US3220933A

US3220933A - Separation of c9 aromatic hydrocarbons

Info

Publication number: US3220933A
Application number: US357599A
Authority: US
Inventors: Emanuel M Amir; William R Edwards
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1964-04-06
Filing date: 1964-04-06
Publication date: 1965-11-30
Anticipated expiration: 1982-11-30

Description

Nov. 30, 1965 E. M. AMlR ETAL 3,220,933

SEPARATION OF C AROMATIC HYDROCARBONS Filed April 6, 1964 LlGHT COMPONENT MAKE-UP SOLVENT EXTRACTIVE -/-DIST|LLAT|ON ZONE HEAVY COMPONENT M-AND P-ETHYLTOLUENE O-ET HY LTOLU ENE MESITYLENE PSEU DOCUM EN E FIG. 2. AND HEAVIER MA KE-UP INVENTORS- SOLVENT 4B EMANUEL M- AMIR,

WILLIAM R.EDWARDS ATTORNEY.

United States Patent 3,220,933 SEPARATION 013 C AROMATIC HYDROCARBONS Emanuel M. Amir, Baytown, Tex., and William R. Edwards, Mountainside, N.J., assignors, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, N ..l., a corporation of Delaware Filed Apr. 6, 1964, Ser. No. 357,599 6 Claims. (Cl. 20239.5)

This application is a continuation-in-part of Serial No. 38,067, filed June 22, 1960, entitled, Separation of C Aromatic Hydrocarbons, for Emanuel M. Amir and William R. Edwards, now abandoned.

The present invention is directed to a method for separating aromatic hydrocarbons. More particularly, the invention is concerned with the extractive distillation of C aromatic hydrocarbons. In its more specific aspects, the invention is concerned with separating C aromatic hydrocarbons under vapor-liquid contacting conditions with the polyalkyl esters of an aromatic polycarboxylic acid having adjacent carboxylic groups.

Some 0,, isomers of aromatic hydrocarbons boil so closely together that it has been extremely diflicult, if not impossible, from a practical viewpoint to separate these particular aromatic hydrocarbons. An expensive superfractionation of the isomers can result in separating the isomers into a mand p-ethyltoluene fraction and a mesitylene, o-ethyltoluene fraction. However, the desired C isomers are mesitylene and o-cthyltoluene as separate fractions. In accordance with the present invention, these C isomers of aromatic hydrocarbons are separated by extractive distillation techniques even though their respective boiling points are very close together as illustrated in Table I below.

Table 1 BR, F. m-Ethyltoluene 322.4 p-Ethyltoluene 323.6 Mesitylene 328.5 o-Ethyltoluene 329.3

It is noted from Table I that the boiling points of these isomers are within a few degrees Fahrenheit of each other. The boiling point difference between mesitylene and oethytoluene is less than one degree Fahrenheit. Using the extractive distillation techniques, which will be more fully described hereinafter, an enrichment of the mesitylene fraction and the o-ethytoluene fraction can be made. Furthermore, according to another aspect of the present invention, separation of the mand p-ethyltoluene can be made from the mesitylene by an easy and practical extractive distillation. Throughout the description of the present invention, the designation mand p-ethyltoluene is to be considered as a mixture of the isomers recovered as a single, separate fraction.

In accordance with one aspect of the present invention, an extractive distillation solvent has been found which will separate o-ethyltoluene from the lower boiling C aromatic hydrocarbons consisting of a mixture of mand p-ethyltoluene, mesitylene and o-ethyltoluene. It has been found that if a mixture of the C aromatic hydrocarbons is contacted with a polyalkyl ester of an aromatic polycarboxylic acid having adjacent carboxylic groups under vapor-liquid contacting conditions to form an overhead Table II -OOOR -C O O R dlalkyl-o-phthalate C O O R COOR dla1ky1-3-nitro-o-phthalate -C 0 O R OzN COOR dia1ky1-4-nltro-o-phthalate C O O R COOR trialkyl hemlmellitate COOR ROOC COOR trialkyl trlmellitate ROOC COOR ROOC COOR tetraalkyl pyromellitate where R is a C to C alkyl group and preferably a C te. C alkyl group.

In separating o-ethyltolucne in the extractive distillation process, the desirable polyalkyl esters of aromatic acids are di'methyl-o-phthalate, dipropyl-3-nitrophthalate, tripropyl hemimellitate, timethyl trimellitate, and tetran-p-ropylpyromellitate and the like. It was further found in separating o-et-hyltoluene in the extractive distillation process that those compounds having three .adjacent substitutions on the aromatic ring were extractive solvents of a different order of magnitude than those having two adjacent substitutions. Thus, the dialkyl-3- nitro-o-phthalates and trialkyl hemimellitates are superior to dialkyl-4-nitro-o-phthalates and triallryl trimellitates, respectively. The solvent-to hydrocarbon ratios used in the separation of o-ethyltoluene from the lower boiling C aromatic hydrocarbons may suitably range from 1:3 to 10:1 under vapor-liquid extractive distillation con ditions.

The present invention is considered to be the finding of suitable solvents to separate the isomers of C aromatic hydrocarbons. The extractive distillation techniques used in the separation of the C isomers are those known to persons skilled in the art of carrying out extractive distillation operations. Thus, as is known in carrying out extractive distillations, the solvent used in an extractive distillation process must have a higher boiling point than the highest boiling product which is to be obtained in a separate fraction. In addition, there must be complete miscibility between the solvent and the materials being separated at the temperatures existing in the extractive distillation tower. It has been found, according to the present invention, that in the extractive distillation of the C isomers the polyalkyl esters of the polycarboxylic acids alter the vapor-pressure characteristics of the C aromatic hydrocarbons so that the. relative volatilities are altered. Thus, the relative volatilities are altered to the extent that in the presence of the polyalkyl esters of the polycarboxylic acids having adjacent carboxylic groups the isomers may be separated into enriched fractions by using the techniques used in extractive distillation which are similar to that of a regular distillation. The operating conditions in a regular distillation are such that the temperature is dependent upon variables governed mainly by the pressure and the boiling point of the materials being separated. However, in an extractive distillation, such as in the extractive distillation of C aromatics, the key to the extractive distillation is the specific solvent such as the particular polyalkyl ester of the polycarboxylic acid used since it is the choice of solvent which alters the relative volatilities of the close boiling materials which enables the separation in the first place. The ratio of the particular solvent to hydrocarbon employed, and in a minor extent the engineering factors to be used in a specific separation, has an eifect on the conditions; but these elTec-ts are known to those skilled in the art. The temperature per se, of course, is not critical to the separation; however, it is obvious that a temperature above the boiling point of the close boiling materials must be used to obtain an overhead fraction and the vapor-liquid conditions. The pressure employed is normally atmospheric, but a superatmospheric or subatmospheric pressure may be used in a particular operation since pressure, as such, is not critical to the separation but merely changes operating conditions.

It was further found in accordance with another aspect of the invention that mand p-ethyltoluene could be separated from mesitylene by extractive distillation techniques using a C to C dialkyl ester of o-phthalic acid. It is quite unusual that the same material will be an extractive solvent for two separations of an isomer mixture. Thus, the finding that the C to C dialkyl esters of ophthalic acid can separate mand p-ethyltoluene from mesitylene as well as separate mesitylene and oethyltoluene is quite unexpected. For example, if a tetraalkyl pyromellitate is used, no separation of mand pethyltoluene from mesitylene can be obtained. Since pethyltoluene and mesitylene have boiling points which are within almost 5 F. of each other, a commercial regular distillation between these fractions would be extremely diflicult. However, according to this aspect of the present invention, the mand p-ethyltoluene fraction may be separated from mesitylene by extractive distillation techniques very easily and inexpensively. The preferred solvent is dimethyl-o phthalate, and the solventto-hydrocarbon ratios employed may suitably range from 1:3 to :1 and preferably 2:1 under the vapor-liquid extractive distillation conditions.

I In finding that the C to C dialkyl esters of o-phthalic acid would enable the separation of mand p-ethyltoluene from mesitylene, as well as enabling the separation of mesitylene and o-ethyltoluene into separate fractions by extractive distillation techniques, another aspect of the present invention is a simplified process whereby these fractions can be separated one from the other by using the same solvent in extractive distillation zones while utilizing a single stripper for separating the solvent from the hydrocarbon, thereby eliminating one solvent stripping tower. According to the simplified process, a mixture of the C isomers consisting of mand p-ethyltoluene, mesitylene and o-ethyltoluene may be separated in high purities by using two extractive distillations in series without an intermediate solvent stripper using the same solvent followed by a simple distillation to recover the solvent and to separate the heavier C aromatic hydrocarbon as a high purity fraction if this is desired. It isomers heavier than o-ethyltoluene are present, they may be separated by a second simple distillation. The solvents used in this simplified process are the C to C dialkyl esters of o-phthalic acid, the preferred solvent being d-im-ethyl-o-phthalate.

The present invention will be further illustrated by reference to the drawings in which:

FIG. 1 is a schematic flow diagram illustrating a simple extractive distillation unit; and

FIG. 2 is a flow diagram of the simplified process for separating C isomers of aromatic hydrocarbons.

Referring now to the drawings and particularly to FIG. 1, the drawings have reference to a conventional extractive distillation apparatus which may be used according to the present invention to make a separation of o-ethyltoluene from the lower boiling C aromatic isomers. The feed mixture consisting of about 25 mol percent p-ethyltoluene, about 50 mol percent rn-ethyltoluene, about 12 mol percent mesitylene, and about 13 mol percent o-ethyltoluene is introduced into extractive zone 11 by feed line 12. The distillation zone 11 is a distillation tower provided with internal vapor-liquid contacting means such as bell cap trays or other equivalent means. The extractive distillation zone is also provided with means (not shown) for introducing reflux back into the tower and auxiliary condensing and cooling means. Further, the extractive distillation zone 11 is provided with means illustrated by steam coil 13 to control the conditions within the tower. The desired solvent, which may be tetra-n-propylpyromellitate, is introduced by line 14 in countercurrent flow to the distillation zone. An overhead fraction which is enriched in the lower boiling C isomers, namely mand p-ethyltoluene and mesitylene, is withdrawn by line 15. The bottoms fraction is recovered by line 16 which is enriched in o-ethyltoluene. The bottoms fraction is discharged by line 16 into a distillation zone 17 which may be a simple distillation column containing internal vapor-liquid contacting means such as bell cap trays and the like. The distillation zone 17 is equipped with the usual means for inducing reflux and provided with auxiliary condensing and cooling means and provided with internal heating means illustrated by steam coil 18. In the distillation zone 17 -a simple distillation of the o-ethyltoluene from the solvent takes place, and the o-ethyltoluene fraction is recovered by line 19 while the solvent, tetra-n-propylpyromellitate, is recovered by line 20. The solvent is circulatedthrough line 20 into solvent line 14 for reintroduction into the extractive distillation zone 11. Makeup solvent may be introduced into line 14 by opening valve 21. The extractive distillation zone 11 may be maintained under a wide range of operating conditions depending on the specific polyalkyl ester of the present invention employed and on the degree of purity desired for the o-ethyltoluene fraction. Byway of illustration, however, temperatures at'the top of the extractive distillation zone 11 may be within the range of 325 to 330 F. at atmospheric pressure with a solvent-tohydrocarbon ratio of 2:1 to obtain a fraction substantially depleted of o-ethyltoluene overhead.

As is known to those skilled in extractive"distillations, the extractivedistillation process is one wherein the fractions are enriched rather than obtaining ideal separations. Accordingly, while it is not shown in FIG. 1, it will be understood that facilities will be provided for removing any of the solvent carried over in the overhead fraction from the extractive distillation zone 11. Such facilities may include a stripping zone such as described in the description of distillation zone 17.

It was further found in accordance with the present invention that the C to C dialkyl esters of o-phthalic acid were unique in separating mand p-ethyltoluene from mesitylene. Accordingly, a simplified process may be accomplished by utilizing the scheme illustrated in FIG. 2 which illustrates apparatus for separating mand p-ethyltoluene, rnesitylene and o-ethyltoluene as separate fractions. In accordance with this aspect of the present invention, the preferred solvent is dimethyl-o-phthalate although other dialkyl esters having 1 to 4 carbon atoms in the alkyl group may be employed. The C feed may include all the C aromatic isomers, namely m-ethyltoluene, p-ethyltoluene, rnesitylene, o-ethyltoluene and pseudocumene, which is introduced into extractive distillation zone 30 by feed line 31. The extractive distillation zone 30 is suitably a distillation tower provided with the usual internal vapor-liquid contacting means and other auxiliary equipment. The solvent, dimethyl-o-phthalate, is introduced by line 32 near the upper portion of the extractive distillation zone 30. The conditions within the extractive distillation zone are controlled by means illustrated by steam coil 33, the conditions within the extractive distillation zone 30 being controlled in a manner known to those skilled in the art. For example, the conditions would be such that the temperatures at the top of this zone 30 would be within the range of about 320 to 325 F. at atmospheric pressure. An overhead fraction enriched in mand p-ethyltoluene is withdrawn from extractive distillation zone 30 by line 34. The bottoms fraction is withdrawn from zone 30 by line 35.

The bottoms fraction which would contain the C isomers heavier than mand p-ethyltoluene and the solvent is introduced into a second extractive distillation zone 36. Additional solvent is introduced near the upper portion of the second extractive distillation zone 36 by line 37. Extractive distillation zone 36 is similar to that of zone 30 and is provided by the usual internal vaporliquid contacting means as well as means for inducing reflux and auxiliary condensing and cooling means and the like. A suitable means for maintaining the conditions within extractive distillation zone 36 is illustrated by steam coil 38. The overhead fraction eriched in mesitylene is withdrawn from zone 36 by line 39. The bottoms fraction is removed by line 40.

The bottoms fraction from zone 36 is enriched in oethyltoluene and the heavier C isomers along with the solvent, dimethyl-o-phthalate. This bottoms fraction is introduced into a stripper column 41 wherein an ordinary distillation is carried out to separate the solvent from the remaining C aromatic isomers. The stripper column 41 may be a tower provided with similar construction as the

extractive distillation zones

30 and 36. Accordingly, besides the other apparatus modifications, the conditions within the stripping tower 41 are maintained by means such as illustrated by steam coil 42. In the simple distillation carried out in stripping tower 41, the aromatic hydrocarbons and other materials having a boiling point lower than the solvent are removed overhead by line 43 while the solvent is recovered by line 44. The solvent may be recycled by line 45 and reintroduced by

lines

32 and 37 into

extractive distillation zones

30 and 36, respectively. The solvent may be purged from the system 6 through line 46' by operating valve 47, and makeup solvent introduced by line 48 which is controlled by valve49.

The overhead fraction'from the stripper column 41 may be introduced into a second distillation tower 50 to obtain the o-ethyltoluene isomer in high purity. Distillation tower 50" may be provided similarly as stripper tower 41, and the conditions therein controlled by means illustrated by steam coil 51. The overhead fraction, namely o-ethyltoluene, is recovered by line 52 while the bottoms fraction is recovered by line 53 which may contain pseudocumene and other heavier components.

To illustrate the present invention, a binary mixture of 1,3,5-trimethylbenzene (.mesitylene) and 1-methyl-2-ethylbenzene (o-ethyltoluene) is extractively distilled in the presence of a solvent having the nature described herein with separation of the mixture into its component parts. While these particular aromatic hydrocarbons boil at 328.5 and 329.3 F. in the absence of a solvent at atmospheric pressure, a good separation is possible in utiliz ing a polyalkyl ester of polycarboxylic acid having adjacent carboxylic groups according to the present invention.

For example, in a plate column the relative volatilities of mand p ethyltoluene and rnesitylene to o-ethyltoluene were determined. Without a solvent, the relative volatility of mand p-ethyl-toluene to o-ethyltoluene is 1.06 and for rnesitylene to o-ethyltoluene is 1.009; whereas with tetra-n-propylpyromellitate as a solvent, the relative volatility of mand p-ethyltoluene to o-ethyltoluene is 1.18 and for rnesitylene to o-ethyltoluene is 1.12.

In a 50 plate Oldershaw column equipped with means for introducing preheated solvent on the top plate, a mixture of rnesitylene and ethyltoluene isomers is charged to the still pot and the hydrocarbons set on total reflux. Hourly samples of overhead and bottoms were taken until the analyses of the samples from hour to hour remained constant. The relative volatilities of the compounds were then determined, and the data indicate that when the solvent is a C to C dialkyl-o-phthalate, not only separation of the rnesitylene and the o-ethyltoluene is obtainable, but also separation of mand p-ethyltoluene and rnesitylene is obtainable. For example, the relative volatility of p-ethyltoluene to rnesitylene without the solvent is 1.058 and is increased to 1.1215 when dimethyl-ophthalate is present as the solvent in about 50 mol percent concentration.

The present invention is quite important and useful and allows the obtaining of numerous advantages in that separation of the particular C aromatic hydrocarbons in purified condition is important. These particular C aromatic hydrocarbons heretofore have not been separated practically, whereas now it is possible to obtain a good separation under vapor-liquid separating conditions. The particular compounds in purified condition are important and useful as chemicals and in the manufacture of other chemicals.

The nature and objects of the present invention having been completely described and illustrated, what we wish to claim as new and useful and secure by Letters Patent 1. A method for separating C aromatic hydrocarbons which comprises extractively distilling a mixture of hydrocarbons containing mand p-ethyltoluene, rnesitylene and o-ethyltoluene by contacting with a C to C dialkyl ester of o-phthalic acid in a first extractive distillation zone whereby an overhead fraction enriched in mand p-ethyltoluene and a bottoms fraction enriched in mesitylene and o-ethyltoluene are formed, contacting said bottoms fraction with additional amounts of said ester in a second extractive distillation zone under extractive distillation conditions whereby a second overhead fraction enriched in rnesitylene and a bottoms fraction enriched in o-ethyltoluene are formed, and recovering said o-ethyltoluene from said second bottoms fraction by simple distillation.

2. A method according to claim 1 wherein said ester is dimethyl-o-phthalate.

3. A method for separating o-ethyltoluene from a mixture containing the lower boiling C aromatic hydrocarbons which comprises contacting said mixture in an extraction zone under extractive distillation conditions with a polyalkyl ester of an aromatic polycarboxylic acid having adjacent carboxylic groups and having 1 to 8 carbon atoms in the alkyl groups to form an overhead fraction and a bottoms fraction, and recovering o-ethyltoluene from said bottoms fraction.

4. A method in accordance with claim 3 wherein said ester is tetra-n-propylpyromellitate.

5. A method for separating mand p-ethyltoluene from a mixture containing the higher boiling C aromatic hydrocarbons which comprises contacting said mixture under extractive distillation conditions with a C to C dialkyl ester of o-phthalic acid whereby an overhead fraction enriched in mand p-ethyltoluene is formed.

6. A method in accordance with claim 5 wherein said ester is dimethyl-o-phthalate.

References Cited by the Examiner UNITED STATES PATENTS 2,721,170 10/1955 Johnson 202-395 2,763,604 9/1956 Dorsey et a1 20239.S

NORMAN YUDKOFF, Primary Examiner.

Claims

1. A METHOD FOR SEPARATING C9 AROMATIC HYDROCARBONS WHICH COMPRISES EXTRACTIVELY DISTILLING A MIXTURE OF HYDROCARBONS CONTAINING M- AND P-ETHYLTOLUENE, MESITYLENE AND O-ETHYLTOLUENE BY CONTACTING WITH A C1 TO C4 DIALKYL ESTER OF O-PHTHALIC ACID IN A FIRST EXTRACTIVE DISTILLATION ZONE WHEREBY AN OVERHEAD FRACTION ENRICHED IN M- AND P-ETHYLTOLUENE AND A BOTTOMS FRACTION ENRICHED IN MESITYLENE AND O-ETHYLTOLUENE ARE FORMED, CONTACTING SAID BOTTOMS FRACTION WITH ADDITIONAL AMOUNTS OF SAID ESTER IN A SECOND EXTRACTIVE DISTILLATION ZONE UNDER EXTRACTIVE DISTILLATION CONDITIONS WHEREBY A SECOND OVERHEAD FRACTION ENRICHED IN MESITYLENE AND A BOTTOMS FRACTION ENRICHED IN O-ETHYLTOLUENE ARE FORMED, AND RECOVERING SAID O-ETHYLTOLUENE FROM SAID SECOND BOTTOMS FRACTION BY SIMPLE DISTILLATION.