US3467723A

US3467723A - Process for the purification of aromatic hydrocarbons

Info

Publication number: US3467723A
Application number: US479814A
Authority: US
Inventors: Yutaro Matsumoto; Tadashi Hisatsune; Tsutomu Sasaki; Fumiaki Mitsudo; Yasuhiro Yamauchi
Original assignee: Yawata Chemical Industry Co Ltd
Current assignee: Nippon Steel Chemical and Materials Co Ltd
Priority date: 1965-08-16
Filing date: 1965-08-16
Publication date: 1969-09-16
Anticipated expiration: 1986-09-16

Description

p 1969 YUTARO MATSUMOTO ETAL 3,467,723

PROCESS FOR THE PURIFICATION OF AROMA'IIC HYDROCARBONS Filed Aug. 16, 1965 FIG.I 9

F l G 2 FIG. 3

I N VE N TORS Yu/aro Ma/sumo/o Tadash/ H/sa fsune Tsu/omu .Sasak/ Fum/aki Mi/sudo Yasuh/ro Yamaucm' United States Patent 3,467,723 PROCESS FOR THE PURIFICATION OF AROMATIC HYDROCARBONS Yutaro Matsumoto, Tadashi Hisatsune, Tsutomu Sasaki,

Fumiaki Mitsudo, and Yasuhiro Yamauchi, Kitakyushu, Japan, assignors to Yawata Chemical Industry C0,, Ltd., Tokyo, Japan Filed Aug. 16, 1965, Ser. No. 479,814 Int. Cl. C07c 7/04, /00

US. Cl. 260-674 2 Claims ABSTRACT OF THE DISCLOSURE Process for obtaining high purity aromatic hydrocarbons from aromatic hydrocarbon oils contaminated by non-aromatic hydrocarbons, comprising two distillation steps, wherein the first step said aromatic hydrocarbon oils are distilled by adding high-boiling point hydrocarbon oils mainly composed of saturated non-aromatic hydrocarbons, said non-aromatic hydrocarbons forming no azeotropic mixtures with aromatic hydrocarbons to' be purified and having boiling point at least C. higher than that of the latter, whereby a distillate containing substantially no contaminating matter is obtained and in the second step said distillate is subjected to known distillation procedures to remove low boiling point contaminating matters.

This invention relates to a process for purifying and separating preferably very pure aromatic hydrocarbons by distillation from a mixture containing mainly of aromatic hydrocarbons, such as, benzene, toluene, and xylene together with non-aromatic hydrocarbons as impurities, for example, from a coke oven light oil, an oil gas light oil, and the like.

It is impossible by only a rectification to obtain extremely high-pure aromatic hydrocarbons from desulfurization products of a coke oven light oil or an oil gas light oil, in particular, the hydrogenated purification product thereof (for example, to produce high-pure benzene having a freezing point above 54 C.; hereinafter, the invention is explained mainly referring to the purification of, in particular, benzene as aromatic hydrocarbons).

That is, in such a rectification procedure, a raw oil is first charged in a first distilling column, whereby a benzene fraction containing benzene and non-aromatic hydrocarbons having boiling points comparatively adjacent that of benzene but containing no toluene and high boiling point aromatic compounds is obtained from the top of the distilling column, while an oil mixture containing toluene and higher point hydrocarbons is recovered from the bottom. The benzene fraction is rectified further in a second rectification column, whereby the aromatic hydrocarbons having boiling points lower than that of benzene and a small amount of benzene are distilled off from the top of the column and benzene is recovered from the bottom.

In such a rectification, considerable proportions of nheptane and methylcycloheptane are intermingled in the benzene fraction withdrawn from the first column for separating the benzene fraction from the raw oil although the boiling points of them are fairly different from that of benzene. Particularly, almost the greater part of nheptane is intermingled in the benzene in the step. Therefore, in the case of obtaining benzene from the bottom of the subsequent second distilling column by rectifying the benzene fraction in the second column, the cyclo hexane, methylcyclopentane, etc., intermingled in the benzene fraction in the first column can be removed in a considerable extent in a rectifying oolumn having a practical number of plates and further, non-aromatic hydro- 3,467,723 Patented Sept. 16, 1969 carbons having boiling point lower than those of the above components can be completely removed. While, the removal of n-heptane and methylcyclohexane from benzene is completely impossible.

It is practically impossible to carry out the rectification in the first column such that the non-aromatic hydrocarbons, such as, n-heptane and methylcyclohexane are not intermingled in the benzene fraction since the plate' number of the rectifying column becomes extremely larger and the reflux ratio is also extremely increased. This is because if the concentration of the aromatic compounds is increased, the active coefficients of n-heptane and methylcyclohexane become very large, whereby the relative volatility of benzene to these hindrance materials approaches 1.

For overcoming these difficulties, a process has already been provided for removing the non-aromatic hydrocarbons from the benzene fraction by adding a suitable foreign materials to the benzene fraction to conduct a socalled azeotropic distillation or extractive distillation. They are, for example, as follows.

(1) Purification process of benzene wherein the azeotropic distillation is conducted by adding in the benzene fraction a foreign component that scarcely forms an azeotropic mixture with benzene but easily forms an azeotropic mixture with the non-aromatic hydrocarbons, such as ketones or nitrils.

(2) Purification process of benzene wherein the extractive distillation is conducted by adding into the benzene fraction a comparatively non-volatile foreign component that makes, however, the non-aromatic hydrocarbons very volatile.

However, it is very difficult to obtain with a high yield a high-pure benzene having a freezing point higher than 54 C. by such known processes caused by the presence of saturated hydrocarbons, the separation of which from benzene being difficult.

That is, the application of the azeotropic distillation or extractive distillation to the purification of the benzene fraction is uprofitable since, while cyclohexane, methylcyclopentane and aromatic hydrocarbons having boiling point lower than those of them can be easily removed from the benzene fraction, the removal of nheptane, methylcyclohexane, etc., is very difiicult, and even if it might be possible, the use of an extremely large amount of additives is required, or the yield for benzene greatly.

(3) Purification process of benzene disclosed by US. Patent 2,581,344 wherein a first step azetotropic distillation is conducted by adding hydrocarbons having boil ing point of -85 C. (e.g., cyclohexane) to a coke oven light oil containing a small amount of non-aromatic hydrocarbons that cannot be separated by a simple distillation to form an azeotropic mixture with benzene, whereby obtaining from the top of the distillation column the azeotropic mixture of, e.g., benzene and cyclohexane, and then a second step azeotropic rectification is carried out by adding a polar azeotropic-mixture-forming agent, such as, methanol or acetone, whereby removing the azeotropic mixture from the top of the column and recovering from the bottom pure benzene.

However, this process requires complicated steps and procedures. That is, since in the first distilling step the added hydrocarbon having a boiling point of 7585 C. forms an azeotropic mixture with benzene, a severe procedure such as an azeotropic distillation must be applied in the second distilling step in order to obtain high-pure benzene by separating benzene from the azetropic mixture. Moreover, since a small amount of the polar azeotropic-mixture-forming agent is present in the benzene obtained from the bottom of the distilling column in the second distilling step, the agent must be removed by washing with water. Furthermore, the separation of the hydrocarbons having a boiling point of 75-85 C. added in the first step and the polar azeotropic-mixture-forming agent is necessary in the process. Thus, the process requires complicated steps and procedures and hence is unprofitable one in economy and industry.

The inventors have studied variously the purification of aromatic hydrocarbons, such as, benzene, toluene, xylene, etc., by distillation and have come to the consideration that if, in the case of separating a benzene fraction from a raw oil by distillation, the purification can be conducted such that high-boiling point non-aromatic hydrocarbons, such as, n-heptane and methylcyclohexane are not intermingled in the distillates, the separation of the low-boiling point non-aromatic hydrocarbons, such as, methylcyclopentane and cyclohexane can sufficiently be conducted by a rectification and further if a known azeotropic distillation or extractive distillation is combined with such a rectification by adding a polar material, higher quality benzene can very easily be obtained. As the results of investigations based on this consideration, the inventors have found the separation of methylhexane and n heptane can be carried out very effectively by distilling the oil mixture under the presence of a hydrocarbon oil that does not form an azeotropic mixture with benzene and has a specific boiling point.

That is, according to the process of this invention, in the case of recovering high pure aromatic hydrocarbons by distilling aromatic hydrocarbon oils containing mainly the aromatic hydrocarbons, such as, benzene, toluene, xylene and the like together with impurities such as paraffinic and naphthenic non-aromatic hydrocarbons having boiling higher than but very adjacent the boiling points of the aromatic hydrocarbons to be purified, the aromatic hydrocarbon oils are distilled under the presence of high boiling point hydrocarbon oils mainly consisting of paraffinic and/or naphthenic hydrocarbons forming no azeotropic mixtures with the aromatic hydrocarbons to be purified and having boiling points at least C. higher than those of the aromatic hydrocarbons to be purified to give aromatic hydrocarbon fractions containing almost no non-aromatic hydrocarbons having boiling points higher than but adjacent the boiling points of the aromatic hydrocarbons to be purified, and then by distilling the aromatic hydrocarbon fraction in a conventional manner, the high quality aromatic hydrocarbons are obtained.

The invention will be explained referring to the case of obtaining benzene although the invention shall not be limited to this case by any means.

That is, in the case of recovering pure benzene by the process of this invention, non-aromatic hydrocarbons having boiling points adjacent but higher than that of benzene, such as, n-heptane and methylhexane are separated from benzene by distilling a benzene fraction containing the non-aromatic hydrocarbons under the presence of high-boiling point hydrocarbon oils mainly consisting of parafiinic and/or naphthenic hydrocarbons forming no azeotropic mixtures with benzene and having boiling points at least 20 C. higher than that of benzene.

For example, in the distilling procedure for separating benzene from a raw oil in accordance with the process of this invention, a suitable amount of above specific hydrocarbon oils is supplied into the system from a proper portion between the top of the column and the position for feeding a feed stock. Thus, the relative volatilities of benzene to n-heptane and methylcyclohexane is increased, therefore, the greater part of n-heptane and methylcyclohexane are removed from the bottom of column while the benzene fraction containing almost no nheptane and methylcyclohexane is obtained from the top of column. By treating thus obtained benzene fraction (containing low-boiling point non-aromatic hydrocarbons, such as, methylcyclopentane and cyclohexane) according to a known process, such as, rectification, azeotropic distillation, and extractive distillation, high-pure benzene can be easily obtained. Moreover, it is possible to remove further n-heptane and methylcyclohexane, if necessary, by distilling again thus recovered benzene fraction with the addition of the above-mentioned specific hydrocarbon oils.

As the aromatic hydrocarbon raw materials used in this invention, there are oils containing mainly the aromatic hydrocarbons, such as, benzene, toluene, xylene and the like and as impurities the non-aromatic hydrocarbons, such as, paraffinic hydrocarbons and naphthenic hydrocarbons but the oils containing larger amount of the aromatic hydrocarbons are more preferable as the raw oils of this invention. As such oils, there are a purified light oil prepared by hydrogenating under pressure a coke oven light oil or oil gas light oil and a purified light oil prepared by treating a coke oven light oil or oil gas light oil with sulfuric acid followed by washing. Furthermore, the raw oil in this invention may be a benzenecontaining oil from petroleums.

As the petroleum raw materials, there are, for example, denatured gasoline and by-product gasoline by high-temperature thermal decomposition. The denatured gasoline may be used in situ as the raw material in this invention since it scarcely contains sulfur, nitrogen and oxygen and also scarcely contains impurities causing clogging of apparatus, such as, olefins, diolefins, acetylene and the like, but in the case of using the by-product gasoline, sulfur, nitrogen, oxygen and the above-mentioned clogging substances must be removed prior to use.

In the process of this invention a high-boiling point hydrocarbon oil mainly consisting of paraffinic and/or naphthenic hydrocarbons forming no azeotropic mixtures with the aromatic hydrocarbons to be purified and having boiling points at least 20 C. higher than those of hydrocarbons to be purified is added in a row oil or aromatic hydrocarbon oil. The boiling points of these high-boiling point hydrocarbons is not limited to a narrow range but should be determined by the composition of the aromatic hydrocarbons to be purified.

That is, in the case where the hydrocarbon oil to be treated by the process of this invention contains only benzene as the hydrocarbon to be purified or contains besides benzene a small proportions of toluene and xylene, which are unnecessary to be recovered, the high-boiling point hydrocarbon oil to be added in the distillation system in accordance with this invention may be one that does not form an azeotropic mixture and can be easily separated from benzene. But, the high-boiling point hydrocarbon oil forming an azeotropic mixture with benzene and easily separable from benzene must have a boiling point at least 20 C. higher than that of benzene.

On the other hand, in the case where the raw material contains large proportions of toluene and/ or xylene and the toluene and/ or xylene are recovered from the raw material, a saturated hydrocarbon that forms no azeotropic mixture with xylene and can be easily separated from xylene may be selected. However, in this case, the boiling point of the high-boiling point hydrocarbon must be at least 20 C. higher than that of xylene. For example, a high-boiling point saturated hydrocarbon oil of which the boiling point of the first drop is higher than 170 C. is selected. Thus, the high-boiling point saturated hydrocarbon oil may be selected having a boiling point at least 20 C. higher than that of the aromatic hydrocarbons to be purified. In general, the high-boiling point saturated hydrocarbon oil having a boiling point of 250 C. preferably 220 C. gives better results. Further, it is not necessary that the high-boiling point saturated hydrocarbon oil to be added in the system is highly refined one but the content of sulfur is desirably as low as possible and preferably below 5-10 p.p.m. for preventing the product from contamination.

The content of aromatic hydrocarbons in the highboiling point saturated hydrocarbon oil is desirably small but even though the high-boiling saturated hydrocarbon oils contains about 30% of aromatic hydrocarbons, the separation of n-heptane and methylcyclohexane from benzene can be effected by increasing the addition amount of the additive without spoiling the advantages of this invention.

The amount of the high-boiling point saturated hydro: carbon oil to be added in the process of this invention is not limited to a narrow range but considering from effect the amount is at least 0.15 times preferably 0.5-7 times as large as the amount of the raw material. The highboiling point saturated hydrocarbon oil is generally supplied supplementally into the system. Since such addition hydrocarbon oil may be a petroleum fraction available as a name of refined kerosene or the like or may be easily obtained by subjecting the fraction partially to a fractional distillation again, the cost of such additive is very low.

The process of this invention wherein the distillation is carried out under the presence of the high-boiling point hydrocarbon oil may, at a glance, appear similar to a conventional extractive distillation wherein the distillation is carried out with the addition of a polar substance, such as, diethylene glycol, triethylene glycol, phenol, or cresol, but the former is completely different from the latter in mechanism and principle. That is, in the known extractive distillation using a polar substance, the paraffinic and naphthenic hydrocarbons contained as impurities in a raw material are converted into volatile materials by the addition agent and they are distilled out of the top of column while benzene is recovered from the bottom of column together with the addition agent. In such an extractive distillation, by the addition of the polar material to a benzene fraction, benzene is converted into an unvolatile state by the interaction between benzene and the added polar material and the benzene and the added polar material are moved downwardly into the bottom of column while the paraflinic and naphthenic hydrocarbons aredistilled out of the top of column in situ. In this case, the relation of benzene and the parafiinic and naphthenic hydrocarbons contained as impurities is in an extremely unideal system. That is, not only cyclohexane and methylcyclopentane that can be considerably removed by a rectifying column having a practical number of plates but also n-heptane and methylcyclohexane having boiling points higher than that of benzene are converted by the additive into more volatile states than benzene whereby they are removed from benzene. By the procedure can be easily separated cyclohexane, methylcyclopentane and the like having boiling points adjacent and lowerthan that of benzene, while in order to separate n-heptane, methylcyclohexane and the like having boiling points higher than that of benzene by such a procedure, a large amount of expensive solvent and a large amount of steam for heating are required.

On the other hand, in the process of this invention, the relation of benzene and the non-aromatic hydrocarbons contained in the benzene fraction is brought in an almost ideal system by the addition of the high-boiling point saturated hydrocarbon oil, whereby n-heptane and methylcyclohexane are easily separated. In other words, the process of this invention is diflferent from the conventional extractive distilling process in principle as wellas the mechanism in the former is contrary to that of the latter.

As. mentioned above, in the process of this invention,

by bringing the relation of benzene and the non-aromatic hydrocarbons in an almost ideal state, the amount of steam for heating may be small as compared with that in the conventional fractional distillation and other distillations and the additive and solvent may be inexpensive ones. The apparatus for effecting the process of this invention may be simple and further since the additives used in this invention are non-corrosive, the apparatus may be made of an inexpensive material and the operation is very simple.

The invention was explained above about the case of obtaining high-pure benzene, but the invention is not limited to the case only, but can be applied to the case of obtaining toluene and xylene from the fractions thereof and the high-pure hydrocarbons can be easily obtained.

As to toluene, in the case of separating a toluene fraction from a mixture from which a benzene fraction has been removed by distillation contains a considerable amount of non-aromatic hydrocarbons having boiling points higher than that of toluene. However, by applying the process of this invention to the toluene fraction, the paraffinic and naphthenic hydrocarbons having boiling points adjacent but higher than that of toluene can be easily removed from the fraction. Therefore, since the toluene fraction contains almost no non-aromatic hydrocarbons having boiling points higher than that of toluene, high-pure toluene can be easily produced by a fractional distillation.

Moreover, as in the case of obtaining pure benzene, known azeotropic distillation or extractive distillation using a polar material may be combined with the distillation of this invention.

The process of this invention will now be explained further in detail referring to the accompanying drawings, in which FIG. 1 to FIG. 3 are schematic flow diagrams of the preferred embodiments of this invention.

FIG. 1 is a flow diagram showing the case of producing high-pure benzene by treating a raw material containing mainly benzene and toluene, or benzene, toluene, and xylene together with non-aromatic hydrocarbons.

The raw material is introduced into a distilling column through a conduit 1. The raw material is distilled in the column under the presence of a high-boiling point saturated hydrocarbon oil supplied through a conduit 11 and separated into a benzene fraction containing almost no n-heptane and methylcyclohexane and a mixture of a toluene-xylene fraction and the high-boiling points saturated hydrocarbon oil, the former being withdrawn from the top of column and the latter from the bottom of column. The benzene fraction 3 from the top of column is introduced into a distilling column 5 wherein the fraction is separated into a hydrocarbon fraction 6 (containing cyclohexane) having a boiling point lower than that of benzene and a high-pure benzene fraction 7. The fraction 7 is high-quality benzene having a freezing point above 5.4 C.

The fraction 4 obtained from the bottomof distilling column 2 is treated in a distilling column 8 to separate a toluene-Xylene fraction 9 and remaining high-boiling point hydrocarbon oil is withdrawn from conduit 10' and recycled into the distilling column through a conduit 11. Thus recycled high-boiling point hydrocarbon oil may be partially deteriorated or contaminated with high-boiling point impurities in the raw material. Accordingly, a part of the oil is withdrawn from the system through a conduit 13 and the loss is supplemented through a conduit 12, but the amount is very little.

The toluene-xylene fraction obtained through an outlet 9 is fractional-distilled by a usual manner into each component. If it is necessary to recover high-pure toluene from the toluene-xylene fraction obtained from the outlet 9, the process shown in FIG. 2, which is almost same as in FIG. 1, may be adopted.

The fraction obtained from outlet 9 in FIG. 1 is intro duced in a distilling column 22 through a conduit 21 and distilled under the presence of the high-boiling point hydrocarbon oil supplied through a conduit 31, whereby a toluene fraction containing almost no high-boiling point non-aromatic hydrocarbons such as n-octane is obtained from the'top 23 of column, which is further introduced into a distilling column to recover the fraction, and highquality toluene is obtained from the bottom 27 of column. The bottom fraction 24 in distilling column 22 is then fractional-distilled in a distilling column 28, whereby toluene is recovered from the top of column while the high-boiling point hydrocarbon oil 30 separated in the bottom of column is recycled into the distilling column 22 through a conduit 31. A part of the recycled oil may be removed and supplemented through conduits 33 and 32 as in FIG. 1 and the amount may also be obtained as in the cases shown in FIG. 1 and FIG. 2.

The procedures in FIG. 1 and FIG. 2 are explained about the case of using a usual fractionating column as the distilling column 5 or 25, but the column may be replaced with a known azeotropic distillation or extractive distillation using a polar material. In this case, as mentioned above, sutficiently high-quality aromatic compounds can be obtained with a reduced amount of the additive.

The process of this invention can be conducted further by exchanging the process in FIG. 1 and the process in FIG. 2, or in the manner as shown in FIG. 3. The process shown in FIG. 3 is explained about the case of purifying benzene.

A raw material is introduced into a distilling column 42 through a conduit 41 wherein a fraction lighter than benzene is first separated and withdrawn from an outlet 43. The fraction 44 consisting of benzene and hydrocarbons having boiling points higher than that of benzene withdrawn from the bottom of the distilling column 42 is introduced into a distilling column 45, wherein the fraction is distilled under the presence of the high-boiling point hydrocarbon oil supplied through a conduit 51. By the procedure, high-quality benzene is obtained from the top of column. The bottom oil 47 is separated in a subsequent distilling column 48 and a toluene-xylene fraction 49 obtained from top of column is sent further to a subsequent separation step. The bottom oil 50 in the distilling column 48 is recycled into the distilling column 45 through a conduit 51. The removal and supplement of a part of the recycling oil may be done as in the process of FIG. 1 through

conduits

53 and 52 respectively and the amount may also be very small.

As mentioned above, high-quality aromatic hydrocarbons that have never been obtained by a conventional distillation can be obtained by the process of this invention, and in the case of the purification of benzene, highpure benzene having a freezing point above 5.4" C. or purity above 99.8% is obtained by the invention, besides obtaining such high-quality aromatic hydrocarbons,

(I) The reflux ratio may be small and heat power by the presence of the specific hydrocarbon oil is not large,

(II) Since the specific hydrocarbon oil used in this invention is suitably contained in, for example, a coke oven light oil or petroleum light oil, a slightly reconstructed refining process for the light oil can be easily combined in the process of this invention, and

(III) The separation of the low-temperature fraction from the aromatic hydrocarbons to be separated may be conducted by a simple distilling process and high-quality aromatic hydrocarbons can be directly separated.

The present invention will be further described by the following practical examples.

Example 1 The procedure of this example is conducted by the system as shown in FIG. 1. Into the distilling column 2 was supplied 359.21 kg. of a compressed pure hydrogenated oil through a conduit 1. The composition of the hydrogenated oil was as follows:

Kg. Low-boiling point paraflins 2.87 n-Heptane 1.26 Cyclohexane 1.26 Methylcyclohexane 1.40

Kg. Benzene 254.71 Toluene 74.00 Ethylbenzene; xylene 15.45 Other paraffins 8.26

Total 359.21

The column 2 was a distilling column of 6.25 cm. in inside diameter and 2 m. in packed length, packed with Dixone packings. Through a conduit 11 was supplied 851.46 kg. of a high-boiling point saturated petroleum hydrocarbon oil having boiling points of -200 C. The raw oil was distilled in the column under the presence of the saturated hydrocarbon oil with a reflux ratio of 3.5 and the fraction obtained from the top of column was charged in a column 5 of 5 cm. in inside diameter and 1.7 m. in packed length, packed with Dixone packings, wherein the fraction was separated into the following two fractions:

Kg. Distillate 14.51 Fraction 6:

Low-boiling point paraflins 2.43 Cyclohexane 0.89 Benzene 11.19 Still residue 232.45 Fraction 7:

Cyclohexane 0.38 Benzene 232.02 n-Heptane 0.03 Others 0.02

The freezing point of the benzene of the still residue was 5.41 C. The fraction 4 obtained from the bottom of the distilling column 2 was distilled into the toluenexylene fraction 9 in the distilling column 8 packed with Dixone packings having 3.75 cm. in inside diameter and 1 m. in packed length. The amount of the fraction 9 was 107.1 kg. The remaining high-boiling point hydrocarbon oil 12 was recycled to the distilling column 2 through the conduits 10 and 11. The recycled amount was 850.2 kg. and the amount of the removed oil through a conduit 13 was 1.25 kg.

For comparison, the same raw material was continuously distilled by using the same distilling system and at the same reflux ratio without the addition of the high-boiling point saturated hydrocarbon oil through the conduit 11 and thus obtained benzene from the outlet 7 has the freezing point of 5.21 C.

Example 2 The procedure in this example was carried out in the embodiment shown in FIG. 2. In order to recover highquality toluene, 107.1 kg. of the toluene-xylene fraction 9' obtained in the process of FIG. 1 was supplied into the distilling column 22 in FIG. 2 through the conduit 21. The composition of the fraction was as follows:

The distilling column 22 was packed with Dixone packings and the inside diameter of it was 6.25 cm. and the packed length was 2 m. From the conduit 31 was supplied 800 kg. of the high-boiling point saturated hydrocarbon oil having boiling points of 160-200 C. The raw material 21 was distilled under the presence of the hydrocarbon oil with the reflux ratio of 7.5 and the fraction obtained from the top of column was introduced into the column 25 packed with Dixone packings and having 5 cm, in inside diameter and 1.7 m. in packed height, where- 9 in the fraction was separated into the following tw fractions:

The fraction 24 obtained from the bottom of the disstilling column 22 was separated into the xylene fraction 29 in the distilling column 28 packed with Dixone packings and having the inside diameter of 3.75 cm. and the packed height of 1 m. The amount of the fraction 29 was 24.9 kg. The remaining high-boiling point saturated hydrocarbon oil was recycled into the distilling column 22 from the conduit 30 through the conduit 31. From the conduit 33 was withdrawn 1.0 kg. of the hydrocarbon oil.

For comparison, the same raw material was continuously distilled using the same distilling system system with the same reflux ratio without the addition of the high-boiling point saturated hydrocarbon oil through the inlet 31, and the purity of thus obtained toluene was 98.5%.

Example 3 In this example, the system shown in FIG. 3 was conducted.

Into the distilling column 42 was supplied 330.47 kg. of a raw material (compressed pure hydrogenated oil) through the conduit 41 and 9.82 kg. of a fraction 43 having lighter than benzene was obtained. The compositions of them were as follows:

The fraction 44 obtained from the bottom of the column 42 was charged in the distilling column 45 and distilled under the presence of 805.7 kg. of the high-boiling point saturated hydrocarbon oil supplied through the conduit 51 with the reflux ratio of 3.3 into the fraction 46 and the fraction 47. The freezing point of the benzene of the fraction 46 was 5.40 C, The composition of the fraction 46 was as follows:

Fraction 46 216.74 Cyclohexane 0.39 Benzene 216.30 n-Heptane 0.03 Others 0.02

The fraction 47 was introduced into the distilling column 48 and separated into 98.68 kg. of the toluene-xylene fraction 49. The remaining high-boiling point hydrocarbon oil was recycled to the distilling column 45 through the conduit 50.

From the conduit 53 was removed 1.16 kg. of the oil from the system. The dimensions of the distilling columns in this example were as follows:

For comparison, the same procedure was repeated without the addition of the high-boiling point saturated hydrocarbon oil through the conduit 51 and the freezing point of benzene obtained from the conduit 46 was 5.20 C.

Thus, by carrying out the distillation with the addition of the high-boiling point hydrocarbon oil, the high-pure aromatic hydrocarbons can be easily produced.

We claim:

1. A process for obtaining high purity aromatic hydrocarbons by subjecting by distilling in two steps an aromatic hydrocarbon oil mainly composed of aromatic hydrocarbons selected from a group consisting of benzene, toluene and xylene, and further containing as impurities paraffinic and naphthenic non-aromatic hydrocarbons having boiling point adjacent but higher than those of the aromatic hydrocarbons to be purified, the separation of said non-aromatic hydrocarbons being very diflicult by a simple distillation, which comprises supplying said aromatic hydrocarbon oil into a distilling column from a position between the top of the distilling column and a position of supplying the feed stock a high boiling point hydrocarbon mainly composed of at least a member selected from a group consisting of paraflinic hydrocarbons, naphthenic hydrocarbons and a mixture thereof, said high boiling point hydrocarbon forming no azeotropic mixtures with the aromatic hydrocarbon to be purified and having boiling point at least 20 C. higher than that of the aromatic hydrocarbon to be purified in an amount of at least 15 wt. percent of the feed stock, subjecting to the first step distillation said aromatic hydrocarbon under the presence of said high boiling point hydrocarbon oil, thereby to obtain an aromatic hydrocarbon fraction which contains almost no high boiling point impurity hydrocarbons but is still contaminated by low boiling point non-aromatic hydrocarbons from the top part of the distilling column, while said high boiling point hydrocarbon oil and said high boiling point impurity hydrocarbons being transferred to the bottom part of the column and then subjecting to the second distillation according to a conventional method the thus obtained aromatic hydrocarbon fraction in a distillation column to be used for the second distillation to separate said low boiling point non-aromatic hydrocarbons therefrom.

2. A process according to claim 1 wherein the high boiling point hydrocarbon is added in the amount of 50 to 700 wt. percent of the feed stock.

References Cited UNITED STATES PATENTS 2,085,287 6/ 1937 Bailey 260674 2,753,3 87 7/ 1956 Love 260674 FOREIGN PATENTS 600,302 6/ 1960 Canada. 681,434 10/1952 Great Britain.

DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, Assistant Examiner US. Cl. X.R.