1. THE PROCESS FOR PREPARING AN AROMATIC HYDROCARBON FROM A NARROW-BOILING VIRGIN NAPHTH CUT RICH IN NORMAL PARAFFIN WHICH COMPRISES VAPORIZING A STREAM OF SAID NAPHTHA, PASSING SAID VAPOR STREAM UPWARDLY THROUGH AN ADSORPTION ZONE IN COUNTERCURRENT CONTACT WITH A MOVING STREAM OF AN ACTIVATED CARBON PREPARED FROM PETROLEUM COKE AND HIGHLY SELECTIVE FOR ABSORBING NORMAL PARAFFINS TOGETHER WITH AROMATIC HYDROCARBONS IN THE PRESENCE OF BRANCHED PARAFFINS AND NAPHTHENES, REMOVING ABSORBENT AND ADSORBATE FROM THE BOTTOM OF THE ADSORPTION ZONE AND A SEPARATE STREAM ENRICHED IN UNADSORBED BRANCHED PARAFFINS AND NAPHTHENES FROM THE TOP OF SAID ZONE, PASSING THE SAID ADSORBENT AND THE ADSORBATE TO A STRIPPING ZONE WHERE IT PASSES DOWNWARD COUNTERCURRENT TO A STREAM OF HOT HYDROGEN CONTAINING GAS, STRIPPING NORMAL PARAFFIN AND AROMATIC ADSORBATE FROM THE ADSORBENT IN SAID HOT GAS STREAM, AT A TEMPERATURE OF AT LEAST 600* F., COOLING AND RECYCLING SAID STRIPPED ADSORBENT TO THE ADSORPTION ZONE, PASSING SAID HOT GAS STREAM COMPRISING A REACTANT MIXTURE OF HYDROGEN, STRIPPED NORMAL PARAFFIN AND AROMATIC HYDROCARBONS TO A CATALYTIC AROMATIZATION ZONE TO CONVERT SAID NORMAL PARAFFIN TO THE CORRESPONDING AROMATIC HYDROCARBON AND ADDITIONAL HYDROGEN IN THE SUBSTANTIAL ABSENCE OF ADDED GAS EXTRANEOUS TO THE REACTION, AND RECYCLING HOT HYDROGEN CONTAINING GAS FROM SAID AROMATIZATION PROCESS AT A TEMPERATUREIN EXCESS OF 600*F. AS THE HOT STRIPPING GAS IN SAID STRIPPING ZONE.
June 28, 1960 w. K. LEWIS, JR., AL
. T so FaATloMATolL HY'DlOFOllMNi Unn- 9 A 'zo- F T i Aosoabal 2 Sharpen rm: I (is) U \1 Obborneg PROCESS FORTHE PRODUCTION OF BENZENE Warren K. Lewis,'Jr., Elizabeth, and Isidor Kirshenbaum, Union, 'NJ., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Mar. 3 1954, Sen No. 413,814
3 Claims. omits-s5 The present application is a continuation-in-part of our copending application, U.S. Serial No. 254,741, filed November 3, 1951, now abandoned, entitled Process for the Production of Benzene.
This invention relates to a process for the production of aromatic hydrocarbons. More particularly, it relates to a process for the conversion of petroleum hydrocarbon fractions rich in normal pai-aflin and naphthenic hydrocarbons into products rich in aromatics.-
Recentdevelopments in the synthetic chemical industry have enormously increased the demand foraromatic hydrocarbons, especially benzene and toluene. The normal demand for these hydrocarbons has been augmented by very large new demands, relating to the preparation of synthetic rubber and various plastics. The usual sources of supply. for these aromatics have been completely exhausted due jtQ this increased demand,
2,943,039 Patented June 28, 1960 ships and the formation of 'azeotropes are such that it is exceedingly .difiicult to prepare a n-hexane concentrate which is free of methylcyclopentane and benzene. Benzene in an aromatization feed stock is not particularly I harmful, but.methylcyclopen'tane is. There is thus a definite need for a process which is capable of producing-a hexane concentrate suitable for use as an a'romati-' nation feed stock, without requiring the actual isolation of separate pure compounds. g As indicated above, the demand forspecific hydrocarbons available from petroleum sources has brought into being plants containing highly eflicient separating equip ment, capable of resolving the lower boiling fractions of naphtha [into certain pure compounds for which there is a ready market. In many cases, however, it has been found more economic to operate these highly efiicient separating plants in such a manner as to concentrate hy- Such equipment, for example, may be used to separate and a number of procedures for synthesizing "aromatics self in great demand as a raw 'materia l' for the manufacture of synthetic fibers. Various higherboiling alkyl b'enzenes which are easily purified such as onthoxylene or paraxylene have become valuable as raw materials for the manufacture of phthallic acid or terephthallic' acid.
The net result'is that the aromatics and cyclohexane derivatives availablein petroleum and coal tar stocks are no longer able to supplythis demand, and attention has been increasingly directed to alternatefeed stocks for the synthesis of aromatics.
As a result of intensive research in this field, it has been found possible to aromatize many paraflin hydrocarbons containing atleast 6 carbon atoms in the longest straight chain. Thus, the recent development of highly active and selective aromatization catalysts of the alumina/chromia type, promoted with silica and p0tassium oxide such as those described by Kirshenbaum and Gilbert in the co-pendling application Serial No. 106,874, has made the aromatization of normal hexane to benzone very attractive commercially. These catalysts, in
r a low pressure operation at about 900-1100 F. and zero to 100 p.s.i.g., feeding hydrogen at a 1/1 to 4/1 mol ratio, can convert a normal parafiinic stock such as nhexane to a high yield of highly aromatic product with in alkyl pentane and alkyl cyclopentane derivatives. However, crude oils as well as naphtha refinery streams do not contain high concentrations of n-hexane.' C0nsequently it is necessary to process a suitable refinery stream by distillation, extractive distillation or other suitlow coke production, particularly when the feed low I able separation process to concentrate thenormal hexane '5 present.
Engineering developments in'the construction of high etficiency distillation equipment have made such operations possible on a large scale, but they are always costly. In the case of n-hexane, the point relationconcentratesof straight chain hydrocarbons from other fractions consisting largely of branched chain hydrocarbons. The latter, or isoparafiin fractions, can then be used as motor fuel blending, agents of enhanced antiknock value, useful in the preparation of aviation gasoline base stocks. is a typical case where the opera-- tion of the plant so as to get the maximum production of premium quality motor fuel, in terms of barrel octane numbers dictates the operation of the separation equipment so as to give somewhat less than the maximum concentration of isoparaffins and normal paraffins in the separate fractions. l The normal hexane concentrate from such a separation procms maybe considered, by way of illustration,
as a potential feed stock forthe production of'benzene.
Typical compositions of hexane streams which can be prepared whenusing a battery of "50 to pl-ate fractionating columns to effect the s'eparation by simple. dis-l 1 til'l ation' are'shown in 'Iable I.
. p 7 TABLE 1' COMPOSITION'QF HEXANE sr'REAMsXmot) Compound, Vol. Percent A O 2-methylpentane 21. 6 t 12. 0 3-methylpentane 13. 1 12. 6 n-hexane 35. 0 37. 4 methylcyclop n n 22. 1 22. 7 benzene. 8. 2 6. 5 2,4-dimethylpentana 2. 1 cyolnhpxana 6.7
' Stream A is typical of the so-called n-hexane stream obtained when the'fractionating unit is being operated primarily to produce fractions suitable for use in aviation base stock. Stream C, slightly different in composition, represents the product obtained when operating primari ly for the preparation of a cyclohexane concentrate.
1 Both these streams contain branched paraflins and methylcyclopentane as well as normal hexane, in spite of the fact that they are the product of an' eflicientand ex pensive dis-tillationprocess. 7 Before the n-hexane canbe' aromatized to benzene, it is highly desirable to separate it,
from all the pentane derivatives present. The branched 'parafiins, including thev methylpentanes and dimethylpentanes, and methylcyclopentane'all degradeunder aromatizing conditionsto give undesirably high yields of coke and gas.
The aromatization of normal hexane has the advantage of upgrading material which has low octane number and ,1 consequently'not only producesbenzene, but at the same a time removes from the gasoline pool materialhaving' a research octane number of only about 25. The aromatizaon feed. When applied to the readily available n-hexane concentrates the process is expensive, however, primarily because of a tendency towards excessive carbon yields and high gas loss. Thus even the most promising catalysts may give typical carbon yields of 5.5 wt. percent when feeding pure n-hexane in the absence of added hydrogen at 1000 F., 0.4 v./v./hr. and atmospheric pressure to produce about 55 mol percent benzene on feed. At the same time, some benzene is normally lost with the efiluent hydrogen formed in the aromatization process. To recover all ofthis benzene is very expensive, and this loss is increased with an increase in the amount of hydrogen and other light gases formed or recycled in the process.
The tendencies toward both coke and gas formation are materially less when using pure normal parafiin as the feed than when the branched chain paraflin and cyclopentane homologues are present. Thus even at low severities, using the preferred potassium oxide-silica promoted chromia/alumina catalysts with a 2/1 mol ratio hydrogen dilution of the feed, a 35% conversion with n-hexane gives only 0.8% coke on feed as compared to 2.5 for Z-methylpentane or 7% for methylcyclopentane. Coke production from the three hydrocarbons is in the ratio of 1 to 3 to 9, and at the same time aromatic yields fall off markedly, in the ratio of about 1 to 0.33 to 0.25.
It is therefore economical with a mixed C cut to carry out a preliminary separation to remove these homologues, and further concentrate the normal hexane in the feed stock. One of the most satisfactory methods of carrying out such a separation is to use a selective adsorbent capable of separating normal paraflins from isoparaffins and naphthenes. A particularly desirable adsorbent is a form of activated carbon or charcoal which is capable of separating normal paraflins from a mixed virgin naphtha fraction and at the same time recovering the aromatic hydrocarbons present. be useful, however, and asan example of such other adsorbents we refer to U .S. Patent 2,522,426, in which the patentee Black describesa process employing a synthetic calcium aluminum silicate capable of selectively Other absorbents may separating straight chain aliphatics from branched chain and cyclic hydrocarbons.
The selective adsorption process, however, has its own disadvantages, chief among which are the cost and possible side effects of the stripping medium and the cost of heating, cooling and reheating the process stream when it is to be employed finally as a feed stock for aromatization. Economic studies indicate that in the ordinary case where steam is used as the stripping medium in such a plant, most of the investment and operating costs are associated with the stripping step. Furthermore, with the preferred catalysts .of the present invention, water is definitely harmful and the potassium oxide added to the silica-stabilized alumina gel is a strong promoter of the water gas shift reaction involving oxygenated compounds and hydrogen, so that ordinary inert gases" such as steam or flue gas cannot be used for stripping in the feed preparation step without being entirely removed prior to the aromatization step.
It is an object of the present invention to prepare a feed stock for aromatization by selective adsorption without incurring the cost and undesirable effects of using steam or inert gas as a special stripping medium.
It is a further object to recover the normal paraffin and aromatic constituents of. a naphtha feed stream by an adsorption process which requires no separation of the product concentrate from stripping medium.
g It is. also;an obje t to prepare a feed concentrate for aromatization by adsorbing normal paraffins and aromatics together from a mixed hydrocarbon stream and desorbing them together, with a hot hydrogen-containing,
gas, to give a stripper effluent free of gases extraneous to the aromatization reaction.
It is a still further object to prepare such a concentrate by a process in which the cost of cooling a stripper efiluent containing the stripping medium and then reheating the recovered desorbate is avoided, as is the loss of feed with discardedstripping medium. Other advantages of the process of our invention are derived from the fact that when a hydrogen-containing gas is used for stripping a higher stripping temperature may be employed without coking and tar formation on the adsorbent. This results in 'a more efficient desorption process, lower regeneration costs and a longer useful life for the absorbent.
Our invention may be illustrated by reference to a process where normal hexane is concentrated from a C naphtha fraction by selective adsorption on a steamactivated coke prepared from a petroleum acid sludge. The particular form of coke employed, known commercially as Chemico coke, is prepared by the steam activa tion of a carbonaceous residue obtained from a refinery sludge produced in the sulfuric acid treatment of highboiling petroleum oils. This sludge, carbonized to recover its sulfur-containing constituents in the course of acid regeneration, gives as a by-product a light coke which is the source of the particular form of activated petroleum coke herein described. This carbonaceous residue is treated with steam or hot hydrogen at l4001600 F. to remove any residual material other than carbon, along with 15-50% of the original carbon in the coke. Instead of using steam it is also possible to use air or other suitable oxidizing gas. The best results in the use of this material in the process of the present invention are obtained when using a high yield coke where approximately 17% of the original carbon is taken out in the activation step. The resulting material is found to be highly selective for the separation of normal paraflinic hydrocarbons from isoparaflins or naphthenes. At the same time, it also adsorbsaromatic constituents of the original feed stock. Thus, a feed stock which consists entirely of naphthenes, aromatics and paraffin hydrocarbons maybe treated with this material to recover an adsorbate consisting essentially of normal paraffin and aromatic constitutents of the feed, while rejecting unadsorbed material consisting of the branched parafiins and naphthenes present.
, According tojthe present invention the adsorbed material thus concentrated on the surface of the adsorbent coke is desorbed by using as, a stripping medium the hydrogen-rich recycle gas produced by the aromatization of the normal paraffin instead of stripping with steam or other so-called inert gases. The harmful effect of steam as acontaminant in the aromatization process when using the particular catalysts here prefererd appears in a marked decrease in the aromatic yield, obtained in the presence of even small amounts of Water vapor. Thus, over a 22% chromia catalyst promoted with 1.8% of potassium oxide and supported onaniactivated alumina gel containing 5% of silica, a particular hexane-rich feed containing about 48% n-C and'.7.5% methyl cyclopentane gives 53% benzene when aromatized at atmospheric pressure and 1025 F. in a water-free hydrogen atmosphere at 2/1 H /hydrocarbondilution. The same feed, under the same conditions, gives only 41% benzene when 1.15 mol percent OfH O'iS added tothe feed; this falls offto 29% benzenevwhen 6.2 mol percent of B 0 is present. The difierence appears as an increase in olefins and unconverted feed. ,Exactly the same effect appears with pure n l iexane, where conditions which give a 50% yield of benzene in the absenceof watervapor give only 35% benzene when: 6.2 molpercent of- B 0 is present.
' Using n-hexane again for'purposesof illustration, the
' ofthe, process can best be understood by reference to the ttested awi s v A suitable refinery stream rich in n-hexane is supplied through line 1 to adsorption tower 2. This feed passes up through tower 2 countercurrent to a recycled stream of stripped adsorbent added through line 3. The adsorbent together with the adsorbate (primarily n-hexane and benzene) is passed by way of line 4 through heat exchanger 6 into stripping tower 12. The unadsorbed con- .stituents in the feed leave tower 2 by way of line 5.
The stripper or desorption tower 12 which is shown here as a separate vessel may also be located immediately below the adsorption tower with a suitable means for supplying heat to the charged adsorbent, corresponding to exchanger 6, during passage from one zone to the other. In a particularly advantageous modification this supply of heat to the charged adsorbent is arranged so as to release a portion of the adsorbate which passes upward into the adsorption zone, as a reflux stream, below the level at which the fresh feed stream is introduced thereto. This reflux action can be controlled so as to result in an appreciable increase in the selectivity of the adsorption process.
In stripper 12, the heated adsorbent and remaining adsorbate enter downflow, in a direction countercurrent to the flow of a hot hydrogen-rich stripping gas entering the bottom of the tower through line 18. The desorbed n-hexane stream passes from the top of tower 12 together with the stripping gas, by way of line 20. This combined stream consisting essentially of hydrogen, normal hexane and benzene is passed directly to thearomatizing or hydroforming unit 22. The only gases present are reactants or recycled products of the aromatization process, and no oxygen-containing or inert gases are introduced at any point in the system which might cause undesirable side reactions or complicate the problem of ultimate product recovery. 1
Stripped adsorbent is withdrawn through line 14, suitablycooled by heat exchanging vessel 15, and returned by, way of line 3 to the top of tower 2. Spent adsorbent may be removed from time to time or continuously as desired by way of valve 16 and line 17. Makeup adsorbent can be added as necessary through valve 8 and line 9. I r l The adsorption process invessel 2 can be carried out either in the liquid phase or in the vapor phase. Vapor .phase operation is preferred, partly because the separation factors or alpha values between the adsorption tendencies of the different types of hydrocarbons are found to be much greater under these circumstances.
Thus both vessel 2 and vessel 12 may take the form of a bubble cap tower, where the adsorbent fluidized inthe stream of rising gas flows down and over the successive plates in each vessel. This makes possible the'eifective Vapor phase operation also eliminates the expensive separation equipment such as filters or centrifuges which would be required for removing interstitial liquid from the adsorbent in the liquid phase process using suspended small particles. V
The high selectivity of the activatedpetroleum coke 'here recommended for the vapor phase separation of 1 normal from iso-parafiins is illustrated by the data shown in Table II, obtained on a /50 test blend of heptane/isooctane:
TABLE II Se aration alpha) Factor Adsorption Capacity, cc./l00 g.
Phase Temp, F.
Liquid. 80 27 7.6 Vapor i 250 Y 18 Adsorptive capacity is relatively little aifected by the vapor phase operation, and these particular results are i found after 28 cycles of alternating adsorption-desorption over the steam-activated coke. The very high separation,
factor realized here can be further pointed up by contrast with the more usual alpha factor of about 2.1 realized in liquid phase tests using the same binary blend with activated coconut charcoal as the adsorbent.
The adsorption step in vessel 2 is carried out preferably at a temperature only slightly above the dew point of the naphtha vapors introduced through line 1, since selectivity is found to drop as this temperature goes much above the dew point. A temperature of about 1 80-200 F. is found to be satisfactory when feeding a C cut at atmospheric pressure, and this might be changed to about 210-230 F. when feeding a C -C cut, as discussed in more detail below, The desorption in vessel 12 is carried out at a temperature between about 600900 F., preferably 800900 F., depending upon the nature of the various impurities present in the feed stream. Higher stripping temperatures give more rapid. stripping, and the hydrogen-containing gas supplied through line 18 permits the use of such higher temperatures'with minimum coke and gum formation during stripping.
The aromatization or hydroforming unit 22 operates under rather severe operating conditions of low pressure and high temperature. a The pressure range may be 0-250 p.s.i.g., preferably 0-50 p.s.i.g., and the temperature range 650-1150 F. preferably 9001050 F., using a highly active catalyst of the type of molybdena on alumina,
molybdena ona zinc aluminate spinel, promoted chromia on alumina, or the like. The hydrogen introduced with the feed stream through line 20 may be in .the H /feed mol ratio of about 1/1 to 10/1, preferably about 2/1 The hydroformed product from the aromatization zone 22 passes by way of line 23 to fractionating unit 24. In certain cases this unit may consist of a group of fractionating towers, or a single tower may be used as shown in the diagram. The principal product stream 25 consists of benzene, which is higher boiling than the other C hydrocarbons and lower molecular weight materials normally produced in the aromatization process. This benzene product stream, which may'also include smaller amounts of heavier aromatics formed inthe aromatization process, will ordinarily be recovered as a bottoms cut from the A side stream 26 consisting of unreacted hexane and including normal hexenes formed during the aromatization process can also be recovered and recycled directly to hydroforming unit 22 by way of line 20. Stream 26 may also contain a certain amount of benzene, and any remaining C naphthenes separated overhead from the benzene product in fractionator 24, all of which can be recycled to the aromatization process for ultimate conversion to benzene;
A lighter product stream 27 contains small amounts of C and C fractions produced during the aromatization. This stream 27 may also contain the lowest boiling of the C hydrocarbons, including the dimethylbutanes, methylpentanes and small amounts of the corresponding lower boiling branched olefins. The constituents of this stream.which are undesirable in the. aromatization feed The overhead stream 28 consists of a hydrogen-rich recycle gas including also a small amount of C -C hydrocarbons. A portion of this gas is removed from the p Q system by way of line 30. Another portion, containing normally from 60 to 90 mol percent or more of hydrogen, is used as the stripping agent introduced through line 18 into tower 12 and recycled therefrom as a diluent gas for the aromatization process in vessel 22. A suitable heating means, shown diagrammatically as heat exchanger 32 is provided to raise the temperature of this stream to the desired temperature for use as a stripping gas. i
The aromatic stream 25, consisting essentially of hen zene with small amounts of heavier aromatic impurities, can be used as such or further purified by methods well known in the art.
The advantages of our process may be further seen from the following examples and experimental data:
Example 1 N-hexane was passed over a promoted Al O CrO catalyst, having the nominal composition 100 parts by weight of a commercial activated alumina carrier containing 5% of SiO together with catalytic oxides corresponding on an elemental analysis basis relative to the carrier of 11 parts of Cr, 0.86 Ce and 1.13 K. The reaction was carried out at 1000 F.; 0.4 v./v./hr.; p.s.i.g.; and 2/1 H /n-C mol. ratio. Aonce-through yield of benzene of 47 mol. on feed was obtained. The carbon yield was only 2 wt. of feed. .With complete recycle of the C fraction in the product, the benzene yield increased to about 74 mol. percent with the carbon yield being only 3 wt. percent on feed.
In a similar experiment but with no hydrogen recycle the once-throughyield was 56 mol. percent but the carbon yield was 5.5%. On recycle the carbon yield increased to 7% while the aromatics yield was only 71%. Itis thus apparent that in our process the ultimate benzene yield is higher (74 vs. 71 mol. percent) and the carbon yield lower than in conventional aromatization without H2 recycle.
Example 2 TABLE III n-G Conversion, wt. Percent 75 85 95 Yields, wt. Percent:
Aromatic 49 58. 5 68. 5
Carbon 0. 6 0. 9 1. 1 Gel-Liquid Produ Vol. Percent on Feed.; 74 5 70 65 5 Benzene, vol. Percent 4 62 76 Toluene-l-Xylene, vol. PercenL. 0 2 0.2 0 4 Olefins, v01. Percent 14 12 Yield, Percent of Theoretical:
Benzene 72 76 80 These results show the clear advantage of using the highly active catalysts preferred for the practice of the present invention, with a feed stock containing a high proportion of normal paraflin.
The advantage of using H recycle gas in the adsorption stripping step may be seen from thefollowing-example:
Example 3 In commercial operation, 5250 barrels per day of the n-hexane stream, from a refinery battery of 50 100 plate naphtha fractionating unit, having the composition 29.4% methylpentanes, 36.3% n-hexane, 7.3%-benzene,- 1.1% dimethylpentane and the remainder methylcyclopentane and cyclohexane are separated by adsorption on Chemico coke activated to 83% of its initial content. Theproducts formed provide 2260 barrels per day of a n-hexane stream having the composition: 79% n-hexane, 16% benzene and 5% branched paraifins and naphthenes, and 2990 barrels per day of an isoparaffimnaphthene rich fraction containing only 5% n hexane and benzene. This latter stream is blended into the motor gasoline pool while the n-hexane stream is sent to the hydroforming unit. In this adsorption operation, the Chemico coke circulation rate is 155 tons per hour and 330 thousandcu. ft. of H rich (80-95% H recycle gas from the hydroformer is used per hour as stripping agent. The n-hexane, benzene and stripping agent are circulated to the low pressure hydroforrning unit operating at about 6 p.s.i.g., 0.4 v./v./hr. and 1000 F. As a result of. this operation there are formed 1300 barrels/day of benzene (including the benzene in the feed), about 800 barrels per day of a C fraction for recycle to the hydroformer, as well as 30 thousand lbs. of C and C per day.
While the process of our invention has been described specifically with reference to its application to the treatment of a hexane cut for the preparation of benzene, the same principle may be employed to advantage. in the treatment of other normal paraflin fractions such as a heptane cut to be used in the preparation of toluene. The boiling point relationships in the C and C .cuts are somewhat analogous, in that the alkyl cyclopentanes are in both cases much lower boiling than the corresponding cyclohexanes. Thus, a heptane cut chosen on the low boiling side of methylcyclohexane may contain most of the heptane of the original naphtha and most of thealkyl cyclopentanes. i I The selectivity of the preferred activated coke adsorbent is such that the next most readily adsorbed hydrocarbons from this mixture after toluene and heptane are the methylhexanes, which are not nearly as objectionable in the aromatization feed stock as are the methylpentanes in the C cut. To the extent that the methylhexanes can be retained in the adsorbate without contaminating the product undesirably with more highly branched paraffins, the operating conditions of the adsorbing process may be modified in this direction. t
The process of our invention can likewise be applied to other paraffin-rich naphtha fractions, containing significant quantities of normal paraflin. It maybe useful, for example, to prepare both benzene and toluene from a C -C cut rich in n-hexane and n-heptane. It can be similarly employed to prepare xylenes from an octane cut, or in various other combinations to produce any-of the lower-boiling monocyclic aromatics from the corresponding paraffin-naphthene cuts. a i
It will also be understood that while the process has been described primarily in terms of its application to a saturated naphtha fraction such as virgin naphtha, precisely the same treatment may be applied to ahydrocarbon fraction which has been catalytically converted to increase the content of more valuable hydrocarbons. The process may alsobe applied to advantage to-various olefin-containing cuts where the concentration of straight chain aliphatic hydrocarbons is high, including both paraffins and olefins. Where olefin concentration is low it may be preferable to acid treat to remove olefins, avoiding any potential complications due to the tendency of olefins adsorbed on the activated carbon or other adsorbent used to polymerize thereon during the desorption step. The branched chain olefins, which polymerize most readily under the influence of heat, are also the most readily removed'by such an acid treatment. The straight chain olefins such as the normal hexenes or heptenes are good feed stocks for catalytic aromatization,-and will be retained in the aromatization feed stock where such a stream istreated according to the present invention.
Under other conditions it may be desirable .to prehydrogenate an olefinic stream before subjecting it to the catiifl g) alytic aroinatization step. On the other hand, it may be most economical to process the olefinic stream concerned by feeding it directly to the selective adsorption step, making up for any decrease in adsorbent capacity by a corresponding increase in the rate at which spent adsorbent is discharged and fresh make-up added to the adsorbent cycle. This type of operation is particularly favored by the use of the hydrogen-rich stripping gas of the present invention. Any of these methods may be applied to a recycle stream of C hydrocarbons from the final ben zene purification step, as outlined above, or this stream branched chain parafiin-olefin concentrate to be sent to gasoline blending and a straight chain paraflin-olefin concentrate to be recycled directly to the catalytic aromatization.
The particular naphtha chosen as feed for this process may be one derived directly from crude distillation, or it may be any of various refinery process streams. Thus it may, for example, have been subjected to various preliminary treatments, such as hydroforming, isomerization, alkylation, or selective dehydrogenation, to selectively remove or alter the relationship between various hydrocarbons present. The principal requirement is only that the feed stream contain a significant proportion of normal parafiin, and be free of objectionable quantities of constituents which would polymerize during the adsorptiondesorption process in spite of the use of hot hydrogen as the stripping medium.
What is claimed is: v
1. The process for preparing an aromatic hydrocarbon from a narrow-boiling virgin naphtha cut rich in normal parafiin which comprises vaporizing a stream of said naphtha, passing said vapor stream upwardly through an adsorption zone in countercurrent contact with a moving stream of an activated carbon prepared from petroleum coke and highly selective for adsorbing normal parafiins together with aromatic hydrocarbons in the presence of branched paraffins and naphthenes, removing adsorbent and adsorbate from the bottom of the adsorption zone and a separate stream enriched in unadsorbed branched parafiins and naphthenes from the top of said zone, passing the said adsorbent and the adsorbate to a stripping zone where it passes downward countercurrent to a stream of hot hydrogen containing gas, stripping normal parafiin and aromatic adsorbate from the adsorbent in said hot gas stream, at a temperature of at least 600 F., cooling and recycling said stripped adsorbent to the adsorption zone, passing said hot gas stream comprising a reactant mixture of hydrogen, stripped normal parafiin and aromatic hydrocarbons to a catalytic aromatization zone to convert said normal parafiin to the corresponding aromatic hydrocarbon and additional hydrogen in the substantial absence of added gas extraneous to the reaction, and recycling hot hydrogen containing gas from said aromatization process at a temperature in excess of 600 F. p
ing said hydrocarbon stream and feeding said vapors up wardly at substantially atmospheric pressure into an adsorption zone countercurrent to a descending stream of stripped carbon adsorbent whereby said rising vapors assist in maintaining the mobility of said fluidized carbon stream, rejecting from the upper portion of said adsorption zone a raffinate comprising branched parafiins and naphthenic constituents of said feed stream, removing charged adsorbent from the bottom of the adsorption zone and passing said adsorbent with adsorbate downwardly through a stripping zone, at a temperature in 'excess of 600 F. countercurrent to a rising stream of a hot hydrogen-containing stripping gas at a temperature between about 650 and 900 F., whereby said rising vapors assist in maintaining the mobility of the fluidized carbon stream, removing a hot fluid stream of stripped carbon from the bottom of the stripping zone, cooling said fluid stream to approximately the desired adsorption temperature and recycling the thus cooled adsorbent to the adsorption zone, removing overhead from the stripping zone a hot stream of combined hydrogen-containing stripping gas and desorbcd normal parafin and aromatic constituents of the original feed stream, free of other added gases extraneous to the aromatization reaction; passing said combined stream directly to a catalytic aromatization zone in the presence of a potassium oxide-promoted catalyst under severe conditions of low pressure hydro-forming, recovering from said aromatization zone a product rich in aromatics and a gaseous stream rich in hydrogen, reheating at least a portion of said hydrogenrich gas stream as stripping gas to the desorption zone.
3. The process for manufacturing aromatic hydrocarbons which comprises vaporimng a naphtha fraction containing normal parafi'ins in the C3-C boiling range together with aromatic, naphthenic, and isoparafiinic hydrocarbons, passing said vapors at a temperature of about 180-250 F. over an adsorbent carbon highly selective. to the separation of normal from iso'parafiins, thereby selectively adsorbing a hydrocarbon fraction enriched in normal paraifin and aromatic compounds and recovering therefrom a stream of unadsorbed hydrocarbon enriched in naphthenic and isoparafiin compounds, desorbing said n-paraffin and aromatic compounds together by stripping the charged adsorbent with a hot hydrogen-containing recycle gas at a temperature of about 600-900 F., passing the resultant hot reactant stream comprising normal parathnic hydrocarbon and hydrogen into a low pressure catalytic aromatization zone employing a potassium oxide promoted catalyst in the absence of any added gas extraneous to the aromatization reaction, recovering from said aromatization zone a product rich in aromatics and a gaseous stream rich in hydrogen, and recycling at least a portion of said hydrogen-rich gas as stripping gas for the desorption.
References Cited in the file of this patent V UNITED STATES PATENTS