US2348726A - Separation of xylene from hydrocarbon mixtures - Google Patents

Description

May 16, 1944.

' cl R. CLARK SEPARATION OF XYLENE FROM HYDROCARBON MIXTURES Filed Dec 3 ATTORNEYv Patented May 16, 1944 i s'ErARA'rIoN or XYLENE FaoM HYDRO- oAnnoN Mix'rrmas .Charles R. Clark, Springfield Township, Montgomery County, Pa., assignor, by mesne assignments, to Allied Chemical & Dye Corporation, a corporation of New York Application December 3, 1940, Serial No. 368,315

8 Claims. (Cl. 202-42) This invention relates to 'a process for recovering xylene hydrocarbons from mixtures containing them together with other hydrocarbons. In this specification and the'appended claims the term xylene is used in its comprehensive v meaning as referring to aromatic compounds of the benzene series boiling inthe range of temperatures at which the' several xylenes boil. The term as thus used includes the three xylenes: ortho, meta and para, and ethyl benzene.

Numerous hydrocarbon oils are known which contain xylenes in varying proportions. For example, gasoline fractions obtained'by the distillation ,of certain types of petroleum frequently contain substantial proportions of xylene, although mainly consisting of other hydrocarbons.

Oils of petroleum origin having a considerable content of aromatics, including xylene, may vbe treated by well known selective solvent processes to produce fractions rich in aromatics; for example, extraction of suitable fractions of such petion or selective solvent extraction or a combination ofv both of these procedures a xylene product of a high degree of purity. Accordingly, recovery of xylene of a relatively high purity from these sources presents a difcult problem of great industrial importance.

ForA many purposes to which xylene is put, it is desirable to use as pure a material as may be economically available. Direct fractional distillation, because of the dimculties pointed out above, will not effect a recovery of pure xylene,

from oils such as enumerated and in many cases will not give fractions of suitably high xylene content or free from materials which even in small concentrations adversely affect chemical treatment of the xylene or the products produced therefrom.

It is an objectl of this invention to provide a process for azeotropically distilling the xylenecontaining oil in the presence of an azeotropic agent which is particularly eective for sepatroleum oils with sulfur dioxide may yield fractions of increased xylene content. In such cases xylene is accompanied by non-aromatic oils which may be largely parafflnic, naphthenic or olefinic in character. A considerable portion of these oil's cannot be completely separated from the xylene by direct fractional distillation because of the closeness of their boiling points to that of xylene or because they form constant boiling mixtures with xylene. Furthermore, while ordinarily xylene may be separated by direct fractional distillation from light oils produced by the gasification of coal, in some cases the xylene is accompaniedby difflcultly separable non-aromatic oils of the same general character is described, owing to carbonization conditions,` type of coal used or other special circum-y stances. Aiso synthetic hydrocarbon gas mixtures produced by various catalytic processes may contain xylene which, when recovered, -is accompanied by similar diflicultly separable constituents.

By fractional distillation of these oils containing xylene, fractions of increased xylene content may be obtained. These xylene fractions, however, will still contain large amounts of the other constituents of the oil having boiling points in theneighborhood of the boiling point of xylene orl forming mixtures of constant boiling points in the range of temperatures at which xylene distills from the oil. The fact that the oils containingxylene have generally a relatively low content of xylene makes it particularly difficult to recover by fractional distillarating the xylene from like-boiling, non-aromatic compounds contained in the oil to obtain a commercially pure xylene fraction.

It is a'.further object of the invention to provide a process for the separation ofv xylene from fractions of petroleum oil containing xylene and other hydrocarbons of similar boiling range.

In accordance with this invention, xylene in any desired degree of purity is separated from an oil containing it and other hydrocarbons boiling over the same temperature range as xylene by subjecting the oil to azeotropic distillation with rectification of.the vapors in the presence of allyl alcohol. For optimum results in the production of xylene of high purity, the rectification should be carried out so as to maintain a temperature at a point in the zone where rectiiication of the oil vapors takes place, not exceeding 97 C., preferably within the range of 94 to 97 C. The distillation in the preferred condensing vapors driven off while the tempera-l ture in the rectification zone is below said temperature. By operating in accordance with these conditions, a xylene of high purity, after removal 0f allyl alcohol by any desired means, may be carbons to be removed from the xylene.

obtained as a residue in the still or as the separately collected' distillate, the purity being improved the closer said temperature is maintained to 97 C.

The xylene-containing oils treated in accordance with my invention, are first preferably fractionally distilled to produce a'fraction boiling.

within the temperature range 125 to 150 C., for example 136 to 144 C; The fractions utilized preferably contain little, if any, hydrocarbons distilling from the hydrccarbon xylene fraction at temperatures materially above those at which xylene distills therefrom. However, oil fractions containinghigher boiling materials than xylene may be utilized so that such materials remain with the xylene residue at the conclusion of the azeotropic distillation of the xylene fraction to remove hydrocarbons of similar boiling range and after the azeotropic distillation is complete, Xylene may be separated from the higher boiling hydrocarbons by fractional distillation in the absence of an azeotropic agent.

Furthermore, in some cases low boiling hydrocarbons may also be present in the xylene fractions subjected to azeotropic distillation. W` boiling materials may be separated from the xylene by the azeotropic distillation itself. Nevertheless, the presence of such low boiling materials requires the use of larger amounts of allyl alcohol to obtain a xylene product of given purity and, hence, I preferably utilize fractions boiling within the range of 125 to 150? C. because such fractions may ordinarily be azeotropically distilled to produce commercially pure xylene with a minimum of the allyl alcohol azeotropic agent.

The ratio ,of allyl alcohol to the xylcne fraction whichfrmay be employed in carrying out this inventiong-,depends upon the amount and nature of the hydrocarbon impurities in the xylene fraction, thepurity desired in the Xylene residue from the distillation, the proportion of Xyl'ene in the original xylene fraction which is to be recovered in the residue, the procedure used for distillation of the allyl alcohol-xylene mixture, and the equipment employed. The quantity of allyl alcohol used inthe distillation of hydrocarbons from a given quantity of xylene fraction should "be inexcess of that which will form azeotropic `mixtures with the non-xylene hydrocarbons which are to be vaporized and taken over into the distillate. By returning most or all of the allyl alcohol collected from the distillate to the still, for example: as'reilux liquid, the total amount of allyl alcohol necessary Sto effect separation of the non-xylene constituents and the xylene will be materially reduced. If desired, fresh allyl alcoholmay be introduced into the xylene fraction being distilled during the course of the distillation; also the allyl alcohol separated from the distillate may be returned continuously or periodically to thestill or rectication column while the distillation of tlic xylene fraction is progressing.

Regulation of the temperature in the rectication of the vapors so as to maintain the control temperature not above 97 C., preferably Within the range of 94 `to 97 C., is accomplished by maintaining an adequate quantity of allyl alcohol in the still or rectification column, or both,` during the distillation of the vxylene fraction to selectively carry over throughV the rectification treatment the vapors of the hydro- As the distillation proceeds.' temperature readings are Such taken at the top of the rectification column and by supplying additional allyl alcohol when required to prevent this temperature from rising above 97 C. an adequate amount of allyl alcohol will be present during the separation of the non-xylene hydrocarbons from the Xylene fraction. It is not necessary that this point of con-l trol temperature be at the top of the column although this is a satisfactory point for determining this temperature. One skilled in the distillation art will recognize suitable points for maintaining this control temperature in any specific apparatus according to well known distillation principles.

When the azeotropic distillation of the xylene fraction has been carried to the point at which the residue containing xylene has the desired purity with respect to hydrocarbons of similar boiling range to xylene, the residue may be puried further to remove any allyl alcohol it contalns and to remove any other impurities present. For example, when the distillation is carricd to the point at which the xylene constitutes substantially 95 percent or more of the total hydrocarbon content of the residue, the

lowing example.

The apparatus of the drawing 'comprises a still I provided with a heater 2 and connected with a rectification column 3 and a condenser 4 for the vapors leaving the top of the column. By

` means of valves 5 and'6, condensate flowing from condenser 4 is divided in controlled proportions.' one part of the condensate being returned through pipe 'I to the top of column 3 and refluxed in contact with the vapors rising in the column and the other portion of the condensate being withdrawn through pipe 8. Three receivers 9, I0 and II'are connected to pipe 8 through valve-controlled branch pipes I2, I3 and I4 so that the condensate drawn off through pipe 8 may be divided andl distributed as described below to the several receivers.

Receiver I0 is connected by a pipe I5 controlled by valve I6 with a second still I1. Still I1, like still I, isprovided with a heater I8, a rectification column I9, a condenser 20 and pipes 2l and 22 controlled by valves 23 and 2l for return of determined proportions of condensate from condenser 20 to the top of column I9 and withdrawal of the remainder of the condensate through pipe 22. Pipe 22 leads to a separator 25 into which water may be introduced from a Pipe 26.

In carrying out one embodiment of my invention, in the apparatus described above, a petroleum oil produced by catalytic treatment of a petroleum distillate in the presence of hydrogen was a crude material from which the xylene is recovered. This oil had the following characteristics:

Specic gravity at 15.5 C 0.780 Aromatics per cent-- 47-.5 Francis bromlne No. i 0.05

Distillation:

Start degree C 44 End do 2104 The charge of this hydrocarbon mixturev introduced into still I .was boiled in the still and the Aevolved vapors were counter-currently contacted in column 3 with reflux condensate from condenser 4, in which the vapors leaving the top of the column were substantially entirely condensed. Most of the condensate from condenser 1 was returned through valve' 5 and pipe .1. to furnish the reux to the top the column. The remaining small portion of the ycondensate is continuously withdrawn through valve G and pipe 8 and is passed into collecting vessel 9. The distillation was conducted under substantially atmospheric pressure;i. e., the pressure in condenser 4 and at the top of column 3 was substantially atmospheric and the pressure in still I was only enough higher to force the vapor through the rectification column to the condenser.

When the vapor temperature in thetop of column 3 reaches 125 C., the condensate passing through pipe t is diverted to vessel I9. The cut taken in vessel I6 includes the distillate corning over up to and at a temperature of 150 C. at the top of the column. Thereafter the condensate passed forwardthrough pipe B is collected in vessel II and the distillation is continued as long as may be desired. The cuts in vessels 9 and II contain low and high boiling fractions of the original oil. They may be treated or used in any desired manner. If desired, instead of taking off a high boiling fraction and collecting it in vessel II, when the desired fracl tion has been collected in vessel I0 the distillation may be discontinued and the residue left lin the still may be withdrawn therefrom.

With eiiicient rectification of the vapors in column 3 during the fractional distillation of the crude xylene oil, the cut collected in vessel I0 may contain about 54 per cent by weight of xylene, as determined by the specic dispersion method for analyzing hydrocarbon oils described in Industrial 8: Engineering Chemistry. Analytical Edition, vol. 1l. page 614, November l5, 1939.

The xylene fraction collectedy in vessel I0. is subjected to azeotropic distillation which represents the second stage of this example. For this purpose the xylene fraction containing 54 per cent of xylene was introduced in still Il together with anhydrous allyl alcohol in the proportions of 333 volumes of allyl alcohol for every 150 volumes of the xylene fraction. The mixture of allyl alcohol and xylene fraction was distilled with rectification of vapors in column I9 and condensation oi the vapors leaving the top of the column in condenser 2i). Most of the condensate was,returned through pipe 2| to the top of column I9 while the remainder was withdrawn by pipe 22 through the separator 25.

Azeotropes formed with allyl alcohol by the hydrocarbons other than xylene present in the xylene fraction introduced into still -II have boiling points suiciently below that of 'the xylene for them to be preferentially vaporized and by rectication in column I9 to be largely separated from any xylene which is vaporized in still I1 and enters column I9, so long as there is sufficient allyl alcohol present in the vapor and liquid phases in the rectification column. The nonxylene-allyl alcohol azeotropes contain the allyl .alcohol in the proportion oi not-more than 2. parts o alcohol to each part of oil. As pointed out above, the requisite quantity of allyl alcohol umn, preferably at the top of the column not above 977 C. In this example, the requisite quantity of allyl alcohol was supplied Ain the initial charge to the still. The alcohol was vaporized from the still, entered the rectification column as vapor and in part was returned to the column as liquid in condensate from condenser 2U.

If insufficient allyl alcohol is initially introduced in the charge to the still, additional alcohol' sufllclent to maintain the requisite temperature in the rectification column may be supplied as the distillation progresses. This allyl alcohol may be introduced either into the still or into the rectication column itself.

The xylene was in large part returned down the rectification column and retained in the still while the azeotropes of the allyl alcohol with the other hydrocarbons were distilled out and were collected in separator 25. 'I'he loss of xylene from the still to the condensatedrawn off through pipe 22 will depend upon the efficiency with which the vapors are rectiiied. Suicient rectification is employed in order to keep down this loss of xylene.

In carrying out the process of this example, as the distillation continued the temperature of the vapors at the top of `column I9 rose from 80.8 to 96.6 C. and thereafter rose to l37.9 C. The xylene content of the oil obtained from the condensate withdrawn through pipe 22 by'washing the condensate with water to free it of allyl alcohol increased from less than 5 per cent to more than 97 per cent before the temperature at the top ofthe column rose above 96.6 C. The oil obtained from the condensate collected while the temperature was rising from 96.6 to 137.9" C. was approximately 100 per cent xylene. The distillation was discontinued when the temperature of the vapors at the top of column I9 reached 137.9 C. and the oil content of the residue left in the still was substantially 100 per cent xylene: the xylene distilled over as the temperature rose from 96.6 to 137.9 C., combined with to be supplied is that which will maintain the the xylene residue left in the still, represented a yield of more than per cent of the xylene in the original xylene cut as per cent xylene.

By agitating the distillate collected in separator 25 withlwater and lallowing the liquid to remain quiescent for a short time, it separates into two layers. The upper layer is an oil -containing the non-xylene constituentsof the xylene fraction supplied to still I1 and a portion of 'the xylene carried over in the distillation. This oil may be treated or used as desired. By separately recovering and treating the distillate in two portions, two oil fractions may be obtained.

distillate with water may be treated in any of.

the well known manners to recover anhydrous allyl alcohol therefrom and this alcohol utilized in distilling a subsequent batch of xylene fraction.

While I have described my process in conjunction with examples in which the two distillation steps are batch procedures, either or both of these Adistillations advantageously may be carried .out

continuously by well-known continuous distillation procedures suitable for the fractional distillation of mixtures of two or more liquids.

It is, of course, obvious preliminary distillation of a crude xylene oil to obtain a xylene fraction suitable for recovery of xylene therefrom by azeotropic distillation with allyl alcohol need not be carried out in immediate conjunction with the azeotropic distillation of the xylene fraction. The xylene fraction suitable for treatment by the azeotropic distillation step of my process may be produced in one plant, transported to and treated later in another plant to distill it with theallyl alcohol. Nor is my invention limited to any particular procedure for the production of the xylene fraction. My invention contemplates the distillation with the above described allyl alcohol, of any oil fraction containing materials distilling in the range of 125 to 150 C., preferably 136 C. to 144 C. and containing xylene hydrocarbons together with other hydrocarbons which, when the oil is distilled, vaporize therefrom in the same temperaturev range as the xylene and. therefore, are not separable from the xylene by direct fractional distillation, As used in this specification and the appended claims, the term xylene fraction refers to all such xylene-containing oils, whether produced by fractional distillation of xylene-containing materials vor by any other means.

In this specification I have described the azeotropic distillation of xylene fractions as carried out under substantially atmospheric pressure and the temperatures given in the specification and claims are for such pressure. It is, of course, possible to distill the xylene fraction with the alcohol under pressures above or below atmospheric. In i Reference is made to my copending applica` tion Serial No. 361,825 of October 19, 1940, in

which I have disclosed and claimed a process for separating xylene from like-boiling hydrocarbons4 involving azeotropic distillationof oil fractions in the presence of water and a saturated monohydroxy aliphatic alcohol of low molecular weight.

What is claimed is:

1. A process for the treatment of a xylene fraction to separate the xylene from other hydrocarbons lof the group consisting of parafilnic, olenic and naphthenic hydrocarbons contained therein which distill from the xylene fraction in the same temperature range as the xylene distills therefrom which comprises distilling said xylene fraction and 'rectifying the vapors evolved therefrom in the presence of sufficient allyl alcohol to eilect separation of a substantial amount of said other hydrocarbons from said xylene, thereby' leaving a residue enriched in xylene.

2. The process for the recovery of xylene from an oil containing it with non-aromatic hydrocarbons of the group consisting of paraillnic, oleflnic and naphthenic hydrocarbons which comprises fractionally distilling said oil, recovering by the fractional distillation a fraction containing xylene and Anon-aromatic hydrocarbons which distill from said xylene fraction in the same temperature range ,as the xylene distills therefrom, distilling said xylene raction and rectifying the vapors evolved therefrom in the presence of allyl alcohol in amount sumcient to eiect vaporizing and removing from the xylene of azeotropes of said non-aromatic hydrocarbons boiling in the same temperature range as the xylene.

3. The process for the recovery of xylene from an oil containing it with non-aromatic hydrocarbons including parainic hydrocarbons and which may include oleiinic and naphthenic hydrocarbons which comprises fractionally distilling said w oil, recovering by the fractional distillation a fraction boiling within the range of 125 to 150 C. containing xylene and non-aromatic hydrocarbons which distill'from said xylene fraction in the same temperature range as the xylene distills therefrom, distilling said xylene fraction and rectifying the vapors evolved therefrom in the presence of allyl alcohol in amount suiiicient to effect vaporizing and removing from the xylene of azeotropes of said non-aromatic hydrocarbons boiling in the same temperature range as the tion' and rectifying the vapors evolved therefrom in the presence of allyl alcohol while maintaining at a point in the rectiflcation zone a temperature not above 97 C. by maintaining the presence of said allyl alcohol at saidl point and continuing the distillation until the unvaporized residue of the distillation contains at least 95 parts by weight of xylene to every 5 parts by weight of other hydrocarbons which distill from a mixture lof xylene and hydrocarbons in the same Atemperature range as the xylene, thereby vaporizing and removing from the xylene fraction other hydrocarbons present. therein which distillA therefrom in the absence of allyl alcohol in the same temperature range as xylene. i

+o 5. The process for the treatment of an oil fraction boiling within the range of 125 to 150 C. containing xylene and non-aromatic hydrocarbons including parainic hydrocarbons and which may contain oleiinic and naphthenic hy- :iu drocarbons boiling in substantially the same temne the azeotropes of non-aromatic hydrocarbons present therein which distill therefrom in the absence of said allyl alcohol in the same temperature range as the xylene and leaving a hydrocarbon residue of the distillation enriched in xylene.

du 6. The process for the recovery of xylene from a xylene oil fraction boiling in the range of 125 to 150 C. containing xylene and paraflinic hydrocarbons and-which may contain olefinic and naphthenic hydrocarbons boiling in the same |35 temperature range as said xylene, which comprises fractionally distilling said fraction and rectifying the evolved vapors in the presence of allyl alcohol in amount such that thetemperature of the vapors at a point in the rectification zone does not exceed about 97 C., thereby vaporizing and removing from the xylene fraction hydrocarbons which'distill therefrom in the absence of allyl alcohol in the same temperature range as the xylene contained therein.

7. The process for the recovery of xylene which comprises azeotropically distilling in the presence of allyl alcohol a xylene oil fraction containing xylene and non-aromatic hydrocarbons including parailinic hydrocarbons and which may include olenlc and naphthenic hydrocarbons boiling in the absence of said allyl alcohol in the range of 125 to 150 C. while rectifying the vapors in the presence of said allyl alcohol until a temperature within the range of 94 to 97 C. is reached at a point in the rectification zone, thereby vaporizing and removing asdistillate non-xylene hydrocarbons present in the fraction.

8. The process for the recovery of xylene which comprises azeotropically distilling ln the presence of allyl alcohol a xylene oil fraction-contamina xylene and non-aromatic hydrocarbons including paraiinic hydrocarbons and which may include olenic and naphthenic hydrocarbons boilingr4 in the absence of said allyl alcohol in the range of 125 to 150 C. while rectii'ying the vapors inthe presence of said allyl alcohol until a temperature.

within the range of 94 to 97 C. is reached at a point in the rectcation zone, thereby vaporizing and removing as distillate substantially all of the non-xylene hydrocarbons present in the fraction and leaving a hydrocarbon residue, the oil content of which is about 95 per cent xylene.

- CHARLES E. CLARK.

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