Depolymerization process of plastic waste material

Abstract

A process for depolymerizing waste plastic material and producing pyrolysis oil is disclosed that includes specific reactor settings. The arrangement includes a reactor having a recovery loop provided with a centrifugal pump and a shell and tube heat exchanger. The process has high efficiency and versatility, and is easy to expand.

Description

Technical Field

[0001] The present invention relates to the field of depolymerization of plastic waste materials into new products including hydrocarbon oils, which have valuable and useful properties. In one aspect, the present invention relates to a process for converting plastics into liquid hydrocarbons, in particular liquid hydrocarbons for use as hydrocarbon feedstocks. Background Art

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

[0003] One attempt to mitigate this impact is to recycle plastic materials from household and industrial waste, allowing part of these materials to re-enter the production cycle. This would involve further positive outcomes, such as less use of fossil hydrocarbon sources to produce plastic products.

[0004] However, various factors indicate that this solution alone is not sufficient to achieve sustainability goals. In fact, mechanical recycling of plastic materials often produces materials of lower quality, is relatively costly, cumbersome, and is not suitable for municipal waste where some plastics are mixed with a variety of different materials.

[0005] Therefore, most of the plastic waste is either used as thermal energy in facilities such as incinerators, or simply stored in landfills, which, as mentioned, contribute to the degradation of the earth's environment by increasing CO2 emissions and by releasing harmful chemicals.

[0006] In view of the foregoing, many attempts have been made in the past to efficiently reprocess feedstocks of waste plastics into liquid hydrocarbon products having valuable and useful properties, particularly as fuels.

[0007] Thermal depolymerization is a basic process in which plastic waste materials are converted into liquid fuels (pyrolysis products) by thermal degradation and optionally catalytic degradation in the absence of oxygen. Plastic waste is typically first melted in a stainless steel chamber under an inert purge gas such as nitrogen or methane. Then, in the same or a different chamber, further heating and optionally a catalyst are provided to crack the polymer molecules of the molten material into a gaseous state consisting of relatively short hydrocarbon chains.

[0008] The hot pyrolysis gases are then condensed in one or more condensers to produce a hydrocarbon distillate (pyrolysis oil) including linear and branched aliphatic hydrocarbons, cyclic aliphatic hydrocarbons, and aromatic hydrocarbons.

[0009] One of the key aspects of the above depolymerization technology is the difficulty in supplying heat to the molecules of the plastic material, since the plastic melt exhibits high viscosity and the plastic is generally a poor conductor of heat. Although this problem may be ignored in small-scale equipment, the operation of large-scale equipment involves a large number of issues regarding heat introduction, so it cannot be ignored in the case of scale-up.

[0010] According to US patent 9,920,255, heat can be supplied to the depolymerization reactor contents by adding fractions obtained from crude oil as solvent to the plastic melt, thereby reducing the viscosity of the plastic melt solution supplied to the depolymerization reactor relative to the viscosity of the plastic melt. To provide the heat, the solvent is preheated to at least 150° C., preferably 200° C. to 300° C. This solution will also allow reducing the heat transferred by the reactor walls, thereby reducing the risk of overheating of the plastic, and facilitating the stirring of the reactor contents. However, in order to significantly reduce the viscosity, large amounts of solvent are required, which, in addition to reducing the productivity of the plant, are not sufficient to avoid the need to provide heat through the reactor jacket. In addition, the logistics and equipment required for handling and supplying the oil will greatly increase the complexity of the plant.

[0011] U.S. Patent 5,917,102 describes a depolymerization reactor design in which a portion of the reactor contents is conveyed to an external circulation system connected to the reactor as a protection against overheating. The circulation system comprises an oven / heat exchanger for providing heat and a high output pump, in particular a rotary pump, for circulating the reactor contents. In order to avoid corrosion of the pump caused by metal parts entrained in the liquid reactor contents, the reactor design comprises a restricted area called a "riser", which is not subject to turbulence and in which solids can settle. Positioning the discharge point for the external loop at the upper end of the riser allows the liquid to circulate without entraining solids. The main problem with this solution is that the riser structure complicates the reactor design, making it more expensive. In addition, due to the nature of the plastic feedstock and the substances formed during the depolymerization process, the function of the fixed riser structure is easily degraded by fouling caused by materials deposited thereon, thereby requiring frequent shutdowns of the reactor to clean / replace the structure.

[0012] In view of the above, an object of the present invention is to provide a plastic waste depolymerization process characterized by high production of liquid hydrocarbons as pyrolysis products, efficient and smooth heat transfer to the reactor contents, continuous and reliable operability associated with low or no fouling, and easy scale-up. Summary of the invention

[0013] Therefore, one aspect of the present invention is a process for depolymerizing waste plastic material and producing a pyrolysis product, wherein the process comprises the following steps:

[0014] (a) feeding a mixture comprising waste plastic material into a feeding system comprising at least one screw extruder (1) and obtaining a molten plastic material;

[0015] (b) feeding the molten plastic material from the extruder into a depolymerization reactor (2), which is a continuously stirred tank reactor maintained at a temperature of 280 to 600° C. and operated at a pressure of 2.0 to 10 barg, wherein depolymerization occurs to form a gaseous effluent and a liquid effluent;

[0016] (c) directing at least a portion of the liquid effluent produced in the reactor (2) to the charcoal treatment section (6), and feeding the gaseous effluent from the reactor (2) to the condensation unit (3);

[0017] (d) withdrawing at least a portion of the liquid effluent from the reactor (2) and recycling it back to the reactor (2) via a recovery loop comprising a centrifugal pump (4) and a shell and tube heat exchanger (5);

[0018] The process is characterised in that at least 80% of the total heat requirement of step (b) is provided by the shell and tube heat exchanger (5). BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the thermal catalytic process equipment. DETAILED DESCRIPTION

[0020] Preferably, the process is carried out in continuous mode.

[0021] In phase (a), the charging system allows the waste plastic material to be fed to be charged into the reactor (2), preferably in a continuous mode. Care should be taken not to introduce oxygen-containing gases into the system. The barrier to potential oxygen-containing gases can be obtained in different ways, such as nitrogen blanketing or a vacuum system connected to the extruder barrel.

[0022] More specifically, the plastic waste mixture is charged into the feed system of the depolymerization reactor (2) through a hopper or two or more parallel hoppers, and the oxygen present in the gas of the plastic waste material is substantially replaced, for example, by nitrogen purging.

[0023] The process according to the invention is very flexible and can be fed with a wide range of plastic waste compositions including, as an example, heterogeneous mixtures of waste plastic materials in which polyolefins are the most abundant component, but for which further sorting steps are no longer economical. Preferred feedstocks, especially when the pyrolysis products are recycled back to the cracking / refining unit, are plastic waste mixtures in which the polyolefin (PE and PP) content is equal to or higher than 70 wt%.

[0024] The waste plastic material preferably undergoes a pretreatment stage in which it is melted by heating and possibly mixed with an additive, which may be an alkaline material. By means of a melt pretreatment, an inhomogeneous mixture of different kinds of waste plastics can be converted into a large amount of homogeneous plastic composite material. Such a pretreatment is therefore also preferred for the case of pyrolysis without additives.

[0025] The heating temperature in the pretreatment stage is appropriately set according to the type and content of plastic contained in the waste plastic material, so as to suppress the pyrolysis of the plastic material to be treated. Such a temperature is generally in the range of 100° C. to 300° C., preferably 150° C. to 250° C. At a temperature close to 300° C. or higher, HCl in the PVC resin that may be present is eliminated.

[0026] If the waste plastic material is mixed with an alkaline material during the melting / kneading pretreatment process, the gas formed by HCl can be removed via an exhaust system and continuously neutralized or captured. For the melting operation, a common kneading machine, an extruder with a screw, etc. can be applied. The plastic waste is preferably fed into the depolymerization reactor through an extruder.

[0027] The extruder melts the plastic waste, makes it at a high temperature (250-350°C) and injects it into the first depolymerization reactor (2). The extruder can receive the plastic waste cut into small pieces into the feed hopper, convey the material flow in the melting section, and heat the polymer through the combined action of mixing energy and heat supplied by the barrel heater.

[0028] Additives can be selectively incorporated into the melt in order to reduce the corrosiveness of the plastic waste or to increase the yield of pyrolysis products in the reaction section.

[0029] During the extrusion process, one or more degassing steps can be foreseen in order to remove residual moisture present in the product.

[0030] Before being fed to the reactor (2), the melt stream may be filtered to remove solid impurities present in the plastic waste.

[0031] Depending on the amount and particle size of solid impurities, several designs of melt filtration units can be applied.

[0032] It is preferred to use a self-cleaning melt filter that can operate for extended periods of time (days) without requiring human intervention to replace the filter element.

[0033] One preferred design of the melt filter is based on a circular perforated plate as melt filter element, perforated by laser or by machining according to the openings, in which solid contaminants accumulate. The accumulation of impurities may increase the pressure difference over the melt filter. For in-line cleaning of the filter element, a rotating scraper removes the accumulated impurities and directs them to a discharge port, which is opened for a short time to remove process contaminating materials.

[0034] This cycle can be repeated several times (up to several days of operation) without manual intervention or the need to stop production for the time required to replace the filter element.

[0035] Another option for self-cleaning melt filters is based on the application of a continuous filtering metal belt, through which the polymer flow passes. Impurities accumulate on the metal filter, causing a pressure increase. As a result, the blocked filter belt section is pushed out of the polymer passage area and the clean section is then inserted.

[0036] The process is automated and allows operation for extended periods (up to several days) without human intervention or the need to stop production for the time required to change the filter element.

[0037] Any extrusion system may be used, such as a single screw extruder, a twin screw extruder, a twin screw extruder with a gear pump, or a combination thereof.

[0038] In step (b), the depolymerization reactor (2) is a stirred vessel operated at a temperature preferably from 300 to 550°C, more preferably from 350 to 500°C.

[0039] The operating pressure is preferably maintained in the range of 2.5 to 8 barg, more preferably in the range of 3.0 to 7 barg.

[0040] By adopting the above conditions, the melt viscosity of the reactor contents is suitable for uniform mixing by a stirring device. Typically, the melt viscosity measured at a temperature of 400°C is 0.1 to 250 cP, particularly 1 to 100 cP, even more particularly 5 to 50 cP.

[0041] It must be noted that the above melt viscosity values ​​are obtained without the addition of any viscosity reducing agents which may be advantageously dispensed in the process of the present invention.

[0042] However, if desired, the molten mass of waste plastic entering the reactor may preferably be premixed with hydrocarbon oil, preferably recycled oil from a condensation unit, in a dedicated container to facilitate dissolution of the melt into the depolymerization reactor. In this case, the volume ratio of oil / molten mass may be from 0.1:1 to 1:1.

[0043] The depolymerization reactor (2) preferably has a cylindrical part, preferably with a circular bottom. In an alternative embodiment, the bottom of the reactor has a conical or truncated-conical shape.

[0044] Preferably, it has a mixer mounted on the vertical axis of the reactor, the mixer being equipped with a gear motor that rotates the blades of the mixer to maintain the system in a stirred state. The design of the mixer and the power of the motor may vary depending on the reactor contents, volume and shape, however, as a non-limiting example, preferably 0.2 to 4 kW / m 3 , preferably 0.2 to 2 kW / m 3 , more preferably 0.3 to 1.5 kW / m 3 The power input operates the reactor.

[0045] According to the invention, at least 80%, preferably more than 85%, in particular more than 90% of the total heat requirement of step (b) is provided by the shell and tube heat exchanger (5). Thus, the heat supplied to the reactor contents via the reactor wall is less than 10%, preferably less than 5%, more preferably absent. Thus, the reactor (2) does not strictly require a jacket wall for heating the reactor contents. In a specific embodiment, the reactor may be jacketed. In a further preferred embodiment, no heating fluid circulates in the reactor jacket.

[0046] Heat to the external heat exchanger (5) may be provided by any heat transfer fluid suitable for operation at depolymerization temperature or above. Preferably, solar salt or synthetic oil is used. It is highly preferred to use molten solar salt heated to a temperature of 300°C to 570°C.

[0047] In particular, the required heat is exchanged via a shell and tube heat exchanger (5) by flowing the liquid effluent from the reactor (2) within the tubes of the heat exchanger while a heat transfer fluid flows on the process side in the shell.

[0048] The feed circuit (not shown) of the molten salt is constructed in such a way as to prevent leakage of the molten salt. The molten salt is molten solar salt, preferably consisting of a mixture of sodium nitrate and potassium nitrate, even more preferably in a weight ratio of 2:3 to 3:2. The solar salt in turn receives heat from a dedicated furnace, which can be an electric furnace or a fuel furnace. Preferably, the furnace is an electric furnace, more preferably, the electricity comes from a renewable source. In the case of a fuel furnace, part of the recovered oil from the condensing unit (3) can be used to feed the furnace.

[0049] The heat transfer fluid, in particular a molten salt, is circulated into the heat exchanger by using a circulation pump.

[0050] Any available shell and tube heat exchanger may be used, which may be sized according to the common knowledge of a person skilled in the art. In a preferred embodiment, a single shell / single tube pass heat exchanger is used.

[0051] It is also possible to operate with two shell and tube heat exchangers in a series or parallel configuration.

[0052] Preferably, the slurry flows at a velocity of 3-10 m / sec, more preferably 5-8 m / sec, within the tubes of the heat exchanger. It must be noted that velocities below 2 m / sec may result in sedimentation of the slurry.

[0053] The liquid effluent is a slurry of solid material dispersed in a liquid hydrocarbon medium, which is recirculated by a centrifugal pump (4). The type of centrifugal pump that can be used is not limited and any commercially available centrifugal pump having suitable characteristics to pump a liquid effluent having the above-mentioned characteristics can be used.

[0054] It has surprisingly been found that the process of the present invention does not have problems arising from lumps or other solid residues present in the liquid effluent. Without wishing to be bound by theory, this may be due to the fact that the impeller of the centrifugal pump can break up the carbon lumps into fine powder.

[0055] It also constitutes a preferred embodiment of the presence of a coke breaker mounted on the hub of the centrifugal pump shaft.

[0056] According to a preferred embodiment, the portion of the liquid slurry that is recycled to the reactor is withdrawn from a point of the reactor that is different from the withdrawal point of the portion of the liquid slurry that is sent to carbon treatment.

[0057] According to another preferred embodiment, the portion of the liquid slurry that is recycled to the reactor and the portion of the liquid slurry that is sent to the carbon treatment are both withdrawn from the same point and then separated successively.

[0058] The separation between the portion of the liquid slurry directed to the carbon treatment and the portion recycled to the reactor can take place before or after the circulation pump (4). In the latter embodiment, the liquid slurry is first fed into a dedicated container equipped with lower and upper outlet points. The liquid portion directed to the carbon treatment (6) is discharged from the lower outlet point in concentrated form, while the liquid portion to be recycled to the reactor (2) is discharged from the upper outlet point.

[0059] The depolymerization process that takes place within the reactor produces molecules with shortened chain lengths and lower boiling points. This continuously operating chain scission mechanism, especially near the reactor walls, produces an increasingly smaller fraction of molecules that are gaseous at the operating temperature and pressure.

[0060] As a result, the composition within the reactor covers a wide range of hydrocarbons, from methane to heavier products, saturated and olefinic, with straight chain or highly branched structures. Some aromatic products as well as fused ring structures may also be present.

[0061] Those that remain liquid under the operating conditions contribute to the reduction of the liquid mass viscosity. As a result of the depolymerization process and the feed composition, the contents of the reactor (2) can be defined as the coexistence of a liquid slurry phase and a gas phase, wherein solids, in particular carbonaceous and inorganic materials, are dispersed in a liquid hydrocarbon medium.

[0062] At least a portion of the liquid slurry phase is withdrawn from the reactor, preferably from the lower part of the reactor, and constitutes the liquid effluent which is sent to the carbon treatment section (6).

[0063] From an operational point of view, it is preferred that the discharge of the slurry phase from the bottom of the reactor is triggered by detecting, by a density sensor, that the density of the liquid slurry reaches a predetermined value.

[0064] The gas phase of the reactor (2) constitutes the gaseous effluent, which is sent to a condensation unit (3) for further processing.

[0065] The gaseous effluent comprises a mixture of light hydrocarbons which may also include some heavy hydrocarbons and entrained carbon particles. The gaseous effluent is conveyed from the top of the reactor to a condenser (3) which is preferably operated at a pressure slightly below the reactor pressure.

[0066] In order to suppress entrained char, the condenser (3) is preferably designed as a scrubber. The condenser temperature is selected in such a way that the heavy hydrocarbons condense and the light hydrocarbons are released as a gaseous stream. The gaseous stream (H2 and light hydrocarbons) is preferably conveyed to another condensation unit (not shown), preferably operating at a lower temperature than the condensation unit (3) for recovering the oil.

[0067] The operating temperature of the condensation unit (3) can also vary within a wide range, depending on the operating pressure. The temperature relative to atmospheric pressure can be from 20° C. to 200° C., more preferably from 40 to 100° C., in particular from 50 to 90° C. When a higher operating pressure is selected, the temperature range may of course be different.

[0068] When the liquid condensate is subjected to GC analysis, the results are reported by grouping the components according to their retention times using a specific hydrocarbon as an internal retention time reference. The results may show the presence of about 2 wt% or more of compounds having a retention time equal to or less than n-heptane, about 25 wt% or more of compounds having a retention time between n-heptane and n-dodecane, a more abundant fraction of compounds having a retention time higher than n-dodecane and lower than n-octacosane (70 wt% or less), and fractions with longer retention times that may be present in small amounts.

[0069] In a preferred arrangement of the condensation unit (3), a partial condenser is installed on top of the scrubber and operates at a temperature lower than the temperature inside the tower. The condensate flows downward by gravity as reflux to the scrubber. The partial condenser can be installed as a separate piece of equipment or inside the scrubber.

[0070] In an alternative or combined arrangement, a pump recovers the liquid collected at the bottom of the scrubber to the top of the column. The recovered liquid is cooled in a dedicated heat exchanger before being injected into the top of the scrubber as reflux.

[0071] Preferably the condensate of hydrocarbons having more than C7 carbon atoms constitutes the liquid stream which is sent for further processing or to a second depolymerization reactor.

[0072] A further depolymerization reactor may also be present. If present, the second depolymerization reactor is preferably of the same type as the first depolymerization reactor, more preferably a continuous stirred tank reactor equipped with the same recovery loop providing heat to the depolymerization stage by means of a centrifugal pump and a shell and tube heat exchanger.

[0073] The second reactor can be connected in series (sequentially) or in parallel with the first reactor. A sequential arrangement is preferred.

[0074] It is also obvious that one or more of the reactors may be equipped with one or more additional recovery loops each provided with a centrifugal pump and a heat exchanger.

[0075] The depolymerization takes place in the same temperature range, but in order to limit the volatility of the heavy hydrocarbons it is preferably operated at a higher pressure than the first reactor, in particular at a pressure of 3 to 10 barg, preferably 3 to 9 barg, more preferably 3 to 8 barg.

[0076] According to the invention, the depolymerization step (b) may take place in the presence of a catalyst. The latter may be selected from those having depolymerization / cracking catalyst activity in a thermocatalytic process. In particular, it may be selected from metal oxides, heteropolyacids, mesoporous silica, aluminosilicate catalysts such as halloysite and kaolinite, and preferably from zeolites. Among them, particularly preferred zeolites are synthetic Y-type zeolites and ZSM-5.

[0077] In a particularly preferred embodiment, the amount of catalyst feed does not exceed 10 wt%, preferably does not exceed 5 wt%, in particular does not exceed 2 wt% relative to the plastic waste feed.

[0078] In a preferred embodiment, the catalyst is injected into the reactor as a powder dispersed in a hydrocarbon oil, preferably the liquid pyrolysis product (oil) obtained from the condensation unit (3).

[0079] Preferably, the catalyst slurry is prepared in a tank-type continuous stirred vessel, where the catalyst is poured from a dedicated silo to maintain a constant concentration of catalyst in the slurry.

[0080] The catalyst dispersed pyrolysis oil is preferably drained from the condensing unit (3) to keep the slurry level in the tank constant.Once ready, the catalyst slurry can be injected, preferably by a screw pump to keep its level constant.

[0081] The liquid effluent from the reactor (2) is preferably a highly concentrated hydrocarbon slurry which also contains the depolymerization catalyst, if used.

[0082] In another embodiment, the catalyst may be fed during the pre-processing stage of the plastic waste feedstock, or more preferably added to an extruder where it is mixed with the molten feedstock.

[0083] When a second depolymerization reactor is present, its gaseous effluent is sent to another condensation unit for recovering the pyrolysis products in the form of oil.

[0084] Such condensation unit preferably has a similar configuration as condensation unit (3). Preferably, the operating conditions of the condensation unit associated with the second reactor are selected in such a way that it has a lower operating temperature and pressure relative to condensation unit (3).

[0085] In particular, the temperature may be from 20 to 80° C., preferably from 30 to 70° C. The pressure value should preferably be lower than the pressure value of the condensation unit (3) so that the non-condensable gases from unit (3) enter the second condensation unit without further pressurization. The oil recovered from the second condensation unit is generally lighter than the oil recovered from the first condensation unit and may in particular have the following composition (GC determination):

[0086] - about 10-15 wt% of the fraction having a retention time equal to or less than that of n-heptane;

[0087] - about 70-75 wt. % of the fraction having a retention time between n-heptane and n-dodecane, and

[0088] - about 12-20 wt% of products having a retention time higher than n-dodecane and lower than n-octacosane,

[0089] - No trace compounds with longer retention times.

[0090] As mentioned above, the liquid effluent discharged from the pyrolysis reactor (2) and directed to the charcoal treatment section is in the form of a slurry, in particular a highly concentrated slurry, and is preferably discharged continuously. Operating a pressurized reactor makes it possible to easily discharge the concentrated slurry to a lower pressure device without using additional discharge equipment. As mentioned, the slurry can be discharged through the bottom of the reactor or, when present, from a pipeline or container after a centrifugal pump (4).

[0091] In view of the process setting, the flow of the slurry stream is preferably continuous.The carbon content in the slurry may be 10 to 65 wt%, preferably 20 to 40 wt%.

[0092] Suitable and preferred carbon treatment parts that may be associated with the process according to the present invention are described in co-pending applications PCT / EP2021 / 086926 and PCT / EP2021 / 086927, the relevant parts of which are incorporated by reference.

[0093] The char treatment section (6) is typically operated at near atmospheric pressure and higher temperatures (relative to the pyrolyzer) to facilitate the separation between the char and volatiles.

[0094] The volatiles separated in the charcoal treatment section (6) are condensed and recycled back to the depolymerization reactor (2).

[0095] The depolymerization process according to the invention is very efficient since it can produce about 10 wt% of pyrolysis gas, about 80 wt% of pyrolysis oil and about 10 wt% of char. As mentioned above, the process is characterized by high operability and reliability, given that the coking and fouling phenomena associated with the heating of the reactor walls are significantly reduced or completely eliminated.

[0096] Figure 1 Schematic diagram showing a process for supplying a plastic waste melt from an extruder (1) to a reactor (2) provided with an inlet (7) for a catalyst (if used) and a recovery circuit with a centrifugal pump (4) and a shell and tube heat exchanger (5). From the recovery circuit the slurry passes through line (13). The gaseous effluent is collected at the top of the reactor and sent via line (8) to a condensation unit (3) provided with a recovery circuit through which the condensate flows via a circulation pump (9) through a heat exchanger (10) and is recovered to the condensation tower (3). From the top of the tower the gaseous products are collected via line 11 and conveyed to further processing. The pyrolysis oil is collected from line 12 and conveyed to further processing or storage.

[0097] As already mentioned, the preferred use of the main product of the pyrolysis process of the invention is as a hydrocarbon feedstock in a cracking plant as a partial replacement of the oil feedstock. However, other uses, such as fuels, are also conceivable. It must be noted that the process of the invention allows the depolymerization product to be obtained in a simple and reliable process, in which the heat transfer is smooth and efficient.

[0098] Additionally, the fact that the process setup is based on readily available equipment makes the process itself easy to scale up.

[0099] Examples

[0100] Example 1

[0101] The following experimental steps were carried out in a depolymerization device comprising a reactor consisting of a jacketed mechanically stirred vessel equipped with an inlet for plastic waste fed from an extruder, an outlet for the generated gases and an outlet for the charcoal treatment section. The gases exiting the reactor were conveyed to a condensation unit from which non-condensable gases and pyrolysis oil were obtained.

[0102] A thermocouple was placed in the reactor to monitor and record the temperature.

[0103] The reactor is also provided with a recovery loop provided with a centrifugal pump and a shell and tube heat exchanger, through which a portion of the liquid slurry is discharged from the reactor, sent to the heat exchanger and reintroduced into the reactor.

[0104] The shell and tube heat exchanger was provided with a stream of molten solar salt flowing in the shell at a temperature of about 465°C, but no molten solar salt was provided to the reactor jacket.

[0105] The plastic waste feedstock was previously analyzed to check the polyolefin content (97 wt%) and residues containing traces of other common polymers (PET, PS, PA, PU) plus inorganic contaminants.

[0106] The raw material was homogenized and pelletized, then loaded into a hopper to feed an extruder operating at a temperature of 250°C and discharged continuously at 7 kg / h into a depolymerization reactor. The depolymerization reactor was operated at a pressure of 4 barg and a temperature of about 410°C with an average residence time of about 3 hours. The gas phase of the reactor was sent to a condensation unit formed by a cooling / washing tower operating at 50°C and a partial condenser operating at 25°C.

[0107] Another portion of the liquid slurry is sent to the carbon treatment section.

[0108] The process setup was able to run smoothly and continuously for 30 days without operating problems and no fouling was observed on the heat transfer surfaces during subsequent inspections. It must be noted that these results were obtained without further supply of heat to the reactor walls, without further addition of oil to reduce the viscosity of the reactor contents and without duplication of the complex reactor design according to USP 5,917,102. When the process setup was inspected after 30 days of operation, the inner walls of the reactor as well as other parts of the reactor system showed no fouling caused by carbon-adherent particles.

[0109] The condensed oil was analyzed via GC-FID. Due to the very large number of compounds, the analytical results have been reported by grouping the resulting compounds according to their retention times using specific hydrocarbons as internal retention time references. The results are reported in Table 1.

[0110] Comparative Example 1

[0111] Similar experiments were performed using the experimental setup described in Example 1, but also providing a molten solar salt stream to the reactor jacket. The same molten solar salt stream was used in series in the exchanger shell and the reactor jacket at a temperature of about 425°C.

[0112] With this setup, 70% of the total heat requirement is provided by the shell and tube heat exchanger.

[0113] The raw material was homogenized and pelletized, then loaded into a hopper to feed an extruder operating at a temperature of 250°C and discharged continuously at 9 kg / h into a depolymerization reactor. The depolymerization reactor was operated at a pressure of 4 barg and a temperature of about 410°C with an average residence time of about 3 hours. The gas phase of the reactor was sent to a condensation unit formed by a cooling / washing tower operating at 50°C and a partial condenser operating at 25°C.

[0114] When the process setup was inspected after 10 days of operation, it was found that the inner wall of the jacketed reactor was fouled with carbon particles.

[0115] The results of the oil characterization are reported in Table 1.

[0116] Table 1

[0117]

Claims

1. A process for depolymerizing waste plastic materials and producing pyrolysis products, wherein the process comprises the following steps: (a) feeding a mixture comprising waste plastic material into a feeding system comprising at least one screw extruder (1) and obtaining a molten plastic material; (b) feeding the molten plastic material from the extruder into a depolymerization reactor (2), which is a continuously stirred tank reactor maintained at a temperature of 280 to 600° C. and operated at a pressure of 2.0 to 10 barg, wherein depolymerization occurs to form a gaseous effluent and a liquid effluent; (c) directing at least a portion of the liquid effluent produced in the reactor (2) to a charcoal treatment section (6), and feeding the gaseous effluent from the reactor (2) to a condensation unit (3); (d) withdrawing a portion of the liquid effluent from the reactor (2) and recycling it back to the reactor (2) via a recovery loop comprising a centrifugal pump (4) and a shell and tube heat exchanger (5); The process is characterized in that at least 80% of the total heat requirement of step (b) is provided by the shell and tube heat exchanger (5).

2. A process according to any one of the preceding claims, wherein the plastic waste is a mixture of waste materials of which polyolefins are the most abundant component.

3. The process according to any one of the preceding claims, wherein the depolymerization reactor (2) is preferably a stirred vessel operated at a temperature of 300 to 550°C, more preferably 350 to 500°C and at a pressure maintained in the range of 2.5 to 8.0 barg, more preferably in the range of 3.0 to 7.0 barg.

4. A process according to any one of the preceding claims, wherein the melt viscosity of the reactor contents measured at a temperature of 400°C is from 0.1 to 250 cP.

5. Process according to any of the preceding claims, wherein more than 85%, in particular more than 90%, of the heat requirement of step (b) is provided by the shell and tube heat exchanger (5).

6. Process according to any of the preceding claims, wherein heat is provided to the heat exchanger (5) by a heat transfer fluid.

7. A process according to claim 6, wherein the heat transfer fluid is molten solar salt heated to a temperature of 300°C to 570°C.

8. Process according to any one of the preceding claims, wherein the liquid effluent from the reactor (2) flows inside the tubes of the heat exchanger (5), while the heat transfer fluid flows in the shell.

9. The process according to any one of the preceding claims, wherein the liquid effluent is discharged from the lower part of the reactor (2).

10. A process according to any one of the preceding claims, wherein the char content in the liquid effluent is from 10 to 65 wt%, preferably from 20 to 40 wt%.

11. The process according to any one of the preceding claims, wherein the pyrolysis product in the form of oil recovered from the condensation unit (3) has the following composition (GC determination): - about 10-15 wt% of the fraction having a retention time equal to or less than that of n-heptane; - about 70-75 wt. % of the fraction having a retention time between n-heptane and n-dodecane, and About 12-20 wt% of the product has a retention time higher than that of n-dodecane and lower than that of n-octacosane.

12. A process according to any one of the preceding claims, wherein the pyrolysis product in the form of an oil is used as a hydrocarbon feedstock in a cracking unit.

13. The process of claim 1 wherein a coke breaker is mounted on the hub of the centrifugal pump shaft.

14. The process of claim 1 wherein the heat exchanger is a single shell pass / single tube pass heat exchanger.

15. A reactor arrangement for depolymerizing waste plastic material, comprising a depolymerization unit, the depolymerization unit being a continuous stirred tank reactor (2) provided with an outlet for gaseous effluent, an outlet for liquid slurry effluent and a recovery circuit for discharging and reintroducing the liquid effluent into the reactor (2), the recovery circuit comprising a recovery pipe connected to a centrifugal pump (4) and a shell and tube heat exchanger (5).

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