U.S. patent application number 11/829225 was filed with the patent office on 2008-02-21 for xylene isomerization process and apparatus.
Invention is credited to Robert B. Larson, James E. Rekoske, Freddie Sandifer, Patrick J. Silady, Patrick C. Whitchurch.
Application Number | 20080041765 11/829225 |
Document ID | / |
Family ID | 39100364 |
Filed Date | 2008-02-21 |
United States Patent
Application |
20080041765 |
Kind Code |
A1 |
Larson; Robert B. ; et
al. |
February 21, 2008 |
Xylene Isomerization Process and Apparatus
Abstract
Xylene isomerization processes, especially those processes in
which ethylbenzene is also converted, are beneficially affected by
adding benzene to the feed.
Inventors: |
Larson; Robert B.; (Chicago,
IL) ; Rekoske; James E.; (South Kensington, GB)
; Silady; Patrick J.; (Niles, IL) ; Whitchurch;
Patrick C.; (Bossier City, LA) ; Sandifer;
Freddie; (Chicago, IL) |
Correspondence
Address: |
HONEYWELL INTELLECTUAL PROPERTY INC;PATENT SERVICES
101 COLUMBIA DRIVE
P O BOX 2245 MAIL STOP AB/2B
MORRISTOWN
NJ
07962
US
|
Family ID: |
39100364 |
Appl. No.: |
11/829225 |
Filed: |
July 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11299579 |
Dec 12, 2005 |
|
|
|
11829225 |
Jul 27, 2007 |
|
|
|
Current U.S.
Class: |
208/134 |
Current CPC
Class: |
C07C 5/2737 20130101;
C10G 2400/30 20130101; C07C 5/2737 20130101; C07C 15/08
20130101 |
Class at
Publication: |
208/134 |
International
Class: |
C10G 35/04 20060101
C10G035/04 |
Claims
1. An apparatus comprising: a. a reaction assembly having at least
one feedstream inlet in communication with a source of a feed of
ethylbenzene and xylene in a non-equilibrium mixture and at least
one product outlet containing therebetween at least one reaction
zone wherein at least one reaction zone contains catalyst suitable
for the conversion of ethylbenzene and the same or at least one
other reaction zone contains catalyst suitable for the
isomerization of xylenes; and b. a distillation assembly in fluid
communication at an inlet with the product outlet of the reaction
assembly, said distillation assembly having an outlet adapted to
provide a benzene-containing stream, said outlet being in fluid
communication with at least one feedstream inlet of the reaction
assembly.
2. The apparatus of claim 1 wherein the source of a feed of
ethylbenzene and xylene is a unit capable of separating para-xylene
from a mixture of xylenes.
3. The apparatus of claim 2 wherein the distillation assembly has
at least one outlet adapted to provide a xylene-containing stream,
said outlet being in fluid communication with said unit capable of
separating para-xylene from a mixture of xylenes.
4. The apparatus of claim 3 wherein the catalyst in said reaction
assembly comprises pentasil zeolite.
Description
CROSS REFERENCE TO THE APPLICATION
[0001] This application is a Division of copending application Ser.
No. 11/299,579 filed Dec. 12, 2005, the contents of which are
hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to improved processes for the
catalytic isomerization of a non-equilibrium mixture of one or more
xylenes including isomerization processes associated with the
conversion of ethylbenzene and apparatus therefor.
BACKGROUND OF THE INVENTION
[0003] The xylenes, para-xylene, meta-xylene and ortho-xylene, are
important intermediates which find wide and varied application in
chemical syntheses. Para-xylene upon oxidation yields terephthalic
acid which is used in the manufacture of synthetic textile fibers
and resins. Meta-xylene is used in the manufacture of plasticizers,
azo dyes, wood preservers, etc. Ortho-xylene is feedstock for
phthalic anhydride production.
[0004] Xylene isomers from catalytic reforming or other sources
generally do not match demand proportions as chemical
intermediates, and further comprise ethylbenzene which is difficult
to separate or to convert. Para-xylene in particular is a major
chemical intermediate. Adjustment of isomer ratio to demand can be
effected by combining xylene-isomer recovery, such as adsorption
for para-xylene recovery, with isomerization to yield an additional
quantity of the desired isomer. Isomerization converts a
non-equilibrium mixture of the xylene isomers which is lean in the
desired xylene isomer to a mixture which approaches equilibrium
concentrations.
[0005] Various catalysts and processes have been developed to
effect xylene isomerization. Catalysts for isomerization of C.sub.8
aromatics ordinarily are classified by the manner of processing
ethylbenzene associated with the xylene isomers. Ethylbenzene is
not easily isomerized to xylenes, but it normally is converted in
the isomerization unit because separation from the xylenes by
superfractionation or adsorption is very expensive. One approach is
reacting ethylbenzene to form a xylene mixture via conversion to
and reconversion from naphthenes in the presence of a solid acid
catalyst with a hydrogenation-dehydrogenation function. An
alternative, widely-used approach is to dealkylate ethylbenzene to
form principally benzene while isomerizing xylenes to a
near-equilibrium mixture. The former approach enhances xylene yield
by forming xylenes from ethylbenzene, while the latter approach
commonly results in higher ethylbenzene conversion, thus lowering
the quantity of recycle to the para-xylene recovery unit and
concomitant processing costs.
[0006] Crystalline aluminosilicate zeolite-containing catalysts
have become prominent for xylene isomerization. U.S. Pat. No.
3,856,872, for example, teaches xylene isomerization and
ethylbenzene conversion with a catalyst containing ZSM-5, -12, or
-21 zeolite. U.S. Pat. No. 4,626,609 discloses conversion of xylene
isomers using a catalyst comprising a composite which has been
steamed at 200.degree. to 500.degree. C. U.S. Pat. No. 4,899,012
discloses the use of a catalyst containing lead, a Group VIII
metal, a pentasil zeolite and an inorganic-oxide binder to
isomerize xylenes and dealkylate ethylbenzene. Development efforts
continue toward realizing economically attractive isomerization
catalysts with a superior combination of activity, selectivity and
stability.
[0007] Of concern in isomerization processes is the loss of
xylenes. The major loss of xylenes is believed to result from the
disproportionation of xylene to toluene and trimethylbenzene. U.S.
Pat. No. 5,998,688 proposes a xylene isomerization process
preferably using a ZSM-5-containing catalyst with a toluene
co-feed. The patentees state that the feedstock to the
isomerization contains 1 to 25 percent toluene by weight, and that
increasing the toluene concentration minimizes the loss of xylenes
during the ethylbenzene conversion stage. Nevertheless, in the sole
example, the addition of toluene to provide a feed containing 18.7
weight percent toluene, resulted in an increase of trimethylbenzene
which is inconsistent with the objective of reducing xylene ring
loss. The mass ratio of C.sub.9+ to C.sub.8 aromatics more than
doubles between the described control and the toluene-co-feed
examples.
[0008] The presence of toluene in feeds to xylene isomerization
processes has been disclosed by others subsequently. Benzene and
toluene can be present in the non-equilibrium xylene mixtures,
including in commercially operating para-xylene and ortho-xylene
production facilities. Examples III and IV of U.S. Pat. No.
6,576,581 disclose xylene isomerization of a feed containing 1.53
and 0.78 mass-% toluene plus benzene.
[0009] U.S. Pat. No. 6,198,014 discloses a process for isomerizing
C.sub.8 aromatic compounds involving adding hydrogen and a recycle
mixture to the C.sub.8 aromatic feed to the isomerization reactor.
The recycle is said to comprise at least one acyclic C.sub.8
paraffin, at least one C.sub.8 naphthene, at least benzene and at
least toluene and is said to be devoid of C.sub.8 aromatic
compounds and paraffins of 1 to 7 carbons. The patentees state that
the recycle mixture provides benefits of reducing the production of
paraffins, naphthenes and C.sub.9 and higher aromatics as well as
reducing the loss of C.sub.8 aromatics by secondary side reactions
of dismutation, transalkylation, hydrogenation and cracking.
Examples 5 and 6 in the patent use feeds containing, inter alia,
0.1 weight percent benzene and toluene, 2.0 and 2.9 weight percent
respectively.
SUMMARY OF THE INVENTION
[0010] In accordance with this invention, improved processes for
xylene isomerization using molecular sieve-containing isomerization
catalyst are provided, in which processes benzene is added. The
added benzene can beneficially affect one or more isomerization and
ethylbenzene conversion, if ethylbenzene is present, properties
including: (i) enhancing isomerization activity to achieve a closer
approach to equilibrium; (ii) enhancing the conversion of
ethylbenzene; (iii) reducing xylene loss; and (iv) reducing
trimethylbenzene. The beneficial effects are with observed through
comparison with a substantially identical process but without the
additional benzene. A substantially identical process is one that
has approximately the same weighted average bed temperature,
operating pressure and hourly space velocity is the same reactor
using the same catalyst (the activity and selectivity of which have
not been materially changed such as by use or poisoning) at steady
state conditions.
[0011] The isomerization processes of this invention are applicable
to xylene isomerization processes with and without associated
ethylbenzene conversion. Associated ethylbenzene conversion
processes include those in which the ethylbenzene conversion is
conducted during the isomerization as well as those in which the
ethylbenzene conversion is primarily conducted in a different
catalytic reaction zone after, or preferably before, the catalytic
reaction zone for the xylene isomerization. The ethylbenzene
conversion processes can be those in which the ethylbenzene is
converted to xylenes or, preferably, those in which ethylbenzene is
dealkylated. Typically, isomerization and ethylbenzene processes
generate benzene as a by-product, especially where ethylbenzene is
in the feed to the isomerization and dealkylation of ethylbenzene
occurs. In this invention, the benzene concentration is in addition
to any benzene that would be generated in the isomerization and
ethylbenzene conversion process.
[0012] In one broad aspect, the processes of this invention for
isomerizing a feed stream containing a non-equilibrium mixture at
least one xylene, and optionally ethylbenzene, comprise: [0013] a.
providing in at least a portion of the feed stream benzene in an
amount sufficient to beneficially affect at least one of
isomerization and ethylbenzene conversion, if ethylbenzene is
present, often between about 0.5 and 25 mass percent of the feed,
and [0014] b. subjecting the feed stream containing the provided
benzene to isomerization, and optionally ethylbenzene, conversion
conditions to provide a product stream having a redistributed
xylenes ratio.
[0015] Preferably, the feed stream contains ethylbenzene and
ethylbenzene is dealkylated during isomerization.
[0016] Preferred isomerization conditions comprise a temperature of
from about 100.degree. to 600.degree. C., a pressure of from about
10 kPa to 5 MPa, a mass hourly space velocity of from about 0.5 to
100 hr.sup.-. Operation at a temperature of between 150.degree. to
500.degree. C. and at a mass hourly space velocity of from about 1
to 50 hr.sup.-1 is especially favored. Preferred catalysts for
isomerization and for ethylbenzene conversion comprise pentasil
zeolites.
[0017] Preferably, benzene is selectively separated, e.g., by
distillation or selective permeation, from at least a portion of
the product stream is to recover benzene and at least a portion of
the benzene is used as provided benzene.
[0018] The broad aspects of the apparatus of this invention
comprise: [0019] a. a reaction assembly having at least one
feedstream inlet and at least one product outlet containing
therebetween at least one reaction zone wherein at least one
reaction zone contains catalyst suitable for the conversion of
ethylbenzene and the same or at least one other reaction zone
contains catalyst suitable for the isomerization of xylenes; [0020]
b. a distillation assembly in fluid communication at an inlet with
the product outlet of the reaction assembly, said distillation
assembly having an outlet adapted to provide a benzene-containing
stream, said outlet being in fluid communication with at least one
feedstream inlet of the reaction assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic depiction of an apparatus of this
invention in which benzene is recovered from the isomerization
product and a portion is recycled to the reaction assembly. [0022]
FIG. 2 is a schematic depiction of an apparatus of this invention
in which benzene from the isomerization product is recovered and
purified in a distillation assembly associated with recovery of
benzene from feedstreams.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The feed stream for aromatics isomerization is typically a
C.sub.8 aromatics stream from which one or more xylenes have been
removed as product. Usually the C.sub.8 aromatics stream is
prepared by removal of at least one of para- and ortho-xylene from
a fresh C.sub.8 aromatics feed obtained from processes, such as
catalytic reformiing and extraction, for the production and
recovery of aromatics from other hydrocarbons. See, for instance,
Robert A. Meyers, Handbook of Petroleum Refining Processes, Second
Edition, McGraw-Hill, 1997, Part 2. Most commercial facilities
recover from a C.sub.8 aromatics stream at least para-xylene, and
sometimes also ortho-xylene, as products and isomerize the
remaining C.sub.8 aromatics to recover more of the para-xylene, and
ortho-xylene if applicable. Consequently, the feed streams may be
relatively free from non-aromatics and from higher and lower
molecular weight aromatics. Alternatively, the feed stream may
contain naphthenes and other hydrocarbons such as paraffins usually
in an amount up to 30 mass-percent. Preferably the isomerizable
hydrocarbons consist essentially of aromatics, however, to ensure
pure products from downstream recovery processes.
[0024] The feed stocks to the aromatics isomerization process of
this invention comprise non-equilibrium xylene and, most
frequently, ethylbenzene. These aromatic compounds are in a
non-equilibrium mixture, i.e., at least one C.sub.8 aromatic isomer
is present in a concentration that differs substantially from the
equilibrium concentration at isomerization conditions. Thus, a
non-equilibrium xylene composition exists where one or two of the
xylene isomers are in less than equilibrium proportion with respect
to the other xylene isomer or isomers. The xylene in less than
equilibrium proportion may be any of the para-, meta- and
ortho-isomers. As the demand for para- and ortho-xylenes is greater
than that for meta-xylene, usually, the feed stocks will contain
meta-xylene. Generally the mixture will have an ethylbenzene
content of about 1 to about 60 mass-percent, an ortho-xylene
content of 0 to about 35 mass-percent, a meta-xylene content of
about 20 to about 95 mass-percent and a para-xylene content of 0 to
about 30 mass-percent. Usually the non-equilibrium mixture is
prepared by removal of one or more of para-, ortho- and meta-xylene
from a fresh C.sub.8 aromatic mixture obtained from an
aromatics-production process.
[0025] In the processes of this invention benzene is provided to a
xylene-containing feed for isomerization, and optionally
ethylbenzene conversion. As stated above, the amount of benzene
provided is sufficient to obtain one or more beneficial effects in
the isomerization and ethylbenzene conversion, if ethylbenzene is
present.
[0026] In one aspect of the processes of this invention, the
beneficial effect of the provided benzene is an enhancement of
xylene isomerization activity. For processes in which para-xylene
is the sought product, the provided benzene is preferably in an
amount sufficient to increase the mole percentage of para-xylene to
total xylenes by at least about 0.1, and more preferably, by at
least about 0.15, percentage points, as compared to substantially
the same process but not containing the provided benzene. Typically
the benzene is provided in an amount of between about 0.5 and 15,
more preferably between about 1 and 12, mass percent based upon the
mass of the feed.
[0027] In another aspect of the processes of this invention, the
beneficial effect of the provided benzene is an enhancement of the
conversion of ethylbenzene, especially where the conversion is a
dealkylation conversion. The amount of provided benzene is
preferably in an amount sufficient to increase the conversion by at
least about 3, preferably at least about 5, percentage points at
approximately the same weighted average bed temperature as compared
to a substantially identical process but not containing the
provided benzene. For this comparison, the weighted average bed
temperature for the comparative process is sufficient to provide an
ethylbenzene conversion between about 10 and 30 percent for
conversion processes generating xylenes, and between about 60 and
75 percent for ethylbenzene dealkylation processes. Typically the
benzene is provided in an amount of between about 0.5 and 15, more
preferably between about 1 and 12, mass percent based upon the mass
of the feed.
[0028] In yet another aspect of the processes of this invention,
the beneficial effect of the provided benzene is a reduction in
xylene loss. The amount of provided benzene is preferably in an
amount sufficient to reduce the xylene loss by at least about 0.3,
preferably at least about 0.5, percentage points at approximately
the same weighted average bed temperature as compared to a
substantially identical process but not containing the provided
benzene. For this comparison, the weighted average bed temperature
for the comparative process is sufficient to provide an
ethylbenzene conversion between about 10 and 30 percent for
conversion processes generating xylenes, and between about 60 and
75 percent for ethylbenzene dealkylation processes. Typically the
benzene is provided in an amount of between about 0.5 and 25, more
preferably between about 1 and 20, and often between about 5 and
20, mass percent based upon the mass of the feed.
[0029] In a further aspect of the processes of this invention, the
beneficial effect of the provided benzene is a reduction in
trimethylbenzene make. The amount of provided benzene is preferably
in an amount sufficient to reduce the trimethylbenzene make by at
least about 0.1, preferably at least about 0.2, percentage points
at approximately the same weighted average bed temperature as
compared to a substantially identical process but not containing
the provided benzene. For this comparison, the weighted average bed
temperature for the comparative process is sufficient to provide an
ethylbenzene conversion between about 10 and 30 percent for
conversion processes generating xylenes, and between about 60 and
75 percent for ethylbenzene dealkylation processes. Typically the
benzene is provided in an amount of between about 5 and 25, more
preferably between about 10 and 20, mass percent based upon the
mass of the feed.
[0030] The optimal amount of benzene to be provided will depend
upon the nature of the benefit sought to be achieved. For instance,
where increases in isomerization activity and/or ethylbenzene
conversion activity are sought, the optimal benzene provided will
typically range from about 2 to 8 mass percent of the feed. As the
amount of benzene is increased above that amount, the activity
decrease, and at much higher levels of provided benzene, the
activities may be less than that in the absence of benzene. On the
other hand, where the reduction in xylene ring loss is the primary
focus, the optimal amount of provided benzene is often greater,
e.g., greater than about 5 or 7 mass percent. Reduction in
trimethylbenzene co-production usually requires even more provided
benzene. Hence, for a given xylene isomerization process, the
artisan, having the benefit of this invention, can select the
amount of provided benzene to achieve an overall optimal
combination of benefits suitable for that production facility,
production economics and the market demands.
[0031] The multiple benefits that can be achieved using the
processes of this invention can be reaped by the artisan in a
number of ways. For example, a xylene production facility having
fresh catalyst may be operated with greater amounts of provided
benzene to take as a primary benefit a reduction in xylene ring
loss. As the catalyst ages, the amount of benzene provided can be
reduced to achieve a desired isomerization and ethylbenzene
conversion activity. In another mode of capturing value, the
addition of benzene can debottleneck a xylene isomerization
reactor.
[0032] It should also be kept in mind that the optimal amount of
benzene to be provided will depend upon the specific process and
process conditions used as well as the concentration of
ethylbenzene in the feed stream and the extent of conversion of the
ethylbenzene. By way of example, in an isomerization process that
dealkylates 70 percent of the ethylbenzene, more benzene will be
produced than a process that only dealkylates 50 percent of the
ethylbenzene, all other things being constant. Similarly, a feed
containing 20 mass-percent ethylbenzene will generate more benzene
than a feed containing 10 mass-percent ethylbenzene, all other
things being constant.
[0033] In preferred aspects of this invention, little or no toluene
is added to the feed stream. Toluene can reduce the benefits
provided by adding benzene. Preferably, the mass ratio of toluene
to benzene in the feed stream is less than about 1:1. In another
preferred aspect of the invention, toluene is present in an amount
less than about 2 mass percent based upon the entire feed.
[0034] The benzene may be provided prior to the contacting of the
feed stream with the isomerization catalyst, or alternatively, all
or a portion of the added benzene may be added during contacting
with the isomerization catalyst. For instance, the isomerization
zone may be composed of two or more stages containing catalyst and
benzene may be admixed with the fluid passing between stages. Where
one stage primarily effects ethylbenzene conversion and another
stage primarily effects xylene isomerization, the locations at
which benzene is added can contribute to the performance of the
unit. For example, the xylene isomerization may precede the
ethylbenzene conversion and an isomerization enhancing amount of
benzene may be added to the feed to the isomerization and more
benzene added to the isomerization product to achieve a reduction
in xylene ring loss during the ethylbenzene conversion.
[0035] The C.sub.8 aromatics-containing feed stream is contacted
with the isomerization catalyst at suitable xylene-isomerization
conditions. The processes of this invention are broadly applicable
to catalytic xylene isomerization processes. The preferred
processes are those in which the feed stream contains ethylbenzene
and ethylbenzene is reacted, e.g., by dealkylation or by conversion
to xylenes. The catalysts and reaction conditions will be dependent
upon the type of isomerization being conducted and, if ethylbenzene
is to be reacted, the type of reaction for the ethylbenzene.
[0036] The processes often involve a temperature ranging from about
100.degree. to 600.degree. C. or more, preferably in the range of
from about 370.degree. to 500.degree. C. The pressure generally is
from about 100 kPa to 10 MPa, and more usually no more than about 5
MPa. Sufficient catalyst is contained in the isomerization zone to
provide a mass hourly space velocity with respect to the
hydrocarbon feed mixture of from about 0.5 to 100 hr.sup.-1, and
preferably 1 to 50 hr.sup.-1. Hydrogen is usually present in a
hydrogen/hydrocarbon (C.sub.8 aromatics) mole ratio of about 0.5:1
to about 10:1 or more; other inert diluents such as nitrogen, argon
and light hydrocarbons may also be present. Where ethylbenzene
conversion to xylenes is sought, the process conditions also
include the presence of naphthenes, e.g., in amounts of between
about 1 and 15 or more mass percent of the feed. Where multiple
reaction zones are used, process conditions can vary among the
reaction zones. Advantageously, the process conditions such as
temperature and pressure are the same or close in each of the
reaction zones for sake of avoiding heat exchange, compression or
other unit operations.
[0037] The process conditions include the presence of solid,
molecular sieve-containing catalyst. The C.sub.8
aromatics-containing feed stream, preferably in admixture with
hydrogen, is contacted with a molecular sieve-containing catalyst
in an isomerization zone. Contacting may be effected using the
catalyst in a fixed-bed system, a moving-bed system, a
fluidized-bed system, or in a batch-type operation. In view of the
danger of attrition loss of the valuable catalyst and of the
simpler operation, it is preferred to use a fixed-bed system. In
this system, a hydrogen-rich gas and the feed mixture are preheated
by suitable heating means to the desired reaction temperature and
then passed into an isomerization zone containing a fixed bed of
catalyst. The conversion zone may be one or more separate reactors
with suitable means therebetween to ensure that the desired
isomerization temperature is maintained at the entrance to each
zone.
[0038] The reactants may be contacted with the catalyst bed in
either upward-, downward-, or radial-flow fashion, and the
reactants may be in the liquid phase, a mixed liquid-vapor phase,
or a vapor phase when contacted with the catalyst.
[0039] The isomerization product contains xylenes in a
redistributed ratio. Where the feed contains ethylbenzene, the
product will contain a reduced concentration of ethylbenzene as
compared to that in the C.sub.8 aromatics-containing feed stream.
The amount of reduction will depend upon the type of ethylbenzene
conversion. For conversion processes in which xylene are produced,
the amount of ethylbenzene in the isomerization product will be
reduced by at least about 5, preferably between 10 and 50, mass
percent as compared to the amount in the feed. For dealkylation
processes, the amounts of ethylbenzene in the isomerization product
will often be reduced by at least about 20, and sometimes between
about 25 and 90, and most often between about 50 and 80,
mass-percent from the amounts in the feed stream. The ratio of the
xylene isomers in the isomerization product preferably approaches
equilibrium at the conditions of the isomerization, e.g., at least
about 80, and more preferably at least about 90, percent of
equilibrium. Typically, the mass ratio of para-xylene to total
xylenes is between about 0.2:1 to 0.25:1, preferably between about
0.23 to 25:1, say, 0.235 to 25:1.
[0040] The particular scheme employed to recover an isomerized
product from the effluent of the reactors of the isomerization zone
is not deemed to be critical to the instant invention, and any
effective recovery scheme known in the art may be used. Typically,
the reactor effluent is condensed and the hydrogen and
light-hydrocarbon components removed therefrom by flash separation.
The condensed liquid product then is fractionated to remove light
and/or heavy byproducts and obtain the isomerized C.sub.8 aromatics
product which can be recycled for recovery of para-xylene and, if
desired, ortho-xylene.
[0041] In most commercial processes, facilities exit for removal of
benzene from the isomerization product such that the C.sub.8
aromatics-containing stream which is recycled for separation of the
sought xylene isomer contains little, if any benzene. As mentioned
above, benzene is strongly sorbed on sorbent used to separate
para-xylene and is not easily desorbed, thus the removal of benzene
is required. Typically the removal is effected by distillation. In
the processes of this invention, at least a portion of the
benzene-containing stream from this separation is recycled to the
isomerization step as provided benzene. Advantageously, the toluene
to benzene mass ratio in this recycle stream is less than about
1:1, and preferably is less than about 1:4.
[0042] Typical catalysts contain a catalytically-effective amount
of molecular sieve having a pore diameter of from about 4 to 8
.ANG. and a catalytically-effective amount of one or more
hydrogenation metal components. Examples of molecular sieves
include those having Si:Al.sub.2 ratios greater than about 10, and
often greater than about 20, such as the MFI, MEL, EUO, FER, MFS,
MTT, MTW, TON, MOR, UZM-8 and FAU types of zeolites. Pentasil
zeolites such as MFI, MEL, MTW and TON are preferred, and MFI-type
zeolites, such as ZSM-5, silicalite, Borolite C, TS-1, TSZ, ZSM-12,
SSZ-25, PSH-3, and ITQ-1 are especially preferred. The catalysts
may contain hydrogenation metal components and may contain suitable
binder or matrix material such as inorganic oxides and other
suitable materials. The relative proportion of zeolite in the
catalyst may range from about 10 to about 99 mass-percent, with
about 20 to about 90 mass-percent being preferred. There is a
tradeoff between the zeolite content of the catalyst composite and
the pressure, temperature and space velocity of an isomerization
operation in maintaining low xylene ring losses.
[0043] A refractory binder or matrix is typically used to
facilitate fabrication of the isomerization catalyst, provide
strength and reduce fabrication costs. The binder should be uniform
in composition and relatively refractory to the conditions used in
the process. Suitable binders include inorganic oxides such as one
or more of alumina, aluminum phosphate, magnesia, zirconia,
chromia, titania, boria and silica. The catalyst also may contain,
without so limiting the composite, one or more of (1) other
inorganic oxides including, but not limited to, beryllia, germania,
vanadia, tin oxide, zinc oxide, iron oxide and cobalt oxide; (2)
non-zeolitic molecular sieves, such as the aluminophosphates of
U.S. Pat. No. 4,310,440, the silicoaluminophosphates of U.S. Pat.
No. 4,440,871 and ELAPSOs of U.S. Pat. No. 4,793,984; and (3)
spinels such as MgAl.sub.2O.sub.4, FeAl.sub.2O.sub.4,
ZnAl.sub.2O.sub.4, CaAl.sub.2O.sub.4, and other like compounds
having the formula MO-Al.sub.2O.sub.3 where M is a metal having a
valence of 2; which components can be added to the composite at any
suitable point.
[0044] The catalyst may be prepared in any suitable manner. One
method for preparation involves combining the binder and molecular
sieve in a hydrosol and then gelling the mixture. One method of
gelling involves combining a gelling agent with the mixture and
then dispersing the resultant combined mixture into an oil bath or
tower which has been heated to elevated temperatures such that
gellation occurs with the formation of spheroidal particles. The
gelling agents which may be used in this process are hexamethylene
tetraarnine, urea or mixtures thereof. The gelling agents release
ammonia at the elevated temperatures which sets or converts the
hydrosol spheres into hydrogel spheres. The spheres are then
continuously withdrawn from the oil bath and typically subjected to
specific aging and drying treatments in oil and in ammoniacal
solution to further improve their physical characteristics. The
resulting aged and gelled particles are then washed and dried at a
relatively low temperature of about 100.degree. to 150.degree. C.
and subjected to a calcination procedure at a temperature of about
450.degree. to 700.degree. C. for a period of about 1 to 20
hours.
[0045] The combined mixture preferably is dispersed into the oil
bath in the form of droplets from a nozzle, orifice or rotating
disk. Alternatively, the particles may be formed by spray-drying of
the mixture at a temperature of from about 425.degree. to
760.degree. C. In any event, conditions and equipment should be
selected to obtain small spherical particles; the particles
preferably should have an average diameter of less than about 5.0
mm, more preferably from about 0.2 to 3 mm, and optimally from
about 0.3 to 2 mm.
[0046] Alternatively, the catalyst may be an extrudate. A multitude
of different extrudate shapes are possible, including, but not
limited to, cylinders, cloverleaf, dumbbell and symmetrical and
asymmetrical polylobates. The extrudates may be further shaped to
any desired form, such as spheres, by any means known to the
art.
[0047] The catalyst of the present invention may contain a halogen
component, comprising any of fluorine, chlorine, bromine or iodine
or mixtures thereof, with chlorine being preferred. Preferably,
however, the catalyst contains no added halogen other than that
associated with other catalyst components.
[0048] The catalytic composite optimally is subjected to steaming
to tailor its acid activity. The steaming may be effected at any
stage of the zeolite treatment, but usually is carried out on the
composite of zeolite binder prior to incorporation of the
platinum-group metal. Alternatively or in addition to the steaming,
the composite may be washed with one or more of a solution of
ammonium nitrate, a mineral acid, and/or water.
[0049] Prior to addition of the hydrogenation metal component the
composite preferably is ion-exchanged with a salt solution
containing at least one hydrogen-forming cation such as
NH.sub.4.sup.+ or quaternary ammonium. The hydrogen-forming cation
replaces principally alkali-metal cations to provide, after
calcination, the hydrogen form of the zeolite component.
[0050] Hydrogenation metal components are selected from the metals
of Groups 6 to 10 of the Periodic Table (IUPAC), preferably
molybdenum, rhenium and platinum-group metal. Preferred
platinum-group metals include one or more of platinum, palladium,
rhodium, ruthenium, osmium, and iridium. The preferred
platinum-group metals are platinum and palladium, with platinum
being especially preferred. The hydrogenation metal component may
exist within the final catalyst composite as a compound such as an
oxide, sulfide, halide, oxysulfide, etc., or as an elemental metal
or in combination with one or more other ingredients of the
catalyst composite. Where the hydrogenation metal component
comprises platinum-group metal component, it is normally present in
the catalyst in an amount of from about 10 to about 10,000 mass-ppm
(parts per million) of the final catalyst composite, calculated on
an elemental basis, with a level of about 100 to about 2000
mass-ppm being particularly suitable. When using a platinum
component, very low levels of about 200 to 800 mass-ppm of platinum
on the catalyst, on an elemental basis, are favored; levels of less
than about 600 mass-ppm are especially favored and levels of about
300 to about 500 mass-ppm show excellent results. When using a
palladium component, levels of about 400 to 2000 mass-ppm of
palladium on the catalyst, on an elemental basis, are favored and
levels of between about 500 and 1200 mass-ppm are especially
favored. Where the hydrogenation metal comprises molybdenum,
molybdenum is usually present in an amount of 0.1 to 5
mass-percent.
[0051] The hydrogenation metal component may be incorporated into
the catalyst composite in any suitable manner. One method of
preparing a platinum-group metal catalyst involves the utilization
of a water-soluble, decomposable compound of a platinum-group metal
to impregnate the calcined sieve/binder composite. Alternatively, a
platinum-group metal compound may be added at the time of
compositing the sieve component and binder. Yet another method of
effecting a suitable metal distribution is by compositing the metal
component with the binder prior to co-extruding the sieve and
binder. Complexes of platinum-group metals which may be employed
according to the above or other known methods include
chloroplatinic acid, chloropalladic acid, ammonium chloroplatinate,
bromoplatinic acid, platinum trichloride, platinum tetrachloride
hydrate, platinum dichlorocarbonyl dichloride, tetraamineplatinum
chloride, dinitrodiaminoplatinum, sodium tetranitroplatinate (II),
palladium chloride, palladium nitrate, palladium sulfate,
diaminepalladium (II) hydroxide, tetraaminepalladium (II) chloride,
and the like.
[0052] It is within the scope of the present invention that the
catalyst may contain other metal components known to modify the
effect of the platinum-group metal component. Such metal modifiers
may include without so limiting the invention rhenium, tin,
germanium, lead, cobalt, nickel, indium, gallium, zinc, and
mixtures thereof. Catalytically effective amounts of such metal
modifiers may be incorporated into the catalyst by any means known
in the art to effect a homogeneous or stratified distribution.
[0053] After addition of the metal component, the resultant
catalytic composite usually is dried and then calcined, e.g., at a
temperature of from about 400.degree. to about 600.degree. C. in an
air atmosphere for a period of from about 0.1 to 10 hours.
[0054] The calcined composite optimally is subjected to a
substantially water-free reduction step to insure a uniform and
finely divided dispersion of the optional metallic components. The
reduction optionally may be effected on the catalyst as loaded in
the isomerization-process reactor of the present invention prior to
the startup of the isomerization process. Substantially pure and
dry hydrogen (i.e., less than 20 vol. ppm H.sub.2O) preferably is
used as the reducing agent in this step. In some cases the
resulting reduced catalyst composite may also be beneficially
subjected to presulfiding by a method known in the art to
incorporate in the catalyst composite from about 0.01 to about 0.5
mass-percent sulfur, calculated on an elemental basis, into the
catalyst.
[0055] The gradual accumulation of coke and other deactivating
carbonaceous deposits on the catalyst during the operation of the
isomerization process will eventually reduce the activity and
selectivity of the process to a level such that regeneration is
desirable. When the performance of the catalyst has decayed to the
point where it is desired to regenerate the catalyst, the
introduction of the hydrocarbon charge stock into the conversion
zone containing the catalyst is stopped and the conversion zone
purged with a suitable gas stream. Any suitable regeneration method
may be used to restore catalyst activity and selectivity, either in
situ or by unloading and regenerating the catalyst in an off-line
facility.
[0056] With respect to the drawings, FIG. 1 depicts an apparatus
generally designated as 100 for recovery of para-xylene from a
xylene mixture. A feedstream containing a mixture of xylenes,
ethylbenzene and heavier aromatics is supplied by line 102 to
xylene distillation column 104 which provides a bottoms stream
containing heavier aromatics which is withdrawn via line 106. An
overhead from xylene column 104 contains xylenes and ethylbenzene
and is passed via line 108 to para-xylene separation unit 110.
Para-xylene separation unit 110 may be based on a fractional
crystallization process or an adsorptive separation process, both
of which are well known in the art, and preferably is based on the
adsorptive separation process. A para-xylene rich stream is
withdrawn from para-xylene separation unit 110 via line 112 for
further product recovery.
[0057] A non-equilibrium mixture of xylenes and ethylbenzene is
passed via line 114 to isomerization reactor assembly 116.
Isomerization reactor assembly 116 may contain one or more reaction
zones that serve to convert ethylbenzene and isomerize the
non-equilibrium mixture of xylenes. The isomerization product is
passed via line 118 from isomerization reactor assembly 116 to
benzene column 120. Benzene column 120 is adapted to provide an
overhead comprising lights which is removed via line 122 and a
benzene rich fraction that is passed via line 124 to isomerization
reactor assembly 116. A portion of the benzene is removed from line
124 via line 126 so that the concentration of benzene in the feed
to the isomerization reactor assembly is maintained within a
desired range.
[0058] Returning to benzene column 120, a bottoms stream is
withdrawn via line 128 and passed to toluene column 130. The
bottoms stream contains xylenes, remaining ethylbenzene, toluene
and heavier by-products from the isomerization and ethylbenzene
conversion. Toluene is recovered in an overhead fraction that is
withdrawn via line 132 and a bottoms stream containing the C.sub.8
aromatics and other higher boiling hydrocarbons, is passed via line
134 to xylene column 104.
[0059] The apparatus of FIG. 2, which is generally designated as
200, illustrates the use of equipment used to treat portions of the
feedstock for an aromatics unit. An aromatics-containing fraction,
e.g., from a reformer, is passed via line 202 to splitter 204 that
provides an overhead containing toluene and lighter hydrocarbons
which overhead is withdrawn via line 206. Splitter 204 also
provides a bottoms stream containing xylene, ethylbenzene and other
higher boiling hydrocarbons. This bottoms stream is fed to xylene
column 210 via line 208. A bottoms stream containing heavier
hydrocarbons is withdrawn from xylene column 210 via line 214.
[0060] Xylene column 210 provides an overhead containing xylenes
and ethylbenzene that is passed through line 216 to para-xylene
separation unit 218. Para-xylene is withdrawn from the para-xylene
separation unit 218 via line 220, and a non-equilibrium mixture of
xylenes and ethylbenzene is withdrawn via line 222 and passed to
isomerization reactor assembly 224 to provide an isomerized product
that has a reduced amount of ethylbenzene. isomerized product is
then sent to toluene distillation assembly 228 which provides a
bottoms stream containing xylene, ethylbenzene and higher boiling
hydrocarbons and an overhead containing toluene, benzene and lower
boiling hydrocarbons. The bottoms stream is passed via line 230 to
xylene column 210.
[0061] The overhead from toluene distillation assembly 228 is
passed via line 232 to lights column 234. As shown, the overhead
from splitter 204 is also passed to lights column 234 via line 206.
An overhead containing lights, i.e., hydrocarbons having boiling
points less than benzene, is provided by lights column 234 and is
withdrawn via line 236. A bottoms stream containing benzene and
toluene is withdrawn from lights column 234 via line 238 and is
passed to benzene column 240. Benzene column 240 provides a bottoms
stream containing toluene which is removed by line 242. This
bottoms stream may yield a toluene product or be used in a further
reaction to produce additional xylenes such as a disproportionation
or transalkylation reaction.
[0062] The overhead from benzene column 240 contains benzene which
is withdrawn via line 244, with a portion being directed via line
246 to isomerization reactor assembly 224.
EXAMPLES
[0063] The following examples are presented only to illustrate
certain specific embodiments of the invention, and should not be
construed to limit the scope of the invention as set forth in the
claims. There are many possible other variations, as those of
ordinary skill in the art will recognize, which are within the
spirit of the invention. All parts and percentages are by mass
unless otherwise noted or clear from the context.
Example I
[0064] In this example an isomerization catalyst comprising
approximately 400 mass-ppm platinum and about 67 mass percent
MFI-type molecular sieve. The catalyst is prepared as follows:
Steamed and calcined aluminum-phosphate-bound MFI zeolite spheres
are prepared using the method of Example I in U.S. Pat. No.
6,143,941. The pellets are impregnated with an aqueous solution of
tetra-amine platinum chloride to give 0.037 mass-percent platinum
after drying and calcination at 538.degree. C. The calcined
catalyst is reduced in hydrogen at 425.degree. C.
[0065] The catalyst is used in a pilot plant processing
non-equilibrium C.sub.8-aromatic feed having the following
composition: C.sub.8 aromatic component of about 7.3 mass-percent
ethylbenzene, 0.7 mass-percent para-xylene, 69.8 mass-percent
meta-xylene and 22.1 mass-percent ortho-xylene; and benzene for
some runs as shown in Table 1.
[0066] The reaction conditions in the pilot plant include a
pressure of about 1200 kPa gauge, a hydrogen to hydrocarbon mole
ratio of about 4:1, and a weight hourly space velocity of 10
hr.sup.-1. The reactor temperature is varied to provide various
ethylbenzene conversions. A summary of the results are provided in
Table 1. TABLE-US-00001 TABLE 1 Benzene Xylene Product Xylene mass
parts per 100 Temp, p-xylene to Ethylbenzene Ring Loss, Run parts
of xylenes .degree. C. total xylenes Conversion % mass %
TMB/C.sub.8A 1 0 404 23.7 74.6 2.1 0.0079 2 0 393 23.7 68.4 1.66
0.0062 3 0 382 23.6 54.2 1.43 0.0050 4 0 371 23.6 43.8 1.16 0.0041
5 18.9 404 23.7 71.9 1.60 0.0059 6 18.9 393 23.7 61.6 1.18 0.0049 7
18.9 382 23.6 51.7 0.95 0.0043
[0067] At this level of benzene addition, xylene ring loss and
trimethylbenzene coproduction are both reduced as compared to
substantially the same process but without the added benzene. The
ethylbenzene conversion activity is reduced. As will be seen in the
following example, at lower amounts of benzene addition, the
ethylbenzene dealkylation activity is enhanced.
Example II
[0068] In this example, a catalyst of the type described in Example
I is used in a pilot plant. A feed of substantially the same
C.sub.8 aromatic composition as in Example I is used for a number
of runs with out added benzene or toluene and with additional
benzene or toluene as set forth below in Table 2. The pilot plant
is operated at a pressure of about 1200 kPa gauge, a hydrogen to
hydrocarbon mole ratio of about 4:1, and a weight hourly space
velocity of 10 hr.sup.-1.
[0069] A summary of the runs is provided in Table 2. A number of
analyses are conducted at each temperature and composition over a
several hour period after the pilot plant has arrived at steady
state conditions for each temperature and additive level. The
analyses reported below are means. TABLE-US-00002 TABLE 2 pX/X,
Ethylbenzene, Xylene Added Benzene, Added Toluene Nominal Weight
ratio Conversion, Ring Loss, TMB, Run Mass-% Mass-% WABT, .degree.
C. (.times.100) Mass-% % Mass-% 1 0 0 371 23.56 43.6 1.93 0.47 2 0
0 382 23.69 54.6 2.12 0.55 3 0 0 393 23.73 65.5 2.44 0.66 4 0 0 404
23.76 75.7 2.87 0.82 5 5 0 363 23.82 50.8 1.17 0.62 6 5 0 381 23.87
70.4 1.77 0.92 7 5 0 396 23.85 84.3 2.88 1.32 8 8 0 368 23.82 52.5
0.60 0.46 9 8 0 386 23.84 71.9 1.18 0.75 10 8 0 400 23.81 84.4 2.16
1.13 11 0 5 371 23.60 44.3 1.63 0.44 12 0 5 382 23.72 55.7 1.81
0.52 13 0 5 393 23.76 66.9 2.07 0.60 14 0 5 404 23.79 76.9 2.41
0.71 15 0 10 371 23.50 36.5 0.99 0.28 16 0 10 382 23.67 49.5 1.26
0.35 17 0 10 393 23.76 63.7 1.43 0.44 18 0 10 404 23.82 76.1 1.73
0.56 19 0 20 371 23.45 32.4 0.31 0.20 20 0 20 382 23.60 40.2 0.37
0.21 21 0 20 393 23.72 53.3 0.61 0.26 22 0 20 404 23.79 67.8 0.76
0.34
[0070] As can be seen in the foregoing table, toluene at low
concentrations has a little beneficial effect on ethylbenzene
dealkylation and xylene isomerization activity, but that benefit is
lost at higher toluene concentrations. In contrast, the runs with
benzene show a pronounced effect on both ethylbenzene dealkylation
and xylene isomerization activities at low concentrations.
Reference can be made to table 1 where higher concentrations of
benzene are used and at the higher concentrations, activity is
suppressed.
[0071] Due to the increased catalytic activity achieved with the
presence of small amounts of benzene, a lower temperature can
achieve the same ethylbenzene conversion. Comparing equivalent
ethylbenzene conversion, xylene ring loss is less with the presence
of benzene than in the absence of benzene and toluene. As the
amount of benzene is increased, the reduction in xylene loss and
trimethylbenzene make is increased, but with some loss in activity
as compared to lower benzene content feeds.
* * * * *