EP0022883A1

EP0022883A1 - Catalytic cracking and hydrotreating process for producing gasoline from hydrocarbon feedstocks containing sulfur

Info

Publication number: EP0022883A1
Application number: EP19790301428
Authority: EP
Inventors: William Edward Winter; William Lee Schuette
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1979-07-18
Filing date: 1979-07-18
Publication date: 1981-01-28
Also published as: DE2966422D1; EP0022883B1

Abstract

A gas oil or other sulfur containing hydrocarbon feedstock is catalytically cracked in a first stage to produce a cat cracked naphtha product of high olefin content (10-60%), and an intermediate or high boiling component thereof or both is recracked as a feed in a second stage over a zeolite catalyst to saturate the olefins, and hydrodenitrogenate and hydrodesulfurize said cat cracked naphtha. The recracked cat cracked naphtha is then hydrotreated or hydrofined at low to mild severities to provide either a low sulfur gasoline blending component for a gasoline blending pool or a feed suitable for a subsequent catalytic reforming (hydroforming) step in which high octane gasoline is produced.

Hydrodesulfurized cat cracked naphtha produced in this process.

Description

The present invention relates to a catalytic cracking and hydrotreating process for gasoline production from sulfur - containing hydropcarbon feedstocks.
Cracking processes, both thermal and catalytic, have constituted the heart of petroleum refining operations for several decades. The purpose of both types of process is the same, i.e., co break heavy molecular feed componencs into lower boiling, more valuable components. The thermal process, which has now been largely replaced by the more effective catalycic process, accomplishes this result by heat, whereas the catalytic process breaks the large molecules by contact between a heavy feed and an active catalyst at lower temperatures than used in thermal processes. The reaccions which cccur in che cacalycic cracking operation are complex including, noc only carbon-carbon bond scission but isomerization, alkylation, dehydrogenation, etc., and a carbonaceous material, or coke, is inevitably deposited on che Catalyst. The catalyst, in such unit, is regenerated in a separate vessel, i.e., a regenerator, by burning off the coke to restore its accivicy. Commonly, the catalyst is continuously cycled becween che reactor and regenerator as a moving bed without shutdown of either unit.
The economics of che cacalycic cracking unit is a refinery because of ics high degree of flexibility, to a large extent, decermines che produce slate which will be produced by a refinery. Products from the catalytic cracking unit thus provide feed for other units, e.g., alkylacion and polymerization units. Cat cycle stocks are used to make tubes, and gas is employed as fuel in che refinery. However, a major portion of che product of che catalytic cracking units of a given refinery are blended direccly in gasoline blending pools which serve as supplies of motor gasoline. With the phaseouc of lead anti-knock compounds it continues a formidable challenge for the refiner to maintain gasoline pools at the occane levels demanded; and, the problem is aggravated by the depletion of conventional petroleum supplies which creates an increased need to process heavy feedstocks such as residua, unconventional heavy crudes and the like for conversion to gasoline.
Cat cracking feed stocks are provided by atmospheric and vacuum stills, phenol extraction plants and hydrotreaters. The usual feed co a commercial catalytic cracking unit is comprised of a gas oil boiling below about 1050°F (1050°F-), typically a virgin gas oil boiling between about 600°F and 1050°F. In addition, thermally cracked materials are often used as cat cracking feeds. While various conventional types of processing; e.g., cat naphtha reforming and cat naphtha eztraction, might be employed to upgrade cac naphtha octanes and increase the supply of high octane gasoline in the gasoline pool as lead is phased out of gasoline, most are quite expensive; particularly cat naphtha reforming which requires initial hydrotreating of the feed so that it can meet reformer feed specifications.
The bulk of the sulfur in a gasoline blending pool is contributed by cat naphtha, or product of the cat cracking units. The addition of large amounts of sulfur to a gasoline blending pool raises acute problems, particularly in view of the presenc requirements in many countries to meet emission standards for hydrocarbons (HC) and carbon monoxide (CO). Thus automobiles are now equipped wich catalytic converters for the purpose of lowering emissions of CO and HC, but the new standards will also impose restrictions on NO_x emissions, as well as added restrictions on,CO and _HC emissions. Sulfur, however, is a known poison for the more useful, and active "three-way" catalysts ccntemplaced by the auto industry for use in cacalycic converters to meet the 1980's standards. Consequently, the activity and activity- maintenance of the catalysts are suppressed due co the presence of sulfur. Moreover, it has been found that, due to the presence of che sulfur, the catalytic converters emit sulfate, either as a sulfuric acid aerosol or as particulates caused by sulfuric acid corrosion of the metal portions of the exhaust train. The sulfur in gasoline, which is typically present in amounts of about 300 ppm, is oxidized in the combustion chamber of the engine to sulfur dioxide. The cacalycic converter, which is required for lowering che emissions of CO and HC, is thus responsible for the oxidation of sulfur dioxide in the exhaust gas co produce sulfur trioxide which immediately hydrates due co the presence of wacer vapor, one of the combustion products, to form a sulfuric acid aerosol or acid particulates, neither of which is environmentally acceptable.
Whereas cat napththa hydrofining might be employed to produce low sulfur gasoline or a naphtha which can meet reformer feed specifications such treatment would be very expensive for such processes would require considerable hydrogen consumption; and hydrogen is a racher expensive commodity. Hydrogen constitutes a major cost in hydrotreating a cat naphtha because cypically from about 20 co 40 percenc and perhaps 60 percent and higher of che feed is olefinic, and a considerable amount of hydrogen is required for saturation of che olefins. The olefins must be virtually complecely saturated before che cac naphchas can be reformed over a platinum or promoted platinum catalyst, chis requiring generally from about 200 to 400 SCF/B of hydrogen to saturate the olefins typically contained in an incermediace boiling range cat naphtha. Moreover, in addition to che restrictive olefins specifications imposed on a cac naphcha feed, such feed also contains considerable amounts of sulfur and nitrogen, and far more severe hydrotreating of che cat naphcha co bring it in line with sulfur and nitrogen reformer feed specificacions is required than even is necessary in hydrotreating virgin naphtha. In fact, in cac naphtha hydrofining mercaptan reversion reactions, or reactions wherein the hydrogen sulfide by-product reacts wich cat naphtha olefins to form mercaptans is a troublesome problem; for mercapcans cannot be tolerated in significant amounts wichin the feed. Mercaptans must thus be eliminated by hydrofining, or hydrotreating the cat naphtha at severe conditions.
While the degree of olefins saturation with resultanc octane loss can be diminished by proper selection of hydrotreating operating variables, and catalyse type, refiners cannot tolerate even small reductions in cat naphtha octane ratings, particularly now when lead is being phased out of gasoline blending pools; much less the loss in naphcha octane racings caused by further increased olefin saturation resultant from the high severities which now appear necessary in order to lower gasoline sulfur limits within the ranges required.
It is che primary objective of the presenc invention to provide an improved process which will at least in part overcome these and other disadvantages of present catalytic cracking processes, and in fact provide a new and novel multiple stage catalytic cracking process for che cracking of gas oils.
This objective and others are achieved in accordance with the presenc invention embodying a process, an essential feature of which comprises recracking a cracked naphcha feed containing up co about 60 percent, suitably from about 20 to about 40 percent olefins, over a crystalline aluminosilicate zeolite catalyst to further crack che naphtha and sacurate at least about 50 percenc of the olefins, preferably from about 80 percenc to about 100 percent of the olefins, based on che weight of said cracked naphtha feed. Suitably, the cracked naphtha feed is contacted and reacted over che catalysc, without dilution of said feed, at temperature ranging from about 800°F co about 1100°F, preferably from about 900°F co abouc 1030°F, and ac pressure ranging from about 0 co about 50 pounds per square inch gauge (psig), preferably from abouc 5 psig co abouc 20 psig. Reaction at such conditions not only produces significant saturation of che olefins, but also significant hydrodenitrogenation and hydrodesulfurization of said cac naphtha feed.
In its preferred aspects the process is one wherein a conventional sulfur-bearing cat cracker feed, suitably a gas oil, is catalytically cracked, at conventional conditions, in an initial or first stage co provide a cat naphtha product containing generally from about 10 to about 60 percenc, preferably from about 20 co about 40 percent olefins. The cac naphtha produce in whole or in pare is chen recracked, as an undiluted feed, in a subsequent or second catalytic cracking zone over a crystalline aluminosilicate zeolite catalyst. Preferably, the cat naphtha produce of the initial or first stage is splic into fractions inclusive of a low occane, highly olefinic intermediate fraction having a low end boiling point ranging from about 120°F, to 250°F preferably from about 180°F co abouc 220°F, and a high end boiling poin ranging from about 250°F co about 380°F, preferably from about 270°F to about 350°F. A higher boiling fraction having a low end boiling poinc range from ₂bouc 25u°F to about 380°F, preferably from about 270°F to about 350°F, and a higher end boiling point range from abcuc 350°F to about 450°F, preferably from about 400°F to about 430°F can also be obtained. The intermediate or high boiling fraction, or a composition which includes both, may be ucilized as feed and further catalytically cracked, or recracked, in a subsequent stage over a crystalline aluminosilicate zeolite catalyst sufficient to produce significant saturation of the olefins, and hydrodenitrogenstion and hydrodesulfurization of said cat cracked naphtha fraction, or fractions. 'The recracked product is then hydrotreaced, or hydrofined, at mild hydrotracting conditions to provide a low sulfur gasoline of improved octane.
Alternatively, che higher boiling fraction or fraction typically having a low end boiling point ranging from abouc 250°F to about 380°F and a high end boiling point ranging from about 350°F co about 450°F is not recracked because ic is generally of relatively high occane and upgrading of this fraction is not required. The intermediate fraction per se, preferably, is utilized as a feed and further catalytically cracked, or recracked, in a subsequent stage over a crystalline aluminosilicate zeolice catalyst sufficienc to produce significant saturation of the olefins, and hydrodenicrogenacion and hydrodesulfurization of said of said cat cracked naphtha fraction. The recracked produce thereof, is then hydrotreated, or hydrofined, ac mild hydrotreating conditions, and chen reformed over a conventional catalyst at conventional reforming (hydroforming) conditions to provide a low olefin gasoline of improved octane.
It has been found, quite surprisingly, that the recracking of an undiluted cracked naphcha, notably the intermediate of high boiling fractions, over a zeolite catalyst at rather low or mild conditions significantly increases the occane number while reducing the olefin content of the cracked naphtha by saturation cf the olefins, without direcc hydrogen addition. This reduction of olefin content while increasing octane number is indeed surprising. This recracking not only virtually eliminates any necessicy of hydrotreating the cracked naphcha to reduce its olefin content, but also significancly reduces the nicrogen and sulfur concent of the cracked naphtha. In particular, it has been found that recracking reduces the sulfur content of the feed by up to about 75 percent, or higher, based on the weight of the sulfur in the cat cracked naphtha. Thereafter, only a mild hydrotreatement of the cat cracked naphtah product is required co eliminate residual sulfur and thereby render the product susceptable co reforming, if desired, over highly sulfur-sensitive catalyst's to further improve che octane number. This, of course, significantly reduces the capital cost of the required hydrotreater (or hydrofiner) and direct high costs of hydrotreating a cracked naphtha to reforming feed specifications. Furthermore, recracking of the cracked naphtha in this manner prior co hydrotreatment of the cracked naphtha to eliminate olefins minimizes mercapcan reversion reactions wherein olefins normally react with by-product hydrogen sulfide to form mercaptans, any significant amount of which simply cannot be tolerated in a reformer feed.
Various cracking catalysts can be used in cracking the gas oil feed, or feed to the first stage catalytic cracker. Suitable cracking catalysts include conventional silica-based materials. Exemplary of such catalyses are, e.g., amorphous silica-alumina; silica-magnesia; silica- zirconia; conventional clay cracking ca.calysts, and the like. The amorphous gel silica-metal oxide cracking catalyst may furcher be composited with kaolin in amounts of about 10 to 40 wt. % (based on total weight of the composited catalyst) and up to -20 wt. % or more crystalline aluminosilicate zeolice, such as faujasite. A crystalline aluminosilicate zeolite catalyst is required in the second stage catalytic cracker, i.e., for cracking the cat cracked naphtha, or fraction chereof, from the first scage. These catalyses are well known and commercially available. Preferably, the catalyst utilized, particularly in che second stage catalytic cracker is an amorphous silica-alumina cacalyst containing from about 5 to 16 weight percent y-type faujasite, and, optionally 15 to 40 percent kaolin.
Generally, the first and second scage catalytic crackers are operated ac abouc che same absolute conditions of cemperature, pressure, space velocity, and catalyst/oil racio, che runs being initiated by adjusting the feed and cacalyst rates, and the temperature and pressure of the reactor to operating condicions. The cacalytic cracking operation in both stages of cracking is continued au conditions by adjustment of the major process variables, within the ranges described below:
The produce of the first stage catalytic cracker, suitably a cat cracked naphtha obtained by cracking a gas oil, is characterized as a.cracked naphtha having an olefin content ranging from about 10 percent co about 60 percent, more typically from about 20 percent to about 40 percent (by weight) and boiling within the gasoline range, typically from about 65°F co about 430°F (i.e., C₅/430°F) All or a portion of the cat cracked naptha, preferably an intermediate or heavy fraction, or composition which includes both fractions, as previously defined, is splic from the product of said first stage, fed into, and recracked, without dilution, over the crystalline aluminosilicate zeolite catalyse in the second scage catalytic cracker. The recracked product is then subjected to a mild hydrocreacment by contact, with a catalyst comprising a composite of an inorganic oxide base, suitably alumina, and a Group VI-B or Group VIII metal, or both, e.g., a cobalt moly/alumina catalyst, ac conditions given as follows, to provide a gasoline suitable for addition co a gasoline blending pool, co wit:
Alternatively, che recracked produce is subjected to the following mild hydrocreacing conditions so as to provide a naphcha suitable as a feed to a reforming process:
The product from the hydrofiner is subjecced.to reforming, ac reforming condicions, by concacc wich a sulfur-sensitive, noble metal reforming cacalysc co produce a satisfactory high-occane gasoline. Suicably, the reforming run is initiated by injection of hydrogen into che reforming reaccor (or zone) wich che feed ac the desired hydrogen and feed rates, wich adjustment of che temperature and pressure co operating condicions. The run is concinued ac optimum reforming condicions by adjusetment of che major process variables, within the ranges described below:
The catalyse employed in reforming is one comprising a refractory or inorganic oxide supporc material, particularly alumina, which is composiced with a Group VIII noble metal hydrogenation-dehydrogenation component, notably platinum, co which may be added an additional metal, or metals, to promote che activicy and seleccivicy of the catalysts, particularly iridium or rhenium, or both, or component selected from the Group IV metals, Group VI metals, Croup VII mecals, ana Group VIII mecals, e.g., germanium, tin, lead, osmium, ruthenium, rhodium or che like. A h_blogen component, suitably chlorine, is generally added co provide che desired acidicy. These components can be added to a supporc by any of the conventional methods, e,g., by impregnation prior co, following or simultaneously with the impregnation of the noble metal, or halogen components. The metal hydrogenation-dehydrogenation components, or promoters are added to a support in concentration ranging about 0.01 to 3 percent, preferably from about 0.05 to about 1 percenc, based on the weight of the catalysc. A suicable support can contain, e.g., one or more of alumina, bentonite, clay, diatomaceous earth, zeolite, silica, activated carbon, magnesia, zirconia, thoria, and the like; though che most preferred support is alumina to which, if desired, can be added a suitable amount of other refractory carrier materials such as silica, zirconia, magnesia, titania, ecc., usually in a range of about 1 to 20 percent, based on che weight of the support. A preferred support is one having a surface area of mere than 50 m²/g, preferably from about 100 to about 300 m²/g, a bulk density of about 0.3 to 1.0 g/ml, preferably about 0.4 to 0.8 g/ml, an average pore volume of about 0.2 to 1.1 ml/g, preferably about 0.3 to 0.8 ml/g, and an average pore diameter of about 30 to 300Å.
The invention will be more fully understood by reference to the following nonlimiting demonstrations and examples which, present comparative data which illustrate its more salient features. All parts are given in terms of weight unless otherwise specified.
In a first step, a 155°F/413°F cac naphtha fraction was obtained by catalytically cracking a virgin gas oil at conventional conditions over a conventional catalyst co obtain a cat cracked naphtha, hereinafter referred to as Feedstock A, the complete feedstock inspections of which are given in Table I, below.

EXAMPLE 1

Feedstock A was hydrofined in a reactor at 550°F, 400 psig, and 800 SCF/B over a cobalt moly-on-alumina catalyst to produce a low sulfur gasoline blending component having a Bromine number of abouc 5 and 20 wppm sulfur, this requiring abouc 96.4% hydrodesulfurization. The produce has a research octane number (RONC) at 77.3 and a mocor octane number (MONC) of 73.0, this represencing a loss of 5.5 RONC and a 2.2 loss of MONC as a resulc of the hydrofining.

EXAMPLE 2

Feedstock A was recracked in a reactor ac 930°F, 14.7 psia, 13.7 WHSV and ac a catalyst/oil racio of 9.2 over a conventional commercial zeolite cracking catalyst containing crystalline aluminosilicate zeolite, silica alumina gel and clays, and the product then fractionated to provide a light ends cut, and three fractions, i.e., a low boiling 65/200°F fraction, and intermediate 200/430°F fraction and a high boiling 430°F+ fraction, the low boiling and intermediate boiling fractions being characterized in Table II.
The 200/430°F fraction is chen hydrofined at 350°F, 400 psig, 800 SCF/Bbl over a cobalt moly-on-alumina catalyst as in Example 1 co produce a low sulfur gasoline blending componenc having a Bromine No. of abouc 1 and containing 11 wppm sulfur, chis requiring about 95.5% hydrodesulfurization. The resulting produce has occane ratings of 89.4 RONC and 80.9 MONC. Thus, che loss in octane racing for chis low sulfur gasoline blending component is nil as relates co che motor occane number racing, and only 1 occane number as relaces to research occane number.
When che recracked 65/200°F and 200/430°F recracked, hydrofined fraccions are combined, che resulcant producc has an occane number of 88.7 RONC and 80.6 MONC, contains only 20 wppm of sulfur, and represents 80.8 vol. percenc recovered produce, based on the original feed. Octane loss as a result of hydrofining is estimated ac abouc 0.5 RONC, or less.
The following example demonscrates a more preferred embodiment wherein an intermediate fraccion only is recracked.

EXAMPLE 3

Feedstock A was splic inco three fractions, a 65/200°F fraction, a 200/330°F fraction, and a 330°F+ fraccion as defined in Table III.
The 65/200 °F fraction is then treated in a Merox process, afcer admixture with light ends from che 200/330°F fraction which is recracked as defined hereafcer, co pr_o- duce a product of 87.5 RONC and 79.2 MONC with 50 ppm sulfur.
The 20G/330°F fraccion is recracked in a reaccor at 930°F, 14.7 psia, 13.7 WHSV at a cacalysc/oil racio of 9.2 over a conventional commercial zeolite, silica-alumina gel and clays, and che producc then fractionated to provide a 65/200°F fraction which is blended wich the 65/200°F fraction to Merox, a 200/430°F fraccion, and a 430°F+ fraction. The 65/200°F and 200/430°F fraccions are characterized in Table IV.
The 330°F+ fraccion, characterized in Table III, is combined wich che 200/430°F and 430°F+ recracked fractions chen hydrofined over a cobalt moly-on-alumina catalyst at 55C°F, 400 psig and 800 SCF/B. The feed entering che hydrofiner (H/F Feed), the producc therefrom (H/F Producc), and che final product formed by blending che hydrofined product and produce from MEROX is characcerized in Table V.
These daca show chac the occane loss due co hydrofining che final produce is ccnsiderably improved as contrasted with hydrofining the original feed, and chac octane loss as a resulc of che hydrofining approximates only 0.8 RON, wich no loss in MON occane value. Naphtha yield is considerably improved with no greater octane loss due to hydrofining as concrasted with recracking the whole naphtha

EXAMPLE 4

Feedstock A was recracked in a reaccor ac 930°F, 14.7 psia, 13.7 WHSV and ac a catalyst/oil racio of 9.2 over a conventional commercial zeolite cracking catalyst containing crystalline aluminosilicate zeolice, silica alumina gel and clays, and the product then fractionaced to provide three fractions, i.e., a low boiling 65/200°F fraction, an intermediate 200/350°F fraccicn and a high boiling 350/430°F fraction, as characterized in Table VI.
The 200/350°F fraction is then hydrofined over a cobalt moly-on-alumina catalyst at conditions jusc sufficienc to produce a suicable reforming feed, this requiring 98.9% hydrodesulfurization, 50% hydrodenitrogenation, and 70% saturation of the olefins to provide a produce of 89 RONC with less than 1 ppm sulfur, less than 1 ppm nitrogen and a bromine number of less chan 1. In forming chis produce a hydrogen consumpcion of 20 SCF/Bbl is required.
The hydrofined fraction is then reformed over an iridium-promoted platinum cacalysc ac 930°F, 1.0 W/Hr/W, 200 psig ac a hydrogen race of 4800 SCF/Bbl co produce 100 RONC gasoline.
In sharp concrasc, when Feedscock A was splic into fractions wichouc recracking, che composicions given in Table VII were obcained, to wic:
These fractions are thus highly unsaturated as contrasted with similar fractions obtained by recracking Feedstock A, and concain considerably more sulfur and nitrogen. By way of farther ccncrast, however, a portion of the 200/350°F fraction (Table VII) is chen hydrofined over che hydrofining catalyst previously defined at conditions jusc sufficient to achieve 99.6% hydrodesulfurization, 94.1% hydrodenitrogenation and 96% saturation of che olefins to produce a produce suitable for reforming co 100 RONC, i.e., one which contained less than 1 ppm sulfur, less than 1 ppm nitrogen and a bromine number of less chan 1. This produced a product of 75 RONC and required over 150 SCF/Rb1 of hydrogen, well over seven times the amount of hydrogen required to hydrofine che recracked produce.
The recracking of Feedstock A is chus shown co drastically reduce the amount of hydrotreating required to produce a reformer feed, and ic achieves chis at far less severity and wich far less consumption of hydrogen. Moreover, assuming first order desulfurizacion kinecics, 20% less reactor volume is required co achieve 98.8% hydrodesulfurization for the intermediate fraction of recracked Feedstock A than is required to produce 99.6% hydrodesulfurization for che intermediate fraction of raw Feedstock A. It also reduces reforming severicy, or the severity required co produce 100 RONC gasoline.
The following example demonstrates a more preferred embodiment wherein an intermediate fraction only is recracked.

EXAMPLE 5

Anocher portion of che 200/350°F fraction split from Feedstock A, as characterized in Table VII, was recracked ac 930°F, 14.7 psia, 14.3 WHSV and ac a catalyst/oil ratio of 9.1. The product was then split into chree fraccions, a 65/200°F fraction, a 200/350°F fraction, and a 350/430°F fraction as defined in Table VIII.
The 200/35C°F fraction is chen hydrofined over a cobalt moly-on-alumina catalyst at conditions jusc sufficient to produce a suitable reforming feed, this requiring 98.8% hydrodesulfurization, 67% hydrodenitrogenacion, and 63% saturation of the olefins to provide a product of 89.6 RONC with less than 1 ppm sulfur, less than 1 ppm nitrogen and a bromine number of less than one. In forming this product, a hydrogen consumption of 20 to 30 SCF/Bbl is required.
The hydrofined fraction is then reformed over an iridium promoted platinum catalyst at 930°F, 1.0 W/Hr/W, 200 psig ac a hydrogen rate of 4800 SCB/Bb1 to produce 100 RONC gasoline.
These data thus show that recracking the intermediate fraction of a cat naphtha offers definite advancages over recracking the whole cat cracked naphtha. In comparing Example 5 with Example 4 it is thus shown that 85.6 percent of a C₅/430°F product is obtained in recracking an intermediate fraction vis-a-vis the 79.9 percent of C₅/430°F product obtained in recracking the whole of Feedstock A. Moreover, 60.8 percent of a 200/350°F product is obtained in recracking the intermediate fraction vis-a-vis the 48.6 percent of a 200/350°F produce obtained in recracking the whole of Feedstock A. This fraction is particularly suitable as a reformer feed.
The preferred enbodiment, as represented by Example 5, also provides higher selectivity for other relatively high value produces vis-a-vis the embodiment of Example 4; or, conversely, lower selectivity for products of lesser value vis-a-vis the embodiment of Example 4. The data given in Table IX presents comparative data illustrative of the product of such relatively low value by-products as coke, light gases, inclusive of hydrogen and C₁ and C₂ hydrocarbons, and 430°F+ hydrocarbons, in the preceding runs wherein, as in Example 4, the whole of Feedstock A is recracked, and in Example 5 an intermediate boiling feedstock is recracked. The Table also presents the yields of C₃ and C₄ hydrocarbons which were obtained, these products being nearly as valuable as gasoline. The first column of Table IX identifies the by-product, the second column gives the percent yield of the by-product, based on the amount of recracked feed which was treated, and the third column gives the percent yield, based on the amount of original Feedstock A.
The advantages of recracking an intermediate cut vis-a-vis a whole feed are apparent. In considering these data it is noted in particular that che 430°F+ product is of low API gravity, is not desirable for use as heacing oil, and is unsuitable for use as diesel fuel or jec fuel. Only small levels of this 430+ product can be tolerated in gasoline for it contains multi-ring aromatics which cause serious engine deposits.
Table X presents data which illustrates that the preferred embodiment produces higher yields of che C₃ ⁼ and C₄ ⁼ hydrocarbons, which material is a potentially valuable alkylate feed. Analysis of to the C₃ and C₄ hydrocarbons thus shows the following yield of C₃ ⁼ and C₄ ⁼ (and i-C₄) hydrocarbons, based on recracked feed.
The advantages of recracking an intermediate boiling feed are therefore demonstrated, However, the recracking of a heavier fraction, e.g., a 200/430°F fraction, is preferable to recracking a whole fraction, i.e., the 65/430°F fraction for obviously, inter alia, the cracking of a 65/200°F fraction will produce little 200/350°F product for reforming, if any.
It is apparent that various modifications and changes can be made without departing the spirit and scope of che invention.

Conversions of Units

Temperatures expressed in °F are converted to °C by subtracting 32 and deviding by 1.8.
Gas Volumes in Standard Cubic Feet (SCF) are converted to litres by multiplying by 28.32.
Liquid Volumes in Barrels (B or Bbl) are converted to litres by multiplying by 159.0
Mass in pounds (lbs) is converted to kilograms by dividing by 2.20462.
Pressures in pounds per Square inch (psi) are converted to kg/cm² by multiplying by 0.07031. Note "psia" denotes absolute pressure in psi and "psig" denotes gauge pressure in psi.

Claims

1. A process for the production of a high octane gardline characterized by the following steps in combination:

(a) cracking a sulfur-bearing hydrocarbon feed in a first cracking zone over a cracking catalyst at conditions sufficient to obtain a cat cracked naphtha product containing in the range of from 10 percent to 60 percent olefins, based on the weight of said product,

(b) withdrawing said cat cracked naphtha as a product from said first cracking zone,

(c) recracking said cat cracked naphtha product or one or more fractions therof, without dilution with other hydrocarbons, over a crystalline aluminosilicate zeolite catalyst in a second cracking zone to desulfurize said feed and saturate at least 50 percent of said olefins, based on the weight of said cat cracked naphtha, and

(d) hydrotreating or hydrofining the product of said second cracking zcne over a hydrogenation catalyst at hydrofining conditions in a hydroreating or hydrofining zone to hydrodesulfurize said product and reduce the olefin content of said product.

2. A process according to claim 1 characterized in that at leat one of the said fractions of said cat cracked naphtha product comprises an intermediate fraction having a low end boiling point in the range of from 150°F (65.5°C) to 250°F (121.1°C) and a high end boiling point in the range of from 250°F (121.1°C) to 380°F (193,30°C).

3. A process according to claim 1 or claim 2 characterized in that at least one of said fractions of said cat cracked naphtha product comprises a fraction having a low end boiling point in the range of from 180°F (82,2°C to 220°F (104.4°C) and a high end boiling point in the range of from. 400°F (204.4°C) to 430°F (221.1°C).

4. A process according to any one of claims 1-3 characterized in that the said hyrotreating or hydrofining conditions of step (d) are selected to result in a product suitable for addition to a gasoline blending pool.

5. A process according to any one of claims 1-3 characterized in that the said hydrotreating or hydrofining conditions of step (d) are selected to result in a product suitable as a feed for reforming over a sulfur-sensitive noble metal reforming catalyst.

6. A process according to any one of claims 1-5 characterized in that the sulfur-bearing hydrocarbon feed introduced-into said first cracking zcne is a gas oil boiling below 1050°F (565.5°C).

7. A prccess according to any one of claims 1-6 characterized in that from 80 to 100 percent of the olefins of the cat cracked naphtha feed introduced into the second cracking zone are saturated during the reactions in the second cracking zone.

8. A process according to any one of claims 1-7 characterized in that the cat cracked naphtha feed introduced into the second cracking zone is reacted at temperature in the range of from 800°F (426.6°C) to 1100°F (593.3°C) and at a pressure in the range of from 0 to 50 psig (0 to 3.5155 kg/cm gauge).

9. A hydrodesulfurized cat cracked naphta product whenever produced by the process of any one of claims 1-8.