Production of High Octane Gasoline by Hydrocracking Catalytic Cracking Products

This invention relates to the production of high octane gasoline and more particularly to the production of high octane gasoline by hydrocracking highly aromatic fractions obtained from catalytic cracking operations.

Under present conditions, petroleum refineries are finding it necessary to convert increasingly greater proportions of crude to premium fuels such as gasoline and middle distillates such as diesel and jet fuel. Catalytic cracking processes, exemplified by the fluid catalytic cracking (FCC) process and Thermofor catalytic cracking (TCC) process together, account for a substantial fraction of heavy liquids conversion in modern refineries. Both are thermally severe processes which result in a rejection of carbon to coke and to residual fractions; during catalytic cracking high molecular weight liquids disproportionate into relatively hydrogen-rich light liquids and aromatic, hydrogen-deficient heavier distillates and residues.

Catalytic cracking in the absence of hydrogen does not provide significant desulfurization nor is the nitrogen content of the feed selectively rejected with the coke. Both sulfur and nitogen therefore concentrate appreciably in the heavier cracking products. Cracking therefore produces significant quantities of highly aromatic, hydrogen-deficient middle and heavy distillates that have high sulfur and nitrogen levels. Recycling these liquids to the catalytic cracker is often not an attractive option, because they are refractory and difficult to convert and often will impair conversion of the less refractory fresh feed. Generally, the level of heteroatom contaminants increases with the boiling point of the fraction, as shown in Table 1 below which gives the sulfur and nitrogen contents for two typical FCC product fractions, a light cycle oil and an FCC main column bottoms (proportions and percentages by weight, as in the remainder of this specification unless the contrary is stated).

Present market requirements make refractory product streams such as these particularly difficult to dispose of as commercially valuable products. Formerly, the light and heavy cycle oils could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading the light cycle oil was conventionally carried out by a relatively low severity, low pressure catalytic hydro-desulfurization (CHD) unit in which the cycle stock would be admixed with virgin mid-distillates from the same crude blend fed to the catalytic cracker. Further discussion of this technology is provided in the Oil and Gas Journal, May 31, 1982, pp. 87-94.

Currrently, however, the refiner is finding a diminished demand for fuel oil. At the same time, the impact of changes in supply and demand for petroleum has resulted in a lowering of the quality of the crudes available to the refiner; this has resulted in the formation of an even greater quantity of refractory cycle stocks. As a result, the refiner is left in the position of producing increased amounts of poor quality cycle streams from the catalytic cracker while having a diminishing market in which to dispose of these streams.

At many petroleum refineries, the light cycle oil (LCO) from the FCC unit is a significant component of the feed to the catalytic hydrodesulfurization (CHD) unit which produces No. 2 fuel oil or diesel fuel. The remaining component is generally virgin kerosene taken directly from the crude distillation unit. The highly aromatic nature of LCO, particularly when the FCC unit is operated in the maximum gasoline mode, increases operational difficulties for the CHD and can result in a product having marginal properties for No. 2 fuel oil or diesel oil, as measured by cetane numbers and sulfur content.

An alternative market for mid-distillate streams is automotive diesel fuel. However, diesel fuel has to meet a minimum cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. Because cetane number correlates closely and inversely with aromatic content, the highly aromatic cycle oils from the cracker typically with aromatic contents of 80% or even higher have cetane numbers as low as 4 or 5. In order to raise the cetane number of these cycle stocks to a satisfactory level by the conventional CHD technology described above, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.

Because of these problems associated with its use as a fuel, recycle of untreated light cycle oil to the FCCU has been proposed as a method for reducing the amount of LCO. Benefits expected from the recycle of LCO include conversion of LCO to gasoline, backout of kerosene from No. 2 fuel oil and diminished use of cetane improvers in diesel fuel. However, in most cases, these advantages are outweighed by disadvantages, which include increased coke make in the FCC unit, diminished quality of the resultant LCO and an increase in heavy cycle oil and gas.

A typical LCO is such a refractory stock and of poor quality relative to a fresh FCC feed that most refineries do not practice recycle of the untreated LCO to any significant extent. One commonly practiced alternative method for upgrading the LCO is to hydrotreat severely prior to recycle to the catalytic cracker or, alternatively, to hydrotreat severely and feed to a high pressure fuels hydrocracker. In both such cases, the object of hydrotreating is to reduce the heteroatom content to low levels while saturating polyaromatics to increase crackability. Although this does enhance the convertibility of these aromatic streams considerably, the economic penalties derived from high hydrogen consumptions and high pressure processing are severe. In addition, in those instances where the production of gasoline is desired, the naphtha may require reforming to recover its aromatic character and meet octane specifications.

Hydrocracking may be used to upgrade the higher-boiling more refractory products derived from catalytic cracking. The catalytic cracker is used to convert the more easily cracked paraffinic gas oils from the distillation unit while the hydrocracker accepts the dealkylated, aromatic cycle oils from the cracker and hydrogenates and converts them to lighter oils. See Petroleum Refining; Second Ed.; Gary, J. H. and Handwerk, G. E.; Marcel Dekker, N.Y. 1984; pp. 138-151; Modern Petroleum Technology, Fourth Ed., Hobson, G. D., Applied Science Publ. 1973; pp. 309-327. These hydrocracking processes using catalytically cracked feeds either on their own or mixed with virgin feeds have, however, generally been incapable of producing high octane gasoline directly. The reason for this is that they have conventionally been operated at high hydrogen pressures and at relatively high conversion levels so as to maximize the saturation of the aromatics (especially the refractory polynuclear aromatics), removal of heteroatoms in inorganic form and the subsequent conversion of the hydrogenated aromatics to paraffins. While this may produce acceptable diesel fuel (which benefits from the presence of n-paraffins) the octane quality of the gasoline has generally been poor as a consequence of the large quantities of low octane paraffin components. For present day use these gasolines will require extensive reforming with its consequent yield loss in order to conform to market product specifications. To illustrate, U.S. Pat. No. 3,132,090 discloses the use of a two-stage hydrocracking scheme to produce gasoline. However, the octane number of the gasoline using a virgin distillate as charge is reported as 68 (RON+0). An octane of 80 (RON+3) is disclosed for a charge-stock of coker distillate and thermally cracked gas oils. The "high octane" gasolines described in this patent contain 3 ml/gallon of tetraethyl lead (TEL) and are in the range of 70-88 (RON+3). Because TEL adds about 4-6 octane numbers these gasolines have an octane rating on a clear basis (RON+0) in the range of 65-83 (RON+0).

Various low pressure hydrocracking processes have also been described. For example, U.S. Pat. Nos. 3,867,277 and 3,923,640 disclose low pressure hydrocracking processes using various high boiling feedstocks, generally of high (20-40) API gravity. The use of such feeds, coupled with the relatively high levels of conversion in those processes leads to napthas of low octane rating since the alkyl groups present in the feeds come through into the naphtha together with the relatively straight chain paraffins produced by the ring opening and cracking of the aromatics. These processes have therefore been unsatisfactory for the direct production of high octane gasoline.

Other low pressure hydrocracking processes producing aromatic products have been described in the past but their potential for producing high octane gasoline from low value, refractory cracking oils has not been appreciated. For example, U.S. Pat. No. 4,435,275 describes a method for producing aromatic middle distillates such as home heating oil from high gravity feeds under relatively low conversion conditions but with the objective of producing low-sulfur middle distillates, octane numbers of only about 78 (R+0) are reported.

A notable advance is described in patent application Ser. No. 825,294, now U.S. Pat. No. 4,676,887, to which reference is made for details of the process. It was found that highly aromatic, refractory feeds derived from catalytic cracking could be converted directly to high octane gasoline by hydrocracking at relatively low pressures, typically 600-1000 psig (about 4250-7000 kPa. abs.) and with low conversions, typically below 50 weight percent to 385.degree. F.- (195.degree. C.-) products. (All SI equivalents in this specification are rounded off to a convenient figure so as to permit convenient comparison; all pressures quoted in SI units are absolute pressures). By using a highly aromatic feed which has been substantially dealkylated in the catalytic cracking operation, typically with an API gravity of 5-25, the hydrocracking proceeds with only a limited degree of aromatics saturation so that a large quantity of single-ring alkylaromatics (mainly benzene, toluene, xylenes and trimethyl benzenes) are obtained by ring opening of partial hydrogenation products of bicyclic aromatics. The single ring aromatics are not only in the gasoline boiling range but also possess high octane numbers so that a high octane gasoline is produced directly, suitable for blending into the refinery gasoline pool without prior reforming.

In the process described in application Ser. No. 825,294, the feed is a full boiling range fraction from a catalytic cracking operation, for example, a full range light cycle oil (LCO) or a heavy cycle oil (HCO). A typical full range light cycle oil (FRLCO) will have a boiling range of about 400.degree.-750.degree. F. (about 205.degree.-400.degree. C.). Using a full boiling range fraction of that kind, the conversion has to be held at relatively low levels--below 50% and preferably below a certain value related to the hydrogen partial pressure, both in order to produce a high octane gasoline product and to avoid excessive catalyst aging. Although the attainment of high gasoline octane in this way is extremely advantageous, the limitation on conversion represents a process limitation which should, if feasible, be transcended in order to maximize gasoline yield. The present invention enables higher levels of conversion to be employed so that high yields of high octane gasoline are produced directly from the feed in a single pass.

In the process described in Ser. No. 825,294, the high octane gasoline is produced from a relatively lower boiling fraction of the aromatic feed from the catalytic cracking operation. By the use of these light cut feeds, conversion may be raised to higher levels without adversely affecting gasoline octane or catalyst aging rate.

By using a highly aromatic, substantially dealkylated distillate feed with a maximum end point of about 750.degree. F. (400.degree. C.), preferably not more than 700.degree. F. (about 370.degree. C.), and usually not more than about 650.degree. F. (345.degree. C.), the conversion may be raised to higher levels without incurring significant penalties. So, if the end point of the feed is lowered further, for instance, to 600.degree. F. (315.degree. C.) or even lower, conversion may be raised still further. Conversion to gasoline boiling range products will, however, be limited to an 85 percent by weight maximum and preferably will not exceed 65 percent by weight.

It has been found that substantially more high octane gasoline may be made by recycling the unconverted fraction from the low pressure hydrocracking operation back to the catalytic cracking unit. Accordingly, the present invention is directed to a process for the low pressure hydrocracking of the highly aromatic, substantially dealkylated feedstocks as described above to produce a high octane gasoline fraction and an unconverted bottoms fraction which is recycled to the catalytic cracking unit, e.g., the FCCU or TCCU.

The feeds used in the present process may typically be either a full range cycle oil as described in Ser. No. 825,294 or, alternatively, a light cut cycle oil as described in Ser. No. 940,382. If the light cut cycle oils are employed, higher conversion levels may be tolerated in the hydrocracking step, as described in Ser. No. 940,382.

The hydrocracking is operated under low to moderate pressure, typically 400-1000 psig (about 2860-7000 kPa) hydrogen pressure. At the relatively low severity conditions employed temperatures will generally be in the range of 600.degree.-850.degree. F. (315.degree.-455.degree. C.), more typically 700.degree.-800.degree. F. (370.degree.-425.degree. C.), with space velocity adjusted to obtain the desired conversion.

The single FIGURE of the accompanying drawings is a simplified schematic illustration of a process unit for producing gasoline by the present process.

Feedstock

The feeds used in the present process are hydrocarbon fractions which are highly aromatic and hydrogen deficient. They are fractions which have been substantially dealkylated, as by a catalytic cracking operation, for example, in an FCC or TCC unit. It is a characteristic of catalytic cracking that the alkyl groups, generally bulky, relatively large alkyl groups (typically but not exclusively C.sub.5 -C.sub.9 alkyls), which are attached to aromatic moieties in the feed become removed during the course of the cracking. It is these detached alkyl groups which lead to the bulk of the gasoline product from the cracker. The aromatic moieties such as benzene, naphthalene, benzothiophenes, dibenzothiophenenes and polynuclear aromatics (PNAs) such as anthracene and phenanthrene form the high boiling products from the cracker. The mechanisms of acid-catalyzed cracking and similar reactions remove side chains of greater than 5 carbons while leaving behind short chain alkyl groups, primarily methyl, but also ethyl groups on the aromatic moieties. Thus, the "substantially dealkylated" cracking products include those aromatics with small alkyl groups, such as methyl, and ethyl, and the like still remaining as side chains, but with relatively few large alkyl groups, i.e., the C.sub.5 -C.sub.9 groups, remaining. More than one of these short chain alkyl groups may be present, for example, one, two or more methyl groups.

Feedstocks of this type have an aromatic content in excess of 50 wt. percent; for example, 70 wt. percent of 80 wt. percent or more, aromatics. Highly aromatic feeds of this type typically have hydrogen contents below 14 wt. percent, usually below 12.5 wt. percent or even lower, e.g. below 10 wt. percent or 9 wt. percent. The API gravity is also a measure of the aromaticity of the feed, usually being below 30 and in most cases below 25 or even lower, e.g., below 20. In most cases the API gravity will be in the range 5 to 25 with corresponding hydrogen contents from 8.5-12.5 wt. percent. Sulfur contents are typically from 0.5-5 wt. percent and nitrogen from 50-1000 ppmw, more usually 50-650 ppmw.

Suitable feeds for the present process are substantially dealkylated cracking product fractions. Suitable feeds of this type include cycle oils from catalytic cracking units. Full range cycle oils may be used, for example, full range light cycle oils with a boiling range of 385.degree.-750.degree. F. (about 195.degree.-400.degree. C.), e.g., 400.degree.-700.degree. F. (about 205.degree.-370.degree. C.) or, alternatively, cycle oil fractions may be employed such as heavy cycle oil or light cycle oil fractions. The use of cycle oils such as these are described in Ser. Nos. 825,294 and 940,382, to which reference is made for details of such feeds.

When operating with an extended boiling range feed such as a full range light cycle oil as described in Ser. No. 825,294, conversion should be limited so as to avoid excessive catalyst aging as well as to produce a high octane product; a maximum conversion of 50% to lower boiling products and preferably below one twentieth the hydrogen partial pressure (psig) should be observed, as described in Ser. No. 825,294. However, if a light cut cycle oil is used, as described in Ser. No. 940,382, higher conversions may be tolerated. For this reason, lower boiling range fractions of that type are preferred. Thus, cycle oils with end points below 650.degree. F. (345.degree. C.), preferably below 600.degree. F. (315.degree. C.) are preferred. Initial boiling point will usually be 300.degree. F. (150.degree. C.) or higher, e.g. 330.degree. F. (165.degree.) or 385.degree. F. (195.degree. C.). Light cycle oils generally contain from about 60 to 80% aromatics and, as a result of the catalytic cracking process, are substantially dealkylated, as described above. Other examples of suitable feedstocks include the dealkylated liquid products from delayed or fluid bed coking processes.

If a cycle oil fraction is to be used, it may be obtained by fractionation of a FRCO or by adjustment of the cut points on the cracker fractionation column. The light stream will retain the highly aromatic character of the catalytic cracking cycle oils (e.g. greater than 50% aromatics by silica gel separation) but the lighter fractions used will generally exclude the heavier polynuclear aromatics (PNAs--three rings or more) which remain in the higher boiling range fractions so that higher conversions may be attained without excessive catalyst aging or loss of product octane. In addition, the heteroatom contaminants are concentrated in the higher boiling fractions so that the hydrocracking step is operated substantially in their absence and preliminary feed hydrotreating is not necessary.

The use of the dealkylated feeds is a significant feature of the process. It will not produce high octane gasoline from predominantly virgin or straight run oils and which have not been previously dealkylated by processes such as catalytic cracking or coking. If the feed used in the present process has not been previously dealkylated, the large alkyl groups found in the feed will be cracked off during the hydrocracking and will be found in the resulting naphtha fraction. Because these groups are relatively straight chain, a low octane gasoline product will result. Smaller, i.e., C.sub.1 -C.sub.3, alkyl side groups, if present do not appear in the naphtha boiling range products from the hydrocracker (even if conditions are severe enough to remove them) and so they have no effect on product octane. If a mixture of dealkylated and non-dealkylated feedstock is used, the octane number will be intermediate between the octane numbers of the feeds used separately. A mixture of alkylated and dealkylated feedstocks can be used in commercial operation but if so, it is likely that the gasoline will have to be subjected to a reforming process in order to achieve the desired octane.

Catalysts

The catalyst used for the hydrocracking is a bifunctional, heterogeneous, porous solid catalyst possessing acidic and hydrogenation-dehydrogenation functionality. Because the highly aromatic feed contains relatively bulky bicyclic and polycyclic components the catalyst should have a pore size which is sufficiently large to admit these materials to the interior structure of the catalyst where cracking can take place. A pore size of at least about 7.4 A (corresponding to the pore size of the large pore size zeolites X and Y) is sufficient for this purpose but because the end point of the feed is limited, the proportion of bulky, polynuclear aromatics is quite low and for this reason, very large pore sizes greatly exceeding those previously mentioned are not required. Crystalline zeolite catalysts which have a relatively limited pore size range, as compared to the so-called amorphous materials such as alumina or silica-alumina, may therefore be used to advantage in view of their activity and resistance to poisoning. Catalysts having aromatic selectivity, i.e. which will crack aromatics in preference to paraffins are preferred because of the highly aromatic character of the feed.

The preferred hydrocracking catalysts are the crystalline catalysts, generally the zeolites, and, in particular, the large pore size zeolites having a Constraint Index less than 2. For purposes of this invention, the term "zeolite" is meant to represent the class of porotectosilicates, i.e., porous crystalline silicates, that contain silicon and oxygen atoms as the major components. Other components are also present, including aluminum, gallium, iron, boron and the like, with aluminum being preferred in order to obtain the requisite acidity. Minor components may be present separately, in mixtures in the catalyst or intrinsically in the structure of the catalyst.

Zeolites with a silica-to-alumina mole ratio of at least 10:1 are useful, it is preferred to use zeolites having much higher silica-to-alumina mole ratios, i.e., ratios of at least 50:1. The silica-to-alumina mole ratio referred to may be determined by conventional analysis. This ratio is meant to represent, as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other forms within the channels.

A convenient measure of the extent to which a zeolite provides control to molecules of varying sizes to its internal structure is the Constraint Index of the zeolite. Zeolites which provide a highly restricted access to and egress from its internal structure have a high value for the Constraint Index, and zeolites of this kind usually have pores of small size, e.g., less than 5 Angstroms. On the other hand, zeolites which provide relatively free access to the internal zeolite structure have a low value for the Constraint Index and usually pores of large size, e.g., greater than 8 Angstroms. The method by which Constraint Index is determined is described fully in U.S. Pat. No. 4,016,218, to which reference is made for details of the method. A Constraint Index of less than 2 and preferably less than 1 is a characteristic of the hydrocracking catalysts used in the present process.

Constraint Index (CI) values for some typical large pore materials are shown in Table 2 below:

The nature of the CI parameter and the technique by which it is determined admit of the possibility that a given zeolite can be tested under somewhat different conditions and thereby exhibit different Constraint Indices. Constraint Index may vary with severity of operation (conversion) and the presence or absence of binders. Other variables, such as crystal size of the zeolite, the presence of occluded contaminants, etc., may also affect the Constraint Index. It may be possible to so select test conditions, e.g., temperatures, as to establish more than one value for the Constraint Index of a particular zeolite, as with zeolite beta. A zeolite is considered to have a Constraint Index within the specified range if it can be brought into the range under varying conditions.

The large pore zeolites, i.e., those zeolites having a Constraint Index less than 2 have a pore size sufficiently large to admit the vast majority of components normally found in the feeds. These zeolites are generally stated to have a pore size in excess of 7 Angstroms and are represented by zeolites having the structure of, e.g., Zeolite Beta, Zeolite X, Zeolite Y, faujasite, Ultrastable Y (USY), Dealuminized Y (Deal Y), Mordenite, ZSM-3, ZSM-4, ZSM-18 and ZSM-20. Zeolite ZSM-20 resembles faujasite in certain aspects of structure, but has a notably higher silica/alumina ratio than faujasite, as do the various forms of zeolite Y, especially USY and De-AlY. Zeolite Y is the preferred catalyst, and it is preferably used in one of its more stable forms, especially USY or De-AlY.

Although Zeolite Beta has a Constraint Index less than 2, it does not behave exactly like a typical large pore zeolite. Zeolite Beta satisfies the pore size requirements for a hydrocracking catalyst for use in the present process but it is not preferred because of its paraffin-selective behavior.

Because they are aromatic selective and have a large pore size, the amorphous hydrocracking catalysts such as alumina and silica-alumina may be used although they are not preferred.

Zeolite ZSM-4 is described in U.S. Pat. No. 3,923,639; Zeolite ZSM-20 in U.S. Pat. No. 3,972,983; Zeolite Beta in U.S. Pat. Nos. 3,308,069 and Re 28,341; Low sodium Ultrastable Y molecular sieve (USY) is described in U.S. Pat. Nos. 3,293,192 and 3,449,070; Dealuminized Y zeolite (Deal Y) may be prepared by the method found in U.S. Pat. No. 3,442,795; and Zeolite UHP-Y is described in U.S. Pat. No. 4,401,556. Reference is made to these patents for details of these zeolite catalysts.

The catalyst should have some acidic functionality, i.e., an alpha value greater than 1 for the cracking function. The alpha value, a measure of zeolite acidic functionality, is described together with details of its measurement in U.S. Pat. No. 4,016,218 and in J. Catalysis, Vol. VI, pages 278-287 (1966) and reference is made to these for such details. However, because the catalyst is being used in a fixed bed operation with a highly aromatic feed at low hydrogen pressure, it must have a low coking tending in order to reduce aging and for this reason, a low alpha value is preferred. Alpha values between 1 and 200, preferably not more than 100 are preferred, with values not more than 75 e.g. 50 being useful.

Catalyst stability during the extended cycle life is essential and this may be conferred by suitable choice of catalyst structure and composition, especially silica:alumina ratio. This ratio may be varied by initial zeolite synthesis conditions, or by subsequent dealuminization as by steaming or by substitution of frame work aluminum with other trivalent species such as boron, iron or gallium. Because of its convenience, steaming is a preferred treatment. In order to secure satisfactory catalyst stability, high silica:alumina ratios, e.g. over 50:1 are preferred, e.g. about 200:1 and these may be attained by steaming. The alkali metal content should be held at a low value, preferably below 1% and lower, e.g. below 0.5% Na. This can be achieved by successive sequential ammonium exchange followed by calcination.

Improved selectivity and other beneficial properties may be obtained by subjecting the zeolite to treatment with steam at elevated temperatures ranging from 500.degree. to 1200.degree. F. (399.degree.-538.degree. C.), and preferably 750.degree. to 1000.degree. F. (260.degree.-694.degree. C.). The treatment may be accomplished in an atmosphere of 100% steam or an atmosphere consisting of steam and a gas which is substantially inert to the zeolites. A similar treatment can be accomplished by lower temperatures and elevated pressure, e.g. 350.degree. to 700.degree. F. (177.degree.-371.degree. C.) at 10 to about 200 atmospheres.

The zeolites are preferably composited with a matrix comprising another material resistant to the temperature and other conditions employed in the process. The matrix material is useful as a binder and imparts greater resistance to the catalyst for the severe temperature, pressure and reactant feed stream velocity conditions encountered in the process. Useful matrix materials include both synthetic and naturally occurring substances, such as clay, silica and/or metal oxides. The latter may be either naturally occurring or in the form of synthetic gelatinous precipitates or gels including mixtures of silica and metal oxides such as alumina and silica-alumina. The matrix may be in the form of a cogel. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families. Such clays can be used in th raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. The relative proportions of zeolite component and the matrix, on an anhydrous basis, may vary widely with the zeolite content ranging from between about 1 to about 99 wt %, and more usually in the range of about 5 to about 80 wt % of the dry composite. If the feed contains greater than 20% 650.degree. F.+ material, that the binding matrix itself be an acidic material having a substantial volume of large pore size material, not less than 100 A.degree.. The binder is preferably composited with the zeolite prior to treatments such as steaming, impregnation, exchange, etc., in order to preserve mechanical integrity and to assist impregnation with non-exchangeable metal cations.

The original cations associated with each of the crystalline silicate zeolites utilized herein may be replaced by a wide variety of other cations, according to conventional techniques. Typical replacing cations including hydrogen, ammonium and metal cations, including mixtures of these cations. Useful cations include metals such as rare earth metals, e.g., manganese, as well as metals of Group IIA and B of the Periodic Table, e.g., zinc, and group VIII of the Periodic Table, e.g., platinum and palladium, to promote stability (as with the rare earth cations) or a desired functionality (as with the Group VI or VIII metals). Typical ion-exchange techniques are to contact the particular zeolite with a salt of the desired replacing cation. Although a wide variety of salts can be employed, particular preference is given to chlorides, nitrates and sulfates. Representative ion-exchange techniques are disclosed in a wide variety of patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; and 3,140,253.

Following contact with a solution of the desired replacing cation, the zeolite is then preferably washed with water and dried at a temperature ranging from 150.degree. to about 600.degree. F. (65.degree.-315.degree. C.), and thereafter calcined in air, or other inert gas, at temperatures ranging from about 500.degree. to 1500.degree. F. (260.degree.-815.degree. C.) for periods of time ranging from 1 to 48 hours or more.

The hydrocracking catalyst also has a metal component to provide hydrogenation-dehydrogenation functionality. Suitable hydrogenation components include the metals of Groups VIA and VIIIA of the Periodic Table (IUPAC Table) such as tungsten, vanadium, zinc, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal such as platinum or palladium, in an amount between 0.1 and about 25 wt %, normally 0.1 to 5 wt % especially for noble metals, and preferably 0.3 to 3 wt %. This component can be exchanged or impregnated into the composition, using a suitable compound of the metal. The compounds used for incorporating the metal component into the catalyst can usually be divided into compounds in which the metal is present in the cation of the compound and compounds in which it is present in the anion of the compound. Compounds which contain the metal as a neutral complex may also be employed. The compounds which contain the metal in the ionic state are generally used, although cationic forms of the metal, e.g. Pt(NH.sub.3).sub.4.sup.2+, have the advantage that they will exchange onto the zeolite. Anionic complex ions such as vanadate or metatungstate which are commonly employed can however be impregnated onto the zeolite/binder composite without difficulty in the conventional manner since the binder is able to absorb the anions physically on its porous structure. Higher proportions of binder will enable higher amounts of these complex ions to be impregnated. Thus, suitable platinum compounds include chloroplatinic acid and various compounds containing the platinum amine complex. Phosphorus is generally also present in the fully formulated catalyst, as phosphorus is often used in solutions from which base metals, such as nickel, tungsten and molybdenum, are impregnated onto the catalyst.

Base metal components, especially nickel-tungsten and nickel-molybdenum are particularly preferred in the present process.

Process Configuration

A preferred process configuration using a light cut LCO feed to the hydrocracker is illustrated schematically in the drawing. A gas oil or resid feed to an FCC unit 10 is cracked in the FCC unit and the cracking products are fractionated in the cracker fractionator 11 to produce the various hydrocarbon fractions which leave the fractionator in the conventional manner. A full range light cycle oil (FRLCO) is withdrawn from fractionator 11 through draw-off conduit 12 and is subjected to a secondary fractionation in distillation tower 13. The lower boiling fraction with a typical boiling range of 300.degree.-650.degree. F. (150.degree.-345.degree. C.), preferably 330.degree.-600.degree. F. (165.degree.-315.degree. C.), is withdrawn through conduit 14 and this light cut LCO (LCLCO) is then passed to hydrocracker 15. Alternately this fractionation can be done on the main FCC column itself. The higher boiling fraction of the cycle oil withdrawn from the bottom of fractionator 13 may be blended into fuel oil products in the conventional way, either directly or after CHD treatment. In the hydrocracker, the typical hydrocracking reactions take place with saturation of the aromatics and ring opening and cracking to form a hydrocracked product which is rich in monocyclic aromatics in the gasoline boiling range. After hydrogen separation in separator 16, the hydrocracker effluent is fractionated in the conventional manner in distillation tower 17 to form the products including dry gas, gasoline, middle distillate and a bottoms fraction which is recycled to FCCU 10 through recycle conduit 18. The gasoline range product from tower 17 is of high octane rating and is suitable for being blended directly into the refinery gasoline product pool without reforming or other treatment to improve octane number.

Hydrotreating

If an extended boiling range feed including significant amounts of higher boiling, e.g., 600.degree. F.+ (315.degree.C.+) or 650.degree. F.+ (345.degree. C.+) fractions is used for the hydrocracker, it is preferred to carry out a preliminary hydrotreating step before the hydrocracker to effect some saturation of aromatics, especially PNAs, as well as to hydrogenate residual heteroatoms, especially nitrogen and sulfur which are then removed in an interstage separator. However, the hydrocracking step will itself carry out a substantial degree of hydrogenation so that the operation of the catalytic cracking step will be improved by the hydrogenation which takes place there rendering the initial hydrotreating step unnecessary although it may be used if desired.

Although, as stated above, the use of two-stage hydrocracking, i.e. hydrotreating followed by hydrocracking is not preferred with light cut feeds since it represents a needless complication and expense, it may be resorted to if desired, e.g. with heavier feeds or to use existing equipment and catalyst loadings. Preliminary hydrotreating may be carried out with or without interstage separation before the hydrocracking step. If interstage separation is omitted, i.e. cascade operation is employed, the hydrotreating catalyst may simply be loaded on top of the hydrocracking catalyst in the reactor.

Hydrotreating may be useful if the feed has a relatively high heteroatom content since hydrotreating with interstage separation of inorganic nitrogen and sulfur will enable extended cycle life to be obtained in the hydrocracking unit.

The hydrotreating catalyst may be any suitable hydrotreating catalyst, many of which are commercially available. These are generally constituted by a metal or combination of metals having hydrogenation/dehydrogenation activity and a relatively inert, i.e. non-acidic refractory carrier having large pores (20.degree. A or more). Suitable carriers are alumina, silica-alumina or silica and other amorphous, large pore size amorphous solids such as those mentioned above in connection with the hydrocracking catalyst binder materials. Suitable metal components are nickel, tungsten, cobalt, molybdenum, vanadium, chromium, often in such combinations as cobalt-molybdenum or nickel-cobalt-molybdenum. Other metals of Groups VI and VIII of the Periodic Table may also be employed. About 0.1-20 wt percent metal, usually 0.1-10 wt. percent, is typical.

Because the catalyst is relatively non-acidic (although some acidity is necessary in order to open heterocyclic rings to effect hetero atom removal) and because temperature is relatively low, conversion during the hydrotreating step will be quite low, typically below 10 volume percent and in most cases below 5 volume percent. Temperatures will usually be from 600.degree. to 800.degree. F. (315.degree.-425.degree. C.), mostly from 625.degree. to 750.degree. F. (330.degree. to 400.degree. C.). Space velocity (LHSV at 20.degree. C.) will usually be from 0.25 to 4.0 hr..sup.-1, preferably 0.4 to 2.5 hr..sup.-1, the exact space velocity selected being dependent on the extent of hydrotreating desired and the selected operational temperature. Hydrogen pressures of 200-1000 psig (1500-7000 kPa), preferably 400-800 psig (2860-5620 kPa) are typical with hydrogen circulation rates of 500-5000 SCF/Bbl (90-9000 n.l.l..sup.-1) being appropriate. If cascade operation is employed, the hydrotreating pressure will be slightly higher than that desired in the hydrocracking step to allow for bed pressure drop.

The hydrotreating catalyst, like the hydrocracking catalyst, may be disposed as a fixed, fluidized, or moving bed of catalyst, although a downflow, fixed bed operation is preferred because of its simplicity.

When a preliminary hydrotreatment is employed, conditions in the hydrocracking step may be adjusted suitably to maintain the desired overall process objective, i.e. incomplete saturation of aromatics with limited ring opening of hydroaromatic components to form high octane gasoline boiling range products. Thus, if some saturation of bicyclic aromatics such as naphthalene, methyl naphthalenes and benzothiophenes is taken in the hydrotreating step, hydrogen consumption in the hydrocracking step will be reduced so that a lower temperature will result if space velocity is kept constant (since the extent of the exothermic hydrogenation reactions will be less for the same throughput in the second stage). In order to maintain the desired level of conversion (which is dependent on temperature, it may be necessary to decrease space velocity commensurately.

A single stage operation without preliminary hydrotreating is preferred with an LCLCO feed. When the feed contains relatively small proportions of polynuclear aromatics (PNAS) as well as of nitrogen and sulfur containing impurities which can all be handled adequately in a single stage operation. The bulk of the PNA's remain in the higher boiling portion of the cycle oil together with the bulk of the heteroatoms and accordingly do not enter the hydrocracker.

Hydrocracking Conditions

During hydrocracking the objective is to create monocyclic aromatics of high octane value from the aromatics in the feed. Because of this, the degree of saturation during the hydrocracking step must be limited so as to avoid complete hydrogenation of these components. For this reason, relatively low to moderate hydrogen pressures are used, usually not more than 1000 psig (7000 kPa), with minimum pressures usually being about 400 psig (about 2860 kPa), with typical pressures in the range of 600-1000 psig (about 4250-7000 kPa), with the exact pressure selected being dependent upon feed characteristics (aromatic and heteroatom content), catalyst stability and aging resistance and the desired product characteristics. This is particularly the case with light out LCO feeds which are principally composed of bicyclic aromatics such as naphthalene, benzothiophene, etc. where excessive saturation is definitely not desired. Similarly, because ring opening is also to be limited in order to preserve the aromatic character of the gasoline product, severity (temperature, residence time, conversion) is also limited. Conversion to 385.degree. F.- (195.degree. C.-) gasoline should be below 80 volume percent and preferably below 65 volume percent. Although conversion may exceed 75 volume percent, conversion levels between 55 and 70 volume percent are preferred. Because the absence of heterotoms and PNAs from the feed reduces catalyst deactivation from heteroatom and PNA induced inhibition and coking, there is a reduced degree of necessity to relate conversion to hydrogen pressure as with the FRLCO feed (see application Ser. No. 825,294). Pressures between 400 and 1000 psig (2860-7000 kPa), usually in the range 600-1000 psig (4250-7000 kPa) with conversions up to 70 volume percent are preferred. Hydrocracking temperatures are typically up to 850.degree. F. (450.degree. C.) although higher temperatures up to about 900.degree. F. (480.degree. C.) may be employed, commonly with temperature minima of about 600.degree. F. (315.degree. C.) or higher, e.g. 700.degree. F. (370.degree. C.) being a recommended minimum. Space velocity will vary with temperature and the desired level of conversion but will typically be 0.25-2.5 hr..sup.-1, more usually 0.5-1.5 hr..sup.-1 (LHSV, 20.degree. C.). Hydrogen circulation rates of 500-5000 SCF/Bbl (90-900 n.l.l.sup. -1) are suitable.

When operating with a full boiling range feed, e.g., a full range LCO, it is necessary to operate under certain pressure-conversion regimes in order to obtain extended catalyst cycle life between successive regenerations. Hydrogen partial pressures as high as 1000 psig may be used provided that the feedstock conversion to product gasoline per pass is limited to a certain level, generally less than 50% to gasoline boiling range products, e.g., products boiling below 385.degree. F. (195.degree. C.). At pressures of about 1000 psig (7000 kPa), conversions of greater than 50% can be attained if the process is operated at low space velocities. However, such high conversions result in lower gasoline octane numbers. A preferred hydrogen partial pressure is 800 psig (5620 kPa), with 600 psig (4240 kPa) being more preferred. The pressure may be maintained at the level prevalent in the hydrotreater, or even reduced to a lower level. However, in general, for full range light cycle oil, the pressure should be maintained such that conversion to 385.degree. F. wt % liquid will equal or be less than one-twentieth the hydrogen partial pressure (0.5 Hz pressure in psig). The ratio of LHSV from the first stage (when used) to the second stage reactor is between about 0.25 and 2.5, and preferably between 0.5 and 1.5. Temperatures in the second stage with these heavier feeds need to be high and typically they are maintained about 700.degree. F. (about 370.degree. C.), up to a maximum of 900.degree. F. (about 480.degree. C.), the precise temperature requirement being dependent upon the nature of the feed being processed.

With recycle of the hydrocracker bottoms products, i.e., the product boiling above the gasoline range, recycle within the hydrocracker unit to remove higher boiling products is avoided, so that the hydrocracker operates in single pass mode.

Hydrocracker Products

As described above, the objective of the present process is to produce a high octane gasoline directly. The boiling range of the gasoline produced in the hydrocracker will typically be C.sub.5 --385.degree. F. (C.sub.5 --196.degree. C.) (end point) but gasolines of higher or lower end points may be encountered, depending on applicable product specifications, e.g. C.sub.5 --330.degree. F. (C.sub.5 --165.degree. C.) (end point) or C.sub.5 --450.degree. F. (C.sub.5 --232.degree. C.). Minimum target octane number is 85 clear or higher, e.g. 87 (RON+0). In most cases, higher octane ratings are attainable, for example, clear ratings of at least 90 or higher, e.g. 95. In favorable cases, clear octane ratings of 100 or higher may be attained. In all cases, the gasoline boiling range product may be blended directly into the refinery gasoline pool without reforming or other treatment to improve octane.

Hydrocracker Bottoms Recycle

The hydrocracker bottoms fraction is recycled to the catalytic cracking unit where its enhanced crackability as a consequence of its increased hydrogen content will further improve the total gasoline yield, this time by increasing the yield from the cracker. The hydrocracker bottoms may also be combined with the high boiling cut of the cycle oil (from fractionator 13) after it has been hydrotreated, e.g. in a conventional CHD unit to form a fuel oil or diesel fuel or, alternatively, the combined stream can be recycled to the FCCU, as previously described. An improvement in the octane of the cracked gasoline may also be noted.

In an operation of this type, the non-gasoline fraction from the hydrocracker distillation unit, e.g., the 385.degree. F.+ (195.degree. C.+) fraction or the 330.degree. F.+ (105.degree. C.+) fraction is then fed to the cracker together with fresh feed, e.g., sour HGO.

The combination of low pressure hydrocracking and fluid catalytic cracking with recycle of the entire unconverted stream from the low pressure hydrocracking of LCO, or any part of it unexpectedly provides more gasoline at higher octane than either recycle of untreated LCO or recycle of conventionally hydrofined LCO. In addition, when compared to conventional hydrofining more gasoline at higher octane is produced at lower hydrogen consumption. In any of these embodiments, the low pressure hydrocracking-FCC combination is superior to that of recycling untreated or conventionally hydrofined LCO.

In these Examples, the feed was an FCC LCO having the properties shown in Table 2 below, or a non-dealkylated, catalytically hydrodesulfurized virgin mid-distillate having the properties shown in Table 3.

These Examples show the octane improvement in a product gasoline by passing the dealkylated feedstock over a large pore zeolite catalyst under conditions including a pressure of 600 psig. The feed was charged to a two-reactor catalyst system operating in the cascade mode. The catalyst in the first reactor or hydrotreating stage was a NiMo/alumina hydrotreating catalyst (NiMo/Al.sub.2 O.sub.3). The second stage catalyst was selected from the following group:

(1) 0.35% palladium impregnated on rare earth-exchanged Y zeolite (0.35% Pd/REY);

(2) 3% palladium on an extensively dealuminized Y zeolite (3% Pd/Deal Y); and

(3) 1% palladium on Ultrastable Y zeolite (1% Pd/USY).

The hydrocracking was carried out under conditions specified in Table 4, which also gives the results:

It can be seen from Table 4 that 14 to 20 wt % (18-25 vol %) of the LCO chargestock is converted to high octane gasoline. This can be compared to the non-dealkylated CHD feedstock where a low octane gasoline, i.e., 74 (RON+0) is produced. These results show that the process does not produce a high octane gasoline from a non-dealkylated feedstock.

Example 5-6 illustrate the effect of different pressure conditions on the octane number of the product gasolines.

The procedure of Example 1 was followed under the conditions specified in Table 5, which also gives the results:

As illustrated in Table 5, the operation at an inlet hydrogen partial pressure of 1000 psig gave a slightly higher conversion to gasoline, but the gasoline had a lower octane number than the operation run at an inlet hydrogen partial pressure of 600 psig.

Examples 7-8 compare the results of the two-stage cascade reactor system using the LCO feed as described in Example 1, with the hydrotreating (HDT) process alone. The catalysts used for the present invention were NiMo/Al.sub.2 O.sub.3 (first stage) and 0.35% Pd/REY (second stage). The basic procedure of Example 1 was followed under conditions specified in Table 6, which also specifies the results:

It can be seen from Table 6 that the process improves the Diesel Index of the unconverted mid-distillate more than simple hydrotreatment, yet consumes a great deal less in hydrogen. The end result is high octane gasoline plus improved distillate with lower hydrogen consumption than hydrotreatment alone.

Examples 9-10 illustrate the advantages of a combination low pressure hydrocracking/FCC process utilizing the LCO from the FCCU as the hydrocracker feed. The process was carried out using a highly aromatic and hydrogen deficient LCO obtained from a commercial fluid catalytic cracking unit during maximum gasoline mode operation. Table 7 gives the properties of the LCO, as well as those of a sour heavy gas oil (SHGO) used in these experiments as the FCC feed.

The LCO contained 80% aromatics and had a hydrogen content of 9.1%. As a result of its very low cetane quality (cetane index of 21.6 and a diesel index of 3.0), it would require blending with about 60% virgin kerosene followed by CHD treating in order to make a marketable quality No. 2 fuel oil. The LCO was hydrotreated at 600 psig hydrogen pressure over a conventional NiMo/Al.sub.2 O.sub.3 catalyst, resulting in 1100 SCF/bbl hydrogen consumption. Table 5 gives process conditions for preparing the hydrotreated LCO, as well as the product properties. For comparison, the LCO was also subjected to low pressure hydrocracking over the 3% Pd/DeAlY catalyst as of Example 2 using the conditions shown in Table 8, with the results indicated.

During the hydrotreating process (Example 9), little hydrogen gas was consumed in heteroatom removal and aromatics saturation and a negligible amount of conversion to 385.degree. F.- occurred. The diesel index of the hydrotreated LCO is 6.9 versus the 3.0 of the untreated LCO. This is consistent with prior observations that ignition quality, as measured by diesel index or cetane index, is relatively insensitive to hydrogen consumption (see, for example, Oil and Gas Journal, May 31, 1982, pp. 87-94).

When the LCO was subjected to cascade low pressure hydrocracking at 600 psig hydrogen pressure over the two NiMo/Al.sub.2 O.sub.3, Pd/DeAlY (Example 10), the process resulted in formation of 17 vol %, 95 RON gasoline (C.sub.5 --385.degree. F.) at 870 SCF/bbl hydrogen consumption. The unconverted distillate was 87 vol % on charge and had a diesel index of 8.0. The hydrogen content of the unconverted distillate was 10.4%, significantly lower than the 11.2% hydrogen content material obtained from conventional hydrotreating. The acid catalyzed low pressure hydrocracking (LPHC) process renders the unconverted 385.degree. F.+ liquids lower in nitrogen than the conventionally hydrotreated LCO.

An FCC feed made up of Sour Heavy Gas Oil (SHGO) either alone or mixed with LCO (20 wt % mixture of either untreated, hydrotreated or low pressure hydrocracked 385.degree. F.+ LCO obtained as in Examples 9 and 10 and 80 wt % sour heavy gas oil) was charged to a fixed-fluidized bed laboratory scale FCC unit. The properties of the sour heavy gas oil are provided in Table 7 above. The catalytic cracking was carried out using a commercial FCC equilibrium catalyst at 960.degree. F., and 1.0 minute oil on-stream. The FCC catalyst properties and FCC results are provided in Tables 9 and 10, respectively.

The results shown for LCO are the incremental yields backed out by comparing the cracking data of the blends with that from the sour point gas oil alone as follows:

The calculation assumes linear addition of yields for sour heavy gas oil and incremental component: ##EQU1##

Low crackability and an increase in coke make are expected when a highly aromatic LCO is recycled to the FCC. Table 10 shows the cracking data for an equal 60% conversion basis of each blend, as well as for the heavy sour gas oil base material. On an incremental yield basis, 23.3 vol % of C.sub.5 + gasoline was formed from the untreated LCO, while 38.8 vol % was formed from the hydrotreated LCO, and 31.8 vol % resulted from the low pressure hydrocracked material. Adjusting the FCC yields by the volume fraction to be sent to the FCC and adding the gasoline formed during the LPHC operation, a total of 45 vol % gasoline is formed from the LPHC/FCC combination.

Assuming linear blending of the octanes, Table 10 shows the octane is significantly higher from the LPHC/FCC route than from the HDT/FCC route. The LPHC/FCC route produces more gasoline with a higher octane at lower hydrogen consumption than the HDT/FCC combination.

The results show that hydrocracking the LCO at low pressure significantly improves the LCO crackability in the FCCU. This combination of hydrocracking and catalytic cracking provides more gasoline at higher octane with less hydrogen consumption than by hydrotreating-cracking. In addition, the unconverted LCO is an improved mid-distillate suitable for No. 2 fuel oil or the diesel pool, it is not recycled to the FCCU for additional high octane gasoline. Conversion of the LCO to gasoline both directly by the hydrocracking and by FCC recycle of the unconverted LCO, results in kerosene being backed out of the No. 2 fuel oil pool permitting expansion in the jet fuel or No. 1 fuel oil market. Further savings can be realized by the smaller amounts of cetane improver necessary with the higher Diesel Index in the product.

These Examples illustrate the benefits of fractionating the LCO feedstock prior to low pressure hydrocracking. Table 11 provides properties of various cuts of LCO processed:

The various cuts of LCO shown in Table 11 were charged to a two reactor catalyst system operating in the cascade mode. The first catalyst consisted of a conventional NiMo/Al.sub.2 O.sub.3 hydrotreating catalyst. The second catalyst was 1 to 3% palladium impregnated on dealuminized zeolite Y. Results from these operations are shown in Table 12:

As can be seen from Table 12, the 550.degree. F.- and 640.degree. F.- fractions converted substantially more than the full range material, which in turn converted more than the 550.degree. F.+ LCO. In addition, octane numbers of the gasoline from the 550.degree. F.- and 640.degree. F. fractions were higher.

The second stage LHSV was higher for the lower boiling fractions, yet conversions were also higher. Thus, it has been found unexpectedly that low pressure hydrocracking of fractionated LCO produced more gasoline at higher octane using a higher LHSV than the full range LCO.

Commercially, the fractionation of the LCO into a higher boiling fraction, with a 5% point ranging from 550.degree.-700.degree. F., hydrotreatment of the higher boiling fraction, and low pressure hydrocracking of the lower boiling fraction would be typical. Hydrotreating of the higher boiling fraction would proceed by charging the higher boiling LCO fraction alone, or as a mixture of the LCO with a virgin kerosene stream, to a catalytic desulfurization (CHD) unit. Table 13 shows the results of such an operation, compared to LPHC of a full range LCO:

Table 13 shows that split stream LPHC produces more gasoline at higher octane and higher space velocity than full range LPHC. In addition, the unconverted 385.degree. F.+ distillate is of better quality, as measured by diesel index.

It can thus be seen that in contrast to earlier approaches, which attempted to saturate and eliminate aromatics prior to conversion, the present invention has been able to selectively extract the most aromatic constituents of the feedstocks, advantageously using a minimum of hydrogen under pressures that are significantly lower than those used in conventional hydrocracking technology. Naphthas of an unexpectedly high octane, i.e., greater than 87 (RON+0), and in a preferred embodiment greater than 90 (RON+0), that are directly blendable into gasoline pools can be produced. The remaining unconverted products in the higher boiling liquids are less aromatic and make better candidate feedstocks for automotive diesel fuel, because the process selectively removes a significant portion of the addition, savings could be realized by backing out cetane improver. This process combination results in the upgrading of a refractory stream, such as light cycle oil, in a hydrogen efficient fashion without the use of expensive high pressure processing.

This Example illustrates the suitability of certain LCLCO streams for processing in a single stage hydrocracking operation without prior hydrotreating. The feedstock in this Example is similar to that in the Example 19, as shown in Table 14 below.

In both cases, the second stage hydrocracking reactor contained a dealuminized zeolite Y catalyst impregnated with 3.8% Ni and 6.5% Mo. When used, the first reactor contained an equal volume of a conventional NiMo/Al.sub.2 O.sub.3 hydrotreating catalyst. When the first reactor operation was discontinued, it was necessary to reduce feed rate to maintain the same overall LHSV to obtain comparable levels of conversion. Results from these operations are shown in Table 15.

Although the single stage operation shows a slightly lower gasoline yield, this can be compensated for by operating at a slightly higher temperature. Note also that for this particular feedstock there is an octane benefit of almost two numbers for the single stage operation.