US2794841A - Aromatization of naphtha - Google Patents

Description

`lune 4, 1957 R. G. HAY ET AL AROMATIZATION 0F NAPHTHA Filed OIM.. 19. 1954' www United States Patent AROMATIZATION F NAPHTHA Russell G. Hay, Fox Chapel, and Ralph W. Helwig, Oakmont, Pa., assignors to Gulf Research c Development Company, Pittsburgh, Pa., a corporation of Delaware Application October 19, 1954, Serial No. 463,239

1 Claim. (Cl. 2611-668) This invention relates to the preparation of aromatic hydrocarbons and more particularly to a process for the dehydrogenation of petroleum hydrocarbons to form aromatic hydrocarbons and the recovery of the aromaticA compounds from reaction products having lower boiling points.

Aromatic hydrocarbons in the gasoline boiling range are valuable as fuel for internal combustion engines because of their high anti-knock rating, and also are valuable solvents. Specic aromatic hydrocarbons such as benzene, toluene, xylene and ethylbenzene in substantially pure solutions are important raw materials in the synthesis of plastics and other organic compounds. The aromatization of less valuable hydrocarbons of higher hydrogen to carbon ratio than aromatic hydrocarbons is accomplished by passing the non-aromatic hydrocarbons in contact with a suitable catalyst at an elevated temperature at which dehydrogenation and ring closure, if necessary, occur. For example, it has been suggested in the prior art that the non-aromatic hydrocarbons can be passed in contact with a composite of alumina and chromia at temperatures in as wide a range as 750 to 1200 F. to form aromatic hydrocarbons.

The aromatization of non-aromatic hydrocarbons is a highly endothermic reaction; hence, it is necessary to supply large quantities of heat to the reaction to maintain the optimum reaction temperature. Large volumes of hydrogen are formed in the aromatization reaction, which hydrogen, in addition to the other reaction products, causes high velocities of reaction products and entrainment `of substantial amounts of catalyst lines from uidized catalytic aromatization processes.

This invention resides in a uidized catalytic process, and apparatus therefor, for the preparation of aromatic hydrocarbons from non-aromatic hydrocarbons of higher hydrogen to carbon ratio, and the separation and recovery of the hydrocarbons boiling in the range of the aromatic hydrocarbons from the entrained catalyst fines and reaction products of lower boiling point. Throughout this specification, the term aromatic hydrocarbons means benzene and alkylated benzenes such as toluene, Xylene and ethylbenzene. Aromatic hydrocarbons having more than one benzene ring are not includedwithin the scope of the term aromatic hydrocarbons as herein used.

The single figure of the drawings is a schematic ow sheet illustrating a process and apparatus suitable for use in this invention. y

Referring to the drawing, charge stock is delivered from a storage tank through a line 12, pump 14, heat exchanger 16 and furnace 18, to a transfer line 20 from which the charge stock is discharged into a reactor 22 in which a tluidized bed of catalyst is maintained. Suitable charge stocks are normal and branched paraihns and olens of at least six carbon atoms and naphthenic hydrocarbons preferably having six or more carbon atoms in the naphthenic ring. The charge stock can be substantially a single compound, or a mixture of hydrocarbons having the same number of carbon atoms per molecule for the preparation of a specific aromatic compound, as in the aromatization of normal hexane and hexene to produce benzene. Also suitable for charge stocks are mixtures of hydrocarbons of hydrogen to carbon ratios higher than those of aromatic compounds having different numbers of carbon atoms per molecule, and particularly mixtures of petroleum hydrocarbons boiling in the gasoline range to produce .a mixtureof different aromatic hydrocarbons. The term naphtha is employed in this specification to designate'non-aromatic hydrocarbons of the proper boiling point suitable as charge stocks in the process of this invention.

Recycle gas consisting principally of hydrogen from a line 24 in amounts up to about 6000 standard cubic feet per barrel of naphtha is mixed with the charge stock prior to entrance into the furnace 18, wherein the mixture is heated to approximately the reaction temperature. The charge stock discharged from furnace 18 picks up regenerated catalytic discharged from the bottom of a standpipe 26 and delivers it to the fluid bed within vthe reactor 22.

The catalyst within the reactor 22 can be any conventional catalyst for conversion of naphtha to aromatic hydrocarbons in finely divided form in which the major weight proportion of the particles falls within the range of about 20 to 100 microns. Examples of suitable catalysts are composites of alumina and chromia and alumina and ,molybdena The alumina-chromia catalysts, which are preferred for the preparation of benzene, can be prepared by the impregnation with chromium nitrate of an activated alumina, or an activated alumina impregnated with small amounts of silica, followed by thermal decomposition of the chromium nitrate to form an aluminachromia composite. Other suitable alumina-chromia catalysts are prepared by coprecipitation of alumina and chromia followed by washing, drying and calcining. Preferred catalysts are alumina-chromia catalysts contain ing about 5 to 30 percent chromia which are further improved by treatment with sodium hydroxide or potassium hydroxide.

The finely divided catalyst particles are suspended in a uidiz'ed dense phase by the ascending vapors of charge stock, hydrogen-containing gases added to the charge stock, and reaction products. The temperature in the reactor may range from as low as 750 F. to as high as l200 F.; however, itis preferable that the temperature within the reactor be within the range of 900 to 1150 F., and still more desirably between 950 and 1100" F. The pressure is maintained as low as possible consistent with the maintenance of satisfactory throughputs through the reactor to improve the yield of aromatic hydrocarbons. Pressures below about 25 p. s. i. g., and preferably about 5 p. s. i. g., are employed. The space velocity of the charge stock is between 0.1 volume of liquid feed per volume of catalyst per hour and 10 volumes per volume per hour.

Deactivated catalyst is withdrawn from the reactor 22 through a standpipe 28 and discharged into a transfer line 30 in which :the catalyst is transported by a stream of air introduced through Va supply lline 31 from a source, not

shown, to a regenerator 32. A fluidized bed of catalyst i such as fuel oil, into the regenerator through a =line 36 to heat the regenerated catalyst to a temperature sufeiently high .to convey substantial heat to the .reactor in the form of sensible heat of the catalyst. The temperature in the regenerator is maintained at 950 to l250 -Flue gases from the regenerator pass through a cyclone separator 3 8, in which entrained catalyst .particles are separated and returned to. the liuid bed, and are dis- Charged through line 40 to a stack.

.Gaseous reaction products from the uid bed within reactor 22 pass through a separator 42, in which the larger of the entrained catalyst particles are separated from `the reaction products, and then are delivered through line 44 to a scrubbing tower 46. Prior to. their introduction into. the scrubbing tower 46, the reaction products are quenched ,with a relatively cold stream of quench oil from line 48. The mixture of quench oil and reaction produqts enters the bottom of scrubbing tower 46 through a line 5 0. The reaction products, which remain ill the gaseous phase, ow upwardly through the scrubhing tower in countercurrent contact with descending scrubbing oil which cools the reaction products to a temperature suitable for further processing and removes entrained catalyst fines from the reaction products. The scrubbing tower is provided with suitable apparatus such as the bubblecap trays and disk and doughnut trays indicated, diagrammatically for improving the contact between, quench oil and ascending gaseous reaction products.

The quench oil and catalystnes are discharged from the bottom of scrubbing tower 46 through a line 52 and are circulated by means of a pumpv 54 through a cooler 5.6'. A portion of the cooled quench oil discharged from Cooler 56returns via line 48 for quenching reaction products. A second portion of the. cooled quench oil passes :throughY lines 58 and 60 to. a point at approximately the middle of: the scrubbing tower 46. to provide a downward flow ofi liquidin the tower to scrub the reaction products. A third: portion of the quench oil passes through line 62 to a. continuous lter 64, or other suitable apparatus, in which the entrained catalyst particles are separated from the. quench oil. The filtered quench oil is returned through lines 66. and cooler 68- to the top of the scrubbing tower 46. The quench oil introduced into the top oi; the.. scrubbing tower 46 provides catalyst-free reflux in the top of the scrubbing tower which further cools the gaseous reaction products and cleans traces of catalyst which-.may be present from the gases.

T-he gaseous reaction products from scrubbing tower 46;ow through line 70.to a compressor 72- in which the pressure on the gases is raised to a pressure which will facilitate the absorption of the aromatic hydrocarbons in an4 absorber oil. A pressure in the range of 35 to 75 p. s. i. is preferred. The compressed reaction products are then cooled inv a. heat. exchanger 74 and introduced into the. bottom of an absorption tower 76. A stream of* lean absorber oil is introduced into the Itop of the absorber. tower through linev 78A and descends through the tower countercurrently to the gaseous reaction products. The descendingabsorber oil extracts a higher boiling. fraction, herein designated as the aromatic fraction, which includes the aromatic hydrocarbons and other hydrocarbons of approximately the same boiling point as the aromatic hydrocarbons, Ifrom the reaction products of lower boiling-point, and is discharged as a rich oil from the. bottom of absorption tower 76 through a line 80. The reaction. products which are not absorbed by the absorber oil, and which largely consistof hydrogen, are discharged from the. top of the absorption tower 76 throughline 82. into a gas line 84; The gas line 84 is connected withline 24 for the recycle of hydrogen to the reactor, Absorption tower 76 is providedwith bubblecap trays, perforatedftrays, ortotherapparatus-such as packing, toimprove the contact between the liquid andgaseous phases therein.

Rich. absorber` oil is directed through line 80 to a heat exchanger 83. -in which it is heated byheat exchange with lean absorber oil and a then delivered through a line into the top of a stripper 86. Stripper -86 -is equipped with bubble cap trays, perforated trays, packing, or other means for providing etiicient contact between the descending rich oil and ascending gases which strip the aromatic fraction from the rich oil. Lean absorber oil is discharged from the bottom of the stripper 86 through a line 88 to a pump 90. A portion of the lean oil is pumped through a hcaterv92, which serves as a boiler, and then back to the stripper 86 through line 94. Heater 92 provides the. heat necessary for the vaporization of the absorbed aromatic fraction lfrom the rich oil. Another portion of the lean oil passes from pump through a line 96, and heat exchanger 83, wherein it gives up a portion of its sensible heat to the rich absorber oil, and then passes through a line 98 to heat exchanger 16 in which the Ilean oil is further cooled by heat exchange with the charge stock. The cool lean oil from heat exchanger 16 is further cooled in a cooler 100 and delivered through line 78 to the top of the absorber- 7-6 for the absorption of the aromatic fraction from the lower boiling point reaction products. Makeup absorber oil is added to the system as required through line 101.

Gaseous reaction products stripped from the absorption oil in stripper 86 are discharged from the top of the stripper through aline 102 and pass through a cooler 104 to an accumulator 106. Gases separated from the reaction products in the accumulator 106 are delivered through line 108: tothe gas line 84. Liquid reaction products are removed' from the accumulator 106 through a line 1'10and delivered to apparatus, not shown, for separation ofthe. aromatic hydrocarbons from non-aromatic hydrocarbons ot? substantially the same boiling range. The nonaromatic- .hydrocarbons separated from the aromatic hydrocarbons may be recycled through the reactor 22, if desired.

The following specic embodiment of this invention is described' in detailto illustrate the process of this inventi'on for the preparation of benzene 4from a specic charge stock.

A charge stock consisting of approximately 80 percent by volumeof` normal hexane, 13% percent by volume methylcyclopentane, 5 percent by volume isohexanes, and ll/ percent by volume benzene is heated to a temperature of. 1022" F. and is introduced into a reactor which is maintained at the same temperature. The charge stock is passed through the reactor at a liquid space velocity off 1.0 volume per volume of catalyst per hour. The pressure in the reactor is regulated at 5 p. s. i. g. Hydrogen is recycled to the reactor by introduction into the charge stock at the rate of 1500 standard cubic feet per barrel of charge prior to final heating of the charge to the reaction temperature. A lluidized bed of coprecipitated: alumina-chromia catalyst containing approximately 80.percent alumina and 20 percent chromia is maintained inthe reactor. l

Gaseous reaction products together with entrained catalyst fines are withdrawn from the reactor at approximately the reaction temperature and quenched with a quench oil' at a temperature of 150 F. to produce a mixture at a temperature of approximately 500 F. The quench oil'- is a gas-oil of approximately 200 molecular Weight. The quenched mixture is introduced into a tower. in which it isscrubbed with additional quench oil. Gaseous reaction products pass upwardly through the scrubbing-tower and are discharged therefrom at F. Quench oil= and entrained catalyst lines are withdrawn fromA the-bottom of the tower and are cooled at approximately F. A portion of the cooled quench oil withdrawnfromthe bottom of the tower is recirculated continuouslythrough the scrubbing tower. action products are also washed with a catalyst-free quench oil for further cooling and scrubbing of the reaction products.

The scrubbed gaseous reaction products are compressed The gaseous re-v to a pressure of approximately 40 p. s. i. g. and cooled to a temperature of 130 F. The aromatic fraction of the reaction products is absorbed in a light gas-oil of approximately 200 molecular weight. A portion of the unabsorbed gases, which consists principally of hydrogen, is recycled to the charge stock at the rate set forth above.

The rich absorption oil is heated to a temperature of approximately 250 F. and introduced into a stripper tower operated at a pressure of approximately p. s. i. g. A reboiler at the bottom of the stripper provides the heat required for stripping the aromatic fraction from the absorber oil.

The quenching of the reaction products not only cools the reaction products to a temperature suitable for further processing, but provides an ecient separation of entrained catalyst fines from the very large volume of gaseous reaction products. Recirculation of cooled quench oil countercurrent to the ow of gaseous reaction products thoroughly scrubs the gases prior to their discharge from the scrubbing tower. Erosion of the equipment in which the catalyst lines are separated from the reaction products, as well as cooling and other processing equipment, its minimized by the quenching and scrubbing procedure.

The details of the process have been set forth with respect to an embodiment of this invention for the purpose of illustrating a process in detail. It will be appreciated that speciiic operating conditions will depend at least in part upon the details of the design of the equipment. Specific operating conditions will be dependent in some instances upon other operating conditions. For example, the temperature employed in the absorber and stripper will depend upon the operating pressure of those units and their fractionating efliciency. This invention is not limited to such details except as set forth in the appended claim.

What we claim is:

A process for the preparation of aromatic hydrocarbons comprising mixing a recycle gas consisting principally of hydrogen with a naphtha consisting essentially of non-aromatic hydrocarbons having at least 6 carbon atoms per molecule in a ratio of about 1500 to 6000 standard cubic feet of recycle gas per barrel of naphtha to form a charge stock, passing the charge stock through a heated coil to heat the charge stock to a temperature of 900 to 1150 F., introducing hot regenerated aromatization catalyst particles from a regenerator into the heated charge stock to form a suspension of the catalyst in the charge stock, delivering the suspension of the catalyst in the charge stock into the lower end of a reactor containing a fluidized bed of the aromatization catalyst maintained at a temperature of 900 to l F. and a pressure below about 25 p. s. i. g., withdrawing a stream of aromatization catalyst from the uidized bed in the reactor and transferring it to a regenerator, burning carbonaceous deposits from the catalyst in the regenerator, the temperature in the regenerator being higher than the temperature in the reactor and in the range of 950 to 1200 to supply heat to the reactor upon transfer of the regenerated catalyst to the reactor, withdrawing reaction products from the upper surface of the uidized bed and passing them through a separator in which entrain/ed catalyst particles are separated from the reaction products and returned to the iiuidized bed, passing the reaction products from the separator through a line into a scrubbing tower adapted for countercurrent contact of gas and liquid and maintained at substantially atmospheric pressure, withdrawing a slurry of quenching oil and catalyst from the bottom of the scrubbing tower, cooling the withdrawn slurry, discharging a first portion of the cooled slurry into the line for admixture with the reaction products between the separator and the scrubbing tower, returning a second portion of the cooled slurry into the scrubbing tower above the point of admission of the reaction products, further cooling the quenching oil in a third portion of the cooled slurry, separating catalyst from the third portion of the cooled slurry to form a substantially catalyst-free stream of quenching oil, introducing the catalyst-free quenching oil into the scrubbing tower at a point higher than the point of introduction of the second portion of slurry to provide clean reflux further cooling and cleaning the gaseous reaction products in the scrubbing tower, discharging gaseous reaction products from the upper end of the scrubber and compressing them to a pressure in the range of about 35 to 75 p. s. i. g., cooling the compressed gaseous reaction products and passing the min countercurrent contact with an absorber oil to strip aromatic hydrocarbons from the recycle gas, returning recycle gas for admixture with the naphtha, and stripping aromatic hydrocarbons from the absorber oil.

References Cited in the le of this patent UNITED STATES PATENTS Cardwell et al Dec. 22,

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