Note: Descriptions are shown in the official language in which they were submitted.
3~
Para-xylene may he separated from a mixture containing it and other
C8 aromatic hydrocarbons through selective adsorbents, including those dis-
closed in United States Patent 3,761,533, Otani et al granted September Z5,
1973, and assigned to Toray Industries, Inc. Otani et al disclose the advan-
tageous use of certain crystalline metal alumino-silicate sorbents (column 10,
line 49--column 12, line 47). These sorbents may be used for the selective
adsorption of para-xylene.
The present invention provides in an adsorptive-separation process
for the separation of para-xylene from a feed stream containing a mixture of
C8 aromatic hydrocarbons including para-xylene by passing said feed stream into
contact with a crystalline aluminosilicate adsorbent capable of selectively
adsorbing para-xylene from said feed stream, the improvement which comprises
desorbing para-xylene from said adsorbent by contacting said adsorbent with a
desorbent containing }-~lower alkyl)-4-isopropylbenzene, wherein the lower
alkyl group contains 1 or 2 carbon atoms.
In a preferred embodiment, the initial separation o the para-xylene
through adsorption is conducted with the use of a faujasite type zeolite con-
taining potassium cations as disclosed in the aforementioned Otani et al patent.
The faujasite type zeolite should contain individual potassium cations at the
cation exchangeable sites.
Optionally, in accordance with a more specific embodiment of this
invention, the faujasite type zeolite may also contain, in addition to the
potassium cations, Group IA cations other than potassium, or Group IIA, Group
IB, yttrium, lead, zirconium, neodymium, thallium, thorium or hydrogen ions.
The desorbent of the inven~ion may in one embodiment consist
essentially of l-(Cl 2 alkyl)-4-isopropylbenzene together with other aromatic
hydrocarbons, such as isomers of para-cymene, isomers of l-ethyl-4-isopropyl-
benzene and diethylbenzenes.
In a more specific embodiment, the l-(Cl 2 alkyl)-4-isopropylbenzene
may be used in admixture with one or more saturated hydrocarbons, such as paraf-
fins, particularly cycloparaffins, and more particularly decahydronaphthalene.
The present invention relates to an improved process for separation
- 1 - ~
... . . .. ..
8;~
of para-xylene from a feed containing a mixture of C8 aromatic hydrocarbons by
adsorptive separation techniques such as disclosed in the Otani et al patent,
where the para-xylene is selectively adsorbed from the feed by a crystalline
adsorbent. More particularly, the invention relates to the discovery of sur-
prisingly active agents for desorbing para-xylene from a crystalline adsorbent,
preferably a crystalline aluminosilicate adsorbent and ~hereby sharply separat-
ing para-xylene from a mixture of C8 aromatic hydrocarbons containing para~
xylene.
The phras~ "C8 aromatic hydrocarbons" is intended to designate a
mixture which includes at least one other eight-carbon aroma~ic hydrocarbon in
addition to the desired para-xylene. Such other hydrocarbon may be meta-xylene,
ortho-xylene or ethyl-benzene, for example. According to this invention it is
possible with surprising effectiveness to separate para-xylene from a mixture
containing one or several other C8 aromatic hydrocarbons.
Some of the manipulative steps of the adsorptive s~paration process
are disclosed in Otani et al and also in United States Patents 3,686,342 and
3,626,020. Although the adsorptive step has been found to remove the para-
xylene from the C8 aromatic hydrocarbon feed by adsorbing the para -xylene
onto the crystalline aluminosilicate adsorbent, the problem remains as to how
then to desorb the para-xylene, which is the desired product.
It has now been discovered that, through the use of a l-(Cl 2 alkyl~
isopropylbenzene desorbent, an improved result in the overall process is
achieved, as compared to the Otani process.
The process is carried out stepwise on the adsorbent, with repeated
alternating steps of adsorption and desorption. Accordingly, in the adsorption
step, the C8 aromatic feed is contacted with an adsorbent in which some of ~ ;
the previously used desorbent material remains. Para-xylene is selectively
adsorbed on the adsorbent, displacing a part of the remaining desorbent, and
a raffinate material is produced which includes the less selectively adsorbed
compone~ts of the ~eed. The raffinate is removed, together with the displaced
~ 3~
desorbent, which is a l-(Cl_2 alkyl)-4-isopropylbenzene.
In the desorption step, the selectively adsoTbed para-x~lene remain-
ing on the crystalline adsorbent is displaced by contacting the adsorbent with
a l-tCl 2 alkyl)-4-isopropylbenzene desorbent. There thus remains an axtract
which includes the l-(Cl 2 alkyl)-4-isopropylbenzene dlesorbent, and para-xylene.
There are several requirements which are significant in evaluating
the quality of a desorbent. The desorbent material must be capable of dis-
placing para-xylene from the crystalline adso~bent, and it also must be a
compound which can be readily separated from para-xylene. The pors diameter
of the adsorbent utilized is at least 5.6 angs~roms. Preferably, the pore
diameter of the adsorbent is in the range of 5.6-10.0 anastroms.
A second general requirement is that the desorbent must be capable
of being displaced from the crystalline adsorbent by the para -xylene of the
feed itself, to permit the use of a continuous process. If this characteristic
is lacking in the desorbent, it ~ould not be possible to practice a continuous
process with the feed it~slf serving to displace the desorben~.
It should also be noted tha~ this process involves a competitive
adsorption of para-xylene by the desorbent. As mentioned above, para-xylene
is adsorbed on the crystalllne adsorbent, which contains residual desorbent,
and thus displaces a part of the remaining desorbent. It is therefore a sign-
ificant characteristic of the desorbent material that is used in such a sepa-
ratio~ process that it should not significantly interfere with the selective
adsorptive capability of the cTystalline adsorbent.
In determinlng the adsorptive capability of a particular adsorbent
one must consider the selectivity, or the ~-value of the adsorbent for para-
xylene as compared with other C8 aromatic hydrocarbons. Selectivity, or ~-
value, may be defined for two given components as the ra~io of ~he concentra-
tion of t~e two components in the adsorbed phase divided by the ratio of the
same two co~ponents in the unadsorbed phase which is in equilibrium with the
ads~rbed phase.
The selectivity value, a, may be expressed in equation form as
follows:
~wt. percent pxtwt percent x)A ;
px/x twt. percent px/wt percent X)U
wherein px is para-xylene; x is the second component; and A and U represent
the adsorbed and unadsorbed phases, respectively.
When selectivity approaches unity, a condition ls approached where-
in there is no preferential adsorption of para-xylene. As selectivity becomes
greater than unity preferential adsorption of para-xylene develops. It may
therefore be seen that the greater the value of selectivity in the above
equation, the better the adsorption of para-xylene versus the co~peting C8
aromatic hydrocarbons. A relatively small value for selectivity would con-
siderably decrease the yield of para-xylene per unit of adsorbent, and would
therefore tend toward a process which is not economical. On occasion, it may
not be possible to attain the desired degree of purity of para-xylene when the ;
desorbent provides a particularly low a-value. A desorbent material providing
relatively high value of ~, or selectivity, as used in the foregoing equation
has long been desired.
In accordance with the present invention it has been discovered that
para-xylene is more effectively separated by using a l-(Cl 2 alkyl)-4-isopropyl-benzene desorbent as the desorbent in a separation process which utili~es a
crys~alline adsorbent, the desorbent of the present invention having been
found to provide a sharply and unexpectedly advantageous ~-value than para-
diethylbenzene, which was previously stated (in United States Patent 3,686,342)
to be the best desorbent available for such a process.
In addition to providing a sharply bet~er a-value, the l-(Cl 2
alkyl)-4-isopropylbenzene desorbents of the present inventlon admirably meet
the other require~ents for desorbent material. They readily and effectively
d~splaee para-x~lene, previously adsorbed on the crystalline adsorbent, so
_ 4 -
,: : .... :,. . .... .... . .... . .
` 1~83611
that the para-xylene can be recovered from the adsorbent and thereafter sep-
arated from the desorbent itself to pro~ide a highly purified para-xylene
product. The desorbents adsorbed in the adsorbent by displacing para-xylene
are also displaced by para-xylene in ~he feed, so that a continuous process
may be carried into effect,
The l-(Cl 2 alkyl)-4-isopropylbenzene desorbent of the present in-
vention may be used effectively alone. However, it may alkernati~ely be used
in admixture with other compounds to provide a desorbent in accordance with
the present invention. Compounds which may be used include desorbent materials
of the prior art, or isomers of the instant 1-(C1 2 alkyl)-4-isopropylbenzene
desorbent, such as meta-cymene, ortho-cymene, l-ethyl-3-isopropylbenzene or
l-ethyl-2-isopropylbenzene, When the l-~Cl 2 alkyl)-4-isopropylbenzene is
used in admixture with other compounds, at least a few percent ~5% by weight
more or less) of the l-~Cl 2 alkyl)-4-isopropylbenzene desorbent should be
present. Also, whether used alone or in combination with other desorbentsJ
the l-(Cl 2 alkyl)-4-isopropylbenzene desorbent itself may be used as para-
cy~ene alone, or as l-ethyl-4-isopropylbenzene alone, or as a mixture of para-
cymene and l-ethyl-4-isopropylbenzene,
The desorbent of the present invention may also be used with diluents
such as saturated hydrocarbons, including the paraffinic type hydrocarbons
and cycloparaffins. In one or more typical examples of the invention in which
a diluent is used, such diluent may include straight or branched-chain paraf-
fins. These include cycloparaffins such as cyclohexane, cyclopentane and
branched derivatives thereof, as well as decahydronaphthalene and its branched-
chain derivatives.
Suitable adsorbents include the aforementioned Otani et al crystal-
line aluminosilicates. The disclosure of Otani et al in United States Patent
3,761,533 explains the structure and ~akeup of such c~ystalline aluminosilicate
adsorbents~ In accordance with a preferred fo~m of the presen~ in~ention with
3a respect to the adsorbents which a~e utilized in accordance with the invention,
.. . . .
1~33~
the crystalline aluminosil`icate adsorbents are the faujasite type zeolites which
are commonly represented as type X and Y zeolites, and are defined by varying
silica to alumina ratios. Faujasite-type zeolites contain selected cations at
the exhangeable cation sites. For the selective adsorption of para-xylene,
these cations are preferably selected from the group consisting of Group IA,
Group IIA, Group IB metal cations and proton.
A particularly advantageous faujasite-type zeolite in the process
of this invention is one in which potassium cations are present, either alone
or together with other cations selected from Group IA, Group IIA, Group IB,
yttrium, lead, zirconium, neodymiumJ thallium3 thorium or proton.
The drawing of Figure 1 represents a schematic arrangement and flow
diagram illustrating one specific embodiment of ~his invention, showing a
fixed bed apparatus connected for countercurrent flow operations. This drawing
is intended to be illustrative.
Referring to the specific embodiment selected for illustration, the
apparatus comprises three zones, 1 being a desorption zone, 2 being a rec-
tification zone, and 3 being an adsorption zone. Each zone comprises four
colu~ns which are charged with adsorbent. These zones are serially and cir-
cularly interconnec~ed in order. In the desorption zone 1 in which the des-
orption step is conducted, para-xylene adsorbed on an adsorbent is displaced
by contact with a desorbent stream, while simultaneously removing an extract
stream comprising desorbent and para-xylene. In the rectification zone 2, the
adsorbent in this zone is contacted with a reflux stream comprising para-
xylene and desorbent to effect a purification of para-xylene and this stream
is directed to maintain countercurrent operation against a simulated flow of
the adsorbent.
In the adsorption zone 3, para-xylene is adsorbed on the adsorbent ;`
from a feed containing a mixture of C8 aromatic hydrocarbons, ~ith simultaneous
removal of a raffinate stream which contains the less selectively adsorbed
components of the feed, and desorbent.
- 6 -
.
~83611
The individual columns are serially and circularly connected to
each other by means of a relatively small diameter connecting pipe fitted with
a valve, and the valves 9 and 10 which are provided between the desorp~ion
zone and the rectification zone and between the adsorption zone and the de-
sorption zone respectively are closed, while simultaneously all of the other
valves not shown in Figure 1 are opened.
Additionally, all columns are connected to a desorbent ~eed line 4,
an extract withdrawal line 5, a reflux feed line 6, a feed inlet line 7, and a
raffinate withdrawal line 8, wherein the individual connecting embodiment is
not shown in detail in Figure 1.
In operation, the top columns of the desorption, rectification and
adsorption zones are simultaneously transferred to the bottoms of the adsorp- ~ ;
tion, desorption and rectification zones, respectively, at predetermined time
intervals. The transfer is effected by shifting all the points of introduction
and withdrawal of all the lines in~o and from the one column simultaneously in
a downstream direction. Thus, a simulated countercurrent flow system is pro- ~`
vided achieving an effect similar to that of a moving bed type adsorption
process. Therefore, the feed containing a mixture of C8 aromatic hydrocarbons
may be continuously separated to produce both the selectively adsorbed com-
ponent (i.e., para-xylene) and the less selectively adsorbed components (me~a-
xylene, ortho-xylene and ethylbenzene), respectively.
In accordance with the process of the present invention, the tem-
perature should be from about zero to about 350C, preferably about 30C to
about 250& . The pressure should be from about atmospheric pressure to about ~-
40 kg/cm2. Although in theory both liquid and vapor phase operations may be
utilîzed for carrying out the separatory process of the present invention, it
has been found preferable in practice to utilize a liquid phase separation
because of ~he reduced temperature requirements and the possibility of sup-
pressing the unwanted side reactions that may occur in the practice of high
temperature operations.
~183~
In accordance with the process o~ the present invention the raffinate
and the extract streams can be passed into different fractionating facilities,
so that the extract stream may be separated to form a relatively pure desorbent
stream and a relatively pure para-xylene stream. The raffinate stream can
similarly be passed into a fractionating facility in which the raffinate ma-
terial can be separated into a concentrated stream of desorbent material and
another stream containing the less selectivel~ adsorbed feed components. In
accordance with the present invention the relatively purified desorbent stream
from both the raffinate stream and the extract stream may be reused in the
process of the invention.
The desorbent material which has been contacted with the adsorbent
in the desorption step may be displaced by a part of the raffinate stream in
advance of the adsorption step, in order to recover a relatively pure desorbent
stream which may be reused in the process of the invention.
The adsorbent which has selectively adsorbed para-xylene in the
adsorption step may be contacted with a part of the extract stream or an en-
riched para-xylene stream prior to the desorption step, in order to effect
purification of the selectively adsorbed para-xylene.
The raffinate stream comprising the less selectively adsorbed com-
ponents of the feed can be passed into an isomeri~ation zone in which isomer-
ization conditions take place to produce additional amounts of para-xylene.
The combination of isomerization and separation processes thus allows the
possibility of an increased yield of ~he desired para-xylene ~rom the feed
stock based upon the quantity of the C8 aromatic hydrocarbon feed.
In testing various desorbents in the following Examples, the
selectivity of the adsorbent in the presence of desorbent was determined using
a static testing apparatus, and using the procedures which are described in
further detail hereinafter.
EXAMPLES
_
3Q The static ~esting apparatus used in these Examples was a container
-- 8 --
1~836~i
having a volume of 5 ml and was made of stainless steel. 2 gm. of feed contain-
ing a mixture of C8 aromatic hydrocarbons and desorbent, and 2 gm. of adsorbent
were introduced into the container. It was ~hen stoppered and placed into an
oil bath at a predetermined temperature for one hour, attaining an equilibrium
adsorption state. The liquid in the apparatus was sampled for testing with a
microsyringe, and was analyzed by means of gas chromatography. The selectivity
or ~-value for obtaining para-xylene, ~ px/x, was calculated according to the
above equation.
The Examples which are set forth below illustrate particular em-
bodiments of the invention, and are not intended as being other than exemplary.
EX~LE
,
As the adsorbent, a crystalline potassium Y zeolite was used, which
was prepared by the following method. The sodium form of type Y zeolite,
which consisted of 4.8 mole ratio of silica over alumina, was subjected to ion
exchange treatment in which the zeolite was contacted with 5 wt percent
potassium nitrate aqueous solution until the residual sodium cations dropped
to less than 20 percent, based on the total amount of cations within the
zeolite. After completing the ion exchange treatment, the zeolite was dried
at 120C for three hours and was then calcined at 500C ~or three hours.
In order to evaluate the performance of para-cymene or l-ethyl 4-
isopropylbenzene as desorbent materials in the process of the present invention,
the selectivity for para-xylene was determined at 30C by the procedure pre-
viously described above.
The feed mixture used for this test had the following composition:
Normal nonane (n-C9) 1 part by weight
Para-xylene (px) 1 part by weight
Ortho-xylene (ox) 1 part by weight
Ethylbenzene ~eb) 1 part by weight
Para-cymene (pcm) 1 part by weight
3Q or
_ g _
... .
~)836~L~
l-Ethyl-4-iso- 1 part by weight
propylbenzene (eipb)
It was assumed in carrying out this test that the n-C9 was not ad-
sorbed within the adsorbent. Accordingly, the selectivities were calculated
using the above equation. The results are set forth in Table 1.
TABLE
EX. N0. 1
Desorbentpara-cymene l-ethyl-4 para-diethyl-
isopropyl- benzene
benzene
'':
Selectivity
~) for:
~ 1.9 2.2 1.5
mx 5.5 6.1 4.1
,
px 5,3 5.9 4.1
EXAMPLE _2
2Q To compare the present invention with the prior art, a comparison
was made with para-diethylbenzene ~p-deb), which was considered in the prior
art literature to be the preferred desorbent for this purpose,
The adsorbent prepa~ed in accordance with Example 1 was contacted
with a feed mixture having the following ratio in terms of weight:
n-C9:px:mx:ox:eb:p-deb = 1:1:1:1:1:1, namely, one part by weight of
each of the above components was present.
The other conditions used in the present Example were in accordance
with Example 1. The results of this Example 2 are also set forth in Table 1.
It will also be noted from Table 1 that the most difficult component
to separate from para-~ylene is ethylbenzene. Accordingly, this selectivity
- 10 -
.: .. . .. . ..
~083G~
value is of greatest importance.
Table 1 shows that each desorbent of the present inYention provides
a higher ~-value for para-xylene versus ethylbenzene than para-diethylbenzene.
EXAMPLE 3
The same adsorbent as in Example 1 was contacted wi~h a feed of the
following composition, the other parameters of Example 1 being followed except
that a temperature of 170C was used:
n-C9:px:mx:ox:eb:desorbent = ; '
~ 5
The results are tabulated at Table 2.
TABLE 2
EX. N0. 3 4 5
Desorbentpara-cymene l-ethyl-4-mixture of para-
isopropyl-para-cymene diethyl_
benzene and para- benzene
bdenzthenle- . ~ :
Selectivity
~eb 2.0 2.1 1.9 1.8
_
mx 3.1 3.8 3.1 3.0
px 2.7 3.3 2.8 2.8
EXAMPLE_ 4
As desorbent, a mixture of 50% para-cymene and 50% para-diethyl-
benzene was used, based upon the weight of the components. The other test
conditions were the same as used in Example 3.
The results are set forth in Table 2 (see col. 4).
EXAMPLE 5
As desorbent, para-diethylbenzene was used. The adsorbent prepared
~0~33~
in Example 1 was contacted with a feed mixture having the following composition:n-C9:px:mx:ox:eb:p-deb =
1: 1: 1: 1: 1: 5
: The other conditions were the same as in Example 3.
The results are set for~h in Table 2 tsee col. 5).
A superior selectivity for the para-xylene versus ethylbenzene can
be seen for the desorbents of the present invention.
EXAMPLE 6
In this Example, para-cymene was demonstrated to be an effective
desorbent for separa~ion of para-xylene from a mixture of C8 aromatic hydro-
carbons.
The apparatus utilized was that set forth in Figure 1, which was a
fixed bed apparatus through which fluid flow was directed to maintain counter-
current flow operations which simulated a moving bed type operation, as
- previously descrîbed herein.
In the present Example, each of the columns (~hich had an inner
diameter of 25 mm and a height of 2.0 m) was fully packed with potassium
zeolite Y adsorbent tl2-24 mesh) which was prepared in accordance with Example
1. Opening and closing of all the valves was effected by a time-actuated
automatic control apparatus, and the shift interval was programmed for 3.5
minutes.
A feed stock consisting of 20% para-xylene, 40% meta-xylene9 20%
ortho-xylene, and 20% ethylbenzene (each component by weight) was, after heat-
ing to 165C, continuously fed through line 7 at a flow rate of 4.0 kg/hour
and under a pressure of 8 kg/cm2 gage.
A reflux stock consisting of 90% para-xylene and 10% para-cymene
~by weight) that had been previously prepared was heated to 165C and con-
tînuously fed through line 6 at a flow rate of 25.7 kg/hour and a pressure of
10 kg/cm gage.
3Q Para-cymene was provided as the desorbent material and was, after
- - 12 _
.. . . .
~836~
heating to 165C, contim~ously fed through line 4 at a flov rate of 32.0 kg/
hour and at a pressure of 12 kg/cm2 gage.
An extract stream was continuously withdrawn through line 5, in
which stream para-xylene was present at an average concentration of 74.6% by
weight based on the total stream. After para-cymene was separated from the
extract by distillation~ the para-xylene fraction had a purity of 99.4~ by
weight. In the raffinate stream, para-xylene was present at an average concen-
tration of 0.2% by weight based on the total stream.
EXAMPLE 7
T~e Example shows that pure para-xylene was separated effectively
from a mixture of C8 aromatic hydrocarbons when 1-ethyl-4-isopropylbenzene was
employedas the desorbent.
The apparatus and conditions were the same as in Example 6, except
as described below.
A reflux stock consisting of 90% para-xylene and 10% previously
prepared l-ethyl-4-isopropylbenzene by weight was continuously fed at a flow
rate of 25.1 kg/hour.
l-ethyl-4-isopropylbenzene as desorbent material was continuously
fed at a flow rate of 34.4 kg/hour. The shift interval was programmed for
~0 3.2 minutes.
In the extract stream, para-xylene was present at an average concen-
tration of 67.9% by weight, based upon the total stream. After l-ethyl-4-
isopropylbenzene was separated fTom the extract by distillation, the para-
xylene fraction had a purity of 99.4% by weight. In the raffinate stream,
para-xylene was present at an average concentration below 0.2% by weight,
based on the total stream.
Although this invention has been described with reference to par-
ticular apparatus for carrying the process into effect, a variety of different
forms of apparatus, and many different manipulative steps may be used, all
3a taking advantage of the basic discovery that the l-lower alkyl 4-is~propyl-
- 13 -
~836~
benzenes ~alkyl being methyl or ethyl) have surprising eEfectiveness in the
desorptive enrichment of C8 hydrocarbons. For example, the process is not
limited to utilizing the three-zone system described in the Examples, or to
simultaneous shifting of points of introduction or withdrawal of streams~ Any
of the various known techniques or equipment for solids-fluid contacting
operations may be used for either the adsorption step or the desorption stepS
or both, such as a compact moving bed, for example, passed successively through
the adsorption and desorption operations with co-current or countercurrent
contact with the s~reams, fluids or vapors. Accordingly, it will be appreciated
that equivalents may be substituted, that certain features may be used in-
dependently of others, and that st~ps may be reversed, all without departing
from the spirit and scope of the invention.
_ 14 -
