U.S. patent application number 10/062097 was filed with the patent office on 2002-06-27 for process for production of paraxylene.
Invention is credited to Clem, Kenneth Ray, Cox, Graeme Ian, Ferraro, John Michael, Lattner, James Richardson, Osman, Robert Michael, Ou, John Di-Yi.
Application Number | 20020082462 10/062097 |
Document ID | / |
Family ID | 24364911 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020082462 |
Kind Code |
A1 |
Ferraro, John Michael ; et
al. |
June 27, 2002 |
Process for production of paraxylene
Abstract
This invention relates to a process and a chemical plant for the
production primarily of paraxylene. In particular the process and
chemical plant utilize zeolite membranes for enhanced paraxylene
production.
Inventors: |
Ferraro, John Michael;
(Houston, TX) ; Osman, Robert Michael; (Houston,
TX) ; Ou, John Di-Yi; (Houston, TX) ; Cox,
Graeme Ian; (Victoria, AU) ; Lattner, James
Richardson; (Seabrook, TX) ; Clem, Kenneth Ray;
(Humble, TX) |
Correspondence
Address: |
ExxonMobil Chemical Company
P.O. Box 2149
Baytown
TX
77522
US
|
Family ID: |
24364911 |
Appl. No.: |
10/062097 |
Filed: |
January 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10062097 |
Jan 31, 2002 |
|
|
|
08591064 |
Jan 25, 1996 |
|
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Current U.S.
Class: |
585/818 |
Current CPC
Class: |
C07C 15/08 20130101;
C07C 7/005 20130101; Y02P 20/52 20151101; C07C 7/005 20130101; C07C
15/08 20130101 |
Class at
Publication: |
585/818 |
International
Class: |
C07C 007/144 |
Claims
1. A process for recovering paraxylene from a C.sub.8 aromatics
stream containing paraxylene and at least one other isomer of
xylene, ethylbenzene, or mixtures thereof which process comprises:
(a) recovering by means of a paraxylene separation process in a
paraxylene recovery unit a portion of said paraxylene from at least
a portion of said C.sub.8 aromatics stream to produce a first
stream having a reduced paraxylene content and containing a portion
of said other isomers of xylene, said ethylbenzene, or mixtures
thereof; (b) passing at least a portion of said first stream
directly or indirectly to a zeolite membrane unit comprising a
zeolite membrane and optionally isomerisation catalyst under
isomerization conditions, such that the permeate withdrawn through
the zeolite membrane and from the zeolite membrane unit is enriched
in paraxylene when compared to the feed to the zeolite membrane
unit and (c) feeding the permeate directly or indirectly back to
the paraxylene separation process.
2. A process as claimed in claim 1 wherein at least a portion of
said first stream is subjected to an isomerisation process in an
isomerisation unit to produce an isomerate having an enriched
paraxylene content compared to that of first stream; and at least a
portion of the isomerate is passed to the zeolite membrane
unit.
3. A process as claimed in claim 1 wherein the permeate withdrawn
from the zeolite membrane unit is enriched in paraxylene compared
to the equilibrium concentration of paraxylene in a xylenes
equilibrium mixture.
4. A process as claimed in claim 1 wherein the paraxylene recovery
unit comprises a fractional crystallisation unit.
5. A process as claimed in claim 1 wherein the paraxylene recovery
unit comprises an adsorption separation unit.
6. A process as claimed in claim 1 wherein the paraxylene recovery
unit comprises an adsorption separation unit in combination with a
fractional crystallisation unit.
7. A process as claimed in claim 1 wherein the zeolite membrane
unit isomerises metaxylene and orthoxylene to paraxylene.
8. A process as claimed in claim 1 wherein the zeolite membrane
unit converts ethylbenzene to benzene and/or xylenes and/or
C.sub.10 aromatics.
9. A process as claimed in claim 1 wherein the zeolite membrane
unit comprises a zeolite membrane which is active as an
isomerization catalyst.
10. A process as claimed in claim 1 wherein the C8 aromatics feed
(fresh feed) is additionally or alternatively introduced to the
process directly to the isomerisation unit, directly to the zeolite
membrane unit or both.
11. A process for recovering paraxylene from a C.sub.8 aromatics
stream containing paraxylene and at least one other isomer of
xylene, ethylbenzene, or mixtures thereof which process comprises:
(a) recovering by means of a paraxylene separation process in a
paraxylene recovery unit a portion of said paraxylene from at least
a portion of said C.sub.8 aromatics stream to produce a first
stream having a reduced paraxylene content and containing a portion
of said other isomers of xylene, said ethylbenzene, or mixtures
thereof; (b) passing at least a portion of said first stream
directly or indirectly to a zeolite membrane unit comprising a
zeolite membrane and optionally isomerisation catalyst under
isomerization conditions, such that the permeate withdrawn through
the zeolite membrane and from the zeolite membrane unit is enriched
in ethylbenzene compared to the retentate; (c) subjecting at least
a portion of said permeate to an ethylbenzene isomerisation process
in an ethylbenzene isomerisation unit to produce an isomerate
having an enriched paraxylene content compared to that of the
permeate; (d) feeding the isomerate optionally combined with the
retentate back to the paraxylene separation process.
12. A process as claimed in claim 11 wherein the isomerate from (c)
optionally combined with the retentate is subjected to a further
isomerisation process in a second isomerisation unit to produce a
second isomerate having an enriched paraxylene content compared to
the feed to the second isomerisation unit and feeding the second
isomerate back to the paraxylene separation process.
13. A process as claimed in claim 11 which further comprises a
zeolite membrane unit after (c) which produces a permeate enriched
in paraxylene compared to the isomerate or second isomerate.
14. A process as claimed in claim 13 wherein the zeolite membrane
unit introduced after (c) further comprises a xylenes isomerisation
catalyst.
15. A process as claimed in claim 11 wherein there is present
ethylbenzene isomerisation catalyst either as part of the zeolite
membrane or downstream of the zeolite membrane but in close
proximity to the membrane or both.
16. A process as claimed in claim 11 wherein the isomerisation
catalyst in the zeolite membrane unit is located in close proximity
to the membrane and on the permeate side of the membrane.
17. A process as claimed in claim 1 or 11 wherein the zeolite
membrane unit comprises two or more alternating zones of catalyst
and zeolite membrane.
18. A process as claimed in claim 1 or 11 wherein there are two or
more zeolite membrane units with or without isomerisation catalyst
arranged sequentially to each other.
19. A paraxylene recovery plant comprising: (a) paraxylene recovery
unit and (b) a zeolite membrane unit comprising a zeolite membrane
and optionally isomerisation catalyst
20. A plant as claimed in claim 19 which further comprises an
isomerisation unit.
21. A process as claimed in claim 1 which comprises a zeolite
membrane which has been prepared by either the LAI-ISC, S-LAI-ISC
or GEL-LAI-ISC processes.
Description
[0001] This invention relates to a process and a chemical plant for
the production of paraxylene. In particular the process and
chemical plant utilise zeolite membranes for enhanced paraxylene
production.
[0002] In the petrochemical production chain one of the most
important streams is the C.sub.6 to C.sub.8 aromatics stream which
is a source of raw materials for high value downstream products.
From this stream, benzene, toluene and the C.sub.8 aromatics which
are particularly valuable may be obtained. The C.sub.8 aromatics
are orthoxylene, metaxylene, paraxylene and ethylbenzene.
Paraxylene is often the most desirable of the xylenes; however
because the boiling points of ethylbenzene, ortho-, meta- and
paraxylene (hereinafter collectively referred to as "C.sub.8
aromatics") are close, they are difficult to separate by fractional
distillation. As a consequence various alternative methods of
separating paraxylene from C.sub.8 aromatics have been developed.
The most common of such methods are fractional crystallisation
which utilises the difference in freezing points between
ethylbenzene, ortho-, meta- and paraxylene, and selective
adsorption which commonly utilises zeolite materials to selectively
adsorb paraxylene from C.sub.8 aromatics streams; the adsorbed
paraxylene is recovered after desorbing from the zeolite. When
either of these processes are used paraxylene can be recovered in
high yields from the C.sub.8 aromatics stream. The resulting
filtrate from the crystallisation process or the raffinate from the
adsorption process are depleted in paraxylene and contain
relatively high proportions of ethylbenzene, ortho-, and
metaxylene. These streams are typically subjected to further
processing downstream of the crystallisation or adsorption
process.
[0003] Typically one of the additional downstream processes is an
isomerisation process which is used to increase the proportion of
paraxylene in paraxylene depleted streams from such processes as
fractional crystallisation or selective adsorption. The xylenes,
which are predominately ortho- and metaxylene, can be contacted
with an isomerisation catalyst under appropriate temperature and
pressure which results in the conversion of some of the ortho- and
metaxylene to paraxylene. It is also usually necessary to convert
some of the ethylbenzene to prevent it from building up to high
concentrations. A catalyst can be selected to enable conversion of
ethylbenzene to benzene, and/or to orthoxylene through a C.sub.8
naphthene intermediate and/or to C.sub.10 aromatics and benzene via
transalkylation. It may be that the catalyst for conversion of
ethylbenzene to orthoxylene is also a xylenes isomerisation
catalyst in which case the orthoxylene from the ethylbenzene is
converted to an equilibrium mixture of xylenes.
[0004] Prior art processes for making paraxylene have typically
included combinations of isomerization with fractional
crystallisation and/or adsorption separation. The problem with this
combination is that despite improvements in catalyst performance
the isomerisation technology is only able to produce equilibrium or
near-equilibrium mixtures of xylenes and may also be relatively
inefficient for the conversion of ethylbenzene to benzene or
xylenes. The consequence of this is that big recycles of the
xylenes stream back through these processes are needed to ensure
the conversion of the C.sub.8 aromatics stream to paraxylene is
maximised with or without the additional recovery if desired of
orthoxylene and/or metaxylene. There is a need therefore for
improved processes and chemical plants for the production of
paraxylene from C.sub.8 aromatics streams, which in particular
address the problems associated with large recycles and/or low
ethylbenzene conversions.
[0005] Zeolite membranes have been described in the prior art, for
example in U.S. Pat. Nos. 4,699,892, 5,100,596, EP 0481658, EP
0481659, EP 0481660, WO 92/13631, WO 93/00155, WO 94/01209, and WO
9425151. However the prior art does not describe how to use such
membranes in actual C.sub.8 aromatics processing in the
petrochemical cycle nor does the prior art describe how to use such
membranes in combination with existing processes to significantly
enhance their paraxylene production capability.
[0006] The present invention is therefore directed to a chemical
plant and process which offers an improvement over the prior art
for the production of paraxylene from C.sub.8 aromatics streams.
The present invention resides in the specific application of a
zeolite membrane unit and process in a paraxylene or paraxylene
with orthoxylene and/or metaxylene recovery process. This invention
utilises zeolite membranes to continuously separate paraxylene
and/or ethylbenzene from xylenes, or to isomerise ortho- and
metaxylene to paraxylene and/or ethylbenzene to xylenes and
simultaneously or subsequently separate paraxylene from the xylenes
mixture. The use of a zeolite membrane unit and process in for
example a process for paraxylene recovery provides for a
significant improvement in paraxylene production when compared to
conventional paraxylene recovery processes.
[0007] Accordingly the present invention provides a process for
recovering paraxylene from a C.sub.8 aromatics stream containing
paraxylene and at least one other isomer of xylene, ethylbenzene,
or mixtures thereof which process comprises:
[0008] (a) recovering by means of a paraxylene separation process
in a paraxylene recovery unit a portion of said paraxylene from at
least a portion of said C.sub.8 aromatics stream to produce a first
stream having a reduced paraxylene content and containing at least
a portion of said other isomers of xylene, said ethylbenzene, or
mixtures thereof;
[0009] (b) passing at least a portion of said first stream directly
or indirectly to a zeolite membrane unit comprising a zeolite
membrane and optionally isomerisation catalyst under isomerization
conditions, such that the permeate withdrawn through the zeolite
membrane and from the zeolite membrane unit is enriched in is
paraxylene when compared to the feed to the zeolite membrane unit
and
[0010] (c) feeding the permeate directly or indirectly back to the
paraxylene separation process.
[0011] Preferably there is an additional step between (a) and (b)
wherein at least a portion of said first stream is subjected to an
isomerisation process in an isomerisation unit to produce an
isomerate having an enriched paraxylene content compared to that of
the first stream; and it is at least a portion of this isomerate
stream which is passed to the zeolite membrane unit. Most
preferably the permeate withdrawn from the zeolite membrane unit is
enriched in paraxylene compared to the equilibrium concentration of
paraxylene in a xylenes equilibrium mixture.
[0012] The present invention further provides for a paraxylene
recovery plant comprising:
[0013] (a) paraxylene recovery unit, and
[0014] (b) a zeolite membrane unit comprising a zeolite membrane
and optionally isomerisation catalyst.
[0015] Preferably the paraxylene recovery plant comprises an
isomerisation unit in addition to the paraxylene recovery unit and
zeolite membrane unit.
[0016] The paraxylene recovery unit uses separation technology to
produce a paraxylene enriched stream and a paraxylene depleted
stream. Such separation technology includes for example the known
processes of fractional crystallisation, or selective adsorption
using for example molecular sieve adsorbers. The paraxylene
recovery unit may therefore be a fractional crystallisation unit
which utilises the difference in freezing points between
ethylbenzene, ortho-, meta- and paraxylene or it may be a selective
adsorption unit which commonly utilises zeolite materials to
selectively adsorb paraxylene from C.sub.8 aromatics streams; the
adsorbed paraxylene is recovered after desorbing from the zeolite.
The paraxylene recovery unit may also be a combination of such
separation units, or may incorporate other less commonly used
techniques such as fractional distillation.
[0017] Fractional crystallisation units are well known in the art
and are described for example in U.S. Pat. No. 4,120,911.
Commercially available processes include the crystallisation
isofining process, direct contact CO.sub.2 crystallisers, scraped
drum crystallisers, and continuous countercurrent crystallisation
processes. The crystalliser may operate for example in the manner
described in Machell et. al. U.S. Pat. No. 3,662,013. Commercial
fractional crystallisation processes typically recover about 60% to
68% of the paraxylene from the feed to the paraxylene recovery unit
when this feed is an equilibrium or near equilibrium mixture of
xylenes and ethylbenzene. The reason for this is that they are
limited by the formation of a eutectic between paraxylene and
metaxylene. However the actual recovery depends on the composition
of the feed with higher recoveries possible when the paraxylene
content of the feed is higher than the xylenes equilibrium
content.
[0018] Selective adsorption units are also well known in the art
and are described for example in U.S. Pat. Nos. 3,706,812,
3,732,325, 4,886,929, and references cited therein, the disclosures
of which are hereby incorporated by reference. Commercially
available processes include UOP PAREX.TM., and IFP-Chevron
ELUXYL.TM. processes. Commercial molecular sieve selective
adsorption processes may recover higher levels of paraxylene than
fractional crystallisation processes; typically they recover over
90% or more typically over 95% of the paraxylene from the feed to
the paraxylene recovery unit.
[0019] The paraxylene recovery unit produces a paraxylene enriched
stream that usually comprises over 99% and may even be as high as
99.9% paraxylene. The exact amount depends on the process used and
the design and operating conditions of the specific plant. The
balance in this stream being ethylbenzene, ortho-, and metaxylene,
toluene, and C.sub.9 aromatics, paraffin's, naphthenes and possibly
small amounts of other materials. The paraxylene recovery also
produces a paraxylene depleted stream containing the balance of
ethylbenzene, ortho-, and metaxylene, toluene, C.sub.9 aromatics,
paraffins, etc. along with any paraxylene fed to the paraxylene
recovery unit that is not removed in the paraxylene rich stream. It
is this paraxylene depleted stream which is then fed to the
isomerisation unit and/or zeolite membrane unit.
[0020] The C.sub.8 aromatics stream which is used as the feed for
the paraxylene separation unit may come from a variety of sources
in the petrochemical plant. One possible source is from naphtha
reforming. Examples of such processes include Exxon
POWERFORMING.TM., UOP Platforming.TM., IFP Aromizing.TM.. Another
possible source is pyrolysis gasoline from steam cracking processes
although this is likely to be a minor source of such streams. A
further possible source is the UOP Cyclar process for conversion of
C.sub.3/C.sub.4 hydrocarbon streams to aromatics (see for example
U.S. Pat. No. 5,258,563, the disclosure of which is hereby
incorporated by reference). A further possible source is from
toluene disproportionation and/or C.sub.9 aromatics
transalkylation. Examples of such processes include UOP
TATORAY.TM., TORAY TAC9.TM., Mobil Selective Toluene
Disproportionation.TM. (MSTDP), Mobil Toluene
Disproportionation.TM. (MTDP), IFP Xylenes PLUS.TM. and FINA
T2BX.TM.. There are other possible sources of C.sub.8 aromatics
streams. The source of C.sub.8 aromatics stream for the process of
the present invention is not critical and may be a single stream or
may be a combination of streams from any of the above
processes.
[0021] The isomerisation unit may be any of the well known units in
the art such as those described in U.S. Pat. Nos. 4,236,996,
4,163,028, 4,188,282, 4,224,141, 4,218,573, 4,236,996, 4,899,011,
3,856,872 and Re. 30,157, the disclosures of which are hereby
incorporated by reference.
[0022] The isomerisation catalyst may be any of the well known
catalysts for isomerisation units in the art. There are primarily
two types of catalyst system which are used in isomerisation units.
The choice of catalyst has an impact on the overall yield and
structure of the aromatics complex and also on the plant design and
economics. The first type of catalyst is designed to convert
ethylbenzene to xylenes and to isomerise the paraxylene depleted
feed stock to a near equilibrium xylene composition. This type of
catalyst system is generally the choice for aromatics producers
whose objective is to maximise para and ortho-xylene production
from a fixed quantity of feed stock. A second catalyst system is
also designed to isomerise the para xylene depleted feed stock;
however rather than converting ethylbenzene to xylenes, this
catalyst system dealkylates the ethylbenzene to produce benzene.
This catalyst system is often employed when the benzene
requirements are high relative to ortho and para xylene production
or when feed stock availability is not a limiting factor.
[0023] Examples of processes and catalyst systems which include the
capability of converting ethylbenzene to benzene are the Mobil MHTI
(Mobil High Temperature Isomerisation) process and catalyst (see
for example U.S. Pat. Nos. 3,856,871 and 4,638,105, the disclosures
of which are hereby incorporated by reference), the Mobil MHAI
(Mobil High Activity Isomerisation) process and catalyst, the AMOCO
AMSAC process and catalyst and the UOP ISOMAR.TM. I-100 process and
catalyst.
[0024] Examples of processes and catalyst systems which include the
capability of converting ethylbenzene to xylenes are the
IFP/ENGELHARD Octafining and Octafining II processes and catalyst,
and the UOP ISOMAR.TM. I-9 process and catalyst. Other processes
include catalysts capable of converting ethylbenzene to C.sub.10
aromatics. Other processes do not include ethylbenzene
conversion.
[0025] Isomerization units typically use a zeolite or mordenite
type catalyst. Isomerization catalysts known to promote conversion
of ortho and metaxylene to paraxylene include metal promoted
molecular sieves such as for example Pt Promoted ZSM-5, Pt promoted
Mordenite and metal promoted borosilicates etc. Commercial examples
are Mobil MHAI and ISOMAR.TM. I-9 catalyst.
[0026] The isomerization reactor is arranged and effective to
isomerise ortho- and metaxylene to paraxylene at these conditions
and also advantageously to convert ethylbenzene to benzene and/or
xylenes. The term "arranged and effective" is used in this
application to denote that conditions in a process unit are as
described in this specification to include the temperatures,
pressures, space velocities, reaction time, other reactants, and
any other process conditions necessary to achieve the desired
reaction, conversion or separation that is the normal function of
that process unit.
[0027] Operating temperatures are typically in the range of 400 to
900.degree. F. and pressures in the range of 25 to 500 PSIG. The
weight hourly space velocity (WHSV) based on hydrocarbon feed
typically ranges from 0.5 to 20. Most isomerization catalyst
systems require a source of hydrogen which can be introduced to the
isomerization reactor to promote the isomerization reaction that
converts ortho- and metaxylene to paraxylene, to assist in the
conversion of ethylbenzene to benzene and or xylenes and assists
also in the prevention of coking of the isomerisation catalyst.
[0028] In one aspect of the present invention the zeolite membrane
unit is used to selectively separate paraxylene and/or ethylbenzene
from a stream which comprises ethylbenzene and an equilibrium or
near equilibrium mixture of xylenes. In this aspect the zeolite
membrane unit may be located downstream of an isomerisation unit
and does not have an isomerisation catalyst in combination with the
membrane.
[0029] In a further aspect of the present invention, a zeolite
membrane unit utilises an isomerisation catalyst in combination
with the membrane to isomerise ortho- and metaxylene to paraxylene
in co-operation with the selective separation function of the
membrane and may also include the catalytic conversion of
ethylbenzene to benzene or xylenes.
[0030] In this aspect of the present invention the zeolite membrane
may itself be rendered catalytically active for the isomerisation
reaction or an appropriate isomerisation catalyst may be located
proximate to the membrane. By proximate to the membrane is meant
that the catalyst is arranged and effective to isomerise the ortho-
and/or metaxylene and/or ethylbenzene in the material in the
zeolite membrane unit but upstream of the zeolite membrane to
produce paraxylene. The exact amount of paraxylene which is
required to be produced by the isomerisation process in the zeolite
membrane unit depends in part on the properties of the zeolite
membrane used. If the membrane for example has high flux and/or
high selectivity for paraxylene then it may be possible or even
desirable for the isomerisation reaction to produce and maintain
paraxylene at a none equilibrium concentration compared to its
concentration in an equilibrium xylene mixture whilst the membrane
selectively removes paraxylene from the upstream material and into
the permeate. However the isomerisation catalyst in the zeolite
membrane unit should ideally be arranged and effective to produce
and maintain paraxylene, upstream of the membrane and inside the
zeolite membrane unit, at 50% or more, preferably 80% or more, and
most preferably 90% or more of the paraxylene equilibrium
concentration whilst the membrane selectively removes paraxylene
from the upstream side of the membrane and into the permeate.
Depending on membrane properties it may be desirable and preferable
to maintain the paraxylene concentration at or near to equilibrium
for xylenes isomerisation whilst the membrane selectively removes
paraxylene from the retentate into the permeate. Thus the
isomerisation catalyst causes the ortho- and metaxylene to convert
to paraxylene and the paraxylene selectively permeates through the
zeolite membrane to be produced as a permeate stream. Ortho- and
metaxylene less readily pass through the zeolite membrane and tend
to stay on the upstream side in the retentate stream where they can
be further isomerised. The permeate stream from xylenes
isomerisation unit may be fractionated to remove materials boiling
below and above the boiling point of xylenes e.g. benzene, toluene
and C9+ aromatics and then transferred to the paraxylene recovery
unit. If the zeolite membrane unit is particularly efficient at
isomerisation and separation there may theoretically be no
retentate stream as there would be no paraxylene depleted stream to
reject. In practice there will however likely be impurities and
heavier aromatic compounds such as C.sub.9 aromatics which remain
in the retentate stream and must be purged from the zeolite
membrane unit for further treatment. Thus in the zeolite membrane
unit there is a dynamic and coupled process of isomerisation and
separation of xylenes. If the catalytic function is also capable of
converting ethylbenzene to benzene or xylenes then any ethylbenzene
which enters into the retentate stream of the unit is also involved
in this dynamic process with the resulting xylenes entering into
the xylenes isomerisation reactions or the resulting benzene
passing through the membrane into the permeate stream. In this
aspect the zeolite membrane unit may be downstream of an
isomerisation unit or may be used in place of an isomerisation
unit.
[0031] In a further aspect the zeolite membrane is used to
selectively separate ethylbenzene with a small amount of paraxylene
from a paraxylene depleted feedstream as is typically found after a
paraxylene separation process. In this aspect the zeolite membrane
unit is located between the paraxylene separation unit and an
ethylbenzene isomerisation unit. The feed to the isomerisation unit
is enriched in ethylbenzene and improves the efficiency of the
ethylbenzene isomerisation process in this unit. The output from
this isomerisation unit is enriched in paraxylene and passes into a
conventional paraxylene isomerisation unit along with the retentate
from the zeolite membrane unit. In such a process the paraxylene
isomerisation unit is required to convert lower levels of
ethylbenzene and therefore may be operated at lower temperatures
and may in fact be a liquid phase isomerisation unit which has no
ethylbenzene conversion activity. The overall effect of this use of
the zeolite membrane is to enhance the conversion of ethylbenzene
to useful xylenes and to significantly reduce the xylene losses
which usually occur due to the use of high temperature
isomerisation units such as ISOMAR.TM. or MHTI.TM.. A further
modification of this aspect of the present invention is to include
a catalytic function into the zeolite membrane unit. This catalytic
function may be for ethylbenzene conversion and may be located
within the membrane itself. This catalytic function may
advantageously be located proximate to the membrane on the permeate
side of the zeolite membrane. The function of this catalyst is to
catalyse the conversion of ethylbenzene to xylenes. The effect of
this is to deplete the concentration of ethylbenzene on the
permeate side of the membrane and in doing so sets up a
concentration gradient across the membrane which increases the
quantity of ethylbenzene transferred from the retentate stream into
the permeate stream. If the ethylbenzene conversion catalyst in the
zeolite membrane unit is particularly efficient there may be no
need for the ethylbenzene isomerisation unit which is located
downstream of the zeolite membrane unit. In a further embodiment a
second zeolite membrane unit for selective paraxylene separation or
for selective paraxylene separation and isomerisation, may be
located downstream of the paraxylene isomerisation unit. The
permeate stream from xylenes isomerisation unit or the second
zeolite membrane unit if present may be fractionated to remove
materials boiling below and above the boiling point of xylenes e.g.
benzene, toluene and C9+ aromatics and then transferred to the
paraxylene recovery unit. Optionally, the retentate stream may be
combined with the permeate stream and the combined streams
fractionated and transferred to the paraxylene recovery unit for
recovery of a paraxylene rich stream.
[0032] Examples of zeolite membranes which may be used in zeolite
membrane units for the present invention are described in the
following documents. U.S. Pat. No. 5,110,478, the disclosure of
which is hereby incorporated by reference, describes the direct
synthesis of zeolite membranes. The membranes produced in
accordance with the teachings of U.S. Pat. No. 5,110,478 were
discussed in "Synthesis and Characterisation of a Pure Zeolite
Membrane," J. G. Tsikoyiannis and W. Haag, Zeolites (Vol. 12, p.
126., 1992). Such membranes are free standing and are not affixed
or attached as layers to any supports. Zeolite membranes have also
been grown on supports. See e.g. "High temperature stainless steel
supported zeolite (MFI) membranes: Preparation, Module,
Construction and Permeation Experiments," E. R. Geus, H. vanBekkum,
J. A. Moulyin, Microporous Materials, Vol. 1, p. 137, 1993;
Netherlands Patent Application 91011048; European Patent
Application 91309239.1 and U.S. Pat. No. 4,099,692, the disclosures
of which are hereby incorporated by reference. Other literature
describing supported inorganic crystalline molecular sieve layers
includes U.S. Pat. No. 4,699,892; J. C. Jansen et at, Proceedings
of 9th International Zeolite Conference 1992 (in which lateral and
axial orientations of the crystals with respect to the support
surface are described), J. Shi et al, Synthesis of Self-supporting
Zeolite Films, 15th Annual Meeting of the British Zeolite
Association, 1992, Poster Presentation (in which oriented Gmelinite
crystal layers are described); and S. Feng et al, Nature, Apr. 28,
1994, p 834 (which discloses an oriented zeolite X analogue layer),
the disclosures of which are hereby incorporated by reference.
[0033] Further examples of zeolite membranes which may be used in
zeolite membrane units for the present invention are described in
the following documents; International Application WO 94/25151,
U.S. Ser. No. 267,760 filed Jul. 8, 1994, PCT US95/08512, PCT
US95/08514, PCT US95/08513, PCT EP95/02704 and WO94/01209, the
disclosures of which are hereby incorporated by reference. In our
earlier International Application WO 94/25151 we have described a
supported inorganic layer comprising optionally contiguous
particles of a crystalline molecular sieve, the mean particle size
being within the range of from 20 nm to 1 .mu.m. The support is
advantageously porous. When the pores of the support are covered to
the extent that they are effectively closed, and the support is
continuous, a molecular sieve membrane results; such membranes have
the advantage that they may perform catalysis and separation
simultaneously if desired. Preferred zeolite membranes are those
which are prepared by the Inverted In-Situ-Crystallisation (I-ISC)
process, or by using a GEL layer and a Low Alkaline synthesis
solution using the Inverted In-Situ-Crystallisation process
(GEL-LAI-ISC), or by using a Seeding Layer and a
Low-Alkaline-synthesis solution using the Inverted In-Situ
Crystallisation (S-LAI-ISC). These processes are described in U.S.
Ser. No. 267,760 filed Jul. 8, 1994, PCT US95/08512, PCT
US95/08514, PCT US95/08513 and PCT EP95/02704. Zeolite compositions
fabricated using the above described LAI-ISC, GEL-LAI-ISC or
S-LAI-ISC techniques can have dense zeolite layers in which the
zeolite crystals are intergrown such that non-selective permeation
pathways in these as-synthesised zeolite layers are virtually
non-existent. The zeolite membranes described above may be
incorporated into the zeolite membrane unit in the form of a module
such as that described in WO94/01209. It is envisaged that the
zeolite membrane unit will contain at least one zeolite membrane
which may or may not be catalytically active. If the membrane is
not catalytically active for the desired process a suitable
catalyst may be used in combination with the membrane. This
catalyst may be located on the upstream side of the membrane or the
downstream side of the membrane depending on the process and the
nature and purpose of the catalyst. In one embodiment one or more
membranes may be arranged with one or more catalysts to provide
alternating membrane and catalyst regions in the zeolite membrane
unit. In this arrangement the feedstream to the unit may for
example pass through a membrane region with the retentate flowing
to a catalyst containing region and then through a second membrane
region to a second catalyst region. The exact number of membrane
and catalyst regions will depend on the nature of the separations
and catalyst processes desired. The separation and catalyst process
may be substantially the same for each combination of catalyst and
membrane or may be different.
[0034] It should be understood that two or more zeolite membrane
units with or without isomerisation catalyst in close proximity to
the zeolite membrane in each unit may be used in the processes of
the present invention. Reference to zeolite membrane unit in this
specification should also be taken to include embodiments where two
or more zeolite membrane units may be used in sequence to each
other with or without any further intervening processes or process
units.
[0035] The zeolite membrane unit may be installed downstream of an
existing xylenes isomerization reactor or installed as a
replacement of an isomerization reactor in an existing paraxylene
recovery process. The zeolite membrane unit may be added to an
existing process solely for separation of paraxylene from xylenes,
or for both isomerization and separation. The most preferred option
is to have the zeolite membrane unit downstream of a xylenes
isomerisation unit and for the zeolite membrane unit to comprise a
zeolite membrane and an isomerisation catalyst so that it performs
both isomerisation of xylenes, and selective separation of
paraxylene; optionally it also catalyses conversion of ethylbenzene
to xylenes or benzene. If the zeolite membrane unit catalyses
conversion of ethylbenzene to xylenes or benzene then this may
allow less conversion of ethylbenzene in the conventional
isomerisation unit with less xylene losses due to the lower
operating temperature which would be required in the conventional
isomerisation unit.
[0036] It is preferred that the zeolite membrane unit is
incorporated into a conventional xylene recovery loop such as that
shown in FIG. 1 and discussed below. The xylene recovery process is
referred to as a "loop" because xylenes not converted to paraxylene
are recycled to the isomerization unit that is usually a part of
the xylene recovery loop again and again until the xylenes are
converted to paraxylene and removed from the loop via the
paraxylene separation unit. In such a loop orthoxylene may also be
a product which is removed from the loop in the xylene splitter if
desired. Orthoxylene can sometimes be generated by the
isomerisation unit if the feed to that unit has a less than
equilibrium orthoxylene concentration.
[0037] As indicated above the fresh feed for the xylene recovery
loop may come from a variety of sources in the petrochemical cycle.
Fresh feed from, for example, a reformer, which is introduced to
the xylene recovery loop is usually fractionated before
introduction to the paraxylene separation unit to remove materials
boiling below the boiling point of xylenes, and may optionally also
be fractionated to remove at least part of the material boiling
above the boiling point of xylenes. If lower boiling materials are
not removed from the fresh feed, it is introduced to a detoluenizer
tower ("DETOL") which removes toluene and lighter materials by
distillation. The feed is then introduced to either a xylene rerun
tower or splitter. A xylene rerun tower removes C.sub.9+ aromatics
from the feed. A xylene splitter tower in addition removes at least
part of the orthoxylene for subsequent recovery as orthoxylene
product in an orthoxylene rerun tower. The fresh feed in a xylenes
loop is combined with a recycle stream which comes from the xylene
isomerisation unit or in the present invention from the zeolite
membrane unit. The overhead stream from the xylene rerun tower or
splitter is typically a mixture of compounds which includes 0 to 10
wt % non aromatics, 0 to 5 wt % toluene, 5 to 20 wt % ethylbenzene,
0 to 10 wt % C.sub.8 naphthenes, and 70 to 95 wt % xylenes. The
exact composition will depend on the fresh feed and the nature of
the catalysts used in the isomerisation unit and in the zeolite
membrane unit. It should be appreciated that the fresh feed to the
xylenes recovery loop could be a combination of two or more feeds
such as those discussed above. Thus it could be a combination of a
feed from a naphtha reformer with that from a TATORAY.TM. or
MSTDP.TM. unit.
[0038] It should be understood that in the present description when
reference is made to a feed to, or material upstream of the
membrane, in a zeolite membrane unit being at equilibrium in
xylenes this means that it can be a mixture of xylenes which are at
the typical respective concentrations for an equilibrium mixture of
xylenes as known in the art. In the same context by near
equilibrium is meant a composition comprising xylenes in which one
or more of the xylenes present are at their none equilibrium
concentration with respect to the other xylenes present and
includes mixtures where one or more of the xylene isomers are
present at a concentration which is greater than their equilibrium
concentration. Ideally in such mixtures the paraxylene should be
present at 50% or more, preferably 80% or more and most preferably
at 90% or more of the paraxylene equilibrium concentration.
[0039] Other objects and features of the invention are described in
the following detailed description wherein reference is made to the
accompanying figures.
[0040] FIG. 1 shows conventional xylenes loop comprising a
paraxylene separation unit and an isomerisation unit.
[0041] FIG. 2 shows a xylene purification loop utilising a zeolite
membrane unit.
[0042] FIG. 3 is a xylene purification loop utilising a zeolite
membrane unit downstream of an isomerization unit.
[0043] FIG. 4 shows a xylene purification loop utilising an
isomerization unit upstream of a zeolite membrane unit without
isomerisation catalyst and used for separating a paraxylene
enriched stream that is transferred to a paraxylene recovery
unit.
[0044] FIG. 5 shows a xylene purification loop having an
isomerization unit upstream from a zeolite membrane unit without
isomerisation catalyst and used for separating a paraxylene
enriched stream that is then transferred to a xylene splitter.
[0045] FIG. 6 shows a xylene purification loop which utilises an
ethylbenzene membrane separation unit upstream of an ethylbenzene
isomerisation unit which is upstream of an xylene isomerisation
unit.
[0046] In FIG. 1 fresh feed containing xylenes is introduced to a
xylene splitter 2 through fresh feed line 1. A bottom stream 3 is
withdrawn from the xylene splitter 2 containing materials having
boiling points above xylenes and possibly containing orthoxylene.
An overhead stream 4 is withdrawn from the xylene splitter
containing xylenes and ethylbenzene. The overhead stream 4 is fed
to paraxylene recovery unit 5. Xylenes are separated in paraxylene
recovery unit 5 to yield a paraxylene rich stream 6 and a
paraxylene poor stream 7. The paraxylene recovery unit normally
uses fractional crystallisation and/or molecular sieve separation
to separate paraxylene from other stream 4 components. The
paraxylene poor stream 7 is then introduced to an isomerization
(ISOM) unit 8 which contains an isomerization catalyst arranged and
effective to promote isomerization of ortho and metaxylene to
paraxylene and the conversion of ethylbenzenes to benzene and/or
xylenes or other compounds. The isomerate 9 from the isomerisation
unit 8 is then introduced to stabiliser 10. Stabiliser 10 separates
five carbon and lighter compounds from stream 9 through differences
in boiling points. Five carbon and lighter compounds are withdrawn
from stabiliser 10 through line 11 and six carbon and heavier
compounds are withdrawn from stabiliser 10 through line 12. The
stream in line 12 is introduced to a DETOL unit 13 to remove
toluene and lighter compounds through line 14. Xylenes and heavier
materials are withdrawn from DETOL unit 13 through line 15 and
introduced to xylene splitter 2. Normally fresh feed 2 and the
xylenes withdrawn from the DETOL 13 through line 15 are passed
through clay treaters which in order to simplify the figures are
not shown in FIGS. 1 to 6. As an alternative to the splitter 2
described in FIG. 1 a rerun tower may be used. It is to be
understood that when reference is made to a splitter in this
specification that this reference also encompasses the use of a
rerun tower in place of the splitter. For the purposes of this
application the description "splitter" shall be used when
substantial orthoxylene is removed in the bottoms and "rerun" shall
be used when it is not.
[0047] There are a number of disadvantages associated with this
conventional process arrangement for paraxylene recovery. The first
is that this combination of process steps requires a significant
recycle through the loop in order to remove the maximum possible
amounts of paraxylene. This is primarily due to the fact that the
isomerisation unit and processes are only able to produce
equilibrium or near equilibrium mixture of xylenes for the recycle
in the isomerate. A typical concentration of paraxylene in the
isomerate from such a unit is 22 wt %. Another problem is that in
most isomerisation processes there are xylenes losses of up to 4%
or higher. Thus with repeated recycles this loss of xylenes to
undesirable products may be significant.
[0048] FIG. 2 shows a xylene purification loop according to the
present invention. Fresh feed containing xylenes is introduced to a
xylene splitter 21 through fresh feed line 20. A bottom stream 22
is withdrawn from xylene splitter 21 containing materials having
boiling points above xylenes and possibly containing orthoxylene.
An overhead stream 23 is withdrawn from the xylene splitter
containing xylenes and ethylbenzene. The overhead stream 23 is fed
to paraxylene recovery unit 24. Xylenes are separated in paraxylene
recovery unit 24 to yield a paraxylene rich stream 25 and a
paraxylene poor stream 26. The paraxylene recovery unit normally
uses fractional crystallisation and/or molecular sieve separation
to separate paraxylene from other stream 23 components. The
paraxylene poor stream 26 is then introduced to zeolite membrane
unit 27. Zeolite membrane unit 27 includes catalyst arranged and
effective to promote isomerization of ortho- and metaxylene to
paraxylene and the conversion of ethylbenzenes to benzene and/or
xylenes or other compounds. The zeolite membrane also selectively
permits permeation of paraxylene through the membrane relative to
ortho- and metaxylene. A stream enriched in paraxylene is withdrawn
from zeolite membrane unit 27 as permeate stream 28. The remaining
material in retentate stream 29 should ideally be at equilibrium or
near equilibrium in xylenes. The permeate stream 28 and retentate
stream 29 may be treated separately (not shown in FIG. 2) or
combined to form combined feed 30 and introduced to stabiliser 31.
Stabiliser 31 separates five carbon and lighter compounds from
stream 30 through differences in boiling points. Five carbon and
lighter compounds are withdrawn from stabiliser 31 through line 32
and six carbon and heavier compounds are withdrawn from stabiliser
31 through line 33. The stream in line 33 is introduced to a DETOL
unit 34 to remove toluene and lighter compounds through line 35.
Xylenes and heavier materials are withdrawn from DETOL unit 34
through line 36 and introduced to xylene splitter 21. In addition
it is also possible to introduce all or some of the fresh feed
directly into the zeolite membrane unit 27 as indicated at line 37.
This would have the advantage of further increasing the
concentration of paraxylene in the paraxylene recovery unit feed,
because a higher proportion of that feed would be derived from the
zeolite membrane unit product rather than from the fresh feed
having only an equilibrium or near equilibrium paraxylene
concentration. However this approach requires a larger zeolite
membrane unit, and may require a larger retentate stream to purge
heavy aromatics brought in with the fresh feed, so there is an
economic optimum for each application regarding how much, if any of
the fresh feed to route directly to the zeolite membrane unit. For
example in a retrofit situation where the paraxylene recovery unit
capacity is limiting the plant production rate, it would likely be
advantageous to route at least a portion of fresh feed to the
zeolite membrane unit.
[0049] This embodiment of the present invention has a significant
advantage over the conventional process as described in FIG. 1. The
zeolite membrane unit produces a permeate with a greater than
equilibrium amount of paraxylene and a retentate which has an
equilibrium or near equilibrium concentration. The combined
products will have a higher paraxylene content than is possible
with the conventional isomerisation unit, which is limited by
equilibrium. When those streams are recycled to the paraxylene
recovery unit via the xylene splitter the paraxylene concentration
is increased there, increasing per pass paraxylene recovery and
reducing recycle which is a problem with conventional processes. In
this embodiment the zeolite membrane unit is required to relatively
efficiently isomerise xylenes and convert ethylbenzene and requires
a zeolite membrane which has selectivity for paraxylene and
exhibits acceptable flux through the membrane. Such membranes may
be prepared using the LAI-ISC, S-LAI-ISC and GEL-LAI-ISC methods
described above.
[0050] The particularly preferred embodiment of the invention is
shown in FIG. 3. Fresh feed containing xylenes is introduced to
xylene splitter 41 through fresh feed line 40. A bottom stream
containing 9 carbon and heavier compounds and possibly containing
orthoxylene is withdrawn from xylene splitter through line 42. An
overhead stream containing xylenes and ethylbenzene is withdrawn
from xylene splitter 41 through line 43 and introduced to
paraxylene recovery unit 44. Paraxylene is recovered in a
paraxylene rich stream 45 while ortho- and metaxylene are recovered
in paraxylene poor stream 46. The paraxylene poor stream 46 is
introduced to an isomerization unit 47 which contains an
isomerization catalyst arranged and effective to promote
isomerization of ortho and metaxylene to paraxylene and the
conversion of ethylbenzenes to benzene and/or xylenes or other
compounds. Isomerised product is withdrawn through line 48 and
introduced to zeolite membrane unit 49 containing zeolite membrane,
catalyst that both isomerises ortho- and metaxylene to paraxylene
and in which unit there is selective permeation of paraxylene
through the zeolite membrane relative to ortho- and metaxylene. It
should be noted that in this embodiment the catalyst may be
incorporated into the membrane or the membrane itself may be
catalytically active or rendered catalytically active or preferably
it may be located on the upstream or inlet side of the membrane but
in close proximity to the membrane. A stream enriched in paraxylene
is withdrawn from zeolite membrane unit 50 through permeate stream
51. The remaining material leaves in retentate stream 52. Permeate
stream 51 and retentate stream 52 may be treated differently (not
shown in FIG. 3) or they may be combined in line 53 and introduced
to stabiliser 54. Stabiliser 54 separates five carbon and lighter
compounds from stream 53 through differences in boiling points.
Five carbon and lighter compounds are withdrawn from stabiliser 54
through line 55 and six carbon and heavier compounds are withdrawn
from stabiliser 54 through line 56. The stream in line 56 is
introduced to a DETOL unit 57 to remove toluene and lighter
compounds through line 58. Xylenes and heavier materials are
withdrawn from DETOL unit 57 through line 59 and introduced to
xylene splitter 41. In addition it is also possible for some or all
of the fresh feed to be introduced to the isomerisation unit 47 or
to the zeolite membrane unit 50 or to both as indicated at lines 60
and/or 61. The reason for using such a split feed has already been
discussed above in relation to FIG. 2. Diverting fresh feed to
either the isomerisation unit via line 60 or the zeolite membrane
unit via line 61 would have similar benefits in terms of reduced
xylene loop recycle. Using line 61 reduces flow through the
isomerisation unit versus that required if line 60 is used. However
it may still be advantageous to utilise line 60 since then it would
combine with the feed in line 46 and could use the same pumps
and/or reactor preheating equipment. This may be particularly
advantageous if the zeolite membrane unit and the isomerisation
unit operate under similar conditions where isomerisation unit
effluent flows directly to the zeolite membrane unit without the
need for heating, cooling and/or pressure change. In that
situation, stream 53 would typically be cooled by transferring its
heat to the isomerisation unit feed to provide at least part of the
reactor preheat. Such feed/effluent heat exchange systems usually
work at their most efficient when feed and effluent flow rates are
approximately the same. The optimum distribution of fresh feed
amongst lines 40, 60 and 61 will vary depending on plant
constraints and economic factors, and should be determined for each
individual application. Conventional processes without zeolite
membrane units have less flexibility in the routing of the fresh
feed into the xylenes loop.
[0051] This particularly preferred embodiment not only has
significant advantages over the prior art process of FIG. 1 but
also has some significant advantages over that of FIG. 2. The
combination of the isomerisation unit and the zeolite membrane unit
downstream of the isomerisation unit enables the beneficial
attributes of both units to be combined for maximum paraxylene
production. The embodiment in FIG. 2 requires a particularly
efficient zeolite membrane unit, as the feed to this unit which is
derived from the paraxylene recovery unit is significantly depleted
in paraxylene. The paraxylene content may be as low as 1% or less
with the balance being mainly ortho- and metaxylene, ethylbenzene
and minor amounts of other materials. This means that the zeolite
membrane unit must be able to quickly isomerise this feed to
produce the required amount of paraxylene on the upstream side of
the membrane which exact concentration of paraxylene depends on the
membrane properties and in some case will need to be an equilibrium
or near equilibrium mixture of xylenes to achieve maximum
efficiency in the process. Furthermore the zeolite membrane unit
must also efficiently convert ethylbenzene or this will build up in
the xylenes loop. The use of an isomerisation unit in combination
with the zeolite membrane unit in FIG. 3 overcomes these
deficiencies with the embodiment of FIG. 2. Firstly the isomerate
from the isomerisation unit is already enriched in paraxylene and
is typically at equilibrium or near equilibrium with respect to
xylenes. This means that the zeolite membrane unit only has to
maintain the isomerate in or near this state to enable the membrane
to work efficiently. Secondly because the isomerisation unit has
the capability of ethylbenzene conversion, the ability of the
zeolite membrane unit to destroy ethylbenzene, although desirable
is not critical. Typically promoting xylenes equilibrium is easier
that destroying ethylbenzene. The negative aspect of xylene losses
which normally occur in the isomerisation unit is offset by the
greatly reduced recycle needed when using the zeolite membrane unit
in this embodiment. Also if the zeolite membrane unit does have at
least some ethylbenzene conversion capability the isomerisation
unit does not have to do as much of this conversion. This would
allow for the isomerisation unit to be operated under milder
conditions and therefore result in less xylenes loss in the
isomerisation unit. Also because there is no need to for the feed
to the zeolite membrane unit to be brought to equilibrium in this
unit, as is required with some membranes when used in FIG. 2
embodiment, it may need significantly less catalyst and be
significantly smaller in size compared to the zeolite membrane unit
which is required for FIG. 2. In the particularly preferred
embodiment of FIG. 3 when a membrane of selectivity of 5 for
paraxylene/(ortho- and metaxylene) and a flux of greater than 10
Kg/m.sup.2/day is used the predicted level of paraxylene in the
feed leaving the zeolite membrane unit compared to the isomerate
leaving the isomerisation unit is 55 wt % compared to 22 wt % (the
equilibrium concentration if about 10 wt % non-xylenes are
present). This provides for an overall increase in paraxylene
production in the cycle of 50% or more. Suitable membranes for use
in a zeolite membrane unit to provide such improved performance are
described for example in U.S. Ser. No. 267,760 filed Jul. 8, 1994,
PCT US95/08512, PCT US95/08514, PCT US95/08513 and PCT
EP95/02704.
[0052] FIG. 4 shows an embodiment of the invention using a zeolite
membrane to separate paraxylene from a xylene stream. Fresh feed
containing xylenes is introduced to xylene splitter 101 through
fresh feed line 100. Compounds boiling above xylenes and possibly
including orthoxylene are withdrawn through bottom stream 102 and
xylene and lighter boiling compounds are withdrawn as overhead
stream 103. Overhead stream 103 is introduced to isomerization unit
104 through intermediate line 105. The isomerization unit 104
converts ortho- and metaxylene to paraxylene and promotes the
conversion of ethylbenzenes to benzene and/or xylenes or other
compounds. Unconverted ortho- and metaxylene along with paraxylene
are withdrawn from isomerization unit 104 through line 106 and
introduced to stabiliser 107. 5 carbon and lighter boiling
compounds are withdrawn from stabiliser 107 through line 108 with
the balance of material withdrawn through line 109 and introduced
to DETOL unit 110. Toluene and lighter boiling compounds are
withdrawn from DETOL unit 110 through line 111 and xylenes are
withdrawn through line 112 and introduced to zeolite membrane unit
113. Zeolite membrane unit 113 comprises a zeolite membrane that is
arranged and effective to permit selective permeation of paraxylene
relative to ortho- and metaxylene. Most paraxylene is withdrawn
through permeate stream 114 and introduced to paraxylene recovery
unit 115. Most ortho- and metaxylene are withdrawn from zeolite
membrane unit 113 through retentate stream 116 and introduced to
xylene splitter 101. The paraxylene recovery unit 115 separates
paraxylene from ortho- metaxylene. Paraxylene is withdrawn through
paraxylene rich stream 117 and the balance of ortho- and metaxylene
are withdrawn through paraxylene poor stream 118. Paraxylene poor
stream 118 is introduced to isomerization reactor 104 through line
105. Alternatively all or part of overhead stream 103 may be
directed to the zeolite membrane unit 113 via line 119 and 112. The
optimal routing of stream 103 depends on an economic balance
amongst several parameters, namely isomerisation unit, per pass
xylenes losses and ethylbenzene conversion and the zeolite membrane
unit's relative selectivity between paraxylene and ethylbenzene.
Routing steam 103 to zeolite membrane unit 113 has the advantage of
avoiding whatever xylenes losses it would have incurred if it had
been passed through the isomerisation unit 104. However there is a
potential risk in this instance that ethylbenzene will build up in
the xylenes loop. For example if zeolite membrane unit 113 ensured
that all the ethylbenzene was retained in the retentate 116 the
ethylbenzene would be retained in the xylenes loop without removal
or destruction and would build up indefinitely as additional
ethylbenzene is brought into the loop in the fresh feed. However if
the selectivity of the zeolite membrane in the unit was such that a
substantial portion of the ethylbenzene permeated through the
membrane and into stream 114 then that portion of the ethylbenzene
would pass to the isomerisation unit 104 via lines 115, 118 and
105, where some of it would be converted thus limiting its build up
in the xylenes loop. There is also the possibility of an
intermediate case where a portion of the stream 103 passes to the
zeolite membrane unit and the remainder flows to the isomerisation
unit. In this case the flow to the isomerisation unit acts as a
purge to prevent the build up of ethylbenzene to unacceptable
levels if the amount permeating through the membrane in the
membrane unit is insufficient. The optimal balance should be
determined for each specific application and will depend on the
membrane properties and the properties of the xylenes isomerisation
catalyst amongst others.
[0053] Another embodiment of the invention is shown in FIG. 5
wherein xylenes are introduced to xylene splitter 201 through fresh
feed line 200. Compounds boiling above the boiling point of xylenes
and possibly some of the orthoxylene are withdrawn from xylene
splitter 201 through bottoms line 202, xylenes and ethylbenzene are
withdrawn through overhead stream 203 and introduced to paraxylene
recovery unit 204. Paraxylene is withdrawn through paraxylene rich
stream 205 and ortho- and metaxylene are withdrawn through
paraxylene poor stream 206. Paraxylene poor stream 206 is
introduced to an isomerization unit 207 through line 208.
Isomerization unit 207 isomerises ortho- and metaxylene to
paraxylene and promotes the conversion of ethylbenzene to benzene
and/or xylenes or other compounds. The isomerate is withdrawn
through line 209. The isomerate mixture which contains a near
equilibrium mixture of ortho-, meta- and paraxylene is introduced
to stabiliser 210 through line 209. 5 carbon and lighter compounds
are withdrawn through line 211 and heavier boiling compounds are
withdrawn through line 212 and introduced to DETOL unit 213.
Toluene and lighter boiling compounds are withdrawn through line
214; xylenes and heavier materials are withdrawn from the DETOL
unit 213 through line 215. Xylenes and heavier materials in line
215 are introduced to zeolite membrane unit 216 which comprises a
zeolite membrane arranged and effective to permit selective
permeation of paraxylene there through. Most paraxylene is
withdrawn through line 217 and introduced to xylene splitter 201.
Ortho- and metaxylene are withdrawn as retentate stream 218 and
reintroduced to isomerization unit 207 through line 208. However it
will likely be necessary to purge a portion of this stream to
xylene splitter 201 via line 219 to avoid an excessive build up of
C9+ aromatics as they will tend to stay in the retentate and not be
removed in the stabiliser or DETOL units.
[0054] A further embodiment of the invention is shown in FIG. 6. In
this embodiment the zeolite membrane unit has as its primary
function the separation of ethylbenzene from a paraxylene depleted
stream so that this may be passed into an ethylbenzene
isomerisation unit. Thus a feed comprising xylenes and ethylbenzene
300 is passed to a xylene re-run fractionation sequence 312 and
into a paraxylene recovery unit 301 via line 315. Paraxylene is
withdrawn through paraxylene rich stream 302 and ortho- and
metaxylene and ethylbenzene are withdrawn through paraxylene poor
stream 303. Paraxylene poor stream 303 is introduced to a zeolite
membrane unit 304 comprising a zeolite membrane which permits
selective permeation of ethylbenzene and possibly paraxylene
through the zeolite membrane relative to ortho- and metaxylene.
Most of the ethylbenzene and possibly most of the paraxylene is
withdrawn from the zeolite membrane unit 304 through permeate
stream 305 and most of the ortho- and metaxylene are withdrawn
through retentate stream 306. Permeate stream 305 passes into an
ethylbenzene isomerisation unit 307. Ethylbenzene isomerization
unit 307 isomerises ethylbenzene to xylenes. The ethylbenzene
isomerate is withdrawn through line 308. The retentate 306 and
ethylbenzene isomerate 308 are combined to provide a unified feed
309 to the isomerisation unit 310 which may be a liquid phase
xylenes isomerisation unit operating at 200.degree. C. The xylenes
isomerate passes from the isomerisation unit 310 through line 311
to a xylene re-run fractionation sequence 312 which produces a
heavy stream 313, a lights stream 314 and a xylenes recycle 315.
The lights stream may be further treated to a fractionation process
to produce a C8 naphthene recycle 316 to the ethylbenzene
isomerisation unit 307 (shown as dotted lines in the figure). A
possible addition to the process described in FIG. 6 is the
inclusion of a zeolite membrane unit after the isomerisation unit
310 but before the xylene re-run fractionation sequence 312. This
additional zeolite membrane unit would further enrich the stream
311 in paraxylene.
[0055] In this embodiment for a given membrane the predicted
recovery of ethylbenzene on the permeate side of the membrane is 44
wt % compared to a normal ethylbenzene concentration of 6 to 7 wt
%. The ethylbenzene conversion across the ethylbenzene
isomerisation unit is 85% which compares favourably with a
conventional process where the per pass conversion is 42%. Because
a much smaller portion of the xylene loop is subjected to the
severe conditions of the ethylbenzene isomerisation unit there are
lower overall xylene losses. Also because the xylenes isomerisation
unit conditions are less severe than in a combined
ethylbenzene/xylenes isomerisation unit the losses of xylenes in
this unit are significantly lower; 1% compared to 3 to 4%. This
results in an overall yield for paraxylene for this embodiment of
94.5% compared to 84.5% for the conventional xylene loop without
zeolite membrane unit. An additional advantage of this embodiment
is that the amount of hydrogen circulation is dramatically reduced
as the ethylbenzene conversion is in a separate reactor and
hydrogen is not required for the xylenes isomerisation unit. This
may result in a significant savings on the cost of operating a
paraxylene recovery process according to this embodiment compared
to the conventional xylenes loop.
* * * * *