CN118207018A - A wax oil hydrocracking method - Google Patents

Abstract

The invention discloses a wax oil hydrocracking method. Wax oil raw material and hydrogen enter a hydrofining reaction zone; the hydrofining reaction effluent enters a cracking reaction zone and contacts with a hydrocracking catalyst containing a β/ZSM-5 composite molecular sieve; the hydrocracking reaction effluent is separated to obtain a naphtha fraction; the obtained naphtha is passed through a normal isomerization adsorption separation device to obtain components rich in normal alkanes and rich in isoalkanes. The present invention is particularly suitable for producing high-octane gasoline components by hydrocracking of wax oil, which can significantly increase the isoalkanes content and octane number in the gasoline component, thereby achieving the purpose of reducing the diesel-gasoline ratio and increasing the production of ethylene cracking raw materials.

Description

A wax oil hydrocracking method

Technical Field

The invention relates to a wax oil hydrocracking method, which is particularly suitable for the process of producing high-octane gasoline blending components by wax oil hydrocracking, and can achieve the purpose of reducing the diesel-gasoline ratio in refineries and increasing the production of ethylene cracking raw materials.

Background technique

Hydrocracking technology is a clean fuel oil and chemical raw material production technology. It can process low-quality raw materials such as vacuum wax oil, catalytic diesel, coker wax oil, coker diesel, fluidized bed wax oil, and fluidized bed diesel to produce light naphtha, heavy naphtha, jet fuel, diesel, and high-quality tail oil. In recent years, the diesel consumption market has been weak, the diesel-to-gasoline ratio has continued to decline, and the demand for ethylene cracking raw materials has increased year by year. Therefore, the current product structure adjustment direction of my country's refineries is to transform the production of traditional low-value-added fuel oil products into the production of high-value-added special oil products or chemical raw materials. Hydrocracking technology can realize the oil transformation of oil refining enterprises and play a pivotal role in the adjustment of the product structure of the whole plant. The naphtha produced by traditional hydrocracking has a low octane number and cannot be directly used as a gasoline blending component. It needs to be catalytically reformed to produce aromatics or gasoline blending components. The process flow of this method is complicated and will cause the aromatic content in gasoline to exceed the standard.

Since the implementation of the National VI gasoline standard, the requirements for the aromatic content in gasoline have become increasingly stringent, so the content of isoparaffins needs to be increased in the gasoline blending components. If the zeolite composition and type of the hydrocracking catalyst can be changed to achieve efficient conversion of paraffin molecules to isoparaffins and retain them in naphtha, the octane number of naphtha can be significantly improved. Only by developing a molecular sieve with paraffin cracking ability can the above requirements be met.

There are many reports on wax oil hydrocracking methods, but there are few reports on the production of high-octane gasoline by wax oil hydrocracking units. CN110938466A discloses a wax oil hydrocracking method, in which the wax oil raw material is mixed with hydrogen and first enters a hydrorefining reactor for desulfurization, denitrogenation and aromatic saturation reaction; the refined reaction effluent enters the hydrocracking reactor, and the upper bed of the hydrocracking reactor is filled with a hydrocracking catalyst containing a modified Y molecular sieve along the material direction, and the lower catalyst bed is filled with a hydroisomerization catalyst containing a molecular sieve with strong isomerization performance such as β and/or ZSM series; finally, naphtha, jet fuel, diesel and tail oil are obtained. This method can achieve the increase of heavy naphtha with high aromatic potential and improve the quality of jet fuel and diesel products by setting a reasonable catalyst grading method and adjusting the catalyst composition and structure in the graded bed.

CN107345156A discloses a hydrocracking method, wherein the hydrocracking catalyst used in the method uses a modified Y-type molecular sieve, amorphous silicon aluminum and aluminum oxide as carriers, and the wax oil raw material is contacted with the hydrocracking catalyst for hydrocracking reaction to obtain heavy naphtha, diesel, jet fuel and hydrogenated tail oil and other products. The modified Y-type molecular sieve is rich in mesopores, suitable for processing high-drying point wax oil, and can effectively produce fuel oil and chemical raw materials.

Summary of the invention

The technical problem to be solved by the present invention is how to selectively isomerize and crack the paraffins in wax oil into naphtha, and after normal isomerization separation, produce high-quality gasoline blending components and ethylene cracking raw materials.

Among the hydrocracking catalysts, different types of molecular sieves have different cracking selectivity for hydrocarbon molecules. Among the many molecular sieves, ZSM-5 molecular sieve has good cracking selectivity for paraffins and a high secondary cracking ratio; β molecular sieve has strong isomerization ability for paraffins. Therefore, in order to efficiently and selectively crack paraffins in wax oil into naphtha, a single molecular sieve component cannot meet the requirements. If ZSM-5 molecular sieve and β molecular sieve can be combined as cracking active components, the paraffin components in wax oil can be cracked into naphtha to the maximum extent, and most of the paraffins in naphtha can be guaranteed to be isoparaffins, and the octane number of naphtha can also be significantly improved. After the adsorption and separation of normal isoparaffins in naphtha, not only can high-octane gasoline rich in isoparaffins be produced, but also high-quality ethylene cracking raw materials can be produced.

In view of the shortcomings of the prior art, the present invention provides a method for hydrocracking of wax oil, using a hydrocracking catalyst containing a β/ZSM-5 composite molecular sieve. The paraffin components in the wax oil first contact with the β molecular sieve to undergo an isomerization reaction, and then contact the internal ZSM-5 molecular sieve to undergo a deep cracking reaction to generate a naphtha component rich in isomerized paraffins. This method can not only reduce the cracking reaction temperature, but also increase the isomerized paraffin content in the naphtha.

A wax oil hydrocracking method of the present invention comprises the following steps:

(1) The process of wax oil hydrocracking includes hydrofining and hydrocracking reaction functional zones. After the wax oil feedstock is mixed with hydrogen, it enters the hydrofining reaction zone and contacts with the hydrofining catalyst to undergo a hydrofining reaction.

(2) The effluent from the hydrotreating reaction enters the hydrocracking reaction zone and contacts the hydrocracking catalyst containing the β/ZSM-5 composite molecular sieve to undergo a hydrocracking reaction;

(3) The effluent from the hydrocracking reaction is distilled through a fractionation system to obtain dry gas, liquefied gas, naphtha, diesel and tail oil components; the initial distillation point of naphtha is 25~80℃, and the dry point is 155~205℃;

(4) The naphtha obtained in step (3) is subjected to a normal-isomerization adsorption separation unit to obtain components rich in normal alkanes and components rich in isoalkanes.

In the present invention, the naphtha fraction rich in normal alkanes can be directly used as a raw material for steam cracking to produce ethylene, while the component rich in isoalkanes can be used as a high-quality National VI gasoline blending component.

In the present invention, the hydrofining reaction zone usually includes 2 to 5 catalyst beds, each of which is filled with a protective agent and a hydrofining catalyst. The hydrocracking reaction zone usually includes 2 to 5 catalyst beds, each of which is filled with a hydrocracking catalyst containing a β/ZSM-5 composite molecular sieve.

In the present invention, the hydrotreating reaction zone and the hydrocracking reaction zone in step (1) may be one reactor or two or more reactors.

In the present invention, the protective agent can be prepared according to the existing patented method, or an industrial catalyst can be used, such as the FZC and FBN series protective agents developed by Dalian Petrochemical Research Institute.

In the present invention, the hydrotreating catalyst in step (1) can be prepared according to the existing patented method, or an industrial catalyst can be used, such as FF-36, FF-46, FF-76 and FF-86 developed by Dalian Petrochemical Research Institute.

In the present invention, the hydrocracking catalyst described in step (2) includes a carrier, an active metal and a binder. The hydrocracking catalyst has an H-type β/ZSM-5 composite molecular sieve as an acidic component, and the carrier is a mixture of alumina and β/ZSM-5 composite molecular sieve. The mass fraction of the H-type β/ZSM-5 composite molecular sieve in the carrier is 10% to 50%. The binder is usually alumina or silicon oxide. The active metal component is a metal of Group VI, Group VII or Group VIII, a metal oxide or a metal sulfide, more preferably one or more of iron, chromium, molybdenum, tungsten, cobalt, nickel, or their sulfides or oxides. Based on the weight of the catalyst, the active component content is generally 10 to 45wt%.

In the present invention, the specific surface area of the β/ZSM-5 composite molecular sieve is 300-600 m ² /g, the pore size is 7.0-12.0 nm, the pore volume is 0.1-0.5 cm ³ /g, and the infrared acid content is 0.5-1.0 mmol/g. The specific surface area of the catalyst is 200-500 m ² /g, the pore size is 4.0-10.0 nm, the pore volume is 0.06-0.25 cm ³ /g, and the infrared acid content is 0.4-0.8 mmol/g. In the β/ZSM-5 composite molecular sieve, the mass fraction of the ZSM-5 molecular sieve is 20%-50%.

In the present invention, the morphology structure of the composite molecular sieve β/ZSM-5 is a core-shell structure, with the ZSM-5 molecular sieve as the core and the β molecular sieve as the shell.

In the present invention, the composite molecular sieve β/ZSM-5 can be prepared by conventional methods in the art. The present invention provides a typical preparation method of the composite molecular sieve. The specific method comprises the following steps:

(a) First, synthesize ZSM-5 seed crystals: use tetraethyl orthosilicate as silicon source, tetrapropylammonium hydroxide (TPAOH) as template, add water as dissolving medium, the mass ratio of the three is TEOS:H ₂ O:TPAOH=1:1~4:1~6, and record the mixed solution I after the three are fully dissolved; use NaAlO ₂ as aluminum source, use NaOH aqueous solution as reaction medium, NaAlO ₂ :H ₂ O:NaOH=1:1~12:0.5~3 (mass ratio), stir the prepared solution II at 15~45℃ for 1~8 h; add solution I into solution II and stir for 1~8 h, then transfer the mixed solution to a crystallization kettle, crystallize at 110~160℃ for 12~100 h, and obtain nano-scale ZSM-5 molecular sieve;

(b) Secondly, prepare Beta emulsion: use tetraethyl orthosilicate as silicon source, tetrapropylammonium hydroxide (TPAOH) as template, add water as dissolving medium, the mass ratio of the three is TEOS:H ₂ O:TPAOH= 1:1~4:1~6, after the three are fully dissolved, record it as mixed solution III; use NaAlO ₂ as aluminum source, use NaOH aqueous solution as reaction medium, NaAlO ₂ :H ₂ O:NaOH=1:1~15:0.5~4 (mass ratio), the prepared solution IV is stirred at 15~45℃ for 2~8 h; add solution III and ZSM-5 molecular sieve seed to solution IV and stir for 2~8 h, then transfer the mixed solution to a crystallization kettle and crystallize at 110~160℃ for 12~100 h; the solution in the crystallization kettle is filtered, washed with water and dried in an oven at 60~100℃ for 6~14 h; the dried solid is calcined at 400-700°C for 4-12 h to obtain a Na-type β/ZSM-5 composite molecular sieve;

(c) Using ammonium chloride aqueous solution as the ammonium exchange medium at an exchange temperature of 70-95°C, the Na-type β/ZSM-5 composite molecular sieve prepared above is subjected to two ammonium exchanges and calcined at 300-500°C for 2-6 h to obtain an H-type β/ZSM-5 composite molecular sieve.

In the present invention, the silicon-aluminum ratio of the feed in step (a) is controlled to be 15-30.

In the present invention, the silicon-to-aluminum ratio of the feed in step (b) is controlled to be 30-80.

In the present invention, the diesel and tail oil components described in step (3) can be extracted as products, and can also be circulated to the hydrofining reaction zone or the hydrocracking reaction zone for deep cracking to produce the naphtha component in the maximum amount.

In the present invention, the naphtha isomerization separation device described in step (4) includes an adsorption unit, a purge unit and a desorption unit. The typical operating conditions of the adsorption unit are: pressure 0.1-1.8 MPa, temperature 140-360 °C, liquid phase volume space velocity 0.4-5 h ^-1 , adsorption time 10-30 min. The typical operating conditions of the purge unit are: purge pressure 0.1-1.8 MPa, temperature 150-360 °C, volume space velocity 40-140 h ^-1 , purge time 5-20 min. The typical operating conditions of the desorption unit are: pressure 0.3-1.8 MPa, temperature 150-360 °C, volume space velocity 40-140 h ^-1 , desorption time 5-20 min.

In the present invention, the reaction conditions in the hydrotreating reaction zone and the hydrocracking reaction zone are generally as follows: reaction pressure is 5.0-17.0 MPa, preferably 12.0-15.0 MPa; average reaction temperature is 290-440°C, preferably 310-410°C; hydrotreating volume space velocity is 0.2-4.0 h ^-1 , preferably 0.5-2.0 h ^-1 ; hydrocracking volume space velocity is 0.6-5.0 h ^-1 , preferably 1.0-2.0 h ^-1 .

The wax oil hydrocracking method of the present invention can be used in any hydrocracking field. The method of the present invention is particularly suitable for producing high-octane gasoline components by hydrocracking of wax oil, which can significantly increase the isoparaffin content and octane number in the gasoline components, thereby achieving the purpose of reducing the diesel-gasoline ratio and increasing the production of ethylene cracking raw materials.

Compared with the prior art, the method of the present invention has the following beneficial effects:

1. The composite molecular sieve β/ZSM-5 with strong paraffin isomerization and cracking ability is selected as the cracking agent carrier, which can convert the paraffin components in the wax oil into naphtha to the maximum extent. The naphtha produced by this method has a high isoparaffin content and a high octane number.

2. Select a catalyst containing β/ZSM-5 core-shell molecular sieve as a hydrocracking catalyst. By selectively separating the normal and isomerized hydrocarbon molecules in naphtha, adhering to the concept of molecular refining, the normal alkanes produced after adsorption separation can be used as high-quality steam cracking to produce ethylene raw materials, thereby realizing refining transformation.

3. In the present invention, wax oil is used as raw material. After adopting the process, the yield of gasoline and ethylene cracking raw materials is significantly improved, and the yield of diesel is reduced, so the purpose of reducing the diesel-gasoline ratio can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the process flow of the method of the present invention.

FIG. 2 is a SEM photograph of the β/ZSM-5 composite molecular sieve used in the example.

Detailed ways

The wax oil hydrocracking-adsorption separation combined process provided by the present invention is further described below in conjunction with the accompanying drawings. Many equipments are omitted in the drawings, such as pumps, heat exchangers, heating furnaces, air cooling and stripping towers, but they are well known to ordinary technicians in this field.

As shown in FIG1 , the process flow of the hydrocracking-adsorption separation combined device of the present invention is described in detail as follows: the raw oil 1 is mixed with the mixed hydrogen 2 and enters the stage refining reactor 3, and desulfurization, denitrogenation and dearomatization reactions occur through the hydrorefining catalyst bed, and the refined oil 4 enters the hydrocracking reactor 5. The oil 6 generated by the hydrocracking reactor enters the separator 7, the gas discharged from the upper part passes through the desulfurization tower 8, and the obtained circulating hydrogen is pressurized by the circulating hydrogen compressor 9 and mixed with the new hydrogen 10, and the mixed hydrogen 2 enters the hydrorefining reactor. The liquid phase 11 separated at the bottom of the separator 7 enters the fractionation tower 12 to obtain the fractionation tower top gas 13, naphtha 14 and the larger than diesel component 15. The larger than diesel component 15 can be extracted as a product, or it can be recycled to the refining or cracking reactor inlet. 14 Naphtha enters the normal isomerization adsorption separation unit, and after adsorption, purging and desorption engineering, the absorption oil and the desorption oil are obtained. The absorption oil is rich in isoparaffins, and the desorption oil is rich in normal paraffins.

The wax oil hydrocracking method provided by the present invention will be further described below in conjunction with the embodiments, but the present invention is not limited thereto.

Table 1. Raw oil properties.

Table 2 Industrial catalysts.

Table 3 Evaluation conditions.

The raw oil used in the following examples and comparative examples is wax oil, and its properties are shown in Table 1. The first stage refining reactor is filled with FF-76 hydrofining catalyst, and the hydrocracking reactor is filled with hydrocracking catalyst. The distillation range of naphtha in the examples and comparative examples is 25~175℃.

The synthesis method of the β/ZSM-5 composite molecular sieve used in the examples is as follows:

(1) First, a hydrocracking catalyst with a β/ZSM-5 composite molecular sieve as the acid center is prepared. The specific preparation method is as follows:

(a) First, synthesize ZSM-5 seed crystals, and the feed silicon-aluminum ratio is controlled to be 20: tetraethyl orthosilicate is used as the silicon source, tetrapropylammonium hydroxide (TPAOH) is used as the template, and a certain amount of water is added as the dissolving medium. The mass ratio of the three is TEOS: H ₂ O:TPAOH= 1:3:4. After the three are fully dissolved, it is recorded as mixed solution I; NaAlO ₂ is used as the aluminum source, and the aqueous solution of NaOH is used as the reaction medium. NaAlO ₂ :H ₂ O:NaOH=1:6:2 (mass ratio), and the prepared solution II is stirred at 35°C for 4 hours. Solution I is added to solution II and stirred for 4 hours, and then the mixed solution is transferred to a crystallization kettle and crystallized at 140°C for 72 hours to obtain a nano-scale ZSM-5 molecular sieve.

(b) Secondly, prepare Beta emulsion, the feed silicon aluminum ratio is 40: tetraethyl orthosilicate is used as silicon source, tetrapropylammonium hydroxide (TPAOH) is used as template, a certain amount of water is added as dissolving medium, the mass ratio of the three is TEOS: H ₂ O:TPAOH = 1:2:3, and the three are fully dissolved and recorded as mixed solution III; NaAlO ₂ is used as aluminum source, and NaOH aqueous solution is used as reaction medium, NaAlO ₂ :H ₂ O:NaOH=1:5:2 (mass ratio), and the prepared solution IV is stirred at 35℃ for 5 h; solution III and ZSM-5 molecular sieve seed are added to solution IV and stirred for 5 h, and then the mixed solution is transferred to a crystallization kettle and crystallized at 140℃ for 72 h. The solution in the crystallization kettle is filtered, washed with water, and then placed in an oven at 80℃ for 8 h; the dried solid is transferred to a muffle furnace and calcined at 500℃ for 8 h to obtain Na-type β/ZSM-5 composite molecular sieve.

(c) Using an aqueous solution of ammonium chloride as the ammonium exchange medium at an exchange temperature of 85°C, the Na-type β/ZSM-5 composite molecular sieve prepared above was subjected to two ammonium exchanges and calcined at 400°C for 4 h to finally obtain an H-type β/ZSM-5 composite molecular sieve.

The specific surface area of the synthesized H-type β/ZSM-5 composite molecular sieve is 450 ^m2 /g, the average pore size is 9.5 nm, the pore volume is 0.35 ^cm3 /g, and the infrared acid content is 0.72mmol/g. The SEM image of the synthesized β/ZSM-5 composite molecular sieve is shown in Figure 2. The outer surface of the molecular sieve particles is attached with disordered β molecular sieves, while the inside is a typical columnar ZSM-5 molecular sieve.

Example 1

The H-type β/ZSM-5 composite molecular sieve synthesized by the above method and alumina are used as carriers, the mass fraction of the H-type β/ZSM-5 composite molecular sieve in the carrier is 10%, and the active metals Ni and W are loaded, and the active metal content is 25wt%. After calcination at 500°C, NiW/β/ZSM-5-Al ₂ O ₃ hydrocracking catalyst is obtained. The prepared hydrocracking catalyst is loaded into a hydrocracking reactor, and the industrial hydrorefining catalyst FF-76 is loaded into the hydrorefining reactor. Using wax oil as raw material, the process conditions in Table 3 are evaluated to obtain naphtha and diesel products. After the naphtha product passes through the normal isomerization separation device, the properties of all products are analyzed.

Example 2

The H-type β/ZSM-5 composite molecular sieve synthesized by the above method and alumina are used as carriers, the mass fraction of the H-type β/ZSM-5 composite molecular sieve in the carrier is 15%, and the active metals Ni and W are loaded, and the active metal content is 22wt%. After calcination at 500°C, NiW/β/ZSM-5-Al ₂ O ₃ hydrocracking catalyst is obtained. The hydrocracking catalyst prepared above is loaded into a hydrocracking reactor, and the industrial hydrorefining catalyst FF-76 is loaded into the hydrorefining reactor. Using wax oil as raw material, the process conditions in Table 3 are evaluated to obtain naphtha and diesel products. After the naphtha product passes through the normal isomerization separation device, the properties of all products are analyzed.

Example 3

The H-type β/ZSM-5 composite molecular sieve synthesized by the above method and alumina are used as carriers, the mass fraction of the H-type β/ZSM-5 composite molecular sieve in the carrier is 20%, and the active metals Ni and W are loaded, and the active metal content is 30wt%. After calcination at 500°C, NiW/β/ZSM-5-Al ₂ O ₃ hydrocracking catalyst is obtained. The hydrocracking catalyst prepared above is loaded into a hydrocracking reactor, and the industrial hydrorefining catalyst FF-76 is loaded into the hydrorefining reactor. Using wax oil as raw material, the process conditions in Table 3 are evaluated to obtain naphtha and diesel products. After the naphtha product passes through the normal isomerization separation device, the properties of all products are analyzed.

Example 4

The H-type β/ZSM-5 composite molecular sieve synthesized by the above method and alumina are used as carriers, the mass fraction of the H-type β/ZSM-5 composite molecular sieve in the carrier is 25%, and the active metals Ni and W are loaded, and the active metal content is 35 wt%. After calcination at 500°C, NiW/β/ZSM-5-Al ₂ O ₃ hydrocracking catalyst is obtained. The hydrocracking catalyst prepared above is loaded into the hydrocracking reactor, and the industrial hydrorefining catalyst FF-76 is loaded into the hydrorefining reactor. Using wax oil as raw material, the process conditions in Table 3 are evaluated to obtain naphtha and diesel products. After the naphtha product passes through the normal isomerization separation device, the properties of all products are analyzed.

Comparative Example 1

The industrial hydrocracking catalyst FC-52 was loaded into the hydrocracking reactor, and the industrial hydrofining catalyst FF-76 was loaded into the hydrofining reactor. With wax oil as the raw material, the process conditions in Table 3 were evaluated to obtain naphtha and diesel products. After the naphtha product passed through the normal isomerization separation device, the properties of all products were analyzed.

Comparative Example 2

The industrial hydrocracking catalyst FC-76 was loaded into the hydrocracking reactor, and the industrial hydrofining catalyst FF-76 was loaded into the hydrofining reactor. With wax oil as the raw material, the process conditions in Table 3 were evaluated to obtain naphtha and diesel products. After the naphtha product passed through the normal isomerization separation device, the properties of all products were analyzed.

Comparative Example 3

The industrial hydrocracking catalyst FC-52/FC-76 was loaded into the hydrocracking reactor at a grading ratio of 1:1, and the industrial hydrofining catalyst FF-76 was loaded into the hydrofining reactor. With wax oil as the raw material, the process conditions in Table 3 were evaluated to obtain naphtha and diesel products. After the naphtha product passed through the normal isomerization separation device, the properties of all products were analyzed.

Table 4 Experimental results of the examples.

Table 5 Comparative test results.

It can be seen from the experimental results of the comparative examples and the embodiments that the β/ZSM-5 composite molecular sieve of the present invention is used as a carrier. Under the same conversion rate, the naphtha produced by the hydrocracking of wax oil in the embodiment has a high octane number. After normal isomerization separation, the octane number of the residual oil rich in isomer components can reach above 96, which is a high-quality National VI gasoline blending component. When the catalyst in Example 3 is used, the naphtha produced by the hydrocracking of wax oil has a maximum octane number of 97.4 after normal isomerization separation.

Claims (10)

1. A wax oil hydrocracking method, comprising the steps of:

(1) The wax oil hydrocracking process comprises a hydrofining and hydrocracking reaction functional area; wax oil raw materials and hydrogen are mixed and then enter a hydrofining reaction zone, and contact with a hydrofining catalyst to carry out hydrofining impurity removal reaction;

(2) The effluent of the hydrofining reaction enters a cracking reaction zone and contacts with a hydrocracking catalyst which is filled in the hydrocracking reaction zone and contains beta/ZSM-5 composite molecular sieve;

(3) The hydrocracking reaction effluent passes through a separation system to obtain dry gas, liquefied gas, naphtha, diesel oil and tail oil components; wherein the initial boiling point of naphtha is 25-80 ℃ and the dry point is 155-205 ℃;

(4) And (3) passing the naphtha obtained in the step (3) through an n-isomerism adsorption separation device to obtain components rich in n-alkanes and rich in isoparaffins.

2. The wax oil hydrocracking method as claimed in claim 1, wherein the beta/ZSM-5 composite molecular sieve has a core-shell structure, the ZSM-5 molecular sieve has a core, and the beta molecular sieve has a shell; the specific surface area of the composite molecular sieve is 300-600 m ²/g, the pore diameter is 7.0-12.0 nm, the pore volume is 0.1-0.5 cm ³/g, and the infrared acid amount is 0.5-1.0 mmol/g.

3. The wax oil hydrocracking method as claimed in claim 2, wherein the mass fraction of the ZSM-5 molecular sieve in the beta/ZSM-5 composite molecular sieve is 20% -50%.

4. A wax oil hydrocracking method according to claim 2 or 3, wherein the hydrocracking catalyst takes an H-type beta/ZSM-5 composite molecular sieve as an acidic component, and the carrier is a mixture of alumina and the beta/ZSM-5 composite molecular sieve; the mass fraction of the beta/ZSM-5 composite molecular sieve in the carrier is 10% -50%.

5. The wax oil hydrocracking process as claimed in claim 1, wherein the naphtha fraction rich in normal paraffins is used as a raw material for producing ethylene by steam cracking, and the isoparaffin-rich component is used as a blending component for high-quality national sixth gasoline.

6. The wax oil hydrocracking process as claimed in claim 1, wherein the diesel and tail oil components in step (3) are withdrawn as products or recycled to the hydrofining reaction zone or the hydrocracking reaction zone for deep cracking.

7. The wax oil hydrocracking process as claimed in claim 1, wherein the reaction conditions of the hydrofining reaction zone and the hydrocracking reaction zone are: the reaction pressure is 5.0-17.0 MPa, preferably 12.0-15.0 MPa; the average reaction temperature is 290-440 ℃, preferably 310-410 ℃; the volume space velocity of hydrofining is 0.2-4.0 h ^-1, preferably 0.5-2.0 h ^-1; the hydrocracking volume space velocity is 0.6 to 5.0 h ^-1, preferably 1.0 to 2.0 h ^-1.

8. The wax oil hydrocracking process according to claim 1, wherein the naphtha normal isomerism separation unit in step (4) comprises an adsorption unit, a purge unit and a desorption unit; the operating conditions of the adsorption unit are as follows: the pressure is 0.1-1.8 MPa, the temperature is 140-360 ℃, the liquid phase volume airspeed is 0.4-5 h ^-1, and the adsorption time is 10-30 min; the operating conditions of the purge unit are: the purging pressure is 0.1-1.8 MPa, the temperature is 150-360 ℃, the volume airspeed is 40-140 h ^-1, and the purging time is 5-20 min; the operation conditions of the desorption unit are as follows: the pressure is 0.3-1.8 MPa, the temperature is 150-360 ℃, the volume airspeed is 40-140 h ^-1, and the desorption time is 5-20 min.

9. The wax oil hydrocracking process as claimed in claim 1, wherein the preparation process of the beta/ZSM-5 composite molecular sieve comprises the steps of:

(a) First, ZSM-5 seed crystals were synthesized: tetraethyl orthosilicate is used as a silicon source, tetrapropylammonium hydroxide is used as a template agent, water is added as a dissolution medium, the mass ratio of TEOS to H ₂ O to TPAOH=1:1-4:1-6, and the mixture is recorded as a mixed solution I after the TEOS, the TPAOH=1:1-4:1-6 are fully dissolved; naAlO ₂ is used as an aluminum source, naOH aqueous solution is used as a reaction medium, the mass ratio of NaAlO ₂:H₂ O to NaOH is 1:1-12:0.5-3, and the prepared solution II is stirred for 1-8 h at 15-45 ℃; adding the solution I into the solution II, stirring for 1-8 hours, transferring the mixed solution into a crystallization kettle, and crystallizing at 110-160 ℃ for 12-100 hours to obtain a nanoscale ZSM-5 molecular sieve;

(b) Tetraethyl orthosilicate is used as a silicon source, tetrapropylammonium hydroxide is used as a template agent, water is added as a dissolution medium, the mass ratio of TEOS to H ₂ O to TPAOH=1:1-4:1-6, and the mixture solution III is recorded after the TEOS, the TPAOH=1:1-4:1-6 are fully dissolved; naAlO ₂ is used as an aluminum source, naOH aqueous solution is used as a reaction medium, the mass ratio of NaAlO ₂:H₂ O to NaOH is 1:1-15:0.5-4, and the prepared solution IV is stirred for 2-8 h at 15-45 ℃; adding the solution III and ZSM-5 seed crystal into the solution IV, stirring for 2-8 hours, transferring the mixed solution into a crystallization kettle, and crystallizing for 12-100 hours at the temperature of 110-160 ℃; filtering and washing the solution in the crystallization kettle, and drying; roasting the dried solid at 400-700 ℃ for 4-12 hours to obtain the Na-type beta/ZSM-5 composite molecular sieve;

(c) And (3) taking an ammonium chloride aqueous solution as an ammonium exchange medium, wherein the exchange temperature is 70-95 ℃, and carrying out ammonium exchange twice on the Na-type beta/ZSM-5 composite molecular sieve prepared by the method and roasting the Na-type beta/ZSM-5 composite molecular sieve at 300-500 ℃ for 2-6 hours to obtain the H-type beta/ZSM-5 composite molecular sieve.

10. The wax oil hydrocracking method as claimed in claim 9, wherein the step (a) controls the feed silicon to aluminum ratio to be 15-30, and the step (b) controls the feed silicon to aluminum ratio to be 30-80.