U.S. patent application number 09/556298 was filed with the patent office on 2002-06-27 for process for the production of paraxylene that comprises an adsorption stage, a liquid phase isomerization atage and a gas phase isomerization stage with an euo-type zeolite.
Invention is credited to Alario, Fabio, Joly, Jean-Francois, Magne-Drisch, Julia, Merlen, Elisabeth, Minkkinen, Ari.
Application Number | 20020082461 09/556298 |
Document ID | / |
Family ID | 9544776 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020082461 |
Kind Code |
A1 |
Magne-Drisch, Julia ; et
al. |
June 27, 2002 |
Process for the production of paraxylene that comprises an
adsorption stage, a liquid phase isomerization atage and a gas
phase isomerization stage with an euo-type zeolite
Abstract
A process for the production of paraxylene is described that
comprises an adsorption stage (18) that uses toluene as a desorbent
in a simulated moving bed of a feedstock whose ethylbenzene was
separated by distillation (line 3) or by adsorption, an
isomerization stage (26) without hydrogen in liquid phase that is
diluted with toluene from the raffinate produced, a distillation
stage (27) of the raffinate that is isomerized to recover the
toluene (line 29) that is recycled. The separated isomerate is
introduced into a xylene distillation column (9) then recycled, in
adsorption. The separated ethylbenzene is isomerized separately in
gas phase with hydrogen at higher temperature and is distilled (5,
2, 9) in the presence of a catalyst that comprises an
EUO-structural-type zeolite, then recycled in adsorption column
(18). The pure paraxylene is collected as extract (line 22) then
optionally purified by crystallization (17). Application for the
synthesis of terephthalic acid.
Inventors: |
Magne-Drisch, Julia;
(Vilette de Vienne, FR) ; Alario, Fabio; (Neuilly
sur Seine, FR) ; Joly, Jean-Francois; (Lyon, FR)
; Minkkinen, Ari; (Saint nom la Breteche, FR) ;
Merlen, Elisabeth; (Ruil Malmaison, FR) |
Correspondence
Address: |
Millen White Zelano & Branigan P C
2200 Clarendon Blvd
Suite 1400
Arlington
VA
22201
US
|
Family ID: |
9544776 |
Appl. No.: |
09/556298 |
Filed: |
April 24, 2000 |
Current U.S.
Class: |
585/478 ;
585/482; 585/828 |
Current CPC
Class: |
C07C 5/2708 20130101;
C07C 15/067 20130101; C07C 5/2708 20130101; C07C 2529/72 20130101;
C07C 2529/42 20130101; C07C 15/08 20130101 |
Class at
Publication: |
585/478 ;
585/482; 585/828 |
International
Class: |
C07C 005/22; C07C
007/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 22, 1999 |
FR |
99/05.153 |
Claims
1. Process for the production of paraxylene from a feedstock of
aromatic hydrocarbons with eight carbon atoms that comprises
orthoxylene, metaxylene, paraxylene and ethylbenzene, in which
hydrocarbon feedstock (1) is enriched with ethylbenzene in an
enrichment zone (2), a first fraction (3) that for the most part
contains ethylenebenzene is recovered, the first fraction is
isomerized in a catalytic isomerization zone (40) in vapor phase in
the presence of hydrogen with a catalyst, an isomerate is
recovered, the isomerate is distilled in a so-called stabilization
column (5) to eliminate its light fractions, and residual isomerate
(8) of the stabilization column is recycled in enrichment zone (2),
whereby enrichment zone (2) delivers a second fraction (4) that is
distilled in a second distillation column (9), a distillate (10)
that contains the orthoxylene, metaxylene, paraxylene and a minimum
quantity of ethylbenzene is recovered, said distillate (10) is
recycled in at least one adsorption column, an adsorption stage in
a simulated moving bed of a feedstock that comprises said
distillate (10) is produced in the adsorption column that contains
a zeolitic sieve, in the presence of a desorbent, a first fraction
that is high in paraxylene and a second fraction that is low in
paraxylene and that contains desorbent, metaxylene, orthoxylene and
ethylbenzene in a quantity at most equal to 15% by weight beyond
the desorbent are recovered, and one or the other of the following
sequences are produced: either said second fraction is isomerized
in liquid phase in another catalytic isomerization zone (26), the
isomerate is distilled in a distillation column (27), and an
isomerate (30) from which desorbent has essentially been removed is
recovered, or the second fraction is distilled in a distillation
column (27), a fraction that contains metaxylene and an adequate
quantity of desorbent is laterally drawn off (line 45), said
fraction drawn-off laterally in liquid phase into another catalytic
isomerization zone (26) is isomerized at least in part, the
isomerized fraction is introduced (line 37) into same distillation
column (27) below the lateral draw-off point of said column,
optionally a portion (47a) of the fraction that is drawn off
between the lateral draw-off point and the point of introduction of
the isomerized fraction is recycled to carry out a washing, and an
isomerate (30) from which the desorbent is removed is recovered,
and isomerate (30) from which desorbent is removed is recycled in
the adsorption column, the process is characterized in that the
catalyst of the isomerization zone in vapor phase, in ball form or
extrudate form, comprises from 1 to 90% by weight of an
EUO-structural-type zeolite, from 0.01 to 2% by weight of at least
one metal of group VIII of the periodic table, and the addition to
100% by weight of at least one binder, whereby the dispersion of
said metal in the catalyst consists of between 50% and 100%, the
macroscopic distribution coefficient of the metal is between 0.7
and 1.3, and the catalyst exhibits a mechanical resistance such
that the crushing value in the bed is greater than 0.7 MPa.
2. Process according to claim 1, characterized in that the
ethylbenzene content of the second fraction that is low in
paraxylene reaches, beyond desorbent, at most 10% by weight and
preferably 5 to 8% by weight.
3. Process according to one of claims 1 and 2, wherein a fraction
that consists essentially of a desorbent that is recycled at least
in part in the adsorption column is drawn off from distillation
column (27).
4. Process according to one of claims 1 to 3, wherein the
isomerization in liquid phase in the first isomerization zone is
carried out under the following conditions: Temperature less than
300.degree. C., preferably between 200 and 260.degree. C., Pressure
less than 40 bar, preferably between 20 and 30 bar,
Desorbent/isomerization feedstock ratio: less than 15%, preferably
10 to 12% by weight, Zeolitic catalyst: ZSM5; Volumetric flow rate
(V.V.H.) less than 10 h.sup.-1, preferably between 2 and 4
h.sup.-1.
5. Process according to one of claims 1 to 4, wherein the
isomerization in gas phase in the isomerization zone in vapor phase
is carried out under the following conditions: Temperature higher
than 300.degree. C., preferably 350 to 480.degree. C., Pressure
lower than 40 bar, preferably 5 to 20 bar, Hourly volumetric flow
rate: less than 10 h.sup.-1, preferably between 0.5 and 6 h.sup.-1,
Catalyst that contains an EUO-structural-type zeolite and at least
one metal of group VIII, H.sub.2/hydrocarbon ratio that is less
than 10, preferably between 3 and 6.
6. Process according to one of claims 1 to 4, wherein enrichment
zone (2) is a first distillation column (2) into which a mixture of
ethylbenzene, metaxylene, paraxylene and orthoxylene is introduced
and which is regulated such that at least 75% by weight of
ethylbenzene and preferably at least 85% is recovered as
distillate.
7. Process according to one of claims 1 to 5, wherein the
enrichment zone is a specific adsorption zone of a mixture of
ethylbenzene, metaxylene, paraxylene and orthoxylene in a specific
adsorbent in the presence of an adequate desorbent, suitable for
separating said feedstock into the first fraction that contains at
least a large portion of ethylbenzene and preferably approximately
all of the ethylbenzene and of said second fraction.
8. Process according to one of claims 1 to 7, wherein the second
distillation column (9) is operated such that it delivers a residue
that contains orthoxylene and the heaviest hydrocarbons; said
residue is distilled in a fourth so-called rerun distillation
column (12); and a distillate that contains the orthoxylene that is
recycled in the isomerization zone in liquid phase is drawn
off.
9. Process according to one of claims 1 to 8, wherein the catalyst
comprises the EUO-structural-type zeolite and at least one metal of
group VIII of the periodic table in a ratio by weight of 0.05 to 1%
relative to the catalyst.
10. Process according to one of claims 1 to 9, wherein the catalyst
contains an EU1 zeolite.
Description
[0001] The invention relates to a process for the production of
paraxylene that combines an adsorption stage in a simulated moving
bed of a feedstock with eight aromatic carbon atoms and a stage of
isomerization in liquid phase of a fraction that is low in
paraxylene that is recovered from said bed. It pertains
particularly to the synthesis of very pure paraxylene for producing
an intermediate petrochemical, terephthalic acid.
[0002] The composition of the aromatic feedstocks with eight carbon
atoms varies extensively according to their origin. Generally, the
content of para- and orthoxylene isomers is close to 50%, so that a
single process does not make it possible to maximize the production
of paraxylene. It is then necessary to combine an adsorption stage
of the feedstock in a zeolitic sieve that delivers a fraction that
is very high in paraxylene and a fraction that is low in paraxylene
and therefore high in orthoxylene and metaxylene followed by an
isomerization stage of this fraction that is low in paraxylene, as
is described in the patent (G.B. 1420796). During this
isomerization stage, the ratio of isomers at equilibrium is
reestablished since the desirable isomers are produced at the
expense of undesirable isomers.
[0003] Now, taking into account the variety of products that are
introduced into the isomerization zone, the conditions of the
isomerization reaction cannot be optimized. There generally follow
secondary reactions of dismutation of ethylbenzene that lead to the
formation of benzene and heavy aromatic hydrocarbons and
dismutation of xylenes that are transformed into toluene and heavy
aromatic compounds, which complicates the downstream separations
and which reduces the amount of desirable isomers for the
production of p-xylene and therefore the final product yield.
[0004] The prior art is described by patent application FR 2 768
724, which describes a combination of stages for isomerization in
liquid phase of a fraction that is high in metaxylene and
orthoxylene and for isomerization in vapor phase of a fraction that
is high in ethylbenzene. Whereby the isomerization conditions are
not adequately optimized, there also results the formation of
secondary products that interfere downstream with the separation of
isomers by adsorption.
[0005] In addition, a pelletized TPZ-3 catalyst that is used for
the vapor phase isomerization of a feedstock that consists of
ethylbenzene or a feedstock that consists of xylenes is known by
patent application EP-A-51318. This application, however, ignores
the incidence of secondary products in a scheme of processes that
should result in the optimized production of very pure paraxylene
and disregards the influence of the shaping of the catalyst.
[0006] One of the objects of the invention is to eliminate the
drawbacks of the prior art and therefore to optimize the
isomerization reactions of the isomers of xylenes, and thus to
reduce the impurities and to increase the yield of p-xylene
produced.
[0007] Another object is to combine an adsorption stage that uses
in particular the toluene as a desorbent with an isomerization
stage of the xylenes that use toluene as a diluent and a separated
stage for isomerization of the ethylbenzene.
[0008] Another object is to isomerize separately the ethylbenzene
that was previously separated with a suitable catalyst under a
judiciously selected shaping.
[0009] It was noted that by combining an adsorption stage in a
simulated moving bed and a catalytic isomerization stage of the
collected fraction that is low in paraxylene and that contains a
substantial amount of toluene and therefore in liquid phase and
with no hydrogen, good results and a simplified use were observed.
In addition, a substantial savings of distillation equipment was
made. More specifically, the invention relates to a process for the
production of paraxylene from a feedstock of aromatic hydrocarbons
with eight carbon atoms that comprises orthoxylene, metaxylene,
paraxylene and ethylbenzene, in which hydrocarbon feedstock (1) is
enriched with ethylbenzene in an enrichment zone (2), a first
fraction (3) that for the most part contains ethylenebenzene is
recovered, the first fraction is isomerized in a catalytic
isomerization zone (40) in vapor phase in the presence of hydrogen
with a catalyst, an isomerate is recovered, the isomerate is
distilled in a so-called stabilization column (5) to eliminate its
light fractions, and residual isomerate (8) of the stabilization
column is recycled in enrichment zone (2), whereby enrichment zone
(2) delivers a second fraction (4) that is distilled in a second
distillation column (9), a distillate (10) that contains the
orthoxylene, metaxylene, paraxylene and a minimum quantity of
ethylbenzene is recovered, said distillate (10) is recycled in at
least one adsorption column, an adsorption stage in a simulated
moving bed of a feedstock that comprises said distillate (10) is
produced in the adsorption column that contains a zeolitic sieve,
in the presence of a desorbent, a first fraction that is high in
paraxylene and a second fraction that is low in paraxylene and that
contains desorbent, metaxylene, orthoxylene and ethylbenzene in a
quantity at most equal to 15% by weight beyond the desorbent are
recovered, and one or the other of the following sequences are
produced:
[0010] either said second fraction is isomerized in liquid phase in
another catalytic isomerization zone (26), the isomerate is
distilled in a distillation column (27), and an isomerate (30) from
which desorbent has essentially been removed is recovered,
[0011] or the second fraction is distilled in a distillation column
(27), a fraction that contains metaxylene and an adequate quantity
of desorbent are laterally drawn off (line 45), said fraction
drawn-off laterally in liquid phase into another catalytic
isomerization zone (26) is isomerized at least in part, the
isomerized fraction is introduced (line 37) into same distillation
column (27) below the lateral draw-off point of said column,
optionally a portion (47a) of the fraction that is drawn off
between the lateral draw-off point and the point of introduction of
the isomerized fraction is recycled to carry out a washing, and an
isomerate (30) from which the desorbent is removed is recovered,
and isomerate (30) from which desorbent is removed is recycled in
the adsorption column,
[0012] the process is characterized in that the catalyst of the
isomerization zone in vapor phase comprises an EUO-structural-type
zeolite.
[0013] The EUO-structural-type zeolite that is contained in the
catalyst, in particular the EU1 zeolite, the ZSM50 zeolite or the
TPZ-3 zeolite and their process of production are described in the
literature, for example patent EP-B-42226, U.S. Pat. No. 4,640,829
or EP-A-51318, and are incorporated as references in patent
application EP-A-923 987.
[0014] The catalyst in ball form or extrudate form can contain:
[0015] from 1 to 90%, preferably 3 to 60% by weight of at least one
EUO-structural-type zeolite that comprises silicon and at least one
T element that is selected from the group that is formed by
aluminum, iron, gallium and boron, preferably aluminum and boron,
whose Si/T atomic ratio is greater than 5, advantageously between 5
and 100, inclusive, preferably between 5 and 80, inclusive, and
also preferably between 5 and 60, inclusive. Said zeolite is at
least in part in acid form, i.e., in hydrogen form (H.sup.+),
whereby the sodium content is such that the Na/T atomic ratio is
less than 0.5, preferably less than 0.1 and even more preferably
less than 0.02,
[0016] from 0.01 to 2%, inclusive, and preferably from 0.05 to
1.0%, inclusive, by weight, of at least one metal of group VIII of
the periodic table, preferably selected from the group that is
formed by platinum and palladium and even more preferably platinum,
whereby said metal of group VIII is deposited on the zeolite or on
the binder, preferably selectively on the binder and that has a
dispersion that is measured by, for example, chemisorption, for
example by H.sub.2--O.sub.2 titration or by, for example,
chemisorption of carbon monoxide, between 50 and 100%, inclusive,
preferably between 60 and 100%, inclusive, and still more
preferably between 70 and 100%, inclusive. In addition, the
macroscopic distribution coefficient of said metal(s), obtained
from its profile that is determined by Castaing microprobe, whereby
said distribution coefficient is defined as the ratio of the
concentrations of said metal in the core of the grain relative to
the edge of the same grain, is between 0.7 and 1.3, inclusive,
preferably between 0.8 and 1.2, inclusive, optionally from 0.01 to
2%, inclusive, and preferably between 0.05 and 1.0%, inclusive, by
weight, of at least one metal of the group that is formed by groups
IIIA and IVA of the periodic table, preferably selected from the
group that is formed by tin and indium,
[0017] optionally sulfur whose content is such that the ratio of
the number of sulfur atoms to the number of metal atoms of group
VIII that are deposited is between 0.5 and 2, inclusive,
[0018] the addition to 100% by weight of at least one binder,
preferably alumina.
[0019] The catalyst may have a mechanical resistance such that the
crushing value in the bed is greater than 0.7 MPa (Shell
method).
[0020] The toluene can be used as a desorbent in the adsorption
process in a simulated moving bed. It can thus be the diluent that
is required for isomerization in liquid phase of the fraction that
is obtained from the simulated moving bed that essentially contains
the orthoxylene and metaxylene, and the toluene with a limited
quantity of ethylbenzene.
[0021] According to a characteristic of the invention, the
ethylbenzene content of the second fraction that is low in
paraxylene can reach, outside of desorbent, at most 10% by weight
and preferably 5 to 8% by weight.
[0022] It is possible to draw off from the distillation column that
treats the isomerate or that treats the first fraction that is high
in paraxylene a fraction that consists essentially of the desorbent
that is recycled at least in part in the adsorption column.
[0023] The liquid phase isomerization can be carried out under the
following conditions:
[0024] Temperature lower than 300.degree. C., preferably between
200 and 260.degree. C.,
[0025] Pressure lower than 40 bar, preferably between 20 and 30
bar,
[0026] Desorbent/isomerization feedstock ratio: less than 15%,
preferably 10 to 12% by weight,
[0027] Zeolitic catalyst: ZSM5, for example,
[0028] Volumetric flow rate (V.V.H.) less than 10 h.sup.-1,
preferably between 2 and 4 h.sup.-1.
[0029] By thus operating in liquid phase that is preferably diluted
with toluene, at low temperature on any catalyst that can isomerize
the hydrocarbons in liquid phase, the conversion into paraxylene is
promoted, and the dismutation reactions of the ethylbenzene and
xylenes that lead to the formation of heavy hydrocarbons are
avoided.
[0030] The feedstock of aromatic hydrocarbons, low in ethylbenzene,
that is introduced into the adsorption zone in a simulated moving
bed can be obtained from said enrichment zone, which is a
distillation of a hydrocarbon mixture or a zone for adsorption of
this mixture on a specific adsorbent bed.
[0031] According to a first variant, said feedstock comprises a
residue of the first distillation column, into which was introduced
the mixture of ethylbenzene, metaxylene, paraxylene and orthoxylene
and which is regulated such that at least 75% by weight of the
ethylbenzene is recovered as distillate.
[0032] This residue can be introduced into a second distillation
column, and a distillate that contains orthoxylene, metaxylene and
paraxylene is drawn off that is sent into the adsorption column,
and a second residue that contains heavy C.sub.9+ hydrocarbons.
[0033] It is possible to operate the second distillation column
such that it delivers said residue that contains orthoxylene and
the heaviest hydrocarbons; said residue is distilled in a fourth
so-called rerun distillation column (12); and a distillate that
contains orthoxylene that is recycled in the isomerization zone in
liquid phase is drawn off.
[0034] The distillate of the first distillation column that
contains ethylbenzene is isomerized in a catalytic isomerization
zone in vapor phase in the presence of hydrogen, and the second
isomerate that is obtained is distilled in a so-called
stabilization column to eliminate its light fractions then recycled
in the first distillation column.
[0035] The conversion into paraxylene is thus maximized.
[0036] The vapor phase isomerization in the isomerization zone can
be carried out under the following conditions:
[0037] Temperature higher than 300.degree. C., preferably 350 to
480.degree. C.,
[0038] Pressure lower than 40 bar, preferably 5 to 20 bar,
[0039] Hourly volumetric flow rate: less than 10 h.sup.-1,
preferably between 0.5 and 6 h.sup.-1,
[0040] Catalyst that contains an EUO-structural-type zeolite,
[0041] H.sub.2/hydrocarbon ratio that is less than 10, preferably
between 3 and 6.
[0042] Since the xylenes are absent from the isomerization
feedstock, the size of the isomerization unit is small, and the
conversion per pass of ethylbenzene is high. Therefore, the
recycling rate is lower, the overall feedstock volume is lower, and
the catalyst volume is reduced.
[0043] Relative to a conventional isomerization in vapor phase of
the entire fraction that is low in paraxylene that would comprise a
mixture of ethylbenzene and xylenes, the hydrogen recycling will be
small, whereby all of these advantages result in substantial
savings.
[0044] All of the catalysts that are able to isomerize the
hydrocarbons with eight carbon atoms are suitable for this
invention. A catalyst that contains an EUO-structural-type zeolite
and at least one metal of group VIII of the periodic table
(Handbook of Chemistry and Physics, 45th Edition, 1964-1965) are
preferably used in a ratio by weight that is described above. The
EU-1 zeolite and the platinum are preferably used as a metal of
group VIII.
[0045] According to this variant, the ethylbenzene-enriched
fraction is isomerized under optimal conditions, and the quantity
of hydrogen introduced is consequently adjusted, and it is
immaterial that a minimum quantity of xylenes is present in the
isomerization feedstock. The consumption of hydrogen is
consequently reduced to the minimum. The use of a catalyst that
contains an EUO-structural-type zeolite makes it possible to reduce
significantly the parasitic secondary reactions of dismutation,
transalkylation and cracking which result in the formation of
benzene, toluene, heavy hydrocarbons and paraffins and therefore in
improving the overall yield per paraxylene pass.
[0046] Furthermore, all of the isomerizations, one at low
temperature and in toluene liquid phase that works on the xylenes,
the other at high temperature in vapor phase that works
specifically on ethylbenzene, are easier to use and more
selective.
[0047] Thus, everything works toward a greater purity and a higher
yield of paraxylene.
[0048] The effluent that is obtained, after having been introduced
into a stabilization column to remove light gases from it, is
separated by distillation into a distillate that contains benzene
and into a residue that comprises heavier hydrocarbons that are
also produced by dismutation, which can be recycled into the first
distillation column that receives the feedstock.
[0049] According to a second variant of the process that also
promotes the production of pure ethylbenzene, the adsorption
feedstock in a simulated moving bed comprises a fraction that is
low in ethylbenzene and that can result from a specific adsorption
of a mixture of ethylbenzene, metaxylene, paraxylene and
orthoxylene on a specific adsorbent in the presence of an adequate
desorbent, suitable for separating said fraction from another
fraction that contains at least the majority of the ethylbenzene
and preferably approximately all of the ethylbenzene.
[0050] The adsorption of the mixture to recover at least the
majority of the ethylbenzene can be carried out in a simulated
moving bed, preferably at simulated countercurrent, in the presence
of a zeolitic adsorbent that contains at least one element that is
selected from the group of elements K, Rb, Cs, Ba, Ca and Sr, and
optionally water. The conditions of this particular adsorption are
described in, for example, U.S. Pat. Nos. 5,453,560, 4,613,725,
4,108,915, 4,079,094 and 3,943,182.
[0051] The operating conditions of the first distillation column or
those of the specific adsorption of the hydrocarbon mixture for
recovering at least the majority of the ethylbenzene will in
general be such that a fraction is recovered that contains at least
85% by weight of ethylbenzene and preferably at least 90% by
weight, which will then be isomerized in vapor phase in the
presence of hydrogen to maximize the production of paraxylene.
[0052] The invention will be better understood based on the
following figures that illustrate an embodiment of the process
among which:
[0053] FIG. 1 shows a combination of an adsorption stage, an
isomerization stage in liquid phase and a distillation stage of the
isomerate that is obtained; and
[0054] FIG. 2 represents a variant in which the distillation stage
precedes the isomerization stage in liquid phase.
[0055] According to FIG. 1, an aromatic hydrocarbon feedstock 1
that contains ethylbenzene, paraxylene (P-X), metaxylene (M-X) and
orthoxylene (O-X) is introduced with a stabilized isomerization
effluent 8 that is described below in a distillation column 2. This
column delivers a distillate via a line 3 that contains
ethylbenzene and 10% by weight, for example, of paraxylenes and
metaxylenes and a residue via a line 4 that contains a minor
portion of ethylbenzene and xylenes (O-X, M-X, P-X). In addition,
line 4 receives a line 30 that contains a distilled isomerate that
is obtained from an isomerization reactor 26 that is described
below. The mixture that is formed is introduced into a distillation
column 9 (xylene splitter) that delivers a distillate via a line
10, that contains the metaxylene and the paraxylene and a residue
via line 11 that contains orthoxylene.
[0056] The residue in line 11 is introduced into a distillation
column 12 that delivers a residue via a line 15 that contains heavy
hydrocarbons (C.sub.9+) that are produced in particular by the two
isomerization stages. A distillate that contains essentially
orthoxylene is drawn off via a line 13 at the top of column 12 and
can be recycled at the inlet of first isomerization zone 26, in
liquid phase, or else recovered as a pure product via a line 14 if
the distillation columns are consequently regulated.
[0057] The distillate (line 10) of column 9 is introduced at
midheight, for example, of an adsorption column in a simulated
moving bed 18 that contains a zeolitic sieve, Ba-X, for example.
This line 10 comprises little ethylbenzene, but it does contain
paraxylene that is fresh and converted in isomerization reactors
and metaxylene that is fresh and not converted in these
reactors.
[0058] This adsorption column is desorbed by a desorbent, the
toluene that is introduced, for example, at the bottom of the
column via a line 29. Between the two points of introduction of the
feedstock and the desorbent, an extract is drawn off via a line 19
that contains pure paraxylene and desorbent. The latter is
separated into a distillation column 20 and is recycled as
distillate via a line 21 into adsorption column 18.
[0059] The paraxylene that is collected as residue via a line 22
can be recovered with an adequate purity that is close to, for
example, 99.8%, or if this is not the case, purified in at least
one crystallization zone 17 at high temperature, as described in
the patent of the applicant (EP-B-531191 that is incorporated as a
reference). The paraxylene that is recovered via a line 23 then
exhibits a purity that is greater than 99.9%, for example. A mother
liquid that is obtained from a centrifuging stage that is behind
the crystallization stage is collected via a line 16 and recycled
in feedstock line 10 to adsorption column 18, operating at
simulated countercurrent. The latter further delivers a raffinate
(line 24) upstream from the point of introduction of the feedstock
(upstream being defined relative to the circulation of liquid
current 18a that circulates from the closed loop from top to bottom
in the column). This raffinate that contains toluene and metaxylene
is mixed with the contents of line 13 that is high in orthoxylene
and is introduced into isomerization reactor 26 via a line 25. This
line advantageously contains less than 10% by weight of
ethylbenzene relative to the isomerization feedstock and an amount
of toluene that is greater than 10% by weight. Isomerization
reactor 26 that operates with a fixed bed of a ZSM5 zeolitic
catalyst, for example in liquid phase, with no hydrogen at a
volumetric flow rate of 3 h.sup.-1, at 260.degree. C., for example,
and under 30 bar, delivers an isomerization effluent that contains
toluene as diluent and is high in paraxylene. This effluent is
introduced into a distillation column 27 (30 plates, for example),
from which are recovered a light fraction via a line 28, a toluene
fraction recycled via a line 29 in the adsorption column and a
residue that contains the raffinate that is isomerized via a line
30. This residue has a xylene isomer concentration that corresponds
to that of the equilibrium (24/20/56% for P.X./O.X./M.X.) and an
ethylbenzene content that is close to 10% by weight. It is directly
sent hot in distillation column 9 to be fractionated with the
residue of distillation column 2 of the ethylbenzene.
[0060] The distillate of distillation column 2 is then sent via a
line 3 into catalytic isomerization reactor 40 that operates at a
temperature that is close to 370-400.degree. C. (line 40a) and in
vapor phase. To save energy in the distillation costs, it is
possible to tolerate up to 10%, for example, of xylenes in the
distillate and the equivalent of ethylbenzene in the residue of
line 4.
[0061] The ortho-, meta- and paraxylene-isomer-enriched isomerate
that is obtained is stabilized in a so-called stabilization column
5 where gases that are provided by the make-up hydrogen, benzene
and toluene that are formed or provided by the feedstock (line 6),
light hydrocarbons (line 7 that is connected to line 28) and a
column residue (line 8) are separated. The latter is mixed with the
upstream feedstock of distillation column 2 of the fresh supply
that contains ethylbenzene.
[0062] According to FIG. 2, the invention relates to a variant of
the isomerization process that is integrated in particular for the
cases where the toluene content of the raffinate that is obtained
from the adsorption column, even after dilution by the recycling of
orthoxylene (line 13), is greater than 20% by weight and/or when it
is desired to reduce it, about 10 to 12% by weight.
[0063] The description of this FIG. 2 relies on reference figures
of FIG. 1.
[0064] Thus, the raffinate (line 24) that contains toluene and that
is obtained from the adsorption column is introduced into a storage
tank 32 into which orthoxylene recycling line 13 that is obtained
from distillation column 12 also empties. The resulting mixture
(line 34) is sent via a pump 35 to the 15th plate of distillation
column 27 of the toluene that contains, for example, 30 of them.
The column that is operated at a top pressure of 5 bar absolute
delivers a toluene current (line 29) at the 6th plate. A top
distillate that comprises a reflux of the toluene and light
products is at least in part condensed in a condenser 42. A large
portion of the condensate is sent back to the top plate of
distillation column 27 via reflux line 44 that is connected to a
separation tank 43 from which the gases are evacuated at the top
when there are some (line 46). A small portion of the condensate is
drawn off from line 44 via a line 28 and recovers the light
hydrocarbons. Recirculation of the light hydrocarbons with toluene
in the adsorption column is thus avoided.
[0065] In the lower portion of distillation column 27, at the 20th
plate, for example, a liquid composition that comprises 10 to 12%
of toluene and orthoxylene and metaxylene, which becomes the
isomerization feedstock in liquid phase at 200-260.degree. C., is
drawn off laterally via a line 45. This feedstock is pumped via a
pump 47 at the reaction pressure that is required to maintain a
liquid phase at the suitable temperature. In general, this pressure
is less than 30 bar with this rate of toluene. A portion of this
feedstock is recycled just below the draw-off plate of said
feedstock as a washing fluid via a line 47a.
[0066] The isomerization feedstock is first preheated in a heat
exchanger 48 via the isomerization effluent then via a preheating
furnace 49 at 200.degree. C. It is then introduced into
isomerization reactor 26 that contains a ZSM-5 zeolitic catalyst
bed. The reaction effluent or isomerate is cooled in heat exchanger
48 and just two or three plates are reintroduced under current
draw-off plate 45 via a line 37 to be fractionated there and under
the reflux plate (line 47a). An isomerization effluent from which
toluene has been removed is collected as a distillation residue via
line 30 that empties at the inlet of xylene distillation column
9.
[0067] The high temperature level of line 37 reduces the addition
heat that is necessary for reboiling at the bottom of the
column.
[0068] The following example illustrates the invention. It is
carried out according to FIG. 1 but without crystallization.
[0069] The aromatic hydrocarbon feedstock has the following
composition:
1 Toluene 5.0 Ethylbenzene 18.1 P-xylene 18.1 M-xylene 36.1
O-xylene 17.9 C.sub.9+ 4.7 Separation of ethylbenzene (column 2):
Column 150-200 plates Reflux rate 55:1 to distillate Top
temperature 140.degree. C., 1.1 bar Bottom temperature 175.degree.
C.; 2.3 bar Content of ethylbenzene of the distillate of the
column: 90% mol and 10% ethylbenzene in column residue (4).
Separation of xylenes (column 9): Column 70 plates Reflux rate
6-7:1 to distillate Top pressure 4 bar Top condensation 200.degree.
C. temperature Bottom temperature 235.degree. C.; 5.5 bar
Separation of orthoxylene (column 12): Column 30 plates Top
temperature 175.degree. C.; 2 bar Reflux rate 3.1 to the distillate
Adsorption in a simulated moving bed at countercurrent: Sieve Ba-X;
5.5% H.sub.2O measured by the loss due to fire (LOI) at 950.degree.
C. Temperature 170.degree. C. Number of beds 24 Pressure 5 bar
Toluene/feedstock 1.6:1 ratio
[0070] The paraxylene that is recovered as extract exhibits a
purity of 99.8% and a yield of 95%.
[0071] At the outlet of distillation column (27) of 30 plates that
operate at 5 bar at the top according to FIG. 2, which recovered an
orthoxylene recycling, the isomerization feedstock that is drawn
off at the 20th plate has the following composition (% by
weight):
2 Toluene 10.0 Ethylbenzene 4.14 Paraxylene 0.18 Metaxylene 60.03
Orthoxylene 25.20 C.sub.9+ 0.45 Isomerization in liquid phase
Pressure 20 bar Temperature 260.degree. C. Zeolite ZSM5 Volumetric
flow rate 3.0 h.sup.-1
[0072] The isomerization effluent has the following composition (%
by weight):
3 Benzene and light hydrocarbons 0.86 Toluene 10.00 Ethylbenzene
4.14 Paraxylene 19.06 Metaxylene 46.54 Orthoxylene 17.94 C.sub.9+
1.46
[0073] This effluent is recycled in the same distillation column
(27) (FIG. 2) 2 or 3 plates below the draw-off plate of
isomerization feedstock (45). A portion of said draw-off is
recycled just below the draw-off plate of the isomerization
feedstock as reflux. The toluene is drawn off about five plates
below the upper plate of the distillation column at a temperature
of 190.degree. C. The isomerate is recycled in column (9) for
separation of xylenes at the 40th plate.
[0074] The distillate (line 3) of distillation column (2) of the
ethylbenzene that contains about 10% impurities (paraxylene and/or
metaxylene) is isomerized in the isomerization reactor in vapor
phase under the following conditions:
4 Temperature 385.degree. C. Catalyst Pt/EU-1 zeolite of the ratio
Si/Al = 18/binder (alumina) (0.3%/10%/89.7%) Dispersion of the
metal 95% Macroscopic distribution 0.99 coefficient Value of the
crushing in 1.05 MPa the catalyst bed Volumetric flow rate 3.5
h.sup.-1 H.sub.2/hydrocarbon ratio 4:1 Pressure 9 bar
[0075] The isomerization effluent has the following composition (%
by weight):
5 Benzene and light hydrocarbons 0.75 Toluene 0.25 Ethylbenzene
21.45 Paraxylene 18.90 Metaxylene 39.04 Orthoxylene 19.06 C.sub.9+
0.55
[0076] The advantages of the process according to the invention
were compared relative to those of a conventional process that
comprises a distillation (xylene splitter) upstream from an
adsorption in a simulated moving bed and an isomerization in the
presence of hydrogen and in vapor phase of the raffinate that is
obtained. It was calculated that for the same performance levels
(purity and yield) and for the same investment although the device
according to the invention comprises a larger number of pieces of
equipment, the main advantages relate to the reduced consumption of
catalyst and hydrogen utilities. For example, a reduction of at
least 20% of the heating fuel consumption and about 60% of the
electric power consumption in terms of pumps, compressors and air
exchangers was observed. In addition, the hydrogen consumption for
isomerization can represent only 20 to 30% of the amount of
hydrogen that is necessary according to the conventional
process.
[0077] By way of comparison, the preceding example of the invention
was incorporated, except that the isomerization stage in vapor
phase of the feedstock (line 3) is carried out in the presence of
the same catalyst that has approximately the same dispersion of the
metal and approximately the same macroscopic distribution
coefficient but shaped, not extruded with a crushing value in the
bed (shell) of 1.05 MPA, but pellets with a crushing value in a bed
of 0.3 MPa.
[0078] The isomerization effluent produced under the same
isomerization operating conditions has the following composition (%
by weight).
6 Benzene and light hydrocarbons 1.85 Toluene 0.35 Ethylbenzene
22.15 Paraxylene 17.20 Metaxylene 38.70 Orthoxylene 18.00 C.sub.9+
1.75.
[0079] For the entire device according to the invention, the losses
per pass due to the isomerization in vapor phase in the presence of
the pelletized catalyst are therefore 3.60% by weight here against
1.30% by weight with a vapor phase isomerization that comprises the
EU1 catalyst in extrudate form.
[0080] Also, for the same separation capacity of column 12 that
accommodates orthoxylene and the C.sub.9+ that are formed, the
residual quantity of C.sub.9+ that remains with the orthoxylene and
is recycled via line 13 at the inlet of the isomerization in liquid
phase is larger. This excess C.sub.9+ brings about a much faster
deactivation of the isomerization catalyst in liquid phase (reactor
26).
[0081] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples. Also, the preceding specific embodiments are to
be construed as merely illustrative, and not limitative of the
remainder of the disclosure in any way whatsoever.
[0082] The entire disclosure of all applications, patents and
publications, cited above and below, and of corresponding French
application 99/05.153, are hereby incorporated by reference.
[0083] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *