US3776838A

US3776838A - Catalytic cracking of naphthas

Info

Publication number: US3776838A
Application number: US00077480A
Authority: US
Inventors: D Youngblood; D Raynolds
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1970-10-02
Filing date: 1970-10-02
Publication date: 1973-12-04
Anticipated expiration: 1990-12-04
Also published as: DE2148121B2; NL7113477A; CA966795A; FR2111650B1; FI54139C; GB1367306A; JPS51962B1; DE2148121C3; BE773099A; ZA715955B; FR2111650A1; AU458343B2; FI54139B; ES395549A1; IT945974B; AU3323971A; AT310909B; DE2148121A1

Abstract

LOW OCTANE NAPHTHA AND A RECYCLE STREAM OF FULL RANGE NAPHTHA OR HEAVY NAPHTHA ARE CATALYTICALLY CRACKED WITH A ZEOLITE CRACKING CATALYST IN SEPARATE ELONGATED REACTION ZOENS YIELDING PRODUCTS WHICH INCLUDE C4 AND LIGHTER HYDROCARBONS FOR ALKYLATE FEEDSTOCK, C5 AND LIGHTER HYDROCARBONS FOR PETROCHEMICA MANUFACTURE AND NAPHTA HAVING AN INCREASED OCTANE RATING. OPTIONALLY, EITHER OR BOTH OF THE FEEDSTOCKS BEING CRACKED MAY BE SUBJECTED TO FURTHER CRACKING IN A DENSE FLUIDIZED BED OF THE ZEOLITE CATALVST.

Description

De- 4, 1973 D. J. YOUNG-BLOOD ET Alv CATALYTIC CRCKING OF NAPHTHAS Filed oct.

@QQ w United States Patent t 3,77 6,838 CATALYTIC CRACKING F NAPHTHAS Douglas J. Youngblood, Groves, and David L. Raynolds, Nederland, Tex., assignors to Texaco Inc., New York,

Filed Oct. 2, 1970, Ser. No. 77,480 Int. Cl. Cg 37/02 U.S. Cl. 208-74 9 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATED APPLICATION This is related to application Ser. No. 79,479 which discloses the fluid catalytic cracking, in risers, of low octane naphtha in combination with other feedstocks BACKGROUND OF THE INVENTION This invention relates to the catalytic cracking of hydrocarbons. In particular, this invention relates to the fluid catalytic cracking of naphtha having a low octane rating and boiling in the range of 100 to 450 F.

Gasoline is frequently blended from stocks, including naphtha, the octane rating of which has been increased through catalytic reforming. Both virgin and cracked stocks -may be upgraded by reforming operations. Catalytic reformers are usually operated to provide the desired octane improvement with the least conversion of gasoline to saturated butanes and lighter materials.

The gasoline blending pool is maintained by a variety of operations-isobutane and butenes, for example, are charged to alkylation units and light olefins are polymerized to provide high octane blending components while the catalytic cracking of gas oil augments the supply of naphtha as well as providing additional feed for alkylation and polymerization units. Although hydrocracking provides additional quantities of gasoline blending naphthas, the heavy naphtha from hydrocracking often has a relatively low octane number.

Recently the introduction of zeolite cracking catalysts has effected significant improvement in the catalytic cracking operation. When employed for gas oil cracking in existing catalytic cracking units these highly active catalysts have produced increased throughput and improved product quality. In addition, catalytic cracking apparatus, such as disclosed in U.S. Pats. 3,433,733 and 3,448,037, has been developed for use with these improved catalysts to take advantage of their unique properties. This apparatus incorporates the concept of riser cracking wherein a substantial portion of the cracking takes place in elongated reaction zones or risers terminating in a tapered reactor containing a dense phase and a dilute phase of catalyst. In a two riser conguration fresh gas oil is introduced to one riser and a cycle gas oil to the other. A mixture of zeolite catalyst and hydrocarbons passes through each riser under cracking conditions tailored to the particular feedstock and desired products. The eluents from the risers discharge into the reactor for separation of the vaporous reaction mixture and the catalyst, or, optionally, further cracking of either or both :ice

of the streams in the dense phase of catalyst in the reactor.

Although the zeolite catalysts increase the supply of high quality naphtha, yields of the lighter hydrocarbons are substantially lower than from catalytic cracking with amorphous silica-alumina catalysts. In the future, therefore, the supply of isobutane, propylene and butenes for alkylate production and of these and other light hydrocarbons for polymerization and petrochemical manufacture will continually decline. A process which will upgrade naphtha streams for use in gasoline blending and supply additional quantities of C4 and lighter hydrocarbons is highly desirable.

Naphtha is more diicult to crack than gas oil and up to the present time limited success has been obtained in cracking naphtha catalytically. Traditional cracking catalyst, such as silica-alumina, exhibited relatively poor selectivity and activity when employed to crack naphtha resulting in the formation of realtively large amounts of gas and coke and producing small amounts of desirable olens and aromatics. U.S. 3,284,341 discloses a process for the catalytic cracking of naphtha with a silica-alumina catalyst to produce substantial quantities of olens and aromatics by maintaining the space velocity above about 4.5, the pressure betwen 0 and 20 p.s.i.g. and the reaction temperature betwen 1000 and 1200 F.

The new zeolite cracking catalysts are being employed extensively in gas oil cracking operations but their utility for the conversion of naphtha has yet to be fully explored. U.S. 3,247,098 discloses that hydrogen mordenite, a crystalline aluminosilicate, is an extremely active catalyst for the conversion of light naphtha to lighter components together with improving the octane number of the resultant naphtha. The utility of the mordenite aluminosilicate as a cracking catalyst for naphtha was surprising in view of the ineffectiveness of a magnesium faujasite catalyst to satisfactorily crack naphtha. The magnesium faujasite was known to be a highly eifective gas oil cracking catalyst. Copending application Ser. No. 889,714 discloses a process for the catalytic cracking of naphtha with type X or type Y crystalline aluminosilicates yielding substantial quantities of light hydrocarbons to serve as feed for petrochemical, polymer and alkylate manufacture and significant quantities of naphtha having an enhanced octane rating. The development of methods for further enhancing the catalytic cracking of naphtha with zeolite cracking catalysts is highly desirable.

SUMMARY OF THE INVENTION Briey our invention is directed to the catalytic cracking with zeolite catalysts of naphtha boiling in the range of -450 F. to produce substantial quantities of naphtha having an octane rating signicantly higher than the low octane feed and also yielding substantial quantities of light hydrocarbons which may serve as feed for alkylate, polymer and petrochemical manufacture. The feedstocks comprise a low octane fresh naphtha fraction and a recycle stream of full range or heavy naphtha recovered from the effluent from the catalytic cracking reactor. These two streams together with zeolite cracking catalyst are passed through separate elongated reaction zones under cracking conditions and discharge into a reactor vessel where either or both of the streams may be subjected to further cracking in a dense iluidized bed of the zeolite catalyst.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be more readily understood by reference to the accompanying drawing which depicts a llow diagram of a preferred embodiment of the process of the invention.

3 DESCRIPTION OF THE PREFERRED EMBODIMENT Broadly, we have found that a low octane naphtha and a recycle naphtha stream recovered from a catalytic cracking eiiiuent stream can be subjected to catalytic cracking with zeolite cracking catalysts in a fluid catalytic cracking unit employing a multiplicity of elongated reaction zones, hereinafter called risers, wherein the low octane naphtha is introduced into one of the risers and the recyle naphtha into a second riser to produce significant quantities of naphtha having an octane rating significantly higher than the low octane feed and also yielding substantial quantities of light hydrocarbons which may serve as feed for alkylate, polymer and petrochemical manufacture. The effluents from the two risers discharge into a reactor vessel where either or both of the feed streams may be subjected to further cracking in a dense uidized bed of the zeolite catalyst. Operating conditions within the two risers are established to obtain optimum product yield and product from each of the individual streams.

Broadly, our invention contemplates a process for the catalytic cracking of naphtha with a zeolite cracking catalyst in a fluid catalytic cracking unit comprising a reactor, a regenerator and a multiplicity of elongated reaction zones, wherein said reactor contains a dense phase and a dilute phase of said catalyst and said elongated reaction zones terminate at said reactor, which comprises:

(a) Passing a naphtha stream and a zeolite cracking catalyst through a first elongated reaction zone under naphtha cracking conditions,

(b) Discharging the effluent from said first elongated reaction zone into a catalyst phase in said reactor, said euent comprising vaporous reaction mixture and catalyst,

(c) Recovering from the vaporous reaction mixture in said reactor a hydrocarbon fraction boiling between 100 and 450 F.,

(d) Passing the fraction from step (c) and a zeolite cracking catalyst through a second elongated reaction zone under naphtha cracking conditions,

(e) Discharging the eiuent from said second elongated reaction zone into a catalyst phase in said reactor, said effluent comprising vaporous reaction mixture and catalyst, and

(f) Recovering from the vaporous reaction mixtures from said first and second elongated reaction zones in said reactor product streams selected from the group consisting of C4 and lighter hydrocarbons, C5 and lighter hydrocarbons and a hydrocarbon fraction boiling between 100 and 450 F. and having an octane rating higher than said naphtha stream.

The catalyst phase into which the efiluent from either of the risers discharges may be either the dense phase or the dilute phase of catalyst in the reactor which leads to a number of optional embodiments incorporating various combinations or riser cracking and dense phase cracking, i.e., bed cracking.

In the simpliest embodiment, the cracking of both the fresh naphtha and recycle naphtha is restricted to the risers by discharging the effluent from both risers into the dilute phase of catalyst in the reactor vessel. In this situation, the reactor vessel is utilized as a disengaging zone with little or no cracking taking place therein.

In another embodiment, the recycle naphtha is subjected to further cracking in the dense catalyst phase. This is achieved by discharging the eiuent from the fresh naphtha riser into the dilute phase of catalyst and the jected only to riser cracking while the fresh -naphtha is cracked in both the riser and the dense phase of catalyst. The eliiuent from the recycle naphtha riser is discharged directly into the dilute phase of catalyst in the reactor vessel while the elliuent from the fresh naphtha riser is discharged into the dense phase of catalyst, the vaporous reaction mixture from the fresh naphtha riser passes through this dense phase under catalytic cracking conditions effecting an additional conversion of 5 to 30 vol. percent and discharges into the dilute phase of catalyst.

In an especially preferred embodiment, both the fresh naphtha and the recycle naphtha are subjected to riser cracking and bed cracking by discharging the eiuent from both risers into the dense phase of catalyst. The vaporous reaction mixture passes through the dense phase under catalytic cracking conditions to effect an additional conversion of each stream of 5 to 30 vol. percent and discharges into the dilute phase.

In accordance with this invention hydrocarbons boiling in the range of about to 450 F. comprise the fresh naphtha fedestock for this process. Many refinery streams having low economic value may be upgraded by employing the process of our invention. Useful feedstocks are usually highly parainic and include such light hydrocarbon fractions as low octane naphthas, Udex raffinate, low octane naphthas from thermal cracking or hydrocracking operations and straight runs naphthas. As used herein, the term low octane' fresh naphtha refers to these useful feedstocks.

The recycle naphtha is obtained from the efiiuent stream from the catalytic cracking unit usually by means of fractionation. This recycle stream may either be a full range naphtha, i.e., having an initial boiling point (IBI) of about 100-120 F. and an end point- (EP)'of about S50-450 F. or a heavy naphtha, i.e., having an IBP of about 22S-275 F. and an EP of about 350-450 F. As used herein, the term boiling between 100 and 450 F. is used to include the boiling range of a full range naphtha as well as a heavy naphtha.

Products from the process of our invention include naphthas with improved octane ratings, and hydrocarbons boiling below the initial boiling point of the feed which find particular utility as feed streams for petrochemical, polymer gasoline, and alkylate manufacture.

The catalyst employedY in the instant invention is a cracking catalyst of the zeolite type as exemplified by those catalysts where the crystalline'aluminosilicate is dispersed in a siliceous matrix. Among the preferred zeolites which may be usefully employed in the catalyst used in the process of our invention are aluminosilicates of type X or type X, including both the naturallyoccurring and synthetic varieties. Because -`of their extremely high activity these zcolitic materials are composited with a material possessing a substantially lower levelof catalytic activity, a siliceous matrix, which may be of the synthetic, semi-synthetic or natural type. These materials may include silica-alumina, silica-gel, silica-beryllia, silicamagnesia, silica-thoria, or silica-zirconia which have been successfully employed heretofore. In general, the composite crystalline zeolite catalyst comprises about 1 to 50 wt. percent zeolite, about 5 to 50 wt. percent alumina and the remainder silica. The crystalline aluminosilicate portion of the catalyst composition is a natural or synthetic, type X or type Y, alkali metal, crystalline aluminosilicate which has been treated to replace all or at least a substantial portion of the original alkali metal ions with other ions such as hydrogen and/or a metal orcombination of metals such as barium, calcium, magnesium, manganese or rare earth metals, for example, cerium, lanthanum, neodyminum, praseodymium, samarium and `yttrium. The crystalline zeolites contemplated above may be represented by the formula y Mg/IIOI AIZOgIxSlOZyHZO y where M represents hydrogen or a metal, n its valance, x has a value ranging from 2 to 10 and y ranges from to 10, in dehydrated zeolites, y will be substantially 0. In the instant invention the crystalline zeolites are either natural or synthetic zeolite X or zeolite Y. In highly prefrred embodiments M is selected from the group consisting of hydrogen, calcium, magnesium and the rare earth metals.

Since both naphtha and recycle naphtha are more diffcult to crack than gas oil the operating conditions are more severe than those used in gas oil catalytic cracking. Further, the recycle naphtha will usually require more stringent conditions than the fresh naphtha. The operating conditions contemplated herein for either the fresh naphtha riser or the recycle naphtha riser include a temperature of 750-1300 F., preferably 900-1000 F., a conversion per pass of 25-80 volume percent, preferably 30-60 volume percent, and a vapor velocity of 15-50 feet/ second, preferably 20-40 feet/ second. Operating conditions in the fluidized bed in the reactor (the dense phase of catalyst) include a temperature of SOO-1150o F., a conversion of -30 volume percent, and a vapor velocity 0f 0.5-4 feet/second preferably 1.3-2.2 feet/second. As used herein, conversion, naphtha conversion, percent conversion or 115 F. conversion (see Table II) is defined as 100 minus the volume percent of product boiling above 115 F.

Ourinvention may be understood from the following detailed description taken with reference to the accompanying drawing which illustrates and exemplifies a means by Which the process of the present invention may be practiced. By describing one of the embodiments of our invention in this manner, it is not intended to restrict the invention thereby since modifications to the following description may be made within the scope of the claims Without departing from the spirit thereof.

In this description the process is conducted in a iluid catalytic cracking unit employing two risers with fresh naphtha being introduced into one while a recycle naphtha stream introduced into the other. The eflluents from both risers are discharged into the iiuidized bed of catalyst in the reactor so that both feed streams are subjected to riser cracking and bed cracking.

Fresh naphtha obtained from a variety of sources, not shown, is introduced through line into riser 12 where it is contacted with regenerated zeolite cracking catalyst from standpipe 14. The resultant catal /st-in-naphtha vapor passes up riser 12 to reactor 16. Reactor 16 contains a bed of catalyst 18 referred to as the dense phase of catalyst and a vapor space 20 above the catalyst bed which functions as a catalyst disengaging space and is referred to as the dilute phase of catalyst. The eluent from riser 12 discharges into the dense bed of catalyst passing upward through the densev phase of cracking catalyst 18 effecting further conversion of the fresh naphtha stream. Either of two recycle naphtha streams, a heavy naphtha passing through line 22 or a full range naphtha passing through line 24 is introduced through line 26 to riser 28 where it is contacted with regenerated zeolite cracking catalyst from standpipe 30. The resultant catalyst-in-naphtha vapor passes up riser 28 to reactor 16 discharging into the dense phase of catalyst 18. The vapor mixture passes upwardly through the bed of catalyst effecting some further conversion of the recycle naphtha stream. Cracked products disengage from the catalyst in the dilute phase of catalyst 20 above the catalyst dense phase 18. These vapors together with any entrained catalyst pass through cyclone separators, not shown, wherein this catalyst is substantially separated from the vapors and returned to the catalyst bed. Eflluent gases containing the cracked products pass from the cyclone separators through line 32 to fractionator section 34 wherein the vapor mixture is separated into various product streams. These products include: (1) a stream of C4 and lighter hydrocarbons passing through line 36, (2) a light naphtha side stream passing through line 38, (3) a heavy naphtha side stream recovered as a product through line 40, passed to line 22 as a recycle naphtha stream or combined with the light naphtha in line 38 to produce a full range naphtha in line 42 which may be passed through line 24 as a recycle naphtha stream and (6) a bottoms product recovered through line 44.

Catalyst is withdrawn from the bottom of the reactor through slide valves 46 and 48 passing into stripping zone 50 containing baies 52. Steam introduced into the lower portion of stripper 50 removes adsorbed and entrained hydrocarbons from the catalyst as it passes through the stripper. Stripped catalyst is withdrawn from the bottom of stripping zone 50 through spent catalyst standpipe 54 discharging into regenerator 56. The zeolite catalyst forms a dense bed Iwithin regenerator 56 and is regenerated therein by contacting it with air to remove the car-bon from the catalyst surface. Regenerated catalyst is withdrawn from the bottom of regenerator 56 through

standpipes

14 and 30 to supply the hot regenerated catalyst to risers 12 and 28 as hereinbefore described.

The following exempliiies the practice of our invention and its advantages over alternative or prior art practices. A series of five runs is performed with a uidized catalytic cracking unit having two feed risers. In the rst run of the series only one of the risers is employed, while both risers are utilized in the remaining runs. The same zeolite-containing cracking catalyst is employed in all runs. The catalyst consists of a 1:1 weight blend of a zeolite cracking catalyst and a high alumina amorphous cracking catalyst. The zeolite catalyst comprises 18 wt. percent of a type X zeolite in a silica-alumina matrix and has a rare earth content of about 2.9 Wt. percent. The amorphous silica-alumina catalyst has a high alumina content, a surface area of about m.2/g. and pore volume of about 0 .4 cc./g. In each of the runs, the fresh naphtha feed is either a heavy straight run gasoline or a heavy Udex raffinate having properties as set forth in Table I below. The gasoline served as the fresh feed in Runs 1, 2 and 3 whles: the Udex raffinate was the fresh feed in Runs 4 an TABLE I.-FEEDSTOCK ANALYSIS Heavy straight Heavy run Udex gasoline ranate Annina p0in1;, F 12222 isig esearc oeane, cllear 50. 6 17.0

esearc oc ane us 3 cc. TEL l Il1/)95 Y 30G/30% S05/gg FIA-MS, vol. percent:

Aromatics 13. 8 3. 2

Parans 48. 3 86. 5

Naphthenes 37. 9 10. 3

l Octane rating with addition of 3 cc. tetraethyllead per gallom Run 1 is a once-through operation without any recycle naphtha being employed. In the remaining four runs a naphtha stream recovered from the effluent is recycled to the unit. In all instances the feeds are introduced to a riser in the catalytic cracking unit and the eluents discharge into the dilute phase of catalyst in the reactor vessel so that substantially all of the catalytic cracking is taking place in the risers with the reactor vessel acting mainly as a disengaging space. In Run 1 only one of the risers is utilized since no recycle is employed. The heavy Straight run gasoline is introduced to the riser together with the cracking catalyst. In Run 2 both risers are employed with heavy straight run gasoline passing through one riser and a heavy recycle naphtha having a 250 F. IBP passing through the other. Run 3 also is a recycle operation and is in all respects the same as Run 2 except that the recycle naphtha is a full range naphtha having a 115 F. IBP. This full range naphtha is a depentanized naphtha. In Runs 4 and 5 the heavy Udex raiiinate serves as the fresh feed and the recycle naphtha is a heavy naphtha having a 250 F. IBP. Runs 4 and 5 are the same in all respects except that the riser temperature in Run is higher than that in Run 4. The operating conditions and the results obtained in the series of runs are set forth to form a rst mixture consisting essentially of said fresh naphtha stream and said iirst portion of catalyst,

(b) passing reactants consisting essentially of said first mixture through said first elongated reaction zone under naphtha cracking conditions,

(c) discharging the eiuent from said lirst elongated in Table II below. reaction zone into a catalyst phase in said reactor,

TABLE II Run No 1 2 3 4 5.

Type operation- Oncethrough Recyle Recycle Recycle Recycle Charm HSR HSR HSR HU HU gasoline l gasoline gasoline raffinate 2 ratlinate Recycle stream- Full Heavy range Heavy Heavy naphtha naphtha 3 naphtha naphtha (250 (115 F. (250 F. (250 F IBP) IBP) IBP) IBB) Operating conditions:

Temperature, F. 920 920 920 920 1, 000 115 F. conversion, vol. percent 35 46 62 75. 5 74. 5 Yi 1laphthn recycle, vol. percent of fresh feed- 66 100 100 85 Dry gas (C: and 1 tr.), wt. percent 1. 1 1. 3 1. 9 3.5 4. 7 Propane, vol. percent. 6. 2 7. 4 10. 2 10.2 10.1 Propylene, vol. percent 4.1 5.0 6.6 11.0 15. 2 Isobutane, vol. percent 12.4 15.0 20. 3 21.8 18.0 n-Butane, vol. percent 4. 6 5. 6 7.4 5.7 5. 6 Butylenes, vol. percent... 2.2 2. 8 3. 6 8.0 11. 4 DB naphtha, vol. percent. 78. 2 68.4 57.7 48. 3 44. 2 Coke, wt. percent 1. 2 6. 3 7. 5 7. 9 7. 4 Total Ca-C4, vol. percent 29. 5 35. 9 48. 1 56.7 60. 3 Alkylate, vol. percent 4. 10.8 13.4 17.5 30. 0 24. 8 DB naphtha plus alkylate, vol. perce 89. 0 81. 9 75.2 78. 3 69. 0 DB naphtha, RON plus 3 cc. TEL 89. 8 94. 2 98.4 92. 2 90. 6 DB naphtha plus alkylate, RON plus 3 cc. TEL-. 91.9 96. 2 99. 8 97.3 96.1

1 HSR gasoline=Heavy Straight Run gasoline. 2 HU ratlnate=Heavy Udex raffinate.

4 Alkylate obtained from entire yield of isobutanc. l Research octane rating with addition of 3 cc. tetraethyllead per gallon.

Run 1 shows that the research octane of a heavy straight `run gasoline can be substantially increased from 74.8 to 89.8 in a once-through catalytic cracking operation. When combined with alkylate produced by alkylat- Runs 2 and 3 show that by recycling either a heavy 45 naphtha or a full range naphtha substantial increases in the research octane of the naphtha can be achievedfrom 74.8 to 94.2 and 98.4, respectively. In addition higher yields of valuable C3C4 petrochemical feed stocks are produced in a recycle operation. Further, naphtha octanes can be significantly improved by recycling fullrange naphtha over those obtained with heavy naphtha recycle.

temperatures are employed with constant conversion and Runs 4 and 5 demonstrate that when two different riser heavy naphtha recycle, the octane of the low octane feed can be increased signilicantly-from 55.0 to 92.2 and 90.6, respectively. The lower riser temperature operation of Run 4 produces higher yields of higher octane gasoline than obtained in Run 5. On the other hand, with the higher riser temperature of Run 5 more C3-C4 petrochemical feed stocks having a higher olefin content are produced than in Run 4.

We claim: 1. A process for the catalytic cracking of naphtha with zeolite cracking catalyst in a iiuid catalytic cracking unit comprising a reactor, a regenerator and a multiplicity of elongated reaction zones, wherein said reactor contains a dense phase and a dilute phase of said catalyst and said elongated reaction zones terminate at said reactor, which comprises: j

(a) introducing a low octane fresh naphtha stream and a iirst portion of freshly-regenerated zeolite cracking catalyst into a rst elongated reaction zone 75 said efliuent comprising vaporous reaction mixture and catalyst,

(d) recovering from the vaporous reaction mixture in said reactor a hydrocarbon fraction boiling between and 450 F.,

(e) introducing the fraction from step (d) and a second portion of freshly-regenerated zeolite cracking catalyst into a second elongated reaction zone to form a second mixture consisting essentially of said fraction from step (d) and said second portion of catalyst,

(f) passing reactants consisting essentially of said second mixture through said second elongated reaction zone under naphtha cracking conditions,

(g) discharging the eiiluent from said second elongated reaction zone into a catalyst phase in said reactor, said etiiuent comprising vaporousreaction mixture and catalyst, and

(h) recovering from the vaporous reaction mixtures from said first and second elongated reaction zones in said reactor product streams selected from the group consisting of C4 and lighter hydrocarbons, C5 and lighter hydrocarbons and a hydrocarbon fraction boiling between 100 and 450 F. yand having an octane rating higher than said fresh naphtha stream.

2. A process according to claim 1 wherein the naphtha cracking conditions of steps (a) and (d) include a temperature of 750-1300-o F., a conversion of 25-80 volume percent and a vapor velocity of 15-50 feet/second.

3. A process according to claim 2 wherein the efiiuents from the iirst andsecond elongated reaction zones are discharged into a dilute phase of catalyst.

4. A process according to claim 2 wherein the effluent from the first elongated reaction zone is discharged into a dilute phase of catalyst and the eluent from the second elongated reaction zone is discharged into a dense phase of' catalyst, said vaporous reaction mixture from said second elongated reaction zone passing through said dense phase under catalyticl cracking conditions effecting an additional conversion of -30 volume percent and discharging into a dilute phase of catalyst.

5. A process according to claim 2 wherein the effluent from the second elongated reaction zone is discharged into a dilute phase of catalyst and the eiuent from the first elongated reaction zone is discharged into a dense phase of catalyst, said vaporous reaction mixture from said first elongated reaction zone passing through said dense phase under catalytic cracking conditions elTecting an additional conversion of 5-30 volume percent and discharging into a dilute phase of catalyst.

6. A process according to claim 2 wherein the eiuents from the first and second elongated reaction zones are discharged into a dense phase of catalyst, said vaporous reaction mixtures from said irst and second reaction zones passing through said dense phase under catalytic cracking conditions eiecting an additional conversion of 5-30 v01- ume percent and discharging into a dilute phase of catalyst.

7. A process according to claim 4 wherein the catalytic cracking conditions in the dense phase include a temperature of 800-1150" F. and a vapor velocity of 0.5-4 eet/ second.

`8. A process according to claim 5 wherein the catalytic cracking conditions in the dense phase include a ternperature of 8001150 F. and a vapor velocity of 0.5-4 feet/second.

9. A process according to claim 6 wherein the catalytic cracking conditions in the dense phase include a temperature of 800-1150 'F. and a Vapor velocity of 0.5-4 feet/second.

References Cited UNITED STATES PATENTS 3,143,491 8/ 1964 Bergstrom 208-74 2,890,164 6/ 1959 Woertz 208-74 3,424,672 1/ 1969 Mitchell 208--164 3,448,037 6/ 1969 Bunn et al 208-164 3,065,166 11/1962 Hennig 208-67 2,827,422 3/ 1958 Rehbein 208-74 2,921,014 1/ 1960 Marshall 208-74 2,409,353 10/ 1946 IGiuliani et al. 208--120 3,649,522 3/1972 Martin 20S-120 3,679,576 7/ 1972 McDonald 208-74 3,692,667 9/1972 McKinney et al. 208-120 DELBERT E. GAN-T Z, Primary Examiner G. E. SCHMITKONS, Assistant Examiner U.S. Cl. X.R.

208DIG. 2, 77, 120, 164