U.S. patent number RE31,782 [Application Number 06/572,784] was granted by the patent office on 1984-12-25 for xylene isomerization.
This patent grant is currently assigned to Mobil Oil Corporation. Invention is credited to Werner O. Haag, David H. Olson.
United States Patent |
RE31,782 |
Olson , et al. |
December 25, 1984 |
Xylene isomerization
Abstract
A catalytic process is provided for isomerizing an isomerization
feed containing an aromatic C.sub.8 mixture of ethylbenzene and
xylene in which the para-xylene content is less than equilibrium
which comprises contacting said feed, under conversion conditions,
with a catalyst comprising a crystalline aluminosilicate zeolite
having a crystal size of at least about 1 micron, a silica to
alumina ratio of at least about 12 and a constraint index, as
hereinafter defined, within the approximate range of 1 to 12 to
yield an isomerization product characterized by a reduced
ethylbenzene content and an enhanced para-xylene content.
Inventors: |
Olson; David H. (Pennington,
NJ), Haag; Werner O. (Trenton, NJ) |
Assignee: |
Mobil Oil Corporation (New
York, NY)
|
Family
ID: |
27075952 |
Appl.
No.: |
06/572,784 |
Filed: |
January 20, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
913999 |
Jun 9, 1978 |
04159282 |
Jun 26, 1979 |
|
|
Current U.S.
Class: |
585/481 |
Current CPC
Class: |
C07C
5/2775 (20130101); C07C 5/2775 (20130101); C07C
15/08 (20130101) |
Current International
Class: |
C07C
5/27 (20060101); C07C 5/00 (20060101); C07C
005/24 () |
Field of
Search: |
;585/481 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gantz; Delbert E.
Assistant Examiner: Pal; A.
Attorney, Agent or Firm: McKillop; A. J. Gilman; M. G. Wise;
L. G.
Claims
What is claimed is:
1. A process for isomerizing an isomerization feed containing an
aromatic C.sub.8 mixture of ethylbenzene and xylene, in which the
para-xylene content is less than equilibrium which comprises
contacting said feed, under conversion conditions, with a catalyst
comprising a crystalline aluminosilicate zeolite having a crystal
size of at least about 1 micron, a silica to alumina ratio of at
least about 12 and a constraint index within the approximate range
of 1 to 12 to yield an isomerization product characterized by a
reduced ethylbenzene content and an enhanced para-xylene
content.
2. The process of claim 1 wherein said conversion conditions
include a temperature of from about 400.degree. to about
1000.degree. F., a pressure of from about 0 to about 1500 psig and
a weight hourly space velocity of between about 0.5 and about
100.
3. The process of claim 1 wherein said conversion conditions
include a temperature of from about 500.degree. to about
950.degree. F., a pressure of from about 25 to about 1000 psig and
a weight hourly space velocity of between about 1 and about 50.
4. The process of claim 1 wherein said aromatic C.sub.8 mixture
consists essentially of ethylbenzene, para-xylene, meta-xylene and
ortho-xylene.
5. The process of claim 1 wherein said isomerization feed contains
said aromatic C.sub.8 mixture and up to 30 weight percent of
non-aromatic hydrocarbons.
6. The process of claim 1 wherein said crystalline aluminosilicate
zeolite is ZSM-5.
7. The process of claim 1 wherein said crystalline aluminosilicate
zeolite is ZSM-5 predominantly in the hydrogen form.
8. The process of claim 1 wherein the crystal size of said
crystalline aluminosilicate zeolite is within the approximate range
of 1 to 20 microns.
9. The process of claim 1 wherein the crystal size of said
crystalline aluminosilicate zeolite is within the approximate range
of 1 to 6 microns.
10. The process of claim 6 wherein the ZSM-5 is present in
combination with a binder therefor.
11. The process of claim 1 wherein said crystalline aluminosilicate
zeolite is characterized by a xylene sorption capacity greater than
1 gram 100 grams of zeolite and an ortho xylene sorption time of 30
percent of said capacity of greater than 10 minutes, said sorption
capacity and sorption time being measured at 120.degree. C., and a
xylene pressure of 4.5.+-.0.8 mm. of mercury.
12. The process of claim 1 wherein said crystalline aluminosilicate
has combined therewith between about 0.5 and about 40 weight
percent of a difficulty reducible oxide.
13. The process of claim 12 wherein said difficulty reducible oxide
is magnesium oxide. .Iadd.
14. A process according to any of claims 1 to 13 comprising a
further isomerization conversion step wherein the isomerization
feed is subsequently contacted with a small crystal aluminosilicate
zeolite having a crystal size less than about 1 micron. .Iaddend.
.Iadd.15. The process of claim 1 further comprising contacting the
isomerization feed with a sequential bed of ZSM-5 zeolite having a
crystal size less than 1 micron.
.Iaddend. .Iadd.16. A process according to claim 1 comprising a
dual isomerization reactor sequence wherein the aromatic C.sub.8
mixture is subsequently contacted with a small crystal
aluminosilicate zeolite having a crystal size less than about 1
micron. .Iaddend. .Iadd.17. A process according to claim 16 wherein
zeolite catalysts of differing crystal size
are in sequential beds, each comprising HZSM-5..Iaddend. .Iadd.18.
A process according to claim 1 wherein the isomerization conversion
is carried out with the crystalline aluminosilicate zeolite having
a bimodal crystal size distribution falling in two ranges,
comprising a first crystal size range greater than 1 micron and a
second crystal size range less than 1 micron, the greater crystal
size being in major proportion. .Iaddend. .Iadd.19. A process
according to claim 18 comprising the step of contacting the
aromatic C.sub.8 mixture with sequential beds comprising HZSM-5
catalyst having differing crystal size ranges. .Iaddend. .Iadd.20.
A process according to claim 6 wherein the isomerization conversion
is carried out with said crystalline aluminosilicate zeolite having
a bimodal crystal size distribution falling in two ranges, a range
being less than 1 micron and a range being at least 1 micron, with
the greater range being in major proportion..Iaddend. .Iadd.21. A
process according to claim 20 comprising contacting the aromatic
C.sub.8 mixture with sequential reactor beds of said zeolite having
an initial crystal size of at least about 1 micron preceding
zeolite crystals of less than 1 micron size.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for effecting isomerization of
an aromatic C.sub.8 mixture comprising ethylbenzene and xylene in
the presence of a specified crystalline aluminosilicate zeolite
catalyst characterized by a crystal size of at least about 1
micron.
2. Description of the Prior Art
Xylenes are valuable industrial chemicals. They are derived
primarily from such aromatic naphthas as petroleum reformates and
pyrolysis gasolines. The former result from processing petroleum
naphthas over a catalyst such as platinum on alumina at
temperatures which favor dehydrogenation of naphthenes. Pyrolysis
gasolines are liquid products resulting from steam cracking of
hydrocarbons to manufacture ethylene, propylene, etc.
Generally and regardless of aromatic naphtha source, it has been
the practice to subject the liquid hydrocarbon to extraction with a
solvent highly selective for aromatics to obtain an aromatic
mixture of the benzene and alkylated benzenes present in the
aromatic naphtha. The resulting aromatic extract may then be
distilled to separate benzene, toluene and C.sub.8 aromatics from
higher boiling compounds in the extract. Benzene and toluene are
recovered in high purity but the C.sub.8 fraction, containing
valuable para xylene, is a mixture of the three xylene isomers with
ethylbenzene. Techniques are known for separating p-xylene by
fractional crystallization with isomerization of the other two
isomers in a loop to the p-xylene separation. That operation is
hampered by the presence of ethylbenzene. However, a widely used
xylene isomerization technique, "Octafining" can be applied.
Octafining by passing the C.sub.8 aromatics lean in p-xylene and
mixed with hydrogen over platinum on silica-alumina not only
isomerizes xylenes but also converts ethylbenzene, thus preventing
buildup of this compound in the separation-isomerization loop.
The manner of producing p-xylene by a loop including Octafining can
be understood by consideration of a typical charge from reforming
petroleum naphtha. The C.sub.8 aromatics in such mixtures and their
properties are:
______________________________________ Density Freezing Boiling
Lbs./U.S. Point .degree.F. Point .degree.F. Gal.
______________________________________ Ethylbenzene -139.0 277.1
7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene
-13.3 292.0 7.37 ______________________________________
Individual isomer products may be separated from the naturally
occuring mixtures by appropriate physical methods. Ethylbenzene may
be separated by fractional distillation although this is a costly
operation. Ortho xylene may be separated by fractional distillation
and is so produced commercially. Para xylene is separated from the
mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has
been interest in isomerizing the other C.sub.8 aromatics toward an
equilibrium mix and thus increasing yields of the desired xylenes.
Of the xylene isomers, i.e., ortho-, meta and para-xylene,
meta-xylene is the least desired product with ortho- and
para-xylene being the more desired products. Para-xylene is of
particular value being useful in the manufacture of terephthalic
acid which is an intermediate in the manufacture of synthetic
fibers such as "Dacron".
Isomerization processes operate in conjunction with the product
xylene of xylenes separation processes. A virgin C.sub.8 aromatics
mixture is fed to such a processing combination in which the
residual isomers emerging from the product separation steps are
then charged to the isomerizer unit and the effluent isomerizate
C.sub.8 aromatics are recycled to the product separation steps. The
composition of isomerizer feed is then a function of the virgin
C.sub.8 aromatic feed, the product separation unit performance, and
the isomerizer performance.
A typical charge to the isomerizing reactor may contain 17 wt. %
ethylbenzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. %
o-xylene when it is desired to co-produce orthoxylene and
para-xylene. When para-xylene is desired as the sole product, a
typical charge may contain 20 wt. % ethylbenzene, 51 wt. %
m-xylene, 9 wt. % para-xylene and 20 wt. % orthoxylene. The
thermodynamic equilibrium varies slightly with temperature. The
objective in the isomerization reactor is to bring the charge as
near to the equilibrium concentration as may be feasible consistent
with reaction times which do not give extensive cracking and
disproportionation.
In Octafining, ethylbenzene reacts through ethyl cyclohexane to
dimethyl cyclohexanes which in turn equilibrate to xylenes.
Competing reactions are disproportionation of ethylbenzene to
benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane
and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethylbenzene approach to equilibrium concentration in a
C.sub.8 aromatic mixture is related to effective contact time.
Hydrogen partial pressure has a very significant effect on
ethylbenzene approach to equilibrium. Temperature change within the
range of Octafining conditions (830.degree. to 900.degree. F.) has
but a very small effect on ethylbenzene approach to
equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products
relate to % approach to equilibrium. Products formed from
ethylbenzene include C.sub.8 + aphthenes, benzene from cracking
benzene and C.sub.10 aromatics from disproportionation, and total
loss to other than C.sub.8 molecular weight. C.sub.5 and lighter
hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does
ethylbenzene, but they do exhibit different rates of isomerization
and hence, with different feed composition situations the rates of
approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with
contact time. By-products include naphthenes, toluene, C.sub.9
aromatics and C.sub.5 and lighter hydrocracking products.
Ethylbenzene has been found responsible for a relatively rapid
decline in catalyst activity and this effect is proportional to its
concentration in a C.sub.8 aromatic feed mixture. It has been
possible then to relate catalyst stability (or loss in activity) to
feed composition (ethylbenzene content and hydrogen recycle ratio)
so that for any C.sub.8 aromatic feed, desired xylene products can
be made with a selected suitably long catalyst use cycle.
Because of its behavior in the loop manufacture of p-xylene, or
other xylene isomer, ethylbenzene is undesirable in the feed but is
tolerated because of the great expense of removal from mixed
C.sub.8 aromatics. Streams substantially free of ethylbenzene are
available from such processes as transalkylation of aromatics
having only methyl substituents. Thus, toluene can be reacted with
itself (the specific transalkylation reaction sometimes called
"disproportionation") or toluene may be reacted with tri-methyl
benzene in known manner.
A recent development in vapor phase isomerization is described in
U.S. Pat. No. 3,856,872 (Morrison) dated Dec. 24, 1974. It is there
shown that use of a catalyst containing HZSM-5 is very efficient
for isomerization of xylenes at reduced hydrogen flow as compared
with Octafining. The extent of xylene loss is substantially reduced
by this change of catalyst. Concurrently, the mechanism of
ethylbenzene conversion is drastically changed on substitution of,
e.g., NiHZSM-5, for the platinum on silica/alumina of Octafiners.
The Morrison process results in conversion of ethylbenzene by
transalkylation reactions including disproportionation of
ethylbenzene to benzene and diethylbenzene, disproportionation and
ethylation of xylene and the like producing alkyl aromatic
compounds of nine or more carbon atoms (C.sub.9 +) together with
benzene and toluene. Those conversion products are readily
separated in the loop for recovery of p-xylene and isomerization of
o- and m-xylene. In general, loss of xylenes increases as severity
of the isomerizer is increased to enhance the conversion of
ethylbenzene. Xylene isomerization may also be carried out in the
liquid phase, using a similar crystalline aluminosilicate zeolite
catalyst, as described in U.S. Pat. No. 3,856,871.
SUMMARY OF THE INVENTION
In accordance with the present invention, there has been discovered
a process for effecting isomerization of a feed containing an
aromatic C.sub.8 mixture of ethylbenzene and xylene in which the
para-xylene content is less than equilibrium to yield a product of
reduced ethylbenzene content and enhanced para-xylene content by
contact of such mixture, at elevated temperatures, with a catalyst
comprising a crystalline aluminosilicate zeolite having a crystal
size of at least about 1 micron, usually in the approximate range
of 1 to 20 microns and preferably 1 to 6 microns. The crystalline
aluminosilicate zeolite is essentially characterized by a silica to
alumina ratio of at least about 12 and a constraint index within
the approximate range of 1 to 12. In one embodiment, the
crystalline aluminosilicate zeolites employed have a xylene
sorption capacity greater than 1 gram/100 grams of zeolite and an
ortho-xylene sorption time for 30 percent of said capacity of
greater than 10 minutes, where the sorption capacity and sorption
times are measured at 120.degree. C. and a xylene pressure of
4.5.+-.0.8 mm. of mercury.
The present process comprises isomerization of an aromatic C.sub.8
mixture in the presence of the specified catalyst at a temperature
between about 400.degree. and about 1000.degree. F. at a pressure
between about atmospheric and about 1500 psig utilizing a feed
weight hourly space velocity (WHSV) between about 0.5 and about
100. The latter WHSV is based on the weight of catalyst
composition, i.e., total weight of active catalyst and binder
therefor. Preferably, but not necessarily the desired isomerization
is effected in the presence of hydrogen. Hydrogen may be present in
the isomerization zone in an amount between about 0 and about 10
and preferably between about 1 and about 5 moles of hydrogen per
mole of hydrocarbon. The above process conditions include operation
in liquid, vapor or mixed phase.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows, in schematic fashion, a system utilizing a single
isomerization reactor containing crystalline zeolite catalysts of
differing crystalline size.
FIG. 2 shows, in schematic fashion, a system utilizing a dual
isomerization reactor sequence individually containing crystalline
aluminosilicate zeolite catalysts of differing crystallite
size.
FIG. 3 shows, in schematic fashion, a system utilizing an initial
isomerization reactor and a second reactor contained in the recycle
loop using crystalline aluminosilicate zeolite catalysts of
differing crystallite size.
FIG. 4. shows, in schematic fashion, a system utilizing in
combination xylene splitter and ortho xylene towers and an
isomerization reactor containing crystalline zeolite catalysts of
differing crystallite size, which system is particularly applicable
when ortho- and para-xylenes are desired products.
FIG. 5 shows a comparison of xylene selectivities expressed in
terms of the ratio of relative ethylbenzene loss to relative
xylenes loss (f.sub.EB /f.sub.XYL).
DESCRIPTION OF SPECIFIC EMBODIMENTS
It is contemplated that any aromatic C.sub.8 mixture containing
ethylbenzene and xylene may be used as feed to the process of this
invention. Generally, such mixture will have an ethylbenzene
content in the approximate range of 5 to 50 weight percent, an
ortho xylene content in the approximate range of 0 to 35 weight
percent, a meta xylene content in the approximate range of 20 to 95
weight percent and a para xylene content in the approximate range
of 0 to 15 weight percent. The feed in addition to the above
aromatic C.sub.8 mixture may contain non-aromatic hydrocarbons,
i.e., naphthenes and paraffins in an amount up to 30 weight
percent. In a preferred embodiment, the invention provides means to
process a mixture of C.sub.8 aromatics such as that derived from
platinum reforming of a petroleum naphtha to a mixture of reduced
ethylbenzene content and increased content of para-xylene.
In accordance with the present invention, such feed is brought into
contact, under conversion conditions, with a catalyst comprising a
crystalline aluminosilicate zeolite having a crystal size of at
least about 1 micron, a silica to alumina ratio of at least about
12 and a constraint index within the approximate range of 1 to
12.
The zeolites herein described are members of a novel class of
zeolites exhibiting some unusual properties. These zeolites induce
profound transformation of aliphatic hydrocarbons in commercially
desirable yields and are generally highly effective in conversion
reactions involving aromatic hydrocarbons. Although they have
unusually low alumina contents, i.e., high silica to alumina
ratios, they are very active even when the silica to alumina ratio
exceeds 30. The activity is surprising since catalytic activity is
generally attributed to framework aluminum atoms and cations
associated with these aluminum atoms. These zeolites retain their
crystallinity for long periods in spite of the presence of steam at
high temperature which induces irreversible collapse of the
framework of other zeolites, e.g., of the X and A type.
An important characteristic of the crystal structure of this class
of zeolites is that it provides constrained access to, and egress
from the intracrystalline free space by virtue of having a pore
dimension greater than about 5 Angstroms and pore windows of about
a size such as would be provided by 10-membered rings of oxygen
atoms. It is to be understood, of course, that these rings are
those formed by the regular disposition of the tetrahedra making up
the anionic framework of the crystalline aluminosilicate, the
oxygen atoms themselves being bonded to the silicon or aluminum
atoms at the centers of the tetrahedra. Briefly, the preferred type
zeolites useful in this invention possess, in combination: a silica
to alumina mole ratio of at least about 12 and a structure
providing constrained access to the crystalline free space.
The silica to alumina ratio referred to may be determined by
conventional analysis. This ratio is meant to represent as closely
as possible, the ratio in the rigid anionic framework of the
zeolite crystal and to exclude aluminum in the binder or in
cationic or other form within the channels. Although zeolites with
a silica to alumina ratio of at least 12 are useful, it is
preferred to use zeolites having higher ratios of at least about
30. Such zeolites, after activation, acquire an intracrystalline
sorption capacity for normal hexane which is greater than that for
water, i.e., they exhibit "hydrophobic" properties. It is believed
that this hydrophobic character is advantageous in the present
invention.
The type zeolites useful in this invention freely sorb normal
hexane and have a pore dimension greater than about 5 Angstroms. In
addition, the structure must provide constrained access to larger
molecules. It is sometimes possible to judge from a known crystal
structure whether such constrained access exists. For example, if
the only pore windows in a crystal are formed by 8-membered rings
of oxygen atoms, then access to molecules of larger cross-section
than normal hexane is excluded and the zeolite is not of the
desired type. Windows of 10-membered rings are preferred, although,
in some instances, excessive puckering or pore blockage may render
these zeolites ineffective. Twelve-membered rings do not generally
appear to offer sufficient constraint to produce the advantageous
conversions although puckered structures exist such as TMA
Offretite which is a known effective zeolite. Also, structures can
be conceived, due to pore blockage or other cause, that may be
operative.
Rather than attempt to judge from crystal structure whether or not
a zeolite possesses the necessary constrained access, a simple
determination of the "constraint index" may be made by passing
continuously a mixture of an equal weight of normal hexane and
3-methylpentane over a sample of zeolite at atmospheric pressure
according to the following procedure. A sample of the zeolite, in
the form of pellets or extrudate, is crushed to a particle size
about that of coarse sand and mounted in a glass tube. Prior to
testing, the zeolite is treated with a stream of air at
1000.degree. F. for at least 15 minutes. The zeolite is then
flushed with helium and the temperature adjusted between
550.degree. F. and 950.degree. F. to give an overall conversion
between 10% and 60%. The mixture of hydrocarbons is passed at 1
liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon
per volume of zeolite per hour) over the zeolite with a helium
dilution to give a helium to total hydrocarbon mole ratio of 4:1.
After 20 minutes on stream, a sample of the effluent is taken and
analyzed, most conveniently by gas chromatography, to determine the
fraction remaining unchanged for each of the two hydrocarbons.
The "constraint index" is calculated as follows: ##EQU1##
The constraint index approximates the ratio of the cracking rate
constants for the two hydrocarnbons. Zeolites suitable for the
present invention are those having a constraint index in the
approximate range of 1 to 12. Constraint Index (CI) values for some
typical zeolites are:
______________________________________ CAS C.I.
______________________________________ ZSM-5 8.3 ZSM-11 8.7 ZSM-12
2 ZSM-38 2 ZSM-35 4.5 Clinoptilolite 3.4 TMA Offretite 3.7 Beta 0.6
ZSM-4 0.5 H-Zeolon 0.4 REY 0.4 Amorphous Silica-Alumina 0.6
Erionite 38 ______________________________________
It is to be realized that the above constraint index values
typically characterize the specified zeolites but that such are the
cumulative result of several variables used in determination and
calculation thereof. Thus, for a given zeolite depending on the
temperature employed within the aforenoted range of 550.degree. F.
to 950.degree. F., with accompanying conversion between 10% and
60%, the constraint index may vary within the indicated approximate
range of 1 to 12. Likewise, other variables such as the crystal
size of the zeolite, the presence of possible occluded contaminants
and binders intimately combined with the zeolite may affect the
constraint index. It will accordingly be understood by those
skilled in the art that the constraint index, as utilized herein,
while affording a highly useful means for characterizing the
zeolites of interest is approximate, taking into consideration the
manner of its determination; with probability, in some instances,
of compounding variable extremes.
While the above experimental procedure will enable one to achieve
the desired overall conversion of 10 to 60% for most catalyst
samples and represents preferred conditions, it may occasionally be
necessary to use somewhat more severe conditions for samples of
very low activity, such as those having a very high silica to
alumina ratio. In those instances, a temperature of up to about
1000.degree. F. and a liquid hourly space velocity of less than
one, such as 0.1 or less, can be employed in order to achieve a
minimum total conversion of about 10%.
The class of zeolites defined herein is exemplified by ZSM-5,
ZSM-11, ZSM-12, ZSM-35 and ZSM-38 and other similar materials. U.S.
Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated
herein by reference.
ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979,
the entire contents of which are incorporated herein by
reference.
ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449,
the entire contents of which is incorporated herein by
reference.
ZSM-35 is more particularly described in U.S. Pat. No. 4,016,245,
the entire contents of which is incorporated herein by
reference.
ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859,
the entire contents of which is incorporated herein by
reference.
The specific zeolites described, when prepared in the presence of
organic cations, are catalytically inactive, possibly because the
intracrystalline free space is occupied by organic cations from the
forming solution. They may be activated by heating in an inert
atmosphere at 1000.degree. F. for one hour, for example, followed
by base exchange with ammonium salts followed by calcination at
1000.degree. F. in air. The presence of organic cations in the
forming solution may not be absolutely essential to the formation
of this type zeolite; however, the presence of these cations does
appear to favor the formation of this special type of zeolite. More
generally, it is desirable to activate this type catalyst by base
exchange with ammonium salts followed by calcination in air at
about 1000.degree. F. for from about 15 minutes to about 24
hours.
Natural zeolites may sometimes be converted to this type zeolite
catalyst by various activation procedures and other treatments such
as base exchange, steaming, alumina extraction and calcination, in
combinations. Natural minerals which may be so treated include
ferrierite brewsterite, stilbite, dachiardite, epistilbite,
heulandite, and clinoptilolite. The preferred crystalline
aluminosilicates are ZSM-5,ZSM-11,ZSM-12,ZSM-38 and ZSM-35 with
ZSM-5 particularly preferred.
In a preferred aspect of this invention, the zeolites hereof are
selected as those having a crystal framework density, in the dry
hydrogen form, of not substantially below about 1.6 grams per cubic
centimeter. It has been found that zeolites which satisfy all three
of these criteria are most desired because they tend to maximize
the production of gasoline boiling range hydrocarbon products.
Therefore, the preferred zeolites of this invention are those
having a constraint index as defined above of about 1 to about 12,
a silica to alumina ratio of at least about 12 and a dried crystal
density of not less than about 1.6 grams per cubic centimeter. The
dry density for known structures may be calculated from the number
of silicon plus aluminum atoms per 1000 cubic Angstroms, as given,
e.g., on Page 19 of the article on Zeolite Structure by W. M.
Meier. This paper, the entire contents of which are incorporated
herein by reference, is included in "Proceedings of the Conference
on Molecular Sieves, London, April 1967", published by the Society
of Chemical Industry, London, 1968. When the crystal structure is
unknown, the crystal framework density may be determined by
classical pyknometer techniques. For example, it may be determined
by immersing the dry hydrogen form of the zeolite in an organic
solvent which is not sorbed by the crystal. It is possible that the
unusual sustained activity and stability of this class of zeolites
is associated with its high crystal anionic framework density of
not less than about 1.6 grams per cubic centimeter. This high
density, of course, must be associated with a relatively small
amount of free space within the crystal, which might be expected to
result in more stable structures. This free space, however, is
important as the locus of catalytic activity.
Crystal framework densities of some typical zeolites are:
______________________________________ Void Framework Zeolite
Volume Density ______________________________________ Ferrierite
0.28 cc/cc 1.76 g/cc Mordenite .28 1.7 ZSM-5, -11 .29 1.79
Dachiardite .32 1.72 L .32 1.61 Clinoptilolite .34 1.71 Laumontite
.34 1.77 ZSM-4 (Omega) .38 1.65 Heulandite .39 1.69 P .41 1.57
Offretite .40 1.55 Levynite .40 1.54 Erionite .35 1.51 Gmelinite
.44 1.46 Chabazite .47 1.45 A .5 1.3 Y .48 1.27
______________________________________
The crystal size of the synthesized zeolite has been found to be an
important factor affecting the desired isomerization of C.sub.8
aromatic mixtures. The crystal size of the above-described
crystalline aluminosilicate zeolite employed in the process of the
invention is at least about 1 micron, being in the approximate
range of 1-20 microns and particularly in the range of 1-6 microns.
With the use of crystals within such size range, distinctly higher
selectivity of production of the desired para-xylene has been
observed as compared with comparable use of smaller size
crystals.
When synthesized in the alkali metal form, the zeolite is
conveniently converted to the hydrogen form, generally by
intermediate formation of the ammonium form as a result of ammonium
ion exchange and calcination of ammonium form to yield the hydrogen
form. In addition to the hydrogen form, other forms of the zeolite
wherein the original alkali metal has been reduced to less than
about 1.5 percent by weight may be used. Thus, the original alkali
metal of the zeolite may be replaced by ion exchange with other
suitable ions of Groups IB to VIII of the Periodic Table,
including, by way of example, nickel, copper, zinc, palladium,
calcium or rare earth metals.
Generally, however, the zeolite either directly or via initial
ammonium exchange followed by calcination, is preferably hydrogen
exchanged such that a predominate proportion of its exchangeable
cations are hydrogen ions. In general, it is contemplated that more
than 50 percent and preferably more than 75 percent of the cationic
sites of the crystalline aluminosilicate zeolite will be occupied
by hydrogen ions.
In practicing the desired isomerization process, it may be
desirable to incorporate the above-described crystalline
aluminosilicate zeolite in another material resistant to the
temperature and other conditions employed in the process. 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 either 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 zeolite include those of the
montmorillonite and kaolin families, which families includes the
sub-bentonites and the kaolins commonly known as Dixie,
McNamee:Georgia and Florida clays or others in which the main
mineral constituent is halloysite, kaolinite, dickite, nacrite or
anauxite. Such clays can be used in a raw state as originally mined
or initially subjected to calcination, acid treatment or chemical
modification.
In addition to the foregoing materials, the zeolites employed
herein may be composited with a porous matrix material, such as
alumina, silica-alumina, silica-magnesia, silica-zirconia,
silica-thoria, silica-berylia, silica-titania as well as ternary
compositions, such as silica-alumina-thoria,
silica-alumina-zirconia, silica-alumina-magnesia and
silica-magnesia-zirconia. The matrix may be in the form of a cogel.
The relative proportions of zeolite component and inorganic oxide
gel matrix may vary widely with the zeolite content ranging from
between about 1 to about 99 percent by weight and more usually in
the range of about 5 to about 80 percent by weight of the
composite.
In the process of the invention, the feed consisting essentially of
a mixture of C.sub.8 aromatic hydrocarbons including ethylbenzene
and xylene is contacted with the above-described catalyst bed at a
temperature of about 400.degree. F. to about 1000.degree. F. and
preferably between about 500.degree. F. to about 950.degree. F., a
contact time equivalent to or the same as a weight hourly space
velocity (WHSV) of about 0.5 to about 100 and preferably about 1 to
about 50, it being understood that WHSV signifies pounds of feed
per pound of zeolite per hour and at a pressure of about 0 to about
1500 psig, preferably between about 25 and about 1000 psig.
Hydrogen may be co-fed along with the feed to retard catalyst
aging, utilizing a hydrogen/feed mole ratio between about 0 and
about 10.
In one embodiment, the crystalline aluminosilicate zeolites
employed are modified prior to use to exhibit a xylene sorption
capacity greater than 1 gram/100 grams of zeolite and an ortho
zylene sorption time for 30 percent of said capacity of greater
than 10 minutes, said sorption capacity and sorption time being
measured at 120.degree. C. and a xylene pressure of 4.5.+-.0.8 mm.
of mercury. This may be accomplished by combining with the
crystalline aluminosilicate zeolite a small amount, generally in
the range of about 0.5 to about 40 weight percent, preferably of a
difficulty reducible oxide, such as the oxides of phosphorus,
boron, magnesium or combinations thereof and also oxides of
antimony. Modification of the zeolite with the desired oxide or
oxides can readily be effected by contacting the zeolite with a
solution of an appropriate compound of the element to be
introduced, followed by drying and calcining to convert the
compound to its oxide form.
The measurements of hydrocarbon sorption capacities and rates are
conveniently carried out gravimetrically in a thermal balance. In
particular, it has been found that an equilibrium sorption capacity
of xylene, which can be either para, meta, ortho or a mixture
thereof, preferably para-xylene since this isomer reaches
equilibrium within the shortest time of at least 1 gram per 100
grams of zeolite measured at 120.degree. C. and a xylene pressure
of 4.5.+-.0.8 mm. of mercury and an ortho-xylene sorption time for
30 percent of said capacity of greater than 10 minutes (at the same
conditions of temperature and pressure) are required in order to
achieve the desired selective production of para-xylene.
It has been found that zeolites exhibiting very high selectivity
for para-xylene production require a very long time up to and
exceeding a thousand minutes to sorb o-xylene in an amount of 30%
of total xylene sorption capacity. For those materials, it is more
convenient to determine the sorption time for a lower extent of
sorption, such as 5%, 10% or 20% of capacity, and to estimate the
30% sorption time by applying the following multiplication factors
F as illustrated for 5% sorption:
______________________________________ t.sub.0.3 = F t.sub.0.05
Factor (F) to Percent of sorption Estimate 30% capacity Sorption
Time ______________________________________ 5 36 10 9 20 2.2
______________________________________
Representative phosphorus-containing compounds which may be used
include derivatives of groups represented by PX.sub.3, RPX.sub.2,
R.sub.2 PX, R.sub.3 P, X.sub.3 PO, (XO.sub.3)PO, (XO).sub.3 P,
R.sub.3 P.dbd.O, R.sub.3 P.dbd.S, RPO.sub.2, PPS.sub.2,
RP(O)(OX).sub.2, RP(S)(SX).sub.2, R.sub.2 P(O)OX, R.sub.2 P(S)SX,
RP(OX).sub.2, RP(SX).sub.2, ROP(OX).sub.2, RSP(SX).sub.2,
(RS).sub.2 PSP(SR).sub.2, and (RO).sub.2 POP(OR).sub.2, where R is
an alkyl or aryl, such as a phenyl radical and X is hydrogen, R, or
halide. These compounds include primary, RPH.sub.2, secondary,
R.sub.2 PH and tertiary, R.sub.3 P, phosphines such as butyl
phosphine; the tertiary phosphine oxides R.sub.3 PO, such as
tributylphosphine oxide, the tertiary phosphine sulfides, R.sub.3
PS, the primary RP(O)(OX).sub.2, and secondary, R.sub.2 P(O)OX,
phosphonic acids such as benzene phosphonic acid; the corresponding
sulfur derivatives such as RP(S)(SX).sub.2 and R.sub.2 P(S)SX, the
esters of the phosphonic acids such as diethyl phosphonate,
(RO).sub.2 P(O)H, dialkyl alkyl phosphonates, (RO).sub.2 P(O)R, and
alkyl dialkylphosphinates, (RO)P(O)R.sub.2 ; phosphinous acids,
R.sub.2 POX, such as diethylphosphinous acid, primary,
(RO)P(OX).sub.2, secondary, (RO).sub.2 POX, and tertiary,
(RO).sub.3 P, phosphites; and esters thereof such as the monopropyl
ester, alkyl dialkylphosphinites, (RO)PR.sub.2, and dialkyl
alkylphosphonite, (RO).sub.2 PR esters. Corresponding sulfur
derivatives may also be employed including (RS).sub.2 P(S)H,
(RS).sub.2 P(S)R, (RS)P(S)R.sub.2, R.sub.2 PSX, (RS)P(SX).sub.2,
(RS).sub.2 PSX, (RS).sub.3 P, (RS)PR.sub.2 and (RS).sub.2 PR.
Examples of phosphite esters include trimethylphosphite,
triethylphosphite, diisopropylphosphite, butylphosphite; and
pyrophosphites such as tetraethylpyrophosphite. The alkyl groups in
the mentioned compounds contain one to four carbon atoms.
Other suitable phosphorus-containing compounds include the
phosphorus halides such as phosphorus trichloride, bromide, and
iodide, alkyl phosphorodichloridites, (RO)PCl.sub.2, dialkyl
phosphorochloridites, (RO).sub.2 PX, dialkylphosphionochloridites,
R.sub.2 PCl, alkyl alkylphosphonochloridates, (RO)(R)P(O)Cl,
dialkyl phosphinochloridates, R.sub.2 P(O)Cl and RP(O)Cl.sub.2.
Applicable corresponding sulfur derivatives include (RS)PCl.sub.2,
(RS).sub.2 PX, (RS)(R)P)S)Cl and R.sub.2 P(S)Cl.
Preferred phosphorus-containing compounds include diphenyl
phosphine chloride, trimethylphosphite and phosphorus trichloride,
phosphoric acid, ammonium hydrogen phosphites, including dihydrogen
ammonium phosphate and diammonium hydrogen phosphite, phenyl
phosphine oxychloride, trimethylphosphate, diphenyl phosphinous
acid, diphenyl phosphinic acid, diethylchloro thiophosphate, methyl
acid phosphate and other alcohol-P.sub.2 O.sub.5 reaction
products.
Reaction of the zeolite with the phosphorous compound is effected
by contacting the zeolite with such compound. Where the treating
phosphorus compound is a liquid, such compound can be in solution
in a solvent at the time contact with the zeolite is effected. Any
solvent relatively inert with respect to the treating compound and
the zeolite may be employed. Suitable solvents include water and
aliphatic, aromatic or alcoholic liquids. Where the
phosphorus-containing compound is, for example, trimethylphosphite
or liquid phosphorus trichloride, a hydrocarbon solvent such as
n-octane may be employed. The phosphorus-containing compound may be
used without a solvent, i.e., may be used as a neat liquid. Where
the phosphorus-containing compound is in the gaseous phase, such as
where gaseous phosphorus trichloride is employed, the treating
compound can be used by itself or can be used in admixture with a
gaseous diluent relatively inert to the phosphorus-containing
compound and the zeolite such as air or nitrogen or with an organic
solvent, such as octane or toluene.
Prior to reacting the zeolite with the phosphorus-containing
compound, the zeolite may be dried. Drying can be effected in the
presence of air. Elevated temperatures may be employed. However,
the temperature should not be such that the crystal structure of
the zeolite is destroyed.
Heating of the phosphorus-containing catalyst subsequent to
preparation and prior to use is also preferred. The heating can be
carried out in the presence of oxygen, for example, air. Heating
can be at a temperature of about 150.degree. C. However, higher
temperatures. i.e., up to about 500.degree. C. are preferred.
Heating is generally carried out for 3-5 hours but may be extended
to 24 hours or longer. While heating temperatures above about
500.degree. C. can be employed, they are not necessary. At
temperatures of about 1000.degree. C., the crystal structure of the
zeolite tends to deteriorate.
The amount of phosphorus incorporated with the zeolite should be at
least about 0.5 percent by weight. However, it is preferred that
the amount of phosphorus in the zeolite be at least about 2 percent
by weight when the same is combined with a binder, e.g., 35 weight
percent of alumina. The amount of phosphorus can be as high as
about 25 percent by weight or more depending on the amount and type
of binder present. Preferably, the amount of phosphorus added to
the zeolite is between about 0.7 and about 15 percent by
weight.
The amount of phosphorus incorporated with the zeolite by reaction
with elemental phosphorus or phosphorus-containing compound will
depend upon several factors. One of these is the reaction time,
i.e., the time that the zeolite and the phosphorus-containing
source are maintained in contact with each other. With greater
reaction times, all other factors being equal, a greater amount of
phosphorus is incorporated with the zeolite. Other factors upon
which the amount of phosphorus incorporated with the zeolite is
dependent include reaction temperature, concentration of the
treating compound in the reaction mixture, the degree to which the
zeolite has been dried prior to reaction with the
phosphorus-containing compound, the conditions of drying of the
zeolite after reaction of the zeolite with the treating compound,
and the amount and type of binder incorporated with the
zeolite.
Another suitable modifying oxide is that of magnesium.
Representative magnesium-containing compounds include magnesium
acetate, magnesium nitrate, magnesium benzoate, magnesium
proprionate, magnesium 2-ethylhexoate, magnesium carbonate,
magnesium formate, magnesium oxalate, magnesium amide, magnesium
bromide, magnesium hydride, magnesium lactate, magnesium laurate,
magnesium oleate, magnesium plamitate, magnesium silicylate,
magnesium stearate and magnesium sulfide.
Reaction of the zeolite with the treating magnesium compound is
effected by contacting the zeolite with such compound. Where the
treating compound is a liquid, such compound can be in solution in
a solvent at the time contact with the zeolite is effected. Any
solvent relatively inert with respect to the treating magnesium
compound and the zeolite may be employed. Suitable solvents include
water and aliphatic, aromatic or alcoholic liquid. The treating
compound may also be used without a solvent, i.e., may be used as a
neat liquid. Where the treating compound is in the gaseous phase,
it can be used by itself or can be used in admixture with a gaseous
diluent relatively inert to the treating compound and the zeolite
such as helium or nitrogen or with an organic solvent, such as
octane or toluene.
Heating of the magnesium compound impregnated catalyst subsequent
to preparation and prior to use is preferred. The heating can be
carried out in the presence of oxygen, for example, air. Heating
can be at a temperature of about 150.degree. C. However, higher
temperatures, i.e. up to about 500.degree. C. are preferred.
Heating is generally carried out for 1-5 hours but may be extended
to 24 hours or longer. While heating temperatures above about
500.degree. C. may be employed, they are generally not necessary.
At temperatures of about 1000.degree. C., the crystal structure of
the zeolite tends to deteriorate. After heating in air at elevated
temperatures, the oxide form of magnesium is present.
The amount of magnesium oxide incorporated in the zeolite should be
at least about 0.25 percent by weight. However, it is preferred
that the amount of magnesium oxide in the zeolite be at least about
1 percent by weight, particularly when the same is combined with a
binder, e.g., 35 weight percent of alumina. The amount of magnesium
oxide can be as high as about 25 percent by weight or more
depending on the amount and type of binder present. Preferably, the
amount of magnesium oxide added to the zeolite is between about 1
and about 15 percent by weight.
Boron oxide is also an effective modifying component.
Representative boron-containing compounds include boric acid,
trimethylborate, boron hydride, boron oxide, boron sulfide,
butylboron dimethoxide, butylboronic acid, dimethylboric anhydride,
hexamethylborazine, phenylboric acid, triethylborane,
tetramethylammonium borohydride, triphenyl boron and
allylborate.
Reaction of the zeolite with the boron compound is effected by
contacting the zeolite with such compound. Where the treating boron
compound is a liquid, such compound can be in solution in a solvent
at the time contact with the zeolite is effected. Any solvent
relatively inert with respect to the treating compound and the
zeolite may be employed. Suitable solvents include water and
aliphatic, aromatic or alcoholic liquids. Where the
boron-containing compound is, for example, trimethylborate, a
hydrocarbon solvent such as n-octane may be employed. The
boron-containing compound may be used without a solvent, i.e., may
be used as a neat liquid. Where the boron-containing compound is in
the gaseous phase, such as where gaseous diborane is employed, the
treating compound can be used by itself or can be used in admixture
with a gaseous diluent inert to the boron-containing compound and
the zeolite such as nitrogen or helium or with an organic solvent,
such as octane.
Prior to reacting the zeolite with the boron-containing compound,
the zeolite may be dried. Drying can be effected in the presence of
air. Elevated temperatures may be employed. However, the
temperature should not be such that the crystal structure of the
zeolite is destroyed.
Heating of the boron-containing catalyst subsequent to preparation
and prior to use is also preferred. The heating can be carried out
in the presence of oxygen, for example, air. Heating can be at a
temperature of about 150.degree. C. However, higher temperatures,
i.e., up to about 500.degree. C. are preferred. Heating is
generally carried out for 3-5 hours but may be extended to 24 hours
or longer. While heating temperatures above about 500.degree. C.
can be employed, they are not necessary. At temperatures of about
1000.degree. C., the crystal structure of the zeolite tends to
deteriorate.
The amount of boron incorporated with the zeolite should be at
least about 0.2 percent by weight. However, it is preferred that
the amount of boron in the zeolite be at least about 1 percent by
weight when the same is combined with a binder, e.g., 35 weight
percent of alumina. The amount of boron can be as high as about 20
percent by weight or more depending on the amount and type of
binder present. Preferably, the amount of boron added to the
zeolite is between about 1.5 and 10 percent by weight. Without
being limited by any theoretical considerations, it is contemplated
that boron is actually present in the zeolite in an oxidized state,
such as B.sub.2 O.sub.3.
Antimony oxide may also be employed as a modifying component. The
antimony oxide is present as Sb.sub.2 O.sub.3 alone or in admixture
with other antimony oxides with or without metallic antimony or
other antimony compounds being present. In all instances,
regardless of the particular state of oxidation of the antimony,
its content with respect to the zeolite is computed as if it were
present as Sb.sub.2 O.sub.3. Generally, the amount of Sb.sub.2
O.sub.3 in the composite catalyst will be between about 6 and about
40 weight percent and preferably between about 10 and about 35
weight percent. Antimony derivatives which may be used include: the
hydrides SbH.sub.3 ; the halides MX.sub.3, MX.sub.5 (M=Sb, X=F, Cl,
Br, I); organic alkyl and aryl stibines and their oxides R.sub.3
Sb, R.sub.5 SB, R.sub.x SB.dbd.O (R-alkyl or aryl); halogen
derivatives RSbX.sub.2, R.sub.2 SbX, RSbX.sub.4, R.sub.2 SbX.sub.3,
R.sub.3 SbX.sub.2, R.sub.4 SbX; the acids H.sub.3 SbO.sub.3,
HSbO.sub.2, HSb(OH).sub.6 ; organic acids such as RSbO(OH).sub.2,
R.sub.2 SbO. OH, all with R and X defined as above noted. Also
included are organic ethers such as R.sub.2 SbOSbR.sub.2 ; esters
and alcoholates such as Sb(OOCCH.sub.3).sub.3, Sb(OC.sub.4
H.sub.9).sub.3, Sb(OC.sub.2 H.sub.5).sub.3, Sb(OCH.sub.3).sub.3 ;
and antimonyl salts as (SbO)SO.sub.4, (SbO)NO.sub.3, K(SbO)C.sub.4
H.sub.4 O.sub.6, NaSbO.sub.2. 3H.sub.2 O.
In some instances, it may be desirable to modify the crystalline
aluminosilicate zeolite by combining therewith two or more of the
specified oxides. Thus, the zeolite may be modified by prior
combination therewith of oxides of phosphorus and boron, oxides of
phosphorus and magnesium or oxides of magnesium and boron. When
such modification technique is employed, the oxides may be
deposited on the zeolite either sequentially or from a solution
containing suitable compounds of the elements, the oxides of which
are to be combined with the zeolite. The amounts of oxides present
in such instance are in the same range as specified above for the
individual oxides, with the overall added oxide content being
between about 0.5 and about 40 weight percent.
Still another modifying treatment entails steaming of the zeolite
by contact with an atmosphere containing from about 5 to about 100
percent steam at a temperature of from about 250.degree. to about
800.degree. C. for a period of between about 0.25 and about 100
hours and under pressures ranging from the sub-atmospheric to
several hundred atmospheres to reduce the alpha value thereof
determined at 1000.degree. F., following the general procedure
described in the Journal of Catalysis, Volume VI, pages 278-287,
1966, to less than 500 and preferably less than 100 but greater
than zero.
Another modifying treatment involves precoking of the catalyst to
deposit a coating of between about 1 and about 50 and preferably
between about 2 and about 20 weight percent of coke thereon.
Precoking can be accomplished by contacting the catalyst with a
hydrocarbon charge, e.g., toluene, or preferably the hydrocarbon
feed employed in the process, under high severity conditions or
alternatively at a reduced hydrogen to hydrocarbon concentration,
i.e., 0 to 1 mole ratio of hydrogen to hydrocarbon for a sufficient
time to deposit the desired amount of coke thereon.
It is also contemplated that a combination of steaming and
precoking of the catalyst under the above conditions may be
employed to suitably modify the crystalline aluminosilicate zeolite
catalyst.
The conversion process described herein may be carried out as a
batch type, semi-continuous or continuous operation utilizing a
fixed or moving bed catalyst system. The catalyst after use in a
moving bed reactor is conducted to a regeneration zone wherein coke
is burned from the catalyst in an oxygen-containing atmosphere,
e.g., air, at an elevated temperature, after which the regenerated
catalyst is recycled to the conversion zone for further contact
with the charge stock. In a fixed bed reactor, regeneration is
carried out in a conventional manner where an inert gas containing
a small amount of oxygen (0.5-2%) is used initially to burn the
coke in a controlled manner so as to limit the temperature to a
maximum of around 500.degree.-550.degree. C.
In general, the xylene isomerization reaction is carried out in a
fixed bed flow reactor containing the specified crystalline
aluminosilicate catalyst in which the majority of the crystals are
greater than about 1 micron, as determined by electron microscope
examination. In more specific embodiments, the reaction is carried
out with the crystalline aluminosilicate catalyst having a bimodal
crystal size distribution generally falling in the two ranges, less
than about 1 micron and greater than about 1 micron, with the
latter being in major proportion.
In one embodiment, as illustrated in FIG. 1, the catalyst may be in
a single reactor where either the crystalline aluminosilicate
zeolite catalysts of differing crystal size are in sequential beds,
with the catalyst crystals of greater than about 1 micron size
preceding the catalyst crystals of less than about 1 micron size or
the differing crystal size catalysts may be present as a physical
mixture.
In another embodiment, as shown in FIG. 2, the catalysts of
differing crystal size may be in separate reactors maintained at
the same temperature or at different temperatures with the
temperature in the first reactor being higher than in the second
and with the reactor containing the catalyst crystals of greater
than about 1 micron preceding the reactor containing the catalyst
crystals of less than 1 micron size.
In still another embodiment, illustrated in FIG. 3, the crystalline
aluminosilicate zeolite catalyst of greater than about 1 micron
size is contained in a separate reactor which contacts only the
fresh feed, i.e., is situated outside the recycle loop. In this
embodiment, operating conditions, including temperature and space
velocity, are adjusted to convert a major fraction of the
ethylbenzene in this reactor. A second reactor in the recycle loop
contains the crystalline aluminosilicate catalyst having a crystal
size of less than about 1 micron.
In a further embodiment, shown in FIG. 4, the crystalline
aluminosilicate catalyst of greater than about 1 micron crystal
size is contained in a reactor to which a feed having a depleted
ortho and para-xylene content is conducted.
With the aromatic C.sub.8 feed mixture utilized herein containing
ethylbenzene, and xylene, a certain quantity of ethylbenzene has to
undergo conversion in the isomerization reactor to prevent build-up
in the recycle system. Inevitably, some of the xylene is
simultaneously converted to less valuable products. Utilizing the
crystalline aluminosilicate catalyst having a crystal size of at
least about 1 micron described hereinabove, it has been found that
considerably less xylene is converted for a given amount of
ethylbenzene converted, i.e., the selectivity ratio f.sub.EB
/f.sub.XYL is larger, where f.sub.EB is the fraction of
ethylbenzene in the feed that is converted and f.sub.XYL is the
fraction of xylene in the feed that is converted.
Turning now to FIG. 1, a C.sub.8 aromatics feed containing
ethylbenzene, para-xylene, meta-xylene and ortho-xylene is fed
through line 10 to crystallizer 11, from which para-xylene is
removed. Residual is removed from the crystallizer through line 13
to isomerization reactor 14 containing the above-described
crystalline aluminosilicate zeolite catalyst having a crystal size
greater than about 1 micron present in major amount and a smaller
amount of similar catalyst of less than 1 micron crystal size. The
catalysts of varying crystal size may be present as a physical
mixture or the catalyst having a crystal size of greater than about
1 micron should be present as a bed at the point of initial contact
of the incoming stream. The isomerizate product exits from the
reactor through line 15 to stripper 16 where products lighter than
C.sub.8 are removed as overhead through line 17, C.sub.8 and
heavier materials pass from stripper 16 through line 18 to spliter
tower 19 where C.sub.9 and heavier materials are removed through
line 20. C.sub.8 materials removed from the splitter tower through
line 21 are conducted to crystallizer 11 and the cycle
repeated.
In the embodiment of FIG. 2, a C.sub.8 aromatics feed containing
ethylbenzene, para-xylene, meta-xylene and ortho-xylene is fed
through line 30 to crystallizer 31, from which para-xylene is
removed. Residual is removed from the crystallizer through line 33
to isomerization reactor 34 containing the described crystalline
aluminosilicate zeolite catalyst having a crystal size greater than
about 1 micron. The isomerizate product exists from reactor 34
through line 35 to a second isomerization reactor 36 containing the
crystalline aluminosilicate zeolite catalyst having a crystal size
less than about 1 micron. The resulting isomerized product is
removed through line 37 to stripper 38 from which products lighter
than C.sub.8 are removed as overhead through line 39. C.sub.8 and
heavier materials pass from stripper 38 through line 40 to splitter
tower 41 where C.sub.9 and heavier materials are removed through
line 42. C.sub.8 materials removed from the splitter tower through
line 43 are conducted to crystallizer 31 and the cycle
repeated.
In the embodiment of FIG. 3, a C.sub.8 aromatic feed containing
ethylbenzene, para-xylene, meta-xylene and ortho-xylene is fed
through line 50 to isomerization reactor 51 containing the
described crystalline aluminosilicate zeolite catalyst having a
crystal size greater than about 1 micron. The isomerizate product
passes from reactor 51 through line 52 to stripper 53 from which
products lighter than C.sub.8 are removed as overhead through line
54. C.sub.8 and heavier materials pass from stripper 53 through
line 55 to splitter tower 56 from which C.sub.9 and heavier
materials are removed through line 57. C.sub.8 materials removed
from the splitter tower through line 58 are conducted to
crystallizer 59, from which para-xylene is removed through line 60.
Residual is removed from the crystallizer 59 through line 61 to
isomerization reactor 62 containing the crystalline aluminosilicate
zeolite catalyst having a crystal size less than about 1 micron.
The resulting isomerized product is removed through line 63 to
stripper 53 and the cycle repeated.
In the embodiment of FIG. 4, a C.sub.8 aromatic feed containing
ethylbenzene and xylenes is fed through line 70 to xylene splitter
tower 71. The bottoms from the xylene splitter constituted by
o-xylene and C.sub.9 aromatics passes by line 72 to the o-xylene
tower 73 from which o-xylene is taken overhead at line 74 and heavy
ends are removed by line 75. The overhead from xylene splitter
tower 71 is transferred to para-xylene crystallizer 76 through line
77. The crystallizer operates in a manner described in U.S. Pat.
No. 3,662,013 to Machell et al.
Since its melting point is much higher than that of the other
C.sub.8 aromatics, p-xylene is readily separated in the
crystallizer after refrigeration of the stream and a xylene mixture
lean in p-xylene is transferred to an isomerization unit through
line 78. The isomerization charge passes through a heater 79, is
admixed with hydrogen admitted through line 80 and the mixture
introduced to the reactor 81.
Isomerized product from reactor 81 is cooled in heat exchanger 82
and passed to a high pressure separator 83 from which separated
hydrogen can be recycled in the process. The liquid product of the
isomerization passes by line 84 to a stripper 85 from which
light-ends are passed overhead by line 86. The remaining liquid
product constituted by C.sub.8 + hydrocarbons is recycled in the
system by line 87 to the inlet of xylene stripper column 71.
The following examples are illustrative of the process of the
invention.
EXAMPLES 1-3
A feed constituting a blend of ethylbenzene and mixed xylenes,
having the composition set forth in Table I below, was passed over
a catalyst of HZSM-5 characterized by an average crystal size of
about 0.5 micron hereinafter referred to as Catalyst A at a
temperature of 650.degree. F., a pressure of 25 psig and weight
hourly space velocities of 10, 20 and 40. The product compositions
and selectivities obtained in each instance are given in Table
I.
In this and other tables, hereinafter set forth: .DELTA.EB is the
amount of ethylbenzene converted; EB is the amount of ethylbenzene
in the isomerization feed; f.sub.EB =.DELTA.EB/EB.times.100 is the
percent of ethylbenzene converted; .DELTA.XYL is the amount of
xylene converted; XYL is the amount in the isomerization feed;
f.sub.XYL =.DELTA.XYL/XYL.times.100 is the percent xylene
converted.
TABLE I ______________________________________ Example Feed 1 2 3
______________________________________ Temp. (.degree.F.) 650 650
650 WHSV 10 20 40 Product Analysis (wt. %) C.sub.5.sup.- 0.17 0.14
0.16 Benzene 0.04 3.27 2.09 1.23 Toluene 0.07 1.06 0.51 0.29
Ethylbenzene 19.61 13.14 15.40 17.05 p-xylene 9.42 18.32 18.67
17.89 m-xylene 50.38 40.89 41.77 43.07 o-xylene 20.48 17.40 18.01
18.49 C.sub.9.sup.+ -- 5.74 3.41 1.92 f.sub.EB = .DELTA. EB/EB
.times. 100 33.0 21.5 13.1 f.sub.XYL = .DELTA. XYL/XYL .times. 100
4.57 2.27 1.03 f.sub.EB /f.sub.XYL 7.2 9.5 12.7
______________________________________
EXAMPLES 4-6
A blend comprising ethylbenzene and mixed xylenes, having the
composition shown in Table II below, was passed over a catalyst of
HZSM-5 characterized by an average crystal size of about 2 microns,
hereinafter referred to as Catalyst B, at 650.degree. F., 25 psig
and weight hourly space velocities of 5, 10 and 20. The product
compositions and selectivities obtained in each instance are shown
in Table II
TABLE II ______________________________________ Example Feed 4 5 6
______________________________________ Temp. (.degree.F.) 650 650
650 WHSV 5 10 20 Product Analysis (wt. %) 5 C.sub.5.sup.- 1.31 0.65
0.38 Benzene 0.04 3.60 2.59 1.62 Toluene 0.11 1.51 0.86 0.35
Ethylbenzene 18.42 10.40 12.50 14.76 p-xylene 8.92 17.61 17.09
15.19 m-xylene 53.14 42.74 44.56 47.07 o-xylene 19.36 17.67 18.03
18.39 C.sub.9.sup.+ 4.96 3.59 2.23 f.sub.EB = .DELTA. EB/EB .times.
100 43.5 32.1 19.9 f.sub.XYL = .DELTA. XYL/XYL .times. 100 4.21
2.14 0.96 f.sub.EB /f.sub.XYL 10.3 15.0 20.8
______________________________________
EXAMPLES 7-9
A blend comprising ethylbenzene and mixed xylenes, having the
composition shown in Table III below, was passed over a catalyst
HZSM-5, chemically modified to contain 6 weight percent MgO,
hereinafter referred to as Catalyst C, characterized by an average
crystal size of about 2 microns at 650.degree. F. 25 psig and
weight hourly space velocities of 7.7, 15.4 and 30.8. The product
compositions and selectivities obtained in each instance are shown
in Table III.
TABLE III ______________________________________ Example Feed 7 8 9
______________________________________ Temp. (.degree.F.) 650 650
650 WHSV 7.7 15.4 30.8 Product Analysis (wt. %) C.sub.5.sup.- 0.45
0.40 0.26 Benzene 0.04 2.91 2.50 1.37 Toluene 0.07 1.07 0.76 0.26
Ethylbenzene 19.70 13.33 14.18 16.41 p-xylene 9.35 11.41 10.88
10.18 m-xylene 50.48 48.20 48.83 49.78 o-xylene 20.36 19.39 19.57
19.94 C.sub.9.sup.+ 3.09 3.74 1.65 f.sub.EB = .DELTA. EB/EB .times.
100 32.3 28.0 16.6 f.sub.XYL = .DELTA. XYL/XYL .times. 100 1.48
1.13 0.26 f.sub.EB /f.sub.XYL 21.8 24.8 63.8
______________________________________
EXAMPLES 10-12
A feed blend comprising ethylbenzene and mixed xylenes, having the
composition shown in Table IV below, was passed through a catalyst
bed, hereinafter collectively referred to as Catalyst D, the upper
2/3 of which making initial contact with the feed, was HZSM-5having
a crystal size of about 2 microns and the lower 1/3 of which was
HZSM-5 having a crystal size of about 0.5 micron. Reaction
conditions included a temperature of 650.degree. F. a pressure of
25 psig and weight hourly space velocities of 5, 10 and 20. The
product compositions and selectivities obtained in each instance
are shown in Table IV.
TABLE IV ______________________________________ Example Feed 10 11
12 ______________________________________ Temp. (.degree.F.) 650
650 650 WHSV 5 10 20 Product Analysis (wt. %) C.sub.5.sup.- 1.96
0.96 0.64 Benzene 0.05 3.55 2.88 1.79 Toluene 0.20 1.41 0.87 0.40
Ethylbenzene 19.84 12.02 13.72 15.92 p-xylene 9.54 17.85 17.63
16.90 m-xylene 49.50 40.50 41.82 43.13 o-xylene 20.71 17.27 18.04
18.78 C.sub.9.sup.+ 5.34 4.07 2.42 f.sub.EB = .DELTA. EB/EB .times.
100 39.5 30.9 19.6 f.sub.XYL = .DELTA. XYL/XYL .times. 100 5.33
2.98 1.31 f.sub.EB /f.sub.XYL 7.4 10.4 14.9
______________________________________
A comparison of the xylene selectivities of the four catalysts
utilized in the above examples is shown in FIG. 5. Referring more
particularly to this Figure, selectivity is expressed in terms of
the ratio of relative ethylbenzene loss to relative xylenes loss
(f.sub.EB /f.sub.XYL). It will be evident from the plotted results
that there is a marked improvement in selectivity obtained
utilizing Catalysts B, C and D over comparable use of the
conventional catalyst, exemplified by Catalyst A.
It is to be understood that the foregoing description is merely
illustrative of preferred embodiments of the invention of which
many variations may be made by those skilled in the art within the
scope of the following claims without departing from the spirit
thereof.
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