Method and apparatus for depolymerizing plastic materials for hydrocarbon production

Abstract

The present invention relates to a method for depolymerizing waste plastic materials and a reactor suitable for depolymerizing waste plastic materials in said method.

Description

[0001] The present invention relates to a method for depolymerizing waste plastic materials for the production of hydrocarbons and to an apparatus suitable for depolymerizing plastic materials in said method.

[0002] Within the scope of the present invention, "waste plastic materials" means materials that remain as residues after their main use and contain at least partially polymer substances of synthetic origin, such as derived from petrochemicals or consisting of these substances. Waste plastic materials can be obtained from municipal or industrial waste (such as processing or finishing residues of products containing a plastic material part), they can have homogeneous or inhomogeneous chemical properties, and can be recyclable or preferably not further usable in common recycling processes.

[0003] Waste plastic materials have a negative impact on the environment and human health and are one of the most widespread causes of pollution, and also have a negative impact on various human activities that should in principle be independent of the plastic material "supply chain", such as fishing or tourism. Recycling of plastic materials from household and industrial users enables at least a part of these plastics to be reintroduced into the production cycle.

[0004] Although desirable, it is in fact unrealistic to expect a rapid and drastic reduction in the demand for plastic materials for various applications in the near future, nor is it imaginable that the plastic recycling rate can reach 100%. In fact, mechanical recycling of plastic materials produces substances with lower quality characteristics, and as the products are gradually "degraded", the recycling process cannot be carried out indefinitely. There is a limit beyond which only the energy of plastic waste (of poor quality at the end of recycling) can be used. It is very important that this waste can also be converted into useful materials, i.e., that the use of this energy has an optimal yield and that the final residue of plastic waste is converted into an inert substance with a limited volume.

[0005] Municipal waste is a mixture of different substances, which vary according to the place of origin and time. One of the "streams" of municipal waste consists in particular of a mixture of plastic materials, which can include different polymers, including polyethylene terephthalate (PET), polystyrene, polyvinyl chloride (PVC), polypropylene (PP), low-density polyethylene (LDPE) and high-density polyethylene (HDPE).

[0006] To date, the vast majority of these plastics, especially those of municipal waste, are not recyclable and are used as a heat source in energy-intensive devices such as incinerators at very low yields, disposed of in landfills or, in the worst case, scattered in the environment polluting the terrestrial and marine environments.

[0007] In most countries, especially in recent years, the need to recycle plastics is considered extremely important and urgent. As a result, a wide variety of new solutions related to the recycling and reuse of plastic materials have emerged, and these new solutions also generate income.

[0008] In more developed countries, the plastic fraction of municipal waste with residue and recyclable value is mainly mechanically recycled by automated machinery, with the aim of recycling even the plastic waste fraction with the smallest commercial value. Especially in Italy, most of the plastic waste from municipal waste is managed by the CSS (Centri di Selezione Spinta) (Compulsory Waste Selection Centers) managed by the COREPLA supply chain consortium, which is part of the larger CONAI consortium. Currently, COREPLA recycles and reuses approximately 60% of the municipal plastic waste (mainly composed of packaging materials) recovered nationwide. In the factories controlled by COREPLA, the recycling and reuse have reached the feasibility limit because the activities of recycling and reusing plastic-containing municipal waste are no longer economically sustainable after exceeding the indicative limit of 60%.

[0009] As a result, approximately 40% of the original municipal plastic waste becomes "useless or harmful" waste, which, as mentioned above, awaits treatment, for example, in landfills or burned in incinerators (in Italy, this is approximately 400,000 tons / year, out of the plastic waste generated from approximately 1 million tons of municipal waste per year, managed by the consortium).

[0010] This "useless and harmful" material is referred to in this article in a simplified way but for ease of understanding as PLASMIX. Its average composition is shown in Table 1.

[0011] Table 1

[0012]

[0013] This material has no economic value in the market. In fact, if it spreads, it will cause serious environmental problems, damaging the land where it is stored and reducing the value of the land for other purposes (such as tourism).

[0014] In most countries of the world, plastic waste in municipal waste is recycled to some extent, to the extent that the recycled materials have subsequent applicability. Less than 10% of plastic waste is used in incinerators, only utilizing its value as heat generation. However, considering the heat required to heat the feedstock and the inert materials subsequently discharged from the incinerator or to remove the water in the organic stream discharged into the incinerator, the overall efficiency of using plastic waste (or similar plastic waste) as fuel in the incinerator is very low and the cost is extremely high (when present and used, it is also affected by strict regulations on gas emissions). In addition, transporting waste to incinerators by road and rail not only causes traffic problems and damages infrastructure in addition to the cost and pollution of transportation itself.

[0015] Incinerators may have been advantageous at the beginning of industrial development, especially due to the reduction in waste volume and the neutralization of potentially hazardous organic waste, but it has become obsolete compared to other technical solutions and is even inopportune compared to other solutions, representing an extremely expensive technical solution.

[0016] On the other hand, it would also be advantageous to use fuel oil (similar to diesel fuel) to at least partially replace fuels of mineral origin (i.e., produced from petroleum).

[0017] Converting waste plastics, especially those that are no longer recyclable, into useful materials, such as hydrocarbon-like mixtures that can replace fuel oil, would have the following advantages:

[0018] - Environment (reducing or eliminating the spread of worthless plastic waste in the environment, which cannot be disposed of and discharged in a manner that has a relative impact on the environment);

[0019] - Hygiene (avoiding potential pollution of water and food caused by the spread of plastic materials in the environment);

[0020] - Economy (in addition to generating employment and profits through self-sustaining activities, avoiding the disposal of plastic materials, reducing the cost of importing and producing diesel (i.e., basically avoiding the supply cost of raw materials) and being environmentally friendly).

[0021] However, the currently available technologies for recycling waste plastic materials by depolymerization, especially pyrolysis, generally have unsatisfactory yields. In fact, the composition of the hydrocarbon mixtures obtained by current technologies through direct treatment of plastic waste actually requires further treatment in a refinery because they consist of mixtures of hydrocarbons with a composition that varies over time and is not completely chemically stable.

[0022] The object of the present invention is to provide a method for the pyrolysis of waste plastic materials that is substantially free of the disadvantages of the above methods.

[0023] The present invention relates to a method for the pyrolysis of waste plastic materials, wherein the method comprises the following steps:

[0024] i. Loading a mixture comprising waste plastic materials or consisting essentially of waste plastic materials into a feeding system comprising at least one screw extruder (1) in an oxygen-free atmosphere, optionally heating the extruder by means of a molten salt channel in a suitable jacket (1a) located outside the extrusion chamber, the heating temperature being such that the temperature of the material emerging from the extruder is between 150 and 180 °C;

[0025] ii. Directly loading the plastic material from the extruder in the previous step into at least one reactor (2), forming a liquid bath of polymer material therein, and carrying out depolymerization to form a gaseous effluent, wherein the volume of the region available for the gaseous effluent above the liquid bath in the reactor (2) is less than 20% of the total volume, preferably less than 15% of the total volume of the reactor, so as to minimize the residence time of the gas generated within the reactor, and heating the reactor (2) to about 300 to about 400 °C by means of a molten salt stream in a jacket outside the reactor body, the jacket being connected to a system for transporting, heating and storing molten salt (3), the reactor (2) being equipped with a screw conveyor (6) connected to the bottom of the reactor (2) and having a discharge head located above the liquid level of the liquid bath inside the reactor, for removing the solid residue formed after depolymerization.

[0026] The present invention also relates to a reactor (2) having a cylindrical cross-section and a vertical axis, the reactor having: a stirrer (7) with a gear motor installed on the axis of the reactor, wherein the interior of the reactor (2) comprises a lower region available for forming a liquid bath of polymer and an upper region for receiving the gaseous effluent derived from the depolymerization of the liquid bath of polymer, the volume of which is less than 20% of the total volume inside the reactor (2), preferably less than 15% of the total volume of the reactor, so as to minimize the residence time of the gaseous effluent generated inside the reactor; a screw conveyor (6) connected to the bottom of the reactor (2) and having a discharge head above the liquid level of the liquid bath of polymer present inside the reactor; and a jacket (5) outside the reactor body and for heating by means of molten salt, which is connected to a system for transporting, heating and storing molten salt (3).

[0027] The present invention will hereinafter be described with reference to non-limiting embodiments, which are provided for illustrative purposes only and with reference to the drawings showing different aspects and embodiments of the present invention.

[0028] In the drawings:

[0029] Figure 1 A general block diagram of the method according to the present invention is shown;

[0030] Figure 2 shows a schematic view of a reactor (2) according to the present invention;

[0031] Figure 2 a shows a schematic view of a scraper present in a depolymerization reactor according to the present invention;

[0032] Figure 2 b shows a schematic view of a depolymerization reactor with a solid discharge system that can be used in the method according to the present invention;

[0033] Figure 3 shows a schematic view of an extruder (1) with a screw (11) that can be used in the method according to the present invention;

[0034] Figure 4 shows a schematic view of a hopper system that can be used in the method according to the present invention.

[0035] Figure 5 shows a schematic view of a condenser (12) that can be used in the method according to the present invention.

[0036] Figure 6 shows a schematic view of a gas-phase (second) reactor (10) that can be used in the method according to the present invention.

[0037] Figure 7 shows a schematic embodiment of a part of an apparatus for the method according to the present invention, which includes the parts indicated in the previous figures.

[0038] Unless otherwise specified, within the scope of the present invention, the percentages and amounts of components in a mixture refer to the weight of the said components relative to the total weight of the mixture.

[0039] Unless otherwise specified, within the scope of the present invention, stating that a composition "comprises or includes" one or more components or substances means that other components or substances may be present in addition to the one or more specifically mentioned substances.

[0040] Unless otherwise specified, within the scope of the present invention, a range of values indicated for a quantity, such as the weight content of a component, includes the lower and upper limits of the range. For example, if the weight or volume content of component A is expressed as "from X to Y", where X and Y are numerical values, then A can be X or Y or any intermediate value.

[0041] Unless otherwise indicated, within the scope of the present invention, the expression "about" indicates the possibility that the value of a given quantity (such as temperature or pressure) is slightly different from the indicated value, for example, more or less by some percentage, depending on the actual composition of the waste being processed and the final mixture of hydrocarbons to be produced. The technology of the present invention is the result of research and experimentation that have allowed the determination of a method in which the main product of the conversion of a mixture of "waste" plastic materials is a liquid fuel similar to commercial oil and gas.

[0042] Within the scope of the present invention, hydrocarbon mixtures equivalent to diesel fuel, gasoline, or natural gas are conventionally distinguished according to the number of carbon atoms prevalent in the molecules contained in said mixture.

[0043] The method of the present invention is suitable for other applications fully consistent with the production of liquid and possibly gaseous hydrocarbons, for example, for the production of ethylene, which, in the case of full application of the circular economy, can be reused to produce "original" plastic materials.

[0044] The plastic waste that can be fed into the method according to the present invention can have different compositions and origins. In particular, the previously described mixture of commingled plastic waste (PLASMIX, as a non-limiting example) can be used, which is generated in an efficient recycling line for the plastic fraction of municipal waste collection, or the residue of plastic mixtures generated during molding or other industrial processes can also be used.

[0045] In fact, the inventors have surprisingly found that it is possible to provide sufficient economic value to "exhausted" plastic waste, which must be removed after use and possibly after recycling.

[0046] The method according to the present invention may also include a pretreatment step, preferably present in the case where the composition of the raw material, i.e., the waste plastic material, does not allow direct feeding into the "thermal" part of the method based on depolymerization, i.e., into step ii of the method.

[0047] When the composition of the fed plastic material is unknown or varies over time, it is generally best to provide pretreatment in the first step of the whole method, for example, when the feed is scrap / waste, the source and composition of which are usually variable. Similarly, in the case of plastic materials, from a chemical point of view, a simplified pretreatment suitable for direct feeding into a depolymerizer may be advantageous. As a non-limiting example, this simplified pretreatment may include mechanical treatment for adjusting and standardizing the size of the fed plastic material.

[0048] In the feedstock, especially if it is generated from uncontrollable waste (such as PLASMIX, etc.), there may also be a small amount of material flow that cannot be depolymerized by the method of the present invention. Therefore, unless they are used to feed other waste recycling processes, they must be eliminated in advance. These are, for example, metal materials, paper, polymers such as PET, PVC, and polyurethane that generate gas under heating (the content of each of these polymers in the waste plastic material fed into the reactor of the method according to the present invention must be less than 3% (by weight)).

[0049] In addition, by way of non-limiting example, the pretreatment step may include:

[0050] · Separating and recovering recyclable components (glass, metal, paper, cardboard)

[0051] · Controlling shape parameters;

[0052] · Reducing / controlling humidity;

[0053] · Eliminating chemical components that have a negative impact on step ii of the "thermal" method.

[0054] Step ii of the method according to the present invention is a high-temperature pyrolysis depolymerization step of the complex polymer chains present in the waste plastic material to be treated in the absence of oxygen.

[0055] In the presence of oxygen, local combustion will be immediately triggered until the combustibles are exhausted. From a safety perspective, possible oxygen inlet points will cause local heating, which is unacceptable.

[0056] Compared with the pretreatment machine, the "thermal" part of the method involves a separate and more complex system, for which very advanced technical solutions have been developed and applied. In this part, controlled depolymerization will be carried out, and from the perspective of energy consumption, the special technologies used can achieve full autonomy in both the pretreatment part and the depolymerization part.

[0057] In the context of the present invention, the absence or substantially absence of oxygen refers to an environment that does not contain oxygen, that is, the amount of oxygen that may be present is lower than the value detected by analytical techniques that can be used in relevant cases and are known to those skilled in the art.

[0058] The actual amount of oxygen present is zero because oxygen (if present) will burn, causing local combustion. Therefore, there should be no oxygen penetration, otherwise, local combustion will cause unwanted and potentially dangerous local overheating.

[0059] Non-limiting examples of the composition of the material feed fed into step ii of the method according to the present invention (i.e., into the "thermal" stage, i.e., the pyrolysis depolymerization reaction) after pretreatment are shown in Table 2 below (unit = wt%).

[0060] Component Minimum Maximum PET 0.00％ 4.00％ PP and / or PE 75.00％ 100.00％ PVC 0.00％ 3.00％ PS 0.00％ 20.00％ Metal 0.00％ 1.00％ Paper and cardboard 0.00％ 1.00％ Other polymers 0.00％ 1.00％ Humidity 0.00％ 1.00％

[0061] From one kilogram of pure incoming polymeric material, the following total outputs can be obtained (yields from extensive experimental testing; data may vary and also depend on the characteristics of the material to be processed and the management mode of the equipment):

[0062] · 662 g of liquid fuel (diesel-like, specific gravity < 0.85 kg / dm 3 )

[0063] · 86 g of carbon (coal powder)

[0064] · 170 g of lighter liquid fuel (similar to gasoline)

[0065] · 82 g of gaseous fuel.

[0066] In the processing equipment for "plastics waste that cannot be recycled further" (such as in the case of PLASMIX), there may be an additional 20 - 30 g of inert material in the feed inlet, which is recovered into the solid waste depending on the raw material.

[0067] By using the lighter hydrocarbon fractions (fuels) generated by the method itself to generate electrical and thermal energy, the energy consumption according to the method of the present invention is fully met (indicatively but not exclusively, for each kilogram of material processed in the case of PLASMIX, it is equal to 0.26 kWh of electrical energy + 0.6 kWh of thermal energy), so that the method actually requires no external energy supply.

[0068] The main product of the method according to the present invention is liquid fuel, which can be sold as fuel oil or used for the operation of high-efficiency diesel generators.

[0069] The liquid fuel (fully stable) can be easily stored and transported because it is liquid at room temperature and has a very low operating cost due to its flash point.

[0070] Other possible uses of the liquid fuel obtained by the method according to the present invention and with reference to the applications already specified are for the feeding of ships, such as fishing fleets, or for the production of diesel for motor vehicles (possibly after mixing with mineral diesel to increase its density). Among many advantages, the liquid fuel that can be obtained by the method of the present invention is practically sulfur-free, that is, the content of sulfur and / or sulfur substances is below the limit specified for the fuel, and is substantially free of certain polluting substances, such as polycyclic aromatic compounds present in diesel from mineral sources (obtained by processing crude oil).

[0071] The depolymerization method according to the present invention may include a series of solutions capable of ensuring the following effects:

[0072] · It is possible to feed the device with a mixture of waste plastic materials that are compositionally heterogeneous and "contaminated", the mixture being characterized by a plurality of polymeric materials and the substantial presence of non-plastic, rigid elements (sand, stones, metals, glass, wood, etc.);

[0073] · Despite the presence of potentially contaminating elements that are mechanically difficult to control (stones and metals in sensitive areas in the presence of mechanical movement), it is also possible to precisely control the important parameters during the process.

[0074] The method according to the invention allows a series of operations to be carried out in sequence in order to suitably control and maintain the important parameters characterizing the final product, so that said parameters comply with the provisions of the current regulations regarding hydrocarbons, for example see standard EN590.

[0075] In the method according to the invention, the depolymerization reaction in the liquid phase can be carried out in one or more similar or identical reactors connected in parallel. These reactors can have a vertical axis with a cylindrical cross-section, with a circular bottom at the bottom and an upper flat cover having dimensions equal to the entire horizontal cross-section of the reactor, which is useful for the maintenance of the reactor itself. A stirrer with a geared motor is installed on the axis of the reactor.

[0076] After being heated and softened in a suitable power extruder in step i of the method according to the invention, the compositionally heterogeneous polymer mixture that constitutes the waste plastic material fed to the method is fed into these reactors and heated to a temperature of about 300 to about 400 °C. The heating of each reactor is carried out by heat transfer caused by a molten salt stream heated to a temperature of about 450 to about 550 °C and passing through a jacket surrounding the entire reactor.

[0077] The actual depolymerization process takes place along the inner wall of the reactor. When the polymer molecules that are already at the liquid bath temperature approach the wall (at a temperature close to that of the molten salt), they undergo chain breakage due to the temperature. When the length of the single chain is such that its boiling point is lower than the temperature of the liquid bath, at this point it undergoes a phase change, turns into a gas and forms bubbles, which detach from the wall and move towards the surface of the liquid bath of polymeric material. Once they reach the surface, they generate a gas stream consisting essentially of hydrocarbons at a temperature slightly higher than the liquid bath temperature (300 °C - 400 °C).

[0078] The depolymerization process produces a series of hydrocarbons that are gaseous at the operating temperature and pressure. In order to maximize the efficiency of the method, all the hydrocarbons thus produced are used in subsequent applications.

[0079] The method according to the invention produces a heavier fraction of the effluent, hereinafter referred to as "alkanes", which consists of hydrocarbons that are gaseous at a temperature of 430°C to 480°C under atmospheric pressure but are solid or highly viscous at room temperature. This fraction can be recycled in a depolymerization reactor to subject it to a new controlled heating cycle, which then causes further breakage of the polymer chains suitable for further conversion.

[0080] At atmospheric pressure and a temperature of 20 - 30°C, a portion of the gaseous effluent is also produced, as well as a fraction consisting essentially of hydrocarbons, the composition of which ranges from a minimum carbon atom number equal to 5 (C5) to a maximum carbon atom number equal to 16 (C16), depending on the target hydrocarbon mixture which is divided into one or more streams.

[0081] A preferred embodiment of the method according to the invention provides for fractionating the effluent (which becomes liquid at room temperature) from the depolymerization process into two fractions: fraction (A) includes hydrocarbons having a carbon atom number from 9 to 16, corresponding to diesel fuel (reference standard EN590), and fraction (B), corresponding to gasoline. A portion of hydrocarbon (C) is also produced, which is in a gaseous state at a temperature of 20 to 30°C and atmospheric pressure. If not used for other purposes, after being purified and stored in an intermediate gasifier, this fraction can be used to generate heat and electricity to keep the equipment running properly. A portion of (A) and / or (B) can also be used for the same purpose.

[0082] At least one of the gaseous fraction (C) and the fraction (B) of hydrocarbons having a carbon atom number from 5 to 8 is preferably, in whole or at least in part, used to generate at least a portion of the electricity and / or heat energy required for the entire process.

[0083] The entire process can be carried out in a plant of a certain scale to be suitable for converting waste plastic raw materials into hydrocarbons or other raw materials at a single location. This eliminates the need to transport the starting materials, which usually have a very low density and require many trips to be transferred from the collection site to the processing site, and the transportation cost is very high.

[0084] The method according to the invention also allows direct conversion of waste plastic materials at the production site, i.e., a mixture of components of plastic polymers contaminated with other substances, into one or more hydrocarbons having properties compatible with the international standards regulating their entry into the market, without the need for further conversion or adjustment downstream.

[0085] The method according to the invention is simple enough to allow the management of the equipment by unprofessionally trained personnel. Except for the loading operation and maintenance of the raw materials, the process is completely automatically managed without any direct intervention by an operator. The method can also be implemented in a relatively small plant, which can be located directly near the site where the waste to be treated is generated or accumulated.

[0086] As previously mentioned, from an energy perspective, the process can be completely autonomous: in this case, whether it is electrical energy or thermal energy, it will be supplied by the hydrocarbon fractions produced by the equipment itself.

[0087] In addition, the method according to the present invention is very effective in the following aspects:

[0088] - The mass conversion rate, defined as the ratio of the weight of the hydrocarbons produced (meeting international standards and marketable) to the weight of the polymer material charged in the process, can even be higher than 65% in the case of self-consumption of energy, and

[0089] - The energy conversion rate, which is equal to the ratio of the energy contained in the hydrocarbons produced to the potential energy of the materials charged in the process, is close to 90% of the total energy and higher than 75% of the net energy (taking into account the self-consumption of energy).

[0090] Self-consumption of energy means using the hydrocarbons produced in the process as fuel to produce the thermal energy and / or electrical energy used in the process itself.

[0091] A series of solutions ensure that the method according to the present invention can always be operated under optimal operating conditions.

[0092] In particular, the method according to the present invention is carried out by precisely controlling the following parameters:

[0093] · The temperature values and temperature trends (heating, maintaining, and decreasing) to which the material is subjected;

[0094] · The pressure values and pressure trends to which the material is subjected;

[0095] · The residence time of the material in each part of the method; and

[0096] · The composition of the internal atmosphere in each step of the method and thus at each point of the equipment implementing the method.

[0097] In order to control the operating parameters of the method according to the present invention and keep them under optimal conditions, various innovative solutions have been implemented.

[0098] The method provides steps for feeding into the "hot" part of the method, including continuously charging a polymer mixture with mixed components derived from waste plastic materials into a pyrolysis reactor. The method according to the present invention provides a method of heating (pyrolyzing) the material in the absence of oxygen until the polymer chains break.

[0099] Step i of the method according to the invention is preferably carried out first by means of a feeding system which allows continuous feeding without introducing the air present in the external storage environment of the waste plastic material to be fed into the reactor. A barrier against a potentially oxygen-containing atmosphere can be obtained with a series of advantageous measures which can be implemented in series to obtain a synergistic effect.

[0100] More specifically, the mixture, optionally after separating non-plastic materials and reducing their size, is fed into the feeding system of the reactor (2) through a hopper or two or more successive hoppers (4), and the oxygen present in the atmosphere of the incoming material is substantially eliminated inside the hopper.

[0101] The waste plastic material is fed into the reactor through one or more extruders (1) which crush, soften the raw material and push it forward towards the reactor, thus eliminating the gas remaining in the material in the direction of the loading hopper (4).

[0102] The heating of the waste plastic is achieved by sufficient heat supply on the outer surface of the extruder (1), preferably by circulating a heat carrier (molten salt) in a suitable jacket (1a).

[0103] A valve is installed at the end part of the extruder (1) which closes the extruder in case of failure and prevents the internal gas of the reactor from entering the loading hopper and also prevents the gas entering the loading hopper from entering the reactor.

[0104] As described below, a feeding extruder with a specific design sequentially performs a series of functions:

[0105] - Continuously loading the waste plastic, i.e. the plastic mixture to be treated, over time;

[0106] - Heating the mixture to the softening point without causing "cracking" of non-chlorinated polymers;

[0107] - Compressing the mixture and discharging the air contained therein towards the loading hopper;

[0108] - Forming a moving plastic "plug" which separates the atmosphere of the loading hopper from the atmosphere of the reactor;

[0109] - "Cracking" possible chlorinated polymer fractions at about 150 °C and possibly discharging the resulting gases (degassing), which gases may contain most of the chlorine possibly present in the fed plastic mixture;

[0110] - Forming a moving plastic plug which separates the degassing zone from the atmosphere of the reactor at a slightly overpressure (20 - 100 mBar).

[0111] The apparent density of the loaded plastic material can vary widely: from very low densities (less than 50 kg / m 3 ) to 900 kg / m 3 and so on.

[0112] The special design of the metering and feeding screw (11) of the extruder and the design of the first part of the extruder screw ensure a constant material feed.

[0113] As Figure 3 shown, the screw (11) in the initial region (1b) (i.e., the loading region) preferably has a configuration that allows it to move a larger volume relative to the body part equipped with a heating jacket. Preferably, this ability is achieved by using a screw shaft with a reduced size relative to the diameter of the extruder and / or by increasing the pitch in the first part (i.e., the loading step of the extruder).

[0114] After the first feeding section, in the subsequent section (1c) of the extruder, the extruded plastic material is directly conveyed to the reactor (2). The diameter of the shaft of the screw (11) is preferably wider than that of the previous section, and / or the pitch of the extruder screw is reduced in order to crush the plastic material and push it forward in the opposite direction to the direction in which the material is fed, eliminating all the gas trapped in the plastic material.

[0115] This gas from the loading hopper of the extruder must be excluded because it may contain a certain amount of air oxygen that is incompatible with the method of the present invention. In the last part of the extruder, the plastic material is heated to the softening temperature and then kneaded to form a uniform and dense mixture.

[0116] Part of the thermal energy required to heat the polymer mixture during feeding and softening is provided by the friction generated by the extruder (1) between the mixture itself and the mechanical components of the extruder, and part is provided by heating the extruder, preferably by means of a jacket (1a) in which molten salt circulates. This heating solution makes the extruder particularly resistant to the presence of materials such as debris, stones, metals, etc., which may fill the interior of the material due to a distance greater than 5 mm between the screw and the screw wall.

[0117] The purpose of heating the extruder (1) from the outside is also to form a thickened and dense waste plastic material that, once it enters the main reactor, will not float but will immediately sink into the liquid bath. The plastic material inherently has a very low heat transfer coefficient and usually appears in the form of a wrinkled film. If not properly processed in step i of the method according to the present invention, the fed plastic material will tend to float on the liquid bath present inside the reactor, and the heating will be very slow.

[0118] In the part of the extruder closest to the reactor to be fed, the screw (11) is designed to further heat the plastic material to a temperature of at least about 150 °C. The minimum temperature is determined by the softening temperature of the plastic material. In the extruder used in the method according to the invention, the material is heated to about 180 °C, i.e., to a temperature below the cracking temperature of most plastic polymers. However, the cracking must take place inside the main reactor.

[0119] In order to obtain a product with distinct characteristics near the desired value, the cracking must occur as much as possible under the wall conditions of the main reactor. Therefore, the process is carried out so as to obtain as little cracking as possible in the feed extruder. It is believed that at 150 °C, the chlorine-containing plastic fraction is at least partially vaporized, releasing most of the chlorine present in the PVC (polyvinyl chloride) component that may be present in the waste plastic material of the feed. In a preferred embodiment, the extruder (1) is capable of discharging the gas that may thus be produced through a specific degassing duct ( Figure 3 not shown in the figure).

[0120] In the last part of the extruder (1), the dense gas-free plastic material also serves as a sealant to separate the degassing zone from the internal atmosphere of the reactor.

[0121] The depolymerization reactor for the waste plastic material provided by the extruder is a reactor (2) with a cylindrical cross-section of the vertical axis, preferably having a circular bottom and / or preferably having an upper flat cover with the same horizontal cross-section as the reactor, which is useful for maintaining the reactor itself. The agitator (7) is installed on the axis of the reactor and is equipped with a geared motor, which can operate at a speed of 30 - 40 rpm as a non-limiting example.

[0122] The non-uniform mixture of plastic material is in liquid form inside the reactor (2) and is maintained within a temperature range of about 300 °C to 400 °C. The heating of a single reactor is carried out by means of heat transfer caused by a molten salt stream, which is heated to a temperature of 450 °C to 550 °C and circulated through a jacket with a special design that surrounds the entire reactor.

[0123] The depolymerization process mainly takes place along the inner wall of the reactor. When approaching the wall (at a temperature close to the molten salt temperature), the polymer molecules in the liquid bath undergo chain breakage (pyrolysis) due to the effect of temperature. When the length of the single chain is such that the boiling point of the corresponding substance is lower than the temperature of the liquid bath at this point, at the working pressure, the single chain undergoes a phase change, turns into a gas and forms bubbles, separates from the wall and moves towards the surface of the polymer material liquid bath.

[0124] The gas phase formed inside one or more reactors consists of molecules that react with each other (recombine or reform), undergo further breakage or also react with the liquid phase. Once they reach the surface, the substances thus formed produce a gas stream consisting essentially of hydrocarbons at a temperature slightly higher than the liquid bath temperature (300 °C - 400 °C).

[0125] The heat flux present on the walls is affected by the following factors:

[0126] - The heat exchange coefficient of the plastic mixture present in the reactor in liquid form at 300 - 400 °C;

[0127] - The average thickness of the bubble layer generated on the walls and moving towards the bath surface;

[0128] - The thickness of the carbon layer formed on the walls due to the depolymerization process occurring on the walls themselves;

[0129] - The properties of the reactor wall metal, suitable for ensuring that the provided mechanical properties are maintained even in the presence of chemically aggressive substances;

[0130] - The temperature and flow rate of the liquid salt circulating in the reactor jacket;

[0131] - The flow rate of the plastic material in contact with the internal components of the reactor caused by the stirrer.

[0132] Each reactor (2) is characterized by:

[0133] - The height of the liquid bath is preferably at least 500 mm and ensures control of the time for bubbles to pass through the liquid bath;

[0134] - The area above the liquid bath has a minimum volume to minimize the residence time of the gas generated inside the reactor, since the small chamber available for the gas phase above the liquid bath minimizes the change in the gas residence time inside the reactor with variations in the process parameters;

[0135] - A screw extruder (1), which ensures continuous feeding and is located directly above the free surface of the liquid bath;

[0136] - A stirrer (7), used to ensure a controlled movement of the liquid bath contained in the reactor in order to:

[0137] - Keep the liquid bath homogeneous;

[0138] - Facilitate the dissolution of the plastic material loaded by the feed extruder;

[0139] - Keep the temperature variation inside the reactor within a range where the maximum difference between one point and another does not exceed 3 °C;

[0140] - Ensure that the polymer liquid in contact with the inner wall of the reactor maintains a constant flow; and

[0141] - Maintain a layer with a controlled carbon thickness on the inner wall of the reactor.

[0142] In fact, the depolymerization process provides the release of one carbon atom for each pair of breaks in the polymer chains. These carbon atoms are fixed at the positions where the breaks occur and form a layer of carbon along the wall. The stirrer (7) is preferably provided with a suitable scraper (8) with a spring movement system to ensure scraping the continuously formed carbon layer on the reactor wall, thus maintaining a fixed controlled thickness.

[0143] The spring movement system ensures that the scraper has sufficient rigidity to maintain a carbon layer of constant thickness through the scraper, but when the rigid body deforms (slips) between the scraper and the wall, the scraper gives way. For this purpose, the scraper (8) conforming to the shape of the wall is mounted on supports rotating around an axis at a distance of 10 - 30 cm from the reactor wall. The scraper is maintained in abutment relative to stops by springs (9), and the stops can be adjusted to ensure the desired distance from the wall.

[0144] In the presence of rigid foreign objects, the scraper group may move away from the wall, thus overcoming the force of the spring. In the case of forming too much carbon layer, the scraper mounted on the rotor scrapes the coal that may have formed in a series of continuous channels until it reaches the adjustment stop at the desired distance from the wall. The presence of the rigid body may depend on the fact that the plastic material loaded into the reactor may be contaminated by foreign objects (such as sand, stone, metal, glass, etc.) that do not undergo the depolymerization process.

[0145] After continuing the depolymerization process, the integrity of the carbon layer formed on the inner surface of the reactor that may have been lost is subsequently restored. The thickness of this carbon layer can be 0.5 to 20 mm. A carbon layer thickness equal to 5 - 6 mm represents a preferred value for obtaining good protection of the inner wall of the reactor.

[0146] The reactor (2) according to the present invention includes an external jacket (5) through which a liquid salt (i.e., molten salt) flows to ensure the necessary heat supply to the reactor. The structure of the feed circuit of the molten salt to the reactor jacket should be such that in case of an abnormal situation, all the salt falls by gravity into a safety tank of the molten salt located below the reactor. Especially during operation, the salt is pushed into the jacket from below. A series of fins ensures a uniform distribution of the molten salt flow in the jacket and ensures a speed suitable for maximizing the heat transfer coefficient. Once the salt reaches a position slightly higher than the surface of the internal bath solution of the reactor, it collapses into the external chamber and is transported back to the salt tank from there under the action of gravity.

[0147] In a preferred embodiment, in the process according to the invention, the molten salt for heating the reactor (2) and the feed system comprises a binary, ternary, quaternary salt or a mixture thereof, or consists of a binary, ternary, quaternary salt or a mixture thereof, having a melting temperature range of about 100 °C to about 250 °C. Preferably, the molten salt comprises a mixture of sodium nitrate and potassium nitrate, or consists of a mixture of sodium nitrate and potassium nitrate, and even more preferably in a weight ratio range of 2:3 to 3:2.

[0148] In a preferred embodiment of the process according to the invention, the solid residue accumulated in the reactor (2) is conveyed towards the outside of the reactor (2) by a discharge system comprising a screw conveyor (6).

[0149] The reactor is equipped with a screw conveyor (6) which is connected to the bottom of the reactor (2) and the discharge head is located above the liquid bath level inside the reactor. The screw is used to discharge the solid components accumulated during the process inside the reactor. Like the reactor, the screw is heated at a temperature of 450 - 550 °C by a jacket with a molten salt flow. The discharge screw advantageously allows the extraction of the solid residue of the reactor to which it is connected without isolating the reactor from other reactors operating in parallel. It may be necessary to insert a shut-off valve between the reactors, which poses safety problems to the equipment (an abnormal shutdown can lead to a closed chamber, overpressure and explosion hazard). The screw is constructed such that the valve allowing the discharge of the solid residue (while ensuring the separation between the internal atmosphere of the reactor and the inert atmosphere of the management system of the solid residue) is located in an area above the liquid level of the liquid plastic mixture present inside the reactor to ensure its operation under more favorable conditions and easy maintenance, since the valve is not contaminated by the liquid plastic. The said screw is heated by a system using molten salt as a heat carrier.

[0150] The depolymerization process requires the efficient conversion of most of the polymer components of the waste plastic material, i.e., the matrix loaded into the reactor, into gas. The non-polymer components and the carbon released by the cleavage of the polymer molecules accumulate at the bottom of the reactor. The size of the reactor is adjusted so that, in the case of a typical starting matrix composition using PLASMIX, after a period of 4 - 6 days, the accumulation of solid residue within a single reactor accounts for one-third of the liquid bath volume.

[0151] In order to maintain the continuous operation of the process according to the invention under controlled conditions (in the case of multiple pyrolysis reactors) and obtain a solid residue which complies with the specifications of special non-hazardous waste when discharged, a method for managing the individual reactors operating in parallel has been determined so that the solid residue can be emptied after being completely hydrocarbon-free.

[0152] When the solids content of one of the reactors equals the limit amount, the feeding of the plastic material is interrupted by the depolymerization step ii until the polymer components are completely converted into gas and then it continues.

[0153] In a preferred embodiment, the molten salt heating system remains continuously open because the reactor is connected to all the other active reactors and thus prevents the gases from the other active reactors from condensing in the colder reactor. Once the gasification process of the polymer components is completed in the reactor to be emptied, the reactor (2) is emptied by a discharge screw which is equipped with two valves at the discharge end. The valves are opened alternately to allow the solid material to pass through without bringing the atmosphere of the system receiving the solid material into contact with the atmosphere of the reactor. Once the required amount of solid material has been discharged, the exhaust system is blocked and the loading system is reactivated by the extruder. The feeding rate of the active reactors is managed so as to obtain a sufficient total gas flow during the discharge cycle of a single reactor.

[0154] The discharge screws of each reactor are in turn connected to each other by screw conveyors. These conveyors move the discharged solid material which remains at a high temperature (450 °C - 550 °C) and is highly reactive in the air of the cooling box (the carbon released by the depolymerization reaction is absolutely anhydrous and reacts immediately with the oxygen present in the air without a flame).

[0155] The solid is discharged from here into a second tank which is suitable for transporting the solid residues classified as special non-hazardous waste and is treated in accordance with legal regulations. By continuously injecting a nitrogen stream, the entire transport and storage system as well as the loading system into the transport tank are kept in an inert atmosphere.

[0156] As described above, in a preferred embodiment of the present invention, the method according to the present invention comprises a step of preparing the mixture used in step i starting from waste plastic materials by a pretreatment step which comprises separating the elements containing non-plastic materials and, if necessary, reducing the plastic materials to a size not exceeding 50 mm x 50 mm x 4 mm.

[0157] In this embodiment, before using them in step i of the method according to the present invention, the incoming waste plastic materials are preferably mechanically treated by one or more of the following methods to make them suitable for loading in the depolymerization step (and production line):

[0158] · Separating the individual plastic components constituting the incoming mixture (the material is usually in bale form);

[0159] · Reduce the size of the plastic material to less than 100 mm x 100 mm x 4 mm and separate the components with dimensions less than 10 mm x 10 mm x 10 mm, as it consists mainly of sand, soil and glass, i.e., inert components, which are useless for the depolymerization process;

[0160] · Separate the polymer components incompatible with the method (most importantly PET, PVC), which can be achieved by flotation in water, by manual selection on the production line or by an automatic separation optical system. In the case of using flotation in water, the effluent material can be further washed to remove paper and centrifuged to remove most of the water used for flotation and washing;

[0161] · Separate the magnetic and non-magnetic metal parts;

[0162] · Further reduce the size to less than 50 mm x 50 mm x 4 mm; and

[0163] · Dry, where in order to make the most of the thermal energy required to operate the equipment and avoid the formation of oxygen in the depolymerization reactor, the fumes discharged from the heating boiler with molten salt can be used together with a suitable fume / air exchanger to generate the hot air stream required for drying the material.

[0164] As a non-limiting example, the material to be depolymerized is fed into the reactor (2) through three successive feed hoppers (4), and the three feed hoppers are separated by two alternately opened valves respectively. When the valve between the first storage tank and the second storage tank is opened and the valve between the second storage tank and the third storage tank is closed, the material is loaded from the first storage tank into the second storage tank. Once the valve between the first storage tank and the second storage tank is closed, the atmosphere in the second storage tank will be replaced by 99.9% pure nitrogen to eliminate any trace of oxygen. Once this process is completed, the valve between the second tank and the third tank is opened, and the material is transferred to the third storage tank for continuous feeding of the reactor.

[0165] At the end of the transfer operation of the material from the second storage tank to the third storage tank, the valve between the second storage tank and the third storage tank is closed, and a nitrogen stream is introduced into the second storage tank again to eliminate any trace gas that may be contaminated by the internal atmosphere (hydrocarbon) of the reactor before opening the valve between the first tank and the second tank. The gas leaving the second storage tank under the push of nitrogen is sent to the flare stack and processed according to legal regulations.

[0166] In a preferred embodiment, the method according to the present invention further comprises:

[0167] iii. A step of depolymerization in the gas phase, wherein the gaseous fluid exiting the reactor (2) after step ii is fed into at least one secondary reactor (10), where a heating cycle is carried out at 450 to 500 °C for less than 1 second and then maintained at a temperature of 400 °C to 480 °C for about 15 seconds to about 30 seconds.

[0168] In the step of depolymerization in the gas phase, as described above, the gas exiting the reactor (2) of step ii undergoes a second conversion process within the secondary reactor (10). The secondary reactor (10) consists of a tube bundle, and at a temperature of 450 - 550 °C, a controlled molten salt flow passes through the tube bundle on the shell side. The tube bundle applies a very rapid heating cycle (less than one second) to the gas generated by the main reactor and maintains it at a set point temperature of 400 °C to 480 °C for a predetermined time, typically from 15 seconds to 30 seconds. This transformation changes the length distribution of the hydrocarbons present in the gas through reactions of further chain breaking and recombination of the polymer chains, thereby affecting the average value of the hydrocarbons (e.g., producing a lighter mixture centered on the composition of a specific main product according to specific requirements) and the variation (to maximize the yield of the desired product). In this way, the method can be adjusted such that hydrocarbons with a distribution conforming to the hydrocarbon definition are produced to the greatest extent (e.g., in the fields of automotive gasoline, automotive diesel, kerosene, naphtha, diesel fuel for heating, fuel oil) or a specific mixture of liquid and gaseous hydrocarbons is reproduced to determined specifications.

[0169] The subsequent condensation step is preferably carried out in at least two condensers (12) immediately downstream of the secondary reactor (10) to suddenly cool the gas mixture (within less than one second) to arrest the depolymerization process. As a non - limiting example, in the specific case of producing a liquid fuel mixture conforming to EN 590:2013,

[0170] · The first condenser consists of a tube bundle cooled by forced air; the outlet temperature of the gas mixture is about 285 °C. The condensate mainly composed of long - chain hydrocarbons with more than C16 is fed into the primary reactor for further depolymerization reaction;

[0171] · The second condenser consists of a tube bundle cooled by forced air; the outlet temperature of the gas mixture is about 152 °C. The condensate mainly composed of hydrocarbons from C9 to C16 is the main product of the process, and its composition is compatible with EN 590:2013;

[0172] · The third condenser consists of a liquid scrubber. The condensed and cooled liquid is used to wash and cool the incoming gas stream. The outlet temperature of the gas mixture is about 30 °C. The condensate mainly composed of hydrocarbons from C5 to C8 is used for the electrical and thermal energy required to operate the equipment.

[0173] The process according to the invention comprises a thermal cycle which provides an outflow mixture of hydrocarbons which is very stable over time, with considerable advantages over mixtures obtained by known techniques which must be further stabilized to avoid conversion of certain components.

[0174] In one embodiment, the present invention relates to a mixture of hydrocarbons obtainable by the above process, which can be used as a fuel, which is practically free of polycyclic aromatic hydrocarbons (content less than 2% by weight / total weight) and has a very low sulfur content (at least 1-2 orders of magnitude compared to the stricter limits for marine fuels provided for by international standards of 1 January 2020 (e.g. European Guideline 2016 / 802)). As a non-limiting example, in the hydrocarbon mixture obtainable according to the invention, the sulfur content may comply with the reference standard EN590 (not exceeding 10 mg / kg).

[0175] It has also been found that the fuel of the invention obtainable by the process described above contains a high percentage of olefinic compounds having a divinyl type bond (preferably 35 to 45% or 38 to 40% by weight per total weight of the mixture).

[0176] In one embodiment, the present invention relates to a reactor (2) with a cylindrical cross-section and a vertical axis, the reactor having: an agitator (7) equipped with a gear motor mounted on the axis of the reactor, wherein the interior of the reactor (2) includes a lower area for forming a polymer bath and an upper area for receiving gas effluents derived from the depolymerization of the polymer bath, the volume of which is less than 20% of the total volume of the interior of the reactor (2), preferably less than 15% of the total volume of the reactor (2), so as to minimize the residence time of the gas effluents generated inside the reactor; a screw conveyor (6) connected to the bottom of the reactor (2) and with the discharge head located above the liquid level of the polymer bath present inside the reactor; and a jacket (5) for heating outside the reactor body and through molten salt, which is connected to a system for transporting, heating and storing molten salt (3).

[0177] The bottom and walls consist of a double wall. The molten salt used as the heat carrier passes through the chamber between the two walls that make up the side walls and the bottom of the reactor. The salt enters the center of the bottom and rises along the jacket in a series of spaced guides to form a helical path to ensure a uniform distribution of velocity, thereby minimizing the temperature difference between various points of the jacket and ensuring sufficient velocity to ensure a high heat transfer coefficient. Once the salt reaches the top of the side wall at the highest level of the liquid in the reactor, the salt is discharged from the jacket and falls into an external collector, which returns the salt to the molten salt tank. The feed extruder feeds the material directly above the free surface of the internal plastic bath in the reactor. The reactor is designed to minimize the free volume above the free surface of the molten plastic material to minimize the residence time of the gas generated by the depolymerization of the introduced plastic mixture in the reactor. The height of this free area is determined by the height of the feed extruder.

[0178] The following examples are provided to illustrate some embodiments of the present invention without limiting its scope.

[0179] The material balance is shown in Table 3, where the data is reported for a "standard" plant with a PLASMIX processing capacity of 9,000 tons / year (of course, different production capacities are possible).

[0180] Table 3

[0181]

[0182]

[0183] Table 3 – Material balance - data for a PLASMIX processing plant with a capacity of 9,000 tons / year (this data is indicative and also takes into account the dependence on the specific composition of the raw materials).

[0184] Table 4 shows again for indicative purposes the heat and electricity requirements of a PLASMIX processing plant with a capacity of 9,000 tons / year.

[0185]

[0186]

[0187] Different mixtures of waste plastic materials were tested in a plant implementing the method according to the present invention. In particular, many "batches" of the above-mentioned waste commonly referred to as "PLASMIX" were used.

[0188] In any case, the method according to the present invention carried out in this plant ensures the best utilization of the energy content in all types of waste tested.

[0189] The environmental impact of the device according to the invention for implementing the method according to the invention is not significant. In fact, the pretreated liquid effluent is released into the environment at a very limited flow rate, and its "pollution" characteristics are much lower than the legally prescribed limits. At the same time, the gas emissions from the burner also provide heat for the process and power the electric generators that produce the electricity required for the process. And, if sold, its impact is far lower than the legally prescribed limits. And finally, the solid emissions are special non-hazardous wastes (basically the coal ash present in the waste plastics and possible mineral feedstocks, or salts produced by neutralizing the possible acids generated by the untreated plastic components).

[0190] The following provides some data on the economic advantages related to the depolymerization method of waste plastic materials according to the invention.

[0191] For simplicity, a pretreatment device with a full production capacity of 9,000 tons / year (a more conservative assumption) is referred to. Scale factors are possible in cases of different production capacities, especially in cases of larger production capacities.

[0192] The economic advantages obviously depend on different parameters, which in turn depend on the specific location of the device and the characteristics of the waste to be treated. The main assumptions adopted in the analysis reported herein are summarized below.

[0193] The following main data are used:

[0194] · Input of raw materials (referring to the worst plastic waste materials, PLASMIX): 9,000 tons / year (higher production capacities can be easily achieved through multiple units operating in parallel, while lower production capacities may reduce the economic efficiency of the investment)

[0195] · Diesel production: 3,928 tons / year.

[0196] Considering factors including the costs of the various components of the device, civil engineering, personnel, and the estimated average annual income, the method according to the invention is economically convenient and ensures excellent economic returns in the short term in addition to having great advantages from an ecological perspective and in terms of waste upgrading.

[0197] The method according to the invention allows waste plastic materials, even of poor quality, to be converted into excellent hydrocarbon mixtures with a very high yield.

[0198] The method according to the invention is particularly advantageous in terms of environmental protection (it avoids releasing plastic waste into the ocean or on land, or even in landfills), and since it allows the production of products (non-limiting examples: diesel, electricity), it improves the utilization rate of waste plastics and avoids the use of petroleum derivatives.

[0199] The applicability for depolymerizing waste materials is very efficient, approaching 90% before self-consumption and having a net value exceeding 75%. On the other hand, the efficiency of competing technologies (such as used in incinerators or cement plants) is significantly reduced (the overall efficiency of incinerators is usually far lower than 10%).

[0200] The usage test was conducted through an experimental device, which included a three-cylinder aspirated diesel engine with an exhaust volume of 1,028 cm 3 , a stroke of 77.6 mm, a cylinder diameter of 5.0 mm, a compression volume ratio of 17.5:1, a common rail injection system, and a maximum intake pressure of 1,400 bar.

[0201] The test was carried out by supplying the fuel "A" (diesel fraction) of the present invention or standard diesel fuel (IES 590 (standard diesel fuel for motor vehicles)) to the above experimental device, and similar performances in terms of engine performance and NOx, PM, CO, and HC emissions were obtained.

[0202] The following table summarizes the obtained results:

[0203]

[0204]

[0205] Fuel: A = Fuel according to the present invention; C = Diesel IES 590 (comparative example)

[0206] Load = Position of the accelerator pedal

[0207]

[0208]

[0209] It has been found that in certain test scenarios, the level of flue gas generated using the fuel according to the present invention is lower than that generated using the control fuel.

Claims

1. A method for the depolymerization of waste plastic materials, wherein the method comprises the following steps: i. Loading a mixture containing waste plastic materials into a feeding system comprising at least one screw extruder (1) in an oxygen-free atmosphere, and optionally heating the screw extruder by circulating molten salt in a suitable jacket (1a) located outside the extrusion chamber, the heating temperature being such that the temperature of the material emerging from the screw extruder is between 150 and 180 °C; ii. Directly loading the waste plastic materials from the screw extruder in the previous step into a reactor (2), where a liquid bath of polymer material is formed, and performing a high-temperature pyrolytic depolymerization step of the complex polymer chains present in the waste plastic materials in the absence of oxygen, thereby forming a gas effluent, wherein the volume of the area available for the gas effluent above the liquid bath in the reactor (2) is less than 20% of the total volume of the reactor to minimize the residence time of the gas generated within the reactor, and heating the reactor (2) to 300 to 450 °C by means of a molten salt flow in a jacket (5) outside the reactor body, the jacket being connected to a system for transporting, heating, and storing molten salt (3), and the reactor (2) being equipped with a screw conveyor (6) connected to the bottom of the reactor (2) and having a discharge head located above the liquid bath level present inside the reactor for removing the solid residue formed after depolymerization; wherein in step ii, a controlled movement of the liquid bath contained within the reactor is ensured by a stirrer (7) so as to keep the liquid bath homogeneous; to facilitate the dissolution of the waste plastic materials loaded by the screw extruder; to ensure a constant flow of the polymer liquid in contact with the inner wall of the reactor; and to maintain a layer with a controlled carbon thickness on the inner wall of the reactor.

2. The method according to claim 1, wherein the mixture containing waste plastic materials is a mixture consisting mainly of waste plastic materials.

3. The method according to claim 1, wherein the volume of the area available for the gas effluent above the liquid bath is less than 15% of the total volume of the reactor.

4. The method according to any one of claims 1 - 3, wherein the waste plastic materials fed in step i are prepared from waste plastic materials by a pretreatment step comprising separating elements consisting of non-plastic materials and, if present, waste plastic materials incompatible with subsequent processes from polymers suitable for depolymerization.

5. The method according to claim 4, wherein the pretreatment step further comprises reducing the size of the waste plastic materials to not more than 50 mm x 50 mm x 4 mm.

6. The method according to any one of claims 1 - 3, wherein the mixture, optionally after separating non-plastic materials and / or incompatible materials and reducing the size, is fed into the feeding system of the reactor (2) through a hopper or successively two or more hoppers (4), wherein the oxygen present in the atmosphere of the incoming material is substantially eliminated.

7. The method according to any one of claims 1 - 3, further comprises: iii. In the step of depolymerization in the gas phase, the gas effluent leaving the reactor (2) after step ii is fed into at least one secondary reactor (10), where a heating cycle is carried out at a temperature of 450 to 500 °C for less than 1 second, and then maintained at a temperature of 400 °C to 480 °C for a time of 15 to 30 seconds.

8. The method according to any one of claims 1 - 3, wherein the solid residue accumulated in the reactor (2) is conveyed out of the reactor towards the outside by a discharge system including an externally heated screw conveyor (6).

9. The method according to any one of claims 1 - 3, wherein the molten salt comprises a binary, ternary, quaternary salt or a mixture thereof, having a melting temperature range of 100 °C to 250 °C.

10. The method according to claim 9, wherein the molten salt consists of a binary, ternary, quaternary salt or a mixture thereof.

11. The method according to claim 9, wherein the molten salt comprises a mixture of sodium nitrate and potassium nitrate.

12. The method according to claim 11, wherein the molten salt consists of a mixture of sodium nitrate and potassium nitrate.

13. The method according to claim 11, wherein sodium nitrate and potassium nitrate are in a weight / weight ratio range of 2:3 to 3:

2.

14. The method according to any one of claims 1 - 3, wherein at the end of the depolymerization step, the following are obtained: fraction A, which contains hydrocarbons having 9 to 16 carbon atoms of diesel fuel conforming to standard EN 590:2013; fraction B of hydrocarbons having 5 to 8 carbon atoms and / or fraction C of hydrocarbons that are gaseous at atmospheric pressure and a temperature of 20 to 30 °C.

15. The method according to claim 14, wherein at least one of fraction B of hydrocarbons having 5 to 8 carbon atoms and gas fraction C is at least partially used to generate at least a part of the electrical energy and / or thermal energy required for the entire method.

16. The method according to claim 14, wherein at least one of fraction B of hydrocarbons having 5 to 8 carbon atoms and gas fraction C is entirely used to generate at least a part of the electrical energy and / or thermal energy required for the entire method.

17. A reactor (2) for performing a high-temperature pyrolysis step of complex polymer chains present in waste plastic materials from an extruder according to the method of any one of claims 1-16, the reactor having: a cylindrical cross-section and a vertical axis, an agitator (7) equipped with a gear motor mounted on the axis of the reactor, wherein the interior of the reactor (2) includes a lower region adapted to form a polymer liquid bath and an upper region adapted to receive the gaseous effluent derived from the depolymerization of the polymer liquid bath, the volume of the upper region being less than 20% of the total interior volume of the reactor (2) to minimize the residence time of the gaseous effluent generated inside the reactor; a screw conveyor (6) connected to the bottom of the reactor (2) and having a discharge head above the level of the polymer liquid bath present inside the reactor for removing the solid residues formed after depolymerization; and a jacket (5) outside the reactor body and heated by molten salt, which is connected to a system for transporting, heating and storing molten salt (3), wherein the agitator (7) includes a scraper (8) conforming to the shape of the wall, and the scraper is mounted on a support that rotates about an axis spaced 10-30 cm from the wall of the reactor and is maintained in butt against a stop by a spring (9), the stop being adjustable to ensure the desired distance from the wall.

18. The method according to claim 17, wherein the volume of the upper region is less than 15% of the total volume of the reactor (2).

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