Method for processing pyrolysis oil from polyolefin-based plastic

Abstract

The present invention relates to a method for processing pyrolysis oil from polyolefin-based plastic. The method of the present invention avoids wastage of high-added-value alpha olefins and thus improves the yield of alpha olefins by pre-separating pyrolysis oil into multiple streams and then processing the streams and also achieves good yields of lower olefins (such as ethylene, propylene and butadiene), thereby increasing the economic value of the chemical cycle of polyolefin waste plastics.

Description

[0001] Method for processing pyrolysis oil from polyolefin-based plastic

[0002] Technical Field

[0003] The present invention relates to a method for processing pyrolysis oil from polyolefin-based plastic.

[0004] Background Art

[0005] As attention turns to carbon footprint and demand for fuel oil falls globally, quality improvement of pyrolysis oil (py-oil) from polyolefin-based plastics and its utilization as a resource is attracting interest. One feasible route for large-scale utilization is by pyrolysis or catalytic pyrolysis of polyolefins to obtain pyrolysis oil. However, pyrolysis oil obtained by pyrolysis has a wide boiling range and a complex composition, with a broad carbon number distribution (C1 - C70), which has restricted its utilization as a resource. Thus, it is often necessary to further subject pyrolysis oil to processes for quality improvement, such as catalytic cracking, hydrogenation to remove heteroatoms, hydrogenation saturation, separation and refining, to obtain naphtha (consisting mostly of C5 - C12 saturated hydrocarbons) and other hydrocarbons, which are then split by steam cracking into low-carbon olefins (ethylene, propylene and butadiene), which are finally polymerized to form new polyolefin plastics, completing the chemical cycle; in this way, the use of non-renewable resources is reduced, so the carbon footprint is reduced.

[0006] Products of the pyrolysis of polyolefin plastics generally may include a large amount (e.g. 50 - 70 wt%) of straight-chain olefins and are dominated by a-olefins (more than half of the total amount). Thus, the production of a-olefins outside the route of utilization as a resource (polyolefin-based plastics - pyrolysis oil -> naphtha - low-carbon olefins) can increase the economic value of the chemical recovery of polyolefin-based plastics.

[0007] Summary of the Invention

[0008] An objective of the present invention is to provide a method for processing pyrolysis oil from polyolefin-based plastics; by pre-separating the pyrolysis oil into multiple streams, purifying an intermediate stream (C5 - C12) and cracking a heavy stream (above C12), the method avoids wastage of high-added-value a-olefins and thus increases the a-olefin yield, and achieves a good yield of low-carbon olefins (e.g. ethylene, propylene and butadiene). Catalyst and process conditions may also be changed according to different production objectives (low-carbon olefins or a-olefins), to adjust the distribution of cracking products, thus increasing the economic value of chemical recovery of polyolefin-based plastics, increasing process flexibility, and reducing carbon emissions for the entire life cycle of chemicals.

[0009] According to the present invention, the abovementioned objective is achieved through the following embodiments:

[0010] 1. A method for processing pyrolysis oil from polyolefin-based plastic, the method comprising the following steps: step (1): separating the pyrolysis oil to obtain at least the following three streams: stream 1, comprising hydrocarbons with less than 5 carbon atoms; stream 2, comprising hydrocarbons with 5 - 12 carbon atoms; and stream 3, comprising hydrocarbons with more than 12 carbon atoms; optional step (2): optionally hydrogenating stream 2, feeding stream 2 or the optionally hydrogenated stream 2 into a cracking furnace 1 to undergo cracking to obtain a cracked stream 4; and step (3): optionally hydrogenating stream 3, feeding stream 3 or the optionally hydrogenated stream 2 into a cracking furnace 2 to undergo cracking to obtain a cracked stream 5.

[0011] 2. The method according to embodiment 1, wherein the amount of stream 1 is 1 - 10 wt%, preferably 1.5 - 8 wt%, and more preferably 1.5 - 5 wt%, based on the total weight of stream 1 , stream 2 and stream 3.

[0012] 3. The method according to embodiment 1 or 2, wherein the amount of stream 2 is 8 - 40 wt%, preferably 10 - 35 wt%, and more preferably 12 - 30 wt%, based on the total weight of stream 1 , stream 2 and stream 3.

[0013] 4. The method according to any one of embodiments 1 to 3, wherein the amount of stream 3 is 50 - 90 wt%, preferably 60 - 85 wt%, and more preferably 65 - 80 wt%, based on the total weight of stream 1, stream 2 and stream 3.

[0014] 5. The method according to any one of embodiments 1 to 4, wherein the separation in step (1) is performed by a process selected from distillation (e.g. atmospheric or reduced-pressure rectification), flash evaporation, condensing, extraction, absorption, adsorption or a combination thereof.

[0015] 6. The method according to any one of embodiments 1 to 5, wherein the cracking in cracking furnace 1 is performed by catalytic cracking or steam cracking; preferably, steam cracking in cracking furnace 1 is performed at 600 - 900°C or 610 - 800°C or 670 - 700°C; preferably, catalytic cracking in cracking furnace 1 is performed at 400 - 700°C or 510 - 600°C or 530 - 580°C; especially preferably, cracked stream 4 comprises olefins with fewer than 5 carbon atoms, and more preferably, the total amount of olefins with fewer than 5 carbon atoms in cracked stream 4 is at least 35 wt%, or 40 - 55 wt%, based on the total weight of stream 2.

[0016] 7. The method according to any one of embodiments 1 to 6, wherein the cracking in cracking furnace 2 is performed by catalytic cracking or steam cracking; preferably, steam cracking in cracking furnace 2 is performed at 500 - 680°C or 510 - 620°C or 550 - 600°C; preferably, catalytic cracking in cracking furnace 2 is performed at 500 - 700°C or 510 - 600°C; especially preferably, the total amount of hydrocarbons with 5 - 12 carbon atoms in cracked stream 5 is 10 - 40 wt%, in particular 15 - 30 wt%, based on the total weight of stream 3; more preferably, the total amount of a-olefins with 5 or more carbon atoms in cracked stream 5 is 20 - 55 wt%, in particular 30 - 50 wt%, and especially 35 - 45 wt%, based on the total weight of stream 3.

[0017] 8. The method according to any one of embodiments 1 to 7, wherein the pyrolysis oil is derived from polyolefin plastic.

[0018] 9. The method according to any one of embodiments 1 to 8, wherein the pyrolysis oil comprises 40 - 80 wt%, preferably 50 - 75 wt% olefins, and 20 - 60 wt%, preferably 25 - 50 wt% alkanes, based on the total weight of the pyrolysis oil.

[0019] 10. The method according to any one of embodiments 1 to 9, wherein the pyrolysis oil comprises 30 - 70 wt%, preferably 40 - 60 wt% a-olefins, based on the total weight of the pyrolysis oil.

[0020] 11. The method according to any one of embodiments 1 to 10, wherein the total amount of hydrocarbons with 6 - 16 carbon atoms in the pyrolysis oil is 30 - 65 wt%, preferably 35 - 60 wt%, based on the total weight of the pyrolysis oil; more preferably, the total amount of hydrocarbons with 6 - 12 carbon atoms in the pyrolysis oil is 15 - 50 wt%, preferably 18 - 40 wt%, based on the total weight of the pyrolysis oil.

[0021] 12. The method according to any one of embodiments 1 to 11 , wherein stream 2 is fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is fed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is hydrogenated and fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is hydrogenated and fed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

[0022] 13. The method according to any one of embodiments 1 to 5, wherein optional step (2) is not performed, and stream 2 is used directly as feedstock for a-olefins.

[0023] 14. The method according to any one of embodiments 1 to 11 , wherein optional step 2 is not performed; and in step (3), optional hydrogenation is not performed, and stream 3 is passed into cracking furnace 2 to undergo cracking to obtain cracked stream 5.

[0024] 15. The method according to embodiment 14, wherein steam cracking is performed in cracking furnace 2 at a temperature of 510 - 650°C, preferably 530 - 600°C or 550 - 580°C.

[0025] 16. The method according to any one of embodiments 1 to 15, wherein cracked stream 5 is recirculated to separation in step (1).

[0026] Brief Description of the Drawings

[0027] Fig. 1 is a flow chart of a technical solution of the present invention. Fig. 2 is a flow chart of a technical solution of the present invention.

[0028] Detailed Description of Embodiments

[0029] Herein, the yield of a product is calculated as follows: weight of the product / weight of feedstock x 100%. For example, if product 1 of weight a, product 2 of weight b and product 3 of weight c are obtained from feedstock of weight r, then the yields of these three products are, respectively: product 1 yield = a / r x 100%, product 2 yield = b / r x 100%, and product 3 yield = c / r x 100%.

[0030] Herein, the boiling range of naphtha may be 50°C - 210°C, and the number of carbon atoms thereof may be about C5 - C12.

[0031] An a-olefin (also denoted alpha-olefin herein) is a monoolefin with a double bond at an end of the molecular chain, a-olefins are an important feedstock in the production of chemical products such as copolymers, poly a-olefins (PAO), low-molecular-weight fatty acids, plasticizers, surfactants and lubricating oil additives.

[0032] Herein, polyolefin-based plastics are plastics in which the proportion of polyolefin plastics is 50 wt% or more, e.g. 60 wt% or more, or 70 wt% or more, or 80 wt% or more, or 90 wt% or more, or 95 wt% or more, or even 98 wt% or more. Polyolefin-based plastics may be recovered waste plastics or may be a fresh mixture of plastics or a mixture of plastics from another source.

[0033] The present invention relates to a method for processing pyrolysis oil from polyolefin-based plastic, the method comprising the following steps: step (1): separating the pyrolysis oil to obtain at least the following three streams: stream 1, comprising hydrocarbons with fewer than 5 carbon atoms; stream 2, comprising hydrocarbons with 5 - 12 carbon atoms; and stream 3, comprising hydrocarbons with more than 12 carbon atoms; optional step (2): optionally hydrogenating stream 2, feeding stream 2 or the optionally hydrogenated stream 2 into a cracking furnace 1 to undergo cracking to obtain a cracked stream 4; and step (3): optionally hydrogenating stream 3, feeding stream 3 or the optionally hydrogenated stream 2 into a cracking furnace 2 to undergo cracking to obtain a cracked stream 5.

[0034] Fig. 1 is a flow chart of a technical solution of the present invention. Fig. 1 contains dotted- line boxes to indicate steps / operations that can be optionally performed. Fig. 2 is a flow chart of a technical solution of the present invention. Fig. 2 contains dotted-line boxes to indicate steps / operations that can be optionally performed.

[0035] In the present disclosure, the term "pyrolysis" refers to a chemical process which a solid mixture (e.g. solid waste or feedstock made therefrom) containing polymers undergoes at a high temperature, in which chemical bonds are forcibly opened and smaller molecules (including but not limited to hydrocarbons with 1 - 70 carbon atoms, other non-hydrocarbon organic substances, and inorganic substances such as hydrogen sulfide, nitrogen oxides and sulfur oxides, etc.) are produced. For example, low density polyethylene (LDPE) can undergo pyrolysis at a high temperature to produce a mixture of various hydrocarbons.

[0036] A pyrolysis product is a composition that comes from a pyrolysis process. A pyrolysis product might be one or more of pyrolysis gas, pyrolysis oil and pyrolysis wax. The pyrolysis product might be in a gas phase, liquid phase or solid phase at 25°C and 1 atmosphere. A crude pyrolysis product is a product which is obtained directly from a pyrolysis process or which has only undergone operations such as condensing, fractionation or filtration after the pyrolysis process.

[0037] Pyrolysis gas is a composition which is gaseous when measured at 25°C and 1 atmosphere, and at least a portion thereof is obtained from pyrolysis of solid waste at a high temperature such as 300°C - 800°C.

[0038] Pyrolysis oil is a composition which is liquid when measured at 25°C and 1 atmosphere, and at least a portion thereof is obtained from pyrolysis of solid waste at a high temperature such as 300°C - 800°C.

[0039] Pyrolysis wax is a composition which is solid when measured at 25°C and 1 atmosphere, and at least a portion thereof is obtained from pyrolysis of solid waste at a high temperature such as 300°C - 800°C.

[0040] Pyrolysis gas, pyrolysis oil and / or pyrolysis wax generally contain hydrocarbons, such as alkanes, olefins, alkynes, aromatic hydrocarbons and cycloalkanes with different numbers of carbon atoms. Pyrolysis oil and / or pyrolysis wax might also contain other organic substances, water, gums, inorganic salts or other impurities. Pyrolysis gas generally contains one or more of hydrogen, oxygen, nitrogen, nitrogen oxides, sulfur oxides, hydrogen sulfide, ammonia, hydrogen chloride, carbon monoxide, carbon dioxide, methane, ethane, ethylene, acetylene, propane, propylene, propyne, butane, isobutane, 1 -butene, 2-butene, 2-methyl-propene, 1-butyne, 2- butyne and butadiene.

[0041] Pyrolysis can be carried out in the presence of a catalyst. The term "pyrolysis" includes slow pyrolysis, rapid pyrolysis, flash pyrolysis, pyrolysis in the presence of hydrogen, flash catalytic and catalytic pyrolysis. These types of pyrolysis differ in terms of process temperature, heating rate, residence time, reaction atmosphere, feedstock granularity, catalyst type, etc., resulting in different product distributions and qualities.

[0042] In the context of the present invention, the term "pyrolysis oil" is pyrolysis oil derived from pyrolysis of polyolefin-based plastics.

[0043] In an embodiment, the polyolefin-based plastics are plastic waste. In the context of the present invention, the term "plastic waste" refers to plastic material discarded after use, i.e. which has already reached the end of its service life. Plastic waste can be pure polymer plastic waste, mixed plastic waste or film waste. Correspondingly, the term "plastic waste" includes industrial and household plastic waste, agricultural and horticultural plastic materials, such as greenhouse sheeting.

[0044] In some embodiments, polyolefin-based plastics include (for example) polypropylene (PP), polyethylene (PE), polyisobutylene (PIB), and copolymers thereof. Polyolefin-based plastics may incorporate other polymers, including polystyrene, polyamides, polyesters, polycarbonates, epoxy resins, polyurethanes and copolymers (e.g. random or block copolymers) or mixtures thereof. In some embodiments, polyolefin-based plastics include linear low density polyethylene (LLDPE), low density polyethylene (LDPE), medium density polyethylene (MDPE), or high density polyethylene (HDPE). Some embodiments of polymer mixtures include (for example) PP / HDPE, PP / LLDPE and LLDPE / HDPE, as well as ternary mixtures such as PP / HDPE / LLDPE.

[0045] In certain embodiments in which polypropylene is used, the polypropylene is a polypropylene random copolymer, an alternating or segmented copolymer, or a block copolymer containing one or more comonomer selected from ethylene, 1 -propylene, C4-C20-alpha olefins, vinylcyclohexane, vinylcyclohexene, C4-C20-alkadienes, C5-C12-cycloalkadienes and norbornene derivatives, in which the total molar amount of propylene and the comonomer is 100%. Examples of suitable C4-C20-alpha olefins include (but are not limited to) 1-butene, 1- pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1- tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene and 4-methyl-1 -pentene. Examples of suitable C4-C20-alkadienes include, but are not limited to, hexadiene and octadiene. Examples of suitable C5-C12-cycloalkadienes include, but are not limited to, cyclopentadiene, cyclohexadiene and cyclooctadiene. Examples of suitable norbornene derivatives include, but are not limited to, 5-ethylidene-2-norbornene, dicyclopentadiene and methylene-dimethylene- hexahydronaphthalene.

[0046] In certain embodiments, a polyolefin polymer forming the polyolefin-based plastic is selected from polypropylene, polyethylene, and copolymers or mixtures thereof. Plastics may also include other polymers including, but not limited to, polystyrene, polyamides, polyesters, polycarbonates, epoxy resins, polyurethanes, or copolymers or mixtures thereof. In certain embodiments, the total amount of other polymers incorporated in the polyolefin-based plastic is less than 50 wt%, less than 45 wt%, less than 40 wt%, less than 35 wt%, less than 30 wt%, less than 25 wt%, less than 20 wt%, less than 15 wt%, less than 10 wt% or less than 5 wt%, based on the total weight of the polyolefin-based plastic. In some embodiments, the polyolefin-based plastic is a polyolefin plastic. The polyolefin plastics are plastics formed from polyolefin polymers.

[0047] In one embodiment, the pyrolysis oil comprises 40-80 wt% (e.g., 45 wt%, 50 wt%, 60 wt%, 70 wt%, or 75 wt%), preferably 50-75 wt%, or 55-72 wt% olefins, and 20-60 wt% (e.g., 25 wt%, 30 wt%, 40 wt%, 50 wt%, or 55 wt%), preferably 25-50 wt% or 28-40 wt% alkanes, based on the total weight of the pyrolysis oil.

[0048] In one embodiment, the pyrolysis oil comprises 30-70 weight percent (e.g., 35 weight percent, 40 weight percent, 50 weight percent, 60 weight percent, or 65 weight percent), preferably 40-60 weight percent, or 45-58 weight percent alpha olefins, based on the total weight of the pyrolysis oil.

[0049] Herein, alpha olefins include, for example, C4-C20-alpha olefins, or C4-C16-alpha olefins, or C4-C12-alpha olefins or C5-C12-alpha olefins.

[0050] In one embodiment, the total amount of hydrocarbons having a carbon number of 6 to 16 in the pyrolysis oil is 30 to 65 wt% (e.g., 35 wt%, 40 wt%, 50 wt%, 55 wt% or 60 wt%), preferably 35 to 60 wt% or 40 to 55 wt%, based on the total weight of the pyrolysis oil.

[0051] In one embodiment, the total amount of hydrocarbons having a carbon number of 6 to 12 in the pyrolysis oil is 15 to 50 weight percent (e.g., 18 weight percent, 20 weight percent, 22 weight percent, 25 weight percent, 30 weight percent, 40 weight percent, or 45 weight percent), preferably 18 to 40% by weight or 20 to 35% by weight or 20 to 30% by weight, based on the total weight of the pyrolysis oil.

[0052] “Weight percent” has the same meaning as “wt%” and “% by weight” herein.

[0053] Pyrolysis oil can usually be liquid or solid at 15°C. " Being liquid at 15°C" in the present invention means that the pyrolysis oil has a density of at most 1 .0 g / mL at 15°C and 1013 mbar, as determined according to DIN EN ISO 12185, for example, a density of 0.70 to 0.98 g / mL. Of course, if the pyrolysis oil is solid, the pyrolysis oil can melt at an elevated temperature, for example at 50-60°C.

[0054] In step (1) of the method of the present invention, the pyrolysis oil is separated to obtain at least the following three streams: stream 1, comprising hydrocarbons with less than 5 carbon atoms; stream 2, comprising hydrocarbons with 5 - 12 carbon atoms; and stream 3, comprising hydrocarbons with more than 12 carbon atoms.

[0055] The separation in step (1) may be performed by a process selected from distillation (e.g. atmospheric or reduced-pressure rectification), flash evaporation, condensing, extraction, absorption, adsorption or a combination thereof. In a preferred embodiment, the separation in step (1) is carried out by distillation, such as atmospheric distillation or rectification under reduced pressure.

[0056] The distillation is usually carried out at a pressure of 0.9 to 1.1 atm, preferably 0.95 to 1.05 atm, preferably 1 atm. In some embodiments, it may be advantageous to carry out the distillation under reduced pressure. Distillation is usually carried out at a column-bottom temperature of 150°C or higher, for example, 200-400°C or 250-400°C or 300-400°C.

[0057] In one embodiment, the amount of stream 1 is from 1 to 10% by weight (e.g. 1.5%, 2%, 3%, 4%, 5%, 6%, 8% or 10% by weight), preferably from 1.5 to 8%, more preferably from 1.5 to 5% by weight, based on the total weight of stream 1 , stream 2 and stream 3. In one embodiment, the amount of stream 2 is from 8 to 40 weight percent (e.g., 10, 12, 15, 20, 25, 30, or 35 weight percent), preferably from 10 to 35 weight percent, more preferably from 12 to 30 weight percent, based on the total weight of stream 1, stream 2 and stream 3.

[0058] In one embodiment, the amount of stream 3 is from 50 to 90% by weight (e.g., 60%, 65%, 70%, 75%, 80% or 85% by weight), preferably from 60 to 85% by weight, more preferably from 65 to 80% by weight, based on the total weight of stream 1 , stream 2 and stream 3.

[0059] Stream 1 is mainly mixed hydrocarbons smaller than C5, and the boiling range thereof can be, for example, less than 70°C. Stream 2 is predominantly C5-C12 mixed hydrocarbons, and the boiling range thereof can be, for example, 45°C to 220°C. Stream 3 is mainly mixed hydrocarbons larger than C12, and the boiling range thereof can be, for example, 50°C to 350°C.

[0060] In optional step (2) of the present invention, stream 2 is optionally hydrogenated, and fed into cracking furnace 1 to undergo cracking to obtain cracked stream 4.

[0061] As defined herein, hydrogenation, also known as "hydrotreating, hydrogenation processing, hydrogenation operation, in the presence of hydrogen", refers to a series of catalytic chemical process methods, in which a hydrogen reaction is used to remove impurities (such as oxygen, chlorine, bromine, iodine, sulfur, nitrogen, phosphorus, silicon or arsenic), for the purpose of saturating unsaturated functional groups such as double bonds, triple bonds and aromatic rings, breaking carbon-carbon bonds, reducing average molecular weight, and rearranging the molecular structure of the feedstock, or any combination thereof.

[0062] The preferred term "hydrogenation" means that a hydrogen reaction is used to remove impurities (such as oxygen, chlorine, sulfur, nitrogen, phosphorus or arsenic) and / or to saturate unsaturated functional groups such as double bonds, triple bonds and aromatic rings.

[0063] The hydrogenation is usually carried out in the presence of a catalyst. The catalyst usually comprises at least one Group 6 metal component and at least one Group 8, 9 or 10 metal component combined with a support, and is for example a nickel-based catalyst. Exemplary hydrotreating catalysts include NiMo and CoMo, preferably with the aforementioned metal components loaded on a support, including but not limited to alumina, silica, zirconia, titania, zinc oxide, zeolites, molecular sieves, amorphous aluminosilicate solids, carbon materials and solid acids. An exemplary hydrotreating catalyst is NHVIO / AI2O3. Another exemplary hydrotreating catalyst is CoMo / AhOs.

[0064] An exemplary hydrogenation temperature is 180-350°C, or 250-350°C. An exemplary hydrogen partial pressure is 1-10 bar. The mass space velocity (WHSV) of the oil is usually 0.1- 10h'1, or 0.1-5h'1, preferably 0.1-2IT1, more preferably 0.2-1IT1(e.g. 0.2, 0.4, 0.6 or 0.8h'1). An exemplary hydrogen / oil ratio (at normal temperature and normal pressure) is 300-600 L / L, preferably 400-500 L / L. Through hydrogenation, the removal rates of N, S, Cl can reach more than 90%, and bromine values fall by more than 40 per cent.

[0065] In one embodiment, the cracking in cracking furnace 1 is carried out by catalytic cracking or steam cracking.

[0066] "Steam cracking" should be understood to mean a process in which relatively long-chain hydrocarbons, such as hydrogenated or unhydrogenated stream 2, are converted to short-chain hydrocarbons by thermal cracking in the presence of water vapor. Steam cracking can provide ethylene, propylene, butylene and / or butadiene as reaction products. For example, methane, ethane, propane and / or hydrogen can be produced as by-products.

[0067] In one embodiment, in steam cracking, the temperature can be 600-900°C (e.g., 610, 620, 640, 650, 670, 680, 690, 700, 750, 800 or 850°C), preferably 610-800°C or 650-800°C or 610- 700°C or 670-700°C or 670-690°C. The residence time of the feedstock may be from 0.1 to 5 seconds (e.g. 0.5, 1, 2, 3 or 4 seconds), preferably from 1 to 3 seconds.

[0068] In the cracking furnace 1 , the weight ratio of steam to the stream fed to the cracking furnace (also known as the water-to-oil ratio) can be 0.1-0.7 (e.g. 0.15, 0.2, 0.25, 0.3, 0.4, 0.5 or 0.6), 0.15-0.5 or 0.15-0.4. The oil-water mixing volume space velocity (GHSV) may be 0.2-0.8 s-1, or 0.3-0.6 S'1, or 0.35-0.5 S'1.

[0069] The pressure in the steam cracking can be 500-1500 mbar, 700-1200 mbar, or it can be atmospheric pressure.

[0070] Catalytic cracking is carried out in the presence of a catalyst. Usually catalysts include zeolites (such as ZSM and SAPO). In one embodiment, the zeolite comprises a medium-pore zeolite and / or a large-pore zeolite, preferably a medium-pore zeolite. Medium- pore zeolites can be selected from ZSM zeolites, ZRP zeolites and combinations thereof. Large-pore zeolites are Y-type zeolites. The ZSM zeolite is preferably selected from ZSM-5, ZSM-11 , ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48 or mixtures thereof, preferably ZSM-5.

[0071] In one embodiment, the catalyst comprises 2-50 wt%, preferably 5-45 wt%, more preferably 10-40 wt% zeolite; 5-90 wt%, preferably 10-80% by weight, more preferably 20-70% by weight of inorganic oxide, and 0-70% by weight, preferably 5-60% by weight, more preferably 10-50% by weight of clay.

[0072] The inorganic oxide is used as a binder, and is preferably silica (SiC>2) and / or alumina (AhjCh). The clay is used as a carrier and is preferably kaolin and / or halloysite.

[0073] The temperature in catalytic cracking can be 400-700°C (such as 420, 450, 480, 510, 520, 530, 550, 580, 600, 620, 650 or 680°C), preferably 450-680°C or 500-680°C or 510-680°C or 510-650°C or 510-600°C or 530-580°C.

[0074] The residence time of the reactants can be 0.1-5s, or 0.3-3s, or 0.3-2s.

[0075] The pressure in the catalytic cracking can be 500-1500 mbar, 700-1200 mbar, or it can be atmospheric pressure.

[0076] Cracked stream 4 may include olefins (also known as lower olefins) having fewer than 5 carbon atoms, such as ethylene, propylene, and butadiene.

[0077] The total amount of olefins (such as ethylene, propylene, and butadiene) having a number of carbon atoms less than 5 in cracked stream 4 is at least 35 weight percent, or 40 to 55 weight percent, based on the total weight of stream 2.

[0078] In one embodiment, the total amount of olefins (e.g., ethylene, propylene, and butadiene) having a number of carbon atoms less than 5 in cracked stream 4 is at least 35 weight percent, or 40 to 55 weight percent, based on the total weight of cracked stream 4.

[0079] In step (3) of the present invention, stream 3 is optionally hydrogenated, and fed into cracking furnace 2 to undergo cracking to obtain cracked stream 5.

[0080] The hydrogenation in step (3) is as described above for step (2).

[0081] According to the present invention, the cracking in the cracking furnace 2 is carried out by catalytic cracking or steam cracking.

[0082] The description of catalytic cracking in the cracking furnace (2) is as described above for step (2).

[0083] The water-to-oil ratio, pressure, residence time and space velocity of steam cracking in cracking furnace 2 are as described above for step (2). The steam cracking temperature in cracking furnace 2 can be 500-680°C (such as 510, 520, 530, 540, 550, 560, 570, 580, 600, 620, 640 or 660°C), preferably 510-620°C or 550-600°C or 550-590°C or 550-580°C or 550- 570°C.

[0084] It is preferred that the total amount of hydrocarbons having 5 to 12 carbon atoms in the cracked stream 5 is 10 to 40% by weight (e.g. 20%, 30% or 40% by weight), especially 15 to 30% by weight, based on the total weight of stream 3. The total amount of alpha olefins having a number of carbon atoms of 5 and above in the cracked stream 5 is from 20 to 55% by weight (for example 20, 30, 40, 45 or 50% by weight), especially from 30 to 50% by weight, especially from 35 to 45% by weight, based on the total weight of stream 3. The total amount of alpha olefins having a number of carbon atoms of 5 to 12 in the cracked stream 5 is 8 to 20 weight percent (e.g., 9, 10, 11 , 12, 13, 14, 15, or 18 weight percent), or 9 to 18 weight percent, preferably 10-15 wt%, based on the total weight of stream 3.

[0085] The total amount of lower olefins (e.g. ethylene, propylene, and butadiene) in cracked stream 5 may be from 10 to 40 weight percent (e.g., 12, 15, 18, 20, 25, 30, or 35 weight percent), preferably from 12 to 30 weight percent, based on the total weight of stream 3.

[0086] In one embodiment of the present invention, stream 2 is fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

[0087] In one embodiment of the present invention, stream 2 is fed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

[0088] In one embodiment of the present invention, stream 2 is hydrogenated and fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

[0089] In one embodiment of the present invention, stream 2 is hydrogenated and fed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

[0090] In one embodiment of the present invention, optional step (2) is not carried out, and stream 2 is used directly as feedstock for a-olefins. Alpha olefins may be separated by the following methods: rectification, pressure swing adsorption, solvent absorption, low-temperature extraction, membrane separation, etc.

[0091] In one embodiment of the present invention, optional step 2 is not performed, and optional hydrogenation is not performed in step (3), feeding stream 3 into cracking furnace 2 to undergo cracking to obtain cracked stream 5. In this embodiment, stream 2 is used directly as feedstock for alpha olefins. As stated above, alpha olefins may be separated by the following methods: rectification, pressure swing adsorption, solvent absorption, low-temperature extraction, membrane separation, etc.

[0092] In one embodiment, cracked stream 5 is recirculated to separation in step (1).

[0093] According to the present invention, a naphtha stream (C5-C12) is enriched from pyrolysis oil by a pre-separation process, retaining the alpha olefin components therein of high added value; in this way, the economic value of polyolefin plastic pyrolysis oil can be increased, or the yield of lower olefins can be increased.

[0094] Examples

[0095] The invention and its effects are further illustrated by the following examples. These examples are for illustrative purposes and should not be construed as limiting the invention. The proportions or percentages given in the examples, unless otherwise stated, are by weight. The yields in the examples are all single pass yields (i.e. , yields of each product when the process is performed only once), unless otherwise stated. Feedstocks

[0096] The pyrolysis oil used in the examples was derived from polyethylene greenhouse sheeting, with total contents of alkanes, olefins, alpha olefins and hydrocarbons with carbon numbers greater than five as shown in Table 1. The boiling range is shown in Table 2, and the element and metal contents are shown in Table 3. The contents in Table 1 were obtained by mass spectrometry analysis.

[0097] able 1

[0098] As can be seen from Table 1 , pyrolysis oil is generally rich in olefins (67.4 wt%), of which 78.44% are alpha olefins. C6-C12 hydrocarbons account for 23.88 wt% and alpha olefins for

[0099] 14.17 wt%; C6-C16 hydrocarbons account for 47.04 wt% and alpha olefins for 27.03 wt%. able 2

[0100] As can be seen from Table 2, about 44 wt% of the pyrolysis oil is heavy oil with a boiling point > 360°C, while naphtha (boiling point < 210°C) has a content of about 18 wt%, and a diesel stream (210°C < boiling point < 360°C) has a content of about 38 wt%. Example 1 (multiple production of alpha olefins, all streams are not hydrogenated, cracking furnace 1 is not activated, cracking furnace 2 uses steam cracking)

[0101] 500g of pyrolysis oil is rectified in an atmospheric-pressure glass-packed column. The theoretical number of trays is 5, and the reflux ratio is 5. As the distillation progresses, the column bottom temperature increases continuously, until the column top temperature reaches 220°C; the column top temperature is controlled to remain at 220°C, and the column bottom temperature is not increased further. The temperature of the bottom of the column is controlled to not exceed 300°C throughout the process. The substance collected at the column top is recovered at normal temperature using a round-bottomed flask. The non-condensing portion is recovered in a dry ice trap. The cracked stream is rectified to obtain three streams. Stream 1 recovered from the dry ice cold trap accounts for 3 wt%, and is mainly mixed hydrocarbons smaller than C5 (boiling range less than 70°C); stream 2 of other substances collected at the column top accounts for 20 wt%, predominantly being C5-C12 mixed hydrocarbons (boiling range 45°C to 220°C); stream 3 of substances collected at the column bottom accounts for 77 wt%, and is mainly mixed hydrocarbons larger than C12 (boiling range 50°C to 350°C).

[0102] When stream 2 is used as a feedstock for the production of alpha olefins, it is not required to be sent to cracking furnace 1.

[0103] 5ml / min stream 3 goes directly into cracking furnace 2 (steam cracking unit) without hydrogenation, for multiple production of a naphtha stream, providing a feedstock for alpha olefin production. When the operating conditions are atmospheric pressure, a water to oil ratio of 0.2, and steam cracking temperatures of 520°C, 540°C and 560°C, respectively, the composition of the cracked product (cracked stream 5) is shown in Table 4.

[0104] Table 4 - Product yield of cracking furnace 2 (100% being the mass of stream 3)

[0105] Herein, the cracked product yield is calculated as follows:

[0106] The feedstock is weighed (mass recorded as F) and then enters a cracking reactor to undergo a cracking reaction. The products are all in the gas phase at high temperature; when cooled to 0°C , some of the products condense to the liquid phase (mass recorded as L). The rest of the products are non-condensable gases (mass recorded as G). Unless specifically measured, the mass of non-condensable gases is calculated by differential subtraction: G = F - L. Yield of liquid phase in product = L / F x 100%, gas phase yield = 1 - liquid phase yield = (1 - L / F) x 100%; when the gas phase product mass is specifically measured, gas phase yield = G / F x 100%. Yield of C5-C12 stream in liquid phase product = (L / F) x mass percent C5-C12 stream in liquid phase product. Ethylene yield = (1-L / F) x mass percent ethylene in the gas phase product. The other yields in the Examples section are calculated similarly.

[0107] The target product of Example 1 is C5-C12 alpha olefins. Taking the data in Table 4 at a cracking temperature of 560°C as an example, when stream 3 undergoes steam cracking, the C5-C12 alpha olefin single pass yield was 12.90 wt% (stream 3 accounted for approximately 77 wt% of the pyrolysis oil, so the yield on a pyrolysis oil basis was 12.90 wt% x 0.77 = 9.93 wt%); adding this to the C5-C12 alpha olefin yield in the feedstock of 14.17 wt% (Table 1) results in an overall C5-C12 alpha olefin single pass yield in Example 1 of 24.10 wt%. In addition, lower olefins are also produced as by-products, and the single pass yield is 19.01 wt% (= 24.69 wt% x 0.77) based on pyrolysis oil.

[0108] Example 2 (multiple production of lower olefins, all streams are not hydrogenated, cracking furnaces 1 and 2 both use steam cracking)

[0109] The rectification operation and material balance are the same as in Example 1 , and the product distribution when stream 3 passes through cracking furnace 2 is the same as in Example 1.

[0110] 5 ml / min of stream 2 is fed directly to cracking furnace 1 (steam cracking unit) without hydrogenation; at atmospheric pressure, with a water to oil ratio of 0.2, and steam cracking temperatures of 640°C, 660°C and 680°C respectively, the composition of the cracked product (cracked stream 4) is shown in Table 5. Table 5 - Cracking Furnace 1 Product Yield (100% being the mass of stream 2)

[0111] The target product of Example 2 is lower olefins. Taking the data in Table 5 at a cracking temperature of 680°C as an example, when stream 2 undergoes steam cracking in cracking furnace 1 , the single pass yield of lower olefins is 55.18 wt%; at the same time, stream 3 is steam cracked by cracking furnace 2 to yield a cracked product (cracked stream 5) containing certain lower olefins, with a single pass yield of 24.69 wt% (Table 4 , 560°C). The total single pass yield of lower olefins, based on the pyrolysis oil, is the sum of the single pass yields of lower olefins in cracked stream 4 and cracked stream 5: 30.05 wt% (= 55.18 wt% x 0.20 + 24.69 wt% x 0.77).

[0112] Example 3 (multiple production of lower olefins, all streams are not hydrogenated, cracking furnace 1 uses catalytic cracking, and cracking furnace 2 uses steam cracking)

[0113] The rectification operation and material balance are the same as in Example 1, and the product distribution when stream 3 passes through cracking furnace 2 is the same as in Example 1.

[0114] 1.8 ml / min of stream 2 is fed without hydrogenation directly to cracking furnace 1 (catalytic cracking unit), with 0.5 g of ZSM-5 as a catalyst; at atmospheric pressure, and catalytic cracking temperatures of 530°C and 560°C respectively, the composition of the cracked product (cracked stream 4) is shown in Table 6.

[0115] Table 6 (100% being the mass of stream 2) The target product of Example 3 is lower olefins. Taking the data in Table 6 at a cracking temperature of 560°C as an example, when stream 2 undergoes steam cracking in cracking furnace 1 , the single pass yield of lower olefins is 48.98 wt%; at the same time, stream 3 is steam cracked by cracking furnace 2 to yield a cracked product (cracked stream 5) containing certain lower olefins, with a single pass yield of 24.69 wt% (Table 4 , 560°C). Then the sum of the total single pass yields of lower olefins of cracked stream 4 and cracked stream 5, based on pyrolysis oil, is: 28.81 wt% (= 48.98 wt% * 0.20 + 24.69 wt% x 0.77).

[0116] Example 4 (multiple production of lower olefins, stream 2 is hydrogenated, cracking furnaces 1 and 2 both use steam cracking)

[0117] The rectification operation and material balance are the same as in Example 1 , and the product distribution when stream 3 passes through cracking furnace 2 is the same as in Example 1.

[0118] Cracking of unsaturated hydrocarbons easily results in carbon build-up. To reduce this effect, stream 2 is introduced into a hydrogenation unit 1 ; the hydrogen partial pressure is 1 MPa, the hydrogenation temperature is 300°C, a Ni-based catalyst is used, the mass space velocity WHSV of the oil = 0.37 h’1, and the hydrogen to oil ratio (L / L) is 467 : 1. After hydrogenation, the saturated hydrocarbon content is increased (the alkane content is increased, the olefin and aromatic hydrocarbon contents are reduced, and the bromine value is reduced), and the contents of N, S and Cl are reduced. The group composition results before and after hydrogenation are shown in Table 7. able 7 (100% being the mass of stream 2)

[0119] Stream 2 is hydrogenated and then fed to cracking furnace 1 (flow rate 5 ml / min); at atmospheric pressure, a water to oil ratio of 0.2, and steam cracking temperatures of 640°C, 660°C and 680°C respectively, the composition of the cracked product (cracked stream 4) is shown in Table 8.

[0120] The target product of Example 4 is lower olefins. Taking the data in Table 8 at a cracking temperature of 680°C as an example, when stream 2 undergoes steam cracking in cracking furnace 1 , the single pass yield of lower olefins is 45.23 wt%; at the same time, stream 3 is steam cracked by cracking furnace 2 to yield a cracked product (cracked stream 5) containing certain lower olefins, with a single pass yield of 24.69 wt% (Table 4 , 560°C). Then the sum of the total single pass yields of lower olefins of cracked stream 4 and cracked stream 5, based on pyrolysis oil, is: 28.06 wt% (= 45.23 wt% x 0.20 + 24.69 wt% x 0.77).

[0121] Example 5 (multiple production of lower olefins, stream 2 is hydrogenated, cracking furnace 1 uses catalytic cracking, and cracking furnace 2 uses steam cracking)

[0122] The rectification operation and material balance are the same as in Example 1 , and the product distribution when stream 3 passes through cracking furnace 2 is the same as in Example 1. The hydrogenation conditions for stream 2 are the same as in Example 4.

[0123] 1.8 ml / min of stream 2 is fed after hydrogenation to cracking furnace 1 (catalytic cracking unit), with 0.5 g of ZSM-5 as a catalyst; at atmospheric pressure, and catalytic cracking temperatures of 510°C, 530°C and 560°C respectively, the composition of the cracked product (cracked stream 4) is shown in Table 9.

[0124] Table 9 (100% being the mass of stream 2) The target product of Example 5 is lower olefins. Taking the data in Table 9 at a cracking temperature of 560°C as an example, when stream 2 undergoes steam cracking in cracking furnace 1 , the single pass yield of lower olefins is 46.42 wt%; at the same time, stream 3 is steam cracked by cracking furnace 2 to yield a cracked product (cracked stream 5) containing certain lower olefins, with a single pass yield of 24.69 wt% (Table 4 , 560°C). Then the sum of the total single pass yields of lower olefins of cracked stream 4 and cracked stream 5, based on pyrolysis oil, is: 28.30 wt% (= 46.42 wt% * 0.2 + 24.69 wt% x 0.77).

[0125] Comparative Example 1A (multiple production of alpha olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 2)

[0126] The pyrolysis oil is fed directly to the cracking furnace 2 without hydrogenation.

[0127] When the operating conditions are atmospheric pressure, a pyrolysis oil flow rate of 5 ml / min, a water to oil ratio of 0.25, and steam cracking temperatures of 560°C, 570°C, 575°C, 580°C and 590°C, respectively, the composition of the cracked product (cracked stream 5) is shown in Table 10.

[0128] Taking the data in Table 10 at a cracking temperature of 560°C as an example, when the pyrolysis oil is steam cracked, the single pass yield of C5-C12 alpha olefins is 17.16 wt%, and the single pass yield of other by-product lower olefins is 15.15 wt%.

[0129] Table 10 Comparative Example 1B (multiple production of alpha olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 2)

[0130] The pyrolysis oil is fed directly to the cracking furnace 2 without hydrogenation. When the operating conditions are atmospheric pressure, a pyrolysis oil flow rate of 5 ml / min, a steam cracking temperature of 575°C, and water to oil ratios of 0.1 , 0.15, 0.2, 0.25, 0.3 and 0.35, respectively, the composition of the cracked product (cracked stream 5) is shown in Table 11.

[0131] Taking the data in Table 11 at a water to oil ratio of 0.2 as an example, when the pyrolysis oil is steam cracked, the single pass yield of C5-C12 alpha olefins is 13.37 wt%, and the single pass yield of other by-product lower olefins is 20.26 wt%.

[0132] Table 11 Comparative Example 1C (multiple production of alpha olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 2) The pyrolysis oil is fed directly to the cracking furnace 2 without hydrogenation. At atmospheric pressure, a pyrolysis oil flow rate of 5 ml / min, a water to oil ratio of 0.2, and steam cracking temperatures of 540°C, 550°C and 570°C, respectively, the composition of the cracked product (cracked stream 5) is shown in Table 12.

[0133] Taking the data in Table 12 at a cracking temperature of 540°C as an example, when the pyrolysis oil is steam cracked, the single pass yield of C5-C12 alpha olefins is 15.39 wt%, and the single pass yield of other by-product lower olefins is 7.25 wt%.

[0134] Table 12

[0135] Comparative Example 1D (multiple production of alpha olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 2)

[0136] The pyrolysis oil is fed directly to the cracking furnace 2 without hydrogenation. The cracking furnace 2 is a catalytic cracking reactor; when 0.5 g of SAPO-34 is used as a catalyst, with a pyrolysis oil flow rate of 1.8 ml / min, an inert carrier gas flow rate of 250 seem, and cracking temperatures of 500°C, 520°C and 540°C, respectively, at atmospheric pressure, the yield of each product is shown in Table 13.

[0137] Taking the data in Table 13 at a cracking temperature of 540°C as an example, when the pyrolysis oil undergoes catalytic cracking, the single pass yield of C5-C12 alpha olefins is 11.31 wt%, and the single pass yield of other by-product lower olefins is 23.94 wt%.

[0138] Comparative Example 2A (multiple production of lower olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 1)

[0139] 5 ml / min of pyrolysis oil is fed directly to cracking furnace 1 (steam cracking unit) without hydrogenation; at atmospheric pressure, with a water to oil ratio of 0.2, and steam cracking temperatures of 600°C, 620°C and 660°C respectively, the composition of the cracked product is shown in Table 14.

[0140] Table 14

[0141] Comparative Example 2B (multiple production of lower olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 1)

[0142] 1 ml / min of pyrolysis oil is fed to the hydrogenation unit 1 ; the hydrogen partial pressure is 1 MPa, the hydrogenation temperature is 300°C, a Ni-based catalyst is used, WHSV = 0.37 IT1, and the hydrogen to oil ratio (L / L) is 467 : 1.

[0143] The pyrolysis oil is fed to cracking furnace 1 (steam cracking unit) after hydrogenation; at atmospheric pressure, a water to oil ratio of 0.2, a space velocity of 0.43 s-1, and steam cracking temperatures of 600°C, 620°C and 660°C respectively, the composition of the cracked product is shown in Table 15.

[0144] Table 15 Comparative Example 2C (multiple production of lower olefins, but no separation operation is performed, and all of the pyrolysis oil is fed to cracking furnace 1)

[0145] 1.8 ml / min of pyrolysis oil is fed directly into the cracking furnace 1 (catalytic cracking unit), the catalyst is 0.5 g of ZSM-5, the inert carrier gas flow rate is 250 seem, and the pressure is atmospheric pressure; when the catalytic cracking temperatures are 580°C, 600°C, 620°C and 640°C respectively, the composition of the cracked product is shown in Table 16.

[0146] Table 16 Comparison of Example 1 with Comparative Examples 1A-1D

[0147] The results for the highest yields of alpha olefins under the respective operating conditions of Example 1 and Comparative Examples 1A-1D were selected and are summarized in Table 17. As can be seen, the separation operation can significantly improve the overall single pass yield of alpha olefins.

Claims

Claims1. A method for processing pyrolysis oil from polyolefin-based plastic, the method comprising the following steps: step (1): separating the pyrolysis oil to obtain at least the following three streams: stream 1, comprising hydrocarbons with less than 5 carbon atoms; stream 2, comprising hydrocarbons with 5 - 12 carbon atoms; and stream 3, comprising hydrocarbons with more than 12 carbon atoms; optional step (2): optionally hydrogenating stream 2, feeding stream 2 or the optionally hydrogenated stream 2 into a cracking furnace 1 to undergo cracking to obtain a cracked stream 4; and step (3): optionally hydrogenating stream 3, feeding stream 3 or the optionally hydrogenated stream 3 into a cracking furnace 2 to undergo cracking to obtain a cracked stream 5.

2. The method as claimed in claim 1, wherein the amount of stream 1 is 1 - 10 wt%, preferably 1.5 - 8 wt%, and more preferably 1.5 - 5 wt%, based on the total weight of stream 1 , stream 2 and stream 3.

3. The method as claimed in claim 1 or 2, wherein the amount of stream 2 is 8 - 40 wt%, preferably 10 - 35 wt%, and more preferably 12 - 30 wt%, based on the total weight of stream 1, stream 2 and stream 3.

4. The method as claimed in any one of claims 1 to 3, wherein the amount of stream 3 is 50 - 90 wt%, preferably 60 - 85 wt%, and more preferably 65 - 80 wt%, based on the total weight of stream 1 , stream 2 and stream 3.

5. The method as claimed in any one of claims 1 to 4, wherein the separation in step (1) is performed by a process selected from distillation (such as atmospheric or reduced-pressure rectification), flash evaporation, condensing, extraction, absorption, adsorption or a combination thereof.

6. The method as claimed in any one of claims 1 to 5, wherein the cracking in cracking furnace 1 is performed by catalytic cracking or steam cracking; preferably, steam cracking in cracking furnace 1 is performed at 600 - 900°C or 610 - 800°C or 670 - 700°C; preferably, catalytic cracking in cracking furnace 1 is performed at 400 - 700°C or 510 - 600°C or 530 - 580°C; especially preferably, cracked stream 4 comprises olefins with fewer than 5 carbon atoms, and more preferably, the total amount of olefins with fewer than 5 carbon atoms in cracked stream 4 is at least 35 wt%, or 40 - 55 wt%, based on the total weight of stream 2.

7. The method as claimed in any one of claims 1 to 6, wherein the cracking in cracking furnace 2 is performed by catalytic cracking or steam cracking; preferably, steam cracking in cracking furnace 2 is performed at 500 - 680°C or 510 - 620°C or 550 - 600°C; preferably, catalytic cracking in cracking furnace 2 is performed at 500 - 700°C or 510 - 600°C; especially preferably, the total amount of hydrocarbons with 5 - 12 carbon atoms in cracked stream 5 is 10- 40 wt%, in particular 15 - 30 wt%, based on the total weight of stream 3; more preferably, the total amount of a-olefins with 5 or more carbon atoms in cracked stream 5 is 20 - 55 wt%, in particular 30 - 50 wt%, and especially 35 - 45 wt%, based on the total weight of stream 3.

8. The method as claimed in any one of claims 1 to 7, wherein the pyrolysis oil is derived from polyolefin plastic.

9. The method as claimed in any one of claims 1 to 8, wherein the pyrolysis oil comprises 40 - 80 wt%, preferably 50 - 75 wt% olefins, and 20 - 60 wt%, preferably 25 - 50 wt% alkanes, based on the total weight of the pyrolysis oil.

10. The method as claimed in any one of claims 1 to 9, wherein the pyrolysis oil comprises 30 - 70 wt%, preferably 40 - 60 wt% a-olefins, based on the total weight of the pyrolysis oil.

11. The method as claimed in any one of claims 1 to 10, wherein the total amount of hydrocarbons with 6 - 16 carbon atoms in the pyrolysis oil is 30 - 65 wt%, preferably 35 - 60 wt%, based on the total weight of the pyrolysis oil; more preferably, the total amount of hydrocarbons with 6 - 12 carbon atoms in the pyrolysis oil is 15 - 50 wt%, preferably 18 - 40 wt%, based on the total weight of the pyrolysis oil.

12. The method as claimed in any one of claims 1 to 11 , wherein stream 2 is fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is passed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is hydrogenated and fed into cracking furnace 1 to undergo steam cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5; or stream 2 is hydrogenated and fed into cracking furnace 1 to undergo catalytic cracking to obtain cracked stream 4; and stream 3 is fed into cracking furnace 2 to undergo steam cracking to obtain cracked stream 5.

13. The method as claimed in any one of claims 1 to 5, wherein optional step (2) is not performed, and stream 2 is used directly as feedstock for a-olefins.

14. The method as claimed in any one of claims 1 to 11 , wherein optional step 2 is not performed; and in step (3), optional hydrogenation is not performed, and stream 3 is passed into cracking furnace 2 to undergo cracking to obtain cracked stream 5.

15. The method as claimed in claim 14, wherein steam cracking is performed in cracking furnace2 at a temperature of 510 - 650°C, preferably 530 - 600°C or 550 - 580°C.

Images