US2385610A - Process for recovering toluene fdom hydrocarbon mixtures - Google Patents

Description

Sept. 25, 19455.` c. R. CLARK 2,385,610

PROCESSl FOR *RECOVERING TOLUENE FROM HYDROCARBON MIXTURES Filed Nov. 2e, 1943 3 INI/EN TOR. mf/w55 f?. z ,4R/f

GAMME@ /l TTORNEY Patented Sept. 25, 1945 UNITED STATES PATLNT OFFICE 2,385,810 PROCESS FOR REOOVEBING TOLUENE FPOM HYDROCABBON MIX'I'URES Charles B. Clark, Springileld Township, 1M o.' v gomery County, Pa.,l assignor to Allied Chemical Dye Corporation, New York, N. Y., a corporation oi' New York Application November 28, 19h43, Serial No. 511,832 13 Claims. (Cl. 2oz-42) Also synthetic hydrocarbon mixtures `produced by various catalytic processes may contain toluene in substantial quantities.

Complex hydrocarbonoil mixtures comprising non-aromatic hydrocarbons such as parailins. oleiins and naphthenes and containing aromatic compounds, including toluene, may be treated by well-known selective solvents to produce fractions relatively rich in the aromatic compounds. For example, suitable petroleum distillate fractions containing toluene and other aromatic compounds may be extracted with sulfur dioxide to yield fractions containing relatively greater proportions of aromatic compounds, including toluene. In such cases toluene is associated with non-aromatic oils -which may be oleilnic, paraillnic, or naphthenic in character. A considerable proportion of these oils cannot be completely separated !rom tre toluene by direct fractionation because of the proximity of their boiling points to that of toluene or because of their ability to form constant boiling mixtures with toluene. In some cases toluene produced bythe gasification of coal or by various catalytic processes may be associated with diiiicultly separable oils of the same general character.

By fractional distillation of the above types of complex hydrocarbon mixtures fractions relatively high in toluene content may be obtained. These toluene fractions, however, will still contain large proportions of the non-aromatic constituents whose boiling points at any given pres-s sure are in the neighborhood of the boiling point of toluene at the same pressure, or which form with toluene or other constituents of the mixture azeotropes distilling in the range of temperatures at which toluene distills from the mixture. In the specification and claims, I referto such difilcultly separable constituents which distill out over the same temperature range as the rumene and from which the toluene 1s not separable 'ny a conventional distillation as like-boiling constituents, oils, etc.

is desirable to use as pure a material asis economically available. For example, toluene is largely used for the production of TNT, for which purpose a so-called "nitration grade of toluene, which is toluene of relativelyhigh purity, is employed. Conventional fractional distillation will not separate toluene ofvsuillciently high purity for such purposes from complex mixtures of the type described above, e. g., oilsy of petroleum origin.

A process has been, developed whereby toluene l of relatively high purity may be recovered from complex hydrocarbon oil mixtures comprising vtoluene and non-aromatic hydrocarbons from which `the toluene is not separable by a conlthe like-boiling non-aromatic hydrocarbon azeovtropes having, under the distillation' conditions prevailing, vapor pressures suillciently different from those oi' toluene and of any toluene azeotrope present at the distillation temperatureto permit sharp separation of, toluene without excessive rectification.

A second importantfactor in the economical separation of like-boiling non-aromatic hydrocarbons from toluene fractions is the provision of an azeotropic agent which has a high capacity for carrying of! as distillate the like-boiling nonaromatic hydrocarbons; i. e., a compound which with like-boiling non-aromatic hydrocarbons form azeotropes comprising a high ratio of hydrocai-bons to azeotropic agent. While certain azeo- Itropic agents may effectively separate like-boiling non-aromatic hydrocarbons from toluene fractions the oilzagent ratio is so low that large` quantities of the agent must be distilled in proportion to the quantity of toluene fraction being treated; accordingly, apparatus of great size and exorbitant cost must be employed, excessive heat losses are incurred and substantial distillation losses in the tail Bases are incurred.

It is an object oi this invention to provide a process for the recovery oi' toluene from mixtures For many purposes to which toluene is put, it of toluene and non-aromatic hydrocarbonsirom which the toluene` cannot be separatedA by a conventional distillation, which process is capable of producing a toluene of high purity and is particularly economical with respect to the proportion of azeotropic agent necessary.

In accordance with the present invention, a toluene oil containing like-boiling non-aromatic hydrocarbons not separable from the toluene by conventional distillation is iractionally distilled with a nitroparadin boiling within the range of 110 to 125 C., with 2nitropropane (boiling at about 120 C.) or nitroethane (boiling at about 115 C.) These nitroparaiilns form with the like-boiling non-aromatic hydrocarbons of such oils, azeotropes which' distill i'rom the resulting mixtures at temperatures substantially below those at which toluene in substantial quantities distlll. Moreover, the azeotropes of these nitroparailins with such like-boiling non-aromatic hivdrccarbonsA contain a particularly. low ratio of nitroparailins to the like-boiling non-aromatic hydrocarbons, and this property is an important feature in the effectiveness of the separation process oi' the present invention.

Use of nitroethane and 2-nitropropane as azeotropic agents i'or toluene recovery is disclosed in my applications, Serial Nos. 368,864 and 368.665, respectively, of which this application is a continuation-in-part.

In the application of the invention to the re oovery of toluene from oils of moderate boiling range but in which the toluene concentration is low. or from oils oiwide boiling range, it is pre i'erred to iractionally distill the oil in conventional manner to provide an enriched toluene fraction of relatively limited boiling range. though many toluene fractions having end boilingvpoints substantially above the boiling point of toluene. for example. having end boiling points up to about 118 C., may be azeotropically distilled in accordance with the teachings of this invention, the process of the invention is preferably applied to toluene fractions having end points substantially corresponding to the boiling point of toluene, e. g., fractions having end points of about 111 C.

The process o1' the invention is preferably a plied to toluene fractions so cut as to exclude torerunnings which do not contain substantial proportions of toluene. For example, the invention is applied to toluene fractions which preferf ably do not contain substantial proportions of oils normally distilling below 95 C. or more prefer ably are .free from substantial proportions of oils normally distilling below 100 C.v

The term '"tnueneyfraction is used herein to designate any' fraction containing both toluene and other hydrocarbons; which, under the conditions maintained indirect distillation processes as commonly used for distilling oils, distill from the oil fraction within the same temperature range as the toluene.

' tion residue.

associo and the excess may be retained in the distillan general, the amount of the nitroparamn should be sumclent to maintain the distillation temperature not higher than the boiling point of the toluene-nitroparafiin azeotrope at distillation pressure. A rise of temperature above this control temperature indicates a deliciency of nitroparaffin, as may also an abrupt rise vbelow this value.. Should the quantity of azeotropic agent be deficient, additional quantities should be added to overcome the deiiciency. Sufficient 2-nitropropane to maintain a vapor concentration thereof at overhead temperatures between the boiling point of the toluene-nitropar. aiiln azeotrope at the prevailing pressure and about 20 centrigrade degrees therebelow, equivaient to 1 part by volume of liquid nitro. Aparaiiin for each 3 parts by volume of oil condensate, has been found to be adequate for the production of a highly refined toluene. The proportion of nitroethane may advantageously be somewhat greater, e. g., suiilcient to maintain a vapor concentration of nitroethaneequivalent to between l and 2 parts by volume of liquid ntroethanefor each 2 parts by volume of oil condensate. For example, between 3 and 8 volumes oi Z-nitropropane `for each 10 volumes of non-aromatic hydrocarbons in the toluene iracconducted at atmospheric pressure or at pressures higher or lower than aimospheric.` "Since" the results are highly satisfactory at ordinary f pressures, there is normally no occasion for de sin:

By distillation of toluene fractions in the presthereof with toluene by this process and high yields of toluene of. or higher, e. g., 99%-` purity are obtained.

Considerable latitude is permissible in the choice of suitable proportions of nitroparailn to be used in carrying out the present process. Substantial excesses of the nitroparamns may be used viation from such pressures insofar as the sep aration itself is concerned. Lower than atmos pheric pressures may improve the steam con sumption, but this advantage must be balanced against higher equipment cost. It will be appreciated that in the event higher or lower than atmospheric pressures are employed, the temper atures of distillation should lbe raisedor lowered correspondingly.

While the'invention contemplates the production of toluene products of any desired purity, it is preferred to provide sucient nitroparan und to control the distillation so as to yield a toluene product containing at least a 9:1 weight ratio of toluene to like-boiling non-aromatic hydrocar bons, and more preferably to recover toluene o 95% to substantially 100% purity.

In practicing the invention, it is preferred to start with a toluene fraction at least 90% by weight of which distllls at vapor temperatures between about 100 and 111 Ci at atmospheric pressure, said fraction containing between 50% and 90% by Weight of toluene.v be recovered from such fractions in accordance with the present invention by distillins it in the presence of 2-nitropropane or nitroethane in the proportions hereinabove disclosed.

- While it is preferred that the toluene fractions treated in accordance with the invention contain only small quantities, if any, of hydrocarbons distilling from the fractions .tt temperatures above that at which toluene distllls off therefrom, the presence of such higher-boiling hydrocarbons is not excluded. Such hydrocarbons may be left with the toluene product at the conclusion of the distillation process of the invention and thereafter the toluene may be separated from them by conventional fractional distillation. may

The toluene may distillate, or residue, if it le not desired te effect y,

be accomplished in separate apparatus or'in a lower section of the same column if a continuous column Vis employed, or in the same still if a batch still is employed.

The process of the invention'may be conducted in conventional distillation equipment, preferably equipment capable of effecting a relatively sharp fractionation. Thus, for batch operations a simple `boiler provided with a rectifying column may be employed and for continuous operations a column still may be employed. The accompanying drawing illustrates diagrammatically an arrangement ot conventional equipment which may be employed for carrying out the invention.

The following examples illustrate the application, o! the invention to typical complex hydrocarbon mixtures of the type discussed above.

l.Encan/.ple 1.--A complex mixture of petroleum origin having, when fractionally distilled at atmospheric pressure, a vapor temperature range of I.5 C. to 110.5 C. and comprising .67% by volume of toluene and the remainder essentially paralnic, oleilnic and naphthenic hydro-v carbon oils which are not separable from the toluene 'by conventional distillation, is mixed with an equal volume oi' Zfnitropropane. The mixture is charged to a conventional still provided with a rectifying column (the equivalent of 18 theoretical plates) and distilled with return of part of the condensate as reflux (20:1 reflux ratio). The distillation begins at a temperature around- 96 C. and the temperature gradually rises to about 108 C. which is about 11/2-2 centigrade degrees below the boilingv point of the 2-nitropropane-toluene azeotrope (approximate B. P. 110 C. at 760 mm. of mercury, absolute pressure). At this point in the distillation there remains as residue a toluene product comprising about 86%toluene corresponding to about 90% of the toluene in the original complex mixture. By carrying the oil distillation further, i. e., to approximately the 50% point (50% of the oil over as distillate), a residual toluene product of better than 92% purity rep- Iresenting"three-fourths or more of thc toluene initially present in the complex mixture is Droduced. Il' the distillation is carried to the 60% point, about two-thirds of the total toluene content of the complex mixture is recovered as a 95% toluene product. By continuing to the 80% point, about one-third of the total toluene content oi the complex mixture is recovered as a substantially 100% toluene product.

In each of the above three instances, the tolu ene product recovered is associated with 2-nitropropane from which it may be separated by washing with dilute fNaOH solution. 'Ihe nitroproypane maybe recovered from the washing agent by acidification of the washings. 2-nitropropane associated with distillate may be separated from the oils in a similar manner.

In conducting the process as a` batch distillation with only a moderately effective rectifier, intermediate distillate containinboth toluene and non-aromatic oils in substantial quantities may be separated as a second distillate cut and returned to the still with a new batch of complex hydrocarbon mixture. For example, the distillation may be carried to a point at which 40% of the oil has vdistilled over and this distillate, which is relativelylow in toluene, may be collected as the first cut. The distillation then'may be continued until 60% of the mixture has distilled over and this 40% to 60% fraction may be returned with the next charge. The 60% to 100% distillation of the toluene, may be recovered as the toluene product. `This procedure produces a toluene product of good purity without sacrifice of yield, giving, for example, from a 67% tolu'- ene mixture a 90% yield and 95% or higher purity.

Example 2.--A complex mixture4 of'petroleum lorigin having, when fractionally distilled at at mospheric pressure, a vapor temperature range of 107.5o C. to 110.5 C. and comprising'67% by volume of toluene and the remainder essentially parainnic, oleilnic and naphthenic hydrocarbon oilswhich are not separable from the toluene by conventional distillation, is mixed with an equal volume of nitroethane. The Vmixture is charged to a conventional still provided with a rectiiying column (the equivalent. of 1 8 theoretical plates) and distilled with return of part oi' the condensate'as reilux (20:1 reflux ratio). The distillation begins at a temperature around 82 C. and the overhead temperature gradually rises to about l04 C., which ls about 2 degrees below the boiling point of the toluene-nitroethane azeotrope (approximate B. P. 106 C. at 760 mm. o1' mercury, absolute pressure). -Durlng this period of the distillation, nitroethane .and most of the non-aromatic hydrocarbon oils distill oil. Depending upon the effectiveness of f rectiilcation varying small proportions of toluene may also distill oil. Residue in the still comprises more than 90% of the toluene in the original complex mixture and from 4% to 6% of non-aromatic oils; that is, the residue.. is 94% .to 96% pure toluene except for `nitroethane which may be removed by washing with dilute NaOH solution. The nltroethane may be recovered from the washing agent by acidincation ot the washings.

Toluene oi' greater purity may be obtained by carrying the distillation to a slightly higher over.. head temperature. Thus, by carrying the overhead temperature to about 106 C., a toluene product of better than 97% purity is obtained. Further, by separately collecting theportion o! the distillate which comes over after `the nrst 20% of the oil has been distilled oilfandbefore 50% of the oil is distilled off and adding this portion to a lsubsequent still charge, the yield o! toluene may be substantially increased, e. g., yield of toluene of 98% to 99% vpurity may be obtained from a 67% toluene traction.

In general, the process ofv the present invention is eilective to produce toluene yields of '10% or 80% to 90% or mora-with 80% to 99+% of the like-boiling non-aromatic hydrocarbons re-- 'moved from toluene fractions of 50% to 90% toluene content without the use of more than customarily emcient rectiners.

In the claims, the temperatures speciiled for the boiling points are determined 4at atmospheric pressure (760 mm. of mercury). It will be understood that the reference in the claims to a 'hydrocarbon mixture boiling within therange oi I ent process with sulfuric acid and fractionally distilling to separate reaction products, the high est nitration grade of toluene is obtainable.

I daim:

1. A process for the recovery of toluene fromv a complex hydrocarbon mixturel containing toluene andnon-aromatic hydrocarbons not separable from the toluene by conventional distillation of the hydrocarbon mixture, which process comprises fractionally distilling the hydrocarbon mixture with a suiilcient amount of a nitroparai v fin boiling within the range of 110 to 125 C. to

vaporlze and remove as distillate asubstantial proportion 4of said non-aromatic hydrocarbons.

2. The method of yrecovering toluene from a complex hydrocarbon mixture containing toluene and non-aromatic hydrocarbons not separable from the toluene by conventional distillation of the hydrocarbon mixture, which method comprises fractionally distilling.- the hydrocarbon mixture with sufficient nitroethane to vaporize and selectively remove as distillate at least a substantial proportion of said non-aromatic hydr carbons. Y

3. The method of recovering toluene' from a complex hydrocarbon mixture containing toluene Vand like-boiling non-aromatic hydrocarbon oils, Awhich method comprises segregating a toluene oil fraction boiling substantially within the Arange of 95 C. to 118 C.; whereof at least 90% by weight normally distills at vapor temperatures between 100 C. and 111 C. and whereof between 50% and.A

90% by weight consists of toluene, mixing said toluene oil fraction with between 1 and 2 volumes oi' nitroethane foreach 2 volumes of non-aromatic hydrocarbons in said toluene oil fraction,

fractionally distilling the nitroetltiane-oil mix` ture, and separating, as at least one fraction oi low aromatic content, a distillate going Vo at overhead temperatures below the boiling point oi the'toluene-nitroethane azeotrope at the prevailing pressure, thereby leaving behind a product enriched in toluene, the hydrocarbon ,contenty oi said product being at least 95% toluene.

4. 'I'he method of recovering toluene froma 1 complex hydrocarbon mixture conta'tolu= but less than 90% by'weight of f ene andthe balance consists for the mt o! like-boiling hydrocarbons, in said toluene l ene and like-boiling non-aromatic rooar oils, ,which method comprises segresating a r i ene oiliraction boiling substantially .within th :1-

range of 95 C. to118 C., whereof at lea8t9'0% weight normally distills vat vaporA between 100 C. and 111 C.'and more f.

cil fraction with between 1 and 2 volumes oi iw@ .A ethane for each 2- volumes of non-aromatic hydrocarbons in said toluene loil fraction; fraction. ally distilling the nitroethane-oil mixture, a

separating, as at least one fraction of low aromatic content, a distillate going ou at overhead temperatures below the boiling point of the toluene-nitroethane azeotrope at the prevailing pressure, and recovering as residue from such distillation aA toluene productI containing` at least 80% of the toluene and less than 20% oi' the likeboiling-non-arornatic hydrocarbon oils initially present in said toluene'oil fractionthe hydrocarbon content of said product being atleast 95% toluene.

5. A process for the recovery of substantially pure toluene from a complex hydrocarbon mixture, whereof at least 90% by weight normally distills;v at vapor temperatures vbetween 100 C, and 111 C., said mixture containing toluene and paramnic hydrocarbons not separable from the toluene by conventional distillation of the mixture, which comprises fractionally azeotropically disu tillin'g the mixture with suicient nitroethane to vaporize and selectively remove as distillate sub stantially all of the hydrocarbons except toluene present in said mixture, thereby leaving behind a product enriched in toluene, the hydrocarbon content of said product being at least 95% tolucomprises iractionslly distiillng me hydrw,

proportion of said nonwaromatic hydrocsr byweit norm d 1il 6. A process for the recovery of substantially pure toluene from a complex hydrocarbon mixture containing toluene and like-boiling nonaromatic hydrocarbons, which comprises distilling the mixture through a fractionating column in the presence of nitroethane, maintaining in the fractionating column a sumcient concentration of nitroethane so that'the overhead vapor temperature is not above the boiling point of the toluene-nitroethane azeotrope at the prevailing pressure, and conducting the distillation under these conditions until a residue is producedin which the ratio by weight of toluene to like-boiling non-aromatic hydrocarbons ls at least 95 :5.

'1. The method of recovering toluene from a complex hydrocarbon mixturer containing toluene and like-boiling paraiiinic hydrocarbons, said w f with cient 2nitropropane to vapo end-remove as distillate at l e.

a pmx t: l rbon mixture containing tolu-` osid like og non-momotic if o @sweating a tolu- 9 @is process for the recovery oi toluene i @95 @to att% at vapor temneretum we en 100 C. and il1 C. Whom? www1! and we?, by weight consists of toluene,

seis toluene ou fraction with between s and d volumes of 2-nitroproe for each l0 volumes l of nom-aromatic hydrocarbons in said toluene oil to iractionally distilllng the 2-nitroproe-oll mixture, and separating, as at least one fraction of low aromatic content, a distillate going ed at overhead temperatures below the boiling point of the toluene 2nitropropane azeotrope at the prevailing pressure, thereby leaving behind a product enriched in toluene, the hydrocarbon ycontent. oi said product being at least toluene.

10. The process .for the recovery of toluene from. a complex hydrocarbon mixture containing toluene and like-boiling non-aromatic hydrocarbon oils, which process comprises segregating a toluene oil fraction boiling substantially within the range of 95 C. to 118 C., whereof at least 90% by weight normally distills at vapo; temperature between C. and 111 C. and more than 50% 'but less than 90% by weight consists oi toluene and the balance consists for the most part of like-boiling hydrocarbons, mixing said toluene oil fraction with between 3 and 8 volumes asuysimmueof 2-nitiopropane for each 10 volumes of nonaromatic hydrocarbons in said toluene oil fraction, fractionally dlstilling the 2-nitrcpropane-oil mixture, and separating as at least one fraction of low aromatic content, a distillate going o at overhead temperatures below the boiling point of the toluene 2-nitropropane azeotrope at the preture, which comprises fractionally azeotropically distilling the mixture with sumcient 2-nitropropane to vaporize and selectively remove as distillate substantially all of the hydrocarbons except toluene present in said mixture, thereby leaving behind a product enriched in toluene, the hydrocarbon content of said product being n .t least 95% toluene.

12. A pr'ocess for-'the recovery of substantially pure toluene from a complex `hydrocarbon mixture containing toluene and like-boiling nonaromatic hydrocarbons, which comprises diatilling the mixture through a iractionatin column in the presence of 2-nitropropane,=!mlin taining in the fractionating column a suillcient concentration of Z-nitropropane so that the overhead vapor temperature is not above the boiling point of the toluene-2-nitropropane azeotrope at the prevailing pressure, and conducting the distillation under these conditions until a residue is produced in which the ratio by weight of toluene to like-boiling nonaromatic hydrocarbons is at least 99:5.

13. The method of recovering tolueneirom a complex hydrocarbon mixture containing toluene and like-boilingv paratllnic hydrocarbons, vsaid mixture being substantially tree from hydrocar bons normally boiling above 118 Cg, which comprises fractionally distilling said mixture in the presence ot sumcient 2-nitropropane to vaporize and selectively remove as distillate substantially all' o! said like-boiling paramnic hydrocarbons. thereby leaving behind a product enriched in toluene, the hydrocarbon content oi'- said product being at least 95% toluene.

CHARLESRCLARK.

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