Depolymerization process of waste plastic materials

Abstract

Disclosed is a process for depolymerizing waste plastic materials and producing pyrolysis oil with a specific reactor setup. This setup includes a reactor with a recycle circuit equipped with a centrifugal pump and a shell and tube heat exchanger. This process has high efficiency and versatility and can be easily expanded.

Description

Technical Field

[0001] The present disclosure relates to the field of depolymerizing waste plastic materials into new products containing hydrocarbon oils having valuable and useful properties. In one aspect, the present disclosure relates to a process for converting plastics, particularly hydrocarbon raw materials, into liquid hydrocarbons.

Background Art

[0002] There is a rapidly growing recognition that waste plastic materials have an adverse impact on the environment and, as a result, on the health of all life.

[0003] One attempt to mitigate the impact is to recycle plastic materials from household and industrial waste and reintroduce some of these materials into the production cycle. This results in additional positive outcomes such as a reduction in the use of fossil hydrocarbon sources for producing plastic products.

[0004] However, for various reasons, it has been shown that this solution alone is insufficient to achieve sustainability goals. In fact, in the mechanical recycling of plastic materials, substances of relatively low quality are usually produced, which are relatively costly and burdensome, and thus not applicable to certain municipal waste where plastics are mixed with various different materials.

[0005] As a result, most of the waste plastic materials are used as a heat energy source in factories such as incinerators or simply stored in landfills, leading to the deterioration of the global environment due to increased CO 2 emissions and the release of harmful chemicals, as described above.

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

[0007] Pyrolysis is a basic process in which waste plastic materials are converted into liquid fuel (pyrolysis products) by pyrolysis in the absence of oxygen and, if necessary, catalytic decomposition. Usually, the waste plastic material is first melted in a stainless - steel chamber under an inert purge gas such as nitrogen or methane. Next, in the same or a different chamber, additional heat and, if necessary, a catalyst are provided, and the polymer molecules of the molten material are decomposed into a gaseous state composed of relatively short hydrocarbon chains.

[0008] Next, the hot pyrolysis gas is condensed in one or more condensers to produce a hydrocarbon fraction containing straight - chain and branched - chain aliphatic, cycloaliphatic, and aromatic hydrocarbons (pyrolysis oil).

[0009] One of the important aspects in the above - mentioned depolymerization technology is that it is difficult to supply heat to the molecules of the plastic material because the plastic melt exhibits high viscosity and plastics generally have low thermal conductivity. This problem may be negligible in small - scale plants, but it is a significant problem regarding heat introduction in the operation of large - scale facilities and thus cannot be ignored in the case of scale - up.

[0010] According to U.S. Patent No. 9,920,255, heat can be supplied to the contents of the depolymerization reactor by adding a fraction obtained from crude oil as a solvent to the plastic melt, thereby reducing the viscosity of the plastic melt solution supplied to the depolymerization reactor compared to the viscosity of the plastic melt. To provide heat, the solvent is pre - heated to at least 150°C, preferably 200°C - 300°C. This solution also reduces the heat transferred by the reactor wall, reduces the risk of overheating of the plastic, and facilitates agitation of the contents of the reactor. However, a large amount of solvent is required to significantly reduce the viscosity, which not only reduces the productivity of the plant but is also insufficient to avoid the need to supply heat through the reactor jacket. Furthermore, the handling and supply of oil, as well as the logistics and equipment required, significantly increase the complexity of the plant.

[0011] U.S. Patent No. 5,917,102 describes a design of a depolymerization reactor in which a part of the contents of the reactor is conveyed to an external circulation system connected to the reactor to prevent overheating. The circulation system includes an oven / heat exchanger for supplying heat and a high-power pump, particularly a rotary pump, for circulating the contents of the reactor. To avoid erosion of the pump by metal parts entrained in the liquid contents of the reactor, the reactor design includes a limited zone called a "riser" that is not affected by turbulent flow and where solids can settle. Placing the extraction point of the external circuit at the upper end of the riser allows the liquid to be circulated without mixing in solids. The main problem with this solution is that the riser structure complicates the reactor design and increases the cost. Furthermore, due to the nature of the plastic raw material and the substances generated during depolymerization, the function of the fixed riser structure tends to deteriorate due to fouling caused by the substances deposited thereon, so the reactor needs to be frequently stopped to clean / exchange the structure.

[0012] In view of the above, an object of the present disclosure is to provide a depolymerization process for waste plastic materials characterized by high throughput of liquid hydrocarbons as pyrolysis products, efficient and smooth heat transfer to the reactor contents, continuous and reliable operability related to little or no fouling, and scalability.

Summary of the Invention

[0013] Accordingly, one aspect of the present disclosure is a process for depolymerizing a waste plastic material to produce pyrolysis products, the process comprising: supplying a mixture containing the waste plastic material to a supply system including at least one screw extruder (1) to obtain a molten plastic material in step (a); (b) Feed the molten plastic material coming out of the extruder into a depolymerization reactor (2), which is a continuous stirred tank reactor operating under a pressure of 2.0 - 10 barg and maintained at a temperature of 280 - 600 °C, where depolymerization occurs, thereby forming a gaseous effluent and a liquid effluent. (c) Guide at least a part of the liquid effluent produced in the reactor (2) to a char treatment section (6), and supply the gaseous effluent from the reactor (2) to a condensation unit (3). (d) Withdraw a part of the liquid effluent from the reactor (2) and recycle it to the reactor (2) via a recycle circuit including 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 in step (b) is supplied through the shell and tube heat exchanger (5).

Brief Description of the Drawings

[0014]

Figure 1

Embodiments for Carrying out the Invention

[0015] Preferably, this process is carried out in continuous mode.

[0016] In step (a), the input system can feed the waste plastic material into the reactor (2), preferably in continuous mode. Care should be taken not to introduce an atmosphere containing oxygen into the system. Barriers against potentially oxygen-containing atmospheres can be realized in various ways, such as a nitrogen blanket or a vacuum system connected to the barrel of the extruder.

[0017] More specifically, the waste plastic material mixture is fed into the supply system of the depolymerization reactor (2) in parallel by a hopper or two or more hoppers, and the oxygen present in the atmosphere of the waste plastic material is substantially replaced, for example, by a nitrogen purge.

[0018] The process according to the present disclosure is very flexible and can supply a wide range of waste plastic material compositions, for example, heterogeneous mixtures of waste plastic materials in which polyolefins are the most abundant components, but further sorting steps are no longer economical. In particular, when recycling the pyrolysis products to the cracking / purification unit, a preferred raw material is a waste plastic material mixture having a polyolefin (PE and PP) content of 70% wt or more.

[0019] The waste plastic material preferably undergoes a pretreatment step where it is melted by heat and optionally mixed with an additive that is an alkaline material. The melting pretreatment can convert a heterogeneous mixture of different types of waste plastics into a mass of a uniform plastic composite material. Therefore, this pretreatment is also preferred when pyrolysis is performed without an additive.

[0020] The heating temperature in the pretreatment step is appropriately set to a temperature such that pyrolysis of the plastic material to be treated is suppressed, depending on the type and content of the plastic contained in the waste plastic material. Such a temperature is usually in the range of 100°C to 300°C, preferably 150°C to 250°C. At a temperature close to or above 300°C, HCl is removed from the PVC resin that may be present.

[0021] When the waste plastic material is mixed with an alkaline material during the melting / kneading pretreatment, the HCl generated gas can be removed through the exhaust system and then neutralized or captured. For the melting operation, a normal kneader, an extruder with a screw, etc. can be applied. The waste plastic material is preferably fed into the depolymerization reactor by an extruder.

[0022] The extruder melts plastic scraps, heats them to a high temperature (250 - 350 °C), and injects them into the first depolymerization reactor (2). The extruder receives finely chopped plastic scraps into a feed hopper, conveys the flow to a melting section, and heats the polymer by the combined action of mixing energy and heat supplied by barrel heaters.

[0023] Additives can optionally be incorporated into the melt for the purpose of reducing the corrosiveness of plastic scraps or improving the yield of pyrolysis products in the reaction section.

[0024] One or more degassing steps are anticipated during extrusion to remove residual moisture present in the product.

[0025] Before being fed into the reactor (2), the melt stream can be filtered to remove solid impurities present in the waste plastic material.

[0026] Depending on the amount and particle size of the solid impurities, several designs of melt filtration units can be applied.

[0027] The use of a self-cleaning melt filter that can operate for a long period (several days) without manual intervention to replace the filter element is preferred.

[0028] One preferred design of the melt filter uses a circular perforated plate as the melt filtration element, with holes drilled by laser or machining according to the openings where solid contaminants accumulate. When impurities accumulate, the differential pressure across the entire melt filter can increase. To perform in-line cleaning of the filter element, a rotating scraper removes the accumulated impurities and guides them to a discharge port, which is opened for a short time to purge process contaminants.

[0029] This cycle can be repeated multiple times (up to several days of operating time) without the need for manual intervention or stopping production for the time required to replace the filter element.

[0030] Another option for the self-cleaning melt filter is based on the application of a continuous filtering metal band through which the polymer flow passes. Impurities accumulate in the metal filter, causing a pressure increase. Therefore, the clogged filtering band section is pushed out of the polymer passage area, and then a clean section is inserted.

[0031] This process is automated and can operate for long periods (up to several days) without manual intervention, and there is no need to stop production during the time required to replace the filtering element.

[0032] Any extrusion system can be applied, such as a single-screw extruder, a twin-screw extruder, a twin-screw extruder with a gear pump, or a combination thereof.

[0033] In step (b), the depolymerization reactor (2) is a stirred vessel preferably operated at a temperature in the range of 300 to 550 °C, more preferably 350 to 500 °C.

[0034] 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.

[0035] By adopting the above conditions, the melt viscosity of the reactor contents becomes suitable for being uniformly mixed by the stirring device. Generally, the melt viscosity measured at a temperature of 400 °C is in the range of 0.1 to 250 cP, particularly 1 to 100 cP, and even more particularly 5 to 50 cP.

[0036] It should be noted that the above melt viscosity values are obtained without adding a viscosity reducing agent that can be advantageously omitted in the process of the present invention.

[0037] However, if necessary, preferably in a dedicated container, the molten mass of waste plastic entering the reactor can be premixed with a hydrocarbon oil, preferably the recycled oil from the condensation unit, to facilitate the melting and dissolution into the depolymerization reactor. In this case, the volume ratio of oil / molten mass is in the range of 0.1:1 to 1:1.

[0038] The depolymerization reactor (2) preferably has a cylindrical cross-section and preferably has a rounded bottom. In another embodiment, the bottom of the reactor has a conical or frustoconical shape.

[0039] Preferably, a mixer is installed on the vertical axis of the reactor, and a gear motor is provided that can rotate the blades of the mixer to keep the system in a stirred state. The design of the mixer and the power of the motor can vary depending on the contents, volume, and shape of the reactor, but by way of non-limiting example, 0.2 to 4 kW / m 3 Preferably 0.2 to 2 kW / m 3 More preferably 0.3 to 1.5 kW / m 3 It is preferred to operate the reactor with a power input in the range.

[0040] According to the present invention, at least 80%, preferably 85% or more, particularly 90% or more of the total heat requirement in step (b) is supplied through the shell and tube heat exchanger (5). Therefore, the heat supplied to the contents of the reactor by the reactor wall is less than 10%, preferably less than 5%, more preferably absent. As a result, the reactor (2) does not strictly require a jacket wall for heating the contents of the reactor. In certain embodiments, the reactor may be jacketed. In a more preferred embodiment, the heating fluid does not circulate within the reactor jacket.

[0041] The heat to the external heat exchanger (5) can be supplied by any heat transfer fluid suitable for operating at a temperature above the depolymerization temperature. Preferably, solar salt or synthetic oil is used. The use of molten solar salt heated to a temperature in the range of 300°C to 570°C is highly preferred.

[0042] Specifically, the liquid effluent from the reactor (2) flows through the tubes of the heat exchanger, while the heat transfer fluid flows through the process side within the shell, such that the required heat is exchanged through the shell and tube heat exchanger (5).

[0043] The molten salt supply circuit (not shown) is configured to prevent leakage of molten salt. The molten salt is preferably a mixture of sodium nitrate and potassium nitrate, more preferably a molten solar salt with a weight ratio in the range of 2:3 to 3:2. The solar salt receives heat sequentially from a dedicated electric or fuel-fed furnace. Preferably, the furnace is electric, and more preferably, the electricity is supplied from renewable resources. When using a fuel-based furnace, a portion of the oil recovered from the condensation unit (3) can be used to supply the furnace.

[0044] The heat transfer fluid, particularly the molten salt, is circulated to the heat exchanger using a circulation pump.

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

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

[0047] Preferably, within the tubes of the heat exchanger, the slurry flows at a velocity in the range of 3 to 10 m / s, more preferably 5 to 8 m / s. Note that if the velocity is less than 2 m / s, precipitation of the slurry may occur.

[0048] The liquid effluent, which is a slurry of solid materials dispersed in a liquid hydrocarbon medium, is circulated by a centrifugal pump (4). There is no limitation on the type of centrifugal pump that can be used, and any commercially available centrifugal pump with a function suitable for pumping the liquid effluent having the above characteristics can be used.

[0049] Surprisingly, in the process of the present disclosure, it has been found that there are no problems caused by lumps or other solid residues present in the liquid effluent. Without wishing to be bound by theory, this may be because the impeller of the centrifugal pump crushes the lumps of carbon into fine powder.

[0050] It is also a preferred embodiment to install a coke crusher on the hub of the centrifugal pump shaft.

[0051] According to a preferred embodiment, the portion of the liquid slurry recycled to the reactor is withdrawn from a point in the reactor that is different from the withdrawal point of the portion of the liquid slurry sent for char treatment.

[0052] According to another preferred embodiment, both the portion of the liquid slurry recycled to the reactor and the portion of the liquid slurry sent for char treatment are withdrawn from the same point and then sequentially divided.

[0053] The division between the portion of the liquid slurry sent for char treatment and the portion recycled to the reactor can be carried out either before or after the centrifugal pump (4). In this latter embodiment, the liquid slurry is first fed to a dedicated container having a lower outlet point and an upper outlet point. The portion of the liquid sent for char treatment (6) is withdrawn in a concentrated state from the lower outlet point, while the portion of the liquid recycled to the reactor (2) is withdrawn from the upper outlet point.

[0054] By the depolymerization process occurring in the reactor, molecules with shorter chain lengths and lower boiling points are produced. This continuously executed chain-breaking mechanism produces molecules that increasingly become gaseous at the operating temperature and pressure, especially near the walls of the reactor.

[0055] As a result, the composition within the reactor covers a wide range of hydrocarbons, both saturated and olefinic hydrocarbons, from methane to heavier products, having linear or highly branched structures. Aromatic products can also be present, not only condensed ring structures.

[0056] Those that are still in a liquid state under the operating conditions are useful for reducing the mass viscosity of the liquid. As a result of the depolymerization process and the composition of the feedstock, the contents of the reactor (2) can be defined as a coexistence of a liquid slurry phase and a gas phase in which solids, particularly carbonaceous substances and inorganic substances, are dispersed in a liquid hydrocarbon medium.

[0057] At least a part of the liquid slurry phase is withdrawn from the reactor, preferably from the lower part of the reactor, and constitutes the liquid effluent sent to the char treatment section (6).

[0058] From an operational perspective, the withdrawal of the slurry phase from the bottom of the reactor is preferably initiated when the density sensor detects that the density of the liquid slurry has reached a predetermined value.

[0059] The gas phase in the reactor (2) becomes a gaseous effluent and is sent to the condenser unit (3) for further treatment.

[0060] The gaseous effluent contains a mixture of light hydrocarbons, which may also contain heavy hydrocarbons and carbonaceous particles. The gaseous effluent is conveyed from the upper part of the reactor to the condenser (3) and is preferably operated at a pressure slightly lower than the pressure of the reactor.

[0061] The condenser (3) is preferably designed as a scrubber column to suppress the entrained char. The temperature of the condenser is selected such that the heavy hydrocarbons are condensed and the light hydrocarbons are released as a gas stream. The gas stream (H 2 and light hydrocarbons) is preferably conveyed to a further condenser unit (not shown) operating at a temperature lower than that of the condenser unit (3), from which oil is recovered.

[0062] The operating temperature of the condenser unit (3) can vary over a wide range depending on the operating pressure. The temperature can be 20°C to 200°C, more preferably 40°C to 100°C, particularly 50°C to 90°C, based on atmospheric pressure. When a higher operating pressure is selected, the temperature range will of course be different.

[0063] When the liquid condensate is analyzed by GC, specific hydrocarbons are used as the internal retention time reference, and the components are grouped according to the retention time and the results are reported. The results may show that there is about 2 wt% or more of compounds having a retention time equal to or shorter than that of n-heptane, about 25 wt% or more of compounds having a retention time included between n-heptane and n-dodecane, that there are more compounds having a retention time longer than n-dodecane and shorter than n-octacosane (70 wt% or less), and that there may be a small amount of compounds having a longer retention time.

[0064] In a preferred setup of the condensation unit (3), the dephlegmator (partial condenser) is installed above the scrubber and operates at a temperature lower than the temperature inside the column. The condensate flows down by gravity as reflux to the scrubber. The dephlegmator can be installed as an independent facility or inside the scrubber.

[0065] In an alternative or combined setup, the liquid accumulated at the bottom of the scrubber is recycled to the top of the column by a pump. The recycled liquid is cooled in a dedicated heat exchanger and then injected as reflux to the top of the scrubber.

[0066] The hydrocarbon condensate (preferably having 7 or more carbon atoms) constitutes a liquid stream that is sent for further processing or to a second depolymerization reactor.

[0067] There can also be a further depolymerization reactor. If present, the second depolymerization reactor is preferably of the same type as the first depolymerization reactor, and more preferably a continuous stirred tank reactor equipped with the same recycle circuit that supplies heat to the depolymerization stage by means of a centrifugal pump and a shell and tube heat exchanger.

[0068] The second reactor can be connected in series (sequential) or in parallel with the first reactor. The sequential setup is preferred.

[0069] It is also clear that one or more reactors can be equipped with one or more additional recycle circuits, each equipped with a centrifugal pump and a heat exchanger.

[0070] The depolymerization occurs in the same temperature range but is preferably operated at a pressure higher than that of the first reactor, particularly in the range of 3 to 10 barg, preferably 3 to 9 barg, more preferably 3 to 8 barg, in order to limit the volatility of the heavy hydrocarbons.

[0071] According to the present invention, the depolymerization step (b) can be carried out in the presence of a catalyst. The latter can be selected from those active as depolymerization / decomposition catalysts in a thermal catalytic process. In particular, it can be selected from metal oxides, heteropolyacids, mesoporous silica, aluminosilicate catalysts such as halloysite and kaolinite, preferably zeolites. Among these, particularly preferred zeolites are synthetic Y-type zeolites and ZSM-5.

[0072] In a particularly preferred embodiment, the catalyst feed amount is 10% or less, preferably 5% or less, particularly 2% by weight or less based on the waste plastic material feed amount.

[0073] In a preferred embodiment, the catalyst is injected into the reactor as a powder dispersed in a hydrocarbon oil, preferably a liquid pyrolysis product (oil) obtained from the condensation unit (3).

[0074] Preferably, the catalyst slurry is prepared in a pot-type continuous stirring vessel, and the catalyst is poured from a dedicated silo to keep the concentration of the catalyst in the slurry constant.

[0075] The pyrolysis oil for dispersing the catalyst is preferably withdrawn from the condensation unit (3) to keep the slurry level in the pot constant. Once prepared, the catalyst slurry is injected, preferably using a progressive cavity pump to keep the level constant.

[0076] The liquid effluent from the reactor (2) is preferably a high-concentration hydrocarbon slurry containing a depolymerization catalyst.

[0077] In another embodiment, the catalyst may be supplied in the pretreatment stage of the waste plastic raw material, or more preferably, added to an extruder where it is mixed with the molten raw material.

[0078] If a second depolymerization reactor is present, its gaseous effluent is conveyed to a further condensation unit for recovering the oily pyrolysis products.

[0079] This condensation unit preferably has a configuration similar to that of the condensation unit (3). Preferably, the operating conditions of the condensation unit associated with the second reactor are selected to have a lower operating temperature and pressure than those of the condensation unit (3).

[0080] In particular, the temperature can be in the range of 20 to 80 °C, preferably 30 to 70 °C. The pressure value should preferably be lower than that of the condensation unit (3) so that the non-condensable gas from the condensation unit (3) can enter the second condensation unit without further pressurization. The oil recovered from the second condensation unit is usually lighter than the oil recovered from the first condensation unit and can particularly have the following composition (measured by GC). - The fraction with a retention time equal to or less than that of n-heptane contains about 10 to 15 wt%. - About 70 to 75 wt% of the fraction having a retention time contains n-heptane and n-dodecane. Also, - About 12 to 20 wt% of the product has a retention time higher than that of n-dodecane and shorter than that of n-octacosane. - Compounds with a long retention time were not found.

[0081] As described above, the liquid effluent discharged from the pyrolysis reactor (2) and sent to the char treatment section is in the form of a slurry, particularly a high-concentration slurry, and is preferably discharged continuously. When operating the pressurized reactor, it becomes possible to easily discharge the concentrated slurry to a low-pressure device without using an additional extraction device. As described above, the slurry can be discharged from the bottom of the reactor or, if present, from a line or container after the centrifugal pump (4).

[0082] From the perspective of process setup, the flow of the slurry stream is preferably continuous. The char content in the slurry ranges from 10 to 65%, preferably from 20 to 40% wt.

[0083] Suitable and preferred char treatment sections that can be associated with the process according to the present disclosure are described in the co-pending applications PCT / EP2021 / 086926 and PCT / EP2021 / 086927, the relevant portions of which are incorporated by reference.

[0084] The char treatment section (6) is usually operated at approximately atmospheric pressure and high temperature (relative to the pyrolysis device) to facilitate the separation of char and volatile substances.

[0085] The volatile substances separated in the char treatment section (6) are condensed and returned to the depolymerization reactor (2).

[0086] The depolymerization process according to the present invention is very efficient because it can produce approximately 10 wt% of pyrolysis gas, approximately 80 wt% of pyrolysis oil, and approximately 10 wt% of char. As described above, this process is characterized by high operability and reliability considering the fact that the coking and fouling phenomena related to the heating of the reactor wall are significantly reduced or completely eliminated.

[0087] Figure 1 shows a schematic diagram of a process in which a molten waste plastic material in an extruder (1) is supplied to a reactor (2) equipped with a recycling circuit including a catalyst (when used) inlet (7), a centrifugal pump (4), and a shell and tube heat exchanger (5). The slurry is discharged from the recycling circuit via line (13). The gaseous effluent is collected at the top of the reactor and sent via line (8) to a condensation unit (3) which is provided with a recycling circuit. By means of this recycling circuit, the condensate passes through a heat exchanger (10) via a circulation pump (9) and is recycled to the condensation column (3). The gaseous product is collected from the top of the column via line 11 and conveyed for further processing. The pyrolysis oil is collected from line 12 and conveyed for further processing or storage.

[0088] As described above, a preferred use of the main product of the pyrolysis process of the present invention is as a hydrocarbon feedstock that partially replaces the oil feedstock in a cracking plant. However, other uses such as fuels are also being considered. Note that with the process of the present disclosure, depolymerization products can be obtained in a simple and reliable process with smooth and efficient heat transfer.

[0089] Furthermore, since the process setup is based on equipment that is readily available, it is easier to scale up the process itself.

[0090]

Example

[0091] Example 1

[0092] The following experimental steps were carried out in a depolymerization apparatus including a reactor consisting of a jacketed mechanically stirred vessel with an inlet for waste plastic material from an extruder, an outlet for the generated gas, and an outlet for the char treatment section. The gas withdrawn from the reactor was conveyed to a condensation unit where non-condensable gas and pyrolysis oil were obtained.

[0093] A thermocouple is placed inside the reactor to monitor and record the temperature.

[0094] The reactor is also equipped with a recycle circuit having a centrifugal pump and a shell-and-tube heat exchanger, by which a part of the liquid slurry is withdrawn from the reactor, sent to the heat exchanger, and reintroduced into the reactor.

[0095] A flow of molten solar salt flowing through the shell at a temperature of about 465 °C was provided in the shell-and-tube heat exchanger, but no molten solar salt was provided in the reactor jacket.

[0096] The waste plastic raw material was previously analyzed and the polyolefin content (97 wt%) was confirmed, but the residue contained traces of other common polymers (PET, PS, PA, PU) and inorganic contaminants.

[0097] Before loading the raw material into the hopper, it was homogenized, pelletized, and then fed to an extruder operating at a temperature of 250 °C and continuously discharged to the depolymerization reactor at 7 kg / h. The depolymerization reactor was operated at a pressure of 4 barg and a temperature of about 410 °C, and the average residence time was about 3 hours. The gas phase of the reactor was sent to a condensation unit formed by a cooling / scrubber column operating at 50 °C and a degreaser operating at 25 °C.

[0098] The remaining part of the liquid slurry was sent to the char treatment section.

[0099] The process setup operated smoothly and continuously for 30 days without any operational problems, and no fouling was observed on the heat transfer surface during subsequent inspections. These results were obtained without adding additional heat to the reactor wall, without adding additional oil to reduce the viscosity of the reactor contents, and without reverting to the complex reactor design according to US Patent No. P5,917,102. As a result of inspecting the process setup after 30 days of operation, no fouling due to adhering particles of char was observed on the inner wall of the reactor and other parts of the reactor system.

[0100] The condensed oil was analyzed by GC-FID. Since the number of compounds was very large, the analysis results were reported by using specific hydrocarbons as internal retention time standards and grouping the obtained compounds according to their retention times. The results are shown in Table 1.

[0101] Comparative Example 1

[0102] A similar test was carried out using the experimental setup described in Example 1, but a flow of molten solar salt was also provided to the reactor jacket. The same flow of molten solar salt was used in series in both the exchanger shell and the reactor jacket at a temperature of about 425 °C.

[0103] With this setup, 70% of the total heat demand was supplied through the shell-and-tube heat exchanger.

[0104] The raw materials were homogenized and pelletized before being loaded into the hopper, and then fed to an extruder operating at a temperature of 250 °C and continuously discharged to the depolymerization reactor at 9 kg / h. The depolymerization reactor was operated at a pressure of 4 barg and a temperature of about 410 °C, and the average residence time was about 3 hours. The gas phase of the reactor was sent to a condensation unit formed by a cooling / scrubber column operating at 50 °C and a degreaser operating at 25 °C.

[0105] As a result of inspecting the process setup after 10 days of operation, it was found that the inner wall of the jacketed reactor was contaminated with carbonized particles.

[0106] The results of the oil property evaluation are shown in Table 1.

Table 1

Claims

1. A process for depolymerizing a waste plastic material to produce a pyrolysis product, the process comprising: Supplying a mixture containing the waste plastic material to a supply system including at least one screw extruder (1) to obtain a molten plastic material (step (a)); Supplying the molten plastic material exiting the extruder to a depolymerization reactor (2), which is a continuous stirred tank reactor maintained at a temperature of 280 to 600 °C and operating under a pressure of 2.0 to 10 barg, where depolymerization occurs therein, thereby forming a gaseous effluent and a liquid effluent (step (b)); Directing at least a portion of the liquid effluent produced in the reactor (2) to a char treatment section (6) and supplying the gaseous effluent from the reactor (2) to a condensation unit (3) (step (c)); Withdrawing a portion of the liquid effluent from the reactor (2) and recycling it to the reactor (2) via a recycle circuit including a centrifugal pump (4) and the shell and tube heat exchanger (5) (step (d)), The process is characterized in that at least 80% of the total heat requirement of step (b) is supplied through the shell and tube heat exchanger (5).

2. The process according to any one of the preceding claims, wherein the waste plastic material is a mixture of wastes in which polyolefin is the most abundant component.

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

4. The 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 in the range of 0.1 to 250 cP.

5. The process according to any one of the preceding claims, wherein 85% or more, particularly 90% or more, of the heat requirement of step (b) is supplied through the shell and tube heat exchanger (5).

6. The process according to any one of the preceding claims, wherein the heat to the heat exchanger (5) is supplied by a heat transfer fluid.

7. The 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. The liquid effluent from the reactor (2) flows through the tubes of the heat exchanger (5), while the heat transfer fluid flows through the shell, the process according to any one of the preceding claims.

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

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

11. The oily pyrolysis product recovered from the condensation unit (3) is - about 10 to 15 wt% of a fraction having a retention time equal to or less than that of n-heptane, - about 70 to 75 wt% of a fraction having a retention time consisting of n-heptane and n-dodecane, about 12 to 20 wt% of a product having a retention time longer than n-dodecane and shorter than n-octacosane, and has as a composition (GC measurement), the process according to any one of the preceding claims.

12. The oily pyrolysis product is used as a hydrocarbon feedstock in a cracking plant, the process according to any one of the preceding claims.

13. A coke grinder is installed on the hub of the centrifugal pump shaft, the process according to claim 1.

14. The heat exchanger is a single shell pass / single tube pass heat exchanger, the process according to claim 1.

15. A reactor configuration for depolymerizing waste plastic materials, comprising a depolymerization unit which is a continuous stirred tank reactor (2), including an outlet for the gaseous effluent, an outlet for the liquid slurry effluent, and a recycle circuit for withdrawing and reintroducing the liquid effluent into the reactor (2), including a recycle pipe connected to the centrifugal pump (4) and the shell and tube heat exchanger (5).

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