GB2396610A

GB2396610A - Composite solvent comprising a sulfolane derivative, N-formylmorpholine or N-methylpyrrolidone, & C8-11 aromatic for use in extractive distillation

Info

Publication number: GB2396610A
Application number: GB0230056A
Authority: GB
Inventors: Longsheng Tian; Wencheng Tang; Ming Zhao; Yunming Zhang; Zhifeng Bian; Huixia Sun; Shijin Wu
Original assignee: Sinopec Research Institute of Petroleum Processing; China Petroleum and Chemical Corp
Current assignee: Sinopec Research Institute of Petroleum Processing; China Petroleum and Chemical Corp
Priority date: 2002-12-23
Filing date: 2002-12-23
Publication date: 2004-06-30
Anticipated expiration: 2022-12-23
Also published as: GB0230056D0; GB2396610B

Abstract

A composite solvent, for separating aromatics from hydrocarbon mixtures by extractive distillation, comprises a main solvent, 3-39wt% of a solutizer and conditionally a modifier, wherein <SL> <LI>(a) the solutizer is selected from any one of C8-C11 aromatics and the composite solvent contains from 0.01 to 10.0 wt% of a modifier, or <LI>(b) the solutizer is selected from mixtures of any two of C8-C11 aromatics having different numbers of carbon atoms and the composite solvent contains from 0 to 10.0wt% of a modifier, </SL> and wherein one of the main solvent and modifier is a sulfolane derivative, and the other is N-formyl morpholine, or N-methyl pyrrolidone, and the sulfolane derivatives have the following formula: <EMI ID=1.1 HE=33 WI=42 LX=806 LY=1168 TI=CF> <PC>wherein each of R1, R2, R3 and R4 is independently selected from H, C1-C10 alkyl, C7-C12 arylalkyl and C1-C8 alkoxy. The number of carbon atoms of the lowest aromatic in the solutizer should be greater than that of the highest aromatic in the aromatics to be separated. When the composite solvent is used to recover aromatics by extractive distillation, moderate operating conditions may be used to recover the solvent, and a higher yield of aromatics is obtained. By using a modifier whose acidity-basicity balance is opposite to that of the main solvent, the separated aromatics that are obtained are pH neutral. The use of water in the composite solvent or the extractive distillation process is avoided.

Description

GB 2396610 A continuation (74) Agent and/or Address for Service: Gill

Jennings & Every Broadgate House, 7 Eldon Street, LONDON, EC2M 7LH, United Kingdom

A PROCESS FOR SEPARATING AROMATICS BY EXTRACTIVE

DISTILLATION AND A COMPOSITE SOLVENT USED THEREIN

Technical field

s The present invention relates to a composite solvent for separating aromatics from hydrocarbon mixtures by extractive distillation.

Background Art

Most of the processes for separating aromatics Mom hydrocarbon mixtures such as catalytically reformed gasoline or hydrogenated pyrolysis lo gasoline ate liquid-liquid exaction and extractire distillation. which the most commonly used selective solvents in liquid-liquid extraction arc glycerols or sulfolane derivatives. Although the liquid-liquid extraction process can obtain benzene, toluene, and Clew (BIX) product5having high purity with a high yield, the prior liquid-liquid extraction processes are only Is suitable for treating feedstocks containing 30-70 wt% of aromatics When Heating feedstocks contusing more Han 70 wt%, aromatics, raffinate oil has to be added; thereby, more energy is consumed and the efficiency of the device is lowered.

Extractive distillation is a process for separating aromatics from to hydrocarbon mixtures which takes advantage of the solvent having different effects on the relative volatility of various components in hydrocarbon mixture. There is no need to add raffinate oil into hydrocarbon mixtures when feedstocks containing high content of aromatics are treated.

The solvent used is generally pure N-formyl mopholine, N-mcyl 25 pyrrolidone, or sulfolane, etc. For example, US 5,310,480 disclosed a process for separating aromatics by extractive distillation, which uses only one column containing a reboiler and a cyclone to separate aromatics from

non-aromatiCs in Me feedstocl<. The upper section of said column is used for extractive distillation, and the lower section is used for recovering solvent.

order to enhance the selectivity and moderate the severity in solvent recovery, 0.1-20 wt% of water is added into the selective solvents such as s sulfolane derivatives, glycerols, or glycerol ethers, while the procedure of solvent recovery is performed by means of steam stripping. Said patent teaches that the selective solvent for aromatics may be a composite solvent, wherein one component is a polyalkylene glycerol having low molecular weight, and another component is a glycerol ether, and the mutual solubility lo of the two components is 0.1-99 wt% The aromatics obtained Mom this exactive distillation process cannot directly serve as a product for containing water, due to the water-contning solvent and the steam stripping used therein; therefore, fumier distillation is needed. Two liquid phases are readily formed when a feedstock with a low content of aromatics Is is subjected to extractive distillation due to the restriction of Me solubility in Me water- contairung solvent, which thereby affect the normal operation. In addition, the water contained in the system tends to cause corrosion of Me device. CNl262264A disclosed a combined process of exhact-ive distillation and liquid-liqwd extraction, which pre-dishls, Me feedstock to yield a first fraction containing benzene and a second Faction containing small amount of benzene. The first firachon is subjected to exactive distillation to yield the benzene product,. and Me second fraction is mixed with the raffinate obtained Dom extractive distillation and Men is subjected to liquid-liquid 25 extraction to obtain toluene and a small amount of benzene. This process has less restriction on the feedstock, broader applicability, lower energy

consumption, and can recover BTX simultaneously, which is therefore

suitable for expanding and reforming of the existing liquid-liquid extraction devices in the refinery. In said patent, 0.1-15 vat% of C8-C,O aromatics are added into the selective solvent for separating benzene by efflactive distillation as a solutizer to increase He solubility of the solvent, which thereby avoids the adverse effect of water on the recover of benzene, since both the solvent and the system contain no water. However, said invention only puts the situation where o-xylene serves as a solutizer into practice, and is limited within the extractive distillation of a combined process.

In the prior processes of using a single extractive distillation solvent, the lo temperature of the recovery column cannot be too high due to the restriction of the decomposition temperature of the solvent during the recovery of solvent; therefore, He level of vacuum should be raised to ensure a Borough separation of the solvent and aromatics in the rich solvent; as a result, the operation seventy of the recovery column is relatively high. Besides, the Is extractive distillation solvent often contains 0, N. or S atoms since a certain polarity is required to increase the selective solubility of aromatics; therefore, the distillation solvent teas certain weak acidity or basicity, which unpans the aromatic product has-tag corresponding weak acidity or basicity.

Therefore, the product thus obtained has to be subjected to a post treatment 20 to neutralize the acidity or basicity.

Summary of the Invention

The object of the present invention is to provide a composite solvent well in aromatics separation by extractive distillation. Such a composite solvent has a higher solubility and relatively wide boiling range so that the operation 25 conditions solvent recovery c" be moderated Ed the energy consumption is reduced.

Description of Figure

Fig. 1 is a schematic process flow diagram of the exhactive distillation process using the composite solvent according to the present invention.

Detailed Description of the Invention

lathe composite solvent according to Me present invention comprises a main solvent, a solutizer and optionally a modifier. Said solutizer is selected Dom any one or a mixture of any two of C-C,, aromatics having different number of carbon atoms, the content of which is 3-39 wt%; the number of carbon atoms of the lowest aromatic in We solutizer should be greater than that of We highest aromatic in the aromatics to be separated. When the solution is lo selected Tom any one of C-C aromatics, the composite solvent contains 0.01-10.0 At% of the modifier; and when We solutizer is selected from mixtures of any two of Cg-C, aromatics having different number of carbon atoms, the composite solvent contains 0-10.0 wt% of the modifier. Said main solvent and modifier are independently selected from sulfolare Is derivatives, N-forml moTpholine, and N-methyl pyrolidone, provided that the acidity and basicity of the modifier are opposite to those of the main solvent. The main solvent and modifier can be selected Tom sulfolane derivatives, N-fotmyl molpholine, and N-methyl pyIrolidoDe respectively. Said zo sulfolane derivatives have the following fonnula: / Al -;li -

R - UTAH-R3

Wherein, each of R1, R2, R3, R4 is independently selected Dom H. Cl-C'O alkyl, C-C12 alylallcyl, Cl-C8 aLkox$. The preferred solvent is sulfolane, 2 ethyl sulfolane or 2, 4-dimethyl sulfolane.

The solutmer in the composite solvent according to the present invention can increase the solubiliW. The selection thereof depends on the composition of He aromatics to be separated; that is, the solutizer should have a structure similar to that of the aromatics to be separated so as to increase He solubility 5 of the aromatics. However, He solutizer should also differ from the aromatics to be separated so as to facilitate recovery of the solvent.

Generally, all of the aromatic-contng feedstocks contain benzene and toluene; therefore, the solutizer can be selected from any one or mixtures of any two of C'-C', aromatics. However, when the feedstock contains xylem, lo He soluimer should be selected Mom aromatics higher than C,, preferably C9 aromatics or mixtures of C, and CIO aromatics.

Addition of the modifier into the composite solvent can solve the deviation of the separated aromatics from neutrality caused by He main solvent. For instance, when sulfolane is used as a main solvent, the Is separated aromatic product is also weakly acidic as sulfolane is weakly acidic per so, therefore a post treatment of the product Is needed to neutralize its acidity. For the same reason, when weakly basic N-formyl morpholine and N-methyl pyolidone is used as the main solvent, the separated product is also weakly basic; therefore, an acid treatment of the zo product is needed to neutralize basicity thereof. If an extraction solvent having an acdity-basicity opposite to that of He main solvent is added into the composite solvent as a modifier, the separated aromatic product is allowed to be neutral so that the post treatment can be avoided. Specifically speaking, when sulfolane serves as the main solvent, the modifier is preferably selected as Dom N-formyl morpholine and Nmethyl pyTrolidone; when N-formyl morpholine or N-methyl pyrrolidone serve as He main solvent, die modifier .. is preferably sulfolane.

l When the solutizer is selected Mom any one of C-C aromatics, said aromatics refers to one of the aromatics baking the same number of carbon atoms or a moisture of isomers such as C8 aromatics including o-xylene, m-

xylene, p-xylene, ethyl benzene or mixed xylene, C9 aromatics including s isopropyl benzene, kimeyl benzene. The solutizer content in the composite solvent is preferably 5-30 urt /O, and the modifier content is preferably 01-5.0 alit%.

When the soluiizer is selected from mixtures of any two of C8-C, aromatics having different number of carbon atoms, the solutizer content in lo the composite solvent is preferably 5-30 urt /O, and the modifier content is preferably 0-5.0 wt%, more preferably 0.1-5 0 wt%. The mixtures of any two of Cafe, aromatics having different number of carbon atoms in said solutizer are preferably the nuxtures of Cat,, alla Cane aromatics, wherein n is 0, 1, or 2. The C'+n/C9+,' aromatic weight ratio is 1-50;1, preferably 1-20:1, is and more preferably 1.5-15.0:1.

The composite solvent according to the present invention is suitable for the extractve distillation process. Suitable feedstocks for effective distillation are the hydrogenated pyrolysis gasoline Factions reformed oil, coal tar oil, in which the aromatic contents are 15-99 wt%. The feedstocks 20 are preferably hydrocarbon mixtures containing one or two aromatics, such as the feedstocks containing benzene, benzene and toluene, or toluene and xylene. The process for separating aromatics by extractive dishllabon using the composite solvent according to the present invention comprises: 25 introducing the composite solvent to the upper part of an extractive distillation column, introducing the hydrocarbon mixture to We middle of the column,

withdrawing the non-aromatics Mom the top of the extractive distillation column, introducing the rich solvent containing aromatics from the bottom of the extraction column to the middle of a solvent recovery column to separate 5 aromatics and the composite solvent, withdrawing Me aromatics thus obtained from Me top of the recovery column, and introducing the composite solvent withdrawn from Me bottom of the solvent recovery column into the top of exit active distillation column for to recycling.

The operation conditions of the extractive distillation column in the above extractive distillation procedure are: the top pressure is 0.10-0. 30 MPa, the inlet temperature of the lean solvent is 80-130 C, the bottom temperature is 130-180CC, the solvent/feed weight ratio, i.e. the ratio of 5 solvent is 3.0-7.0, the reflex ratio is 0.1-3.0, and the theoretical plate number is 10-40; the top pressure of the recovery column is 0.04-0. 10 MPa, the inlet temperature of the rich solvent is 125-175 C, the bottom temperature is 160 1 90 C, the reflex ratio is 0.5-2 5, and the theoretical plate number is 10-25.

As an example, the process flow whit the separation of benzene by 20 extractive distillation will be described below referring to Fig. 1. The fieedstock for extractive distillation enters heat exchanger 3 via line 1, then enters the middle of extractive distillation column 4 aider heat exchanging to a certain temperature. The composite solvent enters the emotive distillation column Mom the upper part via line 2. By extractive distillation, :5 the non-aromatics are withdrawn from the top via line 5 and enter condenser 6. Pacer condensating and cooling, the non-aromatics enter reflex tank 7, a part of which is reflexed into extractive distillation column 4 via line 9, and

the other part leaves the device as rabbinate oil via line 8. The rich solvent from the bottom of the extractive distillation column 4 enters solvent recovery column 12 via line l1. The gas stream at the top of solvent recovery column 12 enters condenser 14 via line 13 for condensation, and s enters condensate tank 16 ader cooling. Apart of condensate is reflexed into the solvent recovery column via line 18, the other part leaves the device as a benzene product via line 11. The top of condensate tank 16 is connected to vacuum system 15. The lean solvent obtained from the bottom of solvent recovery column 12 enters heat exchanger 3 via line 20 and recycle to 0 extractive distillation column 4 after heat exchange. Both exactire distillation column 4 and solvent recovery column 12 are equipped with reboilers 10 and l9 at the bottom.

In case of the simultaneous recovery of benzene/toluene by extractive distillation, the feedstock is C6-C7 Faction and the flow is the same as the above except that a mixture of benzene/toluene is obtained from the top of the recovery column. Pure benzene and pure toluene can be obeyed by farther rectification separation of Me mixture. In case of separation of xylene Mom Cx Daction, the operation is the same as that for the recovery of benzene except that the solutizer used is the aromatics hider than C,. In 2c case of the simultaneous recovery of toluene/xylene, the operation is the same as that for the simultaneous recovery of benzene/toluene, except that the solutizer used is the aromatics higher than C, lee composite solvent according to the present invention significantly enhances the solubility of the composite solvent owing to the addition of the Z5 solutizer, enabling the composite solvent to}save sufficiently high recovery rate when used to recover aromatics from the feedstock with low aromatic content and thereby enlarges the Applicable area of the device. The addition

of the solutizer can also greatly moderate the operation conditions of the solvent recovery column, which can thereby reduce Me adverse effect of We impurities such as sulfur, nitrogen, oxygen produced by the thermal decomposition of the solvent on the quality of the product. In addition, the s addition of Me modifier can ensure the neutrality of the aromatic products and thereby the post treatment can be omitted.

The composite solvent according to the present invention contains no water and the solvent recovery column does not use stripping water either, so that the aromatic product can be obtained directly without tle water lo separation device, therefore, not only Me cost and energy can be saved, but also the corrosion of the device by water can be avoided.

The following Examples further illustrate this invendon.

EXAMPLE I

Benzene was recovered from the benzene Faction in hydrogenated Is pyrolysis gasoline. The composition of the feedstock used is shown in Table 2. The composite solvent used consisted of sulfolane as the main solvent, o xylene and isopropyl benzene as the solutizers, and N-formyl morpholine as the modifier. The contents of various components in the composite solvent are shown in Table 1. The process flow in Fig. 1 was used. The operation 20 conditions of the extractive distillation column and the solvent recovery column are shown in Table 3. The results on the aromatic separation are shown in Table 4 EXAMPLE 2

as A benzene product was recovered from C6 fraction of reformed oil through the process flow in Fig. 1. The main solvent of the composite solvent was sulfolane, the solutizers revere o-xylene and isopropyl benzene,

and the modifier was N-methyl pyrrolidone. The composition of the composite solvent is shown in Table 1. The composition of the feedstock is shown in Table 2. The operation conditions of the extractive column and the solvent recover column are shown in Table 3 The results on the aromatic 5 separation are shown in Table 4.

Tables 3 and 4 show that We extractive distillation of the reformed oil with lower aromatic content using the composite solvent according to the present invention can not only be steadily operated and controlled under mild conditions but also produce the benzene product having a purity of to 99.99 /O, and a sulfur content of 0.2 ppm with a high recovery rate of 99.6%.

The content of aromatics in the raffinate was as low as 0.32%.

EXAMPLE 3

Benzene was recovered from the benzene Daction in hydrogenated pyrolysis gasoline. The applied feedstock and operation conditions were Is similar to those in Example 1, except that the main solvent in the composite solvent was sulfolane, the solutizers were o-xylene and isopropyl benzene, and no modifier was added. The composition of the composite solvent is shown in Table 1. The results on the aromatic separation are shown Table 4. EXAMPLE; 4

Benzene was recovered from Me benzene Faction of hydrogenated pyrolysis gasoline. The feedstock and operation conditions used were similar to those in Example 1, except that the main solvent in Me composite Is solvent was sulfolane, the solutizer was o-xylene and the modifier was N forroyl morpholine. The composition of the composite solvent is shown in Table 1 The results on Me aromatic separation are shown in Table 4.

COlvIPARATIVE EXAMPLE Benzene was recovered from the benzene Faction of hydrogenated pyrolysis gasoline. The feedstock and operation conditions used were s similar to Example 1, except that the composite solvent described in CN1262264A with sulfolane as the main solvent and o-xylene as the solutizer was used. The composition of the composite solvent is shown in Table 1. The operation conditions of the extractive distillation column and the solvent recovery column are shown in Table 3. The results on the lo aromatic separation are shown in Table 4.

It can be seen from the data of Example 3 and the Comparative Example shown in Table 4 that compared to the composite solvent containing only one solutizing conponent according to CN1262264A, the composite solvent Is containing hero solutizg components according to the present invention could enhance the recovery rate of the benzene product under We same operation parameters. The data of Example 4 show that the addition of the modifier into the composite solvent could reduce the content of sulfur in the benzene product and meet the requirement for neutrality. The data of 20 Examples 1 and 2 show that the composite solvent containing two solutizers and a nodifer offer better effects, which could not only enhance the recovery rate of aromatics but also reduce the content of sailer, and allow the benzene product to be neutral. The Lace amount of acidic substances in the neutral benzene product may not be removed from the neutral benzene 25 product by post treatment, thereby the procedure is saved.

EXAMPLE 5

Benzene and toluene were simultaneously recovered Dom the C6-C fiaction of hydrogenated pyrolysis gasoline. The process flow shown in Fig. 1 was employed for extractive distillation and the obtained mixture of benzene/toluene entered the rectification column and converted to the s benzene and toluene products through rectification. The main solvent in the composite solvent used in the extractive distillation column was Nformyl morpholine, the solutizers were o-xylene and isopropyl benzene, and the modifier was sulfolane. The composition of the composite solvent is shown in Table 1. The composition of the feedstoclc is shown in Table 2. The lo operation conditions of the exhactive distillation column and the solvent recovery column are shown in Table 3. The results on the aromatic separation are shown in Table 5.

It can be seen Dom the data of Tables 3 and 5 that mixed aromatics benzene/toluene having a purity of 99.9% could be obtained under relatively Is mild conditions; a top residual pressure of 0.045 MPa, a top temperature of 72 C, and a bottom temperature of 186 C in the solvent recovery column, and the product thus obtained was neutral. The benzene product and toluene product meet the requirement of high quality standard of GB3405-89 and GB3406-90 respectively. If pure N-formyl morpholine is used as the solvent zo for extractve distillation, Me residual pressure of the recovery column should be 0.038 MPa and the bottom temperature should be 201 C to obtain the same purity of the aromatics and the same recovery rate, and the content of nitrogen in the mixed aromatics will be as high as 2-3 ppm, which will lead to a much higher content of nitrogen in the toluene product and exhibit 25 a relatively high biciW. Thus, a post treatment such as adoptive purification of Me product with clay has to be employed so as to produce a neural product.

Table 1

\ Conposition of Main Solutizer, Modifier, \ composite solvent, wt% wt% \olvent wt% Exmple No \ Sulfolane, o-xylene, Isopropyl N-formyl 1 84.0 11. 0 benzene, morpholine, 4.0 10

. .. Sulfolane, o-xylene, Isopropyl N-methyl 2 75.0 17.0 benzene, pyrolidone _ _ 5 0 3.0

Sulfolane, o-xSrtene, Isopropyl _ 3 85.0 14.0 benzene, _ _ 1.0

Sulfolane, o-xylene, N-fomyl 4 84.0 15.8 morpholine, _ _ _ _ 0.2

N-fomyl xed Isopropyl Sulfolane, 5 mopholin, xylene benzene, _ 86 0 6.0 3. 5 4.5

Comparative Example Sulfolane, o-xylene, _ _ 94.0 6.0

Table 2

Composition of Example 1 lxample 2 j Example 5 feedstock, wt% _.

AIkanes C5 _ 3.56 0.44

C6 4.45 35.40 3 89

Cyclane I_194 _ 16.99 _3.02 Cs 1.51 0.33 C6 9.20 2.85 8.24

C7 2.27 0.88 3.34

MCH 0.83._

Aromatcs C6 79.80 39.88 51.52

C7 _ 0.01 1 _ 29.55

Total 100.00 100.00 100.00

Table 3

Items Example Example Example Comparative 1 2 5 Ex. Theoretical plate number 29 25 3 8 29 c Solvent ratio 4.5 4.0 5.5 4.5 :>. Inlet solvent 90 90 I 15 90 w temperature, TIC Top reflex ratio 0.3 0.3 0.7 0.3 I, Top pressure, MPa 0.1 0.1 0.15 0.1 B_tom temperature, C 132 138 175 130 Theoretical plate number 16 16 16 16 Inlet solvent 131 136 174 130 o temperature, C Top reflux ratio 1.0 1.5 1.5 1. 0 c; Top pressure, MPa 0.045 0.08 0.045 0.045 o Top temperature, C 56 72 72 56 v, Bottom temperature, C 175 170 186 175 Table 4

Items Example 1 Example 2 Example 3 Example 4 Com. Ex.

Benzene product Purity, wt% 99.97 99.99 99.97 99.97 99.97 Non-aromatics, ppm 300 100 290 300 300 Sulfur, ppm 0.2 0.2 0.5 0.3 1 Crystallization point, 5,49 5.50 5.49 5.49 5.49 C Neutrality test Neutral Neutral Weak acid Neutral Weak acid Recovery rate, wt% 99.86 99.60_ 99.87 99.88 99.66 Non-aromatic product Aromatics, wt% 0.54 0.32 0.51 0.47 1.33 Solvent, ppm <1 <1 <1 Al <1 s Table 5

Items ExamDle 5 .. Aromatic product (benzene+toluene) Purity, wt% 99.90 Non-aromatics, ppm logo Nitrogen, ppn1 <0.5 Neutrality test Neutral Recover ram, wt% _ 99.6 Non-aromatic product Aromatics, we% 0.65 Solvent, ppm

Claims

1. A composite solvent for separating aromatics from hydrocarbon mixtures by extractive distillation, comprising 5 a main solvent and a solutizer, wherein said solutizer is selected from any one or mixtures of any two of Ca-C1l aromatics having different number of carbon atoms, the content of which solutizer is 3 to 39 wt%, wherein the number of carbon atoms of the lowest 10 aromatic in the solutizer is greater than that of the highest aromatic in the aromatics to be separated, and wherein when the solutizer is selected from any one of Ca-C1l aromatics, the composite solvent contains from 0.01 to 10.0 wt% of a modifier, 15 and wherein when the solutizer is selected from mixtures of any two of Ca-C1l aromatics having different numbers of carbon atoms, the composite solvent contains from O to lO.0wt% of a modifier, and wherein said main solvent and modifier are 20 independently selected from sulfolane derivatives, N-formyl morpholine, and N-methyl pyrrolidone, provided that the acidity and basicity of the modifier are opposite to those of the main solvent, and said sulfolane derivatives have the following formula: outdo -,1 -R3 wherein each of R1, R2, R3 and R4 is independently 30 selected from H. C1-ClO alkyl, C7-C12 arylalkyl and C1-Ca alkoxy.

2. The composite solvent according to claim 1 wherein the content of said solutizer is 5-30 wt% and the content of said modifier is 0.1-5.0 wt% when said solutizer is 35 selected from any one of Ca-C1l aromatics.

3. The composite solvent according to claim 1 wherein the content of said solutizer is 5-30 wt% and the content of

i 1V said modifier is 0.01-5.0 wt% when the solutizer is selected from mixtures of any two of Ca-C1l aromatics having different number of carbon atoms.

4. The composite Solvent according to claim 1 or 3 S wherein said solutizer is a mixture of C8+n and Cg+n aromatics, and the weight ratio of C8+n/Cg+n aromatic is 1 to 20:1, wherein n is 0, 1 or 2.

5. The composite solvent according to claim 4 wherein the weight ratio of Ca+n/Cg+,, aromatic is 1.5 to 15.0:1.

10

6. The composite solvent according to any one of claims 1 to 3 wherein the main solvent is sulfolane and the modifier is selected from N-formyl morpholine and N-methyl pyrrolidone.

7. The composite solvent according to any one of claims 15 1 to 3 wherein the main solvent is selected from N-formyl morpholine and N-methyl pyrrolidone and the modifier is sulfolane.

8. A process for separating aromatics from a hydrocarbon mixture, comprising: 20 introducing a composite solvent according to any one of claims 1-3 to the upper part of an extractive distillation column; introducing the hydrocarbon mixture to the middle of the column; 25 withdrawing non-aromatics from the top of the extractive distillation column; withdrawing rich solvent containing aromatics from the bottom of the extractive distillation column and introducing the rich solvent to the middle of a solvent 30 recovery column in order to separate aromatics and the composite solvent; withdrawing the aromatics obtained from the top of the recovery column; and.

withdrawing composite solvent from the bottom of the 35 solvent recovery column and introducing it into the top of the extractive distillation column for recycling.

9. The process according to claim 8 wherein the top pressure of said extractive distillation is 0.10 to 0.30 MPa, the inlet temperature of the composite solvent is 80 to 130 C, the bottom temperature is 130 to 180 C, and the 5 solvent ratio is 3.0 to 7.0; the top pressure of the recovery column is 0.04 to 0.10 MPa, the inlet temperature of the rich solvent is 125 to 175 C, and the bottom temperature is 160 to 190 C.

10. The process according to claim 8 wherein said 10 aromatics are benzene, a mixture of benzene and toluene, or a mixture of toluene and xylene.

11. A composite solvent for separating aromatics from hydrocarbon mixtures by extractive distillation, the solvent being substantially as described herein with 15 reference to any one of Examples 1 to 5.

12. A process for separating aromatics from a hydrocarbon mixture substantially as described herein with reference to any one of Examples 1 to 5.