US2419521A - Process for recovery of aromatic hydrocarbons - Google Patents

Description

April 22, '1947.

J. w. wgLDRoN PROCESS FOR -RECOVERY 0F AROIATIC HYD'ROCARBONS Filed oct. 12, 1940 ATTORNEY 'Pnenied Apr. 2z; 1947 n PROCESS FOR RECOVERY OF AROMATIC HYDROCARBONS John W. Waldron, Philadelphia, Pa., assigner, by mesne assignments, to Allied Chemical & Dye Corporation, a corporation of New York Application October 12, 1940, Serial No. 360,864 i This invention relates to a process for recovering as a substantially pure product an aromatic hydrocarbon of the benzene series from oils containing it and containing other hydrocarbons, including oleiins, boilins,r from the oil in the same temperature range as the 'aromatic hydrocarbon boils therefrom.

Numerous hydrocarbon oils are known which contain aromatic compounds of the benzene series in varying proportions together with substantial proportions of olelnic hydrocarbons (by which term I refer to both olefins and dioleflns). For

lexample, gasoline fractions obtained in the treatment of petroleum frequently contain substantial proportions of benzene, toluene and/ or xylene together with oleiins, parafiins and naphthenes. Oils of petroleum origin having a considerable content of aromatics of the benzene series may be treated by well known selective solvent processes, for example extraction with liquid sulfur dioxide, to produce fractions rich in aromatics but which will still contain substantial amounts of olefinic, paraflinic and naphthenic hydrocarbons. A considerable portion of these hydrocarbons cannot be completely separated from the aromatic constituents by fractional distillation` because of the closeness of the boiling points of non-aromatic constituents of the oil to the boiling points of the aromatic constituents or because the non-aromatic constituents form constant boiling mixtures with the aromatic. While the aromatic constituents of light oils produced by the gasification of coal ordinarily may readily be separable and recovered as the substantially pure individual aromatic constituents, in some cases these light oils contain non-aromatic oils, including olens, which are but diiiicultly separable from the aromatic constituents of the oil. Also, synthetic hydrocarbon mixtures produced by various catalytic processes may contain aro- 8 Claims. (Cl. 202-,57)

matic constituents of the benzene series accoml panied by olens and other non-aromatic hydrocarbons diiilcultly separable from the aromatic constituents. y

example, toluene is largely used for the producl tion of TNT, for which purpose a so-called nitration grade of toluene is now preferred. While toluene products containing substantial proportions of certain hydrocarbons other than toluene can be nitrated, the mono-nitro compound made from them must be purified prlor to complete nitration. This materially increases the cost and complexity of the process for making TNT. Furthermore, even though toluene containing substantial quantities of certain other materials is sometimes used for nltration, this is only because the diilculties of preparing a pure toluene have outweighed the disadvantages of using the impure toluene for the production of explosives. Direct fractional distillation will not ecct a recovery of pure toluene from oils such as enumerated above and in many cases will not give iractions of suitably high toluene content or free from materials which even in small concentrations adversely aiect the nitration of the toluene or the nitrated product.

Azeotropic distillation of hydrocarbon oils such as described above in the presence of an added azeotropic agent which forms azeotropes with the several non-aromatic hydrocarbons in the mixture, i. e., the olenic, naphthenic and paralnic hydrocarbons, having lower boiling points than the aromatic constituent of the oil or its'azeotrope, will eiect a selective separation from the oil of the non-aromatic hydrocarbons which, :in the absence of the azeotropic agent, boil from the oil in the same temperature range or at lower temperatures than the aromatic hydrocarbon boils therefrom. By azeotropically distilling the oil to accomplish this separation, the residue of the azeotropic distillation may be subjected to a non-azeotropic, fractional distillation to remove hydrocarbon constituents, aromatic or non-aromatic, of higher boiling range than the desired aromatic constituent. Or by iirst fractionally distilling the original oil in the absence of added azeotropic agent to recover, a cut containing the desired aromatic together with the other hydrocarbons boiling from the oil up to temperatures only a few degrees above the boiling point of the desired aromatic constituent and then azeotropically distilling such a cut under suitable conditions, an oil.residue substantially consisting of the desired aromatic hydrocarbon may be obtained,

I have found, however, that in thus azeotropically distilling oils containing aromatic hydro- 'treating oils in which the ratio of olens to aromatics is relatively high. In distilling such refractory oils, the degree of fractionation both in non-azeotropic and in azeotropic distillation, becomes of critical importance, and the degree of fractionation necessary for recovery of aromatic hydrocarbons in good .vield and substantially free of non-aromatic hydrocarbons may be so great as tomake the operation uneconomical or to exceed the facilities available at a given plant. 1'I'he alternative, with fractionation equipment inadequate for securing high yields of relatively pure aromatic hydrocarbons, is to sacrice yield to purity by taking olf as overhead or intermediate cuts distillate containing considerable amounts of the desired aromatics as well as the non-aromatics which are to be removed from the oil to leave an aromatic residue of the desired purity.

n the other hand, although it is known that treatment of an oil with concentrated sulfuric acid will remove olenic hydrocarbons, such treatment is not satisfactory as a preliminary to azeotropic distillation in the present case. In the z' presence of concentrated sulfuric acid olefins condense with aromatic hydrocarbons and the resultant productsv are removed from the oil with the acid. Consequently, when an oil containing a high ratio of olens to aromatics is treated with strong sulfuric acid, a large part or even all of the aromatics may be removed from the oil. The yield of aromatics in a recovery operation following such treatment would be reduced by th'e amount of aromatics so removed.

It is an object of my invention to provide a process whereby a substantially pure aromatic product may be recovered without undue loss from an oil containing itand containing likeboiling, non-aromatic hydrocarbons, including substantial amounts of olens. (In this specication and in the appended claims, the term like-boiling, non-aromatics is used to denote the non-aromatic hydrocarbons of an oil which boil therefrom in the same temperature range as the aromatic hydrocarbons of the oil boil therefrom, and thus these like-boiling,nonaromatics are diillcultly separable or inseparable from the aromatics by a non-azeotropic fractional distillation of the oil.) It is an object of my invention point where the residue containsv a good yield of the desired aromatic hydrocarbon practically free of paralns and naphth'enes, and the' olefin 'content of the residue has been reduced to such an extent that the olens. may be removed by sulfuric acid with much less loss of aromatics than if the oil containing olens is treated with sulfuric acid prior to azeotropic distillation.

My invention comprises distilling a hydrocar- Y 4 aromatic hydrocarbons. olens to aromatic, with rectication of the vapors evolved in distilling the oil, in the presence of a material forming low boiling azeotropes with the non-aromatic constituents of the oil. By this azeotropic distillation the non-aromatic hydrocarbons are selectively distilled out. leaving aresidue enriched in aromatics. This azeotropic distillation of the oil is continued until the residue of the distillation contains at least one aromatic hydrocarbon, no more than 10 parts by weight of like-boiling parafilnic and naphthenic hydrocarbons to each 90 parts of said aromatic hydrocar- Ibon, anda part only ofthe olens ypresent in the oil which is distilled. The aromatic hydrocarbon of the residue of this azeotropic distillation, accompanied by the olefins not removed from the residue, by the azeotropic distillation, is washed with' concentrated sulfuric acid in amount sunlcient to remove from the aromatic hydrocarbon substantially all of the oleilns. By this process a product is obtainedcontaining one or more contain hydrocarbons boiling above and/or below the boiling range, of the aromatic hydrocarbon present in the oil which is to be recovered in the residue of the distillation. Lower boiling hydrocarbons (aromatic or non-aromatic) will be taken over as distillate in the azeotropic distillation. Higher boiling hydrocarbons which may still remain with the desired aromatic product may be removed therefrom by non-azeotropic distillation to give a product containing the desired aromatic hydrocarbon of increased purity.

The process of my invention is of particular advantage when the hydrocarbon oil containing an aromatic. and like-boiling, non-aromatic hydrocarbons which is to be azeotropically distilled, contains oleflns in amount such that the numerical ratio of the percent of aromatic hydrocarbons in the oil to the bromine number of the oil is less than 250.

In determining the ratio of the percent aromatic hydrocarbons in the oil to the bromine number, the aromatic content of the oil may be determined, for example, by the method of specific dispersion (described in Industrial &

Engineering Chemistry, Analytical edition, 1l (1939), 614-624), due care being taken to remove or allow for interfering substances if present in the oil. The bromine number of the oil may be determined by Franciss method (described in Industrial & Enginering Chemistry, 18 (1926) 821-822).

Further, `in treating an voil containing olefins in' accordance with the process of my invention, the oil preferably is subjected to two distillations; one the above-described azeotropic distillation and the other a fractional distillation (non-azeotropic). In' the azeotropic distillation the likeboiling parainic and naphthenic hydrocarbons and a portion of the olenic hydrocarbons are selectively distilled out leaving an oil residue enriched in th'e desired aromatic hydrocarbon, containing normore than 10 parts by weight of the like-boiling naphthenic and parafnic hydrocarbons to each parts of the desired aromatic hydrocarbon, and containing a part of the olens. In the non-azeotropic fractional distillation, which may either precede or follow the azeotropic distillation, the' aromatic'constituent which' is to bon oil containing aromatic and likeboiling,non 75 be recovered by the process is separated from including a high' ratio of higher boiling hydrocarbons and lower boiling hydrocarbons, if any are present. The combined eiect of these two distiilations is to separate an aromatic hydrocarbon product; largely or entirely freed from higher and lower boiling hydrocarbons and from like-boiling parain and naphthene hydrocarbons and freed from like-boiling olefinic hydrocarbons to such' an extent that the original roleilnfaror'natic ratio isv substantially reduced.

' The aromatic hydrocarbon recovered 'in the residue from theazeotropic distillation, accompanied by a reduced amount of olefinic hydrocarbons, is subjected to treatment with concentrated sulfuric acid, preferably of about 88% to produce toluene of high commercial purity, e. g.,

sis-100% and substantially free of oieflns, which meets all requirements for nitration pure toluene.

While my process may be carried out With the operation I much prefer to rst non-azeotropically distill the original oil, taking off a cut or fracltion containing the aromatic constituent which it is desired to recover from the oil separated from the higher and lower boiling hydrocarbons. By first fractionally distilling the aromatic -oi1 and then azeotropic'ally distilling only that cut in which the desired aromatic constituent is concentrated, the quantity of oil treated in the azeotropic distillation and the amount of azeotropic agent distilled per unit of Vproduct made are reduced and the azeotropic distillation made more economical and more readily operable to recover the desired aromatic constituent accompanied by a part of the olens of the original oil, which oleiins are then removed by the sulfuric acid treatment. f

In the processes of my invention the sulfuric acid treatment removes from the aromatic oil fraction the olefin hydrocarbons contained therein. The 'thus treated aromatic fraction may be neutralized with an alkali and redistilled.

My invention is of particular importance as a means for recovering pure toluene from oils containing it together with other hydrocarbons. The examples described below and illustrative of the invention show its application to the recovery of toluene from petroleum oils.

The accompanying drawing illustrates diagrammatically an apparatus suitable for carryof valves 5 and 6, condensate flowing from condenser 4' is dividedl in controlled proportions. One part of the condensate is returned through pipe 1 to the top of column 3 and reuxed inr contact with the vapors rising inthe column. The other portion of the condensate is withdrawn through pipe 8. Two receivers .9 and I0 I are connected to pipe 8 through valve-controlled branch pipes II and I2 so that the condensate drawn 0H through pipe 8 may be divided and distributed as described below to the two receivers.

Receiver I0 is connected by a pipe I3 controlled by 'valve I4 with a second still I5, Still I5, like still I, is provided with a heater I6, a rectification column I1, a condenser I8 and pipes I9 and 20 controlled by valves 2| and 22 for-return of determined proportions of condensate from condenser IB to the top of column I1 and withdrawal of another portion of the condensate through' pipe 20. Pipe .20 connects with valve-controlled branch pipes 23 and 24. Two separators 25 and 26 are connected to the respective branch pipes 23 and 24. Each of these separators is provided with a water inlet pipe 2'I and 28, respectively. Each separator is provided with a liquid withdrawal pipe 29 andA 30, respectively, for removal from the separator of a top oil layer and Withldrawal pipes 3| and 32, respectively, for withdrawal of a bottom aqeous liquid layer. Pipe 30 le'ads to afliquid treating tank 33'in which liquid withdrawn from separator 26 may be agitated with sulfuric acid introduced to tank 33 through a pipe 34. Tank 33 contains any conventional means, not shown in the drawing, for intimately -mixing the sulfuric acid and liquid introduced into the tank from separator 26 and thenfacilitating separation of the mixture into two layers. Tank 33 is provided with a valve-controlled draw-off' pipe 35 leading from the bottom of the tank.

Example 1;--The oil treated by the process of this example was a petroleum fraction havin the following characteristics:

A charge of this hydrocarbon oil introduced into stillqI was boiled in the still and the evolved vapors were countercurrently contacted in column 3.- with reflux from condenser 4, in whichthe vapors leaving the top of the column were substantially entirelylcondensed. Most of the condensate from condenser 4 `was returned through valve 5 and pipe 'l to the top of the column to furnish the reflux for the column. The remaining small portion of the condensate was continuously withdrawn through valve 6 and pipe 8 and passed into collecting vessel 9 or I0. The

distillation was conducted under substantially atmospheric pressure; i. e., the pressure in condenser 4 and at the to-p of column 3 was substantially atmospheric and the pressure in still I was only lenough higher to force the vapor through the rectification column to the condenser.

When the vapor temperature in the top of column 3 reaches 95 C., the condensate passing through pipe 8 is diverted to vessel I0. The cut taken in vessel I0 includes the distillate coming over up to and ata temperature of l11-C. at the top of the column. After this cut has been taken oi to vessel I0, the distillation may be discontinued and the residue left in the still withdrawn therefrom. The cut. in vessel 9 contains the low boiling fraction of the original oil.

a Francis bromine number of 0.72;v Accordingly,

ge ratio percent aromatic:bromine number'- was This cut was passed through pipe I3 into stilll I5. There was also charged into' still I5 anhydrous methanol in the proportions of 2 parts by volume of methanol for every 1 part by volume ofthe toluene fraction. The mixture of methanol and toluene fraction was distilled with rectification of the vapors in column I1 and condensation of the vapors leaving the top of the column in condenser i8. Most of the condensate was returned through pipe I9 to the top of column Il while the remainder waswithdrawn through pipe 20.

Toluene and methanol form an azeotrope having a boiling pointof 64 C. Hydrocarbons other than toluene present in the toluene fraction introduced into still I5 form azeotropes with methanol which have boiling points below that of the toluene azeotrope. The paraffin hydrocarbons and any smalll amount of naphthene hydrocarbons which may have been present in this oil, formvazeotropes with boiling pointe sumciently below that of the toluene azeotrope for them to be preferentially vaporized and by rectication in column I'I readily separated from the toluene and any toluene-methanol azeotrope which is vaporized in still I5 and enters column I'I solong as there is sufcient methanol present in the vapor and liquid .phases in the rectication column so that the temperature atthe top of column I1 remains po higher than 64 C. Azeotropes of olenic hydrocarbons lpresentI vin the toluene fraction introduced to still I5 will, in substantial part, also be distilled out and carried over with the azeotropes of the paraiiin and naphtheriic hydrocarbons. Any toluene which may be vaporized inA still I5 is largely returned down the rectiiication column and retained in the still while the azeotropes of the paraiiin and naphthenehydrocarbons andrsome of the oleiin hydrocarbons are distilled out and withdrawn through pipe 20. During this initial distillation Stage the condensate withdrawn through pipe is passed through pipe 23 into separator 25. The loss of toluene Vfrom the still to the condensatedrawn olI to separator will' depend upon the elciency with which the vapors are rectified in column I1. Eilicient rectication is employed in order to keep down this loss of toluene.

In carrying out the process of this example the initial distillate drawn off through pipe 2li was free from toluene. As the distillation continued, toluene appeared in the distillate and the proportion of toluene to other hydrocarbons gradually increased. The initial distillate was withdrawn to separator 25 after which a cut containing about 9l1/2% by volume of toluene and the remainder substantially olens (based`on the hydrocarbon content of the cut) was taken oil to separator. 26. The tofuene'content of the distillate drawn on through pipe 20 rose to about 96% (based on the hydrocarbon content of the distillate) while the cut was being taken off to separator 26.

The distillates thus collected in separators 25 and 26 are treated in the separators with Water l to remove methanol `from ythe oil and separated vinto ltwo layers, a top oil layer and a bottom aqueous methanol layer. The top oil layer with..

drawn from separator 25 through pipe 29 is treated orl used as desired. This oil layer con-L tains substantially all 'of the parailinic and naphthenic yconstituents of the original toluene fraction introduced to still I5 which boil .from

the ,toluene fraction in the absence of van azeof .l0 trope at the same or lower temperatures than the toluene boils therefrom.l The oil also con- .A

tains a substantial portion of the olens contained in the original toluene fraction. The

aqueous methanol layer withdrawn from the 1li ofua subsequent batch of toluene fraction in sti I5.

The distillate taken oiI to separator 25 is washed in the separator with water and allowed to form two layers. A bottom aqueous methanol layer= is withdrawn through pipe 32 and may be treated with the aqueous methanol from separator 25 as described above. The top oil layer is withdrawn from the separator throughpipe 30 and introduced into tank 33. In tank 33 the oil was washed successively six times with 66 B sulfuric acid introduced through pipe 34. In eachrof the washings the oil 4was treated with 5% by volume of acid, i. e., a total of 30% by volume of the acid wasused for washing the oil. After each treatment of the oil with acid the acid containing reaction products of olens was allowed to settle to the bottom of the tank and withdrawn throughpipe 35. This acid may be treated as desired 4for recovery of any of its constituents 0r may be discharged as Waste. After washing the oil with acid in tank 33 and withdrawal of the last washing acid, the oil thus puried of olens is withdrawn from the tank through pipe 35, neutralized with an alkali and fractionally distilled, the fraction distilling over at ll0.5 C. to 1l0.7 C. (vapor temperature) being collected separate from the forerunnings and higher boiling constituents. 'I'he fraction thus recovered was 100% toluene. l

In the process of the above example, after the distillation had been carried to the point at which the undistilled residue contained toluene substantially separated from like-boiling parainic and naphthenic hydrocarbons, the distillation was continued and they toluene distilled out of that residue and separately collected from the forerunnings for treatment with sulfuric acid to remove the olens which accompanied the tolu-I ene. -At the conclusion of the distillation of the -toluene there was left ln still I5 methanol containing a small amount of residual undistilled oil. This methanol may |be left in the still and 60 used in distilling a subsequent charge of toluene until the accumulation f impurities in the still residue makes it desirable 'to Withdraw the residue and treat it as desired to recover methanol therefrom for reuse in the distillation of a sub- 05 sequent charge of toluene fraction.

Instead of continuing the distillation of the toluene fraction after the non-toluene hydrocarbons have been distilled over to leave a residue from which these latter hydrocarbons have been separated to the desired degree as in the above example, the distillation may be discontinued when the unvaporized residue has been substantially freed of paramnic'and naphthenic hydrocarbons boiling in the absence of an azeotropic 76 agent in the same temperature range as the bottom of separator 25 may be treated to recover the methanol therein for reuse in the distillation V the determination of its bromine number.

toluene. This residue, after-removing any methano] it may contain by extracting withv water, may be treated with sulfuric acid-to remove the Examinez-Another oil treated by my process had the; following characteristics:

The dark color of this'oil would interfere with Accordingly, to determine this bromine number a sample of the oil was treated by the process de- Specic-gravlty (at 15.5C.) 0.798 l'araiflns.--f per cent-- 44.5 Aromatics dn 34.8' Distillation range:

Start C 62 Dry C 25.4

scribed in Industrial and Engineeringv Chemistry,

analytical edition 11, 622 (1939), to remove conjugated diolens by condensation with maleic anhydride followed by extracting thecondensa'- tion product and excess maleicanhydride from the treated oil sample with 5% aqueous caustici soda solution prior to distilling the treated oil sample as described in this publication. 'I'he brominel number for the oil from which conjugated diolens had thus been removed was determined |by the Francis process referred to above. 'I'he bromine number was 0.33. Accordingly, the numerical value of the ratio percent aromatic over bromine number of the treated oil was 105.5.l

The numerical value of this ratio for the originalkoil prior to removal of the conjugated diolens would have been somewhat less than 105.5 by an amount corresponding to the increased bromine number of the original oil due to its conjugated diolen content as compared with the bromine number 0.33 of the oil after treatment to remove the conjugated diolens. Y

The oil having the above tabulated characteristics was introduced into still I and fractionally distilled in the same manner as theA oil of Example 1. 100.2110.3 being collected separately from-'the The cut coming over in thel range 5 forerunnings and higher boiling fractions -for treatment to recover toluene therefrom. This cuthad a toluene content of 25%. y Francis bromine number of 0.182. Accordingly,

the ratio percent aromatic:bromine number `for this cut was 132. Y

Seventhousand parts by volume of methanol were added to 2710 parts by volume of this cut y.

in still l5 and the mixture distilled through rectification column Il as described above. When the toluene content of the oil in the distillate had risen to 90% by volume, the succeeding distillate was separately collected until the toluene content of the oil in the distillate rom'the top of column I1, after passing through a maximum of 100%, fell below 97%. This portion of the distillate, after extracting the-alcohol by treatment It had al.

olefinlc content which was treated by myy process had the following characteristics: l

Specific gravity (at 15.5.C.) ..-0.916 Parafllns per cent 2.0 Aromatics Y dn `49 Distillation range:

Start C-.. 64 Dry C l349 This ou was fracuonauy distilled as described above, the cut distilling inthe vapor temperature range of E-107.7 C. was taken for, isolation of toluene. The 'cut l.contained 74% toluene by volume and hadV a Francis-bromine number of 0.405. The value for the ratio per cent aromaticzbromine number for the cut was 182.7.

vFive thousand parts by volume of methanol lwere added -to 1250 partsby volume of this cut in still l5 and the mixture distilled through a rectificationcolumny I1 las described above. vWhen the toluene content of the oil in the., distillate had risen to 97 byvolume, the succeeding fractions Awere separately collected until the toluene content of the distillate oil,"afte'r passing through `a maximum of 100%, fell below 99%. 362 parts by volume of distillate oil was collected A total of in this cut, the oil containing 99% by volume of toluene. n y

This oil was washed twice with 66 B. sulfuric `acid and neutralized, a volume of acid equal to 1% of the koil beingused for each washing. The washing loss wasv 3% by volume. The separated oil layer* was distilled and yielded 92% of its volume as a fraction distilling. between 110.0'110.5

C. lvapor ,temperature of highA gradel industrial toluene. Y, j 1

rThe invention described above is of (particular importance as` a. method forthe recovery.` of pure toluene from oil fractions containing itgtogether with substantial quantities of olens` and other non-toluene hydrocarbons -Toluene may be recovered by subjecting to the azetropicidistillation step. of my process an oil containing, in addition to like-boiling, non-aromatic hydrocarbons, lower and higher boiling aromatic and nonaromatic hydrocarbons.Y In azeotropicallydistilling such an roil, the distillation is continued until the residue contains toluene and no more than 10 parts of-like-boiling parainic andnaphthenic hydrocarbons (i. e., parainic and naphthenic hydrocarbons which, in the absence of anA azeotropic agent, boil'from the oil in the `same temperature range as the toluene boils thereffrom).' .The tolueneJ thus separated from these like-boiling hydrocarbons is separated from olefins by treatment with sulfuric acid and may be separated from higher boiling aromatic and nonaromatic hydrocarbons by a non-ameotropic fractional distillation.

It is, nevertheless, of particular advantage in the recovery of toluene to preliminarily distill theoriginal toluene-containing oil taking off a out having an end boiling point but little above the boiling point of toluene; i. e., an end boiling point of 118 C. and preferably of 111 C. By so doing there will be excluded from the toluenev fraction subsequently subjected to azeotropic distillation a large part or all of the non-toluene hydrocarbons boiling from the toluene oil at temperatures above the fboiling point of toluene,

the completeness of their separation depending on the sharpness of fractionation during the distillation and the relative amounts and kinds of the oil constituents. tion ofsuch a fraction a residue is lobtained In the azeotropic distilla-.

largely or wholly freed of. paraillns naph- .thenes By washing such a residue with sulfuric rials boiling higher than toluene* it is economically desirable to exclude forerunnings high in non-toluene hydrocarbons for th reasons pointed out above.f In general, it is preferred to subject to the azeotropic distillation a toluene cut boiling in the range 95 to 118 C., preferably in the range 95 to 111 C.

The process of this invention may likewise be applied to the recovery of other aromatics from oil fractions containing them together with non 'aromatic constituents boiling from the oil fraction in the 'same temperature range as the aromatic hydrocarbon or hydrocarbons which are to be recovered boil therefrom. In recovering benzene it is preferred to subject to azeotropic distillation an oil fraction boiling in the range about 65 to about 80 C. For the recovery of a mixture of xylenes it is preferred to azeotropically distill a xylene fraction boiling in the range about 125 to about 145 C. It is particularly desirable, in recovering aromatic hydrocarbon,` to exclude high boiling hydrocarbons from the oil fraction subjected to the azeotropic distillation step of my process as discussed above in connection with the recovery oftoluene. Accordingly, in allcases I prefer that the oil fraction azeotropically distilled be one having an end boiling point in degrees Centigrade which is not more than 1.06 times the boilingpoint in degrees Centigrade of the aromatic hydrocarbon or hydrocarbons in the oil fraction which are to be recovered in the residue of the azeotropic distillation. Following az'eotropicdistillation of an oil fraction having this end boiling point, thev residue of the azeotropic distillation may be Vafter each washing, the color and volume of the acid sludge as compared with the volume of the acid added, one may judge when the oleflns have 4been removed by the sludge having a light color and being of substantiallythe same volume as the acid. Acids of a concentration of 88% or higher, particularly about 93% acid, may be satisfactorily used foi` Washing the oil at room temperatures.

As azeotropic agents in the distillation of the hydrocarbon fractions, I may employ any material which forms azeotropes with the likeboiling parainic or naphthenic hydrocarbons present in the aromatic fraction. I havefound methanol to be particularly suitable for use as the azeotropic agent, although the invention is not limited to any particular azeotropic agent.

The following materials are representative of.

those which may be used as azeotropic agents in carrying .out the azeotropic distillation step of my invention: aliphatic monohydroxy alcohols containing no more than 5 carbon atoms to the molecule: i. e., methanol, ethanol, primary and secondary propanol, the butanols and the vaxnyl alcohols (aqueous as well as non-aqueous lalcohols may be used); acetic acid and acetic anhydride; pyridine, para-dioxane, ethylene chlorfor every 90 parts by weight of the aromatic hydrocarbon which it is desired to recover.

While I have described my processin conjunction with an example in lwhich two distillation steps are carried lout as batch procedures, either or both of these distillations advantageously. may

be carried out continuously by well known continuous distillation procedures suitable for the fractional distillation of mixtures of two or more liquids. In the azeotropic distillation of the aromatic oil fraction side streams enriched in aromatics as compared with the top product may be drawn on from the fractionating column, particularly from the lower part of the column. The term residue in this specication and the appended claims is used in a broad s ense to include such side streams.

It is, of course, obvious a.v preliminary distillation of a crude aromatic oil to recover a fraction suitable for recovery of the desired 'aromatic therefrom by azeotropic distillation and removal of oleilns by means of sulfuric acid need not be carried out in immediate conjunction with the azeotropic distillation of the aromatic fraction. For example, the aromatic fraction suitable for treatment by the azeotropic distillation andsulfuric acid washing steps of my process may be produced in one plant and transported to and treated later in another plant by these latter two steps.

In this specification I have described the azeotropic distillation of aromatic oil fractions as carried out under substantially atmospheric pressure. It is, of course, possible to carry out either the non-azeotropic fractional distillation or the distillation with the azeotropic agent under pressures above or below atmospheric. In such cases the particular temperaturesl in the distillations lwill correspond to the change in distillation temperatures with the change in pressure. The temperatures given in this specification and in the appended claims are corrected temperatures for one atmosphere pressure (760 mm. of Hg). I have used the termlaromatic hydrocarbon" in a broad sense to include both chemical individuals such as toluene, and aromatic fractions of limited boiling range containing closely related and diillcultly separable aromatic compoimds which are commercially used as a unit such as xylol .which may contain two or more of the isomeric xylenes and ethyl benzene.

from an oil containing the same and containing' v higher, lower and like-boiling,.non-aromatic hydrocarbons, including a substantial proportion of like-boiling olens, which c'omprisesvfractionally distilling said aromatic oil and inthis distillation taking olf an oil fraction having an endl boiling point in degreescentigrade notA more than 1.06 times the boiling point in degrees centigrade of said aromatic hydrocarbon and containing said aromatic hydrocarbon and oleiins in amount such that the numerical value of the ratio per cent aromatic:bromine number for said oil fraction is less than 250, azeotroplcally distilling said oil fraction with fractionation of the distilled vapors in the presence of a material forming low-boiling azeotropes with the non-aromatic hydrocarbons present in said fraction, in said azeotropic distillation taking off as an overhead distillate with said material substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than oleflns and a portion of the oleflns present in said oil fraction, thereby recovering an aromatic hydro carbon fraction containing said aromatic hydrocarbon and a portion only of the olens which were present in said oil fraction, washing said aromatic hydrocarbon fraction with sulfuric acid of a concentration and in an amount such'that th aromatic hydrocarbon fraction is substantially completely freed of olefins, separat-A ing the acid-washed aromatic hydrocarbon fraction from reaction products of the sulfuric acid and-olens, neutralizing the aromatic hydrocarbon fraction with an alkali and fractionally distilling the neutralized fraction and in this distillation collecting separate from forerunnings and higher boiling residue a fraction consisting of nitration-pure aromatic hydrocarbon.

`2. The proces for the recovery of a'nitrationpure aromatic hydrocarbon of the benzene series from an oil containing the same and containing higher, lower and like-boiling, non-aromatic hydrocarbons, including a substantial proportion of like-boiling olefins, which comprises fractionalli7 distilling said aromatic oil and in this distillation taking olf an oil fraction havng an end boiling point in degrees centigrade not more than 1.06 times the boiling pointl in degrees centigrade of said aromatic hydrocarbon. and containing said aromatic hydrocarbon and olefinsin amount such that the numerical value of the ratio per cent aromatic: bromine number for said oil fraction is less than 250, azeotropically distilling said oil fraction with fractionation of the distilled vapors in the presence of an aliphatic monohydroxy a1- cohol containing no more `than 5 carbon atoms to the molecule, said alcohol forming low-boiling azeotropes with `the non-aromatic hydrocarbons present invsaid fraction, in said azeotropic distillationtaking off as an overhead distillatewith said alcohol substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than oleiins and a portion of the olens present in said oil fraction, thereby recoveringan aromatic hydrocarbon fraction containing said aromatic hydrocarbon `and a portion only of the and olens, .neutralizing thel aromatic lhydrocarbon fraction with analkaliandfractionallydistilling the neutralized lfractionfaiid.in this clis-- tillation collecting separate, from forerunnings and-higher boiling residue a fraction consisting of nitratlon-pure aromatic hydrocarbon.

3. The process for the recoveryofz nitratlonpure toluene from an oil` containing the same and containing higher, lower and like-boiling, non-aromatic hydrocarbons, including a substantial proportion of like-boiling oleiins, which comprises fractionally distilling said aromatic oil and in this distillation taking off an oil fraction boiling in the range 95 C. to 111 C. containing toluene, like-boiling non-aromatic hydrocarbons and like-boiling olensin amount such that the numerical value of the ratio per cent aromatic: bromine number for said oil fraction is less than 250, azeotroplcally distilling said oil fraction with fractionation of the distilled `vapors in the presence of methanol, said methanol forming lowboiling azeotropes with the non-aromatic hydrocarbons present in said fraction, in'said azeotropic distillation taking olf as an overhead distillate with said methanol substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than olens and a portion of the oleflns present in said oil fraction, thereby recovering an aromatic hydrocarbon fraction containing said aromatic hydrocarbon and a portion only of the oleflns which were present in said oil fraction, washing said aromaticlhydrocarbon fraction with sulfuric acid of a concentration and in amount such that the aromatic hy- 'drocarbon fraction is substantially completely higher, lower and like-boiling, non-aromatic hydrocarbons, including a substantial proportion of like-boiling olens, which comprises fractlonally distilling said aromatic oil and in this distillation taking oi an oil fraction having an end boil- 'ing point in degrees centigrade not more than 1.06 times the boiling point in' degrees centigrade 'of said aromatic hydrocarbon and containing said aromatic hydrocarbon and oleilns in amount such that the numerical value of the ratio per cent aromatimbromine number for said oil fraction is less than 250, azeotropically distilling said loil fraction With fractionationof the distilled vapors in the presence of a ketone and water, said said fraction.

ketone and Water forming low-boiling azeotropes with the non-aromatic hydrocarbons present in in said azeotropic distillation taking off as an overhead distillate with said ketone and Water substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than oleiins and a portion of the olens present in said .oil fraction, thereby recovering an aromatic hydrocarbon fraction containing said aromatic hydrocarbonv and a portion only of the like-boiling olens which were present in said oil fraction, washing said aromatic hydrocarbon fraction with sulfuric acid of a concentration and in amount such that the aromatic :Miami drocarbons, including a substantial proportion of like-boiling olefins, which comprises fractionally distilling said aromatic oil and in this distillation taking oi an oil `fraction having an end boiling 'point in degrees centigrade. not more than 1.06 times the boiling point in degrees centigrade of said aromatic lhydrocarbon and containing said aromatic hydrocarbon and oleiins in amount such that the numerical value of the ratio per cent aromatimbromine number for said oil fraction is less than 250, azeotropically distilling said oil fraction with fractionation of the distilled vapors in the presence of methyl ethyl ketone and water, said ketone and water forming low-boiling azeotropes with the non-aromatic hydrocarbons present in said fraction, in said azeotropicl distillation taking off as an overhead distillate with said ketone and water substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than olefins and a portion of the olefins present in said oil fraction, thereby recovering an aromatic hydrocarbon fraction containing said aromatic hydrocarbon and a portion only of the oleiins which were present in said -oil fraction, washing said aromatic hydrocarbon fraction with sulfuric acid of a lconcentration and in amountv such that the aromatic hydrocarbon fraction is substantially completely freed of ole fins, separating the acid-washed aromatic hydrocarbon fraction from reaction products of the sulfuric acid and olens, neutralizing the aromatic hydrocarbon fraction with an alkali and fractionaliy distilling the neutralized fraction and in this distillation collecting separate from forerunnings and higher boiling residue a fraction consisting of nitration-pure aromatic hydrocarbon. f

6. The process for the recovery of nitrationpure toluene from an oil containing the same and containing higher, lower and like-boiling, nonaromatic hydrocarbons, including a substantial' proportion of like-boiling olens, which comprises fractionally distilling said aromatic oil and in this distillation taking off an oil fraction boiling in the range 108.2 C. to 111 C. containing toluene, like-boiling non-aromatic hydrocarbons and like-boiling oleflns, azeotropically distilling said oil fraction with fractionation of the distilled vapors in the presence of methanol, said meth-v anol forming low-boiling azeotropes with the non-aromatic hydrocarbons present in said fraction, in `said azeotropic distillation taking off as an overhead distillate with said methanol substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other than oleiins and a portion oi.' the olens present -in Said oil traction, thereby recovering an aromatic hydrocarbon fraction containing said aromatic hydroaromatic hydrocarbon fraction with sulfuric acid 16 o1' a concentration and in amount such that the aromatic hydrocarbon fraction4 is substantially completely freed of oleilns, separating the acidwashed aromatic hydrocarbon fraction from reaction products of the sulfuric acid and olefins, neutralizing the aromaticA hydrocarbon fraction and fractionally distilling the neutralized fraction and in this distillation collecting separate from forerunnings and 4higher boiling residue a fraction consisting. of -nitration-pure aromatic hydrocarbon. I r

7. The process for the recovery of pure toluene from an oil containing the 'same and containing' higher, lower and like-boiling, non-aromatic hydrocarbons, including a substantial proportion of like-boiling olefins, which comprises fractionally distilling said aromatic oil and in this distillation taking off an oil fraction in which toluene is concentrated accompanied by like-boiling, nonaromatic hydrocarbons and like-boiling, oleins, azeotropically distilling said oil fraction with fractionation of the distilled vapors in the presence of methanol, said methanol forming lowboiling azeotropes with the non-aromatic hydrocarbons present in said fraction, in said azeotropic distillation taking of! as-an overhead distillate with said methanol substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons other thanolens and a portion of the oleiins present in said oil fraction, thereby recovering an aromatic hydrocarbon fraction containing said toluene and a portion only of the oleflns which were present in said oil fraction.

-washing said aromatic hydrocarbon fraction with sulfuric acid of a concentration and in amount such that residual oleilns not separated from the toluene by the azeotropic distillation are removed from the aromatic hydrocarbon fraction, neutralizing the acid-washed aromatic hydrocarbon fraction and fractionally distilling the neutralized fraction and in this distillation collecting separate from other constituents of said acid-washed aromatic hydrocarbon fraction a distillate fraction which is substantially free from oleiins and has a speciilc gravity of nitration-grade toluene.

8. The process for the recovery of a pure aromatic hydrocarbon having a boiling point up to and including the boiling points of the xylenes from ani'oil containing the same and -containing higher, lower and like-boiling, non-aromatic hy drocarbons, including a substantial proportion of like-boiling olei-lns, which lcomprises fractionally distilling said aromatic oil and in this distillation taking oil an oil fraction in whch said aromatc hydrocarbon is concentrated accompanied by like-boiling, non-aromatic hydrocarbons and like-boiling oleins, azeotropically distilling said oil fraction which fractionation of the distilled vapors in the presence of an aliphatic alcohol containing 1 to 3 carbon atoms in the molecule, said alcohol forming low-boiling azeotropes with the non-aromatic hydrocarbons present in said fraction. in said azeotropic distillation takingoif as an overhead distillate with said alcohol substantially all of the like-boiling and lower boiling non-aromatic hydrocarbons ,other than oleiins and a portion of the olefins present in saidhoil fraction, thereby recovering anaromatic hydrocarbon fraction containing said aromatic hydrocarbon and a portion only of the olens which were present in said oil fraction, washing said aromatic hydrocarbon fraction with sulfuric acid of a concentration and in amount such that residual olens not separated from said aromatic hydrocarbon by the azeotropc distillation are removed from thearomatic hydrocarbon lfraction, neutralizing the acid-washed aromatic hydrocarbon fraction and fr actionally distilling the neutralized fractionand in this distillation collecting separate from other constituents of said acid- Washed aromatic hydrocarbon fraction a distillate fraction which is substantially free from oleflns and has a .specic gravity of said aromatic hydrocarbon of :titration-grade.

JOHN W. WrrriDRoitI.I

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

OTHER REFERENCES Nat. Bur of Standards J. of Research, 21, 157- 160 (1938) 202-42 miscl.

Mizuta J. Soc. Chem. Inc.. Japan, 35, 470B (1932) 202-42 miscl.

Images