US2844518A

US2844518A - Conversion of hydrocarbons

Info

Publication number: US2844518A
Application number: US416462A
Authority: US
Inventors: Forrest H Blanding; Walter J Schmidt
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1954-03-16
Filing date: 1954-03-16
Publication date: 1958-07-22
Anticipated expiration: 1975-07-22
Also published as: FR1125479A

Description

July 22, 1958 F. H. BLANDING ET AL 2,844,518

' CONVERSION OF HYDROCARBONS Filed March 16, 1954 FORREST H. sunomc INVENTORS BY ATTORNEY WALTER J. SCHMIDT United States Patent CONVERSION OF HYDROCARBONS Forrest H. Blanding and Walter J. Schmidt, Cranford,

Application March 16, 1954, Serial N 0. 416,462

5 Claims. (Cl. 196-50) This invention relates to the catalytic cracking of hydrocarbons to produce lower boiling hydrocarbons such as motor fuels.

.During the catalytic cracking of hydrocarbons, cycle oil is separated from the reaction products and it has been known to recycle part or all of this cycle stock to the catalytic cracking reactor. Recycle cracking on some units has given very poor results, resulting in lower gasoline and higher carbon or coke yields than are obtained by once-through cracking. It has now been learned that by a particular choice of operating conditions and recycle stock, extremely advantageous results can be obtained for recycle cracking.

One of the requirements of this invention is that a hydrocarbon oil which is to be recycled is a material within the boiling range of the feedstock and more specifically that only material is recycled which b'oils between the and 80% distillation temperatures of the feedstock. Material boiling above the final boiling point of the feedstock and in fact material boiling much above the 80% distillation temperature of the feed will be-a highly aromatic material which produces many times more carbon or coke as does material boiling within the smaller range of 1080% of the feedstock. Also material boiling below the initial boiling point of the feed and most material boiling lower than the 10% point of the feed is also quite aromatic and refractory.

Another essential feature of this invention is that the operating conditions be selected to be a cracking severity suflicient to produce no greater than 40-50% by weight conversion, preferably 40 to 49% by weight conversion, to coke and to gasoline and lighter products based on the fresh oil feedstock assuming no recycle was used. It has been found that catalytic craokin'gproduces high relatively uniform selectivity to gasoline when cracking to conversion levels of about 40% or less by weight. When proceeding to :higher conversion :levels and particularly to those above 50% by weight, unde'sirable :re-cracking of the gasoline to carbon or coke and 'gas takes place. By keeping the conversion level on the'fresh feedstock portion of the-total feed .to lower thanabout 50% by weight and, preferably about-40% to 49% by weight, this recracking of gasoline can be avoided.

The cycle stock above referred to is carefully fractionated to recover a fraction corresponding "to '10-"80% of the distillation range of the fresh oil feed-or boiling within this range and theresulting fraction is "the one recycled in the present process. Selecting such a fraction, it is easily possible in the presentinvention byproper control of operating conditions to recycle all :of this material to extinction.

It has been found that the proper severity of operation can automatically be found by the us'e of equations giving the amount of recycle oil as volume percent of fresh oil feed and taking into consideration 'the'weight percent of carbon in the aromatic-ring hydrocarbons of'the fresh feed and the volume percent of the freshfoilfeed boiling in the range of the fraction which is being recycled.

According to this invention conditions are given for producing the highest amount of gasoline per unit of coke produced when recycling relatively large amounts of a selected recycle stock. Most catalytic cracking plants are limited in their ability to burn coke or carbon from the catalyst and with the present invention more gasoline per unit of coke is produced and even though a larger amount of gasoline is produced there is no additional amount of coke or carbon laid down and hence there is no additional burden on the regenerator. This is in. direct contrast to recycling of a bottom fraction which produces a lower ratio of gasoline to coke than the once-through operation.

The essence of the invention is the outstanding improvement obtained and only obtained when all three basic requirements are met. First, the boiling range of the recycle oil must be in the correct range. Second, the severity of the operation must be carefully selected not to overcrack the oil feed stock on the first cracking pass. Third, the recycle rate for cracking to extinction must be in the correct range, dependent on the feed stock, and the recycle stock boiling'range. Provided the first requirement is met, the formulas to be given hereinafter for describing the recycle rates will automatically fix the second important requirement and this is an important feature or this invention.

To describe the latter important feature further, ithas been found that there is a very definite relationship between the severity of the cracking operation and the amount of recycle oil required to crack to extinction all material within a fixed boiling range, In order to increase recy-cle, once all cycle stock in a given boiling range is being recycled, it is necessary to decrease the cracking severity. By thus decreasing the cracking severity per pass, it is possible to achieve higher recycle rateswithont increasing the boiling range of the material recycled.

However, it is the important objective of this invention to achieve the proper recycle rate. If severity is maintained too low, say in the order of 25-30% equivalent conversion in a once-operation, excessive-recycle will be required. This will be costly, and have little benefit in product value. If recycle is too low, cracking severity will in turn be too high and this will result in overcracking and poor product distribution. Maintaining the recycle rate .in

accord with the formulas will thus fix the cracking severity in the proper range.

In the drawing there is shown one form of apparatus adapted to carry out the present invention.

Referring now to the drawing, the reference character 10 diagrammatically designates a reactor for carrying out catalytic cracking of hydrocarbons such as gas oil, .certain crude oils and other relatively high boiling hydrocarbon stocks, preferably boiling between 600 and L000- 1l-00 F. The oil, preferably in a preheated condition, is introduced into the reactor through line 12. Hot regenerated catalyst at a temperature of about 85.0-1l50 F. is introduced into line 12 to line 14. In a fluid catalyst operation the catalyst to oil ratio fed to the reactor 10 may vary between about 5 to 20. The temperature in .the reactor 10 may be between about 850,and 10.00" F. In a fluid process the weight of oil per ,hour .per weight of catalyst (which is expressed as w./hr./w.) in the re-.

actor is usually between about 1 and 10, but may be higher. The catalyst is preferably silica alumina .catalyst but other cracking catalysts such as silica-magnesia, .acid treated bentonitic clays etc. may be used. In fiuidoperations the catalyst is finely divided with the majority of the particles being between about 20 and ,microns.

Ground or microspherical catalyst particles of different size ranges'may'be used. The present invention ,may also be used with fixed bed or moving bed processes.

-Provisi'on=is also madefor recyclinga selected fraction to the reactor IOthroug'h-line 16. It is to be understood that showing of the introduction of oil and catalyst is diagrammatic and other points of introduction of these materials may be used. The amount and selection of the recycle oil passing through line 16 will be hereinafter described in greater detail. Spent catalyst from the reactor is passed through line 18 to regenerator 22 wherein the temperature is maintained between about 1000 F. and 1200 F. to burn off carbonaceous deposits or coke from the catalyst and the regenerated catalyst is then returned to the reactor through line 14.

The. cracked reaction vapors pass overhead from the reactor 10 through line 24 and pass to the lower portion of fractionating tower 26 wherein the reaction products are fractionated to recover desired products.

The vaporous reaction products are fractionated in the fractionator 26 to separate a bottoms fraction which is withdrawn through line 28 and may be discarded from the system via line 30 or may be all or in part passed through line 32 to a second fractionating tower 34. A cycle oil fraction is withdrawn further up the fractionating tower 26 through line 36 and passed to second fractionating tower 34 for further fractionation as will presently be described in greater detail.

A heating oil fraction is preferably withdrawn further up through the tower 26 from trap out tray 38 and through line 42. The overhead vapors are passed through line 44 and through condenser 46 and then through gas-liquid I separator 48 for separating liquids from gases. The gas is withdrawn overhead through line 52 and the liquid is withdrawn from the bottomvof the separator 48 through line 54 and from the system through line 56 as a gasoline cut. A portion of the liquid may be recycled as reflux to the top of the fractionator 26 through line 58 by pump 62.

Returning now to second fractionating tower 34, the cycle oil to be recycled is a carefully selected fraction boiling between about 650 and 9001000 F. depending on the boiling range of the fresh oil feed. The cycle oil introduced into second fractionating tower 34 through lines 32 and 36 is further fractionated to separate a bottoms fraction containing the highest boiling constituents. The bottoms fraction is withdrawn from the system through line 66 and may be used as fuel oil or for any other desired purpose. During fractionation in tower 34 the highest and lowest boiling constituents are separated and the cycle oil to be recycled is withdrawn from trap out tray 67 at an intermediate portion of fractionator 34 through line 68 and recycled to reactor 10 through line 16. A portion of this recycle oil may be withdrawn from the system through line 70, if desired. Lighter vapors from fractionator 34 pass overhead through line 72 to main fractionating tower 26.

Where the fresh oil feed has a boiling range of about 600 to 1050 F., the boiling range of the selected cycle oil stock to be recycled is about 675900 F. Where the fresh oil feed has a boiling range of about 800 to 1100 F., the boiling range of the cycle oil stock to be recycled is about 930 to 1020 F. The cycle oil to be recycled is selected to have a boiling range between the 10% and 80% boiling points of the fresh oil feed or virgin charging stock.

As above pointed out the present invention provides a method of producing the highest amount of gasoline per unit of coke produced. According to the present invention it is desirable to maintain total conversion or conversion to coke, gasoline and lighter products between about 40% and 50% by weight and preferably below 49% by weight on the fresh oil feed, assuming no recycle was used, or in other words on a once-through basis. Under these conditions the amount of cycle oil recycled may amount to between about 40% and 250% by volume on the fresh oil feed. Under these conditions superior production distribution is obtained in heating oils and naphtha yields, and cycle oil stock quality is improved.

The low reactor severity to get low conversion is obtained by having a low reactor catalyst holdup in fluid catalyst operations.

It has been found that there is relationship between the weight percent of carbon in the aromatic ring hydrocarbons in the fresh feed and the amount of cycle oil which should be recycled in order to convert all of the cycle oil into more valuable cracked products, while maintaining superior yields. The following equations give the amount of recycle oil as volume percent of fresh feed, the first equation giving the minimum recycle and the second equation giving the maximum recycle, for recycling cycle stock to extinction.

The percent by volume of recycle oil (minimum) as volume percent of fresh feed where A is the weight percent of the total carbon which is present in aromatic ring hydrocarbons in the fresh feed and B is the volume fraction of the fresh oil feed boiling in the range which is being recycled.

Recycled oil as volume percent on fresh oil feed (maximum) the nomenclature being the same in the two equations. The value of A may vary between about 15 and 30'and the value of B may vary between about 0.3 and 0.7.

As one example for comparing the present invention with a once-through operation the following results are given. A once-through operation gave about gasoline for 5 weight percent of coke so that the ratio of gasoline per unit of coke was 10. A recycle operation conducted in accordance with the above principles produced 57.7% gasoline and only 3.8% coke giving a gasoline to coke ratio of 15. This is an increase of over 50% in the amount of gasoline produced per unit of carbon produced. As most catalytic cracking plants are limited in their ability to burn coke formed on the catalyst this is a most important advance in the catalytic cracking art. In contrast to the results obtained by recycling in the present invention, it is noted that the conventional type of recycle of a bottom fraction boiling above about 800 F. produced a lower gasoline to coke ratio than a oncethrough operation.

The weight percent of carbon in the aromatic ring hydrocarbon compounds in the fresh feed is obtained by a method which is a modification of Dr. Fenskes method reported in the Journal of Institute of Petroleum, volume 36, No. 322, October, 1950. Fenske separates the aromatic containing molecules from other hydrocarbons using silica gel and then obtains the percentage of carbon atoms actually in aromatic ring structure in the aromatic fraction, based on measurements of refractive index and other simple physical measurements of this fraction. While the Fenske method is not entirely precise for analyzing aromatics in petroleum, the Fenske values are closely related to values obtained by more precise but complex and costly methods and thus are useful for the purpose of this invention.

Using the equations above given, the minimum and maximum amounts of recycle will be given for example of a cycle oil stock separated from catalytically cracked products and having a boiling range of about 650 to 900 F. The gas on fresh feed is a West Texas Gas oil of 22% by weight of carbon in the aromatic hydrocarbon rings which has boiling between the 650 and 900 F. temperature levels. Aromatic rings in this example and in all succeeding examples herein disclosed are determined by the previously described Fenske method. Thus A is 22 weight percent andB is 0.7. Using these values in the equations it will be found that the minimum amount of recycle which may be used is about 70% on the fresh oil feed and 128% recycle on the fresh. oil feed as the maximum. Any value between these two limits may be used.

For different feed stocks the minimum recycle rates may vary from about 50 to 72%.

As a specific example a mixed sweet gas oil of Gulf coast origin having the following properties was used:

24.3 API gravity:

Initial boiling point 600 F- at 675 F. at 722 F. at 965" F. at 1000" F. Final boiling point 1050 F. Weight percent of carbon in the aromatic ring compounds of this feed equals 18 In the specific example the boiling range of the cycle oilstock from a. catalytic cracking operation was about 675-900 F. Thefollowing data compare once-through 20 operation (no recycle) with ordinary. recycle operation and with the process as carried out following the teachings of the present invention.

quires a. favorably low amount. of catalyst to effect the the results obtained with the process of thepresent inven-.

tion. In operation 5 the boiling range of the recycle is specific, but the recycle rate is too low, being 23% as compared with the 54% minimum amount for this feed stock required by the invention. Results show only a very small gain in gasoline ratio of from 10.8 to 11.5. This compares with a gain of from 10.8 to 22 when using the process of the present invention. Operation 6 shows" that merely operating at recycle rates in the same range as that of the invention will not give improved results. In operation 6 the entire cyclestock boiling within the range of the feed is recycled at the same recycle rate as in operation 4. Yet gasoline to coke ratio is only 9.8, an order of magnitude lower than the 22 of operation. 4.

Table 1 Operation l. 2 3 4 5 6 7 Type Ordinary Recycle This In- Recycle Recycle This Invention ventlon Vol. percent Recycle on Fresh Feed. 5 33 73. 5 23. 6 74.0 55 Boiling Range of Recycle, F 844-1, 650-1, 100 675-900 675-900 650-1, 100 675-900 Yields of Products: I

Gas, wt. percent 8. 7 9.0 11.1 7. 6 10. 9 10.1 7.8 Vol. percent Motor Gasoline, O4-430 F. on

Fresh feed 53. 4 .51. 7 55. 2 62. 1 68. 8 54.8 59. 6 Vol. percent Heating Oil (430-650 F.), on

Fresh feed 16. 9 17.0 19. 7 22. 2 18. 5 19. 7 21. 6 Coke, wt. percent on fresh feed 4. 9 4.9 7.1 2. 8 5. 1 5. 6 2. 9 Ratio, Gasoline/Coke 10.0 10.6 7.8 22 11.5 9.8 20.7 a Total Conversion by wt. to coke, gasoline and lighter products 60 59 66 64 67 03 63 Total Conversion by weight to coke, gasoline and lighter products calculated on the fresh feed component on a once-through basis 60 56 53 45 59 44 47 Operating Conditions:

Temperatur 930 930 930 930 930 930 930 Catalyst/Oil ratio by wt. 10 10 10 10 10. 2 10. 1 11. 1 rJW 1- 3 2.0 1. 9 12.0 1. 94 12.0 11. 5

In the above table it will be noted that operations 2 and 3 are ordinary recycle operations known in the industry and both include bottoms fraction recycle. In operation No. 2 the conversion is lower when conducted to the same coke yield as is also the gasoline for the same coke. Adding more recycle oil raises the coke further giving a little more gasoline due to cracking of cycle stock. Also higher conversion is obtained. However, the ratio of gasoline to coke is no higher for the recycle operation in operations 2 and 3. Comparing operations 1 and 4 show 4 to give an outstanding advantage.

In operation No. 4 the recycle oil fraction was carefully selected and fractionated to have a boiling range between 675900 F. (within 10-80% boiling range of fresh feed).

The percent by volume of the fresh feed boiling in the range of the cycle oil which is to be recycled is about-- 60%, so that B is 0.6. The formula calculates about 54% recycle as the lower limit and 100 as the upper limit. The actual recycle used in operation No. 4 was 73 which is about in the middle of the range. The gasoline to coke ratio was more than double of any of the operations 1, 2 and 3.

it will be noted that there is a drastic change in reactor severity or catalyst holdup in the operation No. 4 as compared to the other operations. The reactor catalyst holdup corresponding to w./hr./w. is 12 in operation 4 (low reactor severity) comparedto not more than 2 in operations 1, 2 and 3. Because higher w./hr./w. values mean less catalyst in the reactor, operation4 also re- The extremely critical nature of the selection of the cycle stock is emphasized by operations 4 and 6. Also, it should be noted that heating oil yield is higher in operation 4 than in operation 5' or 6, an additional advantage for operation 4. Operation 6also shows that merely the specification of recycle of material in the boiling range of.

the feed is not adequate to gain the advantage. Excese sively high recycle ratios offer no further advantage and are costly. The: minimum recycle rate is exceedingly important. Operation 5 shows the loss for operating at 23.6% recycle which is 30% below the lower limit to be very serious. Yet operation 7, at 55% recycle, just within the specification, shows a gasoline ratio nearly as high as operation 4.

The important point of theinvention is the surprisingly higher gasoline-coke ratios obtained when operated in critical accord with the invention, as compared with values of only about half as great when deviating from the teachings of the invention only to an. extent which might ordinarily be considered minor.

Because aromatic hydrocarbon compounds are difficult to crack, somewhat higher recycle ratios are required todefine the critical condition for more aromatic-feed stock. Virgin gas oils usually range between 15% and 30 wt. percent carbon inthe aromatic ring hydrocarbons by the Fenske method, thus. the minimum recycle for such extremes mightvary from 51-72%. i

The invention is not to be limited to merely the recycle of fractions in the. range of 675-900F'F. as in the foregoing examples or operations. The proper fraction to select to obtain these excellent recycle results varies with the feed stocks. This is further illustrated by the following data. A feed stockboiling from about 800 to 1100 F. was catalytically cracked on a once-through basis. The cycle stock from this operation was then fractionated into narrow boiling fractions and these fractions were catalytically cracked to examine the quality of the material for further cracking as would be done in a recycle operation. The results when cracking all stock at the same Fraction Low, boiling between 730 and 830 F., gave a gasoline/coke ratio of only 7.4. This fraction boils within the same range shown to be very favorable in the preceding Examples 4 and 7. However, it is not in accord with the principles of the invention, as in this case fraction Low boils below the of the feed, even though it does boil partly in the same range as thefee/d. Fraction Medium boiling within the 10-80% temperatures of the feed gives an outstandingly better result of 12.3 gasoline/ coke ratio. Fraction High boiling above the 80% of the feed, gives a gasoline/ coke ratio of only 2.9.

The above results represent only once through cracking results on the cycle stocks. In a recycle operation the fractions are cracked once, then again, and again, until they are finally destroyed. Thus, in a recycle operation, the differences shown in cracking the above fractions Low, Medium and High would be accentuated even more. The above example emphasizes the critical nature of the factors affecting the quality of the cycle stock for further cracking. In a recycle operation, operated in accordance with the previous formulas, the recycle of stock Medium would produce a gasoline/coke ratio of about 22. Fraction High, on the other hand, would produce a gasoline/ coke ratio of less than 5. This shows strikingly how important it is to recycle the proper boiling range material.

What is claimed is:

1. In a catalytic cracking process wherein virgin hydrocarbon oil having a boiling range of about 6001050 F. is cracked in the presence of a catalyst and the vaporous cracked products are continuously fractionated into gasoline, heating oil, cycle oil and bottoms, and cycle oil is to be continuously recycled to the cracking step and the selected cycle oil fraction is cracked to extinction, the

improvement which comprises carrying out the cracking step at a relatively low total conversion to coke, gasoline and lighter products below about 49% by weight calculated on a once through basis, continuously fractionating the cracked products to recover a cycle oil fraction boiling between about 675 and 900 F., recycling at least 50% by volume of said selected cycle oil fraction on the virgin oil feed to the cracking step, the amount of recycle oil being related to the weight per cent of total carbon in the rings of the aromatic ring hydrocarbon compound content of the virgin oil feed and also to the amount of virgin oil feed corresponding in boiling range to that of said selected recycle oil fraction, the amount of recycle oil for a selected set of conditions increasing as the Weight percent total carbon in the rings of the aromatic ring hydrocarbon compound content increases in the virgin oil feed.

2. In a catalytic cracking process wherein a hydrocarbon feed selected from the group consisting of fresh and virgin hydrocarbon is continuously cracked in the presence of catalystand the vaporous cracked products are 8 fractionated into gasoline, heating oil, cycle oil and bottoms, and cycle oil is to be recycled continuously to said cracking step and cracked substantially to extinction, the improvement which comprises carrying out the cracking step at a relatively low total conversion to coke, gasoline and lighter products below about 50% by weight calculated on a once-through basis, continuously fractionating the vaporous cracked products to recover a cycle oil fraction boiling between the 10 and 80% boiling points of the fresh oil feed, continuously recycling said cycle oil fraction to said cracking step and cracking it to extinction, the minimum amount of cycle oil fraction as percent by volume on fresh feed oil beingdetermined according to the expression where A is the weight percent of total carbon in the rings of the aromatic ring hydrocarbons in the fresh oil feed and B is the volume fraction of the fresh feed oil boiling in the range of the cycle oil being recycled.

3. A method according to claim 2 wherein A may vary between about 15 and 30 and B may vary between about 0.3 and 0.7.

4. In a continuous catalytic cracking process wherein a hydrocarbon feed selected from the group consisting of fresh and virgin hydrocarbon oil having a boiling range of about 600l050 Ffis cracked in the presence of a catalyst and the vaporous cracked products are continuously fractionated into gasoline, heating oil, cycle oil and bottoms, and cycle oil is to be continuously recycled to the cracking step, the improvement which comprises carrying out the cracking step at a relatively low total conversion to coke, gasoline and lighter products of about by weight calculated on a once-through basis, continuously fractionating the cracked products to recover a cycle oil fraction boiling between about 675 and 900 F. continuously recycling said cycle oil fraction to the cracking step and cracking it to extinction, the minimum amount of recycle oil as percent by volume on fresh feed oil being determined according to the expression and the maximum amount of the recycle oil as percent by volume on the fresh feed oil being determined according to the expression where A is the weight percent of total carbon in the rings of the aromatic ring hydrocarbons in the fresh oil feed and B is the volume fraction of the fresh feed oil boiling in the range of the cycle oil being recycled and where A varies between about 15 and 30 and B varies between about 0.3 and 0.7.

5. In a catalytic cracking process wherein virgin gas oil having a boiling range of about 600 F. to 1050 F. is cracked in the presence of a silica-alumina catalyst and vaporous cracked products are continuously fractionated into gasoline, heating oil, cycle oil and bottoms and cycle oil is continuously recycled to the cracking step, the improvement which comprises carrying out the cracking step at a relatively low total conversion to coke, gasoline and lighter products at a w./hr./w. of about 12, continuously fractionating the cracked products to recover a cycle oil fraction boiling between about 675 F. and 900 F., recycling at least about by volume of said selected cycle oil fraction on the virgin gas oil feed to the cracking step and cracking the selected cycle oil fraction to extinction.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS 2,245,151 Pier June 10, 1941 2,332,794 Hil-l Oct. 26, 1943 5 2,340,960 Hemminger Feb. 8, 1944 10 2,342,090 Saohanen et a1. Feb. 15, 1944 2,436,257 Hansfor d et -al. Feb. 17, 1-948 OTHER REFERENCES Brown at 211.: Ind. Eng. Chem. 38, 136 (1946). Brooks et al.: The Chemistry of Petroleum Hydrocarbons, vol. 2, page 185, 186 (1 955).

Claims

1. IN A CATALYSTIC CRACKING PROCESS WHEREIN VIRGIN HYDROCARBON OIL HAVING A BOILING RANGE OF ABOUT 600*-1050*F. IS CRACKED IN THE PRESENCE OF A CATALYST AND THE VAPOROUS CRACKED PRODUCTS ARE CONTINUOUSLY FRACTIONATE INTO GASOLINE, HEATING OIL, CYCLE OIL AND BOTTOMS, AND CYCLE OIL IS TO BE CONTINUOUSLY RECYCLED TO THE CRACKING STEP AND THE SELECTED CYCLE OIL FRACTION IS CRACKED TO EXTINCTION, THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE CRACKING STEP AT A RELATIVELY LOW TOTAL CONVERSION TO COKE, GASOLINE AND LIGHTER PRODUCTS BELOW ABOUT 49% BY WEIGHT CALCULATED ON A ONCE THROUGH BASIS, CONTINUOUSLY FRACTIONATING THE CRACKED PRODUCTS TO RECOVER A CYCLE OIL FRACTION BOILING BETWEEN ABOUT 675* AND 900*F., RECYCLING AT LEAST 50% BY VOLUME OF SAID SELECTED CYCLE OIL FRACTION ON THE VIRGIN OIL FEED TO THE CRACKING STEP, THE AMOUNT OF RECYCLE OIL BEING RELATED TO THE WEIGHT PER CENT OF TOTAL CARBON IN THE RINGS OF THE AROMATIC RING HYDROCARBON COMPOUND CONTENT OF THE VIRGIN OIL FEED AND ALSO TO THE AMOUNT OF VIRGIN OIL FEED CORRESPONDING IN BOILING RANGE TO THAT OF SAID SELECTED RECYCLE OIL FRACTION,M THE AMOUNT OF RECYCLE OIL FOR A SELECTED SET OF CONDITIONS INCREASING AS THE WEIGHT PERCENT TOTAL CARBON IN THE RINGS OF THE AROMATIC RING HYDROCARBON COMPOUND CONTENT INCREASES IN THE VIRGIN OIL FEED.