US2352534A - Azeotropic distillation of benzene fractions - Google Patents

Description

June 27, 1944. R. E,` GP-EENBWQGv 2,352,534

AZEOTROPIC DISTILLATION OF BENZENE FRACTIONS Filed May 28. 1945 J TTOFNEY Patented June 27, 1944 FRACTIONS Richard B. Greenburg, Cheltenham Township, Montgomery County, Pa., asslgnor to Allied Chemical & Dye Corporation, New York, N. Y.,

a corporation of New York Application May 28,1943, serial No. isasos (ci. 2oz-42) 3 Claims.

This invention relates to a process for recovering benzene from mixtures containing it and other hydrocarbons. Y

Numerous hydrocarbon oils are known which contain benzene in varying proportions. Gasoline fractions obtained by the distillation of certain types of petroleum or recovered from the products of catalytic reformation processes, frequently contain substantial proportions of benzene, although they consist mainly of other hydrocarbons. Oils of petroleum origin having a considerable content of aromatics, including benzene, may be treated by well known selective solvent processes to produce fractions rich in aromatics; for example, extraction of suitable fractions of such petroleum oils with sulfur dioxide may yield fractions of high benzene content. In such cases benzene is accompanied by non-aromatic hydrocarbons which may be largely oleiinic, paraillnic, or naphthenic in character. A considerable proportion of thesel hydrocarbons cannot be completely separated from the benzene by direct fractional distillation because of the closeness of their boiling points to that of benzene or because they form constant boiling mixtures with benzene. Fln'thermore, while ordinarily benzene is readily separable by direct fractional distillation from light oils produced by the gasification of coal, in some cases the benzene is accompanied by diiiicultly separable. non-aromatic hydrocarbons of the same general character as described, owing to carbonization conditions, type of coal used or other special circumstances. Also synthetic hydrocarbon oil mixtures produced by various catalytic processes may contain benzene which, when recovered, is accompanied by similar diillcultly separable constituents. i

By fractional distillation of these oils contain- 'ing benzene, fractions relatively high in benzene content may be obtained. These benzene fractions, however, will still contain large amounts of the other constituents of the oil having boiling points in the neighborhood of the boiling point of benzene or forming mixtures of constant boiling points in the range of temperatures at which benzene distills from the oil. (In this specification and claims, I refer to "such difllcultly separable constituents, which distill out over the same temperature range as the benzene and from which the benzene is not separable by a conventional distillation, as "like-boiling, non-aromatic hydrocarbons,) Accordingly, recovery of benzene of a relatively high purity from these sources presents adimcult problem of great industrial importance: i

For many purposes to which benzene is put, it is desirable to use as pure a material as may be economically available. For example, benzene is used for the production of pharmaceutical chemicals, dyes and synthetic phenol. For these purposes a pure benzene is preferred. Direct fractional distillation, because of thev diillculties pointed out above, will not effect a recovery of pure benzene separate from like-boiling, nonaromatic hydrocarbons such as enumerated, and in many cases will-not give fractions of suitably high benzene content or free from materials which even in small concentrations adversely affect the processes by which the benzene is treated or the products produced therefrom.

It is an object of my invention to provide a process whereby benzene of any desired vdegree of purity may be recovered from hydrocarbon oil mixtures comprising non-'aromatic hydrocarbons from which the benzene isv not separable by a conventional distillation.

In using myinvention for the treatment of an oil in which the benzene 'concentration is low or when the oil is one containingimaterials of wide boiling range, I prefer Vilrst to fractionally distill the oil to recover therefrom an enriched benzene fraction. Although benzene fractions having an end boiling point substantially above the boiling point of benzene (e. g., a boiling point up to about 85' C.) may be azeotropically distilled in the manner hereinafter described, I prefer the benzene fraction treated by my process to be one having an end lboiling point substantially corresponding to the boiling point of pure benzene,

e. g., an end boiling point of about 80 C. Further, I4 prefer that a benzene fraction recovered by a preliminary distillation of a crude benzene oil be so cut as to exclude therefrom the forerunningswhich do not contain substantial proportions of benzene; for example,l to exclude substantial proportions of oils distilling. below 65 C,

In this speciiication and claims, the term benzene fraction is used broadly to designate any oil fraction containing both benzene and other hydrocarbons which, under the conditions main- The non-aromatic hydrocarbons, which in the absence of the .r1-methyl dioxolane distill from the benzene fraction in the same temperature range as the benzene and hence are not separable therefrom by direct fractional distillation, may be distilled from the benzene fraction in the presence of the 4methyl dioxolane to leave a residue containing benzene of any desired purity with respect to its content of these like-boiling, non-aromatic hydrocarbons. On fractionally distilling a mixture of the 4-methyl dioxolane with a benzene fraction, the material first to distill over is a mixture of low boiling azeotropes consisting chiefly of 4-methyl dioxolane and nonaromatic hydrocarbons. On continued distillation, the none-aromatic hydrocarbons are selectivelylremoved and a residue enriched in benzene is obtained. The distillate will contain 4- methyl dioxolane and the non-aromatic hydrocarbons distilled from the benzene fraction. It may also contain some benzene carried over in the distillation.

Bywashing the distillate with water the 4- methyl dioxolane containedtherein may be separated in an aqueous phase from an oil phase containing the hydrocarbons carried over into the distillate. The aqueous phase may be treated to recover 4-methy1 dioxolane therefrom for return to the distillation and reuse in the azeotropic distillation of additional benzene fraction. 4- methyl dioxolane which may be left in the residue may be removed by washing the residue with water. 'Ihe oil phase thus obtained, containing benzene which has been separated from nonaromatic hydrocarbons in the original .benzene fraction, may be given a conventional acid wash, neutralized and redistilled to further purify the benzene. f

The azeotropic distillation in the presence of the. 4methy1 dioxolane `should be carried out with rectification of the vapors. Suflicient 4- methyl dioxolane should be present to carry over as azeotropes the non-aromatic hydrocarbons so that the'A residue containing benzene is freed to the desiredv degree of these non-aromatic hydrocarbons. The quantity of ll-methyl dioxolane thus required will, of course, `depend upon the amount of benzene fraction distilled, the quantity of non-.aromatic hydrocarbons contained therein, etc. `Any excess of 4-methyl dioxolane present will not materially interfere withthe separation of the non-aromatic hydrocarbons from the benzene, although it is obvious that a large excess is undesirable in that it requires removal and recovery of the 4methyl dioxolane from the benzene productof the process.

As pointed out above, it is highly desirable to recover from benzene fractions a relatively pure benzene product and the process of 'my invention permits of obtaining an enriched benzene residue bons which, in the absence of an azeotropic agent, dlstill from the benzene fraction in the same temperature range as the benzene. My invention contemplates, therefore, continuing the azeotropic distillation of the benzene fraction under the conditions described above until the distillation residue is enriched to a' desired extent in benzene with respect to other, like-boiling hydrocarbons,

i. e., hydrocarbons which will distill from thev benzene fraction, in the absence of an azeotropic agent, in the same temperature range as the benzene distills therefrom. In practicing my invention the distillation preferably is controlled to obtain a benzene residue which contains at least 90 parts by weight of benzene to 10 parts of likeboiling, non-benzene hydrocarbons. When the azeotropic distillation has proceeded to a point at which the unvaporized material contains benzene separated to the desired degree from like-boiling, non-aromatic hydrocarbons, the distillation may be discontinued and the residue washed with water to separate the 4-methyl dioxolane from the hydrocarbon which is thus obtained as a product o f the process, or may .be given any desired nal purification if a higher purity benzene product is desired. Instead of interrupting the azeotropic distillation in the manner described and recovering the benzene in the residue, the distillation may be continued either in the presence or in the absence of ii-methyl dioxolane and the distillate now rich in benzene recovered lseparate from the previous distillate containing the likeboiling, aromatic hydrocarbons. The distillate containing benzene thus obtained may be treated similarly to the method of treatment of the residue by washing with water to remove 4-methyl dioxolane and, if desired, to additionally purify of any desired purity with respect to hydrocarthe benzene thus recovered.

. The benzene fraction treated in accordance with my invention preferably will contain little, if any, hydrocarbons distilling from the hydrocarbon benzene fraction at temperatures materially above those at which benzene distills therefrom. On the other hand, high boiling materials may be present in the benzene separated from like-boiling hydrocarbons by the azeotropic distillation. Once the separation of the benzene from the hydrocarbons of similar boiling range has been effected, however, the benzene may-be separated from the high boiling hydrocarbons by fractional distillation in the absence of an azeotropic agent. Whether or not high boiling constituents should be left in the benzene fraction to be azeotropically distilled or whether, if left in the benzene fraction, they will remain with the benzene after the azeotropic distillation, depends upon a number of factors, among which their boiling range and chemical characteristics are important. If the high boiling constituents form azeotropes with the agent used which have boiling points close to the `temperatures at which benzene distills over, it is preferable to separate such high boiling constituents from the benzene fraction by a direct fractional distillation before azeotropically distilling the benzene fraction.4

When pure benzene is to be recovered it is preferred, the benzene fraction subjectedv to azeotropic distillation inaccordance with my invention be a fraction boiling in the range of 65 C. to C. Such a fraction may be azeotropically distilled by my process and pure benzene obtained with a 'relatively small quantity of azeotropic agent present during the distillation. Pure benzene may be obtained by distilling with an azeotropic agent a Wider boiling benzene fraction but the quantity of azeotropic agent present in the distillation of the benzene fraction of wider boiling range must be substantially increased as compared with the quantity which suillces for distilling the fraction of the narrower boiling range.

Benzene of high purity is desired for nitration andother purposes and the process of my invention provides' a method whereby nitration-grade benzene may be economically recovered from benzene oils. The invention, however, is not limited thereto. Benzene products of lower purity than nitration-grade benzene are industrially used for various purposes, e. g., as solvents. The process of my invention may be used advantageously for recovering such products from hydrocarbon oils of lower benzene content. My process is particularly advantageous when the benzene is to be separated from most of the like-boiling,

non-aromatic hydrocarbons present in an oil fraction together with benzene, e. g., when a product is to be obtained containing benzene and no more than 10 parts by weight of like-boiling, non-aromatic hydrocarbons for every 90 parts by weight f benzene.

My invention will be more particularly described and illustrated by the following example of a process for the azeotropic distillation of a benzene fraction in the presence of 4-methyl di.-

oxolane.

The apparatus used for carrying out the process of this example, diagrammatically illustrated in the accompanying drawing, comprised a still I with heater 2 and rectication colunm 3 of conventional design. A condenser I was provided to which the vaporsfrom the top of the column were led and in which they were cooled and condensed. A pipe 5 was provided for returning condensate from thecondenser to the top of the rectification column to serve as reflux for the column. A second pipe 6 was also provided for drawing off from the condenser a portion of the condensate formed therein. Pipes 1 and 8 were provided for the introduction of liquids into still i. A pipe 9 served for withdrawal of residue from thisstill.

The above-described apparatus was employed for the distillation of a benzene fraction having a boiling range of about 75 C. to about 80 C.

and containing 69% by volume of benzene, the remainder principally consisting of like-boiling paralns.

100 volumes of this benzene fraction and 100 volumes of 4-methyl dioxolane were charged into the still of the apparatus described above. 'Ihis charge was boiled in the still and the vapors passing therefrom to the fractionating column were rectified in this column. The vapors leaving the top of the column were condensed, a part of the condensate was withdrawn as distillate through pipe V6, and the remainder of the condensate returned as reilux through pipe 5 to the top of of the fractionation column. Distillation started with the temperature of' the vapors at the top of the fractionation column at about 76.6

C. Washing the distillate with `water to remove the ii-methyl dioxolane and analyzing the hydrocarbons thus obtained (by the Kattwinkle meth-v After the .benzene was discontinuing the distillation at thepoint when the hydrocarbons being taken over into the distillate were substantially all benzene, a residue is obtained which, by washing with water, yields an oil layer substantially consisting of benzene, e. g. is 98100% benzene. By continuing the distillation, collecting the distillate taken over at about- C. separate from those taken over at lower temperatures, and washing this distillate with water to free it from 4-methyl dioxolane,- a product of substantially 100% benzene is also obtained.

While the example described above employs a batch operation for the distillation of the benzene fraction, this distillation may =be carried out continuously For example. 4methyl dioxolane and benzene fraction may be continuously supplied to a column still in which the Abenzene is concentrated and, flowing to the bottom of the still, is continuously withdrawn therefrom. The

azeotropes of 4-methyl dioxolane and nonaro`` matic hydrocarbons are continuously vaporized and withdrawn as distillate from the top of the distillation column. In a continuously operating process enriched benzene products may be taken-olf from the bottom oi the still or as one or more side streams from the fractionation column below the point of feeding the benzene fraction thereto. 'I'hese modifications are within the scope of my invention.

ISulcient 4-methyl dioxolane should'be supplied to carry over as distillate all of the hydrocarbons to be distilled from the benzene fraction and separated from the benzene. By observation of the vapor temperatures at a suitable point in the vapor rectication column, one may ascertain whether adequate 4-methyl dioxolane is present. Whenever these vapor temperatures tend to exceed about 80-81 C. before the benzene has -been separated to the desired degree from the like-"boiling, non-aromatic hydrocarbons, by supplying additional 4-methyl dioxolane to the still or rectification column the desired separation of the benzene and like-boiling, non: aromatic hydrocarbons may be accomplished. This point of temperature observation in 'the apparatus in which the above example of the process was carried out is preferably the top of the rectification column. One skilled in the art of fractional distillation will recognize for any particular type of apparatus suitable points for this temperature control.

In this specication I have described the azeotropic distillation as being carried out under substantially atmospheric pressure. The temperatures as given herein are corrected temperatures for 1 atmosphere absolute (760 mm. of Hg). When carried out under pressures other than atmospheric, the temperature conditions will diier from those given. In any given case, however, the temperatures will correspond to the change in boiling points of the materials due to the diierence in pressure,

I claim: 1. The process for the recovery of benzene from a benzene fraction containing the same and likeboiling, non-aromatic-hydrocarbons which comprises distilling said benzene fraction and rectifying the distilled ,vapors in the presence .of 4- methyl dioxolane in amount which selectively carries over as distillate said like-boiling, nonaromatic hydrocarbons and leaves a hydrocarbon residue of the distillation enriched in benzene.

2. I'he process for the recovery of benzene whichI comprises distilling a benzene oil fraction boiling-within theirange 85? C. to 80 C. and containing benzene and like-boiling. non-aromatic hydrocarbons and rectii'ying the distilled vapors in the presence of a quantity oi- 4-methyl dioxolane which selectively carries over as distillate said like-boiling, non-aromatic hydrocarbons and leaves hydrocarbon residue of the distillation enriched in benzene. Y

8. '111e process for the recovery of benzene from a benzene fraction containing the same and likeboiling; non-aromatic hydrocarbons 'which comprises fractionally distiliing said lbenzene frac-

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