US2957811A - Segregation of xylene isomers - Google Patents

Description

United States Patent SEGREGATION OF XYLENE ISOMERS Edward M. Geiser, Downers Grove, 11]., assignor, by mesne assignments, to Universal Oil Products Company, Des Plaines, 111., a corporation of Delaware No Drawing. Filed Nov. 5, 1957, Ser. No. 694,505

6 Claims. (Cl. 202-395) This invention relates to a process for the separation and recovery of one or more xylene isomers from a hyisomer from a hydrocarbon mixture comprising C aromatic hydrocarbons which comprises contacting said mixture with a solvent comprising an N-alkyl carboxamide while maintaining said amide in substantially liquid phase to thereby form a rich solvent stream comprising at least one of the xylene isomers present in said mixture dissolved in the solvent and a separate raflinate phase comprising at least one other of the xylene isomers present in said mixture in a proportion greater than its proportion in the mixture and thereafter separating the raflinate phase from the rich solvent stream.

A more specific embodiment of the foregoing process comprises separating at least one of the group selected from ethylbenzene and para-xylene from a hydrocarbon mixture comprising C aromatic hydrocarbon isomers, said process comprising: distilling said hydrocarbon mixture in the presence of dimethylformamide, countercurrently contacting the resulting hydrocarbon vapors with a liquid solvent comprising dimethylformamide and separating an overhead stream comprising a hydrocarbon portion enriched with one of the isomers selected from the group consisting of ethylbenzene and para-xylene.

Charging stocks which maybe utilized in the present separation process include generally hydrocarbon mixturescontaining either exclusively aromatic components or these may be accompanied by other classes of hydrocarbons, such as paraflins, other aromatics, olefins, naphthenes, etc., although preferably, the present separation process is most effectively utilized with a feed stock from which the non-C aromatichydrocarbon components are eliminated or in which these components constitute a minor proportion of the whole. Oneof the preferred feed stocks for use herein is a fraction boiling within the range of from about 120 to about 160 C. which may be derived from certain petroleum conversion products containing aromatic hydrocarbons and more particularly, the aromatic hydrocarbons of C composition. The isomeric xylenes are. particularly predominant in certain gasoline boiling range fractions of petroleum conversion products, such as the product of a reforming process in which aliphatic and naphthenic hydrocarbons undergo dehydrogenation, isomerization and/or cyclization reactions to form their corresponding aromatic analogs.v The desired fraction particularly selected for charging stock in the present process may contain, for example, two or more of the C aromatic isomers, in-

. eluding, ortlio-xylene (having a boiling point of 144 C. at 760 mm; pressure), meta-xylene (having a boiling point of 138 C. at atmospheric pressure), para-xylene (boiling point: 138? C. at 760 mm.) and ethyl benzene (boiling point: 136.2 C. at 760 mm.). The fraction of such a conversion product, boiling within the range of from about 120 to about 145 'C., would contain substantially all of the various C isomers, including azeotropes thereof with other classes of hydrocarbons which may-also be present in such a fraction.

According to the present separation process, a .hydrocarbon mixture containing at least two xylene isomers is contacted, preferably under countercurrent flow conditions with a solvent extractant characterized as an N- alkyl carboxamide under contacting conditions of temperature and pressure that at least one of the fluid phases present in the contacting chamber is in substantially liquid state. The carboxamides herein utilized as solvent-extractants are compoundshaving the following empirical formula:

wherein R and R are selected from the group consisting.

of hydrogen and alkyl .andR is.selected solely from alkyl, each of said alkyl radicals containing from one to four. carbon atoms. The preferred membersof the above group of carboxamides utilized herein as solvent-extractants are compounds which are relatively volatile and.

' the above group in which the total number of carbon atoms in the groups: R, R and R does not exceed about five in number. Particularly preferred herein, especially for a process involving extractive distillation of the hydrocarbon charge stock are the monoand di-methyl and monoand diethyl formamides and acetamides; that is,

compounds .in which the- R, R and R groups are hy-.

drogen, methyl or ethyl. These compounds are sufiiciently volatile to distill over at least in part with the isomer comprising the overhead, and yethave sufficiently high boiling points to provide a liquid solvent phase in the extractive distillation column during the fractionation of a mixture of the carboxamide with the charge stock. It is to be emphasized that all of the members of the above general class of compounds do not produce equivalent results and although all are at least somewhat effecthe solvent may be countercurrently contacted with the hydrocarbon feed stock ina countercurrent-liquid-liquid solvent extraction zone, for example utilizing an absorption type separation procedure wherein the amide solvent as the phase of greatest density is introduced into the upper portion of a suitable liquid-liquid contacting zone and the hydrocarbon feedstock as the phaseof lesser density is introduced into the bottom portion of thecontactingpzone, the resulting streams, being allowed theree after to flow in countercurrentrcontact with.each other.

A so-called rich solvent stream containing as a solute Patented Oct. 25 1960' the component of the feed stock of greatest solubility in the solvent, generally orthoand/or meta-xylene, if present in the hydrocarbon charge, is removed from the lower portion of the extraction zone, while the nonextracted portion of the charge stock, normally referred to as the rafiinate phase, is removed from the upper portion of the contacting zone. Provided a suflicient volume of solvent is charged into the contacting zone, compared to the volume of charge stock, the rich solvent stream may contain substantially all of the meta-xylene and ortho-xylene components of feed stock, if present therein. Similarly, the rafiinate stream is composed predominantly of the relatively insoluble components of the charge stock and if a sufiicient number of contacting stages is provided in the extraction zone, the raffinate may consist. substantially completely of ethyl benzene. The para-xylene component, when present, may be recovered from the extraction zone of the latter type from a plate located. at an intermediate point between the raffinate and extract outlets as a side stream therefrom. In order to provide a cyclic process in which the solvent is recirculated to the contacting zone after removing extracted C isomer therefrom, the rich solvent stream may be separately treated, from example, by subjecting it to fractional distillation (the so-called stripping method of the prior art) in order to recover the extract hydrocarbon therefrom as a vapor overhead substantially enriched in ortho-xylene. In such a stripping operation the rich solvent is heated to a temperature above the boiling point of the dissolved hydrocarbon in the presence of the solvent and at the particular pressure maintained in the stripping zone, the solute vapors fiash overhead from the rich solvent and collect as a liquid condensate in the receiver vessel. The residue remaining in the bottom of the stripping column generally consists of substantially regenerated lean solvent which may thereafter be recirculated to the contacting or solvent extraction zone for reuse therein.

Another means of utilizing the present N-alkyl carboxamide solvents in an extraction procedure comprises introducing the solvent, which may be an aqueous mixture of the same, at a suitable solvent extraction temperature, preferably at least about 20' above the end boiling point of the charge stock into an extractive distillation zone into which the feed stock as a heated liquid or as a vapor is also simultaneously introduced, the resulting extractive distillation forming a liquid phase, rich solvent as. a separate stream from the raffinate phase recovered from the extractive distillation zone as an overhead vapor.

In most instances the alkyl amides herein specified for use as solvent extractants have boiling points substan tially above the boiling point of the charge stock and the maintenance of at least a portion of the solvent in liquid phase can therefore be effected at atmospheric pressure or at a relatively small superatmospheric pressure. In other instances, that is, when the molecular weight of the carboxamide is sufficient to predetermine its boiling point at a relatively high level, it becomes expedient to dilute the carboxamide with water or a lower boiling organic compound to thereby reduce the temperature at which extractive distillation will take place. In most instances, however, the solvent component utilized in any particular process should not exceed about 50% by weight of the total composition because of the sub stantial reduction in the solubility relationships for the hydrocarbon charge stock as the proportion of water or other solvent in the composition increases.

For the extraction of charge stocks boiling above about 110 0., below about 175 thereby including all known azeotropic hydrocarbon mixture with any of the xylene isomers, the extraction temperature (that is, the temperature at which the solvent composition contacts the charge stock) is preferably in an excess of about 87 C. but not substantially greater than 200 C., the required pressure at, these temperatures varyingin accordance with the boiling point of the feed stock and the solvent composition utilized in the process. In general, pressures of from atmospheric to 20 atmospheres or higher are sufficient to maintain at least a portion of the solvent composition in liquid phase during the course of the extraction. In an extractive distillation type of separation procedure (in general, one of the preferred methods of separation for use in the present process), the boiling point of the aliphatic components in the charge stock (if present) are affected least by the presence of the solvent and any aliphatic components in the charge stock, therefore, tend to distill overhead from the extractive distillation zone at approximately the normal boiling point of the particular aliphatic components present in the mixture or at the boiling point of a constant boiling mixture thereof with other aliphatic components present in the mixture. The most volatile portion of the overhead distillate is generally the aliphatic components in the charge stock and in a batch distillation, these are collected and segregated first from the overhead. A higher boiling cut may thereafter be distilled overhead, the fraction thus collected containing a greater proportion of ethylbenzene (if present in the charge stock) than the other xylene isomers. The next fraction which boils at a slightly higher temperature than the previous cut contains a greater proportion of the para-xylene isomer (if present in the charge stock); hence, the next highest boiling cut contains a greater proportion of para-xylene than is present in the charge stock. As successively higher boiling cuts are removed from the extractive distillation zone in the presence of the carboxamide solvent, meta-xylene and thereafter, ortho-xylene, distills overhead, depending upon the boiling point of the solvent composition. Each of the resulting enriched cuts (enriched, that is, with respect to the individual isomer which predominates in the particular cut) may thereafter be redistilled in the presence of the solvent to effect greater purification of the particular isomer predominating in the cut. Although the foregoing batch-type extractive distillation procedure may be utilized for any given charge stock and any given solvent composition, a continuous process involving a multi-stage extractive distillation procedure, effected in a series of distillation columns each having more than two and preferably more than ten theoretical plates may be provided; for this purpose a separate column is provided for each isomer taken overhead in the succession of columns, the bottoms of one column being transferred as feed into the next succeeding distillation column for separation of the next higher boiling isomer. Such a procedure comprises a particularly preferred arrangement for continuously separating a multi-isomer containing hydrocarbon mixture.

The present extractive distillation process involves the continuous separation of an overhead distillate, which, after condensation, separates into two liquid phases: 1) a fraction comprises predominately hydrocarbon enriched with respect to one of the isomers (depending upon the boiling point of the distillate) and in which a relatively minor amount of solvent is dissolved, and (2) a second liquid phase consisting predominantly of solvent in' which a relatively minor amount of hydrocarbon may be dissolved and which, in the case of utilizing an aqueous carboxamide solvent composition, may contain a larger proportion of water than is present in the solvent composition charged to the separation zone. In a preferred manner of operating the present process the layer separating from the condensed overhead consisting predominantly of solvent components is refluxed at least in part to one of the upper trays of the extractive distillation column, thereby providing a descending lean solvent stream which enhances the extractive distillation of the feed stock; alternatively, the solvent layer may be redistilled in a separate column to separate the solvent components and any hydrocarbon which may be present therein for recycle of the solvent constituents in the process. The hydrocarbon layer separated from the overhead distillate of the extractive distillation column is substantially enriched in one of the desired xylene isomers and, as indicated above, may contain relatively minor amounts of the solvent composition dissolved therein. In order to recover the solvent constituents, the hydrocarbon layer may be redistilled, thereby effecting a secondary fractionation of the hydrocarbon components, leaving a residue consisting predominantly of previously dissolved solvent which may be recycled to the extractive distillation stage. In order to recover the minor amount of solvent in the hydrocarbon layer, the latter may also be washed with a stream of water, preferably under countercurrent flow conditions, to extract the dissolved solvent components therefrom.

The present invention is further illustrated with respect to several of its specific embodiments in the following illustrious examples, which however, are not intended to necessarily define the scope of the invention in accordance therewith.

In the following series of runs a mixture of C aromatic hydrocarbons comprising the various isomers of xylene are distilled (l) individually (that is, without the presence TABLE I Distillation of mixed C aromatic hydrocarbon isomers Cut N0 Charge 1 2 3 4 5 Vol. percent of Total 100 4.9 5.0 84.0 3.0 2.

Analysis, Wt. Percent: V

Toluene 0. 2 tr. Ethylbenzene" 5. 5 7. 5 5. 5 a. 2 1. 6 1. 1 p-xylene 27. 0 29. 1 29. 4 28. 4 22. 8 22. 6 m-xylene. 64. 3 63.3 63. 4 65. 4 68. 5 66. 9 o-xylene 3. 0 0. 1 1. 6 3. 0 7. 1 9. 4 p-xylene/m-xylene. 0. 42 0. 46 0. 46 0. 43 0. 33 0. 34

The above data indicate that a slight enrichment of the para-xylene was obtained in the first fractions distilled overhead, but the ratio'of isomer enrichment is relatively minor. 7

In the following run, 75 cc. of di-methyl formamide was mixed with the 150 cc. of xylene isomer charge stock, followed by distillation in the same column as that utilized in the foregoing runs in which the distillation was effected in the absence of non-hydrocarbon entrainers.

TABLE H Azeotropic distillation of C aromatic hydrocarbon mixture in the presence of dimethylformamide Gut No Charge 1 2 3 4 5 Botts.

Temp., 0 IBP-134 134-137 137-140 140-146 146-154 154 V01. Percent of Total 100 0. 6 1. 4 7 2. 0 14.0 4. 3 Analysis, Wt. Percent Hydrocarbons:

Toluene 0. 2 1. 5 t1 Ethyl Me 5.8 14.4 5.0 0.7

p-Xylpne 27.4 35. 4 27. 2 3. 5

m-Xy 62. 2 48. 7 66.8 18. 2 9. 1

o'Xylene 4. 4 1. 0 77. 6 90. 9 Total Overhead Hydrocarbon 100 100.0 71. 5 81. 7 70. 9 1. 1 DMF 28. 5 18. 3 29. 1 98. 0 p-Xylene/m-Xylene 0. 44 0. 73 0. 41 0. 19

of non-hydrocarbon components), (2) with di-methyl formamide under azeotropic conditions, and (3) with an aqueous solution of di-methyl formamide under azeotropic distillation conditions. The distillates were separately collected in various cuts, determined by the boiling point of the cut and the constancy of the boiling point, after which the composition of the hydrocarbon layer (it phase separation takes place in the distillate) was 5() of xylene isomers of reagent grade quality is utilized, the

feed stock containing the following components in their indicated proportions by weight:

Toluene 0.2 Ethylbenzene 5.5 p-Xylene 27.0 m-Xylene 64.3 o-Xylene 3.0 p-Xylene/m-xylene 0.42

The above mixture of isomers was charged into the distillation flask attached to a fractional distillation column of three foot length packed with glass helices. The distillation provided the data indicated in the following Table I.

'The data in the above table indicate that the paraxylene and ethylbenzene components of the charge stock tend to concentrate in the initial lowest boiling azeotropic fractions with dimethylformamide distilled from the mixture and that ortho-xylene tends to predominate in the higher boiling cuts. The data further indicate the feasibility of separating C aromatic hydrocarbon mixtures employing a multi-stage fractionating column having sufiicient reflux of dimethylformamide to produce a continuous countercurrent, downwardly flowing stream of solvent through the fractionating column.

In the following run, to the 150 cc. of mixed xylene isomer charge stock was added cc. of dimethylformamide and 75 cc. of water, followed by azeotropic distillation of the resulting mixture. Analysis of the charge stock and the observed data involved in the distillation are reported in the following Table III.

The above data indicate the desirability of utilizing an aqueous mixture of an dialkylformamide as the extractive solvent, indicating the substantial enrichment of para-xylene (compared to meta-xylene) in the initial overhead cuts from the distillation column.

The use of other dialkyl and monoalkylformamides as well as other mono and dialkyl alkanoic amides which are less soluble in water, but relatively more soluble in the hydrocarbon charge stock indicates that both N-monoalkyl and N,N' dialkylamides may be employed as azeotroping agents. These results further indicate that alkanoic acid amides in which the alkyl group attached to the carbonyl group contains up to 5 carbon atoms may similarly be employed as separating agents in the extractive distillation of mixed xylene isomers.

TABLE 111 Azeotropic distillation of C aromatic hydrocarbon mixture in the presence of aqueous dimethylformamide Out No Charge 1 2 3 4 6 Botts.

Temp, C IBP-134 134-137 137-140 140-146 146-154 154 Vol. Percent of Total 100 27. 4 3.3 53. 2. 2 11.0 3. Analysis, Wt. Percent Hydrocarbons:

Tnlnena 0.2 0.1 Ethylbenzene 5. 16. 8 10. 8

64. 3 47. 2 54. 0 68. 14. 8 o-Xy1ene- 3.0 0. 0 0.0 0. 83.1 100 Total Overhead:

H-O 100 8. 8 75. 3 79. 69. 1 13. 1 DMF+H O 14. 5 21. 30. 9 86. 9 100 5 91. 2 l0. 2 p-Xylene/m-Xylene O. 42 0. 76 0. 65 0. 39 0. 13

Whenthe number of carbon atoms in the carboxamide from the group consisting of ethylbenzene and parais greater than about 6, it is noted that the solubility of the amide in the xylene mixture also substantially increases and the boiling point of the amide increases with increasing molecular weight. The above results indicate that the relatively lower molecular weight carboxamides are preferred herein, because of their desirable lower volatility in the extractive distillation.

I claim as my invention:

1. A process for separating at least one isomer selected from the group consisting of ethylbenzene and para-xylene from a liquid mixture thereof with at least one other C aromatic hydrocarbon, which comprises distilling said liquid mixture in the presence of a carboxamide of the formula wherein R and R are selected from the group consisting of hydrogen and an alkyl group of from one to four carbon atoms and R is an alkyl group of from one to four carbon atoms, countercurrently contacting the resultant hydrocarbon vapors with said carboxamide in liquid form, and removing from the distilling and contacting steps an overhead vapor comprising a hydrocarbon portion enriched with one of the isomers selected xylene.

2. The process of claim 1 further characterized in that the total number of carbon atoms in R, R and R in said formula does not exceed five.

3. The process of claim 2 further characterized in that said carboxamide is selected from the group consisting of monoand di-methyl and monoand di-ethyl formamide.

4. The process of claim 1 further characterized in that said carboxamide is dimethylformamide.

5. The process of claim 4 further characterized in that said .dimethylformamide is an aqueous solution.

6; The process of claim 1 further characterized in that said carboxarnide is N,N-diethylformamide.

References Cited in the file of this patent UNITED STATES PATENTS 2,343,611 Cope et al. Mar. 7, 1944 2,433,751 Friedman Dec. 30, 1947 2,608,519 Deters et al. Aug. 26, 1952 2,690,417 Shalit et al. Sept. 18, 1954 2,799,629 Clough et al. July 16, 1957 OTHER REFERENCES Rossini et al.: Hydrocarbons from Petroleum, Reinhold Publishing Corp., 330 W. 42nd Street, N.Y., N.Y.

(1953), pages 448-450 and 345 needed.

Claims (1)

1. A PROCESS FOR SEPARATING AT LEAST ONE ISOMER SELECTED FROM THE GROUP CONSISTING OF ETHYLBENZENE AND PARA-XYLENE FROM A LIQUID MIXTURE THEREOF WITH AT LEAST ONE OTHER C8 AROMATIC HYDROCARBON, WHICH COMPRISES DISTILLING SAID LIQUID MIXTURE IN THE PRESENCE OF A CARBOXAMIDE OF THE FORMULA