Steam cracking process and system

Abstract

This invention relates to the field of low-carbon olefin preparation, and discloses a steam cracking method and system. The method is implemented in a cracking furnace, which includes a convection section, a gas-liquid separator, and a radiant section. The method includes: contacting the cracking feedstock with steam in the convection section and heating it to a transverse temperature, then conveying it to the gas-liquid separator for gas-liquid separation to obtain gaseous and liquid phase materials; conveying the gaseous material to the radiant section for cracking to obtain low-carbon olefins; wherein the transverse temperature is 350-500°C. Using this method can reduce coking in the furnace tubes of the convection section of the cracking furnace, extend the coking cycle in the radiant section, improve the online rate of the unit, and increase the cracking efficiency of the cracking feedstock.

Description

Technical Field

[0001] This invention relates to the field of preparing low-carbon olefins, and discloses a steam cracking method and a steam cracking system. Background Technology

[0002] To fully utilize crude oil resources and increase the yield of low-carbon olefins, cracking furnaces are typically used to crack various hydrocarbon feedstocks into olefins via steam cracking. Commonly used cracking furnaces include convection and radiant sections. Crude oil generally consists of four components: saturated fractions, aromatic fractions, resins, and asphaltenes. Saturated fractions and asphaltenes represent the most stable and least stable components in crude oil, respectively. Crude oil contains high-molecular-weight non-volatile components with boiling points exceeding 590℃. During preheating in the convection section of a conventional cracking furnace, a small portion of these non-volatile components remains unvaporized. This unvaporized non-volatile component is carried into the radiant section with the mixed gas flow, easily causing coking and deposition, and even clogging the radiant section, affecting the online operation rate of the cracking unit and product output.

[0003] CN101583697A discloses a method for cracking a feedstock containing synthetic oil, the method comprising: 1. hydroprocessing a broad-boiling-range fraction comprising: a generally liquid hydrocarbon fraction boiling in the range of 50℉ to 800℉ and substantially free of residual oil; and b. thermally cracked hydrocarbon liquid boiling in the range of 600℉ to 1050℉ to provide synthetic crude oil boiling in the range of 73℉ to 1070℉, comprising greater than 25 wt% aromatic compounds, greater than 25 wt% cycloalkanes, less than 0.3 wt% S, less than 0.02 wt% asphaltenes and substantially free of residual oil other than asphaltenes; 2. adding the generally liquid hydrocarbon component boiling in the range of 100℉ to 1050℉ to the synthetic crude oil; and 3. cracking the mixture produced by 2 in a cracking furnace to provide a cracked effluent, wherein the cracking furnace includes a radiant coil outlet, and wherein the cracking is carried out under conditions sufficient to obtain a radiant coil outlet temperature greater than that of the optimal radiant coil outlet temperature for cracking the synthetic crude oil alone.

[0004] This method involves mixing crude oil with existing ethylene production feedstocks to dilute the crude oil, improve its cracking performance, and increase olefin conversion. However, this method is limited by the availability of existing ethylene production feedstocks and cannot effectively utilize large quantities of crude oil for the production of low-carbon olefins.

[0005] Besides improving the process to reduce the impact of crude oil and other feedstocks on the cracking furnace tubes, improvements can also be considered in the design of the cracking furnace tubes. As those skilled in the art know, the radiant section of the cracking furnace tubes has the characteristics of high temperature, short residence time, and low hydrocarbon partial pressure, which is conducive to the high selectivity, high capacity, and long cycle operation of ethylene production. Many cracking furnace technology patents adopt two-pass branched diameter or two-pass diameter high-selectivity furnace tubes. The first pass uses small-diameter furnace tubes to achieve rapid heating by utilizing the large specific surface area, while the second pass uses large-diameter furnace tubes to reduce the sensitivity to coking in the later stages of hydrocarbon cracking. Currently, the two-pass high-selectivity radiant furnace tubes used in industrial applications are mainly of type 2-1, 4-1, 5-1, 6-1, 8-1, and U(1-1). High-selectivity furnace tubes have a large specific surface area and a fast heating rate, which is very beneficial to hydrocarbon cracking reactions.

[0006] CN101333147A proposes an ethylene cracking furnace. The furnace tubes are located in the radiant section, and each tube consists of an inlet tube and an outlet tube. The tubes are arranged in two rows in the radiant section, each row forming a tube array plane. The inlet and outlet tubes are located alternately in two different tube array planes and connected at the bottom by a symmetrical U-shaped connector. It is argued that this arrangement allows for the scaling up of the cracking furnace, improves radiant heat transfer efficiency, extends the operating cycle, and reduces product energy consumption.

[0007] CN103992812A describes a pyrolysis furnace in which four rows of burners and two sets of radiant furnace tubes are arranged at the bottom of the radiant section. Each set of radiant furnace tubes is arranged in two rows, so that a total of four rows of radiant furnace tubes are provided in the radiant section. It is believed that this can realize the large-scale pyrolysis furnace and reduce the footprint and investment.

[0008] CN103992813A describes an ethylene cracking furnace, comprising a radiant section, a convection section, a quench heat exchanger, an induced draft fan, and a chimney. The radiant section contains two rows of radiant furnace tubes, including an inlet tube bank formed by a row of inlet tubes and an outlet tube bank formed by a row of outlet tubes. Multiple burners are arranged on both sides of the two rows of radiant furnace tubes, and the burners are configured to asymmetrically supply heat to the radiant furnace tubes, such that the burners closer to the inlet tube bank release more heat than the burners closer to the outlet tube bank. It is believed that such a cracking furnace has a long operating cycle, high product yield, and large production capacity.

[0009] CN104232146A discloses an ethylene cracking furnace, which includes a radiant section coil assembly. This assembly consists of X-shaped radiant coil modules arranged perpendicular to the bottom surface along the length of the furnace body within the radiant section. Each X-shaped radiant coil module comprises four sets of radiant coils, and each set of radiant coils consists of furnace tubes. The four sets of radiant coils are connected to a four-in-one three-dimensional polymerization pipe at the center of the X-shaped radiant coil module as a material outlet. The furthest point from the center of the four sets of radiant coils serves as the material inlet and is connected to an inlet manifold. Bottom burners are located in the gaps between every two adjacent radiant coils. The invention claims that each furnace tube on each independent radiant coil receives uniform heating, extending the service life of the furnace tubes and increasing ethylene production capacity.

[0010] The aforementioned patents focus on the arrangement of furnace tubes within the radiant section of a cracking furnace to ensure a larger number of tubes in the furnace chamber and better radiant heat transfer, allowing the material inside the tubes to heat up rapidly within a very short residence time. However, this approach is unsuitable for crude oil cracking. When crude oil is used directly as a cracking feedstock, it often exhibits incomplete gasification and a tendency to coke, resulting in low yields of low-carbon olefins. This is a major reason why it was not previously used as a cracking feedstock. Furthermore, coking often occurs in the convection section of the cracking furnace, a relatively low-temperature region. Once coking occurs in the convection section, it cannot be removed through online coking methods, often requiring furnace shutdown for manual decoking, significantly impacting the cracking furnace's online time. Summary of the Invention

[0011] The purpose of this invention is to overcome the problems of insufficient gasification, easy coking in the convection section, and low yield of low-carbon olefins when using crude oil cracking to produce low-carbon olefins in the existing technology. This invention provides a steam cracking method and a steam cracking system. This method can reduce coking in the furnace tubes of the convection section of the cracking furnace while maintaining the normal operation of the cracking reaction in the radiant section, avoid the shutdown treatment steps required for coking in the convection section, and extend the coking cycle in the radiant section, thereby enabling the cracking furnace to have a longer operating cycle and a higher online rate.

[0012] To achieve the above objectives, a first aspect of the present invention provides a steam cracking method, the method being carried out in a cracking furnace, the cracking furnace including a convection section, a gas-liquid separator, and a radiation section, the method comprising:

[0013] (1) After the pyrolysis feedstock is contacted with water vapor in the convection section and heated to the cross temperature, it is conveyed to the gas-liquid separator for gas-liquid separation to obtain gas phase material and liquid phase material;

[0014] (2) The gaseous material is transported to the radiation section for pyrolysis to obtain low-carbon olefins;

[0015] The temperature range is 350-500℃.

[0016] A second aspect of the present invention provides a steam cracking system, the system comprising:

[0017] A pyrolysis furnace, comprising a convection section, a gas-liquid separator, and a radiant section connected in series, wherein the material outlet of the convection section is connected to the material inlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator is connected to the material inlet of the radiant section; and

[0018] The convection section is used to contact the pyrolysis feedstock with steam and heat it to a cross temperature. The number of convection section tubes is sufficient to make the cross temperature range from 350 to 500°C.

[0019] The method described in this invention sequentially gasifies, separates gas and liquids, and pyrolyzes the pyrolysis feedstock, controlling the temperature range between 350-500°C. This reduces coking in the pyrolysis furnace tubes, especially in the convection section, avoids the shutdown process required for decoking in the convection section, and extends the coking cycle in the radiant section. Furthermore, separating the light and heavy components of the pyrolysis feedstock and hydrogenating them before introducing them into the pyrolysis furnace further extends the furnace's operating cycle and improves the unit's online rate. Detailed Implementation

[0020] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0021] The first aspect of the present invention provides a steam cracking method, which is carried out in a cracking furnace, the cracking furnace including a convection section, a gas-liquid separator and a radiation section, the method comprising:

[0022] (1) After the pyrolysis feedstock is contacted with water vapor in the convection section and heated to the cross temperature, it is conveyed to the gas-liquid separator for gas-liquid separation to obtain gas phase material and liquid phase material;

[0023] (2) The gaseous material is transported to the radiation section for pyrolysis to obtain low-carbon olefins;

[0024] The temperature range is 350-500℃.

[0025] In this invention, the spanning temperature can be, for example, 350, 360, 380, 400, 420, 430, 435, 440, 445, 450, 455, 460, 465, 470, 475, 480, 490, 500°C, or any range between any two values; preferably, the spanning temperature is 430-480°C.

[0026] The above-mentioned cross-temperature can be achieved by reducing the number of pipe rows in the mixing heating section of the convection section and adjusting the set temperature.

[0027] In this invention, the cracking feedstock is the cracking feedstock in the conventional sense in the art. Specifically, the cracking feedstock may include light naphtha, naphtha, diesel, hydrotreated tail oil, light crude oil, crude oil with a final boiling point higher than 600°C and lower than 700°C, and dehydrated and desalted crude oil.

[0028] Preferably, the pyrolysis feedstock is selected from at least one of diesel oil, hydrotreated tail oil, light crude oil, dehydrated and desalted crude oil, and crude oil with a final boiling point higher than 600°C and lower than 700°C.

[0029] Preferably, the API degree of the pyrolysis feedstock is greater than 18, more preferably greater than 22, for example, it can be 24, 26, 28, 30, 32, 34, 36, 38, 40 or more, or any range between any two values.

[0030] API density is a measure developed by the American Petroleum Institute (API) to express the density of petroleum and petroleum products. API density is obtained by measuring the density of the raw material and then converting it.

[0031] In this invention, low-carbon olefins refer to olefins with fewer carbon atoms, such as olefins with four or fewer carbon atoms.

[0032] To fully utilize the heat from the high-temperature flue gas in the radiant section, the convection section of the pyrolysis furnace is typically equipped with multiple sections for heat recovery. Typically, the convection section may include a feedstock preheating section, a boiler feedwater preheating section, a dilution steam superheating section, an ultra-high pressure steam superheating section, and a mixing heating section. The feedstock preheating section is typically used to preheat the pyrolysis feedstock. The boiler feedwater preheating section is typically used to preheat the boiler feedwater supplied to the steam drum. The dilution steam superheating section is typically used to preheat the dilution steam (i.e., water vapor). The ultra-high pressure steam superheating section is typically used to heat the high-pressure steam from the steam drum to obtain ultra-high pressure steam. The mixing heating section is typically used to heat the pyrolysis feedstock to a temperature range spanning [temperature range missing].

[0033] Preferably, the method further includes: preheating the pyrolysis feedstock in a convection section before mixing the steam with the pyrolysis feedstock to obtain a preheated feedstock.

[0034] Preferably, the temperature of the preheated raw material is 120-300℃, more preferably 150-250℃.

[0035] Preferably, the temperature of the water vapor is 480-560℃, and more preferably 500-540℃.

[0036] Preferably, the weight ratio of the pyrolysis feedstock to steam is 1-4:1, more preferably 1.5-2.5:1.

[0037] Material from the convection section undergoes gas-liquid separation in a gas-liquid separator. The gas phase is sent to the radiation section for pyrolysis, while the liquid phase is discharged directly from the system. The gas-liquid separator utilizes the heat within the furnace to achieve the required temperature.

[0038] The gas-liquid separator can be a flash evaporator, a cyclone separator, or other device with gas-liquid separation function. The cyclone separator can be selected from a volute-type cyclone separator, an axial-flow guide vane-type cyclone separator, a straight-cylinder-type cyclone separator, a cone-type combined cyclone separator, or a direct-flow cyclone separator. The cyclone separator is equipped with internal components, including a skimming cylinder located at the top of the cyclone separator and / or a baffle plate and anti-vortex device at the bottom of the cyclone separator.

[0039] Preferably, the liquid phase volume content at the inlet of the gas-liquid separator is 0.01-5% by volume, more preferably 0.02-2% by volume.

[0040] Preferably, the liquid phase content in the gas phase outlet of the gas-liquid separator is less than 10 g / m³. 3 More preferably below 200 mg / m 3 .

[0041] In the radiant section, the material that has reached the cross-temperature range is pyrolyzed. Preferably, the conditions for the pyrolysis reaction include: the outlet temperature of the radiant section is 780-850℃, more preferably 790-840℃.

[0042] In this invention, the radiant section includes multi-pass furnace tubes, preferably 2-6 passes, and more preferably two passes.

[0043] The two-stage furnace tubes, for example, consist of two parallel vertical inlet tubes in the first stage and one vertical outlet tube in the second stage, forming a 2-1 type radiant furnace tube; or four parallel vertical inlet tubes in the first stage and one vertical outlet tube in the second stage, forming a 4-1 type radiant furnace tube.

[0044] Preferably, the ratio of the inner diameter of the outlet pipe to the inner diameter of the inlet pipe of the multi-pass furnace tube is in the range of: greater than 1 and less than or equal to 2.5.

[0045] In a preferred embodiment of the present invention, the inner diameter of the inlet pipe of the multi-pass furnace tube ranges from 25 mm to 70 mm, more preferably from 40 mm to 65 mm.

[0046] In a preferred embodiment of the present invention, the inner diameter of the outlet pipe of the multi-pass furnace tube ranges from 45 mm to 120 mm, more preferably from 60 mm to 95 mm.

[0047] The radiant section of the furnace tube may also employ enhanced heat transfer elements. These elements can be various known or unknown components, such as spiral blade inserts, twisted ribbon inserts, cross-serrated inserts, coil core inserts, spiral wire porous bodies, spherical substrate inserts, etc., to facilitate heat transfer. Different enhanced heat transfer elements can also be added to different parts of the furnace tube.

[0048] In this invention, the material obtained after pyrolysis in the radiation section can be cooled in a quenching device and separated to obtain low-carbon olefins.

[0049] The quenching device can be a conventional quenching device in the art, such as a quenching boiler. The quenching device can be located outside the cracking furnace.

[0050] Unless otherwise specified, the methods and equipment involved in this invention are conventional in the art.

[0051] A second aspect of the present invention provides a steam cracking system, the system comprising:

[0052] A pyrolysis furnace, comprising a convection section, a gas-liquid separator, and a radiant section connected in series, wherein the material outlet of the convection section is connected to the material inlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator is connected to the material inlet of the radiant section; and

[0053] The convection section is used to contact the pyrolysis feedstock with steam and heat it to a cross temperature. The number of convection section tubes (especially the mixed heating section tube bank) is sufficient to make the cross temperature range 350-500°C.

[0054] The specific structures of the various devices involved have already been described in the first aspect and will not be repeated here. Unless otherwise specified, the devices used are all of conventional structures in this field.

[0055] It should be understood that the system may also include other supporting facilities, such as a quenching device, for cooling the pyrolysis material from the radiant section and separating it into low-carbon olefins. Preferably, the material inlet of the quenching device is connected to the material outlet of the radiant section.

[0056] The present invention will be described in detail below through embodiments.

[0057] In the following examples, the pyrolysis feedstock used was dehydrated and desalted crude oil with a density of 862.4 kg / m³ at 20°C. 3 The API level is approximately 33.

[0058] The cracking furnace used in the following examples is a modified CBL-III cracking furnace purchased from Sinopec. A volute cyclone separator is added to the cracking furnace, the material inlet is connected to the convection section, the gas phase material outlet is connected to the material inlet of the radiation section, and the liquid phase material outlet is connected to the material inlet of the hydrogenation catalytic reactor. The number of two sets of mixing and heating section tubes is reduced. This is the cracking furnace used in Example 1.

[0059] Example 1

[0060] This embodiment illustrates the system and method for steam cracking.

[0061] The system for steam cracking includes a cracking furnace and a quench boiler. The cracking furnace comprises a convection section, a gas-liquid separator (volute cyclone separator), and a radiant section connected in series. The material outlet of the convection section is connected to the material inlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator is connected to the material inlet of the radiant section. The convection section of the cracking furnace includes a feedstock preheating section, a boiler feedwater preheating section, a dilution steam superheating section, an ultra-high pressure steam superheating section, and a mixing heating section. The feedstock preheating section preheats the crude oil; the boiler feedwater preheating section preheats the boiler feedwater supplied to the steam drum; the dilution steam superheating section preheats the dilution steam; the ultra-high pressure steam superheating section heats the high-pressure steam from the steam drum to obtain ultra-high pressure steam; and the mixing heating section heats the crude oil to a specified temperature range. The radiant section is connected to the quench boiler via pipeline and is used to transport the cracking products to the quench boiler for cooling and separation to obtain low-carbon olefins.

[0062] The steam cracking method includes: crude oil at 60°C is vaporized and preheated in a convection section, then enters a gas-liquid separator for gas-liquid separation. The crude oil feed rate is 47,000 kg / h, and the dilution steam rate is 37,600 kg / h. The inlet liquid content of the gas-liquid separator is 0.08% by volume, and the liquid density is 1200 kg / m³. 3 The gas phase density is 0.87 kg / m³. 3 The gas-liquid separator has a separation efficiency of 98%, and the liquid phase density at the gas outlet is 500 mg / m³. 3 .

[0063] The separated gas phase enters the radiant section furnace tubes through the gas-liquid separator gas phase outlet for cracking reaction, while the liquid phase is directly discharged from the system through the liquid phase outlet. The cracking products are transported to the quench boiler for cooling and separation to obtain low-carbon olefins. The radiant section furnace tubes are two-pass tubes with an inlet diameter of 51 mm, an outlet diameter of 73 mm, and a tube length of 26.6 m. The feedstock is heated to the transverse temperature (XOT) of 450℃ in the convection section; the radiant section inlet pressure (XOP) is 0.127 MPaG, the outlet temperature (COT) is 790℃, and the radiant section outlet pressure (COP) is 0.083 MPaG.

[0064] Example 1 reduced the heat exchange tube array in the mixing heating section in the convection section. By reducing the heat exchange between the raw material and steam mixture and the flue gas in the mixing heating section, the cross temperature of the material was reduced, which effectively reduced the cracking reaction in the convection section and reduced coking.

[0065] The pyrolysis furnace operates on a 68-day cycle and undergoes online coking five times a year, with each coking session lasting two days. The main components of the pyrolysis products are shown in Table 1.

[0066] Comparative Example 1

[0067] This comparative example is used to illustrate the reference steam cracking method.

[0068] Cracking was carried out in a steam cracking system consisting of a CBL-III type cracking furnace and a quench boiler connected in series with it. That is, unlike Example 1, the cracking furnace in this system does not contain a gas-liquid separator, and there are two more sets of pipe banks in the mixing heating section of the convection section.

[0069] The procedure was performed according to Example 1, except that the temperature range was 520°C. The results are shown in Table 1.

[0070] The pyrolysis furnace operates on a 65-day cycle, with one coking cleaning of the convection section per year (7 days, plus one shutdown and one start-up). It also performs online coking 5 times, with each coking session lasting 2 days. The main components of the pyrolysis products are shown in Table 1.

[0071] Table 1

[0072] Main product yield (wt%) Example 1 Comparative Example 1 ethylene 23.95 23.01 propylene 12.78 13.02 butadiene 5.45 5.05

[0073] As can be seen from Example 1, the online time of the cracking furnace in Example 1 increased by 10 days within a year, resulting in a significant increase in product output and improved product yield. The output of trienes (ethylene, propylene, and butadiene) alone increased by 7429 tons. Based on a product price of RMB 7000 per ton (a fixed value is used here for ease of calculation due to fluctuating product prices), the increased revenue from trienes alone reached RMB 52.01 million. Furthermore, compared to Example 1, Example 1 saved on start-up, shutdown, and maintenance costs. In other words, the technical solution of this invention achieves better results.

[0074] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A steam pyrolysis method, characterized in that, The method is implemented in a pyrolysis furnace, which includes a convection section, a gas-liquid separator, and a radiation section connected in series. The method includes: (1) After the pyrolysis feedstock is contacted with water vapor in the convection section and heated to the cross temperature, it is conveyed to the gas-liquid separator for gas-liquid separation to obtain gas phase material and liquid phase material; (2) The gaseous material is transported to the radiation section for pyrolysis to obtain low-carbon olefins; The convection section is used to contact the pyrolysis feedstock with steam and heat it to a cross temperature. The number of convection section tubes is sufficient to make the cross temperature range from 350 to 500°C.

2. The method according to claim 1, wherein, The temperature range is 430-480℃.

3. The method according to claim 1, wherein, The API degree of the pyrolysis feedstock is 18 or higher.

4. The method according to claim 3, wherein, The API degree of the pyrolysis feedstock is 22 or higher.

5. The method according to claim 3, wherein, The pyrolysis feedstock is selected from at least one of light naphtha, naphtha, diesel, hydrotreated tail oil, light crude oil, crude oil with a final boiling point above 600°C and below 700°C, and dehydrated and desalted crude oil.

6. The method according to claim 5, wherein, The pyrolysis feedstock is at least one of diesel oil, hydrotreated tail oil, light crude oil, dehydrated and desalted crude oil, and crude oil with a final boiling point higher than 600°C and lower than 700°C.

7. The method according to claim 1, wherein, The method further includes: preheating the pyrolysis feedstock in a convection section before mixing the steam with the pyrolysis feedstock to obtain a preheated feedstock.

8. The method according to claim 7, wherein, The temperature of the preheated raw material is 120-300℃.

9. The method according to claim 8, wherein, The temperature of the preheated raw material is 150-250℃.

10. The method according to claim 1, wherein, The temperature of the water vapor is 480-560℃.

11. The method according to claim 10, wherein, The temperature of the water vapor is 500-540℃.

12. The method according to any one of claims 1-11, wherein, The weight ratio of the pyrolysis feedstock to steam is 1-4:

1.

13. The method according to claim 12, wherein, The weight ratio of the pyrolysis feedstock to steam is 1.5-2.5:

1.

14. The method according to any one of claims 1-11, 13, wherein, The liquid phase volume content at the inlet of the gas-liquid separator is 0.01-5% by volume; and / or The liquid phase content at the gas phase outlet of the gas-liquid separator is less than 10 g / m³. 3 .

15. The method according to claim 14, wherein, The liquid phase volume content at the inlet of the gas-liquid separator is 0.02-2% by volume; and / or The liquid phase content at the gas phase outlet of the gas-liquid separator is less than 200 mg / m³. 3 .

16. The method according to claim 12, wherein, The liquid phase volume content at the inlet of the gas-liquid separator is 0.01-5% by volume; and / or The liquid phase content at the gas phase outlet of the gas-liquid separator is less than 10 g / m³. 3 .

17. The method according to claim 16, wherein, The liquid phase volume content at the inlet of the gas-liquid separator is 0.02-2% by volume; and / or The liquid phase content at the gas phase outlet of the gas-liquid separator is less than 200 mg / m³. 3 .

18. The method according to any one of claims 1-11, 13, 15-17, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 780-850℃.

19. The method according to claim 18, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 790-840℃.

20. The method according to claim 12, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 780-850℃.

21. The method according to claim 20, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 790-840℃.

22. The method according to claim 14, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 780-850℃.

23. The method according to claim 22, wherein, The conditions for the pyrolysis reaction include: the outlet temperature of the radiation section is 790-840℃.

24. A steam pyrolysis system, characterized in that, The system includes: The pyrolysis furnace includes a convection section, a gas-liquid separator, and a radiation section connected in series, wherein the material outlet of the convection section is connected to the material inlet of the gas-liquid separator, and the gas phase outlet of the gas-liquid separator is connected to the material inlet of the radiation section. The convection section is used to contact the pyrolysis feedstock with steam and heat it to a cross temperature. The number of convection section tubes is sufficient to make the cross temperature range from 350 to 500°C.

25. The system according to claim 24, wherein, The system also includes a quenching device connected to the material outlet of the radiant section for cooling and separating the material from the radiant section to obtain low-carbon olefins.