Method for producing low-sulfur and high-carbon-residue desulfurized residual oil

Abstract

The invention discloses a method for producing low-sulfur and high-carbon-residue desulfurized residual oil, which is characterized in that the carbon residue value of a raw material is increased by increasing the slag doping ratio of raw oil, the hydrodesulfurization selectivity is improved by keeping high space velocity and adjusting the bed temperature of each reactor, and the properties of the raw material are improved by blending catalytic diesel oil. And a high-quality low-sulfur petroleum coke raw material is provided for a delayed coking device. According to the present invention, the hydrodesulfurization selectivity is improved by adjusting the raw material composition and the temperature of each reactor by using the existing device, such that the low-sulfur and high-carbon-residue desulfurization residual oil meeting the requirements can be produced, and the long-period operation of the device is not affected.

Description

Technical Field

[0001] The invention belongs to the technical field of petrochemical industry, relates to the field of production of low-sulfur petroleum coke as negative electrode raw material, and specifically relates to a method for producing low-sulfur, high-carbon residual desulfurized residual oil. Background Art

[0002] Petroleum coke is a by-product obtained by delayed coking process using residual oil and heavy oil as raw materials in the process of petroleum refining. With the heavier and inferior quality of crude oil, the output of petroleum coke continues to increase, and high-sulfur coke is the main product. According to the sulfur content classification, the sulfur mass fraction not more than 3.0% is called low-sulfur coke, and the sulfur mass fraction greater than 3.0% is called high-sulfur coke. The use of high-sulfur petroleum coke as a highly polluting product is obviously limited, and the rigid demand of the fuel industry will shift to low-sulfur petroleum coke or alternative fuels. The demand for low-sulfur coke in downstream industries will increase significantly, and the demand for high-sulfur coke will shrink, which will form a situation where low-sulfur coke resources are tight and high-sulfur coke production capacity is in excess. Therefore, the development of a production process suitable for the production of low-sulfur petroleum coke is of great significance for solving the outlet of high-sulfur coke, improving resource utilization, and reducing environmental pollution.

[0003] The raw material processed by the traditional delayed coking process is the vacuum residue oil from the atmospheric and vacuum unit. The sulfur content of this raw material is high. The sulfur content of petroleum coke cannot be controlled below 3.0% by relying solely on the delayed coking process. Therefore, it is a practical method to use the residue oil hydrodesulfurization process to convert the high-sulfur residue oil into desulfurized residue oil with a lower sulfur content for delayed coking to produce low-sulfur petroleum coke.

[0004] CN101020843A discloses a hydrogenation method for catalytic cracking raw materials, wherein a mixture of raw oil and hydrogen contacts a hydrogenation protective agent, a residual oil hydrogenation demetallization agent and a residual oil hydrogenation desulfurization agent in sequence to react, and the reaction product is cooled and separated to obtain hydrogen-rich gas and liquid products. Based on the volume of the overall catalyst, the filling volume percentages of the hydrogenation protective agent, the residual oil hydrogenation demetallization agent and the residual oil hydrogenation desulfurization agent are 2-10 volume%, 5-70 volume% and 20-93 volume% respectively. The invention can be mixed with 5-50% by weight of residual oil in gas oil to produce high-quality catalytic cracking raw materials, which not only broadens the source of catalytic cracking raw materials, but also improves the processing depth of residual oil. The liquid product produced by the method has a carbon residue value of 1.65% by weight, which is a high-quality raw material for catalytic cracking, but as a raw material for producing low-sulfur petroleum coke, the carbon residue value is far from enough.

[0005] CN113563921A discloses a method and system for producing low-sulfur petroleum coke, which comprises: (1) introducing a residual oil raw material into a residual oil hydrogenation reaction system for hydrodesulfurization reaction to obtain hydrogenated wax oil and hydroreduced residue; (2a) introducing the hydrogenated wax oil and a part of the hydroreduced residue into a catalytic cracking device for catalytic cracking reaction to obtain catalytic cracking gasoline, catalytic cracking wax oil and oil slurry; (2b) introducing the remaining part of the hydroreduced residue into a delayed coking device for coking reaction to obtain coker wax oil and low-sulfur petroleum coke; (3) recycling the catalytic cracking wax oil, oil slurry and coker wax oil back into the residual oil hydrogenation reaction system for hydrodesulfurization reaction. The method requires a vacuum tower to be set in the fractionation part of the residual oil hydrogenation reaction system, and the desulfurized residual oil is subjected to vacuum fractionation to obtain the hydroreduced residue with high carbon residue. Since the residual oil hydrogenation reaction system with a vacuum tower is rarely used in domestic industry, the wide application of the process is limited.

[0006] CN109135818A discloses a residual oil desulfurization method and a preparation method and production equipment for petroleum coke. The residual oil desulfurization method comprises the following steps: S1) providing residual oil as a reaction raw material, mixing the residual oil with a pretreatment agent to pretreat the residual oil; wherein the pretreatment agent comprises a metal base and / or a metal carbonate; S2) placing the pretreated mixture obtained in step S1) in an electrostatic field for electrostatic adsorption desulfurization treatment to complete the desulfurization of the residual oil, and obtain residual oil with a low sulfur content. The method requires placing the residual oil and the pretreatment agent in an electrostatic field for desulfurization treatment. Currently, most of the delayed coking devices in refineries are not equipped with such pretreatment equipment, which limits the wide application of the method.

[0007] CN112745948A discloses a method and system for processing heavy crude oil and aromatic fraction oil, comprising: (11) introducing the heavy crude oil into a solvent deasphalting unit for solvent deasphalting; (12) introducing the deasphalted oil into a first hydrogenation unit for hydrogenation reaction, and introducing the obtained liquid phase effluent into a DCC unit for reaction; (2) introducing the aromatic fraction oil into a second hydrogenation unit for hydrogenation saturation and fractionation; (3) introducing the deoiled asphalt and aromatic hydrocarbon stream into a third reaction unit for hydrogenation reaction; (4) fractionating the liquid phase product of the third reaction unit; (51) introducing the second light component into a fourth reaction unit for reaction; and (52) introducing the second heavy component into a delayed coking unit for reaction; or using the second heavy component as a low-sulfur marine fuel oil component. The method involves a solvent deasphalting unit and a DCC unit. At present, there is no industrial application of the DCC unit in domestic refineries, which limits the wide application of the method.

[0008] In summary, the current residue oil hydrodesulfurization process reduces the sulfur content of the residue oil while greatly reducing its residual carbon value, that is, the hydrodesulfurization selectivity is not high, and the desulfurized residue oil with low residual carbon value enters the delayed coking unit to produce petroleum coke. On the one hand, it is not formed and easily causes coking and blockage of the distillation system and the venting system. On the other hand, the empty tower linear velocity of the coke tower is high. Summary of the invention

[0009] The technical problem to be solved by the present invention is: using the existing device, by adjusting the raw material composition and the temperature of each reactor, the hydrodesulfurization selectivity is improved, so as to produce low-sulfur, high-carbon residual desulfurized residual oil that meets the requirements without affecting the long-term operation of the device.

[0010] The technical solution of the present invention is:

[0011] By increasing the slag blending ratio of crude oil to increase the residual carbon value of the raw material, by maintaining a high space velocity and adjusting the bed temperature of each reactor to increase the hydrodesulfurization selectivity, and by blending catalytic diesel to improve the raw material properties, we can not only provide high-quality low-sulfur petroleum coke raw materials for the delayed coking unit, but also ensure the long-term operation of the unit.

[0012] Specifically, a method for producing low-sulfur, high-carbon residual desulfurized residual oil of the present invention comprises the following steps:

[0013] (1) introducing atmospheric residue oil, vacuum heavy wax oil and vacuum residue oil from an atmospheric and vacuum unit and coking wax oil from a delayed coking unit into a residue oil hydrogenation reaction system for hydrogenation desulfurization reaction to obtain hydrogenated naphtha, hydrogenated diesel and desulfurized residue oil; the residue oil hydrogenation reaction system comprises six fixed-bed reactors, and the catalyst gradation of each reactor is as follows: the first reactor, the second reactor, the third reactor and the fourth reactor are filled with protective agent and demetallization catalyst, the fifth reactor is filled with desulfurization catalyst, and the sixth reactor is filled with carbon removal catalyst

[0014] (2a) In order to increase the residual carbon value of the raw material so as to be more conducive to the production of low-sulfur petroleum coke raw materials, the amount of vacuum residue oil added to the raw material is increased to control the residue blending ratio;

[0015] (2b) To improve the properties of the feedstock, some catalytic diesel is added to control the pressure drop of each reactor;

[0016] (2c) To improve the hydrodesulfurization selectivity, the residue oil hydrogenation reaction system maintains high space velocity production, and when the fresh feed amount (i.e., the mixed oil of atmospheric residue oil and vacuum heavy wax oil, vacuum residue oil, catalytic diesel oil and coker wax oil) is insufficient, a certain long circulation amount is maintained;

[0017] (3) The mixed crude oil in step (2c) is pressurized (21-23 MPa) by a hydrogenation feed pump and then enters the reactor for hydrodesulfurization and carbon removal.

[0018] (4) In order to further improve the hydrodesulfurization selectivity, the bed temperature of each reactor is reduced, with an emphasis on reducing the bed temperature of the decarbonization reactor to obtain low-sulfur, high-carbon desulfurized residual oil.

[0019] (5) During the cooling process, the speed of the circulating hydrogen compressor is adjusted to maintain the hydrogen-to-oil ratio between 600-1500 Nm 3 / m 3 , to prevent the hydrogen-to-oil ratio from being too low and the oil from being unevenly distributed and forming hot spots.

[0020] In step (1) of the present invention, after the hydrodesulfurization reaction, the reaction products obtained can be separated to obtain gas, hydrogenated naphtha, hydrogenated diesel and hydrogenated slag.

[0021] Preferably, in step (2a) of the present invention, the residue ratio of the feedstock entering the residue oil hydrogenation reaction system is controlled so that the residue ratio of the mixed feed formed by the atmospheric residue oil, vacuum heavy wax oil, vacuum residue oil, catalytic diesel oil and coker wax oil is greater than 50wt%. When it is lower than this interval, the feed ratio of vacuum residue oil is increased to adjust the carbon residue value of the mixed feed.

[0022] More preferably, the proportion of vacuum residue oil entering the residue oil hydrogenation reaction system is controlled so that the blending ratio of the mixed feed formed by the mixed oil of atmospheric residue oil and vacuum heavy wax oil, vacuum residue oil, catalytic diesel oil and coking wax oil is 50-60wt%, and the sulfur content of the final mixed feed is controlled to be less than 4.2wt%, and the residual carbon value is controlled to be 11.5-13.0wt%. When it is higher than this interval range, the feed proportion of vacuum residue oil is reduced to control the pressure drop of each reactor, and the pressure drop of the first reactor is controlled to increase by less than 20KPa, the pressure drop of the second reactor is controlled to increase by less than 20KPa, the pressure drop of the third reactor is controlled to increase by less than 20KPa, the pressure drop of the fourth reactor is controlled to increase by less than 30KPa, the pressure drop of the fifth reactor is controlled to increase by less than 50KPa, and the pressure drop of the sixth reactor is controlled to increase by less than 70KPa.

[0023] Preferably, in step (2b), the proportion of the catalytic diesel is 5-10wt%. When it is higher than this interval, the flow rate of the catalytic diesel in the mixed feed is reduced to control the temperature rise of each reactor. When it is lower than this interval, the flow rate of the catalytic diesel in the mixed feed is increased to control the pressure drop of each reactor.

[0024] Preferably, in step (2c), the residue oil hydrogenation reaction system is controlled to maintain high space velocity production so that the outlet flow rate of the hydrogenation feed pump is ≮220 t / h to improve the hydrodesulfurization selectivity.

[0025] In step (4) of the present invention, the temperature of the desulfurization reactor is appropriately lowered, with a focus on lowering the temperature of the carbon removal reactor, with the fourth reactor lowering the temperature by 3-5°C, the fifth reactor lowering the temperature by 5-8°C, the sixth reactor lowering the temperature by 8-10°C, and the first, second, and third reactors lowering the temperature by 0-3°C, ultimately making the sulfur content of the desulfurized residual oil less than 0.6wt% and the carbon residue greater than 6.0wt%.

[0026] Preferably, in step (4), the inlet temperature conditions of each reactor include: 330-380°C for the first reactor, 332-382°C for the second reactor, 334-384°C for the third reactor, 339-388°C for the fourth reactor, 341-387°C for the fifth reactor, and 344-389°C for the sixth reactor.

[0027] The beneficial effects of the present invention include:

[0028] The present invention can utilize existing equipment without changing the production system of the existing equipment and without affecting the long-term operation of the equipment, and can produce desulfurized residual oil with low sulfur and high carbon residue, thus saving investment. The bed temperature of each reactor is selectively reduced, which not only reduces hydrogen consumption, but also reduces the load of the circulating hydrogen compressor, saving the consumption of medium-pressure steam and electricity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention is further described below in conjunction with the accompanying drawings.

[0030] Figure 1 is a schematic diagram of the process flow of the present invention, Figure 1 The markings are described as follows:

[0031] 1. Mixture of atmospheric residue oil and vacuum heavy wax oil

[0032] 2. Vacuum residue

[0033] 3. Coking wax oil

[0034] 4. Catalytic diesel

[0035] 5. Circulating oil

[0036] 6. Residue oil hydrogenation reaction system

[0037] 7. First Gas

[0038] 8. Liquid phase reaction products

[0039] 9. Residue oil hydrofractionation system

[0040] 10. Second Gas

[0041] 11. Hydrogenated naphtha

[0042] 12. Hydrogenated diesel

[0043] 13. Desulfurized residual oil.

[0044] Figure 2 for Figure 1 Schematic diagram of the composition of the medium residue oil hydrogenation reaction system 6, Figure 2 The markings are described as follows:

[0045] 14. Mixed feeding

[0046] 15. Raw material buffer tank

[0047] 16. Reaction heating furnace entrance

[0048] 17. Reaction heating furnace

[0049] 18. One-way entrance

[0050] 19. Anti

[0051] 20. Second anti-entrance

[0052] 21. Second Anti

[0053] 22. Three anti-entry

[0054] 23. Three Antis

[0055] 24. Four anti-entrances

[0056] 25. Four Anti-Moral Movements

[0057] 26. Five Anti-Entrance

[0058] 27. Five Antis

[0059] 28. Six Anti-Entrances

[0060] 29. Six Anti-Mobilization Campaign

[0061] 30. Reaction effluent

[0062] 31. High and low pressure separation system. DETAILED DESCRIPTION

[0063] Example 1

[0064] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0065] Unless otherwise specified, the pressure described in the present invention is gauge pressure.

[0066] In the present invention, the slag blending ratio is expressed by the formula: slag blending ratio = vacuum residue oil amount / (atmospheric residue oil amount+vacuum residue oil amount)*100%; the hydrogen-oil ratio is expressed by the formula: hydrogen-oil ratio = mixed hydrogen volume flow rate*hydrogen purity / hydrogenation feed pump outlet volume flow rate.

[0067] See also Figure 1 and Figure 2 A method for producing low-sulfur, high-carbon residual desulfurized residual oil of the present invention comprises:

[0068] (1) introducing a mixed oil 1 of atmospheric residue oil and vacuum heavy wax oil, vacuum residue oil 2, coker wax oil 3, catalytic diesel oil 4, and circulating oil 5 into a residue oil hydrogenation reaction system 6 for hydrodesulfurization and carbon removal to obtain a first gas 7 and a reaction liquid product 8;

[0069] (2a) introducing the reaction liquid phase product into the residue oil hydrogenation reaction system fractionation system 9 for distillation and cutting to obtain a second gas 10, hydrogenated naphtha 11, hydrogenated diesel 12, and desulfurized residue 13;

[0070] (2b) introducing the desulfurized residual oil 13 into a delayed coking unit to produce low-sulfur petroleum coke;

[0071] (3) A small portion of the desulfurized residual oil 13 obtained in step (2a) is circulated back to the residual oil hydrogenation reaction system 9 as circulating oil 5 to maintain a high space velocity.

[0072] (4) The residue oil hydrogenation reaction system 6 is composed of a raw material buffer tank 15, a reaction heating furnace 17, a first reactor 19, a second reactor 21, a third reactor 23, a fourth reactor 25, a fifth reactor 27, a sixth reactor 29 and a high and low pressure separation system 31. After being heated by the reaction heating furnace 17, the mixed feed is sequentially fed into the first reactor 19, the second reactor 21, the third reactor 23, the fourth reactor 25, the fifth reactor 27 and the sixth reactor 29 for hydrogenation desulfurization and residual carbon removal. The reaction effluent 30 is fed into the high and low pressure separation system 31 for gas-liquid separation to obtain a first gas 7 and a reaction liquid phase product 8.

[0073] The mixed oil 1 of atmospheric residue oil and vacuum heavy wax oil used in this example has properties shown in Table 1.

[0074] The properties of vacuum residue 2 used in this example are shown in Table 2.

[0075] The catalytic diesel 4 used in this example has properties shown in Table 3.

[0076] The operating conditions, product yields and main product properties of the residue oil hydrogenation reaction system are shown in Table 4.

[0077] Table 1

[0078]

[0079]

[0080] Table 2

[0081] Crude oil number Vacuum residue 2 <![CDATA[Density (20 °C), kg / m 3 > 1032.6 <![CDATA[Kinematic viscosity (100 °C), mm 2 / s]]> 2164 Carbon residue, weight % 21.85 Nitrogen, weight % 0.26 Sulfur, weight % 5.32 Metal content, ppm nickel 41 vanadium 132

[0082] Table 3

[0083] Crude oil number Catalytic Diesel 4 Sulfur, weight % 0.289 50％，℃ 255.1 90％，℃ 334.1 95％，℃ 347.6

[0084] Table 4

[0085]

[0086]

[0087] It can be seen from Table 4 that the desulfurized residual oil produced by the method of the present invention can meet the index requirements of sulfur content <0.6wt% and residual carbon >6.0wt%.

[0088] Comparative Example 1

[0089] See also Figure 1 The method for producing low-sulfur, high-carbon residual desulfurized residual oil in this comparative example comprises:

[0090] (1) introducing a mixed oil 1 of atmospheric residue oil and vacuum heavy wax oil, vacuum residue oil 2, coker wax oil 3, catalytic diesel oil 4, and circulating oil 5 into a residue oil hydrogenation reaction system 6 for hydrodesulfurization and carbon removal to obtain a first gas 7 and a reaction liquid product 8;

[0091] (2a) introducing the reaction liquid phase product into the residue oil hydrogenation reaction system fractionation system 9 for distillation and cutting to obtain a second gas 10, hydrogenated naphtha 11, hydrogenated diesel 12, and desulfurized residue 13;

[0092] (2b) introducing the desulfurized residual oil into a delayed coking unit to produce low-sulfur petroleum coke;

[0093] (3) A small portion of the desulfurized residual oil obtained in step (2a) is circulated back to the residual oil hydrogenation reaction system as circulating oil 5 to maintain a high space velocity.

[0094] The mixed oil 1 of atmospheric residue oil and vacuum heavy wax oil used in this example has properties shown in Table 1.

[0095] The properties of vacuum residue 2 used in this example are shown in Table 2.

[0096] The catalytic diesel 4 used in this example has properties shown in Table 3.

[0097] The feed composition, operating conditions, product yield and main product properties used in this example are shown in Table 5.

[0098] Table 5

[0099]

[0100] It can be seen from Table 5 that the desulfurized residual oil produced by the comparative example method cannot meet the index requirement of residual carbon>6.0wt%.

[0101] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for producing low-sulfur, high-carbon residue desulfurized residual oil, wherein the sulfur content of the low-sulfur, high-carbon residue desulfurized residual oil is less than 0.6wt%, and the carbon residue is greater than 6.0wt%; characterized in that: The method comprises the following steps: (1) introducing a mixed oil of atmospheric residue oil and vacuum heavy wax oil from an atmospheric and vacuum unit, vacuum residue oil and coking wax oil into a residue oil hydrogenation reaction system for hydrogenation desulfurization reaction to obtain gas, hydrogenated naphtha, hydrogenated diesel and hydrogenated atmospheric residue; the residue oil hydrogenation reaction system comprises six fixed-bed reactors, and the catalyst gradation of each reactor is as follows: the first reactor, the second reactor, the third reactor and the fourth reactor are filled with protective agent and demetallization catalyst, the fifth reactor is filled with desulfurization catalyst, and the sixth reactor is filled with carbon removal catalyst; (2a) increasing the amount of vacuum residue added to the raw material to control the residue ratio, so as to increase the residual carbon value of the raw material and be more conducive to the production of low-sulfur petroleum coke raw material; (2b) To improve the properties of the feedstock, some catalytic diesel is added to control the pressure drop of each reactor; (2c) In order to improve the hydrodesulfurization selectivity, the residue oil hydrogenation reaction system maintains high space velocity production, and when the feed amount of the mixed oil of atmospheric residue oil and vacuum heavy wax oil, vacuum residue oil, coker wax oil and catalytic diesel is insufficient, a certain long circulation amount is maintained; (3) The mixed feed oil in step (2c) is pressurized to 21-23 MPa by a hydrogenation feed pump and then enters a reactor for hydrodesulfurization and carbon removal reaction; (4) reducing the bed temperature of the carbon removal reactor to obtain low-sulfur, high-carbon desulfurized residual oil; (5) During the cooling process, the speed of the circulating hydrogen compressor is adjusted to maintain the hydrogen-to-oil ratio between 600 and 1500 Nm 3 / m 3 , used to prevent hot spots from being generated due to uneven oil distribution caused by too low hydrogen-to-oil ratio.

2. The method according to claim 1, characterized in that: In step (2a), the residue blending ratio in the feedstock entering the residue oil hydrogenation reaction system is controlled so that the residue blending ratio of the mixed feed formed by the mixed oil of the atmospheric residue oil and the vacuum heavy wax oil, the vacuum residue oil and the catalytic diesel oil is greater than 50wt%; when it is lower than this interval, the feed ratio of the vacuum residue oil is increased to adjust the residual carbon value of the mixed feed.

3. The method according to claim 2, characterized in that: The proportion of vacuum residue oil entering the residue oil hydrogenation reaction system is controlled so that the residue blending ratio of the mixed feed formed by the atmospheric residue oil, vacuum heavy wax oil, vacuum residue oil, catalytic diesel oil and coking wax oil is 50-60wt%, and the sulfur content of the final mixed feed is controlled to be less than 4.2wt%, and the residual carbon value is controlled to be 11.5-13.0wt%.

4. The method according to claim 3, characterized in that: When the residue blending ratio is higher than 60wt%, the feed ratio of vacuum residue oil is reduced to control the pressure drop of each reactor, and the pressure drop of the first reactor is controlled to increase by <20KPa, the pressure drop of the second reactor is controlled to increase by <20KPa, the pressure drop of the third reactor is controlled to increase by <20KPa, the pressure drop of the fourth reactor is controlled to increase by <30KPa, the pressure drop of the fifth reactor is controlled to increase by <50KPa, and the pressure drop of the sixth reactor is controlled to increase by <70KPa.

5. The method according to claim 1, characterized in that: In step (2b), the proportion of the catalytic diesel is 5 to 10 wt%.

6. The method according to claim 5, characterized in that: When the proportion of the catalytic diesel is higher than 10wt%, the flow rate of the catalytic diesel in the mixed feedstock is reduced to control the temperature rise of each reactor; when it is lower than this range, the flow rate of the catalytic diesel in the mixed feedstock is increased to control the pressure drop of each reactor.

7. The method according to claim 1, characterized in that: In step (2c), the residue oil hydrogenation reaction system is controlled to maintain high space velocity production so that the outlet flow rate of the hydrogenation feed pump is ≮220 t / h to improve the hydrodesulfurization selectivity.

8. The method according to any one of claims 1 to 7, characterized in that: In step (4), lowering the bed temperature of the carbon removal reactor includes lowering the temperature of the desulfurization reactor and lowering the temperature of the carbon removal reactor.

9. The method according to claim 8, characterized in that: In step (4), the temperature of each reactor is reduced by 3-5°C for the fourth reactor, 5-8°C for the fifth reactor, 8-10°C for the sixth reactor, and 0-3°C for the first, second and third reactors, respectively.

10. The method according to claim 9, characterized in that: In step (4), the inlet temperature conditions of each reactor are: 330-380°C for the first reactor, 332-382°C for the second reactor, 334-384°C for the third reactor, 339-388°C for the fourth reactor, 341-387°C for the fifth reactor, and 344-389°C for the sixth reactor.

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