CA1059928A - Separation of aromatic isomers - Google Patents

Abstract

SEPARATION OF AROMATIC ISOMERS
Abstract of the Disclosure A mixture of C8 and higher aromatics, including para-xylene, meta-xylene, ortho-xylene and ethylbenzene, is separated by an adsorption/desorption procedure to provide rapid recovery of the valuable xylene components. In this process, the aromatic mixture is passed through an adsorption column, preferably containing a certain zeolite adsorbent, in which column the meta-xylene and ortho-xylene pass through relatively uninhibited, whereas the para-xylene and ethylbenzene and other aromatics are adsorbed in the column. The meta-xylene and orthoxylene are removed and distilled to effect separation. The para-xylene/ethylbenzene mixture in the column is then desorbed and separated by conventional procedures. In addition, complete and continuous procedures are also provided for maximum separa-tion of all components by operation of two or more columns in parallel in a system such that adsorption is conducted in one column while desorption is carried out in the other column.

Description

Background of` the Invention Field of the Invention This invention relates to a method for 'he separation of valuable components from a mixture of aromatic compounds and more particularly to a novel process for recovering substantially pure meta-xylene and ortho-xylene from a mixture ^ontaining these components together with ethyl benzene and ?ara-xylene.

., ' ' ~.

Description of the Prior Art Aromatic compounds and particular'y para-xylene, meta-xylene, ortho-xylene and ethylbenzene are well known as very useful materials in the chemical industry but are generally found only in admixture with each other. For example, they are found in substantial quantities in coke oven light oil and certain virgin and reformed petroleum naphthas. Over the years many processes have been attempting to obtain a satisfactory separation of the several components and recover the desired components but none of these processes has been entirely successful. Here-_ tofore, the practice of separating these isomers has been to use che~ical methods or to effect distillation between the isomers.
Distillation, however, is difficult because of the very close boiling points of the components. In addition, various methods have been devised utilizing one or more of the steps of crystal-lization, distillation and adsorption but in general, none of these prior processes have been able to provide a procedure for the substantially complete isolation of all the components in i high purity. While a number of the processes are suitable to provide the para-xylene component in good recovery they have not been able to provide the meta-xylene and ortho-xylene in a form free from the para xylene and ethylbenzene.
The process of the present invention provides a unique procedure which overcomes the disadvantages of the prior art and provides a method for the rapid separation of the valuable compon-ents contained in the Cg+ aromatic mixtures.

It ls accordingly one ob~ect of the present invention to provlde a process for the separatlon of a C8~ aromatics mlxture which contains xylene isomers and ethylbenzene which overcomes or otherwise mitigates the problems of the prior art.
SUMMARY OF THE INVENTION
According to this invention, a novel method is provided for the separation and recovery of aromatic isomers contained ln an aromatic mixture comprising para-xylene, meta-xylene, ortho-xylene and ethylbenzene to separate this mixture into one stream comprislng meta-xylene-and ortho-xylene and a second stream comprising para-xylene and ethylbenzene. The mixture is contacted with a ZSM-5 zeolite adsorbent whereby para-xylene and ethylbenzene are adsorbed and meta-xylene~jand ortho-xylene are not adsorbed. The novel procedure comprises contacting said mixture with sald ZSM-5 zeolite adsorbent maintained -in at least two separate reactors disposed in parallel relationship whereby when adsorption is complete in one reactor and the ortho-xylene and meta-xylene removed, desorption is started while simultaneously beginning adsorption in the second reactor and desorbing the second reactor after adsorption while simultaneously beginning adsorption in the first reactor, recovering the para-xylene and ethylbenzene from the desorption steps and the meta-xylene and ortho-xylene from the adsorption steps.

Brief Description of the Drawing Reference now made to the drawing aocompanying this application in which there are illustrated schematic diagrams of processes for practicing the invention, and in which like reference numerals represent like parts, and wherein Figure 1 is a schematic diagram of one adsorption/desorption procedure for the basic separation process; and Figure 2 represents a schematic diagram of the basic separation process included in an integrated continuous process for the recovery of the para-xylene, ortho-xylene, meta-xylene and ethylbenzene.

- Description of Preferred Embodiments The process of the present invention is concerned with a new procedure for treatment of C8 and higher carbon-containing aromatic mixtures for the recovery of the valuable components contained therein. More particularly, this process is concerned with obtaining a rapid separation and recovery of meta-xylene, ortho-xylene, para-xylene and ethylb~nzene when contained in a mixture thereof. Generally, mixture of this type as obtained from most sources will contain these components in the following concentrations.
COMPONENTS AMOUNTS
Para-xylene 15-40 wt. %
Ethylbenzene 0-15 wt. %
~ Ortho-xylene 0-25 wt. %

2~ Meta-xylene 40-60 wt. %

_ 4 --105~928 This mixture will also generally contain other C8 and Cg and higher paraffinic materials which are mostly aromatic in nature and must also be separated from the above components, although many procedures have been tried, separation of these components by methods such as distlllation with fractionation are unsatisfactory because the boiling points of the components are nearly the same.
According to the process of this invention, a unique method is provided for conducting the separation of the components 1 10 contained in a typical aromatic feed stock whi-ch serves to over-come at least one of the primary obstacles preventing commercial-ization of a process of the type described hereinabove.
AS pointed out previously, while the use of the ZSM-5 materials may be used to effectively obtain a good separation between the close-boiling para-, meta-, and ortho-xylenes and ehtylbenzene, the slowness and thus, inefficiency of the chroma-tographic column used has been a ma~or problem. In the present invention it is proposed to overcome this problem by adsorption followed immediately by desorption which has been found to sub-stantially speed up the process. One embodiment of the inventioninvolves the use of two or more columns operated in a parallel manner so that when adsorption is being conducted in one column, desorption can be conducted in a parallel column under such con-ditions as to ob~ain a continuously operating process with faster results than with use of a single column alone. Accordingly, the basic process of this invention comprises a method for contacting an aromatlc mixture containing para-xylene, ethylbenzene, ortho-xylene and meta-xylene in at least one chromatographic column to effect a separatlon thereo~, and in which system, two or more 10599;Z8 columns are preferably operated in parallel relationship.
Briefly, in this process, the aromatics mixture is contacted with a zeolite which serves to adsorb the para-xylene and ethylbenzene while the meta-xylene and ortho-xylene pass through the column rather uninhibited. Immediately, on removal of the ma~or amount-of ortho-xylene and meta-xylene, this mixture is sent to further processing to effect separation thereof as by distil-lation to provide excellent recovery of these components. In the meantime, the column containing the adsorbed para-xylene and ethyl-benzene is desorbed by methods described hereinafter for prompt _ removal of these components from the column and this mixture is then tr~nsported to means for desorbent recovery and recovery and separation of the ethylbenzene and para-x~rlene as by crystallization and distillation. Thus rapid adsorption/desorption is achieved.
It has been found that conducti~g a process of this type -in combination with a second identical and parallel column provides many advantages in a system designed to ultimately recover- all the valuable components. In this system the two columns are maintained ; in parallel relationship and the feed is introduced into one column at a time by use of, for example, a three-way valve mechanism which will be more fully described in the drawings accompanying the appli-cation. In the system disclosed herein, the feed is introduced into one column wherein the adsorption procedure outlined above is effected and for maximum efficiency, when the meta-xylene and ortho-xylene have passed through the column, desorption is begun immediately and the feed into the system is diverted to the parallel column.

~059928 Thus, while desorption is occurring in one column, adsorption is taking place in the second column. At the end of the cycle in each column the systems are then reversed so that a continuous adsorption/desorption can be carried out in the parallel columns by the use of the valve mechanisms attached thereto.
This system is illustrated in its simplest embodiment ` for example in Figure 1 wherein two columns I and II are shown disposed in parallel relationship to effect the adsorption/
desorption technique which comprises the basic process of the invention. As shown in Figure 1, the aromatic-feed enters through line 1 into the three-way^valve 2. To begin the separation process the valve is manipulated so that the feed passes into line 3 and thus into column 5 which-contains the zeolite adsorption material and sufficient time is allowed for adsorption to occur in the column. In Figure 1, it will be seen that the components pass u? through the column and the unadsorbed materials comprising the ma~or amount of ortho-xylene and meta-xylene are passed out via line 6 to further separation procedures. 'As soon as ; the ortho-xylene and meta-xylene are substantially removed through line 6, desorption is started with the introduction of the desorbent from line 7 through valve 8 with introduction into the column by line 9. As soon as desorption is started in column I or simultaneously therewith, the feed coming through valve 2 is diverted to line 4 so that adsorption can begin in column 12.
Thus, at this point adsorption is taking place in column II and desorption is occurring in column I.

The adsorption is conducked in column 12 as in column 5 with the effluent taken off at line 11 and passed to line 6 for further separation as by distillation. The adsorption is conducted over a zeolite adsorbent as in column 5 and as described in more detail hereinafter.
In operating the system with the two columns, optimum ' efficiency would be obtained by adjustment of feed rates and other conditlons such that as feed is introduced into one column to begin adsorption, desorption can begin in the second column.
Conversely, when adsorption is completed in the first column and desorption in the second eolumn, the feed streams can be reversed to obtain desorption in the first column and adsorption in the second column. If the feed streams are accurately ad~usted, a continuous adsorption/desorption operation can be carried out to achieve continuous separation of ~he feed stream.
~After desorption occurs in either of columns I or II
the éffluent from these desorptions which includes the desorbent as well as para-xylene, ethylbenzene and perhaps minor amounts of ortho-xylene and meta-xylene, is taken from column I by line 13 and column II by line 13' into line 14 and passed to desorbent recovery station 16. At this station the desorbent is recovered by conventional means and removed from the system by line 16 for recycle if desired. Then the ethylbenzene, para-xylene mixture is removed by line 18 for separation and/or recovery by any of various procedures such as crystallization and distillation.
From the above described techniques for use of the parallel columns and effecting the separations, it is to be 1~599Z8 appreciated that a prompt and efficient separatlon between the two most difficult to separate component mixtures is achieved.
Moreover, this separation is effected in an efficient manner for maximum recovery of all the components contained in this mixture.
It is to be appreciated, of course, that more than one set of parallel separatory columns can be used and this invention is considered to include such multiple parallel columns.
This basic separation process is shown in Figure 2 when integrated into one type of a complete and continuous system for recovery of all the components contained in an arornatlc _ mixture so that maximum recovery of all desirable components is obta-ined.
Referring now to Figure 2 in detail where like compo-nents contain the same reference numerals as in Figure l, it will be seen that the feed enters the system thro~gh line l directly or may be combined with a later`recycle from column 29 and is introuduced into columns I and II as described hereinabove-for Figure l. Columns I and II are operated as in Figure l, that i~, ; by the parallel adsorption/desorption procedures so that ultimately there is recovered from line 6 a mixture containing most of the ortho-xylene and meta-xylene in the original feed. This feed stream is passed to a xylene-splitter column where the meta-xylene and the ortho-xylene are separated by distillation with most of the meta-xylene being removed through line l9. The resulting bottoms, comprising ortho-xylene and any other C8 and Cg aromatics, are taken through line 20 to column 21 for further distillation.

_ 9 _ 105992~3 :
In this column heavier boiling Cg+ components are removed through line 23 for discard. The distillate comprising prlmarily ortho-xylene with perhaps some meta-xylene is taken from line 22 and passed to isomerization station 27. At this station any type of isomerization reaction may be conducted but it is highly preferable to conduct a low temperature isomerization reaction with toluene dilution, as more fully described hereinafter to form additional amounts of the xylenes which are taken off through line 28, recycled and introduced into feed line 1 for additional conversion and recovery in accordance with the process. ~
In the meantime, the para-xylene/ethylbenzene mixture removed~from columns I and II with the desorbent in line 14, is sent to a desorbent recovery station where the desorbent is re-moved conventionally via line 17. The remaining mixture in line 18 is forwarded for further processing for the separation of the para-xylene/ethylbenzene. Conventionally a good separation between these two components at the para-xylene recovery station 24 is by a technique such as crystallization which is, of course, well-known to the art and is carried out so that the para-xylene in substantially pure form is recovered through line 25. ~he resulting product comprising primarily ethylbenzene is recycled by line 26 for mixing with the feed in line 1 and introduced into the system. In an alternative procedure the feed in line l and the recycled mixture in line 26 are distilled in column 29 to recover at least a portion of the ethylbenzene from line 30 and the resulting mixture of xylenes fed by line l to the system.

It will be seen from a study of the reaction systems of Figure 2 that a completely integrated and continuous process is provided for the recovery of all the v~luable components contained in the mlxture. Moreover, because of the use of the parallel columns to effect the adsorptionidesorption procedures in the initial step a highly efficlent process is provided.
In the above system of Figure 2 the various distillations, isomerizations and crystallizations may be conducted by means that are well-known to the art or as described herein. Since these are well-known in the art, no necessity is se~n for the purposes of this disclosure to pro~ide further details of such known processes except to point out that the essential novelty herein resides in the adsorption/desorption technique of the initial step and its combination with the other steps in an integrated and cyclic process.
It is reiterated that a basic novelty of this invention resides in the use of the parallel columns with simultaneous adsorption and desorption. Exemplified herein is the basic process of Figure 1 wherein two streams are recovered, one stream comprising ethylbenzene and para-xylene and the other stream comprising essentially the meta-xylene and ortho-xylene. After this initial separation is achieved various procedures may be utilized for operating the process in a continuous manner with many variations available for the recovery of any one of the components in greater excess. Figure 2 as described herein provides a process which is particularly valuable for the production of para-xylene, meta-xylene and ethylbenzene. On the other hand, the process may be incor-por2ted into a system wherein the predominate prodùct desired is lOS9928 , meta-xylene or the predominate product desired is ortho-xylene.
As pointed out above, the adsorption and desorption procedures conducted herein represent the basic novelty of the present invention. In the adsorption process, the feed is introduced into the column or othe~ vessel containing -, the zeolite adsorbent at a temperature preferably about 50-500F. and more-preferably about 100-400~.
The feed is passed through a vessel such as a column con-taining the adsorbent or over a porous bed of the same _ in a conventional manner and in either the liquid or gas phase. -`As the feed passes over the adsorbent, the para-xylene and ethylbenzene are adsorbed within the pores of the zeolite whereas the meta-xylene and ortho-xylene pass through the vessel or over the bed withou~`being adsorbed to any substantial degree.
After the meta-xylene and ortho-xylene have left the reactor, the para-xylene and ethylbenzene are then desorbed from the adsorbent. The desorption may be carried out, for example by heating the adsorbent, reducing the partial pressure of the sorbed material in the vapor surrounding the adsorbent, lowering the total pressure of the system or purging ~ith a suitable inert desorbent gas such as steam, he'ium, nitrogen, aromatic hydrocarbons (e.g. toluene or benzene) or other organic or inorganic compounds. As a result of these de-sorption techniques, the paraxylene and ethylbenzene are eluted in this order in the vapor operation and in reverse order'in the li~uid phase operation. The desorption can be conducted in either the li~1uid or vapor phase or alternatively may be conducted by reduced pressure and/or increased temperature ..

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in the absence of a desorbent material. If a desorbent is used the resul~ing mixture o~ para-xylene and ethylbenzene . .
and desorbent is then passed to a conventional system for desorbent recovery and the desorbent may then be reused in ,' the system.
In the description of Figure 2, an isiomerization step is conducted to convert at least a portion of the mixture to para-xylene, a preferred product. The isomerization step may be conducted in any desired manner for conducting such isomerization as known in the art, but is preferably con-ducted under relatively low temperature with tolene dilution.
It is highly preferred that this isomerization step be conducted as a low temperature isomerization with tolene dilution wherein tolene diluent is added to the systems. This type of isomerization is called LTI herein. ,While LTI is the nreferred,manner of conducting the isomerization, it is to be understood that any of the well-known isomerization techniques can be used in this step so long as ethylbenzene is not produced.
When using the low temperature isomerization stage with tolene dilution it may be carried out in any desired manner but is preferably conducted employing about 5 to 30%
by'weight, preferably 10 to 20% by weight of added tolene, based on the amount of material charged to the isomerization stage, as a diluent to increase selectively in the isomeri-zation of the meta-xylene and ortho-xylene and the formation of, para-xylene. This isomerization reaction may be carried out over any desired catalyst but is preferably carrièd out in the presence of a crystalline alu,minosilicate catalyst which ,has a pore size of greater than 5 Angstrom units such as .

zeolites, X, Y, mordenite, and ZSM-4. Because members of the family of zeolites designated as ZSM-4 possess extraordinary selectivity, such materials are especially preferred.
The low temperature isomerization may be carried out at temperatures between about 250F. and 1000F. and at pressures ranging from ambient pressures or less up to about 2000 psig. In general, the isomerizatlon reaction is ` preferably carrled out at temperatures ranging from about ~50F. to 650F. Within these limits the conditions of temperature and pressure may vary considerably depending upon equilibrium considerations and reaction -lOS~39Z8 conditions. Quite obvlously optimum conditions are those in whichmaximum y~elds of desired isomer products are obtained and hence considerations of temperature and pressure may vary within a range of conversion levels designed to provide the highest selectivity and maximum yield. However, in a preferred operation using the ZSM-4 catalyst, it has been found that controlled isomerizations can be effectively achieved at temperatures below about 600F and a llquid phase operation usin~ sufflcient pressure to maintain the material in a liquid phase. The liquid - 10 phase operation is especially advantageous since high levels of activity and selectivity can be maintained for an extended period of time.
The isomerization reaction can be carried out over a wide range of licuid hourly space velocities (LHSV) within the `i i5 ~ range of 0.05 to 40. Good selectivity is-obtained within these limits.
As pointed out above, the initial separation is carried out in a chromatographic manner utilizing an adsorbent which will adsorb only the para-xylene and ethylbenzene but not the other materials of the mixture. The preferred materials to effect these separations are certain crystalline aluminosilicate zeolite molecular sieves which have the desired properties. Pre-ferred zeolites are the ZSM-5 zeolites described below.
More preferred are ZSM-5 zeolites which have been reacted with certain silanes as described hereinafter.

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The temperature at which the separations are carried out is also lmportant; thus, temperatures ranging ~rom about 100C. to about 250C. should be used. It should be noted that a wider temperature range can be employed but because of the possibllity of catalytic conversion in the zeolite-containlng column, 250C appears to be a suitable upper llmit. A more preferred temperature range is between about 100C to 200C.
Generally, these zeolltic materials allow selective separations to be achieved depending on either the size, shape or polarity of the sorbate molecules. This class of novel crystalline aluminosilicates can generally be stated to have intermediate shape-selective sorption properties. The unique nature,of this novel class of zeolites is characterized by the presence of uniform pore openings which are apparently elliptical ,' rather than circula~r in nature. The effective pore openlngs of this unique class of zeolites have both a ma~or and a minor axes, and it is for this reason that the unusual and novel molecular sieving effect~-are achleved. The unique type of molecular sieving produced has generally been referred to as a "keyhole"
molecular sieving action. From their dynamic molecular sieving properties it would appear that the ma~or and minor axes of the elliptical pore in this family of zeolites have effective sizes o . O
~ of about 7.0 + 0.7A and 5.0 + 0.5A, respectively.

, ' 16 -.
This general family of zeolites are described as ZSM-5 type compositions. In general, they have the characteristic ~-ray diffraction pattern set forth in Table I hereinbelow. ZSM-5 compositions can al80 be identified, in terms of mole ratios of oxides, as follows:
0.9 + 0.2 M20 : W203 : 5-100 Y2 Z H20 wherein M is a cation, n is the ~alence of said cation, W is selected from the group consisting of aluminum and gallium, Y is selected from the group consisting of silicon and germanium, and Z is from 0 to 40. In a more preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides, as follows:
0.9 + 0.2 M20 : A1203 : 5-100 SiO2 : z H20 n . and M is selected from the group consisting-uf a mixture of - 15 alkali metal cations, especially sodium, and tetraalkylammonium cations, the a-lkyl groups of which preferably ~ontain 2-5 carbon atoms.
; In a preferred embodiment of ZSM-5, W is aluminum, Y
ls silicon and the silica/alumina mole ratio is at least 10 and ; 20 ranges up to about ~0.
Members of the family of ZSM-5 zeolites possess a definite distinguishing crystalline structure whose X-ray diffrac-tion pattern shows the significant lines set forth in Table I
following: -. .
TABLE I
Interplanar Spacing d(A) Relative Intensity 11.1 + 0.2 S
10.0 + 0.2 S
7.4 + 0.15 W
7.1 + 0~15 W
6.3 + 0.1 W
6.04 + 0.1 W
5.97 + 0.1 W
5.56 + 0.1 W
5.01 + 0.1 W
4.60 + 0.08 W
~ 4.25 + o.o8 W

3.85 + 0.07 VS
3.71 +~0.05 . --S
-- 3.64 + 0.05 M
3.04 + 0.03 W
2.g9 + 0.02 W
~ 2.94 + 0.02 W
m ese values as well as all other X-ray data were determined by standard techniques. The radiation was the K-alpha doublet of copper, and a scintillation counter spectrometer with a strip ~ .
chart pen recorder was used. The peak heights, I, and the positions as a function o~ 2 times~theta, where theta is the Bragg angle, were read from the spectrometer chart. From the-se, the relative intensitles, 100 I/I, where I is the intensity of the strongest line or peak, and d (obs.), the interplanar spacing in A, corresponding to the recorded lines, were calculated. In Table I the relatlve intensities are given in terms of the symbols S = strong, M = medium, W = weak and VS = very strong.
It should be understood that this X-ray diffraction pattern is characteristic of all the species of ZSM-5 compositions. Ion , exchange of the sodium ion with other cations reveals substantially the same pattern with some minor shifts in interplanar spacing and variation in relative intensity. Other minor ~ariations can occur depending on the silicon to aluminum ratio of the particular sample, as well as if it had been sub~ected to thermal treatment.
Various cation exchanged forms of ZSM-5 have ~een prepared. X-ray powd~r diffraction patterns o~ several of these forms are set forth below in Table II. The ZSM-5 forms set forth below are all aluminosilicates.
TABLE II
~ X-ray Diffraction ZS~-5 Powder in Cation Exchanged Forms d SDacin~s Observed .

As Made HCL NaCL CaC12 REC13 AgNO3 11.15 11.16 - 11.19 11.19 11.19 11.19 10.01 10.03 10.05 10.01 10.06 10.01 9.74 9.78 9.80 9.74 9.79 9.77 -- -- 9.01 9.02 -- 8.99 8.o6 -- -- -- __ __ 7.44 7.46 7.46 7.46 7.40 7.46 7.08 7.07 7.09 7.11 -- 7.09 6.70 6.72 6.73 6.70 6.73 6.73 6.36 6.38 6.38 6.37 6.39 6.37 5.99 6.00 6.01- 5.99 6.02 6.01 - 5.70 5.71 5.73 5.70 5.72 5.72 5.56 5.58 ~ 5.58 5.57 5.59 -5.58 5.37 -- 5.38 5.37 5.38 5.37 ~ ~c : . ... . . . ... . .

lOS99Z8 . .
TABLE II ( cont . ) As Made HCl NaCl CaC12 REC13 AgN03 5.13 5.11 5.14 5.12 5.14 __

4.99 5.01 5.D1 5.01 5.01 5.01 - - _ - 4 74 __ __ __ 4.61 4.62 4 62 4.61 4.63 4.62 -- -- 4.46 4.46 -- 4.46 4.36 4.37 4.37 4.36 4.37 4.37 4.~'26 4.27 4.27 4.26 4.27 4.27 4.08 -- 4.09 4.09 4.09 4.09 4.00 4.01 4.01 4.00 4.01 4.01 3.84 3.85 3.85 3.85 3.86 3.86 3.82 3.82 3.82 3.82 3.83 3.82 3.75 3.75 3.75 3.76 3.76 3.75 3.72 3.72 3.72 3.72 3.72 3.72 3.64 3.65 3.65 3.65 3.65 3.65 __ 3.60 3.60 3.60 3.61 3.60 3.48 3.49 3.49 3.48 3.49 3.49 3.44 3.45 3.45 3.44 3.45 3.45 3.34 3~5 3.36 3.35 3.35 3.35 -3.31- 3.31 3.32 3.31 3.32 3.32 3.25 3.25 3.26 3.25 3.25 3.26 3.17 -_ __ 3.17 3.18 __ 3.13 3.14 3.14 3.14 3.15 3.14 3.05 3.05 - 3.o5 3.04 3.06 3.05 ~2.98 2.98 2.99 2.98 2.99 2.99 __ __ __ - - 2.97 - -__ 2.95 2.95 2.94 2.95 2.95 2.86 2.8I 2.87 2.87 2.87 2.87 2.80 - - - - - - - - - - -2.78 __- __ 2.78 -- 2.78 2.73 2.74 2.74 2.73 2.74 2.74 2.67 - - - - 2.68 - - - -2.66 - - - - 2.65 - - - -2.60 2.61 2.61 2.61 2.61 2.61 __ 2.59 2.59 - - - -2.57 -- 2.57 2.56 -- 2.57 2.50 2.52 - 2.52 2.52 2.52 --2.49 2.49 2.49 2.49 2.49 2.49 4 - - 2.45 2.41 2.42 2.42 2.42 2.42 --2.39 2.40 2.40-~ 2.39 2.40 2.40 __ __ - - -2.38 2.35 2.38 __ 2.33 ~~ 2.33 2.32 2.33 - - 2.30 - - __ __ _ - - 2.24 2.23 2.23 -- 2.20 2.21- 2.20 2.20 --- - 2.18 ~ 2.18 - - - - - -2.17 2.17 - - - -_ 20 -1OS9~2 8 TABLE II (cont.) As Made HCl NaCl CaC12 REC13 AgN03 - - 2.13 - - 2.13 - - - -- - 2.11 2.11 - - 2.11 - -- - - - - - 2.10 2.10 - --- 2. o8 2. o8 -- 2. o8 2. o8 __ _ 2.07 2.07 - - - -_ __ 2.04 .01 2.01 2.01 2.01 2.01 2.
1.99 2.00 1.99 1.99 1.99 1.99 - - - - - - 1.97 1.96 - -.95 1.95 1.95 1.95 1.95 --__ _ __ -- _ 1.94 - --- 1.92 1.92 1.92 1.92 1.92 1 9l - - - - - - 1.9l - -- - 1.88 .87 1.87 1.87 1.87 1.87 1.87 - - 1.86 - - ~
1.84 1.84 -- -- 1.84 1.84 1.83 1.83 1.83 1.83 1.83 _ .82 - - 1.81 - - 1.82 - -1.77 1.77 1.79 1.78 -- 1.77 - 1.76 1.76 1.76 1.76 1.76 1.76 1.75 1.75 _ 1.74 1.74 1.73 _ .71 1.72 1.72 - 1.71 -~ -- 1.70 1.67 1.67 1.67 -- 1.67 1.67 - 1.66 1.66 -- 1.66 1.66 1.66 _ _ 1.65 1.65 - - - _ - - - - 1.64 1.64 - - _ -- 1.6~ 1.63 1.63 1.63 1.62 -- 1.61 1.61 1.61 -- 1.6 .58 - - __ __ _ _ 1.57 1.57 -~ 1.57 1.57 -- -- 1.56 1.56 1.56 --Zeolite ZSM-5 can be suitably prepared by preparing a solution containing tetrapropyl ammonium hydroxide, sodium oxide, - an oxide of aluminum or gallium, an oxide of silica or germanium, and-water and having a composition, in terms of mole ratios of 40 oxides, ~alling within the following ranges:

.
~059928 TABLE III

Particular1y ~road Preferred Preferred OH/SiO 0.07-1.0 0.1-0.8 0.2-0.75 R4N~/(R4N + +Na~) 0.2-0.95 0.3-0.9 -4-0-9 ~ Y2/W23 5-100 10-60 10-40 `~ - wherein R is propyl, W is aluminum or gallium and Y is sillcon or germanium, maintaining the mixture until crystals of the zeolite are formed. Thereafter, the crystals are separated from the liquid and recovered. Typical reaction conditions consist - of heating the foregoing reaction mixture to a temperature of from about 90C. to 200C. for a period of time of from about six hours to 60 days. A more preferred temperature range is from about 100 to 175C. wlth the amount of time at a temperature in such range being from about 12 hours to ~ days.
~ The digestion of the gel particles is carried out until crystals form. The solid product is separated from the reaction ; medium, as by cooling the whole to room temperature, filtering, and water washing.
The foregoing product is dried, e.g. at 230F., for from about 8 to 2~ hours. Of course, milder conditions may be employed if desired, e.g., room temperature under ~acuum.
~SM-5 is preferably formed as` an aluminosilicate.
The composition can be prepared utilizing materials which supply the appropriate oxide. Such compositions include for an alumino-silicate, sodium aluminate, alumina, sodium silicate, silica hydrosol, silica gel, silicic acid, sodium hydroxide and tetra-propylammonium hydroxide. It will be understood that each oxide component utilized in the reaction mixture for preparing a member of the ZSM-5 family can be supplled by one or more initial reactants and they can be mixed together in any order. For ; example, soaium oxide can be supplied by an aqueous solution of sodium hydroxide, or by an aqueous solutlon of sodium silicate;
tetrapropylammonium cation can be supplied by the bromlde salt.
The reaction mixture can be prepared either batchwise or con-t~nuously. Crystal size and crystallization tlme of the ZSM-5 composition will vary with the nature of the reaction mixture employed. The family of ZSM-5 zeolites is disclosed and clalmed in U.S. Patent No. 3,702,886.
The zeolites used in the instant invention can have the original cations associated therewith replaced by a wide variety of other cations according to techniques well-known in the art.
Typical replacing cations would include hydrogen, ammonium and metal cations ~ncluding mixtures of the same.
Typical ion exchange techniques would be to contact the particular zeolite with a salt o~ the desired replacing cation or catlons. Although a wide variety of salts can be ; employed, particular preference is given to chlorides, nitrates . and sulfates.

. - .

.
Representative ion exchange techniques are disclosed in a wide variety o~ patents including United States 3,140,249, - United States 3,140,251 and United States 3~140,253.
Following contact with the salt solution of the desired replaclng catlon, the zeolites are then preferably washed with water and dried at a temperature ranging ~rom 150F. to about - 600F. and thereafter calcined in air or other inert gas at tem-peratures ranging from about 500F. to 1500~. for periods o~ time ranging from 1 to 48 hours or more.
Prior to use, the zeolites should be dehydrated at least partially. This can be done by heating to a temperature in the range of 200 to 600C. in an atmosphere, such as air, nitrogen, etc. and at atmospheric or subatmospheric pressures for between 1 and 48 hours. Dehydration can also be performed at - lower temperatures merely by using a vacù~m,-but a longer time 1s required to obtain a sufficient amount of dehydration.
In practicing the aprocess, it ma~ b~ desired to ....
incorporate the zeollte with another material resistant to the temperatures and other conditions employed in the separation processes. Such matrix materials include synthetic or naturally occurring substances as well as inorganic materials such as clay, silica and/or metal oxides. The latter may be elther naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides.
Naturally occurring clays which can be composited - with the zeolites~include the montmorillonite and kaolin family, which families include the sub-bentonites, and the ..

- 105~Z8 kao'ins commonly known as Dixie McNamee-Georgia and Florida cla-.s or others in which the main mineral constituent is halloy-site, kaolinite, dickite, nacrite, or anauxite. Such clays can be used in the raw state as originally mined or initially sub-~ec~ed to calcination, acid treatment or chemical modlfication.
In addition to the foregoing materials, the ZSM-5 type zeolites can be composited with a porous matrix material such as sil~ca-alumina, sillca-magnesia, silica-zirconia, silica-thoria, sil~ca-beryllia, sillca-titania as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-_ alu~ina-magnesia and sllica-magnesia-zirconia. The matrix can be ~n the~form of a cogel. The relative proportions of finely div~ded crystalline aluminosilicate ZSM-5 and inorganic oxide gel mat~ix vary widely with the crystalllne aluminosilicate content - 15 ran~ing from about 1 to about 99 p-ercent b-~ weight and more usually, particularly when the composite is prepared in the form of beads in the range of about 40 to about 90 percent by weight of the composite.
Another embodiment of thls invention resides in sub-~ecting the zeolite ZSM-5 type to a mild steam treatment carried out at ele~ated temperatures of 800F. to 1500F. and preferably at temperatures of about 1000F. to 1400F. The treatment may be ~ccomplished in an atmosphere of 100 percent steam or in atmosphere consisting of steam and a gas which is substantially inert to the aluminosilicate. The steam treatment apparently pro~ides beneficial properties in the aluminosilicate compositions ar.d can be conducted before, after or in place of the calcination treatment .

Even more highly preferred adsorbents are ZSM-5 zeolites which have been treated or contacted with a silane co~pound as superior results are achieved using these products as adsorbents. The organlc substituted silanes deemed useful in

5 5 the process of the present invention are those of the following general formula:
. R
R~;i , R
Rl wherein, in the above formula, R is an organic radical as de-scrlbed hereinafter and each Rl is also an organic radical such .,, ~
3 10 as those defined below for the group R, a hydrogen atom or a halogen atom such as chlorlne or bromine. Organic radicals which may be ~ or Rl include alkyl of 1 and more preferably up to about 40 carbon atoms, alkyl or aryl carboxylic acid acyl whereln the organic portion of said acyl group contains about 1 to 30 carbon atoms and said aryl group contains about 6 to 24 carbon atoms, aryl groups-o1~ about 6 to 24 carbons, which may also be further substituted, alkaryl and aralkyl groups contalning about 7 up to about 30 carbon atoms. Highly preferred compounds falling within the above structure are those wherein R is alkyl of about 12 to 24 ~20 c2rbon atoms, i.e., the long chained alkyl groups, and each R
is hydrogen or chlorine. Highly preferred silanes are octa-decyltrichlorosilane and dodecyltrichlorosilane. Organic silanes o~ the type useful in the process of the present invention are known in the art and may be prepared by known methods.

,.

~059928 For example, the tetrachloro substituted silane, SiC14, may be prepared by the reaction of chlorine and silica and the resulting product may then be reacted with the desired number of moles of a metal salt of the organic compound containing the radical for R or Rl desired, by heating. Other silanes employed in the process of the present inventlon may be prepared by slmilar procedures, all of which are well known in the art.
The desired silane is then contacted wlth a zeollte of the type described hereinbefore, one requirement of the zeolite belng that lt have an available hydrogen for reaction. The _ silane should be selected so that steric hindrance problems are avoided. Thus in the above formula, R and only two Rl should be organlc radlcals which means that at least one Rl should be hydrogen.
The selected silane and the crystalline alumlnoslllcate - zeollte are contacted in the preferred procedure at an elevated temperature. Preferably, the silane and zeollte ;are contacted on a welght basls of about 1:5 to 5:1, preferably about 1:2 to 1:1, respectively. It is also preferable that a blnder for the zeollte be employed such as, for example, bentonite. For good contact between the reactants~ it is also preferable to employ a reaction medlum. Satlsfactory reaction media lnclude the ethers, aliphatlc hydrocarbohs and halo-substituted aliphatic hydrocarbons of 5 to about 8 carbon atoms, (e.g., n-heptalne), the aromatlc, halo-substltuted aromatic hydrocarbons and nltrogen containlng compounds such as heterocycllcs. A particularly pre~erred medla ls pyrldine.

.

' -- lOS99Z8 The following examples are presented to illustrate the invention but it is not to be considered as limited thereto. In the examples and throughout the specification parts are by weight unless otherwise indicated.

~` Typical preparations of ZSM-5 type zeolites are shown in these examples. Examples 1-3 show the preparation of the hydrogen form ZSM-5 and they involve the use of tetrapropyl-ammonium hydroxide (TPAOH) or bromide (TPABr). Example 4 shows a typical preparation of the hydrogen ~orm ZSM-8 using tetraethyl-ammonium hydroxide (TEAOH). Reaction conditions and results are shown in Table V.

. , .. . ~ ... . . ~ . . . . . .

.
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O E-~ K
~1 0 0~0 ~ ~O ~CO
C~O ~ . . . I
Z 3 ~ 1 ~J L~O ~ ~ O~ O
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bO ~ bl~ 1 O
~0 0 0 ,~ O O O
~7 ~r) o C~
O
~ hm~10 Ooc~u~
td m ~ o . . . ~
æ 1 ~ 0 ~ .~ 0 ~ ~O o ~ o ~o ~ ~:
. a E~ z ~ O N ~ X ~ O ~1 O~ ~1 O~ Cq R ~ Q ~ ~ O
o O O'' . . - . - - O
O 3 IS~ ~D 3 N O

O
-,l cq ~ ~ S~
S
O ~D
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. . I
~ m oa~ ~ 0 o~r o o O N Lr~ ~E
~U R 'C O~ ~ O ~I K ~1 0 N O
Ph ~ ~1~ rl o N
WO E~
. -' ' ~ ~ ~ ~
¢00 ~ C~
E-~~1 ~ ., c~ ~ ~1 ~ I , .
O N
N E-~
~1 0 ~ q:~
- ¢ ~ N ~ - O ~ O~
rl . . I
Z ~1 N O Ct) h L~ O ~ ~1:) 0 J O Lr~ 0 3 15~ ~ C~J ~ X ~ O r~
I ~I O N 1 ~1 ~1 O O ~ Q~
tY~ N N S
~0 Cq , . ~1 o _~

O ~ O
~rl 1~ ^ ^ ff rl o a~ ~ S o ~ o o a~ o ~ 3 0 U~ ) ~ O
`' S C) ~ ~ ~ ~ , O . 0 a) X C) ~a ~ r-l ~1 ~ ~ O O N~d rl Ei ~ ~ O O ~O h ~ Z ~: u~ X
s ~--m ~, .
--- 29 - .

.. . .
-- ... y .~ .. ~ .. ~. ..

105992~3 In this example 30 parts of a ZSM-5 crystalline aluminosilicate zeolite of the type prepared in Examples 1-3 comprising 80 parts ZSM-5 and 20 parts bentonite binder, were refluxed with octadecyltrichlorosilane in a weight ratio of 1:1 in 200 cc normal-heptane solvent for a period of four hours.
- Thereafter the resulting solid product was recovered by decanta-~ tion, the solid washed first with chloroform, t~en with normal-pentane and then dried at a temperature of 125C. for four hours.

The aromatic mixture employed as the feedstock in this example was 100 grams of a mixture containing 12 weight percent ethylbenzene, 25 weight percent para-xylene, 45 weight percent meta-xylene, 15 weight percent ortho-xylene and 3 weight percent of Cg and higher aromatic paraffi~. Thls mixture was initially heated to 350F. and ~hen passed through parallel columns containing ZSM-5 zeolite as the adsorbent. The adsorbent was of the type prepared in Examples 1-3. A stream of steam passed over the mixture at 350F. served as the desorbent.
By use of the apparatus of Figure 1, two parallel columns were operated. By us of the three-wa~ valves the ~eed was first lntroduced into Column I at 350F. As the last of the para-xylene and ortho-xylene left the column, steam desorbent gas was_introduced. Simultaneously feed was started into Column II.
After three passes, rates o~ feed and carrier gas desorbent were adjusted so that as adsorption was completed in each column, desorption could be started and vice-versa.

--\

The components not adsorbed in the adsorption step, mostly ortho- and meta-xylene were sent to a column and distilled.
The adsorbed material eluted with the desorbent was processed to remove the desorbent and the residue, cooled to effect 5 crystallization and recovered the para-xylene.

- ~ EXAMPLES 7-13 These examples will illustrate a continuous cyclic operat~on utilizing a single column containing ZSM-5.
In these examples a mixture of 80% by weight of ZSM-5 10 and 20% by weight of bentonite were sized -30 to +60 mesh and placed in a column having an inside diameter of 0.875 inches and a length of 35 inches. The total weight of sorbent in the column was 259 grams of which 80% or 207 grams was ZSM-5.
In all cases, the conditions utilized were 300F. and 15 atmospheric pressure.
The procedure in all cases involved:
~ charging the C8 aromatic mixture at a rate of 224 cc/hr until 22cc of effluent was obtained, (2) steam was then passed through the column at 224 cc/hr 20 until 9 cc of condensed hydrocarbons were obtained, --(3) desorption was continued at a steam rate of 224 cc/hr until 75 grams of water and C8 aromatics were obtained; and (4) the bed was then purged with nitrogen to remove adsorbed water and the cycle repeated.
The results obtained as well as other operating con-ditions are set f~rth below wherein " effluent" represents the product from step 1 supra; " displacement " the product from step 2 and " adsorbate~'' the product from step 3. The designationCEB
refers to impurities lighter than ethylben7ene.

1~599;~8 .

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td O X
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Cl ~
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O ~I J ~/ ~r r-l J ~ J r ~ 3 ~I J ~I -1 3 ~ _ O U~
~1 ~1 O O N N 3 N 03 N ~'`D3 ~1 15~3 3 N ~ 3 ~I t--~) 3 N ~3 J~ 1~ .... ........ .... .... .... ....
~1~ O O ~) /J~ O N OO~ O O ~~ O O 0 ~ O O ~ CJ~ O O ~1 ~ O O N
h ~ 3 J ~ ~1 ~ 5 ~ .
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cq Y ,1 cd S
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cd ~0 ~ I . . .I . . . I . . .I . . . I . . . I . . . I . . .
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1~ ~1 0 S N ~D ~1 ~ ~O N 1~5N ~N ~O ~1 ~ ~ O ~O ~ O
~D ~D I I I I I I I -o a) Q) a~
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E h :~ r I h ~ S ~i h :~--I h ::~ ~I h :~,~1 h ~ ~I h ) ~ r-l Q. O ~ ~I Q~ O ~ ~I Q, O ~ ~1 ~ O ~ ~I Q~ O ~a ~ P. o ~ ~ P, o ¢ ~ ~ ~ ~ ~ ~ ~ ¢ ~ ~ ~ ~ ~ ~ ~ ¢ ~ ~ ~ ~:
a~
0 ~ O ~I N ~) . t~
X

,.

lOSg928 . , As can be seen from the above results, the combined ethylbenzene and p-xylene content of the effluent was less than 0.5 weight percent. The effluent can be very easily sub~ected to conventional distillation to recover m-xylene and orthoxylene.
The fract$on labeled "displacement" contained less than 1% ethylbenzene and could be fed to a low temperature xylene isomerization unit.
The adsorbate contained only about 11 weight percent of meta- and ortho-xylene combined so that p-xylene can easily be recovered by conventional crystallization techniques with subse-quent recovery of ethylbenzene by fractionation.
In describing the process of this invention, the word ~'~dsorbed" has been used in a relative sense. Thus in the speclfication and claims, the terms " adsorbed'' and "not adsorbed "
should be underst~od to mean " preferenti~lly- adsorbedt' and - " preferentially not adsorbed'' since such adsorptions in chromato-graphic system~ such as thls do not always occ~r to the extent of absolutely complete adsorption.
The invention has been described herein with reference to certain preferred embodiments, however, as obvious variations thereon will become apparent to those skilled in the art the invention is not to be considered as limlted thereto.

. .

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for the separation and recovery of aromatic isomers contained in an aromatic mixture com-prising para-xylene, meta-xylene, ortho-xylene and ethyl-benzene to separate this mixture into one stream comprising meta-xylene and ortho-xylene and a second stream comprising para-xylene and ethylbenzene, by a process which includes contacting said mixture with a ZSM-5 zeolite adsorbent whereby para-xylene and ethylbenzene are adsorbed and meta-xylene and ortho-xylene are not adsorbed, the improve-ment which comprises contacting said mixture with said ZSM-5 zeolite adsorbent maintained in at least two separate reactors disposed in parallel relationship whereby when adsorption is complete in one reactor and the ortho-xylene and meta-xylene removed, desorption is started while sim-ultaneously beginning adsorption in the second reactor and desorbing the second reactor after adsorption while simultaneously beginning adsorption in the first reactor, recovering the para-xylene and ethylbenzene from the de_ sorption steps and the meta-xylene and ortho-xylene from the adsorption steps.

2. A method according to claim 1 wherein said starting aromatic mixture contains about 15-40 weight percent of para-xylene, about 0-15 weight percent ethylbenzene, about 0-25 weight percent ortho-xylene and about 40-60 weight percent meta-xylene.

3. A method according to claim 2 wherein said first adsorption stage is conducted in the presence of a ZSM-5 zeolite adsorber at a temperature of about 50° to 500°F.

4. A method according to claim 3 wherein said para-xylene and ethylbenzene are subjected to crystallization for recovery of para-xylene and ethylbenzene.

5. A method according to claim 3 wherein the mixture of ortho- and meta-xylene is distilled to recover the components.

6. A method according to claim 1 wherein desorption is effected by (1) a reduction in the partial pressure of the sorbed material in the vapor surrounding the adsorbent;
(2) a lowering of the total pressure of the system; (3) purging with an inert gas; (4) heating of the adsorbent, or (5) any combination thereof.

7. In a continuous method for the separation and recovery of aromatic isomers contained in an aromatic mixture com-prising para-xylene, meta-xylene, ortho-xylene and ethyl-benzene to separate this mixture into one stream comprising meta-xylene and ortho-xylene and a second stream comprising para-xylene and ethylbenzene, by a continuous process including contacting said mixture with a ZSM-5 zeolite adsorbent whereby para-xylene and ethylbenzene are adsorbed and meta-xylene and ortho-xylene are not adsorbed, the continuous process which comprises contacting said mixture with ZSM-5 adsorbent maintained in at least two separate columns disposed in parallel relationship whereby when adsorption is complete in one reactor and the ortho-xylene and meta-xylene removed, desorption is started while simultaneously beginning adsorption in the second reactor and desorbing the second reactor after adsorption while simultaneously beginning adsorption in the first reactor, continuously conducting said adsorption/desorption steps, continuously recovering the para-xylene and ethylbenzene from the desorption steps and continuously recovering the meta-xylene and ortho-xylene from the adsorption steps.

8. A method according to claim 7 wherein said starting aromatic mixture contains about 15-40 weight percent of para-xylene, about 0-15 weight percent ethylbenzene, about 0-25 weight percent ortho-xylene and about 40-60 weight percent meta-xylene.

9. A method according to claim 8 wherein said first adsorption stage is conducted in the presence of a ZSM-5 zeolite adsorber at a temperature of about 50°F. to 500°F.

10. A method according to claim g wherein said para-xylene and ethylbenzene are subjected to crystallization for recovery of para-xylene and ethylbenzene.

11. A method according to claim 10 wherein the mixture of ortho- and meta-xylene is distilled to recover the components.

12. A method according to claim 7 wherein desorption is effected by (1) a reduction in the partial pressure of the sorbed material in the vapor surrounding the adsorbent;
(2) a lowering of the total pressure of the system; (3) purging with an inert gas; (4) heating of the adsorbent, or (5) any combination thereof.

13. In a continuous method for the separation and recovery of aromatic isomers contained in an aromatic feed mixture comprising para-xylene, meta-xylene, ortho-xylene and ethylbenzene by a continuous process of contacting said mixture with ZSM-5 zeolite adsorbent whereby para-xylene and ethylbenzene are adsorbed and meta-xylene and ortho-xylene are not adsorbed, the steps, comprising contacting said mixture with a ZSM-5 adsorbent maintained in at least two separate columns disposed in parallel relation-ship whereby when adsorption is complete in one column and the ortho-xylene and meta-xylene removed, desorption is started while simultaneously beginning adsorption in the second column and desorbing in the second column after adsorption, while simultaneously beginning adsorp-tion in the first column, continuously conducting said adsorption/desorption steps, continuously removing the un-adsorbed mixture of meta-xylene and orthoxylene and passing to a distillation station, distilling off the major portion of meta-xylene, passing the residue from the distillation to a column for removal of ortho-xylene and withdrawing the residue; passing the ortho-xylene to an isomerization reactor and isomerizing to form additional meta-xylene and paraxylene and recycling to the aromatic feed; simultan-eously removing the desorbed material from the first and second column and passing to desorbent recovery, recovering the desorbent and recycling to the adsorption/desorption steps, passing the remaining mixture to a crystallization station for separating the para-xylene and providing an ethylbenzene residue, recycling the ethylbenzene residue to mix with the aromatic feed, distilling the resultant mix-ture to recover ethylbenzene and introducing the resultant residue as at least a portion of the aromatic feed.

14. A continuous method according to claim 13 wherein said starting aromatic mixture contains about 16-40 weight percent of para-xylene, about 0-15 weight percent ethyl-benzene, about 0-25 weight percent ortho-xylene and about 40-60 weight percent meta-xylene.

15. A continuous method according to claim 14 wherein said adsorption stages are conducted in the presence of a ZSM-5 zeolite adsorber at a temperature of about 50° to about 500°F.

16. A continuous method according to claim 17 wherein desorption is effected by (1) a reduction in the partial pressure of the sorbed material in the vapor surrounding the adsorbent; (2) a lowering of the total pressure of the system; (3) purging with an inert gas; (4) heating of the adsorbent, or (5) any combination thereof.