A method for increasing aromatics production by catalytic diesel hydroconversion coupled with carbon dioxide

Abstract

The application discloses a method for increasing aromatic hydrocarbon yield by coupling carbon dioxide in catalytic diesel hydrogenation conversion. The method comprises the following steps: a catalytic diesel raw material is introduced into a first reaction zone to contact with a catalyst under a hydrogen condition to perform a hydrofining reaction, so that a first stream is obtained; the first stream is introduced into a second reaction zone to contact with a catalyst under a hydrogen condition to perform a hydrocracking reaction, so that a second stream is obtained; the second stream and a fifth stream circulating from the second stream are introduced into a separation system to be separated, so that dry gas, a light aromatic hydrocarbon stream, a third stream of non-aromatic hydrocarbons and a fourth stream of heavy aromatic hydrocarbons are obtained; the fourth stream is circulated to the first reaction zone; and the third stream is introduced into a third reaction zone to perform a coupling reaction under a CO2 atmosphere, so that a fifth stream is obtained. The method can effectively utilize non-aromatic hydrocarbon components of the catalytic diesel, has a higher monocyclic aromatic hydrocarbon yield while maintaining a high polycyclic aromatic hydrocarbon saturation rate.

Description

Technical Field

[0001] This invention belongs to the field of oil refining technology, and specifically relates to a method for hydrogenating catalytic diesel. Background Technology

[0002] Catalytic cracking units play a crucial role in refining and chemical enterprises. Currently, catalytic cracking remains one of the main methods for refining and chemical enterprises to produce lighter versions of heavy oil. Catalytic cracked diesel accounts for more than one-third of China's total diesel production, characterized by high polycyclic aromatic hydrocarbon content and low cetane number, making it difficult to process. Catalytic diesel products from catalytic cracking processes that produce more isoalkane and deep catalytic cracking are even more inferior, with cetane numbers sometimes below 20, further increasing processing difficulty. The properties of catalytic diesel are extremely poor, and current treatment methods are relatively limited, mainly involving combined processing with hydrotreating technologies, such as hydrorefining after mixing catalytic diesel with straight-run diesel, hydrocracking after mixing catalytic diesel with straight-run wax oil, and direct hydroconversion of catalytic diesel to produce aromatics. However, with the stagnation of diesel demand growth and the implementation of the China VI emission standard, the technology of converting catalytic diesel into light aromatics through hydrocracking has become an ideal way to solve the diesel surplus and low-carbon aromatics shortage, achieving cost reduction and efficiency improvement in the aromatics industry through integrated refining and chemical production.

[0003] CN1955262A discloses a two-stage hydrocracking method for catalytic diesel, which mixes low-quality catalytic diesel with heavy cracking feedstock, performs hydrotreating, and then performs hydrocracking to produce heavy naphtha with high aromatic potential. This technology requires the use of precious metal catalysts containing Pt and Pd, and the aromatic potential of the naphtha product is still relatively low, with insufficient aromatic purity, failing to meet the requirements of an aromatics complex. CN103897731A discloses a catalytic diesel and C... 10 +One method for producing light aromatics from distillate oils involves hydrorefining and hydrocracking, followed by product fractionation. Only fractions below 195°C are fed into the aromatics unit to produce light aromatics and clean gasoline blending components, resulting in a relatively low aromatics yield. CN112322349A discloses a full-conversion method and apparatus for producing light aromatics from catalytic diesel. The heavy tail oil from the bottom of the catalytic diesel after hydrotreating is hydrosaturated in a selective saturation reactor to obtain a single-ring product, which is then returned to the selective conversion reactor, thus achieving full-fraction conversion from catalytic diesel to light aromatics. CN109777514A discloses a hydrocracking-aromatics extraction process for processing catalytic diesel feedstock. By setting up two different hydrorefining reaction zones, the selectivity of monocyclic aromatics during hydrorefining is improved, effectively increasing aromatics production. CN112322349A discloses a method and apparatus for the complete conversion of catalytic diesel to light aromatics. The catalytic diesel undergoes hydrorefining and impurity separation, followed by selective conversion. The resulting mixed aromatics are then separated, and the tail oil is hydrogenated and recycled, thus achieving a full-fraction conversion of catalytic diesel to light aromatics with a high yield of monocyclic aromatics. Existing catalytic diesel hydroconversion technologies primarily focus on the hydrogenation of polycyclic aromatics in the catalytic diesel, without utilizing or treating non-aromatic components (alkanes, cycloalkanes, etc.). Therefore, there is still considerable room for improvement in the yield of aromatic products.

[0004] Among methods for converting alkanes to aromatics, the coupling of carbon dioxide to produce aromatics is a promising technical route. CN116120139A discloses a method for the coupling of light alkanes and carbon dioxide to produce aromatics. By adjusting the partial pressure of carbon dioxide, the H / C ratio of C2-C6 alkane products is controlled, thereby achieving high selectivity in aromatic production. CN115851309A discloses a method for the coupling conversion of naphtha and carbon dioxide on a zeolite molecular sieve catalyst to produce aromatics, with an aromatic selectivity exceeding 72%. However, the above methods are only applicable to light feedstocks with fewer than 12 carbon atoms and are not suitable for diesel fractions with higher carbon numbers or greater complexity. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for increasing aromatic hydrocarbon production through catalytic diesel hydroconversion coupled with carbon dioxide. This method effectively utilizes the non-aromatic components of catalytic diesel, achieving higher monocyclic aromatic hydrocarbon yields while maintaining a high polycyclic aromatic hydrocarbon saturation rate.

[0006] This invention provides a method for enhancing aromatic hydrocarbon production through catalytic diesel hydrotreating coupled with carbon dioxide, comprising the following steps:

[0007] (1) Catalytic diesel feedstock enters the first reaction zone and reacts with the catalyst under hydrogen conditions to obtain the first stream; the first reaction zone undergoes a hydrorefining reaction.

[0008] (2) The first stream obtained in step (1) enters the second reaction zone and reacts with the catalyst under hydrogen conditions to obtain the second stream; the second reaction zone carries out a hydrocracking reaction;

[0009] (3) The second stream obtained in step (2) enters the separation system for separation to obtain dry gas, light aromatic hydrocarbon stream, non-aromatic hydrocarbon third stream and heavy aromatic hydrocarbon fourth stream.

[0010] (4) The fourth stream obtained in step (3) is recycled to the first reaction zone; the third stream obtained in step (3) enters the third reaction zone and reacts under a CO2 atmosphere to obtain the fifth stream; wherein, the third reaction zone undergoes a coupling reaction between CO2 and alkanes;

[0011] (5) The fifth material obtained in step (4) is recycled to the separation system for separation.

[0012] In the above method, the initial boiling point of the catalytic diesel in step (1) is generally 160–240°C, preferably 180–220°C; the final boiling point is generally 320–420°C, preferably 350–390°C; the aromatic content is generally above 50 wt%; and the density of the catalytic diesel feedstock is generally 0.91 g·cm³. -3 above.

[0013] In the above method, the catalytic diesel in step (1) can be a catalytic cracking product obtained by processing any base oil, such as catalytic diesel obtained by processing Middle Eastern crude oil, specifically catalytic diesel components obtained by processing Iranian crude oil, Saudi crude oil, etc.

[0014] The above method, specifically the hydrorefining reaction in step (1) is as follows: catalytic diesel oil is contacted with a hydrorefining catalyst under hydrogenation conditions to produce desulfurization and denitrification reactions. Preferably, the tricyclic and dicyclic aromatic hydrocarbons in the catalytic diesel oil are selectively hydrogenated to retain one aromatic ring to obtain cycloalkyl monocyclic aromatic hydrocarbons.

[0015] The above method, in step (1), comprises a hydrorefining catalyst consisting of a support and a supported hydrogenating metal. The hydrogenating metal typically includes Group VIB and Group VIII metals from the periodic table, with Group VIB metals preferably being tungsten and / or molybdenum, and Group VIII metals preferably being nickel and / or cobalt. Based on the weight of the catalyst, the content of the Group VIB metal component, calculated as oxides, is 10%–35%; the content of the Group VIII metal, calculated as oxides, is 1%–7%. The support can be an inorganic refractory oxide, generally selected from at least one of alumina, silica, amorphous silica-alumina, and titanium oxide. The hydrorefining catalyst can be selected from various existing commercial catalysts, or it can be prepared according to common knowledge in the art as needed.

[0016] In the above method, the hydrocracking reaction in step (2) specifically involves the following: the first stream obtained after hydrorefining catalytic diesel is sequentially contacted with graded hydrocracking catalyst I and hydrocracking catalyst II under hydrogenation conditions. Specifically, a ring-opening cracking reaction of cycloalkyl aromatics occurs on hydrocracking catalyst I to obtain alkylbenzene, and a side-chain cleavage reaction of alkylbenzene further occurs on hydrocracking catalyst II to obtain light aromatics. The hydrocracking reaction in step (2) is a selective conversion process. Hydrocracking catalyst I has excellent ring-opening cracking function and low hydrogenation activity, aiming to perform a ring-opening cleavage reaction on cycloalkyl aromatics containing only one aromatic ring, obtained from the hydrogenation saturation conversion of tricyclic or dicyclic aromatics in step (1), while retaining the aromatic ring, to obtain alkylbenzene, effectively controlling the saturation depth and ring-opening position. Hydrocracking catalyst II has excellent side-chain cleavage cracking function and moderate hydrogenation performance, aiming to perform a side-chain cleavage reaction of alkylbenzene to obtain light aromatics with a lower carbon number.

[0017] In the above method, the hydrocracking catalyst I mentioned in step (2) can be any type of hydrocracking catalyst already available in the prior art, as long as it can achieve the purpose of ring-opening cracking of cycloalkyl aromatics to obtain alkylbenzenes in step (2). Various existing commercial catalysts can be selected, such as the FC series catalysts (e.g., FC-52) developed by the Dalian Petrochemical Research Institute of Sinopec, or it can be prepared according to common knowledge in the field as needed. The hydrocracking catalyst I includes a hydrocracking active metal, a molecular sieve component, and alumina. The hydrocracking active metal typically includes Group VIB and Group VIII metals in the periodic table, wherein the Group VIB metal is preferably tungsten and / or molybdenum, and the Group VIII metal is preferably nickel and / or cobalt. Based on the mass of the catalyst, the content of the Group VIB metal as oxide is 8–20 wt%, the content of the Group VIII metal as oxide is 4–10 wt%, the molecular sieve content is 10–80 wt%, and the alumina content is 10–50 wt%. The molecular sieve can be an acidic molecular sieve, such as a Y-type molecular sieve.

[0018] In the above method, the hydrocracking catalyst II mentioned in step (2) can be any type of hydrocracking catalyst already available in the prior art, as long as it can achieve the purpose of obtaining light aromatics by cleaving the alkylbenzene side chain in step (2). Various existing commercial catalysts can be selected, such as the FC series catalysts (e.g., FC-76) developed by the Dalian Petrochemical Research Institute of Sinopec, or it can be prepared according to common knowledge in the field as needed. The hydrocracking catalyst II includes a hydrocracking active metal, a molecular sieve component, and alumina. The hydrocracking active metal typically includes Group VIB and Group VIII metals from the periodic table, wherein the Group VIB metal is preferably tungsten and / or molybdenum, and the Group VIII metal is preferably nickel and / or cobalt. Based on the mass of the catalyst, the content of the Group VIB metal as oxide is 8–20 wt%, the content of the Group VIII metal as oxide is 4–10 wt%, the content of the molecular sieve is 5–50 wt%, and the content of alumina is 10–80 wt%. The molecular sieve can be an acidic molecular sieve, such as a Y-type molecular sieve.

[0019] The above method, in step (1), the operating conditions for the hydrogenation purification reaction are: the liquid hourly space velocity is generally 0.3 to 4 h⁻¹. -1 The pressure is generally 3.0 to 12.0 MPa, the inlet hydrogen-to-oil volume ratio is generally 300:1 to 3000:1, and the reaction temperature is generally 250 to 430℃.

[0020] The operating conditions for the hydrocracking reaction in step (2) of the above method are: space velocity is generally 0.5 to 4 h⁻¹. -1 The pressure is generally 4.0 to 13.0 MPa, the inlet hydrogen-to-oil volume ratio is generally 300:1 to 1000:1, and the reaction temperature is generally 350 to 430℃.

[0021] The separation system in step (3) of the above method includes gas-liquid separation, adsorption separation, distillation, and extraction of the second stream (and the recycled fifth stream). The second stream (and the recycled fifth stream) first undergoes gas-liquid separation to separate dry gas for release; the liquid phase is extracted to obtain aromatic and non-aromatic streams, and the aromatic stream is distilled to obtain light and heavy aromatic streams. The above gas-liquid separation, extraction, and distillation methods can all adopt methods commonly used in the prior art.

[0022] In the above method, the fourth stream of heavy aromatics in step (4) is recycled to the first reaction zone for further purification and subsequent cracking reaction in the second reaction zone to obtain monocyclic aromatics.

[0023] The above method, in step (4), the coupling reaction of CO2 with alkanes is specifically as follows: the non-aromatic third stream enters the third reaction zone and reacts under the CO2 atmosphere, so that the high carbon number alkane component of the catalytic diesel fuel undergoes a coupling reaction with CO2 to further generate aromatics, and a fifth stream rich in aromatics is obtained.

[0024] In the above method, the CO2-alkane coupling reaction in step (4) occurs in a fixed-bed reactor, and the shaped catalyst used includes a modified molecular sieve. The modified molecular sieve is a molecular sieve obtained by metal ion exchange. The molecular sieve is at least one of the molecular sieves with MFI, BEA, TON, MOR, MEL, and MWW configurations, preferably MFI molecular sieve, and more preferably ZSM-5 molecular sieve. The molecular sieve has a particle size of 400-600 nm, and the mesopore volume with a pore size of 2-8 nm accounts for more than 55% of the total pore volume, preferably 55%-80%, and the Si / Al molar ratio of the molecular sieve is 5-100, preferably 5-50. The molecular sieve can be prepared according to conventional methods in the art as needed, such as the method described in CN102464336B. The metal used for metal ion exchange is preferably selected from at least one of La, Zn, Ga, Fe, Mo, and Cr, and more preferably Zn. In the modified molecular sieve obtained by metal ion exchange, the mass content of the metal, calculated as an element, is 3%-15%. The modified molecular sieve content in the catalyst is 50%–90%, preferably 60%–80%. The catalyst may also contain binders and other components. This invention uses the aforementioned modified molecular sieve as a catalyst, which allows for the regulation of both the pore structure and the acidity of the catalyst's outer surface. This not only effectively improves the activity and selectivity of converting unconverted high-carbon-number alkanes into aromatic products, but also facilitates the diffusion of hydrocarbon molecules, ensuring that the generated aromatic products are retained in the product as much as possible, thus slowing down the catalyst deactivation rate. The carbon dioxide coupling conversion catalyst can be prepared according to the above description and common knowledge in the art, such as by kneading and extrusion molding.

[0025] In the above method, the operating conditions for the CO2-alkane coupling reaction in step (4) are: inlet oil volume hourly space velocity of 0.5–5 h⁻¹. -1 Preferably 1 to 3 hours -1 The partial pressure of carbon dioxide is 4.0–13.0 MPa, preferably 8–12 MPa, and the inlet carbon dioxide mass hourly space velocity is 0.5–5 h⁻¹. -1 Preferably 1 to 3 hours -1 The reaction temperature is 300–500℃, preferably 350–450℃.

[0026] In the above method, the fifth material flow described in step (5) is recycled to the separation system described in step (3) for separation.

[0027] In this invention, the light aromatic hydrocarbons refer to aromatic hydrocarbons with 10 or fewer carbon atoms, and the heavy aromatic hydrocarbons refer to aromatic hydrocarbons with more than 10 carbon atoms.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] (1) This invention couples hydroconversion and carbon dioxide conversion to process inferior catalytic diesel. In the hydroconversion stages of the first and second reaction zones, polycyclic aromatic hydrocarbons (PAHs) in the catalytic diesel are hydrogenated to saturation and cracked into monocyclic aromatic hydrocarbons (MOHs). The resulting stream, rich in light aromatic hydrocarbons, is separated by a separation system to obtain light aromatic hydrocarbon streams, heavy aromatic hydrocarbon streams, and non-aromatic hydrocarbon streams. The non-aromatic hydrocarbon streams are then coupled with carbon dioxide in the third reaction zone for conversion. The non-aromatic components, such as alkanes and cycloalkanes with a wide carbon number distribution in the catalytic diesel, can react with carbon dioxide and be further efficiently converted into aromatics through catalyst and reaction conditions. This aromatic hydrocarbon stream is then recycled to the separation system to separate the heavy aromatic hydrocarbon components and recycle them to the hydroconversion unit. Light aromatic hydrocarbon products are obtained through hydrogenation ring-opening and cracking, solving the problem of excessive heavy aromatic hydrocarbon products after the reaction of heavy alkanes with carbon dioxide. Overall, the entire process effectively utilizes the non-aromatic components of the catalytic diesel, maintaining a high PAH saturation rate while achieving a higher MOH yield.

[0030] (2) The method provided by the present invention deeply couples the catalytic diesel hydroconversion and the conversion of carbon dioxide and alkanes in the process flow. Based on the targeted treatment of raw materials and the improvement of product selectivity, the ideal processing effect is obtained. In addition, the carbon dioxide as raw material can be widely obtained from factory exhaust gas or direct air capture. While improving the selectivity of aromatics and reducing the yield of alkane, carbon dioxide is effectively utilized, which can effectively alleviate a series of problems caused by excessive carbon dioxide content in the atmosphere. Attached Figure Description

[0031] Figure 1 This is a flowchart illustrating the method of the present invention. Detailed Implementation

[0032] The method of the present invention will now be described in detail with reference to the accompanying drawings. Figure 1 This is only a schematic diagram of the process flow; some necessary equipment and containers have been omitted from the diagram.

[0033] like Figure 1As shown, the method for increasing aromatics production through catalytic diesel hydroconversion coupled with carbon dioxide provided by the present invention includes: catalytic diesel feedstock 1 enters a first reaction zone 2 for hydrorefining to obtain a first stream 3; this stream enters a second reaction zone 4 for hydrocracking to obtain a second stream 5; this stream enters a separation system 6 to obtain dry gas 7, light aromatics stream product 8, non-aromatics stream 9, and heavy aromatics stream 11. Heavy aromatics stream 11 is recycled back to the first reaction zone 2 for sequential hydrorefining and hydrocracking reactions. Non-aromatics stream 9 enters a third reaction zone 10 for CO2 coupled conversion to obtain aromatics-rich stream 12, which is recycled back to the separation system 6.

[0034] Specifically, the separation system 6 includes a gas-liquid separator, an aromatics extraction unit, and a distillation unit (not shown in the attached figures) connected in sequence.

[0035] Examples 1-5

[0036] Examples 1-5 all employ the process of this invention, such as... Figure 1 As shown. Catalytic diesel, after being heated in a heater, is mixed with recycled hydrogen and enters the hydrorefining reaction zone for hydrodesulfurization and selective hydrogenation saturation of polycyclic aromatic hydrocarbons. The resulting product oil, rich in cycloalkyl monocyclic aromatic hydrocarbons, is mixed with recycled hydrogen and enters the hydrocracking reaction zone. On hydrocracking catalyst I, ring-opening cracking of cycloalkyl aromatic hydrocarbons occurs to yield alkylbenzenes. Further, on hydrocracking catalyst II, side-chain cleavage of alkylbenzenes occurs to yield light aromatic hydrocarbons. The resulting product oil is separated by a separation system to obtain gas, non-aromatic components, heavy aromatic components, and light aromatic hydrocarbon products. The heavy aromatic components are recycled back to the hydrorefining reaction zone. The non-aromatic components are mixed with carbon dioxide and enter the carbon dioxide coupling conversion zone for a coupling reaction. The resulting product oil, rich in aromatic hydrocarbons, is recycled back to the separation system.

[0037] In the embodiments, the catalyst used in the hydrorefining reaction zone is the commercially available FF-36 hydrotreatment catalyst; the catalyst I used in the hydrocracking reaction zone is the commercially available FC-52 hydrocracking catalyst, and the catalyst II is the commercially available FC-76 hydrocracking catalyst; the catalyst used in the carbon dioxide coupling conversion zone is the modified molecular sieve catalyst A of this invention. The mesoporous molecular sieve is prepared according to the method described in CN102464336B, wherein the modified molecular sieve is a metal ion exchange molecular sieve. Catalyst A is prepared by conventional kneading and extrusion molding method, and the binder is small-pore alumina. The properties of the raw materials are shown in Table 1, the properties of the catalyst are shown in Table 2, and the operating conditions and main product properties are shown in Table 3.

[0038] Comparative Example 1

[0039] Comparative Example 1 used the same process and the same catalytic diesel as the Example. The catalyst used in the hydrorefining reaction zone was the commercially available FF-36 hydrotreating catalyst; the catalysts used in the hydrocracking reaction zone were catalyst I (commercially available FC-52 hydrocracking catalyst) and catalyst II (commercially available FC-76 hydrocracking catalyst); the catalyst used in the carbon dioxide coupling conversion zone was a conventional naphtha-carbon dioxide coupling conversion catalyst, namely modified molecular sieve catalyst B, using a metal ion exchange molecular sieve. Catalyst B was prepared using a conventional kneading extrusion molding method, with small-pore alumina as the binder. The properties of the raw materials are shown in Table 1, the properties of the catalyst are shown in Table 2, and the operating conditions and main product properties are shown in Table 3.

[0040] Comparative Example 2

[0041] Comparative Example 2 used the same catalytic diesel oil as the example for production. The process differed from the example; the non-aromatic components obtained after separation were discharged directly without entering the carbon dioxide coupled reactor, while the heavy aromatic components were recycled to the hydrorefining reaction zone. The catalyst used in the hydrorefining reaction zone of Comparative Example 2 was the commercially available FF-36 hydrotreating catalyst; the catalysts used in the hydrocracking reaction zone were catalyst I (commercially available FC-52 hydrocracking catalyst) and catalyst II (commercially available FC-76 hydrocracking catalyst). The properties of the feedstock are shown in Table 1, and the operating conditions and main product properties are shown in Table 3.

[0042] Table 1 Properties of Raw Materials

[0043]

[0044]

[0045] Table 2 Properties of carbon dioxide coupled conversion catalysts

[0046]

[0047] Table 3 Reaction conditions and product properties

[0048]

[0049]

Claims

1. A method for enhancing aromatic hydrocarbon production through catalytic diesel hydroconversion coupled with carbon dioxide, comprising the following steps: (1) Catalytic diesel feedstock enters the first reaction zone and reacts with the catalyst under hydrogen conditions to obtain the first stream; the first reaction zone undergoes a hydrorefining reaction. (2) The first stream obtained in step (1) enters the second reaction zone and reacts with the catalyst under hydrogen conditions to obtain the second stream; the second reaction zone carries out a hydrocracking reaction; (3) The second stream obtained in step (2) enters the separation system for separation to obtain dry gas, light aromatic hydrocarbon stream, non-aromatic hydrocarbon third stream and heavy aromatic hydrocarbon fourth stream. (4) The fourth stream obtained in step (3) is recycled to the first reaction zone; the third stream obtained in step (3) enters the third reaction zone and reacts under a CO2 atmosphere to obtain the fifth stream; wherein, the third reaction zone undergoes a coupling reaction between CO2 and alkanes; (5) The fifth material obtained in step (4) is recycled to the separation system for separation.

2. The method according to claim 1, characterized in that, The catalytic diesel fuel described in step (1) has an initial boiling point of 160–240°C, preferably 180–220°C; a final boiling point of 320–420°C, preferably 350–390°C; an aromatic content of 50 wt% or more; and a density of 0.91 g·cm³. -3 above.

3. The method according to claim 1, characterized in that, The hydrorefining reaction in step (1) specifically involves: catalytic diesel oil as feedstock is contacted with a hydrorefining catalyst under hydrogenation conditions to produce desulfurization and denitrification reactions. Preferably, the tricyclic and dicyclic aromatic hydrocarbons in the catalytic diesel oil are selectively hydrogenated to retain one aromatic ring to obtain cycloalkyl monocyclic aromatic hydrocarbons.

4. The method according to claim 1 or 3, characterized in that, The hydrorefining catalyst in step (1) includes a support and a supported hydrogenation metal, which includes Group VIB and Group VIII metals in the periodic table. The Group VIB metal is preferably tungsten and / or molybdenum, and the Group VIII metal is preferably nickel and / or cobalt. Preferably, based on the weight of the catalyst, the content of the Group VIB metal component as oxide is 10% to 35% and the content of the Group VIII metal as oxide is 1% to 7%.

5. The method according to claim 1, characterized in that, The hydrocracking reaction described in step (2) is as follows: the first stream obtained after hydrorefining of catalytic diesel is contacted sequentially with graded hydrocracking catalyst I and hydrocracking catalyst II under hydrogen conditions. Among them, the ring-opening cracking reaction of cycloalkyl aromatics occurs on hydrocracking catalyst I to obtain alkylbenzene, and the side chain cleavage reaction of alkylbenzene further occurs on hydrocracking catalyst II to obtain light aromatics.

6. The method according to claim 5, characterized in that, In step (2), the function of hydrocracking catalyst I is to perform a ring-opening cleavage reaction on cycloalkyl aromatics containing only one aromatic ring to obtain alkylbenzene under the premise of retaining the aromatic ring; the function of hydrocracking catalyst II is to perform a side chain cleavage reaction on alkylbenzene to obtain light aromatics with a lower number of carbon atoms.

7. The method according to claim 5 or 6, characterized in that, The hydrocracking catalyst I described in step (2) comprises a hydrocracking active metal, a molecular sieve component, and alumina. The hydrocracking active metal comprises Group VIB and Group VIII metals in the periodic table, wherein the Group VIB metal is preferably tungsten and / or molybdenum, and the Group VIII metal is preferably nickel and / or cobalt. Preferably, based on the mass of the catalyst, the content of the Group VIB metal as oxide is 8-20 wt%, the content of the Group VIII metal as oxide is 4-10 wt%, the content of the molecular sieve is 10-80 wt%, and the content of alumina is 10-50 wt%.

8. The method according to claim 5 or 6, characterized in that, The hydrocracking catalyst II described in step (2) comprises a hydrocracking active metal, a molecular sieve component, and alumina; the hydrocracking active metal comprises Group VIB and Group VIII metals in the periodic table, wherein the Group VIB metal is preferably tungsten and / or molybdenum, and the Group VIII metal is preferably nickel and / or cobalt; preferably, based on the mass of the catalyst, the content of the Group VIB metal as oxide is 8-20 wt%, the content of the Group VIII metal as oxide is 4-10 wt%, the content of the molecular sieve is 5-50 wt%, and the content of alumina is 10-80 wt%.

9. The method according to claim 1, characterized in that, The operating conditions for the hydrogenation purification reaction in step (1) are: liquid hourly space velocity (LISH) of 0.3–4 h⁻¹. -1 The pressure is 3.0–12.0 MPa, the inlet hydrogen-to-oil volume ratio is 300:1–3000:1, and the reaction temperature is 250–430℃.

10. The method according to claim 1, characterized in that, The operating conditions for the hydrocracking reaction in step (2) are: liquid hourly space velocity (LHSV) of 0.5–4 h⁻¹. -1 The pressure is 4.0–13.0 MPa, the inlet hydrogen-to-oil volume ratio is 300:1–1000:1, and the reaction temperature is 350–430℃.

11. The method according to claim 1, characterized in that, The separation system in step (3) includes first separating the second stream and the fifth stream from the recycling process by gas-liquid separation to separate dry gas; the liquid phase is extracted by aromatics to obtain aromatic and non-aromatic streams, and the aromatic stream is distilled to obtain light aromatic streams and heavy aromatic streams.

12. The method according to claim 1, characterized in that, The coupling reaction of CO2 and alkanes in step (4) is as follows: the non-aromatic third stream enters the third reaction zone and reacts under the CO2 atmosphere, so that the high carbon number alkane component of the catalytic diesel fuel undergoes a coupling reaction with CO2 to further generate aromatics, thus obtaining the aromatic-rich fifth stream.

13. The method according to claim 12, characterized in that, The CO2-alkane coupling reaction described in step (4) occurs in a fixed-bed reactor, and the shaped catalyst used includes modified molecular sieves. The molecular sieve is at least one of the molecular sieves with MFI, BEA, TON, MOR, MEL, and MWW configurations, preferably an MFI molecular sieve, and more preferably a ZSM-5 molecular sieve; preferably, the content of the modified molecular sieve in the catalyst is 50% to 90%, more preferably 60% to 80%.

14. The method according to claim 13, characterized in that, The modified molecular sieve is a modified molecular sieve obtained by metal ion exchange; the molecular sieve has a particle size of 400-600 nm, and the mesopore volume with a pore size of 2-8 nm accounts for more than 55% of the total pore volume, preferably 55%-80%, and the Si / Al molar ratio is 5-100, preferably 5-50; and / or, the metal used for metal ion exchange is preferably selected from at least one of La, Zn, Ga, Fe, Mo, and Cr, more preferably Zn; and / or, the mass content of the metal in the modified molecular sieve obtained by metal ion exchange is 3%-15% by element.

15. The method according to claim 1 or 12, characterized in that, The operating conditions for the CO2-alkane coupling reaction in step (4) are: inlet oil volume hourly space velocity of 0.5–5 h⁻¹. -1 Preferably 1 to 3 hours -1 The partial pressure of carbon dioxide is 4.0–13.0 MPa, preferably 8–12 MPa, and the inlet carbon dioxide mass hourly space velocity is 0.5–5 h⁻¹. -1 Preferably 1 to 3 hours -1 The reaction temperature is 300–500℃, preferably 350–450℃.

16. The method according to claim 1, characterized in that, In this invention, the light aromatic hydrocarbons refer to aromatic hydrocarbons with 10 or fewer carbon atoms, and the heavy aromatic hydrocarbons refer to aromatic hydrocarbons with more than 10 carbon atoms.

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