US2444855A - Production of aromatics by thermal cracking - Google Patents

Description

July. 6, 1948- R. MQ sHEPARDsoN ErAl. 2,444,855

PRODUCTION OF AROMATICS BY THERMAL CRACKING v Filed Dec. 27, 1940 FRAcr/aNA Tuve 01.0 HIV Z-Il "dmz Fg n 42 450 @www REFRICERAT//Vc Y 36 y S/TRIPP/L Illl WASH A TOWER /IVAE T Patented July 6,1948

PRODUCTION F AROMATICS BY THERMAL CRACKING Robert M. Shepardson, Cranford, and litoyv L. Mathiasen, Fords, N. J., assignors to Standard Oil Development Company, a corporation of Delaware Application December 27, 1940, Serial No. 371,870

4 Claims. (Cl. 260673.5)

The present invention relates to improvements in the production of aromatics, including toluene, from non-benzenoid hydrocarbons, and more particularly it relates to processes for producing nitration grade toluene, entirely satisfactory for the manufacture of tri-nitro toluene, by cracking a paraffnic or naphthenic gas oil under certain specified conditions, fractionating the cracked products, recovering a fraction boiling within the range of from 200250 F. and subjecting the latter fraction to solvent extraction to recover the desired aromatic. By practicing our invention according to the present disclosure, we have been able to secure toluene having a purity greater than 99 per cent, or, in other words, we have secured toluene of nitratable grade.

One object of our' invention, therefore, is to crack a gas oil thermally, preferably a gas oil having an end boiling point not above approximately 850 F. and containing a preponderance of parafiinic and/or naphthenic constituents.

Another object of our invention is to thermally crack gas oil in a relatively short period of time at a high temperature, thereafter to fractionate the cracked products and to recover a fraction boiling within the range of from about 200250 F. and thereafter selectively dissolving from this fraction very pure toluene which may be separated .from the solvent in known manner.

Another object of our invention is to produce aromatic hydrocarbons from non-benzenoid hydrocarbons.

Other and further objects of our invention will appear from the ensuing description and claims, reference being had to the accompanying draw- The drawing represents schematically a system or combination of apparatus elements in which our invention may be carried into eifect.

Referring in detail to the drawing, a gas oil having an A. P. I. gravity of about 33 and boiling within the range of from about 500 to 750 F. was introduced into the system through line I and pump 2 into line 6 in which this material was combined with approximately 80 mol per cent cf steaznbased on the mixture of steam and hydrocarbons introduced through line 4, thence discharged into a coil 3 disposed in a furnace setting 5 where the feed stock was heated to a temperature of about 1200 F. while maintaining a pressure of about 8 lbs./sq. in. gauge, and in which the feed rate to the coil was about 0.5 volume of cold oil per volume of coil per hour. The oil was permitted to remain in the coil for a period of 2.7 seconds. Under the conditions specied, it later turned out, as will more fully appear hereinafter, that this operation resulted in the production of 45 per cent aromatics in the gasoline. The cracked products were withdrawn from coil 3 through line l0 and thence discharged into a fractionating column |2. A heavy fraction suitable for use as a fuel oil w-as withdrawn from fractionatng column |2 through line 30 and a gas oil fraction through line 3|. This gas oil may be recycled to line 6 by pump 1 and line 8, but we have found this material to be a less satisfactory charge stock than virgin materials and prefer to dispose of this fraction elsewhere. The gasoline and gaseous fractions were withdrawn from the top of column I2 through line 32, thence through condenser I4 and to separating drum I5 by means of line 33. Water was separated in drum I5 and withdrawn through line IB. Gas was removed from separating drum I5 through line I1 and gasoline through line I8, each being compressed or pumped to about lbs./sq. in. gauge by means of compressor I9 and pump 20, after which these fractions were combined in line 23 and discharged into separating drum 2|. This compression was necessary to recover the small quantities of aromatics contained in the gas fraction separated in drum l5 at' atmospheric pressure. Gas was withdrawn from separating drum 2| through line 22 and the distillate through line 24 into coil 35 disposed in furnace setting 3'1, following which the vapors were fractionated in column 26. Material boiling above approximately 250 F. and free of toluene was withdrawn through line 25 and the overhead fraction containing toluene was transferred through line 21 into fractionating column 28. Materials boiling below approximately 200 F. and free of toluene were withdrawn through line 29 and the crude toluenecontaining fraction through line 34. The gaseous fractions leaving the system through lines 22 and 29 were found to be rich in olefin fractions such as ethylene, propylene, and butylene, as well as butadiene and higher conjugated diolens. These materials are particularly valuable for the manufacture of high octane blending agents by polymerization or alkylation, or for lay-product manufacture. The toluene-containing fraction withdrawn from fractionating tower 20 through line 34 was cooled with water in cooler 36 thence discharged through line 38 into a refrigeration zone 40 where it was cooled to a temperature in the neighborhood say of 40 F. The refrigeration may be effected by liquid prop-ane or any other known or convenient method. The refrigerated fraction was then withdrawn from zone 40 through line 42 and discharged into a solvent treating tower 46 at a point near the bottom thereof, as indicated. At the same time, chilled liquid SO2 was discharged into solvent treating tower 46 through line 59 at a point near the top thereof. In the tower the hydrocarbon oil passes upwardly against the downwardly flowing SO2, thus forming a ramnate and an extract fraction. The rafinate portion, which is enriched in paraiiinic constituents and also containing some SO2, was withdrawn from tower 46 through line 52 into stripper 53 in which the SO2 is removed through line 55 and the SO2-free raffinate through line 54. The SO2 from the rafnate thence passes through line 55 to SO2 storage drum 80 from which point it was recycled through line 56 and pump 51 by means of line 58 to the extraction tower 46.

'I'he extract fraction, on the other hand, was withdrawn through line 65 and then discharged into a washing tower. 61 at a point near the top thereof. At the same time, isopentane was discharged into tower 61 through line 68 near the bottom thereof. The effect of the iso-pentane is to displace the non-aromatic hydrocarbons contained in the SO2 extract and substitute isopentane so that the extract at the bottom of the tower will contain SO2. aromatic hydrocarbons, and substantially no non-aromatic hydrocarbons except isopentane. This result is accomplished by a sort of mass action effect wherein the large preponderance of isopentane over the non-benzenoid hydrocarbons in the extract causes substantially the complete removal of the latter from the extract. The advantage of replacing the hydrocarbons originally in the extract with isopentane is that the aromatics and isopentane may eventually be separated from each other by simple distillation due to the fact that there is a rather wide hiatus in the boiling points between the two substances. yThe non-benzenoid hydrocarbons originally in the SO2 extract had a boiling point in close proximity to that of the aromatics and therefore a separation could not be effected by simple distillation. The non-benzenoid hydrocarbons originally in the extract, together with the isopentane, are withdrawn from the top of tower 61 through line 10, from which they may be delivered through coil 1| disposed in furnace setting 63 and thence through line 12 into tower |2| in which isopentane is separated from the higher boiling non-benzenoid hydrocarbons, the isopentane being returned through line |25 to cooler 13 thence to storage drum 18 through line 14. Isopentane for tower 61 is supplied from storage drum 16 through line 11 and pump 8i' thence to line 68. The SO: extract is withdrawn from the bottom of the tower through line 15, delivered to stripper 18 in which the SO2 may be separated from the aromatics, compressed, refrigerated and return through line 19 to storage drum ,80 and returned through line 56, pump 51 and thence line 5l) to tower 46. The aromatic compounds are withdrawn from the solvent recovery system through line 82 and thence intimately mixed with an acid. such as sulfuric acid, introduced through line 92 at point 93, this mixture being discharged into separating drum 90 through line 9|. Acid sludge is withdrawn from separating drum 9D through line 94 and the acid treated oil through line 96 into tower 98. A dilute solution of some alkali such as sodium hydroxide in water was discharged into tower 98 through line |80 and passes downward- '4 ly against the upwardly flowing aromatics. thus neutralizing and washing any acid associated with the said aromatics. The washing medium was withdrawn from tower 98 through line |92. This acid treating step was necessary to .remove the very small quantity of oleflns present in the aromatic extract, these olens being objectionable in nitration grade toluene. The purified aromatics and isopentane mixture was withdrawn from wash tower 98 through line |88 and deliveredto a heating coil 84 where the hydrocarbons are heated to vaporization temperature, thence discharged through line ||2 into a fractionating column H4. Mixed xyienes and a small quantity of toluene are withdrawn from the bottom of fractionating column III through line i I3, the major portion of the toluene passing overhead as vapor through line H5 into the fractionating tower |I1. Puried toluene of better than 99 per cent purity was withdrawn from tower H1 a through line I i6 thence discharged into a cooler H8 and nally through line |28 into a receiving drum |28. Overhead vapors from fractionating column H1 consisting of isopentane, benzene, and a small quantity of toluene pass overhead through line I9 in which they are combined with the isopentane fraction from extraction tower 61. 'I'he combined stream is fractionated in column |2| into isopentane as an overhead fraction for return to the extraction system and a bottoms fraction consisting of small quantities of benzene, toluene and non-benzenoid hydrocarbons. This bottoms fraction is withdrawn from fractionating column |2| through line |22 and combined with the mixed xylene fraction from tower H4, the mixture being transferred through line |23, pump |24 and line |28 to the fractionating system following the cracking operation for recovery of the small quantities of toluene which escaped the extraction or distillation zones in the rst-pass through the equipment.

YIn the preceding description, all of the equipment necessary to eilect the separation of isopentane and/or SO2 from the products has not been illustrated because these means are well known to those skilled in the solvent extraction art. Likewise, the method of refrigerating the SO2 and the hydrocarbons has not been described in detail because various methods are known in the art for accomplishing these results.

In the preceding example, we have set forth a preferred modification of our invention, but it is to be distinctly understood that our invention is not limited to these precise details. For example in the cracking coil 9, we may use the following range of conditions:

Pressure Subatmospheric to lbs` Temperature 1100 F.-1400 F. Time of residence in coil, in`seconds 0.515.0

Although steam is not absolutely necessary in the above described cracking operation, we have found that the use of steam` or any other gaseous medium is desirable as it permits the production of a greater quantity of aromatics under otherwise constant operating conditions. Also, the use of steam appears preferable to gaseous fractions in that it can easily be condensed and removed from the system.

Furthermore, with respect to the washing medium or diluent employed in tower 61, the same need not be isopentane but may be any non-aromatic hydrocarbon which has a diierent boiling point from the aromatics contained in said extract. Instead of using SO2, we may use any solvent which has a greater solvent action for aromatic hydrocarbons than for non-aromatic hydrocarbons. As to the charging stock, it is pointed out that for best results the same should be substantially free of olens.

Furthermore, this invention is not limited to the yproduction of substantially pure toluene, as benzene of equal purity can be produced by charging a fraction boiling from approximately 150 F. to 200 F. to the extraction step. Also, mixed xylenes containing ortho, meta, and para-xylene and ethyl benzene can be produced in high purity by charging a fraction boiling from approximately 260 F. to 310 F. to the extraction step.

To recapitulate, our invention comprises cracking a petroleum oil thermally at high temperatures and during a short residence time in the cracking coil, fractionating and recovering a cut boiling within the range of the desired aromatic, subjecting this latter fraction to solvent extraction with a material such as S02 having a preferential solvent action for aromatics, removing the SO: extract to a washing tower where it is` contracted with a non-benzenoid hydrocarbon having a different boiling point than the desired aromatic so as to form a second extract consisting essentially of SO2, or the like, the desired aromatic andthe washing agent separating the SO2 or other aromatic solvent from the hydrocarbons, and iinally distilling the hydrocarbons to recover the desired aromatic.

What we claim is:

i. A method of producing nitration grade toluene o! over 99% purity from paramnic petroleum gas oil hydrocarbons which comprises heating parainic petroleum gas oil hydrocarbons of about 500 to 700 F. boiling range in the presence of a large amount of steam to a temperature of about 1200 to 1400 F. for a period of about 1/2 to 15 seconds at a pressure of not more than 150 lbs/sq. in., fractionating the reaction products, recovering a fraction boiling within the range of from about 200 F. to 250 F., condensing said fraction, cooling said condensed fraction in a refrigeration zone, treating said cooled fraction with liquid sulfur dioxide to form an extract, washing said extract with isopentane, separating sulfur dioxide from said extract, treating said extract with acid to remove oleiins, neutralizing the acid-treated extract and fractionating the extract to recover nitration grade toluene.

2. A method of producing nitration grade toluene o1' over 99% purity from paramnic petroleum gas oil hydrocarbons boiling within the ing within the range oi from about 200 to 250 F., condensing said fraction, cooling said condensed fraction to a temperature or about 40 F., treating said cooled fraction with liquid sulfur dioxide to form an extract, washing said extract with isopentane, separating sulfur dioxide from said extract, treating said extract with acid to remove olefins, neutralizing the acid-treated extract and fractionating the extract to recover nitration grade toluene. 3. In the process of recovering toluene from a hydrocarbon fraction boiling between about 200 and about 250 F. obtained by thermally steam-cracking paramnic gas oil hydrocarbons boiling within the range from about 500 to 700 F. at temperatures of about 1200 to 1400 F., contact times of about 1/2 to 15 seconds and pressures not higher than 150 lbs/sq. in., the steps comprising condensing said fraction, cooling said condensed fraction in a refrigeration zone, treating said cooled fraction with liquid sulfur dioxide to form an extract, washing said extract with isopentane, separating sulfur dioxide from said extract, treating said extract with acid to remove olefins, neutralizing the acid-treated extract and fractionating the extract to recover nitration grade toluene. i

4. The method set forth in claim 3 wherein said condensed fraction is cooled to a temperature of range of from about 500 to about 700 F. which comprises heating the hydrocarbons in the presence of about mol o! steam to a temperature of about 1200 F. for a'period of about 2.7 seconds at a pressure of about 8 lbs/sq. in.. fractionating the reaction products, recovering a fraction boilabout 40 F.

ROBERT M. SHEPARDSON. ROY L. MATHIASEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,871,694 Ihrig Aug. 16, 1932 1,912,349 Tuttle May 30, 1933 2,133,344 Cooke Oct. 18, 1938 2,139,000 Cohen Dec. 6, 1938 2,143,472 Boultbee Jan. 10, 1939 2,218,495 Balcar Oct. 15, 1940 12,263,557 Greenwalt Nov. 25, 1941 2,288,126 Dunn et al s--- June 3 0, 1942 FOREIGN PATENTS Number Country Date 441,104 Great Britain Jan. 13, 1936 443,545 Great Britain 1936 459,189 Great Britain Jan. 4, 1937 459,595 Great Britain Jan. 11, 1937 482,431 Great Britain Mar. 29, 1938 491,992 Great Britain Sept. 13, 1938 OTHER REFERENCES Groll: Vapor-Phase Cracking, Ind. and Eng. Chem., vol. 25, No. 7, pages 784-797 (14 pages) (pages 789- are especially pertinent) (July 1933).

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