US3159567A - Selective hydrocracking process - Google Patents

Description

Dec. l, 1K964 D. A. YOUNG SELECTIVE HYDROCRACKING PROCESS Filed March 26, 1962 MOA/CYCL /C 25 645 OIL FEE@ .United States Patent O 3,l59,567 @ELECTIVE HYDRCRACKING PRCESS Dean Arthur Young, Yorba Linda, Calif., assigner to Union @il Company of California, Los Angeles, Calif., a corporation of California Filed Mar. 26, T1962, Ser. No. 182,263 Claims. (Cl. 20S- 87) This invention relates to catalytic hydrocracking, and more particularly is concerned with methods for converting highly paraiiinic hydrocarbons in the gas oil boiling range to lower boiling paraffin hydrocarbons, boiling for example in the gasoline or jet fuel ranges. More particularly, the process is concerned with new methods for increasing the selectivity of hydrocracking, i.e., for maximizing the yield of products in the desired boiling range, while minimizing the production of very light hydrocarbons such as methane, ethane, propane, and the like. The process also results in a maximum'production of highoctane isoparaiins, as opposed to the relatively lowoctane normal paratins. Brieiiy stated, the invention comprises the essential step of including with the primary paraiiinic feedstock to the hydrocracking zone, a minor proportion of monocyclic aromatic hydrocarbons. In one specic modification of the process, a gas oil feedstock is rst separated, as by solvent extraction, into a paraliinic ratiinate and an aromatic extract; the lighter fraction of the aromatic extract is subjected to hydrodealkylation to produce naphthalene and a mixture of light monocyclic aromatic hydrocarbons, and at least a portion of this monocyclic fraction is then blended with the parainic ratlnate from the solvent extraction, and the mixture is subjected to catalytic hydrocracking.

It is known in the art that optimum hydrocracking conditions for converting parafnic hydrocarbons differ considerably from the optimum conditions for converting aromatic hydrocarbons. It has hence been proposed in the pastto separate hydrocracking `feedstocks into a relatively aromatic fraction and a relatively paraflinic fraction, and to subject the two fractions to separate hydrocracking under optimum conditions for each fraction. It has now been found however that the eiliciency of cracking of the parainic portion of feed can be further irnproved by incorporating therein a desired proportion of monocyclic aromatic hydrocarbons. It has further been found that polycyclic aromatic hydrocarbons such as naphthalene and the like are undesirable in the paraffin hydrocracking zone because they unduly repress the overall activity of the catalyst. Monocyclic aromatics on the 'other hand do not significantly depress the activity of the catalyst for promoting the desired conversions, but on the contrary have a favorable iniiuence in improving the selectivity of hydrocracking, and improving the octane value of the gasoline produced by increasing the ratio of iso/normal parat-'tins therein.

The inventionmay perhaps be more readily understood from the accompanying drawing, which is a how-sheet illustrating one particular modification thereof. The initial feedstock, consisting for example of a straight-run gas oil boiling between about 40G-800 F., is brought in via line 2 and separated into a relatively aromatic and a relatively non-aromatic fraction in solvent extraction column 4. Any conventional method of separating aromatics from non-aromatic hydrocarbons maybe employed, as for example aZeotr-opic distillation, selective adsorption, extrac- 3,l59,567 Patented Bec. 1, 1964 lCe tive distillation and the like. However, inthe modification illustrated, the feedstock is introduced into the bottom of countercurrent extraction column 4, which is preferably packed with a suitable material such as Raschig rings or the like to facilitate contact between countercurrently owing immiscible liquids. The solvent employed for the extraction may comprise any of the Well known polar compounds which exhibit a selective solvency for aromatic hydrocarbons as opposed to non-aromatic hydrocarbons, and which are suitably low-boiling. Suitable solvents include for example ethanol, methanol, phenol, furfural, ethylene glycol monomethyl ether, acetonitrile, sulfur dioxide and the like.

The solvent is admitted to the top of column 4 via line 6 and passes downwardly, countercurrently to the risingl hydrocarbon stream. The aromatic extract is Withdrawn at the bottom of the column via line 8, and transferred to a small fractionating column 10, from which the volatile solvent is removed as overhead via line 12, condensed and recycled to the top of extraction column 4. kThe stripped aromatic extract, comprising both monocyclic and polycyclic aromatic hydrocarbons, is transferred via. line 13 to a second fractionating column 14, from which a light fraction boiling between about 400 and 550 F. is taken overhead via line l5, while the heavier polycyclic aromatics are withdrawn as bottoms via line 16. This bottoms fraction may if desired be subjected to additional hydrogenation and/ or hydrocracking in facilities not shown.

The overhead fraction in line 15 contains higher alkylated benzenes such as durene, di-isopropyl benzene and the like. It also contains a desired proportion, depending upon the selected end-boiling-point of the fraction, of bicyclic aromatics such as naphthalene, methyl naphthalenes, dimethyl naphthalenes and the like. Where maximum naphthalene production is desired from the subsequent dealkylation step, the end-boiling-point of the overhead is about G-550 F., so as to include most of the methyl and dimethyl 'naphthalenes in the extract, while still excluding the tricyclic hydrocarbons. Where naphthalene production is not a prime consideration a lower end-point in the range of about 425-500 F. may be selected.

In any case, the overhead fraction in line l5 is subjected to dealkylation in hydrodealkylation unit 17. Hydrodealkylation is effected for example by passing the overhead in admixture with steam and 1,000-10,000 scf. of hydrogen per barrel, over a cobalt molybdate-alumina catalyst containing about 1% of NaOH, at about l,050 F. and 1,000 p.s.i.g. Such a dealkylation procedure is described more specifically in U.S. Patent No. 2,734,929, and its eitect is to convert methyl naphthalenes to naphthalene, and higher alkyl benzenes primarily to benzene, toluene and xylenes.

The hydrocarbon product from dealkylation unit 17 is transferred via line 18 to fractionation column 19, from which crude naphthalene is recovered as bottoms via line 20. The overhead fraction comprises benzene, toluene and xylenes, and is transferred at least in part via line 21 to line 2S for use in admixture with the parafnic raflinate as hydrocracking feedstock.

i The rainate from extraction column 4, comprising nou-aromatic hydrocarbons containing a small amount of dissolved solvent, is withdrawn via line 22 and sent to rainate stripping column 24, from which -solvent is recovered overhead via line 26 and recycled to line 5 for reuse in extraction column 4. The bottoms from stripping column 24 is withdrawn via line 2S and mixed with monocyclic aromatics from line 21 as previously described.

In the solvent extraction method illustrated, it will normally be found that most of the naphthenes which may have been present in the feed will be recovered along with the paraiiins in line 28. Broadly speaking, the term paraffin as used herein is intended to include both openchain paraflins and cyclo-paraflins. However it is also contemplated that the conditions of extraction in column 4 can be suitably adjusted, as by increasing the solvent/oil ratio, so that a greater portion, or even the major portion, of the naphthenes can be extracted along with the aromatic hydrocarbons, thus producing an essentially naphthene-free raiiinate.

The mixed hydrocracking feed in line 23 is blended with recycle and fresh hydrogen from lines 30 and 32, and the mixture is then passed into hydrocracking reactor 34 via preheater 36. Hydrocracking proceeds in reactor 34 under the following general conditions:

TABLE 1 Hydrocrackz'ng Condilz'ons The eiiluent from hydrocracker 34 is withdrawn via line 38, condensed and transferred to high pressure separator 40 via condenser 42. Recycle hydrogen is withdrawn via line 30 and recycled as previously described. The liquid condensate in separator 40 is then flashed vialine 44 into low pressure separator 46, from which light hydrocarbon gases are exhausted via line 48, while remaining liquid condensate comprising gasoline and/or jet fuel is withdrawn via line S0, and sent to additional fractionation facilities not shown.

It is not intended that the invention be limited to the details described above. In particular, it is contemplated that pure or mixed monocyclic aromatic hydrocarbons derived from extraneous sources may be employed instead of monocyclic hydrocarbons derived from the particular feed used. Specifically, hydrocarbons such as benzene, toluene, ortho, meta, or para-xylene, ethylbenzene, cymenes, cumene, trimethylbenzenes, tetramethylbenzenes, pentamethylbenzene, and the like may also be used. Any proportion of such aromatic hydrocarbons added to the paraftinic portion of feed will benefit the hydrocracking to some extent, but it is preferred to use about 0.5% to by volume thereof, based on the paraliinic feed. In most cases, it is found that the monocyclic aromatic hydrocarbons are relatively unaffected during the hydrocracking, although some cracking of side-chains may occur, as well as some isomerization and/ or disproportionation.

The paralilnic feedstock used in the hydrocracking step may comprise any mineral oil fraction boiling above about 350 F., and up to about 900 F., which contains less than about 2% by volume, and preferably less than about 0.5 of polycyclic aromatic hydrocarbons. Where the feedstock contains more than about 2% by volume of polycyclic aromatic hydrocarbons, little or no benefit is obtained by adding monocyclic aromatics to the feed in respect to the advantages here sought, namely improved selectivity of hydrocracking and improved isoparaiiin/normal paraiiin ratios in the product. Feedstocks of this character can be obtained by many methods other than that illustrated. Many straight-run gas oils derived from paratiinic crude oils may be found suitable as such, or after a minimal treatment with silica gel or activated carbon to adsorb most of the polycyclic aromatic hydrocarbons. Fractional crystallization may be employed in some instances to crystallize out a desired proportion of the polycyclics. Also, in some instances, the initial solvent extraction step may be controlled, as by reducing the solvent/oil ratio, so as to selectively extract the polycyclic aromatics while leaving the major portion of monocyclic aromatics in the parafiinic raiiinate.

Suitable catalysts for use in the hydrocracking step of this invention may comprise any desired combination of a refractory cracking base with a suitable hydrogenating component. Suitable cracking bases include for example mixtures of two or more difiicultly reducible oxides such as silica-alumina, silica-magnesia, silica-zirconia, alumina-boria, silica-titania, silica-zirconia-titania, acid treated clays and the like. Acidic metal phosphates such as aluminum phosphate may also be used. The preferred cracking bases comprise composites of silica and alumina containing about 50-90% silica; coprecipitated composites of silica, titania, and zirconia containing between 5 and of each component; partially dehydrated, zeolitic, crystallinemolecular sieves, e.g., of the X or Y crystal types, having relatively uniform pore diameters of about 8 to 12 angstroms, and comprising silica, alumina and one or more exchangeable zeolitic cations. Any of these cracking bases may be further promoted by the incorporation of acidic halides such as hydrofluoric acid, boron tritiuoride, silicon tetraiiuoride and the like.

A particularly active and useful class of molecular sieve cracking bases are those having a relatively high SiO2/Al203 ratio, eg., between about 2.5 and 6.0. The most active forms are those wherein the exchangeable zeolitic cations are hydrogen and/or a divalent metal such as magnesium, calcium or zinc. In particular, the Y molecular sieves, wherein the SiO2/Al203 ratio is about 5, are preferred, either in their hydrogen form, or a divalent metal form. Normally, such molecular sieves are prepared iirst in the sodium or potassium form, and the monovalent metal is ion-exchanged out with a divalent metal, or where the hydrogen form is desired, with an ammonium salt followed by heating to decompose the zeolitic ammonium ion and leave a hydrogen ion. It is not necessary to exchange out all of the monovalent metal; the final compositions may contain up to about 6% by weight of Na2O, or equivalent amounts of other monovalent metals. Molecular sieves of this nature are described more particularly in Belgian Patents Nos. 577,642, 598,582, 598,683 and 598,682.

As in the case of the X molecular sieves, the Y sieves also contain pores of relatively uniform diameter in the individual crystals. In the case of X sieves, the pore diameters may range between about 6 and 14 A., depending upon the metal ions present, and this is likewise the case in the Y sieves, although the latter usually are found to have crystal pores of about 9 to l0 A. in diameter.

The foregoing cracking bases are compounded, as by impregnation, with from about 0.5% to 25% (based on free metal) of a Group VIB or Group VIII metal promoter, e.g., an oxide or sulfide of chromium, tungsten, cobalt, nickel, or the'corresponding free metals, or any combination thereof. Alternatively, even smaller proportions, between about 0.05% and 2% of the noble metals, c g., platinum, palladium, rhodium or iridium, may be employed. The oxides and sullides of other transitional metals may also be used, but generally to less advantage than the foregoing.

In the case of zeolitic type cracking bases, it is desirable to deposit the hydrogenating metal thereon by ion exchange. This can be accomplished by digesting the zeolite with an aqueous solution of a suitable compound of the desired metal, wherein the metal is present in a cationic form, and then reducing to form the free metal, as described for example in Belgian Patent No. 598,686.

The following example is cited to demonstrate the beneficial effect of added monocyclic paraflins during hydrocracking of a paraffin feed, and the detrimental effects of polycyclic aromatics:

EXAMPLE In this example, n-decane was subjected to hydrocracking in a series of experiments, with and without Various aromatic hydrocarbons added thereto. The hydrocracking catalyst was composed of 16 to 30 mesh granules of an 89% silica-11% alumina carrier, upon which was impregnated nickel nitrate from an aqueous solution, followed by calcining at 1,000" F. to give 13.9 weight-percent nickel on the finished catalyst. Prior to the runs this catalyst was pre-reduced with hydrogen for 1 hour at 800 F., presuliided with a thiophene-'heptane mixture for 4 hours at 600 F., and halided for 3 hours at 500 F. by treatment with 5 volume-percent trifluorotoluene in heptane. f

The etfect of aromatic hydrocarbons on the hydrocracking was determined by successively hydrocracking the following feedstocks:

n-Decane n-Decane-I-S mol-percent benzene n-Decane-j-S mol-percent mixed Xylenes n-Decane-j-S mol-percent naphthalene The hydrocracking conditions in each case were held constant at 550 F., 1,000 p.s.i.g., 1.0 LHSV, and 20,000 s.c.f. of hydrogen per barrel of feed. The following table shows the effects of the various aromatics upon the hydro'- cracking of n-decane:

TABLE 2 Run No.

5 mol 5 mol 5 mol Total aromatics in feed 0. percent percent percent benzene xylenes naphthalene Conversions, vol. percent oi Iced:

To Cri-C product 20 17 12 0.8 To 120-335" F. product 18 18 18 7. 2 Iso/normal paraiiin ratios in product:

Butaues 3. 2 2. 8 2. 5 2. 6 Pentanes 6. 5 7. 3 8. 4 (b) Hexanes 6. 7 7. 6 10. 7 (b) Aromatics in feed, vol. percent:

Benzene 0. 0 2. 34 0.0 0.0 0. 0 0. 0 0. 13 0. 0 0. 0 0. 0 2. 08 0. 0 0. 0 0. 0 0. 9 0. 0 0.0 0.0 0.09 0.0 0.0 0.0 0.0 2. 94

Total aromatics 0. 0 2. 34 3. 2 2. 94

Aromatics in Product, vol. percent:

0. 1 2. 3 0. 14 0. 0. 0 0. 1 0. 7 1. 0.0 0. 49 0.0 0. 1 1. 09 0. 0.0 0. 45 0. 0 0. 1 0.0 0. 0. 0 0. 0 0. 0 0. Naphthalene 0. 0 0. 0 0. 0 0.

Total aromatics 0.1 2.6 2.87 2.

nCorrected for the monocyclic aromatics added to the feed.

bInsuticient product for analysis. The effects of the aromatic hydrocarbons are readily apparent from the foregoing. Upon addition of benzene to the feed, the undesired production of light C3-C5 parafns decreased from to 17%,-While the desired C-jgasoline fraction remained constant at 18%. The -use of mixed xylenes further lowered the light parain production to 12%, while the 06+ gasoline stayed constant at 18%. In each case, it will be noted that there was a obtained were due to the modified hydrocracking of parafns brought about by the aromatic hydrocarbon, and

signicant increase in the -pentane and vhexane iso/ normal were not caused by the hydrocracking of the aromatics. It Wil-l be noted that naphthalene greatly suppressed all hydrocracking.

Results analogous to those indicated in the foregoing example are obtained when other catalysts and conditions, other feedstocks and other process conditions Within the broad purview of the above disclosure are employed. It is hence not intended lto limit the invention to the details of the example or drawing, but only broadly as defined in the following claims:

I claim:

l. A process for obtaining a high-octane gasoline from a predominantly paraiiin gas oil feedstock boiling above the gasoline range and containing monocyclic and polycyclic aromatic hydrocarbons which comprises:

(A) separating said feedstock into an essentially aromatic portion and an essentially paraffinic portion;

(B) fractionating said aromatic portion to obtain an overhead fraction boiling between about 400 and 550 F., and a heavier bottoms fraction containing poycyclic aromatic hydrocarbons;

(C) subjecting said overhead fraction to hydrodealkylation to convert methyl naphthaleues to naphthalene and higher alkyl benzenes to lower alkyl benzenes;

(D) fractionating the product from said dealkylation to recover naphthalene and a monocyclic hydrocarbon overhead;

(E) blending said paraflinic feed portion with said monocyclic hydrocarbon Ioverhead fraction;

(F) subjecting the resulting blend of parainic feed Vand monocyclic aromatic hydrocarbons to hydrocracking with added hydrogen in the presence of a hydrocracking catalyst comprising a Group VIII metal hydrogenating component deposited upon a refractory cracking base, the conditions of hydrocracking including a temperature between about 400 and 850 F., a pressure between about 400 and 3,000 p.s.i.g., and a liquid hourly space velocity between about 0.5 and 15, said conditions being further correlated with each other so as to effect a substantial hydrocracking of parafiinic hydrocarbons without substantial hydrogenation of said monocyclic aromatic hydrocarbons, and recovering from said hydrocracking a gasoline fraction comprising monocyclic aromatic hydrocarbons and a paraffinic portion relatively richer in isoparains than would be obtained under the same hydrocracking conditions in the absence of monocyclic aromatic hydrocarbons.

2. A process as defined in claim 1 wherein said feed separation step (A) is solvent extraction.

3. A process as delined in claim l wherein said paraffinic portion contains less than about 2% by volume of polycyclic aromatic hydrocarbons.

4. A process as defined in claim 1 wherein about 0.5 to 10% by volume of said monocyclic aromatic overhead fraction is blended With said paranic feed portion, based on the parailnic feed portion.

5. A process for obtaining a high-octane gasoline from a predominantly parainic gas-oil feedstock boiling above the gasoline range and containing at lea-st about 2% by Volume of polycyclie aromatic hydrocarbons, which comprlses: l

(A) subjecting said feedstock to a separatory treatment to effect at least a partial removal of said polycyclic aromatic hydrocarbons and produce a rainate oil containing less than about 2% by volume of polycyclic aromatic hydrocarbons;

(B) blending `said rainate oil with at least about 0.5%

by volume of added monocyclic aromatic hydrocarbon; and (C) subjecting the resulting blend of parainic and monocyclic aromatic hydrocarbons to hydrocracking n with added hydrogen in the presence of a hydroc cracking catalyst comprising a Group VIII metal hydrogenating component deposited upon a refractory cracking base, the conditions of hydrocracking including a temperature between about 400-850 F., a pressure between about 400 and 3,000 p.s.i.g., anda liquid hourly space velocity between about 0.5 and 15, lsaid conditions being further correlated with each other so as to efeot a substantial hydrocracking of parainic hydrocarbons without substantial hydrogenation of said monocyclic aromatic hydrocarbons, and recovering from said hydrocracking a gasoline fraction comprising monocyclic aromatic hydrocarbons and a parafiinic portion relatively richer References Cited in the le of this patent UNITED STATES PATENTS Pier et al June 10, 1941 Tongberg Dec. 8, 1942 Lanning Feb. 3, 1953 Hetzel Ian. 4, 1955 Doumani Peb. 14, 1956 Mertes Mar. 5, 1963

Claims (1)

1. 5. A PROCESS FOR OBTAINING A HIGH-OCTANE GASOLINE FROM A PREDOMINANTLY PARAFFINIC GAS-OIL FEEDSTOCK BOILING ABOVE THE GASOLINE RANGE AND CONTAINING AT LEAST ABOUT 2% BY VOLUME OF POLYCYCLIC AROMATIC HYDROCARBON, WHICH COMPRISES: (A) SUBJECTING SAID FEEDSTOCK TO A SEPARATORY TREATMENT TO EFFECT AT LEAST A PARTIAL REMOVAL OF SAID POLYCYCLIC AROMATIC HYDROCARBONS AND PRODUCE A RAFFINATE OIL CONTAINING LESS THAN ABOUT 2% BY VOLUME OF POLYCYCLIC AROMATIC HYDROCARBONS; (B) BLENDING SAID RAFFINATE OIL WITH AT LEAST ABOUT 0.5% BY VOLUME OF ADDED NOMOCYCLIC AROMATIC HYDROCARBON; AND (C) SUBJECTING THE RESULTING BLEND OF PARAFFINIC AND MONOCYCLIC AROMATIC HYDROCARBONS TO HYDROCRACKING WITH ADDED HYDROGEN IN THE PRESENCE OF A HYDROCRACKING CATALYST CONPRISING A GROUP VIII METAL HYDROGENATING COMPONENET DEPOSITED UPON A REFRACTORY CRACKING BASE, THE CONDITIONS OF HYDROCRACKING INCLUDING A TEMPERATURE BETWEEN ABOUT 400-850* F., A PRESSURE BETWEEN ABOUT 400 AND 3,000 P.S.I.G., AND A LIQUID HOURLY SPACE VELOCITY BETWEEN ABOUT 0.5 AND 15, SAID CONDITIONS BEING FURTHER CORRELATED WITH EACH OTHER SO AS TO EFFECT A SUBSTANTIAL HYDROCRACKING OF PARAFFINIC HYDROCARBONS WITHOUT SUBSTANTIAL HYDROGENATION OF SAID MONOCYCLIC AROMATIC HYDROCARBONS, AND RECOVERING FROM SAID HYDROCRACKING A GASOLINE FRACTION COMPRISING MONOCYCLIC AROMATIC HYDROCARBONS AND A PARAFFINIC PORTION RELATIVELY RICHER IN ISOPARAFFINS THEN WOULD BE OBTAINED UNDER THE SAME HYDROCRACKING CONDITIONS IN THE ABSENCE OF SAID ADDED MONOCYCLIC AROMATIC HYDROCARBONS.

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