Two-Catalyst Hydrocracking Process

The present invention relates to a hydrocarbon conversion process. More particularly, this invention relates to the catalyst hydrocracking of hydrocarbons.

The hydrocracking of hydrocarbons is old and well-known in the prior art. These hydrocracking processes can be used to hydrocrack various hydrocarbon fractions such as reduced crudes, gas oils, heavy gas oils, topped crudes, shale oil, coal extract and tar extract wherein these fractions may or may not contain nitrogen compounds. Modern hydrocracking processes were developed primarily to process feeds having a high content of polycyclic aromatic compounds, which are relatively unreactive in catalytic cracking. The hydrocracking process is used to produce desirable products such as turbine fuel, diesel fuel, and light distillate products such as naphtha and gasoline.

The hydrocracking process is generally carried out in any suitable reaction vessel under elevated temperatures and pressures in the presence of hydrogen and a hydrocracking catalyst so as to yield a product containing the desired distribution of hydrocarbon products.

Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise a hydrogenation component selected from the group consisting of Group VIB metals and Group VIII metals of the Periodic Table of Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts contain an acidic support comprising a crystalline aluminosilicate material such as X-type and Y-type aluminosilicate materials. This crystalline aluminosilicate material is generally suspended in a refractory inorganic oxide such as silica, alumina, or silica-alumina.

Regarding the hydrogenation component, the preferred Group VIB metals are tungsten and molybdenum; the preferred Group VIII metals are nickel and cobalt. The prior art also taught that combinations of metals for the hydrogenation component, expressed as oxides and in the order of preference, are: NiO--WO.sub.3, NiO--MoO.sub.3, CoO--MoO.sub.3, and CoO--WO.sub.3. Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, indium, platinum, chromium, vanadium, niobium, and tantalum.

Reference that disclose hydrocracking catalysts utilizing nickel and tungsten as hydrogenation components teach enhanced hydrocracking activity when the matrix or catalyst support contains silica-alumina. For instance, U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 all to Ward et al., each show at Table V thereof that the lowest hydrocracking activity is achieved when alumina is used in the support instead of a dispersion of silica-alumina in alumina. The lowest hydrocracking activity is indicated by the highest reactor temperature required to achieve 60 vol. % conversion of the hydrocarbon components boiling above a predetermined end point to below that end point.

Similarly, U.S. Pat. No. 3,536,605 to Kittrell et al. teaches the use of silica-alumina in the catalyst support when a nickel- and tungsten-containing hydrogenation component is employed.

U.S. Pat. No. 3,598,719 to White teaches a hydrocracking catalyst that can contain 0 wt. % silica, however, the Patent does not present an example showing the preparation of a catalyst devoid of silica nor does the patent teach the preferential use of nickel and tungsten as hydrogenation metals.

As can be appreciated from the above, there is a myriad of catalysts or catalyst systems known for hydrocracking whose properties vary widely. A catalyst suitable for maximizing naphtha yield may not be suitable for maximizing the yield of turbine fuel or distillate. Further, various reactions; i.e., denitrogenation, hydrogenation, and hydrocracking must be reconciled in a hydrocracking process in an optimum manner to achieve the desired results.

For instance, when a feedstock having a high nitrogen content is exposed to a hydrocracking catalyst containing a high amount of cracking component the nitrogen serves to poison or deactivate the cracking component. Thus, hydrodenitrogenation catalysts do not possess a high cracking activity since they are generally devoid of a cracking component that is capable of being poisoned. Another difficulty is presented when the hydrocracking process is used to maximize naphtha yields from a feedstock containing light catalytic cycle oil which has a very high aromatics content. The saturation properties of the catalyst must be carefully gauged to saturate only one aromatic ring of a polynuclear aromatic compound such as naphthalene in order to preserve desirable high octane value aromatic-containing hydrocarbons for the naphtha fraction. If the saturation activity is too high, all of the aromatic rings will be saturated and subsequently cracked to lower octane value paraffins.

On the other hand, distillate fuels such as diesel fuel or aviation fuel have specifications that stipulate a low aromatics content. This is due to the undesirable smoke production caused by the combustion of aromatics in diesel engines and jet engines.

Prior art processes designed to convert high nitrogen content feedstocks are usually two stage processes wherein the first stage is designed to convert organic nitrogen compounds to ammonia prior to contacting with a hydrocracking catalyst which contained a high amount of cracking component; i.e., a molecular sieve material.

For instance, U.S. Pat. No. 3,923,638 to Bertolacini et al. discloses a two-catalyst process suitable for converting a hydrocarbon containing substantial amounts of nitrogen to saturated products adequate for use as jet fuel. Specifically, the subject patent discloses a process wherein the hydrodenitrogenation catalyst comprises as a hydrogenation component a Group VIB metal and Group VIII metal and/or their compounds and a cocatalytic acidic support comprising a large-pore crystalline aluminosilicate material and refractory inorganic oxide. The hydrocracking catalyst comprises as a hydrogenation component a Group VIB metal and a Group VIII metal and/or their compounds, and an acidic support of large-pore crystalline aluminosilicate material. For both hydrodenitrogenation catalyst and the hydrocracking catalyst, the preferred hydrogenation component comprises nickel and tungsten and/or their compounds and the preferred large-pores crystalline aluminosilicate material is ultrastable, large-pore crystalline aluminosilicate material.

Another two-catalyst hydrocracking process is disclosed in U.S. Pat. No. 4,211,634 to Bertolacini et at. The subject patent discloses a process which employs a first catalyst comprising a specific hydrogenation component comprising nickel and molybdenum or tungsten and as the second catalyst a catalyst comprising a specific hydrogenation component comprising cobalt and molybdenum, each of the catalysts also comprising a cocatalytic acidic cracking component comprising an ultrastable, large-pore crystalline aluminosilicate material dispersed in and suspended throughout a silica-alumina matrix.

The first catalyst in the above disclosed two-catalyst hydrocracking process has been significantly improved as disclosed in application 124,280, filed Nov. 23, 1987 now U.S. Pat. No. 4,820,403, when used to convert light catalytic cycle oils containing a substantial amount of aromatics. Specifically, the subject application, the teachings of which are incorporated by reference discloses a catalyst comprising a combination of a nickel component and a tungsten component coupled with a support component containing an alumina component to the exclusion of any other inorganic refractory oxide. This catalyst system provides increased selectivity towards high octane naphtha with decreased undesirable selectivity towards C.sub.1 to C.sub.5 light gas.

The second stage catalyst disclosed in U.S. Pat. No. 4,211,634 has also been investigated and compared with other catalysts as shown in U.S. Pat. No. 3,649,523 to Bertolacini et al. Specifically, the example accompanying the '523 patent shows that a hydrocracking catalyst containing cobalt and molybdenum supported on a silica-alumina matrix has a much higher hydrocracking activity than a catalyst wherein the support component comprises alumina. The alumina containing catalyst, however, afforded a much higher naphtha yield.

It has now been discovered that when the matrix of the second catalyst in series in a two-catalyst hydrocracking process contains alumina as the sole refractory inorganic oxide component, molybdenum as the sole hydrogenation component, and is employed with a first catalyst wherein the matrix similarly contains alumina as the sole refractory inorganic oxide component, the heavy naphtha yield can be considerably improved with a negligible or no loss in activity.

Hydrocarbon conversion catalysts wherein molybdenum or a Group VIB metal is the sole hydrogenation component are known and disclosed, for instance, in U.S. Pat. No. 3,617,507 (Oettinger et al.) and No. 3,853,747 (Young). The use of such catalysts, however, as the second catalyst in a two-catalyst hydrocracking process is not revealed in the prior art.

An attendant advantage of using the cobalt-free second catalyst is, of course, the less expensive and complicated catalyst preparation procedure, by virtue of the lack of cobalt.

This invention relates to a two-catalyst process for hydrocracking a hydrocarbon feedstock with hydrogen at hydrocracking conversion conditions in a plurality of reaction zones in series.

Specifically, the feedstock is contacted in a first reaction zone with a first reaction zone catalyst comprising a nickel component and a tungsten component deposed on a support component consisting essentially of an alumina component and a crystalline molecular sieve component. The effluent from the first reaction zone effluent is then contacted in a second reaction zone with a second reaction zone catalyst wherein the hydrogenation component consists essentially of a molybdenum component deposed on a support component consisting essentially of an alumina component and a crystalline molecular sieve component.

The hydrocarbon charge stock subject to hydrocracking in accordance with the process of this invention is suitably selected from the group consisting of petroleum distillates, solvent deasphalted petroleum residua, shale oils and coal tar distillates. These feedstocks typically have a boiling range above about 200.degree. F. and generally have a boiling range between 350.degree. to 950.degree. F. More specifically these feedstocks include heavy distillates, heavy straight-run gas oils and heavy cracked cycle oils, as well as fluidized catalytic cracking unit feeds. The process of the invention is especially suitable in connection with handling feeds that include a light catalytic cycle oil. This light catalytic cycle oil generally has a boiling range of about 350.degree. to about 750.degree. F., a sulfur content of about 0.3 to about 2.5 wt %, a nitrogen content of about 0.01 to about 0.15 wt % and an aromatics content of about 40 to about 90 vol. %. The light catalytic cycle oil is a product of the fluidized catalytic cracking process.

Operating conditions to be used in each hydrocracking reaction zone in the process of the invention include an average catalyst bed temperature within the range of about 500.degree. C. to 1000.degree. F., preferably 600.degree. C. to 900.degree. F. and most preferably about 650.degree. to about 850.degree. F., a liquid hourly space velocity within the range of about 0.1 to about 10 volumes hydrocarbon per hour per volume catalyst, a total pressure within the range of about 500 psig to about 5,000 psig, and a hydrogen circulation rate of about 500 standard cubic feet to about 20,000 standard cubic feet per barrel.

The process of the present invention is naphtha selective with decreased production of light gases.

In the process of the invention, each reaction zone can comprise one or a plurality of beds that have intrabed quench to control temperature rise due to the exothermic nature of the hydrocracking reactions. The charge stock may be a liquid, vapor, or liquid-vapor phase mixture, depending on the temperature, pressure, proportion of hydrogen, and particular boiling range of the charge stock processed, the source of the hydrogen being admixed can comprise a hydrogen-rich gas stream obtained from a catalytic reforming unit.

The catalysts used in the process of the present invention comprise a hydrogenation component and a catalyst support.

The hydrogenation component of the first reaction zone catalyst comprises nickel and tungsten and/or their compounds. The nickel and tungsten are present in the amounts specified below. These amounts are based on the total catalytic composite or catalyst weight and are calculated as the oxides, NiO and WO.sub.3. These components are typically present in oxide or sulfide form. In another embodiment of the present invention, the first reaction zone catalyst can additionally comprise a phosphorus component. The amount of phosphorus component is calculated as P.sub.2 O.sub.5 with the ranges thereof also set out below.

Another component of the first reaction zone catalytic composite or catalyst is the support. The support contains a crystalline molecular sieve material and alumina. The preferred alumina is gamma alumina. The crystalline molecular sieve material is present in an amount ranging from about 25 to about 60 wt. %, preferably from about 35 to about 50 wt % based on total support weight.

Preferably, the crystalline molecular sieve material is distributed throughout and suspended in a porous matrix of the alumina. The use of alumina in the first zone catalyst support is in contradistinction to U.S. Pat. Nos. 4,576,711, 4,563,434, and 4,517,073 to Ward et al. and U.S. Pat. No. 3,536,605 to Kittrell et al. which require the presence of silica-alumina matrix material. The use of alumina is preferred in the first zone catalyst because it serves to increase hydrogenation activity as opposed to hydrocracking activity. It is preferable to carry out hydrogenation reactions prior to the hydrocracking reactions because the hydrocracking reaction will take place at a faster rate with hydrogenated reactants.

The hydrogenation component of the second reaction zone catalyst consists essentially of molybdenum and/or its compounds in amounts as set out below. These amounts are based on the total catalytic composite or catalyst weight and are calculated as the oxide MoO.sub.3. In another embodiment of the present invention, the second reaction zone catalyst can additionally comprise a phosphorus component. The amount of phosphorus component is calculated as P.sub.2 O.sub.5 with ranges thereof also set out below.

The second reaction zone support comprise a crystalline molecular sieve component and an alumina component. THe crystalline molecular sieve material is present in an amount ranging from about 10 to 60 wt % and preferably from about 25 to 50 wt %. The preferred alumina is gamma alumina.

In all cases the hydrogenation component may be deposited upon the support by impregnation employing heat-decomposable salts of the above-described elements or any other method known to those skilled in the art. Each of the elements may be impregnated onto the support separately, or they may be co-impregnated onto the support. The composites are subsequently dried and calcined to decompose the salts.

The supports may be prepared by various well-known methods and formed into pellets, beads, and extrudates of the desired size. For example, the crystalline molecular sieve material may be pulverized into finely divided material, and this latter material may be intimately admixed with the refractory inorganic oxide. The finely divided crystalline molecular sieve material may be admixed thoroughly with a hydrosol or hydrogel of the inorganic oxide. Where a thoroughly blended hydrogel is obtained, this hydrogel may be dried and broken into pieces of desired shapes and sizes. The hydrogel may also be formed into small spherical particles by conventional spray drying techniques or equivalent means.

The molecular sieve materials of the invention preferably are selected from the group consisting of faujasite-type crystaline aluminosilicates, and mordenite-type crystalline aluminosilicates. Although not preferred, crystalline aluminosilicates such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, and ZSM-35, and crystalline borosilicate molecular sieves such as AMS-1B can also be used with varying results alone or in combination with the faujasite-type or mordenite-type crystalline aluminosilicate. Also suitable for use are gallosilicates in conjunction with another molecular sieve component. Specifically, application Serial No. 287,399, filed Dec. 20, 1988, discloses a hydrocracking catalyst containing a molecular sieve material present in an amount ranging from about 25 to about 60 wt. % based on the weight of the support component wherein at least about 1 to about 80 wt. % of the sieve material is gallosilicate.

Examples of a faujasite-type crystalline aluminosilicate are low-alkali metal X-type and Y-type aluminosilicate, metal-exchanged X-type crystalline crystalline aluminosilicates, and ultrastable, large-pore crystaline aluminosilicate material. Zeolon is an example of a mordenite-type crystalline aluminosilicate.

An ultrastable, large-pore crystalline aluminosilicate material is represented by Z-14US zeolites which are described in U.S. Pat. Nos. 3,293,192 and 3,449,070. Each of these patents is incorporated by reference herein and made a part hereof. By large-pore material is meant a material that has pores which are sufficiently large to permit the passage thereinto of benzene molecules and larger molecules and the passage therefrom of reaction products. For use in petroleum hydrocarbon conversion processes, it is often preferred to employ a large-pore molecular sieve material having a pore size of at least 5 .ANG. (0.5 nm) to 10 .ANG. (1 nm).

The ultrastable, large-pore crystalline aluminosilicate material is stable to exposure to elevated temperatures. This stability at elevated temperatures is discussed in the aforementioned U.S. Pat. Nos. 3,293,192 and 3,449,070. It may be demonstrated by a surface area measurement after calcination at 1,725.degree. F. In addition, the ultrastable, large-pore crystalline aluminosilicate material exhibits extremely good stability toward wetting, which is defined as the ability of a particular aluminosilicate material to retain surface area or nitrogen-adsorption capacity after contact with water or water vapor. A sodium-form of the ultrastable, large-pore crystalline aluminosilicate material material (about 2.15 wt. % sodium) was shown to have a loss in nitrogen-absorption capacity that is less than 2% per wetting, when tested for stability to wetting by subjecting the material to a number of consecutive cycles, each cycle consisting of a wetting and a drying.

The ultrastable, large-pore crystalline aluminosilicate material that is preferred for the catalytic composition of this invention exhibits a cubic unit cell dimension and hydroxyl infrared bands that distinguish it from other aluminosilicate materials. The cubic unit cell dimension of the preferred ultrastable, large-pore crystalline aluminosilicate is within the range of about 24.20 Angstrom units (.ANG.) to about 24.55 .ANG.. The hydroxyl infrared bands obtained with the preferred ultrastable, large-pore crystalline aluminosilicate material are a band near 3,745 cm.sup.-1 (3,745.+-.5 cm.sup.01), a band near 3,695 cm.sup.-1 (3,690.+-.10 cm.sup.-1), and a band near 3,625 cm.sup.-1 (3,610.+-.15 cm.sup.-1). The band near 3,745 cm.sup.-1 may be found on many of the hydrogen-form and decationized aluminosilicate materials, but the band near 3,695 cm.sup.-1 and the band near 3,625 cm.sup.-1 are characteristic of the preferred ultrastable, large-pore crystalline aluminosilicate material that is used in the catalyst of the present invention.

The ultrastable, large-pore crystalline aluminosilicate material is characterized also by an alkali metal content of less than 1%.

Another example of a crystalline molecular sieve zeolite that can be employed in the catalytic composition of the present invention is a metal-exchanged Y-type molecular sieve. Y-type zeolitic molecular sieves are discussed in U.S. Pat No. 3,130,007. The metal-exchanged Y-type molecular sieve can be prepared by replacing the original cation associated with the molecular sieve by a variety of other cations according to techniques that are known in the art. Ion exchange techniques have been disclosed in many patents, several of which are U.S. Pat Nos. 3,140,249, 3,140,251, and 3,140,253. Specifically, a mixture of rare earth metals can be exchanged into a Y-type zeolitic molecular sieve and such a rare earth metal-exchanged Y-type molecular sieve can be employed suitably in the catalytic composition of the present invention. Specific examples of suitable rare earth metals are cerium, lanthanum, and praseodymium.

As mentioned above, another molecular sieve that can be used in the catalytic compositions of the present invention is AMS-1B crystalline borosilicate, which is described in U.S. Pat. No. 4,269,813, which patent is incorporated by reference herein and made a part hereof.

A suitable AMS-1B crystalline borosilicate is a molecular sieve material having the following composition in terms of mole ratios of oxides:

wherein M is at least one cation having a valence of n, Y is within the range of 4 to about 600, and Z is within the range of 0 to about 160, and providing an X-ray diffraction pattern comprising the following X-ray diffraction lines and assigned strengths:

Mordenite-type crystaline aluminosilicates can be employed in the catalyst of the present invention. Mordenite-type crystalline aluminosilicate zeolites have been discussed in patent art, e.g., by Kimberlin in U.S. Pat. No. 3,247,098, by Benesi, et al., in U.S. Pat. No. 3,281,483, and by Adams, et al., in U.S. Pat. No. 3,299,153. Those portions of each of these patents which portions are directed to mordenite-type aluminosilicates are incorporated by reference and made a part hereof.

The catalysts used in the present invention can be used in any form such as pellets, spheres, extrudates, or other shapes having particular cross sections such as a clover leaf, or "C" shape.

In accordance with the process of the invention, the preferred amounts of catalyst in each respective zone are set out below as a percentage range of the overall amount of catalyst used in the process.

In a preferred embodiment of the present invention the catalyst situated at the downstream portion of the plurality of reaction zones possesses a small nominal size while the remaining upstream portion of the total amount of catalyst possesses a large nominal size greater than the small nominal size catalyst. Specifically, the small nominal size is defined as catalyst particles having a U.S. Sieve mesh size ranging from about 10 to 16 preferably 10 to 12. The large nominal size catalyst preferably ranges from about 5 to about 7 U.S. Sieve mesh size. Further details of this preferred embodiment are disclosed in U.S. SErial No. 160,524, filed on February 26, 1988, the teachings of which are incorporated by reference.

Generally, the small nominal size hydrocracking catalyst is present in an amount ranging from about 5 to 70 wt. % of the total overall amount of catalyst used in this invention. Preferably, this amount ranges from about 10 to about 60 wt. %. Most preferably the second reaction zone consists of three catalyst beds wherein the catalyst in all three beds possesses a U.S. Sieve mesh size of about 10 to 12. In this connection preferably the first reaction zone consists of two catalyst beds wherein the the catalyst in both beds has a nominal particle size of about 5 to about 7 mesh (U.S. Sieve).

The amount of small nominal size hydrocracking catalyst used in the process of the invention can be limited in accordance with the desired overall pressure gradient. This amount can be readily calculated by those skilled in the art as explained in U.S. Pat. Nos. 3,796,655 (Armistead et al.) and 3,563,886 (Carlson et al.)

The present invention is described in further detail in connection with the following Example, it being understood that this example is for purposes of illustration and not limitation.

The two-catalyst hydrocracking process of the invention was compared with an alternative two-catalyst process wherein the alternative second zone catalyst contained a silica-alumina matrix and an additional hydrogenation component i.e., cobalt. In the following Tables, "USY" designates an ultrastable Y zeolite.

The process of the invention was tested in a reactor having reactor beds loaded as set out below:

Another embodiment of the invention was tested in a reactor having beds loaded as set out below:

The comparative process was carried out in a reactor loaded as set out below:

All of the catalyst was mixed with inert alundum to improve flow distribution and maintain better temperature control in a catalyst to alundum weight ratio of about 1:2.

The comparative process and the process in accordance with the invention were used to convert a light catalytic cycle oil feedstock to naphtha and distillate products.

Both the comparative process and the process of the invention test runs were carried out on a "once-through" basis at 1250 psig, and a hydrogen flow rate of 12,000 SCFB. All catalysts used were 1/8 inch nominal size cylindrical extrudates. The weight hourly space velocity was 1.45.

Table 1 below sets out the properties of the feedstock used in each test run.

The following Table 2 sets out the compositions of the respective catalysts.

The following Table 3 sets out the selectivities for both the comparative process and the processes of the invention corrected to the common conditions of 725.degree. F. and 77 wt. % conversion of the material boiling above 380.degree. F. to material boiling below 380.degree. F. These "corrected selectivities" were calculated from "corrected yields." The method and equations used to calculate the "corrected" yields are set out at U.S. Pat. No. 3,923,638 (Bertolacini et al.) the teachings of which are incorporated by reference.

The above Table clearly shows the advantages provided by the process of the invention. The heavy naptha yield is increased by about 1 to 2 wt. %. The increase in heavy naphtha yield occurred at the expense of less valuable products, pentanes and light naphtha. A further advantage afforded by the process of the invention is the increased iso to normal ratio for the pentane fraction.

Table 4 below shows the actual yields in wt. % and catalyst activity as the temperature required to effect 77 wt. % conversion of the feedstock material boiling above 380.degree. F. to material boiling below 380.degree. F.