A hydrocracking method for improving heavy naphtha yield

Abstract

The present invention discloses a hydrocracking method for improving the yield of heavy naphtha, comprising: (1) conducting a hydrorefining reaction on heavy raw oil and hydrogen; (2) feeding the effluent of the hydrorefining reaction from the top to a first hydrocracking reactor for a first hydrocracking reaction, and fractionating the first cracking reactant to obtain a heavy naphtha fraction, a diesel fraction, and a tail oil fraction; (3) circulating the tail oil fraction to a side feed port of the first hydrocracking reactor; (4) feeding the diesel fraction as the first feed to the side feed port of a second hydrocracking reactor; feeding fresh diesel raw material from the top to the second hydrocracking reactor as the second feed; and fractionating the second and first hydrocracking reactants together. The method of the present invention rationally matches the circulating components, reaction conditions, and catalysts through a partitioning setting, processes heavy and low-quality raw oil under a hydrocracking process, and can produce more heavy naphtha and reduce the yield of light hydrocarbons.

Description

Technical Field

[0001] The invention belongs to the field of petroleum refining, and in particular relates to a hydrocracking method for improving the yield of heavy naphtha. Background Art

[0002] In recent years, stringent environmental regulations and economic development in my country have led to a rapid increase in demand for aromatic feedstocks in the oil market. The hydrocracking process converts heavy, low-quality crude oils into lighter products, with heavy naphtha being a key feedstock for aromatics production. Currently, recycling diesel and tail oil fractions back into the hydrocracking reactor is an effective way to increase heavy naphtha production. However, this method can lead to over-cracking of hydrocarbons, increasing the yield of light hydrocarbons.

[0003] CN 110835550B discloses a hydrocracking method for producing chemical raw materials. This method utilizes a preferred hydrorefining catalyst and a hydrocracking catalyst to process heavy crude oil in a single-stage, series, single-pass process. The naphtha yield can reach approximately 33%. This single-pass process also produces some middle distillates and tail oil, limiting the naphtha yield.

[0004] CN 111117702A discloses a hydrocracking method for producing high-yield heavy naphtha and jet fuel fractions. The feedstock is mixed with hydrogen, heated, and sequentially passed through a hydrorefining reaction zone and a hydrocracking reaction zone. The resulting reaction products are separated into a light naphtha fraction, a heavy naphtha fraction, a jet fuel fraction, a diesel fraction, and a tail oil fraction. A portion of the diesel fraction is recycled to the hydrocracking reaction zone. Two hydrocracking catalyst grades are used in the hydrocracking reaction zone. This method maximizes the production of heavy naphtha and jet fuel components at high conversion rates, while maintaining relatively high yields of lower-value-added products such as dry gas, liquefied gas, and light naphtha.

[0005] CN109777512A discloses a hydrocracking method for increasing the yield of heavy naphtha. This method utilizes the concept of hydrocarbon zoning intensification to return the products from the fractionation system to the catalyst beds in the hydrocracking reactor via a series of partial circulation processes, thereby achieving the goal of increasing heavy naphtha production. However, this method still results in over-cracking at the bottom of the hydrocracking reactor, resulting in a high yield of dry gas and liquefied gas. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention provides a hydrocracking method for increasing the yield of heavy naphtha. By utilizing a zoning arrangement and rationally matching circulating components, reaction conditions, and catalysts, the present method processes heavy, low-quality feedstock oil under a hydrocracking process, producing more heavy naphtha while reducing the yield of light hydrocarbons.

[0007] The hydrocracking method for improving the yield of heavy naphtha of the present invention comprises the following contents:

[0008] (1) In the hydrorefining reaction zone, heavy crude oil is mixed with hydrogen and then subjected to a hydrorefining reaction, and the effluent of the hydrorefining reaction enters the hydrocracking reaction zone; wherein a fixed-bed hydrorefining reactor is provided in the hydrorefining reaction zone, in which a hydrorefining catalyst is loaded;

[0009] (2) A first hydrocracking reactor and a second hydrocracking reactor are arranged in the hydrocracking reaction zone, and the effluent of the hydrorefining reaction zone is first fed to the first hydrocracking reactor from the top to perform the first hydrocracking reaction. The first cracking reactant enters the fractionation zone and is fractionated to obtain a heavy naphtha fraction, a diesel fraction and a tail oil fraction; wherein the first hydrocracking reactor is a high-pressure cracking reactor, loaded with a first hydrocracking catalyst, and the first to nth hydrocracking catalyst beds are sequentially arranged along the logistics direction, wherein n ≥ 3, preferably 3, 4 and 5; the second hydrocracking reactor is a medium-pressure cracking reactor, loaded with a second hydrocracking catalyst, and the first to nth hydrocracking catalyst beds are sequentially arranged along the logistics direction, wherein n ≥ 2, preferably 2, 3 and 4;

[0010] (3) the tail oil fraction obtained in step (2) is circulated to the side feed port of the first hydrocracking reactor, wherein the side feed port is arranged between the first and second hydrocracking catalyst beds;

[0011] (4) The diesel fraction obtained in step (2) is fed as the first feed into the side feed port of the second hydrocracking reactor, wherein the side feed port is arranged between the first and second hydrocracking catalyst beds; the fresh diesel feed is mixed with hydrogen as the second feed and fed into the second hydrocracking reactor from the top; the two feeds pass through the hydrocracking catalyst bed from top to bottom to undergo a second hydrocracking reaction, and the second hydrocracking reactants enter the fractionation zone and are fractionated together with the first hydrocracking reactants.

[0012] In the method of the present invention, the heavy feedstock oil described in step (1) is vacuum wax oil, and may also be a mixture of one or more of deasphalted oil, coal tar, and direct and indirect coal liquefaction oil; wherein the mass percentage of vacuum wax oil is 75%-100%; the heavy feedstock oil has a distillation range of 200°C-580°C, a sulfur content of 0.2wt%-2.5wt%, a nitrogen content of 700-3000μg / g, and an aromatics content of 25wt%-55wt%, preferably 25wt%-45wt%.

[0013] In the method of the present invention, the hydrofining reaction in step (1) is mainly hydrodesulfurization, hydrodenitrogenation, hydrosaturation, and hydrodemetallization. The hydrofining reaction conditions are as follows: reaction pressure of 5.0 MPa to 35.0 MPa, preferably 12.0 MPa to 19.0 MPa; average reaction temperature of 200°C to 480°C, preferably 270°C to 450°C; volume space velocity of 0.1 to 15.0 h -1 , preferably 0.2~3.0h- 1 ; The volume ratio of hydrogen to oil is 100 to 2500, preferably 400 to 2000.

[0014] In the method of the present invention, the hydrorefining catalyst described in step (1) can be a conventional hydrorefining catalyst, which includes a carrier and a supported hydrogenation active metal. Based on the weight of the catalyst, it generally includes a Group VIB metal component of the periodic table, such as tungsten and / or molybdenum, calculated as oxide, in an amount of 10wt% to 35wt%, preferably 15wt% to 30wt%; and a Group VIII metal such as nickel and / or cobalt, calculated as oxide, in an amount of 1wt% to 7wt%, preferably 1.5wt% to 6wt%. The carrier is an inorganic refractory oxide, generally selected from alumina, amorphous silica-alumina, silica, titanium oxide, etc. Various existing commercial catalysts can be selected, such as FF-14, FF-24, 3936, 3996, FF-16, FF-26, FF-36, FF-46, FF-56, FF-66, etc., developed by Fushun Research Institute of Petrochemical Industry (FRIPP); HC-K and HC-P produced by UOP can also be prepared as needed according to common knowledge in the art.

[0015] In the method of the present invention, the first hydrocracking reactor and the second hydrocracking reactor are both fixed reactors; the first hydrocracking reaction is as follows: hydrogen partial pressure 6.0-20.0 MPa, preferably 12.0 MPa-19.0 MPa, reaction temperature 280-400°C, volume space velocity 0.5-4h -1 , the hydrogen-to-oil volume ratio is 200~2000; usually the average reaction temperature of the hydrocracking catalyst in the next bed (nth bed) is 2°C~20°C higher than the average reaction temperature of the hydrocracking catalyst in the previous bed (n-1th bed), preferably 3°C~5°C.

[0016] In the method of the present invention, the first hydrocracking catalyst comprises a support and an active metal component supported on the support. The support is primarily composed of a Y-type molecular sieve and alumina, or a Y-type molecular sieve, amorphous silica-alumina, and alumina, or amorphous silica-alumina and alumina. The active metal component is selected from at least two metal components selected from Group VIB and / or Group VIII. The active metal component content is generally 13% to 40% by weight, based on the weight of the catalyst. Preferably, the first hydrocracking catalyst contains 18% to 32% by weight of Group VIB and / or Group VIII metals. The catalyst also contains additives such as one or more of phosphorus, titanium, and zirconium. The modified Y molecular sieve in the hydrocracking catalyst has a unit cell constant of 2.437 to 2.450 nm, a SiO2 / Al2O3 molar ratio of 5 to 25, and a relative crystallinity of 80% to 130%. Based on the weight of the catalyst, the content of the Y-type molecular sieve is generally 30 wt% to 80 wt%. Preferably, the content of the Y-type molecular sieve in the first hydrocracking catalyst is 40 wt% to 70 wt%.

[0017] In the method of the present invention, the filling volume ratio of each catalyst bed (from top to bottom) in the first hydrocracking reaction zone is generally 0.5 <T n :T n-1 <2, n≥2; the Y-type molecular sieve content in each hydrocracking catalyst bed (from top to bottom) generally decreases by 2wt% to 10wt%, and the active metal content in each hydrocracking catalyst bed (from top to bottom) generally increases by 2wt% to 4wt%. Taking three hydrocracking catalyst beds as an example, the total catalyst volume in the first hydrocracking reaction zone is 100%, and the catalyst volume fractions from top to bottom are 13% to 37% for hydrocracking catalyst I, 33% to 67% for hydrocracking catalyst II, and 18% to 35% for hydrocracking catalyst III. The weight fraction of the Y-type molecular sieve in the hydrocracking catalyst I carrier is 55wt%~78wt%, and the weight fraction of the metal component is 18wt%~22wt%. The weight fraction of the Y-type molecular sieve in the hydrocracking catalyst II carrier is 50wt%~70wt%, and the weight fraction of the metal component is 20wt%~26wt%. The weight fraction of the Y-type molecular sieve in the hydrocracking catalyst III carrier is 43wt%~60wt%, and the weight fraction of the metal component is 22wt%~28wt%.

[0018] In the method of the present invention, a fractionating tower is provided in the fractionation zone, and the distillation range of the heavy naphtha obtained in the fractionation zone is 65°C to 180°C, the diesel fraction range is 165°C to 280°C, and the tail oil fraction range is 260°C to 400°C. In the method of the present invention, the fresh diesel feedstock is straight-run diesel and / or catalytically cracked light cycle oil, and the content of straight-run diesel in the fresh diesel feedstock is 85wt% to 100wt%, and the mass fraction of paraffins in the fresh diesel feedstock is 30wt% to 60wt%, preferably 35wt% to 55wt%.

[0019] In the method of the present invention, the second hydrocracking catalyst comprises a support and an active metal component supported on the support. The support is primarily composed of a Y-type molecular sieve and alumina, or a Y-type molecular sieve, amorphous silica-alumina, and alumina, or amorphous silica-alumina and alumina. The active metal component is selected from at least two metal components selected from Group VIB and / or Group VIII. The active metal component content is generally 18% to 50% by weight, based on the weight of the catalyst. Preferably, the first hydrocracking catalyst contains 25% to 40% by weight of the Group VIB and / or Group VIII metal. The catalyst also contains additives such as one or more of phosphorus, titanium, and zirconium. The modified Y molecular sieve in the hydrocracking catalyst has a unit cell constant of 2.432 to 2.465 nm, a SiO2 / Al2O3 molar ratio of 5 to 35, and a relative crystallinity of 80% to 130%. Based on the weight of the catalyst, the content of the Y-type molecular sieve is generally 30 wt% to 80 wt%. Preferably, the content of the Y-type molecular sieve in the second hydrocracking catalyst is 40 wt% to 70 wt%.

[0020] In the method of the present invention, the filling volume ratio of each catalyst bed (from top to bottom) in the second hydrocracking reaction zone is generally 0.2 <T n :T n-1 <1.5, n ≥ 2; the Y-type zeolite content in each hydrocracking catalyst bed (from top to bottom) generally decreases by 5wt% to 10wt%, while the active metal content in each hydrocracking catalyst bed (from top to bottom) generally increases by 4wt% to 7wt%. Taking two hydrocracking catalyst beds as an example, the total volume of the second hydrocracking reaction zone is 100%, and from top to bottom, the volume fraction of hydrocracking catalyst IV is 40% to 65%, and the volume fraction of hydrocracking catalyst V is 30% to 50%. The weight fraction of Y-type zeolite in the hydrocracking catalyst IV carrier is 55wt% to 78wt%, and the weight fraction of the metal component is 28wt% to 32wt%. The weight fraction of Y-type zeolite in the hydrocracking catalyst V carrier is 50wt% to 70wt%, and the weight fraction of the metal component is 32wt% to 36wt%.

[0021] In the method of the present invention, the second hydrocracking reaction conditions are: hydrogen partial pressure of 4.0MPa~14.0MPa, preferably, hydrogen partial pressure of 7.0MPa~12.0MPa, which is 6.0MPa~10.0MPa lower than the hydrogen partial pressure of the first hydrocracking reaction zone, reaction temperature of 230℃~450℃, volume space velocity of 0.5~30h -1 , the hydrogen-to-oil volume ratio is 200~2000.

[0022] In the method of the present invention, the ratio of the fresh feed amounts of the first hydrocracking reactor and the second hydrocracking reactor is 1:5 to 5:1, wherein the fresh feed of the first hydrocracking reactor is the hydrotreating reaction effluent from the hydrotreating reactor, and the fresh feed of the second hydrocracking reactor is one or a mixed feed of straight-run diesel, catalytic cracking light cycle oil, and coker diesel, wherein the mass fraction of the straight-run diesel is 85% to 100%, and the mass fraction of paraffins in the mixed feed is 30% to 60%, preferably 35% to 55%.

[0023] In the method of the present invention, the ratio of the first feed to the second feed in step (4) is 1:2 to 2:1.

[0024] Normally, the diesel fraction and the tail oil fraction are circulated to the hydrocracking reactor to increase the conversion depth and the yield of heavy naphtha. However, under conventional hydrocracking reaction conditions (high temperature and high pressure), the adsorption effect of the hydrocracking catalyst on the heavy component is significantly stronger than that on the lighter component, which can easily lead to the lighter component being unable to complete shallow cracking to produce heavy naphtha products. When the diesel and tail oil fractions are circulated to the hydrocracking reactor, the two fractions will be mixed with the hydrorefined raw materials and then enter the hydrocracking reactor. At this time, the catalyst system and reaction conditions in the cracking reactor are not well suitable for the diesel fraction, which can easily cause excessive cracking reaction, thereby increasing the yield of light hydrocarbons.

[0025] The present invention utilizes two hydrocracking reactors, one high-pressure and one medium-pressure. Based on the differences in adsorption energy of different components on the active sites of the hydrocracking catalyst (aromatics have much greater adsorption energy than cycloalkanes and paraffins), the aromatics-rich fraction reacts in the first hydrocracking reactor (high pressure) and the cycloalkanes- and paraffins-rich fraction reacts in the second hydrocracking reactor (medium pressure). Aromatic components are better adsorbed and reacted under high-pressure conditions, while cycloalkanes and paraffins also adsorb and react well under medium-pressure conditions. Adjusting the content of the various components of the feedstock entering the reactors avoids competitive adsorption between the different components, better matching the catalyst, feedstock, and reaction conditions, and cracking the feedstock into a higher proportion of the heavy naphtha fraction. The invention avoids the problem in the prior art of recycling diesel and tail oil fractions to the hydrocracking reactor to increase the production of heavy naphtha, which causes the paraffin, cycloparaffin and aromatic components to accumulate at the bottom of the hydrocracking reactor, the concentration of paraffin and cycloparaffin components is higher than that of aromatics, and the reactor pressure is high, which easily leads to excessive cracking of paraffin and cycloparaffin. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a flow chart of a hydrocracking method for improving the yield of heavy naphtha.

[0027] Among them, 1-raw oil, 2-hydrorefining reaction zone, 3-hydrorefining reaction product, 4-first hydrocracking reactor, 5-first hydrocracking reactant, 6-fractionation zone, 7-naphtha fraction, 8-diesel fraction, 9-tail oil fraction, 10-second hydrocracking reactor, 11-second hydrocracking reactant, 12-fresh diesel feedstock. DETAILED DESCRIPTION

[0028] The method disclosed in the present invention is described in more detail below with reference to the accompanying drawings and specific embodiments.

[0029] Example 1

[0030] Using mixed feedstock 1 as the feedstock and diesel feedstock as fresh feed to the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reactor were 30% / 40% / 30%; in the second hydrocracking reactor, the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V were 65% / 35%.

[0031] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3 and 4.

[0032] Example 2

[0033] Using mixed feedstock 2 as the feedstock and diesel feedstock as fresh feed for the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reactor were 30% / 40% / 30%; in the second hydrocracking reactor, the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V were 65% / 35%.

[0034] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3 and 4.

[0035] Example 3

[0036] Using mixed feedstock 3 as the feedstock and diesel feedstock as fresh feed to the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reaction zone were 30% / 40% / 30%, while the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V in the second hydrocracking reaction zone were 65% / 35%.

[0037] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3 and 4.

[0038] Example 4

[0039] Using mixed feedstock 1 as the feedstock and diesel feedstock as fresh feed for the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reactor were 30% / 50% / 20%; in the second hydrocracking reactor, the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V were 52% / 48%.

[0040] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3 and 4.

[0041] Example 5

[0042] Using mixed feedstock 2 as the feedstock and diesel feedstock as fresh feed for the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reactor were 30% / 50% / 20%; in the second hydrocracking reactor, the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V were 52% / 48%.

[0043] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3 and 4.

[0044] Example 6

[0045] Using mixed feedstock 3 as the feedstock and diesel feedstock as fresh feed for the second hydrocracking reactor, a catalyst system consisting of FF-66 hydrorefining catalyst, hydrocracking catalyst I, hydrocracking catalyst II, hydrocracking catalyst III, hydrocracking catalyst IV, and hydrocracking catalyst V was selected. All five hydrocracking catalysts were supported on amorphous silica-alumina and modified Y molecular sieve. The volume fractions of hydrocracking catalyst I, hydrocracking catalyst II, and hydrocracking catalyst III in the first hydrocracking reactor were 30% / 50% / 20%; in the second hydrocracking reactor, the volume fractions of hydrocracking catalyst IV and hydrocracking catalyst V were 52% / 48%.

[0046] The specific properties of the feedstock oil are shown in Table 1, the basic properties of the catalyst are shown in Table 2, and the process operating conditions and results are shown in Tables 3-4.

[0047] Comparative Examples 1-3

[0048] Comparative Examples 1-3 used mixed feedstock 1 as the feedstock oil, diesel feedstock as the fresh feed to the second hydrocracking reactor, FF-66 hydrorefining catalyst for the hydrotreating reaction zone, FC-56 for the first hydrocracking reactor, and FC-72 for the second hydrocracking reactor. Comparative process tests were conducted using a conventional once-through process, a two-stage series tail oil partial recycle process, and a two-stage series tail oil full recycle process, respectively, to examine the product quality of each fraction. Specific results are shown in Tables 2-4.

[0049] Table 1 Raw oil properties

[0050]

[0051] Table 2 Catalyst properties

[0052]

[0053] Table 3 Main process conditions of hydrocracking

[0054]

[0055] Table 3 (Continued) Main process conditions for hydrocracking

[0056]

[0057] Table 4 Hydrocracking product distribution and main product quality

[0058]

Claims

1. A hydrocracking method for increasing the yield of heavy naphtha, characterized in that The invention comprises the following contents: (1) in a hydrorefining reaction zone, heavy crude oil is mixed with hydrogen and then subjected to a hydrorefining reaction, and the effluent of the hydrorefining reaction enters a hydrocracking reaction zone; wherein a fixed-bed hydrorefining reactor is provided in the hydrorefining reaction zone, wherein a hydrorefining catalyst is loaded; (2) a first hydrocracking reactor and a second hydrocracking reactor are provided in the hydrocracking reaction zone, the effluent of the hydrorefining reaction is first fed from the top to the first hydrocracking reactor to undergo a first hydrocracking reaction, and the first hydrocracking reactant enters a fractionation zone and is fractionated to obtain a heavy naphtha fraction, a diesel fraction and a tail oil fraction; The first hydrocracking reactor is a high-pressure cracking reactor, loaded with a first hydrocracking catalyst, and the first to nth hydrocracking catalyst beds are sequentially arranged along the logistics direction, wherein n≥3; the second hydrocracking reactor is a medium-pressure cracking reactor, loaded with a second hydrocracking catalyst, and the first to nth hydrocracking catalyst beds are sequentially arranged along the logistics direction, wherein n≥2; (3) the tail oil fraction obtained in step (2) is circulated to the side feed port of the first hydrocracking reactor, wherein the side feed port is arranged at the first and second hydrocracking reactors. between the hydrocracking catalyst beds; (4) the diesel fraction obtained in step (2) is fed as the first feed into the side feed port of the second hydrocracking reactor, wherein the side feed port is arranged between the first and second hydrocracking catalyst beds; the fresh diesel feed is mixed with hydrogen as the second feed and fed into the second hydrocracking reactor from the top; the two feeds pass through the hydrocracking catalyst bed from top to bottom to undergo a second hydrocracking reaction, and the second hydrocracking reactants enter the fractionation zone and are fractionated together with the first hydrocracking reactants; The hydrogen partial pressure of the first hydrocracking reaction is 6.0-20.0 MPa, and the hydrogen partial pressure of the second hydrocracking reaction is 4.0 MPa-14.0 MPa. The hydrogen partial pressure of the second hydrocracking reaction is 6.0 MPa-10.0 MPa lower than that of the first hydrocracking reaction.

2. The method according to claim 1, wherein: The first hydrocracking reactor is provided with the 1st to nth hydrocracking catalyst beds in sequence along the logistics direction, wherein n is 3, 4 or 5; the second hydrocracking reactor is provided with the 1st to nth hydrocracking catalyst beds in sequence along the logistics direction, wherein n is 2, 3 or 4.

3. The method according to claim 1, wherein: The heavy feedstock oil in step (1) is vacuum wax oil, or one or more of vacuum wax oil mixed with deasphalted oil, coal tar, direct coal liquefaction oil and indirect coal liquefaction oil; wherein the mass percentage of vacuum wax oil is 75%-100%; the distillation range of the heavy feedstock oil is 200°C-580°C, the sulfur content is 0.2wt%-2.5wt%, the nitrogen content is 700-3000μg / g, and the aromatics content is 25wt%~55wt%.

4. The method according to claim 1, wherein: The hydrofining reaction conditions of step (1) are as follows: reaction pressure of 5.0 MPa to 35.0 MPa, average reaction temperature of 200°C to 480°C, volume space velocity of 0.1 to 15.0 h -1 , the hydrogen-to-oil volume ratio is 100-2500.

5. The method according to claim 4, characterized in that: The hydrofining reaction conditions of step (1) are as follows: reaction pressure of 12.0 MPa to 19.0 MPa; average reaction temperature of 270°C to 450°C; volume space velocity of 0.2 to 3.0 h -1 ; The volume ratio of hydrogen to oil is 400~2000.

6. The method according to claim 1, wherein: The hydrorefining catalyst in step (1) comprises a carrier and a supported hydrogenation active metal, wherein the carrier is an inorganic refractory oxide, and the inorganic refractory oxide is selected from one or more of alumina, amorphous silica-alumina, silica, and titanium oxide.

7. The method according to claim 1, wherein: The first hydrocracking reactor and the second hydrocracking reactor are both fixed bed reactors; the first hydrocracking reaction conditions are: hydrogen partial pressure of 12.0 MPa to 19.0 MPa, reaction temperature of 280°C to 400°C, volume space velocity of 0.5 to 4 h -1 , the hydrogen-to-oil volume ratio is 200~2000.

8. The method according to claim 1, wherein: In the first and second hydrocracking reactors, respectively, along the logistics direction, the average reaction temperature of the nth hydrocracking catalyst bed is 2° C. to 20° C. higher than the average reaction temperature of the n-1th hydrocracking catalyst bed.

9. The method according to claim 1, wherein: Along the logistics direction, the filling volume ratio of each catalyst bed from top to bottom in the first hydrocracking reactor is 0.5 <T n :T n-1 <2, n≥3.

10. The method according to claim 1, wherein: A fractionation tower is provided in the fractionation zone. The distillation range of the heavy naphtha obtained in the fractionation zone is 65°C to 180°C, the distillation range of the diesel is 165°C to 280°C, and the range of the tail oil fraction is 260°C to 400°C.

11. The method according to claim 1, wherein: The fresh diesel feedstock is straight-run diesel and / or catalytic cracking light cycle oil, and the content of straight-run diesel in the fresh diesel feedstock is 85wt%-100wt%, and the mass fraction of paraffin in the fresh diesel feedstock is 30wt%-60wt%.

12. The method according to claim 1, wherein: Along the logistics direction, the filling volume ratio of each catalyst bed from top to bottom in the second hydrocracking reactor is 0.2 <T n :T n-1 <1.5, n≥2.

13. The method according to claim 1, wherein: The second hydrocracking reaction conditions are: hydrogen partial pressure of 7.0MPa~12.0MPa, reaction temperature of 230℃~450℃, volume space velocity of 0.5~30h -1 , the hydrogen-to-oil volume ratio is 200~2000.

14. The method according to claim 1, wherein: The ratio of the hydrotreating reaction effluent to the fresh diesel feed of the second hydrocracking reactor is 1:5 to 5:

1.

15. The method according to claim 1, wherein: The ratio of the first feed to the second feed in step (4) is 1:2 to 2:1.

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