Process for the depolymerization of waste polyolefin material with improved melt injection

Abstract

A process for recycling waste polyolefin material comprises the following steps: (i) feeding waste polyolefin material into a screw extruder, to give a heat extrusion-processed waste polyolefin material; (ii) conveying the heat extrusion-processed waste polyolefin material in a melt line; (iii) depolymerizing a polyolefin-derived material in a polyolefin depolymerization reactor, (iv) withdrawing a reacted material from the bottom of the polyolefin depolymerization reactor, to give a withdrawn material; (v) conveying the withdrawn material to a recycling pump; (vi) withdrawing a pumped material from the recycling pump; (vii) conveying the pumped material to a heating unit, and optionally to the polyolefin depolymerization reactor (3); (viii) heating the pumped material in the heating unit, to give a heated material; (ix) conveying the heated material to the polyolefin depolymerization reactor; wherein the heat extrusion-processed waste polyolefin material in the melt line is conveyed into one or more of the pumped material and the heated material.

Description

TITLEPROCESS FOR THE DEPOLYMERIZATION OF WASTE POLYOLEFINMATERIAL WITH IMPROVED MELT INJECTIONFIELD OF THE INVENTION

[0001] The present disclosure relates to the field of depolymerization of plastic waste material into new products. In one aspect, the present disclosure relates to a process for process for recycling waste polyolefin material. In another aspect, the present disclosure relates to a plant for recycling waste polyolefin material.BACKGROUND OF THE INVENTION

[0002] Polyolefin material constitutes a substantial part of plastics materials. The awareness that waste polyolefin material has 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 polyolefin materials coming from domestic and industrial users, which allows a part of these materials to be reintroduced into the production cycle. This would involve positive results such as lower use of fossil hydrocarbon sources to produce polyolefin material items.

[0004] However, various factors indicate that this means alone would not suffice. In fact, mechanical recycling of polyolefin materials produces substances with lower quality, is relatively costly and burdensome and not applicable to certain urban waste in which polyolefin is mixed to various different materials.

[0005] As a consequence, a large part of polyolefin 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 the release of hazardous chemicals.

[0006] In view of the above, numerous attempts have been made in the past to efficiently reprocess a feedstock of waste polyolefin materials back into a liquid hydrocarbon product that has valuable and useful properties particularly as a fuel.

[0007] Thermolysis is a basic process whereby plastic waste material is converted to liquid fuel by thermal degradation (cracking) in the absence of oxygen.

[0008] WO2024 / 046897 Al teaches a process for pyrolysis of plastics material. A plastic melt is depolymerized in a tubular reactor, and the product then fed to a separation vessel. Maintenance of tubular depolymerization reactors may, however, be troublesome.

[0009] In an alternative setup, plastic waste may be melted within a stainless steel chamber under an inert purging gas, such as nitrogen. This chamber then heats the molten material to a gaseous state that is drawn into a catalytic converter and cracked to form carbon chain of variable length. Hot pyrolytic gases of the desired carbon length range are then condensed inone or more condensers to yield a hydrocarbon distillate, comprising straight and branched chain aliphatic, cyclic aliphatic and aromatic hydrocarbons. Depending on the composition, the resulting mixture is used in a variety of applications but, in any case, the requirement of product consistency and quality has always to be met. Also in view of the unstable composition of the plastic waste raw material, quality consistency of the recovered oil is difficult to be met.

[0010] In yet another alternative setup, an extruder melts the polyolefin-based secondary raw material feedstock and injects it into a stirred depolymerization reactor. A slurry is agitated in the reactor and recirculated by means of a centrifugal pump, with an external heater providing the pyrolysis heat. The extruder is a device of substantial weight, and the polymer melt needs to be injected to the top of the reactor through a melt line jacketed with hot oil. Such configuration is hardly feasible for a commercial unit because it would require the installation of the extruder and its ancillaries at the same elevation as the reactor’s top. A preferred configuration for a commercial unit positions the extruder on the ground floor and transfers the melt from the extruder to the top of the reactor by means of a long and large melt line jacketed with hot oil. Maintenance of such long and large melt line would be quite troublesome.

[0011] In light of the foregoing, an object of the present disclosure is to provide a depolymerization process that provides high quality depolymerisation products in gaseous and liquid form, with lower energy consumption and in which the feedstock feeding to the reactor is smooth and reliable.SUMMARY OF THE INVENTION

[0012] According to the present invention a process for recycling waste polyolefin material is disclosed. Moreover, a plant for recycling waste polyolefin material is disclosed.

[0013] In one aspect of the disclosure, a process comprises the following steps:(i) feeding waste polyolefin material (XI) into at least one screw extruder ( 1 ), the at least one screw extruder (1) heating the waste polyolefin material (XI), in an oxygen-free atmosphere and to a melting temperature of the waste polyolefin material (XI), to give a heat extrusion-processed waste polyolefin material (X2);(ii) conveying the heat extrusion-processed waste polyolefin material (X2) in a melt line (2);(iii) depolymerizing a polyolefin-derived material (X3) in a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor maintained at a temperature in a range of from 280 C to 600 C and operated under a pressure in a range of from 1 barg to 10 barg;(iv) withdrawing a reacted material from the bottom of the polyolefin depolymerization reactor (3), to give withdrawn material (X4);(v) conveying the withdrawn material (X4) to a recycling pump (4);(vi) withdrawing a pumped material (X5) from the recycling pump (4);(vii) conveying the pumped material (X5) a. to a heating unit (5), b. optionally to the polyolefin depolymerization reactor (3);(viii) heating the pumped material (X5) in the heating unit (5), to give a heated material (X6);(ix) conveying the heated material (X6) to the polyolefin depolymerization reactor (3); wherein the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into one or more of: the pumped material (X5), the heated material (X6).

[0014] In one embodiment, the extruder (1) may be positioned on the ground floor, and may have a much shorter routing, and a smaller diameter can be selected for it. Also, the melt line (2) may be short, and its operation and maintenance may be managed using electrical heaters instead of the expensive hot oil system. Also, maintenance is easier with a short melt line.

[0015] In another aspect of the disclosure, a plant for recycling waste polyolefin material comprises: at least one screw extruder (1), the screw extruder (1) being capable of heating a waste polyolefin material (XI), in an oxygen-free atmosphere, to a melting temperature of the waste polyolefin material (XI); a melt line (2) for conveying, from the at least one screw extruder (1), a heat extrusion-processed waste polyolefin material (X2); a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor capable of depolymerizing a polyolefin-derived material (X3) at a temperature in a range of from 280°C to 600°C and under a pressure in a range of from 1 barg to 10 barg;withdrawing means (9) for withdrawing a reacted material from the polyolefin depolymerization reactor (3); a withdrawn material conveying line (10) for conveying a withdrawn material (X4) from the polyolefin depolymerization reactor (3) to a recycling pump (4); the recycling pump (4); a pumped material conveying line (11) for conveying at least part of the pumped material (X5) from the recycling pump (4) to a heating unit (5) capable of heating the pumped material (X5); an optional pumped material conveying line (12) for conveying part of the pumped material (X5) from the recycling pump (4) to the polyolefin depolymerization reactor (3); the heating unit (5); a heated material conveying line (13) for conveying a heated material (X6) from the heating unit (5) to the polyolefin depolymerization reactor (3); one or more melt lines (14a, 14b, 14c) for conveying the heat extrusion-processed waste polyolefin material (X2) into one or more of: the pumped material conveying line (11); the optional pumped material conveying line (12); the heated material conveying line (13).BRIEF DESCRIPTION OF DRAWINGS

[0016] Fig. l is a schematic view of the process according to an embodiment of the first aspect.

[0017] Fig. 2 is a schematic view of a type of a section that processes a gaseous effluent from the polyolefin depolymerization reactor of an embodiment of the second aspect.DETAILED DESCRIPTION OF THE INVENTION

[0018] The process of the invention, recycling waste polyolefin material, comprises the following steps:(i) feeding waste polyolefin material (XI) into at least one screw extruder ( 1 ), the at least one screw extruder (1) heating the waste polyolefin material (XI), in an oxygen-freeatmosphere and to a melting temperature of the waste polyolefin material (XI), to give a heat extrusion-processed waste polyolefin material (X2);(ii) conveying the heat extrusion-processed waste polyolefin material (X2) in a melt line (2);(iii) depolymerizing a polyolefin-derived material (X3) in a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor maintained at a temperature in a range of from 280 C to 600 C and operated under a pressure in a range of from 1 barg to 10 barg;(iv) withdrawing a reacted material from the bottom of the polyolefin depolymerization reactor (3), to give a withdrawn material (X4);(v) conveying the withdrawn material (X4) to a recycling pump (4);(vi) withdrawing a pumped material (X5) from the recycling pump (4);(vii) conveying the pumped material (X5) a. to a heating unit (5), b. optionally to the polyolefin depolymerization reactor (3);(viii) heating the pumped material (X5) in the heating unit (5), to give a heated material (X6);(ix) conveying the heated material (X6) to the polyolefin depolymerization reactor (3); wherein the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into one or more of: the pumped material (X5), the heated material (X6).

[0019] In one preferred embodiment, the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into the pumped material (X5) through line (14a), or through line (14b), or though both lines (14a) and (14b). In another preferred embodiment, the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into the heated material (X6) through line (14c). In yet another embodiment, the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into both the pumped material (X5) and the heated material (X6).

[0020] In one embodiment, the recycling pump (4) sucks the melt or slurry from the reactor bottom; in order to avoid settling, the external heater (5) is typically installed vertically, and the melt or slurry flows downwards through the tubes. The melt or slurry is then conveyed to the top of the reactor (3). The reactor agitator is designed so that the agitator mixing time is shorter than the residence time resulting from the recycling pump capacity.

[0021] The term “oxygen-free atmosphere” refers to an environment in which the concentration of oxygen is substantially eliminated, to prevent oxidation and other oxygen- related reactions within the reactor (3). This atmosphere is typically achieved by reducing the oxygen concentration to e.g. less than 5 part per million (ppm), through methods such as purging and blanketing with an inert gas.

[0022] In a preferred embodiment, the recycling pump (4) is a centrifugal pump.

[0023] Preferably, the process of the disclosure is carried out in a continuous mode.

[0024] In one embodiment, in step (i), a charging system allows charging waste polyolefin material to be fed into the polyolefin depolymerization reactor (3), preferably in continuous mode. Care should be taken to not introduce oxygen into the system. A barrier to the potentially oxygen-containing atmosphere can be obtained with a series of expedients, such as nitrogen blanketing or a vacuum system connected to a barrel of the extruder (1).

[0025] More specifically, in one embodiment the waste polyolefin material is charged into the extruder (1) ofthe polyolefin depolymerization reactor (3) by means of ahopper, ortwo or more hoppers in parallel, and the oxygen present in the atmosphere of the waste polyolefin material is substantially eliminated inside the hopper(s).

[0026] In one embodiment, the waste polyolefin material (XI) is waste material that comprises polyolefin. The process according to the present disclosure is very flexible and can be fed with a wide range of waste compositions, e.g. with a heterogeneous mixture of waste materials (called Plasmix in Italy), in which polyolefins are the most abundant component, but for which a further sorting step is no longer economical. It is preferred, especially when the pyrolytic product is to be recirculated back to a cracking / refining unit, to depolymerize a waste polyolefin material in which the polyolefin (PE and PP) content is equal to or higher than 70 wt%.

[0027] Preferably, the waste polyolefin material 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 polyolefin materials can be transformed into a mass of uniform plastic composite. Therefore, this pretreatment is also preferable for the case in which the main pyrolytic decomposition is performed without additives.

[0028] In one embodiment, the heating temperature in the pre-treatment stage is appropriately set to a temperature in accordance with the kind and content of the polyolefinmaterial contained in the waste polyolefin material so that pyrolytic decomposition of the polyolefin material to be treated is inhibited. Such a pre-treatment 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.

[0029] In one embodiment, the HC1 forming gas is either removed via a venting system and successively neutralized or trapped if the waste polyolefin 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. Any extrusion systems can be applied, as single screw extruders, twin screw extruders, twin screw extruders with gear pump, or combination of the above.

[0030] In one embodiment, the extruder (1) receives the waste polyolefin material cut in small pieces into the feed hopper, conveys the stream in the melting section and heats the waste polyolefin material by combined action of mixing energy and heat supplied by barrel heaters . The extruder ( 1 ) melts the waste polyolefin material, brings it to a high temperature (250-350 °C) and injects it into at least one line leading to the polyolefin depolymerization reactor (3).

[0031] Optionally, additives can be incorporated in the melt aiming to reduce corrosivity of waste polyolefin material received or to improve conversion process in the reaction section.

[0032] In one embodiment, during the extrusion, one or more degassing steps are used to remove residual humidity present in the product.

[0033] In one embodiment, before being fed into at least one line leading to the polyolefin depolymerization reactor (3), the stream of heat-extrusion processed waste polyolefin material (X2) is filtered in order to remove solid impurities present in the polyolefin material waste. The polyolefin depolymerization reactor (3) preferably has a cylindrical section, preferably with a rounded bottom.

[0034] Preferably, the polyolefin depolymerization reactor (3) 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 2 kW / m3more preferably from 0.3 to 1.5 kW / m3.

[0035] In one embodiment, the heating of the polyolefin depolymerization reactor (3) takes place by means of the thermal transfer induced by a flow of molten salt, heated to a temperature ranging from 300 °C to 570 °C and circulated through a jacket which envelops the whole reactor.

[0036] In one embodiment, the feeding circuit (not shown in the Figures) of the molten salt to the reactor jacket 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. In the latter case, part of the recovered oil from the first condensation unit (6) may be used to feed the furnace. In the alternative, or in combination, the heat can be generated by combustion of gaseous or liquid hydrocarbons. The use of gaseous hydrocarbons is preferred.

[0037] In particular, during operation, the salt is pushed into the jacket from a circulation pump. A series of fins guarantees a homogeneous distribution of the flow of molten salts in the jacket and maximization of the thermal exchange coefficient.

[0038] Preferably, the depolymerization process taking place within the polyolefin depolymerization reactor (3) produces molecules having reduced chain length and low boiling point. This continuously running chain breakage mechanism, particularly close to the reactor walls, produces molecules increasingly smaller part of which, at the operating temperature and pressure, are gaseous. As a result, the composition within the polyolefin depolymerization reactor (3) covers a broad range of hydrocarbons from methane to heavier products, both saturated and olefinic, with linear or highly branched structures. Some aromatic product can be also present as well as fused rings structures.

[0039] Those hydrocarbons that are still liquid at the operating conditions contribute to lowering of the liquid mass viscosity. As a result of the depolymerization process and of the composition of the feed, the content of the polyolefin depolymerization reactor (3) can be defined as coexistence of a liquid slurry phase, in which solid especially carbonaceous substances, and inorganic substances, are dispersed in a liquid hydrocarbon mixture, and a gaseous phase.

[0040] From the operative point of view, the withdrawal of the slurry or melt phase from the bottom of the polyolefin depolymerization reactor (3) is preferably triggered by density sensors detecting the density of the liquid slurry or melt reaching a predetermined value.

[0041] At least a portion of the liquid slurry or melt phase is withdrawn from the bottom of the polyolefin depolymerization reactor (3) and constitutes the withdrawn material (X4) which is sent to the recycling pump (4). From the recycling pump (4) the melt or slurry is recirculated back to the reactor optionally through a heating unit (5) via a pumped material conveying line (11), and optionally an optional pumped material conveying line (12).

[0042] The heat-extrusion-processed waste polyolefin material (X2) from the at least one screw extruder (1) can be conveyed into the pumped material (X5) through the line (14a) and pass through the heating unit (5), to then enter the polyolefin depolymerization reactor(3). Alternatively, or additionally, the heat-extrusion-processed waste polyolefin material (X2) can be conveyed through the line (14c) into the heated material (X6) after the heating unit (5), to then enter the polyolefin depolymerization reactor (3). Conveying, preferably exclusively, the heat-extrusion-processed waste polyolefin material into the heated material (X6) is a preferred alternative because possible inhomogeneities of the melt will not cause clogging within the heat unit (5).

[0043] In yet another alternative, the heat-extrusion-processed waste polyolefin material (X2) can be conveyed through the line (14b) into a partial stream of the pumped material (X5) and into the polyolefin depolymerization reactor (3), i.e. without this stream passing through the heating unit (5).

[0044] By cycling liquid slurry or melt through the recycling pump (4), before conveying the heat-extrusion-processed waste polyolefin material (X2) from a low placed extruder (1) into the pumped material (X5), the heat-extrusion-processed waste polyolefin material (X2) can be transported to the top of the polyolefin depolymerization reactor (3). By cycling liquid slurry or melt through the heating unit (5), and ultimately conveying the heatextrusion-processed waste polyolefin material (X2) into the heated material (X6), the heatextrusion-processed waste polyolefin material (X2) can be transported to the top of the polyolefin depolymerization reactor (3) with an increased temperature. Both lessens the requirement for the screw extruder (1) and melt line (2) to provide and retain sufficient pumping and heat output to the feed into reactor (3).

[0045] In one embodiment, the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed directly into the polyolefin depolymerization reactor (3), through line (14d).

[0046] Preferably, the polyolefin depolymerization reactor (3) is an agitated vessel operated at a temperature in a range of from 300 C to 550 C, preferably in a range of from 350°C to 500 C; a pressure in a range of from 2.0 barg to 8 barg, preferably in a range of from 2.5 barg to 7 barg.

[0047] These reaction conditions have been found to result in an especially efficient process.

[0048] Preferably, the melt line (2) has a length in a range of from: 1 m to 50 m, more preferably in a range of from 1 m to 30 m.

[0049] Preferably, the heat extrusion-processed waste polyolefin material (X2) conveyed into the melt line (2) has a velocity in the pipe in a range from 0.5 to 5.0 cm / s, preferably in a range from 1.0 to 4.0 cm / s, more preferably in a range from 1.5 to 3.5 cm / s;a temperature in a range of from 130 C to 350 C, preferably in a range of from 160 °C to 320 °C; and, prior to combining with the heat extrusion-processed waste polyolefin material (X2) in the melt line (2), the pumped material (X5) and / or the heated material (X6); is conveyed at a velocity in the pipe in a range from 1.0 to 10.0 m / s, preferably in a range from 1.5 to 6.0 m / s, more preferably in a range from 2.0 to 4.0 m / s; a temperature in a range of from 300 C to 500 C; a viscosity in a range of from 0.5cP to 150cP.

[0050] Preferably, feeding (i) comprises heating in two screw extruders (la, lb), to give the heat extrusion-processed waste polyolefin material (X2), more preferably wherein the two screw extruders (la, lb) are operated in parallel.

[0051] Preferably, two recycling pumps (4a, 4b) are operated in parallel.

[0052] Preferably, heating of the polyolefin depolymerization reactor (3) is by thermal transfer induced by a flow of molten salt, heated to a temperature in a range of from 300 °C to 570 °C and circulated within a reactor jacket (3a).

[0053] Preferably, the polyolefin depolymerization reactor (3) operates in the presence of a depolymerization catalyst.

[0054] Preferably, the depolymerizing step (iii) in the polyolefin depolymerization reactor (3) forms a gaseous effluent (X7), and at least a portion of the gaseous effluent (X7) is processed. Processing of the gaseous effluent (X7) comprises the steps of: a) feeding the gaseous effluent (X7) to a first condensation unit (6), to give a gaseous condensation unit effluent (X8) and a liquid condensation unit effluent (X9); b) conveying the gaseous condensation unit effluent (X8) from the first condensation unit (6) to a second condensation unit (7), wherein the second condensation unit (7) condenses at a temperature lower than the temperature of the first condensation unit (6); c) directing the liquid condensation unit effluent (X9) from the first condensation unit (6) to a liquid effluent depolymerization reactor (8), wherein the liquid effluent depolymerization reactor (8) is a continuously stirred tank reactor maintained at a temperature in a range of from 280°C to 600 C and operated under a pressure in a range of from 1 barg to 10 barg, wherein depolymerization takes place in the liquideffluent depolymerization reactor (8), thereby forming a second gaseous effluent (X10) and a second liquid effluent (XI 1); d) withdrawing the second gaseous effluent (X10) from the effluent depolymerization reactor (8) and feeding it to the first condensation unit (6); e) conveying at least a portion of the second liquid effluent (XI 1) from the effluent depolymerization reactor (8) to the polyolefin depolymerization reactor (3).

[0055] The gaseous phase of the polyolefin depolymerization reactor (3) constitutes the gaseous effluent (X7) which may be sent to the first condensation unit (6) for further treatment.

[0056] The gaseous effluent may comprise a mixture of light hydrocarbons which may include some heavy hydrocarbons and char particles entrained. The gaseous effluent (X7) may be conveyed from the reactor top to a first condensation unit (6). The condenser temperature is selected in such a way that the heavy hydrocarbons are condensed and the light hydrocarbons are released as gaseous condensation unit effluent (X8). The gaseous condensation unit effluent (H2 and light hydrocarbons) may be conveyed to s second condensation unit (7) working at a temperature lower than the first condensation unit (6) from which pyrolytic product (X12) is recovered.

[0057] Due to the very high number of compounds in the pyrolytic product (XI 2), the result of the analysis is to be reported by grouping the resulting compounds according to their retention time using specific hydrocarbons as internal retention time references.

[0058] The liquid effluent depolymerization reactor (8) is preferably of the same type as the polyolefin depolymerization reactor (3) and more preferably a continuously stirred tank reactor.

[0059] 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 polyolefin depolymerization reactor (3) and in particular in the range from 2 to 10 barg, preferably from 3 to 9 barg and more preferably from 3 to 8 barg.

[0060] Since the liquid effluent depolymerization reactor (8) is fed with the condensed effluents coming from the polyolefin depolymerization reactor (3) via the first condensation unit (6), reactor (8) contains less impurities and produces less char.

[0061] The second liquid effluent (XI 1) coming from the liquid effluent depolymerization reactor (8) is preferably a highly concentrated hydrocarbon slurry which preferably contains the depolymerization catalyst. It is discharged from the liquid effluent depolymerization reactor (8) and sent back to the polyolefin depolymerization reactor (3) via the second liquid effluent line (16). The same density control in reactor (3) for the withdrawal of the slurryis preferably operated also for reactor (8). The slurry density can be control via available methods such as y-ray measurement or Coriolis densimeter.

[0062] When the operative pressure of the polyolefin depolymerization reactor (3) is lower than the liquid effluent depolymerization reactor (8), the light hydrocarbons of the slurry entering the reactor (3) are expected to vaporize and be extracted with the gaseous effluent produced in reactor (3).

[0063] In a preferred embodiment, in reactor (8), part of the liquid slurry withdrawn from the bottom of the reactor (8) is recirculated, with a recycling pump, back to the reactor top through an external heater.

[0064] Preferably, a pyrolytic product (XI 2) is recovered from the second condensation unit (7).

[0065] Preferably, the liquid effluent depolymerization reactor (8) operates in the presence of a depolymerization catalyst.

[0066] According to the present disclosure, the catalyst may be selected from those active as depolymerization / cracking catalysts in thermocatalytic processes. In particular, it may be selected from aluminosilicates catalysts and preferably from zeolites. Among them, particularly preferred zeolites are synthetic Y-type zeolite and ZSM-5.

[0067] In a particularly preferred embodiment, the amount of catalyst feed is not more than 10 wt.% preferably not more than 5 wt.% and especially not more than 2 wt.% with respect to the polyolefin material waste feed.

[0068] In a preferred embodiment, the catalyst is injected into the second reactor as powder dispersed into a hydrocarbon oil, preferably the liquid condensation unit effluent (X9), or liquid pyrolytic product ( I 2) obtained from condensation unit (6) or (7), preferably from the first condensation unit (6).

[0069] 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.

[0070] The pyrolytic oil dispersing the catalyst is preferably withdrawn from the first condensation unit (6), in order to keep constant the slurry level in the pot.

[0071] Once ready the catalyst slurry can be injected, preferably into the liquid effluent depolymerization reactor (8), preferably by means of a progressive cavity pump in order to keep its level constant.

[0072] The first condensation unit (6) may be either a distillation unit with two or more equilibrium stages, preferably three or more equilibrium stages, or only a condenser with one equilibrium stage. An increased number of equilibrium stages improves the separation of components by their respective boiling points. A condenser with one equilibrium stage reduces the required complexity and cost.

[0073] Preferably, the first condensation unit (6) operates at a pressure lower than that of the depolymerization reactor (3).

[0074] Preferably, the liquid effluent depolymerization reactor (8) is operated at a temperature in a range of from 280 C to 600 C; a pressure higher than that of the polyolefin depolymerization reactor (3), preferably in a range of from 2 to 10 barg.

[0075] This complements the reaction conditions of the polyolefin depolymerization reactor (3), improving the overall output of the process.

[0076] Preferably, a fresh depolymerization catalyst is fed to the liquid effluent depolymerization reactor (8).

[0077] Preferably, fresh catalyst is fed to the liquid effluent depolymerization reactor (8) through conduit (15).

[0078] Preferably, the pyrolytic product (XI 2) is recovered from the second condensation unit (7) in form of an oil, wherein the oil has the following composition, as determined via gas chromatography: about 10 to 15 wt% of a fraction having a retention time equal to or less than n- heptane; about 70 to 75 wt% of a fraction having a retention time comprised by n-heptane and n-dodecane, and about 12 to 20 wt% of product having a retention time higher than that of n-dodecane and lower than that of n-octacosane.

[0079] According to the second aspect, a plant for recycling waste polyolefin material comprises: at least one screw extruder (1), the screw extruder (1) being capable of heating a waste polyolefin material (XI), in an oxygen-free atmosphere, to a melting temperature of the waste polyolefin material (XI); a melt line (2) for conveying, from the at least one screw extruder (1), a heat extrusion-processed waste polyolefin material (X2); a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor capable of depolymerizing a polyolefin-derived material (X3) at a temperature in a range of from 280 °C to 600 °C and under a pressure in a range of from 1 barg to 10 barg;withdrawing means (9) for withdrawing a reacted material from the polyolefin depolymerization reactor (3); a withdrawn material conveying line (10) for conveying a withdrawn material (X4) from the polyolefin depolymerization reactor (3) to a recycling pump (4); the recycling pump (4); a pumped material conveying line (11) for conveying at least part of the pumped material (X5) from the recycling pump (4) to a heating unit (5) capable of heating the pumped material (X5); an optional pumped material conveying line (12) for conveying part of the pumped material (X5) from the recycling pump (4) to the polyolefin depolymerization reactor (3); the heating unit (5); a heated material conveying line (13) for conveying a heated material (X6) from the heating unit (5) to the polyolefin depolymerization reactor (3); one or more melt lines (14a, 14b, 14c) for conveying the heat extrusion-processed waste polyolefin material (X2) into one or more of: the pumped material conveying line (11); the optional pumped material conveying line (12); the heated material conveying line (13).

[0080] Each of• the withdrawn material conveying line (10),• the pumped material conveying line (11),• the optional pumped material conveying line (12) and• the heated material conveying line (13) can, independently of each other, either be: o a melt line, or o a slurry line.

[0081] Preferably, the recycling plant comprises two screw extruders (la, lb), to give the heat extrusion-processed waste polyolefin material (X2), more preferably the two screw extruders (la, lb) are operated in parallel.

[0082] Preferably, the recycling plant comprises two recycling pumps (4a, 4b), more preferably the two recycling pumps (4a, 4b) are operated in parallel.Figures 1 and 2

[0083] Fig. 1 shows a schematic view of an embodiment of the process according to the first aspect. Waste polyolefin material (XI) is fed into two screw extruders (la, lb) which are connected to melt line (2). Melt line (2) conveys heat-extrusion processed waste polyolefin material (X2) from the two extruders into one or more melt lines (14a, 14b, 14c, 14d). Melt line (14a) connects to and feeds into pumped material conveying line (11). Melt line (14b) connects to and feeds into optional pumped material conveying line (12). Melt line (14a) connects to and feeds into heated material conveying line (13). Melt line (14d), if present, leads directly and feeds into polyolefin depolymerization reactor (3).

[0084] Polyolefin depolymerization reactor (3) comprises polyolefin-derived material (depolymerizing) (X3). Gaseous effluent (X7) exits from the upper half of polyolefin depolymerization reactor (3).

[0085] At the bottom of polyolefin depolymerization reactor (3) a withdrawing means (9) connects to withdrawn material conveying line (10) that leads to two recycling pumps (4a, 4b). Withdrawn material (X4) is conveyed from the bottom of the reactor to the recycling pumps (4a, 4b) through withdrawn material conveying line (10). From the recycling pumps (4a, 4b), pumped material (X5) is conveyed to pumped material conveying line (11) and optionally to optional pumped material conveying line (12). Pumped material conveying line (11) connects to heating unit (5) and pumped material (X5) is conveyed to heating unit (5) in one route through pumped material conveying line (11). Heating unit (5) leads to polyolefin depolymerization reactor (3) through heated material conveying line (13) which conveys heated material (X6) from heating unit (5) to polyolefin depolymerization reactor (3). Optional pumped material conveying line (12) leads to heated material conveying line (13) in another route.

[0086] Fig. 2 is a schematic view of a type of a section that in one embodiment processes a gaseous effluent (X7) from polyolefin depolymerization reactor (3). Gaseous effluent (X7) from polyolefin depolymerization reactor (3) is conveyed into a first condensation unit (6). Gaseous condensation unit effluent (X8) is conveyed to a second condensation unit (7). Pyrolytic product (12) is obtained as condensation product from the second condensation unit (7).

[0087] Liquid condensation unit effluent (X9) from first condensation unit (6) is pumped to liquid effluent depolymerization reactor (8). Liquid effluent depolymerization reactor (8) is fitted with conduit (15) that supplies fresh catalyst. Second gaseous effluent (X10) is conveyed from the upper half of liquid effluent depolymerization reactor (8) and into first condensation unit (6). Second liquid effluent (XI 1) is withdrawn at the bottom of liquid effluent depolymerization reactor (8), and is either circulated through a pump and heating means, or is conveyed back to polyolefin depolymerization reactor (3) through second liquid effluent line (16). List of reference signs Screw extruder XI Waste polyolefin material Melt line X2 Heat-extrusion processed waste polyolefin Polyolefin depolymerization reactor material Recycling pump X3 Polyolefin-derived material (depolymerizing) Heating unit X4 Withdrawn material First condensation unit X5 Pumped material Second condensation unit X6 Heated material Liquid effluent depolymerization reactor X7 Gaseous effluent Withdrawing means X8 Gaseous condensation unit effluent Withdrawn material conveying line X9 Liquid condensation unit effluent Pumped material conveying line X10 Second gaseous effluent Optional pumped material conveying line XI I Second liquid effluent Heated material conveying line X12 Pyrolytic product Melt line(s) Conduit Second liquid effluent line

Claims

CLAIMS1. A process for recycling waste polyolefin material, the process comprising the following steps:(i) feeding waste polyolefin material (XI) into at least one screw extruder ( 1 ), the at least one screw extruder (1) heating the waste polyolefin material (XI), in an oxygen-free atmosphere and to a melting temperature of the waste polyolefin material (XI), to give a heat extrusion-processed waste polyolefin material (X2);(ii) conveying the heat extrusion-processed waste polyolefin material (X2) in a melt line (2);(iii) depolymerizing a polyolefin-derived material (X3) in a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor maintained at a temperature in a range of from 280 C to 600°C and operated under a pressure in a range of from 1 barg to 10 barg;(iv) withdrawing a reacted material from the bottom of the polyolefin depolymerization reactor (3), to give a withdrawn material (X4);(v) conveying the withdrawn material (X4) to a recycling pump (4);(vi) withdrawing a pumped material (X5) from the recycling pump (4);(vii) conveying the pumped material (X5) a. to a heating unit (5), b. optionally to the polyolefin depolymerization reactor (3);(viii) heating the pumped material (X5) in the heating unit (5), to give a heated material(X6);(ix) conveying the heated material (X6) to the polyolefin depolymerization reactor (3); wherein the heat extrusion-processed waste polyolefin material (X2) in the melt line (2) is conveyed into one or more of: the pumped material (X5), the heated material (X6).

2. The process according to claim 1, wherein the polyolefin depolymerization reactor (3) is an agitated vessel operated ata temperature in a range of from 300 C to 550 C, preferably in a range of from 350°C to 500 C; a pressure in a range of from 2.0 barg to 8 barg, preferably in a range of from 2.5 barg to 7 barg.

3. The process according to any of claims 1 or 2, wherein the melt line (2) has a length in a range of from 1 m to 50 m, preferably in a range of from 1 m to 30 m.

4. The process according to any of the preceding claims 1 to 3, wherein the heat extrusion- processed waste polyolefin material (X2) conveyed into the melt line (2) has a velocity in the pipe in a range from 0.5 to 5.0cm / s, preferably in a range from 1.0 to 4.0 cm / s, more preferably in a range from 1.5 to 3.5 cm / s; a temperature in a range of from 130 C to 350°C, preferably in a range of from 160 °C to 320 °C; and, prior to combining with the heat extrusion-processed waste polyolefin material (X2) in the melt line (2), the pumped material (X5) and / or the heated material (X6) is conveyed at a velocity in the pipe in a range from 1.0 to lO.Om / s, preferably in a range from 1.5 to 6.0 m / s, more preferably in a range from 2.0 to 4.0 m / s; a temperature in a range of from 300°C to 500 C; a viscosity in a range of from 0.5cP to 150cP.

5. The process according to any of the preceding claims 1 to 4, wherein feeding (i) comprises heating in two screw extruders (la, lb), to give the heat extrusion-processed waste polyolefin material (X2), preferably wherein the two screw extruders (la, lb) are operated in parallel.

6. The process according to any of the preceding claims 1 to 5, wherein the heat extrusion- processed waste polyolefin material (X2) in the melt line (2) is conveyed, preferably exclusively, into the heated material (X6).

7. The process according any of the preceding claims 1 to 6, wherein the polyolefin depolymerization reactor (3) operates in the presence of a depolymerization catalyst.

8. The process according to any of the preceding claims 1 to 7, wherein the depolymerizing step (iii) in the polyolefin depolymerization reactor (3) forms a gaseous effluent (X7), and at least a portion of the gaseous effluent (X7) is processed, wherein processing of the gaseous effluent (X7) comprises the steps of: a) feeding the gaseous effluent (X7) to a first condensation unit (6), to give a gaseous condensation unit effluent (X8) and a liquid condensation unit effluent (X9); b) conveying the gaseous condensation unit effluent (X8) from the first condensation unit (6) to a second condensation unit (7), wherein the second condensation unit (7) condenses at a temperature lower than the temperature of the first condensation unit (6); c) directing the liquid condensation unit effluent (X9) from the first condensation unit (6) to a liquid effluent depolymerization reactor (8), wherein the liquid effluent depolymerization reactor (8) is a continuously stirred tank reactor maintained at a temperature in a range of from 280 C to 600°C and operated under a pressure in a range of from 1 barg to 10 barg, wherein depolymerization takes place in the liquid effluent depolymerization reactor (8), thereby forming a second gaseous effluent (X10) and a second liquid effluent (XI 1); d) withdrawing the second gaseous effluent (X10) from the effluent depolymerization reactor (8) and feeding it to the first condensation unit (6); e) conveying at least a portion of the second liquid effluent (XI 1) from the effluent depolymerization reactor (8) to the polyolefin depolymerization reactor (3).

9. The process according to claim 8, wherein a pyrolytic product (X12) is recovered from the second condensation unit (7).

10. The process according any of claims 8 and 9, wherein the liquid effluent depolymerization reactor (8) operates in the presence of a depolymerization catalyst.

11. The process according to any of claims 8 to 10, wherein the first condensation unit (6) operates at a pressure lower than that of the depolymerization reactor (3).

12. The process according to any of claims 8 to 11, wherein the liquid effluent depolymerization reactor (8) is operated at a temperature in a range of from 280 C to 600°C;a pressure higher than that of the polyolefin depolymerization reactor (3), preferably in a range of from 2 to 10 barg.

13. The process according to any of claims 8 to 12, wherein a fresh depolymerization catalyst is fed to the liquid effluent depolymerization reactor (8).

14. The process according to claims 8 to 13, wherein the pyrolytic product (X12) is recovered from the second condensation unit (7) in form of an oil, wherein the oil has the following composition, as determined via gas chromatography: about 10 to 15 wt% of a fraction having a retention time equal to or less than n- heptane; about 70 to 75 wt% of a fraction having a retention time comprised by n-heptane and n-dodecane, and about 12 to 20 wt% of product having a retention time higher than that of n- dodecane and lower than that of n-octacosane.

15. A plant for recycling waste polyolefin material, the recycling plant comprising: at least one screw extruder (1), the screw extruder (1) being capable of heating a waste polyolefin material (XI), in an oxygen-free atmosphere, to a melting temperature of the waste polyolefin material (XI); a melt line (2) for conveying, from the at least one screw extruder (1), a heat extrusion-processed waste polyolefin material (X2); a polyolefin depolymerization reactor (3), wherein the polyolefin depolymerization reactor (3) is a continuously stirred tank reactor capable of depolymerizing a polyolefin-derived material (X3) at a temperature in a range of from 280 °C to 600 °C and under a pressure in a range of from 1 barg to 10 barg; withdrawing means (9) for withdrawing a reacted material from the polyolefin depolymerization reactor (3); a withdrawn material conveying line (10) for conveying a withdrawn material (X4) from the polyolefin depolymerization reactor (3) to a recycling pump (4); the recycling pump (4);a pumped material conveying line (11) for conveying at least part of the pumped material (X5) from the recycling pump (4) to a heating unit (5) capable of heating the pumped material (X5); an optional pumped material conveying line (12) for conveying part of the pumped material (X5) from the recycling pump (4) to the polyolefin depolymerization reactor(3); the heating unit (5); a heated material conveying line (13) for conveying a heated material (X6) from the heating unit (5) to the polyolefin depolymerization reactor (3); - one or more melt lines (14a, 14b, 14c) for conveying the heat extrusion-processed waste polyolefin material (X2) into one or more of: the pumped material conveying line (11); the optional pumped material conveying line (12); the heated material conveying line (13).

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