US2391555A - Production of dienes - Google Patents

Description

Dec. 25, M DE slM'O ETAL PRODUCT ION OF DIENES Filed June 16, 1941 |36 l lOl 95 lnvzn'rors. Martin Dz Simo Rober? M. Roba-is Patented Dec. 25, 1945 UNITED STATES PATENT OFFICE,

PRODUCTION F DIENES Martin de Simo, Piedmont, and Robert M. Roberts, Berkeley, Calii., assig'nors to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application June 16, 1941, Serial No. 398,265

4 Claims. (Cl. 260680) genation of corresponding oleflnes, by dehalogenation or dehydrohalogenation of corresponding halogen derivatives, dehydration of corresponding alcohols, etc. 'Ihese methods, however, have certain disadvantages, for example, the cost involved in preparing the starting materials` It is also known that cracking of hydrocarbons often yields minor proportions of dienes, particularly of butadiene. 'Ihe proportion of the diene in the cracked products is, however, small and it is associated with major amounts of saturated and ethylenic hydrocarbons having nearly equal vapor pressures. Thus, the recovery of the dienes is diicult and their yields are small.

It is an object of this invention to provide a produce hydrocarbon fractions rich in diandv mono-olenes and aromatic hydrocarbons, from which the dienes can be separated in a simple manner. Another purpose is to pyrolyze, i. e., to

crack, selected cracked naphthas in order to raise their octane numbers and to produce large amounts of normally gaseous diolenes rather than mono-olenes. Still further objects include simplied procedures, decreased cost in the manufacture of dienes, and other improvements found hereinafter.

We have found that when pyrolyzng cracked gasoline distillates-in the vapor phase at temperatures above 600 C., gases may be obtained which contain very large proportions of dienes, much larger than those obtainable by cracking under similar conditions of straight-run hydrocarbon oils including gasolines, and that selected fractions of such gases may comprises almost 50% Dentadienes.

Accordingly, our invention comprises pyroliz, ing cracked gasoline distillates so as not only to raise materially the4 octane numbers of resulting oils, etc., at temperatures below 600 C., separating from the cracked product a gasoline distilt late and further pyrolyzing the latter in the va` por phase at temperatures above 600 C.

The pre-cracking may be conducted either in 'the liquid, mixed or vapor phases, depending on the results desired. Suitable pre-cracking temperatures should be below 600 C. and may in general be between 400 and'600 C. Pressures may range from atmospheric to 5000 p. s. i. and more. If desired, the cracking may be 'conducted in the presence of catalysts, preferably catalysts which do not promote cyclization oflolerines such as activated alumina, zeolites, silicates,

, etc.

` oleiinic double bonds may be estimated by the bromine number which is determined, for example, by the method of Uhrig and Levin, Indus trial and Engineering Chemistry, Analytical edltion, 13 90, 1941. The content of aromatics may be calculated from its bromine number and specic'dispersion by a method such as that described by Grosse and Wackher, Industrial and Engineering Chemistry, Analytical edition, 11 614, 1939.

In general, higher cracking temperatures and pressures, and particularly higherresidence times favor the production of aromatics. Under more moderate conditions of temperature, time and pressure, mono-olenes are formed in larger amounts. Thus cracking at relatively low temperatures, i. e., below about 540 C. (1000 Ft), gives in general relatively low proportions of aromatics but may result in high contents of olenes.

Therefore, cracked gasoline distillates obtained in this way are in general preferred over those are especially suitable as the feed which is recracked.

The presence of aromatic hydrocarbons in the pre-cracked gasoline distillate does not by itself reduce its suitability. 'Howeven aromatics are normally formed at the expense of olenes and thus a high content of aromatics is usually accompanied by a relatively low content of olenes. Moreover, aromatics do not on cracking contribute tothe formation of dioleflnes. Therefore, pre-cracked distillates having aromatic contents below about 30%, and preferably below about 15%, are particularly suitable.

Relatively low boiling lean distillates, particu- 'larly those boiling up to about 120 C., are preferred, as they give in general higher yields of dienesrthan higher boiling distillates from the same source. This may be due to the fact that in general relatively low boilingfractions have higher bromine numbers, i. e., higher olene contents than the heavier fractions from the same source.

Suitable pre-cracked gasoline distillates may contain minor proportions of C4 and C5 hydrocarbon, but the main portions should consist of Ce and higher hydrocarbons which yield C4 and C5 dienes by breaking of carbon-carbon bonds, rather than by dehydrogenation, the latter requiring conditions different from those necessary for optimum diene production by carboncarbon scission.

Cracked gasolines, unless refined, generally contain highly unstable components, such as those known as gum-forming compounds, e. g., polyolenes, certain branched and other highly reactive oleines,l styrenes, etc., which, upon recrackng, are apt to form tars and coke. Likewise, nitrogen, sulfur, oxygen, etc., derivatives, particularly the so-called petroleum bases, may also favor cokeformation ii.' present in excessive amounts. These objectionable components may be removed by any suitable means prior to recracking, such as by extraction with a liquid having greater solvent power for aromatics than for paraflinic hydrocarbons, for example, liquid SO2, nitrobenzene, phenol, cresol, methyl acetate, phenyl acetate, methyl or ethyl cellosolve, furfural, acetone, aniline, cresylic acids, dichlorethyl ether, etc., alone or in combination with an anti-- solvent such as propane, under conditions to cause mono-olefines to remain in the resulting raffinate; or by treatment with adsorption agents such as clay, bauxite, iullers earth, diatomaceous earth, silica gel, etc., preferably at elevated temtemperature to which the cracking stock is heated, is above 600 C., preferably 650-850 C.

The resulting cracked products are rapidly quenched and then fractionally distilled, preferably under conditions to produce separate C4 and C5 fractions, and from them the dienes, which constitute substantial proportions thereof, are isolated by conventional methods. Other products obtained in this process include, besides the dienes, tar, tar-free naphtha or middle oil, or both, and gases lighter than naphtha ycontaining high percentages of mono-olefines.

These products may be withdrawn, treated and used as may be convenient. Certain portions of them may, however, advantageously be utilized within the process.

The composition of the cracked naphtha obtained in this process varies with the intensity of the cracking, i. e., it may substantially retain the composition of the cracking feed, or it may have become much more aromatic.

If the cracking is relatively superficial, i. e.,

' the gasication per pass is of the order of 15%- peratures below incipient cracking; or by suli furiel acid or phosphoric acid treatment at suitable concentrations to produce a sludge containing the undesirable constitutents which can be separated without materially ailecting monoolenes, etc. f

. When compared with the vapor phase cracking above 600 C. of straight-run gasolines, it is found that the described pre-cracked gasolines give a much larger yield of dioleilnes, and particularly pentadienes.

When compared with the vapor phase cracking of heavier-than-gasoline hydrocarbons, our process gives a higher concentration of dienes in the C4, and particularly in the C5, fraction and, furthermore, coking and carbon formation is greatly decreased so that the cracking operationis very appreciably facilitated.

Thus, according to this invention, a cracking stock consisting mainly of cracked gasoline distillates, and preferably a mixed phase cracked gasoline having a bromine number above 40 and containing less than 307`aromatics, is cracked in the vapor phase at temperatures above about 600 C. and pressures not substantially above atmospheric for a time to gasify and transform in one or several passes a. maior portion of said gasoline into normally gaseous products.

The vapor phase cracking to produce the dienes is conducted so as to gasify 15%-85% of the feed per pass. We define as gasied that part of the cracked products which consists of molecules having less than six carbon atoms.

The cracking temperature, i. e.l the maximumv 50%, the ungasied portion of the cracking stock is but little aromatized, and may therefore be re-cracked and treated like the original cracking stock, for example, by recirculation as part of said stock. Or else a reformed gasoline of high octane number may be recovered from it.

If the cracking is deep, i. e., the gasification isof the order of 50 %-85%, the naphtha fraction of the cracked products may contain large amounts of aromatic hydrocarbons which are formed by secondary reactions such as polymerization, condensation, cyclization, etc., of the primary products. In this case, it is advantageous to separate valuable aromatic hydrocarbons such as benzene, toluene, styrene, etc., from the naphtha fraction and then, if desired, to further crack remaining non-aromatic components. 'I'his latter procedure, i. e., the deep cracking, is often preferable because the yields of diolenes in the gaseous products are higher and, simultaneously,

'liquid products are produced which are valuable -These hydrocarbons may be desirable in themselves. On the other hand, they may interfere. for example, in the cracking because of their tendency to polymerize, or in the recovery o! aromatics with the aid of selective solvents because of their solubility therein which comes close to that of aromatics. It is thus often desirable to separate and eliminate them.

The cracking in this process is conducted at pressures of about atmospheric orbelow, and preferably not substantially above 10D-150 lbs. p. s. i. Thus, pressures above atmospheric sufficient to overcome the resistance of conduits, condensers and other equipment which follow the cracking zone may be maintained therein. If desired, the cracking may be conducted in the presence of steam, nitrogen and other substantially inert gases and the concentration of such gases may vary from traces up to 80% of the mixture.

The heating of the hydrocarbonsto the cracking temperature may be achieved by contact with a heated surface, for example, in an externally heated coil, or in a preheated brick checker work: or by admixture of hot, substantially inert gases; or by a. combination of 'both methods. The rate of heating should be as high as possible to avoid undesirable side reactionsv at the intermediate temperatures. In order to avoid destruction of dienes formed in the cracking, the cracked product should be quenched rapidly from the cracking temperature down to safe temperatures, preferably below about 400 C.

In order to prevent excessive coke formation in the cracking zone, it is often preferred that this zone be constructed of non-ferrous metals or alloys containing less than predominating amounts of iron. Materials which do not cause rapid coke formation,'such as copper-containing alloys, e. g1., aluminumor phosphor-bronze, or refractory ceramics, are thus preferred in the construction of surfaces which are exposed to hydrocarbons at cracking temperatures.

The velocity at which the gases pass through the cracking zone depends on the exact result desired, the temperature and pressure of cracking, and t-he dimensions of the cracking Zone. The space velocity necessary to produce the desired degree of gasicationincreases rapidly as the cracking temperature is raised. The real velocity of the gaseous hydrocarbons in the cracking zone is very diflicult to determine accurately because the volume of the cracking stock changes rapidly under the cracking conditions. Therefore, in dening this invention, we prefer to utilize the concept of liquid space velocity 'which is the liquid volume of the feed delivered to the cracking zone per volume of said zone per hour. The cracking zone is defined as the space occupied by gas in which the temperature is within C. of the maximum cracking temcaratura rately measured and, for a given pressure, temperature and dilution, determines the real space velocity oi the vapors.

Our process is illustrated in the accompanying drawing which represents a simplified flow diagram of an embodiment of this invention.

For simplicity, the drawing does not show pumps, tanks, heat exchangers, valves, by-passes, vents, reboilers, condensers, coolers, and other auxiliaries, the proper placement of which will be at once evident to those skilled in the art.

A hydrocarbon oil is introduced from storage, not shown, through line I into cracking and 'distilling zone 2, wherein a cracked gasoline distillate is prepared and withdrawn through line S. The cracked gasoline so obtained is preferably refined in reiining zone 4 where it is treated to remove gum-forming and non-hydrocarbon components. The refined cracked gasoline distillate is then withdrawn through line 5. Suitable recycle stock obtained inthe process as hereinafter described may be admitted through, Eine 8 into line 5.

The mixed liquids are led through' line 5 into cracking coil l in furnace 9 in which the desired conditions of temperature. pressure and residence time are maintained, for example, .at 700 C., atmospheric Apressure and ,a liquid space velocity of 300, so as to eect a gasification of about The cracked products which leave reaction coil 7 through line Il are quenched with suitable quenching iluid introduced through line I2 from a source not shown, and are separated from tars and other relatively heavy materials in Iractionator I3. The lighter products, including gases and naphtha, are Vtaken overhead through vapor line l5. The heavier products are withdrawn from the system through line I 1.

The products in line I5 pass into stabilizer I9, wherein an overhead fraction consisting essentially of C5 and lighter components is separated from a heavier naphtha fraction; the latter, substantially free of Cs and lighter compol nents, is taken through line 2| and may go Liquid space velocity can be accuthrough line 23 to storage not shown, or preferably may serve as recycle stock which is returned through lines 25 and 8 to the cracking coil l. If the cracking stock is composed of hydrocarbons substantially heavier than C5, it is advantageous to also eliminate from the recycle stock those components formed during cracking which are lighter than the original' cracking stock. This can be done by withdrawing an appropriate side stream from fractionator I9 through line 20.

If desired, the polyolefines contained in this naphtha fraction may be separated in zone 2l, for example, by extraction with a suitable complex-forming reagent, e. g., CuCla, AgCl, SO2, etc., or by polymerization with or without suitable catalysts such as poly-oxy acids, e. g., HzSOa HaPOr, etc., or Friedel-Crafts catalysts, or by the'Diehls-Alder reaction involving the use of maleic anhydride-type reagents, etc.

The overhead C5 and lighter fraction from stabilizer I9 is taken through conduit 29 to a fractionating system wherein .it is subjected repeatedly to fractional distillation. In column 3la bottom fraction is produced comprising predominantly hydrocarbons having five carbon atoms, which is taken through line 33, and a lighter fraction substantially free from' Cs components. The latter is taken overhead through line 35 to fractionator 3l, where a bottom fraction comprising predominantly hydrocarbons having mostly four carbon atoms is produced and withdrawn through Iline 39. Remaining gases substantially free from C4 and heavier components are withdrawn overhead through line-SI;

'I'hus, separate C: and C4 fractions are taken through lines 33 and 39. respectively.

The C4 fractionwhich contains butadiene is led through line 39 to-column 43, wherein itis subjected to extractive distillation with a suitable solvent having a, relatively high boiling range and greater solvent power for butadiene than for lbutylenes and butanes. This liquid is supplied to the top of extractor 43 through line 4 5 and is contacted with the ascending C4 vapors so as to produce a fat solvent charged with substantially pure-butadiene. 'Ihis fat solvent is eliminated from the bottom of the column 43 through line 41. This fat liquid is separated by fractional distillation in fractionator B9 into the lean solvent and butadiene. The butadiene so produced is withdrawn through overhead line 5I while the lean solvent is drawn ol at the bottom through line 65 and is returned to extractor Q3. 'Ihe butylenes and butanes separated from the butadiene in column i3 are withdrawn through line 53. The large proportion of olefines in this fraction makes it valuable' for the manufacture of polymer or alkylate gasoline.

The C5 fraction, rich in pentadiene, is given a treatment similar to that of the C4 fraction. Line 33 leads it to column 55, wherein it is extracted in the-vapor phase by a. lean solvent supplied through line 51. The fat solvent is drawn oil through line 52, and the residual pentanes and amylenes are withdrawn through line 6|. The' fat solvent is fractionally distilled in fractionator i3 to isolate pentadienes which are withdrawn through line 65, while the lean solvent is recirculated through line 51.

Suitable solvents for the vapor phase extraction of dienes include, for example, alcohols such as butyl alcohols, amyl alcohols, cyclohexanol, etc.; aldehydes such as furfural, acrolein, crotonaldehyde, etc.; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, acetophenone, cyclohexanone; ethers such as di-isopropyl ether, dichlorethyl ether, dioxane; organic nitrogen bases such as -aniline, toluidine, phenylhydrazine, ethylene diamine, ethanol diamine, phenols such as phenol, cresol, xylenols; esters such as ethylene chlorhydrin, d-imethyl phthalate, diethyl tartrate, ethylene dichloride, lactic acid nitrile, acetonitrile, di-isopropyl or allyl sulfate, ethyl borate or ethyl orthosilicate; other organic derivatives of mineral acids such as nitrobenzene, nitromethane, nitroethane, and many other polar compounds.

The recovered pentadienes usually vcomprise a mixture of isoprene (2-methy1 butadiene), piperylene '(n-pentadiene) and cyclopentadiene. This mixture may be either withdrawn through line 61 or, if desired, may be further fractionated to separate the several C5 dienes from one another by leading it through line 69 to zone 1|, where the cyclopentadene is dimerized by subjecting the mixture to moderate' heat in the liquid phase. 'I'he resulting product is fractionally distilled, the dimer forming the distillation residue which is withdrawn through line 13 to be withdrawn through line 15, or to be regenerated by exposure to high temperature and vacuum in a depolymerizing zone 19, t0 which line 11 leads. The vapors of the a-cyclic pentadienes are removed from the dimerization zone 1| through line 8|, and may be further separated in fractionator 83 to produce isoprene which is taken overhead through line 85 and piperylene, which is obtained at the bottom, through line 81. i

The process as illustrated in Fig. 1 can also be carried out in a different manner if desired. The cracking in coil 1 may be intensied, for example, by reducing the space velocity to about 20 under the same conditions of pressure and temperatureI so as to obtain a gasication of about 65%. The naphtha fraction produced under these circumstances and taken through line 2| from fractionator |9 contains aromatic hydrocarbons.

These valuable hydrocarbons may be separated and the recycle stock recovered by convenient methods, such as fractional and extractive distillation, or liquid-liquid extraction, or both. For example, the liquid withdrawn through line 2| is led through line 89, and preferably through zone 21 where it is freed of polyoleines, to fractionator 9| wherein it is split into a narrow Cs overhead fraction taken through line 93 and a bottom fraction of C1 and heavier components withdrawn through line 95. This line leads to fractionator` 91, wherein another narrow C1 overhead fraction is separated and taken through line 99, and a bottom fraction substantially free from C1 hydrocarbons which is withdrawn through line |0|, either to be discarded or further treated, as will be described.

The C5 and C1 fractions thus separated are rich in benzenel and toluene, respectively.

The C5 fraction is conductedthrough line 93 to column |03 wherein it issubjected to. extractive distillation witha lean selective solvent for aromatics introduced through pipe |05. The fat solvent rich in benzene leaves through line |01 and the residual Cs hydrocarbons are withdrawn overhead through line-|09 or may be returned to the cracking coil 1 through lines 25 and 8. The fat solvent is fractionally distilled in fractionator so that the lean solvent is recovered as the bottom fraction and returned through line |05 to extractor |03, and the benzene is obtained overhead through line H3.

The gaseous C1 fraction is conducted through line 99 to column ||5 wherein it is extracted with a lean solvent for aromatics introduced through line |I1. The fat slvent rich in toluene leaves through line H9, and the residual C'z hydrocarbons are withdrawn overhead through line |2| and may be returned to the cracking coil 1 throughlines 25 and 8. The fat solvent is fractionated in fractionator |23, so that the lean solvent is recovered as the bottom fraction and is returned through line I1 to extractor I l5. The toluene is obtained overhead through line |25.

Suitable solvents for the recovery of aromatics yby extractive distillation are, for example, phenol, cresylic acids, alkyl phenol mixtures, aniline, alkyl anilines, diphenyl amine, ditolyl amines, carbitols (diethylene glycol mono-ethers) such as methyl, ethyl and propyl carbitols, chlorinated dialkyl ethers such as beta-beta-dichlorethyl ether, nitrobenzene, nitrotoluene, nitroxylenes,

naphthols, alkyl naphthols, benzo phenone, phe.

nyl tolyl ketone, diphenyl ketone, alkyl phthalates such as dimethyl phthalate, alkyl salicylates such asmethyl salicylate, benzyl alcohol, benz chloline, hydroxy quinolines, 5nitro quinoline, tetrahydrofurfuryl alcohol, furfural alcohol, furfural, the mono glycerol ethers, such as 1methoxy glycerol, 2-methoxy glycerol, l-ethoxy glycerol, 2-ethoxy glycerol, 1propoxy glycerol, 2-propoxy glycerol, 1isopropoxy glycerol, 2-isopropoxy glycerol; the glycerol di-ethers such as 1,2-dimethoxy glycerol, 1,3-di-methoxy glycerol, 1,2- di-ethoxy glycerol, 1,3-di-ethoxy glycerol, 1,2-dipropoxy glycerol, 1,3-di-propoxy glycerol, 1,2-diisopropoxy glycerol, and 1,3-di-isopropoxy glycerol; the mixed di-glycerol ether esters such as l-ethoxy, 2-methoxy glycerol, 1-methoxy, 3pro poxy glycerol, and l-ethoxy, 2-isopropoxy glycerol.

The heavier than C1 bottom fraction from frac- Vtionator 91 obtained through line |0| contains aromatics. The latter are preferably separated by liquid-liquid extraction with a liquid having greater solvent power for aromatics than for olenic hydrocarbons, for example, liquid SO2. For this purpose, the hydrocarbon liquid is delivered through line |0| to extractor |25 wherein it is extracted with liquid SO: enteringr through conduit |28. The ra-filnate passes through line |30 to a stripper |32 wherein it is stripped of dissolved SO: and thence is preferably returned to the cracking coil 1 through lines |34 and 8. Il'he extract containing aromatics is conveyed from extractor |26 through line |38 into separator |38 wherein a bottom fraction comprising mainly SO2-free aromatics and an overhead fraction of SO: are produced. The aromatics are obtained through conduit |50 while the SO2 is taken overhead through line |42 to be liquefied in cooler asuntos I together with the 80: taken from stripper |32 through line |46. The liquefied SO2-ls then il'introduced through conduit |29 into extractor ther solvents suitable for the liquid-liquid extraction are, for example, nitrobenzene, methyl acetate, phenyl acetate, methyl or ethyl cel1o solve. furfural, acetone, aniline, phenol, cresylic acids, dichlorethyl ether, antimony trichloride, SO: benzene mixtures, or combinations of solvents and anti-solvents such'as phenol or cresol with propane, etc.

If desired, the charge in line 89 may by-pass the fractionators 9| and 91 through a by-pass not shown and all the aromatics separated by liquid-liquid extraction in extractor |28, as described: or alternatively, if desired. the liquidliquid extraction step may be replaced by further fractional and extractive distillation of the several fractions.

The separation of butaand pentadienes may be conducted as described above by extractive distillation or other vapor phase extraction with similar solvents; or by azeotropical distillation in the presence of anhydrous ammonia; methylamine, CO2, lower alcohols, etc.: or by liquidliquid-extraction with acetone, furfural, aniline,

phenol, etc.: or through formation of complex compounds with copper or silver salts, SO2, etc.: or by polymerization under the influence of heat, pressure, light, or catalysts such as Na, K, H2804. HaPOa, P203, AlCla. etc.; or a combination thereof.

It may sometimes be preferred to separate the dienes or the aromatica or both, without pre- .fractionation or following a degree of fractionation less thorough than described. Alternatively,

jit may also sometimes b'e desirableto produce a From the Ungasied portion, a gasoline was recovered amounting to 51.8% of the charge and having an ASTM octane number of 75.

A cracked gasoline distillate (ASTM boiling range 42-115 C.) produced by liquid phase cracking in a Dubbs cracking unit of California sas oil was recracked ina -phosphor-bronze tube at a temperature of 750 C. and space velocity of 128. Gasiflcation was 64%. Analysis of the recovered gas was as follows:

i Per cent C: and lighter components 39.3 CiHe butadiene 5.3 NC4Ha 8.4 Iso-C4118 4.5 C4H1o .4 vCsHi; pentadienes 8.3 05H10 l 16.4 CsHiz I 17.4

We claim as our invention:

sharper fractionation before further recovery or to use directly fractions rich in dienes or aromatics. or both, produced by very sharp fractionation. A

The following examples further illustrate our invention:

A cracked gasoline distillate obtained by liquid phase cracking of a California crude and having a boiling range of 98 to 205 C., an ASTM octane number of 65, a bromine number of 57 and an aromatic content of 15% was cracked in a bronze tubefllled with quartz chips at 750- C.

with a liquid space velocity of-182. Gasiilcation was 40%. The gaseous products had 'the following 1.A In the process of producing dioleflns the steps comprising cracking a straight run hydrocarbon oil selected from the group'consisting of naphtha, kerosene and gas oil at a temperature below 54.0 C., fractionating the resulting cracked products to produce a cracked distillate consistingof Ce and higher hydrocarbons boiling below about 120 C., having a bromine number above and an aromatic content below 15%, treating said distillate to remove gum forming components, pyrolyzing said cracked distillate and a recycle oil produced in said process in theabsence of metals containing predominating lamounts of iron at a temperature between 650 C. and 850 C. under conditions to gasify 15-85% thereof, whereby dioleflns are formed and separating from the resulting cracked products at least the following fractions: a dioleiln fraction, said recycle oil which is substantially free of tan'y and normally gaseous hydrocarbons and contains less than 30% aromatica, and a tarry fraction.

2.\ The process of claim 1 wherein said recycle oil is substantially free of polyolens.

3. In the process of producing diolens the steps comprising cracking a straight run hydrocarbon oil selected from the group consisting of naphtha, kerosene and gas oil at a temperature below 600 C., fractionating the resulting cracked products to produce a cracked distillate consisting of Cs-and higher hydrocarbons boiling below about C., having'a bromine number above 40 and an aromatic content below 30%. treating said cracked distillate to remove gum forming components, recracking said cracked distillate 'at a temperature between 650 C. and 850 C. under conditions to gasify 15% to 85% thereof, whereby diolefins are formed and separating said dioleiins. and aromatica from each other and the other resulting cracked products.

4. The process of claim 3 wherein said cracked distillate has a bromine number above 60.

MARTIN Dl SMO. ROBERT M. ROBERTS.

Images