US2382755A - Catalytic conversion of hydrocarbon oils - Google Patents

Description

lAug 14, C w. TYSON i CATALYTIC CONVERSION OF HYDROCARBON OILS Filed May 24, 1941 CAS /NLET IGAS INLET ntlj ff l q.

T NEA /zvc can. I f VERT/CAL REAQTOA i o Sir/blu: cAs 25 33 Patented Aug. 14,1945

CATALY'TIC CONVERSION F IIYDBO- CARBON GILS charles w. Tyson, summit, N. J., minor to ration of Delaware Standard (lilV Development Company, a corpo- Applicatlon May 24, 1941, Serial No. 394,963

18 Claims.

This invention relates to the catalytic conversion of hydrocarbon oils and pertains more particularly to a process for treating crude petroleum oils to form lower boiling motor fuel products.l

One of the important objects of the present invention is to provide a process for cracking oils in the presence of a catalyst in finely-divided form which is capable of converting a greater proportion of the original oil into motor fuel products than is possible according to present processes.

It has heretofore been proposed to form motor fuel by cracking petroleum oil in the presence of a catalyst. According to one general method of procedure, the catalyst in finely-divided state is mixed with the oil vapors to be cracked and the resulting mixture passed through a cracking zone. The present invention deals particularly with a process of this type.

When operating in this manner, the catalyst more or less rapidly becomes contaminated with carbonaceous deposits which impair the activity of the catalyst for bringing about the desired cracking. It is necessary, therefore, to regenerate the catalyst at frequent intervals to restore its activity. The regeneration may .be accomplished by controlled oxidation, in which the catalyst is treated with an oxidizing gas at a temperature which will burn oil the carbonaceous deposits without permanently injuring the activity of the catalyst.

Since the time required for eiecting the regeneration is non-productive, it has been the practice in the past to utilize as an oil for the cracking process a relatively clean condensate stock so that the amount of coke formed during cracking can be held to a minimum. This method of operating requires that the original crude petroleum be initially subjected to a distilling operation so as to distill or vaporize the charging stock employed for catalytic cracking purposes. It also necessitated the rejection of a residual fraction of the crude oil which could not be va.- porized without decomposition.

`The process forming the present invention makes it possible to utilize the residual unvaporizable portion of the crudeoil which has been previously rejected as well 4as the clean condensate portions previously employed for catalytic cracking purposes. The invention will' be better understood from the more detailed description hereinafter in which reference will be made to the illustration of an apparatus suitable for carrying the invention into effect.

Referring to the drawing, the reference character I0 designates a charge line through which the oil to be processed is introduced into the equipment. This oil is preferably a residual stock, such as a. crude oil, topped crude, reduced crude, or the like. For example. it may be the total crude fraction or a crude petroleum fraction from which the gasoline .has been removed or a fraction from' which the gasoline and a portion or all of the kerosene or gas oil has been removed. The oil charged through line ID is forced by means of pump II through a suitable heat exchanger I2 in which the oil may be preheated in any desired manner. The amount of preheating should be controlled to maintaina major portion of the oil in liquid phase. The heat exchanger I2 may be located, if desired, so as to exchange heat between the incoming stream and hot oil products or catalyst.

After passing through the heat exchanger I2,

Athere is introduced through line i3 into the stream oi oil to be processed a nely-divided natural or activated clays or synthetic absorptive products of the same or different chemical composition. For example, synthetic gel catalysts consisting of silica-alumina, silicaemagnesia, boron oxide-alumina and silica-zirconia have been found to be eiective for this purpose. In order to introduce the catalyst powder into the oil stream, it is necessary to impose a pressure on the powder suilicient to overcome the pressure existing in the stream of oil. According to the drawing, the necessary' pressure is developed by constructing line I3 in the form of a vertical column or standpipe having a. height suilicient to develop a static pressure at the bottom thereof adequate to overcome the pressure of the oil. In order to prevent thecatalyst from bridging across the column or standpipe I3, a iiuidizing medium such as an inert gas may be introduced at one or mlore spacedpoints through lines I4, IE, I6 and l.

The bottom portion of the standpipe I3 may l be provided with a valve I8 for regulating the amount of powdered material introduced into the oil vapors. A suitable inert gas such as steam may be alsointroduced into the conduit I3 below Y accompanying drawing which is a diagrammatic valve I8 through line I9 to maintain the powder As a typical example, where the amount of unvaporizable oil amounts to about of the crude, the amount of powdered material introduced should be from 100% to 450% by weight of the unvaporizable portion. Where undistilled crude is employed in the process. this amount will correspond in such case from 10% to 45% by weight of the fresh feed introduced through line I0.

The amount of powder to be introduced in,`

each particular case should be controlled to completely absorb or adsorb all the unvaporizable constituents without becoming tacky or sticky and will depend on the character of the feed, the absorptive properties of the catalyst, and other factors. In ordinary cases, when employing either absorptlve clays or synthetic gels, the amount will be somewhere within the above limits.

The mixture of catalyst and oil formed in line I0 passes to a separating chamber 2,0 in which gases and vapors separate from a liquid residue containing the catalyst. The gases and vapors will consist of the uidizing gas mixed with the powder and hydrocarbon vapors formed during preheating the oil in heat exchanger I2. According to one method of operating, the amount of heat imparted to the oil during its passage through the heat exchanger I2 may be 'suiilcient to vaporize a portion of the oil, but

it will be necessary to retain a suicient amount of oil in unvaporlzed form to form a liquid slurry in the separator 20 which may be readily pumped by means of a liquid pump and which will form a suilicient amount of vapors during 'the subsequent treatment to serve as a carrier for the powder contained in the slurry and thus prevent the powder from separating out in the vaporizing coil.

Vapors and gases separated in the separator 20 pass overhead through line 2I and'are treated as hereinafter described. The liquid slurry separated in the separator 20 is withdrawn through line 22 and is forced by means of a pump 23 to a heating and vaporizing coil 24 in which the slurry is heated to a temperature suilicient to vaporize all constituents vaporizable Without substantial decomposition. The pump 23 may be automatically controlled by a liquid level controller (not numbered) provided in the separatory chamber 20 whereby the liquid level in the chamber may be maintained between desired limits. As previously mentioned. the amount of powder present should be sufiicient to absorb the unvaporizable portion of the original charge without becoming sticky or tacky. Under proper conditions the products leaving the coil 24 will consist of oil vapors and dry powder.

The temperature of the oil obtained during passage through the vaporizing coil 24 may be of the order of from 700 F. to 1000" F. The suspension of vapors and powder may be passed from the heating coil 2| through transfer line 26 into the bottom section of a vertical reactor 26.

The velocity of the vapors passing through the vertical reactor 26 should be controlled so as to permit a partial settling of the powdered material introduced therein. In other words, the velocity of the gases passing through the vertical column should be insufiicient to cause the powdered material to travel through the reactor at substantially the same rate as the oil vapors. As a result the density of the mixture within the vertical column 26 will be materially higher than the density of the stream passing thereto and the time required for passage of powder through the reactor will be materially longer than the time required for passage of oil vapors. Under normal conditions the average density of the gas-powder mixture within the vertical tower should be at Aleast twice the density of the stream passing thereto.

The time of residence of the powder in the vertical chamber 26 may be controlled by regulating the velocity oi the vapors, the size of the particles, and by other factors so as to convert the residual material absorbed within the powder into oil vapors suitable for further cracking treatment. If desired, additional heat may be supplied to the mixture of oil vapors and powder passing through the reactor 26.

In lieu of passing the total products from the vaporizing coil 2li to chamber 26, the vapors'may be iirst segregated from the powder and only the powder passed to the coking chamber 26. To this end a part or all of the products from the transfer line 25 may be passed through line 21 to a cyclone separator 28 or other equivalent separators for separating solids from gases. Vapors separated in cyclone separator 28 are removed overhead through line 29 and may be combined with the gas stream in line 2 I.

The powder separated in cyclone separator 28 containing the unvaporized residue absorbed therein discharges through conduit 30 into a hopper 3I from whence it is fed through pipe 32 into transfer line 25 leading to the coking vessel '26. When operating in this manner, a heating,

stripping and carrier gas from an external source may be used for carrying the powder through the coking vessel. Such gas may be introduced through line 33 and may comprise steam, carbon dioxide, nitrogen, spent combustion gases, or the like. If desired, a small amount of oxygen may be admitted with the carrier gas which may be used to burn a portion of the residual oil contained in the powder to supply additional heat.

According to the embodiment hereinbefore described, the powder for absorbing the unvaporizable residue is added to the oil and the resulting mixture passed through the vaporizing zone. In some cases it may be desirable to add the powder to the oil stream to absorb the unvaporizable residue after vaporization of the bulk of the oil has been accomplished. When operating in this manner, a part or all of the powder from standpipe I3 may be passed to the outlet of the vaporizing coil 24. For simplicity I have shown a branch line 3l yleading from the standpipe Il to the line 2'I. In practical operation it will be necessary to provide eithera gravity flow or some transfer means such as gas for carrying the powder to the line 21. In some cases the heat exchanger I2 may bein the form of a vaporizing coil. In such case the powder may be injected into the outlet of the heat exchanger and the resulting mixture passed through line 39 directly to the v cyclone separator 28.

As a further alternative the absorbent powder may be mixed with the residue after separation of a part or all of the vapors. For example, the powder may be passed from standpipe I3 through conduits 34 and 3B to the draw-oi! line 22 leading from the separator 2li.

The suspension of vapors and the finely-divided catalyst containing the coke resulting from the distilling and vaporizing treatment within the vertical tower 26'is transferred through line 31 and is admixed with gases and vapors in line 2l which leads to a main cracking chamber 39. However, before passing into the cracking chamber, an additional supply of fresh catalyst is mixed with the oil vapors and gases. This additional catalyst may be supplied from a standpipe 39 connected with vapor line .2|. The catalyst in standpipe 39 may be maintained in suitable fiuidized condition by introducing a suitable fluidizing gas through spaced inlet lines 40, 4| and 42. The amount of fresh catalytic material introduced through standpipe 39 may vary from 2`to 10 parts of catalyst per part of oil by weight. This catalyst is preferably of the same composition as the powder introduced into the fresh oil through line I3 since the two materials become intermingled.

The material introduced into the oil vapors Vin line 2l is preferably somewhat coarser than the material added to the fresh oil and may be of different chemical composition. In the latter case the material combined with the original oil will contain fines formed by attrition of the coarser catalyst combined with the oil vapors.

The cracking chamber 39 is preferably in the form of a vertical tower with vapors passing upwardly against the yforce of gravity. The velocity of the oil vapors passing through the cracking I chamber 39 is also preferably controlled so that the average time of passage of the catalyst powder er than the time required for the passage of the oil vapors.

The suspension of cracked products and catalyst after passing through the cracking chamber 39 passes through line 43 to a primary cyclone separator 44 wherein the bulk o f the powdered catalyst separates from the cracked vapors. The catalyst so separated discharges through line 49 to a catalyst hopper 46.

Vapors from the primary separator pass through line 41 to a secondary cyclone separator 48 wherein further separation of powder from vapors is accomplished. Powder from the second cyclone separator 48 discharges through line 49 into the catalyst hopper 46. Vapors from the secondary separator 48 may pass through line 50 to a third cyclone separator l wherein .inal purification and separation is carried out. Finely-divided catalyst from the third cyclone 5l discharges through line 52 to catalyst hopper 49. g

The cracked vapors after passing through the cyclone separators 44. 48 and 5l pass through line E3 to a fractionating tower 54 in which the products are fractionated to condense insuillciently cracked constituents as reflux condensate. Vapors remaining uncondensed in the fractionating tower 54 and consisting primarily of normally gaseous hydrocarbons and motor fuel vapors pass overhead through line 55 to a condenser |56 in winch the motor fuel vapors are condensed. Products from the condenser 58 pass to a product receiver 51 in which the liquid distillate separates from uncondensed gases. Gases separated in the receiver It are withdrawn therefrom through line Ilaand may be passed to a suitable gas recovery system (not shown) for removing entrained motor fuel products contained therein. The liquid motor fuel distillate separated in the receiver 51 may be removed therefrom through line i9 and may be subjected to any further finishing treatmentde.- sired for the production of the final market product.

Returning again to the catalyst hopper 49, the catalyst collected therein discharges into a standpipe I9 into which a fiuidizing medium may be introduced through one or more branch lines B9, Il and 92. The height of the standpipe I3 should 4be suilicient to develop a pressure at the bottom thereof adequate to feed the catalyst into a stream of air or other regenerating gas ing' troduced into the system through line 33. The standDlDe 59 is preferably provided with a suitablevalve 94 for controlling the amount of catalyst introduced into the regenerating gas. The mixture of regenerating gas and catalyst formed in line 63 is passed into a regenerating chamberf 65 which is also preferably in the form of a vertif cal tower through which thesuspension passes upwardly against lthe force of gravity. The velocity of the gas passing through thel regenerator 89 is controlled to maintain the powder within the regenerating chamber for a period suillcient to remove the carbonaceous deposits formed thereon during /the cracking operartion. After passing through the regenerating chamber 35, the suspension of regenerating gas and regenerated catalyst passes through line 86 to a primary cyclone separator 91 which serves to remove the bulk of the coarser particles of regenerated cata- I lyst. The catalyst separated from the regenerating gas in the primary cyclone separator 91 discharges into a catalyst hopper 68 from which itpasses into the standpipel 39 leading to the oil vapor line 2|.

In order to control the temperature within the regenerating chamber 85, it may also be desirable to cool a portion of the regenerated catalyst and recirculate it to the regenerating chamber to absorb heat therein. To this end a portion of the regenerated catalyst in the hopper 68 may be passed through standpipe 69 and discharged through valve 10 into a stream of air or other carrier gas introduced through line 1l. Standpipe 69 may be aerated by injecting a fiuidizing gas'by means of connections 40a, 4|a and 42a. The resulting mixture then passes through line 12 to a. cooler 13 in which the temperature of the suspension is cooled prior to being returned to the regenerating chamber 95.

'I'he gases from the primary cyclone separator 61 containing a portion of the finer catalyst particles pass through line 14 to a secondary cyclone separator ..15 in which further separation of entrained powder from the regenerating gas is accomplished.

Gases from the second cyclone separator 15 may be passed through line 16 to a third cyclone separator 11 for further purification. Gases from the third cyclone separator 11 may be passed through line 18 to an electrical precipitator 19 for final purication. Gases from the electrical preclpitator 19 may be vented from the system through li'ne 80. These gases may Ibe passed, if desired, to suitable heat recovery system for recovering energy in the form of heat or pressure before being rejected to the atmosphere.

The iiner portions of the catalyst separated in the secondary and tertiary cyclones and electrical precipitator are collected in a catalyst hopper 8i. A portion of this fine powder discharges into the standpipe I3 and is remixed with the fresh oil introduced into the process. Any excess of the fine powder collected in the hopper 8| may be discharged through standpipe 82 into the standpipe 39 leading to the oil vapor -stream in line 2|. For example, line B2 may be provided with an automatic valve 82 controlled by the level of catalytic material in the hopper 8| so as to maintain the catalyst level within the hopper at the desired point.

One of the features of the present invention is to utilize the finer portion of the catalyst powder for contacting with the fresh feed. By utilizing this portion of the catalyst, any inorganic ma-V terial present in the crude becomes deposited on the finer particles of the catalyst, and since this y material is first to be carried out of the system and lost, no permanent harm is done. For example, the amount of iine powder employed for admixing with the initial feed may amount to from 5 to 10% of the total amount of the material employed in the process. Consequently, any inorganic material present in the crude will be collected or accumulated on this relatively small proportion of the 'total catalyst used. If, for example, the amount of inorganic material is of the order of 50 pounds per thousand barrels of reduced crude, which is a typical example, and the rate of catalyst lost per day is of the order of one pound per barrel of oil treated, the amount of catalyst contamination rwith inorganic deposits will not exceed more than about 4% by weight on the catalyst. This contamination takes place only on the finest 5-10% of the particles in the system which are the first to be lost. Therefore, the main bulk of the catalyst remains uncontaminated. As a. result this contamination will not materially affect the activity of the catalyst for bringing about the desired reaction.

Another important advantage of the present invention is that the coke formed during the vaporization of the heavy residues does not collect on the walls of the heating coil and reaction chamber but is absorbed within a relatively small amount of the total catalyst employed in the process.

A further important feature of the invention is the fact that all of the heating is carried out in the presence of catalytic material so that the amount of thermal cracking is reduced to a minimum.

Having described the preferred embodiment of the invention, it will be understood that it embraces such other variations and modifications as come within the spirit and scope thereof.

What is desired to be protected by Letters Patent is:

l. A process for the conversion of residual oils containing constituents unvaporizable without substantial decomposition under normal pressure conditions which comprises adding to Said oil an absorptive powder in an amount sufficient to completely absorb said unvaporizable constituents and form discrete particles of non-adhesive powder, passing the resulting mixture through a heating zone, heating said mixture during passage through said zone to a temperature sufficient to convert said oil into vapors and coke, thereafter passing the resulting mixture of vapors and powder containing said coke through a cracking zone, contacting said mixture within said cracking zone with a cracking catalyst separate and independent from said first-named absorptive contact material to crack said vapors into motor fuel constituents and segregating the motor fuel from the cracked products.

2. A process fer the conversion of residual oils containing constituents unvaporizable without substantial decomposition which comprises adding to said residual oil an absorptive powder in an amount suflicient to completely absorb said unvaporizable constituents and form discrete particles of non-adhesive powder, passing the resulting mixture through a heating zone, heating said mixture during passage therethrough to a temperature sufficient to convert said oil into vapors and coke, thereafter passing the mixture of vapors and powder containing said coke from said heating zone to a cracking zone, admixing with the stream of products passing through said cracking zone fresh cracking catalyst in powder f orm. maintaining said mixture within said cracking zone for a period suicient to obtain substantial cracking of said vapors, thereafter separating the powder from said cracked products and fractionating the cracked products to separate a motor fuel therefrom.

3. In .the process defined in claim 2, the further improvement which comprises subjecting the powder separated from said cracked products to a regenerative treatment to remove carbonaceous and coke deposits formed thereon during the heating, coking yand cracking treatments. thereafter segregating the regenerated powder into a relatively coarse fraction and a relatively finer fraction, passing said coarser fraction to said cracking zone and mixing said finer fraction with said first-named residual oil.

4. A process for the conversion of residual oils containing constituents unvaporizable without substantial decomposition under normal pressure conditions which comprises adding to said oil an absorptive powder in an amount sufficient to completely absorb said unvaporizable constitu- 'ents to form discrete particles of non-adhesive powder, passing the mixture of powder and oil `upwardly Athrough an elongated heating zone maintained at a temperature suilicient to convert said oil into vapors and coke, regulating the velocity of the oil passing through said heating zone to maintain said powder therein for a period sufiicient to completely convert said unvaporizable constituents into vapors and coke, thereafter passing the resulting mixture of vapors and powder containing said coke through a cracking zone, contacting said mixture Within said cracking zone with an additional cracking catalyst in powder form, maintaining said vapors within said cracking zone for a period sufficient to produce substantial cracking thereof, thereafter separating the powder from said cracked products and fractionating the cracked products to segregate a. motor fuel fraction therefrom.

5. A process for the conversion of residual oils containing vaporizable constituents and constituents unvaporizable without substantial decomposition under normal pressure conditions which comprises adding to said oil an absorptive powder in an amount sufficient to completely absorb said unvaporizable constituents, passing the resulting mixture through a vaporizlng zone maintained at a temperature sufficient to vaporize said vaporizable constituents, thereafter passing the powder containing the unvaporizable constituents absorbed thereon through a heating zone maintained at a temperature sufficient to decompose said unvaporizable constituents into vapors and coke, thereafter passing said mixture of vaporc and powder containing said coke through a .the vapors formed in said vaporizing zone with the vapors and powder formed in said heating zone prior to passing the latter to said cracking zone.

7. In the process defined by claim 5, the further improvement which comprises separating products removed from said vaporizing zone into vapors and powder and passing said powder to said heating zone.

8. In the process defined in claim 5, the further improvement which comprises separating the products from said vaporizing zone into vapors and solids, suspending said solids in an extraneous gas and passing the resulting mixture of powder and gas through said heating zone.

9. In the process defined in claim 5, the further improvement which comprises separating the products from said vaporizing zone into vapors and powder, mixing said powder so separated with an oxidizing gas, passing the resulting mixture of oxidizing gas and powder through said heating zone and controlling the oxygen concentration of said oxidizing gas to partially burn-unvaporizable constituents absorbed on said powder to supply additional heat to said heating zone.

10. A process for the conversion of residual oils containing vaporizable constituents and constituents unvaporizable without substantial decomposition under normal pressure conditions which comprises adding to said oil an absorptive powder in an amount sunicient to completely absorb said unvaporizable constituents, passing they resulting mixture of powder and oil through avaporizing zone, controlling the temperature in said vaporizing zone to vaporize said vaporizable constituents, thereafter passing the products from said vaporizing zone through an additional heating zone, maintaining said heating zone under temperature conditions sufficient to decompose unvaporizable constituents absorbed on said powder into vapors and coke, passing the products from said last-named heating zone through a. cracking zone, contacting the products in said last-named cracking zone with an additional cracking catalyst in powder form but composed of coarser particles than said first-named absorptive powder, maintaining the oil vapors within said cracking zone for a period sumcient to produce a substantial cracking into motor fuel constituents, thereafter segregating the powder from the cracked products, fractionating the cracked products to separaie a motor fuel fraction, regenerating the powder so separated to remove combustible deposits formed thereon during the heating and cracking treatments, segregating the regenerated powder into a relatively coarse fraction and a relatively of said oil, passing the resulting mixture `of oil ,and powder through a vaporizing zone, maintaining said mixture within said zone for a period sumcient to vaporize said vaporizable constituents and form a stream of vapors and relatively dry powder, separating the powder from the oil vapor, passing the resulting powder containing the unvaporizable constituents absorbed thereon through a coking zone, maintaining said coking zone at a temperature sumcient to convert said unvaporizable constituents absorbed in said powder into vapors and coke, combining said lastnamed vapors with vapors liberated during said varporing treatment,'passing the resulting vapors through a cracking zone, contacting said vapors with additional cracking catalyst in powder form within said last-named cracking zone, separating the powder from said cracked products, fractionating the cracked products to separate a'motor fuel fraction therefrom, subjecting said powder to vregenerative treatment to remove combustible deposits formedthereon during said coking and cracking treatment.' combining a portion of said regenerated catalyst with said fresh residual oil and passing the remainder thereof to said cracking zone.

12. In the process deilned by claim 11, thev I further improvement which comprises passing said powder upwardly through said coking zone and controlling the velocity of the stream passing through said coking zone to retain said powder therein for a period sufilcient to convert said unvaporizable constituents into vapors and a coke residue.

18. A process for the conversion of residual oils containing vaporizable constituents and constituents unvaporizable without substantial decomposition which comprises heating said oi1` to vaporize a portion thereof, thereafter adding a finely-divided absorbent powder to said heated oil ilne fraction, mixing said iine fraction with said i in an amount suilicient to completely absorb the unvaporizable constituents th'ereof and form a suspension of vapors and powder containing the unvaporized oil, separating the last-named vapors from the powder, passing the powder so separated through a cokingv zone, maintaining said coking zone at a temperature suicient to convert said unvaporized oil absorbed in said powder into vapors and coke, passing the vapors and powder containing said coke through a cracking zone independent of said coking zone. contacting said mixture within said cracking zone with an active cracking catalyst, maintaining said cracking zone at a temperature suicient to convert a substantial portion of said vapors into motor fuel constituents and lthereafter separating the motor fuel fraction from the cracked products.

14. In the process defined by claim 13, the further improvement which comprises combining the vapors separated from said powder prior to said kcoking treatment with the vapors and powder passing to said cracking zone.

. 15. A process for the conversion of residual oils containing constituents unvaporizable without substantial decomposition which comprises mixing said oil with a finely-divided absorbentpowder, adding suiiicient powder to said oil to completely absorb all unvaporizable constituents contained therein, heating said oil while in contact with said powder to convert said oil into vapors and a solid residue, passing the resulting vapors and said powder containing the solid residue through a, cracking zone, contacting said mixture while in said cracking zone with' an active cracking catalyst in finely-divided form, maintaining said oil vapors within said cracking zone for a period suiiicient to convert a substantial portion thereof into motor fuel, thereafter separating the cracked products from the catalyst and powder, and fractionating the cracked products to segregate a motor fuel fraction therefrom.

16. In the process defined by claim 15, the further improvement which comprises regenerating the finely-divided absorbent powder and catalyst separated from the cracked products and remixing the regenerated material with the oii undergoing treatment. A 17. In the process defined by claim 11, the further improvement which comprises contacting said powder within the coking zone with an oxidizing gas to maintain said temperature.

18. In a process for cracking residual oils containing inorganic impurities in the presence of a finely divided catalyst having different particle sizes wherein the catalyst is continuously circulated through a cracking zone in which it is contacted with oil to be cracked and through a regenerating zone in which it is contacted with a regenerating gas to burn combustible deposits therefrom, the catalyst being subsequently separated from the regenerating gas, and wherein a portion of the finer particles of said catalyst is continuously expelled from the system; the meth'- od of reducing the amount of inorganic impurities accumulated on said catalyst being circulated which comprises separating the regenerated catalyst into a coarser fraction and a ilner fraction, initially contacting said residual oil containing said impurities with at least a portion of said ilner fraction in an amount sufficient to absorb the higher boiling constituents of said residual oil containing said inorganic impurities, and thereafter contacting said oil with said coarser frac'- tion of regenerated catalyst while at active cracking temperature and for a period suillcient to 20 obtain the desired cracking thereof.

CHARUES W. TYSON.

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