US2727077A - Production of aromatic hydrocarbons from virgin naphtha - Google Patents

Description

Dec. 13, 1955 N. FRAGEN ET A1.

PRODUCTION OF AROMATIC HYDROCARBONS FROM VIRGIN NAPHTHA Filed Aug. l0, 1951 Suk. mm? am n m & n m WM N N m o P. W F H T sv m n 0. r vm m k A f r Nm M S wxd/ QN AIIII|\1||. si mi United States PatentO PRODUCTION OF AROMATIC HYDROCARBONS FROM VIRGIN NAPHTHA Nathan Fragen, Hammond, Ind., and Mark C. Hopkins,

Texas City, Tex., asslgnors, by direct and mesne assignments, to The American Oil Company Application August 10, 1951, Serial No. 241,246

8 Claims. (Cl. 260-668) This invention relates to the production of an aromatic hydrocarbon from virgin. naphtha and it pertains more particularly to an improved method for increasing the amount of benzene obtainable from a given naphtha charge by a new combination of fractionation, dehydrogenation, isomerization and recycling steps.

In virgin naphtha, such as straight run and natural gasolines, the amount of benzene is usually quite small but there are considerably larger amounts of cyclohexane and methylcyclopentane. These three components of the virgin naphtha may be concentrated by fractionation into a heart cut boiling in the range of about 150 F. to 185 F. The cyclohexane component of the heart cut may be directly dehydrogenated to produce additional benzene, but the methylcyclopentane component must be isomerized to cyclohexane before it can be converted to benzene. It has been proposed to rst treat a naphtha heart cut containing both cyclohexane and methylcyclopentane with an isomerization catalyst to convert a substantial amount of the methylcyclopentane to cyclohexane and then subject the resulting hydrocarbon mixture to dehydrogenation (U. S. 2,288,866), such procedure having been alleged to improve the eifectiveness of the dehydrogenation catalyst by diminishing requirements of catalyst regeneration and replacement. It has now been found that the isomerization of such a heart eut prior to dehydrogenation presents serious operating difficulties, particularly when the virgin naphtha contains appreciable amounts of benzene. An object of this invention is to provide a distinctively dilferent method of operation which will avoid the operating diiiiculties and disadvantages of said prior process. A further object is to provide a method of operation whereby the size and cost of the isomerization equipment will be greatly reduced, the effectiveness of the isomerization step will be remarkably increased, and the effectiveness of the dehydrogenation catalyst will likewise be enhanced with the overall result of obtaining a phenomenal increase in total benzene yield at lower operating costs than was heretofore believed possible.

A more specic object of the invention is to provide a process wherein a minimum amount of benzene is present in the charge to the isomerization step whereby the isomerization of methylcyclopentane to cyclohexane may be effected more rapidly and to a greater extent with minimum side reactions, such as alkylation. A further object is to provide a process which will enable charging to the dehydrogenation step a.total hydrocarbon mixture which contains a higher percentage of naphthenes (both methylcyclopentane and cyclohexane) than was present in the initial heart, cut. Still another object is to provide an improved method and means for eliminating paraihnic hydrocarbons from a recycle system in order to effect ultimate conversion of substantially all of the methylcyclopentane as well as the cyclohexane into benzene. Other objects of the invention will be apparent as the detailed description of the invention'proceeds.

In practicing the invention, the heart cut (about 150 lin conjunction with the accompanying drawings which r6 .1C-e

F. to about 185 F.) of virgin naphtha is supplemented and enriched with a recycle stream which is rich in both cyclohexane and methylcyclopentane, which is substantially free from benzene and from which hydrocarbons boiling below about 157 F. have been eliminated. This enriching step may actually increase the percentage of methylcyclopentane in the total dehydrogenation charge stream and it always increases the per cent of cyclohexane. It will thus be seen that there is no elimination of methylcyelopentane from the original heart cut component of the dehydrogenation charge; the dehydrogenation charge desirably contains considerable amounts of methylcyclopentane particularly when the dehydrogenation is effected in the so-called hydroforrning operation by contact with an alumina molybdena catalyst (or other known dehydrogenation catalyst having isomerization properties) since some isomerization of methylcyclopentane to cyclohexane is effected in such hydroforming step itself which minimizes the amount of isomerization which must be effected in the separate isomerization step. The term molybdena as employed herein has the meaning commonly ascribed to it in the hydroforrning catalyst art, i. e. it means molybdenum oxide just as chromia means chromium oxide, alumina means aluminum oxide etc.

The effluent stream from the dehydrogenation (hydroforming) step is again fractionated to obtain a heart cut having a true boiling point range from about F. to F. and this second heart cut is extractively distilled with a selective solvent, such as phenol, to remove substantially pure benzene and give an overhead or raiiinate stream which consists chiefly of methylcyciopentane but which contains less than 5% and preferably less than 3% of benzene and which is much smaiier in volume than the original heart cut. The isomerization of this fraction may be eifected under known conditions with known catalysts, a preferred catalyst being an aluminum chloride-hydrocarbon complex which is too low in catalyst activity to effect extensive conversion of the paramnic hydrocarbons which are present. The isomerization unit may be relatively small because the charge thereto is only about one-half to one-third the volume of the initial heart cut and the isomerization is eifected more rapidly and with greater ultimate yields and with less by-product formation (by alkylation, etc.) because of the absence of benzene which was removed by the extractive distillation ste r1libe product stream from the isomerization unit is then fractionated by simple distillation or stripping to remove normal hexane and lighter hydrocarbons (some of which are methyl pentanes resulting from a small amount of isomerization of n-hexane) from a recycle stream which consists chiey of cyclohexane and methylcyclopentane. The very small amount of benzene in this distillation or stripping step enables a more effective separation than would be obtained in the presence of larger amounts of benzene. The recycle stream is employed as a supplement for enriching the initial heart cut and providing an improved total charge for the dehydrogenation step.

From the above description, it will be seen that the process of this invention improves the effectiveness and the efficiency of both the hydroformer unit and the isomerization unit, but the most remarkable advantage of this process is that from a given amount of an initial heart cut it results in the obtaining of about 40% more benzene than could be obtained by the prior process of isomerizing initial heart cut to convert methylcyclopentane to cyclo hexane prior to subjecting the initial heart cut to dehydrogenation.

The invention will be more clearly understood from the following detailed description of a specific example read form a part of this `specification and which illustrate a schematic dow diagram of a plant for processing 17,130 barrels per day of straight run and natural gasolines. I

The virgin naphtha in this example is a mixture of straight run and natural' gasolines containing 1.2% benzene (all percentages are'by volume)l,"3.9%methylcyclopentane, 2.8% cyclohexane and 92.1% of parafnic hydrocarbons which may be considered as inerts in this proces-s. This virgin naphtha is charged 'by' line 10 to fractionator 11 for the removal of hydrocarbons boiling below about 150 F. (all boiling points'recited are true boiling points at atmospheric pressure). Fractionator 11 may'be a conventional tower containing about 65 trays and provided with the required reboiler-and reux means (not shown) and in this example about 5,629 barrels per day of light naphtha is taken overhead through line 12. l.

The bottoms from tower 11 are introduced by line 13 to fractionator 14 which also has 65 trays. This fractionator like all other fractionators hereinafter described is provided with conventional heating means at its base and reux means at its top. Tower 14 is operated to take overhead the rst heart cut boiling from about 150 F. to about 185 F. and to remove as bottoms' through line 15 about 8,449 barrels per day of heavy naphtha. The heart cut which is taken overhead through line 16 in amounts of about 3,052 barrels per day Vcontains 6.2% benzene, 19.6% methylcyclopentane, 147.2% cyclohexane and 60.0% paramnic or inert hydrocarbons.

Instead of subjecting this heart cut to isomerization for converting methylcyclopentane 4to cyclohexane, the heart cut is supplemented or enriched by a recycle stream in amounts of about 1021 barrels per day to give a total dehydrogenation charging stock of 4,073. barrels per day containing 5.0% benzene, 20.4% rnethylcyclopentane,

25.2% cyclohexane and 49.4% paraiinic hydrocarbons.

It will thus be observed that in this example the `charge to the dehydrogenation unit contained a higher percentage of methylcyclopentane, a higher percentage of cyclohexane, a lower percentage of benzene 4and. a lower percentage of paraninic hydrocarbons than were contained in the initial heart cut.

The dehydrogenation may be effected by the use of any known dehydrogenation catalysts underknown conditions for said catalysts, but it is preferred to etect the dehydrogenation in a so-called hydroformer `unit 17. The hydroformer may be of the fixed bed, moving bed or uid type and the catalyst employed therein is preferably molybdena on alumina, although other known hydroforming catalysts, such as chromia or other group VI oxides on alumina, cobalt molybdate, nickel tungstate, etc., may be z employed. The hydroforming is usually eiected with molybdena alumina catalyst at a temperature yin the-range of about'850 to 1050" F., under a pressure inthe range ofabout 100 to 500 p. s. i., with a'h'ydrog'en recirculation rate of 1000 to 10,000 cubic feet 'per' bar-reland a space velocity of 0.2 to 2 volumes yofv'charging stock yper hour per Volume of catalyst. -Othe`rknowndehydrogena tion processes may be employed, such as hydrofining, autofining, platforming (U. S. 2,479,1'09-`l0), etc., Vthe specific operating conditions being chosen inany instance to correspond with the catalyst employed. Since no invention is claimed in the dehydrogenation step per se,

Y this step does not require description in any .further detail` However, the invention is partieularly'applicable to the use or' hydroforrning catalyst and operating conditions for effecting the dehydrogenationbecause of theimprovement in the quality of other products producedfan'd the conversion to benzene of certain components lof the feed other than hydro-aromatical l' i Y l The product stream from the dehydrogenation step, i. e. from hydroformer unit 17, is 'passedfby line 18 to fractionating column 19, which in this example is provided with 55 trays with conventional/redux and r'eboiler.'Y In this example, 3,185 barrels'per vv'day' offhydroformer eilluent is thus fractionated, the total-'hydro'frmei'emuent y containing about 30.3% benzene, 23.6% methylcyclopentane, 6.4% cyclohexane, and .39.1% parainic or inert hydrocarbons. In this example, 287 barrels per day of light naphtha boiling below'about 150 F. is taken overhead from tower 19 through line 20. The bottoms from tower 19 are passed by line 21 to tower 22 which is provided with trays and 'is operated to take overhead a second heart cut boiling in the range of about F. to 185V F.,"in amountshf abou t"2`,29r8 barrels per day. Thus, about 600' barrels per day of heavy naphtha is withdrawn from the base of tower 22'by line 23."V

The second'heart cut .is passed"by'line'24 to about the 20th or 25th tray ofextractive distillation column 25 into which an extractive distillation solvent, such as phenol is introduced through' line at about the 40th or 50th plate in a 65 plate column. 'The extractive distillation column is operated with a phenol to charge ratio of about 2:1 to 10:1, e. g. about 8:1, so that substantially all of the benzene is 'carried with thephenol to the bottom of the tower which is provided with va suitable reboiler whereby rsubstantially all hydrocarbons, other than benzene, are vapo'n'zed and Vdischarged from the top of the column. The phenol-benzene solution is withdrawn from the base of the extractive distillation column through line 27 to stripper 28 from which about 965 barrels per day of benzene is recovered through line 29. The lean phenol is recycled from the bottom of the stripper to the upper part of the extractive distillation column by line 26. For simplicity, reflux means, reboilers, heat exchangers, etc. have been omitted from the drawings and more detailed description of the extractive distillation system is not required because such systems are well known to those skilled in the art.

The overhead from extractiv'e distillation column 2S .amounts to about 1,333 .barrels per day and contains about 1.5% benzene, about 56.4% methylcyclopentane,

'15 .3% cyclohexane and 26.8% parainic or inert hydrolysts are well'known in the art (note U. S. 2,250,118).

In this example', the catalyst is an aluminum chloridearomatic hydrocarbon vcomplex, although aluminum chloride complexes may be prepared vfrom parainic, oleiinic, and/or naphthenic hydrocarbons. The hydrocarbon content of `the complex may be in the range of .30 to 40% ormore .by weight, for example about 35% .by weight. The aluminum chloride-hydrocarbon complex is preferably. milder kin activity than the conventional aluminum*chloride-hydrocarbon complex catalyst used for effecting disproportionation and/ or isomerization of paranic hydrocarbons. 'The temperature employed for eiecting .the isomerization of methylcyclopentane to cyclohexane is lower than that employed for isomerizing or disproportioning parain hydrocarbons and is preferably in therange of about to 220 F. Lower temperatures favor the desired equilibrium for-obtaining maximum conversion of methylcyclopentane to cyclohexane, but lowertemperatures likewise resultin lower reaction rates.' vHigher temperatures improve reaction rates @but .lead to unfayorable equilib'ri'umconditions (less methylcyclopentane conversion) and undulyihigh temperatures :mayllead to a substantial amount of paraffin conversion which in turn requires more make-up aluminum chloride for maintaining the desired catalyst activity. Because ofthe mild reaction conditions, no hydrogen is required vlorsuppressing 'side reactions. A small amountv of :.HClipromotermay be remployed but is not always necessary. No invention is claimed in the isomerization step per se and it should be understood that any known isomerization catalysts may be employed under their known operating conditions. The invention is particularly applicable to the use or catalysts such as aluminum halide-hydrocarbon complexes because with such catalysts the presence of excessive amounts of aromatics in the charging stocks inhibits the isomerization; the extractive distillation step for removing benzene immediately precedes the isomerization step and thus provides an ideal charge for this type of isomerization catalyst.

The isomerization etiuent product stream is introduced by line 32 to fractionating or stripping column 33 which may be provided with about 65 trays and with the usual reboiler at its base. The stream introduced into tower 33 may contain about 1.5% benzene, about 23.4% methylcyclopentane, 47.7% cyclohexane and 27.4% paratn hydrocarbons. The normal hexane and lighter hydrocarbons are removed overhead from column 33 through line 34 at the rate of about 285 barrels per day. The bottoms from column 33 are recycled through line 35 to supply the diluent for the rst heart cut in line 16. This recycle stream contains about 1.4% benzene, 22.5% methylcyclopentane, 57.9% cyclohexane and 18.2% para'inic hydrocarbons. It will be noted that this recycle stream is actually richer in methylcyclopentane than the initial heart cut itself but it contains less benzene and inert parain hydrocarbons and consists chiefly of cyclohexane.

By the improved combination of steps hereinabove described, it will be seen that substantially all of the methylcyclopentane and cyclohexane which was present in the initial virgin naphtha is ultimately converted into benzene so that in this particular example 965 barrels per day of benzene is obtained. With the same amount of the same charging stock employed in a system wherein isomerization of the methylcyclopentane in the initial heart cut is etected prior to the hydroforming or dehydrogenation step in a once through operation as heretofore practiced, only 685 barrels per day of benzene is produced. This remarkable increase in benzene recovery is obtained without any corresponding increase in operating cost or detriment to either the hydroforming or isomerization unit operation; on the contrary, the hydroformer catalyst retains peak activity because the charge enrichment step increases the relative amount of cyclohexane while decreasing the amount of parajln hydrocarbons, the isomerization unit operates at peak effectiveness because of the absence of large amounts of benzene and the relatively small amount of parajiin hydrocarbons, and the removal of normal hexane and lighter hydrocarbons (in column 33) can be effected without the loss of appreciable benzene because this separation step is eiected at a portion of the system wherein benzene concentration is small.

While the invention is primarily directed to the production of benzene from a virgin naphtha heart cut boiling in the range of 150 to 185 F., the invention may also be applied to the production of toluene from a virgin naphtha fraction boiling in the range of about 200 F. to 240 F. and corresponding fractions for higher boiling aromatics. Other known means such as solvent extraction, silica gel adsorption, etc., may be used instead of extractive distillation for removing benzene from the second heart cut. Other modifications and alternatives will be apparent from the above description to those skilled in the art.

We claim:

1. 'Ihe method of obtaining benzene from a virgin naphtha containing cyclohexane and methylcyclopentane, which method comprises fractionating said naphtha to obtain a first heart cut boiling in the range of F. to 185 F., enriching said first heart cut with a recycle stream which is richer in cyclohexane than said rst heart cut, contacting said diluted iirst heart cut with a hydroforming catalyst under conditions to convert most of the cyclohexane to benzene, fractionating the product from the hydroforming step to obtain a second heart cut boiling in the range of about 150 F. to 185 F., recovering benzene from said second heart cut, isomerizing that portion of the second heart cut from which benzene has been removed to convert at least a part of the methylcyclopentane contained therein to cyclohexane, removing components lower boiling than about 157 F. from isomerization products, and recycling components of the isomerization products boiling above 157 F. to serve as a supplement to the charge in the enriching step.

2. The method of claim 1 wherein the hydroforming catalyst is a group VI metal oxide supported on alumina and the hydroforming conditions include a temperature in the range of 850 F. to 1050 F., pressure in the range of 100 to 500 p. s. i. g. and a hydrogen recycle rate in the range of about 1000 to 5000 cubic feet per barrel.

3. The method of claim 1 wherein the isomerization is eected with an aluminum halide hydrocarbon complex at a temperature in the range of about 160 F. to about 220 F.

4. The method of claim 3 wherein the aluminum halidehydrocarbon complex is a complex of aluminum chloride and an aromatic hydrocarbon.

5. The method of claim 3 wherein the benzene is recovered from the second heart cut by extractive distillation.

6. The method of obtaining benzene from a narrow cut naphthenic virgin naphtha in approximately the benzene boiling range with a hydrogenative reforming catalyst under conditions favoring dehydrogenation of hydroaromatc naphthene hydrocarbons to aromatics, thereby converting substantially all of the hydroaromatc naphthenes to aromatics, fractionating the dehydrogenation eluent to obtain a second narrow cut fraction of approximately the benzene boiling range, recovering benzene from said second fraction, isomerizing that portion of said second fraction from which benzene has been removed to convert at least a portion of the methylcyclopentane contained therein to cyclohexane, separating out a fraction rich in cyclohexane from the isomerization products, and recirculating said cyclohexane fraction to contact with the hydrogenative reforming catalyst in admixture with fresh naphtha feed.

7. The method of claim 6 in which the hydrogenative reforming catalyst is a group VI metal oxide supported on alumina.

8. The method of claim 7 wherein the hydrogenative reforming catalyst is a molybdena alumina catalyst.

References Cited in the file of this patent UNITED STATES PATENTS 2,241,393 Danner May 13, 1941 2,350,834 Sensel et al. June 6, 1944 2,373,674 Crawford et al. Apr. 17, 1945 2,375,573 Meier c May 8, 1945 2,423,176 Cole July 1, 1947

Claims (1)

1. THE METHOD OF OBTAINING BENZENE FROM A VIRGIN NAPHTHA CONTAINING CYCLOHEXANE AND METHYLCLOPTANE, WHICH METHOD COMPRISES FRACTIONATING SAID NAPHTHA TO OBTAIN A FIRST HEART CUT BOILING IN THE RANGE OF 150* F. TO 185* F., ENRICHING SAID FIRST HEART CUT WITH A RECYCLE STREAM WHICH IS RICHER IN CYCLOHEXANE THAN SAID FIRST HEART CUT, CONTACTING SAID DILUTE FIRST HEART CUT WITH A HYDROFORMING CATALYST UNDER CONDITIONS TO CONVERT MOST OF THE CYCLOHEXANE TO BENZENE, FRACTIONING THE PRODUCT FROM THE HYDROFORMING STEP TO OBTAIN A SECOND HEAT CUT BOILING IN THE RANGE OF ABOUT 150* F. TO 185* F., RECOVERING BENZENE FROM SAID SECOND HEART CUT, ISOMERIZING THAT PORTION OF THE SECOND HEART CUT FROM WHICH BENZENE HAS BEEN REMOVED TO COVERT AT LEAST A PART OF THE METHYLCYCLOPENTANE CONTAINED THEREIN TO CYCLOHEXANE, REMOVING COMPONENTS LOWER BOILING THAN ABOUT 157* F. FROM ISOMERIZATION PRODUCTS, AND RECYCLING COMPONENTS OF THE ISOMERIZATION PRODUCTS BOILING ABOVE 157* F. TO SERVE AS A SUPPLEMENT TO THE CHARGE IN THE ENRICHING STEP.

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