A method for recovering aromatic hydrocarbons from wide-cut gasoline

Abstract

A method for recovering aromatic hydrocarbons from wide-cut gasoline, which is mainly used for wide-cut gasoline with an end point of the distillation range within 180 °C and a low content of light aromatic hydrocarbons such as benzene or toluene. The specific method includes contacting gasoline with a solvent in a liquid-liquid extraction column for countercurrent extraction. The first rich solvent obtained at the bottom of the liquid-liquid extraction column is sent to an extractive distillation column. The gasoline fraction containing light aromatic hydrocarbons separated from the top of the extractive distillation column is returned to the liquid-liquid extraction column. The second rich solvent obtained at the bottom of the extractive distillation column is sent to a solvent recovery column to obtain mixed aromatic hydrocarbons and lean solvent, and the lean solvent is recycled. The raffinate obtained at the top of the liquid-liquid extraction column is sent out of the unit as non-aromatic raffinate after removing the solvent by water washing. The mixed aromatic hydrocarbons obtained at the top of the solvent recovery column are sent to an aromatic hydrocarbon refining unit and successively pass through a benzene column, a toluene column, and an xylene column to obtain benzene (B), toluene (T), C8 aromatic hydrocarbons (X), and C9+ aromatic hydrocarbons.

Description

Technical Field

[0001] The present invention relates to a method for recovering aromatics from a wide-boiling gasoline with a low light aromatics content by using a solvent. Specifically, it is a method for recovering aromatics through a combined unit technology of liquid-liquid extraction, extractive distillation, and aromatics rectification. Background Art

[0002] The wide-boiling gasoline described in the present invention refers to a gasoline fraction with a boiling range of 60-180°C. Of course, it can also be a gasoline fraction with a lower or higher initial boiling point and a lower or higher final boiling point. The gasoline with a boiling range of 60-180°C contains benzene-toluene-xylene (BTX), C9 aromatics, and some C10 aromatics, among which C9 aromatics and some C10 aromatics are mainly alkylbenzenes. As is well known, BTX is one of the most important basic organic chemical raw materials, with a large demand and a wide range of applications; C9 aromatics and C10 aromatics, which are mainly alkylbenzenes, can be relatively easily converted into BTX through disproportionation and transalkylation, and are suitable for maximizing the production of BTX. Therefore, the gasoline fraction with a boiling range of 60-180°C has greater industrial value for separating aromatics.

[0003] The main methods for recovering aromatics from wide-boiling gasoline are liquid-liquid extraction and extractive distillation. Among them, the liquid-liquid extraction method is a method for separating aromatics from non-aromatics by using the different solubilities of solvents for each component of hydrocarbons, and the extractive distillation method is a method for separating aromatics by using the different effects of solvents on the relative volatilities of each component of hydrocarbons. The selective solvents used in both methods are roughly glycols, sulfolane, N-formylmorpholine, etc. At present, in industry, these two extraction processes are mainly applied to reformed gasoline, steam-cracked hydrogenated gasoline, and coal tar raw materials, and the main solvent is sulfolane.

[0004] With the continuous improvement of gasoline standards and environmental awareness, as well as the accelerating process of oil conversion, the technical demand for recovering aromatic products from secondary processed gasoline such as catalytic cracking and hydrocracking is increasing day by day. The composition and properties of these secondary processed gasoline are closely related to the processing technology and crude oil properties adopted, and there are also significant differences among different units. Among them, there is a large category of gasoline with moderate aromatic content, less light aromatics, and more heavy aromatics. When processing such raw materials, the existing sulfolane liquid-liquid extraction process has the following problems: 1) Using the traditional sulfolane liquid-liquid extraction process, although most of the non-aromatics are separated from the top of the liquid-liquid extraction tower, there are still some non-aromatics that are close to the properties of aromatics and are difficult to be separated by liquid-liquid extraction, which will remain in the rich solvent at the bottom of the liquid-liquid extraction tower; 2) Using the traditional extractive distillation process, due to the wide fraction range of the raw material, the low content of light aromatics such as benzene and toluene, and the wide distribution of non-aromatics, the operating temperature at the bottom of the extractive distillation tower needs to be higher than 180 °C to distill out all the non-aromatics in the raw material; however, a tower bottom temperature above 180 °C will cause the accelerated decomposition of sulfolane, increase the solvent loss, and the device cannot operate in a long cycle; in addition, due to the wide distribution of non-aromatics in the raw material, the separation difficulty of extractive distillation will also increase, resulting in a decrease in the purity of the recovered mixed aromatics, and ultimately affecting the purity of aromatic products, especially toluene products, which cannot meet the requirement of the national standard for premium products with a purity of ≥ 99.9%; 3) The solubility of sulfolane solvent in C9+ aromatics is relatively low, and there are always problems of low recovery rate and low purity of C9+ aromatics when using the existing liquid-liquid extraction process.

[0005] Currently, there are few process patents for recovering aromatics from wide-fraction gasoline with low light aromatic content.

[0006] Patent CN1209327C discloses a method for recovering aromatics by extraction and extractive distillation. This method is mainly used to recover benzene, toluene, and C8 aromatics from hydrocarbon mixtures with high aromatic content. The specific approach is to first pre-fractionate the hydrocarbon mixture to obtain a benzene fraction, a toluene fraction, and a C8 aromatic fraction, send the toluene fraction to a liquid-liquid extraction tower for extraction, and send the benzene fraction to an extractive distillation tower for extractive distillation. This method not only needs to cut out the benzene fraction and toluene fraction through pre-fractionation, but also the recovered product from the recovery tower is still a mixed aromatic, and it is necessary to distill again to obtain products such as benzene and toluene, with high energy consumption and no practical application value. In addition, this patent is more suitable for raw materials with relatively high aromatic content, especially benzene content. It is clearly stated in claim 2 that the benzene content in the benzene fraction > 50%, and it is not applicable to raw materials with a benzene content of less than 7%.

[0007] Patent CN100355866C discloses a method for separating aromatics and non-aromatics, which is achieved through a parallel extractive distillation and liquid-liquid extraction hybrid operation system. This method divides the raw material into two streams and feeds them into the extractive distillation column and the liquid-liquid extraction column simultaneously for separation, and is applicable to the renovation and expansion of liquid-liquid extraction devices. However, when the raw material fraction is relatively wide, such as in the C6-C9 fraction, the extractive distillation column cannot meet the separation requirements, resulting in unqualified quality of toluene products and C8 aromatic products.

[0008] Patent CN105308155A discloses a method for treating heavy hydrocarbons from an extraction solvent, which cross-connects an independent and complete liquid-liquid extraction process with a complete extractive distillation process to achieve the treatment of wide-fraction raw materials. This method feeds a part of the lean solvent obtained from the recovery column of the extractive distillation process into the liquid-liquid extraction process, and uses the method of back-extraction to purify the solvent, thereby ensuring the normal operation of the extractive distillation process. However, this process flow is too complex and still cannot solve the problems encountered in treating wide-fraction gasoline.

[0009] Patent CN108690658A discloses a method for recovering aromatics from oils with low aromatic content, including steps such as pre-fractionation, extraction, back-extraction, distillation, and water washing. This method is applicable to raw materials with an aromatic content of 5-10%. The pre-fractionation column obtains a pre-benzene fraction and a benzene fraction. The benzene fraction goes to the extraction column to separate non-aromatics and obtain a rich solvent. The pre-benzene fraction serves as the back-extraction agent for the rich solvent, and the aromatics and the solvent are separated through back-extraction, and then the aromatics and the pre-benzene fraction are obtained through distillation. The purity of the aromatic products of this method is relatively low, a pre-fractionation column is added, and the energy consumption is also relatively high.

[0010] Patent US3361664 discloses a method for separating aromatics and non-aromatics by extractive distillation using sulfolane solvent. This method is aimed at wide-fraction raw materials with a low light aromatic content, and adopts the operation of negative pressure in the extractive distillation column or adding stripping water to solve the problem of too high bottom temperature of the column. However, both negative pressure and adding stripping water in this method will reduce the separation efficiency of the extractive distillation column. In particular, adding stripping water is likely to form a gas-liquid-liquid three-phase zone on the tray, seriously affecting the separation effect of the extractive distillation column. This method cannot obtain high-purity and high-yield toluene products, nor can it solve the problem of low C9+ aromatic yield.

[0011] Patent US3844902 discloses a liquid-liquid extraction - extractive distillation hybrid process. This method cross-connects an independent and complete liquid-liquid extraction process with an extractive distillation process. The raw material first enters the extractive distillation column, and the rich solvent at the bottom of the column goes to the solvent recovery column, and the material obtained from the top of the extractive distillation column goes to the liquid-liquid extraction part for treatment. The liquid-liquid extraction part includes a liquid-liquid extraction column, an extractive distillation column, a solvent recovery column, etc. The process flow involved in this patent is complex, with high energy consumption and no implementation feasibility.

[0012] Patent US4428829 discloses a method for recovering aromatic hydrocarbons from gasoline fractions. A prefractionation column is provided before the extraction process, and two extraction columns are set in the extraction process to separately process light and heavy fractions. The process flow of this method is too complex, with high energy consumption and no feasibility for implementation. Summary of the Invention

[0013] Aiming at the deficiencies of the prior art, the object of the present invention is to provide a method for recovering aromatic hydrocarbons from a wide-cut gasoline with a low content of light aromatic hydrocarbons. This method is particularly suitable for treating catalytic cracking gasoline, hydrocracking gasoline, etc.

[0014] To achieve the object of the present invention, a method for recovering aromatic hydrocarbons from a wide-cut gasoline with a low content of light aromatic hydrocarbons is characterized by comprising the following steps:

[0015] (1) The wide-cut gasoline raw material enters the liquid-liquid extraction column from the middle and lower parts, and the solvent enters the liquid-liquid extraction column from the upper part. The two contact countercurrently. The non-aromatic raffinate is obtained at the top of the column, and the first rich solvent is obtained at the bottom of the column.

[0016] (2) The first rich solvent enters the upper part of the extractive distillation column, and the second rich solvent is obtained at the bottom of the column.

[0017] (3) The second rich solvent enters the solvent recovery column, and the mixed aromatic hydrocarbons are obtained at the top of the column, and the lean solvent is obtained at the bottom of the column.

[0018] (4) The mixed aromatic hydrocarbons enter the aromatic distillation section, and benzene, toluene, xylene products and C9+ aromatic hydrocarbons are obtained through the benzene column, toluene column and xylene column respectively. Brief Description of the Drawings

[0019] Figure 1 It is a process flow diagram for recovering aromatic hydrocarbons from wide-cut gasoline provided by the present invention. Detailed Embodiments

[0020] The present invention will be further described in detail below with reference to the drawings and embodiments. Through these descriptions, the features and advantages of the present invention will become more clearly defined.

[0021] The special term "exemplary" here means "serving as an example, embodiment or illustration". Any embodiment described here as "exemplary" does not have to be construed as superior to or better than other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings do not have to be drawn to scale unless otherwise specified.

[0022] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0023] In the present invention, the term "C6+" refers to having 6 or more carbons, and the term "C6+ aromatic hydrocarbon" refers to an aromatic hydrocarbon having 6 or more carbons.

[0024] In the present invention, the term "C9+" refers to having 9 or more carbons, and the term "C9+ aromatic hydrocarbon" refers to an aromatic hydrocarbon having 9 or more carbons.

[0025] With the continuous improvement of environmental awareness and the accelerating process of oil conversion, the technical demand for recovering aromatic hydrocarbon products from secondary processed gasoline such as catalytic cracking and hydrocracking gasoline is increasing day by day. In response to this demand, the present invention specifically proposes an efficient liquid-liquid extraction separation combined process. The specific method adopted in the present invention is that the wide-cut gasoline raw material enters the liquid-liquid extraction tower from the middle and lower parts, and the solvent enters the liquid-liquid extraction tower from the upper part. The two are in countercurrent contact. The raffinate oil substantially free of aromatic hydrocarbons is withdrawn from the top of the tower, and the first rich solvent containing a small amount of non-aromatic hydrocarbons is withdrawn from the bottom of the tower; the raffinate oil enters the water washing tower, and after removing the solvent, it is sent out of the device as non-aromatic raffinate oil, and the washed water enters the water vapor stripping tower; the first rich solvent enters the upper part of the extractive distillation tower, and the gasoline fraction containing light aromatic hydrocarbons is obtained at the top of the tower and sent back to the liquid-liquid extraction tower as the reflux liquid of the liquid-liquid extraction tower. The second rich solvent is obtained at the bottom of the extractive distillation tower; the second rich solvent enters the solvent recovery tower, the mixed aromatic hydrocarbons are obtained at the top of the tower, and the lean solvent is obtained at the bottom and recycled; the mixed aromatic hydrocarbons enter the aromatic hydrocarbon refining unit, and benzene, toluene, xylene and C9+ aromatic hydrocarbons are obtained through the benzene tower, toluene tower and xylene tower respectively.

[0026] According to an embodiment of the present invention, benzene product is withdrawn from the side line of the benzene tower, and the benzene obtained at the top of the tower is used as stripped benzene and returned to the extractive distillation tower as the purification and stripping medium. According to another embodiment of the present invention, a part of the toluene obtained at the top of the toluene tower is used as stripped toluene and returned to the extractive distillation tower as the purification and stripping medium. This feature of the present invention - returning part of the benzene product or toluene product separated from the obtained mixed aromatic hydrocarbon product by rectification to the lower section of the extractive distillation tower reduces the partial pressure of the extractive distillation tower and has the same effect as raising the operating temperature or lowering the tower pressure. At the same time, introducing benzene or toluene into the extractive distillation tower can also displace heavy non-aromatic hydrocarbons that are difficult to separate and improve the separation effect of the extractive distillation tower. This method does not introduce new components, does not increase the separation difficulty, and reduces the operating temperature of the extractive distillation tower and improves the purity of the mixed aromatic hydrocarbons by a simple method.

[0027] According to an embodiment of the present invention, another feature of the present invention is the use of a solvent that has good solubility for C9+ aromatics. Sulfolane, alkyl sulfolane, 1,3-dimethyl-2-imidazolidinone (DMI), glycols, and glycol ethers are all commonly used solvents for separating aromatics from non-aromatics. Among them, the solvents with the best solubility are alkyl sulfolane, DMI, and glycol ether solvents; the solvent with the best selectivity for non-aromatics and aromatics is sulfolane. The present invention precisely utilizes the differences in selectivity and solubility of these solvents and selects sulfolane as the solution for the liquid-liquid extraction column. Further, in addition to the main solvent sulfolane, the solution may also include a certain proportion of a co-solvent, which increases the solubility of C9+ aromatics on the basis of basically not reducing the selectivity, thereby improving the ability to process wide-cut gasoline. After the solvent is matched with the corresponding extraction temperature, it can have good solubility and selectivity for C9+ aromatics while not reducing the separation efficiency of C6-C8 aromatics.

[0028] According to an embodiment of the present invention, the solvent includes the main solvent sulfolane with a concentration of 70-100 wt%, preferably 80-90 wt%. The solvent also includes a co-solvent selected from one or more of DMI, methyl sulfolane, dimethyl sulfolane, and polyglycol ethers, and the polyglycol ethers are selected from one or more of triethylene glycol monomethyl ether, tetraethylene glycol methyl ether, tetraethylene glycol monomethyl ether, pentaethylene glycol methyl ether, and pentaethylene glycol monomethyl ether. The selection principle of the co-solvent is that it has good solubility for C9+ aromatics, has a certain selectivity for non-aromatics and aromatics, and has a boiling point at least 50 °C higher than the dry point of the raw material to reduce the difficulty of solvent recovery and avoid solvent loss. In a preferred embodiment of the present invention, the solvent may contain 0.5-3.0 wt% of water and / or 0-2.0 wt% of C9+ hydrocarbons.

[0029] The present invention is applicable to wide-cut gasoline with a low light aromatic content and gasoline fractions with a dry point within 180 °C, preferably various secondary processed gasoline after desulfurization and denitrification treatment, where the benzene content is 0.2-7 wt% and the total aromatic content is greater than 30 wt%, such as catalytic cracking gasoline, catalytic pyrolysis gasoline, and hydrocracking gasoline. Specifically, it may be a C5-C11 wide cut or a hydrocarbon fraction of some carbon numbers therein, preferably a C6-C9 fraction.

[0030] According to an embodiment of the present invention, the gasoline fraction enters the liquid-liquid extraction column from the middle and lower parts, and the solvent enters the liquid-liquid extraction column from the upper part. The two contact countercurrently, and the non-aromatic raffinate is obtained at the top of the column, and the first rich solvent is obtained at the bottom of the column.

[0031] According to an embodiment of the present invention, the raffinate obtained at the top of the liquid-liquid extraction column enters the water washing column, and after removing the solvent, it is sent out of the device as the non-aromatic raffinate, and the washed water enters the water vapor stripping column.

[0032] According to an embodiment of the present invention, the first rich solvent enters the upper part of the extractive distillation column. A gasoline fraction containing light aromatics is obtained at the top of the column and sent as a reflux washing liquid to the liquid-liquid extraction column, and a second rich solvent is obtained at the bottom of the column; stripped benzene or stripped toluene enters from the lower part of the extractive distillation column.

[0033] According to an embodiment of the present invention, the second rich solvent enters the solvent recovery column. Mixed aromatics are obtained at the top of the column, and lean solvent is obtained at the bottom and recycled.

[0034] According to an embodiment of the present invention, the temperature and pressure of the water stripping column are controlled. A small amount of hydrocarbons are obtained at the top of the column and returned to the extractive distillation column, and most of the remaining water vapor enters the solvent recovery column as stripping gas.

[0035] According to an embodiment of the present invention, the mixed aromatics enter the aromatics rectification section, and benzene (B), toluene (T), C8 aromatics (X), and C9+ aromatics are obtained through the benzene column, toluene column, and xylene column respectively.

[0036] According to an embodiment of the present invention, the top pressure of the liquid-liquid extraction column is 0.3 - 1.2 MPaG, the number of theoretical plates is 5 - 30, preferably 0.5 - 0.9 MPaG. The solvent inlet tower temperature is 80 - 120 °C, the raw material inlet tower temperature is 40 - 80 °C, the reflux washing liquid inlet tower temperature is 40 - 80 °C, the bottom temperature of the column is 60 - 180 °C, preferably 70 - 120 °C. The mass ratio of the reflux washing liquid to the wide-cut gasoline raw material, i.e., the backwashing ratio, is 0.6 - 1.5, and the mass ratio of the solvent to the wide-cut gasoline raw material, i.e., the solvent ratio, is 1 - 6:1, preferably 3 - 5:1.

[0037] According to an embodiment of the present invention, the number of theoretical plates of the water washing column is 2 - 10, the top pressure of the column is 0.1 - 0.8 MPaG, the top temperature of the column is 30 - 60 °C, and the mass ratio of the washing water entering the water washing column to the raffinate is 0.1 - 0.5:1, preferably 0.2 - 0.3:1.

[0038] According to an embodiment of the present invention, the top pressure of the extractive distillation column is -0.05 - 0.15 MPaG, preferably 0 - 0.08 MPaG, the bottom temperature of the column is 150 - 180 °C, preferably 170 - 178 °C, and the number of theoretical plates is 10 - 30. The first rich solvent is fed in the upper section of the column, and stripped benzene or stripped toluene is fed in the lower part of the column. The theoretical plates are numbered from small to large from top to bottom. If the number of theoretical plates of the column is 10, then stripped benzene or stripped toluene is fed on any plate below the 6th plate or at the bottom of the column. The mass ratio of stripped benzene to the wide-cut gasoline raw material is 0 - 0.2:1, and the mass ratio of stripped toluene to the wide-cut gasoline raw material is 0 - 0.25:1. Only one of stripped benzene and stripped toluene is required. When it is necessary to improve the purity of toluene product, toluene is preferably used as the stripping medium.

[0039] According to an embodiment of the present invention, the number of theoretical plates of the solvent recovery column is 12 - 30, the reflux ratio is 0.3 - 1.0, the absolute pressure at the top of the column is 10 - 80 kPa, and the bottom temperature of the column is 160 - 180 °C, preferably 170 - 178 °C. The second rich solvent is fed into the middle of the column, and water or water-containing solvent from the water stripping column is fed below the feeding position of the second rich solvent, preferably at the bottom of the column. The mass ratio of water or water-containing solvent from the water stripping column to the second rich solvent, i.e., the stripping water ratio, is 0.1 - 0.3.

[0040] According to an embodiment of the present invention, the water stripping column operates at atmospheric pressure, the pressure at the top of the column is 0 - 0.15 MPaG, the number of theoretical plates is 2 - 8, and the bottom temperature of the column is 100 - 135 °C. The heat source at the bottom of the column comes from the heat exchange of the lean solvent at the bottom of the solvent recovery column.

[0041] According to an embodiment of the present invention, the number of theoretical plates of the benzene column is 10 - 60, the reflux ratio is the ratio of the reflux flow rate at the top of the column to the feed flow rate of the benzene column, preferably 0.8 - 2.0:1, the pressure at the top of the column is 0.05 - 0.1 MPaG, and the bottom temperature of the column is 120 - 160 °C. A part of the benzene withdrawn from the top of the benzene column is refluxed, and a part is sent to the extractive distillation column as stripping benzene, and benzene products are withdrawn from the side line.

[0042] According to an embodiment of the present invention, the number of theoretical plates of the toluene column is 10 - 60, the reflux ratio is the ratio of the reflux flow rate at the top of the column to the feed flow rate of the toluene column, preferably 1.0 - 5.0, the pressure at the top of the column is 0.05 - 0.1 MPaG, and the bottom temperature of the column is 140 - 180 °C. Toluene products are obtained at the top of the column, a part is refluxed, a part is taken out as a product, and another part can enter the extractive distillation column as stripping toluene as needed. The bottom material of the column enters the C8 aromatics column. The number of theoretical plates of the C8 aromatics column is 30 - 100, the reflux ratio is the ratio of the reflux flow rate at the top of the column to the feed flow rate of the C8 aromatics column, preferably 1.0 - 5.0, the pressure at the top of the column is 0.05 - 0.2 MPaG, and the bottom temperature of the column is 160 - 210 °C. C8 aromatics products are obtained at the top of the column, and C9+ aromatics are at the bottom.

[0043] According to an embodiment of the present invention, the toluene column and the xylene column (i.e., the C8 aromatics column) can increase the operating pressure of any column according to the contents of toluene and C8 aromatics in the raw material to facilitate the thermal coupling between the distillation columns. The present invention will not elaborate on this here. In addition, a solvent regeneration column can be added in the extraction part to purify the solvent, and a de-olefination facility, such as clay, can be added in the distillation part to remove trace unsaturated hydrocarbons. However, the above all belong to well-known technical processes and have no influence on the concept of the present invention.

[0044] The present invention will be further described below with reference to the accompanying drawings.

[0045] Figure 1In it, the wide - cut gasoline feedstock exchanges heat with the solvent and then enters the middle - lower part of the liquid - liquid extraction column 101 through pipeline 1. The solvent enters the upper part of the liquid - liquid extraction column 101 through pipeline 4. After counter - current extraction, the raffinate oil is discharged from the top of column 101 through pipeline 2 and enters the middle - lower part of the water - washing column 102. The first rich solvent obtained at the bottom of column 101 enters the upper part of the extractive distillation column 103 through pipeline 3. The washing water enters column 102 through pipeline 7. The non - aromatic raffinate oil product is obtained at the top of column 102, and the water containing solvent at the bottom enters the water - vapor stripping column 105 through pipeline 8. The benzene fraction obtained from the top of the benzene column 106 enters the extractive distillation column 103 through pipeline 13. The gasoline fraction containing benzene and non - aromatics obtained at the top of column 103 is used as the reflux washing liquid and enters the lower part of the liquid - liquid extraction column through pipeline 5. The second rich solvent containing aromatics at the bottom of column 103 enters the solvent recovery column 104 through pipeline 12. The mixed aromatics are obtained at the top of column 104, and the lean solvent at the bottom enters column 105 through pipeline 17 as a heat source, then exchanges heat with pipeline 3 through pipeline 18, and after exchanging heat with the feedstock, enters column 101 through pipeline 4. The water obtained from the water pocket of the reflux drum at the top of column 104 enters column 102 through pipeline 7. The water obtained from pipeline 8 and pipeline 11 enters the water - vapor stripping column 105. A small amount of hydrocarbons at the top of column 105 enters pipeline 10 through pipeline 16, and the remaining water or water - containing solvent enters the bottom of column 104. The mixed aromatics enter the benzene column 106 through pipeline 15. The stripped benzene obtained at the top enters column 103 through pipeline 13. The benzene product is obtained from the side line of column 106 through pipeline 20, and the bottom stream enters the toluene column 107. The toluene product is obtained at the top of column 107, and the stripped toluene obtained at the top can enter column 103 through pipeline 24. The bottom stream of column 107 enters the C8 aromatics column 108. The C8 aromatics product is obtained at the top of column 108, and the C9+ aromatics product is obtained at the bottom.

[0046] The present invention adopts the process of returning the stripped benzene or stripped toluene to the extractive distillation column, which greatly reduces the operating temperature at the bottom of the extractive distillation column, breaks through the limitation of the low benzene content in the feedstock on the temperature of the extractive distillation column, and at the same time displaces the difficult - to - separate non - heavy aromatics from the bottom of the extractive distillation column. It not only maintains a high product yield but also improves the purity of C8 aromatics and C9+ aromatics. At the same time, in the present invention, the ratio of the main solvent and the co - solvent in the solvent is flexibly adjusted according to the C9+ aromatics content in the feedstock, increasing the treatment capacity for the C9+ fraction, and high - purity and high - yield C9+ aromatics products can be obtained. The biggest advantage of the present invention is that there is almost no limitation on the composition of the feedstock gasoline fraction, expanding the distillation range of the extraction feedstock. There is no need to set up a fractionation column for the upstream incoming material. The de - pentanized oil from the upstream device can directly adopt the process of the present invention for processing. If there is a fractionation column in the upstream device, the side - line gasoline products with various distillation ranges can be arbitrarily adjusted according to actual needs, and the present invention can efficiently process them and obtain various products with high purity and high yield.

[0047] The present invention will be further described in detail below through examples, but the present invention is not limited thereto.

[0048] At 20 °C, the solubility data of different solvents for C9 aromatics as a model are shown in Table 1.

[0049] Table 1 Solubility of Solvents for C9 Aromatics

[0050] Item C9 Aromatics (g / 100g solvent) Sulfolane 35 80wt% Sulfolane + 20wt% Triethylene Glycol Monomethyl Ether >70 80wt% Sulfolane + 20wt% Tetraethylene Glycol Monomethyl Ether >70 80wt% Sulfolane + 20wt% DMI >70 80wt% Sulfolane + 20wt% Methylsulfolane >70

[0051] As can be seen from the results shown in Table 1, the solubility of sulfolane in C9 aromatics is relatively low. After adding a cosolvent, the solubility of the solvent in C9 aromatics is significantly enhanced.

[0052] Example 1

[0053] According to the Figure 1 process, aromatics are recovered from the gasoline fraction. The raw material contains 2.4 wt% benzene, 13.4 wt% C9 aromatics, and 45 wt% total aromatics. The detailed hydrocarbon composition is shown in Table 2. The proportion of benzene stripped in the extractive distillation column is 12% of the extractive raw material. The solvent is 85 wt% sulfolane + 15 wt% triethylene glycol monomethyl ether. The main operating conditions of each column are shown in Table 3, and the results are shown in Table 4. Using the method of the present invention, the yields of benzene and toluene are 99.8%, the yield of C8 aromatics is 99.7%, and the yield of C9+ aromatics is 98.6%; the purity of benzene reaches 99.98 wt%, the purity of toluene reaches 99.93 wt%, the purity of the C8 aromatics product reaches 99.1 wt%, and the purity of the C9+ aromatics reaches 98.5 wt%. The bottom temperature of the extractive distillation column is 177 °C, and the separation effect is good.

[0054] Example 2

[0055] According to the Figure 1 process, aromatics are recovered from the gasoline fraction. The raw material contains 1.9 wt% benzene, 20.1 wt% C9 aromatics, and 59.1 wt% total aromatics. The detailed hydrocarbon composition is shown in Table 2. The proportion of toluene fraction stripped in the extractive distillation column is 20% of the extractive raw material. The solvent is 85 wt% sulfolane + 15 wt% tetraethylene glycol monomethyl ether. The main operating conditions of each column are shown in Table 3, and the results are shown in Table 4. Using the method of the present invention, the yields of benzene and toluene are 99.8%, the yield of C8 aromatics is 99.8%, and the yield of C9+ aromatics is 99.0%; the purity of benzene reaches 99.99 wt%, the purity of toluene reaches 99.94 wt%, the purity of the C8 aromatics product reaches 99.1 wt%, and the purity of the C9+ aromatics reaches 99.2 wt%. By returning the toluene fraction, the bottom temperature of the extractive distillation column is 177 °C, and the purities of benzene, toluene, and C9+ aromatics are higher than those in Example 1.

[0056] Example 3

[0057] According to the Figure 1The process also has good effects on recovering aromatics from gasoline fractions with a distillation range of 60 - 150°C. The raw material has a benzene content of 2.5 wt% and a total aromatics content of 50.8 wt%. The detailed hydrocarbon composition is shown in Table 2. The proportion of benzene stripped from the extractive distillation column is 5% of the extractive raw material quantity. The solvent is sulfolane. The main operating conditions of each column are shown in Table 3, and the results are shown in Table 4. The yields of benzene, toluene, and C8 aromatics are 99.8%; the purity of benzene reaches 99.99 wt%, the purity of toluene reaches 99.92 wt%, the purity of the C8 aromatics product is 99.2 wt%, and the bottom temperature of the extractive distillation column is 177°C, with good separation effect.

[0058] Table 2 Composition of gasoline fraction

[0059]

[0060] Table 3 Main operating conditions

[0061]

[0062]

[0063]

[0064] Table 4 Separation results

[0065]

[0066]

[0067] Comparative Example 1

[0068] According to Figure 1 the process for recovering aromatics from gasoline fractions, the raw material is the same as that in Example 1. The detailed hydrocarbon composition is shown in Table 1. The solvent is sulfolane. There is no benzene or toluene stripping in the extractive distillation column. The main operating conditions of each column are shown in Table 5, and the results are shown in Table 4. The yield of benzene is 99.8%, the yield of toluene is 99.7%, the yield of C8 aromatics is 99.7%, and the yield of C9+ aromatics is 96.8%; the purity of benzene is 99.99 wt%, the purity of toluene is 99.85 wt%, the purity of the C8 aromatics product is 98.6 wt%, and the purity of C9+ aromatics is 98.8%. The bottom temperature of the extractive distillation column is 199°C.

[0069] Comparing with Example 1, it can be seen that the purity and yield of benzene change little, the purity of toluene drops by 0.08 wt%, failing to meet the requirement of 99.9 wt% for the first-class high-quality toluene in the national standard, the purity of C8 aromatics drops by 0.5 wt%, and the yield of C9+ aromatics drops by 1.8%. The bottom temperature of the extractive distillation column is close to 200°C, and the solvent is prone to decomposition, so the device cannot operate normally for a long time.

[0070] Comparative Example 2

[0071] According to Figure 1 The process for recovering aromatics from gasoline fraction is carried out with the raw material being the same as that in Example 1. The detailed hydrocarbon composition is shown in Table 1. The solvent is 85 wt% sulfolane + 15 wt% triethylene glycol monomethyl ether. There is no benzene or toluene stripping in the extractive distillation column. The main operating conditions of each column are shown in Table 5, and the results are shown in Table 4. The benzene yield is 99.8%, the toluene yield is 99.7%, the C8 aromatics yield is 99.7%, and the C9+ aromatics yield is 98.0%; the benzene purity is 99.99 wt%, the toluene purity is 99.87 wt%, the C8 aromatics product purity is 98.8 wt%, and the C9+ aromatics purity is 98.7%. The bottom temperature of the extractive distillation column is controlled at 198 °C.

[0072] Compared with Example 1, the toluene purity decreases by 0.06 wt%, the C8 aromatics purity decreases by 0.3 wt%, and the C9+ aromatics yield decreases by 0.6%. The bottom temperature of the extractive distillation column is close to 200 °C and it cannot operate normally. It can be seen that the stripping medium can effectively reduce the bottom temperature of the extractive distillation column by more than 20 °C, playing a role in protecting the solvent.

[0073] Compared with Comparative Example 1, in the sulfolane solvent, adding a cosolvent increases the C9+ aromatics yield by 1.2%, indicating that adding a cosolvent can effectively improve the solubility of C9+ aromatics and increase the C9+ aromatics yield.

[0074] Comparative Example 3

[0075] According to Figure 1 The process for recovering aromatics from gasoline fraction is carried out with the raw material being the same as that in Example 3. The detailed hydrocarbon composition is shown in Table 1. The solvent is sulfolane. There is no benzene stripping in the extractive distillation column. The main operating conditions of each column are shown in Table 5, and the results are shown in Table 4. The yields of benzene, toluene, and C8 aromatics are 99.8%; the benzene purity reaches 99.99 wt%, the toluene purity reaches 99.92 wt%, the C8 aromatics product purity is 99.1 wt%, and the bottom temperature of the extractive distillation column is 185 °C.

[0076] Compared with Example 3, the product yield and purity in Comparative Example 3 change little, but without benzene stripping, the bottom temperature of the extractive distillation column exceeds 180 °C, and it is impossible to protect the solvent from decomposition, and the device is difficult to operate in a long cycle.

[0077] Table 5 Main operating conditions

[0078]

[0079]

[0080] The conditions of the solvent recovery column, water washing column, water vapor stripping column, benzene column, toluene column, and C8 aromatics column are the same as those in Table 2.

[0081] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", "front", "rear", "left", "right", etc. is the orientation or positional relationship based on the working state of the present invention. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0082] The present invention has been described above in conjunction with preferred embodiments, but these embodiments are merely exemplary and only serve an illustrative purpose. On this basis, various substitutions and improvements can be made to the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for recovering aromatic hydrocarbons from a wide-boiling gasoline with a low content of light aromatic hydrocarbons, characterized in that, It includes the following steps: (1) The wide-cut gasoline feedstock enters the liquid-liquid extraction column from the middle and lower parts, and the solvent enters the liquid-liquid extraction column from the upper part. The two contact countercurrently. Raffinate oil free of aromatics is obtained at the top of the column, and the first rich solvent is obtained at the bottom of the column. The solvent includes the main solvent sulfolane; (2) The first rich solvent enters the upper part of the extractive distillation column, and the second rich solvent is obtained at the bottom of the column; (3) The second rich solvent enters the solvent recovery column, mixed aromatics are obtained at the top of the column, and the lean solvent is obtained at the bottom of the column; (4) The mixed aromatics enter the aromatics rectification section, and benzene, toluene, xylene products and C9+ aromatics are obtained through the benzene column, toluene column and xylene column respectively; Among them, benzene products are withdrawn from the side line of the benzene column, and the benzene obtained at the top of the column is used as stripping benzene and returned to the extractive distillation column as a purification and stripping medium; or A part of the toluene obtained at the top of the toluene column is used as stripping toluene and returned to the extractive distillation column as a purification and stripping medium.

2. The method according to claim 1, characterized in that The raffinate oil free of aromatics obtained at the top of the liquid-liquid extraction column enters the water washing column, is sent out of the device after removing the solvent as the raffinate oil free of aromatics, and the washed water enters the water stripping column.

3. The method according to claim 1, wherein The gasoline fraction containing light aromatics obtained at the top of the extractive distillation column is sent to the liquid-liquid extraction column as the return washing liquid of the liquid-liquid extraction column.

4. The method according to claim 1, wherein The solvent further includes a co-solvent selected from one or more of 1,3-dimethyl-2-imidazolidinone, methyl sulfolane, dimethyl sulfolane and polyglycol ethers.

5. The method according to claim 4, wherein The polyglycol ethers are selected from one or more of triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether and pentaethylene glycol monomethyl ether.

6. The method according to claim 1, wherein The concentration of the main solvent in the solvent is 70-100 wt%.

7. The method according to claim 1, wherein The solvent contains 0.5-3.0 wt% of water and / or 0-2.0 wt% of C9+ hydrocarbons.

8. The method according to claim 1, characterized in that, The top pressure of the liquid-liquid extraction column is 0.3-1.2 MPaG, the bottom temperature is 60-180 °C, and the mass ratio of the solvent to the wide-cut gasoline feedstock is 1-6:

1.

9. The method according to claim 3, characterized in that, The mass ratio of the water washing water entering the water washing column to the raffinate oil is 0.1-0.5:

1.

10. The method according to claim 1, characterized in that, The number of theoretical plates of the extractive distillation column is 10-30, the top pressure is -0.05-0.15 MPaG, and the bottom temperature is 150-180 °C.

11. The method according to claim 1, wherein The number of theoretical plates of the solvent recovery column is 12-30, the reflux ratio is 0.3-1.0, the absolute pressure at the top of the column is 10-80 kPa, and the bottom temperature is 160-180 °C.

12. The method according to claim 1, wherein The mass ratio of the stripping benzene to the wide-cut gasoline feedstock is 0.01-0.3:

1.

13. The method according to claim 1, wherein The mass ratio of the stripping toluene to the wide-cut gasoline feedstock is 0.01-0.25:

1.

14. The method according to claim 1, characterized in that, The wide-cut gasoline feedstock is a gasoline fraction with a dry point within 180 °C, in which the benzene content is 0.2-7 wt% and the total aromatics content is greater than 30 wt%.

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