

[0117] Fig. 1 represent a schematic view of the process in which plastic waste melt in the extruder (1) is supplied to reactor (2) which is provided with catalyst (if used) inlet (7 ) and a recycling circuit with centrifugal pump (4) and shell and tube heat exchanger (5). From the recycling circuit, via line (13) slurry. Gaseous effluents are collected at the reactor top and sent via line (8) to condensation unit (3) which is provided with a recycling circuit by which the condensate via a circulation pump (9) flows through the heat exchanger (10) and is recycled to the condensation column (3). From the top of the column via line 11 gaseous product is collected and conveyed to further processing. From the line 12, pyrolytic oil is collected and conveyed to further processing or storage.

[0118] As already mentioned, the preferred use of the main product of the pyrolytic process of the disclosure is as hydrocarbon feedstock partially replacing oil feedstock in cracking plants. However, other uses, such as fuel, are also contemplated. It has to be noted that the process of the present disclosure allows obtaining the depolymerization product with a simple and reliable process where the heat transfer is smooth and efficient. [0119] In addition, the fact that the process set-up is based on equipment that is readily available makes the process itself susceptible of scaling up.

[0120] EXAMPLES

[0121] The following experimental steps have been carried out in a depolymerization apparatus according to Fig. 1 of WO2022/13633 comprising two reactors connected in series, which were mechanically agitated vessels (and jacketed for heating). The first reactor was provided with an inlet for the plastic waste coming from the extruder feed, and an outlet for the generated gases. The gases withdrawn from the reactor are conveyed to a condensation unit from which an incondensable gas and a pyrolytic oil are obtained. Thermocouples are positioned into the reactor to monitor and record the temperatures. The oil collected from the condensation unit is fed to a second depolymerization reactor, provided also with an inlet for catalyst feeding. The catalyst was fed into the reactor as solid slurry by mixing with part of the same oil from the condensation section.

[0122] The second reactor was also provided with an outlet line in order to recycle part of the reactor content to the first depolymerization reactor.

[0123] The plastic feedstock was homogeneized and pelletized before the loading in the hopper needed to feed the extruder which worked at a temperature of 290°C and discharged continuously into the depolymerization reactor at 4 kg/h. The first depolymerization reactor was operated at a pressure of 3 barg and at temperature of about 408°C while the average residence time was about 3h. The gaseous phase of the reactor was sent to a condensation unit formed by a cooling/scrubber column working at 80°C and a dephlegmator working at 25°C. Then, the oil stream was fed into the second vessel operated at 398°C and 5 barg. Average residence time in this case was about 105 minutes. In this second reactor, a sample of H-USY Zeolite type (CBV 400 - CAS number 1318-02-1 ex Zeolyst International) was tested. The catalyst was fed into the second pyrolizer in such an amount to get a ratio of 6wt% with respect to the reactive phase mass. [0124] During operation, an aliquot of the char containing hydrocarbon slurry was withdrawn from the first reactor and cooled at room temperature obtaining a waxy material which was then mixed with n-hexane to obtain a slurry. A portion of the slurry was centrifuged using a Centrifuge Model PK131R manufactured by ALC. The liquid portion was siphoned and the wet solid phase dried at 80°C. From the calculation, it resulted that the slurry withdrawn from the reactor was composed of about 30%wt of char (solid phase) and 70%wt of liquid hydrocarbon while the wet cake after centrifugation was composed by 45% of char and 55% of liquid. It was estimated that if a decanter draining centrifuge would be used, a wet cake with a 66%wt of char and 34% of liquid hydrocarbon is expected to be obtained.

Based on this, the estimated heat duty of the char drying section when fed with the char containing slurry discharged from the reactor (comparative) would be 144 kW/t , while if the char drying section would be fed with the wet cake after centrifugation step (inventive) the heat duty would drop to 41 kW/t.

Claims

CLAIMS What is claimed is:

1. A process for depolymerizing waste plastic material and producing a pyrolytic product, wherein said process comprises the following steps:

2. The process according to any of the preceding claims in which plastic waste is a mixture of waste materials in which polyolefins are the most abundant component.

3. The process according to any of the preceding claims in which the depolymerization reactor (2) is preferably an agitated vessel operated at temperature ranging from 300 to 550°C and more preferably from 350 to 500°C. and under a pressure kept in the range 2.5 to 8.0 barg, more preferably in the range 3.0 to 7.0 barg.

4. The process according to any of the preceding claims in which the melt viscosity of the reactor content, measured at a temperature of 400°C, ranges from 0.1 to 250cP particularly from 1 to 100 cP and even more particularly from 5 to 50 cP.

5. The process according to any of the preceding claims in which the char content in the char containing hydrocarbon slurry ranges from 10 to 65% preferably from 20 to 55%wt..

6. The process according to any of the preceding claims in which temperature of the char containing hydrocarbon slurry subject to the centrifugation treatment ranges from room temperature to 600°C, preferably from 100 to 550°C, more preferably from 150 to 500°C and especially from 250 to 450°C.

7. The process according to any of the preceding claims in which the centrifugation treatment must be carried out under conditions such that at least 50% wt, preferably at least 60% wt, more preferably at least 70% wt and especially at least 75%wt of liquid hydrocarbon portion is removed from the char containing slurry.

8. The process according to any of the preceding claims in which the centrifugation treatment is carried out with a decanter centrifuge.

9. The process according to claim 8 in which the centrifugation treatment is carried out with a vertical decanter centrifuge.

10. The process according to any of the preceding claims in which the hydrocarbon liquid separated with the centrifugation is sent to a further depolymerization stage.

11. The process according to claim 10 in which part of the liquid separated from the centrifuge (6a) is sent to a collecting vessel (6b) from which, through a pump (6c) and line (6d) is sent to a further depolymerization stage.

12. The process according to any of the preceding claims in which a cooler is installed on line (6e) so that the liquid entering line (13) lowers the temperature of the char containing slurry discharged from the reactor (2).

13. The process according to any of the preceding claims in which the char containing hydrocarbon slurry discharged from the reactor (2) is diluted with a light hydrocarbon oil before being centrifuged.

14. The process according to any of the preceding claims in which the wet cake obtained after the first centrifugation step is diluted with light hydrocarbon oil and subject to a further centrifugation step.

15. The process according to any of the preceding claims in which the pyrolytic product in form of oil is used as hydrocarbon feedstock in cracking plants.