US2548032A

US2548032A - Process for increased olefin recovery in a petroleum refinery

Info

Publication number: US2548032A
Application number: US56453A
Authority: US
Inventors: Donald M Little
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1948-10-25
Filing date: 1948-10-25
Publication date: 1951-04-10
Anticipated expiration: 1968-04-10

Description

prxl l0, 1951 D. M. LITTLE l PROCESS EOR INOREASED OLEEIN RECOVERY 1N A PETROLEUM REEINERY Filed oct. 25, 1948 Patented Apr. 10, 1951 PROCESS FOR INCREASED OLEFIN` RECOV- ERY IN A PETRGLEUM'REFINERY i Donald M. Little, Bartlesville, one., assigner to Phillips Delaware Petroleum Company, a corporation of Application October 25, 1948, Serial No. 56,453

6 Claims. l

This invention relates to thetreatment of hydrocarbons. In one of its more specirlc aspects it relates to the separation of C5 and lighter by" drocarbons from a cracking furnace eiuent. In still another of its more specific aspects it relates to the recovery of C3, C4 and C5 oleiins from a mixture of hydrocarbons containing the same. In `still another of its more specic aspects it relates to van improved process for the recovery of Ca, C4. and C5 olens from a cracking furnace eiiluent. In yet another of its more specific aspects it relates to an improved method of operating an integrated refining process comprising oleiin feed preparation, alkylation and recovery of products.

The processes of reforming, polyforming and thermal cracking of light oil stocks for the production of gasoline are widely used in the petroleum industry. `In these processes an appreciable quantity of olenic material is formed. Cx, Ci and C5 olens are used in the well-known processes of alkylation, isomerization, chemical synthesis, etc. Many naphtha reforming and/or polyforming units were built or put into operation before it became advantageous to recover olefins formed during the reforming and/or polyforming processes. In the usual reforming or polyforming unit the furnace eilluent is separated into a fuel oil fraction, a gas oil fraction, a light oil side draw fraction, a gasoline fraction and a gaseous fraction containing substantially C5 and lighter hydrocarbons. The gasoline fraction is stabilized to provide a gasoline fraction suitable for motor fuel or for blending purposes. Many existing units were designed merely to perform the operation of stabilizing gasoline with no thought of recovering Ca, C4 and C5 olens to feed an alkylation unit, with the result that many Ci and C5 olens are contained in the stabilized gasoline fraction. The usual existing stabilizer will not make a good separation between the olefin-containing fraction and the C6 and heavier hydrocarbons which predominate in gasoline because the stabilizer will not handle the liquid load necessary for sufficient fractionation. The stabilizer-Will not handle the reflux necessary to recover lall the available C4 and C5 olens. As a result, C4 and C5 olens are lost for alkylation, isomerization, chemical synthesis, etc. The usual separation and recovery system, used in conjunction with a naphtha fraction reforming or polyforming operation or with -a light oil cracking operation, contains a .gasoline accumulation zone, usually referred to as a gasoline accumulator, The gasoline and most of the lighter hydrocarbons formed in the cracking f operation are fed into this gasoline accumulation zone. In most operations a gas comprised predominantly of C5 and lighter hydrocarbons lis taken overhead from the gasoline accumulator; however, the accumulator acts only as a flash zone and no sharp separ-ation is made. The gasoline stabilizer is fed with the liquid in the gasoline accumulator which contains appreciable quantities of C3, Ci and C5 oleflns. The gasoline stabilizer is Operated so as to take overhead las many of the C3, C4 and C5 olens as possible. I have found that by introducing a normally gaseous hydrocarbon fraction into a gasoline accumulator, or into the feed to an accumulator, one can recover more of the Ca, C4 and C5 olefins in the overhead from the gasoline stabilizer and the gasoline accumulator, thereby making more olens available for recovery in a light hydrocarbon n recovery system,

It is an object of this invention to provide an improved method of separating a mixture of hydrocarbons.

`Another object of this invention is to provide an improved method of recovering C3, Ci and C5 oleflns from the effluent loi a naphtha reformer or polyformer or from a light oil cracking furnace.

It is another object of this invention to provide an improved method of operating a gasoline recovery system so as to recover more Cs, C4 and VC5 olefins. Y Y y Still another object of this invention is to provide ,anY improved process of operating an integrated refining operation comprising olefin fecd preparation, alkylation and recovery of unreactecl hydrocarbons so as to increase the amcunt of Cs, Ci and C5 oleflns available for alkylation.

Other objects of my invention will become apparent, to one skilled in the art, from this disclosure.

My invention is particularly applicable to a process wherein a naphtha fraction, such as a straight run naphtha fraction., is reformed or polyforlned. Also, it is applicable to processes Where other light oil stocks are cracked so as to `form gasoline and lighter hydrocarbons. rlhe drawing is a schematic flow sheet diagrammatically showing a preferred embodilllent of my invention. The following discussion of vmy invention applies to reforming-or polyforrning a straight run naphtha fraction, as

a preferred example of my invention. A straight run naphtha fraction is fed to a reformer or polyformerv represented by heating zone 2 through line 3. A ley-pass line 4 is provided so that part of the feed may be preheated in heat exchanger G by interchange with the effluent from heating zone 2. As will be appreciated, this heat interchange is purely optional and only acts to cool the furnace eluent to the temperature desired for the first fractionation step. The furnace effluent is fed to a depropanizer represented by fractionation zone I which operates to carry overhead a portion of the light hydrocarbons produced in the reforming or polyforming furnace. In carrying` out my invention fractionation zone I8 is operated at a top temperature of from 190C F. to 230 F., and at a pressure of from 300 to 360 pounds per square inch gauge. The vapors from the top of fractionation zone I0 are cooled and/or condensed in condenser I4 and passed to accumulator I8. predominantly of H2 and CH4 is vented from accumulator I8. Part of the liquid hydrocarbon fraction in accumulator I8 is passed back to fractionation zone I0 as reflux and the remainder, which is a hydrocarbon fraction comprised predominantly of C2, Cs and C4 hydrocarbons, is passed through line 8 to a light hydrocarbon recovery system tied into the system or to an extraneous light hydrocarbon stream. A hydrocarbon fraction comprised predominantly of C4 and heavier hydrocarbons is Withdrawn from the bottom portion of fractionation zone I0 as bottoms product and passed through line 9 to a ash zone 26. This bottoms product is heated in heating zone 22 before entering flash zone 2'6. The fuel oil formed in heating zone 2 and present in the effluent from heating zone 2 is withdrawn from flash zone 26 through line I5. The hydrocarbons lighter than the fuel oil fraction are taken overhead from flash zone 26 and are passed to fractionation zone 30 through line I3. Fractionation zone 30 is employed to separate gas oil from the gasoline formed in the cracking operation. A clay tower to polymerize dioleflns and gum forming constituents and a polymer separator (tower and separator now shown) can be used in conjunction with fractionation zone 30 to separate out polymers formed in the cracking operation and in the clay tower. This, of course, is optional at this stage of the process. Fractionation zone 30 is operated with a top temperature of from 375 to 450 F., and at a pressure of from 50 to 10 pounds per square inch gauge. tionation zone 30 are condensed and cooled in condendenser 34. Part of the liquifled gases from the top of fractionation zone 30 are used as reflux and the remainder are'passed through line I2 into a gasoline accumulation zone represented by accumulator 38. If a clay tower and polymer separator are used to treat the overhead vapors from fractionation zone 30, fractionation zone 30 can be refluxed with the liquid hydrocarbons in gasoline accumulator 38, that is, accumulator 38 may serve as a reflux accumulator for fractionation zone 30. The hydrocarbon fraction taken overhead from fractionation zone 30 and passed into gasoline accumulator 38 contains the gasoline and light hydrocarbons not taken overhead in fractionation zone IG which were formed in the cracking operation. Substantially all of the C4 and C5 olens which were formed in the cracking operation are contained inl the fraction passed to accumulator 38. A light hydrocarbon fraction, such as a hydrocarbon fraction comprised predominantly of C3 hydrocarbons, is added either to the feed line I2 to accumulator 38 or directly to accumulator 38. Either method of adding the light hy- A gaseous fraction comprised The gases leaving the top of fracdrocarbon fraction will give the desired results. A separation of vapor and liquid present in accumulator 38 is made. An overhead fraction comprised predominantly of C5 and lighter hydrocarbons is passed through line 2'I to a light hydrocarbon recovery system. A liquid hydrocarbon fraction is withdrawn from the bottom portion of accumulator 38 and passed through line 25 to a gasoline stabilization zone or stabilizer represented by fractionation zone 42. Gasoline accumulator 38, is operated at a pressure of from 8 to 16 pounds per square inch gauge and at a temperature from F. to 120 F. and operates primarily as a liquid-vapor separator, or flash zone, with no attempt at fractionating. The liquid hydrocarbon fraction Withdrawn as bottoms from accumulator 38 contains substantially all of the gasoline formed in the cracking operation and some Cs,-C4 and C5 olefins along with other C5 and lighter hydrocarbons. Stabilized gasoline is withdrawn from the bottom portion of fractionation zone 42 through line 29. Hydrocarbon vapors containing a substantial portion cf the C5, and lighter hydrocarbons contained in the feed to fractionation zone 42 are withdrawn overhead from fractionation zone 42 and condensed in condenser 46 which discharges to reflux accumulator 50. Part of the condensed hydrocarbons are used to reflux fractionation zone 42 and the remainder are passed through line 3i to a light hydrocarbon recovery system. The gasoline stabilizer represented by fractionation zone 42I is operated With a top ternperature of from 140 F. to 180 F. and at a pressure of from 5 to 13 pounds per square inch absolute. The withdrawn overhead hydrocarbon fractions from fractionation zone IIJ, separation zone or gasoline accumulator 3B and fractionation zone or gasoline stabilizer 42 are passed to a light hydrocarbon recovery zone which is represented on the diagrammatic flow sheet, by fractionation zones 54, 60 and 66. The light hydrocarbon recovery system just referred to is used to recover C3, C4 and C5 oleflns to use as reactants in an alkylation process represented by alkylation reactors 12, acid settling zone 16 and acid stripping zone 80. In fractionation zone 54 a cut is made between C5 hydrocarbons and gasoline present in the feed streams to the light hydrocarbon recovery system. C5 and heavier hydrocarbons carried over in the overhead from fractionation zone 42 and accumulator 38 are removed as bottoms product from fractionation zone 54 via line 39. The overhead from fractionation zone 54 containing C5 and lighter hydrocarbons is passed through line 31 to fractionation zone 60. The overhead from fractionation zone 54 may be deethanized if desired (means to do this not shown on the flow diagram). C3. C4 and C5 oleflns are taken overhead from fractionation zones 60 and 66 Via

lines

58 and 62 respectively, and are passed through line 55 to alkylation reactors 12 which are charged with isoparafiins through line 51, with new acid through lines 59 and 69, and with recycle acid through lines and 69 and/or 6I and 69. If desired, C3 and C4 olens may be withdrawn from the system through lines 43 and 5I and/or 43, 56 and 53. C5 olefins may be Withdrawn from the system via lines 45 and 49 and/or 45, 52 and 53, if desired. A. C5 bottoms fraction from fractionation zone 60 is passed via line 4I to fractionation zone 66. Pentane bottoms from fractionation zone 66 is removed via line 41. Acid settling zone 16 fed through line 63 and acid stripping zone 80 fed through line 61 are used to separate alkylate and unreacted parailins from the reaction mixture.

A mixture of alkylate and unconverted reactants is withdrawn from acid stripping zone 8D through line TI and passed to fractionation zone 84. The alkylate formed in the alkylation reactors is withdrawn from the bottom portion of fractionation zone 82B through line l1. The unconverted parailns are recovered as represented by side cuts taken oli` through lines 'I3 and 15. A light hydrocarbon fraction comprised pref dominantly of C3 hydrocarbons is taken overhead from fractionation zone 85. This light hydrocarbon fraction may be recycled to the gasoline accumulation zone 38 through line I9 entering line I1. t may also be passed directly to the overhead from fractionation zgne 30 by passing it to line I2. If desired, the overhead from fractionation zone 84 may be withdrawn from the system through line 8l. It is to be understood that the foregoing discussion of my invention is given only as a preferred method of operation and the scope of my invention is not to be unduly limited by said foregoing discussion. The drawing is only a diagrammatic ow sheet used in order to enable me to clearly point out my invention. One skilled in the art will appreciate that various modifications and arrangements of valves, pumps, auxiliary equipment, etc. can be made for any particular plant carrying out my invention.

Example The temperatures, pressures, quantities, equipment, etc. referred to in the following example are only illustrative of my invention and are not to be deemed to unduly narrow the scope of inlet pressure was 2-200 pounds per square inch gauge and the inlet temperature of the straightrun naphtha was 746 F., the straight-run naphtha having been preheated by the furnace efliuent in a heat exchanger represented by heat exchanger 6. The temperature of the furnace eiiluent was 970 F. The furnace effluent was.

cooled to 490 F. and passed into a depropanizer represented by fractionation zone I0. Said depropanizer was operated at a top pressure of 330 pounds per square inch gauge and a top temperature of 210 F. The pressure drop over the depropanizer was 5 pounds per square inch. A gaseous fraction comprised predominantly of H2 and CHl was vented from accumulator I8 at a rate of 2081 M. C. FJD. The depropanizer was refluxed with a liquid hydrocarbon stream from accumulator I8 at a rate of 5,325 B./D. A hydrocarbon fraction comprised predominantly of Cz, C3 and C4 hydrocarbons was withdrawn from accumulator I8 through line 8 as overhead product. A `hydrocarbon fraction comprised predominantly of C4 and heavier hydrocarbons was withdrawn from the bottom of the depropanizer at a rate of 10,000 B./D. and passed through line 9 to a flash zone 26. lI'his bottoms product was heated from 546 F. to 692 F. in a feed heater represented by heating zone 22. Flash zone 26 was operated at a pressure of 65 pounds per square inch gauge and at a temperature of 670 F. A fuel oil product was withdrawn from the CTI bottom of flash zone 26 at a rate of 228 B./D. and passed through line I5 to storage. The overhead gas from flash zone 26 was passed to fractionation zone 30 through line I3. Fractionation zone 30 was operated at a top temperature of 423 F. and at a top pressure of 61 pounds per square inch gauge. The pressure drop across the fractionator represented by fractionation zone 30 was approximately 5 pounds per square inch. A side stream consisting of hydrocarbons boilingv in the kerosene range was withdrawn from fractionation zone 30 at the rate of 653 B./D. Gasoil was Withdrawn from the bottom of fractionation zone 30 at the rate of 975 B./D. Gasoline and lighter hydrocarbons were withdrawn overhead from fractionation zone 30 and passed through a clay tower operated at 59 pounds per square inch gauge and then passed to a polymer separator. The clay tower and polymer separator are not shown on the diagrammatic flow sheet. The conventional clay tower converted diolens and gum-forming constituents to polymers which were removed from the bottom 0E; the polymer separator at a rate of 192 B./D. "The polymer separator was operated at the top temperature of 353 F. and at a pressure of 43 pounds per square inch gauge. The polymer-free gasoline and light hydrocarbons were passed to a gasoline accumulator represented by accumulator 38. VGasoline accumulator 38 was operated at a temperature of 102 F. and at -a pressure of l2 pounds per square inch gauge. Fractionation zone 30 was reiiuxed at a rate of 5380 B./D. with liquid hydrocarbons withdrawn from accumulator 38. A gaseous hydrocarbon fraction was withdrawn overhead from accumulator 38 at a rate of 663 B./D. and had the following composition:

Gasoline accumulator overhead C1 76.2 C2 169.7 C2' 22.5 C3 137.2 C3l 41.8 C4 71.6 -C4, 13.9 n-.C/L C5' 21.9 C5 39.1 06+ 47.1 Total am) A liquid hydrocarbon fraction was withdrawn from the bottom of gasoline accumulator 38 and passed to a gasoline stabilizer represented by fractionation zone d2 at a rate of 3790 B./D. This feed to the gasoline stabilizer had the following composition: Y

Bottoms from gasoline accumulator The gasoline stabilizer represented by fractionation zone 42' was operated with a top temperature of 160 F. and at a top pressure of 9 pounds per square inch absolute. An overhead product was Withdrawn from fractionation zone 42 through line 3| at a rate 359 B./D. and had the following composition:

Overhead from gasoline stabilizer C2 29 Cz' 3 C3 69 C3' 19 C4 79 -C4' 10 Irl-C4' 23 C5 45 C5 82 06+ 0 Total 359 Stabilized gasoline was withdrawn from the bottom of fractionation zone 42 at the rate of 3431 B./D. through the line 29. This stabilized gasoline had the following composition:

Stabilizer gasoline "5 B./ D. C4 48 -C4' 8 I1-C4l 1 C5' 'l2 C5 99 C6+ 3,190

Ttal 3,431

The overhead from gasoline accumulator 38 was combined with the overhead from the gasoline stabilizer represented by fractionation zone 42 and passed to a light hydrocarbon recovery system represented by fractionation zones 54, and 66. The combined streams gave a total of 1023 B./D. of light hydrocarbons. This light hydrocarbon feed stream had the following composition:

Gasoline accumulator overhead plus gasoline Thus 197 B./D. of C3, C4 and C5 olefins were available to be recovered in the light hydrocarbon recovery system.

A similar test was then run in which a C3 hydrocarbon stream was added to gasoline accumulator 38 through line Il at a rate of 500 B./D. The same amount of straight run naphtha was reformed, fractionating and heating conditions remained the same and the same amount of fuel,

oil, gas oil, kerosene, polymer and overhead from the de-propanizer were withdrawn from the system. All towers were refluxed at the same rate.

On overhead gas was withdrawn from the gasoline accumulator 38 at a rate 1,162 B./D. This overhead gas stream now had the following composition:

Gasoline accumulator overhead-Cs added C1 76.2 C2 193.6 C2' 25.1 C3 466.9 C3' 53.2 C4 107.2 i-C4' 20.3 Il-C4' 32.3 C5' 35.7 C5 62.4 Cs+ 89.5

TOtal 1,162.4

A liquid hydrocarbon stream was withdrawn from the bottom of gasoline accumulator 38 and fed to a gasoline stabilizer, represented by fractionation zone 42, through line 25 at a rate of 3,790 B./D. This gasoline stabilizer feed stream now had the following composition:

Bottoms from gasoline accumulator- C3 added C2 19 C2' 2 C3 139 C3' 15 C4 113 -C4' 15' n-C4' 33 C5' 112 C5 172 C6+ 3,170

Total 3,790

The overhead from the gasoline stabilizer now had the following composition and was withdrawn at the rate of 498 B./D.

Overhead from gasoline stabilizer-C3 added C2 19 C2l 2 C3 139 C3' 15 C4 78 -C4I 10 11-C4l 23 C5' 52 C5 90 C6+ 70 Ttal 498 The stabilized gasoline withdrawn from the bottom of the gasoline stabilizer now had the following composition and was withdrawn at a rate of 3292 B./D.

Stabilized gasoline- C3 added C4 35 -C4' 5 )f1-C4' 10 C5' 60 C5 82 C6+ 3,100

TOtal 3,292

The overhead from the gasoline stabilizer and the overhead from the gasoline accumulator now totaled 1660 B./'D. and had the following composition.

Gasoline accumulator overhead plus. gasoline stabiliser overhead- Ca added B./D. C1 76 C2 213 Cz' 27 C3 606 C' 68 C4 185 -C4 30 Il-Ci' 55 C5l 88 C5 152 l 06+ 160 'IOtal 1,660

Thus the addition of the C3 hydrocarbon stream to gasoline accumulator 38 made available 241 B/D. of C3, C4 and C5 oleiins for recovery in the light hydrocarbon recovery system.

y,The addition of C3 hydrocarbons to the gasoline accumulator increased the recovery of C3, C4 and C5 oleiins by 44 B./D., and by 22 per cent.

As will be evident to those skilled in the art, various modifications of this invention can be made, or followed, in thelight of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

I claim:

l. An improved process for recovering C3, C4 and C5 olefins from a mixture of hydrocarbons containing the same in admixture with other hydrocarbons, said mixture being a thermally cracked straight run naphtha fraction, which comprises passing said mixture `to a first fractionation zone operating under a pressure of from 300 to 360A pounds per square inch gauge with a top temperature of from 190 F. to 230 withdrawing overhead from said first fractionation zone a hydrocarbon fraction comprised predominantly of C2, C3 and C4 hydrocarbons, withdrawe ing a hydrocarbon fraction as bottoms from said rst fractionation zone, passing said withdrawn bottoms to a second fractionation zone operating under a pressure of from 50 to '70 pounds per square inch gauge and with a top temperature of from 375 F. to 450 F., withdrawing overhead from said second fractionation Zone a gaseous hydrocarbon fraction, at least partially condensing same and introducing into a portion of same a hydrocarbon fraction 'comprised predominantly of Ca hydrocarbons, passing a resulting hydrocarbon mixture into an accumulation zone opLrating under a pressure of from 8 to 16 pounds per square inch gauge and at a temperature oi' from 80 F. to 120 F., withdrawing a gaseous hydrocarbon fraction overhead from said` accumulation zone, 'withdrawing a liquid hydrocarbon fraction from the bottom portion of said accumulation zone and passing same to a third fractionation zone operating under a pressure of from 5 to 13 pounds per square inch absolute and at a top temperature of from 140 F. to 180 F., withdrawing a hydrocarbon fraction overhead from said third fractionation zone, passing said hydrocarbon fractions, Withdrawn overhead from said iirst fractionation zone, said accumu lation Zone and said third fractionation zone, to a fourth fractionation zone and recovering therefrom Cs, C4 and C5 olens.

2. An improved process for recovering C3, C4

and C5 olens from a mixture of hydrocarbons constituting effluent from a light oil cracking furnace, which comprises first fractionation zone, withdrawing overhead from said first Afractionation zone a hydrocarbon fractionv comprised predominantly of Cz, C3 and C4 hydrocarbons, withdrawing a hydrocarbon fraction from the bottom portion of said first fractionation zone comprised predominantly of C4 and heavier hydrocarbons, passing said hydrocarbon fraction withdrawn from the bottom portion of said first fractionation zone to a second fractionation zone, withdrawing overhead from said second fractionation zone a gaseous hydrocarbon fraction containing gasoline and lighter hydrocarbons not taken overhead in said rst fractionation zone, at least partially condensing same and passing a portion ofsaid hydrocarbon fraction withdrawn overhead from said second fractionation zone to an accumulation zone, introducing into said accumulation zone a light hydrocarbon fraction containing C3 hydrocarbons and minor amounts of other light hydrocarbons, withdrawing a gaseous hydrocarbon fraction overhead from said accumulation zone comprised predominantly of C5 and lighter hydrocarbons, withdrawing a liquid hydrocarbon fraction from the bottom portion of said accumulation zone containing C6 and heavier hydrocarbons and containing appreciable quantities of C5 and. lighter hydrocarbons, passing said liquid hydrocarbon fraction Withdrawn from the bottom portion of said accumulation zone to a third fractionation zone, withdrawing overhead from said third fractionation zone a hydrocarbon fraction containing a substantial portion of the C5 and lighter hydrocarbons contained in the above Vmentioned hydrocarbon fraction passed to said third fractionation zone, passing said hydrocarbon fractions, withdrawn overhead from said rst fractionation zone, said accumulation zone and said third fractionation zone, to a fourth fractionation zone and recovering therefrom C3, C4 and C5 olens.

3. An improved process for recovering C3, C4 and C5 olens from a mixture of hydrocarbons constituting eiiiuent from a naphtha fraction cracking furnace, which comprises passing said mixture to a first fractionation zone, withdrawing overhead from said first fractionation zone a hydrocarbon fraction comprised predominantly of C2, C3 andC4 hydrocarbons, withdrawing a hydrocarbon fraction from the bottom portion of said first fractionation zone comprised predominantly of C4 and heavier hydrocarbons, passing said hydrocarbon fraction withdrawn from the bottom portion of said first fractionation zone to a second fractionation zone, with- .rawing overhead from said second fractionation zone a gaseous hydrocarbon fraction containing gasoline and lighter hydrocarbons not taken overhead in said first fractionation zone, at least partially condensing same and passing a portion cf said hydrocarbon fraction withdrawn overhead from said second fractionation zone to an accumulation zone, introducing into said accumulation zone a light hydrocarbon fraction containing C3 hydrocarbons and minor amounts of other light hydrocarbons, withdrawing a gaseous hydrocarbon fraction overhead from said accumulation zone comprised predominantly of C5 and lighter hydrocarbons, withdrawing a liquid hydrocarbon fraction from the bottom portion of said accumulation zone containing Cs and heavier hydrocarbons and containing apprepassing said mixture to a.

ciable quantities of C5 and lighter hydrocarbons, passing said liquid hydrocarbon fraction withdrawn from the bottom portion of said accumulation zone to a third fractionation zone, withdrawing overhead from said third fractionation zone a hydrocarbon fraction containing a substantial portion of the C5 and lighter hydrocarbons contained in the above mentioned hydrocarbon fraction passed to said third fractionation zone, passing said hydrocarbon fractions, withdrawn overhead from said firstl fractionation zone, said accumulation zone and said third fractionation zone, to a fourth fractionation Zone and recovering therefrom C3, C4 and C5 olefins, alkylating said C3, C4 and C5 olefins recovered from the above mentioned three hydrocarbon fractions, passing a resulting alkylation reaction product to a fth fractionation zone, withdrawing overhead from said fifth fractionation zone a hydrocarbon fraction containing C3 hydrocarbons and minor amounts of other light hydrocarbons, and recycling said hydrocarbon fraction withdrawn overhead from said fifth fractionation zone to said accumulation zone.

4. The improved process of claim 2 wherein said light-oil is a naphtha fraction.

5. An improved process for recovering C3, C4 and C5 olens from a mixture of hydrocarbons constituting eiiiuent from a light-oil cracking furnace, which comprises passing said mixture to a rst fractionation zone, withdrawing overhead from said first fractionation zone a hydrocarbon fraction comprised predominantly of C2, C3 and C4 hydrocarbons, withdrawing a hydrocarbon fraction from the bottom portion of said first fractionation Zone comprised predominantly of C4 and heavier hydrocarbons, passing said hydrocarbon fraction withdrawn from the bottom portion of said first fractionation zone to a second fractionation zone, withdrawing overhead from said second fractionation Zone a gaseous hydrocarbon fraction containing gasoline and lighter hydrocarbons not taken overhead in said rst fractionation zone, removing polymer forming constituents from same, at least partially condensing said resulting gasoline and lighter hydrocarbons and passing resulting hydrocarbons into an accumulation zone, introducing into said accumulation zone a light hydrocarbon fraction containing C3 hydrocarbons and minor amounts of other light hydrocarbons, withdrawing a gaseous hydrocarbon fraction overhead from said accumulation zone comprised predominantly of C5 and lighter hydrocarbons, withdrawing a liquid hydrocarbon fraction from the bottom portion of said accumulation Zone containing C5 and heavier hydrocarbons and containing appreciable quantities of C5 and lighter hydrocarbons, passing said liquid hydrocarbon fraction withdrawn from the bottom portion of said accumulation zone to a third fractionation zone, withdrawing overhead from said third fractionation zone a hydrocarbon fraction containing a substantial portion of the C5 and lighter hydrocarbons contained in the above-mentioned hydrocarbon fraction passed to said third fractionation zone, passing said hydrocarbon fractions, withdrawn overhead from said first fractionation zone, said accumulation zone and said third fractionation Zone, to a fourth fractionation Zone and recovering therefrom C3, C4 and C5 olens.

6. The improved process of claim 5 wherein said light-oil is a naphtha fraction.

DONALD M. LITTLE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,901,863 Beardsley et al. Mar. 2l, 1933 2,387,309 Sweeney Oct. 23, 1945 2,408,753 Burk Oct. 8, 1946 2,442,440 Scoville June 1, 1948

Claims

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