Composite solvent and method for separating styrene from hydrocarbon mixture

Abstract

A composite solvent for separating styrene from a hydrocarbon mixture comprises 35-80% by mass of a main solvent and 20-65% by mass of a cosolvent, the main solvent is sulfolane, and the cosolvent is 1, 3-dimethyl-2-imidazolinone and / or gamma-butyrolactone. The composite solvent is used as an extraction agent to extract, rectify and separate styrene from a hydrocarbon mixture, the operation temperature of an extraction rectifying tower kettle can be effectively reduced, styrene polymerization is reduced, energy consumption is reduced, and the yield and purity of styrene are high.

Description

Technical Field

[0001] The present invention relates to a composite solvent and a method for separating styrene from a hydrocarbon mixture, in particular to a composite solvent and a method for recovering styrene from a hydrocarbon mixture by extraction and rectification. Background Art

[0002] Extractive distillation is a well-known technology that separates the raw materials by adding a third component (called an extractant or solvent) to the raw material solution to change the relative volatility of the original components. The boiling point of the extractant is generally much higher than the boiling points of the components in the raw material solution, and it does not form a constant boiling liquid with the components. Extractive distillation is often used to separate solutions with very small differences in boiling points (volatility) of the components. Ordinary distillation can hardly separate such a system, but the relative volatility can be changed under the action of the solvent to achieve separation.

[0003] Styrene monomer is an important basic organic chemical raw material, and is also widely used in coatings, pesticides, medicines, etc. According to statistics, the pyrolysis gasoline produced as a by-product of the ethylene cracking unit contains 3-5% by mass of styrene. For a million-ton ethylene unit, the styrene content in the pyrolysis gasoline is about 20,000-30,000 tons / year. In the existing process of processing pyrolysis gasoline, this part of styrene is hydrogenated into ethylbenzene and exists in xylene. This ethylbenzene-rich hydrocarbon mixture is not ideal for supplying downstream paraxylene units as raw materials. If super-distillation is used to separate ethylbenzene and then dehydrogenated into styrene, the process is long, energy consumption is high, and it is also unreasonable.

[0004] The most difficult to separate from styrene in pyrolysis gasoline is o-xylene, the boiling point difference between the two is only 0.8℃, and the relative volatility is only 1.04. Ordinary distillation requires thousands of theoretical plates. It is a mature technology to recover styrene from pyrolysis gasoline using extractive distillation technology. The key technology of the extractive distillation process is the performance of the solvent, which mainly includes selectivity, boiling point and control of the operating temperature of the extractive distillation process, which has a vital impact on the technical and economic indicators of the separation process, and also affects whether the device can operate stably for a long period of time. The excellent selectivity of the solvent can greatly change the relative volatility of styrene / o-xylene, which determines whether high-purity styrene can be obtained; controlling the lower operating temperature can reduce some factors that are not conducive to the long-term operation of the device caused by styrene polymerization and increase the yield of styrene products.

[0005] USP3763015 discloses a method for separating styrene from steam cracking gasoline using an inhibitor and extractive distillation. The method extracts and distills steam cracking gasoline in the presence of a polar solvent and a nitrogen-containing inhibitor, then recovers styrene from the solvent, and then reacts the recovered styrene with a sufficient amount of nitric acid to remove impurities in the styrene, and then redistills to remove impurities to obtain substantially colorless pure styrene. The polar solvent is selected from dimethylacetamide, dimethylformamide, N-methylpyrrolidone and dimethyl sulfoxide, and the inhibitor used is hydroquinone and p-tert-butylcatechol. However, the preferred dimethylacetamide solvent has a low boiling point (165°C) and a small difference in boiling point with o-xylene and styrene, resulting in high process energy consumption.

[0006] CN1225081A discloses a method for separating styrene from pyrolysis gasoline by extractive distillation. The solvent of the extractive distillation is divided into two parts. The solvent is mainly composed of a first part selected from propylene carbonate, cyclopentane sulfone, methyl carbitol, 1-methyl-2-pyrrolidone, etc. or a mixture thereof but not including water, and a second part composed of water. The first part of the solvent enters from the upper part of the extractive distillation tower, and the second part of the solvent is added from the bottom of the extractive distillation tower.

[0007] CN1962013A discloses a solvent for extracting and rectifying styrene from a steam cracking hydrocarbon mixture, comprising 15-60% by mass of a sulfone compound and 40-45% by mass of a nitrogen-containing compound, wherein the sulfone compound is selected from dimethyl sulfoxide or cyclopentane sulfone, and the nitrogen-containing compound is selected from at least one of succinonitrile, dimethylacetamide, N-methyl-2-pyrrolidone, 2-pyrrolidone, ethylpiperazine or N-formylmorpholine. The solvent can make the pH value of the solvent about 7, and improve the relative volatility of o-xylene to styrene. When used for recovering styrene from the C8 fraction of cracked gasoline, the solvent has the advantages of convenient separation and being conducive to inhibiting styrene polymerization.

[0008] CN101468938A discloses a composite solvent for extractive distillation separation of styrene, comprising 62-90% by mass of a main solvent and 10-38% by mass of a co-solvent, wherein the main solvent is selected from sulfone compounds, pyrrolidone compounds, glycol compounds or a mixture of any two or three thereof, and the co-solvent is selected from C9-C12 aromatic light, amides containing 2-6 carbon atoms or a mixture thereof.

[0009] CN103360200A discloses a composite solvent for extracting and rectifying styrene from a hydrocarbon mixture, comprising 80-99 mass % of a main solvent, 0.01-19 mass % of a co-solvent and 0.1-2.0 mass % of water, wherein the main solvent is a sulfone compound and the co-solvent is methoxyphenol. The composite solvent is used in combination with a polymerization inhibitor to reduce the dosage of the polymerization inhibitor and reduce the polymerization rate of styrene.

[0010] CN102452891A discloses a method for extracting and distilling styrene from a hydrocarbon mixture, comprising introducing a hydrocarbon mixture from the middle of an extractive distillation tower, introducing a solvent from the upper part of the extractive distillation tower, and adding styrene to the bottom of the extractive distillation tower. After extractive distillation, the raffinate is discharged from the top of the extractive distillation tower, and the rich solvent rich in styrene is discharged from the bottom of the extractive distillation tower, enters a solvent recovery tower, and the styrene is discharged from the top of the recovery tower. The lean solvent discharged from the bottom of the recovery tower is recycled back to the upper part of the extractive distillation tower for reuse, and the solvent is one or more of cyclopentane, diethylene glycol, triethylene glycol, tetraethylene glycol, N,N-dimethylacetamide and N-formylmorpholine. The method can effectively reduce the bottom temperature of the extractive distillation tower, significantly reduce styrene polymerization, and extend the operation cycle of the device, but the addition of pure styrene at the bottom of the extractive distillation tower reduces the processing capacity and increases energy consumption.

[0011] The above existing technologies all use composite solvents to try to maintain the high selectivity of the solvent system, but the effect is general in actual tests. For example, cyclopentane is used as the main solvent, and dimethylacetamide, N-methyl-2-pyrrolidone or N-formylmorpholine are used as co-solvents. The selectivity is reduced compared with pure cyclopentane. Some single solvents, such as tetraethylene glycol, have a higher boiling point, resulting in a higher operating temperature. It is well known that the effect of temperature on the polymerization rate of styrene is very large. When the temperature exceeds 130°C, the polymerization of styrene is significantly accelerated. When the temperature exceeds 140°C, the addition of inhibitors cannot effectively reduce the polymerization. In the process of extractive distillation, the styrene content at the bottom of the extractive distillation tower is high and the operating temperature is high, so polymerization is most likely to occur. Selecting a suitable solvent to reduce the bubble point temperature of the low styrene solvent system of the extractive distillation tower is the most fundamental means to solve the polymerization of styrene. Summary of the invention

[0012] The purpose of the present invention is to provide a composite solvent and method for recovering styrene from a hydrocarbon mixture. The composite solvent is used as an extractant to extract and distill styrene from a hydrocarbon mixture, which can effectively reduce the operating temperature of the extractive distillation tower kettle, reduce styrene polymerization, reduce energy consumption, and the styrene yield and purity are both high.

[0013] The composite solvent for separating styrene from a hydrocarbon mixture provided by the present invention comprises 35-80% by mass of a main solvent and 20-65% by mass of an auxiliary solvent, wherein the main solvent is sulfolane and the auxiliary solvent is 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone.

[0014] The present invention adopts a composite solvent in which cyclopentane is used as a main solvent and 1,3-dimethyl-2-imidazolidinone and / or gamma-butyrolactone is used as a co-solvent to extract and distill styrene from a hydrocarbon mixture, which can effectively reduce the bubble point temperature of the system, thereby reducing the operating temperature of the bottom of the extraction distillation tower during the extraction distillation process, reducing the polymerization of styrene, reducing energy consumption, and achieving high styrene yield and purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The figure is a schematic diagram of the process flow of the method for separating styrene from a hydrocarbon mixture of the present invention. DETAILED DESCRIPTION

[0016] The polymerization test results of the sulfolane solution with a styrene content of 15% show that when the temperature is 130°C, the styrene loss rate is about 1% by mass, and when the temperature exceeds 140°C, the styrene loss rate reaches more than 3% by mass. As the polymerization experiment temperature increases, the styrene loss rate increases rapidly, so a lower temperature can reduce the loss of styrene caused by polymerization, improve the yield of styrene, and facilitate long-term operation of the device. The inventors of the present invention have found that compared with a mixed solution of styrene and pure sulfolane, a mixed solution of a composite solvent with styrene and sulfolane as the main solvent and 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone as a co-solvent has a lower bubble point temperature, that is, using sulfolane as the main solvent, 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone as a co-solvent, when extracting and distilling styrene from a hydrocarbon mixture, the operating temperature at the bottom of the extractive distillation tower can be reduced, significantly reducing the polymerization of styrene, thereby improving the yield of styrene and reducing the energy consumption of the device. The inventors of the present invention also found that in a system in which a composite solvent comprising styrene, o-xylene and cyclopentane sulfone is used as the main solvent and 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone is used as the co-solvent and is used as the extractant, the relative volatility of styrene to o-xylene is close to that in the system in which cyclopentane sulfone is used as the extractant, indicating that the separation effect of the composite solvent on styrene and o-xylene is not significantly reduced compared with cyclopentane sulfone as the extractant.

[0017] The composite solvent for separating styrene from a hydrocarbon mixture provided by the present invention comprises 35-80% by mass of a main solvent and 20-65% by mass of a co-solvent, wherein the main solvent is cyclopentane sulfone, and the co-solvent is 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone. Preferably, when the co-solvent is 1,3-dimethyl-2-imidazolidinone, the composite solvent comprises 55-80% by mass of a main solvent and 20-45% by mass of a co-solvent; when the co-solvent is γ-butyrolactone, the composite solvent comprises 60-80% by mass of a main solvent and 20-40% by mass of a co-solvent; when the co-solvent is 1,3-dimethyl-2-imidazolidinone and γ-butyrolactone, the mass ratio of 1,3-dimethyl-2-imidazolidinone to γ-butyrolactone is 0.8-2:1.

[0018] The hydrocarbon mixture of the present invention is preferably a C8 fraction of pyrolysis gasoline, wherein the styrene content is 5-75% by mass, and the C8 aromatic and non-aromatic hydrocarbon content is 25-95% by mass.

[0019] The method for separating styrene from a hydrocarbon mixture provided by the present invention comprises introducing a hydrocarbon mixture containing styrene from the middle of an extractive distillation tower, introducing the composite solvent of the present invention from the top of the extractive distillation tower, performing extractive distillation, discharging a raffinate from the top of the extractive distillation tower, discharging a rich solvent containing styrene from the bottom of the extractive distillation tower, entering a solvent recovery tower, performing distillation separation, discharging styrene from the top of the solvent recovery tower, and discharging the composite solvent from the bottom of the solvent recovery tower and returning to the extractive distillation tower for recycling.

[0020] The number of theoretical plates of the extractive distillation tower is 30-70, the reflux ratio of the tower top is 1-4, the tower bottom temperature is 120-135° C., and the tower top pressure is 8-20 kPa.

[0021] The number of theoretical plates of the solvent recovery tower is 20-50, the reflux ratio of the top of the tower is 1-4, the temperature of the bottom of the tower is 125-140° C., and the pressure of the top of the tower is 8-20 kPa.

[0022] The method of the present invention is further described below in conjunction with the accompanying drawings. Figure 1In the process, the hydrocarbon mixture and the composite solvent are heat-exchanged and then enter the middle of the extractive distillation tower 101 through pipeline 1, and the composite solvent enters the upper part of the tower 101 through pipeline 2. After extractive distillation, the raffinate is discharged from the top of the tower 101 through pipeline 3, and after condensation and cooling, enters the reflux tank 102. A part of the raffinate returns to the tower 101 through pipeline 4, and the rest is sent out of the device through pipeline 5; the rich solvent rich in styrene is discharged from the bottom of the tower 101 and enters the upper part of the solvent recovery tower 103 through pipeline 6. The stripping water is injected into the lower part of the recovery tower 103 through pipeline 11. Styrene is discharged from the top of the tower 103 through pipeline 7, and after condensation and cooling, enters the reflux tank 104. A part of the styrene returns to the tower 103 through pipeline 8, and the rest is sent out of the device through pipeline 9. The lean solvent obtained at the bottom of the tower 103 returns to the tower 101 through pipeline 10.

[0023] The present invention is further described below by way of examples, but the present invention is not limited thereto.

[0024] The relative volatility of o-xylene and styrene described in the example is determined by headspace gas chromatography. The sample to be tested is added to an Agilent 7694 headspace injector. When gas-liquid equilibrium is reached, the gas and liquid phase compositions in the injector are determined by 7890 chromatographic analysis, and the relative volatility of the components is calculated by formula (1).

[0025] α ij =(yi / y j ) / (x i / x j ) Formula (1)

[0026] In formula (1), α is the relative volatility of styrene to o-xylene when gas-liquid equilibrium is reached, y is the gas phase composition, x is the liquid phase composition, i represents the light component, i.e. styrene, and j represents the heavy component, i.e. o-xylene.

[0027] Example 1

[0028] A hydrocarbon mixture raw material A was prepared by mixing 50% by mass of styrene and 50% by mass of o-xylene, and a composite solvent composed of 50% by mass of main solvent sulfolane and 50% by mass of auxiliary solvent 1,3-dimethyl-2-imidazolidinone was prepared. The composite solvent and the hydrocarbon mixture raw material were mixed at a mass ratio of 4:1 (i.e., solvent ratio) to obtain a mixed liquid. The gas phase and liquid phase compositions at gas-liquid equilibrium were measured at 80°C. The relative volatility of styrene relative to o-xylene was calculated according to formula (1). The results are shown in Table 1. The bubble point temperature of the mixed liquid of the composite solvent containing 10% styrene at 20 kPa was measured. The results are shown in Table 1.

[0029] Example 2

[0030] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 35% by mass of the main solvent cyclopentane sulfone and 65% by mass of the auxiliary solvent 1,3-dimethyl-2-imidazolidinone. The results are shown in Table 1.

[0031] Example 3

[0032] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 75% by mass of the main solvent cyclopentane sulfone and 25% by mass of the auxiliary solvent 1,3-dimethyl-2-imidazolidinone. The results are shown in Table 1.

[0033] Example 4

[0034] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of the auxiliary solvents 1,3-dimethyl-2-imidazolidinone and γ-butyrolactone, and the mass ratio of 1,3-dimethyl-2-imidazolidinone to γ-butyrolactone was 1:1. The results are shown in Table 1.

[0035] Example 5

[0036] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent sulfolane and 50% by mass of the auxiliary solvent γ-butyrolactone. The results are shown in Table 1.

[0037] Comparative Example 1

[0038] The relative volatility of styrene to o-xylene of raw material A in solvent sulfolane and the bubble point temperature of the solvent sulfolane mixture containing 10% styrene at 20 kPa were tested according to the method of Example 1. The results are shown in Table 1.

[0039] Comparative Example 2

[0040] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of the auxiliary solvent triethylene glycol monomethyl ether. The results are shown in Table 1.

[0041] Comparative Example 3

[0042] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of the auxiliary solvent diethylene glycol monomethyl ether. The results are shown in Table 1.

[0043] Comparative Example 4

[0044] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 75% by mass of the main solvent cyclopentane sulfone and 25% by mass of the auxiliary solvent diethylene glycol monomethyl ether. The results are shown in Table 1.

[0045] Comparative Example 5

[0046] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of the auxiliary solvent diethylene glycol monoethyl ether. The results are shown in Table 1.

[0047] Comparative Example 6

[0048] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of the auxiliary solvent N-methylpyrrolidone. The results are shown in Table 1.

[0049] Comparative Example 7

[0050] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 75% by mass of the main solvent cyclopentane sulfone and 25% by mass of the auxiliary solvent diethylene glycol. The results are shown in Table 1.

[0051] Comparative Example 8

[0052] The relative volatility of styrene to o-xylene in the composite solvent and the bubble point temperature of the composite solvent mixture containing 10% styrene at 20 kPa of raw material A were tested according to the method of Example 1, except that the composite solvent consisted of 75% by mass of the main solvent cyclopentane sulfone and 25% by mass of the auxiliary solvent p-diethylbenzene. The results are shown in Table 1.

[0053] Table 1

[0054]

[0055]

[0056] As can be seen from Table 1, the composite solvents of Examples 1 to 5 of the present invention maintain a relatively high relative volatility of styrene to o-xylene, i.e., a relatively high styrene selectivity, relative to the comparative example, while making the mixed solution of styrene and the composite solvent have a lower bubble point temperature.

[0057] Example 6

[0058] This example follows Figure 1 The process of separating styrene from a hydrocarbon mixture is shown in Table 2. The raw material 1 has a styrene content of 35% by mass, C8 aromatics of 63% by mass, and non-aromatics of 2% by mass. The specific composition is shown in Table 2. The composite solvent includes 50% by mass of the main solvent cyclopentane sulfone and 50% by mass of 1,3-dimethyl-2-imidazolidinone. The bottom temperature of the extractive distillation tower is 131°C. The yield of styrene is 99%, and the purity of styrene reaches 99.8%. The main operating conditions are shown in Table 3, and the results are shown in Table 4.

[0059] Example 7

[0060] This example follows Figure 1 The process of separating styrene from a hydrocarbon mixture is shown in Table 2. The raw material 1 has a styrene content of 35% by mass, C8 aromatics of 63% by mass, and non-aromatics of 2% by mass. The specific composition is shown in Table 2. The composite solvent includes 65% by mass of the main solvent cyclopentane sulfone and 35% by mass of the auxiliary solvent 1,3-dimethyl-2-imidazolidinone. The bottom temperature of the extractive distillation tower is 132°C. The yield of styrene is 99%, and the purity of styrene reaches 99.8%. The main operating conditions are shown in Table 3, and the results are shown in Table 4.

[0061] Example 8

[0062] This example follows Figure 1 The process of separating styrene from a hydrocarbon mixture is shown in Table 2. The raw material 2 has a styrene content of 53% by mass, C8 aromatics of 45% by mass, and non-aromatics of 2% by mass. The specific composition is shown in Table 2. The composite solvent is 75% by mass of the main solvent sulfolane + 25% by mass of the auxiliary solvent γ-butyrolactone. The bottom temperature of the extractive distillation tower is 132°C. The yield of styrene is 98.5%, and the purity of styrene reaches 99.8%. The main operating conditions are shown in Table 3, and the results are shown in Table 4.

[0063] Comparative Example 9

[0064] This example follows Figure 1The process of separating styrene from a hydrocarbon mixture is shown in Table 2. The raw material 1 has a styrene content of 35% by mass, C8 aromatics of 63% by mass, and non-aromatics of 2% by mass. The specific composition is shown in Table 2. The solvent is cyclopentane sulfone, the bottom temperature of the extractive distillation tower is 140°C, the yield of styrene is 96%, and the purity of styrene reaches 99.8%. Styrene polymerization is aggravated and long-term operation is impossible. The main operating conditions are shown in Table 3 and the results are shown in Table 4.

[0065] Comparative Example 10

[0066] This example follows Figure 1 The process of separating styrene from a hydrocarbon mixture is shown in Table 2. The raw material 1 has a styrene content of 35% by mass, C8 aromatics of 63% by mass, and non-aromatics of 2% by mass. The specific composition is shown in Table 2. The solvent is cyclopentane sulfone, and stripping styrene is injected into the bottom of the tower to cool down. The mass ratio of stripping styrene to solvent is 0.01, the temperature at the bottom of the extractive distillation tower is 133°C, the yield of styrene is 99%, and the purity of styrene reaches 99.8%. The main operating conditions are shown in Table 3, and the results are shown in Table 4.

[0067] Compared with Example 6, the purity and yield of styrene in Comparative Example 10 did not change much, but the energy consumption increased.

[0068] Table 2

[0069]

[0070] Table 3

[0071]

[0072] Table 4

[0073]

Claims

1. A composite solvent for separating styrene from a hydrocarbon mixture, comprising 35-80% by mass of a main solvent and 20-65% by mass of a co-solvent, wherein the main solvent is sulfolane and the co-solvent is 1,3-dimethyl-2-imidazolidinone and / or γ-butyrolactone.

2. The composite solvent according to claim 1, characterized in that The invention comprises 55-80 mass % of a main solvent and 20-45 mass % of an auxiliary solvent, wherein the auxiliary solvent is 1,3-dimethyl-2-imidazolidinone.

3. The composite solvent according to claim 1, characterized in that The invention comprises 60-80 mass % of a main solvent and 20-40 mass % of an auxiliary solvent, wherein the auxiliary solvent is gamma-butyrolactone.

4. The composite solvent according to claim 1, characterized in that When the co-solvent is 1,3-dimethyl-2-imidazolidinone and γ-butyrolactone, the mass ratio of 1,3-dimethyl-2-imidazolidinone to γ-butyrolactone is 0.8 to 2:

1.

5. A method for separating styrene from a hydrocarbon mixture, comprising introducing a hydrocarbon mixture containing styrene from the middle of an extractive distillation tower, introducing the composite solvent according to claim 1 from the top of the extractive distillation tower, performing extractive distillation, discharging a raffinate from the top of the extractive distillation tower, discharging a rich solvent containing styrene from the bottom of the extractive distillation tower, entering a solvent recovery tower, performing distillation separation, discharging styrene from the top of the solvent recovery tower, and discharging the composite solvent from the bottom of the solvent recovery tower and returning to the extractive distillation tower for recycling.

6. The method according to claim 5, characterized in that The mass ratio of the composite solvent to the hydrocarbon mixture containing styrene is 3-6.

7. The method according to claim 5, characterized in that The number of theoretical plates of the extractive distillation tower is 30-70, the reflux ratio of the tower top is 1-4, the tower bottom temperature is 120-135° C., and the tower top pressure is 8-20 kPa.

8. The method according to claim 5, characterized in that The number of theoretical plates of the solvent recovery tower is 20-50, the reflux ratio of the top of the tower is 1-4, the temperature of the bottom of the tower is 125-140° C., and the pressure of the top of the tower is 8-20 kPa.

9. The method according to claim 5, characterized in that The styrene content in the styrene-containing hydrocarbon mixture is 5 to 75% by mass.

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