U.S. patent application number 11/797983 was filed with the patent office on 2007-09-13 for separator, reactor, and method for producing aromatic carboxylic acids.
Invention is credited to Keiichi Akimoto, Kazuto Kobayashi, Noritaka Matsumoto, Motoki Numata, Hiroyuki Osora, Yoshio Seiki, Hiroaki Shimazu, Yoshiyuki Takeuchi.
Application Number | 20070213557 11/797983 |
Document ID | / |
Family ID | 31980552 |
Filed Date | 2007-09-13 |
United States Patent
Application |
20070213557 |
Kind Code |
A1 |
Seiki; Yoshio ; et
al. |
September 13, 2007 |
Separator, reactor, and method for producing aromatic carboxylic
acids
Abstract
In a process for producing an aromatic carboxylic acid, it is a
dehydrating process which can achieve compactification of a step
for removing water from a mixture of acetic acid and water produced
in the production process, and which can reduce the consumed
energy. In a production process of an aromatic carboxylic acid
having an oxidation reaction step for producing a slurry of an
aromatic carboxylic acid by carrying out liquid-phase oxidation
reaction of an alkyl aromatic compound with an oxygen-containing
gas in a solvent containing acetic acid in the presence of an
oxidation catalyst, at least a portion of a mixture containing
acetic acid and water produced in the production steps is separated
into a permeable gas mainly comprising water and nonpermeable
substances mainly comprising acetic acid, using a separation
membrane having water selectivity.
Inventors: |
Seiki; Yoshio; (Hiroshima,
JP) ; Kobayashi; Kazuto; (Hiroshima, JP) ;
Takeuchi; Yoshiyuki; (Hiroshima, JP) ; Osora;
Hiroyuki; (Hiroshima, JP) ; Akimoto; Keiichi;
(Tokyo, JP) ; Matsumoto; Noritaka; (Tokyo, JP)
; Numata; Motoki; (Kitakyusyu, JP) ; Shimazu;
Hiroaki; (Matsuyama, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
31980552 |
Appl. No.: |
11/797983 |
Filed: |
May 9, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10525971 |
Sep 27, 2005 |
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PCT/JP03/11122 |
Aug 29, 2003 |
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11797983 |
May 9, 2007 |
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Current U.S.
Class: |
562/410 ;
202/82 |
Current CPC
Class: |
Y10S 159/27 20130101;
B01D 3/4205 20130101; B01D 3/143 20130101; C07C 51/47 20130101;
B01D 53/268 20130101; C07C 51/44 20130101; B01D 3/14 20130101; C07C
51/265 20130101; B01J 2219/00006 20130101; Y10S 159/28 20130101;
B01J 8/009 20130101; C07C 51/44 20130101; Y10S 159/03 20130101;
C07C 51/47 20130101; C07C 51/265 20130101; C07C 63/26 20130101;
C07C 63/26 20130101; C07C 63/26 20130101 |
Class at
Publication: |
562/410 ;
202/082 |
International
Class: |
C07C 51/16 20060101
C07C051/16; B01D 1/28 20060101 B01D001/28 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2002 |
JP |
2002-255663 |
Aug 30, 2002 |
JP |
2002-255664 |
Claims
1-7. (canceled)
8. A method of producing an aromatic carboxylic acid comprising an
oxidation reaction step in which an alkyl aromatic compound is
subjected to liquid-phase oxidation reaction using an
oxygen-containing gas in a solvent containing acetic acid in the
presence of an oxidation catalyst to produce a slurry of said
aromatic carboxylic acid; a solid-liquid separation step in which
said slurry is separated into a reaction mother liquid and an
aromatic carboxylic acid cake; and a step of separating at least a
portion of a mixture of acetic acid and water produced during said
steps into a permeable gas mainly comprising water and nonpermeable
substances mainly comprising acetic acid by use of a separation
membrane having selectivity for water.
9. The method of producing an aromatic carboxylic acid of claim 8
wherein at least a portion of the mixture fed to said separation
membrane is a gas.
10. The method of producing an aromatic carboxylic acid of claim 8
wherein said mixture of acetic acid and water further contains
methyl acetate, and wherein using said separation membrane having
selectivity for water, at least a portion of said mixture is
separated into said permeable gas, which mainly comprises water,
and said nonpermeable substances, which mainly comprises acetic
acid and further containing methyl acetate as another main
component.
11. The method of producing an aromatic carboxylic acid of claim 10
wherein said mixture is produced in said oxidation reaction step,
wherein using said separation membrane having selectivity for
water, at least a portion of said mixture is separated into said
permeable gas, which mainly comprises water, and said nonpermeable
substances, which mainly comprises acetic acid and methyl acetate,
and wherein said nonpermeable substances are at least partially
returned to said oxidation reaction step.
12. The method of producing an aromatic carboxylic acid of claim 10
wherein at least a portion of a mix of acetic acid, a methyl
acetate as a byproduct, and water, said mix being produced in a
production process, is supplied into a distillation column, wherein
at least a portion of the acetate in said mix is recovered from a
bottom of said distillation column, wherein at least a portion of
said mix is produced from a top of said distillation column as said
mixture containing acetic acid, methyl acetate and water, wherein
using said separation membrane having selectivity for water, at
least a portion of said mixture is separated into said permeable
gas, which mainly comprises water, and said nonpermeable
substances, which mainly comprises acetic acid and methyl
acetate.
13. The method of producing an aromatic carboxylic acid of claim 12
wherein a portion of said mixture produced from the top of said
distillation column is returned to said distillation column, and
the remainder of said mixture is separated, using said separation
membrane having selectivity for water, into said permeable gas,
which mainly comprises water, and said nonpermeable substances,
which mainly comprises acetic acid and methyl acetate.
14. The method of producing an aromatic carboxylic acid of claim 12
wherein said nonpermeable substances are returned to said oxidation
reaction step.
15. The method of producing an aromatic carboxylic acid of claim 8
wherein using a separation membrane having selectivity for water,
said permeable gas, which mainly comprises water, is further
separated into a permeable gas mainly comprising water and
nonpermeable substances mainly comprising acetic acid.
16. The method of producing an aromatic carboxylic acid of any of
claim 15, wherein one of said separation membranes that is provided
upstream from the other is one that is higher in the permeating
speed, and the other is one that is higher in the separation
ability.
17. The method of producing an aromatic carboxylic acid of claim 8
wherein said separation membrane or said separation membranes are
made of an inorganic material.
18. The method of producing an aromatic carboxylic acid of claim 17
wherein said separation membrane or said separation membranes
comprise an inorganic porous member carrying in pores thereof a
silica gel obtained by hydrolyzing an alkoxysilane containing
ethoxy groups or methoxy groups.
19. The method of producing an aromatic carboxylic acid of claim 8
wherein said alkyl aromatic compound is paraxylene, and said
aromatic carboxylic acid is terephthalic acid.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing an
aromatic carboxylic acid by subjecting alkyl substituents or a
partially oxidized aromatic alkyl compound to liquid-phase
oxidation using an oxygen-containing gas, and a reactor used in
this method.
BACKGROUND ART
[0002] Aromatic carboxylic acids are usually produced in an
oxidation reactor by subjecting alkyl aromatic compounds as raw
materials to liquid-phase oxidation in a solvent containing acetic
acid in the presence of a catalyst containing a heavy metal
compound and a bromine compound by use of a gas containing oxygen
in the form of molecules. During this oxidizing reaction, water is
produced. Reaction vapor discharged from the oxidation reactor and
a mother liquid of aromatic carboxylic acid slurry contain reaction
water. Such reaction vapor and mother liquid, the latter being
obtained by separating aromatic carboxylic acids from the slurry
produced, are typically dehydrated and distilled to recover acetic
acid, which is recycled as a solvent for oxidation reaction.
[0003] Since water and acetic acid are difficult to separate from
each other and because a dilute aqueous solution of acetic acid in
particular has a specific volatility near 1, in order to separate
acetic acid from water by dehydration and distillation so that the
concentration of acetic acid in the distilled water will not exceed
1 weight percent, it is necessary to increase the number of stages
of the distilling column and/or increase the reflux ratio. This
increases the size of the dehydration/distillation column, which in
turn pushes up its cost as well as the facility cost. Also, because
the latent heat of vaporization of water is high, reboiler loads of
distillation increases by increasing the reflux ratio. Various
measures have been reported for avoiding this problem. For example,
patent document 1 reports a method of reducing distillation loads
by combining distillation with extraction. Patent document 2
reports that it is possible to reduce the reflux ratio and energy
consumption by azeotropic distillation using an azeotropic agent in
the dehydration/distillation column.
[0004] Also known are methods of dehydration in which a separation
membrane such as a reverse osmosis membrane is used in distillation
(patent documents 3 and 4). Patent document 3 proposes to remove
water and alcohol by separation using a membrane in producing an
aromatic carboxylic acid. With this arrangement, in which
distillation is combined with separation using a membrane, it is
possible to reduce loads on the dehydration/distillation column, so
that an aliphatic carboxylic acid, a solvent, can be recovered with
high efficiency.
[0005] The alcohol mentioned in patent document 5 is an unnecessary
byproduct produced when recovering acetic acid by hydrolyzing an
aliphatic carboxylic acid ester such as methyl acetate, which is a
byproduct produced in the system. It is separated together with
water by a membrane.
[0006] Patent document 1: JP patent publication 7-53443
[0007] Patent document 2: WO96-06065
[0008] Patent document 3: JP patent publication 2001-328957
[0009] Patent document 4: WO02-50012
[0010] Patent document 5: JP patent publication 2001-328957
DISCLOSURE OF THE INVENTION
[0011] As is apparent from patent documents 1 and 2, extraction
agents and azeotropic agents are needed in the extraction and
azeotropic methods, respectively. This complicates the
dehydration/distillation steps. The methods disclosed in these
references are therefore not sufficiently streamlined.
[0012] In an arrangement in which a separator membrane is used in
distillation, such as disclosed in patent documents 3 and 4, it is
an essential requirement to use a reverse osmosis membrane made of
an organic material in the separation step. But organic
high-molecular membranes, which are typically used as
acid-resistant separator membranes that can selectively separate
water from an aqueous solution containing organic acids, have a
drawback in that their heat resistance is so poor that they can be
used only at relatively low temperatures.
[0013] Further, because it is an effective means for minimizing the
loss of an acetic acid solvent to reuse methyl acetate in the
oxidation reaction step, the consumption of acetic acid will
increase if alcohol is discharged out of the system by hydrolyzing
an aliphatic carboxylic acid ester, as disclosed in patent document
5.
[0014] Still further, in this prior art, since any components that
have not permeated the membrane is returned into the distillation
column, components that have permeated the membrane are diluted at
the top of the distillation column. This increases the amount of
vapor and the amount of fluid flow in the distillation column,
which in turn makes it necessary to increase the size of the
distillation column and the area of the separator membrane. Also,
it is necessary to re-heat the components that have not permeated
the membrane and have been returned into the column. This requires
additional energy consumption.
[0015] A general object of the invention is therefore to avoid all
of the abovementioned problems, and its particular object is to
provide a process for producing an aromatic carboxylic acid in
which the step of removing water from a mixture of acetic acid and
water that is produced during the process can be carried out using
a facility of a reduced size with reduced energy consumption.
[0016] The present inventors sought ways to achieve this object and
found out that by using a specific separator membrane, a mixture of
acetic acid and water produced during a manufacturing process of an
aromatic carboxylic acid can be efficiently separated into water
and acetic acid, using a compact device with minimum energy
consumption.
[0017] Specifically, the invention provides a separation system
comprising:
[0018] a distillation column into which a mixture of a first
component mainly comprising water and a second component mainly
comprising nonaqueous substances is adapted to be supplied; a
separator including a separation membrane for separating overhead
vapor discharged from a top of the distillation column into a
permeable vapor which mainly comprises the first component and a
nonpermeable vapor which mainly comprises the second component by
allowing only a selected portion of the overhead vapor to permeate
the separation membrane; and a reflux unit for cooling a portion of
the overhead vapor into a liquid and returning the liquid thus
obtained into an upper portion of the distillation column.
[0019] With this arrangement, by returning a portion of the
overhead vapor into the distillation column through the reflux
unit, it is possible to reduce the concentration of a
high-boiling-point component in the overhead vapor (either the
first or second component). Thus, the separation membrane has only
to separate overhead vapor of which the concentration of a
high-boiling-point component has decreased. This makes it possible
to reduce the concentration of high-boiling-point components in the
vapor that has permeated the separation membrane to a required
level.
[0020] In the second invention, the distillation column includes
fluid beds.
[0021] In the third invention, there is provided a separation
system comprising: a distillation column into which a mixture of a
first component mainly comprising water and a second component
mainly comprising nonaqueous substances is adapted to be supplied;
a first separator including a first separation membrane for
separating overhead vapor discharged from a top of the distillation
column into a first permeable vapor which mainly comprises the
first component and a first nonpermeable vapor which mainly
comprises the second component by allowing only a selected portion
of the overhead vapor to permeate the first separation membrane;
and a second separator including a second separation membrane for
separating the first permeable vapor into a second permeable vapor
mainly comprising the first component and higher in the
concentration of the first component than the first permeable
vapor, and a second nonpermeable vapor which mainly comprises the
second component by allowing only a selected portion of the first
permeable vapor to permeate the second separation membrane.
[0022] After the overhead vapor has been separated into the first
permeable vapor and the first nonpermeable vapor, the first
permeable vapor is further separated into the second permeable
vapor and the second nonpermeable vapor. Thus, most of the portion
of the second component that may permeate the first separation
membrane will be separated as the second nonpermeable vapor by the
second separation membrane. Thus, it is possible to obtain a
permeable condensed vapor high in the concentration of the first
component as the second nonpermeable vapor.
[0023] According to the fourth invention, there is provided a
reactor system comprising: a reactor for producing an aromatic
carboxylic acid and water from an alkyl aromatic compound in a
solvent containing acetic acid, and for generating a vapor mixture
of a solvent and water; a first separation membrane for separating
the vapor mixture, which is discharged from the reactor, into a
first permeable vapor mainly comprising a first component and a
first nonpermeable vapor mainly comprising a second component; a
second separation membrane for separating the first permeable
vapor, which is discharged from the first separation membrane, into
a second permeable vapor mainly comprising the first component and
a second nonpermeable vapor mainly comprising the second component;
and a return passage for condensing and returning the first
nonpermeable vapor and the second nonpermeable vapor into the
reactor.
[0024] Any nonaqueous components that remain in the first permeable
vapor without being separated by the first separation membrane are
separated by the second separation membrane, recovered as the
second nonpermeable vapor, and returned into the reactor. Thus, it
is possible to reduce the water concentration in the reactor to
less than a predetermined level, thereby accelerating reaction.
[0025] According to the fifth invention, solvent containing acetic
acid is acetic acid, the alkyl aromatic compound is paraxylene, and
the aromatic carboxylic acid is terephtahlic acid.
[0026] The reactor according to the sixth invention further
comprises gas-liquid separators each provided between one of the
first and second separation membranes and the return passage for
separating terephthalic acid from the first and second nonpermeable
vapors.
[0027] According to the seventh invention, the separation membrane
or the first and second separation membranes comprise an inorganic
porous member carrying in pores thereof a silica gel obtained by
hydrolyzing an alkoxysilane containing ethoxy groups or methoxy
groups.
[0028] According to the eighth invention, there is provided a
method of producing an aromatic carboxylic acid comprising an
oxidation reaction step in which an alkyl aromatic compound is
reacted with a oxygen-containing gas in a solvent containing acetic
acid in the presence of an oxidation catalyst to produce a slurry
of the aromatic carboxylic acid; a solid-liquid separation step in
which the slurry is separated into a reaction mother liquid and an
aromatic carboxylic acid cake; and a step of separating at least a
portion of a mixture of acetic acid and water produced during the
steps into a permeable gas mainly comprising water and nonpermeable
substances mainly comprising acetic acid, using a separation
membrane capable of separating water.
[0029] In the step for producing an aromatic carboxylic acid using
acetic acid as a solvent, water is produced as a byproduct during
oxidation reaction. In distilling and separating acetic acid and
water, since the evaporative latent heat of water is high, huge
energy is needed. By separating the mixture into a permeable gas
mainly comprising water and a nonpermeable substances mainly
comprising acetic acid, using a membrane capable of separating
water, i.e. a membrane that passes gaseous H.sub.2O molecules but
is less likely to pass nonpermeable substances mainly comprising
acetic acid, it is possible to reduce the energy needed to separate
acetic acid from water.
[0030] According to the ninth invention, in the arrangement of the
first invention, at least a portion of the mixture fed to the
separation membrane is a gas. Since the substance that has
permeated the separation membrane according to the invention is a
gas, if the mixture is also a gas, it permeates the separation
membrane more efficiently. By keeping the temperature of the
mixture higher than the boiling point of acetic acid at the
operation pressure when fed to the separation membrane,
substantially the entire mixture can be supplied in the form of a
gas. Thus, it is possible to separate a greater amount of mixture
in a shorter period of time.
[0031] According to the tenth invention, the mixture of acetic acid
and water contains methyl acetate, and at least a portion of the
mixture is separated into a permeable gas mainly comprising water
and nonpermeable substances mainly comprising acetic acid and
methyl acetate. Since a separation membrane which can separate
water is less likely to pass methyl acetate, it is present in the
nonpermeable substances which mainly comprise acetic acid. Methyl
acetate can thus be recovered together with acetic acid. This
reduces the energy necessary to separate a mixture of acetic acid,
methyl acetate and water into water and a mixture of acetic acid
and methyl acetate.
[0032] According to the 11.sup.th invention, in the third
invention, the nonpermeable substances are at least partially
returned to the oxidation reaction step. The nonpermeable
substances, i.e. the substances that have not permeated the
separation membrane, mainly comprise acetic acid and further
contain methyl acetate. They scarcely contain water. On the other
hand, oxidation reaction needs acetic acid as a solvent. Thus, by
returning the nonpermeable substances as a solvent for oxidation
reaction, acetic acid contained in the nonpermeable substances can
be effectively used. Also, by recovering the methyl acetate
contained in the nonpermeable substances, which is one of the
byproducts of the oxidation reaction, in the oxidation step, it is
possible to suppress the production of methyl acetate due to the
equilibrium reaction of acetic acid, thereby reducing the loss of
the solvent.
[0033] According to the 12.sup.th invention, in the third
invention, before membrane separation, the mixture is supplied into
the distillation column, and at least part of acetic acid is
recovered form the bottom of the column, and at least part of the
mixture of acetic acid, methyl acetate and water which is
discharged from the top of the column is supplied to the separation
membrane, which is capable of separating water.
[0034] The larger the carboxylic acid production plant, the greater
the amount of a mixture that has to be separated. In such a case,
the mixture should be fed into a small distillation column
beforehand to produce an overhead component having a reduced acetic
acid content. By separating such an overhead component with a
separation membrane, it is possible to reduce the energy required
for separation.
[0035] According to the 13.sup.th invention, in the fifth
invention, part of the mixture discharged from the top of the
column is returned into the distillation column, and part of it is
supplied to the separation membrane.
[0036] By returning part of the overhead component, its acetic acid
content further decreases. Thus, by separating it through the
separation membrane, it is possible to further reduce the energy
for separation.
[0037] According to the 14.sup.th invention, in the fifth or sixth
invention, the nonpermeable substances are returned to the
oxidation reaction step. Thus, the methyl acetate contained in the
nonpermeable substances can be recovered in the oxidation reaction
step. This suppresses the production of methyl acetate during the
equilibrium reaction of acetic acid, thereby reducing the loss of
the solvent.
[0038] According to the 15.sup.th invention, the permeable gas
mainly comprising water is further separated into a permeable gas
mainly comprising water and nonpermeable substances mainly
comprising acetic acid, using a separation membrane capable of
separating water.
[0039] By providing the two separation membranes, the second
separation membrane removes any organic components that have not
been removed by the first separation membrane. High-purity water is
thus obtained.
[0040] According to the 16.sup.th invention, in the eighth
invention, one of the separation membranes that is provided
upstream from the other is one that is higher in the permeating
speed, and the other is one that is higher in the separation
ability. With this arrangement, it is possible to separate the
mixture of water and acetic acid in large amounts and with high
purity.
[0041] According to the 17.sup.th invention, in the first
invention, the separation membrane is made of an inorganic
material. Since the membrane is formed of an inorganic material, it
is durable and high in the ability to separate water.
[0042] According to the 18.sup.th invention, in the ninth
invention, the separation membrane or the separation membranes
comprise an inorganic porous member carrying in pores thereof a
silica gel obtained by hydrolyzing an alkoxysilane containing
ethoxy groups or methoxy groups.
[0043] By using silica gel, it is possible to obtain water higher
in purity.
[0044] According to the 19.sup.th invention, the alkyl aromatic
compound is paraxylene, and the aromatic carboxylic acid is
terephthalic acid. Today, among aromatic carboxylic acids,
terephthalic acid is being produced in the greatest amount. Thus,
plants for manufacturing terephthalic acid are increasing in size
year after year. The present invention is most advantageously
applicable to such plants.
[0045] According to the present invention, it is possible to reduce
the size of systems for recovering solvents such as distillation
columns and to reduce the energy consumed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a diagrammatic view of a separation system of a
first embodiment according to the present invention for separating
a solution mixture,
[0047] FIG. 2 is a diagrammatic view of similar separation system
of a second embodiment of the present invention,
[0048] FIG. 3 is a diagrammatic view of a third embodiment of the
present invention, which is a reactor system,
[0049] FIGS. 4A-4C are diagrammatic views of modified examples of
the third embodiment, and
[0050] FIG. 5 schematically shows a separation membrane.
BEST MODE FOR EMBODYING THE INVENTION
[0051] The aromatic alkyl compound used in the invention is an
alkyl benzene such as mono, di or trialkyl benzene, which is to be
converted to an aromatic carboxylic acid such as aromatic
monocarboxylic acid, aromatic dicarboxylic acid or aromatic
tricarboxylic acid by liquid-phase oxidation, and include alkyl
benzenes having their alkyl groups partially oxidized. The present
invention is applicable especially to the production of a
terephthalic acid. The aromatic alkyl compound used in the
invention as the preferable raw material is a paraxylene.
[0052] Now description is made on how a terephthalic acid is
produced by oxidizing a paraxylene.
[0053] Acetic acid as the solvent used in the invention is used in
an amount 2 to 6 times in weight of the paraxylene, the raw
material. This acetic acid solvent may contain a small amount, i.e.
not more than 15 percent by weight, of water.
[0054] In order to oxidize the paraxylene in a liquid phase, a gas
containing molecular oxygen is used, which is typically air because
air can be used at a low cost with a simple facility. Air may be
diluted or enriched with oxygen.
[0055] In oxidizing the paraxylene, a catalyst containing cobalt
(Co), manganese (Mn) and bromine (Br) as its constituent elements
is usually used.
[0056] The paraxylene is oxidized in a liquid phase by continuously
supplying a gas containing molecular oxygen at 140-230 degrees
Celsius, preferably 150-210 degrees Celsius in the presence of a
catalyst in the acetic acid solvent. The pressure during the
oxidation step has to be at least high enough for the mixture to be
capable of maintaining a liquid phase at the reaction temperature,
and is typically in the range of 0.2 to 5 MPa, preferably 1 to 2
MPa.
[0057] The reactor is typically a tank having an agitator. But an
agitator is not an essential element. For example, it may be a
bubble tower. It has a port for a molecular oxygen-containing gas
at its lower portion.
[0058] A molecular oxygen-containing gas that has been supplied
into the reactor through its port is used for oxidation, and leaves
the reactor as an exhaust gas carrying a large amount of solvent
vapor.
[0059] The reaction product obtained in the reaction step is formed
into a reaction slurry, of which the temperature and the pressure
is reduced to suitable levels in a crystallization step to obtain a
terephthalic acid slurry. The crystallization is carried out in 1
to 6 steps, preferably in 2 to 4 steps. In the crystallization
step, flush cooling is employed. The final step is preferably
carried out in a reduced-pressure, boiled state.
[0060] Typically, the slurry that has been subjected to
crystallization is subjected to a solid-liquid separation step and
a drying step to recover a terephthalic acid. The terephthalic acid
thus recovered may consist of a low-purity terephthalic acid and a
medium-purity terephthalic acid. But the low-purity terephthalic
acid may be further refined to a high-purity terephthalic acid. In
this case, after replacing the acetic acid solvent with an aqueous
solvent in a solvent replacement step, the low-purity terephthalic
acid may be directly sent to the refining step, while bypassing the
drying step.
[0061] In the solvent recovering step, the mother liquid that has
been aliquoted in the solid-liquid separation step, and oxidized
exhaust gas or its condensate are refined, typically distilled, to
obtain acetic acid.
[0062] Using a separation membrane having preference for water, at
least part of a mixture of acetic acid, methyl acetate byproducts
and reaction water produced in the oxidation step, solid-liquid
separation step and solvent recovering step is separated into a
permeable gas of which the major component is water and a
nonpermeable substances of which acetic acid and methyl acetate are
main components to recover methyl acetate and acetic acid.
[0063] Separation membranes 8, 46, 84 and 86 (FIGS. 1-3) according
to the present invention, which have preference for water, are
membranes that preferentially pass water (H.sub.2O) molecules. That
is, when a mixture of water and organic compounds is fed through
such membranes, water can more easily permeate them than the
organic compound molecules. More specifically, these separation
membranes have a vapor-acetic acid separation coefficient .alpha.
in the range of about 200 to 500 if the vapor concentration is 20
to 40 weight percent. (The separation coefficient .alpha. is
expressed by .alpha.={(1-Y)/Y}/{(1-X)/X}, where X is the molar
fraction of the nonpermeative components before permeating the
membrane, and Y is the molar fraction of the nonpermeative
components after permeating the membrane.)
[0064] Such separation membranes can separate e.g. a mixture of
acetic acid and water into a permeable gas consisting essentially
of aqueous substances of which the major component is water and
nonpermeable substances mainly comprising nonaqueous substances
such as acetic acid are main components.
[0065] In the preferred embodiment, mixtures fed through separation
membranes are gases. Such gases include gases discharged from the
oxidation reactor and gases obtained during
dehydration/distillation treatment in the solvent recovering step.
Such gases further include gases obtained by directly supplying
exhaust gases from the oxidation reactor directly into a
distillation column and dehydrating them, and gases obtained by
cooling, while releasing pressure, a condensate obtained by
condensing at least part of the abovementioned gases.
[0066] By feeding such a gas mixture of acetic acid and water
through one of the separation membranes of the invention, the gas
mixture can be separated into a permeable gas consisting
essentially of aqueous substances and a nonpermeable gas consisting
essentially of nonaqueous substances.
[0067] If such a gas mixture contains an organic compound or an
inert gas in addition to acetic acid and water, when it permeates
one of the separation membranes of the invention, such an organic
compound or inert gas will preferentially remain in the
nonpermeable substances, which consist essentially of nonaqueous
substances, because such an organic compound or inert gas is less
permeable through the membrane.
[0068] Any of the separation membranes according to the present
invention, which is designated by 111 in FIG. 5, is made of an
inorganic material. Specifically, it comprises an inorganic porous
member 112 such as a porous ceramic substrate, about 1 mm thick,
and a silica gel carrying layer 113 comprising a silica gel
membrane about 10 micrometers thick. The separation membrane may be
a flat, tubular or otherwise shaped member. The silica gel is not
limited but is preferably one obtained by hydrolyzing an
alkoxysilane containing ethoxy groups or methoxy groups because it
improves preference for water.
[0069] To such a separation membrane, water (H.sub.2O) is
preferentially adsorbed by --OH groups in the silica gel carrying
layer 113, thereby inhibiting other components from finding their
way into the pores of the silica gel carrying layer 113. The water
adsorbed by the --OH groups moves in the pores and permeates the
silica gel carrying layer 113. Thus, the --OH groups in the silica
gel carrying layer 1113 serve to selectively separate and remove
water in vapor. As a result, the separation membrane reveals
preference for water.
[0070] Now embodiments of the invention are described. Throughout
the specification, the aqueous component mainly comprising water is
referred to as the "first component", and the nonaqueous component
mainly comprising acetic acid, methyl acetate and the like is
referred to as the "second component".
EMBODIMENT 1
[0071] FIG. 1 shows Embodiment 1 of the present invention, which is
a separation system for separating a solution of a mixture of water
and acetic acid.
[0072] Typically, the separation system of Embodiment 1 is used to
remove water produced by oxidation reaction when producing a
terephthalic acid by oxidizing, in a liquid phase, a paraxylene as
the raw material using air in a reaction solvent containing acetic
acid in the presence of an oxidation catalyst.
[0073] The separation system of Embodiment 1 includes a
distillation column 1 having a plurality of fluidized beds such as
shelves in the interior thereof. To an upper portion of the column,
78 weight percent of a liquid-phase aqueous solution of acetic acid
(liquid-phase feed) is supplied through an upper supply pipe 2, and
to a lower portion of the column, 87 weight percent of an aqueous
solution of acetic acid and a small amount of nitrogen are supplied
as a vapor-phase feed through a lower supply pipe 3. Thus, a
mixture of the first and second components is supplied into the
distillation column 1.
[0074] An overhead vapor supply pipe 4 is connected to the top of
the distillation column 1 such that overhead vapor from the column
1 is introduced into the supply pipe 4. The pipe 4 branches to a
first branch pipe 5 and a second branch pipe 6. In Embodiment 1,
overhead vapor is distributed to the first branch pipe 5 and the
second branch pipe 6 in the ratio of e.g. 9 to 1. A superheater 7
for superheating the overhead vapor is mounted to the downstream
end of the first branch pipe 5. Provided further downstream from
the superheater 7 is a separator 8 including a separation membrane
8a for separating the overhead vapor into a permeable vapor mainly
comprising steam (first component) and nonpermeable substances
mainly comprising acetic acid vapor (second component).
[0075] In this embodiment, the separation membrane 8a is made of an
inorganic material. It passes water or water vapor relatively
freely but scarcely passes acetic acid or acetic acid vapor.
[0076] The second branch pipe 6 includes a reflux unit 9 comprising
a condenser 10 for cooling and liquefying the overhead vapor
flowing into the pipe 6, a gas-liquid separator 11 for separating
the thus cooled overhead vapor into gas and liquid, and a
liquid-phase pump 13 for returning the separated liquid into the
distillation column 1 through a return pipe 12. Gas separated in
the gas-liquid separator 11 is discharged through a discharge pipe
14.
[0077] The vapor that has permeated the separation membrane 8a of
the separator 8 flows into a vapor introducing pipe 15 connected to
the separator 8, and is cooled and liquefied in a condenser 16
provided in the pipe 16. The thus cooled vapor is separated into
gas and liquid in a gas-liquid separator 17 connected to the pipe
15. The gas separated in the separator 17 is discharged through a
discharge pipe 18 and a vacuum pump 20 into a gas discharge pipe
21. The liquid separated in the separator 17 is discharged through
a discharge pipe 19 and a liquid-phase pump 22 into a liquid
discharge pipe 23.
[0078] Nonpermeable vapor that has not permeated the separation
membrane 8a of the separator 8 flows into a nonpermeable vapor
introducing pipe 24 connected to the separator 8, and is cooled and
liquefied in a condenser 25 provided in the pipe 24. The thus
cooled vapor is separated into gas and liquid in a gas-liquid
separator 26 connected to the pipe 24. Gas separated in the
separator 26 is discharged through a discharge pipe 27 and a
pressure valve 28 into the vacuum pump 20 and the gas discharge
pipe 21. Liquid separated in the separator 26 is discharged through
a liquid-phase pump 29 and a first acetic acid discharge pipe
30.
[0079] To the bottom of the distillation column 1, a second acetic
acid discharge pipe 31 is connected through which the bottom layer
of the liquid in the distillation column 1, which is high in the
acetic acid concentration, is discharged. Part of the liquid
flowing into the pipe 31 flows into a circulating pipe 33 extending
from an intermediate portion of the pipe 31 to the distillation
column 1, is reheated in a reboiler 32 provided in the pipe 33, and
is returned into the distillation column 1.
[0080] Separation steps performed by the separating system of
Embodiment 1 are now described.
[0081] First, the liquid-phase feed A and the vapor-phase feed B
are fed through the upper supply pipe 2 and lower supply pipe 3,
respectively, into the distillation column 1. Since the
liquid-phase feed A falls in the distillation column 1 and the
vapor-phase feed B rises in the column 1, they will contact each
other in the column 1. Part of the liquid discharged through the
second acetic acid discharge pipe 31 is heated by the reboiler 32
and returned into the distillation column 1 near its bottom through
the circulating pipe 33.
[0082] The liquid-phase feed A is a liquid substance of which
primary components are water and acetic acid. The vapor-phase feed
B is a gaseous substance or a liquid substance that is gasified in
the column 1 and primarily comprises water and acetic acid. The
vapor-phase feed B also includes a substance that remains liquid in
the column but is gasified when heated by the reboiler 32 or the
reboiler 74 in Embodiment 2.
[0083] When the feeds A and B are fed into the column, they will be
mixed together such that water concentration is higher near the top
of the distillation column 1 and the acetic acid concentration is
higher near the bottom of the column 1.
[0084] Overhead vapor (of which the water concentration is
relatively high) flows into the overhead vapor introducing pipe 4,
and is then distributed into the first branch pipe 5 and the second
branch pipe 6 in the proportion of 9 to 1.
[0085] Overhead vapor that has been introduced into the second
branch pipe 6 is returned into the distillation column 1 by the
reflux unit 9. The vapor thus returned into the column 1 further
increases the water concentration and thus reduces the acetic acid
concentration near the top of the column 1.
[0086] Overhead vapor introduced into the first branch pipe 5 is
superheated in the superheater 7 (to prevent the overhead vapor
from being liquefied before reaching the separation membrane 8a),
and introduced into the separator 8.
[0087] The thus superheated overhead vapor is separated into a
permeable vapor mainly comprising water and a nonpermeable vapor
mainly comprising acetic acid.
[0088] The reflux unit 9 serves to reduce the acetic acid
concentration of the overhead vapor introduced into the first
branch pipe 5 to a substantially constant value (about 62 weight
percent). The acetic acid concentration of the permeable vapor that
has permeated the separation membrane 8a thus decreases to less
than 1 weight percent.
[0089] The permeable vapor is cooled in the condenser 16 and mostly
liquefied. After removing nitrogen gas and other gases mixed in the
liquid in the gas-liquid separator 17, the liquid is fed by the
liquid-phase pump 22 and recovered.
[0090] The nonpermeable vapor is cooled in the condenser 25 and
mostly liquefied. After removing nitrogen gas and other gases mixed
in the liquid in the gas-liquid separator 26, the liquid is fed by
the liquid-phase pump 29 and recovered.
[0091] Gaseous components removed from the vapor in the gas-liquid
separators 17 and 26 are sucked by the vacuum pump 20 and
discharged from the system. The pressure valve 28 prevents the
nonpermeable vapor from flowing toward the gas-liquid separator 17
even if the nonpermeable vapor pressure is higher than the
permeable vapor pressure.
[0092] The permeable vapor thus produces water containing not more
than 1 weight percent of acetic acid, while the nonpermeable vapor
produces a liquid comprising not less than 93 weight percent of
acetic acid. The liquid produced from the permeable vapor is useful
in the plant. Or even if it is discarded, it will not contaminate
the environment because it is practically pure water. The
nonpermeable vapor and the liquid discharged from the lower portion
of the distillation column 1 have enough purity as solvents to be
used in the process. When discharged from the top of the
distillation column 1, the overhead vapor contains methyl acetate,
which is a byproduct produced during oxidation reaction. It is
separated as the nonpermeable vapor together with acetic acid in
the separator 8. The nonpermeable gas, which contains acetic acid
and methyl acetate, is recovered and reused in the oxidation step.
This reduces the consumption of acetic acid.
[0093] In Embodiment 1, the reflux unit 9 is provided to reduce the
acetic acid concentration of the overhead vapor. Thus, separation
of the overhead vapor can be carried out so as to meet the
requirements of the user according to the separation capability of
the separation membrane 8a. The separated liquid needs not be
returned into the distillation column 1. This makes it possible to
use a smaller distillation column 1 and saves energy.
[0094] In Embodiment 1, both the liquid-phase feed and the
vapor-phase feed are supplied into the distillation column 1. But
only one of them may be supplied.
[0095] In Embodiment 1, a mixed solution is distilled in the
distillation column 1. But if it is desired to reduce the size of
the entire separation system, the distillation column 1 may be
replaced with an evaporating can.
EMBODIMENT 2
[0096] FIG. 2 shows Embodiment 2 of the present invention, which is
a separation system for separating a solution of a mixture of water
and acetic acid.
[0097] Like the system of Embodiment 1, the system of Embodiment 2
is typically used to remove water produced by oxidation reaction
when producing a terephthalic acid by oxidizing, in a liquid phase,
a paraxylene as the raw material using air in a reaction solvent
containing acetic acid in the presence of an oxidation
catalyst.
[0098] Like the system of Embodiment 1, the system of Embodiment 2
includes a distillation column 41 having a plurality of fluid beds
such as shelves in the interior thereof. To an upper portion of the
column, 78 weight percent of a liquid-phase aqueous solution of
acetic acid (liquid-phase feed A) is supplied through an upper
supply pipe 42, and to a lower portion of the column, 87 weight
percent of an aqueous solution of acetic acid and a small amount of
nitrogen are supplied as a vapor-phase feed B through a lower
supply pipe 43.
[0099] To the top of the distillation column 41, an overhead vapor
introducing pipe 44 is connected into which overhead vapor
spontaneously flows from the top of the distillation column 41. To
the downstream end of the pipe 44, a superheater 45 for
superheating the overhead vapor is connected. Downstream from the
superheater 45, a first separator 46 is provided which includes a
first separation membrane 46a for separating the overhead vapor
into a first permeable vapor mainly comprising steam and a first
nonpermeable vapor mainly comprising acetic acid vapor.
[0100] The first separation membrane 46a is identical to the
separation membrane 8a of Embodiment 1.
[0101] The first permeable vapor, which has permeated the first
separation membrane 46a, is introduced into a first permeable gas
introducing pipe 47 connected to the first separator 46. The first
permeable gas introducing pipe 47 is provided with a second
separator 48 including a second separation membrane 48a for
separating the first permeable vapor into a second permeable vapor
containing, as its major component, a first component of the first
permeable vapor in a higher concentration than the first permeable
vapor, and into a second nonpermeable vapor containing, as its
major component, a second component of the first permeable vapor.
The second separation membrane 48a is identical to the first
separation membrane 46a. A superheater (not shown) may be provided
between the first separator 46 and the second separator 48 to
superheat the first permeable vapor.
[0102] The first nonpermeable vapor, i.e. the vapor that has not
permeated the first separation membrane 46a of the first separator
46, is introduced into a first nonpermeable vapor introducing pipe
49 connected to the first separator 46. The pipe 49 is provided
with a condenser 50 for cooling and liquefying the first
nonpermeable vapor. The thus liquefied first nonpermeable gas is
then separated into gas and liquid in a gas-liquid separator 51
connected to the pipe 49. The gas separated in the separator 51 is
introduced into a discharge pipe 52 connected to the separator 51,
while the liquid separated in the separator 51 is introduced into a
discharge pipe 53 connected to the separator 51. The discharge pipe
52 is connected through a pressure valve 54 to a vacuum pump 55 and
a gas discharge pipe 56. The discharge pipe 53 is connected through
a first liquid-phase pump 57 and a second liquid-phase pump 58 to a
first acetic acid discharge pipe 70.
[0103] The second nonpermeable vapor, i.e. the vapor that has not
permeated the second separation membrane 48a of the second
separator 48, is introduced into a second nonpermeable vapor
introducing pipe 59 connected to the second separator 48. The pipe
59 is provided with a condenser 60 for cooling and liquefying the
second nonpermeable vapor. The thus cooled and liquefied vapor is
separated into gas and liquid in a gas-liquid separator 61
connected to the pipe 59. Gas separated in the separator 61 flows
into a discharge pipe 62 connected to the separator 61, while
liquid separated in the separator 61 flows into a discharge pipe 63
connected to the separator 61. The discharge pipe 62 is connected
through a pressure valve 64 to the vacuum pump 55. The discharge
pipe 63 is connected to the second liquid-phase pump 58.
[0104] The second permeable vapor, i.e. the vapor that has
permeated the second separation membrane 48a of the second
separator 48, is introduced into a second permeable vapor
introducing pipe 65 connected to the separator 48, and is cooled
and liquefied in a condenser 66 provided in the pipe 65. The thus
cooled and liquefied second permeable vapor is separated into gas
and liquid in a gas-liquid separator 67 connected to the pipe 65.
Gas separated in the separator 67 is introduced into the discharge
pipe 68 connected to the separator 67, while liquid separated in
the separator 67 flows into a discharge pipe 69 connected to the
separator 67. The discharge pipe 68 connects to the vacuum pump 55.
The discharge pipe 69 connects to a water discharge pipe 72 through
a third liquid-phase pump 71.
[0105] To the bottom of the distillation column 41, a second acetic
acid discharge pipe 73 is connected through which the lowermost
layer of the liquid in the distillation column 41, which is high in
the acetic acid concentration, is discharged. Part of the liquid
flowing into the pipe 73 flows into a circulating pipe 75 extending
from an intermediate portion of the pipe 73 to the distillation
column 1, is reheated in a reboiler 74 provided in the pipe 75, and
returned into the distillation column 41.
[0106] Now the operation of the system of Embodiment 2 is
described.
[0107] First, the liquid-phase feed and the vapor-phase feed are
fed through the upper supply pipe 42 and lower supply pipe 43,
respectively, into the distillation column 41. Since the
liquid-phase feed falls in the distillation column 41, and the
vapor-phase feed rises in the column 41, they contact each other in
the column 41. Part of the liquid discharged through the second
acetic acid discharge pipe 73 is heated by the reboiler 74 and
returned into the distillation column 41 near its bottom through
the circulating pipe 75.
[0108] When the feeds are fed into the column, they are distributed
in the column such that the water concentration is higher near the
top of the distillation column 41 and the acetic acid concentration
is higher near its bottom.
[0109] Overhead vapor (of which the water concentration is
relatively high) flows into the overhead vapor introducing pipe 44,
is superheated in the superheater 45 (to prevent the overhead vapor
from being liquefied before reaching the separation membrane 46a),
and is introduced into the first separator 46.
[0110] The thus superheated overhead vapor is separated into a
first permeable vapor mainly comprising water and a first
nonpermeable vapor mainly comprising acetic acid.
[0111] In Embodiment 2, in order to increase the water
concentration of the overhead vapor to a certain extent, the
liquid-phase feed is introduced into the upper portion of the
distillation column 41. Still, the acetic acid concentration of the
first permeable vapor is sufficiently high (about 68 percent by
weight).
[0112] The first permeable vapor is introduced into the second
separator 48, and separated into a second permeable vapor mainly
comprising water, and a second nonpermeable vapor mainly comprising
acetic acid.
[0113] Since the acetic acid concentration of the first permeable
vapor is about 5 percent by weight as described above, the acetic
acid content of the vapor that has permeated the second separation
membrane 48a decreases to less than 1 percent by weight.
[0114] The second permeable vapor is cooled in the condenser 66 and
mostly liquefied. After removing nitrogen gas and other gases mixed
in the liquid in the gas-liquid separator 67, the liquid is fed by
the third liquid-phase pump 71 and recovered.
[0115] The first separation membrane 46a of the first separator 46
and the second separation membrane 48a of the second separator 48
are both capable of separating water. In order to ensure both high
separation speed and high separation ability, the first separation
membrane 46a is preferably one which is high in permeating speed,
while the second separation membrane 48a is preferably one high in
separation ability.
[0116] The second nonpermeable vapor is cooled in the condenser 60
and mostly liquefied. After removing nitrogen gas and other gases
mixed in the liquid in the gas-liquid separator 61, the liquid is
fed by the second liquid-phase pump 58 and recovered.
[0117] The first nonpermeable vapor, which has not permeated the
first separator 46, is cooled in the condenser 50 and mostly
liquefied. After removing nitrogen gas and other gases mixed in the
liquid in the gas-liquid separator 51, the liquid is fed by the
first and second liquid-phase pumps 57 and 58 and recovered.
[0118] The second permeable vapor thus produces water containing
not more than 1 percent by weight of acetic acid, while the first
nonpermeable vapor and the second nonpermeable vapor produce a
liquid containing 95 percent by weight of acetic acid. A liquid
discharged from the bottom of the distillation column 41 contains
98 percent by weight of acetic acid. The liquid produced from the
second permeable vapor is useful in the plant. Or even if it is
discarded, it will not contaminate the environment because it is
practically pure water. The first and second nonpermeable vapor and
the liquid discharged from the bottom of the distillation column 41
have enough purity as solvents to be used in the process. The
overhead vapor discharged from the top of the distillation column
41 contains methyl acetate, which is produced during oxidation
reaction. It is separated as the nonpermeable vapor together with
acetic acid in the first and second separators 46 and 48. The
nonpermeable vapors, which contain acetic acid and methyl acetate,
are recovered and reused in the oxidation step. This reduces the
consumption of acetic acid.
[0119] In Embodiment 2, the first and second separators 46 and 48
are provided to separate the overhead vapor in the first separator
46, and then re-separate the first permeable gas, i.e. vapor that
has permeated the first separation membrane 46a with the second
separation membrane 48a to obtain the second permeable vapor. Thus,
the liquid obtained from the second permeable vapor has improved
purity. The liquids obtained from the first nonpermeable vapor,
i.e. vapor that has not permeated the first separation membrane
46a, and the second nonpermeable vapor, i.e. vapor that has not
permeated the second separation membrane 48a, also have enough
purity. Since the liquids produced from these separated vapors are
all high in purity, they do not have to be returned into the
distillation column 41. This makes it possible to use a smaller
distillation column 41 and save energy.
[0120] In Embodiment 2, both the liquid-phase feed and the
vapor-phase feed are supplied into the distillation column 41. But
only the liquid-phase feed may be supplied.
[0121] In Embodiment 2, a mixed solution is distilled in the
distillation column 41. But if it is desired to reduce the size of
the entire separation system, the distillation column 41 may be
replaced with an evaporating can.
[0122] In Embodiment 2, the first permeable vapor, i.e. the vapor
that has permeated the first separation membrane 46a, is
re-separated with the second separation membrane 48a. But according
to the capacity of the separation membranes, the concentration of
the mixed solution and other conditions, a different arrangement
may be employed. For example, the first nonpermeable vapor, i.e.
the vapor that has not permeated the first separation membrane 46a,
may be re-separated with the second separation membrane 48a.
[0123] In Embodiment 2, too, the reflux unit 9 of Embodiment 1 may
be used.
EMBODIMENT 3
[0124] FIG. 3 shows Embodiment 3, which is a reactor system for
synthesizing terephthalic acid according to the present
invention.
[0125] This reactor system includes a reactor 81 filled with an
oxidation catalyst for paraxylene (such as a cobalt compound). To
the reactor 81, a raw material supply pipe 82 is connected through
which paraxylene as the raw material, an acetic acid solvent, and
an oxidation catalyst are supplied into the reactor. To the top of
the reactor 81, a reaction vapor discharge pipe 83 is connected
through which reaction vapor produced in the reactor is discharged.
Air as an oxidant is supplied into the reactor 81 through an
oxidant supply pipe 101.
[0126] To the reaction vapor discharge pipe 83, a first separator
84 including a first separation membrane 84a is connected. The
first separation membrane 84a is identical to the separation
membrane 8a of Embodiment 1. Thus, it passes a first component
which mainly comprises steam, but does not pass a second component
containing acetic acid vapor and other organic components.
[0127] The interior of the reactor 81 is kept at 1-2 MPa and at a
temperature of 100 to 200 degrees Celsius. The vapor supplied
through the reaction vapor discharge pipe 83 to the first
separation membrane 84a contains steam, acetic acid vapor and vapor
of other organic components, besides gaseous components derived
from the air supplied and gaseous components produced during the
reaction.
[0128] First permeable vapor, i.e. the vapor that has permeated the
first separation membrane 84a of the first separator 84 (of which
the major component is water), is introduced into a first permeable
vapor introducing pipe 85 connected to the separator 84. To the
first permeable vapor introducing pipe 85, a second separator 86
including a second separator membrane 86a is connected. The second
separation membrane 86a is identical to the first separation
membrane 84a. A superheater (not shown) may be provided between the
first and second separators 84 and 86 to superheat the first
permeable vapor. The second separator 86 separates the first
permeable vapor into a second nonpermeable vapor comprising a
nonaqueous substance of which the major component is the solvent in
the first permeable vapor and a second permeable vapor mainly
comprising water.
[0129] First nonpermeable vapor, i.e. the vapor that has not
permeated the first separation membrane 84a of the first separator
84 (of which main components are acetic acid solvent, other organic
components and gas components derived from the air supplied and
reaction gas components) is introduced into a first nonpermeable
vapor introducing pipe 87 connected to the separator 84. The pipe
87 is provided with a condenser 88 and a pressure regulating valve
89.
[0130] Second permeable vapor, i.e. the vapor that has permeated
the second separation membrane 86a of the second separator 86 (of
which the major component is water) is introduced into a second
permeable vapor introducing pipe 90 connected to the separator 86.
The pipe 90 is provided with a condenser 91 and a liquid-phase pump
92.
[0131] Second nonpermeable vapor, i.e. the vapor that has not
permeated the second separation membrane 86a of the second
separator 86 (of which main components are acetic acid solvent,
other organic components and gas components derived from the air
supplied and reaction gas components) is introduced into a second
nonpermeable vapor introducing pipe 93 connected to the separator
86. The pipe 93 is provided with a condenser 94 and a pressure
regulating valve 95.
[0132] The first and second nonpermeable vapor introducing pipes 87
and 93 are connected to gas-liquid separators 96a and 96b,
respectively. Gas components (such as oxygen, nitrogen, carbon
dioxide and carbon monoxide in Embodiment 3) separated in the
separators 96a and 96b are discharged through a gas discharge pipe
97 connected to the separators 96a and 96b, while liquid components
(acetic acid solvent and other organic components) separated in the
separators 96a and 96b are returned into the reactor 81 through a
liquid discharge pipe 98 connected to the separators 96a and 96b.
The first and second nonpermeable vapor introducing pipes 87 and
93, condensers 88 and 94, gas-liquid separators 96a and 96b and
liquid discharge pipe 98 form a return passage. A liquid-phase pump
99 may be provided in the return passage as shown.
[0133] To a lower portion of the reactor 81, a product discharge
pipe 100 is connected through which an acetic acid slurry of
terephthalic acid present in the form of a liquid in the reactor 81
is discharged.
[0134] In Embodiment 3, to the reaction vapor discharge pipe 83,
through which vapor produced in the reactor 81 flows, a
distillation unit 121 may be connected as shown in FIG. 4A. In this
arrangement, vapor discharged from the reactor 81 is supplied into
the distillation unit 121 and distilled therein to recover an
acetic acid component. Also, gas containing a smaller amount of
acetic acid is discharged from the top of the distillation unit
121. The gas thus produced is supplied to the first separator 84.
The acetic acid component recovered from the distillation unit 121
is fed into the liquid discharge pipe 98 and thus returned into the
reactor 81. Thus, the distillation unit 121 serves to reduce the
acetic acid component in the vapor to be supplied to the first
separator 84.
[0135] In Embodiment 3, to the reaction vapor discharge pipe 83,
through which the vapor produced in the reactor 81 flows, a
condenser 122 may be connected as shown in FIG. 4B. The condenser
122 serves to condense any condensable components contained in the
vapor discharged from the reactor 81. Any components that are not
condensed by the condenser 122 are fed into the gas discharge pipe
97.
[0136] Any components condensed in the condenser 122 are at least
partially evaporated in an evaporator 124, and the evaporated
components are fed optionally to the first separator 84 through a
heater. Any components that are not evaporated in the evaporator
124 are fed to the gas-liquid separator 96a. The condenser 122 and
the evaporator 124 make it possible to treat any noncondensable
components contained in the vapor discharged from the reactor 81
while bypassing the first separator 84.
[0137] The evaporator 124 is not limited provided it can at least
partially evaporate the liquid condensed in the condenser 122. For
example, if the liquid condensed in the condenser 122 is under
pressure, the evaporator 124 is preferably a flush tank, a tank
which is kept at a lower pressure than in the condenser 122.
[0138] In Embodiment 3, as shown in FIG. 4C, the distillation unit
121 shown in FIG. 4A may be connected to the reaction vapor
discharge pipe 83, through which the vapor discharged from the
reactor 81 flows, and further the condenser 122 shown in FIG. 4B
may be connected to a discharge pipe extending from the
distillation unit 121. This arrangement has both of the functions
of the arrangements of FIGS. 4A and 4B.
[0139] Now the operation of Embodiment 3 or the reactor system
according to the present invention is described.
[0140] A paraxylene solution is supplied into the reactor 81
through the raw material supply pipe 82 together with an acetic
acid solvent. Air as an oxidant is supplied through the oxidant
supply pipe 101. In the reactor 81, paraxylene is oxidized by the
action of a catalyst, producing terephthalic acid and water.
Through the reaction vapor discharge pipe 83, a vapor mixture
(about 150-200 degrees Celsius) of water, acetic acid solvent,
other organic components, gas components derived from the supplied
air, and reaction gas components is discharged.
[0141] The vapor mixture discharged from the reactor 81 is
introduced into the first separator 84 through the reaction vapor
discharge pipe 83, and is separated by the first separation
membrane 84a into the first permeable vapor, of which the major
component is water, and the first nonpermeable vapor, of which main
components are acetic acid solvent, other organic components,
gaseous components derived from the supplied air, and reaction gas
components.
[0142] In Embodiment 3, as in Embodiment 1, the separation ability
of the first separation membrane 84a is such that the first
permeable vapor inevitably contains certain amounts of acetic acid
solvent, other orgnic compounds, gasesous components derived from
the supplied air, and reaction gas components.
[0143] However, the first permeable vapor is fed to the second
separator 86 through the first permeable vapor introducing pipe 85,
and separated again by the second separation membrane 86a into the
second permeable vapor, of which the major component is water and
the second nonpermeable vapor, of which main components are acetic
acid solvent, other organic components, gaseous components derived
from the supplied air, and reaction gas components. Thus, the
second permeable vapor is practically pure water. The second
permeable vapor is introduced into the second permeable vapor
introducing pipe 90, liquiefied in the condenser 91, fed under
pressure by the liquid-phase pump 92 and recovered in the form of
water.
[0144] The first and second nonpermeable vapors, of which main
components are acetic acid solvent, other organic components,
gaseous components derived from the supplied air, and reaction gas
components, are introduced into the fist and second nonpermeable
vapor introducing pipes 87 and 93, condensed in the condensers 88
and 94, and fed into the gas-liquid separators 96a and 96b,
respectively.
[0145] The first separation membrane 84a of the first separator 84
and the second separation membrane 86a of the second separator 86
are both capable of separating water. In order to ensure both high
separation speed and separation ability, the first separation
membrane 84a is preferably one which is high in permeating speed,
while the second separation membrane 86a is preferably one which is
high in separation ability.
[0146] In the gas-liquid separators 96a and 96b, gas components
mixed in the first nonpermeable vapor and the second nonpermeable
vapor (which mainly comprise gaseous components derived from the
supplied air and reaction gas components) are separated and
discharged through the gas discharge pipe 97. Liquid components
(which mainly comprise acetic acid solvent and other organic
components) are discharged through the liquid discharge pipe 98,
optionally pressurized in the liquid-phase pump 99, and returned
into the reactor 81 (i.e. to the oxidation step) through the raw
material supply pipe 82.
[0147] Terephthalic acid produced in the reaction during the
oxidation step is discharged through the product discharge pipe in
the form of an acetic acid slurry and recovered. It may be refined
to obtain high-purity terephthalic acid.
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