US 2921014 A
Description (OCR text may contain errors)
Jan. 12, 1960 J. A. MARSHALL.
PRODUCTION OF' GASOLINE BY CATALYTIC CRACKING Filed May 16, 1956 INVENTOR:
JOHN A. MARSHALL m/ f HIS ATTORNEY m cmmu Iwmmm UnitedStates Patent O PRODUCTIDN F GASOLINE BY CATALYTIC CRACKING `lohn A. Marshall, Ferguson, Mo., assignor to Shell Development Company,`New York, N.Y., a corporation of Delaware Application May 16, 1956, Serial No. 585,318
1A Claim. (Cl. 20S-L74) This invention relates to the production of motor gasoline by the-catalytic cracking of hydrocarbon oils boiling essentially abovelgasoline. More speciiically, it relates to an improved method of operation whereby increased yields of-gasoline may be obtained without loss in gasoline quality.
In theV catalytic cracking of hydrocarbon oils as commercially -practiced lto produce motor gasoline as the primary product, the conversion is :adjusted to afford the maximum yield ofgasoline which is of the order of 25- 40% depending upon the feed and other factors. This gasoline is satisfactory 'for use in the premium grade lmotor gasoline marketed today and is in fact the primary component of such gasoline but itis borderline and would notvbe satisfactory as a major constituent of premium grade gasoline if the octane number of suchfgasolinewere much increased. A
The yield ofl gasoline can be increased somewhat by recycling part of the unconverted or partially converted oil, i.e. so-called catalytic gas oil and a small recycle issometimes used in commercial practice. While recycling ncreases the yieldsomewhat it has the disadvantage of de-, creasing the octane number of the gasoline.
It has recently been found that the yield of gasoline may be greatly increased by effecting the catalytic cracking operation in separate stages with separation offthegasoline produced in the tirst stage prior to further cracking of the oil in the subsequent stage. In this process it is found that the yield of gasoline is greatly dependent upon the ratio of the conversion in the irst stage to that in the second stage. On theother hand, it is necessary to maintain the iirst stageconversion at at least about 40% since at lower iirst stage conversions the octane number of the gasoline is reduced and this Ais especially the case if an appreciable recycle` is applied and/or the cracking further object of the invention is to allow the catalytic cracking to be effected' in three or more stages without ioss in octane number of the gasoline produced. A further object of the invention is to allow a large recycle ratio to be employed in staged catalytic cracking without suffering a loss in theoctane number of thegasoline produced.
The process of the invention broadly stated is a catalytic cracking process for the conversion of hydrocarbon oil boiling essentially above gasoline to `produce catalytically cracked motorgasoline wherein the oil to be cracked is contacted in separate and Vdistinct cracking stages under cracking conditions with a solid cracking catalyst and pfice product' is removed by separate fractionation of the hydrocarbon 'reaction mixture from the separate cracking stages. The hydrocarbon mixture from the4 first stage of cracking is fractionated under conditions to remove as overhead product a hydrocarbon fraction having a iinal boilingfpoint above about 266 F. but below thefiinal boiling of the finished motor gasoline. The resulting bottoms product containing insufliciently converted or unconverted oil along with a part Aof the catalytically cracked gasoline produced in the first stage cracking boil# ing abovethe above-mentioned boilingl point is subjected to furthercracking in a subsequent stage. The-hydrocarbonreactionmixture from thelast stage of catalytic cracking isfractionated under conditions'to-remove as overhead producta gasoline vhaving the final boiling point of motorgasoline. In this way'there is Vproduced a light and afheavyvgasoline both of which have high octane numbersfand which may be blended to producey gasoline of the -motor gasoline end point having a high octane number.
In .amodiiication of the process, catalytic gas oil from the last cracking stage is recycled tothe iirst cracking stage. In a further modification, the first stage cracking iseffected in a riser reactor using a powdered cracking catalyst.
A preferred embodiment ofy the invention fis illustrated diagrammatically in the accompanying drawing. Referring to the drawing, the fresh feed tobe cracked is introduced by lline`1 tothe bottom of a4 riser reactor 2.wherein it is contactedwithr hot freshly regenerated'catalyst from standpipe-3. .The'amount'of catalyst is suiiicient to maintaina cracking temperature in the riser reactor between about 900?. and about 1000 F. The diameter of the riser reactor is such that the vapor velocity is suicient to maintainthe catalyst as a `dispersion in the vapors and passes concurrently with the vapors.
Theiirst stage conversion is controlledby adjustment of the temperature and the velocity in the riser. As the velocity is Iincreased, eg. by adding. steam,.inert gas, or recycled catalytic gas oil, the contact time is decreased and also the density of the catalyst dispersion is decreased. In order to produce a large yield of gasoline of superior quality, the conversion isadjusted in the range of about 35-60%, but preferably close to 0.60 times the overall conversion based on the fresh feed. When it is not desired to maintain the octane number, i.e. prevent loss of octane number, the conversion is generally under 40% and may be as', low as l5-20%.
The reactionmixture from .the riserV reactor .is passed directly into a cyclone separator system consisting of first stage and second stage cyclones 4 and 5 `to quickly separate the catalyst from the oil vapors. The separated catalyst is passed byl-ine' to a second stage reactor 7 which'may bea conventional iiuidized catalyst reactor.
vPrior-.to `any further cracking of the oil, the product from the -first stage of cracking is fractionated in a first stage1fractionator'8. Fractionator 8 is controlled to remove 'as`overhead product all material boiling below .a specifiedy end point -whichis -below the end point of the f gasoline being produced by the process. .Premium grade i motor gasoline normally has an ASTM finalV boiling point or end point of about 425-43091?. The end point of the product taken overhead from Afractionating column 8 will be appreciably below this, e.g. at least 10"-13. below and may be as low as 266 F. The lower the end point of the overhead product in this range,-the greater'is the octane number and lesser isV the volume of the'overhead product.
The bottom product from fractionating column 8 contains everything boiling above the above-mentioned end point, including unconverted oil. This bottom product is passed by line 9 to the second stage reactor 7 wherein it is further cracked to produce additional amounts of gasoline.
In the arrangement shown the catalyst after use in the rst stage reactor is reused in the second stage reactor. Itis then passed by line 10 to a catalyst regeneration system 11 which normally includes provisions for stripping.
The temperature in the second stage reactor may be maintained between about 850 and about 1000 F. In the system illustrated it will be somewhat below that in the riser reactor. The catalyst hold-up, and hence the space velocity, in the second stage reaction 7 is maintained 'sufficient to effect a considerable further conversion to gasoline so that the overall conversion, based on the fresh feed is above about 60%.
The hydrocarbon vapors from the second stage reactor are passed by line 12 to a separate fractionator 13. This fractionator is operated in the conventional manner to remove as overhead product all material having the nal boiling point of motor gasoline, e.g. up to 430 F. end point. The product from fractionator 13 contains everything boiling above the motor gasoline range. This material may be withdrawn and recycled in whole or in part. If it is to be further cracked it would normally be recycled by line 14 to the second stage cracking'reactor. In a preferred embodiment of the invention, however, itis recycled by line 15 to the riser reactor 2.
The overhead products from fractionators 8 and 13, after condensing and separation of gas which is sent to a conventional gas recovery plant, are blended. After stabilizing in the conventional manner there is thus produced a high octane gasoline of correct nal boiling point and front end volatility.
In general, two stages of cracking are sufficient. However, if desired, a third cracking stage .may be included either between or following the two stages described. In this case the rst stage conversion will ordinarily be adjusted between about 25 and 40%. The present invention is of particular vvalue in this case in preventing the severe drop in octane number of the blended gasoline which would otherwise occur.
As pointed out above, while a small recycle is often permissible in previous commercial practice, extensive recycling resulted in a significant decline in the octane number of the gasoline. By recycling to the riser reactor in EXAMPLE In the following example it is desired to produce a good yield of motor gasoline having an ASTM nal boiling point of 425-430 F. and an F-l clear octane number of at least 96 from a feed stock having the properties listed in the following Table I.
Table I Gravity, API 28.1 Molecular weight ca, 340 Ramsbottom carbon, percent W 0.30 Sulfur, percent w 0.30 Saturates, percent w 7l Monocyclic aromatics, percent w 10 Polycyclic aromatics, percent w 16 Resins, percent w 3 The catalyst is a plant equilibrium silica-alumina cracking catalyst passing a 100 mesh standard sieve and having an available surface area of about 102 m.2/ g.
When cracking this feed stock with this catalyst under optimum conditions using the conventional once-through operation the yield of debutanized gasoline is 40.4% v. at a conversion of 57.0% v.
This feed stock was cracked in a pilot plant in a two stage operation in which gasoline of the desired end point was separated by fractionation after each stage. In a comparable case the operation was carried out according to the present invention in which case the product from the first stage cracking was fractionated to remove as overhead product the material boiling only up to 400 F. and the total bottom product was passed to the second stage of cracking. The operating conditions and the yields are shown in the following Table II.
Table II Second Stage Second Stage Feed, All Feed, All Material From Material From First Stage First Stage First Stage Boiling Above Boiling Above 425 F., Gaso 400 F. line End Point Pressure, p.s.l.g 8 8 8 8 16 16 Temperature, F 951 953 952 953 950 950 Charge Rate, g.p.h.. 0.358 0.353 0.358 0.351 0.360 0.300 C/O Ratio. 3. 92 2. 89 3. 08 3. 35 3. 79 5.06 Space Velocity, W/hr./W 8.67 8. 8. 67 8. 50 1. 89 1. 90 Inlet Oil Mol Fraction 0. 255 0. 26 0. 26 0. 25 0.38 0. 37 Conversion, Percent v. by precision distllatio 32. 8 27. 8 32. 9 34. 7 46. 3 43. 8
Per- Per- Per- Per- Per- Per- Per- Per- Per- Per- Per- Percent v. cent w. cent v cent W cent v cent w cent v cent w cent v cent w cent v. cent W. Yields:
Hydrogen (s.e.f./bbl.) (32. 2) 0. 0 (25. 8) 0. 0 (30. 0) 0. 1 (22. 0) 0. 0 (75. 2) 0. l (79. 0) 0. 1 Methane 1.2 0.4 1. 4 0. 5 1.2 0.4 1.3 0.4 3.3 1. 2 3.0 l. 0 Ethene 0.7 0.3 1.0 0.4 0.8 0.3 0.8 0.4 1.2 0.5 0.7 0.3 Ethane-- 0.9 0.4 1.2 0.5 0.9 0.4 1.3 0.5 1.8 0.8 1.8 0.7 Prope11e 3.6 2. 1 3.3 1.0 .3.6 2.1 4. 5 2.7 2.8 1. 7 5.3 3.2 Propane 1.2 0.7 1.2 0.7 1.0 0.6 1.4 0.8 3.1 1.8 2.7 1.5 Bntpnns 5.9 4.1 4. 5 3. 0 5. 4 3. 7 6. l 4.1 7. 0 4. 8 6. 7 4. 6 Is0butane 1. 7 1. 0 0. 3 0. 2 1.6 1. 0 1. 5 0. 9 4. 1 2. 6 4. 7 0 0.2 0.1 0.1 0.1 0.4 0.3 0.8 0.5 2.4 1.6 1.1 0.8 PPIIPUPQ 4. 3 3. 2 3. 3 2. 4 3. 5 2. 6 4. 0 3.0 5. 4 4. 0 3. 8 2. 8 Isopentane 0. 8 0.6 0.6 0. 4 0. 9 0.6 0.8 0.6 3. 5 2. 5 L. 6 1.9 Pentane- 0. 3 0. 2 0.1 0. 1 0. l 0. 1 0. 2 0. 1 0. 5 0. 3 O. 5 0. 4 Cn-425 F 20. 5 l 18. 3 18. 6 16. 8 21. 8 19. 3 21. 5 18. 7 21. 3 18. 9 19. l 17. 4 425-630 F. 31. 2 32. 1 30. 9 32.3 630 F+ 67. 2 67. 6 72. 2 72. 2 67. 1 67. 5 65. 3 66. 1 22. 7 23. 5 25. 8 26. 3 Carbon. 1. 0 0. 8 1.0 1. 2 3. 6 3. 7
l (Jl-400 F.
The results shown in Table II, after adjustment to the same total carbon make through the use of established correlations, are shown in the following Table III.
Considering the results it will be seen that when effecting the cracking in two stages in which end point gasoline was separated after each stage the volume yield of gasoline is 46.0% as compared to 40.4% for conventional catalytic cracking. However, the F-l clear octane number of the gasoline was only 94.8 which does not meet the requirements. When operating according to the invention on the other hand, the yield of debutanized gasoline was 43.4% which is still a substantial improvement in yield over the conventional operation while the F-l clear octane number of the product was about 96.2. This gasoline therefore is not only obtained in a high yield but also meets the octane requirement.
What is claimed is:
In the catalytic cracking of a hydrocarbon oil boiling essentially above gasoline to produce catalytically cracked motor gasoline having a specified ASTM final boiling point wherein the oil to be cracked is contacted in separate and distinct cracking stages under cracking conditions with a solid cracking catalyst and cracked products are removed by separate fractionation of the hydrocarbon reaction mixtures from the separate stages, the improvement which comprises effecting the first stage cracking at a temperature between about 900 F. and about 1000" F. with the catalyst maintained dispersed in the oil vapors, under conditions of severity giving about 35 to 60% conversion and close to 0.6 times the total conversion for the two cracking stages fractionating the hydrocarbon mixture from the first stage of cracking under conditions to remove as overhead product a hydrocarbon fraction having a iinal boiling point above about 266 F. but at least 25 F. below the final boiling point of the motor gasoline product from a residuum fraction containing insufficiently converted or unconverted oil along with a part of the catalytically cracked gasoline produced in the rst stage boiling in the range between about 266 F. and the final boiling point of the motor gasoline product, subjecting said residuum fraction, to further cracking in a subsequent stage to an overall conversion of at least fractionating the hydrocarbon reaction mixture from the last stage of catalytic cracking under conditions to remove as overhead product a cracked gasoline having the ASTM final boiling point of said motor gasoline product and blending the resulting gasoline with the above said overhead product to produce specified end point motor. gasoline.
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