A method for producing steam cracking furnace feedstock by hydroisomerization of high-boiling waxy fraction cracked from waste plastics

Abstract

This application discloses a method for preparing steam cracking furnace feedstock from high-boiling-point waxy fractions of waste plastic pyrolysis via hydroisomerization, comprising the following steps: a) Pre-treating the high-boiling-point fractions of waste plastic pyrolysis and then feeding them into a hydroisomerization reactor for hydroisomerization, controlling the isomerization rate at 65-90% and the cracking rate at less than 20%, to obtain hydroisomerization products; b) Feeding the hydroisomerization products obtained in step a) into a selective hydrocracking reactor packed with NiMo / modified Y zeolite catalyst for selective hydrocracking, controlling the single-pass conversion rate at 55-75%, to obtain hydrocracking products; c) Separating the hydrocracking products obtained in step b) by distillation to obtain C5-C66 products. 10 The fraction is used as feedstock for a steam cracking furnace, C 12 ~C 40 Unconverted components are recycled back to the hydroisomerization reactor. This method solves the problem of C content in the high-boiling fraction of waste plastic pyrolysis. 20 ~C 40 Long-chain n-alkanes exhibit poor selectivity in direct cracking and require direct hydrocracking of C5~C2. 10 Technical problems such as low fraction yield and pipeline blockage by high-freezing-point wax.

Description

Technical Field

[0001] This application relates to the fields of waste plastic resource utilization and petrochemical technology, and in particular to a method for producing high-quality feedstock for a steam cracking furnace by hydroisomerization of high-boiling-point waxy fractions from waste plastics. Background Technology

[0002] The high-boiling-point fraction (>350℃) of the cracked oil produced from the thermal cracking of waste plastics (especially polyethylene) contains a large amount of carbon. 20 ~C 40 Long-chain n-alkanes (paraffinic hydrocarbons) with wax content as high as 40-70 wt% and a freezing point above 50℃. These high-boiling-point waxy fractions present numerous technical challenges in practical applications.

[0003] The existing technology has the following problems: (1) High-pour-point waxy components are prone to clogging pipelines and catalyst beds during pretreatment; (2) C 20 ~C 40 Long-chain n-alkanes are directly fed into a steam cracking furnace, resulting in poor cracking selectivity and an ethylene + propylene yield of only about 35%; (3) If directly hydrocracking is performed (high conversion rate), the products are mainly C1~C4 light hydrocarbons and C5~C4 light hydrocarbons. 10 Low fraction yield (25~35wt%); (4) Traditional isomer dewaxing is for lubricating oil base oil, the product is high viscosity isoalkanes, which are not suitable as raw materials for steam cracking furnace.

[0004] Several invention patents exist regarding the treatment of heavy fractions from pyrolysis of waste plastics. For example, CN114901781B (Chevron USA, authorized on February 13, 2024) discloses a circular economy method for converting plastic waste into polyethylene and lubricating oil through a crude oil unit and an isomerization dewaxing unit. This patent sends the heavy fraction from pyrolysis of waste plastics to the isomerization dewaxing unit, with the target product being lubricating oil base oil, rather than steam cracking furnace feedstock; it does not disclose the combined hydroisomerization-selective hydrocracking process for producing C5~C from high-wax (wax content 40~70%) high-boiling-point fractions from pyrolysis of waste plastics. 10 The route for steam cracking feed.

[0005] CN114846117B (Chevron USA, authorized announcement date 2023-12-12) discloses a circular economy method for converting plastic waste into polypropylene and lubricating oil through FCC and isomerization dewaxing units in oil refineries. This patent involves treating the heavy fraction of pyrolyzed waste plastics with FCC, and then sending the heavy fraction to an isomerization dewaxing unit to produce lubricating oil. While the target product is lubricating oil, it does not involve the preparation of high-quality feedstock for steam cracking furnaces.

[0006] CN119032151A (Chevron USA, Publication Date 2024-11-26) discloses a circular economy method for producing polyethylene and base oil from plastic waste via a refinery hydrocracking unit. This patent application blends waste plastics with petroleum and then hydrocrackles the mixture, sending the heavy fraction to an isomerization dewaxing unit to produce base oil. It does not disclose the process of "hydroisomerization (Pt-SAPO-11) → selective hydrocracking (NiMo / Y zeolite) → C5~C-rich isoalkanes". 10 The complete technical route of "steam cracking feed".

[0007] Current technologies and processes suffer from the following problems: the isomerization treatment of heavy fractions from waste plastic pyrolysis targets lubricating oil base oils, rather than steam cracking feedstocks, as the two have drastically different requirements for product distribution (lubricating oils require high molecular weight isoalkanes, while steam cracking feedstocks require C5~C4). 10 (Light alkanes); There is no specific hydroisomerization strategy designed for the waxy components (40-70 wt%) in the high-boiling fractions of waste plastic pyrolysis. Traditional isomerization dewaxing (with an isomerization rate close to 100%) results in products with excessively short carbon chains, generating large amounts of C1-C4 light hydrocarbons and C5-C4 light hydrocarbons. 10 The target fraction yield is low; the stepwise conversion strategy of "moderate isomerization (preserving the carbon chain) → subsequent selective cracking to the target fraction" is lacking, making it impossible to efficiently convert C... 20 ~C 40 Long-chain n-alkanes are converted into C5~C chains rich in isoalkanes. 10 The lack of online monitoring and automatic control methods for wax accumulation in high-boiling-point waxy fractions from waste plastic pyrolysis during transportation and processing poses a risk of pipeline blockage.

[0008] Therefore, a new technical solution is needed to address the problem of efficient resource utilization of high-boiling-point waxy fractions from waste plastic pyrolysis. Summary of the Invention

[0009] The purpose of this application is to overcome the aforementioned shortcomings of the existing technology and provide a method for producing high-quality feedstock for steam cracking furnaces by hydroisomerization of high-boiling-point waxy fractions from waste plastic pyrolysis. By designing an integrated process route of "mild hydrorepurification → hydroisomerization (moderate isomerization rate 65-90%) → selective hydrocracking," the method aims to remove C from high-boiling-point fractions (>350℃, wax content 40-70 wt%) from waste plastic pyrolysis. 20 ~C 40 Long-chain n-alkanes are efficiently converted into C5~C chains rich in isoalkanes. 10 The distillate fraction, as a high-quality feedstock for steam cracking furnaces, achieves an ethylene + propylene yield of 55-62% during steam cracking, with C5-C6 fractions. 10With a fraction yield of 50-60 wt% and a catalyst coking cycle of >12 months, this method solves the problem of efficient resource utilization of high-boiling-point waxy fractions from waste plastic pyrolysis. It overcomes the long-standing technical bias in the field that "isomerization dewaxing should pursue a high isomerization rate (close to 100%) to maximize the reduction of the freezing point." By controlling the isomerization rate at 65-90% (which is usually not considered by those skilled in the art due to technical bias), it achieves efficient conversion of high-waxy waste plastic pyrolysis fractions into high-quality feedstock for steam pyrolysis furnaces.

[0010] To achieve the above objectives, this application provides a method for preparing steam cracking furnace feedstock by hydroisomerization of high-boiling-point waxy fractions from waste plastic pyrolysis, comprising the following steps: a) After pretreatment, the high-boiling-point fraction of waste plastic pyrolysis is fed into a hydroisomerization reactor filled with Pt-SAPO-11 / Al2O3 bifunctional catalyst to carry out hydroisomerization reaction. The isomerization rate is controlled at 65~90% and the cracking rate is less than 20% to obtain hydroisomerization products. The hydroisomerization reaction is carried out at a temperature of 290–350 °C, a pressure of 2–6 MPa, and a liquid hourly space velocity of 0.5–3.0 h⁻¹. -1 The hydrogen-to-oil volume ratio is 100~600 Nm 3 / m 3 ; b) The hydroisomerization product obtained in step a) is fed into a selective hydrocracking reactor packed with NiMo / modified Y zeolite catalyst to carry out selective hydrocracking reaction. The single-pass conversion rate is controlled at 55~75% to obtain hydrocracking product. The selective hydrocracking reaction is carried out at a temperature of 340–390 °C, a pressure of 6–14 MPa, and a liquid hourly space velocity of 0.3–2.0 h⁻¹. -1 ; c) The hydrocracking products obtained in step b) are separated by distillation to obtain C5~C 10 The fraction is used as feedstock for a steam cracking furnace, C 12 ~C 40 Unconverted components are recycled back to the hydroisomerization reactor in step a); The high-boiling-point fraction of the waste plastic pyrolysis is a waste plastic pyrolysis fraction with a boiling point greater than 350℃ and a wax content greater than 40 wt%.

[0011] Preferably, the preprocessing includes the following steps: The high-boiling-point fraction of the waste plastic pyrolysis is kept warm and transported at 40~90℃. It first passes through a macroporous Al2O3 protective bed at 140~160℃ to remove heavy metals and solid particles, and then passes through an alkaline fixed bed at 190~210℃ to remove organochlorine, so that the chlorine content in the export stream is reduced to below 300 ppm.

[0012] Preferably, the high-boiling-point fraction of the waste plastic pyrolysis is mixed with vacuum gas oil at a mass ratio of 1:9 to 4:6 before entering the pretreatment stage.

[0013] Preferably, in step a), the freezing point of the hydroisomerization product is reduced to below 0°C.

[0014] Preferably, in step c), C5~C 10 The fraction contains 50-70 wt% isoalkanes, 15-30 wt% n-alkanes, and less than 5 wt% aromatics.

[0015] Preferably, in step c), the C separated by the distillation column 10 ~C 15 The middle fraction is recycled to the selective hydrocracking reactor in step b), or used as a component in jet fuel blending.

[0016] Preferably, an online freezing point monitor is installed on the circulating logistics pipeline. When the freezing point of the circulating logistics is higher than 20~40℃, the hydroisomerization reaction temperature in step a) is automatically increased by 3~7℃.

[0017] Preferably, in the Pt-SAPO-11 / Al2O3 bifunctional catalyst, the Pt loading is 0.3~1.0 wt%, and the SAPO-11 molecular sieve content is 20~50 wt%.

[0018] Preferably, in the NiMo / modified Y zeolite catalyst, the loading of Ni is 2-5 wt%, the loading of Mo is 10-20 wt%, and the content of modified Y zeolite is 30-60 wt%.

[0019] Preferably, in step c), C5~C 10 When the fraction is used as feedstock for a steam cracking furnace, the total yield of ethylene and propylene from steam cracking is greater than 50 wt%.

[0020] In this application, vacuum gas oil (VGO) is the 350-500℃ fraction obtained from petroleum vacuum distillation, mainly composed of C. 15 ~C 35 Alkanes and aromatics are used in catalytic cracking or hydrocracking.

[0021] C5~C 10 The fraction refers to a mixture of hydrocarbons with 5 to 10 carbon atoms, with a boiling point range of approximately 30 to 170°C, and is a high-quality feedstock for steam cracking furnaces.

[0022] C 12 ~C 40 Distillate fractions refer to mixtures of hydrocarbons with 12 to 40 carbon atoms, mainly long-chain alkanes.

[0023] DCS is short for Distributed Control System. In this application, the DCS control system receives real-time signals from the online freezing point analyzer and automatically adjusts the heating control valve of the hydroisomerization reactor to achieve closed-loop automatic control of the reaction temperature.

[0024] The beneficial effects of this application include, but are not limited to: (1) C5~C 10 Significantly improved fraction yield: This application achieves a C5~C6 fraction yield through a stepwise conversion strategy of "moderate isomerization (65~90%) → selective hydrocracking". 10 The fraction yield is 50-60 wt%, which is about 27 percentage points higher than the 25-35 wt% of direct hydrocracking. This improvement is due to the intentional retention of 10-35% of long-chain n-alkanes in this application, providing a "carbon chain library" for subsequent selective cracking, and avoiding the problem of excessively short carbon chains and the generation of a large amount of C1-C4 light hydrocarbons caused by traditional high isomerization rates (close to 100%).

[0025] (2) Significantly improved ethylene + propylene yield from steam cracking: The product of this application contains 55-65 wt% isoalkanes, 20-28 wt% n-alkanes, and <3 wt% aromatics. Composition optimization results in an ethylene + propylene yield of 55-62 wt% during steam cracking, which is about 20 percentage points higher than the 35-40% yield of direct hydrocracking, and higher than C 20 ~C 40 The direct cracking rate of long-chain n-alkanes increased by approximately 25 percentage points from 35%.

[0026] (3) Extended catalyst coking cycle: This application adopts a stepwise conversion strategy, with hydroisomerization carried out under relatively mild conditions (290~350℃, 2~6 MPa), and a cracking rate of <20%, avoiding severe coking caused by a one-time high conversion rate under high temperature and high pressure. The catalyst coking cycle is >12 months, which is about 10 months longer than the 3~6 months of direct hydrocracking, significantly reducing the catalyst replacement frequency and operating costs.

[0027] (4) Effectively solve the pipeline blockage problem: This application reduces the product freezing point to below -15℃ through hydroisomerization. At the same time, an online freezing point monitor and an automatic temperature control system are set up. When the freezing point of the circulating material is higher than 20~40℃, the hydroisomerization reaction temperature is automatically increased by 3~7℃ to prevent wax from accumulating and crystallizing in the circulating pipeline and to ensure the long-term stable operation of the device.

[0028] (5) Moderate hydrogen consumption: The hydrogen consumption of this application is 250~350 Nm 3 / t, between the levels of traditional isomerization dewaxing (200~300 Nm) 3 / t) and direct hydrocracking (400~600 Nm 3 Between / t), while ensuring high C5~C 10 While increasing yield, hydrogen consumption is kept within a reasonable range.

[0029] (6) Strong adaptability of raw materials: This application can process high-boiling-point fractions of high-wax waste plastics with a wax content of 40~70 wt%, and can also process waste plastics pyrolysis high-boiling-point fractions with vacuum gas oil at a mass ratio of 1:9~4:6, adapting to different raw material supply conditions. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the process flow for a method of producing high-quality feedstock for a steam cracking furnace by hydroisomerization of high-boiling-point waxy fractions from waste plastics, as described in this application.

[0031] The reference numerals and their names in the figure are as follows: 1-Feedstock tank; 2-Pressure-reducing gas oil storage tank; 3-Mixing valve; 4-Protective bed; 5-Alkaline fixed bed; 6-Hydrogen isomerization reactor; 7-Hot high-pressure separator; 8-Hydrogen compressor; 10-Selective hydrocracking reactor; 11-Distillation column; 12-Fuel gas system; 13-Steam cracking furnace; 14-First circulation pump; 15-Second circulation pump; 16-Online pour point analyzer; 17-DCS control system. Detailed Implementation

[0032] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0033] Unless otherwise specified, all raw materials and reagents used in this application are commercially purchased and used directly without processing. The instruments and equipment used adopt the manufacturer's recommended scheme and parameters.

[0034] According to one embodiment, the method for producing high-quality feedstock for a steam cracking furnace by hydroisomerization of high-boiling-point waxy fractions from waste plastic pyrolysis, provided in this application, includes the following steps: Step a): The high-boiling-point fraction from the pyrolysis of waste plastics, after pretreatment, is fed into a hydroisomerization reactor packed with a Pt-SAPO-11 / Al2O3 bifunctional catalyst. The reaction is carried out at a temperature of 290~350℃, a pressure of 2~6 MPa, and a liquid hourly space velocity of 0.5~3.0 h⁻¹. -1 Hydrogen-to-oil volume ratio 100~600 Nm 3 / m 3 Hydrogenation is carried out under specific conditions, with the isomerization rate controlled at 65-90% and the cracking rate below 20%.

[0035] Optionally, the reaction temperature is selected from any value or a range of any two of 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, and 350℃.

[0036] Optionally, the reaction pressure is selected from any value or a range of any two of 2 MPa, 2.5 MPa, 3 MPa, 3.5 MPa, 4 MPa, 4.5 MPa, 5 MPa, 5.5 MPa, and 6 MPa.

[0037] Optionally, the liquid time space velocity is selected from 0.5 h. -1 0.8 h -1 1.0 h -1 1.2 h -1 1.5 h -1 1.8 h -1 2.0 h -1 2.5 h -1 3.0 h -1 Any value in the range or any combination of both.

[0038] Optionally, the hydrogen-to-oil volume ratio is selected from 100 Nm 3 / m 3 150 Nm 3 / m 3 200 Nm 3 / m 3 250 Nm 3 / m 3 300Nm 3 / m 3 350 Nm 3 / m 3 400 Nm 3 / m 3 450 Nm 3 / m 3 500 Nm 3 / m 3 550 Nm 3 / m 3 600 Nm 3 / m 3 Any value in the range or any combination of both.

[0039] Optionally, the isomerization rate is selected from any value of 65%, 68%, 70%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, or a range of any two of these values.

[0040] Step b): The hydroisomerization product obtained in step a) is fed into a selective hydrocracking reactor packed with NiMo / modified Y zeolite catalyst, and the reaction is carried out at a reaction temperature of 340~390℃, a reaction pressure of 6~14 MPa, and a liquid hourly space velocity of 0.3~2.0 h⁻¹. -1 Selective hydrocracking is carried out under certain conditions, with the single-pass conversion rate controlled at 55-75%.

[0041] Optionally, the reaction temperature is selected from any value or a range of any two of 340℃, 345℃, 350℃, 355℃, 360℃, 365℃, 370℃, 375℃, 380℃, 385℃, and 390℃.

[0042] Optionally, the reaction pressure is selected from any value of 6 MPa, 7 MPa, 8 MPa, 9 MPa, 10 MPa, 11 MPa, 12 MPa, 13 MPa, 14 MPa, or a range of any two of these values.

[0043] Optionally, the liquid time space velocity is selected from 0.3 h. -1 0.5 h -1 0.8 h -1 1.0 h -1 1.2 h -1 1.5 h -1 1.8 h -1 2.0 h -1 Any value in the range or any combination of both.

[0044] Optionally, the single-pass conversion rate is selected from any value of 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, or a range of any two of these values.

[0045] Step c): The hydrocracking products obtained in step b) are separated by distillation to obtain C5~C66 products. 10 Distillate fractions are used as high-quality feedstock for steam cracking furnaces, C 12 ~C 40 The unconverted component is recycled back to the hydroisomerization reactor in step a).

[0046] The high-boiling-point fraction of the waste plastic pyrolysis is a waste plastic pyrolysis fraction with a boiling point greater than 350℃ and a wax content greater than 40 wt%.

[0047] Optionally, the pretreatment includes: conveying the high-boiling-point fraction of the waste plastic pyrolysis at a temperature of 40~90℃, first removing heavy metals and solid particles through a macroporous Al2O3 protective bed at a temperature of 140~160℃, and then removing organochlorines through an alkaline fixed bed at a temperature of 190~210℃, so that the chlorine content in the effluent is reduced to below 300 ppm.

[0048] Optionally, the heat preservation and conveying temperature is selected from any value or a range of any two of 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, and 90℃.

[0049] Optionally, the operating temperature of the macroporous Al2O3 protective bed is selected from any value of 140℃, 145℃, 150℃, 155℃, and 160℃, or a range of any two.

[0050] Optionally, the operating temperature of the alkaline fixed bed is selected from any value of 190℃, 195℃, 200℃, 205℃, 210℃, or a range of any combination of both.

[0051] Optionally, the chlorine content in the exported stream is reduced to below 200 ppm; more preferably, it is reduced to below 150 ppm.

[0052] Optionally, the high-boiling-point fraction of the waste plastic pyrolysis is mixed with vacuum gas oil at a mass ratio of 1:9 to 4:6 before entering the pretreatment process. Optionally, the mass ratio is selected from any value among 1:9, 1:8, 1:7, 2:8, 3:7, and 4:6.

[0053] Optionally, in step a), the freezing point of the hydroisomerization product is lowered to below 0°C. Preferably, the freezing point is lowered to below -5°C; more preferably, to below -10°C; and most preferably, to below -15°C.

[0054] Optionally, in step c), C5~C 10 The fraction contains 50-70 wt% isoalkanes, 15-30 wt% n-alkanes, and less than 5 wt% aromatics. Preferably, the isoalkanes content is 55-65 wt%; more preferably, 58-62 wt%. Preferably, the n-alkanes content is 20-28 wt%; more preferably, 22-26 wt%. Preferably, the aromatics content is less than 3 wt%; more preferably, less than 2 wt%.

[0055] Optionally, in step c), the C separated by the distillation column 10 ~C 15 The middle fraction is recycled to the selective hydrocracking reactor in step b), or used as a component in jet fuel blending.

[0056] Optionally, an online freezing point monitor is installed on the circulating material pipeline. When the freezing point of the circulating material exceeds 20-40℃, the hydroisomerization reaction temperature in step a) is automatically increased by 3-7℃. Optionally, the freezing point threshold is selected from any value among 20℃, 22℃, 25℃, 28℃, 30℃, 32℃, 35℃, 38℃, and 40℃. Optionally, the temperature increase range is selected from any value among 3℃, 4℃, 5℃, 6℃, and 7℃.

[0057] Optionally, in the Pt-SAPO-11 / Al2O3 bifunctional catalyst, the Pt loading is 0.3~1.0 wt%, and the SAPO-11 molecular sieve content is 20~50 wt%. Optionally, the Pt loading is selected from any value or a range of any combination of 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, and 1.0 wt%. Optionally, the SAPO-11 molecular sieve content is selected from any value or a range of any combination of 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, and 50 wt%.

[0058] Optionally, in the NiMo / modified Y zeolite catalyst, the Ni loading is 2-5 wt%, the Mo loading is 10-20 wt%, and the modified Y zeolite content is 30-60 wt%. Optionally, the Ni loading is selected from any value or a range of any two of 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, and 5 wt%. Optionally, the Mo loading is selected from any value or a range of any two of 10 wt%, 12 wt%, 14 wt%, 15 wt%, 16 wt%, 18 wt%, and 20 wt%. Optionally, the modified Y zeolite content is selected from any value or a range of any two of 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, and 60 wt%.

[0059] Preferably, the loading of Pt is 0.4~0.8 wt%; more preferably, it is 0.5~0.7 wt%.

[0060] Preferably, the content of SAPO-11 molecular sieve is 30-45 wt%; more preferably, it is 35-40 wt%.

[0061] Preferably, the Ni loading is 2.5 to 4 wt%; more preferably, it is 3 to 3.5 wt%.

[0062] Preferably, the Mo loading is 12-18 wt%; more preferably, it is 14-16 wt%.

[0063] Preferably, the content of the modified Y zeolite is 35-55 wt%; more preferably, it is 40-50 wt%.

[0064] Optionally, in step c), C5~C 10 When the fraction is used as a high-quality feedstock for a steam cracking furnace, the total yield of ethylene and propylene from steam cracking is greater than 50 wt%. Preferably, the total yield is greater than 53 wt%; more preferably, greater than 55 wt%; and most preferably, greater than 57 wt%.

[0065] like Figure 1 As shown, the process flow of the method for producing high-quality feedstock for steam cracking furnace by hydroisomerization of high-boiling-point waxy fractions from waste plastics according to this application includes the following units: (1) Raw material pretreatment unit: High-boiling-point fractions from waste plastic pyrolysis are output from raw material tank 1 and optionally mixed with depressurized gas oil from depressurized gas oil storage tank 2 in mixing valve 3 at a mass ratio of 1:9 to 4:6. The mixed raw material is conveyed under heat preservation conditions of 40 to 90°C. The heat preservation conveying adopts jacketed insulation pipe or electric heat tracing pipe; the jacketed insulation pipe has a jacket layer on the outer wall of the pipe, and hot water (40 to 90°C) or steam is introduced into the jacket to maintain the material temperature; the electric heat tracing pipe has electric heating tape wrapped around the outer wall of the pipe to maintain the material temperature by electric heating; optionally, temperature sensors are set at key nodes of the pipeline and linked with DCS control system 17 to realize automatic control of the heat preservation temperature. After being kept warm, the raw material first passes through a protective bed 4 (filled with macroporous Al2O3, operating temperature 140~160℃) to remove heavy metals and solid particles, and then passes through an alkaline fixed bed 5 (operating temperature 190~210℃) to remove organic chlorine, so that the chlorine content in the export stream is reduced to below 300 ppm.

[0066] (2) Hydroisomerization unit: The purified waxy fraction is fed into hydroisomerization reactor 6, which is packed with a Pt-SAPO-11 / Al2O3 bifunctional catalyst. The reaction is carried out at a temperature of 290~350℃, a reaction pressure of 2~6 MPa, and a liquid hourly space velocity of 0.5~3.0 h. -1 Hydrogen-to-oil volume ratio 100~600 Nm 3 / m 3Hydroisomerization is carried out under controlled conditions, with the isomerization rate controlled at 65-90% and the cracking rate below 20%. The hydrogen required for the reaction is supplied by circulating hydrogen from hydrogen compressor 8, and the consumed fresh hydrogen is replenished by a fresh hydrogen replenishment device to maintain a stable hydrogen partial pressure in the system. The reaction products enter the hot high-pressure separator 7, the top gas phase is circulated to the hydroisomerization reactor 6 via hydrogen compressor 8, and the bottom liquid phase is sent to the selective hydrocracking reactor 10.

[0067] (3) Selective hydrocracking unit: The hydroisomerization product is fed into the selective hydrocracking reactor 10, which is packed with NiMo / modified Y zeolite catalyst. The reaction is carried out at a temperature of 340~390℃, a pressure of 6~14 MPa, and a liquid hourly space velocity of 0.3~2.0 h⁻¹. -1 Selective hydrocracking is carried out under certain conditions, with the single-pass conversion rate controlled at 55-75%.

[0068] (4) Distillation and Separation Unit: Hydrocracking products are fed into distillation column 11 for separation. The top C1~C4 components are sent to fuel gas system 12 or hydrogen recovery unit, and C5~C4 components are extracted from the side stream. 10 The fraction is fed to steam cracking furnace 13 as high-quality feed, and C is extracted from the side stream 2. 10 ~C 15 The middle fraction is recycled via the first circulation pump 14 to the selective hydrocracking reactor 10 or used as a jet fuel blending component, with the bottom C... 12 ~C 40 The unconverted components are recycled back to the hydroisomerization reactor 6 via the second circulation pump 15.

[0069] (5) Online monitoring and automatic control unit: An online freezing point analyzer 16 is installed on the circulating logistics pipeline to monitor the freezing point of the circulating logistics in real time. When the freezing point is higher than 20~40℃, the reaction temperature of the hydroisomerization reactor 6 is automatically increased by 3~7℃ through the DCS control system 17 to prevent the wax from accumulating and crystallizing in the circulating pipeline and causing blockage.

[0070] In the examples, the catalyst was prepared as follows: (1) Alkaline fixed-bed dechlorination agent Sodium carbonate, magnesium oxide, and calcium oxide were thoroughly mixed with alumina powder (10 wt% sodium carbonate, 3.5 wt% magnesium oxide, and 8.0 wt% calcium salt), and deionized water was added to knead and shape the mixture. The mixture was then extruded into cylindrical particles with a diameter of 3 mm and a length of 5 mm. The particles were dried at 120°C for 12 h and calcined at 500°C for 4 h to obtain the alkaline fixed-bed dechlorination agent.

[0071] (2) Pt-SAPO-11 / Al2O3 bifunctional catalyst SAPO-11 molecular sieve (silicon:aluminum:phosphorus molar ratio Si:Al:P=0.1:1.0:1.0, hydrothermal synthesis, pore size 0.63 nm) was mixed with γ-Al2O3 powder at a mass ratio of 35:65. Deionized water and guar gum powder were added and kneaded to form a granule. The granules were extruded into strips with a diameter of 1.5 mm, dried at 120 °C for 12 h, and calcined at 500 °C for 4 h to obtain the SAPO-11 / Al2O3 support. Chloroplatinic acid (H2PtCl6) solution was impregnated onto the above support by an equal volume method, with a Pt loading of 0.5 wt%. The mixture was dried at 120 °C for 12 h and calcined at 500 °C for 3 h to obtain the catalyst precursor. Before use, the precursor was reduced at 350 °C for 4 h in a hydrogen atmosphere to obtain the Pt-SAPO-11 / Al2O3 bifunctional catalyst.

[0072] (3) Pt-ZSM-23 / Al2O3 catalyst The same Pt-SAPO-11 / Al2O3 bifunctional catalyst is used, except that the SAPO-11 molecular sieve is replaced with ZSM-23 molecular sieve.

[0073] (4) NiMo / Modified Y Zeolite Catalyst: NaY zeolite (silicon-to-aluminum ratio 5.5) was subjected to ammonium exchange in ammonium nitrate solution (80℃, 2 h, repeated 3 times), followed by ion exchange with rare earth mixed salts (La(NO3)3 and Ce(NO3)3, La:Ce molar ratio 1:1) to introduce rare earth elements to improve thermal stability; after hydrothermal treatment (550℃, 100% steam, 2 h), dealuminization modification was performed to increase the silicon-to-aluminum ratio to 12, resulting in modified Y zeolite; the modified Y zeolite was mixed with γ-Al2O3 powder at a mass ratio of 45:55, deionized water and guar gum powder were added and kneaded into shape, extruded into strip-shaped particles with a diameter of 1.5 mm, dried at 120℃ for 12 h, and calcined at 550℃ for 4 h to obtain modified Y zeolite / Al2O3 support; a mixed solution of ammonium molybdate and nickel nitrate was loaded onto the above support by an equal volume impregnation method, with a Mo loading of 15%. The catalyst precursor was obtained by drying at 120℃ for 12 h and calcining at 500℃ for 4 h with 3 wt% Ni loading and 120℃ for 12 h. Before use, it was pre-sulfurized at 350℃ for 4 h in a hydrogen atmosphere containing H2S (5 vol%) to obtain the NiMo / modified Y zeolite catalyst.

[0074] The analysis and testing methods used in this embodiment are as follows: 1. Isomerization and cracking rate determination: An Agilent 7890B gas chromatograph equipped with a flame ionization detector (FID) and an HP-PONA column (50 m × 0.2 mm × 0.5 μm) was used to analyze the content of n-alkanes, isoalkanes, and C1–C4 light hydrocarbons in the feed and products. Chromatographic conditions: Injector temperature 300℃, detector temperature 320℃, column temperature programmed (50℃ held for 5 min, increased to 300℃ at 3℃ / min, held for 20 min).

[0075] Isomerization rate = (feed n-alkane content - product n-alkane content) / feed n-alkane content × 100%; Cracking rate = product C1~C4 content / total feed mass × 100%.

[0076] 2. Single-pass conversion determination: An Agilent 7890B gas chromatograph equipped with an FID detector and an HP-5 column (30 m × 0.32 mm × 0.25 μm) was used to analyze C in the feed and product. 12 ~C 40 The content of the distillate.

[0077] Single-pass conversion rate = (feed C) 12 ~C 40 Content - Product C 12 ~C 40 Content) / Feed C 12 ~C 40 Content × 100%.

[0078] 3. Freezing point determination: The freezing point shall be determined in accordance with GB / T 510-2018 "Determination of Freezing Point of Petroleum Products". The sample shall be placed in a test tube and cooled in a cooling bath at a rate of 1℃ / min. Observe once every 1℃ decrease. When crystals appear on the sample surface and the sample is tilted at 45° and held at a constant temperature for 5 seconds without flowing, record this temperature as the freezing point.

[0079] 4. Determination of distillate composition: An Agilent 7890B gas chromatograph equipped with an FID detector and an HP-PONA column was used to analyze C5~C6 fractions. 10 The content of isoalkanes, n-alkanes and aromatics in the fraction.

[0080] 5. Chlorine content determination: The combustion-microcoulometric method is used. The sample is burned at a high temperature of 800~900℃ in a nitrogen-oxygen mixed gas stream. The chlorine is converted into chloride and enters the silver acetic acid ion titration cell. The chlorine content is determined by coulometric titration.

[0081] 6. Determination of heavy metal content: The content of heavy metals such as nickel and vanadium was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7900).

[0082] 7. Steam cracking experiment: A small-scale laboratory steam cracking apparatus was used, with a cracking temperature of 840℃, a residence time of 0.5s, and a steam / hydrocarbon mass ratio of 0.6. The ethylene and propylene contents in the cracking products were analyzed. Total yield of ethylene + propylene = (mass of ethylene + mass of propylene) / mass of feed × 100%.

[0083] Example 1 (Standard Operating Condition Basic Example) Raw materials: High-boiling-point fraction (>350℃) from pyrolysis of waste polyethylene plastics, wax content 55 wt%, freezing point 62℃, chlorine content 1500 ppm, nickel content 5 ppm, vanadium content 8 ppm.

[0084] Step 1: Convey the material at 65℃ through a macroporous Al2O3 protective bed (10 m³). 3 After passing through an alkaline fixed bed (particle size 3-5 mm, pore size 80 nm, 150℃), the nickel content decreased to 0.8 ppm and the vanadium content decreased to 1 ppm. 3 The dechlorination agent consists of 10 wt% sodium carbonate, 3.5 wt% magnesium oxide, and 8.0 wt% calcium salt, loaded on an alumina carrier. After 200°C, the chlorine content drops to 150 ppm.

[0085] Step 2: Pt-SAPO-11 / Al2O3 catalyst (loading amount 50 m) 3 The reaction mixture consisted of 0.5 wt% Pt loading, 35 wt% SAPO-11 content, and γ-Al₂O₃ as the Al₂O₃ support (specific surface area 200 m² / g). The reaction temperature was 320℃, the pressure was 4 MPa, and the liquid hourly space velocity was 1.5 h⁻¹. -1 Hydrogen-to-oil ratio 350 Nm 3 / m 3 The isomerization rate was 78%, the cracking rate was 9%, and the product freezing point was -15℃.

[0086] Step 3: NiMo / modified Y zeolite catalyst (packing amount 60 m³) 3 The reaction mixture contained 3 wt% Ni, 15 wt% Mo, and 45 wt% modified Y zeolite. The modification method was rare earth element (La, Ce) ion exchange (silicon-to-aluminum ratio 1:2). The reaction temperature was 365℃, the pressure was 10 MPa, and the liquid hourly space velocity was 1.0 h⁻¹. -1 The single-trip conversion rate is 65%.

[0087] Step 4: After separation in a distillation column (80 trays, reflux ratio 3:1), C5~C10 Yield 55 wt% (62 wt% isoalkanes, 25 wt% n-alkanes, 2 wt% aromatics), C1-C4 yield 8 wt%, C 10 ~C 15 Yield 12 wt%, C 12 ~C 40 25 wt% of the unconverted component is recycled back to the hydroisomerization reactor.

[0088] Steam cracking results: cracking temperature 840℃, residence time 0.5 s, steam / hydrocarbon mass ratio 0.6, ethylene yield 33 wt%, propylene yield 24 wt%, total yield of ethylene + propylene 57 wt%.

[0089] Hydrogen consumption statistics: Hydrogen consumption of the hydroisomerization unit is 120 Nm³. 3 / t, selective hydrocracking unit hydrogen consumption 180 Nm 3 / t, total comprehensive hydrogen consumption 300 Nm 3 / t (based on high-boiling-point fraction of waste plastic pyrolysis feed).

[0090] Catalyst operating cycle: After 14 months of continuous operation, the isomerization rate of the hydroisomerization catalyst decreased from 78% to 75%, while the cracking rate increased from 9% to 11%; the single-pass conversion rate of the selective hydrocracking catalyst decreased from 65% to 62%. The catalyst activity remained good, and no pipeline blockage incidents occurred.

[0091] Example 2 (Extended Example under High Wax Content Conditions) Raw materials: High-boiling-point fraction (>350℃) from pyrolysis of waste polyethylene plastics, wax content 68 wt%, freezing point 70℃, chlorine content 2200 ppm, nickel content 6 ppm, vanadium content 10 ppm.

[0092] Step 1: After being conveyed at 75℃, the nickel content is reduced to 0.9 ppm and the vanadium content is reduced to 1.2 ppm after passing through a macroporous Al2O3 protective bed (150℃); after passing through an alkaline fixed bed (200℃), the chlorine content is reduced to 180 ppm.

[0093] The composition and preparation method of the dechlorination agent used in the alkaline fixed bed are the same as step one of Example 1.

[0094] Step 2: Pt-SAPO-11 / Al2O3 catalyst (Pt loading 0.6 wt%, SAPO-11 content 40 wt%), reaction temperature 335℃, pressure 5 MPa, liquid hourly space velocity 1.2 h⁻¹ -1 Hydrogen-to-oil ratio 400 Nm 3 / m 3 The isomerization rate was 82%, the cracking rate was 12%, and the product freezing point was -12℃.

[0095] Step 3: NiMo / modified Y zeolite catalyst (Ni loading 3.5 wt%, Mo loading 16 wt%, modified Y zeolite content 50 wt%), reaction temperature 375℃, pressure 11 MPa, liquid hourly space velocity 0.8 h⁻¹ -1 The one-way conversion rate is 68%.

[0096] Step 4: After distillation separation, C5~C 10 Yield 58 wt% (65 wt% isoalkanes, 23 wt% n-alkanes, 1.5 wt% aromatics), C1~C4 yield 10 wt%, C 10 ~C 15 Yield 10 wt%, C 12 ~C 40 22 wt% of the unconverted component was recycled back to the hydroisomerization reactor.

[0097] Steam cracking results: ethylene yield 35 wt%, propylene yield 25 wt%, total yield of ethylene and propylene 60 wt%.

[0098] Catalyst operating cycle: After 13 months of continuous operation, the catalyst activity remained good and no pipeline blockage accidents occurred.

[0099] Example 3 (Alternative Example of Blending Operation) Raw materials: High-boiling-point fraction of pyrolyzed polyethylene waste plastic (wax content 55 wt%) is mixed with conventional vacuum gas oil at a mass ratio of 3:7. The mixture has a wax content of 38 wt%, a freezing point of 45℃, and a chlorine content of 1200 ppm.

[0100] Step 1: After being conveyed at 60℃, the chlorine content is reduced to 120 ppm after passing through a macroporous Al2O3 protective bed (150℃) and an alkaline fixed bed (200℃).

[0101] The composition and preparation method of the dechlorination agent used in the alkaline fixed bed are the same as step one of Example 1.

[0102] Step 2: Pt-SAPO-11 / Al2O3 catalyst (Pt loading 0.5 wt%, SAPO-11 content 35 wt%), reaction temperature 305℃, pressure 3.5 MPa, liquid hourly space velocity 1.8 h⁻¹ -1 Hydrogen-to-oil ratio 300 Nm 3 / m 3 The isomerization rate was 73%, the cracking rate was 7%, and the product freezing point was -18℃.

[0103] Step 3: NiMo / modified Y zeolite catalyst (Ni loading 3 wt%, Mo loading 15 wt%, modified Y zeolite content 45 wt%), reaction temperature 355℃, pressure 9 MPa, liquid hourly space velocity 1.3 h⁻¹ -1 The single-trip conversion rate is 62%.

[0104] Step 4: After distillation separation, C5~C 10 Yield 52 wt% (58 wt% isoalkanes, 28 wt% n-alkanes, 2.5 wt% aromatics), C1–C4 yield 7 wt%, C 10 ~C 15 Yield 13 wt%, C 12 ~C 40 28 wt% of the unconverted component was recycled back to the hydroisomerization reactor.

[0105] Steam cracking results: ethylene yield 32 wt%, propylene yield 24 wt%, total yield of ethylene and propylene 56 wt%.

[0106] Catalyst operating cycle: After 15 months of continuous operation, the catalyst activity remained good and no pipeline blockage accidents occurred.

[0107] Example 4 (Verification Example of Online Freezing Point Monitoring and Automatic Temperature Control) Raw materials: Same as in Example 1.

[0108] Based on Example 1, an online freezing point analyzer 16 (model: PAC Corporation CPM5000, measurement range -60~+80℃, accuracy ±0.5℃) was installed on the circulating material pipeline (from the outlet of the second circulating pump 15 to the inlet of the hydroisomerization reactor 6) to monitor the freezing point of the circulating material in real time.

[0109] Experimental Procedure: During the 8th month of continuous operation, due to fluctuations in the wax content of the feedstock (increasing from 55 wt% to 62 wt%), the online freezing point analyzer 16 detected that the freezing point of the circulating stream rose from 25℃ to 32℃ (above the 30℃ set threshold). The DCS control system 17 automatically increased the reaction temperature of the hydroisomerization reactor 6 by 5℃ from 320℃ to 325℃. After 24 hours of operation, the freezing point of the circulating stream dropped to 25℃ (below the 30℃ set threshold), and the system automatically returned to normal operation at 320℃. No pipeline blockage occurred throughout the entire process, and the catalyst activity remained unaffected.

[0110] Technical results: The online freezing point monitoring and automatic temperature control system has been verified to effectively prevent wax from accumulating and crystallizing in the circulation pipeline, thus ensuring the long-term stable operation of the equipment.

[0111] Example 5 (Optimization Example of Middle Distillate Recycling) Raw materials: Same as in Example 1.

[0112] Based on Example 1, the C separated from distillation column 11 10 ~C 15 The middle fraction (accounting for 12 wt% of the total product) is entirely recycled to the selective hydrocracking reactor 10 via the first circulation pump 14, instead of being sent out as a jet fuel blending component.

[0113] Experimental results: C5~C before cycling 10 Yield 55 wt%, C 10 ~C 15 Distillate yield 12 wt%; C5~C after recycling 10 The yield increased to 58 wt%, C 10 ~C 15 The fraction yield decreased to 6 wt%. The liquid hourly space velocity (LHSV) of the selective hydrocracking reactor 10 decreased from 1.0 h⁻¹. -1 Reduced to 0.9 h -1 (Due to the increased feed rate), the single-pass conversion rate remains at 65%.

[0114] Technical effect: Through C 10 ~C 15 The middle fraction is recycled to a selective hydrocracking reactor for further cracking into C5-C6. 10 The distillate fraction can increase the yield of the target product by 3 percentage points, reduce the by-products of the middle distillate, and improve the utilization rate of raw materials.

[0115] Comparative Example 1 (direct hydrocracking, without hydroisomerization) Raw materials: Same as in Example 1.

[0116] The feedstock from Example 1 (after pretreatment) was directly fed into a hydrocracking reactor (loaded with NiMo / Y zeolite catalyst, 60 m³). 3 Operating conditions: reaction temperature 400℃, pressure 14 MPa, liquid hourly space velocity 1.0 h⁻¹ -1 Hydrogen-to-oil ratio 500 Nm 3 / m 3 .

[0117] Result: C5~C 10 The yield was only 28 wt% (45 wt% isoalkanes, 35 wt% n-alkanes, and 3 wt% aromatics), while the yield of C1-C4 light hydrocarbons was as high as 35 wt%. 10 ~C 15 Yield 15 wt%, C 12 ~C 40 Unconverted components: 22 wt%. Total yield of ethylene + propylene from steam cracking: 37 wt%.

[0118] After four months of operation, the catalyst became deactivated due to waxy coking, and the conversion rate dropped from 72% to 45%. Dissection of the reactor revealed that the upper 10% of the catalyst bed was blocked by wax, and the catalyst surface was severely coked (coke content 15 wt%). The circulation pipeline experienced multiple instances of wax crystallization blockage, requiring shutdown for cleaning.

[0119] Comparative explanation: Compared with Example 1 of this application, the direct hydrocracking route C5~C 10 The yield was about 27 percentage points lower, the yield of ethylene + propylene from steam cracking was about 20 percentage points lower, and the catalyst operating cycle was shortened by about 10 months, which fully verified the technical superiority of the stepwise conversion strategy of "hydroisomerization → selective hydrocracking" in this application.

[0120] Comparative Example 2 (Traditional isomerization dewaxing, isomerization rate close to 100%) Raw materials: Same as in Example 1.

[0121] The raw material (after pretreatment) from Example 1 was fed into an isomerization dewaxing reactor (50 m³) packed with Pt-ZSM-23 / Al2O3 catalyst. 3 Operating conditions: reaction temperature 360℃, pressure 6 MPa, liquid hourly space velocity 1.0 h⁻¹ -1 Hydrogen-to-oil ratio 400 Nm 3 / m 3 The goal is to achieve a high isomerization rate (close to 100%).

[0122] Results: Isomerization rate 98%, cracking rate 35%, product freezing point -20℃. After distillation separation, C5~C 10 Yield 18 wt% (75 wt% isoalkanes, 5 wt% n-alkanes, 2 wt% aromatics), with C1-C4 light hydrocarbon yields as high as 45 wt%. 10 ~C 15 Yield 15 wt%, C 15 + High viscosity isoalkanes yield 22 wt% (suitable as a base oil for lubricating oils, but not suitable as feedstock for steam cracking furnaces).

[0123] Comparative explanation: Although traditional isomerization dewaxing achieves a high isomerization rate (98%) and a low freezing point (-20℃), excessive isomerization and cracking result in lower C5~C6 concentrations. 10 The yield was only 18 wt%, while the yield of C1-C4 light hydrocarbons was as high as 45 wt%, making it unsuitable as a feedstock for steam cracking furnaces. This verifies that the "moderate isomerization (65-90%) → selective hydrocracking" strategy of this application overcomes the technical bias in the field that "isomerization dewaxing should pursue a high isomerization rate".

[0124] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for preparing steam cracking furnace feedstock by hydroisomerization of high-boiling-point waxy fractions from waste plastic pyrolysis, characterized in that... Includes the following steps: a) After pretreatment, the high-boiling-point fraction of waste plastic pyrolysis is fed into a hydroisomerization reactor filled with Pt-SAPO-11 / Al2O3 bifunctional catalyst to carry out hydroisomerization reaction. The isomerization rate is controlled at 65~90% and the cracking rate is less than 20% to obtain hydroisomerization products. The hydroisomerization reaction is carried out at a temperature of 290–350 °C, a pressure of 2–6 MPa, and a liquid hourly space velocity of 0.5–3.0 h⁻¹. -1 The hydrogen-to-oil volume ratio is 100~600 Nm 3 / m 3 ; b) The hydroisomerization product obtained in step a) is fed into a selective hydrocracking reactor packed with NiMo / modified Y zeolite catalyst to carry out selective hydrocracking reaction. The single-pass conversion rate is controlled at 55~75% to obtain hydrocracking product. The selective hydrocracking reaction is carried out at a temperature of 340–390 °C, a pressure of 6–14 MPa, and a liquid hourly space velocity of 0.3–2.0 h⁻¹. -1 ; c) The hydrocracking products obtained in step b) are separated by distillation to obtain C5~C 10 The fraction is used as feedstock for a steam cracking furnace, C 12 ~C 40 Unconverted components are recycled back to the hydroisomerization reactor in step a); The high-boiling-point fraction of the waste plastic pyrolysis is a waste plastic pyrolysis fraction with a boiling point greater than 350℃ and a wax content greater than 40 wt%.

2. The method according to claim 1, characterized in that, The preprocessing includes the following steps: The high-boiling-point fraction of the waste plastic pyrolysis is kept warm and transported at 40~90℃. It first passes through a macroporous Al2O3 protective bed at 140~160℃ to remove heavy metals and solid particles, and then passes through an alkaline fixed bed at 190~210℃ to remove organochlorine, so that the chlorine content in the export stream is reduced to below 300 ppm.

3. The method according to claim 1, characterized in that, Before entering the pretreatment stage, the high-boiling-point fraction of the waste plastic pyrolysis is mixed with vacuum gas oil at a mass ratio of 1:9 to 4:

6.

4. The method according to claim 1, characterized in that, In step a), the freezing point of the hydroisomerization product drops below 0°C.

5. The method according to claim 1, characterized in that, In step c), C5~C 10 The fraction contains 50-70 wt% isoalkanes, 15-30 wt% n-alkanes, and less than 5 wt% aromatics.

6. The method according to claim 1, characterized in that, In step c), the C separated by the distillation column 10 ~C 15 The middle fraction is recycled to the selective hydrocracking reactor in step b), or used as a component in jet fuel blending.

7. The method according to claim 1, characterized in that, An online freezing point monitor is installed on the circulating logistics pipeline. When the freezing point of the circulating logistics is higher than 20~40℃, the hydroisomerization reaction temperature in step a) is automatically increased by 3~7℃.

8. The method according to claim 1, characterized in that, In the Pt-SAPO-11 / Al2O3 bifunctional catalyst, the Pt loading is 0.3~1.0 wt%, and the SAPO-11 molecular sieve content is 20~50 wt%.

9. The method according to claim 1, characterized in that, In the NiMo / modified Y zeolite catalyst, the loading of Ni is 2~5 wt%, the loading of Mo is 10~20 wt%, and the content of modified Y zeolite is 30~60 wt%.

10. The method according to claim 1, characterized in that, In step c), C5~C 10 When the fraction is used as feedstock for a steam cracking furnace, the total yield of ethylene and propylene from steam cracking is greater than 50 wt%.

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