US2767126A - Catalytic cracking process and apparatus - Google Patents

Description

Oct. 16, 1956 T. RICE 2,767,126

CATALYTIC CRACKING `PROCESS AND APPARATUS Filed Marph 2s, 195s United States Patent CATALYTIC CRACKING PROCESS AND APPARATUS Theodore Rice, Penn Township, Allegheny County, Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Application March 23, 1953, Serial No. 344,078

11 Claims. (Cl. 196-52) This invention relates to a uid catalytic cracking process and apparatus and more particularly to an improved process and apparatus for the catalytic conversion of hydrocarbons.

Hydrocarbon charge stocks containing appreciable amounts of deleterious materials, such as sulfur compounds and/or nitrogen bases, are not ordinarily desirable as charge stocks for conventional catalytic cracking operations because these charge stocks, under conventional catalytic cracking conditions, normally tend to deposit upon the cracking catalyst excessive amounts of catalyst contaminant. The catalyst deposits tend to build up on the cracking catalyst at an alarming rate, with the result that the cracking catalyst may quickly be poisoned and the cracking reaction adversely aected or the cracked products may be contaminated with the undesirable contaminants.

In accordance with the invention, a hydrocarbon charge stock containing appreciable amounts of deleterious materials, such as sulfur compounds and/ or nitrogen bases, is contacted in a tirst reaction zone under relatively mild cracking conditions with partially regenerated uidized cracking catalyst obtained from an upper portion of a regeneration Zone; and a hydrocarbon charge stock substantially free of objectionable materials, such as sulfur compounds and/or nitrogen bases, and which may, in fact, comprise the cracked products comprising the eiuent from the iirst reaction zone, is contacted in a second reaction zone under conventional cracking conditions with regenerated tiuidized cracking catalyst obtained from a lower portion of said regeneration zone.

By operating in accordance with my invention, the above-noted ditiiculties are avoided by removing in the rst reaction zone, maintained under relatively mild cracking conditions, the objectionable materials from such a charge stock, and thereafter passing the remainder or" the charge stock to a second reaction zone where conventional catalytic cracldng conditions are maintained. When the charge oil contains large amounts of sulfur, H28 vapors are released and heavy combustible deposits not vaporized are deposited on the cracking catalyst in the first reaction zone. Since the HzS vapors pass overhead from the rst reaction Zone and may be detrimental to natural catalysts in subsequent cracking operations, it may be advisable to remove HzS vapors from the etiiuent. This can be done by means known in the art, as for example, by passing the effluent carrying the H28 vapors through a vapor-liquid separator. Treating charge stocks containing large amounts of nitrogen bases in the first reaction zone will result inrdeposits of combustible nitrogen-containing compounds on the cracking catalyst and some formation of nitrogen and ammonia vapors. ri`he nitrogen and ammonia vapors are not detrimental to subsequent cracking operations and need not be removed from the eiuent in the rst reaction zone. When the charge stock contains both sulfur and nitrogen compounds, volatile products as well as combustible deposits containing both of these compounds are formed in the iirst reaction zone. For simplicity, these combustible deposits resulting from the cracking of the deleterious compounds noted, in particular sulfur compounds and/ or nitrogen bases, as well as the combustible hydrocarboi naceous deposits generally formed on the catalyst during catalytic cracking, sometimes known `as coke, will be termed hereinafter contaminant Y it is important in the practice of my invention that (l) cracking catalyst employed in the first reaction zone, wherein mild cracking conditions are maintained, be partially regenerated cracking catalyst obtained from the upper portion of the regenerator; (2) cracking catalyst employed in the second 'or conventional catalytic'cracking zone be regenerated cracking catalyst obtained from a lower portion of said regenerator; (3) spent catalyst from each of the reaction Zones be continuously passed to the upper part of said regenerator; and (4) an oxidizing gas, like air, be passed upwardly through the base of the regenerator for the purpose of regenerating the catalyst therein.

By operating in this manner, and, as will be explained in detm'l hereinafter, as the catalyst moves downwardly from the upper portion of the regenerator and is regenerated by the oxidizing gas moving upwardly, the oxygen in the oxidizing gas is substantially consumed with the result that the gas in the uppermost portion of the regenerator contains little or no oxygen and after-burning is avoided.

in obtaining partially regenerated catalyst from an upper zone of the regenerator for the first reactor and regenerated catalyst from a lower zone of the regenerator for the second reactor, an etcient and reliable means of obtaining sumcient amounts of the proper catalyst for each of the reaction zones is assured. Since the cracking catalyst passed to the tirst reaction zone is to be employed under relatively mild cracking conditions only, and thus need not be unduly active, it is apparent that, by obtaining partially regenerated catalyst from the upper part of the regeneration zone, I am able to control the supply of such catalyst as well as the level of the contaminant thereon. Similarly, the catalyst needed in the second or conventional catalytic cracking Zone is preferably a relatively active cracking catalyst substantially free of contarninant, and by obtaining the catalyst for such latter Vzone from a lower portion of the regenerator I am able to control the amount and quality of catalyst passed t0 such latter zone. Y

The novel apparatus employed in carrying out my process comprises a regenerator provided with a plurality of regeneration Zones, superposed one upon another; a iirst and second reactor; means for removing a partially regenerated uidized cracking catalyst from one of the upper regeneration zones, admixing it with a charge stock containing appreciable amounts of objectionable materials, such as the aforementioned sulfur compounds and/ or nitrogen bases, and moving the resulting mixture to the rst reactor; means for removing a regenerated tluidizing cracking catalyst from one of the lower regeneration zones, admixing it with a charge stock containing substantially less objectionable materials than are present in the charge stock passed to the rst reactor, and moving the resulting mixture to the second reactor; means for removing cracked products from each of the two reactors; and means for continuously recycling catalyst contaminated with carbonaceous material from each of the two reactors to the regenerator.

Any conventional cracking catalyst, natural or synthetic, can be employed in the process of my invention, as for example silica-alumina cracking catalyst comprising an acid-activated halloysite such as Filtrol SR, montmorillonite clay such as Filtrol D, synthetic silica-alumina cracking catalyst such as synthetic composites containing 3 between 80 to 90 percent of silica and 10 to 2O percent of alumina, and so forth. The aforementioned catalysts should be of conventional particle size used in iuid catalytic cracking systems. Y n u ln order toV illustratethe process and apparatusrof my invention, reference should be had to the accompanying 'figure which is aV diagrammatic representation-of a fluid `-conversionsystem in accordance with my invention. The accompanying ligure is hereby incorporated into my invention and made a part thereof. Y A charge stock comprising a heavy gas oil containing iappreciable amounts of sulfur compounds and/o1' nitrogen bases is introduced into the system through line 12. The chargeV stock has been preheated (by means not shown) to a temperature'rsomewhat below the cracking Y temperature maintained in reactor 22.Y If desired, the charge stock in line V12 can be joined by recycle oil from line 13. The combined feed enters line ,14 and is joined therein by partially regenerated cracking catalyst particles, such as silica-alumina cracking catalyst, entering through valve i6 `from standpipe 18 extending upwardly through the Vbase of regenerator 20 into upper portion A thereof.- The partially Vregenerated cracking catalystV entering standpipe 18 is at a temperature somewhat above the Vcracking temperature present in reactor 22 and serves to heat the mixture of charge stock and recycle oil to this cracking temperature. The mixture of partially regenerated catalyst, charge stock and recycle oil, if any, passes upwardly through line 14 into the base of reactor 22, `which contains a dense phase bed of silica-alumina cracking catalyst particles.

Within reactor 22 the mixture passes through grid 24 and the feed is mildly cracked throughtcontact with the catalyst particles in reactor 22. The dense phase uidized Vbedof catalyst particles within reactor 22 is maintained at a temperature in the range of about 750 to about 900 F. and a pressure of about 15 pounds per square inch for a time suicient to obtain mild cracking of the total charge `and the release of a major portion of the sulfur and/or nitrogen in the'charge in the form of hydrogen suliide, combustible catalyst deposits, etc. The cracked products, together with the uncracked eiluent, are withdrawn from reactor 22 ythrough cyclone separator 26, which returns entrained catalyst tothe dense phase catalyst bed in reactor 22, and; are ledthrough line 28 to a ractionator '(not shown.) where they subsequently undergo such addi- ,tional refining and processing as is readily apparent to one skilled in the art. Y

The rate of introduction of feed and catalyst into reactor 22 and the rate of withdrawal of products and contaminated catalyst from reactor 22 is preferably so regulated that the average level of contaminant upon the catalyst in reactor 22 is maintained preferably in the range of about 0.5 to about 1.2 percent by weight. The withdrawal of contaminated catalyst from reactor 22 is effected through stripper 30. The contaminated catalyst passes through stripper 30 or strippers of other designs known to those familiar with the art, and is stripped of entrained hydrocarbons by an inert gas such as steam entering stripper 30 through steam line and nozzle 32.

From stripper 30 the contaminated catalyst passes through valve 34 and line 36 to regenerater 20. Regenerator is at a lower level than reactor 22 so that iow from reactor22 to regenerator 20 can be readily effected without the necessity of adding appreciable quantities of aerating gas to line 36. However, some aerating gas can be added to Vfacilitate proper flow of the contaminated catalyst from reactor 22 to regenerator 20.

Regenerator 20 comprises a two-zone regenerator with upper zone'A being located above a perforated'plate member or grid 40 and lower zone B being located below grid 40.- The contaminated Vcatalyst from line 36 enters zone AV and is regenerated by oxygen-containing regeneratinggas-leaving zone B. This oxygen-containing Yregenerating gas, originally comprised an oxygen-rich gas,

lyst containing preferably in the rangeV of about 0.1 tol about 0.6 weightper cent of contaminant. A Grid 40 re- Y distributes catalyst and lregenerating gas in their move-V ment through the regenerator and thus provides a means for eliminating channeling and permitting intimate contact betWeent-he catalystA andthe regeneratingV gas. In this way temperatures throughout Vthe Vregenerator are easilymaintained at a temperature in the range 0f 950. to about ll25 F., and temperatures above about ll50 F., which may occur in conventional single-zone regenerators because of hot spots and which mayV result in permanent deactivation of the catalyst, are avoided. Redistribution, moreover, provides for rapid disengagement and removal of nitrogen gases and H28 vapors that might have been occluded from reactor 22. The regeneration effected in regeneration zone B is more severe than that eiected in regeneration zone A, and accordingly a major portion of the oxygen content of the regenerating gas is consumed in regeneration zone B.

As a result'of the foregoing, the regenerating gasV entering regeneration zone A has a relatively low concentraf tion of oxygen. This concentration of oxygen should be suiicientV to insure that the contaminant level on' the spent catalyst from reactor 22 and reactor 62, hereinafter noted, is reduced to a value of about 0.4 to about 1.0 percent by Weight in regeneration zone A.

However,v the oxygen content of the regenerating gas in regeneration zone A should be sufficiently low, preferably less than about 2 percent, as to eliminate the danger of after-burning in the upper part of regenerator 20.

After-burning can occur when a combustible mixture t exists in the section of the regenerator known as the dilute phase, which is the region above the more dense uid bed where the concentration of the catalyst particles is relatively low. In the regeneration of Vthe spent catalyst, the burning of the carbon results in the formation of combustion products such as carbon monoxide, carbon dioxide and water and, in addition, any oxygen notwconsumed during regeneration is also present. When' the'V oxygen and carbon monoxide are in the proper proportions, the mixture is readily combustible. The gas may then burn in the top of the regenerator or in the flue gas ducts, resulting in very high temperature, above about 1100 F., which is extremely damaging to the cyclone separators and the ue gas ducts, and may Valso affect the activity of the catalyst. After-burning can usually be avoided if the oxygen content of the combustion gas in the dilute phase of zone A is kept below about 2 percent.

By operating in accordance with my invention, the amount of oxygen present in the oxidizing gas in the `dilute phase of zone A is easily maintained below about 2 percent. The oxidizing gas moving upwardly from zone B mayV contain more than 2 percent oxygen but the excess oxygen, or that amount :over 2 percent, is consumed in burning off combustible `deposits on the catalyst surface in zone A. Thus, by regenerating the contaminated catalyst in the regenerator while maintaining the catalyst inV a plurality-,of beds, the excess oxygen which would ordinarily be present in the upper portion lof the regenerator Y is substantially consumed in an upper catalyst bed, and 'after-'burning is avoided. t e n The regeneration ue gases yare Withdrawn fromV :re-

generator 20 through cyclone Aseparator 50 and line "52;V

yCyclone Iseparator 50 returns regenerating catalyst -to the dense phase fbed of catalyst in regenerator 20. Y

While standpipe 18 extends Yupwardly through the base of regenerator 20 into upper 'zoneV A for the purposeof 'withdrawing partially regenerated catalyst preferably con- :taining about 0.4 rto about 1.0 Weight percent of contaminant -for use in reactor V2-2, a second standpipe 54 extends upwardly through the base of regenerator 20 into` lower zone B for the purpose of withdrawing regenerated catalyst containing preferably about 0.1 :to about 0.6 weight percent of contaminant for Vuse 'in reactor 62, Where charge stocks containing little or no deleterious materials, Isuch Yas objectionable -sulfur and/or nitrogen compounds, are caitalytically cracked.

A charge stock comprising a heavy gas oil containing substantially less `of the deleterious materials present in the charge stock passed rto reactor 22, which has previously been heated (by means not shown) to `a temperature somewhat below the cracking temperature maintained in reactor 62, is introduced into line 56. This charge `stock may be joined, if desired, by recycle cil and/or desulfurized :oil through line 57, particularly that obtained as a result of cracking in reactor 22. The combined charge in line 58 is joined by regenerated silicaalumina cracking catalyst containing preferably about 0.1 to about 0.6 `weight percent of contaminant from standpipe 54 through valve 60. The regenerated catalyst =from standpipe 54 is yat .a temperature somewhat @above the cracking temperature maintained in reactor 62 :and serves to heat the mixture `of total charge to this cracking tempera/ture. The mixture of regenerated catalyst and charge passes upwardly yfrom line 58 into the base of reactor 62. Reactor 62 contains a dense phase bed of silica-alumina cracking catalyst particles.

Within reactor 62 the mixture passes through grid 64 and .the feed is catalytically cracked through contact with the catalyst particles in reactor 62. The dense phase -uidized bed of catalyst particles within reactor 62 is maintained at cracking temperatures in the range of labout 800 lto `about llO0 F., such as about 900 F. As ya result of contact with the catalyst particles in reactor 62 the feed is -catalytically cracked to relatively low-boiling hydrocarbons ysuch as gasoline boiling range hydrocarbons, butanes, butenes, propane fand propylene. Simultaneously, contaminant is deposited upon the silicaalumina cracking catalyst. The cracked products `are withdrawn from reactor 62 through cyclone separator 66, which returns entrained catalyst to the dense phase catalyst bed in reactor 62, and the cracked products then pass through line 2S, Where they join the products from reactor 22, to a fractionator (not shown). If desired, the cracked products from reactor 62 -may be :sent to a fractionator `other than that to which the products from reactor 22 are sent. In the fractionator, these cracked products undergo such additional refining and processing as is readily apparent to lone skilled in the art.

The rate of introduction of feed and catalyst into reactor 62 and the rate :and withdrawal of cracked products and contaminated catalyst from reactor 62 preferably `are so regulated that the average level Lof contaminant upon the catalyst in reactor 62 is maintained in the range `of about 0.5 to about 1.2 percent by weight. 'The withdrawal :of contaminated catalyst from reactor 62 is effected :through .stripper 68 or strippers of other designs known to those familiar with the art. The contaminated catalyst passes through stripper I68 and is stripped of entrained hydrocarbons by an incr-t gas such as steam entering stripper 68 through steam line Iand nozzle 70.

From stripper 68 fthe contaminated catalyst passes through valve 72 'and line 74 to regenerator 20. Regenerator 20 is at `a lower level than reactor 62 so that flow from reactor 62 to regenerator 20 can be readily elected without the necessity of adding appreciable quantities of aora-ting gas to line 74. However, 'some aerating gas can be added to facilitate proper ow of the contaminated catalyst from reactor 62 to regenerator 20. Operating conditions in regenerator 20 are maintained |as previously described.

In order to illustrate a .specific embodiment in accordis contacted with cracking catalyst in reactor 22 iat `a temperature in the range of lapproximately 750 to ab out 900 F. and 4a pressure of about 15 pound-s per square inch gauge for a time sulicient to obtain mild cracking therein. As a result of such cracking, substantially rall of the desirable gas 'oil fractions :are vaporized .and the heavy cornponents of the gas oil charge are mildly cracked, resulting in it'ne formation of lighter hydrocarbons, which are removed from the reactor along with the vapor-ized gas oil fractions, and deposits of heavy combustible material on the catalyst therein.

ln reactor 62, a gas toil charge stock, relatively free .of deleterious materials found in the charge stock entering reactor 22, and having the following inspection data:

Characterization factor 11.8 ASTM distillation, F. (D-158):

Initial boiling Vpointdend point 60G-1050 is catalytically cracked ata temperature of about 900 F. :and a pressure about 15 pounds per square inch gauge, while maintaining a catalyst to oil ratio of about 10:1 'and la 'space velocity of about 1.5 (weight per hour of oil per weight :of catalyst). As Ia result tof cracking in reactor 62, relatively low-boiling hydrocarbons such #as gasoline boiling range hydrocarbons, butanes, butenes, propane and prepylene ,are :obtained in the eiiluent therefrom land carhfonaceous contaminant is deposited on the cracking catalyst.

While l have disclosed specific operating conditions which are preferably maintained in reactors 22 and 62, it is understood that these conditions are to be considered illustrative only and not limitative. The operating conditions necessary in practicing my invention are not critical but may be varied over a wide range and may be considered to be dependent yon many variables, such as the composition of the charge stocks employed, the cracked products desired, a balance of economic costs involved, and so forth. For example, in reactor 22, the temperature may vary from about 700 to about 1000 F., the pressure from about atmospheric to pounds per square inch gauge or more but preferably .at atmospheric pressure, and the space velocity may range from about 5 to l5; while in reactor 62 the temperature may range from about 800u to about 1l00 F., the pressure from about atmospheric to about 100 pounds per square inch gauge or more, and the space velocity may range from about 0.5 to about 10 or more. lt has been found that the poisoning effect of H28 is a function of its partial pressure and time of contact. Therefore, it is preferred to operate at low pressures and high space velocities in reactor 22 to minimize deactivation of the catalyst. The catalyst oil ratio employed in reactors 22 and 62 may be as low las 1:1 or as high as 15:1 4or more.

'While the character of the invention has been described in detail and examples given, this has been done by way of illustration only, and the invention is not t-o be considered to be so limited. Numerous modifications and variations of my invention may be apparent to one skilled in the art, and these modicati-ons should be construed as included within the scope of the claims appended hereto. For example, stan-dpipes 18 and 54 need not be located as shown in the drawing but may penetrate the base of regenerator 20 at other points as well, and, in fact, may even run exteriorly of regenerator 20 Ybefore penetrating zones A and B, respectively. If desired, regenerator 20 may be located at a higher level than either of reactors- 22 andj 62,.-or both, and additional aerating gas maybe introduced into lines 36and 74 for the purpose of'aiding the movement `of spent catalyst fromir'reactors 22 and 62 to regenerator 20. in addition, the regenerator may include ymore than Vtwo regenerator zones ifdesired.

The fluid'catalytic process and apparatus of my invention permit Ythe processing of distillates having lan appreciable amount of deleterious materials, such as sulfur compounds and/or nitrogen bases, without materially aiecting the activity of the cracking catalyst employed inthe conventional catalytic cracking zone and at the same time permit higher yield of valuable low boiling point hydrocarbons Yto be obtained while simultaneously decreasing both the deposition of undesirable contaminant upon'the catalyst in said cracking zone and the yield of undesirable low boiling point, gaseous hydrocarbons.

While I have described my invention by reference to certain improvements thereof, the same is not limited thereto except as deiined by the appended claims.

I claim:

A iluid catalytic cracking process which comprises regenerating contaminated fluidized cracking catalyst particles by removing the contaminant by oxidative combustion from the catalyst particles in a regenerator provided with a plurality of regeneration zones, superposed one upon another; in which the amount of contaminant present on the cracking catalyst in an upper zone is greater than that which is present on the cracking catalyst in arlower zone thereof, the amount of contaminant in said upper zone being about 0.4 to about 1.0 weight percent .and the amount `of contaminant in said lower Zone being about 0.1 to about 0.6 weight percent; admixing a hydrocarbon charge. stock containing-appreciable amounts of deleterious materials with partially regenerated cracking catalyst obtained from an upper regeneration zone; subjecting such mixture to relatively mild.

cracking conditions in a rst reaction zone for .a time sufficient to -release a major portion of said deleterious materials in said iirst reaction zone and deposit contaminant on the cracking catalyst therein; admixing a hydro-V carbon charge stock containing substantially less of the deleterious materials than is present in the charge stock passed to iirst reaction zone with a regenerated cracking catalyst obtained from a lower regeneration zone; subjecting said last named mixture to elevated temperatures in `a Vsecond reaction zone, thereby forming a relatively lower boiling hydrocarbon product and depositing contaminant on the catalyst particles therein; continuously recycling contaminated catalyst from each of the two reaction zones to the regenerator and continuously withdrawing the cracked product from each of the two reaction zones.

2. A process in accordance with claim 1 in which the charge stock entering the iirst reaction zone contains appreciable -amounts of deleterious sulfur and nitrogencontaining compounds.

3. A process in accordance with claim 1 in which the charge stock entering the rst reaction zone contains appreciable amounts of deleterious sulfur-containing compounds. Y

4. A process in accordance with claim ,1 in which the charge stock entering the first reaction Zone contains appreciable amounts of deleterious nitrogen-containing compounds.

aoK

5. A process Ain accordance with claim '1; in which the chargeY stock entering Vtheecond reaction zone; comprises theefliuent from the irst reaction zojne. y

`6.1A process in accordanceA with claim 1 in which. an oxygen-containing gas is 'passed `upwardly through the regenerator and the catalystV being regenerated in passed downwardly through 'the regenerator.

g 7. A iluidV cracking unit comprising a regenerator provided with a plurality of regeneration zones, superposed one upon another; means for separating an upper regeneration zone from a lower regeneration zone and permitting movement of catalyst and regenerating gas there-V through; a reactor;-means for removing a partially regenerated fluidized cracking catalyst from an upper regeneration zone, admixing it with a hydrocarbon charge stock, moving Vthe resulting mixture to the reactor; Vmeans for removing cracked products from the reactor; means for removing a regenerated iluidized cracking catalyst from a lower regeneration zone; means for continuously recycling contaminated catalyst from the reactor to` an upper regeneration zone; and means for introducing an oxygenfcontaining gas upwardly through the base of the regenerator.

8j A Huid cracking unit comprising a regenerator provided with a pluralityao'f regeneration zones, superposed one upon another; means for separating an upper regeneration zone from a lower regeneration zone and permitting movement of catalyst and regenerating gas therethrough; a first reactor and a second reactor; means for removing a partially regenerated fluidized cracking catalyst from an upper regeneration zone, admixing it with a hydrocarbon charge stock containing appreciable amounts of deleten'- ous materials, and moving the resulting mixture to the first reactor; means for removing a regenerated catalyst from a lower regeneration zone, admixing it with a hydrocarbon charge stock containing substantially less of the objectionable materials than is present in the charge stock passed to the iirst reaction zone, and moving the resulting mixture to the second reactor; means for removing racked products from each of the two reactors; means for continuously recycling contaminated catalyst from each of the two reactors to an upper-,regeneration zone; and means for introducing an oxygen-containing gas upwardly through the base of the regenerator.

9. A fluid catalytic cracking unit in accordance with claim 8 in which the zones in the regienerator are separated by at least one grid.

10. A uid cracking unit intaccordance with claimV 8 in which the regenerator is provided with Vat least two regeneration zones.

ll. A fluid catalytic cracking unit in accordance with claim 8 in which the means for removing catalyst from the regeneration zones in the regenerator include standpipes extending upwardly through the baseof the regen- Y erator into the upper and lower regeneration zones, re-

spectively.

References Cited in the iile of this patent y UNITED STATES VPATENTS 2,416,730 Arveson Mar. 4, 1947 2,432,644 Alther Dec. 16, 1947 2,444,990 Hemminger Y July 13, 1948 2,465,255 Moorman Mar. 22, 1949 2,494,614 Grote' Jan. 17, 1950

Claims (1)

1. A FLUID CATALYTIC CRACKING PROCESS WHICH COMPRISES REGENERATING CONTAMINATED FLUIDIZED CRACKING CATALYST PARTICLES BY REMOVING THE CONTAMINANT BY OXIDATIVE COMBUSTION FROM THE CATALYST PARTICLES IN A REGENERATOR PROVIDED WITH A PLURALITY OF REGENERATION ZONE, SUPERPOSED ONE UPON ANOTHER, IN WHICH THE AMOUNT OF CONTAMINANT PRESENT ON THE CRACKING CATALYST IN AN UPPER ZONE IS GREATER THAN THAT WHICH IS PRESENT ON THE CRACKING CATALYST IN A LOWER ZONE THEREOF, THE AMOUNT OF CONTAMINANT IN SAID UPPER ZONE BEING ABOUT 0.4 TO ABOUT 1.0 WEIGHT PERCENT AND THE AMOUNT OF CONTAMINANT IN SAID LOWER ZONE BEING ABOUT 0.1 TO ABOUT 0.6 WEIGHT PERCENT; ADMIXING A HYDROCARBON CHARGE STOCK CONTAINING APPRECIABLE AMOUNTS OF DELETERIOUS MATERIALS WITH PARTIALLY REGENERATED CRACKING CATALYST OBTAINED FROM AN UPPER REGENERATION ZONE; SUBJECTING SUCH MIXTURE TO RELATIVELY MILD CRACKING CONDITIONS IN A FIRST REACTION ZONE FOR A TIME SUFFICIENT TO RELEASE A MAJOR PORTION OF SAID DELEFERIOUS MATERIALS IN SAID FIRST REACTION ZONE AND DEPOSIT CONTAMINANT ON THE CRACKING CATALYST THEREIN; ADMIXING A HYDROCARBON CHARGE STOCK CONTAINING SUBSTANTIALLY LESS OF THE DELETERIOUS MATERIALS THAN IS PRESENT IN THE CHARGE STOCK PASSED TO FIRST REACTION ZONE WITH A REGENERATION CRACKING CATALYST OBTAINED FROM A LOWER REGENERATION ZONE; SUBJECTING SAID LAST NAMED MIXTURE TO ELEVATED TEMPERATURES IN A SECOND REACTION ZONE, THEREBY FORMING A RELATIVELY LOWER BOILING HYDROCARBON PRODUCT AND DEPOSITING CONTAMINANT ON THE CATALYST PARTICLES THEREIN; CONTINUOUSLY RECYCLING CONTAMINATED CATALYST FROM EACH OF THE TWO REACTION ZONES TO THE REGENERATOR AND CONTINUOUSLY WITHDRAWING THE CRACKED PRODUCT FROM EACH OF THE TWO REACTION ZONES.

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