Process for the separation of ethylbenzene

A process for separating ethylbenzene from a feed mixture comprising ethylbenzene and at least one xylene isomer. The mixture is contacted with an adsorbent comprising a barium cation exchanged type-X zeolite containing from about 3.0 to about 6.0 wt. % H.sub.2 O. The unadsorbed portion of the feed mixture is removed from the adsorbent and the ethylbenzene recovered by desorption with benzene.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The field of art to which the claimed invention pertains is solid-bed 
adsorptive separation. More specifically, the claimed invention relates to 
a process for the separation of ethylbenzene from a feed mixture 
comprising ethylbenzene and at least one xylene isomer which process 
employs a solid adsorbent and desorbent combination which effects 
selective removal of ethylbenzene from the feed mixture. 
2. Background Information 
It is well known in the separation art that certain crystalline 
aluminosilicates can be used to separate hydrocarbons species from 
mixtures thereof. In particular, the separation of normal paraffins from 
branched chained paraffins can be accomplished by using the type A zeolite 
which have pore openings from 3 to about 5 Angstroms. Such a separation 
process is disclosed for example in U.S. Pat. Nos. 2,985,589 and 
3,201,491. These adsorbents allow a separation based on the physical size 
differences in the molecules by allowing the smaller or normal 
hydrocarbons to be passed into the cavities within the crystalline 
aluminosilicate adsorbent, while excluding the larger or branched chain 
molecules. 
U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in 
which larger pore diameter zeolites such as the type X or type Y 
structured zeolites can be used to separate olefinic hydrocarbons. 
In addition to separating hydrocarbon types, the type X or type Y zeolites 
have also been employed in processes to separate para-xylene from a feed 
mixture comprising para-xylene and at least one other xylene isomer by 
selectively adsorbing para-xylene over the other xylene isomers. Such 
processes are disclosed in U.S. Pat. Nos. 3,732,325; 3,997,620; 4,029,717; 
4,031,155; and 4,255,607. These references disclose various adsorbent 
(including Ba and K exchanged X-type zeolites) and desorbent combinations 
which effect selectivity for xylenes over ethylbenzene, but none teach 
benzene as a desorbent material, optimum water content of the adsorbent, 
nor, of course, ethylbenzene as the extract component. 
Ethylbenzene is used as a raw material in the production of styrene 
monomer. Ethylbenzene can be and is commercially produced from the 
alkylation of benzene with ethylene. The cost of and competing demands for 
necessary benzene and ethylene feed streams have, however, prompted new 
efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams 
which already contain ethylbenzene. Such feed streams for instance, 
include C.sub.8 aromatic extracts produced by a typical solvent extraction 
process from a pyrolysis gasoline or from a naphtha which has been 
reformed with a platinum-halogen-containing catalyst. Additionally C.sub.8 
aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by 
fractionation without solvent extraction contain varying amounts of 
ethylbenzene. 
Ethylbenzene can, of course, be separated from the xylene isomers by 
fractionation but because its boiling point is within about 4.degree. F. 
of that of para-xylene, the fractionation can be achieved only with the 
more intricate super-fractionators. Typical ethylbenzene fractionators 
contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to 
feed ratio. The process of our invention therefore offers a competitive 
alternative to the separation of ethylbenzene by super-fractionation. 
U.S. Pat. No. 3,943,182 discloses a process for extracting ethylbenzene 
from admixture with xylenes which employs a type X zeolite having only 
Group I-A ions at exchangeable cationic sites, and which contains about 
0.02 to 2.5 wt. % water. Benzene is mentioned as a possible but not 
preferred desorbent. 
The present invention is based on the surprising discovery that, in 
contradistinction to much of the teaching in the art, an X type zeolite 
adsorbent having Ba exchanged cations is selective for ethylbenzene when 
the adsorbent contains an amount of water within a specified range and 
when benzene is employed as the desorbent. Furthermore, the adsorbent 
having such composition is superior to the adsorbent of U.S. Pat. No. 
3,943,182 for the separation of ethylbenzene from admixture with xylenes. 
SUMMARY OF THE INVENTION 
It is, accordingly, a broad objective of our invention to provide a process 
for the separation of ethylbenzene from a feed mixture comprising 
ethylbenzene and at least one xylene isomer. 
In brief summary, our invention is, in one embodiment, a process for 
separating ethylbenzene from a feed mixture comprising ethylbenzene and at 
least one xylene isomer which process comprises contacting said mixture 
with an adsorbent comprising a type X structured zeolite containing barium 
cations at exchangeable cationic sites. The adsorbent also contains from 
about 3.0 to 6.0 wt. % H.sub.2 O measured by loss on ignition at 
350.degree. C. The contacting is effected at ethylbenzene adsorption 
conditions. The unadsorbed portion of the feed mixture is removed from the 
adsorbent and the adsorbed ethylbenzene is recovered from the adsorbent by 
contacting the adsorbent with benzene at desorption conditions. 
Other embodiments and objects of the present invention encompass details 
about feed mixtrues, adsorbents, desorbents, and operating conditions all 
of which are hereinafter disclosed in the following discussion of each of 
these facets of the present invention. 
DESCRIPTION OF THE INVENTION 
Feed mixtures which can be utilized in the process of this invention will 
comprise ethylbenzene and at least one xylene isomer. Specifically, the 
feed mixture may contain ethylbenzene and para-xylene or meta-xylene or 
ortho-xylene. Possible feed mixtures can as well contain, in addition to 
ethylbenzene, any two xylene isomers or all three xylene isomers. The more 
typical feed mixtures will either be a mixture containing ethylbenzene and 
all three of the xylene isomers or a mixture containing ethylbenzene along 
with para-xylene and meta-xylene. Ortho-xylene which has a boiling point 
of about 6.degree. F. higher than that of the nearest other C.sub.8 
aromatic (metal-xylene) can be separated by conventional fractionation 
techniques and hence may be previously removed from a feed mixture prior 
to its being charged to the process of this invention. Ortho-xylene 
fractionator towers for example will contain about 100 to 150 actual trays 
and will operate with about a 5-8 to 1 reflux to feed ratio. 
Mixtures containing substantial quantities of ethylbenzene and xylene 
isomers generally are produced by reforming and isomerization processes. 
In reforming processes, a naphtha feed is contacted with a 
platinum-halogen-containing catalyst at severities selected to produce an 
effluent containing C.sub.8 aromatic compounds which can be subsequently 
separated using the method of this invention. Xylene isomerization 
processes generally isomerize a xylene mixture deficient in one or more 
isomers to give an effluent containing approximately equilibrium 
quantities of the C.sub.8 aromatic isomers which effluent can then be 
separated using the method of this invention. Generally the C.sub.8 
aromatics in such effluent stream will be concentrated by solvent 
extraction processes or by fractionation prior to being introduced into 
the process of this invention. 
The equilibrium compositions of the xylene isomers and ethylbenzene at 
various temperatures are shown in Table 1 below. 
TABLE 1 
______________________________________ 
Equilibrium C.sub.8 Aromatic Compositions* 
______________________________________ 
Temperature, .degree.C. 
327 427 527 
Mole percent of isomers 
Ethylbenzene 6 8 11 
Para-Xylene 22 22 21 
Meta-Xylene 50 48 45 
Ortho-Xylene 22 22 23 
______________________________________ 
*Based on API sources 
Since the ethylbenzene boils at about the same temperature as the 
meta-xylene and para-xylene isomers, fractionation methods are impractical 
for separating the ethylbenzene from meta- and para-xylene. 
Feed mixtures may also contain small quantities of nonaromatics such as 
straight or branched chain paraffins, cycloparaffins, or olefinic 
materials. However, since separation of ethylbenzene from a feed mixture 
by selective adsorption of the ethylbenzene present in the feed mixture on 
a zeolite adsorbent apparently takes place because of a rather delicate 
acidity/basicity difference between ethylbenzene and the adsorbent 
compared to the xylene isomers and the adsorbent, it is preferred that 
these contaminants, especially olefins, be less than about 20 vol. % of 
the feed mixture passed into the process and more preferably less than 
about 10 vol. %. 
To separate ethylbenzene from a feed mixture containing ethylbenzene and at 
least one xylene isomer, the mixture is contacted with the adsorbent and 
ethylbenzene is more selectively adsorbed and retained by the adsorbent 
while the less selectively adsorbed xylene isomer is removed from the 
interstitial void spaces between the particles of adsorbent and the 
surface of the adsorbent. The adsorbent containing the more selectively 
adsorbed ethylbenzene is referred to as a "rich" adsorbent--rich in the 
more selectively adsorbed ethylbenzene. 
The more selectively adsorbed feed component is commonly referred to as the 
extract component of the feed mixture, while the less selectively adsorbed 
component is referred to as the raffinate component. Fluid streams leaving 
the adsorbent comprising an extract component and comprising a raffinate 
component are referred to, respectively, as the extract stream and the 
raffinate stream. As previously mentioned, the feed mixture can obtain 
more than one xylene isomer and it will therefore be recognized that all 
of the xylene isomers present in the feed mixture will be less selectively 
adsorbed with respect to ethylbenzene. Thus the raffinate stream will 
contain as raffinate components all of the xylene isomers appearing in the 
feed mixture and the extract stream will contain ethylbenzene as the 
extract component. 
Although it is possible by the process of this invention to produce high 
purity (98% or greater, expressed as a percent of C.sub.8 aromatics 
present) ethylbenzene at high recoveries, it will be appreciated that an 
extract component is never completely adsorbed by the adsorbent, nor is a 
raffinate component completely non-adsorbed by the adsorbent. Therefore, 
small amounts of a raffinate component can appear in the extract stream 
and, likewise, small amounts of an extract component can appear in the 
raffinate stream. The extract and raffinate streams then are further 
distinguished from each other and from the feed mixture by the ratio of 
the C.sub.8 aromatic isomers appearing in the particular stream. More 
specifically, the ratio of the more selectively adsorbed ethylbenzene to 
the less selectively adsorbed xylene isomer will be highest in the extract 
stream, next highest in the feed mixture and lowest in the raffinate 
stream. Likewise, the ratio of the less selectively adsorbed xylene isomer 
to the more selectively adsorbed ethylbenzene will be highest in the 
raffinate stream, next highest in the feed mixture and lowest in the 
extract stream. 
The adsorbent can be contained in one or more chambers where through 
programmed flow into and out of the chamber separation of the isomers is 
effected. The adsorbent may be contacted with a desorbent material which 
is capable of displacing the adsorbed extract component from the 
adsorbent. Alternatively, the adsorbed extract component could be removed 
from the adsorbent by purging or by increasing the temperature of the 
adsorbent or by decreasing the pressure of the chamber or vessel 
containing the adsorbent or by a combination of these means. 
The adsorbent may be employed in the form of a dense compact fixed bed 
which is alternately contacted with the feed mixture and a desorbent 
material (hereinafter described). In the simplest embodiment of the 
invention the adsorbent is employed in the form of a single static bed in 
which case the process is only semi-continuous. A set of two or more 
static beds may be employed in fixed-bed contacting with appropriate 
valving so that the feed mixture is passed through one or more adsorbent 
beds while the desorbent material is passed through one or more of the 
other beds in the set. The flow of feed mixture and desorbent material may 
be either up or down through the adsorbent. Any of the conventional 
apparatus employed in static bed fluid-solid contacting may be used. 
Moving bed or simulated moving bed systems, however, have a much greater 
separation efficiency than fixed adsorbent bed systems and are therefore 
preferred. 
Specifically, one preferred processing flow scheme which can be utilized to 
effect the process of this invention are those known in the art as 
simulated moving-bed countercurrent systems. One such system includes the 
flow scheme described in U.S. Pat. No. 2,985,589 issued to D. B. 
Broughton, incorporated herein by reference. This patent generally 
describes the processing sequence involved in a particular simulated 
moving-bed countercurrent solid-fluid contacting process. 
Another embodiment of a simulated moving bed flow system suitable for use 
in the process of the present invention is the co-current high efficiency 
simulated moving bed process disclosed in our assignee's U.S. Pat. No. 
4,402,832, incorporated by reference herein in its entirety. 
Adsorption and desorption in the process of our invention could be 
conducted both in the vapor phase or liquid phase or one operation may be 
conducted in the vapor phase and the other in the liquid phase. Operating 
pressures and temperatures for adsorption and desorption might be the same 
or different. 
Preferred operating conditions for both adsorption and desorption include a 
temperature within the range of from about 20.degree. to about 200.degree. 
C. and a pressure selected to maintain liquid phase throughout the bed of 
adsorbent. 
The desorbent materials which can be used in the various processing schemes 
employing a given adsorbent will vary depending on the type of operation 
employed. The term "desorbent material" as used herein means any fluid 
substance capable of removing a selectively adsorbed isomer from the 
adsorbent. In the swingbed system in which the selectively adsorbed isomer 
is removed from the adsorbent by a purge stream, gaseous hydrocarbons such 
as methane, ethane, etc. or other types of gases such as nitrogen or 
hydrogen may be used at elevated temperatures or reduced temperatures or 
reduced pressure or both to effectively purge the adsorbed isomer from the 
adsorbent. 
However, in processes which are generally operated at substantially 
constant pressures and temperatures to insure liquid phase, the desorbent 
material relied upon must be judiciously selected in order that it may 
displace the adsorbed isomer from the adsorbent with reasonable mass flow 
rates and also without unduly preventing the adsorbed isomer from 
displacing the desorbent in a following adsorption cycle. 
Desorbent materials should additionally be substances which are easily 
separable from the feed mixture that is passed into the process. In 
desorbing the preferentially adsorbed component of the feed, both 
desorbent and the extract component are removed from adsorbent in 
admixture. Without a method of separation of these two materials, the 
purity of the extract component of the feed stock would not be very high 
since it would be diluted with desorbent. It is contemplated that any 
desorbent material used will have a substantially different average 
boiling point than that of the feed mixture. More specifically, 
"substantially different" shall mean that the difference between the 
average boiling points shall be at least 20.degree. F. The boiling range 
of the desorbent material could be higher or lower than that of the feed 
mixture. The use of a desorbent material having a substantially different 
average boiling point than that of the feed allows separation of desorbent 
material from feed components in the extract and raffinate streams by 
simple fractionation or other methods thereby permitting reuse of 
desorbent material in the process. 
In the preferred isothermal, isobaric, liquid-phase embodiment of the 
process of our invention, proper selection of a desorbent material is 
critical to the successful operation of the process. Adsorptive 
selectivity of the particular adsorbent employed in our process for 
ethylbenzene with respect to xylene isomers as well as reasonable mass 
flow rates appears only when certain apparently unique desorbent materials 
are employed. 
We have discovered a surprising effectiveness of benzene for a desorbent 
material in uniquely satisfying all of the above criteria when used with a 
barium cation exchanged X-type zeolite adsorbent discussed in greater 
detail hereinbelow. 
One can appreciate that certain characteristics of adsorbents are highly 
desirable, if not absolutely necessary, to the successful operation of a 
selective adsorption process. Among such characteristics are: adsorption 
capacity for some volume of an extract component per volume of adsorbent; 
the selective adsorption of an extract component with respect to a 
raffinate component and the desorbent; and sufficiently fast rates of 
adsorption and desorption of the extract component to and from the 
adsorbent. 
Capacity of the adsorbent for adsorbing a specific volume of an extract 
component (ethylbenzene in the process of our invention) is of course, a 
necessity; without such capacity the adsorbent is useless for adsorptive 
separation. Furthermore, the higher the adsorbent's capacity for an 
extract component the better is the adsorbent. Increased capacity of a 
particular adsorbent makes it possible to reduce the amount of adsorbent 
needed to separate the extract component contained in a particular charge 
rate of feed mixture. A reduction in the amount of adsorbent required for 
a specific adsorptive separation reduces the cost of the separation 
process. It is important that the good initial capacity of the adsorbent 
be maintained during actual use in the separation process over some 
economically desirable life. 
The second necessary adsorbent characteristic is the ability of the 
adsorbent to separate components of the feed or, in other words, that the 
adsorbent possess adsorptive selectivity, (B), for one component as 
compared to another component. Selectivity can be expressed not only for 
one feed as compared to another but can also be expressed between any feed 
mixture component and the desorbent. The selectivity, (B), as used 
throughout this specification is defined as the ratio of the two 
components of the adsorbed phase over the ratio of the same two components 
in the unadsorbed phase at equilibrium conditions. 
Selectivity is shown as equation 1 below: 
##EQU1## 
where C and D are two components of the feed represented in volume percent 
and the subscripts A and U represent the adsorbed and unadsorbed phases 
respectively. The equilibrium conditions as defined here were determined 
when the feed passing over a bed of adsorbent did not change composition 
after contacting the bed adsorbent. In other words, there was no net 
transfer of material occurring between the unadsorbed and adsorbed phases. 
As can be seen where the selectivity of two components approaches 1.0 there 
is no preferential adsorption of one component by the adsorbent. As the 
(B) becomes less than or greater than 1.0 there is a preferential 
selectivity by the adsorbent of one component. When comparing the 
selectivity of the adsorbent of one component C over component D, A (B) 
larger than 1.0 indicates preferential adsorption of component C within 
the adsorbent. A (B) less than 1.0 would indicate that component D is 
preferentially adsorbed leaving an unadsorbed phase richer in component C 
and an adsorbed phase richer in component D. The preferred selectivity for 
an extract component is about 2.0. Desorbents ideally would have a 
selectivity equal to about 1 or slightly less than 1. 
The third important characteristic is the rate of exchange of the adsorbed 
isomer with the desorbent or, in other words, the relative rate of 
desorption of the adsorbed isomer. This characteristic relates directly to 
the amount of desorbent that must be employed in the process to recover 
the adsorbed isomer from the adsorbent. 
In order to test various adsorbents to measure the characteristics of 
adsorptive capacity and selectivity, a dynamic testing apparatus may be 
employed. The apparatus consists of an adsorbent chamber of approximately 
70 cc volume having inlet and outlet portions at opposite ends of the 
chamber. The chamber is contained within a temperature control means and, 
in addition, pressure control equipment is used to operate the chamber at 
a constant predetermined pressure. Chromatographic analysis equipment can 
be attached to the outlet line of the chamber and used to analyze "on 
stream" the effluent stream leaving the adsorbent chamber. 
A pulse test, performed using this apparatus and the following general 
procedure, is used to determine selectivities and other data for various 
adsorbent systems. The adsorbent is filled to equilibrium with a 
particular desorbent by passing the desorbent material through the 
adsorbent chamber. At a convenient time, a pulse of feed containing known 
concentrations of a nonadsorbed parafinnic tracer (n-nonane for instance) 
and of the particular C.sub.8 aromatic isomers all diluted in desorbent is 
injected for a duration of several minutes. Desorbent flow is resumed, and 
the tracer and the aromatic isomers are eluted as in a liquid-solid 
chromatographic operation. The effluent can be analyzed by onstream 
chromatographic equipment and traces of the envelopes of corresponding 
component peaks developed. Alternatively, effluent samples can be 
collected periodically and later analyzed separately by gas 
chromatography. 
From information derived from the chromatographic traces, adsorbent 
performance can be rated in terms of capacity index for an extract 
component, selectivity for one C.sub.8 aromatic isomer with respect to the 
other, and the rate of desorption of extract component by the desorbent. 
The capacity index may be characterized by the distance between the center 
of the peak envelope of the selectively adsorbed C.sub.8 aromatic isomer 
and the peak envelope of the tracer component or some other known 
reference point. It is expressed in terms of the volume in cubic 
centimeters of desorbent pumped during this time interval. Selectivity, 
(B), for the non-adsorbed isomer with respect to the adsorbed isometer may 
be characterized by the ratio of the distance between the center of the 
non-adsorbed isomer peak envelope and the tracer peak envelope (or other 
reference point) to the corresponding distance for the other (adsorbed) 
isomer. The rate of exchange of an adsorbed isomer with the desorbent can 
generally be characterized by the width of the peak envelopes at half 
intensity. The narrower the peak width the faster the desorption rate. The 
desorption rate can also be characterized by the distance between the 
center of the tracer peak envelope and the disappearance of a selectively 
adsorbed isomer which has just been desorbed. This distance is again the 
volume of desorbent pumped during this time interval. 
To translate this type of data into a practical separation process requires 
actual testing of the best system in a continuous simulated moving bed 
liquid-solid contacting device. The general operating principles of a 
countercurrent type of such a device have been previously described and 
are found in Broughton U.S. Pat. No. 2,985,589. A specific laboratory-size 
apparatus utilizing these principles is described in deRosset et al U.S. 
Pat. No. 3,706,812. The equipment comprises multiple adsorbent beds with a 
number of access lines attached to distributors within the beds and 
terminating at a rotary distributing valve. At a given valve position, 
feed and desorbent are being introduced through two of the lines and 
raffinate and extract are being withdrawn through two more. All remaining 
access lines are inactive and when the position of the distributing valve 
is advanced by one index all active positions will be advanced by one bed. 
This simulates a condition in which the adsorbent physically moves in a 
direction countercurrent to the liquid flow. Additional details on the 
above-mentioned adsorbent testing apparatus and adsorbent evaluation 
techniques may be found in the paper "Separation of C.sub.8 Aromatics by 
Adsorption" by A. J. deRosset, R. W. Neuzil, D. J. Korous and D. H. 
Rosback presented at the American Chemical Society, Los Angeles, Calif., 
Mar. 28 through Apr. 2, 1971. 
Adsorbents used in the process of this invention are generally referred to 
as the crystalline aluminosilicates or molecular sieves and can comprise 
both the natural and synthetic aluminosilicates. Particular crystalline 
aluminosilicates encompassed by the present invention include crystalline 
aluminosilicate cage structures in which the alumina and silica tetrahedra 
are intimately connected in an open three dimensional network. The 
tetrahedra are cross-linked by the sharing of oxygen atoms with spaces 
between the tetrahedra occupied by water molecules prior to partial or 
total dehydration of this zeolite. The dehydration of the zeolite results 
in crystals interlaced with cells having molecular dimensions. Thus, the 
crystalline aluminosilicates are often referred to as molecular sieves 
when the separation which they effect is dependent essentially upon 
distinction between molecule sizes as, for instance, when normal paraffins 
are separated from isoparaffins by using a particular crystalline 
aluminosilicate. In the process of this invention, however, the term 
molecular sieves is not strictly suitable since the separation of specific 
C.sub.8 aromatic isomers is dependent on differences in the 
electrochemical attraction of different isomer configurations rather than 
pure physical size differences of the isomer molecules. 
In hydrated form, the crystalline aluminosilicates generally encompass 
those zeolites represented by the Formula 1 below: 
FORMULA 1 
EQU M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O 
where M is a cation which balances the electrovalence of the tetrahedra and 
is generally referred to as an exchangeable cationic site, n represents 
the valence of the cation, w represents the moles of SiO.sub.2 and y 
represents the moles of water. The cations may be any one of a number of 
cations which will be hereinafter described in detail. 
The type X structured and type Y structured zeolites as discussed in this 
specification shall include crystalline aluminosilicates having a three 
dimensional interconnected cage structure and can specifically be defined 
by U.S. Pat. Nos. 2,882,244 and 3,130,007. The terms "type X structured" 
and "type Y structured" zeolites as used herein shall include all zeolites 
which have a general structure as represented in the above two cited 
patents and additionally shall specifically include those crystalline 
aluminosilicates produced from either of the zeolites described in U.S. 
Pat. Nos. 2,882,244 and 3,130,007 as starting materials by various 
exchange techniques or thermal treatments or combinations thereof to in 
any way modify the properties (such as pore diameter of cell size) of the 
type X or type Y zeolites starting material. In the most limiting sense 
only these terms refer to zeolite X and zeolite Y. 
The type X structured zeolites can be represented in terms of mole oxides 
as represented in Formula 2 below: 
FORMULA 2 
EQU (0.9.+-.0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5.+-.0.5)SiO.sub.2 :yH.sub.2 O 
where M represents at least one cation having a valence of not more than 3, 
n represents the valence of M and Y is a value up to about 9 depending 
upon the identity of M and the degree of hydration of the crystalline 
structure. 
The type Y structured zeolite can be represented in terms of the mole 
oxides for the sodium form as represented by Formula 3 below: 
FORMULA 3 
EQU (0.9+0.2)Na.sub.2 O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 O 
where w is a value of greater than about 3 up to 8, and y may be any value 
up to about 9. 
The term "exchangeable cationic sites" for the type X and type Y zeolites 
generally refers to the sites occupied by sodium cations present in the 
type X and type Y zeolites as indicated in Formula 2 and Formula 3 above 
and which can be replaced or exchanged with other cations to modify the 
properties of these zeolites. 
Cationic or base exchange methods are generally known to those familiar 
with the field of crystalline aluminosilicate production. They are 
generally performed by contacting the sodium form of the zeolite with an 
aqueous solution of the soluble salt of the cation or cations desired to 
be placed upon the zeolite. After the desired degree of exchange takes 
place the sieves are removed from the aqueous solution, washed and dried 
to a desired water content. It is contemplated that cation exchange 
operations may take place by using individual solutions of the desired 
cations to be placed on the zeolite or by using an exchange solution 
containing a mixture of cations, where two or more desired cations are to 
be placed on the zeolite. 
When singular cations are exchanged upon a zeolite the singular cations can 
comprise anywhere from 5 up to 75 wt. % on a relative volatile free basis 
of the zeolite depending upon the molecular weight of the material 
exchanged upon the zeolite. It is contemplated that when single ions are 
placed upon the zeolite that they may be on the zeolite in concentrations 
of from about 1% to about 100% of the original cations present (generally 
sodium) upon the zeolite prior to its being ion-exchanged. By knowing the 
empirical formula including the silica to alumina ratio of the zeolite 
used, its water content, and the percentage of binder used if any, it is 
possible to calculate the percentage of ion exchange that has taken place. 
When two or more cations are placed upon the zeolite there are two 
parameters in which one can operate in order to effectively produce a 
zeolite having the maximum selective properties. One of the parameters is 
the extent of the zeolite ion exchange which is determined by the length 
of time, temperature and cation concentration. The other parameter is the 
ratio of individual cations placed on the zeolite. In instances in which 
two cations are exchanged upon the zeolite the weight ratio of these two 
respective components upon the zeolite can vary anywhere from about less 
than one up to about one hundred depending upon the molecular weight of 
the cations. 
We have discovered the surprising and unexpected superiority of barium 
cation exchanged type X structured zeolite as an adsorbent for the process 
of the present invention when used in combination with benzene as a 
desorbent material. Preferably the zeolite will also contain potassium 
cations at exchangeable cation sites to achieve optimum rates of the 
transfer of benzene for the various components in the feed stream. 
In the process of this invention we have additionally found a particular 
amount of water present on the zeolite adsorbent, as measured by loss on 
ignition at a certain temperature which is critical to the performance of 
the adsorbent. The amount of water present on the adsorbent is critical 
because too much water can decrease adsorptive capacities. On the other 
hand if the adsorbent is excessively dry ethylbenzene selectivity is 
decreased and the transfer rates are too slow. In this specification, the 
volatile matter (water) content of the zeolitic adsorbent is determined by 
first weighing the adsorbent and thereafter contacting the adsorbent in a 
high temperature furnace at a temperature of from about 350.degree. C. to 
about 500.degree. C. under an inert purge gas stream such as nitrogen for 
a period of time sufficient to achieve a constant weight. The sample is 
then cooled under an inert atmosphere and weighed to determine the 
difference in weight between the adsorbent before it was passed into the 
oven and afterwards. The difference in weight is calculated as a loss on 
ignition (LOI) and represents the volatile matter present on or within the 
adsorbent. 
The exact mechanics by which water changes the adsorbent's selectivity for 
ethylbenzene with respect to the xylene isomers is not fully understood, 
but it is thought that in some way it increases the acidity of the 
adsorbent. Adsorbent water content is therefore an important process 
variable especially in continuous processing where the tendency might be 
for the adsorbent to pick up or lose water via the feed mixture and 
desorbent material with time. The water content of the adsorbent can be 
from about 3.0 to about 6.0 wt. % water as measured by loss on ignition at 
350.degree. C. without destroying ethylbenzene selectivity.

EXAMPLE 1 
The following example is presented to illustrate, via the above discussed 
pulse test apparatus, the ability of the process of our invention to 
effect separation of ethylbenzene from all three xylene isomers. This 
example presents results of six pulse tests all using a barium cation 
exchanged X-type zeolite and benzene desorbent. In all but the first test 
the water content of the adsorbent was in the range of from about 3.0 to 
about 6.0 wt. %. Three of the six adsorbents tested also contained 
potassium cations, as preferred. 
The testing apparatus was an adsorbent chamber containing approximately 70 
cc of each adsorbent and was contained within a heat-control means in 
order to maintain essentially isothermal operations through the column. 
For each pulse test the column was maintained at a temperature of 
150.degree. C. and a pressure of 60 psig to maintain liquid-phase 
operations. Gas chromatographic analysis equipment was attached to the 
column effluent stream in order to determine the composition of the 
effluent material at given time intervals. The feed mixture employed for 
each test contained 5 vol. % ethylbenzene, 5 vol. % para-xylene, 5 vol. % 
meta-xylene, 5 vol. % ortho-xylene, 5 vol. % n-nonane which was used as a 
tracer and 75 vol. % desorbent material. The desorbent material employed 
was a mixture of 30 vol. % benzene and 70 vol. % normal heptane. 
The operations taking place were as follows. The desorbent was run 
continuously at a nominal liquid hourly space velocity (LHSV) of 1.0 which 
amounted to about 1.17 cc per minute feed rate of desorbent. At some 
convenient time interval the desorbent is stopped and the feed is run for 
a ten-minute interval at 1 LHSV. The desorbent stream is then resumed at 1 
LHSV and continued to pass into the adsorbent column until all the feed 
C.sub.8 aromatics have been eluted from the column by observing the 
chromatograph generated by the effluent material leaving the adsorption 
column. The sequence of operations usually takes about an hour. The 10 
minute pulse of feed and subsequent desorption may be repeated in sequence 
as often as is desired. 
From information derived from the chromatographic traces selectivities of 
the adsorbents for ethylbenzene with respect to the xylene isomers (E/P, 
E/M and E/O) were calculated for each pulse test. Peak envelope widths and 
retention volumes were also measured. The results for the six pulse tests, 
1 through 6, are shown in Table 1 below. 
TABLE 1 
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Exchanged 
22 wt. % BaO + 
Cation 6.5 wt. % K.sub.2 O 30 wt. % BaO 
Wt. # H.sub.2 O 
Dry 3.0 4.0 4.5 5.0 6.0 
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Widths, cc 
n-C.sub.9 
8.3 8.5 8.7 8.7 9.7 10.7 
EB 15.6 17.6 14.5 11.4 13.6 17.7 
p-X 16.6 13.3 13.1 14.4 19.5 18.9 
m-X 15.4 12.7 13.7 14.2 18.1 20.4 
o-X 17.0 12.4 12.9 13.3 17.2 21.1 
Ret. Vols., cc 
EB 27.7 31.4 24.3 18.7 24.2 18.1 
p-X 30.7 20.0 14.4 9.4 13.7 7.9 
m-X 21.0 12.6 10.6 9.0 12.4 10.1 
o-X 29.6 20.1 15.6 11.5 16.7 10.8 
Bs 
E/P 0.9 1.57 1.69 1.99 1.77 2.29 
E/M 1.32 2.49 2.29 2.08 1.95 1.79 
E/O 0.94 1.56 1.56 1.63 1.45 1.68 
______________________________________ 
It is first immediately apparent that the dry adsorbent does not even 
exhibit selectivity for ethylbenzene and is thus of no utility in the 
separation of the present invention. Furthermore, data for the use of 
barium cation exchanged X-type zeolite with desorbents other than benzene 
is not presented, since, as discussed above, it is known in the art that 
such adsorbent/desorbent combinations exhibit selectivity for one or more 
of the xylenes over ethylbenzene. The data, of course, does show a 
consistent selecivity for ethylbenzene of the adsorbent/desorbent/water 
content combinations of the present invention. 
It should be further noted that the peak widths for the xylene isomers are 
lower for the preferred potassium cation containing adsorbent which is 
indicative of a desirable more rapid rate of removal of these isomers from 
the adsorbent by the desorbent. 
EXAMPLE 2 
The feasibility of separating ethylbenzene from a feed mixture comprising 
ethylbenzene and at least one xylene isomer by selective adsorption of 
ethylbenzene, which was demonstrated by pulse test results, was confirmed 
by continuous testing in a laboratory-sized embodiment of the 
countercurrent simulated moving bed apparatus described above and referred 
to as a carousel unit. 
The adsorption column of the carousel unit was packed with 462 ml. of 
barium and potassium exchanged X-type zeolite containing 4-5 wt. % water 
and divided into twenty-four beds. The feed composition comprised a 
C.sub.8 aromatic and n-octane mixture further described in the following 
Table 2. The desorbent comprised pure benzene. Other operating parameters 
were as follows: 
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A/F 2.0 (based on selective pore 
volume = 12% of bed volume) 
.theta., hrs 0.75 
L.sub.1 /A 1.15 
L.sub.2 /A 0.37 
L.sub.3 /A 1.31 
L.sub.4 /A -1.76 
Temperature 130.degree. C. 
______________________________________ 
where A is the selective pore volume circulating per hour, F is the feed 
rate; .theta. is the valve cycle time of the moving bed system; and 
L.sub.1, L.sub.2, L.sub.3 and L.sub.4 are the liquid flow rates into zones 
I, II, III and IV, respectively, less the void volume of each respective 
zone. 
The results of the carousel test run are given in the following Table 2. 
TABLE 2 
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Carousel Results of Ethylbenzene Separation 
Averages from samples acquired over a two-hour period. - 
Feed Extract Raffinate 
Wt. % Wt. % Wt. % 
______________________________________ 
Ethylbenzene 
24.1 99.3 1.4 
n-Octane 4.1 -- 5.4 
p-Xylene 17.4 0.2 22.3 
m-Xylene 39.6 0.4 51.1 
o-Xylene 14.8 0.1 19.8 
______________________________________ 
Ethylbenzene Extraction Efficiency = 95.5% 
The effectiveness of the present invention in an actual simulated moving 
bed environment is clearly demonstrated by the data of Table 2.