U.S. patent application number 16/769203 was filed with the patent office on 2021-06-10 for method of recovering paraxylene in a pressure swing adsorption unit with varying hydrogen purge flow direction.
The applicant listed for this patent is BP CORPORATION NORTH AMERICA INC.. Invention is credited to Brian BENJAMIN.
Application Number | 20210170323 16/769203 |
Document ID | / |
Family ID | 1000005447084 |
Filed Date | 2021-06-10 |
United States Patent
Application |
20210170323 |
Kind Code |
A1 |
BENJAMIN; Brian |
June 10, 2021 |
METHOD OF RECOVERING PARAXYLENE IN A PRESSURE SWING ADSORPTION UNIT
WITH VARYING HYDROGEN PURGE FLOW DIRECTION
Abstract
A method of recovering paraxylene in a pressure swing adsorption
unit with varying hydrogen purge pressures. The pressure swing
adsorption zone is adapted to adsorb and desorb paraxylene based on
the cycling of partial pressure in the zone. A first hydrogen purge
is fed concurrent to the feed. A second hydrogen purge is
countercurrent to the feed.
Inventors: |
BENJAMIN; Brian; (Chicago,
IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BP CORPORATION NORTH AMERICA INC. |
Houston |
TX |
US |
|
|
Family ID: |
1000005447084 |
Appl. No.: |
16/769203 |
Filed: |
December 5, 2018 |
PCT Filed: |
December 5, 2018 |
PCT NO: |
PCT/US2018/064097 |
371 Date: |
June 2, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62594750 |
Dec 5, 2017 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2259/40079
20130101; B01D 2259/40045 20130101; C07C 7/005 20130101; B01D
2257/7027 20130101; C07C 5/2732 20130101; B01D 2259/40081 20130101;
C07C 7/14 20130101; B01D 2259/40056 20130101; B01D 2256/24
20130101; B01D 53/047 20130101; C07C 7/12 20130101 |
International
Class: |
B01D 53/047 20060101
B01D053/047; C07C 7/12 20060101 C07C007/12; C07C 5/27 20060101
C07C005/27; C07C 7/14 20060101 C07C007/14; C07C 7/00 20060101
C07C007/00 |
Claims
1. A process for the recovery of a paraxylene product from a
mixture of C8 aromatic hydrocarbons, the process comprising:
feeding a C8-rich aromatic hydrocarbon mixture to a pressure swing
adsorption zone to form a paraxylene-rich stream and a
paraxylene-lean stream, the pressure swing adsorption zone being
adapted to adsorb and desorb paraxylene based on the cycling of
partial pressure in the zone, the cycling of partial pressure in
the zone using a first hydrogen purge flowing in a cocurrent
direction relative to a feed direction and a second hydrogen purge
flowing in a countercurrent direction relative to the feed
direction.
2. The process of claim 1, wherein an intermediate hydrogen purge
occurs between the first and second hydrogen purge and flows
cocurrent relative to the feed direction.
3. The process of claim 1, where an intermediate hydrogen purge
occurs between the first and second hydrogen purge and flows
countercurrent relative to the feed direction.
4. The process of claim 1, wherein the first hydrogen purge is
introduced at a pressure between 200 and 400 psia.
5. The process of claim 1, wherein the second hydrogen purge is
introduced at a pressure between 40 and 400 psia.
6. The process of claim 1, further comprising isomerizing the
paraxylene-lean stream to form an isomerate having a paraxylene
concentration greater than the paraxylene concentration of the
paraxylene-lean stream.
7. The process of claim 1, further comprising crystallizing the
paraxylene-rich stream to form a paraxylene product stream and a
2.sup.nd paraxylene-lean stream.
8. The process of claim 7, isomerizing the 2.sup.nd paraxylene-lean
stream in a 2.sup.nd isomerization zone to form a 2.sup.nd
isomerate having a paraxylene concentration greater than the
paraxylene concentration in the 2.sup.nd paraxylene-lean
stream.
9. The process of claim 8, further comprising converting
ethylbenzene to other compounds in the 2.sup.nd isomerization
zone.
10. The process of claim 9, further comprising feeding the
isomerate, the 2.sup.nd isomerate, and a mixed xylene feed stream
comprising C7, C8, and C9+ aromatic hydrocarbons to a fractionation
zone, and separating the C8 aromatic hydrocarbons from other
components in the fractionation zone to form said C8-rich aromatic
hydrocarbon mixture.
11. The process of claim 1, wherein the pressure swing adsorption
zone comprises a plurality of adsorption vessels each containing an
adsorbent, each vessel being controlled in a sequence of batch
operations comprising the first hydrogen purge and the second
hydrogen purge, the sequence of operations gf each vessel being
offset in time from the sequence of operations in the other vessels
such that said plurality of vessels operate together in a
pseudo-continuous manner.
Description
[0001] The present teachings relate generally to processes for
recovering paraxylene, and in particular, to processes utilizing
pressure swing adsorption for recovering paraxylene.
BACKGROUND
[0002] Paraxylene is a chemical intermediate that is oxidized to
form terephthalic acid, which is a precursor to polyester.
[0003] Paraxylene is typically manufactured and recovered from
streams comprising "mixed xylenes." In the industry, mixed xylenes
refer to a narrow boiling distillation heart cut of C8 aromatic
hydrocarbons comprising the three xylene isomers orthoxylene,
metaxylene, and paraxylene, as well as the structural isomer
ethylbenzene. Mixed xylenes may also contain non-aromatic compounds
with boiling points close to the xylenes. These mainly comprise C9
paraffins and naphthenes. Mixed xylenes generally also contain low
levels of toluene and C9 and higher aromatics present due to their
imperfect separation in the distillation towers used to produce the
mixed xylenes heart cut. Mixed xylenes are typically obtained from
a reformate of the refinery catalytic reformer unit or another unit
used to produced mixed xylenes, such as a non-selective toluene
disproportionation (TDP) unit, a selective toluene
disproportionation (STDP) unit, a non-selective or selective
toluene alkylation unit, a toluene/aromatic C9-plus transalkylation
(TA) unit or an aromatic C9-plus transalkylation unit.
[0004] Paraxylene manufacturing units typically have three sections
in a recycle loop: 1) a reaction section comprising a xylene
isomerization catalyst and an ethylbenzene conversion catalyst; and
2) a fractionation section for separating byproducts produced in
the reaction section and/or present in the fresh feed; and a 3) a
paraxylene recovery section for recovering paraxylene from a
mixture of xylene isomers and ethylbenzene. A recycle returns a
paraxylene-lean stream formed as a reject filtrate in the
paraxylene recovery section to the reaction section.
[0005] The isomerization catalyst returns a paraxylene-lean stream
to its near equilibrium ratio of 1:2:1
(paraxylene:metaxylene:orthoxylene). The ethylbenzene conversion
catalyst is also present because it is not practical to remove
ethylbeneze by distillation because its boiling point is very close
to the xylene isomers. Thus, ethylbenzene must be converted to
xylenes or to byproducts that can be easily separated by
distillation to prevent its build-up in the loop. For example,
ethylbenzene isomerization-type catalysts (also known as naphthene
pool catalysts) have the ability to convert a portion of the
ethylbenzene to xylene isomers via C8 naphthene intermediates.
Ethylbenzene dealkylation-type catalysts convert ethylbenzene
primarily via reaction with hydrogen to form benzene and ethane.
Ethylbenzene transalkylation-type catalysts convert ethylbenzene
primarily by the transfer of the ethyl group to another ethyl
benzene or to a xylene.
[0006] All of these catalysts produce by-products from the
ethylbenzene conversion reactions and/or side reactions that must
be separated in the fractionation section. These by-products
include benzene, toluene, and C9-plus aromatics. The fractionation
zone also removes C9-plus aromatics and other heavies present in
the feed.
[0007] Two known methods for recovering paraxylene in the
paraxylene recovery section are crystallization and selective
adsorption. Selective adsorption processes include the UOP Parex
process described in R A Meyers (editor) Handbook of Petroleum
Refining Processes, Third Edition (2004) and the Axens Eluxyl
process described in G Ash, et al, Oil and Gas Technology, 49 (5),
541-549 (2004). However, crystallization is often preferred to
selective adsorption because it leads to overall process energy
savings. Although xylene isomers and ethylbenzene have undesirably
similar boiling points (making distillation difficult), they have
dramatically different melting points. Pure paraxylene freezes at
56.degree. F. (13.degree. C.), pure metaxylene freezes at
-54.degree. F. (-48.degree. C.), pure orthoxylene freezes at
-13.degree. F. (-25.degree. C.) and pure ethylbenzene freezes at
-139.degree. F. (-95.degree. C.).
[0008] In a typical crystallization zone for recovering paraxylene,
liquid paraxylene is crystallized from a feedstream comprising the
xylene isomers and ethylbenzene. The paraxylene is generally caused
to crystallize by cooling the feedstream to a temperature below the
freezing point of the paraxylene but preferably above the freezing
point of the other components in the feedstream. More particularly,
the temperature is selected to seek to optimize the crystallization
of paraxylene, for example by selecting a temperature at which
paraxylene freezes but which is above the eutectic temperature (the
eutectic temperature is the temperature at which a xylene isomer
other than paraxylene begins to co-crystallize). The
paraxylene-metaxylene and paraxylene-orthoxylene eutectic
temperatures can be close depending on the composition within the
crystallizer, so either metaxylene or orthoxylene may be the first
isomer to begin to co-crystallize. For non-selective feedstocks,
the eutectic temperature is typically around -88.degree. F.
(-67.degree. C.) to around -94.degree. F. (-70.degree. C.).
[0009] The low temperatures required to crystallize paraxylene from
xylene mixtures are typically achieved by a cascaded vapour
compression refrigerant system using a Deep Refrigerant. A Deep
Refrigerant is defined as one for which it is generally not
possible, or not economic, to compress its vapour or gas to a
pressure level where it can be condensed by air or water cooling.
Ethylene is a Deep Refrigerant, because its critical temperature is
49.degree. F. (9.5.degree. C.), and its critical pressure is 50.76
bar. Thus, for most places on earth, for at least part of the year,
ethylene is a gas above its critical temperature at ambient
temperature, and it is not possible to condense ethylene via air or
water cooling. When used as a refrigerant, ethylene is usually
condensed by transferring heat to a High Level Refrigerant. A High
Level Refrigerant is defined as one for which it is possible to
condense its vapour against air or water. Thus, a cascaded
ethylene/propylene, ethylene/propane, or ethylene/ammonia
refrigeration system can be used to achieve the low temperatures
required for paraxylene crystallization.
[0010] Effluent from the crystallization zone contains paraxylene
solids dispersed in a mother liquor, and it will typically
therefore be necessary to separate these solids in one or more
solid-liquid separation devices, such as centrifuges. Separation of
the effluent produces a filtrate and a relatively paraxylene-rich
cake. The cake obtained by separating the effluent from the
crystallization stage contains paraxylene crystals with adhered
mother liquor that contains ethylbenzene, other xylene isomers,
unrecovered paraxylene and other components of the feedstream. To
improve the purity, the cake is typically further processed in one
or more reslurry zones in which the cake is equilibrated with a
diluent stream comprising liquid paraxylene to provide a slurry.
The reslurry effluent is separated in a solid-liquid separator to
form a relatively pure paraxylene solid product and a filtrate that
may be recycled or used in other parts of the process.
[0011] Another method for recovering paraxylene from mixed xylenes
is known as pressure swing adsorption and is disclosed, for
example, in U.S. Pat. Nos. 6,573,418, 6,600,083, 6,627,783,
6,689,929, and 7,271,305. In a pressure swing adsorption unit, a
vapor phase containing mixed xylenes is fed at elevated temperature
and pressure to a bed of fixed adsorbent containing a selective
molecular sieve. Paraxylene and ethylbenzene are preferentially
adsorbed to the sieve. The remaining stream is rich in metaxylene
and orthoxylene and passes out of the pressure swing adsorption
unit. The pressure is then lowered and paraxylene and ethylbenzene
are desorbed to form a paraxylene and ethylbenzene rich effluent
stream. This effluent may be then sent to a crystallization zone
for recovery of the paraxylene.
[0012] Prior pressure swing adsorption units were effective in
separating and recovering paraxylene. However, one of the variable
costs of the prior methods was to compress hydrogen for circulation
to the pressure swing adsorption unit. There remains a need to
develop cost efficient processes for manufacturing paraxylene.
SUMMARY
[0013] According to one aspect of the invention, a process for the
recovery of a paraxylene product from a mixture of C8 aromatic
hydrocarbons is provided. The process includes feeding a C8-rich
aromatic hydrocarbon mixture to a pressure swing adsorption zone to
form a paraxylene-rich stream and a paraxylene-lean stream. The
pressure swing adsorption zone is adapted to adsorb and desorb
paraxylene based on the cycling of pressure in the zone. The
cycling of pressure in the zone uses a first hydrogen purge that
travels in the cocurrent direction relative to the xylene feed in
the zone and a second hydrogen purge that travels in the
countercurrent direction relative to the xylene feed in the zone.
Intermediate hydrogen purges traveling in either the cocurrent or
countercurrent direction relative to the xylene feed may also be
utilized.
[0014] Other aspects of the invention will be apparent to those
skilled in the art in view of the description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1a shows a process flow diagram for manufacturing
paraxylene according to one prior art method.
[0016] FIG. 1b shows a process flow diagram for manufacturing
paraxylene in accordance with one embodiment of the present
invention.
[0017] FIG. 2 shows a process flow diagram of a pressure swing
adsorption zone in accordance with one embodiment of the present
invention
[0018] FIG. 3 shows a process flow diagram of a paraxylene recovery
zone in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION
[0019] By way of general introduction, a process for recovering
paraxylene is disclosed. The process includes a pressure swing
adsorption zone for separating paraxylene and ethylbenzene from
metaxylene and orthoxylene. The present invention improves the
recovery and purity of the paraxylene rich stream leaving the
pressure swing adsorption zone. This results in cost savings,
reducing impure recycles around the unit thus saving energy and
reducing undesirable xylene losses across reactors in the
design.
[0020] Referring now to FIG. 1a and FIG. 1b, a process for the
recovery of paraxylene product is shown generally at 10. FIG. 1a
illustrates a prior art process for manufacturing paraxylene, and
FIG. 1b illustrates a process in accordance with the present
invention. A fresh feed 12 including mixed xylenes is fed to a
fractionation zone including a xylene recovery distillation column
20 for separation of C8-rich aromatic hydrocarbon mixture from
other components. In one embodiment, the mixed xylene fresh feed
comprises paraxylene, orthoxylene, metaxylene, as well as the
structural isomer ethylbenzene. In other embodiments, the fresh
feed also comprises C7 and C9+ aromatic compounds, as well as
non-aromatic compounds such as C9 paraffins and naphtenes.
Typically, the mixed xylene fresh feed 12 is formed as reformate of
a refinery catalytic reformer unit, or another unit used to
produced mixed xylenes, such as a non-selective toluene
disproportionation (TDP) unit, a selective toluene
disproportionation (STDP) unit, a non-selective or selective
toluene alkylation unit, a toluene/aromatic C9-plus transalkylation
(TA) unit or an aromatic C9-plus transalkylation unit. The mixed
xylene fresh feed 12 is typically at least 90 wt % mixed xylenes.
In some embodiments, the mixed xylene fresh feed 12 is at least 95
wt %, 98 wt %, or 99 wt % mixed xylenes.
[0021] In some embodiments, a second fresh feed mixed xylene
containing stream 14 is also fed the column 20. The second mixed
xylene containing stream 14 is typically a heavier cut of reformate
containing a higher concentrations of C9+ compounds, and is fed
lower on the column 20 than the first fresh feed 12. The second
mixed xylene stream 14 typically contains at least 10 wt % of C9+
compounds. In some embodiments, the mixed xylene fresh feed 12 is
at least 15 wt %, 20 wt %, 25 wt %, 35 wt %, or 50 wt % of C9+
compounds.
[0022] At least one other xylene containing stream resulting from
recycle loops in the process 10 is fed to the column 20. In the
embodiment shown, two such feeds 16, 18 are shown. Those skilled in
the art will appreciate that other configurations of the recycled
feed are also possible. The xylene containing feed stream 16 and
xylene containing feed stream 18 typically contain proportionally
less ethylbenzene than the mixed xylene feed streams 12, 14. The
feed streams 16 and 18 further comprise benzene. In some
embodiments, the feed streams contain at least 1 wt % or 2 wt %
benzene. In other embodiments, feed stream 16 contains at least 5
wt % benzene.
[0023] The xylene recovery column 20 is configured to separate the
feed streams 12, 14, 16, 18 into one or more streams comprising a
C8-rich aromatic hydrocarbon mixture, a stream containing C7-
compounds, and a stream containing C9+ compounds. In the embodiment
shown in FIG. 1a or FIG. 1b, a first sidedraw stream 22 comprises a
liquid phase C8-rich aromatic hydrocarbon mixture, while a second
sidedraw stream 24 comprising a vapor phase C8-rich aromatic
hydrocarbon mixture. The first sidedraw stream 22 is withdrawn at
location on the column above the second sidedraw stream 24. The
feed stream 18 is introduced to the column above the vapor phase
sidedraw stream 24 so that gaseous components in the feed stream 18
do not exit through the vapor phase sidedraw stream 24. The liquid
phase sidedraw phase is pressurized by pump 23. The vapor phase
sidedraw stream is condensed by condenser 26 and the resulting
condensate is pressured by pump 27. The pressurized condensate of
the vapor phase sidedraw stream and the pressurized liquid phase
sidedraw stream are combined to form a combined C8-rich aromatic
hydrocarbon mixture stream 28.
[0024] An overhead product stream 30 is withdrawn from the top of
the column 20 and comprises C7- compounds including benzene,
toluene, and ethane. The overhead product stream 30 is partially
condensed by condenser 32 and the condenser effluent is separated
into liquid and gaseous components in flash drum 34. The liquid
phase is partially returned to the column 30 as a reflux stream 36b
and partially removed from the process via stream 36a. The gaseous
components are removed from the process as a light co-product
stream 38.
[0025] A bottoms product stream 40 is removed from the bottom of
the column 20 and comprises C9+ compounds including
trimethylbenzene and methylethylbenzene. A portion of the bottoms
product is recovered as a bottoms co-product stream 42, while
another portion 44 of the bottoms product stream 40 is reboiled by
reboiler furnace 46 and returned to the column 20. The reboiler
furnace 46 provides for the elevated temperature of the column 20
which operates in a temperature gradient, for example, between
500.degree. F. (260.degree. C.) and 50.degree. F. (10.degree. C.)
and a pressure of 15-80 psia.
[0026] In the prior art process shown in FIG. 1a, the combined
C8-rich aromatic hydrocarbon mixture stream 28 is fed to a
paraxylene recovery zone 72. However, in the present invention
illustrated in FIG. 1b, at least a portion of the C8-rich aromatic
hydrocarbon mixture 28 recovered from the fractionation zone is
pre-heated by furnace 50 and one or more heat exchangers (not
shown) and delivered via line 51 to a pressure swing adsorption
zone 52. In the pressure swing adsorption zone 52, the C8-rich
aromatic hydrocarbon mixture is fed at elevated temperature and
pressure to a bed of fixed adsorbent containing a selective
molecular sieve. Paraxylene and ethylbenzene are preferentially
adsorbed to the sieve. The remaining stream is rich in metaxylene
and orthoxylene and passes out of the pressure swing adsorption
unit as paraxylene-lean stream 54. The partial pressure is then
lowered and paraxylene and ethylbenzene are desorbed to form a
paraxylene-rich and ethylbenzene-rich effluent stream 56. The
configuration and operation of the pressure swing adsorption zone
is more fully described below and in reference to FIG. 2. Stream 56
is then sent to a condenser 57 and a flash drum 62 where a C8 rich
stream 70 is formed along with a hydrogen rich stream 64. Stream 64
is then sent to compressor 66 to be compressed and sent back to
pressure swing adsorption zone 52.
[0027] Pressurized hydrogen purge gas streams are fed through lines
60a and 60b from compressor 66 to the pressure swing adsorption
zone 52. In one embodiment, a first hydrogen purge gas, through
line 60a, is fed cocurrent to the feed direction with stream 54 as
its destination. In another embodiment, the first hydrogen purge,
through line 60a, is fed cocurrent to the feed direction with
stream 51 as its destination. The feeding of the first hydrogen
purge through 60a cocurrent to the feed takes advantage of the fact
that at the end of the feed step the material near where the feed
was exiting the bed is enriched in metaxylene and orthoxylene. By
having a first cocurrent hydrogen purge, this remaining metaxylene
and orthoxylene can be removed while minimizing the paraxylene and
ethylbenzene being lost thus increasing the purity and recovery of
the paraxylene and ethylbenzene and improving the overall
performance of the pressure swing adsorption section.
[0028] In one embodiment, a second hydrogen purge is fed, through
line 60b, countercurrent to the feed direction with stream 56 as
its destination. In another embodiment, an intermediate hydrogen
purge is introduced between the first and second hydrogen purge in
a direction cocurrent to the feed through line 60a. In another
embodiment, an intermediate hydrogen purge is introduced between
the first and second hydrogen purge in a direction countercurrent
to the feed through line 60b. The paraxylene-rich and
ethylbenzene-rich effluent 70 exiting the flash drum 62 is fed to a
paraxylene recovery zone 72. In one embodiment, a C8-rich aromatic
hydrocarbon mixture feed 74 to the paraxylene recovery zone 72
comprises a second portion of the combined C8-rich aromatic
hydrocarbon mixture stream 28 exiting the column 20 and bypasses
the pressure swing adsorption unit 52. In one embodiment, the
second feed 74 comprises at least 10 wt % of the combined
paraxylene-rich and ethylbenzene-rich stream 28. In other
embodiments, the second feed 74 comprises at least 20 wt %, at
least 30 wt %, at least 40 wt %, at least 50 wt %, at least 60 wt
%, at least 70 wt %, at least 80 wt %, or at least 90 wt % of the
combined C8-rich aromatic hydrocarbon mixture stream 28.
[0029] The paraxylene recovery zone 72 operates to produce a
paraxylene product 76 and to recycle a paraxylene-lean stream 78
for further processing. In one embodiment, the paraxylene recovery
zone 72 is configured to recover paraxylene product through a
selective adsorption process. In another embodiment, the paraxylene
recovery zone 72 is configured is configured as to recover
paraxylene through a crystallization process. One particular
crystallization process is described below in reference to FIG.
3.
[0030] The paraxylene-lean stream 54 exiting the pressure swing
adsorption unit 52 is fed to an isomerization reactor 80. The
isomerization reactor 80 is a packed bed reactor containing a bed
of an isomerization catalyst for converting metaxylene and
orthoxylene to paraxylene at an approximately equilibrium ratio of
1:2:1 (paraxylene:metaxylene:orthxylene). In one embodiment,
hydrogen 82 is added to the paraxylene-lean stream 54 upstream of
the isomerization reactor 80. In another embodiment, the
paraxylene-lean 54 contains enough hydrogen after exiting the
pressure swing adsorption unit 52 that make-up hydrogen is not
added to the feed 54. In the embodiment shown, the isomerate stream
91 is fed to a condenser 120 and a flash drum 122. where a C8 rich
stream 126 is formed along with a hydrogen rich stream 124. Stream
124 is then sent to compressor 66 to be compressed and sent back to
pressure swing adsorption zone 52. Bottom stream 126 is sent to
pump 128. Then all or a portion of the pump effluent may be sent
via stream 130 directly to furnace 50 or all or a portion of the
pump effluent may be sent via stream 132 to drum 94.
[0031] The second paraxylene-lean stream 78 exiting the paraxylene
recovery zone 72 is mixed with fresh hydrogen 84 and pre-heated
with a furnace 86 and/or one or more heat exchangers (not shown).
The preheated mixture 88 is fed to the additional isomerization
reactor 90. The isomerization reactor 90 contains a isomerization
catalyst for converting metaxylene and orthoxylene to paraxylene at
an approximately equilibrium ratio of 1:2:1
(paraxylene:metaxylene:orthxylene). In some embodiments, the
isomerization reactor 90 also contains an ethylbenzene conversion,
catalyst such as dealkylation catalyst for converting ethylbenzene
to benzene and ethane. Suitable isomerization catalysts and
ethylbenzene catalysts are disclosed, for example, in U.S. Pat.
Nos. Re 31,782, 4,899,011, and 6,518,472.
[0032] In the embodiment shown, the isomerate stream 92 from the
additional isomerization reactor 90 is fed to a high temperature
separator 94 where the stream is flashed. A liquid-rich phase
bottom stream from the high temperature separator 94 is one of the
xylene containing feed stream 18 to the column 20. A vapor-rich
phase stream 95 exiting high temperature separator 94 is sent to a
low temperature separator 96 where the vapor-rich phase stream is
flashed. A liquid-rich phase stream exiting the low temperature
separator 96 is another of the xylene containing feed stream 16 fed
to the column. A vapor phase stream 98 exiting the low temperature
separator 96 comprises hydrogen, ethane, and other light components
and may be recycled and used as a source for streams 82 and/or 84
or be used for fuel.
[0033] FIG. 2 shows one embodiment of the pressure swing adsorption
zone 52 according to the present invention. The pressure swing
adsorption zone 52 comprises one or more vessels 100a, 100b, 100c,
100d, containing a paraxylene selective adsorbent. In the
embodiment shown, there are four vessels, but those skilled in the
art will recognize that other configurations are also possible,
such as configurations with different numbers of vessels, e.g. 3
vessels, 5 vessels, 6 vessels, 7 vessels, 8 vessels, etc. In one
embodiment, the paraxylene selective adsorbent is a non-acidic,
medium pore, molecular sieve. In one embodiment, the molecular
sieve is of the MFI structure type and the process is operated in
the vapor phase at elevated temperatures and pressures wherein the
temperature is substantially isothermal. Adsorbents useful in the
present invention are based on molecular sieves that selectively
adsorb paraxylene within the channels and pores of the molecular
sieve while not effectively adsorbing metaxylene and orthoxylene Ca
isomers (i.e., total exclusion of the larger metaxylene and
orthoxylene or having much slower adsorption rates compared to
paraxylene).
[0034] Molecular sieves are ordered porous crystalline materials,
typically formed from silica, alumina, and phosphorus oxide (PO4)
tetrahedra, that contain a crystalline structure with cavities
interconnected by channels. The cavities and channels within the
crystalline structure are uniform in size and may permit selective
separation of hydrocarbons based upon molecular dimensions.
Generally, the term "molecular sieve" includes a wide variety of
natural and synthetic crystalline porous materials which typically
are based on silica tetrahedra in combination with other
tetrahedral oxide materials such as aluminum, boron, titanium,
iron, gallium, and the like. In these structures networks of
silicon and elements such as aluminum are cross-linked through
sharing of oxygen atoms. Substitution of elements such as aluminum
or boron for silicon in the molecular sieve structure produces a
negative framework charge which must be balanced with positive ions
such as alkali metal, alkaline earth metal, ammonium or hydrogen.
Molecular sieve structures also may be formed based on phosphates
in combination with other tetrahedrally substituted elements such
as aluminum.
[0035] Adsorbents useful in this invention should not possess
catalytic isomerization or conversion activity with respect to the
Ca aromatic feedstream. Thus, suitable molecular sieves should be
non-acidic. If an element such as aluminum or gallium is
substituted in the molecular sieve framework, the sieve should be
exchanged with a non-acidic counter-ion, such as sodium, to create
a non-acidic sieve adsorbent.
[0036] Examples of molecular sieves suitable as adsorbents useful
in this invention include zeolitic materials containing pore
dimensions in the range of 5 to 6 angstroms (10 meter), typically
5.1 to 5.7 angstroms, and preferably 5.3 to 5.6 angstroms, as
measured in cross axes of the pore. This range typically is
referred to as "medium pore" and typically contains 10-ring
tetrahedra structures. Typical examples of medium pore molecular
sieves include those with MFI and MEL framework structures as
classified in Meier and Olson, "Atlas of Zeolite Structure Types,"
International Zeolite Association (1987), incorporated herein by
reference in its entirety. A small pore molecular sieve, such as A
zeolite, which contains 8-ring structures does not have a
sufficiently large pore opening to effectively adsorb para-xylene
within the sieve. Most large pore molecular sieves, such as
mordenite, Beta, LTL, or Y zeolite, that contain 12-ring structures
do not adsorb para-xylene selectively with respect to ortho- and
meta-xylenes. However, several 12 ring structures, having a smaller
effective pore size, for example due to puckering, are potentially
useful in the invention, such as structure types MTW (e.g., ZSM-12)
and ATO (e.g., ALPO-31).
[0037] Specific examples of molecular sieves include ZSM-5 (MFI
structure type) and ZSM-11 (MEL structure type) and related
isotypic structures. Since suitable adsorbents should not be
catalytically reactive to components in the feedstream, the
preferable adsorbent useful in this invention is silicalite (MFI
structure type), an essentially all silica molecular sieve, which
contains minimal amounts of aluminum or other substituted elements.
Typically, the silica/alumina ratio of suitable silicalite is above
200 and may range above 1000 depending on the contaminant level of
aluminum used in the sieve's preparation. Other MFI and MEL sieves
may be use to the extent they are made non-catalytically active.
Other potentially useful adsorbents include structure types MTU,
FER EUO, MFS, TON, AEL, ATO, NES, and others with similar pore
sizes.
[0038] A molecular sieve which is not catalytically reactive will
typically exhibit less than 10% conversion of paraxylene to
metaxylene and orthoxylene, and in some embodiments, less than 5%,
and in other embodiments less than 1%, at the temperature of
operation for the process of the invention.
[0039] The C8-rich aromatic hydrocarbon mixture enters pressure
swing adsorption zone 52 through xylene header 102 and is
introduced into vessels 100a, 100b, 100c, 100d through respective
feed control valves 102a, 102b, 102c, 102d. The first hydrogen
purge, from 60a, enters the vessels 100a, 100b, 100c, 100d through
hydrogen header 106 and high pressure hydrogen feed control valves
106a, 106b, 106c, 106d respectively. The second hydrogen purge gas,
from 60a, enters the vessels 100a, 100b, 100c, 100d through
hydrogen header 112 and hydrogen feed control valves 112a, 112b,
112c, 112d, respectively. The third hydrogen purge gas, from 60b,
enters the vessels 100a, 100b, 100c, 100d through hydrogen header
104 and hydrogen feed control valves 104a, 104b, 104c, 104d,
respectively.
[0040] The pressure swing adsorption zone 52 also comprises a
paraxylene and ethylbenzene collection header 108 and a set of
outlet control valves 108a, 108b, 108c, 108d for removing a
paraxylene-rich and ethylbenzene-rich stream 56 from each of the
vessels 100a, 100b, 100c, 100d, 100e, 100f, respectively. The
pressure swing adsorption zone 52 also comprises a metaxylene and
orthoxylene collection header 110 and a set of outlet control
valves 110a, 110b, 110c, 110d, 110e, 110f for removing the first
paraxylene-lean stream 54 from the vessels 100a, 100b, 100c, 100d,
respectively. The pressure swing adsorption zone 52 also comprises
a feed recirculation header 114 which can direct flow back to the
xylene header 102 through a set of control valves 114a, 114b, 114c,
114d.
[0041] The vessels 100a, 100b, 100c, 100d in the pressure swing
adsorption zone 52 are operated in a sequence of operations, the
sequence of operations in each vessel being offset in time from the
sequence of operations in the other vessels such that the vessels
operate together in a pseudo-continuous manner.
[0042] The sequence of operations are now described with reference
to the first vessel 100a. All the valves are controlled
automatically by a control system (not shown). The valves are
maintained closed unless they are described as being opened below
for a particular operation.
[0043] In the first operation, designated "FEED", C8-rich aromatic
hydrocarbon stream is introduced through feed header 102 and feed
control valve 102a to vessel 100a at elevated pressure. The
paraxylene and ethylbenzene molecules adsorb to the adsorbent,
while the metaxylene and orthoxylene molecules are blown through
the bed and leave the process through the outlet control valve 110a
and the metaxylene and orthoxylene collection header 110.
[0044] In the second operation, designated "HP1," (hydrogen purge
1) the high pressure purge gas, from 60a (FIG. 1b), is fed through
header 106 and control valve 106a to sweep the bed. This hydrogen
displaces some of the metaxylene and orthoxylene left in the void
space of the bed and continues to flow out through control valve
110a and header 110 after the FEED operation is complete. This
allows the paraxylene and ethylbenzene to be extracted in a later
operation without being contaminated by metaxylene and
orthoxylene.
[0045] In the third operation, designated "HP2", (hydrogen purge 2)
the high pressure purge gas, from 60a (FIG. 1b), is fed through
header 112 and 112a to sweep the bed. This hydrogen displaces the
remaining metaxylene and orthoxylene left in the void space of the
bed. In the act of removing the last remaining metaxylene and
orthoxylene some paraxylene and ethylbenzene also begins to be
displaced from the bed. Thus, the HP2 step leaves the bed through
valve 114a to the feed recirculation header 114 and back to the
xylene feed header 102. This allows the remaining metaxylene and
orthoxylene to be removed from the bed without losing the
paraxylene and ethylbenzene that go with it to the metaxylene and
orthoxylene collection header 110. This improves the recovery and
purity of the final products.
[0046] In the fourth operation, designated "HP3" (hydrogen purge
3), the hydrogen stream, from 60b (FIG. 1b), is fed through
hydrogen header 104 and valve 104a into vessel 100a at the
desorption pressure and sweeps the bed. This sweeping of the bed
drops the partial pressure of paraxylene and ethylbenzene. This
causes desorption of paraxylene and ethylbenzene from the
adsorbent. The paraxylene and ethylbenzene rich stream then leaves
the bed via valve 108a and on to the paraxylene and ethylbenzene
collection header 108.
[0047] All four vessels 100a, 100b, 100c, 100d go through this same
cycle of four operations, but at any given time, each vessel is at
a different stage of the cycle. The system is designed and operated
such that one of vessels is always in the FEED operation so that
the feed to the pressure swing adsorption zone 52 as a whole is
constant. Table 1 illustrates one embodiment of a sequence of the
four operations for the pressure swing adsorption zone 52 in which
plurality of vessels operate together in a pseudo-continuous
manner. The Table illustrates four time periods and shows which
operation is being performed in each vessel at each time period. A
typical time period is from 5 seconds to about 120 seconds. Those
skilled in the art will recognize that the sequence in Table 1 is
exemplary and other sequences are also possible to carry out the
invention.
TABLE-US-00001 TABLE 1 Sequence of Pressure Swing Adsorption
Operations Time Period Vessel a Vessel b Vessel c Vessel d 1 Feed
HP3 HP2 HP1 2 HP1 Feed HP3 HP2 3 HP2 HP1 Feed HP3 4 HP3 HP2 HP1
Feed
[0048] FIG. 3 illustrates one embodiment of the paraxylene recovery
zone 72 in accordance with the present invention. The
paraxylene-rich and orthoxylene-rich effluent 70 enters a
crystallization zone comprising a first crystallization zone
comprising a first crystallizer 202 and a second crystallization
zone comprising a second crystallizer 216. In one embodiment, the
effluent 70 comprises at least 25 wt % paraxylene. In other
embodiments, the effluent 70 comprises less than 75 wt %, less than
85 wt %, or less than 95 wt % paraxylene. The first crystallizer
202 is temperature controlled to operate to solidify paraxylene in
the effluent 70. In one embodiment, the first crystallization zone
operates at a temperature greater than -40.degree. F. (-40.degree.
C.). In another embodiment, the first crystallization zone operates
at a temperature greater than -30.degree. F. (-34.4.degree. C.).
The first crystallization zone typically operates a temperature
between 40.degree. F. (4.4.degree. C.) and -40.degree. F.
(40.degree. C.). The first crystallizer effluent 204 is withdrawn
from the first crystallizer 202 and sent to a first solid-liquid
separator 206. The first solid-liquid separator 206 separates a
paraxylene-lean filtrate stream 208 from a first paraxylene-rich
cake stream 210. One portion 212 of the paraxylene-lean filtrate
stream may be recycled to the first crystallizer 202, while another
portion 214 of the paraxylene-lean filtrate stream is sent to a
second crystallizer 216. The second crystallizer 216 is temperature
controlled to further solidify any remaining paraxylene. The second
crystallization zone operates at a temperature less than the first
crystallization zone. In one embodiment, the second crystallization
zone operates at a temperature less than -70.degree. F.
(-56.7.degree. C.). In another embodiment, the second
crystallization zone operates at a temperature less than
-90.degree. F. (-67.8.degree. C.). In another embodiment, the
second crystallization zone operates at a temperature less than
-110.degree. F. (-78.9.degree. C.). The second crystallization zone
typically operates at a temperature between -30.degree. F.
(-34.4.degree. C.) and -130.degree. F. (-90.degree. C.). The second
crystallizer effluent 218 is withdrawn from second crystallizer 216
and introduced to a second solid-liquid separator 220. In one
embodiment, the effluent 218 exiting the second crystallizer is
colder than -95.degree. F. (-70.6.degree. C.). The second
solid-liquid separator 220 separates a second paraxylene-rich cake
222 from a second paraxylene-lean filtrate stream 224. A portion
228 of the filtrate stream 224 may be recycled to the crystallizer
216, while another portion 78 of the paraxylene-lean filtrate
stream 224 is recycled for further processing as described above
and in reference to FIG. 1b.Hhhh
[0049] The first and second solid-liquid separator 202, 216 may be
any solid-liquid separation devices known in the art, such as
centrifuges, rotary pressure filters, rotary vacuum filters, or
filter columns. In one particular embodiment, the first
solid-liquid separator 206 comprises a pusher centrifuge and the
second solid-liquid separator 220 comprises a screen bowl
centrifuge. In one embodiment, the second solid-liquid separator
220 removes an additional paraxylene-lean filtrate 226 before
withdrawing the second paraxylene-lean filtrate 224. The additional
filtrate 226 is higher in paraxylene concentration than the second
paraxylene-lean filtrate 224 and is recycled to the second
crystallizer 216.
[0050] The first paraxylene-rich cake 210 and the second
paraxylene-rich cake 222 enters one or more reslurrying zones for
removing any remaining impurities. The embodiment of FIG. 3 shows
two reslurrying zones, each having a reslurry drum 224, 240. The
paraxylene-rich cake 210 from the first solid-liquid separator is
fed to either or both of the first reslurry drum 224 and the second
reslurry drum 240 through streams 226 and 242, respectively. The
second paraxylene-rich cake 222 is fed to the first reslurry drum
224. The paraxylene-rich cake(s) are reslurried in the first
reslurry drum 224 with reslurrying fluids to remove impurities from
the paraxylene crystals and the effluent 231 from the first
reslurry drum 224 is sent to the third solid-liquid separator 232.
The third solid-liquid separator 232 separates the effluent 231
into a third paraxylene-rich cake 238 and a third paraxylene-lean
filtrate stream 234. A portion 228 of the third paraxylene-lean
filtrate stream 234 is recycled to the first reslurry drum 224 as a
reslurrying fluid, and another portion 236 may be recycled to the
first crystallizer 202 for further recovery of paraxylene.
[0051] The third paraxylene-rich cake 238 is fed to a second
reslurry drum 240 for further reslurrying with one or more
reslurrying fluids for removing impurities from the paraxylene
crystals. The effluent 248 from the second reslurry drum 240 is fed
to a fourth solid-liquid separator 250. The fourth solid-liquid
separator 250 separates the effluent 248 into a fourth
paraxylene-rich cake 252 and a fourth paraxylene-lean filtrate
stream 254. A portion 244 of the fourth paraxylene-lean filtrate
stream 254 is recycled to the second reslurry drum 244 as a
reslurrying fluid, and another portion 230 of the fourth
paraxylene-lean filtrate stream 254 may be recycled to the first
reslurry drum 224 for use as a reslurrying fluid.
[0052] The third and fourth solid-liquid separator 232, 250 may be
any solid-liquid separation devices known in the art, such as
centrifuges, rotary pressure filters, rotary vacuum filters, or
filter columns. The fourth solid-liquid separator 250 may also be a
wash column. Suitable filter columns are disclosed, for example, in
U.S. Pat. Nos. 7,812,206, 8,211,319, and 8,530,716, and 8,962,906,
Suitable wash columns are disclosed, for example, in U.S. Pat. Nos.
4,734,102 and 4,735,781. In one particular embodiment, the third
solid-liquid separator 232 comprises a pusher centrifuge and the
fourth solid-liquid separator 250 comprises a pusher centrifuge. In
one embodiment, the fourth solid-liquid separator 250 removes an
additional paraxylene-lean filtrate 246 before withdrawing the
fourth paraxylene-lean filtrate 254. The additional filtrate 246 is
higher in paraxylene concentration than the fourth paraxylene-lean
filtrate 254 and is recycled to the second reslurry drum 240.
[0053] The fourth paraxylene-rich cake 252 is fed to a melt drum
256. The fourth paraxylene-rich cake is completely melted and a
paraxylene product stream 76 is recovered. A portion 258 of the
melted paraxylene may be recycled to the fourth solid-liquid
separator 250 in order to wash impurities from the cake. In one
embodiment, the paraxylene product 76 is at least 99 wt %
paraxylene. In other embodiment, the paraxylene product is at least
99.5 wt %, 99.6 wt %, 99.7 wt %, or 99.8 wt % paraxylene.
[0054] The use of a pressure swing adsorption zone with an
additional isomerization zone allows for less total mass being fed
to the paraxylene recovery zone, because a significant portion of
the metaxylene and orthoxylene in the system is recycled through
stream 54 (FIG. 1b). In one embodiment, the ratio of the total mass
of the paraxylene-rich stream entering the paraxylene recovery zone
to the total mass of the paraxylene-rich product stream is less
than 6. In other embodiments, the ratio of the total mass of the
paraxylene-rich stream entering the paraxylene recovery zone to the
total mass of the paraxylene-rich product stream is less than 5,
less than 4, less than 3, or less than 2. In other embodiments, the
ratio of the total mass of the recycle stream 78 (FIG. 1b) to the
total mass of the paraxylene-rich product stream 76 is less than 5,
less than 3, or less than 2. The feed to the paraxylene recovery
zone also contains a higher concentration of paraxylene compared to
systems not having a pressure swing adsorption zone. This is
because the pressure swing adsorption zone allows for paraxylene
concentrations greater than the equilibrium concentration resulting
from the isomerization reaction.
[0055] According to another aspect of the invention, a method for
retrofitting a system for recovering paraxylene is provided.
According to the retrofitting method, the pressure swing adsorption
zone 52 (FIG. 1b) is added to a pre-existing system (FIG. 1a) not
having a pressure swing adsorption zone. At least a first portion
51 of the combined C8-rich aromatic hydrocarbon mixture stream 28
is routed to the pressure swing adsorption zone 52 to form a
paraxylene-rich intermediate stream 56 (which is flashed in drum 62
to form stream 70) before being fed to the paraxylene recovery zone
72. The retrofit method may also comprise adding the secondary
isomerization zone 80 to a pre-existing system where there was no
previous secondary isomerization zone 80. The retrofit method may
also include adding the bypass stream 74 so that a second portion
of the combined C8-rich aromatic hydrocarbon mixture stream 28
routes directly to the paraxylene recovery zone 72, bypassing the
pressure swing adsorption zone 52. The amount of C8-rich aromatic
hydrocarbon mixture stream bypassed through bypass stream 74 is
dependent upon the throughputs of the pressure swing adsorption
zone 52 and the pre-existing equipment. In one embodiment, the
pre-existing equipment does not have to be re-sized as a result of
the retrofit, which allows increased recovery of paraxylene without
significant capital expenditures. By enriching the combined stream
28 in paraxylene prior to its delivery to the paraxylene recovery
zone and adding isomerization capacity, the retrofit method allows
for increased recovery of paraxylene product compared to the
pre-existing system. In one embodiment, the amount of a paraxylene
product recovered by the retrofitted system increases without
increasing the throughput of the primary isomerization zone 90. In
another embodiment, the amount of paraxylene product recovered
increases without increasing the amount of hydrogen fed to the
system. In another embodiment, the amount of paraxylene product
recovered increases without increasing the amount of the
refrigeration duty of the crystallization zone. In another
embodiment, the amount of paraxylene product recovered increases
without increasing the amount of the furnace duty 86 of the primary
isomerization zone. In another embodiment, the amount of paraxylene
product recovered increases without increasing the amount of the
furnace duty 46 of the fractionation zone.
[0056] The foregoing detailed description and the accompanying
drawings have been provided by way of explanation and illustration,
and are not intended to limit the scope of the appended claims.
Many variations in the presently preferred embodiments illustrated
herein will be apparent to one of ordinary skill in the art, and
remain within the scope of the appended claims and their
equivalents.
[0057] It is to be understood that the elements and features
recited in the appended claims may be combined in different ways to
produce new claims that likewise fall within the scope of the
present invention. Thus, whereas the dependent claims appended
below depend from only a single independent or dependent claim, it
is to be understood that these dependent claims can, alternatively,
be made to depend in the alternative from any preceding
claim--whether independent or dependent--and that such new
combinations are to be understood as forming a part of the present
specification.
* * * * *