Method for preparing pyrolytic oil from waste plastics

Abstract

The invention provides a method for preparing pyrolytic oil from waste plastics, which comprises the following steps: (S1) feeding a waste plastic raw material into a first reactor, carrying out primary pyrolysis, and discharging upper gaseous flow generated by pyrolysis through the upper part of the first reactor; (S2) feeding the upper gaseous stream of the first reactor into one or more distillation columns, separating the upper gaseous stream according to boiling points to obtain a stream containing C5-12 light oil (LO), a stream containing C13-22 intermediate oil (MO) and a stream containing heavy oil (HO) above C23, respectively; (S3) feeding a portion of the intermediate oil (MO)-containing stream separated from the distillation column to a second reactor for secondary pyrolysis and feeding the gaseous stream produced by pyrolysis to the distillation column; and (S4) recycling the heavy oil (HO)-containing stream separated from the distillation column to the first reactor.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0110204, filed on August 23, 2023, the entire content of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for preparing pyrolysis oil from waste plastics, and more particularly, to a method for preparing light hydrocarbon oil from waste plastics with high yield. Background Art

[0004] Recently, the development and use of plastics with physical properties required for various applications and purposes have been increasing. A large amount of energy is required from crude oil extraction to plastic preparation, and a large amount of carbon is emitted during this process. In addition, when plastics used for various products are discarded, it also causes environmental pollution and huge treatment costs. Therefore, the recycling of waste plastics has become an important social issue.

[0005] Generally, methods for recycling waste plastics (resins) include mechanical recycling, chemical recycling, and thermal recycling. Mechanical recycling is a method of crushing and screening collected waste plastics, separating waste plastics by type, melting waste plastics using an extruder, and mixing waste plastics with a certain proportion of new materials or adding functional additives when granulating waste materials to prepare resin products. Chemical recycling is a method of using various chemical means to extract only specific polymers or to recycle and repolymerize pure single molecules. Thermal recycling is a method of burning waste plastics and recovering waste plastics as heat energy.

[0006] In particular, compared with the incineration of waste plastics, chemical recycling can reduce greenhouse gases and has recently received much attention in the development of alternative fuels.

[0007] For example, when waste plastics such as polyethylene or polypropylene materials are pyrolyzed by heating at a certain temperature, a gaseous stream containing a mixture of non-condensable gas and liquid oil can be generated, and highly viscous residual wax that has not been completely decomposed can be discharged. According to the increasing importance of liquid oil in the pyrolysis products as a raw material for producing petrochemical products, research has been actively carried out to increase the yield of liquid oil.

[0008] The liquid oil produced by the pyrolysis of waste plastics, i.e., pyrolysis oil, is generally a mixed oil containing 5-12 light hydrocarbon oil (such as naphtha) and long-chain hydrocarbon oil. When such a mixture contains a large amount of high-boiling components, the yield of high-value light hydrocarbon oil is limited, and the amount of heavy oil components produced and the residual wax discharged are excessive.

[0009] To solve these problems, methods have been proposed for carrying out a catalytic cracking contact decomposition reaction during the pyrolysis of waste plastics; and, methods such as using a contactor (i.e., a contact heat exchanger) to condense the heavy hydrocarbon (i.e., long-chain hydrocarbon) components in the pyrolysis products of waste plastics and then recycling the components to a pyrolysis reactor to subject these components to the pyrolysis process again. However, these methods have limitations in improving the pyrolysis efficiency of waste plastics and are therefore only used for the recycling of mixed oil.

[0010] In particular, the contactor can be disposed above the pyrolysis reactor to only improve the selectivity for pyrolysis oil components within a specific boiling point range (e.g., long-chain hydrocarbons at 240°C to 280°C).

[0011] Therefore, there is a need to develop a technology that can improve the pyrolysis process of waste plastics and improve the yield of high-quality light hydrocarbon oil. SUMMARY OF THE INVENTION

[0012] TECHNICAL PROBLEM

[0013] To solve the problems mentioned in the background art, an object of the present invention relates to a method for improving the yield of light hydrocarbon oil by connecting one or more distillation towers in series above a reactor for pyrolyzing waste plastics to replace a contactor (heat exchanger), separating the high-boiling components in the pyrolysis products into middle oil (MO) and heavy oil (HO), and then performing additional pyrolysis.

[0014] TECHNICAL SOLUTION

[0015] In general, a method for preparing pyrolysis oil from waste plastics includes: (S1) feeding a waste plastic raw material into a first reactor, performing primary pyrolysis, and discharging the upper gaseous stream generated by the pyrolysis through the upper part of the first reactor; (S2) feeding the upper gaseous stream of the first reactor into one or more distillation towers, separating the upper gaseous stream according to the boiling point to respectively obtain a stream containing C 5-12 light oil (LO), a stream containing C 13-22 middle oil (MO), and a stream containing C 23 heavy oil (HO) of C and above; (S3) feeding a part of the stream containing middle oil (MO) separated in the distillation tower into a second reactor, performing secondary pyrolysis, and feeding the gaseous stream generated by the secondary pyrolysis in the second reactor into the distillation tower; and (S4) recycling the stream containing heavy oil (HO) separated in the distillation tower to the first reactor.

[0016] ADVANTAGEOUS EFFECTS

[0017] According to the present invention, in one or more distillation columns connected in series with a first reactor for pyrolyzing waste plastic raw materials, the pyrolyzed gaseous stream is separated according to boiling points. Then, a part of the middle oil (MO) in the separated components is subjected to secondary pyrolysis in a second reactor, while the heavy oil (HO) is recycled to the first reactor, so that the yield of high-quality light oil (LO) can be improved.

[0018] In addition, since the pyrolysis in the first reactor and the pyrolysis in the second reactor are both carried out under the condition of gradually increasing temperature, the decomposition efficiency of waste plastics is improved, so that the yield of high-quality light hydrocarbon oil can be increased, and the emission of residual wax that is not fully utilized can be minimized.

[0019] In addition, by adjusting the upper operating temperature of the distillation column to a predetermined range, the proportion of the light oil (LO) component in the gaseous stream separated in the distillation column is increased, and in the process of exchanging heat between the gaseous stream and water in a condenser connected to the upper part of the distillation column to obtain a liquid phase, the water fed into the condenser can be converted into steam by recovering the waste heat of the gaseous stream.

[0020] In addition, using the light hydrocarbon oil obtained from the pyrolysis of waste plastics can reduce the greenhouse gas emissions caused when feeding raw materials to the petrochemical process, can improve the process efficiency (such as energy conservation), and since no harmful gases are generated during the treatment of waste plastics, it can be environmentally friendly. Description of the Drawings

[0021] Figure 1 The waste plastic pyrolysis process of an embodiment of the present invention is shown.

[0022] Figure 2 The waste plastic pyrolysis process of a comparative example is shown. Detailed Description

[0023] The terms and words used in the specification and claims of the present invention should not be construed restrictively as having conventional or dictionary meanings, but should be understood as having meanings and concepts conforming to the technical concept of the present invention based on the principle that the inventor can appropriately define the terms in order to describe their own invention in the best way.

[0024] As used in this application, the meaning of "comprising" or "containing" specifically defines specific characteristics, regions, integers, steps, operations, elements, and / or components, and does not exclude other specific characteristics, regions, integers, steps, operations, elements, and / or components.

[0025] The term "stream" used in this application can refer to the fluid stream in a process or the fluid flowing through the pipeline itself. Specifically, the stream can refer to the fluid itself flowing through the pipeline connecting various devices or the fluid stream. In addition, the fluid can include any one or more components of gas, liquid, and solid.

[0026] The term "C n " in this application refers to all hydrocarbons having n carbon atoms. For example, "C 5-12 " refers to all hydrocarbon molecules having 5 to 12 carbon atoms.

[0027] The term "liquid oil" used in this application refers to the product obtained by condensing the gaseous stream obtained in the pyrolysis step into a liquid phase, and can also be referred to as "liquid distillate oil".

[0028] In addition, the "pressure" involved in this application refers to the absolute pressure measured based on a complete vacuum.

[0029] According to an embodiment of the present invention, a method for preparing pyrolysis oil from waste plastics includes: (S1) performing primary pyrolysis on the waste plastic raw material; (S2) separating the pyrolyzed gaseous stream according to the boiling point; (S3) performing secondary pyrolysis on the separated intermediate oil portion; and (S4) recycling the separated heavy oil to the pyrolysis of the raw material.

[0030] As Figure 1 shown, this method can be carried out by using a process system, which includes: a first reactor 10 into which the waste plastic raw material is fed; one or more distillation towers 20 connected in series with the first reactor; a second reactor 11 connected to the distillation tower 20; and a residue treatment reactor 30.

[0031] Hereinafter, each step of the method for preparing pyrolysis oil from waste plastics of the present invention will be described in detail with reference to the accompanying drawings.

[0032] First, the waste plastic raw material is fed into the first reactor 10 for primary pyrolysis, and the gaseous stream generated by pyrolysis is discharged from the upper part of the first reactor 10 (S1).

[0033] The waste plastics can include natural polymers, synthetic polymers, or mixtures thereof. The synthetic polymers can include thermoplastic resins such as polyethylene, polypropylene, and polystyrene. In addition, the thermoplastic resin can also be a mixture of other types of resins such as PVC and PET, thermosetting resins, etc.

[0034] The waste plastics (such as the above materials) can be pretreated through processes including crushing, washing, drying, and melting after collection and screening. The pretreatment process can be carried out by conventional methods in the art.

[0035] For example, the size of the shredded waste plastic is not particularly limited, but generally, the waste plastic can be shredded to a size range of 0.5 cm to 6.0 cm. Then, the washed and dried shredded waste plastic material is put into a tubular melter (such as an extruder) and then melted. The extruder has an extrusion function of melting and kneading. For example, it can be a twin-screw extruder. When the waste plastic is a thermoplastic resin, such as polyethylene, polypropylene, or a mixture thereof, the melting temperature can be 120°C to 350°C, or 150°C to 250°C, but is not limited thereto.

[0036] The waste plastic melt obtained through the pretreatment process is fed into the first reactor for pyrolysis.

[0037] The first pyrolysis reactor applicable to the present invention can be a stirred tank reactor equipped with a stirrer. The stirrer is not particularly limited as long as it can sufficiently stir the waste plastic melt fed as a raw material, and can be, for example, a spiral ribbon type or an anchor type. Advantageously, a gap of about 5 mm to 1 cm is maintained with the inner wall of the reactor to maximize the stirring of the waste plastic and heat transfer through the reactor wall. In addition, the reactor can operate in a batch mode or a continuous mode. In addition, the reactor can be purged with nitrogen to maintain an anaerobic or low-oxygen atmosphere while carrying out the pyrolysis reaction of the waste plastic melt.

[0038] The waste plastic melt is fed into the first reactor equipped with a stirrer, and while the stirrer is operating, it is heated to carry out pyrolysis of the waste plastic melt.

[0039] The heating of the waste plastic can be carried out by passing high-temperature / high-pressure steam, hot water, heat transfer fluid, etc. through a jacket provided outside the reactor to transfer high-temperature heat to the waste plastic, but is not particularly limited thereto.

[0040] In one embodiment of the present invention, the primary pyrolysis in the first reactor can be carried out at a temperature of 400°C to 450°C. The waste plastic raw material is mainly a thermoplastic resin. For example, considering that the waste plastic raw material can be a mixture containing polyethylene with a number average molecular weight of 10,000 to 500,000 (specifically 100,000 to 300,000) or polypropylene with a number average molecular weight of 5,000 to 300,000 (specifically 10,000 to 200,000), the pyrolysis reaction is advantageously carried out within 400°C to 450°C, specifically within 420°C to 430°C. When the primary pyrolysis temperature is lower than 400°C, the pyrolysis rate may be slow. When the pyrolysis temperature is higher than 450°C, the pyrolysis rate may be fast, but excessive solid carbides (such as carbon) may be produced due to the high temperature.

[0041] In addition, the primary pyrolysis in the first reactor can be carried out at a pressure of 0.9 bar to 1.1 bar for 1 hour to 5 hours, especially 2 hours to 4 hours. The time of the primary pyrolysis reaction can be determined by considering the time when the production rate of light hydrocarbons with 12 or fewer carbon atoms increases within the temperature range of 400°C to 450°C after the start of the waste plastic pyrolysis reaction.

[0042] When the waste plastic raw material is pyrolyzed in the first reactor 10, a gaseous stream containing C 1-4 gas components, C 5-12 light hydrocarbons (such as naphtha) and long-chain hydrocarbon components (such as C 13-22 hydrocarbons and C 23 hydrocarbon components with 10 or more carbon atoms) is discharged from the upper part of the reactor, while the high-viscosity residual wax does not evaporate and remains in the lower part of the reactor.

[0043] Subsequently, the gaseous stream discharged from the upper part of the first reactor 10 is fed into one or more distillation towers 20 for separation according to the boiling point (S2).

[0044] One or more distillation towers 20 can be arranged in series above the first reactor 10, and the pyrolysis products transferred from the first reactor can be separated into various components according to the boiling point by selective heating and condensation. For example, the stream fed into each distillation tower can be heated by the heat provided by the reactor connected to the lower part of the tower, the steam generated by heating can move to the upper part of the tower and then be discharged, and the discharged steam can be partially or completely condensed in the condenser connected to the upper part of the tower. A part of the condensate is recycled to the tower.

[0045] Different from the existing waste plastic pyrolysis process where a contactor (heat exchanger) is applied to the pyrolysis reactor, in the present invention, one or more distillation towers are arranged at the upper part of the first reactor to separate the pyrolysis products according to the boiling point. The high-boiling oil discharged from the middle or lower part of the distillation tower can be selectively recycled to the first reactor or fed into the second reactor for additional pyrolysis reaction, thereby improving the yield of light oil (LO).

[0046] In addition, in the present invention, the carbon range of the desired pyrolysis oil (distilled oil) can be changed by adjusting the upper operating temperature of the distillation tower.

[0047] Specifically, when the upper operating temperature of the distillation tower increases, the carbon range of the pyrolysis oil moves to high boiling points, and when the upper operating temperature of the distillation tower decreases, the carbon range of the pyrolysis oil moves to low boiling points, thereby increasing the yield of light oil (such as naphtha).

[0048] For example, by adjusting the upper temperature of the distillation tower to 220°C to 300°C, a distillate containing more than 80% of C 5-12 low-boiling hydrocarbons and C13-22 A gaseous stream of medium-boiling hydrocarbons. At the same time, when the upper temperature of the distillation column is adjusted to 150 °C to less than 220 °C, a gaseous stream containing more than 70% of C 5-12 A gaseous stream of low-boiling hydrocarbons. The gaseous stream can be cooled, condensed, and converted into liquid oil by heat exchange with water connected to the upper part of the distillation column, and the uncondensed gas components (e.g., C 1-4 Hydrocarbons) can be discharged from the upper part. In this case, some components contained in the condensed liquid oil, i.e., medium-boiling or high-boiling components, can flow back to the upper part of the distillation column and move to the lower part of the distillation column. As a result, C 5-12 Light oil (LO) can be discharged from the upper trays of the column, C 13-22 Middle oil (MO) can be discharged from the middle trays of the column, and C 23 Above (e.g., C 23-40 Heavy oil (HO)) can be discharged from the lower trays of the column.

[0049] In addition, a dividing-wall distillation column (DWC) including two sections divided by a central dividing wall can also be used as the distillation column 10. A stream containing C 5-12 Light hydrocarbons is separated through the upper part of the DWC, a stream containing C 13-22 Middle hydrocarbons is separated through the side part of the DWC, and a stream containing C 23 High-boiling hydrocarbons above is separated through the lower part of the DWC.

[0050] At the same time, when multiple distillation columns 10 are provided, they can be operated by setting the condensers connected to the upper parts of the respective columns to different temperatures. That is, the carbon range of the pyrolysis oil (distilled oil) is changed by setting the upper temperature of the upstream column to be relatively high and the upper temperature of the downstream column to be relatively low. For example, the upper temperature of the upstream column can be adjusted to 220 °C to 300 °C, and the upper temperature of the downstream column can be adjusted to 150 °C to less than 220 °C. As described above, the upper gaseous streams discharged from the upstream column and the downstream column can be condensed and then discharged as liquid oil, or a part of the upper gaseous stream can flow back to the upper parts of the respective columns. In addition, C 13-22 Middle oil (MO) or C 23 Heavy oil (HO) above can be selectively discharged through the middle and lower parts of the respective columns.

[0051] In this way, the gaseous stream generated by the primary pyrolysis of the waste plastic raw material can be separated according to the boiling point in one or more distillation columns to obtain a stream containing C 5-12 Light oil (LO), a stream containing C 13-22 Middle oil (MO), and a stream containing C 23 Heavy oil (HO) above.

[0052] In addition, in the process where the gaseous stream exchanges heat with water to obtain a liquid phase in a condenser connected to the upper part of the distillation column, the water fed into the condenser is converted into steam by recovering the waste heat of the gaseous stream.

[0053] In order to generate steam by recovering this preheat, in the present invention, hot water at 60°C to 100°C is used as the heat exchange water in the condenser provided in the upper part of the distillation column. The temperature of the water can be adjusted according to the composition of the pyrolysis oil required.

[0054] In particular, when multiple distillation columns are arranged as described above, the condenser of the upstream column operates at a high temperature, and the condenser of the downstream column operates at a low temperature, so that steam can be generated at different pressures. For example, when the operating temperature of the upper part of the distillation column is set relatively high and the carbon range of the pyrolysis oil moves to high boiling points, high-pressure steam can be generated by the heat absorption of the condenser connected to the upper part of the distillation column 20. At the same time, when the operating temperature of the upper part of the distillation column is set relatively low and the carbon range of the pyrolysis oil moves to low boiling points, low-pressure steam can be generated.

[0055] Thereafter, a part of the stream containing C 13-22 middle oil (MO) separated from the distillation column 20 is fed into the second reactor 11 for secondary pyrolysis, and the gaseous stream generated by the secondary pyrolysis is fed into the distillation column 20 (S3).

[0056] The second reactor 11 can be arranged in series with the distillation column 20. That is, the first reactor 10 is arranged on one side of the distillation column 20, and the second reactor 11 is arranged on the other side of the distillation column 20, so that the first reactor, the distillation column, and the second reactor can be arranged in series in sequence.

[0057] The pyrolysis of the middle oil (MO) part in the second reactor can be carried out at a higher temperature than the pyrolysis of the waste plastic raw material carried out in the first reactor. That is, in the present invention, in order to improve the decomposition efficiency of waste plastics, it is advantageous to carry out the pyrolysis in the first reactor and the pyrolysis in the second reactor under gradually increasing temperature conditions.

[0058] In an embodiment of the present invention, the temperature of the secondary pyrolysis in the second reactor 11 can be in the range of 420°C to 450°C, and when the secondary pyrolysis is carried out at a higher temperature than the primary pyrolysis, the yield of light oil (LO) generated according to the decomposition of the middle oil (MO) can be further increased. When the secondary pyrolysis temperature is lower than 420°C, it is difficult to induce sufficient pyrolysis of the middle oil (MO), and when the secondary pyrolysis temperature is higher than 450°C, the formation of solid carbides such as carbon can be accelerated.

[0059] In addition, the secondary pyrolysis can be carried out at a pressure of 0.9 bar to 1.1 bar for 1 hour to 3 hours, especially 1.5 hours to 2.5 hours. The secondary pyrolysis reaction time can be determined by considering the time when the production rate of light hydrocarbons with 12 or fewer carbon atoms starts to increase after the decomposition reaction of the middle oil (MO) begins.

[0060] Through the secondary pyrolysis, the sufficient pyrolysis of C 13-22 obtained from the waste plastic raw materials can be induced, and C 13-22 in the middle oil (MO) can be converted to have a lower carbon number, so that the yield of C 5-12 light oil (LO) finally obtained can be increased.

[0061] At the same time, the stream of heavy oil (HO) containing C 23 above separated from the distillation column 20 is recycled to the first reactor 10 (S4).

[0062] The heavy oil (HO) contained in the gaseous product obtained by the primary pyrolysis of the waste plastic raw materials is a high-boiling hydrocarbon with a boiling point of 360 °C or higher, and can be discharged from the lower part of the distillation column. The stream containing the high-boiling heavy oil can be transferred to the first reactor and pyrolyzed again to be converted into C 5-12 light oil (LO) and C 13-22 middle oil (MO).

[0063] In addition, each of the low-level residues of the first reactor 10 and the second reactor 11 can be fed into the residue treatment reactor 30, and additional pyrolysis is carried out.

[0064] The residue treatment reactor 30 can include: an upper part for carrying out the pyrolysis reaction, and a lower part for discharging the high-viscosity residue wax.

[0065] The lower residue of the first reactor and the lower residue of the second reactor can be fed into the upper part of the residue treatment reactor, and additional pyrolysis can be carried out. In this case, the additional pyrolysis is preferably carried out at a temperature higher than the pyrolysis in the first reactor or the second reactor.

[0066] In addition, the additional pyrolysis in the residue treatment reactor is carried out at a pressure of 0.9 bar to 1.1 bar for 1 hour to 3 hours, especially 1.5 hours to 2.5 hours. The additional pyrolysis reaction time is determined by considering the time when the production rate of low-boiling hydrocarbons increases after the decomposition reaction of the residue begins.

[0067] The pyrolysis oil obtained from the above pyrolysis process can contain a high fraction of high-value-added C 5-12 light oil (LO). In addition, relative to the total weight of the waste plastic raw materials, the finally obtained C5-12 The content of light oil (LO) can be 50% to 70% by weight, especially 50% to 60% by weight. The C obtained in high yield can be 5-12 condensed for light hydrocarbons and then effectively used as high-quality fuel oil.

[0068] In particular, the present invention finally obtains C 5-12 The boiling point of the light hydrocarbon fuel oil is 0°C to 230°C, especially 30°C to 216°C, the kinematic viscosity at 40°C is 0.3 cSt to 1.0 cSt, especially 0.4 cSt to 0.9 cSt, and the flash point is above -80°C (for example, -40°C). Therefore, it can be effectively used as a petrochemical raw material.

[0069] According to the present invention described above, in one or more distillation towers connected in series with the first reactor for pyrolyzing waste plastic raw materials, the pyrolyzed gaseous stream is separated according to the boiling point, and then, the middle oil (MO) part of the separated components is subjected to secondary pyrolysis in the second reactor, while the heavy oil (HO) is recycled to the first reactor, so that the yield of high-quality light oil (LO) can be improved.

[0070] In addition, since the pyrolysis in the first reactor and the pyrolysis in the second reactor are carried out under the condition of gradually increasing temperature, the decomposition efficiency of waste plastics is improved, so that the yield of high-quality light hydrocarbon oil can be increased, and the emission of residual wax that is not fully utilized can be minimized.

[0071] In addition, the upper operating temperature of the distillation tower is adjusted to a predetermined range, so that the fraction of the light oil (LO) component in the gaseous stream separated in the distillation tower can be increased, and in the process of exchanging heat between the gaseous stream and water in the condenser connected to the upper part of the distillation tower to obtain a liquid phase, the water fed into the condenser is converted into steam by recovering the waste heat of the gaseous stream.

[0072] In addition, using the light hydrocarbon oil obtained from the pyrolysis of waste plastics can reduce the greenhouse gas emissions caused when feeding raw materials to the petrochemical process, can improve the process efficiency (such as energy saving), and since no harmful gases are generated during the treatment of waste plastics, it can be environmentally friendly.

[0073] Examples

[0074] Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are provided for illustrative purposes of the present invention. It is obvious to those skilled in the art that various modifications and changes can be made without departing from the scope and spirit of the present invention, and the scope of the present invention is not limited thereto.

[0075] Example 1:

[0076] As Figure 1As shown, the pyrolysis of waste plastics is carried out using a process system including a first reactor 10, a distillation column 20, a second reactor 11, and a residue treatment reactor 30.

[0077] First, 100 parts by weight of waste plastics containing polyethylene (PE) and polypropylene (PP) in a weight ratio of 6:4 are melt-fed into the stirred first reactor 10. The reactor 10 is heated by an external heating device. When the temperature reaches 430 °C, a primary pyrolysis reaction is carried out for 4 hours while maintaining a constant temperature.

[0078] The gaseous stream discharged from the upper part of the first reactor 10 is fed into the distillation column 20 and separated into its components according to the boiling point. Specifically, the column is operated at an upper temperature of 210 °C. The gaseous stream is discharged from the upper part, cooled to 25 °C in a condenser to obtain condensed liquid light oil (LO), and the uncondensed gas components are discharged. Subsequently, a stream containing middle oil (MO) is obtained from the middle trays of the column, and a lower stream containing heavy oil (HO) is excluded from the lower trays of the column. 5-12 A stream containing middle oil (MO) is obtained from the middle trays of the column, and a lower stream containing heavy oil (HO) is excluded from the lower trays of the column. 13-22 A stream containing middle oil (MO) is obtained from the middle trays of the column, and a lower stream containing heavy oil (HO) is excluded from the lower trays of the column. 23-40 A lower stream containing heavy oil (HO) is excluded from the lower trays of the column.

[0079] A part of the middle oil (MO) separated from the distillation column 20 is fed into the second reactor 11, and a secondary pyrolysis is carried out at 430 °C for 2 hours. The gaseous stream generated by the secondary pyrolysis is recycled to the distillation column 20.

[0080] Meanwhile, the lower discharge stream of the distillation column 20 is recycled to the first reactor 10 for further pyrolysis.

[0081] In addition, each of the lower residues of the first reactor 10 and the second reactor 11 is fed into the residue treatment reactor 30, and an additional pyrolysis is carried out at 450 °C for 2 hours. The upper gaseous stream generated is recycled through the lower part of the distillation column 20, and the residue remaining in the lower part is discharged.

[0082] Example 2:

[0083] The same process as in Example 1 is carried out, except that a part of the middle oil (MO) is subjected to secondary pyrolysis at 440 °C for 2 hours in the second reactor 11.

[0084] Example 3:

[0085] The same process as in Example 1 is carried out, except that a part of the middle oil (MO) is subjected to secondary pyrolysis at 450 °C for 2 hours in the second reactor 11.

[0086] Comparative Example 1:

[0087] As Figure 2As shown, pyrolysis of waste plastics is carried out using a process system including a first reactor 10, a distillation column 20, and a residue treatment reactor 30. Specifically, the same process as in Example 1 is carried out, except that a part of the middle oil (MO) separated in the distillation column 20 is not subjected to secondary pyrolysis.

[0088] Table 1 shows the composition of the pyrolysis products obtained based on the feed in the examples and comparative examples analyzed by GC-MS (weight % based on the raw material feed amount).

[0089] [Table 1]

[0090]

[0091] As can be seen from Table 1, in Examples 1 to 3, the pyrolysis gas stream is separated according to the boiling point in one or more distillation columns connected in series with the first reactor for pyrolyzing the waste plastic raw material, and then a part of the middle oil (MO) in the separated components is subjected to secondary pyrolysis in the second reactor. Compared with Comparative Example 1 where secondary pyrolysis was not carried out, the yield of low-boiling light oil (LO) was improved.

[0092] Furthermore, in Examples 2 and 3 where primary pyrolysis and secondary pyrolysis were carried out under the condition of gradually increasing temperature, compared with Example 1 where primary pyrolysis and secondary pyrolysis were carried out at the same temperature, the yield of light oil was further improved.

Claims

1. A method for preparing pyrolysis oil from waste plastics, the method comprising: (S1) feeding the waste plastic raw material into a first reactor, performing primary pyrolysis, and discharging an upper gaseous stream generated by the pyrolysis through an upper portion of the first reactor; (S2) feeding the upper gaseous stream of the first reactor into one or more distillation towers, and separating the upper gaseous stream of the first reactor according to boiling point to obtain a gaseous stream containing C 5-12 Light oil (LO) stream, containing C 13-22 Intermediate oil (MO) stream and containing C 23 Heavy oil (HO) streams above; (S3) feeding a portion of the stream containing the intermediate oil (MO) separated in the distillation tower to a second reactor for secondary pyrolysis, and feeding a gaseous stream generated by the secondary pyrolysis to the distillation tower; as well as (S4) recycling the stream containing heavy oil (HO) separated in the distillation column to the first reactor.

2. The method according to claim 1, wherein: The first reactor, the distillation column, and the second reactor are connected in series.

3. The method according to claim 1, wherein: The stream containing light oil (LO) is separated into a gas phase in the upper part of the distillation column, and then exchanges heat with water in a condenser and is recovered as liquid light oil (LO).

4. The method according to claim 3, wherein: The water fed to the condenser is hot water at 60° C. to 100° C., and the waste heat of the gas phase is recovered and converted into steam through the heat exchange.

5. The method according to claim 1, wherein: The upper temperature of the distillation tower is adjusted to 150°C to below 220°C to discharge the distilled water containing more than 70% C 5-12 The upper gaseous stream of the distillation column of light hydrocarbons.

6. The method according to claim 1, wherein: The pyrolysis in the first reactor is performed at a temperature lower than the temperature of the pyrolysis in the second reactor.

7. The method according to claim 6, wherein: The pyrolysis in the first reactor is carried out at 400°C to 450°C.

8. The method according to claim 6, wherein: The pyrolysis in the second reactor is carried out at 420°C to 450°C.

9. The method according to claim 1, wherein: Each of the lower residue of the first reactor and the lower residue of the second reactor is fed to a residue treatment reactor, subjected to additional pyrolysis, and a gaseous stream generated by the additional pyrolysis in the residue treatment reactor is recycled to the distillation column.

10. The method according to claim 9, wherein: The additional pyrolysis in the residue treatment reactor is performed at a temperature higher than the pyrolysis temperature of the first reactor or the second reactor.

11. The method according to claim 1, wherein: The content of the light oil (LO) is 50 wt % to 70 wt % relative to the total weight of the waste plastic raw material.

12. The method according to claim 1, wherein: The waste plastic raw material is a mixture containing polyethylene (PE) or polypropylene (PP).

13. The method according to claim 1, wherein: The waste plastic raw material is fed into the first reactor after undergoing a pretreatment process including crushing, washing, drying and melting.

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