US2370412A - Preparation of unsaturated hydrocarbons - Google Patents

Description

Feb- 27, 1945 R. c. MORRIS Erm.

PREPARATION 0F UNSATURATED HYDROCARBONS 2 Sheets-Sheet 2 Filed June 15. 1942 Patented Feb. 27, 1945 PREPARATION OF UNSATURATED HYDROCARBON S Rupert C. Morris and Robert J. Moore, Berkeley,

Calif., assignors to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application Junell, 19442, Serial No. 447,138

4 Claims.

This invention relates to a process for the preparation of unsaturated hydrocarbons from petro leum distillates and, more particularly, aromatics such as benzene, toluene and Xylene, and diolefines such a"s butadiene, isoprene, and other homologous diolenes.

It has heretofore been proposed to separate toluene or other aromatics from natural or cracked petroleum distillates by concentrating the aromatics by Way of distillation to obtain a fraction containing upwards of 5%' aromatics, and by extracting the resulting concentratewhile in the vapor phase with a suitable selective solvent to obtain pure or substantiallypure aromatics, and a raffinate substantially freel from toluene.

It has also been proposed to subject this raf'- n nate to various treatments in order to produce therefrom more useful products. Thus,` for -example, it has been proposed to subject the aromatic-free rafnate to vvapor phase cracking at a temperature above about 1100 F. whereby aromatics and diolenes are produced; and to recover these valuable hydrocarbons.

It is an object of this invention to improve the yields of valuable hydrocarbons having conjugated double bonds, and particularly of toluene, and of butadiene and pentadiene which may be obtained by thermal cracking of aromatic-free rafnates obtained in the recovery of toluene or other aromatics from concentrates derived from natural or synthetic petroleum distillates. Further objects will be found in the improvements hereinafter disclosed.

This invention is based on the discovery that subjecting to an isomerizing treatment the nonaromatic hydrocarbons normally associated 'with and boiling close to the aromatics, prior to thermally cracking them, greatly improves the yields of valuable hydrocarbons obtained therefrom.

According to the present invention, at least a portion of the substantially aromatic-free raffinate obtained after recovery of the aromatic from the,l concentrate is subjected to an isomerizing treatment followed by thermal cracking in the vapor phase above about 1000 F., whereby an unusually high proportion of aromatics and dienes is formed. These hydrocarbons are then separated, and preferably from the remainder of the cracked product a naphtha fraction may be produced which is recycled through the system.

A hydrocarbon fraction to be Suitable for the purpose of this invention is one in which has been concentrated the major amount and preferably substantially the entire amount of a 'single aromatic hydrocarbon or a single group of metameric aromatic hydrocarbons, e. g., benzene, or toluene or Xylenes and ethyl benzene, etc., which amount was originally contained in the hydrocarbonfdistillate under treatment.- This fraction or concen-` trate should have a boiling range as narrow as possible consistent with the requirement of containing the major portion of the aromatic, and it should be substantially free of components boiling above or below the boiling range over which the aromatic boils when the4 fraction is subjectedy to a close fractional distillation. In producing these suitable concentrate fractions, highlyeffl cient fractional distillation equivalent at least to 20 theoretical plates at a reflux ratio of 2: 1 should be employed. Such narrow fractions are preferably produced by. continuous fractionation 'and if so produced will, in general, upon being subjected to analytical distillation, show atleast boiling within the following ranges:

Analytical distillation as herein delined is a laboratory batch distillation operated over 30 equilibrium plates and at a reilux ratio of at least 20: 1.

For the purpose of our invention the composition of the concentratesmay be said to consist of 3 component parts: (1) an aromatic or a group of metameric aromatic hydrocarbons, (2) nonaromatic hydrocarbons which form upon cracking large amounts of dienes, and 3) non-aromatic hydrocarbons which do not give such high yields of dienes. These three components -cannot be separated from each-other by simple fractional distillation, i. e., distillation conducted in the absence of added separating agents, and in order to effect a separation it is necessary to distill the concentrates in the presence of a selective solvent for one of the components of the aromatic hydrocarbons. However, after separation of aromatics, simple fractional distillation is capable of separating the two kinds of non-aromatic hydrocarbons, the lower boiling fraction containing those which give originally lower yields of dienes and are particularly improved by our invention.

`It will be apparent that narrower fractions than above-described may be used with advantage but will give lower overall yields of the desired dienes as calculated on the total distillate. The inclusion of small amounts of higher or lower boiling hydrocarbons, howevenwill give inferior results, the extent of depreciation depending upon the amount of these hydrocarbons included. Such variations may be produced, for example, by less emcient fractionation of the petroleum distillate when producing the narrow boiling fractions.

Fractions boiling in the above range and particularly in the preferred range, allow the separation of pure aromatics practically free of other hydrocarbons and boiling within 1 F.

The nature of the invention will be better understood from the following detailed description, taken together with the drawings forming a part of this specification which illustrate certain preferred embodiments thereof, in which:

Figure I is a ow diagram illustrating a particularly preferred form of the improved process; and

Figure II is a flow -diagram illustrating the isomerization step of this process.

Figure III is a flow-diagram illustrating the general steps of the process.

For simplicity the drawings do not show pumps, heat exchangers, valves, by-passes, vents, reboilers, condensers, and other auxiliaries, the proper placement of which will be atonce evident to those skilled in the art.

Referring to Figure I, illustrating a particularly preferred form of this'invention: A hydrocarbon feed containing toluene and a large proportion of non-aromatic hydrocarbons whose-boiling temperatures range from below to above that of toluene is introduced at I from Va source not shown into fractional distillation zone 3. kThis feed may be of any provenience, natural or synthetic, cracked or straight-run'. In order to facilitate the isomerization step of this process, it is advantageous that the feed be substantially free of olefines; thus straight-run'gasoline which contains appreciable quantities of toluene is preferred. Toluene-rich naphthas produced in the process, as hereinafter described, may be added thereto through line 5. In the fractionating zone 3 a concentrate containing most of the toluene of the feed and having a boiling range as narrow as practicably possible is produced and taken through line I while substantially toluene-free higher and lowerboiling portions are withdrawn through lines 9 and II. The toluene concenabout 190 F. and 245 F. and preferably between about 205 F. and 230 F., toluene boiling at 231.4"

F. Hydrocarbons having nearby boiling points form azeotropes with toluene and the boiling range of these azeotropes varies with the nature of these hydrocarbons, being in general comprised in the above limits. It is usually desirable to produce a concentrate having a toluene content of 5% and preferably 8% or more, and this may require adjusting the boiling range of the concentrate to the nature of the particular feed. It was found that from an East Texas straight-run gasoline a concentrate containing from 8% to over 10% of toluene and boiling from about 206 F. to about 230 F. could be obtained, while from West Texas straight-run gasoline a concentrate containing from 10% to over 15% of toluene and boiling from about 208 F. to 228 F. could be obtained.

The toluene concentrate hereinabove described is led through line '1 into separation zone I3 wherein substantially pure toluene is recovered and a substantially toluene-free fraction is obtained. This latter fraction will hereinafter be called toluene-free ramnate or "raffinate" for short. This raffinate may contain small amounts of aromatics of the order of 1% or 2% because of practical limits upon the degree of separation which can be attained. This small proportion does not interfere with the subsequent steps of this process. Larger proportions of toluene such as those which are originally present in the .toluene concentrate, e. g., 5% and up, are detrimental, particularly in the isomerization. In the latter operation, toluene is retained by the catalyst and inactivates it rapidly, for example, by dilution or complex formation in the case of liquid catalysts or of sludge formation in the case of solid catalysts.

A cracked toluene concentrate produced in the process as will be described may be introduced through line I5 and treated in toluene recovery zone I3 concurrently with the above-mentioned toluene concentrate. The separation of toluene from the raffinate will be more fully discussed hereinafter.

The toluene is Withdrawn.l through line Il and the raffinate is led through conduit I9 to be isomerized and cracked. Prior to isomerization the raflinate may be fractionated in fractional distillation zone 2 I, so as to separate fractions which are most benefited by-isomerization from those that may advantageously by-pass the isomerization step of this process, to be directly subjected to cracking conditions.

It is often advantageous to separate from the raflinate a relatively high boiling fraction boiling above about 210 F. and preferably above 215 F., which is led directly into cracking zone 2l through lines 23 and 25. The remaining lower boiling portion of the raffinate, or ifthis fractionation step is omitted, the entire raiinate, is conducted through line 29 into isomerization zone 3l The isomerization step of this process is designed to so modify the structure of hydrocarbons contained in the raffinate that on subsequent cracking they produce larger amounts of aromatics and dienes than could otherwise be obtainable. It has been found that conditions under which isomerization takes place substantially without cracking or dehydrogenation are especially suitable. In such an operation the composition of a mixture of hydrocarbons always shifts towards a state of equilibrium between the components, and it is preferred to continue the operation until this equilibrium is substantially reached. This equilibrium composition may vary very appreciably with temperature, so that isomerization of a hydrocarbon mixture at a high temperature may produce the opposite change from isomerization at low temperature. Thus isomerization should be conducted at a temperature below about 800 F. and preferably 400 F. The isomerization step will be more fully discussed hereinafter and a preferred embodiment will be described with reference to Figure II.

The isomerized railinate may advantageously be combined with the unisomerized fraction obor quenched at 33. vThe time of cracking at these high temperatures is governed so as to gasify about; 30% to 80% of the feed per pass, gasification `being dened as conversion of the liquid feed to molecules having 5 or less 'carbon atoms. The degree of gasification which is a measure of intensity of cracking affects greatly the nature of cracked products. If the cracking is relatively mild, i. e., corresponding to a gasification below about 45%, the normally liquid portion of the cracked. products will comprise a predominant proportion of unchanged raffinate, while the gases, particularly the C4 and Cs portion, comprise major amounts of dienes. If, on the contrary, the cracking is relatively severe, i. e., corresponds to a gasification above about 65%', the liquid portion comprises major amounts of aromatics, particularly toluene, which are apparently formed mainly by secondary reactions from the olenes first produced. A1; the same time the amount of dienes formed is slightly higher, their concentration in the respective fractions substantially being unchanged, and the proportion `of these fractions in the gasied products being greatly reduced. Upon increasing cracking intensities, particularly in the intermediate range, i. e., upon increasing gasications from about 45% to 65%, the amount of unchanged raffinate in the liquid product decreases markedly vand the proportion of aromaticsincreases.

As compared with the unintensied stock, at any given gasification or cracking conditions the conversion to valuable hydrocarbons having conjugated double bonds is unusually high under otherwise comparable conditions.

Of particular advantage is the high concentration of C4 and C5 dienes in the respective C4 and C5v fractions. Thus. for example, the amount of pentadienes found in a C5 fraction may be as high as 60%. This greatly facilitates the recovery of the dienes, as it is relatively simple to separate Cir-andY C5 fractions but it is diicult and costly to subsequently separate the dienes from the other hydrocarbons having nearly equal vapor pressures.

The heating of the hydrocarbons to the cracking temperature may be achieved by contact with a heated solid surface, for example in an externally heated coil, or may be realized by admixture of hot, substantially inert gases such as hue gases, carbon dioxide, hydrocarbon gases, steam, etc., or by a combination of both methods.

In order to prevent excessive coke formation it is often desirable to avoid the contact of ferrous metals with the hydrocarbon at cracking temperatures. Copper-containing alloys such as aluminum or phosphor-bronze, or refractory ceramic materials do not cause rapid coking when used in the construction of surfaces exposed to hydrocarbons at cracking temperature.

Cracking according to this invention is preferably purely thermal, i. e., non-catalytic, and is estimate correctly because of volume variations duringcracking, butshould not exceed about '0.1 second to avoid cracking the .butadiene formed. The liquid space velocity. is, however, easy to estimate and, for given temperatures, pressures .and admixtures, determines the actual vapor space velocity and cracking time. Liquid space velocity is the volume of liquid delivered per hour to the cracking zone,.per volume unit of said zone. The cracking zone is defined as the space occupied by gases in which the tempera* ture is within 20 C. of the peak temperature.

4The liquid space velocity necessary to obtain a given gasication depends on-the temperature and pressure in the cracking coil. At 775 C. and atmospheric pressure, for example, a liquid space velocity of about 60 results in gasiiication of about 60%.

Resumingl the description .of Figure I, the cracked and quenched products are led through line 35 into a fractional distillation Zone 37 wherein several fractions required for the recovery of the several valuable hydrocarbons, as well as a recycle naphtha, are produced. Gases` lighter than C4 which are rich in olelnes may be with'- drawn through line 39 and tars and heavy madienes are led through line 43 to zone 45 wherein the dienes are advantageously recovered, .for ex'- ample, by extractive or azeo-tropcal distillation,

. or by solvent extraction by means of a polar solconducted in the absence of catalyst, be itl added catalysts such as gaseous halogenated compounds, solid catalytic metals or oxides, or catalysts preformed in situ, e. g., lustrous carbon or pyrophoric metals.

The cracking is advantageously conducted at pressures of about atmospheric or below and preferably not'I substantially above 100-150 lbs. p. s. i. Pressures not substantially in excess of vthose required to overco-me the back pressure of the cracking unit are desirable.

The actual time for which the vapors are subjected to cracking conditions is very diflicult-to vent having greater solvent power for dienesthan for mono-olenes and saturated hydrocarbons, or

by selective polymerization, or by complex or com pound formation, or by a combination of these methods, etc.

A naphtha substantially free of C5 and lighter hydrocarbons is withdrawn' from zone 37 through line 4l'. The composition of this cracked naphtha depends greatly on the severity of cracking conditions as has already been mentioned.

If the cracking is mild, this cracked naphtha predominates in unchanged cracking stock and may advantageously be recycled to cracking zone 2l through lines 41 and 24. Y

If the cracking is severe the cracked naphtha contains toluene in addition to other non-aromatic hydrocarbons, and it is advantageous to recycle it tothe toluenerecovery System, i. e., Zones 3 and I3. This may be eiected through lines Q17, 49 and 5. Alternately, a toluene concentrate may be prepared by fractional distillation or the naphtha in zone 31,' higher and lower boiling components being withdrawn through lines 5l and 53. This cracked toluene concentrate `may be recycled directly to separation zone I3, through lines ill, 49 and l5.

The cracked naphtha or cracked toluene concentrate may contain substantial amounts of olefines, and particularly polyolefines, producedin the cracking. Olcnes have a polarity intermediate between that of paraiiinic hydrocarbons and of toluene. This makes it more difficult to separate olenes, and especially polyolefines, than to separate parafns from toluene in separation zone t3'. I Furthermore, once separated from the toluene, olelines form part of the toluene-free raflinate and interfere with the proper functioning of manyv isomerization catalysts 'in zone 3l. To overcome this difficulty the raffinate may be freed of oleiines after being separated from the toluene; or'as preferred, the olenes may be separated from the cracked naphtha or concentrate prior to the'recovery of toluene. The latter can be done in zone 55 by methods which eliminate or transform preferentially polyolef'lnes` or poly-and mono-olefines without substantially aecting aromatics and paraflins. Such methods are, for example, absorption in dilute acids such as sulfuric acid or complex-forming reagents, e. g., Cu2Cl2, AgCl, etc.; hydrogenation in the presence of a suitable catalyst such as nickel, copper, etc.; polymerization followed by fractionation, Vespecially catalytic polymerization, in the presence of suitable catalysts such as solid phosphoric acid or one of those listed in National Petroleum News of November 20, 1935, page 45 and following, or in U. S. Patent 2,171,207; partial oxidation in the presence of a catalyst suchas silver, etc., followed by fractional distillation, etc.; or a combination thereof.

The resulting substantially olene-free cracked product containing toluene may then be conducted through line -51 and recycled through either line E5 or line l5, as previously described, to

being both highly effective and economical, is a.

preferred method.

Extractive distillation may be performed by means of two columns, the polar solvent being introduced at the top of the first one while vapors containing toluene are fed at intermediate points and toluene-rich solvent is obtained at the bottom. In the second column the toluene-rich solvent is stripped of toluene which is obtained overhead and the regenerated solvent obtained as bottom` product may be fed to the first column. Suitable refluxing and reboilingr means are used in both columns to insure efficient separation.

The relative height on the first column of inlets for the two toluene concentrates of this process, i. e., the one obtained directly from separation zone 3 through line 1 and the cracked concentrate which may be obtained through line i5, depends upon their contents of toluene. The richer one will always be introduced below the poorer one for best results.

The most practical polar solvents suitable for extractive distillation are phenol, cresols, various mixtures of alkyl phenols, aniline, alkyl anilines, diphenyl amine, ditolyl amines, carbitols (diethylene glycol mono ethers) such as methyl, ethyl, and pro=pyl carbitols, chlorinated dialkyl ethers such as beta-beta-dichlorethyl ether, nitrobenzene, nitrotoluene, nitroxylenes, naphthols, alkyl naphtho-ls, benzo phenone, phenyl tolyl ketone, diphenyl ketone, alkyl phthalates such as dimethyl phthalate. alkyl salicylates such as methyl salicylate, benzyl alcohol, benz chlorides,

,ethoxy 2 e methoxy glycerol', 1,.'.s`-diethoxy`y glycerol, 1,2-di-propoxy glycerol, 1,3-di-propoxy glycerol, 1,2-di-isopropoxy glycerol, and 1,3-di-isopropoxy glycerol, themixed di-glycerol ether esters such as 1- glycerol, 1 methoxy 3- propoxy glycerol, and l-ethoxy-Z-isopropoxy glycerol.

It has been already pointed out that the isomerization step is essential to this process and is performed in zone 3| on at least a part of the rainate prior to cracking. Isomerizing condi- .tions capable of transforming naphthenes having 5 carbon atom rings into hydrocarbons having 6 carbon atom rings, e. g., methyl cyclopentaneto cyclohexane, have been found suitable for .the purpose of this invention. 'I'his isomerizing step will now be described more fully. ,HIsomerization may be effected by contacting the hydrocarbons with a suitable catalyst'under conditions depending on the nature of the catalyst. Thus, for example, molybdenum sulfide or oxide may act as an isomerization catalyst in the vapor phase at 400 C. to 450 C. in the presence of a large proportion of hydrogen. It is preferred, however, to utilize at a lower temperature Friedel-Crafts catalysts, e. g., the halides of Al, Zr, etc., in the presence of a hydrogen halide promoter and, if desired, hydrogen.

Pieferable isomerization catalysts are those comprising anhydrous aluminum chloride and/or aluminum bromide. Of these, aluminum chloride is preferred since it is less expensive, easier to employ and has much less tendency than alu- -minum'bromide to cause degradation of hydrocarbons. Other acid-acting catalysts of the FriedelCrafts type may also be employed if desired in conjunction with the aluminum chloride.

The catalyst may, if desired, be employed as a solid, either in suspension in the liquid reaction mixture or as a fixed bed. In the latter case it is preferably employed as lumps, granules or pellets of suitable size, preferably deposited upon or mixed with a suitable solid support. Particularly effective catalysts are produced when aluminum chloride is supported upon or intimately mixed with one of the various preferably adsorptive siliceous and/or aluminous materials of natural or synthetic origin which contain an appreciable amount of firmly-bound water. Suitable materials of this category are, for example, the naturally-occurring minerals and clays such as pipe clay, bauxite, fullers earth, bentonite, Florida earth, meerschaum, kaolin, infusorial earth, Kieselguhr, diatomaceous earth and the like; the various treated clays and clay-like materials such as Tonsil, Celite, Sil-O-Cel, Terrana and the like; and artificially prepared materials such as Activated Alumina, silica gel, the artificial permutites and the like. These materials are preferably heated in a dry atmosphere at a moderately elevated temperature, such, for instance, as about 400 C. to 800 C.I until they substantially cease to give offv water. Although these materials produce preferred catalysts when combined with aluminum chloride. other supporting materials s uch as activated carbon, coke. crushed brick, pumice and the like may also be used. v l

While solid catalysts may be suitably employed, an improved and advantageous method has been developed whereby the isomerization treatment may be effected in a continuous manner using liquid catalysts. Particularly suitable liquid catalysts are those of the Gustavson (C. Z. 1903 II, vi113) and Ansolvo (German Patent No.

' liquid catalyst.

535,473) acid types` wherein. aluminum chloride. is in combination. with various organic'compounds such organic.I acids, ethers, esters, hydrocarbons, etc., and those of; themetal double-salt type in which the aluminum chloride is in loose:

lysts may be preparedv which melt to free-flowing liquids at very moderate temperaturesand may be pumped through the reaction system with or countercurrent to the hydrocarbon fraction to be isomerized. The use of liquidcatalysts in the isomerization hasl delniteV advantages; The raffinate to be treated often contains small amounts of aromatic and/or oleiinic hydrocarbons. These hydrocarbon types, if present, are generally quite detrimental when employing fixed beds of catalysts. When, employing fluid catalysts, hydrocarbon. fractions containing small. amounts of olefines ory aromatic hydrocarbons may be treated without. diiculty, though large amounts of. either'are always. detrimental. These vliquid catalysts, moreover, have` ai considerably milder action and in` general; are much less prone to cause degradation ofihydrocarbons. Also,.they allow the: isomerization to be effected in simple apparatus in, a continuous. and most practical manner.

A ow diagram of a suit-able.,isomerization uniti employingA this' principle; is illustrated in thel at.- tached Figure II., Referring to Figure II, the hydrocarbon to. be treated is fed through line 29 into a surge tank 6 I` and from there Via pump 62.Y to a circulating pump 63 wherein it is mixedwith a liquid catalyst andA promoter and circulated through a heater 64 and time tank 65, and back to the circulating pump 631via a' valved line 66; The time tank 65 is preferably equipped with ,perforated baiiles or other means to insure thorough contact between the hydrocarbon and All or a portion of the material passing through time-tank 65 is withdrawnto a separating device 6-1 whereinthe liquid' catalyst is separated from the bulk of the hydrocarbon'. The liquid catalyst fromv separator l'lV is recycled to the circulating pump B3 Via valve 68- and line A 68. A portion of thepartially spent liquid catalystA may be continuously or intermittently removed if desired via valved outlet 69 andan equivalent amount of fresh catalyst maybe added via pipe l0. The hydrocarbon separated from the catalyst in separator 61 is passed through a cooler 7l and filter 12 wherein any particles of suspended catalysts are removed, and is then passed to tank 73.

The assembly of apparatus illustrated diagrammatically in Figure II is adapted for the use of liquid catalysts of the above-mentioned Gustavson or Ansolvo acid types. The catalyst may be prepared in any conventional' manner and supplied to the system, for instance, Via line 10. For the sake of completeness the preparation ofthe catalyst is also shown in the'flow diagram of Figure II. The apparatus provided therefor and' which may be considerably modied if desired, comprises a mixing vessel 14 interconnected in a circulatory system via circulating pump l5 to a small chamber 16. Aluminum. chloride or other catalyst is charged to the mixing vessel 14 and a quantity of the organic material to bev reacted therewithto. form the complex. catalyst is` introducedY via. line lil. may be any one or a mixture of a great:number of organicA compounds.

Incase the: complex catalyst is one of the Gustavson type'(C`..Z. 1903 II, 1113)` the liquidadded may be an aromatic-hydrocarbon or a hydrocarbon mixture rich in. aromatic hydrocarbons. Aromatic hydrocarbons, as. a` class, are generally suitable. Thus,.for example, benzol'and the various alkyl derivatives thereof. may be used. Particularly' suitable catalysts of this" type may be prepared from the: soz-called bright stocksy or Edeleanu extracts obtained. by the extraction of aromatic-containing: distillates: with sulfur dioxide.. These catalysts arefoundito contain' approximately twice.I as much aluminum chloride, for: example, asf. catalystsY of the same general type. prepared from benzol or toluol. They' therefore have a much longer-- life. Catalysts of` this type are described in U. S. Patents Nos.` 1,671,517?, 1,586,357, 1,999,345, and British Patent No. '7.,112 (1914). Complexes: prepared with extracts or unextractedk distillates are also useful;

The isomerization treatmentis preferably executedin the-presence of a. hydrogen' halide pro.- moter such, for example, as: hydrogen chloride. The amount of promoter preferably employedd'epends upon the. activity.v ofV the catalyst and1 the prevailing temperature, and may vary from a smallamountsuch as about 0.05% upto several per cent. Inonepreferred' method of" operation, whereinl the hydrocarbon. fraction istreated in a continuous manner in the liquid phase, a con-A siderableexcess of hydrogen' chloride (for exam- I ple, a partial pressure of hydrogen chloride vapor reaction zone. Thus, in FigureII- the product.

stored in tank 83- from which' it isy conducted to' the cracking step ofthis process' through line4 25'.

The hydrogen chloridepromoter separated from the productl in` the stripping" column isv recycled back tov-the isomerization system via line 88. The hydrogen halide (and hydrogen', if'this is used) recycled: to the system is preferablyv passed-l throughy ar drier 86' containing' a suitable drying agent such as CaClz, B205, activated alumina', or the like'. Small amounts of excess gas may be vented from' time to time byvalved outlet 81'; Make-uphydrogen chlorideis vsupplied'to the system from any'conyenient sourcesuch as a highbe carried. tothe reaction Zone,v dissolved inane of the. reactants, or maybe generatedin the reaction.zone from; added materials. which under the. reactionconditions react orA decompose to-libF erate hydrogen halide. Suitable materials which may be added for this purpose are, for example, organic halides such as organic alkyl halides, organic hydroxy compounds such as the various phenols and alcohols, inorganic salts containing molecularly-bound hydrogen halide such as PbSO4-2HC1, CuSOr-ZHCI and the like.

The isomerization with Friedel-Crafts catalysts may be effected over a wide range of temperatures depending upon the space velocity, concentration of promoter employed and upon the activity of the catalyst. Although temperatures ranging from about room temperature up to about 500 F. may be employed, the isomerization may be effected economically at quite moderate temperatures such, for example, as from about 160 F. to 400 F. In some cases, especially when the catalyst is particularly active or the temperature is somewhat high, it may be found that the isomerization is accompanied by appreciable amounts of side reactions. This may be easily remedied, if it occurs, by slightly decreasing the temperature or concentration of promoter, or if desired, by eiiecting the isomerization treatment in the presence of hydrogen. Hydrogen, if employed, may be conveniently recycled with the hydrogen halide promoter. Although the use of hydrogen is not generally necessary, provision for its use is shown in Figure II. Thus, if hydrogen is recycled along with the prometer, make-up hydrogen may be added from cylinder 85.

Referring now to Figure III which illustrates an application of this process in which one concentrate, e. g., the benzene or the toluene or the xylene concentrate, or any two concentrates, or the three concentrates simultaneously, are being treated. It will not be described in regard to the last-mentioned case, it being understood that by proper variation of the ow scheme any other case may be realized.

A hydrocarbon distillate containing aromatics is admitted from storage not shown through line 9| to fractional*distillation-zone 92. A suitable distillate produced in the process may be admitted thereto through line 93. In this fractionation zone 92 the admitted distillates are fractionated to produce the desired narrow boiling concentrates and separate them from the remainder of the distillate. The concentrates may be conducted through lines 94, B5 and 98 to aromatic separation zone 91. The remainder may be withdrawn through lines 98, 99, and 10|. If an aromatic is present in traces only it may by-pass the aromatic separation zone 91. Thus, for example, the benzene cut. may be conducted through lines |02 to line |04 in which the benzene-free benzene raffinate may be obtained from separation zone 91. In both cases (i. e., whether the benzene has been separated or not) this cut may be advantageously separated in fractional distillation column |05 into two substantial fractions, a bottom portion boiling above 170 F., which is withdrawn through line |06, Vand an overhead fraction obtained through line 41 and conducted to line 44.

The other aromatic-free raiiinate may be obtained through lines |08 and |09 which lead, as well as line |04, to isomerization zone ||0.

A fractionation step to separate the raflinate into two portions, both forming a substantial part thereof, one higher and one lower boiling, and to by-pass the latter around the isomerization zone may be effected also on the toluene and Axylene raffinate. In zone ||0 the three rafiinates are isomerized under conditions substantially as described above for toluene. Theymay be oommingled through line |I| and isomerized together, but it is preferred to effect the isomerization separately because of different optimum condltions of isomerization of the several raillnates.

The isomerized rainates or their fractions are withdrawn from zone 0 by lines ||2, ||3 and I|4, respectively, and are advantageously commingled with the higher boiling part of the rafflnate which has by-passed the isomerization zone. Thus, for example, the part of the benzene raflinate boiling above 170 F., which has been obtained through line |06 from fractional distillation column |05, is commingled with the isomerized part boiling below 170 F., conducted through line ||2. `In the cracking zone ||5 three fractions are cracked under conditions similar to those previously described for the toluene concentrate in cracking coils H6, |`|1 and IIB. It is preferred to crack the fractions under independently controlled conditions of time and temperature, as their thermal stability is different, the lower boiling ones being less stable and requiring greater liquid space velocities for the same gasification. If desired, the several concentrates may be combined by means of line ||9 and cracked together in one coil.

The cracked products after rapid quenching are treated in separation zone |20 to separate therefrom dienes which may be withdrawn through line |2I, an aromatic-rich naphtha having a broad boiling range taken through line 93 to zone 92, or one or more cracked concentrates taken through lines ||2, |23 and |24 to zone 91; and other hydrocarbons may be withdrawn at |25 or |26.

Oleiines which may interfere with the separation of aromatics or cracking may be separated in separation zone- |21 as described above.

The following examples may further serve to illustrate this invention.

lExample I Isomerized Unisomcrized riilnate raiinatc Liquid space velocity 0l 70 Gasitlcation 59 53 Butadienc produced wt. on feed. 4, 7 3.2 Peutadiene produced wt. on

feed 6.4 l. 3 Butadiene produced wt. of gas phase 8.0 Pentadiene produced wt. of gasA phase 10.9 47.1i Butadlene produced wt. of C4 cut- 40. 3 2li. li Pentadiene produced wt. of C; cut-. to. 0 52. :i

Example II A xylene-iree xylene ra'flinate obtained from a West Texas naphtha boiled from F. to 142 F. One part of it was cracked at 1382 F. without any further treatment, and another part was isomerized in the presence of toluene-aluminum chloride catalyst at 176 F. at a liquid space velocity of 1 volume of ranate per volume of catalyst per hour and in the presence of 0.1% HC1, and then cracked under the same conditions as the first part.

The former yielded 12.8% of dienes in the gas phase, while the latter yielded 16%.

We claim as our invention:

1. An improved process for manufacturing conjugated diolens from a relatively narrow boiling hydrocarbon fraction boiling within the range of 150 to 290 F. from which substantially all the aromatics have been removed, comprising the steps of fractionally distilling said fraction to produce higher boiling and lower boiling portions, isomerizing the lower boiling portion, combining the isomerized product with said higher boiling portion, cracking the combined product in the vapor phase above 1000 F. for a time to gasify 20% to 80% thereof, whereby substantial amounts of conjugated diolefns are produced, and separating said diolefins from the resulting products.

2. The process of claim 1 wherein said narrow boiling fraction boils between 156 and 185 F. and from which substantially all of the benzene has been removed.

3. The process of claim 1 wherein said narrow boiling fraction boils between 195 and 245 F. from which substantially all of the toluene has been removed.

4. The process of claim 1 wherein said narrow boiling `fraction boils between about 260 and 300 F. from which substantially all the Xylenes and ethyl benzene have been removed.

RUPERT C. MORRIS. ROBERT J. MOORE.

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