US2950240A

US2950240A - Selective cracking of aliphatic hydrocarbons

Info

Publication number: US2950240A
Application number: US766619A
Authority: US
Inventors: Paul B Weisz
Original assignee: Socony Mobil Oil Co Inc
Current assignee: ExxonMobil Oil Corp
Priority date: 1958-10-10
Filing date: 1958-10-10
Publication date: 1960-08-23
Anticipated expiration: 1977-08-23

Description

nit-ed States Filed on. 10, was, $93. No. 766,619

6 Claims. (Cl. 208-70) This invention relates to a process foreffecting selective cracking of aliphatic hydrocarbons including normal and isoaliphatics from a mixture of the same with cyclic hydrocarbons. Such hydrocarbon mixtures are normally present in petroleum and selected fractions of petroleum distillates or processing streams including for example, mixtures of normal paraffins, normal olefins, isoparafiins, iso-olefins, naphthenes, aromatics, alkyl-aromatics, etc.

It is well known that the above mixtures of hydrocarbons may undergo catalytic cracking in thepresence of cracking catalysts such as synthetic composites of silica and alumina or acid activated naturally occurring clays. It is also well known that cracking in the presence of these catalysts proceeds with dilferent relative ease by difierent classes of hydrocarbons, namely in the order of decreasing ease of cracking:

(1) Alkyl-aromatics, particularly with alkyl groups of two or more carbon atoms to dealkylated cracking products;

(2) Naphthenes;

(3) Iso-aliphatics;

(4) Normal aliphatics.

The process of the present invention involves a cracking process which is unique in the order of reactivity toward the different classes of hydrocarbons as compared to the above-noted familiar processes.

In one embodiment, the present invention provides a continuous process which preferentially cracks normal aliphatic hydrocarbons, i.e. normal olefins or normal parafiins, from a hydrocarbon mixture or petroleum charge stock containing iso-alipnatic hydrocarbon and cyclic hydrocarbon components to the substantial exclusion of cracking of said components Whether they be naphthenic or aromatic and regardless of the type and size of alkyl groups contained therein.

In accordance with the process of the invention, the charge stock initially containing normal aliphatic, isoaliphatic and cyclic hydrocarbons is introduced into a continuous system comprising a catalytic de-isornerization zone and a catalytic cracking zone, the latter containing as catalyst a crystalline zeolite having rigid threedimensional networks bearing catalytic surfaces active in hydrocarbon cracking and having uniform interstitial dimensions sufiiciently large to sorb normal aliphatic components contained in the charge but sufiiciently small to exclude the hydrocarbon components of larger molecular size, i.e. iso-aliphatic and cyclic hydrocarbons. Under such conditions, the normal aliphatic hydrocarbon components of the charge mixture capable of entering the interior pore structure of the above catalyst undergo cracking to lighter highly oletinic materials while the iso-aliphatic and cyclic hydrocarbon components, be-

cause of their larger molecular size are unable to pene-- trate the porous catalyst structure and accordingly undergo little or no catalytic cracking. The product emerging from the cracking zone containing uncracked charge components and products resulting from cracking of the normal aliphatic components is conducted to the deisomerization zone where iso-aliphatic components are converted to normal aliphatic hydrocarbons. The prod- "atent cycles.

Patented Aug. 23, 1960 net from the de-isomerization operation is then recycled to the cracking zone Where the normal aliphatic hydrocarbons produced during the de-isomerization step undergo cracking. The above cyclic cracking and de-isomerization operations are repeated any desired number of Generally, it is preferred to continue the recycle operation until the charge is substantially free of aliphatic hydrocarbons leaving a useful concentrate of cyclic hydrocarbons. The products resulting from the selective cracking step prior to de-isomerization, may be subjected to a suitable intermediate separation operation, such as by fractionation or other feasible means, wherein light materials produced, i.e. those boiling below the initial charge stock are removed and the remaining materials, of equal or higher boiling range than the charge stock, are then subjected to de-isomerization and thereafter recycled in the above continuous cracking and deisomerization system until the desired extent of concentration of cyclic hydrocarbons is achieved.

The process of the invention is useful for a number of applications. For example, jet fuels or other high energy fuels require a low aliphatic hydrocarbon content. Thus, the process applied to kerosine range petroleum fractions achieves selective cracking of aliphatic hydrocarbons leaving an energy-rich fuel of cyclic compounds which may be used as obtained or after being subjected to hydrogenation. Also, the process described herein is useful in production of aromatic oils and solvents. For such purpose suitable boiling range petroleum fractions are subjected to catalytic cracking of aliphatic hydrocarbon components to obtain a cyclic concentrate. The light materials resulting from cracking with the crystalline zeolite catalyst employed in such step are highly olefinic in character. Separation of these light olefinic materials from the heavier components bebefore recycling the latter accordingly aflords products susceptible to a wide variety of useful applications. Such products may be employed, for example, in the formation of high octane alkylate; they also may be used for the alkylation of benzene to form curnene or other alkyl benzenes; or they may be polymerized to liquid fuels or to form plastics such as polyethylene and polypropylene.

The charge material conducted to the present process is a mixture of normal aliphatic hydrocarbons, iso-aliphatic hydrocarbons and cyclic hydrocarbons. The normal aliphatic hydrocarbons may be normal parafiins or normal olefins or a mixture of the two. Likewise, iso-aliphatic hydrocarbons contained in the charge mixture may be iso-parafiins or iso-olefins or a mixture of the two. Cyclic hydrocarbons include naphthenes or aromatics or a mixture of the two. 'It will thus be apparent that the charge material may be a relatively simple hydrocarbon mixture or a complex mixture of hydrocarbons which may contain minor proportions of other materials such as sulfur, nitrogen and oxygencontaining components. Representative of the latter type mixtures is petroleum and various fractions thereof. A preferred charge for use in the present process is a petroleum fraction characterized by an aliphatic, isoaliphatic and cyclic hydrocarbon content and having an approximate boiling range of 200 to 600 F. and in particular, a fraction of to 320 F. boiling range obtained from a 98 octane number reformate characterized by an approximate boiling range of 180 to 400 F.

it is contemplated that the reaction conditions heretofore used in effecting catalytic cracking will be employed in the cracking step of the present process. Thus, catalytic cracking with the crystalline size-selective zeolite catalyst described herein may be carried out by contacting a hydrocarbon charge at catalytic cracking conditions employing a temperature within the approximate range of 700 F."to l2 Fjand preferably between about 800 F. and about 1050 F. and under a pressure ranging from sub-atmospheric pressure up 'to' several hundred atmospheres. The liquid hourly space velocity of the charge mayrange from 0.2 to 4.0 and preferably T from 0.5 to 2.0. The catalyst may be usedas pellets in a fixed bed operation or they may be used in a compact moving bed operation or in a fluidized operation. The contact time of the hydrocarbon charge with the catalyst is adjusted in any case according to the conditions, the r particular charge stock and the particular results desired to give a substantial amount of c'racking of thenormal aliphatic hydrocarbon components to lower boiling products.

, pattern. Such structure contains a large number of small cavities interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size. It is essential for the present process that the zeolite catalyst have a uniform pore structure permitting entry into the interior thereof of normal aliphatic hydrocarbons but excluding entry of iso-aliphatic hydrocarbons, cyclic hydrocarbons and other components characterized by a molecular size greater than'that of,

normal aliphatics. Generally, a catalyst fulfilling the above requirements has a uniform pore size of about Angstrom units. Thus, if the pore size is materially smaller than 5 Angstrom units, the pores are too small to permit entry into the crystalline catalyst structure of normalaliphatics. If the pores are materially larger, the

large hydrocarbon molecules, i.e. is'o-aliphatics and cyclic hydrocarbons are able to enter the pores of the catalyst silicon is equal to tWo.

; initially in the sodium form of the crystal.

. 4 Such molecular sieves consist fundamentally of a three-dimensional tetrahedral structure of silicon and aluminum. These tetrahedra are joined by sharing oxygen atoms in such a manner that the ratio of atoms of oxygen to the total number of atoms of aluminum and The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This equilibrium'can be expressed by the formula wherein the ratio of A1 to the number of the various cations such as Ca, Sr, Na K; or Li is equal to unity. One cation may be exchanged either in entirety or partially by another cation utilizing ion exchange techniques. The spaces between the terahedra are occupied by molecules of water prior to dehydration;

' The above molecular sieves are ordinarily prepared The sodium ion in such form may, as desired, be exchanged for other alkali metal or other alkaline earth metal cations. In general, the process of preparation involves heating, in aqueous solution, an appropriate mixture of oxides, or

of material Whose chemical composition canbe comfor periods of 15 minutes to 90 hours or more.

pletely represented as a mixture of oxides Na O, A1 0 Si0 and H 0 at a'temperature of approximately 100 C. The product which crystallizes Within this hot mixture is separated therefrom and Water Washed until the water in equilibrium with thezeolite has a pH Within the range of 9 to 12. The material is thereafter activated by heating until dehydration is attained. 7

it is to be noted that the sodium form of theabove zeolite, i.e. MolecularSieve 4A is catalytically inactive for use as a cracking catalyst in the process of the present invention; However, such inactive form ;may be rendered ln'ghly active in catalytically selectively cracking normal aliphatic hydrocarbons in the present process by the sodium after crystallization.

structure so that selective catalytic cracking of the nor- 7 where Me is a-metalcation,

V V. x

is the number of exchangeable metal cations of valence n, x'is also the number of aluminum ions combined in By base-exchanging the sodium aluminosilicate salt with a calcium ion-containing solution, the salt becomes catalytically active in selectively cracking normal aliphatic hydrocarbons and has a resulting uniform .pore diameter of about 5 j Angstrom units.

While calcium aluminosilicate characterized by uniform 'pores of approximately 5 Angstroms in diameter, i.e.

Molecular Sieve 5A is preferred for use as the catalyst in the cracking step of the present'process, it is con-.

mgnesium, capable of sorbing at least 10 percent of their the form of aluminate, y is the numberof silicon atoms,

and z is the number of Water molecules, removal of is a'n'umber from 1 to 5 and usually from 1 to .2. Zeolites having the above characteristics have. sometimes been referred to as molecular sieves. 'At the present time, there are commercially available molecular sieves having channels of about 5 Angstroms in diam;

' eter. 'Such material known as,M0leci1larfSieve 5A is Weight of n-hexane but negligible amounts of iso-hexanes when brought into contact with liquid hexane at substan tially atmospheric pressure and a temperature of about 25 C. are suitable for use in the cracking step of the present process. The sodium form of'the above zeolite exchanged With calcium or other of the above-indicated divalent metals possesses. larger pores than the unexchanged material. An unusual characteristic of the calcium or other divalent metal exchanged zeolite is that the opening of the poresis not accomplished progressively as the sodium ions are replaced by calcium or other divalent ions but isprcduced'withinafairly narrow range essentially the crystalline calcium aluminosilicate prepared bybase-exchange ofa crystalline sodium alurnino "silicate having channels of about 4 Angstroms in diamofcomposition. When the exchange is 25 percent or less, the substance possesses substantiallythe same pore characteristics as the sodium forrnof the zeolite, namely a pore diameter of about 4 fmgstrom units, However, When the exchange'isin substantial excess of about 25 -percent, the pore characteristics become those of the cal- :cium form of the zeolite, i.e. a pore diameter of about i 5 Angstrom units.

7 Accordingly, ib-Wlll be appreicated that thecrystalline aluminosilicate zeolite employed as cracking catalyst in the process of the present invention may be a mixed salt of sodium and calcium or of sodium and one or more of the other above-indicated suitable divalent metal ions, it being essential that the pore diameter of the resulting composition be sufliciently large to admit normal aliphatic hydrocarbons and sufliciently small to exclude hydrocarbons having a molecular size exceeding that of the aliphatics.

It is also contemplated that the size-selective catalyst may be a composite comprising a major proportion of the above-described crystalline zeolite and a minor proportion of one or more materials which may act as promoters or activators or otherwise enhance the desired catalytic conversion while the crystalline zeolite is responsible for the major size-selective characteristics. In this regard, the crystalline zeolite may suitably be composited with activated clays, vanadia, thoria, zirconia, molybdena, silica, alumina or combinations thereof. Such composites may be prepared by impregnation of the crystalline zeolite with the added substance or by mechanical admixture of the two materials usually followed by shaping of the resulting composite into particles by casting, pelleting, extrusion or other suitable means.

The product from the cracking zone is conducted to a de-isomerization zone. As an aid in the understanding of the action taking place in the de-isomerization zone, it is to be recognized that thermodynamic equilibria between normal and isomeric aliphatics requires substantial quantities of both of these components to be present in the equilibrium product. Thus, an isomerization reaction will efiect a net shift to isomeric products when the feed stock is rich in normal aliphatic hydrocarbons, or a net conversion to normal aliphatics when the feed stock is rich in isomer components. The latter will be the case in the process of the present invention since the selective cracking action taking place in the cracking zone, and removal of light cracked products, results in an iso-enriched product which is conducted to the de-isomerization zone. It will accordingly be understood that this zone has been termed dc-isomerization zone to characterize the unconventional method of use as well as resultant chemical conversion, although the de-isomerization step is carried out under conditions and in the presence of catalysts conventionally employed in isomerization reactions. Thus, the catalyst employed may be a Friedel-Crafts type catalyst such as aluminum chloride, aluminum bromide, aluminum iodide, aluminum fluoride, boron trifluoride, iron chloride, complexes thereof with organic and inorganic compounds and other halogencontaining catalysts such as hydrogen fluoride. Generally, however, it is preferred to employ, as a catalyst, a metal-acidic oxide type isomerization catalyst which atfords substantial de-isomerization without appreciable material losses by cracking reactions. Suitable catalysts of such type include generally, metals of group VIII of the periodic system such as cobalt, nickel, platinum, and palladium supported upon carriers such as silica, alumina or composites thereof. Also, an isomerization catalyst made up of a mechanical mixture of a platinum metal deposited on alumina and a silica-alumina cracking componet may be utilized as the catalyst. Isomerization using the latter mechanically mixed catalyst is described in my co-pending application Serial Number 633,189 filed January 9, 1957, now US. Patent No. 2,892,003.

Generally, the de-isomerization step will be carried out at a temperautre between about 650 F. and about 900 F., at a liquid hourly space velocity between about 0.5 and about 20, employing a hydrogen to hydrocarbon molar ratio of between about 0.5 and about 20, and a pressure of between about 50 p.s.i.g. and about 1000 p.s.i.g. All of the aforementioned variables are interrelated. As a practical matter, the temperature of operation is generally fixed as a result of primary choice with respect to other variables and the desired conversion level.

In the de-isomerization zone, the iso-aliphatic componeuts contained in the charge are converted to normal aliphatic hydrocarbons. The products resulting from the cracking zone prior to introduction to the de-isomerization zone are suitably separated into light materials boil ing below the range of the initial charge stock and ma terials of equal or higher boiling range than the initial charge stock. The latter are subjected to de-isomerization and thereafter recycled to the cracking zone. The lighter materials removed are highly olefinic and are suitable for use in a variety of applications described hereinabove.

A schematic flow diagram of the process of the present invention is shown in the attached figure. Referring more particularly to such figure, the hydrocarbon feed initially containing normal aliphatic hydrocarbons, iso-aliphatic hydrocarbons, and cyclic hydrocarbons is conducted through

lines

10 and 11 to cracking zone 12. Selective catalytic cracking of the normal aliphatic hydrocarbon components takes place in the cracking zone in the presence of the above-described crystalline size-selective zeolite catalyst maintained under catalytic cracking conditions. The product emerges from the cracking zone through line 13 and is conducted to a fractionating column 14. Light materials, highly olefim'c in content, boiling below the initial charge stock are removed as overhead through line 15. Materials having a boiling point at least equal to that of the initial charge are removed as bottoms from fractionating column 14 through line 16 and conducted through line 17 to de-iscmerization zone 18 wherein the iso-aliphatic hydrocarbon components are converted in the presence of a suitable isomerization catalyst to normal aliphatic hydrocarbons. The product from the de-isomerization zone is recycled through

lines

19, 20, 21 and 11 to the cracking zone 12. When a product of desired cyclic hydrocarbon content is obtained, the same is removed from the system by adjusting valve 22 permitting all or a portion of the product stream flowing through line 16 to be withdrawn through conduit 23.

The following example will serve to illustrate the process of the invention without limiting the same:

A petroleum distillate containing approximately 35 percent by weight aliphatic and approximately 65 percent by weight cyclic constituents and boiling in the range of 400 to 500 F. is used as the charge stock. The aliphatic component of such charge contains about 20 percent by weight of normal parafl ins and about 15 percent by weight of iso-paraflins. The charge in the ratio of 1 part of fresh stock to 3 parts of recycle stock is subjected to cracking in the presence of a catalyst of Molecular Sieve 5A at a temperature of 900 F. and a liquid hourly space velocity of /2 based on fresh feed. The product resulting from cracking is then distilled. The fraction boiling substantially below 400 F., constituting about 32 weight percent of the distillation charge, is continuously withdrawn. This product consists essentially of gasoline and gas range hydrocarbons, primarily light normal paraifins and olefins. The fraction boiling above 400 F. is divided into two streams. stream constituting 67 weight percent of such fraction serves as the recycle charge. Before returning such recycle charge to the cracking zone, the same is passed through a de-isomerization zone containing a platinum silica-alumina isomerization catalyst at a temperature of 770 F., a total pressure of 300 p.s.i.g. with admixture of hydrogen in a molar ratio of 6 to l in relation to the hydrocarbon flow rate. The remaining 23 weight percent of the fraction boiling above 400 F. is withdrawn as cyclic concentrate product, containing less than 10 percent by weight of total paraffins.

It will be understood that the above description is merely illustrative of preferred embodiments of the invention, of which many variations may be made within One the scope of thefollowing claims by those skilledin the art 'without' departing from the spirit thereof.

.;{Iclaim: f -1. A continuous method for selectively cracking aliphatic hydrocarbons from a mixture of thesame with cyclic hydrocarbons and obtaining a resultant cyclic hydrocarbon concentrate which comprises conducting a charge initially containing normal aliphatic, iso-aliphatic and cyclic hydrocarbon components in a continuous system to a cracking zone maintained under catalytic crack- Ying'conditions and containing a cracking catalyst comprising a crystalline zeolite having a uniform pore struc ture of sutficient size to afford entry into the catalyst structure of normal aliphatic hydrocarbon components while" excluding hydrocarbon components of larger molecular size whereby said normal aliphatic hydrocarbon components are selectively cracked, subjecting the resulting mixture of cracked and uncrackcd' components to de-isomerization in the presenceof an isomerizatio-n catalyst and under catalytic isomerization conditions whereby iso-aliphatic components are converted to normal aliphatic hydrocarbons, recycling the resulting product to said cracking zone, repeating the aforesaid treatment on a continuous cyclic basis to yield a concentrate of cyclic hydrocarbons andremoving said concentrate'from said continuous system. i

2. A continuous method for selectively cracking sufiicient size to afiord entry into the catalyst structure of normal aliphatic hydrocarbon components while excluding hydrocarbon com onents of larger molecular size whereby said normal aliphatic hydrocarbon components are selectively cracked, subjecting the'resulting productmixture to fractionatiomremoving as overhead the lighter materials having a boiling point below that;

of the initial charge mixture, removing as bottoms a frac- 8 under catalytic cracking conditions with a cracking'catalyst comprisinga crystalline zeolite havinga pore size of 7 about Angstrom units-in diameter whereby said normal aliphatic hydrocarbon components are selectively cracked, subjecting the resulting product mixture to fractionation, removing as overhead the lighter materials having aboiling point-below that of the initial charge mixture, removing as bottoms a fraction having a boiling point at least equal to that of the initial charge mixture, subjecting said bottoms to de-isomerization in the presence of an isomerization catalyst andrunder catalytic isomerization conditions Wherebyiso-alipha'tic components are converted to normal, aliphatichydrocarbons, recycling the product from de-isomerization to said cracking zone and'repeating the aforesaid treatment on a continuous cyclic basis to yield a concentrate of cyclic hydrocarbons and removing said concentrate from'saidcontinuous system.

5, A continuous 1 method for'selectively cracking aliphatic hydrocarbons from a' mixture of the same with cyclic hydrocarbons and obtaininga resultant cyclic hy drocarbon concentrate which comprises contacting a charge initially containing'normal aliphatic, 'iso-aliphatic and cyclic hydrocarbon components under catalytic cracking conditions, with a crackingicatalyst comprising a crystalline calcium aluminosilicate having a pore size of about 5 Angstroms in diameter whereby said normal aliphatic hydrocarbon components are selectively cracked, subjecting the resultant mixture of cracked and uncracked components to 'de-isomerization in the presence of an isomerization catalyst and under catalytic isomen'zation conditions whereby iso aliphatic components are converted to normal aliphatic hydrocarbona'recycling the resulting tion having a boiling point 'at least equal to that of the V 7 initial charge mixture, subjecting said bottoms to dc-isomerization the p esence of an isomerization catalyst and under catalytic iscmerization' conditions whereby isoaliphatic components are converted to normal aliphatic.

product'to' said' cracking zone, repeating the aforesaid treatment on'a continuous cyclic basis to yield a'concentrate of cyclic'hydrocarbons and removing said concentrate from said continuous system,

6. A continuousmethod for selectively cracking aliphatic'hydrocarbons' from a: mixture'of the same with cyclic hydrocarbons and obtaining a resultant cyclic hydrocarbon concentrate which comprises contacting a charge initially containing normal. aliphatic,'iso'-aliphatic and cyclic hydrocarbon components under catalytic cracking conditions with a" cracking catalyst comprising a crystalline calcium aluminosilicate having a pore size of about 5 Angstroms in diameter whereby said normal aliphatic hydrocarbon components are selectively cracked, subjecting the resulting product mixture to fractionation, removing as 'overhead the lighter'materials having a boiling point mate boiling range of 290 to 600 F. whichlcomprises contacting said mixture in a cracking'zone maintained under catalytic crackin conditions with a cracking catalyst comprising a crystalline zeolite having a pore tion characterized by a normal aliphatic, an iso-aliphatic,

and cyclic hydrocarbon content and having an approximate boiling range of 200 to 600 P. which comprises contacting said mixture in a cracking zone maintained below that of the initial charge mixture, removing as bottoms a fraction having a boiling point at least equal to that of the initial charge mixture, subjecting said bottoms to de-isomerization in the presence of an isomerization catalyst and under catalytic isomerization conditions whereby iso aliph'atic components are converted to normal aliphatic, hydrocarbons, recycling the product from de-isomerization to said'cracking zone andrepeating' the aforesaid treatment on a continuous cyclic basis toyield a concentrate oi cyclic'hydrocarbo'ns and removing said concentrate from said continuous System11 ReferencesCited' in thefile of this patent UNiTnD STATES PATENTS 1,840,450 7 Jaeger an. a Ian. 12, 1932 2,326,779 Houdry Aug. 17, 1943 u 2,469,733 V May 10, 1949 2,666,022 Johnson Jan. 12', 1954 if, 7 FOREIGN PAT S] Great Britain, Sept. 15, 1954 715,474 g p q Q N a OTHER REFERENCES .lsonierizatiori ci Pure Hydrocarbonsf". Egloff et al., publishen Reinhold Publishing Corp, 1942; New York, N .Y. pages 232 to 235 V

Claims

1. A CONTINUOUS METHOD FOR SELECTIVELY CRACKING ALIPHATIC HYDROCARBONS FROM A MIXTURE OF THE SAME WITH CYCLIC HYDROCARBONS AND OBTAINING A RESULTANT CYCLIC HYDROCARBON CONCENTRATE WHICH COMPRISES CONDUCTING A CHARGE INITIALLY CONTAINING NORMAL ALIPHATIC, ISO-ALIPHATIC AND CYCLIC HYDROCARBON COMPONENTS IN A CONTINUOUS SYSTEM TO A CRACKING ZONE MAINTAINED UNDER CATALYTIC CRACKING CONDITIONS AND CONTAINING A CRACKING CATALYTIC COMPRISING A CRYSTALLINE ZEOLITE HAVING A UNIFORM PORE STRUCTURE OF SUFFICIENT SIZE TO AFFORD ENTRY INTO THE CATALYST STRUCTURE OF NORMAL ALIPHATIC HYDROCARBON COMPONENTS WHILE EXCLUDING HYDROCARBON COMPONENTS OF LARGER MOLECULAR SIZE WHEREBY SAID NORMAL ALIPHATIC HYDROCARBON COMPONENTS ARE SELECTIVELY CRACKED, SUBJECTING THE RESULTING MIXTURE OF CRACKED AND UNCRACKED COMPONENTS TO DE-ISOMERIZATION IN THE PRESENCE OF AN ISOMERIZATION CATALYST AND UNDER CATALYTIC ISOMERIZATION CONDITIONS WHEREBY ISO-ALIPHATIC COMPONENTS ARE CONVERTED TO NORMAL ALIPHATIC HYDROOCARBONS, RECYCLING THE RESULTING PRODUCT TO SAID CRACKING ZONE, REPEATING THE AFORESAID TREATMENT ON A CONTINUOUS CYCLIC BASIS TO YIELD A CONCENTRATE OF CYCLIC HYDROCARBONS AND REMOVING SAID CONCENTRATE FROM SAID CONTINUOUS SYSTEM.