US3684665A - Process for recovering styrene and xylenes from cracked oil by extractive distillation with a dealkyl acetamide - Google Patents

Abstract

AROMATIC HYDROCARBONS AR RECOVERED FROM CRACKED OIL BY EXTRACTIVE-DISTILLATION. STYRENE, PARTICULARLY, IS DIRECTLY OBTAINED FROM THE CRACKED OIL BY EXTRACTIVE-DISTILLATION.

Description

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GENERAL FIELD OF THE INVENTION The present invention relates to a process for recovering aromatic hydrocarbons contained in cracked oil obtained as by-products from the production of ethylene by thermal degradation (cracking) of naphtha. Further, the present invention relates to a process for obtaining styrene and Xylene from the cracked oil directly by extractivedistillation using solvents defined hereunder.

Ethylene, being one of the most important intermediate products of the petro-chemical industry, is produced by heat-degradation of a refinery gas, a natural gas, or a naphtha. However, cracking of naphtha is the main method for producing ethylene in most locations in the world, other than those having an abundance of natural gas.

'In the cracking of naphtha, naphtha is thermally cracked at about 800 C., and ethylene is produced at a yield of about Ztl-35% by weight. At the same time, a plentiful amount of by-product called cracked oil is obtained. The oil consists of many different components, Because it contains a large amount of unsaturated hydrocarbons such as oleins, dienes, acetylene derivatives and the like, it is very difcult to separate and rene a specific component directly from the cracked oil. (In the present invention the term unsaturated hydrocarbons means unsaturated hydrocarbons having an oleiinic double bond or an acetylenic triple bond, but does not mean the double 'bond of an aromatic ring.) However, as a large amount of the cracked oil is formed and it contains a large amount of aromatic hydrocarbons, it is a very important matter to utilize these aromatic hydrocarbons for reducing the cost of ethylene.

DISCUSSION OF THE PRIOR AtRT Heretofore, in aromatic hydrocarbons such as benzene, toluene and Xylene contained in the cracked oil obtained by ethylene production have been recovered by, in the first step, hydrogenating unsaturated hydrocarbons such as mono olens and dienes to form saturated hydrocarbons, and then separating aromatic hydrocarbons by extraction. Namely, at rst, hydrocarbons having up to carbon atoms and hydrocarbons having more than 9 carbon atoms were removed by distillation from the cracked oil, the resulting mixture was hydrogenated to convert unsaturated hydrocarbons such as olens, dienes and acetylene derivatives, and then aromatic hydrocarbons were separated from the mixture by extraction using solvents such as diethyleneglycol, sulfolane and N-methylpyrrolidone. Benzene, toluene, xylene and other aromatic hydrocarbons were separated by further distillation of the resulting hydrocarbons. Benzene and toluene thus recovered did not have any problem as to quality; however,

3,684,665 Patented Aug. 15, 1972 ice with regard to Xylene quality, about 40-50% by Weight ethylbenzene was contained in this xylene. Such xylene might be unsuitable as a raw material for production of P-Xylene. The composition of this xylene, namely a composition of aromatic hydrocarbons having 8 carbon atoms depends on the condition of thermal degradation of the naphtha. However, ordinarily cracked oil before hydrogenation has about 30-40% by Weight styrene instead of ethylbenzene. Styrene contained in the cracked oil is converted into ethylbenzene by hydrogenation. Moreover, attempting to obtain more ethylene by cracking of the naphtha under more severe cracking conditions has a tendency to increase the styrene content in the cracked oil. Therefore, it is diicult to obtain Xylene of good quality, namely xylene containing high concentration of P-Xylene, from cracked oil by prior art methods. Further, in the prior art methods it is necessary to use a large quantity of hydrogen gas because of hydrogenation of the styrene that is contained in the cracked oil.

SUMMARY OF THE INVENTION A novel and advantageous process is provided for recovering aromatic hydrocarbons particularly styrene and xylene from the cracked oil, which consists of, in order to concentrate C-8 aromatic hydrocarbon, separating hydrocarbons having up to about 5, more preferably 7, carbon atoms and more than 9 carbon atoms from the cracked oil by fractional distillation, producing a fraction containing Xylene at the top of the distillation column and a residue containing styrene at the bottom of the distillation column by extraetive-distillation, and, in necessity, hydrogenating the fraction containing the Xylene.

BRIEF DESCRIPTION `OF THE DRAWING The attached drawing is a schematic illustration of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION It is an object of this invention to recover aromatic hydrocarbons especially Xylenes and styrene from the cracked oil obtained from the cracking of naphtha. It is also an object of this invention to obtain Xylenes containing p-Xylene of high concentration from a fraction of hydrocarbons having 8 carbon atoms contained in the cracked oil. It is another object of this invention to obtain styrene directly from the cracked oil. The other objects of this invention will appear in further detail hereinafter.

The present invention relates to a process for recovering aromatic hydrocarbons from cracked oil which consists of (l) removing hydrocarbons having up to about 5, more preferably about 7, carbon atoms and more than 9 carbon atoms from the cracked oil by fractional dis tillation, (2) obtaining the fraction mainly consisting of Xylene from the top of the distillation column and the residue substantially consisting of styrene and solvent from the bottom of the distillation column by extractivedistillation of the middle fraction of step (l), and (3) in necessity, hydrogenating the fraction containing xylenes obtained from step (2).

`In the present invention, it is sometimes preferable that the present invention be carried out by: (1) separating hydrocarbons having about 5 carbon atoms or less from the cracked oil by fractional distillation, (2) separating benzene, toluene (as a fraction having a lower boiling temperature than Xylenes or styrene) and hydrocarbons having at least about 9 carbon atoms (as a residue having a higher boiling temperature than xylenes or styrene) by fractional distillation, (3) obtaining the fraction of xylenes from the top of the distillation column and styrene from the bottom of the distillation column by extractive-distillation of the fraction containing xylenes and styrene obtained from step (2) and (4) hydrogenating the fraction of xylenes.

According to the present invention benzene, toluene, Xylene and styrene can be recovered from the cracked oil eiciently and economically.

The cracked oil employed according to the present invention may consist of by-products from a plant producing ethylene by heat degradation (cracking) of naphtha at about 700-l000 C. Since the cracked oil consists of multiple components, it is diflcult to state its composition critically because various compositions are created under different cracking conditions. However, in the present invention, all cracked oils from which have been removed hydrocarbons having up to 5 carbon atoms are considered to be useful. It is not necessary to remove these hydrocarbons completely, but the total retained contents of these hydrocarbons should not be in excess of about 1% by weight. As above mentioned, t is not possible to set forth the composition of the cracked oil critically, however, normally a cracked oil containing about 30-80% by weight aromatic hydrocarbons may be used. Of course, there is no intention to limit the present invention with respect to the contents above mentioned. The cracked oils are readily available commercially as by-products from plants producing olefins (ethylene) by cracking of naphtha.

The step of removing hydrocarbons having up to 5 (7) carbon atoms and more than 9 carbon atoms from the cracked oil is usually carried out by fractional distillation. All of the known distillation operations are useful in the present invention. Normally, the distillation operation is carried out at a temperature of about 70 C., under a normal atmospheric pressure, with a reux ratio of about 2, preferably at a temperature of 135 C. and 150 C. of the converted value of normal atmospheric pressure, and with a reflux ratio of 3. In this distillation procedure, two distillation columns are preferably used.

With regard to the step of separating aromatic hydrocarbons as main components from such cracked oil, namely separating the fraction having a lower boiling temperature than Xylene or styrene and the residue having a higher boiling temperature than Xylene or styrene from the cracked oil, the operating condition of the distillation step is set up and controlled at the discretion of the operator. However, it is preferable that this fraction contains a few parts of an aromatic hydrocarbon having 8 carbon atoms and that the residue consists of hydrocarbons having at least 9 carbon atoms.

In this distillation operation, when one distillation column is used, the fraction having a lower boiling temperature is recovered from the top of the column, the fraction of desired aromatic hydrocarbons is recovered from the middle of the column, and the residue is re- I covered from the bottom of the column. Of course, it is possible to use two or more distillation columns. The distillation is carried out under atmospheric pressure or reduced pressure. Normally, the distillation operation 1s carried out at a temperature of 70 C. under a normal atmospheric pressure, and with a reux ratio of 2. However, these operating conditions are not intended to limit the scope of the present invention.

The desired product fraction thus obtained consists of aromatic hydrocarbons having 8 carbon atoms as its main components. However, as benzene and toluene are separable easily from the fraction, it is possible to include a great amount of benzene and toluene in the fraction, although these amounts may be greater than the amounts of Xylene and styrene. However, it is preferable that the total amount of benzene and toluene should not exceed 10% by weight. However, it is not desired that any great amount of hydrocarbons having at least 9 carbon atoms be contained in this fraction, because they are an obstacle to subsequent extractive-distillation. The total amount of these hydrocarbons should be less than about 1% by weight.

The separation of xylene and styrene from the fraction is successfully carried out by eXtractive-distillation. The extractive-distillation step is carried out by feeding the fraction obtained from the above mentioned operations into a column at the middle of the column and also feeding a solvent, hereunder dened, into the column near the top of the column, and then the fraction containing concentrated xylene is recovered from the top of the column and concentrated styrene is recovered from the bottom of the column with solvent. One or more distillation columns are used in the extract-ive-distillation, however, it is preferable to use two distillation columns.

Suitable solvents include a dialkyl acetamide such as dimethylacetamide, a dialkylsulfoxide such as dimethylsulfoxide, an alkylene carbonate such as ethylene carbonate and propylene carbonate, a lactone such as 'ybutyrolactone and methyl-fy-butyrolactone, a lactum such as e-caprolactum, phenol, an alkylphenol, a salicylic acid alkylester, aniline, an alkylaniline, a phthalic acid alkylester, a tetraalkylurea, an N,Ndialkyl carbamicester, a glycol monoalkylether such as diethyleneglycol monoalkylether and triethyleneglycol monoalkylether and N- methylpyrrolidone.

In the above mentioned alkyl substituted compounds, it is preferable to use compounds having alkyl group of 1 to 3 carbon atoms.

One to 50 mol of solvent, preferably 2-10 mol of solvent, is used as an extractive-distillation solvent based on one mol of styrene contained in the feed.

In the extractive-distillation step, it is preferable to use a polymerization inhibitor such as hydroquinone for preventing thermal polymerization of styrene. Further, the temperature and pressure should be controlled in the extractive-distillation step; it may be below 120 C. at the bottom (boiler) of the distillation column, preferably within the range of about -110 C. The pressure should be determined according to the structure of the distillation column and the nature of the solvent. However, the pressure may usually be within the range of about 10-200 mm. Hg absolute, preferably within the range of about 20-100 rnm. 'Hg absolute at the top of the column. Virtually any known inhibitor may be used, such as hydroquinone, tert-butylcatechol, phenothiozine, sulfur, etc. The amount of inhibitor employed in the distillation step may be 10-500 p.p.m. based on the weight of the feed, or even more. The preferable reux ratio is within the range of 10-20.

It is considered surprising that styrene can be obtained from cracked oil directly according to the present invention.

In the extractive-distillation step of the present inwention, when using a solvent selected from the group consisting of a dialkyl acetamide such as dimethylacetamide, a dialkylsulfoxide such as dimethylsulfoxide, an alkylene carbonate such as ethylene carbonate and propylene carbonate, a lactone such as q/-butyrolactone and methyl-'ybutyrolactone, a lactum such as e-caprolactum, phenol, an alkylphenol, a salicylic acid alkylester, aniline, an alkylaniline, a phthalic acid alkylester, a tetraalkylurea, an N,N-dialkyl carbamicester, a glycol monoalkylether such as diethyleneglycol monoalkylether and triethyleneglycol monoalkylether and N-methylpyrrolidone styrene can be obtained at a high purity and at a high yield ratio.

Heretofore, styrene has been produced by dehydrogenation of ethylbenzene of high purity and distillation of the resulting products under reduced pressure. Ethylbenzene has been obtained by reaction of ethylene and benzene, or separated from fractions of Xylenes by distillation. It would be thought to be particularly advantageous to recover ethylbenzene from the xylene fraction since great quantities of the xylene fraction can be readily obtained from the patro-chemical industry. However, in fact, because the boiling points of p-Xylene and m-Xylene are close to that of ethylbenzene, it is sometimes necessary to use a distillation column having as many as 350 plates for separating ethylbenzene from the xylene fraction. Therefore, such method has a drawback both technically and economically.

On the other hand, the method which consists of transforming ethylbenzene contained in the xylene fraction into styrene by dehydrogenation and recovering styrene thus obtained by distillation has also been examined. However, in this method, Xylenes, especially O-Xylene (boiling point 144.4 C.) are very close to the boiling point (1\45.2 C.) of styrene, and it is virtually impossible to separate styrene from the resulting mixture by normal distillation. Therefore, the technique of separating styrene from a xylene fraction gives an unsolved problem for practical use at this time. As to this technique, some improved methods have been proposed as follows:

(1) The method of separating styrene by extraction using a silver salt aqueous solution (2) The method of separating styrene by azeotropic distillation (3) The method of separating styrene by extractivedistillation using dialkylforrnamide especially dimethylformamide as a solvent (4) The method of separating styrene by polymerization of styrene in the Xylene fraction However, these methods have still presented problems. The method (l) is expensive because of the silver salt; the method (2) has a low separation etlciency; the method (3) is troublesome because styrene and xylene respectively form azeotropics with dimethylformamide and so it is virtually impossible to recover styrene and xylene from solvent by distillation; the method (4) is actually impractical.

However, styrene can be -obtained from the xylene fraction easily by the extractive-distillation using the above mentioned solvents.

In the present invention, the residue thus obtained consists essentially of styrene and solvent. Styrene can be separated easily by normal distillation of the residue under reduced pressure. It is preferable that the pressure be controlled within the range of about 20-100 mm. Hg absolute at the top of the column. In order to prevent thermal polymerization of the styrene, it is preferable that the temperature be kept below 120 C. at any plate of the column. It is also preferable to use an inhibitor in this distillation as well as in the extractive-distillation. Styrene can be recovered from the top of the column as a fraction.

In the extractive-distillation step, the fraction taken from the top of the column consists mainly of xylene but toluene, benzene and other saturated and unsaturated aliphatic hydrocarbons may be present. The fraction, as it is or after washing with Water, may be hydrogenated by using a catalyst such as Mo, Co, Ni and combinations thereof. The hydrogenation reaction may be carried out under a pressure of -200 kg./cm.2 absolute and at a temperature of 10D-400 C. In this hydrogenation procedure it may be advisable to use a mixture with xylene fraction of extractive-distillation and the lower fraction and/or the residue obtained from the distillation of the cracked oil.

According to the present invention, aromatic hydrocarbons such as benzene, toluene, xylene and styrene can be recovered from the cracked oil successfully. Especially according to the present invention, xylene having good quality (containing a high concentration of P-xylene and a low concentration of ethylbenzene) can be obtained, and also styrene having high purity can be obtained from cracked oil directly. P-xylene is an important raw material of terephthalic acid, and styrene is also important as a raw material in plastics. The present invention should contribute to reduce the costs of these products greatly.

Some examples of the process of the present invention will now be described.

Example l Cracked oil of depentanized bottom product containmg 7 .5% by weight styrene was obtained from the commercial plants of naphtha cracking. Lower fractions whose boiling temperatures of up to 130 C. and higher fractions of beyond '150 C. were removed by fractional distillation of said cracked oil.

The remaining oil was extractively distilled, using two distillation columns. The first column which has plates number of 50 was operated under 70 mm. Hg absolute at 60 C. using a reflux ratio of l2, and the second column which has plates number 60 was operated under 20 mm. Hg absolute at 45 C. using a redux ratio of 15. The oil was fed into the middle plate of the first column and dimethylacetamide in the same amount of the oil (containing 300 ppm. of tert-butylcatechol) was supplied from the top of the column. The residue from the bottom of the first column was fed into the middle plate of the second column, and dimethylacetamide in a quantity of `0.5 times the amount of the residue was supplied to the top of the secondl column. The fractions from the top of the second column were recycled to the middle of the irst column, and the mixture of styrene and the solvent was obtained from the bottom cf the second column.

Said mixture was fractionated under a reduced pressure at C. with a reliux ratio of 3, styrene Was thus obtained from the top of the column having 99.2% purity and 86% yield ratio based on the amount of styrene contained in the cracked oil.

The fraction from the top of the lirst extractive-distillation column and the lower fraction were mixed together. The mixture was washed with water, and was hydrogenated under 40 atm. at 330 C. in the presence of Co-Mo catalysts. Xylene contained in the reaction mixture thus obtained was composed of 21% by weight ethylbenzene, 18% by Weight P-xylene, 36% by weight m-xylene, and 25% by Weight O-xylene. Comparably, xylene without separating styrene contained 47% by weight ethylbenzene and 12% P-xylene.

Example 2 The same operation as Example 1 was carried out, except using 4-methyl-y-butylolactone instead of using dimethylacetamide of Example 1 as the solvent.

Styrene was recovered at 98% purity and with a 75% yield ratio based on the cracked oil.

Xylenes contained in the reaction mixture were composed of 25% by weight ethylbenzene, 17% by weight P-xylene, 34% by weight m-xylene and 24% by weight O-Xylene.

Example 3 A cracked oil containing 7% by weight styrene was obtained by fractional distillation of by-product of a plant producing ethylene after removing hydrocarbons having up to 5 carbon atoms. The lower fractions having boiling temperatures of up to C. under 150 rnm. Hg absolute were separated from the cracked oil by fractional distillation and the higher fractions having boiling temperatures of at least 80 C. under 43 mm. Hg were separated by fractional distillation. Fractions of hydrocarbons having 8 carbon atoms as the main portion were obtained.

Said fractions were extractively-distilled. Two distillation columns were used for extractive-distillation. The rst column, composed of 50 plates, vwas operated under 50 mm. Hg absolute at 55 C. using reflux ratio of 15, and the second column, composed of 60 plates was operated under 20 mm. Hg absolute at 45 C. using a reflux ratio of v20.

Said fractions were fed into the middle plate of the first column and dimethylsulfoxide (containing 500 p.p.m.

in cracked oil. The fractions from the top of the rst column were hydrogenated under 50 atm. pressure at 380 C. in the presence of Co-Mo catalysts. The xylene fraction thus obtained was composed of 20% by weight ethylbenzene, 18% by weight P-xylene, 36% by weight rnxylene and 26% by vweight O-xylene.

On the other hand, the xylenes contained in the reaction mixture hydrogenated without separating styrene by extractive-distillation contained 48% by weight ethylbenzene and 12% by weight P-xylene.

Example 7 The same operation of Example 6 was applied except using 'y-butylolactone instead of dimethylacetamide as the solvent.

Styrene was recovered with 99.2% purity and at an 85% yield ratio.

Xylene contained in the reaction mixture were composed of 21% by weight ethylbenzene, 18% by weight P- xylene, 36% by weight m-xylene and 25% by weight O-xylene.

Examples 8-24 Styrene and xylene were separated from each other and from various compositions consisting of styrene and xylene by extractive-distillation using various solvents. In these extractive-distillations, each distillation column had a capacity of 2.5 meters height and 30 mm. inner diameter,` and was filled up with 3 x 3 mm. wire nettings.

Each composition was supplied from a feed plate set up at 1.8 meter height of each column, and also each solvent was supplied from a feed plate set up at 2.4 meters height of each column.

Xylene was recovered from the top of the column as a fraction, and styrene was recovered from the bottom of the column as residue.

These examples produced results as shown in table.

In this table, each percentage of purity of hydrocarbon represents the concentrate ratio except of solvent.

To be sure, in these examples, solvents were mainly recovered from the bottom of the columns together with styrene as residue.

We claim:

1. The method for separating xylene and styrene from a C3 fraction containing primarily xylene and styrene 10 comprising extractively distilling said C3 fraction with a dialkyl acetamide; recovering a xylene fraction in the form of a vapor, removing the styrene in a solvent solution of the dialkyl acetamide and rectifying the solvent solution to separate the styrene from the dialkyl acetamide.

2. The method according to claim 1 wherein said extraction distilling is conducted in the presence of an eective amount of an inhibitor sucient to prevent polymerization of said styrene in said solvent solution.

3. The method according to claim 1 wherein 1-50 mols of said dialkyl acetamide is used per mol of styrene in said C3 fraction.

4. The method according to claim 1 wherein said dialkyl acetamide is dimethyl acetamide.

5. The method according to claim 2 wherein said inhibitor is a member selected from the class consisting of hydroquinone, tert-butylcatechol, phenothiozine, sulfur and mixtures thereof.

6. The method according to claim 1 wherein said xylene containing fraction is thereafter hydrogenated whereby aliphatic unsaturated impurities are hydrogenated and thereafter separating said impurities from said xylene.

References Cited UNITED STATES PATENTS 2,385,235 9/ 1945 Schneider 203-64 3,018,228 1/ 1962 Cornell 203-58 2,959,626 11/ 1960 Krausse et al 203-84 3,209,044 9/ 1965 Meek et al. 260-669 A 3,227,632 1/ 1966 Schmalenbach et al. 208-313 3,272,723 9/ 1966 Fannin 203-57 3,328,267 6/ 1967 Mller 203-60' 3,388,531 6/1968 Bolles 203-57 3,415,739 10/ 1968 Eisenlohr et al. 208-313 3,445,537 5/ 1969 Luther et al 208-313 WILBUR L. BASCOMB, In., Primary Examiner U.S. Cl. X.R.

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